18F-Flortaucipir PET/MRI Correlations in Nonamnestic and Amnestic Variants of Alzheimer Disease

Synthesizing images of tau pathology from cross-modal neuroimaging using deep learning

Given the prevalence of dementia and the development of pathology-specific disease-modifying therapies, high-value biomarker strategies to inform medical decision-making are critical. In vivo tau-PET is an ideal target as a biomarker for Alzheimer's disease diagnosis and treatment outcome measure. However, tau-PET is not currently widely accessible to patients compared to other neuroimaging methods. In this study, we present a convolutional neural network (CNN) model that imputes tau-PET images from more widely available cross-modality imaging inputs. Participants (n = 1192) with brain T1-weighted MRI (T1w), fluorodeoxyglucose (FDG)-PET, amyloid-PET and tau-PET were included. We found that a CNN model can impute tau-PET images with high accuracy, the highest being for the FDG-based model followed by amyloid-PET and T1w. In testing implications of artificial intelligence-imputed tau-PET, only the FDG-based model showed a significant improvement of performance in classifying tau positivity and diagnostic groups compared to the original input data, suggesting that application of the model could enhance the utility of the metabolic images. The interpretability experiment revealed that the FDG- and T1w-based models utilized the non-local input from physically remote regions of interest to estimate the tau-PET, but this was not the case for the Pittsburgh compound B-based model. This implies that the model can learn the distinct biological relationship between FDG-PET, T1w and tau-PET from the relationship between amyloid-PET and tau-PET. Our study suggests that extending neuroimaging's use with artificial intelligence to predict protein specific pathologies has great potential to inform emerging care models.

A review of the flortaucipir literature for positron emission tomography imaging of tau neurofibrillary tangles

Alzheimer's disease is defined by the presence of β-amyloid plaques and neurofibrillary tau tangles potentially preceding clinical symptoms by many years. Previously only detectable post-mortem, these pathological hallmarks are now identifiable using biomarkers, permitting an in vivo definitive diagnosis of Alzheimer's disease. 18F-flortaucipir (previously known as 18F-T807; 18F-AV-1451) was the first tau positron emission tomography tracer to be introduced and is the only Food and Drug Administration-approved tau positron emission tomography tracer (Tauvid™). It has been widely adopted and validated in a number of independent research and clinical settings. In this review, we present an overview of the published literature on flortaucipir for positron emission tomography imaging of neurofibrillary tau tangles. We considered all accessible peer-reviewed literature pertaining to flortaucipir through 30 April 2022. We found 474 relevant peer-reviewed publications, which were organized into the following categories based on their primary focus: typical Alzheimer's disease, mild cognitive impairment and pre-symptomatic populations; atypical Alzheimer's disease; non-Alzheimer's disease neurodegenerative conditions; head-to-head comparisons with other Tau positron emission tomography tracers; and technical considerations. The available flortaucipir literature provides substantial evidence for the use of this positron emission tomography tracer in assessing neurofibrillary tau tangles in Alzheimer's disease and limited support for its use in other neurodegenerative disorders. Visual interpretation and quantitation approaches, although heterogeneous, mostly converge and demonstrate the high diagnostic and prognostic value of flortaucipir in Alzheimer's disease.

The Sporadic Early-onset Alzheimer's Disease Signature Of Atrophy: Preliminary Findings From The Longitudinal Early-onset Alzheimer's Disease Study (LEADS) Cohort

INTRODUCTION

Magnetic resonance imaging (MRI) research has advanced our understanding of neurodegeneration in sporadic early-onset Alzheimer's disease (EOAD) but studies include small samples, mostly amnestic EOAD, and have not focused on developing an MRI biomarker.

METHODS

We analyzed MRI scans to define the sporadic EOAD-signature atrophy in a small sample (n = 25) of Massachusetts General Hospital (MGH) EOAD patients, investigated its reproducibility in the large longitudinal early-onset Alzheimer's disease study (LEADS) sample (n = 211), and investigated the relationship of the magnitude of atrophy with cognitive impairment.

RESULTS

The EOAD-signature atrophy was replicated across the two cohorts, with prominent atrophy in the caudal lateral temporal cortex, inferior parietal lobule, and posterior cingulate and precuneus cortices, and with relative sparing of the medial temporal lobe. The magnitude of EOAD-signature atrophy was associated with the severity of cognitive impairment.

DISCUSSION

The EOAD-signature atrophy is a reliable and clinically valid biomarker of AD-related neurodegeneration that could be used in clinical trials for EOAD.

HIGHLIGHTS

We developed an early-onset Alzheimer's disease (EOAD)-signature of atrophy based on magnetic resonance imaging (MRI) scans. EOAD signature was robustly reproducible across two independent patient cohorts. EOAD signature included prominent atrophy in parietal and posterior temporal cortex. The EOAD-signature atrophy was associated with the severity of cognitive impairment. EOAD signature is a reliable and clinically valid biomarker of neurodegeneration.

Tau: a biomarker of Huntington's disease

Developing effective treatments for patients with Huntington's disease (HD)-a neurodegenerative disorder characterized by severe cognitive, motor and psychiatric impairments-is proving extremely challenging. While the monogenic nature of this condition enables to identify individuals at risk, robust biomarkers would still be extremely valuable to help diagnose disease onset and progression, and especially to confirm treatment efficacy. If measurements of cerebrospinal fluid neurofilament levels, for example, have demonstrated use in recent clinical trials, other proteins may prove equal, if not greater, relevance as biomarkers. In fact, proteins such as tau could specifically be used to detect/predict cognitive affectations. We have herein reviewed the literature pertaining to the association between tau levels and cognitive states, zooming in on Alzheimer's disease, Parkinson's disease and traumatic brain injury in which imaging, cerebrospinal fluid, and blood samples have been interrogated or used to unveil a strong association between tau and cognition. Collectively, these areas of research have accrued compelling evidence to suggest tau-related measurements as both diagnostic and prognostic tools for clinical practice. The abundance of information retrieved in this niche of study has laid the groundwork for further understanding whether tau-related biomarkers may be applied to HD and guide future investigations to better understand and treat this disease.

The Adult Neurogenesis Theory of Alzheimer's Disease

Alzheimer's disease starts in neural stem cells (NSCs) in the niches of adult neurogenesis. All primary factors responsible for pathological tau hyperphosphorylation are inherent to adult neurogenesis and migration. However, when amyloid pathology is present, it strongly amplifies tau pathogenesis. Indeed, the progressive accumulation of extracellular amyloid-β deposits in the brain triggers a state of chronic inflammation by microglia. Microglial activation has a significant pro-neurogenic effect that fosters the process of adult neurogenesis and supports neuronal migration. Unfortunately, this "reactive" pro-neurogenic activity ultimately perturbs homeostatic equilibrium in the niches of adult neurogenesis by amplifying tau pathogenesis in AD. This scenario involves NSCs in the subgranular zone of the hippocampal dentate gyrus in late-onset AD (LOAD) and NSCs in the ventricular-subventricular zone along the lateral ventricles in early-onset AD (EOAD), including familial AD (FAD). Neuroblasts carrying the initial seed of tau pathology travel throughout the brain via neuronal migration driven by complex signals and convey the disease from the niches of adult neurogenesis to near (LOAD) or distant (EOAD) brain regions. In these locations, or in close proximity, a focus of degeneration begins to develop. Then, tau pathology spreads from the initial foci to large neuronal networks along neural connections through neuron-to-neuron transmission.

The heterogeneity of asymmetric tau distribution is associated with an early age at onset and poor prognosis in Alzheimer's disease

PURPOSE

Left-right asymmetry, an important feature of brain development, has been implicated in neurodegenerative diseases, although it's less discussed in typical Alzheimer's disease (AD). We sought to investigate whether asymmetric tau deposition plays a potential role in AD heterogeneity.

METHODS

Two independent cohorts consisting of patients with mild cognitive impairment due to AD and AD dementia with tau PET imaging were enrolled [the Alzheimer's Disease Neuroimaging Initiative (ADNI) cohort with 18F-Flortaucipir, the Shanghai Memory Study (SMS) cohort with 18F-Florzolotau]. Based on the absolute global tau interhemispheric differences, each cohort was divided into two groups (asymmetric versus symmetric tau distribution). The two groups were cross-sectionally compared in terms of demographic, cognitive characteristics, and pathological burden. The cognitive decline trajectories were analyzed longitudinally.

