Researchers have long debated whether genetics, the environment or a combination play a role in both SIDS and Parkinson's. The studies two on Parkinson's and one on SIDS support theories that genes are a culprit in at least some cases.
The studies appear in Wednesday's Journal of the American Medical Association.
The Parkinson's studies, from Duke University, offer evidence of a genetic link to late-onset Parkinson's disease cases that develop after age 50 and the most common form of the nervous-system disorder. Most previous genetic research has suggested genes play a role in the much rarer cases that begin earlier in adulthood.
The SIDS study, led by researchers from Mayo Clinic and Baylor College of Medicine, found evidence that a gene defect linked to a heart condition known as long QT syndrome may be involved in about 2 percent of U.S. cases, or at least 50 of the nearly 3,000 SIDS deaths nationwide each year.
That's fewer than suggested in an attention-grabbing Italian study published in 1998 that linked long QT syndrome with SIDS. But the earlier study was disputed by many SIDS experts and, unlike the new research, did not involve molecular evidence, said Dr. Henry Krous, a San Diego pathologist and prominent SIDS researcher.
Parkinson's, which results from nerve-cell damage in the brain, causes muscle tremors and stiffness and affects more than 1 million Americans. Some have suggested that environmental factors may trigger the damage, including infections and certain chemicals and pesticides.
The new research focuses on genes that also may be important, said Duke scientist Dr. Jeffery Vance, who was involved in both studies.
One found evidence that three genetic variations in the tau gene could make some people prone to developing late-onset Parkinson's. Tau is a protein that helps maintain brain cell structure and defects have been linked to Alzheimer's and some rare Parkinson-like ailments.
The scientists examined blood samples from 1,056 people from families with at least one case of the disease.
The other Duke study, involving 174 families with several afflicted members, suggests that many genes may be linked to late-onset Parkinson's. It also bolsters earlier suggestions that a gene is linked to early-onset cases.
Dr. J. William Langston, scientific director of the Parkinson's Institute in Sunnyvale, Calif., called the research "very solid" but said it's unclear if the conclusions would apply to the 80 percent of Parkinson patients without a family history of the disease.
Dr. Robert Nussbaum, a National Institutes of Health geneticist, said the studies "represent important contributions to the overall goal of understanding Parkinson's disease."
"It is long, long overduthat we stop arguing about genetic versus environment. The disease is almost certainly the result of both kinds of factors," Nussbaum said.
SIDS, the unexplained death of an otherwise healthy baby, has been linked to stomach-sleeping. The incidence has dropped dramatically since a mid-1990s campaign urging parents to put their infants to sleep on their backs.
Some researchers think subtle brain-stem abnormalities may make some babies more susceptible to stomach-sleeping risks. Whether long QT syndrome does, too, is uncertain, Krous said.
Long QT syndrome can cause abnormal heart rhythms and sudden cardiac arrest, and has previously been linked to a gene called SCN5A. It can be treated with beta blocker drugs.
In the SIDS study, researchers examined heart tissue collected from autopsies of 93 SIDS babies in Arkansas. Two were found to have mutations in the SCN5A gene, which regulates activity of electrically charged molecules in the heart that are involved in prompting a normal heartbeat.
The study "does not establish if long QT unequivocally caused the babies' deaths," but it is powerful evidence that should prompt further research, Krous said.
While the Italian study prompted routine EKGs for all Italian newborns, a single EKG may not detect long QT syndrome, said study co-author Dr. Jeffrey Towbin of Baylor.
He said it makes more sense to routinely ask pregnant women about long QT syndrome and to monitor babies of those with a strong family history.
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