Determination of mutagenicity in tissues of transgenic mice following exposure to 1,3-butadiene and N-ethyl-N-nitrosourea

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.

In vivo studies in the mouse to define a threshold for the genotoxicity of EMS and ENU

The presence of ethyl methanesulfonate (EMS) in tablets of a HIV medication triggered non-clinical studies into the dose response for mutation analysis after chronic dosing. Although there are a multitude of in vitro and in vivo studies on the genotoxic activity of EMS, no lifetime carcinogenicity studies, repeat dose mutation data or exposure analysis are available to serve as a solid basis for risk assessment. For alkylators like EMS it is generally assumed that the dose response for mutagenicity (and by default for carcinogenicity) is linear - indicating that no 'safe' dose does exist. A recent in vitro genotoxicity study [S.H. Doak, G.J. Jenkins, G.E. Johnson, E. Quick, E.M. Parry, J.M. Parry, Mechanistic influences for mutation induction curves after exposure to DNA-reactive carcinogens, Cancer Res. 67 (2007) 3904-3911] provided evidence, however, that the dose-response curve for mutagenic and clastogenic activity of EMS was thresholded - in contrast to ethylnitrosourea (ENU) tested in parallel. For risk assessment we sought to verify the existence of a threshold for mutagenic and clastogenic activity in vivo using the micronucleus test (MNT) and gene mutation test (MutaMouse), with the aim to provide reassurance to the patients that their exposure to EMS did not carry a toxicological risk. Dose levels ranging from 1.25 to 260mg/(kgday) were applied for up to 28 days. As reference we included ENU at doses of 1.1-22mg/(kgday). Our studies showed that daily doses of EMS up to 25mg/(kgday) (bone marrow, GI tract) and 50mg/(kgday) (liver) did not induce mutations in the lacZ gene in the three organs tested. Doses of EMS up to 80mg/(kgday) did not induce micronuclei in mouse bone marrow. Only at higher dose levels the genotoxic activity of EMS became apparent. Dose fractionation of EMS (28 times 12.5mg/kg versus a single high dose 380mg/kg) in the MutaMouse study provided further convincing evidence for the thresholded dose response of EMS and showed that no accumulation below the threshold was occurring. For ENU no threshold was apparent and dose fractionation indicated additivity. However, there are arguments that a threshold in the dose region of about 0.4mg/(kgday) ENU might exist.

Evaluation of cancer tests of 1,3-butadiene using internal dose, genotoxic potency, and a multiplicative risk model

In cancer tests with 1,3-butadiene (BD), the mouse is much more sensitive than the rat. This is considered to be related to the metabolism of BD to the epoxide metabolites, 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane, and 1,2-epoxy-3,4-butanediol. This study evaluates whether the large difference in outcome in cancer tests with BD could be predicted quantitatively on the basis of the concentration over time in blood (AUC) of the epoxide metabolites, their mutagenic potency, and a multiplicative cancer risk model, which has earlier been used for ionizing radiation. Published data on hemoglobin adduct levels from inhalation experiments with BD were used for the estimation of the AUC of the epoxide metabolites in the cancer tests. The estimated AUC of the epoxides were then weighed together to a total genotoxic dose, by using the relative genotoxic potency of the respective epoxide inferred from in vitro hprt mutation assays using EB as standard. The tumor incidences predicted with the risk model on the basis of the total genotoxic dose correlated well with the earlier observed tumor incidences in the cancer tests. The total genotoxic dose that leads to a doubling of the tumor incidences was estimated to be the same in both species, 9 to 10 mmol/Lxh EB-equivalents. The study validates the applicability of the multiplicative cancer risk model to genotoxic chemicals. Furthermore, according to this evaluation, different epoxide metabolites are predominating cancer-initiating agents in the cancer tests with BD, the diepoxide in the mouse, and the monoepoxides in the rat.

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.

Genotoxic effects of butadiene in mouse lung cells detected by an ex vivo micronucleus test

Lung fibroblasts from BD-exposed mice have been analysed for the occurrence of micronuclei. Primary cultures set up 24h after the end of exposure were treated with cytochalasin B and micronuclei scored in binucleate cells. A three-fold statistically significant increase of micronucleated cells was detected after exposure to 500ppm, the lowest tested concentration. A linear dose effect relationship was observed between 500 and 1300ppm. Immunofluorescent staining of kinetochore proteins was applied to distinguish between acentric micronuclei produced by chromosome breaks and micronuclei containing a centromeric region, most likely induced by chromosome loss. A statistically significant increase of both types of MN in 1300ppm-exposed females and a significant increase in centromeric MN in 500ppm-exposed males were detected. These data demonstrate that an intermediate of BD metabolism with a potential for clastogenic and aneugenic effects is active in lung cells after inhalation exposure. These effects can play a role in the initiation and promotion of BD-induced lung tumours.

