Patient-specific hiPSC-derived cardiomyocytes indicate allelic and contractile imbalance as pathogenic factor in early-stage Hypertrophic Cardiomyopathy.

Hypertrophic Cardiomyopathy (HCM) is often caused by heterozygous mutations in β-myosin heavy chain (MYH7, β-MyHC). In addition to hyper- or hypocontractile effects of HCM-mutations, heterogeneity in contractile function (contractile imbalance) among individual cardiomyocytes was observed in end-stage HCM-myocardium. Contractile imbalance might be induced by burst-like transcription, leading to unequal fractions of mutant versus wildtype mRNA and protein in individual cardiomyocytes (allelic imbalance). Until now it is not known if allelic and contractile imbalance are present early in HCM-development or rather occur in response to disease-associated remodeling. To address this question, we used patient-specific human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) with heterozygous MYH7-mutations R723G and G741R as models of early-stage HCM without secondary adaptions upon disease progression. R723G-hiPSC-CMs showed typical HCM-markers like hypertrophy and myofibrillar disarray. Using RNA-FISH and allele-specific single-cell-PCR, we show for both cell lines that MYH7 is transcribed in bursts. Highly variable mutant vs. wildtype MYH7-mRNA fractions in individual HCM-hiPSC-CMs indicated allelic imbalance. HCM-hiPSC-CM-lines showed functional alterations like slowed twitch contraction kinetics and reduced calcium sensitivity of myofibrillar force generation. A significantly larger variability in force generation or twitch parameters of individual HCM-hiPSC-CMs compared to WT-hiPSC-CMs indicated contractile imbalance. Our results with early-stage hiPSC-CMs strongly suggest that burst-like transcription and allelic imbalance are general features of CMs, which together with mutation-induced changes of sarcomere contraction could induce contractile imbalance in heterozygous CMs, presumably aggravating development of HCM. Genetic or epigenetic approaches targeting functional heterogeneity in HCM could lead to promising future therapies, in addition to myosin modulation.