Cumulative increases in circulating mtDNA as a potential biomarker of brain injury in rugby union: a pilot study

Objectives: Rugby is a high contact sport that can lead to head contact events, especially in forward players, triggering inflammatory processes. Mitochondrial DNA, released during injury, may act as a proinflammatory signal. This study aimed to assess levels of three mitochondrial DNA forms—mitochondrial (Fraction 1), protein bound (Fraction 2), and naked (Fraction 3) - across a rugby season and correlate them with neuroinflammatory markers, blood parameters, and head impact exposure. Design: Observational, longitudinal. Methods: Thirteen male professional rugby players were monitored across two matches in the 2023–24 season. Blood samples were collected before (T0, T2), immediately after (T1, T3) each match, and one month post season (T4). Mitochondrial DNA levels and integrity were quantified, along with neuroinflammatory markers and pro-inflammatory cytokines, hematochemical parameters, immune receptor expression and monocyte distribution. Results: Mitochondrial DNA levels increased progressively from T0 to T4, particularly in forward players. Post-match, Fraction 2 and 3 mitochondrial DNA levels were elevated, with a peak at T4. Interleukin-6 and interleukin-8 rose after both matches, while tumor necrosis factor-alpha increased only at T3. Neurofilament light chain levels spiked post-match but normalized afterward. Baseline mitochondrial DNA correlated with several hematological and metabolic markers, and immune cell subsets. Global Positioning System data linked mitochondrial DNA levels with high powered actions and contact intensity, especially in forward players. Conclusions: Repeated head impacts in rugby lead to sustained mitochondrial DNA elevation, suggesting its potential as an early biomarker of cell damage and neuroinflammation. This may aid in preventing sports- related neurodegeneration.