A minimally invasive biomarker for sensitive and accurate diagnosis of Parkinson’s disease | Acta Neuropathologica Communications

124 serum samples (82 PD, 42 HC) and 131 saliva samples (83 PD, 48 HC) were examined by αSyn-SAA using the RT-QuIC platform (Table 1). PD blood donors had a mean age of 69.21 years (range 44–88) and 45 males (54.9%); HC blood donors had a mean age of 66.55 years (range 44–81) and 11 males (26.2%) (Table 1). The PD saliva donors had a mean age of 69.58 years (range 49–87) and 46 males (55.4%), and HC saliva donors had a mean age of 64.71 years (range 30–81) and 14 males (29.2%) (Table 1).

Detection of αSyn^D seeding activity in serum from PD and HC subjects

We modified the immunoprecipitation-based αSyn-SAA protocol with RT-QuIC [30] to detect αSyn^D seeding activities in serum and saliva samples. The αSyn-SAA reproducibility of different batches of recombinant αSyn protein was verified with 14 biopsy skin samples from known PD and healthy control subjects (7 each) (Supplementary Fig. 1). Representative RT-QuIC ThT fluorescence curves for blinded saliva and serum samples, including 10 PD and 10 HC each, indicated that the saliva and serum samples of patients with PD had overall higher ThT fluorescence readings than HC samples (Fig. 1).

Representative ThT Fluorescence Curves of αSyn RT-QuIC Assays of Serum or Saliva Samples. A. Representative curves of ThT fluorescence readings over time for αSyn^D RT-QuIC assays of serum samples from 10 PD and 10 HC subjects. B. Representative curves of ThT fluorescence readings over time for αSyn^D RT-QuIC assays of saliva samples from 10 PD and 10 HC subjects. All samples were coded and blinded for the RT-QuIC assays. The ThT fluorescence readings at the endpoint (93.35 h) were normalized to percentages of the maximal fluorescence reading (260,000) and used to measure the relative αSyn^D seeding activities in the respective samples. Orange lines: curves for PD samples; black lines: curves for HC subjects

αSyn-SAA examination of 124 serum samples from 82 patients with PD (63 probable PD, 19 possible PD) and 42 HC subjects revealed 80.49% sensitivity, 90.48% specificity, and 0.9006 accuracy [AUC of ROC (same below), 95% CI, 0.8472–0.9539, p < 0.0001] for diagnosis of PD compared with clinical diagnosis (Fig. 2-A & B, Table 2). For serum samples from patients with probable PD, the sensitivity, specificity, and accuracy were 79.37%, 90.48%, and 0.8857 (95% CI, 0.8212-9502, p < 0.0001), respectively (Supplementary Fig. 2-A & B).

Comparison of αSyn^D Seeding Activity in Serum or Saliva Samples from Patients with PD and Healthy Controls (HC) by αSyn-SAA. Scatter graphs of RT-QulC endpoint ThT fluorescence intensities (αSyn^D seeding activities) in serum samples (A) or saliva samples (C) from patients with PD and HC subjects. Graphed are the average of the endpoint ThT fluorescence in quadruplicate wells of 124 serum samples (42 HC, 82 PD) or 131 saliva samples (48 HC, 83 PD) in RT-QuIC assays as a percentage of the maximum fluorescence (%ThT fluorescence). ThT fluorescence cutoff: serum, 52,105; saliva, 62,613. **** p < 0.0001. ROC curves for αSyn^D seeding activities in 124 serum samples (B) or 131 saliva samples (D) from patients with PD and HC subjects. SE, standard error. 95% CI, 95% confidence interval.

Detection of αSyn^D seeding activity in saliva from PD and HC subjects

αSyn-SAA examination of 131 saliva samples from 83 PD (24 probable PD and 59 possible PD) and 48 HC subjects achieved 74.70% sensitivity, 97.92% specificity, and 0.8966 accuracy (95% CI, 0.8454–0.9478, p < 0.0001) for diagnosis of patients with PD (Fig. 2-C & D, Table 2). For saliva samples from probable PD cases, the sensitivity, specificity, and accuracy were 79.66%, 97.92%, and 0.9054 (95% CI, 0.8484-09623, p < 0.0001), respectively (Supplementary Fig. 2-C & D).

PD diagnosis based on αSyn^D seeding activities in both serum and saliva

We hypothesized that PD patients with negative serum αSyn-SAA are likely to have positive saliva αSyn-SAA and vice versa. To test this hypothesis, we evaluated serum and saliva αSyn-SAA data from the “ss-subset” composed of 48 patients with PD (34 probable PD, 14 possible PD) and 26 HC subjects who provided both blood and saliva during the same visits and compared performance in PD diagnosis when the αSyn-SAA data from the two sample types were used alone or together. When the serum αSyn-SAA data were used alone, 85.42% sensitivity, 92.31% specificity, and 0.9623 accuracy (95% CI, 0.9258–0.9989, p < 0.0001) were achieved for PD diagnosis (Fig. 3-A & B); for probable PD, the sensitivity, specificity, and accuracy were 85.29%, 92.31%, and 0.9615 (95% CI, 0.9210-1.000, p < 0.0001), respectively (Supplementary Fig. 3-A & B). In comparison, when the saliva αSyn-SAA data were used alone, 75.00% sensitivity, 92.31% specificity, and 0.9046 accuracy (95% CI, 0.8392–0.9701, p < 0.0001) were achieved for PD diagnosis (Fig. 3-C & D, Table 2); for probable PD cases, the sensitivity, specificity, and accuracy were 76.47%, 92.31%, and 0.8824 (95% CI, 0.7971-0.9676, p < 0.0001), respectively (Supplementary Fig. 3-C & D).

