In vivo function of VDR in gene expression-VDR knock-out mice

Abstract

Vitamin D exerts many biological actions through nuclear vitamin D receptor (VDR)-mediated gene expression. The transactivation function of VDR is activated by binding 1 α,25-dihydroxyvitamin D 3[1 α,25(OH) 2D 3], an active form of vitamin D. Conversion from 25(OH)D 3 is finely regulated in kidney by 25(OH)D 3 1 α-hydroxylase[25(OH)D 1 α-hydroxylase], keeping serum levels of 1 α,25(OH) 2D 3 constant. Deficiency of vitamin D and mutations in the genes like VDR (type II genetic rickets) are known to cause rickets like lowered serum calcium, alopecia and impaired bone formation. However, the molecular basis of vitamin D–VDR system in the vitamin D action in intact animals remained to be established. In addition, the 1 α-hydroxylase gene from any species had not yet been cloned, irrespective of its biological significance and putative link to the type I genetic rickets. We generated VDR-deficient mice (VDR KO mice). VDR KO mice grew up normally until weaning, but after weaning they developed abnormality like the type II rickets patients. These results demonstrated indispensability of vitamin D–VDR system in mineral and bone metabolism only in post-weaning life. Using a newly developed cloning system, we cloned the cDNA encoding a novel P450 enzyme, mouse and human 1 α-hydroxylase. The study in VDR KO mice demonstrated the function of liganded VDR in the negative feed-back regulation of 1 α,25(OH) 2D 3 production. Finally, from the analysis of type I rickets patients, we found missense genetic mutations in 1 α-hydroxylase, leading to the conclusion that this gene is responsible for the type I rickets.