Next-generation gene targeting in the mouse for functional genomics

PHEXL222P Mutation Increases Phex Expression in a New ENU Mouse Model for XLH Disease

PhexL222P mouse is a new ENU mouse model for XLH disease due to Leu to Pro amino acid modification at position 222. PhexL222P mouse is characterized by growth retardation, hypophosphatemia, hypocalcemia, reduced body bone length, and increased epiphyseal growth plate thickness and femur diameter despite the increase in PHEXL222P expression. Actually, PhexL222P mice show an increase in Fgf23, Dmp1, and Mepe and Slc34a1 (Na-Pi IIa cotransporter) mRNA expression similar to those observed in Hyp mice. Femoral osteocalcin and sclerostin and Slc34a1 do not show any significant variation in PhexL222P mice. Molecular dynamics simulations support the experimental data. P222 might locally break the E217-Q224 β-sheet, which in turn might disrupt inter-β-sheet interactions. We can thus expect local protein misfolding, which might be responsible for the experimentally observed PHEXL222P loss of function. This model could be a valuable addition to the existing XLH model for further comprehension of the disease occurrence and testing of new therapies.

An evolutionary machine learning for pulmonary hypertension animal model from arterial blood gas analysis

Pulmonary hypertension (PH) is a rare and fatal condition that leads to right heart failure and death. The pathophysiology of PH and potential therapeutic approaches are yet unknown. PH animal models' development and proper evaluation are critical to PH research. This work presents an effective analysis technology for PH from arterial blood gas analysis utilizing an evolutionary kernel extreme learning machine with multiple strategies integrated slime mould algorithm (MSSMA). In MSSMA, two efficient bee-foraging learning operators are added to the original slime mould algorithm, ensuring a suitable trade-off between intensity and diversity. The proposed MSSMA is evaluated on thirty IEEE benchmarks and the statistical results show that the search performance of the MSSMA is significantly improved. The MSSMA is utilised to develop a kernel extreme learning machine (MSSMA-KELM) on PH from arterial blood gas analysis. Comprehensively, the proposed MSSMA-KELM can be used as an effective analysis technology for PH from arterial Blood gas analysis with an accuracy of 93.31%, Matthews coefficient of 90.13%, Sensitivity of 91.12%, and Specificity of 90.73%. MSSMA-KELM can be treated as an effective approach for evaluating mouse PH models.

Preclinical Evaluation of the Oncolytic Vaccinia Virus TG6002 by Translational Research on Canine Breast Cancer

Oncolytic virotherapy is a promising therapeutic approach for the treatment of cancer. TG6002 is a recombinant oncolytic vaccinia virus deleted in the thymidine kinase and ribonucleotide reductase genes and armed with the suicide gene FCU1, which encodes a bifunctional chimeric protein that efficiently catalyzes the direct conversion of the nontoxic 5-fluorocytosine into the toxic metabolite 5-fluorouracil. In translational research, canine tumors and especially mammary cancers are relevant surrogates for human cancers and can be used as preclinical models. Here, we report that TG6002 is able to replicate in canine tumor cell lines and is oncolytic in such cells cultured in 2D or 3D as well as canine mammary tumor explants. Furthermore, intratumoral injections of TG6002 lead to inhibition of the proliferation of canine tumor cells grafted into mice. 5-fluorocytosine treatment of mice significantly improves the anti-tumoral activity of TG6002 infection, a finding that can be correlated with its conversion into 5-fluorouracil within infected fresh canine tumor biopsies. In conclusion, our study suggests that TG6002 associated with 5-fluorocytosine is a promising therapy for human and canine cancers.

Comparative oncology: Integrating human and veterinary medicine

Cancer constitutes the major health problem both in human and veterinary medicine. Comparative oncology as an integrative approach offers to learn more about naturally occurring cancers across different species. Canine models have many advantages as they experience spontaneous disease, have many genes similar to human genes, five to seven-fold accelerated ageing compared to humans, respond to treatments similarly as humans do and health care levels second only to humans. Also, the clinical trials in canines could generate more robust data, as their spontaneous nature mimics real-life situations and could be translated to humans.

