Blood β-synuclein is related to amyloid PET positivity in memory clinic patients

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.

Early increase of the synaptic blood marker β-synuclein in asymptomatic autosomal dominant Alzheimer's disease

INTRODUCTION

β-synuclein is a promising blood marker to track synaptic degeneration in Alzheimer's disease (AD) but changes in preclinical AD are unclear.

METHODS

We investigated serum β-synuclein in 69 cognitively unimpaired mutation non-carriers, 78 cognitively unimpaired AD mutation carriers (asymptomatic AD), and 31 symptomatic mutation carriers from the Dominantly Inherited Alzheimer Network.

RESULTS

β-synuclein levels were already higher in asymptomatic AD mutation carriers compared to non-carriers and highest in symptomatic carriers. Longitudinal trajectories 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.

DISCUSSION

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.

HIGHLIGHTS

Blood β-synuclein levels were already higher in asymptomatic Alzheimer's disease (AD) mutation carriers. Blood β-synuclein levels were highest in symptomatic AD mutation carriers. Blood β-synuclein levels start to rise 11 years before symptom onset. Rise of β-synuclein precedes axonal degeneration, brain atrophy, and cognitive decline. β-synuclein levels gradually increased with declining cognition.

Alzheimer's disease neuropathology and its estimation with fluid and imaging biomarkers

Alzheimer's disease (AD) is neuropathologically characterized by the extracellular deposition of the amyloid-β peptide (Aβ) and the intraneuronal accumulation of abnormal phosphorylated tau (τ)-protein (p-τ). Most frequently, these hallmark lesions are accompanied by other co-pathologies in the brain that may contribute to cognitive impairment, such as vascular lesions, intraneuronal accumulation of phosphorylated transactive-response DNA-binding protein 43 (TDP-43), and/or α-synuclein (αSyn) aggregates. To estimate the extent of these AD and co-pathologies in patients, several biomarkers have been developed. Specific tracers target and visualize Aβ plaques, p-τ and αSyn pathology or inflammation by positron emission tomography. In addition to these imaging biomarkers, cerebrospinal fluid, and blood-based biomarker assays reflecting AD-specific or non-specific processes are either already in clinical use or in development. In this review, we will introduce the pathological lesions of the AD brain, the related biomarkers, and discuss to what extent the respective biomarkers estimate the pathology determined at post-mortem histopathological analysis. It became evident that initial stages of Aβ plaque and p-τ pathology are not detected with the currently available biomarkers. Interestingly, p-τ pathology precedes Aβ deposition, especially in the beginning of the disease when biomarkers are unable to detect it. Later, Aβ takes the lead and accelerates p-τ pathology, fitting well with the known evolution of biomarker measures over time. Some co-pathologies still lack clinically established biomarkers today, such as TDP-43 pathology or cortical microinfarcts. In summary, specific biomarkers for AD-related pathologies allow accurate clinical diagnosis of AD based on pathobiological parameters. Although current biomarkers are excellent measures for the respective pathologies, they fail to detect initial stages of the disease for which post-mortem analysis of the brain is still required. Accordingly, neuropathological studies remain essential to understand disease development especially in early stages. Moreover, there is an urgent need for biomarkers reflecting co-pathologies, such as limbic predominant, age-related TDP-43 encephalopathy-related pathology, which is known to modify the disease by interacting with p-τ. Novel biomarker approaches such as extracellular vesicle-based assays and cryptic RNA/peptides may help to better detect these co-pathologies in the future.

Biomarkers of synaptic degeneration in Alzheimer's disease

Synapse has been considered a critical neuronal structure in the procession of Alzheimer's disease (AD), attacked by two pathological molecule aggregates (amyloid-β and phosphorylated tau) in the brain, disturbing synaptic homeostasis before disease manifestation and subsequently causing synaptic degeneration. Recently, evidence has emerged indicating that soluble oligomeric amyloid-β (AβO) and tau exert direct toxicity on synapses, causing synaptic damage. Synaptic degeneration is closely linked to cognitive decline in AD, even in the asymptomatic stages of AD. Therefore, the identification of novel, specific, and sensitive biomarkers involved in synaptic degeneration holds significant promise for early diagnosis of AD, reducing synaptic degeneration and loss, and controlling the progression of AD. Currently, a range of biomarkers in cerebrospinal fluid (CSF), such as synaptosome-associated protein 25 (SNAP-25), synaptotagmin-1, growth-associated protein-43 (GAP-43), and neurogranin (Ng), along with functional brain imaging techniques, can detect variations in synaptic density, offering high sensitivity and specificity for AD diagnosis. However, these methods face challenges, including invasiveness, high cost, and limited accessibility. In contrast, biomarkers found in blood or urine provide a minimally invasive, cost-effective, and more accessible alternative to traditional diagnostic methods. Notably, neuron-derived exosomes in blood, which contain synaptic proteins, show variations in concentration that can serve as indicators of synaptic injury, providing an additional, less invasive approach to AD diagnosis and monitoring.

