Effect of dose fractionation on the ethylnitrosourea induction of specific-locus mutations in mouse spermatogonia

The molecular underpinnings of fertility: Genetic approaches in Caenorhabditis elegans

The study of mutations that impact fertility has a catch-22. Fertility mutants are often lost since they cannot simply be propagated and maintained. This has hindered progress in understanding the genetics of fertility. In mice, several molecules are found to be required for the interactions between the sperm and egg, with JUNO and IZUMO1 being the only known receptor pair on the egg and sperm surface, respectively. In Caenorhabditis elegans, a total of 12 proteins on the sperm or oocyte have been identified to mediate gamete interactions. Majority of these genes were identified through mutants isolated from genetic screens. In this review, we summarize the several key screening strategies that led to the identification of fertility mutants in C. elegans and provide a perspective about future research using genetic approaches. Recently, advancements in new technologies such as high-throughput sequencing and Crispr-based genome editing tools have accelerated the molecular, cell biological, and mechanistic analysis of fertility genes. We review how these valuable tools advance our understanding of the molecular underpinnings of fertilization. We draw parallels of the molecular mechanisms of fertilization between worms and mammals and argue that our work in C. elegans complements fertility research in humans and other species.

Mouse Genome Informatics (MGI) Resource: Genetic, Genomic, and Biological Knowledgebase for the Laboratory Mouse

The Mouse Genome Informatics (MGI) Resource supports basic, translational, and computational research by providing high-quality, integrated data on the genetics, genomics, and biology of the laboratory mouse. MGI serves a strategic role for the scientific community in facilitating biomedical, experimental, and computational studies investigating the genetics and processes of diseases and enabling the development and testing of new disease models and therapeutic interventions. This review describes the nexus of the body of growing genetic and biological data and the advances in computer technology in the late 1980s, including the World Wide Web, that together launched the beginnings of MGI. MGI develops and maintains a gold-standard resource that reflects the current state of knowledge, provides semantic and contextual data integration that fosters hypothesis testing, continually develops new and improved tools for searching and analysis, and partners with the scientific community to assure research data needs are met. Here we describe one slice of MGI relating to the development of community-wide large-scale mutagenesis and phenotyping projects and introduce ways to access and use these MGI data. References and links to additional MGI aspects are provided.

Current strategies for mutation detection in phenotype-driven screens utilising next generation sequencing

Mutagenesis-based screens in mice are a powerful discovery platform to identify novel genes or gene functions associated with disease phenotypes. An N-ethyl-N-nitrosourea (ENU) mutagenesis screen induces single nucleotide variants randomly in the mouse genome. Subsequent phenotyping of mutant and wildtype mice enables the identification of mutated pathways resulting in phenotypes associated with a particular ENU lesion. This unbiased approach to gene discovery conducts the phenotyping with no prior knowledge of the functional mutations. Before the advent of affordable next generation sequencing (NGS), ENU variant identification was a limiting step in gene characterization, akin to ‘finding a needle in a haystack’. The emergence of a reliable reference genome alongside advances in NGS has propelled ENU mutation discovery from an arduous, time-consuming exercise to an effective and rapid form of mutation discovery. This has permitted large mouse facilities worldwide to use ENU for novel mutation discovery in a high-throughput manner, helping to accelerate basic science at the mechanistic level. Here, we describe three different strategies used to identify ENU variants from NGS data and some of the subsequent steps for mutation characterisation.

Mouse ENU Mutagenesis to Understand Immunity to Infection: Methods, Selected Examples, and Perspectives

Infectious diseases are responsible for over 25% of deaths globally, but many more individuals are exposed to deadly pathogens. The outcome of infection results from a set of diverse factors including pathogen virulence factors, the environment, and the genetic make-up of the host. The completion of the human reference genome sequence in 2004 along with technological advances have tremendously accelerated and renovated the tools to study the genetic etiology of infectious diseases in humans and its best characterized mammalian model, the mouse. Advancements in mouse genomic resources have accelerated genome-wide functional approaches, such as gene-driven and phenotype-driven mutagenesis, bringing to the fore the use of mouse models that reproduce accurately many aspects of the pathogenesis of human infectious diseases. Treatment with the mutagen N-ethyl-N-nitrosourea (ENU) has become the most popular phenotype-driven approach. Our team and others have employed mouse ENU mutagenesis to identify host genes that directly impact susceptibility to pathogens of global significance. In this review, we first describe the strategies and tools used in mouse genetics to understand immunity to infection with special emphasis on chemical mutagenesis of the mouse germ-line together with current strategies to efficiently identify functional mutations using next generation sequencing. Then, we highlight illustrative examples of genes, proteins, and cellular signatures that have been revealed by ENU screens and have been shown to be involved in susceptibility or resistance to infectious diseases caused by parasites, bacteria, and viruses.

