C9Orf72 (Chromosome 9 Open Reading Frame 72) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.
The C9orf72[1] gene (Chromosome 9 Open Reading Frame 72) encodes a
protein involved in autophagy, endosomal trafficking, and immune regulation. A GGGGCC hexanucleotide repeat[2] expansion (HRE) in the first intron of C9orf72[2] expansion in
C9ORF72 is the cause of chromosome 9p21-linked ALS-FTD[3]. Neuron. 2011;72(2):257-268.
[doi:10.1016/j.neuron.2011.09.010]https://pubmed.ncbi.nlm.nih.gov/21944778/)" title="[Renton AE, Majounie E, Waite A, et al. A hexanucleotide
repeat[2] expansion in C9ORF72 is the cause of chromosome 9p21-linked ALS-FTD[3]. Neuron. 2011;72(2):257-268.
[doi:10.1016/j.neuron.2011.09.010]https://pubmed.ncbi.nlm.nih.gov/21944778/)">[1] is the most common genetic cause of both amyotrophic
lateral sclerosis (ALS) and Frontotemporal Dementia (FTD), accounting for approximately 30–40% of familial ALS and 20–30% of familial FTD cases in
European populations (Renton et al., 2011; DeJesus-Hernandez et al.,
2011). The discovery of this mutation in 2011 fundamentally reshaped understanding of the [ALS-FTD[1] associates with SMCR8 and WDR41 to regulate the autophagy-lysosome pathway. Acta Neuropathol Commun. 2016;4(1):51.
[doi:10.1186/s40478-016-0324-5]https://pubmed.ncbi.nlm.nih.gov/27103069/)" title="[Sullivan PM, Zhou X, Robber AM, et al. The ALS/FTLD associated
protein C9orf72[1] associates with SMCR8 and WDR41 to regulate the autophagy-lysosome pathway. Acta Neuropathol Commun. 2016;4(1):51.
[doi:10.1186/s40478-016-0324-5]https://pubmed.ncbi.nlm.nih.gov/27103069/)">[3] spectrum] and established a molecular link between these two
conditions.
¶ Gene Structure and Genomic Context
The C9orf72[1] gene is located on chromosome 9p21.2 and spans approximately 27 kb with 12 exons. The gene produces three transcript variants through alternative splicing, encoding two protein isoforms: a long isoform (481 amino acids) and a short isoform (222 amino acids). The pathogenic GGGGCC repeat is located in intron 1, between non-coding exons 1a and 1b.
The C9orf72[1] protein forms a complex with SMCR8 and WDR41, functioning as a guanine nucleotide exchange factor (GEF) for Rab GTPases. Key physiological roles include:
C9orf72[1] is widely expressed throughout the central nervous system, with highest levels in the [cerebral cortex, hippocampus, cerebellum, and [motor [neurons). It is also highly expressed in myeloid lineage cells, including microglia).
¶ Inheritance and Penetrance
The expansion follows autosomal dominant inheritance with age-dependent, incomplete penetrance:
- ~50% penetrance by age 58
- ~90% penetrance by age 80
- Rare cases of apparent non-penetrance in elderly carriers
- Anticipation (earlier onset in successive generations) has been observed but is not consistent
Three non-mutually exclusive mechanisms contribute to C9orf72[1]-mediated neurodegeneration:
The repeat expansion reduces C9orf72[1] protein levels through:
- Epigenetic silencing: CpG hypermethylation of the C9orf72[1] promoter region
- Heterochromatin formation: Trimethylation of histone H3 at lysine 9 (H3K9me3) and lysine 27 (H3K27me3)
- Transcriptional repression: G-quadruplex and R-loop structures impede transcription
Reduced C9orf72[1] leads to impaired autophagy, lysosomal dysfunction, and
dysregulated immune responses. C9orf72[1] knockout mice develop splenomegaly, lymphadenopathy, and
systemic autoimmunity, underscoring the protein's role in immune homeostasis (O'Rourke et al., 2016).
