Role of DNA topoisomerase II in chromosome dynamics in mammalian cells

ICRF‐193, a bis‐(2,6‐dioxopiperazine) derivative and a non‐cleavable‐complex‐forming‐type topoisomerase II inhibitor, inhibited cell division but allowed cells to traverse the cell cycle, leading to the accumulation of polyploid cells with 8C complements or more of DNA. Analysis of the mechanism of how cell division is inhibited by ICRF‐193 revealed that: (1) replication of DNA was inhibited only at terminal stages; (2) CDC2 kinase was activated and cells enter absence‐of‐chromosome‐segregation (‘ACS’) M‐phase, where chromosomes are not fully condensed and are not separated, but other mitotic events, such as nuclear‐envelope breakdown and cytoskeletal reorganization forming the spindle apparatus, take place, i.e. chromosome dynamics could be uncoupled from the other mitotic events which are normally co‐ordinated with the former in mitosis; (3) cells successfully exit from mitosis to the next G1‐phase to continue the cell cycle; (4) progression through ‘ACS’ M‐phase appears to be lethal to the cells. All of these observations could be accounted for by inactivation of topoisomerase II activity of the cells caused by the drug. ICRF‐193 was thus shown to be a valuable agent in elucidation of the role of topoisomerase II in genetic processes in vivo.