CHICAGO -- Most cases of Parkinson's disease are not due to a genetic defect but are caused by other factors that are probably environmental, according to a landmark study of more than 17,000 twins.
"For the first time, today we can say that for people with Parkinson's disease diagnosed after age 50, it's most commonly caused by environmental factors," said Dr. Caroline M. Tanner of the Parkinson's Institute in Sunnyvale, Calif., who led the study. It is published in Wednesday's Journal of the American Medical Association.
The environmental factors are unknown but may include chemical exposures, diet and smoking -- the last of which paradoxically seems to lessen the risk of developing Parkinson's, she added in a teleconference call Tuesday.
A genetic cause is most common in the approximately 10 percent of people with the disease who are diagnosed before age 50, she said.
At least 1 million Americans have Parkinson's, Tanner said, making it second only to Alzheimer's disease in frequency as a degenerative disease of the brain and nerves.
Parkinson causes slow deterioration of the nerves' ability to control the muscles. It usually starts with small tremors, then progresses to a shuffling gait and increasing weakness. There is no cure, and drug therapy tends to lose effectiveness over time.
In 1997, researchers identified a long-sought gene defect that can cause a form of Parkinson's, but the proportion of cases arising from the defect was unknown before this study.
In the new study, researchers tracked down more than 17,000 men enrolled in a World War II-era twin registry. The researchers found 161 twin pairs in which at least one brother had Parkinson's disease and data on the pairs was complete.
Among the 161 sets, there were 16 in which Parkinson's had struck before age 50.
Of the four sets of identical twins, who have exactly the same genetic material, both brothers had Parkinson's.
Among the 12 fraternal pairs, who share only half their genetic material, there were only two in which both brothers had Parkinson's.
Researchers calculated that if one twin developed the disease by age 50, the other was six times more likely to get it if he were an identical twin than if he were a fraternal twin.
Tanner said environmental factors most likely to play a role in typical Parkinson's include exposure to chemicals such as pesticides and herbicides, diet and tobacco smoking. The apparent protective effect of smoking was found in the twins and in previous research, Tanner said.
She and Dr. J. William Langston, president of the Parkinson's Institute and senior author, said the protection is probably real, perhaps caused by smoking's stimulation of the liver to produce enzymes that neutralize some toxin that would otherwise provoke Parkinson's.
"But there are about 2,000 chemicals in cigarette smoke, so we still have a big job ahead of us in finding what chemicals might actually be protective," Langston said. "And we don't recommend smoking to prevent Parkinson's disease."
An expert not involved in the study said it presents a compelling reason to search for better treatments, since the drugs now available may lessen symptoms but don't really attack the disease or prevent disability.
And surgery is promising but still largely experimental, noted Dr. Jeffrey L. Cummings of the University of California, Los Angeles. In an editorial accompanying the study, he recommended that the new findings should refocus research on environmental triggers for typical Parkinson's disease and genetic influences in early-onset disease.