Chromosome breakage is primarily responsible for the micronuclei induced by 1,4-dioxane in the bone marrow and liver of young CD-1 mice

An integrated assessment of the 1,4-dioxane cancer mode of action and threshold response in rodents

1,4-Dioxane is an environmental contaminant that has been shown to cause cancer in rodents after chronic high dose exposures. We reviewed and integrated information from recently published studies to update our understanding of the cancer mode of action of 1,4-dioxane. Tumor development in rodents from exposure to high doses of 1,4-dioxane is preceded by pre-neoplastic events including increased hepatic genomic signaling activity related to mitogenesis, elevation of Cyp2E1 activity and oxidative stress leading to genotoxicity and cytotoxicity. These events are followed by regenerative repair and proliferation and eventual development of tumors. Importantly, these events occur at doses that exceed the metabolic clearance of absorbed 1,4-dioxane in rats and mice resulting in elevated systemic levels of parent 1,4-dioxane. Consistent with previous reviews, we found no evidence of direct mutagenicity from exposure to 1,4-dioxane. We also found no evidence of CAR/PXR, AhR or PPARα activation resulting from exposure to 1,4-dioxane. This integrated assessment supports a cancer mode of action that is dependent on exceeding the metabolic clearance of absorbed 1,4-dioxane, direct mitogenesis, elevation of Cyp2E1 activity and oxidative stress leading to genotoxicity and cytotoxicity followed by sustained proliferation driven by regenerative repair and progression of heritable lesions to tumor development.

Mechanistic Considerations in 1,4-Dioxane Cancer Risk Assessment

The risk assessment of many carcinogens involves extrapolation across large exposure differences between the dose levels used in animal studies and the much lower human exposures. This is true for 1,4-dioxane which has a consistent liver carcinogenic effect in both genders of rats and mice. These data have been applied to risk assessment assuming a linear low dose extrapolation in some cases but non-linear or threshold models have been used in others. This choice hinges on our understanding of the 1,4-dioxane cancer mechanism. While 1,4-dioxane is not genotoxic in standard test batteries and has non-linear toxicokinetics, the mechanism for its carcinogenic effect remains unknown and is an active area of research. This review summarizes the possible modes of action for this chemical, data gaps and application to risk assessment. We find that the cytotoxicity/hyperplasia and metabolic saturation hypotheses do not explain the carcinogenic response and do not take into account 1,4-dioxane's induction of its own metabolism, leading to less likelihood for saturation during chronic exposure. There is evidence for other mechanisms, especially oxidative stress associated with the induction of CYP2E1 and in vivo genotoxicity that is not seen in vitro. The dose response for these effects needs further exploration compared to the time course and dose response for 1,4-dioxane-induced carcinogenesis. An additional consideration is the manner in which these 1,4-dioxane effects may augment naturally occurring and disease-related processes that contribute to the increasing rate of human liver cancer. These factors add to the rationale for using a non-threshold linear approach for extrapolating to low dose for this carcinogen, which is consistent with the default for carcinogens which do not have a clearly defined mode of action.

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

In vivo effects of 1,4-dioxane on genotoxic parameters and behavioral alterations in Drosophila melanogaster

1,4-Dioxane (DXN) is used as solvent in different consumer products including cosmetics, paints, surfactants, and waxes. In addition, DXN is released as an unwanted contaminating by-product as a result of some reactions including ethoxylation of alcohols, which occurs with in personal care products. Consequently, DXN pollution was detected in drinking water and is considered as an environmental problem. At present, the genotoxicity effects attributed to DXN are controversial. The present study using an in vivo model organism Drosophila melanogaster aimed to determine the toxic/genotoxic, mutagenic/recombinogenic, oxidative damage as evidenced by ROS production, phenotypic alterations as well as behavioral and developmental alterations that are closely related to neuronal functions. Data demonstrated that nontoxic DXN concentration (0.1, 0.25, 0.5, or 1%) induced mutagenic (1%) and recombinogenic (0.1, 0.25, or 0.5%) effects in wing spot test and genotoxicity in hemocytes using comet assay. The nontoxic concentrations of DXN (0.1, 0.25, 0.5, or 1%) significantly increased oxidative stress, climbing behavior, thermal sensivity and abnormal phenotypic alterations. Our findings show that in contrast to in vitro exposure, DXN using an in vivo model Drosophila melanogaster this compound exerts toxic and genotoxic effects. Data suggest that additional studies using other in vivo models are thus warranted.

