The hierarchical progression of tau tangles in Alzheimer’s disease—from the entorhinal cortex to the neighboring hippocampus and out into the great neocortical beyond—is emblazoned in the minds of AD researchers, in part due to Heiko and Eva Braak’s neuropathological staging scheme. The advent of ever-more-sensitive tau-PET tracers has made it possible to recapitulate this staging in living people. Now, researchers led by Pedro Rosa-Neto of McGill University in Montreal have correlated in vivo tau staging with all manner of imaging, fluid, and cognitive biomarkers. On April 25 in Nature Aging, they published more support for the idea that amyloid accumulation is a prerequisite for tangle progression beyond Braak stage III, when cognitive symptoms arise. They found that CSF biomarkers of phospho-tau start to rise as early as Braak stage II, just as tangles started to creep out of the entorhinal cortex. Their study uncovers a more nuanced, tangle-based view of biomarker trajectories in AD.
- Progression of tau tangles, as per PET, broadly followed Braak neuropathological staging.
- Amyloid plaque accumulation was necessary for Braak III or later.
- CSF p-tau biomarkers emerged in Braak stage II, with p-tau231 changing first.
“This is a very strong dataset and provides a nice framework to assess the overall progression of amyloid-related changes, soluble tau changes, fibrillar tau changes and their relationships to clinical and cognitive status,” wrote David Holtzman of Washington University in St. Louis. “It should be useful in design of clinical trials, assessing treatments, and in relating previous pathological studies to the in vivo situation in living people.” Cliff Jack, Mayo Clinic, Rochester, Minnesota, called the study “extremely well done.”
Holtzman noted that while similar studies have been done before, first author Joseph Therriault and colleagues used the PET tracer MK6240, which allows for very accurate determination of Braak staging (Mar 2016 news).
By the time memory problems arise in a person with AD, amyloid and tau pathology have been doing their dirty work for decades. This long preclinical phase of AD prompted the creation of diagnostic criteria that are based on the pathobiology of the disease (Apr 2018 news). Researchers commonly use fluid and imaging biomarkers to infer the pathological goings-on within the brain during life, while the distribution of amyloid plaques and neurofibrillary tau tangles, i.e., CERAD score and Braak staging, respectively, in brain samples serve as neuropathological gold standards postmortem (Mirra et al., 1991; Braak and Braak, 1991; Braak and Braak, 1995).
Might it be possible to stage tangle progression while people are alive? Previously, Rosa-Neto’s group had reported that [18F]MK6240, which binds tangles with subnanomolar affinity, detects their accumulation in people who were still in the presymptomatic phase of AD (Pascoal et al., 2021). In the current study, Therriault devised a PET-based version of Braak staging for 324 participants in the TRIAD cohort, tying these stages to AD biomarkers. This cohort included 179 cognitively unimpaired people, 80 with mild cognitive impairment, and 65 with AD.
Based on tracer distribution, Therriault and colleagues assigned each participant a PET-based Braak stage ranging from 0 to VI, in keeping with the neuropathological scheme. Therriault said he was surprised at how well the PET-based measures in the living cohort aligned with classical Braak staging.
People in stage 0 had no detectable tangle accumulation. In stage I, tangles were limited to the transentorhinal cortex, and in stage II they had spread into the neighboring hippocampus. In stages III and IV, tangle burden increased in these earlier stage regions, and also appeared in the temporal neocortex. Tangles cropped up in the association cortices in stage V, and extended into the primary sensory areas by stage VI.
PET Brings Braak Staging to Life. The distribution of tau tracer uptake increases hierarchically according to Braak stage. [Courtesy of Therriault et al., Nature Aging, 2022.]
By and large, tau pathology proceeded in a hierarchical fashion, meaning that tangles in later Braak stage regions only appeared in people who had tau tangles in earlier Braak stage regions, as well.
