Pharmacodynamic resistance to warfarin associated with a Val66Met substitution in vitamin K epoxide reductase complex...

The gene encoding vitamin K epoxide reductase complex subunit 1 (VKORC1), a component of the enzyme that is the therapeutic target site for warfarin, has recently been identified. In order to investigate the relationship betweenVKORC1 and warfarin dose response, we studied the VKORC1 gene (VKORC1) in patients with warfarin resistance. From a study group of 820 patients, we identified 4 individuals who required more than 25 mg of warfarin daily for therapeutic anticoagulation. Three of these had serum warfarin concentrations within the therapeutic range of 0.7-2.3 mg/l and showed wild-type VKORC1 sequence. The fourth warfarin resistant individual had consistently high (> or =5.7 mg/l) serum warfarin concentrations, yet had no clinically discernible cause for warfarin resistance. VKORC1 showed a heterozygous 196G-->A transition that predicted aVal66Met substitution in the VKORC1 polypeptide. This transition was also identified in 2 asymptomatic family members who had never received warfarin. These individuals had normal vitamin-K dependent coagulation factor activities and undetectable serum PIVKAII and vitamin K1 2,3 epoxide suggesting that their basal vitamin K epoxide reductase activity was not adversely affected by the VKORC1 Val66Met substitution. The association between a nucleotide transition in VKORC1 and pharmacodynamic warfarin resistance supports the hypothesis that VKORC1 is the site of action of warfarin and indicates thatVKORC1 sequence is an important determinant of the warfarin dose response.