Huntington?s disease (HD) is a progressive autosomal dominant neurodegenerative disorder caused by a polyglutamine expansion in the HD gene product, huntingtin (Htt). Mechanisms of HD cellular pathogenesis remain elusive, with no disease-modifying treatments for HD available today. Studies using cellular and mouse models of HD have revealed disruptions in many cellular processes and functions, likely mediated by abnormal protein interactions of polyQ-expanded Htt. Transcriptional dysregulation is widely implicated in HD pathogenesis, and epigenetic regulation of transcription was found to play a role in transcriptional abnormalities in HD. Arginine methylation is an important post-translational modification of proteins with an emerging role in neurodegeneration. In addition to providing a key level of epigenetic regulation, this modification found on many non-histone RNA-binding proteins mediates RNA processing and splicing. However, despite the fact that both gene transcription and RNA processing abnormalities have been widely associated with HD, there is very little information on the role of arginine methylation in HD pathogenesis. This project is a continuation of our previous findings that arginine dimethylation of endogenous substrates is impaired in HD models due to aberrant interactions of mutant Htt with the enzyme that carries out these modifications, PRMT5. Our findings of the PRMT5/Htt functional interaction raise the possibility that mutant Htt exerts its effects on transcription and RNA processing via direct interaction with histone-modifying enzymes, and by modulating their activity. We think this warrants further studies evaluating dimethylation of arginine, and the modifying enzymes, as potential new therapeutic targets for HD. Arginine methylation is a reversible modification, and several enzymes capable of arginine demethylation have been described. Thus, if arginine methylation of certain key targets is decreased in HD, there is a potential for reversal using the inhibitors of demethylases, several of which are available today, and some are being tested for cancer therapies. In Aim 1, we will assess proteome-wide changes in protein arginine methylation in HD, and will identify key players, focusing on histones and RNA-binding proteins involved in RNA processing and splicing. We will use novel methods of quantitative mass spectrometry, and our novel HD iPS cell lines, as well as human HD brain. In Aim 2, we will evaluate if Htt is directly involved in the methylation of the key substrates, identified in Aim 1. In Aim 3 we will evaluate approaches to reverse methylation deficiencies, and will test if such intervention is beneficial for neuronal survival. Taken together, these studies will provide novel information of the changes in arginine methylation of proteins in HD models and will elucidate whether abnormalities in epigenetic regulation of transcription and RNA processing represent a unifying pathogenic mechanism yielding novel therapeutic targets. The outcomes of these studies will reveal novel enzymatic targets, methyl-modifying enzymes, modulated by Htt, potentially providing a proof of principle towards novel molecular targets for preclinical therapeutic studies.