Abstract
Background:
Associations of plasma total tau levels with future risk of AD have been described.
Objective:
To examine the extent to which plasma tau reflects underlying AD brain pathology in cognitively healthy individuals.
Methods:
We examined cross-sectional associations of plasma total tau with 11C-Pittsburgh Compound-B (PiB)-PET and 18F-Flortaucipir (FTP)-PET in middle-aged participants at the community-based Framingham Heart Study.
Results:
Our final sample included 425 participants (mean age 57.6± 9.9, 50% F). Plasma total tau levels were positively associated with amyloid-β deposition in the precuneus region (β±SE, 0.11±0.05; p = 0.025). A positive association between plasma total tau and tau PET in the rhinal cortex was suggested in participants with higher amyloid-PET burden and in APOE ɛ4 carriers.
Conclusions:
Our study highlights that plasma total tau is a marker of amyloid deposition as early as in middle-age.
INTRODUCTION
The past decade has seen rapid development in biomarkers for Alzheimer’s disease (AD) and related disorders (ADRDs). The possibility of screening people for future risk of AD at a preclinical stage by using blood biomarkers is moving closer to clinical application [1]. Established AD diagnostic biomarkers approved for clinical use include PET (amyloid and tau) and cerebrospinal fluid (CSF) Aβ42, tau, and p-tau [2]. More recently, plasma Aβ40/42, tau, p-217, and p-181, have been proposed as complementary biomarkers, due to their strong diagnostic accuracy and capacity to predict disease progression [1, 3].
However, how well these markers reflect underlying brain pathology when measured in the blood in cognitively healthy individuals, remains unknown. Plasma total tau is a promising biomarker for AD and has been examined in a variety of clinical scenarios. Prior research has identified that plasma total tau levels are higher in patients with AD [4] and can improve AD diagnostic accuracy when combined with CSF markers [5]. Plasma total tau associates with cortical glucose uptake, thinning of the temporal lobe, and memory deficits in aging individuals [6], as well as cognitive function, hippocampal volumes, and risk for incident dementia in large studies [7]. Recently, associations of plasma total tau with markers of early motor circuit disruption, including grip strength and fast walk speed have been described [8].
Although plasma total tau has been associated with AD [4, 5] and cognitive decline [6], it remains unclear whether plasma total tau levels reflect ongoing AD-pathological processes or non-specific neurodegeneration. Prior work has suggested that total tau levels may, in fact, reflect neuronal injury from other causes unrelated to AD [9]. Studies have demonstrated that plasma total tau increases in response to traumatic brain injury [10] and other conditions associated with cognitive impairment [11]. Importantly, plasma total tau has demonstrated relatively weak correlations with CSF total tau, and plasma total tau levels may reflect peripheral sources of tau rather than brain tau levels [12]. In the present study, we examined associations of plasma total tau with brain amyloid and tau accumulation measured by PET in middle aged participants from the large community-based Framingham Heart Study.
METHODS
Study sample
The FHS is a community-based, prospective study spanning 3 generations of participants from Framingham, Massachusetts. It began in 1948 with the recruitment of 5,209 participants in the Original Cohort (Gen1), which has since been re-examined once every 2 years [13]. The FHS Offspring Cohort (Gen2) commenced in 1971 with the enrollment of 5,124 participants, either offspring of Gen1 or their spouses and have had 10 quadrennial examinations completed to date [14]. In 2002, 4,095 grandchildren of the Gen1 cohort were enrolled into the Gen3 cohort, now studied across 3 examination cycles [15]. As a callback at the examination cycle 3, a subset of the Gen3 cohort were recruited for and completed their first amyloid and tau PET scans for the study. At the same time, a selected sample of Gen2 cohort participants who attended their 9th examination cycle were invited to undergo amyloid and tau PET scans. The current analyses include Gen 2 and Gen 3 participants who underwent PET imaging between 2016 and present. Eligibility for the PET imaging study included age 35 to 74 years at the time of the examination, attendance at the most recent FHS examination (Gen 3 exam 3 and Gen 2 exam 9), previous FHS study brain MRI, and absence of clinical stroke and dementia, and other neurological conditions such as multiple sclerosis that could impede MRI volume measures. The study did not specifically exclude participants with possible cognitive decline or mild cognitive impairment (MCI). FHS participants undergo routine cognitive screening and comprehensive monitoring for continual surveillance of dementia [16]. For the purposes of this analysis, we included all individuals with available PET and with plasma tau data obtained at the previous exam.