A new Otogelin ENU mouse model for autosomal-recessive nonsyndromic moderate hearing impairment

Approximately 10 % of the population worldwide suffers from hearing loss (HL) and about 60 % of persons with early onset HL have hereditary hearing loss due to genetic mutations. Highly efficient mutagenesis in mice with the chemical mutagen, ethylnitrosourea (ENU), associated with relevant phenotypic tools represents a powerful approach in producing mouse models for hearing impairment. A benefit of this strategy is to generate alleles to form a series revealing the full spectrum of gene function in vivo. It can also mimic the range of human mutations and polymorphisms for HL. In the course of a genome ENU mutagenesis program, we selected a new mouse model for hearing defect based on a dysmorphological screen. We identified by gene mapping the mutation responsible for this phenotype and characterized it at the histological level of the inner ear and evaluated the vestibule by following the recommendations of the standard operating procedures, IMPReSS. We have identified and characterized a new recessive allele of the otogelin gene, Otog^vbd/vbd, due to a homozygous one base pair substitution at the splice donor site of intron 29. This mutation leads to a frame-shift and a premature stop codon. We observed a decrease in the amount of sensory cells in the maculae of Otog^vbd/vbd mice as well as an apparent drastically decreased density to almost absence of the otoconial membrane. Compared to Otog^tm1Prs and twister, the two other existing otogelin alleles, the detailed analysis of Otog^vbd/vbd revealed that these mice share some common behavioural characteristics either with Otog^tm1Prs or twister whereas the fine vestibular phenotype and the hearing defect are different. Our results emphasize the importance of detecting and characterizing a new allele of a gene in order to get comprehensive information about the gene function.

Germline mutation rates and the long-term phenotypic effects of mutation accumulation in wild-type laboratory mice and mutator mice

The germline mutation rate is an important parameter that affects the amount of genetic variation and the rate of evolution. However, neither the rate of germline mutations in laboratory mice nor the biological significance of the mutation rate in mammalian populations is clear. Here we studied genome-wide mutation rates and the long-term effects of mutation accumulation on phenotype in more than 20 generations of wild-type C57BL/6 mice and mutator mice, which have high DNA replication error rates. We estimated the base-substitution mutation rate to be 5.4 × 10⁻⁹ (95% confidence interval = 4.6 × 10⁻⁹–6.5 × 10⁻⁹) per nucleotide per generation in C57BL/6 laboratory mice, about half the rate reported in humans. The mutation rate in mutator mice was 17 times that in wild-type mice. Abnormal phenotypes were 4.1-fold more frequent in the mutator lines than in the wild-type lines. After several generations, the mutator mice reproduced at substantially lower rates than the controls, exhibiting low pregnancy rates, lower survival rates, and smaller litter sizes, and many of the breeding lines died out. These results provide fundamental information about mouse genetics and reveal the impact of germline mutation rates on phenotypes in a mammalian population.

Defective craniofacial development and brain function in a mouse model for depletion of intracellular inositol synthesis

myo-Inositol is an essential biomolecule that is synthesized by myo-inositol monophosphatase (IMPase) from inositol monophosphate species. The enzymatic activity of IMPase is inhibited by lithium, a drug used for the treatment of mood swings seen in bipolar disorder. Therefore, myo-inositol is thought to have an important role in the mechanism of bipolar disorder, although the details remain elusive. We screened an ethyl nitrosourea mutant mouse library for IMPase gene (Impa) mutations and identified an Impa1 T95K missense mutation. The mutant protein possessed undetectable enzymatic activity. Homozygotes died perinatally, and E18.5 embryos exhibited striking developmental defects, including hypoplasia of the mandible and asymmetric fusion of ribs to the sternum. Perinatal lethality and morphological defects in homozygotes were rescued by dietary myo-inositol. Rescued homozygotes raised on normal drinking water after weaning exhibited a hyper-locomotive trait and prolonged circadian periods, as reported in rodents treated with lithium. Our mice should be advantageous, compared with those generated by the conventional gene knock-out strategy, because they carry minimal genomic damage, e.g. a point mutation. In conclusion, our results reveal critical roles for intracellular myo-inositol synthesis in craniofacial development and the maintenance of proper brain function. Furthermore, this mouse model for cellular inositol depletion could be beneficial for understanding the molecular mechanisms underlying the clinical effect of lithium and myo-inositol-mediated skeletal development.

ENU-induced phenovariance in mice: inferences from 587 mutations

Background

We present a compendium of N-ethyl-N-nitrosourea (ENU)-induced mouse mutations, identified in our laboratory over a period of 10 years either on the basis of phenotype or whole genome and/or whole exome sequencing, and archived in the Mutagenetix database. Our purpose is threefold: 1) to formally describe many point mutations, including those that were not previously disclosed in peer-reviewed publications; 2) to assess the characteristics of these mutations; and 3) to estimate the likelihood that a missense mutation induced by ENU will create a detectable phenotype.

