Cleidocranial dysplasia (CCD) spectrum disorder is a skeletal dysplasia that represents a clinical continuum ranging from classic CCD (triad of delayed closure of the cranial sutures, hypoplastic or aplastic clavicles, and dental abnormalities) to mild CCD to isolated dental anomalies without the skeletal features. Most individuals come to diagnosis because they have classic features. At birth, affected individuals typically have abnormally large, wide-open fontanelles that may remain open throughout life. Clavicular hypoplasia can result in narrow, sloping shoulders that can be opposed at the midline. Moderate short stature may be observed, with most affected individuals being shorter than their unaffected sibs. Dental anomalies may include supernumerary teeth, eruption failure of the permanent teeth, and presence of the second permanent molar with the primary dentition. Individuals with CCD spectrum disorder are at increased risk of developing recurrent sinus infections, recurrent ear infections leading to conductive hearing loss, and upper-airway obstruction. Intelligence is typically normal.

Camurati-Engelmann disease (CED) is characterized by hyperostosis of the long bones and the skull, proximal muscle weakness, limb pain, a wide-based, waddling gait, and joint contractures. Facial features such as macrocephaly, frontal bossing, enlargement of the mandible, proptosis, and cranial nerve impingement resulting in facial palsy are seen in severely affected individuals later in life.

Pycnodysostosis is characterized by short-limbed short stature, typical facial appearance (convex nasal ridge and small jaw with obtuse mandibular angle), osteosclerosis with increased bone fragility, acroosteolysis of the distal phalanges, delayed closure of the cranial sutures, and dysplasia of the clavicle. In affected individuals, the facial features become more prominent with age, likely due to progressive acroosteolysis of the facial bones, but can usually be appreciated from early childhood, particularly the small jaw and convex nasal ridge. Additional features include dental and nail anomalies. Intelligence is typically normal with mild psychomotor difficulties reported in some individuals.

Trichodentoosseous syndrome (TDO) is an autosomal dominant disorder with complete penetrance characterized by abnormalities involving hair, teeth, and bone (summary by Nguyen et al., 2013).

Primary hyperoxaluria type 1 (PH1) is caused by a deficiency of the liver peroxisomal enzyme alanine:glyoxylate-aminotransferase (AGT), which catalyzes the conversion of glyoxylate to glycine. When AGT activity is absent, glyoxylate is converted to oxalate, which forms insoluble calcium oxalate crystals that accumulate in the kidney and other organs. Individuals with PH1 are at risk for recurrent nephrolithiasis (deposition of calcium oxalate in the renal pelvis / urinary tract), nephrocalcinosis (deposition of calcium oxalate in the renal parenchyma), or end-stage renal disease (ESRD). Age at onset of symptoms ranges from infancy to the sixth decade. Approximately 10% of affected individuals present in infancy or early childhood with nephrocalcinosis, with or without nephrolithiasis, and failure to thrive related to renal failure. The majority of individuals with PH1 present in childhood or early adolescence, usually with symptomatic nephrolithiasis and normal or reduced kidney function. The remainder of affected individuals present in adulthood with recurrent renal stones and a mild-to-moderate reduction in kidney function. The natural history of untreated PH1 is one of progressive decline in renal function as a result of calcium oxalate deposits in kidney tissue and complications of nephrolithiasis (e.g., obstruction and infection) with eventual progression to oxalosis (widespread tissue deposition of calcium oxalate) and death from ESRD and/or complications of oxalosis.

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.

A rare genetic disease characterized by childhood onset of bilateral progressive sensorineural hearing loss, ocular anomalies (myopia, cataract, retinitis pigmentosa), central and peripheral nervous system features (dementia, epilepsy, ataxia, peripheral neuropathy), ectodermal features (skin atrophy, alopecia, dental caries), and skeletal anomalies (bone cysts, joint stiffness, scoliosis, kyphosis). Laboratory examination may reveal elevated cerebrospinal fluid protein.

