Genome-wide, large-scale production of mutant mice by ENU mutagenesis

Mouse models for human deafness: current tools for new fashions

Mouse models are one of the major tools used for discovery and characterization of genes for non-syndromic deafness in humans. The similarities between the mouse and human genomes, and between the physiology and morphology of their auditory systems, are striking. This article describes the latest mouse models, including spontaneous, 'knockout' and ENU (N-ethyl-N-nitrosourea)-induced mutants, and the recent discovery of modifier genes that are involved in mouse deafness; this discovery is leading the search for genetic modifiers for human disorders.

Mouse models for neurological disease

The mouse has many advantages over human beings for the study of genetics, including the unique property that genetic manipulation can be routinely carried out in the mouse genome. Most importantly, mice and human beings share the same mammalian genes, have many similar biochemical pathways, and have the same diseases. In the minority of cases where these features do not apply, we can still often gain new insights into mouse and human biology. In addition to existing mouse models, several major programmes have been set up to generate new mouse models of disease. Alongside these efforts are new initiatives for the clinical, behavioural, and physiological testing of mice. Molecular genetics has had a major influence on our understanding of the causes of neurological disorders in human beings, and much of this has come from work in mice.

Using mouse models to dissect the genetics of obesity

Mice have proved to be powerful models for understanding obesity in humans and farm animals. Single-gene mutants and genetically modified mice have been used successfully to discover genes and pathways that can regulate body weight. For polygenic obesity, the most common pattern of inheritance, many quantitative trait loci (QTLs) have been mapped in crosses between selected and inbred mouse lines. Most QTL effects are additive, and diet, age and gender modify the genetic effects. Congenic, recombinant inbred, advanced intercross, and chromosome substitution strains are needed to map QTLs finely, to identify the genes underlying the traits, and to examine interactions between them.

Genetic contributions to addiction

Even the most extreme environmentalists along the nature-nurture continuum in psychology now acknowledge that genes often contribute to individual differences in behavior. Behavioral traits are complex, reflecting the aggregate effects of many genes. These genetic effects are interactive, inter se and with the environments in which they are expressed. Human studies of addictive behaviors have clearly implicated both environmental and genetic influences. This review selects drug dependence as a paradigmatic addiction, and further, concentrates on the extensive literature with genetic animal models. Both traditional studies with inbred strains and selected lines and studies exploiting the new molecularly based technologies of the genomics era are discussed. Future directions for further contribution of animal models studies to our understanding of the brain dysregulations characteristic of addictions are identified.

Complexities of cancer research: mouse genetic models

Cancer susceptibility is a complex interaction of an individual's genetic composition and environmental exposures. Huge strides have been made in understanding cancer over the past 100 yr, from recognition of cancer as a genetic disease, to identification of specific carcinogens, isolation of oncogenes, and recognition of tumor suppressors. A tremendous amount of knowledge has accumulated about the etiology of cancer. Cancer genetics has played a significant role in these discoveries. Analysis of high-risk familial cancers has led to the discovery of new tumor suppressor genes and important cancer pathways. These families, however, represent only a small fraction of cancer in the general population. Most cancer is instead probably the result of an intricate interaction of polymorphic susceptibility genes with the sea of environmental exposures that humans experience. Although the central cadre of cancer genes is known, little is understood about the peripheral genes that likely comprise the polymorphic susceptibility loci. The challenge for cancer genetics is therefore to move forward from the mendelian genetics of the rare familial cancer syndromes into the field of quantitative trait loci, susceptibility factors, and modifier genes. By identifying the genes that modulate an individual's susceptibility to cancer after an environmental exposure, researchers will be able to gain important insights into human biology, cancer prevention, and cancer treatment. This article summarizes the current state of quantitative trait genetic analysis and the tools, both proven and theoretical, that may be used to unravel one of the great challenges in cancer genetics.

Transgenic systems in drug discovery: from target identification to humanized mice

Pharmaceutical companies are faced with the challenge that only approximately 10% of compounds tested in costly clinical trials eventually become a new drug. Investment in early discovery research can decrease this attrition in late-stage R&D and focus resources on the best targets. Transgenic technology influences decision-making in target identification, target validation, and can also provide better models for human diseases, as well as models designed to alert researchers early about potential issues with drug metabolism and toxicity. Here we review how transgenic technology can reduce the late-stage attrition by increasing the quality of both the target and the compound.

