Abetalipoproteinemia typically presents in infancy with failure to thrive, diarrhea, vomiting, and malabsorption of fat. Hematologic manifestations may include acanthocytosis (irregularly spiculated erythrocytes), anemia, reticulocytosis, and hemolysis with resultant hyperbilirubinemia. Malabsorption of fat-soluble vitamins (A, D, E, and K) can result in an increased international normalized ratio (INR). Untreated individuals may develop atypical pigmentation of the retina that may present with progressive loss of night vision and/or color vision in adulthood. Neuromuscular findings in untreated individuals including progressive loss of deep tendon reflexes, vibratory sense, and proprioception; muscle weakness; dysarthria; and ataxia typically manifest in the first or second decades of life.

Pantothenate kinase-associated neurodegeneration (PKAN) is a type of neurodegeneration with brain iron accumulation (NBIA). The phenotypic spectrum of PKAN includes classic PKAN and atypical PKAN. Classic PKAN is characterized by early-childhood onset of progressive dystonia, dysarthria, rigidity, and choreoathetosis. Pigmentary retinal degeneration is common. Atypical PKAN is characterized by later onset (age >10 years), prominent speech defects, psychiatric disturbances, and more gradual progression of disease.

Alpha-mannosidosis encompasses a continuum of clinical findings from mild to severe. Three major clinical subtypes have been suggested: A mild form recognized after age ten years with absence of skeletal abnormalities, myopathy, and slow progression (type 1). A moderate form recognized before age ten years with presence of skeletal abnormalities, myopathy, and slow progression (type 2). A severe form manifested as prenatal loss or early death from progressive central nervous system involvement or infection (type 3). Individuals with a milder phenotype have mild-to-moderate intellectual disability, impaired hearing, characteristic coarse features, clinical or radiographic skeletal abnormalities, immunodeficiency, and primary central nervous system disease – mainly cerebellar involvement causing ataxia. Periods of psychiatric symptoms are common. Associated medical problems can include corneal opacities, hepatosplenomegaly, aseptic destructive arthritis, and metabolic myopathy. Alpha-mannosidosis is insidiously progressive; some individuals may live into the sixth decade.

Mucopolysaccharidosis type I (MPS I) is a progressive multisystem disorder with features ranging over a continuum of severity. While affected individuals have traditionally been classified as having one of three MPS I syndromes (Hurler syndrome, Hurler-Scheie syndrome, or Scheie syndrome), no easily measurable biochemical differences have been identified and the clinical findings overlap. Affected individuals are best described as having either a phenotype consistent with either severe (Hurler syndrome) or attenuated MPS I, a distinction that influences therapeutic options. Severe MPS I. Infants appear normal at birth. Typical early manifestations are nonspecific (e.g., umbilical or inguinal hernia, frequent upper respiratory tract infections before age 1 year). Coarsening of the facial features may not become apparent until after age one year. Gibbus deformity of the lower spine is common and often noted within the first year. Progressive skeletal dysplasia (dysostosis multiplex) involving all bones is universal, as is progressive arthropathy involving most joints. By age three years, linear growth decreases. Intellectual disability is progressive and profound but may not be readily apparent in the first year of life. Progressive cardiorespiratory involvement, hearing loss, and corneal clouding are common. Without treatment, death (typically from cardiorespiratory failure) usually occurs within the first ten years of life. Attenuated MPS I. Clinical onset is usually between ages three and ten years. The severity and rate of disease progression range from serious life-threatening complications leading to death in the second to third decade, to a normal life span complicated by significant disability from progressive joint manifestations and cardiorespiratory disease. While some individuals have no neurologic involvement and psychomotor development may be normal in early childhood, learning disabilities and psychiatric manifestations can be present later in life. Hearing loss, cardiac valvular disease, respiratory involvement, and corneal clouding are common.

GNPTAB-related disorders comprise the phenotypes mucolipidosis II (ML II) and mucolipidosis IIIa/ß (ML IIIa/ß), and phenotypes intermediate between ML II and ML IIIa/ß. ML II is evident at birth and slowly progressive; death most often occurs in early childhood. Orthopedic abnormalities present at birth may include thoracic deformity, kyphosis, clubfeet, deformed long bones, and/or dislocation of the hip(s). Growth often ceases in the second year of life; contractures develop in all large joints. The skin is thickened, facial features are coarse, and gingiva are hypertrophic. All children have cardiac involvement, most commonly thickening and insufficiency of the mitral valve and, less frequently, the aortic valve. Progressive mucosal thickening narrows the airways, and gradual stiffening of the thoracic cage contributes to respiratory insufficiency, the most common cause of death. ML IIIa/ß becomes evident at about age three years with slow growth rate and short stature; joint stiffness and pain initially in the shoulders, hips, and fingers; gradual mild coarsening of facial features; and normal to mildly impaired cognitive development. Pain from osteoporosis becomes more severe during adolescence. Cardiorespiratory complications (restrictive lung disease, thickening and insufficiency of the mitral and aortic valves, left and/or right ventricular hypertrophy) are common causes of death, typically in early to middle adulthood. Phenotypes intermediate between ML II and ML IIIa/ß are characterized by physical growth in infancy that resembles that of ML II and neuromotor and speech development that resemble that of ML IIIa/ß.

