Nicole Edgar, Etienne Sibille
Center for Neuroscience, Department of Psychiatry,
University of Pittsburgh, Pittsburgh, PA, USA
Genetically engineered animal; Genetically modified organism; GMO; Transgenic animal
A genetically modified animal is one whose genetic material has been altered by the use of genetic engineering or recombinant DNA technology. In biomedical sciences, genetically modified animals are typically generated for the purpose of studying the function of a particular gene.
One of the main goals of the field of genetics is to classify and functionally characterize individual genes. The investigative approach to studying genes in living organisms has principallybeen divided into three strategies: (1) analysis of natural variation, (2) random mutagenesis, and (3) targeted mutagenesis and transgenesis (Rudolph and Mohler 1999). Analysis of natural variation (e.g., spontaneous mutations) and random mutagenesis (e.g., chemical or irradiation) are the primary approaches of ► forward genetics in which the genetic cause (► genotype) of an altered or abnormal ► phenotype is investigated. However, with random mutagenesis, many chromosomal loci are often targeted and it is difficult to trace any phenotype back to a specific genetic origin. The development of ► reverse genetic approaches, in which a particular gene is altered and the phenotype is investigated, provided tools to investigate specific gene function in a more targeted manner (Brusa 1999). Since the development of these tools in the 1980s and 1990s, their use in the field of biomedical research and pharmacology has been substantial owing to the ability to develop suitable animal models of specific diseases, to genetically dissect the underlying mechanisms of disease, and to identify and verify molecular targets of pharmacological agents.
In the past two decades, a variety of techniques have been developed to introduce genetic modifications in various species for specific research purposes. Among the most targeted species are Drosophila melanogaster, Caenorhabdi-tis elegans, and mice, which have each played integral roles in identifying genes involved in development, aging, cell differentiation, and other major biological functions.
Other genetically modified animals that have been developed include xenopus, zebrafish, rabbits, pigs, and cows. More recently, transgenic and ► knockout rats have been developed, which will allow more extensive research in the neurosciences because of their extensive use in behavioral paradigms (Abbott 2004). In addition, the first transgenic primate disease model (for Huntington's disease) was recently created (Yang et al. 2008). While a wide variety of genetically modified organisms have been created to date for numerous research purposes, techniques for genetically modifying mice are the most advanced and the most applicable to the field of psychopharmacology, which represent the main focus of the remainder of this article.
While the term "transgenic" has grown to include any type of genetically modified animal, the traditional definition of a ► transgenic organism is one containing foreign DNA, whether from the same species or a different one. The expression of foreign DNA in a mouse is a valuable technique, since it allows for the investigation of the functional role of this gene in a living organism. For instance, transgenic mice overexpressing a particular gene are often generated to analyze exaggerated pheno-types. The expression of a human gene or a mutated gene in mice is also often used to explore gene function, particularly in the context of a specific disease.
There are several ways to create a transgenic mouse; however, all methods consist of first designing a DNA fragment, or "genetic construct," which contains the gene of interest (GOI) and other features necessary for the expression of this gene in a mammalian system (e.g., gene promoter, enhancer, polyA signal, etc.). The traditional transgenic method consists of physically injecting the trans-genic construct into the nucleus of a fertilized egg (pro-nuclear microinjection), allowing it to develop in vitro to the blastocyst stage, and then implanting the egg into a ► pseudopregnant female (Fig 1a). The embryos must then be screened for the presence of the transgene and unlike the production of knockouts, the transgene typically occurs in an all or none fashion, with the embryo either containing the transgene in every cell or in no cells at all. Alternatively, transgenic animals can be produced by viral infection of the fertilized embryo (see below) or transfection of embryonic stem cells (ES) with the gene of interest (Dale and von Schantz 2002). While the transgen-ic approach is fast and efficient, limitations of the technology include: (1) The GOI may randomly integrate into the genome, which can result in expression in ectopic sites, interference with the endogenous gene, or severe
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