A polymorphism in the VKORC1 gene is associated with an interindividual variability in the dose-anticoagulant effect of warfarin

Effect of VKORC1 haplotypes on transcriptional regulation and warfarin dose

BACKGROUND

The management of warfarin therapy is complicated by a wide variation among patients in drug response. Variants in the gene encoding vitamin K epoxide reductase complex 1 (VKORC1) may affect the response to warfarin.

METHODS

We conducted a retrospective study of European-American patients receiving long-term warfarin maintenance therapy. Multiple linear-regression analysis was used to determine the effect of VKORC1 haplotypes on the warfarin dose. We determined VKORC1 haplotype frequencies in African-American, European-American, and Asian-American populations and VKORC1 messenger RNA (mRNA) expression in human liver samples.

RESULTS

We identified 10 common noncoding VKORC1 single-nucleotide polymorphisms and inferred five major haplotypes. We identified a low-dose haplotype group (A) and a high-dose haplotype group (B). The mean (+/-SE) maintenance dose of warfarin differed significantly among the three haplotype group combinations, at 2.7+/-0.2 mg per day for A/A, 4.9+/-0.2 mg per day for A/B, and 6.2+/-0.3 mg per day for B/B (P<0.001). VKORC1 haplotype groups A and B explained approximately 25 percent of the variance in dose. Asian Americans had a higher proportion of group A haplotypes and African Americans a higher proportion of group B haplotypes. VKORC1 mRNA levels varied according to the haplotype combination.

CONCLUSIONS

VKORC1 haplotypes can be used to stratify patients into low-, intermediate-, and high-dose warfarin groups and may explain differences in dose requirements among patients of different ancestries. The molecular mechanism of this warfarin dose response appears to be regulated at the transcriptional level.

Warfarin: a case history in pharmacogenetics

Both hereditary and environmental factors contribute to inter-individual variability in drug response. The considerable interest in the role of genes has to be balanced with the contribution of external influences. Warfarin provides a useful case study of the need to integrate both genetic and non-genetic information when selecting the right dose for a patient. This article discusses the latest data on genotype and warfarin sensitivity and the efforts to incorporate this information into normograms. Exploring the genetics of warfarin response will lead not only to safer prescribing but elucidate the mechanism of action of warfarin and enable the development of new anticoagulant drugs.

A novel functional VKORC1 promoter polymorphism is associated with inter-individual and inter-ethnic differences in warfarin sensitivity

Warfarin, a commonly prescribed anticoagulant, exhibited large inter-individual and inter-ethnic differences in the dose required for its anticoagulation effect. Asian populations, including Chinese, require a much lower maintenance dose than Caucasians, for which the mechanisms still remain unknown. We determined DNA sequence variants in CYP2C9 and VKORC1 in 16 Chinese patients having warfarin sensitivity (< or = 1.5 mg/day, n = 11) or resistance (> or = 6.0 mg/day, n = 5), 104 randomly selected Chinese patients receiving warfarin, 95 normal Chinese controls and 92 normal Caucasians. We identified three CYP2C9 variants, CYP2C9*3, T299A and P382L, in four warfarin-sensitive patients. A novel VKORC1 promoter polymorphism (-1639 G > A) presented in the homozygous form (genotype AA) was found in all warfarin-sensitive patients. The resistant patients were either AG or GG. Among the 104 randomly selected Chinese patients receiving warfarin, AA genotype also had lower dose than the AG/GG genotype (P < 0.0001). Frequencies of AA, AG and GG genotypes were comparable in Chinese patients receiving warfarin (79.7, 17.6 and 2.7%) and normal Chinese controls (82, 18 and 0%), but differed significantly from Caucasians (14, 47 and 39%) (P < 0.0001). The promoter polymorphism abolished the E-box consensus sequences and dual luciferase assay revealed that VOKRC1 promoter with the G allele had a 44% increase of activity when compared with the A allele. The differences in allele frequencies of A/G allele and its levels of VKORC1 promoter activity may underscore the inter-individual differences in warfarin dosage as well as inter-ethnic differences between Chinese and Caucasians.

Pharmacogenetics of oral anticoagulants

Understanding the basis for the observed wide interindividual variation in the dose requirement for the oral anticoagulants should facilitate the safer use of these widely prescribed drugs. Factors that include patient age, body weight and concomitant medications are well known to affect dose requirement and in a large number of recent pharmacogenetic studies, it has been demonstrated that the common variant CYP2C9 alleles are also good predictors of dose requirement, although they fail to account for all the observed interindividual variability. Since pharmacokinetic factors seem to be major contributors to the variability, the possibility that additional CYP2C9 polymorphisms or other polymorphisms affecting anticoagulant disposition could also predict dose is discussed. In addition, the relevance with regard to dose requirement of polymorphism in the recently discovered vitamin K epoxide reductase gene, which encodes the target gene for oral anticoagulants, is considered.