Amyloid and tau PET imaging
11C-Pittsburgh Compound B (PiB) Aβ and 18F-Flortaucipir (FTP) tau PET imaging were conducted on two scanners: first, a Siemens ECAT HR+ scanner (3D mode; 63 image planes; 15.2 cm axial field of view; 5.6 mm transaxial resolution; and 2.4 mm slice interval), as described previously [17] and a Discovery GE . GE Discovery images are smoothed with a 6 mm Gaussian filter to harmonize inter-camera data. Because of the use of two cameras, statistical models were adjusted for camera type (see Statistical analysis). Briefly, PiB PET images were acquired with a 10 to 15 mCi bolus injection followed by a 60 min dynamic acquisition. FTP PET images were obtained across 80 to 100 min using 4×5 min frames after a single 9 to 11 mCI bolus injection. PiB and FTP images were co-registered to a structural T1-weighted brain MRI using SPM8. FreeSurfer v6.0 was used to derive regions of interest [18]. PiB retention was expressed as the distribution volume ratio using the cerebellar cortex as a reference region [19]. FTP retention was expressed as the standardized uptake value ratio using the cerebellar cortex as a reference. PET data were evaluated without partial volume correction given the relatively young age of the sample with minimal atrophy. A PiB summary measure of frontal, lateral, and retrosplenial regions (FLR), was derived from the mean of superior frontal, inferior frontal, rostral middle frontal, rostral anterior cingulate, medial orbitofrontal, inferior and middle temporal, inferior parietal, and precuneus regions [19]. Since this is a middle-aged cohort and amyloid levels are lower than in elderly populations, we used non-PVC values in the FLR region to define groups based on amyloid levels using decile values. The high amyloid group corresponded to the top decile group and low amyloid were those in the remaining deciles. Regional precuneus PiB was also examined separately given the region’s susceptibility to early Aβ accumulation [20]. FTP retention was evaluated in regions with vulnerability to early tau deposition including the rhinal cortex [21], a temporal global measure estimated as the average of superior, inferior, medial, banks of superior temporal, fusiform, entorhinal, amygdala, parahippocampal regions) [22], and a global cortical measure defined as the average of 34 FreeSurfer cortical regions: the banks of the superior temporal sulcus, caudal anterior cingulate, caudal middle frontal, cuneus, entorhinal, frontal pole, fusiform, inferior parietal, inferior temporal, insula, isthmus cingulate, lateral occipital, lateral orbitofrontal, lingual, medial orbitofrontal, middle temporal, paracentral, parahippocampal, pars opercularis, pars orbitalis, pars triangularis, pericalcarine, postcentral, posterior cingulate, precentral, precuneus, rostral anterior cingulate, rostral middle frontal, superior frontal, superior parietal, superior temporal, supramarginal, temporal pole, and transverse temporal.
Quantification of plasma total tau
Blood samples were obtained following an overnight fast at examination cycle 8 (2005–2008) for the Gen2 cohort and examination cycle 2 (2008–2011) for the Gen3 cohort. Samples were immediately centrifuged, aliquoted, and stored at –80°C and plasma total tau measured using a Simoa Tau 2.0 Kit and a Simoa HD-1 analyzer (Quanterix, MA) as described previously [7]. The limit of detection of the assay is 0.019 pg/mL. The assay uses a set of monoclonal antibodies reacting to both normal and phosphorylated tau and can detect all tau isoforms. The analytical range was between 0.06 and 360 pg/mL. The intra-assay coefficient of variation was 4.1%, and the inter-assay coefficient of variation was 7.5%. As an additional quality control, we included 292 phantom samples, which corresponded to duplicates marked with dummy participant identification codes, used to confirm assay precision. A subsample (471 of 6,417) was excluded with suboptimal correspondence between phantom and original samples across 6 consecutive days of running the assays. These samples, all from Gen3 participants, were excluded from further analysis. Comparison of Gen3 participants who were included and excluded based on t-tau assay quality was previously reported [7]. For all analyses, plasma tau values were log transformed as done previously [7].
