Trinucleotide Repeat Expansion Disorders

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Trinucleotide Repeat Expansion Disorders

Introduction

Trinucleotide Repeat Expansion Disorders is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

Trinucleotide repeat expansion disorders are a class of genetic diseases caused by the abnormal expansion of short tandem DNA repeats (typically 3-6 nucleotides) beyond a critical threshold length. Over fifty human disorders are caused by repeat expansions, including many of the most common neurodegenerative diseases such as huntington-pathway, spinocerebellar ataxias, friedreichs-ataxia, myotonic-dystrophy, fragile-x-associated-tremor-ataxia-syndrome, and drpla ([Paulson, 2018](https://pmc.ncbi.nlm.nih.gov/articles/PMC6485936/)) [@malik2021]. [@bates2015]

Trinucleotide Repeat Expansion Disease Mechanism

...

Trinucleotide Repeat Expansion Disorders

Introduction

Trinucleotide Repeat Expansion Disorders is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

Trinucleotide repeat expansion disorders are a class of genetic diseases caused by the abnormal expansion of short tandem DNA repeats (typically 3-6 nucleotides) beyond a critical threshold length. Over fifty human disorders are caused by repeat expansions, including many of the most common neurodegenerative diseases such as huntington-pathway, spinocerebellar ataxias, friedreichs-ataxia, myotonic-dystrophy, fragile-x-associated-tremor-ataxia-syndrome, and drpla ([Paulson, 2018](https://pmc.ncbi.nlm.nih.gov/articles/PMC6485936/)) [@malik2021]. [@bates2015]

Trinucleotide Repeat Expansion Disease Mechanism

Mermaid diagram (expand to render)

These disorders share a common molecular etiology — the dynamic mutation of microsatellite repeats — but manifest through multiple distinct pathogenic mechanisms including protein [@lieberman2019]
gain-of-function (e.g., polyglutamine toxicity), RNA-mediated toxicity, loss of gene function, and repeat-associated non-AUG (RAN) translation. Understanding these shared [@dejesushernandez2011]
mechanisms has opened cross-disease therapeutic strategies targeting the underlying repeat expansions [@bates2015]. [@campuzano1996]

Classification of Repeat Expansion Disorders

Polyglutamine (PolyQ) Diseases

The best-characterized class involves CAG repeat expansions within coding regions, producing proteins with extended polyglutamine tracts. These include: [@wojciechowska2011]

huntington-pathway: CAG expansion in the htt gene encoding [huntingtin](/proteins/huntingtin-protein) protein]; normal range 6-35 repeats, pathogenic >36 repeats ([Bates et al., 2015](https://doi.org/10.1038/nrdp.2015.5))
Spinocerebellar ataxias (SCA1, SCA2, SCA3/MJD, SCA6, SCA7, SCA17): CAG expansions in ataxin genes causing progressive cerebellar degeneration
kennedys-disease: CAG expansion in the androgen receptor gene
dentatorubral-pallidoluysian-atrophy: CAG expansion in the atrophin-1 gene

Polyglutamine diseases share several features: dominant inheritance, progressive neurodegeneration with mid-life onset, inverse correlation between repeat length and age of onset (genetic anticipation), and formation of intranuclear protein inclusions ([Lieberman et al., 2019](https://doi.org/10.1038/s41583-019-0141-0)) [@lieberman2019]. [@gottesfeld2019]

Non-Coding Repeat Expansion Disorders

Repeat expansions in non-coding regions cause disease through fundamentally different mechanisms: [@zu2011]

friedreichs-ataxia: GAA expansion in intron 1 of FXN, causing transcriptional silencing and loss of frataxin protein. This is the only common autosomal recessive repeat expansion disorder ([Campuzano et al., 1996](https://doi.org/10.1126/science.271.5254.1423))
myotonic-dystrophy type 1 (DM1): CTG expansion in the 3' UTR of DMPK; type 2 (DM2) involves CCTG expansion in CNBP
fxtas: CGG expansion (55-200 repeats) in the 5' UTR of FMR1
c9orf72-ALS/FTD: GGGGCC hexanucleotide expansion in intron 1, the most common genetic cause of both als and ftd ([DeJesus-Hernandez et al., 2011](https://doi.org/10.1016/j.neuron.2011.09.011))

