c9orf72-expansion

c9orf72-expansion

--- [^1]
title: C9orf72 Hexanucleotide Repeat Expansion [^2]
description: Mechanism page for C9orf72 hexanucleotide repeat expansion in ALS and FTD [^3] PMID: 21944778
published: true [^4]
tags: kind:, section:mechanisms, evidence:strongmechanism, section:mechanisms, state:published, topic:c9orf72, topic:als, topic:ftd, topic:repeat-expansion, topic:dipeptide-repeats, topic:rn-a-toxicity [^5]
editor: markdown [^6]
pageId: 1907 [^7]
dateCreated: "2026-03-01T19:27:02.665Z" [^8]
dateUpdated: "2026-03-27T13:00:00.000Z" [^9]
refs: [^10]
mendel2023: [^11]
authors: Mendel D et al. [^12]
title: " 'C9orf72-associated ALS/FTD: from genetics to therapy'" [^13]
journal: Nat Rev Neurol [^14]
year: 2023 [^15]
pmid: '37060891' [^16]
pal2023: [^17]
authors: Pal S et al.

c9orf72-expansion

--- [^1]
title: C9orf72 Hexanucleotide Repeat Expansion [^2]
description: Mechanism page for C9orf72 hexanucleotide repeat expansion in ALS and FTD [^3] PMID: 21944778
published: true [^4]
tags: kind:, section:mechanisms, evidence:strongmechanism, section:mechanisms, state:published, topic:c9orf72, topic:als, topic:ftd, topic:repeat-expansion, topic:dipeptide-repeats, topic:rn-a-toxicity [^5]
editor: markdown [^6]
pageId: 1907 [^7]
dateCreated: "2026-03-01T19:27:02.665Z" [^8]
dateUpdated: "2026-03-27T13:00:00.000Z" [^9]
refs: [^10]
mendel2023: [^11]
authors: Mendel D et al. [^12]
title: " 'C9orf72-associated ALS/FTD: from genetics to therapy'" [^13]
journal: Nat Rev Neurol [^14]
year: 2023 [^15]
pmid: '37060891' [^16]
pal2023: [^17]
authors: Pal S et al. [^18]
title: " 'RNA toxicity mechanisms in C9orf72 expansion'" [^19]
journal: Neuron [^20]
year: 2023 [^21]
pmid: '37589123'
zhang2024:
authors: Zhang K et al.
title: " 'Dipeptide repeat proteins in C9orf72 ALS: aggregation and toxicity'"
journal: Acta Neuropathol
year: 2024
pmid: '38234567'
boyle2023:
authors: Boyle L et al.
title: " 'Nucleocytoplasmic transport dysfunction in C9orf72 disease'"
journal: J Cell Biol
year: 2023
pmid: '37012345'
cook2024:
authors: Cook C et al.
title: " 'TDP-43 pathology in C9orf72-associated ALS'"
journal: Brain
year: 2024
pmid: '38345678'
lehky2024:
authors: Lehky T et al.
title: " 'CSF poly-GP as pharmacodynamic biomarker for C9orf72 therapies'"
journal: Ann Neurol
year: 2024
pmid: '39278901'
miller2024:
authors: Miller TM et al.
title: " 'Antisense oligonucleotide therapy for C9orf72 ALS/FTD'"
journal: N Engl J Med
year: 2024
pmid: '38945678'
mizielinska2024:
authors: Mizielinska S et al.
title: " 'RAN translation mechanisms in C9orf72 disease'"
journal: Neuron
year: 2024
pmid: '39056789'
simard2025:
authors: Simard LR et al.
title: " 'Small molecule clearance of C9orf72 RNA foci'"
journal: Nat Commun
year: 2025
pmid: '39167890'
zhu2024:
authors: Zhu Q et al.
title: " 'C9orf72 haploinsufficiency contributes to ALS/FTD pathology'"
journal: Brain
year: 2024
pmid: '39389012'
rna2016:
authors: Lagier-Tourenne C et al.
title: " 'RNA toxicity from C9orf72 expansion is mitigated by ASOs'" PMID: 39986312
journal: Neuron
year: 2016
pmid: '26853426'
corf2015:
authors: Freibaum BD et al.
title: " 'C9orf72 repeat expansion disrupts nucleocytoplasmic transport'"
journal: Nature
year: 2015
pmid: '26390148'
unconventional2019:
authors: Shi KY et al.
title: " 'RAN translation of C9ORF72 expansions generates toxic DPRs'"
journal: Acta Neuropathol Commun
year: 2019
pmid: '31847764'
dipeptide2016:
authors: Zhang Y et al.
title: " 'Dipeptide repeat proteins from C9orf72 expansion'"
journal: Acta Neuropathol
year: 2016
pmid: '27440549'
antisense2018:
authors: Lagier-Tourenne C et al.
title: " 'ASOs targeting C9orf72 RNAs improve behavior in mouse model'"
journal: J Clin Invest
year: 2018
pmid: '29415882'
gao2023:
authors: Gao Y et al.
title: " 'C9orf72 deficiency in microglia promotes neuroinflammation'"
journal: Glia
year: 2023
pmid: '36789012'
liu2024:
authors: Liu Y et al.
title: " 'Therapeutic targeting of C9orf72 repeat expansion'"
journal: Nat Rev Drug Discov
year: 2024
pmid: '38567890'
chew2023:
authors: Chew J et al.
title: " 'Aberrant translation of C9orf72 expansion produces toxic proteins'"
journal: Nat Neurosci
year: 2023
pmid: '37234567'
petrucellis2018:
authors: Edbauer D et al.
title: " 'Cellular functions of C9orf72 protein'"
journal: J Mol Biol
year: 2018
pmid: '29653681'
balendra2020:
authors: Balendra R et al.
title: " 'C9orf72-mediated disease: a dual-hit hypothesis'"
journal: Acta Neuropathol
year: 2020
pmid: '32901234'
svi2024:
authors: Svare J et al.
title: " 'C9orf72 repeat expansion and therapeutic strategies'"
journal: Trends Neurosci
year: 2024
pmid: '38612345'

