Genetic Risk Factor For Alzheimer's Disease Research Articles

Three major effects of APOEε4 on Aβ immunotherapy induced ARIA

The targeting of amyloid-beta (Aβ) plaques therapeutically as one of the primary causes of Alzheimer’s disease (AD) dementia has been an ongoing effort spanning decades. While some antibodies are extremely promising and have been moved out of clinical trials and into the clinic, most of these treatments show similar adverse effects in the form of cerebrovascular damage known as amyloid-related imaging abnormalities (ARIA). The two categories of ARIA are of major concern for patients, families, and prescribing physicians, with ARIA-E presenting as cerebral edema, and ARIA-H as cerebral hemorrhages (micro- and macro-). From preclinical and clinical trials, it has been observed that the greatest genetic risk factor for AD, APOEε4, is also a major risk factor for anti-Aβ immunotherapy-induced ARIA. APOEε4 carriers represent a large population of AD patients, and, therefore, limits the broad adoption of these therapies across the AD population. In this review we detail three hypothesized mechanisms by which APOEε4 influences ARIA risk: (1) reduced cerebrovascular integrity, (2) increased neuroinflammation and immune dysregulation, and (3) elevated levels of CAA. The effects of APOEε4 on ARIA risk is clear, however, the underlying mechanisms require more research.

Estradiol improves behavior in FAD transgenic mice that express APOE3 but not APOE4 after ovariectomy

Increasing evidence suggests that female individuals have a higher Alzheimer’s disease (AD) risk associated with post-menopausal loss of circulating estradiol (E2). However, clinical data are conflicting on whether E2 lowers AD risk. One potential contributing factor is APOE. The greatest genetic risk factor for AD is APOE4, a factor that is pronounced in female individuals post-menopause. Clinical data suggests that APOE impacts the response of AD patients to E2 replacement therapy. However, whether APOE4 prevents, is neutral, or promotes any positive effects of E2 is unclear. Therefore, our goal was to determine whether APOE modulates the impact of E2 on behavior and AD pathology in vivo. To that end, mice that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce Aβ42 were ovariectomized at either 4 months (early) or 8 months (late) and treated with vehicle or E2 for 4 months. In E3FAD mice, we found that E2 mitigated the detrimental effect of ovariectomy on memory, with no effect on Aβ in the early paradigm and only improved learning in the late paradigm. Although E2 lowered Aβ in E4FAD mice in the early paradigm, there was no impact on learning or memory, possibly due to higher Aβ pathology compared to E3FAD mice. In the late paradigm, there was no effect on learning/memory and Aβ pathology in E4FAD mice. Collectively, these data support the idea that, in the presence of Aβ pathology, APOE impacts the response to E2 supplementation post-menopause.

DNA tetrahedron-based dual-signal fluorescence detection of apoE4 gene sites on a microplate reader.

As a neurodegenerative disorder, Alzheimer's disease (AD) is characterized by cognitive dysfunction and behavioral impairment. Among the various genetic risk factors for AD, apoE4 gene plays a pivotal role in the onset and progression of AD, and detection of apoE4 gene holds significance for prevention and early diagnosis of AD. Herein, dual-signal fluorescence detection of fragments associated with apoE ε4 allele near codon 112 (Tc1) and codon 158 (Tc2) was achieved using DNA tetrahedron nanostructure (DTN). The Förster resonance energy transfer (FRET) process in the DTN was initiated in which the nucleic acid intercalating dye thiazole orange (TO) served as the donor and the cyanine dyes of cyanine3 (Cy3) and cyanine5 (Cy5) at the two vertices of DTN served as the acceptors. In the presence of Tc1 and Tc2, the FRET process between TO and the cyanine dyes was hindered by the enzymatic cleavage reaction, which ensures the dual-signal fluorescence assay of apoE4 gene sites. The limit of detection for Tc1 and Tc2 was estimated to be 0.82nM and 0.77nM, respectively, and the whole assay was accomplished within 1h on a microplate reader. The proposed method thus possesses the advantages of easy operation, short detection time, and high-throughput capability.

