Transgenic Animals

Gene targeting has been used to identify the function of hundreds of mouse genes. One dramatic example was the deletion of the Lim-1 gene by Richard Behringer and colleagues. The mice carrying this deletion died during embryonic development because of a complete lack of brain and head structure development. This demonstrated that the Lim-1 gene was critical for head development.

The gene responsible for sex determination in mice was also identified thanks to the use of transgenic animals. When a gene called Sry (sex-determining region of the Y chromosome) was microinjected into mouse embryos, the resulting transgenic mice were all male. Indeed, even in the mice that had two X chromosomes and were thus genetically female, the presence of the Sry gene was sufficient to cause them to develop testes and led to complete sex reversal. This clearly demonstrated that the Sry gene alone was responsible for sex determination.

Gene targeting is also being exploited by scientists to create models of human disease. For instance, mutations have been made in the mouse version of the cystic fibrosis transmembrane conductance regulator gene. Although mice with the mutated gene do not develop the devastating symptoms of cystic fibrosis in their lungs, they do develop the intestinal and pancreatic duct defects associated with the disease and thereby provide a model to study at least part of the disease. Transgenic mice overexpressing the amyloid precursor protein form deposits in the brain that resemble the amyloid plaques found in Alzheimer's patients. Mouse models such as these can potentially be used to test drug therapies and to learn more about the progression of the disease.

One important application of transgenic technology is the generation of transgenic livestock as "bioreactors." Key human genes have been introduced into sheep, cows, goats, and pigs so that the human protein is secreted into the milk of the transgenic animal. In theory, large quantities of the human protein can be produced in the animal's milk and subsequently purified for use in medical therapies. An early example of this technology by John Clark and colleagues was the production of transgenic sheep expressing the human blood-clotting factor IX needed by many patients with hemophilia. These researchers placed the human factor IX gene under the control of a piece of sheep DNA that normally turns on the beta-lactoglobulin gene in the mammary tissue. Though the sheep secreted factor IX into their milk, the levels of the protein were very small. With advances in the efficiency of creating and expressing genes in transgenic farm animals, therapeutic proteins can now be isolated.