Absence of Novel CYP4F2 and VKORC1 Coding Region DNA Variants in Patients Requiring High Warfarin Doses

Abstract

Objective:Warfarin is an FDA-approved oral anticoagulant for long-term prevention of thromboembolism. Substantial inter-individual variation in dosing requirements and the narrow therapeutic index of this widely-prescribed drug make safe initiation and dose stabilization challenging. Single nucleotide polymorphisms (SNPs) occurring in CYP2C9, VKORC1, and CYP4F2 genes are known to impact dose, and VKORC1 and CYP4F2 polymorphisms are associated with higher therapeutic dose requirements in our cohort. However, the most advanced regression models using personal, clinical, and genetic factors to predict individual stable dose account for only 50% to 60% of the observed variability in stable theapeutic dose in Caucasians. Design and Methods:In this study, we used DNA sequence analysis to determine whether additional variants in CYP4F2 and VKORC1 gene coding regions contribute to variable dosing requirements among individuals for whom the actual dose was the highest relative to regression modelpredicted dose. Results and Conclusions:No novel DNA variants in the coding regions of these genes were identified among subjects requiring high warfarin doses, suggesting that other factors yet to be defined contribute to variability in warfarin dose requirements in this subset of our cohort. Al though warfarin is the most commonly used drug for chronic prevention of thromboembolic events and stroke, it also ranks high among drugs that cause serious adverse events. 1,2 The narrow therapeutic index coupled with substantial inter-individual variability in warfarin dose requirement puts patients at increased risk for hemorrhagic events, and the variability in dose requirements has been attributed to inter-individual differences in medical, personal, and genetic factors. Some factors known to contribute to this variability include valve replacement, diabetes, body surface area (BSA), age, and DNA variants in three enzymes whose activity impacts vitamin K-dependent synthesis of coagulation factors: CYP2C9, VKORC1, and CYP4F2.3-7 Regression models that incorporate these factors have been developed to predict stable dose in order to improve clinical management (eg, time in thereapeutic range) and reduce thrombotic and hemorrhagic events. However, because these algorithms collectively explain only 50% to 60% of dose variability in Caucasians, 8,9 efforts are continuing to identify additional genetic and clinical factors that systematically impact warfarin dose requirements.

Effect of Genetic Variants, Especially CYP2C9 and VKORC1, on the Pharmacology of Warfarin

“…37,42,43 Depending on the study population, clinical factors may explain up to 20% of warfarin dose requirements. 44 However, the common, large interindividual dose requirements (e.g., in one study ranging from 7 to 280 mg per week 45 ) is not explained cumulatively by the majority of these uncommon though plausible factors with modest effect size. For instance, amiodarone can reduce warfarin dose requirement by approximately 18% 38 but is used by < 10% of warfarin users, and in a principal component analysis of 245 patients, of whom 21 took amiodarone, it was not found to be a statistically significant determinant.…”

VKORC1 Asp36Tyr geographic distribution and its impact on warfarin dose requirements in Egyptians

“…For example, it is relatively common in Ethiopians with minor allele frequency (MAF) of 15%, and Ashkenazi Jews (MAF 4%), less common in Israeli Jews (MAF 1.5%) and Arab Muslims in Israel (MAF 1%), and has a MAF of 0.5% in Sephardic, Yemenite, and North African Jews (10,(14)(15)(16)(17)(18). On the other hand, it was absent in over 700 non-Jewish Caucasian controls, 180 Israelis of Druze descent, 220 Han Chinese, 240 Southeast Indians and 213 South African individuals (17,(19)(20)(21)(22).…”

Identification and Functional Assessment of a New CYP2C9 Allelic Variant CYP2C9*59

“…This study was approved by the Institutional Ethical Committee of Beijing Hospital and informed written consent was obtained from the patient during blood sample collection. The genomic DNA extracted from the peripheral blood cells of the subject, using the TIANamp Blood DNA Midi Kit (TIANGEN, Beijing, China), was used for the polymerase chain reaction amplification of the promoters or exons of the CYP2C9, VKORC1, and CYP4F2 genes according to the methods reported previously (Yuan et al, 2005;Burmester et al, 2011;Dai et al, 2014). To avoid sequencing errors, bidirectional sequencing was performed for the putatively mutated sites.…”