Apolipoprotein E and central nervous system disorders: Reviews of clinical findings
Masatoshi Takeda, MD, PhD, Department of Psychiatry, Osaka University Graduate School of Medicine, 2-2 Yamadaoka, Suita City, Osaka 565-0871, Japan. Email: email@example.com
Dementia is a major health problem in developed countries with over 25 million people affected worldwide and probably over 75 million people at risk during the next 20 years. Alzheimer's disease (AD) is the most frequent cause of dementia (50–70%), followed by vascular dementia (30–40%), and mixed dementia (15–20%). AD pathogenesis is still to be elucidated but it is believed to be the complex interaction between genetic and environmental factors in later life. Three causative genes for familial AD have been identified: amyloid precursor protein, presenilin-1, and presenilin-2. There are 150 genes involved with increased neuronal vulnerability to premature death in the AD brain. Among these susceptibility genes, the apolipoprotein E (ApoE) gene is the most prevalent as a risk for AD pathogenic process in which complex interactions between genetic and environmental factors are involved, leading to a cascade of pathogenic events converging in final pathways to premature neuronal death. Some of these mechanisms are common to several neurodegenerative disorders that differ depending upon the genes affected and the involvement of environmental conditions.
ApoE is a key lipoprotein in lipid and cholesterol metabolism and it is also the major risk gene for AD and many other central nervous system disorders. The pathogenic role of ApoE-4 is still to be clarified; however, diverse evidence suggests that ApoE may play pleiotropic functions in dementia and central nervous system disorders.
THE NUMBER OF dementia patients has increased significantly due to extended life spans. Dementia occurs in 6–15% of the elderly, causing a social problem in Japan and in other countries.1 At present, clinicians are expected to attend 2 million dementia patients with behavioral and psychological problems of dementia, which causes a serious burden to the patients as well as to the caregivers.2 Pharmacological treatment, mainly by anti-psychotics, works with some patients,3–5 but is not always useful. Some opinions recommend refraining from prescribing anti-psychotics for dementia with behavioral and psychological problems of dementia,6 and Chinese herbal medicine is recommended by some opinion leaders.7–9 Behavioral therapy,10 aromatherapy11 and animal-assisted therapy12 are sometimes useful. Different strategies are recommended by the clinical settings either at home13 or in institutions,14,15 targeting each type of dementia, including Alzheimer's disease (AD),2 frontotemporal dementia,16 diffuse Lewy body disease,17 and vascular dementia.18
Considering the limited effectiveness of these interventions, researchers are now more interested in implementing early diagnosis,19 disease-modifying therapeutics,20,21 and even prevention of dementia.22 The concept of mild cognitive impairment (MCI) and subjective cognitive impairment (SCI) have been proposed to stimulate the understanding of the earlier stage, or prodromal stage of dementia,1 in which the interaction in the aging brain of genetic factors and lifestyle, influencing environmental factors, is the major issue to be elucidated.23 Apolipoprotein E (ApoE) is recognized as the most powerful genetic risk factor for dementia, and it is also a key player in lipid metabolism. We believe that the clinical findings related to ApoE should be taken into consideration, in order to reconcile gene and environmental interactions in the pathogenesis of dementia, including AD. The pathogenic role of ApoE-4 is still to be clarified. The biological functions of ApoE will be reviewed with regard to dementia and other central nervous system (CNS) disorders.
Apolipoprotein E gene
The three major isoforms of human ApoE(19q13.2) (ApoE-2, ApoE-3, ApoE-4) are coded by the ε2, ε3, and ε4 alleles. Differences in the amino acid sequence at sites A (residue 112) and B (residue 158) of the ApoE molecule distinguish the ApoE-2 (Cys/Cys), ApoE-3 (Cys/Arg), and ApoE-4 (Arg/Arg) isoforms.24,25ApoE-3 is the most frequent isoform (wild-type), and ApoE-4 differs from ApoE-3 in a Cys-to-Arg change at position 112 (ApoE-4/Cys112Arg). ApoE-2 (Arg158Cys) is the most common isoform of the four different mutations at the E2 position with isoelectric focusing. The other three ApoE-2 isoforms are E2(Lys146Gln), E2(Arg145Cys), and E2(Arg136Ser).26 The ApoE gene encodes a 299-amino acid polypeptide (Mw 34 200). This gene is in close proximity with the APOC1, APOC2 and GPI genes in the same region of 19q.27 Sequence haplotype variation in 5.5 kb of genomic DNA encompassing the whole ApoE locus and adjoining flanking regions revealed the existence of 22 diallelic sites defining 31 distinct haplotypes. Sequence analysis suggested that haplotypes defining the ApoE-3 and ApoE-2 alleles were derived from the ancestral ApoE-4 and that the ApoE-3 group of haplotypes had increased in frequency, relative to ApoE-4, over the past 200 000 years. Substantial heterogeneity is present in the three classes of sequence haplotypes, with interpopulation differences in the sequence variation underlying the protein isoforms, probably explaining conflicting results when interpreting phenotypic associations with variation in the common protein isoforms.28,29
The ApoE alleles show a peculiar distribution throughout the world.30 The ApoE-3 allele is the most frequent in all human societies, especially in populations with a long-established agricultural economy, such as those of the Mediterranean basin, where the allele frequency is 0.849–0.898. ApoE-4 is the ancestral allele, with a frequency that still remains higher in Pygmies (0.407), Khoi San (0.370), Papuans (0.368), Lapps (0.310), some Native Americans (0.280), Australian Aborigines (0.260), and Aborigines of Malaysia (0.240) where an economy of foraging still exists, or food supply is scarce or sporadically available. The frequency of the ApoE-2 allele fluctuates with no apparent trend (0.145–0.02) and is absent in Native Americans and very low (<1%) in southern Europeans.30–32
