GENE THERAPY

Gene therapy involves supplying a functional gene to cells lacking that function, with the aim of correcting a genetic disorder or acquired disease. Gene therapy can be broadly divided into two categories. The first is alteration of germ cells, that is, sperm or eggs, which results in a permanent genetic change for the whole organism and subsequent generations. This “germ line gene therapy” is considered by many to be unethical in human beings. The second type of gene therapy, “somatic cell gene therapy”, is analogous to an organ transplant. In this case, one or more specific tissues are targeted by direct treatment or by removal of the tissue, addition of the therapeutic gene or genes in the laboratory, and return of the treated cells to the patient. Clinical trials of somatic cell gene therapy began in the late 1990s, mostly for the treatment of cancers and blood, liver, and lung disorders.

The history of human gene therapy is, however, not a particularly happy one. The effect of introducing a gene into cells rarely promotes more than small transient relief from the symptoms of the disease being treated. Worse still, there have been highly publicized cases where gene therapy trial patients have suffered as a consequence of the treatment itself. For example, in 1999 an 18-year-old gene therapy trial volunteer from Philadelphia died following a gene therapy trial. In addition, one of the few success stories of human gene therapy—the treatment of severe combined immune deficiency, X-SCID—has turned out to have unforeseen consequences. Bone marrow cells were taken from patients suffering from this disease and treated with a virus to introduce a functional copy of the defective gene. When the modified bone marrow cells were returned to patients, their immune systems were functional once more. However, some patients treated this way subsequently developed leukaemia, which most likely arises as a result of random insertion of a section of DNA into the human genome with the consequent disruption of nearby gene function.