In vivo mutagenicity of ethylene oxide at the hprt locus in T-lymphocytes of B6C3F1 lacI transgenic mice following inhalation exposure

Systematic review of the scientific evidence on ethylene oxide as a human carcinogen

Ethylene oxide is a highly reactive chemical primarily used as an intermediate in chemical production and as a sterilant of medical equipment and food products; it also is produced endogenously as a result of physiological processes. We conducted a systematic review of the potential carcinogenicity of inhaled ethylene oxide in humans using methods that adhere to PRIMSA guidelines and that incorporate aspects from the Institute of Medicine (IOM) (now the National Academy of Medicine) as well as several US Environmental Protection Agency (EPA) frameworks for systematic reviews. After a comprehensive literature search and selection process, study quality was evaluated following a method adapted from the EPA Toxic Substances Control Act (TSCA) framework. The literature screening and selection process identified 24 primary studies in animals or humans and more than 50 mechanistic studies. Integrating epidemiological, animal, and mechanistic literature on ethylene oxide and cancer according to the IOM framework yielded classifications of suggestive evidence of no association between ethylene oxide and stomach cancer, breast cancer and lymphohematopoietic malignancies at human relevant exposures. However, we acknowledge that there is additional uncertainty in the classification for lymphohematopoietic malignancies owing to a paucity of evidence for specific types of these tumors, each of which is a distinct disease entity of possibly unique etiology.

Human genetic risk of treatment with antiviral nucleoside analog drugs that induce lethal mutagenesis: The special case of molnupiravir

This review considers antiviral nucleoside analog drugs, including ribavirin, favipiravir, and molnupiravir, which induce genome error catastrophe in SARS-CoV or SARS-CoV-2 via lethal mutagenesis as a mode of action. In vitro data indicate that molnupiravir may be 100 times more potent as an antiviral agent than ribavirin or favipiravir. Molnupiravir has recently demonstrated efficacy in a phase 3 clinical trial. Because of its anticipated global use, its relative potency, and the reported in vitro "host" cell mutagenicity of its active principle, β-d-N4-hydroxycytidine, we have reviewed the development of molnupiravir and its genotoxicity safety evaluation, as well as the genotoxicity profiles of three congeners, that is, ribavirin, favipiravir, and 5-(2-chloroethyl)-2'-deoxyuridine. We consider the potential genetic risks of molnupiravir on the basis of all available information and focus on the need for additional human genotoxicity data and follow-up in patients treated with molnupiravir and similar drugs. Such human data are especially relevant for antiviral NAs that have the potential of permanently modifying the genomes of treated patients and/or causing human teratogenicity or embryotoxicity. We conclude that the results of preclinical genotoxicity studies and phase 1 human clinical safety, tolerability, and pharmacokinetics are critical components of drug safety assessments and sentinels of unanticipated adverse health effects. We provide our rationale for performing more thorough genotoxicity testing prior to and within phase 1 clinical trials, including human PIG-A and error corrected next generation sequencing (duplex sequencing) studies in DNA and mitochondrial DNA of patients treated with antiviral NAs that induce genome error catastrophe via lethal mutagenesis.

Genotoxicity as a toxicologically relevant endpoint to inform risk assessment: A case study with ethylene oxide

The purpose of the present investigation is to analyze the in vivo genotoxicity dose-response data of ethylene oxide (EO) and the applicability of the derived point-of-departure (PoD) values when estimating permitted daily exposure (PDE) values. A total of 40 data sets were identified from the literature, and benchmark dose analyses were conducted using PROAST software to identify a PoD value. Studies employing the inhalation route of exposure and assessing gene or chromosomal mutations and chromosomal damage in various tissues were considered the most relevant for assessing risk from EO, since these effects are likely to contribute to adverse health consequences in exposed individuals. The PoD estimates were screened for precision and the values were divided by data-derived adjustment factors. For gene mutations, the lowest PDE was 285 parts per trillion (ppt) based on the induction of lacI mutations in the testes of mice following 48 weeks of exposure to EO. The corresponding lowest PDE value for chromosomal mutations was 1,175 ppt for heritable translocations in mice following 8.5 weeks of EO exposure. The lowest PDE for chromosomal aberrations was 238 ppt in the mouse peripheral blood lymphocytes following 48 weeks of inhalation exposure. The diverse dose-response data for EO-induced genotoxicity enabled the derivation of PoDs for various endpoints, tissues, and species and identified 238 ppt as the lowest PDE in this retrospective analysis.

Analysis of Biomarkers of DNA Damage and Mutagenicity in Mice Exposed to Acrylonitrile

Acrylonitrile (ACN), which is a widely used industrial chemical, induces cancers in the mouse via unresolved mechanisms. For this report, complementary and previously described methods were used to assess in vivo genotoxicity and/or mutagenicity of ACN in several mouse models, including (i) female mice devoid of cytochrome P450 2E1 (CYP2E1), which yields the epoxide intermediate cyanoethylene oxide (CEO), (ii) male lacZ transgenic mice, and (iii) female (wild-type) B6C3F1 mice. Exposures of wild-type mice and CYP2E1-null mice to ACN at 0, 2.5 (wild-type mice only), 10, 20, or 60 (CYP2E1-null mice only) mg/kg body weight by gavage for 6 weeks (5 days/week) produced no elevations in the frequencies of micronucleated erythrocytes, but induced significant dose-dependent increases in DNA damage, detected by the alkaline (pH >13) Comet assay, in one target tissue (forestomach) and one nontarget tissue (liver) of wild-type mice only. ACN exposures by gavage also caused significant dose-related elevations in the frequencies of mutations in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) reporter gene of T-lymphocytes from spleens of wild-type mice; however, Hprt mutant frequencies were significantly increased in CYP2E1-null mice only at a high dose of ACN (60 mg/kg) that is lethal to wild-type mice. Similarly, drinking water exposures of lacZ transgenic mice to 0, 100, 500, or 750 ppm ACN for 4 weeks caused significant dose-dependent elevations in Hprt mutant frequencies in splenic T-cells; however, these ACN exposures did not increase the frequency of lacZ transgene mutations above spontaneous background levels in several tissues from the same animals. Together, the Comet assay and Hprt mutant frequency data from these studies indicate that oxidative metabolism of ACN by CYP2E1 to CEO is central to the induction of the majority of DNA damage and mutations in ACN-exposed mice, but ACN itself also may contribute to the carcinogenic modes of action via mechanisms involving direct and/or indirect DNA reactivity.

