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).

Hypophosphatasia is characterized by defective mineralization of growing or remodeling bone, with or without root-intact tooth loss, in the presence of low activity of serum and bone alkaline phosphatase. Clinical features range from stillbirth without mineralized bone at the severe end to pathologic fractures of the lower extremities in later adulthood at the mild end. While the disease spectrum is a continuum, seven clinical forms of hypophosphatasia are usually recognized based on age at diagnosis and severity of features: Perinatal (severe): characterized by pulmonary insufficiency and hypercalcemia. Perinatal (benign): prenatal skeletal manifestations that slowly resolve into one of the milder forms. Infantile: onset between birth and age six months of clinical features of rickets without elevated serum alkaline phosphatase activity. Severe childhood (juvenile): variable presenting features progressing to rickets. Mild childhood: low bone mineral density for age, increased risk of fracture, and premature loss of primary teeth with intact roots. Adult: characterized by stress fractures and pseudofractures of the lower extremities in middle age, sometimes associated with early loss of adult dentition. Odontohypophosphatasia: characterized by premature exfoliation of primary teeth and/or severe dental caries without skeletal manifestations.

The Al-Awadi/Raas-Rothschild/Schinzel phocomelia syndrome (AARRS) is a rare autosomal recessive disorder characterized by severe malformations of upper and lower limbs with severely hypoplastic pelvis and abnormal genitalia. The disorder is believed to represent a defect of dorsoventral patterning and outgrowth of limbs (summary by Kantaputra et al., 2010).

Osteogenesis imperfecta (OI) is a connective tissue disorder characterized clinically by bone fragility and increased susceptibility to fractures. Osteogenesis imperfecta type IX (OI9) is a severe autosomal recessive form of the disorder (summary by van Dijk et al., 2009).

A rare multiple congenital malformations/dysmorphic syndrome characterized by osteogenesis imperfecta with multiple prenatal bone fractures, joint laxity, severe microcephaly, and bilateral cataracts. Additional reported manifestations include dysmorphic facial features (such as blue sclerae, hypertelorism, and low-set ears), lissencephaly, hydrocephalus, and cardiac and genital anomalies. The syndrome is lethal <i>in utero</i> or shortly after birth. There have been no further descriptions in the literature since 1978.

Osteogenesis imperfecta is a connective tissue disorder characterized by bone fragility and low bone mass. OI type VII is an autosomal recessive form of severe or lethal OI (summary by Barnes et al., 2006).

Yunis-Varon syndrome (YVS) is a severe autosomal recessive disorder characterized by skeletal defects, including cleidocranial dysplasia and digital anomalies, and severe neurologic involvement with neuronal loss. Enlarged cytoplasmic vacuoles are found in neurons, muscle, and cartilage. The disorder is usually lethal in infancy (summary by Campeau et al., 2013).

Osteogenesis imperfecta (OI) is a connective tissue disorder characterized by bone fragility and low bone mass. Due to considerable phenotypic variability, Sillence et al. (1979) developed a classification of OI subtypes based on clinical features and disease severity: OI type I, with blue sclerae (166200); perinatal lethal OI type II, also known as congenital OI (166210); OI type III, a progressively deforming form with normal sclerae (259420); and OI type IV, with normal sclerae (166220). Most forms of OI are autosomal dominant with mutations in one of the 2 genes that code for type I collagen alpha chains, COL1A1 (120150) and COL1A2 (120160). Cabral et al. (2007) described a form of autosomal recessive OI, which they designated OI type VIII, characterized by white sclerae, severe growth deficiency, extreme skeletal undermineralization, and bulbous metaphyses.

Osteogenesis imperfecta (OI) comprises a group of connective tissue disorders characterized by bone fragility and low bone mass. The disorder is clinically and genetically heterogeneous. OI type X is an autosomal recessive form characterized by multiple bone deformities and fractures, generalized osteopenia, dentinogenesis imperfecta, and blue sclera (Christiansen et al., 2010).

Bent bone dysplasia syndrome-1 (BBDS1) is a perinatal lethal skeletal dysplasia characterized by poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones (Merrill et al., 2012). Genetic Heterogeneity of Bent Bone Dysplasia Syndrome BBDS2 (620076) is caused by mutation in the LAMA5 gene (601033) on chromosome 20q13.

Osteogenesis imperfecta type XVI (OI16) is characterized by prenatal onset of multiple fractures of ribs and long bones, blue sclerae, decreased ossification of the skull, and severe demineralization. Heterozygous family members may exhibit recurrent fractures with minimal trauma, osteopenia, and blue sclerae (Keller et al., 2018; Lindahl et al., 2018).

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 with or without polydactyly, see SRTD1 (208500).

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).

ACCIID is characterized by arthrogryposis, cleft palate, craniosynostosis, micrognathia, short stature, and impaired intellectual development. Seizures and bony abnormalities (severe slenderness of the ribs and tubular bones and perinatal fractures) have been observed (Mizuguchi et al., 2018).

A restrictive dermopathy that has material basis in homozygous or compound heterozygous mutation in the ZMPSTE24 gene on chromosome 1p34.

Meckel syndrome-14 (MKS14) is a lethal disorder characterized by occipital encephalocele, postaxial polydactyly of the hands and feet, and polycystic kidneys. Stillbirth has been reported, as well as death within hours in a live-born affected individual (Shaheen et al., 2016; Ridnoi et al., 2019). For a general phenotypic description and discussion of genetic heterogeneity of Meckel syndrome, see MKS1 (249000).

Bent bone dysplasia syndrome-2 (BBDS2) is characterized by defects in both the axial and appendicular skeleton, with radiographic findings of undermineralized bone and a distinct angulation of the mid femoral shaft. Extraskeletal features include facial dysmorphisms, abnormally formed ears with tags, widely spaced nipples, and atrial septal defects. Abnormalities of muscle function are suggested by the presence of elbow fusions, ulnar flexion contractions at the wrist, and bilateral talipes equinovarus, as well as failure to mount a respiratory effort at birth (Barad et al., 2020). For a general phenotypic description and discussion of genetic heterogeneity of bent bone dysplasia syndrome, see BBDS1 (614592).