RESULTS

Fourteen (23.3%) and 42 (48.3%) patients in the ADNI and SMS cohorts showed an asymmetric tau distribution, respectively. An asymmetric tau distribution was associated with an earlier age at disease onset (proportion of early-onset AD: ADNI/SMS/combined cohorts, p = 0.093/0.026/0.001) and more severe pathological burden (i.e., global tau burden: ADNI/SMS cohorts, p < 0.001/= 0.007). And patients with an asymmetric tau distribution were characterized by a steeper cognitive decline longitudinally (i.e., the annual decline of Mini-Mental Status Examination score: ADNI/SMS/combined cohorts, p = 0.053 / 0.035 / < 0.001).

CONCLUSIONS

Asymmetry in tau deposition, which may be associated with an earlier age at onset, more severe pathological burden, and a steeper cognitive decline, is potentially an important characteristic of AD heterogeneity.

APOE ε4 influences medial temporal atrophy and tau deposition in atypical Alzheimer's disease

INTRODUCTION

Apolipoprotein E (APOE) ε4 is an important genetic risk factor for typical Alzheimer's disease (AD), influencing brain volume and tau burden. Little is known about its influence in atypical presentations of AD.

METHODS

An atypical AD cohort of 140 patients diagnosed with either posterior cortical atrophy or logopenic progressive aphasia underwent magnetic resonance imaging and positron emission tomography. Linear mixed effects models were fit to assess the influence of APOE ε4 on cross-sectional and longitudinal regional metrics.

RESULTS

At baseline, APOE ε4 carriers had smaller hippocampal and amygdala volumes and greater tau standardized uptake volume ratio in the hippocampus and entorhinal cortex compared to non-carriers while longitudinally, APOE ε4 non-carriers showed faster rates of atrophy and tau accumulation in the entorhinal cortex, with faster tau accumulation in the hippocampus.

DISCUSSION

APOE ε4 influences patterns of neurodegeneration and tau deposition and was associated with more medial temporal involvement, although there is evidence that non-carriers may be catching up over time.

Medial Temporal Lobe Tau Aggregation Relates to Divergent Cognitive and Emotional Empathy Abilities in Alzheimer's Disease

BACKGROUND

In Alzheimer's disease (AD), the gradual accumulation of amyloid-β (Aβ) and tau proteins may underlie alterations in empathy.

OBJECTIVE

To assess whether tau aggregation in the medial temporal lobes related to differences in cognitive empathy (the ability to take others' perspectives) and emotional empathy (the ability to experience others' feelings) in AD.

METHODS

Older adults (n = 105) completed molecular Aβ positron emission tomography (PET) scans. Sixty-eight of the participants (35 women) were Aβ positive and symptomatic with diagnoses of mild cognitive impairment, dementia of the Alzheimer's type, logopenic variant primary progressive aphasia, or posterior cortical atrophy. The remaining 37 (22 women) were asymptomatic Aβ negative healthy older controls. Using the Interpersonal Reactivity Index, we compared current levels of informant-rated cognitive empathy (Perspective-Taking subscale) and emotional empathy (Empathic Concern subscale) in the Aβ positive and negative participants. The Aβ positive participants also underwent molecular tau-PET scans, which were used to investigate whether regional tau burden in the bilateral medial temporal lobes related to empathy.

RESULTS

Aβ positive participants had lower perspective-taking and higher empathic concern than Aβ negative healthy controls. Medial temporal tau aggregation in the Aβ positive participants had divergent associations with cognitive and emotional empathy. Whereas greater tau burden in the amygdala predicted lower perspective-taking, greater tau burden in the entorhinal cortex predicted greater empathic concern. Tau burden in the parahippocampal cortex did not predict either form of empathy.

CONCLUSIONS

Across AD clinical syndromes, medial temporal lobe tau aggregation is associated with lower perspective-taking yet higher empathic concern.

TREM2 risk variants are associated with atypical Alzheimer's disease

Alzheimer's disease (AD) has multiple clinically and pathologically defined subtypes where the underlying causes of such heterogeneity are not well established. Rare TREM2 variants confer significantly increased risk for clinical AD in addition to other neurodegenerative disease clinical phenotypes. Whether TREM2 variants are associated with atypical clinical or pathologically defined subtypes of AD is not known. We studied here the clinical and pathological features associated with TREM2 risk variants in an autopsy-confirmed cohort. TREM2 variant cases were more frequently associated with non-amnestic clinical syndromes. Pathologically, TREM2 variant cases were associated with an atypical distribution of neurofibrillary tangle density with significantly lower hippocampal NFT burden relative to neocortical NFT accumulation. In addition, NFT density but not amyloid burden was associated with an increase of dystrophic microglia. TREM2 variant cases were not associated with an increased prevalence, extent, or severity of co-pathologies. These clinicopathological features suggest that TREM2 variants contribute to clinical and pathologic AD heterogeneity by altering the distribution of neurofibrillary degeneration and tau-dependent microglial dystrophy, resulting in hippocampal-sparing and non-amnestic AD phenotypes.

Neuropathology and emerging biomarkers in corticobasal syndrome

Corticobasal syndrome (CBS) is a clinical syndrome characterised by progressive asymmetric limb rigidity and apraxia with dystonia, myoclonus, cortical sensory loss and alien limb phenomenon. Corticobasal degeneration (CBD) is one of the most common underlying pathologies of CBS, but other disorders, such as progressive supranuclear palsy (PSP), Alzheimer's disease (AD) and frontotemporal lobar degeneration with TDP-43 inclusions, are also associated with this syndrome.In this review, we describe common and rare neuropathological findings in CBS, including tauopathies, synucleinopathies, TDP-43 proteinopathies, fused in sarcoma proteinopathy, prion disease (Creutzfeldt-Jakob disease) and cerebrovascular disease, based on a narrative review of the literature and clinicopathological studies from two brain banks. Genetic mutations associated with CBS, including GRN and MAPT, are also reviewed. Clinicopathological studies on neurodegenerative disorders associated with CBS have shown that regardless of the underlying pathology, frontoparietal, as well as motor and premotor pathology is associated with CBS. Clinical features that can predict the underlying pathology of CBS remain unclear. Using AD-related biomarkers (ie, amyloid and tau positron emission tomography (PET) and fluid biomarkers), CBS caused by AD often can be differentiated from other causes of CBS. Tau PET may help distinguish AD from other tauopathies and non-tauopathies, but it remains challenging to differentiate non-AD tauopathies, especially PSP and CBD. Although the current clinical diagnostic criteria for CBS have suboptimal sensitivity and specificity, emerging biomarkers hold promise for future improvements in the diagnosis of underlying pathology in patients with CBS.

Dissecting the clinical heterogeneity of early-onset Alzheimer's disease

Early-onset Alzheimer's disease (EOAD) is a rare but particularly devastating form of AD. Though notable for its high degree of clinical heterogeneity, EOAD is defined by the same neuropathological hallmarks underlying the more common, late-onset form of AD. In this review, we describe the various clinical syndromes associated with EOAD, including the typical amnestic phenotype as well as atypical variants affecting visuospatial, language, executive, behavioral, and motor functions. We go on to highlight advances in fluid biomarker research and describe how molecular, structural, and functional neuroimaging can be used not only to improve EOAD diagnostic acumen but also enhance our understanding of fundamental pathobiological changes occurring years (and even decades) before the onset of symptoms. In addition, we discuss genetic variation underlying EOAD, including pathogenic variants responsible for the well-known mendelian forms of EOAD as well as variants that may increase risk for the much more common forms of EOAD that are either considered to be sporadic or lack a clear autosomal-dominant inheritance pattern. Intriguingly, specific pathogenic variants in PRNP and MAPT-genes which are more commonly associated with other neurodegenerative diseases-may provide unexpectedly important insights into the formation of AD tau pathology. Genetic analysis of the atypical clinical syndromes associated with EOAD will continue to be challenging given their rarity, but integration of fluid biomarker data, multimodal imaging, and various 'omics techniques and their application to the study of large, multicenter cohorts will enable future discoveries of fundamental mechanisms underlying the development of EOAD and its varied clinical presentations.

