Different Substitutions at the Same β-Myosin Residue Underlie Divergent Molecular Phenotypes and Distinct Cardiomyopathies

In genetic cardiomyopathies, a frequently described phenomenon is how similar mutations in one protein can lead to discrete clinical phenotypes. One example is illustrated by two mutations in beta myosin heavy chain (MYH7) that are linked to hypertrophic cardiomyopathy (HCM) (Ile467Val, I467V) and left ventricular non-compaction (LVNC) (Ile467Thr, I467T). To investigate how these missense mutations lead to independent diseases, we studied the molecular effects of each mutation using recombinant human MYH7 Subfragment 1 (S1) in in vitro assays. Both HCM-I467V and LVNC-I467T S1 mutations exhibited similar mechanochemical functions, including unchanged actin-activated maximal ATPase and enhanced actin velocity. Molecular dynamics simulations revealed that both I467T and I467V allosterically disrupt interactions between myosin and ADP, which may contribute to the enhanced actin velocity observed in these mutations. However, I467V and I467T had distinct effects on the single turnover rate of myosin. HCM-I467V S1 showed no change in single ATP turnover rate while LVNC-I467T reduced the single turnover rate by 50%. These contrasting molecular effects may lead to contractile dysregulation that initiates LVNC-associated signaling pathways that progress the phenotype. Taken together, our results suggest that phenotypic complexity likely originates at the biochemical level.