Single gene mutation muddying Parkinson's risk forecasts
PORTLAND, Ore. – A peculiar form of a gene mutation known to increase a person's risk for Parkinson's disease is puzzling doctors about how to counsel patients who have the anomaly.
A study by researchers at the Oregon Health & Science University School of Medicine's Parkinson Center of Oregon, the University of Washington School of Medicine and the New York State Department of Health, Wadsworth Center, raises concerns about whether patients testing positive for a single mutation of the parkin gene, rather than the two mutations typically required for developing Parkinson's, can be accurately informed about their risks of developing the disease or passing it on to their children.
The study represents "a call for getting more information about the gene," said John "Jay" G. Nutt, M.D., OHSU professor of neurology, and physiology and pharmacology, and Parkinson center director. "What are the clinical implications of finding this gene?"
What's alarmed doctors is that in the clinical setting, the single mutation appears to be common: 18 percent of patients with early-onset Parkinson's disease – those diagnosed before age 40 – tested positive for parkin gene mutations, and of that group, 70 percent had only one mutation.
"It was formerly thought that two mutations caused the disease, that it was recessive, that you had to get one copy of the gene from each parent" to develop the disease, said Nutt, also director of the Parkinson's Disease Research, Education, and Clinical Center (PADRECC) at the Portland Veterans Affairs Medical Center.
These latest findings, however, have baffled Parkinson's disease scientists.
"This raises all sorts of red flags for testing for parkin. How are you going to use this information? How are you going to counsel patients? What does this really mean?" Nutt said.
Whatever the cause, Nutt believes counseling single-mutation patients about their risks of developing Parkinson's disease will be difficult at best.
"You just don't know," Nutt said. "Clearly, if you have two copies, then we think you're at high risk for developing Parkinson's disease. What would be the risk for your child? If your mate doesn't have abnormal genes, you would say your risk is very low." But the study "raises questions about how you would interpret any gene testing," he added. "It makes counseling basically impossible."
Although parkin mutations are the most commonly identified cause of Parkinson's disease, the number of Americans with early-onset Parkinson's disease that has the parkin mutation is unknown. Previous reports established a range of between 16 percent and 49 percent, but the estimates were based on special populations selected for genetic studies, so the rates may have been inflated.
In addition, despite the large percentage of individuals with early-onset Parkinson's who were parkin-positive, none of 96 controls had any mutation. "So exactly how common it is, I think, is still not known," Nutt said.
So far, symptoms of early-onset Parkinson's disease are the same for individuals with or without the parkin mutation. In fact, parkin cases are distinguishable only by molecular testing.
"At this point, there are no clinical implications as far as treatment of the patient," Nutt said.
In the meantime, researchers will continue to seek a better understanding of how the single mutation is inherited and the frequency with which it occurs among carriers. They are working closely with Haydeh Payami, Ph.D., former OHSU professor of genetics and neurology who is helping Nutt track some 700 Parkinson's patients as head of the NeuroGenetics Research Program at the New York State Department of Health's Wadsworth Center.
Nutt's collaborators included: Payami, research scientist, Wadsworth Center, and adjunct professor of neurology, Albany Medical College; Stephen T. Gancher, M.D., adjunct associate professor of neurology, OHSU School of Medicine; Parvoneh Poorkaj Navas, Ph.D., research assistant professor of psychiatry and behavioral sciences, UW; Thomas D. Bird, M.D., professor of neurology, UW; Ellen J. Steinbart, research nurse, Alzheimers Disease Research Center, UW; and Gerard D. Schellenberg, Ph.D., research professor of medicine in the Division of Gerontology and Geriatric Medicine, and adjunct research professor of neurology, UW.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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