Gene Therapy Defined

• It is an approach to treating disease by either modifying the expressions of an individual's genes or correction of abnormal genes. By administration of DNA rather than a drug, many different diseases are currently being investigated as candidates for gene therapy. These include cystic fibrosis, cardiovascular disease, infectious diseases such as AIDS and cancer.

Gene Therapy vs. Cell Therapy

Gene Therapy and Cell Therapy are overlapping fields of biomedical research with similar therapeutic goals. Gene Therapy can be defined as the use of genetic material (usually deoxyribonucleic acid - DNA) to manipulate a patient's cells for the treatment of an inherited or acquired disease. Cell Therapy can be defined as the infusion or transplantation of whole cells into a patient for the treatment of an inherited or acquired disease.

The concept of Gene Therapy was introduced in the late 1970s after the development of recombinant DNA technology. At this time, many approaches for Gene Therapy are being evaluated in animal models of human diseases and in clinical trials. While there have been no completely successful applications of Gene Therapy for human disease, considerable progress has been made. Compared to Gene Therapy, Cell Therapy is an older discipline, dating back to the first blood transfusions in the 1940's, and proceeding through organ and bone marrow transplantation in the 1960s and 70s, to the more modern adoptive transfer of lymphocytes to treat cancer and the potential to use stem cells to repair damaged organs in the future.

A classic example of Gene Therapy is the efforts to correct hemophilia. Hemophilia A and hemophilia B are caused by deficiencies of the clotting factors factor VIII (FVIII) and factor IX (FIX) respectively. FVIII and FIX are made in the liver and secreted into the blood where they have critical roles in the formation of clots at the sites of vessel injury. Mutations in the FVIII or FIX genes prevent clot formation, and patients with hemophilia are at a severe risk of bleeding to death. Using disabled virus carriers, researchers have been able to introduce normal FVIII and FIX genes into the muscle and liver of animal models of hemophilia, and in the case of FIX, human patients. The transferred genes function, and in the animal models produce enough protein to correct the bleeding problems. The initial studies in humans have been cautious, but they have demonstrated that the procedure is safe and that the transferred gene makes FIX. Gene Therapy for hemophilia could provide a cost effective alternative to the repeated need for hemophilia patients to be injected with recombinant FVIII or FIX, as well as avoid the complications of contaminating proteins or pathogens in the injected proteins. A recurring obstacle that has been identified in the animal studies is that when FVIII or FIX is made in animals that have never had these proteins, the hosts immune system can recognize the new FVIII or FIX as a foreign protein and will develop antibodies against it. An active area of research currently is to determine how best to deal simultaneously with the gene transfer and the immune response.

Currently the most common Cell Therapy (other than blood transfusions) is bone marrow transplantation. Bone marrow transplantation is the treatment of choice for many kinds of leukemia and lymphoma, and is used to treat many inherited disorders ranging from the relatively common thalassemias (deficiencies of alpha-globin or beta-globin, the components of hemoglobin) to more rare disorders like Severe Combined Immune Deficiency (SCID the "Bubble Boy" disease). The key to bone marrow transplantation is the identification of a good "immunological matched" donor. The patient's bone marrow cells are then destroyed by chemotherapy or radiation, and cells from the matched donor are infused. The most primitive bone marrow cells, called stem cells then find their way to the bone marrow where the replicate to increase their number (self renew) and also proliferate and mature producing normal numbers of donor derived blood cells in the circulation of the patient in a few weeks. Unfortunately, not all patients have a good "immunological match". In addition, up to a third (depending on several factors including the disease) of bone marrow grafts fail to fully repopulate the patient, and the destruction of the host bone marrow can be lethal, particularly in very ill patients. These factors combine to hold back the obvious potential of bone marrow transplantation.

Gene Therapy and Cell Therapy overlap in the treatment of SCID. The two types of SCID that have been treated by Gene therapy are ADA-SCID, caused by disabling mutations in the Adenosine Deaminase gene, and X-SCID, caused by disabling mutations in the IL-2 receptor gamma chain gene, also called the common gamma chain ( c). ADA or c deficient patients have no T-lymphocytes (the cells that recognize foreign proteins0 and few or dysfunctional B-cells (the cells that make antibodies). SCID patients are therefore unable to mount an immune response to common pathogens, and unless treated usually die early in life from severe infections. The treatment of choice for thee patients is a bone marrow transplant from the patent with the best immunological match. If there is not a matched parent (~25% of the time) or the transplant is unsuccessful (~25% of the time) these patients are candidates for gene therapy. Gutted viruses containing the ADA or c genes are introduced into the patient's bone marrow cells and the treated cells are returned to the patient. In some recent cases of ADA deficient SCID, the infusion was preceded by a mild depletion of the patient's bone marrow cells. In these early studies, it was clearly demonstrated that bone marrow stem cells were marked with the new gene, and that the transferred gene made either ADA or c. In several ADA SCID patients that also received mild bone marrow depletion, enough ADA producing T and B cells emerged that these patients no longer need the supplemental injection of purified ADA enzyme. These results will be discussed at the Advances in Clinical Gene therapy session on Sunday morning. In the XSCID patients, 10/11 children began to produce functional T-cells and developed antibodies when vaccinated against the common childhood diseases. Recently two of these patients have developed a T-cell leukemia that is associated with the insertion of the c gene into a known leukemia gene, resulting in a moratorium on further attempts to perform gene therapy for X-SCID.

Another example of Cell and Gene Therapy overlapping is in the use of T-lymphocytes to treat cancer. Many tumors are recognized as foreign by the patient's T-cells, but these T-cells do not expand their numbers fast enough to kill the tumor. T-cells found in the tumor can be grown outside the body to very high numbers and then infused into the patient, often causing a dramatic reduction in the size of the tumor. This treatment is especially effective for tumors that have spread, as the tumor specific lymphocytes will track them down where ever they are. The addition of gene to the T-cells can allow specific T-cells that may be more effective tumor killers to be selected, and a second gene that can be used to kill the expanded T-cells after they have done their job or if an adverse event develops were among the first human Gene Therapy trails performed.