A molecular signature in blood identifies early Parkinson's disease

Abstract

Background: The search for biomarkers in Parkinson's disease (PD) is crucial to identify the disease early and monitor the effectiveness of neuroprotective therapies. We aim to assess whether a gene signature could be detected in blood from early/mild PD patients that could support the diagnosis of early PD, focusing on genes found particularly altered in the substantia nigra of sporadic PD.

Results: The transcriptional expression of seven selected genes was examined in blood samples from 62 early stage PD patients and 64 healthy age-matched controls. Stepwise multivariate logistic regression analysis identified five genes as optimal predictors of PD: p19 S-phase kinase-associated protein 1A (odds ratio [OR] 0.73; 95% confidence interval [CI] 0.60-0.90), huntingtin interacting protein-2 (OR 1.32; CI 1.08-1.61), aldehyde dehydrogenase family 1 subfamily A1 (OR 0.86; 95% CI 0.75-0.99), 19 S proteasomal protein PSMC4 (OR 0.73; 95% CI 0.60-0.89) and heat shock 70-kDa protein 8 (OR 1.39; 95% CI 1.14-1.70). At a 0.5 cut-off the gene panel yielded a sensitivity and specificity in detecting PD of 90.3 and 89.1 respectively and the area under the receiving operating curve (ROC AUC) was 0.96. The performance of the five-gene classifier on the de novo PD individuals alone composing the early PD cohort (n = 38), resulted in a similar ROC with an AUC of 0.95, indicating the stability of the model and also, that patient medication had no significant effect on the predictive probability (PP) of the classifier for PD risk. The predictive ability of the model was validated in an independent cohort of 30 patients at advanced stage of PD, classifying correctly all cases as PD (100% sensitivity). Notably, the nominal average value of the PP for PD (0.95 (SD = 0.09)) in this cohort was higher than that of the early PD group (0.83 (SD = 0.22)), suggesting a potential for the model to assess disease severity. Lastly, the gene panel fully discriminated between PD and Alzheimer's disease (n = 29).

Conclusions: The findings provide evidence on the ability of a five-gene panel to diagnose early/mild PD, with a possible diagnostic value for detection of asymptomatic PD before overt expression of the disorder.

Figure 1

The receiver operating characteristic curve (ROC) of the logistic regression model for discriminating between early PD and control. The blue line depicts integrative specificity and sensitivity for the PP derived from five variables (SKP1A, HIP2, ALDH1A1, PSMC4 and HSPA8 transcriptional expression levels, AUC = 0.96) of the early/mild PD vs healthy control subject cohorts. At a cut-off point of 0.5 it was possible to distinguish between PD individuals and healthy controls with sensitivity and specificity values of 90.3% and 89.1% respectively. The red line shows the performance of the classifier on the 38 de novo, non medicated PD individuals alone from the early PD cohort, which resulted in a similar ROC with an AUC of 0.95, indicating the stability of the logistic model.

Figure 2

Predictive probability (PP) for PD in early PD subjects compared to advanced PD, AD and healthy control groups. a) The distribution of the PP values of the early/mild PD, advanced PD, AD and healthy cohorts derived from the logistic regression analysis are depicted. The box plots represent 50% of the cases, with the median (horizontal bold line) and the 1st and 3rd quartile values (bottom and top of the box, respectively). The bottom and top whiskers show the lowest and the highest datum within 1.5x interquartile range (IQR, the range between the 1^st and 3^rd quartile) from bottom and top of the box, respectively. Outliers are denoted by black dots. Beyond the cut-off of PP = 0.5 the subjects were considered as having PD. b) The performance of the classifier across the entire data set. Blood samples are ordered by their PP for PD. 86 out of 93 individuals that ranked in the upper panel (>0.5) are PD. 86 out of 92 individuals that ranked in the lower panel (< 0.5) are controls. PD = Parkinson’s disease. AD = Alzheimer’s disease.

Figure 3

QRT-PCR assessment of the relative transcript expression levels in the marker panel. The box plots depict the natural logarithms of the relative gene expression levels (calculated by dividing the QRT-PCR values by the geometric mean of the HKs ACTB, ALAS1 and GAPDH raw quantities) for the individual five genes in blood samples of 62 early stage PD patients (Early PD; H&Y = 1.40 (SD = 0.56)), 30 PD patients at relatively advanced stage (Advanced PD; H&Y = 3.07 (SD = 0.81)), 29 AD patients and 64 healthy age-matched subjects (Control). Outliers are denoted by black dots. The significance was calculated by one-way ANOVA, with post-Hoc Tukey analysis. *p<0.05 vs control; ~p<0.05 vs early PD. In the text, expression level changes (percentage) refer to relative gene expression levels and not to the natural logarithms.