Reduced lymphocyte longevity and homeostatic proliferation in lamin B receptor-deficient mice results in profound and progressive lymphopenia

The lamin B receptor (LBR) is a highly unusual inner nuclear membrane protein with multiple functions. Reduced levels are associated with decreased neutrophil lobularity, whereas complete absence of LBR results in severe skeletal dysplasia and in utero/perinatal lethality. We describe a mouse pedigree, Lym3, with normal bone marrow and thymic development but profound and progressive lymphopenia particularly within the T cell compartment. This defect arises from a point mutation within the Lbr gene with only trace mutant protein detectable in homozygotes, albeit sufficient for normal development. Reduced T cell homeostatic proliferative potential and life span in vivo were found to contribute to lymphopenia. To investigate the role of LBR in gene silencing in hematopoietic cells, we examined gene expression in wild-type and mutant lymph node CD8 T cells and bone marrow neutrophils. Although LBR deficiency had a very mild impact on gene expression overall, for common genes differentially expressed in both LBR-deficient CD8 T cells and neutrophils, gene upregulation prevailed, supporting a role for LBR in their suppression. In summary, this study demonstrates that LBR deficiency affects not only nuclear architecture but also proliferation, cell viability, and gene expression of hematopoietic cells.

Creating a "hopeful monster": mouse forward genetic screens

One of the most straightforward approaches to making novel biological discoveries is the forward genetic screen. The time is ripe for forward genetic screens in the mouse since the mouse genome is sequenced, but the function of many of the genes remains unknown. Today, with careful planning, such screens are within the reach of even small individual labs. In this chapter we first discuss the types of screens in existence, as well as how to design a screen to recover mutations that are relevant to the interests of a lab. We then describe how to create mutations using the chemical N-ethyl-N-nitrosourea (ENU), including a detailed injection protocol. Next, we outline breeding schemes to establish mutant lines for each type of screen. Finally, we explain how to map mutations using recombination and how to ensure that a particular mutation causes a phenotype. Our goal is to make forward genetics in the mouse accessible to any lab with the desire to do it.

Utilization of a mutagenesis screen to generate mouse models of hyperaldosteronism

Primary aldosteronism is considered to be responsible for almost 10% of all cases of arterial hypertension. The genetic background of this common disease, however, has been elucidated only for the rare familial types, whereas in the large majority of sporadic cases, underlying mechanisms still remain unclear. In an attempt to define novel genetic loci involved in the pathophysiology of primary aldosteronism, a mutagenesis screen after treatment of mice with the alkylating agent N-ethyl-N-nitrosourea was established for the parameter aldosterone. As the detection method we used a time-resolved fluorescence immunoassay that allows the measurement of aldosterone in very small murine sample volumes. Based on this assay, we first determined the normal aldosterone values for wild-type C3HeB/FeJ mice under baseline conditions [92 ± 6 pg/ml for females (n = 69) and 173 ± 16 pg/ml for males (n = 55)]. Subsequently, aldosterone measurement was carried out in more than 2800 F(1) offspring of chemically mutagenized C3HeB/FeJ mice, and values were compared with aldosterone levels from untreated animals. Persistent hyperaldosteronism (defined as levels +3 sd above the mean of untreated animals) upon repeated measurements was present in seven female and two male F(1) offspring. Further breeding of these founders gave rise to F(2) pedigrees from which eight lines with different patterns of inheritance of hyperaldosteronism could be established. These animals will serve for detailed phenotypic and genetic characterization in the future. Taken together, our data demonstrate the feasibility of a phenotype-driven mutagenesis screen to detect and establish mutant mouse lines with a phenotype of chronic hyperaldosteronism.