Genotoxic effects and molecular docking of 1,4-dioxane: combined protective effects of trans-resveratrol

In this study, the protective effects of trans-resveratrol (t-resv) against 1,4-dioxane-induced toxicity in meristematic cells were investigated. For this purpose, Allium test was used and the alterations in all experimental groups were examined by using physiological, cytogenetic, biochemical, and anatomical parameters. In order to elucidate the toxicity mechanism, interactions of 1,4-dioxane and intracellular antioxidant molecules were investigated by molecular docking. As a result of the analysis, it was determined that 1,4-dioxane causes serious abnormalities in Allium cepa meristematic cells. In 1,4-dioxane-treated group, germination percentage was regressed 1.6 times, root length was reduced 12.7 times, and weight gain was decreased 7.7 times compared to control group. T-resv administration with 1,4-dioxane resulted in an improvement in physiological parameters and reduced the relative injury rate from 0.4 to 0.16. Mitotic index (MI), micronucleus (MN), and chromosomal abnormality (CAs) frequencies were investigated as cytogenetic parameters. 1,4-Dioxane decreased MI index, and increased CAs and MN frequency. In addition, it was determined by the comet test that 1,4-dioxane caused deterioration in DNA integrity. T-resv treatment was found to cause a dose-dependent improvement in genotoxic effects. Changes in the antioxidant system in all experimental groups were determined by measuring malondialdehyde (MDA) and glutathione (GSH) levels, superoxide dismutase (SOD), and catalase (CAT) enzyme activities. 1,4-Dioxane caused alterations in all tested parameters, causing deterioration in the oxidant/antioxidant balance in the cell. A 200-mg/L t-resv+1,4-dioxane treatment caused a 1.9-fold decrease in MDA level which is indicator of lipid peroxidation compared to only 1,4-dioxane-treated group. The mechanism of the disruption in antioxidant/oxidant dynamics and genetic integrity was elucidated by molecular docking analysis of 1,4-dioxane with antioxidant molecules and DNA. In 1,4-dioxane treatment group, anatomical changes such as cell deformation, flattened cell nucleus, and thickening of cortex cell wall were observed. The frequency of these changes decreased with t-resv administration. As a result, it was determined that 1,4-dioxane caused a versatile toxicity in A. cepa meristematic cells, while t-resv was found to have a dose-dependent protective feature against 1,4-dioxane-induced toxicity.

A 90-day drinking water study in mice to characterize early events in the cancer mode of action of 1,4-dioxane

Studies demonstrate that with sufficient dose and duration, 1,4-dioxane (1,4-DX) induces liver tumors in laboratory rodent models. The available evidence aligns with a threshold-dependent, tumor promotion mode of action (MOA). The MOA and key events (KE) in rats are well developed but less so in the mouse. Therefore, we conducted a 90-day drinking water study in female mice to evaluate early KE at 7, 28, and 90 days. Female B6D2F1/Crl mice consumed drinking water containing 0, 40, 200, 600, 2000 or 6000 ppm 1,4-DX. 1,4-DX was detected in blood at 90-days of exposure to 6000 ppm, but not in the other exposure groups, indicating a metabolic clearance threshold between 2000 and 6000. Early events identified in this study include glycogen-like vacuolization, centrilobular hypertrophy, centrilobular GST-P staining, apoptosis, and pan-lobular increase in cell proliferation observed after 90-days of exposure to 6000 ppm 1,4-DX. There was minimal evidence of hepatotoxicity over the duration of this study. These findings demonstrate a previously unreported direct mitogenic response following exposures exceeding the metabolic clearance threshold of 1,4-DX. Collectively, the information generated in this study supports a threshold MOA for the development of liver tumors in mice after exposure to 1,4-DX.

