Capitalizing on large-scale mouse mutagenesis screens

Time to synchronize our clocks: Connecting developmental mechanisms and evolutionary consequences of heterochrony

Heterochrony, defined as a change in the timing of developmental events altering the course of evolution, was first recognized by Ernst Haeckel in 1866. Haeckel's original definition was meant to explain the observed parallels between ontogeny and phylogeny, but the interpretation of his work became a source of controversy over time. Heterochrony took its modern meaning following the now classical work in the 1970-80s by Steven J. Gould, Pere Alberch, and co-workers. Predicted and described heterochronic scenarios emphasize the many ways in which developmental changes can influence evolution. However, while important examples of heterochrony detected with comparative morphological methods have multiplied, the more mechanistic understanding of this phenomenon lagged conspicuously behind. Considering the rapid progress in imaging and molecular tools available now for developmental biologists, this review aims to stress the need to take heterochrony research to the next level. It is time to synchronize the different levels of heterochrony research into a single analysis flow: from studies on organismal-level morphology to cells to molecules and genes, using complementary techniques. To illustrate how to achieve a more comprehensive understanding of phyletic morphological diversification associated with heterochrony, we discuss several recent case studies at various phylogenetic scales that combine morphological, cellular, and molecular analyses. Such a synergistic approach offers to more fully integrate phylogenetic and ontogenetic dimensions of the fascinating evolutionary phenomenon of heterochrony.

Reduced Anthocyanins in Petioles codes for a GST anthocyanin transporter that is essential for the foliage and fruit coloration in strawberry

The red color of the foliage and fruit in strawberry comes from anthocyanins stored in the vacuole; however, how this anthocyanin accumulation is regulated remains unclear. A reduced anthocyanin in petioles (rap) mutant was identified in an N-ethyl-N-nitrosourea (ENU) mutagenized population of YW5AF7, a white-fruited variety of the wild strawberry Fragaria vesca. The causative mutation was identified to be a premature stop codon in a glutathione S-transferase (GST) gene. In addition to the foliage coloration, RAP also mediates fruit pigmentation and acts downstream of the fruit-specific transcription factor FvMYB10. Among all eight GST genes in the same subfamily, RAP is most abundantly expressed in the ripening fruit. Expression analysis and transient expression assays demonstrated that RAP is the principal transporter of anthocyanins among the paralogs. Moreover, domain-swap experiments showed that both the N- and C-terminals of RAP are essential for the binding capability of anthocyanins. In addition, transient knock-down of RAP resulted in reduced fruit coloration in cultivated strawberry. Collectively, our results demonstrate that RAP encodes the principal GST transporter of anthocyanins in the strawberry foliage and fruit, and it could be modified to alter the fruit color in strawberry.

Genome-wide chemical mutagenesis screens allow unbiased saturation of the cancer genome and identification of drug resistance mutations

Drug resistance is an almost inevitable consequence of cancer therapy and ultimately proves fatal for the majority of patients. In many cases, this is the consequence of specific gene mutations that have the potential to be targeted to resensitize the tumor. The ability to uniformly saturate the genome with point mutations without chromosome or nucleotide sequence context bias would open the door to identify all putative drug resistance mutations in cancer models. Here, we describe such a method for elucidating drug resistance mechanisms using genome-wide chemical mutagenesis allied to next-generation sequencing. We show that chemically mutagenizing the genome of cancer cells dramatically increases the number of drug-resistant clones and allows the detection of both known and novel drug resistance mutations. We used an efficient computational process that allows for the rapid identification of involved pathways and druggable targets. Such a priori knowledge would greatly empower serial monitoring strategies for drug resistance in the clinic as well as the development of trials for drug-resistant patients.

High-throughput discovery of novel developmental phenotypes

Approximately one-third of all mammalian genes are essential for life. Phenotypes resulting from knockouts of these genes in mice have provided tremendous insight into gene function and congenital disorders. As part of the International Mouse Phenotyping Consortium effort to generate and phenotypically characterize 5,000 knockout mouse lines, here we identify 410 lethal genes during the production of the first 1,751 unique gene knockouts. Using a standardized phenotyping platform that incorporates high-resolution 3D imaging, we identify phenotypes at multiple time points for previously uncharacterized genes and additional phenotypes for genes with previously reported mutant phenotypes. Unexpectedly, our analysis reveals that incomplete penetrance and variable expressivity are common even on a defined genetic background. In addition, we show that human disease genes are enriched for essential genes, thus providing a dataset that facilitates the prioritization and validation of mutations identified in clinical sequencing efforts.

Obesity genetics in mouse and human: back and forth, and back again

Obesity is a major public health concern. This condition results from a constant and complex interplay between predisposing genes and environmental stimuli. Current attempts to manage obesity have been moderately effective and a better understanding of the etiology of obesity is required for the development of more successful and personalized prevention and treatment options. To that effect, mouse models have been an essential tool in expanding our understanding of obesity, due to the availability of their complete genome sequence, genetically identified and defined strains, various tools for genetic manipulation and the accessibility of target tissues for obesity that are not easily attainable from humans. Our knowledge of monogenic obesity in humans greatly benefited from the mouse obesity genetics field. Genes underlying highly penetrant forms of monogenic obesity are part of the leptin-melanocortin pathway in the hypothalamus. Recently, hypothesis-generating genome-wide association studies for polygenic obesity traits in humans have led to the identification of 119 common gene variants with modest effect, most of them having an unknown function. These discoveries have led to novel animal models and have illuminated new biologic pathways. Integrated mouse-human genetic approaches have firmly established new obesity candidate genes. Innovative strategies recently developed by scientists are described in this review to accelerate the identification of causal genes and deepen our understanding of obesity etiology. An exhaustive dissection of the molecular roots of obesity may ultimately help to tackle the growing obesity epidemic worldwide.

