Project Summary Myotonic dystrophy type 1 (DM1), the most common form of adult onset muscular dystrophy, is an incurable neuromuscular disorder. Its genetic origin is a triplet (CTG) repeat expansion in the 3'-untranslated region (UTR) of the dystrophia myotonica protein kinase (DMPK) gene. No treatment options exist to delay disease progression. Strong evidence supports a gain-of-function role for the expanded RNA transcript (rCUGexp) and it is considered the toxic agent that causes DM1. The rCUGexp sequesters important proteins, inhibiting their normal function. Chief among these proteins is muscleblind-like protein 1 (MBNL1), a key regulator of alternative splicing. Its sequestration leads to the mis-splicing of >100 pre-mRNAs and many of the symptoms of DM1. The overall goal of this proposal is to discover novel therapeutic approaches and to identify and develop agents that target dCTGexp to inhibit its transcription and rCUGexp, if formed, to liberate sequestered protein. These agents may serve ultimately as new lead therapeutic agents for DM1. Agents that function well in cell culture models will be assessed for their drug-like abilities (e.g., ADME-tox) and suitable leads will be tested in sophisticated animal models scoring both phenotypic improvements (e.g., cardiac function) and correlating this with target-based activity (levels of relevant spliced RNAs). The specific aims of the proposal are: (1) Discovery and Development of Small Molecules that Bind dCTGexp and rCUGexp, (2) Development of Self-Assembling Therapeutics Targeting dCTGexp and rCUGexp, (3) Polymer Approaches to Targeting dCTGexp and rCUGexp, and (4) Evaluation of Promising Agents in Cellular Assays and Drosophila and Mouse Models of DM1.