The induction of recessive mutations in mouse primordial germ cells with N-ethyl-N-nitrosourea

Transgene mutagenesis in the testicular cells of Muta™Mouse treated transplacentally with N-ethyl-N-nitrosourea at the primordial germ cell stages: Comparisons with the specific-locus test and the intragenic gene-recombination assay

N-Ethyl-N-nitrosourea (ENU) induces recessive mutations (RM) at a high frequency in male mouse primordial germ cells (PGCs)　in a dose-dependent and stage-specific manner when administered during embryonic development as confirmed by a specific locus test (SLT) (Shibuya et al., 1993, 1996 [1,2]). ENU also induces intragenic recombination (IGR) in the p^un allele at E10.5 in PGCs of male mice (Shibuya et al., 2022 [3]). In this study, the induced mutant frequencies (MF) in testicular cells of male Muta™Mousetreated at the same developmental stages of PGCs were determined with a positive selection system (MM/PS). Although the mutant frequencies　of MM/PS were consistently lower than for the SLT/RM, they showed similar stage-specificity and dose-dependency. Expressed as a linear equation, the correlation coefficient on the MF from SLT and MM/PS was extremely high (r² = 0.920).

Intragenic recombination within the pun allele of the pink-eyed dilution locus in pre-melanocytes and primordial germ cells of embryonic mice treated with N-ethyl-N-nitrosourea

The forward or reverse processes of intragenic recombination (IGR), which occur through the addition or deletion of duplicated homologous exons of the p^un allele in Pun mice, was observed in vivo, after introducing an homozygous p^un allele in a C57BL/6 background. We assessed the frequency of IGR upon N-ethyl-N-nitrosourea (ENU) treatment of pre-melanocytes (PMCs: somatic cells) and primordial germ cells (PGCs: germ cells) of embryonic mice at 10.5 days of development (E10.5). We simultaneously examined IGR and other mutations at the p locus of PMCs responsible for coat color in the offspring obtained by crossing p^un/p^un with p^un/P mice. The frequencies of both spontaneous and ENU-induced IGR were markedly higher than that of the recessive mutation (RM) in PMCs obtained from crossing C57BL/6 and PW strains (Shibuya et al., 1982). ENU also induces IGR at a higher frequency in PGCs at E10.5, which was observed in the next generation. These results indicate that ENU, which preferentially induces gene mutations through base substitution, also induces IGR at a high frequency in the p^un allele in both somatic and germ cells of embryonic mice at the E10.5 developmental stage.

Improvement of ENU Mutagenesis Efficiency Using Serial Injection and Mismatch Repair Deficiency Mice

ENU mutagenesis is a powerful method for generating novel lines of mice that are informative with respect to both fundamental biological processes and human disease. Rapid developments in genomic technology have made the task of identifying causal mutations by positional cloning remarkably efficient. One limitation of this approach remains the mutation frequency achievable using standard treatment protocols, which currently generate approximately 1–2 sequence changes per megabase when optimized. In this study we used two strategies to attempt to increase the number of mutations induced by ENU treatment. One approach employed mice carrying a mutation in the DNA repair enzyme Msh6. The second strategy involved injection of ENU to successive generations of mice. To evaluate the number of ENU-induced mutations, single mice or pooled samples were analyzed using whole exome sequencing. The results showed that there is considerable variability in the induced mutation frequency using these approaches, but an overall increase in ENU-induced variants from one generation to another was observed. The analysis of the mice deficient for Msh6 also showed an increase in the ENU-induced variants compared to the wild-type ENU-treated mice. However, in both cases the increase in ENU-induced mutation frequency was modest.

Poly (ADP-ribose) polymerase-1 deficiency does not affect ethylnitrosourea mutagenicity in liver and testis of lacZ transgenic mice

