Adeno-associated virus-mediated microdystrophin expression protects young mdx muscle from contraction-induced injury

Duchenne muscular dystrophy (DMD) is the most common inherited lethal muscle degenerative disease. Currently there is no cure. Highly abbreviated microdystrophin cDNAs were developed recently for adeno-associated virus (AAV)-mediated DMD gene therapy. Among these, a C-terminal-truncated Delta R4-R23/Delta C microgene (Delta R4/Delta C) has been considered as a very promising therapeutic candidate gene. In this study, we packaged a CMV.Delta R4/Delta C cassette in AAV-5 and evaluated the transduction and muscle contractile profiles in the extensor digitorum longus muscles of young (7-week-old) and adult (9-month-old) mdx mice. At similar to 3 months post-gene transfer, 50-60% of the total myofibers were transcluced in young mdx muscle and the percentage of centrally nucleated myofibers was reduced from similar to 70% in untreated mdx muscle to similar to 22% in microdystrophin-treated muscle. Importantly, this level of transduction protected mdx muscle from eccentric contraction-induced damage. In contrast, adult mdx muscle was more resistant to AAV-5 transduction, as only similar to 30% of the myofibers were transcluced at 3 months postinfection. This transduction yielded marginal protection against eccentric contraction-induced injury. The extent of central nucleation was also more difficult to reverse in adult mdx muscle (from similar to 83% in untreated to similar to 58% in treated). Finally, we determined that the Delta R4/Delta C microdystrophin did not significantly alter the expression pattern of the endogenous full-length dystrophin in normal muscle. Neither did it have any adverse effects on normal muscle morphology or contractility. Taken together, our results suggest that AAV-mediated Delta R4/Delta C microdystrophin expression represents a promising approach to rescue muscular dystrophy in young mdx skeletal muscle.