Perturbation of the Akt/Gsk3- signalling pathway is common to Drosophila expressing expanded untranslated CAG, CUG and AUUCU repeat RNAs

Discipline of Genetics, School of Molecular and Biomedical Sciences and ARC Special Research Centre for the Molecular Genetics of Development, University of Adelaide, Adelaide SA 5005, Australia.
Human Molecular Genetics (Impact Factor: 6.39). 07/2011; 20(14):2783-94. DOI: 10.1093/hmg/ddr177
Source: PubMed


Recent evidence supports a role for RNA as a common pathogenic agent in both the 'polyglutamine' and 'untranslated' dominant expanded repeat disorders. One feature of all repeat sequences currently associated with disease is their predicted ability to form a hairpin secondary structure at the RNA level. In order to investigate mechanisms by which hairpin-forming repeat RNAs could induce neurodegeneration, we have looked for alterations in gene transcript levels as hallmarks of the cellular response to toxic hairpin repeat RNAs. Three disease-associated repeat sequences--CAG, CUG and AUUCU--were specifically expressed in the neurons of Drosophila and resultant common transcriptional changes assessed by microarray analyses. Transcripts that encode several components of the Akt/Gsk3-β signalling pathway were altered as a consequence of expression of these repeat RNAs, indicating that this pathway is a component of the neuronal response to these pathogenic RNAs and may represent an important common therapeutic target in this class of diseases.

