Trinucleotide Repeat Expansion Disorders Research Articles

The Role of NRF2 in Trinucleotide Repeat Expansion Disorders

The Role of NRF2 in Trinucleotide Repeat Expansion Disorders

Huntington disease-like 2: insight into neurodegeneration from an African disease.

Huntington disease (HD)-like 2 (HDL2) is a rare genetic disease caused by an expanded trinucleotide repeat in the JPH3 gene (encoding junctophilin 3) that shows remarkable clinical similarity to HD. To date, HDL2 has been reported onlyin patients with definite or probable African ancestry. A single haplotype background is shared by patients with HDL2 from different populations, supporting a common African origin for the expansion mutation. Nevertheless, outside South Africa, reports of patients with HDL2 in Africa are scarce, probably owing to limited clinical services across the continent. Systematic comparisons of HDL2 and HD have revealed closely overlapping motor, cognitive and psychiatric features and similar patterns of cerebral and striatal atrophy. The pathogenesis of HDL2 remains unclear but it is proposed to occur through several mechanisms, including loss of protein function and RNA and/or protein toxicity. This Review summarizes our current knowledge of this African-specific HD phenocopy and highlights key areas of overlap between HDL2 and HD. Given the aforementioned similarities in clinical phenotype and pathology, an improved understanding of HDL2 could provide novel insights into HD and other neurodegenerative and/or trinucleotide repeat expansion disorders.

Repeat-associated non-AUG translation induces cytoplasmic aggregation of CAG repeat-containing RNAs

CAG trinucleotide repeat expansions cause several neurodegenerative diseases, including Huntington's disease and spinocerebellar ataxia. RNAs with expanded CAG repeats contribute to disease in two unusual ways. First, these repeat-containing RNAs may agglomerate in the nucleus as foci that sequester several RNA-binding proteins. Second, these RNAs may undergo aberrant repeat-associated non-AUG (RAN) translation in multiple frames and produce aggregation-prone proteins. The relationship between RAN translation and RNA foci, and their relative contributions to cellular dysfunction, are unclear. Here, we show that CAG repeat-containing RNAs that undergo RAN translation first accumulate at nuclear foci and, over time, are exported to the cytoplasm. In the cytoplasm, these RNAs are initially dispersed but, upon RAN translation, aggregate with the RAN translation products. These RNA-RAN protein agglomerates sequester various RNA-binding proteins and are associated with the disruption of nucleocytoplasmic transport and cell death. In contrast, RNA accumulation at nuclear foci alone does not produce discernable defects in nucleocytoplasmic transport or cell viability. Inhibition of RAN translation prevents cytoplasmic RNA aggregation and alleviates cell toxicity. Our findings demonstrate that RAN translation-induced RNA-protein aggregation correlates with the key pathological hallmarks observed in disease and suggest that cytoplasmic RNA aggregation may be an underappreciated phenomenon in CAG trinucleotide repeat expansion disorders.

Loss of Corneal Nerves and Corneal Haze in Patients with Fuchs’ Endothelial Corneal Dystrophy with the Transcription Factor 4 Gene Trinucleotide Repeat Expansion

