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The discovery that repeat expansions in the C9orf72 gene are a frequent cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) has revolutionized our understanding of these diseases. Substantial headway has been made in characterizing C9orf72-mediated disease and unravelling its underlying aetiopathogenesis. Three main disease mechanisms have been proposed: loss of function of the C9orf72 protein and toxic gain of function from C9orf72 repeat RNA or from dipeptide repeat proteins produced by repeat-associated non-ATG translation. Several downstream processes across a range of cellular functions have also been implicated. In this article, we review the pathological and mechanistic features of C9orf72-associated FTD and ALS (collectively termed C9FTD/ALS), the model systems used to study these conditions, and the probable initiators of downstream disease mechanisms. We suggest that a combination of upstream mechanisms involving both loss and gain of function and downstream cellular pathways involving both cell-autonomous and non-cell-autonomous effects contributes to disease progression.

Mutations in the TARDBP gene, which encodes the TDP-43 protein, account for only 3–5% of familial cases of amyotrophic lateral sclerosis and less than 1% of cases that are apparently idiopathic. However, the discovery of neuronal inclusions of TDP-43 as the neuropathological hallmark in the majority of cases of amyotrophic lateral sclerosis has transformed our understanding of the pathomechanisms underlying neurodegeneration. An individual TARDBP mutation can cause phenotypic heterogeneity. Most mutations lie within the C-terminus of the TDP-43 protein. In pathological conditions, TDP-43 is mislocalised from the nucleus to the cytoplasm, where it can be phosphorylated, cleaved, and form insoluble aggregates. This mislocalisation leads to dysfunction of downstream pathways of RNA metabolism, proteostasis, mitochondrial function, oxidative stress, axonal transport, and local translation. Biomarkers for TDP-43 dysfunction and targeted therapies are being developed, justifying cautious optimism for personalised medicine approaches that could rescue the downstream effects of TDP-43 pathology.

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases that exist on a clinico-pathogenetic spectrum, designated ALS/FTD. The most common genetic cause of ALS/FTD is expansion of the intronic hexanucleotide repeat (GGGGCC) n in C9orf72 . Here, we investigate the formation of nucleic acid secondary structures in these expansion repeats, and their role in generating condensates characteristic of ALS/FTD. We observe significant aggregation of the hexanucleotide sequence (GGGGCC) n , which we associate to the formation of multimolecular G-quadruplexes (mG4s) by using a range of biophysical techniques. Exposing the condensates to G4-unfolding conditions leads to prompt disassembly, highlighting the key role of mG4-formation in the condensation process. We further validate the biological relevance of our findings by detecting an increased prevalence of G4-structures in C9orf72 mutant human motor neurons when compared to healthy motor neurons by staining with a G4-selective fluorescent probe, revealing signal in putative condensates. Our findings strongly suggest that RNA G-rich repetitive sequences can form protein-free condensates sustained by multimolecular G-quadruplexes, highlighting their potential relevance as therapeutic targets for C9orf72 mutation-related ALS/FTD. A common genetic cause of Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is expansion of the intronic hexanucleotide repeat (GGGGCC) n in C9orf72 . Here the authors reveal that the RNA (GGGGCC) n expansion repeat associated with ALS/FTD can generate condensates in the absence of proteins, highlighting the potential relevance of targeting RNA-structures to treat neurodegenerative diseases.

Intronic GGGGCC repeat expansions in C9orf72 are the most common known cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), which are characterised by degeneration of cortical and motor neurons, respectively. Repeat expansions have been proposed to cause disease by both the repeat RNA forming foci that sequester RNA‐binding proteins and through toxic dipeptide repeat proteins generated by repeat‐associated non‐ATG translation. GGGGCC repeat RNA folds into a G‐quadruplex secondary structure, and we investigated whether targeting this structure is a potential therapeutic strategy. We performed a screen that identified three structurally related small molecules that specifically stabilise GGGGCC repeat G‐quadruplex RNA. We investigated their effect in C9orf72 patient iPSC‐derived motor and cortical neurons and show that they significantly reduce RNA foci burden and the levels of dipeptide repeat proteins. Furthermore, they also reduce dipeptide repeat proteins and improve survival in vivo , in GGGGCC repeat‐expressing Drosophila . Therefore, small molecules that target GGGGCC repeat G‐quadruplexes can ameliorate the two key pathologies associated with C9orf72 FTD/ALS. These data provide proof of principle that targeting GGGGCC repeat G‐quadruplexes has therapeutic potential. Synopsis Small molecules targeting G‐quadruplex GGGGCC repeat RNA are effective at ameliorating disease phenotypes in C9orf72 patient neurons, and in vivo phenotypes in C9orf72 flies. Therefore, targeting expanded GGGGCC RNA could be an effective therapeutic strategy for C9orf72 ALS and FTD. FRET based screen identifies small molecules that specifically bind to C9orf72 repeat RNA G‐quadruplexes. Small molecules reduce RNA foci and dipeptide repeat proteins (DPRs) in C9orf72 patient neurons. G‐quadruplex GGGGCC binding small molecule improves survival and reduces levels of the toxic DPR poly‐GR in C9orf72 flies. Provides proof of principle for targeting GGGGCC RNA G‐quadruplexes in C9orf72 FTD/ALS. Graphical Abstract Small molecules targeting G‐quadruplex GGGGCC repeat RNA are effective at ameliorating disease phenotypes in C9orf72 patient neurons, and in vivo phenotypes in C9orf72 flies. Therefore, targeting expanded GGGGCC RNA could be an effective therapeutic strategy for C9orf72 ALS and FTD.

