APOE and Alzheimer disease: a major gene with semi-dominant inheritance

Molecular Pathogenesis and Interventional Strategies for Alzheimer's Disease: Promises and Pitfalls

Alzheimer's disease (AD) is a debilitating disorder characterized by age-related dementia, which has no effective treatment to date. β-Amyloid depositions and hyperphosphorylated tau proteins are the main pathological hallmarks, along with oxidative stress, N-methyl-d-aspartate (NMDA) receptor-mediated excitotoxicity, and low levels of acetylcholine. Current pharmacotherapy for AD only provides symptomatic relief and limited improvement in cognitive functions. Many molecules have been explored that show promising outcomes in AD therapy and can regulate cellular survival through different pathways. To have a vivid approach to strategize the treatment regimen, AD physiopathology should be better explained considering diverse etiological factors in conjunction with biochemical disturbances. This Review attempts to discuss different disease modification approaches and address the novel therapeutic targets of AD that might pave the way for new drug discovery using the well-defined targets for therapy of the disease.

Genetic architecture of neurodegenerative dementias

Molecular genetics has been an invaluable tool to help understand the molecular basis of neurodegenerative dementias. In this review, we provide an overview of the genetic architecture underlying some of the most prevalent causes of dementia, including Alzheimer's dementia, frontotemporal lobar degeneration, Lewy body dementia, and prion diseases. We also discuss the complexity of the human genome and how the novel technologies have revolutionized and accelerated the way we screen the variety of our DNA. Finally, we also provide some examples about how this genetic knowledge is being transferred into the clinic through personalized medicine. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Longitudinal Maintenance of Cognitive Health in Centenarians in the 100-plus Study

IMPORTANCE

Some individuals who reach ages beyond 100 years in good cognitive health may be resilient against risk factors associated with cognitive decline. Exploring the processes underlying resilience may contribute to the development of therapeutic strategies that help to maintain cognitive health while aging.

OBJECTIVE

To identify individuals who escape cognitive decline until extreme ages and to investigate the prevalence of associated risk factors.

DESIGN, SETTING, AND PARTICIPANTS

The 100-plus Study is a prospective observational cohort study of community-based Dutch centenarians enrolled between 2013 and 2019 who were visited annually until death or until participation was no longer possible. The centenarians self-reported their cognitive health, as confirmed by a proxy. Of the 1023 centenarians approached for study inclusion, 340 fulfilled the study criteria and were included in analyses. Data analysis was performed from April 2019 to December 2019.

MAIN OUTCOMES AND MEASURES

Cognition was assessed using the Mini-Mental State Examination (MMSE). To identify centenarians who escape cognitive decline, this study investigated the association of baseline cognition with survivorship and cognitive trajectories for at least 2 years of follow-up using linear mixed models, adjusted for sex, age, and education. This study investigated the prevalence of apolipoprotein E (APOE) genotypes and cardiovascular disease as risk factors associated with cognitive decline.

RESULTS

At baseline, the median age of 340 centenarians was 100.5 years (range, 100.0-108.2 years); 245 participants (72.1%) were female. The maximum survival estimate plateaued at 82% per year (95% CI, 77% to 87%) across centenarians who scored 26 to 30 points on the baseline MMSE (hazard ratio, 0.56; 95% CI, 0.42 to 0.75; P < .001), suggesting that an MMSE score of 26 or higher is representative of both cognitive and physical health. Among the 79 centenarians who were followed up for 2 years or longer, those with baseline MMSE score less than 26 experienced a decline in MMSE score of 1.68 points per year (95% CI, -2.45 to -0.92 points per year; P = .02), whereas centenarians with MMSE scores of 26 or higher at baseline experienced a decline of 0.71 point per year (95% CI, -1.08 to -0.35 points per year). For 73% of the centenarians with baseline MMSE scores of 26 or higher, no cognitive changes were observed, which often extended to ensuing years or until death. It is estimated that this group is representative of less than 10% of Dutch centenarians. In this group, 18.6% carried at least 1 APOE-ε4 allele, compared with 5.6% of the centenarians with lower and/or declining cognitive performance.

CONCLUSIONS AND RELEVANCE

Most centenarians who scored 26 or higher on the MMSE at baseline maintained high levels of cognitive performance for at least 2 years, in some cases despite the presence of risk factors associated with cognitive decline. Investigation of this group might reveal the processes underlying resilience against risk factors associated with cognitive decline.

The Medical Genome Reference Bank contains whole genome and phenotype data of 2570 healthy elderly

Population health research is increasingly focused on the genetic determinants of healthy ageing, but there is no public resource of whole genome sequences and phenotype data from healthy elderly individuals. Here we describe the first release of the Medical Genome Reference Bank (MGRB), comprising whole genome sequence and phenotype of 2570 elderly Australians depleted for cancer, cardiovascular disease, and dementia. We analyse the MGRB for single-nucleotide, indel and structural variation in the nuclear and mitochondrial genomes. MGRB individuals have fewer disease-associated common and rare germline variants, relative to both cancer cases and the gnomAD and UK Biobank cohorts, consistent with risk depletion. Age-related somatic changes are correlated with grip strength in men, suggesting blood-derived whole genomes may also provide a biologic measure of age-related functional deterioration. The MGRB provides a broadly applicable reference cohort for clinical genetics and genomic association studies, and for understanding the genetics of healthy ageing.

Healthspan and healthy aging are areas of research with potential socioeconomic impact. Here, the authors present the Medical Genome Reference Bank (MGRB) which consist of over 4,000 individuals aged 70 years and older without a history of the major age-related diseases and report on results from whole-genome sequencing and association analyses.

Sex difference in Alzheimer's disease: An updated, balanced and emerging perspective on differing vulnerabilities

Sex biology influences Alzheimer's disease (AD). Sex differences exist in the epidemiologic, imaging, biomarker, and pathology studies of this uniquely human condition. The mandate to understand sex differences in major diseases like AD is important for many reasons. First, AD is the most common neurodegenerative condition and a devastating disease-experienced as an insidious and progressive erosion of memory, cognition, and other brain functions. Second, since true sex differences in AD exist, their precise understanding could reveal what protects one sex or makes the other vulnerable-and this knowledge could inform development of new therapeutic approaches to benefit both sexes. Third, AD develops in the aging brain in a milieu of decreased circulating gonadal hormones. Thus, how sex-specific depletion affects the brain along with how replacement of androgens in men and estrogens and progestins in women alters vulnerability to AD are relevant questions, with clinical implications in a future of personalized medicine. This review will highlight advances in sex differences in AD in human populations with a focused perspective on epidemiology, biomarkers, and clinical trials. A thorough and concise overview of sex differences reviewed here indicates varying vulnerabilities in men and women. This review examines several lines of recent and strong evidence that collectively indicate the following: (1) men die faster with AD, (2) more women live with AD, (3) both sexes show similar risk of developing AD until advanced ages when women show increased risk, (4) both sexes show largely similar AD biomarker burden with notable exceptions for higher tau levels in subgroups of women with high amyloid, (5) women show brain reserve and resilience to tau pathology, (6) both sexes are vulnerable to the genetic risk of carrying APOE4, with women showing higher risk, and (7) neither sex has shown clear benefit of hormone replacement for AD or dementia risk in randomized clinical trials to date.

Health Outcome Prioritization in Alzheimer's Disease: Understanding the Ethical Landscape

BACKGROUND

Dementia has been described as the greatest global health challenge in the 21st Century on account of longevity gains increasing its incidence, escalating health and social care pressures. These pressures highlight ethical, social, and political challenges about healthcare resource allocation, what health improvements matter to patients, and how they are measured. This study highlights the complexity of the ethical landscape, relating particularly to the balances that need to be struck when allocating resources; when measuring and prioritizing outcomes; and when individual preferences are sought.

OBJECTIVE

Health outcome prioritization is the ranking in order of desirability or importance of a set of disease-related objectives and their associated cost or risk. We analyze the complex ethical landscape in which this takes place in the most common dementia, Alzheimer's disease.

METHODS

Narrative review of literature published since 2007, incorporating snowball sampling where necessary. We identified, thematized, and discussed key issues of ethical salience.