A versatile model for investigating the protective effects of Ceratonia siliqua pod extract against 1,4-dioxane toxicity

In this study, the toxic effects of 1,4-dioxane, a common contaminant, and the protective property of Ceratonia siliqua L. pod extract (Cspe) against this toxicity are aimed to be demonstrated with a versatile model. For this purpose, Allium toxicity test was used and six different experimental groups were formed. While the control group was germinated in tap water, the application groups were germinated in mediums containing 750 mg/L Cspe, 1500 mg/L Cspe, 100 mg/L 1,4-dioxane, 750 mg/L Cspe+100 mg/L 1,4-dioxane, and 1500 mg/L Cspe+100 mg/L 1,4-dioxane. Each group was germinated in related solution for 72 h and alterations in physiological, biochemical, genetic, and anatomical parameters were investigated. Germination percentage, relative injury rate, root length, and weight gain parameters were examined as physiological parameters, and no significant difference was observed in the control group and only-Cspe-treated groups. In groups treated with 100 mg/L 1,4-dioxane, germination percentage, root length, and weight gain were significantly decreased, and the relative injury rate reached the highest value as 0.48. It was determined that all physiological parameters improved in the groups where Cspe and 1,4-dioxane treated together, and the relative injury rate decreased to 0.22 in the group treated with 1500 mg/L Cspe+1,4-dioxane. Genotoxic effects were tested by the micronucleus and chromosomal abnormality frequency, and statistically insignificant micronucleus formation was found in control group and Cspe-treated groups. Micronucleus frequency were found to be 58.00 ± 12.12 and 31.00 ± 07.38 in 1,4-dioxane and 1500 mg/L Cspe+1,4-dioxane-treated groups, respectively. This result showed that the application of 1500 mg/L Cspe had a 46.5% reduction in the frequency of 1,4-dioxane-induced micronucleus and had a protective effect on genomic integrity. It has been found that 1,4-dioxane application induces lipid peroxidation and increases malondialdehyde level 4.5 times compared with control group. Oxidative stress, which was proved by increased malondialdehyde levels in 1,4-dioxane-treated group caused induction of superoxide dismutase and catalase enzymes, and it was determined that enzyme activities increased by 1.99 and 4.9 times, respectively, compared with the control group. Cspe treatment with 1,4-dioxane caused a significant decrease in malondialdehyde level, superoxide dismutase, and catalase enzyme activities, indicating that oxidative stress formation in the cells was repressed. Abnormalities such as cell deformation, cell wall thickening, and flattened cell nuclei were seen in 1,4-dioxane-treated group in the cross sections of root tips, and the frequency of these abnormalities decreased with Cspe application. As a result, it was determined that 1,4-dioxane caused a versatile toxicity in the test material Allium cepa, whereas Cspe application had a dose-dependent protective feature against toxicity in all tested parameters.

Removal of 1,4-dioxane from landfill leachate by a rotating advanced oxidation contactor equipped with activated carbon/TiO2 composite sheets

A rotating advanced oxidation contactor (RAOC) equipped with activated carbon (AC)/TiO₂ composite sheets for 1,4-dioxane removal from biologically treated landfill leachate (BTLL) was developed. The performance of the RAOC in 1,4-dioxane removal was compared to that of a TiO₂ slurry reactor by evaluating the removal efficiencies in pure water (PW) and the BTLL. In the TiO₂ slurry reactor, 1,4-dioxane was hardly degraded in the BTLL during 66 h of treatment because of strong inhibition by coexisting substances in the BTLL. In contrast, the RAOC successfully removed 1,4-dioxane from the BTLL by 89% through adsorption and by 81% through photocatalysis during treatment for 66 h. The ratio of the rate constants for degrading 1,4-dioxane in the BTLL and PW by the RAOC was two orders of magnitude higher than that for a TiO₂ slurry reactor. This shows that the RAOC greatly mitigated the inhibition by coexisting substances in the BTLL. The electrical energy required for 1,4-dioxane degradation in the BTLL by the RAOC was much lower than that required for degradation by the TiO₂ slurry reactor. The results show that the RAOC equipped with AC/TiO₂ composite sheets effectively removed 1,4-dioxane from BTLL.

