Apolipoprotein E represents a potent gene-based therapeutic target for the treatment of sporadic Alzheimer's disease

Current biomarkers and treatment strategies in Alzheimer disease: An overview and future perspectives

Alzheimer's disease (AD), a progressive degenerative disorder first identified by Alois Alzheimer in 1907, poses a significant public health challenge. Despite its prevalence and impact, there is currently no definitive ante mortem diagnosis for AD pathogenesis. By 2050, the United States may face a staggering 13.8 million AD patients. This review provides a concise summary of current AD biomarkers, available treatments, and potential future therapeutic approaches. The review begins by outlining existing drug targets and mechanisms in AD, along with a discussion of current treatment options. We explore various approaches targeting Amyloid β (Aβ), Tau Protein aggregation, Tau Kinases, Glycogen Synthase kinase-3β, CDK-5 inhibitors, Heat Shock Proteins (HSP), oxidative stress, inflammation, metals, Apolipoprotein E (ApoE) modulators, and Notch signaling. Additionally, we examine the historical use of Estradiol (E2) as an AD therapy, as well as the outcomes of Randomized Controlled Trials (RCTs) that evaluated antioxidants (e.g., vitamin E) and omega-3 polyunsaturated fatty acids as alternative treatment options. Notably, positive effects of docosahexaenoic acid nutriment in older adults with cognitive impairment or AD are highlighted. Furthermore, this review offers insights into ongoing clinical trials and potential therapies, shedding light on the dynamic research landscape in AD treatment.

HDL-like-Mediated Cell Cholesterol Trafficking in the Central Nervous System and Alzheimer's Disease Pathogenesis

The main aim of this work is to review the mechanisms via which high-density lipoprotein (HDL)-mediated cholesterol trafficking through the central nervous system (CNS) occurs in the context of Alzheimer’s disease (AD). Alzheimer’s disease is characterized by the accumulation of extracellular amyloid beta (Aβ) and abnormally hyperphosphorylated intracellular tau filaments in neurons. Cholesterol metabolism has been extensively implicated in the pathogenesis of AD through biological, epidemiological, and genetic studies, with the APOE gene being the most reproducible genetic risk factor for the development of AD. This manuscript explores how HDL-mediated cholesterol is transported in the CNS, with a special emphasis on its relationship to Aβ peptide accumulation and apolipoprotein E (ApoE)-mediated cholesterol transport. Indeed, we reviewed all existing works exploring HDL-like-mediated cholesterol efflux and cholesterol uptake in the context of AD pathogenesis. Existing data seem to point in the direction of decreased cholesterol efflux and the impaired entry of cholesterol into neurons among patients with AD, which could be related to impaired Aβ clearance and tau protein accumulation. However, most of the reviewed studies have been performed in cells that are not physiologically relevant for CNS pathology, representing a major flaw in this field. The ApoE4 genotype seems to be a disruptive element in HDL-like-mediated cholesterol transport through the brain. Overall, further investigations are needed to clarify the role of cholesterol trafficking in AD pathogenesis.

Longitudinal Basal Forebrain Degeneration Interacts with TREM2/C3 Biomarkers of Inflammation in Presymptomatic Alzheimer's Disease

Cholinergic inputs originating from the peripheral nervous system regulate the inflammatory immune responses of macrophages during clearance of blood-based pathogens. Because microglia are involved in clearing amyloid and tau pathology from the central nervous system, we hypothesized that cholinergic input originating from the basal forebrain might similarly regulate inflammatory immune responses to these pathologies in the aging brain. To explore this hypothesis, we leveraged the Alzheimer's Disease Neuroimaging Initiative dataset. Cognitively normal older male and female human adults were differentiated according to the relative concentration of phosphorylated tau and amyloid in their cerebrospinal fluid, yielding neurotypical and preclinical, cognitively healthy, subgroups. We then tracked these two groups longitudinally with structural MRI and biomarkers of inflammation, including soluble sTREM2 levels in the CSF and complement C3 expression in the blood transcriptome. Longitudinal loss of basal forebrain volume was larger in the preclinical compared with the neurotypical subgroup. Across preclinical adults, loss of basal forebrain volume was associated with greater longitudinal accumulation of sTREM2 and higher peripheral blood C3 expression. None of these relationships were attributable to degeneration in the whole-brain gray matter volume. Preclinical APOE e4 carriers exhibited the largest loss of basal forebrain volume and highest C3 expression. Consistent with the known anti-inflammatory influence of the peripheral cholinergic pathways on macrophages, our findings indicate that a loss of central cholinergic input originating from the basal forebrain might remove a key check on microglial inflammation induced by amyloid and tau accumulation.SIGNIFICANCE STATEMENT In the peripheral nervous system, cholinergic modulation holds the reactivity of macrophages to blood-based pathogens in check, promoting clearance while preventing runaway inflammation and immune-triggered cell death. Microglia are the brain's resident macrophages and play an important role in clearing accumulated amyloid and tau from neurons. Here, we demonstrate that a loss of cholinergic integrity in the CNS, indexed by longitudinal decreases of basal forebrain volume, interacts with multiple biomarkers of inflammation in cognitively normal older adults with abnormal amyloid and tau pathology. These interactions were not detected in cognitively normal older adults with "neurotypical" levels of amyloid and tau. An age-related loss of cholinergic neuromodulation may remove key checks on microglial reactivity to amyloid and tau.

