Combined measure of salivary alpha-synuclein species as diagnostic biomarker for Parkinson’s disease

Abstract

Parkinson’s disease (PD) diagnosis is still vulnerable to bias, and a definitive diagnosis often relies on post-mortem neuropathological diagnosis. In this regard, alpha-synuclein (αsyn)-specific in vivo biomarkers remain a critical unmet need, based on its relevance in the neuropathology. Specifically, content changes in αsyn species such as total (tot-αsyn), oligomeric (o-αsyn), and phosphorylated (p-αsyn) within the cerebrospinal fluid (CSF) and peripheral fluids (i.e., blood and saliva) have been proposed as PD biomarkers possibly reflecting the neuropathological outcome. Here, we measured the p-αsyn levels in the saliva from 15 PD patients along with tot-αsyn, o-αsyn and their ratios, and compared the results with those from 23 healthy subjects (HS), matched per age and sex. We also calculated the optimal cutoff values for different αsyn species to provide information about their capability to discriminate PD from HS. We found that p-αsyn was the most abundant alpha-synuclein species in the saliva. While p-αsyn concentration did not differ between PD and HS when adjusted for total salivary proteins, the ratio p-αsyn/tot-αsyn was largely lower in PD patients than in HS. Moreover, the concentration of o-αsyn was increased in the saliva of PD patients, and tot-αsyn did not differ between PD and HS. The ROC curves indicated that no single αsyn form or ratio could provide an accurate diagnosis of PD. On the other hand, the ratio of different items, namely p-αsyn/tot-αsyn and o-αsyn, yielded more satisfactory diagnostic accuracy, suggesting that the combined measure of different species in the saliva may show more promises as a diagnostic means for PD.

Evaluating the Diagnostic Potential of Combined Salivary and Skin Biomarkers in Parkinson’s Disease

“…In our study, PD patients exhibited significantly higher mean levels of salivary oligomeric α-syn compared to HSs, corroborating findings coming from previous stud-ies using similar methodological approaches on different biological fluids, including saliva [9,11,12,23,24], plasma [21] or CSF [39]. The detection of elevated salivary oligomeric α-syn levels in PD patients as assessed by ELISA is especially noteworthy, as it highlights a potential diagnostic biomarker for PD derived from an easily accessible, non-invasively sampled biological fluid with a straightforward detection method.…”

“…The earliest studies, which focused on the analysis of cerebrospinal fluid (CSF), demonstrated that concentrations of oligomeric α-syn in the CSF are higher 2 of 15 in PD patients compared to control subjects, as determined by immunoassays such as ELISA [13,14,[16][17][18][19][20]. More recently, these findings have been confirmed using more accessible biological fluids, including plasma [21,22] and saliva [9,11,12,23,24], suggesting that the increase in oligomeric α-syn in these samples may serve as a potential molecular biomarker of PD. At the same time, the detection of intra-neural pathologically phosphorylated isoforms of α-syn deposits in the skin has been shown to reliably distinguish patients with PD from healthy controls as well as from those with atypical parkinsonism [25][26][27][28].…”

Salivary Biomarkers for Parkinson’s Disease: A Systematic Review with Meta-Analysis

“…In contrast, Goldman et al [42] and Chahine et al [38] observed an increasing tendency. Another two studies performed ROC analyses, which did not reach satisfactory results (AUC 0.523, sensitivity 40.0 or 52.5%, specificity 51.6 or 78.3%) [36,40]. In summary, despite discrepancies in levels, salivary t-α-syn seems to be a reliable candidate for PD biomarker based on our meta-analysis.…”

“…Nevertheless, substantial discrepancies in specificity are confusing; indeed, in a study by Sabaei et al [44], patients affected by Alzheimer's Disease (AD) had even lower levels of t-α-syn than PD patients. On the other hand, as many as seven studies revealed no significant differences in salivary t-α-syn between the diagnostic groups [36,[38][39][40][41][42]46]. Among them, most studies revealed a decreasing tendency in t-α-syn levels in PD patients.…”

“…On the other hand, seven studies investigated salivary o-α-syn levels. In six studies, salivary o-α-syn levels were significantly elevated in PD groups compared to healthy controls [36,40,[45][46][47][48]. Some authors performed ROC analyses of o-α-syn as a potential biomarker of PD [36,40,45,48].…”