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.

A review of the genetic and related effects of 1,3-butadiene in rodents and humans

In this paper, the metabolism and genetic toxicity of 1,3-butadiene (BD) and its oxidative metabolites in humans and rodents is reviewed with attention to newer data that have been published since the latest evaluation of BD by the International Agency for Research on Cancer (IARC). The oxidative metabolism of BD in mice, rats and humans is compared with emphasis on the major pathways leading to the reactive intermediates 1,2-epoxy-3-butene (EB), 1,2:3, 4-diepoxybutane (DEB), and 3,4-epoxy-1,2-butanediol (EBdiol). Results from recent studies of DNA and hemoglobin adducts indicate that EBdiol may play a more significant role in the toxicity of BD than previously thought. All three metabolites are capable of reacting with macromolecules, such as DNA and hemoglobin, and have been shown to induce a variety of genotoxic effects in mice and rats as well as in human cells in vitro. DEB is clearly the most potent of these genotoxins followed by EB, which in turn is more potent than EBdiol. Studies of mutations in lacI and lacZ mice and of the Hprt mutational spectrum in rodents and humans show that mutations at G:C base pairs are critical events in the mutagenicity of BD. In-depth analyses of the mutational spectra induced by BD and/or its oxidative metabolites should help to clarify which metabolite(s) are associated with specific mutations in each animal species and which mutational events contribute to BD-induced carcinogenicity. While the quantitative relationship between exposure to BD, its genotoxicity, and the induction of cancer in occupationally exposed humans remains to be fully established, there is sufficient data currently available to demonstrate that 1,3-butadiene is a probable human carcinogen.

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.

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.

Characterization of the reactivity, regioselectivity, and stereoselectivity of the reactions of butadiene monoxide with valinamide and the N-terminal valine of mouse and rat hemoglobin

Occupational exposure to 1,3-butadiene (BD) has been monitored by measuring the level of hemoglobin N-terminal valine adduct formation with the primary reactive metabolite, butadiene monoxide (BMO). However, mechanistic details concerning the relative reactivity, regioselectivity, and stereospecificity of BMO with the N-terminal valine of hemoglobin are lacking. In the studies presented here, L-valinamide was used as a model for the N-terminal valine of hemoglobin to compare the nucleophilic reactivity, regioselectivity, and stereoselectivity of the reaction both in aqueous solution and within a protein microenvironment. Four products produced by the reaction of L-valinamide with racemic BMO (two pairs of diastereomers produced by reactions at C-1 and C-2 of the epoxide moiety) were synthesized, purified, and characterized by (1)H NMR and GC/MS. These four reaction products were used as analytical standards for kinetic studies of the reaction of valinamide with BMO at physiological pH (7.4) and temperature (37 degrees C). The results show that the adducts formed by reaction at C-2 were formed at a ratio of approximately 2:1 compared to the adducts formed by reaction at C-1. The stereoisomers of each respective regioisomer were produced with similar rates of formation. The reaction of BMO with the N-terminal valine of hemoglobin was also studied in vitro using intact erythrocytes from Sprague-Dawley rats and B6C3F1 mice. After cleavage of the N-modified valine by the N-alkyl Edman degradation procedure using pentafluorophenylisothiocyanate (PFPITC), a novel procedure was developed that allowed GC/MS detection and quantitation of the four expected products by silylation of the PFPTH-valine-BMO derivatives. The hemoglobin results contrast with the valinamide results in that the reaction of BMO with the N-terminal valine residue in both rat and mouse hemoglobin produced mostly C-1 adducts. The rates obtained with rat hemoglobin were much slower than the rates obtained with mouse hemoglobin or with valinamide. These results, and the finding that the reaction with rat hemoglobin produced a higher ratio of C1:C2 adducts in comparison with the reaction with mouse hemoglobin, indicate the importance of measuring all four adducts when comparing the relative rates of adduct formation both with model compounds and among different species.

