11. Use of Genetically Modified Stem Cells in Experimental …

To date, only nonembryonic human stem cells have been used in cell-based gene therapy studies. The inherent limitations of these stem cells, as discussed below, have prompted scientists to ponder and explore whether human embryonic stem cells might overcome the current barriers to the clinical success of cell-based gene therapies.

Gene therapy is a relatively recent, and still highly experimental, approach to treating human disease. While traditional drug therapies involve the administration of chemicals that have been manufactured outside the body, gene therapy takes a very different approach: directing a patient's own cells to produce and deliver a therapeutic agent. The instructions for this are contained in the therapeutic transgene (the new genetic material introduced into the patient). Gene therapy uses genetic engineeringthe introduction or elimination of specific genes by using molecular biology techniques to physically manipulate genetic materialto alter or supplement the function of an abnormal gene by providing a copy of a normal gene, to directly repair such a gene, or to provide a gene that adds new functions or regulates the activity of other genes.

Clinical efforts to apply genetic engineering technology to the treatment of human diseases date to 1989. Initially, gene therapy clinical trials focused on cancer, infectious diseases, or disorders in which only a single gene is abnormal, such as cystic fibrosis. Increasingly however, efforts are being directed toward complex, chronic diseases that involve more than one gene. Prominent examples include heart disease, inadequate blood flow to the limbs, arthritis, and Alzheimer's disease.

The potential success of gene therapy technology depends not only on the delivery of the therapeutic transgene into the appropriate human target cells, but also on the ability of the gene to function properly in the cell. Both requirements pose considerable technical challenges.

Gene therapy researchers have employed two major strategies for delivering therapeutic transgenes into human recipients (see Figure 11.1. Strategies for Delivering Therapeutic Transgenes into Patients). The first is to "directly" infuse the gene into a person. Viruses that have been altered to prevent them from causing disease are often used as the vehicle for delivering the gene into certain human cell types, in much the same way as ordinary viruses infect cells. This delivery method is fairly imprecise and limited to the specific types of human cells that the viral vehicle can infect. For example, some viruses commonly used as gene-delivery vehicles can only infect cells that are actively dividing. This limits their usefulness in treating diseases of the heart or brain, because these organs are largely composed of nondividing cells. Nonviral vehicles for directly delivering genes into cells are also being explored, including the use of plain DNA and DNA wrapped in a coat of fatty molecules known as liposomes.

Figure 11.1. Strategies for Delivering Therapeutic Transgenes into Patients.

( 2001 Terese Winslow)

The second strategy involves the use of living cells to deliver therapeutic transgenes into the body. In this method, the delivery cellsoften a type of stem cell, a lymphocyte, or a fibroblastare removed from the body, and the therapeutic transgene is introduced into them via the same vehicles used in the previously described direct-gene-transfer method. While still in the laboratory, the genetically modified cells are tested and then allowed to grow and multiply and, finally, are infused back into the patient.

Gene therapy using genetically modified cells offers several unique advantages over direct gene transfer into the body and over cell therapy, which involves administration of cells that have not been genetically modified. First, the addition of the therapeutic transgene to the delivery cells takes place outside the patient, which allows researchers an important measure of control because they can select and work only with those cells that both contain the transgene and produce the therapeutic agent in sufficient quantity. Second, investigators can genetically engineer, or "program," the cells' level and rate of production of the therapeutic agent. Cells can be programmed to steadily churn out a given amount of the therapeutic product. In some cases, it is desirable to program the cells to make large amounts of the therapeutic agent so that the chances that sufficient quantities are secreted and reach the diseased tissue in the patient are high. In other cases, it may be desirable to program the cells to produce the therapeutic agent in a regulated fashion. In this case, the therapeutic transgene would be active only in response to certain signals, such as drugs administered to the patient to turn the therapeutic transgene on and off.

To date, about 40 percent of the more than 450 gene therapy clinical trials conducted in the United States have been cell-based. Of these, approximately 30 percent have used human stem cellsspecifically, blood-forming, or hematopoietic, stem cellsas the means for delivering transgenes into patients.

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