Genetically altered mice: phenotypes, no phenotypes, and Faux phenotypes

The role of ATP1A3 gene in epilepsy: We need to know more

The ATP1A3 gene, which encodes the Na⁺/K⁺-ATPase α3 catalytic subunit, plays a crucial role in both physiological and pathological conditions in the brain, and mutations in this gene have been associated with a wide variety of neurological diseases by impacting the whole infant development stages. Cumulative clinical evidence suggests that some severe epileptic syndromes have been linked to mutations in ATP1A3, among which inactivating mutation of ATP1A3 has been intriguingly found to be a candidate pathogenesis for complex partial and generalized seizures, proposing ATP1A3 regulators as putative targets for the rational design of antiepileptic therapies. In this review, we introduced the physiological function of ATP1A3 and summarized the findings about ATP1A3 in epileptic conditions from both clinical and laboratory aspects at first. Then, some possible mechanisms of how ATP1A3 mutations result in epilepsy are provided. We think this review timely introduces the potential contribution of ATP1A3 mutations in both the genesis and progression of epilepsy. Taken that both the detailed mechanisms and therapeutic significance of ATP1A3 for epilepsy are not yet fully illustrated, we think that both in-depth mechanisms investigations and systematic intervention experiments targeting ATP1A3 are needed, and by doing so, perhaps a new light can be shed on treating ATP1A3-associated epilepsy.

Modulating the Metabolic Phenotype of Cancer Microenvironment

This chapter provides a brief overview of the methods to study and modulate the metabolic phenotype of the tumor microenvironment, including own research work to demonstrate the impact that metabolic shifts in the host have on cancer. Firstly, we briefly discuss the relevance of using animal models to address this topic, and also the importance of acknowledging that animals have diverse metabolic phenotypes according to species, and even with strain, age or sex. We also present original data to highlight the impact that changes in metabolic phenotype of the microenvironment have on tumor progression. Using an acute leukemia mouse xenograft model and high-fat diet we show that a shift in the host metabolic phenotype, induced by high-fat feeding, significantly impacts on tumor progression. The mechanism through which this occurs involves a direct effect of the increased levels of circulating lipoproteins in both tumor and non-neoplastic cells.

Pathology Principles and Practices for Analysis of Animal Models

Over 60% of NIH extramural funding involves animal models, and approximately 80% to 90% of these are mouse models of human disease. It is critical to translational research that animal models are accurately characterized and validated as models of human disease. Pathology analysis, including histopathology, is essential to animal model studies by providing morphologic context to in vivo, molecular, and biochemical data; however, there are many considerations when incorporating pathology endpoints into an animal study. Mice, and in particular genetically modified models, present unique considerations because these modifications are affected by background strain genetics, husbandry, and experimental conditions. Comparative pathologists recognize normal pathobiology and unique phenotypes that animals, including genetically modified models, may present. Beyond pathology, comparative pathologists with research experience offer expertise in animal model development, experimental design, optimal specimen collection and handling, data interpretation, and reporting. Critical pathology considerations in the design and use of translational studies involving animals are discussed, with an emphasis on mouse models.

Endothelial Cell Development and Its Application to Regenerative Medicine

The formation and remodeling of a functional circulatory system is critical for sustaining prenatal and postnatal life. During embryogenesis, newly differentiated endothelial cells require further specification to create the unique features of distinct vessel subtypes needed to support tissue morphogenesis. In this review, we explore signaling pathways and transcriptional regulators that modulate endothelial cell differentiation and specification, as well as applications of these processes to stem cell biology and regenerative medicine. We also summarize recent technical advances, including the growing utilization of single-cell sequencing to study vascular heterogeneity and development.

Show and tell: disclosure and data sharing in experimental pathology

Reproducibility of data from experimental investigations using animal models is increasingly under scrutiny because of the potentially negative impact of poor reproducibility on the translation of basic research. Histopathology is a key tool in biomedical research, in particular for the phenotyping of animal models to provide insights into the pathobiology of diseases. Failure to disclose and share crucial histopathological experimental details compromises the validity of the review process and reliability of the conclusions. We discuss factors that affect the interpretation and validation of histopathology data in publications and the importance of making these data accessible to promote replicability in research.