Persistence of N7-(2,3,4-trihydroxybutyl)guanine adducts in the livers of mice and rats exposed to 1,3-butadiene

Liquid chromatography (LC) in combination with tandem mass spectrometry (MS/MS) and stable isotope methodology was employed for the analysis of the N7-guanine (Gua) adducts derived from 1,2:3, 4-diepoxybutane (BDO2) a reactive metabolite of 1,3-butadiene (BD). Two diastereomeric forms of N7-(2,3,4-trihydroxybutyl)guanine (THBG) were identified in the livers of both mice and rats. One of the diastereomers [(+/-)-THBG] was formed by reaction of DNA with (+/-)-BDO2, and the other diastereomer (meso-THBG) was formed by reaction of DNA with meso-BDO2. There was significantly more (+/-)-THBG and meso-THBG in the liver DNA of the mice when compared with those of the rats during the 10 days of exposure to BD and the 6 days of postexposure that were monitored. There was a 2-fold excess of (+/-)-THBG over meso-THBG in the rat liver at all the time points. In the mouse liver after 10 days of exposure to BD, the (+/-)-THBG (3.9 adducts/10(6) normal bases) was also present in an almost 2-fold excess over meso-THBG (2.2 adducts/10(6) normal bases). However, 6-days after exposure to BD, (+/-)-THBG (1.2 adducts/10(6) normal bases) and meso-THBG (1.0 adduct/10(6) normal bases) were present in almost equal amounts in the mouse liver. Furthermore, there was an almost 5-fold excess of the two THBG diastereomers in the mouse liver DNA 6 days after exposure to BD when compared with rat liver DNA. The half-lives of (+/-)-THBG and meso-THBG appeared to be slightly longer in mouse liver (4.1 and 5.5 days, respectively) than in rat liver (3.6 and 4.0 days, respectively). The apparent persistence of these adducts in the mouse may contribute to the increased susceptibility of this species to BD-induced carcinogenesis. It is possible that (+/-)-THBG and meso-THBG could have also been derived from the reaction of DNA with the hydrolysis product of BDO2, 1,2-dihydroxy-3,4-epoxybutane (DHEB). Surprisingly, a vast majority of the studies in which the mutagenic and carcinogenic potential of BDO2 have been examined have only employed the commercially available (+/-)-BDO2. In light of the present findings, additional studies will be required to determine the potency of meso-BDO2 and the DHEB that is the precursor to meso-THBG as mutagens and carcinogens.

Epoxybutene-hemoglobin adducts in rats and mice: dose response for formation and persistence during and following long-term low-level exposure to butadiene

Measurement of specific adducts to hemoglobin can be used to establish the dosimetry of electrophilic compounds and metabolites in experimental animals and in humans. The purpose of this study was to investigate the dose response for adduct formation and persistence in rats and mice during long-term low-level exposure to butadiene by inhalation. Adducts of 3,4-epoxy-1-butene, the primary metabolite of butadiene, with N-terminal valine in hemoglobin were determined in male B6C3F1 mice and male Sprague-Dawley rats following exposure to 0, 2, 10, or 100 ppm of 1,3-butadiene, 6 h/day, 5 days/week for 1, 2, 3, or 4 weeks. Blood samples were collected from groups of five mice and three rats at the end of each week during the 4 weeks of exposure and weekly for 3 weeks following the end of the 4-week exposure period. The increase and decrease, respectively, of the adduct levels during and following the end of the 4-week exposure followed closely the theoretical curve for adduct accumulation and removal for rats and mice, thereby demonstrating that the adducts are chemically stable in vivo and that the elimination follows the turnover of the red blood cells. The adduct level increased linearly with butadiene exposure concentration in the mice, whereas a deviation from linearity was observed in the rats. For example, after exposure to 100 ppm butadiene, the epoxybutene-hemoglobin adduct levels were about four times higher in mice than in rats; at lower concentrations of butadiene, the species difference was less pronounced. Blood concentrations of epoxybutene, estimated from hemoglobin adduct levels, were in general agreement with reported concentrations in mice and rats exposed by inhalation to 62.5 ppm. These studies show that adducts of epoxybutene with N-terminal valine in hemoglobin can be used to predict blood concentration of epoxybutene in experimental animals.

