Most cases of Parkinson's disease are not caused by a defective gene, but rather by exposure to as yet unknown chemicals in the environment, California scientists reported today.
The discovery should provide some comfort to family members of Parkinson's victims who fear for their own future health, said the research team from the Parkinson's Institute in Sunnyvale. The study also suggests that research should focus on potential environmental causes, such as pesticides and herbicides, they added.
Genetics is a factor, however, in the relatively small number of patients--less than 10%--whose familial Parkinson's begins under the age of 50. Their disease is caused by a gene that has been identified.
Based on previous studies with small numbers of twins, scientists have long suspected that genetics did not play an important role in the disease, which affects more than a million Americans. The new study of nearly 20,000 white male twins who fought in World War II seems to confirm that definitively.
Dr. Caroline M. Tanner and her colleagues at the Parkinson's Institute report in today's Journal of the American Medical Assn. that the disorder most commonly affected only one member of a twin pair, whether the pair consisted of 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 D. 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, said 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."
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.
