Fibrodysplasia ossificans progressiva (FOP) is characterized by congenital bilateral hallux valgus malformations and early-onset heterotopic ossification, which may be spontaneous or precipitated by trauma including intramuscular vaccinations. Painful, recurrent soft-tissue swellings (flare-ups) may precede localized heterotopic ossification. Heterotopic ossification can occur at any location, but typically affects regions in close proximity to the axial skeleton in the early/mild stages, before progressing to the appendicular skeleton. This can lead to restriction of movement as a result of ossification impacting joint mobility. Problems with swallowing and speaking can occur with ossification affecting the jaw, head, and neck, and restriction of the airway and breathing may lead to thoracic insufficiency syndrome.

Pompe disease is classified by age of onset, organ involvement, severity, and rate of progression. Infantile-onset Pompe disease (IOPD; individuals with onset before age 12 months with cardiomyopathy) may be apparent in utero but more typically onset is at the median age of four months with hypotonia, generalized muscle weakness, feeding difficulties, failure to thrive, respiratory distress, and hypertrophic cardiomyopathy. Without treatment by enzyme replacement therapy (ERT), IOPD commonly results in death by age two years from progressive left ventricular outflow obstruction and respiratory insufficiency. Late-onset Pompe disease (LOPD; including: (a) individuals with onset before age 12 months without cardiomyopathy; and (b) all individuals with onset after age 12 months) is characterized by proximal muscle weakness and respiratory insufficiency; clinically significant cardiac involvement is uncommon.

Idiopathic pulmonary hemosiderosis is a respiratory disease due to repeated episodes of diffuse alveolar hemorrhage without any underlying apparent cause, most often in children. Anemia, cough, and pulmonary infiltrates on chest radiographs are found in majority of the patients.

Leigh syndrome is a clinical diagnosis based primarily on characteristic brain imaging findings associated with progressive and severe neurodegenerative features with onset within the first months or years of life, sometimes resulting in early death. Affected individuals usually show global developmental delay or developmental regression, hypotonia, ataxia, dystonia, and ophthalmologic abnormalities, such as nystagmus or optic atrophy. The neurologic features are associated with the classic findings of T2-weighted hyperintensities in the basal ganglia and/or brainstem on brain imaging. Leigh syndrome can also have detrimental multisystemic affects on the cardiac, hepatic, gastrointestinal, and renal organs. Biochemical studies in patients with Leigh syndrome tend to show increased lactate and abnormalities of mitochondrial oxidative phosphorylation (summary by Lake et al., 2015). Genetic Heterogeneity of Nuclear Leigh Syndrome Leigh syndrome is a presentation of numerous genetic disorders resulting from defects in the mitochondrial OXPHOS complex. Accordingly, the genes implicated in Leigh syndrome most commonly encode structural subunits of the OXPHOS complex or proteins required for their assembly, stability, and activity. Mutations in both nuclear and mitochondrial genes have been identified. For a discussion of genetic heterogeneity of mitochondrial Leigh syndrome, see MILS (500017). Nuclear Leigh syndrome can be caused by mutations in nuclear-encoded genes involved in any of the mitochondrial respiratory chain complexes: complex I deficiency (see 252010), complex II deficiency (see 252011), complex III deficiency (see 124000), complex IV deficiency (cytochrome c oxidase; see 220110), and complex V deficiency (see 604273) (summary by Lake et al., 2015). Some forms of combined oxidative phosphorylation deficiency (COXPD) can present as Leigh syndrome (see, e.g., 617664). Leigh syndrome may also be caused by mutations in components of the pyruvate dehydrogenase complex (e.g., DLD, 238331 and PDHA1, 300502). Deficiency of coenzyme Q10 (607426) can present as Leigh syndrome.

Cerebrotendinous xanthomatosis (CTX) is a lipid storage disease characterized by infantile-onset diarrhea, childhood-onset cataract, adolescent- to young adult-onset tendon xanthomas, and adult-onset progressive neurologic dysfunction (dementia, psychiatric disturbances, pyramidal and/or cerebellar signs, dystonia, atypical parkinsonism, peripheral neuropathy, and seizures). Chronic diarrhea from infancy and/or neonatal cholestasis may be the earliest clinical manifestation. In approximately 75% of affected individuals, cataracts are the first finding, often appearing in the first decade of life. Xanthomas appear in the second or third decade; they occur on the Achilles tendon, the extensor tendons of the elbow and hand, the patellar tendon, and the neck tendons. Xanthomas have been reported in the lung, bones, and central nervous system. Some individuals show cognitive impairment from early infancy, whereas the majority have normal or only slightly impaired intellectual function until puberty; dementia with slow deterioration in intellectual abilities occurs in the third decade in more than 50% of individuals. Neuropsychiatric symptoms such as behavioral changes, hallucinations, agitation, aggression, depression, and suicide attempts may be prominent. Pyramidal signs (i.e., spasticity) and/or cerebellar signs almost invariably become evident between ages 20 and 30 years. The biochemical abnormalities that distinguish CTX from other conditions with xanthomas include high plasma and tissue cholestanol concentration, normal-to-low plasma cholesterol concentration, decreased chenodeoxycholic acid (CDCA), increased concentration of bile alcohols and their glyconjugates, and increased concentrations of cholestanol and apolipoprotein B in cerebrospinal fluid.

Clinical features of achondrogenesis type 1B (ACG1B) include extremely short limbs with short fingers and toes, hypoplasia of the thorax, protuberant abdomen, and hydropic fetal appearance caused by the abundance of soft tissue relative to the short skeleton. The face is flat, the neck is short, and the soft tissue of the neck may be thickened. Death occurs prenatally or shortly after birth.

The autosomal dominant TRPV4 disorders (previously considered to be clinically distinct phenotypes before their molecular basis was discovered) are now grouped into neuromuscular disorders and skeletal dysplasias; however, the overlap within each group is considerable. Affected individuals typically have either neuromuscular or skeletal manifestations alone, and in only rare instances an overlap syndrome has been reported. The three autosomal dominant neuromuscular disorders (mildest to most severe) are: Charcot-Marie-Tooth disease type 2C. Scapuloperoneal spinal muscular atrophy. Congenital distal spinal muscular atrophy. The autosomal dominant neuromuscular disorders are characterized by a congenital-onset, static, or later-onset progressive peripheral neuropathy with variable combinations of laryngeal dysfunction (i.e., vocal fold paresis), respiratory dysfunction, and joint contractures. The six autosomal dominant skeletal dysplasias (mildest to most severe) are: Familial digital arthropathy-brachydactyly. Autosomal dominant brachyolmia. Spondylometaphyseal dysplasia, Kozlowski type. Spondyloepiphyseal dysplasia, Maroteaux type. Parastremmatic dysplasia. Metatropic dysplasia. The skeletal dysplasia is characterized by brachydactyly (in all 6); the five that are more severe have short stature that varies from mild to severe with progressive spinal deformity and involvement of the long bones and pelvis. In the mildest of the autosomal dominant TRPV4 disorders life span is normal; in the most severe it is shortened. Bilateral progressive sensorineural hearing loss (SNHL) can occur with both autosomal dominant neuromuscular disorders and skeletal dysplasias.

The spectrum of ASAH1-related disorders ranges from Farber disease (FD) to spinal muscular atrophy with progressive myoclonic epilepsy (SMA-PME). Classic FD is characterized by onset in the first weeks of life of painful, progressive deformity of the major joints; palpable subcutaneous nodules of joints and mechanical pressure points; and a hoarse cry resulting from granulomas of the larynx and epiglottis. Life expectancy is usually less than two years. In the other less common types of FD, onset, severity, and primary manifestations vary. SMA-PME is characterized by early-childhood-onset progressive lower motor neuron disease manifest typically between ages three and seven years as proximal lower-extremity weakness, followed by progressive myoclonic and atonic seizures, tremulousness/tremor, and sensorineural hearing loss. Myoclonic epilepsy typically begins in late childhood after the onset of weakness and can include jerking of the upper limbs, action myoclonus, myoclonic status, and eyelid myoclonus. Other findings include generalized tremor, and cognitive decline. The time from disease onset to death from respiratory complications is usually five to 15 years.

PLOD1-related kyphoscoliotic Ehlers-Danlos syndrome (kEDS) is an autosomal recessive generalized connective tissue disorder characterized by hypotonia, early-onset kyphoscoliosis, and generalized joint hypermobility in association with skin fragility and ocular abnormality. Intelligence is normal. Life span may be normal, but affected individuals are at risk for rupture of medium-sized arteries. Adults with severe kyphoscoliosis are at risk for complications from restrictive lung disease, recurrent pneumonia, and cardiac failure.

COL1A1/2 osteogenesis imperfecta (COL1A1/2-OI) is characterized by fractures with minimal or absent trauma, variable dentinogenesis imperfecta (DI), and, in adult years, hearing loss. The clinical features of COL1A1/2-OI represent a continuum ranging from perinatal lethality to individuals with severe skeletal deformities, mobility impairments, and very short stature to nearly asymptomatic individuals with a mild predisposition to fractures, normal dentition, normal stature, and normal life span. Fractures can occur in any bone but are most common in the extremities. DI is characterized by gray or brown teeth that may appear translucent, wear down, and break easily. COL1A1/2-OI has been classified into four types based on clinical presentation and radiographic findings. This classification system can be helpful in providing information about prognosis and management for a given individual. The four more common OI types are now referred to as follows: Classic non-deforming OI with blue sclerae (previously OI type I). Perinatally lethal OI (previously OI type II). Progressively deforming OI (previously OI type III). Common variable OI with normal sclerae (previously OI type IV).

Lysinuric protein intolerance (LPI) typically presents after an infant is weaned from breast milk or formula; variable findings include recurrent vomiting and episodes of diarrhea, episodes of stupor and coma after a protein-rich meal, poor feeding, aversion to protein-rich food, failure to thrive, hepatosplenomegaly, and muscular hypotonia. Over time, findings include: poor growth, osteoporosis, involvement of the lungs (progressive interstitial changes, pulmonary alveolar proteinosis) and of the kidneys (progressive glomerular and proximal tubular disease), hematologic abnormalities (normochromic or hypochromic anemia, leukopenia, thrombocytopenia, erythroblastophagocytosis in the bone marrow aspirate), and a clinical presentation resembling the hemophagocytic lymphohistiocytosis/macrophagic activation syndrome. Hypercholesterolemia, hypertriglyceridemia, and acute pancreatitis can also be seen.

Fukuyama congenital muscular dystrophy (FCMD) is characterized by hypotonia, symmetric generalized muscle weakness, and CNS migration disturbances that result in changes consistent with cobblestone lissencephaly with cerebral and cerebellar cortical dysplasia. Mild, typical, and severe phenotypes are recognized. Onset typically occurs in early infancy with poor suck, weak cry, and floppiness. Affected individuals have contractures of the hips, knees, and interphalangeal joints. Later features include myopathic facial appearance, pseudohypertrophy of the calves and forearms, motor and speech delays, intellectual disability, seizures, ophthalmologic abnormalities including visual impairment and retinal dysplasia, and progressive cardiac involvement after age ten years. Swallowing disturbance occurs in individuals with severe FCMD and in individuals older than age ten years, leading to recurrent aspiration pneumonia and death.

Collagen VI-related dystrophies (COL6-RDs) represent a continuum of overlapping clinical phenotypes with Bethlem muscular dystrophy at the milder end, Ullrich congenital muscular dystrophy (UCMD) at the more severe end, and a phenotype in between UCMD and Bethlem muscular dystrophy, referred to as intermediate COL6-RD. Bethlem muscular dystrophy is characterized by a combination of proximal muscle weakness and joint contractures. Hypotonia and delayed motor milestones occur in early childhood; mild hypotonia and weakness may be present congenitally. By adulthood, there is evidence of proximal weakness and contractures of the elbows, Achilles tendons, and long finger flexors. The progression of weakness is slow, and more than two thirds of affected individuals older than age 50 years remain independently ambulatory indoors, while relying on supportive means for mobility outdoors. Respiratory involvement is not a consistent feature. UCMD is characterized by congenital weakness, hypotonia, proximal joint contractures, and striking hyperlaxity of distal joints. Decreased fetal movements are frequently reported. Some affected children acquire the ability to walk independently; however, progression of the disease results in a loss of ambulation by age ten to eleven years. Early and severe respiratory insufficiency occurs in all individuals, resulting in the need for nocturnal noninvasive ventilation (NIV) in the form of bilevel positive airway pressure (BiPAP) by age 11 years. Intermediate COL6-RD is characterized by independent ambulation past age 11 years and respiratory insufficiency that is later in onset than in UCMD and results in the need for NIV in the form of BiPAP by the late teens to early 20s. In contrast to individuals with Bethlem muscular dystrophy, those with intermediate COL6-RD typically do not achieve the ability to run, jump, or climb stairs without use of a railing.

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). Patients with a clinical diagnosis of Beemer-Langer syndrome have been found to carry mutations in the IFT80 gene (611177); see SRTD2, 611263. For a discussion of genetic heterogeneity of short-rib thoracic dysplasia, see SRTD1 (208500).

