House Mouse, Mus musculus

The house mouse, Mus musculus, has been collected and bred by mouse “fanciers” for hundreds of years, and some of the purebreds developed by fanciers are used today as standards in scientific studies. The house mouse is the leading mammalian model, and it is more like a human than we may care to admit (see Figure 1-12). The mouse genome is almost the size of the human genome and encodes essentially the same number of genes, 99% of which have homologs in the human. In fact, much of the mouse’s genome is syntenic with ours, meaning that whole blocks of genes occur in the same order in both species (see Figure 8-4).

Compared with other model organisms, mice are more cumbersome to work with in every way. They are larger, of course, but they also have a generation time of about 8 to 10 weeks and produce, on average, only 3 to 14 pups per litter. These statistics are attractive when compared with other mammals, but pale in comparison with other model organisms. Colonies of mice are also costly to maintain, and they simply cannot be dealt with in the numbers needed to perform large genetic screens, as with other model organisms. However, unlike the nematode and fruit fly, mice have biological systems that have no parallel in lower animal models, such as the immune and skeletal systems, or are simply better models for studies of complex systems such as the cardiovascular system, endocrine system, and many others. The mouse is a model for human disease, including cancer, in virtually all of these systems.

Early Studies of the Mouse as a Model Organism

Genetic studies in mice began in the early 1900s, when selection and breeding were the main methods of obtaining progeny with the desired traits. These early studies produced a general model that explained coat coloring in all other fur-bearing mammals. Mice and rats also have a long history in nutritional studies, especially in the identification of vitamins and the symptoms caused by vitamin deficiencies in the diet. Use of mice as a human disease model was pioneered by Clarence Cook Little. In the 1920s he developed an inbred mouse strain, C57BL/6 (commonly known as Black6), which eventually became the mouse strain used to determine the genome sequence. Little also founded the Jackson Laboratory in Bar Harbor, Maine, a center for mouse genetics that also serves as a public repository of mouse models for scientific research.

Life Cycle

The X and Y chromosomes determine sex in mice, as in humans. Fertilization gives rise to a blastocyst containing some undifferentiated cells bunched together in the inner cell mass, the source of embryonic stem cells. Gestation is complete within 19 to 21 days and gives rise to a litter of 3 to 14 pups. Sexual maturity requires about 6 weeks for females and 8 weeks for males, but breeding can take place in as short a time as 35 days. Mice live for 1 to 2 years, and females can produce about 5 to 10 litters, mainly in their first year of life.

Genetic Techniques

Mutagenesis Inbreeding over many generations has produced many useful strains of mutant mice. Adding mutagenic chemicals to the food supply also facilitates development of mutant strains.

Introduction of DNA Foreign DNA can be injected directly into the nucleus of fertilized eggs, followed by implantation of the eggs in the oviduct of the female recipient. Random integration occurs with high frequency. The recombinant DNA used typically has a mouse promoter that directs expression of a reporter gene, such as lacZ or GFP (green fluorescent protein), so that expression of the transgene can be followed during development. About half of the transgenic mice contain recombinant DNA in the germ line and therefore pass on the recombinant gene to future generations.

Gene Knockouts Targeted knockouts for mouse disease models are constructed in embryonic stem cells (Figure A-9). The stem cells are extracted from the inner cell mass of the blastocyst and grown in culture. Cultured stem cells are then transformed with linear DNA containing a mutated copy of the gene under study, along with genes for neomycin resistance (neor) and thymidine kinase (tk). Homologous DNA flanks the mutated gene (genemut in Figure A-9) and the neor gene, such that homologous integration replaces the wild-type gene with the mutant gene plus the neor gene. On the other hand, DNA that inserts randomly results in integration of the entire DNA fragment, including tk.

Figure A-9: The construction of a knockout mouse.

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To select for cells with the desired gene knockout, two steps are required. Selection for neor, using neomycin, kills all cells that fail to integrate transformed DNA. Selection against tk, using the antiviral gancyclovir, kills cells with DNA integrated by nonhomologous recombination, because these cells contain tk and thymidine kinase converts gancyclovir to a toxin that kills these cells. Only cells that contain gene knockouts produced by homologous recombination survive both selections. Engineered stem cells are injected into a host wild-type blastocyst-stage embryo. This results in formation of an embryo that is a chimera of host wild-type and donor engineered cells. Resulting chimeras are bred for germ-line transmission of the genetic modification. The F1 (first-generation) heterozygous mice are then crossed to obtain F2 wild-type, heterozygous, and homozygous offspring, in the expected Mendelian 1:2:1 ratio. Selective breeding results in homozygous knockout mice. Methods have been worked out to preserve valuable, and hard to obtain, mouse strains by cryopreservation of sperm or egg cells.

More recently, flippase–flippase recognition target (FLP-FRT) and/or Cre-lox technology have been developed in mice and allow tissue-specific and cell type– specific genome editing, instead of whole body genome alterations. The mouse has also been used for more recent gene inactivation methodologies that include RNAi and novel techniques such as transcription activator–like effector nucleases (TALENs) and CRISPR.

The Mouse as a Model Organism Today

Human Disease The mouse is an important model for studying human disease and aging. Disease models can be derived by inbreeding or by producing knockouts of known disease genes. Mouse models of human disease include cancer, atherosclerosis, hypertension, metabolic diseases, immune disorders, type 1 and type 2 diabetes, Down syndrome, Alzheimer disease, glaucoma, osteoporosis, obesity, epilepsy, Lou Gehrig disease (amyotrophic lateral sclerosis), Huntington disease, blood disorders, and many others.

Mapping of Mutant Genes Mutant genes can be identified more quickly in the mouse than can mutant genes in humans. Closely related strains of mice (e.g., Mus musculus and Mus spretus) can be crossed to produce hybrids that usually contain different sequences at polymorphic positions, enabling researchers to use linkage analysis to develop detailed genetic maps and locate a disease gene.

Behavior Mouse models exist for certain types of behavior, including alcoholism, drug addiction, anxiety disorders, and aggressive behavior.

Mammalian Development Transgenic mice are used to study the location and timing of expression of particular genes at various stages of development. In addition, models exist for studying certain human developmental disorders, including cleft lip and cleft palate.

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