The 5'-GNC site for DNA interstrand cross-linking is conserved for diepoxybutane stereoisomers

DNA Adducts in Cancer Chemotherapy

DNA adducting drugs, including alkylating agents and platinum-containing drugs, are prominent in cancer chemotherapy. Their mechanisms of action involve direct interaction with DNA, resulting in the formation of DNA addition products known as DNA adducts. While these adducts are well-accepted to induce cancer cell death, understanding of their specific chemotypes and their role in drug therapy response remain limited. This perspective aims to address this gap by investigating the metabolic activation and chemical characterization of DNA adducts formed by the U.S. FDA-approved drugs. Moreover, clinical studies on DNA adducts as potential biomarkers for predicting patient responses to drug efficacy are examined. The overarching goal is to engage the interest of medicinal chemists and stimulate further research into the use of DNA adducts as biomarkers for guiding personalized cancer treatment.

Effects of GSTT1 Genotype on the Detoxification of 1,3-Butadiene Derived Diepoxide and Formation of Promutagenic DNA-DNA Cross-Links in Human Hapmap Cell Lines

Smoking is a leading cause of lung cancer, accounting for 81% of lung cancer cases. Tobacco smoke contains over 5000 compounds, of which more than 70 have been classified as human carcinogens. Of the many tobacco smoke constituents, 1,3-butadiene (BD) has a high cancer risk index due to its tumorigenic potency and its abundance in cigarette smoke. The carcinogenicity of BD has been attributed to the formation of several epoxide metabolites, of which 1,2,3,4-diepoxybutane (DEB) is the most toxic and mutagenic. DEB is formed by two oxidation reactions carried out by cytochrome P450 monooxygenases, mainly CYP2E1. Glutathione-S-transferase theta 1 (GSTT1) facilitates the conjugation of DEB to glutathione as the first step of its detoxification and subsequent elimination via the mercapturic acid pathway. Human biomonitoring studies have revealed a strong association between GSTT1 copy number and urinary concentrations of BD-mercapturic acids, suggesting that it plays an important role in the metabolism of BD. To determine the extent that GSTT1 genotype affects the susceptibility of individuals to the toxic and genotoxic properties of DEB, GSTT1 negative and GSTT1 positive HapMap lymphoblastoid cell lines were treated with DEB, and the extent of apoptosis and micronuclei (MN) formation was assessed. These toxicological end points were compared to the formation of DEB-GSH conjugates and 1,4-bis-(guan-7-yl)-2,3-butanediol (bis-N7G-BD) DNA-DNA cross-links. GSTT1 negative cell lines were more sensitive to DEB-induced apoptosis as compared to GSTT1 positive cell lines. Consistent with the protective effect of GSH conjugation against DEB-derived apoptosis, GSTT1 positive cell lines formed significantly more DEB-GSH conjugate than GSTT1 negative cell lines. However, GSTT1 genotype did not affect formation of MN or bis-N7G-BD cross-links. These results indicate that GSTT1 genotype significantly influences BD metabolism and acute toxicity.

Cross-linking by epichlorohydrin and diepoxybutane correlates with cytotoxicity and leads to apoptosis in human leukemia (HL-60) cells

The bifunctional alkylating agents epichlorohydrin (ECH) and diepoxybutane (DEB) have been linked to increased cancer risks in industrial workers. These compounds react with DNA and proteins, leading to genotoxic effects. We used the comet assay to monitor formation of cross-links in HL-60 cells treated with ECH, DEB, and the structurally related anti-cancer drug mechlorethamine (HN2). We report a time- and dose-dependent cytotoxicity that correlated with cross-linking activity, following the order HN2 > DEB > ECH. The rate of cross-link repair also varied with drug, with ECH-induced lesions the fastest to repair. High drug doses led to the formation of saturating amounts of HN2 cross-links that were repaired inefficiently. DEB and ECH produced fewer overall cross-links, but some were also resistant to repair. These persistent cross-links may activate cell-cycle arrest to allow repair of damage, with prolonged arrest triggering apoptosis. Quantitative reverse transcription polymerase chain reaction experiments revealed that treatment of HL-60 cells with DEB and ECH results in up-regulation of several genes involved in the intrinsic (mitochondrial) apoptosis pathway, including BAX, BAK1, CASP-9, APAF-1, and BCL-2. These findings contribute to our understanding of the principles underlying the carcinogenic potentials of these xenobiotics.