Opsismodysplasia (OPSMD) is a rare skeletal dysplasia involving delayed bone maturation. Clinical signs observed at birth include short limbs, small hands and feet, relative macrocephaly with a large anterior fontanel, and characteristic craniofacial abnormalities including a prominent brow, depressed nasal bridge, a small anteverted nose, and a relatively long philtrum. Death in utero or secondary to respiratory failure during the first few years of life has been reported, but there can be long-term survival. Typical radiographic findings include shortened long bones with delayed epiphyseal ossification, severe platyspondyly, metaphyseal cupping, and characteristic abnormalities of the metacarpals and phalanges (summary by Below et al., 2013 and Fradet and Fitzgerald, 2017).

Congenital fiber-type disproportion is a condition that primarily affects skeletal muscles, which are muscles used for movement. People with this condition typically experience muscle weakness (myopathy), particularly in the muscles of the shoulders, upper arms, hips, and thighs. Weakness can also affect the muscles of the face and muscles that control eye movement (ophthalmoplegia), sometimes causing droopy eyelids (ptosis). Individuals with congenital fiber-type disproportion generally have a long face, a high arch in the roof of the mouth (high-arched palate), and crowded teeth.\n\nIndividuals with congenital fiber-type disproportion may have joint deformities (contractures) and an abnormally curved lower back (lordosis) or a spine that curves to the side (scoliosis). Approximately 30 percent of people with this disorder experience mild to severe breathing problems related to weakness of muscles needed for breathing. Some people who experience these breathing problems require use of a machine to help regulate their breathing at night (noninvasive mechanical ventilation), and occasionally during the day as well. About 30 percent of affected individuals have difficulty swallowing due to muscle weakness in the throat. Rarely, people with this condition have a weakened and enlarged heart muscle (dilated cardiomyopathy).\n\nThe severity of congenital fiber-type disproportion varies widely. It is estimated that up to 25 percent of affected individuals experience severe muscle weakness at birth and die in infancy or childhood. Others have only mild muscle weakness that becomes apparent in adulthood. Most often, the signs and symptoms of this condition appear by age 1. The first signs of this condition are usually decreased muscle tone (hypotonia) and muscle weakness. In most cases, muscle weakness does not worsen over time, and in some instances it may improve. Although motor skills such as standing and walking may be delayed, many affected children eventually learn to walk. These individuals often have less stamina than their peers, but they remain active. Rarely, people with this condition have a progressive decline in muscle strength over time. These individuals may lose the ability to walk and require wheelchair assistance.

While the majority of individuals with Costello syndrome share characteristic findings affecting multiple organ systems, the phenotypic spectrum is wide, ranging from a milder or attenuated phenotype to a severe phenotype with early lethal complications. Costello syndrome is typically characterized by failure to thrive in infancy as a result of severe postnatal feeding difficulties; short stature; developmental delay or intellectual disability; coarse facial features (full lips, large mouth, full nasal tip); curly or sparse, fine hair; loose, soft skin with deep palmar and plantar creases; papillomata of the face and perianal region; diffuse hypotonia and joint laxity with ulnar deviation of the wrists and fingers; tight Achilles tendons; and cardiac involvement including: cardiac hypertrophy (usually typical hypertrophic cardiomyopathy), congenital heart defect (usually valvar pulmonic stenosis), and arrhythmia (usually supraventricular tachycardia, especially chaotic atrial rhythm/multifocal atrial tachycardia or ectopic atrial tachycardia). Relative or absolute macrocephaly is typical, and postnatal cerebellar overgrowth can result in the development of a Chiari I malformation with associated anomalies including hydrocephalus or syringomyelia. Individuals with Costello syndrome have an approximately 15% lifetime risk for malignant tumors including rhabdomyosarcoma and neuroblastoma in young children and transitional cell carcinoma of the bladder in adolescents and young adults.

The autosomal dominant TRPV4 disorders (previously considered to be clinically distinct phenotypes before their molecular basis was discovered) are now grouped into neuromuscular disorders and skeletal dysplasias; however, the overlap within each group is considerable. Affected individuals typically have either neuromuscular or skeletal manifestations alone, and in only rare instances an overlap syndrome has been reported. The three autosomal dominant neuromuscular disorders (mildest to most severe) are: Charcot-Marie-Tooth disease type 2C. Scapuloperoneal spinal muscular atrophy. Congenital distal spinal muscular atrophy. The autosomal dominant neuromuscular disorders are characterized by a congenital-onset, static, or later-onset progressive peripheral neuropathy with variable combinations of laryngeal dysfunction (i.e., vocal fold paresis), respiratory dysfunction, and joint contractures. The six autosomal dominant skeletal dysplasias (mildest to most severe) are: Familial digital arthropathy-brachydactyly. Autosomal dominant brachyolmia. Spondylometaphyseal dysplasia, Kozlowski type. Spondyloepiphyseal dysplasia, Maroteaux type. Parastremmatic dysplasia. Metatropic dysplasia. The skeletal dysplasia is characterized by brachydactyly (in all 6); the five that are more severe have short stature that varies from mild to severe with progressive spinal deformity and involvement of the long bones and pelvis. In the mildest of the autosomal dominant TRPV4 disorders life span is normal; in the most severe it is shortened. Bilateral progressive sensorineural hearing loss (SNHL) can occur with both autosomal dominant neuromuscular disorders and skeletal dysplasias.

Myhre syndrome is a connective tissue disorder with multisystem involvement, progressive and proliferative fibrosis that may occur spontaneously or following trauma or surgery, mild-to-moderate intellectual disability, and in some instances, autistic-like behaviors. Organ systems primarily involved include: cardiovascular (congenital heart defects, long- and short-segment stenosis of the aorta and peripheral arteries, pericardial effusion, constrictive pericarditis, restrictive cardiomyopathy, and hypertension); respiratory (choanal stenosis, laryngotracheal narrowing, obstructive airway disease, or restrictive pulmonary disease), gastrointestinal (pyloric stenosis, duodenal strictures, severe constipation); and skin (thickened particularly on the hands and extensor surfaces). Additional findings include distinctive craniofacial features and skeletal involvement (intrauterine growth restriction, short stature, limited joint range of motion). To date, 55 individuals with molecularly confirmed Myhre syndrome have been reported.

A rare primary bone dysplasia disorder with characteristics of severe short stature, coarse facies, thoracolumbar kyphoscoliosis and enlarged joints with contractures. Psychomotor delay and intellectual disability may also be associated. Radiographic features include flat vertebral bodies, lacy ossification of the metaphyses of long bones and iliac crests, and marked sclerosis of the skull base.

Brown-Vialetto-Van Laere syndrome is a rare autosomal recessive neurologic disorder characterized by sensorineural hearing loss and a variety of cranial nerve palsies, usually involving the motor components of the seventh and ninth to twelfth (more rarely the third, fifth, and sixth) cranial nerves. Spinal motor nerves and, less commonly, upper motor neurons are sometimes affected, giving a picture resembling amyotrophic lateral sclerosis (ALS; 105400). The onset of the disease is usually in the second decade, but earlier and later onset have been reported. Hearing loss tends to precede the onset of neurologic signs, mostly progressive muscle weakness causing respiratory compromise. However, patients with very early onset may present with bulbar palsy and may not develop hearing loss until later. The symptoms, severity, and disease duration are variable (summary by Green et al., 2010). Genetic Heterogeneity of Brown-Vialetto-Van Laere Syndrome See also BVVLS2 (614707), caused by mutation in the SLC52A2 gene (607882) on chromosome 8q.

Thanatophoric dysplasia (TD) is a short-limb skeletal dysplasia that is usually lethal in the perinatal period. TD is divided into subtypes: TD type I is characterized by micromelia with bowed femurs and, uncommonly, the presence of craniosynostosis of varying severity. TD type II is characterized by micromelia with straight femurs and uniform presence of moderate-to-severe craniosynostosis with cloverleaf skull deformity. Other features common to type I and type II include: short ribs, narrow thorax, relative macrocephaly, distinctive facial features, brachydactyly, hypotonia, and redundant skin folds along the limbs. Most affected infants die of respiratory insufficiency shortly after birth. Rare long-term survivors have been reported.

Syndromic microphthalmia-9 (MCOPS9), also referred to as pulmonary hypoplasia-diaphragmatic hernia-anophthalmia-cardiac defect, is characterized by bilateral clinical anophthalmia, pulmonary hypoplasia/aplasia, cardiac malformations, and diaphragmatic defects. The phenotype is variable, ranging from isolated clinical anophthalmia or microphthalmia to complex presentations involving the cardiac, pulmonary, diaphragmatic, and renal systems. At its most severe, infants are born without pulmonary structures and die soon after birth (Marcadier et al., 2015).

Carnitine palmitoyltransferase II (CPT II) deficiency is a disorder of long-chain fatty-acid oxidation. The three clinical presentations are lethal neonatal form, severe infantile hepatocardiomuscular form, and myopathic form (which is usually mild and can manifest from infancy to adulthood). While the former two are severe multisystemic diseases characterized by liver failure with hypoketotic hypoglycemia, cardiomyopathy, seizures, and early death, the latter is characterized by exercise-induced muscle pain and weakness, sometimes associated with myoglobinuria. The myopathic form of CPT II deficiency is the most common disorder of lipid metabolism affecting skeletal muscle and the most frequent cause of hereditary myoglobinuria. Males are more likely to be affected than females.

Mitochondrial phosphate carrier deficiency (MPCD) is an autosomal recessive disorder characterized by onset of cardiorespiratory insufficiency soon after birth. Patients usually require intervention in the neonatal period. The disorder may result in death in infancy, although those that survive have stabilization or amelioration of symptoms with age. Most affected individuals have hypotonia, delayed motor development, and exercise intolerance, but cognitive development is normal. Laboratory studies typically show increased serum lactate, although this may not be present. Muscle biopsy shows abnormal mitochondria and lipid accumulation. There is phenotypic variability likely depending on the location of the mutation (summary by Bhoj et al., 2015).

Most characteristically, Aicardi-Goutières syndrome (AGS) manifests as an early-onset encephalopathy that usually, but not always, results in severe intellectual and physical disability. A subgroup of infants with AGS present at birth with abnormal neurologic findings, hepatosplenomegaly, elevated liver enzymes, and thrombocytopenia, a picture highly suggestive of congenital infection. Otherwise, most affected infants present at variable times after the first few weeks of life, frequently after a period of apparently normal development. Typically, they demonstrate the subacute onset of a severe encephalopathy characterized by extreme irritability, intermittent sterile pyrexias, loss of skills, and slowing of head growth. Over time, as many as 40% develop chilblain skin lesions on the fingers, toes, and ears. It is becoming apparent that atypical, sometimes milder, cases of AGS exist, and thus the true extent of the phenotype associated with pathogenic variants in the AGS-related genes is not yet known.

The signs and symptoms of myofibrillar myopathy vary widely among affected individuals, typically depending on the condition's genetic cause. Most people with this disorder begin to develop muscle weakness (myopathy) in mid-adulthood. However, features of this condition can appear anytime between infancy and late adulthood. Muscle weakness most often begins in the hands and feet (distal muscles), but some people first experience weakness in the muscles near the center of the body (proximal muscles). Other affected individuals develop muscle weakness throughout their body. Facial muscle weakness can cause swallowing and speech difficulties. Muscle weakness worsens over time.\n\nOther signs and symptoms of myofibrillar myopathy can include a weakened heart muscle (cardiomyopathy), muscle pain (myalgia), loss of sensation and weakness in the limbs (peripheral neuropathy), and respiratory failure. Individuals with this condition may have skeletal problems including joint stiffness (contractures) and abnormal side-to-side curvature of the spine (scoliosis). Rarely, people with this condition develop clouding of the lens of the eyes (cataracts).\n\nMyofibrillar myopathy is part of a group of disorders called muscular dystrophies that affect muscle function and cause weakness. Myofibrillar myopathy primarily affects skeletal muscles, which are muscles that the body uses for movement. In some cases, the heart (cardiac) muscle is also affected.

Isolated complex I deficiency is a rare inborn error of metabolism due to mutations in nuclear or mitochondrial genes encoding subunits or assembly factors of the human mitochondrial complex I (NADH: ubiquinone oxidoreductase) and is characterized by a wide range of manifestations including marked and often fatal lactic acidosis, cardiomyopathy, leukoencephalopathy, pure myopathy and hepatopathy with tubulopathy. Among the numerous clinical phenotypes observed are Leigh syndrome, Leber hereditary optic neuropathy and MELAS syndrome (see these terms).

X-linked myopathy with excessive autophagy (XMEA) is an X-linked recessive skeletal muscle disorder characterized by childhood onset of progressive muscle weakness and atrophy primarily affecting the proximal muscles. While onset is usually in childhood, it can range from infancy to adulthood. Many patients lose ambulation and become wheelchair-bound. Other organ systems, including the heart, are clinically unaffected. Muscle biopsy shows intracytoplasmic autophagic vacuoles with sarcolemmal features and a multilayered basal membrane (summary by Ramachandran et al., 2013; Kurashige et al., 2013, and Ruggieri et al., 2015). Danon disease (300257), caused by mutation in the LAMP2 gene (309060) on chromosome Xq24, is a distinct disorder with similar pathologic features.