Divergent Cortical Tau Positron Emission Tomography Patterns Among Patients With Preclinical Alzheimer Disease

Importance

Characterization of early tau deposition in individuals with preclinical Alzheimer disease (AD) is critical for prevention trials that aim to select individuals at risk for AD and halt the progression of disease.

Objective

To evaluate the prevalence of cortical tau positron emission tomography (PET) heterogeneity in a large cohort of clinically unimpaired older adults with elevated β-amyloid (A+).

Design, Setting, and Participants

This cross-sectional study examined prerandomized tau PET, amyloid PET, structural magnetic resonance imaging, demographic, and cognitive data from the Anti-Amyloid Treatment in Asymptomatic AD (A4) Study from April 2014 to December 2017. Follow-up analyses used observational tau PET data from the Alzheimer's Disease Neuroimaging Initiative (ADNI), the Harvard Aging Brain Study (HABS), and the Wisconsin Registry for Alzheimer's Prevention and the Wisconsin Alzheimer's Disease Research Center (together hereinafter referred to as Wisconsin) to evaluate consistency. Participants were clinically unimpaired at the study visit closest to the tau PET scan and had available amyloid and tau PET data (A4 Study, n = 447; ADNI, n = 433; HABS, n = 190; and Wisconsin, n = 328). No participants who met eligibility criteria were excluded. Data were analyzed from May 11, 2021, to January 25, 2022.

Main Outcomes and Measures

Individuals with preclinical AD with heterogeneous cortical tau PET patterns (A+T cortical+) were identified by examining asymmetrical cortical tau signal and disproportionate cortical tau signal relative to medial temporal lobe (MTL) tau. Voxelwise tau patterns, amyloid, neurodegeneration, cognition, and demographic characteristics were examined.

Results

The 447 A4 participants (A+ group, 392; and normal β-amyloid group, 55), with a mean (SD) age of 71.8 (4.8) years, included 239 women (54%). A total of 36 individuals in the A+ group (9% of the A+ group) exhibited heterogeneous cortical tau patterns and were further categorized into 3 subtypes: asymmetrical left, precuneus dominant, and asymmetrical right. A total of 116 individuals in the A+ group (30% of the A+ group) showed elevated MTL tau (A+T MTL+). Individuals in the A+T cortical+ group were younger than those in the A+T MTL+ group (t61.867 = -2.597; P = .03). Across the A+T cortical+ and A+T MTL+ groups, increased regional tau was associated with reduced hippocampal volume and MTL thickness but not with cortical thickness. Memory scores were comparable between the A+T cortical+ and A+T MTL+ groups, whereas executive functioning scores were lower for the A+T cortical+ group than for the A+T MTL+ group. The prevalence of the A+T cortical+ group and tau patterns within the A+T cortical+ group were consistent in ADNI, HABS, and Wisconsin.

Conclusions and Relevance

This study suggests that early tau deposition may follow multiple trajectories during preclinical AD and may involve several cortical regions. Staging procedures, especially those based on neuropathology, that assume a uniform trajectory across individuals are insufficient for disease monitoring with tau imaging.

Rates of longitudinal change in 18 F-flortaucipir PET vary by brain region, cognitive impairment, and age in atypical Alzheimer's disease

Introduction

Longitudinal positron emission tomography (PET) studies of tau accumulation in Alzheimer's disease (AD) have noted reduced increases or frank decreases in tau signal. We investigated how such reductions related to analytical confounds and disease progression markers in atypical AD.

Methods

We assessed regional and interindividual variation in longitudinal change on ¹⁸F‐flortaucipir PET imaging in 24 amyloid beta (Aβ)+ patients with atypical, early‐onset amnestic or non‐amnestic AD plus 62 Aβ– and 132 Aβ+ Alzheimer's Disease Neuroimaging Initiative (ADNI) participants.

Results

In atypical AD, ¹⁸F‐flortaucipir uptake slowed or declined over time in areas with high baseline signal and older, more impaired individuals. ADNI participants had reduced longitudinal change in early Braak stage regions relative to late‐stage areas.

Discussion

Results suggested radioligand uptake plateaus or declines in advanced neurodegeneration. Further research should investigate whether results generalize to other radioligands and whether they relate to changes of the radioligand binding site structure or accessibility.

Multiparametric magnetic resonance imaging and positron emission tomography findings in neurodegenerative diseases: Current status and future directions

Neurodegenerative diseases (NDDs) are characterized by progressive neuronal loss, leading to dementia and movement disorders. NDDs broadly include Alzheimer's disease, frontotemporal lobar degeneration, parkinsonian syndromes, and prion diseases. There is an ever-increasing prevalence of mild cognitive impairment and dementia, with an accompanying immense economic impact, prompting efforts aimed at early identification and effective interventions. Neuroimaging is an essential tool for the early diagnosis of NDDs in both clinical and research settings. Structural, functional, and metabolic imaging modalities, including magnetic resonance imaging (MRI) and positron emission tomography (PET), are widely available. They show encouraging results for diagnosis, monitoring, and treatment response evaluation. The current review focuses on the complementary role of various imaging modalities in relation to NDDs, the qualitative and quantitative utility of newer MRI techniques, novel radiopharmaceuticals, and integrated PET/MRI in the setting of NDDs.

Impaired visual search in posterior cortical atrophy vs. typical Alzheimer's disease

BACKGROUND

Posterior cortical atrophy (PCA) is a neurocognitive disorder characterized by difficulty localizing in space. Recognizing PCA is important because it is usually missed early in its course and may result from a number of neurological disorders other than Alzheimer's disease (AD).

OBJECTIVE

This study aimed to clarify whether impaired visual search tasks of spatial localization distinguished patients with PCA from those with other more typical dementias as well as from healthy control (HC) subjects.

METHODS

Twelve patients meeting neuroimaging-supported Consensus Criteria for PCA, 12 comparably advanced patients with amnestic-predominant typical AD (tAD), and 24 HC participants were compared on tests of untimed and timed visual search, spatial neglect, mental rotation, environmental orientation, visuospatial construction, and face recognition.

RESULTS

Only abnormalities in untimed and timed visual search and environmental orientation distinguished the PCA patients from both the tAD group and the HC group without also distinguishing the tAD patients from HC's. The PCA patients also had a tendency to greater difficulty scanning left hemispace compared to HC's. Visuospatial constructions, although worse in PCA, and face recognition were impaired in both dementia groups.

CONCLUSIONS

These findings support the concept of PCA as a disorder of spatial processing and localization, indicating that visual search tasks are particularly sensitive and specific for detecting PCA and distinguishing it from more typical dementia syndromes.

Tau PET Imaging with [18F]PM-PBB3 in Frontotemporal Dementia with MAPT Mutation

BACKGROUND

Flortaucipir (AV-1451) and pyridinyl-butadienyl-benzothiazole 3 (PBB3) are newly developed and commonly used positron emission tomography (PET) tracers to detect tau deposition in tauopathies, including frontotemporal dementia (FTD). [18F]PM-PBB3, as a second-generation compound, has not been described in FTD so far.

OBJECTIVE

We aim to explore the in vivo performance of [18F]PM-PBB3 tau PET in an FTD case caused by microtubule-associated protein tau (MAPT) mutation and compare the binding to different tau strains between AV-1451 and PBB3.

METHODS

We reported the clinical and FDG, [18F]AV45 amyloid and [18F]PM-PBB3 tau PET findings in a patient with FTD of P301L MAPT mutation. Based on our results and published data, we summarized and compared the different utilities of tau PET tracers of AV-1451 and PBB3 in FTD with MAPT mutation.

RESULTS

The patient demonstrated slightly diffuse [18F]PM-PBB3 tau deposition in cerebral lobes especially in the left frontal lobe overlapping with the hypometabolic region detected by FDG PET. From our analysis of 35 FTD patients with MAPT mutation who underwent tau PET, AV-1451 was positive in all (n = 11) patients with mutations known to cause three and four repeat (3R/4R) tau deposition and in 14.3% (n = 2/14) of 4R tauopathies, while positive PBB3 retention was found in all patients with both 3R/4R (n = 2) and 4R (n = 8) tau.