Relationship between exposure to treosulfan and its monoepoxytransformer - An insight from population pharmacokinetic study in pediatric patients before hematopoietic stem cell transplantation

Treosulfan (TREO), a structural analog of busulfan, is currently studied as a myeloablative agent in conditioning regimens before hematopoietic stem cell transplantation in pediatric patients. High exposure to TREO (>1650 mg∗h/mL) might be related to early toxicity, especially skin toxicity and mucositis. The aim of the present study was to investigate a potential relationship between exposure to TREO and its monoepoxytransformer (S,S-EBDM), as well as variability of the pharmacokinetics of these entities by means of a population pharmacokinetic approach with a non-linear mixed-effects analysis. The study included data from 14 children with malignant and non-malignant diseases treated with TREO in daily doses 10-14 g/m². The parent-metabolite population pharmacokinetic model was developed in NONMEM 7.3 software. Upon the constructed model, an extensive simulation was performed to assess the correlation between exposure to TREO and S,S-EBDM. It was found that TREO and S,S-EBDM pharmacokinetics was best described with 2-compartmental and 1-compartmental linear models, respectively. The vast majority (>65%) of TREO was transformed to S,S-EBDM. Overall, a considerable interpatient variability of pharmacokinetic parameters was observed, especially the clearance of S,S-EBDM. A weak correlation was found between the exposure to TREO and S,S-EBDM (r = 0.1681, p < 0.0001). Also, patients with an exposure to TREO above 1650 mg∗h/mL were most likely to have also a high exposure to S,S-EBDM (35.38 μM∗h vs. 43.14 μM∗h, p < 0.0001). In summary, a parent-metabolite population pharmacokinetic model for TREO and S,S-EBDM was developed for the first time. It was shown that there is a weak correlation between exposure to TREO and S,S-EBDM. Therefore therapeutic drug monitoring of not only prodrug but also its active epoxide might be needed.

Improvement of genetic stability in lymphocytes from Fanconi anemia patients through the combined effect of α-lipoic acid and N-acetylcysteine

Fanconi Anemia (FA) is a rare genetic disorder, characterized by progressive bone marrow failure and increased predisposition to cancer. Despite being highly heterogeneous, all FA patients are hypersensitive to alkylating agents, in particular to 1,2:3,4-diepoxybutane (DEB), and to oxidative damage. Recent studies point to defective mitochondria in FA cells, which is closely related with increased production of reactive oxygen species (ROS) and concomitant depletion of antioxidant defenses, of which glutathione is a well-known biomarker.The objective of the present work is to evaluate the putative protective effect of α-lipoic acid (α-LA), a mitochondrial protective agent, and N-acetylcysteine (NAC), a direct antioxidant and a known precursor for glutathione synthesis, in spontaneous and DEB-induced chromosome instability (CI) in lymphocyte cultures from FA patients.For that purpose, lymphocyte cultures from 15 FA patients and 24 healthy controls were pre-treated with 20 μM α-LA, 500 μM NAC and α-LA plus NAC at the same concentrations, and some of them were exposed to DEB (0.05 μg/ml). A hundred metaphases per treatment were scored to estimate the relative frequency of spontaneous and DEB-induced chromosome breakage.The obtained results revealed that a cocktail of α-LA and NAC can drastically improve the genetic stability in FA lymphocytes in vitro, decreasing CI by 60% and 80% in cultures from FA patients and FA mosaic/chimera patients, respectively. These results suggest that the studied cocktail can be used as a prophylactic approach to delay progressive clinical symptoms in FA patients caused by CI, which can culminate in the delay of the progressive bone marrow failure and early cancer development.