Impact of age and apolipoprotein E ε4 status on regional white matter hyperintensity volume and cognition in healthy aging.

White matter hyperintensity (WMH) volume is a neuroimaging marker of lesion load related to small vessel disease that has been associated with cognitive aging and Alzheimer's disease (AD) risk. The present study sought to examine whether regional WMH volume mediates the relationship between APOE ε4 status, a strong genetic risk factor for AD, and cognition and if this association is moderated by age group differences within a sample of 187 healthy older adults (APOE ε4 status [carrier/non-carrier] = 56/131). After we controlled for sex, education, and vascular risk factors, ANCOVA analyses revealed significant age group by APOE ε4 status interactions for right parietal and left temporal WMH volumes. Within the young-old group (50-69 years), ε4 carriers had greater right parietal and left temporal WMH volumes than non-carriers. However, in the old-old group (70-89 years), right parietal and left temporal WMH volumes were comparable across APOE ε4 groups. Further, within ε4 non-carriers, old-old adults had greater right parietal and left temporal WMH volumes than young-old adults, but there were no significant differences across age groups in ε4 carriers. Follow-up moderated mediation analyses revealed that, in the young-old, but not the old-old group, there were significant indirect effects of ε4 status on memory and executive functions through left temporal WMH volume. These findings suggest that, among healthy young-old adults, increased left temporal WMH volume, in the context of the ε4 allele, may represent an early marker of cognitive aging with the potential to lead to greater risk for AD.

Association of PM2.5 Exposure and Alzheimer Disease Pathology in Brain Bank Donors-Effect Modification by APOE Genotype.

Fine particulate matter (PM2.5) exposure has been found to be associated with Alzheimer disease (AD) and is hypothesized to cause inflammation and oxidative stress in the brain, contributing to neuropathology. The APOE gene, a major genetic risk factor of AD, has been hypothesized to modify the association between PM2.5 and AD. However, little prior research exists to support these hypotheses. This study investigates the association between traffic-related PM2.5 and AD hallmark pathology, including effect modification by APOE genotype, in an autopsy cohort. A cross-sectional study was conducted using brain tissue donors enrolled in the Emory Goizueta AD Research Center who died before 2020 (n = 224). Donors were assessed for AD pathology including the Braak stage, Consortium to Establish a Registry for AD (CERAD) score, and combined AD neuropathologic change (ABC) score. Traffic-related PM2.5 concentrations were modeled for the metro-Atlanta area during 2002-2019 with a spatial resolution of 200-250 m. One-year, 3-year, and 5-year average PM2.5 concentrations before death were matched to participants' home address. We assessed the association between traffic-related PM2.5 and AD hallmark pathology and effect modification by APOE genotype, using adjusted ordinal logistic regression models. Among the 224 participants, the mean age of death was 76 years, and 57% had at least 1 APOE ε4 copy. Traffic-related PM2.5 was significantly associated with the CERAD score for the 1-year exposure window (odds ratio [OR] 1.92; 95% CI 1.12-3.30) and the 3-year exposure window (OR 1.87; 95% CI 1.01-3.17). PM2.5 was also associated with higher Braak stage and ABC score albeit nonsignificantly. The strongest associations between PM2.5 and neuropathology markers were among those without APOE ε4 alleles (e.g., for the CERAD score and 1-year exposure window, OR 2.31; 95% CI 1.36-3.94), though interaction between PM2.5 and APOE genotype was not statistically significant. Our study found traffic-related PM2.5 exposure was associated with the CERAD score in an autopsy cohort, contributing to epidemiologic evidence that PM2.5 affects β-amyloid deposition in the brain. This association was particularly strong among donors without APOE ε4 alleles. Future studies should further investigate the biological mechanisms behind this association.

Report of the APOE4 National Institute on Aging/Alzheimer Disease Sequencing Project Consortium Working Group: Reducing APOE4 in Carriers is a Therapeutic Goal for Alzheimer's Disease.