Adult Refsum disease (ARD is associated with elevated plasma phytanic acid levels, late childhood-onset (or later) retinitis pigmentosa, and variable combinations of anosmia, polyneuropathy, deafness, ataxia, and ichthyosis. Onset of symptoms ranges from age seven months to older than age 50 years. Cardiac arrhythmia and heart failure caused by cardiomyopathy are potentially severe health problems that develop later in life.

Werner syndrome is characterized by the premature appearance of features associated with normal aging and cancer predisposition. Individuals with Werner syndrome develop normally until the end of the first decade. The first sign is the lack of a growth spurt during the early teen years. Early findings (usually observed in the 20s) include loss and graying of hair, hoarseness, and scleroderma-like skin changes, followed by bilateral ocular cataracts, type 2 diabetes mellitus, hypogonadism, skin ulcers, and osteoporosis in the 30s. Myocardial infarction and cancer are the most common causes of death; the mean age of death in individuals with Werner syndrome is 54 years.

Mucopolysaccharidosis type I (MPS I) is a progressive multisystem disorder with features ranging over a continuum of severity. While affected individuals have traditionally been classified as having one of three MPS I syndromes (Hurler syndrome, Hurler-Scheie syndrome, or Scheie syndrome), no easily measurable biochemical differences have been identified and the clinical findings overlap. Affected individuals are best described as having either a phenotype consistent with either severe (Hurler syndrome) or attenuated MPS I, a distinction that influences therapeutic options. Severe MPS I. Infants appear normal at birth. Typical early manifestations are nonspecific (e.g., umbilical or inguinal hernia, frequent upper respiratory tract infections before age 1 year). Coarsening of the facial features may not become apparent until after age one year. Gibbus deformity of the lower spine is common and often noted within the first year. Progressive skeletal dysplasia (dysostosis multiplex) involving all bones is universal, as is progressive arthropathy involving most joints. By age three years, linear growth decreases. Intellectual disability is progressive and profound but may not be readily apparent in the first year of life. Progressive cardiorespiratory involvement, hearing loss, and corneal clouding are common. Without treatment, death (typically from cardiorespiratory failure) usually occurs within the first ten years of life. Attenuated MPS I. Clinical onset is usually between ages three and ten years. The severity and rate of disease progression range from serious life-threatening complications leading to death in the second to third decade, to a normal life span complicated by significant disability from progressive joint manifestations and cardiorespiratory disease. While some individuals have no neurologic involvement and psychomotor development may be normal in early childhood, learning disabilities and psychiatric manifestations can be present later in life. Hearing loss, cardiac valvular disease, respiratory involvement, and corneal clouding are common.

Mucolipidosis IV (MLIV) is an ultra-rare lysosomal storage disorder characterized by severe psychomotor delay, progressive visual impairment, and achlorhydria. Individuals with MLIV typically present by the end of the first year of life with delayed developmental milestones (due to a developmental brain abnormality) and impaired vision (resulting from a combination of corneal clouding and retinal degeneration). By adolescence, all individuals with MLIV have severe visual impairment. A neurodegenerative component of MLIV has become more widely appreciated, with the majority of individuals demonstrating progressive spastic quadriparesis and loss of psychomotor skills starting in the second decade of life. About 5% of individuals have atypical MLIV, manifesting with less severe psychomotor impairment, but still exhibiting progressive retinal degeneration and achlorhydria.

Initial symptoms of multiple sulfatase deficiency (MSD) can develop from infancy through early childhood, and presentation is widely variable. Some individuals display the multisystemic features characteristic of mucopolysaccharidosis disorders (e.g., developmental regression, organomegaly, skeletal deformities) while other individuals present primarily with neurologic regression (associated with leukodystrophy). Based on age of onset, rate of progression, and disease severity, several different clinical subtypes of MSD have been described: Neonatal MSD is the most severe with presentation in the prenatal period or at birth with rapid progression and death occurring within the first two years of life. Infantile MSD is the most common variant and may be characterized as attenuated (slower clinical course with cognitive disability and neurodegeneration identified in the 2nd year of life) or severe (loss of the majority of developmental milestones by age 5 years). Juvenile MSD is the rarest subtype with later onset of symptoms and subacute clinical presentation. Many of the features found in MSD are progressive, including neurologic deterioration, heart disease, hearing loss, and airway compromise.

