Question: 

The major cause of familial hypertrophic cardiomyopathy (fHCM) are mutations in sarcomeric proteins. 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).

Methods: 

We measured active and passive force, force-calcium relations, and cross-bridge kinetics in cardiomyocytes of the left ventricle (LV) from explanted hearts of two patients with the ß-myosin mutation R723G. In addition, we determined the ratio of mutated vs. wildtype ß-MHC-mRNA and the status of phosphorylation of several sarcomeric proteins.

Results: 

Our measurements revealed reduced maximum force generation of myocytes with mutation 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 were found to contain the mutation, just as seen in M. soleus. Secondly, similar to previous findings for failing heart, gel electrophoresis of the fHCM cardiac tissue showed reduced phosphorylation of troponins I and T, myosin binding protein C, and myosin light chain 2 compared to donor tissue. Adjustment of phosphorylation of TnI and MyBPC 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.

Conclusion: 

(1) Typical adaptations commonly seen in end stage heart failure like increased calcium-sensitivity due to changes in protein phosphorylation may be obscured by the primary effects of fHCM related mutations. (2) To identify primary functional effects of a mutation in myocardial tissue, posttranslational modifications like protein phosphorylation and morphological changes must be taken into account.