Poly (ADP-ribose) polymerase-1 (Parp1) has been implicated in DNA base excision repair, single- and double-strand break repair pathways, as well as in cell death by apoptosis or necrosis. We used Parp1(-/-) lacZ plasmid-based transgenic mice to investigate whether Parp1 deficiency influences the in vivo mutagenic and clastogenic response to the alkylating agent N-ethyl-N-Nitrosourea (ENU) in somatic and germ-cell tissues. The comparison of the lacZ mutant frequencies (MFs) between Parp1(+/+) and Parp1(-/-) mice showed that the ablation of Parp1 does not affect the spontaneous or ENU-induced MFs in liver and testis. In addition, the spectrum of the ENU-induced mutations was not dependent on the Parp1 status, given that similar spectra, consisting mostly of point mutations and a small fraction of deletions/insertions, wereobserved in organs of both Parp1(-/-) and Parp1(+/+) mice. Sequencing of point mutations revealed a consistent significant increase in A:T --> T:A base substitutions, typically induced by ENU. Overall, we observed that neither the frequency nor the spectrum of ENU-induced mutations demonstrated a specificity that could be attributed to the Parp1 impairment in mice organs. The analysis of micronucleus frequency in peripheral blood reticulocytes showed that ENU was clastogenic in both Parp1(-/-) and Parp1(+/+) mice and had a strong cytotoxic effect in Parp1(-/-) mice only. The present data suggest that, at a whole-organism level, Parp1-independent repair mechanisms may be operative in the removal of ENU-induced DNA lesions or that highly damaged cells may be preferentially committed to death when Parp1 is inactivated.

ENU mutagenesis as a tool for understanding lung development and disease

ENU (N-ethyl-N-nitrosourea) is a chemical mutagen that randomly induces point mutations in DNA. Since the 1990s ENU has been successfully used as a means to obtain mouse mutants using both gene-driven (reverse genetics) and phenotype-driven (forward genetics) approaches. A high-efficiency ENU approach results in approx. 25 functional mutations per genome; most of these will result in hypomorphic alleles. Our group has recently begun using ENU mutagenesis as a tool for understanding lung development and disease. In collaboration with other groups at MRC Harwell, we have undertaken a screen for recessive mutations affecting mouse lung development. We are currently pursuing two lines identified from this screen, Hel (head, eye and lung) and RecBA17. Both these lines exhibit lung defects and we believe that by studying the phenotypes and identifying the causative mutations, we may also shed light on lung disease pathogenesis. In collaboration with Bill Cookson and Miriam Moffatt, we are also taking a gene-driven approach for understanding asthma. Using the Harwell ENU sperm archive, we have recovered mouse lines harbouring mutations in the asthma-susceptibility genes Phf11 (PHD finger protein 11) and Dpp10 (dipeptidylpeptidase 10). Functional analyses of these alleles are currently under way.

ENU mutagenesis, a way forward to understand gene function

Arguably, the main challenge for contemporary genetics is to understand the function of every gene in a mammalian genome. The mouse has emerged as a model for this task because its genome can be manipulated in a number of ways to study gene function or mimic disease states. Two complementary genetic approaches can be used to generate mouse models. A reverse genetics or gene-driven approach (gene to phenotype) starts from a known gene and manipulates the genome to create genetically modified mice, such as knockouts. Alternatively, a forward genetics or phenotype-driven approach (phenotype to gene) involves screening mice for mutant phenotypes without previous knowledge of the genetic basis of the mutation. N-ethyl-N-nitrosourea (ENU) mutagenesis has been widely used for both approaches to generate mouse mutants. Here we review progress in ENU mutagenesis screening, with an emphasis on creating mouse models for human disorders.

Ethylnitrosourea-induced apoptosis in primordial germ cells of the rat fetus

Ethylnitrosourea (ENU) is a simple alkylating agent. It induces gene mutations in fetal primordial germ cells (PGCs), and a high incidence of congenital malformations is also found in the offspring of male mice treated with ENU at the embryonic stage. It is also reported that decreases in the fertility rate and weights of the testis and ovary were found in the offspring from dams treated with ENU. In this study, we analyzed the occurrence of apoptotic cell death and the expression of p53 protein which is thought to play an important role in the DNA damage-induced apoptosis after administration of ENU to pregnant rats on day 13 of gestation to obtain a clue for clarifying the toxic effect of ENU on PGCs. Apoptotic cells increased in PGCs in fetal gonads from 3 h after treatment. The number of apoptotic PGCs peaked at 6 h and gradually decreased towards 24 h after treatment. On the other hand, p53-positive PGCs increased from I h after treatment, prior to the induction of apoptosis. The number of p53-positive PGCs peaked at 3 h and returned to the control level at 24 h after treatment. These results suggest that ENU induces apoptosis in rat fetal PGCs immediately after its administration to dams and excess cell death by apoptosis may have a close relation to the later occurrence of decreases in the fertility rate and gonadal weight. Moreover, a possible involvement of p53 is suggested in the ENU-induced apoptosis in PGCs.