Summary: Reproducibility of findings in experiments using model organisms has recently become a source of concern, particularly for translational science. We discuss factors affecting the interpretation and reliability of experimental pathology findings in the mouse, and how disclosure and transparent reporting are crucial for replicability.

Double transduction of a Cre/LoxP lentiviral vector: a simple method to generate kidney cell-specific knockdown mice

In a lentivirus-based gene delivery system, the incorporated gene is continuously expressed for a long time. In this study, we devised a simple way to knock down a specific gene in a kidney cell-specific pattern in adult mice by lentivirus-assisted transfer of short hairpin RNA (shRNA). Kidney collecting duct (CD)-specific aquaporin-3 (AQP3)-knockdown mice were generated by consecutive injection of Hoxb7-Cre-expressing lentivirus (LV-Hoxb7 Cre) and loxP-AQP3 shRNA-expressing lentivirus (LV-loxP shAQP3) in adult C57BL6/J mice. LV-Hoxb7 Cre was designed to express mCherry, while LV-loxP shAQP3 was designed with a floxed enhanced green fluorescent protein (EGFP)-tagged stop sequence, and thus EGFP would be expressed only in the absence of Cre recombination. In mice treated with LV-Hoxb7 Cre alone, mCherry protein expression, which indicates the presence of Cre recombinase, occurred only in CD cells. However, LV-loxP shAQP3 injection alone resulted in an increase in EGFP expression in all kidney cells, indicating the transcription of the floxed region. When LV-Hoxb7 Cre and LV-loxP shAQP3 were sequentially transduced, EGFP expression was attenuated while mCherry expression was sustained in CD cells, demonstrating a CD cell-specific recombination of the floxed region. AQP3 expression in mice injected with LV-Hoxb7 Cre or LV-loxP shAQP3 alone did not differ, but consecutive injection of LV-Hoxb7 Cre and LV-loxP shAQP3 significantly reduced AQP3 expression in CD cells. However, the expression levels of AQP3 were not altered in other cell types. Double transduction of Cre- and loxP-based lentivirus can easily generate kidney cell-specific knockdown mice, and this method might be applicable to other species.

Pathobiology of aging mice and GEM: background strains and experimental design

The use of induced and spontaneous mutant mice and genetically engineered mice (and combinations thereof) to study cancers and other aging phenotypes to advance improved functional human life spans will involve studies of aging mice. Genetic background contributes to pathology phenotypes and to causes of death as well as to longevity. Increased recognition of expected phenotypes, experimental variables that influence phenotypes and research outcomes, and experimental design options and rationales can maximize the utility of genetically engineered mice (GEM) models to translational research on aging. This review aims to provide resources to enhance the design and practice of chronic and longevity studies involving GEM. C57BL6, 129, and FVB/N strains are emphasized because of their widespread use in the generation of knockout, transgenic, and conditional mutant GEM. Resources are included also for pathology of other inbred strain families, including A, AKR, BALB/c, C3H, C57L, C58, CBA, DBA, GR, NOD.scid, SAMP, and SJL/J, and non-inbred mice, including 4WC, AB6F1, Ames dwarf, B6, 129, B6C3F1, BALB/c,129, Het3, nude, SENCAR, and several Swiss stocks. Experimental strategies for long-term cross-sectional and longitudinal studies to assess causes of or contributors to death, disease burden, spectrum of pathology phenotypes, longevity, and functional healthy life spans (health spans) are compared and discussed.

In-depth metabolic phenotyping of genetically engineered mouse models in obesity and diabetes

The world-wide prevalence of obesity and diabetes has increased sharply during the last two decades. Accordingly, the metabolic phenotyping of genetically engineered mouse models is critical for evaluating the functional roles of target genes in obesity and diabetes, and for developing new therapeutic targets. In this review, we discuss the practical meaning of metabolic phenotyping, the strategy of choosing appropriate tests, and considerations when designing and performing metabolic phenotyping in mice.

