Concurrent tau pathologies in frontotemporal lobar degeneration with TDP‐43 pathology

AimsAccumulating evidence suggests that patients with frontotemporal lobar degeneration (FTLD) can have pathologic accumulation of multiple proteins, including tau and TDP-43. This study aimed to determine the frequency and characteristics of concurrent tau pathology in FTLD with TDP-43 pathology (FTLD-TDP).MethodsThe study included 146 autopsy-confirmed cases of FTLD-TDP and 55 cases of FTLD-TDP with motor neuron disease (FTLD-MND). Sections from the basal forebrain were screened for tau pathology with phospho-tau immunohistochemistry. For cases with tau pathology on the screening section, additional brain sections were studied to establish a diagnosis. Genetic analysis of C9ORF72, GRN, and MAPT was performed on select cases.ResultsAmong 201 cases, we found 72 cases (36%) with primary age-related tauopathy (PART), 85 (42%) with aging-related tau astrogliopathy (ARTAG), 45 (22%) with argyrophilic grain disease (AGD), and 2 cases (1%) with corticobasal degeneration (CBD). Patients with ARTAG or AGD were significantly older than those without these comorbidities. One of the patients with FTLD-TDP and CBD had C9ORF72 mutation and relatively mild tau pathology, consistent with incidental CBD.ConclusionThe coexistence of TDP-43 and tau pathologies was relatively common, particularly PART and ARTAG. Although rare, individual patients with FTLD can have multiple concurrent proteinopathies. The absence of TDP-43-positive astrocytic plaques may suggest that CBD and FTLD-TDP were independent disease processes in the two patients with both tau and TDP-43 pathologies. It remains to be determined if mixed cases represent a unique disease process or two concurrent disease processes in an individual.