Before the doubts, the promises. The more that is understood of the causes of illness, the greater the potential for prevention. The decline in cigarette smoking in Western countries, for example, will eventually reduce the incidence of cancer in the developed world. Conversely, tobacco consumption in developing countries is increasing by some 2 per cent a year, so they can expect to see a corresponding increase in the disease. Preventive knowledge is effective only if acted on, and there is still little grasp of what makes people act as they do and how they can be persuaded to do otherwise. Any form of prevention that depends on behavioral change will, most likely, continue to be overshadowed in science and technology that are now shaping the future of medicine. Nowhere is this more evident than in new approaches to inherited disease.
Many illnesses are caused by a defect in a single gene: a single segment of the hereditary material or DNA. Genetic techniques are already used to diagnose such disorders. If undamaged copies of the defective gene could be introduces into the patient's body, the activity controlled by that gene would be restored to normal. This is the basis of gene therapy, itself now on the verge of entering routine practice.
In cystic fibrosis, for example, a defective gene causes patients to produce copious quantities of an abnormal mucus that makes the airways susceptible to various lethal respiratory diseases. In principle, the gene therapist might remove some cells from the lungs and breathing passages, insert normal genes into them, and then return them to the body. Such an approach is feasible for blood of bone marrow cells, for the airways, it is impracticable. An alternative strategy is to repair the defect in situ.
This has already been done by exploiting a virus that colonizes the cells of the airways. Suitably doctored to render it harmless, the virus acts as a carrier for a normal version of the abnormal gene. Getting viruses into the lungs presents no problem; an inhaled aerosol will take them where they are needed. The beneficial effects of the new gene should last as long as the viral colonies continue to thrive -- and in the handful of patients on whom this experiment has been tried, this is what appears to happen. An alternative way of introducing the genes is to wrap them in fatty envelopes, call liposomes, and blow them in an aerosol into the nose. The long-term benefits of both these strategies remain to be seen.
In some cases gene therapy could turn out to be far simpler, and require nothing more complicated than an injection of the normal of missing gene directly into the tissues in which it is required. This might even apply to Duchenne muscular dystrophy, an incurable disease in which certain muscles become progressively more wasted and weaker. Muscle cells are able to absorb genetic material. If DNA containing the gene required to overcome the disease is introduced into the affected muscles, they may take it up and start to manufacture the missing protein.
Other inherited diseases -- of which there are several thousands with a familiarity ranging from sickle-cell anemia and muscular dystrophy to Tay-Sachs and Lesch-Nyan -- will demand other strategies. Some are the consequence of single gene defects, whereas others are result of several such errors. Many hitherto incurable or even untreatable illnesses will soon be candidates for gene therapy. The technique is not limited to inherited disorders. The body makes many natural products able to fight disease: substances such as interferon and interleukin. Gene therapy could be used to increase the output of these substances, or even persuade cells that do not normally make them to do so.
|A Sickle-shaped Red Blood Cell||Red Blood Cell|
The hereditary material of each human probably comprises some 100,000 genes, only a tiny proportion of which have been identified. But if the ambitions of a scheme called the Human Genome Project are realized, scientists will have mapped the lot. It will then be possible to identify the defect underlying every inherited disorder and, given sufficient ingenuity and resources, correct it.