Cholesterol and Dementia: A Long and Complicated Relationship

BACKGROUND

There is a huge demand for efficient strategies for maintaining cognitive wellbeing with age, especially in the context of population aging. Dementia constitutes the main reason for disability and dependency in the elderly. Identification of potential risk and protective factors, as well as determinants of conversion from MCI to dementia, is therefore crucial. In case of Alzheimer's disease, the most prevalent dementia syndrome amongst the members of modern societies, neurodegenerative processes in the brain can begin many years before first clinical symptoms appear. First functional changes typically mean advanced neuron loss, therefore, the earliest possible diagnosis is critical for implementation of promising early pharmaceutical interventions.

OBJECTIVE

The study aimed to discuss the relationships between both circulating and brain cholesterol with cognition, and explore its potential role in early diagnosis of cognitive disorders.

METHODS

Literature review.

RESULTS

The causal role of high cholesterol levels in AD or MCI has not been confirmed. It has been postulated that plasma levels of 24(S)-OHC can potentially be used as an early biochemical marker of altered cholesterol homeostasis in the CNS. Some studies brought conflicting results, finding normal or lowered levels of 24(S)-OHC in dementia patients compared to controls. In spite of decades of research on the relationship between cholesterol and dementia, so far, no single trusted indicator of an early cognitive deterioration has been identified.

CONCLUSION

The current state of knowledge makes the use of cholesterol markers of cognitive decline in clinical practice impossible.

Apolipoprotein E isoform dependently affects Tat-mediated HIV-1 LTR transactivation

Background

Apolipoprotein E (ApoE) is the major carrier protein that mediates the transport and delivery of cholesterol and other lipids in the brain. Three isoforms of ApoE (ApoE2, ApoE3, ApoE4) exist in humans, and their relative expression levels impact HIV-1 infection, HIV-1/AIDS disease progression, and cognitive decline associated with HIV-1-associated neurocognitive disorder. Because HIV-1 Tat, a viral protein essential for HIV-1 replication, can bind to low-density lipoprotein receptor-related protein 1 (LRP1) that controls ApoE uptake in the brain, we determined the extent to which different isoforms of ApoE affected Tat-mediated HIV-1 LTR transactivation.

Methods

Using U87MG glioblastoma cells expressing LTR-driven luciferase, we determined the extent to which LRP1 as well as ApoE2, ApoE3, and ApoE4 affected Tat-mediated HIV-1 LTR transactivation.

Results

A specific LRP1 antagonist and siRNA knockdown of LRP1 both restricted significantly Tat-mediated LTR transactivation. Of the three ApoEs, ApoE4 was the least potent and effective at preventing HIV-1 Tat internalization and at decreasing Tat-mediated HIV-1 LTR transactivation. Further, Tat-mediated LTR transactivation was attenuated by an ApoE mimetic peptide, and ApoE4-induced restriction of Tat-mediated LTR transactivation was potentiated by an ApoE4 structure modulator that changes ApoE4 into an ApoE3-like phenotype.

Conclusions

These findings help explain observed differential effects of ApoEs on HIV-1 infectivity and the prevalence of HAND in people living with HIV-1 infection and suggest that ApoE mimetic peptides and ApoE4 structure modulator might be used as a therapeutic strategy against HIV-1 infection and associated neurocognitive disorders.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1129-1) contains supplementary material, which is available to authorized users.

Fatty acid metabolism in carriers of apolipoprotein E epsilon 4 allele: is it contributing to higher risk of cognitive decline and coronary heart disease?

