CSF biomarker panels. Focus on synaptic pathology

Abstract

Fluid biomarkers of neuropathological features are important clinical tools in diagnostics and patient monitoring of neurodegenerative diseases. For the most prevalent cause of dementia, Alzheimer’s disease (AD), several biomarkers have been introduced in the clinic reflecting the underlying pathophysiology features of amyloid-β deposition, tau pathology with hyperphosphorylation, and neurodegeneration (ATN). Together the biomarkers have been demonstrated to have >90% sensitivity and specificity for the early stages of AD. However, in the field of biomarker research, one area which has gained recent attention is biomarkers reflective of synaptic pathology (degeneration and dysfunction of the synapse), which is an early and central part of the pathophysiology of many neurodegenerative diseases, including AD, and clinically relevant since synaptic function is the foundation of cognition. Synaptic biomarkers are thus of interest not only in the routine clinical assessment of neurodegenerative diseases to facilitate diagnosis, disease staging, and progression, but especially to monitor the efficacy and endpoints of treatments in drug trials which commonly aim to halt or reduce synaptic damage.

The main goals of this thesis were to develop and optimize methods for quantifying biomarkers of synaptic pathology, to evaluate their potential in AD and across neurodegenerative diseases, and to examine concordance and discordance between biomarker results and other measures of synaptic dysfunction. The work focused on multiplexed mass spectrometric (MS) methods that enable quantification with high specificity and sensitivity of a range of potential biomarkers with different functions and localizations. These methods were then used not only to compare the biomarkers’ diagnostic and disease monitoring potential but also to reinforce the credibility of the results of proteins with similar outcomes, i.e., validating general and specific pathological patterns across and within neurodegenerative diseases. A synaptic panel assay was successfully established, quantifying 17 synaptic proteins, including several SNARE proteins, neurogranin, synucleins, neuronal pentraxins, and 14-3-3 proteins. Together with an in-house-established MS assay quantifying SNAP-25 and synaptotagmin-1, the panel method was used to study synaptic proteins across neurodegenerative diseases in several studies included in this thesis.

One of the main findings is that out of the potential synaptic biomarkers, several of them showed specifically higher concentrations in the AD continuum in contrast to other neurodegenerative diseases. Indicating that higher levels of synaptic proteins are possibly generally a specific feature of AD and thus a marker of AD-specific synaptic dysfunction mechanisms compared with other neurodegenerative diseases. The possible exemption to this seems to be 14-3-3 ζ/δ, of which higher levels across neurodegenerative diseases might indicate that it is a general biomarker of synaptic degeneration mechanisms. Particularly, SNAP-25, neurogranin, and β-synuclein, as well as 14-3-3 ζ/δ, seem to be promising AD biomarkers able to both predict disease progression as well as cognitive decline. For SNAP-25, it was also shown that a newly developed Single molecule array (Simoa) assay for SNAP-25 quantification can be used interchangeably with other previously established methods. Furthermore, this thesis work demonstrated that the neuronal pentraxins are present at lower concentrations across neurodegenerative diseases, indicative of synaptic dysfunction and degeneration mechanisms equally affected across diseases. The neuronal pentraxins were also found to be associated with cognitive status in AD dementia and Parkinson’s disease, the latter in which they were also associated with cognitive decline and the progression of motor symptoms and might be useful to predict disease severity. This thesis establishes that the neuronal pentraxins are possible prognostic and monitoring biomarkers for synaptic dysfunction/degeneration that associate with cognitive and motor symptoms across neurodegenerative diseases. Additionally, novel differences in synaptic proteins were found in both parkinsonian disorders and genetic frontotemporal dementia (FTD), with differential synaptic impairment represented by different synaptic proteins. Interestingly, multiple abnormalities were shown in the symptomatic patients with MAPT mutations indicating specific synaptic dysfunction in regard to the underlying proteinopathy found in each genetic FTD mutation. The results demonstrate that differential patterns of synaptic protein alterations across neurodegenerative diseases exist, probably due to differences in synaptic pathology mechanisms. In conclusion, several of the studied synaptic proteins show promise as possible complements to other CSF and imaging markers as diagnostic, prognostic, stage, or monitoring biomarkers of cognitive decline and synaptic pathology. Furthermore, this thesis provided novel insight into synaptic pathology in neurodegenerative diseases. A better understanding of the mechanistic pathways of synaptic dysfunction across and between diseases may thus contribute to improving diagnostics and potentially also to the development of new therapeutic strategies targeting said pathways. The work included in this thesis demonstrates the importance of MS-based biomarker discovery, allowing for the simultaneous quantification and exploration of multiple biomarkers leading to knowledge that can drive the development of biomarkers as well as new, highly precise methods and increase the availability of biomarker quantification.