Osteomesopyknosis is a nonmalignant sclerosing bone dysplasia of the axial skeleton. Osteosclerosis is limited to the axial skeleton, pelvis, and proximal long bones. The condition is often an incidental radiologic finding in a young adult complaining of back pain. Patients have normal height and laboratory examinations are within normal limits (summary by Yao and Camacho, 2014).

SOST-related sclerosing bone dysplasias include sclerosteosis and van Buchem disease, both disorders of progressive bone overgrowth due to increased bone formation. The major clinical features of sclerosteosis are progressive skeletal overgrowth, most pronounced in the skull and mandible, and variable syndactyly, usually of the second (index) and third (middle) fingers. Affected individuals appear normal at birth except for syndactyly. Facial distortion due to bossing of the forehead and mandibular overgrowth is seen in nearly all individuals and becomes apparent in early childhood with progression into adulthood. Hyperostosis of the skull results in narrowing of the foramina, causing entrapment of the seventh cranial nerve (leading to facial palsy) with other, less common nerve entrapment syndromes including visual loss (2nd cranial nerve), neuralgia or anosmia (5th cranial nerve), and sensory hearing loss (8th cranial nerve). In sclerosteosis, hyperostosis of the calvarium reduces intracranial volume, increasing the risk for potentially lethal elevation of intracranial pressure. Survival of individuals with sclerosteosis into old age is unusual, but not unprecedented. The manifestations of van Buchem disease are generally milder than sclerosteosis and syndactyly is absent; life span appears to be normal.

Pyknoachondrogenesis is a lethal skeletal osteochondrodysplasia characterized by severe generalized osteosclerosis. The disease is very rare and only five cases (four males and one female) have been reported in the literature so far. Pyknoachondrogenesis may be detected prenatally due to the extreme shortening of the limbs and hydrops fetalis, or is recognized at birth. The main clinical manifestations include a large head, palpebral edema, a flat nose, low-set ears, a short neck, a short and wide trunk, a prominent abdomen, and severe micromelic dwarfism. Etiology remains unknown. Pyknoachondrogenesis has a lethal outcome, either prenatally or during the early neonatal period.

Raine syndrome (RNS) is a neonatal osteosclerotic bone dysplasia of early and aggressive onset that usually results in death within the first few weeks of life, although there have been some reports of survival into childhood. Radiographic studies show a generalized increase in the density of all bones and a marked increase in the ossification of the skull. The increased ossification of the basal structures of the skull and facial bones underlies the characteristic facial features, which include narrow prominent forehead, proptosis, depressed nasal bridge, and midface hypoplasia. Periosteal bone formation is also characteristic of this disorder and differentiates it from osteopetrosis and other known lethal and nonlethal osteosclerotic bone dysplasias. The periosteal bone formation typically extends along the diaphysis of long bones adjacent to areas of cellular soft tissue (summary by Simpson et al., 2009). Some patients survive infancy (Simpson et al., 2009; Fradin et al., 2011).

Osteopetrosis (OPT) is a life-threatening disease caused by subnormal osteoclast function, with an incidence of 1 in 250,000 births. The disease usually manifests in the first few months of life with macrocephaly and frontal bossing, resulting in a characteristic facial appearance. Defective bone remodeling of the skull results in choanal stenosis with concomitant respiratory problems and feeding difficulties, which are the first clinical manifestation of disease. The expanding bone encroaches on neural foramina, leading to blindness, deafness, and facial palsy. Complete visual loss invariably occurs in all untreated patients, and hearing loss is estimated to affect 78% of patients with OPT. Tooth eruption defects and severe dental caries are common. Calcium feedback hemostasis is impaired, and children with OPT are at risk of developing hypocalcemia with attendant tetanic seizures and secondary hyperparathyroidism. The most severe complication of OPT, limiting survival, is bone marrow insufficiency. The abnormal expansion of cortical and trabecular bone physically limits the availability of medullary space for hematopoietic activity, leading to life-threatening cytopenia and secondary expansion of extramedullary hematopoiesis at sites such as the liver and spleen (summary by Aker et al., 2012). Genetic Heterogeneity of Autosomal Recessive Osteopetrosis Other forms of autosomal recessive infantile malignant osteopetrosis include OPTB4 (611490), which is caused by mutation in the CLCN7 gene (602727) on chromosome 16p13, and OPTB5 (259720), which is caused by mutation in the OSTM1 gene (607649) on chromosome 6q21. A milder, osteoclast-poor form of autosomal recessive osteopetrosis (OPTB2; 259710) is caused by mutation in the TNFSF11 gene (602642) on chromosome 13q14, an intermediate form (OPTB6; 611497) is caused by mutation in the PLEKHM1 gene (611466) on chromosome 17q21, and a severe osteoclast-poor form associated with hypogammaglobulinemia (OPTB7; 612301) is caused by mutation in the TNFRSF11A gene (603499) on chromosome 18q21. Another form of autosomal recessive osteopetrosis (OPTB8; 615085) is caused by mutation in the SNX10 gene (614780) on chromosome 7p15. A form of autosomal recessive osteopetrosis associated with renal tubular acidosis (OPTB3; 259730) is caused by mutation in the CA2 gene (611492) on chromosome 8q21. OPTB9 (620366) is caused by mutation in the SLC4A2 gene (109280) on chromosome 7q36. Autosomal dominant forms of osteopetrosis are more benign (see OPTA1, 607634).

