Alpha-Synuclein Oligomers as a Biomarker Candidate for Parkinson's Disease and Disorders
Authors: El-Agnaf Omar 
Published: July 2019

Developing effective treatments for neurodegenerative diseases is one of the greatest medical challenges of the 21st century. Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are very common neurological disorders of the elderly (Jankovic et al., 2008). Although many of these clinical entities have been recognized for more than a hundred years, it is only during the past fifteen years that the molecular events that precipitate the diseases have begun to be understood. Mutations in the α-synuclein gene cause early-onset PD, often associated with dementia, and one an α-synuclein mutation segregated with pure DLB (with no Alzheimer's pathology). Neuropathologically these diseases are characterized by the presence of Lewy bodies, intraneuronal inclusions mostly composed of α-synuclein protein fibrils, cementing the notion that this protein has a central role in Lewy body diseases (LBD). Despite the progress that has been made in understanding the underlying disease mechanisms of LBD, there remains an urgent need to develop methods for use in diagnosis. The development of reliable surrogate markers for the presence and abundance of α-synuclein lesions (Lewy bodies) in the brain would naturally facilitate a more streamlined work-up during the early care of PD and DLB patients, and importantly, allow for the biologically guided evaluation of future drug trials aimed at neuroprotection in the synucleinopathies. Recently, we generated new antibodies specific for different forms of α-synuclein such as oligomeric-α-synuclein (o-α-synuclein), phosphorylated-α-synuclein at serine 129 (p-S129-α-synuclein) or total-α-synuclein (t-α-synuclein), to develop highly specific and sensitive ELISA systems to assess the levels of these species in biological fluids. Next, we utilized our assays to explore the potential use of α-synuclein species as biomarkers for PD in cerebrospinal fluid (CSF) samples from a cross-sectional cohort of 46 PD patients and 48 age-matched healthy controls. Thereafter, we validated our results in large longitudinal DATATOP cohort (n = 242) over two-year follow-up period). In our study we also investigated the potential predictive role of CSF Alzheimer's disease (AD) core biomarkers (Aβ42, total tau, p-tau). In our study we also investigated the added predictive value of CSF AD core biomarkers (Aβ42, total tau, p-tau) to CSF α-synuclein species. Interestingly, a strong positive correlation between the changes in CSF t- and o-α-synuclein levels was noted. Such a strong correlation clearly suggests that the increase of CSF t-α-synuclein levels at the early stages of the disease lead to the formation and accumulation of the pathogenic species of o-α-synuclein in the brain that cause neuronal cell death. It has been shown that soluble o-α-synuclein are elevated in brain homogenates from PD and DLB compared to normal brains (Sharon, et al., 2003; Paleologou, et al., 2009). Together with the results from our previous studies, we provide strong evidence for the important role of o-α-synuclein in the pathogenesis of PD and other synucleinopathies (El-Agnaf et al., 2003). Consistent with our recent study (Parnetti, et al., 2014), we also found that the change in CSF Aβ42 levels correlated with the change in UPDRS-mental scores, suggesting that CSF Aβ42 may aid in predicting cognitive decline in PD patients. It's worth mentioning that one of the many important features that distinguished the DATATOP study from other studies is that the untreated PD subjects enrolled in the DATATOP study were at a very early stage of the disease; a stage estimated to be earlier than most of the other clinical trials. At baseline, PD patients presented with minimal disability and were followed until just prior to the initiation of the dopamine replacement therapy, and this is reflected in the low H&Y scores registered for these patients. This limited motor disability may explain the lack of an association between the baseline or follow-up CSF biomarkers and disease severity. However, the significant changes of CSF α-synuclein levels we observed in the DATATOP cohort over approximately two years of disease progression, suggest that a correlation between CSF α-synuclein levels and clinical parameters will become apparent with longer follow-up. However, our findings still need to be further validated in large-scale, prospective and well-controlled studies, especially those that include subjects with neuroimaging-supported definite PD and healthy controls such as the on-going Parkinson's Progression Markers Initiative. Synucleinopathies and tauopathies show significant clinical and pathological overlap, making the early diagnosis of PD more challenging. This overlap necessitates a combination of measurements of CSF α-synuclein species with AD key biomarkers to improve diagnostic accuracy. Our cross-sectional analyses revealed a positive correlation between CSF t-α-synuclein and t-tau and p-tau in the PD group. Our findings confirm and extend the observations by Toledo and his colleagues who reported a positive correlation between CSF t-α-synuclein and tau levels in PD, AD and controls (Toledo et al., 2013). These data suggest that the elevation of CSF tau and t-α-synuclein reflect synaptic degeneration and axonal loss. To our knowledge, this is the first study that combines measurements of different species of α-synuclein, along with AD biomarkers, and showed significant changes of CSF α-synuclein species in large longitudinal study at the early stages of PD. Taken together, these data suggest that the levels of t-, o- and p-S129-α-synuclein species change over the natural course of the disease, and thus might aid in monitoring disease progression and severity. This supports our hypothesis that changes in CSF α-synuclein species are a more direct measure of the formation of LBs and Lewy neurites, which are the hallmarks of PD pathology. Our findings study demonstrated that quantification of α-synuclein species in CSF has strong potential value as a tool not only for PD diagnosis but also for presymptomatic screening of high-risk individuals who are good candidates for neuroprotective treatment.

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