ObjectiveSeventy percent of Fuchs’ endothelial corneal dystrophy (FECD) cases are caused by an intronic trinucleotide repeat expansion in the transcription factor 4 gene (TCF4). The objective of this study was to characterize the corneal subbasal nerve plexus and corneal haze in patients with FECD with (RE+) and without the trinucleotide repeat expansion (RE−) and to assess the correlation of these parameters with disease severity.DesignCross-sectional, single-center study.ParticipantsFifty-two eyes of 29 subjects with a modified Krachmer grade of FECD severity from 1 to 6 were included in the study. Fifteen of the 29 subjects carried an expanded TCF4 allele length of ≥ 40 cytosine-thymine-guanine repeats (RE+).Main Outcomes MeasuresIn vivo confocal microscopy assessments of corneal nerve fiber length (CNFL), corneal nerve branch density, corneal nerve fiber density (CNFD), and anterior corneal stromal backscatter (haze); Scheimpflug tomography densitometry measurements of haze in anterior, central, and posterior corneal layers.ResultsUsing confocal microscopy, we detected a negative correlation between FECD severity and both CNFL and CNFD in the eyes of RE+ subjects (Spearman ρ = −0.45, P = 0.029 and ρ = −0.62, P = 0.0015, respectively) but not in the eyes of RE− subjects. Additionally, CNFD negatively correlated with the repeat length of the expanded allele in the RE+ subjects (Spearman ρ = −0.42, P = 0.038). We found a positive correlation between anterior stromal backscatter and severity in both the RE+ and RE− groups (ρ = 0.60, P = 0.0023 and ρ = 0.44, P = 0.024, respectively). The anterior, central, and posterior Scheimpflug densitometry measurements also positively correlated with severity in both the RE+ and RE− groups (P = 5.5 × 10−5, 2.5 × 10−4, and 2.9 × 10−4, respectively, after adjusting for the expansion status in a pooled analysis. However, for patients with severe FECD (Krachmer grades 5 and 6), the posterior densitometry measurements were higher in the RE+ group than in the RE− group (P < 0.05).ConclusionsLoss of corneal nerves in FECD supports the classification of the TCF4 trinucleotide repeat expansion disorder as a neurodegenerative disease. Haze in the anterior, central, and posterior cornea correlate with severity, irrespective of the genotype. Quantitative assessments of corneal nerves and corneal haze may be useful to gauge and monitor FECD disease severity in RE+ patients.

Benefits of aerobic exercise in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a multisystem trinucleotide repeat expansion disorder characterized by the misregulated alternative splicing of critical mRNAs. Previous work in a transgenic mouse model indicated that aerobic exercise effectively improves splicing regulation and function in skeletal muscle. In this issue of the JCI, Mikhail et al. describe the safety and benefits of applying this approach in individuals affected by DM1. A 12-week aerobic exercise program improved aerobic capacity and mobility, but not by the mechanism observed in transgenic mice. Here, we consider the possible reasons for this disparity and review other salient findings of the study in the context of evolving DM1 research.

Diagrammatic approaches to RNA structures with trinucleotide repeats

Trinucleotide repeat expansion disorders are associated with the overexpansion of (CNG) repeats on the genome. Messenger RNA transcripts of sequences with greater than 60–100 (CNG) tandem units have been implicated in trinucleotide repeat expansion disorder pathogenesis. In this work, we develop a diagrammatic theory to study the structural diversity of these (CNG)n RNA sequences. Representing structural elements on the chain’s conformation by a set of graphs and employing elementary diagrammatic methods, we have formulated a renormalization procedure to re-sum these graphs and arrive at a closed-form expression for the ensemble partition function. With a simple approximation for the renormalization and applied to extended (CNG)n sequences, this theory can comprehensively capture an infinite set of conformations with any number and any combination of duplexes, hairpins, multiway junctions, and quadruplexes. To quantify the diversity of different (CNG)n ensembles, the analytical equations derived from the diagrammatic theory were solved numerically to derive equilibrium estimates for the secondary structural contents of the chains. The results suggest that the structural ensembles of (CNG)n repeat sequence with n ∼60 are surprisingly diverse, and the distribution is sensitive to the ability of the N nucleotide to make noncanonical pairs and whether the (CNG)n sequence can sustain stable quadruplexes. The results show how perturbations in the form of biases on the stabilities of the various structural motifs, duplexes, junctions, helices, and quadruplexes could affect the secondary structures of the chains and how these structures may switch when they are perturbed.

Chronic Exposure to Cadmium and Antioxidants Does Not Affect the Dynamics of Expanded CAG•CTG Trinucleotide Repeats in a Mouse Cell Culture System of Unstable DNA.

