C9RAN translation: A potential therapeutic target for the treatment of amyotrophic lateral sclerosis and frontotemporal dementia

Mayo Clinic Florida, Department of Neuroscience , Jacksonville, FL 32224 , USA.
Expert Opinion on Therapeutic Targets (Impact Factor: 5.14). 07/2013; 17(9). DOI: 10.1517/14728222.2013.818659
Source: PubMed


A hexanucleotide (GGGGCC) repeat expansion within a non-coding region of the C9ORF72 gene is the most common mutation associated with both frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Elucidating how these expanded repeats (GGGGCCexp) cause 'c9FTD/ALS' has since become an important goal of the FTD/ALS field. GGGGCCexp transcripts aggregate into discrete nuclear structures, termed RNA foci. This phenomenon, observed in various repeat expansion disorders, is associated with RNA-binding protein sequestration. Of note, recent findings show that GGGGCCexp transcripts also succumb to an alternative fate: repeat-associated non-ATG translation (RAN translation). This unconventional mode of translation, which occurs in the absence of an initiating codon, results in the production of polyGA, polyGP and polyGR peptides. Antibodies generated against these peptides detect high molecular weight, insoluble material in brain homogenates, as well as neuronal inclusions throughout the central nervous system of c9FTD/ALS cases. Given that both foci formation and RAN translation in c9FTD/ALS require the synthesis of GGGGCCexp RNA, therapeutic strategies that target these transcripts and result in their neutralization or degradation could effectively block these two potential pathogenic mechanisms and provide a much needed treatment for c9FTD/ALS.

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    • "These findings suggest that nuclear aggregation of PR peptides initiates a cascade of events leading to neurodegeneration via global perturbation in RNA processing and posttranscriptional regulation of gene expression. RESULTS RAN Translated Poly-PR Proteins Are Neurotoxic In vivo, pathogenic intronic G4C2 repeat expansions undergo RAN translation on both sense and antisense transcripts using all reading frames, giving rise to five dipeptide repeat proteins (DRPs) of presumably variable length (antisense: poly-PR and poly-PA; sense: poly-GA and poly-GR; both sense: poly-GP) (Ash et al., 2013; Gendron et al., 2013b; Mori et al., 2013; Zu et al., 2013). DRPs form aggregates in C9 patients in different neurons, including those not affected by disease (Gendron et al., 2013a; Zu et al., 2013). "
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    ABSTRACT: Expanded GGGGCC (G4C2) nucleotide repeats within the C9ORF72 gene are the most common genetic mutation associated with both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Sense and antisense transcripts of these expansions are translated to form five dipeptide repeat proteins (DRPs). We employed primary cortical and motor neuron cultures, live-cell imaging, and transgenic fly models and found that the arginine-rich dipeptides, in particular Proline-Arginine (PR), are potently neurotoxic. Factors that anticipated their neurotoxicity included aggregation in nucleoli, decreased number of processing bodies, and stress granule formation, implying global translational dysregulation as path accountable for toxicity. Nuclear PR aggregates were also found in human induced motor neurons and postmortem spinal cord tissues from C9ORF72 ALS and ALS/FTD patients. Intronic G4C2 transcripts, but not loss of C9ORF72 protein, are also toxic to motor and cortical neurons. Interestingly, G4C2 transcript-mediated neurotoxicity synergizes with that of PR aggregates, suggesting convergence of mechanisms.
    Neuron 12/2014; 84(6):1213-1225. DOI:10.1016/j.neuron.2014.12.010 · 15.05 Impact Factor
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    • "Third, our current study suggests that binding of mutant C9orf72 to trimethylated lysine residues within histones H3 and H4 causes repression of the C9orf72 gene, an event that may play an important role in the development of c9FTD/ALS. Taken together, early studies suggest a C9orf72 loss of function, both through neurotoxic processes and epigenetic changes, as a disease mechanism, and demonstrate that a better understanding of the C9orf72 protein function is crucial in order to identify therapeutic targets and to develop effective treatment strategies [21]. Additional research to examine DNA and histone methylation as a part of c9FTD/ALS pathogenesis may also lead to the identification of novel therapeutic targets for ALS and FTD. "
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    ABSTRACT: Individuals carrying (GGGGCC) expanded repeats in the C9orf72 gene represent a significant portion of patients suffering from amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Elucidating how these expanded repeats cause "c9FTD/ALS" has since become an important goal of the field. Toward this end, we sought to investigate whether epigenetic changes are responsible for the decrease in C9orf72 expression levels observed in c9FTD/ALS patients. We obtained brain tissue from ten c9FTD/ALS individuals, nine FTD/ALS cases without a C9orf72 repeat expansion, and nine disease control participants, and generated fibroblastoid cell lines from seven C9orf72 expanded repeat carriers and seven participants carrying normal alleles. Chromatin immunoprecipitation using antibodies for histone H3 and H4 trimethylated at lysines 9 (H3K9), 27 (H3K27), 79 (H3K79), and 20 (H4K20) revealed that these trimethylated residues bind strongly to C9orf72 expanded repeats in brain tissue, but not to non-pathogenic repeats. Our finding that C9orf72 mRNA levels are reduced in the frontal cortices and cerebella of c9FTD/ALS patients is consistent with trimethylation of these histone residues, an event known to repress gene expression. Moreover, treating repeat carrier-derived fibroblasts with 5-aza-2-deoxycytidine, a DNA and histone demethylating agent, not only decreased C9orf72 binding to trimethylated histone residues, but also increased C9orf72 mRNA expression. Our results provide compelling evidence that trimethylation of lysine residues within histones H3 and H4 is a novel mechanism involved in reducing C9orf72 mRNA expression in expanded repeat carriers. Of importance, we show that mutant C9orf72 binding to trimethylated H3K9 and H3K27 is detectable in blood of c9FTD/ALS patients. Confirming these exciting results using blood from a larger cohort of patients may establish this novel epigenetic event as a biomarker for c9FTD/ALS.
    Acta Neuropathologica 10/2013; 126(6). DOI:10.1007/s00401-013-1199-1 · 10.76 Impact Factor
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    ABSTRACT: Friedreich ataxia (FRDA) is a devastating neurodegenerative disease caused by mutations in the frataxin gene (FXN). Frataxin is an essential protein which localizes to the mitochondria and is required for the synthesis of iron-sulfur clusters and heme. Most individuals with FRDA are homozygous for trinucleotide GAA.TTC repeat expansions in intron 1 of FXN. The instability of these GAA.TTC repeats, the formation of non-B DNA GAA.TTC structures, and accompanying epigenetic changes lead to reduced FXN transcript and frataxin protein. This 'loss of frataxin' is considered the main driver of disease pathology with mitochondria-rich tissues such as the heart and the brain most affected. While our understanding of FRDA etiology has advanced in recent years, exactly how reduced frataxin leads to disease remains largely unknown. Most therapeutic strategies aim to increase frataxin, yet there are other underlying aspects of the molecular pathology that could impact disease progression and severity. These include RNA toxicity due to antisense RNAs, dysregulated splicing and microRNAs, and repeat-associated protein toxicity via RAN translation. Here we review the diverse array of molecular events that have been shown to influence clinical outcome in FRDA. We also examine additional pathogenic factors from other trinucleotide repeat diseases which could be potentially important in FRDA.
    Discovery medicine 01/2014; 17(91):25-35. · 3.63 Impact Factor
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