Mutation Types and Aging Differently Affect Revertant Fiber Expansion in Dystrophic Mdx and Mdx52 Mice

Skeletal muscles have been shown to display sporadic dystrophin-positive revertant fibers, both in animal models of DMD as well as in Duchenne patients. These dystrophin-positive fibers arise from spontaneous exon skipping events (alternative splicing of the mRNA) and lead to the production of an in-frame truncated dystrophin protein. The revertant events are thought to arise within a subset of muscle precursor cells which proliferate in response to the ongoing muscle degeneration and the expansion the clusters of revertant fibers is known to be dependent on active muscle regeneration. The mechanisms by which revertant fibers arise and expand are not understood, and in the present study, Toshifumi Yokoto et al. (Echigoya Y, Lee J, Rodrigues M, Nagata T, Tanihata J, et al. (2013) Mutation Types and Aging Differently Affect Revertant Fiber Expansion in Dystrophic Mdx and Mdx52 Mice. PLoS ONE 8(7): e69194. doi:10.1371/journal.pone.0069194) examined the effects of two types of mutation (mdx mice containing a nonsense mutation in exon 23 and mdx52 mice containing deletion mutation of exon 52) and aging on revertant fibers expansion and muscle regeneration. Their results demonstrate that revertant fiber expansion and muscle regeneration is influenced by both ageing and the specific type of mutation.

Abstract: Duchenne muscular dystrophy (DMD), one of the most common and lethal genetic disorders, and the mdx mouse myopathies are caused by a lack of dystrophin protein. These dystrophic muscles contain sporadic clusters of dystrophin expressing revertant fibers (RFs), as detected by immunohistochemistry. RFs are known to arise from muscle precursor cells with spontaneous exon skipping (alternative splicing) and clonally expand in size with increasing age through the process of muscle degeneration/regeneration. The expansion of revertant clusters is thought to represent the cumulative history of muscle regeneration and proliferation of such precursor cells. However, the precise mechanisms by which RFs arise and expand are poorly understood. Here, to test the effects of mutation types and aging on RF expansion and muscle regeneration, we examined the number of RFs in mdx mice (containing a nonsense mutation in exon 23) and mdx52 mice (containing deletion mutation of exon 52) with the same C57BL/6 background at 2, 6, 12, and 18 months of age. Mdx mice displayed a significantly higher number of RFs compared to mdx52 mice in all age groups, suggesting that revertant fiber expansion largely depends on the type of mutation and/or location in the gene. A significant increase in the expression and clustering levels of RFs was found beginning at 6 months of age in mdx mice compared with mdx52 mice. In contrast to the significant expansion of RFs with increasing age, the number of centrally nucleated fibers and embryonic myosin heavy chain-positive fibers (indicative of cumulative and current muscle regeneration, respectively) decreased with age in both mouse strains. These results suggest that mutation types and aging differently affect revertant fiber expansion in mdx and mdx52 mice.