The FLNB disorders include a spectrum of phenotypes ranging from mild to severe. At the mild end are spondylocarpotarsal synostosis (SCT) syndrome and Larsen syndrome; at the severe end are the phenotypic continuum of atelosteogenesis types I (AOI) and III (AOIII) and Piepkorn osteochondrodysplasia (POCD). SCT syndrome is characterized by postnatal disproportionate short stature, scoliosis and lordosis, clubfeet, hearing loss, dental enamel hypoplasia, carpal and tarsal synostosis, and vertebral fusions. Larsen syndrome is characterized by congenital dislocations of the hip, knee, and elbow; clubfeet (equinovarus or equinovalgus foot deformities); scoliosis and cervical kyphosis, which can be associated with a cervical myelopathy; short, broad, spatulate distal phalanges; distinctive craniofacies (prominent forehead, depressed nasal bridge, malar flattening, and widely spaced eyes); vertebral anomalies; and supernumerary carpal and tarsal bone ossification centers. Individuals with SCT syndrome and Larsen syndrome can have midline cleft palate and hearing loss. AOI and AOIII are characterized by severe short-limbed dwarfism; dislocated hips, knees, and elbows; and clubfeet. AOI is lethal in the perinatal period. In individuals with AOIII, survival beyond the neonatal period is possible with intensive and invasive respiratory support. Piepkorn osteochondrodysplasia (POCD) is a perinatal-lethal micromelic dwarfism characterized by flipper-like limbs (polysyndactyly with complete syndactyly of all fingers and toes, hypoplastic or absent first digits, and duplicated intermediate and distal phalanges), macrobrachycephaly, prominant forehead, hypertelorism, and exophthalmos. Occasional features include cleft palate, omphalocele, and cardiac and genitourinary anomalies. The radiographic features at mid-gestation are characteristic.

Pfeiffer syndrome is an autosomal dominant craniosynostosis syndrome with characteristic anomalies of the hands and feet. Three clinical subtypes, which have important diagnostic and prognostic implications, have been identified. Type 1, the classic syndrome, is compatible with life and consists of craniosynostosis, midface deficiency, broad thumbs, broad great toes, brachydactyly, and variable syndactyly. Type 2 consists of cloverleaf skull with Pfeiffer hands and feet, together with ankylosis of the elbows. Type 3 is similar to type 2 but without cloverleaf skull. Ocular proptosis is severe, and the anterior cranial base is markedly short. Various visceral malformations have been found in association with type 3. Early demise is characteristic of types 2 and 3 (Cohen, 1993). Cohen and Barone (1994) further tabulated the findings in the 3 types of Pfeiffer syndrome.

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.

A multiple congenital anomalies syndrome with characteristics of poliosis, distinct facial features (epicanthal folds, hypertelorism, posterior rotation of ears, prominent philtrum, high-arched palate) and congenital anomalies/malformations of the eye (blue sclera), cardiopulmonary (atrial septal defect, prominent thoracic and abdominal veins) and skeletal (clinodactyly, syndactyly of the fingers and second and third toes) systems. There have been no further descriptions in the literature since 1980.

LTBP4-related cutis laxa is characterized by cutis laxa, early childhood-onset pulmonary emphysema, peripheral pulmonary artery stenosis, and other evidence of a generalized connective tissue disorder such as inguinal hernias and hollow visceral diverticula (e.g., intestine, bladder). Other manifestations can include pyloric stenosis, diaphragmatic hernia, rectal prolapse, gastrointestinal elongation/tortuosity, cardiovascular abnormality, pulmonary hypertension, hypotonia and frequent pulmonary infections. Bladder diverticula and hydronephrosis are common. Early demise has been associated with pulmonary emphysema.

Cytochrome P450 oxidoreductase deficiency (PORD) is a disorder of steroidogenesis with a broad phenotypic spectrum including cortisol deficiency, altered sex steroid synthesis, disorders of sex development (DSD), and skeletal malformations of the Antley-Bixler syndrome (ABS) phenotype. Cortisol deficiency is usually partial, with some baseline cortisol production but failure to mount an adequate cortisol response in stress. Mild mineralocorticoid excess can be present and causes arterial hypertension, usually presenting in young adulthood. Manifestations of altered sex steroid synthesis include ambiguous genitalia/DSD in both males and females, large ovarian cysts in females, poor masculinization and delayed puberty in males, and maternal virilization during pregnancy with an affected fetus. Skeletal malformations can manifest as craniosynostosis, mid-face retrusion with proptosis and choanal stenosis or atresia, low-set dysplastic ears with stenotic external auditory canals, hydrocephalus, radiohumeral synostosis, neonatal fractures, congenital bowing of the long bones, joint contractures, arachnodactyly, and clubfeet; other anomalies observed include urinary tract anomalies (renal pelvic dilatation, vesicoureteral reflux). Cognitive impairment is of minor concern and likely associated with the severity of malformations; studies of developmental outcomes are lacking.

