Biomarker evidence of neurodegeneration in mid-life former rugby players

Abstract

Repetitive head impacts and traumatic brain injuries in contact sports, such as rugby union, are associated with increased risk of neurodegenerative diseases such as Alzheimer's disease and chronic traumatic encephalopathy. Advances in fluid and imaging biomarkers are transforming dementia diagnosis but have not been systematically applied to individuals previously exposed to head impacts during rugby participation. We used biomarkers, including those with sensitivity and specificity for early Alzheimer's pathology to explore neurodegenerative risk in mid-life elite retired rugby players with significant repetitive head impact exposure. Plasma neurofilament light, glial fibrillary acid protein, amyloid-β (Aβ)42, Aβ40 and phospho-tau217 were quantified using ultrasensitive single molecule array digital enzyme-linked immunosorbent assay (SiMoA) in former elite rugby players as well as age/sex-matched unexposed controls. 3 T MRI and neuropsychology assessments were performed, with National Institute for Neurological Disorders and Stroke criteria used to ascertain the presence of traumatic encephalopathy syndrome. Regression models were used to relate plasma/imaging biomarkers to clinical phenotype. Individual-levels analyses were performed for fluid and imaging metrics, based on control biomarker distributions. Biomarker data from two aligned un-exposed Alzheimer's cohorts were included to contextualize our findings. Two hundred ex-rugby players (median age 44 years, 90% males) and 33 unexposed controls were assessed. Twenty-four (12%) ex-players fulfilled criteria for traumatic encephalopathy syndrome but none had dementia. Plasma phospho-tau217 concentrations were 17.6% higher in ex-rugby players than controls (95% confidence interval 3.7-33.3, P = 0.047). A total of 46 (23.1%) ex-players had elevated phospho-tau217 at the individual level; as did 18 (9.0%) players in relation to raised plasma neurofilament light. Ex-players' concentrations were lower than in unexposed adults with late onset Alzheimer's disease (n = 69). Ex-players also showed significantly reduced volumes in the frontal/cingulate cortex on voxel-based morphometry at the group level; with reduced white matter and lower hippocampal volume associated with longer career durations within ex-players. Trauma-associated white matter changes measured with diffusion tensor imaging were uncommon in ex-players (4.6%). Traumatic encephalopathy syndrome was significantly more common in ex-players with elevated phospho-tau217, while those with raised plasma neurofilament light had significantly more anxiety and depressive symptoms. Frontal brain volumes correlated negatively with neurofilament light (r = -0.21, P = 0.010), and hippocampal volumes correlated negatively with phospho-tau217 (r = -0.19, P = 0.024). Elite rugby participation is associated with abnormal fluid and neuroimaging neurodegeneration biomarkers in mid-life. These include elevated phospho-tau217, which may indicate amyloid-dependent tau pathology. The results provide support for using state-of-the-art neurodegenerative biomarkers in the evaluation of long-term effects of sports head impact exposure.

Keywords: CTE; TBI; concussion; dementia; head injury; sports.

Figure 1

Participant demographics, head injury history and elite career duration. (A) Histogram of study participant numbers stratified by group (controls or ex-players, vertically), and by age horizontally. (B) Histogram showing career duration in years (elite play) within ex-players. (C) Box plot of concussion count (without loss of consciousness) in ex-players stratified by position.

Figure 2

Plasma biomarkers of neurodegeneration. (A) Group level comparisons between unexposed controls, ex-players, healthy unexposed older adults, and unexposed people with late onset Alzheimer’s disease (AD). Dotted line separating ex-players/controls and AD/older controls indicates that analyses were run on the same platform with the same assay, but using different kits, hence direct comparisons and statistical tests are precluded, and groups are shown side-by-side for indicative purposes. Box plots show median and interquartile ranges with individual data points plotted. On regression, a significant group difference was present for plasma p-tau₂₁₇ but no other biomarkers. (B) Venn diagram within the ex-players group only, showing individual-level abnormalities in fluid biomarkers (defined by values beyond the healthy control group 97.5th centile. Percentages shown are of all those ex-players with any biomarker abnormality (n = 65). Aβ42:40 ratio = amyloid-beta 42:40 ratio; GFAP = glial fibrillary acidic protein; NfL = neurofilament light; p-tau217 = phosphorylated tau 217.

Figure 3

Brain volume associations of former elite rugby participation. [A(i)] Left: Voxel-based morphometry group comparison of grey matter brain volumes between ex-rugby players and healthy volunteers. TFCE-corrected significance map shown (one-tailed t-test; higher significance indicated by light blue colour). Right: Box plot showing medians and interquartile ranges of normalized grey matter volume, with individual data points plotted. [B(i)] Left: Significance map of regression relating career duration and lower grey matter volume within ex-rugby players. Right: Scatter plot showing years of play against normalized volume within this region at the individual level; linear regression line is shown in blue with 95% confidence intervals in grey. (ii) Significance map of regression relating career duration and lower white matter volume within ex-rugby players. Right: Scatter plot demonstrating the effect. (iii) Significance map of regression relating career duration and higher CSF volume within ex-rugby players. Right: Scatter plot demonstrating the effect. TFCE = threshold-free cluster enhancement.

Figure 4

Diffusion tensor imaging white matter microstructure in ex-players. Fractional anisotropy values in three regions of interest in ex-rugby players and unexposed healthy volunteers, within corpus callosum (left), corticospinal tracts (middle) and whole white matter skeleton (right). Box plots show median and interquartile ranges with individual data points plotted. Points in red represent data below the 2.5th centile of the healthy control distribution.