More than 30 human disorders are caused by the expansion of simple sequence DNA repeats, among which triplet repeats remain the most frequent. Most trinucleotide repeat expansion disorders affect primarily the nervous system, through mechanisms of neurodysfunction and/or neurodegeneration. While trinucleotide repeat tracts are short and stably transmitted in unaffected individuals, disease-associated expansions are highly dynamic in the germline and in somatic cells, with a tendency toward further expansion. Since longer repeats are associated with increasing disease severity and earlier onset of symptoms, intergenerational repeat size gains account for the phenomenon of anticipation. In turn, higher levels of age-dependent somatic expansion have been linked with increased disease severity and earlier age of onset, implicating somatic instability in the onset and progression of disease symptoms. Hence, tackling the root cause of symptoms through the control of repeat dynamics may provide therapeutic modulation of clinical manifestations. DNA repair pathways have been firmly implicated in the molecular mechanism of repeat length mutation. The demonstration that repeat expansion depends on functional DNA mismatch repair (MMR) proteins, points to MMR as a potential therapeutic target. Similarly, a role of DNA base excision repair (BER) in repeat expansion has also been suggested, particularly during the removal of oxidative lesions. Using a well-characterized mouse cell model system of an unstable CAG•CTG trinucleotide repeat, we tested if expanded repeat tracts can be stabilized by small molecules with reported roles in both pathways: cadmium (an inhibitor of MMR activity) and a variety of antioxidants (capable of neutralizing oxidative species). We found that chronic exposure to sublethal doses of cadmium and antioxidants did not result in significant reduction of the rate of trinucleotide repeat expansion. Surprisingly, manganese yielded a significant stabilization of the triplet repeat tract. We conclude that treatment with cadmium and antioxidants, at doses that do not interfere with cell survival and cell culture dynamics, is not sufficient to modify trinucleotide repeat dynamics in cell culture.

Bisulfite Treatment of CG-Rich Track of Trinucleotide Repeat Expansion Disorder: Make the Sequence Less CG Rich.

Background:Trinucleotide repeat (TNR) expansion is a kind of mutation with instability in the number of microsatellite repeats. This nature of mutation leads to the different kinds of neurological and neuromuscular disorders; among them, fragile-X syndrome is the main cause of intellectual disability in which the increasing number of CGG TNR in 5' untranslated region is the main reason for epigenetic silencing of Fragile X mental retardation 1 gene. The aim of this study is to decrease the CG content of the candidate region to facilitate amplification by conventional polymerase chain reaction (PCR). Bisulfite treatment of the genomic DNA results in conversion of unmethylated cytosine to uridine and may overcome the diagnostic pitfalls.Materials and Methods:The whole blood DNA was extracted and bisulfite treated. Then any simplification in PCR process of desire sequence were assayed through following conventional PCR using specifically designed primers for converted sequence. Bisulfite-treated PCR product of a nearby sequence confirmed our results as a conversion control.Results:Both the control and the candidate sequences undergoing bisulfite treatment were successfully amplified by PCR.Conclusions:Decreasing the GC content of the sequence by bisulfite treating could be a new approach to overcome difficulties in amplifying GC-rich sequences.

THE INTERACTION OF miR-4258, miR-3960, miR-211-3p AND miR-3155b WITH mRNAs GENES OF NON-POLYGLUTAMINE TRINUCLEOTIDE DISORDERS