Comprehensive analysis of DNA adducts (DNA adductome analysis) in the liver of rats treated with 1,4-dioxane

1,4-Dioxane is a genotoxic carcinogen, and its mutagenic properties were recently observed in the liver of guanine phosphoribosyl transferase (gpt) delta transgenic rats. However, the mechanisms of its genotoxicity remain unclear. We analyzed DNA adduct formation in rat livers following 1,4-dioxane treatment. After administering 1,4-dioxane in drinking water at doses of 0, 20, 200, and 5,000 ppm, liver adduct formation was analyzed by DNA adductome analysis. Adducts in treated rat livers were dose-dependently increased compared with those in the control group. Principal component analysis-discriminant analysis (PCA-DA) clearly revealed two clusters of DNA adducts, associated with 0 ppm and low-dose (20 ppm) 1,4-dioxane-treatment versus middle- and high-dose (200, 5,000 ppm)-treated rats. After confirming the intensity of each adduct, three adducts were screened as characteristic of 1,4-dioxane treatment. Two of the three candidates contained thymine or cytidine/uracil moieties. Another candidate was identified as 8-oxo-dG based on mass fragmentation together with high-resolution accurate-mass (HRAM) mass spectrometry data. Oxidative stress responses may partly explain the mechanisms of increased mutations in the liver of gpt delta rats following 1,4-dioxane treatment.

1,4-Dioxane as an emerging water contaminant: State of the science and evaluation of research needs

1,4-Dioxane has historically been used to stabilize chlorinated solvents and more recently has been found as a contaminant of numerous consumer and food products. Once discharged into the environment, its physical and chemical characteristics facilitate migration in groundwater, resulting in widespread contamination of drinking water supplies. Over one-fifth of U.S. public drinking water supplies contain detectable levels of 1,4-dioxane. Remediation efforts using common adsorption and membrane filtration techniques have been ineffective, highlighting the need for alternative removal approaches. While the data evaluating human exposure and health effects are limited, animal studies have shown chronic exposure to cause carcinogenic responses in the liver across multiple species and routes of exposure. Based on this experimental evidence, the U.S. Environmental Protection Agency has listed 1,4-dioxane as a high priority chemical and classified it as a probable human carcinogen. Despite these health concerns, there are no federal or state maximum contaminant levels for 1,4-dioxane. Effective public health policy for this emerging contaminant requires additional information about human health effects, chemical interactions, environmental fate, analytical detection, and treatment technologies. This review highlights the current state of knowledge, key uncertainties, and data needs for future research on 1,4-dioxane.

Cyclic Ether Contaminant Removal from Water Using Nonporous Adaptive Pillararene Crystals via Host-Guest Complexation at the Solid-Solution Interface

The removal of soluble cyclic ether contaminants, such as dioxane and THF, produced in industrial chemical processes from water is of great importance for environmental protection and human health. Here we report that nonporous adaptive crystals of perethylated pillar[5]arene (EtP5) and pillar[6]arene (EtP6) work as adsorbents for cyclic ether contaminant removal via host-guest complexation at the solid-solution interface. Nonporous EtP6 crystals have the ability to adsorb dioxane from water with the formation of 1:2 host-guest complex crystals, while EtP5 crystals cannot. However, both guest-free EtP5 and EtP6 crystals remove THF from water with EtP5 having a better capacity. This is because EtP5 forms a 1:2 host-guest complex with THF via host-guest complexation at the solid-solution interface while EtP6 forms a 1:1 host-guest complex with THF. EtP6 also shows the ability to selectively remove dioxane from water even in the presence of THF. Moreover, the reversible transitions between nonporous guest-free EtP5 and EtP6 structures and guest-loaded structures make them highly recyclable.

