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Updated: Feb 13, 2026
Imaging Mismatch Repair and Cellular Responses to DNA Damage in Bacillus subtilis
Published on: February 8, 2010
GAA•TTC repeat expansion in human cells is mediated by mismatch repair complex MutLγ and depends upon the
Anasheh Halabi1, Kayla T B Fuselier2, Ed Grabczyk2
1Division of Neurology, Department of Neurosciences, University of California, San Diego, CA 92103, USA.
Abstract:
DNA repeat expansion underlies dozens of progressive neurodegenerative disorders. While the mechanisms driving repeat expansion are not fully understood, increasing evidence suggests a central role for DNA mismatch repair. The mismatch repair recognition complex MutSβ (MSH2-MSH3) that binds mismatched bases and/or insertion/deletion loops has previously been implicated in GAA•TTC, CAG•CTG and CGG•CCG repeat expansion, suggesting a shared mechanism. MutSβ has been studied in a number of models, but the contribution of subsequent steps mediated by the MutL endonuclease in this pathway is less clear. Here we show that MutLγ (MLH1-MLH3) is the MutL complex responsible for GAA•TTC repeat expansion. Lentiviral expression of shRNA targeting MutL nuclease components MLH1, PMS2, and MLH3 revealed that reduced expression of MLH1 or MLH3 reduced the repeat expansion rate in a human Friedreich ataxia cell model, while targeting PMS2 did not. Using splice-switching oligonucleotides we show that MLH3 isoform 1 is active in GAA•TTC repeat expansion while the nuclease-deficient MLH3 isoform 2 is not. MLH3 isoform switching slowed repeat expansion in both model cells and FRDA patient fibroblasts. Our work indicates a specific and active role for MutLγ in the expansion process and reveals plausible targets for disease-modifying therapies.
Insights
DNA mismatch repair, specifically MutLγ (MLH1-MLH3), drives repeat expansion in neurodegenerative disorders like Friedreich
Area of Science:
- Molecular Biology
- Genetics
- Neuroscience
Background:
- DNA repeat expansions cause numerous progressive neurodegenerative diseases.
- The DNA mismatch repair system, particularly MutSβ (MSH2-MSH3), is implicated in repeat expansion.
- The role of downstream MutL endonucleases in this process remains unclear.
Purpose of the Study:
- To identify the specific MutL endonuclease complex involved in GAA•TTC repeat expansion.
- To investigate the functional significance of MLH3 isoforms in repeat expansion.
- To explore potential therapeutic targets for repeat expansion disorders.
Main Methods:
- Utilized lentiviral shRNA to reduce expression of MutL endonuclease components (MLH1, PMS2, MLH3).
- Employed splice-switching oligonucleotides to modulate MLH3 isoform activity.
- Assessed repeat expansion rates in a Friedreich ataxia cell model and patient fibroblasts.
Main Results:
- Reduced expression of MLH1 or MLH3 significantly decreased GAA•TTC repeat expansion rates.
- Targeting PMS2 did not affect repeat expansion.
- Active MLH3 isoform 1, but not nuclease-deficient isoform 2, drives GAA•TTC repeat expansion; isoform switching reduced expansion.
Conclusions:
- MutLγ (MLH1-MLH3) is the specific MutL complex responsible for GAA•TTC repeat expansion.
- MLH3 isoform activity is critical for repeat expansion, presenting a potential therapeutic target.
- This study elucidates a key mechanism in repeat expansion disorders and suggests novel therapeutic strategies.
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