A six-letter DNA sequence, GGGGCC, repeated hundreds or even thousands of times inside the C9orf72 gene is the most common known genetic cause of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Healthy people carry fewer than about 30 of these repeats, but expansion carriers harbor anywhere from dozens to several thousand copies, and the mutation damages motor neurons through at least three converging mechanisms rather than one simple pathway. Understanding how a single repeat expansion can wreak so much havoc on the nervous system has reshaped how researchers think about neurodegenerative disease and opened new, if still imperfect, avenues for treatment.
How the C9orf72 Protein Normally Works
Before diving into what goes wrong, it helps to know what C9orf72 does in a healthy cell. The protein acts as a kind of traffic coordinator inside cells, physically interacting with a family of small signaling molecules called Rab proteins that direct cargo through the cell’s internal transport and recycling systems. Laboratory experiments have shown that C9orf72 associates with several of these Rab proteins and colocalizes with structures involved in autophagy, the process cells use to break down and recycle damaged components.1Human Molecular Genetics. C9ORF72, implicated in amytrophic lateral sclerosis and frontotemporal dementia, regulates endosomal trafficking More specifically, C9orf72 helps regulate the very first step of autophagy by guiding one of the key initiation complexes to the right place in the cell. When C9orf72 levels are experimentally reduced in neurons, autophagy slows down and clumps of a waste-disposal marker protein called p62 accumulate, a pattern strikingly similar to what is seen in patient brain tissue.2PubMed Central. The C9orf72 protein interacts with Rab1a and the ULK1 complex to regulate initiation of autophagy In short, the normal C9orf72 protein helps keep the cell’s cleanup crew running. Lose half of it, and garbage starts piling up.
Three Roads to Neuron Damage
The field has settled on a framework in which the repeat expansion inflicts harm through three overlapping routes, often called loss of function, RNA toxicity, and dipeptide repeat protein toxicity. No single route seems sufficient to explain the full disease on its own; instead, they interact in ways that are still being untangled.
Loss of Normal C9orf72 Protein
The expanded repeat region can fold into unusual DNA structures called G-quadruplexes. These structures physically block the cellular machinery that reads the gene, reducing how much C9orf72 messenger RNA the cell produces.3Nucleic Acids Research. G-Quadruplexes as pathogenic drivers in neurodegenerative disorders On top of that, the DNA around the expansion tends to become chemically modified through a process called methylation, further dialing down gene expression. Larger repeat expansions correlate with heavier methylation of the gene’s promoter region, and heavier methylation correlates with lower protein output.4PubMed Central. The C9orf72 repeat size correlates with onset age of disease, DNA methylation and transcriptional downregulation of the promoter The result is that expansion carriers end up with roughly half the normal amount of C9orf72 protein, a state called haploinsufficiency. With autophagy and intracellular trafficking already compromised, neurons become less able to deal with the toxic products the same mutation is simultaneously generating.
Toxic RNA That Gets Stuck in the Nucleus
The expanded repeats are still transcribed into RNA, even if less efficiently. That repeat-laden RNA does not behave normally. Instead of being exported to the cytoplasm and put to work, it folds into tight tangles and accumulates as clumps called RNA foci inside the nucleus. Studies using patient-derived neurons have found that roughly a third of cells containing the expansion harbor these intranuclear foci.5Neuron. RNA Toxicity from the ALS/FTD C9ORF72 Expansion Is Mitigated by Antisense Intervention Making matters worse, the gene is read in both directions, producing sense and antisense transcripts, each of which forms its own set of foci in patient brain tissue.6PubMed Central. RAN proteins and RNA foci from antisense transcripts in C9ORF72 ALS and frontotemporal dementia
These RNA foci are not just inert blobs. They act like molecular flypaper, sequestering proteins that the cell needs for other jobs. One key protein trapped by the foci is hnRNP-H, a factor involved in processing other RNA messages. When RNA-binding proteins like hnRNP-H are soaked up by repeat RNA, normal gene regulation throughout the cell starts to go sideways.7Cell Reports. Expanded Hexanucleotide Repeats in C9ORF72 Form Toxic RNA Foci and Associate with RNA-Binding Proteins in Amyotrophic Lateral Sclerosis and Frontotemporal Dementia Antisense RNA foci appear to be particularly damaging in motor neurons: their presence in motor neurons tracks with mislocalization of TDP-43, the protein whose aggregation is considered the hallmark signature of ALS neurodegeneration.8PubMed Central. Antisense RNA foci in the motor neurons of C9ORF72-ALS patients are associated with TDP-43 proteinopathy
Dipeptide Repeat Proteins Made from “Untranslatable” RNA
Perhaps the most surprising discovery in this field was that the repeat RNA gets translated into protein even though it sits in a non-coding region of the gene. Through a noncanonical mechanism called repeat-associated non-AUG (RAN) translation, both the sense and antisense transcripts are read in all three possible reading frames, producing six different dipeptide repeat proteins (DPRs).6PubMed Central. RAN proteins and RNA foci from antisense transcripts in C9ORF72 ALS and frontotemporal dementia Research has identified a specific start codon with a favorable surrounding sequence that initiates translation of at least one of these species, poly-GA, and knockout of a non-canonical translation factor called eIF2A impairs that production.9PubMed Central. Translation of dipeptide repeat proteins from the C9ORF72 expanded repeat is associated with cellular stress
Of the six DPR species, the two that contain arginine, poly-GR and poly-PR, are by far the most toxic. They carry dense positive charges that let them stick promiscuously to cellular components. But they behave differently from each other. Poly-PR tends to migrate into the nucleolus because proline spaces the arginine charges in a way that favors nuclear entry, whereas poly-GR, whose glycine backbone is more flexible, largely stays in the cytoplasm and binds tightly to a smaller number of molecular targets.10iScience. Structural basis for differential subcellular localization and interactome of C9orf72-encoded arginine-rich dipeptide repeats These distinct behaviors mean each arginine-rich DPR damages the cell in its own way, even though both originate from the same repeat expansion.
