Scientists have suggested that people with type A blood groups are at greater risk of having a stroke before they turn 60 than those with other blood types, but say further studies are needed to clarify this.
The most familiar blood types are A and B, which can be present together as AB, individually as A or B, or not present at all, as O.
Yet even within these major blood types, there are subtle variations arising from mutations in the genes responsible, reports Science Alert.
In a study published in 2022, genomics researchers uncovered a clear relationship between the gene for the A1 subgroup and early onset stroke.
Researchers compiled data from 48 genetic studies, which included roughly 17 000 people with a stroke and nearly 600 000 non-stroke controls. All participants were between 18 and 59.
A genome-wide search revealed two locations strongly associated with an earlier risk of stroke. One coincided with the spot where genes for blood type sit.
A second analysis of specific types of blood-type gene then found people whose genome coded for a variation of the A group had a 16% higher chance of a stroke before 60, compared with a population of other blood types.
For those with a gene for group O1, the risk was lower by 12%.
The researchers noted, however, that the additional risk of stroke in people with type A blood 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 from the University of Maryland.
“But it may have something to do with blood-clotting factors like platelets and cells that line the blood vessels as well as other circulating proteins, all of which play a role in the development of blood clots.”
While the study findings may seem alarming – that blood type could change early stroke risk – let’s put these results into context.
Each year in the US, just under 800 000 people experience a stroke. Most of these, around three out of every four, occur in people 65 and older, with risks doubling every decade after the age of 55.
Also, the people included in the study lived in North America, Europe, Japan, Pakistan, and Australia, with people of non-European ancestry only making up 35% of participants. Future studies with a more diverse sample could help clarify the significance of the results.
“We clearly need more follow-up studies to clarify the mechanisms of increased stroke risk,” Kittner said.
Another key finding of the study came from comparing people who had a stroke, before 60, with those who had a stroke after 60.
For this, the researchers used a dataset of about 9 300 people over 60 who had a stroke, and some 25 000 controls over 60 who didn’t have a stroke.
They found that the increased risk of stroke in the type A blood group became insignificant in the late-onset stroke group, suggesting that strokes that happen early in life may have a different mechanism from those that occur later on.
Strokes in younger people are less likely to be caused by a build-up of fatty deposits in the arteries (atherosclerosis) and more likely to be caused by factors to do with clot formation, the authors said.
The study, published in Neurology, also found that people with type B blood were around 11% more likely to have a stroke compared with non-stroke controls, regardless of their age.
Previous studies suggest that the part of the genome that codes for blood type, called the ‘ABO locus’, is associated with coronary artery calcification, which restricts blood flow, and heart attack.
The genetic sequence for A and B blood types have also been associated with a slightly higher risk of blood clots in veins, called venous thrombosis.
Study details
Contribution of Common Genetic Variants to Risk of Early-Onset Ischaemic Stroke
Thomas Jaworek, Huichun Xu, Brady J. Gaynor, John W. Cole, et al.
Published in Neurology on 18 October 2022
Abstract
Background and Objectives
Current genome-wide association studies of ischaemic stroke have focused primarily on late-onset disease. As a complement to these studies, we sought to identify the contribution of common genetic variants to risk of early-onset ischaemic stroke.
Methods
We performed a meta-analysis of genome-wide association studies of early-onset stroke (EOS), ages 18–59 years, using individual-level data or summary statistics in 16,730 cases and 599,237 non-stroke controls obtained across 48 different studies. We further compared effect sizes at associated loci between EOS and late-onset stroke (LOS) and compared polygenic risk scores (PRS) for venous thromboembolism (VTE) between EOS and LOS.
Results
We observed genome-wide significant associations of EOS with 2 variants in ABO, a known stroke locus. These variants tag blood subgroups O1 and A1, and the effect sizes of both variants were significantly larger in EOS compared with LOS. The odds ratio (OR) for rs529565, tagging O1, was 0.88 (95% confidence interval [CI]: 0.85–0.91) in EOS vs 0.96 (95% CI: 0.92–1.00) in LOS, and the OR for rs635634, tagging A1, was 1.16 (1.11–1.21) for EOS vs 1.05 (0.99–1.11) in LOS; p-values for interaction = 0.001 and 0.005, respectively. Using PRSs, we observed that greater genetic risk for VTE, another prothrombotic condition, was more strongly associated with EOS compared with LOS (p = 0.008).
Discussion
The ABO locus, genetically predicted blood group A, and higher genetic propensity for venous thrombosis are more strongly associated with EOS than with LOS, supporting a stronger role of prothrombotic factors in EOS.
See more from MedicalBrief archives:
Blood type could predict stroke risk before age 60 – Maryland study
Non-O blood groups associated with higher heart attack risk
Blood types not linked to Covid-19 incidence and severity — Large US analysis