Novel approaches for identifying genes regulating lymphocyte development and function

The draft sequence of the human and mouse genomes provides an unparalleled opportunity for understanding the genetic control of immune-cell development. Strategies can begin with a gene sequence and pursue a putative immune-system function by employing mRNA-expression profiling or creating gene knockouts in embryonic stem cells. The latter can be produced by utilising the Cre/Lox system, a tetracycline operon, a gene-trap method or chemical mutagenesis. Alternatively, mutant phenotypes (derived using the mutagen ethylnitrosourea) can be traced back to gene sequences.

Mouse models for the genetic dissection of atopy

Atopic disorders are complex traits with a significant contribution of heritability. Several mouse models for atopic phenotypes such as allergen-induced and intrinsic asthma, atopic dermatitis and immediate cutaneous hypersensitivity have been used to systematically dissect the genetics of these diseases. Many of the chromosomal regions identified in genome-wide screens colocalize with linkage regions of autoimmune phenotypes suggesting common regulatory genes in combination (or addition) to disease-specific genes. Moreover, many of these regions correspond to regions found in the human system, thus strengthening the likelihood of a gene contributing to the phenotype. The role of individual candidate genes has been vigorously pursued in genetically modified mice by destruction or overexpression of the candidate gene. The complex interactions of multiple contributing genes may further be elucidated in randomly mutated mouse strains. Thus mouse models can contribute largely to the elucidation of disease genes.

A gene-driven approach to the identification of ENU mutants in the mouse

The construction of parallel archives of DNA and sperm from mice mutagenized with ethylnitrosurea (ENU) represents a potentially powerful and rapid approach for identifying point mutations in any gene in the mouse genome. We provide support for this approach and report the identification of mutations in the gene (Gjb2) encoding connexin 26, using archives established from the UK ENU mutagenesis program.

ENU mutagenesis reveals a highly mutable locus on mouse Chromosome 4 that affects ear morphogenesis

Chemical mutagenesis followed by screening for abnormal phenotypes in the mouse holds much promise as a method for revealing gene function. This method is particularly well-suited for discovering genes involved in hearing or balance function, as these defects are relatively easy to screen for in the mouse. We report here the inner ear abnormalities and genetic localization of seven new dominant mutations created by ENU mutagenesis. All seven mutant stocks were identified because of circling and/or head-weaving behavior, which is an indication of balance dysfunction. Investigation of the inner ears of the seven mutant stocks revealed very similar lateral and posterior semicircular canal defects. Studies of the development of the canals in one mutant stock revealed that the affected canals showed reduced outgrowth and delayed canal fusion. Physiological studies performed in one mutant stock showed raised average compound-action-potential thresholds of approximately 10-20 dB sound pressure level (SPL) (depending on frequency), indicating a mild hearing impairment, although scanning electron microscopy performed in several of the mutant stocks revealed no obvious structural defects in the organ of Corti. All seven mutations mapped to the proximal portion of Chromosome (Chr) 4, near the centromere. On the basis of their similar phenotype and map location, we suggest that the seven mutant genes may be allelic and represent a highly mutable locus on Chr 4 that may be particularly susceptible to ENU-induced mutation on the BALB/c genetic background.

Genetically engineered mice for studies of stress-related clinical conditions

Genetically engineered mice with a specific deletion of targeted genes provide a novel and useful tool to study the endogenous mechanisms underlying aberrant behaviour. In this review we take the stress hormone (hypothalamic-pituitary-adrenocortical) system as an example to demonstrate how refined molecular technologies have allowed to target individual genes involved in stress hormone regulation. We describe different gene targeting methods: the generation of "conventional" knock-out mice enables us to delete a gene of interest in every cell of the body. Equally important for the studies of gene function in the mouse is the use of tissue-specific regulatory systems that allow gene inactivation to be restricted to specific tissues and, in some cases, to specific time points during development, such as the "conditional" knock-out, or the application of antisense techniques. Importantly, deletion of individual genes is not providing animal models for certain psychiatric disorders as these are caused by a manifold of minor changes in a series of so-called susceptibility genes. However, these gene targeting methods have become valuable tools to dissect the functions of individual components of complex biological systems in behavioural neuroscience: genetically engineered animals help to unravel the complex interactions and correlations between individual genes, hormonal regulation and behaviour, the most complex form of biological organization.

Beethoven, a mouse model for dominant, progressive hearing loss DFNA36

Despite recent progress in identifying genes underlying deafness, there are still relatively few mouse models of specific forms of human deafness. Here we describe the phenotype of the Beethoven (Bth) mouse mutant and a missense mutation in Tmc1 (transmembrane cochlear-expressed gene 1). Progressive hearing loss (DFNA36) and profound congenital deafness (DFNB7/B11) are caused by dominant and recessive mutations of the human ortholog, TMC1 (ref. 1), for which Bth and deafness (dn) are mouse models, respectively.