How Neurons Break Down at the Cellular Level
The three toxic streams described above converge on several cellular systems at once, and the damage compounds over time.
Nuclear-Cytoplasmic Transport Goes Awry
A healthy cell maintains a strict border between the nucleus and the cytoplasm, shuttling molecules back and forth through pores in the nuclear envelope. The repeat expansion disrupts this gatekeeper system. In patient motor cortex tissue, a key pore-associated protein called RanGAP1 forms abnormal clumps instead of distributing smoothly around the nuclear rim, and these clumps colocalize with repeat RNA.11PubMed Central. The C9ORF72 repeat expansion disrupts nucleocytoplasmic transport When trafficking between the nucleus and cytoplasm breaks down, proteins that should be in the nucleus end up outside it and vice versa, setting off a cascade of dysfunction in gene regulation and protein quality control.
Stress Granules That Refuse to Dissolve
Under stress, cells form temporary clusters of stalled RNA and proteins called stress granules. These are normally reversible. But arginine-rich DPRs undergo a process called liquid-liquid phase separation and trigger spontaneous stress granule assembly even without an external stressor.12PubMed Central. Phase Separation of C9orf72 Dipeptide Repeats Perturbs Stress Granule Dynamics Recent work has shown that these DPRs bind a critical stress granule scaffold protein called G3BP1 about a thousand times more strongly than RNA does, essentially hijacking granule assembly and driving a slow transition from liquid-like droplets into hardened, aggregate-like states that can recruit ALS-linked proteins such as TDP-43.13PubMed Central. C9orf72-linked arginine-rich dipeptide repeats aggravate pathological phase separation of G3BP1 This conversion from reversible droplet to irreversible aggregate may be one bridge between DPR toxicity and TDP-43 pathology, the two dominant features found together in patient tissue.
Mitochondrial Damage and Oxidative Stress
Poly-GR, the arginine-rich DPR that largely stays in the cytoplasm, has been found to preferentially bind to proteins of the mitochondrial ribosome, the machinery mitochondria use to make their own essential components. The result is compromised energy production, increased oxidative stress, and DNA damage inside motor neurons, with the oxidative burden growing worse with age.14PubMed Central. Poly(GR) in C9ORF72-Related ALS/FTD Compromises Mitochondrial Function and Increases Oxidative Stress and DNA Damage in iPSC-Derived Motor Neurons Motor neurons are especially energy-hungry cells, so mitochondrial dysfunction hits them particularly hard.
Nucleolar Stress Comes Early
One intriguing finding is that the nucleolus, the structure inside the nucleus responsible for building ribosomes, begins to shrink in motor neurons before TDP-43 has even mislocalized and before RNA foci are abundant. This suggests that nucleolar stress may be among the earliest events in the disease process, not a late consequence of it.15PubMed Central. Nucleolar stress in C9orf72 and sporadic ALS spinal motor neurons precedes TDP-43 mislocalization Poly-PR, which preferentially homes to the nucleolus, may be partly responsible. In cell models, poly-PR self-assembly inside the nucleus has been shown to directly trigger condensation and aggregation of TDP-43, offering a mechanistic link between DPR accumulation and the hallmark proteinopathy of ALS.16iScience. C9orf72 poly-PR forms anisotropic condensates causative of nuclear TDP-43 pathology
Why Motor Neurons Are Especially Vulnerable
ALS selectively ravages motor neurons despite the repeat expansion being present in every cell of the body. Part of the answer lies in excitotoxicity. Motor neurons from C9orf72 expansion carriers have been shown to express more of a specific glutamate receptor subunit that permits calcium to flood into the cell. This shift toward calcium-permeable receptors was found to be specific to motor neurons and absent in cortical neurons from the same patients.17Nature Communications. C9ORF72 repeat expansion causes vulnerability of motor neurons to Ca2+-permeable AMPA receptor-mediated excitotoxicity Excess calcium triggers a chain of events including enzyme activation and mitochondrial overload that can kill the cell outright.
