TISSUE RESISTANCE TO 1,25-DIHYDROXYVITAMIN-D WITHOUT A MUTATION OF THE VITAMIN-D RECEPTOR GENE

OBJECTIVE Hereditary vitamin D resistant rickets (HVDRR) is characterized by severe rickets and is often accompanied by alopecia. Mutations in the gene encoding the vitamin D receptor have been found in this condition. In a patient with the characteristic phenotype we have investigated the functional defect and sequenced the gene to seek a mutation. DESIGN Patient and control cell lines prepared from skin fibroblasts and peripheral brood lymphocytes were used to measure binding of 1,25(OH)(2)D-3 and to isolate vitamin D receptor mRNA. VDR cDNA was sequenced and transfected into receptor defective cells. PATIENT A child with alopecia diagnosed as having rickets due to resistance to 1,25(OH)(2)D-3. MEASUREMENTS Cytosolic binding and nuclear association of 1,25(OH)(2)D-3 were determined in patient and control cells, and functional response to 1,25(OH)(2)D-3 assessed by measurement of 24-hydroxylase activity. VDR mRNA was prepared, reverse transcribed, and cDNA sequenced. VDR cDNA was also transfected into VDR-deficient CV-1 cells and functional response to 1,25(OH)(2)D-3 assessed by co-transfection with a chloramphenicol acetyltransferase (CAT) reporter plasmid. RESULTS VDR from the patient were able to bind 1,25(OH)(2)D-3 but showed no nuclear localization resulting in an absence of functional response to 1,25(OH)(2)D-3. Sequencing revealed that the VDR coding region was normal. Expression studies of the patient's VDR showed functionally normal VDR as evidenced by normal transactivation in the presence of 1,25(OH)(2)D-3. CONCLUSION These data indicate a new cause of tissue resistance to 1,25(OH)(2)D-3 which occurs in the absence of mutations in the coding region of VDR gene and which is characterized by defective nuclear localization of this receptor.