Ghosal hematodiaphyseal dysplasia (GHDD) is an autosomal recessive disorder characterized by increased bone density with predominant diaphyseal involvement and aregenerative corticosteroid-sensitive anemia (summary by Genevieve et al., 2008).

Ossification of the posterior longitudinal ligament of the spine (OPLL) is a common degenerative spinal disorder that causes severe neurologic dysfunction in middle-aged and elderly populations. This ectopic ossification results in compression of the spinal cord and nerve root by the ossified ligament. Histologic studies of OPLL suggest that OPLL develops through a process of endochondral ossification (summary by Nakajima et al., 2016).

Desmosterolosis is a rare autosomal recessive disorder characterized by multiple congenital anomalies and elevated levels of the cholesterol precursor desmosterol in plasma, tissue, and cultured cells (summary by Waterham et al., 2001).

Autosomal recessive osteopetrosis-5 (OPTB5) is a form of infantile malignant osteopetrosis, characterized by defective osteoclast function resulting in decreased bone resorption and generalized osteosclerosis. Defective resorption causes development of densely sclerotic fragile bones and progressive obliteration of the marrow spaces and cranial foramina. Marrow obliteration is associated with extramedullary hematopoiesis and hepatosplenomegaly, and results in anemia and thrombocytopenia, whereas nerve entrapment accounts for progressive blindness and hearing loss. Other major manifestations include failure to thrive, pathologic fractures, and increased infection rate. Most affected children succumb to severe bone marrow failure and overwhelming infection in the first few years of life (summary by Quarello et al., 2004).

The spectrum of CLCN7-related osteopetrosis includes infantile malignant CLCN7-related autosomal recessive osteopetrosis (ARO), intermediate autosomal osteopetrosis (IAO), and autosomal dominant osteopetrosis type II (ADOII; Albers-Schönberg disease). ARO. Onset is at birth. Findings may include: fractures; reduced growth; sclerosis of the skull base (with or without choanal stenosis or hydrocephalus) resulting in optic nerve compression, facial palsy, and hearing loss; absence of the bone marrow cavity resulting in severe anemia and thrombocytopenia; dental abnormalities, odontomas, and risk for mandibular osteomyelitis; and hypocalcemia with tetanic seizures and secondary hyperparathyroidism. Without treatment maximal life span in ARO is ten years. IAO. Onset is in childhood. Findings may include: fractures after minor trauma, characteristic skeletal radiographic changes found incidentally, mild anemia, and occasional visual impairment secondary to optic nerve compression. Life expectancy in IAO is usually normal. ADOII. Onset is usually late childhood or adolescence. Findings may include: fractures (in any long bone and/or the posterior arch of a vertebra), scoliosis, hip osteoarthritis, and osteomyelitis of the mandible or septic osteitis or osteoarthritis elsewhere. Cranial nerve compression is rare.

