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Nancy L. Pedersen, Ph.D. Background: The cause of Parkinson's disease is considered to involve both genetic and environmental factors as well as interactions between these-the so called, gene-environment interaction. Mutations in several genes have been linked to Parkinson's disease. Although they are important for understanding the mechanism and process of the disease, they do not appear to explain most late-onset Parkinson's cases. To evaluate the role of genetics in Parkinson's disease, these investigators performed an epidemiologic study in which they screened over 14,000 pairs of twins from Sweden for Parkinson's disease in telephone interviews. Seven hundred and sixty-four individuals were identified as either having Parkinson's or parkinsonian symptoms, or use of antiparkinsonian medication. Advance: Of the 382 pairs of twins, 7 pairs were found in which both had evidence of Parkinson's disease. These data agree with other twin studies that determined that "genetic effects are of little importance in Parkinson's disease." The results of almost no genetic influence for Parkinson's are quite notable in this study, given the considerably higher heritability estimates reported for other disorders such as Alzheimer's disease, asthma, and Type I diabetes. Implication: One explanation of the lack of genetic influence on Parkinson's disease in light of the number of genes that have been linked to the disease is the gene-environment interaction. In other words, specific genes may increase the risk of Parkinson's disease only in individuals exposed to a certain risk factor. This study's results suggest that if genetic effects are important, they are only expressed once sufficient environmental factors are present. As Dr. Ken Olden, Director, NIEHS has said, "Genes load the gun. The environment pulls the trigger." Citation: Wirdefeldt K, Gatz M, Schalling M, Pedersen NL. No evidence for heritability of Parkinson disease in Swedish twins. Neurology. 2004 Jul 27;63(2):305-11. |
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