Mouse mutagenesis with the chemical supermutagen ENU

The generation and analysis of germline mutations in the mouse is one of the cornerstones of modern biological research. The chemical supermutagen N-ethyl-N-nitrosourea (ENU) is the most potent known mouse mutagen and can be used to generate point mutations throughout the mouse genome. The progeny of ENU-mutagenized males can be screened for autosomal dominant phenotypes, or they can be used to generate multigeneration pedigrees to screen for autosomal recessive traits. The introduction of balancer chromosomes into the breeding scheme can allow for the selective capture of mutations in a specific chromosomal region. More recent work has demonstrated that the use of animals that already have a mutation of interest can lead to the successful isolation of additional mutations that modify the original mutant phenotype. Further, modern molecular techniques ensure that mutations can be readily identified. We describe here the procedures for mutagenizing male mice with ENU and explain the various types of screens that can be performed for different kinds of induced mutations. The currently published research on ENU mutagenesis in the mouse has only scratched the surface of what is possible with this powerful technique, and further work is certain to deepen our knowledge of the role of the individual components of the mouse genome and the myriad relationships between them.

Using ENU mutagenesis for phenotype-driven analysis of the mouse

The use of mutagenesis in invertebrates to generate phenotypic variants has a long and productive history. Despite the conclusive demonstration by Russell and colleagues in the 1970s that the chemical N-ethyl-N-nitrosourea (ENU) is a highly effective mutagen in mice, the application of phenotypic-driven mutagenesis as a method to study mammalian biology proceeded slowly. With the development of tools for genomic analysis, the task of positional cloning ENU-induced mutations has become quite feasible, and this approach has recently been widely applied and highly productive. It has specifically lived up to its theoretical utility as means to provide insight into the biological roles of genes that is not biased by presumptions of their function. While the power of this approach is indisputable, the effort necessary for its success remains substantial, requiring careful attention to aspects including ENU treatment, mouse husbandry, screen assay design, genetic mapping, positional cloning, and mutation validation. In this chapter we discuss practical aspects of implementing a phenotype-driven analysis of an ENU-mutagenized mouse population.

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.

Next-generation gene targeting in the mouse for functional genomics

In order to elucidate ultimate biological function of the genome, the model animal system carrying mutations is indispensable. Recently, large-scale mutagenesis projects have been launched in various species. Especially, the mouse is considered to be an ideal model to human because it is a mammalian species accompanied with well-established genetic as well as embryonic technologies. In 1990's, large-scale mouse mutagenesis projects firstly initiated with a potent chemical mutagen, N-ethyl-N-nitrosourea (ENU) by the phenotype-driven approach or forward genetics. The knockout mouse mutagenesis projects with trapping/conditional mutagenesis have then followed as Phase II since 2006 by the gene-driven approach or reverse genetics. Recently, the next-generation gene targeting system has also become available to the research community, which allows us to establish and analyze mutant mice carrying an allelic series of base substitutions in target genes as another reverse genetics. Overall trends in the large-scale mouse mutagenesis will be reviewed in this article particularly focusing on the new advancement of the next-generation gene targeting system. The drastic expansion of the mutant mouse resources altogether will enhance the systematic understanding of the life. The construction of the mutant mouse resources developed by the forward and reverse genetic mutagenesis is just the beginning of the annotation of mammalian genome. They provide basic infrastructure to understand the molecular mechanism of the gene and genome and will contribute to not only basic researches but also applied sciences such as human disease modelling, genomic medicine and personalized medicine.

Genetic dissection of Toll-like receptor signaling using ENU mutagenesis

Forward genetics has led to many discoveries and particularly in the field of Toll-like receptors (TLRs), it has played an important role in identifying key components involved in the innate sensing of pathogens. With the mouse genome fully sequenced and the ability to generate many mutant phenotypes through random germline mutagenesis, forward genetics has become an efficient means by which to identify key components involved in our immune response. In this chapter I provide a practical guide for performing germline mutagenesis in mice. I focus on the application of this technology to the identification of genes involved in TLR signaling.

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.

ENU mutagenesis in mice

Forward genetics has led to many "breakthrough" discoveries, and with the mouse genome almost fully sequenced, the creation of phenotypes through random germline mutagenesis has become an efficient means by which to find the function of yet undescribed genes. In this chapter, we will provide a practical guideline for performing germline mutagenesis in mice. In particular, we will focus on the application of this technology to identify genes that are essential to innate immune defense.