RESULTS

Eight areas of ethical salience for outcome prioritization emerged: 1) Public health and distributive justice, 2) Scarcity of resources, 3) Heterogeneity and changing circumstances, 4) Knowledge of treatment, 5) Values and circumstances, 6) Conflicting priorities, 7) Communication, autonomy and caregiver issues, and 8) Disclosure of risk.

CONCLUSION

These areas highlight the difficult balance to be struck when allocating resources, when measuring and prioritizing outcomes, and when individual preferences are sought. We conclude by reflecting on how tools in social sciences and ethics can help address challenges posed by resource allocation, measuring and prioritizing outcomes, and eliciting stakeholder preferences.

Is Alzheimer's Disease a Liver Disease of the Brain?

Clinical specialization is not only a force for progress, but it has also led to the fragmentation of medical knowledge. The focus of research in the field of Alzheimer's disease (AD) is neurobiology, while hepatologists focus on liver diseases and lipid specialists on atherosclerosis. This article on AD focuses on the role of the liver and lipid homeostasis in the development of AD. Amyloid-β (Aβ) deposits accumulate as plaques in the brain of an AD patient long before cognitive decline is evident. Aβ generation is a normal physiological process; the steady-state level of Aβ in the brain is determined by balance between Aβ production and its clearance. We present evidence suggesting that the liver is the origin of brain Aβ deposits and that it is involved in peripheral clearance of circulating Aβ in the blood. Hence the liver could be targeted to decrease Aβ production or increase peripheral clearance.

Multimodal MRI of grey matter, white matter, and functional connectivity in cognitively healthy mutation carriers at risk for frontotemporal dementia and Alzheimer's disease

BACKGROUND

Frontotemporal dementia (FTD) and Alzheimer's disease (AD) are associated with divergent differences in grey matter volume, white matter diffusion, and functional connectivity. However, it is unknown at what disease stage these differences emerge. Here, we investigate whether divergent differences in grey matter volume, white matter diffusion, and functional connectivity are already apparent between cognitively healthy carriers of pathogenic FTD mutations, and cognitively healthy carriers at increased AD risk.

METHODS

We acquired multimodal magnetic resonance imaging (MRI) brain scans in cognitively healthy subjects with (n=39) and without (n=36) microtubule-associated protein Tau (MAPT) or progranulin (GRN) mutations, and with (n=37) and without (n=38) apolipoprotein E ε4 (APOE4) allele. We evaluated grey matter volume using voxel-based morphometry, white matter diffusion using tract-based spatial statistics (TBSS), and region-to-network functional connectivity using dual regression in the default mode network and salience network. We tested for differences between the respective carriers and controls, as well as for divergence of those differences. For the divergence contrast, we additionally performed region-of-interest TBSS analyses in known areas of white matter diffusion differences between FTD and AD (i.e., uncinate fasciculus, forceps minor, and anterior thalamic radiation).

RESULTS

MAPT/GRN carriers did not differ from controls in any modality. APOE4 carriers had lower fractional anisotropy than controls in the callosal splenium and right inferior fronto-occipital fasciculus, but did not show grey matter volume or functional connectivity differences. We found no divergent differences between both carrier-control contrasts in any modality, even in region-of-interest analyses.

CONCLUSIONS

Concluding, we could not find differences suggestive of divergent pathways of underlying FTD and AD pathology in asymptomatic risk mutation carriers. Future studies should focus on asymptomatic mutation carriers that are closer to symptom onset to capture the first specific signs that may differentiate between FTD and AD.

Genetic perturbations of disease risk genes in mice capture transcriptomic signatures of late-onset Alzheimer's disease

BACKGROUND

New genetic and genomic resources have identified multiple genetic risk factors for late-onset Alzheimer's disease (LOAD) and characterized this common dementia at the molecular level. Experimental studies in model organisms can validate these associations and elucidate the links between specific genetic factors and transcriptomic signatures. Animal models based on LOAD-associated genes can potentially connect common genetic variation with LOAD transcriptomes, thereby providing novel insights into basic biological mechanisms underlying the disease.

METHODS

We performed RNA-Seq on whole brain samples from a panel of six-month-old female mice, each carrying one of the following mutations: homozygous deletions of Apoe and Clu; hemizygous deletions of Bin1 and Cd2ap; and a transgenic APOEε4. Similar data from a transgenic APP/PS1 model was included for comparison to early-onset variant effects. Weighted gene co-expression network analysis (WGCNA) was used to identify modules of correlated genes and each module was tested for differential expression by strain. We then compared mouse modules with human postmortem brain modules from the Accelerating Medicine's Partnership for AD (AMP-AD) to determine the LOAD-related processes affected by each genetic risk factor.

RESULTS

Mouse modules were significantly enriched in multiple AD-related processes, including immune response, inflammation, lipid processing, endocytosis, and synaptic cell function. WGCNA modules were significantly associated with Apoe-/-, APOEε4, Clu-/-, and APP/PS1 mouse models. Apoe-/-, GFAP-driven APOEε4, and APP/PS1 driven modules overlapped with AMP-AD inflammation and microglial modules; Clu-/- driven modules overlapped with synaptic modules; and APP/PS1 modules separately overlapped with lipid-processing and metabolism modules.

CONCLUSIONS

This study of genetic mouse models provides a basis to dissect the role of AD risk genes in relevant AD pathologies. We determined that different genetic perturbations affect different molecular mechanisms comprising AD, and mapped specific effects to each risk gene. Our approach provides a platform for further exploration into the causes and progression of AD by assessing animal models at different ages and/or with different combinations of LOAD risk variants.

Polygenic risk scores for Alzheimer's disease, and academic achievement, cognitive and behavioural measures in children from the general population

OBJECTIVE

Several studies report a polygenic component of risk for Alzheimer's disease. Understanding whether this polygenic signal is associated with educational, cognitive and behavioural outcomes in children could provide an earlier window for intervention.

METHODS

We examined whether polygenic risk scores (PRS) at varying P-value thresholds in children from the Avon Longitudinal Study of Parents and Children were associated with academic achievement, cognitive and behavioural measures in childhood and adolescence.

RESULTS

We did not detect any evidence that the genome-wide significant PRS (5x10-8) were associated with these outcomes. PRS at the highest P-value threshold examined (P ≤ 5x10-1) were associated with lower academic achievement in adolescents (Key Stage 3; β: -0.03; 95% confidence interval: -0.05, -0.003) but the effect was attenuated when single nucleotide polymorphisms (SNPs) associated with educational attainment were removed. These PRS were associated with lower IQ (β: -0.04; 95% CI: -0.07, -0.02) at age 8 years with the effect remaining after removing SNPs associated with educational attainment.

CONCLUSIONS

SNPs mediating the biological effects of Alzheimer's disease are unlikely to operate early in life. The evidence of association between PRS for Alzheimer's disease at liberal thresholds and cognitive measures suggest shared genetic pathways between Alzheimer's disease, academic achievement and cognition.

Strengths and Weaknesses of the Gray Mouse Lemur (Microcebus murinus) as a Model for the Behavioral and Psychological Symptoms and Neuropsychiatric Symptoms of Dementia

To face the load of the prevalence of Alzheimer’s disease in the aging population, there is an urgent need to develop more translatable animal models with similarities to humans in both the symptomatology and physiopathology of dementia. Due to their close evolutionary similarity to humans, non-human primates (NHPs) are of primary interest. Of the NHPs, to date, the gray mouse lemur (Microcebus murinus) has shown promising evidence of its translatability to humans. The present review reports the known advantages and limitations of using this species at all levels of investigation in the context of neuropsychiatric conditions. In this easily bred Malagasy primate with a relatively short life span (approximately 12 years), age-related cognitive decline, amyloid angiopathy, and risk factors (i.e., glucoregulatory imbalance) are congruent with those observed in humans. More specifically, analogous behavioral and psychological symptoms and neuropsychiatric symptoms of dementia (BPSD/NPS) to those in humans can be found in the aging mouse lemur. Aged mouse lemurs show typical age-related alterations of locomotor activity daily rhythms such as decreased rhythm amplitude, increased fragmentation, and increased activity during the resting-sleeping phase of the day and desynchronization with the light-dark cycle. In addition, sleep deprivation successfully induces cognitive deficits in adult mouse lemurs, and the effectiveness of approved cognitive enhancers such as acetylcholinesterase inhibitors or N-methyl-D-aspartate antagonists is demonstrated in sleep–deprived animals. This result supports the translational potential of this animal model, especially for unraveling the mechanisms underlying dementia and for developing novel therapeutics to prevent age-associated cognitive decline. In conclusion, actual knowledge of BPSD/NPS-like symptoms of age-related cognitive deficits in the gray mouse lemur and the recent demonstration of the similarity of these symptoms with those seen in humans offer promising new ways of investigating both the prevention and treatment of pathological aging.