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

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

Mutagenicity of DNA adducts derived from ethylene oxide exposure in the pSP189 shuttle vector replicated in human Ad293 cells

Ethylene oxide (EO) is a widely used chemical intermediate also formed endogenously from ethylene metabolism. Despite conflicting epidemiological evidence, EO is classified by the IARC as a human carcinogen. The mutagenicity and carcinogenicity of EO is attributed to direct reaction with DNA and formation of multiple 2-hydroxyethyl (HE) DNA adducts. However, the actual lesions responsible for the reported mutagenicity of EO have not been established. This study used the supF mutation assay to investigate the biological relevance of low levels of EO-induced DNA adducts in human Ad293 cells, with respect to the type and level of each HE adduct present. Initial experiments were conducted using pSP189 plasmid containing up to 290 N7-HEGuanine (N7-HEG) adducts/10(6) nucleotides, which far exceeds that typically detected in human DNA. No other HE-lesions were detectable using our validated LC-MS/MS assay. Replication in cells failed to produce a statistically significant increase in relative mutation frequency, above background rates in the solvent control. Furthermore, the mutation spectrum compiled for EO-treated plasmid (10-2000muM) did not differ significantly from the spontaneous distribution, suggesting EO is not strongly mutagenic in this system. Under refined reaction conditions using higher EO concentrations capable of inducing detectable levels of N1-HEdA, O(6)-HEdG and N3-HEdU along with N7-HEG, there was a significant dose-related increase in relative mutation frequency above background (3.76- and 5.30-fold at 10 and 30mM, respectively). EO treatment appeared associated with an elevated frequency of GC-->CG mutations and the occurrence of substitutions at AT base pairs. Additionally, there was a distinct GC-->TA mutational hotspot in the 10mM EO spectrum. Overall, the results suggest a certain level of promutagenic adducts must be attained before mutations become detectable above background, indicating that N7-HEG is not a promutagenic lesion, and support a role for the minor products of DNA hydroxyethylation in the generation of base substitutions by EO.

A Framework for Human Relevance Analysis of Information on Carcinogenic Modes of Action

The human relevance framework (HRF) outlines a four-part process, beginning with data on the mode of action (MOA) in laboratory animals, for evaluating the human relevance of animal tumors. Drawing on U.S. EPA and IPCS proposals for animal MOA analysis, the HRF expands those analyses to include a systematic evaluation of comparability, or lack of comparability, between the postulated animal MOA and related information from human data sources. The HRF evolved through a series of case studies representing several different MOAs. HRF analyses produced divergent outcomes, some leading to complete risk assessment and others discontinuing the process, according to the data available from animal and human sources. Two case examples call for complete risk assessments. One is the default: When data are insufficient to confidently postulate a MOA for test animals, the animal tumor data are presumed to be relevant for risk assessment and a complete risk assessment is necessary. The other is the product of a data-based finding that the animal MOA is relevant to humans. For the specific MOA and endpoint combinations studied for this article, full risk assessments are necessary for potentially relevant MOAs involving cytotoxicity and cell proliferation in animals and humans (Case Study 6, chloroform) and formation of urinary-tract calculi (Case Study 7, melamine). In other circumstances, when data-based findings for the chemical and endpoint combination studied indicate that the tumor-related animal MOA is unlikely to have a human counterpart, there is little reason to continue the risk assessment for that combination. Similarly, when qualitative considerations identify MOAs specific to the test species or quantitative considerations indicate that the animal MOA is unlikely to occur in humans, such hazard findings are generally conclusive and further risk assessment is not necessary for the endpoint-MOA combination under study. Case examples include a tumor-related protein specific to test animals (Case Study 3, d-limonene), the tumor consequences of hormone suppression typical of laboratory animals but not humans (Case Study 4, atrazine), and chemical-related enhanced hormone clearance rates in animals relative to humans (Case Study 5, phenobarbital). The human relevance analysis is highly specific for the chemical-MOA-tissue-endpoint combination under analysis in any particular case: different tissues, different endpoints, or alternative MOAs for a given chemical may result in different human relevance findings. By providing a systematic approach to using MOA data, the HRF offers a new tool for the scientific community's overall effort to enhance the predictive power, reliability and transparency of cancer risk assessment.

Propylene oxide: genotoxicity profile of a rodent nasal carcinogen

Propylene oxide (PO) is a DNA-reactive genotoxic agent; that is, it reacts with DNA to produce lesions in the genetic material. PO also induces tumors in rodents, although only at high concentrations and at portals of entry. This review of PO's genotoxicity profile is organized according to endpoints measured, that is, nonmutational or mutational endpoints, and as to whether the results were from in vitro or in vivo studies. In addition to results of experimental studies, PO's genotoxicity for humans is assessed by reviewing results of published biomarker studies. The weight of evidence indicates that although it is genotoxic, PO's potency as a DNA-reactive mutagen is weak. Other aspects of PO's overall tissue toxicities are also reviewed, with attention to glutathione (GSH) depletion and its consequences, that is, cell proliferation, death, and necrosis. These toxic tissue responses occur in the same anatomical regions in rodents as do the PO-induced tumors. Furthermore, some of these tissue toxicities can produce effects that may either augment PO's DNA-reactive mutagenicity or be genotoxic in themselves, not dependent on PO's DNA reactivity. Although its DNA reactivity may be a necessary component of PO's overall genotoxicity and rodent carcinogenicity, it is likely not sufficient, and the associated tissue toxicities, which are rate-limiting, also seem to be required. This complex mode of action has implications for estimations of PO's cancer potential in humans, especially at low exposure concentrations.