Enhanced Diagnostic Accuracy for PD Using αSyn^D Seeding Activities in Both Serum and Saliva Samples from a Subset of Patients with PD and Healthy Control (HC) by αSyn-SAA. A. Scatter graph of αSyn^D seeding activities (RT-QuIC endpoint ThT fluorescence intensity) of serum samples in a subset of PD and HC subjects with paired serum and saliva samples. Scatter graph was plotted based on the average of the endpoint ThT fluorescence in quadruplicate wells as a percentage of the maximum fluorescence (%ThT fluorescence) in RT-QuIC assay of serum samples from 48 patients with PD and 26 HC in a subset of PD and HC subjects with both serum and saliva samples (termed serum-saliva subset or ss-subset). ThT fluorescence cutoff: 52,105. **** p < 0.0001. B. ROC curve and AUC for serum αSyn^D seeding activity comparisons between patients with PD and HC subjects in the ss-subset. ROC curve and AUC value were obtained based on αSyn^D seeding activity in serum samples from the patients with PD and HC of the ss-subset shown in panel A. C. Scatter graph of RT-QulC endpoint ThT fluorescence intensity (αSyn^D seeding activity) of saliva samples from patients with PD and HC in the ss-subset. Scatter graph was plotted based on αSyn^D seeding activities in saliva samples from the patients with PD and HC of the ss-subset shown in panel A. ThT fluorescence cutoff: 62,613. **** p < 0.0001. D. ROC curve and AUC for saliva αSyn^D seeding activity comparisons between the patients with PD and HC in a ss-subset. ROC curve and AUC value were obtained based on αSyn^D seeding activities in saliva of the patients with PD and HC of the ss-subset shown in panel C. E. 3-D Plot to identify optimal cutoff values for serum and saliva for maximum diagnostic accuracy for PD in the ss-subset shown in A and C. PD diagnostic accuracy was plotted against the RT-QuIC endpoint ThT fluorescence cutoff values of both serum and saliva in a 3-D plot, which identified the optimal endpoint ThT fluorescence cutoff settings to achieve maximal diagnostic accuracy for PD as 52,960 for serum and 66,800 for saliva. The accuracy values were calculated by varying the cutting off values for both serum and saliva with the definition that a patient was considered positive for PD only when the endpoint ThT fluorescence of both serum and saliva samples exceeded their respective cutoff values. F. ROC curve and AUC for PD diagnosis based on αSyn^D seeding activities in both serum and saliva of patients with PD and HC in the ss-subset. ROC curve and AUC were obtained based on calculated sensitivity and specificity values when varying the ThT fluorescence cutoff values for both serum and saliva. The sensitivity and specificity values were calculated based on the same definition of PD positivity as described in panel E. R analysis of the paired serum and saliva αSyn^D seeding activity data of the ss-subset was in agreement with the ROC analysis. **** p < 0.001. SE, standard error. 95% CI, 95% confidence interval

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For combined serum and saliva αSyn-SAA data analysis, a patient was PD-positive if either serum or saliva αSyn-SAA was positive, and a patient was PD-negative when αSyn-SAAs were negative in both sample types. 3-D plotting with values of accuracy, serum cutoff, and saliva cutoff (Fig. 3E) revealed 52,960 and 66,800 (ThT fluorescence units) as the optimal cutoff values for serum and saliva samples, respectively. With these optimal cutoff values, 95.83% sensitivity, 96.15% specificity, and 93.75% accuracy were achieved for PD diagnosis (Table 2). If the specificity was set at 100%, 91.67% sensitivity and 91.67% accuracy were still attained. We also generated a ROC curve for the combined serum-saliva αSyn^D seeding activity data, which showed an accuracy of 0.98 (AUC of ROC, 95% CI, 0.96-1.0, p < 0.001) (Fig. 3F). The cumulative RT-QuIC ThT fluorescence kinetic curves displaying the mean and standard deviation (SD) over time of serum or saliva samples from patients with probable PD, patients with possible PD, and healthy controls of the ss-subset are shown in Supplementary Fig. 4.

Taken together, these results demonstrate that the diagnostic accuracy for PD using serum and saliva αSyn-SAA data together is much better than using αSyn-SAA data from either sample type alone (Table 2).

