Poor recognition of O6-isopropyl dG by MGMT triggers double strand break-mediated cell death and micronucleus induction in FANC-deficient cells

An unexpected butadiene diolepoxide-mediated genotoxicity implies alternative mechanism for 1,3-butadiene carcinogenicity

1,3-Butadiene (BD) is abundant in combustion products such as cigarette smoke. While BD has been classified as a known human carcinogen, a long-standing question is the identity of the ultimate carcinogenic metabolite in humans. We hypothesize that 3,4-epoxybutane-1,2-diol (EBD) may play a critical role in human carcinogenesis due to its high bioavailability. We utilized a differential toxicity assay for BD metabolites and newly synthesized EBD analogs in a series of isogenic chicken cells lacking specific DNA repair proteins to address the mode of action of BD genotoxicity and infer a mode of action. Surprisingly, as with the diepoxide 1,2:3,4-diepoxybutane (DEB), the monoepoxide EBD showed remarkable toxicity to cells deficient in Fanconi anemia (FANC) genes. This observation suggests that EBD may be transformed into a bifunctional metabolite and forms interstrand cross-links. EBD and its analog with a hydroxy substituent at C1 were found to be highly toxic to FANCD2-deficient chicken and human cells. The Results suggest that EBD may be transformed to a bifunctional epoxy aldehyde, perhaps by alcohol dehydrogenase, to which the observed FANC sensitivity could be attributed. The implications of this study are very important in considering mechanisms by which EBD may cause leukemia and lymphoma in humans exposed to BD.

Potential Doxorubicin-Mediated Dual-Targeting Chemotherapy in FANC/BRCA-Deficient Tumors via Modulation of Cellular Formaldehyde Concentration

Doxorubicin (DOX) is a widely used classical broad-spectrum anticancer drug. The major mechanism of DOX-mediated anticancer activity at clinically relevant concentrations is believed to be via DNA double-strand breaks due to topoisomerase IIα. However, other mechanisms by which DOX causes cytotoxicity have been proposed, including formaldehyde-dependent virtual interstrand cross-linking (ICL) formation. In this study, a method was established whereby cytotoxicity caused by virtual ICL derived from DOX is turned on and off using a cell culture system. Using this strategy, DOX-mediated cytotoxicity in Fanconi anemia group gene (FANC)/breast cancer susceptibility gene (BRCA)-deficient cells increased up to 70-fold compared to that in cells proficient in DNA repair pathways by increasing intracellular formaldehyde (FA) concentration. This approach also demonstrated that cytotoxicity introduced by DOX-mediated FA-dependent virtual ICL is completely independent of the toxicity induced by topoisomerase II inhibition at the cellular level. The potential of dual-targeting by DOX treatment was verified using an acid-specific FA donor. Overall, anticancer therapy targeting tumors deficient in the FANC/BRCA pathway may be possible by minimizing DOX-induced toxicity in normal cells.