Project Summary Microsatellite expansions cause over 50 neurodegenerative and neuromuscular diseases, with CAG repeat expansions being one of the most common classes of disease-causing repeats. CAG repeat expansion diseases, which include Huntington’s disease (HD), spinal and bulbar muscular atrophy (SBMA), and spinocerebellar ataxia (SCA) types 1, 2, 3, 6, 7 and 12, share the production of CAG expansion RNAs and in most cases, the expression of toxic polyglutamine containing proteins. Despite these shared features, there are no cross-disease therapeutic approaches and there is a lack a clear understanding of the precise molecular mechanisms responsible for disease pathogenesis. While current therapeutic approaches focus mostly on treating symptoms, there are multiple available targets in the pathogenic cascade for these diseases: the CAG expansion containing gene, expansion transcripts, and/or expansion proteins. Therapeutic approaches in preclinical development that target this cascade have been largely gene-specific, offering hope for one disease but not for the larger family of CAG repeat diseases. Our approach of designing therapeutic strategies that selectively target abundance of CAG RNAs has the potential to alleviate the underlying mechanisms and impact all downstream aspects of the pathogenic cascade across multiple CAG repeat expansion diseases. Our central hypothesis for the parent grant is that targeting CAG expansions with small molecules will provide therapeutic efficacy across multiple CAG SCAs. To this end, we have already identified multiple small molecules that selectively reduce levels of CAG transcripts across SCA1, SCA3 and SCA7 patient-derived fibroblasts and SCA1 mice providing proof-of-concept for a cross-disease small molecule approach for CAG expansion SCAs. In the parent grant, we will leverage our established pipeline to identify FDA-approved small molecules and natural products with therapeutic efficacy across CAG SCAs. This supplement is focused on complementary studies to investigate the therapeutic potential of our lead candidate small molecules in Huntington’s disease models and in combination with protein regulating small molecules. By expanding the group of diseases for our novel, RNA targeting small molecules, Daphne will gain valuable training experience while providing key information as to whether CAG repeat selective compounds could provide therapeutic benefit beyond the SCAs. Furthermore, while compounds that target SCA proteins, most notably ATXN3, and the huntingtin protein have been published, the possibility of an additive effect of small molecules that target both protein and RNA has yet to be considered for CAG expansion diseases. Therefore, Daphne’s investigation of combination therapeutics for CAG SCAs and the broader group of CAG expansion diseases has the potential to uncover promising strategies that maximize therapeutic benefit whilst minimizing off target effects of any individual small molec...