Embryo toxicity and teratogenicity of formaldehyde

C-14 formaldehyde crosses the placenta and enters fetal tissues. The incorporated radioactivity is higher in fetal organs (i.e., brain and liver) than in maternal tissues. The incorporation mechanism has not been studied fully, but formaldehyde enters the single-carbon cycle and is incorporated as a methyl group into nucleic acids and proteins. Also, formaldehyde reacts chemically with organic compounds (e.g., deoxyribonucleic acid, nucleosides, nucleotides, proteins, amino acids) by addition and condensation reactions, thus forming adducts and deoxyribonucleic acid-protein crosslinks. The following questions must be addressed: What adducts (e.g., N-methyl amino acids) are formed in the blood following formaldehyde inhalation? What role do N-methyl-amino adducts play in alkylation of nuclear and mitochondrial deoxyribonucleic acid, as well as mitochondrial peroxidation? The fact that the free formaldehyde pool in blood is not affected following exposure to the chemical does not mean that formaldehyde is not involved in altering cell and deoxyribonucleic acid characteristics beyond the nasal cavity. The teratogenic effect of formaldehyde in the English literature has been sought, beginning on the 6th day of pregnancy (i.e., rodents) (Saillenfait AM, et al. Food Chem Toxicol 1989, pp 545-48; Martin WJ. Reprod Toxicol 1990, pp 237-39; Ulsamer AG, et al. Hazard Assessment of Chemicals; Academic Press, 1984, pp 337-400; and U.S. Department of Health and Human Services. Toxicological Profile of Formaldehyde; ATSDR, 1999 [references 1-4, respectively, herein]). The exposure regimen is critical and may account for the differences in outcomes. Pregnant rats were exposed (a) prior to mating, (b) during mating, (c) or during the entire gestation period. These regimens (a) increased embryo mortality; (b) increased fetal anomalies (i.e., cryptochordism and aberrant ossification centers); (c) decreased concentrations of ascorbic acid; and (d) caused abnormalities in enzymes of mitochondria, lysosomes, and the endoplasmic reticulum. The alterations in enzymatic activity persisted 4 mo following birth. In addition, formaldehyde caused metabolic acidosis, which was augmented by iron deficiency. Furthermore, newborns exposed to formaldehyde in utero had abnormal performances in open-field tests. Disparities in teratogenic effects of toxic chemicals are not unusual. For example, chlorpyrifos has not produced teratogenic effects in rats when mothers are exposed on days 6-15 (Katakura Y, et al. Br J Ind Med 1993, pp 176-82 [reference 5 herein]) of gestation (Breslin WJ, et al. Fund Appl Toxicol 1996, pp 119-30; and Hanley TR, et al. Toxicol Sci 2000, pp 100-08 [references 6 and 7, respectively, herein]). However, either changing the endpoints for measurement or exposing neonates during periods of neurogenesis (days 1-14 following birth) and during subsequent developmental periods produced adverse effects. These effects included neuroapoptosis, decreased deoxyribonucleic acid and ribonucleic acid synthesis, abnormalities in adenylyl cyclase cascade, and neurobehavioral effects (Johnson DE, et al. Brain Res Bull 1998, pp 143-47; Lassiter TL, et al. Toxicol Sci 1999, pp 92-100; Chakraborti TK, et al. Pharmacol Biochem Behav 1993, pp 219-24; Whitney KD, et al. Toxicol Appl Pharm 1995, pp 53-62; Chanda SM, et al. Pharmacol Biochem Behav 1996, pp 771-76; Dam K, et al. Devel Brain Res 1998, pp 39-45; Campbell CG, et al. Brain Res Bull 1997, pp 179-89; and Xong X, et al. Toxicol Appl Pharm 1997, pp 158-74 [references 8-15, respectively, herein]). Furthermore, the terata caused by thalidomide is a graphic human example in which the animal model and timing of exposure were key factors (Parman T, et al. Natl Med 1999, pp 582-85; and Brenner CA, et al. Mol Human Repro 1998, pp 887-92 [references 16 and 17, respectively, herein]). Thus, it appears that more sensitive endpoints (e.g., enzyme activity, generation of reactive oxygen species, timing of exposure) for the measurement of toxic effects of environmental agents on embryos, fetuses, and neonates are more coherent than are gross terata observations. The perinatal period from the end of organogenesis to the end of the neonatal period in humans approximates the 28th day of gestation to 4 wk postpartum. Therefore, researchers must investigate similar stages of development (e.g., neurogenesis occurs in the 3rd trimester in humans and neonatal days occur during days 1-14 in rats and mice, whereas guinea pigs behave more like humans). Finally, screening for teratogenic events should also include exposure of females before mating or shortly following mating. Such a regimen is fruitful inasmuch as environmental agents cause adverse effec

