Spinocerebellar ataxia type 12: clues to pathogenesis

Spinocerebellar ataxias: from pathogenesis to recent therapeutic advances

Spinocerebellar ataxia is a phenotypically and genetically heterogeneous group of autosomal dominant-inherited degenerative disorders. The gene mutation spectrum includes dynamic expansions, point mutations, duplications, insertions, and deletions of varying lengths. Dynamic expansion is the most common form of mutation. Mutations often result in indistinguishable clinical phenotypes, thus requiring validation using multiple genetic testing techniques. Depending on the type of mutation, the pathogenesis may involve proteotoxicity, RNA toxicity, or protein loss-of-function. All of which may disrupt a range of cellular processes, such as impaired protein quality control pathways, ion channel dysfunction, mitochondrial dysfunction, transcriptional dysregulation, DNA damage, loss of nuclear integrity, and ultimately, impairment of neuronal function and integrity which causes diseases. Many disease-modifying therapies, such as gene editing technology, RNA interference, antisense oligonucleotides, stem cell technology, and pharmacological therapies are currently under clinical trials. However, the development of curative approaches for genetic diseases remains a global challenge, beset by technical, ethical, and other challenges. Therefore, the study of the pathogenesis of spinocerebellar ataxia is of great importance for the sustained development of disease-modifying molecular therapies.

Generation of a human induced pluripotent stem cell line JHUi004-A with heterozygous mutation for spinocerebellar ataxia type 12 using genome editing

Spinocerebellar ataxia type 12 (SCA12) is caused by a CAG expansion mutation in PPP2R2B, a gene encoding brain-specific regulatory units of protein phosphatase 2A (PP2A); while normal alleles carry 4 to 31 triplets, the disease alleles carry 43 to 78 triplets. Here, by CRISPR/Cas9n genome editing, we have generated a human heterozygous SCA12 iPSC line with 73 triplets for the mutant allele. The heterozygous SCA12 iPSCs have normal karyotype, express pluripotency markers and are able to differentiate into the three germ layers.

Role of Bβ1 overexpression in the pathogenesis of SCA12

BACKGROUND

Spinocerebellar ataxia type 12 (SCA12) is a neurodegenerative disease caused by a CAG/CTG repeat expansion at the PPP2R2B locus.

OBJECTIVE

We investigated how the CAG repeat expansion within the PPP2R2B 7B7D transcript influences the expression of Bβ1 and a potential protein containing a long polyserine tract.

METHODS

Transcript and protein expression were measured using quantitative PCR (qPCR) Role of Bβ1 overexpression in the pathogenesis of SCA12 and Western blot, respectively, in an SK-N-MC cell model that overexpresses the full-length PPP2R2B 7B7D transcript. The apoptotic effect of a protein containing a long polyserine tract on SK-N-MC cells was evaluated using caspase 3/7 activity.

RESULTS

The CAG repeat expansion increases the expression of the PPP2R2B 7B7D transcript, as well as Bβ1 protein, in an SK-N-MC cell model in which the full-length PPP2R2B 7B7D transcript is overexpressed. The CAG repeat expansion within the 7B7D transcript is translated into a long polyserine tract that triggers apoptosis in SK-N-MC cells.

CONCLUSIONS

The SCA12 mutation leads to overexpression of PPP2R2B Bβ1 and to expression of a protein containing a long polyserine tract; both these effects potentially contribute to SCA12 pathogenesis. © 2024 International Parkinson and Movement Disorder Society.

Integration of graph network with kernel SVM and logistic regression for identification of biomarkers in SCA12 and its diagnosis

Spinocerebellar ataxia type 12 is a hereditary and neurodegenerative illness commonly found in India. However, there is no established noninvasive automatic diagnostic system for its diagnosis and identification of imaging biomarkers. This work proposes a novel four-phase machine learning-based diagnostic framework to find spinocerebellar ataxia type 12 disease-specific atrophic-brain regions and distinguish spinocerebellar ataxia type 12 from healthy using a real structural magnetic resonance imaging dataset. Firstly, each brain region is represented in terms of statistics of coefficients obtained using 3D-discrete wavelet transform. Secondly, a set of relevant regions are selected using a graph network-based method. Thirdly, a kernel support vector machine is used to capture nonlinear relationships among the voxels of a brain region. Finally, the linear relationship among the brain regions is captured to build a decision model to distinguish spinocerebellar ataxia type 12 from healthy by using the regularized logistic regression method. A classification accuracy of 95% and a harmonic mean of precision and recall, i.e. F1-score of 94.92%, is achieved. The proposed framework provides relevant regions responsible for the atrophy. The importance of each region is captured using Shapley Additive exPlanations values. We also performed a statistical analysis to find volumetric changes in spinocerebellar ataxia type 12 group compared to healthy. The promising result of the proposed framework shows that clinicians can use it for early and timely diagnosis of spinocerebellar ataxia type 12.