Findings

In the context of an ENU mutagenesis program for C57BL/6J mice, a total of 185 phenotypes were tracked to mutations in 129 genes. In addition, 402 incidental mutations were identified and predicted to affect 390 genes. As previously reported, ENU shows strand asymmetry in its induction of mutations, particularly favoring T to A rather than A to T in the sense strand of coding regions and splice junctions. Some amino acid substitutions are far more likely to be damaging than others, and some are far more likely to be observed. Indeed, from among a total of 494 non-synonymous coding mutations, ENU was observed to create only 114 of the 182 possible amino acid substitutions that single base changes can achieve. Based on differences in overt null allele frequencies observed in phenotypic vs. non-phenotypic mutation sets, we infer that ENU-induced missense mutations create detectable phenotype only about 1 in 4.7 times. While the remaining mutations may not be functionally neutral, they are, on average, beneath the limits of detection of the phenotypic assays we applied.

Conclusions

Collectively, these mutations add to our understanding of the chemical specificity of ENU, the types of amino acid substitutions it creates, and its efficiency in causing phenovariance. Our data support the validity of computational algorithms for the prediction of damage caused by amino acid substitutions, and may lead to refined predictions as to whether specific amino acid changes are responsible for observed phenotypes. These data form the basis for closer in silico estimations of the number of genes mutated to a state of phenovariance by ENU within a population of G3 mice.

Human germline genetic modification: scientific and bioethical perspectives

The latest mammalian genetic modification technology offers efficient and reliable targeting of genomic sequences, in the guise of designer genetic recombination tools. These and other improvements in genetic engineering technology suggest that human germline genetic modification (HGGM) will become a safe and effective prospect in the relatively near future. Several substantive ethical objections have been raised against HGGM including claims of unacceptably high levels of risk, damage to the status of future persons, and violations of justice and autonomy. This paper critically reviews the latest GM science and discusses the key ethical objections to HGGM. We conclude that major benefits are likely to accrue through the use of safe and effective HGGM and that it would thus be unethical to take a precautionary stance against HGGM.

Site-specific integration and tailoring of cassette design for sustainable gene transfer

Integrative gene transfer methods are limited by variable transgene expression and by the consequences of random insertional mutagenesis that confound interpretation in gene-function studies and may cause adverse events in gene therapy. Site-specific integration may overcome these hurdles. Toward this goal, we studied the transcriptional and epigenetic impact of different transgene expression cassettes, targeted by engineered zinc-finger nucleases to the CCR5 and AAVS1 genomic loci of human cells. Analyses performed before and after integration defined features of the locus and cassette design that together allow robust transgene expression without detectable transcriptional perturbation of the targeted locus and its flanking genes in many cell types, including primary human lymphocytes. We thus provide a framework for sustainable gene transfer in AAVS1 that can be used for dependable genetic manipulation, neutral marking of the cell and improved safety of therapeutic applications, and demonstrate its feasibility by rapidly generating human lymphocytes and stem cells carrying targeted and benign transgene insertions.

Colitis locus on chromosome 2 impacting the severity of early-onset disease in mice deficient in GPX1 and GPX2

BACKGROUND

Genetic background has a profound effect on inflammatory bowel disease. The Gpx1 and Gpx2 double knockout (GPX1/2-DKO) mice on a mixed C57BL/6 (B6) and 129S1/SvimJ (129) background exhibit spontaneous ileocolitis. The DKO mice on a B6 background have mild ileocolitis. We characterized the 129 DKO mice to identify a genetic locus affecting disease severity.

METHODS

We backcrossed B6;129 DKO mice to 129 and analyzed for ileocolitis penetrance and severity at N5, N7, and N10. By correlating disease severity with single-nucleotide polymorphism (SNP) markers, we identified a colitis locus.

RESULTS

As early as 9 days of age, 129 DKO N5 and N10 mice showed disease signs and morbidity. The N10 DKO mice had the severest colitis with nearly complete penetrance and high morbidity compared with other generations or backgrounds. 129 DKO mice had elevated colonic KC and SAA3 expression, shorter colon length, and cecal E. coli overgrowth compared to B6 DKO mice. Analysis of the B6 loci in 129 N5, N7, and N10 cohorts pointed to a region of chromosome 2: 119 Mbp contributing to mild symptoms.