High-throughput selection of retrovirus producer cell lines leads to markedly improved efficiency of germ line-transmissible insertions in zebra fish

Vesicular stomatitis virus glycoprotein G-pseudotyped mouse retroviral vectors have been used as mutagens for a large-scale insertional mutagenesis screen in the zebra fish. To reproducibly generate high-titer virus stocks, we devised a method for rapidly selecting cell lines that can yield high-titer viruses and isolated a producer cell line that yields virus at a high titer on zebra fish embryos. Virus produced from this line, designated GT virus, is nontoxic following injection of zebra fish blastulae and efficiently infects embryonic cells that give rise to the future germ line. Using GT virus preparations we generated roughly 500,000 germ line-transmissible proviral insertions in a population of 25,000 founder fish in about 2 months. The GT virus contains a gene trap, and trap events can be detected in the offspring of almost every founder fish. We discuss potential applications of this highly efficient method for generating germ line-transmissible insertions in a vertebrate

Unravelling novel intracellular pathways in cell-based assays

The pharmaceutical industry is currently facing several challenges to identify and develop novel drug targets. Traditional drug discovery focussed on a small number of well-characterized gene products. Recently, this picture has changed with the completion of the draft sequence of the human genome, which has led to the identification of thousands of novel genes with unknown or poorly understood function. To cope with this overwhelming number of potential drug target candidates, new strategies for the elucidation of gene function, as well as their involvement in intracellular pathways, are required.

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.

Large-scale mutational analysis for the annotation of the mouse genome

After sequencing the human and mouse genomes, the annotation of these sequences with biological functions is an important challenge in genomic research. A major tool to analyse gene function on the organismal level is the analysis of mutant phenotypes. Because of its genetic and physiological similarity to man, the mouse has become the model organism of choice for the study of genetic diseases. In addition, there is at the moment no other vertebrate for which versatile techniques to manipulate the genome are as well developed. Several mouse mutagenesis projects have provided the proof-of-principle that a systematic and comprehensive mutagenesis of every gene in the mammalian genome will be feasible. An exhaustive functional annotation of the mammalian genome can only be achieved in a combination of phenotype- and gene-driven approaches in large- and small-scale academic and private projects. Major challenges will be to develop standardised phenotyping protocols for the clinical and pathological characterisation of mouse mutants, the improvement of mutation detection methods and the dissemination of resources and data. Beyond gene annotation, it will be necessary to understand how gene functions are integrated into the complex network of regulatory interactions in the cell.

Functional annotation of mammalian genomic DNA sequence by chemical mutagenesis: a fine-structure genetic mutation map of a 1- to 2-cM segment of mouse chromosome 7 corresponding to human chromosome 11p14-p15

Eleven independent, recessive, N-ethyl-N-nitrosourea-induced mutations that map to a approximately 1- to 2-cM region of mouse chromosome (Chr) 7 homologous to human Chr 11p14-p15 were recovered from a screen of 1,218 gametes. These mutations were initially identified in a hemizygous state opposite a large p-locus deletion and subsequently were mapped to finer genomic intervals by crosses to a panel of smaller p deletions. The 11 mutations also were classified into seven complementation groups by pairwise crosses. Four complementation groups were defined by seven prenatally lethal mutations, including a group (l7R3) comprised of two alleles of obvious differing severity. Two allelic mutations (at the psrt locus) result in a severe seizure and runting syndrome, but one mutation (at the fit2 locus) results in a more benign runting phenotype. This experiment has added seven loci, defined by phenotypes of presumed point mutations, to the genetic map of a small (1-2 cM) region of mouse Chr 7 and will facilitate the task of functional annotation of DNA sequence and transcription maps both in the mouse and the corresponding human 11p14-p15 homology region.

The genetic architecture of quantitative traits

Phenotypic variation for quantitative traits results from the segregation of alleles at multiple quantitative trait loci (QTL) with effects that are sensitive to the genetic, sexual, and external environments. Major challenges for biology in the post-genome era are to map the molecular polymorphisms responsible for variation in medically, agriculturally, and evolutionarily important complex traits; and to determine their gene frequencies and their homozygous, heterozygous, epistatic, and pleiotropic effects in multiple environments. The ease with which QTL can be mapped to genomic intervals bounded by molecular markers belies the difficulty in matching the QTL to a genetic locus. The latter requires high-resolution recombination or linkage disequilibrium mapping to nominate putative candidate genes, followed by genetic and/or functional complementation and gene expression analyses. Complete genome sequences and improved technologies for polymorphism detection will greatly advance the genetic dissection of quantitative traits in model organisms, which will open avenues for exploration of homologous QTL in related taxa.

