Novel CSF β-synuclein-specific assays signal early synaptic degeneration in Alzheimer's disease.

BACKGROUND: Beta-synuclein (β-syn), measured at N-terminal epitopes, is an emerging cerebrospinal fluid (CSF) biomarker for synaptic degeneration in Alzheimer's disease (AD). Targeting the mid-region or C-terminus of β-syn may enhance analytical specificity due to the distinct structures of these regions across the synuclein protein family, unlike targeting the N-terminus, which is conserved across the family. This study aimed to confirm that β-syn is a promising CSF biomarker in AD, using novel assays designed to target different regions of β-syn, to investigate whether these regions are differentially affected in AD. METHODS: We developed two novel CSF β-syn-specific ELISAs targeting mid-region and C-terminus epitopes and assessed their analytical performance. Using these novel assays in combination with the established N-terminus ELISA, we analyzed a proof-of-concept cohort comprising biomarker-confirmed AD (n = 25) and non-AD subjects (n = 25) and a larger clinical cohort (n = 160) from the Amsterdam Dementia Cohort, wich included 41 individuals with subjective cognitive decline (SCD, controls; AD biomarker negative; 64.3 ± 3.3 years, 23 females), 39 with SCD (AD biomarker positive; 65.7 ± 3.1 years, 17 females), 40 with mild cognitive impairment due to AD (MCI-AD; 66.2 ± 2.9 years, 20 females), and 40 with AD dementia (AD-dem; 65.3 ± 3.4 years, 20 females). RESULTS: Both the mid-region and C-terminus assays demonstrated reliable analytical performance. All assays consistently detected β-syn in all clinical samples above their limits of detection, with a good average intra-assay coefficient of variation (range of the three assays: 2.7-6.5%CV) in the proof-of-concept cohort and clinical cohort (range of the three assays: 3.9-7.5%CV). CSF β-syn levels, with all the assays, were significantly elevated in all the AD groups compared with the controls in both cohorts. The diagnostic performance of the assays for distinguishing AD patients from controls was comparable (Delong's p > 0.05, AUC 0.71-0.80). Notably, mid-region β-syn significantly differentiated SCD-AD patients from AD-dem patients (p = 0.035) and MCI-AD patients at a trend level. Only mid-region and C-terminal levels correlated with MMSE scores (mid-region rho = -0.22, p = 0.006; C-terminal rho = -0.19, p = 0.016; N-terminus rho = -0.14, p = 0.069). CONCLUSION: Our novel assays demonstrated good analytical and clinical performance. CSF β-syn reliably indicates early synaptic degeneration in AD. The mid-region assay uniquely differentiated SCD-AD from AD-dem, showing promise for early disease detection.