Literature Review
Peripheral plasma levels of the CNS protein tau are chronically elevated after traumatic brain injury (TBI) and appear to correlate with the severity of postconcussive symptoms, according to a report published online ahead of print August 3 in JAMA Neurology.
If these findings are confirmed, tau may be the first biomarker that is sensitive and specific to persistent TBI-related symptoms. The results also suggest that “months to years after the primary brain injury, there may be a continuation of secondary injuries with residual axonal degeneration and blood–brain barrier disruptions in this population that may contribute to the maintenance of postconcussive disorder symptoms and affect symptom severity,” said Anlys Olivera, PhD, of the National Institute of Nursing Research in Bethesda, Maryland, and her associates.
Tau stabilizes the structure of the axonal cytoskeleton. It is elevated in the CSF and the peripheral blood (albeit in extremely low concentrations) of patients with severe TBI, professional boxers, and athletes who sustain concussions. The extremely low levels of tau in the peripheral blood have been difficult to measure until the recent development of an ultrahigh-sensitivity immunoassay technology. Using this innovation, the researchers were able to examine for the first time the associations between plasma tau levels and the frequency and severity of deployment-related TBIs.
Over a two-year period, Dr. Olivera and her associates assessed tau levels in 70 members of the military who self-reported one or more TBIs and 28 military control subjects without TBI who were matched for age, sex, race, time since deployment, and number of deployments. Almost all participants in the TBI group had been injured at least 18 months previously. The most common causes of TBI were blows to the head, exposure to blasts, vehicular crashes, and sports-related concussions.
Total tau was significantly increased in the TBI group (mean level, 1.13 pg/mL), compared with the control group (0.63 pg/mL). Total tau also increased with increasing severity of the initial brain injury, with increasing number of TBIs, and with increasing severity of present-day postconcussive symptoms. These associations, moreover, were independent of symptoms of post-traumatic stress disorder and depression, which were prevalent in the TBI group, the investigators said.
Tau is not only a marker of brain injury, it also can contribute to secondary injury processes such as inflammation, which makes it a potential target for therapy. If the findings of this study are confirmed and extended to demonstrate a direct mechanistic relationship between TBI and tau aggregation, treatments such as the direct delivery of proteasomes “would be invaluable, considering the dearth of treatments for TBIs and chronic [postconcussive disorder] symptoms,” Dr. Olivera and her associates said.
Among the study limitations cited by the investigators are lack of neuroimaging and neuropsychologic data.
—Mary Ann Moon