Cardiac Myosin-Binding Protein C Mutations and Hypertrophic Cardiomyopathy

Background— Mutations in the MYBPC3 gene, encoding cardiac myosin-binding protein C (cMyBP-C), are a frequent cause of familial hypertrophic cardiomyopathy. In the present study, we investigated whether protein composition and function of the sarcomere are altered in a homogeneous familial hypertrophic cardiomyopathy patient group with frameshift mutations in MYBPC3 (MYBPC3 mut). Methods and Results— Comparisons were made between cardiac samples from MYBPC3 mutant carriers (c. 2373dupG, n=7; c. 2864₂865delCT, n=4) and nonfailing donors (n=13). Western blots with the use of antibodies directed against cMyBP-C did not reveal truncated cMyBP-C in MYBPC3 mut. Protein expression of cMyBP-C was significantly reduced in MYBPC3 mut by 33±5%. Cardiac MyBP-C phosphorylation in MYBPC3 mut samples was similar to the values in donor samples, whereas the phosphorylation status of cardiac troponin I was reduced by 84±5%, indicating divergent phosphorylation of the 2 main contractile target proteins of the β-adrenergic pathway. Force measurements in mechanically isolated Triton-permeabilized cardiomyocytes demonstrated a decrease in maximal force per cross-sectional area of the myocytes in MYBPC3 mut (20. 2±2. 7 kN/m 2) compared with donor (34. 5±1. 1 kN/m 2). Moreover, Ca 2+ sensitivity was higher in MYBPC3 mut (pCa 50 =5. 62±0. 04) than in donor (pCa 50 =5. 54±0. 02), consistent with reduced cardiac troponin I phosphorylation. Treatment with exogenous protein kinase A, to mimic β-adrenergic stimulation, did not correct reduced maximal force but abolished the initial difference in Ca 2+ sensitivity between MYBPC3 mut (pCa 50 =5. 46±0. 03) and donor (pCa 50 =5. 48±0. 02). Conclusions— Frameshift MYBPC3 mutations cause haploinsufficiency, deranged phosphorylation of contractile proteins, and reduced maximal force-generating capacity of cardiomyocytes. The enhanced Ca 2+ sensitivity in MYBPC3 mut is due to hypophosphorylation of troponin I secondary to mutation-induced dysfunction.