Biological function of ApoE
The best known effect of ApoE is the regulation of lipid metabolism; however, in addition to its role in the transport of cholesterol and the metabolism of lipoprotein particles,33 ApoE may be involved in many other physiological and pathological processes, including immunoregulation, nerve regeneration, activation of lipolytic enzymes (hepatic lipase, lipoprotein lipase, lecithin : cholesterol acyltransferase), ligand for several cell receptors, neuronal homeostasis, and tissue repair.29
ApoE is essential for the normal catabolism of triglyceride-rich lipoprotein constituents. The interaction of ApoE and the low-density lipoprotein (LDL) receptor controls the removal of ApoE-rich lipoproteins (very low-density lipoprotein [VLDL], chylomicron remnants, intermediate density proteins) and determines the homeostasis of cholesterol and triglycerides.31 Some studies indicate that ApoE polymorphism variation may explain 14–17% of the genetic variability of plasma cholesterol concentrations.31,34,35
The three ApoE isoforms have different affinities for the LDL receptor, ApoE-3 and ApoE-4, showing similar affinities and ApoE-2 exhibiting a defective binding activity. ApoE plays a critical role in lipoprotein metabolism and plasma lipid homeostasis through its high-affinity binding to the LDL-receptor family. In solution, ApoE is an oligomeric protein, and the C-terminal domain causes ApoE's aggregation. The aggregation property presents a major difficulty for structural determination of this protein. Using protein engineering techniques, Fan et al. identified a monomeric, biologically active ApoE C-terminal domain mutant. This mutant replaces five bulky hydrophobic residues in the region of residues 253–289 with either smaller hydrophobic or polar/charged residues (F257A, W264R, V269A, L279Q, V287E). These residues are critical for aggregation but may not be important for maintaining the structure, stability, and lipid-binding activity of this ApoE domain, suggesting that ApoE may use different epitopes for its aggregation property, helical structure/stability, and lipid-binding activity.36
ApoE-2-containing remnants and VLDL particles are slowly removed from the plasma and induce an upregulation of the liver LDL receptor and subsequent low concentration of plasma cholesterol. VLDL-ApoE-4 particles are removed from plasma faster than VLDL-ApoE-3 particles, inducing a downregulation of the LDL receptor, and thus the VLDL-ApoE-4 phenotype is associated with higher concentration of circulating cholesterol.31
The ApoE genotype is an important determinant of plasma and CSF ApoE and lipid levels. The ApoE-2 allele is associated with high concentrations of ApoE and the ApoE-4 allele with lower ApoE levels.31 Some authors have found association between ApoE-2 and ApoE-4 and high levels of plasma triglycerides as well as an association of ApoE-3 and low levels of triglycerides in the general population.35
It has also been suggested that individuals carrying the ApoE-4 genotype had a significantly greater increase in triglycerides accompanied by an increase in bodyweight, suggesting that obese individuals with an ApoE-4 allele might be at increased risk for developing hypertriglyceridemia and atherosclerosis; however, recent studies in the AD population clearly indicate that: (i) cholesterol levels are markedly increased in ApoE-4/4 carriers; (ii) triglyceride levels are the lowest in ApoE-4/4; and (iii) ApoE-4/4 carriers show a high atherogenic activity. These differences probably reflect the influence of endogenous factors interacting with ApoE to induce an alteration in lipid metabolism in patients with AD.37–39
The ApoE-4/4 and ApoE-3/4 genotypes have also been associated with high systolic blood pressure levels.40 It has been suggested that the ApoE-2 allele may exert a protective effect on coronary atherosclerosis,41,42 and that the ApoE-4 allele increases the risk of myocardial infarction and atherosclerosis.31,42 After studying the association of ApoE with birthweight, Garces et al. suggested that the interaction of the ApoE genotype and birthweight may be an important determinant of atherosclerosis.43
Sullivan et al. studied the pattern of ApoE expression in the CNS. Immunocytochemistry on brain sections from three human ApoE targeted replacement mouse lines, wild-type mice, African green monkeys, and humans, and showed a predominantly glial pattern of ApoE expression. The levels of human ApoE protein in the hippocampus and frontal cortex were similar between targeted replacement mice and non-demented human tissue. Within a given brain region, the levels of ApoE were very similar amongst all three isoforms, which contrasts sharply with plasma, where ApoE2 levels are 16-fold higher than ApoE3 and ApoE4 levels. Across brain regions, cerebellar ApoE levels were significantly higher than cerebral ApoE levels.44 In the human brain, ApoE-4 dose correlates inversely with dendritic spine density in the hippocampus.45 ApoE is expressed at high levels in hepatocytes, macrophages, fibroblasts and astrocytes. Neurons also express ApoE at lower levels than astrocytes in response to various physiological and pathological conditions, including excitotoxic stress. Neuronal expression of ApoE is regulated by a diffusible factor or factors released from astrocytes, and this regulation depends on the activity of the extracellular signal-regulated kinase (Erk) pathways in neurons.46 For many years, alterations in ApoE and defects in the ApoE gene have been associated with dysfunctions in lipid metabolism, cardiovascular disease, and atherosclerosis. An enormous number of studies, however, clearly documented the role of ApoE-4 as a risk factor for AD.