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Available from: Deon J Venter, Mar 08, 2015
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    • "Drosophila models of expanded repeat diseases have been described that specifically investigate the intrinsic toxicity of both translated and untranslated expanded repeat sequences (Lawlor et al., 2011; van Eyk et al., 2011, 2012; Samaraweera et al., 2013). In one study (Lawlor et al., 2011), a single line of Drosophila expressing untranslated CAG was identified with a marked degenerative phenotype (whereas multiple other random insertion lines of the same transgene had no such phenotype). "
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    ABSTRACT: Repeat sequences that are expanded in copy number are the basis for ~20 dominantly inherited neurodegenerative diseases, including Huntington’s Disease. Despite some of the responsible genes being identified as long as 20 years ago, the identity and nature of the disease-causing pathogenic pathway remains a gap in knowledge for these diseases. This understanding is essential for rational approaches to delay onset, slow progression or ultimately effect cure. We have previously hypothesized that an RNA-based pathogenic pathway has a causal role in the dominantly inherited unstable expanded repeat neurodegenerative diseases. In support of this hypothesis we, and others, have characterized rCAG.rCUG100 repeat double-strand RNA (dsRNA) as a previously unidentified agent capable of causing pathogenesis in a Drosophila model of neurodegenerative disease. Dicer, Toll and autophagy pathways have distinct roles in this Drosophila dsRNA pathology. Dicer-dependence is accompanied by cleavage of rCAG.rCUG100 repeat double-strand RNA down to r(CAG)7 21-mers. Among the ‘molecular hallmarks’ of this pathway that have been identified in Drosophila, some [i.e. r(CAG)7 and elevated TNF] correlate with observations in affected people (e.g. HD, ALS) or in related animal models [i.e. autophagy]. The Toll pathway is activated in the presence of repeat-containing double-stranded RNA and toxicity is also dependent on this pathway. How might the endogenously expressed dsRNA mediate Toll-dependent toxicity in neuronal cells? Endogenous RNAs are normally shielded from Toll pathway activation as part of the mechanism to distinguish ‘self’ from ‘non-self’ RNAs. This typically involves post-transcriptional modification of the RNA. Therefore, it is likely that rCAG.rCUG100 repeat double-strand RNA has a characteristic property that interferes with or evades this normal mechanism of shielding. We predict that repeat expansion leads to an alteration in RNA structure and/or form that perturbs RNA mod
    Full-text · Article · Sep 2013 · Frontiers in Molecular Neuroscience
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    • "Multiple independent transgenic insertions were initially generated for each construct (rCUG∼100, rCAG∼100 or rCAA∼100, named alphabetically, ‘A’ through to ‘K’) and used to create lines carrying sets of 4 independent transgene insertions (4xrCAG∼100, 4xrCUG∼100 or 4xrCAA∼100, named numerically, ‘line 1’ onwards) [20]. We have previously shown that, in the case of CUG or CAG expression, these lines give comparable levels of transgene expression, so that phenotypic outcomes can be compared in each case [17], [20]. CAA expression transgenes appear to be present at a lower steady state level [17], possibly due to their inability to form stable secondary structures. "
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    ABSTRACT: Expanded DNA repeat sequences are known to cause over 20 diseases, including Huntington's disease, several types of spinocerebellar ataxia and myotonic dystrophy type 1 and 2. A shared genetic basis, and overlapping clinical features for some of these diseases, indicate that common pathways may contribute to pathology. Multiple mechanisms, mediated by both expanded homopolymeric proteins and expanded repeat RNA, have been identified by the use of model systems, that may account for shared pathology. The use of such animal models enables identification of distinct pathways and their 'molecular hallmarks' that can be used to determine the contribution of each pathway in human pathology. Here we characterise a tergite disruption phenotype in adult flies, caused by ubiquitous expression of either untranslated CUG or CAG expanded repeat RNA. Using the tergite phenotype as a quantitative trait we define a new genetic system in which to examine 'hairpin' repeat RNA-mediated cellular perturbation. Further experiments use this system to examine whether pathways involving Muscleblind sequestration or Dicer processing, which have been shown to mediate repeat RNA-mediated pathology in other model systems, contribute to cellular perturbation in this model.
    Preview · Article · Jun 2012 · PLoS ONE
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    • "A recent study shows that modifier of mdg4 [mod(mdg4)] transcript levels is altered in Drosophila heads upon expression of CAG-, CUG- and AUUCU-repeat RNA and that mod(mdg4) is an enhancer of CAG- and CUG-repeat toxicity (28). We did not find that mod(mdg4) met our criteria in generating gene lists; however, we used only brain tissue and not heads with eye tissue included. "
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    ABSTRACT: Spinocerebellar ataxia type 3 is one of the polyglutamine (polyQ) diseases, which are caused by a CAG-repeat expansion within the coding region of the associated genes. The CAG repeat specifies glutamine, and the expanded polyQ domain mutation confers dominant toxicity on the protein. Traditionally, studies have focused on protein toxicity in polyQ disease mechanisms. Recent findings, however, demonstrate that the CAG-repeat RNA, which encodes the toxic polyQ protein, also contributes to the disease in Drosophila. To provide insights into the nature of the RNA toxicity, we extracted brain-enriched RNA from flies expressing a toxic CAG-repeat mRNA (CAG100) and a non-toxic interrupted CAA/G mRNA repeat (CAA/G105) for microarray analysis. This approach identified 160 genes that are differentially expressed specifically in CAG100 flies. Functional annotation clustering analysis revealed several broad ontologies enriched in the CAG100 gene list, including iron ion binding and nucleotide binding. Intriguingly, transcripts for the Hsp70 genes, a powerful suppressor of polyQ and other human neurodegenerative diseases, were also upregulated. We therefore tested and showed that upregulation of heat shock protein 70 mitigates CAG-repeat RNA toxicity. We then assessed whether other modifiers of the pathogenic, expanded Ataxin-3 polyQ protein could also modify the CAG-repeat RNA toxicity. This approach identified the co-chaperone Tpr2, the transcriptional regulator Dpld, and the RNA-binding protein Orb2 as modifiers of both polyQ protein toxicity and CAG-repeat RNA-based toxicity. These findings suggest an overlap in the mechanisms of RNA and protein-based toxicity, providing insights into the pathogenicity of the RNA in polyQ disease.
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