| Genetic Modification of Animals |
Dolly the lamb stole the headlines as the first example of livestock cloned from DNA of an adult animal. But the real breakthrough came with Polly, the first transgenic lamb. Born the year after Dolly, Polly was given a human gene that encodes blood-clotting factor IX, the protein missing in people with one form of hemophilia. Harvesting such proteins from transgenic livestock is one goal of this research. The road to Polly and subsequent transgenic animals began with research using genetically altered mice. Along the way, technologies for cloning animals, modifying DNA, and targeting expression of proteins to specific tissues were developed. Someday, human gene therapy - supplying genes to patients with missing or altered proteins - may become common practice. However, significant challenges remain. Moreover, risks and ethical concerns must be addressed.
Antithrombin III (AT-III) is an example of a pharmaceutical produced in transgenic livestock. A normal level of AT-III keeps the formation of blood clots under control. Patients with AT-III deficiency may have thromboembolic problems beginning in early adulthood, particularly clots in the legs and pulmonary embolism. Providing therapeutic AT-III can reduce clotting risks in such patients. Other therapeutic proteins being considered for production by transgenic animals include human hemoglobin, human serum albumin, tissue plasminogen activator (used to treat stroke), human alpha-1-antitrypsin (alpha-1-antitrypsin deficiency can cause life-threatening emphysema), various vaccine proteins, and monoclonal antibodies.
For some time, mice have been genetically altered to exhibit human genetic disease. To generate such animal models normal genes in mice are inactivated using "knockout" technology, or altered by replacement of the normal gene with a mutated counterpart. Mouse disease models now exist for cystic fibrosis, beta-thalassaemia, atherosclerosis, retinoblastoma, and Duchenne muscular dystrophy. Such animal models allow researchers to test therapeutic compounds and study the molecular basis of given diseases.
Knockout technology, as well as other genetic engineering approaches, depends on the ability to target genes for insertion into particular locations within the host chromosome. To do this, a region on the chromosome is identified and DNA homologous to that region is engineered into a cloning vector. The newly inserted sequence can then be disrupted by insertion of a selected gene; for example, a marker gene encoding antibiotic resistance. Once cells take up the DNA, homologous recombination on either side of the marker gene allows it to be precisely inserted into the chromosome. At the same time, some or all of the target gene on the chromosome is deleted (Fig. 4).
Gene knockout in pigs is being studied as an avenue for transplanting animal organs into humans. A major cause of tissue rejection is an immune reaction to the carbohydrate galactose-a-1,3,-galactose on the surface of non-human cells. Deletion of the a-1,3,-galatosyltransferase gene may allow the production of animals lacking this surface marker.
As researchers recognized the potential of transgenic livestock for the production of human therapeutics and transplant tissue, farmers recognized the contributions that genetic engineering might make to the economics of livestock production. Cows might be produced that could grow more muscle mass, require less feed, produce more milk, or be leaner. The composition of milk could be changed; for example, casein could be over-expressed to provide increased cheese production. Lactose might be removed from milk for lactose-intolerant consumers. Disease resistant animals could reduce the use of antibiotics. Poultry with less fat content and eggs with lower cholesterol are other goals.