Transgenic Animals

A transgenic animal is one in which there has been a deliberate modification of the genome -- as opposed to a spontaneous mutation -- using recombinant DNA technology. The foreign DNA introduced must be transmitted through the germ line, so that every cell of the animal, including its germ cells, carries the same modified genetic material. Many transgenic animals -- mice, rats, rabbits, pigs, sheep, cows, fowl and fish -- have been produced through rDNA technology, for a wide range of purposes: identifying the function of specific factors in complex homeostatic systems (medical research); use as responsive test animals in toxicology; mammalian developmental genetics; analysing the regulation of gene expression at the level of a whole animal (molecular biology); targeted production of pharmaceutical proteins and drug/product-efficacy testing (the pharmaceutical industry); production of specific proteins generally (biotechnology); genetically engineered hormones to increase milk yield or meat production, and genetic engineering of livestock/aquaculture physiology and anatomy; cloning procedures to reproduce specific bloodlines; and developing animals specially created for xenografting.

1. Transgenic mice and cancer research. Modifying transgenic mice with a particular oncogene (cancer-causing gene) lets researchers develop a specific type of cancer in the lab and study the relationship between that oncogene and cancer development, including, in principle, treatment and prevention. In Philip Leder's laboratory at Harvard (USA), a transgenic mouse model for investigating breast cancer was developed this way, analysing whether the oncogenes myc and ras led to breast cancer in mice carrying them.
2. Transgenic farm animals. Researchers have increasingly focused on producing transgenic farm animals for greater benefit -- improving meat production in some, and milk yield/quality or disease-free status in others. At the start of the twentieth century, a dairy cow gave 2,000 to 3,000 litres of milk a year; today, a Holstein cow averages around 6,000 litres, and the best animals give 8,000-10,000 litres. Likewise, a hen a century ago laid about 70 eggs a year, while today's best breeds lay up to 250 -- improvements credited in part to the advent of biotechnology.
3. Transgenic cattle for food production. Among the few studies applying transgenic technology to cattle for food, researchers introduced extra copies of bovine beta- or kappa-casein genes into dairy cattle and studied the effect on milk production and composition; the transgenic offspring showed an 8-20% increase in beta-casein, and roughly a two-fold increase in kappa-casein.
4. Transgenic cattle for human therapeutic production. A second application of genetically modified cattle is producing human therapeutic proteins -- human lactoferrin, human alpha-lactalbumin, human serum albumin and human bile-salt-stimulated lipase have all been expressed in cow's milk, since the mammary gland in dairy cows is an excellent natural protein-production factory. A single transgenic cow, for instance, could be more than sufficient to supply the world's annual requirement of Factor IX (plasma thromboplastin component), used to treat haemophilia. In 1990, Tracy -- the first such transgenic cow -- was born in Scotland, able to produce a human therapeutic protein in her milk. Antibodies are also used for many human clinical applications, including treatment of infectious disease, cancer, transplanted-organ rejection and autoimmune disease, and as antitoxins; to make a human antibody product this way, genetically modified cows are immunised with a vaccine containing the relevant disease agent.
5. Transgenic sheep. Gene-transfer technology applied to sheep has produced transgenic sheep able to achieve better growth and meat production, as well as serving as bioreactors -- the human growth-hormone gene has been introduced into sheep to promote growth and meat production. Separately, the bacterial genes cysE and cysM, involved in biosynthesis of the amino acid cysteine (needed to form the keratin protein in wool), have been cloned and introduced into sheep to increase wool production and improve wool quality. …