Cloning of human 25-hydroxyvitamin D-1α-hydroxylase and mutations causing vitamin D-dependent rickets type 1

Abstract

The secosteroid hormone, 1,25-dihydroxyvitamin D [1,25(OH)2D], plays a crucial role in normal bone growth, calcium metabolism, and tissue differentiation. The key step in the biosynthesis of 1,25-(OH)2D is its 1α- hydroxylation from 25-hydroxyvitamin D (25-OHD) in the kidney. Because its expression in the kidney is very low, we cloned and sequenced cDNA for 25- OHD-1α-hydroxylase (P450c1α) from human keratinocytes, in which lα- hydroxylase activity and mRNA expression can be induced to be much greater. P450c1α mRNA was expressed at much lower levels in human kidney, brain, end testis. Mammalian cells transfected with the cloned P450c1α cDNA exhibit robust 1α-hydroxylase activity. The identity of the 1,25(OH)2D3 product synthesized in transfected cells was confirmed by HPLC and gas chromatography-mass spectrometry. The gene encoding P450c1α was localized to chromosome 12, where the lα-hydroxylase deficiency syndrome, vitamin D- dependent rickets type 1 (VDDR-1), has been localized. Primary cultures of human adult and neonatal keratinocytes exhibit abundant lα-hydroxylase activity, whereas those from a patient with VDDR-1 lacked detectable activity. Kerotinocyte P450c1α cDNA from the patient with VDDR-1 contained deletion/frameshift mutations either at codon 211 or at codon 231, indicating that the patient was a compound heterozygote for two null mutations. These findings establish the molecular genetic basis of VDDR-1, establish a novel means for its study in keratinocytes, and provide the sequence of the key enzyme in the biological activation of vitamin D.