Review of reproductive and developmental toxicity of 1,3-butadiene

Toxic doses of 1,3-butadiene (BD) have been reported to cause reproductive and/or developmental toxicity. Regardless of the strain used, mice were always affected by BD at lower doses than rats, an expected observation, based on well recognized differences in pharmacokinetic (PK) parameters in these two species. Because the mouse is particularly sensitive to BD in comparison with other laboratory species, and there are important functional and anatomical differences between humans and mice, the NOELs and LOELs identified for BD for various reproductive endpoints in mice may not be relevant to human reproductive risk. In mice, the LOELs for reproductive endpoints include developmental toxicity at 200 ppm, genotoxic effects at 500 ppm (mouse spot test), ovarian atrophy in females at 6.25 ppm (carcinogenicity study), reduced testicular weights at 200 ppm and testicular atrophy at 625 ppm BD in males (carcinogenicity studies), low incidences of abnormal sperm heads at 1000 and 5000 ppm BD (sperm head morphology study), small reversible increases in resorption at 1250/1300 ppm or 5000 ppm (dominant lethal studies), and other possible sequelae of genotoxicity resulting from exposure of male mice at 12.5 ppm BD and higher (dominant lethal study). When available, the much higher NOELs and LOELs of other species tested for the same endpoints should be considered. For example, maternal and developmental NOELs for BD in the rat were 200 and 1000 ppm, respectively, and 40 ppm in the mouse. Likewise, exposure of cohabited pairs of rats, guinea pigs and rabbits or of female dogs to BD concentrations as high as 6700 ppm for 8 months did not impair fertility or cause testicular or ovarian atrophy in these species. Thus, consideration of these remarkable species-dependent differences in toxicity is necessary. In addition, there are alternative scientific interpretations for some of the mouse studies and this review attempts to address these areas. For example, it may be incorrect to categorize results indicating weak in vivo genotoxic effects in male mice (sperm head morphology and dominant lethal studies) at 12.5 ppm BD and higher as reproductive effects because concentrations of BD as high as 5000 ppm did not affect mating, fertility or live litter sizes, even in this sensitive species. Similarly, it may be inappropriate to identify the ovary as a target organ for reproductive risk since the ovarian atrophy in mice was identified after completion of the normal reproductive life and after more than 15 months of exposure. Neither ovarian nor testicular atrophy occurred in Sprague-Dawley rats after exposure to BD concentrations as high as 8000 ppm for 105 (females) or 111 (males) weeks.

A physiological toxicokinetic model for 1,3-butadiene in rodents and man: blood concentrations of 1,3-butadiene, its metabolically formed epoxides, and of haemoglobin adducts--relevance of glutathione depletion

A physiological toxicokinetic (PT) model is presented describing disposition and metabolism of 1,3-butadiene (BU) and 1,2-epoxy-3-butene (BMO) in rat, mouse and man, and of 1,2:3,4-diepoxybutane (BDI) in mice. It contains formation of BMO and BDI, intrahepatocellular first-pass hydrolysis of BMO, conjugation of BMO with glutathione (GSH) and GSH-turnover in the liver. Tissue:air partition coefficients of BU and BMO were determined experimentally. Haemoglobin (HB) adducts of BMO in rodents following exposure to BU were simulated and compared with published data. The model is compared with those published earlier. An attempt was made to compare the carcinogenic potential of BU in mice and rats with respect to the carcinogenic potentials of both epoxides.

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

1,3-Butadiene (BD) is a carcinogen that is bioactivated to at least two genotoxic metabolites. In the present article, we review briefly our previous studies on the in vivo mutagenicity and mutational spectra of BD in bone marrow and extend these studies to examine the effect of exposure time (5-days vs. 4-week exposure to 625 ppm BD used in previous studies) on the lacI mutant frequency in the bone marrow. Inhalation exposure to BD at 625 ppm and 1,250 ppm mutagenic in vivo, inducing an increase in the transgene mutant and mutation frequency in the bone marrow. Analysis of the mutational spectrum in BD-exposed and air control mice demonstrated that BD exposure induced an increased frequency of mutations at A:T base pairs. There was no difference in the lacI mutant frequency determined in the bone marrow between a short-term exposure to BD (5 days) and a longer-term exposure (4 weeks). These data taken together demonstrate that inhalation exposure to BD induces in vivo somatic cell mutation.

A strategy for the application of transgenic rodent mutagenesis assays

The past several years have seen an enormous increase in the development and use of transgenic animal models to measure mutations in specific inserted reporter genes. These systems provide gene mutation data in vivo in a wide range of relevant tissues. Numerous laboratories are now using these systems with consistent results. This paper describes the unique niche that transgenic mutagenesis systems can fill in product development and registration strategies. In addition to tissue-specific mechanistic studies, transgenic assays are available to follow up mutagenic effects demonstrated in Salmonella, Escherichia coli, mouse lymphoma (L5178Y) cells, or other in vitro systems.

