Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
Review
. 2016 Sep 15:1647:19-29.
doi: 10.1016/j.brainres.2016.04.004. Epub 2016 Apr 6.

There has been an awakening: Emerging mechanisms of C9orf72 mutations in FTD/ALS

Aaron D Gitler  1 , Hitomi Tsuiji  2
Affiliations
Review

There has been an awakening: Emerging mechanisms of C9orf72 mutations in FTD/ALS

Aaron D Gitler et al. Brain Res. .

Abstract

The discovery of C9orf72 mutations as the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) has awakened a surge of interest in deciphering how mutations in this mysterious gene cause disease and what can be done to stop it. C9orf72 harbors a hexanucleotide repeat, GGGGCC, in a non-coding region of the gene and a massive expansion of this repeat causes ALS, FTD, or both (FTD/ALS). Many questions lie ahead. What does this gene normally do? What is the consequence of an enormous GGGGCC repeat expansion on that gene's function? Could that hexanucleotide repeat expansion have additional pathological actions unrelated to C9orf72 function? There has been tremendous progress on all fronts in the quest to define how C9orf72 mutations cause disease. Many new experimental models have been constructed and unleashed in powerful genetic screens. Studies in mouse and human patient samples, including iPS-derived neurons, have provided unprecedented insights into pathogenic mechanisms. Three major hypotheses have emerged and are still being hotly debated in the field. These include (1) loss of function owing to decrease in the abundance of C9orf72 protein and its ability to carryout its still unknown cellular role; (2) RNA toxicity from bidirectionally transcribed sense (GGGGCC) and antisense (GGCCCC) transcripts that accumulate in RNA foci and might sequester critical RNA-binding proteins; (3) proteotoxicity from dipeptide repeat proteins produced by an unconventional form of translation from the expanded nucleotide repeats. Here we review the evidence in favor and against each of these three hypotheses. We also suggest additional experiments and considerations that we propose will help clarify which mechanism(s) are most important for driving disease and therefore most critical for considering during the development of therapeutic interventions. This article is part of a Special Issue entitled SI:RNA Metabolism in Disease.

Keywords: ALS, FTD; C9orf72; Dipeptide repeat protein; RNA.

PubMed Disclaimer

Figures

Figure 1
Figure 1. C9orf72 mutations: three proposed pathomechanisms
A) The C9orf72 gene harbors a polymorphic hexanucleotide (GGGGCC) repeat in a non-coding region of the gene. Large expansions of this nucleotide repeat cause c9FTD/ALS. There are currently three major hypotheses to explain how such repeat expansions could be pathogenic. B) The large GGGGCC repeat expansion could cause a downregulation in C9orf72 gene expression by interfering with transcription, leading to a decrease in C9orf72 protein and a loss of C9orf72’s function. C) RNA transcripts harboring C9orf72 repeat expansions are produced by both sense and antisense transcription, resulting in the accumulation of nuclear or cytoplasmic foci of GGGGCC RNA as well as the antisense GGCCCC RNA, which could cause the sequestration of essential RNA-binding proteins (RBP), including splicing factors, leading to defects in pre-mRNA splicing by an RNA toxicity mechanism. D) Sense and antisense repeat RNAs are substrates for an unconventional form of translation to generate a series of dipeptide repeat proteins, which accumulate in the brain and spinal cord of C9orf72 mutation carriers and may cause disease by dipeptide repeat protein toxicity mechanism. Figure adapted from (Ling et al., 2013).
Figure 2
Figure 2. Additional experiments to test C9orf72 loss of function
A) Mice have been generated in which the β–galactosidase gene replaces exons 2–6 of one of the C9orf72 alleles (Suzuki et al., 2013). These mice could be intercrossed to generate homozygous mutant mice (Atanasio et al., 2016; O’Rourke et al., 2016) and, together with their heterozygous littermates, extensively analyzed for any effects on pathological phenotypes, survival and cognitive or motor behavioral impairments. B) Crossing transgenic mice containing a human BAC with a fragment of the C9orf72 locus harboring ~500 GGGGCC repeats (e.g., Peters et al., 2015) to the C9orf72 knockout mice will test if disease is accelerated by reducing wild type C9orf72 function. C) Injecting the C9orf72 transgene (Chew et al., 2015) into the central nervous system of C9orf72 WT, +/−, or −/− animals will test if disease features are accelerated by the reduction of wild type C9orf72. D) iPS derived from c9FTD/ALS patients have been reported to exhibit phenotypic differences from control neurons, including glutamate excitotoxicity, sensitivity to ER stress, and alterations in electrical activity. If these phenotypes are due to loss of C9orf72 function, then increasing C9orf72 levels should mitigate them and lowering C9orf72 levels should worsen them. E) C9orf72 may function as a guanine nucleotide exchange factor (GEF) to regulate Rab GTPase activity. Rabs orchestrate multiple steps of membrane trafficking within cells and it will be important to define which Rab and thus which trafficking step C9orf72 regulates.
Figure 3
Figure 3. Additional experiments to test C9orf72 RNA toxicity
A) Drosophila has been used to disentangle the contributions of C9orf72 RNA toxicity and dipeptide repeat proteins (Mizielinska et al., 2014). Flies expressing a GGGGCC expanded repeat produce RNA foci and dipeptide repeat proteins (DPR), and exhibit neurodegenerative phenotypes (e.g., rough eye). Engineering stop codons into the GGGGCC transgene maintains RNA foci but abolishes DPR production, and mitigates the degenerative phenotypes. B) The new viral vector transgenic mouse model (Chew et al., 2015) could be used in a way similar to the fly experiments, to test relative roles of RNA and DPRs towards neurodegenerative phenotypes. Constructs could be generated that have Stop codons interrupting the repeats or flanking the repeats, in order to prevent translation but preserve RNA foci formation. These mice could be assessed for pathological features (TDP-43, RNA foci, DPRs) as well as neurodegeneration and cognitive deficits.
Figure 4
Figure 4. Additional experiments to test C9orf72 dipeptide repeat protein toxicity
A) To specifically block RAN translation will require elucidating RAN translation mechanisms and identifying RAN translation-specific regulators. These putative regulators will be new targets for the development of small molecule inhibitors to specifically inhibit RAN translation. B) The development of positron emission tomography (PET) ligands to detect DPR pathology in vivo would allow longitudinal studies of C9orf72 mutation carriers to help resolve the role of DPRs in disease pathogenesis and to eventually be used in clinical trial settings to assess efficacy of candidate therapeutics.
See this image and copyright information in PMC

