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

Alpha-synuclein (α-syn) seed-amplification-assay (SAA) is highly accurate in cerebrospinal fluid and skin, but collection requires a dedicated invasive or minor surgical procedure and is not collected at population scale.

Four previous studies have reported α-syn seeding activity in gastrointestinal biopsies, with sample sizes of 7–23 patients, single anatomical regions, and widely varying positivity rates.

Immunohistochemistry for α-syn in mucosal biopsies is limited by sparse, unevenly distributed pathology, the absence of a standardized scoring system, and its inability to establish whether detected aggregates are seeding-competent.

Whether gastrointestinal seeding activity varies by anatomical region, disease duration, disease severity or severity of motor and GI symptoms is unknown.

In the largest gastrointestinal α-Syn SAA series to date (96 participants), biopsies obtained during routine endoscopy identified PD with 97.50-100% specificity and moderate sensitivity (48.65% ileum, 64.29% sigmoid).

Sigmoid SAA-positivity was duration dependent, rising from 40% at <5 years to 94% beyond 10 years (adjusted OR 2.82 per 5 years), and disease duration was the only independent correlate once H&Y stage was included in the model.

Constipation, intestinal inflammation, and sigmoid mucosal dysbiosis tracked PD severity but did not distinguish sigmoid SAA-positive from SAA-negative participants.

Ileal SAA-positivity was constant across disease duration and severity, indicating that positivity was not uniform along the gastrointestinal tract.

In the identical PD participant’s sigmoid tissue, SAA identified α-Syn pathology in 28 of 37 patients compared with 10 of 37 by proteinase K-resistant phospho-Ser129 immunohistochemistry.

Sigmoid SAA-positivity was detected in 5 of 7 participants with MSA, with positive cases confined to the parkinsonian subtype (MSA-P) and negative cases to the cerebellar subtype (MSA-C).

Strain typing by α-Syn SAA enables differentiation of PD from MSA cases using sigmoid biopsies.

Colonoscopy is already performed at scale in the age group at-risk of PD, so mucosal biopsies are an accessible matrix for α-Syn seeding assays and for biomarker-based enrichment of clinical trials.

Because yield depends on both sampling site and disease duration, prospective validation against cerebrospinal fluid must specify region and stage; a negative result from a single site cannot currently exclude PD.