Characterizing brain tau and cognitive decline along the amyloid timeline in Alzheimer's disease.

Recent longitudinal PET imaging studies have established methods to estimate the age at which amyloid becomes abnormal at the level of the individual. Here we recontextualized amyloid levels into the temporal domain to better understand the downstream Alzheimer's disease processes of tau neurofibrillary tangle (NFT) accumulation and cognitive decline. This cohort study included a total of 601 individuals from the Wisconsin Registry for Alzheimer's Prevention and Wisconsin Alzheimer's Disease Research Center that underwent amyloid and tau PET, longitudinal neuropsychological assessments and met clinical criteria for three clinical diagnosis groups: cognitively unimpaired (n = 537); mild cognitive impairment (n = 48); or dementia (n = 16). Cortical 11C-Pittsburgh compound B (PiB) distribution volume ratio (DVR) and sampled iterative local approximation were used to estimate amyloid positive (A+; global PiB DVR > 1.16 equivalent to 17.1 centiloids) onset age and years of A+ duration at tau PET (i.e. amyloid chronicity). Tau PET burden was quantified using 18F-MK-6240 standardized uptake value ratios (70-90 min, inferior cerebellar grey matter reference region). Whole-brain and region-specific approaches were used to examine tau PET binding along the amyloid timeline and across the Alzheimer's disease clinical continuum. Voxel-wise 18F-MK-6240 analyses revealed that with each decade of A+, the spatial extent of measurable tau spread (i.e. progressed) from regions associated with early to late NFT tau stages. Regional analyses indicated that tau burden in the entorhinal cortex was detectable, on average, within 10 years of A+ onset. Additionally, the entorhinal cortex was the region most sensitive to early amyloid pathology and clinical impairment in this predominantly preclinical sample. Among initially cognitively unimpaired (n = 472) individuals with longitudinal cognitive follow-up, mixed effects models showed significant linear and non-linear interactions of A+ duration and entorhinal tau on cognitive decline, suggesting a synergistic effect whereby greater A+ duration, together with a higher entorhinal tau burden, increases the likelihood of cognitive decline beyond their separable effects. Overall, the amyloid time framework enabled a spatiotemporal characterization of tau deposition patterns across the Alzheimer's disease continuum. This approach, which examined cross-sectional tau PET data along the amyloid timeline to make longitudinal disease course inferences, demonstrated that A+ duration explains a considerable amount of variability in the magnitude and topography of tau spread, which largely recapitulated NFT staging observed in human neuropathological studies. By anchoring disease progression to the onset of amyloid, this study provides a temporal disease context, which may help inform disease prognosis and timing windows for anti-amyloid therapies.

Characterizing the emergence of amyloid and tau burden in Down syndrome.

INTRODUCTION: Almost all individuals with Down syndrome (DS) will develop neuropathological features of Alzheimer's disease (AD). Understanding AD biomarker trajectories is necessary for DS-specific clinical interventions and interpretation of drug-related changes in the disease trajectory. METHODS: A total of 177 adults with DS from the Alzheimer's Biomarker Consortium-Down Syndrome (ABC-DS) underwent positron emission tomography (PET) and MR imaging. Amyloid-beta (Aß) trajectories were modeled to provide individual-level estimates of Aß-positive (A+) chronicity, which were compared against longitudinal tau change. RESULTS: Elevated tau was observed in all NFT regions following A+ and longitudinal tau increased with respect to A+ chronicity. Tau increases in NFT regions I-III was observed 0-2.5 years following A+. Nearly all A+ individuals had tau increases in the medial temporal lobe. DISCUSSION: These findings highlight the rapid accumulation of amyloid and early onset of tau relative to amyloid in DS and provide a strategy for temporally characterizing AD neuropathology progression that is specific to the DS population and independent of chronological age. HIGHLIGHTS: Longitudinal amyloid trajectories reveal rapid Aß accumulation in Down syndrome NFT stage tau was strongly associated with A+ chronicity Early longitudinal tau increases were observed 2.5-5 years after reaching A.

White matter microstructure associations to amyloid burden in adults with Down syndrome.

