Project Summary Since we discovered repeat associated non-ATG (RAN) RAN translation in 2011, we and others have shown that RAN proteins accumulate in nine different expansion disorders. These proteins, which can be expressed from both sense and antisense expansion transcripts, accumulate in disease-relevant human tissues including spinocerebellar ataxia type 8 (SCA8) and Huntington disease (HD). We now have evidence that polySer and polyLeu RAN proteins accumulate in a group of spinocebellar ataxias (SCA1, 2, 3, 6 and 7) in which the CAG·CTG expansion mutations are located in polyGln open reading frames. Additionally, we have developed AAV and small molecule approaches to inhibit RAN translation. We will use these tools and genetic approaches to test our central hypotheses that RAN protein pathology is a common feature shared across polyglutamine encoding CAG·CTG expansion disorders and that inhibiting the PKR pathway will reduce RAN protein levels and mitigate disease. We will address our central hypothesis in three specific aims (1) To test the hypothesis that RAN proteins contribute to spinocerebellar ataxias (SCAs) caused by polyglutamine encoding CAG·CTG repeat expansion mutations. (2) To test the hypothesis that SCA and HD RAN proteins are toxic and PKR inhibition will decrease RAN protein levels and improve cellular phenotypes in HD and SCA3 iPSC derived cells (3) : To test the hypothesis that RAN proteins contribute to HD and SCA3 phenotypes in mice independent of polyGln effects using genetic and pharmacological approaches. Taken together these specific aims will determine the contribution of RAN proteins to HD,SCA3 and CAG·CTG repeat expansion disorders and characterize PKR inhibition as a potential therapeutic approach for this large class of devastating repeat expansion diseases.