Novel cerebrospinal fluid and blood tau biomarkers in Alzheimer's disease and other neurodegenerative diseases

Tau is a protein predominantly expressed in neurons in the central nervous system (CNS), where it binds microtubules (MT), playing a major role regulating their dynamics and architecture. Under pathological conditions, tau detaches from MTs, leading to neuronal dysfunction and cell death. These events are the cornerstone of a group of diseases called tauopathies. In this context, Alzheimer’s disease (AD) emerges as the most prevalent cause of dementia worldwide, accounting for 60 to 80% of all cases. Two common landmarks for all tauopathies are the abnormal phosphorylation and truncation of tau protein, which are highly relevant from a fluid biomarker perspective, as the resulting tau species leak from brain into cerebrospinal fluid (CSF) and blood, where they can be quantified as indicators of pathogenic processes in brain. At the time this PhD was started, only two fluid biomarker tau species were clinically validated for AD diagnosis: phosphorylated tau at threonine 181 (p-tau181) and total-tau (t-tau), both measured in CSF. While these biomarkers have consistently shown high performance assisting the clinical diagnosis of AD, their broad implementation worldwide has been severely hampered by the perceived invasiveness associated with lumbar puncture. Similarly, imaging biomarkers have failed to become a feasible alternative due to their lack of scalability and high costs. Thus, measuring tau biomarkers in blood has been a long-sought goal in the field of fluid biomarkers, as blood collection does provide the accessibility, cost-effectiveness and scalability required in clinical settings. Our hypothesis was that the potential of tau protein as a fluid biomarker both in terms of disease staging and mechanistic understating was not yet fully uncovered. Therefore, the aim of this thesis was the identification and quantification of novel tau species (including different phosphorylated tau forms and fragments) in CSF and blood, generating clinically relevant tools that would improve our understanding of AD and other neurodegenerative diseases. First, we developed and validated two novel immunoassays capable of quantifying p-tau181 and p-tau231 in blood, demonstrating that both are highly specific biomarkers of AD (both during clinical and preclinical stages), capable of nearly perfect discrimination of neuropathology-confirmed AD from other neurodegenerative diseases, and showing high association with AD progression. Thus, this work has greatly contributed to the previously elusive goal of a worldwide implementation of fluid biomarkers through simple blood-testing. Another notable contribution of this thesis was the development of a biomarker capable of staging preclinical AD. While there are biomarkers capable of identifying incipient AD pathology during preclinical stages with high accuracy, they cannot determine whether AD cases are at the beginning or the end of the asymptomatic phase. In this context, the novel CSF biomarker p-tau235 was shown to stage preclinical AD on the basis of neuropathological findings found in post-mortem-confirmed AD brain. Our findings indicate that the combined measurement of CSF p-tau235 and p-tau231 can stratify preclinical AD into early and late cases. Finally, we investigated if tau fragments of different lengths provide meaningful advantages in terms of early detection of AD and availability in blood. Our results suggest that N-terminal t-tau forms increase early in the AD continuum, being more suitable for early detection, whereas mid-region t-tau assays provide a better diagnostic performance when discriminating AD dementia from non-AD dementia cases. Most notably, N-terminal assays appear to offer a superior diagnostic performance in blood, where we were able to demonstrate that they successfully identify AD cases. In conclusion, the work included in this thesis demonstrates that tau protein as a fluid biomarker (both phosphorylated and non-phosphorylated), can reflect a much wider variety of subtle but meaningful aspects of brain pathology than previously understood, and that the uses of tau as a fluid biomarker expand not only from asymptomatic to symptomatic AD, but also to other tauopathies and acute neurological conditions

N-terminal and mid-region tau fragments as fluid biomarkers in neurological diseases

