Hypertrophic cardiomyopathy: studies on hiPSC cardiomyocytes suggest that cell to cell allelic and contractile imbalance contribute to disease development from the beginning

Hypertrophic cardiomyopathy (HCM) often associated with heterozygous mutations in sarcomeric proteins. One of the consequences is altered contractile function of cardiomyocytes (CMs). We previously noted highly variable force generation among individual cardiomyocytes isolated from HCM-patient’s myocardium. We also observed stochastic, burst-like transcription of wildtype and mutant alleles in individual nuclei, and unequal ratios of mutant vs. wildtype mRNA from cell to cell. Our data altogether suggest that discontinuous transcription generates unequal fractions of mutated protein from cell to cell, resulting in unequal contraction of the CMs (contractile imbalance).

In this study, we asked whether variable allelic transcription and variable contractile function from cell to cell already exist from the beginning of the disease, in addition to effects of the mutation on sarcomere function, instead of alternatively resulting from disease progression in HCM-patients.

We used HCM-patient-derived induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) with MYH7-mutation R723G as model for an early, fetal-like cardiomyocyte stage of development. After 35 days of cultivation, R723G-HiPSC-CMs showed morphological changes like hypertrophy and myofibrillar disarray consistent with an HCM phenotype. RNA-fluorescence in situ hybridization and allele-specific single cell RT-PCR revealed burst-like transcription and unequal expression of mutant and wildtype mRNA from cell-to-cell (allelic imbalance), similar to previous data from R723G-HCM-patient’s cardiac tissue. Calcium sensitivity of R723G-hiPSC-CM-derived myofibrils was reduced, and relaxation kinetics were slowed down compared to WT-hiPSC-myofibrils. Twitch relaxation of vital R723G-hiPSC-CMs compared to WT-hiPSC-CMs was also slowed down, and frequently arrhythmic events were recorded in R723G-CMs. Interestingly, variability of twitch kinetics and of calcium sensitivity from cell to cell was substantially increased among R723G- compared to WT-hiPSC-CMs, suggesting variable contractile function from cell to cell also in this cellular model. Similar observations were also made with patient-derived hiPSC-CMs carrying MYH7-mutation G741R.

Transcriptional and functional analyses on fetal-like hiPSC-cardiomyocytes derived from HCM-patients suggest that variable allelic expression and variable contractile function among cardiomyocytes are present from the beginning in patients with HCM. Thus, these factors might contribute to the development of HCM hallmarks, in addition to the heterozygous mutation.