α-Synuclein Seeding Activity in Fixed Antemortem Skin Biopsies From Patients With Synucleinopathies

α-Synuclein seed amplification assays can detect seed-competent pathological α-synuclein in fresh and frozen biospecimens, but their performance in fixed antemortem skin biopsies is less well defined. We applied an optimized real-time quaking-induced conversion assay to paraformaldehyde-fixed skin biopsy sections from participants with Parkinson's disease (PD), dementia with Lewy bodies (DLB), multiple system atrophy (MSA), and controls. Fixed tissue retained measurable α-synuclein seeding activity, although amplification was delayed and the apparent 50% seeding dose was lower than in unfixed or autopsy skin. In this selected proof-of-concept cohort, SAA positivity was observed in 70% of PD, 80% of DLB, 80% of MSA, and 20% of control samples; exploratory ROC analysis yielded an AUC of 0.86. Seeding readouts correlated with Hoehn and Yahr stage but not significantly with MoCA score or pS129 α-synuclein burden. Fixed skin biopsies can therefore provide complementary morphological and functional measures of peripheral α-synuclein pathology.