A physiological perspective on the neuroscience of eating

I present the thesis that 'being physiological,' i.e., analyzing eating under conditions that do not perturb, or minimally perturb, the organism's endogenous processes, should be a central goal of the neuroscience of eating. I describe my understanding of 'being physiological' based on [i] the central neural-network heuristic of CNS function that traces back to Cajal and Sherrington, [ii] research on one of the simpler problems in the neuroscience of eating, identification of endocrine signals that control eating. In this context I consider natural meals, physiological doses and ranges, and antagonist studies. Several examples involve CCK. Next I describe my view of the cutting edge in the molecular neuroscience of eating as it has evolved from the discovery of leptin signaling through the application of optogenetic and pharmacogenetic methods. Finally I describe some novel approaches that may advance the neuroscience of eating in the foreseeable future. I conclude that [i] the neuroscience of eating may soon be able to discern 'physiological' function in the operation of CNS networks mediating eating, [ii] the neuroscience of eating should capitalize on methods developed in other areas of neuroscience, e.g., improved methods to record and manipulate CNS function in behaving animals, identification of canonical regional circuits, use of population electrophysiology, etc., and [iii] subjective aspects of eating are crucial aspects of eating science, but remain beyond mechanistic understanding.

A phenotype survey of 36 mutant mouse strains with gene-targeted defects in glycosyltransferases or glycan-binding proteins

The consortium for functional glycomics (CFG) was a large research initiative providing networking and resources for investigators studying the role of glycans and glycan-binding proteins in health and disease. Starting in 2001, six scientific cores were established to generate data, materials and new technologies. By the end of funding in 2011, the mouse phenotype core (MPC) submitted data to a website from the phenotype screen of 36 mutant mouse strains deficient in a gene for either a glycan-binding protein (GBP) or glycosyltransferase (GT). Each mutant strain was allotted three months for analysis and screened by standard phenotype assays used in the fields of immunology, histology, hematology, coagulation, serum chemistry, metabolism and behavior. Twenty of the deficient mouse strains had been studied in other laboratories, and additional tests were performed on these strains to confirm previous observations and discover new data. The CFG constructed 16 new homozygous mutant mouse strains and completed the initial phenotype screen of the majority of these new mutant strains. In total, >300 phenotype changes were observed, but considering the over 100 assays performed on each strain, most of the phenotypes were unchanged. Phenotype differences include abnormal testis morphology in GlcNAcT9- and Siglec-H-deficient mice and lethality in Pomgnt1-deficient mice. The numerous altered phenotypes discovered, along with the consideration of the significant findings of normality, will provide a platform for future characterization to understand the important roles of glycans and GBPs in the mechanisms of health and disease.

Animal models in virus research: their utility and limitations

Viral diseases are important threats to public health worldwide. With the number of emerging viral diseases increasing the last decades, there is a growing need for appropriate animal models for virus studies. The relevance of animal models can be limited in terms of mimicking human pathophysiology. In this review, we discuss the utility of animal models for studies of influenza A viruses, HIV and SARS-CoV in light of viral emergence, assessment of infection and transmission risks, and regulatory decision making. We address their relevance and limitations. The susceptibility, immune responses, pathogenesis, and pharmacokinetics may differ between the various animal models. These complexities may thwart translating results from animal experiments to the humans. Within these constraints, animal models are very informative for studying virus immunopathology and transmission modes and for translation of virus research into clinical benefit. Insight in the limitations of the various models may facilitate further improvements of the models.

The gene or not the gene--that is the question: understanding the genetically engineered mouse phenotype

Embryonic stem cells have had a significant impact on understanding gene function and gene interactions through the use of genetically engineered mice. However, the genetic context (ie, mouse strain) in which these modifications in alleles are made may have a considerable effect on the phenotypic changes identified in these mice. In addition, tissue- and time-specific gene expression systems may generate unanticipated outcomes. This article discusses the history of embryonic stem cells, reviews how mouse strain can affect phenotype (using specific examples), and examines some of the caveats of conditional gene expression systems.

