Depurination of benzo[a]pyrene-diolepoxide treated DNA

Understanding the importance of low-molecular weight (ethylene oxide- and propylene oxide-induced) DNA adducts and mutations in risk assessment: Insights from 15 years of research and collaborative discussions

The interpretation and significance of DNA adduct data, their causal relationship to mutations, and their role in risk assessment have been debated for many years. An extended effort to identify key questions and collect relevant data to address them was focused on the ubiquitous low MW N7-alkyl/hydroxyalkylguanine adducts. Several academic, governmental, and industrial laboratories collaborated to gather new data aimed at better understanding the role and potential impact of these adducts in quantifiable genotoxic events (gene mutations/micronucleus). This review summarizes and evaluates the status of dose-response data for DNA adducts and mutations from recent experimental work with standard mutagenic agents and ethylene oxide and propylene oxide, and the importance for risk assessment. This body of evidence demonstrates that small N7-alkyl/hydroxyalkylguanine adducts are not pro-mutagenic and, therefore, adduct formation alone is not adequate evidence to support a mutagenic mode of action. Quantitative methods for dose-response analysis and derivation of thresholds, benchmark dose (BMD), or other points-of-departure (POD) for genotoxic events are now available. Integration of such analyses of genetox data is necessary to properly assess any role for DNA adducts in risk assessment. Regulatory acceptance and application of these insights remain key challenges that only the regulatory community can address by applying the many learnings from recent research. The necessary tools, such as BMDs and PODs, and the example datasets, are now available and sufficiently mature for use by the regulatory community. Environ. Mol. Mutagen. 60: 100-121, 2019. © 2018 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Non-enzymatic depurination of nucleic acids: factors and mechanisms

Depurination has attracted considerable attention since a long time for it is closely related to the damage and repair of nucleic acids. In the present study, depurination using a pool of 30-nt short DNA pieces with various sequences at diverse pH values was analyzed by High Performance Liquid Chromatography (HPLC). Kinetic analysis results showed that non-enzymatic depurination of oligodeoxynucleotides exhibited typical first-order kinetics, and its temperature dependence obeyed Arrhenius’ law very well. Our results also clearly showed that the linear relationship between the logarithms of rate constants and pH values had a salient point around pH 2.5. Interestingly and unexpectedly, depurination depended greatly on the DNA sequences. The depurination of poly (dA) was found to be extremely slow, and thymine rich sequences depurinated faster than other sequences. These results could be explained to some extent by the protonation of nucleotide bases. Moreover, two equations were obtained based on our data for predicting the rate of depurination under various conditions. These results provide basic data for gene mutagenesis and nucleic acids metabolism in acidic gastric juice and some acidic organelles, and may also help to rectify some misconceptions about depurination.

Correlation between base-excision repair gene polymorphisms and levels of in-vitro BPDE-induced DNA adducts in cultured peripheral blood lymphocytes

In vitro benzo[a]pyrene diol epoxide (BPDE)-induced DNA adducts in cultured peripheral lymphocytes have been shown to be a phenotypic biomarker of individual’s DNA repair phenotype that is associated with cancer risk. In this study, we explored associations between genotypes of base-excision repair genes (PARP1 Val762Ala, APEX1 Asp148Glu, and XRCC1 Arg399Gln) and in vitro BPDE-induced DNA adducts in cultured peripheral blood lymphocytes in 706 cancer-free non-Hispanic white subjects. We found that levels of BPDE-induced DNA adducts were significantly higher in ever smokers than in never smokers and that individuals with the Glu variant genotypes (i.e., Asp/Glu and Glu/Glu) exhibited lower levels of BPDE-induced DNA adducts than did individuals with the common Asp/Asp homozygous genotype (median RAL levels: 32.0 for Asp/Asp, 27.0 for Asp/Glu, and 17.0 for Glu/Glu, respectively; P_trend = 0.030). Further stratified analysis showed that compared with individuals with the common APEX1-148 homozygous Asp/Asp genotype, individuals with the APEX1-148Asp/Glu genotype or the Glu/Glu genotype had a lower risk of having higher-level adducts (adjusted OR = 0.60, 95% CI: 0.36–0.98 and adjusted OR = 0.47, 95% CI: 0.26–0.86, respectively; P_trend = 0.012) among smokers. Such an effect was not observed in non-smokers. However, there was no significant interaction between the APEX1 Asp148Glu polymorphism and smoking exposure in this study population (P = 0.512). Additional genotype-phenotype analysis found that the APEX1-148Glu allele had significantly increased expression of APEX1 mRNA in 270 Epstein-Barr virus-transformed lymphoblastoid cell lines, which is likely associated with more active repair activity. Our findings suggest that the functional APEX1-148Glu allele is associated with reduced risk of having high levels of BPDE-induced DNA adducts mediated with high levels of mRNA expression.

