Analysis of cardiac function and morphology in genetic reference population of BXD strains reveals associated eQTLs and candidate causal and modifier genes
Authors: Orgil Buyan‐Ochir , Xu Fuyi , Munkhsaikhan Undral , Li Ning , Alberson Neely R. , Johnson Jason , Towbin Jeffrey A. , Lu Lu , Purevjav Enkhsaikhan 
Published: May 2022

IntroductionThe murine recombinant inbred (RI) family of BXD strains derived from crosses between C57BL/6J (B6) and DBA/2J (D2) mice are the largest genetic reference population (GRP). The parental D2 mouse is a natural model of hypertrophic cardiomyopathy (HCM). Thus, the BXD family represents an applicable rodent model for phenotype‐genotype studies of complex cardiomyopathy traits.HypothesisGenetic background affects an expression of cardiomyopathy traits.MethodsEchocardiography, electrocardiography (ECG) and blood pressure measurements were performed in 44 BXD strains (N>5/sex) at 4‐5 months of age. Cardiac traits (heart function, mass, rhythm, and arrhythmias) and blood pressure values were then associated with heart transcriptome, and expression quantitative trait loci (eQTL) mapping was performed.ResultsMore than 2‐fold variance in ejection fraction (EF%), fractional shortening (FS%), left ventricular (LV) volumes, internal dimensions (ID) and mass, as well as in thickness of LV posterior wall (LVPW), and interventricular septum (IVS) was found by echocardiography among BXDs. Varied abnormal heart rhythms including bradycardia, atrioventricular block, premature atrial or ventricular complexes, ventricular tachycardia and sick sinus syndrome were recorded among BXD strains using ECG. Parental D2 strains had significantly prolonged QT interval (62.82±9.42 ms and 53.42±1.73 ms, respectively) compared to B6 parental strains. A significant variability in systolic and diastolic BP between BXD strains was also recorded.Among male BXDs, a significant QTL of 91.0 to 97.4 Mb (LRS=18.50) was identified on chromosome (Chr) 8 that is associated with LV volumes and IDs during systole and diastole. eQTL mapping for each of the 131 genes at the QTL interval of Chr 8 identified 6 genes (Coq9, Ndrg4, Crnde, Irx3, Rpgrip1l, and Rbl2) being cis‐regulated. Among those genes, Ndrg4was found to be positively correlated with LV volumes at systole and diastole and ID traits, and negatively correlated with the cardiomyocyte size (r = ‐0.588, p = 0.025). In female BXDs, a significant QTL with LRS=20.1 was found on Chr 7 of 40.2 Mb that is associated with LVPW at diastole, where Josd2is a strong candidate gene that is cis‐regulated and significantly correlated with LVPW trait.ConclusionsOur study found variable traits of echocardiographic, electrocardiographic and blood pressure parameters are segregated among the BXD family. The cardiovascular traits significantly vary between BXD lines suggesting an influence of genetic background on expression of those traits. Traits such as reduced EF%, FS%, and LVPW and increased LV volumes are seen in patients with dilated cardiomyopathy (DCM). Increased LV mass, LV thickness, LVPW, and IVS mimic traits seen in patients with HCM. Varied abnormal heart rhythms detected in BXD strains simulate cardiac conduction abnormalities in humans. Multiple associated eQTLs defined in BXDs demonstrate that the BXD family of RI strains is a suitable model to map gene variants and identify causal genes and genetic modifiers that influence cardiovascular phenotypes including cardiomyopathies.