MedGen

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 (ALP). Biallelic ALPL pathogenic variants often result in severe hypophosphatasia that can result in stillbirth without mineralized bone, while heterozygous ALPL pathogenic variants are more likely to manifest as modest, mild, or even asymptomatic disease. Regardless of the number of ALPL pathogenic variants, many individuals with hypophosphatasia suffer from pain, disability, and reduced quality of life. Variability of clinical manifestations is common in both childhood and adult forms of hypophosphatasia and even occurs within affected families. 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 restrictive lung disease, respiratory failure, vitamin B6-dependent seizures, hypercalcemia with high morbidity, and mortality 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 ALP activity Severe childhood (juvenile): Variable presenting features progressing to rickets Mild childhood: Present later in childhood without rachitic disease, low bone mineral density for age, increased risk of fracture, and premature loss of primary teeth with intact roots Adult: Characterized by osteomalacia and stress fractures and pseudofractures of the lower extremities in middle age, sometimes associated with early loss of adult dentition. Adults with hypophosphatasia may also have significant bone pain and pronounced non-skeletal disease, with muscle weakness, dental problems, and reduced quality of life. Odontohypophosphatasia: Characterized by premature exfoliation of primary teeth and/or severe dental caries without skeletal manifestations [from GeneReviews]

Zellweger spectrum disorder (ZSD) is a phenotypic continuum ranging from severe to mild. While individual phenotypes (e.g., Zellweger syndrome [ZS], neonatal adrenoleukodystrophy [NALD], and infantile Refsum disease [IRD]) were described in the past before the biochemical and molecular bases of this spectrum were fully determined, the term "ZSD" is now used to refer to all individuals with a defect in one of the ZSD-PEX genes regardless of phenotype. Individuals with ZSD usually come to clinical attention in the newborn period or later in childhood. Affected newborns are hypotonic and feed poorly. They have distinctive facies, congenital malformations (neuronal migration defects associated with neonatal-onset seizures, renal cysts, and bony stippling [chondrodysplasia punctata] of the patella[e] and the long bones), and liver disease that can be severe. Infants with severe ZSD are significantly impaired and typically die during the first year of life, usually having made no developmental progress. Individuals with intermediate/milder ZSD do not have congenital malformations, but rather progressive peroxisome dysfunction variably manifest as sensory loss (secondary to retinal dystrophy and sensorineural hearing loss), neurologic involvement (ataxia, polyneuropathy, and leukodystrophy), liver dysfunction, adrenal insufficiency, and renal oxalate stones. While hypotonia and developmental delays are typical, intellect can be normal. Some have osteopenia; almost all have ameleogenesis imperfecta in the secondary teeth. [from GeneReviews]

Vitamin D-dependent rickets type 2A (VDDR2A) is caused by a defect in the vitamin D receptor gene. This defect leads to an increase in the circulating ligand, 1,25-dihydroxyvitamin D3. Most patients have total alopecia in addition to rickets. VDDR2B (600785) is a form of vitamin D-dependent rickets with a phenotype similar to VDDR2A but a normal vitamin D receptor, in which end-organ resistance to vitamin D has been shown to be caused by a nuclear ribonucleoprotein that interferes with the vitamin D receptor-DNA interaction. For a general phenotypic description and a discussion of genetic heterogeneity of rickets due to disorders in vitamin D metabolism or action, see vitamin D-dependent rickets type 1A (VDDR1A; 264700). [from OMIM]

Classic Schinzel-Giedion syndrome (SGS), an ultra-rare multisystem disorder caused by gain-of-function pathogenic variants in a SETBP1 mutational hot spot, is characterized by global neurodevelopmental impairment leading to moderate-to-profound intellectual disability, epilepsy (often refractory to treatment), hypotonia, spasticity, dysautonomia, hearing loss, and cerebral visual impairment. Other findings can include poor weight gain often associated with gastroesophageal reflux disease, chronic vomiting, constipation, gastroparesis, and/or feeding intolerance. Structural malformations can involve the heart, skeleton, kidney and urinary tract, genitalia, and brain. Anomalies of the liver, spleen, and/or pancreas are less common. Other features may include neuroepithelial neoplasia, severely disrupted sleep, choanal stenosis, inguinal hernia, sensitive skin, and increased risk of infection. To date, more than 50 individuals have been reported with molecularly confirmed classic SGS. Atypical SGS, reported in five individuals to date, is caused by pathogenic SETBP1 variants in proximity to – but not within – the mutational hot spot. The broad spectrum of clinical features of variable severity partially overlaps with classic SGS; however, this spectrum does not include risk for neuroepithelial neoplasia to date. [from GeneReviews]

