Specificity of mutagenesis by 4-aminobiphenyl. A possible role for N-(deoxyadenosin-8-yl)-4-aminobiphenyl as a premutational lesion

Influence of arylamine N-acetyltransferase, sex, and age on 4-aminobiphenyl-induced in vivo mutant frequencies and spectra in mouse liver

One model for cancer initiation by 4-aminobiphenyl (ABP) involves N-oxidation by cytochrome P450 CYP1A2 followed by O-conjugation by N-acetyltransferase(s) NAT1 and/or NAT2 and decomposition to a DNA-binding nitrenium ion. We recently observed that neonatal ABP exposure produced liver tumors in male but not in female mice, and that NAT deficiency reduced liver tumor incidence. However, ABP-induced liver tumor incidence did not correlate with liver levels of N-(deoxyguanosin-8-yl)-ABP adducts 24 hr after exposure. In this study, we compared in vivo ABP-induced DNA mutant frequencies and spectra between male and female wild-type and NAT-deficient Muta™Mouse using both the tumor-inducing neonatal exposure protocol and a 28-day repetitive dosing adult exposure protocol. ABP produced an increase in liver DNA mutant frequencies in both neonates and adults. However, we observed no sex or strain differences in mutant frequencies in neonatally exposed mice, and higher frequencies in adult females than males. Neonatal ABP exposure of wild-type mice increased the proportion of G-T transversions in both males and females, while exposure of Nat1/2(-/-) mice produced increased G-T transversions in males and a decrease in females, even though females had higher levels of N-(deoxyguanosin-8-yl)-4-ABP adducts. There was no correlation of mutant frequencies or spectra between mice dosed as neonates or as adults. These results suggest that observed sex- and NAT-dependent differences in ABP-induced liver tumor incidence in mice are not due to differences in either mutation rates or mutational spectra, and that mechanisms independent of carcinogen bioactivation, covalent DNA binding and mutation may be responsible for these differences.

Quantitative changes in endogenous DNA adducts correlate with conazole in vivo mutagenicity and tumorigenicity

The mouse liver tumorigenic conazole fungicides triadimefon and propiconazole have previously been shown to be in vivo mouse liver mutagens in the Big Blue™ transgenic mutation assay when administered in feed at tumorigenic doses, whereas the nontumorigenic conazole myclobutanil was not mutagenic. DNA sequencing of the mutants recovered from each treatment group as well as from animals receiving control diet revealed that propiconazole- and triadimefon-induced mutations do not represent general clonal expansion of background mutations, and support the hypothesis that they arise from the accumulation of endogenous reactive metabolic intermediates within the liver in vivo. We therefore measured the spectra of endogenous DNA adducts in the livers of mice from these studies to determine if there were quantitative or qualitative differences between mice receiving tumorigenic or nontumorigenic conazoles compared to concurrent control animals. We resolved and quantitated 16 individual adduct spots by (32)P postlabelling and thin layer chromatography using three solvent systems. Qualitatively, we observed the same DNA adducts in control mice as in mice receiving conazoles. However, the 13 adducts with the highest chromatographic mobility were, as a group, present at significantly higher amounts in the livers of mice treated with propiconazole and triadimefon than in their concurrent controls, whereas this same group of DNA adducts in the myclobutanil-treated mice was not different from controls. This same group of endogenous adducts were significantly correlated with mutant frequency across all treatment groups (P = 0.002), as were total endogenous DNA adduct levels (P = 0.005). We hypothesise that this treatment-related increase in endogenous DNA adducts, together with concomitant increases in cell proliferation previously reported to be induced by conazoles, explain the observed increased in vivo mutation frequencies previously reported to be induced by treatment with propiconazole and triadimefon.

Quantification of N-(deoxyguanosin-8-yl)-4-aminobiphenyl adducts in human lymphoblastoid TK6 cells dosed with N-hydroxy-4-acetylaminobiphenyl and their relationship to mutation, toxicity, and gene expression profiling

