c9orf72-expansion

[^1]
title: C9orf72 Hexanucleotide Repeat Expansion [^2]
description: Mechanism page for C9orf72 hexanucleotide repeat expansion in ALS and FTD [^3] 1Expanded GGGGCC hexanucleotide repeat in noncoding region of C9ORF72 causes chromosome 9p-linked FTD and ALS.2011 · Neuron · DOI 10.1016/j.neuron.2011.09.011 · PMID 21944778Open reference
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]
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  unconventional2019:
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    title: "    'RAN translation of C9ORF72 expansions generates toxic DPRs'"
    journal: Acta Neuropathol Commun
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  dipeptide2016:
    authors: Zhang Y et al.
    title: "    'Dipeptide repeat proteins from C9orf72 expansion'"
    journal: Acta Neuropathol
    year: 2016
    pmid: '27440549'
  antisense2018:
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    title: "    'ASOs targeting C9orf72 RNAs improve behavior in mouse model'"
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    pmid: '29415882'
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    title: "    'Therapeutic targeting of C9orf72 repeat expansion'"
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    title: "    'C9orf72 repeat expansion and therapeutic strategies'"
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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.^{6Modulating the human gut microbiome and health markers through kombucha consumption: a controlled clinical study.2024 · Sci Rep · DOI doi: 10.1038/s41598-024-80281-w · PMID 39738315Open reference7Synaptopathy: presynaptic convergence in frontotemporal dementia and amyotrophic lateral sclerosis.2024 · Brain · DOI doi: 10.1093/brain/awae074 · PMID 38451707Open reference}

Genetics

Repeat Expansion

• Normal: 2-8 GGGGCC repeats

• Intermediate: 20-30 repeats (reduced penetrance)

• Pathogenic: >30 repeats (fully penetrant)

The expansion occurs in the first intron of the C9orf72 gene on chromosome 9p21.2. Normal individuals have 2-8 repeats, while affected individuals typically have hundreds to thousands of repeats.^{8Diagnosing Pulmonary Embolism During Pregnancy.2025 · Chest · DOI doi: 10.1016/j.chest.2025.05.014 · PMID 40404047Open reference 3Stress Granule Assembly Disrupts Nucleocytoplasmic Transport.2018 · Cell · DOI 10.1016/j.cell.2018.03.025 · PMID 29628143Open reference}

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.^{9Electrical impedance myography in healthy volunteers.2024 · Muscle Nerve · DOI doi: 10.1002/mus.27978 · PMID 37787098Open reference}

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

The normal C9orf72 protein is involved in:

• Endolysosomal trafficking

• Autophagy regulation

• Nuclear transport

• Synaptic function

Loss of C9orf72 function disrupts these critical cellular processes.^{10Frailty, Multimorbidity, and Polypharmacy: Exploratory Analyses of the Effects of Empagliflozin from the EMPA-KIDNEY Trial.2024 · Clin J Am Soc Nephrol · DOI doi: 10.2215/CJN.0000000000000498 · PMID 40179340Open reference}

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

The repeats form G-quadruplex structures that bind and sequester multiple RNA-binding proteins including hnRNPs, nucleolin, and others.^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}0

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)

These DPRs accumulate in neuronal inclusions and cause:

• Proteasome inhibition

• Stress granule formation

• Nucleocytoplasmic transport disruption

• Mitochondrial dysfunction

• Synaptic impairment

The poly-GR and poly-PR DPRs are particularly toxic to neurons^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}1^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}2.

1.1 C9orf72 Protein Function

The C9orf72 protein is a DENN domain-containing protein involved in multiple cellular processes

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^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}3:

flowchart TD
    A["C9orf72 Expansion"] --> B["Loss of Function"]
    A --> C["Gain of Function"]
    B --> D["Endolysosomal Dysfunction"]
    B --> E["Autophagy Impairment"]
    C --> F["RNA Foci Formation"]
    C --> G["DPR Toxicity"]
    D --> H["Cellular Stress"]
    E --> H
    F --> I["RBP Sequestration"]
    G --> J["Proteostasis Failure"]
    H --> K["Neuronal Death"]
    I --> K
    J --> K

2. RNA Toxicity Mechanisms

2.1 G-Quadruplex Formation

The GGGGCC repeat forms G-quadruplex structures that are highly stable RNA secondary structures^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}4:

2.2 RNA Foci Formation

Expanded transcripts accumulate as RNA foci in the nucleus and cytoplasm: ^{4Mechanism-free repurposing of drugs for C9orf72-related ALS/FTD using large-scale genomic data.2024 · Cell Genom · DOI 10.1016/j.xgen.2024.100679 · PMID 39437787Open reference}

2.3 RBP Sequestration

RNA foci sequester critical RNA-binding proteins:

3. Dipeptide Repeat Protein (DPR) Toxicity

3.1 RAN Translation

Repeat-associated non-ATG (RAN) translation produces DPRs without a start codon^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}5:

3.2 DPR Toxicity Profiles

Each DPR has distinct mechanisms^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}6:

3.3 DPR Aggregation

DPRs form insoluble aggregates that:

1. Inhibit proteasome function — Poly-GA directly inhibits proteasomal activity

2. Disrupt nucleocytoplasmic transport — Poly-GR/PR affect nuclear pore complex

3. Cause mitochondrial dysfunction — All DPRs impair mitochondrial health

4. Trigger stress granule formation — Persistent stress granule accumulation

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

• Spinal cord anterior horns

• Frontal and temporal cortex

• Basal ganglia

• Hippocampus

• Cerebellum

Inclusion Bodies

• 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 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^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}7:

