No abstract

Etoposide (ETO), a drug used for the treatment of human tumors, is associated with the development of secondary malignancies. Recently, therapeutic strategies have incorporated chemosensitizing agents to improve the tumoral response to this drug. ETO creates DNA double-strand breaks (DSB) via inhibition of DNA topoisomerase II (Top2). To repair DSB, homologous recombination (HR) and non-homologous end-joining (NHEJ), involving D-NHEJ (dependent of the catalytic subunit of DNA-dependent protein kinase, DNA-PKcs) and B-NHEJ (backup repair pathway) are activated. We evaluated the progression of the DNA damage induced by the Top2 poison ETO in G2 phase of human HeLa cells after chemical inhibition of DNA-PKcs with NU7026. Compared to ETO treatment alone, this combined treatment resulted in a twofold higher rate of chromatid breaks and exchanges when analysis was performed in the following metaphase. Moreover, when analysis was performed in the second metaphase following treatment, increases in the percentage of micronuclei with H2AX (biomarker for DSB) foci in binucleated cells and dicentric chromosomes were seen. In post-mitotic G1 phase, a close association between unresolved DSB and meiotic recombination 11 homolog A (MRE11) signals was observed, demonstrating the contribution of MRE11 in the DSB repair by B-NHEJ. Hence, chemical inhibition of DNA-PKcs impaired both D-NHEJ and HR repair pathways, altering the maintenance of chromosomal integrity and cell proliferation. Our results suggest that the chemosensitizing effectiveness of the DNA-PKcs inhibitor and the survival rate of aberrant cells may contribute to the development of therapy-related tumors.

The poisoning of Topoisomerase II (Top2) has been found to be useful as a therapeutic strategy for the treatment of several tumors. The mechanism of Top2 poisons involves a drug-mediated stabilization of a Top2-DNA complex, termed Top2 cleavage complex (Top2cc), which maintains a 5' end of DNA covalently bound to a tyrosine from Top2 through a phosphodiester group. Drug-stabilized Top2cc leads to Top2-linked-DNA breaks, which are believed to mediate their cytotoxicity. Several time-consuming or cell type-limiting assays have been used in the past to study drug-stabilized Top2cc. Here, we describe a flow cytometry-based method that allows a rapid assessment of drug-induced Top2cc, which is suitable for high throughput analysis in almost any kind of human cell. The analyses of the drug-induced Top2cc in the cell cycle context and the possibility to track its removal are additional benefits from this methodology. © 2017 by John Wiley & Sons, Inc.