CHROMOSOME RECOMBINATION AND DEFECTIVE GENOME SEGREGATION INDUCED IN CHINESE-HAMSTER CELLS BY THE TOPOISOMERASE-II INHIBITOR VM-26

We found that 4'-demethylepipodophyllotoxinthenylidene-beta-D-glucoside (VM-26; Teniposide), which specifically inhibits the enzyme DNA topoisomerase II, induces the formation of quadriradial chromosomes in Chinese hamster ovary cells. VM-26 traps topoisomerase II molecules when they are covalently integrated into DNA during their reaction. Quadriradial chromosomes are formed by reciprocal exchange of double-stranded DNA between single chromatids of two different chromosomes. Using synchronised cells, we found that they were formed after a single replication cycle in the presence of VM-26 at a low concentration (0.008-mu-M), which does not affect DNA replication, and occurred in 50% of the mitotic cells at a concentration of 0.16-mu-M. They were also formed when VM-26 was present for only 1.5 h before mitosis, after the completion of S-phase DNA replication. Chromatids bearing a translocated segment of another chromatid, which were derived from recombined chromosomes, were observed in late metaphase cells. Segregation of the daughter genomes was defective in many mitotic cells, probably because chromatids with two or no centromeres and kinetochores, formed from chromosomes recombined between their centromeres, could not be segregated. In the light of evidence that topoisomerase II molecules covalently integrated in DNA are trapped and therefore more abundant in the presence of VM-26, and that this enzyme can effect recombination of double-stranded DNA in vitro, we interpret these observations as evidence that topoisomerase II can mediate chromosome recombination in vivo.