ENU mutagenesis identifies mice modeling Warburg Micro Syndrome with sensory axon degeneration caused by a deletion in Rab18

Mutations in the gene of RAB18, a member of Ras superfamily of small G-proteins, cause Warburg Micro Syndrome (WARBM) which is characterized by defective neurodevelopmental and ophthalmological phenotypes. Despite loss of Rab18 had been reported to induce disruption of the endoplasmic reticulum structure and neuronal cytoskeleton organization, parts of the pathogenic mechanism caused by RAB18 mutation remain unclear. From the N-ethyl-N-nitrosourea (ENU)-induced mutagenesis library, we identified a mouse line whose Rab18 was knocked out. This Rab18(-/-) mouse exhibited stomping gait, smaller testis and eyes, mimicking several features of WARBM. Rab18(-/-) mice were obviously less sensitive to pain and touch than WT mice. Histological examinations on Rab18(-/-) mice revealed progressive axonal degeneration in the optic nerves, dorsal column of the spinal cord and sensory roots of the spinal nerves while the motor roots were spared. All the behavioral and pathological changes that resulted from abnormalities in the sensory axons were prevented by introducing an extra copy of Rab18 transgene in Rab18(-/-) mice. Our results reveal that sensory axonal degeneration is the primary cause of stomping gait and progressive weakness of the hind limbs in Rab18(-/-) mice, and optic nerve degeneration should be the major pathology of progressive optic atrophy in children with WARBM. Our results indicate that the sensory nervous system is more vulnerable to Rab18 deficiency and WARBM is not only a neurodevelopmental but also neurodegenerative disease.

Exploiting the power of LINE-1 retrotransposon mutagenesis for identification of genes involved in embryonic stem cell differentiation

Identifying the genes or epigenetic factors that control the self-renewal and differentiation of stem cells is critical to understanding the molecular basis of cell commitment. Although a number of insertional mutagenesis vectors have been developed for identifying gene functions in animal models, the L1 retrotransposition system offers additional advantages as a tool to disrupt genes in embryonic stem cells in order to identify their functions and the phenotypes associated with them. Recent advances in producing synthetic versions of L1 retrotransposon vector system and the optimization of techniques to accurately identify retrotransposon integration sites have increased their utility for gene discovery applications. We have developed a novel episomal, nonviral L1 retrotransposon vector using scaffold/matrix attachment regions that provides stable, sustained levels of retrotransposition in cell cultures without being affected by epigenetic silencing or from some of the common problems of vector integration. This modified vector contains a GFP marker whose expression occurs only after successful gene disruption events and thus the cells with disrupted genes can be easily picked for functional analysis. Here we present a method to disrupt gene function in embryonic stem cells that aid in the identification of genes involved in stem cell differentiation processes. The methods presented here can be easily adapted to the study of other types of cancer stem cells or induced pluripotent stem cells using the L1 retrotransposon as an insertional mutagen.

Interrogating congenital heart defects with noninvasive fetal echocardiography in a mouse forward genetic screen

BACKGROUND

Congenital heart disease (CHD) has a multifactorial pathogenesis, but a genetic contribution is indicated by heritability studies. To investigate the spectrum of CHD with a genetic pathogenesis, we conducted a forward genetic screen in inbred mice using fetal echocardiography to recover mutants with CHD. Mice are ideally suited for these studies given that they have the same four-chamber cardiac anatomy that is the substrate for CHD.

METHODS AND RESULTS

Ethylnitrosourea mutagenized mice were ultrasound-interrogated by fetal echocardiography using a clinical ultrasound system, and fetuses suspected to have cardiac abnormalities were further interrogated with an ultrahigh-frequency ultrasound biomicroscopy. Scanning of 46 270 fetuses revealed 1722 with cardiac anomalies, with 27.9% dying prenatally. Most of the structural heart defects can be diagnosed using ultrasound biomicroscopy but not with the clinical ultrasound system. Confirmation with analysis by necropsy and histopathology showed excellent diagnostic capability of ultrasound biomicroscopy for most CHDs. Ventricular septal defect was the most common CHD observed, whereas outflow tract and atrioventricular septal defects were the most prevalent complex CHD. Cardiac/visceral organ situs defects were observed at surprisingly high incidence. The rarest CHD found was hypoplastic left heart syndrome, a phenotype never seen in mice previously.

CONCLUSIONS

We developed a high-throughput, 2-tier ultrasound phenotyping strategy for efficient recovery of even rare CHD phenotypes, including the first mouse models of hypoplastic left heart syndrome. Our findings support a genetic pathogenesis for a wide spectrum of CHDs and suggest that the disruption of left-right patterning may play an important role in CHD.

Genome-wide mouse mutagenesis reveals CD45-mediated T cell function as critical in protective immunity to HSV-1

Herpes simplex encephalitis (HSE) is a lethal neurological disease resulting from infection with Herpes Simplex Virus 1 (HSV-1). Loss-of-function mutations in the UNC93B1, TLR3, TRIF, TRAF3, and TBK1 genes have been associated with a human genetic predisposition to HSE, demonstrating the UNC93B-TLR3-type I IFN pathway as critical in protective immunity to HSV-1. However, the TLR3, UNC93B1, and TRIF mutations exhibit incomplete penetrance and represent only a minority of HSE cases, perhaps reflecting the effects of additional host genetic factors. In order to identify new host genes, proteins and signaling pathways involved in HSV-1 and HSE susceptibility, we have implemented the first genome-wide mutagenesis screen in an in vivo HSV-1 infectious model. One pedigree (named P43) segregated a susceptible trait with a fully penetrant phenotype. Genetic mapping and whole exome sequencing led to the identification of the causative nonsense mutation L3X in the Receptor-type tyrosine-protein phosphatase C gene (Ptprc(L3X)), which encodes for the tyrosine phosphatase CD45. Expression of MCP1, IL-6, MMP3, MMP8, and the ICP4 viral gene were significantly increased in the brain stems of infected Ptprc(L3X) mice accounting for hyper-inflammation and pathological damages caused by viral replication. Ptprc(L3X) mutation drastically affects the early stages of thymocytes development but also the final stage of B cell maturation. Transfer of total splenocytes from heterozygous littermates into Ptprc(L3X) mice resulted in a complete HSV-1 protective effect. Furthermore, T cells were the only cell population to fully restore resistance to HSV-1 in the mutants, an effect that required both the CD4⁺ and CD8⁺ T cells and could be attributed to function of CD4⁺ T helper 1 (Th1) cells in CD8⁺ T cell recruitment to the site of infection. Altogether, these results revealed the CD45-mediated T cell function as potentially critical for infection and viral spread to the brain, and also for subsequent HSE development.

Interactions between Twist and other core epithelial-mesenchymal transition factors are controlled by GSK3-mediated phosphorylation

A subset of transcription factors classified as neural crest 'specifiers' are also core epithelial-mesenchymal transition regulatory factors, both in the neural crest and in tumour progression. The bHLH factor Twist is among the least well studied of these factors. Here we demonstrate that Twist is required for cranial neural crest formation and fate determination in Xenopus. We further show that Twist function in the neural crest is dependent upon its carboxy-terminal WR domain. The WR domain mediates physical interactions between Twist and other core epithelial-mesenchymal transition factors, including Snail1 and Snail2, which are essential for proper function. Interaction with Snail1/2, and Twist function more generally, is regulated by GSK-3-β-mediated phosphorylation of conserved sites in the WR domain. Together, these findings elucidate a mechanism for coordinated control of a group of structurally diverse factors that function as a regulatory unit in both developmental and pathological epithelial-mesenchymal transitions.