ApoE in Alzheimer's disease (AD)
In 1993 Allen Roses and co-workers found a clear association between ApoE genotypes and AD, demonstrating that the frequency of the ApoE-4 allele was significantly higher in late-onset Alzheimer's disease (LOAD).47 Since then, many studies have confirmed this, reporting an increased frequency of the ApoE-4 allele in AD and the association of the ApoE-4 allele with LOAD and sporadic forms of AD. A protective effect of ApoE-2 for LOAD has also been proposed48 and confirmed.49 There is also a significant lowering of age at onset for subjects with ApoE-4/4 as compared to other ApoE genotypes.50 ApoE-4 promotes arteriosclerosis and is less frequent in centenarians than in controls, and ApoE-2, which was associated with type III and type IV hyperlipemia, is more frequent in people with higher longevity rates.51 The risk for AD increases from 20% to 90% and mean age at onset decreases from 84 to 68 years with an increasing number of ApoE-4 alleles, confirming the dosage effect of the ApoE-4 allele, which in ApoE-4/4 homozygotes anticipates the age at onset to their 60s. The combination of low head circumference and ApoE-4 is also a strong predictor of early-onset AD.52
ApoE is found in amyloid plaques and neurofibrillary tangles (NFT) in AD brains. The accumulation of potentially pathogenic C-terminally truncated fragments of ApoE depends on both the isoform and the cellular source of ApoE. Neuron-specific proteolytic cleavage of ApoE-4 is associated with increased phosphorylation of tau and may play a key role in the development of AD-related neuronal deficits.53 Hippocampal ApoE levels correlate with NFT formation, especially in ApoE-3/3 autopsy samples, but not in ApoE-4 carriers.54 Monocyte-derived macrophages exhibit a significantly greater increase in nitric oxide production during immune activation in AD patients with the ApoE-4 allele. Enhanced macrophage responsiveness and increased production of nitric oxide in ApoE-4 may predispose the CNS to an increased potential for nitration and nitrosation, consistent with the reduction-oxidation imbalance and neuroinflammatory state observed in AD.55
ApoE may affect NFT and β-amyloid peptide (BAP) deposition in AD.56 ApoE-4-related proteins may interfere with binding of tau to microtubules, altering tau glycation and phosphorylation.57 The presence of ApoE-4 increases the odds ratio for cerebral amyloid angiopathy; and ApoE-4 is strongly associated with increased BAP deposition in AD.58–60 The oxidized form of purified ApoE-4 shows a higher affinity binding to synthetic BAP and MAP2 than the ApoE-3 isoform, and probably ApoE may affect microtubule function and BAP accumulation in AD.56,61 Carriers of ApoE-2 and ApoE-4 alleles are also more prone to recurrent cerebral amyloid angiopathy than ApoE-3/3 carriers.62 AD ApoE-4 carriers show reduced glucose metabolism in selected brain regions.63 There is also an ApoE-related cognitive decline in AD patients, which is more accelerated in subjects with the ApoE-4/4 genotypes. ApoE-related differences in serum ApoE levels,64,65 blood pressure values66 and lymphocyte apoptosis67,68 have been demonstrated in AD. ApoE-4/4 patients are also the worst responders to different treatments.26 ApoE-4 carriers also show a poorer brain metabolism.63,69,70
The ApoE-4 genotype is accompanied by lower metabolic activity in the nucleus basalis of Meynert neurons in AD and controls.71 Dubelaar et al. used the size of the Golgi apparatus as an indicator of metabolic activity to show that control subjects harboring the ApoE-4 allele have reduced neuronal metabolism and show more neurons with smaller Golgi apparatus size compared with ApoE-4 non-carriers. As the disease progresses into later stages of AD (Braak V-VI stages) neuronal metabolism strongly diminishes, resulting in neurons with extremely small Golgi apparatus size, irrespective of ApoE genotype.71
ApoE-4 may influence AD pathology interacting with APP metabolism and BAP accumulation, enhancing hyperphosphorylation of tau protein and NFT formation, reducing choline acetyltransferase activity, increasing oxidative processes, modifying inflammation-related neuroimmunotrophic activity and glial activation, altering lipid metabolism, lipid transport and membrane biosynthesis in sprouting and synaptic remodeling, and inducing neuronal apoptosis.
