A novel hierarchical framework elucidating regional differences in α-synuclein and tau co-pathology in military veterans with parkinsonism

Neuropathologic features diagnostic of parkinsonian disorders infrequently occur in isolation; hyperphosphorylated tau (p-tau) and amyloid plaques are often observed in combination with α-synuclein deposition. Co-pathologies in neurodegenerative diseases are now recognized as the norm rather than an exception, but existing neuropathological assessment tools do not capture the complexity of concurrent co-pathologies. Characterization of this co-pathology is critical, as it has the potential to identify synergistic mechanisms. We developed a hierarchical cytoarchitectural classification system, which we applied to an autopsy series of military veterans with parkinsonism (n = 26), focusing on Lewy and neurofibrillary pathologies. We defined co-pathology as Type A (co-morbid), Type B (co-regional), Type C (co-cellular), or Type D (co-aggregate). The regional distributions of each co-pathology subtype were assessed using double-label immunohistochemistry in the frontal cortex, hippocampal formation, and midbrain. The frontal cortex demonstrated only subtypes A-C (no co-aggregates), whereas the midbrain and hippocampus showed all subtypes of copathology (A-D). In summary, we show marked differences in the prevalence and levels of mixed α-synuclein and tau pathology in this cohort. Our classification system has the potential to be applied broadly for the study of co-pathology in neurodegenerative disorders.