Brain-derived tau secreted into CSF and blood consists of different N-terminal and mid-domain fragments, which may have a differential temporal course and thus, biomarker potential across the Alzheimer's disease continuum or in other neurological diseases. While current clinically validated total-tau (t-tau) assays target mid-domain epitopes, comparison of these assays with new biomarkers targeting N-terminal epitopes using the same analytical platform may be important to increase the understanding of tau pathophysiology. We developed three t-tau immunoassays targeting specific N-terminal (NTA and NTB t-tau) or mid-region (MR t-tau) epitopes, using single molecule array technology. After analytical validation, the diagnostic performance of these biomarkers was evaluated in CSF and compared with the Innotest t-tau (and as proof of concept, with N-p-tau181 and N-p-tau217) in three clinical cohorts (n = 342 total). The cohorts included participants across the Alzheimer's disease continuum (n = 276), other dementia (n = 22), Creutzfeldt-Jakob disease (n = 24), acute neurological disorders (n = 18) and progressive supranuclear palsy (n = 22). Furthermore, we evaluated all three new t-tau biomarkers in plasma (n = 44) and replicated promising findings with NTA t-tau in another clinical cohort (n = 50). In CSF, all t-tau biomarkers were increased in Alzheimer's disease compared with controls (P < 0.0001) and correlated with each other (rs = 0.53-0.95). NTA and NTB t-tau, but not other t-tau assays, distinguished amyloid-positive and amyloid-negative mild cognitive impairment with high accuracies (AUCs 84% and 82%, P < 0.001) matching N-p-tau217 (AUC 83%; DeLong test P = 0.93 and 0.88). All t-tau assays were excellent in differentiating Alzheimer's disease from other dementias (P < 0.001, AUCs 89-100%). In Creutzfeldt-Jakob disease and acute neurological disorders, N-terminal t-tau biomarkers had significantly higher fold changes versus controls in CSF (45-133-fold increase) than Innotest or MR t-tau (11-42-fold increase, P < 0.0001 for all). In progressive supranuclear palsy, CSF concentrations of all t-tau biomarkers were similar to those in controls. Plasma NTA t-tau concentrations were increased in Alzheimer's disease compared with controls in two independent cohorts (P = 0.0056 and 0.0033) while Quanterix t-tau performed poorly (P = 0.55 and 0.44). Taken together, N-terminal-directed CSF t-tau biomarkers increase ahead of standard t-tau alternatives in the Alzheimer's disease continuum, increase to higher degrees in Creutzfeldt-Jakob disease and acute neurological diseases and show better potential than Quanterix t-tau as Alzheimer's disease blood biomarkers. For progressive supranuclear palsy, other tau biomarkers should still be investigated

Head-to-head comparison of plasma p-tau181, p-tau231 and glial fibrillary acidic protein in clinically unimpaired elderly with three levels of APOE4-related risk for Alzheimer's disease

Plasma phosphorylated tau (p-tau) and glial fibrillary acidic protein (GFAP) both reflect early changes in Alzheimer's disease (AD) pathology. Here, we compared the biomarker levels and their association with regional β-amyloid (Aβ) pathology and cognitive performance head-to-head in clinically unimpaired elderly (n = 88) at three levels of APOE4-related genetic risk for sporadic AD (APOE4/4 n = 19, APOE3/4 n = 32 or non-carriers n = 37). Concentrations of plasma p-tau181, p-tau231 and GFAP were measured using Single molecule array (Simoa), regional Aβ deposition with 11C-PiB positron emission tomography (PET), and cognitive performance with a preclinical composite. Significant differences in plasma p-tau181 and p-tau231, but not plasma GFAP concentrations were present between the APOE4 gene doses, explained solely by brain Aβ load. All plasma biomarkers correlated positively with Aβ PET in the total study population. This correlation was driven by APOE3/3 carriers for plasma p-tau markers and APOE4/4 carriers for plasma GFAP. Voxel-wise associations with amyloid-PET revealed different spatial patterns for plasma p-tau markers and plasma GFAP. Only higher plasma GFAP correlated with lower cognitive scores. Our observations suggest that plasma p-tau and plasma GFAP are both early AD markers reflecting different Aβ-related processes