New approaches to the representation and analysis of phenotype knowledge in human diseases and their animal models

The systematic investigation of the phenotypes associated with genotypes in model organisms holds the promise of revealing genotype-phenotype relations directly and without additional, intermediate inferences. Large-scale projects are now underway to catalog the complete phenome of a species, notably the mouse. With the increasing amount of phenotype information becoming available, a major challenge that biology faces today is the systematic analysis of this information and the translation of research results across species and into an improved understanding of human disease. The challenge is to integrate and combine phenotype descriptions within a species and to systematically relate them to phenotype descriptions in other species, in order to form a comprehensive understanding of the relations between those phenotypes and the genotypes involved in human disease. We distinguish between two major approaches for comparative phenotype analyses: the first relies on evolutionary relations to bridge the species gap, while the other approach compares phenotypes directly. In particular, the direct comparison of phenotypes relies heavily on the quality and coherence of phenotype and disease databases. We discuss major achievements and future challenges for these databases in light of their potential to contribute to the understanding of the molecular mechanisms underlying human disease. In particular, we discuss how the use of ontologies and automated reasoning can significantly contribute to the analysis of phenotypes and demonstrate their potential for enabling translational research.

Internet and Print Resources to Facilitate Pathology Analysis When Phenotyping Genetically Engineered Rodents

Genetically engineered mice and rats are increasingly used as models for exploring disease progression and mechanisms. The full spectrum of anatomic, biochemical, and functional changes that develop in novel, genetically engineered mouse and rat lines must be cataloged before predictions regarding the significance of the mutation may be extrapolated to diseases in other vertebrate species, including humans. A growing list of reference materials, including books, journal articles, and websites, has been produced in the last 2 decades to assist researchers in phenotyping newly engineered rodent lines. This compilation provides an extensive register of materials related to the pathology component of rodent phenotypic analysis. In this article, the authors annotate the resources they use most often, to allow for quick determination of their relevance to research projects.

Ulcerative typhlocolitis associated with Helicobacter mastomyrinus in telomerase-deficient mice

Telomerase deficiency induces early senescence and defects in proliferating cell populations, but in mice it has not been associated with inflammatory bowel disease. Genetically engineered mice lacking either telomerase reverse transcriptase (TERT) or telomerase RNA were examined for chronic diarrhea and wasting. Affected mice had pasty stools, thickened nondistensible colon walls, and contracted ceca. Histologically, the cecal mucosa was largely replaced by inflammatory infiltrate consisting of plasma cells, neutrophils, lymphocytes, and macrophages with marked widespread fibrosis and ulceration. Remaining epithelium was disorganized and hyperplastic, with multifocal dysplasia. Colonic mucosa was markedly hyperplastic with similar inflammation and epithelial dysplasia. Multifocal adenomatous hyperplasia, but no inflammation, was present in the small intestine. Microaerophilic spiral bacteria with 16S rRNA gene sequences identical to Helicobacter mastomyrinus were isolated from the colon and cecum. Severe granulomatous typhlocolitis without epithelial dysplasia developed in germ-free recombination-activating gene (RAG) knockout (KO) recipients of CD4+ T cells and inoculated with cecal contents from affected TERT KO mice and in specific pathogen-free recipient RAG KO mice and interleukin-10 KO mice inoculated with H mastomyrinus. Typhlocolitis in mice given H mastomyrinus was more severe than in mice given Helicobacter hepaticus. Telomerase-deficient mice are susceptible to helicobacter-associated typhlocolitis. H mastomyrinus causes severe disease in susceptible mouse strains.

Hypothalamic-pituitary-adrenal axis abnormalities in response to deletion of 11beta-HSD1 is strain-dependent

