Abstract / Project Summary From a spontaneous founder mutant, we have established several lines of mice that naturally develop heart disease (HD), which rapidly progresses to heart failure and premature death. Characterization of affected mutants revealed eosinophilia in multiple organs, including the heart, lungs, and spleen. Affected mice develop eosinophilic myocarditis (EM) that leads to extensive fibrosis and right ventricular dilated cardiomyopathy, with most mice dying by ~15-weeks. Clinically, many of the pathologies in mutant mice coincide with those realized in patients diagnosed with hypereosinophilic syndrome (HES) or eosinophilic granulomatosis and polyangiitis (EGPA, aka Churg-Strauss Syndrome), two rare eosinophil-associated diseases with a worse prognosis when accompanied by HD. We have maintained the EM/HD trait on an inbred A/J strain background (i.e., A/JHD) for many generations, so we suspected that heritability of the mutant trait is controlled by a small set of genes. The EM/HD disease could also be transferred to irradiated A/J mice by adoptive transfer of splenocytes from affected mice, implicating a role for the immune system. Because little is known regarding the genes involved in most eosinophilic diseases, we sought to map EM/HD in this mutant. For linkage studies, we systematically backcrossed and intercrossed proven A/JHD mutant males (i.e. previously sired affected offspring) with 4 inbred strains and identified SJL/J (SJ) mice as the best strain–cross combination to reproduce EM/HD for mapping. Using QTL analysis of SJ-derived F2 and N2 populations, HD was mapped to 3 highly significant loci (named Emhd1-3 for ‘eosinophilic myocarditis to heart disease’). Linkage regions included a recessive variant on Chr5 (Emhd1) that is necessary but not sufficient for disease, and 2 separate dominant loci on Chr17 (Emhd2 and Emhd3). Recently, we found that using a 3-generation N2/F2 mating scheme with A/JHD and SJ breeders could further improve the rate of producing recombinants that developed HD. With this experience and knowledge gained from reconstituting EM/HD in several inbred strains, along with a more efficient N2/F2 breeding strategy, we now propose to recapitulate the oligogenic trait in select CC strains. Our working hypothesis is that the genetic diversity of CC lines will help refine linkage intervals, identify any additional QTLs and determine the importance of heterozygous regions (i.e., do they house a dominant variant?) to the trait. The primary goals of this grant are to validate QTLs and better understand the overall heritability. Specifically, we will reestablish EM/HD in recombinants generated from select CC lines that carry the desired strain alleles at critical sites of interest. All N2/F2 CC-recombinants will be phenotyped for EM/HD and an equal number of affected and unaffected CC-recombinants genotyped, and QTL and haplotype analyses performed. We expect the CC lines will instill the genetic diversity ...