Discovery Genetics - The History and Future of Spontaneous Mutation Research

Historically, spontaneous mutations in mice have served as valuable models of heritable human diseases, contributing substantially to our understanding of both disease mechanisms and basic biological pathways. While advances in molecular technologies have improved our ability to create mouse models of human disease through targeted mutagenesis and transgenesis, spontaneous mutations continue to provide valuable research tools for discovery of novel genes and functions. In addition, the genetic defects caused by spontaneous mutations are molecularly similar to mutations in the human genome and, therefore often produce phenotypes that more closely resemble those characteristic of human disease than do genetically engineered mutations. Due to the rarity with which spontaneous mutations arise and the animal intensive nature of their genetic analysis, large-scale spontaneous mutation analysis has traditionally been limited to large mammalian genetics institutes. More recently, ENU mutagenesis and new screening methods have increased the rate of mutant strain discovery, and high-throughput DNA sequencing has enabled rapid identification of the underlying genes and their causative mutations. Here, we discuss the continued value of spontaneous mutations for biomedical research.

Systematic approaches to toxicology in the zebrafish

As the current paradigms of drug discovery evolve, it has become clear that a more comprehensive understanding of the interactions between small molecules and organismal biology will be vital. The zebrafish is emerging as a complement to existing in vitro technologies and established preclinical in vivo models that can be scaled for high-throughput. In this review, we highlight the current status of zebrafish toxicology studies, identify potential future niches for the model in the drug development pipeline, and define the hurdles that must be overcome as zebrafish technologies are refined for systematic toxicology.

Technical approaches for mouse models of human disease

The mouse is the leading organism for disease research. A rich resource of genetic variation occurs naturally in inbred and special strains owing to spontaneous mutations. However, one can also obtain desired gene mutations by using the following processes: targeted mutations that eliminate function in the whole organism or in a specific tissue; forward genetic screens using chemicals or transposons; or the introduction of exogenous transgenes as DNAs, bacterial artificial chromosomes (BACs) or reporter constructs. The mouse is the only mammal that provides such a rich resource of genetic diversity coupled with the potential for extensive genome manipulation, and is therefore a powerful application for modeling human disease. This poster review outlines the major genome manipulations available in the mouse that are used to understand human disease: natural variation, reverse genetics, forward genetics, transgenics and transposons. Each of these applications will be essential for understanding the diversity that is being discovered within the human population.

Phenotypic and genetic analysis of the cerebellar mutant tmgc26, a new ENU-induced ROR-alpha allele

ROR-alpha is an orphan nuclear receptor, inactivation of which cell-autonomously blocks differentiation of cerebellar Purkinje cells with a secondary loss of granule neurons. As part of our ENU mutagenesis screen we isolated the recessive tmgc26 mouse mutant, characterized by early-onset progressive ataxia, cerebellar degeneration and juvenile lethality. Detailed analysis of the tmgc26-/- cerebella revealed Purkinje cell and granule cell abnormalities, and defects in molecular layer interneurons and radial glia. Chimera studies suggested a cell-autonomous effect of the tmgc26 mutation in Purkinje cells and molecular layer interneurons, and a non-cell-autonomous effect in granule cells. The mutation was mapped to a 13-Mb interval on chromosome 9, a region that contains the ROR-alpha gene. Sequencing of genomic DNA revealed a T-to-A transition in exon 5 of the ROR-alpha gene, resulting in a nonsense mutation C257X and severe truncation of the ROR-alpha protein. Together, our data identify new roles for ROR-alpha in molecular layer interneurons and radial glia development and suggest tmgc26 as a novel ROR-alpha allele that may be used to further delineate the molecular mechanisms of ROR-alpha action.

Metabolic syndrome components in murine models

Animal models have enriched understanding of the physiological basis of metabolic disorders and advanced identification of genetic risk factors underlying the metabolic syndrome (MetS). Murine models are especially appropriate for this type of research, and are an excellent resource not only for identifying candidate genomic regions, but also for illuminating the possible molecular mechanisms or pathways affected in individual components of MetS. In this review, we briefly discuss findings from mouse models of metabolic disorders, particularly in light of issues raised by the recent flood of human genome-wide association studies (GWAS) results. We describe how mouse models are revealing that genotype interacts with environment in important ways, indicating that the underlying genetics of MetS is highly context dependant. Further we show that epistasis, imprinting and maternal effects each contribute to the genetic architecture underlying variation in metabolic traits, and mouse models provide an opportunity to dissect these aspects of the genetic architecture that are difficult if not impossible to ascertain in humans. Finally we discuss how knowledge gained from mouse models can be used in conjunction with comparative genomic methods and bioinformatic resources to inform human MetS research.

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.

Discovery of candidate disease genes in ENU-induced mouse mutants by large-scale sequencing, including a splice-site mutation in nucleoredoxin

An accurate and precisely annotated genome assembly is a fundamental requirement for functional genomic analysis. Here, the complete DNA sequence and gene annotation of mouse Chromosome 11 was used to test the efficacy of large-scale sequencing for mutation identification. We re-sequenced the 14,000 annotated exons and boundaries from over 900 genes in 41 recessive mutant mouse lines that were isolated in an N-ethyl-N-nitrosourea (ENU) mutation screen targeted to mouse Chromosome 11. Fifty-nine sequence variants were identified in 55 genes from 31 mutant lines. 39% of the lesions lie in coding sequences and create primarily missense mutations. The other 61% lie in noncoding regions, many of them in highly conserved sequences. A lesion in the perinatal lethal line l11Jus13 alters a consensus splice site of nucleoredoxin (Nxn), inserting 10 amino acids into the resulting protein. We conclude that point mutations can be accurately and sensitively recovered by large-scale sequencing, and that conserved noncoding regions should be included for disease mutation identification. Only seven of the candidate genes we report have been previously targeted by mutation in mice or rats, showing that despite ongoing efforts to functionally annotate genes in the mammalian genome, an enormous gap remains between phenotype and function. Our data show that the classical positional mapping approach of disease mutation identification can be extended to large target regions using high-throughput sequencing.

