Abundant transcriptomic alterations in the human cerebellum of patients with a C9orf72 repeat expansion.

The most prominent genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) is a repeat expansion in the gene C9orf72. Importantly, the transcriptomic consequences of the C9orf72 repeat expansion remain largely unclear. Here, we used short-read RNA sequencing (RNAseq) to profile the cerebellar transcriptome, detecting alterations in patients with a C9orf72 repeat expansion. We focused on the cerebellum, since key C9orf72-related pathologies are abundant in this neuroanatomical region, yet TDP-43 pathology and neuronal loss are minimal. Consistent with previous work, we showed a reduction in the expression of the C9orf72 gene and an elevation in homeobox genes, when comparing patients with the expansion to both patients without the C9orf72 repeat expansion and control subjects. Interestingly, we identified more than 1000 alternative splicing events, including 4 in genes previously associated with ALS and/or FTLD. We also found an increase of cryptic splicing in C9orf72 patients compared to patients without the expansion and controls. Furthermore, we demonstrated that the expression level of select RNA-binding proteins is associated with cryptic splice junction inclusion. Overall, this study explores the presence of widespread transcriptomic changes in the cerebellum, a region not confounded by severe neurodegeneration, in post-mortem tissue from C9orf72 patients.

Investigating Repeat Expansions in NIPA1, NOP56, and NOTCH2NLC Genes: A Closer Look at Amyotrophic Lateral Sclerosis Patients from Southern Italy.

The discovery of hexanucleotide repeats expansion (RE) in Chromosome 9 Open Reading frame 72 (C9orf72) as the major genetic cause of amyotrophic lateral sclerosis (ALS) and the association between intermediate repeats in Ataxin-2 (ATXN2) with the disorder suggest that repetitive sequences in the human genome play a significant role in ALS pathophysiology. Investigating the frequency of repeat expansions in ALS in different populations and ethnic groups is therefore of great importance. Based on these premises, this study aimed to define the frequency of REs in the NIPA1, NOP56, and NOTCH2NLC genes and the possible associations between phenotypes and the size of REs in the Italian population. Using repeat-primed-PCR and PCR-fragment analyses, we screened 302 El-Escorial-diagnosed ALS patients and compared the RE distribution to 167 age-, gender-, and ethnicity-matched healthy controls. While the REs distribution was similar between the ALS and control groups, a moderate association was observed between longer RE lengths and clinical features such as age at onset, gender, site of onset, and family history. In conclusion, this is the first study to screen ALS patients from southern Italy for REs in NIPA1, NOP56, and NOTCH2NLC genes, contributing to our understanding of ALS genetics. Our results highlighted that the extremely rare pathogenic REs in these genes do not allow an association with the disease.

Reversal of C9orf72 mutation-induced transcriptional dysregulation and pathology in cultured human neurons by allele-specific excision.

Efforts to genetically reverse C9orf72 pathology have been hampered by our incomplete understanding of the regulation of this complex locus. We generated five different genomic excisions at the C9orf72 locus in a patient-derived induced pluripotent stem cell (iPSC) line and a non-diseased wild-type (WT) line (11 total isogenic lines), and examined gene expression and pathological hallmarks of C9 frontotemporal dementia/amyotrophic lateral sclerosis in motor neurons differentiated from these lines. Comparing the excisions in these isogenic series removed the confounding effects of different genomic backgrounds and allowed us to probe the effects of specific genomic changes. A coding single nucleotide polymorphism in the patient cell line allowed us to distinguish transcripts from the normal vs. mutant allele. Using digital droplet PCR (ddPCR), we determined that transcription from the mutant allele is upregulated at least 10-fold, and that sense transcription is independently regulated from each allele. Surprisingly, excision of the WT allele increased pathologic dipeptide repeat poly-GP expression from the mutant allele. Importantly, a single allele was sufficient to supply a normal amount of protein, suggesting that the C9orf72 gene is haplo-sufficient in induced motor neurons. Excision of the mutant repeat expansion reverted all pathology (RNA abnormalities, dipeptide repeat production, and TDP-43 pathology) and improved electrophysiological function, whereas silencing sense expression did not eliminate all dipeptide repeat proteins, presumably because of the antisense expression. These data increase our understanding of C9orf72 gene regulation and inform gene therapy approaches, including antisense oligonucleotides (ASOs) and CRISPR gene editing.

