CAG Repeat Expansion Research Articles

Factors influencing cognitive function in patients with Huntington's disease from China: A cross-sectional clinical study.

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disorder caused by CAG repeats expansion. Cognitive decline contributes to the loss of daily activity in manifest HD. We aimed to examine the cognition status in a Chinese HD cohort and explore factors influencing the diverse cognitive domains. A total of 205 participants were recruited in the study with the assessment by neuropsychological batteries, including the mini-mental state examination (MMSE), Stroop test, symbol digit modalities test (SDMT), trail making test (TMT), verbal fluency test (VFT), and Hopkins verbal learning test-revised, as well as motor and psychiatric assessment. Pearson correlation and multiple linear regression models were applied to investigate the correlation. Only 41.46% of patients had normal global function first come to our center. There was a significantly difference in MMSE, Stroop test, SDMT, TMT, and VFT across each stage of HD patients (p<.05). Apathy of PBA-s was correlated to MMSE, animal VFT and Stroop-interference tests performance. Severity of motor symptoms, functional capacity, age, and age of motor symptom onset were correlated to all neuropsychological scores, whereas education attainment and diagnostic delay were correlated to most neuropsychological scores except TMT. Severity of motor symptoms, functional capacity, and education attainment showed independent predicting effect (p<.05) in diverse cognitive domains. Cognitive impairment was very common in Chinese HD patients at the first visit and worse in the patients in advanced phase. The severity of motor symptoms and functional capacity were correlated to the diverse cognitive domains.

Memantine administration prevented chorea movement in Huntington’s disease: a case report

BackgroundHuntington’s disease is an autosomal dominant inherited disorder characterized by personality changes (such as irritability and restlessness) and psychotic symptoms (such as hallucinations and delusions). When the personality changes become noticeable, involuntary movements (chorea) also develop. The disease is caused by the CAG repeat expansion in the coding region of the HTT gene, and the diagnosis is based on the presence of this expansion. However, there is currently no effective treatment for the progression of Huntington’s disease and its involuntary motor symptoms. Herein, we present a case in which memantine was effective in treating the chorea movements of Huntington’s disease.Case presentationA 75-year-old Japanese woman presented to the hospital with involuntary movements of Huntington’s disease that began when she was 73 years old. In a cerebral blood flow test (N-isopropyl-p-iodoamphetamine–single-photon emission computed tomography), decreased blood flow was observed in the precuneus (anterior wedge) and posterior cingulate gyrus. Usually, such areas of decreased blood flow are observed in patients with Alzheimer’s-type dementia. So, we administered memantine for Alzheimer’s-type dementia, and this treatment suppressed the involuntary movements of Huntington’s disease, and the symptoms progressed slowly for 7 years after the onset of senility. In contrast, her brother died of complications of pneumonia during the course of Huntington’s disease.ConclusionsWe recorded changes in parameters such as the results of the N-isopropyl-p-iodoamphetamine–single-photon emission computed tomography and gait videos over 7 years. Treatment with memantine prevented the chorea movement and the progression of Huntington’s disease. We believe this record will provide clinicians with valuable information in diagnosing and treating Huntington’s disease.

CAG repeat expansion in the Huntington's disease gene shapes linear and circular RNAs biogenesis.

Alternative splicing (AS) appears to be altered in Huntington's disease (HD), but its significance for early, pre-symptomatic disease stages has not been inspected. Here, taking advantage of Htt CAG knock-in mouse in vitro and in vivo models, we demonstrate a correlation between Htt CAG repeat length and increased aberrant linear AS, specifically affecting neural progenitors and, in vivo, the striatum prior to overt behavioral phenotypes stages. Remarkably, a significant proportion (36%) of the aberrantly spliced isoforms are not-functional and meant to non-sense mediated decay (NMD). The expanded Htt CAG repeats further reflect on a previously neglected, global impairment of back-splicing, leading to decreased circular RNAs production in neural progenitors. Integrative transcriptomic analyses unveil a network of transcriptionally altered micro-RNAs and RNA-binding proteins (Celf, hnRNPs, Ptbp, Srsf, Upf1, Ythd2) which might influence the AS machinery, primarily in neural cells. We suggest that this unbalanced expression of linear and circular RNAs might alter neural fitness, contributing to HD pathogenesis.

Mitochondrial Dysfunction and Decreased Cytochrome c in Cell and Animal Models of Machado-Joseph Disease.