Apolipoprotein E (ApoE) is a protein playing a pivotal role in lipid homeostasis since it regulates cholesterol, triglyceride and phospholipid metabolism in the blood and the brain. APOE gene regulates the expression of this protein and has three different alleles: ε2, ε3 and ε4. Carrying an APOE4 allele is recognised as a genetic risk factor of late-onset Alzheimer’s disease (LOAD) and coronary heart disease (CHD). Consuming fatty fish, rich in long chain omega-3 fatty acids (LC omega-3), seems to be associated with risk reduction of developing LOAD and CHD but this link seems not to hold in APOE4 carriers, at least in LOAD. In CHD trials, APOE4 carriers supplemented with LC omega-3 were categorized as differential responders to the treatment with regards to CHD risk markers. This is potentially because fatty acid metabolism is disturbed in APOE4 carriers compared to the non-carriers. More specifically, homeostasis of LC omega-3 is disrupted in carriers of APOE4 allele and this is potentially because they β-oxidize more LC omega-3 than the non-carriers. Therefore, there is a potential shift in fatty acid selection for β-oxidation towards LC omega-3 which are usually highly preserved for incorporation into cell membranes.

APOE ε4: the most prevalent yet understudied risk factor for Alzheimer's disease

Brain pathology of Alzheimer's diseases (AD) and the genetics of autosomal dominant familial AD have been the "lamp posts" under which the AD field has been looking for therapeutic targets. Although this approach still remains valid, none of the compounds tested to date have produced clinically meaningful results. This calls for developing complementary therapeutic approaches and AD targets. The allele ε4 of apolipoprotein E4 (APOE ε4), is the most prevalent genetic risk factor for sporadic AD, and is expressed in more than half of the AD patients. However, in spite of its genetic prominence, the allele APOE ε4 and its corresponding protein product apoE4 have been understudied. We presently briefly discuss the reasons underlying this situation and review newly developed AD therapeutic approaches that target apoE4 and which pave the way for future studies.

Long-term probucol therapy continues to suppress markers of neurovascular inflammation in a dietary induced model of cerebral capillary dysfunction

BACKGROUND

Probucol has been shown to prevent cerebral capillary disturbances characterized by blood-to-brain extravasation of plasma derived proteins and neurovascular inflammation in mice maintained on western-styled diets for 12 weeks. However the effect of probucol on capillary integrity in aging models with capillary dysfunction is not known.

METHODS

Wild-type C57BL6 mice were randomized to a low-fat (LF); saturated-fat (SFA); or SFA + Probucol diet for up to12 months of intervention.

RESULTS

Mice fed the LF diet had substantially greater parenchymal abundance of plasma derived IgG and apo B lipoproteins at 12 months, compared to LF mice at 3 months of intervention. Markers of neurovascular inflammation were also greater at 12 months in LF fed mice compared to LF mice at 3 months. The SFA diet exacerbated the aging induced parenchymal abundance of IgG and of apo B lipoproteins and neurovascular inflammation at 12 months. The SFA effects were associated with increased production of intestinal lipoprotein amyloid-β (Aβ). The co-provision of probucol with the SFA completely abolished heightened inflammation at 12 months. Probucol attenuated SFA-induced capillary permeability but had only a modest inhibitory effect on parenchymal retention of apoB lipoproteins. The improvements in markers of inflammation and capillary integrity because of probucol correlated with enterocytic genesis of chylomicron Aβ.

CONCLUSION

In this long-term feeding study, probucol profoundly suppressed dietary SFA induced disturbances in capillary integrity but had a more modest effect on age-associated changes.

Apolipoprotein E and lipid homeostasis in the etiology and treatment of sporadic Alzheimer's disease

The discovery that the apolipoprotein E (apoE) ε4 allele is genetically linked to both sporadic and familial late-onset Alzheimer's disease (AD) raises the possibility that a dysfunction of the lipid transport system could seriously affect lipid homeostasis in the brain of AD subjects. The presence of the ε4 allele has been associated with lower levels of apoE in both serum and brain tissues of normal and AD subjects. In an attempt to reverse the apoE deficit in AD, we identified and characterized several apoE inducer agents using a low-throughput in vitro screening assay. The most promising of these compounds is called probucol. Administration of probucol, an old cholesterol-lowering drug, in a pilot trial in mild-to-moderate sporadic AD led to a significant increase in cerebrospinal fluid (CSF) apoE levels and a decrease in CSF in both phosphorylated tau 181 and beta-amyloid 1-42 concentrations without significant modifications of lipid hydroperoxide levels.