In vivo genotoxicity of 1,4-dioxane evaluated by liver and bone marrow micronucleus tests and Pig-a assay in rats

1,4-Dioxane, used widely as a solvent in the manufacture of chemicals and as a laboratory reagent, induced liver adenomas and carcinomas in mice and rats, and nasal tumors in rats in several long-term studies. 1,4-Dioxane has been reported to be non-genotoxic in vitro, and there is no clear conclusion concerning its in vivo genotoxicity in rodents. In the present study, we investigated the ability of 1,4-dioxane to induce micronuclei in the liver and bone marrow of rats. For the liver micronucleus test, we performed the juvenile animal method and two methods using partial hepatectomy (PH), dosing before PH or dosing after PH. We also evaluated the in vivo mutagenicity of 1,4-dioxane by Pig-a gene mutation assay using rat peripheral blood. As a result, all methods of liver micronucleus test showed an increase in the frequency of micronucleated hepatocytes by 1,4-dioxane. The dosing before PH, a suitable method for detecting structural chromosome aberration inducers, showed the clearest response for micronucleated hepatocytes induction among the three methods. This finding is consistent with a previous report that 1,4-dioxane induces mainly chromosome breakage in the liver. Negative results were obtained in the bone marrow micronucleus test and Pig-a gene mutation assay in our study. These results suggested that 1,4-dioxane is clastogenic in the liver but not genotoxic in the bone marrow of rats.

In vivo positive mutagenicity of 1,4-dioxane and quantitative analysis of its mutagenicity and carcinogenicity in rats

1,4-Dioxane is a widely used synthetic industrial chemical and its contamination of drinking water and food is a potential health concern. It induces liver tumors when administered in the drinking water to rats and mice. However, the mode of action (MOA) of the hepatocarcinogenicity of 1,4-dioxane remains unclear. Importantly, it is unknown if 1,4-dioxane is genotoxic, a key consideration for risk assessment. To determine the in vivo mutagenicity of 1,4-dioxane, gpt delta transgenic F344 rats were administered 1,4-dioxane at various doses in the drinking water for 16 weeks. The overall mutation frequency (MF) and A:T- to -G:C transitions and A:T- to -T:A transversions in the gpt transgene were significantly increased by administration of 5000 ppm 1,4-dioxane. A:T- to -T:A transversions were also significantly increased by administration of 1000 ppm 1,4-dioxane. Furthermore, the DNA repair enzyme MGMT was significantly induced at 5000 ppm 1,4-dioxane, implying that extensive genetic damage exceeded the repair capacity of the cells in the liver and consequently led to liver carcinogenesis. No evidence supporting other MOAs, including induction of oxidative stress, cytotoxicity, or nuclear receptor activation, that could contribute to the carcinogenic effects of 1,4-dioxane were found. These findings demonstrate that 1,4-dioxane is a genotoxic hepatocarcinogen and induces hepatocarcinogenesis through a mutagenic MOA in rats. Because our data indicate that 1,4-dioxane is a genotoxic carcinogen, we estimated the point of departure of the mutagenicity and carcinogenicity of 1,4-dioxane using the no-observed effect-level approach and the Benchmark dose approach to characterize its dose-response relationship at low doses.

Expanding the environmental applications of metal (Al, Ti, Mn, Fe) doped graphene: adsorption and removal of 1,4-dioxane

The potential applications of Al, Ti, Mn and Fe-doped graphene for environmental remediation of 1,4-dioxane (a critical pollutant and toxic compound) are analyzed in detail in the framework of density functional theory calculations. 1,4-Dioxane is a highly mobile and soluble pollutant and developing new strategies for its adsorption and subsequent removal becomes an important issue. All the systems were fully optimized and analyzed in their most stable spin states. The results determined that the proposed doped-graphene materials enhance the interaction with 1,4-dioxane compared to intrinsic graphene, with adsorption energies in the range of 1.2-1.6 eV. The high stability of the adsorbent-dioxane interactions is fully discussed in terms of chemical metal-dioxane binding, charge transfer and long-range interactions. The adsorbent-dioxane adsorption is also accompanied by changes in the electronic structure with respect to the isolated substrates, which are larger for Mn and Fe as dopants. Ab initio molecular dynamics simulations also show that the adsorbent-adsorbate interactions remain strong at room temperature (300 K). Finally, implicit/explicit solvent methodologies were implemented to get insights into the effects of aqueous environments on the adsorption strength, which shows the high stability of interaction in water, sorting the sorption efficiency as AlG ≈ FeG > MnG ≈ TiG. From these new insights, Al, Ti, Mn and Fe-doped graphene emerge as new potential materials to be applied in technologies related to the removal of 1,4-dioxane.