People with atypical presentations of AD deviated from this hierarchy. Some of them developed tau pathology in stage III regions before extensive tangles had built up in their stage II areas. This skewed pattern of progression dovetails with findings from a recent study by scientists in Elizabeth Mormino’s lab at Stanford University, California, who identified a subset of people with preclinical AD who had more tau in cortical regions than they did in the stage II Braak regions of the medial temporal lobe (Apr 2022 news on Young et al., 2022).
The Montreal researchers also charted the longitudinal progression of Braak staging in a subset of 163 participants who underwent follow-up scans. They found that at one- and two-year follow-up, participants either remained in the same Braak stage, or had progressed to the next. Only two people progressed through two Braak stages over two years.
How did amyloid accumulation correlate with PET-based Braak staging? The researchers found that about half of people in stage 0, I, or II were amyloid-positive as per PET. By Braak stage III, two-thirds of participants were positive for amyloid, and many of those who were negative were near the threshold for brain-wide positivity, i.e., they had some regional amyloid deposition. All participants in stage IV and beyond were amyloid-positive, and the researchers found that their amyloid burden increased up to Braak stage IV before leveling off. They came to similar conclusions when using the CSF Aβ42/40 ratio as a gauge of amyloid accumulation instead of PET. In all, these findings support the concept that amyloid accumulation is required for tau tangles to move beyond the medial temporal lobe (mTL).
Therriault also tracked CSF concentrations of four phospho-tau epitopes. Differences first emerged in Braak stage II, with small, yet detectable rises in CSF p-tau217, p-tau231, p-tau235, and p-tau181 compared to Braak stage 0. These markers ramped up significantly in Braak stage III, when tangles first appear outside of the MTL. By Braak stage IV, all four forms of p-tau started to plateau.
Interestingly, p-tau231, followed closely by p-tau217, increased the most in earlier Braak stages, while p-tau181 ramped up robustly in later Braak stages. When measured in the plasma, p-tau231 was significantly elevated by Braak II, whereas p-tau181 started to trend upward in Braak II but was not significantly up until Braak IV. Researchers in Oskar Hansson’s group at the University of Lund, Sweden, had reported similar correlations in the BioFinder cohort (Aug 2019 conference news). The early rise in p-tau231 meshes with recent findings from the ALFA cohort, which also found that this particular p-tau form rises before all others, in some cases even before brain-wide amyloid positivity, as had been reported previously (Feb 2021 news).
What about cognition? Memory problems surfaced by Braak stage II, while executive function, language, and visuospatial domains remained normal. Differences in composite measures, such as the Montreal Cognitive Assessment, did not emerge until Braak IV. Scores on the clinical dementia rating scale started to worsen in Braak III-IV, consistent with mild cognitive impairment. Most people in stage V had mild dementia, while those in stage VI had mild to moderate disease.
Therriault noted that while, on average, cognition declined with increasing Braak stage, this relationship varied from person to person. This may be because health and lifestyle factors influence cognitive decline, not just AD pathogenesis. Therriault believes the PET-based Braak staging will allow researchers to better distinguish between these risk factors, and also to investigate those that boost cognitive resilience in the face of a given pathological burden.
Pulling their data together, the researchers adapted the well-known AD staging curves described by Jack and colleagues at Mayo Clinic, Rochester (Feb 2013 conference news). Instead of plotting against time on the x-axis, as Jack had done, Therriault and colleagues plotted against PET-based Braak stage (see image below). Whether anchored in time or Braak stage, the ordering of biomarker curves looked remarkably similar.
The scientists hope that using tau PET as an unbiased anchor of pathological disease stage will allow them to validate new biomarkers, and to more thoroughly understand how existing biomarkers relate to the anatomical progression of tangles.—Jessica Shugart
- Tau PET Aligns Spread of Pathology with Alzheimer’s Staging
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- From Near and Far, Aβ Beckons Tau to Tangle in the Cortex
- Move Over CSF, P-Tau in Blood Also Tells Us There’s Plaque in the Brain
- Earliest of Them All: Blood P-Tau231 Assay Flags Pre-Amyloid Alzheimer’s
- HAI—Sharper Curves: Revamping a Biomarker Staging Model
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