Mitotic bypass via an occult cell cycle phase following DNA topoisomerase II inhibition in p53 functional human tumor cells.

Cells cycle checkpoints guard against the inapproriate commitment to critical cell events such as mitosis. The bisdioxxopiperazzine ICRF-193, a catalytic inhibitor of DNA topoisomerase II causes a reversible stalling of the exit of cells from G(2) at the decatenation checkpoint (DC) and can generate tetraploidy via the compromising of chromosome segregation and mitotic failure. We have addressed an alternative origin-endocycle entry-for the tetraploidisation step in ICRF-193 exposed cells. Here we show that DC-proficient p53-functional tumor cells can undergo a transition to tetraploidy and subbsequent aneuploidy via an initial bypass of mitosis and the mitotic spindle checkpoint. DC-deficient SV4-tranformed cells move exclusively through mitosis to tetraploidy. In p53-functional tumor cells, escape through mitosis is enhanced by dominant negative p53 co-expression. The mitotic bypass transition phase (termed G(2)(endo)) disconnects cyclin B1 degradation from nuclear envelope breakdown and allows cells to evade the action of Taxol. G(2)(endo) constitutes a novel and alternative cell cycle phase-lasting some 8 h-with distinct molecular motifs at its boundaries for G(2) exit and subsequent entry into a delayed G(1) tetraploid state. The result challenge the paradigm that checkpoint breaching leads directly to abnormal ploidy states via mitosis alone. We further propose that the induction of bypass could: facilitate the covert development of tetraploidy in p53 functional cancers, lead to a misinterpretation of phase allocation during cell cycle arrest and contribbute to tumor cell drug resistance.