The R249Q and R146N Hypertrophic Cardiomyopathy Myosin Mutations Decrease Muscle Force and Power Generation

Abstract

Hypertrophic cardiomyopathy (HCM) is an inheritable disease in humans that causes the heart's ventricular walls to thicken. There are over 200 identified mutations that cause HCM, and the generally proposed hypothesis is that HCM mutations increase heart muscle contractility. To test this hypothesis, we expressed two HCM myosin mutations, R249Q and R146N, in Drosophila and performed skinned indirect flight muscle (IFM) fiber mechanics to measure power, force and muscle kinetics. The R146N and R249Q mutant fibers significantly (p<0.05) decreased power generation by 45% and 67%, respectively, from control fibers. The decrease in power by R146N resulted from a decrease in force generation because isometric tension and work production decreased, 47% and 42%, respectively, proportional to power reduction. R249Q fibers produced less power because both force and muscle speed decreased. Isometric tension decreased by 37% and the frequency at which maximum power generation occurred was 20% lower. Muscle mechanics were performed on mutant and control fibers from two-hour-old Drosophila, prior to degradation of myofilament structure, which started at two-days-old and increased with age. Electron microscopy showed that degradation was more pronounced in the R146N line. Decreased power output from the fibers caused a 60% decrease in two-day-old Drosophila flight ability for both lines. Molecular modeling suggests R249Q could be interfering with communication between the nucleotide and thin filament binding sites, while R146N may alter the N-terminal domain's interactions with the lever arm. Overall, our results contradict the increased contractility HCM hypothesis and instead suggest ventricular hypertrophy is a compensatory response to a decrease in heart muscle power generating ability. Supported by NIH grants R01 AR055611 to D.M.S. and R01 GM322433 to S.I.B.