INTRODUCTION: Individuals with Down syndrome (DS) are at an increased risk of developing Alzheimer's Disease (AD). One of the early underlying mechanisms in AD pathology is the accumulation of amyloid protein plaques, which are deposited in extracellular gray matter and signify the first stage in the cascade of neurodegenerative events. AD-related neurodegeneration is also evidenced as microstructural changes in white matter. In this work, we explored the correlation of white matter microstructure with amyloid load to assess amyloid-related neurodegeneration in a cohort of adults with DS. METHODS: In this study of 96 adults with DS, the relation of white matter microstructure using diffusion tensor imaging (DTI) and amyloid plaque burden using [11C]PiB PET were examined. The amyloid load (AßL) derived from [11C]PiB was used as a global measure of amyloid burden. AßL and DTI measures were compared using tract-based spatial statistics (TBSS) and corrected for imaging site and chronological age. RESULTS: TBSS of the DTI maps showed widespread age-by-amyloid interaction with both fractional anisotropy (FA) and mean diffusivity (MD). Further, diffuse negative association of FA and positive association of MD with amyloid were observed. DISCUSSION: These findings are consistent with the white matter microstructural changes associated with AD disease progression in late onset AD in non-DS populations.

Corrigendum: Basal Forebrain Cholinergic Neurons: Linking Down Syndrome and Alzheimer's Disease.

[This corrects the article DOI: 10.3389/fnagi.2021.703876.].

Basal Forebrain Cholinergic Neurons: Linking Down Syndrome and Alzheimer's Disease.

Down syndrome (DS, trisomy 21) is characterized by intellectual impairment at birth and Alzheimer's disease (AD) pathology in middle age. As individuals with DS age, their cognitive functions decline as they develop AD pathology. The susceptibility to degeneration of a subset of neurons, known as basal forebrain cholinergic neurons (BFCNs), in DS and AD is a critical link between cognitive impairment and neurodegeneration in both disorders. BFCNs are the primary source of cholinergic innervation to the cerebral cortex and hippocampus, as well as the amygdala. They play a critical role in the processing of information related to cognitive function and are directly engaged in regulating circuits of attention and memory throughout the lifespan. Given the importance of BFCNs in attention and memory, it is not surprising that these neurons contribute to dysfunctional neuronal circuitry in DS and are vulnerable in adults with DS and AD, where their degeneration leads to memory loss and disturbance in language. BFCNs are thus a relevant cell target for therapeutics for both DS and AD but, despite some success, efforts in this area have waned. There are gaps in our knowledge of BFCN vulnerability that preclude our ability to effectively design interventions. Here, we review the role of BFCN function and degeneration in AD and DS and identify under-studied aspects of BFCN biology. The current gaps in BFCN relevant imaging studies, therapeutics, and human models limit our insight into the mechanistic vulnerability of BFCNs in individuals with DS and AD.

Neurofibrillary tau depositions emerge with subthreshold cerebral beta-amyloidosis in down syndrome.

INTRODUCTION: Adults with Down syndrome are genetically predisposed to develop Alzheimer's disease and accumulate beta-amyloid plaques (Aß) early in life. While Aß has been heavily studied in Down syndrome, its relationship with neurofibrillary tau is less understood. The aim of this study was to evaluate neurofibrillary tau deposition in individuals with Down syndrome with varying levels of Aß burden. METHODS: A total of 161 adults with Down syndrome (mean age = 39.2 (8.50) years) and 40 healthy, non-Down syndrome sibling controls (43.2 (12.6) years) underwent T1w-MRI, [C-11]PiB and [F-18]AV-1451 PET scans. PET images were converted to units of standardized uptake value ratios (SUVrs). Aß burden was calculated using the amyloid load metric (AßL); a measure of global Aß burden that improves quantification from SUVrs by suppressing the nonspecific binding signal component and computing the specific Aß signal from all Aß-carrying voxels from the image. Regional tau was assessed using control-standardized AV-1451 SUVr. Control-standardized SUVrs were compared across Down syndrome groups of Aß-negative (A-) (AßL < 13.3), subthreshold A+ (13.3 ≤ AßL < 20) and conventionally A+ (AßL ≥ 20) individuals. The subthreshold A + group was identified as having significantly higher Aß burden compared to the A- group, but not high enough to satisfy a conventional A + classification. RESULTS: A large-sized association that survived adjustment for chronological age, mental age (assessed using the Peabody Picture Vocabulary Test), and imaging site was observed between AßL and AV-1451 within each Braak region (p < .05). The A + group showed significantly higher AV-1451 retention across all Braak regions compared to the A- and subthreshold A + groups (p < .05). The subthreshold A + group showed significantly higher AV-1451 retention in Braak regions I-III compared to an age-matched sample from the A- group (p < .05). DISCUSSION: These results show that even the earliest detectable Aß accumulation in Down syndrome is accompanied by elevated tau in the early Braak stage regions. This early detection of tau can help characterize the tau accumulation phase during preclinical Alzheimer's disease progression in Down syndrome and suggests that there may be a relatively narrow window after Aß accumulation begins to prevent the downstream cascade of events that leads to Alzheimer's disease.

PET measurement of longitudinal amyloid load identifies the earliest stages of amyloid-beta accumulation during Alzheimer's disease progression in Down syndrome.