Factors affecting somatic mutation frequencies in vivo

The factors that influence the spontaneous mutant frequencies in mammalian tissues have been ranked on the basis of data from our laboratory together with published data. Some of the data come from the endogenous hprt and Dlb-1 loci, but most come from transgenic mice carrying the bacterial lacI and lacZ genes in recoverable lambda phage vectors. Since there is evidence that these bacterial loci are selectively neutral, the mutant frequency observed is the integral of the mutation rates from the formation of the zygote. The factors that affect the inferred mutation rate, in decreasing order of importance are: site of integration of the transgene, age, tissue, and strain. Insufficient data exist to determine the influence of gender (probably small) and inter-laboratory variables (probably at least as important as age). The two most surprising results are (1) that about half of all mutations arise during development (and half of these in utero) and (2) that most somatic tissues, whether queiscent or actively proliferating, have similar mutant frequencies and similar increases during adult life.

1,3-Butadiene working group report

During the Workshop in North Carolina, the in vivo metabolism, adduct formation and genotoxicity data available from rodent and human exposure to 1,3-butadiente (BD) were reviewed and they are summarized in the present report. BD is metabolized by cytochrome P-450-dependent monoxygenases to the primary metabolite 1,2-epoxybutene-3 (epoxybutene, EB). EB is subjected to further metabolism: oxidation to 1,2:3,4-diepoxybutane (DEB), hydrolysis to 3-butene-1,2-diol and conjugation to glutathione. The first pathway seems to prevail in mice while the latter is characteristic for rats and possibly for humans. Species differences exist in adduct formation of the monoepoxide to hemoglobin, for which the following pattern has been found: mice > rats > humans. Genotoxity of BD was found in mice with all applied tests; however, negative results were obtained in rats. In exposed humans, the cytogenetic studies in peripheral blood lymphocytes did not show genotoxic effects, although one report described elevated hprt variant levels in peripheral blood lymphocytes of exposed workers. It was concluded that the presently available data are insufficient for the application of the parallelogram model to estimate genetic risk for humans. As an alternative approach, a tentative estimate of the doubling dose for induction of hprt mutations in somatic cells of mice and men was performed and the calculated values were surprisingly similar, i.e. 9000 ppmh. However, this estimate is burdened with a number of caveats which were discussed in detail. The working group identified a series of urgent research needs to provide the appropriate data for the application of the parallelogram model, such as identification of metabolic pathways in different rodent species and humans, metabolic studies in mice, rats and humans considering metabolic polymorphisms, studies of adducts to DNA and hemoglobin especially of DEB and other butadiene metabolites in rodents and humans, studies of mutational spectra (mutational fingerprinting) in somatic and germinal cells, confirmation of the human hprt mutation data, conformation of the rodent malformation data, dose-response studies in rodent germ cell tests and studies on repair kinetics of mono-adducts induced by EB as opposed to repair of cross-links produced by DEB. Finally, it was suggested that the original parallelogram consisting of data from somatic cell studies in rodents and humans plus studies of heritable effects in rodents to extrapolate to germ cell risk for humans should be supplemented with studies in sperm of experimental animals and exposed men.

Molecular and genetic toxicology of 1,3-butadiene

During the last 9 years, there have been many studies published concerning the mutagenic potential of butadiene in mammalian systems, including alterations at the molecular level. Butadiene has tested positive in several mouse in vivo and in vitro assays, but has generally tested negative in rat studies. Most of these studies are cytogenetic and include positive data in mice for chromosomal aberrations, micronucleus formation, and sister chromatid exchanges. Butadiene also induces mutations in lung, spleen, and bone marrow of transgenic mice. The positive bone marrow cytogenetic and transgenic data may be significant in view of the increased lymphohematopoietic malignancies observed in mice and probably in humans. In addition, butadiene causes mutations in the K-ras protooncogene and in the p53 tumor suppressor gene in mouse studies. Mutations in these genes are associated with oncogenesis in humans as well as in rodents. Also, positive mutagenicity data have been obtained in a pilot study of workers exposed to butadiene. Positive dominant lethal studies in rodents suggest that exposure to butadiene can result in germ cell mutation and heritable risk. These mutagenicity and molecular data suggest that butadiene is both a somatic and germ cell mutagen in mammals, possibly including humans.

