Acid-specific formaldehyde donor is a potential, dual targeting cancer chemotherapeutic/chemo preventive drug for FANC/BRCA-mutant cancer

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.

The failure of two major formaldehyde catabolism enzymes (ADH5 and ALDH2) leads to partial synthetic lethality in C57BL/6 mice

Background

Exogenous formaldehyde is classified by the IARC as a Category 1 known human carcinogen. Meanwhile, a significant amount of endogenous formaldehyde is produced in the human body; as such, formaldehyde-derived DNA and protein adducts have been detected in animals and humans in the absence of major exogenous formaldehyde exposure. However, the toxicological effects of endogenous formaldehyde on individuals with normal DNA damage repair functions are not well understood. In this study, we attempted to generate C57BL/6 mice deficient in both Adh5 and Aldh2, which encode two major enzymes that metabolize endogenous formaldehyde, in order to understand the effects of endogenous formaldehyde on mice with normal DNA repair function.

Results

Due to deficiencies in both ADH5 and ALDH2, few mice survived past post-natal day 21. In fact, the survival of pups within the first few days after birth was significantly decreased. Remarkably, two Aldh2 ^-/- /Adh5 ^-/- mice survived for 25 days after birth, and we measured their total body weight and organ weights. The body weight of Aldh2 ^-/- /Adh5 ^-/- mice decreased significantly by almost 37% compared to the Aldh2 ^-/- /Adh5 ^+/- and Aldh2 ^-/- /Adh5 ^+/+ mice of the same litter. In addition, the absolute weight of each organ was also significantly reduced.

Conclusion

Mice deficient in both formaldehyde-metabolizing enzymes ADH5 and ALDH2 were found to develop partial synthetic lethality and mortality shortly after birth. This phenotype may be due to the accumulation of endogenous formaldehyde. No serious phenotype has been reported in people with dysfunctional, dominant-negative ALDH2*2 alleles, but it has been reported that they may be highly susceptible to osteoporosis and neurodegenerative diseases. It is important to further investigate these diseases in individuals with ALDH2*2 alleles, including an association with decreased metabolism, and thus accumulation, of formaldehyde.

DNA-protein crosslink formation by endogenous aldehydes and AP sites

Covalent binding between proteins and a DNA strand produces DNA-protein crosslinks (DPC). DPC are one of the most deleterious types of DNA damage, leading to the blockage of DNA replication and transcription. Both DNA lesions and endogenous products with carbonyl functional groups can produce DPC in genomic DNA under normal physiological conditions. For example, formaldehyde, the most abundant endogenous human carcinogen, and apurinic/apyrimidinic (AP) sites, the most common type of endogenous DNA lesions, has been shown to crosslink proteins and/or DNA through their carbonyl functional groups. Unfortunately, compared to other types of DNA damage, DPC have been less studied and understood. However, a recent advancement has allowed researchers to determine accurate yields of various DNA lesions including formaldehyde-derived DPC with high sensitivity and specificity, paving the way for new developments in this field of research. Here, we review the current literature and remaining unanswered questions on DPC formation by endogenous formaldehyde and various aldehydic 2-deoxyribose lesions.