Gene expression profiles that are anchored to phenotypic endpoints may lead to the identification of signatures that predict mutagenicity or carcinogenicity. The study presented here describes the analysis of DNA adducts in the human TK6 lymphoblastoid cell line after exposure to N-hydroxy-4-aminobiphenyl, a mutagenic metabolite of 4-aminobiphenyl. A validated nano-LC microelectrospray mass spectrometry assay is reported for the detection and quantification of N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-ABP), the principal DNA adduct of 4-aminobiphenyl. Limits of quantification, based on a signal-to-noise ratio of 10:1, are determined to correspond to approximately 27 fg of dG-C8-ABP injected on-column. The assay has been used to measure the steady-state levels of the adduct in the human TK6 lymphoblastoid cell line as a function of dose (0.5, 1.0, and 10.0 microM) and time (2, 6, and 27 h) after exposure to N-hydroxy-4-aminobiphenyl. The levels of dG-C8-ABP adducts in the cells, ranging from 18 to 500 adducts in 10(9) nucleotides, were then correlated to cell toxicity, induced mutation at the TK (thymidine kinase) and HPRT loci, and gene expression profiling through microarray analysis. Cell cultures were evaluated for toxicity by growth curve extrapolation, mutation assays were performed on the HPRT and TK loci, and gene expression profiles were generated by analyses using microarray technology. In the mutation assay analysis, as the toxicant concentration increased, there was an increase in mutation fraction, indicating a direct correlation to metabolite dosing level and mutations occurring at these two loci. Statistical analysis of the gene expression data determined that a total of 2250 genes exhibited statistically significant changes in expression after treatment with N-OH-AABP (P < 0.05). Among the genes identified, 2245 were up-regulated, whereas 5 genes that had functions in cell survival and cell growth and, hence, could be indicators of toxicity, were down-regulated relative to controls. The results demonstrate the value of anchoring gene expression patterns to phenotypic markers, such as DNA adduct levels, toxicity, and mutagenicity.

4-Aminobiphenyl induces liver DNA adducts in both neonatal and adult mice but induces liver mutations only in neonatal mice

The mechanisms underlying the susceptibility of neonatal mice to genotoxic carcinogens were investigated by analyzing the DNA adducts and mutations induced in the livers of neonatal and adult Big Blue transgenic mice by 4-aminobiphenyl (4-ABP), a potent human and rodent carcinogen. Neonatal and adult mice were treated with a regimen of 4-ABP known to induce tumors in neonatal mice. Animals were sacrificed 1 day after the last treatment for DNA adduct analysis and 8 weeks after the last treatment for analysis of lacI and cII mutant frequency (MF). N-(Deoxyguanosin-8-yl)-4-ABP was the major DNA adduct identified in the livers of the 4-ABP-treated mice and levels of this adduct were significantly higher in treated animals than in the controls for both the neonates and adults. Adduct levels for adult females (44.0 +/- 4.8 adducts/10(6) nucleotides) were higher than in neonatal females (25.9 +/- 2.2 adducts/10(6) nucleotides), while adduct levels in adult males (13.5 +/- 2.0 adducts/10(6) nucleotides) were lower than in neonatal males (33.8 +/- 4.1 adducts/10(6) nucleotides). 4-ABP treatment significantly increased the liver cII MFs in both sexes of neonatal mice but not in adult mice. Sequence analysis of cII mutant DNA revealed that 4-ABP induced a unique spectrum of mutations in neonatal mice, characterized by a high frequency of G:C-->T:A transversion, while the mutation spectrum in 4-ABP-treated adults was similar to that of control mice. Our results indicate that DNA adduct formation by 4-ABP depends as much on sex as it does on age, whereas the conversion of DNA adducts into mutations differed with animal age. These observations suggest that neonates are more sensitive than adults to genotoxic carcinogens because the relatively high levels of cell division in the developing animal facilitate the conversion of DNA damage into mutation. Supplementary material for this article can be found on the International Journal of Cancer website at http://www.interscience.wiley.com/jpages/0020-7136/suppmat/index.html

Biomonitoring of arylamines and nitroarenes

Arylamines and nitroarenes are very important intermediates in the industrial manufacture of dyes, pesticides and plastics, and are significant environmental pollutants. The metabolic steps of N-oxidation and nitroreduction to yield N-hydroxyarylamines are crucial for the toxic properties of arylamines and nitroarenes. Nitroarenes are reduced by microorganisms in the gut or by nitroreductases and aldehyde dehydrogenase in hepatocytes to nitrosoarenes and N-hydroxyarylamines. N-Hydroxyarylamines can be further metabolized to N-sulphonyloxyarylamines, N-acetoxyarylamines or N-hydroxyarylamine N-glucuronide. These highly reactive intermediates are responsible for the genotoxic and cytotoxic effects of this class of compounds. N-Hydroxyarylamines can form adducts with DNA, tissue proteins, and the blood proteins albumin and haemoglobin in a dose-dependent manner. DNA and protein adducts have been used to biomonitor humans exposed to such compounds. All these steps are dependent on enzymes, which are present in polymorphic forms. This article reviews the metabolism of arylamines and nitroarenes and the biomonitoring studies performed in animals and humans exposed to these substances.

