| Study pins Parkinson's on chemicals Pesticides, herbicides are prime suspects, researchers report By Thomas H. Maugh 11 Los Angeles Times Most cases of Parkinson's disease are caused by exposure to chemicals in the environment, rather than by a defective gene, scientists say. The discovery should provide some comfort to family members of Parkinson's victims who fear for their own health, said the research team from the Parkinson's Institute in Sunnyvale, Calif. The study suggests that research should focus on potential environmental causes, such as pesticides and herbicides. Genetics is a factor, however, in the fewer than 10 percent of patients whose Parkinson's begins under the age of 50. Their disease is caused by a gene that already has been identified. Based on previous studies with small numbers of twins, scientists long have suspected that genetics did not play a major role in the disease, which affects more than I million Americans. The new study of nearly 20,000 white male twins who fought in World War II seems to confirm that suspicion. Dr. Caroline Tanner and her colleagues at the Parkinson's Institute report in today's Journal of the American Medical Association that the disorder usually affected only one member of a twin pair, regardless of whether they were identical or fraternal twins. If the disease were genetic in origin, both members of a pair of identical twins-who share all their genes-would be expected to develop it. This "landmark study ... provides guidance that is extremely important," said Dr. Michael Walker of the National Institute of Neurological Diseases and Stroke. "For patients over the age of 50, it means that we are going to have to look elsewhere for causes." But the study of the younger patients with a familial form of the disease will remain important, added Dr. Neal Hermanowicz, medical director of the movement disorders program at Good Samaritan Hospital in Los Angeles. "Any time you have a gene for a disease, whether it applies to all cases or not, it gives you a huge leg up in understanding the disease process." How the disease works Parkinson's disease results from the death of certain brain cells that secrete dopamine, a chemical messenger used for controlling movements. The major symptoms include tremor, stiffness of muscles and bradykinesia, or slowness of movement. It is most commonly treated with drugs that replace the lost dopamine. Transplants of fetal tissues that secrete dopamine also have been helpful. Tremors sometimes can be controlled by pallidotomies, in which a small section of the brain is destroyed, or by implanting electrodes. But there is no cure, and the disease usually is fatal. An estimated 60,000 people develop Parkinson's each year, and that number is expected to climb as the general population grows older. Where the idea began, The idea that Parkinson's might be caused by chemicals in the environment got a major boost in 1982 when Dr. J. William Langston, now president of the institute and a co-author of the current paper, discovered several young people who developed Parkinson's symptoms literally overnight after using tainted heroin. He found that the symptoms were caused by a contaminant called MPTP, which bears a strong chemical similarity to many pesticides and other environmental chemicals. Two years ago, researchers also discovered that the disease could be caused, at least in some Italian and Greek families, by defects in the gene for a protein called alpha-synuclein. Many authorities who previously had supported an environmental cause flip-flopped back to favoring a genetic origin, Hermanowicz said. To clear up the confusion, Tanner and her colleagues studied 19,842 surviving white male twins who are enrolled in the National Academy of Sciences/National Research Council World War 11 Veterans Twin Registry. They identified 193 individuals who had confirmed cases of Parkinson's disease. Among those who developed the disorder after the age of 50, the likelihood that their twin brother also would have Parkinson's was no greater than the risk for the population at- large. Such results normally mean that there is little or no genetic contribution to the disease. |
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