C9orf72 is a human gene on chromosome 9. It is best known because an expanded repeat in the gene is a major genetic cause of amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and mixed ALS-FTD.
The name originally meant "chromosome 9 open reading frame 72". Modern gene resources describe it as the C9orf72-SMCR8 complex subunit. The gene is active in several tissues and is especially important in neurons, including motor neurons and neurons in the cerebral cortex.
Normal Function
C9orf72 provides instructions for making a protein involved in normal cell processes. MedlinePlus Genetics describes the protein as abundant in nerve cells in the outer layers of the brain and in motor neurons in the brain and spinal cord.
The exact range of C9orf72 functions is still being studied. Current work links it with RNA handling, protein production, intracellular transport, autophagy, endosomal trafficking, and the handling of cellular material inside membrane-bound compartments.
Repeat Expansion
The clinically important C9orf72 change is a hexanucleotide repeat expansion. The repeated sequence is GGGGCC, sometimes written as G4C2. A small number of repeats can be part of normal variation. A much larger repeat expansion can be pathogenic.
This expansion is not a simple spelling error in the gene. It can affect cells in more than one way. Proposed mechanisms include reduced normal C9orf72 protein function, toxic RNA effects, abnormal dipeptide repeat proteins, disturbed RNA processing, impaired protein handling, and stress within neurons.
Motor neurons and frontal or temporal brain networks appear especially vulnerable, which helps explain the link with ALS and FTD.
Associated Conditions
C9orf72 repeat expansion is associated mainly with:
- Amyotrophic lateral sclerosis, a motor neuron disease causing progressive weakness and loss of motor function.
- Frontotemporal dementia, a group of disorders affecting behaviour, executive function, language, personality, and social judgement.
- ALS-FTD, where features of both conditions occur in the same person.
GeneReviews describes C9orf72-FTD/ALS as most often involving FTD and upper and lower motor neuron disease, although atypical presentations can occur. Age at onset is often between 50 and 64 years, but reported ranges are much wider.
Clinical Features
The presentation is variable. Some people develop mainly motor symptoms. Others develop cognitive, language, behavioural, or psychiatric symptoms first.
ALS features can include:
- Progressive limb weakness.
- Bulbar symptoms such as speech or swallowing difficulty.
- Muscle wasting or fasciculations.
- Stiffness, spasticity, or brisk reflexes.
- Respiratory weakness in advanced disease.
FTD features can include:
- Personality and behaviour change.
- Loss of judgement or social awareness.
- Apathy, disinhibition, or compulsive behaviour.
- Language difficulty.
- Executive dysfunction, including planning and problem solving.
- Memory problems in some cases.
C9orf72-related disease can differ between relatives carrying the same expansion. One family member may develop ALS, another may develop FTD, and another may remain unaffected into later life.
Inheritance
C9orf72-FTD/ALS is usually inherited in an autosomal dominant pattern. This means a person with a pathogenic expansion has a 1 in 2 chance of passing it to each child.
Penetrance is age-related and incomplete. In plain terms, inheriting the expansion increases risk substantially, but it does not guarantee that symptoms will appear by a particular age. Genetic counselling is therefore important before predictive testing in an unaffected person.
Genetic Testing
Testing for C9orf72 repeat expansion is a specialist genetic test. Standard sequencing panels may not detect repeat expansions reliably, so the requested method matters.
Testing may be considered when a person has ALS, FTD, ALS-FTD, a strong family history, or a clinical pattern suggestive of inherited neurodegenerative disease. Results can affect family counselling, eligibility for research, and future trial options, but they can also have major psychological and insurance implications.
Research
C9orf72 became central to ALS and FTD research after separate 2011 studies identified the repeat expansion as a cause of chromosome 9p-linked ALS and FTD. Since then, research has focused on how the expansion damages neurons and whether targeted therapies can reduce toxic RNA, toxic proteins, or downstream cellular stress.
Approaches under investigation include antisense oligonucleotides, gene-silencing strategies, disease biomarkers, and treatments aimed at common pathways in ALS and FTD. These remain specialist research areas rather than routine cures.
See Also
References
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