Diaphanospondylodysostosis is a rare, recessively inherited, perinatal lethal skeletal disorder. The primary skeletal characteristics include small chest, abnormal vertebral segmentation, and posterior rib gaps containing incompletely differentiated mesenchymal tissue. Consistent craniofacial features include ocular hypertelorism, epicanthal folds, depressed nasal bridge with short nose, and low-set ears. The most commonly described extraskeletal finding is nephroblastomatosis with cystic kidneys, but other visceral findings have been described in some cases (summary by Funari et al., 2010).

Niemann-Pick disease type C (NPC) is a slowly progressive lysosomal disorder whose principal manifestations are age dependent. The manifestations in the perinatal period and infancy are predominantly visceral, with hepatosplenomegaly, jaundice, and (in some instances) pulmonary infiltrates. From late infancy onward, the presentation is dominated by neurologic manifestations. The youngest children may present with hypotonia and developmental delay, with the subsequent emergence of ataxia, dysarthria, dysphagia, and, in some individuals, epileptic seizures, dystonia, and gelastic cataplexy. Although cognitive impairment may be subtle at first, it eventually becomes apparent that affected individuals have a progressive dementia. Older teenagers and young adults may present predominantly with apparent early-onset dementia or psychiatric manifestations; however, careful examination usually identifies typical neurologic signs.

Pontocerebellar hypoplasia (PCH) refers to a group of severe neurodegenerative disorders affecting growth and function of the brainstem and cerebellum, resulting in little or no development. Different types were classified based on the clinical picture and the spectrum of pathologic changes. PCH type 1 is characterized by central and peripheral motor dysfunction associated with anterior horn cell degeneration resembling infantile spinal muscular atrophy (SMA; see SMA1, 253300); death usually occurs early. Genetic Heterogeneity of Pontocerebellar Hypoplasia Also see PCH1B (614678), caused by mutation in the EXOSC3 gene (606489); PCH1C (616081), caused by mutation in the EXOSC8 gene (606019); PCH1D (618065), caused by mutation in the EXOSC9 gene (606180); PCH1E (619303), caused by mutation in the SLC25A46 gene (610826); PCH1F (619304), caused by mutation in the EXOSC1 gene (606493); PCH2A (277470), caused by mutation in the TSEN54 gene (608755); PCH2B (612389), caused by mutation in the TSEN2 gene (608753); PCH2C (612390), caused by mutation in the TSEN34 gene (608754); PCH2D (613811), caused by mutation in the SEPSECS gene (613009); PCH3 (608027), caused by mutation in the PCLO gene (604918); PCH4 (225753), caused by mutation in the TSEN54 gene; PCH5 (610204), caused by mutation in the TSEN54 gene; PCH6 (611523), caused by mutation in the RARS2 gene (611524); PCH7 (614969), caused by mutation in the TOE1 gene (613931); PCH8 (614961), caused by mutation in the CHMP1A gene (164010); PCH9 (615809), caused by mutation in the AMPD2 gene (102771); PCH10 (615803), caused by mutation in the CLP1 gene (608757); PCH11 (617695), caused by mutation in the TBC1D23 gene (617687); PCH12 (618266), caused by mutation in the COASY gene (609855); PCH13 (618606), caused by mutation in the VPS51 gene (615738); PCH14 (619301), caused by mutation in the PPIL1 gene (601301); PCH15 (619302), caused by mutation in the CDC40 gene (605585); PCH16 (619527), caused by mutation in the MINPP1 gene (605391); and PCH17 (619909), caused by mutation in the PRDM13 gene (616741) on chromosome 6q16.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

The X-linked otopalatodigital (X-OPD) spectrum disorders, characterized primarily by skeletal dysplasia, include the following: Otopalatodigital syndrome type 1 (OPD1). Otopalatodigital syndrome type 2 (OPD2). Frontometaphyseal dysplasia type 1 (FMD1). Melnick-Needles syndrome (MNS). Terminal osseous dysplasia with pigmentary skin defects (TODPD). In OPD1, most manifestations are present at birth; females can present with severity similar to affected males, although some have only mild manifestations. In OPD2, females are less severely affected than related affected males. Most males with OPD2 die during the first year of life, usually from thoracic hypoplasia resulting in pulmonary insufficiency. Males who live beyond the first year of life are usually developmentally delayed and require respiratory support and assistance with feeding. In FMD1, females are less severely affected than related affected males. Males do not experience a progressive skeletal dysplasia but may have joint contractures and hand and foot malformations. Progressive scoliosis is observed in both affected males and females. In MNS, wide phenotypic variability is observed; some individuals are diagnosed in adulthood, while others require respiratory support and have reduced longevity. MNS in males results in perinatal lethality in all recorded cases. TODPD, seen only in females, is characterized by a skeletal dysplasia that is most prominent in the digits, pigmentary defects of the skin, and recurrent digital fibromata.

X-linked infantile spinal muscular atrophy (XL-SMA) is characterized by congenital hypotonia, areflexia, and evidence of degeneration and loss of anterior horn cells (i.e., lower motor neurons) in the spinal cord and brain stem. Often congenital contractures and/or fractures are present. Intellect is normal. Life span is significantly shortened because of progressive ventilatory insufficiency resulting from chest muscle involvement.

VACTERL describes a constellation of congenital anomalies, including vertebral anomalies, anal atresia, congenital cardiac disease, tracheoesophageal fistula, renal anomalies, radial dysplasia, and other limb defects; see 192350. Cases of familial VACTERL with hydrocephalus (H) have been reported with suggestion of autosomal recessive or X-linked inheritance (see 314390). Other patients thought to have VACTERL-H, including 2 unrelated infants reported by Porteous et al. (1992), had been found to have Fanconi anemia (see 227650). Porteous et al. (1992) suggested that chromosomal breakage studies should be performed in all cases of VACTERL/VACTERL-H to rule out Fanconi anemia. Alter et al. (2007) noted that a VATER phenotype had been reported in Fanconi anemia of complementation groups A (227650), C (227645), D1 (605724), E (600901), F (603467), and G (614082). X-linked VACTERL-H is also associated with mutations in the FANCB gene (300515).

Thymomas are low-grade epithelial cancers of the thymus. Familial occurrence of thymoma is rare.

A rare genetic skin disorder with characteristics of very early-onset of progressive skin thickening over the entire body (except for eyelids, neck and ears), progressively limited joint mobility with gradual freezing of joints and eventual severe chest and abdomen movement restriction, manifesting with restrictive pulmonary disease, which may lead to death. Additional features include severe growth restriction and osteoporosis. There have been no further descriptions in the literature since 1974.

Clinical features of atelosteogenesis type 2 (AO2) include rhizomelic limb shortening with normal-sized skull, hitchhiker thumbs, small chest, protuberant abdomen, cleft palate, and distinctive facial features (midface retrusion, depressed nasal bridge, epicanthus, micrognathia). Other typical findings are ulnar deviation of the fingers, gap between the first and second toes, and clubfoot. AO2 is usually lethal at birth or shortly thereafter due to pulmonary hypoplasia and tracheobronchomalacia. However, it exists in a continuous phenotypic spectrum with diastrophic dysplasia, and long-term survivors have been reported.

Congenital myopathy-1B (CMYO1B) is an autosomal recessive disorder of skeletal muscle characterized by severe hypotonia and generalized muscle weakness apparent soon after birth or in early childhood with delayed motor development, generalized muscle weakness and atrophy, and difficulty walking or running. Affected individuals show proximal muscle weakness with axial and shoulder girdle involvement, external ophthalmoplegia, and bulbar weakness, often resulting in feeding difficulties and respiratory insufficiency. Orthopedic complications such as joint laxity, distal contractures, hip dislocation, cleft palate, and scoliosis are commonly observed. Serum creatine kinase is normal. The phenotype is variable in severity (Jungbluth et al., 2005; Bharucha-Goebel et al., 2013). Some patients show symptoms in utero, including reduced fetal movements, polyhydramnios, and intrauterine growth restriction. The most severely affected patients present in utero with fetal akinesia, arthrogryposis, and lung hypoplasia resulting in fetal or perinatal death (McKie et al., 2014). Skeletal muscle biopsy of patients with recessive RYR1 mutations can show variable features, including multiminicores (Ferreiro and Fardeau, 2002), central cores (Jungbluth et al., 2002), congenital fiber-type disproportion (CFTD) (Monnier et al., 2009), and centronuclear myopathy (Wilmshurst et al., 2010). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Autosomal recessive myosin storage congenital myopathy-7B (CMYO7B) is a skeletal muscle disorder characterized by the onset of scapuloperoneal muscle weakness in early childhood or young adulthood. Affected individuals have difficulty walking, steppage gait, and scapular winging due to shoulder girdle involvement. The severity and progression of the disorder is highly variable, even within families. Most patients develop respiratory insufficiency, nocturnal hypoventilation, and restrictive lung disease; some develop hypertrophic cardiomyopathy. Additional features include myopathic facies, high-arched palate, scoliosis, and muscle wasting with thin body habitus. Serum creatine kinase may be normal or elevated. Skeletal muscle biopsy shows variable findings, including myosin storage disease, type 1 fiber predominance, centralized nuclei, and multiminicore disease (Onengut et al., 2004; Tajsharghi et al., 2007; Beecroft et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Autosomal recessive severe infantile nemaline myopathy-5A (NEM5A) is a skeletal muscle disorder characterized by symptom onset soon after birth or in early infancy. Affected infants show axial hypotonia, stiffness, rigid spine with progressive kyphosis, pectus deformities, and contractures or limited movement of the large joints. Some patients show transient tremors. There is muscle atrophy and poor gross motor development. Respiratory insufficiency develops in the first years of life, often leading to death. Muscle biopsy shows nemaline rods (Johnston et al., 2000; Geraud et al., 2021). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM2 (256030).

A rare developmental defect with connective tissue involvement characterized by multiple joint dislocations, flattened facial appearance, abnormal palmar creases, laryngotracheomalacia, and pulmonary hypoplasia. Additional signs may include a bifid tongue, micrognathia, non-immune hydrops fetalis, and brain dysplasia. The disease is lethal shortly after birth due to respiratory insufficiency.

Silverman-Handmaker dyssegmental dysplasia (DDSH) is a lethal autosomal recessive skeletal dysplasia with anisospondyly and micromelia. Individuals with DDSH also have a flat face, micrognathia, cleft palate and reduced joint mobility, and frequently have an encephalocele. The endochondral growth plate is short, the calcospherites (spherical calcium-phosphorus crystals produced by hypertrophic chondrocytes) are unfused, and there is mucoid degeneration of the resting cartilage (summary by Arikawa-Hirasawa et al., 2001).

Rhizomelic chondrodysplasia punctata type 1 (RCDP1), a peroxisome biogenesis disorder (PBD) has a classic (severe) form and a nonclassic (mild) form. Classic (severe) RCDP1 is characterized by proximal shortening of the humerus (rhizomelia) and to a lesser degree the femur, punctate calcifications in cartilage with epiphyseal and metaphyseal abnormalities (chondrodysplasia punctata, or CDP), coronal clefts of the vertebral bodies, and cataracts that are usually present at birth or appear in the first few months of life. Birth weight, length, and head circumference are often at the lower range of normal; postnatal growth deficiency is profound. Intellectual disability is severe, and the majority of children develop seizures. Most affected children do not survive the first decade of life; a proportion die in the neonatal period. Nonclassic (mild) RCDP1 is characterized by congenital or childhood cataracts, CDP or infrequently, chondrodysplasia manifesting only as mild epiphyseal changes, variable rhizomelia, and milder intellectual disability and growth restriction than classic RCDP1.

Sengers syndrome is an autosomal recessive mitochondrial disorder characterized by congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy, exercise intolerance, and lactic acidosis. Mental development is normal, but affected individuals may die early from cardiomyopathy (summary by Mayr et al., 2012). Skeletal muscle biopsies of 2 affected individuals showed severe mtDNA depletion (Calvo et al., 2012).

Microcephalic osteodysplastic primordial dwarfism type I (MOPD1) is a severe autosomal recessive skeletal dysplasia characterized by dwarfism, microcephaly, and neurologic abnormalities, including mental retardation, brain malformations, and ocular/auditory sensory deficits. Patients often die in early childhood (summary by Pierce and Morse, 2012).

Triosephosphate isomerase deficiency (TPID) is an autosomal recessive multisystem disorder characterized by congenital hemolytic anemia, and progressive neuromuscular dysfunction beginning in early childhood. Many patients die from respiratory failure in childhood. The neurologic syndrome is variable, but usually includes lower motor neuron dysfunction with hypotonia, muscle weakness and atrophy, and hyporeflexia. Some patients may show additional signs such as dystonic posturing and/or spasticity. Laboratory studies show intracellular accumulation of dihydroxyacetone phosphate (DHAP), particularly in red blood cells (summary by Fermo et al., 2010).

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction. Patients present clinically with onset of variable muscle weakness between infancy and adulthood. These disorders have been classified according to the location of the defect: presynaptic, synaptic, and postsynaptic. Endplate acetylcholinesterase deficiency is an autosomal recessive congenital myasthenic syndrome characterized by a defect within the synapse at the neuromuscular junction (NMJ). Mutations in COLQ result in a deficiency of acetylcholinesterase (AChE), which causes prolonged synaptic currents and action potentials due to extended residence of acetylcholine in the synaptic space. Treatment with ephedrine may be beneficial; AChE inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

The neuronal ceroid lipofuscinoses (NCL; CLN) are a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by the intracellular accumulation of autofluorescent lipopigment storage material in different patterns ultrastructurally. The clinical course includes progressive dementia, seizures, and progressive visual failure (Mole et al., 2005). For a discussion of genetic heterogeneity of neuronal ceroid lipofuscinosis, see CLN1 (256730).

