Assessment of 1,3-butadiene mutagenicity in the bone marrow of B6C3F1 lacI transgenic mice (Big Blue): a review of mutational spectrum and lacI mutant frequency after a 5-day 625 ppm 1,3-butadiene exposure

Evaluation of in vivo mutagenesis for assessing the health risk of air pollutants

Various kind of chemical substances, including man-made chemical products and unintended products, are emitted to ambient air. Some of these substances have been shown to be mutagenic and therefore to act as a carcinogen in humans. National pollutant inventories (e.g., Pollutant Release and Transfer Registration in Japan) have estimated release amounts of man-made chemical products, but a major concern is the release of suspended particulate matter containing potent mutagens, for example, polycyclic aromatic hydrocarbons and related compounds generated by the combustion of fossil fuel, which are not estimated by PRTR system. In situ exposure studies have revealed that DNA adducts in the lung, and possibly mutations in germline cells are induced in rodents by inhalation of ambient air, indicating that evaluating in vivo mutations is important for assessing environmental health risks. Transgenic rodent systems (Muta, Big Blue, and gpt delta) are good tools for analyzing in vivo mutations induced by a mixture of chemical substances present in the environment. Following inhalation of diesel exhaust (used as a model mixture), mutation frequency was increased in the lung of gpt delta mice and base substitutions were induced at specific guanine residues (mutation hotspots) on the target transgenes. Mutation hotspots induced by diesel exhaust were different from those induced by benzo[a]pyrene, a typical mutagen in ambient air, but nearly identical to those induced by 1,6-dinitropyrene contained in diesel exhaust. Comparison between mutation hotspots in the TP53 (p53) gene in human lung cancer (data extracted from the IARC TP53 database) and mutations we identified in gpt delta mice showed that G to A transitions centered in CGT and CGG trinucleotides were mutation hotspots on both TP53 genes in human lung cancers and gpt genes in transgenic mice that inhaled diesel exhaust. The carcinogenic potency (TD50 value) of genotoxic carcinogen was shown to be correlated with the in vivo mutagenicity (total dose per increased mutant frequency). These results suggest that the mutations identified in transgenic rodents can help identify environmental mutagens that cause cancer.

Polymerase Bypass of N(6)-Deoxyadenosine Adducts Derived from Epoxide Metabolites of 1,3-Butadiene

N(6)-(2-Hydroxy-3-buten-1-yl)-2'-deoxyadenosine (N(6)-HB-dA I) and N(6),N(6)-(2,3-dihydroxybutan-1,4-diyl)-2'-deoxyadenosine (N(6),N(6)-DHB-dA) are exocyclic DNA adducts formed upon alkylation of the N(6) position of adenine in DNA by epoxide metabolites of 1,3-butadiene (BD), a common industrial and environmental chemical classified as a human and animal carcinogen. Since the N(6)-H atom of adenine is required for Watson-Crick hydrogen bonding with thymine, N(6)-alkylation can prevent adenine from normal pairing with thymine, potentially compromising the accuracy of DNA replication. To evaluate the ability of BD-derived N(6)-alkyladenine lesions to induce mutations, synthetic oligodeoxynucleotides containing site-specific (S)-N(6)-HB-dA I and (R,R)-N(6),N(6)-DHB-dA adducts were subjected to in vitro translesion synthesis in the presence of human DNA polymerases β, η, ι, and κ. While (S)-N(6)-HB-dA I was readily bypassed by all four enzymes, only polymerases η and κ were able to carry out DNA synthesis past (R,R)-N(6),N(6)-DHB-dA. Steady-state kinetic analyses indicated that all four DNA polymerases preferentially incorporated the correct base (T) opposite (S)-N(6)-HB-dA I. In contrast, hPol β was completely blocked by (R,R)-N(6),N(6)-DHB-dA, while hPol η and κ inserted A, G, C, or T opposite the adduct with similar frequency. HPLC-ESI-MS/MS analysis of primer extension products confirmed that while translesion synthesis past (S)-N(6)-HB-dA I was mostly error-free, replication of DNA containing (R,R)-N(6),N(6)-DHB-dA induced significant numbers of A, C, and G insertions and small deletions. These results indicate that singly substituted (S)-N(6)-HB-dA I lesions are not miscoding, but that exocyclic (R,R)-N(6),N(6)-DHB-dA adducts are strongly mispairing, probably due to their inability to form stable Watson-Crick pairs with dT.

