Beta-Adrenergic Stimulation and MYH7 G256E Mutant Gene Dosage Drive Hypertrophic Cardiomyopathy Phenotype Penetrance

Hypertrophic cardiomyopathy (HCM) is the most prevalent genetic heart disorder, characterized by significant phenotypic variability even among individuals with identical MYH7 mutations. This study aims to elucidate factors contributing to this variability and identify drivers of phenotype penetrance. We compared the baseline phenotypes of a highly penetrant MYH7 H251N mutation and the variably penetrant MYH7 G256E mutation and investigated the impact of adding beta-adrenergic stimulation and homozygosity on disease phenotype penetrance using cardiomyocytes from an isogenic line of human induced pluripotent stem cells (hiPSC-CMs).

Isogenic hiPSCs with MYH7 H251N and MYH7 G256E mutations were generated using CRISPR/Cas9 technology and differentiated into cardiomyocytes (CMs). Single-cell RNA sequencing (scRNAseq) and functional analysis of contractile function revealed consistent HCM phenotype presentation in H251N CMs, whereas G256E CMs exhibited a subtle and more variable phenotype. Beta-adrenergic stimulation induced a distinct metabolic stress response in G256E CMs, characterized by impaired mitochondrial ATP upregulation. Increasing mutant gene dosage from hetero- to homozygosity led to consistent increase in hypertrophic and structural gene expression changes in G256E CMs at RNA and protein levels. These changes were distinct from the changes observed with stress response. Importantly, homozygous G256E CMs exhibited a hypercontractile functional and disorganized structural phenotype. Across multiple experimental conditions, we identified consistent increase in cardiomyocyte specific transcriptomic markers such as NPPB, APOE, PDLIM3 and ANKRD1.

Our study highlights the use of a variably penetrant MYH7 mutation to investigate factors that influence HCM phenotype penetrance. Specifically, we found that mutant gene dosage and beta-adrenergic stimulation induce distinct HCM disease phenotypes, providing novel insights into mechanisms that may contribute to variable disease expression in HCM.