Plasma p‐tau217 and APOE genotype: Prodromal Alzheimer's disease staging

Abstract

Introduction

Plasma phosphorylated tau (p‐tau)217 levels and apolipoprotein E (APOE) ε4 genotype are used for prodromal identification of individuals at high risk for dementia. We used baseline samples from cognitively normal subjects in the Risk Reduction for Alzheimer's Disease (rrAD) clinical trial (NCT02913664) to investigate the relationship between these two markers in older adults with additional vascular risk factors.

Methods

We measured APOE genotype, plasma p‐tau217, and standard neuroimaging metrics in 400 rrAD participants, aged 60 to 84 years, with sedentary lifestyle, hypertension, and hypercholesterolemia.

Results

Plasma p‐tau217 was 57% higher in subjects with at least one APOE ε4 allele, increased with age, was higher in males, and negatively correlated with brain volume measures by magnetic resonance imaging.

Discussion

Plasma p‐tau217 demonstrated elevation in the APOE ε4 genotype in cognitively normal older adults. These data also suggest that brain volume shrinkage in males is related to elevated p‐tau217. APOE genotyping will be important for clinical practice using p‐tau217 quantification.

Highlights

• Cognitively healthy subjects with cardiovascular risk factors and the apolipoprotein E (APOE) ε4 genotype had markedly higher blood levels of phosphorylated tau (p‐tau)217 than non‐ε4 carriers.

• Males had higher p‐tau217 levels than females, irrespective of APOE genotype.

• p‐tau217 levels were negatively correlated with brain volume measures in cognitively healthy males.

1. INTRODUCTION

Alzheimer's disease (AD) is the most prevalent cause of dementia and has been projected to affect > 150 million people by 2050. ¹ AD‐related neurodegenerative processes lead to progressive cognitive decline and interfere with the ability to function independently. Given that the pathological changes of AD begin decades before the onset of clinical symptoms, identifying individuals at high risk for AD and related dementias (ADRD) during this prodromal stage is critical for effective therapeutic intervention.

Apolipoprotein E (apoE) is a protein that, in the brain, is localized within glial cells where it plays a role in lipid homeostasis, neuronal development, and brain repair. ² apoE exists in three isoforms: ε2, ε3, and ε4. ² The ε2 isoform induces neuroprotection against developing AD, the ε3 form is the most common (> 50% of the population), and possession of ε4 increases the risk of AD. Although allele frequencies differ among non‐Hispanic White, Black, Hispanic, and Japanese populations, the ε4 isoform confers elevated AD risk across all major ancestral groups. ³ Importantly, recent studies indicate that carrying two copies of ε4 represents a genetic cause for late‐onset AD. ⁴

Blood‐based biomarkers for AD offer a minimally invasive and cost‐effective alternative to cerebrospinal fluid analysis and neuroimaging, making them ideal for large‐scale screening and clinical trial recruitment. ⁵ Recent data indicate that plasma levels of phosphorylated tau (p‐tau)217 are highly accurate in the identification of AD, both in its early and late stages, and even for prodromal identification. ⁶ , ⁷ , ⁸ For example, plasma p‐tau217 levels increased longitudinally and correlated with brain atrophy and clinical deterioration in two AD‐related cohorts, ⁶ and provided an accurate marker for amyloid beta (Aβ) pathology. ⁹ The US Food and Drug Administration has recently approved the Lumipulse G pTau217/β‐Amyloid_1–42 plasma ratio as a blood test for AD. ¹⁰ Whether blood biomarkers for AD can be used to identify prodromal AD in older adults with cardiovascular risk factors for AD (i.e., hypertension and hypercholesterolemia) was the focus of this study. Here, using baseline data (n = 400) from the 2‐year Risk Reduction for Alzheimer's Disease (rrAD) clinical trial, we examined whether cognitively normal older adults with vascular risk factors who possess the APOE ε4 allele exhibit elevated plasma p‐tau217 relative to non‐ε4 carriers.

RESEARCH IN CONTEXT

1. Systematic review: The authors reviewed the literature using traditional (e.g., PubMed) sources, meeting abstracts, and presentations. The importance of apolipoprotein E (APOE) genotype and risk for Alzheimer's disease (AD) has been highlighted in many studies and in current reviews, as has the utility of the phosphorylated tau (p‐tau)217 blood biomarker for the disease. These relevant citations are appropriately referenced.

2. Interpretation: Our findings show the importance of the APOE genotype combined with plasma p‐tau217 in quantifying the pathophysiology of AD and aiding in its prodromal identification, including in individuals with vascular risk factors.