A critical review in the literature provides convincing support to the hypothesis of ApoE as a major player in AD pathogenesis and risk of dementia. The major facts demonstrating that ApoE is associated with AD can be summarized as follows: (i) increased frequency of the ApoE-4 allele in AD and protective effect of ApoE-2; (ii) association of ApoE-4 with an anticipation of the age-at-onset; (iii) negative influence of ApoE on cognitive performance; (iv) deleterious associations of ApoE-4 with other genes as potential risk factors for AD; (v) ApoE and sex differences in AD; (vi) association of ApoE with BAP and tau in AD pathology; (vii) ApoE and alterations in lipid metabolism; (viii) ApoE and neuroendocrine function in AD; (ix) ApoE and behavior; (x) ApoE and brain atrophy; (xi) ApoE and survival; and (xii) ApoE in other CNS disorders.29,72
Association of ApoE with BAP and tau in AD pathology
The ApoE-4 isoform binds to BAP more rapidly than the ApoE-3 isoform;73 and the ApoE-4 allele is strongly associated with increased senile plaques but not NFT in AD and in the AD Lewy body variant.58 However, isoform-specific differences have been identified in the binding of ApoE to microtubule-associated protein tau, which forms NFT, and to BAP, a major component of senile plaques.56 Other studies have reported that the presence of ApoE-4 is significantly associated with both BAP and NFT in autopsy brains,74 but the effect is differentially modified by age and gender. For instance, the effect of ApoE-4 on NFT is noted at ages 80 and above, but not between ages 60 and 79, in both genders, whereas the association between the ApoE-4 allele and senile plaques for women is found only from ages 60 to 79, but not above 80 years, with no age difference in men.75 The amount of deposited BAP40 is significantly increased in AD brain samples with ApoE-4 allele and also in cases with the -491 AA genotype independent of ApoE-4 status, suggesting that the association between increased BAP load and alleles of the ApoE promoter polymorphism is independent of ApoE genotype.76
In animal models, overexpression of human ApoE-4 in transgenic mice led to an increase in plaque formation, with the association of the ApoE-4 isoform with APP and BAP in the plaques, a decrease in presynaptic terminals, and an increase in tau phosphorylation and in surrounding gliosis, all these events corresponding with major neuropathological hallmarks of AD. ApoE reduces BAP levels by 20–80% in cell cultures. ApoE may function independently of BAP, and conformational changes in its molecular structure might contribute to neurodegeneration. Characterization of the 3-D structure of ApoE shows four helix bundles in between the amino and carboxy terminal of the molecule. ApoE-4 is the most unstable isoform in terms of protein folding; ApoE-2 folds in the most stable conformation; and ApoE-3 shows an intermediate stability. The molten globule conformation linked to greater stability is acquired most often by ApoE-4 than ApoE-3 or ApoE-2, and ApoE-4 exhibits the highest tendency among these three proteins to form molten globules whose conformational features may lead to increased degradation, alterations in cell signaling, increased binding to lipids, modifications in protein–protein interactions, increased membrane binding, changes in transport through membranes, and modified interactivity with cellular receptors.77 The structural changes in ApoE-4 seem to be related to an interaction between Arg112 and Arg61 with Glu225 that does not occur in ApoE-3 owing to the presence of a Cys residue at position 112.74 Wild-type ApoE-4 seems to be associated with higher BAP production, more extensive disruption of the cytoskeleton, and increased lysosomal cleavage.78–80 Astrocytes appear to play a critical role in the clearance of BAP in the brain following migration to areas of the brain rich in neurotoxic deposits. A receptor-specific uptake seems to mediate internalization and degradation, but defects in these steps associated with ApoE may impair clearance, thus favoring further accumulation of BAP and the appearance of neurodegenerative events. Expression of ApoE-3 in a transgenic model decreased the BAP load in a dose-dependent manner in PDAPP mice at 12–15 months of age, and expression of ApoE-4 led to increased deposition of BAP in these PDAPP/ApoE-knockout mice. ApoE-2 induced a marked decrease in BAP accumulation.78 So, it appears that ApoE polymorphic variants affect the amount of BAP deposited in the brain, and ApoE is able to reduce γ-secretase cleavage of APP, lowering BAP levels. In neuronal and non-neuronal cell lines, ApoE treatment reduced BAP40 by 60–80% and BAP42 to a lesser extent (20–30%) in the conditioned media. ApoE treatment resulted in an accumulation of APP-C-terminal fragments in cell extracts and a marked reduction of APP intracellular domain-mediated signaling, consistent with diminished γ-secretase processing of APP. All three isoforms of ApoE had similar effects on BAP and APP-C-terminal fragments, and the effects were independent of the LDL receptor family.
