Therapeutic Discovery Determinants of Mitotic Catastrophe on Abrogation of the G2 DNA Damage Checkpoint by UCN-01

Genotoxic stress such as ionizing radiation halts entry into mitosis by activation of the G2 DNA damage checkpoint. The CHK1 inhibitor 7-hydroxystaurosporine (UCN-01) can bypass the checkpoint and induce unscheduled mitosis in irradiated cells. Precisely, how cells behave following checkpoint abrogation remains to be defined. In this study, we tracked the fates of individual cells after checkpoint abrogation, focusing in particular on whether they undergo mitotic catastrophe. Surprisingly, while a subset of UCN-01–treated cells were immediately eliminated during the first mitosis after checkpoint abrogation, about half remained viable and progressed into G1. Both the delay of mitotic entry and the level of mitotic catastrophewere dependent on the dose of radiation. Although the level of mitotic catastrophe was specific for different cell lines, it could be promoted by extending the mitosis. In supporting this idea, weakening of the spindle-assembly checkpoint, by either depleting MAD2 or overexpressing the MAD2-binding protein p31, suppressed mitotic catastrophe. Conversely, delaying of mitotic exit by depleting either p31 or CDC20 tipped the balance toward mitotic catastrophe. These results underscore the interplay between the level of DNA damage and the effectiveness of the spindle-assembly checkpoint in determining whether checkpoint-abrogated cells are eliminated during mitosis. Mol Cancer Ther; 10(5); 784–94. 2011 AACR.