FANCD2-Associated Nuclease 1 Partially Compensates for the Lack of Exonuclease 1 in Mismatch Repair

Area of Science:

• Molecular Biology
• Genetics
• Cancer Research

Background:

• Germline mutations in mismatch repair (MMR) genes (MSH2, MSH6, MLH1, PMS2) are linked to hereditary cancers and microsatellite instability.
• Mutations in EXO1, the sole exonuclease implicated in MMR, do not cause familial cancer and exhibit a weaker mutator phenotype.

Purpose of the Study:

• To investigate potential functional redundancy with EXO1 in MMR.
• To identify novel nucleases involved in DNA repair pathways.

Main Methods:

• Analysis of the MLH1 interactome to identify interacting proteins.
• Biochemical assays to characterize FAN1's enzymatic properties and MMR activity.
• In vivo studies using cell lines deficient in EXO1 and FAN1 to assess MMR defects and drug resistance.

Main Results:

• FAN1 was identified as a novel enzyme with biochemical properties similar to EXO1.
• FAN1 efficiently substitutes for EXO1 in MMR assays, with its function modulated by MLH1 interaction.
• Cells lacking both EXO1 and FAN1 show MMR defects, resistance to MNU and TG, and an amplified mutational profile, indicating redundant roles in an antimutagenic pathway.
• The MMR defect in FAN1/EXO1-deficient cells is less severe than in MSH6 or MLH1 deficient cells.

Conclusions:

• Eukaryotic cells possess nucleases like FAN1 that provide functional redundancy with EXO1 in MMR.
• FAN1 and EXO1 act in a common antimutagenic pathway, but other compensatory mechanisms exist for EXO1 loss.
• The findings shed light on the complex mechanisms underlying DNA repair and genome stability.