α-Synuclein real-time quaking-induced conversion in the submandibular glands of Parkinson's disease patients

Background: Identification of a peripheral biomarker is a major roadblock in the diagnosis of PD. Immunohistological identification of p-serine 129 α-synuclein in the submandibular gland tissues of PD patients has been recently reported. Objective: We report on a proof-of-principle study for using an ultra-sensitive and specific, real-time quaking-induced conversion assay to detect pathological α-synuclein in the submandibular gland tissues of PD patients. Methods: The α-synuclein real-time quaking-induced conversion assay was used to detect and quantify pathological α-synuclein levels in PD, incidental Lewy body disease, and control submandibular gland tissues as well as in formalin-fixed paraffin-embedded sections. Results: We determined the quantitative seeding kinetics of pathological α-synuclein present in submandibular gland tissues from autopsied subjects using the α-synuclein real-time quaking-induced conversion assay. A total of 32 cases comprising 13 PD, 3 incidental Lewy body disease, and 16 controls showed 100% sensitivity and 94% specificity. Interestingly, both PD and incidental Lewy body disease tissues showed 100% concordance for elevated levels of pathological α-synuclein seeding activity compared to control tissues. End-point dilution kinetic analyses revealed that the submandibular gland had a wide dynamic range of pathological α-synuclein seeding activity. Conclusions: Our results are the first to demonstrate the utility of using the real-time quaking-induced conversion assay on peripherally accessible submandibular gland tissues and formalin-fixed paraffin-embedded tissue sections to detect PD-related pathological changes with high sensitivity and specificity. Additionally, the detection of seeding activity from incidental Lewy body disease cases containing immunohistochemically undetected pathological α-synuclein demonstrates the α-synuclein real-time quaking-induced conversion assay's potential utility for identifying prodromal PD in submandibular gland tissues.