Evaluating Properties of Dystrophin and Delivery Methods of rAAV Gene Therapy for Duchenne Muscular Dystrophy

Abstract

Duchenne muscular dystrophy (DMD) is a recessive muscle wasting disease caused by a deleterious mutation in the gene encoding the dystrophin protein. Dystrophin is an integral component the dystrophin-glycoprotein complex (DGC) that stabilizes the sarcolemma and allows transmission of mechanical force in striated muscle. Recombinant adeno-associated viral (rAAV) vectors have shown promise as a method for delivering therapeutic genes to dystrophic muscles. Vectors expressing miniaturized, or micro-, dystrophin proteins have repeatedly demonstrated rescue of rodent dystrophic animal models as well as improvement in larger dystrophic animal models. However, current micro-dystrophin constructs do not restore full, wild type function to transduced skeletal muscles. To improve the functionality of micro-dystrophin, we designed novel constructs and evaluated rAAV vector-treated dystrophic mice expressing these micro-dystrophins. We observed an improvement in functionality in two novel micro-dystrophins when compared to a previously established construct serving as our standard. We also examined the consequences of ablating micro-dystrophin expression in a mouse model. After determining that adult skeletal muscle falls into a dystrophic condition by three months after ablation, we concluded that rAAV vector-mediated gene therapy for DMD may require persistent expression of micro-dystrophin for life. We expanded on a previously reported immunosuppressive regimen in order to allow readministration of rAAV vectors in both dystrophic and wild type mice. Additionally, rAAV vectors effectively transduced striated muscle tissues after repeated, systemic delivery into wild type mice at doses that would be therapeutic for neuromuscular diseases. In order to further understand the tropism and properties of AAV serotypes that exhibit a high degree of tropism for skeletal muscle, we compared their transduction properties in mice and canine animal models. We found that AAV serotypes 6, 8, and 9 all poorly transduce myogenic satellite cells. We also determined that AAV8 transduces mouse and canine skeletal muscle at a lower efficiency than AAV serotypes 6 and 9. Yet, serotypes 6 and 9 exhibited similar transduction when administered into the jugular vein of canines at sub-saturating doses. These results expand on several aspects of rAAV-mediated gene therapy for DMD involving the a) design and functionality of the therapeutic construct, b) consequences of lost expression of micro-dystrophin, c) immune responses in relation to repeat transduction of rAAV vectors, and d) properties of AAV serotypes and methods of delivery.