rs16930609 — CYP2R1

CYP2R1 rs16930609 — When the First Step Falters

Vitamin D from sunlight or diet is biologically inert until activated by two sequential hydroxylation steps in the liver and kidneys. The first step — the conversion of vitamin D3 to 25-hydroxyvitamin D (25(OH)D)¹¹ 25-hydroxyvitamin D (25(OH)D)
Also called calcidiol; the main storage and transport form of vitamin D and the standard measure of vitamin D status on a blood test. Normal range is 30–100 ng/mL (75–250 nmol/L) — is performed primarily by the enzyme CYP2R1 (cytochrome P450 family 2 subfamily R member 1), expressed mainly in the liver. This first activation step sets the ceiling for everything downstream: the kidneys can only convert 25(OH)D to the active hormone calcitriol if there is enough 25(OH)D to work with.

rs16930609 sits approximately 2 kilobases upstream of the CYP2R1 coding sequence on chromosome 11p15.2. It is part of a haplotype block that encompasses several functionally important CYP2R1 promoter-region variants including rs10741657 and rs2060793. The C allele (minor allele, approximately 6% frequency in Europeans) tags this haplotype and is associated with modestly reduced CYP2R1 activity, leading to lower circulating 25(OH)D. Genome-wide association studies consistently identify the CYP2R1 locus as one of the four major genetic determinants of circulating vitamin D status, alongside GC (vitamin D binding protein), DHCR7/NADSYN1 (skin synthesis), and CYP24A1 (degradation).

The Mechanism

The rs16930609 variant lies in the upstream regulatory region of CYP2R1. While it does not alter the CYP2R1 protein directly, it is in linkage disequilibrium²² linkage disequilibrium
Two variants are in LD when they tend to be inherited together more often than expected by chance — knowing one allele predicts the other with regulatory variants (notably rs10741657 and rs2060793, both in the gene's promoter region) that influence how much CYP2R1 protein is produced. Less CYP2R1 protein means lower 25-hydroxylation capacity: the same dietary vitamin D or UVB-derived vitamin D3 produces less circulating 25(OH)D.

In knockout mouse studies³³ knockout mouse studies
Zhu et al. 2013 — mice with the Cyp2r1 gene deleted had more than 50% reduction in serum 25(OH)D3, confirming CYP2R1 as the dominant 25-hydroxylase in vivo, eliminating CYP2R1 alone cut circulating 25(OH)D by more than half, establishing it as the major hepatic 25-hydroxylase. This enzymatic bottleneck means that genetic variation reducing CYP2R1 efficiency translates directly into lower vitamin D status, regardless of sun exposure or dietary intake.

The Evidence

The original SUNLIGHT consortium GWAS⁴⁴ original SUNLIGHT consortium GWAS
Wang TJ et al. Common genetic determinants of vitamin D insufficiency: a genome-wide association study. Lancet, 2010 identified the CYP2R1 locus (proxied by rs10741657, in strong LD with rs16930609) at genome-wide significance (p = 3.3 × 10⁻²⁰). A composite genetic risk score combining CYP2R1 with GC and DHCR7/NADSYN1 variants showed that participants in the highest genetic risk quartile had 2.47-fold increased odds of vitamin D insufficiency (< 75 nmol/L).

A haplotype analysis in 2,868 elderly Swedish men⁵⁵ haplotype analysis in 2,868 elderly Swedish men
Bjork A et al. Haplotypes in the CYP2R1 gene are associated with levels of 25(OH)D and bone mineral density, but not with other markers of bone metabolism (MrOS Sweden). PLoS One, 2018 (the MrOS Sweden cohort) directly genotyped rs16930609 alongside seven other CYP2R1 variants and found that CYP2R1 haplotypes containing this SNP were associated with 25(OH)D differences of 4.6–18.5% between haplotype groups (p < 0.05), as well as significant differences in bone mineral density. When analyzed as an individual SNP, rs16930609 alone did not reach statistical significance for 25(OH)D (p = 0.18), confirming that its effects are best captured at the haplotype level.

In 2,897 healthy Han Chinese subjects, Zhang et al. 2013⁶⁶ Zhang et al. 2013
Zhang Z et al. An analysis of the association between the vitamin D pathway and serum 25-hydroxyvitamin D levels in a healthy Chinese population. J Bone Miner Res, 2013 identified the CYP2R1 haplotype AAGA (rs7936142–rs12794714–rs2060793–rs16930609) as a genetic risk factor for lower 25(OH)D concentration, supporting cross-ethnic replication of this locus.

A meta-analysis of 16 studies (52,417 participants)⁷⁷ meta-analysis of 16 studies (52,417 participants)
Duan L et al. Effects of CYP2R1 gene variants on vitamin D levels and status: a systematic review and meta-analysis. Gene, 2018 concluded that CYP2R1 variants (principally rs10741657 and closely related SNPs in the same haplotype block) significantly predict vitamin D deficiency risk (OR 1.09, p = 0.002), with stronger effects in Caucasian populations (OR ~1.3). CYP2R1 variants have been shown to explain approximately 4.8–9.8% of the variance in baseline 25(OH)D concentration in women.

Practical Implications

Carriers of the C allele at rs16930609 have an enzymatic disadvantage at the first step of vitamin D activation. The same sun exposure or dietary vitamin D intake produces less circulating 25(OH)D compared to AA homozygotes. For heterozygotes (AC), the effect is mild and easily addressed by ensuring consistent vitamin D intake and routine monitoring. For the rare CC homozygotes, the reduction in 25-hydroxylation capacity is more pronounced and warrants higher supplementation targets.

Critically, standard vitamin D blood tests measure 25(OH)D — exactly the metabolite whose production is impaired in C allele carriers. This means the blood test is the right tool to use: if supplementation is adequate, 25(OH)D will normalize regardless of genotype. The key is choosing a supplement dose sufficient to overcome the enzymatic limitation.

Interactions

rs16930609 is in linkage disequilibrium with rs10741657 and rs2060793, the best-characterized CYP2R1 promoter-region SNPs. Users who have data on any of these variants carry equivalent information about CYP2R1 25-hydroxylation capacity.

The CYP2R1 locus interacts functionally with the GC locus (rs2282679, rs4588, rs7041) and the DHCR7/NADSYN1 locus (rs12785878). A person carrying a CYP2R1 haplotype associated with lower 25-hydroxylation efficiency alongside a GC low-transport isoform will have compounded reductions in total circulating 25(OH)D. Similarly, DHCR7 variants that reduce skin synthesis further limit the substrate available for CYP2R1 to convert.

CYP2R1 variants do not appear to significantly modify the response to vitamin D3 supplementation — meaning supplementation effectively bypasses the enzymatic limitation when dosed adequately. This is mechanistically logical: once 25(OH)D is formed (whether from sun, diet, or supplements), the downstream pathways are unaffected by this variant.