Non-homologous end joining as an important mutagenic process in cell cycle-arrested cells

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• Cells can acquire mutations even without external mutagens, a process termed replication-independent mutation.
• These mutations are hypothesized to arise from the repair of spontaneous DNA damage.
• The role of DNA double-strand break (DSB) repair pathways in this process remains largely unexplored.

Purpose of the Study:

• To investigate the contribution of DNA double-strand break (DSB) repair pathways, specifically non-homologous end joining (NHEJ), to replication-independent mutations.
• To determine if defects in NHEJ alter the frequency of these spontaneous mutations.

Main Methods:

• Utilized Saccharomyces cerevisiae (yeast) with frameshift alleles to study mutations.
• Assessed the impact of deleting genes essential for NHEJ (Ku70, DNA ligase IV) on mutation frequency.
• Sequenced mutation sites to characterize the nature of NHEJ-dependent events.
• Tested diploid strains and strains deficient in homologous recombination (RAD52, RAD54).

Main Results:

• Deletion of NHEJ genes (Ku70 or DNA ligase IV) reduced replication-independent mutation frequency by 50% in haploid cells.
• NHEJ-dependent mutations were predominantly small deletions in mononucleotide repeats, similar to DNA polymerase slippage errors.
• These findings were consistent in both haploid and diploid cells, provided NHEJ was not repressed.
• Homologous recombination pathways (RAD52, RAD54) did not significantly account for these mutations.

Conclusions:

• The non-homologous end joining (NHEJ) pathway is a significant contributor to replication-independent frameshift mutations in resting cells.
• While NHEJ promotes large-scale chromosomal stability, it increases the frequency of small-scale mutations.
• This trade-off may have implications for understanding carcinogenesis, as unrepaired DNA damage can lead to mutations.