Mutagenic properties of dimethylaniline isomers in mice as evaluated by comet, micronucleus and transgenic mutation assays

Background The carcinogenic potential of dimethylaniline (DMA) isomers in rodents and humans has been previously reported, and there is sufficient evidence for the carcinogenicity of 2,6-DMA in experimental animals. The target organ of carcinogenesis of 2,6-DMA is the nasal cavity. In the current study, six DMA isomers, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-DMA, were evaluated for mutagenic properties. Results Male ddY mice (3/group) were treated intragastrically (i.g.) with 200 mg/kg of one of the six DMAs, and a comet assay was performed on samples of bone marrow, kidney, liver and lung at 3 and 24 h after the treatment. Positive responses were observed in the kidney, liver and lungs of mice from all of the DMA treatment groups after 3 h and in the bone marrow of mice treated with either 3,4- or 3,5-DMA after 3 h; however, these effects were diminished at the 24 h time point. The micronucleus induction in the bone marrow was analysed in the same mouse at 24 h after the treatment. No induction of micronucleated polychromatic erythrocytes was observed after treatment with any of the DMAs. Male transgenic Muta™ mice (five/group) were treated i.g. with 2,5-, 2,6- or 3,5-DMA at 100 mg/kg bw weekly for 4 weeks, and the lacZ and the cII mutation frequencies were examined in the nasal cavity, liver and bone marrow at 7 days after the last treatment. Statistically significant increases in the mutation frequencies of the lacZ and/or cII genes were observed in the nasal cavity of 2,5-DMA or 2,6-DMA treated mice. Sequence analysis showed increased incidences of AT to GC and GC to TA mutations in the nasal tissues. Conclusions These findings suggest that the carcinogenic activities of DMAs are associated with mutagenic events. Electronic supplementary material The online version of this article (10.1186/s41021-018-0106-3) contains supplementary material, which is available to authorized users.


Background
The structures of six dimethylaniline (DMA) isomers, a benzene-ring with two methyl functional groups and one amino functional group, are shown in Fig. 1. The carcinogenic potential of some DMAs has been previously reported. In a two-year carcinogenicity study, male and female rats that were fed diets containing 2,6-DMA at 3000 ppm showed significant increases in the incidences of adenocarcinomas or carcinomas of the nasal cavity and of the papillary adenomas of the nasal cavity, relative to control animals [1]. Rhabdomyosarcoma, a rare tumour of the nasal cavity, was observed in rats of each sex. Additionally, increased incidence of subcutaneous fibromas and fibrosarcomas in male and female rats and an increased incidence of neoplastic nodules of the livers of female rats were observed. In two-stage nasal carcinogenesis models using male rats, the tumour-promoting activity of 2,6-DMA was evident [2][3][4]. Another carcinogenicity study demonstrated that 2,4-DMA induced pulmonary tumours in female mice and that 2,5-DMA led to an increase in subcutaneous fibromas and fibrosarcoma in male rats and in vascular tumours in male mice [5].
Humans can be exposed to DMAs via various sources. For example, 2,6-DMA exists in tobacco leaves and is detected in tobacco smoke [6] and may also enter the environment through the degradation of some pesticides [1]. Some anaesthetics contain a DMA moiety; in particular, lidocaine is known to be metabolized to 2,6-DMA. After lidocaine treatment, 2,6-DMA-haemoglobin adducts and 2,6-DMA-DNA adducts in the urinary bladder were increased in humans [7,8]. The use of permanent hair dye is associated with a level of 3,5-DMA-haemoglobin adducts [9]. Importantly, two epidemiologic studies conducted with six isomers of DMA or 2,6-DMA showed that levels of haemoglobin adducts of 2,6-and 3,5-DMA in peripheral blood samples were significantly associated with an increased risk of bladder cancer in humans [9,10].
Although a wealth of in vitro data demonstrating the genotoxicity of DMAs exists, there are currently no reports of in vivo gene mutation assays for any of DMA isomers. A transgenic mouse model constructed to assay mutations in the target organs of carcinogenicity is useful to evaluate if chemical-induced carcinogenesis is related to mutagenic events. In the present study, the transgenic mouse mutation assay was conducted using Muta™ mice to assess in vivo mutagenicity of 2,5-, 2,6or 3,5-DMA in the nasal cavity, liver and bone marrow. In addition, a comet assay was performed with the bone marrow, kidney, liver and lung of ddY mice at 3 and 24 h after treatment with the isomers 2,3-, 2,4, 2,5-, 2,6-, 3,4-and 3,5-DMA. The comet assay is useful to evaluate a broad spectrum of types of DNA damage. This paper reports the results of the transgenic mouse mutation assay and comet assay for DMAs. A bone marrow micronucleus assay in the ddY mice, and a peripheral blood micronucleus assay in the Muta™ mice were also conducted, the results of which are briefly reported in this paper.

