Anne M. Fagan

Toward defining the preclinical stages of Alzheimer’s disease: Recommendations from the National Institute on Aging-Alzheimer's Association workgroups on diagnostic guidelines for Alzheimer's disease

The pathophysiological process of Alzheimers disease (AD) is thought to begin many years before the diagnosis of AD dementia. This long “preclinical” phase of AD would provide a critical opportunity for therapeutic intervention; however, we need to further elucidate the link between the pathological cascade of AD and the emergence of clinical symptoms. The National Institute on Aging and the Alzheimers Association convened an international workgroup to review the biomarker, epidemiological, and neuropsychological evidence, and to develop recommendations to determine the factors which best predict the risk of progression from “normal” cognition to mild cognitive impairment and AD dementia. We propose a conceptual framework and operational research criteria, based on the prevailing scientific evidence to date, to test and refine these models with longitudinal clinical research studies. These recommendations are solely intended for research purposes and do not have any clinical implications at this time. It is hoped that these recommendations will provide a common rubric to advance the study of preclinical AD, and ultimately, aid the field in moving toward earlier intervention at a stage of AD when some disease‐modifying therapies may be most efficacious.

Clinical and Biomarker Changes in Dominantly Inherited Alzheimer's Disease

BACKGROUND The order and magnitude of pathologic processes in Alzheimers disease are not well understood, partly because the disease develops over many years. Autosomal dominant Alzheimers disease has a predictable age at onset and provides an opportunity to determine the sequence and magnitude of pathologic changes that culminate in symptomatic disease. METHODS In this prospective, longitudinal study, we analyzed data from 128 participants who underwent baseline clinical and cognitive assessments, brain imaging, and cerebrospinal fluid (CSF) and blood tests. We used the participants age at baseline assessment and the parents age at the onset of symptoms of Alzheimers disease to calculate the estimated years from expected symptom onset (age of the participant minus parents age at symptom onset). We conducted cross-sectional analyses of baseline data in relation to estimated years from expected symptom onset in order to determine the relative order and magnitude of pathophysiological changes. RESULTS Concentrations of amyloid-beta (Aβ)(42) in the CSF appeared to decline 25 years before expected symptom onset. Aβ deposition, as measured by positron-emission tomography with the use of Pittsburgh compound B, was detected 15 years before expected symptom onset. Increased concentrations of tau protein in the CSF and an increase in brain atrophy were detected 15 years before expected symptom onset. Cerebral hypometabolism and impaired episodic memory were observed 10 years before expected symptom onset. Global cognitive impairment, as measured by the Mini-Mental State Examination and the Clinical Dementia Rating scale, was detected 5 years before expected symptom onset, and patients met diagnostic criteria for dementia at an average of 3 years after expected symptom onset. CONCLUSIONS We found that autosomal dominant Alzheimers disease was associated with a series of pathophysiological changes over decades in CSF biochemical markers of Alzheimers disease, brain amyloid deposition, and brain metabolism as well as progressive cognitive impairment. Our results require confirmation with the use of longitudinal data and may not apply to patients with sporadic Alzheimers disease. (Funded by the National Institute on Aging and others; DIAN ClinicalTrials.gov number, NCT00869817.).

Inverse relation between in vivo amyloid imaging load and cerebrospinal fluid Aβ42 in humans

Amyloid‐β42 (Aβ42) appears central to Alzheimers disease (AD) pathogenesis and is a major component of amyloid plaques. Mean cerebrospinal fluid (CSF) Aβ42 is decreased in dementia of the Alzheimers type. This decrease may reflect plaques acting as an Aβ42 “sink,” hindering transport of soluble Aβ42 between brain and CSF. We investigated this hypothesis.