Chemistry and structural biology of DNA damage and biological consequences

The formation of adducts by the reaction of chemicals with DNA is a critical step for the initiation of carcinogenesis. The structural analysis of various DNA adducts reveals that conformational and chemical rearrangements and interconversions are a common theme. Conformational changes are modulated both by the nature of adduct and the base sequences neighboring the lesion sites. Equilibria between conformational states may modulate both DNA repair and error-prone replication past these adducts. Likewise, chemical rearrangements of initially formed DNA adducts are also modulated both by the nature of adducts and the base sequences neighboring the lesion sites. In this review, we focus on DNA damage caused by a number of environmental and endogenous agents, and biological consequences.

Protective effect of acetyl-l-carnitine and α-lipoic acid against the acute toxicity of diepoxybutane to human lymphocytes

The biotransformation and oxidative stress may contribute to 1,2:3,4-diepoxybutane (DEB)-induced toxicity to human lymphocytes of Fanconi Anemia (FA) patients. Thus, the identification of putative inhibitors of bioactivation, as well as the determination of the protective role of oxidant defenses, on DEB-induced toxicity, can help to understand what is failing in FA cells. In the present work we studied the contribution of several biochemical pathways for DEB-induced acute toxicity in human lymphocyte suspensions, by using inhibitors of epoxide hydrolases, inhibitors of protective enzymes as glutathione S-transferase and catalase, the depletion of glutathione (GSH), and the inhibition of protein synthesis; and a variety of putative protective compounds, including antioxidants, and mitochondrial protective agents. The present study reports two novel findings: (i) it was clearly evidenced, for the first time, that the acute exposure of freshly isolated human lymphocytes to DEB results in severe GSH depletion and loss of ATP, followed by cell death; (ii) acetyl-l-carnitine elicits a significant protective effect on DEB induced toxicity, which was potentiated by α-lipoic acid. Collectively, these findings contribute to increase our knowledge of DEB-induce toxicity and will be very useful when applied in studies with lymphocytes from FA patients, in order to find out a protective agent against spontaneous and DEB-induced chromosome instability.

[Cross-linked nucleic acids: formation, structure, and biological function]

Published data on the main types of reagents capable of introducing covalent interstrand cross links into nucleic acids (NA) are summarized in the present review. The reactivity of cross-linking agents, their preferred binding sites, and methods of determining the cross-link localization in a duplex are discussed. Cell response to DNA cross linking, namely, the blocking of replication and transcription, the initiation of reparation processes, and apoptotic death of the cell, are analyzed, as well as the use of cross-linking reagents in therapy and molecular biology.

Molecular dosimetry of 1,2,3,4-diepoxybutane-induced DNA-DNA cross-links in B6C3F1 mice and F344 rats exposed to 1,3-butadiene by inhalation

1,3-Butadiene (BD) is an important industrial and environmental chemical classified as a human carcinogen based on epidemiologic studies in occupationally exposed workers and animal studies in laboratory rats and mice. BD is metabolically activated to three epoxides that can react with nucleophilic sites in biomolecules. Among these, 1,2,3,4-diepoxybutane (DEB) is considered the ultimate carcinogen due to its high genotoxicity and mutagenicity attributed to its ability to form DNA-DNA cross-links. Our laboratory has developed quantitative high-performance liquid chromatography-muESI(+)-tandem mass spectrometry methods for two DEB-specific DNA-DNA cross-links, 1,4-bis-(guan-7-yl)-2,3-butanediol (bis-N7G-BD) and 1-(guan-7-yl)-4-(aden-1-yl)-2,3-butanediol (N7G-N1A-BD). This report describes molecular dosimetry analysis of these adducts in tissues of B6C3F1 mice and F344 rats exposed to a range of BD concentrations (0-625 ppm). Much higher (4- to 10-fold) levels of DEB-DNA cross-links were observed in mice compared with rats exposed to the same BD concentrations. In both species, bis-N7G-BD levels were 1.5- to 4-fold higher in the liver than in other tissues examined. Interestingly, tissues of female animals exposed to BD contained higher concentrations of bis-N7G-BD adducts than tissues of male animals, which is in accord with previously reported differences in tumor incidence. The molecular dosimetry data presented herein suggest that species and gender differences observed in BD-induced cancer are directly related to differences in the extent of BD metabolism to DEB. Furthermore, a rat model of sensitivity to BD may be more appropriate than a mouse model for assessing human risk associated with BD exposure, because rats and humans seem to be similar with respect to DEB formation.