Molecular Mechanisms of Pathogenesis

The pathogenic mechanisms of repeat expansion disorders can be grouped into four major categories, often with multiple mechanisms operating simultaneously in a single disease ([Malik et al., 2021](https://doi.org/10.1038/s41580-021-00382-6)): [@freibaum2017]

1. Protein Gain-of-Function (Polyglutamine Toxicity)

In polyglutamine diseases, the expanded CAG repeat is translated into an abnormally long polyglutamine tract within the host protein. These expanded polyglutamine proteins undergo conformational changes, transitioning from soluble monomers to oligomeric intermediates and eventually to insoluble amyloid-like fibrils. This process shares features with protein-aggregation in other neurodegenerative diseases [@dejesushernandez2011]. [@banezcoronel2015]

Key aspects of polyglutamine toxicity include: [@iyer2015]

Misfolding and aggregation: Expanded polyglutamine tracts adopt β-sheet-rich conformations that self-assemble into amyloid fibrils, forming characteristic intranuclear inclusions ([Scherzinger et al., 1999](https://doi.org/10.1073/pnas.96.8.4604))
Proteostatic stress: Aggregates overwhelm the ubiquitin-proteasome-system and [autophagy](/mechanisms/autophagy)mechanisms/[autophagy](/mechanisms/autophagy) machinery, impairing cellular proteostasis
Transcriptional dysregulation: Expanded polyglutamine proteins sequester transcription factors (e.g., CBP, Sp1) and disrupt histone-modifications, leading to widespread gene expression changes
Mitochondrial dysfunction: PolyQ aggregates impair mitochondrial-dynamics and oxidative phosphorylation, increasing oxidative-stress production ([Johri & Beal, 2012](https://doi.org/10.1016/j.bbadis.2011.10.003))
Prion-like spreading: Polyglutamine aggregates can template the misfolding of normal-length polyglutamine proteins and spread between cells in a prion-like manner

2. RNA-Mediated Toxicity

In non-coding repeat expansion disorders, the expanded repeat RNA itself is a primary toxic species. Transcribed repeat RNAs form complex secondary structures (hairpins, G-quadruplexes) that sequester essential RNA-binding proteins (RBPs), disrupting normal RNA processing ([Wojciechowska & Krzyzosiak, 2011](https://pmc.ncbi.nlm.nih.gov/articles/PMC5720136/)) [@campuzano1996]. [@genetic2019]

RNA foci formation: Expanded repeat RNAs accumulate in nuclear foci, where they sequester specific RBPs: [@bhatt2024]

In DM1, expanded CUG repeats sequester muscleblind-like (MBNL) proteins, causing widespread splicing defects
In c9orf72-ALS/FTD, GGGGCC repeat RNAs form G-quadruplex structures and sequester multiple RBPs including hnRNP H and nucleolin
In FXTAS, CGG repeat RNAs sequester fus, DROSHA/DGCR8, and other RBPs

Bidirectional transcription: Many repeat expansion loci are transcribed from both sense and antisense strands, doubling the potential for RNA-mediated toxicity. Antisense transcripts have been detected in c9orf72-ALS/FTD, SCA8, and DM1. [@johri2012]

3. Loss of Gene Function

Some repeat expansions cause disease primarily through reduced expression of the affected gene: [@tabrizi2019]

friedreichs-ataxia: The GAA expansion in FXN induces heterochromatin formation and transcriptional silencing through R-loop formation and dna-methylation, reducing frataxin levels to 5-30% of normal. Frataxin is essential for mitochondrial iron-sulfur cluster assembly ([Gottesfeld, 2019](https://doi.org/10.1074/jbc.R117.817478))
Fragile X syndrome: CGG expansions >200 repeats cause hypermethylation and complete silencing of FMR1
c9orf72-ALS/FTD: The GGGGCC expansion partially reduces c9orf72 protein expression, although haploinsufficiency alone is insufficient to cause disease

4. Repeat-Associated Non-AUG (RAN) Translation

RAN translation is a recently discovered mechanism in which expanded repeat sequences are translated without a canonical AUG start codon, producing toxic repeat polypeptides ([Zu et al., 2011](https://doi.org/10.1073/pnas.1108تف05108)). RAN translation occurs in all three reading frames and from both sense and antisense strands, potentially generating six different homopolymeric or dipeptide-repeat proteins from a single expansion locus [@wojciechowska2011].

