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

Clonality, trafficking, and molecular alterations among Hprt mutant T lymphocytes isolated from control mice versus mice treated with N-ethyl-N-nitrosourea

Mutations in T lymphocytes (T-cells) are informative quantitative markers for environmental mutagen exposures, but risk extrapolations from rodent models to humans also require an understanding of how T-cell development and proliferation kinetics impact mutagenic outcomes. Rodent studies have shown that patterns in chemical-induced mutations in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) gene of T-cells differ between lymphoid organs. The current work was performed to obtain knowledge of the relationships between maturation events during T-cell development and changes in chemical-induced mutant frequencies over time in differing immune compartments of a mouse model. A novel reverse transcriptase-polymerase chain reaction based method was developed to determine the specific T-cell receptor beta (Tcrb) gene mRNA expressed in mouse T-cell isolates, enabling sequence analysis of the PCR product that then identifies the specific hypervariable CDR3 junctional region of the expressed Tcrb gene for individual isolates. Characterization of spontaneous Hprt mutant isolates from the thymus, spleen, and lymph nodes of control mice for their Tcrb gene expression found evidence of in vivo clonal amplifications of Hprt mutants and their trafficking between tissues in the same animal. Concurrent analyses of Hprt mutations and Tcrb gene rearrangements in different lymphoid tissues of control versus N-ethyl-N-nitrosourea-exposed mice permitted elucidation of the localization and timing of mutational events in T-cells, establishing that mutagenesis occurs primarily in the pre-rearrangement replicative period in pre-thymic/thymic populations. These findings demonstrate that chemical-induced mutagenic burden is determined by the combination of mutagenesis and T-cell clonal expansion, processes with roles in immune function and in the pathogenesis of autoimmune disease and cancer.

Analysis of DNA Adducts and Mutagenic Potency and Specificity in Rats Exposed to Acrylonitrile

Acrylonitrile (ACN), which is a widely used industrial chemical, induces cancers in multiple organs/tissues of rats by unresolved mechanisms. For this report, evidence for ACN-induced direct/indirect DNA damage and mutagenesis was investigated by assessing the ability of ACN, or its reactive metabolite, 2-cyanoethylene oxide (CEO), to bind to DNA in vitro, to form select DNA adducts [N7-(2'-oxoethyl)guanine, N²,3-ethenoguanine, 1,N⁶-ethenodeoxyadenosine, and 3,N⁴-ethenodeoxycytidine] in vitro and/or in vivo, and to perturb the frequency and spectra of mutations in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) gene in rats exposed to ACN in drinking water. Adducts and frequencies and spectra of Hprt mutations were analyzed using published methods. Treatment of DNA from human TK6 lymphoblastoid cells with [2,3-¹⁴C]-CEO produced dose-dependent binding of ¹⁴C-CEO equivalents, and treatment of DNA from control rat brain/liver with CEO induced dose-related formation of N7-(2'-oxoethyl)guanine. No etheno-DNA adducts were detected in target tissues (brain and forestomach) or nontarget tissues (liver and spleen) in rats exposed to 0, 3, 10, 33, 100, or 300 ppm ACN for up to 105 days or to 0 or 500 ppm ACN for ∼15 months; whereas N7-(2'-oxoethyl)guanine was consistently measured at nonsignificant concentrations near the assay detection limit only in liver of animals exposed to 300 or 500 ppm ACN for ≥2 weeks. Significant dose-related increases in Hprt mutant frequencies occurred in T-lymphocytes from spleens of rats exposed to 33-500 ppm ACN for 4 weeks. Comparisons of "mutagenic potency estimates" for control rats versus rats exposed to 500 ppm ACN for 4 weeks to analogous data from rats/mice treated at a similar age with N-ethyl-N-nitrosourea or 1,3-butadiene suggest that ACN has relatively limited mutagenic effects in rats. Considerable overlap between the sites and types of mutations in ACN-exposed rats and butadiene-exposed rats/mice, but not controls, provides evidence that the carcinogenicity of these epoxide-forming chemicals involves corresponding mutagenic mechanisms.

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.