Animals
The comet assay was performed using male ddY mice obtained from Japan SLC (Shizuoka, Japan). The bone marrow micronucleus assay was also conducted in these animals. The gene mutation assay was performed with male Muta™ mice, supplied by Covance Research Products (PA, USA). The peripheral blood micronucleus assay was also conducted in these animals. All animals were housed in a room maintained at 20-24°C and 55-65% humidity with a 12 h light-dark cycle, fed commercial pellets (Oriental Yeast Industries Co., Tokyo, Japan) and given tap water ad libitum. Animal experiments were performed in accordance with the recommendations of the ethics committee of the institution. Dose (100, 200 mg/kg) were set based on the reported mouse LD 50 of DMAs (250-1070 mg/kg) [19].

Comet assay in ddY mice
Three ddY mice/group were treated with 0 (vehicle: olive oil) or 200 mg/kg of one of the six isomers of DMA by oral gavage at a volume of 10 ml/kg body weight. The animals were sacrificed by cervical dislocation at 3 h or 24 h after treatment, and the bone marrow, liver, kidney and lung were collected and immediately processed for the comet assay. The tissues were washed in saline, minced and suspended at a concentration of 1 g/ml in ice cold homogenizing buffer (HBSS with 20 mM EDTA, 10% DMSO, pH 7.5) and gently homogenized. The femoral bone marrow samples collected at 24 h after treatment were divided into two portions. One portion was suspended in chilled homogenizing buffer and used for the comet assay. The other portion was used for the micronucleus assay. The cell suspensions were appropriately diluted in chilled homogenizing buffer and subjected to the comet assay.
Normal melting point agarose (NMA: 1%, 250 μl) was placed on slides and dried overnight. The cell suspensions (10 μl) and 75 μl of 0.5% low melting point agarose (LMA) (37°C) were placed onto the 1% NMA. The slides were covered with a cover slip and placed on a chilled plate to allow complete solidification of agarose. Finally, 75 μl of 0.5% LMP agarose was quickly layered in the same manner after removing the cover slip. The slides were immersed in freshly made ice-cold lysis solution (2.5 M NaCl, 100 mM EDTA, 10 mM Tris (pH 10.0), 10% DMSO and 1% Triton X-100) in the dark at 4°C for at least 60 min. The slides were then placed in a horizontal electrophoresis tank containing electrophoresis buffer (300 mM NaOH and 1 mM EDTA; pH 13.0 and higher) for 10 min, allowing salt equilibration and further DNA denaturation before electrophoresis at 1 V/cm, 300 mA for 20 min. The slides were washed three times (10 min each) with chilled neutralizing buffer (400 mM Tris, pH 7.5). Following the third wash, the slides were stained with 20 μg/ml EtBr and covered with a cover slip. To prevent drying, the slides were stored in a humidified container until microscopic examination. The same electrophoresis unit and power supply were used throughout the study.
The slides were examined using a fluorescent microscope (Olympus, Tokyo Japan). All slides were coded and examined blindly. A total of 500 randomly selected cells were examined per sample (animal). The comets were classified into five categories according to the report by Kobayashi et al. [20], [Type 1: without tail (no damage), Type 2: with a small tail, Type 3: with tail showing evident migration, Type 4: with definite tale showing a consistent amount of fragments, Type 5: Almost all of the DNA is present in the tail (severe genetic damage)] depending on the fraction of DNA migrated out into the tail.
Statistical analysis was made on the incidences of type I cells against corresponding control by the Student's t-test.

Micronucleus assay in the bone marrow of ddY mice
A portion of the cell suspension from the bone marrow of the above-described ddY mice was smeared onto a clean glass slide. After air-drying, the bone marrow smears were fixed and stained with Giemsa. For each animal, 1000 polychromatic erythrocytes (PCE) were examined for the presence of micronuclei. The ratio of PCE to normochromatic erythrocytes (NCE) was determined.