Mitochondrial dysfunction has been described in many neurodegenerative disorders; however, there is less information regarding mitochondrial deficits in Machado-Joseph disease (MJD), a polyglutamine (polyQ) disorder caused by CAG repeat expansion in the ATXN3 gene. In the present study, we characterized the changes in mitochondrial function and biogenesis markers in two MJD models, CMVMJD135 (MJD135) transgenic mice at a fully established phenotype stage and tetracycline-regulated PC6-3 Q108 cell line expressing mutant ataxin-3 (mATXN3). We detected mATXN3 in the mitochondrial fractions of PC6-3 Q108 cells, suggesting the interaction of expanded ATXN3 with the organelle. Interestingly, in both the cerebella of the MJD135 mouse model and in PC6-3 Q108 cells, we found decreased mitochondrial respiration, ATP production and mitochondrial membrane potential, strongly suggesting mitochondrial dysfunction in MJD. Also, in PC6-3 Q108 cells, an additional enhanced glycolytic flux was observed. Supporting the functional deficits observed in MJD mitochondria, MJD135 mouse cerebellum and PC6-3 Q108 cells showed reduced cytochrome c mRNA and protein levels. Overall, our findings show compromised mitochondrial function associated with decreased cytochrome c levels in both cell and animal models of MJD.

Aberrant splicing of mutant huntingtin in Huntington's disease knock-in pigs

Huntington's disease (HD) is an autosomal-dominant inherited neurodegenerative disease caused by a CAG repeat expansion in exon1 of the huntingtin gene (HTT). This expansion leads to the production of N-terminal mutant huntingtin protein (mHtt) that contains an expanded polyglutamine tract, which is toxic to neurons and causes neurodegeneration. While the production of N-terminal mHtt can be mediated by proteolytic cleavage of full-length mHtt, abnormal splicing of exon1-intron1 of mHtt has also been identified in the brains of HD mice and patients. However, the proportion of aberrantly spliced exon1 mHTT in relation to normal mHTT exon remains to be defined. In this study, HTT exon1 production was examined in the HD knock-in (KI) pig model, which more closely recapitulates neuropathology seen in HD patient brains than HD mouse models. The study revealed that aberrant spliced HTT exon1 is also present in the brains of HD pigs, but it is expressed at a much lower level than the normally spliced HTT exon products. These findings suggest that careful consideration is needed when assessing the contribution of aberrantly spliced mHTT exon1 to HD pathogenesis, and further rigorous investigation is required.

Investigating Neuron Degeneration in Huntington's Disease Using RNA-Seq Based Transcriptome Study.

Huntington's disease (HD) is a progressive neurodegenerative disorder caused due to a CAG repeat expansion in the huntingtin (HTT) gene. The primary symptoms of HD include motor dysfunction such as chorea, dystonia, and involuntary movements. The primary motor cortex (BA4) is the key brain region responsible for executing motor/movement activities. Investigating patient and control samples from the BA4 region will provide a deeper understanding of the genes responsible for neuron degeneration and help to identify potential markers. Previous studies have focused on overall differential gene expression and associated biological functions. In this study, we illustrate the relationship between variants and differentially expressed genes/transcripts. We identified variants and their associated genes along with the quantification of genes and transcripts. We also predicted the effect of variants on various regulatory activities and found that many variants are regulating gene expression. Variants affecting miRNA and its targets are also highlighted in our study. Co-expression network studies revealed the role of novel genes. Function interaction network analysis unveiled the importance of genes involved in vesicle-mediated transport. From this unified approach, we propose that genes expressed in immune cells are crucial for reducing neuron death in HD.

High throughput compound screening in neuronal cells identifies statins as activators of ataxin 3 expression

The spinocerebellar ataxias (SCA) comprise a group of inherited neurodegenerative diseases. SCA3 is the most common form, caused by the expansion of CAG repeats within the ataxin 3 (ATXN3) gene. The mutation results in the expression of an abnormal protein, containing long polyglutamine (polyQ) stretches. The polyQ stretch confers a toxic gain of function and leads to misfolding and aggregation of ATXN3 in neurons. Thus, modulators of ATXN3 expression could potentially ameliorate the pathology in SCA3 patients. Therefore, we generated a CRISPR/Cas9 modified ATXN3-Exon4-Luciferase (ATXN3-LUC) genomic fusion- and control cell lines to perform a reporter cell line-based high-throughput screen comprising 2640 bioactive compounds, including the FDA approved drugs. We found no unequivocal inhibitors of, but identified statins as activators of the LUC signal in the ATXN3-LUC screening cell line. We further confirmed that Simvastatin treatment of wild type SK-N-SH cells increases ATXN3 mRNA and protein levels which likely results from direct binding of the activated sterol regulatory element binding protein 1 (SREBP1) to the ATXN3 promotor. Finally, we observed an increase of normal and expanded ATXN3 protein levels in a patient-derived cell line upon Simvastatin treatment, underscoring the potential medical relevance of our findings.

