Plasma p-tau231: a new biomarker for incipient Alzheimer's disease pathology

Abstract

The quantification of phosphorylated tau in biofluids, either cerebrospinal fluid (CSF) or plasma, has shown great promise in detecting Alzheimer's disease (AD) pathophysiology. Tau phosphorylated at threonine 231 (p-tau231) is one such biomarker in CSF but its usefulness as a blood biomarker is currently unknown. Here, we developed an ultrasensitive Single molecule array (Simoa) for the quantification of plasma p-tau231 which was validated in four independent cohorts (n = 588) in different settings, including the full AD continuum and non-AD neurodegenerative disorders. Plasma p-tau231 was able to identify patients with AD and differentiate them from amyloid-β negative cognitively unimpaired (CU) older adults with high accuracy (AUC = 0.92-0.94). Plasma p-tau231 also distinguished AD patients from patients with non-AD neurodegenerative disorders (AUC = 0.93), as well as from amyloid-β negative MCI patients (AUC = 0.89). In a neuropathology cohort, plasma p-tau231 in samples taken on avergae 4.2 years prior to post-mortem very accurately identified AD neuropathology in comparison to non-AD neurodegenerative disorders (AUC = 0.99), this is despite all patients being given an AD dementia diagnosis during life. Plasma p-tau231 was highly correlated with CSF p-tau231, tau pathology as assessed by [¹⁸F]MK-6240 positron emission tomography (PET), and brain amyloidosis by [¹⁸F]AZD469 PET. Remarkably, the inflection point of plasma p-tau231, increasing as a function of continuous [¹⁸F]AZD469 amyloid-β PET standardized uptake value ratio, was shown to be earlier than standard thresholds of amyloid-β PET positivity and the increase of plasma p-tau181. Furthermore, plasma p-tau231 was significantly increased in amyloid-β PET quartiles 2-4, whereas CSF p-tau217 and plasma p-tau181 increased only at quartiles 3-4 and 4, respectively. Finally, plasma p-tau231 differentiated individuals across the entire Braak stage spectrum, including Braak staging from Braak 0 through Braak I-II, which was not observed for plasma p-tau181. To conclude, this novel plasma p-tau231 assay identifies the clinical stages of AD and neuropathology equally well as plasma p-tau181, but increases earlier, already with subtle amyloid-β deposition, prior to the threshold for amyloid-β PET positivity has been attained, and also in response to early brain tau deposition. Thus, plasma p-tau231 is a promising novel biomarker of emerging AD pathology with the potential to facilitate clinical trials to identify vulnerable populations below PET threshold of amyloid-β positivity or apparent entorhinal tau deposition.

Keywords: Alzheimer’s disease; Biomarkers; Blood; Braak; Preclinical; Tau; p-tau181; p-tau217; p-tau231.

Fig. 1

Plasma p-tau231 concentration identifies individuals across the AD spectrum and neuropathologically confirmed AD cases. The box-and-whisker plots (left side) show plasma phospho-tau231 (p-tau231) concentrations across groups. For the box-and-whisker plots, the horizontal bar shows the median, and the upper and lower boundaries show the 25th and 75th percentiles, respectively. P values indicate the results of the analysis of variance models with Tukey’s post hoc multiple comparisons at ** P < 0.05. The figure also displays the corresponding ROC curves in the four cohorts studied (right side) to separate individuals with AD dementia from the other groups. The AUC values of the ROC curves indicate the overall biomarker performance across groups, with 0.5 indicating no difference from chance and 1.0 a biomarker with specificity and sensitivity of 100%. a, b In the discovery cohort (n = 38), plasma p-tau231 concentrations accurately discriminated AD from CU elderly Aβ- controls. c, d In the TRIAD validation cohort (n = 313), AD dementia individuals had higher plasma p-tau231 levels than all other groups. e, f In the primary care clinical cohort (n = 190), AD dementia individuals had higher plasma p-tau231 than CU but not than MCI individuals. g, h In the neuropathology cohort (n = 47) clinical cohort, AD individuals had higher plasma p-tau231 than non-AD dementias

Fig. 2

Plasma p-tau231 concentration correlates with PET and CSF tau and Aβ. The brain maps show the results of linear regressions false discovery rate corrected for multiple comparisons at P < 0.05, whereas the plots show the results of Spearman rank correlation between biomarkers. a Plasma p-tau231 concentrations were associated with [¹⁸F]MK-6240 SUVR across the cortex with the highest association in the temporal and posterior cingulate cortices as well as with b CSF p-tau231 concentrations. c Plasma p-tau231 correlates with [¹⁸F]AZD4694 SUVR across the cortex and with d CSF Aβ_1-42 concentrations

Fig. 3

Plasma p-tau231 increases earlier than plasma p-tau181 as a function of brain Aβ deposition. The lines represent weighted regression models showing the association between z-scored plasma p-tau231 and p-tau181 concentrations as a function of centiloid Aβ PET levels in CU and MCI individuals (P < 0.0001). The vertical dashed lines represent the inflection point of the plasma p-tau231 (red), plasma p-tau181 (blue), as well as the centiloid threshold of Aβ positivity (black; centiloid = 22). The graph suggests that plasma p-tau231 raises before plasma p-tau181 with the increase of brain Aβ levels

Fig. 4

Plasma p-tau231 increases earlier that plasma p-tau181 and CSF p-tau217 as a function of Aβ PET quartiles. The horizontal grey bars shows the mean and 95% confidence intervals (CI) of plasma p-tau231 and p-tau181 as well as CSF p-tau217 in elderly individuals (CU, MCI, AD) segregated Aβ PET quartiles. Plasma p-tau231 showed abnormally increased levels from Aβ PET quartile 2 to 4; plasma p-tau181 in quartile 4, and CSF in quartiles 3–4

Fig. 5

Plasma p-tau231 concentration segregates individuals across the entire Braak stage spectrum. The bars show the mean and the standard error of the mean. P values indicate the results of the analysis of variance models with Tukey’s post hoc multiple comparisons at ** P < 0.05. The figure shows that plasma p-tau231 segregates individuals across all Braak stage, whereas plasma p-tau181 did not differentiate individuals who were in Braak stage 0 from those in Braak stage I–II, suggesting plasma p-tau231 raises early in the AD pathophysiological phase