A Human Embryonic Stem Cell Model of Aβ-Dependent Chronic Progressive Neurodegeneration

We describe the construction and phenotypic analysis of a human embryonic stem cell model of progressive Aβ-dependent neurodegeneration (ND) with potential relevance to Alzheimer’s disease (AD). We modified one allele of the normal APP locus to directly express a secretory form of Aβ40 or Aβ42, enabling expression from this edited allele to bypass the normal amyloidogenic APP processing pathway. Following neuronal differentiation, edited cell lines specifically accumulate intracellular aggregated/oligomeric Aβ, exhibit a synaptic deficit, and have an abnormal accumulation of endolysosomal vesicles. Edited cultures progress to a stage of overt ND. All phenotypes appear at earlier culture times for Aβ42 relative to Aβ40. Whole transcriptome RNA-Seq analysis identified 23 up and 70 down regulated genes (differentially expressed genes) with similar directional fold change but larger absolute values in the Aβ42 samples suggesting common underlying pathogenic mechanisms. Pathway/annotation analysis suggested that down regulation of extracellular matrix and cilia functions is significantly overrepresented. This cellular model could be useful for uncovering mechanisms directly linking Aβ to neuronal death and as a tool to screen for new therapeutic agents that slow or prevent human ND.

Multi-omics signature of brain amyloid deposition in asymptomatic individuals at-risk for Alzheimer's disease: The INSIGHT-preAD study

BACKGROUND

One of the biggest challenge in Alzheimer's disease (AD) is to identify pathways and markers of disease prediction easily accessible, for prevention and treatment. Here we analysed blood samples from the INveStIGation of AlzHeimer's predicTors (INSIGHT-preAD) cohort of elderly asymptomatic individuals with and without brain amyloid load.

METHODS

We performed blood RNAseq, and plasma metabolomics and lipidomics using liquid chromatography-mass spectrometry on 48 individuals amyloid positive and 48 amyloid negative (SUVr cut-off of 0·7918). The three data sets were analysed separately using differential gene expression based on negative binomial distribution, non-parametric (Wilcoxon) and parametric (correlation-adjusted Student't) tests. Data integration was conducted using sparse partial least squares-discriminant and principal component analyses. Bootstrap-selected top-ten features from the three data sets were tested for their discriminant power using Receiver Operating Characteristic curve. Longitudinal metabolomic analysis was carried out on a subset of 22 subjects.

FINDINGS

Univariate analyses identified three medium chain fatty acids, 4-nitrophenol and a set of 64 transcripts enriched for inflammation and fatty acid metabolism differentially quantified in amyloid positive and negative subjects. Importantly, the amounts of the three medium chain fatty acids were correlated over time in a subset of 22 subjects (p < 0·05). Multi-omics integrative analyses showed that metabolites efficiently discriminated between subjects according to their amyloid status while lipids did not and transcripts showed trends. Finally, the ten top metabolites and transcripts represented the most discriminant omics features with 99·4% chance prediction for amyloid positivity.

INTERPRETATION

This study suggests a potential blood omics signature for prediction of amyloid positivity in asymptomatic at-risk subjects, allowing for a less invasive, more accessible, and less expensive risk assessment of AD as compared to PET studies or lumbar puncture. FUND: Institut Hospitalo-Universitaire and Institut du Cerveau et de la Moelle Epiniere (IHU-A-ICM), French Ministry of Research, Fondation Alzheimer, Pfizer, and Avid.

Latent Classes of Cognitive Functioning Among Depressed Older Adults Without Dementia

OBJECTIVE

Use latent class analysis (LCA) to identify patterns of cognitive functioning in a sample of older adults with clinical depression and without dementia and assess demographic, psychiatric, and neurobiological predictors of class membership.

METHOD

Neuropsychological assessment data from 121 participants in the Alzheimer's Disease Neuroimaging Initiative-Depression project (ADNI-D) were analyzed, including measures of executive functioning, verbal and visual memory, visuospatial and language functioning, and processing speed. These data were analyzed using LCA, with predictors of class membership such as depression severity, depression and treatment history, amyloid burden, and APOE e4 allele also assessed.

RESULTS

A two-class model of cognitive functioning best fit the data, with the Lower Cognitive Class (46.1% of the sample) performing approximately one standard deviation below the Higher Cognitive Class (53.9%) on most tests. When predictors of class membership were assessed, carrying an APOE e4 allele was significantly associated with membership in the Lower Cognitive Class. Demographic characteristics, age of depression onset, depression severity, history of psychopharmacological treatment for depression, and amyloid positivity did not predict class membership.

CONCLUSION

LCA allows for identification of subgroups of cognitive functioning in a mostly cognitively intact late life depression (LLD) population. One subgroup, the Lower Cognitive Class, more likely to carry an APOE e4 allele, may be at a greater risk for subsequent cognitive decline, even though current performance on neuropsychological testing is within normal limits. These findings have implications for early identification of those at greatest risk, risk factors, and avenues for preventive intervention.

Association of apolipoprotein E genotypes with epilepsy risk: A systematic review and meta-analysis

OBJECTIVE

The objective of this study was to identify the association between certain genotypes or alleles of the APOE (Apolipoprotein E) gene and the epilepsy risk.

METHODS

All studies on human APOE genotypes associated with epilepsy were included. Separate meta-analyses were conducted between the patients with epilepsy and the control group from the following three aspects: ε4 carriers or ε2 carriers vs ε3/ε3 (the ε2/ε4 genotype was excluded), ε4 carriers vs ε2 carriers, and five genotypes vs ε3/ε3. The subgroup analysis was conducted on the ethnicity, the control group was healthy or not, and type of epilepsy.

RESULTS

Nine studies with 2210 individuals were included. Compared with ε3/ε3 genotype, ε4 carriers increased the epilepsy risk (odds ratios [ORs]: 1.27; 95% confidence intervals [CI]: 1.01 to 1.59; P = 0.042), while ε2 carriers had no association with epilepsy risk (OR: 0.88; 95% CI: 0.66 to 1.18; P = 0.184). The risk of epilepsy was 1.45 times greater in ε4 carriers compared with ε2 carriers (OR: 1.45; 95% CI: 1.02 to 2.04; P = 0.037). When the number of APOE ε4 allele increased, the ORs increased progressively (no ε4 alleles, OR: 0.88, 95% CI: 0.66 to 1.18; one ε4 allele, OR: 1.25, 95% CI: 0.99 to 1.57; two ε4 alleles, OR: 1.84, 95% CI: 0.83 to 4.10). Apolipoprotein E ε4 carriers had a higher epilepsy risk in the population without primary diseases (OR: 1.43; 95% CI: 1.09 to 1.88), and a higher risk in Asian populations (OR: 1.67; 95% CI: 1.12 to 2.49).

CONCLUSIONS

Apolipoprotein E ε4 allele genotype was associated with an increased epilepsy risk, which was more prominent in the Asian and the population without primary diseases. These findings may be used to guide the directions of prevention and treatment on epilepsy. Larger clinical studies are needed.

Spatial navigation deficits - overlooked cognitive marker for preclinical Alzheimer disease?

Detection of incipient Alzheimer disease (AD) pathophysiology is critical to identify preclinical individuals and target potentially disease-modifying therapies towards them. Current neuroimaging and biomarker research is strongly focused in this direction, with the aim of establishing AD fingerprints to identify individuals at high risk of developing this disease. By contrast, cognitive fingerprints for incipient AD are virtually non-existent as diagnostics and outcomes measures are still focused on episodic memory deficits as the gold standard for AD, despite their low sensitivity and specificity for identifying at-risk individuals. This Review highlights a novel feature of cognitive evaluation for incipient AD by focusing on spatial navigation and orientation deficits, which are increasingly shown to be present in at-risk individuals. Importantly, the navigation system in the brain overlaps substantially with the regions affected by AD in both animal models and humans. Notably, spatial navigation has fewer verbal, cultural and educational biases than current cognitive tests and could enable a more uniform, global approach towards cognitive fingerprints of AD and better cognitive treatment outcome measures in future multicentre trials. The current Review appraises the available evidence for spatial navigation and/or orientation deficits in preclinical, prodromal and confirmed AD and identifies research gaps and future research priorities.

