Nov. 5 (UPI) -- Researchers have identified a specific and common variant of a gene that plays a role in the severity of chronic traumatic encephalopathy based on a study of 86 brains of football players.
The findings, which were published Saturday in Acta Neuropathologica Communications, could explain why similar levels of head trauma can cause some of them to suffer more drastic symptoms of CTE.
Athletes who developed CTE pathology with the risk variation of TMEM106B were 2.5 times more likely to have dementia and increased levels of p-tau, according to researchers at the Boston University School of Medicine and Veterans Administration Boston Healthcare System.
They based their findings on a study from a sub-group of more than 600 brains donated by family members of military service members, football players and other contact-sport athletes over 10 years.
"In order to understand how genetic variants influence disease or severity, you need hundreds of cases," study co-senior author Dr. Ann McKee, a professor of neurology and pathology director of the BU CTE Center, said in a press release. "I expect to find a flood of new genes that influence the pathology of CTE. This is just the beginning."
McKee, who was named one of Time magazine's 100 Most Influential People of 2018, said "we've reached a critical mass" in the number of brains donated.
CTE can only be diagnosed by studying a person's brain after death.
The brain bank includes Aaron Hernandez, the former New England Patriots star who committed suicide in 2017 while in prison for murder. He was found to have one of the most severe cases of CTE among athletes whose brains were studied by BU researchers. His family has sued the NFL, accusing the league of failing to protect him from brain injury.
The study group included the brains of some well-known athletes, but they declined to name them.
The brain bank first started recruiting brain donations from families of deceased athletes in 2008 in coordination with the Concussion Legacy Foundation and its founder, former professional wrestler Chris Nowinski.
On Friday, the foundation reported that 147 colleges have had former football players diagnosed with CTE, including 26 colleges with at least three former players who developed the condition. The University of Georgia had the most cases with nine.
Researchers for years have wanted to know why repeated head injuries can lead to a range of CTE symptoms among individual college and professional athletes.
With the newly discovered genetic association between CTE and TMEM106B, they hope someday to predict which people, while they are still alive, are most likely to develop severe symptoms of CTE.
CTE symptoms include dementia and the buildup of phosphorylated tau protein, a feature also associated with other neurodegenerative disorders, including ALS, Alzheimer's and Parkinson's.
"We focused on this gene because of its role in altering brain pathology in other neurological diseases and its role in inflammatory responses," said co-senior author Dr. Thor Stein, a neuropathologist at VA Boston Healthcare System, and the Alzheimer's Disease Center neuropathology core associate director.
They analyzed TMEM106B and evidence of CTE in 86 Caucasian football players.
The researchers said athletes are good to study because of the frequency of their head injuries compared with people who sustain more random head injuries from military-related blasts or other various traumas.
On the participants, the team performed genetic sequencing and identified two distinct categories of TMEM106B genetic variatio. A major genetic variation was found in about 60 percent of people and a minor genetic variation in the remaining 40 percent.
The studied different traits known to be associated with CTE symptoms, including the loss of synapses, dementia before death and p-tau accumulation throughout the brain.
Comparing the results with data from people who did not have CTE, the team found that TMEM106B does not influence whether a person develops CTE.
But among those diagnosed with CTE, the 60 percent with the major genetic variation were significantly more likely to have higher accumulation of p-tau, more severe brain inflammation associated and dementia during their lifetimes.
"We're at a place where we clearly know that there's a link between contact sports and CTE, but we don't yet have a good sense of the magnitude of that relationship," said co-first author Jesse Mez, a member of the Alzheimer's Disease Center and CTE Center.
The TMEM106B gene directs cells to produce proteins of the same name, which are thought to play a role in the formation of cell organelles responsible for digesting enzymes. Too much TMEM106B, including the result of genetic variation, can make enzyme digestion less efficient.
Because inside brain cells play an important role in cleaning the brain of damage-related proteins like p-tau, abnormal enzyme digestion could result in the hallmark buildup of the damaging proteins in CTE cases.
The researchers suspect a number of genes contribute to the severity of CTE.