C9orf72 Hexanucleotide Expansion — Discovery and Mechanism
In the fall of 2011, two independent research groups published simultaneously what would become one of the most significant genetic discoveries in the history of neurodegenerative disease research. A repetitive sequence — a six-letter unit of DNA repeated hundreds or thousands of times where it should appear only a few times — in a gene called C9orf72 turned out to be the most common known cause of both familial ALS and familial frontotemporal dementia. The discovery did not just add a gene to a list. It restructured the conceptual landscape of motor neuron disease.
What they did
Both research teams — one led by Rosa Rademakers and colleagues at Mayo Clinic (DeJesus-Hernandez et al.), the other by Alan Renton and colleagues at the National Institutes of Health (Renton et al.) — had been studying familial forms of ALS and frontotemporal dementia. A subset of affected families had disease that mapped to chromosome 9, but the specific mutation responsible had eluded researchers for years. The causative variant was not a point mutation, a deletion, or a standard insertion — it was a repeat expansion, a class of mutation that is technically difficult to detect and characterize.
Both teams applied genome-wide linkage analysis, whole-genome sequencing, and targeted validation assays to families with chromosome 9-linked ALS-FTD. The repeat expansion — a GGGGCC hexanucleotide motif in the first intron of C9orf72 — was present in affected individuals and absent or at low copy number in unaffected controls. Healthy individuals typically carry fewer than 30 repeats; affected individuals carry hundreds to thousands.
The two studies were published back to back in the same issue of Neuron. The independent simultaneous discovery — from two different groups, using overlapping but distinct patient cohorts and analytical approaches — constituted strong immediate replication of the finding.
What they found
The GGGGCC hexanucleotide repeat expansion in the first intron of C9orf72 is the most common single genetic cause of familial ALS identified to date. It accounts for approximately 40% of familial ALS in populations of Northern European descent. It is also the most common known genetic cause of familial frontotemporal dementia. This single discovery explained more familial ALS than all previously known ALS genes combined.
The expansion produces disease through at least three proposed mechanisms, which are not mutually exclusive and may all contribute simultaneously. First, the expanded repeat is transcribed into long repetitive RNA that forms hairpin structures and sequesters RNA-binding proteins, disrupting normal RNA metabolism — a loss-of-function effect on the proteins that would normally interact with these sequences. Second, the repeat RNA undergoes repeat-associated non-ATG (RAN) translation, producing dipeptide repeat (DPR) proteins — poly-GA, poly-GR, poly-PR, poly-GP, and poly-PA — that are toxic to neurons, particularly at high concentrations. Third, reduced C9orf72 protein expression itself (haploinsufficiency) may impair cellular waste-clearing mechanisms.
The discovery also unified two diseases that had previously seemed separate. ALS and frontotemporal dementia (FTD) — a form of dementia affecting personality, behavior, and executive function — had long been observed to co-occur in some patients and families. The C9orf72 expansion explained why: both diseases can result from the same underlying genetic mutation, and they represent different points on a shared disease spectrum. This ALS-FTD spectrum is now central to how the field thinks about both diseases.
The toxic DPR proteins, particularly poly-GP, can be measured in cerebrospinal fluid as pharmacodynamic biomarkers — a direct readout of whether a treatment is reducing production of the toxic species. ASO therapies targeting C9orf72 have used poly-GP CSF levels as a primary outcome measure in early trials.
Why it matters
This discovery set off a decade of research into RNA-mediated toxicity and DPR biology that continues today. It produced multiple therapeutic targets — the expanded RNA, the DPR proteins, and the C9orf72 protein itself — and has led to antisense oligonucleotide (ASO) programs aimed at silencing the expanded repeat allele. Drugs targeting C9orf72 (BIIB078, WVE-004) have entered clinical trials, following the same ASO strategy that eventually produced tofersen for SOD1-ALS.
For the broader MND research community, the C9orf72 discovery demonstrated that common genetic causes of apparently sporadic disease could still be found — that the era of major ALS genetics discoveries was not over with the identification of SOD1. It also cemented the view that ALS and FTD are not coincidentally related but biologically connected, which has influenced clinical trial design, patient selection, and the way cognitive and behavioral symptoms are assessed in ALS patients.
What this means for PLS
C9orf72 is not a major cause of PLS. The hexanucleotide expansion has occasionally been reported in PLS patients — it appears in case series of individuals with apparent PLS, including in a review of therapeutic trial considerations in PLS. But it is not enriched in PLS cohorts in the way it is in familial ALS, and it is not considered a primary cause of the PLS phenotype.
This distinction matters. C9orf72 ALS can be UMN-predominant, and a patient who carries the expansion and presents with slowly progressive spastic features could receive a clinical diagnosis of PLS before the expansion is identified. Finding C9orf72 in a patient labeled as PLS would reframe the case as C9orf72-associated MND with a UMN-predominant phenotype — a meaningful distinction for genetic counseling (autosomal dominant inheritance, 50% risk to first-degree relatives) and potentially for therapy eligibility.
This is one reason why genetic testing — specifically including C9orf72 repeat expansion analysis — is worth considering in PLS patients with family history, younger onset, or co-occurring cognitive or behavioral changes. The yield in unselected sporadic adult PLS is low, but in the right clinical context, finding a C9orf72 expansion changes the diagnosis and the family's risk picture entirely.
How this connects
The C9orf72 story is best understood alongside the SOD1 story — together they represent the two dominant genetic forces in ALS research, one producing a protein-folding problem (SOD1) and the other producing toxic RNA and DPR proteins (C9orf72). Both have enabled ASO-based drug programs; SOD1-ALS has already yielded an approved therapy (tofersen), while C9orf72-targeting ASOs remain in trials. The connection to TDP-43 is also important: most C9orf72-ALS patients show TDP-43 pathology despite the distinct primary mutation, illustrating how multiple genetic causes converge on the same downstream pathological protein. For the overall genetic landscape of PLS, the Genetics Research hub provides context for why C9orf72 looms large in ALS but is a minor player in PLS. For investigational therapies including those targeting C9orf72, see the Investigational Treatments page.
Citation
DeJesus-Hernandez M, et al. Expanded GGGGCC hexanucleotide repeat 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
Renton AE, et al. A hexanucleotide repeat expansion in C9ORF72 is the cause of chromosome 9p21-linked ALS-FTD. Neuron. 2011;72(2):257–268. doi:10.1016/j.neuron.2011.09.010