SORL1 genetic variants and Alzheimer disease risk: a literature review and meta-analysis of sequencing data

Massive parallel sequencing recently allowed the identification of three genes carrying a higher burden of rare, protein-truncating and missense predicted damaging variants in Alzheimer disease (AD) cases as compared to controls: TREM2, SORL1, and ABCA7. SORL1 encodes SorLA, a key protein involved in the processing of the amyloid-beta (Aβ) precursor protein (APP) and the secretion of the Aβ peptide, the aggregation of which triggers AD pathophysiology. Common SORL1 single nucleotide polymorphisms had originally been associated with AD with modest odds ratios (ORs). The association of AD with rare SORL1 coding variants has been demonstrated at the gene level by aggregating protein-truncating (PTV) and rare predicted damaging missense variants. In addition to the loss of SorLA function induced by PTVs, a few missense variants were studied in vitro, showing diverse degrees of decreased SorLA function and leading to increased Aβ secretion. However, the exact functional consequences of most of the missense variants remain to be determined as well as corresponding levels of AD risk. Hereby we review the evidence of the association of SORL1 common and rare variants with AD risk and conduct a meta-analysis of published data on SORL1 rare variants in five large sequencing studies. We observe a significant enrichment in PTVs with ORs of 12.29 (95% confidence interval = [4.22-35.78]) among all AD cases and 27.50 [7.38-102.42] among early-onset cases. Rare [minor allele frequency (MAF) < 1%] and ultra-rare (MAF < 10^-4) missense variants that are predicted damaging by 3/3 bioinformatics tools also show significant associations with corresponding ORs of 1.87 [1.54-2.28] and 3.14 [2.30-4.28], respectively. Per-domain analyses show significant association with the APP-binding CR cluster class A repeats and the Aβ-binding VPS10P domains, as well as the fibronectin type III domain, the function of which remains to be specified. These results further support a critical role for SORL1 rare coding variants in AD, although functional and segregation analyses are required to allow an accurate use in a clinical setting.

The new genetic landscape of Alzheimer's disease: from amyloid cascade to genetically driven synaptic failure hypothesis?

A strong genetic predisposition (60–80% of attributable risk) is present in Alzheimer’s disease (AD). In view of this major genetic component, identification of the genetic risk factors has been a major objective in the AD field with the ultimate aim to better understand the pathological processes. In this review, we present how the genetic risk factors are involved in APP metabolism, β-amyloid peptide production, degradation, aggregation and toxicity, innate immunity, and Tau toxicity. In addition, on the basis of the new genetic landscape, resulting from the recent high-throughput genomic approaches and emerging neurobiological information, we propose an over-arching model in which the focal adhesion pathway and the related cell signalling are key elements in AD pathogenesis. The core of the focal adhesion pathway links the physiological functions of amyloid precursor protein and Tau with the pathophysiological processes they are involved in. This model includes several entry points, fitting with the different origins for the disease, and supports the notion that dysregulation of synaptic plasticity is a central node in AD. Notably, our interpretation of the latest data from genome wide association studies complements other hypotheses already developed in the AD field, i.e., amyloid cascade, cellular phase or propagation hypotheses. Genetically driven synaptic failure hypothesis will need to be further tested experimentally within the general AD framework.

Apolipoprotein E and Alzheimer disease: pathobiology and targeting strategies

Polymorphism in the apolipoprotein E (APOE) gene is a major genetic risk determinant of late-onset Alzheimer disease (AD), with the APOE*ε4 allele conferring an increased risk and the APOE*ε2 allele conferring a decreased risk relative to the common APOE*ε3 allele. Strong evidence from clinical and basic research suggests that a major pathway by which APOE4 increases the risk of AD is by driving earlier and more abundant amyloid pathology in the brains of APOE*ε4 carriers. The number of amyloid-β (Aβ)-dependent and Aβ-independent pathways that are known to be differentially modulated by APOE isoforms is increasing. For example, evidence is accumulating that APOE influences tau pathology, tau-mediated neurodegeneration and microglial responses to AD-related pathologies. In addition, APOE4 is either pathogenic or shows reduced efficiency in multiple brain homeostatic pathways, including lipid transport, synaptic integrity and plasticity, glucose metabolism and cerebrovascular function. Here, we review the recent progress in clinical and basic research into the role of APOE in AD pathogenesis. We also discuss how APOE can be targeted for AD therapy using a precision medicine approach.

Sex modulates the ApoE ε4 effect on brain tau deposition measured by 18F-AV-1451 PET in individuals with mild cognitive impairment

The strongest genetic risk factor for Alzheimer's disease (AD) is the Apolipoprotein E type 4 allele (ApoE ε4). The interaction between sex and ApoE ε4 carrier status on AD risk remains an area of intense investigation. We hypothesized that sex modulates the relationship between ApoE ε4 carrier status and brain tau deposition (a quantitative endophenotype in AD) in individuals with mild cognitive impairment (MCI).

Methods: Preprocessed ¹⁸F-AV-1451 tau and ¹⁸F-AV-45 amyloid PET images, T1-weighted structural magnetic resonance imaging (MRI) scans, demographic information, and cerebrospinal fluid (CSF) total tau (t-tau) and phosphorylated tau (p-tau) measurements from 108 MCI subjects in the Alzheimer's Disease Neuroimaging Initiative (ADNI) database were included. After downloading pre-processed images from ADNI, an iterative reblurred Van Cittertiteration partial volume correction (PVC) method was applied to all PET images. MRIs were used for PET spatial normalization. Regions of interest (ROIs) were defined in standard space, and standardized uptake value ratio (SUVR) images relative to cerebellum were computed. ApoE ε4 by sex interaction analyses on ¹⁸F-AV-1451 and CSF tau (t-tau, p-tau) were assessed using generalized linear models. The association between ¹⁸F-AV-1451 SUVR and CSF tau (t-tau, p-tau) was assessed.

Results: After applying PVC and controlling for age, education level and global cortical ¹⁸F-AV-45 SUVR, we found that the entorhinal cortex, amygdala, parahippocampal gyrus, posterior cingulate, and occipital ROIs exhibited a significant ApoE ε4 by sex interaction effect (false discovery rate P < 0.1) among MCI individuals. We also found a significant ApoE ε4 by sex interaction effect on CSF t-tau and p-tau. ¹⁸F-AV-1451 SUVR in the 5 ROIs with ApoE ε4 by sex interaction was significantly correlated with CSF p-tau and t-tau.

Conclusions: Our findings suggest that women are more susceptible to ApoE ε4-associated accumulation of neurofibrillary tangles in MCI compared to males. Both CSF tau (p-tau, t-tau) and brain tau PET are robust quantitative biomarkers for studying ApoE ε4 by sex effects on brain tau deposition in MCI participants.

Dementia in Women

Despite the progress made in the understanding of the etiology, neuropathophysiology, and treatment of different types of dementia, such disorders continue to pose huge health problems worldwide. The differential effect of the burden of disease on women is just being realized. These effects range from direct effects of the disease processes themselves, the way women experience the disease, and the caregiver burden. This article provides a brief overview of the available information on this topic.