APOE genotype
DNA was obtained using 5–10 ml of whole blood. APOE genotype was determined by PCR as previously described [23]. We defined APOE ɛ4 carrier status as positive when participants had at least one copy of the ɛ4 allele.
Statistical analysis
Baseline demographic and clinical covariates were obtained at the time of the plasma tau measurements which is the main exposure in our study, corresponding to the 9th examination cycle in the Gen2 cohort and the 2nd examination cycle for the Gen3 cohort (See Quantification of plasma total tau). Data normality was visually ascertained. Aβ PET retention in the FLR and precuneus were skewed and were natural log transformed to normalize their distributions. All PET variables were standardized prior to analyses. Associations between plasma total tau levels with regional Aβ and tau PET retention were evaluated with linear regression models adjusting for age, sex, time from blood draw to PET examination, and PET camera type. Interactions between median age (<57 versus≥57), APOE ɛ4 status (carrier versus non-carrier), and amyloid burden (high versus low groups, defined above) and plasma tau were evaluated with linear regression. When significant interactions were observed, stratified analyses were performed using linear regression adjusted for the aforementioned covariates. Statistical tests were 2-sided and the criterion for significance was set at p-value of < 0.05 for the main analyses. For interaction tests, a p-value threshold of p < 0.10 was used considering power limitations due to a relatively small sample size. Our study focused on 5 regions associated with early vulnerability to AD (two for PiB and three for FTP). Due to the relatively small number of outcome regions examined no FDR-correction was applied. Stage-1 hypertension was defined as having a systolic blood pressure at least 140, diastolic blood pressure at least 90, or being on anti-hypertension medication (self-reported or obtained from prescriptions data when available). Diabetes was defined as fasting plasma glucose level≥126 mg/dl, random plasma glucose level≥200 mg/dl and/or current use of a glucose-lowering medication. Analyses were performed using SAS version 9.4.
RESULTS
Our final sample consisted of 425 participants (68 Gen 2, 357 Gen 3) with both plasma total tau and PET data. The mean age at the time of PET evaluation was 57.6 (SD±9.9), and 50% of the participants were female. The mean time between blood collection and PET examination was 10 years (SD±2.3). Mean plasma total tau level was 4.1pg/mL (SD±1.9). Demographic and clinical characteristics, including the number of participants who underwent both tau and amyloid PET are shown in Table 1. A comparison of demographic and clinical characteristics of those included in the PET study versus the rest of the cohort who attended Gen3 exam 2 or Gen2 exam (when plasma tau was measured) is shown in Supplementary Table 1. Enrolled participants were younger, less hypertense, less diabetic, and more highly educated.
Demographic and Clinical Characteristics
Plasma total tau levels were associated with Aβ deposition in the precuneus region in multivariate models adjusted for age, sex, the time from blood to PET examination, and camera used (β±SE, 0.11±0.05; p = 0.025) (Table 2). No significant associations were found with other brain regions evaluated. The association remained significant when performed in the subsample of participants who completed both amyloid and tau PET (β±SE, 0.13±0.05; p = 0.01; n = 313) (Supplementary Table 2).
Associations between plasma tau and PET measures (n = 425)
Standardized βs presented. All models adjusted for age, age squared, sex, time from plasma to PET, and camera. Aβ, amyloid-β. Significant associations highlighted in bold.
