FIGURE 15–16 How Gene Therapy Can Be Used Gene therapy uses normal genes to add to or replace defective genes or to boost a normal function like immunity. Interpret Visuals How is the virus in this diagram being used?

If an individual is suffering from a missing or defective gene, can we replace that gene with a healthy one and fix the problem? The experimental field of gene therapy is attempting to answer that question. Gene therapy is the process of changing a gene to treat a medical disease or disorder. In gene therapy, an absent or faulty gene is replaced by a normal, working gene. This process allows the body to make the protein or enzyme it needs, which eliminates the cause of the disorder.

The idea of using gene therapy to cure disease arose from the major advances in molecular biology made in the past 20 years, including the Human Genome Project. Figure 15–16 shows one of the ways in which researchers have attempted to carry out gene therapy. To deliver the correct, or therapeutic, gene to the affected, or target, cells, researchers first engineer a virus that cannot reproduce or cause harmful effects. They place DNA containing the therapeutic gene into the modified virus, and then they infect the patient's cells with it. In theory the virus will insert the healthy gene into the target cell and correct the defect. The challenge, however, is to deliver a gene that works correctly over the long term. For all the promise it holds, in most cases gene therapy remains a high-risk experimental procedure. For gene therapy to become an accepted treatment, we need more reliable ways to insert working genes and to ensure that the DNA used in the therapy does no harm.

Genetic Testing If two prospective parents suspect they are carrying the alleles for a genetic disorder such as cystic fibrosis (CF), how could they find out for sure? Because the CF allele has slightly different DNA sequences from its normal counterpart, genetic tests using labeled DNA probes can distinguish it. Like many genetic tests, the CF test uses specific DNA sequences that detect the complementary base sequences found in the disease-causing alleles. Other genetic tests search for changes in cutting sites of restriction enzymes. Some use PCR to detect differences between the lengths of normal and abnormal alleles. Genetic tests are now available for diagnosing hundreds of disorders.

FIGURE 15–17 A Brave Volunteer Gene therapy can be risky. In 1999, 18-year-old Jesse Gelsinger volunteered for a gene therapy experiment designed to treat a genetic disorder of his liver. He suffered a massive reaction from the viruses used to carry genes into his liver cells, and he died a few days later. Jesse's case makes clear that experiments with gene therapy must be done with great caution.

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