MUSCLE FUNCTION & HOMEOSTASIS / MOLECULAR THERAPEUTIC APPROACHES: P.63 AAV.CAPN3 gene therapy for Limb-girdle muscular dystrophy type 2A (LGMD2A): Phenotypic improvements in the calpain3 (CAPN3)-null mouse

Abstract

LGMD2A is caused by mutations in CAPN3 gene. CAPN3 loss of function is shown to play a role in disease pathogenicity, which might be improved by CAPN3 gene therapy. rAAVrh74.tMCK.CAPN3 vector was systemically delivered to young (YC, 6-10 week-old) and old cohorts (OC, 21-23 week-old) of CAPN3-KO mice. Each cohort included two treatment groups receiving low dose (LD; 3E12 vg, n=12) and high dose (HD; 6E12 vg, n=12) of the vector and age-matched untreated (UT) control groups (UT-YC, n=13;UT-OC, n=15). Outcome measures were performed at 20 weeks post-injection. AAV.CAPN3 improved treadmill performance in both cohorts, at both LD and HD subgroups (p<0.0001). There was no significant age-related effect on treadmill activity and the average performance of the treatment groups, HD vs. LD did not differ significantly. In the OC however, females (n=5) from the HD group run longer distance than males from same group (n=7; p<0.01). In the LD and HD treatment groups from OC, in vivo muscle contractility assay showed an improvement in both Twitch (p=0.001) and Tetanic Max responses (p=0.047) at the endpoint. The average of percent change between baseline and endpoint tetanic responses increased in the treated-YC while there was a decline in the UT cohort (p=0.002). Histological analyses of the skeletal muscle (gastroc, tibialis anterior, quadriceps and triceps) showed remodeling of muscle, a switch to fatigue resistant oxidative fibers in both cohorts along with fiber size increase. In-house toxicology studies in the HD cohort revealed no organ tissue abnormalities; specifically, there was no histopathological evidence of cardiotoxicity. AAV.CAPN3 gene therapy improved the LGMD2A-related phenotype in the CAPN3-KO mouse model in both low and high doses tested, independent of age at the time of vector-administration. Improvements shown were supported with the absence of cardiotoxicity, enhancing the efficacy and safety of AAV.CAPN3 vector as a potential gene therapy for LGMD2A.