Characterization of the DNA damage in 6-thioguanine-treated cells

6-Thioguanine (TG) incorporation into DNA has been associated with cytotoxicity and DNA damage in Chinese hamster ovary (CHO) and murine leukemia L1210 cells. According to alkaline elution analysis, single-strand breaks (SSB) occur in both cell types. DNA-protein and interstrand crosslinks are prominent features of TG effects in L1210, CEM, and HL-60 but not CHO cells. To assess which DNA strand experiences SSB in CHO cells, the cells were synchronized by growth to confluence (late g 1, S). The cells were then diluted into fresh medium so that they underwent a round of division during a subsequent 16-hr interval. They were treated with TG during this first cell cycle, and mitotic cells were harvested at the end of the first cycle using colcemid. SSB were determined in parental DNA (radiolabeled with thymidine during growth to confluence), TG-containing DNA (radiolabeled with [ 14C]TG during drug exposure), and daughter DNA (labeled with thymidine during the second cell cycle). SSB occurred in TG-containing DNA late in the second cell cycle after drug exposure and in the DNA synthesized from a TG-DNA template (daughter DNA). This observation is consistent with the known delayed cytotoxicity and chromosomal aberrations seen in CHO cells. The SSB suggest relatively normal elongation of DNA containing TG but altered synthesis and/or ligation from a TG-DNA template. This premise was tested in synchronized CHO cells. The DNA strand incorporating TG elongated naturally; however, DNA elongation was impaired in the cell cycle following TG treatment. The results are consistent with SSB in daughter DNA synthesized from a TG-DNA template due to inability to elongate the newly-synthesized strand.