Melorheostosis (MEL) is characterized by 'flowing' hyperostosis of the cortex of tubular bones. The lesions are usually asymmetric and involve only 1 limb or correspond to a particular sclerotome. They may be accompanied by abnormalities of adjacent soft tissue, including joint contractures, sclerodermatous skin lesions, muscle atrophy, or hemangiomas (review by Hellemans et al., 2004). The designation combines root words meaning 'limb,' 'flow,' and 'bone.' Melorheostosis may sometimes be a feature of Buschke-Ollendorff syndrome (BOS; 166700), a benign disorder which is caused by mutation in the LEMD3 gene (607844). Although germline or somatic LEMD3 mutations had been postulated to cause isolated melorheostosis (Butkus et al., 1997; Debeer et al., 2003; Happle, 2004; Hellemans et al., 2004), several studies have not been able to prove this (Hellemans et al., 2004; Mumm et al., 2007; Zhang et al., 2009).

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 cases of OI are autosomal dominant with mutations in 1 of the 2 genes that code for type I collagen alpha chains, COL1A1 (120150) and COL1A2 (120160). Martinez-Glez et al. (2012) described osteogenesis imperfecta type XIII, an autosomal recessive form of the disorder characterized by normal teeth, faint blue sclerae, severe growth deficiency, borderline osteoporosis, and an average of 10 to 15 fractures a year affecting both upper and lower limbs and with severe bone deformity.

Osteosclerotic metaphyseal dysplasia (OSMD) is a rare condition characterized by distinctive radiographic changes, including osteosclerosis localized predominantly to the metaphyses of the long bones. The shafts of the long bones are osteopenic. Laboratory abnormalities include elevated alkaline phosphatase levels in some, but not all, patients. Elevated urinary pyridinoline and deoxypyridinoline levels, markers of osteoclastic activity, have also been reported (Nishimura and Kozlowski, 1993; Kasapkara et al., 2013; Guo et al., 2017). Patients with OSMD have been described who also show hypotonia, developmental delay, seizures, and later-onset spastic paraplegia; however, OSMD resulting from mutation in the LRRK1 gene does not appear to include these neurologic features (Nishimura and Kozlowski, 1993; Kasapkara et al., 2013; Guo et al., 2017). Reviews Howaldt et al. (2020) reviewed published reports of LRRK1-associated OSMD, and noted that patients typically present with recurrent pathologic fractures and osteosclerosis at multiple skeletal sites, predominantly at the metaphyses and vertebral bodies. Variable degrees of osteosclerosis of ribs and skull and of Erlenmeyer flask deformity of the femurs have been observed.

About half of all people with trichothiodystrophy have a photosensitive form of the disorder, which causes them to be extremely sensitive to ultraviolet (UV) rays from sunlight. They develop a severe sunburn after spending just a few minutes in the sun. However, for reasons that are unclear, they do not develop other sun-related problems such as excessive freckling of the skin or an increased risk of skin cancer. Many people with trichothiodystrophy report that they do not sweat.\n\nTrichothiodystrophy is also associated with recurrent infections, particularly respiratory infections, which can be life-threatening. People with trichothiodystrophy may have abnormal red blood cells, including red blood cells that are smaller than normal. They may also have elevated levels of a type of hemoglobin called A2, which is a protein found in red blood cells. Other features of trichothiodystrophy can include dry, scaly skin (ichthyosis); abnormalities of the fingernails and toenails; clouding of the lens in both eyes from birth (congenital cataracts); poor coordination; and skeletal abnormalities including degeneration of both hips at an early age.\n\nMothers of children with trichothiodystrophy may experience problems during pregnancy including pregnancy-induced high blood pressure (preeclampsia) and a related condition called HELLP syndrome that can damage the liver. Babies with trichothiodystrophy are at increased risk of premature birth, low birth weight, and slow growth. Most children with trichothiodystrophy have short stature compared to others their age. \n\nThe signs and symptoms of trichothiodystrophy vary widely. Mild cases may involve only the hair. More severe cases also cause delayed development, significant intellectual disability, and recurrent infections; severely affected individuals may survive only into infancy or early childhood.\n\nIntellectual disability and delayed development are common in people with trichothiodystrophy, although most affected individuals are highly social with an outgoing and engaging personality. Some people with trichothiodystrophy have brain abnormalities that can be seen with imaging tests. A common neurological feature of this disorder is impaired myelin production (dysmyelination). Myelin is a fatty substance that insulates nerve cells and promotes the rapid transmission of nerve impulses.\n\nIn people with trichothiodystrophy, tests show that the hair is lacking sulfur-containing proteins that normally gives hair its strength. A cross section of a cut hair shows alternating light and dark banding that has been described as a "tiger tail."\n\nTrichothiodystrophy, commonly called TTD, is a rare inherited condition that affects many parts of the body. The hallmark of this condition is hair that is sparse and easily broken. 

