GENETIC-DEFECTS OF THE 1,25-DIHYDROXYVITAMIN-D3 RECEPTOR

Target organ resistance to steroid hormone action is known to produce clinical disorders ranging from testicular feminization in the case of androgen resistance to hypocalcemic vitamin D-resistant ricets (HVDRR) in the case of 1,25-dihydroxyvitamin D3. The etiologic basis of these disorders is thought to be genetic mutations in the gene encoding receptors for these hormones. We investigated this possibility by analyzing the vitamin D receptor (VDR) protein, mRNA, and DNA from patients with HVDRR. This autosomal recessive disease of children is characterized by early onset rickets, hypocalcemia, hyperparathyroidism, and elevated levels of 1,25-(OH)2D3. Cells from patients fall into three general classes of molecular defects: (i) decreased or absent hormone binding; (ii) decreased affinity of VDR for DNA, or; (iii) defective nuclear translocation or retention. Analysis of the DNA and/or mRNA from these cells has identified missense mutations in the DNA binding (zinc finger) domain and a nonsense mutation in the steroid binding domain of VDR. The mutations were individually recreated in wild type VDR and the expressed mutant protein behaved biochemically identically to the patient receptor. Further studies have shown that the receptor is unable to interact with the specific hormone response element (HRE) of the osteocalcin gene and activate appropriate transcription. Rapid diagnostic genotyping of these mutations is possible with either restriction digestion or allele-specific oligonucleotide hybridization. Analysis of these naturally occurring, disease producing mutations of a gene regulatory protein should provide insight into the key amino acid residues of the protein and the mechanism by which steroids modulate gene transcription.