Transgenic mouse mutation assay in Muta™ mice
Based on the number of ortho substitution of aniline, 2,5-[1], 2,6- [2], and 3,5-DMA (0) were selected and subjected to the transgenic mouse mutation assay. Groups of five Muta™ mice were administered 2,5-, 2,6or 3,5-DMA by gavage once a week for 4 weeks at a volume of 10 ml/kg, at 100 mg/kg. Separate groups of the vehicle control (olive oil) were treated and maintained in the same manner. The liver, entire nasal cavity and bone marrow were collected immediately after sacrificing the animals 7 days after the last treatment. The tissue samples were quickly frozen in liquid nitrogen and then stored at − 80°C until analysis. The genomic DNA was extracted from each tissue by the standard phenol/chloroform method as previously reported [21]. Briefly, homogenized tissues were incubated with RNase and proteinase K, and genomic DNA was extracted using a phenol/chloroform mixture and chloroform followed by ethanol precipitation and then dissolved in TE-4 buffer (10 mM Tris-HCl at pH 8.0 containing 4 mM EDTA).
The bacteriophage lambda vectors were recovered by in vitro packaging reactions. The DNA solution was gently mixed with the Transpack packaging extract (Stratagene) and incubated at 37°C for 1.5 h twice. The positive selections for lacZ and cII mutants were performed as previously reported [21] with a slight modification. As for the lacZ model, the phage solution was absorbed to E. coli C (lac − , galE − ) at room temperature for 20-30 min. For the titration, appropriately diluted E. coli solutions were mixed with LB top agar (containing 10 mM MgSO 4 ) plated onto dishes containing bottom agar and the number of plaques formed were counted. The remaining phage-E. coli solution was mixed with LB top agar containing P-gal and plated as described. The plates were incubated overnight at 37°C for selecting lacZ mutants. As for the cII selection, the phage solution was absorbed to E. coli G1225 (hfl − ) at room temperature for 20 min. Appropriately diluted E. coli solutions were mixed with LB top agar (containing 10 mM MgSO 4 ) and plated onto dishes containing bottom agar and incubated at 37°C for 24 h for titration. For the selection of cII mutants, the remaining phage-E. coli solutions were plated similarly but incubated at 25°C for 48 h. The mutant frequency was calculated by the number of plaques on the selection plates divided by the total plaques.

DNA sequence analysis
Sequencing of cII mutants was conducted as follows. The entire lambda cII region was amplified directly from mutant plaques by Taq DNA polymerase with primers P1; 5′-AAAAAGGGCATCAAATTAAACC-3′ and P2; 5′-CCGAAGTTGAGTATTTTTGCTGT-3′, as previously reported [21]. PCR products for 446 bp, involving the entire (294 bp) cII gene, were purified with a Micro-Spin S-400 HR column (Amersham) and then used for a sequencing reaction with the Ampli Taq cycle sequencing kit (Perkin-Elmer Biosystem, Tokyo Japan) using the primer P1. The reaction product was purified by ethanol precipitation and analysed by the ABI PRISM™ 310 Genetic Analyzer (Perkin-Elmer Biosystems, Tokyo, Japan).

Micronucleus assay in the peripheral blood of Muta™ mice
Blood samples were collected from the tail vein of the Muta™ mice used in the transgenic mouse mutation assay at 48 h after the first treatment and examined for micronucleated reticulocytes (MNRETs) using acridine orange supravital staining. In this experiment, 1000 reticulocytes (RETs) per animal were examined for the incidence of micronucleus.

Statistical analysis
The significance of differences in the mutant frequency between the treated and control groups was analysed using Student's t-test. Significance was indicated by P values < 0.05.

Comet assay in ddY mice
The distribution pattern of DNA damage in terms of DNA migration among the different treatment groups is expressed as the percent of cells in the five comet classes from Type 1 to Type 5, as presented in Figs. 2 and 3. At 3 h, the numbers of comet cells classified as Type 3 and/ or Type 4 were increased in the kidney and lung for all of the DMAs. Type 2 and/or Type 3 cells were also increased in the liver for all DMA treatment groups and in the bone marrow for 3,4-and 3,5-DMA (Fig. 2). Among six DMA isomers, incidence of DNA damage was higher after treatment with 2,3-, 2,4-and 2,5-DMA in the kidney and 2,5-DMA in the liver. Incidence of DNA damage was lower in the lungs of mice treated with 3,4-and 3,5-DMA. At 24 h after treatment, the DNA damage was recovered in mice from all treatment groups (Fig. 3). Detailed data were available in Additional file 1.