Trinucleotide repeat expansion disorders constitute a group of dominantly inherited neurological diseases that are incurable and ultimately fatal. In the present work, miRNA binding sites were predicted by the MirTarget program. It was given characteristics of miRNAs binding sites in 5' and 3' UTR mRNAs genes of non-polyglutamine trinucleotide disorders with CGG, GCC, CUG repeats. Binding sites of 2567 miRNAs with mRNAs of 17494 human genes were determined. 206 genes with nucleotide repeats, mRNAs of which are bind with miRNA in the 5'UTR and 3'UTR, were observed. From thus, 2668 miRNAs binding sites are located in the 5'UTR, 3853 – in the 3'UTR with ΔG/ΔGm values equal to 85 % and more. It was found that 34 gene’s mRNA having trinucleotide (CGG\GCC\CUG) repeats were targets for miR-4258, miR-3960 miR-211-3p and miR-3155b. miR-4258 binds to mRNA of ADARB1, C11orf87 and CBFB genes with free binding energy - 93 kJ/mole and ΔG/ΔGm 91%, to mRNA of ARHGEF7, BCR, BRSK2 and C9orf91 genes with free binding energy - 91 kJ/mole and ΔG/ΔGm 89%. miR-3960 binds in GCC repeats to mRNA of ABCC1 and BLMH genes with free binding energy - 116 kJ/mole. miR-211-3p and miR-3155b interact with mRNA of ACACA and ANKRD13D genes in 5’-3’untranslated regions. Studying binding characteristics of miRNA and genes will help identify association of miRNAs with genes with trinucleotide repeats for recommending for the diagnosis of nucleotide repeat expansion disorders.

Quantifying Structural Diversity of CNG Trinucleotide Repeats using Diagrammatic Algorithms

Trinucleotide repeat expansion disorders (TREDs) exhibit complex mechanisms of pathogenesis, some of which have been attributed to RNA transcripts of overexpanded CNG repeats, resulting in possibly a gain-of-function. In this paper, we aim to probe the structures of these expanded transcript by analyzing the structural diversity of their conformational ensembles. We used graphs to catalog the structures of an NG-(CNG)16-CN oligomer and grouped them into subensembles based on their characters and calculated the structural diversity and thermodynamic stability for these ensembles using a previously described graph factorization scheme. Our findings show that the generally assumed structure for CNG repeats--a series of canonical helices connected by two-way junctions and capped with a hairpin loop--may not be the most thermodynamically favorable, and the ensembles are characterized by largely open and less structured conformations. Furthermore, a length-dependence is observed for the behavior of the ensembles9 diversity as higher-order diagrams are included, suggesting that further studies of CNG repeats are needed at the length scale of TREDs onset to properly understand their structural diversity and how this might relate to their functions.

Real Time Videomicroscopy and Semiautomated Analysis of Brain Cell Culture Models of Trinucleotide Repeat Expansion Diseases.

Proper brain function requires the coordinated and intricate interaction between neuronal and glial cells. Like many other neurological conditions, trinucleotide repeat expansion disorders are likely initiated by the synergistic combination of abnormalities hitting different brain cell types, which ultimately disrupt brain function and lead to the onset of neurological symptoms. Understanding how trinucleotide repeat expansions affect the phenotypes and physiology of neurons and glia is fundamental to improve our understanding of disease mechanisms in the brain and shape the design of future therapeutic interventions.Here we describe a protocol for semiautomated videomicroscopy analysis of cultured brain cells, maintained under suitable and controlled conditions. Through real-time monitoring of basic cell phenotypes (such as proliferation, cell morphology, differentiation, and migration) this method provides an accurate primary assessment of the impact of the repeat expansion on the physiology of neurons and glia. The versatility of the system, the automated image acquisition and the semiautomated processing of the data collected allow rapid phenotypic analysis of individual cell types, as well as the investigation of cell-cell interactions. The stability of the acquisition system provides reproducible and robust results. The raw data can be easily exported to other software to perform more sophisticated imaging analysis and statistical tests. In summary, the methods described offer versatile, reproducible, and time-effective means to dissect the impact of the repeat expansion on different brain cell types and on intercellular interactions.

Structural insights into synthetic ligands targeting A-A pairs in disease-related CAG RNA repeats.

The trinucleotide repeat expansion disorders (TREDs) constitute of a group of >40 hereditary neurodegenerative human diseases associated with abnormal expansion of repeated sequences, such as CAG repeats. The pathogenic factor is a transcribed RNA or protein whose function in the cell is compromised. The disorders are progressive and incurable. Consequently, many ongoing studies are oriented at developing therapies. We have analyzed crystal structures of RNA containing CAG repeats in complex with synthetic cyclic mismatch-binding ligands (CMBLs). The models show well-defined interactions between the molecules in which the CMBLs mimic nucleobases as they form pseudo-canonical base pairs with adenosine residues and engage in extensive stacking interactions with neighboring nucleotides. The binding of ligands is associated with major structural changes of the CAG repeats, which is consistent with results of biochemical studies. The results constitute an early characterization of the first lead compounds in the search for therapy against TREDs. The crystallographic data indicate how the compounds could be further refined in future biomedical studies.

