Gene therapy is the addition of new genes to a patient's cells to replace missing or malfunctioning genes. Researchers typically do this using a virus to carry the genetic cargo into cells, because that’s what viruses evolved to do with their own genetic material.

The treatment, which was first tested in humans in 1990, can be performed inside or outside of the body. When it’s done inside the body, doctors may inject the virus carrying the gene in question directly into the part of the body that has defective cells. This is useful when only certain populations of cells need to be “fixed.” For example, researchers are using it to try to treat Parkinson's disease, because only part of the brain must be targeted. This approach is also being used to treat eye diseases and hemophilia, an inherited disease that leads to a high risk for excess bleeding, even from minor cuts.

Early in-the-body gene therapies used a virus called adenovirus—the virus behind the common cold—but the agent can cause an immune response from the body, putting a patient at risk of further illness. Today, researchers use a virus called adeno-associated virus, which is not known to cause any disease in humans. In nature, this agent needs to hitch a ride with an adenovirus, because it lacks the genes required to spread itself on its own. To produce an adeno-associated virus that can carry a therapeutic gene and live on its own, researchers add innocuous DNA from adenovirus during preparation.

In-the-body gene therapies often take advantage of the natural tendency of viruses to infect certain organs. Adeno-associated virus, for example, goes straight for the liver when it is injected into the bloodstream. Because blood-clotting factors can be added to the blood in the liver, this virus is used in gene therapies to treat hemophilia.

In out-of-the-body gene therapy, researchers take blood or bone marrow from a patient and separate out immature cells. They then add a gene to those cells and inject them into the bloodstream of the patient; the cells travel to the bone marrow, mature and multiply rapidly, eventually replacing all of the defective cells. Doctors are working on the ability to do out-of-the-body gene therapy to replace all of a patient's bone marrow or the entire blood system, as would be useful in sickle-cell anemia—in which red blood cells are shaped like crescents, causing them to block the flow of blood.

Out-of-the-body gene therapy has already been used to treat severe combined immunodeficiency—also referred to as SCID or boy-in-the-bubble syndrome—where patients are unable to fight infection and die in childhood. In this type of gene therapy, scientists use retroviruses, of which HIV is an example. These agents are extremely good at inserting their genes into the DNA of host cells. More than 30 patients have been treated for SCID, and more than 90 percent of those children have been cured of their disorder—an improvement over the 50 percent chance of recovery offered by bone marrow transplants.

A risk involved with retroviruses is that they may stitch their gene anywhere into DNA, disrupting other genes and causing leukemia. Unfortunately, five of the 30 children treated for SCID have experienced this complication; four of those five, however, have beaten the cancer. Researchers are now designing delivery systems that will carry a much lower risk of causing this condition.

Although there are currently no gene therapy products on the market in the U.S., recent studies in both Parkinson's disease and Leber congenital amaurosis, a rare form of blindness, have returned very promising results. If these results are borne out, there could be literally hundreds of diseases treated with this approach.