Association between polygenic risk score of Alzheimer’s disease and plasma phosphorylated tau in individuals from the Alzheimer’s Disease Neuroimaging Initiative

BACKGROUND: Recent studies suggest that plasma phosphorylated tau181 (p-tau181) is a highly specific biomarker for Alzheimer’s disease (AD)-related tau pathology. It has great potential for the diagnostic and prognostic evaluation of AD, since it identifies AD with the same accuracy as tau PET and CSF p-tau181 and predicts the development of AD dementia in cognitively unimpaired (CU) individuals and in those with mild cognitive impairment (MCI). Plasma p-tau181 may also be used as a biomarker in studies exploring disease pathogenesis, such as genetic or environmental risk factors for AD-type tau pathology. The aim of the present study was to investigate the relation between polygenic risk scores (PRSs) for AD and plasma p-tau181. METHODS: Data from the Alzheimer’s Disease Neuroimaging Initiative (ADNI) was used to examine the relation between AD PRSs, constructed based on findings in recent genome-wide association studies, and plasma p-tau181, using linear regression models. Analyses were performed in the total sample (n = 818), after stratification on diagnostic status (CU (n = 236), MCI (n = 434), AD dementia (n = 148)), and after stratification on Aβ pathology status (Aβ positives (n = 322), Aβ negatives (n = 409)). RESULTS: Associations between plasma p-tau181 and APOE PRSs (p = 3e−18–7e−15) and non-APOE PRSs (p = 3e−4–0.03) were seen in the total sample. The APOE PRSs were associated with plasma p-tau181 in all diagnostic groups (CU, MCI, and AD dementia), while the non-APOE PRSs were associated only in the MCI group. The APOE PRSs showed similar results in amyloid-β (Aβ)-positive and negative individuals (p = 5e−5–1e−3), while the non-APOE PRSs were associated with plasma p-tau181 in Aβ positives only (p = 0.02). CONCLUSIONS: Polygenic risk for AD including APOE was found to associate with plasma p-tau181 independent of diagnostic and Aβ pathology status, while polygenic risk for AD beyond APOE was associated with plasma p-tau181 only in MCI and Aβ-positive individuals. These results extend the knowledge about the relation between genetic risk for AD and p-tau181, and further support the usefulness of plasma p-tau181 as a biomarker of AD

Blood phospho-tau in Alzheimer disease: analysis, interpretation, and clinical utility

Well-authenticated biomarkers can provide critical insights into the biological basis of Alzheimer disease (AD) to enable timely and accurate diagnosis, estimate future burden and support therapeutic trials. Current cerebrospinal fluid and molecular neuroimaging biomarkers fulfil these criteria but lack the scalability and simplicity necessary for widespread application. Blood biomarkers of adequate effectiveness have the potential to act as first-line diagnostic and prognostic tools, and offer the possibility of extensive population screening and use that is not limited to specialized centres. Accelerated progress in our understanding of the biochemistry of brain-derived tau protein and advances in ultrasensitive technologies have enabled the development of AD-specific phosphorylated tau (p-tau) biomarkers in blood. In this Review we discuss how new information on the molecular processing of brain p-tau and secretion of specific fragments into biofluids is informing blood biomarker development, enabling the evaluation of preanalytical factors that affect quantification, and informing harmonized protocols for blood handling. We also review the performance of blood p-tau biomarkers in the context of AD and discuss their potential contexts of use for clinical and research purposes. Finally, we highlight outstanding ethical, clinical and analytical challenges, and outline the steps that need to be taken to standardize inter-laboratory and inter-assay measurements

CSF biomarkers and plasma p-tau181 as predictors of longitudinal tau accumulation: Implications for clinical trial design