A Review of Ocular Movement Abnormalities in Hereditary Cerebellar Ataxias

Cerebellar ataxias are a wide heterogeneous group of disorders that may present with fine motor deficits as well as gait and balance disturbances that have a significant influence on everyday activities. To review the ocular movements in cerebellar ataxias in order to improve the clinical knowledge of cerebellar ataxias and related subtypes. English papers published from January 1990 to May 2022 were selected by searching PubMed services. The main search keywords were ocular motor, oculomotor, eye movement, eye motility, and ocular motility, along with each ataxia subtype. The eligible papers were analyzed for clinical presentation, involved mutations, the underlying pathology, and ocular movement alterations. Forty-three subtypes of spinocerebellar ataxias and a number of autosomal dominant and autosomal recessive ataxias were discussed in terms of pathology, clinical manifestations, involved mutations, and with a focus on the ocular abnormalities. A flowchart has been made using ocular movement manifestations to differentiate different ataxia subtypes. And underlying pathology of each subtype is reviewed in form of illustrated models to reach a better understanding of each disorder.

Bidirectional Transcription at the PPP2R2B Gene Locus in Spinocerebellar Ataxia Type 12

BACKGROUND

Spinocerebellar ataxia type 12 (SCA12) is a neurodegenerative disease caused by expansion of a CAG repeat in the PPP2R2B gene.

OBJECTIVE

In this study, we tested the hypothesis that the PPP2R2B antisense (PPP2R2B-AS1) transcript containing a CUG repeat is expressed and contributes to SCA12 pathogenesis.

METHODS

Expression of PPP2R2B-AS1 transcript was detected in SCA12 human induced pluripotent stem cells (iPSCs), iPSC-derived NGN2 neurons, and SCA12 knock-in mouse brains using strand-specific reverse transcription polymerase chain reaction. The tendency of expanded PPP2R2B-AS1 (expPPP2R2B-AS1) RNA to form foci, a marker of toxic processes involving mutant RNAs, was examined in SCA12 cell models by fluorescence in situ hybridization. The apoptotic effect of expPPP2R2B-AS1 transcripts on SK-N-MC neuroblastoma cells was evaluated by caspase 3/7 activity. Western blot was used to examine the expression of repeat associated non-ATG-initiated translation of expPPP2R2B-AS1 transcript in SK-N-MC cells.

RESULTS

The repeat region in the PPP2R2B gene locus is bidirectionally transcribed in SCA12 iPSCs, iPSC-derived NGN2 neurons, and SCA12 mouse brains. Transfected expPPP2R2B-AS1 transcripts induce apoptosis in SK-N-MC cells, and the apoptotic effect may be mediated, at least in part, by the RNA secondary structure. The expPPP2R2B-AS1 transcripts form CUG RNA foci in SK-N-MC cells. expPPP2R2B-AS1 transcript is translated in the alanine open reading frame (ORF) via repeat-associated non-ATG translation, which is diminished by single-nucleotide interruptions within the CUG repeat and MBNL1 overexpression.

CONCLUSIONS

These findings suggest that PPP2R2B-AS1 contributes to SCA12 pathogenesis and may therefore provide a novel therapeutic target for the disease. © 2023 International Parkinson and Movement Disorder Society.

Bidirectional transcription at the PPP2R2B gene locus in spinocerebellar ataxia type 12

OBJECTIVE

Spinocerebellar ataxia type 12 (SCA12) is a neurodegenerative disease caused by expansion of a CAG repeat in the PPP2R2B gene . Here we tested the hypothesis that the PPP2R2B antisense ( PPP2R2B-AS1 ) transcript containing a CUG repeat is expressed and contributes to SCA12 pathogenesis.

