Arrest of Replication Fork Progression at Sites of Topoisomerase II-Mediated DNA Cleavage in Human Leukemia CEM Cells Incubated with VM-26

Recent studies have shown that the anticancer drugs VM-26 and mitoxantrone stabilize preferentially the binding of topoisomerase IIα to replicating compared to nonreplicating DNA. To further understand the mechanisms by which cleavable complex-forming topoisomerase II inhibitors interfere with DNA replication, we examined the effects of VM-26 on this process in human leukemia CEM cells. Both the inhibition of DNA synthesis and cell survival were directly related to the total amount of drug-stabilized cleavable complexes formed in VM-26-treated cells. DNA chain elongation was also inhibited in a concentration-dependent fashion in these cells, which suggested that VM-26-stabilized cleavable complexes interfered with the movement of DNA replication forks. To test this hypothesis directly, we monitored replication fork progression at a specific site of VM-26-induced DNA cleavage. A topoisomerase II-mediated cleavage site was detected in the first exon of the c-myc gene in VM-26-treated cells. This cleavage site was downstream of a putative replication origin located in the 5‘ flanking region of the gene. Replication forks, which moved through this region of the c-myc gene in the 5‘ to 3‘ direction, were specifically arrested at this site in VM-26-treated cells, but not in untreated or aphidicolin-treated cells. These studies provide the first direct evidence that a VM-26-stabilized topoisomerase II−DNA cleavable complex acts as a replication fork barrier at a specific genomic site in mammalian cells. Furthermore, the data support the hypothesis that the replication fork arrest induced by cleavable complex-forming topoisomerase II inhibitors leads to the generation of irreversible DNA damage and cytotoxicity in proliferating cells.