Effectiveness of the liver micronucleus assay using juvenile mice

This study investigated the effectiveness of the liver micronucleus (MN) assay using juvenile mice. Therefore, we analyzed various hepatic cytochrome P450 (CYP)- mediated activities of ethoxyresorufin O-deethylation, pentoxyresorufin O-dealkylation, tolbutamide hydroxylation, bufuralol 1’-hydroxylation, aniline hydroxylation and midazolam 4-hydroxylation by CYP1A, CYP2B, CYP2C, CYP2D, CYP2E and CYP3A, respectively, in non-treated male ICR mice aged between 3 and 8 weeks. The enzyme efficiency levels in 3- and 4-week-old mice were approximately similar to or higher than those in 8-week-old mice, except for CYP1A and CYP2E in 3- and 4-week-old mice, respectively. Since these results suggest that juvenile mice have sufficient activities for most CYP enzymes, we also conducted a liver MN assay using diethylnitrosamine (DEN), a rodent hepatocarcinogen, on male ICR mice aged between 3 and 6 weeks. A peripheral blood (PB) MN assay was performed simultaneously in 4-week-old mice. Assays incorporating DEN produced positive results in 3- and 4-week-old mice and showed a dose-dependent increase in the micronucleated hepatocyte frequencies at 4 weeks. Both the liver MN assay in 5- and 6-week-old mice and the PB MN assay had negative results when using DEN. These results suggest that 3- and 4-week-old mice have micronuclei-inducing potential in the liver to detect genotoxic compounds using the liver MN assay.

Oxidized and Si-doped graphene: emerging adsorbents for removal of dioxane

The adsorption properties of oxidized graphene (GO) and Si-doped graphene (SiG) towards 1,4-dioxane were theoretically characterized.

Differences in the origins of kinetochore-positive and kinetochore-negative micronuclei: A live cell imaging study

Micronuclei (MNi) are extensively used to evaluate genotoxicity and chromosomal instability. Classification of kinetochore-negative (K-MNi) and kinetochore-positive micronuclei (K+MNi) improves the specificity and sensitivity of the micronucleus (MN) test; however, the fundamental differences in the origins of K-MNi and K+MNi have not been addressed due to the limitations of traditional methods. In the current study, HeLa CENP B-GFP H2B-mCherry cells were constructed in which histone 2B (H2B) and centromere protein B (CENP B) were expressed as fusion proteins to monomeric Cherry (mCherry) and EGFP, respectively. MNi were identified using H2B-mCherry; K+MN contained CENP B-GFP, while K-MN did not. Long-term live cell imaging was conducted to examine MN formation in the dual-color fluorescent HeLa cells. The results suggested that K-MNi were derived from kinetochore-negative displaced chromosomes (K-DCs), kinetochore-negative lagging chromosomes (K-LCs) and fragments of broken chromosome bridges (CBs) during late mitotic stages. The results also indicated that K+MNi are derived from kinetochore-positive displaced chromosomes (K+DCs), kinetochore-positive lagging chromosomes (K+LCs), and fragments of broken CBs. Different aberrant chromosomes emerged during mitosis at different frequencies and developed into K-MNi and/or K+MNi in the daughter cells at different rates. K+LCs formed K+MNi at a higher frequency than K+DCs, and K-LCs formed K-MNi at a higher rate than K-DCs; however, broken CBs transformed into K-MNi and/or K+MNi. In summary, these results show that K-MNi and K+MNi have different origins in HeLa cells and that each mechanism of MN formation contributes differently to the overall number of K-MNi and K+MNi.

