Forced myofiber regeneration promotes dystrophin gene transfer and improved muscle function despite advanced disease in old dystrophic mice

Duchenne muscular dystrophy (DMD) is caused by defects in the dystrophin ge ne. In young dystrophic mdx mice, immature regenerating myofibers represent the principal substrate for adenovirus vector (AdV)-mediated dystrophin ge ne transfer. However, in DMD patients immature regenerating myofibers are g enerally sparse. Such a situation also exists in old mdx mice, which may re present a more realistic model. Therefore, here we have used old mdx mice ( of 14- to 17 months of age) to test the hypothesis that one-time administra tion of a myonecrotic agent can transiently re-establish a population of im mature myofibers susceptible to AdV-mediated dystrophin gene transfer. This strategy led to upregulation of the coxsackie/adenovirus attachment recept or by means of induction of regenerating myofibers, significantly augmented AdV-mediated dystrophin gene expression, and enhanced force-generating cap acity. In addition, it led to an increased resistance to contraction-induce d injury compared with untreated controls. The latter protective effect was positively correlated with the number of dystrophin-expressing myofibers ( r = 0.83, P < 0.05). Accordingly, the risk:benefit ratio associated with th e sequential use of forced myofiber regeneration and AdV-mediated dystrophi n gene transfer was favorable in old mdx mice despite advanced disease. The se findings have implications for the potential applicability of AdV-mediat ed gene therapy to DMD and other muscle diseases in which immature regenera ting myofibers are lacking.