Investigation of RNA-mediated pathogenic pathways in a Drosophila model of expanded repeat disease.

Abstract

Expansion of a repeat sequence beyond a pathogenic range has been identified as the cause of a group of neurodegenerative diseases known as the expanded repeat diseases. Disease-associated repeat tracts have been found both within the coding region of genes, such as the CAG repeat coding for polyglutamine, or within noncoding regions. Despite the identification of the mutation involved in these diseases, the mechanism by which this type of mutation leads to cell death remains unclear. There is a substantial amount of evidence to suggest that RNA-mediated toxicity plays a role in pathogenesis of both the polyglutamine diseases and the untranslated dominant expanded repeat diseases. A common feature of the expanded repeats involved in each of these diseases is the ability of the repeat-containing RNA to form a hairpin secondary structure and therefore it has been predicted that similar mechanisms may be responsible for initiating cellular dysfunction and death in each case. This study uses a Drosophila model to investigate the intrinsic, RNA-mediated toxicity of three repeat sequences (CUG, CAG and AUUCU) associated with degeneration in human disease. Using a combination of hypothesis-driven and nonbiased approaches, early changes elicited in response to neuronal expression of these expanded repeat tracts have been investigated. A hypothesis of a role for RNA editing in CAG repeat pathogenesis was explored using this Drosophila model. Microarray and proteomic approaches were also utilised to identify pathways which are perturbed by the expression of these repeat sequences. The results described in this thesis demonstrate a degree of sequence- and context-independent toxicity of expanded repeat RNA in this model, suggesting that this kind of effect may also be a component of pathogenesis in the disease situation. Pathways commonly perturbed in response to expression of these RNA species may represent particularly valuable therapeutic targets, since preventing this type of effect could provide positive outcomes in a number of diseases.