Deletion of the N-Terminus of Myosin Essential Light Chain (N-ELC) in the Background of HCM-A57G Mutation in Double Mutant Mice Rescues Hypercontractile Myosin Phenotype

Abstract

Cardiac pathologic remodeling including fibrosis, diastolic dysfunction and increased Ca2+ sensitivity of force has been observed in mice expressing the A57G mutation in myosin ventricular ELC (MYL3 gene) with clinical phenotype of hypertrophic cardiomyopathy (HCM). Our previous investigation showed that the lack of the unique 43 amino acid-long N-terminus ELC (N-ELC) in >7-month-old Δ43 mice results in non-pathological cardiac hypertrophy. Papillary muscles (PM) from ∼8 mo-old Δ43 hearts were subjected to mantATP-chase assay and compared to those from age-matched WT-ELC and A57G mice. The data showed that Δ43 fibers favor the super-relaxed (SRX) state of myosin with increased population of slowly cycling heads compared to PM from age-matched WT-ELC mice. The opposite was true for A57G mice whose PM demonstrated a pathologic hypercontractile behavior with significantly inhibited SRX. We then tested the effect of N-ELC deletion in A57G mice on the SRX in offspring resulting from breeding Δ43 with A57G mice (Δ43xA57G) and expressing 0-22% of Δ43 and 0-84% of A57G. Single nucleotide turnover assays on PM from all Δ43xA57G animals showed stabilization of the SRX state that was reminiscent of Δ43 alone, and different from all other groups of mice. Interestingly, echocardiographic evaluation of ∼8-month-old Δ43xA57G mice revealed a decrease in cardiac hypertrophy (decrease in LV mass) compared to A57G and Δ43 animals. Myocardial performance index was also improved in Δ43xA57G crosses compared to HCM-A57G mice, and not different compared to WT or Δ43 mice. Our studies suggest that the deletion of N-ELC in the background of HCM-ELC mutation may be beneficial in reversing some of the pathological myosin motor phenotypes associated with HCM. Supported by NIH R01HL123255 and R56HL146133 (DSC).