Uncovering Effects of the Familial Hypertrophic Cardiomyopathy (fHCM) Related ß-Myosin Mutation Arg723gly in Human Cardiomyocytes

Abstract

Mutations in sarcomeric proteins have been identified as the major cause of fHCM. The primary functional effects of these mutations at the sarcomere level of cardiomyocytes, however, are still largely unknown. To address this point we studied cardiomyocytes of the left ventricle (LV) from explanted hearts of two patients with ß-myosin (ß-MHC) mutation R723G (Enjuto et al., JMCC 2000). Our measurements revealed reduced maximum force generation of myocytes but unchanged calcium-sensitivity. In contrast, previous studies on slow skeletal muscle fibers expressing ß-MHC with the same mutation showed reduced calcium-sensitivity and increased maximum force. To address this discrepancy we first determined the expression of mutated and wildtype ß-MHC-mRNA in LV tissue. 62±2% of total ß-MHC-mRNA was found to contain the mutation, just as seen in M. soleus. Secondly, similar to previous findings for failing human heart, gel electrophoresis of the fHCM cardiac tissue showed reduced phosphorylation of troponins I (TnI) and T, myosin binding protein C (MyBP-C), and myosin light chain 2 compared to donor tissue. Adjustment of phosphorylation of TnI and MyBP-C in donor and HCM myocytes by treatment with protein kinase A (PKA), however, uncovered an originally masked reduction in calcium-sensitivity while maximum force was not affected by PKA treatment. Electron microscopy showed reduced myofibrillar density in cardiac tissue samples of the patients which may account for the reduced force. In conclusion, our study reveals that secondary, adaptational processes, triggered by the fHCM-related mutation, can obscure primary effects.