Human apoE isoforms differentially regulate brain amyloid-β peptide clearance

Human apoE4 increases the concentration of soluble Aβ in the brain by impairing its clearance. Clearing the Debris in Alzheimer’s Disease The strongest risk factor for developing the common sporadic form of Alzheimer’s disease (AD) that occurs in old age is the ε4 allele encoding apolipoprotein E4 (apoE4). Two ε4 alleles can lower the age of onset of AD by 10 to 15 years. In contrast, the ε2 allele decreases the risk of developing this neurodegenerative disorder. APOE is important for lipoprotein metabolism, but how it might be involved in AD has remained unclear. It has been suggested that the apoE4 isoform might somehow help to drive accumulation of the peptide amyloid-β (Aβ), which forms amyloid plaques in the brain that contribute to neuronal death and are the characteristic hallmark of AD. In a tour de force study in humans and mice, Holtzman and his team at Washington University in St. Louis now show that apoE4 contributes to Aβ accumulation in the brain not by affecting Aβ synthesis but by affecting its clearance. First, the authors looked at the Aβ concentration in the cerebrospinal fluid (CSF) of cognitively normal individuals under age 70 carrying different APOE genotypes. They found that those with the ε4/ε4 genotype had a much lower CSF Aβ concentration than did those with the protective ε2/ε3 genotype. A CSF Aβ concentration of less than 500 pg/ml is an indication that Aβ peptide is accumulating in the brain and thus is not moving into the CSF. Next, the researchers analyzed imaging data using a dye called Pittsburgh compound B that binds to amyloid plaques in the brain and showed that those individuals with the ε4/ε4 genotype bound more dye than did those with the other APOE genotypes. They then moved to a mouse model of AD in which the mice expressed one of the three human apoE isoforms. They measured Aβ concentrations in the interstitial fluid of these mice using in vivo microdialysis and then looked at stained hippocampal sections from these mice. They found greater Aβ concentrations in both interstitial fluid and the hippocampus in mice expressing the human apoE4 isoform than in animals expressing either the E3 or E2 isoforms. They discovered that this difference in Aβ concentration between the mice carrying different APOE genotypes was present in young as well as aged mice, suggesting that it predates the appearance of amyloid plaques. They then measured clearance of Aβ from the interstitial fluid of young mice and showed that those with the human apoE4 isoform were less able to clear Aβ than those with the apoE2 or apoE3 isoforms. The researchers showed that processing of the amyloid precursor protein and generation of the Aβ peptide did not vary according to genotype, lending credence to the hypothesis that apoE4 affects clearance of Aβ but not its synthesis. This thorough study sheds new light on how apoE4 is implicated in AD and highlights the Aβ clearance pathway as a new target for developing drugs to slow or even halt the accumulation of amyloid plaques in patients with AD. The apolipoprotein E (APOE) ε4 allele is the strongest genetic risk factor for late-onset, sporadic Alzheimer’s disease (AD). The APOE ε4 allele markedly increases AD risk and decreases age of onset, likely through its strong effect on the accumulation of amyloid-β (Aβ) peptide. In contrast, the APOE ε2 allele appears to decrease AD risk. Most rare, early-onset forms of familial AD are caused by autosomal dominant mutations that often lead to overproduction of Aβ42 peptide. However, the mechanism by which APOE alleles differentially modulate Aβ accumulation in sporadic, late-onset AD is less clear. In a cohort of cognitively normal individuals, we report that reliable molecular and neuroimaging biomarkers of cerebral Aβ deposition vary in an apoE isoform–dependent manner. We hypothesized that human apoE isoforms differentially affect Aβ clearance or synthesis in vivo, resulting in an apoE isoform–dependent pattern of Aβ accumulation later in life. Performing in vivo microdialysis in a mouse model of Aβ-amyloidosis expressing human apoE isoforms (PDAPP/TRE), we find that the concentration and clearance of soluble Aβ in the brain interstitial fluid depends on the isoform of apoE expressed. This pattern parallels the extent of Aβ deposition observed in aged PDAPP/TRE mice. ApoE isoform–dependent differences in soluble Aβ metabolism are observed not only in aged but also in young PDAPP/TRE mice well before the onset of Aβ deposition in amyloid plaques in the brain. Additionally, amyloidogenic processing of amyloid precursor protein and Aβ synthesis, as assessed by in vivo stable isotopic labeling kinetics, do not vary according to apoE isoform in young PDAPP/TRE mice. Our results suggest that APOE alleles contribute to AD risk by differentially regulating clearance of Aβ from the brain, suggesting that Aβ clearance pathways may be useful therapeutic targets for AD prevention.

APOE predicts amyloid‐beta but not tau Alzheimer pathology in cognitively normal aging

To examine interactions of apolipoprotein E (APOE) genotype with age and with in vivo measures of preclinical Alzheimer disease (AD) in cognitively normal aging.

P-glycoprotein deficiency at the blood-brain barrier increases amyloid-β deposition in an Alzheimer disease mouse model

Accumulation of amyloid-beta (Abeta) within extracellular spaces of the brain is a hallmark of Alzheimer disease (AD). In sporadic, late-onset AD, there is little evidence for increased Abeta production, suggesting that decreased elimination from the brain may contribute to elevated levels of Abeta and plaque formation. Efflux transport of Abeta across the blood-brain barrier (BBB) contributes to Abeta removal from the brain. P-glycoprotein (Pgp) is highly expressed on the luminal surface of brain capillary endothelial cells and contributes to the BBB. In Pgp-null mice, we show that [I]Abeta40 and [I]Abeta42 microinjected into the CNS clear at half the rate that they do in WT mice. When amyloid precursor protein-transgenic (APP-transgenic) mice were administered a Pgp inhibitor, Abeta levels within the brain interstitial fluid significantly increased within hours of treatment. Furthermore, APP-transgenic, Pgp-null mice had increased levels of brain Abeta and enhanced Abeta deposition compared with APP-transgenic, Pgp WT mice. These data establish a direct link between Pgp and Abeta metabolism in vivo and suggest that Pgp activity at the BBB could affect risk for developing AD as well as provide a novel diagnostic and therapeutic target.

