Lack of uncoupling of S phase and mitosis after irradiation in p53^- human lymphoblast cell lines

It has been shown that p53^- human colorectal cancer cells arrest after DNA damage in a G₂-like state and may then undergo DNA synthesis without intervening mitosis (Waldman et al., Nature 381, 713-716, 1996). To further clarify the role of p53 in the regulation of the G₂/M-phase checkpoint, we have studied cells of three closely related human lymphoblastoid cell lines (TK6, WTK1 and TK6E6, an HPV16 E6-transfected TK6 line) with differing p53 status. The cells were irradiated with 1.5-12 Gy γ rays with or without 2 mM caffeine. There was no evidence of uncoupling of DNA synthesis and mitosis after irradiation in the p53-cell lines, WTK1 and TK6E6, suggesting that this uncoupling may not be a universal phenomenon. The apparent formation of tetraploid cells after irradiation of cells of the p53^- WTK1 line was due to the occurrence of a G₂-phase block in a pre-existing tetraploid population. These results support the conclusion that control of the G₂/M-phase checkpoint after irradiation may differ among different cell types.