In Vivo Cardiac Myosin Binding Protein C Gene Transfer Rescues Myofilament Contractile Dysfunction in Cardiac Myosin Binding Protein C Null Mice

Abstract

Background Decreased expression of cardiac myosin binding protein C (cMyBPC) in the heart has been implicated as a consequence of mutations in cMyBPC that lead to abnormal contractile function at the myofilament level, thereby contributing to the development of hypertrophic cardiomyopathy in humans. It has not been established whether increasing the levels of cMyBPC in the intact heart can improve myofilament and in vivo contractile function and attenuate maladaptive remodeling processes because of reduced levels of cMyBPC. Methods and Results We performed in vivo gene transfer of cMyBPC by direct injection into the myocardium of cMyBPC-deficient (cMyBPC−/−) mice, and mechanical experiments were conducted on skinned myocardium isolated from cMyBPC−/− hearts 21 days and 20 weeks after gene transfer. Cross-bridge kinetics in skinned myocardium isolated from cMyBPC−/− hearts after cMyBPC gene transfer were significantly slower compared with untreated cMyBPC−/− myocardium and were comparable to wild-type myocardium and cMyBPC−/− myocardium that was reconstituted with recombinant cMyBPC in vitro. cMyBPC content in cMyBPC−/− skinned myocardium after in vivo cMyBPC gene transfer or in vitro cMyBPC reconstitution was similar to wild-type levels. In vivo echocardiography studies of cMyBPC−/− hearts after cMyBPC gene transfer revealed improved systolic and diastolic contractile function and reductions in left ventricular wall thickness. Conclusions This proof-of-concept study demonstrates that gene therapy designed to increase expression of cMyBPC in the cMyBPC-deficient myocardium can improve myofilament and in vivo contractile function, suggesting that cMyBPC gene therapy may be a viable approach for treatment of cardiomyopathies because of mutations in cMyBPC.

Impact of the Myosin Modulator Mavacamten on Force Generation and Cross‐Bridge Behavior in a Murine Model of Hypercontractility

“…Thus, the differential impact of Myk461 on k rel observed here is likely related to inherent differences in XB behavior between KO and WT myocardium. It is known that cMyBPC plays a dominant role in strain‐dependent XB detachment from actin and that XBs are destabilized in the absence of cMyBPC, thus resulting in faster rates of force decay 30, 37, 54. Perhaps Myk461 counters this effect by altering myosin head conformation in a manner that increases XB stability and/or slows XB turnover in KO myocardium, partly explaining the reduced force deficits induced by Myk461 in KO myocardium—a hypothesis that requires further validation using molecular studies specifically geared towards elucidating the contributions of cMyBPC on Myk461‐mediated modulation of XB behavior.…”

“…Perhaps Myk461 counters this effect by altering myosin head conformation in a manner that increases XB stability and/or slows XB turnover in KO myocardium, partly explaining the reduced force deficits induced by Myk461 in KO myocardium—a hypothesis that requires further validation using molecular studies specifically geared towards elucidating the contributions of cMyBPC on Myk461‐mediated modulation of XB behavior. Nevertheless, the Myk461‐induced slowing in k rel in the KO myocardium would be expected to reduce overall ATP turnover rate and tension cost as XB detachment was shown to correlate well with tension cost,56, 57 which may improve systolic function in vivo and reduce pathological hypertrophy and remodeling in models of hypertrophy induced by reductions in cMyBPC 30, 32. Notably, a slowed k rel and increased XB “ on ” time following Myk461 incubation may underlie our observation that Myk461‐induced force generation reductions were not as pronounced in the KO myocardium (Table 2; Figure 3).…”

“…Functionally, decreased incorporation of cMyBPC in human HCM samples expressing cMyBPC truncation mutations led to accelerated XB kinetics at submaximal Ca 2+ activations, in part, contributing to cardiac hypercontractility and hypertrophy 27. Mouse models of reduced cMyBPC expression or cMyBPC ablation also exhibit accelerated rates of XB recruitment and XB detachment rates at submaximal Ca 2+ activations,28, 29, 30 in part because reduced expression of cMyBPC radially displaces the myosin heads closer to actin, thereby effectively reducing the distance between actin and myosin and increasing the probability of acto‐myosin XB interactions 31. However, the impact of Myk461 on accelerated XB behavior induced by reduced cMyBPC expression has yet to be investigated.…”

Alterations in sarcomere function modify the hyperplastic to hypertrophic transition phase of mammalian cardiomyocyte development

“…There have been multiple groups that have also demonstrated that reversal of established sarcomeric cardiomyopathy in the adult animal was not successful (8,9,11). In contrast, one study used a viral strategy to express MYBPC3 protein in the adult MYBPC3-null heart and reported partial improvement in systolic function and a reduction in heart mass 21 days after viral injection (47). However, the mice in this study were injected with a MYBPC3-expressing viral vector at a range of ages spanning 8 to 26 weeks and were then analyzed together.…”

Targeting the sarcomere to correct muscle function

“…In this regard, another obvious target is myosin binding protein C, which is the most commonly mutated protein in hypertrophic cardiomyopathy. Because most mutations cause disease through haploinsufficiency, gene therapy would be expected to be most effective in such conditions [107][108][109] . In skeletal and cardiac muscle, the balance between contraction and relaxation is readily shifted by physiological mechanisms…”