Exploring the Potential of Small Molecule-Based Therapeutic Approaches for Targeting Trinucleotide Repeat Disorders.

In recent years, neurological disorders have globally become a leading cause of disability and death. Neurological disorders are very common in both high- and low-income countries, and the number of patients is predicted to increase in the coming decades. Disorders caused by the expanded trinucleotide repeats (CAG, CGG, CCG, CTG, CUG, GAA, and GCN) in the genome, also described as trinucleotide repeat expansion disorders (TREDs), comprise of the major class of neurological diseases. Various TREDs have different modes of pathogenesis, but the severity and time of onset of disease depends on the trinucleotide repeat numbers. Numerous therapeutic strategies, including symptomatic treatment, blockage of mutant protein synthesis, targeting the toxic protein aggregates and degradation of RNA transcripts have been developed for the treatment of these diseases. However, various limitations to these therapeutic strategies have been reported, and therefore, researchers are exploring different avenues of therapeutics development. One of the recent developments include targeting the expanded repeats with small molecules. Small molecule binds with the secondary/tertiary structure of RNA (like bulges, loops, and hairpins) irrespective of its sequences. Altogether, small molecule-based therapeutics may have the advantage over others to be able to overcome the hurdles of the blood-brain barrier, poor absorption, and allergic reactions. In this review, we have summarized various TREDs and envisage the potential of small molecule-based therapeutics for targeting these hitherto incurable neurological disorders.

Diagnostic Utility of Genome-wide DNA Methylation Testing in Genetically Unsolved Individuals with Suspected Hereditary Conditions

Conventional genetic testing of individuals with neurodevelopmental presentations and congenital anomalies (ND/CAs), i.e., the analysis of sequence and copy number variants, leaves a substantial proportion of them unexplained. Some of these cases have been shown to result from DNA methylation defects at a single locus (epi-variants), while others can exhibit syndrome-specific DNA methylation changes across multiple loci (epi-signatures). Here, we investigate the clinical diagnostic utility of genome-wide DNA methylation analysis of peripheral blood in unresolved ND/CAs. We generate a computational model enabling concurrent detection of 14 syndromes using DNA methylation data with full accuracy. We demonstrate the ability of this model in resolving 67 individuals with uncertain clinical diagnoses, some of whom had variants of unknown clinical significance (VUS) in the related genes. We show that the provisional diagnoses can be ruled out in many of the case subjects, some of whom are shown by our model to have other diseases initially not considered. By applying this model to a cohort of 965 ND/CA-affected subjects without a previous diagnostic assumption and a separate assessment of rare epi-variants in this cohort, we identify 15 case subjects with syndromic Mendelian disorders, 12 case subjects with imprinting and trinucleotide repeat expansion disorders, as well as 106 case subjects with rare epi-variants, a portion of which involved genes clinically or functionally linked to the subjects' phenotypes. This study demonstrates that genomic DNA methylation analysis can facilitate the molecular diagnosis of unresolved clinical cases and highlights the potential value of epigenomic testing in the routine clinical assessment of ND/CAs.

