Molecular Characterization of a Novel MYH7 Mutation Q222H in a Patient with Severe Dilated Cardiomyopathy

Mutations in the cardiac myosin heavy chain gene (MYH7) are known to cause both hypertrophic and dilated cardiomyopathy (HCM and DCM), and the use of recombinant human cardiac myosin has enabled the study of the precise biochemical and biophysical effects of these mutations on myosin function at the molecular level. Here we report a case of a patient with severe, early-onset DCM who carries a previously unreported Q222H mutation in MYH7. This mutation affects a residue at the distal end of the ATP binding pocket of cardiac myosin. The Q222H mutant myosin exhibited significantly reduced steady-state actin-activated ATPase activity and a decreased rate of ATP binding in transient kinetic studies. It also showed a ∼50% reduction in actin gliding velocity in an in vitro motility assay. Single-nucleotide turnover assays were performed comparing Q222H mutant myosin 2-headed constructs with either a long S2 tail capable of forming a folded-back auto-inhibited state or a short S2 tail that cannot stably fold back and sequester heads. These studies 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 did not differ significantly between WT and the Q222H mutant myosins. Taken together, the Q222H mutation exerts substantial effects on the motor properties of the cardiac myosin catalytic domain, rather than affecting the availability of myosin heads in the sarcomere. Further transient kinetic assays are underway to delineate precise steps in the actin-myosin ATPase cycle affected by the mutation. Future work also includes comparing these effects with the Q222K mutation that has been reported to cause HCM, to gain further insights into the molecular basis for phenotypic determination of cardiomyopathy.