Résumé :
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Genetic defects in Calpain 3 leads to Limb-Girdle Muscular Dystrophy type 2A (LGMD2A). There is no treatment for this disease to date. We previously evaluated the potential of recombinant adeno-associated virus (rAAV) vectors for gene therapy in a murine model for LGMD2A. Efficient and stable transgene expression was obtained in the skeletal muscle after intramuscular and loco-regional administration. Moreover, its presence resulted in improvement of the histological features and in therapeutic efficacy at physiological level.In the present study, we performed intravenous administration with the same AAV vector and observed death of some animals This observation led us to perform an anatomo-pathological investigation. Large areas of fibrosis were detected in the heart, probably related to unwanted activity of calpain 3. Following these results, we developed new AAV vectors with skeletal muscle restricted expression. Results with this new generation of vectors showed that they were indeed able to prevent the cardiac toxicity.Besides the primary defect in LGMD2A, secondary deficiency in calpain 3 has been observed in a number of muscle diseases, including Tibial Muscular Dystrophy (TMD). TMD is a dominant disorder caused by mutations in the Cterminus of titin. To define whether the correction of the calpain 3 secondary deficiency would improve the course of the disease, we injected an AAV vector expressing calpain 3 in the muscle of a TMD murine model (a knock-in model carrying the most common titin mutation in Finland) that we have generated. Unexpectedly, histological analysis of the injected muscles showed an increase of the level of fibers with centrally located nuclei, indicating the worsening of the presentation. It should be noted that injection of the calpain 3 AAV vector in wild-type muscle does not induce any abnormality. Considering this result, we crossed the TMD model with a calpain 3 deficient model to obtain double heterozygous mice. Histological analysis of the muscles showed that the TMD phenotype was corrected, demonstrating a participation of calpain 3 in the pathogenesis of this disease. The secondary reduction in calpain 3 observed in TMD is probably due to an increased turn-over and therefore an over-activation of the protein that is deleterious. The data also indicates a role of the M-line titin in controlling the calpain 3 proteolytic activity. Inhibition of calpain 3 activity may therefore be a new therapeutic avenue for TMD.
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