Transgenic mutation assay in Muta™ mice
The MFs in nasal tissues, liver and bone marrow induced by 2,5-, 2,6-and 3,5-DMA are shown in Tables 2, 3 and 4. The MFs of the lacZ genes in 2,5and 2,6-DMA-treated mice, and the cII genes in 2,5-DMA-treated mice were significantly increased in the nasal tissues. The MF of the cII genes in the 2,6-DMA treatment group was also increased in the nasal tissues, but the change was not statistically significant. No changes were found on the MFs of the lacZ and cII genes in the liver for 2,5-, 2,6-and 3,5-DMAs. As for the bone marrow, an increase in the MF of the lacZ genes was observed in mice treated with 2,5-DMA, but these increases are not considered biologically significant because total plaques of four animals in the control group were inadequate for evaluation.
DNA sequence analysis Table 5 shows a summary of the cII mutation spectra induced in the nasal tissues by 2,5-and 2,6-DMA. The sequence analysis showed an increased incidence of AT to GC transitions and GC to TA transversions. At CpG sites, transition of C to T, observed in the control group, was reduced by 2,6-DMA treatment.

Discussion
In the comet assay conducted in ddY mice, DNA damage was observed in the lung, kidney and liver for all of the DMAs and in the bone marrow of mice treated with 3,4-and 3,5-DMA at the 3 h timepoint. However, these types of damage were recovered in all of the mice at 24 h. Przybojewska et al. (1999) reported that DNA damage was observed in the liver cells of B6C3F1 male mice exposed to 2,4-DMA (i.p. injection) at 100 and 200 mg/kg bw 16 h after administration [22]. In another   These results indicate that DMA isomers cause DNA damage in various organs. In the current comet assay, all of the DNA damage was recovered within 24 h. This is inconsistent with a study conducted by Sasaki et al. (1999), in which a positive result was observed at 24 h in the lungs, similar to that described above [23]. The dosage difference (200 mg/kg vs 350 mg/kg) might affect the difference in recovery. DNA damage caused by DMAs in the lung may persist under certain conditions. It should also be noted that 2,4-DMA has been shown to cause pulmonary tumours in female mice [5].
In the current in vivo micronucleus assays, no changes in the frequency of micronuclei were observed for all tested isomers of DMAs in the bone marrow or peripheral blood in mice. In two previous bone marrow micronucleus assays [24,25], treatment with 2,6-DMA did not increase the frequency of micronuclei in the bone marrow of mice and did not change the PCE/NCE ratio in ICR mice, which is consistent with our results. Although in vitro chromosomal aberration tests indicated clastogenicity of 2,3-, 2,4-, 2,6-and 3,5-DMA with or without the addition of S9 [11,[14][15][16], it can be concluded that DMAs are not clastogenic in haematopoietic systems in vivo at 200 mg/kg (20-80% of LD 50 ). The difference between in vitro and in vivo results might be explained by the concentration of active metabolites in the bone marrow because the PCE/NCE ratio was not changed.
In bacterial reverse mutation assays, positive or weak positive findings were observed for six isomers of DMA when S9 mix was added [11][12][13]15]. DMAs undergo metabolic N-hydroxylation, and all six N-hydroxy-DMAs were mutagenic in S. typhimurium TA100 [26]. After N-hydroxylation, DMAs are further metabolized, and formation of haemoglobin adducts and DNA adducts has been observed in several previously conducted studies [7,[26][27][28]. The DNA adduct in the nasal cavity, the major target organ of carcinogenicity, has been shown to Table 3 Mutation frequency in the lacZ and cII genes from the liver of Muta™ mice treated with 2,5-, 2,6-and 3,5-DMAs a Data with at least 100,000 total plaques were evaluated by the statistical analysis and a parenthesis indicates exclusion be higher than in other organs such as the liver, urinary bladder and testes after administration of 2,6-DMA [8,29,30]. Tydén et al. (2004) showed that the capacity of CYP enzymes to activate DMAs in the nasal mucosa was higher than other tissues, such as the liver and forestomach [31]. In the current transgenic mutation assay using Muta™ mice, the MFs of the lacZ and/or cII genes were significantly increased in the nasal cavity of mice treated with 2,5-and 2,6-DMA, but not in the liver. Our findings suggest that the carcinogenic activities in the nasal cavity are related to