The hypertrophic cardiomyopathy-causing W792R and T1075 mutations in cardiac myosin binding protein-C generate cardiac dysfunction in mice

Background: Mutations in the MYBPC3 gene, which encodes the contractile regulatory protein cardiac myosin binding protein-C (cMyBP-C), account for approximately 40% of known cases of hypertrophic cardiomyopathy (HCM). Therefore, elucidating the pathogenicity of HCM-causing cMyBP-C mutations is critical for understanding how they contribute to the development of HCM. Objective: Our study aimed to determine the functional effects of two distinct HCM-causing mutations in MYBPC3-encoded cMyBP-C, a missense mutation in the C6 domain (W792R) and a C-terminal truncation of the C9/C10 domain (T1075 fs/5). Methods and Results: Cardiac specific transgenic mouse models were developed using a Tet-Off inducible system allowing for the controlled expression of W792R and T1075 cMyBP-C on the MYBPC3 null background. Functional cMyBP-C Tet-off (FT) mice were generated by crossing tetracycline transactivator mice with responder mice carrying the W792R and T1075 transgenes, which were compared to FT-WT controls. Short-axis M-mode echocardiography identified depressed percent ejection fraction and fractional shortening and increased left ventricular chamber size in hearts from FT-W792R and FT-T1075 mice compared to FT-WT controls. SDS-PAGE using cardiac myofibrillar fractions demonstrated a reduction in the expression of total W792R full-length and T1075 truncated cMyBP-C transgenic proteins compared to WT transgenic cMyBP-C. Immunofluorescence analysis of cardiac tissue sections revealed that the WT and W792R transgenic proteins localized in the classic cMyBP-C doublet pattern within cardiac sarcomeres. Intriguingly, the T1075 transgenic protein localized at the Z-lines within cardiac sarcomeres, suggesting a contrast in the localization patterns of cMyBP-C missense-mutated and truncated proteins. Conclusions: These results demonstrate that the W792R and T1075 cMyBP-C mutations generate cardiac contractile dysfunction in mice, although the molecular mechanisms of dysfunction differ between the two mutations, in that W792R is a missense mutation with normal localization of cMyBP-C and T1075 is a truncation mutation with reduced expression and altered sites of binding of cMyBP-C.