Using multimodal MRI imaging with VBM, SBM and DTI, we demonstrate that higher 18F-AV1451 binding in tau-prone regions affected by PSP was correlated with changes in grey and white matter. We interpret these findings in terms of a correlation between tau pathology and structural integrity while recognizing important caveats below.
In comparing PSP patients with controls independently of 18F-AV1451 binding, extensive structural impairment was observed, with GM volume loss affecting frontotemporal regions, basal ganglia, midbrain and cerebellum, CTh reduction in left entorhinal and fusiform gyri, and DTI changes in the corpus callosum, internal capsule, corona radiata, and posterior thalamic radiations, among others. These results are in keeping with the previous observations [12,13,14, 17, 18], suggesting a key role of grey and white matter changes in the pathophysiology of PSP, especially the PSP-Richardson syndrome [26].
We also observed that 18F-AV1451 in high-binding regions for PSP (i.e., thalamus, putamen, and cerebellar dentate gyrus) was correlated with GM volume loss mainly in anterior regions, i.e., SMA, superior and middle frontal and precentral gyri, while increased PET binding was related to cortical thinning in posterior regions (e.g., lateral-occipital, fusiform gyrus and precuneus). In addition, 18F-AV1451 BPND was associated with altered DTI in motor tracts (corona radiata, internal capsule), posterior thalamic radiations and SLF, the latter being particularly involved in spatial attention, oculomotor function, and motor behaviour [27]. Higher 18F-AV1451 binding in the thalamus and dentate gyrus was also related to white matter disruption in the body of corpus callosum, whose fibres pass through the corona radiata to reach the brain surface, the cingulum (which is densely connected to the thalamus and spinothalamic tract) and superior fronto-occipital fasciculus. These cross-modal association shed light onto the intricate in vivo relationship between the integrity of connecting fibres and 18F-AV1451 binding. Due to the cross-sectional design of our study, it is still unclear whether microstructural damage is a direct consequence of tau pathology in the white matter (as observed post mortem) or consecutive to Wallerian degeneration [28]. However, considering the GM volume loss and DTI signal impairment taking place in similar anterior regions in relation to higher 18F-AV1451 binding, our results suggest that white matter degeneration is at least partly secondary to GM volume loss. Using a sparse canonical correlation analysis between DTI and 18F-AV1451 PET imaging, Sintini et al. recently demonstrated that higher 18F-AV1451 binding in cerebellar dentate, red nucleus and subthalamic nucleus was associated with altered DTI in sagittal stratum, corona radiata and superior cerebellar peduncle [29].
At variance with GM volume and DTI changes observed in anterior brain regions, increased 18F-AV1451 binding was associated with cortical thinning in more posterior (parieto-occipital) areas. These findings are of particular interest as CTh reduction in PSP is usually described in frontal regions, with little changes observed in longitudinal studies [16]. Therefore, with the present correlational analyses, we were able to tackle more subtle posterior cortical changes in relationship to a PET proxy assessing tau deposition.
In addition to these three regions (i.e., putamen, thalamus, and dentate gyrus), we performed correlational analyses of MRI changes with midbrain, frontal lobe, and global cortical 18F-AV1451 binding, as they are PSP-prone regions for tau deposition. Although unspecific white matter changes involving corpus callosum and motor tracts were related to higher 18F-AV1451 uptake in these ROIs, there was no significant cortical thinning in relationship to it. In addition, higher frontal and global 18F-AV1451 was negatively correlated with frontal GM volume loss. This highlights the fact that the three high-binding 18F-AV1451 regions we initially selected (i.e. thalamus, putamen, and dentate gyrus) have specific relationships with structural GM loss, as remote effects on GM involving both cortical thinning and volume loss are specifically related to higher 18F-AV1451 binding in diencephalic and cerebellar regions. One must insist that these are spatiotemporal relationships using a cross-sectional approach and that it does not necessarily involves causality. However, considering our modest cohort and a relatively stringent significance threshold, we believe that this brings novel insights into the complex relationship of a tau PET proxy with structural changes.
With regard to multimodal imaging correlates of clinical motor (PSP-RS) or cognitive (ACER) impairment, our results pointed out to a lack of correlation with 18F-AV1451 binding. This is in accordance with Schonhaut et al. [7] but contrasts with Smith et al. [9], who found a correlation between pallidal 18F-AV1451 binding and PSP-RS using Spearman correlations. One explanation could be the relatively small sample sizes or early disease stage of our patients. In addition, we assessed correlation using linear regressions adjusting for age and gender. In fact, when performing a Spearman correlation instead, we observed a trend between higher midbrain AV1451 and higher PSPRS score (rho = 0.34, p = 0.13). In addition, lower 18F-AV1451 binding in the dentate gyrus was associated at the trend level with higher ACER (rho = − 0.32, p = 0.16). In AD, 18F-AV1451 binding in the temporal cortex was negatively related to temporal (especially its medial portion) CTh change [30]. In addition, Bejanin et al. [31] showed that tau deposition was related in a region-specific manner to cognitive decline. That 18F-AV1451 is a more sensitive marker of paired helical filaments than straight filamentous might also play a role.
Conversely, we did find that lower cognitive performance according to ACER score was significantly associated with reduced GM volume in mediotemporal regions, right superior frontal gyrus, thalamus, and pallidum. In addition, lower ACER correlated with impaired diffusion metrics in the corpus callosum, cingulum, motor tracts, and SLF. These highlight the prominent role of frontal dysfunction in PSP [32, 33], as these white matter tracts are densely connected to the frontal lobe [34]. In a previous study, Piattella et al. showed that MMSE score in PSP was correlated with whole white matter mean FA [18].
While PSP-RS score was not associated with specific cortical thinning, we observed altered GM and DTI in bilateral temporal and right occipital regions, highlighting the close relationship between atrophy and white matter degeneration according to motor impairment.
Our study is not without limitations. First, our results would require confirmation in larger and independent samples. Second, the present relationship between 18F-AV1451 and structural imaging is based on a cross-sectional design: longitudinal studies are, therefore, required to fully assess the spatial and temporal interplay between grey/white matter integrity and 18F-AV1451 in PSP, and test causal models of pathology in humans. Moreover, due to the relatively modest sample size, most of our correlational results were uncorrected for multiple comparisons. We, however, used a stringent significance threshold (p < 0.001) and extent cluster threshold. In addition, 18F-AV1451 off-target binding is a caveat to our interpretation. Neuromelanin-containing cells in the substantia nigra and monoamine oxidase in the striatum are bound by this ligand [35, 36]. However, we recently showed that off-target binding in basal ganglia, cortex or adjacent white matter is not sufficient an explanation, as postmortem data did not show relevant neuromelanin-containing cells [6]. The very high clinic–pathological correlations in PSP–Richardson syndrome preclude TDP43 [11] or concurrent AD pathology as an alternative explanation, noting that the six patients in our study who have come to post-mortem examination were confirmed as PSP without significant dual pathology.
In conclusion, we present evidence of the in vivo association between structural brain integrity and 18F-AV1451 in PSP. Longitudinal studies would be helpful to determine the dynamic changes occurring for both tau deposition and grey/white matter changes. Tau PET probes more specific and sensitive for straight filaments would also enable confirmation of the complex interplay between cortical tau aggregation, grey/white matter degeneration and disease progression in PSP.