METHODS

Expression of PPP2R2B-AS1 transcript was detected in SCA12 human induced pluripotent stem cells (iPSCs), iPSC-derived NGN2 neurons, and SCA12 knock-in mouse brains using strand-specific RT-PCR (SS-RT-PCR). The tendency of expanded PPP2R2B-AS1 ( expPPP2R2B-AS1 ) RNA to form foci, a marker of toxic processes involving mutant RNAs, was examined in SCA12 cell models by fluorescence in situ hybridization. The toxic effect of expPPP2R2B-AS1 transcripts on SK-N-MC neuroblastoma cells was evaluated by caspase 3/7 activity. Western blot was used to examine the expression of repeat associated non-ATG-initiated (RAN) translation of expPPP2R2B-AS1 transcript in SK-N-MC cells.

RESULTS

The repeat region in PPP2R2B gene locus is bidirectionally transcribed in SCA12 iPSCs, iPSC-derived NGN2 neurons, and SCA12 mouse brains. Transfected expPPP2R2B-AS1 transcripts are toxic to SK-N-MC cells, and the toxicity may be mediated, at least in part, by the RNA secondary structure. The expPPP2R2B-AS1 transcripts form CUG RNA foci in SK-N-MC cells. expPPP2R2B-AS1 transcript is translated in the Alanine ORF via repeat-associated non-ATG (RAN) translation, which is diminished by single nucleotide interruptions within the CUG repeat, and MBNL1 overexpression.

INTERPRETATION

These findings suggest that PPP2R2B-AS1 contributes to SCA12 pathogenesis, and may therefore provide a novel therapeutic target for the disease.

Clinical and Preclinical Neuroimaging Changes in Spinocerebellar Ataxia Type 12: A Study of Three Chinese Pedigrees

BACKGROUND

Spinocerebellar ataxia type 12 (SCA12) is a rare SCA subtype with unclear clinical and imaging features. Also, the radiological changes in prodromal and early stages remain unknown.

METHODS

Ten symptomatic and two pre-symptomatic cases from three Chinese pedigrees received clinical assessments and imaging studies including routine magnetic resonance imaging (MRI), diffusion kurtosis imaging (DKI), and positron emission tomography (PET) using 18F-flurodeoxyglucose (FDG) to investigate glucose metabolism in brain and 18F-vesicle monoamine transporter 2 (VMAT2) to inspect the integrity of the dopaminergic neuron. Seventy-two healthy individuals were recruited as controls in the quantitative FDG-PET analysis. Imaging parameters were compared between symptomatic and presymptomatic cases with different disease durations.

RESULTS

Patients displayed prominent action tremor, moderate ataxia, and subtle parkinsonism with poor levodopa-response. MRI showed extensive but heterogeneous cerebral atrophy, which was most evident in the frontoparietal lobes. Cerebellar atrophy was apparent in later stages. DKI detected impaired fibers in the cerebellar peduncles. In both symptomatic and pre-symptomatic cases, PET-CT showed an earlier FDG decline than atrophic changes in multiple regions, and the frontoparietal lobes were the earliest and most severe. However, the VMAT2 density were normal in the putamen and caudate nucleus of most cases (7/8).

CONCLUSIONS

We first found that hypometabolism in the cerebral cortex, but not cerebellum, is an early and prominent change in SCA12. The integrity of presynaptic dopaminergic neurons remains largely spared during the whole disease process.

Clinical, Radiological, and Genetic Profile of Spinocerebellar Ataxia 12: A Hospital-Based Cohort Analysis

Introduction:

Spinocerebellar ataxia type-12 (SCA12) is a rare form of SCA, most commonly reported from the Indian Agarwal and related families. In this study we describe the clinical, genetic, and radiological characteristics of a sizeable cohort of genetically proven SCA12.

Methods:

A retrospective chart-review of the genetically confirmed SCA12 patients from our centre. The demographic, clinical, and investigation findings were reviewed. Correlation of expanded repeats length with various demographic and clinical features were studied.

Results:

A total of 49 patients (34 males, 42 families) were included of which 79.6% belonged to Agarwal community. The mean age at onset and age at presentation were 46.38 ± 11.7 years and 53.16 ± 12.78 years respectively. The most common initial symptom was tremor (73.5%), followed by ataxia (18.4%). At presentation, 95.9% of the patients had tremor with predominant distribution in the bilateral upper limbs (85.7%). At presentation, 73.5% of patients had ataxia and 22.4% had cognitive dysfunction. The mean CAG repeat length in PPP2R2B in the expanded allele was 53.26 ± 6.10 (40–72). The lowest pathogenic expanded repeat sizes in PPP2R2B recorded in our cohort was 40 & 42 repeats from two patients with a consistent clinical phenotype. Another unusual phenotype was the presence of prominent myoclonus. There was no significant correlation between the age at onset of symptoms and the repeat size of CAG repeat.

