Muscle weakness correlates with muscle atrophy and precedes the development of inclusion body or rimmed vacuoles in the mouse model of DMRV/hIBM.

Distal myopathy with rimmed vacuoles (DMRV), also called hereditary inclusion body myopathy (hIBM), is characterized clinically by weakness and atrophy that initially involves the distal muscles and pathologically by the presence of rimmed vacuoles (RVs) or intracellular protein deposits in myofibers. It is caused by mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene that is important in sialic acid synthesis. Recently, we generated a mouse model (Gne(-/-)hGNED176VTg) that exhibits muscle weakness and pathological changes similar to DMRV patients. To gain better understanding of the pathomechanism of DMRV, we determined temporal changes in the overall motor performance of this model mouse for DMRV in correlation with the structure and function of isolated skeletal muscles and muscle pathology. These DMRV mice exhibited muscle weakness, decreased whole muscle mass and cross-sectional area (CSA), and reduced contractile power in an age-related manner. Single-fiber CSA further supported the finding of muscle atrophy that involved both type I and type II fibers. These results suggest that atrophy is highly correlated with reduced production of force at young age, both in vivo and ex vivo, thereby implicating the important role of atrophy in the pathomechanism of DMRV. In older age, and particularly in gastrocnemius muscles, RVs and intracellular inclusions were seen in type IIA fibers, further aggravating reduction of force and specific increase in twitch-tetanus ratio.