Genotoxic sensitivity of the developing hematopoietic system

Genotoxic sensitivity seems to vary during ontogenetic development. Animal studies have shown that the spontaneous mutation rate is higher during pregnancy and infancy than in adulthood. Human and animal studies have found higher levels of DNA damage and mutations induced by mutagens in fetuses/newborns than in adults. This greater susceptibility could be due to reduced DNA repair capacity. In fact, several studies indicated that some DNA repair pathways seem to be deficient during ontogenesis. This has been demonstrated also in murine hematopoietic stem cells. Genotoxicity in the hematopoietic system has been widely studied for several reasons: it is easy to assess, deals with populations cycling also in the adults and may be relevant for leukemogenesis. Reviewing the literature concerning the application of the micronucleus test (a validated assay to assess genotoxicity) in fetus/newborns and adults, we found that the former show almost always higher values than the latter, both in animals treated with genotoxic substances and in those untreated. Therefore, we draw the conclusion that the genotoxic sensitivity of the hematopoietic system is more pronounced during fetal life and decreases during ontogenic development.

The frequency of Pig-a mutant red blood cells in rats exposed in utero to N-ethyl-N-nitrosourea

The Pig-a assay has been developed as a rapid sensitive measure of gene mutation in adult rats; however, no data exist on its ability to detect mutation following in utero exposures or in neonatal animals. Pregnant Sprague-Dawley rats were treated daily on gestational days 12-18 with oral doses of 0, 6, or 12 mg/kg/day N-ethyl-N-nitrosourea (ENU); following parturition, the offspring and dams were monitored over a period of 5 months for the frequency of CD59-deficient erythrocytes as a marker of Pig-a mutation. Significant dose-related increases in Pig-a mutant red blood cells (RBCs) were observed in ENU-treated dams. However, only very weak increases in RBC Pig-a mutant frequency (MF) were noted in offspring treated in utero with the lower ENU dose. The higher ENU dose produced extremely variable responses in the offspring as a function of age, even among littermates, ranging from a steady low or moderately high Pig-a MF to a rapidly increasing or decreasing Pig-a MF. The manifestation kinetics of Pig-a mutant RBCs in the offspring suggest that the change from predominantly hepatic to predominantly bone marrow erythropoiesis that occurs during early development may have contributed to this variability. Our results indicate that using the RBC Pig-a model for mutation detection in animals treated in utero may require analysis of multiple offspring from the same litter to account for potential "jack pot" effects, and that detection of the earliest treatment effect (i.e., in neonates using the hepatic RBC fraction) may require optimization of blood processing.

Transplacental drug transfer and frequency of Tk and Hprt lymphocyte mutants and peripheral blood micronuclei in mice treated transplacentally with zidovudine and lamivudine

In previous studies, we have shown that zidovudine (3'-azido-3'-deoxythymidine; AZT), but not lamivudine [(-)2',3'-dideoxy-3'-thiacytidine; 3TC], is genotoxic when administered to neonatal mice, and that 3TC when coadministered with AZT does not alter the responses observed with AZT alone (Von Tungeln et al. [2002] Carcinogenesis 23:1427-1432). We now have investigated the transplacental transfer of these drugs and the induction of mutants and micronuclei in the neonatal offspring. From gestational day 12 until parturition, female C57BL/6N and C57BL/6N/Tk(+/-) mice, which had been mated to male C3H/HeNMTV mice, were treated daily by gavage with AZT, 3TC, or a combination of AZT and 3TC. In both dams and fetuses, AZT was found at much higher levels than its metabolites, AZT 5'-glucuronide and 3'-azido-3'-deoxythymidine. In the neonates, AZT and the mixture of AZT and 3TC caused a decrease in the percentage of reticulocytes (RETs) and an increase in the percentage of micronucleated RETs and micronucleated normochromatic erythrocytes. When assessed 3 weeks after birth, AZT and the combination of AZT and 3TC increased the thymidine kinase (Tk) mutant frequency in male mice; at 5 weeks, 3TC increased the Tk mutant frequency in female mice. The increase in Tk mutants in mice treated with AZT and the mixture of AZT and 3TC was associated with loss of the wild-type (Tk(+)) allele (loss of heterozygosity; LOH) and a pattern of discontinuous LOH. These data indicate that AZT, 3TC, and the combination of AZT and 3TC are transplacental mutagens and that the increase in mutants resulting from AZT is due mainly to large-scale genetic alterations.