Similar articles

See all similar articles

Cited by

  • Divergence, Convergence, and Therapeutic Implications: A Cell Biology Perspective of C9ORF72-ALS/FTD.
    Tang X, Toro A, T G S, Gao J, Chalk J, Oskarsson B, Zhang K. Tang X, et al. Mol Neurodegener. 2020 Jun 8;15(1):34. doi: 10.1186/s13024-020-00383-7. Mol Neurodegener. 2020. PMID: 32513219 Free PMC article. Review.
  • Poly(GP) proteins are a useful pharmacodynamic marker for C9ORF72-associated amyotrophic lateral sclerosis.
    Gendron TF, Chew J, Stankowski JN, Hayes LR, Zhang YJ, Prudencio M, Carlomagno Y, Daughrity LM, Jansen-West K, Perkerson EA, O'Raw A, Cook C, Pregent L, Belzil V, van Blitterswijk M, Tabassian LJ, Lee CW, Yue M, Tong J, Song Y, Castanedes-Casey M, Rousseau L, Phillips V, Dickson DW, Rademakers R, Fryer JD, Rush BK, Pedraza O, Caputo AM, Desaro P, Palmucci C, Robertson A, Heckman MG, Diehl NN, Wiggs E, Tierney M, Braun L, Farren J, Lacomis D, Ladha S, Fournier CN, McCluskey LF, Elman LB, Toledo JB, McBride JD, Tiloca C, Morelli C, Poletti B, Solca F, Prelle A, Wuu J, Jockel-Balsarotti J, Rigo F, Ambrose C, Datta A, Yang W, Raitcheva D, Antognetti G, McCampbell A, Van Swieten JC, Miller BL, Boxer AL, Brown RH, Bowser R, Miller TM, Trojanowski JQ, Grossman M, Berry JD, Hu WT, Ratti A, Traynor BJ, Disney MD, Benatar M, Silani V, Glass JD, Floeter MK, Rothstein JD, Boylan KB, Petrucelli L. Gendron TF, et al. Sci Transl Med. 2017 Mar 29;9(383):eaai7866. doi: 10.1126/scitranslmed.aai7866. Sci Transl Med. 2017. PMID: 28356511 Free PMC article.
  • Novel genes associated with amyotrophic lateral sclerosis: diagnostic and clinical implications.
    Chia R, Chiò A, Traynor BJ. Chia R, et al. Lancet Neurol. 2018 Jan;17(1):94-102. doi: 10.1016/S1474-4422(17)30401-5. Epub 2017 Nov 16. Lancet Neurol. 2018. PMID: 29154141 Free PMC article. Review.
  • Mitophagy regulation in aging and neurodegenerative disease.
    Banarase TA, Sammeta SS, Wankhede NL, Mangrulkar SV, Rahangdale SR, Aglawe MM, Taksande BG, Upaganlawar AB, Umekar MJ, Kale MB. Banarase TA, et al. Biophys Rev. 2023 Apr 4;15(2):239-255. doi: 10.1007/s12551-023-01057-6. eCollection 2023 Apr. Biophys Rev. 2023. PMID: 37124925 Free PMC article. Review.
  • Role of C9orf72 hexanucleotide repeat expansions in ALS/FTD pathogenesis.
    Geng Y, Cai Q. Geng Y, et al. Front Mol Neurosci. 2024 Jan 22;17:1322720. doi: 10.3389/fnmol.2024.1322720. eCollection 2024. Front Mol Neurosci. 2024. PMID: 38318532 Free PMC article. Review.
See all "Cited by" articles

References

    1. Aguzzi A, Gitler AD. A template for new drugs against Alzheimer’s disease. Cell. 2013;154:1182–4. - PubMed
    1. Almeida S, et al. Modeling key pathological features of frontotemporal dementia with C9ORF72 repeat expansion in iPSC-derived human neurons. Acta Neuropathol. 2013;126:385–99. - PMC - PubMed
    1. Ash PE, et al. Unconventional translation of C9ORF72 GGGGCC expansion generates insoluble polypeptides specific to c9FTD/ALS. Neuron. 2013;77:639–46. - PMC - PubMed
    1. Atanasio A, et al. C9orf72 ablation causes immune dysregulation characterized by leukocyte expansion, autoantibody production, and glomerulonephropathy in mice. Sci Rep. 2016;6:23204. - PMC - PubMed
    1. Banez-Coronel M, et al. RAN Translation in Huntington Disease. Neuron. 2015;88:667–77. - PMC - PubMed