Combined oxidative phosphorylation deficiency type 3 is an extremely rare clinically heterogenous disorder described in about 5 patients to date. Clinical signs included hypotonia, lactic acidosis, and hepatic insufficiency, with progressive encephalomyopathy or hypertrophic cardiomyopathy.

Immunodeficiency-54 is an autosomal recessive primary immunodeficiency characterized by severe intra- and extrauterine growth retardation, microcephaly, decreased numbers of natural killer (NK) cells, and recurrent viral infections, most often affecting the respiratory tract and leading to respiratory failure. Affected individuals also have adrenal insufficiency requiring corticosteroid replacement therapy and may have an increased susceptibility to cancer. Laboratory studies of patient cells showed a DNA repair defect (summary by Gineau et al., 2012).

The nephronophthisis (NPH) phenotype is characterized by reduced renal concentrating ability, chronic tubulointerstitial nephritis, cystic renal disease, and progression to end-stage renal disease (ESRD) before age 30 years. Three age-based clinical subtypes are recognized: infantile, juvenile, and adolescent/adult. Infantile NPH can present in utero with oligohydramnios sequence (limb contractures, pulmonary hypoplasia, and facial dysmorphisms) or postnatally with renal manifestations that progress to ESRD before age 3 years. Juvenile NPH, the most prevalent subtype, typically presents with polydipsia and polyuria, growth retardation, chronic iron-resistant anemia, or other findings related to chronic kidney disease (CKD). Hypertension is typically absent due to salt wasting. ESRD develops at a median age of 13 years. Ultrasound findings are increased echogenicity, reduced corticomedullary differentiation, and renal cysts (in 50% of affected individuals). Histologic findings include tubulointerstitial fibrosis, thickened and disrupted tubular basement membrane, sporadic corticomedullary cysts, and normal or reduced kidney size. Adolescent/adult NPH is clinically similar to juvenile NPH, but ESRD develops at a median age of 19 years. Within a subtype, inter- and intrafamilial variability in rate of progression to ESRD is considerable. Approximately 80%-90% of individuals with the NPH phenotype have no extrarenal features (i.e., they have isolated NPH); ~10%-20% have extrarenal manifestations that constitute a recognizable syndrome (e.g., Joubert syndrome, Bardet-Biedl syndrome, Jeune syndrome and related skeletal disorders, Meckel-Gruber syndrome, Senior-Løken syndrome, Leber congenital amaurosis, COACH syndrome, and oculomotor apraxia, Cogan type).

The spectrum of DCTN1-related neurodegeneration includes Perry syndrome, distal hereditary motor neuronopathy type 7B (dHMN7B), frontotemporal dementia (FTD), motor neuron disease / amyotrophic lateral sclerosis (ALS), and progressive supranuclear palsy. Some individuals present with overlapping phenotypes (e.g., FTD-ALS, Perry syndrome-dHMN7B). Perry syndrome (the most common of the phenotypes associated with DCTN1) is characterized by parkinsonism, neuropsychiatric symptoms, hypoventilation, and weight loss. The mean age of onset in those with Perry syndrome is 49 years (range: 35-70 years), and the mean disease duration is five years (range: 2-14 years). In most affected persons, the reported cause/circumstance of death relates to sudden death/hypoventilation or suicide.

The spectrum of MECP2-related phenotypes in females ranges from classic Rett syndrome to variant Rett syndrome with a broader clinical phenotype (either milder or more severe than classic Rett syndrome) to mild learning disabilities; the spectrum in males ranges from severe neonatal encephalopathy to pyramidal signs, parkinsonism, and macroorchidism (PPM-X) syndrome to severe syndromic/nonsyndromic intellectual disability. Females: Classic Rett syndrome, a progressive neurodevelopmental disorder primarily affecting girls, is characterized by apparently normal psychomotor development during the first six to 18 months of life, followed by a short period of developmental stagnation, then rapid regression in language and motor skills, followed by long-term stability. During the phase of rapid regression, repetitive, stereotypic hand movements replace purposeful hand use. Additional findings include fits of screaming and inconsolable crying, autistic features, panic-like attacks, bruxism, episodic apnea and/or hyperpnea, gait ataxia and apraxia, tremors, seizures, and acquired microcephaly. Males: Severe neonatal-onset encephalopathy, the most common phenotype in affected males, is characterized by a relentless clinical course that follows a metabolic-degenerative type of pattern, abnormal tone, involuntary movements, severe seizures, and breathing abnormalities. Death often occurs before age two years.

Long-chain hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency and trifunctional protein (TFP) deficiency are caused by impairment of mitochondrial TFP. TFP has three enzymatic activities – long-chain enoyl-CoA hydratase, long-chain 3-hydroxyacyl-CoA dehydrogenase, and long-chain 3-ketoacyl-CoA thiolase. In individuals with LCHAD deficiency, there is isolated deficiency of long-chain 3-hydroxyacyl-CoA dehydrogenase, while deficiency of all three enzymes occurs in individuals with TFP deficiency. Individuals with TFP deficiency can present with a severe-to-mild phenotype, while individuals with LCHAD deficiency typically present with a severe-to-intermediate phenotype. Neonates with the severe phenotype present within a few days of birth with hypoglycemia, hepatomegaly, encephalopathy, and often cardiomyopathy. The intermediate phenotype is characterized by hypoketotic hypoglycemia precipitated by infection or fasting in infancy. The mild (late-onset) phenotype is characterized by myopathy and/or neuropathy. Long-term complications include peripheral neuropathy and retinopathy.

Lethal congenital contracture syndrome-3 (LCCS3) is a severe autosomal recessive form of arthrogryposis characterized by multiple joint contractures with muscle wasting and atrophy (Narkis et al., 2007). For a general phenotypic description and a discussion of genetic heterogeneity of LCCS, see LCCS1 (253310).

Pulmonary surfactant metabolism dysfunction-2 (SMDP2) is a rare autosomal dominant disease associated with progressive respiratory insufficiency and lung disease with a variable clinical course. The pathophysiology of the disorder is postulated to involve intracellular accumulation of a structurally defective SPC protein (Thomas et al., 2002). For a general phenotypic description and a discussion of genetic heterogeneity of pulmonary surfactant metabolism dysfunction, see SMDP1 (265120).

Disease caused by homozygous mutation in the prosaposin gene (PSAP) on chromosome 10q22. The disease is genetically distinct from Krabbe disease. Clinical features include onset in infancy with respiratory and neurologic involvement.

SLC25A24 Fontaine progeroid syndrome is a multisystem connective tissue disorder characterized by poor growth, abnormal skeletal features, and distinctive craniofacial features with sagging, thin skin, and decreased subcutaneous fat suggesting an aged appearance that is most pronounced in infancy and improves with time. Characteristic radiographic features include turribrachycephaly with widely open anterior fontanelle, craniosynostosis, and anomalies of the terminal phalanges. Cardiovascular, genitourinary, ocular, and gastrointestinal abnormalities may also occur. To date, 13 individuals with a molecularly confirmed diagnosis of SLC25A24 Fontaine progeroid syndrome have been described.

Congenital disorders of glycosylation (CDGs) are a genetically heterogeneous group of autosomal recessive disorders caused by enzymatic defects in the synthesis and processing of asparagine (N)-linked glycans or oligosaccharides on glycoproteins. Type I CDGs comprise defects in the assembly of the dolichol lipid-linked oligosaccharide (LLO) chain and its transfer to the nascent protein. These disorders can be identified by a characteristic abnormal isoelectric focusing profile of plasma transferrin (Leroy, 2006). For a discussion of the classification of CDGs, see CDG1A (212065).

Emery-Dreifuss muscular dystrophy (EDMD) is characterized by the clinical triad of: joint contractures that begin in early childhood; slowly progressive muscle weakness and wasting initially in a humero-peroneal distribution that later extends to the scapular and pelvic girdle muscles; and cardiac involvement that may manifest as palpitations, presyncope and syncope, poor exercise tolerance, and congestive heart failure along with variable cardiac rhythm disturbances. Age of onset, severity, and progression of muscle and cardiac involvement demonstrate both inter- and intrafamilial variability. Clinical variability ranges from early onset with severe presentation in childhood to late onset with slow progression in adulthood. In general, joint contractures appear during the first two decades, followed by muscle weakness and wasting. Cardiac involvement usually occurs after the second decade and respiratory function may be impaired in some individuals.

G6PC3 deficiency is characterized by severe congenital neutropenia which occurs in a phenotypic continuum that includes the following: Isolated severe congenital neutropenia (nonsyndromic). Classic G6PC3 deficiency (severe congenital neutropenia plus cardiovascular and/or urogenital abnormalities). Severe G6PC3 deficiency (classic G6PC3 deficiency plus involvement of non-myeloid hematopoietic cell lines, additional extra-hematologic features, and pulmonary hypertension; known as Dursun syndrome). Neutropenia usually presents with recurrent bacterial infections in the first few months of life. Intrauterine growth restriction (IUGR), failure to thrive (FTT), and poor postnatal growth are common. Other findings in classic and severe G6PC3 deficiency can include inflammatory bowel disease (IBD) resembling Crohn's disease, and endocrine disorders (growth hormone deficiency, hypogonadotropic hypogonadism, and delayed puberty).

Any autosomal dominant Emery-Dreifuss muscular dystrophy in which the cause of the disease is a mutation in the SYNE2 gene.

Myofibrillar myopathy-6 is an autosomal dominant severe neuromuscular disorder characterized by onset in the first decade of rapidly progressive generalized and proximal muscle weakness, respiratory insufficiency, cardiomyopathy, and skeletal deformities related to muscle weakness. Muscle biopsy shows fiber-type grouping, disruption of the Z lines, and filamentous inclusions, and sural nerve biopsy shows a neuropathy, often with giant axonal neurons. Most patients are severely affected by the second decade and need cardiac transplant, ventilation, and/or a wheelchair (summary by Jaffer et al., 2012). For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy (MFM), see MFM1 (601419).

Like all CDGs, which are caused by a shortage of precursor monosaccharide phosphate or deficiencies in the glycosyltransferases required for lipid-linked oligosaccharide precursor (LLO) synthesis, CDG Ij is caused by a defect in the formation of DPAGT1, the first dolichyl-linked intermediate of the protein N-glycosylation pathway. For a general discussion of CDGs, see CDG1A (212065).

CDG IIe is caused by a mutation that impairs the integrity of the conserved oligomeric Golgi (COG) complex and alters Golgi trafficking, resulting in the disruption of multiple glycosylation pathways. For a general discussion of CDGs, see CDG1A (212065).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (Beltran-Valero de Bernabe et al., 2004). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

MDDGB2 is an autosomal recessive congenital muscular dystrophy associated with impaired intellectual development and mild structural brain abnormalities (Yanagisawa et al., 2007). It is part of a group of similar disorders, collectively known as 'dystroglycanopathies,' resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) (Godfrey et al., 2007). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

HUPRA syndrome is a severe autosomal recessive multisystem disorder characterized by onset in infancy of progressive renal failure leading to electrolyte imbalances, metabolic alkalosis, pulmonary hypertension, hypotonia, and delayed development. Affected individuals are born prematurely (summary by Belostotsky et al., 2011).

SUCLG1-related mitochondrial DNA (mtDNA) depletion syndrome, encephalomyopathic form with methylmalonic aciduria is characterized in the majority of affected newborns by hypotonia, muscle atrophy, feeding difficulties, and lactic acidosis. Affected infants commonly manifest developmental delay / cognitive impairment, growth retardation / failure to thrive, hepatopathy, sensorineural hearing impairment, dystonia, and hypertonia. Notable findings in some affected individuals include hypertrophic cardiomyopathy, epilepsy, myoclonus, microcephaly, sleep disturbance, rhabdomyolysis, contractures, hypothermia, and/or hypoglycemia. Life span is shortened, with median survival of 20 months.

The Mardini-Nyhan association comprises uni- or bilateral lung agenesis, complex cardiac defects, particularly total anomalous pulmonary venous return (TAPVR), and thumb abnormalities (summary by Hastings et al., 2009).

Multiple mitochondrial dysfunctions syndrome-1 (MMDS1) is a severe autosomal recessive disorder of systemic energy metabolism, resulting in weakness, respiratory failure, lack of neurologic development, lactic acidosis, and early death (Seyda et al., 2001). Genetic Heterogeneity of Multiple Mitochondrial Dysfunctions Syndrome See also MMDS2 (614299), caused by mutation in the BOLA3 gene (613183) on chromosome 2p13; MMDS3 (615330), caused by mutation in the IBA57 gene (615316) on chromosome 1q42; MMDS4 (616370), caused by mutation in the ISCA2 gene (615317) on chromosome 14q24; MMDS5 (617613), caused by mutation in the ISCA1 gene (611006) on chromosome 9q21; MMDS6 (617954), caused by mutation in the PMPCB gene (603131) on chromosome 7q22; MMDS7 (620423), caused by mutation in the GCSH gene (238330) on chromosome 16q23; MMDS8 (251900), caused by mutation in the FDX2 gene (614585) on chromosome 19p13; MMDS9A (617717) and MMDS9B (620887), both caused by mutation in the FDXR gene (103270) on chromosome 17q25.