Altered Cholesterol Intracellular Trafficking and the Development of Pathological Hallmarks of Sporadic AD

Compared to the rare familial early onset Alzheimer's disease (AD) that results from gene mutations in AbPP and presenilin-1, the pathogenesis of sporadic AD is much more complex and is believed to result from complex interactions between nutritional, environmental, epigenetic and genetic factors. Among those factors, the presence APOE4 is still the single strongest genetic risk factor for sporadic AD. However, the exact underlying mechanism whereby apoE4 contributes to the pathogenesis of sporadic AD remains unclear. Here, we discuss how altered cholesterol intracellular trafficking as a result of apoE4 might contribute to the development of pathological hallmarks of AD including brain deposition of amyloid beta (Ab), neurofibrillary tangles, and synaptic dysfunction.

Role of LDL cholesterol and endolysosomes in amyloidogenesis and Alzheimer's disease

The pathogenesis of late-onset sporadic Alzheimer's disease (AD) is believed to result from complex interactions between nutritional, environmental, epigenetic and genetic factors. Among those factors, altered circulating cholesterol homeostasis, independent of the APOE genotype, continues to be implicated in brain deposition of amyloid beta protein (Aβ) and the pathogenesis of AD. It is believed that trafficking of amyloid beta precursor protein (AβPP) into endolysosomes appears to play a critical role in determining amyloidogenic processing of AβPP because this is precisely where two enzymes critically important in AβPP metabolism are located; beta amyloid converting enzyme (BACE-1) and gamma secretase enzyme. We have shown that elevated levels of LDL cholesterol promote AβPP internalization, disturb neuronal endolysosome structure and function, and increase Aβ accumulation in neuronal endolysosomes. Here, we will further discuss the linkage between elevated levels of LDL cholesterol and AD pathogenesis, and explore the underlying mechanisms whereby elevated levels of plasma LDL cholesterol promote amyloidogenesis.

Effect of APOE polymorphism on obesity and lipid profile in women of differing reproductive status

The aim of this study was to investigate whether the effect of apolipoprotein E polymorphism (APOE) on somatic and lipid risk parameters varies in women of differing reproductive status. We analyzed 447 Slovak women aged between 39 and 90 years. APOE genotypes were determined by PCR-RFLP. Regression analysis confirmed the effect of the APOE genotype on the levels of LDL-cholesterol, apolipoprotein B (apoB), nonHDL-cholesterol and on the three atherogenic indices: apoB-to-apoA1, TC-to-HDLcholesterol, LDL-C-to-HDL-cholesterol. Here, lower mean levels were registered in the E2 carriers than in the E3 and E4 subgroups. However, the impact of menopausal status on lipid parameters was not confirmed. Bonferroni correction showed that systolic blood pressure was significantly lower in the E4 carriers compared to the E3 group (P=0.017). Univariate analysis of covariance revealed a significant interaction between the menopausal group and the APOE group, and their common effect on waist-to-hipratio (WHR). Bonferroni correction in early postmenopausal women showed that the mean WHR values were significantly different between E2 and E4 groups (P=0.008). This study demonstrates that the E*2 allele has a protective effect against higher blood lipid levels. Moreover, the results suggest that E*2 could have a partial negative effect on WHR in early postmenopausal Slovak women.

Proteolytic cleavage of apolipoprotein E4 as the keystone for the heightened risk associated with Alzheimer's disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disease characterized by microscopic lesions consisting of beta-amyloid plaques and neurofibrillary tangles (NFTs). The majority of cases are defined as sporadic and are likely caused by a combination of both genetic and environmental factors. Of the genetic risk factors identified, the 34 kDa protein, apolipoprotein (apo) E4, is of significant importance as APOE4 carriers account for 65%–80% of all AD cases. Although apoE4 plays a normal role in lipoprotein transport, how it contributes to AD pathogenesis is currently unknown. One potential mechanism by which apoE4 contributes to disease risk is its propensity to undergo proteolytic cleavage generating N- and C-terminal fragments. The purpose of this review will be to examine the mechanisms by which apoE4 contributes to AD pathogenesis focusing on the potential loss or gain of function that may occur following cleavage of the full-length protein. In this context, a discussion of whether targeting apoE4 therapeutically is a rationale approach to treating this disease will be assessed.

