A missense mutation within the C6-domain of cMyBP-C results in cardiac enlargement and depressed ventricular function

Cardiac myosin-binding protein-C (cMyBP-C) plays a key role in determining the force and kinetics of myocardial contraction, primarily by binding to either myosin subfragment-2 or actin or both. By modulating the probability of myosin cross-bridge binding to actin, cMyBP-C normally acts to govern the kinetics of force development and relaxation. To determine the effects on cardiac morphology and function due to a novel cMyBP-C W792R missense mutation associated with clinical hypertrophic cardiomyopathy in humans. We generated cMyBP-C-W792R heterozygous and homozygous knock-in mice to investigate the structural and functional phenotypes associated with a missense mutation within the C6-domain of cMyBP-C. Compared to wild-type, homozygous W792R knock-in hearts exhibited significant ventricular and atrial dilation. While the W792R mutant protein was expressed at equivalent levels as in wild-type mice, the extent of phosphorylation of the mutant cMyBP-C protein was markedly reduced. Measurements of steady-state force and cross-bridge cycling kinetics showed that homozygous cMyBP-C W792R skinned myocardium exhibited a significant increase in the Ca2+-sensitivity of force and accelerated cross-bridge cycling kinetics (ktr) at maximal levels of Ca2+-activation, compared to WT myocardium. Cardiac function, as assessed by transthoracic echocardiography, revealed depressed indices of systolic and diastolic ventricular function in the homozygous W792R KI mice. These results demonstrate that a mutation within the mid-region of cMyBP-C can disrupt the dynamic regulation between myosin and actin that is normally governed by cMyBP-C, thereby leading to aberrant cardiac enlargement and commensurate diminution of in vivo ventricular function.