We then explored the effect of having high amyloid deposition on the aforementioned associations. In this middle-aged cohort, 41 participants met our criteria for high Aβ deposition (top decile for non-PVC PiB uptake in the FLR region). From the 41 participants with high Aβ levels, 26 also had tau PET data. A modification effect by amyloid status was suggested on the associations of plasma total tau and tau PET deposition in the rhinal cortex (p = 0.08) (Supplementary Table 3). Exploratory analyses in stratified groups did not identify significant associations between plasma total tau and tau PET binding in this region (β±SE, 0.38±0.34; p = 0.28; n = 26), likely due to the small number of individuals in this middle-aged sample with high Aβ deposition (Supplementary Table 4). An interaction was observed when examining a modification effect by APOE ɛ4 status in the association of plasma total tau levels and tau tracer uptake in the rhinal cortex (p = 0.03) (Supplementary Table 5). An exploratory analysis stratified by APOE ɛ4 did not identify a significant association between plasma total tau and tau PET binding in the rhinal region in APOE ɛ4 carriers (β±SE, 0.20±0.14; p = 0.15; n = 73), likely due to the small number of participants in this subgroup (Supplementary Table 6). No interactions were observed with age (Supplementary Table 7).
DISCUSSION
In our study, plasma total tau levels were associated with precuneus amyloid deposition in a middle-aged community-based cohort. In the early phases of AD, pathological abnormalities are predominantly located in posterior cortical regions of the brain. The precuneus has been identified as one of the earliest affected regions in AD, together with the posterior cingulate, the retrosplenial cortex, and the lateral posterior parietal cortex [24]. Amyloid deposition in these regions occurs in parallel with hypometabolism [25], and changes in neural connectivity in the default mode network [26]. Studies have highlighted the key role of the precuneus as a pivotal element in the default mode network and an area vulnerable to AD pathology [27]. In patients with mild cognitive impairment, reductions in functional connectivity in the precuneus are observed [28] and disconnection of the precuneus has been proposed as an early event preceding brain atrophy, which becomes prominent at later disease stages [29]. Notably, in a recent randomized trial, repetitive transcranial magnetic stimulation of the precuneus for 24 months in mild to moderate AD patients was associated with slower cognitive decline [30], opening the possibility to further exploring the effect of early interventions targeting the precuneus at earlier stages in people at risk. Our finding that plasma total tau levels associate with amyloid accumulation in the precuneus when measured in a middle-aged community cohort suggests that, despite the possibility that plasma total tau levels can rise in response to unrelated neurodegenerative processes, elevated plasma total tau levels likely reflect early amyloid deposition. As an interpretation of the identified precuneus effect, we found that an increase in 1 standard deviation units (SDU) in log plasma tau was equivalent to approximately 3.7 years of brain aging, which was found by dividing the beta estimate of our main model (b = 0.11) by the estimate from the regression of STDZ(log(precuneus))∼age (b = 0.03).
Although there is evidence that amyloid and tau pathology can occur independently [31], there is growing evidence that amyloid precedes tau pathology in the progression from cognitively normal to AD [32–35]. This aspect may help explain why total tau was associated with precuneus amyloid deposition but not tau-PET deposition. Although we anticipated associations between plasma total tau with amyloid FLR, no significant associations were detected. A possibility is that our sample was too young to detect an association, however no interaction by age was noted when we stratified by median age of 57. Given that amyloid deposition is a marker of AD risk, we stratified participants based on amyloid levels (top decile of non-PVC FLR versus bottom deciles) and studied the modification effect of amyloid deposition in the association of plasma tau levels with amyloid and tau accumulation. An interaction of amyloid deposition was observed for the association of plasma tau with tau PET binding in the rhinal cortex. Although a change in directionality was suggested, no significant associations were observed in groups stratified by amyloid deposition, potentially due to small sample size, limiting our power to detect an association. If such a trend exist should be evaluated in a future study with a larger number of participants with elevated amyloid levels.
The progression of amyloid and tau accumulation in aging is strongly dependent on underlying APOE ɛ4 genotype, a recent study using aggregated cohorts from BioFINDER, Seoul, AVID, UCSF, and ADNI (n = 1667) identified different tau accumulation trajectories based on APOE ɛ4 genotypes, suggesting the existence of subtypes rather than a single continuum in the patterns of progression [36]. Prior work indicates that APOE ɛ4 is associated with an earlier age of onset in patients with amnestic presentation (most of AD patients), while APOE ɛ4 negative for non-amnestic etiologies [37–39]. A recent study indicated that APOE ɛ4 negative carriers have a younger symptom onset, more overall tau burden and more right sided accumulation and worse executive function, APOE ɛ4 carriers where the most frequently associated with AD pathology and medial temporal lobe affectation in particular and followed the classical Braak staging. We found a modification effect by APOE ɛ4 for the association of plasma total tau and tau PET levels in the rhinal region, which is consistent with a predominant MTL trajectory in APOE ɛ4 carriers [36] and the fact that the rhinal region has recently been identified as the initial location of MTL tau spread [21].