There has been increasing interest in a potential role for fatty acids in adversely affecting organismal substrate utilization and contributing to the cardiovascular complications in insulin resistance. Fatty acids have already been implicated in regulating the expression of a number of genes in resident cells of the vessel wall. In this regard, it has been demonstrated that oleic acid increases ApoE secretion from macrophages at a locus involving post-translational glycosylation.81
ApoE in other forms of dementia and CNS Disorders
The distribution of ApoE genotypes clearly differs among different CNS disorders, with an accumulation of the ApoE-4 allele in dementia, especially in AD and mixed-type dementia (MXD). In early-onset AD (EOAD), the ApoE-3/4 and ApoE-4/4 genotypes account for 54.35% of the cases, and the presence of the ApoE-4 allele is more frequent in women (52.42%) than in men (35.47%). In late-onset AD (LOAD), the ApoE-4 allele is present in 55.88% of the cases (55.87% in women and 55.92% in men). According to these results, the frequency of the ApoE-4 allele is similar in women with EOAD and LOAD, but significantly higher in men with LOAD as compared with EOAD men; however, women and men show an identical distribution in LOAD. Integrating both types of age-related AD phenotypes (EOAD+LOAD), the presence of the ApoE-4 alleles accounts for 51.38% of AD cases (54.38% in women and 45.43% in men).29
In pure vascular dementia (VD), secondary to severe cardiovascular and cerebrovascular disorders (e.g. stroke, atrial fibrillation, hypertension), the ApoE-4 allele is present in 37.60% of the cases, with a relative distribution similar in women (39.08%) and men (35.57%), but significantly different from the distribution pattern seen in AD. The highest accumulation of ApoE-4 carriers is observed in MXD (53.01%), with a distribution in women (58.76%) and men (45.10%) similar to that detected in AD; however, the ApoE-4/4 genotype is over-represented in both women (12.55%) and men (13.41%) with MXD. In this regard, patients with MXD exhibit the highest frequency of the ApoE-4/4 as compared with any other cluster or pathological group in the Spanish population.29,37,39
In AD patients with history of cerebrovascular disorders (excluding stroke), such as chronic cerebrovascular insufficiency, migraine, hypotension or dizziness, a high frequency of ApoE-4 was also found (44.71%), with identical distribution of the ApoE-4/4 genotype in women and men. Nevertheless, in patients with cerebrovascular disorders without cognitive impairment, the frequency of the ApoE-4 allele (24.65%) was similar to that of controls (24.76%), suggesting that the risk of developing dementia in patients with chronic cerebrovascular disorders may be associated with the presence of the ApoE-4 allele. In patients with different CNS disorders, including Parkinson's disease, schizophrenia, depression, anxiety and epilepsy, an increased frequency of the ApoE-4 allele was detected (41.13%), with a similar distribution in women and men, probably indicating that the ApoE-4 allele might represent a factor of brain vulnerability in different medical conditions. Finally, we found a low frequency of ApoE-4 (24.99%) in patients with stroke, practically the same as in controls and in patients with cerebrovascular disorders without cognitive deterioration. Surprisingly, a high frequency of ApoE-4 was also observed in patients with anxiety (39%), diabetes (40%) and hypertension (36%). The highest frequencies of the ApoE-4/4 genotype in decreasing order were identified in MXD, diabetes, VD, headache, and AD. The fact that patients with stroke and/or cerebrovascular disorders without cognitive impairment show a frequency of ApoE-4 similar to controls (20–30%) together with the evidence that patients with MXD and AD represent the population with the highest frequency of ApoE-4 (50–60%) suggests that the inheritance of ApoE-4 is an important risk factor in dementia in general, and that the presence of ApoE-4 in patients with cerebrovascular disorders and/or stroke may be determinant for these patients to develop dementia as a secondary event following cerebrovascular damage.29,37,39
Vascular dementia and cerebrovascular disorders
The frequency of the ApoE-4 allele has been found to be increased in vascular dementia (VD).82–84 In early reports it was suggested that the increased plasma cholesterol concentrations and resulting atherosclerosis associated with ApoE-4 might contribute to VD.82 Wieringa et al.85 found a higher frequency of ApoE-4 in multi-infarct dementia, but the increased prevalence of the ApoE-4 allele was not related to serum lipid levels, and they concluded that the hypothesis that the onset of multi-infarct dementia may be precipitated by ApoE-4's mediation of higher serum cholesterol levels was not supported. Some authors did not find a great difference in ApoE-4 allele frequency between AD and VD.86,87
A high frequency of the ApoE-2 allele was observed in patients with cerebral amyloid angiopathy-related hemorrhage, suggesting that patients with ApoE-2 may be protected from parenchymal AD but may be susceptible to rupture of amyloid-laden vessels.88,89 Lin et al. reported that ApoE-4 plays no significant role in the development of ischemic cerebrovascular disease and VD, but that ApoE-2 has a protective effect with regard to the development of ischemic cerebrovascular disorders and VD for Taiwanese–Chinese subjects younger than 65 years.90 Greenberg et al. also found association between ApoE-2 and vasculopathy in cerebral amyloid angiopathy, postulating that ApoE-2 and ApoE-4 might promote hemorrhage through separate mechanisms: ApoE-4 by enhancing amyloid deposition, and ApoE-2 by promoting rupture. ApoE-2 is also a risk factor for early recurrence of cerebral amyloid angiopathy.91 Others have reported that possession of ApoE-4 does not by itself confer an increased risk of cerebral amyloid angiopathy but may be associated with reduced longevity even in the absence of AD or cerebral hemorrhage.92 The ApoE-2 allele may influence the therapeutic response in some cases. For instance, there is evidence that the efficacy of i.v. tissue plasminogen activator in patients with acute ischemic stroke may be enhanced in those carrying the ApoE-2 allele.93