Multimodal techniques for diagnosis and prognosis of Alzheimer's disease

Alzheimers disease affects millions of people around the world. Currently, there are no treatments that prevent or slow the disease. Like other neurodegenerative diseases, Alzheimers disease is characterized by protein misfolding in the brain. This process and the associated brain damage begin years before the substantial neurodegeneration that accompanies dementia. Studies using new neuroimaging techniques and fluid biomarkers suggest that Alzheimers disease pathology can be detected preclinically. These advances should allow the design of new clinical trials and early mechanism-based therapeutic intervention.

Pittsburgh Compound B Imaging and Prediction of Progression From Cognitive Normality to Symptomatic Alzheimer Disease

OBJECTIVE To determine whether preclinical Alzheimer disease (AD), as detected by the amyloid-imaging agent Pittsburgh Compound B (PiB) in cognitively normal older adults, is associated with risk of symptomatic AD. DESIGN A longitudinal cohort study of cognitively normal older adults assessed with positron emission tomography (PET) to determine the mean cortical binding potential for PiB and followed up with annual clinical and cognitive assessments for progression to very mild dementia of the Alzheimer type (DAT). SETTING The Alzheimers Disease Research Center, Washington University, St Louis, Missouri. PARTICIPANTS One hundred fifty-nine participants with a mean age of 71.5 years with a Clinical Dementia Rating (CDR) of 0 on a PET PiB scan at baseline. MAIN OUTCOME MEASURE Progression from CDR 0 to CDR 0.5 status (very mild dementia). RESULTS Twenty-three participants progressed to CDR 0.5 at follow-up assessment (range, 1-5 assessments after PET PiB). Of these, 9 also were diagnosed with DAT. Higher mean cortical binding potential values for PiB (hazard ratio, 4.85; 95% confidence interval, 1.22-19.01; P = .02) and age (hazard ratio, 1.14; 95% confidence interval, 1.02-1.28; P = .03) predicted progression to CDR 0.5 DAT. The CDR 0.5 DAT group showed decline in 3 cognitive domains (episodic memory, semantic memory, and visuospatial performance) and had volume loss in the parahippocampal gyrus (includes entorhinal cortex) compared with individuals who remained at CDR 0. CONCLUSION Preclinical AD as detected by PET PiB is not benign, as it is associated with progression to symptomatic AD.

Expression of human apolipoprotein E reduces amyloid-β deposition in a mouse model of Alzheimer's disease

The epsilon4 allele of apolipoprotein E (apo E) is associated with an increased risk for developing Alzheimers disease (AD). This may be due to interactions between apo E and the amyloid-beta protein (Abeta). To assess the effects of human apo E isoforms on Abeta deposition in vivo, we bred apo E3 and apo E4 hemizygous (+/-) transgenic mice expressing apo E by astrocytes to mice homozygous (+/+) for a mutant amyloid precursor protein (APPV717F) transgene that develop age-dependent AD neuropathology. All mice were on a mouse apo E null (-/-) background. By nine months of age, APPV717F+/-, apo E-/- mice had developed Abeta deposition, and, as reported previously, the quantity of Abeta deposits was significantly less than that seen in APPV717F+/- mice expressing mouse apo E. In contrast to effects of mouse apo E, similar levels of human apo E3 and apo E4 markedly suppressed early Abeta deposition at nine months of age in APPV717F+/- transgenic mice, even when compared with mice lacking apo E. These findings suggest that human apo E isoforms decrease Abeta aggregation or increase Abeta clearance relative to an environment in which mouse apo E or no apo E is present. The results may have important implications for understanding mechanisms underlying the link between apo E and AD.

APOE4 Allele Disrupts Resting State fMRI Connectivity in the Absence of Amyloid Plaques or Decreased CSF Aβ42

Identifying high-risk populations is an important component of disease prevention strategies. One approach for identifying at-risk populations for Alzheimers disease (AD) is examining neuroimaging parameters that differ between patients, including functional connections known to be disrupted within the default-mode network. We have previously shown these same disruptions in cognitively normal elderly who have amyloid-β (Aβ) plaques [detected using Pittsburgh Compound B (PIB) PET imaging], suggesting neuronal toxicity of plaques. Here we sought to determine if pathological effects of apolipoprotein E ε4 (APOE4) genotype could be seen independent of Aβ plaque toxicity by examining resting state fMRI functional connectivity (fcMRI) in participants without preclinical fibrillar amyloid deposition (PIB−). Cognitively normal participants enrolled in longitudinal studies (n = 100, mean age = 62) who were PIB− were categorized into those with and without an APOE4 allele and studied using fcMRI. APOE4 allele carriers (E4+) differed significantly from E4− in functional connectivity of the precuneus to several regions previously defined as having abnormal connectivity in a group of AD participants. These effects were observed before any manifestations of cognitive changes and in the absence of brain fibrillar Aβ plaque deposition, suggesting that early manifestations of a genetic effect can be detected using fcMRI and that these changes may antedate the pathological effects of fibrillar amyloid plaque toxicity.