Next-generation sequencing of vertebrate experimental organisms

Next-generation sequencing technologies are revolutionizing biology by allowing for genome-wide transcription factor binding-site profiling, transcriptome sequencing, and more recently, whole-genome resequencing. While it is currently not possible to generate complete de novo assemblies of higher-vertebrate genomes using next-generation sequencing, improvements in sequence read lengths and throughput, coupled with new assembly algorithms for large data sets, will soon make this a reality. These developments will in turn spawn a revolution in how genomic data are used to understand genetics and how model organisms are used for disease gene discovery. This review provides an overview of the current next-generation sequencing platforms and the newest computational tools for the analysis of next-generation sequencing data. We also describe how next-generation sequencing may be applied in the context of vertebrate model organism genetics.

Lineage-specific biology revealed by a finished genome assembly of the mouse

The mouse (Mus musculus) is the premier animal model for understanding human disease and development. Here we show that a comprehensive understanding of mouse biology is only possible with the availability of a finished, high-quality genome assembly. The finished clone-based assembly of the mouse strain C57BL/6J reported here has over 175,000 fewer gaps and over 139 Mb more of novel sequence, compared with the earlier MGSCv3 draft genome assembly. In a comprehensive analysis of this revised genome sequence, we are now able to define 20,210 protein-coding genes, over a thousand more than predicted in the human genome (19,042 genes). In addition, we identified 439 long, non-protein-coding RNAs with evidence for transcribed orthologs in human. We analyzed the complex and repetitive landscape of 267 Mb of sequence that was missing or misassembled in the previously published assembly, and we provide insights into the reasons for its resistance to sequencing and assembly by whole-genome shotgun approaches. Duplicated regions within newly assembled sequence tend to be of more recent ancestry than duplicates in the published draft, correcting our initial understanding of recent evolution on the mouse lineage. These duplicates appear to be largely composed of sequence regions containing transposable elements and duplicated protein-coding genes; of these, some may be fixed in the mouse population, but at least 40% of segmentally duplicated sequences are copy number variable even among laboratory mouse strains. Mouse lineage-specific regions contain 3,767 genes drawn mainly from rapidly-changing gene families associated with reproductive functions. The finished mouse genome assembly, therefore, greatly improves our understanding of rodent-specific biology and allows the delineation of ancestral biological functions that are shared with human from derived functions that are not.

A chemical mutagenesis screen to identify modifier genes that interact with growth hormone and TGF-beta signaling pathways

We describe a phenotype-driven mutagenesis screen in which mice carrying a targeted mutation are bred with ENU-treated males in order to provide a sensitized system for detecting dominant modifier mutations. The presence of initial mutation renders the screening system more responsive to subtle changes in modifier genes that would not be penetrant in an otherwise wild type background. We utilized two mutant mouse models: 1) mice carrying a mutation in growth hormone releasing hormone receptor (Ghrhr) (denoted 'lit' allele, Ghrhr(lit)), which results in GH deficiency; and 2) mice lacking Smad2 gene, a signal transducer for TGF-beta, an important bone growth factor. The Smad2(-/-) mice are lethal and Ghrhr(lit/lit) mice are dwarf, but both Smad2(+/-) and Ghrhr(lit/)(+) mice exhibit normal growth. We injected 6-7 weeks old C57BL/6J male mice with ENU (100 mg/kg dose) and bred them with Ghrhr(lit/)(+) and Smad2(+/-) mice. The F1 mice with Ghrhr(lit/)(+) or Smad2(+/-) genotype were screened for growth and skeletal phenotypes. An outlier was identified as >3 SD units different from wild type control (n=20-30). We screened about 100 F1 mice with Ghrhr(lit/)(+) and Smad2(+/-) genotypes and identified nine outliers. A backcross established heritability of three mutant lines in multiple generations. Among the phenotypic deviants, we have identified a mutant mouse with 30-40% reduced bone size. The magnitude of the bone size phenotype was amplified by the presence of one copy of the disrupted Ghrhr gene as determined by the 2-way ANOVA (p<0.02 for interaction). Thus, a new mouse model has been established to identify a gene that interacts with GH signaling to regulate bone size. In addition, the sensitized screen also demonstrated higher recovery of skeletal phenotypes as compared to that obtained in the classical ENU screen in wild type mice. The discovery of mutants in a selected pathway will provide a valuable tool to not only to discover novel genes involved in a particular process but will also prove useful for the elucidation of the biology of that process.

Isolated sagittal and coronal craniosynostosis associated with TWIST box mutations

Craniosynostosis, the premature fusion of one or more cranial sutures, affects 1 in 2,500 live births. Isolated single-suture fusion is most prevalent, with sagittal synostosis occurring in 1/5,000 live births. The etiology of isolated (nonsyndromic) single-suture craniosynostosis is largely unknown. In syndromic craniosynostosis, there is a highly nonrandom pattern of causative autosomal dominant mutations involving TWIST1 and fibroblast growth factor receptors (FGFRs). Prior to our study, there were no published TWIST1 mutations in the anti-osteogenic C-terminus, recently coined the TWIST Box, which binds and inhibits RUNX2 transactivation. RUNX2 is the principal master switch for osteogenesis. We performed mutational analysis on 164 infants with isolated, single-suture craniosynostosis for mutations in TWIST1, the IgIIIa exon of FGFR1, the IgIIIa and IgIIIc exons of FGFR2, and the Pro250Arg site of FGFR3. We identified two patients with novel TWIST Box mutations: one with isolated sagittal synostosis and one with isolated coronal synostosis. Kress et al. [2006] reported a TWIST Box "nondisease-causing polymorphism" in a patient with isolated sagittal synostosis. However, compelling evidence suggests that their and our sequence alterations are pathogenic: (1) a mouse with a mutation of the same residue as our sagittal synostosis patient developed sagittal synostosis, (2) mutation of the same residue precluded TWIST1 interaction with RUNX2, (3) each mutation involved nonconservative amino acid substitutions in highly conserved residues across species, and (4) control chromosomes lacked TWIST Box sequence alterations. We suggest that genetic testing of patients with isolated sagittal or coronal synostosis should include TWIST1 mutational analysis.

Comparative genetic analysis: the utility of mouse genetic systems for studying human monogenic disease

One of the long-term goals of mutagenesis programs in the mouse has been to generate mutant lines to facilitate the functional study of every mammalian gene. With a combination of complementary genetic approaches and advances in technology, this aim is slowly becoming a reality. One of the most important features of this strategy is the ability to identify and compare a number of mutations in the same gene, an allelic series. With the advent of gene-driven screening of mutant archives, the search for a specific series of interest is now a practical option. This review focuses on the analysis of multiple mutations from chemical mutagenesis projects in a wide variety of genes and the valuable functional information that has been obtained from these studies. Although gene knockouts and transgenics will continue to be an important resource to ascertain gene function, with a significant proportion of human diseases caused by point mutations, identifying an allelic series is becoming an equally efficient route to generating clinically relevant and functionally important mouse models.

