Alpha-synuclein misfolding as a fluid biomarker for Parkinson's disease and synucleinopathies measured with the iRS platform

Misfolding and aggregation of alpha-synuclein (Syn) plays a key role in the pathophysiology of Parkinsons disease (PD). It induces cellular and axonal damage already in the early stages of the disease. Despite considerable advances in PD diagnostics by Syn seed-amplification assays (SAAs), an early and differential diagnosis of PD still represents a major challenge. Here, we extended the immuno-infrared sensor (iRS) platform technology from Alzheimers disease (AD), in which {beta}-amyloid misfolding was monitored as a fluid biomarker towards Syn misfolding in PD. Using the iRS platform technology, we analyzed cerebrospinal fluid (CSF) from two independent cohorts, a discovery and a validation cohort comprising clinically diagnosed PD (n=57), atypical Parkinsonian disorders with Syn pathology (multiple system atrophy (MSA), n= 5) or Tau pathology (corticobasal degeneration (CBD), n=5, progressive supranuclear palsy (PSP) n=9), and further disease controls (frontotemporal dementia (FTD) n=7 and other, n=51). In the discovery cohort, an AUC of 0.90, 95 %-CL 0.85 - 0.96 is obtained for the differentiation of PD/MSA vs. all controls, and in the validation cohort, an AUC of 0.86, 95 %-CL 0.80 - 0.93, respectively. In the combined dataset, the Syn misfolding classifies PD/MSA from controls with an AUC of 0.90 (n=134, 95 %-CL 0.85 - 0.96). Using two threshold values instead of one identified people in the continuum between clearly unaffected (low misfolding group) and affected by PD/MSA (high misfolding group) with an intermediate area in between. The controls versus PD/MSA in the low vs. high misfolding group were classified with 97% sensitivity and 92% specificity. The spectral data showed misfolding in CSF from an -helical/random-coil secondary structure of Syn in controls to {beta}-sheet enriched secondary structures in PD and MSA patients. In subgroups, the iRS platform implied a potential for stratifying patients with overlapping clinical symptoms. With high accuracy, the iRS Syn misfolding platform provides a novel diagnostic tool using body fluids for differential, biological classification of the Syn associated disorders PD and MSA. The iRS platform, indicating directly and fast all Syn conformers, is complementary to the Syn SAAs, which also uses Syn misfolding as a fluid biomarker. However, monomers amplify competent misfolded conformers in SAA over time. The iRS platform opens new avenues for the stratification of PD by a body-fluid analysis and follow-ups in the continuum of healthy to clinically impaired individuals.