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We have evaluated the diagnostic potential of the seeding amplification assay (SAA) in detecting α-synuclein seeding activity in postmortem brain and cerebrospinal fluid (CSF) samples from patients with primary and co-pathology α-synucleinopathies. Moreover, we investigated potential SAA positivity in control samples which may suggest unrecognized co-pathology. A total of 15 brain and 14 CSF samples with definite dementia with Lewy bodies (DLB, n = 6), Alzheimer´s disease with amygdala Lewy body (AD/ALB, n = 3), and patients with concomitant Creutzfeldt-Jakob disease and Lewy body pathology (CJD/LBP, n = 6) comorbidity were tested for α-synuclein seeding activity using SAA assay utilizing recombinant α-synuclein (WT) with N-terminal His-tag. Control samples consisted of other neurodegenerative diseases (n = 17 for brain and n = 18 for CSF samples) and healthy corneal donors (n = 17). The analysis of seeding activity in brain samples suggested 100% sensitivity and 91.2% specificity. Five out of 34 brain control samples gave a positive SAA outcome. However, upon reevaluation, two of these samples were reclassified as Alzheimer´s disease (AD) with synucleinopathy co-pathology. The analysis of CSF also suggested 100% sensitivity and 94.4% specificity, although dilution of some samples was necessary to decrease the effect of inhibitors. We report a good performance of the SAA not only in postmortem samples from primary synucleinopathies with advanced pathology, but also in co-pathology synucleinopathies with isolated Lewy bodies in the amygdala in AD cases. Our findings highlight the importance of careful diagnostic evaluation in AD patients, where co-existing synucleinopathy may otherwise go unrecognized.

Real-time quaking-induced conversion assay (RT-QuIC) exploits templating activity of pathogenic prion protein for ultrasensitive detection of prions. We have utilized second generation RT-QuIC assay to analyze matching post-mortem cerebrospinal fluid and skin samples of 38 prion disease patients and of 30 deceased neurological controls. The analysis of cerebrospinal fluid samples led to 100% sensitivity and 100% specificity, but some samples had to be diluted before the analysis to alleviate the effect of present RT-QuIC inhibitors. The analysis of the corresponding skin samples provided 89.5% sensitivity and 100% specificity. The median seeding dose present in the skin was one order of magnitude higher than in the cerebrospinal fluid, despite the overall fluorescent signal of the skin samples was comparatively lower. Our data support the use of post-mortem cerebrospinal fluid for confirmation of prion disease diagnosis and encourage further studies of the potential of skin biopsy samples for intra-vitam prion diseases´ diagnostics.