Hereditary vitamin D–resistant rickets (HVDRR) is a rare recessive genetic disorder caused by mutations in the vitamin D receptor (VDR). In this study, we examined the VDR in a young girl with clinical features of HVDRR including rickets, hypophosphatemia, and elevated serum 1,25(OH)2D. The girl also had total alopecia. Two mutations were found in the VDR gene: a nonsense mutation (R30X) in the DNA-binding domain and a unique 3-bp in-frame deletion in exon 6 that deleted the codon for lysine at amino acid 246 (ΔK246). The child and her mother were both heterozygous for the 3-bp deletion, whereas the child and her father were both heterozygous for the R30X mutation. Fibroblasts from the patient were unresponsive to 1,25(OH)2D3 as shown by their failure to induce CYP24A1 gene expression, a marker of 1,25(OH)2D3 responsiveness. [3H]1,25(OH)2D3 binding and immunoblot analysis showed that the patient's cells expressed the VDRΔK246 mutant protein; however, the amount of VDRΔK246 mutant protein was significantly reduced compared with wildtype controls. In transactivation assays, the recreated VDRΔK246 mutant was unresponsive to 1,25(OH)2D3. The ΔK246 mutation abolished heterodimerization of the mutant VDR with RXRα and binding to the coactivators DRIP205 and SRC-1. However, the ΔK246 mutation did not affect the interaction of the mutant VDR with the corepressor Hairless (HR). In summary, we describe a patient with compound heterozygous mutations in the VDR that results in HVDRR with alopecia. The R30X mutation truncates the VDR, whereas the ΔK246 mutation prevents heterodimerization with RXR and disrupts coactivator interactions
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