RESUMEN
BACKGROUND: Few manuscripts have reported phenotypes of skeletal muscle myopathies caused by mutations in the head region of slow/cardiac beta-myosin heavy chain (MyHCI). Among the patients, some of them showed the phenotype of skeletal muscle weakness with the obvious clinical features of cardiomyopathy while others showed pure skeletal muscle weakness with no symptoms of cardiac involvement. Genotype-phenotype relationship regarding the effect of a mutation on MyHCI is complex. Questions regarding why some mutations cause cardiomyopathy or skeletal muscle disorders alone or a combination of both still need to be answered. More findings in genetic variation are needed to extend knowledge of mutations in the MYH7 gene linked to skeletal muscle disorders. CASE PRESENTATION: Here we present a female adult patient with a phenotype of childhood onset of muscular disorders and predominant involvement of thigh muscles with biopsy showing intrasarcoplasmic inclusion bodies. Whole exome sequencing showed that variant c.1370 T > G (p.Ile457Arg) in the MYH7 gene is a missense mutation possibly linked to the clinical findings. Our patient likely shows an uncharacteristic myosin storage myopathy associated with respiratory and cardiac involvement linked to a missense mutation in the head of MyHCI. CONCLUSIONS: Given this mutation is located within the motor domain of MyHCI, this might affect the regulation of myosin mechano-chemical activity during the contractile cycle. Consequently, this potentially damaging effect can be easily amplified within the network of ~ 300-myosin molecules forming the thick filament and therefore become cumulatively deleterious, affecting, in turn, the overall organization and performance of sarcomere.