Genetic evidence that the human CYP2R1 enzyme is a key vitamin D 25-hydroxylase

TLDR

Vitamin D activation requires 1α‑hydroxylation by CYP27B1 in the kidney and 25‑hydroxylation in the liver, but the hepatic enzyme remained unidentified for over three decades. The study examined a patient with severe vitamin D deficiency to determine the genetic basis of impaired 25‑hydroxylation. A homozygous exon 2 transition in CYP2R1 replaced a conserved leucine with proline at residue 99, abolishing 25‑hydroxylase activity and confirming CYP2R1 as the liver enzyme responsible for vitamin D activation.

Abstract

The synthesis of bioactive vitamin D requires hydroxylation at the 1α and 25 positions by cytochrome P450 enzymes in the kidney and liver, respectively. The mitochondrial enzyme CYP27B1 catalyzes 1α-hydroxylation in the kidney but the identity of the hepatic 25-hydroxylase has remained unclear for >30 years. We previously identified the microsomal CYP2R1 protein as a potential candidate for the liver vitamin D 25-hydroxylase based on the enzyme's biochemical properties, conservation, and expression pattern. Here, we report a molecular analysis of a patient with low circulating levels of 25-hydroxyvitamin D and classic symptoms of vitamin D deficiency. This individual was found to be homozygous for a transition mutation in exon 2 of the CYP2R1 gene on chromosome 11p15.2. The inherited mutation caused the substitution of a proline for an evolutionarily conserved leucine at amino acid 99 in the CYP2R1 protein and eliminated vitamin D 25-hydroxylase enzyme activity. These data identify CYP2R1 as a biologically relevant vitamin D 25-hydroxylase and reveal the molecular basis of a human genetic disease, selective 25-hydroxyvitamin D deficiency.