The Mouse Ascending: Perspectives for Human-Disease Models

FOCUS ON DEVELOPMENT AND DISEASE COMMENTARY The mouse ascending: perspectives for human-disease models Nadia Rosenthal1 and Steve Brown2 The laboratory mouse is widely considered the model organism of choice for studying the diseases of humans, with whom they share 99% of their genes. A distinguished history of mouse genetic experimentation has been further advanced by the development of powerful new tools to manipulate the mouse genome. The recent launch of several international initiatives to analyse the function of all mouse genes through mutagenesis, molecular analysis and phenotyping underscores the utility of the mouse for translating the information stored in the human genome into increasingly accurate models of human disease. Mice and humans share most physiologi- China and Japan, where the first ‘fancy’ mouse Mice with various immunodeficiencies are par- cal and pathological features: similarities in breeds were developed. By the late 1800s, these ticularly valuable for studying tumour growth nervous, cardiovascular, endocrine, immune, mutants attracted the attention of mouse col- and infectious diseases, and can act as hosts to musculoskeletal and other internal organ lectors and distributors in Europe and the human tissues and cells6. Metabolic, physiolog- systems have been extensively documented. United States. Unusual coat colours and other ical and behavioural stresses can be tested on Comparative analyses of mouse and human variations in visible features attracted Victorian mice models, the results of which can be com- genomes have provided insight into our mouse fanciers to these animals, many of pared directly with human clinical informa- common features and have guided powerful which are the direct forebears of today’s stand- tion. Through these advances, the lowly mouse, genomic manipulations in the mouse to gener- ard laboratory mouse strains. The economics once vilified as vermin and agricultural pests, ate models of human pathologies. These mice of supplying inbred strains to mouse fanciers has now emerged as the premier mammalian can then be subjected to biomedical experi- was soon dwarfed by a burgeoning investment model system for biomedical exploration. mentation not possible in patients. As more is in mammalian genetics, as researchers noted learned about human diseases that mice do not a striking resemblance between the charac- New tricks of the mouse trade usually contract, a growing armamentarium of teristics of some mouse mutants and certain The mouse’s current appeal as a model for experimental approaches is being applied to human diseases. human disease has its origins in the histori- ‘humanize’ mouse physiology and mimic our To many scientists in the 1930s, mice were cal selection and breeding initiatives used clinical manifestations. the obvious choice for genetic experimenta- for producing offspring with specific traits. How did the mouse ascend to such promi- tion2. Small, docile, rapid and prolific breed- Numerous mutants were generated by expos- nence in biological research? Compared with ers, they were also readily available from the ing mice to radiation or DNA-damaging other mammals that contract our diseases, the fancy mouse collectors of the day. Since then, chemicals1,2. Although rapid and relatively mouse is cheap, easy to maintain and straight- the mouse has attracted the attention of biolo- inexpensive, these early approaches intro- forward to breed in captivity . Mice also have gists and biomedical researchers, and a new bil- duced multiple unknown genomic altera- a long history of cohabitation with humans1. lion-dollar industry has sprung up in response, tions that required extensive follow-up work Our chequered relationship with the mouse breeding and shipping over 25 million mice to to locate and characterize. More recently, has its roots in the battle over agriculture and research laboratories each year. Repositories researchers have generated single targeted food stores in Africa more than 10,000 years routinely distribute hundreds of unique inbred mutations using an array of innovative genetic ago. The subsequent domestication of the mouse strains3 to laboratories around the world, technologies that produce specific genomic house mouse is likely to have originated in as well arranging the transport of genetically changes5. For example, transgenic mice, in engineered mutants prone to different cancers, which the gene of interest is injected into a heart disease, hypertension, diabetes, obesity, fertilized mouse egg, produce offspring with 1Mouse Biology Unit, EMBL Monterotondo Outstation, via Ramarini 32, 00016, osteoporosis, glaucoma, blindness and deaf- extra copies of the transgene; this transgene Monterotondo, Rome, Italy. ness, neuropathologies such as