Enemy at the gates: forward genetics of the mouse antiviral response

The environment and the genetic constitution of both the pathogen and the host influence the severity and the outcome of viral infections. Whereas identification of the host component in humans remains challenging, recent progress in defining genes through analysis of mouse models of infection presenting natural or chemically induced variation in host susceptibility mark a fruitful period of gene discovery. This includes recognition that UNC93B1, which encodes an endocytic protein, is a susceptibility gene, providing an unexpected entry point to our understanding of the response against herpesvirus infection. By contrast, elucidation of alternative mechanisms of host resistance against mouse cytomegalovirus in inbred mouse strains has led to new insights regarding molecular recognition of the infected cells by natural killer cell MHC class I receptors. In addition, the conservation of genetic and functional aspects between mouse and human is enabling a rational pursuit of potential cures. With the continuous development of resources for experimental investigation of the genome, the production of new mutant alleles, and the phenotypic characterization of new models of infection, we predict that mouse genetic models will make an increasing contribution to our understanding of the genetic puzzle of host response to virus infection.

N-ethyl-N-nitrosourea (ENU) mutagenesis and male fertility research

Male infertility affects about 1 in 25 men in the western world. Conversely, there is an urgent requirement for additional male-based contraceptives, yet progress in both areas has been severely hampered by a lack of knowledge of the biochemistry and physiology of male reproductive function. It is only through a thorough knowledge of these processes that we can hope to insightfully regulate male reproductive function. Without doubt, mouse models will form an important foundation in any future process. In recent years, the chemical mutagen N-ethyl-N-nitrosourea (ENU) has been used widely to identify genes essential for a range of biological systems including male infertility. These studies have shown random mutagenesis is an attractive means of identifying key genes for male fertility. This technique has distinct, but complementary advantages compared to knockout technologies. Specifically, it allows the removal of researcher bias whereby only pre-conceived genes are tested for function; it produces mice with a guaranteed phenotype and allows for the production of allelic series of mice to dissect all aspects of gene function. ENU mouse mutagenesis programs will enable advances in the diagnosis and treatment of human male infertility and ultimately aid in the development of novel male-based contraceptives.

A repository of ENU mutant mouse lines and their potential for male fertility research

Many of the proteins and their encoding genes involved in spermatogenesis are unknown, making the specific diagnosis and treatment of infertility in males difficult and highlighting the importance of identifying new genes that are involved in spermatogenesis. Through genome-wide chemical mutagenesis using N-ethyl-N-nitrosourea (ENU) and a three-generation breeding scheme to isolate recessive mutations, we have identified mouse lines with a range of abnormalities relevant to human male fertility. Abnormal phenotypes included hypospermatogenesis, Sertoli cell-only (SCO) seminiferous tubules, germ-cell arrest and abnormal spermiogenesis and were accompanied, in some, with abnormal serum levels of reproductive hormones. In total, from 65 mouse lines, 14 showed a reproductive phenotype consistent with a recessive mutation. This study shows that it is feasible to use ENU mutagenesis as an effective and rapid means of generating mouse models relevant to furthering our understanding of human male infertility. Spermatozoa and genomic DNA from all mouse lines, including those with abnormal reproductive tract parameters, have been cryopreserved for the regeneration of lines as required. This repository will form a valuable resource for the identification and analysis of key regulators of multiple aspects of male fertility.

Cause and Effect Considerations in Diagnostic Pathology and Pathology Phenotyping of Genetically Engineered Mice (GEM)

Over the next several decades, biology is embarking on its most ambitious project yet: to annotate the human genome functionally, prioritizing and focusing on those genes relevant to development and disease. Model systems are fundamental prerequisites for this task, and genetically engineered mice (GEM) are by far the most accessible mammalian system because of their anatomical, physiological, and genetic similarity to humans. The scientific utility of GEM has become commonplace since the technology to produce them was established in the early 1980s. Conceptually, however, an efficiently coordinated high-throughput approach that permits correlation between newly discovered genes, functional properties of their protein products, and biological relevance of these products as drug targets has yet to be established. The discipline of veterinary anatomical pathology (hereafter referred to as pathology) is not immune to this requirement for evolution and adaptation, and to address relationships and tissue consequences between tens of thousands of genes and their cognate proteins, novel interdisciplinary technologies and approaches must emerge. Although many of the techniques of pathology are well established, in the context of pathology's contribution to functional annotation of the genome, several conceptually important and unresolved issues remain to be addressed. While an ever-increasing arsenal of genetic and molecular tool-sets are available to evaluate and understand the function of genes and their pathophysiological mechanisms, pathology will continue to play an essential role in confirming cause and effect relationships of gene function in development and disease. This role will continue to be dependent on keen observation, a systematic but disciplined approach, expert knowledge of strain-dependent anatomical differences and incidental lesions, and relevant tissue-based evidence. Miniaturization and high-throughput adaptation of these methods must also continue so that they can complement parallel phenotyping efforts, provide pathology-based data in pace with concurrent phenotyping efforts, and continue to find new utility in the collective effort of functional annotation.

Identification of mutations from phenotype-driven ENU mutagenesis in mouse chromosome 7

We have used the new high-throughput mutation-scanning technique temperature-gradient capillary electrophoresis (TGCE) for the identification of point mutations induced by N-ethyl-N-nitrosourea (ENU) in the mouse genome. TGCE detects the presence of heteroduplex molecules formed between a wild-type gene segment and the corresponding homologous segment containing an induced mutation or a naturally occurring single nucleotide polymorphism (SNP). Partially denatured heteroduplex molecules are resolved from homoduplexes by virtue of their differential mobilities during capillary electrophoresis conducted in a finely controlled temperature gradient. Simultaneous heteroduplex analysis of 96 amplicons ranging from 150 to 600 bp in size is achieved in approximately 45 min without the need for predetermining the melting profile of each fragment. Initially, we exploited known mouse mutations to develop TGCE protocols for analyzing unpurified PCR samples amplified from crude tail-DNA preparations. TGCE was then applied to the rapid identification of three new ENU-induced mutations recovered from regional mutagenesis screens of a segment of mouse Chromosome 7. Enzyme assays and quantitative reverse transcription-PCR (qRT-PCR) methods validated these new mutations. Our data demonstrate that rapid mutation scanning with TGCE, followed by sequence verification only of detected positives, is an efficient approach to the identification of point mutations in the mouse genome.

