Base excision repair intermediates as topoisomerase II poisons

Area of Science:

• Molecular Biology
• DNA Repair Mechanisms
• Enzymology

Background:

• Abasic sites are frequent DNA lesions from endogenous/environmental damage and base excision repair (BER).
• Intact abasic sites can poison topoisomerase II, increasing DNA cleavage.
• Most cellular abasic sites are processed BER intermediates with strand breaks.

Purpose of the Study:

• To investigate the effects of processed abasic sites on human topoisomerase IIalpha activity.
• To determine if BER intermediates containing abasic sites can lead to permanent DNA damage.

Main Methods:

• Utilized model BER intermediates with abasic sites and proximal strand breaks (nicks or flaps).
• Assessed DNA cleavage by human topoisomerase IIalpha using these model substrates.
• Quantified the enhancement of DNA scission by abasic sites within BER intermediates.

Main Results:

• Processed abasic sites, including 5 eal- or 3 eal-nicked abasic sites and deoxyribosephosphate flaps, acted as suicide substrates for topoisomerase IIalpha.
• Abasic sites adjacent to nicks significantly enhanced topoisomerase II-mediated DNA scission (approximately 10-fold) compared to simple nicked DNA.
• These findings indicate that topoisomerase II processes BER intermediates into potentially permanent DNA breaks.

Conclusions:

• Processed abasic sites within BER intermediates can be converted into permanent double-stranded DNA breaks by topoisomerase II.
• Topoisomerase II plays a critical role in processing DNA lesions that arise during base excision repair.
• This mechanism highlights a pathway for generating genomic instability from common DNA damage and repair intermediates.