title: Chronic exposure to sublethal doses of radiation mimetic Zeocin selects for clones deficient in homologous recombination.
abstract: DNA double-strand breaks (DSBs) are highly toxic lesions leading to genome variability/instability. The balance between homologous recombination (HR) and non-homologous end-joining (NHEJ), two alternative DSB repair systems, is essential to ensure genome maintenance in mammalian cells. Here, we transfected CHO hamster cells with the pcDNA3.1/Zeo plasmid, and selected transfectants with Zeocin, a bleomycin analog which produces DSBs. Despite the presence of a Zeocin resistance gene in pcDNA3.1/Zeo, Zeocin induced 8-10 gamma-H2AX foci per cell. This shows that the Zeocin resistance gene failed to fully detoxify cells treated with Zeocin, and that during selection cells were submitted to a chronic sublethal DSB stress. Selected clones show decreases in both spontaneous and induced intrachromosomal HR. In contrast, in an in vitro assay, these clones show an increase in NHEJ products specific to the KU86 pathway. We selected cells, in the absence of pcDNA3.1/Zeo, with low and sublethal doses of Zeocin, producing a mean 8-10 gamma-H2AX foci per cell. Newly selected clones exhibited similar phenotypes: HR decrease accompanied by an increase in KU86-dependent NHEJ efficiency. Thus chronic exposure to sublethal numbers of DSBs selects cells whose HR versus NHEJ balance is altered. This may well have implications for radio- and chemotherapy, and for management of environmental hazards.
authors.affiliation: UMR CNRS/CEA 217, DSV, DRR, CEA, 18 route du panorama, 92265 Fontenay aux Roses, Cédex, France.
authors.forename: Fabien
authors.initials: F
authors.lastname: Delacôte
authors.forename: Ludovic
authors.initials: L
authors.lastname: Deriano
authors.forename: Sarah
authors.initials: S
authors.lastname: Lambert
authors.forename: Pascale
authors.initials: P
authors.lastname: Bertrand
authors.forename: Yannick
authors.initials: Y
authors.lastname: Saintigny
authors.forename: Bernard S
authors.initials: BS
authors.lastname: Lopez
mesh_terms: D000818:Animals; D001761:Bleomycin / Q000008:administration & dosage / Q000633:toxicity*; D016466:CHO Cells; D002999:Clone Cells; D006224:Cricetinae; D003412:Cricetulus; D053903:DNA Breaks, Double-Stranded; D004260:DNA Repair; D066298:In Vitro Techniques; D011995:Recombination, Genetic; D014162:Transfection
chemical_list: D001761:Bleomycin; C105427:Zeocin