Transgenic Animals are Good Medicine
Modern medicine has come a long way, but many people can honestly claim it hasn't yet come far enough. Serious illnesses such as diabetes, hemophilia, emphysema, or heart problems now often can be treated to some degree, but medicines for these conditions are frequently expensive. Other conditions may not be fatal if left untreated, but can certainly be inconvenient; lactose intolerance, a condition in which the sugars in cow's milk cannot be digested, is an example of these nuisance conditions.
In an attempt to solve these problems, scientists are using a wonderful application of molecular biology by creating transgenic animals. Transgenic animals have had a foreign gene inserted into their genome. (A gene is a sequence of DNA that codes for a protein and passes on inherited information; a genome is the complete set of genetic information for a species.)
Transgenic animals are frequently created by a method known as microinjection. In microinjection, foreign DNA is injected by a small glass-needled syringe into an egg that has just been fertilized by a sperm. This allows the foreign DNA to become integrated into the recipient's genome. Many copies of the foreign gene often will become included in the animal's genome. The process is easier to describe than it is to carry out; problems with microinjection include difficulty in getting large numbers of eggs from an animal and the procedure is of a delicate nature. Transgenic animals thus are expensive to make, but are relatively easy to care for and can be bred to create more animals with the desired gene.(The rate of viable offspring is low, however, that's probably the greatest problem the technique now faces.)
Transgenic animals whose genome has been altered to produce pharmaceuticals offer some advantages over traditional methods of drug production. For example, although microorganisms such as bacteria generate many drugs, sometimes drug proteins useful to humans require changes to their structure after they have been synthesized. Bacteria can't be engineered to make these changes, but animal cells can do it.
In the past, molecular farming of drugs from transgenic livestock such as pigs, sheep, and cows required the death of the animal so the drug could be harvested from their blood. Human growth hormone and the insulin needed by diabetics are some of the drugs that have been produced this way. Since those early experiments, the focus has shifted to producing useful proteins secreted into the milk of transgenic livestock.
When a transgene---the microinjected foreign DNA---is introduced, sometimes it contains a promoter. Promoters are the part of a gene that acts as a switch to regulate when a gene product is made. It is possible to introduce a milk protein promoter connected to a gene coding for a specific drug. Milk protein promoters are tissue specific, meaning they turn on only in the mammary glands, not the blood. Drug production in the blood could make the animal sick, and the drug also could be contaminated by germs present in the animal's system. Getting a drug by milking an animal is more convenient, more humane, and safer than harvesting it from blood.
And it works. Among the many drug products already produced from the milk of transgenic animals are some waiting for final FDA approval. Examples include Factor IX, a blood-clotting factor deficient in hemophiliacs; an alpha antitrypsin, the shortage of which makes people more likely to get emphysema; and tissue plasminogen activator, which dissolves blood clots.
Transgenic technology may benefit sick people, but in the future it also could be applied to other problems. Manipulation of milk proteins could create a milk that lactose-intolerant people could drink, or a baby formula identical to human mother's milk, or even less expensive cheese.