CONCLUSIONS

GPX1/2-DKO mice on 129 genetic background have the most aggressive colitis compared to B6;129 and B6 colonies. A B6 locus significantly contributing the resistance resides on chromosome 2: 119 Mbp. This region coincides with cytokine-deficiency-induced colitis susceptibility, Cdcs3, identified in the resistant B6 and sensitive C3H/HeJBir (C3Bir) with IL-10 deficiency. A three-way SNP analysis between 129, B6, and C3Bir locus points the major candidate genes to B2m, Dnajc17, Duox2, Pla2g4b, Pla2g4e, Pla2g4f and Slc30a4.

Mouse models of cancer

Genetically engineered mouse models have significantly contributed to our understanding of cancer biology. They have proven to be useful in validating gene functions, identifying novel cancer genes and tumor biomarkers, gaining insight into the molecular and cellular mechanisms underlying tumor initiation and multistage processes of tumorigenesis, and providing better clinical models in which to test novel therapeutic strategies. However, mice still have significant limitations in modeling human cancer, including species-specific differences and inaccurate recapitulation of de novo human tumor development. Future challenges in mouse modeling include the generation of clinically relevant mouse models that recapitulate the molecular, cellular, and genomic events of human cancers and clinical response as well as the development of technologies that allow for efficient in vivo imaging and high-throughput screening in mice.

Dog models of naturally occurring cancer

Studies using dogs provide an ideal solution to the gap in animal models for natural disease and translational medicine. This is evidenced by approximately 400 inherited disorders being characterized in domesticated dogs, most of which are relevant to humans. There are several hundred isolated populations of dogs (breeds) and each has a vastly reduced genetic variation compared with humans; this simplifies disease mapping and pharmacogenomics. Dogs age five- to eight-fold faster than do humans, share environments with their owners, are usually kept until old age and receive a high level of health care. Farseeing investigators recognized this potential and, over the past decade, have developed the necessary tools and infrastructure to utilize this powerful model of human disease, including the sequencing of the dog genome in 2005. Here, we review the nascent convergence of genetic and translational canine models of spontaneous disease, focusing on cancer.

ENU-based gene-driven mutagenesis in the mouse: a next-generation gene-targeting system

As a new mouse mutant resource, the RIKEN ENU-based gene-driven mutagenesis system in the mouse has been available to the research community since 2002. By using random base-substitution mutagenesis with ENU, a new reverse genetics infrastructure has been developed as a next-generation gene-targeting system. The construction of a large-scale mutant mouse library and high-throughput mutation discovery systems were the keys making it practically feasible. The RIKEN mutant mouse library consists of ~ 10,000 G1 mice, within which 100-150 mutant strains have been established based on users' requests every year. Use of the system is very simple: users 1) download an application form from our web site and send to us, and 2) design the PCR primers for the target gene. Then, we screen the RIKEN mutant mouse library and report all the detected mutations to the user. From among the allelic series of discovered mutations, users decide which mutant strain(s) to analyze and request the live mutant strain for functional studies of the target gene. Users have been reporting various functional mutations in the RIKEN mutant mouse library: e.g., missense, knockout-type and even functional non-coding mutations. In the near future, next-generation re-sequencing systems should drastically enhance the utility of the ENU-based gene-driven mutagenesis not only for the mouse but also for other species.

