A Sensitized ENU Mutagenesis Screen for Thrombosis Modifiers Identifies Suppressor Variants in Non-mutagenized Parental Generations Due to Antithrombotic Selective Pressures

Thrombosis is a leading cause of morbidity and mortality. We used a mouse forward genetic ENU screen to identify genomic variants that suppress F5L/L Tfpi+/- lethal thrombosis. Surviving F5L/L Tfpi+/- mice from our Modifier of Factor 5 Leiden 16 (MF5L16) ENU line were subjected to whole-genome sequencing analysis. This revealed that instead of an ENU-induced mutation, four mutations introduced from our F5L/L breeding stock were responsible for survival, which we named sMF5L1-4 for spontaneous Modifier of Factor 5 Leiden. In our colony, F5L/L female breeders carrying all four sMF5L mutations produced more litters and offspring than breeders with three or fewer mutations (p<0.006). Genotyping of 13 additional MF5L lines demonstrated that the four sMF5L mutations were present in all lines and were consistently associated with survival. Of these four mutations, a single G to A intergenic variant on Chromosome 18 (Chr18A, sMF5L4), was most significantly associated with survival (p=0.003), with [~]15% penetrance for conferring the survival phenotype. Furthermore, platelet aggregation was significantly reduced in Chr18A mice, suggesting an additional mechanism by which Chr18A could suppress lethal thrombosis. Comparative transcriptomics analysis of livers from Chr18A mice versus wildtype littermate controls revealed a small number of differentially expressed genes both known and unknown to affect thrombosis. In summary, we have identified four variants exerting a significant selective breeding advantage along with antithrombotic effects. Superimposing our mutagenesis screen on a selective background illustrates the interplay of natural strain background variants and de novo ENU mutations in suppressing F5L/L Tfpi+/- lethal thrombosis.