Alzheimer's disease (AD) is the most common neurodegenerative disorder and one of the leading causes of disability worldwide. The apolipoprotein E4 gene (APOE4) is the strongest genetic risk factor for AD. In 2023, the APOE4 National Institute on Aging/Alzheimer's Disease Sequencing Project working group came together to gather data and discuss the question of whether to reduce or increase APOE4 as a therapeutic intervention for AD. It was the unanimous consensus that cumulative data from multiple studies in humans and animal models support that lowering APOE4 should be a target for therapeutic approaches for APOE4 carriers. ANN NEUROL 2024.

MUTUAL INFORMATION OF NEUROIMAGES AND GENETIC VARIANTS TO STUDY THE DETERMINANTS OF BRAIN AGING

Abstract Understanding the genetic determinants of brain aging is important because advanced biological age of the brain is a risk factor for Alzheimer’s disease (AD). Single-nucleotide polymorphisms (SNPs) are genetic variations with effects on brain aging; mapping their influence on brain structure can elucidate the genetic correlates of AD. We adopted an information theoretic approach to this by computing the mutual information (MI) between (A) 22 AD-risk SNPs and (B) the MRI intensities of the cortex in 6,000 UK Biobank participants (~50% females) aged 54 to 84 years. These cortex-wide calculations quantified how SNPs impact brain structure. As expected, the APOE-ε4 allele, the strongest genetic risk factor for AD, exhibited significantly (p < 0.025) higher MI with MRI intensity, when compared to the average MI expected for a null distribution model, in the medial parietal lobe, pars orbitalis, and left inferior occipital lobe, three brain structures responsible for memory recall, language processing, and visual perception. These findings reflect the memory loss, decreased speech, and reduced peripheral vision experienced by individuals with AD. The ABCA7 allele, a risk factor for late- onset AD, had significantly (p < 0.025) higher MI in the inferior temporal sulcus and angular gyrus. These structures are responsible for visual recognition and attention, two functions that are severely impaired by AD. Other findings reveal similar relationships between AD-risk SNPs and brain structure. Our findings illustrate how mapping the MI between MRI intensities and SNPs provides insight into genetic influences on brain structures linked to AD symptoms.

In Silico Design of Natural Inhibitors of ApoE4 from the Plant Moringa oleifera: Molecular Docking and Ab Initio Fragment Molecular Orbital Calculations.

Alzheimer's disease (AD) is a neurological disease, and its signs and symptoms appear slowly over time. Although current Alzheimer's disease treatments can alleviate symptoms, they cannot prevent the disease from progressing. To accurately diagnose and treat Alzheimer's disease, it is therefore necessary to establish effective methods for diagnosis. Apolipoprotein E4 (ApoE4), the most frequent genetic risk factor for AD, is expressed in more than half of patients with AD, making it an attractive target for AD therapy. We used molecular docking simulations, classical molecular mechanics optimizations, and ab initio fragment molecular orbital (FMO) calculations to investigate the specific interactions between ApoE4 and the naturally occurring compounds found in the plant Moringa Oleifera. According to the FMO calculations, quercetin had the highest binding affinity to ApoE4 among the sixteen compounds because its hydroxyl groups generated strong hydrogen bonds with the ApoE4 residues Trp11, Asp12, Arg15, and Asp130. As a result, we proposed various quercetin derivatives by introducing a hydroxyl group into quercetin and studied their ApoE4 binding properties. The FMO data clearly showed that adding a hydroxyl group to quercetin improved its binding capacity to ApoE4. Furthermore, ApoE4 Trp11, Asp12, Arg15, and Asp130 residues were discovered to be required for significant interactions between ApoE4 and quercetin derivatives. They had a higher ApoE4 binding affinity than our previously proposed epicatechin derivatives. Accordingly, the current results evaluated using the ab initio FMO method will be useful for designing potent ApoE4 inhibitors that can be used as a candidate agent for AD treatment.