PolyGR and polyPR knock-in mice reveal a conserved neuroprotective extracellular matrix signature in C9orf72 ALS/FTD neurons.

Dipeptide repeat proteins are a major pathogenic feature of C9orf72 amyotrophic lateral sclerosis (C9ALS)/frontotemporal dementia (FTD) pathology, but their physiological impact has yet to be fully determined. Here we generated C9orf72 dipeptide repeat knock-in mouse models characterized by expression of 400 codon-optimized polyGR or polyPR repeats, and heterozygous C9orf72 reduction. (GR)400 and (PR)400 knock-in mice recapitulate key features of C9ALS/FTD, including cortical neuronal hyperexcitability, age-dependent spinal motor neuron loss and progressive motor dysfunction. Quantitative proteomics revealed an increase in extracellular matrix (ECM) proteins in (GR)400 and (PR)400 spinal cord, with the collagen COL6A1 the most increased protein. TGF-ß1 was one of the top predicted regulators of this ECM signature and polyGR expression in human induced pluripotent stem cell neurons was sufficient to induce TGF-ß1 followed by COL6A1. Knockdown of TGF-ß1 or COL6A1 orthologues in polyGR model Drosophila exacerbated neurodegeneration, while expression of TGF-ß1 or COL6A1 in induced pluripotent stem cell-derived motor neurons of patients with C9ALS/FTD protected against glutamate-induced cell death. Altogether, our findings reveal a neuroprotective and conserved ECM signature in C9ALS/FTD.

eIF5 stimulates the CUG initiation of RAN translation of poly-GA dipeptide repeat protein (DPR) in C9orf72 FTLD/ALS.

Tandem GGGGCC repeat expansion in C9orf72 is a genetic cause of frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Transcribed repeats are translated into dipeptide repeat proteins via repeat-associated non-AUG (RAN) translation. However, the regulatory mechanism of RAN translation remains unclear. Here, we reveal a GTPase-activating protein, eukaryotic initiation factor 5 (eIF5), which allosterically facilitates the conversion of eIF2-bound GTP into GDP upon start codon recognition, as a novel modifier of C9orf72 RAN translation. Compared to global translation, eIF5, but not its inactive mutants, preferentially stimulates poly-GA RAN translation. RAN translation is increased during integrated stress response, but the stimulatory effect of eIF5 on poly-GA RAN translation was additive to the increase of RAN translation during integrated stress response, with no further increase in phosphorylated eIF2α. Moreover, an alteration of the CUG near cognate codon to CCG or AUG in the poly-GA reading frame abolished the stimulatory effects, indicating that eIF5 primarily acts through the CUG-dependent initiation. Lastly, in a Drosophila model of C9orf72 FTLD/ALS that expresses GGGGCC repeats in the eye, knockdown of endogenous eIF5 by two independent RNAi strains significantly reduced poly-GA expressions, confirming in vivo effect of eIF5 on poly-GA RAN translation. Together, eIF5 stimulates the CUG initiation of poly-GA RAN translation in cellular and Drosophila disease models of C9orf72 FTLD/ALS.

Resting-state fMRI functional connectome of C9orf72 mutation status.

OBJECTIVE: The resting-state functional connectome has not been extensively investigated in amyotrophic lateral sclerosis (ALS) spectrum disease, in particular in relationship with patients' genetic status. METHODS: Here we studied the network-to-network connectivity of 19 ALS patients carrying the C9orf72 hexanucleotide repeat expansion (C9orf72+), 19 ALS patients not affected by C9orf72 mutation (C9orf72-), and 19 ALS-mimic patients (ALSm) well-matched for demographic and clinical variables. RESULTS: When compared with ALSm, we observed greater connectivity of the default mode and frontoparietal networks with the visual network for C9orf72+ patients (P = 0.001). Moreover, the whole-connectome showed greater node degree (P < 0.001), while sensorimotor cortices resulted isolated in C9orf72+. INTERPRETATION: Our results suggest a crucial involvement of extra-motor functions in ALS spectrum disease. In particular, alterations of the visual cortex may have a pathogenic role in C9orf72-related ALS. The prominent feature of these patients would be increased visual system connectivity with the networks responsible of the functional balance between internal and external attention.