CONCLUSIONS

[18F]PM-PBB3 tau PET assisted the diagnosis of FTD with P301L MAPT mutation, and might be useful in the in vivo detection of both 3R/4R and 4R tau domains in the brain of FTD with MAPT mutation.

Memory Resilience in Alzheimer Disease With Primary Progressive Aphasia

OBJECTIVE

To determine whether memory is preserved longitudinally in primary progressive aphasia (PPA) associated with Alzheimer disease (AD) and to identify potential factors that maintain memory despite underlying neurofibrillary degeneration of mediotemporal memory areas.

METHODS

Longitudinal memory assessment was done in 17 patients with PPA with autopsy or biomarker evidence of AD (PPA-AD) and 14 patients with amnestic dementia of the Alzheimer type with AD at autopsy (DAT-AD).

RESULTS

In PPA-AD, episodic memory, tested with nonverbal items, was preserved at the initial testing and showed no decline at retesting 2.35 ± 0.78 years later, at which time symptoms had been present for 6.26 ± 2.21 years. In contrast, language functions declined significantly during the same period. In DAT-AD, both verbal memory and language declined with equal severity. Although imaging showed asymmetric left-sided mediotemporal atrophy in PPA-AD, autopsy revealed bilateral hippocampo-entorhinal neurofibrillary degeneration at Braak stages V and VI. Compared to DAT-AD, however, the PPA-AD group had lower incidence of APOE ε4 and of mediotemporal TAR DNA-binding protein 43 (TDP-43) pathology.

CONCLUSIONS

Memory preservation in PPA is not just an incidental finding at onset but a core feature that persists for years despite the hippocampo-entorhinal AD neuropathology that is as severe as that of DAT-AD. Asymmetry of mediotemporal atrophy and a lesser impact of APOE ε4 and of TDP-43 on the integrity of memory circuitry may constitute some of the factors underlying this resilience. Our results also suggest that current controversies on memory in PPA-AD reflect inconsistencies in the diagnosis of logopenic PPA, the clinical variant most frequently associated with AD.

CLINICALTRIALSGOV IDENTIFIER

NCT00537004 and NCT03371706.

Benson's Disease or Posterior Cortical Atrophy, Revisited

BACKGROUND

D. Frank Benson and colleagues first described the clinical and neuropathological features of posterior cortical atrophy (PCA) from patients in the UCLA Neurobehavior Program.

OBJECTIVE

We reviewed the Program's subsequent clinical experience with PCA, and its potential for clarifying this relatively rare syndrome in comparison to the accumulated literature on PCA.

METHODS

Using the original criteria derived from this clinic, 65 patients with neuroimaging-supported PCA were diagnosed between 1995 and 2020.

RESULTS

On presentation, most had visual localization complaints and related visuospatial symptoms, but nearly half had memory complaints followed by symptoms of depression. Neurobehavioral testing showed predominant difficulty with visuospatial constructions, Gerstmann's syndrome, and Balint's syndrome, but also impaired memory and naming. On retrospective application of the current Consensus Criteria for PCA, 59 (91%) met PCA criteria with a modification allowing for "significantly greater visuospatial over memory and naming deficits." There were 37 deaths (56.9%) with the median overall survival of 10.3 years (95% CI: 9.6-13.6 years), consistent with a slow neurodegenerative disorder in most patients.

CONCLUSION

Together, these findings recommend modifying the PCA criteria for "relatively spared" memory, language, and behavior to include secondary memory and naming difficulty and depression, with increased emphasis on the presence of Gerstmann's and Balint's syndromes.

Progressive Supranuclear Palsy and Corticobasal Degeneration

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are neurodegenerative tauopathies with neuronal and glial lesions composed of tau that is composed predominantly of isomers with four repeats in the microtubule-binding domain (4R tau). The brain regions vulnerable to pathology in PSP and CBD overlap, but there are differences, particularly with respect to distribution of neuronal loss, the relative abundance of neuronal and glial lesions, the morphologic features of glial lesions, and the frequency of comorbid pathology. Both PSP and CBD have a wide spectrum of clinical manifestations, including disorders of movement and cognition. Recognition of phenotypic diversity in PSP and CBD may improve antemortem diagnostic accuracy, which tends to be very good for the most common presentation of PSP (Richardson syndrome), but poor for the most characteristic presentation of CBD (corticobasal syndrome: CBS). Development of molecular and imaging biomarkers may improve antemortem diagnostic accuracy. Currently, multidisciplinary symptomatic and supportive treatment with pharmacological and non-pharmacological strategies remains the standard of care. In the future, experimental therapeutic trials will be important to slow disease progression.

Imaging of Tau Pathology in Neurodegenerative Diseases: An Update

Pathological accumulated misfolded tau underlies various neurodegenerative diseases and associated clinical syndromes. To diagnose those diseases reliable before death or even at early stages, many different tau-specific radiotracers have been developed in the last decade to be used with positron-emission-tomography. In contrast to amyloid-β imaging, different isoforms of tau exist further complicating radiotracer development. First-generation radiotracers like [¹¹C]PBB3, [¹⁸F]AV1451 and [¹⁸F]THK5351 have been extensively investigated in vitro and in vivo. In Alzheimer's disease (AD), high specific binding could be detected, and evidence of clinical applicability recently led to clinical approval of [¹⁸F]flortaucipir ([¹⁸F]AV1451) by the FDA. Nevertheless, absent or minor binding to non-AD tau isoforms and high off-target binding to non-tau brain structures limit the diagnostic applicability especially in non-AD tauopathies demanding further tracer development. In vitro assays and autoradiography results of next-generation radiotracers [¹⁸F]MK-6240, [¹⁸F]RO-948, [¹⁸F]PM-PBB3, [¹⁸F]GTP-1 and [¹⁸F]PI-2620 clearly indicate less off-target binding and high specific binding to tau neurofibrils. First in human studies have been conducted with promising results for all tracers in AD patients, and also some positive experience in non-AD tauopathies. Overall, larger scaled autoradiography and human studies are needed to further evaluate the most promising candidates and support future clinical approval.

Association of APOE4 and Clinical Variability in Alzheimer Disease With the Pattern of Tau- and Amyloid-PET

OBJECTIVE

To assess whether Alzheimer disease (AD) clinical presentation and APOE4 relate to the burden and topography of β-amyloid (Aβ) and tau pathologies using in vivo PET imaging.

METHODS

We studied 119 Aβ-positive symptomatic patients aged 48-95 years, including 29 patients with logopenic variant primary progressive aphasia (lvPPA) and 21 with posterior cortical atrophy (PCA). Pittsburgh compound B (PiB)-Aβ and flortaucipir (tau)-PET standardized uptake value ratio (SUVR) images were created. General linear models assessed relationships between demographic/clinical variables (phenotype, age), APOE4, and PET (including global cortical and voxelwise SUVR values) while controlling for disease severity using the Clinical Dementia Rating Sum of Boxes.

RESULTS

PiB-PET binding showed a widespread cortical distribution with subtle differences across phenotypes and was unrelated to demographic/clinical variables or APOE4. Flortaucipir-PET was commonly elevated in temporoparietal regions, but showed marked phenotype-associated differences, with higher binding observed in occipito-parietal areas for PCA, in left temporal and inferior frontal for lvPPA, and in medial temporal areas for other AD. Cortical flortaucipir-PET binding was higher in younger patients across phenotypes (r = -0.63, 95% confidence interval [CI] -0.72, -0.50), especially in parietal and dorsal prefrontal cortices. The presence of APOE4 was associated with a focal medial temporal flortaucipir-SUVR increase, controlling for all other variables (entorhinal: + 0.310 SUVR, 95% CI 0.091, 0.530).

CONCLUSIONS

Clinical phenotypes are associated with differential patterns of tau but not amyloid pathology. Older age and APOE4 are not only risk factors for AD but also seem to affect disease expression by promoting a more medial temporal lobe-predominant pattern of tau pathology.

