377. AAV-Mediated Transfer of FKRP Shows Therapeutic Efficacy in a Murine Model of Limb-Girdle Muscular Dystrophy Type 2i, but Requires Tight Control of Gene Expression

Limb Girdle Muscular Dystrophies (LGMD) type 2I, a recessive autosomal muscular dystrophy, is caused by mutations in the Fukutin Related Protein (FKRP) gene. It has been proposed that FKRP, whose function remains unclear, is a participant in α-dystroglycan (αDG) glycosylation, which is important to ensure the cell/matrix anchor of muscle fibers. A knock-in mouse model of LGMD2I was generated to express the most frequent mutation (L276I) encountered in patients. The introduction of the mutation did not alter the expression of FKRP, neither at transcriptional nor at translational levels, but did alter its function since abnormal glycosylation of αDG was observed. In this model, skeletal muscles were functionally impaired from 2 months of age and a moderate dystrophic pattern was evident by histology starting from 6 months of age. Gene transfer with a rAAV2/9 vector expressing Fkrp restored the biochemical defects, corrected the histological abnormalities and improved the resistance to eccentric stress in the mouse model was obtained. However, injection of high doses of the vector induced a decrease of αDG glycosylation and laminin binding. Finally, we showed that intravenous injection of the rAAV-Fkrp vector into a dystrophic mouse model suffering of dystroglycanopathy due to skeletal muscle-specific Fukutin (Fktn) knock-out caused toxicity. The dose-dependent worsening of the dystrophic phenotype suggests requirement for a precise control of its expression.