3. Future directions: Future studies can examine whether the efficacy of lifestyle interventions that alter vascular risk factors and/or the trajectory of AD are modulated by the presence of elevated p‐tau217 and APOE ε4 genotype. Examples include: (a) does exercise reduce the risk of AD for only those without the APOE ε4 genotype? and (b) does exercise reduce the progression of AD pathology via reduction of p‐tau217, especially in ε4‐expressing males?

2. METHODS

2.1. Subjects

Subjects included in this study were those selected for blood biomarker analyses from the rrAD trial ¹¹ , ¹² testing the hypothesis that exercise combined with intensive pharmacological reduction of vascular risk factors over a period of 2 years provides greater benefits for neurocognitive function than either intervention alone. This study was approved by the human subjects committees at Pennington Biomedical Research Center, The University of Texas Southwestern Medical Center, University of Kansas Medical Center, and Washington University School of Medicine. This study included 400 participants who consented for blood draw, of whom 355 also consented to DNA analysis.

Please see Supplemental Methods for supporting information on subject details, genotyping, plasma biomarkers, magnetic resonance imaging (MRI) methods for brain volume, kidney function, and statistical analyses.

3. RESULTS

Table S1 in supporting information shows participant characteristics for the n = 400 rrAD participants, with 60% being female and 27.5% carrying an APOE ε4 allele (n = 110). Table S2 in supporting information shows final mean/median values for all biomarkers used in the continuous measures, including plasma p‐tau217, plus cognitive scores and neuroimaging results taken at baseline upon enrollment in the rrAD trial. Based on the validated p‐tau217 cutoff (0.3 pg/mL), ⁹ 42.5% of participants were classified as Aβ+. APOE ε4 carriers were significantly more likely to be Aβ+ (P < 0.0001), with Cohen κ = 0.279 (P < 0.0001), indicating fair agreement (66.0%) between APOE ε4 carrier status and Aβ classification. Analysis of covariance (ANCOVA) examined the relationships among p‐tau217, sex, and APOE genotype, with age and estimated glomerular filtration rate (eGFR) as covariates (39 participants had low eGFR scores). p‐tau217 was negatively correlated with eGFR (P < 0.01; Table S3 in supporting information) and brain volume across all six regions of interest (all P < 0.001; Table S4 in supporting information).

After accounting for age (F = 14.33, P < 0.0002) and eGFR (F = 20.87, P < 0.0001), carriers of at least one APOE ε4 allele exhibited 57% higher plasma p‐tau217 levels compared to non‐carriers (F = 55.05, P < 0.0001; Figure 1A). Using a Dunn post hoc test, we found that p‐tau217 levels were significantly lower in ε2/ε3 carriers compared to ε4/ε4 carriers (p = 0.0003). Levels were lower in ε3/ε3 versus ε4/ε4 carriers (p = 0.0055) and ε3/ε4 carriers (p = 0.0055). However, no significant difference was observed between ε3/ε4 and ε4/ε4 carriers (p = 0.925), confirming the elevated p‐tau217 levels in even single ε4 allele carriers. Males exhibited significantly higher levels of plasma p‐tau217 than females (F = 16.77, p < 0.0001; Figure 1B) with either ε4 absent or present.

FIGURE 1.

Plasma p‐tau217 levels measured in rrAD participants as a function of APOE genotype and sex. A, p‐tau217 levels, based upon APOE allele number, show increases from ε2/ε3 to ε4/ε4. B, The mean p‐tau217 levels are elevated in males versus females regardless of whether they carry APOE ε4 versus no APOE ε4. C, Aggregated data shown in (A) but divided by the 0.3pg/ml p‐tau217 cut‐off value for potential Aβ+ (left side) and Aβ– (right side) individuals. Each dot represents an rrAD subject at baseline. Aβ, amyloid beta; APOE, apolipoprotein E; p‐tau, phosphorylated tau; rrAD, Risk Reduction for Alzheimer's Disease trial.

Using a cutoff of p‐tau217 > 0.3 pg/mL (150/353; 42.5%), participants were classified as Aβ+. APOE ε4 carriers were significantly more likely to be Aβ+ than non‐carriers (63.6% vs. 32.9%; χ²[1] = 29.233, p < 0.0001; Figure 1C). Repeating the primary ANCOVA and binary generalized linear models within Aβ− and Aβ+ subgroups produced consistent results with the pooled analyses: APOE ε4 carrier status was associated with higher p‐tau217 (continuous and binary models), but there was no longer an APOE ε4 × sex interaction (see Table S5 in supporting information).