Patient-centered connectivity-based prediction of tau pathology spread in Alzheimer's disease

In Alzheimer's disease (AD), the Braak staging scheme suggests a stereotypical tau spreading pattern that does, however, not capture interindividual variability in tau deposition. This complicates the prediction of tau spreading, which may become critical for defining individualized tau-PET readouts in clinical trials. Since tau is assumed to spread throughout connected regions, we used functional connectivity to improve tau spreading predictions over Braak staging methods. We included two samples with longitudinal tau-PET from controls and AD patients. Cross-sectionally, we found connectivity of tau epicenters (i.e., regions with earliest tau) to predict estimated tau spreading sequences. Longitudinally, we found tau accumulation rates to correlate with connectivity strength to patient-specific tau epicenters. A connectivity-based, patient-centered tau spreading model improved the assessment of tau accumulation rates compared to Braak stage-specific readouts and reduced sample sizes by ~40% in simulated tau-targeting interventions. Thus, connectivity-based tau spreading models may show utility in clinical trials.

Multimodal in vivo and postmortem assessments of tau in Lewy body disorders

We compared regional retention of ¹⁸F-flortaucipir between 20 patients with Lewy body disorders (LBD), 12 Alzheimer's disease patients with positive amyloid positron emission tomography (PET) scans (AD+Aβ) and 15 healthy controls with negative amyloid PET scans (HC-Aβ). In LBD subjects, we compared the relationship between ¹⁸F-flortaucipir retention and cerebrospinal fluid (CSF) tau, cognitive performance, and neuropathological tau at autopsy. The LBD cohort was stratified using an Aβ42 cut-off of 192 pg/mL to enrich for groups likely harboring tau pathology (LBD+Aβ = 11, LBD-Aβ = 9). ¹⁸F-flortaucipir retention was higher in LBD+AB than HC-Aβ in five, largely temporal-parietal regions with sparing of medial temporal regions. Higher retention was associated with higher CSF total-tau levels (p = 0.04), poorer domain-specific cognitive performance (p = 0.02-0.04), and greater severity of neuropathological tau in corresponding regions. While ¹⁸F-flortaucipir retention in LBD is intermediate between healthy controls and AD, retention relates to cognitive impairment, CSF total-tau, and neuropathological tau. Future work in larger autopsy-validated cohorts is needed to define LBD-specific tau biomarker profiles.

MRI and flortaucipir relationships in Alzheimer's phenotypes are heterogeneous

OBJECTIVE

To assess the relationships between MRI volumetry and [18 F]flortaucipir PET of typical and atypical clinical phenotypes of Alzheimer's disease, by genarian (age by decade).

METHODS

Five-hundred and sixty-four participants including those with typical (n = 86) or atypical (n = 80) Alzheimer's dementia and normal controls (n = 398) underwent apolipoprotein E genotyping, MRI, flortaucipir, and 11 C-PiB; all 166 Alzheimer's participants were beta-amyloid positive and all controls were beta-amyloid negative. Grey matter volume and flortaucipir standard uptake value ratios were calculated for hippocampus, entorhinal cortex, and neocortex. Ratios of hippocampal-to-neocortical and entorhinal-to-neocortical volume and flortaucipir uptake were also calculated. Linear regression models assessed relationships among regional volume, flortaucipir uptake, and ratios and phenotypes, within three genarians (50-59, 60-69, and 70+). Voxel-level analyses were also performed.

RESULTS

For 50-59 greater medial temporal atrophy and flortaucipir uptake was observed in the typical compared with atypical phenotype. The typical phenotype also showed greater frontal neocortex uptake with the voxel-level analysis. For 60-69 and 70+ there was greater hippocampal volume loss in the typical compared with atypical phenotype while only the 60-69, but not the 70+ group, showed a difference in hippocampal flortaucipir uptake. We also observed a pattern for higher neocortical flortaucipir uptake to correlate with younger age decade for both phenotypes.

INTERPRETATION

MRI volumetry versus flortaucipir PET relationships differ across Alzheimer's clinical phenotypes, and also within phenotype across age decades. This suggests that there is potential risk of masked effects by not accounting for genarian in participants with beta-amyloid and tau-positive biomarker defined Alzheimer's disease.

Progressive supranuclear palsy: Advances in diagnosis and management

Progressive supranuclear palsy (PSP) is a complex clinicopathologic disease with no current cure or disease modulating therapies that can only be definitively confirmed at autopsy. Growing understanding of the phenotypic diversity of PSP has led to expanded clinical criteria and new insights into etiopathogenesis that coupled with improved in vivo biomarkers makes increased access to current clinical trials possible. Current standard-of-care treatment of PSP is multidisciplinary, supportive and symptomatic, and several trials of potentially disease modulating agents have already been completed with disappointing results. Current ongoing clinical trials target the abnormal aggregation of tau through a variety of mechanisms including immunotherapy and gene therapy offer a more direct method of treatment. Here we review PSP clinicopathologic correlations, in vivo biomarkers including MRI, PET, and CSF biomarkers. We additionally review current pharmacologic and non-pharmacologic methods of treatment, prior and ongoing clinical trials in PSP. Newly expanded clinical criteria and improved specific biomarkers will aid in identifying patients with PSP earlier and more accurately and expand access to these potentially beneficial clinical trials.

Early-onset Alzheimer Disease and Its Variants

PURPOSE OF REVIEW

Early-onset Alzheimer disease (AD) is defined as having an age of onset younger than 65 years. While early-onset AD is often overshadowed by the more common late-onset AD, recognition of the differences between early- and late-onset AD is important for clinicians.

RECENT FINDINGS

Early-onset AD comprises about 5% to 6% of cases of AD and includes a substantial percentage of phenotypic variants that differ from the usual amnestic presentation of typical AD. Characteristics of early-onset AD in comparison to late-onset AD include a larger genetic predisposition (familial mutations and summed polygenic risk), more aggressive course, more frequent delay in diagnosis, higher prevalence of traumatic brain injury, less memory impairment and greater involvement of other cognitive domains on presentation, and greater psychosocial difficulties. Neuroimaging features of early-onset AD in comparison to late-onset AD include greater frequency of hippocampal sparing and posterior neocortical atrophy, increased tau burden, and greater connectomic changes affecting frontoparietal networks rather than the default mode network.

SUMMARY

Early-onset AD differs substantially from late-onset AD, with different phenotypic presentations, greater genetic predisposition, and differences in neuropathologic burden and topography. Early-onset AD more often presents with nonamnestic phenotypic variants that spare the hippocampi and with greater tau burden in posterior neocortices. The early-onset AD phenotypic variants involve different neural networks than typical AD. The management of early-onset AD is similar to that of late-onset AD but with special emphasis on targeting specific cognitive areas and more age-appropriate psychosocial support and education.

Prospective longitudinal atrophy in Alzheimer's disease correlates with the intensity and topography of baseline tau-PET

β-Amyloid plaques and tau-containing neurofibrillary tangles are the two neuropathological hallmarks of Alzheimer's disease (AD) and are thought to play crucial roles in a neurodegenerative cascade leading to dementia. Both lesions can now be visualized in vivo using positron emission tomography (PET) radiotracers, opening new opportunities to study disease mechanisms and improve patients' diagnostic and prognostic evaluation. In a group of 32 patients at early symptomatic AD stages, we tested whether β-amyloid and tau-PET could predict subsequent brain atrophy measured using longitudinal magnetic resonance imaging acquired at the time of PET and 15 months later. Quantitative analyses showed that the global intensity of tau-PET, but not β-amyloid-PET, signal predicted the rate of subsequent atrophy, independent of baseline cortical thickness. Additional investigations demonstrated that the specific distribution of tau-PET signal was a strong indicator of the topography of future atrophy at the single patient level and that the relationship between baseline tau-PET and subsequent atrophy was particularly strong in younger patients. These data support disease models in which tau pathology is a major driver of local neurodegeneration and highlight the relevance of tau-PET as a precision medicine tool to help predict individual patient's progression and design future clinical trials.