APOE4-specific changes in Aβ accumulation in a new transgenic mouse model of Alzheimer disease

APOE4 is the greatest risk factor for Alzheimer disease (AD) and synergistic effects with amyloid-β peptide (Aβ) suggest interactions among apoE isoforms and different forms of Aβ accumulation. However, it remains unclear how the APOE genotype affects plaque morphology, intraneuronal Aβ, soluble Aβ42, and oligomeric Aβ (oAβ), particularly in vivo. As the introduction of human APOE significantly delays amyloid deposition in transgenic mice expressing familial AD (FAD) mutations (FAD-Tg), 5xFAD-Tg mice, which exhibit amyloid deposition by age 2 months, were crossed with apoE-targeted replacement mice to produce the new EFAD-Tg mice. Compared with 5xFAD mice, Aβ deposition was delayed by ∼4 months in the EFAD mice, allowing detection of early changes in Aβ accumulation from 2-6 months. Although plaque deposition is generally greater in E4FAD mice, E2/E3FAD mice have significantly more diffuse and E4FAD more compact plaques. As a first report in FAD-Tg mice, the APOE genotypes had no effect on intraneuronal Aβ accumulation in EFAD mice. In E4FAD mice, total apoE levels were lower and total Aβ levels higher than in E2FAD and E3FAD mice. Profiles from sequential three-step extractions (TBS, detergent, and formic acid) demonstrated that the lower level of total apoE4 is reflected only in the detergent-soluble fraction, indicating that less apoE4 is lipoprotein-associated, and perhaps less lipidated, compared with apoE2 and apoE3. Soluble Aβ42 and oAβ levels were highest in E4FAD mice, although soluble apoE2, apoE3, and apoE4 levels were comparable, suggesting that the differences in soluble Aβ42 and oAβ result from functional differences among the apoE isoforms. Thus, APOE differentially regulates multiple aspects of Aβ accumulation.

Lipid metabolism and glial lipoproteins in the central nervous system

Lipoproteins in the central nervous system (CNS) are not incorporated from the blood but are formed mainly by glial cells within the CNS. In addition, cholesterol in the CNS is synthesized endogenously because the blood-brain barrier segregates the CNS from the peripheral circulation. Apolipoprotein (apo) E is a major apo in the CNS. In normal condition, apo E is secreted from glia, mainly from astrocytes, and forms cholesterol-rich lipoproteins by ATP-binding cassette transporters. Subsequently, apo E-containing glial lipoproteins supply cholesterol and other components to neurons via a receptor-mediated process. Recent findings demonstrated that receptors of the low density lipoprotein (LDL) receptor family not only internalize lipoproteins into the cells but also, like signaling receptors, transduce signals upon binding the ligands. In this review, the regulation of lipid homeostasis will be discussed as well as roles of lipoproteins and functions of receptors of LDL receptor family in the CNS. Furthermore, the relation between lipid metabolism and Alzheimer's disease (AD) is discussed.

Introducing Human APOE into Aβ Transgenic Mouse Models

Apolipoprotein E (apoE) and apoE/amyloid-β (Aβ) transgenic (Tg) mouse models are critical to understanding apoE-isoform effects on Alzheimer's disease risk. Compared to wild type, apoE^−/− mice exhibit neuronal deficits, similar to apoE4-Tg compared to apoE3-Tg mice, providing a model for Aβ-independent apoE effects on neurodegeneration. To determine the effects of apoE on Aβ-induced neuropathology, apoE^−/− mice were crossed with Aβ-Tg mice, resulting in a significant delay in plaque deposition. Surprisingly, crossing human-apoE-Tg mice with apoE^−/−/Aβ-Tg mice further delayed plaque deposition, which eventually developed in apoE4/Aβ-Tg mice prior to apoE3/Aβ-Tg. One approach to address hAPOE-induced temporal delay in Aβ pathology is an additional insult, like head injury. Another is crossing human-apoE-Tg mice with Aβ-Tg mice that have rapid-onset Aβ pathology. For example, because 5xFAD mice develop plaques by 2 months, the prediction is that human-apoE/5xFAD-Tg mice develop plaques around 6 months and 12 months before other human-apoE/Aβ-Tg mice. Thus, tractable models for human-apoE/Aβ-Tg mice continue to evolve.

