Suppression of the yeast elongation factor Spt4 ortholog reduces expanded SCA36 GGCCUG repeat aggregation and cytotoxicity

Abstract

A hexanucleotide GGCCTG repeat expansion in intron 1 of the nucleolar protein 56 gene causes spinocerebellar ataxia type 36 (SCA36), which is a relatively pure cerebellar ataxia with progressive motor neuron involvement. In this study SCA36 cell models were generated by introducing expanded GGCCTG/CAGGCC repeats into cultured Neuro2A cells. Sense (GGCCUG)exp but not antisense (CAGGCC)exp RNA foci were detected in the cells, consistent with observations in autopsied brains of patients with SCA36. Glycine-proline dipeptide repeat (DPR) formation due to repeat-associated non-ATG translation rarely occurred in cells expressing expanded GGCCTG repeats; in contrast, cells harboring expanded c9orf72 GGGGCC/GGCCCC repeats robustly expressed DPR proteins. There are currently no effective treatments for microsatellite repeat expansion diseases including SCA36. In order to identify potentially useful therapies, we screened five candidate chemical compounds for their ability to diminish the toxicity of expanded SCA36 repeats and evaluated whether small interfering RNA-mediated silencing of Supt4a/Supt5, the murine ortholog of the yeast transcriptional elongation factor Spt4/Spt5, has therapeutic potential based on RNA foci quantification and cytotoxicity assays. Supt4a knockdown and erythromycin treatment suppressed the formation of (GGCCUG)exp RNA foci and DPR protein formation via regulation of (GGCCUG)exp mRNA, thereby ameliorating the cytotoxicity in SCA36 cell models. These data provide a basis for developing effective therapeutic strategies for the treatment of SCA36 and other repeat expansion disorders.