Anti-Correlated Cerebrospinal Fluid Biomarker Trajectories in Preclinical Alzheimer’s Disease

INTRODUCTION

Preclinical Alzheimer’s disease (AD) has been defined according to the progressive appearance of pathophysiological specific stages triggered by amyloid-β (Aβ), i.e., decreasing Aβ₄₂ in cerebrospinal fluid (CSF), followed by brain injury neurodegeneration, i.e., abnormal levels of tau protein and cortical and hippocampal atrophy, which ultimately results in subtle cognitive decline, though not yet so severe as to meet criteria for mild cognitive impairment (MCI) [1]. However, accumulating evidence has challenged this temporally ordered model of the evolution of the underlying pathophysiological processes that lead to AD. In the context of autosomal dominant early-onset AD, cross-sectional data inspection showed evidence of CSF total tau abnormalities more or less coinciding with CSF Aβ₄₂ abnormalities suggestive of increased amyloid aggregation [2, 3]. More recently, using probabilistic event modeling in a mixed group of cognitively intact (CI), MCI, and AD subjects, Young et al. showed that CSF tau may become abnormal before amyloidosis [4].

Similarly, several lines of evidence suggest that tau abnormalities in medial temporal lobe and limbic regions without amyloidosis may be age-associated (i.e., a so-called PART syndrome) [5]. Findings that a group of CI subjects had CSF tau but not amyloid abnormalities were consistent with this view [6]. Indeed, tau pathology in the form of abnormally phosphorylated protein has been documented to start in early young adulthood and even childhood [7, 8]. It has also been recently suggested that the initial appearance of brain injury biomarkers (in cognitively normal subjects) may not be dependent on β-amyloidosis. Based on their observations of a subgroup of CI people showing evidence of widespread neuronal injury (glucose hypometabolism and hippocampal atrophy) in absence of amyloid aggregation (PIB- positron emission tomography (PET)) [9, 10], it was hypothesized that this group had a suspected non-AD pathophysiology (SNAP).

In this study we sought to expand and refine the recent study of Mattsson et al. [11]. They identified three temporal patterns of CSF amyloid burden in CI subjects in ADNI: 1) subjects with normal levels of amyloid at baseline that remained longitudinally stable (hereafter termed Aβ₄₂ stable normal); 2) subjects with normal levels of amyloid at baseline consistently declining longitudinally (termed Aβ₄₂ declining); and 3) subjects with abnormal amyloid levels at baseline that consistently remained abnormal over time (termed Aβ₄₂ abnormal). Here we extend this work by carefully examining CSF p-tau trajectories in these Aβ₄₂-classified groups and the attendant interplay between these biomarkers.

MATERIALS AND METHODS

RESULTS

DISCUSSION

In a subgroup of clinically CI individuals who demonstrated declining CSF Aβ₄₂ levels, p-tau levels in CSF demonstrated coincident increases, suggesting that slopes were tightly coupled. Importantly, at least four subjects in the declining subgroup of the ten for whom data were available, eventually demonstrated abnormal amyloid levels (“amyloid positivity”), i.e., they progressed to preclinical AD. Covarying with Aβ₄₂ decline was a p-tau increase, which began at normal levels and ended at abnormal levels. Aβ₄₂ slope accounted for 47% of the variance in p-tau slope increases. These findings suggest that both CSF biomarkers are changing at similar rates in individuals who are converting or may be converting from CI to preclinical AD status. Of course, correlations do not equate with causality and it thus might be possible that some underlying disease processes account for both amyloid and p-tau changes independently.

In this Aβ₄₂ declining group, p-tau and Aβ₄₂ interactions also had a significant effect on hippocampus atrophy. This suggests that neurodegenerative processes involving cell death or neuronal loss in medial temporal lobe were commencing and required interactions between Aβ₄₂ and p-tau. Additionally, baseline Aβ₄₂ level had an impact on precuneus thickness, as has been reported previously [23–25].

The Aβ₄₂ stable normal group demonstrated increases in p-tau. However the slope was considerably more gradual than in the Aβ₄₂ declining group. Thus, such increases may be age associated [26]. Based on the high proportion of individuals carrying the neuroprotective APOE E2 variant in this stable group, it is unlikely that a significant number will advance to AD. The Aβ₄₂ abnormal group demonstrated a steep increase in p-tau, while Aβ₄₂ levels remained relatively stable but abnormal. In this case it may be argued that while Aβ₄₂ levels have reached an asymptote, p-tau related neurodegeneration may be ongoing and accelerated [27]. As APOE E4 carriers were disproportionately observed in this Aβ₄₂ abnormal group, it is possible that this group was more advanced in terms of AD-related neurobiological progression because of E4-driven earlier age of onset.

