rs10766197 — CYP2R1

CYP2R1 rs10766197 — A Second Switch on the Vitamin D Activation Gate

Your liver performs the first critical step in making vitamin D biologically useful: converting vitamin D3 (cholecalciferol) from sun exposure or supplements into 25-hydroxyvitamin D¹¹ 25-hydroxyvitamin D
25(OH)D, also called calcidiol — the circulating storage form measured in blood tests, with a half-life of about 3 weeks (25(OH)D). This hydroxylation is carried out by the enzyme CYP2R1, encoded by the CYP2R1 gene on chromosome 11p15. The rs10766197 variant sits in the gene's promoter region²² promoter region
The promoter is the DNA sequence upstream of a gene that controls how much mRNA — and thus how much protein — the cell produces, altering how much 25-hydroxylase enzyme the liver makes.

The Mechanism

The A allele at rs10766197 reduces transcriptional activity of the CYP2R1 promoter, resulting in lower enzyme levels in hepatocytes. With less 25-hydroxylase available, a given dose of vitamin D3 — whether from sun exposure, food, or supplements — is converted to 25(OH)D less efficiently. The effect is additive: each A allele incrementally reduces conversion capacity. Unlike rs10741657 (another CYP2R1 regulatory variant 7 kb upstream), rs10766197 appears to affect gene expression through a partially independent mechanism, and studies often include both in genetic risk scores because they jointly explain more variance in 25(OH)D levels than either alone.

The Evidence

The clearest demonstration of rs10766197's functional relevance came from a pair of randomized trials in Denmark. Nissen et al. (2015)³³ Nissen et al. (2015)
Nissen J et al. Common variants in CYP2R1 and GC genes are both determinants of serum 25-hydroxyvitamin D concentrations after UVB irradiation and vitamin D3 fortification. Am J Clin Nutr, 2015 studied 92 participants undergoing UVB treatment and 201 families consuming vitamin D3-fortified bread and milk over 6 months. Both rs10741657 and rs10766197 independently predicted 25(OH)D responses, with A-allele carriers achieving significantly lower circulating vitamin D under identical exposure conditions.

In 758 Danish children and adults, Nissen et al. (2014)⁴⁴ Nissen et al. (2014)
Nissen J et al. Common Variants in CYP2R1 and GC Genes Predict Vitamin D Concentrations in Healthy Danish Children and Adults. PLoS ONE, 2014 built a four-allele genetic risk score combining rs10766197 (A allele) and rs10741657 with two GC variants. Individuals carrying no risk alleles had 25(OH)D levels 16.5% higher than those carrying all four — a clinically meaningful difference separating sufficiency from insufficiency in many cases.

A randomized controlled trial in 253 Iranian adolescent girls⁵⁵ randomized controlled trial in 253 Iranian adolescent girls
Bahrami A et al. A genetic variant in the cytochrome P450 family 2 subfamily R member 1 determines response to vitamin D supplementation. Clin Nutr, 2018 found that GG homozygotes were twice as likely as A-allele carriers to achieve adequate 25(OH)D response to high-dose supplementation (50,000 IU/week for 9 weeks; OR=2.1, 95% CI 1.0–4.2, p=0.03). A parallel study of 1,787 healthy non-Hispanic whites aged 45–75⁶⁶ 1,787 healthy non-Hispanic whites aged 45–75
Barry EL et al. Genetic variants in CYP2R1, CYP24A1, and VDR modify the efficacy of vitamin D3 supplementation for increasing serum 25-hydroxyvitamin D levels in a randomized controlled trial. Am J Clin Nutr, 2014 confirmed that rs10766197 genotype modified supplementation efficacy, with GG carriers achieving the greatest absolute rise in 25(OH)D per unit of D3 taken.

Beyond vitamin D levels, the A allele has been associated with downstream disease risk. In a case-control study of 116 multiple sclerosis patients, Martinez-Hernandez et al. (2021)⁷⁷ Martinez-Hernandez et al. (2021)
Martinez-Hernandez A et al. Polymorphisms CYP2R1 rs10766197 and CYP27B1 rs10877012 in Multiple Sclerosis. J Immunol Res, 2021 found the A allele was more common in MS patients than controls (37.9% vs 30.5%, p=0.05), with GA+AA genotypes conferring OR=1.67 (95% CI 1.05–2.64, p=0.02) for MS in a dominant model.

Practical Implications

If you carry the A allele, your liver produces less CYP2R1 enzyme, meaning a larger fraction of any vitamin D3 you ingest — from sunlight, food, or supplements — passes through without being activated. To achieve the same target 25(OH)D level (typically 40–60 ng/mL / 100–150 nmol/L), you may need meaningfully higher D3 doses than the general population. The only reliable way to calibrate your personal dose is to test, supplement, and retest.

In populations with very low sun exposure or darker skin (which reduces cutaneous synthesis), this variant becomes especially relevant: you start with less vitamin D substrate and convert a smaller fraction of it.

Interactions

CYP2R1 rs10766197 sits adjacent to rs10741657 (approximately 7 kb away) and both target the same gene's regulatory region. Studies consistently include both in vitamin D genetic risk scores because they carry partially independent effects on 25(OH)D levels. Individuals who carry risk alleles at both SNPs face the largest reduction in hepatic hydroxylase output.

Downstream, if a person also carries variants in the GC gene (rs4588, rs7041, rs2282679) that reduce VDBP binding affinity, or VDR variants (rs1544410) that reduce receptor sensitivity, the compounded effect spans the entire vitamin D activation cascade: less made, less transported, less recognized. Each layer independently raises the case for testing and personalized dosing.