Phosphorylated α-synuclein in CSF and plasma does not reflect synucleinopathy

We developed a highly sensitive and specific single-molecule array (Simoa) Homebrew assay for quantification of phosphorylated α-synuclein at serine 129 (pS129 α-syn) and evaluated its performance in human cerebrospinal fluid (CSF) and plasma. Using a cohort of patients with Parkinson's disease (PD), Alzheimer's disease (AD), and neurological controls with available CSF α-synuclein seed amplification assay (synSAA) outcome, we examined pS129 α-syn alongside N-terminal and C-terminal α-syn proteoforms. Our results showed that pS129 α-syn concentration was about 1% and 0.001% of the other α-syn species in CSF and plasma, respectively. We found no correlation between pS129 α-syn and synSAA outcome, indicating that soluble pS129 α-syn in CSF and plasma does not reflect presence of synucleinopathy. Interestingly, pS129 α-syn and other α-syn forms were significantly increased in AD compared to PD and controls, supporting the role of α-syn as biomarker of synaptic degeneration in AD.