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.

Focusing on the genetics of hearing: you ain't heard nothin' yet

The complexity of genetic pathways for hearing is beginning to be amenable to unraveling by systematic functional genomic analysis. Genome-wide mutagenesis studies in the mouse are beginning to shed further light on the structure and regulation of the machinery of hearing.

The fallacy of behavioral phenotyping without standardisation

Behavioral phenotyping of mutant mice is a new and challenging task for the behavioral neuroscientist. Therefore, standardisation of the experimental conditions is required to permit comparisons between the results of experiments within and between laboratories. Once mutation-induced behavioral changes have been identified, phenotyping of mouse mutants should be performed along a systematic trajectory, which allows for an in-depth characterisation of the mutant under investigation.

Full-speed mammalian genetics: in vivo target validation in the drug discovery process

The completion of the Human Genome Project has signaled the beginning of the post-genome era, with a corresponding shift in focus from the sequencing and identification of genes to the exploration of gene function. A rate-limiting step in deriving value from this gene sequence information is determining the potential pharmaceutical applications of genes and their encoded proteins. This validation step is crucial for focusing efforts and resources on only the most promising targets. Strategies using reverse mouse genetics provide excellent methods for validating potential targets and therapeutic proteins in vivo in a mammalian model system.

Molecular characterization of Pax6(2Neu) through Pax6(10Neu): an extension of the Pax6 allelic series and the identification of two possible hypomorph alleles in the mouse Mus musculus

Phenotype-based mutagenesis experiments will increase the mouse mutant resource, generating mutations at previously unmarked loci as well as extending the allelic series at known loci. Mapping, molecular characterization, and phenotypic analysis of nine independent Pax6 mutations of the mouse recovered in mutagenesis experiments is presented. Seven mutations result in premature termination of translation and all express phenotypes characteristic of null alleles, suggesting that Pax6 function requires all domains to be intact. Of major interest is the identification of two possible hypomorph mutations: Heterozygotes express less severe phenotypes and homozygotes develop rudimentary eyes and nasal processes and survive up to 36 hr after birth. Pax6(4Neu) results in an amino acid substitution within the third helix of the homeodomain. Three-dimensional modeling indicates that the amino acid substitution interrupts the homeodomain recognition alpha-helix, which is critical for DNA binding. Whereas cooperative dimer binding of the mutant homeodomain to a paired-class DNA target sequence was eliminated, weak monomer binding was observed. Thus, a residual function of the mutated homeodomain may explain the hypomorphic nature of the Pax6(4Neu) allele. Pax6(7Neu) is a base pair substitution in the Kozak sequence and results in a reduced level of Pax6 translation product. The Pax6(4Neu) and Pax6(7Neu) alleles may be very useful for gene-dosage studies.

Sensitized polygenic trait analysis

Genetic variation in many biological processes and evolutionary adaptations is caused by polygenes--genes that act in combination to affect a particular trait. Despite the recent identification of several polygenes, many remain to be found, suggesting that new experimental and analytical methods are needed to facilitate their discovery. Here we discuss sensitized polygenetic trait analysis, a method that has emerged recently for simplifying the genetic analysis of polygenic traits. The method uses a known single gene mutation in linkage testing crosses to 'sensitize' the analysis. By increasing the frequency of affected individuals in segregating populations, linkages are more readily detected. This method has considerable potential, especially given the increasing variety of mutations that can be used to sensitize the genetic analysis of polygenic traits.

Mutant laboratory mice with abnormalities in hair follicle morphogenesis, cycling, and/or structure: annotated tables

Numerous transgenic, targeted mutagenesis (so-called knockouts), conditional (so-called "gene switch") and spontaneous mutant mice develop abnormal hair phenotypes. The number of mice that exhibit such abnormalities is increasing exponentially as genetic engineering methods become routine. Since defined abnormalities in hair follicle morphogenesis, cycling and/or structure in such mutant mice provide important clues to the as yet poorly understood functional roles of many gene products, it is useful to summarize and classify these mutant mice according to their hair phenotype. This review provides a corresponding, annotated table of mutant mice with hair abnormalities, classifying the latter into 6 categories, 1) abnormally low number of hair follicles, 2) disorders of hair morphogenesis, 3) of hair follicle cycling, 4) of hair follicle structure 5) of sebaceous gland structure, and 6) hair growth disorders as a consequence of immunological abnormalities. This annotated table should serve as a useful source of reference for anyone who is interested in the molecular controls of hair growth, for investigators who are looking for mouse models to explore or compare the functional activities of their gene of interest, and for comparing the hair phenotype of newly generated mouse mutants with existing ones.

