Point Mutations in the Human Vitamin D Receptor Gene Associated with Hypocalcemic Rickets

Hypocalcemic vitamin D--resistant rickets is a human genetic disease resulting from target organ resistance to the action of 1,25-dihydroxyvitamin D$_{3}$. Two families with affected children homozygous for this autosomal recessive disorder were studied for abnormalities in the intracellular vitamin D receptor (VDR) and its gene. Although the receptor displays normal binding of 1,25-dihydroxyvitamin D$_{3}$ hormone, VDR from affected family members has a decreased affinity for DNA. Genomic DNA isolated from these families was subjected to oligonucleotide-primed DNA amplification, and each of the nine exons encoding the receptor protein was sequenced for a genetic mutation. In each family, a different single nucleotide mutation was found in the DNA binding domain of the protein; one family near the tip of the first zinc finger (Gly$\rightarrow $Asp) and one at the tip of the second zinc finger (Arg$\rightarrow $Gly). The mutant residues were created in vitro by oligonucleotide directed point mutagenesis of wildtype VDR complementary DNA and this cDNA was transfected into COS-1 cells. The produced protein is biochemically indistinguishable from the receptor isolated from patients.