Utilization of fluid-based biomarkers as endpoints in disease-modifying clinical trials for Alzheimer's disease: a systematic review

BACKGROUND

Clinical trials in Alzheimer's disease (AD) had high failure rates for several reasons, including the lack of biological endpoints. Fluid-based biomarkers may present a solution to measure biologically relevant endpoints. It is currently unclear to what extent fluid-based biomarkers are applied to support drug development.

METHODS

We systematically reviewed 272 trials (clinicaltrials.gov) with disease-modifying therapies starting between 01-01-2017 and 01-01-2024 and identified which CSF and/or blood-based biomarker endpoints were used per purpose and trial type.

RESULTS

We found that 44% (N = 121) of the trials employed fluid-based biomarker endpoints among which the CSF ATN biomarkers (Aβ (42/40), p/tTau) were used most frequently. In blood, inflammatory cytokines, NFL, and pTau were most frequently employed. Blood- and CSF-based biomarkers were used approximately equally. Target engagement biomarkers were used in 26% (N = 72) of the trials, mainly in drugs targeting inflammation and amyloid. Lack of target engagement markers is most prominent in synaptic plasticity/neuroprotection, neurotransmitter receptor, vasculature, epigenetic regulators, proteostasis and, gut-brain axis targeting drugs. Positive biomarker results did not always translate to cognitive effects, most commonly the small significant reductions in CSF tau isoforms that were seen following anti-Tau treatments. On the other hand, the positive anti-amyloid trials results on cognitive function were supported by clear effect in most fluid markers.

CONCLUSIONS

As the field moves towards primary prevention, we expect an increase in the use of fluid-based biomarkers to determine disease modification. Use of blood-based biomarkers will rapidly increase, but CSF markers remain important to determine brain-specific treatment effects. With improving techniques, new biomarkers can be found to diversify the possibilities in measuring treatment effects and target engagement. It remains important to interpret biomarker results in the context of the trial and be aware of the performance of the biomarker. Diversifying biomarkers could aid in the development of surrogacy biomarkers for different drug targets.

Monitoring synaptic pathology in Alzheimer's disease through fluid and PET imaging biomarkers: a comprehensive review and future perspectives

Alzheimer's disease (AD) is currently constrained by limited clinical treatment options. The initial pathophysiological event, which can be traced back to decades before the clinical symptoms become apparent, involves the excessive accumulation of amyloid-beta (Aβ), a peptide comprised of 40-42 amino acids, in extraneuronal plaques within the brain. Biochemical and histological studies have shown that overaccumulation of Aβ instigates an aberrant escalation in the phosphorylation and secretion of tau, a microtubule-binding axonal protein. The accumulation of hyperphosphorylated tau into intraneuronal neurofibrillary tangles is in turn correlated with microglial dysfunction and reactive astrocytosis, culminating in synaptic dysfunction and neurodegeneration. As neurodegeneration progresses, it gives rise to mild clinical symptoms of AD, which may eventually evolve into overt dementia. Synaptic loss in AD may develop even before tau alteration and in response to possible elevations in soluble oligomeric forms of Aβ associated with early AD. These findings largely rely on post-mortem autopsy examinations, which typically involve a limited number of patients. Over the past decade, a range of fluid biomarkers such as neurogranin, α-synuclein, visinin-like protein 1 (VILIP-1), neuronal pentraxin 2, and β-synuclein, along with positron emission tomography (PET) markers like synaptic vesicle glycoprotein 2A, have been developed. These advancements have facilitated the exploration of how synaptic markers in AD patients correlate with cognitive impairment. However, fluid biomarkers indicating synaptic loss have only been validated in cerebrospinal fluid (CSF), not in plasma, with the exception of VILIP-1. The most promising PET radiotracer, [11C]UCB-J, currently faces significant challenges hindering its widespread clinical use, primarily due to the necessity of a cyclotron. As such, additional research geared toward the exploration of synaptic pathology biomarkers is crucial. This will not only enable their extensive clinical application, but also refine the optimization process of AD pharmacological trials.