Repeat expansions in the human genome are responsible for more than thirty neurodegenerative and neuromuscular diseases, including the dominantly inherited diseases myotonic dystrophy types 1 and 2 (DM1 and DM2), Huntington's disease, and the spinocerebellar ataxias (SCAs). The repeats associated with DM1, d(CTG)n, and DM2, d(CCTG)n, lie in untranslated regions of their host gene. The repeat expansions reach dramatic sizes of thousands of repeats and generate gain-of-function """"""""toxic"""""""" CUG repeat RNAs that accumulate in nuclear foci and interact with CUG repeat binding proteins involved in control of tissue-specific pre-mRNA splicing. Similarly, the SCA type 10 (SCA10) d(ATTCT)n repeat expansions occur in an untranslated intron and reach dramatic sizes. This, plus our preliminary results, suggests that dominantly inherited SCA10 disease, like DM1 and DM2, may be a toxic RNA disease. In contrast, knock down of SCA10 gene expression in neurons increases cell death, suggesting that SCA10 may be a loss-of-function disease. Thus, it is entirely unclear how the expanded ATTCT repeat leads to SCA10 disease. It is unclear that the AUUCU-repeat-containing intronic RNA is stable after splicing, and the possibility of an AUUCU binding protein is provocative and could uncover a new class of proteins that regulate pre-mRNA splicing. Lack of understanding of how the d(ATTCT)n repeat expansion alters tissue-specific gene expression is a critical problem, because until this information becomes available, it will not be possible to develop new therapeutic strategies. The long-term goal of this research is to determine the biological pathways impacted by the ATTCT repeat expansion to cause SCA10 disease. The central hypothesis for the proposed research is that SCA10 disease is manifested by a toxic RNA effect that interferes with correct splicing of brain-specific pre-mRNAs. The central hypothesis will be tested by pursuing the following specific aims: 1) localize the RNAs transcribed from SCA10 d(ATTCT)n repeat expansions; 2) identify proteins that interact with the expanded SCA10 RNA. ? ? ?