Wu, W., Wang, M., Mussfeldt, T. and Iliakis, G. Enhanced Use of Backup Pathways of NHEJ in G₂ in Chinese Hamster Mutant Cells with Defects in the Classical Pathway of NHEJ. Radiat. Res. 170, 512–520 (2008).

In higher eukaryotes DNA double-strand breaks (DSBs) are repaired by homologous recombination repair (HRR) or non-homologous end joining (NHEJ). In addition to the DNA-PK dependent pathway of NHEJ (D-NHEJ), cells employ a backup pathway (B-NHEJ) using DNA ligase III and PARP1. We have reported previously that mouse embryo fibroblasts (MEFs) defective in D-NHEJ show enhanced repair of DSBs in G₂ not reflecting a contribution of HRR. Here we extend these studies to Chinese hamster mutant cells with defects in the DNA-PKcs, Ku80 or XRCC4 components of D-NHEJ or in the XRCC2 and XRCC3 components of HRR. Using cell sorting to separate cells at defined times after irradiation, we measure repair of DSBs with pulsed-field gel electrophoresis in unperturbed G₁- and G₂-phase cells. Wild-type cells and mutants of XRCC2 and XRCC3 repair DSBs with similar efficiency in G₁ and G₂. Mutants of DNA-PKcs, Ku80 and XRCC4 show more pronounced repair in G₂ than in G₁. These and previously published results provide support for the notion that the increased efficacy of DSB repair in G₂ reflects the enhanced function of B-NHEJ, which may be a general feature of rodent cells that also holds for human cells.