Mitochondrial encephalo-cardio-myopathy due to <i>TMEM70</i> mutation is characterized by early neonatal onset of hypotonia, hypetrophic cardiomyopathy and apneic spells within hours after birth accompanied by lactic acidosis, hyperammonemia and 3-methylglutaconic aciduria.

Keppen-Lubinsky syndrome (KPLBS) is a rare disorder characterized by severely delayed psychomotor development, hypertonia, hyperreflexia, generalized lipodystrophy giving an aged appearance, and distinctive dysmorphic features, including microcephaly, prominent eyes, narrow nasal bridge, and open mouth (summary by Masotti et al., 2015).

Geleophysic dysplasia, a progressive condition resembling a lysosomal storage disorder, is characterized by short stature, short hands and feet, progressive joint limitation and contractures, distinctive facial features, progressive cardiac valvular disease, and thickened skin. Intellect is normal. Major findings are likely to be present in the first year of life. Cardiac, respiratory, and lung involvement result in death before age five years in approximately 33% of individuals with ADAMTSL2-related geleophysic dysplasia.

Multiple mitochondrial dysfunctions syndrome-2 (MMDS2) with hyperglycinemia is a severe autosomal recessive disorder characterized by developmental regression in infancy. Affected children have an encephalopathic disease course with seizures, spasticity, loss of head control, and abnormal movement. Additional more variable features include optic atrophy, cardiomyopathy, and leukodystrophy. Laboratory studies show increased serum glycine and lactate. Most patients die in childhood. The disorder represents a form of 'variant' nonketotic hyperglycinemia and is distinct from classic nonketotic hyperglycinemia (NKH, or GCE; 605899), which is characterized by significantly increased CSF glycine. Several forms of 'variant' NKH, including MMDS2, appear to result from defects of mitochondrial lipoate biosynthesis (summary by Baker et al., 2014). For a general description and a discussion of genetic heterogeneity of multiple mitochondrial dysfunctions syndrome, see MMDS1 (605711).

Pulmonary surfactant metabolism dysfunction-5 (SMDP5) is an autosomal recessive lung disorder manifest clinically and pathologically as pulmonary alveolar proteinosis (PAP). PAP is a rare lung disease characterized by the ineffective clearance of surfactant by alveolar macrophages. This results in the accumulation of surfactant-derived lipoproteinaceous material in the alveoli and terminal bronchioles, causing respiratory failure (summary by Greenhill and Kotton, 2009). For a general phenotypic description and a discussion of genetic heterogeneity of pulmonary surfactant metabolism dysfunction, see SMDP1 (265120).

Congenital myopathy-10A (CMYO10A) is a severe autosomal recessive skeletal muscle disorder characterized by generalized hypotonia, respiratory insufficiency, and poor feeding apparent from birth. Decreased fetal movements may be observed. More variable features include high-arched palate, distal joint contractures, foot deformities, scoliosis, areflexia, and dysphagia. Many patients show eventration of the diaphragm. Affected individuals become ventilator-dependent in the first months or years of life and never achieve walking; many die in childhood (Logan et al., 2011). Patients with more damaging mutations in the MEGF10 gene, including nonsense or frameshift null mutations, show the more severe phenotype (CMYO10A), whereas those with missense mutations affecting conserved cysteine residues in the EGF-like domain show the less severe phenotype with later onset of respiratory failure and minicores on muscle biopsy (CMYO10B) (Croci et al., 2022). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Hyperglycinemia, lactic acidosis, and seizures (HGCLAS) is a severe autosomal recessive disorder characterized by onset of hypotonia and seizures associated with increased serum glycine and lactate in the first days of life. Affected individuals develop an encephalopathy or severely delayed psychomotor development, which may result in death in childhood. The disorder represents a form of 'variant' nonketotic hyperglycinemia and is distinct from classic nonketotic hyperglycinemia (NKH, or GCE; 605899), which is characterized by significantly increased CSF glycine. Several forms of 'variant' NKH, including HGCLAS, appear to result from defects of mitochondrial lipoate biosynthesis (summary by Baker et al., 2014).

Granulomatosis with polyangiitis, formerly termed Wegener granulomatosis, is a systemic disease with a complex genetic background. It is characterized by necrotizing granulomatous inflammation of the upper and lower respiratory tract, glomerulonephritis, vasculitis, and the presence of antineutrophil cytoplasmatic autoantibodies (ANCAs) in patient sera. These ANCAs are antibodies to a defined target antigen, proteinase-3 (PR3, PRTN3; 177020), which is present within primary azurophil granules of neutrophils (PMNs) and lysozymes of monocytes. On cytokine priming of PMNs, PR3 translocates to the cell surface, where PR3-ANCAs can interact with their antigens and activate PMNs. PMNs from patients with active GPA express PR3 on their surface, produce respiratory burst, and release proteolytic enzymes after activation with PR3-ANCAs. The consequence is a self-sustaining inflammatory process (Jagiello et al., 2004).

Primary coenzyme Q10 (CoQ10) deficiency is usually associated with multisystem involvement, including neurologic manifestations such as fatal neonatal encephalopathy with hypotonia; a late-onset slowly progressive multiple-system atrophy-like phenotype (neurodegeneration with autonomic failure and various combinations of parkinsonism and cerebellar ataxia, and pyramidal dysfunction); and dystonia, spasticity, seizures, and intellectual disability. Steroid-resistant nephrotic syndrome (SRNS), the hallmark renal manifestation, is often the initial manifestation either as isolated renal involvement that progresses to end-stage renal disease (ESRD), or associated with encephalopathy (seizures, stroke-like episodes, severe neurologic impairment) resulting in early death. Hypertrophic cardiomyopathy (HCM), retinopathy or optic atrophy, and sensorineural hearing loss can also be seen.

EXOSC3 pontocerebellar hypoplasia (EXOSC3-PCH) is characterized by abnormalities in the posterior fossa and degeneration of the anterior horn cells. At birth, skeletal muscle weakness manifests as hypotonia (sometimes with congenital joint contractures) and poor feeding. In persons with prolonged survival, spasticity, dystonia, and seizures become evident. Within the first year of life respiratory insufficiency and swallowing difficulties are common. Intellectual disability is severe. Life expectancy ranges from a few weeks to adolescence. To date, 82 individuals (from 58 families) with EXOSC3-PCH have been described.

Brown-Vialetto-Van Laere syndrome-2 (BVVLS2) is an autosomal recessive progressive neurologic disorder characterized by early childhood onset of sensorineural deafness, bulbar dysfunction, and severe diffuse muscle weakness and wasting of the upper and lower limbs and axial muscles, resulting in respiratory insufficiency. Some patients may lose independent ambulation. Because it results from a defect in riboflavin metabolism, some patients may benefit from high-dose riboflavin supplementation (summary by Johnson et al., 2012; Foley et al., 2014). For discussion of genetic heterogeneity of Brown-Vialetto-Van Laere syndrome, see BVVLS1 (211530).

Lethal congenital contracture syndrome-4 (LCCS4) is a severe form of neuromuscular arthrogryposis characterized by contractures leading to various degrees of flexion or extension limitations evident at birth (Markus et al., 2012). For a general phenotypic description and discussion of genetic heterogeneity of LCCS, see LCCS1 (253310).

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Mitochondrial DNA depletion syndrome-11 is an autosomal recessive mitochondrial disorder characterized by onset in childhood or adulthood of progressive external ophthalmoplegia (PEO), muscle weakness and atrophy, exercise intolerance, and respiratory insufficiency due to muscle weakness. More variable features include spinal deformity, emaciation, and cardiac abnormalities. Skeletal muscle biopsies show deletion and depletion of mitochondrial DNA (mtDNA) with variable defects in respiratory chain enzyme activities (summary by Kornblum et al., 2013). For a discussion of genetic heterogeneity of autosomal recessive mtDNA depletion syndromes, see MTDPS1 (603041).

Congenital myopathy-2A (CMYO2A) is an autosomal dominant disorder of the skeletal muscle characterized by infantile- or childhood-onset myopathy with delayed motor milestones and nonprogressive muscle weakness. Of the patients with congenital myopathy caused by mutation in the ACTA1 gene, about 90% carry heterozygous mutations that are usually de novo and cause the severe infantile phenotype (CMYO2C; 620278). Some patients with de novo mutations have a more typical and milder disease course with delayed motor development and proximal muscle weakness, but are able to achieve independent ambulation. Less frequently, autosomal dominant transmission of the disorder within a family may occur when the ACTA1 mutation produces a phenotype compatible with adult life. Of note, intrafamilial variability has also been reported: a severely affected proband may be identified and then mildly affected or even asymptomatic relatives are found to carry the same mutation. The severity of the disease most likely depends on the detrimental effect of the mutation, although there are probably additional modifying factors (Ryan et al., 2001; Laing et al., 2009; Sanoudou and Beggs, 2001; Agrawal et al., 2004; Nowak et al., 2013; Sewry et al., 2019; Laitila and Wallgren-Pettersson, 2021). The most common histologic finding on muscle biopsy in patients with ACTA1 mutations is the presence of 'nemaline rods,' which represent abnormal thread- or rod-like structures ('nema' is Greek for 'thread'). However, skeletal muscle biopsy from patients with mutations in the ACTA1 gene can show a range of pathologic phenotypes. These include classic rods, intranuclear rods, clumped filaments, cores, or fiber-type disproportion, all of which are nonspecific pathologic findings and not pathognomonic of a specific congenital myopathy. Most patients have clinically severe disease, regardless of the histopathologic phenotype (Nowak et al., 2007; Sewry et al., 2019). ACTA1 mutations are the second most common cause of congenital myopathies classified histologically as 'nemaline myopathy' after mutations in the NEB gene (161650). ACTA1 mutations are overrepresented in the severe phenotype with early death (Laing et al., 2009). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM2 (256030).

Congenital myasthenic syndromes are genetic disorders of the neuromuscular junction (NMJ) that are classified by the site of the transmission defect: presynaptic, synaptic, and postsynaptic. CMS8 is an autosomal recessive disorder characterized by prominent defects of both the pre- and postsynaptic regions. Affected individuals have onset of muscle weakness in early childhood; the severity of the weakness and muscles affected is variable (summary by Maselli et al., 2012). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Multiple mitochondrial dysfunctions syndrome-3 (MMDS3) is an autosomal recessive severe neurodegenerative disorder characterized by loss of previously acquired developmental milestones in the first months or years of life. Some affected patients have normal development in early infancy before the onset of symptoms, whereas others show delays from birth. Features included loss of motor function, spasticity, pyramidal signs, loss of speech, and cognitive impairment. The disease course is highly variable: some patients die of respiratory failure early in childhood, whereas some survive but may be bedridden with a feeding tube. Less commonly, some patients may survive and have a stable course with motor deficits and mild or even absent cognitive impairment, although there may be fluctuating symptoms, often in response to infection. Other variable features include visual problems and seizures. Brain imaging shows diffuse leukodystrophy in the subcortical region, brainstem, cerebellum, and spinal cord. Laboratory studies tend to show increased lactate and CSF glycine, and decreased activity of mitochondrial complexes I and II, although these findings are also variable. There may be additional biochemical evidence of mitochondrial dysfunction (summary by Liu et al., 2018). For a general description and a discussion of genetic heterogeneity of multiple mitochondrial dysfunctions syndrome, see MMDS1 (605711).

The features of Ehlers-Danlos syndrome spondylodysplastic type 2 (EDSSPD2) include an aged appearance, developmental delay, short stature, craniofacial disproportion, generalized osteopenia, defective wound healing, hypermobile joints, hypotonic muscles, and loose but elastic skin (Okajima et al., 1999). For a discussion of genetic heterogeneity of the spondylodysplastic type of Ehlers-Danlos syndrome, see 130070.

A rare subtype of axonal hereditary motor and sensory neuropathy characterised by early-onset axial hypotonia, generalised muscle weakness, absent deep tendon reflexes and decreased muscle mass. Electromyography reveals decreased motor nerve conduction velocities with markedly reduced sensory and motor amplitudes. There is evidence the disease is caused by homozygous or compound heterozygous mutation in the TRIM2 gene on chromosome 4q.

Asparagine synthetase deficiency (ASD) mainly presents as a triad of congenital microcephaly, severe developmental delay, and axial hypotonia followed by spastic quadriplegia. Low cerebrospinal fluid (CSF) asparagine level can help the clinician in differentiating this disorder from others. In most cases age of onset of apnea, excessive irritability, and seizures is soon after birth. Affected individuals typically do not acquire any developmental milestones. Spastic quadriplegia can lead to severe contractures of the limbs and neurogenic scoliosis. Feeding difficulties (gastroesophageal reflux disease, frequent vomiting, swallowing dysfunction, and gastroesophageal incoordination) are a significant problem in most affected individuals. A majority have cortical blindness. MRI findings are nonspecific but may include generalized atrophy and simplified gyral pattern.

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (see CED1, 218330). For a discussion of genetic heterogeneity of short-rib thoracic dysplasia, see SRTD1 (208500).

