>Genetic Tools

One of the reasons why mice are widely used as model organisms is because of the many genetic tools that are available to be used to create mouse models of human disease.

Reverse Genetics

With reverse genetics techniques, a specific gene is targeted within the mouse genome and the mouse is observed for any phenotypes that could provide insight into the gene’s function. In the 1980’s, the development of gene targeting technologies in mouse embryonic stem (ES) cells to create transgenic mice dramatically transformed the landscape of mouse genetics. The gene targeting process begins with the introduction of a transgene into ES cells, which, through the process of homologous recombination, leads to mutations or alterations in the target gene in the genome. The cells are then injected into developing mouse embryos, which are implanted into pseudopregnant female mice to generate chimeric progeny. The mice are screened for germ line integrations to confirm that the target gene sequence was altered. This advance in technology has led to important discoveries of gene function in mammalian biology and disease.

Forward Genetics

Forward genetics techniques, in contrast to reverse genetics, aim to discover new genes involved in specific processes by mutagenizing the genome and screening for a particular phenotype. Forward genetic screens are typically done in lower model organisms due to cost and time constraints, though they have been used in mice. One forward genetic approach is to use inbred mouse strains to map causative natural variations. Instead of relying on natural variation, ENU has been used as a powerful mutagen to perform large-scale phenotype-driven screens in the mouse.

Insertional Mutagenesis

Insertional mutagenesis is a genetic tool that is used in a variety of model organisms. Retroviral-mediated insertional mutagenesis has been used in mice to identify mutations in oncogenes that are involved in cancer. Transposons have also been successfully used in the mouse for insertional mutagenesis.

Genetic Mosaics

Conditional gene knockout or expression using the Cre/loxP or FRT/FLP recombination systems has been a successful strategy used to confer spatial and temporal specificity to gene activation or inactivation. This approach has been used widely in mice to model the loss of tumor suppressors or the activation of oncogenes in sporadic cancers. Clones of mutant cells can be marked by Cre-mediated deletion or activation of GFP or LacZ reporters.

Humanized Mice

Although the mouse has been a good in vivo model for understanding human disease, there are still some limitations in that the results of mouse experiments don’t always correlate with what is observed in humans. Recent efforts have aimed at humanizing the mouse by reconstituting human tissues, cells, or genes within the mouse. One strategy designed to humanize a mouse is to put human genes in mouse ES cells that give rise to mice that express the human genes. Mice that carry human genes can provide experimental models to directly study the activities of the human gene and to test therapeutic agents that target the human gene products. Another strategy used to produce humanized mice is to allow engraftment of human tissues or cells in immunodeficient mice to repopulate the immune system.