ARTS: a web-based tool for the set-up of high-throughput genome-wide mapping panels for the SNP genotyping of mouse mutants

Genome-wide mapping in the identification of novel candidate genes has always been the standard method in genetics and genomics to correlate a clinically interesting phenotypic trait with a genotype. However, the performance of a mapping experiment using classical microsatellite approaches can be very time consuming. The high-throughput analysis of single-nucleotide polymorphisms (SNPs) has the potential of being the successor of microsatellite analysis routinely used for these mapping approaches, where one of the major obstacles is the design of the appropriate SNP marker set itself. Here we report on ARTS, an advanced retrieval tool for SNPs, which allows researchers to comb freely the public mouse dbSNP database for multiple reference and test strains. Several filters can be applied in order to improve the sensitivity and the specificity of the search results. By employing the panel generator function of this program, it is possible to abbreviate the extraction of reliable sequence data for a large marker panel including several different mouse strains from days to minutes. The concept of ARTS is easily adaptable to other species for which SNP databases are available, making it a versatile tool for the use of SNPs as markers for genotyping. The web interface is accessible at http://andromeda.gsf.de/arts.

Smoking-gene interaction and disease development: relevance to pancreatic cancer and atherosclerosis

There is little doubt that cigarette smoking remains a major environmental health risk that humans are facing in the twenty-first century. Cigarette smokers are more likely to develop many forms of diseases than nonsmokers, including cancers and vascular diseases. With the availability of the human genome sequence, we become more aware of the genetic contributions to these common diseases, especially the interactive relations between environmental factors (e.g., smoking) and genes on disease susceptibility, development, and prognosis. Although smoking is responsible for up to 30% of pancreatic cancers and about 10% of cases are ascribed to genetic reasons, some genetic variants do not predispose carriers to disease development unless they are exposed to a specific adverse environment such as smoking. This smoke-gene interaction could potentially be responsible for most of the cases. Certain polymorphisms in genes such as CYP1A1 have been shown particularly sensitive to smoking-induced pathogenesis, including pancreatic cancer and atherosclerosis. We found that individuals with CYP1A1 CC genotype had a more than three fold increase in risk for severe coronary atherosclerosis when they smoked. Patients with endothelial nitric oxide synthase (eNOS) intron 4 27 repeat homozygotes were more likely to develop severe coronary stenosis when they smoked. On the other hand, DNA variants at the eNOS gene also dictate how smoking affects the expression of eNOS. We showed that GSTM1 deficiency was not involved in smoking-induced vascular diseases, but p53 polymorphisms tended to modify the disease severity in smokers. We are still at an early stage of defining the pairs and mechanisms of smoke-gene interaction, and this etiologic mechanism may hold great potential for risk assessment, treatment strategy, and prognostic predictions.

Analysis of human neurological disorders using mutagenesis in the mouse

The mouse continues to play a vital role in the deciphering of mammalian gene function and the modelling of human neurological disease. Advances in gene targeting technologies have facilitated the efficiency of generating new mouse mutants, although this valuable resource has rapidly expanded in recent years due to a number of major random mutagenesis programmes. The phenotype-driven mutagenesis screen at the MRC Mammalian Genetics Unit has generated a significant number of mice with potential neurological defects, and our aim has been to characterize selected mutants on a pathological and molecular level. Four lines are discussed, one displaying late-onset ataxia caused by Purkinje cell loss and an allelic series of three tremor mutants suffering from hypomyelination of the peripheral nerve. Molecular analysis of the causative mutation in each case has provided new insights into functional aspects of the mutated proteins, illustrating the power of mutagenesis screens to generate both novel and clinically relevant disease models.

Implementing large-scale ENU mutagenesis screens in North America

A step towards annotating the mouse genome is to use forward genetics in phenotype-driven screens to saturate the genome with mutations. The purpose of this article is to highlight the new projects in North America that are focused on isolating mouse mutations after ENU mutagenesis and phenotype screening.

A novel mouse model of hypogonadotrophic hypogonadism: N-ethyl-N-nitrosourea-induced gonadotropin-releasing hormone receptor gene mutation

An autosomal-recessive mutation that causes hypogonadotrophic hypogonadism was isolated during an N-ethyl-N-nitrosourea mutagenesis screen in mice. Affected males had micropenis and small, undescended testes with spermatogenesis arrested at the pachytene stage of meiosis, leading to sterility. Androgen-sensitive organs were small and immature. Affected females were externally normal but sterile with small ovaries due to an arrest at the secondary stage of folliculogenesis, and the uterus and oviducts were thin and immature. Circulating reproductive hormones were significantly decreased in affected males and females. There was also a dramatic reduction in the numbers of FSH- and LH-producing gonadotrophs. Meiotic mapping of the mutation and candidate gene sequencing determined that the N-ethyl-N-nitrosourea-induced lesion is in the third transmembrane domain of the GnRH receptor gene (Gnrhr). In vitro studies indicate that the mutant receptor is not coupled to the plasma membrane signal transduction system. Moreover, this mutant cannot be rescued with defined GnRH receptor pharmacoperones (pharmacological chaperones), an approach that rescues many other misfolded mutants. The mutant GnRH receptor was also shown to exert a dominant-negative effect on wild-type receptor function, indicating that the mutant receptor is unable to fold properly and likely misrouted within the cell, not reaching the plasma membrane. Surprisingly, Gnrhr mutant transcripts were significantly up-regulated in the pituitaries of Gnrhr mutants, revealing a previously unknown autoregulatory feedback loop. This is the first report of a mouse with a Gnrhr loss of function mutation. These GnRH-insensitive mice provide a novel animal model for the study of human idiopathic hypogonadotrophic hypogonadism.

Three ENU-induced neurological mutations in the pore loop of sodium channel Scn8a (Na(v)1.6) and a genetically linked retinal mutation, rd13

The goal of The Jackson Laboratory Neuroscience Mutagenesis Facility is to generate mouse models of human neurological disease. We describe three new models obtained from a three-generation screen for recessive mutations. Homozygous mutant mice from lines nmf2 and nmf5 exhibit hind limb paralysis and juvenile lethality. Homozygous nmf58 mice exhibit a less severe movement disorder that includes sustained dystonic postures. The mutations were mapped to the distal region of mouse Chromosome (Chr) 15. Failure to complement a mutant allele of a positional candidate gene, Scn8a, demonstrated that the mutations are new alleles of Scn8a. Missense mutations of evolutionarily conserved residues of the sodium channel were identified in the three lines, with the predicted amino acid substitutions N1370T, I1392F, and L1404H. These residues are located within the pore loop of domain 3 of sodium channel Na(v)1.6. The lethal phenotypes suggest that the new alleles encode proteins with partial or complete loss of function. Several human disorders are caused by mutation in the pore loop of domain 3 of paralogous sodium channel genes. Line nmf5 contains a second, independent mutation in the rd13 locus that causes a reduction in cell number in the outer nuclear layer of the retina. rd13 was mapped to the distal 4 Mb of Chr 15. No coding or splice site mutations were detected in Pde1b, a candidate gene for rd13. The generation of three independent Scn8a mutations among 1100 tested G3 families demonstrates that the Scn8a locus is highly susceptible to ENU mutagenesis. The new alleles of Scn8a will be valuable for analysis of sodium channel physiology and disease.

