Double-stranded RNA as a pathogenic agent in a Drosophila model of dominant expanded repeat diseases.

Abstract

The expansion of tandem repeat sequences beyond a pathogenic threshold is responsible for a series of neurodegenerative diseases known as dominantly inherited expanded repeat diseases. A number of these diseases are caused by the expansion of a CAG repeat tract in the coding region of various genes and are termed polyglutamine diseases. In these cases the polyglutamine tract is thought to contribute to pathogenesis. Several other clinically indistinguishable diseases however, are caused by the expansion of various repeat sequences in untranslated regions of genes. As expanded repeat RNAs are present in each of these cases, these RNAs have been proposed as a common pathogenic agent. Increasing evidence now exists for bi-directional transcription across the expanded repeat sequence of disease genes, leading to toxicity. The products of bi-directional transcription are predicted to form complementary double-stranded RNA. This study uses a Drosophila model of bi-directional transcription to determine the physical properties of the RNA and the downstream pathways that could contribute to pathogenesis in these diseases. Expression of complementary repeat sequences predicted to form double-stranded RNA was toxic in this model and caused age-dependent neurodegeneration. This toxicity was dependent on several components of the RNA biogenesis pathway, including Dicer-2. The abundance of rCAG 21mer RNA and an altered miRNA profile were identified as biomarkers of this pathway. Microarray analysis identified genes involved in redox regulation, immune activation, cellular signalling and neurotransmission as novel candidates of pathogenesis. Furthermore, activation and signalling via the Toll pathway was required for pathogenesis indicating that an elevated immune response contributes to toxicity. Glial expression of double-stranded RNA caused severe neurodegeneration suggestive of non-autonomous toxicity as the cause of neuronal dysfunction. The identification of pathogenic pathways and molecular biomarkers are a critical step in developing therapeutic interventions for these diseases.