A rare, primary bone dysplasia characterized by severe growth retardation, short stature, cortical thickening and medullary stenosis of long bones, delayed closure of the anterior fontanelle, absent diploic space in the skull bones, prominent forehead, macrocephaly, dental anomalies, eye problems (hypermetropia and pseudopapilledema), and hypocalcemia due to hypoparathyroidism, sometimes resulting in convulsions. Intelligence is normal.

Hereditary hypophosphatemic rickets is a disorder related to low levels of phosphate in the blood (hypophosphatemia). Phosphate is a mineral that is essential for the normal formation of bones and teeth.\n\nIn most cases, the signs and symptoms of hereditary hypophosphatemic rickets begin in early childhood. The features of the disorder vary widely, even among affected members of the same family. Mildly affected individuals may have hypophosphatemia without other signs and symptoms. More severely affected children experience slow growth and are shorter than their peers. They develop bone abnormalities that can interfere with movement and cause bone pain. The most noticeable of these abnormalities are bowed legs or knock knees. These abnormalities become apparent with weight-bearing activities such as walking. If untreated, they tend to worsen with time.\n\nOther signs and symptoms of hereditary hypophosphatemic rickets can include premature fusion of the skull bones (craniosynostosis) and dental abnormalities. The disorder may also cause abnormal bone growth where ligaments and tendons attach to joints (enthesopathy). In adults, hypophosphatemia is characterized by a softening of the bones known as osteomalacia.\n\nAnother rare type of the disorder is known as hereditary hypophosphatemic rickets with hypercalciuria (HHRH). In addition to hypophosphatemia, this condition is characterized by the excretion of high levels of calcium in the urine (hypercalciuria).\n\nResearchers have described several forms of hereditary hypophosphatemic rickets, which are distinguished by their pattern of inheritance and genetic cause. The most common form of the disorder is known as X-linked hypophosphatemic rickets (XLH). It has an X-linked dominant pattern of inheritance. X-linked recessive, autosomal dominant, and autosomal recessive forms of the disorder are much rarer.

Obesity due to mutation in the MC4R gene is the most common cause of monogenic obesity. Patients have early-onset severe obesity and hyperphagia (Farooqi et al., 2003).

Autosomal recessive osteopetrosis-9 (OPTB9) is characterized by increased bone density and bone fragility, as well as renal failure. Vision may be compromised due to compression of the optic nerve secondary to osteopetrotic stenosis of the optic nerve canal (Xue et al., 2022). For a general phenotypic description and discussion of genetic heterogeneity of autosomal recessive osteopetrosis, see OPTB1 (259700).

Craniometadiaphyseal osteosclerosis with hip dysplasia (CMDOH) is characterized by macrocephaly, cranial hyperostosis, and vertebral endplate sclerosis. Other frequent findings include hip dysplasia, heart malformations, variable developmental delay, and hematologic anomalies including anemia and pancytopenia. Bone biopsy shows evidence of increased osteoblast and reduced osteoclast function at the growth plate resorption zone, resulting in coarse trabeculae (Terhal et al., 2023). For syndromes with overlapping features, see osteopetrosis, autosomal recessive (OPTB1; 259700) and dominant (OPTA1; 607634), and osteopathia with cranial sclerosis (OSCS; 300373).