Characterization of a mutation in the lens-specific MP70 encoding gene of the mouse leading to a dominant cataract

During an ethylnitrosourea mutagenesis screen, Aey5, a new mouse mutation exhibiting an autosomal dominant congenital cataract was isolated. The cataractous phenotype is visible at the eye opening and progresses to a nuclear and zonular cataract at 2 months of age with no difference in onset or severity between heterozygous and homozygous mutants. Histological analysis revealed that fiber cell differentiation continues at the lens bow region, but the cell nuclei do not degrade normally and remain in the deeper cortex. Further, the lens nucleus has clefts of various sizes while the remainder of the eye was morphologically normal. The mutation was mapped to chromosome 3 between the markers D3Mit101 and D3Mit77 near the connexin encoding genes Gja5 and Gja8. Sequence analysis revealed no differences in the Gja5 gene, but identified a T-->C mutation at position 191 in the Gja8 gene, which was confirmed by an additional Mva 12691 restriction site in the genomic DNA of homozygous mutants. This mutation results in Val-->Ala substitution at codon 64 of connexin50 (Cx50) also known as lens membrane protein 70 (MP70). Aey5 represents the second dominant mouse cataract mutant affecting Cx50, a membrane protein preferentially expressed in the lens. Since both mutations affect similar regions in the first extracellular domain this region appears to be critically important for its function in lens transparency.

Efficient gene transfer into the embryonic mouse brain using in vivo electroporation

Mouse genetic manipulation has provided an excellent system to characterize gene function in numerous contexts. A number of mutants have been produced by using transgenic, gene knockout, and mutagenesis techniques. Nevertheless, one limitation is that it is difficult to express a gene in vivo in a restricted manner (i.e., spatially and temporally), because the number of available enhancers and promoters which can confine gene expression is limited. We have developed a novel method to introduce DNA into in/exo utero embryonic mouse brains at various stages by using electroporation. More than 90% of operated embryos survived, and more than 65% of these expressed the introduced genes in restricted regions of the brain. Expression was maintained even after birth, 6 weeks after electroporation. The use of fluorescent protein genes clearly visualized neuronal morphologies in the brain. Moreover, it was possible to transfect three different DNA vectors into the same cells. Thus, this method will be a powerful tool to characterize gene function in various settings due to its high efficiency and localized gene expression.

Chromosomal microdeletions: dissecting del22q11 syndrome

Identifying the genes that underlie the pathogenesis of chromosome deletion and duplication syndromes is a challenge because the affected chromosomal segment can contain many genes. The identification of genes that are relevant to these disorders often requires the analysis of individuals that carry rare, small deletions, translocations or single-gene mutations. Research into the chromosome 22 deletion (del22q11) syndrome, which encompasses DiGeorge and velocardiofacial syndrome, has taken a different path in recent years, using mouse models to circumvent the paucity of informative human material. These mouse models have provided new insights into the pathogenesis of del22q11 syndrome and have established strategies for research into chromosomal-deletion and -duplication syndromes.

The molecular neurobiology of stress--evidence from genetic and epigenetic models

Knowledge of the genetic and molecular events underlying the neuroendocrine and behavioural sequelae of the response to stress has advanced rapidly over recent years. The response of an individual to a stressful experience is a polygenic trait, but also involves non-genetic sources of variance. Using a combination of top-down (quantitative trait locus [QTL] and microarray analysis) and bottom-up (gene targeting, transgenesis, antisense technology and random mutagenesis) strategies, we are beginning to dissect the molecular players in the mediation of the stress response. Given the wealth of the data obtained from mouse mutants, this review will primarily focus on the contributions made by transgenesis and knockout studies, but the relative contribution of QTL studies and microarray studies will also be briefly addressed. From these studies it is evident that several neuroendocrine and behavioural alterations induced by stress can be modelled in mouse mutants with alterations in hypothalamic-pituitary-adrenal axis activity or other, extrahypothalamic, neurotransmitter systems known to be involved in the stress response. The relative contribution of these models to understanding the stress response and their limitations will be discussed.

Taking a functional genomics approach in molecular medicine

The elucidation of genetic components of human diseases at the molecular level provides crucial information for developing future causal therapeutic intervention. High-throughput genome sequencing and systematic experimental approaches are fuelling strategic programs designed to investigate gene function at the biochemical, cellular and organism levels. Bioinformatics is one important tool in functional genomics, although showing clear limitations in predicting ab initio gene structures, gene function and protein folds from raw sequence data. Systematic large-scale data-set generation, using the same type of experiments that are used to decipher the function of single genes, are being applied on entire genomes. Comparative genomics, establishment of gene catalogues, and investigation of cellular and tissue molecular profiles are providing essential tools for understanding gene function in complex biological networks.