Investigating the role of ketogenic conditions on cholesterol metabolism and neuroinflammation in APOE4 carriers

AbstractBackgroundThe study of genetic risk factors in Alzheimer’s Disease (AD) in conjunction with lifestyle factors is critical in order to fully understand manifestations of disease phenotypes. APOE4, the primary genetic risk factor for AD, is a mutant implicated in dysfunctional lipid and cholesterol transport. We have demonstrated that APOE4‐expressing oligodendrocytes produce a multitude of dysfunctional cholesterol synthesis conditions including decreased cholesterol transport, increased cholesterol accumulation, and reduced levels of myelination (Blanchard et al. 2022). One avenue to study the intersection of genetic and lifestyle factors is to explore APOE mutants with particular dietary conditions that may implicate neuro‐inflammatory phenotypes. The role of cholesterol metabolic dysfunctionality in APOE4 carriers, taking into account the diverse diets that humans consume, has yet to be studied.MethodIn order to investigate this, we developed a 3D multi‐cellular integrated brain tissue (miBrain) technology that contains iPSC‐derived neurons, endothelial cells, astrocytes, pericytes, microglia, and oligodendrocytes. This model has the ability to recapitulate physiological conditions of disease with great accuracy. To study different dietary conditions with our miBrain technology, we employed ketogenic vs. glucose media conditions on APOE4‐carrying cell types within the miBrain model.ResultWe have found that ketogenic conditions promote a neuro‐inflammatory phenotype in APOE4 carriers. This is shown by our observed differences in cholesterol accumulation, levels of myelination, and transcriptomics, between APOE3 and APOE4 miBrain models treated with ketogenic energy sources. We are confirming these findings using in vivo models.ConclusionKetogenic conditions may exacerbate the cholesterol abnormalities in APOE4 carriers, through promoting neuro‐inflammatory conditions that give rise to AD pathogenesis. Findings from these studies highlight therapeutic and diagnostic strategies to reduce the risk of developing AD in APOE4 carriers.

Transcriptomic Profiling Across Age, APOE and Sex for Models of Alzheimer’s Disease at a Single Cell Resolution

AbstractBackgroundAdvanced age, apolipoprotein E4(APOE4), and female sex are well‐established risk factors for Alzheimer’s Disease(AD). Almost two‐thirds of people with AD are women. The APOE4 genotype, the strongest genetic risk factor for AD, is associated with higher AD risk in women than in men between ages 55 to 70. We propose to study the interplay between these risk factors by analyzing single‐nucleus RNA‐sequencing(snRNA‐seq) data of the hippocampus of human APOE4 and APOE3 knock‐in(KI) female and male mice across age.MethodsA sex‐balanced cohort of APOE3‐KI and APOE4‐KI homozygous mice were sacrificed at 6 (young), 12 (adult), and 18 (old) months of age(Figure1A). Single nuclei were prepared from the hippocampus from each mouse and sequenced. Demultiplexed fastq files were aligned to a custom reference genome mm10‐1.2.0, using Cellranger v2.0.1. Quality assurance, count normalization, clustering, cell type identification, and differentially expressed gene(DEG) analysis were performed using Seurat4.0.ResultsOur dataset consists of a total of 527,395 cells covering 27,153 genes. A total of 36 cell clusters were determined with six major brain cell types including astrocytes, microglia, oligodendrocytes, oligodendrocyte precursor cells, and excitatory and inhibitory neurons (Figure1B). We investigated the abundance of each cell type and found that cellular composition is consistent across ages, APOE genotypes, and sexes for all cell types except astrocytes. Astrocyte abundance is significantly higher in old females compared to old males(padj = 0.025, Tukey’s multiple comparison test) and in APOE4 young males compared to APOE3 young males (padj = 0.042)(Figure1C).Sex and age stratified differential gene expression analysis comparing APOE4 versus APOE3 astrocytes reveals many shared DEGs between young and adult males but not with old males, while females have few consistent DEGs across the three age groups(Figure1D). We identified several DEGs associated with plasma membrane assembling and post‐synaptic transmission and signaling that are upregulated in APOE4 old females versus APOE3 old females while downregulated in APOE4 young females, young and adult male compared to their APOE3 counterparts.ConclusionWe identify clear interplay between age, APOE genotype, and sex in mouse models of AD and investigate the interaction and convergence of these risk factors at a single cell resolution.

