Genetic and clinical predictors of warfarin dose requirements in African Americans

Genome-wide association study identifies genetic determinants of warfarin responsiveness for Japanese

Warfarin is a commonly used anticoagulant, whose dose needs to be determined for each individual patient owing to large inter-individual variability in its therapeutic dose. Although several clinical and genetic variables influencing warfarin dose have been identified, uncovering additional factors are critically important for safer use of warfarin. Through a genome-wide association study, we identified single-nucleotide polymorphism (SNP) rs2108622 [cytochrome P450, family 4, subfamily F, polypeptide 2 (CYP4F2)] as a genetic determinant of warfarin responsiveness for Japanese. Stratifying subjects who have been pre-classified according to the genotypes of SNP rs10509680 [cytochrome P450, family 2, subfamily C, polypeptide 9 (CYP2C9)] and SNP rs9923231 [vitamin K epoxide reductase complex subunit 1 (VKORC1)], based on their genotypes of rs2108622 allowed identification of subjects who require higher dose of warfarin. Incorporating genotypes of rs2108622 into a warfarin dosing algorithm that considers age, body surface area, status of amiodarone co-administration and genotypes of SNPs in the CYP2C9 and VKORC1 genes improved the model's predictability to 43.4%. In this study, the association of CYP4F2 with warfarin dose of the Japanese has been established for the first time. Besides, a warfarin dosing algorithm that incorporates genotypes of rs2108622 and amiodarone co-administration status was suggested for the Japanese. Our study also implied that common SNPs other than those in the CYP2C9, VKORC1 and CYP4F2 genes that show strong effect on the therapeutic warfarin dose might not exist.

Gamma-glutamyl carboxylase and its influence on warfarin dose

Via generation of vitamin K-dependent proteins, gamma-glutamyl carboxylase (GGCX) plays a critical role in the vitamin K cycle. Single nucleotide polymorphisms (SNPs) in GGCX, therefore, may affect dosing of the vitamin K antagonist, warfarin. In a multi-centered, cross-sectional study of 985 patients prescribed warfarin therapy, we genotyped for two GGCX SNPs (rs11676382 and rs12714145) and quantified their relationship to therapeutic dose. GGCX rs11676382 was a significant (p=0.03) predictor of residual dosing error and was associated with a 6.1% reduction in warfarin dose (95% CI: 0.6%-11.4%) per G allele. The prevalence was 14.1% in our predominantly (78%) Caucasian cohort, but the overall contribution to dosing accuracy was modest (partial R2 = 0.2%). GGCX rs12714145 was not a significant predictor of therapeutic dose (p = 0.26). GGCX rs11676382 is a statistically significant predictor of warfarin dose, but the clinical relevance is modest. Given the potentially low marginal cost of adding this SNP to existing genotyping platforms, we have modified our non-profit website (www.WarfarinDosing.org) to accommodate knowledge of this variant.

Worldwide allele frequency distribution of four polymorphisms associated with warfarin dose requirements

The aim of the study is to improve our understanding of the worldwide allele frequency distribution of four genetic polymorphisms known to influence warfarin dosing (VKORC1 rs9923231, CYP2C9 rs1799853, CYP2C9 rs1057910 and CYP4F2 rs2108622). These four polymorphisms were genotyped in the Human Genome Diversity Project-Centre Etude Polymorphism Humain (HGDP-CEPH) worldwide sample (N=963), as well as in a sample of individuals of European, East Asian and South Asian ancestry living in Canada (N=316). The VKORC1 rs9923231 single nucleotide polymorphism (SNP) showed an extensive geographic differentiation, with the derived T allele appearing at very high allele frequencies in East Asian populations. Results from several tests of positive selection indicate that this unusual distribution may be the result of positive selection in East Asia. Understanding the worldwide distribution of markers determining warfarin dosing is important for the future application of pharmacogenomic-based algorithms to different population groups.

Implementing genotype-guided antithrombotic therapy

Genotyping has the potential to improve the efficacy and safety of major antithrombotic drugs. For warfarin, the stable maintenance dose varies from 1-10 mg/day. The VKORC1 -1639G>A allele and the CYP2C9*2 and *3 alleles (cumulative frequency: 90% in Asians, 65% in Europeans and 20% in Africans), explain 45% of response variability in European and 30% in African populations. The large clinical trials COAG and EU-PACT will define the extent to which pharmacogenetic dosing affects the safety and efficacy of warfarin and coumarin derivatives. The platelet inhibitor clopidogrel requires activation by the CYP2C19 enzyme. CYP2C19*2 and *3 alleles (cumulative frequency: 20-50%) produce null enzyme activity, and their presence attenuates platelet inhibition and increases cardiovascular events. The US FDA-mandated drug labeling recognizes the relevance of genotyping in the selection and dosing of both warfarin and clopidogrel.