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.

Mutagenicity testing with transgenic mice. Part II: Comparison with the mouse spot test

The mouse spot test, an in vivo mutation assay, has been used to assess a number of chemicals. It is at present the only in vivo mammalian test system capable of detecting somatic gene mutations according to OECD guidelines (OECD guideline 484). It is however rather insensitive, animal consuming and expensive type of test. More recently several assays using transgenic animals have been developed. From data in the literature, the present study compares the results of in vivo testing of over twenty chemicals using the mouse spot test and compares them with results from the two transgenic mouse models with the best data base available, the lacI model (commercially available as the Big Blue(R) mouse), and the lacZ model (commercially available as the Mutatrade mark Mouse). There was agreement in the results from the majority of substances. No differences were found in the predictability of the transgenic animal assays and the mouse spot test for carcinogenicity. However, from the limited data available, it seems that the transgenic mouse assay has several advantages over the mouse spot test and may be a suitable test system replacing the mouse spot test for detection of gene but not chromosome mutations in vivo.

Mutagenicity testing with transgenic mice. Part I: Comparison with the mouse bone marrow micronucleus test

As part of a larger literature study on transgenic animals in mutagenicity testing, test results from the transgenic mutagenicity assays (lacI model; commercially available as the Big Blue(R) mouse, and the lacZ model; commercially available as the Mutatrade markMouse), were compared with the results on the same substances in the more traditional mouse bone marrow micronucleus test. 39 substances were found which had been tested in the micronucleus assay and in the above transgenic mouse systems. Although, the transgenic animal mutation assay is not directly comparable with the micronucleus test, because different genetic endpoints are examined: chromosome aberration versus gene mutation, the results for the majority of substances were in agreement. Both test systems, the transgenic mouse assay and the mouse bone marrow micronucleus test, have advantages and they complement each other. However, the transgenic animal assay has some distinct advantages over the micronucleus test: it is not restricted to one target organ and detects systemic as well as local mutagenic effects.

Mutagenic spectrum of butadiene-derived N1-deoxyinosine adducts and N6,N6-deoxyadenosine intrastrand cross-links in mammalian cells

Reactive metabolites of 1,3-butadiene, including 1,2-epoxy-3-butene (BDO), 1,2:3,4-diepoxybutane (BDO(2)), and 3,4-epoxy-1,2-butanediol (BDE), form both stable and unstable base adducts in DNA and have been implicated in producing genotoxic effects in rodents and human cells. N1 deoxyadenosine adducts are unstable and can undergo either hydrolytic deamination to yield N1 deoxyinosine adducts or Dimroth rearrangement to yield N(6) adducts. The dominant point mutation observed at AT sites in both in vivo and in vitro mutagenesis studies using BD and its epoxides has been A --> T transversions followed by A --> G transitions. To understand which of the butadiene adducts are responsible for mutations at AT sites, the present study focuses on the N1 deoxyinosine adduct at C2 of BDO and N(6),N(6)-deoxyadenosine intrastrand cross-links derived from BDO(2). These lesions were incorporated site-specifically and stereospecifically into oligodeoxynucleotides which were engineered into mammalian shuttle vectors for replication bypass and mutational analyses in COS-7 cells. Replication of DNAs containing the R,R-BDO(2) intrastrand cross-link between N(6) positions of deoxyadenosine yielded a high frequency (59%) of single base substitutions at the 3' adducted base, while 19% mutagenesis was detected using the S,S-diastereomer. Comparable studies using the R- and S-diastereomers of the N1 deoxyinosine adduct gave rise to approximately 50 and 80% A --> G transitions with overall mutagenic frequencies of 59 and 90%, respectively. Collectively, these data establish a molecular basis for A --> G transitions that are observed following in vivo and in vitro exposures to BD and its epoxides, but fail to reveal the source of the A --> T transversions that are the dominant point mutation.

