The mismatch repair system is required for S-phase checkpoint activation
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Updated: Sep 28, 2026
Visualizing and Quantifying Endonuclease-Based Site-Specific DNA Damage
Published on: August 21, 2021
Kevin D Brown1, Abhilasha Rathi, Ravindra Kamath
1Department of Biochemistry and Molecular Biology and the Stanley S. Scott Cancer Center, Louisiana State University Health Sciences Center, New Orleans, Louisiana 70112, USA.
Abstract:
Defective S-phase checkpoint activation results in an inability to downregulate DNA replication following genotoxic insult such as exposure to ionizing radiation. This 'radioresistant DNA synthesis' (RDS) is a phenotypic hallmark of ataxia-telangiectasia, a cancer-prone disorder caused by mutations in ATM. The mismatch repair system principally corrects nucleotide mismatches that arise during replication. Here we show that the mismatch repair system is required for activation of the S-phase checkpoint in response to ionizing radiation. Cells deficient in mismatch repair proteins showed RDS, and restoration of mismatch repair function restored normal S-phase checkpoint function. Catalytic activation of ATM and ATM-mediated phosphorylation of the protein NBS1 (also called nibrin) occurred independently of mismatch repair. However, ATM-dependent phosphorylation and activation of the checkpoint kinase CHK2 and subsequent degradation of its downstream target, CDC25A, was abrogated in cells lacking mismatch repair. In vitro and in vivo approaches both show that MSH2 binds to CHK2 and that MLH1 associates with ATM. These findings indicate that the mismatch repair complex formed at the sites of DNA damage facilitates the phosphorylation of CHK2 by ATM, and that defects in this mechanism form the molecular basis for the RDS observed in cells deficient in mismatch repair.
Insights
The mismatch repair system is crucial for activating the S-phase checkpoint after DNA damage. Defects in this system lead to radioresistant DNA synthesis (RDS), a hallmark of ataxia-telangiectasia.
Area of Science:
- Molecular Biology
- Genetics
- Cellular Biology
Background:
- Defective S-phase checkpoint activation prevents downregulating DNA replication after genotoxic stress.
- This radioresistant DNA synthesis (RDS) is characteristic of ataxia-telangiectasia, a cancer-prone disorder linked to ATM mutations.
- The mismatch repair system corrects nucleotide mismatches during DNA replication.
Purpose of the Study:
- To investigate the role of the mismatch repair system in S-phase checkpoint activation following ionizing radiation.
- To elucidate the molecular mechanisms by which mismatch repair influences DNA damage response pathways.
Main Methods:
- Analysis of S-phase checkpoint activation in cells deficient in mismatch repair proteins.
- Assessment of ATM and NBS1 phosphorylation.
- Investigation of CHK2 phosphorylation and CDC25A degradation.
- In vitro and in vivo binding assays for MSH2-CHK2 and MLH1-ATM interactions.
Main Results:
- Cells deficient in mismatch repair proteins exhibited RDS, which was restored upon functional repair.
- ATM and NBS1 phosphorylation occurred independently of mismatch repair.
- ATM-dependent CHK2 activation and CDC25A degradation were abrogated in mismatch repair-deficient cells.
- MSH2 was found to bind CHK2, and MLH1 associated with ATM.
Conclusions:
- The mismatch repair system is essential for ionizing radiation-induced S-phase checkpoint activation.
- The mismatch repair complex at DNA damage sites facilitates CHK2 phosphorylation by ATM.
- Defects in this mismatch repair-ATM-CHK2 pathway underlie the RDS observed in mismatch repair-deficient cells.
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