4. DISCUSSION

We found that the AD blood biomarker p‐tau217 was positively correlated with age in our cohort of cognitively normal subjects with cardiovascular risk factors, and APOE ε4 carrier status exhibited 57% higher plasma levels of p‐tau217 than non‐carriers. To our knowledge, no prior work has established a mechanistic link between tau phosphorylation sites and ε4 expression, but the presence of this gene, along with an age of > 60 years, places these individuals within the prodromal stage of AD. ¹³ This is particularly relevant given that two copies of the APOE ε4 allele have been reported to confer a risk for incident AD equivalent to an additional 10 years of aging after age 50. ¹⁴ , ¹⁵

It was recently reported that a plasma p‐tau217 level of 0.29pg/mL was characteristic of mild cognitive impairment (MCI) patients, and a level of 0.72pg/mL was observed in AD patients. ¹⁵ Our cohort had a general p‐tau217 level of 0.35pg/mL, but APOE ε4 carriers exhibited 0.47 pg/mL. These data indicate that APOE genotype is important in defining cutoffs for disease staging, as is sex, where we found males to have significantly higher p‐tau217 levels than females. Higher p‐tau217 levels in males may reflect sex‐specific vulnerability to tau phosphorylation, differences in microglial activation, or structural reserve, ² but further mechanistic studies are warranted. Currently, it is suggested that APOE genotyping be part of routine clinical workup for people with memory impairment ² , ¹⁴ , ¹⁶ not only because of the risk factor for AD, but for an important side effect of amyloid antibody therapy—that is, amyloid‐related imaging abnormalities—which is related to mini‐strokes and microbleeds in ε4 carriers. ¹⁷ Identification of a substantial Aβ+ subgroup within our cognitively normal cohort aligns with estimates for preclinical AD prevalence in the seventh decade of life. The stronger APOE ε4 associations observed within the Aβ+ subgroup support the interpretation that elevated p‐tau217 is, in part, driven by early amyloid‐associated tau pathology.

We observed a consistent negative correlation between plasma p‐tau217 levels and brain volume across all regions examined. Interestingly, the volume of these six brain regions was consistently smaller in males than in females (data not shown), yet plasma levels of p‐tau217 were higher in males. It may be that the male brain is hyper‐phosphorylating tau more aggressively than the female brain and thereby causing neurodegeneration/brain shrinkage. However, this hypothesis requires investigation. Although body mass index, blood pressure, and lipid profiles are known contributors to dementia risk, they did not significantly influence p‐tau217 levels after covariate adjustment (Table S3).

There is no cognitive impairment in the rrAD cohort, as enrollment was based on a normal cognitive status. However, rrAD participants with two copies of APOE ε4 were in the “MCI range” of the study by Thijssen et al., ¹⁵ in which p‐tau217 was measured in those with MCI. Several studies suggest the utility of APOE genotyping at an early stage for identification of at‐risk patients. For example, a case–control study confirmed that the rare Christchurch APOE ε3 mutation prevented a woman with autosomal dominant AD from becoming demented at the typical age. ¹⁸ Also, in mice, this mutation suppressed the Aβ‐induced tau seeding that is characteristic of AD ¹⁹ and reduced APOE‐mediated toxicity and tau phosphorylation. ²⁰ A recent review ² highlights how APOE genotype interacts with AD pathology and biomarkers (both central and peripheral), indicating a promising future for monitoring APOE‐targeted therapies through fluid biomarkers like p‐tau217. A substantial fraction of our cognitively normal cohort exhibited plasma p‐tau217 levels above the operational cutoff for probable Aβ positivity (≈ 40%). Because APOE ε4 carriers were significantly overrepresented among Aβ+ participants, APOE genotype influences the distribution of blood‐based biomarkers in screening samples and should therefore be considered when defining cutoffs and when stratifying participants for clinical trials. Future clinical trials will benefit from testing subject participants both for p‐tau217 and APOE ε4 at baseline, as this may affect the therapeutic efficacy of the intervention. This dual approach could not only improve subject stratification for therapeutic trials but also allow for a more personalized medicine approach in which interventions are tailored to an individual's specific genetic and biomarker profile.

CONFLICT OF INTEREST STATEMENT

The authors have no conflicts of interest to report. Author disclosures are available in the supporting information.

CONSENT STATEMENT

All human subjects provided informed consent.

Supporting information

Supporting Information

Supporting Information

Stowe AM, Kahn B, Ballesteros A, et al. Plasma p‐tau217 and APOE genotype: Prodromal Alzheimer's disease staging. Alzheimer's Dement. 2026;18:e70279. 10.1002/dad2.70279