The formation and biological significance of N7-guanine adducts

DNA alkylation or adduct formation occurs at nucleophilic sites in DNA, mainly the N7-position of guanine. Ever since identification of the first N7-guanine adduct, several hundred studies on DNA adducts have been reported. Major issues addressed include the relationships between N7-guanine adducts and exposure, mutagenesis, and other biological endpoints. It became quickly apparent that N7-guanine adducts are frequently formed, but may have minimal biological relevance, since they are chemically unstable and do not participate in Watson Crick base pairing. However, N7-guanine adducts have been shown to be excellent biomarkers for internal exposure to direct acting and metabolically activated carcinogens. Questions arise, however, regarding the biological significance of N7-guanine adducts that are readily formed, do not persist, and are not likely to be mutagenic. Thus, we set out to review the current literature to evaluate their formation and the mechanistic evidence for the involvement of N7-guanine adducts in mutagenesis or other biological processes. It was concluded that there is insufficient evidence that N7-guanine adducts can be used beyond confirmation of exposure to the target tissue and demonstration of the molecular dose. There is little to no evidence that N7-guanine adducts or their depurination product, apurinic sites, are the cause of mutations in cells and tissues, since increases in AP sites have not been shown unless toxicity is extant. However, more research is needed to define the extent of chemical depurination versus removal by DNA repair proteins. Interestingly, N7-guanine adducts are clearly present as endogenous background adducts and the endogenous background amounts appear to increase with age. Furthermore, the N7-guanine adducts have been shown to convert to ring opened lesions (FAPy), which are much more persistent and have higher mutagenic potency. Studies in humans are limited in sample size and differences between controls and study groups are small. Future investigations should involve human studies with larger numbers of individuals and analysis should include the corresponding ring opened FAPy derivatives.

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll and Peto, 1981). An important and prevalent class of potent carcinogenic compounds present in he environment is polycyclic aromatic hydrocarbons (PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive.

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll & Peto, 1981). An important and prevalent class of potent carcinogenic compounds present in the environment is polycyclic aromatic hydrocarbons (PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll & Peto, 1981). An important and prevalent class of potent carcinogenic compounds present in the environment is polycyclic aromatic hydrocarbons (PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive.

DNA–carcinogen interaction: covalent DNA-adducts of benzo(a)pyrene 7, 8-dihydrodiol 9, 10-epoxides studied by biochemical and biophysical techniques

Exposure to various chemicals, either due to occupation or lifestyle, is considered to be a major contributing factor to tumour formation in man (Higginson, 1969; Doll and Peto, 1981). An important and prevalent class of potent carcinogeniccompounds present in the environment is polycyclic aromatic hydrocarbons(PAHs), which are found in various petroleum and combustion products derived from heat and power generation and motor vehicle exhausts (Baum, 1978). Furthermore, since PAHs are generally formed by pyrolysis of organic matters such as tobacco smoking and certain procedures of food preparation, the PAH exposure to humans is extensive.

Induction of base damages representing a high risk site for double-strand DNA break formation in genomic DNA by exposure of cells to DNA damaging agents

DNA repair is known as a defense mechanism against genotoxic insults. However, the most lethal type of DNA damages, double-strand DNA breaks (DSBs), can be produced by DNA repair. We have previously demonstrated that when long patch base excision repair attempts to repair a synthetic substrate containing two uracils, the repair produces DSBs (Vispe, S. and Satoh, M. S. (2000) J. Biol. Chem. 275, 27386-27392 and Vispe, S., Ho, E. L., Yung, T. M., and Satoh, M. S. (2003) J. Biol. Chem. 278, 35279-35285). In this synthetic substrate, the two uracils are located on the opposite DNA strands (separated by an intervening sequence stable at 37 degrees C) and represent a high risk site for DSB formation. It is not clear, however, whether similar high risk sites are also induced in genomic DNA by exposure to DNA damaging agents. Thus, to investigate the mechanisms of DSB formation, we have modified the DSB formation assay developed previously and demonstrated that high risk sites for DSB formation are indeed generated in genomic DNA by exposure of cells to alkylating agents. In fact, genomic DNA containing alkylated base damages, which could represent high risk sites, are converted into DSBs by enzymes present in extracts prepared from cells derived from clinically normal individuals. Furthermore, DSBs are also produced by extracts from cells derived from ataxia-telangiectasia patients who show cancer proneness due to an impaired response to DSBs. These results suggest the presence of a novel link between base damage formation and DSBs and between long patch base excision repair and human diseases that occur due to an impaired response to DSB.