Mutational spectrum of 1,3-butadiene and metabolites 1,2-epoxybutene and 1,2,3,4-diepoxybutane to assess mutagenic mechanisms

1,3-Butadiene (BD) is a multisite carcinogen and is mutagenic in multiple tissues of B6C3F1 mice. BD is bioactivated to at least three directly mutagenic metabolites: 1,2-epoxybutene (EB), 1,2-epoxy-3,4-butanediol (EBD), and 1,2,3,4-diepoxybutane (DEB). However, the contribution of these individual metabolites to the carcinogenicity and in vivo mutatidnal spectrum of BD is uncertain. To assess the role of two BD metabolites EB and DEB in the in vivo mutagenicity of the parent compound BD, we examined the in vitro mutational spectra of EB and DEB in human and rodent cells. We also examined the in vivo mutagenicity and mutational spectrum of inhaled EB in the lung. In the bone marrow and spleen of B6C3F1 laci transgenic mice, BD-induced an increased frequency of the identical class of point mutations at A:T base pairs: AT-->GC transitions and AT-->TA transversions. BD exposure also induced an increased frequency of GC-->AT transitions in the spleen that was not observed in bone marrow, demonstrating tissue-specific differences in mutation spectrum. Exposure of Rat2 laci transgenic cells and human TK6 lymphoblasts to EB-induced an increased frequency of AT-->TA transversions. DEB exposure induced an increased frequency of AT-->TA transversions and partial deletions at hprt in human cells. In Rat laci transgenic cells, DEB was not mutagenic at laci but induced an increased frequency of micronuclei. In contrast to inhaled BD, inhaled DEB and EB were not mutagenic in the bone marrow or spleen. However, EB was mutagenic in the lungs. In the lung of mice, EB-induced specific increases in GC-->AT transitions, AT-->TA transversions, and deletion events. AT-->TA transversions are the most consistent mutation observed across biological systems following in vivo exposure to BD or in vitro exposures to EB and DEB. Although, BD exposure in mice induces chromosomal alterations and single base substitutions, the specific BD metabolite that induces the genetic events leading to tumors is uncertain. At present, it appears that only DEB can effectively induce this range of mutagenic events at levels of this metabolite that occur in the blood of mice exposed to BD. Detailed investigations to identify relevant biomarkers of BD exposure and response, particularly DNA adducts or lesions, that can be biologically linked to the range of genotoxic events known to occur in mice exposed to BD are needed.

Synthesis and characterization of nucleosides and oligonucleotides bearing adducts of butadiene epoxides on adenine n(6) and guanine n(2)

Butadiene is a major industrial chemical whose genotoxic effects are attributed to the reaction of its oxidized metabolites, butadiene monoepoxide (BDO) and butadiene diepoxide (BDO2), with DNA. Nucleosides and oligonucleotides containing regio- and stereochemically specific adducts of BDO and the BDO2-related compound, butene 3,4-diol 1,2-epoxide (BDE), on guanine [(2R)- and (2S)-N(2)-(1-hydroxy-3-buten-2-yl) and (2R,3R)- and (2S,3S)-N(2)-(2,3,4-trihydroxybut-1-yl), respectively] and on adenine [(2R)- and (2S)-N(6)-(1-hydroxy-3-buten-2-yl) and (2R,3R)- and (2S,3S)-N(6)-(2,3,4-trihydroxybut-1-yl), respectively] have been prepared by nonbiomimetic routes. For guanine adducts, 2-fluoro-O(6)-(trimethylsilylethyl)-2'-deoxyinosine was treated with (2R)- and (2S)-2-amino-3-buten-1-ol to give the BDO adducts and with (2R,3R)- and (2S,3S)-1-amino-2,3,4-butanetriol to produce the BDE adducts; the adducted oligonucleotides were prepared from 11-mer oligonucleotides containing the halopurine. Adenine adducts were prepared in a similar fashion using 6-chloropurine 2'-deoxyriboside as the reactive purine component.