Conclusion:

SCA12 is not confined to a single ethnicity. Upper limb tremor and ataxia were the most common presentation. Unusual presentation may cause diagnostic confusion especially when recorded in patients from non-Aggarwal families.

Gastrointestinal disorders in hyperkinetic movement disorders and ataxia

BACKGROUND

Gastrointestinal (GI) disorders have been thoroughly investigated in hypokinetic disorders such as Parkinson's disease, but much less is known about GI disorders in hyperkinetic movement disorders and ataxia. The aim of this review is to draw attention to the GI disorders that are associated with these movement disorders.

METHODS

References for this systematic review were identified by searches of PubMed through May 2020. Only publications in English were reviewed.

RESULTS

Data from 249 articles were critically reviewed, compared, and integrated. The most frequently reported GI symptoms overall in hyperkinetic movement disorders and ataxia are dysphagia, sialorrhea, weight changes, esophago-gastritis, gastroparesis, constipation, diarrhea, and malabsorption. We report in detail on the frequency, characteristics, pathophysiology, and management of GI symptoms in essential tremor, restless legs syndrome, chorea, and spinocerebellar ataxias. The limited available data on GI disorders in dystonias, paroxysmal movement disorders, tardive dyskinesias, myoclonus, and non-SCA ataxias are also summarized.

CONCLUSION

The purpose of our systematic review is to draw attention that, although primarily motor disorders, hyperkinetic movement disorders and ataxia can involve the GI system. Raising awareness about the GI symptom burden in hyperkinetic movement disorders and ataxia could contribute to a new research interest in that field, as well as improved patient care.

Use of single guided Cas9 nickase to facilitate precise and efficient genome editing in human iPSCs

Cas9 nucleases permit rapid and efficient generation of gene-edited cell lines. However, in typical protocols, mutations are intentionally introduced into the donor template to avoid the cleavage of donor template or re-cleavage of the successfully edited allele, compromising the fidelity of the isogenic lines generated. In addition, the double-stranded breaks (DSBs) used for editing can introduce undesirable "on-target" indels within the second allele of successfully modified cells via non-homologous end joining (NHEJ). To address these problems, we present an optimized protocol for precise genome editing in human iPSCs that employs (1) single guided Cas9 nickase to generate single-stranded breaks (SSBs), (2) transient overexpression of BCL-XL to enhance survival post electroporation, and (3) the PiggyBac transposon system for seamless removal of dual selection markers. We have used this method to modify the length of the CAG repeat contained in exon 7 of PPP2R2B. When longer than 43 triplets, this repeat causes the neurodegenerative disorder spinocerebellar ataxia type 12 (SCA12); our goal was to seamlessly introduce the SCA12 mutation into a human control iPSC line. With our protocol, ~ 15% of iPSC clones selected had the desired gene editing without "on target" indels or off-target changes, and without the deliberate introduction of mutations via the donor template. This method will allow for the precise and efficient editing of human iPSCs for disease modeling and other purposes.

Generation of a human induced pluripotent stem cell line JHUi003-A with homozygous mutation for spinocerebellar ataxia type 12 using genome editing

Spinocerebellar ataxia type 12 (SCA12) is caused by a CAG expansion mutation in PPP2R2B, a gene encoding a brain-specific regulatory unit of protein phosphatase 2A (PP2A); while normal alleles carry 4 to 31 triplets, the disease alleles carry 43 to 78 triplets. Here, by CRISPR/Cas9 genome editing, we have generated a human homozygous SCA12 iPSC line with 69 and 72 triplets for each allele. The homozygous SCA12 iPSCs have normal karyotype, express pluripotency markers and are able to differentiate into the three germ layers.

Protein phosphatase 2A B55β limits CD8+ T cell lifespan following cytokine withdrawal

How T cells integrate environmental cues into signals that limit the magnitude and length of immune responses is poorly understood. Here, we provide data that demonstrate that B55β, a regulatory subunit of protein phosphatase 2A, represents a molecular link between cytokine concentration and apoptosis in activated CD8+ T cells. Through the modulation of AKT, B55β induced the expression of the proapoptotic molecule Hrk in response to cytokine withdrawal. Accordingly, B55β and Hrk were both required for in vivo and in vitro contraction of activated CD8+ lymphocytes. We show that this process plays a role during clonal contraction, establishment of immune memory, and preservation of peripheral tolerance. This regulatory pathway may represent an unexplored opportunity to end unwanted immune responses or to promote immune memory.