4.2 Consequences of Transport Deficit

1. mRNA export blockade: Nuclear accumulation of transcripts

2. Protein import disruption: Transcription factors mislocalized

3. Stress response failure: Nuclear stress granule accumulation

4. DNA damage response: Impaired DNA repair machinery

5. TDP-43 Pathology

5.1 TDP-43 Mislocalization

TDP-43 (TAR DNA-binding protein 43) is the signature pathology in C9orf72-ALS^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}8: ^{5Progranulin as a therapeutic target in neurodegenerative diseases.2022 · Trends Pharmacol Sci · DOI 10.1016/j.tips.2021.11.015 · PMID 35039149Open reference}

5.2 Relationship to C9orf72 Mechanisms

flowchart TD
    A["C9orf72 Expansion"] --> B["RNA Foci"]
    A --> C["DPR Production"]
    B --> D["TDP-43 Sequestration"]
    C --> E["Nuclear Pore Damage"]
    D --> F["TDP-43 Mislocalization"]
    E --> F
    F --> G["Splicing Dysregulation"]
    F --> H["Stress Granule Formation"]
    G --> I["ALS Pathology"]
    H --> I

6. Clinical Trials and Therapeutic Development

6.1 Antisense Oligonucleotide Trials

Key learnings from trials^{2Amyotrophic lateral sclerosis caused by hexanucleotide repeat expansions in C9orf72: from genetics to therapeutics.2025 · Lancet Neurol · DOI 10.1016/S1474-4422(25)00026-2 · PMID 39986312Open reference}9:

• 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^{3Stress Granule Assembly Disrupts Nucleocytoplasmic Transport.2018 · Cell · DOI 10.1016/j.cell.2018.03.025 · PMID 29628143Open reference}0:

6.3 Emerging Therapeutic Approaches

7. Microglial Involvement

7.1 C9orf72 in Microglia

C9orf72 is highly expressed in microglia, and deficiency promotes neuroinflammation^{3Stress Granule Assembly Disrupts Nucleocytoplasmic Transport.2018 · Cell · DOI 10.1016/j.cell.2018.03.025 · PMID 29628143Open reference}1:

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

8.2 Inclusion Body Types

9. Diagnostic and Biomarker Advances

9.1 Genetic Testing Algorithms

flowchart TD
    A["Clinical Suspicion"] --> B["C9orf72 Testing"]
    B --> C{"Repeat Size"}
    C -->|"<30"| D["Negative"]
    C -->|"30-50"| E["Intermediate"]
    C -->|">30"| F["Pathogenic"]
    F --> G["Family Counseling"]
    E --> H["Monitor"]
    D --> I["Other Testing"]

9.2 Fluid Biomarkers

10. Therapeutic Rationale by Mechanism

10.1 Target-Focused Strategies

10.2 Combination Therapy Rationale

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

1. ASO + small molecule: Target both RNA and DPR toxicity

2. Anti-DPR antibody + ASO: Clear existing and prevent new DPRs

3. Gene therapy + immunotherapy: Restore function + remove pathology

Brain Atlas Resources

• Allen Human Brain Atlas: C9orf72 expression

• Allen Cell Types Data Portal

• BrainSpan Developmental Transcriptome

See Also

• Genes Index

• ALS

• FTD

• ALS-FTD Spectrum

• C9orf72 Gene

• TDP-43 Proteinopathy

• RNA Metabolism in Neurodegeneration

• Protein Aggregation

External Links

• NCBI Gene: C9orf72

• OMIM: 614260

• ALS Association

• ALS Therapy Development Institute

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:

1. Loss of Function: Reduced C9orf72 protein expression due to DNA hypermethylation and transcriptional silencing, disrupting endolysosomal trafficking, autophagy, and synaptic function

2. RNA Toxicity: Expanded GGGGCC repeat transcripts form G-quadruplex structures that sequester RNA-binding proteins, causing nucleolar stress, splicing disruption, and nucleocytoplasmic transport impairment

3. DPR Toxicity: Repeat-associated non-ATG (RAN) translation produces five dipeptide repeat proteins (poly-GA, GP, GR, PR, PA) that inhibit proteasome function, disrupt nuclear pores, cause mitochondrial dysfunction, and trigger stress granule formation

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

Multiple therapeutic approaches are in development:

• ASOs targeting C9orf72 transcripts (Phase 1-3)

• Small molecules modulating G-quadruplex structures

• Gene therapy restoring C9orf72 expression

• Immunotherapy targeting DPRs

The identification of CSF poly-GP as a pharmacodynamic biomarker enables monitoring of target engagement in clinical trials, advancing the translation of mechanism-based therapies for C9orf72-associated disease.

C9orf72 Pathogenesis Mechanisms

flowchart TD
    A["C9orf72 GGGGCC Repeat Expansion"] --> B["Loss of Function"]
    A --> C["RNA Toxicity"]
    A --> D["DPR Protein Toxicity"]
    B --> E["Reduced C9orf72 Protein"]
    E --> F["Impaired Autophagy"]
    E --> G["Endolysosomal Dysfunction"]
    C --> H["RNA Foci Formation"]
    H --> I["RBP Sequestration"]
    I --> J["Splicing Disruption"]
    D --> K["Poly-GA: Proteasome Inhibition"]
    D --> L["Poly-GR/PR: Nuclear Pore Disruption"]
    F --> M["Protein Aggregate Accumulation"]
    J --> N["TDP-43 Mislocalization"]
    L --> N
    M --> O["Motor Neuron Degeneration"]
    N --> O