Functional Effects of the β-Myosin Mutation Arg453Cys in Familial Hypertrophic Cardiomyopathy

Abstract

In Familial Hypertrophic Cardiomyopathy (FHC) 1/3 of the patients are affected by mutations in the ß-myosin heavy chain (ß-MyHC), the myosin isoform of the ventricle and of slow skeletal muscle fibers in humans. Yet, not much is known about (i) direct effects of specific ß-MyHC-mutations on acto-myosin function, and (ii) how these mutations may trigger development of the FHC-phenotype. To address these questions, we analyzed the effects of the ß-MyHC-mutation R453C on contractile properties of slow (type I) fibers from the M. soleus of a severely affected FHC patient. We found an about 12% higher isometric force, 15% faster rate constant of force redevelopment (ktr), 10% higher isometric ATPase activity and essentially unchanged tension cost. Together with only slightly higher fiber stiffness in rigor, a main part of the increase in isometric force appears to be due to altered cross-bridge cycling kinetics, specifically an increase in fapp, the rate constant for the cross-bridge transition into force generating states. Currently we investigate whether increased force generated per myosin head e.g., by a larger y0-value also contributes. Relative quantification of mutated vs. wildtype ß-MyHC-mRNA of the heterozygous patient revealed a fraction of about 35% for the R453C-mRNA. Assuming similar abundance of mutated MyHC at the protein level, as we had found in other FHC-mutations, about a third of the ß-myosin heads in the sarcomeres carry the mutation. Thus, force contribution of the mutated myosin head population is about 40% increased. A larger variability in pCa50 among individual fibers with mutation R453C vs. control fibers, as we had seen for other FHC-mutations, suggests unequal expression of mutated myosin. We hypothesize that unequal expression of mutated myosin in individual cardiomyocytes causes imbalanced force generation and initiates functional impairment of the myocardium in FHC.