Scottish scientists have discovered for the first time exactly how a single sporting or car accident brain injury can develop years later into life-changing dementia.
The University of Glasgow team of neuroscientists was headed by Dr Willie Stewart, who has campaigned for greater safety in rugby and football, and last year scanned the brain of former Premier League ace turned football pundit Alan Shearer.
In a "landmark" study, they have shown how just a single traumatic brain injury can generate a defective form of a common neural protein that can go on to spread right through the brain.
The result is memory deficits, neuronal damage, and even eventual full-blown dementia.
Working with specialists in Italy, the Glasgow team analysed brain specimens from patients surviving a year or more after a single, severe brain injury.
They looked for so-called "Tau" proteins, which are abundant in human brain tissue, but when in a "defective" form can no longer function properly and are associated with progressive, degenerative brain disease
The team found deposits of abnormal tau proteins were "much more widespread" in brain-injured patients than in normal brains.
They found the same type of abnormal tau in injured mice, which, over time, spread from the site of injury to other regions of the brain altogether.
Importantly, they also found that abnormal tau protein injected into the brains of healthy mice then appeared to "seed" and spread.
Dr Stewart, who believes legendary West Bromwich striker Jeff Astle was killed by progressive brain damage linked to trauma, said the "landmark" finding could lead to new treatments.
He said: "We've recognised for many decades that brain injury is one of the strongest environmental risk factors for dementia.
"More recently we are recognising sportsmen and women may be at increased risk of dementia.
"One of the striking features when we look at the brains of people who've been exposed to brain injury is the abnormal deposition of tau.
"What we've never really understood is how you get from a single brain injury, such as being hit by a car, or repeated brain injury, such as participating in sport, through to a widespread degenerative brain disease and a brain filled with tau.
"We can detect these deposits of tau quite early on after brain injury and they seem to be quite remote from areas that are related to memory and so forth, but when we see patients who have survived many more years those tau proteins appear to have spread through the whole brain.
"The question has always been how does this happen, how does the whole brain get affected, with consequent effects on memory and function?
"What we saw in mice was this protein spreading throughout the brain when we let them survive a number of months, and importantly when we took samples of these diseased brains from mice and injected them into mice who had never been exposed to brain injury those mice developed the same problems.
"So it appears this abnormal protein can 'seed' or spread through the brain.
"It's very similar to what's been described for CJD, where an infective agent can spread through the brain.
"It's like a grain of sand in an oyster.
"We need tau proteins for our brains to function, but they can become abnormally folded, and that abnormal protein can them transmit its abnormal shape to other tau proteins and generate this evolving pathology.
"If this spreading tau protein is part of what's generating the late dementia, perhaps one way to try and prevent it -- short of avoiding brain injury -- might be to try and target that tau protein and stop it from spreading.
"It gives us a novel way of challenging dementia."
Dr Elisa Zanier, who led the Italian team, based at the Mario Negri Institute for Pharmacological Research in Milan, added: "Traumatic brain injury is a leading cause of death and disability in young adults.
"Even in milder cases, it represents a risk factor for dementia. Understanding the mechanism linking an acute mechanical event to a progressive, degenerative brain disease would help the development of new therapies."
The study is published in the journal Brain.
In Europe, more than five million people live with moderate or severe traumatic brain injury.
Last year, in a separate study, a team guided by Dr Stewart produced a study which found that heading a modern-day football just 20 times causes brain impairment.
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