XRCC1 promotes replication restart, nascent fork degradation and mutagenic DNA repair in BRCA2-deficient cells

Area of Science:

• DNA repair mechanisms
• Cancer genomics
• Cell cycle regulation

Background:

• Homologous recombination/end joining (HR/HEJ)-deficient cancers, often driven by BRCA mutations, rely on alternative DNA double-strand break repair pathways like alternative non-homologous end joining/microhomology-mediated end joining (alt-EJ/MMEJ).
• The roles of XRCC1, a key alt-EJ factor, in HR-deficient cancers and during replication stress remain unclear, despite its synthetic lethality with BRCA2 deficiency when depleted alongside other alt-EJ factors like PARP1 and POLQ.

Purpose of the Study:

• To elucidate the function of XRCC1 in DNA repair pathways within BRCA-deficient cancer cells, particularly under replication stress.
• To investigate the interplay between BRCA2, XRCC1, and alt-EJ/MMEJ in maintaining genome stability.
• To explore the clinical relevance of XRCC1 and MRE11 expression in HR-deficient breast cancer.

Main Methods:

• In vitro biochemical assays to characterize the XRCC1-POLQ-MRE11 repair complex.
• Cellular studies assessing DNA repair, replication fork dynamics, and chromosome aberrations in BRCA2-deficient cells with and without XRCC1.
• Analysis of The Cancer Genome Atlas (TCGA) data to correlate XRCC1 and MRE11 gene expression with patient survival in HR-deficient breast cancer.

Main Results:

• XRCC1 forms an active repair complex with POLQ and MRE11 that promotes alt-EJ activity post-replication stress.
• BRCA2 normally suppresses XRCC1-mediated alt-EJ at stalled replication forks.
• In BRCA2-deficient cells, XRCC1 facilitates MRE11-dependent fork resection and restart, enhancing replication completion but increasing genome instability.
• High XRCC1 and MRE11 expression correlates with poorer survival in HR-deficient breast cancer patients.

Conclusions:

• XRCC1 plays a critical role in promoting alt-EJ/MMEJ and MRE11-dependent DNA processing in BRCA2-deficient cells, contributing to replication restart and chromosome aberrations.
• BRCA2 acts as a suppressor of XRCC1-mediated alt-EJ at stalled forks.
• The findings reveal novel functions for XRCC1 and alt-EJ/MMEJ in HR-deficient cancers and suggest potential therapeutic targets, including POLQ and PARP inhibitors.