Quantitative trait loci that determine plasma lipids and obesity in C57BL/6J and 129S1/SvImJ inbred mice

The plasma lipid concentrations and obesity of C57BL/6J (B6) and 129S1/SvImJ (129) inbred mouse strains fed a high-fat diet containing 15% dairy fat, 1% cholesterol, and 0.5% cholic acid differ markedly. To identify the loci controlling these traits, we conducted a quantitative trait loci (QTL) analysis of 294 (B6 x 129) F(2) females fed a high-fat diet for 14 weeks. Non-HDL cholesterol concentrations were affected by five significant loci: Nhdlq1 [chromosome 8, peak centimorgan (cM) 38, logarithm of odds [LOD] 4.4); Nhdlq4 (chromosome 10, cM 70, LOD 4.0); Nhdlq5 (chromosome 6, cM 0) interacting with Nhdlq4; Nhdlq6 (chromosome 7, cM 10) interacting with Nhdlq1; and Nhdlq7 (chromosome 15, cM 0) interacting with Nhdlq4. Triglyceride (TG) concentrations were affected by three significant loci: Tgq1 (chromosome 18, cM 42, LOD 3.2) and Tgq2 (chromosome 9, cM 66) interacting with Tgq3 (chromosome 4, cM 58). Obesity measured by percentage of body fat mass and body mass index was affected by two significant loci: Obq16 (chromosome 8, cM 48, LOD 10.0) interacting with Obq18 (chromosome 9, cM 65). Knowing the genes for these QTL will enhance our understanding of obesity and lipid metabolism.

Mouse limb deformity mutations disrupt a global control region within the large regulatory landscape required for Gremlin expression

The mouse limb deformity (ld) mutations cause limb malformations by disrupting epithelial-mesenchymal signaling between the polarizing region and the apical ectodermal ridge. Formin was proposed as the relevant gene because three of the five ld alleles disrupt its C-terminal domain. In contrast, our studies establish that the two other ld alleles directly disrupt the neighboring Gremlin gene, corroborating the requirement of this BMP antagonist for limb morphogenesis. Further doubts concerning an involvement of Formin in the ld limb phenotype are cast, as a targeted mutation removing the C-terminal Formin domain by frame shift does not affect embryogenesis. In contrast, the deletion of the corresponding genomic region reproduces the ld limb phenotype and is allelic to mutations in Gremlin. We resolve these conflicting results by identifying a cis-regulatory region within the deletion that is required for Gremlin activation in the limb bud mesenchyme. This distant cis-regulatory region within Formin is also altered by three of the ld mutations. Therefore, the ld limb bud patterning defects are not caused by disruption of Formin, but by alteration of a global control region (GCR) required for Gremlin transcription. Our studies reveal the large genomic landscape harboring this GCR, which is required for tissue-specific coexpression of two structurally and functionally unrelated genes.

A twist code determines the onset of osteoblast differentiation

Runx2 is necessary and sufficient for osteoblast differentiation, yet its expression precedes the appearance of osteoblasts by 4 days. Here we show that Twist proteins transiently inhibit Runx2 function during skeletogenesis. Twist-1 and -2 are expressed in Runx2-expressing cells throughout the skeleton early during development, and osteoblast-specific gene expression occurs only after their expression decreases. Double heterozygotes for Twist-1 and Runx2 deletion have none of the skull abnormalities observed in Runx2(+/-) mice, a Twist-2 null background rescues the clavicle phenotype of Runx2(+/-) mice, and Twist-1 or -2 deficiency leads to premature osteoblast differentiation. Furthermore, Twist-1 overexpression inhibits osteoblast differentiation without affecting Runx2 expression. Twist proteins' antiosteogenic function is mediated by a novel domain, the Twist box, which interacts with the Runx2 DNA binding domain to inhibit its function. In vivo mutagenesis confirms the antiosteogenic function of the Twist box. Thus, relief of inhibition by Twist proteins is a mandatory event precluding osteoblast differentiation.

Moving forward with chemical mutagenesis in the mouse

The study of genetic variation in mice offers a powerful experimental platform for understanding gene function. Complex trait analysis, gene-targeting and gene-trapping technologies, as well as insertional and chemical mutagenesis approaches are becoming increasingly sophisticated and provide a variety of options for cataloguing gene activities and interactions. In this review we discuss fundamental and practical concepts related to chemical mutagenesis and we highlight the growing list of strategies for performing mutagenesis screens in mice. Gene-driven and diverse types of phenotype-driven screens provide several options for the recovery of the invaluable variety of alleles generated by chemical mutagenesis. The unique advantages offered using chemical mutagenesis compare favourably to and complement the spectrum of approaches available for functional annotation of the mammalian genome.

ENU large-scale mutagenesis and quantitative trait linkage (QTL) analysis in mice: novel technologies for searching polygenetic determinants of craniofacial abnormalities

Discrepancies in size and shape of the jaws are the underlying etiology in many orthodontic and orthognathic surgery patients. Genetic factors combined with environmental interactions have been postulated to play a causal or contributory role in these craniofacial abnormalities. Along with the soon-to-be-available complete human and mouse genomic sequence data, mouse mutants have become a valuable tool in the functional mapping of genes involved in the development of human maxillofacial dysmorphologies. We review two powerful methods in such efforts: N-ethyl-N-nitrosourea (ENU) large-scale mutagenesis and quantitative trait linkage (QTL) analysis. The former aims at producing a plethora of novel variants of particular trait(s), and ultimately mapping the point mutations responsible for the appearance of these new traits. In contrast, the latter applies intensive breeding and mapping techniques to identify multiple loci (and, subsequently, genes) contributing to the phenotypic difference between the tested strains. A prerequisite for either approach to studying variations in the traits of interest is the application of effective mouse cephalometric phenotype analysis and rapid DNA mapping techniques. These approaches will produce a wealth of new data on critical genes that influence the size and shape of the human face.

