Exo1 independent DNA mismatch repair involves multiple compensatory nucleases

Area of Science:

• Molecular Biology
• Genetics
• Cancer Research

Background:

• DNA mismatch repair (MMR) is crucial for genomic stability and preventing cancer.
• Loss of MMR function leads to resistance to DNA damaging agents and is implicated in Lynch syndrome.
• Exo1 is the primary 5'→3' exonuclease involved in removing the nicked strand during MMR.

Purpose of the Study:

• To investigate the role of Exo1 nuclease activity in MMR.
• To identify alternative nucleases that can compensate for Exo1 loss in MMR.
• To understand the impact of Exo1 deficiency on hematopoietic stem cell (HSC) behavior under genotoxic stress.

Main Methods:

• Utilized nuclease-dead Exo1 mutant (Exo1(mut)) mouse embryonic fibroblasts (MEFs) and HSCs.
• Assessed sensitivity to temozolomide (TMZ) and O6-benzylguanine (BG).
• Employed an MMR reporter plasmid for in vitro mismatch repair assays.
• Performed gene expression analysis and shRNA-mediated silencing of candidate nucleases (Artemis, Fan1, Mre11).

Main Results:

• Exo1(mut) cells were sensitive to TMZ/BG treatment, indicating a role for Exo1 nuclease activity.
• Exo1(mut) MEFs could repair G:T mismatches in vitro, suggesting Exo1 is not essential for all MMR.
• Unlike other MMR-deficient models, Exo1(mut) HSCs did not exhibit a survival advantage after TMZ/BG treatment.
• Artemis, Fan1, and Mre11 were upregulated in Exo1(mut) cells and their silencing impaired MMR and increased TMZ/BG resistance.

Conclusions:

• Nuclease activity is required for optimal MMR function.
• A compensatory network of nucleases (Artemis, Fan1, Mre11) can maintain MMR proficiency in the absence of Exo1 nuclease activity.
• This compensatory mechanism impacts cellular responses to DNA damaging agents like temozolomide.