Paget disease of bone-5 is an autosomal recessive, juvenile-onset form of Paget disease, a disorder of the skeleton resulting from abnormal bone resorption and formation. Clinical manifestations include short stature, progressive long bone deformities, fractures, vertebral collapse, skull enlargement, and hyperostosis with progressive deafness. There is phenotypic variability, with some patients presenting in infancy, while others present later in childhood (summary by Naot et al., 2014). For discussion of genetic heterogeneity of Paget disease of bone, see 167250.

Thiamine-responsive megaloblastic anemia syndrome (TRMA) is characterized by megaloblastic anemia, progressive sensorineural hearing loss, and diabetes mellitus. Onset of megaloblastic anemia occurs between infancy and adolescence. The anemia is corrected with thiamine treatment, but the red cells remain macrocytic and anemia can recur if treatment is withdrawn. Progressive sensorineural hearing loss often occurs early and can be detected in toddlers; hearing loss is irreversible and may not be prevented by thiamine treatment. The diabetes mellitus is non-type I in nature, with age of onset from infancy to adolescence. Thiamine treatment may reduce insulin requirement and delay onset of diabetes in some individuals.

Maternally inherited diabetes-deafness syndrome (MIDD) is a mitochondrial disorder characterized by onset of sensorineural hearing loss and diabetes in adulthood. Some patients may have additional features observed in mitochondrial disorders, including pigmentary retinopathy, ptosis, cardiomyopathy, myopathy, renal problems, and neuropsychiatric symptoms (Ballinger et al., 1992; Reardon et al., 1992; Guillausseau et al., 2001). The association of diabetes and deafness is observed with Wolfram syndrome (see 222300), Rogers syndrome (249270), and Herrmann syndrome (172500), but all 3 of these disorders have other clinical manifestations.

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). The hallmark of CLN3 is the ultrastructural pattern of lipopigment with a 'fingerprint' profile, which can have 3 different appearances: pure within a lysosomal residual body; in conjunction with curvilinear or rectilinear profiles; and as a small component within large membrane-bound lysosomal vacuoles. The combination of fingerprint profiles within lysosomal vacuoles is a regular feature of blood lymphocytes from patients with CLN3 (Mole et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of CLN, see CLN1 (256730).

Aceruloplasminemia is characterized by iron accumulation in the brain and viscera. The clinical triad of retinal degeneration, diabetes mellitus (DM), and neurologic disease is seen in individuals ranging from age 30 years to older than 70 years. The neurologic findings of movement disorder (blepharospasm, grimacing, facial and neck dystonia, tremors, chorea) and ataxia (gait ataxia, dysarthria) correspond to regions of iron deposition in the brain. Individuals with aceruloplasminemia often present with anemia prior to onset of DM or obvious neurologic problems. Cognitive dysfunction including apathy and forgetfulness occurs in more than half of individuals with this condition.

A retinitis pigmentosa that has material basis in variation in the chromosome region 1p21.3-p13.3.

Any retinitis pigmentosa in which the cause of the disease is a mutation in the PRPF8 gene.

PNPLA6 disorders span a phenotypic continuum characterized by variable combinations of cerebellar ataxia; upper motor neuron involvement manifesting as spasticity and/or brisk reflexes; chorioretinal dystrophy associated with variable degrees of reduced visual function; and hypogonadotropic hypogonadism (delayed puberty and lack of secondary sex characteristics). The hypogonadotropic hypogonadism occurs either in isolation or as part of anterior hypopituitarism (growth hormone, thyroid hormone, or gonadotropin deficiencies). Common but less frequent features are peripheral neuropathy (usually of axonal type manifesting as reduced distal reflexes, diminished vibratory sensation, and/or distal muscle wasting); hair anomalies (long eyelashes, bushy eyebrows, or scalp alopecia); short stature; and impaired cognitive functioning (learning disabilities in children; deficits in attention, visuospatial abilities, and recall in adults). Some of these features can occur in distinct clusters on the phenotypic continuum: Boucher-Neuhäuser syndrome (cerebellar ataxia, chorioretinal dystrophy, and hypogonadotropic hypogonadism); Gordon Holmes syndrome (cerebellar ataxia, hypogonadotropic hypogonadism, and – to a variable degree – brisk reflexes); Oliver-McFarlane syndrome (trichomegaly, chorioretinal dystrophy, short stature, intellectual disability, and hypopituitarism); Laurence-Moon syndrome; and spastic paraplegia type 39 (SPG39) (upper motor neuron involvement, peripheral neuropathy, and sometimes reduced cognitive functioning and/or cerebellar ataxia).