The shared ancestry between the C9orf72 hexanucleotide repeat expansion and intermediate-length alleles using haplotype sharing trees and HAPTK.

The C9orf72 hexanucleotide repeat expansion (HRE) is a common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The inheritance is autosomal dominant, but a high proportion of subjects with the mutation are simplex cases. One possible explanation is de novo expansions of unstable intermediate-length alleles (IAs). Using haplotype sharing trees (HSTs) with the haplotype analysis tool kit (HAPTK), we derived majority-based ancestral haplotypes of HRE samples and discovered that IAs containing ≥18-20 repeats share large haplotypes in common with the HRE. Using HSTs of HRE and IA samples, we demonstrate that the longer IA haplotypes are largely indistinguishable from HRE haplotypes and that several ≥18-20 IA haplotypes share over 5 Mb (>600 markers) haplotypes in common with the HRE haplotypes. These analysis tools allow physical understanding of the haplotype blocks shared with the majority-based ancestral haplotype. Our results demonstrate that the haplotypes with longer IAs belong to the same pool of haplotypes as the HRE and suggest that longer IAs represent potential premutation alleles.

Assessment of risk of ALS conferred by the GGGGCC hexanucleotide repeat expansion in C9orf72 among first-degree relatives of patients with ALS carrying the repeat expansion.

OBJECTIVES: We aimed to estimate the age-related risk of ALS in first-degree relatives of patients with ALS carrying the C9orf72 repeat expansion. METHODS: We included all patients with ALS carrying a C9orf72 repeat expansion in The Netherlands. Using structured questionnaires, we determined the number of first-degree relatives, their age at death due to ALS or another cause, or age at time of questionnaire. The cumulative incidence of ALS among first-degree relatives was estimated, while accounting for death from other causes. Variability in ALS risk between families was evaluated using a random effects hazards model. We used a second, distinct approach to estimate the risk of ALS and FTD in the general population, using previously published data. RESULTS: In total, 214 of the 2,486 (9.2%) patients with ALS carried the C9orf72 repeat expansion. The mean risk of ALS at age 80 for first-degree relatives carrying the repeat expansion was 24.1%, but ranged between individual families from 16.0 to 60.6%. Using the second approach, we found the risk of ALS and FTD combined was 28.7% (95% CI 17.8%-54.3%) for carriers in the general population. CONCLUSIONS: On average, our estimated risk of ALS in the C9orf72 repeat expansion was lower compared to historical estimates. We showed, however, that the risk of ALS likely varies between families and one overall penetrance estimate may not be sufficient to describe ALS risk. This warrants a tailor-made, patient-specific approach in testing. Further studies are needed to assess the risk of FTD in the C9orf72 repeat expansion.

Analysis of normal C9orf72 repeat length as possible disease modifier in amyotrophic lateral sclerosis.

The C9orf72 hexanucleotide repeat (HR) expansion is the main genetic cause of amyotrophic lateral sclerosis (ALS), with expansion size from 30 to >4000 units. Normal C9orf72 HR length is polymorphic (2-23 repeats) with alleles >8 units showing a low frequency in the general population. This study aimed to investigate if the normal C9orf72 HR length influences C9orf72 gene expression and acts as disease modifier in ALS patients negative for C9orf72 mutation (ALS-C9Neg). We found that the distribution of HR alleles was similar in 325 ALS-C9Neg and 303 healthy controls. Gene expression analysis in blood revealed a significant increase of total C9orf72 and V3 mRNA levels in ALS-C9Neg carrying two long alleles (L/L; ≥8 units) compared to patients homozygous for the 2-unit short allele (S/S). However, HR allele genotypes (L/L, S/L, S/S) correlated with no clinical parameters. Our data suggest that normal C9orf72 HR length does not act as disease modifier in ALS-C9Neg despite increasing gene expression.

The ALS/FTD-related C9orf72 hexanucleotide repeat expansion forms RNA condensates through multimolecular G-quadruplexes.

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.

Senataxin helicase, the causal gene defect in ALS4, is a significant modifier of C9orf72 ALS G4C2 and arginine-containing dipeptide repeat toxicity.

