Pharmacogenetics of Oral Anticoagulation - VKORC1-Haplotypes Determine the Inter-Individual and Inter-Ethnical Variability.

Purpuse: Coumarins are known for their high inter-individual and inter-ethnical variability of the dose-response association (i.e. significant higher maintenance dose in Africans and lower dose in Chinese). Recently, common SNPs of the VKORC1 gene (encoding the molecular target of coumarins, vitamin K epoxide reductase) were shown to be associated with lower warfarin dose requirement (c.-1639G>A [rs17878363], c.173+1000C>T [rs9934438]). In order to clarify whether these SNPs are part of more extended haplotypes, we established a comprehensive haplotype map of the entire VKORC1 gene locus. The association of VKORC1-haplotypes with coumarin dose requirement was analysed in different ethnical populations and patient cohorts.

Methods: In 200 German blood donors the VKORC1 gene locus was analysed by direct sequencing. VKORC1 genotype data in 7 additional populations from Africa, Asia and Europe were retrieved from internet databases. Haplotype frequencies were inferred using EHplus and FAMHAP. Unselected patients receiving phenprocoumon (n=61) and patient cohorts of European origin with either increased coumarin sensitivity (n=14, weekly phenprocoumon dose <6mg) or partial coumarin resistance (n=36, weekly phenprocoumon dose >42 mg or warfarin dose>70mg) were typed for VKORC1 haplotypes.

Results: In 200 controls we identified 28 SNPs within the VKORC1 gene. 6 SNPs formed 3 main haplotypes covering more than 99% of the genetic variability of VKORC1 (VKORC1*2: 42%, VKORC1*3: 38%, and VKORC1*4: 20%). SNPs associated with low warfarin dose (rs17878363, rs9934438) were in complete linkage disequilibrium with the VKORC1*2 haplotype. Haplotype frequency in Africans and Chinese differed significantly from the European sample (for VKORC1*2: Europeans 42%, Chinese 95%, Africans 14%). In 61 unselected patients receiving phenprocoumon c.-1639AA genotype patients (homozygous VKORC1*2) required less phenprocoumon (1.40 mg/d) than AG (2.12 mg/d) and GG patients (3.02 mg/d).13 of 14 patients (93%) with increased coumarin sensitivity but none of the patients with partial coumarin resistance were c.-1639AA. Vice versa, the c.-1639G allele (present in the non VKORC1*2 haplotypes) was found homozygous in 31 patients (86%) with partial coumarin resistance but in none of the patients with increased coumarin sensitivity.

Conclusions: Three main haplotypes represent more than 99% of VKORC1’s genetic variability in Europeans. VKORC1 haplotypes are strongly associated with the inter-individual and inter-ethnical variability of oral anticoagulation. In future studies, VKORC1-haplotype testing may provide a clinically relevant predictor of coumarin dosing and bleeding risk in oral anticoagulation.