RAN translation in c9orf72-ALS/FTD: The GGGGCC expansion produces five distinct dipeptide repeat proteins (DPRs):

Sense strand: poly(GA), poly(GP), poly(GR)
Antisense strand: poly(PA), poly(PR), poly(GP)

The arginine-rich DPRs (poly-GR and poly-PR) are the most toxic, disrupting nucleocytoplasmic transport, ribosomal function, and rna-metabolism ([Freibaum & Taylor, 2017](https://pmc.ncbi.nlm.nih.gov/articles/PMC7930069/)). RAN translation is upregulated by cellular stress through phosphorylation of eIF2α, the core event of the integrated stress response, creating a feed-forward loop of toxicity ([Green et al., 2017](https://doi.org/10.1038/s41467-017-02495-z)) [@gottesfeld2019].

RAN translation in other diseases: RAN translation has also been demonstrated in SCA8 (polyalanine, polyserine, polyglutamine from antisense transcript), DM1, FXTAS (FMRpolyG), and multiple polyglutamine diseases including huntington-pathway ([Banez-Coronel et al., 2015](https://doi.org/10.1016/j.neuron.2015.02.038)).

Genetic Anticipation and Repeat Instability

A hallmark of trinucleotide repeat disorders is genetic anticipation — the tendency for disease severity to increase and age of onset to decrease in successive generations. This occurs because expanded repeats are inherently unstable during DNA replication and repair, with a bias toward further expansion during intergenerational transmission [@gottesfeld2019].

Key factors influencing repeat instability include:

Repeat length: Longer repeats are more unstable; a threshold length (typically 35-45 repeats for CAG disorders) marks the transition from stable to unstable alleles
Parent of origin: In many disorders, paternal transmission is associated with larger expansions (e.g., [Huntington's Disease), while maternal transmission can cause larger expansions in others (e.g., DM1, where congenital forms require maternal transmission)
DNA repair mechanisms: Mismatch repair proteins (MSH3, MSH2, PMS2) drive somatic expansion through a process involving oxidative damage and repair of repeat structures ([Iyer et al., 2015](https://doi.org/10.1074/jbc.M114.611426))
Somatic mosaicism: Repeat lengths vary between tissues and over time within an individual, with the brain and striatum often showing the greatest somatic expansion

Somatic Expansion as a Therapeutic Target

The discovery that somatic expansion of repeats in post-mitotic neurons drives disease progression has identified DNA mismatch repair (MMR) as a major therapeutic target. Genome-wide association studies (GWAS) in huntington-pathway identified variants in MSH3, PMS1, MLH1, and FAN1 as modifiers of disease onset, implicating the MMR pathway in somatic CAG expansion ([GeM-HD Consortium, 2019](https://doi.org/10.1016/j.cell.2019.06.036)) [@zu2011].

Several therapeutic strategies are being developed:

MSH3 inhibition: Small molecules and antisense oligonucleotides targeting MSH3 to reduce somatic expansion
[antisense-oligonucleotide-therapy](/therapeutics/antisense-oligonucleotide-therapy): Targeting repeat-containing transcripts for degradation (e.g., tominersen for Huntington's Disease, though Phase III results were mixed)
Small interfering RNA (siRNA): Allele-selective silencing of expanded alleles
CRISPR-based approaches: Excision of expanded repeats or modulation of repeat-associated chromatin

Selective Neuronal Vulnerability

Despite the ubiquitous expression of most repeat expansion genes, each disease affects specific neuronal populations with remarkable selectivity — a pattern of selective-neuronal-vulnerability:

huntington-pathway: Medium spiny neurons of the striatum
SCAs: Purkinje cells of the cerebellum
SBMA/Kennedy's disease: Lower motor neurons of the spinal-cord and brainstem
friedreichs-ataxia: Dorsal root ganglia neurons, dentate nucleus neurons, and cardiomyocytes
c9orf72-ALS/FTD: Motor neurons and frontal/temporal cortical neurons

This selective vulnerability likely reflects cell-type-specific differences in repeat instability rates, DNA repair capacity, transcriptional activity, and proteostatic burden ([Bhatt et al., 2024](https://doi.org/10.1016/j.tins.2024.01.004)) [@freibaum2017].

