Co-occurrence of multiple pathologies in a case of frontotemporal dementia with TBK1 mutation: first in vivo detection of alpha-synuclein and tau co-pathology.

We present the case of a 74-year-old woman with behavioral variant frontotemporal dementia (bvFTD) linked to a pathogenic TANK-binding kinase 1 (TBK1) mutation (c.1349_1352del; p.Ile450Lysfs*15). During clinical workup, the patient underwent comprehensive biomarker analysis, including tau positron emission tomography (PET) and cerebrospinal fluid (CSF) seed amplification assay (SAA) for α-synuclein (αSyn). While CSF biomarkers for Alzheimer's disease were normal, the αSyn SAA was clearly positive, indicating misfolded αSyn aggregates. Tau PET revealed increased [F]PI-2620 uptake in the basal ganglia. Genetic testing confirmed autosomal dominant TBK1-associated FTD. The patient's condition deteriorated over the following year, with rapid cognitive decline and the emergence of cortical signs. Post-mortem neuropathological analysis confirmed multiple proteinopathies: FTLD-TDP43 (subtype A), Lewy body disease (limbic type, Braak stage 5), argyrophilic grain disease (AGD), aging-related tau astrogliopathy (ARTAG), and primary age-related tauopathy (PART). This is the first reported TBK1-FTD case with in vivo detection of αSyn pathology via SAA and in vivo monitoring of tau pathology. The case expands the clinical and neuropathological spectrum of TBK1-associated FTD. Our findings support a broader interpretation of TBK1-associated neurodegeneration and highlight the importance of multimodal diagnostic approaches that integrate molecular, genetic, imaging, and neuropathological tools. This case also underscores the utility of αSyn SAA and tau PET in detecting co-pathologies that may otherwise remain clinically silent and illustrates the need for further studies exploring the molecular cross-talk between TBK1, tau, and αSyn pathologies.