Huntington's Disease Pathogenesis: Two Sequential Components

Historically, Huntington's disease (HD; OMIM #143100) has played an important role in the enormous advances in human genetics seen over the past four decades. This familial neurodegenerative disorder involves variable onset followed by consistent worsening of characteristic abnormal movements along with cognitive decline and psychiatric disturbances. HD was the first autosomal disease for which the genetic defect was assigned to a position on the human chromosomes using only genetic linkage analysis with common DNA polymorphisms. This discovery set off a multitude of similar studies in other diseases, while the HD gene, later renamed HTT, and its vicinity in chromosome 4p16.3 then acted as a proving ground for development of technologies to clone and sequence genes based upon their genomic location, with the growing momentum of such advances fueling the Human Genome Project. The identification of the HD gene has not yet led to an effective treatment, but continued human genetic analysis of genotype-phenotype relationships in large HD subject populations, first at the HTT locus and subsequently genome-wide, has provided insights into pathogenesis that divide the course of the disease into two sequential, mechanistically distinct components.

Application of CRISPR-Cas9-Mediated Genome Editing for the Treatment of Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a debilitating multisystemic disorder, caused by expansion of a CTG microsatellite repeat in the 3' untranslated region of the DMPK (dystrophia myotonica protein kinase) gene. To date, novel therapeutic approaches have focused on transient suppression of the mutant, repeat-expanded RNA. However, recent developments in the field of genome editing have raised the exciting possibility of inducing permanent correction of the DM1 genetic defect. Specifically, repurposing of the prokaryotic CRISPR (clustered regularly interspaced short palindromic repeats)-Cas9 (CRISPR-associated protein 9) system has enabled programmable, site-specific, and multiplex genome editing. CRISPR-based strategies for the treatment of DM1 can be applied either directly to patients, or indirectly through the ex vivo modification of patient-derived cells, and they include excision of the repeat expansion, insertion of synthetic polyadenylation signals upstream of the repeat, steric interference with RNA polymerase II procession through the repeat leading to transcriptional downregulation of DMPK, and direct RNA targeting of the mutant RNA species. Potential obstacles to such therapies are discussed, including the major challenge of Cas9 and guide RNA transgene/ribonuclear protein delivery, off-target gene editing, vector genome insertion at cut sites, on-target unintended mutagenesis (e.g., repeat inversion), pre-existing immunity to Cas9 or AAV antigens, immunogenicity, and Cas9 persistence.

Systematic screening identifies a 2-gene signature as a high-potential prognostic marker of undifferentiated pleomorphic sarcoma/myxofibrosarcoma

The Cancer Genome Atlas (TCGA) Research Network confirmed that undifferentiated pleomorphic sarcoma (UPS) and myxofibrosarcoma (MFS) share a high level of genomic similarities and fall into a single spectrum of tumour. However, no molecular prognostic biomarkers have been identified in UPS/MFS. In this study, by extracting data from TCGA-Sarcoma (SARC), we explored relapse-related genes, their prognostic value and possible mechanisms of the dysregulations. After systematic screening, ITGA10 and PPP2R2B were included to construct a 2-gene signature. The 2-gene signature had an AUC value of 0.83 and had an independent prognostic value in relapse-free survival (RFS) (HR: 2.966, 95%CI: 1.995-4.410 P < .001), and disease-specific survival (DSS) (HR: 2.283, 95%CI: 1.358-3.835, P = .002), as a continuous variable. Gene-level copy number alterations (CNAs) were irrelevant to their dysregulation. Two CpG sites (cg15585341 and cg04126335) around the promoter of ITGA10 showed strong negative correlations with ITGA10 expression (Pearson's r < -0.6). Transcript preference was observed in PPP2R2B expression. The methylation of some CpG sites in two gene body regions showed at least moderate positive correlations (Pearson's r > .4) with PPP2R2B expression. Besides, the 2-gene signature showed a moderate negative correlation with CD4 + T cell infiltration. High-level CD4 + T cell infiltration and neutrophil infiltration were associated with significantly better RFS. Based on these findings, we infer that the 2-gene signature might be a potential prognostic marker in patients with UPS/MFS. Considering the potential benefits of immunotherapy for UPS/MFS patients, it is imperative to explore the predictive value of this signature in immunotherapeutic responses in the future.