Spastic paraplegia 15 (SPG15), typically an early-onset complex hereditary spastic paraplegia, is characterized by progressive spasticity that begins in the lower extremities and is associated with several manifestations resulting from central and peripheral nervous system dysfunction. While onset of spasticity is typically in mid- to late childhood or adolescence (i.e., between ages 5 and 18 years), other manifestations, such as developmental delay or learning disability, may be present earlier, often preceding motor involvement. Individuals with adult onset have also been reported.

A rare multiple congenital anomalies/dysmorphic syndrome with characteristics of profound intellectual disability, choreoathetosis, progressive spastic diplegia, progressive tapetoretinal degeneration with loss of retinal vessels and glomerulopathy resulting in death late in the first or early in the second decade of life. Absence of the cerebellar granular layer has been reported.

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 lipopigment patterns observed most often in CLN5 comprise mixed combinations of 'granular,' 'curvilinear,' and 'fingerprint' profiles. The clinical course includes progressive dementia, seizures, and progressive visual failure (Mole et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of CLN, see CLN1 (256730).

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 lipopigment pattern seen most often in CLN1 is referred to as granular osmiophilic deposits (GROD). The patterns most often observed in CLN2 and CLN3 are 'curvilinear' and 'fingerprint' profiles, respectively. CLN4, CLN5, CLN6, CLN7, and CLN8 show mixed combinations of granular, curvilinear, fingerprint, and rectilinear profiles. The clinical course includes progressive dementia, seizures, and progressive visual failure (Mole et al., 2005). Zeman and Dyken (1969) referred to these conditions as the 'neuronal ceroid lipofuscinoses.' Goebel (1995) provided a comprehensive review of the NCLs and noted that they are possibly the most common group of neurodegenerative diseases in children. Mole et al. (2005) provided a detailed clinical and genetic review of the neuronal ceroid lipofuscinoses. Genetic Heterogeneity of Neuronal Ceroid Lipofuscinosis See also CLN2 (204500), caused by mutation in the TPP1 gene (607998) on chromosome 11p15; CLN3 (204200), caused by mutation in the CLN3 gene (607042) on 16p12; CLN4 (162350), caused by mutation in the DNAJC5 gene (611203) on 20q13; CLN5 (256731), caused by mutation in the CLN5 gene (608102) on 13q22; CLN6A (601780) and CLN6B (204300), both caused by mutation in the CLN6 gene (606725) on 15q21; CLN7 (610951), caused by mutation in the MFSD8 gene (611124) on 4q28; CLN8 (600143) and the Northern epilepsy variant of CLN8 (610003), both caused by mutation in the CLN8 gene (607837) on 8p23; CLN10 (610127), caused by mutation in the CTSD gene (116840) on 11p15; CLN11 (614706), caused by mutation in the GRN gene (138945) on 17q21; CLN13 (615362), caused by mutation in the CTSF gene (603539) on 11q13; and CLN14 (611726), caused by mutation in the KCTD7 gene (611725) on 7q11. CLN9 (609055) has not been molecularly characterized. A disorder that was formerly designated neuronal ceroid lipofuscinosis-12 (CLN12) is now considered to be a variable form of Kufor-Rakeb syndrome (KRS; 606693).

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.

Late-onset retinal degeneration (LORD) is an autosomal dominant disorder characterized by onset in the fifth to sixth decade with night blindness and punctate yellow-white deposits in the retinal fundus, progressing to severe central and peripheral degeneration, with choroidal neovascularization and chorioretinal atrophy (Hayward et al., 2003).

Retinitis pigmentosa with or without skeletal anomalies (RPSKA) is characterized by retinal degeneration, brachydactyly, craniofacial abnormalities, short stature, and neurologic defects. Night blindness occurs around 10 years of age, followed by restriction of visual fields. Brachydactyly affects primarily the distal phalanges. Craniofacial abnormalities include frontal bossing, downslanting palpebral fissures, large columella, hypoplastic nares, micrognathia, and large low-set ears (summary by Xu et al., 2017).

Spastic paraplegia 11 (SPG11) is characterized by progressive spasticity and weakness of the lower limbs frequently associated with the following: mild intellectual disability with learning difficulties in childhood and/or progressive cognitive decline; peripheral neuropathy; pseudobulbar involvement; and increased reflexes in the upper limbs. Less frequent findings include: cerebellar signs (ataxia, nystagmus, saccadic pursuit); retinal degeneration; pes cavus; scoliosis; and parkinsonism with characteristic brain MRI features that include thinning of the corpus callosum. Onset occurs mainly during infancy or adolescence (range: age 1-31 years) and in rare cases as late as age 60 years. Most affected individuals become wheelchair bound one or two decades after disease onset.