Nemaline myopathy-9 is an autosomal recessive muscle disorder characterized by onset of muscle weakness in early infancy. The phenotype is highly variable, ranging from death in infancy due to lack of antigravity movements, to slowly progressive distal muscle weakness with preserved ambulation later in childhood. Muscle biopsy shows typical rod-like structure in myofibers (summary by Gupta et al., 2013). For a discussion of genetic heterogeneity of nemaline myopathy, see 161800.

Tubular aggregates in muscle, first described by Engel (1964), are structures of variable appearance consisting of an outer tubule containing either one or more microtubule-like structures or amorphous material. They are a nonspecific pathologic finding that may occur in a variety of circumstances, including alcohol- and drug-induced myopathies, exercise-induced cramps or muscle weakness, and inherited myopathies. Tubular aggregates are derived from the sarcoplasmic reticulum (Salviati et al., 1985) and are believed to represent an adaptive mechanism aimed at regulating an increased intracellular level of calcium in order to prevent the muscle fibers from hypercontraction and necrosis (Martin et al., 1997; Muller et al., 2001). Genetic Heterogeneity of Tubular Aggregate Myopathy See also TAM2 (615883), caused by mutation in the ORAI1 gene (610277) on chromosome 12q24.

Combined oxidative phosphorylation defect type 20 is a rare mitochondrial oxidative phosphorylation disorder characterized by variable combination of psychomotor delay, hypotonia, muscle weakness, seizures, microcephaly, cardiomyopathy and mild dysmorphic facial features. Variable types of structural brain anomalies have also been reported. Biochemical studies typically show decreased activity of mitochondrial complexes (mainly complex I).

Centronuclear myopathy-5 (CNM5) is an autosomal recessive congenital myopathy characterized by severe neonatal hypotonia with respiratory insufficiency and difficulty feeding. Some patients die in infancy, and some develop dilated cardiomyopathy. Children show severely delayed motor development (summary by Agrawal et al., 2014). For a discussion of genetic heterogeneity of centronuclear myopathy, see CNM1 (160150).

Any primary ciliary dyskinesia in which the cause of the disease is a mutation in the CCDC151 gene.

Pontocerebellar hypoplasia type 1C is a severe autosomal recessive neurodegenerative disorder characterized by severe muscle weakness and failure to thrive apparent in the first months of life. Affected infants showed delayed psychomotor development, often with visual and hearing impairment, and may die of respiratory failure. Brain imaging typically shows cerebellar hypoplasia, hypoplasia of the corpus callosum, and immature myelination (summary by Boczonadi et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

PURA-related neurodevelopmental disorders include PURA syndrome, caused by a heterozygous pathogenic sequence variant in PURA, and 5q31.3 deletion syndrome, caused by a genomic 5q31.3 deletion encompassing all or part of PURA. PURA-related neurodevelopmental disorders are characterized by moderate-to-severe neurodevelopmental delay with absence of speech in most and lack of independent ambulation in many. Early-onset problems can include hypotonia, hypothermia, hypersomnolence, feeding difficulties, excessive hiccups, recurrent central and obstructive apneas, epileptic seizures, abnormal nonepileptic movements (dystonia, dyskinesia, and dysconjugate eye movements), and abnormal vision. Congenital heart defects, urogenital malformations, skeletal abnormalities, and endocrine disorders occur, but are less common.

Congenital myasthenic syndrome-19 (CMS19) is an autosomal recessive disorder resulting from a defect in the neuromuscular junction, causing generalized muscle weakness, exercise intolerance, and respiratory insufficiency. Patients present with hypotonia, feeding difficulties, and respiratory problems soon after birth, but the severity of the weakness and disease course is variable (summary by Logan et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Autosomal recessive progressive external ophthalmoplegia with mitochondrial DNA deletions-2 (PEOB2) is a mitochondrial disorder characterized by adult onset of progressive external ophthalmoplegia, exercise intolerance, muscle weakness, and signs and symptoms of spinocerebellar ataxia, such as impaired gait and dysarthria. Some patients may have respiratory insufficiency. Laboratory studies are consistent with a defect in mtDNA replication (summary by Reyes et al., 2015). For a discussion of genetic heterogeneity of autosomal recessive PEO, see PEOB1 (258450).

Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) clinically characterized by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Treatment with cholinesterase inhibitors or amifampridine may be helpful (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) clinically characterized by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Patients may show a favorable response to amifampridine (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Fast-channel congenital myasthenic syndrome (FCCMS) is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the AChR channel, specifically from abnormally brief opening and activity of the channel, with a rapid decay in endplate current and a failure to reach the threshold for depolarization. Treatment with pyridostigmine or amifampridine may be helpful; quinine, quinidine, and fluoxetine should be avoided (summary by Sine et al., 2003 and Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) clinically characterized by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Treatment with acetylcholinesterase inhibitors or amifampridine may be helpful (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Fast-channel congenital myasthenic syndrome (FCCMS) is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the acetylcholine receptor channel, specifically from abnormally brief opening and activity of the channel, with a rapid decay in endplate current and a failure to reach the threshold for depolarization. Treatment with pyridostigmine or amifampridine may be helpful; quinine, quinidine, and fluoxetine should be avoided (summary by Sine et al., 2003 and Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Slow-channel congenital myasthenic syndrome (SCCMS) is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the AChR channel, specifically from prolonged opening and activity of the channel, which causes prolonged synaptic currents resulting in a depolarization block. This is associated with calcium overload, which may contribute to subsequent degeneration of the endplate and postsynaptic membrane. Treatment with quinine, quinidine, or fluoxetine may be helpful; acetylcholinesterase inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized clinically by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Treatment with cholinesterase inhibitors or amifampridine may be helpful (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

An asphyxiating thoracic dystrophy that has material basis in homozygous mutation in the CEP120 gene on chromosome 5q23.

CLIFAHDD is a congenital disorder characterized by congenital contractures of the limbs and face, resulting in characteristic facial features, hypotonia, and variable degrees of developmental delay. All reported cases have occurred de novo (summary by Chong et al., 2015).

Interstitial lung and liver disease is an autosomal recessive disorder characterized by onset of respiratory insufficiency and progressive liver disease in infancy or early childhood. Pathologic examination of lung lavage is consistent with pulmonary alveolar proteinosis (summary by Hadchouel et al., 2015).

A type of arthrogryposis with characteristics of congenital cleft palate, microcephaly, craniostenosis and arthrogryposis. Additional features include facial dysmorphism. Velopharyngeal insufficiency with difficulties in swallowing, increased secretion of the nose and throat, prominent occiput, generalised muscular hypotonia with mild cyanosis and no spontaneous movements, seizures, torticollis, areflexia, intellectual disability, hypertrichosis of the lower extremities, and scleroedema are also observed. The disease often leads to early death. Transmission is autosomal recessive. No new cases have been described since 1983.

Early-onset vitamin B6-dependent epilepsy-1 (EPEO1) is an autosomal recessive neurologic disorder characterized by onset of seizures in the neonatal period or first months of life. The seizures show favorable response to treatment with activated vitamin B6 (pyridoxal 5-prime-phosphate; PLP) and/or pyridoxine. However, most patients show delayed psychomotor development (Darin et al., 2016). Genetic Heterogeneity of Early-Onset Epilepsy EPEO2 (618832) is caused by mutation in the SETD1A gene (611052) on chromosome 16p11. EPEO3 (620465) is caused by mutation in the ATP6V0C gene (108745) on chromosome 16p13. EPEO4 (266100) is caused by mutation in the ALDH7A1 gene (107323) on chromosome 5q23. EPEO5 (615400) is caused by mutation in the CNTN2 gene (190197) on chromosome 1q32.

GDACCF is an intellectual disability syndrome apparent soon after birth with neonatal hypotonia, poor feeding, and respiratory insufficiency followed by delayed psychomotor development and intellectual disability with poor speech. Brain imaging shows aplasia or hypoplasia of the corpus callosum. Affected individuals have variable dysmorphic facial features, and some may have dysplastic, cystic kidneys or mild cardiac defects (summary by Stevens et al., 2016).

Any congenital myasthenic syndrome in which the cause of the disease is a mutation in the SLC18A3 gene.

Autosomal recessive limb-girdle muscular dystrophy-21 (LGMDR21) is characterized by progressive limb-girdle weakness with age of onset ranging from congenital to adult. Muscle imaging shows a specific and selective pattern of fatty muscle degeneration (summary by Servian-Morilla et al., 2020). For a discussion of genetic heterogeneity of autosomal recessive LGMD, see LGMDR1 (253600).

PEBAT is an autosomal recessive neurodevelopmental disorder characterized by severely delayed psychomotor development apparent soon after birth or in infancy, profound intellectual disability, poor or absent speech, and seizures. Most patients are never able to walk due to hypotonia or spasticity. Brain imaging shows cerebral and cerebellar atrophy, thin corpus callosum, and secondary hypomyelination. The disorder shows progressive features, including microcephaly, consistent with a neurodegenerative process (summary by Miyake et al., 2016; Flex et al., 2016).

Distal arthrogryposis with impaired proprioception and touch is an autosomal recessive neurologic disorder characterized by loss of certain mechanosensation modalities resulting in ataxia, difficulty walking, dysmetria, muscle weakness and atrophy, and progressive skeletal contractures. Patients have onset of symptoms in early childhood (summary by Chesler et al., 2016 and Delle Vedove et al., 2016).

Hydrops, lactic acidosis, and sideroblastic anemia (HLASA) is an autosomal recessive multisystem disorder characterized by the onset of hydrops in utero. The severity of the hydrops and the disorder in general is highly variable. At birth, affected infants usually show poor growth, lactic acidosis, pulmonary hypertension with hypoxic respiratory insufficiency, and sideroblastic anemia. More variable features may include hepatosplenomegaly or cholestasis, hypoglycemia, pancreatic insufficiency, and micropenis or hypospadias. Death in infancy may occur. Those who survive tend to have resolution of lactic acidosis and anemia, but may show developmental delay and sensorineural deafness (summary by Riley et al., 2020).

Lipid storage myopathy due to FLAD1 deficiency is an autosomal recessive inborn error of metabolism that includes variable mitochondrial dysfunction. The phenotype is extremely heterogeneous: some patients have a severe disorder with onset in infancy and cardiac and respiratory insufficiency resulting in early death, whereas others have a milder course with onset of muscle weakness in adulthood. Some patients show significant improvement with riboflavin treatment (summary by Olsen et al., 2016).

Pseudo-TORCH syndrome-2 (PTORCH2) is an autosomal recessive multisystem disorder characterized by antenatal onset of intracranial hemorrhage, calcification, brain malformations, liver dysfunction, and often thrombocytopenia. Affected individuals tend to have respiratory insufficiency and seizures, and die in infancy. The phenotype resembles the sequelae of intrauterine infection, but there is no evidence of an infectious agent. The disorder results from inappropriate activation of the interferon (IFN) immunologic pathway (summary by Meuwissen et al., 2016). For a discussion of genetic heterogeneity of PTORCH, see PTORCH1 (251290).

MDCCAID is an autosomal recessive form of muscular dystrophy with onset of progressive muscle weakness in early childhood. Almost all patients also have early-onset cataracts, most have impaired intellectual development of varying severity, and some have seizures (summary by Wiessner et al., 2017 and Osborn et al., 2017).

Neurodevelopmental disorder with progressive microcephaly, spasticity, and brain anomalies (NDMSBA) is an autosomal recessive neurodevelopmental disorder characterized by infantile onset of progressive microcephaly and spasticity and severe global developmental delay resulting in profoundly impaired intellectual development and severely impaired or absent motor function. More variable features include seizures and optic atrophy. Brain imaging may show myelinating abnormalities and white matter lesions consistent with a leukoencephalopathy, as well as structural anomalies, including thin corpus callosum, gyral abnormalities, and cerebral or cerebellar atrophy. Some patients die in early childhood (summary by Falik Zaccai et al., 2017 and Hall et al., 2017).

MDDGC14 is an autosomal recessive form of muscular dystrophy characterized by onset in early childhood of mild proximal muscle weakness. Some patients may have additional features, such as mild intellectual disability or seizures. It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (summary by Carss et al., 2013). Some patients with GMPPB mutations may show features consistent with a congenital myasthenic syndrome (see, e.g., CMS1A; 601462), such as fatigability and decremental compound muscle action potential response to repetitive nerve stimulation; these patients may show a positive therapeutic response to treatment with pyridostigmine (Belaya et al., 2015). For a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type C, see MDDGC1 (609308).

CAKUTHED is an autosomal dominant highly pleiotropic developmental disorder characterized mainly by variable congenital anomalies of the kidney and urinary tract, sometimes resulting in renal dysfunction or failure, dysmorphic facial features, and abnormalities of the outer ear, often with hearing loss. Most patients have global developmental delay (summary by Heidet et al., 2017 and Slavotinek et al., 2017).

NELABA is a severe autosomal recessive metabolic disorder characterized by onset at birth of progressive encephalopathy associated with increased serum lactate. Affected individuals have little or no psychomotor development and show brain abnormalities, including cerebral atrophy, cysts, and white matter abnormalities. Some patients die in infancy (summary by Habarou et al., 2017).