The effects of ApoE e4 genotype on plasma p‐tau181 concentrations on clinically diagnosed Alzheimer’s disease (AD) compared to controls

AbstractBackgroundThe association between ApoE e4, a powerful genetic risk factor for AD, and beta‐amyloid (Aβ) pathology has been reported, but the evidence for the correlation with tau pathology is not as clear. The current study examined the relationship between ApoE e4 allele and plasma concentrations of p‐tau181 in patients with clinically diagnosed AD compared with an age‐matched controls with normal cognitive function.MethodWe examined 302 EDTA plasma samples of patients assessed at the UBC Hospital Clinic for Alzheimer’s disease. The cohort was clinically characterized as controls (n = 59) if no significant cognitive impairment was found, and AD (n = 243) if they fulfilled the NIA‐AA criteria (McKhann 2011). The plasma ptau‐181 levels were measured using an ADx‐developed p‐tau181‐specific Simoa assay. ApoE genotyping was performed on extracted DNA from blood samples. We examined the association of presence of ApoE e4 allele and plasma p‐tau181 concentrations using two‐by‐two ANOVA using SPSS 29.ResultThe plasma concentrations of p‐tau181 in AD who are ApoE e4 carriers (75.4 pg/ml) were slightly higher than but not significantly different from in AD non‐ ApoE4 carriers (70.1 pg/ml) (p = 0.64). The plasma levels of ptau‐181 in control cases with and without the ApoE allele were 24.8 and 24 pg/ml respectively (p = 1.000). There was no statistical significant interaction between the effects of AD and ApoE e4 carrier status on plasma p‐tau181 (F (1, 298) = 0.21, p = 0.65, ηp2 = 0.001).ConclusionWhile the plasma p‐tau181 concentrations were significantly higher in AD groups compared to controls (p<0.001) it was no significant effect of ApoE e4 genotype on the levels of plasma p‐tau181 in the control or AD groups (p = 0.94, p = 0.21). This suggests that the ApoE e4 carrier status does not affect plasma p‐tau181 concentrations significantly, independent of the AD clinical status.

Novel APOE Mutation in a Moroccan Subject Suffering from Alzheimer Disease: A Case Study and Exploration of Pathogenic Implication.

Alzheimer disease (AD) is a major public health concern worldwide. It is a severe neurodegenerative disease that primarily affects the elderly and causes significant brain cell death. According to the most complete scientific research, the APOE gene, which encodes the APOE protein, maybe the key to identifying the likely cause of delayed AD. The development of plaques and tangles, as well as increased amyloid (amyloid-β) levels and deposition, have been linked to APOE4. Pathogenic mutations in this gene can impact how beta-amyloid deposits and how they are cleared from the body. In this study, we report a novel pathogenic mutation, Arg160Leu, in APOE that was identified in a Moroccan patient. The magnetic resonance imaging of this 67-year-old woman revealed hippocampal shrinkage, and the results of her cognition testing revealed that she is suffering from severe AD. The current study may increase awareness of the genetic risk factors for AD caused by APOE4 mutations.

A cell therapy approach to restore microglial Trem2 function in a mouse model of Alzheimer's disease.

Alzheimer's disease (AD) remains one of the grand challenges facing human society. Much controversy exists around the complex and multifaceted pathogenesis of this prevalent disease. Given strong human genetic evidence, there is little doubt, however, that microglia play an important role in preventing degeneration of neurons. For example, loss of function of the microglial gene Trem2 renders microglia dysfunctional and causes an early-onset neurodegenerative syndrome, and Trem2 variants are among the strongest genetic risk factors for AD. Thus, restoring microglial function represents a rational therapeutic approach. Here, we show that systemic hematopoietic cell transplantation followed by enhancement of microglia replacement restores microglial function in a Trem2 mutant mouse model of AD.

