Altered C10 domain in cardiac myosin binding protein-C results in hypertrophic cardiomyopathy

Aims: A 25-base pair deletion in the cardiac myosin binding protein-C (cMyBP-C) gene (MYBPC3), proposed to skip exon 33, modifies the C10 domain (cMyBP-C^ΔC10mut) and is associated with hypertrophic cardiomyopathy (HCM) and heart failure, affecting approximately 100 million South Asians. However, the molecular mechanisms underlying the pathogenicity of cMyBP-C^ΔC10mutin vivo are unknown. We hypothesized that expression of cMyBP-C^ΔC10mut exerts a poison polypeptide effect leading to improper assembly of cardiac sarcomeres and the development of HCM. Methods and results: To determine whether expression of cMyBP-C^ΔC10mut is sufficient to cause HCM and contractile dysfunction in vivo, we generated transgenic (TG) mice having cardiac-specific protein expression of cMyBP-C^ΔC10mut at approximately half the level of endogenous cMyBP-C. At 12 weeks of age, significant hypertrophy was observed in TG mice expressing cMyBP-C^ΔC10mut (heart weight/body weight ratio: 4.43 ± 0.11 mg/g non-transgenic (NTG) vs. 5.34 ± 0.25 mg/g cMyBP-C^ΔC10mut, P < 0.05). Furthermore, haematoxylin and eosin, Masson's trichrome staining, as well as second-harmonic generation imaging revealed the presence of significant fibrosis and a greater relative nuclear area in cMyBP-C^ΔC10mut hearts compared with NTG controls. M-mode echocardiography analysis revealed hypercontractile hearts (EF: 53.4%±2.9% NTG vs. 66.4% ± 4.7% cMyBP-C^ΔC10mut; P < 0.05) and early diastolic dysfunction (E/E′: 28.7 ± 3.7 NTG vs. 46.3 ± 8.4 cMyBP-C^ΔC10mut; P < 0.05), indicating the presence of an HCM phenotype. To assess whether these changes manifested at the myofilament level, contractile function of single skinned cardiomyocytes was measured. Preserved maximum force generation and increased Ca²⁺-sensitivity of force generation were observed in cardiomyocytes from cMyBP-C^ΔC10mut mice compared with NTG controls (EC₅₀: 3.6 ± 0.02 μM NTG vs. 2.90 ± 0.01 μM cMyBP-C^ΔC10mut; P < 0.0001). Conclusion: Expression of cMyBP-C protein with a modified C10 domain is sufficient to cause contractile dysfunction and HCM in vivo.