Duchenne muscular dystrophy (DMD) is an X-linked myopathy caused by mutations or deletions in the dystrophin gene. DMD is a common form of muscular dystrophy, affecting about 1 in every 5000 boys. DMD is a severe disease, causing progressive muscle wasting that leads to loss of ambulation and premature death in affected individuals. Despite having understood the genetic defect in DMD for the past 20 years, no approved therapy exists that has been shown to ultimately alter disease outcome. We have pioneered the development of a novel therapy for DMD based on the overexpression of GALGT2, a gene that encodes a glycosylation enzyme that alters sugars on the skeletal muscle membrane in order to boost the expression of proteins that ameliorate disease. This approach has shown therapeutic efficacy in three different mouse models of muscular dystrophy, including the mdx mouse model for DMD, suggesting that GALGT2 gene therapy may be useful in multiple forms of the disease. GALGT2 overexpression can also protect wild type muscles from injury and may therefore have therapeutic usefulness that extends beyond neuromuscular disorders. In light of these proof of concept studies demonstrating therapeutic efficacy, we have developed gene therapy vectors for use in human clinical trials. Intra-arterial delivery to the hindlimb muscles has shown functional correction in the mdx mouse and sustained expression in the non-human primate. This work has helped move us toward our goal of performing the first GALGT2 gene therapy clinical trial for DMD. The work proposed here will, for the first time, assess the therapeutic efficacy of GALGT2 in protecting the heart muscle, a muscle that greatly affects DMD morbidity and mortality. It will also test a new generation AAV vector that allows high expression in both skeletal muscle and heart to determine if single dose vascular delivery can be used to treat the whole DMD patient. Second, it will test GALGT2 gene therapy in a severe large animal model of DMD. This addresses issues of the scalability of gene therapy to the human and more rigorously tests its therapeutic value. Third, it will describe a new pathway that regulates the expression of endogenous muscle Galgt2 gene expression, opening up new approaches to exploit this important gene for therapy development.