Cell-type-specific regulation of APOE and CLU levels in human neurons by the Alzheimer’s disease risk gene SORL1

SORL1 is implicated in the pathogenesis of Alzheimer's disease (AD) through genetic studies. To interrogate the roles of SORL1 in human brain cells, SORL1-null induced pluripotent stem cells (iPSCs) were differentiated to neuron, astrocyte, microglial, and endothelial cell fates. Loss of SORL1 leads to alterations in both overlapping and distinct pathways across cell types, with the greatest effects in neurons and astrocytes. SORL1 loss induces a neuron-specific reduction in apolipoprotein E (APOE) and clusterin (CLU) and altered lipid profiles. Analyses of iPSCs derived from a large cohort reveal a neuron-specific association between SORL1, APOE, and CLU levels, a finding validated in postmortem brain.Enhancement of retromer-mediated trafficking rescues tau phenotypes observed in SORL1-null neurons but does not rescue APOE levels. Pathway analyses implicate transforming growth factor β (TGF-β)/SMAD signaling in SORL1 function, and modulating SMAD signaling in neurons alters APOE RNA levels in a SORL1-dependent manner. Taken together, these data provide a mechanistic link between strong genetic risk factors for AD.

O-GlcNAcylation regulates extracellular signal-regulated kinase (ERK) activation in Alzheimer's disease.

Aberrant activation of Extracellular Signal-Regulated Kinase (ERK) signaling is associated with Alzheimer's disease (AD) pathogenesis. For example, enhanced ERK signal activation mediated by Apolipoprotein E4 (APOE4), which is a critical genetic risk factor for AD, increases the transcription of amyloid precursor protein (APP). We hypothesize that O-linked N-acetylglucosamine (O-GlcNAc) regulates the phosphorylation and activation of ERK. O-GlcNAc is a single sugar post-translational modification that dynamically cycles on and off proteins in response to nutrient changes by the action of the enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. However, O-GlcNAc quickly returns to a baseline level after stimulus removal (called O-GlcNAc homeostasis). We did a serum reactivation time-course followed by western blot in SH-SY5Y neuroblastoma cells after long-term O-GlcNAcase (OGA) inhibition by Thiamet-G (TMG) treatment, O-GlcNAc transferase (OGT) knock-down (KD) and OGA KD. Brain tissues of C57BL6/J mice and 5XFAD Alzheimer's disease mice intra-peritoneally injected with TMG for 1 month and C57BL6/J mice intra-peritoneally injected with TMG for 6 months were also used for western blot. We found that ERK1/2 phosphorylation at Thr 202/Tyr204 and Thr183/Tyr185 (p-ERK) are amplified and hence ERK1/2 are activated after long-term OGA inhibition in SH-SY5Y cells. In addition to pharmacological treatment, genetic disruption of O-GlcNAc by OGT KD and OGA KD also increased p-ERK in SH-SY5Y cells suggesting O-GlcNAc homeostasis controls ERK signaling. To determine how O-GlcNAc regulates p-ERK, we probed the expression of phosphorylated mitogen-activated protein kinase-kinase (p-MEK) which phosphorylates and activates ERK and Dual specificity phosphatase-4 (DUSP4) which dephosphorylates and inactivates ERK in SH-SY5Y cells. p-MEK increases in TMG treated and OGT KD cells whereas total DUSP4 decreases in OGT KD and OGA KD cells with serum reactivation time course. Next, we probed the role of OGA inhibition in regulating ERK activation using mice brain-tissue samples. Interestingly, 6-month intra-peritoneal TMG injection in C57BL/6J mice showed an increase in amplitude of p-ERK and APP protein levels, indicating long-term OGA inhibition potentially contributes to AD progression. Furthermore, 1-month TMG injection was sufficient to increase the amplitude of p-ERK in 5XFAD AD mice brains suggesting AD phenotype contributes to the acceleration of ERK activation mediated by OGA inhibition. Together, these results indicate that disruptions to O-GlcNAc homeostasis amplify ERK signal activation in AD.