Consequences of Metabolic Disruption in Alzheimer's Disease Pathology

Alzheimer's disease (AD) is an irreversible, progressive disease that slowly destroys cognitive function, such as thinking, remembering, and reasoning, to a level that one cannot carry out a daily living. As people live longer, the risk of developing AD has increased to 1 in 10 among people who are older than 65 and to almost 1 in 2 among those who are older than 85 according to a 2019 Alzheimer's Association report. As a most common cause of dementia, AD accounts for 60-80% of all dementia cases. AD is characterized by amyloid plaques and neurofibrillary tangles, composed of extracellular aggregates of amyloid-β peptides and intracellular aggregates of hyperphosphorylated tau, respectively. Besides plaques and tangles, AD pathology includes synaptic dysfunction including loss of synapses, inflammation, brain atrophy, and brain hypometabolism, all of which contribute to progressive cognitive decline. Recent genetic studies of sporadic cases of AD have identified a score of risk factors, as reported by Hollingworth et al. (Nat Genet 43:429-435, 2001) and Lambert et al. (Nat Genet 45:1452-1458, 2013). Of all these genes, apolipoprotein E4 (APOE4) still presents the biggest risk factor for sporadic cases of AD, as stated in Saunders et al. (Neurology 43:1467-1472, 1993): depending on whether you have 1 or 2 copies of APOE4 allele, the risk increases from 3- to 12-fold, respectively, in line with Genin et al. (Mol Psychiatry 16:903-907, 2011). Besides these genetic risk factors, having type 2 diabetes (T2D), a chronic metabolic disease, is known to increase the AD risk by at least 2-fold when these individuals age, conforming to Sims-Robinson et al. (Nat Rev Neurol 6:551-559, 2010). Diabetes is reaching a pandemic scale with over 422 million people diagnosed worldwide in 2014 according to World Health Organization. Although what proportion of these diabetic patients develop AD is not known, even if 10% of diabetic patients develop AD later in their life, it would double the number of AD patients in the world. Better understanding between T2D and AD is of paramount of importance for the future. The goal of this review is to examine our current understanding on metabolic dysfunction in AD, so that a potential target can be identified in the near future.

Alzheimer Disease Pathogenesis: Insights From Molecular and Cellular Biology Studies of Oligomeric Aβ and Tau Species

Alzheimer disease (AD) represents an oncoming epidemic that without an effective treatment promises to exact extraordinary human and financial burdens. Studies of pathogenesis are essential for defining targets for discovering disease-modifying treatments. Past studies of AD neuropathology provided valuable, albeit limited, insights. Nevertheless, building on these findings, recent studies have provided an increasingly rich harvest of genetic, molecular and cellular data that are creating unprecedented opportunities to both understand and treat AD. Among the most significant are those documenting the presence within the AD brain of toxic oligomeric species of Aβ and tau. Existing data support the view that such species can propagate and spread within neural circuits. To place these findings in context we first review the genetics and neuropathology of AD, including AD in Down syndrome (AD-DS). We detail studies that support the existence of toxic oligomeric species while noting the significant unanswered questions concerning their precise structures, the means by which they spread and undergo amplification and how they induce neuronal dysfunction and degeneration. We conclude by offering a speculative synthesis for how oligomers of Aβ and tau initiate and drive pathogenesis. While 100 years after Alzheimer's first report there is much still to learn about pathogenesis and the discovery of disease-modifying treatments, the application of new concepts and sophisticated new tools are poised to deliver important advances for combatting AD.

The Alzheimer's Prevention Initiative Generation Program: Study design of two randomized controlled trials for individuals at risk for clinical onset of Alzheimer's disease

Introduction

Alzheimer's disease (AD) pathology, including the accumulation of amyloid beta (Aβ) species and tau pathology, begins decades before the onset of cognitive impairment. This long preclinical period provides an opportunity for clinical trials designed to prevent or delay the onset of cognitive impairment due to AD. Under the umbrella of the Alzheimer's Prevention Initiative Generation Program, therapies targeting Aβ, including CNP520 (umibecestat), a β-site-amyloid precursor protein cleaving enzyme-1 (BACE-1) inhibitor, and CAD106, an active Aβ immunotherapy, are in clinical development in preclinical AD.

Methods

The Alzheimer's Prevention Initiative Generation Program comprises two pivotal (phase 2/3) studies that assess the efficacy and safety of umibecestat and CAD106 in cognitively unimpaired individuals with high risk for developing symptoms of AD based on their age (60-75 years), APOE4 genotype, and, for heterozygotes (APOE ε2/ε4 or ε3/ε4), elevated brain amyloid. Approximately, 3500 individuals will be enrolled in either Generation Study 1 (randomized to cohort 1 [CAD106 injection or placebo, 5:3] or cohort 2 [oral umibecestat 50 mg or placebo, 3:2]) or Generation Study 2 (randomized to oral umibecestat 50 mg and 15 mg, or placebo [2:1:2]). Participants receive treatment for at least 60 months and up to a maximum of 96 months. Primary outcomes include time to event, with event defined as diagnosis of mild cognitive impairment due to AD and/or dementia due to AD, and the Alzheimer's Prevention Initiative preclinical composite cognitive test battery. Secondary endpoints include the Clinical Dementia Rating Sum of Boxes, Repeatable Battery for the Assessment of Neuropsychological Status total score, Everyday Cognition Scale, biomarkers, and brain imaging.

Discussion

The Generation Program is designed to assess the efficacy, safety, and biomarker effects of the two treatments in individuals at high risk for AD. It may also provide a plausible test of the amyloid hypothesis and further accelerate the evaluation of AD prevention therapies.

Multimodal Neuroimaging in Alzheimer's Disease: Early Diagnosis, Physiopathological Mechanisms, and Impact of Lifestyle

Over the last ten years, we have conducted research in Alzheimer's disease (AD) using multimodal neuroimaging techniques to improve diagnosis, further our understanding of the pathological mechanisms underlying the disease, and support the development of innovative non-pharmacological preventive strategies. Our works emphasized the interest of hippocampal subfield volumetry in early diagnosis and the need for further development in this field including optimization, standardization, and automatization of the techniques. Also, we conducted several studies in cognitively intact at-risk elderly (e.g., subjective cognitive decline patients and APOE4 carriers) to better identify biomarkers associated with increased risk of developing AD. Regarding the physiopathological mechanisms, specific multimodal neuroimaging techniques allowed us to highlight the relevance of diaschisis, the mismatch between neurodegeneration and local Aβ deposition and the regional variation in the mechanisms underlying structural or functional alterations. Further works integrating other biomarkers known to play a role in the physiopathology of AD (tau, TDP-43, inflammation, etc.) in a longitudinal design would be useful to get a comprehensive understanding of their relative role, sequence, and causal relationships. Our works also highlighted the relevance of functional connectivity in further understanding the specificity of cognitive deficits in AD and how connectivity differentially influences the propagation of the different AD biomarkers. Finally, we conducted several studies on the links between lifestyle factors and neuroimaging biomarkers to unravel mechanisms of reserve. Further efforts are needed to better understand which lifestyle factor, or combination of factors, impact on AD pathology, and when, to help translating our knowledge to training programs that might prevent or delay brain and cognitive changes leading to AD dementia.

Different Cognitive Frailty Models and Health- and Cognitive-related Outcomes in Older Age: From Epidemiology to Prevention

Frailty, a critical intermediate status of the aging process that is at increased risk for negative health-related events, includes physical, cognitive, and psychosocial domains or phenotypes. Cognitive frailty is a condition recently defined by operationalized criteria describing coexisting physical frailty and mild cognitive impairment (MCI), with two proposed subtypes: potentially reversible cognitive frailty (physical frailty/MCI) and reversible cognitive frailty (physical frailty/pre-MCI subjective cognitive decline). In the present article, we reviewed the framework for the definition, different models, and the current epidemiology of cognitive frailty, also describing neurobiological mechanisms, and exploring the possible prevention of the cognitive frailty progression. Several studies suggested a relevant heterogeneity with prevalence estimates ranging 1.0–22.0% (10.7–22.0% in clinical-based settings and 1.0–4.4% in population-based settings). Cross-sectional and longitudinal population-based studies showed that different cognitive frailty models may be associated with increased risk of functional disability, worsened quality of life, hospitalization, mortality, incidence of dementia, vascular dementia, and neurocognitive disorders. The operationalization of clinical constructs based on cognitive impairment related to physical causes (physical frailty, motor function decline, or other physical factors) appears to be interesting for dementia secondary prevention given the increased risk for progression to dementia of these clinical entities. Multidomain interventions have the potential to be effective in preventing cognitive frailty. In the near future, we need to establish more reliable clinical and research criteria, using different operational definitions for frailty and cognitive impairment, and useful clinical, biological, and imaging markers to implement intervention programs targeted to improve frailty, so preventing also late-life cognitive disorders.

Cases in Precision Medicine: When Patients Present With Direct-to-Consumer Genetic Test Results

A couple is planning to start a family, and they decide to order a 23andMe test after reading about the company's carrier screening test and the new BRCA1/BRCA2 test. They bring the results to their internist for advice on how to proceed. Given the rise in public interest in human genetics and precision medicine, direct-to-consumer genetic testing is becoming increasingly popular, and clinicians should expect patients to present the results of these tests more frequently. This article uses a case scenario to provide information about what the results of these tests mean, and what they do not mean.

