| Factor V serves as the nonenzymatic cofactor for Factor Xa to produce thrombin. A Factor V variant called Factor V Leiden (FVL) is the most common known inherited risk factor for venous thrombosis, though incomplete penetrance is observed in people inheriting FVL. FVL is resistant to degradation by Activated Protein C (APC), resulting in increased thrombin. In vitro studies suggest that wildtype FV can also serve as a cofactor for APC in the inactivation of Factor VIIIa. We used mouse models of FV and FVL to demonstrate the in vivo relevance of the wildtype FV APC cofactor activity on thrombosis. Additionally, we used a synthetic lethal genetic interaction between FVL homozygosity (FVQ/Q) and heterozygous tissue factor pathway inhibitor deficiency (TFPI+/- ) in mice as a phenotype in a sensitized ENU mutagenesis screen to identify dominant modifier genes contributing to FVL penetrance. As conceptual proof, it was demonstrated that: [1] loss of one tissue factor allele; or [2] a suite of dominantly acting genes in the DBA mouse strain suppress the lethal phenotype. In the mutagenesis screen, analysis of 6677 G1 offspring identified 92 FVQ/Q TFPI+/- mice surviving to weaning, with 13 exhibiting successful transmission of a putative suppressor mutation. These findings have important implications for the genetic regulation of hemostatic balance.;While characterizing gene-targeted mice deficient in SerpinB2 , we observed mice which were small at birth (sml). Sml was initially attributed to SerpinB2 deficiency. However, analysis of SerpinB2 deficient mice derived from two additional, independent ES cell clones revealed no growth abnormalities. Using recombination observed between sml and the SerpinB2 locus, sml was mapped to a 2.78 Mb interval ∼12 cM (30 Mb) proximal to SerpinB2. Sequencing of candidate genes revealed a nonsense mutation in insulin receptor substrate 1, Irs1. Analysis of ES cell DNA demonstrated that the Irs1 mutation arose spontaneously in the original cell culture used for gene targeting. This study suggests that new mutations arising during ES cell culture and linked to the targeted allele could result in incorrect assignment of phenotype and may account for a subset of discordant results for experiments independently targeting the same gene. |
