Comparison of the mutational spectra of the lacZ transgene in four organs of the MutaMouse treated with benzo[a]pyrene: target organ specificity

The functional mutational landscape of the lacZ gene

The lacZ gene of Escherichia coli encodes β-galactosidase (β-gal), a lactose metabolism enzyme of the lactose operon. Previous chemical modification or site-directed mutagenesis experiments have identified 21 amino acids that are essential for β-gal catalytic activity. We have assembled over 10,000 lacZ mutations from published studies that were collected using a positive selection assay to identify mutations in lacZ that disrupted β-gal function. We analyzed 6,465 independent lacZ mutations that resulted in 2,732 missense mutations that impaired β-gal function. Those mutations affected 492 of the 1,023 lacZ codons, including most of the 21 previously known residues critical for catalytic activity. Most missense mutations occurred near the catalytic site and in regions important for subunit tetramerization. Overall, our work provides a comprehensive and detailed map of the amino acid residues affecting the structure and catalytic activity of the β-gal enzyme.

Chemically induced mutations in a MutaMouse reporter gene inform mechanisms underlying human cancer mutational signatures

Transgenic rodent (TGR) models use bacterial reporter genes to quantify in vivo mutagenesis. Pairing TGR assays with next-generation sequencing (NGS) enables comprehensive mutation pattern analysis to inform mutational mechanisms. We used this approach to identify 2751 independent lacZ mutations in the bone marrow of MutaMouse animals exposed to four chemical mutagens: benzo[a]pyrene, N-ethyl-N-nitrosourea, procarbazine, and triethylenemelamine. We also collected published data for 706 lacZ mutations from eight additional environmental mutagens. We report that lacZ gene sequencing generates chemical-specific mutation signatures observed in human cancers with established environmental causes. For example, the mutation signature of benzo[a]pyrene, a carcinogen present in tobacco smoke, matched the signature associated with tobacco-induced lung cancers. Our results suggest that the analysis of chemically induced mutations in the lacZ gene shortly after exposure provides an effective approach to characterize human-relevant mechanisms of carcinogenesis and propose novel environmental causes of mutation signatures observed in human cancers.

Marc A. Beal, Matt J. Meier et al. use a transgenic rodent model with NGS to inform mutational signature analyses. They analyze lacZ mutations from the bone marrow of MutaMouse animals exposed to chemical mutagens. They report that lacZ sequencing generates chemical-specific mutation signatures observed in human cancers with established environmental causes.

Toxicokinetics of benzo[a]pyrene in humans: Extensive metabolism as determined by UPLC-accelerator mass spectrometry following oral micro-dosing

Benzo[a]pyrene (BaP), is a known human carcinogen (International Agency for Research on Cancer (IARC) class 1). The remarkable sensitivity (zepto-attomole ¹⁴C in biological samples) of accelerator mass spectrometry (AMS) makes possible, with de minimus risk, pharmacokinetic (PK) analysis following [¹⁴C]-BaP micro-dosing of humans. A 46 ng (5 nCi) dose was given thrice to 5 volunteers with minimum 2 weeks between dosing and plasma collected over 72 h. [¹⁴C]-BaP_eq PK analysis gave plasma T_max and C_max values of 1.25 h and 29-82 fg/mL, respectively. PK parameters were assessed by non- compartment and compartment models. Intervals between dosing ranged from 20 to 420 days and had little impact on intra-individual variation. DNA, extracted from peripheral blood mononuclear cells (PBMCs) of 4 volunteers, showed measurable levels (LOD ~ 0.5 adducts/10¹¹ nucleotides) in two individuals 2-3 h post-dose, approximately three orders of magnitude lower than smokers or occupationally-exposed individuals. Little or no DNA binding was detectable at 48-72 h. In volunteers the allelic variants CYP1B1^*1/*⁎1, ^*1/*3 or ^*3/*3 and GSTM1^*0/0 or ^*1 had no impact on [¹⁴C]-BaP_eq PK or DNA adduction with this very limited sample. Plasma metabolites over 72 h from two individuals (one CYP1B1^*1/*1 and one CYP1B1^*3/*3) were analyzed by UPLC-AMS. In both individuals, parent [¹⁴C]-BaP was a minor constituent even at the earliest time points and metabolite profiles markedly distinct. AMS, coupled with UPLC, could be used in humans to enhance the accuracy of pharmacokinetics, toxicokinetics and risk assessment of environmental carcinogens.

