Dystrophin Replacement

Project Summary Gene therapy for Duchenne muscular dystrophy (DMD) is a promising method to treat this devastating disorder, and 3 companies have recently initiated human clinical trials of AAV/micro-dystrophin infusion. This approach has shown significant potential in pre-clinical studies, and early data from the trials are encouraging. It is hoped that DMD gene therapy will be effective in greatly slowing the development of dystrophy in young boys. Nonetheless, there remain a number of critical issues related to current vector delivery outcomes, several of which will be addressed in this renewal application. Our proposed studies are a natural progression of our previous lines of investigation. For example, in previous grant cycles we performed extensive structure/function studies of dystrophin that led to miniaturized clones that we chose to name “micro- dystrophin” (µDys). Our groups also developed compact, yet powerful muscle-specific gene regulatory cassettes from the MCK gene that enabled µDys expression from within AAV vectors. We also showed for the first time that AAV could be used for systemic gene delivery to muscles of adult animals. These studies led to extensive studies on methods and outcomes of systemic delivery to large animal models for DMD and facilitated the recently initiated human clinical trials. Here we propose to extend these studies by focusing on several critical unresolved issues related to DMD gene therapy. Initially, the relative importance of dystrophin expression in myogenic stem cells and mature myofibers will be explored. These studies will include a focus on muscle maintenance & regeneration. Also, current clinical trials are typically focused on very young DMD boys, yet it remains unclear whether any aspects of the dystrophic phenotype can be reversed in older boys by expression of dystrophin, a question we will address in aging and old animal models of DMD. Furthermore, issues remain surrounding the efficiency and safety of AAV-mediated dystrophin delivery to widely dispersed muscles, stems cells and the liver at tolerable and affordable vector doses. Consequently, we will explore vector modifications that could increase gene delivery efficiency, safety and persistence. We continue to believe that gene therapy offers real hope for treating DMD, which is among the most common human genetic disorders. Our proposed studies have significant potential to improve both the efficacy and safety of ongoing therapies, with the hope that a robust and safe treatment can emerge for boys and young men with this disorder. Finally, we note that enhancements to gene therapy for DMD should be generally applicable to many forms of muscular dystrophy and other disorders of striated muscle.