Genetic Heterogeneity of Alzheimer's Disease: Embracing Research Partnerships

Studies on the genetics of Alzheimer’s disease (AD) have revealed the complexity and heterogeneity of the disease. All our studies have supported this evidence and contribute to the current understanding of the genetic architecture of AD. This report reviews the success of our investigations, focusing on the implications and importance of the genetics of AD, and demonstrates the relevance of research strategies embracing partnerships.

The effect of ApoE ε4 on longitudinal brain region-specific glucose metabolism in patients with mild cognitive impairment: a FDG-PET study

While the ApoE ε4 allele is a known risk factor for mild cognitive impairment (MCI) and Alzheimer's disease, brain region specific effects remain elusive. In this study, we investigate whether the ApoE ε4 allele exhibits brain region specific effects in longitudinal glucose uptake among patients with MCI from the Alzheimer's Disease Neuroimaging Initiative (ADNI). Preprocessed FDG PET images, MRIs, and demographic information were downloaded from the ADNI database. An iterative reblurred Van Cittertiteration method was used for partial volume correction (PVC) on all PET images. Structural MRIs were used for PET spatial normalization and region of interest (ROI) definition in standard space. Longitudinal changes in ROI FDG standardized uptake value ratio (SUVR) relative to cerebellum in 24 ApoE ε4 carriers and 24 age-matched ApoE ε4 non-carriers were measured for up to 84-months (median 72 months, SD = 11.2 months) and compared using a generalized linear mixed effects model controlling for gender, education, baseline age, and follow-up period. Additionally, voxelwise analysis was performed by implementing a paired t-test comparing matched baseline and 72 month FDG SUVR images in ApoE carriers and non-carriers separately. Results with PVC were compared with ones from non-PVC based analysis. After applying PVC, the superior fontal, parietal, lateral temporal, medial temporal, caudate, thalamus, and post-cingulate, and amygdala regions had greater longitudinal decreases in FDG uptake in ApoE ε4 carriers with MCI compared to non-carriers with MCI. Similar forebrain and limbic clusters were found through voxelwise analysis. Compared to the PVC based analysis, fewer significant ApoE-associated regions and clusters were found in the non-PVC based PET analysis. Our findings suggest that the ApoE ε4 genotype is associated with a longitudinal decline in glucose uptake in 8 forebrain and limbic brain regions in the context of MCI. In conclusion, this 84-months longitudinal FDG PET study demonstrates a novel ApoE ε4-associated brain-region specific glucose metabolism pattern in patients with MCI. Partial volume correction improved FDG PET quantification.

Identification of genetic heterogeneity of Alzheimer's disease across age

The risk of APOE for Alzheimer's disease (AD) is modified by age. Beyond APOE, the polygenic architecture may also be heterogeneous across age. We aim to investigate age-related genetic heterogeneity of AD and identify genomic loci with differential effects across age. Stratified gene-based genome-wide association studies and polygenic variation analyses were performed in the younger (60-79 years, N = 14,895) and older (≥80 years, N = 6559) age-at-onset groups using Alzheimer's Disease Genetics Consortium data. We showed a moderate genetic correlation (r_g = 0.64) between the two age groups, supporting genetic heterogeneity. Heritability explained by variants on chromosome 19 (harboring APOE) was significantly larger in younger than in older onset group (p < 0.05). APOE region, BIN1, OR2S2, MS4A4E, and PICALM were identified at the gene-based genome-wide significance (p < 2.73 × 10^-6) with larger effects at younger age (except MS4A4E). For the novel gene OR2S2, we further performed leave-one-out analyses, which showed consistent effects across subsamples. Our results suggest using genetically more homogeneous individuals may help detect additional susceptible loci.

Cerebrospinal Fluid Biomarkers for the Diagnosis of Prodromal Alzheimer's Disease in Amnestic Mild Cognitive Impairment

Background/Aims

Disease-modifying therapy for Alzheimer's disease (AD) has led to a need for biomarkers to identify prodromal AD and very early stage of AD dementia. We aimed to identify the cutoff values of cerebrospinal fluid (CSF) biomarkers for detecting prodromal AD.

Methods

We assessed 56 patients with amnestic mild cognitive impairment (aMCI) who underwent lumbar puncture. Additionally, 87 healthy elderly individuals and 34 patients with AD dementia served as controls. Positron emission tomography was performed using florbetaben as a probe. We analyzed the concentration of Aβ_1–42, total tau protein (t-Tau), and tau protein phosphorylated at threonine 181 (p-Tau¹⁸¹) in CSF with INNOTEST enzyme-linked immunosorbent assay.

Results

For the detection of prodromal AD in patients with aMCI, the cutoff values of CSF Aβ_1–42, t-Tau, and p-Tau¹⁸¹ were 749.5 pg/mL, 225.6 pg/mL, and 43.5 pg/mL, respectively. To discriminate prodromal AD in patients with aMCI, the t-Tau/Aβ_1–42 and ­p-Tau¹⁸¹/Aβ_1–42 ratios defined cutoff values at 0.298 and 0.059, respectively.

Conclusions

CSF biomarkers are very useful tools for the differential diagnosis of prodromal AD in aMCI patients. The concentration of CSF biomarkers is well correlated with the stages of the AD spectrum.

Shared genetic architecture between metabolic traits and Alzheimer's disease: a large-scale genome-wide cross-trait analysis

A growing number of studies clearly demonstrate a substantial link between metabolic dysfunction and the risk of Alzheimer's disease (AD), especially glucose-related dysfunction; one hypothesis for this comorbidity is the presence of a common genetic etiology. We conducted a large-scale cross-trait GWAS to investigate the genetic overlap between AD and ten metabolic traits. Among all the metabolic traits, fasting glucose, fasting insulin and HDL were found to be genetically associated with AD. Local genetic covariance analysis found that 19q13 region had strong local genetic correlation between AD and T2D (P = 6.78 × 10- 22), LDL (P = 1.74 × 10- 253) and HDL (P = 7.94 × 10- 18). Cross-trait meta-analysis identified 4 loci that were associated with AD and fasting glucose, 3 loci that were associated with AD and fasting insulin, and 20 loci that were associated with AD and HDL (Pmeta < 1.6 × 10- 8, single trait P < 0.05). Functional analysis revealed that the shared genes are enriched in amyloid metabolic process, lipoprotein remodeling and other related biological pathways; also in pancreas, liver, blood and other tissues. Our work identifies common genetic architectures shared between AD and fasting glucose, fasting insulin and HDL, and sheds light on molecular mechanisms underlying the association between metabolic dysregulation and AD.

Retinoic Acid Enhances Apolipoprotein E Synthesis in Human Macrophages

Apolipoprotein E (ApoE) represents a pivotal target in Alzheimer's disease (AD) and is modulated through retinoic acid (RA), an endogenous neuroprotective and anti-inflammatory compound. A major source of ApoE are microglia, which are pathologically activated in AD. Activated microglia are known to block RA signaling. This suggests a vicious cycle between inflammation, RA signaling, and ApoE homeostasis in AD pathogenesis. To test this hypothesis, we investigated effects of RA and proinflammatory activation on ApoE synthesis in primary human macrophage-derived microglial-like cells. Our results indicate that proinflammatory activation attenuates ApoE synthesis, an effect blocked by RA.

Apolipoprotein E gene in physiological and pathological aging

INTRODUCTION

The genetic background plays a role on longevity. The distribution of the apolipoprotein E gene (APOE) variants (ε2, ε3, ε4) may differ across age groups, especially in the oldest old and despite geographical and ethnic specificities. Since the ε4 variant is associated with Alzheimer's disease (AD), it might represent an opportunity for exploring the relationship of APOE with physiological and pathological aging.

AIM

To explore the role played by APOE genotype/alleles on physiological and pathological brain aging.

MATERIALS AND METHODS

The study was conducted in a cohort of centenarians (n = 106), and two cohorts of octogenarians (without cognitive decline, n = 351 controls; and with AD, n = 294).

