Evaluation of the role of RNA toxicity in SCA2 pathogenesis using genome editing in patient iPSCs

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Spinocerebellar ataxia type 2 (SCA2) is a devastating neurodegenerative disease caused by a CAG repeat expansion in the gene ataxin-2 (ATXN2). The CAG repeat is translated into a polyglutamine (polyQ) tract in the mutant ATXN2 protein that has neurotoxic properties. Current therapeutic efforts are focused at suppressing the expression of the mutant ATXN2 protein, or targeting downstream pathways of neurotoxicity. We and others have shown that mutant CAG/CTG repeat-containing transcripts also contribute to pathogeneses of several repeat diseases, including Huntington's disease (HD), Huntington's disease-like 2 (HDL2) and spinocerebellar ataxia type 3 and 8 (SCA3 and SCA8). Therefore, in addition to targeting the neurotoxicity of mutant proteins, a successful therapy may require targeting the mutant transcripts and/or pathways downstream of the mutant transcripts. Our preliminary evidence supports the idea that in SCA2, in addition to toxic ATXN2 protein, both sense ATXN2 RNA and a transcript antisense to ATXN2 (ATXN2-AS) containing an expanded CUG repeat, contribute to SCA2 pathogenesis. We therefore propose to use genome editing approach to modify normal and SCA2 iPSC line into novel, isogenic iPSC lines that model either protein or RNA-induced mechanisms of SCA2 pathogenesis. These lines will be used to (1) further test the novel hypothesis of SCA2 by which mutant ATXN2/ATXN2-AS transcripts disrupt RNA processing in SCA2, including RNA export, rRNA metabolism and splicing, and (2) determine the relative contribution of RNA neurotoxicity to SCA2 pathogenesis. The results obtained from this study will not only facilitate our understanding of the increasingly complex pathogenesis of SCA2, but also that of other CAG/CTG repeat diseases in which both bi-directionally encoded transcripts and protein contribute to pathology. The results will help guide future development of SCA2 therapy, as well as therapies of other microsatellite repeat expansion diseases.