Gene therapy is the use of genes engineered for treating disease. The first human gene therapy was approved for clinical trial in the United States in May1989. Because this powerful technique is still in the experimental stages, each country has its own approval process, designed to protect the patient, the health workers, and the public. In the United States, each procedure must be approved by the National Institutes of Health's Recombinant DNA Advisory Committee, by the Food and Drug Administration, and by the director of the National Institutes of Health. Gene therapy begins with the isolation of agene that can have a therapeutic effect, such as producing a protein in patients who lack the protein-producing gene or who have defective genes. In thelaboratory, the desired gene is cut out of a cell's deoxyribonucleic acid (DNA) with enzymes. It is then inserted into somatic (functional) cells removedfrom the patient, and the treated cells are returned to the patient's body. An alternative approach is to insert the gene into disabled (harmless) viral or bacterial genetic material, which serve as vectors. These molecular delivery trucks carrying the attached gene are then injected directly into the patient, where they seek and enter somatic cells. In some cases, the virus itselfis altered to make many copies of the gene. Ideally the gene should be targeted to an exact location in the cell's DNA. Viral vectors are primarily retroviruses, whose genetic material is ribonucleic acid (RNA) instead of DNA. These viruses have the potential to be permanently integrated into host cells' DNA. Techniques also exist for altering the individual's germ (reproductive) cells, which not only treat the individual but are inherited by the next generation. Such techniques are already used in plants and animals. However, this approach raises ethical dilemmas when applied to humans because permanent genetic changes (eugenics) have been attempted to harm people or eliminate groupsconsidered inferior or undesirable. Two early extensive gene therapy trialsfocused on severe combined immunodeficiency (SCID) and malignant melanoma. This rare disease inhibits immune system functioning. In a well-publicized case, a teenager named David lived for several years in a plastic bubble to protect him from infection. Some instances of SCID result from a genetic mutation that prevents production of the protein adenosine deaminase (ADA), which protects immune system white cells called lymphocytes. In September 1990, Drs. R. Michael Blaese and W. French Anderson at the U.S. National Institutes of Health performed the world's first gene therapy on a four-year-old with this condition. A normal gene for ADA was inserted into a virusand allowed to enter lymphocytes that were withdrawn from her body. Then shewas injected with the altered cells. During the next year-and-a-half, she hadseveral series of injections, along with conventional treatment. In continued studies of young SCID patients, the cells appeared to induce production ofADA, reducing infections and allowing the children to have normal lives. There were no side effects. However, because most of these children were also given a standard enzyme treatment (due to ethical considerations), scientists could not conclusively say that it was the gene therapy that caused the improvements. Melanoma is a type of often-fatal skin cancer. Since 1991, the National Cancer Institute's Dr. Steven A. Rosenberg has been studying treatment of the disease using TIL (tumor-infiltrating lymphocytes) cells taken from the patient's cancerous tumor. These cells normally enter a tumor and produce the protein called tumor necrosis factor. But often the tumor isn't destroyed. Scientists insert a gene into the TIL cells that boosts production of tumor necrosis factor. The cells are injected into the patients, and the genes function for a short period of time.
Then, as a safety feature, the injected cells die. Cystic fibrosis. A genetictreatment for this lung disease was approved in 1992. Dr. Ronald Crystal, ofthe National Institutes of Health inserted a needed gene into an inactive cold virus that the patients inhale. The gene then entered the lung and functioned to prevent the production of the mucus that blocks a patient's breathing.In follow-up studies, liposomes--hollow spheres of fat molecules formed in solution--were used as vectors to prevent side effects possibly caused by theviral vectors, like inflamed lungs and swollen nostrils. Familial hypercholesterolemia. Patients with this condition lack a gene for disposing of harmfullow-density lipoprotein cholesterol, allowing it to build up in their bodies.People lacking both copies of the gene usually die from a heart attack in their early teens. Someone with only one copy suffers from severe coronary disease. Scientists at several medical centers are studying insertion of the needed gene into cells from a patient's liver, then injecting the cells into theperson's body. Studies are under way on genetic therapies for a host of illnesses, including Hemophilia B, AIDS, liver failure, leukemia, brain tumors, several cancers, arthritis, and sickle-cell anemia.
Despite ongoing advances, gene therapy remains a highly experimental procedure. Geneticists must overcome the dilemma of ensuring that a sufficient numberof the therapeutic genes reach the proper cells to become functional. Most likely, viral vectors will play a key role in accomplishing this goal. Although viruses are extremely efficient at entering cells, the difficulty has beento get them to enter the proper cells. In 1998, scientists from Harvard Medical School announced a technique that can essentially build a bridge with twoproteins. The bridge enables the virus to bind to and enter a specific cell.If proven effective, this approach would allow gene therapists to pinpoint cells for therapy. In addition to retroviruses as vectors, scientists have beenstudying adenoviruses vectors to target nondividing cells, which retroviruses cannot do. However, viruses and liposomes, which adhere to cells like tumors and insert genes into them, have drawbacks as vectors. Viruses can infect many types of cells and cause immune responses or become inserted in the wrongcell. Liposomes also have a slight possibility of infecting germ cells, producing heritable changes. As a result, researchers continue to look for new vectors like plasmids (circular DNA packages) and human artificial chromosomes(HACs). These chromosomes are less likely to trigger an immune response likeviruses and are not able to seep into and affect other tissues because they can only be inserted into cells that have been removed from the body. Althoughit is still too early to tell, HACs may be viable vectors for gene therapiesin blood diseases, like hemophilia and sickle-cell anemia.
An alternative approach for delivery genes has been developed by two scientists at Northwestern University. The gene "gun" is powered by pressurized helium that injects microscopic gold "bullets" coated with genes. In animal studies focusing on skin cells around tumors, they found that the surrounding cellsbegan producing immune cells, called cytokines, which are what the genes areencoded for. The therapy shrunk tumors and lengthened the lives of the mice.Unlike viral vectors, this approach does not seek to permanently integrate the gene into the cell's DNA. This would be a disadvantage for inherited disease because the "mutant" genes that cause the disease would eventually becomedominant again. However, the gene gun may be useful in disease such as cancerbecause of the reduced likelihood of prolonged or permanent side effects like those associated with traditional chemotherapy.
Although many obstacles must be overcome, gene therapy has shown much promisein animal studies. In 1998 researchers from Stanford University announced agene therapy that provides proteins to inhibit inflammation in multiple sclerosis in mice. Gene therapy may also prove to be especially effective in many types of cancer, which often has a strong genetic basis, such as genemutations that cause cancers to grow and spread. One approach uses gene-modified cancer cells as vaccines. Cancer cells are extracted from tumors removedfrom the body, then scientists insert the corrective genes and reinsert theminto the patients' tumors, essentially immunizing them with their own tumor cells. Although good result have been obtained in animals, the therapy has yetto be proven effective in humans.