Transgenic animals are animals (most commonly mice) that have had a foreign gene deliberately inserted into their genome. Such animals are most commonly created by the micro-injection of DNA into the pronuclei of a fertilised egg which is subsequently implanted into the oviduct of a pseudopregnant surrogate mother. This results in the recipient animal giving birth to genetically modified offspring. The progeny are then bred with other transgenic offspring to establish a transgenic line. Transgenic animals can also be created by inserting DNA into embryonic stem cells which are then micro-injected into an embryo which has developed for five or six days after fertilisation, or infecting an embryo with viruses that carry a DNA of interest. This final method is commonly used to manipulate a single gene, in most cases this involves removing or 'knocking out' a target gene. The end result is what is known as a ‘knockout’ animal.
A transgenic animal is one whose genome has been altered by the transfer of a gene or genes from another species or breed.
Transgenic animals are routinely used in the laboratory as models in biomedical research. Over 95 per cent of those used are genetically modified rodents, predominantly mice. They are important tools for researching human disease, being used to understand gene function in the context of disease susceptibility, progression and to determine responses to a therapeutic intervention. Mice have also been genetically modified to naturally produce human antibodies for use as therapeutics. Seven out of the eleven monoclonal antibody drugs approved by the FDA between 2006 and 2011 were derived from transgenic mice. Transgenic farm animals are also being explored as a means to produce large quantities of complex human proteins for the treatment of human disease. Such therapeutic proteins are currently produced in mammalian cell-based reactors, but this production process is expensive. In 2008, for example, the building of a new cell-based manufacturing facility for one therapeutic protein was estimated to cost over US$500 million. A cheaper option would be to develop a means to produce recombinant proteins in the milk, blood or eggs of transgenic animals. Progress in this area, however, has been slow to-date. Only two biomedical products have so far received regulatory approval. The first is human antithrombin III, a therapeutic protein produced in the milk of transgenic goats, which is used to prevent clots in patients with hereditary antithrombin deficiency receiving surgery or undergoing childbirth. A relatively small herd of goats (about 80) can supply enough human antithrombin III for all of Europe. The second product is a recombinant human C12 esterase inhibitior produced in the milk of transgenic rabbits. This is used to treat hereditary angiodema, a rare genetic disorder which causes blood vessels in the blood to expand and cause skin swellings.
The ability to produce transgenic animals is reliant on a number of components. One of the first things needed to generate transgenic animals is the ability to transfer embryos. The first successful transfer of embryos was achieved by Walter Heape in Angora rabbits in 1891. Another important component is the ability to manipulate the embryo. In vitro manipulation of embryos in mice was first reported in the 1940s using a culture system. What is also vital is the ability to manipulate eggs. This was made possible through the efforts of Ralph Brinster, attached to the University of Pennsylvania, who in 1963 devised a reliable system to culture eggs, and that of Teh Ping Lin, based at the California School of Medicine, who in 1966 outlined a technique to micro-inject fertilised mouse eggs which enabled the accurate insertion of foreign DNA. The first genetic modification of animals was reported in 1974 by the virologist Rudolph Jaenisch, then at the Salk Institute, and the mouse embryologist Beatrice Mintz at Fox Chase Cancer Center. They demonstrated the feasibility of modifying genes in mice by injecting the SV40 virus into early-stage mouse embryos. The resulting mice carried the modified gene in all their tissues. In 1976, Jaenisch reported that the Moloney Murine Leukemia Virus could also be passed on to offspring by infecting an embryo. Four years later, in 1980, Jon Gordon and George Scango together with Frank Ruddle, announced the birth of a mouse born with genetic material they had inserted into newly fertilised mouse eggs. By 1981 other scientists had reported the successful implantation of foreign DNA into mice, thereby altering the genetic make-up of the animals. This included Mintz with Tim Stewart and Erwin Wagner at the Fox Chase Cancer Center in Philadelphia; Brinster and Richard Palmiter at the University of Washington, Seattle; and Frank Costantini and Elizabeth Lacy at Oxford University. Such work laid the basis for the creation of transgenic mice genetically modified to inherit particular forms of cancer. These mice were generated as a laboratory tool to better understand the onset and progression of cancer. The advantage of such mice is that they provide a model which closely mimics the human body. Such mice are not only used to gain greater insight into cancer but also to test experimental drugs.