Mutagenic Replication of N2-Deoxyguanosine Benzo[a]pyrene Adducts by Escherichia coli DNA Polymerase I and Sulfolobus solfataricus DNA Polymerase IV

Benzo[a]pyrene, a potent human carcinogen, is metabolized in vivo to a diol epoxide that reacts with the N²-position of guanine to produce N²-BP-dG adducts. These adducts are mutagenic causing G to T transversions. These adducts block replicative polymerases but can be bypassed by the Y-family translesion synthesis polymerases. The mechanisms by which mutagenic bypass occurs is not well-known. We have evaluated base pairing structures using atomic substitution of the dNTP with two stereoisomers, 2'-deoxy-N-[(7R,8S,9R,10S)-7,8,9,10-tetrahydro-7,8,9-trihydroxybenzo[a]pyren-10-yl]guanosine and 2'-deoxy-N-[(7S,8R,9S,10R)-7,8,9,10-tetrahydro-7,8,9-trihydroxybenzo[a]pyren-10-yl]guanosine. We have examined the kinetics of incorporation of 1-deaza-dATP, 7-deaza-dATP, 2'-deoxyinosine triphosphate, and 7-deaza-dGTP, analogues of dATP and dGTP in which single atoms are changed. Changes in rate will occur if that atom provided a critical interaction in the transition state of the reaction. We examined two polymerases, Escherichia coli DNA polymerase I (Kf) and Sulfolobus solfataricus DNA polymerase IV (Dpo4), as models of a high fidelity and TLS polymerase, respectively. We found that with Kf, substitution of the nitrogens on the Watson-Crick face of the dNTPs resulted in decreased rate of reactions. This result is consistent with a Hoogsteen base pair in which the template N²-BP-dG flipped from the anti to syn conformation. With Dpo4, while the substitution did not affect the rate of reaction, the amplitude of the reaction decreased with all substitutions. This result suggests that Dpo4 bypasses N²-BP-dG via Hoogsteen base pairs but that the flipped nucleotide can be either the dNTP or the template.

Mitochondrial DNA exhibits resistance to induced point and deletion mutations

The accumulation of somatic mitochondrial DNA (mtDNA) mutations contributes to the pathogenesis of human disease. Currently, mitochondrial mutations are largely considered results of inaccurate processing of its heavily damaged genome. However, mainly from a lack of methods to monitor mtDNA mutations with sufficient sensitivity and accuracy, a link between mtDNA damage and mutation has not been established. To test the hypothesis that mtDNA-damaging agents induce mtDNA mutations, we exposed Muta^TMMouse mice to benzo[a]pyrene (B[a]P) or N-ethyl-N-nitrosourea (ENU), daily for 28 consecutive days, and quantified mtDNA point and deletion mutations in bone marrow and liver using our newly developed Digital Random Mutation Capture (dRMC) and Digital Deletion Detection (3D) assays. Surprisingly, our results demonstrate mutagen treatment did not increase mitochondrial point or deletion mutation frequencies, despite evidence both compounds increase nuclear DNA mutations and demonstrated B[a]P adduct formation in mtDNA. These findings contradict models of mtDNA mutagenesis that assert the elevated rate of mtDNA mutation stems from damage sensitivity and abridged repair capacity. Rather, our results demonstrate induced mtDNA damage does not readily convert into mutation. These findings suggest robust mitochondrial damage responses repress induced mutations after mutagen exposure.

Characterizing Benzo[a]pyrene-induced lacZ mutation spectrum in transgenic mice using next-generation sequencing

BACKGROUND

The transgenic rodent mutation reporter assay provides an efficient approach to identify mutagenic agents in vivo. A major advantage of this assay is that mutant reporter transgenes can be sequenced to provide information on the mode of action of a mutagen and to identify clonally expanded mutations. However, conventional DNA sequence analysis is laborious and expensive for long transgenes, such as lacZ (3096 bp), and is not normally implemented in routine screening.

METHODS

We developed a high-throughput next-generation sequencing (NGS) approach to simultaneously sequence large numbers of barcoded mutant lacZ transgenes from different animals. We collected 3872 mutants derived from the bone marrow DNA of six Muta™Mouse males exposed to the well-established mutagen benzo[a]pyrene (BaP) and six solvent-exposed controls. Mutants within animal samples were pooled, barcoded, and then sequenced using NGS.