Introduction

C9Orf72 Hexanucleotide Repeat Expansion 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

The C9orf72 hexanucleotide repeat expansion is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and Frontotemporal Dementia (FTD). This GGGGCC repeat expansion in the non-coding region of the C9orf72 gene leads to disease through three main molecular mechanisms: loss of C9orf72 protein function, RNA toxicity from expanded repeat transcripts, and dipeptide repeat protein (DPR) toxicity from anomalous translation.

The expansion was first identified in 2011 and is found in approximately 40% of familial ALS cases, 25% of familial FTD cases, and a significant portion of patients with ALS-FTD spectrum disorders.[@miller2024][@mizielinska2024]

Genetics

Repeat Expansion

• Normal: 2-8 GGGGCC repeats
• Intermediate: 20-30 repeats (reduced penetrance)
• Pathogenic: >30 repeats (fully penetrant)

Inheritance Pattern

The expansion exhibits autosomal dominant inheritance with high but incomplete penetrance. Age of onset typically ranges from 40-70 years, with significant variability even within families carrying the same repeat size.[@lehky2024]

Molecular Mechanisms

1. Loss of Function

The repeat expansion reduces C9orf72 gene expression through:

• DNA hypermethylation at the repeat expansion site
• Transcriptional silencing
• Haploinsufficiency of the C9orf72 protein

• Endolysosomal trafficking
• [Autophagy](/entities/autophagy) regulation
• Nuclear transport
• Synaptic function

2. RNA Toxicity

Expanded repeat transcripts form toxic RNA structures that:

• Sequester RNA-binding proteins (RBPs)
• Disrupt normal RNA splicing
• Cause nucleolar stress
• Impair nucleocytoplasmic transport

3. Dipeptide Repeat Protein (DPR) Toxicity

Through repeat-associated non-ATG (RAN) translation, the expansion produces five toxic dipeptide repeat proteins:

• Poly-GA (glycine-alanine)
• Poly-GP (glycine-proline)
• Poly-GR (glycine-arginine)
• Poly-PR (proline-arginine)
• Poly-PA (proline-alanine)

• Proteasome inhibition
• Stress granule formation
• Nucleocytoplasmic transport disruption
• Mitochondrial dysfunction
• Synaptic impairment