Oxidized, deaminated cytosines are a source of C --> T transitions in vivo

The most common base substitution arising from oxidative damage of DNA is a GC --> AT transition. In an effort to determine the oxidized lesion(s) that gives rise to this mutation, the mutagenicity of three oxidized cytosines, 5-hydroxycytosine, 5-hydroxyuracil, and uracil glycol, were investigated in Escherichia coli. An M13 viral genome was constructed to contain a single oxidized cytosine at a specific site. Replication in vivo of the single-stranded genomes yielded mutation frequencies of 0.05%, 83%, and 80% for 5-hydroxycytosine, 5-hydroxyuracil, and uracil glycol, respectively. The predominant mutation observed was C --> T. A model for C --> T oxidative mutagenesis is suggested in which initial cytosine oxidation is followed by deamination to a poorly repaired uracil derivative that is strongly miscoding during replication.

Mutational and DNA binding specificity of the carcinogen 2-amino-3, 8-dimethylimidazo[4,5-f]quinoxaline

The mutagenic specificity of 2-amino-3,8-dimethylimidazo[4, 5-f]quinoxaline (MeIQx), a food-borne mutagen and carcinogen, was studied. Plasmid pK19 was modified by photolysis with the 2-azido form of the carcinogen. High pressure liquid chromatography confirmed that the photoactivated azide formed primarily C8 and N2 guanyl adducts. Transformation of modified pK19 into excision repair competent Escherichia coli resulted in dose-dependent increases in genotoxicity and in mutagenesis within the lacZalpha target sequence. Upon induction of the SOS response, a 20-fold increase in mutation frequency over background was observed. A mutational spectrum for MeIQx, generated by sequencing 125 independent mutants, revealed base substitutions (41%), frameshifts (54%), and complex mutations (5.6%); >90% of the mutations occurred at G-C base pairs. Two hotspots were evident at runs of three or five G-C base pairs; approximately 60% of the mutations occurred at the hotspot sites. The hotspot at position 2532 produced mainly base substitutions, while that at position 2576 gave exclusively frameshift mutations. A polymerase inhibition assay mapped the sites of MeIQx adducts. Arrest sites were primarily at or one base 3' to a guanine residue, which correlated well with the distribution of mutations. No direct correlation was seen, however, between intensity of modification and hotspots for mutation.

Effects of DNA adduct structure and distribution on the mutagenicity and genotoxicity of two platinum anticancer drugs

cis-Diamminedichloroplatinum(II) (cis-DDP) and cis,trans,cis-ammine(cyclohexylamine)-dibutyratodichloroplatinu m(IV) (ACDDP) are anticancer drugs that bind to DNA, forming replication blocking adducts. ACDDP, probably manifests its cytotoxicity through the metabolite cis-ammine(cyclohexylamine)dichloroplatinum(II) (ACDP). The biological effects of ACDP and cis-DDP were compared by studying polymerase inhibition in vitro and mutagenesis and genotoxicity in vivo in the duplex genome of bacteriophage M13mp18 replicated in Escherichia coli. cis-DDP and ACDP adducts were equally genotoxic within the statistical error limits of the analysis. Survival of genomes platinated by either drug, increased by threefold in cells pretreated with u.v. irradiation to induce the SOS functions of the host. In the M13mp18 lacZ' gene fragment, the mutagenicity of ACDP was lower than that of cis-DDP; the difference was approximately twofold at a dose of two adducts per 370 base-pair mutational target. Mutagenesis by both compounds was SOS-dependent. The structural basis for lower mutagenicity of ACDP is proposed to be its reduced reactivity at d(ApG) sites. This effect is attributed to an orientational isomerism that precludes the formation of one of two possible DNA adducts at d(ApG) residues. The types of mutations induced for both drugs were similar, but they occurred with different distributions. Both compounds induced primarily G-->T transversions at d(GpG) sites whereas G-->A transitions and A-->T transversions, many at d(ApG), d(GpNpG), and d(GpG) sites, were also well represented. The mapping of DNA adducts by DNA synthesis inhibition revealed excellent correlation between the location of DNA lesions and the sites of mutations. Analysis of the distribution of mutations and the distribution of potential platination sites revealed no sequence-dependent mutation hotspots; i.e. mutagenesis was random throughout the lacZ' region of the M13mp18 bacteriophage genome. These results offer insights into the molecular mechanism of mutagenicity of platinum anticancer drugs.