Exposure of hematopoietic stem cells to ethylene oxide during processing represents a potential carcinogenic risk for transplant recipients

Stem cells for transplantation are obtained from bone marrow, umbilical cord blood, and peripheral blood. A rare complication of hematopoietic stem cell transplantation is donor cell-derived leukemia (DCL). The donors remain cancer free and the causes of these DCL are unknown. Stem cells must repopulate the bone marrow and then give rise to all hematopoietic cells for the rest of the transplant recipient's life. No procedure is acceptable that might introduce precancerous or cancerous mutations in cells performing such a critical function. Medical disposable sets consisting of bags, tubing sets and freezing containers are used to collect, purify and store stem cells. Sterilization of disposables with ethylene oxide is widespread, even though those sets unavoidably retain residual amounts of ethylene oxide which is a potent, direct-acting mutagen and clastogen that has been demonstrated to induce hematopoietic cancer in mice, rats and human beings. Potential exposure levels to ethylene oxide during processing under proposed US FDA guidelines for residual ethylene oxide would be biologically active and present a significant risk factor for DCL. For direct-acting mutagens, there is no recognized "no effect" dose using currently accepted cancer risk assessment models. The safety concerns with ethylene oxide can be eliminated by the use of alternative technologies including electron beam, gamma irradiation, or steam for the sterilization of all products used for stem cell processing and storage.

Age-, gender-, and species-dependent mutagenicity in T cells of mice and rats exposed by inhalation to 1,3-butadiene

Experiments were performed: (i) to investigate potential age- and gender-dependent differences in mutagenic responses in T cells following exposures of B6C3F1 mice and F344 rats by inhalation for 2 weeks to 0 or 1250 ppm butadiene (BD), and (ii) to determine if exposures for 2 weeks to 62.5 ppm BD produce a mutagenic effect in female rats. To evaluate the effect of age on mutagenic response, mutant manifestation curves for splenic T cells of female mice exposed at 8-9 weeks of age were defined by measuring Hprt mutant frequencies (MFs) at multiple time points after BD exposure using a T cell cloning assay and comparing the resulting mutagenic potency estimate (calculated as the difference of areas under the mutant manifestation curves of treated versus control animals) to that reported for female mice exposed to BD in the same fashion beginning at 4-5 weeks of age. The shapes of the mutant T cell manifestation curves for spleens were different [e.g., the maximum BD-induced MFs in older mice (8.0+/-1.0 [S.D.]x10(-6)) and younger mice (17.8+/-6.1 x 10(-6)) were observed at 8 and 5 weeks post-exposure, respectively], but the mutagenic burden was the same for both age groups. To assess the effect of gender on mutagenic response, female and male rodents were exposed to BD at 4-5 weeks of age and Hprt MFs were measured when maximum MFs are expected to occur post-exposure. The resulting data demonstrated that the pattern for mutagenic susceptibility from high-level BD exposure is female mice>male mice>female rats>male rats. Exposures of female rats to 62.5 ppm BD caused a minor but significant mutagenic response compared with controls (n=16/group; P=0.03). These results help explain part of the differing outcomes/interpretations of data in earlier Hprt mutation studies in BD-exposed rodents.

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.

In vivo mutagenicity and mutation spectrum in the bone marrow and testes of B6C3F1 lacI transgenic mice following inhalation exposure to ethylene oxide

The lacI mutant frequency and mutation spectrum were determined in the bone marrow and testes of B6C3F1 lacI transgenic mice exposed by inhalation to ethylene oxide (EO). Groups of male transgenic lacI B6C3F1 mice were exposed to 0, 25, 50, 100 or 200 p.p.m. EO for up to 48 weeks (6 h/day, 5 days/week) and were killed at 12, 24 or 48 weeks of EO exposure for determination of lacI mutant frequency. In the bone marrow, the lacI mutant frequency was significantly increased at the two highest exposure levels (100 and 200 p.p.m.) and at the 48 week exposure time point. The shape of the exposure-response curve for lacI mutant frequency in the bone marrow was non-linear. DNA sequence analysis of the bone marrow mutation spectrum revealed that only AT-->TA transversions occurred at an increased frequency in EO-exposed mice: 25.4% in EO-exposed mice for 48 weeks (200 p.p.m.) compared with 1.4% in air controls. In testes, the lacI mutant frequency was increased at a single exposure level of 200 p.p.m. for 24 weeks. At 48 weeks, the lacI mutant frequency in testes was significantly increased to an equal degree at 25, 50 and 100 p.p.m. EO but not at 200 p.p.m. Analysis of the testes mutation spectrum in air control mice and in mice exposed to 200 p.p.m. EO for 48 weeks revealed that no single mutational type occurred at an increased frequency. In the testes, there was a small increase across all mutational types that was sufficient to increase the overall lacI mutation frequency although not significant individually. The mutation spectrum in testes of EO-exposed mice also revealed that the increased lacI mutant frequency observed at 25 or 50 p.p.m. EO was not due to an increase in mutant siblings (clonality). These data demonstrate that inhalation exposure to EO for up to 48 weeks produces distinct mutagenic responses in bone marrow and testes.