Kinetics of DNA alkylation, depurination and hydrolysis of anti diol epoxide of benzo(a)pyrene and the effect of cadmium on DNA alkylation

Anti benzo[a]pyrene diol epoxide (BPDE) alkylates guanines of DNA at N7 in the major groove and at the exocyclic amino group in the minor groove. In this report we investigated the rates of BPDE hydrolysis, DNA alkylation and subsequent depurination of BPDE-adducted pBR322 DNA fragment using polyacrylamide gel electrophoresis. Preincubation studies showed that it hydrolyzed completely in triethanolamine buffer in <2 min. The depurination kinetics showed that a fraction of the N7 alkylated guanine depurinated rapidly; however a significant amount of N7 guanine alkylation remained stable to spontaneous depurination over a 4-h period. Similar results were obtained for the hydrolysis and alkylation rates of syn isomer but it required nearly 500 times more concentration to induce similar levels of N7 guanine alkylation. Cadmium ion strongly inhibited the N7 guanine alkylation of both isomers. But the minor groove alkylation was not affected as demonstrated by postlabeling assay which confirmed the presence of heat-and cadmium-stable minor groove adducts in BPDE-treated calf thymus DNA. Based on these and our earlier findings, we propose a mechanism for the synergistic effect of cadmium in chemically induced carcinogenesis.

Formation of stable adducts and absence of depurinating DNA adducts in cells and DNA treated with the potent carcinogen dibenzo[a,l]pyrene or its diol epoxides

Polycyclic aromatic hydrocarbons (PAH) are widespread environmental contaminants, and some are potent carcinogens in rodents. Carcinogenic PAH are activated in cells to metabolites that react with DNA to form stable covalent DNA adducts. It has been proposed [Cavalieri, E. L. & Roger, E. G. (1995) Xenobiotica 25, 677-688] that unstable DNA adducts are also formed and that apurinic sites in the DNA resulting from unstable PAH adducts play a key role in the initiation of cancer. The potent carcinogen dibenzo[a,l]pyrene (DB[a, l]P) is activated in cells to (+)-syn- and (-)-anti-DB[a,l]P-11, 12-diol-13,14-epoxide (DB[a,l]PDE), which have been shown to form stable adducts with DNA. To evaluate the importance of unstable PAH adducts, we compared stable adduct formation to apurinic site formation. Stable DB[a,l]PDE adducts were determined by 33P-postlabeling and HPLC. To measure apurinic sites they were converted to strand breaks, and these were monitored by examining the integrity of a particular restriction fragment of the dihydrofolate reductase gene. The method easily detected apurinic sites resulting from methylation by treatment of cells or DNA with dimethyl sulfate or from reaction of DNA with DB[a,l]P in the presence of horseradish peroxidase. We estimate the method could detect 0.1 apurinic site in the 14-kb fragment examined. However, apurinic sites were below our limit of detection in DNA treated directly with (+)-syn- or (-)-anti-DB[a,l]PDE or in DNA from Chinese hamster ovary B11 cells so treated, although in these samples the frequency of stable adducts ranged from 3 to 10 per 14 kb. We also treated the human mammary carcinoma cell line MCF-7 with DB[a,l]P and again could not detect significant amounts of unstable adducts. These results indicate that the proportion of stable adducts formed by DB[a,l]P activated in cells and its diol epoxides is greater than 99% and suggest a predominant role for stable DNA adducts in the carcinogenic activity of DB[a,l]P.

