Dec. 22 (UPI) -- Head injury and post-traumatic stress disorder, or PTSD, may increase the risk of developing Alzheimer's disease and related dementias among people who carry a certain genetic variant.
That's according to a large-scale study of U.S. military veterans published Thursday in Alzheimer's & Dementia, the journal of the Alzheimer's Association.
According to the study, PTSD, traumatic brain injury, and a genetic risk factor called apolipoprotein E (APOE) epsilon 4 allele all showed strong associations with Alzheimer's disease and related dementias.
And the prevalence of Alzheimer's associated with both disorders increased with the number of inherited APOE epsilon 4 alleles.
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The research was led by Mark W. Logue, a statistician in the National Center for PTSD at the VA Boston Healthcare System.
Logue, who is an Army veteran and associate professor at Boston University, said in a news release that he was most surprised to see such clear evidence of a link between PTSD and head trauma on dementia risk.
"I've worked in Alzheimer's disease genetics for over a decade now, and I was used to seeing a clear impact of APOE ε4 on Alzheimer's risk," Logue said in the release.
"However, in this cohort, the effects of PTSD and head injury were just as clear and looked similar to the effect of inheriting ε4 from one of your parents."
Researchers said their findings illustrate the importance of including a person's PTSD and traumatic brain injury history when interpreting the results of genetic testing for dementias -- to end up with more accurate risk assessment for the complex, progressive neurological disorder, especially among veterans.
For their work, the researchers analyzed data from the VA's Million Veteran Program, among the world's largest databases of health and genetic information.
The scientists noted that late-onset Alzheimer's disease is the most common form of dementia and affects an estimated 10% of the U.S. population. And its estimated prevalence increases with age -- from 3% between ages 65 and 74 to 32% for people ages 85 or older.
They said the Department of Veterans Affairs has placed a high priority on clinical research into Alzheimer's disease and related dementias because of the aging population of veterans it serves: with 40% of U.S. veterans now above the age of 75.
The scientists said military service may increase Alzheimer's risk in veterans, citing previous large-scale studies that have estimated that veterans with PTSD face roughly a 50% to 60% greater risk of developing dementia than people without the disorder.
And previous research indicates the risk for dementia may be even greater among people with a history of traumatic brain injury, they said.
Among genetic factors that also influence a person's risk of developing dementia, the apolipoprotein E (APOE) epsilon 4 allele is the strongest risk factor, they said.
But the strength of the association depends on a person's ancestry. Each APOE epsilon 4 allele inherited roughly quadruples Alzheimer's risk in people of European ancestry, but doubles risk among African Americans.
So, the VA scientists set out to explore how Alzheimer's disease-associated genetic variants might modify dementia risk associated with PTSD and traumatic brain injury.
First, they found a greater percentage of dementia in veterans with PTSD and in those with traumatic brain injury, relative to veterans without these disorders. And they found higher dementia rates among veterans who had inherited the genetic variant.
Logue and his team then looked for interactions between the genetic variant, PTSD, and TBI using a mathematical model, a news release explained.
They found an increased dementia risk due to PTSD and traumatic brain injury in veterans of European ancestry who inherited the APOE epsilon 4 allele genetic variant.
In veterans of African ancestry, the impact of PTSD didn't vary as a function of the genetic variant, the release said. But the traumatic brain injury effect and interaction with the APOE epsilon 4 allele was even stronger among these veterans.