Alzheimer's disease genetics: current knowledge and future challenges

Alzheimer's disease (AD) is highly heritable, but genetically complex. Recently, three large-scale genome-wide association studies have made substantial breakthroughs in disentangling the genetic architecture of the disease. These studies combined include data from over 43 000 independent individuals and provide compelling evidence that variants in four novel susceptibility genes (CLU, PICALM, CR1, BIN1) are associated with disease risk. These findings are tremendously exciting, not only in providing new avenues for exploration, but also highlighting the potential for further gene discovery when larger samples are analysed. Here we discuss progress to date in identifying risk genes for dementia, ways forward and how current findings are refining previous ideas and defining new putative primary disease mechanisms.

Pharmacogenetic Study on the Effect of Rivastigmine on PS2 and APOE Genes in Iranian Alzheimer Patients

Background/Aims

Alzheimer disease (AD) is a complex and genetically heterogeneous disorder, and certain genes such as PS2 and APOE4 contribute to the development of AD. Due to its heterogeneity, AD-predisposing genes could vary in different populations. Moreover, not all AD patients will respond to the same therapy. We specifically investigated the effect ofrivastigmine (Exelon) on PS2 and APOE genes in Iranian AD patients.

Methods

A total of 100 AD patients, 67 patients with sporadic AD (SAD) and 33 patients with familial AD (FAD), receiving rivastigmine therapy and 100 healthy controls were studied. PCR-RFLP was used for genotyping of PS2 and APOE.

Results

We found a positive association between the PS2 –A allele and SAD patients (p^c = 0.01), and the PS2 +A/–A genotype was significantly more frequent in SAD than FAD patients (p^c = 0.009). The APOE4 allele was associated with total AD, SAD and FAD (p^c = 0.000002). Patients with the PS2 +A/–A genotype and bigenic genotypes of +A/–A·∊3/∊3 and +A/–A·∊3/∊4 were the best responders to Exelon therapy, and those with the PS2 +A/+A and APOE ∊3/∊4 genotypes were the worst responders.

Conclusion

Our findings suggest that the PS2 and APOE4 alleles and genotypes affect both AD risk and response to rivastigmine therapy.

Function and comorbidities of apolipoprotein e in Alzheimer's disease

Alzheimer's disease (AD)—the most common type of dementia among the elderly—represents one of the most challenging and urgent medical mysteries affecting our aging population. Although dominant inherited mutation in genes involved in the amyloid metabolism can elicit familial AD, the overwhelming majority of AD cases, dubbed sporadic AD, do not display this Mendelian inheritance pattern. Apolipoprotein E (APOE), the main lipid carrier protein in the central nervous system, is the only gene that has been robustly and consistently associated with AD risk. The purpose of the current paper is thus to highlight the pleiotropic roles and the structure-function relationship of APOE to stimulate both the functional characterization and the identification of novel lipid homeostasis-related molecular targets involved in AD.

APOE and cholesterol homeostasis in Alzheimer's disease

Converging evidence from clinical and pathological studies indicate the presence of important relationships between the ongoing deterioration of brain lipid homeostasis, vascular changes and the pathophysiology of sporadic Alzheimer's disease (AD). These associations include the recognition of cholesterol transporters apolipoprotein E (APOE), APOC1 and APOJ as major genetic risk factors for common AD and observations associating risk factors for cardiovascular disease such as high midlife plasma cholesterol, diabetes, stroke, obesity and hypertension to dementia. Moreover, recent clinical findings lend support to the notion that progressive deterioration of cholesterol homeostasis in AD is a central player in the disease pathophysiology and is, therefore, a potential therapeutic target for disease prevention.

Advances and perspectives from genetic research: development of biological markers in Alzheimer's disease

Despite important recent advances, a full understanding of the (genetic) etiology of Alzheimer's disease (AD) is still a long way off. Large collaborative efforts are ongoing, as well as the exploration of various sources of genetic variation. Evidence supports the view that Mendelian early-onset familial forms of AD are caused by rare and usually highly penetrant mutations in three genes (APP, PSEN1 and PSEN2). Considering sporadic late-onset AD (LOAD), the APOE epsilon4 allele is by far the best-established risk gene. Recently published large-scale genome-wide analyses point to additionally relevant genetically associated loci, particularly CLU, PICALM and CR1. These susceptibility loci support existing hypotheses about the amyloid, lipid, chaperone and chronic inflammatory mechanisms in AD pathogenesis, and are therefore likely to provide the basis for the development of hypothesis-driven novel biomarker candidates. Additional genes, listed online in AlzGene (e.g., GAB2 or SORL1) have repeatedly shown risk effects in LOAD, and may be true risk genes, but this is much less certain. New epigenetic research provided some evidence that DNA modifications maybe involved in LOAD (e.g., post-mortem studies described both hypo- and hyper-methylation in AD-related susceptibility genes). With respect to biomarkers, elderly nondemented APOE epsilon4 carriers demonstrated distinct cerebrospinal fluid biomarker signatures and alterations of brain glucose metabolism similar to those observed in AD. Future research should evaluate the usefulness of newly detected AD risk genes and epigenetic changes as potential biomarkers towards genetic profiling of AD or for correlation with endophenotypes and therapeutic outcome.