Behavioural phenotyping of mouse mutants

Behavioural phenotyping of mouse mutants is not a goal in itself but serves to characterise the behavioural effects of naturally occurring or experimentally induced mutations. Genetically engineered mouse mutants are valuable tools to elucidate the genetic control of behaviour and the interaction between genetic and environmental factors. However, a prerequisite for their use is the ability to assess different elements of behaviour. To this end, a battery of tests, which should be flexible enough to meet the needs of a particular study, should be used to characterise the behavioural phenotype. Detailed and extensive information about the effects of gene mutations is crucial for model building and model evaluation. Model building is an iterative process, switching between experimental data and theory formation. In order to facilitate this process and to allow comparison of results within and between laboratories, the standardisation of breeding, housing, and testing conditions is essential. The development and standardisation of sensitive, valid behavioural tests which are suited to phenotype mouse mutants is both a responsibility and a challenge to investigators of mouse behaviour.

Mouse-based phenogenomics for modelling human disease

The powerful and wide-ranging genetic tools available in the laboratory mouse make it the major experimental model for studying mammalian gene function in vivo and modelling human disease traits. Large-scale random mutagenesis approaches, either gene-driven or phenotype-driven, promise to identify new clinically relevant phenotypes and their associated genes. Development of appropriate tools for assessing clinical phenotypes in mice is a crucial component of these endeavours, as is the establishment of the infrastructure for archiving and distribution of the growing mutant resource to the community. Integrated, multidisciplinary programs will be needed to fully exploit the power of the mouse in molecular medicine.

A radiation hybrid transcript map of the mouse genome

Expressed-sequence tag (EST) maps are an adjunct to sequence-based analytical methods of gene detection and localization for those species for which such data are available, and provide anchors for high-density homology and orthology mapping in species for which large-scale sequencing has yet to be done. Species for which radiation hybrid-based transcript maps have been established include human, rat, mouse, dog, cat and zebrafish. We have established a comprehensive first-generation-placement radiation hybrid map of the mouse consisting of 5,904 mapped markers (3,993 ESTs and 1,911 sequence-tagged sites (STSs)). The mapped ESTs, which often originate from small-EST clusters, are enriched for genes expressed during early mouse embryogenesis and are probably different from those localized in humans. We have confirmed by in situ hybridization that even singleton ESTs, which are usually not retained for mapping studies, may represent bona fide transcribed sequences. Our studies on mouse chromosomes 12 and 14 orthologous to human chromosome 14 show the power of our radiation hybrid map as a predictive tool for orthology mapping in humans.

Gene-trap mutagenesis: past, present and beyond

Although at least 35,000 human genes have been sequenced and mapped, adequate expression or functional information is available for only approximately 15% of them. Gene-trap mutagenesis is a technique that randomly generates loss-of-function mutations and reports the expression of many mouse genes. At present, several large-scale, gene-trap screens are being carried out with various new vectors, which aim to generate a public resource of mutagenized embryonic stem (ES) cells. This resource now includes more than 8,000 mutagenized ES-cell lines, which are freely available, making it an appropriate time to evaluate the recent advances in this area of genomic technology and the technical hurdles it has yet to overcome.

Mutations of the mouse Twist and sy (fibrillin 2) genes induced by chemical mutagenesis of ES cells

A prior phenotype-based screen of mice derived from ethylmethanesulfonate-mutagenized embryonic stem cells yielded two mouse limb defect mutants. Animals heterozygous for the polydactyly ems (Pde) mutation display preaxial polydactyly of the hindlimbs, and homozygous syndactyly ems (sne) animals are characterized by a fusion of the middle digits of their hindlimbs and sometimes forelimbs. We now report that Pde is a new allele of the basic helix-loop-helix protein gene Twist. Sequencing the full-length cDNA and several hundred basepairs of genomic DNA upstream of the coding region failed to reveal a mutation, suggesting that the lesion may be in a regulatory element of the gene. sne is a new fused phalanges (fp) allele of the shaker-with-syndactylism deletion complex (sy), and we show that the genomic lesion is a small deletion removing an entire exon, coincident with the insertion of the 3' end of a LINE element belonging to the TF subfamily.