INTRODUCTION: Adults with Down syndrome (DS) are predisposed to Alzheimer's disease (AD) and reveal early amyloid beta (Aß) pathology in the brain. Positron emission tomography (PET) provides an in vivo measure of Aß throughout the AD continuum. Due to the high prevalence of AD in DS, there is need for longitudinal imaging studies of Aß to better characterize the natural history of Aß accumulation, which will aid in the staging of this population for clinical trials aimed at AD treatment and prevention. METHODS: Adults with DS (N = 79; Mean age (SD) = 42.7 (7.28) years) underwent longitudinal [C-11]Pittsburgh compound B (PiB) PET. Global Aß burden was quantified using the amyloid load metric (AßL). Modeled PiB images were generated from the longitudinal AßL data to visualize which regions are most susceptible to Aß accumulation in DS. AßL change was evaluated across Aß(-), Aß-converter, and Aß(+) groups to assess longitudinal Aß trajectories during different stages of AD-pathology progression. AßL change values were used to identify Aß-accumulators within the Aß(-) group prior to reaching the Aß(+) threshold (previously reported as 20 AßL) which would have resulted in an Aß-converter classification. With knowledge of trajectories of Aß(-) accumulators, a new cutoff of Aß(+) was derived to better identify subthreshold Aß accumulation in DS. Estimated sample sizes necessary to detect a 25% reduction in annual Aß change with 80% power (alpha 0.01) were determined for different groups of Aß-status. RESULTS: Modeled PiB images revealed the striatum, parietal cortex and precuneus as the regions with earliest detected Aß accumulation in DS. The Aß(-) group had a mean AßL change of 0.38 (0.58) AßL/year, while the Aß-converter and Aß(+) groups had change of 2.26 (0.66) and 3.16 (1.34) AßL/year, respectively. Within the Aß(-) group, Aß-accumulators showed no significant difference in AßL change values when compared to Aß-converter and Aß(+) groups. An Aß(+) cutoff for subthreshold Aß accumulation was derived as 13.3 AßL. The estimated sample size necessary to detect a 25% reduction in Aß was 79 for Aß(-) accumulators and 59 for the Aß-converter/Aß(+) group in DS. CONCLUSION: Longitudinal AßL changes were capable of distinguishing Aß accumulators from non-accumulators in DS. Longitudinal imaging allowed for identification of subthreshold Aß accumulation in DS during the earliest stages of AD-pathology progression. Detection of active Aß deposition evidenced by subthreshold accumulation with longitudinal imaging can identify DS individuals at risk for AD development at an earlier stage.

Cognitive indicators of transition to preclinical and prodromal stages of Alzheimer's disease in Down syndrome.

INTRODUCTION: There is a critical need to identify measures of cognitive functioning sensitive to early Alzheimer's disease (AD) pathophysiology in Down syndrome to advance clinical trial research in this at-risk population. The objective of the study was to longitudinally track performance on cognitive measures in relation to neocortical and striatal amyloid beta (Aß) in non-demented Down syndrome. METHODS: The study included 118 non-demented adults with Down syndrome who participated in two to five points of data collection, spanning 1.5 to 8 years. Episodic memory, visual attention and executive functioning, and motor planning and coordination were assessed. Aß was measured via [C-11] Pittsburgh Compound-B (PiB) PET. RESULTS: PiB was associated with level and rate of decline in cognitive performance in episodic memory, visual attention, executive functioning, and visuospatial ability in models controlling for chronological age. DISCUSSION: The Cued Recall Test emerged as a promising indicator of transition from preclinical to prodromal AD.

Amyloid accumulation in Down syndrome measured with amyloid load.

INTRODUCTION: Individuals with Down syndrome (DS) show enhanced amyloid beta (Aß) deposition in the brain. A new positron emission tomography (PET) index of amyloid load (AßL ) was recently developed as an alternative to standardized uptake value ratios (SUVrs) to quantify Aß burden with high sensitivity for detecting and tracking Aß change.1. METHODS: AßL was calculated in a DS cohort (N = 169, mean age ± SD = 39.6 ± 8.7 years) using [C-11]Pittsburgh compound B (PiB) PET imaging. DS-specific PiB templates were created for Aß carrying capacity (K) and non-specific binding (NS). RESULTS: The highest values of Aß carrying capacity were found in the striatum and precuneus. Longitudinal changes in AßL displayed less variability when compared to SUVrs. DISCUSSION: These results highlight the utility of AßL for characterizing Aß deposition in DS. Rates of Aß accumulation in DS were found to be similar to that observed in late-onset Alzheimer's disease (AD; ≈3% to 4% per year), suggesting that AD progression in DS is of earlier onset but not accelerated.