Bietti crystalline dystrophy (BCD) is a chorioretinal degeneration characterized by the presence of yellow-white crystals and/or complex lipid deposits in the retina and (to a variable degree) the cornea. Progressive atrophy and degeneration of the retinal pigment epithelium (RPE) / choroid lead to symptoms similar to those of other forms of retinal degeneration that fall under the category of retinitis pigmentosa and allied disorders, namely: reduced visual acuity, poor night vision, abnormal retinal electrophysiology, visual field loss, and often impaired color vision. Marked asymmetry between eyes is not uncommon. Onset is typically during the second to third decade of life, but ranges from the early teenage years to beyond the third decade. With time, loss of peripheral visual field, central acuity, or both result in legal blindness in most if not all affected individuals.

BBS9 is an autosomal recessive disorder characterized by obesity, polydactyly, renal anomalies, retinopathy, and mental retardation (Abu-Safieh et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

Axial spondylometaphyseal dysplasia (SMDAX) is characterized by postnatal growth failure, including rhizomelic short stature in early childhood that evolves into short trunk in late childhood, and thoracic hypoplasia that may cause mild to moderate respiratory problems in the neonatal period and later susceptibility to airway infection. Impaired visual acuity comes to medical attention in early life and vision rapidly deteriorates. Retinal changes are diagnosed as retinitis pigmentosa or pigmentary retinal degeneration on funduscopic examination and as cone-rod dystrophy on electroretinogram. Radiologic hallmarks include short ribs with flared and cupped anterior ends, mild spondylar dysplasia, lacy iliac crests, and metaphyseal irregularities essentially confined to the proximal femora (summary by Suzuki et al., 2011).

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. The lipopigment pattern seen most often in CLN2 consists of 'curvilinear' profiles (Mole et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of CLN, see CLN1 (256730).

Syndrome with characteristics of progressive pigmentary retinal degeneration (with nyctalopia and visual field restriction), cystic macular degeneration and angle closure glaucoma. It has been described in seven members of one family. Patients also have hyperopia and nanophthalmos. The mode of transmission is autosomal recessive.

Bardet-Biedl syndrome is an autosomal recessive and genetically heterogeneous ciliopathy characterized by retinitis pigmentosa, obesity, kidney dysfunction, polydactyly, behavioral dysfunction, and hypogonadism (summary by Beales et al., 1999). Eight proteins implicated in the disorder assemble to form the BBSome, a stable complex involved in signaling receptor trafficking to and from cilia (summary by Scheidecker et al., 2014). Genetic Heterogeneity of Bardet-Biedl Syndrome BBS2 (615981) is caused by mutation in a gene on 16q13 (606151); BBS3 (600151), by mutation in the ARL6 gene on 3q11 (608845); BBS4 (615982), by mutation in a gene on 15q22 (600374); BBS5 (615983), by mutation in a gene on 2q31 (603650); BBS6 (605231), by mutation in the MKKS gene on 20p12 (604896); BBS7 (615984), by mutation in a gene on 4q27 (607590); BBS8 (615985), by mutation in the TTC8 gene on 14q32 (608132); BBS9 (615986), by mutation in a gene on 7p14 (607968); BBS10 (615987), by mutation in a gene on 12q21 (610148); BBS11 (615988), by mutation in the TRIM32 gene on 9q33 (602290); BBS12 (615989), by mutation in a gene on 4q27 (610683); BBS13 (615990), by mutation in the MKS1 gene (609883) on 17q23; BBS14 (615991), by mutation in the CEP290 gene (610142) on 12q21, BBS15 (615992), by mutation in the WDPCP gene (613580) on 2p15; BBS16 (615993), by mutation in the SDCCAG8 gene (613524) on 1q43; BBS17 (615994), by mutation in the LZTFL1 gene (606568) on 3p21; BBS18 (615995), by mutation in the BBIP1 gene (613605) on 10q25; BBS19 (615996), by mutation in the IFT27 gene (615870) on 22q12; BBS20 (619471), by mutation in the IFT172 gene (607386) on 9p21; BBS21 (617406), by mutation in the CFAP418 gene (614477) on 8q22; and BBS22 (617119), by mutation in the IFT74 gene (608040) on 9p21. The CCDC28B gene (610162) modifies the expression of BBS phenotypes in patients who have mutations in other genes. Mutations in MKS1, MKS3 (TMEM67; 609884), and C2ORF86 also modify the expression of BBS phenotypes in patients who have mutations in other genes. Although BBS had originally been thought to be a recessive disorder, Katsanis et al. (2001) demonstrated that clinical manifestation of some forms of Bardet-Biedl syndrome requires recessive mutations in 1 of the 6 loci plus an additional mutation in a second locus. While Katsanis et al. (2001) called this 'triallelic inheritance,' Burghes et al. (2001) suggested the term 'recessive inheritance with a modifier of penetrance.' Mykytyn et al. (2002) found no evidence of involvement of the common BBS1 mutation in triallelic inheritance. However, Fan et al. (2004) found heterozygosity in a mutation of the BBS3 gene (608845.0002) as an apparent modifier of the expression of homozygosity of the met390-to-arg mutation in the BBS1 gene (209901.0001). Allelic disorders include nonsyndromic forms of retinitis pigmentosa: RP51 (613464), caused by TTC8 mutation, and RP55 (613575), caused by ARL6 mutation.

