Plasma epidermal growth factor levels predict cognitive decline in Parkinson disease

Abstract

Objective: Most people with Parkinson disease (PD) eventually develop cognitive impairment (CI). However, neither the timing of onset nor the severity of cognitive symptoms can be accurately predicted. We sought plasma-based biomarkers for CI in PD.

Methods: A discovery cohort of 70 PD patients was recruited. Cognitive status was evaluated with the Mattis Dementia Rating Scale-2 (DRS) at baseline and on annual follow-up visits, and baseline plasma levels of 102 proteins were determined with a bead-based immunoassay. Using linear regression, we identified biomarkers of CI in PD, that is, proteins whose levels correlated with cognitive performance at baseline and/or cognitive decline at follow-up. We then replicated the association between cognitive performance and levels of the top biomarker, using a different technical platform, with a separate cohort of 113 PD patients.

Results: Eleven proteins exhibited plasma levels correlating with baseline cognitive performance in the discovery cohort. The best candidate was epidermal growth factor (EGF, p < 0.001); many of the other 10 analytes covaried with EGF across samples. Low levels of EGF not only correlated with poor cognitive test scores at baseline, but also predicted an 8-fold greater risk of cognitive decline to dementia-range DRS scores at follow-up for those with intact baseline cognition. A weaker, but still significant, relationship between plasma EGF levels and cognitive performance was found in an independent replication cohort of 113 PD patients.

Interpretation: Our data suggest that plasma EGF may be a biomarker for progression to CI in PD.

Figure 1. Expression of proteins with plasma levels associated with cognitive performance

Heat map represents plasma expression of 11 proteins whose levels correlated with baseline cognition (age-adjusted DRS scores), as well as 2 additional proteins (CD40 and EGF-R, purple text) with receptor activity for CD40 ligand and EGF, the top 2 proteins in our screen (red text). Of note, CD40 and EGF-R levels were not correlated with cognition, and plasma levels may not reflect the cell-bound fraction of these receptors. Hierarchical clustering (dendrogram on right side) indicates that many of the 11 cognition-associated proteins co-vary, with CD40 ligand, EGF, PAI-1, Thrombospondin-1, and PDGF representing one cluster with particularly correlated expression. However, HBEGF and EGF do not co-vary across samples, although they may act on the same receptor. Rows in the heat map represent expression levels of each protein (labeled on left side), with red denoting higher expression, blue denoting lower expression, and grey denoting intermediate expression. Columns in the heat map represent individual patients, with columns marked with green bars (top) indicating individuals with baseline age-adjusted DRS≤5 (PDD-range), and those marked with gold bars indicating individuals with baseline age-adjusted DRS>5 (non-PDD-range). Individuals with baseline PDD-range cognition almost uniformly had low expression for EGF and co-expressed proteins (yellow box). Green arrows (top) indicate individuals with baseline non-PDD-range cognition who converted to PDD-range cognition during follow-up.

Figure 2. EGF as a biomarker for cognitive impairment in PD

(a) EGF plasma levels were robust across technical platforms. Duplicate aliquots of 24 plasma samples were quantified for EGF levels by multiplex immunoassay and by traditional ELISA. Readings by multiplex immunoassay (EGF-RBM, X-axis) were plotted against readings by ELISA (EGF-ELISA, Y-axis). Cross-platform correspondence was excellent, as indicated by an r² value of 0.76. Values are shown in pg/mL. (b) Low plasma EGF levels were predictive of conversion from non-PDD-range cognitive performance (age-adjusted DRS>5) to PDD-range cognitive performance (age-adjusted DRS≤5). Follow-up DRS scores were available for 49/54 patients with non-PDD-range baseline cognition. Survival curves of time to PDD-range cognitive performance are shown for those with the lowest quartile of EGF expression (quartile 1, red line), as well as those in EGF quartiles 2 (blue line), 3 (green line), and 4 (purple line). Outcomes for EGF quartile 1 differed significantly from the other 3 quartiles, demonstrating an eightfold higher risk of conversion to PDD-range cognition (p<0.001, hazard ratio 8.34, 95% CI 4.26-122.90) with a median time-to-conversion of 14 months. (c) Numbers of individuals (N), EGF values (median, full range in pg/mL) for each EGF quartile are shown among those patients with baseline age-adjusted DRS>5. In addition, the number of individuals converting to an age-adjusted DRS≤5 within the follow-up period is shown for each EGF quartile; follow-up periods (median months, IQR = interquartile range) were similar among the quartiles.

Figure 3. Correlation of plasma EGF and age-adjusted DRS score in an independent replication cohort

(a) Graphical representation of multivariate model used in replication cohort. In the second cohort of 113 PD patients, gender and UPDRS motor scores differed significantly between those individuals with PDD-range cognitive performance (more likely to be male and have high UPDRS motor scores), and those with non-PDD range cognitive performance. Taking into account these effects, low plasma EGF levels were predictive of lower DRS scores in women across all ranges of UPDRS motor scores, and men with low UPDRS motor scores. Red denotes females, and blue denotes males. Solid lines denote individuals with low UPDRS motor scores, and dashed lines denote individuals with high UPDRS motor scores. (b) A forward stepwise approach was used to determine the final multivariate linear regression model, which specified plasma EGF values as the independent variable, with gender, UPDRS motor scores (UPDRS), and interaction terms as covariates.