Do CYP2R1 genetic variants lower circulating 25-hydroxyvitamin D?

Genetic variations in the CYP2R1 gene are major determinants of circulating vitamin D levels due to the enzyme's central role in hepatic vitamin D metabolism.

Clinical and genetic evidence

• Large-scale genome-wide association studies (GWAS) identify CYP2R1 polymorphisms as key genetic drivers of vitamin D status. Specifically, the rs10741657 variant is strongly linked to lower circulating 25-hydroxyvitamin D [25(OH)D].
• Carriers of the G allele (AG or GG genotypes) show a significant reduction of approximately 2 to 4 nmol/L in serum 25(OH)D per allele, leading to a higher risk of clinical deficiency and a decreased response to therapeutic cholecalciferol supplementation.

Enzymatic and molecular mechanisms

• The CYP2R1 gene encodes a microsomal cytochrome P450 enzyme predominantly expressed in hepatocytes, which acts as the primary physiological vitamin D 25-hydroxylase.
• Using NADPH-cytochrome P450 reductase, CYP2R1 catalyzes the insertion of an oxygen atom at the C-25 position of both vitamin D2 and D3 to yield 25(OH)D, which is subsequently hydroxylated by CYP27B1 in the kidney to form active 1,25-dihydroxyvitamin D.
• Biochemical assays show that CYP2R1 possesses a very low Michaelis constant ($K_m$) for vitamin D, giving it an approximately 17-fold higher catalytic efficiency than other potential hydroxylases like CYP27A1.
• Rare homozygous loss-of-function mutations in CYP2R1 cause vitamin D-dependent rickets type 1B, characterized by a near-total absence of circulating 25(OH)D, confirming that alternative hydroxylases cannot compensate for its loss.

Bottom line

• Polymorphisms in CYP2R1 alter the expression or function of the body's primary, high-affinity hepatic 25-hydroxylase, directly limiting the metabolic conversion of vitamin D into its main circulating form, 25-hydroxyvitamin D.