A new subtype of frontotemporal lobar degeneration with FUS pathology

TLDR

Frontotemporal dementia is a heterogeneous clinical syndrome whose most common neuropathology involves TDP‑43 or tau aggregates, while FUS mutations have been linked to familial amyotrophic lateral sclerosis and associated with FUS protein accumulations. The study aimed to determine whether FUS protein underlies the atypical FTLD‑U subtype of frontotemporal dementia. The authors examined post‑mortem brain tissue from 15 sporadic aFTLD‑U cases using FUS immunohistochemistry and protein blotting to assess neuronal and glial inclusions. All 15 cases exhibited FUS‑positive neuronal and glial inclusions, increased insoluble FUS protein, and no FUS mutations, indicating that FUS is the pathological protein in this subgroup and supporting the close relationship between FTD and ALS.

Abstract

Frontotemporal dementia (FTD) is a clinical syndrome with a heterogeneous molecular basis. The neuropathology associated with most FTD is characterized by abnormal cellular aggregates of either transactive response DNA-binding protein with Mr 43 kDa (TDP-43) or tau protein. However, we recently described a subgroup of FTD patients, representing around 10%, with an unusual clinical phenotype and pathology characterized by frontotemporal lobar degeneration with neuronal inclusions composed of an unidentified ubiquitinated protein (atypical FTLD-U; aFTLD-U). All cases were sporadic and had early-onset FTD with severe progressive behavioural and personality changes in the absence of aphasia or significant motor features. Mutations in the fused in sarcoma (FUS) gene have recently been identified as a cause of familial amyotrophic lateral sclerosis, with these cases reported to have abnormal cellular accumulations of FUS protein. Because of the recognized clinical, genetic and pathological overlap between FTD and amyotrophic lateral sclerosis, we investigated whether FUS might also be the pathological protein in aFTLD-U. In all our aFTLD-U cases (n = 15), FUS immunohistochemistry labelled all the neuronal inclusions and also demonstrated previously unrecognized glial pathology. Immunoblot analysis of protein extracted from post-mortem aFTLD-U brain tissue demonstrated increased levels of insoluble FUS. No mutations in the FUS gene were identified in any of our patients. These findings suggest that FUS is the pathological protein in a significant subgroup of sporadic FTD and reinforce the concept that FTD and amyotrophic lateral sclerosis are closely related conditions.