RESULTS

We identified 1652 mutant sequences from 1006 independent mutations that underwent clonal expansion. This deep sequencing analysis of mutation spectrum demonstrated that BaP causes primarily guanine transversions (e.g. G:C → T:A), which is highly consistent with previous studies employing Sanger sequencing. Furthermore, we identified novel mutational hotspots in the lacZ transgene that were previously uncharacterized by Sanger sequencing. Deep sequencing also allowed for an unprecedented ability to correct for clonal expansion events, improving the sensitivity of the mutation reporter assay by 50 %.

CONCLUSION

These results demonstrate that the high-throughput nature and reduced costs offered by NGS provide a sensitive and fast approach for elucidating and comparing mutagenic mechanisms of various agents among tissues and enabling improved evaluation of genotoxins.

Benzo pyrene-induced DNA adducts and gene expression profiles in target and non-target organs for carcinogenesis in mice

BACKGROUND

Gene expression changes induced by carcinogens may identify differences in molecular function between target and non-target organs. Target organs for benzo[a]pyrene (BaP) carcinogenicity in mice (lung, spleen and forestomach) and three non-target organs (liver, colon and glandular stomach) were investigated for DNA adducts by 32P-postlabelling, for gene expression changes by cDNA microarray and for miRNA expression changes by miRNA microarray after exposure of animals to BaP.

RESULTS

BaP-DNA adduct formation occurred in all six organs at levels that did not distinguish between target and non-target. cDNA microarray analysis showed a variety of genes modulated significantly by BaP in the six organs and the overall gene expression patterns were tissue specific. Gene ontology analysis also revealed that BaP-induced bioactivities were tissue specific; eight genes (Tubb5, Fos, Cdh1, Cyp1a1, Apc, Myc, Ctnnb1 and Cav) showed significant expression difference between three target and three non-target organs. Additionally, several gene expression changes, such as in Trp53 activation and Stat3 activity suggested some similarities in molecular mechanisms in two target organs (lung and spleen), which were not found in the other four organs. Changes in miRNA expression were generally tissue specific, involving, in total, 21/54 miRNAs significantly up- or down-regulated.

CONCLUSIONS

Altogether, these findings showed that DNA adduct levels and early gene expression changes did not fully distinguish target from non-target organs. However, mechanisms related to early changes in p53, Stat3 and Wnt/β-catenin pathways may play roles in defining BaP organotropism.

Benzo[a]pyrene, aflatoxine B₁ and acetaldehyde mutational patterns in TP53 gene using a functional assay: relevance to human cancer aetiology

Mutations in the TP53 gene are the most common alterations in human tumours. TP53 mutational patterns have sometimes been linked to carcinogen exposure. In hepatocellular carcinoma, a specific G>T transversion on codon 249 is classically described as a fingerprint of aflatoxin B₁ exposure. Likewise G>T transversions in codons 157 and 158 have been related to tobacco exposure in human lung cancers. However, controversies remain about the interpretation of TP53 mutational pattern in tumours as the fingerprint of genotoxin exposure. By using a functional assay, the Functional Analysis of Separated Alleles in Yeast (FASAY), the present study depicts the mutational pattern of TP53 in normal human fibroblasts after in vitro exposure to well-known carcinogens: benzo[a]pyrene, aflatoxin B₁ and acetaldehyde. These in vitro patterns of mutations were then compared to those found in human tumours by using the IARC database of TP53 mutations. The results show that the TP53 mutational patterns found in human tumours can be only partly ascribed to genotoxin exposure. A complex interplay between the functional impact of the mutations on p53 phenotype and the cancer natural history may affect these patterns. However, our results strongly support that genotoxins exposure plays a major role in the aetiology of the considered cancers.

Polycyclic aromatic hydrocarbons and digestive tract cancers: a perspective

Cancers of the colon are most common in the Western world. In majority of these cases, there is no familial history and sporadic gene damage seems to play an important role in the development of tumors in the colon. Studies have shown that environmental factors, especially diet, play an important role in susceptibility to gastrointestinal (GI) tract cancers. Consequently, environmental chemicals that contaminate food or diet during preparation become important in the development of GI cancers. Polycyclic aromatic hydrocarbons (PAHs) are one such family of ubiquitous environmental toxicants. These pollutants enter the human body through consumption of contaminated food, drinking water, inhalation of cigarette smoke, automobile exhausts, and contaminated air from occupational settings. Among these pathways, dietary intake of PAHs constitutes a major source of exposure in humans. Although many reviews and books on PAHs and their ability to cause toxicity and breast or lung cancer have been published, aspects on contribution of diet, smoking and other factors toward development of digestive tract cancers, and strategies to assess risk from exposure to PAHs have received much less attention. This review, therefore, focuses on dietary intake of PAHs in humans, animal models, and cell cultures used for GI cancer studies along with epidemiological findings. Bioavailability and biotransformation processes, which influence the disposition of PAHs in body and the underlying causative mechanisms of GI cancers, are also discussed. The existing data gaps and scope for future studies is also emphasized. This information is expected to stimulate research on mechanisms of sporadic GI cancers caused by exposure to environmental carcinogens.

