Viewpoint: Trangenic Animals as 'Alternatives' to Animal Use
Jon Gordon, M.D., Ph.D.
Superficially, transgenic animal testing might appear to increase animal use. However, closer examination of this issue indicates that in many circumstances, transgenic technology can actually lead to a reduction in the use of larger animals and primates, as well as a decrease in the numbers of animals required to attain specific scientific objectives.
Transgenic animals are animals that carry new genes. These genes are usually inserted by microinjection of purified DNA into the pronucleus of the fertilized egg. When inserted in this way, the new genetic material becomes integrated into the chromosomal DNA of the host organism, and transmitted as a heritable trait to succeeding generations of progeny. These "transgenes," which may be derived from any species (e.g., human genes in transgenic mice) can be efficiently expressed, with the tissue distribution of expression determined by discrete elements close to the gene coding sequence.
These elements, loosely referred to as "promoters" or "enhancers" can be grafted by recombinant DNA technology to heterologous genes, thereby to target expression of the latter to a site specific for the promoter/enhancer.
Thus, a promoter/enhancer element specific for liver expression can be linked to a gene for growth hormone, which is normally expressed in the pituitary gland. The resulting transgenic animal will then produce growth hormone from its liver. This technology has extraordinary power for elucidating fundamental mechanisms of gene regulation, and for performing insightful and useful genetic engineering trials.
In many respects, transgenic mice and rats underscore the immeasurable value of research involving the intact organism for obtaining subtle information in crucial areas, such as fundamentals of gene regulation, physiology and developmental genetics. However, because transgenic technology is efficient, it can lead to reductions in total animals used for specific projects and can provide rodent models for disease states which could only be studies in larger animals before now.
There are many cases in which transgenic technology has allowed the creation of novel rodent models for studying human disease, which have substituted for models that previously existed only in larger animals. One example is hepatitis B transgenic mice. Several investigators have demonstrated that expression of hepatitis B surface antigen in transgenic mouse liver can induce a disease state resembling chronic active hepatitis, and that transgenic mice with this disorder develop hepatocellular carcinoma. Prior to the advent of this model, the chimpanzee was one of the leading models for hepatitis B studies. Other animal models, such as the duck were cumbersome. Another case involves mouse models for HIV-related pathology. While transgenic mice that express HIV genes clearly do not develop AIDS, they do exhibit some disorders that are highly similar to conditions characteristic of AIDS-related disease in humans. As an example, overexpression of some HIV genes in transgenic mice induces lesions of the skin that closely resemble Kaposi's sarcoma. Previously, the chimpanzee was considered one of the most representative AIDS models. It is anticipated that these transgenic mice will allow a shift of at least some AIDS studies to rodents; an eventuality that could significantly reduce primate use.
Perhaps the best example of an emerging transgenic model for a disease which could previously only be studies in larger species relates to Alzheimer's disease (AD). This disorder has no known counterpart in rodents. However, lesions reminiscent of AD occur in older primates and long-term maintenance of primates has long been necessary for studies of AD. Now, several laboratories have shown that introduction of some portions of the amyloid protein precursor (APP) gene into transgenic mice produces a disorder highly similar to AD. These mice offer enormous promise for the study of the etiology and pathogenesis of AD, and could replace the primate model.
Another important area where transgenic animals represent an "alternative" to animal usage is in their potential for reducing the number of animals needed for a variety of studies. For example, transgenic mice have been produced to serve as sensitive "living tests" for the presence and activity of mutagens in the environment. The strategy for these animals is to introduce genes that provide the opportunity for an in vitro assay. Mutation of the transgenes can be detected with high specificity and great sensitivity because the transgenic tissues allows for convenient screening of large numbers of substances. These transgenic animals can be studies so efficiently, fewer are needed for detection of mutagens. Moreover, the sensitivity of such animals as detection systems can reduce experimental exposure of test animals to toxic or irritating doses of potentially mutagenic chemicals, thus offering a significant refinement of current procedures.
Other transgenic mice have been produced that manifest an excessive sensitivity to carcinogens. Because the presence of carcinogens in the environment may be detected more easily using such animals, the total number of animals needed to determine carcinogen hazards is reduced. It is possible that in the near future animals will be produced that are extra sensitive to toxicity of drugs and/or their metabolites, thereby greatly streamlining the efficiency of the animal testing phase in trials of new drugs.
It is difficult to foresee all future applications of transgenic technology to biomedical research. It is clear, however, that these animals will have a major impact on the study of birth defects, autoimmune disease, aging, cancer, allergy and infectious disease and environmental health. In many or all of these areas, the elegance and efficiency with which transgenic models can provide the data necessary to reach a definitive conclusion will reduce animal use and lead to a progressive transition of animal models from the larger species to the rodent. As such, transgenic technology represents an attractive alternative to previously employed methods of animal use.
Jon Gordon received his Ph.D. from Yale University in 1978, and his M.D. from Yale in 1980. During his subsequent postdoctoral fellowship with Frank Ruddle at Yale, he developed the microinjection methods of inserting genes into intact animals. Later, he and Dr. Ruddle demonstrated germ line transmission of these gens, and coined the term "transgenic" to describe these animals. Dr. Gordon is G. Harold and Leila Y. Mathers Professor, Molecular Biology of Aging, and professor of obstetrics and gynecology at Mt. Sinai School of Medicine, New York.