Study of trinucleotide CAG-repeats expansion in androgen receptor gene among patients with suspected Kennedy’s syndrome

Aim. To perform a molecular genetic study of CAG-repeat expansion in androgen receptor gene AR in individuals with suspected spinal and bulbar muscular atrophy (Kennedy’s syndrome). Methods. Clinical and genealogical, method of differential diagnosis, DNA isolation and purification, molecular genetic: polymerase chain reaction, electrophoresis in agarose gel. Results. A molecular genetic study of trinucleotide CAG-repeats expansion in androgen receptor gene in 30 people with suspected Kennedy’s syndrome was performed. In 5 probands of the study group, 38 CAG repeats (the upper limit of the norm) were established and in 27 examined patients, the number of CAG repeats did not exceed 37 (the norm). Among the examined group of patients, was found a family in which three men had 49 CAG repeats in the AR gene, which confirmed the presence of Kennedy’s syndrome. Conclusions. Kennedy’s syndrome is a rare X-linked recessive disease that requires the development of specific biomarkers to clarify the pathogenic process and facilitate early diagnosis.

Establishment of human-induced pluripotent stem cell GZHMCi0011-A from peripheral blood mononuclear cells from a volunteer with 14/63 CAG repeats of the ATXN3 mutation

Spinocerebellar ataxia type 3 (SCA3) is a genetic degeneration disease of the nervous system with ataxia as the main clinical manifestation, and the most frequent subtype of SCA3 is known to be caused by CAG repeat expansions of more than 55 units in ATXN3. In this study, we used peripheral blood mononuclear cells (PBMCs) from a volunteer with 14/63 CAG repeats in ATXN3 to generate induced pluripotent stem cells (iPSCs), which will be a good model for studying SCA3.

The Huntington's Disease Gene in an Italian Cohort of Patients with Bipolar Disorder.

Huntington's disease (HD) is characterized by motor, cognitive and psychiatric manifestations and caused by an expansion of CAG repeats over 35 triplets on the huntingtin (HTT) gene. However, expansions in the range 27-35 repeats (intermediate allele) can be associated with pathological phenotypes. The onset of HD is conventionally defined by the onset of motor symptoms, but psychiatric disturbances can precede the motor phase by up to twenty years. The aims of the present study are to identify HD patients in the pre-motor phase of the disease among patients diagnosed with bipolar disorders and evaluate any differences between bipolar patients carrying the normal HTT allele and patients with the expanded HTT gene. We assessed the HTT genotype in an Italian cohort of 69 patients who were affected by either type 1 or type 2 bipolar disorder. No patient was found to be a carrier of the pathological HTT allele, but 10% of bipolar subjects carried an intermediate allele. Carriers of the intermediate allele were older at the onset of psychiatric symptoms than non-carriers. The pathological HTT gene was not associated with bipolar disorder, while we found a higher frequency of the intermediate allele among the bipolar population with respect to healthy controls. The identification of this subset of bipolar subjects has implications for the clinical management of patients and their family members and promotes further investigation into possible pathological mechanisms common to both HD and bipolar disorder.

Longitudinal imaging highlights preferential basal ganglia circuit atrophy in Huntington's disease.