Association of sleep with cognition and beta amyloid accumulation in adults with Down syndrome.

Adults with Down syndrome have an increased risk for both disordered sleep and Alzheimer's disease (AD). In the general population, disrupted sleep has been linked to beta amyloid accumulation, an early pathophysiologic feature of AD. In this study, the association among sleep, beta amyloid, and measures of AD-related cognitive decline was examined in 47 non-demented adults with Down syndrome (aged 26-56 years). Sleep was measured using actigraphy over 7 nights. Pittsburgh Compound B positron emission tomography was used to assess global and striatal beta amyloid burden. Participants had the following clinical AD status: 7 (15%) mild cognitive impairment and 40 (85%) cognitively unaffected. Average length of night-time awakenings was significantly positively associated with striatal beta amyloid and decreased cognitive performance in executive functioning and motor planning and coordination. Findings suggest that disrupted sleep is associated with beta amyloid accumulation and cognitive features of preclinical AD in Down syndrome. Early identification and treatment of sleep problems could be a lifestyle intervention that may delay beta amyloid accumulation and cognitive decline in this AD vulnerable group.

Patterns of glucose hypometabolism in Down syndrome resemble sporadic Alzheimer's disease except for the putamen.

INTRODUCTION: Adults with Down syndrome (DS) are predisposed to Alzheimer's disease (AD) and the relationship between cognition and glucose metabolism in this population has yet to be evaluated. METHODS: Adults with DS (N = 90; mean age [standard deviation] = 38.0 [8.30] years) underwent [C-11]Pittsburgh compound B (PiB) and [F-18]fluorodeoxyglucose (FDG) positron emission tomography scans. Associations among amyloid beta (Aß), FDG, and measures of cognition were explored. Interregional FDG metabolic connectivity was assessed to compare cognitively stable DS and mild cognitive impairment/AD (MCI-DS/AD). RESULTS: Negative associations between Aß and FDG were evident in regions affected in sporadic AD. A positive association was observed in the putamen, which is the brain region showing the earliest increases in Aß deposition. Both Aß and FDG were associated with measures of cognition, and metabolic connectivity distinguished cases of MCI-DS/AD from cognitively stable DS. DISCUSSION: Associations among Aß, FDG, and cognition reveal that neurodegeneration in DS resembles sporadic AD with the exception of the putamen, highlighting the usefulness of FDG in monitoring neurodegeneration in DS.

In Vivo Characterization and Quantification of Neurofibrillary Tau PET Radioligand 18F-MK-6240 in Humans from Alzheimer Disease Dementia to Young Controls.

Tau PET imaging has potential for elucidating changes in the deposition of neuropathological tau aggregates that are occurring during the progression of Alzheimer disease (AD). This work investigates in vivo kinetics, quantification strategies, and imaging characteristics of a novel tau PET radioligand 18F-MK-6240 in humans. Methods: Fifty-one individuals ranging from cognitively normal young controls to persons with dementia underwent T1-weighted MRI as well as 11C-PiB and 18F-MK-6240 PET imaging. PET data were coregistered to the MRI, and time-activity curves were extracted from regions of interest to assess 18F-MK-6240 kinetics. The pons and inferior cerebellum were investigated as potential reference regions. Reference tissue methods (Logan graphical analysis [LGA] and multilinear reference tissue method [MRTM2]) were investigated for quantification of 18F-MK-6240 distribution volume ratios (DVRs) in a subset of 19 participants. Stability of DVR methods was evaluated using truncated scan durations. SUV ratio (SUVR) estimates were compared with DVR estimates to determine the optimal timing window for SUVR analysis. Parametric SUVR images were used to identify regions of potential off-target binding and to compare binding patterns with neurofibrillary tau staging established in neuropathology literature. Results: SUVs in the pons and the inferior cerebellum indicated consistent clearance across all 51 subjects. LGA and MRTM2 DVR estimates were similar, with LGA slightly underestimating DVR compared with MRTM2. DVR estimates remained stable when truncating the scan duration to 60 min. SUVR determined 70-90 min after injection of 18F-MK-6240 indicated linearity near unity when compared with DVR estimates and minimized potential spill-in from uptake outside the brain. 18F-MK-6240 binding patterns in target regions were consistent with neuropathological neurofibrillary tau staging. Off-target binding regions included the ethmoid sinus, clivus, meninges, substantia nigra, but not the basal ganglia or choroid plexus. Conclusion:18F-MK-6240 is a promising PET radioligand for in vivo imaging of neurofibrillary tau aggregates in AD with minimal off-target binding in the human brain.