Background and objectives: Warfarin inhibits vitamin K epoxide reductase, of which microsomal epoxide hydrolase is a putative member. Several studies have found signals of association with warfarin maintenance dose in the EPHX1 gene. The aim of this study was to determine the effects of EPHX1 variants on warfarin maintenance dose in a multiethnic Asian population.
Methods: We sequenced the exons of EPHX1 using PCR and direct sequencing in 279 patients consisting of three major ethnic groups receiving maintenance warfarin with a stable international normalized ratio. The effects of EPHX1 variants were assessed using multiple linear regression.
Results: An association between an intronic SNP rs1877724 and warfarin maintenance dose was found, with homozygous variant carriers requiring approximately 0.5 mg/day lower than wild type and heterozygotes after adjustment for covariates. However, its contribution is small, explaining only an additional 0.8% of the dose variability. Rare variants were pooled but there was no association between their presence and warfarin maintenance dose. However, the presence of noncoding rare SNPs was significantly associated with warfarin maintenance dose.
Conclusion: Despite a significant finding in rs1877724, which concurs with an earlier study, overall, genetic variants in EPHX1 do not have a clinically significant impact on warfarin dose requirements in our population.