Huntington's disease is caused by a CAG repeat expansion in the Huntingtin gene (HTT), coding for polyglutamine in the Huntingtin protein, with longer CAG repeats causing earlier age of onset. The variable 'Age' × ('CAG'-L), where 'Age' is the current age of the individual, 'CAG' is the repeat length and L is a constant (reflecting an approximation of the threshold), termed the 'CAG Age Product' (CAP) enables the consideration of many individuals with different CAG repeat expansions at the same time for analysis of any variable and graphing using the CAG Age Product score as the X axis. Structural MRI studies have showed that progressive striatal atrophy begins many years prior to the onset of diagnosable motor Huntington's disease, confirmed by longitudinal multicentre studies on three continents, including PREDICT-HD, TRACK-HD and IMAGE-HD. However, previous studies have not clarified the relationship between striatal atrophy, atrophy of other basal ganglia structures, and atrophy of other brain regions. The present study has analysed all three longitudinal datasets together using a single image segmentation algorithm and combining data from a large number of subjects across a range of CAG Age Product score. In addition, we have used a strategy of normalizing regional atrophy to atrophy of the whole brain, in order to determine which regions may undergo preferential degeneration. This made possible the detailed characterization of regional brain atrophy in relation to CAG Age Product score. There is dramatic selective atrophy of regions involved in the basal ganglia circuit-caudate, putamen, nucleus accumbens, globus pallidus and substantia nigra. Most other regions of the brain appear to have slower but steady degeneration. These results support (but certainly do not prove) the hypothesis of circuit-based spread of pathology in Huntington's disease, possibly due to spread of mutant Htt protein, though other connection-based mechanisms are possible. Therapeutic targets related to prion-like spread of pathology or other mechanisms may be suggested. In addition, they have implications for current neurosurgical therapeutic approaches, since delivery of therapeutic agents solely to the caudate and putamen may miss other structures affected early, such as nucleus accumbens and output nuclei of the striatum, the substantia nigra and the globus pallidus.

Gelation of cytoplasmic expanded CAG RNA repeats suppresses global protein synthesis.

RNA molecules with the expanded CAG repeat (eCAGr) may undergo sol-gel phase transitions, but the functional impact of RNA gelation is completely unknown. Here, we demonstrate that the eCAGr RNA may form cytoplasmic gel-like foci that are rapidly degraded by lysosomes. These RNA foci may significantly reduce the global protein synthesis rate, possibly by sequestering the translation elongation factor eEF2. Disrupting the eCAGr RNA gelation restored the global protein synthesis rate, whereas enhanced gelation exacerbated this phenotype. eEF2 puncta were significantly enhanced in brain slices from a knock-in mouse model and from patients with Huntington's disease, which is a CAG expansion disorder expressing eCAGr RNA. Finally, neuronal expression of the eCAGr RNA by adeno-associated virus injection caused significant behavioral deficits in mice. Our study demonstrates the existence of RNA gelation inside the cells and reveals its functional impact, providing insights into repeat expansion diseases and functional impacts of RNA phase transition.

Arginine methylation of RNA-binding proteins is impaired in Huntington's disease.

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion in the HD gene, coding for huntingtin protein (HTT). Mechanisms of HD cellular pathogenesis remain undefined and likely involve disruptions in many cellular processes and functions presumably mediated by abnormal protein interactions of mutant HTT. We previously found HTT interaction with several protein arginine methyl-transferases (PRMT) enzymes. Protein arginine methylation mediated by PRMT enzymes is an important post-translational modification with an emerging role in neurodegeneration. We found that normal (but not mutant) HTT can facilitate the activity of PRMTs in vitro and the formation of arginine methylation complexes. These interactions appear to be disrupted in HD neurons. This suggests an additional functional role for HTT/PRMT interactions, not limited to substrate/enzyme relationship, which may result in global changes in arginine protein methylation in HD. Our quantitative analysis of striatal precursor neuron proteome indicated that arginine protein methylation is significantly altered in HD. We identified a cluster highly enriched in RNA-binding proteins with reduced arginine methylation, which is essential to their function in RNA processing and splicing. We found that several of these proteins interact with HTT, and their RNA-binding and localization is affected in HD cells likely due to a compromised arginine methylation and/or abnormal interactions with mutant HTT. These studies reveal a potential new mechanism for disruption of RNA processing in HD, involving a direct interaction of HTT with methyl-transferase enzymes and modulation of their activity, and highlighting methylation of arginine as potential new therapeutic target for HD.

A chlorzoxazone–folic acid combination improves cognitive affective decline in SCA2-58Q mice

Spinocerebellar ataxia type 2 (SCA2) is a polyglutamine disorder caused by a pathological expansion of CAG repeats in ATXN2 gene. SCA2 is accompanied by cerebellar degeneration and progressive motor decline. Cerebellar Purkinje cells (PCs) seem to be primarily affected in this disorder. The majority of the ataxia research is focused on the motor decline observed in ataxic patients and animal models of the disease. However, recent evidence from patients and ataxic mice suggests that SCA2 can also share the symptoms of the cerebellar cognitive affective syndrome. We previously reported that SCA2-58Q PC-specific transgenic mice exhibit anxiolytic behavior, decline in spatial memory, and a depressive-like state. Here we studied the effect of the activation of the small conductance calcium-activated potassium channels (SK channels) by chlorzoxazone (CHZ) combined with the folic acid (FA) on the PC firing and also motor, cognitive and affective symptoms in SCA2-58Q mice. We realized that CHZ-FA combination improved motor and cognitive decline as well as ameliorated mood alterations in SCA2-58Q mice without affecting the firing rate of their cerebellar PCs. Our results support the idea of the combination therapy for both ataxia and non-motor symptoms in ataxic mice without affecting the firing frequency of PCs.