Studies on the Small Body Size Mouse Developed by Mutagen N-Ethyl-N-nitrosourea

Mutant mouse which show dwarfism has been developed by N-ethyl-N-nitrosourea (ENU) mutagenesis using BALB/c mice. The mutant mouse was inherited as autosomal recessive trait and named Small Body Size (SBS) mouse. The phenotype of SBS mouse was not apparent at birth, but it was possible to distinguish mutant phenotype from normal mice 1 week after birth. In this study, we examined body weight changes and bone mineral density (BMD), and we also carried out genetic linkage analysis to map the causative gene(s) of SBS mouse. Body weight changes were observed from birth to 14 weeks of age in both affected (n = 30) and normal mice (n = 24). BMD was examined in each five SBS and normal mice between 3 and 6 weeks of age, respectively. For the linkage analysis, we produced backcross progeny [(SBS × C57BL/6J) F₁ × SBS] N₂ mice (n = 142), and seventy-four microsatellite markers were used for primary linkage analysis. Body weight of affected mice was consistently lower than that of the normal mice, and was 43.7% less than that of normal mice at 3 weeks of age (P < 0.001). As compared with normal mice at 3 and 6 weeks of age, BMD of the SBS mice was significantly low. The results showed 15.5% and 14.1% lower in total body BMD, 15.3% and 8.7% lower in forearm BMD, and 29.7% and 20.1% lower in femur BMD, respectively. The causative gene was mapped on chromosome 10. The map order and the distance between markers were D10Mit248 - 2.1 cM - D10Mit51 - 4.2 cM - sbs - 0.7 cM - D10Mit283 - 1.4 cM - D10Mit106 - 11.2 cM - D10Mit170.

Comparison of the genetic effects of equimolar doses of ENU and MNU: while the chemicals differ dramatically in their mutagenicity in stem-cell spermatogonia, both elicit very high mutation rates in differentiating spermatogonia

Mutagenic, reproductive, and toxicity effects of two closely related chemicals, ethylnitrosourea (ENU) and methylnitrosourea (MNU), were compared at equimolar and near-equimolar doses in the mouse specific-locus test in a screen of all stages of spermatogenesis and spermiogenesis. In stem-cell spermatogonia (SG), ENU is more than an order of magnitude more mutagenic than MNU. During post-SG stages, both chemicals exhibit high peaks in mutation yield when differentiating spermatogonia (DG) and preleptotene spermatocytes are exposed. The mutation frequency induced by 75mgMNU/kg during this peak interval is, to date, the highest induced by any single-exposure mutagenic treatment - chemical or radiation - that allows survival of the exposed animal and its germ cells, producing an estimated 10 new mutations per genome. There is thus a vast difference between stem cell and differentiating spermatogonia in their sensitivity to MNU, but little difference between these stages in their sensitivity to ENU. During stages following meiotic metaphase, the highest mutation yield is obtained from exposed spermatids, but for both chemicals, that yield is less than one-quarter that obtained from the peak interval. Large-lesion (LL) mutations were induced only in spermatids. Although only a few of the remaining mutations were analyzed molecularly, there is considerable evidence from recent molecular characterizations of the marker genes and their flanking chromosomal regions that most, if not all, mutations induced during the peak-sensitive period did not involve lesions outside the marked loci. Both ENU and MNU treatments of post-SG stages yielded significant numbers of mutants that were recovered as mosaics, with the proportion being higher for ENU than for MNU. Comparing the chemicals for the endpoints studied and additional ones (e.g., chromosome aberrations, toxicity to germ cells and to animals, teratogenicity) revealed that while MNU is generally more effective, the opposite is true when the target cells are SG.

Efficient and fast targeted production of murine models based on ENU mutagenesis

Mice with targeted genetic alterations are the most effective tools for deciphering organismal gene function. We generated an ENU-based parallel C3HeB/FeJ sperm and DNA archive characterized by a high probability to identify allelic variants of target genes as well as high efficiencies in allele retrieval and model revitalization. Our archive size of over 17,000 samples contains approximately 340,000 independent alleles (20 functional mutations per individual sample). Based on an estimated number of approximately 30,000 mouse genes, the parallel sperm/DNA archive should permit the identification and recovery of ten or more alleles per average target gene which translates to a calculated 99% success rate in the discovery of five allelic variants for any given average gene. The low rate of unrelated ENU-induced passenger mutations has no practical impact on the analysis of the allele-specific phenotype at the G3 generation because of dilution and free segregation of such unrelated passenger mutations. To date 39 mouse models representing 33 different genes have been recovered from our archive using in vitro fertilization techniques. The generation time for a murine model heterozygous for a mutation in a gene of interest is less than 2 months, i.e., three to four times faster compared with current embryonic stem-cell-based technologies. We conclude that ENU-based targeted mutagenesis is a powerful tool for the fast and high-throughput production of murine gene-specific models for biomedical research.