Neuropathologic basis of in vivo cortical atrophy in the aphasic variant of Alzheimer's disease

The neuropathologic basis of in vivo cortical atrophy in clinical dementia syndromes remains poorly understood. This includes primary progressive aphasia (PPA), a language-based dementia syndrome characterized by asymmetric cortical atrophy. The neurofibrillary tangles (NFTs) and amyloid-ß plaques (APs) of Alzheimer's disease (AD) can cause PPA, but a quantitative investigation of the relationships between NFTs, APs and in vivo cortical atrophy in PPA-AD is lacking. The present study measured cortical atrophy from corresponding bilateral regions in five PPA-AD participants with in vivo magnetic resonance imaging scans 7-30 months before death and acquired stereologic estimates of NFTs and dense-core APs visualized with the Thioflavin-S stain. Linear mixed models accounting for repeated measures and stratified by hemisphere and region (language vs. non-language) were used to determine the relationships between cortical atrophy and AD neuropathology and their regional selectivity. Consistent with the aphasic profile of PPA, left language regions displayed more cortical atrophy (P = 0.01) and NFT densities (P = 0.02) compared to right language homologues. Left language regions also showed more cortical atrophy (P < 0.01) and NFT densities (P = 0.02) than left non-language regions. A subset of data was analyzed to determine the predilection of AD neuropathology for neocortical regions compared to entorhinal cortex in the left hemisphere, which showed that the three most atrophied language regions had greater NFT (P = 0.04) and AP densities (P < 0.01) than the entorhinal cortex. These results provide quantitative evidence that NFT accumulation in PPA selectively targets the language network and may not follow the Braak staging of neurofibrillary degeneration characteristic of amnestic AD. Only NFT densities, not AP densities, were positively associated with cortical atrophy within left language regions (P < 0.01) and right language homologues (P < 0.01). Given previous findings from amnestic AD, the current study of PPA-AD provides converging evidence that NFTs are the principal determinants of atrophy and clinical phenotypes associated with AD.

Topographical Heterogeneity of Alzheimer's Disease Based on MR Imaging, Tau PET, and Amyloid PET

Alzheimer’s disease (AD) patients are known to have heterogeneous clinical presentation and pathologic patterns. We hypothesize that AD dementia can be categorized into subtypes based on multimodal imaging biomarkers such as magnetic resonance imaging (MRI), tau positron emission tomography (PET), and amyloid PET. We collected 3T MRI, ¹⁸F-THK5351 PET, and ¹⁸F-flutemetamol (FLUTE) PET data from 83 patients with AD dementia [Clinical Dementia Rating (CDR) ≤1] and 60 normal controls (NC), and applied surface-based analyses to measure cortical thickness, THK5351 standardized uptake value ratio (SUVR) and FLUTE SUVR for each participant. For the patient group, we performed an agglomerative hierarchical clustering analysis using the three multimodal imaging features on the vertices (n = 3 × 79,950). The identified AD subtypes were compared to NC using general linear models adjusting for age, sex, and years of education. We mapped the effect size within significant cortical regions reaching a corrected p-vertex <0.05 (random field theory). Our surface-based multimodal framework has revealed three distinct subtypes among AD patients: medial temporal-dominant subtype (MT, n = 44), parietal-dominant subtype (P, n = 19), and diffuse atrophy subtype (D, n = 20). The topography of cortical atrophy and THK5351 retention differentiates between the three subtypes. In the case of FLUTE, three subtypes did not show distinct topographical differences, although cortical composite retention was significantly higher in the P type than in the MT type. These three subtypes also differed in demographic and clinical features. In conclusion, AD patients may be clustered into three subtypes with distinct topographical features of cortical atrophy and tau deposition, although amyloid deposition may not differ across the subtypes in terms of topography.

Longitudinal tau PET in ageing and Alzheimer's disease

See Hansson and Mormino (doi:10.1093/brain/awy065) for a scientific commentary on this article.Our objective was to compare different whole-brain and region-specific measurements of within-person change on serial tau PET and evaluate its utility for clinical trials. We studied 126 individuals: 59 cognitively unimpaired with normal amyloid, 37 cognitively unimpaired with abnormal amyloid, and 30 cognitively impaired with an amnestic phenotype and abnormal amyloid. All had baseline amyloid PET and two tau PET, MRI, and clinical assessments. We compared the topography across all cortical regions of interest of tau PET accumulation rates and the rates of four different whole-brain or region-specific meta-regions of interest among the three clinical groups. We computed sample size estimates for change in tau PET, cortical volume, and memory/mental status indices for use as outcome measures in clinical trials. The cognitively unimpaired normal amyloid group had no observable tau accumulation throughout the brain. Tau accumulation rates in cognitively unimpaired abnormal amyloid were low [0.006 standardized uptake value ratio (SUVR), 0.5%, per year] but greater than rates in the cognitively unimpaired normal amyloid group in the basal and mid-temporal, retrosplenial, posterior cingulate, and entorhinal regions of interest. Thus, the earliest elevation in accumulation rates was widespread and not confined to the entorhinal cortex. Tau accumulation rates in the cognitively impaired abnormal amyloid group were 0.053 SUVR (3%) per year and greater than rates in cognitively unimpaired abnormal amyloid in all cortical areas except medial temporal. Rates of accumulation in the four meta-regions of interest differed but only slightly from one another. Among all tau PET meta-regions of interest, sample size estimates were smallest for a temporal lobe composite within cognitively unimpaired abnormal amyloid and for the late Alzheimer's disease meta-region of interest within cognitively impaired abnormal amyloid. The ordering of the sample size estimates by outcome measure was MRI < tau PET < cognitive measures. At a group-wise level, observable rates of short-term serial tau accumulation were only seen in the presence of abnormal amyloid. As disease progressed to clinically symptomatic stages (cognitively impaired abnormal amyloid), observable rates of tau accumulation were seen uniformly throughout the brain providing evidence that tau does not accumulate in one area at a time or in start-stop, stepwise sequence. The information captured by rate measures in different meta-regions of interest, even those with little topographic overlap, was similar. The implication is that rate measurements from simple meta-regions of interest, without the need for Braak-like staging, may be sufficient to capture progressive within-person accumulation of pathologic tau. Tau PET SUVR measures should be an efficient outcome measure in disease-modifying clinical trials.

NIA-AA Research Framework: Toward a biological definition of Alzheimer's disease

In 2011, the National Institute on Aging and Alzheimer’s Association created separate diagnostic recommendations for the preclinical, mild cognitive impairment, and dementia stages of Alzheimer’s disease. Scientific progress in the interim led to an initiative by the National Institute on Aging and Alzheimer’s Association to update and unify the 2011 guidelines. This unifying update is labeled a “research framework” because its intended use is for observational and interventional research, not routine clinical care. In the National Institute on Aging and Alzheimer’s Association Research Framework, Alzheimer’s disease (AD) is defined by its underlying pathologic processes that can be documented by postmortem examination or in vivo by biomarkers. The diagnosis is not based on the clinical consequences of the disease (i.e., symptoms/signs) in this research framework, which shifts the definition of AD in living people from a syndromal to a biological construct. The research framework focuses on the diagnosis of AD with biomarkers in living persons. Biomarkers are grouped into those of β amyloid deposition, pathologic tau, and neurodegeneration [AT(N)]. This ATN classification system groups different biomarkers (imaging and biofluids) by the pathologic process each measures. The AT(N) system is flexible in that new biomarkers can be added to the three existing AT(N) groups, and new biomarker groups beyond AT(N) can be added when they become available. We focus on AD as a continuum, and cognitive staging may be accomplished using continuous measures. However, we also outline two different categorical cognitive schemes for staging the severity of cognitive impairment: a scheme using three traditional syndromal categories and a six-stage numeric scheme. It is important to stress that this framework seeks to create a common language with which investigators can generate and test hypotheses about the interactions among different pathologic processes (denoted by biomarkers) and cognitive symptoms. We appreciate the concern that this biomarker-based research framework has the potential to be misused. Therefore, we emphasize, first, it is premature and inappropriate to use this research framework in general medical practice. Second, this research framework should not be used to restrict alternative approaches to hypothesis testing that do not use biomarkers. There will be situations where biomarkers are not available or requiring them would be counterproductive to the specific research goals (discussed in more detail later in the document). Thus, biomarker-based research should not be considered a template for all research into age-related cognitive impairment and dementia; rather, it should be applied when it is fit for the purpose of the specific research goals of a study. Importantly, this framework should be examined in diverse populations. Although it is possible that β-amyloid plaques and neurofibrillary tau deposits are not causal in AD pathogenesis, it is these abnormal protein deposits that define AD as a unique neurodegenerative disease among different disorders that can lead to dementia. We envision that defining AD as a biological construct will enable a more accurate characterization and understanding of the sequence of events that lead to cognitive impairment that is associated with AD, as well as the multifactorial etiology of dementia. This approach also will enable a more precise approach to interventional trials where specific pathways can be targeted in the disease process and in the appropriate people.