Recombinant retroviral production and infection of B cells

The transgenic expression of genes in eukaryotic cells is a powerful reverse genetic approach in which a gene of interest is expressed under the control of a heterologous expression system to facilitate the analysis of the resulting phenotype. This approach can be used to express a gene that is not normally found in the organism, to express a mutant form of a gene product, or to over-express a dominant-negative form of the gene product. It is particularly useful in the study of the hematopoietic system, where transcriptional regulation is a major control mechanism in the development and differentiation of B cells, reviewed. Mouse genetics is a powerful tool for the study of human genes and diseases. A comparative analysis of the mouse and human genome reveals conservation of synteny in over 90% of the genome. Also, much of the technology used in mouse models is applicable to the study of human genes, for example, gene disruptions and allelic replacement. However, the creation of a transgenic mouse requires a great deal of resources of both a financial and technical nature. Several projects have begun to compile libraries of knock out mouse strains (KOMP, EUCOMM, NorCOMM) or mutagenesis induced strains (RIKEN), which require large-scale efforts and collaboration. Therefore, it is desirable to first study the phenotype of a desired gene in a cell culture model of primary cells before progressing to a mouse model. Retroviral DNA integrates into the host DNA, preferably within or near transcription units or CpG islands, resulting in stable and heritable expression of the packaged gene of interest while avoiding transcriptional silencing. The genes are then transcribed under the control of a high efficiency retroviral promoter, resulting in a high efficiency of transcription and protein production. Therefore, retroviral expression can be used with cells that are difficult to transfect, provided the cells are in an active state during mitosis. Because the structural genes of the virus are contained within the packaging cell line, the expression vectors used to clone the gene of interest contain no structural genes of the virus, which both eliminates the possibility of viral revertants and increases the safety of working with viral supernatants as no infectious virions are produced. Here we present a protocol for recombinant retroviral production and subsequent infection of splenic B cells. After isolation, the cultured splenic cells are stimulated with Th derived lymphokines and anti-CD40, which induces a burst of B cell proliferation and differentiation. This protocol is ideal for the study of events occurring late in B cell development and differentiation, as B cells are isolated from the spleen following initial hematopoietic events but prior to antigenic stimulation to induce plasmacytic differentiation.

Creating a "hopeful monster": mouse forward genetic screens

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

Mouse mutagenesis and disease models for neuropsychiatric disorders

In this chapter, mutant mouse resources which have been developed by classical genetics as well as by modern large-scale mutagenesis projects are summarized. Various spontaneous and induced mouse mutations have been archived since the rediscovery of Mendel's genetics in 1900. Moreover, genome-wide, large-scale mutagenesis efforts have recently been expanding the available mutant mouse resources. Forward genetics projects using ENU mutagenesis in the mouse were started in the mid-1990s. The widespread adoption of reverse genetics, using knockouts and conditional mutagenesis based on gene-targeting technology, followed. ENU mutagenesis has now evolved to provide a further resource for reverse genetics, with multiple point mutations in a single gene and this new approach is described. Researchers now have various options to obtain mutant mice: point mutations, transgenic mouse strains, and constitutional or conditional knockout mice. The established mutant strains have already contributed to modeling human diseases by elucidating the underlying molecular mechanisms as well as by providing preclinical applications. Examples of mutant mice, focusing on neurological and behavioral models for human diseases, are reviewed. Human diseases caused by a single gene or a small number of major genes have been well modeled by corresponding mutant mice. Current evidence suggests that quantitative traits based on polygenes are likely to be associated with a range of psychiatric diseases, and these are now coming within the range of modeling by mouse mutagenesis.

Mouse models in male fertility research

Limited knowledge of the genetic causes of male infertility has resulted in few treatment and targeted therapeutic options. Although the ideal approach to identify infertility causing mutations is to conduct studies in the human population, this approach has progressed slowly due to the limitations described herein. Given the complexity of male fertility, the entire process cannot be modeled in vitro. As such, animal models, in particular mouse models, provide a valuable alternative for gene identification and experimentation. Since the introduction of molecular biology and recent advances in animal model production, there has been a substantial acceleration in the identification and characterization of genes associated with many diseases, including infertility. Three major types of mouse models are commonly used in biomedical research, including knockout/knockin/gene-trapped, transgenic and chemical-induced point mutant mice. Using these mouse models, over 400 genes essential for male fertility have been revealed. It has, however, been estimated that thousands of genes are involved in the regulation of the complex process of male fertility, as many such genes remain to be characterized. The current review is by no means a comprehensive list of these mouse models, rather it contains examples of how mouse models have advanced our knowledge of post-natal germ cell development and male fertility regulation.

Genome-wide ENU mutagenesis for the discovery of novel male fertility regulators

The completion of genome sequencing projects has provided an extensive knowledge of the contents of the genomes of human, mouse, and many other organisms. Despite this, the function of most of the estimated 25,000 human genes remains largely unknown. Attention has now turned to elucidating gene function and identifying biological pathways that contribute to human diseases, including male infertility. Our understanding of the genetic regulation of male fertility has been accelerated through the use of genetically modified mouse models including knockout, knock-in, gene-trapped, and transgenic mice. Such reverse genetic approaches however, require some fore-knowledge of a gene's function and, as such, bias against the discovery of completely novel genes and biological pathways. To facilitate high throughput gene discovery, genome-wide mouse mutagenesis via the use of a potent chemical mutagen, N-ethyl-N-nitrosourea (ENU), has been developed over the past decade. This forward genetic, or phenotype-driven, approach relies upon observing a phenotype first, then subsequently defining the underlining genetic defect. Mutations are randomly introduced into the mouse genome via ENU exposure. Through a controlled breeding scheme, mutations causing a phenotype of interest (e.g., male infertility) are then identified by linkage analysis and candidate gene sequencing. This approach allows for the possibility of revealing comprehensive phenotype-genotype relationships for a range of genes and pathways i.e. in addition to null alleles, mice containing partial loss of function or gain-of-function mutations, can be recovered. Such point mutations are likely to be more reflective of those that occur within the human population. Many research groups have successfully used this approach to generate infertile mouse lines and some novel male fertility genes have been revealed. In this review, we focus on the utility of ENU mutagenesis for the discovery of novel male fertility regulators.