The spectrum of TP53 mutations in bladder carcinoma

The mutational spectrum for the TP53 gene was investigated in a large series of bladder tumors and bladder tumor cell lines. Tumors and cell lines were screened for the presence of TP53 point mutations by single-strand conformational polymorphism analysis followed by direct sequencing. Mutations were detected in 16 of 88 (18%) tumors and 4 of 14 cell lines (28%). In total, twelve missense mutations, one nonsense mutation, three deletions, and two insertions were identified by direct sequencing. Of the thirteen point mutations sequenced, only one was a transition at a CpG site, whereas five G:C-->T:A transversions were found, suggesting a major role for exogenous mutagens in bladder tumorigenesis. Tumors were also examined for loss of heterozygosity (LOH) on chromosome arm 17p. LOH of one or more markers on 17p was detected in 31% of tumors. All eight tumors with a TP53 mutation from patients informative at TP53 had LOH, whereas nine tumors with LOH at TP53 did not have an identified mutation. Three tumors had LOH on 17p at sites distal to the TP53 locus but retained both TP53 alleles, suggesting the involvement of another tumor suppressor gene on 17p in bladder tumorigenesis in some tumors.

The action of 4-hydroxyaminobiphenyl in Escherichia coli: cytotoxic and mutagenic effects in DNA repair deficient strains

The cytotoxic and mutagenic effects of 4-hydroxyaminobiphenyl (N-OH-ABP) were studied using Escherichia coli strains with different repair capacities. N-OH-ABP was equally cytotoxic for uvrA and recA mutants as well as in wild-type cells while polA mutant strains proved particularly sensitive to its toxicity. In contrast, the mutation frequency in the uvrA strains tested was elevated to 30-400-fold the wild-type values. We suggest that aminobiphenyl-DNA adducts responsible for mutation are repaired by UVR endonuclease but different pathways exist for removal of DNA lesions responsible for bacterial killing. From the 32P-postlabeling analysis, it was concluded that ABP-DNA adducts can be relatively rapidly repaired in wild-type strains, while persisting in the uvrA strains.

Mutagenicity and genotoxicity of the major DNA adduct of the antitumor drug cis-diamminedichloroplatinum(II)

The mutagenicity and genotoxicity of cis-[Pt(NH3)2[d(GpG)-N7(1),-N7(2)]] (G*G*), the major DNA adduct of the antitumor drug cisplatin, has been investigated in Escherichia coli. A duplex bacteriophage M13 genome was constructed to contain the G*G* adduct at a specific site in the (-) strand. The singly platinated duplex genome exhibited a survival of 22% relative to that of the unplatinated control genomes, and this value rose to 38% in cells treated with ultraviolet light to induce the SOS response. Singly platinated single-stranded genomes were also produced. Replication of the single- and double-stranded genomes in vivo yielded SOS-dependent, targeted mutations at frequencies of 1.3% and 0.16%, respectively. The mutagenic specificity of G*G* in both single- and double-stranded DNA was striking in that 80-90% of the mutations occurred at the 5'-platinated G. Approximately 80% of the mutations were G-->T transversions at that site. A model of mutagenesis is presented to explain this mutational specificity with respect to current understanding of platinum-DNA adduct structure.

Products formed from the in vitro reaction of metabolites of 3-aminochrysene with calf thymus DNA

3-Aminochrysene, a mutagenic geometric isomer of the mutagenic and carcinogenic aromatic amine 6-aminochrysene, has been synthesized and its metabolic activation studied by characterization of the products formed from the reaction of metabolites with calf thymus DNA. DNA adducts produced by 3-aminochrysene via N-oxidation were examined by preparing 3-nitrosochrysene and incubating the nitroso derivative with calf thymus DNA in the presence of ascorbic acid (to generate the N-hydroxy derivative) at pH 5. The major adduct, as determined by 1H-NMR and thermospray-mass spectrometry of the modified nucleoside obtained after enzymatic hydrolysis of the modified DNA, was N-(deoxyguanosin-8-yl)-3-aminochrysene. Thus, the reaction of N-hydroxy-3-aminochrysene with DNA differs from that of N-hydroxy-6-aminochrysene, which had previously been shown to generate N-(deoxyguanosin-8-yl)-6-aminochrysene, 5-(deoxyguanosin-N2-yl)-6-aminochrysene and N-(deoxyinosin-8-yl)-6- aminochrysene as major adducts. 32P-Postlabeling analysis of DNA treated with 3-aminochrysene in the presence of liver microsomes from rats pretreated with phenobarbital indicated an adduct pattern identical to that seen with DNA that had been treated with 3-nitrosochrysene and ascorbic acid. However, DNA treated with 3-aminochrysene (3-AC) in the presence of liver microsomes from rats pretreated with 3-methylcholanthrene contained a major adduct that was chromatographically distinct from N-(deoxyguanosin-8-yl)-3-aminochrysene.