BBS2 is an autosomal recessive ciliopathy characterized by retinal degeneration, polydactyly, renal disease, hypogonadism, obesity, dysmorphic features, and variable degrees of cognitive impairment (Innes et al., 2010). Mutation in the BBS2 gene is the third most frequent cause of BBS, accounting for approximately 8% of cases (Zaghloul and Katsanis, 2009). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

BBS4 is a rare multisystemic disorder characterized primarily by retinal dystrophy, obesity, polydactyly, and renal dysfunction that accounts for less than 3% of BBS (Katsanis et al., 2002). Anosmia has been described in patients with BBS4 (Iannaccone et al., 2005), as well as polydactyly confined to the hands (Carmi et al., 1995). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

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).

Senior-Løken syndrome is a rare disorder characterized by the combination of two specific features: a kidney condition called nephronophthisis and an eye condition known as Leber congenital amaurosis.\n\nNephronophthisis causes fluid-filled cysts to develop in the kidneys beginning in childhood. These cysts impair kidney function, initially causing increased urine production (polyuria), excessive thirst (polydipsia), general weakness, and extreme tiredness (fatigue). Nephronophthisis leads to end-stage renal disease (ESRD) later in childhood or in adolescence. ESRD is a life-threatening failure of kidney function that occurs when the kidneys are no longer able to filter fluids and waste products from the body effectively.\n\nLeber congenital amaurosis primarily affects the retina, which is the specialized tissue at the back of the eye that detects light and color. This condition causes vision problems, including an increased sensitivity to light (photophobia), involuntary movements of the eyes (nystagmus), and extreme farsightedness (hyperopia). Some people with Senior-Løken syndrome develop the signs of Leber congenital amaurosis within the first few years of life, while others do not develop vision problems until later in childhood.

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).

Autosomal recessive childhood-onset severe retinal dystrophy is a heterogeneous group of disorders affecting rod and cone photoreceptors simultaneously. The most severe cases are termed Leber congenital amaurosis, whereas the less aggressive forms are usually considered juvenile retinitis pigmentosa (summary by Gu et al., 1997). Mutation in TULP1 can also cause a form of autosomal recessive retinitis pigmentosa (RP14; 600132). For a general phenotypic description and a discussion of the genetic heterogeneity of Leber congenital amaurosis, see LCA1 (204000); for retinitis pigmentosa, see 268000.

An autosomal recessive muscular dystrophy caused by mutations in the POMGNT1 gene. It is associated with characteristic brain and eye malformations, profound mental retardation, and death usually in the first years of life.

HSD10 mitochondrial disease (HSD10MD) most commonly presents as an X-linked neurodegenerative disorder with highly variable severity and age at onset ranging from the neonatal period to early childhood. The features are usually multisystemic, consistent with mitochondrial dysfunction. Some affected males have a severe infantile form associated with cardiomyopathy that may result in death in early childhood, whereas other rare patients may have juvenile onset or even atypical presentations with normal neurologic development. More severely affected males show developmental regression in infancy or early childhood, often associated with early-onset intractable seizures, progressive choreoathetosis and spastic tetraplegia, optic atrophy or retinal degeneration resulting in visual loss, and mental retardation. Heterozygous females may show non-progressive developmental delay and intellectual disability, but may also be clinically normal. Although the diagnosis can be aided by the observation of increased urinary levels of metabolites of isoleucine breakdown (2-methyl-3 hydroxybutyrate and tiglylglycine), there is not a correlation between these laboratory features and the phenotype. In addition, patients do not develop severe metabolic crises in the neonatal period as observed in other organic acidurias, but may show persistent lactic acidosis, most likely reflecting mitochondrial dysfunction (summary by Rauschenberger et al., 2010; Zschocke, 2012). In a review of this disorder, Zschocke (2012) noted that although it was originally thought to be an inborn error of branched-chain fatty acid and isoleucine metabolism resulting from decreased HSD17B10 dehydrogenase activity (HSD17B10 'deficiency'), subsequent studies have shown that the HSD17B10 gene product has additional functions and also acts as a component of the mitochondrial RNase P holoenzyme, which is involved in mitochondrial tRNA processing and maturation and ultimately mitochondrial protein synthesis. The multisystemic features of HSD10MD most likely result from the adverse effect of HSD17B10 mutations on mitochondrial function, rather than from the effects on the dehydrogenase activity (see PATHOGENESIS).

