Phospho-specific plasma p-tau181 assay detects clinical as well as asymptomatic Alzheimer's disease

Abstract

Objective: Plasma phosphorylated-tau-181 (p-tau181) reliably detects clinical Alzheimer's disease (AD) as well as asymptomatic amyloid-β (Aβ) pathology, but is consistently quantified with assays using antibody AT270, which cross-reacts with p-tau175. This study investigates two novel phospho-specific assays for plasma p-tau181 and p-tau231 in clinical and asymptomatic AD.

Methods: Plasma p-tau species were quantified with Simoa in 44 AD patients, 40 spouse controls and an independent cohort of 151 cognitively unimpaired (CU) elderly who underwent Aβ-PET. Simoa plasma Aβ42 measurements were available in a CU subset (N = 69). Receiver operating characteristics and Aβ-PET associations were used to evaluate biomarker validity.

Results: The novel plasma p-tau181 and p-tau231 assays did not show cross-reactivity. Plasma p-tau181 accurately detected clinical AD (area under the curve (AUC) = 0.98, 95% CI 0.95-1.00) as well as asymptomatic Aβ pathology (AUC = 0.84, 95% CI 0.76-0.92), while plasma p-tau231 did not (AUC = 0.74, 95% CI 0.63-0.85 and 0.61, 95% CI 0.52-0.71, respectively). Plasma p-tau181, but not p-tau231, detected asymptomatic Aβ pathology more accurately than age, sex and APOE combined (AUC = 0.64). In asymptomatic elderly, correlations between plasma p-tau181 and Aβ pathology were observed throughout the cerebral cortex (ρ = 0.40, p < 0.0001), with focal associations within AD-vulnerable regions, particularly the precuneus. The plasma Aβ42/p-tau181 ratio did not reflect asymptomatic Aβ pathology better than p-tau181 alone.

Interpretation: The novel plasma p-tau181 assay is an accurate tool to detect clinical as well as asymptomatic AD and provides a phospho-specific alternative to currently employed immunoassays.

Figure 1

Specificity of the novel versus commercial p‐tau181 Simoa assay. Synthetic peptides representing different phosphorylation statuses of T175 and T181 of the tau protein were tested in the ADx prototype assay (top panel) and in the commercial Quanterix test kit (Simoa^® p‐tau181 v.2 Advantage Kit, bottom panel), respectively. Peptide concentrations ranged between 2 pg/mL and 800 pg/mL (X axis). The assay readouts (average enzyme per beads signal) are shown on the Y axis. Peptide concentrations below the detection limit are not shown.

Figure 2

Alterations in plasma p‐tau181 and p‐tau231 levels in clinical and asymptomatic AD. Box‐ and whisker plots show differences in plasma phosphorylated‐tau (p‐tau)181 (blue, left) and p‐tau231 (orange, right) levels between Alzheimer's disease (AD) dementia patients and spouse controls of the memory clinic cohort (A) and between Aβ‐positive (A+) and Aβ‐negative (A‐) CU elderly of the F‐PACK cohort (B). The middle line of the box represents the median while the lower and upper line denote the 25th and 75th percentiles. Whiskers represent the range. Individual datapoints are presented on top of the plot. P values were obtained using Mann–Whitney U tests and are shown if significant after multiple comparison correction (significance level α = 0.05/2 = 0.03). ROC curves of plasma p‐tau181 and p‐tau231 are shown with AD diagnosis (C, E) or PET‐based evidence of Aβ pathology (Centiloid > 23.5) (D,F) as the standard‐of‐truth, either unadjusted (C,D) or adjusted (E,F) for age, sex and APOE. The demographic model (black) includes age, sex and APOE genotype as predictors without inclusion of any p‐tau biomarker. Areas under the curve (AUCs) with 95% confidence intervals (CIs) are superimposed on the plot.

Figure 3

Relationship between novel plasma p‐tau biomarkers and cerebral Aβ burden in asymptomatic elderly. (A) Plasma phosphorylated‐tau (p‐tau)181 levels (blue) were plotted against amyloid‐β (Aβ)‐PET Centiloids and Spearman rank correlations were calculated in the total F‐PACK cohort (N = 151). Linear fits are shown on top of the plot, either unadjusted (blue) or adjusted for age, sex and APOE‐ε4 carrier status (red). (B) Parametric maps of the APOE‐adjusted regional relationships between plasma p‐tau181 levels and Centiloids in the total F‐PACK cohort calculated through voxelwise multivariate linear regression models. The significance threshold was set at a cluster‐level whole‐brain family‐wise error (FWE) threshold of P_FWE < 0.05 with voxel‐level set at P _uncorrected  < 0.001. Thresholded maps were superimposed on the left and right hemisphere of the PALS cortical surface (PALS‐B12) using CARET v5.65 (Van Essen Lab, http://brainvis.wustl.edu). (C) Scatterplot of plasma p‐tau231 and Aβ‐PET CLs with calculated Spearman rank correlations in the total F‐PACK cohort with linear fits shown on top, either unadjusted (orange) or adjusted for age, sex and APOE.

Figure 4

Alterations in plasma Aβ42, p‐tau181, p‐tau231 and their ratios in asymptomatic AD. Box‐ and whisker plots of the plasma amyloid‐β (Aβ42)/phosphorylated‐tau (p‐tau)181 ratio (A, green) and p‐tau181 (B, blue) in the F‐PACK cohort stratified by Aβ‐PET status are shown (N = 69). F‐PACK subjects were Aβ‐positive (A+) when Centiloid >23.5. The middle line of the box represents the median while the lower and upper line denote the 25th and 75th percentiles. Whiskers represent the range. Individual data points are presented on top of the plot. P values were obtained using Mann–Whitney U tests (significance level α = 0.05 corresponding to an uncorrected p = 0.05/5 biomarkers = 0.01). (B) Receiver operating characteristic (ROC) curves of plasma p‐tau181, p‐tau231, Aβ42 and their ratios are shown with Aβ‐positivity as the standard‐of‐truth. Areas under the curve (AUCs) with 95% confidence intervals (CIs) are superimposed on the plot.

Figure 5

Relationship of the plasma Aβ42/p‐tau181 ratio and p‐tau181 alone with global cerebral Aβ burden. Plasma amyloid‐β (Aβ)42/phosphorylated‐tau (p‐tau)181 ratio (A, green) and p‐tau181 alone (B, blue) were plotted against Aβ‐PET Centiloids and Spearman rank correlations were calculated in the F‐PACK subset with available Aβ42 measurements (N = 69). Linear fits are shown on top of the plot.