Mutagenicity of zidovudine, lamivudine, and abacavir following in vitro exposure of human lymphoblastoid cells or in utero exposure of CD-1 mice to single agents or drug combinations

Experiments were performed to investigate the impact of zidovudine (AZT), lamivudine (3TC), and abacavir (ABC) on cell survival and mutagenicity in two reporter genes, hypoxanthine-guanine phosphoribosyltransferase (HPRT) and thymidine kinase (TK), using cell cloning assays for assessing the effects of individual drugs/drug combinations in (1) TK6 human lymphoblastoid cells exposed in vitro and (2) splenic lymphocytes from male CD-1 mice exposed transplacentally on days 12-18 of gestation. In TK6 cells, dose-related increases in HPRT and TK mutant frequencies were found following 3 days of exposure to AZT or 3TC alone (33, 100, or 300 microM), or to equimolar amounts of AZT-3TC. Compared with single drug exposures, AZT-3TC coexposures generally yielded enhanced elevations in HPRT and TK mutant frequencies. Mutagenicity experiments with ABC alone, or in combination with AZT-3TC, were complicated by the extreme cytotoxicity of ABC. Exposure of cells either to relatively high levels of AZT-3TC short-term (100 microM, 3 days), or to peak plasma-equivalent levels of AZT-3TC for an extended period (10 microM, 30 days), resulted in similar drug-induced mutagenic responses. Among sets of mice necropsied on days 13, 15, or 21 postpartum, Hprt mutant frequencies in T-cells were significantly elevated in the AZT-only (200 mg/kg bw/day) and AZT-3TC (200 mg AZT + 100 mg 3TC/kg bw/day) groups at 13 days of age. These results suggest that the mutagenicity by these nucleoside analogs is driven by cumulative dose, and raises the question of whether AZT-3TC has greater mutagenic effects than AZT alone in perinatally exposed children.

Mutagenicity at the Hprt locus in T cells of female mice following inhalation exposures to low levels of 1,3-butadiene

A study was conducted to test the hypothesis that repeated low level exposures to 1,3-butadiene (BD), approaching the OSHA occupational threshold for this chemical, produce a significant mutagenic response in mice. Female B6C3F1 mice (4-5 weeks of age) were exposed by inhalation for 2 weeks (6 h/day, 5 days/week) to 0 or 3 ppm BD, and then necropsied at 4 weeks after the cessation of exposures to measure the frequency of mutations (MF) at the Hprt locus using the T-lymphocyte clonal assay. At necropsy, T cells were isolated from spleen and cultured in the presence of mitogen, growth factors, and a selection agent. Cells were scored for growth on days 8-9 after plating to determine cloning efficiencies (CEs) and Hprt MFs. There was a marginal but significant reduction in the growth of splenic T cells from mice exposed to 3 ppm (n=27) compared with control mice (n=24) (P=0.004), suggesting the occurrence of BD-induced cytotoxicity at this low exposure concentration. In addition, the average Hprt MF in mice exposed to 3 ppm BD [1.54+/-0.82 (S.D.)x10(-6)] was significantly increased by 1.6-fold over the average control value of 0.96+/-0.51 (S.D.)x10(-6) (P=0.004). Comparisons of these data to earlier Hprt mutagenicity studies of mice exposed to high concentrations of BD (where significant mutagenic but not cytotoxic effects were observed) indicate that the ability to detect the cytotoxic and mutagenic responses of T cells to low levels of BD was enhanced by using a much larger sample size than usual for both the control and treatment groups. Additional analyses of the quantitative relationships between CE and MF demonstrated that CE had no significant effect upon MF values in sham-exposed control mice or mice exposed to low-level BD. Furthermore, the approaches for assessing the impact of CE and clonality on Hprt MFs in these control and BD-exposed mice were applied with the same rigor as in in vivo Hprt mutagenicity studies in human children. The overall study results support the conclusion that short-term low-level BD exposure is mutagenic in the mouse.