Spinocerebellar Ataxia 8 (SCA8) appears unique among triplet repeat expansion-induced neurodegenerative diseases because the predicted gene product is a noncoding RNA. Little is currently known about the normal function of SCA8 in neuronal survival or how repeat expansion contributes to neurodegeneration. To investigate the molecular context in which SCA8 operates, we have expressed the human SCA8 noncoding RNA in Drosophila. SCA8 induces late-onset, progressive neurodegeneration in the Drosophila retina. Using this neurodegenerative phenotype as a sensitized background for a genetic modifier screen, we have identified mutations in four genes: staufen, muscleblind, split ends, and CG3249. All four encode neuronally expressed RNA binding proteins conserved in Drosophila and humans. Although expression of both wild-type and repeat-expanded SCA8 induce neurodegeneration, the strength of interaction with certain modifiers differs between the two SCA8 backgrounds, suggesting that CUG expansions alter associations with specific RNA binding proteins. Our demonstration that SCA8 can recruit Staufen and that the interaction domain maps to the portion of the SCA8 RNA that undergoes repeat expansion in the human disease suggests a specific mechanism for SCA8 function and disease. Genetic modifiers identified in our SCA8-based screens may provide candidates for designing therapeutic interventions to treat this disease.
Bibliographical noteFunding Information:
We are grateful to D. St. Johnston, A. Brand, M. Baylies and the Bloomington Stock Center for fly stocks and reagents. We thank W. Theurkauf, D. Ish-Horowicz, S. Bullock and B. Cha for advice on RNA injection and N. Watson for help with microscopy. We thank S. Lindquist, T. Orr-Weaver, J.T. Littleton, R. Fehon, D. Doroquez and members of the Rebay lab for advice and critical comments. Confocal and Scanning Electron Microscopy were performed in the W.M. Keck Foundation Biological Imaging Facility at the Whitehead Institute. M. D. K. acknowledges financial support from NIH (RO1NS42256-02) and I. R. acknowledges financial support from the Burroughs Wellcome Fund.