Benzo[a]pyrene diol epoxide-DNA cis adduct formation through a trans chlorohydrin intermediate

Alkylation of DNA by 7r,8t-dihydroxy,9t,10t-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE) forms mainly trans adducts (with respect to the C-9/10 positions). We recently described a halide-catalyzed pathway that preferentially generates cis adducts and now report that the trans chlorohydrin of anti-BPDE (trans-BPDCH) is an intermediate in the chloride-catalyzed reaction. trans-BPDCH was synthesized, and both it and anti-BPDE were reacted with deoxyadenosine as a model DNA nucleophile. The stereochemistry and yields of deoxyadenosine adducts were determined as a function of chloride concentration. In the absence of salt, the fraction of cis adducts obtained from anti-BPDE and trans-BPDCH are 0.33 and 0.67, respectively. Adding sodium chloride increases the fraction of cis adducts (and consequently decreases the fraction of trans adducts), with the midpoint of the increase for both substrates at approximately 35-40 mM chloride. The chloride-dependent curves for BPDE and BPDCH converge at 1 M chloride, where the fraction of cis adducts is 0.88. Chloride also increases the total yield of cis adducts with either substrate, whereas the yield of trans adducts from the chlorohydrin is not significantly changed. These results support a mechanism by which chloride ion undergoes nucleophilic addition to the benzylic C-10 position of anti-BPDE. This generates a trans halohydrin that alkylates DNA with inversion of configuration to form a cis adduct. This pathway may have biological significance because chlorohydrins could form in serum or in cells with relatively high intracellular concentrations of chloride.

On the mechanism of preferential incorporation of dAMP at abasic sites in translesional DNA synthesis. Role of proofreading activity of DNA polymerase and thermodynamic characterization of model template-primers containing an abasic site

DNA polymerase preferentially incorporate dAMP opposite abasic sites (A-rule). The mechanism of the A-rule can be studied by analyzing three dissected stages of the reaction including (i) initial nucleotide insertion, (ii) proofreading excision of the inserted nucleotide and (iii) extension of the nascent primer terminus. To assess the role of the stage (ii) in the A-rule, kinetic parameters of the proofreading excision of primer terminus nucleotides opposite abasic sites were determined using E.coli DNA polymerase I Klenow fragment. The relative efficiency of the excision (Vmax/Km) revealed that removal of A was the least favored of the four nucleotides, but the differences in the efficiencies between excision of A and the other nucleotides was less than 2-fold. In addition, in an attempt to reconcile kinetic data associated with the stage (i) or (ii), the differences in free energy changes (delta delta G degrees) for the formation of model template-primer termini containing XN pairs (X = abasic site, N = A, G, C or T) were determined by temperature dependent UV-melting measurements. The order of delta delta G degrees was XG > XA = XC > or = XT, with delta delta G degrees being 0.5 kcal/mol for the most stable XG and the least stable XT. Based on these data, the role of the stage (ii) and energetic aspects of the A-rule are discussed.

Synthesis and damage specificity of a novel probe for the detection of abasic sites in DNA

The abasic site (apurinic/apyrimidinic site) is the most common lesion in DNA and is suggested to be an important intermediate in mutagenesis and carcinogenesis. We have recently reported a novel assay for the detection and quantitation of abasic sites in DNA [Kubo, K., Ide, H., Wallace, S. S., & Kow, Y. W. (1992) Biochemistry 31, 3703-3708]. In this assay, the aldehyde group in an abasic site is first modified by a probe bearing a biotin residue, called the Aldehyde Reactive Probe (ARP) and then the tagged biotin is quantified by an ELISA-like assay. However, in the previous study, ARP was prepared only in a crude form, and no solid chemical data concerning the structure and specificity of ARP were reported. In this study, an improved method for the preparative synthesis of ARP has been established, and its structure has been unambiguously characterized using spectroscopic means. In order to elucidate the specificity of ARP to DNA damages, ARP was incubated with a variety of damaged bases or nucleosides and the reaction mixtures were analyzed by HPLC. Of the 14 compounds tested for their reactivity to ARP, 2-deoxyribose (a model compound for an abasic site) and 5-formyluracil reacted with ARP. Interestingly, compounds bearing a formamide group such as formamidopyrimidine and deoxyribosylformamide did not react with ARP, indicating that ARP is specific to damages having an alkyl or allyl aldehyde group. Furthermore, the ability of ARP synthesized by the defined chemical route to detect abasic sites has been substantiated using natural DNA containing abasic sites. Potential applications and limitations of the ARP assay are discussed.

