Gene therapy is a recombinant DNA process in which cells are taken from the patient, altered by adding genes, and replaced in the patient. The genes then provide the genetic codes for proteins the patient is lacking. Nonreproductive cells are used in gene therapy, so there is no carryover of inserted genes to the next generation.
In the early 1990s, gene therapy was used to correct a deficiency of the enzyme adenosine deaminase (ADA). Blood cells called lymphocytes were removed from the bone marrow of two children, then genes for ADA production were inserted into the cells using viruses as vectors. Finally, the cells were reinfused in the bodies of the children. Once established in the bodies, the gene-altered cells began synthesizing the enzyme ADA. Thus, the deficiency was removed and the disease resolved.
Approximately 2,000 single-gene defects are believed to exist, and patients with these defects may be candidates for gene therapy. A multilayered review system now exists to ensure the safety of gene therapy proposals. Many aspects must be considered before approval is granted for gene therapy experiments.