Enhanced Spontaneous and Benzo(a)pyrene-Induced Mutations in the Lung of Nrf2-Deficient gpt Delta Mice

The lung is an organ that is sensitive to mutations induced by chemicals in ambient air, and transgenic mice harboring guanine phosphoribosyltransferase (gpt) gene as a target gene are a well-established model system for assessing genotoxicity in vivo. Transcription factor Nrf2 mediates inducible and constitutive expression of cytoprotective enzymes against xenobiotics and mutagens. To address whether Nrf2 is also involved in DNA protection, we generated nrf2+/−::gpt and nrf2−/−::gpt mice. The spontaneous mutation frequency of the gpt gene in the lung was approximately three times higher in nrf2-null (nrf2−/−) mice than nrf2 heterozygous (nrf2+/−) and wild-type (nrf2+/+) mice, whereas in the liver, the mutation frequency was higher in nrf2−/− and nrf2+/− mice than in nrf2+/+ wild-type mice. By contrast, no difference in mutation frequency was observed in testis among the three genotypes. A single intratracheal instillation of benzo(a)pyrene (BaP) increased the lung mutation frequency 3.1- and 6.1-fold in nrf2+/− and nrf2−/− mice, respectively, compared with BaP-untreated nrf2+/− mice, showing that nrf2−/− mice are more susceptible to genotoxic carcinogens. Surprisingly, mutation profiles of the gpt gene in BaP-treated nrf2+/− mice was substantially different from that in BaP-untreated nrf2−/− mice. In nrf2−/− mice, spontaneous and BaP-induced mutation hotspots were observed at nucleotides 64 and 140 of gpt, respectively. These results thus show that Nrf2 aids in the prevention of mutations in vivo and suggest that Nrf2 protects genomic DNA against certain types of mutations. [Cancer Res 2007;67(12):5643–8]

Oxidized metabolites from benzo[a]pyrene, benzo[e]pyrene, and aza-benzo[a]pyrenes. A computational study of their carbocations formed by epoxide ring opening reactions

A DFT study aimed at understanding structure-reactivity relationships and fluorine substitution effects on carbocation stability in benzo[a]pyrene (BaP), benzo[e]pyrene (BeP), and aza-benzo[a]pyrene (aza-BaP) derivatives are reported. The relative energies of the resulting carbocations are examined and compared, taking into account the available biological activity data on these compounds. O-Protonation of the epoxides and diol epoxides leads to carbocation formation by barrierless processes. Charge delocalization modes in the resulting carbocations were deduced via NPA-derived changes in charges, and fluorine substitution effects were analyzed on the basis of charge density at different carbocation positions. Thus, fluorine substitution at sites bearing negative charge generated inductive destabilization of the carbocation, whereas a fluorine atom at a ring position which presented significant positive charge density produced a less pronounced destabilization due to fluorine p-pi back-bonding. Protonation reactions were also studied for the azaBaPs. In selected cases, the covalent adducts generated via bond formation with the exocyclic nitrogen of cytosine were computed and relative energies and geometries of the resulting adducts were examined.

Co-mutagenic activity of arsenic and benzo[a]pyrene in mouse skin

Exposure to inorganic arsenic in drinking water is linked to skin, lung and bladder cancer in humans. The mechanism of arsenic-induced cancer is not clear, but exposure to arsenic and polycyclic arylhydrocarbons (PAH) is more carcinogenic than exposure to either type of carcinogen alone. Arsenic can also generate reactive oxygen species, suggesting that oxidation of DNA may play a role in carcinogenesis. Oxidization of guanosines in polyG tracts is known to cause frameshift mutations, and such events can be detected in situ using the G11 placental alkaline phosphatase (PLAP) transgenic mouse model, which reports frameshift mutations in a run of 11 G:C basepairs by generating cells containing heat-resistant alkaline phosphatase activity. PAH can also induce frameshift mutations. In the study described here, FVB/N mice carrying the G11 PLAP transgene were crossed to C57Bl/6 mice. Half of the hybrid mice were given drinking water with sodium arsenite (10 mg/L) for 10 weeks. Half of the arsenic treated mice were also exposed to benzo[a]pyrene (BaP) by skin painting (500 nmol/week) for 8 weeks. Another group of mice was exposed to BaP but not arsenic. The effect on frameshift mutation was assessed by staining sections of skin tissue to detect cells with PLAP activity. Arsenic alone had no significant effect. On average, mice given BaP alone had approximately three times more PLAP-positive (PLAP+) cells. By contrast, mice exposed to both arsenic and BaP exhibited 10-fold more PLAP+ cells in the skin, and these cells were often arranged in large clusters, suggesting derivation from stem cells. Whereas combined treatment produced more PLAP+ cells, stable BaP adduct levels and arsenic burdens were not higher in mice exposed to both agents compared to mice exposed to either one agent or the other.

