BRCA1- and BRCA2-Deficient Cells Are Sensitive to Etoposide-Induced DNA Double-Strand Breaks via Topoisomerase II

Abstract

The function of BRCA1 and BRCA2 in DNA repair could affect the sensitivity of cells to cytotoxic agents, and would therefore be an important component of planning therapy for breast and ovarian cancers. Previously, both BRCA1-and BRCA2-deficient tumors were shown to be sensitive to mitomycin C, and the mechanism was presumed to be a defect in the repair of interstrand crosslinks by homologous recombination. Here, we show that both BRCA1 and BRCA2 determine the sensitivity to the cytotoxic drug, etoposide, using genetic complementation of BRCA-deficient cells. Etoposide is known to bind to topoisomerase II and prevent the resolution of the ''cleavable complex,'' in which one DNA duplex is passed through a second duplex. The specificity of this BRCAdependent sensitivity was confirmed by the use of aclarubicin, which is a catalytic inhibitor of topoisomerase II and prevents the formation of the cleavable complex. In the presence of aclarubicin, the differential sensitivity of BRCA-proficient and BRCA-deficient cells was lost. Thus, etoposide requires the presence of topoisomerase II to show specific sensitization in the absence of the function of BRCA1 or BRCA2. We conclude that homologous recombination is used in the repair of DNA damage caused by topoisomerase II poisons. Overall, these results suggest that etoposide is a potentially useful drug in the treatment of BRCA-deficient human cancers. [Cancer Res 2007;67(15):7078-81]

Utility of DNA Repair Protein Foci for the Detection of Putative BRCA1 Pathway Defects in Breast Cancer Biopsies

“…Conversely, not all triple-negative sporadic breast cancers will be BRCA1 defective(14). It remains to be established whether BRCA1 defects, as defined by our foci assay, are predictors of clinical sensitivity to chemotherapeutics such as platinum compounds or topoisomerase II inhibitors(26, 27). While our pilot study is too small to address this question, the observed radiographic responses to neoadjuvant doxorubicin in patients with BRCA1 dysfunctional cancers are entirely consistent with this hypothesis (Taghian et al unpublished data).…”

Cancer Resistance to Type II Topoisomerase Inhibitors

“…The relevance of homologous recombination (HR) in fixing TOPO2-mediated DNA damage in mammalian cells is less evident as compared to the above mentioned NHEJ DNA repair system, though mutants defective in the HR machinery do show high sensitivity to etoposide [22,23]; since treatment with etoposide activates both NHEJ and HR systems [24], defects in both pathways might be exploited for a therapeutic approach based on the synthetic lethality phenomenon. Interestingly, recent preclinical experiments suggest that simultaneous inhibition of both NHEJ and HR pathways is a more effective strategy (as compared to inhibition of each pathway separately) to synergize with TOPO2 poisons [25].…”

Riccardin D, a novel macrocyclic bisbibenzyl, induces apoptosis of human leukemia cells by targeting DNA topoisomerase II

“…Etoposide has been used as a drug of first choice for treatment of various cancers including leukemia [31]. However, its wide therapeutic usage is often hindered by its limitations, which include acquired MDR and doselimiting toxicities [32].…”