mutagenic events. This is the first report showing a positive response in the nasal tissue by the transgenic mouse mutation assay. It is important to use the target organs of carcinogenesis for analysis in this assay. Sequence analysis showed an increased incidence of AT to GC transitions and GC to TA transversions in the nasal tissues of mice treated with 2,5-and 2,6-DMA. Multiplicity of 2,6-DMA or 3,5-DMA adducts have been found by in vitro investigations [27,28]. The dA (2′-deoxyadenosine) adduct was the most abundant product of the reaction of N-acetoxy-2,6-DMA with DNA [28], while the dG (2′-deoxyguanosine) adduct was the major product of the reaction of N-acetoxy-3,5-DMA with DNA [27]. In the current study, transition of C to T, normally observed in the control group, was reduced at CpG sites by 2,6-DMA treatment, which suggested a biologically significant increase in the MF. It is possible that adenine adducts may induce AT to GC transitions and are thus involved in the carcinogenic activity observed as a result of 2,6-DMA treatment, although the structure of the DMA-adducts have not been identified. Because the AT to GC transition is relatively rare type of mutation, it can be used as a molecular signature after confirmation in other systems.
Although the MFs of the lacZ and/or cII genes were increased by 2,5-and 2,6-DMA treatment, no changes were observed in the nasal tissues as a result of 3,5-DMA treatment. This result may suggest that the MFs of lacZ gene were not statistically evaluated because total plaques of 4 animals in the control group were less than 100,000 a Data with at least 100,000 total plaques were evaluated by the statistical analysis and a parenthesis indicates exclusion ortho-substituted DMAs are more mutagenic than meta-substituted DMA in the nasal cavity of rats. Marques et al. (1997) also showed that the position of methyl substitutes in DMAs influenced the mutagenic potential in S. typhimurium TA 100 and the stability of adduct formations [26]. In the current comet assay, meta-substituted DMAs, 3,4-and 3,5-DMAs, and four ortho-substituted isomers showed differing activity of DNA damage. That is to say, only 3,4-and 3,5-DMA caused DNA damage in the bone marrow, while they were shown to cause relatively weak DNA damage in the lung. As for 2,4-and 2,6-DMA, a species difference in metabolic pathways was found between rats and dogs, and general toxicity for repeated dosing was stronger for 2,4-DMA in rats and for 2,6-DMA in dogs [32]. Because isomer differences and species differences likely play a role in DMA toxicity, further investigation is necessary to gain a better understanding of the relationship between DMAs and carcinogenicity in humans.

Conclusions
DMA isomers caused DNA damage in various organs, as evaluated by the comet assay, and isomer-specific activity was found. In the in vivo gene mutation assay, 2,5-and 2,6-DMA increased gene mutations in the nasal cavity and increased the incidence of AT to GC transitions and GC to TA transversions, suggesting that carcinogenic activities of DMAs are associated with mutagenic events. The pre-existing data for genotoxicity of DMAs are conflicting and not clearly demonstrated the genotoxicity of DMAs. The current result of the transgenic mouse mutation assay emphasized an importance of the in vivo test at the target organ for carcinogenesis.

Additional file
Additional file 1: Table S1. Types of comet induced by DMAs at 3 and 24 h after the treatment in bone marrow, liver, kidney and lung of ddY mouse. (XLS 123 kb)

Funding
This study was supported by the Ministry of Health, Labour and Welfare, Japan.

Availability of data and materials
All data generated or analyzed during this study are included in this published article.
Authors' contributions AK and TS performed the experiments and collected the data. MM mainly wrote the manuscript. AH, MH, and MH made discussions and suggestions on the manuscript. TS designed the experiment and is responsible for the final manuscript. All authors read and approved the final manuscript.

Ethics approval and consent to participate
The ethical approval for the animal experiment was obtained from the institutional animal ethics committee before conducting the experiment. Animal experiment has been done following the guidance provided by the committee, with a careful attention to animal welfares. No human subject was used.

Consent for publication
Not applicable.