Additional features of Meier-Gorlin syndrome can include difficulty feeding and a lung condition known as pulmonary emphysema or other breathing problems.\n\nMost people with Meier-Gorlin syndrome have distinctive facial features. In addition to being abnormally small, the ears may be low-set or rotated backward. Additional features can include a small mouth (microstomia), an underdeveloped lower jaw (micrognathia), full lips, and a narrow nose with a high nasal bridge.\n\nSome people with Meier-Gorlin syndrome have other skeletal abnormalities, such as unusually narrow long bones in the arms and legs, a deformity of the knee joint that allows the knee to bend backwards (genu recurvatum), and slowed mineralization of bones (delayed bone age).\n\nMeier-Gorlin syndrome is a condition primarily characterized by short stature. It is considered a form of primordial dwarfism because the growth problems begin before birth (intrauterine growth retardation). After birth, affected individuals continue to grow at a slow rate. Other characteristic features of this condition are underdeveloped or missing kneecaps (patellae), small ears, and, often, an abnormally small head (microcephaly). Despite a small head size, most people with Meier-Gorlin syndrome have normal intellect.\n\nAbnormalities in sexual development may also occur in Meier-Gorlin syndrome. In some males with this condition, the testes are small or undescended (cryptorchidism). Affected females may have unusually small external genital folds (hypoplasia of the labia majora) and small breasts. Both males and females with this condition can have sparse or absent underarm (axillary) hair.

Abnormalities in sexual development may also occur in Meier-Gorlin syndrome. In some males with this condition, the testes are small or undescended (cryptorchidism). Affected females may have unusually small external genital folds (hypoplasia of the labia majora) and small breasts. Both males and females with this condition can have sparse or absent underarm (axillary) hair.\n\nMeier-Gorlin syndrome is a condition primarily characterized by short stature. It is considered a form of primordial dwarfism because the growth problems begin before birth (intrauterine growth retardation). After birth, affected individuals continue to grow at a slow rate. Other characteristic features of this condition are underdeveloped or missing kneecaps (patellae), small ears, and, often, an abnormally small head (microcephaly). Despite a small head size, most people with Meier-Gorlin syndrome have normal intellect.\n\nSome people with Meier-Gorlin syndrome have other skeletal abnormalities, such as unusually narrow long bones in the arms and legs, a deformity of the knee joint that allows the knee to bend backwards (genu recurvatum), and slowed mineralization of bones (delayed bone age).\n\nMost people with Meier-Gorlin syndrome have distinctive facial features. In addition to being abnormally small, the ears may be low-set or rotated backward. Additional features can include a small mouth (microstomia), an underdeveloped lower jaw (micrognathia), full lips, and a narrow nose with a high nasal bridge.\n\nAdditional features of Meier-Gorlin syndrome can include difficulty feeding and a lung condition known as pulmonary emphysema or other breathing problems.

Arboleda-Tham syndrome (ARTHS) is an autosomal dominant disorder with the core features of impaired intellectual development, speech delay, microcephaly, cardiac anomalies, and gastrointestinal complications (summary by Kennedy et al., 2019).

Chitayat syndrome (CHYTS) is a rare condition characterized by respiratory distress presenting at birth, bilateral accessory phalanx resulting in shortened index fingers with ulnar deviation, hallux valgus, and characteristic facial features including prominent eyes, hypertelorism, depressed nasal bridge, full lips, and upturned nose (summary by Balasubramanian et al., 2017).

Cardiac-urogenital syndrome is characterized by partial anomalous pulmonary venous return in association with tracheal anomalies, pulmonary hypoplasia, congenital diaphragmatic hernia, thyroid fibrosis, thymic involution, cleft spleen, penoscrotal hypospadias, and cryptorchidism (Pinz et al., 2018).

Megacystis-microcolon-intestinal hypoperistalsis syndrome-2 (MMIHS2) is characterized by prenatal bladder enlargement, neonatal functional gastrointestinal obstruction, and chronic dependence on total parenteral nutrition and urinary catheterization. The majority of cases have a fatal outcome due to malnutrition and sepsis, followed by multiorgan failure (summary by Wang et al., 2019). For a discussion of genetic heterogeneity of MMIHS, see 249210.

DEGCAGS syndrome is an autosomal recessive syndromic neurodevelopmental disorder characterized by global developmental delay, coarse and dysmorphic facial features, and poor growth and feeding apparent from infancy. Affected individuals have variable systemic manifestations often with significant structural defects of the cardiovascular, genitourinary, gastrointestinal, and/or skeletal systems. Additional features may include sensorineural hearing loss, hypotonia, anemia or pancytopenia, and immunodeficiency with recurrent infections. Death in childhood may occur (summary by Bertoli-Avella et al., 2021).

Hyper-IgE syndrome-4A with recurrent infections (HIES4A) is an autosomal dominant immunologic disorder characterized by recurrent, mainly sinopulmonary infections associated with increased serum IgE. The phenotype is variable, even within families. Some patients have onset of symptoms in early childhood and develop complications, including bronchiectasis or hemoptysis, whereas others have later onset of less severe infections. Immunologic workup usually shows normal leukocyte levels, although some patients may demonstrate alterations in lymphocyte subsets, including T cells. Affected individuals also have variable skeletal abnormalities, including high-arched palate, hyperextensible joints, scoliosis, and bone fractures. The IL6ST mutations are loss-of-function, although the truncated mutant proteins are expressed and interfere with the wildtype protein in a dominant-negative manner by disrupting IL6 (147620) and IL11 (147681) signaling (summary by Beziat et al., 2020). For a discussion of genetic heterogeneity of hyper-IgE syndrome, see HIES1 (147060).