Conversely, results from a meta-analysis of nearly 17,000 cases of ischemic stroke in adults younger than 60 years showed that having type O blood reduced the risk for EOS by 12%.
In addition, the associations with risk were significantly stronger in EOS than in those with late-onset stroke (LOS), pointing to a stronger role for prothrombotic factors in younger patients, the researchers noted.
“What this is telling us is that maybe what makes you susceptible to stroke as a young adult is the blood type, which is really giving you a much higher risk of clotting and stroke compared to later onset,” coinvestigator Braxton Mitchell, PhD, professor of medicine and epidemiology and public health at the University of Maryland, Baltimore, said in an interview.
The findings were published online in Neurology.
The genome-wide association study (GWAS) was done as part of the Genetics of Early Onset Ischemic Stroke Consortium, a collaboration of 48 different studies across North America, Europe, Japan, Pakistan, and Australia. It assessed early onset ischemic stroke in patients aged 18-59 years.
Researchers included data from 16,927 patients with stroke. Of these, 5,825 had a stroke before age 60, defined as early onset. GWAS results were also examined for nearly 600,000 individuals without stroke.
Results showed two genetic variants tied to blood types A and O emerged as highly associated with risk for early stroke.
Researchers found that the protective effects of type O were significantly stronger with EOS vs. LOS (odds ratio [OR], 0.88 vs. 0.96, respectively; P = .001). Likewise, the association between type A and increased EOS risk was significantly stronger than that found in LOS (OR, 1.16 vs. 1.05; P = .005).
Using polygenic risk scores, the investigators also found that the greater genetic risk for venous thromboembolism, another prothrombotic condition, was more strongly associated with EOS compared with LOS (P = .008).
Previous studies have shown a link between stroke risk and variants of the ABO gene, which determines blood type. The new analysis suggests that type A and O gene variants represent nearly all of those genetically linked with early stroke, the researchers noted.
While the findings point to blood type as a risk factor for stroke in younger people, Dr. Mitchell cautions that “at the moment, blood group does not have implications for preventive care.”
“The risk of stroke due to blood type is smaller than other risk factors that we know about, like smoking and hypertension,” he said. “I would be much more worried about these other risk factors, especially because those may be modifiable.”
He noted the next step in the study is to assess how blood type interacts with other known risk factors to raise stroke risk.
“There may be a subset of people where, if you have blood type A and you have some of these other risk factors, it’s possible that you may be at particularly high risk,” Dr. Mitchell said.
More research needed on younger patients
In an accompanying editorial, Jennifer Juhl Majersik, MD, associate professor of neurology at the University of Utah, Salt Lake City, and Paul Lacaze, PhD, associate professor and head of the public health genomics program at Monash University, Australia, noted that the study fills a gap in stroke research, which often focuses mostly on older individuals.