Identifying genetic modifiers of familial amyotrophic lateral sclerosis (ALS) may reveal targets for therapeutic modulation with potential application to sporadic ALS. GGGGCC (G4C2) repeat expansions in the C9orf72 gene underlie the most common form of familial ALS, and generate toxic arginine-containing dipeptide repeats (DPRs), which interfere with membraneless organelles, such as the nucleolus. Here we considered senataxin (SETX), the genetic cause of ALS4, as a modifier of C9orf72 ALS, because SETX is a nuclear helicase that may regulate RNA-protein interactions involved in ALS dysfunction. After documenting that decreased SETX expression enhances arginine-containing DPR toxicity and C9orf72 repeat expansion toxicity in HEK293 cells and primary neurons, we generated SETX fly lines and evaluated the effect of SETX in flies expressing either (G4C2)58 repeats or glycine-arginine-50 [GR(50)] DPRs. We observed dramatic suppression of disease phenotypes in (G4C2)58 and GR(50) Drosophila models, and detected a striking relocalization of GR(50) out of the nucleolus in flies co-expressing SETX. Next-generation GR(1000) fly models, that show age-related motor deficits in climbing and movement assays, were similarly rescued with SETX co-expression. We noted that the physical interaction between SETX and arginine-containing DPRs is partially RNA-dependent. Finally, we directly assessed the nucleolus in cells expressing GR-DPRs, confirmed reduced mobility of proteins trafficking to the nucleolus upon GR-DPR expression, and found that SETX dosage modulated nucleolus liquidity in GR-DPR-expressing cells and motor neurons. These findings reveal a hitherto unknown connection between SETX function and cellular processes contributing to neuron demise in the most common form of familial ALS.

The molecular mechanisms of spinocerebellar ataxias for DNA repeat expansion in disease.

Spinocerebellar ataxias (SCAs) are a heterogenous group of neurodegenerative disorders which commonly inherited in an autosomal dominant manner. They cause muscle incoordination due to degeneration of the cerebellum and other parts of nervous system. Out of all the characterized (>50) SCAs, 14 SCAs are caused due to microsatellite repeat expansion mutations. Repeat expansions can result in toxic protein gain-of-function, protein loss-of-function, and/or RNA gain-of-function effects. The location and the nature of mutation modulate the underlying disease pathophysiology resulting in varying disease manifestations. Potential toxic effects of these mutations likely affect key major cellular processes such as transcriptional regulation, mitochondrial functioning, ion channel dysfunction and synaptic transmission. Involvement of several common pathways suggests interlinked function of genes implicated in the disease pathogenesis. A better understanding of the shared and distinct molecular pathogenic mechanisms in these diseases is required to develop targeted therapeutic tools and interventions for disease management. The prime focus of this review is to elaborate on how expanded 'CAG' repeats contribute to the common modes of neurotoxicity and their possible therapeutic targets in management of such devastating disorders.

Pearls & Oy-sters: Familial Verbal Auditory Agnosia Due to C9orf72 Repeat Expansion.

Chromosome 9 open reading frame 72 (C9orf72) gene pathogenic variants have been typically associated with frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), but recent studies suggest their involvement in other disorders. This report describes a family with an autosomal dominant pattern of inheritance of progressive verbal auditory agnosia due to GGGGCC repeat expansion in C9orf72. A 60-year-old right-handed male truck driver presented with slowly progressive poor speech perception for 8 years, which became most troublesome when receiving verbal orders over the phone. He had difficulty recognizing single-syllable spoken words beyond his hearing loss but had no problem understanding complex written language. He had a heterozygous pathogenic variant carrying 160 hexanucleotide repeats in the C9orf72 gene. His family history included his deceased mother with similar symptoms that had progressed over 30 years, as well as his older brother and youngest sister who experienced speech perception difficulty beginning in their early fifties. His asymptomatic younger brother had a heterozygous 2 repeat in the C9orf72 gene, while his symptomatic youngest sister had a heterozygous 159 repeat. The patient and his sister exhibited more pronounced cortical thinning in the frontotemporoparietal areas. The discrepancy observed between the distribution of atrophy and the presentation of symptoms in patients with C9orf72 pathogenic repeat expansion may be attributable to the slow progression of their clinical course over time. The variable symptom presentation of C9orf72 pathogenic repeat expansion highlights the importance of considering this pathogenic variant as a potential cause of autosomal dominant degenerative brain diseases beyond FTD and ALS.

