Duchenne muscular dystrophy (DMD) is a severe muscle-wasting disorder caused by mutations in the dystrophin gene, without curative treatment yet available. This study is the first to provide the overall safety profile and therapeutic dose of a recombinant adeno-associated virus vector, serotype 8 (rAAV8) carrying a modified U7snRNA sequence. This sequence promotes exon skipping to restore a functional in-frame dystrophin transcript when injected into the forelimb by locoregional transvenous perfusion. Eighteen Golden Retriever Muscular Dystrophy (GRMD) dogs were exposed to increasing doses of GMP-manufactured vector. Treatment was well tolerated in all, and no acute or delayed adverse effect, including systemic and immune toxicity, was detected. There was a dose relationship for the amount of exon skipping with up to 80% of myofibers expressing dystrophin at the highest dose. Similarly, histological, nuclear magnetic resonance pathological indices and strength improvement responded in a dose-dependent manner. The systematic comparison of effects using different independent methods, allowed to define a minimum threshold of dystrophin expressing fibers (>33% for structural measures and >40% for strength) under which there was no clear-cut therapeutic effect. Altogether, these results support the concept of a phase 1/2 trial of locoregional delivery into upper limbs of nonambulatory DMD patients.