Model organisms--A historical perspective

Much of our knowledge on heredity, development, physiology and the underlying cellular and molecular processes is derived from the studies of model, or reference, organisms. Despite the great variety of life, a common base of shared principles could be extracted by studying a few life forms, selected based on their amenability to experimental studies. Very briefly, the origins of a few model organisms are described, including E. coli, yeast, C. elegans, Drosophila, Xenopus, zebrafish, mouse, maize and Arabidopsis. These model organisms were chosen because of their importance and wide use, which made them systems of choice for genome-wide studies. Many of their genomes were between the first to be fully sequenced, opening unprecedented opportunities for large-scale transcriptomics and proteomics studies.

Validation of adequate endogenous reference genes for the normalisation of qPCR gene expression data in human post mortem tissue

Gene expression analyses based on messenger RNA (mRNA) profiling require accurate data normalisation. When using endogenous reference genes, these have to be validated carefully. Therefore, we examined the transcript stability of 10 potential reference genes using quantitative real-time polymerase chain reaction: beta actin, 18S rRNA, glyceraldehyde-3-phosphate dehydrogenase, TATA box-binding protein, hypoxanthine phosphoribosyl-transferase I, beta-2-microglobulin, hydroxymethylbilane synthase, succinate dehydrogenase complex, subunit A, cyclophilin A and ubiquitin C. The aim of the current study was to assess which reference genes show stable mRNA levels in human post mortem cardiac muscle, skeletal muscle and brain tissue. Considering cardiac muscle tissue, CYCA and TBP were identified as the most stable while in skeletal muscle tissue, SDHA and TBP, and in brain tissue, SDHA and HMBS turned out to be the most stable. Furthermore, we recommend a minimum of four carefully validated endogenous control genes for reliable data normalisation in human post mortem tissue. Parameters influencing the stability of transcript amounts were found to be mainly the post mortem interval in cardiac muscle and skeletal muscle tissue and the donor's cause of death in skeletal muscle and brain samples. Further parameters like gender, age at death and body mass index were found to influence mRNA quantities in skeletal muscle only. The set of stable control genes identified in this study may be used in further studies if the composition of the samples is similar to the one used here.

Phenotype-driven mouse ENU mutagenesis screens

In the past decade, forward genetic screens in the mouse have come into their own as a practical method for exploring the genetic basis of many biological processes. By looking directly for disruption in a process of interest, genetic screens have always been powerful, but completion of the genome sequence has made mouse forward genetic screens practical, as well. The sequenced genome means we can map and sequence more efficiently than before, so small focused screens are now within the reach of even small labs. N-Ethyl-N-nitrosourea (ENU) is the preferred mutagen in forward genetic screens, because it is extremely potent in the premeiotic male germ line, where it induces point mutations. This last point is crucial, as point mutations lead to all classes of mutations (e.g., null, hypomorphs, neomorphs, antimorphs, and hypermorphs), which is why forward genetic screens can implicate a gene in a particular process when a targeted deletion may not. Point mutations often mimic human disease states, yielding highly relevant animal models. Since mammals reproduce, lactate, behave, develop, and protect themselves from infection differently from other vertebrates, mammalian forward genetic screens are uniquely informative. In fact, in the past decade, forward genetics has uncovered mutations demonstrating that certain genes exist only in mammals, that specific mechanisms function only in mammals, and that particular biological processes may exist only in mammals; hence screens focused on these processes have identified unsuspected genes. As powerful as the approach is, forward genetics remains a method for the committed; the process of screening requires organization and tenacity. This chapter is intended to help those who are ready to make the commitment by providing practical advice. To this end I detail the issues surrounding screen design and screen execution, as well as mutation identification and confirmation.