Large-scale gene expression profiling has added another layer: neuron populations that are more resistant to the repeat expansion tend to have higher baseline activity of protein-homeostasis pathways, suggesting that neurons already well-equipped to handle misfolded or aggregated proteins survive longer, while those with thinner margins of quality control succumb first.18bioRxiv. Differential neuronal vulnerability to C9orf72 repeat expansion driven by Xbp1 transcription signature Combined with the enormous energy demands and extreme physical length of motor neurons, these factors create a perfect storm of vulnerability.
The Role of Immune Cells in Neurodegeneration
Neuron death in C9orf72 ALS is not purely a cell-intrinsic problem. C9orf72 turns out to be expressed at its highest levels not in neurons but in myeloid immune cells, including the brain’s resident immune cells, microglia. When C9orf72 is lost or reduced in mice, microglia develop swollen waste-processing compartments and shift into a chronically activated, pro-inflammatory state that worsens with age, producing elevated levels of inflammatory signaling molecules.19PubMed Central. C9orf72 is required for proper macrophage and microglial function in mice The inflammatory profile of these C9orf72-deficient mice resembles what is seen in C9orf72 ALS patient tissue more closely than it resembles sporadic ALS, pointing to a mutation-specific inflammatory environment.20JCI Insight. Microglia and C9orf72 in neuroinflammation and ALS and frontotemporal dementia
Studies using microglia grown from patient-derived stem cells have confirmed this at a human level. Microglia carrying the C9orf72 expansion show impaired ability to engulf and clear debris, along with an exaggerated inflammatory response when stimulated, reinforcing the idea that immune dysfunction is a genuine contributor to neurodegeneration in these patients, not just a bystander effect.21PubMed Central. Cell-autonomous immune dysfunction driven by disrupted autophagy in C9orf72-ALS iPSC-derived microglia contributes to neurodegeneration
The Repeat Itself Keeps Growing
One under-appreciated aspect of C9orf72 disease is that the repeat expansion is not fixed. It continues to change size, both across generations and within the body of a single person over their lifetime. A mouse model engineered to carry 96 human G4C2 repeats revealed two distinct modes of expansion: small, incremental increases driven by the cell’s mismatch repair machinery, and large jumps triggered when DNA breaks occur near the repeat, relying on a different repair pathway. In living animals, the gradual mode was the primary driver of repeat growth in tissues over time.22PubMed Central. Somatic and intergenerational G4C2 hexanucleotide repeat instability in a human C9orf72 knock-in mouse model
This somatic instability has striking consequences. Autopsies of ALS and FTD patients have shown that repeat sizes vary enormously between tissues within the same person. In some cases, non-neural tissues carried fewer than 100 repeats while neural tissues harbored expansions 20 to 40 times larger.23Human Molecular Genetics. Extensive size variability of the GGGGCC expansion in C9orf72 in both neuronal and non-neuronal tissues in 18 patients with ALS or FTD This means that a blood test showing a modest expansion could be hiding a far larger one in the brain, and it raises the possibility that somatic expansion itself contributes to disease progression and the selective vulnerability of the nervous system.
Epigenetic Modification as a Double-Edged Sword
The methylation that silences the C9orf72 gene creates a paradox. On one hand, it reduces the amount of functional C9orf72 protein, which compromises autophagy. On the other hand, silencing the mutant gene also means less repeat RNA and fewer dipeptide repeat proteins are produced. Evidence suggests the second effect may outweigh the first, at least in some patients. In people with FTD carrying the expansion, higher promoter methylation was associated with later age at death and longer disease duration, and also with smaller repeat sizes.24PubMed Central. Hypermethylation of repeat expanded C9orf72 is a clinical and molecular disease modifier This finding supports the idea that the toxic gain-of-function products, the RNA foci and DPRs, may be more damaging than the loss of C9orf72 protein itself, and that the body’s attempt to silence the mutant gene is at least partly protective.