A high frequency of ApoE-4 has been found in Lewy body dementia94,95 where ApoE-4 carriers also showed a greater neuritic degeneration in hippocampal CA2-3 regions. The ApoE-2/3 genotype has been associated with significantly earlier age of onset of Huntington's disease.95 ApoE-2/2 has been associated with frontotemporal dementia, but the rarity of this genotype recommends being cautious in the interpretation of results.96 In Chamorros with amyotrophic lateral sclerosis/parkinsonism dementia complex, the ApoE-4 allele was not found to be associated with this form of dementia and the presence of the ApoE-3 allele did not reveal any protective effect against NFT formation in this population.97 Itabashi et al. compared the distribution of ApoE genotypes in a necropsy series of AD and other dementias (Parkinson's disease with dementia, progressive supranuclear palsy, Lewy body dementia, polyglucosan-body disease, Pick's disease, dementia+hydrocephalus, Wernicke–Korsakoff syndrome)98 and found no major differences in the distribution of the ApoE-4 allele in AD and the other dementias, suggesting that the ApoE-4 allele is not predictive of AD. An increased frequency of the ApoE-4 allele was reported in bulbar-onset motor neuron disease,99 but this could not be replicated in another study.100 No association has been found between ApoE-4 and the incidence or the age of onset of sporadic or autosomal dominant amyotrophic lateral sclerosis.101
Senile plaques in Down syndrome are particularly large in ApoE-4 carriers and less abundant than in AD, suggesting that pathology in Down syndrome is due to increased amyloid production and deposition with ApoE-4 probably increasing senile plaque initiation.102 In patients with Down syndrome, ApoE-2 was associated with increased longevity and decreased frequency of dementia.103 No ApoE-4/4 was seen in Down syndrome cases in some studies;103 in contrast, others could not find significant differences in the distribution of ApoE genotypes between AD and Down syndrome.104 In general terms, it appears that the frequency of the ApoE-4 allele in Down syndrome does not differ from that of the general population and that ApoE-2 may exert a protective effect.105
Several studies attempted to associate ApoE-4 with schizophrenia. Harrington et al. reported an increased frequency of ApoE-4 in schizophrenia,106 but subsequent studies in different populations failed to replicate this finding.107–113 However, ApoE-4 was associated with an early onset of schizophrenia,109,114 with a reduced outcome of positive symptoms,115,116 and with a worse prognosis in women,112 but these results could not be replicated by others.110,117 Some authors found that ApoE-3 might increase the risk of schizophrenia,118 but this finding could not be confirmed.119 Both early-onset schizophrenia120 and a poor response to neuroleptics were associated with ApoE-2.121 In a recent study, no differences in ApoE allele or genotype frequencies were observed in schizophrenia, although a possible association between schizophrenia in men and the ApoE-2/3 genotype was postulated.122 In patients with paraphrenia or late-onset schizophrenia, Howard et al. found comparable frequencies of the ApoE-4 allele to that found in centenarians.123
The ApoE genotype was related to the incidence of psychiatric symptomatology.124 The presence of one ApoE-4 allele conferred a 2.5-fold risk and the presence of two ApoE-4 alleles conferred a 5.6-fold risk for development of delusions; however, no association was found for depressive symptoms or behavioral disturbances in some studies;121 in contrast, others have found a small increment of psychiatric symptoms and aberrant behaviors in AD patients with ApoE-4.125
Some authors suggest that increased levels of ApoE in the frontal cortex of schizophrenics may be associated with the pathology of schizophrenia and that antipsychotic drugs decrease ApoE levels as part of their therapeutic action.126
The ApoE-4 allele was associated with significantly faster progression of disability and more extensive axonal damage in patients with multiple sclerosis,127 but some studies found that ApoE-4 and/or the -491 A/T ApoE promoter polymorphism were not associated with a more rapid course of multiple sclerosis.128 ApoE-4 was also associated with slightly earlier disease onset, but it does not constitute a risk factor for multiple sclerosis.129–132 Niino et al. found no relation between ApoE and multiple sclerosis in Japan.133
N-acetylaspartate (NAA) is exclusively present in mature neurons, and it appears decreased in multiple sclerosis, reflecting neuronal loss, axonal loss, and generalized neuronal dysfunction. Multiple sclerosis patients with ApoE-4(+) exhibit a higher degree of disability and a lower NAA : creatine ratio than patients with ApoE-4(−)(244). ApoE-4(+) carriers have more relapses and have a 5-fold higher rate of annual brain volume loss compared to ApoE-4(−) carriers. ApoE-4(+) carriers also show an increase in individual lesions on magnetic resonance imaging. In contrast, ApoE-2 carriers show the lowest annual volume loss of brain volume.127 These results by Enzinger et al. clearly demonstrate the negative influence that ApoE-4 exerts on brain volume, contributing to increasing brain atrophy in multiple sclerosis.127
It has been reported that ApoE-4 may negatively influence recovery in patients with head injury. Teasdale et al. found that ApoE-4(+) carriers were more likely to have an unfavorable outcome 6 months after injury than ApoE-4(−) carriers.134 ApoE-4(+) patients also have more difficulties with memory than matched patients without ApoE-4. The performance of ApoE-4(+) carriers is poor regardless of severity of injury, whereas performance in ApoE-4(−) carriers worsens in parallel with more severe injury.135 In patients with mild to moderate traumatic brain injury the ApoE-4 allele also affects short-term recovery.136 The frequency of the ApoE-4 allele was also found to be increased in patients with prolonged post-traumatic unawareness who did not recover consciousness. In addition, ApoE-4 was associated with BAP deposition following head injury. CSF ApoE and BAP levels decrease after traumatic brain injury, whereas CSF S100B levels increase. There is also a correlation between injury severity and the decrease in BAP after brain injury.137
The ApoE-4 allele does not function as a risk factor that influences the development of AD lesions in Parkinson's disease.138 The ApoE-4 allele frequency in Parkinson's disease patients with dementia (0.068) and in those without dementia (0.13) does not greatly differ from controls (0.102), indicating that the biological basis of dementia in Parkinson's disease may differ from that of AD (254). In general, ApoE-4 was not associated with Parkinson's disease in the Caucasian population.139 However, the age at onset of Parkinson's disease appears to be significantly earlier in ApoE-3/4 and ApoE-4/4 carriers than in patients with the ApoE-3/3 genotype.140
