A dominant autosomal warfarin-resistance gene was determined on chromosome 7 in house mice. Three VKORC1 sequence variants mediating resistance to anticoagulants seem to be widely distributed: Tyr139Cys (Y139C), Leu128Ser (L128S) and a group of linked sequence variants Arg12Trp/Ala26Ser/Ala48Thr/Arg61Leu (spretus introgression).
House mice carrying the homozygous Y139C sequence variant were found to be highly resistant to warfarin and bromadiolone.
Research in the UK with a strain of house mice carrying the homozygous Leu128Ser sequence variant showed such mice to be resistant to warfarin, and presumably other first-generation anticoagulants. Some individuals also survived choice and no-choice trials with bromadiolone and difenacoum. It seems that the mutation enables some of the mice to stabilize their vitamin K metabolism even when consuming anticoagulants of high potency over prolonged periods.
The third VKORC1 sequence variant (Arg12Trp/Ala26Ser/Ala48Thr/Arg61Leu) has probably been transferred from Mus spretus, a species found in the Iberian peninsula and north Africa, to Mus musculus by inter-specific hybridisation and then quickly spread over long distances, presumably by freight transportation of individuals. Although the phenotypic effect must still be verified, it is known to be associated with a substantial loss of anticoagulant efficacy against first-generation anticoagulants (e.g. warfarin, coumatetralyl), as well as the second-generation compounds bromadiolone and most probably difenacoum.
Studies revealed that sex and modifier genes, which influence the expression of the resistance gene, affect the penetrance of the genotype in house mice. Variations in metabolism and clearance are also known to influence the efficacy of anticoagulants, probably associated with detoxification by the enzyme complex cytochrome P450.