Introduction Clinical trials targeting tau in Alzheimer's disease (AD) need to recruit individuals at risk of tau accumulation. Here, we studied cerebrospinal fluid (CSF) biomarkers and plasma phosphorylated tau (p-tau)181 as predictors of tau accumulation on positron emission tomography (PET) to evaluate implications for trial designs.Methods We included older individuals who had serial tau-PET scans, baseline amyloid beta (A beta)-PET, and baseline CSF biomarkers (n = 163) or plasma p-tau181 (n = 74). We studied fluid biomarker associations with tau accumulation and estimated trial sample sizes and screening failure reductions by implementing these markers into participant selection for trials.Results P-tau181 in CSF and plasma predicted tau accumulation (r > 0.36, P 0.37, P Discussion Clinical trials testing tau-targeting therapies may benefit from using fluid biomarkers to recruit individuals at risk of tau aggregation.</p

Associations of Fully Automated CSF and Novel Plasma Biomarkers With Alzheimer Disease Neuropathology at Autopsy

Objective: To study cerebrospinal fluid (CSF) biomarkers of Alzheimer's disease (AD) analyzed by fully automated Elecsys immunoassays in comparison to neuropathologic gold standards, and compare their accuracy to plasma phosphorylated tau (p-tau181) measured using a novel Simoa method.Methods: We studied ante-mortem Elecsys-derived CSF biomarkers in 45 individuals who underwent standardized post-mortem assessments of AD and non-AD neuropathologic changes at autopsy. In a subset of 26 participants, we also analysed ante-mortem levels of plasma p-tau181 and neurofilament light (NfL). Reference biomarker values were obtained from 146 amyloid-PET-negative healthy controls (HC).Results: All CSF biomarkers clearly distinguished pathology-confirmed AD dementia (N=27) from HC (AUCs=0.86-1.00). CSF total-tau (t-tau), p-tau181, and their ratios with Aβ1-42, also accurately distinguished pathology-confirmed AD from non-AD dementia (N=8; AUCs=0.94-0.97). In pathology-specific analyses, intermediate-to-high Thal amyloid phases were best detected by CSF Aβ1-42 (AUC[95% CI]=0.91[0.81-1]), while intermediate-to-high CERAD neuritic plaques and Braak tau stages were best detected by CSF p-tau181 (AUC=0.89[0.79-0.99] and 0.88[0.77-0.99], respectively). Optimal Elecsys biomarker cut-offs were derived at 1097/229/19 pg/ml for Aβ1-42, t-tau, and p-tau181. In the plasma subsample, both plasma p-tau181 (AUC=0.91[0.86-0.96]) and NfL (AUC=0.93[0.87-0.99]) accurately distinguished pathology-confirmed AD (N=14) from HC. However, only p-tau181 distinguished AD from non-AD dementia cases (N=4; AUC=0.96[0.88-1.00]), and showed a similar, though weaker, pathologic specificity for neuritic plaques (AUC=0.75[0.52-0.98]) and Braak stage (AUC=0.71[0.44-0.98]) as CSF p-tau181.Conclusions: Elecsys-derived CSF biomarkers detect AD neuropathologic changes with very high discriminative accuracy in-vivo. Preliminary findings support the use of plasma p-tau181 as an easily accessible and scalable biomarker of AD pathology.</p

Effects of pre-analytical procedures on blood biomarkers for Alzheimer's pathophysiology, glial activation, and neurodegeneration

Introduction: We tested how tube types (ethylenediaminetetraacetic acid [EDTA], serum, lithium heparin [LiHep], and citrate) and freeze-thaw cycles affect levels of blood biomarkers for Alzheimer's disease (AD) pathophysiology, glial activation, and neuronal injury.Methods: Amyloid beta (A beta)42, A beta 40, phosphorylated tau181 (p-tau181), glial fibrillary acidic protein, total tau (t-tau), neurofilament light, and phosphorylated neurofilament heavy protein were measured using single molecule arrays.Results: LiHep demonstrated the highest mean value for all biomarkers. Tube types were highly correlated for most biomarkers (r > 0.95) but gave significantly different absolute concentrations. Weaker correlations between tube types were found for A beta 42/40 (r = 0.63-0.86) and serum t-tau (r = 0.46-0.64). Freeze-thaw cycles highly influenced levels of serum A beta and t-tau (PDiscussion: The same tube type should be used in research studies on blood biomarkers. Individual concentration cut-offs are needed for each tube type in all tested biomarkers despite being highly correlated. Serum should be avoided for A beta 42, A beta 40, and t-tau. Freeze-thaw cycles > 3 should be avoided for p-tau181.</p