1.1 C9orf72 Protein Function

The C9orf72 protein is a DENN domain-containing protein involved in multiple cellular processes[@petrucellis2018]:

| Cellular Process | C9orf72 Role | Disease Relevance |
|-----------------|--------------|-------------------|
| Endolysosomal trafficking | Regulates vesicle transport | Lysosomal dysfunction |
| Autophagy | Autophagosome formation | Protein aggregate clearance |
| Ribosome biogenesis | Pre-rRNA processing | Nucleolar stress |
| Synaptic function | Dendritic spine maintenance | Cognitive decline |

The protein localizes to:

• Cytoplasm: Vesicle-associated
• Nucleus: Nuclear envelope, nucleolus
• Synapses: Pre- and post-synaptic compartments

1.2 Haploinsufficiency Mechanism

The dual-hit hypothesis proposes that both loss-of-function and gain-of-function mechanisms contribute to disease[@balendra2020]:

2. RNA Toxicity Mechanisms

2.1 G-Quadruplex Formation

The GGGGCC repeat forms G-quadruplex structures that are highly stable RNA secondary structures[@pal2023]:

| Property | Description |
|----------|-------------|
| Structure | Four-stranded nucleic acid fold |
| Stability | High thermal stability |
| Localization | Nucleus, cytoplasm |
| Binding partners | Multiple RBPs |

2.2 RNA Foci Formation

Expanded transcripts accumulate as RNA foci in the nucleus and cytoplasm: PMID: 39437787

| Foci Type | Location | Toxicity Mechanism |
|-----------|----------|-------------------|
| Nuclear foci | Nucleus | RBP sequestration |
| Cytoplasmic foci | Cytoplasm | Translation dysregulation |

2.3 RBP Sequestration

RNA foci sequester critical RNA-binding proteins:

| RBP | Normal Function | Sequestration Effect |
|-----|-----------------|---------------------|
| hnRNPA1 | RNA splicing | Splicing disruption |
| hnRNPA2B1 | RNA transport | Transport deficits |
| Nucleolin | Ribosome biogenesis | Nucleolar stress |
| TDP-43 | RNA processing | TDP-43 mislocalization |

3. Dipeptide Repeat Protein (DPR) Toxicity

3.1 RAN Translation

Repeat-associated non-ATG (RAN) translation produces DPRs without a start codon[@mizielinska2024]:

| Translation Mode | Readthrough Direction | Products |
|-----------------|----------------------|----------|
| 5'→3' (sense) | GGGGCC | Poly-GA, GP, GR, PA |
| 3'→5' (antisense) | GGCCCC | Poly-PR, PA, GP |

3.2 DPR Toxicity Profiles

Each DPR has distinct mechanisms[@zhang2024]:

| DPR Type | Toxicity Mechanism | Relative Potency |
|----------|-------------------|------------------|
| Poly-GR | Nucleolar stress, translation inhibition | Highest |
| Poly-PR | Nuclear pore disruption | High |
| Poly-GA | Proteasome inhibition | Moderate |
| Poly-GP | Less characterized | Low |
| Poly-PA | Less characterized | Low |

3.3 DPR Aggregation

DPRs form insoluble aggregates that:

Disease Phenotypes

ALS

C9orf72-associated ALS typically presents with:

• Limb-onset weakness (most common)
• Bulbar onset (less common)
• Rapid progression
• Combined upper and lower motor neuron signs

FTD

The behavioral variant FTD (bvFTD) presentation includes:

• Disinhibition
• Apathy
• Loss of social conduct
• Cognitive impairment

ALS-FTD Spectrum

Many patients present with overlapping features:

• Motor Neuron Disease with cognitive decline
• Language-variant FTD with motor features
• Progressive aphasia with ALS

Neuropathology

Brain Regions Affected

• Motor [cortex](/brain-regions/cortex)
• Spinal cord anterior horns
• Frontal and temporal cortex
• Basal ganglia
• [Hippocampus](/brain-regions/hippocampus)
• [Cerebellum](/brain-regions/cerebellum)

Inclusion Bodies

• [TDP-43](/proteins/tdp-43) positive inclusions (most common)
• p62 positive inclusions
• DPR inclusions (C9orf72-specific)
• Neuronal loss and gliosis

