The role of nonhomologous end joining and homologous recombination in the clonogenic bystander effects of mammalian cells after exposure to counted 10 MeV protons and 4.5 MeV alpha-particles of the PTB microbeam.

We have studied the dependence of clonogenic bystander effects on defects in the pathways of DNA double-strand break (DSB) repair and on linear energy transfer (LET). The single-ion microbeam of the Physikalisch-Technische Bundesanstalt (PTB) was used to irradiate parental Chinese hamster ovary cells or derivatives deficient in nonhomologous end joining (NHEJ) or homologous recombination (HR) in the G1-phase of the cell cycle. Cell nuclei were targeted with 10 MeV protons (LET = 4.7 keV/microm) or 4.5 MeV alpha-particles (LET = 100 keV/microm). During exposure, the cells were confluent, allowing signal transfer through both gap junctions and diffusion. When all cell nuclei were targeted with 10 MeV protons, approximately exponential survival curves were obtained for all three cell lines. When only 10% of all cell nuclei were targeted, a significant bystander effect was observed for parental and HR-deficient cells, but not for NHEJ-deficient cells. For all three cell lines, the survival data after exposure of all cell nuclei to 4.5 MeV alpha-particles could be fitted by exponential curves. When only 10% of all cell nuclei were targeted, significant bystander effects were obtained for parental and HR-deficient cells, whereas for NHEJ-deficient cells a small, but significant, bystander effect was observed only at higher doses. The data suggest that bystander cell killing is a consequence of un- or misrejoined DSB which occur in bystander cells during the S-phase as a result of the processing of oxidative bistranded DNA lesions. The relative contributions of NHEJ and HR to the repairing of DSB in the late S/G2-phase may affect clonogenic bystander effects.