Genetics and genomics of behavioral and psychiatric disorders

Psychiatric conditions are to some degree under genetic influences. Despite the application of advanced genetic and molecular biological technologies, the genetic bases of the human behavioral traits and psychiatric diseases remains largely unresolved. Conventional genetic linkage approaches have not yielded definitive results, possibly because of the absence of objective diagnostic tests, the complex nature of human behavior or the incomplete penetrance of psychiatric traits. However, recent studies have revealed some genes of interest using multifaceted approaches to overcome these challenges. The approaches include using families in which specific behaviors segregate as a mendelian trait, utilization of endophenotypes as biological intermediate traits, identification of psychiatric disease phenotypes in genomic disorders, and the establishment of mouse models.

Manipulation of the mouse genome: a multiple impact resource for drug discovery and development

Few would deny that the pharmaceutical industry's investment in genomics throughout the 1990s has yet to deliver in terms of drugs on the market. The reasons are complex and beyond the scope of this review. The unique ability to manipulate the mouse genome, however, has already had a positive impact on all stages of the drug discovery process and, increasingly, on the drug development process too. We give an overview of some recent applications of so-called 'transgenic' mouse technology in pharmaceutical research and development. We show how genetic manipulation in the mouse can be employed at multiple points in the drug discovery and development process, providing new solutions to old problems.

A mutation in Af4 is predicted to cause cerebellar ataxia and cataracts in the robotic mouse

The robotic mouse is an autosomal dominant mutant that arose from a large-scale chemical mutagenesis program. It has a jerky, ataxic gait and develops adult-onset Purkinje cell loss in the cerebellum in a striking region-specific pattern, as well as cataracts. Genetic and physical mapping of the disease locus led to the identification of a missense mutation in a highly conserved region of Af4, a putative transcription factor that has been previously implicated in leukemogenesis. We demonstrate that Af4 is specifically expressed in Purkinje cells, and we hypothesize that the expression of mutant Af4 leads to neurodegeneration. This function was not identified through knock-out studies, highlighting the power of phenotype-driven mutagenesis in the mouse to identify new pathways involved in neurological disease.

Genetic influence on cigarette-induced cardiovascular disease

Cigarette smoking as an addictive habit has accompanied human beings for more than 4 centuries. It is also one of the most potent and prevalent environmental health risks human beings are exposed to, and it is responsible for more than 1000 deaths each day in the United States. With recent research progress, it becomes clear that cigarette smoking can cause almost all major diseases prevalent today, such as cancer or heart disease. These detrimental effects are not only present in active smokers who choose the risk, but also to innocent bystanders, as passive smokers, who are exposed to cigarettes not-by-choice. While the cigarette-induced harm to human health is indiscriminate and severe, the degree of damage also varies from individual to individual. This intersubject variability in cigarette-induced pathologies is partly mediated by genetic variants of genes that may participate in detoxification process, eg, cytochrome P450 (CYP), cellular susceptibility to toxins, such as p53, or disease development. Through population studies, we have learned that certain CYP1A1 variants, such as Mspl polymorphism, may render the carriers more susceptible to cigarette-induced lung cancer or severe coronary atherosclerosis. The endothelial nitric oxide synthase intron 4 rare allele homozygotes are more likely to have myocardial infarction if they also smoke. In vitro experimental approach has further demonstrated that cigarettes may specifically regulate these genes in genotype-dependent fashion. While we still know little about genetic basis and molecular pathways for cigarette-induced pathological changes, understanding these mechanisms will be of great value in designing strategies to further reduce smoking in targeted populations, and to implement more effective measures in prevention and treatment of cigarette-induced diseases.

Mutagenesis strategies for identifying novel loci associated with disease phenotypes

The systematic identification of the function of all the genes in the mammalian genome is one of the major scientific challenges for the 21st century. A comprehensive insight into mammalian gene function will illuminate our understanding of the genetic bases of disease. Mouse mutagenesis is a powerful tool for the study of mammalian gene function. Most recently, a number of approaches employing the chemical mutagen ethylnitrosourea (ENU) have been utilised by mouse geneticists to deliver a substantial new collection of mouse disease models. The growing mouse mutant archive provides a powerful resource for the identification of novel genes involved with human genetic disease.

Genetics of dark skin in mice

Chemical mutagenesis in the mouse is a powerful approach for phenotype-driven genetics, but questions remain about the efficiency with which new mutations ascertained by their phenotype can be localized and identified, and that knowledge applied to a specific biological problem. During a global screen for dominant phenotypes in about 30,000 animals, a novel class of pigmentation mutants were identified by dark skin (Dsk). We determined the genetic map location, homozygous phenotype, and histology of 10 new Dsk and 2 new dark coat (Dcc) mutations, and identified mutations in Agouti (Met1Leu, Dcc4), Sox18 (Leu220ter, Dcc1), Keratin 2e (Thr500Pro, Dsk2), and Egfr (Leu863Gln, Dsk5). Cutaneous effects of most Dsk mutations are limited to melanocytes, except for the Keratin 2e and Egfr mutations, in which hyperkeratosis and epidermal thickening precede epidermal melanocytosis by 3-6 wk. The Dsk2 mutation is likely to impair intermediate filament assembly, leading to cytolysis of suprabasal keratinocytes and secondary hyperkeratosis and melanocytosis. The Dsk5 mutation causes increased tyrosine kinase activity and a decrease in steady-state receptor levels in vivo. The Dsk mutations represent genes or map locations not implicated previously in pigmentation, and delineate a developmental pathway in which mutations can be classified on the basis of body region, microscopic site, and timing of pigment accumulation.

Tools for targeted manipulation of the mouse genome

In the postgenomic era the mouse will be central to the challenge of ascribing a function to the 40,000 or so genes that constitute our genome. In this review, we summarize some of the classic and modern approaches that have fueled the recent dramatic explosion in mouse genetics. Together with the sequencing of the mouse genome, these tools will have a profound effect on our ability to generate new and more accurate mouse models and thus provide a powerful insight into the function of human genes during the processes of both normal development and disease.

Random mutagenesis in the mouse as a tool in drug discovery

The flood of raw information generated by large-scale data acquisition technologies in genomics, microarrays and proteomics is changing the early stages of the drug discovery process. Although many more potential drug targets are now available compared with the pre-genomics era, knowledge about the physiological context in which these targets act--information crucial to both discovery and development--is scarce. Random mutagenesis strategies in the mouse provide scalable approaches for both the gene-driven validation of candidate targets in vivo and the discovery of new physiological pathways by phenotype-driven screens.