Seed Amplification Assay-Based Detection of Alpha-Synuclein Aggregates Outperforms Immunohistochemical Analysis in the Intestinal Mucosal Biopsies of Parkinson's Disease and Multiple System Atrophy Participants

Objective: To determine whether an α–synuclein (α–Syn) seed amplification assay (SAA) detects seeding–competent aggregates in endoscopic mucosal biopsies, differs by gastrointestinal region, registers pathology more frequently than immunohistochemistry (IHC), and associates with Parkinson disease (PD) diagnosis, disease duration or severity. Design: This cross–sectional, proof–of–concept diagnostic accuracy study analyzed duodenum, ileum, and sigmoid biopsies from PD participants, age– and sex–matched healthy controls (HC), plus exploratory multiple system atrophy (MSA) subtypes [parkinsonian (MSA–P) and cerebellar (MSA–C)]. Ileal and sigmoid PD and HC biopsies were assessed using proteinase K–resistant phospho–Ser129 IHC. SAA positivity was evaluated against clinical and biological measures. Results: In PD, SAA detected seeding competent α–Syn aggregates with high observed specificity (97.5–100%) and region–dependent sensitivity (18/37, 48.65% in ileum and 36/56, 64.29% in sigmoid). Sigmoid SAA positivity increased significantly with PD disease duration (40.00% at <5 years, 73.33% at 5–10 years, and 93.75% at >10 years), independent of motor severity, whereas ileal positivity remained stable across disease stages. In 37 paired sigmoid biopsies, SAA detected α–Syn aggregates 2.8–fold more frequently than IHC. Sigmoid seeding activity was detected in all five MSA–P, but not in two MSA–C. IHC did not detect aggregates in any MSA. Conclusions: In PD, α–Syn SAA detected seeding–competent α–Syn aggregates in endoscopic mucosal biopsies with high observed specificity, moderate sensitivity, and approximately 2.8–fold higher detection rate than IHC in sigmoid. Positive SAA in MSA patients supports GI involvement across synucleinopathies. SAA detection in PD differed by intestinal region and disease duration, supporting further validation of intestinal SAA as a PD biomarker.