Hprt mutant frequencies in splenic T-cells of male F344 rats exposed by inhalation to propylene

Propylene is a major industrial intermediate and atmospheric pollutant to which humans are exposed by inhalation. In this study, 6-week-old male F344 rates were exposed to 0, 200, 2000, or 10,000 ppm propylene by inhalation for 4 weeks (6 h/day, 5 days/week), and mutant frequencies were determined in the Hprt gene of splenic T-lymphocytes. Twenty milligrams of cyclophosphamide monohydrate (CPP)/kg bw, given on the penultimate day of propylene exposure, was used as a positive control mutagen. Rats (n = 8/group) were necropsied for isolation of T-cells 8 weeks after the last dose, a sampling time that produced peak spleen Hprt mutant frequencies (Mfs) in a preliminary mutant manifestation study using CCP treatment. Hprt Mfs were measured via the T-cell cloning assay, which was performed without knowledge of the animal treatment groups. Mean Hprt Mfs were significantly increased over control values (mean Mf = 5.24 +/- 1.55 (SD) x 10(-6)) in CPP-treated rats (10.37 +/- 4.30 x 10(-6), P = 0.007). However, Hprt Mfs in propylene-exposed rats were not significantly increased over background, with mean Mfs of 4.90 +/- 1.84 x 10(-6) (P = 0.152), 5.05 +/- 3.70 x 10(-6) (P = 0.895), and 5.95 +/- 2.49 x10(-6) (P = 0.500) for animals exposed to 200, 2000, or 10,000 ppm propylene, respectively. No significant increase in F344 rat or B6C3F1 mouse cancer incidence was reported in the National Toxicology Program carcinogenicity studies of propylene across this same exposure range. Taken together, these findings support the conclusion that inhalation exposure of rats to propylene does not cause mutations or cancer.

Induction and persistence of micronuclei, sister-chromatid exchanges and chromosomal aberrations in splenocytes and bone-marrow cells of rats exposed to ethylene oxide

Studies on the induction and persistence of ethylene oxide (EO) induced chromosomal alterations in rat bone-marrow cells and splenocytes following in vivo exposure were carried out. Rats were exposed to ethylene oxide either chronically by inhalation (50-200ppm, 4 weeks, 5 days/week, 6h/day) or acutely by intraperitoneal injection (i.p.) at dose levels of 50-100ppm.Spontaneous- and induced-frequencies of micronuclei (MN), sister-chromatid exchanges (SCEs) and chromosomal aberrations were determined in rat bone-marrow cells, and in splenocytes following in vitro mitogen stimulation. Unstable chromosomal aberrations were studied in whole genome using standard Giemsa staining technique and fluorescence in situ hybridisation using probe for chromosome #2 was employed to detect chromosome translocations. Following chronic exposure, the cytogenetic analyses were carried out at days 5 and 21 in rat splenocytes, to study the induction and persistence of sister-chromatid exchanges. Following chronic exposure, ethylene oxide was effective in inducing SCEs, and markedly cells with high frequency SCEs were observed and they in-part persisted until day 21 post-exposure. However, no significant effect was observed in rat splenocytes for induction of MN and chromosomal aberrations. Following acute exposure, both SCEs and MN were increased significantly in rat bone-marrow cells as well as splenocytes.In conclusion, this study indicates that ethylene oxide at the concentrations employed by intraperitoneal injection or inhalation in adult rats is mutagenic and can induce both SCEs and MN.

Transplacental mutagenicity of N-ethyl-N-nitrosourea at the hprt locus in T-lymphocytes of exposed B6C3F1 mice

Previous studies have compared age-related differences in total mutagenic burden in mice of differing age (preweanling, weanling, or young adult) after single intraperitoneal (i.p.) injections of ethylnitrosourea (ENU). The purpose of the present investigation was to determine the effects of time elapsed since treatment on the frequency of hprt mutant T-cells (Mf) from mice treated transplacentally with single acute vs. multiple split doses of ENU. To this end, pregnant C57BL/6 mice (n = 13-16/group), which had been bred to C3H males, were given i.p. injections of 40 mg ENU/kg bw in a single dose on day 18 of gestation, in a split dose of 6 mg ENU/kg bw on days 12 through 18 of gestation, or DMSO vehicle alone. Groups of pups were necropsied on days 10, 13, 15 (single dose only), 17, 20, 40, and 70 postpartum for T-cell isolations and hprt Mf measurements using the T-cell cloning assay. The time required to reach maximum Mfs in T-cells isolated from thymus of transplacentally treated animals was 2 weeks, the same time span as previously observed after ENU treatment of adult, weanling, and preweanling mice. Mfs in T-cells isolated from spleens of control animals averaged 2.1 +/- 0.3 (SE) x 10(-6). In spleens of mice treated transplacentally with ENU in a single dose, Mfs reached a maximum at 15 days postpartum [84.7 +/- 15.8 (SE) x 10(-6)] and decreased to lower but still elevated levels at 40 days postpartum. In spleens of mice treated transplacentally with ENU in a split dose, Mfs reached a maximum at 13 days postpartum [74.0 +/- 16.3 (SE) x 10(-6)] and decreased to background levels at 40 days postpartum. The areas under the curves describing the change in hprt Mfs over time for ENU-treated vs. control mice estimate the mutagenic potency for transplacental single- and split-dose exposures to be 1.9 and 0.8 x 10(3), respectively. Comparison of the mutagenic potency estimates for mice exposed to ENU in utero to 4-week-old mice given a similar dose of the same lot number of ENU indicates that the mouse is more susceptible to ENU-induced mutagenesis during fetal life.

