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Several months ago, 11-year-old Hillary DeSmarais of Fort Worth, Texas, attended a party in her honor. People brought presents, laughed, and gave the healthy girl hugs. It wasn't her birthday; the party was at a hospital. At the age of six, Hillary had been diagnosed with acute lymphoblastic leukemia.
Hillary could easily have lost her battle with cancer. Not because she didn't get good care: Her doctors acted quickly, putting her on a cocktail of powerful chemotherapy drugs. This treatment helps cure some 80 percent of leukemia patients. But one of the drugs, mercaptopurine, can produce toxic effects.
After a few weeks of treatment, Hillary's white blood cell counts had dropped so low that she had virtually no immune system. Her doctors had to stop treatment to let her immunity recover, and so began a terrible dance for the little girl: Hillary would be on the drugs for two to three weeks, and her white blood cell count would plummet. Then she'd go off for a week to give her immunity a chance to return. "It was scary," says her mother, Araceli, a pediatric nurse. "Every time she went off treatment, she'd get leg pain. I kept thinking it was cancer in her bone marrow."
Three frightening months later, Hillary finally caught a break. Her doctor called William Evans, research director at St. Jude Children's Hospital in Memphis. Evans had helped uncover a genetic breakthrough: One in ten Americans have a bad version of a gene that produces the enzyme called TPMT. This enzyme helps break down certain drugs, including mercaptopurine. People with bum copies of the gene have virtually no TPMT. "When they take mercaptopurine," explains Evans, "it's as if you're giving them up to ten times too much."
Sure enough, a blood test showed that Hillary had a bad copy of the gene. Her dosage was slashed, and she remained on finely tuned doses of mercaptopurine for two and a half years. Last December at her party, Hillary was celebrating five years of being cancer-free -- the official definition of a survivor.
Hillary DeSmarais is among a growing number of people who are reaping the benefits of pharmacogenomics, a new field of personalized medicine. Because of their genes, people like Hillary are uniquely sensitive to certain drugs; for others, those same drugs have little effect. The promise of personalized medicine is that by testing a patient's DNA, doctors will be able to identify these genetic variations. "You can lower the dose of the drug, you can raise it or you can avoid it altogether," says Richard Weinshilboum, a professor of molecular pharmacology at the Mayo Clinic and a leading pharmacogenomic researcher. "This is one of the first places where the genomic revolution is having an impact on medical practice."
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