The mismatch repair system is required for S-phase checkpoint activation

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.