Widespread dysregulation of mRNA splicing implicates RNA processing in the development and progression of Huntington's disease.

In Huntington's disease (HD), a CAG repeat expansion mutation in the Huntingtin (HTT) gene drives a gain-of-function toxicity that disrupts mRNA processing. Although dysregulation of gene splicing has been shown in human HD post-mortem brain tissue, post-mortem analyses are likely confounded by cell type composition changes in late-stage HD, limiting the ability to identify dysregulation related to early pathogenesis. To investigate gene splicing changes in early HD, we performed alternative splicing analyses coupled with a proteogenomics approach to identify early CAG length-associated splicing changes in an established isogenic HD cell model. We report widespread neuronal differentiation stage- and CAG length-dependent splicing changes, and find an enrichment of RNA processing, neuronal function, and epigenetic modification-related genes with mutant HTT-associated splicing. When integrated with a proteomics dataset, we identified several of these differential splicing events at the protein level. By comparing with human post-mortem and mouse model data, we identified common patterns of altered splicing from embryonic stem cells through to post-mortem striatal tissue. We show that widespread splicing dysregulation in HD occurs in an early cell model of neuronal development. Importantly, we observe HD-associated splicing changes in our HD cell model that were also identified in human HD striatum and mouse model HD striatum, suggesting that splicing-associated pathogenesis possibly occurs early in neuronal development and persists to later stages of disease. Together, our results highlight splicing dysregulation in HD which may lead to disrupted neuronal function and neuropathology. This research is supported by the Lee Kong Chian School of Medicine, Nanyang Technological University Singapore Nanyang Assistant Professorship Start-Up Grant, the Singapore Ministry of Education under its Singapore Ministry of Education Academic Research Fund Tier 1 (RG23/22), the BC Children's Hospital Research Institute Investigator Grant Award (IGAP), and a Scholar Award from the Michael Smith Health Research BC.

Machine learning in Huntington’s disease: exploring the Enroll-HD dataset for prognosis and driving capability prediction

BackgroundIn biomedicine, machine learning (ML) has proven beneficial for the prognosis and diagnosis of different diseases, including cancer and neurodegenerative disorders. For rare diseases, however, the requirement for large datasets often prevents this approach. Huntington’s disease (HD) is a rare neurodegenerative disorder caused by a CAG repeat expansion in the coding region of the huntingtin gene. The world’s largest observational study for HD, Enroll-HD, describes over 21,000 participants. As such, Enroll-HD is amenable to ML methods. In this study, we pre-processed and imputed Enroll-HD with ML methods to maximise the inclusion of participants and variables. With this dataset we developed models to improve the prediction of the age at onset (AAO) and compared it to the well-established Langbehn formula. In addition, we used recurrent neural networks (RNNs) to demonstrate the utility of ML methods for longitudinal datasets, assessing driving capabilities by learning from previous participant assessments.ResultsSimple pre-processing imputed around 42% of missing values in Enroll-HD. Also, 167 variables were retained as a result of imputing with ML. We found that multiple ML models were able to outperform the Langbehn formula. The best ML model (light gradient boosting machine) improved the prognosis of AAO compared to the Langbehn formula by 9.2%, based on root mean squared error in the test set. In addition, our ML model provides more accurate prognosis for a wider CAG repeat range compared to the Langbehn formula. Driving capability was predicted with an accuracy of 85.2%. The resulting pre-processing workflow and code to train the ML models are available to be used for related HD predictions at: https://github.com/JasperO98/hdml/tree/main.ConclusionsOur pre-processing workflow made it possible to resolve the missing values and include most participants and variables in Enroll-HD. We show the added value of a ML approach, which improved AAO predictions and allowed for the development of an advisory model that can assist clinicians and participants in estimating future driving capability.

Mutant HTT does not affect glial development but impairs myelination in the early disease stage.

