Role of Homologous Recombination in the Alpha-Particle-Induced Bystander Effect for Sister Chromatid Exchanges and Chromosomal Aberrations

Abstract

The bystander effect for sister chromatid exchanges (SCEs) and chromosomal aberrations was examined in hamster cell lines deficient in either DNA-PKcs (V3 cells, deficient in nonhomologous end joining, NHEJ) or RAD51C (irs3 cells, deficient in homologous recombination, HR). Cells synchronized in G0/G1 phase were irradiated with very low fluences of alpha particles such that < 1% of the nuclei were traversed by an alpha particle. Wild-type cells showed a prominent bystander response for SCE induction; an even greater effect was observed in V3 cells. On the other hand, no significant induction of SCE was observed in the irs3 RAD51C-deficient bystander cells irradiated at various stages in the cell cycle. Whereas a marked bystander effect for chromosomal aberrations occurred in V3 cells, the induction of chromosomal aberrations in irs3 bystander cells was minimal and similar to that of wild-type cells. Based on these findings, we hypothesize that HR is essential for the induction of SCE in bystander cells; however, HR is unable to repair the DNA damage induced in NHEJ-deficient bystander cells that leads to either SCE or chromosomal aberrations.

DNA-PKcs and ATM influence generation of ionizing radiation-induced bystander signals

“…ATM is reported to play a role in telomere maintenance as well (Pandita, 2002;Denchi and de Lange, 2007). In addition to DNA-PKcs and ATM's well-established roles in repair and intracellular signaling, (Nagasawa et al, 2003(Nagasawa et al, , 2005Collis et al, 2005;Lavin and Kozlov, 2007), our findings indicate a role for these proteins in intercellular signaling of the ionizing radiation-induced BSE. We designed a cell transfer strategy that enables us to differentiate between the generation versus the reception of bystander signals.…”

“…Given that the multiple markers that have been used to study the BSE, that is, micronuclei formation (Yang et al, 2007), clonogenic survival (O'NeillMehlenbacher et al, 2007), apoptosis (Grifalconi et al, 2007) and sister chromatid exchange (SCE) (Nagasawa et al, 2005) (Figure 1), are themselves considered to be detrimental, it has been assumed that the BSE is harmful to neighboring cells. However, it has also been proposed that the BSE may actually be beneficial at a tissue level; cells exposed to a bystander signal are more radioresistant to subsequent IR-induced damage indicative of an adaptive response (Iyer and Lehnert, 2002).…”

“…Previous studies investigating how DNA repair status influences the BSE include several by Nagasawa et al, who demonstrated that cells deficient in DNA repair proteins tend to exhibit large bystander responses following a-particle irradiation (Nagasawa et al, 2005). These authors speculated that cells experiencing defective repair of DNA damage induced by direct irradiation, display an increased bystander response likely due to increased production of ROS (Nagasawa and Little, 2002).…”

Low doses of alpha particles do not induce sister chromatid exchanges in bystander Chinese hamster cells defective in homologous recombination

“…Previously we reported that spontaneous SCE frequencies for the V79-derived rad51c cell line irs3 were 0.16 SCE per chromosome, approximately 50% lower than wild-type V79 cells [23]. Following irradiation with very low doses of α-particles (<2.6 mGy, <2% of cells directly hit), SCEs were not significantly increased above background levels in irs3 cells whereas SCE frequencies increased significantly 30-40% above background in both wild-type V79 and non-homologous end joining (NHEJ)-deficient V3 (deficient in DNA-PKcs) CHO cells.…”

“…In addition, mainly chromatid-type aberrations were induced in the bystander cells (i.e., cells neighboring directly-irradiated cells) when cultures of V79, V-3, and xrs5 (deficient in Ku70/80) cells were irradiated in G0/G1-phase, which suggested increased levels of single-stranded DNA damage in the bystander cells [23][24][25]. Similarly, the spectrum of hprt mutations induced in bystander cells following low dose α-particle irradiation were nearly all point mutations, in contrast to directly irradiated cells for which total and partial gene deletions predominated [26,27].…”