HEREDITARY 1,25-DIHYDROXYVITAMIN D-RESISTANT RICKETS DUE TO AN OPAL MUTATION CAUSING PREMATURE TERMINATION OF THE VITAMIN-D-RECEPTOR

Mutations in the vitamin D receptor (VDR) gene have been shown to caus e hereditary vitamin D-resistant rickets (HVDRR), The patient in this study is a young French-Canadian boy with no known consanguinity in hi s family, The child exhibited the clinical characteristics of HVDRR wi th early onset rickets, hypocalcemia, secondary hyperparathyroidism, a nd elevated 1,25-dihydroxyvitamin D (1,25(OH)(2)D) levels as well as t otal alopecia, Fibroblasts were cultured from a skin biopsy of the pat ient and used to assess the VDR, Northern blot analysis showed that a normal size VDR transcript was expressed; however, [H-3] 1,25(0H)(2)D- 3-binding levels were very low and Western blot analysis failed to det ect any VDR protein, Total resistance to 1,25(OH)(2)D-3 was demonstrat ed by the failure of the cultured fibroblasts to induce the transcript ion of the 25-hydroxyvitamin D-24-hydroxylase gene when treated with h igh concentrations of 1,25(OH)(2)D-3. Analysis of the DNA sequence rev ealed a unique C to T base change corresponding to nucleotide 218 of t he VDR cDNA, This single base substitution changes the codon for argin ine (CGA) to an opal stop codon (TGA), resulting in the truncation of the VDR at amino acid 30, The Arg30stop mutation prematurely terminate s translation and deletes 398 amino acids including most of the zinc f ingers as well as the entire ligand-binding domain, Restriction fragme nt length polymorphism analysis of a DdeI restriction site created by the mutation showed that the parents were heterozygous for the mutant allele. In conclusion, the Arg30stop mutation truncates the VDR and le ads to a hormone-resistant condition,which is the molecular basis of H VDRR in this patient.