Effects of apolipoprotein E-epsilon4 and -epsilon2 in amnestic mild cognitive impairment and dementia in Shanghai: SCOBHI-P

OBJECTIVE

To determine apolipoprotein E (APOE)-epsilon4 and -epsilon2 frequencies and risk of mild cognitive impairment (MCI) and dementia in Shanghai, China.

METHODS

A total of 34 MCI and 34 dementia cases were recruited from an urban Memory Disorders Clinic and 32 controls were recruited from a residential community served by the clinic. Apolipoprotein E was genotyped using standard methods.

RESULTS

Among controls, frequencies were epsilon2, 0.11; epsilon3, 0.84; and epsilon4, 0.05; among MCI, 0.05, 0.77, and 0.18; and for dementia, 0.02, 0.84, and 0.15, respectively. In education-adjusted models, the odds ratio (OR) = 5.6 for dementia (95% CI = 1.09-29.3) and 4.7 for MCI (95% CI = 0.90-25.2) associated with any epsilon4 allele. The epsilon2 allele was inversely associated with dementia (OR = 0.12, 95% CI = 0.013-0.997) and MCI (OR = 0.38, 95% CI = 0.08-1.61).

CONCLUSIONS

APOE-epsilon4 increases and -epsilon2 decreases the risk of dementia vs normal cognition. Similar trends were observed for amnestic mild cognitive impairment (aMCI).

Brain lipid metabolism, apolipoprotein E and the pathophysiology of Alzheimer's disease

Brain lipid homoeostasis is critical during neurodevelopment, repair after traumatic brain injury and for the maintenance of efficient neurotransmission. Several neurodegenerative disorders occur as a direct result of neuronal lipid dysfunction and underlying disease processes that are associated with Alzheimer's disease (AD) also appear to be related to an imbalance in brain lipid homeostasis. In support of this latter hypothesis, recent genome wide association studies have confirmed and extended the now widely reproduced association between the epsilon4 allele of the apolipoprotein E gene (APOE) and late onset AD. Even in populations with low APOE epsilon4 allele frequency, gene dosage of APOE epsilon4 increases the age-adjusted relative risk for developing the more common late onset form of AD. A major role for apolipoprotein E (apoE) in the brain is to maintain a constant supply of neuronal lipids for rapid and dynamic membrane synthesis thus ensuring efficient neurotransmitter release and the propagation of action potentials. Additionally, apoE synthesized primarily by glia is critical for the elimination of toxic brain-derived Abeta peptides. In addition to apoE isoform, the overall levels of apoE appear to be important determinants for brain Abeta clearance. Susceptibility to AD in APOE epsilon4 carriers may occur early since brain activity and the accumulation of Abeta in brain parenchyma both appear well in advance of disease onset. Given the pivotal role apoE plays in maintaining neuronal membrane homeostasis, elevating the levels of apoE in brain may be a viable therapeutic strategy for the prevention and/or treatment of AD.

Will testing for apolipoprotein E assist in tailoring dementia risk reduction? A review

This paper aims to systematically review the influence of apolipoprotein E (ApoE) on the effects of potentially modifiable mid and late life risk factors for dementia. Scopus, Medline, PubMed, PsycINFO, and HuGE databases were searched up to November 2008. Two independent reviewers selected 94 articles from 13,122 results. Results suggest the deleterious effect of current smoking is limited only to persons without ApoE epsilon 4 (4 out of 4 studies), ApoE epsilon 4 increases the risk of dementia associated with greater fat consumption, particularly saturated fats (3 out of 4 studies), and increases the protective effect against dementia associated with HRT use (3 out of 5 with one of the non-significant studies suggesting a trend). There was evidence that ApoE does not modify the risk of dementia associated with measures of, and treatments for CVD, other dietary factors, and estradiol levels. There was inconsistent or contradictory evidence for other environmental factors reviewed. There is insufficient evidence for the recommendation of ApoE testing to assist with tailoring risk reduction recommendations for dementia.

