Enhanced myofilament calcium sensitivity aggravates abnormal calcium handling and diastolic dysfunction in patient-specific induced pluripotent stem cell-derived cardiomyocytes with MYH7 mutation

Hypertrophic cardiomyopathy (HCM), the most common inherited heart disease, is frequently caused by mutations in the β-cardiac myosin heavy chain gene (MYH7). Abnormal calcium handling and diastolic dysfunction are archetypical features of HCM caused by MYH7 gene mutations. However, the mechanism of how MYH7 mutations leads to these features remains unclear, which inhibits the development of effective therapies. Initially, cardiomyocytes were generated from induced pluripotent stem cells from an eight-year-old girl diagnosed with HCM carrying a MYH7(C.1063 G>A) heterozygous mutation(mutant-iPSC-CMs) and mutation-corrected isogenic iPSCs(control-iPSC-CMs) in the present study. Next, we compared phenotype of mutant-iPSC-CMs to that of control-iPSC-CMs, by assessing their morphology, hypertrophy-related genes expression, calcium handling, diastolic function and myofilament calcium sensitivity at days 15 and 40 respectively. Finally, to better understand increased myofilament Ca²⁺ sensitivity as a central mechanism of central pathogenicity in HCM, inhibition of calcium sensitivity with mavacamten can improveed cardiomyocyte hypertrophy. Mutant-iPSC-CMs exhibited enlarged areas, increased sarcomere disarray, enhanced expression of hypertrophy-related genes proteins, abnormal calcium handling, diastolic dysfunction and increased myofilament calcium sensitivity at day 40, but only significant increase in calcium sensitivity and mild diastolic dysfunction at day 15. Increased calcium sensitivity by levosimendan aggravates cardiomyocyte hypertrophy phenotypes such as expression of hypertrophy-related genes, abnormal calcium handling and diastolic dysfunction, while inhibition of calcium sensitivity significantly improves cardiomyocyte hypertrophy phenotypes in mutant-iPSC-CMs, suggesting increased myofilament calcium sensitivity is the primary mechanisms for MYH7 mutations pathogenesis. Our studies have uncovered a pathogenic mechanism of HCM caused by MYH7 gene mutations through which enhanced myofilament calcium sensitivity aggravates abnormal calcium handling and diastolic dysfunction. Correction of the myofilament calcium sensitivity was found to be an effective method for treating the development of HCM phenotype in vitro.