Objective: To determine the sensitivity for Parkinson’s disease (PD) diagnosis of seed amplification assay (SAA) for α-synuclein (α-syn) in formalin-fixed paraffin-embedded (FFPE) skin specimens and compare results with pathologic skin α-syn detected by immunohistochemistry (IHC).

Background: SAA is a promising method to detect pathologic α-syn in PD. While robust SAA assays have been developed for CSF and fresh frozen skin, protocols on FFPE specimens are less well established. Their optimization would allow for application of SAA on large samples of already-fixed antemortem and premortem skin specimens available in existing biobanks. A comparison within-subject to pathologic α-syn detected by IHC methods, as well as to CSF SAA for α-synuclein, would further inform the utility of α-syn SAA on FFPE tissue.

Method: This pilot study included a subsample of the Systemic Synuclein Sampling Study (S4), a multi-center, cross-sectional, observational study to evaluate α-syn pathology in tissues and biofluids in PD and healthy controls (HC). S4 obtained CSF from participants and applied SAA; maximal fluorescence values were used to designate subjects as +/- for CSF SAA. S4 also obtained 3-mm skin cervical paravertebral region punch biopsies. FFPE skin sections were stained with 5C12 monoclonal antibody. Digital images of slides were interpreted by neuropathologists blinded to diagnosis, and participants were rated +/- in skin for pathologic α-syn (IHC+). FFPE slides from sample subsets enriched for skin IHC+, IHC-, and HC, were subjected to SAA, and each subject was designated as +/- for skin SAA based on amplification onset time-to-threshold (blinded to IHC results). The PD samples were selected based on known IHC and CSF positivity: 11 were IHC+ CSF+, 4 IHC- CSF+, and 3 IHC- CSF-. Skin IHC, skin SAA, and CSF SAA results were compared within-subject.

Results: The sample included 18 PD participants (4 early, 7 moderate, 7 advanced), and 2 HC. 12/18 (70%) of PD participants were skin SAA+; of the 6 participants that were skin SAA-, 1 was IHC+, and 3 were CSF SAA+. All those with skin SAA+ had CSF SAA+.

Conclusion: SAA for α-syn in FFPE skin is feasible and detected all but 1 specimen that was IHC+. While CSF SAA maintains higher sensitivity for PD diagnosis compared to either skin α-syn detected by SAA or IHC in FFPE skin, diagnostic tests for PD in peripheral tissues are promising.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/utility-of-seed-amplification-assay-for-detecting-%ce%b1-synuclein-in-formalin-fixed-paraffin-embedded-skin-samples-in-parkinsons-disease/