Neuropathology of the hippocampus in FTLD-Tau with Pick bodies: a study of the BrainNet Europe Consortium

Frontotemporal lobar degeneration with Pick bodies (Pick's disease) is characterized by the presence of tau immunoreactive spherical structures in the cytoplasm of neurones. In view of confusion about the molecular pathology of Pick's disease, we aimed to evaluate the spectrum of tau pathology and concomitant neurodegeneration-associated protein depositions in the characteristically affected hippocampus.

We evaluated immunoreactivity (IR) for tau (AT8, 3R, 4R), α-synuclein, TDP43, p62, and ubiquitin in the hippocampus, entorhinal and temporal cortex in 66 archival cases diagnosed neuropathologically as Pick's disease.

Mean age at death was 68.2 years (range 49-96). Fifty-two (79%) brains showed 3R immunoreactive spherical inclusions in the granule cells of the dentate gyrus. These typical cases presented mainly with the behavioural variant of frontotemporal dementia, followed by progressive aphasia, mixed syndromes or early memory disturbance. α-Synuclein IR was seen only in occasional spherical tau-positive inclusions, TDP-43 IR was absent, and 4R IR was present only as neurofibrillary tangles in pyramidal neurones. Aβ IR was observed in 16 cases; however, the overall level of Alzheimer's disease-related alterations was mainly low or intermediate (n = 3). Furthermore, we identified six cases with unclassifiable tauopathy.

(i) Pick's disease may occur also in elderly patients and is characterized by a relatively uniform pathology with 3R tau inclusions particularly in the granule cells of dentate gyrus; (ii) even minor deviation from these morphological criteria suggests a different disorder; and (iii) immunohistological revision of archival cases expands the spectrum of tauopathies that require further classification.