Hypertrophic Cardiomyopathy: Variable Expression of Myosin-Binding Protein C from Cell-To-Cell and Functional Imbalance among Individual Cardiomyocytes

Hypertrophic cardiomyopathy (HCM) has been related to mutations in sarcomeric proteins. Mainly affected are β-myosin heavy chain (β-MyHC) and myosin binding protein C (cMyBP-C). A common mechanism how different mutations trigger HCM development is not clear. We asked whether for an HCM-patient with cMyBP-C-mutation c.927-2A>G, which creates a premature stop-codon (i) functional imbalance among individual cardiomyocytes exists, and (ii) expression of cMyBP-C from cell-to-cell is variable, as seen in our previous work on HCM-related β-MyHC-mutations. To test this we treated individual cardiomyocytes with the heterozygous MyBP-C-mutation and donor cardiomyocytes with PP1 and PKA, and recorded force-pCa relations. This was followed by immunofluorescent double labelling of cMyBP-C and α-actinin. Maximum isometric force of patient cardiomyocytes was reduced by 25% and Ca++-sensitivity was increased compared to controls. Interestingly, a large cell-to-cell variation of force at submaximal activation levels was found, with some cells being essentially indistinguishable from controls, while others generated substantially increased force suggesting reduced levels of cMyBP-C. Western blot revealed no truncated protein in the tissue. Fluorescence intensity ratio (IMyBPC/IAlpha-Actinin) was reduced by 35% for patient's cardiomyocytes and significantly more variable from cell-to-cell compared to donors. The appearance of cMyBP-C was patchier in some patient cardiomyocytes, as already seen before in histology of patients’ cardiac tissue. This suggests variable abundance of wildtype-cMyBP-C in patient cardiomyocytes. Since no truncated protein was found, presumably the mutation causes mRNA-decay and only the wildtype-allele produces functional protein. If, as we propose, transcription of cMyBP-C occurs in a random, burst-like fashion independent for both alleles, cMyBPC-protein could be highly variable from cell-to-cell, causing functional variability as seen here. In the long run, such functional imbalance from cell-to-cell would result in structural distortions like cellular/myofibrillar disarray, a hallmark of HCM.