Clinical correlation of αSyn^D seeding activities in serum or saliva samples

We examined correlations between the αSyn-SAA status in serum or saliva samples of patients with PD with clinical features and demographic factors (Supplementary Table 1). When comparing serum αSyn-SAA positivity between PD and HC subjects, significant differences were found for Schwab & England scale (p < 0.001), PDQ-39 scores and some sub-scores [total (p < 0.05), mobility (p = 0.041), ADL (p < 0.001), and cognitive impairment (p = 0.006)], HAM-D (p = 0.025), self-reported hyposmia (p = 0.001) and constipation (p < 0.001) ( Supplementary Table 1). For saliva αSyn-SAA positivity in PD and HC subjects, the p-value findings were similar, except that significant difference was also found for age (p = 0.003) but not PDQ-39 mobility (p = 0.11) or HAM-D (p = 0.078) (Supplementary Table 1). A subset of saliva donors (34 PD and 22 HC) from our “saliva biomarker study” cohort, with a more limited clinical dataset, was only included in the analysis for age, age at diagnosis, disease duration, sex, mH&Y, MoCA, and MSD-UPDRS Part 3 (Supplementary Table 1).

Clinical correlations with αSyn^D seeding activities in serum or saliva samples from patients with PD were also examined by Pearson’s correlation analysis. Serum αSyn^D seeding activities of patients with PD correlated significantly with MoCA (p = 0.04, inversely) and HAM-D (p = 0.03, positively), and weakly with PDQ-39 cognitive impairment (p = 0.07, positively) (Fig. 4, Supplementary Table 2). No significant correlation was found with hyposmia (p = 0.11), RBD (p = 0.21), mH&Y (p = 0.69), constipation (p = 0.98), or any other features examined (Supplementary Table 2). Saliva αSyn^D seeding activities of patients with PD correlated significantly with age at diagnosis (p = 0.02, inversely) and RBD (p = 0.04, inversely) (Fig. 5). No significant correlation was found between saliva αSyn^D seeding activities with MoCA (p = 0.35), mH&Y (p = 0.70), hyposmia (p = 0.63), constipation (p = 0.50) or any other features (Supplementary Table 2). No significant differences were found in clinical features between αSyn-SAA positive and αSyn-SAA negative PD participants for either serum or saliva samples.

Subgroup analyses by sex or age (< 70 years or ≥ 70 years) were also performed (Supplementary Tables 3 & 4). The age of 70 years was chosen to divide the age groups into two for this binary analysis for two reasons: (1) 70 is the mean age of onset for PD patients in general and the approximate mean age of our PD cohorts, and (2) age 70 would divide the cohorts into two subgroups that allow for meaningful subgroup statistical analysis. For sex subgroup analyses of patients with PD, serum αSyn^D seeding activities correlated inversely with MoCA in males (p = 0.01) but not in females (p = 0.70), weakly positively with PDQ-39 cognitive impairment in females (p = 0.07) but not in males (p = 0.27), and positively with HAM-D score in females (p = 0.04) but not in males (p = 0.36) (Supplementary Table 3); saliva αSyn^D seeding activities correlated inversely with age at diagnosis in males (p = 0.04) but not in females (p = 0.13) (Supplementary Table 4).

For age subgroup analyses of patients with PD, serum αSyn^D seeding activities correlated weakly inversely with MoCA in the ≥ 70 age group (p = 0.07) but not in the < 70 age group (p = 0.15), positively with HAM-A in the < 70 age group (p = 0.04) but not in the ≥ 70 age group (p = 0.40), positively with HAM-D in the < 70 age group (p = 0.01) but not in the ≥ 70 age group (p = 0.92), positively with orthostatic hypotension by vitals in the < 70 age group (p = 0.01) but not in the ≥ 70 age group (p = 0.31), inversely with ESS in the ≥ 70 age group (p = 0.03) but not in the < 70 age group (p = 0.92), and inversely with RBD in the ≥ 70 age group (p = 0.03) but not in the < 70 age group (p = 0.56) (Supplementary Table 3); saliva αSyn^D seeding activities correlated inversely with age at diagnosis in the < 70 age group (p = 0.01) but not in the ≥ 70 age group (p = 0.72), and inversely with Schwab & England in the ≥ 70 age group (p = 0.02) but not in the < 70 age group (p = 0.11) (Supplementary Table 4).

Serum αSyn^D Seeding Activities Correlate with MoCA and HAM-D among Patients with PD. αSyn^D seeding activities (endpoint ThT fluorescence as a percentage of the maximum reading) in serum samples correlate inversely with MoCA score (A), positively with HAM-D score (B), and weakly positively with PDQ-39 cognitive impairment score (D), but not with modified Hoehn & Yahr (mH&Y) (C) of patients with PD. Linear regression lines with 95% confidence interval (gray shade) are shown

Saliva αSyn^D Seeding Activities Correlate with Age at Diagnosis and RBD among Patients with PD. αSyn^D seeding activities (endpoint ThT fluorescence as a percentage of the maximum reading) in saliva samples correlate inversely with RBD status (A) and age at diagnosis (B), but not with modified Hoehn & Yahr (mH&Y) (C) or MoCA (D) of patients with PD. Linear regression lines with 95% confidence interval (gray shade) are shown for B-D