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). Genetic Heterogeneity of Asphyxiating Thoracic Dysplasia SRTD1 has been mapped to chromosome 15q13. See also SRTD2 (611263), caused by mutation in the IFT80 gene (611177); SRTD3 (613091), caused by mutation in the DYNC2H1 gene (603297); SRTD4 (613819), caused by mutation in the TTC21B gene (612014); SRTD5 (614376), caused by mutation in the WDR19 gene (608151); SRTD6 (263520), caused by mutation in the NEK1 gene (604588); SRTD7 (614091), caused by mutation in the WDR35 gene (613602); SRTD8 (615503), caused by mutation in the WDR60 gene (615462); SRTD9 (266920), caused by mutation in the IFT140 gene (614620); SRTD10 (615630), caused by mutation in the IFT172 gene (607386); SRTD11 (615633), caused by mutation in the WDR34 gene (613363); SRTD13 (616300), caused by mutation in the CEP120 gene (613446); SRTD14 (616546), caused by mutation in the KIAA0586 gene (610178); SRTD15 (617088), caused by mutation in the DYNC2LI1 gene (617083); SRTD16 (617102), caused by mutation in the IFT52 gene (617094); SRTD17 (617405), caused by mutation in the TCTEX1D2 gene (617353); SRTD18 (617866), caused by mutation in the IFT43 gene (614068); SRTD19 (617895), caused by mutation in the IFT81 gene (605489); SRTD20 (617925), caused by mutation in the INTU gene (610621); and SRTD21 (619479), caused by mutation in the KIAA0753 gene (617112). See also SRTD12 (Beemer-Langer syndrome; 269860).

Amyotrophic lateral sclerosis-24 (ALS24) is a fatal neurodegenerative disease characterized by adult-onset loss of motor neurons (Brenner et al., 2016).

Hypomyelinating leukodystrophy-14 (HLD14) is an autosomal recessive neurodevelopmental disorder characterized by hypotonia, almost complete lack of motor or cognitive skills, and absent language development. Additional features include spasticity and intractable seizures; many patients also have perceptive hearing loss and/or blindness. Most patients require tube feeding or ventilatory support, and most die in the first years of life. Brain imaging shows hypomyelination, small caudate and putamen, and cerebral and cerebellar atrophy (summary by Hamilton et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of hypomyelinating leukodystrophy, see 312080.

Lethal congenital contracture syndrome-5 (LCCS5) is an autosomal recessive disorder characterized by decreased fetal movements, joint contractures, hypotonia, skeletal abnormalities with thin bones, and brain and retinal hemorrhages (Koutsopoulos et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of LCCS, see LCCS1 (253310).

Congenital hypomyelinating neuropathy (CHN) is characterized clinically by onset of hypotonia at birth, areflexia, distal muscle weakness, and very slow nerve conduction velocities (often less than 10 m/s). Warner et al. (1997, 1998) noted that pathologic findings on sural nerve biopsies show hypomyelination of most or all fibers. Based on these findings, CHN is considered to be a result of congenital impairment in myelin formation. There has been some controversy and difficulty in differentiating congenital hypomyelination from Dejerine-Sottas syndrome (DSS; 145900) because there is considerable overlap in clinical presentation. Based on pathologic findings of sural nerve biopsies (the absence of active myelin breakdown and the paucity of the onion bulbs in CHN and the presence of demyelination/remyelination and an abundance of well-organized onion bulbs in DSS; see Balestrini et al., 1991), CHN is considered to result from a congenital impairment in myelin formation, whereas DSS is thought to be due to aberrant demyelination and subsequent remyelination of the peripheral nerve. There is also variation in the prognosis of patients diagnosed with CHN. In patients with CHN, Harati and Butler (1985) showed correlation of morbidity and mortality with the presence/absence of onion bulbs: patients with few onion bulbs died in early infancy, usually because of difficulty in swallowing and respiration after birth. Patients with atypical onion bulbs survived but were affected with severe motor and sensory impairment. These differences in outcome may represent genetic heterogeneity such that mutations in essential early myelin gene(s) cause a severe phenotype, whereas mutations in other, possibly later acting gene(s), such as MPZ, lead to a less severe outcome. Genetic Heterogeneity of Congenital Hypomyelinating Neuropathy See also CHN2 (618184), caused by mutation in the MPZ gene (159440) on chromosome 1q23; and CHN3 (618186), caused by mutation in the CNTNAP1 gene (602346) on chromosome 17q21.

Immunodeficiency-100 with pulmonary alveolar proteinosis and hypogammaglobulinemia (IMD100) is primarily a lung disorder characterized by onset of respiratory insufficiency due to pulmonary alveolar proteinosis (PAP) in the first months of life. Affected individuals may have normal respiratory function at birth. Development of the disorder appears to be influenced or triggered by viral infection, manifest as progressive respiratory insufficiency, confluent consolidations on lung imaging, and diffuse collection of periodic acid-Schiff (PAS)-positive material in pulmonary alveoli associated with small and nonfoamy alveolar macrophages. Patients also have hypogammaglobulinemia, leukocytosis, and splenomegaly. Many patients die of respiratory failure in infancy or early childhood; hematopoietic stem cell transplantation (HSCT) is curative. The pathogenesis may be related to abnormal function of alveolar macrophages, resulting in decreased catabolism of surfactant (summary by Cho et al., 2018). Magg et al. (2021) determined that the disorder results from a gain-of-function effect that particularly affects B cells and monocytes.

Pontocerebellar hypoplasia type 1D (PCH1D) is a severe autosomal recessive neurologic disorder characterized by severe hypotonia and a motor neuronopathy apparent at birth or in infancy. Patients have respiratory insufficiency, feeding difficulties, and severely delayed or minimal gross motor development. Other features may include eye movement abnormalities, poor overall growth, contractures. Brain imaging shows progressive cerebellar atrophy with relative sparing of the brainstem (summary by Burns et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Stress-induced childhood-onset neurodegeneration with variable ataxia and seizures (CONDSIAS) is an autosomal recessive neurodegenerative disorder with onset in the first years of life following normal early development. Patient have cyclic episodic deterioration in response to stress, such as infection or febrile illness. The severity is highly variable: some patients develop seizures early in life that are associated with loss of developmental milestones and early sudden death in childhood, whereas others present at a later age with muscle weakness, gait ataxia, impaired speech, more subtle clinical deterioration, and cognitive decline. Neurologic involvement includes gait ataxia, cerebellar signs associated with cerebellar atrophy, generalized brain atrophy, impaired intellectual development, hearing loss, and peripheral neuropathy (summary by Ghosh et al., 2018).

Congenital hypomyelinating neuropathy-3 is an autosomal recessive neurologic disorder characterized by onset of neurogenic muscle impairment in utero. Affected individuals present at birth with severe hypotonia, often causing respiratory insufficiency or failure and inability to swallow or feed properly. They have profoundly impaired psychomotor development and may die in infancy or early childhood. Those that survive are unable to sit or walk. Sural nerve biopsy shows hypomyelination of the nerve fibers, and brain imaging often shows impaired myelination and cerebral and cerebellar atrophy. Nerve conduction velocities are severely decreased (about 10 m/s) or absent due to improper myelination (summary by Vallat et al., 2016 and Low et al., 2018). For a discussion of genetic heterogeneity of CHN, see CHN1 (605253).

Fibrosis, neurodegeneration, and cerebral angiomatosis (FINCA) is characterized by severe progressive cerebropulmonary symptoms, resulting in death in infancy from respiratory failure. Features include malabsorption, progressive growth failure, recurrent infections, chronic hemolytic anemia, and transient liver dysfunction. Neuropathology shows increased angiomatosis-like leptomeningeal, cortical, and superficial white matter vascularization and congestion, vacuolar degeneration and myelin loss in white matter, as well as neuronal degeneration. Interstitial fibrosis and granuloma-like lesions are seen in the lungs, and there is hepatomegaly with steatosis and collagen accumulation (Uusimaa et al., 2018).

The fetal akinesia deformation sequence (FADS) refers to a clinically and genetically heterogeneous constellation of features including fetal akinesia, intrauterine growth retardation, arthrogryposis, and developmental anomalies, including lung hypoplasia, cleft palate, and cryptorchidism (Vogt et al., 2009). It shows phenotypic overlap with the lethal form of multiple pterygium syndrome (see 253290). For a general phenotypic description and a discussion of genetic heterogeneity of FADS, see 208150.

Fatal infantile hypertonic myofibrillar myopathy is a severe autosomal recessive muscular dystrophy with onset in the first weeks of life after a normal neonatal period. Affected infants show rapidly progressive muscular rigidity of the trunk and limbs associated with increasing respiratory difficulty resulting in death before age 3 years (summary by Del Bigio et al., 2011).

Developmental and epileptic encephalopathy-71 (DEE71) is characterized by early neonatal refractory seizures, respiratory failure, structural brain abnormalities and cerebral edema, with death within weeks after birth. Glutamine levels are significantly increased (z score 3.2-11.7). Three patients have been described (summary by Rumping et al., 2019).

Combined oxidative phosphorylation deficiency-37 is an autosomal recessive multisystem disorder apparent at birth or in the first months of life. Affected individuals have hypotonia, failure to thrive, and neurodegeneration with loss of developmental milestones, as well as liver dysfunction. Some patients may have hypertrophic cardiomyopathy, loss of vision and hearing, and/or seizures. Mitochondrial respiratory dysfunction is apparent in liver and skeletal muscle tissue. Most patients die in childhood (summary by Zeharia et al., 2016). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Neurodevelopmental disorder with central and peripheral motor dysfunction (NEDCPMD) is an autosomal recessive neurologic disorder with a highly variable phenotype. At the severe end of the spectrum, patients may have hypotonia apparent from birth, necessitating mechanical respiration and tube-feeding, and global developmental delay with absence of reaction to touch and no eye contact. At the mild end of the spectrum, patients may present with infantile-onset progressive ataxia and demyelinating peripheral neuropathy. The disorder is caused by mutation in the NFASC gene, which has several neuronal- and glial-specific transcripts. The variable clinical phenotype may be caused by several factors, including the severity of the mutation, the selective involvement of distinct isoforms by pathogenic variants, and the presence of genetic modifiers (summary by Monfrini et al., 2019).

Congenital myopathy-19 (CMYO19) is an autosomal recessive skeletal muscle disorder characterized by infantile-onset of progressive muscle weakness and atrophy associated with scoliosis, variably impaired walking, and dysmorphic facial features (Feichtinger et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Congenital myopathy-8 (CMYO8) is an autosomal dominant disorder of the skeletal muscle characterized by hypotonia and delayed motor development apparent from infancy or childhood, resulting in difficulties walking or loss of ambulation within the first few decades. Affected individuals show respiratory insufficiency, high-arched palate, and scoliosis; external ophthalmoplegia may also be present. Skeletal muscle biopsy shows cores and myofibrillar disorganization (Lornage et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Primary ciliary dyskinesia-42 (CILD42) is an autosomal recessive disorder characterized by a defect in motile cilia and ciliary clearance resulting in the onset of respiratory insufficiency soon after birth, and associated with recurrent upper and lower respiratory infections with chronic progressive lung disease. Other more variable features may include infertility and mild hydrocephalus. Patients with this form of the disorder do not have situs abnormalities. The disorder is considered to be a type of ciliopathy known as 'reduced generation of multiple motile cilia' (RGMC) (summary by Boon et al., 2014). For a discussion of genetic heterogeneity of primary ciliary dyskinesia, CILD1 (244400).

Pseudo-TORCH syndrome-3 (PTORCH3) is an autosomal recessive disorder of immune dysregulation and neuroinflammation apparent from early infancy. Affected individuals have developmental delay with acute episodes of fever and multisystemic organ involvement, including coagulopathy, elevated liver enzymes, and proteinuria, often associated with thrombotic microangiopathy. Brain imaging shows progressive intracranial calcifications, white matter abnormalities, and sometimes cerebral or cerebellar atrophy. Laboratory studies show abnormal elevation of interferon (IFN)-stimulated gene (ISG) transcripts consistent with a type I interferonopathy. The phenotype resembles the sequelae of intrauterine infection, but there is usually no evidence of an infectious agent. The disorder results from defects in negative regulation of the interferon immunologic pathway. Death in early childhood is common (summary by Duncan et al., 2019 and Gruber et al., 2020). For a discussion of genetic heterogeneity of PTORCH, see PTORCH1 (251290).

Rajab interstitial lung disease with brain calcifications-1 (RILDBC1) is an autosomal recessive multisystem disorder with a highly variable phenotype. Most patients present in infancy or early childhood with poor growth and interstitial lung disease, which may lead to death. Some may also have liver, skeletal, and renal abnormalities, and most have intracranial calcifications on brain imaging. Some may have early impaired motor development, but most have normal cognitive development (summary by Xu et al., 2018). Genetic Heterogeneity of Rajab Interstitial Lung Disease with Brain Calcifications Also see Rajab interstitial disease with brain calcifications-2 (RILDBC2; 619013), caused by mutation in the FARSA gene (602918).