Lung-specific mutagenicity and mutational spectrum in B6C3F1 lacI transgenic mice following inhalation exposure to 1,2-epoxybutene

1,3-Butadiene (BD) is carcinogenic and mutagenic in B6C3F1 mice. BD inhalation induces an increased frequency of specific base substitution mutations in the bone marrow and spleen of B6C3F1 lacI transgenic mice. BD is bioactivated to at least three mutagenic metabolites: 1,2-epoxybutene (EB), 1,2-epoxy-3,4-butanediol (EBD), and 1,2,3,4-diepoxybutane (DEB), however, the contribution of these individual metabolites to the in vivo mutational spectrum of BD is uncertain. In the present study, lacI transgenic mice were exposed by inhalation (6h per day, 5 days per week for 2 weeks) to 0 or 29.9ppm of the BD metabolite, EB to assess its contribution to the in vivo mutational spectrum of BD. No increase in lacI mutant frequency was observed in the bone marrow or spleen of EB-exposed mice. The lack of mutagenicity in the bone marrow or spleen likely relate to insufficient levels of EB reaching these tissues. The lacI mutant frequency was increased 2.7-fold in the lungs of EB-exposed mice (mean+/-S.D., 9.9+/-3.0x10(-5)) compared to air control mice (3.6+/-0.7x10(-5)). DNA sequence analysis of 65 and 66 mutants from the lungs of air control and EB-exposed mice, respectively, revealed an increase in the frequency of two categories of base substitution mutation and deletions. Like mice exposed to BD, EB-exposed mice had an increased frequency of A:T-->T:A transversions. However, in contrast to the BD mutational spectra, G:C-->A:T transitions at 5'-CpG-3' sequences, occurred with increased frequency in the EB-exposed mice. The increased frequency of deletions as well as the induction of two tandem mutations and a tandem deletion in the lungs of EB-exposed mice are also inconsistent with previous mutational spectra from BD-exposed mice or EB-exposed cells in culture. We hypothesize that the direct in vivo mutagenicity and further in situ metabolism of EB in the lungs of EB-exposed mice played a prominent role in the generation of the current mutational spectrum.

Recent advances in the protocols of transgenic mouse mutation assays

Transgenic mutation assays were developed to detect gene mutations in multiple organs of mice or rats. The assays permit (1) quantitative measurements of mutation frequencies in all tissues/organs including germ cells and (2) molecular analysis of induced and spontaneous mutations by DNA sequencing analysis. The protocols of recently developed selections in the lambda phage-based transgenic mutation assays, i.e. cII, Spi(-) and 6-thioguanine selections, are described, and a data set of transgenic mutation assays, including those using Big Blue and Muta Mouse, is presented.

Butadiene-induced intrastrand DNA cross-links: a possible role in deletion mutagenesis

To initiate studies designed to identify the mutagenic spectrum associated with butadiene diepoxide-induced N(2)-N(2) guanine intrastrand cross-links, site specifically adducted oligodeoxynucleotides were synthesized in which the adducted bases were centrally located within the context of the human ras 12 codon. The two stereospecifically modified DNAs and the corresponding unmodified DNA were ligated into a single-stranded M13mp7L2 vector and transfected into Escherichia coli. Both stereoisomeric forms (R, R and S,S) of the DNA cross-links resulted in very severely decreased plaque-forming ability, along with an increased mutagenic frequency for both single base substitutions and deletions compared with unadducted DNAs, with the S,S stereoisomer being the most mutagenic. Consistent with decreased plaque formation, in vitro replication of DNA templates containing the cross-links by the three major E. coli polymerases revealed replication blockage by both stereoisomeric forms of the cross-links. The same DNAs that were used for replication studies were also assembled into duplex DNAs and tested as substrates for the initiation of nucleotide excision repair by the E. coli UvrABC complex. UvrABC incised linear substrates containing these intrastrand cross-links with low efficiency, suggesting that these lesions may be inefficiently repaired by the nucleotide excision repair system.