Preferential repair and strand-specific repair of benzo[a]pyrene diol epoxide adducts in the HPRT gene of diploid human fibroblasts

If excision repair-proficient human cells are allowed time for repair before onset of S phase, the premutagenic lesions formed by (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy- 7,8,9,10-tetrahydrobenzo[a]pyrene (benzo[a]pyrene diol epoxide, BPDE) are lost from the transcribed strand of the hypoxanthine (guanine) phosphoribosyltransferase (HPRT) gene faster than from the nontranscribed strand. No change in strand distribution is seen with repair-deficient cells. These results suggest strand-specific repair of BPDE-induced DNA damage in human cells. To test this, we measured the initial number of BPDE adducts formed in each strand of the actively transcribed HPRT gene and the rate of repair, using UvrABC excinuclease in conjunction with Southern hybridization and strand-specific probes. We also measured the rate of loss of BPDE adducts from the inactive 754 locus. The frequencies of adducts formed by exposure to BPDE (1.0 or 1.2 microM) in either strand of a 20-kilobase fragment that lies entirely within the transcription unit of the HPRT gene were similar; the frequency in the 14-kilobase 754 fragment was approximately 20% lower. The rates of repair in the two strands of the HPRT fragment differed significantly. Within 7 hr after treatment with 1.2 microM BPDE, 53% of the adducts had been removed from the transcribed strand, but only 26% from the nontranscribed strand; after 20 hr, these values were 87% and 58%, respectively. In contrast, only approximately 14% of the BPDE adducts were lost from the 754 locus in 20 hr, a value even lower than the rate of loss from the overall genome (i.e., 38%). These results demonstrate strand-specific and preferential repair of BPDE adducts in human cells. They suggest that the heterogeneous repair of BPDE adducts in the human genome cannot be accounted for merely by the greatly increased rate of the repair specific to the transcribed strand of the active genes, and they point to a role for the chromatin structure.

Inactivation and repair of double-stranded DNA damaged by the aziridinyl nitroimidazole RSU 1069

Incubation of RSU 1069 in the presence of biologically active double-stranded phi X174 DNA resulted in, depending on pH, ionic strength and concentration of drug, inactivation of the DNA. A variety of lesions are induced including a high number of single-strand breaks and alkali-labile lesions, which are at most partly lethal. The main inactivating damage consists probably of base damage, induced by alkylation. A considerable part of the damage induced by RSU 1069 can be repaired by the various repair enzymes of the bacterial host of the phi X174 DNA. Finally the damage (pattern) depends considerably on the ionic composition of the reaction solution, which can be explained by an equilibrium model presented in this paper.

Interaction of RSU 1069 and 1137 with DNA in vitro. Biological implications and mechanistic aspects

We have examined the capacity of the nitroimidazole aziridine antitumour drug RSU 1069 to react with DNA in vitro in order to get a better understanding of its mechanism of action. Moreover, we have utilized biologically active phi X174 DNA to investigate the biological relevance of the chemical DNA modification induced by the drug. Incubation of RSU 1069 in the presence of single-stranded phi X174 DNA resulted in extensive inactivation of the DNA, which is dependent on the concentration of drug and temperature. Only about 2% of the inactivating damage can be attributed to strand breakage. The main damage most probably consists of base damage, of which a part is non-lethal and alkali-labile which in turn can be converted into lethal lesion and subsequently into a break applying a post-incubation alkali treatment. Furthermore, from the dependence of the inactivation and also the formation of breaks on pH and ionic strength, it is concluded that the reaction most probably takes place between a protonated RSU 1069 and a negative DNA coil and that the damage pattern reflects the difference in reactivity of RSU 1069 with the phosphate groups and the bases in DNA. Comparison between RSU 1069 and its ring-open hydrolysis product RSU 1137 revealed that (lethal) damage induced in the DNA must be ascribed to the alkylating properties of the aziridine moiety.