Congenital myopathy-17 (CMYO17) is an autosomal recessive muscle disorder. Affected individuals present at birth with hypotonia and respiratory insufficiency associated with high diaphragmatic dome on imaging. Other features include poor overall growth, pectus excavatum, dysmorphic facies, and renal anomalies in some. The severity of the disorder is highly variable: some patients may have delayed motor development with mildly decreased endurance, whereas others have more severe hypotonia associated with distal arthrogryposis and lung hypoplasia, resulting in early death (summary by Watson et al., 2016 and Lopes et al., 2018). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

The phenotypic spectrum of GARS1-associated axonal neuropathy ranges from GARS1 infantile-onset SMA (GARS1-iSMA) to GARS1 adolescent- or early adult-onset hereditary motor/sensory neuropathy (GARS1-HMSN). GARS1-iSMA. Age of onset ranges from the neonatal period to the toddler years. Initial manifestations are typically respiratory distress, poor feeding, and muscle weakness (distal greater than proximal). Weakness is slowly progressive, ultimately requiring mechanical ventilation and feeding via gastrostomy tube. GARS1-HMSN. Age of onset is most commonly during the second decade (range eight to 36 years). Initial manifestations are typically muscle weakness in the hands sometimes with sensory deficits. Lower limb involvement (seen in ~50% of individuals) ranges from weakness and atrophy of the extensor digitorum brevis and weakness of toe dorsiflexors to classic peroneal muscular atrophy with foot drop and a high steppage gait.

Neuronal ceroid lipofuscinosis-15 (CLN15) is characterized by severe global developmental delay apparent in infancy or early childhood. Affected individuals have hypotonia with impaired motor development, respiratory insufficiency, and feeding difficulties requiring intervention. Intellectual and speech development is also delayed, and most have visual defects, including cortical visual blindness, nystagmus, and esotropia. The disorder is progressive, as manifest by developmental regression consistent with neurodegeneration. Although overt seizures are not observed, some patients may have episodic hypertonia or apnea, and EEG may show nonspecific abnormalities. Brain imaging shows unique diffusion restriction signal abnormalities affecting the brainstem, cerebellum, and corticospinal tracts. Early death may occur (summary by Polovitskaya et al., 2020).

Muscular dystrophy, congenital hearing loss, and ovarian insufficiency syndrome (MDHLO) is an autosomal recessive systemic disorder characterized by progressive muscle weakness, sensorineural hearing loss, and endocrine abnormalities, mainly primary amenorrhea due to ovarian insufficiency. Features of the disorder appear soon after birth, although endocrine anomalies are not noted until puberty. The severity of the phenotype is variable: some patients may lose ambulation and have significant respiratory insufficiency, whereas others retain the ability to walk (Foley et al., 2020).

Infantile hypotonia with psychomotor retardation and characteristic facies-3 is a severe autosomal recessive neurodevelopmental disorder with onset at birth or in early infancy. Most affected individuals show very poor, if any, normal psychomotor development, poor speech, and inability to walk independently (summary by Bhoj et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of infantile hypotonia with psychomotor retardation and characteristic facies, see IHPRF1 (615419).

Primary coenzyme Q10 (CoQ10) deficiency is usually associated with multisystem involvement, including neurologic manifestations such as fatal neonatal encephalopathy with hypotonia; a late-onset slowly progressive multiple-system atrophy-like phenotype (neurodegeneration with autonomic failure and various combinations of parkinsonism and cerebellar ataxia, and pyramidal dysfunction); and dystonia, spasticity, seizures, and intellectual disability. Steroid-resistant nephrotic syndrome (SRNS), the hallmark renal manifestation, is often the initial manifestation either as isolated renal involvement that progresses to end-stage renal disease (ESRD), or associated with encephalopathy (seizures, stroke-like episodes, severe neurologic impairment) resulting in early death. Hypertrophic cardiomyopathy (HCM), retinopathy or optic atrophy, and sensorineural hearing loss can also be seen.

Combined D-2- and L-2-hydroxyglutaric aciduria (D2L2AD) is an autosomal recessive neurometabolic disorder characterized by neonatal-onset encephalopathy with severe muscular weakness, intractable seizures, respiratory distress, and lack of psychomotor development resulting in early death. Brain imaging shows abnormalities including enlarged ventricles, delayed myelination, and germinal layer cysts (Muntau et al., 2000). See also isolated L-2-hydroxyglutaric aciduria (236792) and isolated D-2-hydroxyglutaric aciduria (see 600721).

Carey-Fineman-Ziter syndrome-1 (CFZS1) is a multisystem congenital disorder characterized by hypotonia, Moebius sequence (bilateral congenital facial palsy with impairment of ocular abduction), Pierre Robin complex (micrognathia, glossoptosis, and high-arched or cleft palate), delayed motor milestones, and failure to thrive. More variable features include dysmorphic facial features, brain abnormalities, and intellectual disability. It has been postulated that many clinical features in CFZS1 may be secondary effects of muscle weakness during development or brainstem anomalies (summary by Pasetti et al., 2016). Di Gioia et al. (2017) determined that CFZS1 represents a slowly progressive congenital myopathy resulting from a defect in myoblast fusion. Genetic Heterogeneity of Carey-Fineman-Ziter Syndrome Carey-Fineman-Ziter syndrome-2 (CFZS2) is caused by mutation in the MYMX gene (619912) on chromosome 6p21.

Stuve-Wiedemann syndrome is an autosomal recessive disorder characterized by bowing of the long bones and other skeletal anomalies, episodic hyperthermia, respiratory distress, and feeding difficulties usually resulting in early death (Dagoneau et al., 2004). See also 'classic' Schwartz-Jampel syndrome type 1 (SJS1; 255800), a phenotypically similar but genetically distinct disorder caused by mutation in the HSPG2 gene (142461) on chromosome 1p36. Genetic Heterogeneity of Stuve-Wiedemann Syndrome Stuve-Wiedemann syndrome-2 (STWS2; 619751) is caused by mutation in the IL6ST gene (600694) on chromosome 5q11.

Pontocerebellar hypoplasia type 17 (PCH17) is a severe autosomal recessive developmental disorder characterized by neonatal hypotonia, severe feeding difficulties, and respiratory insufficiency. Brain imaging shows cerebellar and brainstem hypoplasia. Most affected individuals die in infancy. Those who survive show variable developmental delay. Other features of the disorder include distal hypertonia, poor overall growth, visual defects, autonomic problems, dysmorphic features, and seizures (Coolen et al., 2022). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

An instance of renal tubular dysgenesis that is caused by a modification of the individual's genome.

Primordial dwarfism-immunodeficiency-lipodystrophy syndrome (PDIL) is characterized by pre- and postnatal growth restriction, with extreme microcephaly, short stature, and absence of subcutaneous fat. There is also significant hematologic/immune dysfunction, with hypo- or agammaglobulinemia, as well as lymphopenia, anemia, and thrombocytopenia, and most affected individuals succumb to infection in early childhood (Parry et al., 2020).

Congenital myopathy-4B (CMYO4B) is an autosomal recessive disorder of the skeletal muscle characterized by the onset of muscle weakness in infancy or early childhood. The severity and pattern of muscle weakness varies, but most affected individuals show congenital contractures, delayed motor development, hypotonia, generalized muscle weakness, and weakness of the proximal limb muscles and neck muscles, resulting in difficulty walking or inability to walk. Affected individuals have respiratory insufficiency due to muscle weakness, which may be life-threatening. Other common features include myopathic facies, chest deformities, distal joint laxity, and scoliosis. Variable histologic findings on skeletal muscle biopsy are observed, including nemaline rods, type 1 fiber predomination, and centralized nuclei (Tan et al., 1999; Lehtokari et al., 2008). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Autosomal recessive congenital myopathy-2B (CMYO2B) is a disorder of the skeletal muscle characterized by severe hypotonia with lack of spontaneous movements and respiratory insufficiency, usually leading to death in infancy or early childhood (Agrawal et al., 2004). However, longer survival has also been reported, likely due to the type of mutation and extent of its impact (O'Grady et al., 2015). Mutations in the ACTA1 gene can cause a range of skeletal muscle diseases. About 90% of patients with ACTA1 mutations carry heterozygous mutations, usually de novo (CMYO2A; 161800), whereas 10% of patients carry biallelic ACTA1 mutations (CMYO2B) (Nowak et al., 2007). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Juvenile amyotrophic lateral sclerosis-27 (ALS27) is an autosomal dominant disorder characterized by early childhood-onset lower extremity spasticity manifesting as toe walking and gait abnormalities, followed by progressive lower motor neuron-mediated weakness without sensory signs or symptoms (Mohassel et al., 2021). For a discussion of genetic heterogeneity of amyotrophic lateral sclerosis, see ALS1 (105400).

Classic congenital myopathy-22A (CMYO22A) is an autosomal recessive muscle disorder characterized by onset of muscle weakness in utero or soon after birth. Early features may include fetal hypokinesia, breech presentation, and polyhydramnios. Affected individuals are born with severe hypotonia and require respiratory and feeding assistance. Those who survive the neonatal period show a 'classic' phenotype of congenital myopathy with delayed motor development, difficulty walking, proximal muscle weakness of the upper and lower limbs, facial and neck muscle weakness, easy fatigability, and mild limb contractures or foot deformities. Some have persistent respiratory insufficiency; dysmorphic facial features may be present (Zaharieva et al., 2016). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Neurodevelopmental disorder with intracranial hemorrhage, seizures, and spasticity (NEDIHSS) is an autosomal recessive disorder characterized by prenatal or neonatal onset of intracranial hemorrhage, usually with ventriculomegaly and calcifications, resulting in parenchymal brain damage. Some affected individuals have symptoms incompatible with life and die in utero. Those that survive show profound global developmental delay with almost no motor or cognitive skills, hypotonia, spasticity, and seizures. Other features may include facial dysmorphism, retinal vascular abnormalities, and poor overall growth. The pathogenesis of the disease likely results from dysfunction of vascular endothelial cells in the brain (Lecca et al., 2023).

Autosomal recessive limb-girdle muscular dystrophy-28 (LGMDR28) is characterized by progressive muscle weakness affecting the proximal and axial muscles of the upper and lower limbs. The age at onset is highly variable, usually in the first decade, although onset in the fourth decade has also been reported. The disorder can be rapidly progressive or show a slower course. Most patients have limited ambulation or become wheelchair-bound within a few decades, and respiratory insufficiency commonly occurs. Laboratory studies show increased serum creatine kinase and elevated fasting blood glucose levels, although cholesterol is normal. EMG shows a myopathic pattern; muscle biopsy is generally unremarkable, but can show nonspecific myopathic or dystrophic features (Yogev et al., 2023; Morales-Rosado et al., 2023). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

Autosomal dominant nemaline myopathy-5C (NEM5C) is a relatively mild skeletal muscle disorder with wide clinical variability, even within families. Affected individuals develop symptoms of muscle weakness in the first or second decades; those with earlier onset tend to have a more severe disease course. Features include difficulty walking on the heels, waddling gait, proximal muscle weakness affecting the upper and lower limbs, and Gowers sign. Additional features may include myopathic facies, high-arched palate, scoliosis or kyphosis, and ankle weakness. Patients remain ambulatory into late adulthood. Skeletal muscle biopsy shows hypotrophy of type 1 fibers, hypertrophy of type 2 fibers, fiber size variation, and myofibrillar disorganization. Nemaline rods in type 1 fibers are often observed, but are not always present (Konersman et al., 2017; Holling et al., 2022). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM2 (256030).

Oculopharyngeal muscular dystrophy-2 (OPMD2) is an autosomal dominant muscle disorder characterized by early-onset ptosis, ophthalmoplegia, dysphagia, variable respiratory insufficiency, and proximal limb muscle weakness. Most patients have onset in the first years of life, although rare patients have onset in their teens. The disorder is slowly progressive and the severity is highly variable; the most severely affected individuals lose ambulation and may require tube-feeding or noninvasive ventilation (Kim et al., 2022). For a discussion of genetic heterogeneity of OPMD, see OPMD1 (164300).

Spinal muscular atrophy (SMA) is characterized by muscle weakness and atrophy resulting from progressive degeneration and irreversible loss of the anterior horn cells in the spinal cord (i.e., lower motor neurons) and the brain stem nuclei. The onset of weakness ranges from before birth to adulthood. The weakness is symmetric, proximal > distal, and progressive. Before the genetic basis of SMA was understood, it was classified into clinical subtypes based on maximum motor function achieved; however, it is now apparent that the phenotype of SMN1-associated SMA spans a continuum without clear delineation of subtypes. With supportive care only, poor weight gain with growth failure, restrictive lung disease, scoliosis, and joint contractures are common complications; however, newly available targeted treatment options are changing the natural history of this disease.

Infection-induced acute encephalitis-12 (IIAE12) is an autosomal recessive disorder characterized by episodic acute encephalopathy associated with infections and febrile illness. Patients present with neurologic symptoms in the first months or years of life, usually after normal early development. Brain imaging in the acute episodes shows restriction on diffusion-weighted imaging (DWI) and T2-weighted abnormalities in the periventricular and subcortical regions, which progresses to cavitation of previously affected areas. The long-term outcome is highly variable, even within families, ranging from death to severe neurologic deficits to normal outcomes (Shashi et al., 2023). For a discussion of genetic heterogeneity of susceptibility to acute infection-induced encephalopathy, see 610551.

Primary ciliary dyskinesia-53 (CILD53) is an autosomal recessive disorder characterized by randomization of the left-right body asymmetry and respiratory symptoms (Hjeij et al., 2023). For a discussion of genetic heterogeneity of primary ciliary dyskinesia, see CILD1 (244400).