Human APOE isoform-dependent effects on brain beta-amyloid levels in PDAPP transgenic mice

To investigate the role of human apolipoprotein E (apoE) on Abeta deposition in vivo, we crossed PDAPP mice lacking mouse Apoe to targeted replacement mice expressing human apoE (PDAPP/TRE2, PDAPP/TRE3, or PDAPP/TRE4). We then measured the levels of apoE protein and Abeta peptides in plasma, CSF, and brain homogenates in these mice at different ages. We also quantified the amount of brain Abeta and amyloid burden in 18-month-old mice. In young PDAPP/TRE4 mice that were analyzed at an age before brain Abeta deposition, we observed a significant decrease in the levels of apoE in CSF and brain when compared with age-matched mice expressing either human E2 or E3. The brain levels of Abeta42 in PDAPP/TRE4 mice were substantially elevated even at this very early time point. In older PDAPP/TRE4 mice, the levels of insoluble apoE protein increased in parallel to the dramatic rise in brain Abeta burden, and the majority of apoE was associated with Abeta. In TRE4 only mice, we also observed a significant decrease in the level of apoE in brain homogenates. Since the relative level of apoE mRNA was equivalent in PDAPP/TRE and TRE only mice, it appears that post-translational mechanisms influence the levels of apoE protein in brain (E4 < E3 << E2), resulting in early and dramatic apoE isoform-dependent effects on brain Abeta levels (E4 >> E3 > E2) that increase with age. Therapeutic strategies aimed at increasing the soluble levels of apoE protein, regardless of isoform, may effectively prevent and (or) treat Alzheimer's disease.

Genetics of Alzheimer's disease: recent advances

Alzheimer's disease is a progressive neurodegenerative disorder with high prevalence in old age. It is the most common cause of dementia, with a risk reaching 50% after the age of 85 years, and with the increasing age of the population it is one of the biggest healthcare challenges of the 21st century. Genetic variation is an important contributor to the risk for this disease, underlying an estimated heritability of about 70%. Alzheimer's genetics research in the 1990s was successful in identifying three genes accounting for most cases of early-onset disease with autosomal dominant inheritance, and one gene involved in the more common late-onset disease, which shows complex inheritance patterns. Despite the presence of significant remaining genetic contribution to the risk, the identification of genes since then has been elusive, reminiscent of most other complex disorders. In the past decade there have been significant efforts towards a systematic evaluation of the multiple genetic association studies for Alzheimer's disease, while the first genome-wide association studies are now being reported with promising results. As sample sizes grow through new collections and collaborative efforts, and as new technologies make it possible to test alternative hypotheses, it is expected that new genes involved in the disease will soon be identified and confirmed. The gene discoveries of the 1990s have taught us a lot about Alzheimer's disease pathogenesis, providing many therapeutic targets that are currently at various stages of testing for future clinical use. As new genes become known and the biological pathways leading to disease are further explored, the possibility of prevention and successful personalized treatment is becoming tangible, providing hope for the millions of patients with Alzheimer's disease and their caregivers.

Apolipoprotein E and cholesterol in aging and disease in the brain

Cholesterol can be detrimental or vital, and must be present in the right place at the right time and in the right amount. This is well known in the heart and the vascular system. However, in the CNS cholesterol is still an enigma, although several of its fundamental functions in the brain have been identified. Brain cholesterol has attracted additional attention owing to its close connection to ApoE, a key polymorphic transporter of extracellular cholesterol in humans. Indeed, both cholesterol and ApoE are so critical to fundamental activities of the brain, that the brain regulates their synthesis autonomously. Yet, similar control mechanisms of ApoE and cholesterol homeostasis may exist on either sides of the blood-brain barrier. One indication is that the APOE ε4 allele is associated with hypercholesterolemia and a proatherogenic profile on the vascular side and with increased risk of Alzheimer's disease on the CNS side. In this review, we draw attention to the association between cholesterol and ApoE in the aging and diseased brain, and to the behavior of the ApoE4 protein at the molecular level. The attempt to correlate in vivo and in vitro observations is challenging but crucial for developing future strategies to address ApoE-related aberrations in cholesterol metabolism selectively in the brain.