Molecular Mechanisms of Dystrophin Expression in Ameliorated Phenotypes

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Abstract Duchenne muscular dystrophy (DMD) typically results from mutations in the DMD gene that disrupt the open reading frame, resulting in no dystrophin protein, whereas the milder Becker muscular dystrophy (BMD) typically results from mutations that allow expression of a partially functional dystrophin protein. This observation has led to the development of therapies intended to result in expression of internally-deleted, BMD-like dystrophin proteins. Despite the availability of four such commercial therapies for a subset of patients, and the transformative promise of microdystrophin gene therapies that are on the horizon, there are fundamental unanswered questions about the relationship of partial dystrophin expression to patient function—questions that bear on our ability to assess the benefits of therapies, to offer prognosis to families, and to guide approaches to retreatment in gene replacement therapies, among other issues. Furthermore, it will be critical to understand the distinctions between endogenous, lifelong expression of partially functional dystrophins, and therapeutic dystrophins delivered postnatally. Our long-term goal is to understand a fundamental question: How much dystrophin protein—and what kind of partially functional dystrophin, expressed at what time—is enough? Our objective in this project is two-fold. First, we seek to understand in finer detail the molecular parameters of dystrophin expression that result in phenotype amelioration. Second, we seek to develop methods for full-length dystrophin expression from the endogenous DMD gene, which is a possibility for a for a subset of patients, and has the potential for improved outcomes in comparison to microdystrophin. Our central hypothesis is that multiple mechanisms exist to account for variations in disease severity, and that understanding these mechanisms will lead to a better understanding of the durability and long-term benefits of dystrophin restoration therapies. Our rationale is that a detailed exploration of these mechanisms will lead to a better understanding of the long-term outcomes expected from novel gene therapies for DMD.