Comparison of hprt and lacI mutant frequency with DNA adduct formation in N-hydroxy-2-acetylaminofluorene-treated Big Blue rats

N-Hydroxy-2-acetylaminofluorene (N-OH-AAF) is the proximate carcinogenic metabolite of the powerful rat liver carcinogen 2-acetylaminofluorene. In this study, transgenic Big Blue(R) rats were used to examine the relationship between in vivo mutagenicity and DNA adduct formation by N-OH-AAF in the target liver compared with that in nontarget tissues. Male rats were given one, two, or four doses of 25 mg N-OH-AAF/kg body weight by i.p. injection at 4-day intervals, and groups of treated and control rats were euthanized up to 10 weeks after beginning the dosing. Mutant frequencies were measured in the spleen lymphocyte hprt gene, and lacI mutant frequencies were determined in the liver and spleen lymphocytes. At 6 weeks after beginning the dosing, the hprt mutant frequency in spleen lymphocytes from the four-dose group was 16.5 x 10(-6) compared with 3.2 x 10(-6) in control animals. Also at 6 weeks, rats given one, two, or four doses of N-OH-AAF had lacI mutant frequencies in the liver of 97.6, 155.6, and 406.8 x 10(-6), respectively, compared with a control frequency of 25.7 x 10(-6); rats given four doses had lacI mutant frequencies in spleen lymphocytes of 55.8 x 10(-6) compared with a control frequency of 20.4 x 10(-6). Additional rats were evaluated for DNA adduct formation in the liver, spleen lymphocytes, and bone marrow by (32)P-postlabeling. Adduct analysis was conducted 1 day after one, two, and four treatments with N-OH-AAF, 5 days after one treatment, and 9 days after two treatments. N-(Deoxyguanosin-8-yl)-2-aminofluorene was the major DNA adduct identified in all the tissues examined. Adduct concentrations increased with total dose to maximum values in samples taken 1 day after two doses, and remained essentially the same after four doses. In samples taken after four doses, adduct levels were 103, 28, and 7 fmol/microg of DNA in liver, spleen lymphocytes, and bone marrow, respectively. The results indicate that the extent of both DNA adduct formation and mutant induction correlates with the organ specificity for N-OH-AAF carcinogenesis in the rat. Environ. Mol. Mutagen. 37:195-202, 2001. Published 2001 Wiley-Liss, Inc.

Strategies and testing methods for identifying mutagenic risks

The evolution of testing strategies and methods for identification of mutagenic agents is discussed, beginning with the concern over potential health and population effects of chemical mutagens in the late 1940s that led to the development of regulatory guidelines for mutagenicity testing in the 1970s and 1980s. Efforts to achieve international harmonization of mutagenicity testing guidelines are summarized, and current issues and needs in the field are discussed, including the need for quantitative methods of mutagenic risk assessment, dose-response thresholds, indirect mechanisms of mutagenicity, and the predictivity of mutagenicity assays for carcinogenicity in vivo. Speculation is offered about the future of mutagenicity testing, including possible near-term changes in standard test batteries and the longer-term roles of expression profiling of damage-response genes, in vivo mutagenicity testing methods, and models that better account for differences in metabolism between humans and laboratory model systems.

Carcinogenicity and genotoxicity of ethylene oxide: new aspects and recent advances

Long-term inhalation studies in rodents have presented unequivocal evidence of experimental carcinogenicity of ethylene oxide, based on the formation of malignant tumors at multiple sites. However, despite a considerable body of epidemiological data only limited evidence has been obtained of its carcinogenicity in humans. Ethylene oxide is not only an important exogenous toxicant, but it is also formed from ethylene as a biological precursor. Ethylene is a normal body constituent; its endogenous formation is evidenced by exhalation in rats and in humans. Consequently, ethylene oxide must also be regarded as a physiological compound. The most abundant DNA adduct of ethylene oxide is 7-(2-hydroxyethyl)guanine (HOEtG). Open questions are the nature and role of tissue-specific factors in ethylene oxide carcinogenesis and the physiological and quantitative role of DNA repair mechanisms. The detection of remarkable individual differences in the susceptibility of humans has promoted research into genetic factors that influence the metabolism of ethylene oxide. With this background it appears that current PBPK models for trans-species extrapolation of ethylene oxide toxicity need to be refined further. For a cancer risk assessment at low levels of DNA damage, exposure-related adducts must be discussed in relation to background DNA damage as well as to inter- and intraindividual variability. In rats, subacute ethylene oxide exposures on the order of 1 ppm (1.83 mg/m3) cause DNA adduct levels (HOEtG) of the same magnitude as produced by endogenous ethylene oxide. Based on very recent studies the endogenous background levels of HOEtG in DNA of humans are comparable to those that are produced in rodents by repetitive exogenous ethylene oxide exposures of about 10 ppm (18.3 mg/m3). Experimentally, ethylene oxide has revealed only weak mutagenic effects in vivo, which are confined to higher doses. It has been concluded that long-term human occupational exposure to low airborne concentrations to ethylene oxide, at or below current occupational exposure limits of 1 ppm (1.83 mg/m3), would not produce unacceptable increased genotoxic risks. However, critical questions remain that need further discussions relating to the coherence of animal and human data of experimental data in vitro vs. in vivo and to species-specific dynamics of DNA lesions.

Biomarkers of exposure and effect as indicators of potential carcinogenic risk arising from in vivo metabolism of ethylene to ethylene oxide

The purposes of the present study were: (i) to investigate the potential use of several biomarkers as quantitative indicators of the in vivo conversion of ethylene (ET) to ethylene oxide (EO); (ii) to produce molecular dosimetry data that might improve assessment of human risk from exogenous ET exposures. Groups (n = 7/group) of male F344 rats and B6C3F1 mice were exposed by inhalation to 0 and 3000 p. p.m. ET for 1, 2 or 4 weeks (6 h/day, 5 days/week) or to 0, 40, 1000 and 3000 p.p.m. ET for 4 weeks. N:-(2-hydroxyethyl)valine (HEV), N:7-(2-hydroxyethyl) guanine (N7-HEG) and HPRT: mutant frequencies were assessed as potential biomarkers for determining the molecular dose of EO resulting from exogenous ET exposures of rats and mice, compared with background biomarker values. N7-HEG was quantified by gas chromatography coupled with high resolution mass spectrometry (GC-HRMS), HEV was determined by Edman degradation and GC-HRMS and HPRT: mutant frequencies were measured by the T cell cloning assay. N7-HEG accumulated in DNA with repeated exposure of rodents to 3000 p.p.m. ET, reaching steady-state concentrations around 1 week of exposure in most tissues evaluated (brain, liver, lung and spleen). The dose-response curves for N7-HEG and HEV were supralinear in exposed rats and mice, indicating that metabolic activation of ET was saturated at exposures >/=1000 p.p.m. ET. Exposures of mice and rats to 200 p.p.m. EO for 4 weeks (as positive treatment controls) led to significant increases in HPRT: mutant frequencies over background in splenic T cells from exposed rats and mice, however, no significant mutagenic response was observed in the HPRT: gene of ET-exposed animals. Comparisons between the biomarker data for both unexposed and ET-exposed animals, the dose-response curves for the same biomarkers in EO-exposed rats and mice and the results of the rodent carcinogenicity studies of ET and EO suggest that too little EO arises from exogenous ET exposure to produce a significant mutagenic response or a carcinogenic response under standard bioassay conditions.