Response of phage T4 polynucleotide kinase toward dinucleotides containing apurinic sites: design of a 32P-postlabeling assay for apurinic sites in DNA

We have examined the capacity of bacteriophage T4 polynucleotide kinase (EC 2.7.1.78) to phosphorylate the partially depurinated products of d-ApA, namely, d-SpA and d-ApS (where S represents an apurinic deoxyribose group). It was observed that the enzyme acted only on the latter isomer. Since molecules of this type (d-NpS) are the sole apurinic site containing products resulting from the combined digestion of lightly depurinated DNA by snake venom phosphodiesterase and calf alkaline phosphatase [Weinfeld, M., Liuzzi, M., & Paterson, M. C. (1989) Nucleic Acids Res. 17, 3735-3745], we were able to devise a postlabeling assay for these biologically important DNA lesions. The method offers several advantages, including (a) elimination of the need for prelabeled DNA, (b) high (femtomole range) sensitivity, and (c) nearest-neighbor analysis of bases 5' to apurinic/apyrimidinic sites. Using this assay, we obtained a value for the rate of depurination of form I pRSVneo plasmid DNA, incubated at pH 5.2 at 70 degrees C, of approximately 3.3 apurinic sites per plasmid molecule per hour. This value compares favorably with previously published data of others, acquired by alternative approaches. The rate of depurination of poly(dA), treated in a similar fashion, was found to be approximately 1 base per 10(3) nucleotides per hour.

Sequence specificity in the reaction of benzopyrene diol epoxide with DNA

Benzopyrene diol epoxide (BPDE; (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene), the ultimate carcinogen derived from the polycyclic hydrocarbon benzo[a]pyrene, reacts principally with the guanine bases in DNA. Nineteen double stranded, self-complementary oligonucleotides, containing deoxyguanosine in various sequence contexts, were each treated with tritium labelled BPDE. The extent of reaction was determined by releasing the BPDE-guanine adduct with acid, isolating it by chromatography on a reverse-phase column, and estimating it by its radioactivity. Oligonucleotides containing an isolated guanine, such as AAGTACTT, were little affected by BPDE. Reactivity was increased where the guanine was flanked by another guanine on the same strand (e.g. TACCTAGGTA) or on the complementary strand (e.g. TATTCGAATA), and was highest in mixed G-C sequences such as ATCCGGAT. The results should help predict major sites of attack of BPDE on cellular proto-oncogenes.

Involvement of DNA polymerases in the repair of DNA damage by benzo[a]pyrene in cultured Chinese hamster cells

Mechanisms for induction of single-strand scissions in DNA by S9-activated benzo[a]pyrene (B[a]P) and their repair in cultured Chinese hamster V79 cells were investigated with inhibitors of DNA-repair synthesis using alkaline sucrose gradient sedimentation analysis. The marked induction of single-strand scissions in DNA was observed following 3 h treatment of V79 cells with 5 micrograms/ml of B[a]P. These DNA lesions were repaired to the control level within 4 h after removal of B[a]P. The simultaneous addition of inhibitors of DNA-repair synthesis. 1-beta-D-arabinofuranosylcytosine (araC) plus hydroxyurea with B[a]P did not increase the formation of DNA single-strand scissions. When these inhibitors were added after removal of B[a]P, however, they significantly blocked the rejoining of DNA-strand scissions. On the other hand, when aphidicolin, a specific inhibitor of DNA polymerase alpha, was used instead of araC, a partial inhibition of the rejoining was observed, and further addition of 2',3'-dideoxythymidine, an inhibitor of DNA polymerase beta, augmented the inhibitory effect. These results indicate that B[a]P-induced single-strand scissions of DNA in V79 cells could be repaired mostly by excision repair which involved DNA polymerase alpha and a non-alpha polymerase, presumably polymerase beta.

Kinds of mutations formed when a shuttle vector containing adducts of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9, 10-tetrahydrobenzo[a]pyrene replicates in human cells

We have investigated the kinds of mutations induced when a shuttle vector containing covalently bound residues of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9, 10-tetrahydrobenzo[a]pyrene (BPDE) replicates in human cells. A human embryonic kidney cell line, 293, was used as the eukaryotic host. The target gene for mutation analysis, supF, codes for a tyrosine suppressor tRNA and is strategically located between the origin of replication of the plasmid in Escherichia coli and the gene for a selectable marker, so that the possibility of recovering supF mutants containing gross rearrangements is low. The frequency of supF mutants obtained when untreated plasmid replicated in 293 cells was 1.4 X 10(-4). The frequency with BPDE-treated plasmid increased linearly as a function of the number of adducts, with 16 adducts per plasmid giving 38 X 10(-4). Polyacrylamide gel and agarose gel electrophoresis analysis of 137 plasmids with mutations in the supF gene indicated that 70% (21/30) from untreated plasmids contained deletions or insertions or showed altered gel mobility, whereas only 28% (30/107) of those derived from BPDE-treated plasmids contained such alterations. Of the 86 unequivocally independent mutants derived from BPDE-treated plasmids that were analyzed by sequencing, the majority (60/86) exhibited base substitutions. Mutants exhibiting frameshifts (insertions or deletions of one, two, or four base pairs) were also found, but they were a minority (11/86). In the progeny of BPDE-treated plasmids 61/71 base substitutions observed were transversions, with 45/61 G X C----T X A. Examination of the location of BPDE-induced mutations among the 85 base pairs in the structure of the tRNA revealed that 30% of the base substitutions occurred at two sites and 44% of the rest occurred at five other hot spots. Only 20% of all these base changes involved a site in which a guanine containing a BPDE adduct is predicted to be labile--i.e., a guanine that has a pyrimidine to its 5' side.