Huntington is often engineered to be expressed in a tis- 2MRC Mammalian Genetics Unit, Harwell, Oxfordshire, OX11 0RD, UK. or Lou Gehrig disease, and behavioural dis- sue-specific pattern or to be inducible with [email protected] turbances including anxiety and depression4,5. certain drugs. NATURE CELL BIOLOGY VOLUME 9 | NUMBER 9 | SEPTEMBER 2007 993 © 2007 Nature Publishing Group COMMENTARY FOCUS ON DEVELOPMENT AND DISEASE Critical Frt loxP 5′ Homology exon 3′ Homology arm arm SA-βgeo-pA ES cell Homologous recombination ES-cell gene locus ‘Floxed’ gene SA-βgeo-pA Drug selection Injection into blastocyst Germline offspring Chimeric offspring Figure 1 Large-scale strategy for mutagenesis of the mouse genome: a conditional knockout vector is created for each gene and introduced into mouse embryonic stem (ES) cells. The exon of the target gene is flanked by DNA motifs (loxP) that are recognized by a specific bacterial Cre recombinase. Homologous recombination between the cellular gene and the knockout vector flanking DNA sequences (green and yellow) results in the incorporation of the vector into the host ES-cell genome. The presence of a drug-resistance and marker gene (β-geo; purple) in the knockout vector allows targeted ES cells to survive drug selection, but these markers can be removed by a second recombinase (Flp) that recognizes a unique sequence (known as FRT). Mutant ES cells are injected into a host mouse blastocyst, which, when implanted into a foster mother, gives rise to chimeric offspring. This effect can be seen in their coat colour. Incorporation of mutant ES cells into germ cells results in germ-line transmission of the ES-cell genome to subsequent offspring. Alternative methods for mutagenesis rely carrying the target gene, a mutation is induced ES-cell mutations can be readily transformed on the manipulation of mouse embryonic by recombination in a spatially and tempo- into mice by using blastocyst injection, and stem (ES) cells. At present, a number of muta- rally controlled way (Fig. 2). Because most the mutation is activated by crossing with the genesis strategies based on ES cells are used, common human diseases are acquired later desired Cre recombinase driver strain. all of which use homologous recombination to in life, the generation of new mouse models As a complementary strategy, forward genet- alter genes in their original location, produc- of human disease currently relies heavily on ics — where the observation of a phenotype ing either ‘knock outs’ to cripple gene func- this conditional mutagenesis technology that is followed by the identification of the respon- tion or ‘knock ins’ to introduce an altered gene limits gene mutation to specific tissues or to a sible genetic loci — has been valuable for version. Knockout technology is extraordinar- specific life stage. discovering new and unexpected gene–func- ily powerful and generally produces loss-of- To meet the growing demand for conditional tion relationships. In common with the early function mutations, which disrupt the earliest mutant models, internationally coordinated random-mutagenesis strategies, more recent role of a gene in embryogenesis, but this often initiatives have been established for the sys- methods for unbiased, random mutation in confounds the analysis of gene dysfunction tematic generation of mouse mutants on a large the mouse rely on post-mutagenesis detec- at later stages of development or adulthood, scale using various strategies7,8,9 (Table 1). The tion of the sequence change, which has now which is when many human diseases are man- majority of these initiatives are committed to been streamlined with the advent of positional ifest. Further refinements to these manipula- the production of mutant mouse ES-cell lines, cloning, haplotype maps, single-nucleotide tions include the conditional induction of each of which carries an altered — or ‘floxed’ polymorphisms (SNPs) and high-throughput genomic changes (Fig. 1). By crossing a mouse (Fig. 1) — allele of a particular gene that har- genome sequencing. Newly developed reagents bearing a recombinase effector gene with one bours Cre recombinase sites. These mutant for mutagenesis include the reconstructed 994 NATURE CELL BIOLOGY VOLUME 9 | NUMBER 9 | SEPTEMBER 2007 © 2007 Nature Publishing Group FOCUS ON DEVELOPMENT AND DISEASE COMMENTARY Table 1 International mouse genetics and genomics initiatives Consortium Mission and aims URL European Conditional Mouse EUCOMM Genome-wide mouse mutagenesis www.eucomm.org Mutagenesis Programme www.nih.gov/science/models/mouse/knock- KOMP Knock-out Mouse Project Genome-wide mouse mutagenesis out/ North Americam Conditional NorCOMM Genome-wide mouse mutagenesis www.norcomm.org Mouse Mutagenesis Programme http://www.informatics.jax.org/mgihome/

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