Recent work demonstrated that C9orf72[1] HRE specifically impairs
[microglial/cell-types/microgliaphagocytic and inflammatory responses (Nature Neuroscience,
2025.
The expanded repeat is bidirectionally transcribed, generating both sense (GGGGCC) and antisense (CCCCGG) repeat-containing RNAs that:
- Form nuclear RNA foci through liquid-liquid phase separation and multimolecular G-quadruplex (mG4) structures (Raguseo et al., 2023)
- Sequester RNA-binding proteins including hnRNP-H, hnRNP-A3, ALYREF, and nucleolin, disrupting normal RNA metabolism
- Cause nucleolar stress by disrupting rRNA biogenesis
- Impair [nucleocytoplasmic transport] by sequestering Ran-GTPase pathway components
The expanded repeat undergoes repeat-associated non-ATG ([RAN translation) in all six reading frames, producing five dipeptide repeat proteins (DPRs):
| DPR |
Reading Frame |
Toxicity |
Key Mechanisms |
| Poly-GA |
Sense |
Moderate |
[Proteasome] impairment, p62 sequestration |
| Poly-GP |
Sense + Antisense |
Low (biomarker) |
CSF biomarker for target engagement |
| Poly-GR |
Sense |
High |
Nucleolar stress, [DNA damage], mitochondrial dysfunction |
| Poly-PR |
Antisense |
High |
[Nucleocytoplasmic transport] disruption, [stress granule] dynamics |
| Poly-PA |
Antisense |
Low |
Less characterized |
Arginine-containing DPRs (poly-GR, poly-PR) are the most toxic, disrupting ribosome biogenesis, [stress granule] dynamics, DNA damage repair, and heterochromatin structure (Ash et al., 2013; Kwon et al., 2014).
Increasing evidence suggests these mechanisms act synergistically. Loss of C9orf72[1] function impairs autophagy-mediated clearance of DPRs and
TDP-43 aggregates, while RNA foci and DPRs further compromise proteostasis. TDP-43 pathology, the hallmark of ALS/FTD
neuropathology, is found in affected neurons but is thought to be a downstream consequence rather than a direct effect of the repeat expansion.
C9orf72[1]-ALS accounts for 5–10% of sporadic and 30–40% of familial ALS:
- Mean age of onset: 54–58 years (earlier than non-C9 ALS)
- Both bulbar and spinal onset patterns
- Often associated with cognitive/behavioral impairment
- Faster progression compared to some other genetic ALS subtypes
C9orf72[1]-FTD accounts for 5–10% of sporadic and 20–30% of familial FTD:
- Behavioral variant FTD (bvFTD) is the most common presentation
- Non-fluent/agrammatic variant of primary progressive aphasia also occurs
- Mean age of onset: 58–63 years
- Psychotic features (delusions, hallucinations) more common than in non-C9 FTD
Many C9orf72[1] carriers develop overlapping features across the
[ALS-FTD[3] continuum]:
- Motor Neuron Disease with concurrent cognitive/behavioral decline
- Progressive behavioral changes followed by motor symptoms
- This clinical overlap reflects shared TDP-43 proteinopathy]
C9orf72[1] expansions have also been reported in:
C9orf72[1] repeat expansions are the most common known genetic cause of ALS and FTD worldwide, though prevalence varies by ancestry:
| Population |
Familial ALS (%) |
Sporadic ALS (%) |
Familial FTD (%) |
| European |
30–40 |
5–10 |
20–30 |
| North American |
25–35 |
5–8 |
15–25 |
| East Asian |
2–5 |
<2 |
3–5 |
| African |
Rare |
Rare |
Rare |
The expansion likely arose from a common founder in Northern Europe approximately 1,500 years ago (Smith et al., 2013).
