Development of a novel method for the quantification of tyrosine 39 phosphorylated α- and β-synuclein in human...

Parkinson's disease (PD) is the second most prevalent neurodegenerative disorder. Biomarkers that can help monitor the progression of PD or response to disease-modifying agents will be invaluable in making appropriate therapeutic decisions. Further, biomarkers that could be used to distinguish PD from other related disorders with PD-like symptoms will be useful for accurate diagnosis and treatment. C-Abl tyrosine kinase is activated in PD resulting in increased phosphorylation of the tyrosine residue at position 39 (Y39) of α-synuclein (α-syn) (pY39 α-syn), which contributes to the death of dopaminergic neurons. Because pY39 α-syn may be pathogenic, monitoring pY39 α-syn could allow us to diagnose presymptomatic PD and help monitor disease progression and response to treatment. We sought to investigate if increased phosphorylation of pY39 α-syn can be detected in the cerebrospinal fluid (CSF) of PD patients by targeted mass spectrometry.

Here, we report a two-step enrichment method in which phosphotyrosine peptides were first enriched with an anti-phosphotyrosine antibody followed by a second round of enrichment by titanium dioxide (TiO₂) beads to detect EGVLpYVGSK sequence derived from tyrosine 39 region of α- and β-synuclein (αβ-syn). Accurate quantification was achieved by adding a synthetic heavy version of pY39 αβ-syn peptide before enzymatic digestion.

Using the developed enrichment methods and optimized parallel reaction monitoring (PRM) assays, we detected pY39 αβ-syn peptide in human CSF and demonstrated that the ratio of pY39 αβ-syn to Y39 αβ-syn was significantly increased in the CSF of patients with PD.

We anticipate that this optimized two-step enrichment-based PRM detection method will help monitor c-Abl activation in PD patients and can also be used to quantify other phosphotyrosine peptides of low abundance in biological samples.