Overview of mutation assays in transgenic mice for routine testing

There is scientific and regulatory interest in using mutation assays in transgenic mice in safety assessments for new chemicals and drugs. Currently these assays are in the process of being validated, and protocols for routine testing are being defined. Some of the issues and results to date with regard to assay validation include reproducibility of the assay results (they are qualitatively reproducible), relevance of the test system (the transgene closely approximates an endogenous mammalian gene as a mutational target for the limited number of compounds tested), and the predictivity of the assay for heritable effects (unknown at this time) or carcinogenicity (the assays show good positive predictivity for carcinogenicity; the negative predictivity of the assay requires further investigation). Definition of appropriate study protocols for routine testing requires that applicable statistical methods are available and that the experimental parameters that affect the detection of mutations are known. Progress made in identifying these parameters is discussed. A proposal is made for the custom design of routine safety studies, which is based on the anticipated use of each individual test agent. A working group has been formed to conduct some of the studies still required for validation of these assays.

Statistical design and analysis of mutation studies in transgenic mice

We have been working on identifying sources of variability in data from transgenic mouse mutation assays in order to develop appropriate statistical methods and designs for routine studies. Data from our lab and elsewhere point to the presence of significant animal-to-animal variability, which must be taken into account in statistical hypothesis tests. Here, the usual Cochran-Armitage (CA) test for trend in mutant frequencies, which takes the transgene as the experimental unit, and a generalized Cochran-Armitage test (GCA), which takes the animal as the experimental unit, are contrasted in computer simulations that help to quantify the differences between these statistical tests. The simulations report the statistical power of each test to detect treatment group differences, and their type I error rates. We find in general that the GCA test performs poorly compared to the CA test when it is appropriate to take the transgene as the experimental unit, and the study also uses a small number of animals. However, the CA test performs poorly in small group-size studies when the animal is the appropriate experimental unit. Extensions of the computer simulations allow for identification of cost-effective experimental designs. The results emphasize that the benefits of using additional animals in these mutation studies can be realized without substantial increases in costs. Here we illustrate the methods for liver studies in our lab. These methods can be used to derive optimal experimental designs for any combination of spontaneous mutant frequency and animal-to-animal variability.

Development of a cloning assay with high cloning efficiency to detect induction of 6-thioguanine-resistant lymphocytes in spleen of adult mice following in vivo inhalation exposure to 1,3-butadiene

A cloning assay with high cloning efficiency has been developed to detect spontaneous and induced 6-thioguanine-resistant T-lymphocytes (HPRT mutants) from the spleen of adult mice. The mean cloning efficiency in untreated male mice of 20-22 weeks old was 34.5 +/- 11.2% (SD) and the corresponding mutant frequency 0.7 +/- 0.8 (SD) x 10(-6). The cloning efficiencies obtained in this study are substantially higher than those reported previously by other investigators. Using this assay, it could be demonstrated that inhalation exposure of mice to 200, 500 or 1300 ppm of 1,3-butadiene for 6 h/day on 5 consecutive days caused a statistically significant induction of 6-thioguanine-resistant mutations in T-lymphocytes from spleens of adult mice exposed to 1300 ppm. The exposure to 1300 ppm resulted in a three-fold increase of the spontaneous mutant frequency. The mutant frequency after exposure to 500 ppm was higher than the control but the increase was not significant.

Transgenic animal models for measuring mutations in vivo

Transgenic animal models for measuring mutations provide a powerful tool for rapidly assessing tissue-specific mutations following in vivo treatment. These models are based on the insertion into the rodent genome of the Escherichia coli lacI (lac repressor) or lacZ (beta-galactosidase) genes that serve as targets for mutations. Following in vivo treatment of animals, genomic DNA is isolated from various tissues and the target gene is packaged into lambda-phage heads; the lambda-phage are used to infect E. coli in order to produce plaques. Mutations in the target gene are then detected using colorimetric or selective procedures. In this review methods are discussed for producing these transgenic models, the target genes used, gene rescue techniques, sequencing of isolated mutants, and parameters that affect dosing regimens and design of studies. We also present a summary of data published to date with these systems and present our conclusions and proposed directions for future research.

Statistical tests of significance in transgenic mutation assays: considerations on the experimental unit

When significant animal-to-animal variability is present in binary response data, the usual statistical tests applied to such data do not always operate correctly. In transgenic mouse mutation data, some evidence of significant animal-to-animal variability already exists, suggesting that conventional statistical methods may not be appropriate. Here, we describe an alternative statistical method that treats the animal as the experimental (or statistically independent) unit, and contrast results of its application with those from methods that take the transgene as the experimental unit. Using data from two publications that report experimental results for individual animals, the transgene-based and animal-based analyses can yield very different interpretations of the experimental data. The performance of animal-based statistical methods should be improved by conducting future experiments with enough animals to adequately address animal-to-animal variability.