Genome-wide ENU mutagenesis to reveal immune regulators

A complete list of molecular components for immune system function is now available with the completion of the human and mouse genome sequences. However, identification and functional annotation of genes involved in immunological processes require a discovery methodology that can efficiently and broadly analyze the complex interplay of these components in vivo. Our recent experience indicates that genome-wide chemical mutagenesis in the mouse is an extremely powerful methodology for the identification of genes required for complex immunological processes.

Mouse genomics: making sense of the sequence

Interpretation of the human genome sequence relies on studies of model genetic organisms. Mouse genetics and genomics will help to identify all the genes, and to determine their function.

Intraocular pressure in genetically distinct mice: an update and strain survey

BACKGROUND

Little is known about genetic factors affecting intraocular pressure (IOP) in mice and other mammals. The purpose of this study was to determine the IOPs of genetically distinct mouse strains, assess the effects of factors such as age, sex and time of day on IOP in specific strain backgrounds, and to assess the effects of specific candidate gene mutations on IOP.

RESULTS

Based on over 30 studied mouse strains, average IOP ranges from approximately 10 to 20 mmHg. Gender does not typically affect IOP and aging results in an IOP decrease in some strains. Most tested strains exhibit a diurnal rhythm with IOP being the highest during the dark period of the day. Homozygosity for a null allele of the carbonic anhydrase II gene (Car2n) does not alter IOP while homozygosity for a mutation in the leptin receptor gene (Leprdb) that causes obesity and diabetes results in increased IOP. Albino C57BL/6J mice homozygous for a tyrosinase mutation (Tyrc-2J) have higher IOPs than their pigmented counterparts.

CONCLUSIONS

Genetically distinct mouse strains housed in the same environment have a broad range of IOPs. These IOP differences are likely due to interstrain genetic differences that create a powerful resource for studying the regulation of IOP. Age, time of day, obesity and diabetes have effects on mouse IOP similar to those in humans and other species. Mutations in two of the assessed candidate genes (Lepr and Tyr) result in increased IOP. These studies demonstrate that mice are a practical and powerful experimental system to study the genetics of IOP regulation and disease processes that raise IOP to harmful levels.

Gene targeting reveals a role for the glutamate receptors mGluR5 and GluR2 in learning and memory

This work suggests that class I mGluRs are involved in long-term potentiation (LTP) at CA1 synapses within the hippocampus. Our data support a pathway linking class I-mGluRs with PKC and src to enhance the open probability of the NMDAR channel. This leads to LTP of the NMDAR, but not the AMPAR. We are currently analyzing double mGluR1 X mGluR5 knockouts with Collingridge for a loss of the LTP induction switch [Nature 368 (1994) 740.]. This induction of LTP of the NMDAR is necessary for "spatial" learning and memory to occur, since mice lacking the mGluR5 are deficient in the Morris water maze and context-dependent fear conditioning. We postulate that AMPARs may provide negative feedback inhibition to the NMDAR. Hence, in null mutants lacking the AMPAR subtype, GluR2, LTP in the CA1 region of hippocampal slices was markedly enhanced (twofold) and non-saturating, whereas neuronal excitability and paired-pulse facilitation were normal. The ninefold increase in Ca(2+) permeability, in response to kainate application, suggests one possible mechanism for enhanced LTP. Enhanced LTP could result from enhanced AMPAR channel conductance or increased recruiting of previously silent synapses. Since the GluR2 null mutants showed reduced exploration and impaired motor coordination, we could make no conclusion about its role in learning and memory. Future work will be directed to inducible deletion of GluR2 only in CA1 after development is complete. These results support the correlation between LTP and learning and memory.

Efficient chromosomal transposition of a Tc1/mariner- like transposon Sleeping Beauty in mice

The presence of mouse embryonic stem (ES) cells makes the mouse a powerful model organism for reverse genetics, gene function study through mutagenesis of specific genes. In contrast, forward genetics, identification of mutated genes responsible for specific phenotypes, has an advantage to uncover novel pathways and unknown genes because no a priori assumptions are made about the mutated genes. However, it has been hampered in mice because of the lack of a system in which a large-scale mutagenesis and subsequent isolation of mutated genes can be performed efficiently. Here, we demonstrate the efficient chromosomal transposition of a Tc1/mariner-like transposon, Sleeping Beauty, in mice. This system allows germ-line mutagenesis in vivo and will facilitate certain aspects of phenotype-driven genetic screening in mice.

Protein--protein interaction maps: a lead towards cellular functions

The availability of complete genome sequences now permits the development of tools for functional biology on a proteomic scale. Several experimental approaches or in silico algorithms aim at clustering proteins into networks with biological significance. Among those, the yeast two-hybrid system is the technology of choice to detect protein-protein interactions. Recently, optimized versions were applied at a genomic scale, leading to databases on the web. However, as with any other 'genetic' assay, yeast two-hybrid assays are prone to false positives and false negatives. Here we discuss these various technologies, their general limitations and the potential advances they make possible, especially when in combination with other functional genomics or bioinformatics analyses.