Detailed review of transgenic rodent mutation assays

Induced chromosomal and gene mutations play a role in carcinogenesis and may be involved in the production of birth defects and other disease conditions. While it is widely accepted that in vivo mutation assays are more relevant to the human condition than are in vitro assays, our ability to evaluate mutagenesis in vivo in a broad range of tissues has historically been quite limited. The development of transgenic rodent (TGR) mutation models has given us the ability to detect, quantify, and sequence mutations in a range of somatic and germ cells. This document provides a comprehensive review of the TGR mutation assay literature and assesses the potential use of these assays in a regulatory context. The information is arranged as follows. (1) TGR mutagenicity models and their use for the analysis of gene and chromosomal mutation are fully described. (2) The principles underlying current OECD tests for the assessment of genotoxicity in vitro and in vivo, and also nontransgenic assays available for assessment of gene mutation, are described. (3) All available information pertaining to the conduct of TGR assays and important parameters of assay performance have been tabulated and analyzed. (4) The performance of TGR assays, both in isolation and as part of a battery of in vitro and in vivo short-term genotoxicity tests, in predicting carcinogenicity is described. (5) Recommendations are made regarding the experimental parameters for TGR assays, and the use of TGR assays in a regulatory context.

In vivo mutagenesis induced by benzo[a]pyrene instilled into the lung of gpt delta transgenic mice

Benzo[a]pyrene (B[a]P) is a ubiquitous airborne pollutant whose mutagenicity has been evaluated previously by oral and intraperitoneal administration to experimental animals. In this study, mutagenesis in the lungs, the target organ of air pollutants, was examined after a single intratracheal instillation of 0-2 mg B[a]P into gpt delta transgenic mice. Intratracheal injection of B[a]P resulted in a statistically significant and dose-dependent increase in gpt mutant frequency as measured by 6-thioguanine selection. The mutant frequencies at B[a]P doses of 0.5, 1, and 2 mg were 2.8, 4.2, and 6.8 times higher than the frequency seen in nontreated mice (0.60 +/- 0.13 x 10(-5)). The most frequent mutations induced by B[a]P treatment were G:C-->T:A transversions, which are characteristic of B[a]P mutagenesis in other models, and single-base deletions of G:C base pairs. To characterize the hotspots of B[a]P-induced mutations in the gpt gene, we analyzed sequences adjacent to the mutated G:C base pairs. Guanine bases centered in the nucleotide sequences CGT, CGA, and CGG were the most frequent targets of B[a]P. Our results indicate that intratracheal instillation of B[a]P into gpt delta mice causes a dose-dependent increase in gpt mutant frequency in the lung, and that the predominant mutation induced is G:C-->T:A transversion.

Approaches to carcinogenic risk assessment for polycyclic aromatic hydrocarbons: a UK perspective

This paper reviews the approaches to carcinogenic risk assessment of polycyclic aromatic hydrocarbons (PAHs) in air pollution with emphasis on high potency PAHs such as dibenzo[a,l]pyrene (DB[a,l]P). The potency of DB[a,l]P may be 100-fold greater than benzo[a]pyrene (B[a]P); thus the B[a]P surrogate approach currently used to monitor for compliance with UK air pollution standards may not be appropriate. It is suggested that an approach based on potency equivalency factors (PEFs) could be developed to include highly potent PAHs provided an appropriate reference data set for relevant PAHs using a route acceptable for inhalation risk assessment is selected. Available data suggest that intratracheal administration of low doses of PAHs to rats is likely to simulate the kinetics of inhalation exposure to PAHs in a feasible manner. The use of a measure of total DNA adducts as an endpoint, which correlates well with lung tumourigenicity, would provide surrogate data for setting PEFs without the need for long-term bioassays in rodents. Further, dose-response studies using intratracheal administration of a range of PAHs singly and in combination to assess additivity are required to develop a PEF system for inhalation PEFs derived from DNA adduct measurements.

