Susceptibility to any particular type of ischemic stroke may not be genetically determined

Sunday, February 08, 2004

Midway through the first study to screen the entire human genome for genes that increase the risk of ischemic stroke, researchers have found that susceptibility to any particular type of ischemic stroke is not likely to be genetically determined.

A stroke, or "brain attack," happens when blood flow to the brain is interrupted. Doctors have long classified stroke into two main types, hemorrhagic or ischemic, and into a number of subtypes based on what they believe caused the stroke. Hemorrhagic strokes are caused by a burst blood vessel in the brain. Ischemic strokes are caused by blood clots that form and obstruct a blood vessel. Ischemic strokes are subtyped according to where the blood clot forms and where it causes obstruction.

The multicenter, North American "Siblings With Ischemic Stroke Study," or SWISS, was designed to identify genes that increase the risk of ischemic stroke. The DNA of pairs of siblings who've each suffered an ischemic stroke is being screened for risk genes. The hypothesis was that these genes would predispose to specific subtypes. It followed that most people in the study would have had the same type of stroke that their sibling had.

"We came up with a surprising result," says Mayo Clinic neurologist Dr. James Meschia, who is the principal investigator for SWISS. "In the first 100 sibling pairs, we did not find an association where the stroke subtype in one sibling influenced the subtype in another sibling. What does this mean? The simplest interpretation is that genetic risk factors for ischemic stroke likely impart a risk for all types of ischemic stroke and not just one."

Meschia suggests that certain risk genes may predispose to conditions that influence blood-vessel constriction and blood-clot formation, which underlie the development of all types of ischemic stroke.

Meschia offers another possible explanation for not finding a genetic susceptibility to a particular subtype of ischemic stroke. Classifying by subtype may not be very accurate. He says, for example, patients who've had a stroke can have carotid artery stenosis and atrial fibrillation. Since both of these conditions can play a role in the process of having a stroke, it's difficult to say which condition might have caused it.

Meschia presented this interim data from SWISS at the American Stroke Association's 29th International Stroke Conference Feb. 5-7 in San Diego. SWISS is sponsored by the National Institute of Neurological Disorders and Stroke.

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