Distinct Molecular Signatures Of HCM- And DCM-causing Mutations At The Same Residue In The Transducer Of Cardiac Myosin

Mutations in cardiac myosin cause both hypertrophic (HCM) and dilated cardiomyopathy (DCM), and the clinical phenotype ultimately depends on the precise effect of the mutation on the function of myosin. Using recombinant human cardiac myosin, we studied the molecular effects of a novel Q222H mutation in MYH7 found in a patient with severe, early-onset DCM, and compared the effects to the Q222K mutation that was previously reported to cause HCM. The Q222 residue is located within the transducer of the head domain near the ATP binding pocket. The Q222H (DCM) mutant myosin exhibited significantly reduced steady-state actin-activated ATPase activity and a decreased rate of ATP binding in transient kinetic studies, while Q222K (HCM) mutant protein showed an increase in ATPase activity and ATP binding kinetics. In vitro motility assays showed 43% reduction and 32% increase in actin gliding velocity in Q222H and Q222K mutations, respectively. Finally, single nucleotide turnover assays revealed that the fraction of myosin with a slow ATP hydrolysis rate (super relaxed state, or SRX) thought to represent myosin in an autoinhibited state was significantly reduced in Q222K and increased in Q222H. Taken together, the Q222H/K mutations exerted contrasting effects on the motor properties of the cardiac myosin catalytic domain. Cellular characterization and molecular dynamic simulations of cardiac myosin to assess the structural impact of mutant residues are underway. These results highlight how subtle structural differences between cardiomyopathy-causing mutations can lead to significant differences in cardiac myosin motor activity and thus phenotypic expression of cardiomyopathy.