Vitamin D-dependent rickets type II. Defective induction of 25-hydroxyvitamin D3-24-hydroxylase by 1,25-dihydroxyvitamin D3 in cultured skin fibroblasts.

The study investigated the induction of 25-hydroxyvitamin D3-24-hydroxylase (24-OHase) by 1,25-dihydroxyvitamin D3 in skin fibroblasts from 10 normal subjects and 4 patients with vitamin D-dependent rickets type II (DD II). In normal fibroblasts, 24-OHase induction was detectable at low concentrations of 1,25(OH)2D3, while fibroblasts from DD II patients showed abnormal induction. Two patients' fibroblasts required much higher concentrations of 1,25(OH)2D3 for detectable induction, correlating with their partial in vivo response to calciferols. The other two patients showed no induction and no calcemic response. The study concluded that measuring 24-OHase induction is a sensitive test for identifying defects in the 1,25(OH)2D pathway and predicting response to calciferol therapy.