Duchenne Muscular Dystrophy (DMD) is a degenerative muscle wasting disease caused by mutations in the dystrophin gene. The absence of dystrophin protein at the muscle sarcolemma results in increased muscle susceptibility to contraction-induced damage as well as dysregulation of secondary signaling pathways, many of which remain poorly understood. Despite advances in treatment strategies under investigation aimed at restoration of dystrophin expression including viral delivery of mini-dystrophin, read-through of translation stop codons, and exon skipping to restore the reading frame, there is no cure for DMD, and the identification of therapies that improve pathology independent of dystrophin would be of significant value to patients. In this vain, genetic modifiers of DMD are emerging as potential therapeutic targets. Our lab recently identified Jagged1 and Pitpna as genetic modifiers of DMD pathology in a Golden Retriever Muscular Dystrophy (GRMD) dog colony in which two exceptional ?escaper? dogs exhibited a drastically milder phenotype than typical GRMD dogs despite being dystrophin-deficient. Normally, GRMD dogs show a severe phenotype similar to human DMD including early progressive muscle degeneration, fibrosis, and premature death often within the first 2 years of life due to cardiopulmonary failure. Gene expression analyses of the escaper dogs compared to severely affected GRMD dogs and control animals revealed that Jagged1 overexpression (OE) and decreased Pitpna expression were hallmarks of the mild phenotype, which including maintained ambulation and normal lifespan. In subsequent genetic studies of dystrophin-deficient zebrafish, we demonstrated that modulation of Jagged1 and Pitpna prevents manifestation of the dystrophic muscle phenotype, increases long-term survival, and improves swim performance. In primary myoblasts derived from normal and DMD patients, we also showed that Jagged1 overexpression and Pitpna inhibition impact AKT/PTEN signaling and improve myoblast fusion. Given this positive data, we will now extend our research of Jagged1 and Pitpna modulation into the well-characterized mdx5cv mouse model of DMD. We will also investigate genetic and pharmacological inhibition of PDE10A, which we have shown to elicit decreased Pitpna expression and improve dystrophic pathology in dystrophin-deficient zebrafish. The long-term goal in this project is to assess and validate the therapeutic potential of the genetic modifiers Jagged1 and Pitpna to ameliorate DMD pathology, and to identify a viable pharmacologic modulator to advance into clinical studies. To accomplish this, we will implement experiments in accordance to the following three specific aims: 1) Determine the effect of reduced Pitpna expression on dystrophic pathology in mdx5cv mice, 2) Characterize the functional role of Jagged1 overexpression in mdx5cv mice, and 3) Assess the therapeutic potential of PDE10A inhibition on dystrophic pathology in primary human muscle cells, zebrafish, and mouse models of DMD.
In the Golden Retriever model of Duchenne Muscular Dystrophy (DMD), we identified Jagged1 and Pitpna as genetic modifiers of dystrophic pathology, whose overexpression and inhibition, respectively, elicit positive outcomes to disease progression. In genetic studies of dystrophin-deficient zebrafish, we demonstrated that modulation of Jagged1 and Pitpna prevents manifestation of the dystrophic muscle phenotype, increases long- term survival, and improves swim performance. Our studies described herein aim to extend this research into a well-characterized mouse model of DMD to test our hypothesis that increased Jagged1 expression and/or Pitpna inhibition, which will be tested both genetically and pharmacologically by PDE10A inhibition, can ameliorate dystrophic symptoms, thus creating a parallel therapeutic strategy to dystrophin-replacement therapies to improve DMD patient outcomes.
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