gene and repeat-expansion disease · BEGINNER LESSON
C9orf72
C9orf72 is a gene in which a GGGGCC repeat expansion is a major known genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). In affected people, the repeat can expand from fewer than 30 copies to hundreds or thousands, and both repeat size and the gene’s normal expression can influence disease.
What it is and what is measured
C9orf72 is studied mainly by measuring the length of a GGGGCC hexanucleotide repeat expansion, the amount of C9orf72 expression from the normal (wild-type) allele, and disease-related outcomes such as ALSFRS-R scores, survival, or signs of cell damage. A hexanucleotide is a sequence of six DNA letters repeated many times. In people without the expansion, the repeat is usually under 30 copies; in affected patients it may reach hundreds to thousands.
[1] C9orf72-related amyotrophic lateral sclerosis-frontotemporal ... [2] Wild-type C9orf72 expression is a genetic modifier of C9-ALS ...
How it can cause disease
The sources describe three overlapping ways the expansion may harm cells. First, loss of function means the normal C9orf72 protein is reduced. Second, the expanded repeat RNA can form toxic RNA foci, which are visible clumps of RNA in cells. Third, the repeat can be translated by repeat-associated non-ATG (RAN) translation, a process that makes dipeptide repeat proteins (DPRs) without the usual start codon; these DPRs include species such as poly-GR and poly-PR. Studies in cell, neuron, rodent, and postmortem tissue models link DPRs and repeat expansions to DNA damage, repair defects, R-loops, mitochondrial dysfunction, oxidative stress, and suppression of ataxia-telangiectasia mutated (ATM) signaling.
[1] C9orf72-related amyotrophic lateral sclerosis-frontotemporal ... [3] Microglial Dysfunction Induced by C9ORF72 Dipeptide Repeat Proteins: Biomarker and Therapeutic Perspectives - PubMed
Why the normal allele matters too
Variation on the wild-type, meaning unexpanded and non-mutant, C9orf72 allele can change how much normal C9orf72 is made in motor neurons. In the reported data, certain common and rare variants formed regulatory haplotypes that stratified patients into subgroups with different survival trajectories; higher wild-type expression was associated with slower disease progression. This suggests that both the mutant repeat and the healthy copy can matter in the same person.
[2] Wild-type C9orf72 expression is a genetic modifier of C9-ALS ...
Limitations and cautions
The evidence base is not complete. One review notes that many studies focus on DPRs, while loss-of-function and repeat RNA expansion mechanisms have been less directly tested. Another limit is that associations between wild-type expression and outcome come from grouped patient data, so they describe population trends rather than predicting a single person’s course. The sources also show that DNA damage can both result from the expansion and, in some settings, promote further expansion through mismatch repair (MMR) and homology-directed repair (HDR) pathways.
[1] C9orf72-related amyotrophic lateral sclerosis-frontotemporal ... [2] Wild-type C9orf72 expression is a genetic modifier of C9-ALS ...
Compact mechanism summary
The repeat expansion can create toxic RNA and DPRs, reduce normal C9orf72 function, and destabilize DNA repair and cell stress systems. These effects can feed into one another rather than act alone.
[1] C9orf72-related amyotrophic lateral sclerosis-frontotemporal ... [2] Wild-type C9orf72 expression is a genetic modifier of C9-ALS ... [3] Microglial Dysfunction Induced by C9ORF72 Dipeptide Repeat Proteins: Biomarker and Therapeutic Perspectives - PubMed
SEE THE IDEA
C9orf72 repeat expansion and disease-linked effects
Repeat length rises from normal counts to very large expansions.
Expanded RNA can form foci; RAN translation can produce DPRs such as poly-GR and poly-PR.
ATM signaling, DNA repair, mitochondria, and oxidative balance can be disrupted.
These linked changes are associated with ALS and FTD progression.
The repeat expansion can trigger several overlapping cell effects that contribute to disease.
A simple path from repeat expansion to molecular stress and clinical disease.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.