Induction of DNA crosslinks in vitro upon reduction of the nitroimidazole-aziridines RSU-1069 and RSU-1131

The interaction of the nitroimidazole-aziridines RSU-1069 and RSU-1131, as parent or radiation-reduced species, with plasmid DNA in aqueous solution at pH 7 results in strand breakage. The yields of DNA single strand breaks (ssb), "alkali-labile" damage and DNA crosslinks induced by these alkylating agents have been assessed. It is shown that DNA crosslinks are induced only by the reduced nitro-compounds. RSU-1069, as parent or reduced compound, is more efficient at producing these effects than the equivalent form of RSU-1131. Further, RSU-1069 is about 2 X more susceptible to nucleophilic attack by inorganic phosphate and deoxynucleotides than RSU-1131. RSU-1069 also shows greater selectivity for reaction with the nucleotide base moiety than does the less-reactive monomethyl analogue, RSU-1131. The yields of ssb and "alkali-labile" damaged sites induced by the two agents reflect their respective chemical reactivities and appear largely to determine their aerobic cytotoxicities. In contrast, the yield of DNA crosslinks induced by the reduced compounds appears to correspond rather better with the observed hypoxic cytotoxicities. From these findings it is suggested that the induction of DNA crosslinks by these agents may play a major role in their effectiveness as hypoxia-selective cytotoxins.

Origins of stereospecificity in DNA damage by anti-benzo[a]pyrene diol-epoxides. A molecular modelling study

A general computational procedure for the modelling of intercalated DNA-ligand complexes has been developed, and is used here to model intercalated complexes of the (+)-anti and (-)-anti enantiomers of benzo[a]pyrene diol-epoxide (BPDE) with cytosine-3',5'-guanosine double-stranded DNA sequences (dCpG). Results are presented indicating differences between the behaviours of the two enantiomers which have implications for the understanding of the stereospecificity of DNA strand breakage by benzo[a]pyrene diol-epoxides.

Excretion of benzo[a]pyrene-Gua adduct in the urine of benzo[a]pyrene-treated rats

A benzo[a]pyrene(BP)-Gua adduct was extracted in the urine of rats treated with BP. Some (0.15%) of the administered dose of BP was excreted as BP-Gua within 48 h. A double labelling experiment demonstrated that the excreted product contained both a BP and a Gua moiety. Partially hepatectomized rats treated with [14C]Gua during the regenerative phase were injected with [3H]BP and the urine collected and processed by chromatographic procedures. The adduct had similar chromatographic properties to the adduct released from human PLC/5 cells treated with 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) and co-chromatographed with 7-BPDE-Gua released from BPDE-adducted DNA under aqueous conditions. Detection and quantitation of BP-Gua offers an alternative, non-invasive method of monitoring individuals exposed to carcinogenic polycyclic aromatic hydrocarbons (PAHs).

Interaction of the aziridine moiety of RSU-1069 with nucleotides and inorganic phosphate. Implications for alkylation of DNA

The aziridine moiety of RSU-1069 (1-(2-nitro-1-imidazolyl)-3-(1-aziridino-2-propanol] alkylates inorganic phosphate and a range of nucleotides in aqueous solutions of pH 7.0. From the rate constants obtained and a study of the adducts formed it is demonstrated that phosphate is a major target on nucleotides at neutral pH, although additional sites may exist particularly on dGMP and dAMP. From the dependence of reactivity on pH and the influence of ionic strength, it is established that the protonated aziridine is the reactive species and that hydrolysis is insignificant when compared to the rate of phosphorylation. The reaction kinetics detailed in this study are discussed in terms of DNA alkylation and strand breakage effected by the aziridine moiety of RSU-1069.