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.

Formation of DNA adducts and induction of mutagenic effects in rats following 4 weeks inhalation exposure to ethylene oxide as a basis for cancer risk assessment

Ethylene oxide (EO) is mutagenic in various in vitro and in vivo test systems and carcinogenic in rodents. EO forms different adducts upon reaction with DNA, N7-(2-hydroxyethyl)guanine (N7-HEG) being the main adduct. The major objectives of this study were: (a) to determine the formation and persistence of N7-HEG adducts in liver DNA of adult male rats exposed to 0, 50, 100 and 200 ppm by inhalation (4 weeks, 5 days/week, 6 h/day) and (b) to assess dose-response relationships for Hprt gene mutations and various types of chromosomal changes in splenic lymphocytes.N7-HEG adducts were measured 5, 21, 35 and 49 days after cessation of exposure. By extrapolation, the mean concentrations of N7-HEG immediately after cessation of exposure ('day 0') to 50, 100 and 200 ppm were calculated as 310, 558 and 1202 adducts/10(8) nucleotides, respectively, while the mean concentration in control rats was 2.6 adducts/10(8) nucleotides. At 49 days, N7-HEG values had returned close to background levels. The mean levels of N-(2-hydroxyethylvaline) adducts in haemoglobin were also determined and amounted 61.7, 114 and 247 nmol/g globin, respectively. Statistically significant linear relationships were found between mean N7-HEG levels ('day 0') and Hprt mutant frequencies at expression times 21/22 and 49/50 days and between mean N7-HEG ('day 0') and sister-chromatid exchanges (SCEs) or high frequency cells (HFC) measured 5 days post-exposure. At day 21 post-exposure, SCEs and HFCs in-part persisted and were significantly correlated with persistent N7-HEG adducts. No statistically significant dose effect relationships were observed for induction of micronuclei, nor for chromosome breaks or translocations. In conclusion, this study indicates that following sub-chronic exposure, EO is only weakly mutagenic in adult rats. Using the data of this study to predict cancer risk in man resulting from low level EO exposures in conjunction with other published data, i.e., those on (a) genotoxic effects of EO in humans and rats, (b) DNA binding of other carcinogens, (c) natural background DNA binding and (d) genotoxic potency of low energy transfer (LET) radiation, it is not expected that long term occupational exposure to airborne concentrations of EO at or below 1 ppm EO produces an unacceptable increased risk in man.

Relationships between exposure, cell loss and proliferation, and manifestation of Hprt mutant T cells following treatment of preweanling, weanling, and adult male mice with N-ethyl-N-nitrosourea

Experiments were performed to characterize the age-related patterns of appearance and frequency of hypoxanthine-guanine phosphoribosyl transferase (Hprt) mutant T lymphocytes in thymus and spleen following exposure of preweanling (12-day-old), weanling (22-day-old), and young adult (8-week-old) male B6C3F1 mice to ethylnitrosourea (ENU). Mice were given single i.p. injections of 0 or 40 mg ENU/kg and then groups of animals were necropsied from 2 h to 116 days after treatment to examine the relationships between exposure, cell loss and proliferation, and the frequency of Hprt mutant T cells in thymus and spleen. Hprt mutant frequency (Mf) data for thymus of ENU-exposed (0, 11.7, 35, 58, or 72 mg/kg, or five weekly doses of 1.7 mg/kg i.p.) male C57BL/6 mice (12- or 62-week-old), obtained during an earlier study of spleen cells [I.M. Jones, K. Burkhart-Schultz, C.L. Strout, T.L. Crippen, Factors that affect the frequency of thioguanine-resistant lymphocytes in mice following exposure to ethylnitrosourea, Environ. Mutagen, 9 (1987) 317-329.], were compared to results in B6C3F1 mice. Isolated T cells were cultured in the presence of mitogen, growth factor, and 6-thioguanine to detect Hprt mutants. The time required to achieve maximum Mfs in thymus was uniformly found at 2 weeks after ENU treatment, while the times needed to reach peak values in spleen were proportional to animal age at treatment. These data indicate that age-related differences in the appearance of Hprt mutant cells in spleen are largely defined by the physiologically based, age-dependent trafficking of mutant cells from or through the thymus. Three modes of handling the resulting Hprt Mf data were evaluated: (i) comparing the Mfs at a single time point, (ii) comparing the maximum Mfs observed, and (iii) comparing the change in Mfs over time (or the mutant T cell 'manifestation' curves in treated vs. control mice) in each age group post-exposure. Measuring the Mfs in spleen at multiple time points after cessation of exposure and integrating the frequency of mutants as a function of time appeared to be the superior method for comparing mutagenic responses in different age groups. Some of the underlying assumptions of this approach, as well as its strengths and weaknesses, are discussed.