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare autosomal recessive disorder characterized by the presence of hypophosphatemia secondary to renal phosphate wasting, radiographic and/or histologic evidence of rickets, limb deformities, muscle weakness, and bone pain. HHRH is distinct from other forms of hypophosphatemic rickets in that affected individuals present with hypercalciuria due to increased serum 1,25-dihydroxyvitamin D levels and increased intestinal calcium absorption (summary by Bergwitz et al., 2006). [from OMIM]

Wiedemann-Rautenstrauch syndrome (WDRTS) is a rare autosomal recessive neonatal progeroid disorder characterized by intrauterine growth retardation, failure to thrive, short stature, a progeroid appearance, hypotonia, and variable mental impairment (summary by Toriello, 1990). Average survival in WDRTS is 7 months, although survival into the third decade of life has been reported (Akawi et al., 2013). [from OMIM]

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

Vitamin D hydroxylation-deficient rickets type 1B (VDDR1B) is caused by a defect in vitamin D 25-hydroxylation (Molin et al., 2017). The major function of vitamin D is to maintain calcium and phosphate levels in the normal range to support metabolic functions, neuromuscular transmission, and bone mineralization. Disorders of vitamin D metabolism or action lead to defective bone mineralization and clinical features including intestinal malabsorption of calcium, hypocalcemia, secondary hyperparathyroidism, increased renal clearance of phosphorus, and hypophosphatemia. The combination of hypocalcemia and hypophosphatemia causes impaired mineralization of bone that results in rickets and osteomalacia (summary by Liberman and Marx, 2001). Rickets can occur because of inadequate dietary intake or sun exposure or because of genetic disorders. Vitamin D3 (cholecalciferol) is taken in the diet or synthesized in the skin from 7-dehydrocholesterol by ultraviolet irradiation. For vitamin D to be active, it needs to be converted to its active form, 1,25-dihydroxyvitamin D3. Vitamin D is transported in the blood by the vitamin D binding protein (DBP; 139200) to the liver, where vitamin D 25-hydroxylase (CYP2R1; 608713) is the key enzyme for 25-hydroxylation. Vitamin D 25(OH)D3, the major circulating form of vitamin D, is then transported to the kidney, where 25(OH)D3 is hydroxylated at the position of carbon 1 of the A ring, resulting in the active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (summary by Christakos et al., 2010). [from OMIM]

Enlarged parietal foramina are characteristic symmetric, paired radiolucencies of the parietal bones, located close to the intersection of the sagittal and lambdoid sutures, caused by deficient ossification around the parietal notch. Enlarged parietal foramina are usually asymptomatic. Meningeal, cortical, and vascular malformations of the posterior fossa occasionally accompany the bone defects and may predispose to epilepsy. In a minority of individuals, headaches, vomiting, or intense local pain are sometimes associated with the defects, especially on application of mild pressure to the unprotected cerebral cortex. [from GeneReviews]

Vitamin D-dependent rickets is a disorder of bone development that leads to softening and weakening of the bones (rickets). There are several forms of the condition that are distinguished primarily by their genetic causes: type 1A (VDDR1A), type 1B (VDDR1B), and type 2A (VDDR2A). There is also evidence of a very rare form of the condition, called type 2B (VDDR2B), although not much is known about this form.

The signs and symptoms of vitamin D-dependent rickets begin within months after birth, and most are the same for all types of the condition. The weak bones often cause bone pain and delayed growth and have a tendency to fracture. When affected children begin to walk, they may develop abnormally curved (bowed) legs because the bones are too weak to bear weight. Impaired bone development also results in widening of the areas near the ends of bones where new bone forms (metaphyses), especially in the knees, wrists, and ribs. Some people with vitamin D-dependent rickets have dental abnormalities such as thin tooth enamel and frequent cavities. Poor muscle tone (hypotonia) and muscle weakness are also common in this condition, and some affected individuals develop seizures.

Hair loss (alopecia) can occur in VDDR2A, although not everyone with this form of the condition has alopecia. Affected individuals can have sparse or patchy hair or no hair at all on their heads. Some affected individuals are missing body hair as well.

In vitamin D-dependent rickets, there is an imbalance of certain substances in the blood. An early sign in all types of the condition is low levels of the mineral calcium (hypocalcemia), which is essential for the normal formation of bones and teeth. Affected individuals also develop high levels of a hormone involved in regulating calcium levels called parathyroid hormone (PTH), which leads to a condition called secondary hyperparathyroidism. Low levels of a mineral called phosphate (hypophosphatemia) also occur in affected individuals. Vitamin D-dependent rickets types 1 and 2 can be grouped by blood levels of a hormone called calcitriol, which is the active form of vitamin D; individuals with VDDR1A and VDDR1B have abnormally low levels of calcitriol and individuals with VDDR2A and VDDR2B have abnormally high levels. [from MedlinePlus Genetics]

An abnormally increased width of the cranial fontanelles and sutures. [from HPO]