1,25-Dihydroxyvitamin D Resistance, Rickets, and Alopecia: Analysis of Receptors and Bioresponse in Cultured Fibroblasts from Patients and Parents

Abstract To investigate further the cellular defects of vitamin D-dependent rickets type II with alopecia, we studied 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptors and the response to 1,25-(OH)2D3 in cultured skin fibroblasts from rachitic patients. Our studies included cells from four affected patients from three kindreds and their parents and cells from five normal subjects. We measured total 1,25-(OH)2D3 receptor binding in cell extracts and the capacity of 1,25-(OH)2D3 to induce the enzyme 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) as a marker of functional repsonse. In normal fibroblasts, the 1,25-(OH)2D3 maximal binding capacity was 52 ± 5 fmol/100 ng DNA (mean ± SE), and the apparent dissociation constant (Kd) was 0.05 ± 0.01 nM. The maximal induced 24-hydroxylase activity after 1,25-(OH)2D3 treatment was 11.5 ± 1 fmol/106 cells-30 min, and the dose of 1,25-(OH)2D3 that achieved half-maximal induction was 2.3 ± 0.3 nM. Fibroblasts from all four rachitic patients had the same defect: n...