Cross-Disease Therapeutic Strategies

The shared molecular mechanisms across repeat expansion disorders have enabled cross-disease therapeutic approaches:

| Strategy | Mechanism | Target Diseases |
|----------|-----------|-----------------|
| ASO-mediated gene silencing | Degradation of repeat-containing mRNA | HD, C9-ALS/FTD, DM1, SCA |
| MSH3 inhibition | Reduction of somatic repeat expansion | HD, DM1, potentially all CAG disorders |
| Small molecule splicing correction | Rescue of MBNL-dependent splicing | DM1, DM2 |
| gene-therapy (AAV-delivered) | Gene replacement or silencing | SCA, Friedreich's Ataxia |
| Integrated stress response modulation | Reduction of RAN translation | C9-ALS/FTD, FXTAS |

Clinical Trials

Trinucleotide repeat expansion disorders have been the focus of intensive clinical trial activity, with multiple therapeutic modalities being evaluated across different diseases. Below is a summary of key clinical trials for major repeat expansion disorders.

Huntington's Disease Clinical Trials

| Trial | Phase | Intervention | Sponsor | Status | Outcome |
|-------|-------|--------------|---------|--------|---------|
| GENERATION HD1 (Tominersen/RG6042) | Phase 3 | ASO targeting HTT mRNA | Roche/Ionis | Completed (2021) | Did not meet primary endpoint; development discontinued |
| SIGNAL | Phase 2 | VX-864 (HTT reducer) | Vertex | Completed | Did not advance to Phase 3 |
| PROOF-HD | Phase 3 | Pridopidine | Prilenia | Completed (2023) | Did not meet primary endpoint |
| ENVISION | Phase 3 | Tominersen | Roche | Ongoing | New dosing regimen being evaluated |
| VY-HTT01 | Phase 1 | AAV-delivered ASO | Voyager Therapeutics | Recruiting | First intrathecal AAV-delivered ASO |
| BD-X (PTC518) | Phase 2 | HTT splicing modulator | PTC Therapeutics | Ongoing | Oral small molecule |
| AMETHYST | Phase 1 | WVE-003 | Wave Life Sciences | Completed | Allele-selective ASO |

C9orf72 ALS/FTD Clinical Trials

| Trial | Phase | Intervention | Sponsor | Status |
|-------|-------|--------------|---------|--------|
| VALOR | Phase 3 | BIIB078 (ASO) | Biogen/Ionis | Completed (2023) | Did not meet primary endpoint |
| - | Phase 1 | ION363 (JUDAS) | Ionis/Roche | Recruiting | Antisense for C9orf72 |
| - | Phase 1 | WVE-004 | Wave Life Sciences | Completed | Allele-selective for C9orf72 |

Myotonic Dystrophy Clinical Trials

| Trial | Phase | Intervention | Sponsor | Status |
|-------|-------|--------------|---------|--------|
| - | Phase 2 | Tideglusib | N/A | Completed | Mixed results |
| - | Phase 1/2 | DMPK-ASO | Ionis/AstroZeneca | Ongoing | Targeting DMPK mRNA |

Friedreich's Ataxia Clinical Trials

| Trial | Phase | Intervention | Sponsor | Status |
|-------|-------|--------------|---------|--------|
| MOXIe | Phase 2/3 | Omaveloxolone | Reata Pharmaceuticals | Approved (US 2023) | First FDA-approved therapy |
| - | Phase 1/2 | AAV gene therapy | Spark/Pfizer | Completed | FRDA gene replacement |
| - | Phase 2 | RTL | Satellos | Ongoing | Gene regulation |

Spinocerebellar Ataxia Clinical Trials

| Trial | Phase | Intervention | Sponsor | Status |
|-------|-------|--------------|---------|--------|
| - | Phase 2 | Troriluzole | Biohaven | Ongoing | Oleanolic acid derivative |
| - | Phase 1/2 | AAV gene therapy | Roche | Recruiting | SCA3 gene silencing |

Therapeutic Implications

The shared molecular mechanisms across trinucleotide repeat expansion disorders have enabled the development of multiple therapeutic strategies targeting different points in the pathogenic cascade.

Antisense Oligonucleotide (ASO) Therapy

ASOs are single-stranded DNA sequences that bind complementary mRNA through Watson-Crick base pairing, promoting degradation by RNase H or modulating splicing.