Genetic and Neuroimaging Analysis of SIGMAR1 for Frontotemporal Dementia.

BACKGROUND: Recently, Sigma nonopioid intracellular receptor 1 (SIGMAR1) variants have been shown harboring C9orf72 pathogenic repeat expansions in some frontotemporal dementia (FTD) cases. However, no SIGMAR1 genotype analysis has been reported in a cohort absent of C9orf72 pathogenic repeat expansions to date. OBJECTIVE: The present study investigated the contribution of SIGMAR1 independent of C9orf72 gene status to FTD spectrum syndromes. METHODS: We directly sequencing the entire coding region and a minimum of 50 bp from each of the flanking introns of SIGMAR1 gene in 82 sporadic FTD patients (female: male = 42 : 40) and 417 controls. For the patient carrying SIGMAR1 variant, a follow-up 3T MR imaging was performed in the study. RESULTS: Gene sequencing of SIGMAR1 revealed a rare 3'UTR nucleotide variation rs192856872 in a male patient with semantic dementia independent of C9orf72 gene status. The MR imaging showed asymmetrical atrophy in the anterior temporal lobes and the degeneration extends caudally into the posterior temporal lobes as the disease progresses. ESEFinder analysis showed new SRSF1 and SRSF1-IgM-BRCA1 binding sites with significant scores, which is predicted to affect normal splicing. CONCLUSION: We found a novel SIGMAR1 variant independent of C9orf72 gene status associated with semantic dementia phenotype.

Aspects of degradation and translation of the expanded C9orf72 hexanucleotide repeat RNA.

An hexanucleotide repeat expansion mutation in the non-coding region of C9orf72 gene causes frontotemporal dementia and amyotrophic lateral sclerosis. This mutation is estimated to be the most frequent genetic cause of these currently incurable diseases. Since the mutation causes autosomal dominantly inherited diseases, disease cascade essentially starts from the expanded DNA repeats. However, molecular disease mechanism is inevitably complex because possible toxic entity for the disease is not just functional loss of translated C9ORF72 protein, if any, but potentially includes bidirectionally transcribed expanded repeat containing RNA and their unconventional repeat-associated non-AUG translation products in all possible reading frames. Although the field learned so much about the disease since the identification of the mutation in 2011, how the expanded repeat causes a particular type of fronto-temporal lobe dominant neurodegeneration and/or motor neuron degeneration is not yet clear. In this review, we summarize and discuss the current understandings of molecular mechanism of this repeat expansion mutation with focuses on the degradation and translation of the repeat containing RNA transcripts.

Globally reduced N6-methyladenosine (m6A) in C9ORF72-ALS/FTD dysregulates RNA metabolism and contributes to neurodegeneration.

Repeat expansion in C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here we show that N6-methyladenosine (m6A), the most prevalent internal mRNA modification, is downregulated in C9ORF72-ALS/FTD patient-derived induced pluripotent stem cell (iPSC)-differentiated neurons and postmortem brain tissues. The global m6A hypomethylation leads to transcriptome-wide mRNA stabilization and upregulated gene expression, particularly for genes involved in synaptic activity and neuronal function. Moreover, the m6A modification in the C9ORF72 intron sequence upstream of the expanded repeats enhances RNA decay via the nuclear reader YTHDC1, and the antisense RNA repeats can also be regulated through m6A modification. The m6A reduction increases the accumulation of repeat RNAs and the encoded poly-dipeptides, contributing to disease pathogenesis. We further demonstrate that, by elevating m6A methylation, we could significantly reduce repeat RNA levels from both strands and the derived poly-dipeptides, rescue global mRNA homeostasis and improve survival of C9ORF72-ALS/FTD patient iPSC-derived neurons.

Short tandem repeat expansions in sporadic amyotrophic lateral sclerosis and frontotemporal dementia.