Recommended protocols for the liver micronucleus test: Report of the IWGT working group

At the 6th International Workshop on Genotoxicity Testing (IWGT), the liver micronucleus test working group discussed practical aspects of the in vivo rodent liver micronucleus test (LMNT). The group members focused on the three methodologies currently used, i.e., a partial hepatectomy (PH) method, a juvenile/young rat (JR) method, and a repeated-dose (RD) method in adult rodents. Since the liver is the main organ that metabolizes chemicals, the LMNT is expected to detect clastogens, especially those that need metabolic activation in the liver, and aneugens. Based on current data the three methods seem to have a high sensitivity and specificity, but more data, especially on non-genotoxic but toxic substances, would be needed to fully evaluate the test performance. The three methods can be combined with the micronucleus test (MNT) using bone marrow (BM) and/or peripheral blood (PB). The ability of the PH method to detect both clastogens and aneugens has already been established, but the methodology is technically challenging. The JR method is relatively straightforward, but animal metabolism might not be fully comparable to adult animals, and data on aneugens are limited. These two methods also have the advantage of a short testing period. The RD method is also straightforward and can be integrated into repeated-dose (e.g. 2 or 4 weeks) toxicity studies, but again data on aneugens are limited. The working group concluded that the LMNT could be used as a second in vivo test when a relevant positive result in in vitro mammalian cell genotoxicity tests is noted (especially under the condition of metabolic activation), and a negative result is observed in the in vivo BM/PB-MNT. The group members discussed LMNT protocols and reached consensus about many aspects of test procedures. However, data gaps as mentioned above remain, and further data are needed to fully establish the LMNT protocol.

The rat gut micronucleus assay: a good choice for alternative in vivo genetic toxicology testing strategies

The in vivo bone marrow (BM) micronucleus assay is one of the three tests in the standard test battery to assess the genotoxic potential of a pharmaceutical candidate. In some cases, depending on results of in vitro studies, the route of administration or the degree of systemic exposure, in vivo assessment of genotoxicity in the BM alone may not be sufficient. Based on the potential for high gut exposures to orally administered compounds with low systemic exposures as well as the potential susceptibility of rapidly dividing cells of the intestinal tissues, we have developed a modified technique for evaluating micronuclei formation in both the duodenum and colon of rats based on earlier publications. Adult male Sprague Dawley rats were treated once daily for 2 days with either vehicle control or with the test articles acetyl salicylic acid (ASA), carbendazim (CAR), cyclophosphamide (CP), dimethylhydrazine (DMH), mitomycin C (MMC) or vinblastine sulfate (VIN). The duodenum, colon, and BM were harvested, processed, and analyzed for micronucleus induction. Results from these studies demonstrated differences in the susceptibility for different test compounds in the three tissues tested. When MMC and VIN were dosed by different routes at the same dose levels both compounds produced positive results in all three tissues by intraperitoneal injection but not oral administration. These studies suggest that overall the GI micronucleus assay might be a useful tool for clastogenic and aneugenic compounds that are expected to produce high sustained concentrations in the gastrointestinal tract with little systemic exposure.

Micronucleus assays in rodent tissues other than bone marrow

This report updates previous reviews that were conducted as part of the third and fourth International Workshops on Genetic Toxicology Testing of micronucleus (MN) assays in rodent tissues other than bone marrow. Tissues discussed here are liver, lung, skin, colon, spleen, testes and foetal/neonatal tissues with transplacental exposure. Previous reviews have been updated to include literature published after 2000. In addition to the previously described tissues, MN assays in bladder, buccal mucosal cells, stomach and vagina are also included. MN assays using tissues other than bone marrow are critical for risk assessments, for in situ evaluation and for studies of systemic genotoxic effects and modes of action. Protocols for the majority of assays in tissues other than bone marrow have not yet been well standardised and validated for regulatory application, and further development is needed to support regulatory studies.