Measuring Disease Before Symptoms Appear
One of the six DPR species, poly-GP, has emerged as a promising biomarker. Because it is produced in relatively large quantities and is chemically stable, it can be detected in cerebrospinal fluid. An ultrasensitive assay has achieved 100% specificity and 100% sensitivity for distinguishing C9orf72 expansion carriers from non-carriers, with the lowest signal from a carrier still running about eightfold higher than controls.25PubMed Central. Development of a sensitive trial-ready poly(GP) CSF biomarker assay for C9orf72-associated frontotemporal dementia and amyotrophic lateral sclerosis Critically, poly-GP has been detected in the spinal fluid of expansion carriers who have no symptoms yet, suggesting that DPR production begins years before disease onset and can serve as an early marker as well as a way to measure whether a therapy is hitting its target.26PubMed Central. Poly-GP in cerebrospinal fluid links C9orf72-associated dipeptide repeat expression to the asymptomatic phase of ALS/FTD
Therapeutic Approaches and Their Limits
Because the toxic RNA and DPRs flow directly from the repeat expansion, the most obvious strategy is to destroy the repeat-containing RNA before it can do harm. Antisense oligonucleotides (ASOs) are short synthetic DNA-like molecules designed to bind and trigger destruction of a specific RNA target. In preclinical mouse models, a single injection of a C9orf72-targeting ASO produced sustained reductions in RNA foci and DPR levels and improved behavioral deficits, establishing proof of concept for the approach.27PubMed Central. Gain of Toxicity from ALS/FTD-Linked Repeat Expansions in C9ORF72 Is Alleviated by Antisense Oligonucleotides Targeting GGGGCC-Containing RNAs
Translating that success to humans, however, has been harder. BIIB078 (tadnersen), the first ASO targeting C9orf72 repeat-containing transcripts to reach clinical trials, was designed to degrade the toxic RNA via an enzyme-based cleavage mechanism while preserving the normal protein-coding messenger RNA.28Cell. Multi-omic and neuropathological evaluation of BIIB078 antisense oligonucleotide treatment in C9orf72-associated ALS In its Phase 1 trial, the drug did not reduce neurofilament levels, a marker of ongoing neuronal damage, and showed no clinical benefit relative to placebo, leading to discontinuation of its development.29The Lancet Neurology. Safety and pharmacokinetics of BIIB078 in adults with C9orf72-associated amyotrophic lateral sclerosis: a phase 1, randomised, placebo-controlled trial Other ASOs and related strategies remain in development, and the failure of one specific molecule does not invalidate the underlying approach, but it underscored how far the field still has to go.
Alternative strategies are being explored in parallel. Small molecules that bind specifically to the G-quadruplex structures formed by the repeat RNA have shown the ability to reduce RNA foci and rescue cell death in both cell lines and fruit fly models.30PubMed Central. Selective C9orf72 G-Quadruplex-Binding Small Molecules Ameliorate Pathological Signatures of ALS/FTD Models At the most radical end of the spectrum, CRISPR-based gene editing has been used to delete the expanded repeat entirely from patient-derived stem cells, eliminating RNA foci and restoring normal methylation patterns without disrupting C9orf72 protein expression.31bioRxiv. CRISPR-Cas9 targeted deletion of the C9orf72 repeat expansion mutation corrects cellular phenotypes in patient-derived iPS cells Both of these are far from the clinic, but they demonstrate that multiple angles of attack are feasible in principle.
Spreading Between Cells
There is growing evidence that DPRs do not stay put in the cells that produce them. Using cell-culture platforms that included spinal motor neurons derived from C9orf72-ALS patients, researchers found that DPRs can spread from cell to cell through both vesicle-dependent and vesicle-independent pathways.32PubMed Central. Cell-to-Cell Transmission of Dipeptide Repeat Proteins Linked to C9orf72-ALS/FTD If this happens in living patients, it could help explain why the disease progresses along anatomically connected regions of the nervous system over time, a pattern clinicians have long observed but struggled to explain mechanistically. Cell-to-cell DPR transmission would also mean that even neurons not carrying the expansion could be poisoned by their neighbors.
Modeling the Disease Outside the Human Body
Studying a disease that unfolds over years inside the human spinal cord presents obvious practical challenges. Patient-derived stem cell technologies have partially bridged this gap by allowing researchers to grow motor neurons and even more complex tissue structures from the cells of expansion carriers. Neuromuscular organoids, three-dimensional mini-tissues containing motor neurons, muscle fibers, and supporting cells, grown from C9orf72-ALS patient cells have been shown to develop muscle weakness, visible defects at the nerve-muscle junction, and loss of the insulating cells that normally wrap nerve fibers.33Cell Reports. Modeling spinal neuromuscular pathologies in C9orf72 ALS using patient-derived organoids These organoid models reproduce multiple disease features in a dish, giving researchers a human-relevant platform to test drugs and probe disease mechanisms that would be inaccessible in animal models.