Mechanisms of mutation induction in germ cells of the mouse as assessed by the specific locus test

Mouse germ cell specific locus mutagenesis data and a molecular characterization of mutant alleles have been reviewed to arrive at an understanding of the mechanism of mutation induction in mammals. (a) The spermatogenic stage specificity for the sensitivity to mutation induction by 20 chemical mutagens is considered. (b) The effects of a saturable repair process and its recovery over time are examined for the mutagenic efficiency of ethylnitrosourea. (c) The mutagenic events following methylnitrosourea and chlorambucil are shown to be mainly deletions. In contrast the mutations recovered after ethylnitrosourea treatment are almost exclusively base pair substitutions. (d) It is emphasized that to date very few specific locus experiments have been designed to test for mutagenic events outside the interval stem cell spermatogonia-mature spermatozoa. A specific locus mutation has recently been shown to be due to loss of heterozygosity via mitotic recombination in an early zygote stage and suggests a broader range of possible mechanisms of mutation when these stages are considered. (e) With the cloning of all 7 marker loci mutation analysis at the molecular level will allow a more direct assessment of the mutation process in future studies.

Life cycle of the mammalian germ cell: implication for spontaneous mutation frequencies

A brief history of the developmental life cycle of the mammalian germ cell, from fertilization to gametogenesis in the mature gonad, is presented. The differences between gametogenesis in the mature gonad of males and females are also described with regard to properties that may affect their susceptibilities to mutation. It is emphasized that any historical control background rate of necessity will include mutations that occur in germinal tissue at all stages of development and differentiation, although it is not always possible to determine at what stage of germline development a spontaneous mutation has occurred. Studies of induced mutations suggest that the impact on the molecular level and the distribution of mutations among the F1 and F2 progeny may be partly determined by the stage and sex of germ cells in which spontaneous mutations occur. In summary, historical control rates should only be considered the sum total of mutations that occur during the entire life of the individual and cannot represent the control values of any individual germ cell stage. Nonetheless, it is certainly important and valid to use historical control data for calculating human risk, because the primary use of the estimation of mutant frequencies is to access the potential impact of agents in increasing the genetic load in the human population.

Dose-dependent induction of recessive mutations with N-ethyl-N-nitrosourea in primordial germ cells of male mice

Using a specific locus test, we previously found that N-ethyl-N-nitrosourea (ENU) induces recessive mutations at a relatively high rate in male mouse primordial germ cells (PGC) at 8.5, 10.5 and 13.5 days of development (G8.5, G10.5 and G13.5). A large difference was observed on the induced mutation rate between 30 and 50 mg/kg ENU in 10.5-day PGC. We therefore carried out specific locus tests to ascertain whether ENU induces recessive mutations in a dose-dependent manner in G8.5 and G10.5 PGC. We also gave multiple doses of 25 mg/kg ENU using an 18-h interval, the approximate doubling time of PGC at these developmental stages, to test for an additive effect on the induced mutations rate. A dose-dependent induction of recessive mutations by ENU was observed in both G8.5 and G10.5 PGC, and multiple dosing of 25 mg/kg ENU showed an additive effect. Comparing these results to data on spermatogonial stem cells, we conclude the capacity to repair ENU-induced premutagenic damages is less effective in male mouse PGC at these developmental stages than in spermatogonial stem cells.

Mutagenicity of anticancer drugs in mammalian germ cells

The evidence for mammalian germ cell mutagenicity induced by anticancer drugs is summarized. Primary attention is paid to the three major mouse germ cell mutagenicity tests- the dominant lethal, heritable translocation, and morphological specific locus tests- from which most germ cell mutagenicity data historically have been obtained. Of the 21 anticancer drugs reviewed, 16 have been tested in one or more of these three tests; with all 16 tested in the most common germ cell test, the male dominant lethal test, and 9 of the 16 also tested in the female dominant lethal test. The patterns of germ cell stage specificity for most of the anticancer drugs are similar, and generally resemble the patterns seen with other types of chemicals; however, some of the patterns are unique. For example, 2 of the 8 chemicals shown to induce dominant lethal mutations in female oocytes, do not induce dominant lethal mutations in male germ cells (adriamycin and platinol). Ten of the 16 chemicals tested in the dominant lethal test were positive in post-meiotic stages (spermatids through mature sperm), and seven also induced reciprocal translocations and/or specific locus mutations in post-meiotic stages. This propensity to induce mutations in post-meiotic stages has been observed with most mutagens. However, 5 of the anticancer drugs also induced dominant lethal mutations in spermatocytes (meiotic prophase cells) and one of them, 6-mercaptopurine, uniquely induced dominant lethal mutations exclusively in preleptotene spermatocytes. Finally, three of the anticancer drugs (melphalan, mitomycin C, procarbazine) are members of a very select group of chemicals shown to induce specific locus mutations in spermatogonial stem cells of mice. The implications for human risk are discussed.

