DNA mismatch repair-induced double-strand breaks.

Area of Science:

• Molecular Biology
• Genetics
• DNA Repair

Background:

• Escherichia coli dam mutants show increased sensitivity to cytotoxic agents such as cisplatin and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG).
• Mismatch repair (MMR) deficiency confers tolerance to these agents.
• Previous studies indicated MMR-mediated double-strand breaks (DSBs) in dam recB(Ts) cells treated with cisplatin.

Purpose of the Study:

• To investigate the mechanisms of double-strand break (DSB) formation in Escherichia coli dam mutants exposed to DNA damaging agents.
• To elucidate the roles of DNA replication and mismatch repair (MMR) in DSB formation and cytotoxicity.

Main Methods:

• Pulse field gel electrophoresis (PFGE) to detect double-strand breaks (DSBs).
• Treatment of dam mutants (dam recB(Ts) and dam recB(Ts) ada ogt) with cisplatin and MNNG.
• Analysis of DSB formation in relation to DNA replication and MMR status.

Main Results:

• The majority of cisplatin-induced DSBs in dam recB(Ts) cells require DNA replication, suggesting replication fork collapse at MMR-processed nicks/gaps.
• MNNG induced dose- and MMR-dependent DSBs in dam recB(Ts) ada ogt cells, independent of DNA replication.
• Two distinct mechanisms for DSB formation were proposed: replication-independent (overlapping repair tracts) and replication-dependent (replication fork collapse at O(6)-methylguanine undergoing MMR futile cycling).
• Fast-growing cells with more replication origins showed increased sensitivity to MNNG, supporting the role of replication-dependent DSBs in cytotoxicity.

Conclusions:

• DNA damaging agents induce DSBs through distinct replication-dependent and replication-independent pathways in Escherichia coli.
• Replication fork collapse at MMR-processed lesions contributes significantly to MNNG-induced cytotoxicity.
• The interplay between DNA replication, MMR, and DNA damage dictates the cytotoxic outcome.