Measurement of HPRT mutations in splenic lymphocytes and haemoglobin adducts in erythrocytes of Lewis rats exposed to ethylene oxide

Young adult male Lewis rats were exposed to ethylene oxide (EO) via single intraperitoneal (i.p.) injections (10-80 mg kg-1) or drinking water (4 weeks at concentrations of 2, 5, and 10 mM) or inhalation (50, 100 or 200 ppm for 4 weeks, 5 days week-1, 6 h day-1) to measure induction of HPRT mutations in lymphocytes from spleen by means of a cloning assay. N-ethyl-N-nitrosourea (ENU) and N-(2-hydroxyethyl)-N-nitrosourea (HOENU) were used as positive controls. Levels of N-(2-hydroxyethyl)valine (HOEtVal) adducts in haemoglobin (expressed in nmol g-1 globin) were measured to determine blood doses of EO (mmol kg-1 h, mM h). Blood doses were used as a common denominator for comparison of mutagenic effects of EO administered via the three routes. The mean HPRT mutant frequency (MF) of the historical control was 4.3 x 10(-6). Maximal mean MFs for ENU (100 mg kg-1) and HOENU (75 mg kg-1) were 243 x 10(-6) and 93 x 10(-6), respectively. In two independent experiments, EO injections led to a statistically significant dose-dependent induction of mutations, with a maximal increase in MF by 2.3-fold over the background. Administration of EO via drinking water gave statistically significant increases of MFs in two independent experiments. Effects were, at most, 2.5-fold above the concurrent control. Finally, inhalation exposure also caused a statistically significant maximal increase in MF by 1.4-fold over the background. Plotting of mutagenicity data (i.e., selected data pertaining to expression times where maximal mutagenic effects were found) for the three exposure routes against blood dose as common denominator indicated that, at equal blood doses, acute i.p. exposure led to higher observed MFs than drinking water treatment, which was more mutagenic than exposure via inhalation. In the injection experiments, there was evidence for a saturation of detoxification processes at the highest doses. This was not seen after subchronic administration of EO. The resulting HPRT mutagenicity data suggest that EO is a relatively weak mutagen in T-lymphocytes of rats following exposure(s) by i.p. injection, in drinking water or by inhalation.

Mutagenicity of the racemic mixtures of butadiene monoepoxide and butadiene diepoxide at the Hprt locus of T-lymphocytes following inhalation exposures of female mice and rats

The purpose of this study was to determine if Hprt mutant frequency (Mf) data from rodents exposed directly to individual epoxy metabolites of 1,3-butadiene (BD) can be used to identify the relative significance of each intermediate in the mutagenicity of BD in mice vs. rats. To this end, the relative contributions of the racemic mixtures of BD monoepoxide (BDO) and BD diepoxide (BDO(2)) to BD-induced mutagenicity was investigated by exposing mice and rats to selected concentrations of BDO and BDO(2) (i.e., 2.5 and 4.0 ppm, respectively) and comparing the mutagenic potency of each intermediate to that of BD (at 62.5 ppm) when comparable blood levels of metabolites are achieved (in the mouse). Female B6C3F1 mice and F344 rats (4-5 weeks old) were exposed to rac-BDO (0, 2.5, or 25 ppm) or (+/-)-BDO(2) (0, 2, 4 ppm) by inhalation for 4 weeks (6 h/day, 5 days/week), and then groups of control and exposed animals (n=3-12/group) were necropsied at multiple time points post-exposure for measuring Hprt Mfs in splenic lymphocytes (via the T-cell cloning assay) and estimating mutagenic potencies (represented by the difference in the areas under the mutant T-cell 'manifestation' curves of treated vs. control animals). The resulting Mf data, along with the extant metabolism data, suggest that at lower BD exposures (</=62.5 ppm) (+/-)-BDO(2) is a major contributor to the mutagenicity of BD in mice, whereas other metabolites and stereochemical configurations are responsible for mutations in BD-exposed rats and for the incremental mutagenic effects at higher BD exposures in mice. These studies indicate that additional work is needed to determine more definitively the relative contributions of these and other metabolites and stereochemical forms to BD-induced mutagenicity. Also, the novel approach of measuring mutagenic potencies as the change in Hprt Mfs over time in T-cells of exposed vs. control animals, as used in this study, can be valuable for predicting the potential role of these intermediates in each species.

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.

A comparative study of in vivo mutation assays: analysis of hprt, lacI, cII/cI and as mutational targets for N-nitroso-N-methylurea and benzo[a]pyrene in Big Blue mice

We have compared the response of the native hprt gene and the lacI, cII, and cI transgenes in Big Blue B6C3F1 mice following treatment with either N-nitroso-N-methylurea (MNU) or benzo[a]pyrene (BaP). Three weeks after mutagen treatment splenic T cells were isolated from the animals, and samples were either cultured to measure mutation at the native hprt locus or used to extract genomic DNA for transgene mutation analysis. Phage rescued from extracted DNA were plated in the presence of 5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside (X-gal) to score lacI mutations, or plated on a hflAB lawn to score cII and cI mutants. With MNU hprt mutant frequency increased in a dose-related, sublinear manner up to 78-fold above background at the highest dose tested (20 mg/kg). In comparison, the lacI transgene yielded only a 3.1-fold increase at this dose, and the cII and cI transgenes did not show any increase. With 150 mg/kg BaP a 5.8- and 8.7-fold increase in mutant frequency was observed at hprt and lacI, respectively, while only a 1.3-fold increase was observed at cII. DNA sequencing revealed an increase in GC-->TA transversions among the cII mutants, suggesting that the increase was related to BaP exposure. No significant increase in cI mutant frequency was observed. Therefore, the order of mutation assay sensitivity was hprt>lacI>cII/cI with MNU, and hprt approximately lacI> cII/cI with BaP. While the hflAB selection system offers significant advantages with respect to cost and effort when compared to the lacI assay, additional evaluation of its sensitivity is warranted.

