Different fates of camptothecin-induced replication fork-associated double-strand DNA breaks in mammalian cells

The S phase cytotoxicity of camptothecin (CPT) requires both the formation of a covalent topoisomerase I-DNA complex and ongoing DNA replication. The interaction of DNA synthesis and the drug-induced complexes results in the production of DNA double-strand breaks (DSBs) concentrated in replicating DNA. These DSBs are likely to be extremely cytotoxic lesions and are likely to account for the S phase specificity of CPT. Here we show that a brief exposure to CPT results in replication-associated DSBs and, once formed, the fate of these DNA DSBs is different in human and Chinese hamster cell lines. In hamster CHO-KI, even at supra-lethal concentrations, CPT-induced DSBs in nascent DNA disappear within 5 h of drug removal. Those CHO-KI cells in S phase during treatment with toxic doses of CPT arrive at mitosis within 18 h, with potentially lethal chromatid aberrations. In human cells, CPT-induced DSBs are long lived, and are still detectable at least 24 h after drug removal. After toxic doses of CPT to S phase human cells, mitosis does not occur within 72 h of drug removal and there is an extended, perhaps permanent, cycle arrest in S/G2, possibly due to the presence of unrepaired DNA DSBs. These data, and the greater sensitivity of hamster than human cells to low doses of CPT, suggests that, besides the generation of replication fork-associated DNA DSBs, subsequent processing/repair of these lesions may modulate the sensitivity of cells to this important anti-tumour drug.