Inter-individual differences in hypothalamic-pituitary-adrenal (HPA) axis activity underlie differential vulnerability to neuropsychiatric and metabolic disorders, although the basis of this variation is poorly understood. 11beta-Hydroxysteroid dehydrogenase type 1 (11beta-HSD1) has previously been shown to influence HPA axis activity. 129/MF1 mice null for 11beta-HSD1 (129/MF1 HSD1(-/-)) have greatly increased adrenal gland size and altered HPA activity, consistent with reduced glucocorticoid negative feedback. On this background, concentrations of plasma corticosterone and adrenocorticotrophic hormone (ACTH) were elevated in unstressed mice, and showed a delayed return to baseline after stress in HSD1-null mice with reduced sensitivity to exogenous glucocorticoid feedback compared to same-background genetic controls. In the present study, we report that the genetic background can dramatically alter this pattern. By contrast to HSD1(-/-) mice on a 129/MF1 background, HSD1(-/-) mice congenic on a C57Bl/6J background have normal basal plasma corticosterone and ACTH concentrations and exhibit normal return to baseline of plasma corticosterone and ACTH concentrations after stress. Furthermore, in contrast to 129/MF1 HSD1(-/-) mice, C57Bl/6J HSD1(-/-) mice have increased glucocorticoid receptor expression in areas of the brain involved in glucocorticoid negative feedback (hippocampus and paraventricular nucleus), suggesting this may be a compensatory response to normalise feedback control of the HPA axis. In support of this hypothesis, C57Bl/6J HSD1(-/-) mice show increased sensitivity to dexamethasone-mediated suppression of peak corticosterone. Thus, although 11beta-HSD1 appears to contribute to regulation of the HPA axis, the genetic background is crucial in governing the response to (and hence the consequences of) its loss. Similar variations in plasticity may underpin inter-individual differences in vulnerability to disorders associated with HPA axis dysregulation. They also indicate that 11beta-HSD1 inhibition does not inevitably activate the HPA axis.

Adipose tissue, adipocytes and the circadian timing system

Circadian clocks time the daily occurrence of multiple aspects of behaviour and physiology. Through studies of chronic misalignment between our internal clocks and the environment (e.g. during shift work), it has long been postulated that disruption of circadian rhythms is detrimental to human health. Recent advances in understanding of the cellular and molecular basis of mammalian circadian timing mechanisms have identified many key genes involved in circadian rhythm generation and demonstrated the presence of clocks throughout the body. Furthermore, clear links between sleep, circadian rhythms and metabolic function have been revealed, and much current research is studying these links in more detail. Here, we review the evidence linking circadian rhythms, clock genes and adipose biology. We also highlight gaps in our understanding and finally suggest avenues for future research.

Mammary tumor phenotypes in wild-type aging female FVB/N mice with pituitary prolactinomas

Prolactin-secreting pituitary adenomas are common spontaneous lesions in aging FVB females. Prolactin-secreting pituitary proliferations play a significant role in mouse mammary tumorigenesis generally producing adenosquamous carcinomas. Since genetically engineered FVB mice are frequently used to study mammary tumor biology, we have examined a cohort of 64 aging wild-type FVB/N females to establish the prevalence and the nature of spontaneous mammary and pituitary tumors. Tissues from mammary and pituitary glands were studied by histopathology and immunohistochemistry. Of the 64 examined mice, 20 had pituitary tumors and 20 had mammary tumors. Mammary and pituitary tumors were associated in 17 mice. All pituitary tumors were prolactin-positive by immunohistochemistry and classified as prolactinomas. Fourteen mammary tumors, including 12 cases with and 2 without concurrent prolactinomas, were adenocarcinomas with different combinations of epithelial growth patterns. Five mice with prolactinomas had mammary tumors characterized by the epithelial-mesenchymal transition (EMT) phenotype. Estrogen receptor alpha (ERalpha)-positivity was observed for 14 of the 18 mammary tumors tested, including both adenocarcinomas with nuclear immunoreactivity and EMT-phenotype tumors with both nuclear and cytoplasmic immunoreactivity. No immunoreactivity for the progesterone receptor was observed. This study confirms that spontaneous prolactinomas and mammary tumors are both common and significantly associated lesions in FVB mice. Parity and age represented risk factors for the development of these tumors. Compared with previous reports, prolactinoma-associated mammary tumors displayed a broader morphologic spectrum, including cases with the EMT phenotype. The elevated number of prolactinoma-associated and ERalpha-positive mammary tumors opens intriguing possibilities concerning the role of ERalpha cytoplasmic localization during EMT tumorigenesis.

'One medicine---one pathology': are veterinary and human pathology prepared?

