A novel missense mutation in the MYH7 gene causes an uncharacteristic phenotype of myosin storage myopathy: a case report

Dominant myosin storage myopathy mutations disrupt striated muscles in Drosophila and the myosin tail-tail interactome of human cardiac thick filaments

Myosin storage myopathy (MSM) is a rare skeletal muscle disorder caused by mutations in the slow muscle/β-cardiac myosin heavy chain (MHC) gene. MSM missense mutations frequently disrupt the tail's stabilizing heptad repeat motif. Disease hallmarks include subsarcolemmal hyaline-like β-MHC aggregates, muscle weakness, and, occasionally, cardiomyopathy. We generated transgenic, heterozygous Drosophila to examine the dominant physiological and structural effects of the L1793P, R1845W, and E1883K MHC MSM mutations on diverse muscles. The MHC variants reduced lifespan and flight and jump abilities. Moreover, confocal and electron microscopy revealed that they provoked indirect flight muscle breaks and myofibrillar disarray/degeneration with filamentous inclusions. Incorporation of GFP-myosin enabled in situ determination of thick filament lengths, which were significantly reduced in all mutants. Semiautomated heartbeat analysis uncovered aberrant cardiac function, which worsened with age. Thus, our fly models phenocopied traits observed among MSM patients. We additionally mapped the mutations onto a recently determined, 6 Å resolution, cryo-EM structure of the human cardiac thick filament. The R1845W mutation replaces a basic arginine with a polar-neutral, bulkier tryptophan, while E1883K reverses charge at critical filament loci. Both would be expected to disrupt the core and the outer shell of the backbone structure. Replacing L1793 with a proline, a potent breaker of α-helices, could disturb the coiled-coil of the myosin rod and alter the tail-tail interactome. Hence, all mutations likely destabilize and weaken the filament backbone. This may trigger disease in humans, while potentially analogous perturbations are likely to yield the observed thick filament and muscle disruption in our fly models.

A recurrent single-amino acid deletion (p.Glu500del) in the head domain of ß-cardiac myosin in two unrelated boys presenting with polyhydramnios, congenital axial stiffness and skeletal myopathy

BACKGROUND

Alterations in the MYH7 gene can cause cardiac and skeletal myopathies. MYH7-related skeletal myopathies are extremely rare, and the vast majority of causal variants in the MYH7 gene are predicted to alter the rod domain of the of ß-cardiac myosin molecule, resulting in distal muscle weakness as the predominant manifestation. Here we describe two unrelated patients harboring an in-frame deletion in the MYH7 gene that is predicted to result in deletion of a single amino acid (p.Glu500del) in the head domain of ß-cardiac myosin. Both patients display an unusual skeletal myopathy phenotype with congenital axial stiffness and muscular hypertonus, but no cardiac involvement.

RESULTS

Clinical data, MRI results and histopathological data were collected retrospectively in two unrelated boys (9 and 3.5 years old). Exome sequencing uncovered the same 3-bp in-frame deletion in exon 15 (c.1498_1500delGAG) of the MYH7 gene of both patients, a mutation which deletes a highly conserved glutamate residue (p.Glu500del) in the relay loop of the head domain of the ß-cardiac myosin heavy chain. The mutation occurred de novo in one patient, whereas mosaicism was detected in blood of the father of the second patient. Both boys presented with an unusual phenotype of prenatal polyhydramnios, congenital axial stiffness and muscular hypertonus. In one patient the phenotype evolved into an axial/proximal skeletal myopathy without distal involvement or cardiomyopathy, whereas the other patient exhibited predominantly stiffness and respiratory involvement. We review and compare all patients described in the literature who possess a variant predicted to alter the p.Glu500 residue in the ß-cardiac myosin head domain, and we provide in-silico analyses of potential effects on polypeptide function.

CONCLUSION

The data presented here expand the phenotypic spectrum of mutations in the MYH7 gene and have implications for future diagnostics and therapeutic approaches.