Autosomal recessive spinocerebellar ataxia-12 is a neurologic disorder characterized by onset of generalized seizures in infancy, delayed psychomotor development with mental retardation, and cerebellar ataxia. Some patients may also show spasticity (summary by Mallaret et al., 2014).

Nephronophthisis can occur as part of separate syndromes that affect other areas of the body; these are often referred to as nephronophthisis-associated ciliopathies. For example, Senior-Løken syndrome is characterized by the combination of nephronophthisis and a breakdown of the light-sensitive tissue at the back of the eye (retinal degeneration); Joubert syndrome affects many parts of the body, causing neurological problems and other features, which can include nephronophthisis.\n\nAbout 85 percent of all cases of nephronophthisis are isolated, which means they occur without other signs and symptoms. Some people with nephronophthisis have additional features, which can include liver fibrosis, heart abnormalities, or mirror image reversal of the position of one or more organs inside the body (situs inversus).\n\nNephronophthisis eventually leads to end-stage renal disease (ESRD), a life-threatening failure of kidney function that occurs when the kidneys are no longer able to filter fluids and waste products from the body effectively. Nephronophthisis can be classified by the approximate age at which ESRD begins: around age 1 (infantile), around age 13 (juvenile), and around age 19 (adolescent).\n\nNephronophthisis is a disorder that affects the kidneys. It is characterized by inflammation and scarring (fibrosis) that impairs kidney function. These abnormalities lead to increased urine production (polyuria), excessive thirst (polydipsia), general weakness, and extreme tiredness (fatigue). In addition, affected individuals develop fluid-filled cysts in the kidneys, usually in an area known as the corticomedullary region. Another feature of nephronophthisis is a shortage of red blood cells, a condition known as anemia.

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).

X-linked congenital retinoschisis (XLRS) is characterized by symmetric bilateral macular involvement with onset in the first decade of life, in some cases as early as age three months. Fundus examination shows areas of schisis (splitting of the nerve fiber layer of the retina) in the macula, sometimes giving the impression of a spoke wheel pattern. Schisis of the peripheral retina, predominantly inferotemporally, occurs in approximately 50% of individuals. Affected males typically have 20/60 to 20/120 vision. Visual acuity often deteriorates during the first and second decades of life but then remains relatively stable until the fifth or sixth decade.

Bardet-Biedl syndrome-17 (BBS17) is an autosomal recessive ciliopathy characterized by retinitis pigmentosa, cognitive impairment, obesity, renal dysfunction, and hypogenitalism. Polydactyly, most often postaxial, is also a primary feature of BBS; in BBS17, mesoaxial polydactyly, with fused or Y-shaped metacarpals, is a distinct manifestation (Deffert et al., 2007; Schaefer et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is an autosomal recessive disorder with congenital muscular dystrophy resulting in muscle weakness early in life and brain and eye anomalies. It is usually associated with delayed psychomotor development and shortened life expectancy. The phenotype includes the alternative clinical designations Walker-Warburg syndrome (WWS) and muscle-eye-brain disease (MEB). The disorder represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as dystroglycanopathies (summary by Stevens et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

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). For a discussion of genetic heterogeneity of short-rib thoracic dysplasia, see SRTD1 (208500).

Bardet-Biedl syndrome-16 (BBS16) is an autosomal recessive ciliopathy characterized by retinal degeneration, obesity, renal disease, and cognitive impairment. Although polydactyly is considered a primary feature of BBS overall, it has not been reported in any BBS16 patient (Billingsley et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

Any retinitis pigmentosa in which the cause of the disease is a mutation in the PRPF4 gene.

Developmental and epileptic encephalopathy-28 (DEE28) is an autosomal recessive severe neurologic disorder characterized by the onset of refractory seizures in the first months of life. Affected individuals have severe axial hypotonia and profoundly impaired psychomotor development. More severely affected patients have acquired microcephaly, poor or absent visual contact, and retinal degeneration; early death may occur (summary by Mignot et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Any mitochondrial DNA depletion syndrome in which the cause of the disease is a mutation in the OPA1 gene.

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).

Individuals with biallelic APOB-related familial hypobetalipoproteinemia (APOB-FHBL) may present from infancy through to adulthood with a range of clinical symptoms including deficiency of fat-soluble vitamins and gastrointestinal and neurologic dysfunction. Affected individuals typically have plasma total cholesterol, LDL cholesterol, and apo B levels below the fifth centile for age and sex. Acanthocytosis, elevated liver enzymes, and hyperbilirubinemia may also be found. The most common clinical findings are hepatomegaly, steatorrhea, and failure to thrive / growth deficiency. In the absence of treatment, affected individuals can develop atypical pigmentation of the retina; progressive loss of deep tendon reflexes, vibratory sense, and proprioception; muscle pain or weakness; dysarthria; ataxia; tremors; and steatohepatitis, fibrosis, and rarely, cirrhosis of the liver. Individuals with a heterozygous, typically truncating pathogenic variant in APOB are usually asymptomatic with mild liver dysfunction and hepatic steatosis. However, about 5%-10% of individuals with heterozygous APOB-FHBL develop relatively more severe nonalcoholic steatohepatitis requiring medical attention and occasionally progressing to cirrhosis, albeit very rarely.