The American Medical Association and the American Veterinary Medical Association have recently approved resolutions supporting 'One Medicine' or 'One Health' that bridge the two professions. The concept is far from novel. Rudolf Virchow, the Father of Modern Pathology, and Sir William Osler, the Father of Modern Medicine, were outspoken advocates of the concept. The concept in its modern iteration was re-articulated in the 1984 edition of Calvin Schwabe's 'Veterinary Medicine and Human Health.' The veterinary and medical pathology professions are steeped in a rich history of 'One Medicine,' but they have paradoxically parted ways, leaving the discipline of pathology poorly positioned to contribute to contemporary science. The time has come for not only scientists but also all pathologists to recognize the value in comparative pathology, the consequences of ignoring the opportunity and, most importantly, the necessity of preparing future generations to meet the challenge inherent in the renewed momentum for 'One Medicine.' The impending glut of new genetically engineered mice creates an urgent need for prepared investigators and pathologists.

Functional genomics and schizophrenia: endophenotypes and mutant models

This article summarizes the rationale, methods, and results of gene discovery programs in schizophrenia research and describes functional methods of investigating potential candidate genes. It focuses next on the most prominent current candidate genes and describes (1) evidence for their association with schizophrenia and research into the function of each gene; (2) investigation of the clinical phenotypes and endophenotypes associated with each gene, at the levels of psychopathologic, neurocognitive, electrophysiologic, neuroimaging, and neuropathologic findings; and (3) research into the ethologic, cognitive, social, and psychopharmacologic phenotype of mutants with targeted deletion of each gene. It examines gene-gene and gene-environment interactions. Finally, it looks at future directions for research.

Tshz1 is required for axial skeleton, soft palate and middle ear development in mice

Members of the Tshz gene family encode putative zinc fingers transcription factors that are broadly expressed during mouse embryogenesis. Tshz1 is detected from E9.5 in the somites, the spinal cord, the limb buds and the branchial arches. In order to assess the function of Tshz1 during mouse development, we generated Tshz1-deficient mice. Tshz1 inactivation leads to neonatal lethality and causes multiple developmental defects. In the craniofacial region, loss of Tshz1 function leads to specific malformations of middle ear components, including the malleus and the tympanic ring. Tshz1(-/-) mice exhibited Hox-like vertebral malformations and homeotic transformations in the cervical and thoracic regions, suggesting that Tshz1 and Hox genes are involved in common pathways to control skeletal morphogenesis. Finally, we demonstrate that Tshz1 is required for the development of the soft palate.

A role for Connexin43 during neurodevelopment

Connexin43 (Cx43) is the predominant gap junction protein expressed in premitotic radial glial cells and mature astrocytes. It is thought to play a role in many aspects of brain development and physiology, including intercellular communication, the release of neuroactive substances, and neural and glial proliferation and migration. To investigate the role of Cx43 in brain physiology, we generated a conditional knockout (cKO) mouse expressing Cre recombinase driven by the human GFAP promoter and a floxed Cx43 gene. The removal of Cx43 from GFAP-expressing cells affects the behavior of the mice and the development of several brain structures; however, the severity of the phenotype varies depending on the mouse background. One mouse subline, hereafter termed Shuffler, exhibits cellular disorganization of the cortex, hippocampus, and cerebellum, accompanied by ataxia and motor deficits. The Shuffler cerebellum is most affected and displays altered distribution and lamination of glia and neurons suggestive of cell migration defects. In all Shuffler mice by postnatal day two (P2), the hippocampus, cortex, and cerebellum are smaller. Disorganization of the ventricular and subventricular zone of the cortex is also evident. Given that these are sites of early progenitor cell proliferation, we suspect production and migration of neural progenitors may be altered. In conclusion, neurodevelopment of Shuffler/Cx43 cKO mice is abnormal, and the observed cellular phenotype may explain behavioral disturbances seen in these animals as well as in humans carrying Cx43 mutations.