- Repeat-primed PCR: Screening test that detects the presence of an expansion but cannot precisely size large repeats
- Southern blot: Gold standard for repeat sizing; shows characteristic smear pattern due to somatic instability
- Genetic testing is recommended for all ALS and FTD patients, particularly those with family history or overlapping ALS-FTD[3] features
- [Neurofilament light chain (NfL: Elevated in CSF and blood plasma; correlates with disease progression
- Poly-GP DPR in CSF: Research biomarker for target engagement in clinical trials; reduced by ASO treatment
- TDP-43 pathology: Present at autopsy; not yet available as a fluid biomarker
- Plasma biomarkers: p-tau217 and GFAP help distinguish co-morbid AD pathology
- MRI: Symmetric frontotemporal atrophy (in contrast to asymmetric atrophy in sporadic FTD); thalamic and cerebellar involvement
- FDG-PET: Frontal, temporal, and parietal hypometabolism
- DTI: White matter tract degeneration in frontal and temporal regions
Postmortem examination reveals:
- TDP-43 inclusions: Characteristic p62-positive, TDP-43-positive neuronal cytoplasmic inclusions in cortex and spinal cord
- DPR inclusions: p62-positive, TDP-43-negative inclusions containing dipeptide repeat proteins, particularly in cerebellum and hippocampus
- RNA foci: Sense and antisense RNA foci in neuronal and glial nuclei
- Neuronal loss: In motor cortex, [frontal cortex, temporal cortex, hippocampus, and spinal anterior horn
ASO therapy is the most advanced therapeutic strategy for C9orf72[1]-ALS/FTD, targeting repeat-containing RNA transcripts:
| Agent |
Developer |
Status |
Notes |
| BIIB078 (tadnersen) |
Biogen/Ionis |
Discontinued (2022) |
Phase I showed no clinical benefit |
| WVE-004 |
Wave Life Sciences |
Discontinued (2023) |
Reduced poly-GP by 51% but no clinical benefit in FOCUS-C9 trial |
| Afinersen |
Ionis |
Phase I |
Targeting both sense and antisense transcripts |
The failure of early ASO programs has prompted rethinking of therapeutic strategies, including dual-targeting approaches and earlier intervention in presymptomatic carriers (Ito et al., 2024; Ludolph & Wiesenfarth, 2025).
- CRISPR-CasRx: RNA-targeting CRISPR systems to degrade both sense and antisense repeat-containing transcripts (The Lancet Neurology, 2025)
- Small molecules: Compounds targeting RNA G-quadruplex structures to destabilize toxic RNA foci
- Immunotherapy: Antibodies targeting specific DPR species
- autophagy modulation: Enhancing clearance of DPR aggregates and TDP-43 inclusions
- Gene therapy: AAV-based delivery of repeat-targeting sequences
The ATLAS trial (NCT04856982) and similar studies are evaluating biomarker-guided intervention in presymptomatic C9orf72[1] carriers, with NfL elevation as a trigger for treatment initiation.
¶ Animal and Cellular Models
- C9orf72[1] knockout mice: Develop immune dysregulation and autoimmunity; mild neurodegeneration
- BAC transgenic mice: Carry the human repeat expansion; develop RNA foci, DPR pathology, and progressive neurodegeneration
- Drosophila models: Express expanded repeats or individual DPRs; useful for modifier screens
- iPSC-derived [motor neurons/cell-types/motor-[neurons): From patient fibroblasts; recapitulate RNA foci, DPR production, and TDP-43 mislocalization
The study of C9Orf72 (Chromosome 9 Open Reading Frame 72) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