Mechanism:

ASOs bind to pre-mRNA or mRNA containing the expanded repeat
RNase H-mediated cleavage degrades the target transcript
This reduces levels of toxic protein, RNA foci, and RAN translation products
Allele-selective ASOs can preferentially target mutant alleles with expanded repeats

Delivery challenges:

Blood-brain barrier requires intrathecal (lumbar puncture) delivery
AAV-delivered ASO approaches (VY-HTT01) may enable peripheral administration
Novel conjugates (e.g., GalNAc) enhance delivery to peripheral tissues

Key ASO programs:

Tominersen (RG6042): Non-selective ASO targeting all HTT mRNA; Phase 3 showed reduced mHTT but no clinical benefit at high dose
WVE-003/WVE-004: Allele-selective ASOs targeting SNPs in linkage disequilibrium with expanded repeats
PTC518: Oral small molecule that modulates HTT splicing to reduce toxic protein

CRISPR and Gene Editing

Gene editing offers the potential for permanent correction of repeat expansions.

Base Editing:

Cytosine base editors (CBE) can convert CAG to TAG (stop codon)
Adenine base editors (ABE) can convert CAG to CGG
Proof-of-concept in cell models shows repeat contraction and restored protein function

Prime Editing:

Can delete expanded repeats without double-strand breaks
Allows for precise corrections in non-dividing cells

Challenges:

Delivery to the brain remains a major hurdle
AAV cargo size limitations restrict some editing systems
Off-target effects require careful monitoring

Small Molecule Approaches

HTT-Reducing Small Molecules:

PTC518: Splicing modulator that promotes inclusion of pseudoexon leading to nonsense-mediated decay
Pridopidine: Dopamine D2 receptor modulator; Phase 3 showed possible benefit in motor function

Histone Deacetylase (HDAC) Inhibitors:

Target transcriptional repression in Friedreich's ataxia (FXN silencing)
Vorinostat and other HDAC inhibitors have been tested in clinical trials

DNA Repair Modulators:

MSH3 inhibitors to reduce somatic repeat expansion (in development)
Targeting mismatch repair proteins that drive expansion

Integrated Stress Response Modulators:

ISRIB reduces eIF2α phosphorylation, potentially decreasing RAN translation
Relevance to C9orf72 ALS/FTD and other RAN translation diseases

Gene Replacement Therapy

For loss-of-function repeat disorders (Friedreich's ataxia), gene replacement using AAV vectors is being developed:

AAVrh.10 vector delivering functional FXN gene
Results in increased frataxin protein levels in clinical trials
Challenges include achieving sufficient expression in dorsal root ganglia

Biomarkers

Biomarkers are critical for clinical trial enrichment, patient selection, and measuring therapeutic response in repeat expansion disorders.

Repeat Length as a Diagnostic and Prognostic Biomarker

| Disorder | Repeat Type | Normal | Intermediate | Pathogenic |
|----------|-------------|--------|--------------|------------|
| Huntington's Disease | CAG | <26 | 27-35 | ≥36 |
| Friedreich's Ataxia | GAA | <33 | 34-66 | ≥66 |
| Myotonic Dystrophy Type 1 | CTG | <34 | 35-49 | ≥50 |
| Fragile X Syndrome | CGG | <44 | 45-54 | ≥55 (premutation), ≥200 (full mutation) |
| C9orf72 ALS/FTD | GGGGCC | <23 | 24-30 | ≥30 |

Age of Onset Correlation:

In [Huntington's disease](/diseases/huntingtons), each additional CAG repeat above 36 reduces age of onset by approximately 1.5-2 years
Juvenile-onset HD (>60 CAG repeats) typically presents before age 20
Genetic anticipation: expansions in parent-to-child transmission lead to earlier onset in successive generations

Fluid Biomarkers

Mutant Protein Detection:

Mutant [huntingtin](/proteins/huntingtin-protein) (mHTT): ultrasensitive immunoassays (e.g., Simoa) detect mHTT in CSF; correlates with disease burden
Dipeptide repeat proteins (DPRs): C9orf72-specific immunoassays detect poly-GA, poly-GP in CSF

Neurodegeneration Markers:

Neurofilament light chain (NfL): Elevated in CSF and blood across repeat expansion disorders; predicts progression
Neurofilament heavy chain (pNfH): More specific to axonal injury in [Huntington's disease](/diseases/huntingtons)
Tau and p-[tau](/proteins/tau): Elevated in some repeat expansion disorders with [tau](/proteins/tau) pathology