Comparison of mutant frequencies and types of mutations induced by thiotepa in the endogenous Hprt gene and transgenic lacI gene of Big Blue rats

The utility of the lacI transgene of Big Blue rats as a reporter of in vivo mutation was evaluated by comparing the frequency and types of mutations induced by thiotepa in the transgene and the endogenous Hprt gene. Transgenic rats were given i.p. injections of 1.4 mg/kg of thiotepa three times per week over a period of 4 weeks (a total dose of 16.8 mg/kg); 1 week after the last injection, mutation assays were performed on spleen lymphocytes isolated from the animals. Thiotepa treatment increased the lacI mutant frequency from 34.8 +/- 4.1 x 10(-6) in control animals to 140.9 +/- 64.8 x 10(-6) (p = 0.0020) and the Hprt mutant frequency from 3.5 +/- 1.5 x 10(-6) to 41.1 +/- 23.2 x 10(-6) (p = 0.0028). Sequence analysis of lacI mutant DNA and Hprt mutant cDNA produced similar overall mutation patterns: G:C-->T:A transversion was the most common base pair substitution (32% of independent mutations in the lacI gene and 28% of Hprt mutations), and deletions and insertions accounted for 34% of mutations in the lacI gene and 28% in the Hprt gene. The majority of thiotepa-induced base pair substitutions in the Hprt gene occurred with the mutated purine on the non-transcribed DNA strand, while no strand-related bias was found for mutations in the lacI gene. Substitutions at G:C base pairs in the lacI gene, but not in the Hprt gene, were found disproportionately in CpG sites. In addition, multiplex polymerase chain reaction analysis of genomic DNA from the Hprt mutants indicated that 34% had relatively large deletions; none of these deletions was detected by the cDNA analysis. The results indicate that the frequency of thiotepa-induced mutants in Big Blue rats was 2.8-fold greater in the lacI gene than in the Hprt gene. Although the Hprt gene recovered a fraction of large deletions not found among the lacI mutants, the effects of transcription-coupled DNA repair in the Hprt gene and the targeting of base pair substitutions to G:C base pairs in CpG sites may have contributed to the higher mutant frequencies induced by thiotepa in the lacI transgene compared with the Hprt gene.

Effect of target gene CpG content on spontaneous mutation in299 transgenic mice

Transgenic mutation assays utilizing bacterial target genes display a high frequency of spontaneous mutation at CpG sequences. This is believed to result from the fact that: (1) the prokaryotic genes currently being used as transgenic mutation targets have a high CpG content and (2) these sequences are methylated by mammalian cells to produce 5-methylcytosine (5MC), a known promutagenic base. To study the effect of CpG content on the frequency and type of spontaneous mutation, we have synthesized an analogue of the bacterial lacI target gene (mrkII) that contains a reduced number of CpG sequences. This gene was inserted into a lambda vector and used to construct transgenic mice that undergo vector rescue from genomic DNA upon in vitro packaging. Results on spontaneous mutation frequency and spectrum have been collected and compared to those observed at the lacI gene in Big Blue transgenic mice. Spontaneous mutations at the mrkII gene occurred at a frequency in the mid-10-5 range and were predominantly base pair substitutions, similar to results seen in Big Blue. However, mrkII mutations were distributed toward the carboxyl end of the gene instead of the bias toward the amino terminus seen in lacI. Unexpectedly, 23% of the spontaneous mrkII mutations were GC-->AT transitions at CpG sequences (compared to 32% in lacI), despite the reduction in CpG number from 95 in lacI to only 13 in mrkII. Nine of the CpG bases undergoing transition mutations in mrkII have not been recorded previously as spontaneous sites in Big Blue. Therefore, substantial reduction of the number of CpG sequences in the lacI transgene did not significantly reduce the rate of spontaneous mutation or alter the contribution of CpG-related events. This suggests that other factors are also operating to establish frequency and composition of spontaneous mutations in transgenic targets.

Assessment of the in vivo mutagenicity of ethylene oxide in the tissues of B6C3F1 lacI transgenic mice following inhalation exposure

In the present study, the lacI mutant frequency was determined in the tissues of B6C3F1 lacI transgenic mice exposed by inhalation to ethylene oxide (EO). Groups of 15 male transgenic lacI B6C3F1 mice were exposed to either 0, 50, 100, or 200 ppm EO for 4 weeks (6 h/day, 5 days/week) and were sacrificed at 0, 2, or 8 weeks after the last EO exposure. The lacI transgene was recovered from lung, bone marrow, spleen, and germ cells for determination of the lacI mutant frequency. The tissues selected for analysis were tumor target site tissues in chronic bioassays (lung tumors and lymphomas) and germ cells. The lacI mutant frequency in lung was significantly increased at 8 weeks post exposure to 200 ppm EO (6.2 +/- 2.2 vs. 9.1 +/- 1.5. p = 0.046). In contrast, the lacI mutant frequency in spleen and bone marrow at 2 and 8 weeks was not significantly increased in mice exposed to 200 ppm EO. The lacI mutant frequencies in male germ cells for 200 ppm EO-exposed mice were not increased compared to air controls at 2 and 8 weeks post-exposure. In a spleen cell fraction two of three EO-exposed mice at the 200 ppm exposure level demonstrated an elevated lacI mutant frequency. The increased lacI mutant frequency in these animals was likely due to mutant siblings that contained background G:C --> A:T transitions at CpG sites. These results demonstrate that a 4-week inhalation exposure to EO is mutagenic in lung. However, EO did not increase the frequency of mutations recovered at the lacI transgene in other tissues examined under the conditions used in the present studies. Since the mutational spectrum for EO in other systems consists of an increased proportion of large deletions, the lack of a mutagenic response in the tissues examined is likely due to the lack of recovery of large deletions in lambda-based shuttle vector systems. These data indicate that a primary mechanism of EO-induced mutagenicity in vivo is likely through the induction of deletions, not specific point mutations.