Inhibition of DNA topoisomerase II may trigger illegitimate recombination in living cells: experiments with a model system

We have developed a plasmid test system to study recombination in vitro and in mammalian cells in vivo, and to analyze the possible role of DNA topoisomerase II. The system is based on a plasmid construct containing an inducible marker gene ccdB ("killer" (KIL) gene) whose product is lethal for bacterial cells, flanked by two different potentially recombinogenic elements. The plasmids were subjected to recombinogenic conditions in vitro or in vivo after transient transfection into COS-1 cells, and subsequently transformed into E. coli which was then grown in the presence of the ccdB gene inducer. Hence, all viable colonies contained recombinant plasmids since only recombination between the flanking regions could remove the KIL gene. Thus, it was possible to detect recombination events and to estimate their frequency. We found that the frequency of topoisomerase II-mediated recombination in vivo is significantly higher than in a minimal in vitro system. The presence of VM-26, an inhibitor of the religation step of the topoisomerase II reaction, increased the recombination frequency by 60%. We propose that cleavable complexes of topoisomerase II are either not religated, triggering error-prone repair of the DNA breaks, or are incorrectly religated resulting in strand exchange. We also studied the influence of sequences known to contain preferential breakpoints for recombination in vivo after chemotherapy with topoisomerase II-targeting drugs, but no preferential stimulation of recombination by these sequences was detected in this non-chromosomal context.