Most cases of Parkinson's disease not inherited, study says

SUNNYVALE, California (CNN) -- A new study suggests most cases of Parkinson's disease are not inherited, therefore suggesting environmental causes.

The study followed almost 20,000 white male twins enrolled in the National Academy of Sciences/National Research Council World War II Veteran Twins Registry.

Researchers found no evidence to suggest the disease is genetically linked in most cases.

However, genetics do appear to be important when the disease begins before age 50. Only about 15 percent of all Parkinson's cases are in younger people.

Researchers say the study addresses a 150-year-old debate regarding whether Parkinson's disease is inherited.

"This is a very important study, which has the power to answer the question of heredity in Parkinson's disease," said Dr. Jorges Juncos of Emory University School of Medicine. "Therefore, this study suggests environment plays a major role in the development of Parkinson's."

The study is published in this week's Journal of the American Medical Association.

Researchers say this study will likely open the door to looking for environmental causes. Many environmental influences have been studied, though none have been implicated. Past studies have focused on pesticides, herbicides and some illicit drugs.

"Following the recent identification of a gene related to Parkinson's disease prompted enthusiasm into genetic research," Juncos said. "Despite this latest finding, genetics research will continue because it may help us understand the physiology of the disease, which could lead to better treatments."

Almost one million Americans have Parkinson's disease, a degenerative, neurological disease. The disease is characterized by tremor, rigidity and erratic body movements.

Treatment includes various drugs as well as brain surgeries that target the part of the brain causes the abnormal movements.