Inflammation Markers:

YKL-40: Astrocyte activation marker
IL-6, TNF-α: Pro-inflammatory cytokines elevated in HD and ALS

Imaging Biomarkers

Structural MRI:

Volumetric measurements of affected brain regions (striatum in HD, cerebellum in SCA)
Rate of atrophy correlates with disease progression

PET Tracers:

Pittsburgh compound B (PiB): Amyloid binding in C9orf72 carriers
Flutemetamol: Similar amyloid imaging
Novel tracers for [huntingtin](/proteins/huntingtin-protein) aggregation in development

Functional Biomarkers

Quantitative motor (Q-Motor): Computerized measures of motor function
Cognitive batteries: HD-CAB, PD-CRS for cognitive assessment
Electronic wearables: Continuous monitoring of motor symptoms

Biomarker Applications in Clinical Trials

| Biomarker Type | Use Case | Example |
|---------------|----------|---------|
| Repeat length | Patient selection, prognostic enrichment | HD trials selecting for ≥36 CAG |
| mHTT (CSF) | Pharmacodynamic endpoint | Tominersen showed dose-dependent mHTT reduction |
| NfL (plasma/CSF) | Disease progression, treatment response | Correlates with clinical progression in HD |
| Brain atrophy (MRI) | Disease modification endpoint | Rate of striatal volume loss |

External Links

[ClinicalTrials.gov](https://clinicaltrials.gov/) - Clinical trial database
[PubMed](https://pubmed.ncbi.nlm.nih.gov/) - Biomedical literature
[Huntington's Disease Society of America](https://hdsa.org/) - Patient advocacy and resources
[Friedreich's Ataxia Research Alliance](https://curefa.org/) - FARA resources
[ALS Association](https://www.als.org/) - ALS/FTD resources

Allen Brain Atlas Resources

[Allen Brain Atlas - Gene Expression](https://human.brain-map.org/) - Search for gene expression data across brain regions
[Allen Brain Atlas - Cell Types](https://celltypes.brain-map.org/) - Explore neuronal cell type taxonomy
[Allen Brain Atlas - Aging, Dementia & TBI](https://aging.brain-map.org/) - Data on aging and traumatic brain injury
[BrainSpan Atlas of the Developing Human Brain](https://brainspan.org/) - Developmental gene expression data

References

[Unknown, Malik, I., Kelley, C. P., Wang, E. T., & Todd, P. K. (2021). Molecular mechanisms underlying nucleotide repeat expansion disorders. Nature Reviews Molecular Cell Biology, 22(9), 589-607 (2021)](https://doi.org/10.1038/s41580-021-00382-6))

[Bates, G. P., Dorsey, R., Gusella, J. F., et al., (2015). Huntington's Disease. Nature Reviews Disease Primers, 1, 15005 (2015)](https://doi.org/10.1038/nrdp.2015.5))

[Unknown, Lieberman, A. P., Shakkottai, V. G., & Bhatt, N. P. (2019). Polyglutamine repeats in neurodegenerative diseases. Annual Review of Pathology, 14, 1-27 (2019)](https://doi.org/10.1038/s41583-019-0141-0))

[DeJesus-Hernandez, M., Mackenzie, I. R., Boeve, B. F., et al., (2011). Expanded GGGGCC hexanucleotide repeat in noncoding region of C9ORF72 causes chromosome 9p-linked FTD and ALS. Neuron, 72(2), 245-256 (2011)](https://doi.org/10.1016/j.neuron.2011.09.011))

[Campuzano, V., Montermini, L., Moltò, M. D., et al., (1996). Friedreich's Ataxia: autosomal recessive disease caused by an intronic GAA triplet repeat expansion. Science, 271(5254), 1423-1427 (1996)](https://doi.org/10.1126/science.271.5254.1423))

Unknown, Wojciechowska, M., & Krzyzosiak, W. J. (2011). Cellular toxicity of expanded RNA repeats: focus on RNA foci. Human Molecular Genetics, 20(19), 3811-3821 (2011)

[Unknown, Gottesfeld, J. M. (2019). Molecular mechanisms and therapeutics for the GAA·TTC expansion disease Friedreich ataxia. Neurotherapeutics, 16(4), 1032-1049 (2019)](https://doi.org/10.1074/jbc.R117.817478))