Initial studies did not find association between ApoE-4 and other amyloid-forming diseases, including Creutzfeldt–Jakob disease, familial amyloidotic polyneuropathy, and Down syndrome. Subsequent studies concluded that ApoE-4 might be a major susceptibility factor for Creutzfeldt–Jakob disease.141
Increased frequency of ApoE-4 has been found in patients with inclusion body myositis.142 The probability of moderate to severe sleep-disordered breathing (apnea/hypopnea) was reported to be significantly higher in ApoE-4(+) carriers, independent of age, sex, body mass index, and ethnicity.142 Patients with primary dystonia harboring the ApoE-4 genotype tend to have an earlier age at onset than ApoE-4(−) carriers.143
Copin et al. have reported that two ApoE-promoter SNP previously associated with AD also modified the primary open-angle glaucoma (POAG) genotype. ApoE(-219G) is associated with increased optic nerve damage,144 and ApoE(-491T), interacting at a highly significant level with a SNP in the myoclin gene (MYOC) promoter (MYOC-1000G), is associated with increased intra-ocular pressure and with limited effectiveness of intra-ocular pressure-lowering treatments in patients with POAG. Some studies have speculated with an increased frequency of glaucoma in AD patients; however, the studies of Copin et al.145 have been criticized by Bunce et al.,146 and Ressiniotis et al.147 reported that ApoE is not a risk factor for developing POAG, even in patients with normal tension glaucoma. Other studies also indicate that the ApoE genotype does not constitute a risk factor for developing POAG.147
Although there is no apparent association of particular ApoE genotypes with macular degeneration,148 the inheritance of specific ApoE alleles has been linked to the incidence of age-related macular degeneration (ARMD). The ApoE-4 allele appears to be protective, or at least, to delay the age at diagnosis of the disease, whereas the ApoE-2 allele appears to have a modifier effect by bringing forward the mean age of disease diagnosis.149,150 ApoE is an intrinsic component of drusen, the hallmark of ARMD. Age-related alteration of lipoprotein biosynthesis and processing at the levels of the retinal pigment epithelium, where ApoE can be locally synthesized, and/or Bruch membrane might be a significant contributing factor in drusen formation and ARMD pathogenesis.151 ApoE has also been implicated in pupil dilation, and a hypersensitive pupil dilation response to tropicamide was reported in cognitively normal individuals with the ApoE-4 allele.152 Hypersensitivity responses of the pupil to the cholinergic agonist pilocarpine and the antagonist tropicamide have also been reported in AD,153,154 but these findings could not always be replicated.155
Estrogen use was associated with less cognitive decline among 2716 women (>65 years) who did not have the ApoE-4 allele, but not among women who had at least one ApoE-4 allele,156 probably indicating that ApoE-4(+) carriers under estrogen regimens may have a higher risk of cognitive deterioration.
The ApoE-4 allele frequency is not increased in familial non-insulin-dependent diabetes mellitus (NIDDM), despite the presence of ApoE in the pancreatic islet amyloid in NIDDM.157 In China, Liu et al.158 found that: (i) the heparan sulfate proteoglycan (HSPG) T allele is a risk factor for the development of severe diabetic nephropathy in type 2 diabetic patients; (ii) the ApoE-2 allele is a risk factor for the occurrence of type 2 diabetes mellitus in the Chinese general population; and (iii) the co-inheritance of HSPG-T/ApoE-2 confers a higher risk of type 2 diabetes mellitus progression to diabetic nephropathy in Chinese.158 In the Japanese population, the ApoE-2 is a prognostic risk factor for both the onset and progression of diabetic nephropathy in type 2 diabetes.159
Herpes simplex virus type 1 (HSV1) is present in certain regions of the brain in a high proportion of elderly subjects and patients with AD. It has been reported by Itzhaki and co-workers that the combination of HSV1 in the brain, and carriage of the ApoE-4 allele, was a strong risk factor for AD.160–163 Corder et al. also showed that HIV-infected subjects with the ApoE-4 allele have excess dementia and peripheral neuropathy, postulating that long-term survivors of HIV infection with ApoE-4 may be at high risk for dementia and that gene–viral interaction may speed AD pathogenesis.164 Tursen et al. recently reported that the presence of ApoE-2/3, high-density lipoprotein (HDL)-cholesterol levels and the absence of the ApoE-3/3 genotype can be regarded as risk factors for superficial fungal disease, especially dermatophytosis.165
Cardiopulmonary bypass induces a rise in cytokine release by activated monocytes. ApoE-4 and TNFB polymorphisms (TNFB-A329G) are risk factors for atherosclerosis. The presence of TNFB*A329G and ApoE-4 is associated with significantly higher releases of IL8 and TNFA, prolonged intubation, and increased transfusion in patients undergoing coronary artery bypass grafting, relative to patients without genetic variants.166
The ApoE-2 allele seems to be associated with the lowest reproductive efficiency and the ApoE-3 with the highest. The different total cholesterol levels associated with ApoE genotypes could have an effect on steroidogenesis and as a consequence determine the observed differential fertility.167
Physical activity improves lipid levels by altering triglyceride metabolism, and ApoE facilitates triglyceride clearance by mediating lipoprotein binding to hepatic receptors. Thompson et al. studied the influence of ApoE variants on lipid and physiological response to exercise training in the USA.168 This prospective study demonstrates that the serum lipid response to exercise training differs by ApoE genotype in a pattern consistent with known metabolic differences among the variants. TG were slightly higher in ApoE-2/3, whereas LDL-cholesterol was lower. TG decreased by 11% with training for the entire cohort, and 7%, 12%, and 14% for ApoE-2/3, ApoE-3/3 and ApoE-3/4, respectively. LDL-cholesterol did not change in the cohort, but decreased slightly in ApoE-2/3 and ApoE-3/3 subjects, and increased 4% in the ApoE-3/4 group. Total cholesterol/HDL and LDL/HDL decreased with training in ApoE-2/3 and ApoE-3/3, but increased in ApoE-3/4. The ApoE genotype also affected the increase in aerobic capacity produced by exercise training possibly via undefined effects on nerve and skeletal muscle function.168 In another study, no association was found between ApoE and maximal oxygen uptake levels either in the sedentary state or in response to exercise training.169 In summary, all these studies globally indicate that ApoE-related polymorphic variants, especially the ApoE-4 allele, represent a biological disadvantage for brain function and lipid metabolism.
