DNA repeat expansion · BEGINNER LESSON
G4C2 repeat
A G4C2 repeat is a hexanucleotide DNA sequence made of GGGGCC units that can expand abnormally in the C9orf72 gene. These repeats can fold into G-quadruplex structures, which are four-stranded shapes stabilized by guanine stacking and potassium ions.
What it is and what is measured
A G4C2 repeat is a short DNA sequence unit, GGGGCC, repeated many times. Scientists measure how many repeat units are present, because unusually large expansions are associated with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). In the sources, healthy individuals are described as carrying fewer than 20 repeat units, while patient expansions are much larger. The repeat is found in a noncoding intronic region of the C9orf72 gene, meaning it lies inside a segment that is copied into RNA but does not directly encode a protein.
[1] Crystal structures of distinct parallel and antiparallel DNA G ... - PMC [2] New roles for canonical transcription factors in repeat ... [3] Unraveling the Molecular Basis for G‐Quadruplex‐Binders to ALS/FTD‐Associated G4C2 Repeats of the C9orf72 Gene
How it can act in cells
Expanded G4C2 repeats can be transcribed, which means DNA is copied into RNA by RNA polymerase II (RNAPII). The resulting repeat-containing RNA can form foci, or visible RNA accumulations, that may trap RNA-binding proteins. The repeat RNA can also undergo repeat-associated non-AUG or repeat-associated non-ATG translation, a nonstandard way of making peptide chains without the usual start signal, producing dipeptide repeat proteins that may be toxic. The sources also note that the repeat can reduce normal C9orf72 gene product levels, so both gain-of-function toxicity and loss of normal function are considered.
[2] New roles for canonical transcription factors in repeat ... [3] Unraveling the Molecular Basis for G‐Quadruplex‐Binders to ALS/FTD‐Associated G4C2 Repeats of the C9orf72 Gene
G-quadruplex structure and why shape matters
The repeat is rich in guanine (G), and guanine-rich DNA or RNA can fold into G-quadruplexes, often shortened to G4s. A G-tetrad is a square arrangement of four guanines linked by Hoogsteen hydrogen bonds; stacked tetrads are stabilized by potassium ions (K+). The sources report that four DNA G4C2 repeats can adopt parallel or antiparallel G4 topologies. Because the repeat can fold in more than one way, its structure is polymorphic, and that shape diversity is thought to influence disease-related processes such as transcription problems, chromatin changes, or repeat instability.
[1] Crystal structures of distinct parallel and antiparallel DNA G ... - PMC [3] Unraveling the Molecular Basis for G‐Quadruplex‐Binders to ALS/FTD‐Associated G4C2 Repeats of the C9orf72 Gene
Limitations and uncertainty
The exact pathway from G4C2 expansion to ALS or FTD is still not fully resolved. The sources describe several proposed mechanisms, including RNA foci, toxic dipeptide repeat proteins, disrupted RNA polymerase activity, altered chromatin structure, and R-loops, which are hybrid structures formed when DNA and nascent RNA pair with each other. These are supported by experimental and patient-derived observations, but no single mechanism is established as the only cause.
[1] Crystal structures of distinct parallel and antiparallel DNA G ... - PMC [2] New roles for canonical transcription factors in repeat ... [3] Unraveling the Molecular Basis for G‐Quadruplex‐Binders to ALS/FTD‐Associated G4C2 Repeats of the C9orf72 Gene
SEE THE IDEA
From expanded repeat to cellular effects
More GGGGCC repeat units in C9orf72 than usual
DNA or RNA can form parallel or antiparallel G-quadruplex shapes
RNA foci, toxic dipeptide repeats, transcription disruption, or repeat instability
Observed in ALS and FTD-associated C9orf72 expansion
Expanded repeat can fold into structured nucleic acids that may alter cell processes and contribute to disease-linked effects.
A repeat expansion is measured by copy number; its structure and downstream cellular effects are the key mechanistic steps.
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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.