Molecular Insights Into The Phenotypic Heterogeneity Observed In Two Different Murine Models Of Hypertrophic Cardiomyopathy (HCM)

Background: Phenotypic variability in HCM has long been appreciated to correlate with genotype. However, the molecular drivers underpinning these differences remain poorly understood. We hypothesized that the variable HCM phenotypes observed in 2 different widely used HCM mouse models (R403Q-MyHC, R92W-TnT) result from complex interactions between a variety of cardiac cell types. We performed unbiased assessments of gene expression and chromatin accessibility in mutant and littermate control hearts at an early disease stage, when MyHC-mutants have higher LV-mass and normal LV-function, while TnT-mutants have similar LV-mass, hypercontractility and diastolic dysfunction, compared to littermate-controls. Methods: Nuclei were isolated from hearts of 5-week-old mutant and control animals and subjected to the standard linked ATAC and gene expression profiling pipeline from 10X Genomics w/o enrichment/sorting. Clustering, differential gene expression and transcription factor (TF) motif enrichment was performed using Seurat, ArchR; SCENIC was used to predict TF (Regulon)-activation from gene expression data and Cellchat for cellular crosstalk. Results: Integrated single-nuclei data-based clustering identified all cardiac cell types. Both mutant ventricular(V)-myocytes had similar dysregulation (80-90% agreement) of gene expression and TF enrichment, with matching of 23/30 top TF-motifs, and agreement with gene expression-based TF-prediction. Both mutant V-myocytes had activation of SWI/SNF complex TFs (Jun, Fos, Smarc family), however activation of SREBF-1/2, Atf5, Creb3l2, Egr2 (drivers of fibroblast activation), and Klf2, Klf4, Bcl6b (drivers of shear stress-mediated gene expression in endothelial cells (ECs)) was only present in TnT-mutants. Activation of VCAM signaling from V-myocytes-to-ECs, and PDGF signaling to VSMCs is predicted in both mutants, but TGF-β signaling to fibroblasts, endothelin-1 signaling to V-myocytes, was only predicted in TnT-mutants. Conclusions: Mutant sarcomeric proteins activate the SWI/SNF complex that drives V-myocyte hypertrophy at early disease stage. Mutation-specific activation of TF-programs in non-myocyte cells are important drivers of cardiac phenotypic variability in HCM.