Monday, January 18, 2016

Biomarker for diffuse axonal injury in concussions

Researchers have taken another step toward identifying a biomarker for diffuse axonal injury (DAI) in patients with a concussion.

They used immunohistochemistry (IHC) to highlight a cleaved protein called α-II spectrin N-terminal fragment (SNTF), which signals damaged axons in brain tissue, from human donors who had a severe traumatic brain injury (TBI) and from animal models of mild TBI (mTBI).

A test that detects axonal damage could eventually be used to identify the 20% of patients with concussion who experience persistent symptoms, said study author Douglas H. Smith, MD, professor, neurosurgery, and director, Center for Brain Injury and Repair, University of Pennsylvania, Philadelphia.

In patients with a concussion coming to the emergency department, "we want to find out if there was brain damage and if so, what kind of brain damage," Dr Smith told Medscape Medical News.
Knowing that DAI is the pathology at play could help guide research to study "the first ever" potential treatments for concussion, he said...

Accumulation of the amyloid precursor protein (APP) is the current "gold standard" approach for the diagnosis of DAI, but only a subset of the total population of axons in a given white matter tract accumulates APP after a TBI.

While most people who sustain a concussion recover normal homeostasis in the brain, in patients who have persistent symptoms, the axons may be too damaged. "In some cases, the axons eventually rupture and spill this fragment of a protein that is cleaved by this calcium-activated enzyme out into the brain, and then it gets into the blood," said Dr Smith...

The new study complements earlier blood biomarker studies. Researchers first identified SNTF in the blood of concussion patients in emergency departments. That marker was found "in the exact same 20% of patients who later on, when we evaluated them 3 months after their injury, had persistent neurocognitive dysfunction," said Dr Smith.

They then looked at blood samples from professional hockey players with a sports-related concussion. "Again, we saw an increase [in SNTF] in the ones with worse symptoms, and it stayed high as these symptoms persisted, and when they felt they could go back to play, it went back down."

The new study investigated SNTF in the brain rather than the blood. Researchers used IHC to compare SNTF in 18 postmortem samples from patients who had experienced a severe TBI and survived for less than 7 days with SNTF in 16 age-matched controls...

Identifying this pathology provides insight into a more comprehensive approach to the neuropathologic assessment of DAI, said Dr Smith. He used the analogy of a suspected myocardial infarction, wherein doctors routinely test for troponins in the blood, which reveals not only that the heart muscle is degenerating but also something about the process.

"This is not like many current methods to simply diagnose concussion in general — this test shows us if there is irreversible damage in the brain and how that damage occurs, specifically providing a diagnosis of diffuse axonal injury," said Dr Smith...

Another issue is the definition of mTBI, a label often used synonymously with concussion, said Dr Conidi. "I believe that any time you can find an objective measure of brain damage, it's no longer a mild traumatic brain injury; it's a traumatic brain injury," he said. "I don't know about mTBI; I don't even know how to define it."

Dr Smith agreed. "While concussion is alternatively called 'mild TBI', there is nothing mild about it for some individuals," he said. "Indeed, for them, 'mild TBI' is an oxymoron. Potentially, new tools such as blood biomarkers will help distinguish patients who will fully recover from those who won't in the near term. This may lead to new diagnostic terms to describe these differences in injury severity."

1 comment:

  1. Victoria E. Johnson, William Stewart, Maura T. Weber, D. Kacy Cullen, Robert Siman, Douglas H. Smith. SNTF immunostaining reveals previously undetected axonal pathology in traumatic brain injury. Acta Neuropathologica.
    January 2016, Volume 131, Issue 1, pp 115-135.


    Diffuse axonal injury (DAI) is a common feature of severe traumatic brain injury (TBI) and may also be a predominant pathology in mild TBI or “concussion”. The rapid deformation of white matter at the instant of trauma can lead to mechanical failure and calcium-dependent proteolysis of the axonal cytoskeleton in association with axonal transport interruption. Recently, a proteolytic fragment of alpha-II spectrin, “SNTF”, was detected in serum acutely following mild TBI in patients and was prognostic for poor clinical outcome. However, direct evidence that this fragment is a marker of DAI has yet to be demonstrated in either humans following TBI or in models of mild TBI. Here, we used immunohistochemistry (IHC) to examine for SNTF in brain tissue following both severe and mild TBI. Human severe TBI cases (survival <7d; n = 18) were compared to age-matched controls (n = 16) from the Glasgow TBI archive. We also examined brains from an established model of mild TBI at 6, 48 and 72 h post-injury versus shams. IHC specific for SNTF was compared to that of amyloid precursor protein (APP), the current standard for DAI diagnosis, and other known markers of axonal pathology including non-phosphorylated neurofilament-H (SMI-32), neurofilament-68 (NF-68) and compacted neurofilament-medium (RMO-14) using double and triple immunofluorescent labeling. Supporting its use as a biomarker of DAI, SNTF immunoreactive axons were observed at all time points following both human severe TBI and in the model of mild TBI. Interestingly, SNTF revealed a subpopulation of degenerating axons, undetected by the gold-standard marker of transport interruption, APP. While there was greater axonal co-localization between SNTF and APP after severe TBI in humans, a subset of SNTF positive axons displayed no APP accumulation. Notably, some co-localization was observed between SNTF and the less abundant neurofilament subtype markers. Other SNTF positive axons, however, did not co-localize with any other markers. Similarly, RMO-14 and NF-68 positive axonal pathology existed independent of SNTF and APP. These data demonstrate that multiple pathological axonal phenotypes exist post-TBI and provide insight into a more comprehensive approach to the neuropathological assessment of DAI.