Molecular pathomechanism of distal myopathy with rimmed vacuoles

Distal myopathy with rimmed vacuoles (DMRV) and hereditary inclusion body myopathy (HIBM) are now known to be the same disease and are caused by mutations in tile GNE gene that encodes a bifunctional protein with two enzymatic activities: UDP-GlcNAc2-epimerase (GNE) and ManNAc kinase (MNK). GNE catalyzes the rate-limiting step in the sialic acid biosynthesis and MNK catalyzes the next step. So far, we have found homozygous or compound heterozygous mutations in 55 unrelated Japanese DMRV patients. Among them, c.1714G>C (p.V572L) mutation is the most common, accounting for 57% of the mutant alleles. The same mutation was recently identified also in Korean DMRV patients, raising the possibility of the presence of a common founder. We have also found that cardiac involvement is not very rare and is found in 18% of patients, albeit degree of severity widely varies; in some patients, it can result in sudden death. The length of time when patients become non ambulatory is diverse. The severity of clinical symptoms also varies widely, as evidenced by the presence of an asymptomatic homozygote harboring of p.D176V, the second most common mutation among Japanese patients. Patients' fibroblasts and myotubes are hyposialylated and this hyposialylation can be recovered by adding GNE metabolite, ManNAc, or sialic acid per se, NeuAc. Accordingly, the sialylation status in the skeletal muscle tissue is also greatly altered especially in fibers with rimmed vacuoles, suggesting the tight association between hyposialylation and the formation of rimmed vacuoles. However, we still do not know why hyposialylation leads to the formation of rimmed vacuoles. To further elucidate the pathomechanism and to develop a therapy of DMRV, we need to produce mouse model mouse for this disease.