Cardiac myosin heavy chain isoform exchange alters the phenotype of cTnT-related cardiomyopathies in mouse hearts

Familial hypertrophic cardiomyopathy, FHC, is a clinically heterogeneous, autosomal-dominant disease of the cardiac sarcomere leading to extensive remodeling at both the whole heart and molecular levels. The remodeling patterns are mutation-specific, a finding that extends to the level of single amino acid substitutions at the same peptide residue. Here we utilize two well-characterized transgenic FHC mouse models carrying independent amino acid substitutions in the TM-binding region of cardiac troponin T (cTnT) at residue 92. R92Q and R92L cTnT domains have mutation-specific average peptide conformation and dynamics sufficient to alter thin filament flexibility and cross-bridge formation and R92 mutant myocytes demonstrate mutation-specific temporal molecular remodeling of Ca²⁺ kinetics and impaired cardiac contractility and relaxation. To determine if a greater economy of contraction at the crossbridge level would rescue the mechanical defects caused by the R92 cTnT mutations, we replaced the endogenous murine α-myosin heavy chain (MyHC) with the β-MyHC isoform. While β-MyHC replacement rescued the systolic dysfunction in R92Q mice, it failed to rescue the defects in diastolic function common to FHC-associated R92 mutations. Surprisingly, a significant component of the whole heart and molecular contractile improvement in the R92Q mice was due to improvements in Ca²⁺ homeostasis including SR uptake, [Ca2+]_i amplitude and phospholamban phosphorylation. Our data demonstrate that while genetically altering the myosin composition of the heart bearing a thin filament FHC mutation is sufficient to improve contractility, diastolic performance is refractory despite improved Ca²⁺ kinetics. These data reveal a previously unrecognized role for MyHC isoforms with respect to Ca²⁺ homeostasis in the setting of cardiomyopathic remodeling and demonstrate the overall dominance of the thin filament mutation in determining the degree of diastolic impairment at the myofilament level.