Mavacamten rescues increased myofilament calcium sensitivity and dysregulation of Ca2+ flux caused by thin filament hypertrophic cardiomyopathy mutations.

Thin filament hypertrophic cardiomyopathy (HCM) mutations increase myofilament Ca2+- sensitivity and alter Ca2+ handling and buffering. The myosin inhibitor mavacamten reverses the increased contractility caused by HCM thick filament mutations, and we here test its effect on HCM thin filament mutations where the mode of action is not known. Mavacamten (250 nM) partially reversed the increased Ca2+ sensitivity caused by HCM mutations cTnT R92Q and cTnI R145G in in vitro ATPase assays. The effect of mavacamten was also analysed in cardiomyocyte models of cTnT R92Q and cTnI R145G containing cytoplasmic and myofilament specific Ca2+ sensors. While mavacamten rescued the hypercontracted basal sarcomere length, the reduced fractional shortening did not improve with mavacamten. Both mutations caused an increase in peak systolic Ca2+ detected at the myofilament, and this was completely rescued by 250 nM mavacamten. Systolic Ca2+ detected by the cytoplasmic sensor was also reduced by mavacamten treatment although only R145G increased cytoplasmic Ca2+. There was also a reversal of Ca2+ decay time prolongation caused by both mutations at the myofilament but not in the cytoplasm. We thus show that mavacamten reverses some of the Ca2+-sensitive molecular and cellular changes caused by the HCM mutations, particularly altered Ca2+ flux at the myofilament. The reduction of peak systolic Ca2+ as a consequence of mavacamten treatment represents a novel mechanism by which the compound is able to reduce contractility, working synergistically with its direct effect on the myosin motor.