Pathogenic short tandem repeat (STR) expansions cause over 20 neurodegenerative diseases. To determine the contribution of STRs in sporadic amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), we used ExpansionHunter, REviewer, and polymerase chain reaction validation to assess 21 neurodegenerative disease-associated STRs in whole-genome sequencing data from 608 patients with sporadic ALS, 68 patients with sporadic FTD, and 4703 matched controls. We also propose a data-derived outlier detection method for defining allele thresholds in rare STRs. Excluding C9orf72 repeat expansions, 17.6% of clinically diagnosed ALS and FTD cases had at least one expanded STR allele reported to be pathogenic or intermediate for another neurodegenerative disease. We identified and validated 162 disease-relevant STR expansions in C9orf72 (ALS/FTD), ATXN1 [spinal cerebellar ataxia type 1 (SCA1)], ATXN2 (SCA2), ATXN8 (SCA8), TBP (SCA17), HTT (Huntington's disease), DMPK [myotonic dystrophy type 1 (DM1)], CNBP (DM2), and FMR1 (fragile-X disorders). Our findings suggest clinical and pathological pleiotropy of neurodegenerative disease genes and highlight their importance in ALS and FTD.

Inflammasome-Mediated Neuronal-Microglial Crosstalk: a Therapeutic Substrate for the Familial C9orf72 Variant of Frontotemporal Dementia/Amyotrophic Lateral Sclerosis.

Intronic G4C2 hexanucleotide repeat expansions (HRE) of C9orf72 are the most common cause of familial variants of frontotemporal dementia/amyotrophic lateral sclerosis (FTD/ALS). G4C2 HREs in C9orf72 undergo non-canonical repeat-associated translation, producing dipeptide repeat (DPR) proteins, with various deleterious impacts on cellular homeostasis. While five different DPRs are produced, poly(glycine-arginine) (GR) is amongst the most toxic and is the only DPR to accumulate in the associated clinically relevant anatomical locations of the brain. Previous work has demonstrated the profound effects of a poly (GR) model of C9orf72 FTD/ALS, including motor impairment, memory deficits, neurodegeneration, and neuroinflammation. Neuroinflammation is hypothesized to be a driving factor in the disease course; microglia activation is present prior to symptom onset and persists throughout the disease. Here, using an established mouse model of C9orf72 FTD/ALS, we investigate the contributions of the nod-like receptor pyrin-containing 3 (NLRP3) inflammasome in the pathogenesis of FTD/ALS. We find that inflammasome-mediated neuroinflammation is increased with microglial activation, cleavage of caspase-1, production of IL-1ß, and upregulation of Cxcl10 in the brain of C9orf72 FTD/ALS mice. Excitingly, we find that genetic ablation of Nlrp3 significantly improved survival, protected behavioral deficits, and prevented neurodegeneration suggesting a novel mechanism involving HRE-mediated induction of innate immunity. The findings provide experimental evidence of the integral role of HRE in inflammasome-mediated innate immunity in the C9orf72 variant of FTD/ALS pathogenesis and suggest the NLRP3 inflammasome as a therapeutic target.

The DAXX tax: C9orf72 DNA repeat expansions drive gain- and loss-of-function pathology in c9FTD/ALS.

In this issue of Neuron, Liu et al.1 identify DAXX as a C9orf72 hexanucleotide repeat expansion DNA-binding protein that initiates epigenetic modifications and chromatin remodeling, contributing to C9orf72 haploinsufficiency by inhibiting its stress-inducible expression and mediating both loss- and toxic gain-of-function pathology.

Advances in Nucleotide Repeat Expansion Diseases: Transcription Gets in Phase.

Unstable DNA repeat expansions and insertions have been found to cause more than 50 neurodevelopmental, neurodegenerative, and neuromuscular disorders. One of the main hallmarks of repeat expansion diseases is the formation of abnormal RNA or protein aggregates in the neuronal cells of affected individuals. Recent evidence indicates that alterations of the dynamic or material properties of biomolecular condensates assembled by liquid/liquid phase separation are critical for the formation of these aggregates. This is a thermodynamically-driven and reversible local phenomenon that condenses macromolecules into liquid-like compartments responsible for compartmentalizing molecules required for vital cellular processes. Disease-associated repeat expansions modulate the phase separation properties of RNAs and proteins, interfering with the composition and/or the material properties of biomolecular condensates and resulting in the formation of abnormal aggregates. Since several repeat expansions have arisen in genes encoding crucial players in transcription, this raises the hypothesis that wide gene expression dysregulation is common to multiple repeat expansion diseases. This review will cover the impact of these mutations in the formation of aberrant aggregates and how they modify gene transcription.