563. Correction of Glycogen Storage Disease Type II by Systemic Delivery of an AAV2/8 Vector Containing a Muscle-Specific Promoter

Abstract

Glycogen storage disease type II (GSD-II; Pompe disease; MIM 232300) is a genetic muscle disorder caused by a deficiency of acid-|[alpha]|-glucosidase (GAA; acid maltase; EC 3.2.1.20), which results in glycogen accumulation, primarily involving the heart and skeletal muscle. Previously, we provided evidence that an AAV2/6 vector containing a muscle-specific creatine kinase (MCK) promoter leads to persistent, high-level human GAA expression and glycogen clearance in the injected muscle in an immunocompetent GAA-KO mouse model. Muscle-restricted expression of hGAA provoked only a humoral (not cellular) immune response. We hypothesized that systemic administration of an AAV2/8 vector containing a MCK promoter could provide long-term efficacy in multiple muscles in GAA-KO mice. We intravenously administered either a low dose (1x1011 vg, n=3) or high dose (1x1012 vg, n=4) of the MCK-containing AAV2/8 vector in adult GAA-KO mice. At 18 weeks post-injection, GAA activity was significantly elevated only for the high-dose group to greater than the level for normal mice in heart (1.5-fold increase) and skeletal muscle (3.2-fold increase). The glycogen content was reduced by 46% and 50% in heart and quadriceps respectively, when compared to untreated, age-matched GAA-KO mice. Periodic acid-Schiff staining of glycogen confirmed correction of glycogen accumulation in individual myofibers of heart and quadriceps of AAV-treated mice. Low-level GAA activity was observed in liver and diaphragm, which is most likely related to lower activity of the MCK promoter in those tissues, because higher copy number of vector genome were present in liver (109.3 vg/cell) and diaphragm (1.63 vg/cell) than that in heart (0.47 vg/cell) and quadriceps (0.99 vg/cell) as determined by quantitative real-time PCR. In order to characterize transgene expression in different types of muscle fibers, an AAV vector encoding GFP under control of the MCK promoter was administered intravenously (1x1012 vg). Expression of GFP was detected in heart, quadriceps and soleus muscles, but not in EDL (extensor digitorum longus) muscle at 18 weeks after vector injection. Fiber-typing confirmed that soleus muscle consists mainly of type I (slow) myofibers, while EDL consists of type IIb (fast) myofiber in GAA-KO mice. In summary, persistent correction of glycogen storage in type I fibers of the heart and skeletal muscle was achieved with an AAV2/8 vector containing a MCK promoter in GAA-KO mice. This approach could be considered for muscle-targeted gene therapy in Pompe disease.