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AAGGG repeat expansions trigger repeat-dependent synaptic dysregulation in human CANVAS Neurons [CANVAS_Control_Isogenic]

ABSTRACT: Cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS) is a late onset, recessively inherited neurodegenerative disorder caused by a biallelic, non-reference pentameric CCCTT(AAGGG) repeat expansion within the second intron of replication factor complex subunit 1 (RFC1). While rare compound heterozygous CANVAS cases harbor RFC1 loss-of-function mutations alongside monoallelic repeat-expansions, RFC1 expression is maintained at normal levels in the presence of biallelic expanded repeats. To investigate how these pentameric repeats cause disease, we generated CANVAS patient iPSC derived neurons and utilized calcium imaging and transcriptomic analysis to define repeat elicited gain-of-function and loss-of-function contributions to neuronal toxicity. AAGGG repeat expansions do not alter neuronal RFC1 splicing, expression, or DNA repair pathway functions. In reporter assays, AAGGG repeats are translated into pentapeptide repeat proteins that selectively accumulate in patient brains. However, neither these proteins nor repeat RNA foci were detected in iNeurons and overexpression of these repeats in isolation did not induce neuronal toxicity. CANVAS iNeurons exhibit defects in neuronal development and diminished synaptic connectivity that is rescued by CRISPR deletion of a single expanded allele. Importantly, these deficits were not replicated by knockdown of RFC1 in control neurons and showed incomplete or no rescue upon over-expression of RFC1. These findings support a repeat-dependent and RFC1-independent mechanism as the underlying cause of neurodegeneration in CANVAS, with important implications for therapeutic development in this currently untreatable condition.

ORGANISM(S): Homo sapiens

PROVIDER: GSE248342 | GEO | 2024/09/01

REPOSITORIES: GEO

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Project description:HuntingtonÂs disease (HD) is a devastating neurodegenerative disease, with out effective treatment. Despite significant advances, our understanding of how an expanded CAG repeat in the amino terminus of the large ubiquitously expressed HD gene product remains incomplete. Augmented adult neurogenesis in response to acute and chronic injury, including HuntingtonÂs disease, has been demonstrated, but its role development, disease, and recovery is largely unknown, as are the factors controlling it. The HD gene product, huntingtin (htt) is know to interact with a number of transcription factors which subsequently influence gene expression. Understanding the factors involved in controlling neurogenesis in response to disease would bridge a significant knowledge gap and have tremendous therapeutic potential. We propose to identify transcripts responsible for CAG repeat facilitated neurogenesis. Our hypothesis is that expanded CAG repeats in the HuntingtonÂs disease gene facilitates neurogenesis by altering transcription of genes critical in neurogenesis. We have developed a model in which mouse embryonic stem cells (ESC) with expanded CAG repeats have facilitated neurogenesis. In this model more ESC with expanded CAG repeats transition to neuronal precursors and then develop into mature neurons more rapidly than control ESC. We propose to compare the gene expression profiles between ESC with expanded CAG repeats and control ESCs on days 4 and 6 of neuronal differentiation. Initial studies indicate that key transcriptional differences are occurring at these time points. Control ECS and a an ESC line with150 CAG repeats will be differentiated in triplicate, and RNA isolated form each replicate. It is anticipated that comparison of gene expression between the control and CAG repeat lines at each time point will identify transcripts involved in CAG repeat facilitated neurogenesis. Observed differences at day 4 may be more relevant to the transition from ESC to neuronal precursor whereas differences at day 6 may be more relevant to the neuronal precursor to neuron transition.

Project description:Dominantly inherited expanded repeat neurodegenerative diseases are typically caused by the expansion of existing variable copy number tandem repeat sequences in otherwise unrelated genes. Repeats located in translated regions encode polyglutamine that is thought to be the toxic agent, however in several instances the expanded repeat is in an untranslated region, necessitating multiple pathogenic pathways or an alternative common toxic agent. As numerous clinical features are shared by several of these diseases, and expanded repeat RNA is a common intermediary, RNA has been proposed as a common pathogenic agent. Various forms of repeat RNA are toxic in animal models, by multiple distinct pathways. In Drosophila, repeat-containing double-stranded RNA (rCAG.rCUG~100) toxicity is dependent on Dicer processing evident with the presence of single-stranded rCAG7, which have been detected in affected HD brains. Microarray analysis of Drosophila rCAG.rCUG~100 repeat RNA toxicity revealed perturbation of several pathways including innate immunity. Recent reports of elevated circulating cytokines prior to clinical onset, and age-dependent increased inflammatory signaling and microglia activation in the brain, suggest that immune activation precedes neuronal toxicity. Since the Toll pathway is activated by certain forms of RNA, we assessed the role of this pathway in RNA toxicity. We find that rCAG.rCUG~100 activates Toll signaling and that RNA toxicity is dependent on this pathway. The sensitivity of RNA toxicity to autophagy further implicates innate immune activation. Expression of rCAG.rCUG~100 was therefore directed in glial cells and found to be sufficient to cause neuronal dysfunction. Non-autonomous toxicity due to expanded repeat-containing double-stranded RNA mediated activation of innate immunity is therefore proposed as a candidate pathway for this group of human genetic diseases. The heads from newly eclosed male Drosophila were used for RNA extraction and profiling on Affymetrix Drosophile Genome 2.0 microarrays. Nine samples were analysed, representing control and experimental lines. Two independent lines of rCAG.rCUG~100 double-stranded RNA were analysed in triplicate. These were compared to 4xUAS control analysed in triplicate. All transgenes were expressed using the elavII-GAL4 pan-neuronal driver. Candidates were selected from the pool of transcripts which showed a 'present' call in all samples. T-tests were performed on raw values to determine samples that showed a significant difference with a P-value < 0.05.

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AAGGG repeat expansions trigger repeat-dependent synaptic dysregulation in human CANVAS Neurons [CANVAS_Control_Isogenic]

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