Leber congenital amaurosis with early-onset deafness (LCAEOD) is an autosomal dominant syndrome manifesting as early-onset and severe photoreceptor and cochlear cell loss. Some patients show extinguished responses on electroretinography and moderate to severe hearing loss at birth (Luscan et al., 2017).

An atypical form of Usher syndrome, here designated type IV (USH4), is an autosomal recessive disorder characterized by late onset of retinitis pigmentosa and usually late-onset of progressive sensorineural hearing loss without vestibular involvement (summary by Khateb et al., 2018). For a discussion of genetic heterogeneity of Usher syndrome, see 276900.

Cerebellar, ocular, craniofacial, and genital syndrome (COFG) is characterized by moderate to severe developmental delay and impaired intellectual development, severe cerebellar hypoplasia, a noticeably short forehead, medially sparse/flared and laterally extended eyebrows, corneal dystrophy, underdeveloped labioscrotal folds, and tufts of hair extruding from the lactiferous ducts with breast and nipple underdevelopment. Additional features such as pontine involvement, retinal degeneration, anteverted nares, and low-set ears have been variably observed (Rad et al., 2019).

Leber congenital amaurosis-19 (LCA19) is characterized by reduced vision in early childhood and severely reduced responses of both rods and cones on electroretinography (Yi et al., 2019). For a general description and a discussion of genetic heterogeneity of LCA, see 204000.

Liberfarb syndrome is a progressive disorder involving connective tissue, bone, retina, ear, and brain. Patients exhibit severe short stature and scoliosis with thoracic kyphosis and lumbar hyperlordosis. Severe joint laxity results in dislocations of elbows, hips, and knees. Eye findings are consistent with early-onset retinal degeneration, and there is moderate to severe early-onset hearing loss. Microcephaly is apparent by school age, and patients exhibit developmental delay and intellectual deficits (Peter et al., 2019). Clinical variability has been observed, with some patients presenting differences in the severity and location of skeletal dysplasia involvement as well as variation in other features of the syndrome (Girisha et al., 2019; Zhao et al., 2019).

SHILCA is characterized by early-onset retinal degeneration in association with sensorineural hearing loss, short stature, vertebral anomalies, and epiphyseal dysplasia, as well as motor and intellectual delay. Delayed myelination, leukoencephalopathy, and hypoplasia of the corpus callosum and cerebellum have been observed on brain MRI (Bedoni et al., 2020).

Neuronal ceroid lipofuscinosis-6A (CLN6A) is an autosomal recessive neurodegenerative disorder with a variable age at onset in the first years of life after normal early development. Affected individuals have progressive decline of neurologic function, including visual deterioration in most, cognitive impairment, loss of motor function, and seizures. As with all CLNs, CLN6A is characterized pathologically by the intracellular accumulation of autofluorescent lipopigment storage material in different patterns ultrastructurally. The lipopigment patterns observed most often in CLN6A comprises mixed combinations of 'curvilinear' and 'fingerprint' profiles (summary by Sharp et al., 2003; Mole et al., 2005). For a discussion of genetic heterogeneity of CLN, see CLN1 (256730).

GNB5-related neurodevelopmental disorder (GNB5-NDD) is characterized by a spectrum of neurodevelopmental phenotypes that range from severe-to-profound intellectual disability (ID; 31/41 reported individuals), to mild-to-moderate ID (5/41), to normal intellect with severe language disorder (5/41, one extended family). A unique and specific feature of GNB5-NDD – regardless of neurodevelopmental phenotype – is nearly universal bradycardia caused by sinoatrial node dysfunction (sick sinus syndrome). Most individuals with severe and profound ID have a developmental and epileptic encephalopathy with focal seizures or epileptic spasms, as well as visual impairment (central or retinal) with nystagmus, difficulty feeding, and gastroesophageal reflux disease. The risk of early mortality is increased.

Combined oxidative phosphorylation deficiency-59 (COXPD59) may present as a lethal infantile form of Leigh syndrome (see 256000) or as a milder disorder with hypertrophic cardiomyopathy, lactic acidosis, attention deficit-hyperactivity disorder (ADHD) and survival into adulthood (summary by Amarasekera et al., 2023). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).