DNA repair and chromosomal alterations

Area of Science:

• Molecular Biology
• Genetics
• DNA Repair Mechanisms

Background:

• Mutagenic agents cause DNA lesions, which, if unrepaired or misrepaired, lead to chromosomal aberrations (CAs).
• Various DNA repair pathways, including nucleotide excision repair (NER), base excision repair (BER), non-homologous end joining (NHEJ), homologous recombination repair (HRR), and cross-link repair (FANC), are involved in maintaining genomic stability.
• DNA double-strand breaks (DSBs) are critical lesions, particularly after ionizing radiation, and are primarily repaired by NHEJ and HRR.

Purpose of the Study:

• To investigate the relative contributions of NHEJ and HRR pathways in repairing DNA double-strand breaks (DSBs) and their role in causing chromosomal aberrations (CAs).
• To explore the involvement of different DNA repair pathways in response to specific mutagenic agents like ionizing radiation, benzo(alpha)pyrene-7,8-diol-9,10-epoxide (BPDE), and acetaldehyde.
• To understand how the cell cycle phase influences the activity of NHEJ and HRR in DNA repair and aberration formation.

Main Methods:

• Utilized Chinese hamster ovary (CHO) mutant cells deficient in either NHEJ or HRR.
• Exposed cells to ionizing radiation, BPDE, and acetaldehyde at different cell cycle phases (G1, S, G2).
• Analyzed the frequencies of chromatid-type aberrations to infer the repair pathways involved and their mis-repair components.

Main Results:

• In NHEJ-deficient cells irradiated in G1, HRR repaired unrepaired DSBs in S phase with significant mis-repair, leading to CAs.
• In HRR-deficient cells, NHEJ repaired DSBs in S phase with minimal mis-repair, as indicated by lower CA frequencies.
• NER and HRR were crucial for protection against BPDE-induced damage, while HRR and FANC pathways were important for acetaldehyde-induced damage. All pathways ultimately converge to DSBs for CA formation.

Conclusions:

• NHEJ operates throughout the cell cycle, while HRR is primarily active in S and G2 phases.
• The study revealed unexpected roles and mis-repair efficiencies for NHEJ and HRR when the other pathway is deficient.
• Different mutagenic agents engage specific DNA repair pathways, but DSBs are the common precursor to chromosomal aberrations, involving both NHEJ and HRR.