Diepoxybutane activates the mitochondrial apoptotic pathway and mediates apoptosis in human lymphoblasts through oxidative stress

Diepoxybutane (DEB) is the most potent metabolite of the environmental chemical 1,3-butadiene (BD), which is prevalent in petrochemical industrial areas. BD is a known mutagen and human carcinogen, and possesses multi-systems organ toxicity. We recently reported that DEB-induced cell death in TK6 lymphoblasts was due to the occurrence of apoptosis, and not necrosis. In this study, we investigated the molecular mechanisms responsible for DEB-induced apoptosis in these cells. Bax and Bak were found to be over-expressed and activated, and the mitochondrial trans-membrane potential was attenuated in cells undergoing DEB-induced apoptosis. Cytochrome c was depleted from the mitochondria of TK6 cells undergoing apoptosis, and was released into the cytosol in Jurkat T-lymphoblasts exposed to the same concentrations of DEB. Executioner caspase 3 was deduced to be activated by initiator caspase 9. DEB-induced reactive oxygen species (ROS) formation, and the ROS scavenger N-acetyl-L-cysteine effectively blocked DEB-induced apoptosis in TK6 cells. Collectively, these results demonstrate that the mitochondrial apoptotic pathway is activated to mediate DEB-induced apoptosis in human TK6 lymphoblasts. These results further demonstrate that DEB-induced apoptosis is also mediated by the DEB-induced generation of ROS. This is the first report to examine the mechanism of DEB-induced apoptosis in human lymphoblasts.

DNA interstrand cross-linking by epichlorohydrin

Epichlorohydrin (ECH), an important industrial chemical, is a bifunctional alkylating agent with the potential to form DNA cross-links. Occupational exposure to this suspect carcinogen leads to chromosomal aberrations, and ECH has been shown previously to undergo reaction with DNA in vivo and in vitro. We used denaturing polyacrylamide gel electrophoresis to monitor the possible formation of interstrand cross-links within DNA oligomers by ECH and the related compound, epibromohydrin (EBH). Although both compounds did indeed form cross-links between deoxyguanosine residues, EBH was a more efficient cross-linker than ECH. The optimal pH for cross-linking also varied, with ECH more efficient at pH 5.0 and EBH more efficient at pH 7.0. Both agents were relatively flexible in the sequences targeted, with comparable efficiencies for 5'-GGC and 5'GC sites. Furthermore, interstrand cross-linking by the two optical isomers of ECH correlated with their relative cytotoxicities, with R-ECH about twice as potent as S-ECH.

HPLC−ESI+-MS/MS Analysis of N7-Guanine−N7-Guanine DNA Cross-Links in Tissues of Mice Exposed to 1,3-Butadiene