Huntington's disease (HD) is caused by expanded CAG repeats in the huntingtin gene (HTT) and is characterized by late-onset neurodegeneration that primarily affects the striatum. Several studies have shown that mutant HTT can also affect neuronal development, contributing to the late-onset neurodegeneration. However, it is currently unclear whether mutant HTT impairs the development of glial cells, which is important for understanding whether mutant HTT affects glial cells during early brain development. Using HD knock-in mice that express full-length mutant HTT with a 140 glutamine repeat at the endogenous level, we analyzed the numbers of astrocytes and oligodendrocytes from postnatal day 1 to 3 months of age via Western blotting and immunocytochemistry. We also performed electron microscopy, RNAseq analysis, and quantitative RT-PCR. The numbers of astrocytes and oligodendrocytes were not significantly altered in postnatal HD KI mice compared to wild type (WT) mice. Consistently, glial protein expression levels were not significantly different between HD KI and WT mice. However, at 3 months of age, myelin protein expression was reduced in HD KI mice, as evidenced by Western blotting and immunocytochemical results. Electron microscopy revealed a slight but significant reduction in myelin thickness of axons in the HD KI mouse brain at 3 months of age. RNAseq analysis did not show significant reductions in myelin-related genes in postnatal HD KI mice. These data suggest that cytoplasmic mutant HTT, rather than nuclear mutant HTT, mediates myelination defects in the early stages of the disease without impacting the differentiation and maturation of glial cells.

Genome-wide screening in pluripotent cells identifies Mtf1 as a suppressor of mutant huntingtin toxicity

Huntington’s disease (HD) is a neurodegenerative disorder caused by CAG-repeat expansions in the huntingtin (HTT) gene. The resulting mutant HTT (mHTT) protein induces toxicity and cell death via multiple mechanisms and no effective therapy is available. Here, we employ a genome-wide screening in pluripotent mouse embryonic stem cells (ESCs) to identify suppressors of mHTT toxicity. Among the identified suppressors, linked to HD-associated processes, we focus on Metal response element binding transcription factor 1 (Mtf1). Forced expression of Mtf1 counteracts cell death and oxidative stress caused by mHTT in mouse ESCs and in human neuronal precursor cells. In zebrafish, Mtf1 reduces malformations and apoptosis induced by mHTT. In R6/2 mice, Mtf1 ablates motor defects and reduces mHTT aggregates and oxidative stress. Our screening strategy enables a quick in vitro identification of promising suppressor genes and their validation in vivo, and it can be applied to other monogenic diseases.

Transcriptional and Histone Acetylation Changes Associated with CRE Elements Expose Key Factors Governing the Regulatory Circuit in the Early Stage of Huntington's Disease Models.

Huntington's disease (HD) is a disorder caused by an abnormal expansion of trinucleotide CAG repeats within the huntingtin (Htt) gene. Under normal conditions, the CREB Binding Protein interacts with CREB elements and acetylates Lysine 27 of Histone 3 to direct the expression of several genes. However, mutant Htt causes depletion of CBP, which in turn induces altered histone acetylation patterns and transcriptional deregulation. Here, we have studied a differential expression analysis and H3K27ac variation in 4- and 6-week-old R6/2 mice as a model of juvenile HD. The analysis of differential gene expression and acetylation levels were integrated into Gene Regulatory Networks revealing key regulators involved in the altered transcription cascade. Our results show changes in acetylation and gene expression levels that are related to impaired neuronal development, and key regulators clearly defined in 6-week-old mice are proposed to drive the downstream regulatory cascade in HD. Here, we describe the first approach to determine the relationship among epigenetic changes in the early stages of HD. We determined the existence of changes in pre-symptomatic stages of HD as a starting point for early onset indicators of the progression of this disease.

Loss of TDP-43 promotes somatic CAG repeat expansion in Huntington’s disease knock-in mice

TAR binding protein 43 (TDP-43) is normally present in the nucleus but mislocalized in the cytoplasm in a number of neurodegenerative diseases including Huntington’s disease (HD). The nuclear loss of TDP-43 impairs gene transcription and regulation. However, it remains to be investigated whether loss of TDP-43 influences trinucleotide CAG repeat expansion in the HD gene, a genetic cause for HD. Here we report that CRISPR/Cas9 mediated-knock down of endogenous TDP-43 in the striatum of HD knock-in mice promoted CAG repeat expansion, accompanied by the increased expression of the DNA mismatch repair genes, Msh3 and Mlh1, which have been reported to increase trinucleotide repeat instability. Furthermore, suppressing Msh3 and Mlh1 by CRISPR/Cas9 targeting diminished the CAG repeat expansion. These findings suggest that nuclear TDP-43 deficiency may dysregulate the expression of DNA mismatch repair genes, leading to CAG repeat expansion and contributing to the pathogenesis of CAG repeat diseases.