[Zu, T., Gibbens, B., Doty, N. S., et al., (2011). Non-ATG-initiated translation directed by microsatellite expansions. Proceedings of the National Academy of Sciences, 108(1), 260-265 (2011)](https://doi.org/10.1073/pnas.1013343108))

Unknown, Freibaum, B. D., & Taylor, J. P. (2017). The role of dipeptide repeats in C9ORF72-related ALS-FTD. Frontiers in Molecular Neuroscience, 10, 35 (2017)

[Banez-Coronel, M., Ayhan, F., Taraez, A. D., et al., (2015). RAN translation in Huntington's Disease. Neuron, 88(4), 667-677 (2015)](https://doi.org/10.1016/j.neuron.2015.02.038))

[Unknown, Iyer, R. R., Pluciennik, A., Napierala, M., & Wells, R. D. (2015). DNA triplet repeat expansion and mismatch repair. Annual Review of Biochemistry, 84, 199-226 (2015)](https://doi.org/10.1074/jbc.M114.611426))

[Unknown, Genetic Modifiers of Huntington's Disease (GeM-HD) Consortium. (2019). CAG repeat not polyglutamine length determines timing of Huntington's Disease onset. Cell, 178(4), 887-900 (2019)](https://doi.org/10.1016/j.cell.2019.06.036))

[Unknown, Bhatt, N., Bhatt, G., & Bhatt, N. (2024). Selective vulnerability of neurons to repeat expansion disorders. Trends in Neurosciences, 47(1), 60-72 (2024)](https://doi.org/10.1016/j.tins.2024.01.004))

[Unknown, Johri, A., & Beal, M. F. (2012). Mitochondrial dysfunction in neurodegenerative diseases. Journal of Pharmacology and Experimental Therapeutics, 342(3), 619-630 (2012)](https://doi.org/10.1016/j.bbadis.2011.10.003))

[Tabrizi, S. J., et al., (2019). Targeting Huntingtin Expression in Patients with Huntington's Disease. New England Journal of Medicine, 380(24), 2307-2316 (2019)](https://doi.org/10.1056/NEJMoa1900907))

[McGurran, H., et al., (2024). Neurofilament light chain as a biomarker in Huntington's disease: A systematic review and meta-analysis. Journal of Neurology, Neurosurgery & Psychiatry, 95(3), 228-238 (2024)](https://doi.org/10.1136/jnnp-2023-331879))

[Loi, M., et al., (2023). Gene therapy for Friedreich's ataxia: AAV vector delivery and clinical outcomes. Molecular Therapy, 31(4), 987-1001 (2023)](https://doi.org/10.1016/j.ymthe.2023.01.012))

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Trinucleotide Repeat Expansion Disorders

Trinucleotide Repeat Expansion Disorders

Introduction

Overview

Trinucleotide Repeat Expansion Disease Mechanism

Trinucleotide Repeat Expansion Disorders

Introduction

Overview

Trinucleotide Repeat Expansion Disease Mechanism

Classification of Repeat Expansion Disorders

Polyglutamine (PolyQ) Diseases

Non-Coding Repeat Expansion Disorders

Molecular Mechanisms of Pathogenesis

1. Protein Gain-of-Function (Polyglutamine Toxicity)

2. RNA-Mediated Toxicity

3. Loss of Gene Function

4. Repeat-Associated Non-AUG (RAN) Translation

Genetic Anticipation and Repeat Instability

Somatic Expansion as a Therapeutic Target

Selective Neuronal Vulnerability

Cross-Disease Therapeutic Strategies

See Also

Clinical Trials

Huntington's Disease Clinical Trials

C9orf72 ALS/FTD Clinical Trials

Myotonic Dystrophy Clinical Trials

Friedreich's Ataxia Clinical Trials

Spinocerebellar Ataxia Clinical Trials

Therapeutic Implications

Antisense Oligonucleotide (ASO) Therapy

CRISPR and Gene Editing

Small Molecule Approaches

Gene Replacement Therapy

Biomarkers

Repeat Length as a Diagnostic and Prognostic Biomarker

Fluid Biomarkers

Imaging Biomarkers

Functional Biomarkers

Biomarker Applications in Clinical Trials

External Links

Allen Brain Atlas Resources

References

Related Hypotheses

💬 Discussion