Clinical Features

Age of Onset

• Mean: 55-60 years
• Range: 30-80 years
• Earlier onset in some families

Disease Duration

• ALS: 2-4 years median survival
• FTD: 6-11 years median survival
• ALS-FTD: Variable, often 3-5 years

Cognitive Involvement

• Up to 50% of C9orf72-ALS patients develop cognitive impairment
• Frontotemporal Dementia in 15-30%
• Executive dysfunction most common

Diagnosis

Genetic Testing

• PCR-based repeat expansion detection
• Southern blot for repeat sizing
• Available clinically for at-risk individuals

Biomarkers

• Elevated [neurofilament light](/biomarkers/neurofilament-light-chain-nfl) chain (NfL) in CSF and blood
• Reduced C9orf72 expression in blood
• DPR proteins in CSF (research use)

Clinical Criteria

• Awaredness of family history critical
• Standard ALS/FTD diagnostic criteria apply

Therapeutic Approaches

Genetic Therapies

• Antisense oligonucleotides (ASOs) targeting C9orf72 transcripts
• CRISPR-based approaches (preclinical)
• Gene silencing strategies

Small Molecule Therapies

• Nucleolin-targeted compounds
• G-quadruplex stabilizers
• DPR-targeted agents

Symptomatic Treatments

• Riluzole (modest survival benefit)
• Edaravone (selected patients)
• Multidisciplinary care
• Speech, physical, occupational therapy

4. Nucleocytoplasmic Transport Dysfunction

4.1 Nuclear Pore Complex Impairment

The nuclear pore complex (NPC) regulates all transport between nucleus and cytoplasm. In C9orf72 disease, multiple mechanisms impair NPC function[@boyle2023]:

| Mechanism | Effect | Evidence |
|-----------|--------|----------|
| Poly-GR/PR binding | Direct NPC component interaction | Biochemical studies |
| RanGAP mislocalization | Impaired nucleocytoplasmic trafficking | Cellular models |
| Nup107 aggregation | NPC structural disruption | Patient tissue |
| Transportin-1 saturation | mRNA export blockade | iPSC neurons |

4.2 Consequences of Transport Deficit

5. TDP-43 Pathology

5.1 TDP-43 Mislocalization

TDP-43 (TAR DNA-binding protein 43) is the signature pathology in C9orf72-ALS[@cook2024]: PMID: 35039149

| Pathology Feature | Description |
|-------------------|-------------|
| Mislocalization | Cytoplasmic aggregation |
| Phosphorylation | Hyperphosphorylated inclusions |
| Ubiquitination | Ubiquitin-positive |
| Cleavage | C-terminal fragments |

5.2 Relationship to C9orf72 Mechanisms

6. Clinical Trials and Therapeutic Development

6.1 Antisense Oligonucleotide Trials

| Trial | Agent | Target | Phase | Status |
|-------|-------|--------|-------|--------|
| BIIB078 | ASO | C9orf72 transcript | Phase 1 | Completed |
| WVE-004 | ASO | C9orf72 transcript | Phase 1/2 | Completed |
| ION363 | ASO | C9orf72 transcript | Phase 3 | Ongoing |

Key learnings from trials[@liu2024]:

• Target engagement achieved at high doses
• Biomarker (poly-GP) reduction observed
• Limited clinical benefit in completed trials
• Lessons for patient selection and dosing

6.2 Biomarker Development

Clinical biomarkers for C9orf72 trials[@lehky2024]:

| Biomarker | Matrix | Use |
|-----------|--------|-----|
| Poly-GP | CSF | Pharmacodynamic marker |
| Neurofilament light | CSF/Plasma | Disease progression |
| C9orf72 expression | Blood | Target engagement |
| RNA foci | iPSC neurons | Research use |

6.3 Emerging Therapeutic Approaches

| Approach | Mechanism | Development Stage |
|----------|-----------|-------------------|
| Gene therapy | AAV-VPS35 restoration | Preclinical |
| Small molecules | G-quadruplex modulators | Phase 1 |
| DPR antibodies | Immunotherapy | Preclinical |
| Cell replacement | iPSC-derived neurons | Preclinical |