Unraveling innate immunity using large scale N-ethyl-N-nitrosourea mutagenesis

With the mouse genome almost entirely sequenced and readily accessible to all who wish to examine it, the challenge across most biological disciplines now lies in the decipherment of gene and protein function rather than in the realm of gene identification per se. In the field of innate immunity, forward genetic methods have repeatedly been applied to identify key sensors, adapters, and effector molecules. However, most spontaneous mutations that affect innate immune function have been mapped and cloned, and the need for new monogenic phenotypes has been felt evermore keenly. N-Ethyl-N-nitrosourea (ENU) mutagenesis is an efficient tool for the creation of aberrant monogenic innate immune response phenotypes. In this review, we will discuss the potential of the forward genetic approach and ENU mutagenesis to identify new genes and new functions of known genes related to innate immunity.

The art and design of genetic screens: mouse

Humans are mammals, not bacteria or plants, yeast or nematodes, insects or fish. Mice are also mammals, but unlike gorilla and goat, fox and ferret, giraffe and jackal, they are suited perfectly to the laboratory environment and genetic experimentation. In this review, we will summarize the tools, tricks and techniques for executing forward genetic screens in the mouse and argue that this approach is now accessible to most biologists, rather than being the sole domain of large national facilities and specialized genetics laboratories.

Transmitted mutational events induced in mouse germ cells following acrylamide or glycidamide exposure

An increase in the germ line mutation rate in humans will result in an increase in the incidence of genetically determined diseases in subsequent generations. Thus, it is important to identify those agents that are mutagenic in mammalian germ cells. Acrylamide is water soluble, absorbed and distributed in the body, chemically reactive with nucleophilic sites, and there are known sources of human exposure. Here we review all seven published studies that assessed the effectiveness of acrylamide or its active metabolite, glycidamide, in inducing transmitted reciprocal translocations or gene mutations in the mouse. Major conclusions were (a) acrylamide is mutagenic in spermatozoa and spermatid stages of the male germ line; (b) in these spermatogenic stages acrylamide is mainly or exclusively a clastogen; (c) per unit dose, i.p. exposure is more effective than dermal exposure; and (d) per unit dose, glycidamide is more effective than acrylamide. Since stem cell spermatogonia persist and may accumulate mutations throughout the reproductive life of males, assessment of induced mutations in this germ cell stage is critical for the assessment of genetic risk associated with exposure to a mutagen. The two specific-locus mutation experiments which studied the stem cell spermatogonial stage yielded conflicting results. This discrepancy should be resolved. Finally, it is noted that no experiments have studied the mutagenic potential of acrylamide to increase the frequency of transmitted mutational events following exposure in the female germ line.

Genetic Mapping and ENU Mutagenesis

ENU mutagenesis is a potent means to generate novel mutations in the mouse, and the further investigation of these mutations can be logistically demanding. Determination of the map position of a mutation early in its characterization can be extremely useful. We describe how the use of interval haplotype analysis can facilitate this with even small numbers of affected progeny.

An N-ethyl-N-nitrosourea mutagenesis screen for epigenetic mutations in the mouse

The mammalian epigenetic phenomena of X inactivation and genomic imprinting are incompletely understood. X inactivation equalizes X-linked expression between males and females by silencing genes on one X chromosome during female embryogenesis. Genomic imprinting functionally distinguishes the parental genomes, resulting in parent-specific monoallelic expression of particular genes. N-ethyl-N-nitrosourea (ENU) mutagenesis was used in the mouse to screen for mutations in novel factors involved in X inactivation. Previously, we reported mutant pedigrees identified through this screen that segregate aberrant X-inactivation phenotypes and we mapped the mutation in one pedigree to chromosome 15. We now have mapped two additional mutations to the distal chromosome 5 and the proximal chromosome 10 in a second pedigree and show that each of the mutations is sufficient to induce the mutant phenotype. We further show that the roles of these factors are specific to embryonic X inactivation as neither genomic imprinting of multiple genes nor imprinted X inactivation is perturbed. Finally, we used mice bearing selected X-linked alleles that regulate X chromosome choice to demonstrate that the phenotypes of all three mutations are consistent with models in which the mutations have affected molecules involved specifically in the choice or the initiation of X inactivation.