Since the mid-1980s transgenic mice have become a key model for investigating disease. Mice are the model of choice not only because there is extensive analysis of its completed genome sequence, but its genome is similar to the human. Moreover, physiologic and behavioural tests performed on mice can be extrapolated directly to human disease. Robust and sophisticated techniques are also easily available for the generic manipulation of mouse cells and embryos. Another advantage of mice is the fact that they have a short reproduction cycle. Other transgenic species, such as pig, sheep and rats are also used, but their use in pharmaceutical research has so far been limited due to technical constraints. Recent technological advances, however, are laying the foundation for wider adoption of the transgenic rat.
Transgenic rodents play a number of critical roles in drug discovery and development. Importantly, they enable scientists to study the function of specific genes at the level of the whole organism which has enhanced the study of physiology and disease biology and facilitated the identification of new drug targets. Due to their similarity in physiology and gene function between humans and rodents, transgenic rodents can be developed to mimic human disease. Indeed, an array of transgenic mice models have been produced for this purpose. Mice are being used as models, for example, to study obesity, heart disease, diabetes, arthritis, substance abuse, anxiety, ageing, Alzheimer's disease and Parkinson's disease. They are also used to study different forms of cancer. In addition, transgenic pigs are being investigated as a source of organs for transplants, which if proven clinically safe could overcome some of the severe donor organ shortages.
Transgenic animals: timeline of key events
|1929||Jackson Memorial Laboratories established to develop inbred strains of mice to study the genetics of cancer and other diseases||Jackson Memorial Laboratoroies|
|1974||First publication on inserting foreign DNA into mice||Jaenisch, Mintz||Salk Institute, Fox Chase Institute for Cancer Research|
|September 1980||First transgenic mice made with recombinant DNA announced||Barbosa, Gordon, Plotkin, Ruddle, Scangos||Yale University|
|November 1980||Technique published using fine glass micropipettes to inject DNA directly into the nuclei of cultured mammalian cells. High efficiency of the method enables investigators to generate transgenic mice containing random insertions of exogenous DNA.||Capecchi||University of Utah|
|November 5, 1981||First report of successful nuclear integration and germ-line transmission of foreign DNA into laboratory mice||Constantini, Lacy||Oxford University, Yale University|
|December 1982||Giant mice made with the injection of rat growth hormone||Brinster, Palmiter||University of Pennsylvania, University of Washington Seattle|
|1983||Course started in the molecular embyology of mice||Costantini, Hogan, Lacy||Cold Spring Harbour Laboratory, NIMR, Sloan Kettering Cancer Research Center, Columbia University|
|1985||First transgenic mice created with with genes coding for both the heavy and light chain domains in an antibody.||Kohler, Rusconi||Max-Planck Institute|
|November 6, 1987||Publication of gene targeting technique for targetting mutations in any gene||Thomas, Capecchi||University of Utah|
|1988||Patent application filed for a method to create transgenic mice for the production of human antibodies||Bruggeman, Caskey, Neuberger, Surani, Teale, Waldmann, Williams||Laboratory of Molecular Biology, Babraham Institute, Cambridge University|
|April 1988||OncoMouse patent granted||Leder, Stewart||Harvard University|
|1994||First transgenic mice strains reported for producing human monoclonal antibodies||Bruggemann, Green, Lonsberg, Neuberger||Cell Genesys, GenPharm, Laboratory of Molecular Biology|
|January 1996||First cloning of a mammal||Wilmut||Roslin Institute|
|July 5, 1996||Dolly the sheep, the first cloned mammal, was born||Wilmut||Roslin Institute|
|February 14, 2003||Dolly the sheep, the first cloned mammal died||Wilmut||Roslin Institute|
|September 2006||First fully human monoclonal antibody drug approved||Agensys, Amgen|
|2007||Nobel Prize for Physiology for Medicine awarded for their discoveries enabling germline gene modification in mice by the use of embryonic stem cells||Capecchi, Evans, Smithies||University of North Carolina, University of Utah|
|September 23, 2015||Beijing Genomics Institute announced the sale of the first micropigs created with the help of the TALENs gene-editing technique at Shenzhen International Biotech Leaders Summit||Beijing Genomics Institute|
|October 5, 2015||US Scientists report use of CRISPR to modify 60 genes in pig embryos in first step to create pig organs suitable for use in humans||Church||Harvard University|
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