Comparison of the mutations at Hprt exon 3 of T-lymphocytes from B6C3F1 mice and F344 rats exposed by inhalation to 1,3-butadiene or the racemic mixture of 1,2:3,4-diepoxybutane

Experiments were conducted to define the spectra of mutations occurring in Hprt exon 3 of T-cells isolated from spleens of female B6C3F1 mice and F344 rats exposed by inhalation to 1,3-butadiene (BD) or its reactive metabolite, (+/-)-diepoxybutane (DEB). Hprt mutant frequencies (Mfs) in BD-exposed (1250 ppm for 2 weeks or 625 ppm for 4 weeks; 6 h/day, 5 days/week) and DEB-exposed (2 or 4 ppm for 4 weeks or 5 ppm for 6 weeks; 6 h/day, 5 days/week) mice and rats were significantly increased over concurrent control values. Mutant T-cell colonies from control and treated animals were screened for mutations in Hprt exon 3 using PCR amplification of genomic DNA and denaturing gradient gel electrophoresis, followed by sequence analysis. Exon 3 mutations were found at the following frequencies: 20/394 (5%) in control mice, 56/712 (8%) in BD-exposed mice, 59/1178 (5%) in BD-exposed rats, 66/642 (10%) in DEB-exposed mice, and 51/732 (7%) in DEB-exposed rats. Mutations in exposed animals included base substitutions, small deletions (1 to 74 bp), and small insertions (1 to 8 bp), with base substitutions predominating. Among the types of base substitutions observed in mice, the proportions of G.C-->A.T transitions (p=0.035, Fisher's Exact Test) and G.C-->C.G transversions (p=0.05) were significantly different in control vs. BD-exposed animals. Given the small number of exon 3 mutants analyzed, there was a high degree of overlap in the mutational spectra between BD-exposed mice and rats, between BD- and DEB-exposed mice, and between BD- and DEB-exposed rats in terms of the sites with base substitutions, the mutations found at those mutated sites, the relative occurrence of the most frequently observed base substitutions, and the occurrence of a consistent strand bias for the most frequently observed base substitutions. The spectra data suggest that adduction of both G.C and A.T bps is important in the induction of in vivo mutations by BD metabolites in exposed mice and rats.

Mutagenic potential of guanine N2 adducts of butadiene mono- and diolepoxide

To explore the role of guanine N(2) adducts of stereoisomeric butadiene metabolites in butadiene-induced mutagenesis, 11-mer deoxyoligonucleotides were prepared containing adducts of (R)- and (S)-monoepoxide and (R,R)- and (S,S)-diolepoxide. These adducted oligonucleotides were utilized in both in vivo and in vitro experiments designed to examine the mutagenic potency of each and their replication by Escherichia coli polymerases. Each of the four adducted deoxyoligonucleotides was ligated into a single-stranded M13mp7L2 vector and transfected into E. coli. The resulting plaques were screened for misincorporation at position 2 of the N-ras 12 codon. Although the mutagenic frequencies were low, different relative mutagenicities of the various stereoisomers were discernible. In addition, the biological effects of each adduct on the three major E. coli polymerases were determined via primer extension assays. The adducted 11-mers were ligated into a 60-mer linear DNA molecule to provide a sufficiently long template for primer elongation. All four guanine adducts were determined to be blocking to each of the three polymerases via primer extension assays.

Mutagenicity of 1,3-butadiene at the Hprt locus of T-lymphocytes following inhalation exposures of female mice and rats

The species specific response to 1,3-butadiene (BD), an important industrial chemical, was investigated by determining the influence of exposure duration and exposure concentration on the mutagenicity of BD in mice and rats and by defining the spectra of mutations in the Hprt gene T-cell mutants from control and BD-exposed mice. Female B6C3F1 mice and F344 rats (4-5 weeks old) were exposed by inhalation to 0, 20, 62.5, or 625 ppm of BD for up to 4 weeks (6 h/day, 5 days/week). Groups of control and exposed animals (n=4-12/group) were necropsied at multiple time points after exposure and the T-cell cloning assay was used to measure Hprt mutant frequencies in lymphocytes isolated from spleen. Mutant clones collected from control and BD-exposed mice were propagated and analyzed by RT-PCR to produce Hprt cDNA for sequencing. In animals necropsied 4 weeks after 2 or 4 weeks of BD exposure (0 or 625 ppm), the rate of accumulation of mutations was greater in mice than in rats. Supra-linear dose-response curves were observed in BD-exposed mice, indicating a higher efficiency of mutant induction at lower concentrations of BD. The mutagenic potency estimates (represented by the differences in the areas under the mutant T-cell 'manifestation' curves of treated vs. control animals) in mice were 11 and 61 following 4 weeks of exposures to 62.5 and 625 ppm of BD, respectively, while mutant frequencies (Mfs) in rats were significantly increased only at 625 ppm BD (mutagenic potency of 7). Molecular analysis of Hprt cDNA from expanded T-cell clones from control and BD-exposed mice demonstrated an increased frequency of mutants in exposed animals that likely contain large deletions in the Hprt gene (P=0.016). These data indicate that both exposure duration and exposure concentration are important in determining the magnitude of mutagenic response to BD, and that mutagenic and carcinogenic properties of BD in mice may be related more to the ability of its metabolites to cause chromosomal deletions than to produce point mutations.