Updated APOE and TOMM40 rs2075650 genotype effects on cognition: a UK Biobank study

AbstractBackground58‐79% of late‐onset Alzheimer’s disease (AD) is due to genetic heritability. The epsilon 4 (ε4) variant of Apolipoprotein E (APOE) is the strongest genetic risk factor for AD, yet only accounts for 20% of AD heritability. In addition, the APOE ε2 or ε3 variants appear to have either a protective or neutral effect on AD risk respectively. Translocase of Outer Mitochondrial Membrane 40 (TOMM40) is next to APOE and may modulate different APOE variant effects, such as with the Single Nucleotide Polymorphism (SNP) at rs2075650. Thus, we explored main and interaction effects of TOMM40 and APOE on longitudinal measures of executive function and memory.Method10,347 adults (40‐70y) in the UK Biobank were genotyped for APOE and TOMM40 ‘650. Executive function and memory were tested at baseline and twice over a period of 6‐10 years. Main effects within APOE haplotypes and TOMM40 SNP genotypes (A,G) were analyzed, as well as interactions between these genetic markers.ResultHaving the TOMM40 ‘650 G risk variant was related to better baseline cognitive scores regardless of APOE haplotype (ps <.05), but a greater decline over time in APOE4 carriers (ps <.05) The effects of the APOE haplotype were strongly modulated by sex. Men vs. women had less protective or more detrimental effects based on APOE2 or APOE4 haplotype respectively.ConclusionThe data suggests that TOMM40 modulates APOE, regardless of its haplotype. Future studies should stratify APOE by haplotype to re‐test our findings. In addition, more research needs to be done on the effect of having what we term a “mixed” APOE haplotype (carrying a copy of both the protective and risk gene variants).

APOEε4 and risk of Alzheimer's disease - time to move forward.

The inheritance of Apolipoprotein E4 (APOEε4) brings the highest genetic risk of Alzheimer's disease (AD), arguably the highest genetic risk in human pathology. Since the discovery of the association, APOE protein isoforms have been at the center of tens of thousands of studies and reports. While, without a doubt, our knowledge about the normal physiological function of APOE isoforms in the brain has increased tremendously, the questions of how the inheritance of the APOEε4 allele translates into a risk of AD, and the risk is materialized, remain unanswered. Moreover, the knowledge about the risk associated with APOEε4 has not helped design a meaningful preventative or therapeutic strategy. Animal models with targeted replacement of Apoe have been generated and, thanks to the recent NIH/NIA/Alzheimer's disease Association initiative, are now freely available to AD researchers. While helpful in many aspects, none of the available models recapitulates normal physiological transcriptional regulation of the human APOE gene cluster. Changes in epigenetic regulation of APOE alleles in animal models in response to external insults have rarely been if ever, addressed. However, these animal models provide a useful tool to handle questions and investigate protein-protein interactions with proteins expressed by other recently discovered genes and gene variants considered genetic risk factors of AD, like Triggering Receptor expressed on Myeloid cells 2 (TREM2). In this review, we discuss genetic and epigenetic regulatory mechanisms controlling and influencing APOE expression and focus on interactions of APOE and TREM2 in the context of microglia and astrocytes' role in AD-like pathology in animal models.

Reduced binding of apoE4 to complement factor Hpromotes amyloid-β oligomerization and neuroinflammation.

The APOE4 variant of apolipoprotein E (apoE) is the most prevalent genetic risk allele associated with late-onset Alzheimer's disease (AD). ApoE interacts with complement regulator factor H (FH), but the role of this interaction in AD pathogenesis is unknown. Here we elucidate the mechanism by which isoform-specific binding of apoE to FH alters Aβ1-42-mediated neurotoxicity and clearance. Flow cytometry and transcriptomic analysis reveal that apoE and FH reduce binding of Aβ1-42 to complement receptor 3 (CR3) and subsequent phagocytosis by microglia which alters expression of genes involved in AD. Moreover, FH forms complement-resistant oligomers with apoE/Aβ1-42 complexes and the formation of these complexes is isoform specific with apoE2 and apoE3 showing higher affinity to FH than apoE4. These FH/apoE complexes reduce Aβ1-42 oligomerization and toxicity, and colocalize with complement activator C1q deposited on Aβ plaques in the brain. These findings provide an important mechanistic insight into AD pathogenesis and explain how the strongest genetic risk factor for AD predisposes for neuroinflammation in the early stages of the disease pathology.