Objectives The use of clinical staging in the fatal neurodegenerative disease amyotrophic lateral sclerosis would have value in optimising future therapeutic trials. We aimed to use previous clinical trial data to determine the length of time patients spend in each of four proposed stages, its range and transition patterns to subsequent stages. Methods Using databases from two multicentre clinical trials, patients were retrospectively staged through the trial course. At each stage we assessed whether patients then progressed to an earlier, consecutive or later stage or death. Duration spent in each stage before progression to a later stage was calculated. Results There were 725 patients. No patients moved to an earlier stage. More patients at stages 1, 2 and 3 progressed to the consecutive stage rather than skipping a stage. 59.3% of patients at Stage 1 progressed to Stage 2, 54.0% of patients at Stage 2 progressed to Stage 3, 42.3% of patients at Stage 3 progressed to Stage 4 and 47.0% of Stage 4 patients progressed to death. Transition times between stages had a median duration of 3 to 7 months for stages 2 to 4. Discussion We have shown using trial data that transition times between stages are short. Use of stage duration as an endpoint might allow a shorter trial duration. We have shown face validity in this system as most patients progress through consecutive stages, and none revert to earlier stages. Furthermore, we have shown the system is reliable across populations and therefore has content validity.

An intronic GGGGCC expansion in C9orf72 is the most common known cause of both frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). The repeat expansion leads to the generation of sense and antisense repeat RNA aggregates and dipeptide repeat (DPR) proteins, generated by repeat-associated non-ATG translation. The arginine-rich DPR proteins poly(glycine-arginine or GR) and poly(proline-arginine or PR) are potently neurotoxic and can localise to the nucleolus when expressed in cells, resulting in enlarged nucleoli with disrupted functionality. Furthermore, GGGGCC repeat RNA can bind nucleolar proteins in vitro. However, the relevance of nucleolar stress is unclear, as the arginine-rich DPR proteins do not localise to the nucleolus in C9orf72 -associated FTLD/ALS (C9FTLD/ALS) patient brain. We measured nucleolar size in C9FTLD frontal cortex neurons using a three-dimensional, volumetric approach. Intriguingly, we found that C9FTLD brain exhibited bidirectional nucleolar stress. C9FTLD neuronal nucleoli were significantly smaller than control neuronal nucleoli. However, within C9FTLD brains, neurons containing poly(GR) inclusions had significantly larger nucleolar volumes than neurons without poly(GR) inclusions. In addition, expression of poly(GR) in adult Drosophila neurons led to significantly enlarged nucleoli. A small but significant increase in nucleolar volume was also observed in C9FTLD frontal cortex neurons containing GGGGCC repeat-containing RNA foci. These data show that nucleolar abnormalities are a consistent feature of C9FTLD brain, but that diverse pathomechanisms are at play, involving both DPR protein and repeat RNA toxicity.

A hexanucleotide repeat expansion in the C9orf72 gene is a common genetic cause of ALS and FTD. The repeats are translated into five different dipeptide repeat proteins (DPRs). In this issue, Lehmer et al (2017) demonstrate that one of these DPRs, poly(GP), can be measured in the CSF of individuals with C9orf72 mutations. In conjunction with the findings from another recent study (Gendron et al , 2017), these DPR biomarkers may prove to be extremely valuable in the quest for effective therapies for C9FTD/ALS. Graphical Abstract Isaacs and colleagues discuss two recent studies showing how a dipeptide repeat protein, poly(GP), can be measured in the CSF of individuals with C9orf72 mutations and used as a valuable biomarker in the quest for effective therapies for C9FTD/ALS.

