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 1 alpha-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-1 alpha-
hydroxylase (
P450c1 alpha) from human keratinocytes, in which 1 alpha-
hydroxylase activity and
mRNA expression can be induced to be much greater.
P450c1 alpha mRNA was expressed at much lower levels in human kidney, brain, and testis. Mammalian cells transfected with the cloned
P450c1 alpha cDNA exhibit robust 1 alpha-
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
P450c1 alpha was localized to chromosome 12, where the 1 alpha-
hydroxylase deficiency syndrome,
vitamin D-dependent
rickets type 1 (VDDR-1), has been localized. Primary cultures of human adult and neonatal keratinocytes exhibit abundant 1 alpha-
hydroxylase activity, whereas those from a patient with VDDR-1 lacked detectable activity. Keratinocyte
P450c1 alpha 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.