Force, Ca-Sensitivity and Contractile Efficiency in Human Myocardium Expressing a Truncated Cardiac Myosin Binding Protein-C

We have investigated contractile parameters of ventricular myocardium in samples from a patient diagnosed with hypertrophic cardiomyopathy (HCM) caused by a truncation mutation in MYBPC3, the gene encoding cardiac myosin binding protein C (cMyBP-C). The mutation truncates the protein in the C7 domain resulting in the loss of 408 residues. Our earlier work has shown that the truncated protein is not stably expressed and the disease is likely to be mediated by cMyBP-C haploinsufficiency.We measured Ca2+-sensitivity, isometric force generation, and myosin ATPase activity in tissue flash frozen in liquid nitrogen and subsequently stored in dry ice at −80°C before chemical demembranation. ATPase activity within the myocardium was measured simultaneously with force, using a fluorimetric technique and a linked-enzyme assay. Healthy human cardiac ventricular tissue served as control.The mutant tissue exhibited an increased Ca2+-sensitivity (pCa50 in control: 5.98±0.02 (n = 12); mutant: 6.52±0.07 (n=6), p<0.001) whereas the maximum isometric tension was reduced in mutant compared to control (control: 18.5±3.0 kN.m−2 (n = 26); mutant 8.6±0.8 kN.m−2 (n=7), p<0.05). There was no difference in the ATPase activity in maximally Ca2+-activated tissue between the two groups (control, 131±20μM.s−1 (n=26); mutant, 127±9μM.s−1 (n=7), p=0.87). The dependence of ATPase activity on force was linear, with a slope (tension cost) of 7.32±0.97μM.m2.kN−1.s−1 (mutant, n=6) and 3.46±0.87 μM. m2.kN−1.s−1 (control, n=10), p = 0.01. The increased tension cost of the mutant sarcomeres may cause energetic compromise, which has been suggested to play an important role in the development of the HCM phenotype. Increased Ca2+sensitivity has been reported in other investigations on HCM myocardium, and may be a direct effect of cMyBP-C haploinsufficiency or reflect compensatory changes.Work supported by the British Heart Foundation.