An intronic GGGGCC repeat expansion in C9orf72 is a common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. The repeats are transcribed in both sense and antisense directions to generate distinct dipeptide repeat proteins, of which poly(GA), poly(GR), and poly(PR) have been implicated in contributing to neurodegeneration. Poly(PR) binding to RNA may contribute to toxicity, but analysis of poly(PR)-RNA binding on a transcriptome-wide scale has not yet been carried out. We therefore performed crosslinking and immunoprecipitation (CLIP) analysis in human cells to identify the RNA binding sites of poly(PR). We found that poly(PR) binds to nearly 600 RNAs, with the sequence GAAGA enriched at the binding sites. In vitro experiments showed that poly(GAAGA) RNA binds poly(PR) with higher affinity than control RNA and induces the phase separation of poly(PR) into condensates. These data indicate that poly(PR) preferentially binds to poly(GAAGA)-containing RNAs, which may have physiological consequences.

A hexanucleotide expansion in C9orf72 is a common cause of the fatal neurodegenerative disorder amyotrophic lateral sclerosis. We have found evidence in a Drosophila model that neurotoxicity is mediated by dipeptide repeat (DPR) proteins generated by repeat-associated non-ATG translation. Here we aimed to evaluate in models of amyotrophic lateral sclerosis caused by the C9orf72 mutation (C9orf72-ALS) whether DPR proteins cause nucleolar dysfunction and whether novel small molecules that bind C9orf72-repeat RNA reduce DPR formation and neurotoxicity. We assessed nucleolar function in in-vivo Drosophila models. Nucleolar size was measured with immunofluorescence and confocal microscopy, using automated image analysis. Human induced pluripotent stem cells (iPSCs) from patients with C9orf72-ALS and from healthy controls were taken through neural induction and patterning to derive spinal motor neuron populations. Disease phenotypes were measured with fluorescence in-situ hybridisation for the typical RNA foci seen in C9orf72-ALS patients. Small molecules binding to C9orf72-repeat RNA were fed to C9orf72 Drosophila and applied to the human iPSC-derived spinal motor neurons to evaluate rescue of disease phenotypes. DPR proteins colocalised with nucleoli in C9orf72-Drosophila brain tissue, with effects on nucleolar morphology. C9orf72-Drosophila had significantly reduced egg-to-adult viability (p<0·05), and the bioavailability of the small molecules in Drosophila was investigated. Emerging evidence suggests that nucleolar dysfunction is a key mechanism in C9orf72-ALS. It is crucial that this finding is validated in relevant disease models to rapidly translate findings into promising therapeutic targets. The high prevalence of C9orf72-ALS makes use of targeted therapies a compelling strategy. These experiments might provide novel mechanistic insights into a common form of amyotrophic lateral sclerosis and deliver preclinical data on an exciting therapeutic approach. RB is a Leonard Wolfson Clinical Research Training Fellow and is funded by a Wellcome Trust Clinical Research Training Fellowship (107196/Z/14/Z).

It is well established that L-NAME, a generic NOS inhibitor, stimulates neurogenesis in the dentate gyrus of the adult rat and corticosterone reduces it. These experiments explore the interaction between L-NAME and corticosterone. L-NAME (50 mg/kg), as expected, increased proliferation, but also lowered plasma corticosterone levels. However, the stimulating action of L-NAME depends on the presence of rhythmic changes in plasma corticosterone, as it is abolished in rats treated with a subcutaneous implant of corticosterone, which flattens the diurnal rhythm. Adrenalectomized rats implanted with corticosterone also failed to respond to L-NAME. Giving them a single daily injection of corticosterone (2 mg/kg) in an attempt to replicate the diurnal rhythm restored the sensitivity of the progenitor cells to L-NAME. The mechanism for this result remains to be investigated. Excess corticosterone given by daily injection (40/mg/kg) reduced proliferation but did not alter the response to L-NAME, even though this occurred from a lower baseline. nNOS was demonstrable only in the inner (proliferative) layer of the dentate gyrus in control rats, and did not alter following excess corticosterone treatment. iNOS was detectable at low levels in control rats, but was increased markedly following corticosterone. eNOS was evident throughout the dentate gyrus, and also increased after corticosterone (particularly in the hilus). Aminoguanidine (100 mg/kg/day; an iNOS antagonist) significantly increased proliferation in corticosterone-treated rats (40 mg/kg/day) but not in controls without additional corticosterone, confirming that iNOS plays a role in corticosterone-regulated neurogenesis. Corticosterone may thus act on progenitor cells in part at least through increased nitric oxide (NO) formation. The effects of reduced NO on neurogenesis may rely on a dual mechanism: corresponding reductions in plasma corticosterone and increased induction of iNOS (and/or eNOS) within the dentate gyrus. The possibility that NO acts downstream of glucocorticoids in the dentate gyrus is suggested.