Association between polygenic risk score of Alzheimer's disease and plasma phosphorylated tau in individuals from the Alzheimer's Disease Neuroimaging Initiative

Polygenic risk for AD including APOE was found to associate with plasma p-tau181 independent of diagnostic and Aβ pathology status, while polygenic risk for AD beyond APOE was associated with plasma p-tau181 only in MCI and Aβ-positive individuals. These results extend the knowledge about the relation between genetic risk for AD and p-tau181, and further support the usefulness of plasma p-tau181 as a biomarker of AD.\nRecent studies suggest that plasma phosphorylated tau181 (p-tau181) is a highly specific biomarker for Alzheimer's disease (AD)-related tau pathology. It has great potential for the diagnostic and prognostic evaluation of AD, since it identifies AD with the same accuracy as tau PET and CSF p-tau181 and predicts the development of AD dementia in cognitively unimpaired (CU) individuals and in those with mild cognitive impairment (MCI). Plasma p-tau181 may also be used as a biomarker in studies exploring disease pathogenesis, such as genetic or environmental risk factors for AD-type tau pathology. The aim of the present study was to investigate the relation between polygenic risk scores (PRSs) for AD and plasma p-tau181.\nData from the Alzheimer's Disease Neuroimaging Initiative (ADNI) was used to examine the relation between AD PRSs, constructed based on findings in recent genome-wide association studies, and plasma p-tau181, using linear regression models. Analyses were performed in the total sample (n = 818), after stratification on diagnostic status (CU (n = 236), MCI (n = 434), AD dementia (n = 148)), and after stratification on Aβ pathology status (Aβ positives (n = 322), Aβ negatives (n = 409)).\n), while the non-APOE PRSs were associated with plasma p-tau181 in Aβ positives only (p = 0.02).\nCONCLUSIONS\nBACKGROUND\nMETHODS\nRESULT

Cerebrospinal fluid p-tau231 as an early indicator of emerging pathology in Alzheimer's disease

Background: Phosphorylated tau (p-tau) epitopes in cerebrospinal fluid (CSF) are accurate biomarkers for a pathological and clinical diagnosis of Alzheimer's disease (AD) and are seen to be increased in preclinical stage of the disease. However, it is unknown if these increases transpire earlier, prior to amyloid-beta (Aβ) positivity as determined by position emission tomography (PET), and if an ordinal sequence of p-tau epitopes occurs at this incipient phase. Methods: We measured CSF concentrations of p-tau181, p-tau217 and p-tau231 in 171 participants across the AD continuum who had undergone Aβ ([18F]AZD4694) and tau ([18F]MK6240) position emission tomography (PET) and clinical assessment. Findings: All CSF p-tau biomarkers were accurate predictors of cognitive impairment but CSF p-tau217 demonstrated the largest fold-changes in AD patients in comparison to non-AD dementias and cognitively unimpaired individuals. CSF p-tau231 and p-tau217 predicted Aβ and tau to a similar degree but p-tau231 attained abnormal levels first. P-tau231 was sensitive to the earliest changes of Aβ in the medial orbitofrontal, precuneus and posterior cingulate before global Aβ PET positivity was reached. Interpretation: We demonstrate that CSF p-tau231 increases early in development of AD pathology and is a principal candidate for detecting incipient Aβ pathology for therapeutic trial application