Efficient generation and mapping of recessive developmental mutations using ENU mutagenesis

Treatment with N-ethyl-N-nitrosourea (ENU) efficiently generates single-nucleotide mutations in mice. Along with the renewed interest in this approach, much attention has been given recently to large screens with broad aims; however, more finely focused studies have proven very productive as well. Here we show how mutagenesis together with genetic mapping can facilitate the rapid characterization of recessive loci required for normal embryonic development. We screened third-generation progeny of mutagenized mice at embryonic day (E) 18.5 for abnormalities of organogenesis. We ascertained 15 monogenic mutations in the 54 families that were comprehensively analyzed. We carried out the experiment as an outcross, which facilitated the genetic mapping of the mutations by haplotype analysis. We mapped seven of the mutations and identified the affected locus in two lines. Using a hierarchical approach, it is possible to maximize the efficiency of this analysis so that it can be carried out easily with modest infrastructure and resources.

ENU mutagenesis: analyzing gene function in mice

With the completion of the human genome, sequence analysis of gene function will move into the center of future genome research. One of the key strategies for studying gene function involves the genetic dissection of biological processes in animal models. Mouse mutants are of particular importance for the analysis of disease pathogenesis and transgenic techniques, and gene targeting have become routine tools. Recently, phenotype-driven strategies using chemical mutagenesis have been the target of increasing interest. In this review, the current state of ENU mutagenesis and its application as a systematic tool of genome analysis are examined.

Significance of the perigametic interval as a major source of spontaneous mutations that result in mosaics

An earlier analysis showed that a significant percentage of spontaneous specific-locus mutations in mice are recovered as mosaics and that the spontaneous mutation rate per cell cycle is probably higher for those mutations that produce mosaics than for those that produce whole-body mutants. The finding that the average germline composition of the mosaics was approximately 50% supported the suggestion that single-strand DNA alterations during the perigametic interval constitute the major source of spontaneous mosaics. Here, alternative origins of 50% germline mosaicism are examined. Supporting the earlier hypothesis is the finding that spontaneous mutations that are recovered as clusters constitute a different array of types from those giving rise to singletons, and the evidence from interspecies comparisons that a unique component of the life cycle, probably meiosis, makes a major contribution to spontaneous mutations. Biological factors associated with the perigametic interval were examined in an effort to suggest explanations for the observations that 1) the spontaneous mutation rate in that interval is high relative to that characterizing any mitotic cell cycle, 2) the types of mutations appear to be different from those arising during mitotic divisions, and 3) the spontaneous mutation rate for males is higher than that for females. It is concluded that the higher yield from the perigametic interval is consistent with what is known about methylation status in development of both sexes and with repair capacity in the male germline. For both parameters, differences between the sexes during their respective perigametic intervals may be at least partly responsible for the fact that the spontaneous mutation rate of mammalian females is lower than that of males.

The mutagenic activity of ethylnitrosourea at low doses in spermatogonia of the mouse as assessed by the specific-locus test

Ethylnitrosourea is the most efficient chemical mutagen in spermatogonial stem cells of the mouse and its mutagenic activity has been intensively studied. The pertinent specific-locus mutation test results for a discussion of low dose-effect studies have been summarized and indicate: (1) A threshold dose response best characterizes the relationship between dose and mutation rate. (2) The reduced effectiveness of ethylnitrosourea in the low dose range is likely due to a saturable repair process. (3) The recovery of the saturable repair process as assessed in fractionated dose experiments is long (ca. 168 h). The dynamics of stem cell spermatogonia suggests a long time interval before the cell population passes through at least one cell division and this may be relevant to an interpretation of the fractionation effects. (4) There is a slight but important discrepancy between the predicted and observed mutagenic activity of ethylnitrosourea in the low dose range. This is interpreted to be due to the differences between a mathematical abstraction and the biological realities of the system being studied.

