Novel mutations in the 1 alpha-hydroxylase (P450cl) gene in three familieswith pseudovitamin D-deficiency rickets resulting in loss of functional enzyme activity in blood-derived macrophages

Pseudovitamin D-defiency rickets (PDDR) is an autosomal recessive disorder characterized by hypocalcemia, rickets (which are resistant to treatment wi th vitamin D), and low or undetectable serum levels of 1,250dihydroxyvitami n D (1,25(OH)(2)D), The symptoms are corrected with 1,25(OH)(2)D treatment, and the disease is now believed to result from a defect in the cytochrome P450 component (P450c1; CYP27B1) of the renal 25-hydroxyvitamin D-1 alpha-h ydroxylase (1-OHase), We have studied genomic DNA from three families with PDDR and have identified the same homozygous mutation in the P450c1 gene in two of the index cases, causing a frameshift in exon 8, resulting in a pre mature stop codon in the heme-binding domain. The two cases in the third ki ndred were compound heterozygotes with missense mutations in exons 6 and 9, We have also identified a C/T polymorphism in intron 6 of the P450c1 genom ic DNA, Interferon gamma-inducible 1-OHase activity in blood-derived macrop hages was shown by 1,25(OH)(2)D synthesis in all control cells tested (37-1 84 fmol/h/10(6) cells) and those from the PDDR family parents (34-116 fmol/ h/10(6) cells) but was totally absent from the patients' cells, indicating a defect in their macrophage 1-OHase, similar to the presumed renal defect. The assumption of similarity between the renal and macrophage P450c1 was s upported by our ability to clone a 514 bp sequence, including the heme-bind ing region of the macrophage P450c1 cDNA from controls, which was identical to that published for both the renal and keratinocyte P450c1 cDNAs.