Absolute quantification of the synaptic blood marker β-synuclein by targeted mass spectrometry ranks synaptic degeneration after amyloid deposition in autosomal dominant Alzheimer's disease

Introduction: Synaptic degeneration is a major hallmark of Alzheimer´s disease (AD) and the pathological correlate of memory impairment. Monitoring of synaptic changes in AD patients is therefore of importance for diagnosis, prognosis and assessing disease progression but also to track beneficial effects on synaptic loss in clinical trials. The protein β-Synuclein is a promising and easily accessible blood marker to track synaptic degeneration in AD but changes in preclinical AD are unclear. The Dominantly Inherited Alzheimer Network (DIAN) observational study is a worldwide multicenter initiative to longitudinally study dominantly inherited AD and provides the unique opportunity to study blood β-synuclein levels in very early (asymptomatic) AD and assess the timing and relation of β-synuclein changes to other pathological alterations.

Methods: We used immunoprecipitation and multiple reaction monitoring mass spectrometry (IP-MS) with an Eksigent MicroLC200 and a QTRAP6500 for the absolute quantification of β-synuclein in serum samples of 178 participants of the DIAN study including 69 cognitively unimpaired mutation non-carriers (NC), 78 cognitively unimpaired AD mutation carriers (asymptomatic AD, aMC) and 31 symptomatic MC (sMC). Changes of β-synuclein serum levels were compared and correlated with cerebrospinal fluid (CSF) biomarkers (Aβ42/40, tTau, pTau variants), amyloid deposition ([11C]-Pittsburgh Compound B - positron emission tomography (PiB-PET)), brain atrophy (magnetic resonance imaging (MRI)) and metabolism ([18F]-fluorodeoxyglucose - PET (FDG-PET)), axonal degeneration (serum NfL) and cognitve impairment (MMSE, Clinical Dementia Rating sum-of-boxes (CDR-SB), DIAN cognitive composite). Longitudinal trajectories were estimated using the expected years to symptom onset (EYO).

Results: Blood β-synuclein levels were already higher in asymptomatic autosomal dominant AD (ADAD) mutation carriers (median 7.29pg/mL, interquartile range (IQR) 5.89-8.59) compared with non-carriers (5.25pg/mL, IQR 4.63-6.66) and highest in sMC (14.4pg/mL, IQR 11.7-18.4). Longitudinal trajectories based on the EYO and correlation analyses indicated that β-synuclein levels start to rise after amyloid deposition preceding axonal degeneration, brain atrophy and hypometabolism and cognitive decline. β-synuclein levels were associated with cognitive impairment and gradually increased with declining cognition. Thus, our data rank synaptic degeneration as one of the earliest events in the ADAD pathophysiological cascade as it is close to the start of amyloid deposition and it supports a retrograde degeneration mechanism starting in the synapse.

Conclusion: Our study supports the use of blood β-synuclein to track synaptic changes in preclinical AD and as a surrogate marker for cognitive impairment which might be used in early diagnosis and to support patient selection and monitoring of treatment effects in clinical trials of putative AD-modifying therapies.