Plasma NfL is associated with the APOE ε4 allele, brain imaging measurements of neurodegeneration, and lower recall memory scores in cognitively unimpaired late-middle-aged and older adults

BackgroundPlasma neurofilament light (NfL) is an indicator of neurodegeneration and/or neuroaxonal injury in persons with Alzheimer’s disease (AD) and a wide range of other neurological disorders. Here, we characterized and compared plasma NfL concentrations in cognitively unimpaired (CU) late-middle-aged and older adults with two, one, or no copies of the APOE ε4 allele, the major genetic risk factor for AD. We then assessed plasma NfL associations with brain imaging measurements of AD-related neurodegeneration (hippocampal atrophy and a hypometabolic convergence index [HCI]), brain imaging measurements of amyloid-β plaque burden, tau tangle burden and white matter hyperintensity volume (WMHV), and delayed and total recall memory scores.MethodsPlasma NfL concentrations were measured in 543 CU 69 ± 9 year-old participants in the Arizona APOE Cohort Study, including 66 APOE ε4 homozygotes (HM), 165 heterozygotes (HT), and 312 non-carriers (NC). Robust regression models were used to characterize plasma NfL associations with APOE ε4 allelic dose before and after adjustment for age, sex, and education. They were also used to characterize plasma NfL associations with MRI-based hippocampal volume and WMHV measurements, an FDG PET-based HCI, mean cortical PiB PET measurements of amyloid-β plaque burden and meta-region-of-interest (meta-ROI) flortaucipir PET measurements of tau tangle burden, and Auditory Verbal Learning Test (AVLT) Delayed and Total Recall Memory scores.ResultsAfter the adjustments noted above, plasma NfL levels were significantly greater in APOE ε4 homozygotes and heterozygotes than non-carriers and significantly associated with smaller hippocampal volumes (r =  − 0.43), greater tangle burden in the entorhinal cortex and inferior temporal lobes (r = 0.49, r = 0.52, respectively), and lower delayed (r =  − 0.27), and total (r =  − 0.27) recall memory scores (p < 0.001). NfL levels were not significantly associated with PET measurements of amyloid-β plaque or total tangle burden.ConclusionsPlasma NfL concentrations are associated with the APOE ε4 allele, brain imaging biomarkers of neurodegeneration, and less good recall memory in CU late-middle-aged and older adults, supporting its value as an indicator of neurodegeneration in the preclinical study of AD.

Neurostructural and neurocognitive correlates of APOE ε4 in youth bipolar disorder

Bipolar disorder (BD) is a clinical risk factor for Alzheimer's disease (AD). Apolipoprotein E ε4 (APOE ε4), a genetic risk factor for AD, has been associated with brain structure and neurocognition in healthy youth. We evaluated whether there was an association between APOE ε4 with neurostructure and neurocognition in youth with BD. Participants included 150 youth (78 BD:19 ε4-carriers, 72 controls:17 ε4-carriers). 3T-magnetic resonance imaging yielded measures of cortical thickness, surface area, and volume. Regions-of-interest (ROI) and vertex-wise analyses of the cortex were conducted. Neurocognitive tests of attention and working memory were examined. Vertex-wise analyses revealed clusters with a diagnosis-by-APOE ε4 interaction effect for surface area (p = 0.002) and volume (p = 0.046) in pars triangularis (BD ε4-carriers > BD noncarriers), and surface area (p = 0.03) in superior frontal gyrus (controls ε4-carriers > other groups). ROI analyses were not significant. A significant interaction effect for working memory (p = 0.001) appeared to be driven by nominally poorer performance in BD ε4-carriers but not control ε4-carriers; however, post hoc contrasts were not significant. APOE ε4 was associated with larger neurostructural metrics in BD and controls, however, the regional association of APOE ε4 with neurostructure differed between groups. The role of APOE ε4 on neurodevelopmental processes is a plausible explanation for the observed differences. Future studies should evaluate the association of APOE ε4 with pars triangularis and its neurofunctional implications among youth with BD.