There are many conditions that must be met in order to allow gene therapy to be possible. First, the details of the disease process must be understood. Of course, scientists must know exactly what gene is defective, but also when and at what level that gene would normally be expressed, how it functions, and what the regenerative possibilities are for the affected tissue. Not all diseases can be treated by gene therapy. It must be clear that replacement of the defective gene would benefit the patient. For example, a mutation that leads to a birth defect might be impossible to treat, because irreversible damage will have already occurred by the time the patient is identified. Similarly, diseases that cause death of brain cells are not well suited to gene therapy: Although gene therapy might be able to halt further progression of disease, existing damage cannot be reversed because brain cells cannot regenerate. Additionally, the cells to which DNA needs to be delivered must be accessible. Finally, great caution is warranted as gene therapy is pursued, as the body's response to high doses of viral vectors can be unpredictable. On September 12, 1999, Jesse Gelsinger, an eighteen-year-old participant in a clinical trial in Philadelphia, became unexpectedly ill and died from side effects of liver administration of adenovirus. This tragedy illustrates the importance of careful attention to safety regulations and extensive experiments in animal model systems before moving to human clinical trials.
Muscular Dystrophies. Duchenne and other recessive muscular dystrophies are well suited in many ways for gene therapy. These are loss-of-function recessive genetic diseases caused by mutations in the dystrophin gene or in genes for other structural muscle proteins. The normal levels of these pro-
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