Mosaicism: the embryo as a target for induction of mutations leading to cancer and genetic disease

Mosaicism, the existence of "patches" of cells with a genetic constitution that differs from that of other cells of an organism, has been observed in both germinal and somatic tissues of several species, including humans. Mutational events occurring during early embryogenesis can give rise to an organism with a significant number of cells with the mutant genotype in one or more tissues. If this event occurs in a precursor of the germ cells, the mutation can be transferred to subsequent generations. In the F1 generation, this event will usually be perceived as a de novo germinal mutation rather than a transmitted variant allele, unless significant effort is directed toward detecting the mosaicism. Similarly, mutations in oncogenes and tumor-suppressor genes in proliferating somatic cells can generate populations of cells that are at increased risk of transforming into tumor cells. The number of potential preneoplastic cells is larger when the mutagenic event occurs in early development than if it occurs in the mature adult. Experimental data confirm that treatment of the developing embryo or fetus with carcinogenic and mutagenic agents increases the cancer incidence in these animals and the frequency of mutations in the offspring of the animals that were exposed in utero. The available data are conclusive that the developing organism is at risk from exposure to mutagenic and carcinogenic agents. However, the data are insufficient to estimate the level of risk associated with exposures in utero, relative to either the background (spontaneous) level of risk or risk associated with similar exposures to the adult organism.(ABSTRACT TRUNCATED AT 250 WORDS)

Lower mutation frequencies are induced by ENU in undifferentiated embryonic cells than in differentiated cells of the mouse in vitro

The pluripotent embryonic carcinoma cells of line P19 established from undifferentiated cells of the early mouse embryo and their differentiated progeny, the epithelioid ectoderm-like EPI-7 cells, were investigated for the induction of mutations at the HPRT locus by the alkylating agent N-ethyl-N-nitrosourea (ENU). We showed that the cytotoxic effects of ENU after a 5-h treatment were lower in undifferentiated P19 cells than in differentiated EPI-7 cells. The IC50 values of ENU in the two cell lines amounted to 0.6 mg/ml and 0.09 mg/ml for P19 and EPI-7 cells, respectively. The induction of 6-thioguanine-resistant mutants by ENU (1.0 mg/ml) determined after an expression time of 8 days for both cell lines resulted in similar mutation frequencies. Using expression times of 8 days for P19 and 11.75 days for EPI-7 cells, taking into account the longer generation time of differentiated EPI-7 cells (13.7 +/- 3.6 h) in comparison to undifferentiated P19 cells (9.3 +/- 0.9 h), ENU induced significantly higher mutant frequencies in EPI-7 cells (4865 mutants/10(6) cells) than in P19 cells (282 mutants/10(6) cells). Our results and data from the literature on UV irradiation-induced repair support the idea that the induction of lower mutation frequencies in embryonic cells may correlate with different proliferation capacities, cell cycle parameters and/or different mechanisms of DNA repair in embryonic stem cells and differentiated cells, respectively.

Congenital malformations in the offspring of male mice treated with ethylnitrosourea at the embryonic stage

Ethylnitrosourea (ENU) was i.p. injected into ICR female mice at 25 or 50 mg/kg on day 10 of gestation, and male newborns treated with ENU at the embryonic stage were obtained. The treated males were aged for 10 weeks and then mated to untreated females of the same strain. The male germ cells used in the copulation correspond to primordial germ cells (PGC) at the time of ENU treatment. The F1 offspring were sampled as fetuses on day 18 of gestation and inspected for external and skeletal malformations. Evidence of F1 teratogenesis due to the mutagenized PGC was obtained when the fetuses from the males treated with 25 mg ENU/kg were inspected the frequencies of skeletally malformed fetuses in the ENU-treated series and the control being, respectively, 1.0% (14/1,360) and 0.3% (3/1,017). The fetuses from 50 mg/kg-treated males did not show a significant increase in malformations, most probably reflecting a high vulnerability of PGC to the killing effect of ENU. The findings in this study suggest that germ cell stage at PGC is at risk for the induction of congenital malformations by environmental chemicals.