RESULTS

No significant differences in genotype/allele distributions were observed comparing controls to centenarians. The prevalence of ε2/ε3, ε3/ε3, ε3/ε4 and ε4/ε4 genotypes were significantly different in centenarians compared to AD. The prevalence of ε2 and ε3 alleles were significantly higher in centenarians, whereas the ε4 was less frequent. The ε4 allele was positively associated with AD, whereas a negative association was found for ε2 and ε3 alleles.

CONCLUSIONS

Our study indicates that ε4 allele is strongly associated with AD. APOE significantly affects AD risk, but apparently not longevity.

Genome-wide analysis of genetic predisposition to Alzheimer's disease and related sex disparities

BACKGROUND

Alzheimer's disease (AD) is the most common cause of dementia in the elderly and the sixth leading cause of death in the United States. AD is mainly considered a complex disorder with polygenic inheritance. Despite discovering many susceptibility loci, a major proportion of AD genetic variance remains to be explained.

METHODS

We investigated the genetic architecture of AD in four publicly available independent datasets through genome-wide association, transcriptome-wide association, and gene-based and pathway-based analyses. To explore differences in the genetic basis of AD between males and females, analyses were performed on three samples in each dataset: males and females combined, only males, or only females.

RESULTS

Our genome-wide association analyses corroborated the associations of several previously detected AD loci and revealed novel significant associations of 35 single-nucleotide polymorphisms (SNPs) outside the chromosome 19q13 region at the suggestive significance level of p < 5E-06. These SNPs were mapped to 21 genes in 19 chromosomal regions. Of these, 17 genes were not associated with AD at genome-wide or suggestive levels of associations by previous genome-wide association studies. Also, the chromosomal regions corresponding to 8 genes did not contain any previously detected AD-associated SNPs with p < 5E-06. Our transcriptome-wide association and gene-based analyses revealed that 26 genes located in 20 chromosomal regions outside chromosome 19q13 had evidence of potential associations with AD at a false discovery rate of 0.05. Of these, 13 genes/regions did not contain any previously AD-associated SNPs at genome-wide or suggestive levels of associations. Most of the newly detected AD-associated SNPs and genes were sex specific, indicating sex disparities in the genetic basis of AD. Also, 7 of 26 pathways that showed evidence of associations with AD in our pathway-bases analyses were significant only in females.

CONCLUSIONS

Our findings, particularly the newly discovered sex-specific genetic contributors, provide novel insight into the genetic architecture of AD and can advance our understanding of its pathogenesis.

Neuronal Apolipoprotein E4 Expression Results in Proteome-Wide Alterations and Compromises Bioenergetic Capacity by Disrupting Mitochondrial Function

Apolipoprotein (apo) E4, the major genetic risk factor for Alzheimer's disease (AD), alters mitochondrial function and metabolism early in AD pathogenesis. When injured or stressed, neurons increase apoE synthesis. Because of its structural difference from apoE3, apoE4 undergoes neuron-specific proteolysis, generating fragments that enter the cytosol, interact with mitochondria, and cause neurotoxicity. However, apoE4's effect on mitochondrial respiration and metabolism is not understood in detail. Here we used biochemical assays and proteomic profiling to more completely characterize the effects of apoE4 on mitochondrial function and cellular metabolism in Neuro-2a neuronal cells stably expressing apoE4 or apoE3. Under basal conditions, apoE4 impaired respiration and increased glycolysis, but when challenged or stressed, apoE4-expressing neurons had 50% less reserve capacity to generate ATP to meet energy requirements than apoE3-expressing neurons. ApoE4 expression also decreased the NAD+/NADH ratio and increased the levels of reactive oxygen species and mitochondrial calcium. Global proteomic profiling revealed widespread changes in mitochondrial processes in apoE4 cells, including reduced levels of numerous respiratory complex subunits and major disruptions to all detected subunits in complex V (ATP synthase). Also altered in apoE4 cells were levels of proteins related to mitochondrial endoplasmic reticulum-associated membranes, mitochondrial fusion/fission, mitochondrial protein translocation, proteases, and mitochondrial ribosomal proteins. ApoE4-induced bioenergetic deficits led to extensive metabolic rewiring, but despite numerous cellular adaptations, apoE4-expressing neurons remained vulnerable to metabolic stress. Our results provide insights into potential molecular targets of therapies to correct apoE4-associated mitochondrial dysfunction and altered cellular metabolism.

Amyloid-β and Tau at the Crossroads of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia characterized neuropathologically by senile plaques and neurofibrillary tangles (NFTs). Early breakthroughs in AD research led to the discovery of amyloid-β as the major component of senile plaques and tau protein as the major component of NFTs. Shortly following the identification of the amyloid-β (Aβ) peptide was the discovery that a genetic mutation in the amyloid precursor protein (APP), a type1 transmembrane protein, can be a cause of autosomal dominant familial AD (fAD). These discoveries, coupled with other breakthroughs in cell biology and human genetics, have led to a theory known as the "amyloid hypothesis", which postulates that amyloid-β is the predominant driving factor in AD development. Nonetheless, more recent advances in imaging analysis, biomarkers and mouse models are now redefining this original hypothesis, as it is likely amyloid-β, tau and other pathophysiological mechanism such as inflammation, come together at a crossroads that ultimately leads to the development of AD.

Dissociable effects of the apolipoprotein-E (APOE) gene on short- and long-term memories

Short- and long-term memory performance as a function of apolipoprotein-E (APOE) genotype was examined in older, healthy individuals using sensitive and comparable tasks to provide a more detailed description of influences of the ε4 allele (highest genetic risk factor for Alzheimer's disease) on memory. Older heterozygous and homozygous ε4 carriers and noncarriers performed 2 tasks of memory. Both tasks allowed us to measure memory for item identity and locations, using a sensitive, continuous measure of report. Long-term memory for object locations was impaired in ε4/ε4 carriers, whereas, paradoxically, this group demonstrated superior short-term memory for locations. The dissociable effects of the gene on short- and long-term memory suggest that the effect of genotype on these two types of memories, and their neural underpinnings, might not be co-extensive. Whereas the long-term memory impairment might be linked to preclinical Alzheimer's disease, the short-term memory advantage may reflect an independent, phenotypical effect of this allele on cognition.

Blood-Brain Barrier: From Physiology to Disease and Back

The blood-brain barrier (BBB) prevents neurotoxic plasma components, blood cells, and pathogens from entering the brain. At the same time, the BBB regulates transport of molecules into and out of the central nervous system (CNS), which maintains tightly controlled chemical composition of the neuronal milieu that is required for proper neuronal functioning. In this review, we first examine molecular and cellular mechanisms underlying the establishment of the BBB. Then, we focus on BBB transport physiology, endothelial and pericyte transporters, and perivascular and paravascular transport. Next, we discuss rare human monogenic neurological disorders with the primary genetic defect in BBB-associated cells demonstrating the link between BBB breakdown and neurodegeneration. Then, we review the effects of genes underlying inheritance and/or increased susceptibility for Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, and amyotrophic lateral sclerosis (ALS) on BBB in relation to other pathologies and neurological deficits. We next examine how BBB dysfunction relates to neurological deficits and other pathologies in the majority of sporadic AD, PD, and ALS cases, multiple sclerosis, other neurodegenerative disorders, and acute CNS disorders such as stroke, traumatic brain injury, spinal cord injury, and epilepsy. Lastly, we discuss BBB-based therapeutic opportunities. We conclude with lessons learned and future directions, with emphasis on technological advances to investigate the BBB functions in the living human brain, and at the molecular and cellular level, and address key unanswered questions.

Brain and cognitive correlates of subjective cognitive decline-plus features in a population-based cohort

Background

Subjective cognitive decline (SCD) consists of self-perceived decline in cognition over time. The occurrence of specific additional features in SCD (so-called SCDplus) confers a higher risk of future cognitive decline. However, it is not known whether SCDplus patients have a distinct cognitive and neuroimaging profile. Therefore, we aimed to study the associations between SCDplus features and cognitive and neuroimaging profiles in a population-based cohort.

Methods

A total of 2670 individuals from the ALFA cohort underwent clinical, cognitive, and MRI (n = 532) explorations. Subjects were classified as self-reporting cognitive decline (SCD) or not self-reporting cognitive decline (non-SCD). Within the SCD group, participants were also classified according to the number of SCDplus features they met (SCD+, > 3; SCD–, ≤ 3).