With respect to cognition, our findings in the whole group of CI suggest that immediate and delayed memory and speed of processing changes were predicted by increases in p-tau, supporting the already reported finding that p-tau may be more directly associated with behavioral memory performance than Aβ₄₂ [28]. However, we must acknowledge that overall cognition was inconsistently related to CSF changes in our study sample. This should be considered into the context of the very subtle cognitive impairment usually seen in CI individuals, and the possible lack of sensitivity of the traditional neuropsychological tests employed in detecting those subtle variations in cognition. Last, it is plausible that CSF effects on cognition may be mediated by brain atrophy and thus not directly related to Aβ₄₂ or p-tau variations per se.

Total tau did not demonstrate the same type of changes that p-tau showed in our study. T-tau seems to be a general marker of damage to cortical axons, whereas p-tau appears to be more characteristic of AD [29]. In the same line, the ₄₂ amino acid form of the Aβ peptide (Aβ₄₂) is particularly considered the driving force in the AD disease process as it is the most abundant species of Aβ₄₂ in amyloid plaques [30]. Therefore, reduction of CSF Aβ₄₂ seems to be a consequence of the aggregation of this Aβ₄₂ isoform into plaques [31].

Several caveats to our study must be acknowledged. First, the sample sizes for each Aβ₄₂ derived subgroup were small. However, lumbar puncture is considered an invasive procedure and less widely available, factors that limit enrollment in CSF studies. Second, longitudinal follow-up may be considered as short (4 years of follow-up). Future studies should confirm our conclusions with longer follow-up. We have reviewed the literature to ascertain sample size and missing data issues in longitudinal CSF studies in aging and preclinical AD (See Supplementary Table 5). Briefly, three studies [11, 32] in the same database had same rates of missing data, and one very recent study (Baltimore Aging) did have a slightly greater sample: 20/21 individuals with three or more follow-ups, as opposed to 31 individuals at three years of follow-up and 13 at four years in our study [33]. Thus, our study is the second largest with longitudinal CSF data in aging and preclinical AD. Furthermore, we also detail in the Supplementary Material the procedure we followed for handling missing data. Additionally, and as with every study of this type, we do not know the slope of Aβ₄₂ or p-tau before baseline. Third, the age range of our sample (from 62 to 90 years) may have some role in our findings, though it was a forced covariate in our regressions.

For the purposes of this study, we assumed that Aβ₄₂ can initiate neurodegeneration of the Alzheimer-type in older individuals. Support for this view comes from early onset familial autosomal dominant mutations in PS1, PS2, and APP, all of which directly induce amyloid misprocessing. Nevertheless, and as noted in the introduction, we are aware of so called SNAP cases in which neurodegeneration may be independent from or precede amyloid abnormalities [9, 10], that tau aggregation may occur in the absence of amyloidosis in so-called PART [5, 34], and that early tau changes may be characteristic of admixed CI, MCI, and AD subpopulations. More broadly, it is important to appreciate that for the specific purposes of this study, we began our work from a premise in which amyloid changes precede tau changes. However, this premise itself is debatable [35, 36]. Whether one accepts it, rejects it, or is agnostic as we are, there are interesting, and in our view draw unexpected implications that go beyond biomarker staging. Thus, and as described, we have shown tight temporal coupling between p-tau and Aβ₄₂. However, recent in vivo tau imaging in CI controls has indicated that fibrillary tau is generally restricted to medial temporal lobe/inferior temporal regions, while Aβ₄₂ aggregates reside primarily in neocortex [25]. This in turn suggests that there is little spatial overlap between the two. In our view, determining the extent and nature of rapid, pathological molecular cross-talk between aberrantly phosphorylated tau and presumptively toxic Aβ₄₂ species will be an increasingly important area of investigation. Indeed, several studies have underscored the importance of the interaction between Aβ₄₂ and tau to accelerate neurofibrillary disease and induce neurodegeneration [27, 37]. Additionally, both in vitro and preclinical work in transgenic mouse models of tau aggregation also suggests that Aβ₄₂ may accelerate tau proto-aggregation or aggregation [38–40].

The primacy of amyloid in initiating neurodegenerative events, while arguable, continues to have traction in research studies and clinical trials. Notions about: 1) the amyloid cascade [41]; 2) amyloid as an initiating event in AD (as in Jack and colleagues model of staging); and 3) use of amyloid to define so-called preclinical AD including in A4 study [42] hold much sway in the field. Moreover, several recent meta-analyses of Aβ₄₂ in the preclinical and prodromal stages of AD (both in JAMA 2015) highlight its continuing relevance for the field. Indeed, multiple clinical trials target only amyloid [43, 44]. We view our data as a potential corrective, in that it provides anomalous data demonstrating that coincident with consistent declines in CSF Aβ₄₂ in a subsample of healthy individuals, increases in p-tau were observed.

In sum, Aβ₄₂ and p-tau show anti-correlated longitudinal trajectories in CI subjects converting to preclinical AD, and both factors impact hippocampal atrophy. Both Aβ₄₂ and p-tau and their interaction are of obvious importance in understanding the development of neuropathological processes leading to the manifestation of preclinical AD, and therefore both proteins may be important targets for clinical prevention trials in individuals at an incipient state of AD.