Acetazolamide affects breathing differently in ICR and C57 mice

Acetazolamide (ACZ) administration was compared on ventilation in outbred male ICR Swiss Webster (ICR) and inbred C57BL/6J (C57) mice, used in development of transgenic strains. We hypothesized that in both strains ACZ would affect breathing similarly. Mice received intraperitoneally vehicle and the next week ACZ (40 mg/kg), and were exposed to air for 90 min, followed by 5-min exposure to 10% O(2), air for 15 min, and to 5 min of 5% CO(2) in O(2). Ventilation was evaluated using plethysmography. ACZ stimulated ventilation in both stains exposed to air. C57 mice minimally increased frequency and tidal volume, whereas ICR mice markedly increased frequency. Strain differences in the ventilatory pattern in response to hypoxia and hypercapnia occurred. ACZ-treated ICR mice decreased hypoxic responsiveness to 50% of vehicle values, whereas ACZ had no effect in C57 mice. ACZ decreased hypercapnic ventilatory responsiveness in both strains. Differential effects of ACZ breathing in these two strains suggest that genetic factors modulate its effect on breathing.

Strain background effects and genetic modifiers of hearing in mice

Genetic modifiers can be detected in mice by looking for strain background differences in inheritance or phenotype of a mutation. They can be mapped by analyses of appropriate linkage crosses and congenic lines, and modifier genes of large effect can be identified by positional-candidate gene testing. Inbred strains of mice vary widely in onset and severity of age-related hearing loss (AHL), an important consideration when assessing hearing in mutant mice. At least 8 mapped loci and a mitochondrial variant (mt-Tr) are known to contribute to AHL in mouse strains; one locus (ahl) has been identified as a variant of the cadherin 23 gene (Cdh23(753A/G)). This variant also was shown to modify hearing loss associated with the Atp2b2(dfw-2J) and Mass1(frings) mutations. The hearing modifier (Moth1) of tubby (Tub(tub)) mutant mice was shown to be a strain variant of the Mtap1a gene. Human hearing modifiers include DFNM1, which suppresses recessive deafness DFNB26, and a nuclear gene that modulates the severity of hearing loss associated with a mitochondrial mutation. Recently, a variant of the human ATP2B2 gene was shown to exacerbate hearing loss in individuals homozygous for a CDH23 mutation, similar to the Atp2b2(dfw-2J)-Cdh23(753A/G) interaction affecting hearing in mice. Because modifier genes and digenic inheritance are not always distinguishable, we also include in this review several examples of digenic inheritance of hearing loss that have been reported in both mice and humans.

Potential and limitations of genetic manipulation in animals

Over the last decade, sequencing and characterisation of the mouse genome has been accompanied by unparalleled advances in functional genomics. In the context of drug action, we analyse the strengths and limitations of classical mutagenesis and gene targeting techniques, as well as alternative approaches such as chemical mutagenesis, gene trap, recombineering, transposon-mediated mutagenesis, chromosomal engineering, viral transgenesis and RNA interference. This review also focuses on the emerging importance of genetic manipulation in other species and related logistical issues of experimental work using mutants.:

Microbial considerations in genetically engineered mouse research

Microbial infections have long been of concern to scientists using laboratory rodents because of their potential to confound and invalidate research. With the explosion of genetically engineered mice (GEM), new concerns over the impact of microbial agents have emerged because these rodents in many cases are more susceptible to disease than their inbred or outbred counterparts. Moreover, interaction between microbe and host and the resulting manifestation of disease conceivably differ between GEM and their inbred and outbred counterparts. As a result, infections may alter the GEM phenotype and confound interpretation of results and conclusions about mutated gene function. In addition, because GEM are expensive to produce and maintain, contamination by pathogens or opportunists has severe economic consequences. This review addresses how microbial infections may influence phenotype, how immunomodulation of the host as the result of induced mutations may modify host susceptibility to microbial infections, how novel host:microbe interactions have led to the development of new animal models for disease, how phenotype changes have led to the discovery of new pathogens, and new challenges associated with prevention and control of microbial infections in GEM. Although the focus is on naturally occurring infections, extensive literature on the use of GEM in studies of microbial pathogenesis also exists, and the reader is referred to this literature if microbial infection is a suspected culprit in phenotype alteration.