What is gene therapy?
Genes, part of the DNA in cells, can be thought of as "master switches" that control chemical processes in the body. The idea behind gene therapy is to give cells a gene which functions in a normal way in order to override or replace a gene that is functioning abnormally.

Viruses are very good at getting into cells, so scientists take viruses, remove the disease parts, and use them to carry a new gene part into cells.

A serious problem discovered
Researchers had used gene therapy to create a cure for X-linked severe combined immunodeficiency (SCID), a rare disorder in which the immune system, which fights disease, is severely defective. The added gene stimulated growth of missing immune cells.

Unfortunately, the added gene seems to have caused a leukemia-like cancer to develop in two of the children who had their SCID treated this way. Both children have had their cancer successfully treated with chemotherapy, but what will happen to them in the future is not known.

FDA puts studies on hold
The U.S. Food and Drug Administration (FDA) on January 14, 2003, placed on "clinical hold" all active gene therapy studies (no matter what disorder is being studied) using the same kind of virus (a retrovirus) and treatment method as the SCID gene therapy. The 27 studies affected by this FDA action are unable to enroll new patients.

Virus used may be the culprit
Some researchers suspect that the virus carrying the new gene landed next to a gene for leukemia and made it function abnormally. If this is true, then other therapies using different genes may not carry the cancer risk that SCID therapy seems to cause.

Gene therapy too promising to abandon
To date, the FDA has found no evidence of leukemia caused by gene therapy in other studies. The FDA has stated that some of the studies, such as those trying to treat fatal or life-threatening disorders that have no other treatments, could be restarted after being evaluated, with patients in the studies being informed about the two children in the SCID study who developed cancer. The FDA would like to restart the SCID gene therapy as well, since it offers the only real hope for cure for children with the disorder.

The FDA plans to hold advisory committee meetings in 2003 to review the cancer cases in the SCID study and to study retrovirus use in gene therapy.

Information for this article was taken from:
- U.S. Food and Drug Administration. "FDA places temporary halt on gene therapy trials using retroviral vectors in blood stem cells." Press release, January 14, 2003.
- CNN.com. "Gene therapy studies on 'bubble boy disease' halted." January 15, 2003.
- Boyce, N. "The cost of a cure." U.S. News & World Report, January 27/February 3, 2003, p. 43.

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