Gene Therapy

GENE THERAPY

The goal of gene therapy is to treat disease through insertion of the normal, cloned DNA for a gene into the somatic cells of a patient who has a defect in that gene as a result of a disease-causing mutation. Because somatic gene therapy changes only the targeted somatic cells, the change is not passed on to the next generation. [Note: In germline gene therapy, it is the germ cells that are modified, and so the change is passed on. A long-standing moratorium on germline gene therapy is in effect world-wide.] There are two types of gene transfer: 1) ex vivo, in which cells from the patient are removed, transduced, and returned; and 2) in vivo, in which the cells are directly transduced. Both types require use of a vector (viral or nonviral) to deliver the DNA into the target cell. Challenges of gene therapy include development of vectors, achievement of long-lived expression, and prevention of side effects such as an immune response. The first successful gene therapy (1990) involved two patients with severe combined immunodeficiency disease (SCID) caused by mutations to the gene for adenosine deaminase. It utilized mature T lymphocytes transduced ex vivo with a viral vector (Figure 33.25). Since 1990, only a small number of patients (with a variety of disorders, such as hemophilia, cancers, and certain types of blindness) have been treated with gene therapy, with varying degrees of success.

Figure 33.25 Gene therapy for SCID caused by adenosine deaminase deficiency. [Note: Bone marrow stem cells and a modified retroviral vector are now used.]