Novel transgenic rat for in vivo genotoxicity assays using 6-thioguanine and Spi- selection

Transgenic rodents are valuable models for investigating the genotoxicity of chemicals in vivo. Here, we report the establishment of a novel transgenic rat for genotoxicity analysis. In this model, about 10 copies of lambdaEG10 DNA carrying the gpt gene of E. coli and the red/gam genes of lambda phage are integrated per haploid genome of Sprague-Dawley rats at position 4q24-q31. After recovery of lambdaEG10 phage, point mutations in the gpt gene and deletions in the red/gam genes are identified by 6-thioguanine and Spi(-) selection, respectively. To examine the suitability of these rats for performing in vivo mutagenicity assays, rats were treated with single intraperitoneal injections of ethylnitrosourea (ENU; 100 mg/kg) or benzo[a]pyrene (B[a]P; 62.5 and 125 mg/kg), and the mutant frequencies (MFs) in the liver were determined 7 days after the treatment. ENU enhanced the gpt MF about 7-fold over the control while it did not significantly increase the Spi(-) MF. B[a]P increased both the gpt and Spi(-) MFs several-fold in a dose-dependent manner. To examine the kinetics of MF, ENU was administered (50 mg/kg/day for 5 successive days) and gpt MFs in the liver were determined 7, 21, 35, and 70 days after the last injection. The MF increased to 8-fold and 13-fold over the control at 7 and 35 days, respectively, after the last injection and then slightly declined at 70 days. These kinetics are similar to those reported for ENU-treated lacZ transgenic mice. This novel transgenic rat could be useful for investigating species differences between rats and mice in their response to genotoxic agents.

Effect of 10-aza-substitution on benzo[a]pyrene mutagenicity in vivo and in vitro

Benzo[a]pyrene (BaP), an environmental carcinogen, shows genotoxicity after metabolic transformation into the bay-region diol epoxide, BaP-7,8-diol 9,10-epoxide. 10-Azabenzo[a]pyrene (10-azaBaP), in which a ring nitrogen is located in the bay-region, is also a carcinogen and shows mutagenicity in the Ames test in the presence of the rat liver microsomal enzymes. In order to evaluate the effect of aza-substitution on in vivo genotoxicity, BaP and 10-azaBaP were assayed for their in vivo mutagenicity using the lacZ-transgenic mouse (MutaMouse). BaP was potently mutagenic in all of the organs examined (liver, lung, kidney, spleen, forestomach, stomach, colon, and bone marrow), as described in our previous report, whereas, 10-azaBaP was slightly mutagenic only in the liver and colon. The in vitro mutagenicities of BaP and 10-azaBaP were evaluated by the Ames test using liver homogenates prepared from several sources, i.e. CYP1A-inducer-treated rats, CYP1A-inducer-treated and non-treated mice, and humans. BaP showed greater mutagenicities than 10-azaBaP in the presence of a liver homogenate prepared from CYP1A-inducer-treated rodents. However, 10-azaBaP showed mutagenicities similar to or more potent than BaP in the presence of a liver homogenate or S9 from non-treated mice and humans. These results indicate that 10-aza-substitution markedly modifies the nature of mutagenicity of benzo[a]pyrene in both in vivo and in vitro mutagenesis assays.

Mutational spectra of benzo[a]pyrene and MeIQx in rpsL transgenic zebrafish embryos

To evaluate the rpsL transgenic zebrafish (Brachydanio rerio) mutation assay, we treated the embryos with benzo[a]pyrene (B[a]P) (10 microg/ml) or 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) (300 microg/ml) for 16h and determined the mutation spectra. These treatments were previously reported to induce mutant frequencies that were 4.3 and 2.4 times the control value, respectively. In the B[a]P-treated group, half of the mutations were single base substitutions, 74% of which occurred at G:C base pairs. Among G:C base pair substitutions, G:C to T:A and G: C to C:G transversions were predominant, suggesting that B[a]P induced mutations in zebrafish embryos by mechanisms previously described in mammalian tissues. In the MeIQx-treated group, about 60% of the mutations were deletions. Some specific mutations were found, but the compound primarily amplified the background mutation level; improvement in the conditions of treatment may be required for elucidating MeIQx-mutagenesis in this system. This study showed that transgenic zebrafish may be a useful tool for detecting mutagens in aquatic environments and for elucidating mutagenic mechanisms.