In this longitudinal study of tau change and MRI atrophy, we examined the associations between baseline FTP binding level and subsequent neurodegeneration (FTP accumulation and MRI cortical atrophy) over time across MCI patients. In the MCI progression, we observed intimated link between FTP accumulation and cortical brain atrophy on the voxelwise analyses, especially in bilateral temporoparietal regions. And longitudinal FTP change has mediated the relationship between baseline FTP uptake and subsequent atrophy across MCI participants. Finally, we found that longitudinal FTP change, but not baseline FTP uptake or longitudinal cortical atrophy, was significantly correlated with the annualized cognitive change as assessed by MMSE.
Previous neuroimaging studies examining the association between longitudinal tau change and cortical atrophy in healthy elders and AD patients, and it remained unclear what happened and which association in symptomatic patients with MCI. Our study extended this gap and the main finding was that the spatial topology of FTP accumulation and its annualized change rate (6–11% per year) in MCI, which was predominantly middle-inferior temporal cortex. This is in line with previous neuroimaging and pathologic studies in MCI neurodegeneration showing the similar topology and spread of tau pathology [24, 25]. It was worth noting that the annualized FTP percentage rate for MCI group in this work was rather than for healthy elders (2–3% per year) [15]. Considering the sensitivity of FTP and the spread pattern of FTP accumulation may be different in various clinical symptoms (i.e., stable MCI, developing MCI or reversed MCI) [26, 27], it remained to confirmed that the generalizability of our observed rates of longitudinal FTP accumulation in MCI group. We further used complementary voxelwise and ROI-based analyses to reveal the association between longitudinal FTP change and atrophy. The annualized rate of FTP accumulation and cortical atrophy were considerably stronger and were present across all prior regions. There was a moderate association between the two spatial patterns (Fig. 2 and Fig. 3). Emerging evidence suggested that FTP pathology may present before neurodegeneration starts and may propagate its effects on cognition ability through structural pathology [28, 29]. Thus, it might be expected that the retention of tau may increase the risk of brain atrophy, in that the higher tau accumulation rate and magnitude, the downstream regions related to severe atrophy. This is consistent with the previous cross-sectional finding that the rate of tau accumulation correlated with atrophy pattern over time [6, 12].
Evidence from mediation models had emerged a plausible hypothesis that baseline cortical FTP uptake had a delayed and indirect, longitudinal FTP change-mediated association with longitudinal cortical atrophy. Previous longitudinal MRI data had estimated that baseline tau pathology, rather than amyloid pathology, was a major driver of local neurodegeneration and highlight the relevance of baseline tau uptake level as a precision way to predict individual patient’s cortical atrophy [14]. We observed that baseline tau uptake was more closely correlated with longitudinal tau change, beyond the longitudinal cortical atrophy. We also found that those individuals with high rates of tau change had elevated baseline tau uptake and atrophy, rather than the lower patients. These observations expand on previous findings from longitudinal studies by providing evidence that baseline tau level as an important role regarding not only the longitudinal brain atrophy but also longitudinal tau accumulation. Previous clinicopathological studies both PET and cerebrospinal fluid data suggested that synergy tau and atrophy were correlated with brain dysfunction and cognitive deficits [14, 30, 31]. Together, growing evidence suggested that baseline tau uptake may reflect the presence of subsequent tau change, which had a predictive ability to cortical atrophy over time. These findings had potential ramifications for clinical trials in that based-tau measurements might be superior to track pathology trajectories during MCI progression. Contrary to expectations, we observed that clinical cognitive change as assessed by MMSE and CDRSB was only associated with longitudinal cortical FTP increase. There were no significant correlations between clinical decline and baseline tau uptake, similar in longitudinal cortical atrophy. These weak correlations may be related to the small sample size or intrinsic noise interferences both clinical assessments and imaging [14].
A strength of this prospective study was that we used longitudinal FTP PET and structural MRI imaging in individuals across MCI spectrum. There were some limitations that should be pointed to appropriately interpret our results. Firstly, the small sample size may had limited our statistical power and prevented generalization, and especially to explore the patterns of longitudinal tau change and cortical atrophy (uncorrected P value). Secondly, the MCI patients of this study is collected from ADNI database, which may limit the appropriate follow-up visits (only two-time points) and further time points would enable a more detailed characterization of neurodegeneration trajectories. Finally, our cohort only included the progression of MCI stage and would not be extrapolated to more serve stage (AD or dementia), and also lack amyloid PET to determine the more comprehensive pathological trajectories.