CAG Repeat Not Polyglutamine Length Determines Timing of Huntington's Disease Onset

Variable, glutamine-encoding, CAA interruptions indicate that a property of the uninterrupted HTT CAG repeat sequence, distinct from the length of huntingtin's polyglutamine segment, dictates the rate at which Huntington's disease (HD) develops. The timing of onset shows no significant association with HTT cis-eQTLs but is influenced, sometimes in a sex-specific manner, by polymorphic variation at multiple DNA maintenance genes, suggesting that the special onset-determining property of the uninterrupted CAG repeat is a propensity for length instability that leads to its somatic expansion. Additional naturally occurring genetic modifier loci, defined by GWAS, may influence HD pathogenesis through other mechanisms. These findings have profound implications for the pathogenesis of HD and other repeat diseases and question the fundamental premise that polyglutamine length determines the rate of pathogenesis in the "polyglutamine disorders."

PPP2R2B hypermethylation causes acquired apoptosis deficiency in systemic autoimmune diseases

Chronic inflammation causes target organ damage in patients with systemic autoimmune diseases. The factors that allow this protracted response are poorly understood. We analyzed the transcriptional regulation of PPP2R2B (B55ß), a molecule necessary for the termination of the immune response, in patients with autoimmune diseases. Altered expression of B55ß conditioned resistance to cytokine withdrawal-induced death (CWID) in patients with autoimmune diseases. The impaired upregulation of B55ß was caused by inflammation-driven hypermethylation of specific cytosines located within a regulatory element of PPP2R2B preventing CTCF binding. This phenotype could be induced in healthy T cells by exposure to TNF-α. Our results reveal a gene whose expression is affected by an acquired defect, through an epigenetic mechanism, in the setting of systemic autoimmunity. Because failure to remove activated T cells through CWID could contribute to autoimmune pathology, this mechanism illustrates a vicious cycle through which autoimmune inflammation contributes to its own perpetuation.

Spinocerebellar ataxia

The spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of autosomal dominantly inherited progressive disorders, the clinical hallmark of which is loss of balance and coordination accompanied by slurred speech; onset is most often in adult life. Genetically, SCAs are grouped as repeat expansion SCAs, such as SCA3/Machado-Joseph disease (MJD), and rare SCAs that are caused by non-repeat mutations, such as SCA5. Most SCA mutations cause prominent damage to cerebellar Purkinje neurons with consecutive cerebellar atrophy, although Purkinje neurons are only mildly affected in some SCAs. Furthermore, other parts of the nervous system, such as the spinal cord, basal ganglia and pontine nuclei in the brainstem, can be involved. As there is currently no treatment to slow or halt SCAs (many SCAs lead to premature death), the clinical care of patients with SCA focuses on managing the symptoms through physiotherapy, occupational therapy and speech therapy. Intense research has greatly expanded our understanding of the pathobiology of many SCAs, revealing that they occur via interrelated mechanisms (including proteotoxicity, RNA toxicity and ion channel dysfunction), and has led to the identification of new targets for treatment development. However, the development of effective therapies is hampered by the heterogeneity of the SCAs; specific therapeutic approaches may be required for each disease.

The central role of DNA damage and repair in CAG repeat diseases

Diseases such as Huntington's disease and certain spinocerebellar ataxias are caused by the expansion of genomic cytosine-adenine-guanine (CAG) trinucleotide repeats beyond a specific threshold. These diseases are all characterised by neurological symptoms and central neurodegeneration, but our understanding of how expanded repeats drive neuronal loss is incomplete. Recent human genetic evidence implicates DNA repair pathways, especially mismatch repair, in modifying the onset and progression of CAG repeat diseases. Repair pathways might operate directly on repeat sequences by licensing or inhibiting repeat expansion in neurons. Alternatively, or in addition, because many of the genes containing pathogenic CAG repeats encode proteins that themselves have roles in the DNA damage response, it is possible that repeat expansions impair specific DNA repair pathways. DNA damage could then accrue in neurons, leading to further expansion at repeat loci, thus setting up a vicious cycle of pathology. In this review, we consider DNA damage and repair pathways in postmitotic neurons in the context of disease-causing CAG repeats. Investigating and understanding these pathways, which are clearly relevant in promoting and ameliorating disease in humans, is a research priority, as they are known to modify disease and therefore constitute prevalidated drug targets.