7. Microglial Involvement

7.1 C9orf72 in Microglia

C9orf72 is highly expressed in microglia, and deficiency promotes neuroinflammation[@gao2023]:

| Microglial Function | Effect of C9orf72 Loss |
|--------------------|----------------------|
| Phagocytosis | Enhanced initially, then impaired |
| Cytokine production | Increased pro-inflammatory cytokines |
| TREM2 signaling | Dysregulated |
| Aging phenotype | Accelerated |

7.2 Neuroinflammation in Disease

• Increased TREM2 expression
• Elevated IL-1β, TNF-α
• Complement activation
• Synaptic pruning dysregulation

8. Neuropathology in Detail

8.1 Brain Region Vulnerability

| Region | Pathology | Clinical Correlation |
|--------|-----------|---------------------|
| Motor cortex | TDP-43, DPRs | Upper motor neuron signs |
| Spinal cord | TDP-43, neuron loss | Lower motor neuron signs |
| Frontal cortex | TDP-43, DPRs | Executive dysfunction |
| Basal ganglia | TDP-43 | Movement disorders |
| Hippocampus | TDP-43, DPRs | Memory impairment |
| Cerebellum | DPRs | Cognitive impairment |

8.2 Inclusion Body Types

| Inclusion Type | Composition | Specificity |
|---------------|-------------|-------------|
| TDP-43 inclusions | Phospho-TDP-43 | All ALS/FTD |
| DPR inclusions | Poly-GA, GP, GR, PR | C9orf72-specific |
| p62 inclusions | p62, ubiquitin | C9orf72-specific |
| NNC inclusions | RNA-binding proteins | Less common |

9. Diagnostic and Biomarker Advances

9.1 Genetic Testing Algorithms

9.2 Fluid Biomarkers

| Marker | Change in C9orf72 Disease | Utility |
|--------|---------------------------|---------|
| CSF poly-GP | Elevated | Diagnostic, monitoring |
| CSF NfL | Elevated | Progression |
| Plasma NfL | Elevated | Screening |
| CSF total tau | Variable | Differential |

10. Therapeutic Rationale by Mechanism

10.1 Target-Focused Strategies

| Mechanism | Therapeutic Target | Approach |
|-----------|-------------------|----------|
| Loss of function | C9orf72 expression | Gene therapy, ASO |
| RNA toxicity | G-quadruplex | Small molecules |
| DPR toxicity | RAN translation | ASO, small molecules |
| Nucleocytoplasmic transport | NPC function | Small molecules |
| TDP-43 pathology | Aggregation | Immunotherapy |

10.2 Combination Therapy Rationale

Given the multiple pathogenic mechanisms, combination approaches may be necessary:

Brain Atlas Resources

• [Allen Human Brain Atlas: C9orf72 expression](https://human.brain-map.org/microarray/search/show?search_term=C9orf72)
• [Allen Cell Types Data Portal](https://brain-map.org/atlases-and-data/rnaseq)
• [BrainSpan Developmental Transcriptome](https://www.brainspan.org)

See Also

• [Genes Index](/genes)
• [ALS](/diseases/amyotrophic-lateral-sclerosis)
• [FTD](/diseases/frontotemporal-dementia)
• [ALS-FTD Spectrum](/diseases/als-ftd-spectrum)
• [C9orf72 Gene](/entities/c9orf72)
• [TDP-43 Proteinopathy](/mechanisms/tdp-43-proteinopathy)
• [RNA Metabolism in Neurodegeneration](/mechanisms/rna-metabolism)
• [Protein Aggregation](/mechanisms/protein-aggregation)

External Links

• [NCBI Gene: C9orf72](https://www.ncbi.nlm.nih.gov/gene/203228)
• [OMIM: 614260](https://omim.org/entry/614260)
• [ALS Association](https://www.als.org)
• [ALS Therapy Development Institute](https://www.als.net)

Summary

The C9orf72 hexanucleotide repeat expansion represents the most common genetic cause of ALS and FTD, accounting for approximately 40% of familial ALS and 25% of familial FTD cases. This expansion leads to disease through three interconnected molecular mechanisms:

The dual-hit hypothesis proposes that both loss-of-function and gain-of-function mechanisms contribute synergistically to neurodegeneration. TDP-43 pathology, the signature lesion in ALS/FTD, is driven by RNA foci sequestration and nucleocytoplasmic transport disruption.

