Perturbed Length-Dependent Activation in Human Hypertrophic Cardiomyopathy With Missense Sarcomeric Gene Mutations

Rationale: High-myofilament Ca2+ sensitivity has been proposed as a trigger of disease pathogenesis in familial hypertrophic cardiomyopathy (HCM) on the basis of in vitro and transgenic mice studies. However, myofilament Ca2+ sensitivity depends on protein phosphorylation and muscle length, and at present, data in humans are scarce. Objective: To investigate whether high myofilament Ca2+ sensitivity and perturbed length-dependent activation are characteristics for human HCM with mutations in thick and thin filament proteins. Methods and Results: Cardiac samples from patients with HCM harboring mutations in genes encoding thick (MYH7, MYBPC3) and thin (TNNT2, TNNI3, TPM1) filament proteins were compared with sarcomere mutation-negative HCM and nonfailing donors. Cardiomyocyte force measurements showed higher myofilament Ca2+ sensitivity in all HCM samples and low phosphorylation of protein kinase A (PKA) targets compared with donors. After exogenous PKA treatment, myofilament Ca2+ sensitivity was similar (MYBPC3(mut), TPM1(mut), sarcomere mutation-negative HCM), higher (MYH7(mut), TNNT2(mut)), or even significantly lower (TNNI3(mut)) compared with donors. Length-dependent activation was significantly smaller in all HCM than in donor samples. PKA treatment increased phosphorylation of PKA-targets in HCM myocardium and normalized length-dependent activation to donor values in sarcomere mutation-negative HCM and HCM with truncating MYBPC3 mutations but not in HCM with missense mutations. Replacement of mutant by wild-type troponin in TNNT2(mut) and TNNI3(mut) corrected length-dependent activation to donor values. Conclusions: High-myofilament Ca2+ sensitivity is a common characteristic of human HCM and partly reflects hypophosphorylation of PKA targets compared with donors. Length-dependent sarcomere activation is perturbed by missense mutations, possibly via posttranslational modifications other than PKA hypophosphorylation or altered protein-protein interactions, and represents a common pathomechanism in HCM.