GGC Repeat Expansion and Exon 1 Methylation of XYLT1 Is a Common Pathogenic Variant in Baratela-Scott Syndrome

Baratela-Scott syndrome (BSS) is a rare, autosomal-recessive disorder characterized by short stature, facial dysmorphisms, developmental delay, and skeletal dysplasia caused by pathogenic variants in XYLT1. We report clinical and molecular investigation of 10 families (12 individuals) with BSS. Standard sequencing methods identified biallelic pathogenic variants in XYLT1 in only two families. Of the remaining cohort, two probands had no variants and six probands had only a single variant, including four with a heterozygous 3.1 Mb 16p13 deletion encompassing XYLT1 and two with a heterozygous truncating variant. Bisulfite sequencing revealed aberrant hypermethylation in exon 1 of XYLT1, always in trans with the sequence variant or deletion when present; both alleles were methylated in those with no identified variant. Expression of the methylated XYLT1 allele was severely reduced in fibroblasts from two probands. Southern blot studies combined with repeat expansion analysis of genome sequence data showed that the hypermethylation is associated with expansion of a GGC repeat in the XYLT1 promoter region that is not present in the reference genome, confirming that BSS is a trinucleotide repeat expansion disorder. The hypermethylated allele accounts for 50% of disease alleles in our cohort and is not present in 130 control subjects. Our study highlights the importance of investigating non-sequence-based alterations, including epigenetic changes, to identify the missing heritability in genetic disorders.

Group Testing Approach for Trinucleotide Repeat Expansion Disorder Screening.

Fragile X syndrome (FXS, OMIM #300624) is an X-linked condition caused by trinucleotide repeat expansions in the 5' UTR (untranslated region) of the fragile X mental retardation 1 (FMR1) gene. FXS testing is commonly performed in expanded carrier screening and has been proposed for inclusion in newborn screening. However, because pathogenic alleles are long and have low complexity (>200 CGG repeats), FXS is currently tested by a single-plex electrophoresis-resolved PCR assay rather than multiplexed approaches like next-generation sequencing or mass spectrometry. In this work, we sought an experimental design based on nonadaptive group testing that could accurately and reliably identify the size of abnormally expanded FMR1 alleles of males and females. We developed a new group testing scheme named StairCase (SC) that was designed to the constraints of the FXS testing problem, and compared its performance to existing group testing schemes by simulation. We experimentally evaluated SC's performance on 210 samples from the Coriell Institute biorepositories using pooled PCR followed by capillary electrophoresis on 3 replicates of each of 3 pooling layouts differing by the mapping of samples to pools. The SC pooled PCR approach demonstrated perfect classification of samples by clinical category (normal, intermediate, premutation, or full mutation) for 90 positives and 1800 negatives, with a batch of 210 samples requiring only 21 assays. Group testing based on SC is an implementable approach to trinucleotide repeat expansion disorder testing that offers ≥10-fold reduction in assay costs over current single-plex methods.

Deep sequencing of mitochondrial genomes reveals increased mutation load in Friedreich's ataxia

ObjectiveFriedreich's ataxia (FRDA) is an autosomal recessive trinucleotide repeat expansion disorder caused by epigenetic silencing of the frataxin gene (FXN). Current research suggests that damage and variation of mitochondrial DNA (mtDNA) contribute to the molecular pathogenesis of FRDA. We sought to establish the extent of the mutation burden across the mitochondrial genome in FRDA cells and investigate the molecular mechanisms connecting FXN downregulation and the acquisition of mtDNA damage.MethodsDamage and mutation load in mtDNA of a panel of FRDA and control fibroblasts were determined using qPCR and next‐generation MiSeq sequencing, respectively. The capacity of FRDA and control cells to repair oxidative lesions in their mtDNA was measured using a quantitative DNA damage assay. Comprehensive RNA sequencing gene expression analyses were conducted to assess the status of DNA repair and metabolism genes in FRDA cells.ResultsAcute or prolonged downregulation of FXN expression resulted in a significant increase in mtDNA damage that translated to a significant elevation of mutation load in mtDNA. The predominant mutations identified throughout the mtDNA were C>T, G>A transitions (P = 0.007). Low FXN expression reduced capacity to repair oxidative damage in mtDNA. Downregulation of FXN expression strongly correlated (r = 0.73) with decreased levels of base excision repair (BER) DNA glycosylase NTHL1.InterpretationDownregulation of FXN expression in FRDA cells elevates mtDNA damage, increases mutation load of the mitochondrial genome, and diminishes DNA repair capacity. Progressive accumulation of mtDNA mutations in vulnerable FRDA patient cells reduces mitochondrial fitness ultimately leading to cell death.