Different ApoE genotypes confer specific phenotypic profiles to AD patients. Some of these profiles may add risk or benefit when the patients are treated with conventional drugs, and in many instances the clinical phenotype demands the administration of additional drugs, which increase the complexity of therapeutic protocols. From studies designed to define ApoE-related AD phenotypes,29,37,72,170–175 several confirmed conclusions can be drawn: (i) the age-at-onset is 5–10 years earlier in approximately 80% of AD cases harboring the ApoE-4/4 genotype; (ii) the serum levels of ApoE are lowest in ApoE-4/4, intermediate in ApoE-3/3 and ApoE-3/4, and highest in ApoE-2/3 and ApoE-2/4; (iii) serum cholesterol levels are higher in ApoE-4/4 than in the other genotypes; (iv) HDL-cholesterol levels tend to be lower in ApoE-3 homozygotes than in ApoE-4 allele carriers; (v) LDL-cholesterol levels are systematically higher in ApoE-4/4 than in any other genotype; (vi) triglyceride levels are significantly lower in ApoE-4/4; (vii) nitric oxide levels are slightly lower in ApoE-4/4; (viii) serum Aβ levels do not differ between ApoE-4/4 and the other most frequent genotypes (ApoE-3/3, ApoE-3/4); (ix) blood histamine levels are dramatically reduced in ApoE-4/4 as compared with the other genotypes; (x) brain atrophy is markedly increased in ApoE-4/4>ApoE-3/4>ApoE-3/3; (xi) brain mapping activity shows a significant increase in slow wave activity in ApoE-4/4 from early stages of the disease; (xii) brain hemodynamics, as reflected by reduced brain blood flow velocity and increased pulsatility and resistance indices, is significantly worse in ApoE-4/4 (and in ApoE-4 carriers, in general, as compared with ApoE-3 carriers); (xiii) lymphocyte apoptosis is markedly enhanced in ApoE-4 carriers; (xiv) cognitive deterioration is faster in ApoE-4/4 patients than in carriers of any other ApoE genotype; (xv) occasionally, in approximately 3–8% of the AD cases, the presence of some dementia-related metabolic dysfunctions (e.g. iron, folic acid, vitamin B12 deficiencies) accumulate more in ApoE-4 carriers than in ApoE-3 carriers; (xvi) some behavioral disturbances (bizarre behaviors, psychotic symptoms), alterations in circadian rhythm patterns (e.g. sleep disorders), and mood disorders (anxiety, depression) are slightly more frequent in ApoE-4 carriers; (xvii) aortic and systemic atherosclerosis is also more frequent in ApoE-4 carriers; (xviii) liver metabolism and transaminase activity also differ in ApoE-4/4 with respect to other genotypes; (xix) blood pressure (hypertension) and other cardiovascular risk factors also accumulate in ApoE-4; and (xx) ApoE-4/4 carriers are the poorest responders to conventional drugs. These 20 major phenotypic features clearly illustrate the biological disadvantage of ApoE-4 homozygotes and the potential consequences that these patients may experience when they receive pharmacological treatment.170–176
AD is a multifactorial and complex disorder in which over 150 different genes distributed across the human genome may be involved. Among AD-causing genes, APP, PS1, and PS2 mutations in part explain AD pathogenesis, however Mendelian mutations in those three genes only account for less than 10% of AD cases, indicating that many other networking mechanisms must be involved in neurodegeneration and premature neuronal death in AD. ApoE-related polymorphic variants (ApoE-4 allele) represent the most significant susceptibility genetic defect in AD, contributing to neuronal dysfunction in approximately 30–40% of AD cases. The precise mechanism by which ApoE affects neurodegeneration is still unclear. ApoE-4 is a genetic risk factor of cognitive impairment in many neurodegenerative disorders, including AD and other types of dementia.