The in vivo mutagenicity and mutational spectrum at the lacI transgene recovered from the spleens of B6C3F1 lacI transgenic mice following a 4-week inhalation exposure to 1,3-butadiene

1,3-Butadiene (BD) is carcinogenic and mutagenic in B6C3F1 mice. We determined the lacI mutant frequency and mutational spectrum in spleen following inhalation exposure to BD at levels that are known to induce tumors. B6C3F1 lacI transgenic mice were exposed to air or to 62.5, 625, or 1250 ppm BD for 4 weeks (6 h/day, 5 days/week) and euthanized 14 days after the last exposure. BD increased the lacI mutant frequency in spleen at all levels of BD examined. In BD-exposed mice, an increased frequency of G:C-->A:T transitions occurred at non-5'-CpG-3' sites. Exposure to BD in B6C3F1 lacI transgenic mice also increased the frequency of base substitution mutations that occurred at A:T base pairs when compared to air controls. The increased frequency of specific mutations at G:C base pairs in spleen was not observed in our previous studies in bone marrow and indicates tissue-specific differences in the BD-induced mutational spectrum. These data demonstrate that in vivo transgenic mouse mutagenicity assays can identify tissue-specific mutagenicity and mutational spectrum responses of genotoxic carcinogens at exposure levels that are known to induce tumors.

Comparison of induction of hprt mutations by 1,3-butadiene and/or its metabolites 1,2-epoxybutene and 1,2,3,4-diepoxybutane in lymphocytes from spleen of adult male mice and rats in vivo

Induction of hprt mutations by 1,3-butadiene (BD) and its metabolites 1,2-epoxybutene (EB) and 1,2,3,4-diepoxybutane (DEB) was studied in lymphocytes from spleens of 6- to 14-week-old mice and 10- to 11-week-old rats. For unknown reasons, results from experiments with mice that received inhalation exposure to BD were quite variable. In the first experiment, mice were exposed for 5 days to 200, 500 or 1300 ppm and this resulted in a statistically significant, dose-dependent, induction of mutations. When the experiment was repeated and an extra expression time for mutations was included, it was not possible to detect induction of mutations. In a third experiment, a 6-day exposure to 500 ppm was mutagenic when mice with zero mutants were not excluded from the statistical analysis of the data. The monofunctional metabolite EB appeared to be mutagenic in mice (3 x 33 and 3 x 100 mg/kg), but not in rats (3 x 33 and 100 mg/kg or 30 days drinking water with 0.1, 0.3, or 1.0 mM EB). Contrary to expectations, there was no induction of mutations in mice and rats exposed to the bifunctional metabolite DEB (mice, 3 x 7, 21, 3 x 14, or 42 mg/kg; rats, 20 or 40 mg/kg or 30 days drinking water with 0.3 or 1 mM DEB), although in our earlier studies with mice and rats, DEB treatment significantly enhanced frequencies of micronuclei in splenocytes and in early spermatids of mice and rats. Some of these results differ from findings reported by other investigators. It is now becoming evident that these differences are, to a large extent, due to differences in age of the animals at the time of treatment. For example, the mutagenic potency of BD, EB and DEB was stronger in preweanling mice or 4-week-old mice than in 8- to 12-week-old adult mice.