1,3-butadiene (BD) is a major industrial chemical used in rubber and plastics production and is recognized as an animal and human carcinogen. Although the exact mechanism of BD-induced carcinogenesis is unknown, chemical reactions of epoxide metabolites of BD with DNA to form nucleobase adducts are likely to contribute to multistage carcinogenesis. Among BD-derived epoxy metabolites, 1,2:3,4-diepoxybutane (DEB) appears to be the most genotoxic and carcinogenic, probably because of its bifunctional nature. Initial DNA alkylation by DEB produces N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)guanine monoadducts, which can then be hydrolyzed to N7-(2',3',4'-trihydroxy-1'-yl)guanine or can react with another site in double-stranded DNA to form 1,4-bis(guan-7-yl)-2,3-butanediol (bis-N7G-BD) cross-links. While (2',3',4'-trihydroxy-1'-yl)guanine lesions have been previously quantified in vivo, they cannot be used as a biomarker of DEB because the same lesions are also formed by another, more prevalent BD metabolite, 1,2-epoxy-3,4-butanediol. In contrast, bis-N7G-BD can only be formed from DEB, potentially providing a specific biomarker of DEB formation. We have developed a quantitative HPLC-ESI+-MS/MS method for measuring racemic and meso forms of bis-N7G-BD in DNA extracted from tissues of BD-exposed laboratory animals. In our approach, bis-N7G-BD adducts are released from DNA as free bases by neutral thermal hydrolysis, purified by solid-phase extraction, and subjected to HPLC-ESI+-MS/MS analysis. Selected reaction monitoring is performed by following the loss of a guanine moiety from protonated molecules of bis-N7G-BD and the formation of protonated guanine under collision-induced dissociation. Quantitative analysis of racemic and meso forms of bis-N7G-BD is based on isotope dilution with the corresponding 15N-labeled internal standards. The lower limit of quantification of our current method is 10-20 fmol/0.1 mg of DNA. The accuracy and precision of the new method were determined by spiking control mouse liver DNA with racemic and meso forms of bis-N7G-BD (10 fmol each), followed by sample processing and HPLC-ESI+-MS/MS analysis. Calculated amounts of racemic and meso forms of bis-N7G-BD were within 20% of the theoretical value (9.7 +/- 2 and 9.2 +/- 1.9 fmol, respectively, N = 4). DNA extracted from liver and lung tissues of mice exposed to 625 ppm butadiene for 5 days contained 3.2 +/- 0.4 and 1.8 +/- 0.5 racemic adducts per 10(6) guanines, respectively, while the amounts of meso-bis-N7G-BD were below the detection limits of our method (1 per 10(7) guanines). Control animals did not contain either bis-N7G-BD lesion. Sensitive and specific quantitative methods for bis-N7G-BD analysis developed in this work provide a unique biomarker of DEB-induced DNA alkylation following exposure to BD.

Structure of the 1,4-Bis(2'-deoxyadenosin-N(6)-yl)-2S,3S-butanediol intrastrand DNA cross-link arising from butadiene diepoxide in the human N-ras codon 61 sequence

The 1,4-bis(2'-deoxyadenosin-N(6)-yl)-2S,3S-butanediol intrastrand DNA cross-link arises from the bis-alkylation of tandem N(6)-dA sites in DNA by R,R-butadiene diepoxide (BDO(2)). The oligodeoxynucleotide 5'-d(C(1)G(2)G(3)A(4)C(5)X(6)Y(7)G(8)A(9)A(10)G(11))-3'.5'-d(C(12)T(13)T(14)C(15)T(16)T(17)G(18)T(19)C(20)C(21)G(22))-3' contains the BDO(2) cross-link between the second and third adenines of the codon 61 sequence (underlined) of the human N-ras protooncogene and is named the (S,S)-BD-(61-2,3) cross-link (X,Y = cross-linked adenines). NMR analysis reveals that the cross-link is oriented in the major groove of duplex DNA. Watson-Crick base pairing is perturbed at base pair X(6).T(17), whereas base pairing is intact at base pair Y(7).T(16). The cross-link appears to exist in two conformations, in rapid exchange on the NMR time scale. In the first conformation, the beta-OH is predicted to form a hydrogen bond with T(16) O(4), whereas in the second, the beta-OH is predicted to form a hydrogen bond with T(17) O(4). In contrast to the (R,R)-BD-(61-2,3) cross-link in the same sequence (Merritt, W. K., Nechev, L. V., Scholdberg, T. A., Dean, S. M., Kiehna, S. E., Chang, J. C., Harris, T. M., Harris, C. M., Lloyd, R. S., and Stone, M. P. (2005) Biochemistry 44, 10081-10092), the anti-conformation of the two hydroxyl groups at C(beta) and C(gamma) with respect to the C(beta)-C(gamma) bond results in a decreased twist between base pairs X(6).T(17) and Y(7).T(16), and an approximate 10 degrees bending of the duplex. These conformational differences may account for the differential mutagenicity of the (S,S)- and (R,R)-BD-(61-2,3) cross-links and suggest that stereochemistry plays a role in modulating biological responses to these cross-links (Kanuri, M., Nechev, L. V., Tamura, P. J., Harris, C. M., Harris, T. M., and Lloyd, R. S. (2002) Chem. Res. Toxicol. 15, 1572-1580).