Mouse mutagenesis on target

Large-scale mutagenesis of the mouse genome is an essential task associated with the Human Genome Project. The two opposing schools of direct and reverse genetics have demonstrated comparable advantages, and yet large numbers of mutant lines have mostly been the prerogative of direct genetics. An improved gene-trapping resource now brings reverse genetics one step closer.

Functional analysis of secreted and transmembrane proteins critical to mouse development

We describe the successful application of a modified gene-trap approach, the secretory trap, to systematically analyze the functions in vivo of large numbers of genes encoding secreted and membrane proteins. Secretory-trap insertions in embryonic stem cells can be transmitted to the germ line of mice with high efficiency and effectively mutate the target gene. Of 60 insertions analyzed in mice, one-third cause recessive lethal phenotypes affecting various stages of embryonic and postnatal development. Thus, secretory-trap mutagenesis can be used for a genome-wide functional analysis of cell signaling pathways that are critical for normal mammalian development and physiology.

The Wheels mutation in the mouse causes vascular, hindbrain, and inner ear defects

In a screen for mouse mutations with dominant behavioral anomalies, we identified Wheels, a mutation associated with circling and hyperactivity in heterozygotes and embryonic lethality in homozygotes. Mutant Wheels embryos die at E10.5-E11.5 and exhibit a host of morphological anomalies which include growth retardation and anomalies in vascular and hindbrain development. The latter includes perturbation of rhombomeric boundaries as detected by Krox20 and Hoxb1. PECAM-1 staining of embryos revealed normal formation of the primary vascular plexus. However, subsequent stages of branching and remodeling do not proceed normally in the yolk sac and in the embryo proper. To obtain insights into the circling behavior, we examined development of the inner ear by paint-filling of membranous labyrinths of Whl/+ embryos. This analysis revealed smaller posterior and lateral semicircular canal primordia and a delay in the canal fusion process at E12.5. By E13.5, the lateral canal was truncated and the posterior canal was small or absent altogether. Marker analysis revealed an early molecular phenotype in heterozygous embryos characterized by perturbed expression of Bmp4 and Msx1 in prospective lateral and posterior cristae at E11.5. We have constructed a genetic and radiation hybrid map of the centromeric portion of mouse Chromosome 4 across the Wheels region and refined the position of the Wheels locus to the approximately 1.1-cM region between D4Mit104 and D4Mit181. We have placed the locus encoding Epha7, in the Wheels candidate region; however, further analysis showed no mutations in the Epha7-coding region and no detectable changes in mRNA expression pattern. In summary, our findings indicate that Wheels, a gene which is essential for the survival of the embryo, may link diverse processes involved in vascular, hindbrain, and inner ear development.

Fishing for lymphoid genes

Thymic organogenesis and T-cell lymphopoiesis are crucial interdependent processes that establish a functional vertebrate immune system. The current understanding of vertebrate thymic development during embryogenesis remains incomplete and would benefit from novel approaches. The zebrafish Danio rerio is a powerful developmental and genetic system for the dissection of early events in the ontogeny of the immune system. Forward genetic screens have uncovered genes involved in hematopoiesis, and specific screens are being designed to examine the genes that regulate T-cell development and the origin of the thymus. Studies of the zebrafish should improve our understanding of lymphoid development in vertebrates.

Transposition and gene disruption in the male germline of the mouse

We have tested a synthetic, functional, transposon called Sleeping Beauty for use in mice as a germline insertional mutagen. We describe experiments in which mutagenic, polyadenylation-site trapping, transposon vectors were introduced into the germline of mice. When doubly transgenic males, expressing the Sleeping Beauty transposase gene (SB10) and harboring poly(A)-trap transposon vectors, were outcrossed to wild-type females, offspring were generated with new transposon insertions. The frequency of new transposon insertion is roughly two per male gamete. These new insertions can be passed through the germline to the next generation and can insert into or near genes. We have generated a preliminary library of 24 mice harboring 56 novel insertion sites, including one insertion into a gene represented in the EST database and one in the promoter of the galactokinase (Gck) gene. This technique has promise as a new strategy for forward genetic screens in the mouse or functional genomics.

Genomics: saviour or millstone?

The Keystone Symposium on the impact of Genomics Development was held in Santa Fe, New Mexico, from 2 to 7 February 2001.