Twisting right to left: A…A mismatch in a CAG trinucleotide repeat overexpansion provokes left-handed Z-DNA conformation.

Conformational polymorphism of DNA is a major causative factor behind several incurable trinucleotide repeat expansion disorders that arise from overexpansion of trinucleotide repeats located in coding/non-coding regions of specific genes. Hairpin DNA structures that are formed due to overexpansion of CAG repeat lead to Huntington’s disorder and spinocerebellar ataxias. Nonetheless, DNA hairpin stem structure that generally embraces B-form with canonical base pairs is poorly understood in the context of periodic noncanonical A…A mismatch as found in CAG repeat overexpansion. Molecular dynamics simulations on DNA hairpin stems containing A…A mismatches in a CAG repeat overexpansion show that A…A dictates local Z-form irrespective of starting glycosyl conformation, in sharp contrast to canonical DNA duplex. Transition from B-to-Z is due to the mechanistic effect that originates from its pronounced nonisostericity with flanking canonical base pairs facilitated by base extrusion, backbone and/or base flipping. Based on these structural insights we envisage that such an unusual DNA structure of the CAG hairpin stem may have a role in disease pathogenesis. As this is the first study that delineates the influence of a single A…A mismatch in reversing DNA helicity, it would further have an impact on understanding DNA mismatch repair.

MicroRNAs in CAG trinucleotide repeat expansion disorders: an integrated review of the literature.

MicroRNAs are small RNAs involved in gene silencing. They play important roles in transcriptional regulation and are selectively and abundantly expressed in the central nervous system. A considerable amount of the human genome is comprised of tandem repeating nucleotide streams. Several diseases are caused by above-threshold expansion of certain trinucleotide repeats occurring in a protein-coding or non-coding region. Though monogenic, CAG trinucleotide repeat expansion disorders have a complex pathogenesis, various combinations of multiple coexisting pathways resulting in one common final consequence: selective neurodegeneration. Mutant protein and mutant transcript gain of toxic function are considered to be the core pathogenic mechanisms. The profile of microRNAs in CAG trinucleotide repeat disorders is scarcely described, however microRNA dysregulation has been identified in these diseases and microRNA-related intereference with gene expression is considered to be involved in their pathogenesis. Better understanding of microRNAs functions and means of manipulation promises to offer further insights into the pathogenic pathways of CAG repeat expansion disorders, to point out new potential targets for drug intervention and to provide some of the much needed etiopathogenic therapeutic agents. A number of disease-modifying microRNA silencing strategies are under development, but several implementation impediments still have to be resolved. CAG targeting seems feasible and efficient in animal models and is an appealing approach for clinical practice. Preliminary human trials are just beginning.

The role of the immune system in triplet repeat expansion diseases.

Trinucleotide repeat expansion disorders (TREDs) are a group of dominantly inherited neurological diseases caused by the expansion of unstable repeats in specific regions of the associated genes. Expansion of CAG repeat tracts in translated regions of the respective genes results in polyglutamine- (polyQ-) rich proteins that form intracellular aggregates that affect numerous cellular activities. Recent evidence suggests the involvement of an RNA toxicity component in polyQ expansion disorders, thus increasing the complexity of the pathogenic processes. Neurodegeneration, accompanied by reactive gliosis and astrocytosis is the common feature of most TREDs, which may suggest involvement of inflammation in pathogenesis. Indeed, a number of immune response markers have been observed in the blood and CNS of patients and mouse models, and the activation of these markers was even observed in the premanifest stage of the disease. Although inflammation is not an initiating factor of TREDs, growing evidence indicates that inflammatory responses involving astrocytes, microglia, and the peripheral immune system may contribute to disease progression. Herein, we review the involvement of the immune system in the pathogenesis of triplet repeat expansion diseases, with particular emphasis on polyglutamine disorders. We also present various therapeutic approaches targeting the dysregulated inflammation pathways in these diseases.