Two-year inhalation study of carcinogenicity and chronic toxicity of 1,4-dioxane in male rats

Carcinogenicity and chronic toxicity of 1,4-dioxane were examined by inhalation exposure of 50 male F344 rats to 1,4-dioxane vapor at 0 (clean air), 50, 250, or 1250 ppm (v/v) for 6 h/day, 5 days/wk, and 104 wk. Survival rates of 250 and 1250 ppm-exposed groups were decreased near the end of the 2-yr exposure period, due probably to the occurrence of malignant tumors. A statistically significant but marginal decrement of terminal body weight (<10%) was found in the 1250 ppm-exposed group, suggesting slight systemic toxicity. Significant changes in plasma levels of AST, ALT, ALP, and gamma-GTP and relative weight of the liver occurred in the 1250 ppm-exposed group. Dose-dependent and statistically significant increases in incidences of nasal squamous cell carcinomas, hepatocellular adenomas, and peritoneal mesotheliomas were found primarily in the 1250 ppm-exposed group. The incidences of renal cell carcinomas, fibroadenomas in the mammary gland, and adenomas in the Zymbal gland were also increased dose-dependently. Preneoplastic lesions occurred in the nasal cavity and liver of the 1,4-dioxane-exposed groups. As nonneoplastic lesions, the significantly increased incidences of nuclear enlargement, atrophy, and respiratory metaplasia in the nasal cavity were noted at 50 ppm and above. A LOAEL (lowest observed adverse effect level) was determined at 50 ppm for the nasal endpoint of general chronic toxicity. This study provides clear evidence of carcinogenicity for 1,4-dioxane in male rats. A cytotoxic-proliferative and in vivo genotoxic mode of action is suggested to operate in 1,4-dioxane-induced carcinogenesis.

Carcinogenicity studies of 1,4-dioxane administered in drinking-water to rats and mice for 2 years

The carcinogenicity of 1,4-dioxane was examined by giving groups of 50 F344/DuCrj rats and 50 Crj:BDF(1) mice of each sex 1,4-dioxane in the drinking-water for 2 years. The concentrations of 1,4-dioxane were 0 (control), 200, 1000 and 5000 ppm (wt./wt.) for rats and 0, 500, 2000 and 8000 ppm for mice. The highest dose levels did not exceed the maximum tolerated dose. In the rat, there was significant induction of nasal squamous cell carcinomas in females and hepatocellular adenomas and carcinomas in males and females, peritoneal mesotheliomas in males, and mammary gland adenomas in females. In the mouse, there was significant induction of hepatocellular tumors in males and females. Two nasal tumors occurring in the 8000 ppm-dosed groups were spontaneously rare and, thus, were attributed to 1,4-dioxane exposure. The present studies provided clear evidence of carcinogenicity in rats and mice. Lifetime cancer risk of humans exposed to 1,4-dioxane through drinking-water was quantitatively estimated with a non-threshold approach by application of a linearized multistage model to dose-carcinogenic response relationships, in addition to a threshold approach for estimation of the tolerable daily intake using no-observed- or lowest-observed-adverse-effect levels of the carcinogenic responses and uncertainty factors.

Multiple origins of spontaneously arising micronuclei in HeLa cells: direct evidence from long-term live cell imaging

Although micronuclei (MNi) are extensively used to evaluate genotoxic effects and chromosome instability, the most basic issue regarding their origins has not been completely addressed due to limitations of traditional methods. Recently, long-term live cell imaging was developed to monitor the dynamics of single cell in a real-time and high-throughput manner. In the present study, this state-of-the-art technique was employed to examine spontaneous micronucleus (MN) formation in untreated HeLa cells. We demonstrate that spontaneous MNi are derived from incorrectly aligned chromosomes in metaphase (displaced chromosomes, DCs), lagging chromosomes (LCs) and broken chromosome bridges (CBs) in later mitotic stages, but not nuclear buds in S phase. However, most of bipolar mitoses with DCs (91.29%), LCs (73.11%) and broken CBs (88.93%) did not give rise to MNi. Our data also show directly, for the first time, that MNi could originate spontaneously from (1) MNi already presented in the mother cells; (2) nuclear fragments that appeared during mitosis with CB; and (3) chromosomes being extruded into a minicell which fused with one of the daughter cells later. Quantitatively, most of MNi originated from LCs (63.66%), DCs (10.97%) and broken CBs (9.25%). Taken together, these direct evidences show that there are multiple origins for spontaneously arising MNi in HeLa cells and each mechanism contributes to overall MN formation to different extents.