Complex genetic interactions between DNA polymerase β and the NHEJ ligase.

Mammalian cells possess multiple pathways for repairing various types of DNA damage. Although the molecular mechanisms of each DNA repair pathway have been analyzed by biochemical analysis and cell biological analysis, interplay between different pathways has not been fully elucidated. In this study, using human Nalm-6-mutant cell lines, we analyzed the relationship between the base excision repair factor DNA polymerase beta (POL beta) and DNA ligase IV (LIG4), which is essential for DNA double-strand break (DSB) repair by non-homologous end-joining (NHEJ). We found that cells lacking both POL beta and LIG4 grew significantly more slowly than either single mutant, indicating cooperative functions of the two proteins in normal cell growth. To further investigate the genetic interaction between POL beta and LIG4, we examined DNA damage sensitivity of the mutant cell lines. Our results suggested that NHEJ acts as a backup pathway for repairing alkylation damage (when converted into DSBs) in the absence of POL beta. Surprisingly, despite the critical role of POL beta in alkylation damage repair, cells lacking POL beta exhibited increased resistance to camptothecin (a topoisomerase I inhibitor that induces DNA single-strand breaks), irrespective of the presence or absence of LIG4. A LIG4-independent increased resistance associated with POL beta loss was also observed with ionizing radiation; however, cells lacking both POL beta and LIG4 were more radiosensitive than either single mutant. Taken together, our findings provide novel insight into the complex interplay between different DNA repair pathways.