Expression of Myosin Storage Myopathy Mutations in Drosophila Disrupts Skeletal and Cardiac Muscle Structure and Function

Myosin storage myopathy (MSM) is a predominantly autosomal dominant, congenital skeletal muscle disorder caused by missense mutations in the slow muscle/β-cardiac myosin heavy chain (MHC) rod. MSM is characterized by subsarcolemmal accumulations of β-MHC that have a hyaline appearance. Cases of MSM with cardiomyopathy have been reported. The mutations disrupt hydropathy or charge of residues in the heptad repeat, altering interactions that stabilize the rod's coiled-coil dimers and thick filaments.We have previously demonstrated in vitro that purified mutant MHC showed a reduced ability to form myosin filaments and that L1793P and E1883K filaments were less stable in limited proteolysis experiments. We have now generated heterozygous Drosophila MSM models to perform physiological, structural, and biophysical analysis of the effects of L1793P, R1845W, and E1883K mutations on diverse muscles. Flight and jump ability were highly compromised in mutant heterozygotes. Confocal and electron microscopy revealedsevere disruption of indirect flight musclemorphology and myofibrillar disarray/degeneration with hyaline-like inclusions in pupal muscles. We are currently performing super-resolution confocal microscopy to identify possible in vivo thick filament length discrepancies. Further, cryo-electron microscopy three-dimensional image reconstruction of native thick filaments will explore potential ultrastructural aberrations in filament backbones and molecular packing. Semi-automated heartbeat analysis of mutant heterozygotes revealed restrictive cardiac physiology and diastolic dysfunction, which worsened with age. Lifespans of MSM mutants were reduced relative to control. Thus, our heterozygous Drosophila models phenocopy afflicted patients’ skeletal and cardiac muscle structure and function. Going forward, our novel fly models will be used to screen for in vivo modifiers of mutant muscle performance