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).

X-linked recessive hypophosphatemic rickets (XLHRR) is a form of X-linked hypercalciuric nephrolithiasis, which comprises a group of disorders characterized by proximal renal tubular reabsorptive failure, hypercalciuria, nephrocalcinosis, and renal insufficiency. These disorders have also been referred to as the 'Dent disease complex' (Scheinman, 1998; Gambaro et al., 2004). For a general discussion of Dent disease, see 300009.

Dent disease, an X-linked disorder of proximal renal tubular dysfunction, is characterized by low molecular weight (LMW) proteinuria, hypercalciuria, and at least one additional finding including nephrocalcinosis, nephrolithiasis, hematuria, hypophosphatemia, chronic kidney disease (CKD), and evidence of X-linked inheritance. Males younger than age ten years may manifest only LMW proteinuria and/or hypercalciuria, which are usually asymptomatic. Thirty to 80% of affected males develop end-stage renal disease (ESRD) between ages 30 and 50 years; in some instances ESRD does not develop until the sixth decade of life or later. The disease may also be accompanied by rickets or osteomalacia, growth restriction, and short stature. Disease severity can vary within the same family. Males with Dent disease 2 (caused by pathogenic variants in OCRL) may also have mild intellectual disability, cataracts, and/or elevated muscle enzymes. Due to random X-chromosome inactivation, some female carriers may manifest hypercalciuria and, rarely, renal calculi and moderate LMW proteinuria. Females rarely develop CKD.

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).

Vitamin D-dependent rickets type 2B with normal vitamin D receptor (VDDR2B) is an unusual form of rickets due to abnormal expression of a hormone response element-binding protein that interferes with the normal function of the vitamin D receptor. Vitamin D-dependent rickets type 2A (VDDR2A) is caused by mutation in the vitamin D receptor gene (VDR; 601769), and most patients have alopecia in addition to rickets. 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).

Any Fanconi syndrome in which the cause of the disease is a mutation in the SLC34A1 gene.

Hyperphosphatemic familial tumoral calcinosis (HFTC) is characterized by: Ectopic calcifications (tumoral calcinosis) typically found in periarticular soft tissues exposed to repetitive trauma or prolonged pressure (e.g., hips, elbows, and shoulders); and Painful swellings (referred to as hyperostosis) in the areas overlying the diaphyses of the tibiae (and less often the ulna, metacarpal bones, and radius). The dental phenotype unique to HFTC includes enamel hypoplasia, short and bulbous roots, obliteration of pulp chambers and canals, and pulp stones. Less common are large and small vessel calcifications that are often asymptomatic incidental findings on radiologic studies but can also cause peripheral vascular insufficiency (e.g., pain, cold extremities, and decreased peripheral pulses). Less frequently reported findings include testicular microlithiasis and angioid streaks of the retina.

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare autosomal recessive metabolic disorder characterized by the progressive deposition of basic calcium phosphate crystals in periarticular spaces, soft tissues, and sometimes bone (Chefetz et al., 2005). The biochemical hallmark of tumoral calcinosis is hyperphosphatemia caused by increased renal absorption of phosphate due to loss-of-function mutations in the FGF23 (605380) or GALNT3 (601756) gene. The term 'hyperostosis-hyperphosphatemia syndrome' (HHS) is sometimes used when the disorder is characterized by involvement of the long bones associated with the radiographic findings of periosteal reaction and cortical hyperostosis. Although some have distinguished HHS from FTC by the presence of bone involvement and the absence of skin involvement (Frishberg et al., 2005), Ichikawa et al. (2010) concluded that the 2 entities represent a continuous spectrum of the same disease, best described as familial hyperphosphatemic tumoral calcinosis. HFTC is considered to be the clinical converse of autosomal dominant hypophosphatemic rickets (ADHR; 193100), an allelic disorder caused by gain-of-function mutations in the FGF23 gene and associated with hypophosphatemia and decreased renal phosphate absorption (Chefetz et al., 2005; Ichikawa et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of HFTC, see 211900.