Skin Seed Amplification Assay differentiates Multiple System Atrophy from Parkinson's Disease

BackgroundMisdiagnoses of multiple system atrophy (MSA) and Parkinsons disease (PD) remain common due to overlapping clinical features. ObjectiveTo develop and assess a skin -synuclein seed amplification assay (Syn-SAA) for detecting pathological -synuclein and distinguishing MSA from PD. MethodsIn this blinded, cross-sectional study conducted across two laboratories, 308 skin biopsies from 117 participants were analyzed using a standardized Syn-SAA protocol. The cohort included 42 PD, 31 MSA, 30 controls, and 14 with progressive supranuclear palsy (PSP). Biopsies were obtained from up to four anatomical sites and positivity was defined as [≥]75% positive replicates. Kinetic parameters were analyzed to differentiate PD and MSA, and clinical features were correlated with Syn-SAA positivity. Fibril morphology from amplified skin samples was compared to postmortem brain using proteinase K digestion and electron microscopy. ResultsSyn-SAA detected synucleinopathy in 86% of PD and 77% of MSA patients, with 90% specificity versus controls and 86% versus PSP. ROC analysis showed that maximum fluorescence (Fmax, AUC = 0.93) and lag time (AUC = 0.91) distinguished PD from MSA at the biopsy level. A 50% mean Fmax cutoff across positive samples accurately differentiated synucleinopathies with 90% sensitivity and 85% specificity. Syn-SAA positivity correlated with RBD in both disorders and with autonomic features in MSA. Amplified fibrils displayed distinct structural and proteolytic profiles between PD and MSA, consistent with brain-derived aggregates. ConclusionSkin Syn-SAA detects pathological -synuclein in living patients with high accuracy and distinguishes MSA from PD based on assay kinetics and fibril strain properties.