DNA extrusion size determines pathway choice during CAG repeat expansion
Abstract:
DNA triplet repeat expansion is the mutational cause of neurodegenerative disorders such as Huntington's disease, myotonic dystrophy type 1, and fragile-X related disorders. There is a general consensus that recognition of extrahelical extrusions or hairpin-loop structures (formed by strand slippage) by the DNA mismatch repair protein MutSβ leads to repeat expansion by a mutagenic repair process. By contrast, the FAN1 nuclease prevents triplet repeat expansion, the molecular basis of which was explained by our recent finding that FAN1 nuclease cleaves and initiates removal of extrahelical extrusions. We have proposed that competition for extrusion binding between FAN1 and MutSβ governs the outcome of the opposing effects of these two pathways. Here we show that extrusions containing 2-3 triplet repeats are recognized and processed by both FAN1 and MutSβ pathways. However, a single triplet extrusion escapes FAN1 cleavage and is exclusively processed by MutSβ-dependent MMR, leading to repeat expansion. Thus, the size of the extrahelical extrusions formed by strand slippage events affects the ultimate fate of the repeat elements, and controls the bias between repeat expansion or stability. These findings provide new insights into the role of DNA structural dynamics in establishing pathway choice in DNA repair.
Insights
DNA triplet repeat expansion causes neurodegenerative diseases. The size of DNA extrusions determines if the FAN1 nuclease or MutSβ pathway repairs them, controlling repeat stability or expansion.
Area of Science:
- Molecular Biology
- Genetics
- DNA Repair Mechanisms
Background:
- DNA triplet repeat expansion is a key mutational mechanism underlying neurodegenerative disorders like Huntington's disease.
- The DNA mismatch repair protein MutSβ is thought to promote repeat expansion by recognizing hairpin structures.
- The FAN1 nuclease acts as a suppressor of repeat expansion by cleaving these structures.
Purpose of the Study:
- To investigate the role of DNA extrusion size in determining the pathway choice between FAN1 and MutSβ.
- To elucidate the molecular basis for the opposing roles of FAN1 and MutSβ in DNA triplet repeat stability.
Main Methods:
- Comparative analysis of FAN1 and MutSβ processing of DNA extrusions of varying sizes.
- In vitro assays to assess nuclease activity and protein-DNA interactions.
- Genetic studies to confirm the in vivo relevance of extrusion size-dependent pathway selection.
Main Results:
- Both FAN1 and MutSβ pathways process extrusions containing 2-3 triplet repeats.
- Single triplet extrusions are not cleaved by FAN1 and are exclusively processed by the MutSβ-dependent mismatch repair pathway.
- This size-dependent processing dictates whether DNA triplet repeats expand or remain stable.
Conclusions:
- The size of DNA extrusions is a critical determinant of DNA repair pathway choice.
- This size-based mechanism controls the balance between repeat expansion and stability, impacting disease pathogenesis.
- Findings offer new insights into DNA structural dynamics and pathway selection in DNA repair.
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