The effect of the interval between dose applications on the observed specific-locus mutation rate in the mouse following fractionated treatments of spermatogonia with ethylnitrosourea

Our earlier analyses have suggested an apparent threshold dose-response for ethylnitrosourea-induced specific-locus mutations in treated spermatogonia of the mouse to be due to a saturable repair process. In the current study a series of fractionated-treatment experiments was carried out in which male (102 x C3H)F1 mice were exposed to 4 x 10, 2 x 40. 4 x 20 or 4 x 40 mg ethylnitrosourea per kg body weight with 24 h between applications; 4 x 40 mg ethylnitrosourea per kg body weight with 72 h between dose applications; and 2 x 40, 4 x 20 and 4 x 40 mg ethylnitrosourea per kg body weight with 168 h between dose applications. For all experiments with 24-h intervals between dose applications, there was no effect due to dose fractionation on the observed mutation rates, indicating the time interval between dose applications to be shorter than the recovery time of the repair processes acting on ethylnitrosourea-induced DNA adducts. In contrast, a fractionation interval of 168 h was associated with a significant reduction in the observed mutation rate due to recovery of the repair process. However, although reduced, the observed mutation rates for fractionation intervals of 168 h were higher than the spontaneous specific-locus mutation rate. These observations contradict the expectation for a true threshold dose response. We interpret this discrepancy to be due to the differences in the predictions of a mathematical abstraction of experimental data and the complexities of the biological system being studied. Biologically plausible explanations of the discrepancy are presented.

In vivo mutagenesis in the cynomolgus monkey: time course of HPRT mutant frequency at long time points following ethylnitrosourea exposure

We have monitored mutant frequency at the HPRT locus in peripheral blood lymphocytes of cynomolgus monkeys using a clonal assay in which mutants are selected by resistance to 6-thioguanine. Among untreated animals, the mean spontaneous mutant frequency was 2.9 +/- 2.9 x 10(-6) (standard deviation, based on 131 determinations in 33 animals), in good agreement with HPRT mutant frequencies in other species. In four animals treated with a single intraperitoneal dose of 77 mg/kg ethylnitrosourea, mutant frequency increased with time, peaking 70 to 100 days after treatment. Mutant frequency in two of the four animals was monitored at intervals for 6 years, and a second identical treatment was given about 830 days after the first. Mutant frequency again peaked in these two animals 70 days after the second dose and decreased following peak values, declining to a plateau that was higher than the predose mutant frequency in both animals. This pattern was repeated following the second ethylnitrosourea treatment. Fractionating the dose of ethylnitrosourea into five equal daily injections had no effect on mutant frequency in two animals when compared to a single dose.

Temporal and molecular characteristics of mutations induced by ethylnitrosourea in germ cells isolated from seminiferous tubules and in spermatozoa of lacZ transgenic mice

The lacZ transgenic mouse (Muta mouse) model was used to examine the timing of ethylnitrosourea (ENU)-induced mutations in germ cells. The spectrum of mutations was also determined. Animals received five daily treatments with ENU at 50 mg/kg and were sampled at times up to 55 days after treatment. In mixed germ-cell populations isolated from seminiferous tubules, there was little increase in the mutant frequency 5 days after treatment; subsequently, there was a continuous increase until the maximum (17.5-fold above background) was reached by approximately 35 days. In the spermatozoa, an increase in mutant frequency was not seen until 20 days after treatment, with the maximum (4.3-fold above background) being achieved no sooner than approximately 35 days. Based on the timing of sampling, these data demonstrate the detection of both spermatogonial and postspermatogonial, mutations. The most prominent feature of the ENU-induced base-pair mutations in testicular germ cells sampled 55 days after treatment is that 70% are induced in A.T base pairs, compared to only 16% in spontaneous mutations. These findings are consistent with comparable data from ENU studies using assays for inherited germ-cell mutations in mice. This study has demonstrated the utility and potential of the transgenic mouse lacZ model (Muta mouse) for the detection and study of germ-cell mutations and provides guidance in the selection of simplified treatment and sampling protocols.

Isolation and structural characterization of a cDNA clone encoding the human DNA repair protein for O6-alkylguanine

O6-Methylguanine-DNA methyltransferase (MGMT; DNA-O6-methylguanine:protein-L-cysteine S-methyltransferase, EC 2.1.1.63), a unique DNA repair protein present in most organisms, removes the carcinogenic and mutagenic adduct O6-alkylguanine from DNA by stoichiometrically accepting the alkyl group on a cysteine residue in a suicide reaction. The mammalian protein is highly regulated in both somatic and germ-line cells. In addition, the toxicity of certain alkylating drugs in tumor and normal cells is inversely related to the levels of this protein. The cDNA of the human gene, henceforth named MGMT, has been cloned in an expression vector on the basis of its rescue of a methyltransferase-deficient (ada-) Escherichia coli host. A 22-kDa active methyltransferase encoded entirely by the cDNA contains an amino acid sequence of 61 residues that bears 60-65% similarity with segments of E. coli methyltransferase (products of the ada and ogt genes), which encompass the alkyl-acceptor residues. The human cDNA has no sequence similarity with the ada and ogt genes, due in part to differences in codon usage, and shows no detectable homology with E. coli genomic DNA. However, it hybridizes with distinct restriction fragments of human, mouse, and rat DNAs. The lack of methyltransferase observed in many human cell lines is due to the absence of the MGMT gene or to lack of synthesis and/or stability of its 0.95-kilobase poly(A)+ RNA transcript.