Our study has a number of limitations. First, blood draw and PET examination did not occur at the same time and although our models were adjusted by the time between the two measures, the time window represents a potential limitation. Second, the number of participants identified as having high amyloid levels was relatively small and didn’t allow a robust stratification based on amyloid positivity. Additionally, the sample size did not allow for the exploration of ethnic-specific subgroup analyses which should be done in a larger sample. Finally, a consideration is the fact that recent progress in AD biofluid biomarkers identified brain-derived tau as a novel marker that primarily reflects tau isoforms derived from the brain as opposed to other isoforms from peripheral sources [40], as well as plasma phosphorylated (p)-tau 181 and 217 [41] as measures of special interest due to their strong capacity to identify AD risk when compared to plasma total tau. The extent to which plasma total tau reflects vulnerability to AD is, however, a relevant question due to a large body of prior and ongoing work which motivated our study.
Conclusion
We measured plasma total tau and amyloid and tau PET in a middle-aged community cohort at the FHS. Our results show that plasma total tau levels are associated with precuneus amyloid deposition, one of the earliest affected regions in the progression to AD. This study highlights that plasma total tau is a marker of early amyloid deposition, with abnormalities detectable as early as mid-life.
AUTHOR CONTRIBUTIONS
Jaime Ramos-Cejudo (Conceptualization; Investigation; Methodology; Writing – original draft); Matthew Richard Scott (Formal analysis; Investigation; Methodology; Software; Writing – review & editing); Jeremy A. Tanner (Methodology; Resources; Writing – review & editing); Matthew P. Pase (Methodology; Writing – review & editing); Emer R. McGrath (Writing – review & editing); Saptaparni Ghosh (Project administration; Resources); Ricardo S. Osorio (Writing – review & editing); Emma Thibault (Data curation; Resources; Writing – review & editing); Georges El Fakhri (Data curation; Resources; Supervision; Writing – review & editing); Keith A. Johnson (Data curation; Methodology; Resources; Writing – review & editing); Alexa Beiser (Conceptualization; Formal analysis; Funding acquisition; Methodology; Software; Supervision; Writing – review & editing); Sudha Seshadri (Conceptualization; Funding acquisition; Methodology; Supervision; Writing – review & editing).
Footnotes
ACKNOWLEDGMENTS
The authors would like to acknowledge the participants at the Framingham Heart Study.
FUNDING
The Framingham Heart Study is supported by the NHLBI (contract no. N01-HC-25195, no. HHSN268201500001, and no. 75N92019D00031) and the NIH R01AG054076. Dr. Beiser is supported by R01NS017950, NIH/National Institute on Aging (NIA) R01AG054076, NIH UH2 NS100605, NIH RF1AG063507, NIH/NHLBI 2018-AARG-591645, Alzheimer’s Association 75N92019D00031, NIH/NHLBI RF1AG059421-01, NIH/NIA R01AG059725, NIH/NIA R01AG062531-01A1, and NIH AARG-D 2020. Dr. Seshadri is supported by NIH 75N92019D00031 and AG054076. JRC received support from the National Alzheimer’s Disease Coordinating Center (NACC) New Investigator Award, the National Institutes of Health (NIH) R01AG079282-01, NIH R01AG070821, and the VA Cooperative Studies Program. MPP is supported by a National Health and Medical Research Council of Australia Emerging Leader 2 Investigator Grant Fellowship (GTN2009264).
CONFLICT OF INTEREST
The authors have no conflict of interest to report.
DATA AVAILABILITY
The Framingham Heart Study data is publicly available at the BioLINCC and dbGAP repositories and can be accessed at https://biolincc.nhlbi.nih.gov/home/ and 
. PET imaging data is currently being processed and will be made available in the near future. Code used for analysis is available upon reasonable request and for collaboration and reproducibility purposes.