Regional Distribution, Asymmetry, and Clinical Correlates of Tau Uptake on [18F]AV-1451 PET in Atypical Alzheimer's Disease

BACKGROUND

Despite common pathology, Alzheimer's disease (AD) can have multiple clinical presentations which pathological studies suggest result from differences in the regional distribution of tau pathology. Positron emission tomography (PET) ligands are now available that can detect tau proteins in vivo and hence can be used to investigate the biological mechanisms underlying atypical AD.

OBJECTIVE

To assess regional patterns of tau uptake on PET imaging in two atypical AD variants, posterior cortical atrophy (PCA) and logopenic progressive aphasia (lvPPA).

METHODS

Eighteen PCA and 19 lvPPA subjects that showed amyloid-β deposition on PET underwent tau-PET imaging with [18F]AV-1451. Group comparisons of tau uptake in PCA and lvPPA were performed using voxel-level and regional-level analyses. We also assessed the degree of lobar tau asymmetry and correlated regional tau uptake to age and performance on clinical evaluations.

RESULTS

Both syndromes showed diffuse tau uptake throughout all cortical regions, although PCA showed greater uptake in occipital regions compared to lvPPA, and lvPPA showed greater uptake in left frontal and temporal regions compared to PCA. While lvPPA showed predominant left-asymmetric tau deposition, PCA was more bilateral. Younger subjects showed greater tau uptake bilaterally in frontal and parietal lobes than older subjects, and sentence repetition, Boston naming test, simultanagnosia, and visuoperceptual function showed specific regional tau correlates.

CONCLUSION

Tau deposition is closely related to clinical presentation in atypical AD with age playing a role in determining the degree of cortical tau deposition.

The role of age on tau PET uptake and gray matter atrophy in atypical Alzheimer's disease

INTRODUCTION

Little is known about the role of age on neurodegeneration and protein deposition in atypical variants of Alzheimer's disease (AD).

METHODS

Regional tau and β-amyloid positron emission tomography standard uptake value ratios and gray matter volumes were calculated in a cohort of 42 participants with atypical AD. The relationship between regional metrics and age was modeled using a Bayesian hierarchical linear model.

RESULTS

Age was strongly associated with tau uptake across all cortical regions, particularly parietal, with greater uptake in younger participants. Younger age was associated with smaller parietal and lateral temporal volumes. Regional β-amyloid differed little by age. Age showed a stronger association with tau than volume and β-amyloid in all cortical regions. Age was not associated with cognitive performance.

DISCUSSION

Age is an important determinant of severity of cortical tau uptake in atypical AD, with young participants more likely to show widespread and severe cortical tau uptake.

Alzheimer's disease neuroimaging

PURPOSE OF REVIEW

The aim of this study was to discuss the contribution of neuroimaging studies to our understanding of Alzheimer's disease. We now have the capability of measuring both tau and beta-amyloid (Aβ) proteins in the brain, which together with more traditional neuroimaging modalities, has led the field to focus on using neuroimaging to better characterize disease mechanisms underlying Alzheimer's disease.

RECENT FINDINGS

Studies have utilized tau and Aβ PET, as well as [18F]fluorodeoxyglucose PET, and structural and functional MRI, to investigate the following topics: phenotypic variability in Alzheimer's disease , including how neuroimaging findings are related to clinical phenotype and age; multimodality analyses to investigate the relationships between different neuroimaging modalities and what that teaches us about disease mechanisms; disease staging by assessing neuroimaging changes in the very earliest phases of the disease in cognitively normal individuals and individuals carrying an autosomal dominant Alzheimer's disease mutation; and influence of other comorbidities and proteins to the disease process.

SUMMARY

The findings shed light on the role of tau and Aβ, as well as age and other comorbidities, in the neurodegenerative process in Alzheimer's disease. This knowledge will be crucial in the development of better disease biomarkers and targeted therapeutic approaches.

Regional multimodal relationships between tau, hypometabolism, atrophy, and fractional anisotropy in atypical Alzheimer's disease

Alzheimer's disease (AD) can present with atypical clinical forms where the prominent domain of deficit is not memory, that is, atypical AD. Atypical AD patients show cortical atrophy on MRI, hypometabolism on [¹⁸ F]fluorodeoxyglucose (FDG) PET, tau uptake on [¹⁸ F]AV-1451 PET, and white matter tract degeneration on diffusion tensor imaging (DTI). How these disease processes relate to each other locally and distantly remains unclear. We aimed to examine multimodal neuroimaging relationships in individuals with atypical AD, using univariate and multivariate techniques at region- and voxel-level. Forty atypical AD patients underwent MRI, FDG-PET, tau-PET, beta-amyloid PET, and DTI. Patients were all beta-amyloid positive. Partial Pearson's correlations were performed between tau and FDG standardized uptake value ratios, gray matter MRI-volumes and white matter tract fractional anisotropy. Sparse canonical correlation analysis was applied to identify multivariate relationships between the same quantities. Voxel-level associations across modalities were also assessed. Tau showed strong local negative correlations with FDG metabolism in the occipital and frontal lobes. Tau in frontal and parietal regions was negatively associated with temporoparietal gray matter MRI-volume. Fractional anisotropy in a set of posterior white matter tracts, including the splenium of the corpus callosum, cingulum, and posterior thalamic radiation, was negatively correlated with parietal and occipital tau, atrophy and, predominantly, with hypometabolism. These results support the view that tau is the driving force behind neurodegeneration in atypical AD, and that a breakdown in structural connectivity is related to cortical neurodegeneration, particularly hypometabolism.

18F-flortaucipir uptake patterns in clinical subtypes of primary progressive aphasia

We analyzed ¹⁸F-flortaucipir uptake patterns and structural changes in patients with subtypes of primary progressive aphasia (PPA) using ¹⁸F-flortaucipir positron emission tomography and volumetric magnetic resonance imaging. We enrolled 34 consecutive patients with PPA (10 nonfluent/agrammatic PPA [nfvPPA], 18 semantic variant PPA [svPPA], and 6 logopenic variant PPA [lvPPA], as well as 20 healthy controls, and 20 patients with Alzheimer's disease. ¹⁸F-flortaucipir uptake was increased in the frontal cortex and underlying white matter, and subcortical nuclei in the 10 nfvPPA and 8 nfvPPA-amyloid-β (Aβ)- subgroup patients. In the svPPA patients (both the 13 svPPA-Aβ- and 5 svPPA-Aβ+), uptake generally increased in the widespread neocortex with left anterior temporal predominance. ¹⁸F-flortaucipir uptake patterns in the 6 lvPPA and the 5 lvPPA-Aβ+ subgroup patients were similar to those seen in the patients with Alzheimer's disease with mild predominance in the left lateral temporal cortex. Cortical thinning in each PPA subtype corresponded with increased ¹⁸F-flortaucipir uptake. ¹⁸F-flortaucipir uptake patterns and cortical atrophy were distinct and corresponded to areas related to the specific language functions that are impaired in each subtype of PPA.

Greater tau load and reduced cortical thickness in APOE ε4-negative Alzheimer's disease: a cohort study

BACKGROUND

Alzheimer's disease is characterized by aggregated β-amyloid and tau proteins, but the clinical presentations and patterns of brain atrophy vary substantially. A part of this heterogeneity may be linked to the risk allele APOE ε4. The spread of tau pathology is related to atrophy and cognitive decline, but little data exist on the effects of APOE ε4 on tau. The objective of this preliminary study was therefore to test if tau load and brain structure differ by APOE ε4 in Alzheimer's disease.

