No genetic link to late Parkinson's.

A study of World War II veterans indicates that Parkinson's disease that strikes after age 50 doesn't stem from a person's genetic make-up. Some researchers are now turning their attention to potential environmental causes.

Researchers located 161 white men with both a diagnosis of Parkinson's disease and a twin brother who had grown to adulthood. The sample included 71 pairs of identical twins and 90 pairs of fraternal twins. In only 11 identical pairs did both men have the disease, roughly 16 percent. In only 10 pairs of fraternal twins did both brothers have the disease, about 11 percent.

Although these percentages seem to indicate that identical twins are more likely to share the disease, this was true only in men who developed Parkinson's before age 50, says the report in the Jan. 27 Journal of the American Medical Association.

Among men diagnosed with Parkinson's after age 50, the incidence of both twins having the disease was the same for the two groups, about 11 percent, says coauthor Caroline M. Tanner, a neurologist at the Parkinson's Institute in Sunnyvale, Calif.

Among the 12 sets of fraternal twins in which at least one brother had Parkinson's before age 50, in only two cases did both twins have the disease. In contrast, in the four sets of identical twins where at least one was affected by early Parkinson's, all eight men had the disease. Tanner and her colleagues warn that findings from such a small sample need to be validated in larger studies.

The study "suggests that research is best focused on environmental causes for typical Parkinson's disease," says Jeffrey L. Cummings of the University of California, Los Angeles in an accompanying editorial. Tanner plans to probe the men's habits, occupations, diet, and possibly pesticide exposure.

Roughly 1 in 10 cases of Parkinson's occurs before age 50. This study's findings indicate that researchers should seek a genetic cause for early-onset Parkinson's, Cummings says.

The researchers identified the men by combing through records of 19,842 war veterans born between 1917 and 1927. The men were between 64 and 73 when the data were tabulated.

Parkinson's affects at least 1 million people in the United States. Symptoms include muscle rigidity, tremors, slowness of movement, poor balance, and walking problems. The average age of onset is about 60.