Uses of Genetically Modified Animals

The basic idea in making genetically modified animals is to turn animals into specialized protein factories, much like what was described for bacteria in chapter 5. The product made by this technique that may be furthest along in development is a blood-clotting factor produced in milk. However, some companies are trying to genetically engineer animals to produce useful proteins in their urine. This may seem repulsive, but there are some real advantages in producing compounds that are secreted in urine. While only females

Figure 13.2 Genetic Modifications of Embryonic Stem Cells Using Mice of Different Colors. Note that the drawings are not to scale, the DNA, of course being much smaller than the cell, and the blastocysts being much smaller than the mice. A. Embryonic stem cells are obtained from the embryo of a gray-colored mouse. these cells are not actually gray; only the coat of the mouse is gray. The cells are shaded so that they can be distinguished from those of the white mouse. B. A foreign gene is transferred to the embryonic stem cells by electric shock. C. Embryonic stem cells with the foreign gene are injected into an early-stage embryo called a blastocyst. In this example, the blastocysts are derived from a white mouse so that we can distinguish the cells that have the foreign gene from those that do not. D. The blastocysts are then implanted into a surrogate mother that produces offspring. Some of the offspring are made up of a mixture of white-mice cells of the blastocyst and gray-mouse cells with the foreign gene, and for this reason they are called mosaics. If the cells from the gray mouse make the gametes, the sperm or the egg in these mosaics, breeding them will produce mice with the foreign gene in all their cells.

Figure 13.2 Genetic Modifications of Embryonic Stem Cells Using Mice of Different Colors. Note that the drawings are not to scale, the DNA, of course being much smaller than the cell, and the blastocysts being much smaller than the mice. A. Embryonic stem cells are obtained from the embryo of a gray-colored mouse. these cells are not actually gray; only the coat of the mouse is gray. The cells are shaded so that they can be distinguished from those of the white mouse. B. A foreign gene is transferred to the embryonic stem cells by electric shock. C. Embryonic stem cells with the foreign gene are injected into an early-stage embryo called a blastocyst. In this example, the blastocysts are derived from a white mouse so that we can distinguish the cells that have the foreign gene from those that do not. D. The blastocysts are then implanted into a surrogate mother that produces offspring. Some of the offspring are made up of a mixture of white-mice cells of the blastocyst and gray-mouse cells with the foreign gene, and for this reason they are called mosaics. If the cells from the gray mouse make the gametes, the sperm or the egg in these mosaics, breeding them will produce mice with the foreign gene in all their cells.

produce milk, both males and females produce urine, and urine is much less complex chemically than milk, so it may be easier to purify proteins produced in urine instead of milk. So far, experiments have produced genetically modified mice that produce small amounts of human-growth hormone. In addition to medically important proteins, scientists have also engineered animals that secrete other proteins of commercial interest, such as cows that produce silk protein in their milk.

One can imagine that animals could be engineered for innumerable human medical purposes. In fact, pigs have been engineered to produce human hemoglobin for blood transfusions. Other pigs have been genetically modified so as not to express antigens recognized as foreign by humans. The organs of these engineered pigs could potentially be used as transplants in human patients because they would not be rejected. However, so far such applications have not yet been implemented. One danger is that viruses that infect animals could easily and inadvertently enter humans through the organs or blood products from engineered animals. Such viruses may not be transmitted easily now because of natural barriers to transmission, but organ transplant will facilitate transmission. Because we know very little about pig viruses, we do not know the risks of a pig virus causing a major disease in humans. If even a single pig virus caused disease in humans and was easily transmissible, we could be faced with a catastrophic epidemic. Scientists are well aware of these risks and take them very seriously. In short, animal genetic modification is still at an experimental stage. At this point, it is unclear whether genetically modified animals should be or will ever be used as tissue donors, protein factories, or for other purposes.

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