METHODS

Sixty-five β-amyloid-positive patients at the prodromal and dementia stages of Alzheimer's disease were enrolled, including APOE ε4-positive (n = 46) and APOE ε4-negative (n = 19) patients. 18F-AV-1451 positron emission tomography was used to measure tau and brain magnetic resonance imaging (MRI) was used to measure cortical thickness.

RESULTS

Compared with their APOE ε4-positive counterparts, APOE ε4-negative patients had greater tau load and reduced cortical thickness, with the most pronounced effects for both in the parietal cortex. Relative to the overall cortical tau load, APOE ε4-positive patients had greater tau load in the entorhinal cortex. APOE ε4-positive patients also had slightly greater cortical β-amyloid load. There was an interaction between APOE ε4 and 18F-AV-1451 on cortical thickness, with greater effects of 18F-AV-1451 on cortical thickness in APOE ε4-negative patients. APOE ε4 and 18F-AV-1451 were independent predictors of cognition, but showed distinct associations with different cognitive tests.

CONCLUSIONS

APOE genotype may be associated with differences in pathways in Alzheimer's disease, potentially through differential development and spread of tau, as well as through effects on cognitive outcomes involving non-tau-related mechanisms.

The development and validation of tau PET tracers: current status and future directions

Purpose

To provide an overview on positron emission tomography (PET) imaging of tau pathology in Alzheimer’s disease (AD) and other neurodegenerative disorders.

Results

Different classes of tau tracers such as flortaucipir, THK5317, and PBB3 have been developed and utilized in previous clinical studies. In AD, the topographical distribution of tracer binding follows the known distribution of neurofibrillary tangles and is closely associated with neurodegeneration as well as the clinical phenotype of dementia. Significant retention of tracers has also been observed in the frequent site of the 4-repeat (4R) tau isoform deposits in non-AD tauopathies, such as in progressive supranuclear palsy. However, in vitro binding studies indicate that most tau tracers are less sensitive to straight tau filaments, in contrast to their high binding affinity to paired helical filaments of tau (PHF-tau). The first-generation of tau tracers shows off-target binding in the basal ganglia, midbrain, thalamus, choroid plexus, and venous sinus. Off-target binding of THK5351 to monoamine oxidase B (MAO-B) has been observed in disease-associated brain regions linked to neurodegeneration and is associated with astrogliosis in areas of misfolded protein accumulation. The second generation of tau tracers, such as [¹⁸F]MK-6240, is highly selective to PHF-tau with little off-target binding and have enabled the reliable assessment of PHF-tau burden in aging and AD.

Conclusions

Tau PET tracers have enabled in vivo quantification of PHF-tau burden in human brains. Tau PET can help in understanding the underlying cause of dementia symptoms, and in patient selection for clinical trials of anti-dementia therapies.

Distinct [18F]THK5351 binding patterns in primary progressive aphasia variants

PURPOSE

To assess the binding of the PET tracer [¹⁸F]THK5351 in patients with different primary progressive aphasia (PPA) variants and its correlation with clinical deficits. The majority of patients with nonfluent variant (NFV) and logopenic variant (LV) PPA have underlying tauopathy of the frontotemporal lobar or Alzheimer disease type, respectively, while patients with the semantic variant (SV) have predominantly transactive response DNA binding protein 43-kDa pathology.

METHODS

The study included 20 PPA patients consecutively recruited through a memory clinic (12 NFV, 5 SV, 3 LV), and 20 healthy controls. All participants received an extensive neurolinguistic assessment, magnetic resonance imaging and amyloid biomarker tests. [¹⁸F]THK5351 binding patterns were assessed on standardized uptake value ratio (SUVR) images with the cerebellar grey matter as the reference using statistical parametric mapping. Whole-brain voxel-wise regression analysis was performed to evaluate the association between [¹⁸F]THK5351 SUVR images and neurolinguistic scores. Analyses were performed with and without partial volume correction.

RESULTS

Patients with NFV showed increased binding in the supplementary motor area, left premotor cortex, thalamus, basal ganglia and midbrain compared with controls and patients with SV. Patients with SV had increased binding in the temporal lobes bilaterally and in the right ventromedial frontal cortex compared with controls and patients with NFV. The whole-brain voxel-wise regression analysis revealed a correlation between agrammatism and motor speech impairment, and [¹⁸F]THK5351 binding in the left supplementary motor area and left postcentral gyrus. Analysis of [¹⁸F]THK5351 scans without partial volume correction revealed similar results.

CONCLUSION

[¹⁸F]THK5351 imaging shows a topography closely matching the anatomical distribution of predicted underlying pathology characteristic of NFV and SV PPA. [¹⁸F]THK5351 binding correlates with the severity of clinical impairment.

Imaging correlations of tau, amyloid, metabolism, and atrophy in typical and atypical Alzheimer's disease

INTRODUCTION

Neuroimaging modalities can measure different aspects of the disease process in Alzheimer's disease, although the relationship between these modalities is unclear.

METHODS

We assessed subject-level regional correlations between tau on [¹⁸F]AV-1451 positron emission tomography (PET), β amyloid on Pittsburgh compound B PET, hypometabolism on [¹⁸F] fluorodeoxyglucose PET, and cortical thickness on magnetic resonance imaging in 96 participants with typical and atypical Alzheimer's disease presentations. We also assessed how correlations between modalities varied according to age, presenting syndrome, tau-PET severity, and asymmetry.

RESULTS

[¹⁸F]AV-1451 uptake showed the strongest regional correlation with hypometabolism. Correlations between [¹⁸F]AV-1451 uptake and both hypometabolism and cortical thickness were stronger in participants with greater cortical tau severity. In addition, age, tau asymmetry, and clinical diagnosis influenced the strength of the correlation between [¹⁸F]AV-1451 uptake and cortical thickness.

DISCUSSION

These findings support a close relationship between tau and hypometabolism in Alzheimer's disease but show that correlations between neuroimaging modalities vary across participants.

[18 F]AV-1451 tau-PET and primary progressive aphasia

OBJECTIVES

To assess [18 F]AV-1451 tau-PET (positron emission tomography) uptake patterns across the primary progressive aphasia (PPA) variants (logopenic, semantic, and agrammatic), examine regional uptake patterns of [18 F]AV-1451 independent of clinical diagnosis, and compare the diagnostic utility of [18 F]AV-1451, [18 F]-fluorodeoxygluclose (FDG)-PET and MRI (magnetic resonance imaging) to differentiate the PPA variants.

METHODS

We performed statistical parametric mapping of [18 F]AV-1451 across 40 PPA patients (logopenic-PPA = 14, semantic-PPA = 13, and agrammatic-PPA = 13) compared to 80 cognitively normal, Pittsburgh compound B-negative controls, age and gender matched 2:1. Principal component analysis of regional [18 F]AV-1451 tau-PET standard uptake value ratio was performed to understand underlying patterns of [18 F]AV-1451 uptake independent of clinical diagnosis. Penalized multinomial regression analyses were utilized to assess diagnostic utility.

RESULTS

Logopenic-PPA showed striking uptake throughout neocortex, particularly temporoparietal, compared to controls, semantic-PPA, and agrammatic-PPA. Semantic-PPA and agrammatic-PPA showed milder patterns of focal [18 F]AV-1451 uptake. Semantic-PPA showed elevated uptake (left>right) in anteromedial temporal lobes, compared to controls and agrammatic-PPA. Agrammatic-PPA showed elevated uptake (left>right) throughout prefrontal white matter and in subcortical gray matter structures, compared to controls and semantic-PPA. The principal component analysis of regional [18 F]AV-1451 indicated two primary dimensions, a severity dimension that distinguished logopenic-PPA from agrammatic-PPA and semantic-PPA, and a frontal versus temporal contrast that distinguishes agrammatic-PPA and semantic-PPA cases. Diagnostic utility of [18 F]AV-1451was superior to MRI and at least equal to FDG-PET.

INTERPRETATION

[18 F]AV-1451binding characteristics differ across the PPA variants and were excellent at distinguishing between the variants. [18 F]AV-1451binding characteristics were as good or better than other brain imaging modalities utilized in clinical practice, suggesting that [18 F]AV-1451 may have clinical diagnostic utility in PPA. Ann Neurol 2018 Ann Neurol 2018;83:599-611.