Results

The prevalence of SCD in the cohort was 21.4% (55.8% SCD–, 44.2% SCD+). SCD+ subjects performed worse than non-SCD and SCD– subjects in memory and executive function. Among the SCDplus features, confirmation of decline by an informant was the best predictor of worse cognitive performance and lower gray matter volumes.

Conclusions

Our findings show that individuals with SCDplus features have a distinct cognitive and brain volumetric profile similar to that found in Alzheimer’s disease and therefore support the use of the SCDplus concept as an enrichment criterion in population-based cohorts.

Electronic supplementary material

The online version of this article (10.1186/s13195-018-0449-9) contains supplementary material, which is available to authorized users.

The Genetic Diagnosis of Neurodegenerative Diseases and Therapeutic Perspectives

Genetics has led to a new focus regarding approaches to the most prevalent diseases today. Ascertaining the molecular secrets of neurodegenerative diseases will lead to developing drugs that will change natural history, thereby affecting the quality of life and mortality of patients. The sequencing of candidate genes in patients suffering neurodegenerative pathologies is faster, more accurate, and has a lower cost, thereby enabling algorithms to be proposed regarding the risk of neurodegeneration onset in healthy persons including the year of onset and neurodegeneration severity. Next generation sequencing has resulted in an explosion of articles regarding the diagnosis of neurodegenerative diseases involving exome sequencing or sequencing a whole gene for correlating phenotypical expression with genetic mutations in proteins having key functions. Many of them occur in neuronal glia, which can trigger a proinflammatory effect leading to defective proteins causing sporadic or familial mutations. This article reviews the genetic diagnosis techniques and the importance of bioinformatics in interpreting results from neurodegenerative diseases. Risk scores must be established in the near future regarding diseases with a high incidence in healthy people for defining prevention strategies or an early start for giving drugs in the absence of symptoms.

Purinergic signaling in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is characterized by three major histopathological markers: amyloid-β (Aβ) plaques, neurofibrillary tangles and gliosis in the central nervous system (CNS). It is now accepted that neuroinflammatory events in the CNS play a crucial role in the development of AD. This review focuses on neuroinflammatory signaling mediated by purinergic receptors (P1 adenosine receptors, P2X ATP-gated ion channels and G protein-coupled P2Y nucleotide receptors) and how therapeutic modulation of purinergic signaling influences disease progression in AD patients and animal models of AD.

APOE genotypes in Lebanon: distribution and association with hypercholesterolemia and Alzheimer's disease

AIM

We first investigated the distribution of APOE genotypes in Lebanese general population and Alzheimer's disease study (ADS) groups, and compared it with 1000 genomes populations. Then, we assessed eventual association between APOE genotypes, hypercholesterolemia and Alzheimer's disease (AD).

MATERIALS & METHODS

This cross-sectional study was conducted on 591 individuals. Clinical and biological data were collected, DNA was extracted and genotyped using Kompetitive allele specific PCR (KASP™) and PCR-restriction fragment length polymorphis (PCR-RFLP). Results: Prevalence of APOE genotypes in Lebanon was similar to that seen in Asian populations. APOE genotypes were not associated with hypercholesterolemia. A significant difference between APOE genotypes in AD cases versus controls and versus Lebanese general population was seen. Moreover, E4 allele was approximately threefold higher in Alzheimer's disease study patients when compared with the remaining individuals.

CONCLUSION

We established the distribution of APOE genotypes in the Lebanon and showed that in contrast to lipid profile, E4 correlates with AD.

Structural Basis of Tau Interaction With BIN1 and Regulation by Tau Phosphorylation

Bridging integrator-1 (BIN1) gene is associated with an increased risk to develop Alzheimer's disease, a tauopathy characterized by intra-neuronal accumulation of phosphorylated Tau protein as paired helical filaments. Direct interaction of BIN1 and Tau proteins was demonstrated to be mediated through BIN1 SH3 C-terminal domain and Tau (210-240) peptide within Tau proline-rich domain. We previously showed that BIN1 SH3 interaction with Tau is decreased by phosphorylation within Tau proline-rich domain, of at least T231. In addition, the BIN1/Tau interaction is characterized by a dynamic equilibrium between a closed and open conformations of BIN1 isoform 1, involving an intramolecular interaction with its C-terminal BIN1 SH3 domain. However, the role of the BIN1/Tau interaction, and its potential dysregulation in Alzheimer's disease, is not yet fully understood. Here we showed that within Tau (210-240) peptide, among the two proline-rich motifs potentially recognized by SH3 domains, only motif P²¹⁶TPPTR²²¹ is bound by BIN1 SH3. A structural model of the complex between BIN1 SH3 and Tau peptide (213-229), based on nuclear magnetic resonance spectroscopy data, revealed the molecular detail of the interaction. P216 and P219 within the proline-rich motif were in direct contact with the aromatic F588 and W562 of the BIN1 SH3 domain. The contact surface is extended through electrostatic interactions between the positively charged R221 and K224 residues of Tau peptide and those negatively charged of BIN1 SH3, corresponding to E556 and E557. We next investigated the impact of multiple Tau phosphorylations within Tau (210-240) on its interaction with BIN1 isoform 1. Tau (210-240) phosphorylated at four different sites (T212, T217, T231, and S235), contrary to unphosphorylated Tau, was unable to compete with the intramolecular interaction of BIN1 SH3 domain with its CLAP domain. In accordance, the affinity of BIN1 SH3 for phosphorylated Tau (210-240) peptide was reduced, with a five-fold increase in the dissociation constant, from a Kd of 44 to 256 μM. This study highlights the complexity of the regulation of BIN1 isoform 1 with Tau. As abnormal phosphorylation of Tau is linked to the pathology development, this regulation by phosphorylation might have important functional consequences.

Paired Immunoglobulin-like Type 2 Receptor Alpha G78R variant alters ligand binding and confers protection to Alzheimer's disease

Paired Immunoglobulin-like Type 2 Receptor Alpha (PILRA) is a cell surface inhibitory receptor that recognizes specific O-glycosylated proteins and is expressed on various innate immune cell types including microglia. We show here that a common missense variant (G78R, rs1859788) of PILRA is the likely causal allele for the confirmed Alzheimer’s disease risk locus at 7q21 (rs1476679). The G78R variant alters the interaction of residues essential for sialic acid engagement, resulting in >50% reduced binding for several PILRA ligands including a novel ligand, complement component 4A, and herpes simplex virus 1 (HSV-1) glycoprotein B. PILRA is an entry receptor for HSV-1 via glycoprotein B, and macrophages derived from R78 homozygous donors showed significantly decreased levels of HSV-1 infection at several multiplicities of infection compared to homozygous G78 macrophages. We propose that PILRA G78R protects individuals from Alzheimer’s disease risk via reduced inhibitory signaling in microglia and reduced microglial infection during HSV-1 recurrence.

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder resulting from a complex interaction of environmental and genetic risk factors. Despite considerable progress in defining the genetic component of AD risk, understanding the biology of common variant associations is a challenge. We find that PILRA G78R (rs1859788) is the likely AD risk variant from the 7q21 locus (rs1476679) and PILRA G78R reduces PILRA endogenous and exogenous ligand binding. Our study highlights a new immune signaling axis in AD and suggests a role for exogenous ligands (HSV-1). Further, we have identified that reduced function of a negative regulator of microglia and neutrophils is protective from AD risk, providing a new candidate therapeutic target.

Genetic underpinnings in Alzheimer's disease - a review

In this review, we discuss the genetic etiologies of Alzheimer's disease (AD). Furthermore, we review genetic links to protein signaling pathways as novel pharmacological targets to treat AD. Moreover, we also discuss the clumps of AD-m ediated genes according to their single nucleotide polymorphism mutations. Rigorous data mining approaches justified the significant role of genes in AD prevalence. Pedigree analysis and twin studies suggest that genetic components are part of the etiology, rather than only being risk factors for AD. The first autosomal dominant mutation in the amyloid precursor protein (APP) gene was described in 1991. Later, AD was also associated with mutated early-onset (presenilin 1/2, PSEN1/2 and APP) and late-onset (apolipoprotein E, ApoE) genes. Genome-wide association and linkage analysis studies with identified multiple genomic areas have implications for the treatment of AD. We conclude this review with future directions and clinical implications of genetic research in AD.