Recovery of induced mutations for X chromosome-linked muscular dystrophy in mice

We have used elevated levels of plasma creatine phosphokinase activity to identify muscular dystrophy phenotypes in mice and to screen the progeny of chemical mutagen-treated male mice for X chromosome-linked muscular dystrophy mutations. We were not successful in identifying heterozygous carriers of these induced muscular dystrophy mutations in greater than 8000 progeny. However, we were highly successful in identifying three additional alleles of the characterized mdx locus. These alleles of mdx were recovered from various mutagen-treated males and they occur on an X chromosome that carries flanking markers that allow us to follow the mutations in genetic crosses and in the development of corresponding mutant stocks. These alleles have been designated as mdx2Cv, mdx3Cv, and mdx4Cv. Preliminary data show that mice with mdx2Cv and mdx3Cv mutations have muscular dystrophic phenotypes that do not grossly differ from the characterized mdx mutation. These additional mdx mutations expand the value of mouse models of X chromosome-linked muscular dystrophy and potentially define additional sites of mutation that impair dystrophin expression.

Induction of new mutations in a mouse t-haplotype using ethylnitrosourea mutagenesis

N-ethyl-N-nitrosourea (ENU) was used to induce mutations within thetw5-haplotype of the mouse to make possible a further study of gene arrangement int-mutants and to provide potential landmarks for cloning and sequence studies in the region. Two independent mutants were isolated for each of three loci in thet-region, brachyury (T), quaking (qk), and tufted (tf). The newTktalleles produce tailless mice when atctmutation is present intrans. The newqkktalleles are recessive and homozygous lethal. They are viable, male fertile, and cause seizures and quaking when paired with theqkmutation which previously defined the locus. Thetfktmutations are recessive and phenotypically similar to the mutant alleles available in non-tchromosomes. The mutations were induced in thetw5-haplotype at an average per locus frequency of 1 in 1500. Their isolation demonstrates the power of this technique for obtaining the specific mouse mutants that are needed to genetically dissect a complex mammalian system.

Implication of nonlinear kinetics on risk estimation in carcinogenesis

Efforts in estimating carcinogenic risk in humans from long-term exposure to chemical carcinogens have centered on the problem of low-dose extrapolation. For chemicals with metabolites that interact with DNA, it may be more meaningful to relate tumor response to the concentration of the DNA adducts in the target organ rather than to the applied dose. Many data suggest that the relation between tumor response and concentration of DNA adducts in the target organ may be linear. This implies that the nonlinearities of the dose-response curve for tumor induction may be due to the kinetic processes involved in the formation of carcinogen metabolite--DNA adducts. Of particular importance is the possibility that the kinetic processes may show a nonlinear "hockey-stick" like behavior which results from saturation of detoxification or DNA repair processes. The mathematical models typically used for low-dose extrapolation are shown potentially to overestimate risk by several orders of magnitude when nonlinear kinetics are present.

Dose--response curve for ethylnitrosourea-induced specific-locus mutations in mouse spermatogonia

The extreme mutagenic effectiveness of N-ethyl-N-nitrosourea in the mouse has permitted the accumulation of the most extensive dose--response data yet obtained for chemical induction of specific-locus mutations in spermatogonia. In the lower portion of the curve, below a dose of 100 mg/kg, the data fall statistically significantly below a maximum likelihood fit to a straight line. Independent evidence indicates that, over this dose range, ethylnitrosourea reaches the testis in amounts directly proportional to the injected dose. It is concluded that, despite the mutagenic effectiveness of ethylnitrosourea, the spermatogonia are apparently capable of repairing at least a major part of the mutational damage when the repair process is not swamped by a high dose. This finding is important both in basic studied on the mutagenic action of chemicals in mammals and in risk estimation.