People with one of the A blood types are more likely to have a stroke before age 60 than people with other blood types, researchers have found.
Blood types describe the rich variety of chemicals displayed on the surface of our red blood cells. Among the most familiar are those called A and B, which can be present together as AB, separately as A or B, or not at all as O.
Even within these basic blood types, there are subtle variations resulting from mutations in the genes responsible.
Genomic studies have now revealed a clear link between the A1 subgroup gene and early-onset stroke.
The researchers pooled data from 48 genetic studies that included approximately 17,000 people with stroke and nearly 600,000 controls without stroke. All participants were between the ages of 18 and 59.
A genome-wide search revealed two sites strongly associated with earlier stroke risk. One matched where the blood type genes are located.
A second analysis of specific blood type gene types then found that people whose genome coded for the type A variation had a 16% higher chance of having a stroke before age 60 compared to a population of other blood types.
For those with the O1 group gene, the risk was 12 percent lower.
However, the researchers note that the additional risk of stroke in people with blood type A is small, so there is no need for extra vigilance or screening in this group.
“We still don’t know why blood type A would confer a higher risk,” said senior author and vascular neurologist Steven Kittner of the University of Maryland.
“But it probably has something to do with clotting factors like platelets and cells that line blood vessels, as well as other circulating proteins, all of which play a role in the development of blood clots.”
While the study results may seem alarming that blood type can alter early stroke risk, let’s put these results into context.
Each year in the US, just under 800,000 people have a stroke. Most of these events – about three in four – occur in people aged 65 and over, with risks doubling every decade after age 55.
In addition, the people included in the study lived in North America, Europe, Japan, Pakistan and Australia, with people of non-European descent making up only 35 percent of the participants. Future studies with a more diverse sample could help clarify the significance of the results.
“We clearly need more follow-up studies to elucidate the mechanisms of increased stroke risk,” says Kittner.
Another key finding from the study came from comparing people who had a stroke before age 60 with those who had a stroke after age 60.
To do this, the researchers used a data set of about 9,300 people over the age of 60 who had had a stroke and about 25,000 control groups over the age of 60 who had not had a stroke.
They found that the increased risk of stroke in type A blood became insignificant in the late stroke group, suggesting that strokes that occur at an early age may have a different mechanism compared to those that occur later .
Strokes in younger people are less likely to be caused by a buildup of fatty deposits in the arteries (a process called atherosclerosis) and more likely to be caused by factors related to clot formation, the authors said.
The study also found that people with type B blood were about 11 percent more likely to have a stroke compared to stroke-free controls, regardless of their age.
Previous studies have suggested that the part of the genome that codes for blood type, called the “ABO locus,” is linked to coronary artery calcification that restricts blood flow and heart attacks.
The genetic sequence for blood types A and B is also associated with a slightly higher risk of blood clots in the veins, called venous thrombosis.
This article was published in Neurology.