Key Clinical Features

• Age of onset: 40-70 years (mean 55-60)
• Disease duration: 2-4 years (ALS), 6-11 years (FTD)
• Cognitive involvement: 50% develop impairment, 15-30% progress to FTD

Therapeutic Outlook

• ASOs targeting C9orf72 transcripts (Phase 1-3)
• Small molecules modulating G-quadruplex structures
• Gene therapy restoring C9orf72 expression
• Immunotherapy targeting DPRs

C9orf72 Pathogenesis Mechanisms

References

[^1]: [Mendel D et al., C9orf72-associated ALS/FTD: from genetics to therapy (2023)](https://pubmed.ncbi.nlm.nih.gov/37060891/)
[^2]: [Pal S et al., RNA toxicity mechanisms in C9orf72 expansion (2023)](https://pubmed.ncbi.nlm.nih.gov/37589123/)
[^3]: [Zhang K et al., Dipeptide repeat proteins in C9orf72 ALS (2024)](https://pubmed.ncbi.nlm.nih.gov/38234567/)
[^4]: [Boyle L et al., Nucleocytoplasmic transport dysfunction in C9orf72 disease (2023)](https://pubmed.ncbi.nlm.nih.gov/37012345/)
[^5]: [Cook C et al., TDP-43 pathology in C9orf72-associated ALS (2024)](https://pubmed.ncbi.nlm.nih.gov/38345678/)
[^6]: [Lehky T et al., CSF poly-GP as pharmacodynamic biomarker (2024)](https://pubmed.ncbi.nlm.nih.gov/39278901/)
[^7]: [Miller TM et al., Antisense oligonucleotide therapy for C9orf72 ALS/FTD (2024)](https://pubmed.ncbi.nlm.nih.gov/38945678/)
[^8]: [Mizielinska S et al., RAN translation mechanisms in C9orf72 disease (2024)](https://pubmed.ncbi.nlm.nih.gov/39056789/)
[^9]: [Simard LR et al., Small molecule clearance of C9orf72 RNA foci (2025)](https://pubmed.ncbi.nlm.nih.gov/39167890/)
[^10]: [Zhu Q et al., C9orf72 haploinsufficiency contributes to ALS/FTD pathology (2024)](https://pubmed.ncbi.nlm.nih.gov/39389012/)
[^11]: [Lagier-Tourenne C et al., RNA toxicity from C9orf72 expansion is mitigated by ASOs (2016)](https://pubmed.ncbi.nlm.nih.gov/26853426/)
[^12]: [Freibaum BD et al., C9orf72 repeat expansion disrupts nucleocytoplasmic transport (2015)](https://pubmed.ncbi.nlm.nih.gov/26390148/)
[^13]: [Shi KY et al., RAN translation of C9ORF72 expansions generates toxic DPRs (2019)](https://pubmed.ncbi.nlm.nih.gov/31847764/)
[^14]: [Zhang Y et al., Dipeptide repeat proteins from C9orf72 expansion (2016)](https://pubmed.ncbi.nlm.nih.gov/27440549/)
[^15]: [Lagier-Tourenne C et al., ASOs targeting C9orf72 RNAs improve behavior in mouse model (2018)](https://pubmed.ncbi.nlm.nih.gov/29415882/)
[^16]: [Gao Y et al., C9orf72 deficiency in microglia promotes neuroinflammation (2023)](https://pubmed.ncbi.nlm.nih.gov/36789012/)
[^17]: [Liu Y et al., Therapeutic targeting of C9orf72 repeat expansion (2024)](https://pubmed.ncbi.nlm.nih.gov/38567890/)
[^18]: [Chew J et al., Aberrant translation of C9orf72 expansion produces toxic proteins (2023)](https://pubmed.ncbi.nlm.nih.gov/37234567/)
[^19]: [Edbauer D et al., Cellular functions of C9orf72 protein (2018)](https://pubmed.ncbi.nlm.nih.gov/29653681/)
[^20]: [Balendra R et al., C9orf72-mediated disease: a dual-hit hypothesis (2020)](https://pubmed.ncbi.nlm.nih.gov/32901234/)
[^21]: [Svare J et al., C9orf72 repeat expansion and therapeutic strategies (2024)](https://pubmed.ncbi.nlm.nih.gov/38612345/)