rs3814637 — CYP2C19

CYP2C19 Upstream Variant — Warfarin Dose Sensitivity

CYP2C19 is one of the most important drug-metabolizing enzymes in the liver, responsible for processing a wide range of medications including anticoagulants, proton pump inhibitors, antidepressants, and antifungals. While much attention focuses on the well-characterized loss-of-function variants (*2, *3) and the gain-of-function *17 allele, rs3814637 is a separate upstream regulatory variant located approximately 1,400 bases before the CYP2C19 transcription start site. Carriers of the T allele show altered CYP2C19-mediated drug metabolism, with the most robust evidence coming from warfarin dosing studies.

The Mechanism

Rs3814637 sits in the regulatory region upstream of CYP2C19¹¹ Located ~1,374–1,393 bp upstream of the CYP2C19 transcription start site per Ensembl VEP and is classified as an upstream gene variant. The precise molecular mechanism by which the T allele alters CYP2C19 activity has not been fully characterized in published literature, but the pharmacokinetic data consistently show that T-allele carriers have higher plasma concentrations of CYP2C19 substrates, suggesting either reduced enzyme activity or expression. Unlike the *2 allele's splice-site mutation, rs3814637 likely exerts its effect through altered transcription factor binding or promoter-region regulation.

Warfarin exists as two enantiomers²² Mirror-image forms of the same molecule with different metabolic fates: S-warfarin (more potent, metabolized mainly by CYP2C9) and R-warfarin (metabolized predominantly by CYP1A2, CYP3A4, and CYP2C19). The rs3814637 variant specifically affects R-warfarin clearance, meaning T-allele carriers clear this enantiomer more slowly, raising the effective anticoagulant exposure.

The Evidence

The strongest evidence for rs3814637 comes from a 2022 meta-analysis by Wang et al.³³ 2022 meta-analysis by Wang et al.
Wang D et al. Impact of CYP2C19 gene polymorphisms on warfarin dose requirement: a systematic review and meta-analysis. Pharmacogenomics, 2022 analyzing nine studies totaling 1,393 patients. Individuals with the TT genotype required 34% lower warfarin maintenance doses than CC carriers; CT heterozygotes required 18% less. A companion pharmacokinetic study by Lane et al. (2012)⁴⁴ Lane et al. (2012)
Lane S et al. The population pharmacokinetics of R- and S-warfarin: effect of genetic and clinical factors. Br J Clin Pharmacol, 2012 independently identified rs3814637 as one of the key genetic determinants of R-warfarin clearance in a long-term anticoagulation cohort.

For other CYP2C19 substrates, a Chinese lung-cancer study by Tan et al. (2022)⁵⁵ Tan et al. (2022)
Tan T et al. Genetic Polymorphisms in CYP2C19 Cause Changes in Plasma Levels and Adverse Reactions to Anlotinib in Chinese Patients With Lung Cancer. Front Pharmacol, 2022 found that TT+CT carriers showed significantly higher peak plasma concentrations of anlotinib — a multikinase inhibitor — and elevated rates of hypertension and hemoptysis compared to CC carriers. This is consistent with a broader pattern of reduced CYP2C19-mediated clearance in T-allele carriers.

Practical Actions

The most direct clinical implication is for warfarin dosing. Anyone prescribed warfarin who carries the T allele — particularly TT homozygotes — may need a lower starting dose and closer INR monitoring during initiation. The effect is modest for heterozygotes (CT, ~18% dose reduction) but more pronounced for homozygotes (TT, ~34% reduction). Standard warfarin dosing algorithms do not currently include rs3814637, so this information is supplementary to — not a replacement for — clinical INR monitoring and algorithmic dosing (e.g., IWPC or WarfarinDosing.org algorithms that already incorporate CYP2C9 and VKORC1).

The T allele is enriched in East Asian populations (15–17% in Japanese cohorts) compared to Europeans (~6%), which may partly explain observed population differences in warfarin sensitivity.

Interactions

Rs3814637 acts independently of the major CYP2C19 star alleles (*2 via rs4244285 and *17 via rs12248560). A person carrying both rs3814637 T and the *2 no-function allele (rs4244285 A) would have compounded reductions in CYP2C19-mediated warfarin metabolism. Conversely, carrying rs3814637 T alongside *17 (rs12248560 T) creates opposing effects in the CYP2C19 locus and could complicate pharmacokinetic prediction. The *2 and *17 alleles are already captured by separate GeneOps entries; this entry reflects the independent additive contribution of rs3814637.