- [Renton AE, Majounie E, Waite A, et al. A hexanucleotide repeat[2] expansion in C9ORF72 is the cause of chromosome 9p21-linked ALS-FTD[3]. Neuron. 2011;72(2):257-268. [doi:10.1016/j.neuron.2011.09.010]https://pubmed.ncbi.nlm.nih.gov/21944778/)
- [DeJesus-Hernandez M, Mackenzie IR, Boeve BF, et al. Expanded GGGGCC hexanucleotide repeat[2] in noncoding region of C9ORF72 causes chromosome 9p-linked FTD and ALS. Neuron. 2011;72(2):245-256. [doi:10.1016/j.neuron.2011.09.011]https://pubmed.ncbi.nlm.nih.gov/21944779/)
- [Sullivan PM, Zhou X, Robber AM, et al. The ALS/FTLD associated protein C9orf72[1] associates with SMCR8 and WDR41 to regulate the autophagy-lysosome pathway. Acta Neuropathol Commun. 2016;4(1):51. [doi:10.1186/s40478-016-0324-5]https://pubmed.ncbi.nlm.nih.gov/27103069/)
- [Farg MA, Sundaramoorthy V, Sultana JM, et al. C9ORF72, implicated in amytrophic lateral sclerosis and Frontotemporal Dementia, regulates endosomal trafficking. Hum Mol Genet. 2014;23(13):3579-3595. [doi:10.1093/hmg/ddu068]https://pubmed.ncbi.nlm.nih.gov/25294927/)
- [O'Rourke JG, Bogdanik L, Yanez A, et al. C9orf72[1] is required for proper macrophage and microglial function in mice. Science. 2016;351(6279):1324-1329. [doi:10.1126/science.aaf1064]https://pubmed.ncbi.nlm.nih.gov/26842965/)
- [Ash PE, Bieniek KF, Gendron TF, et al. Unconventional translation of C9ORF72 GGGGCC expansion generates insoluble polypeptides specific to c9FTD/ALS. Neuron. 2013;77(4):639-646. [doi:10.1016/j.neuron.2013.02.004]https://pubmed.ncbi.nlm.nih.gov/24153174/)
- [Kwon I, Xiang S, Kato M, et al. Poly-dipeptides encoded by the C9orf72[1] repeats bind nucleoli, impede RNA biogenesis, and kill cells. Science. 2014;345(6201):1139-1145. [doi:10.1126/science.1254917]https://pubmed.ncbi.nlm.nih.gov/25103406/)
- [van Blitterswijk M, DeJesus-Hernandez M, Niemantsverdriet E, et al. Association between repeat sizes and clinical and pathological characteristics in carriers of C9ORF72 repeat expansions (Xpansize-72]. Lancet Neurol. 2013;12(10):978-988. DOI
- [Raguseo F, Wang Y, Li J, et al. The ALS/FTD-related C9orf72[1] hexanucleotide repeat[2] expansion forms RNA condensates through multimolecular G-quadruplexes. Nat Commun. 2023;14(1):8272. [doi:10.1038/s41467-023-43872-1]https://pubmed.ncbi.nlm.nih.gov/38092738/)
- [Smith BN, Newhouse S, Shatunov A, et al. The C9ORF72 expansion mutation is a common cause of ALS+/-FTD in Europe and has a single founder. Eur J Hum Genet. 2013;21(1):102-108. [doi:10.1038/ejhg.2012.98]https://pubmed.ncbi.nlm.nih.gov/23257289/)
- [Cooper-Knock J, Shaw PJ, Kirby J. The widening spectrum of C9ORF72-related disease; genotype/phenotype correlations and potential modifiers of clinical phenotype. Acta Neuropathol. 2014;127(3):333-345. [doi:10.1007/s00401-014-1251-9]https://pubmed.ncbi.nlm.nih.gov/24493408/)
- [Balendra R, Isaacs AM. C9orf72[1]-mediated ALS and FTD: multiple pathways to disease. Nat Rev Neurol. 2018;14(9):544-558. [doi:10.1038/s41582-018-0047-2]https://pubmed.ncbi.nlm.nih.gov/30120348/)
- [Ito D, Bhatt N, Bhatt S. Rethinking antisense oligonucleotide therapeutics for amyotrophic lateral sclerosis. Ann Clin Transl Neurol. 2024;11(12):e52234. [doi:10.1002/acn3.52234]https://onlinelibrary.wiley.com/doi/full/10.1002/acn3.52234)
- [Shi Y, Lin S, Staats KA, et al. Haploinsufficiency leads to neurodegeneration in C9ORF72 ALS/FTD human induced motor neurons:313-325. [doi:10.1038/nm.4490]https://pubmed.ncbi.nlm.nih.gov/29400714/)