Association of folate with hearing is dependent on the 5,10-methylenetetrahdyrofolate reductase 677C-->T mutation

Vascular disease and its risk factors have been associated with the age-related hearing loss. We examined the association of elevated plasma homocysteine and its determinants with hearing levels. Pure-tone air conduction thresholds in 728 individuals with sensorineural hearing loss were not associated with homocysteine, erythrocyte folate and Vitamin B6. Low concentrations of serum folate and Vitamin B12 were associated with better hearing. When folate status was below the median, 5,10-methylenetetrahydrofolate reductase (MTHFR) 677TT homozygotes had similar hearing levels to subjects with a C allele. However, when folate status was above the median, MTHFR 677TT homozygotes had on an average 5 dB (p = 0.037) and 2.6 dB (p = 0.021) lower PTA-high and PTA-low hearing thresholds, respectively, than the subjects with a 677C allele. The relationship between serum folate and hearing thresholds appeared to be dependent on MTHFR 677 genotype (CC, r = 0.13, p = 0.034; TT, r = -0.10, p = 0.291). This supports the hypothesis that a greater one-carbon moiety commitment to de novo synthesis of nucleotides and an increase in formyl-folate derivatives relative to methyl-folate derivatives is protective for hearing.     

The effects of APOE-ε4 on the BOLD response

In the last decade, functional magnetic resonance imaging (fMRI) has emerged as a tool to study changes in brain function associated with a genetic risk for Alzheimer's disease (AD), with a particular focus on the effects of the APOE-ε4 allele. This review compiles the existing literature concerning the effects of APOE genotype on the blood oxygen level dependent (BOLD) response, measured during task-based functional magnetic resonance imaging. While most studies report a significant difference in brain activity between carriers and noncarriers of the ε4 allele, there are inconsistencies in the direction and location of change. These inconsistencies were addressed by examining the effect of task, family history of Alzheimer's disease, and age on the relationship between APOE genotype and the BOLD response, but no clear pattern emerged. The review discusses the interpretation of BOLD differences between ε4 carriers and noncarriers, provides suggestions for future studies, and highlights important limitations of this type of research.     

Memory performance and the apolipoprotein E polymorphism in a community sample of middle-aged adults

The apolipoprotein E genotype (APOE) is an established risk factor for Alzheimer disease, with the age-at-onset occurring earlier in individuals having at least one APOE epsilon 4 allele, relative to the APOE epsilon 3 or APOE epsilon 2 isoforms. Moreover, nondemented older adults with the APOE epsilon 4 allele also show diminished cognitive performance, particularly on tests of learning and memory, and an accelerated decline in memory performance with increasing age. The current investigation extends the study of the APOE epsilon 4 allele and cognitive performance to healthy, middle-aged adults. A community sample of 220 non-Hispanic Caucasian men and women, aged 24-60 (average age = 46), were genotyped for the APOE polymorphism and completed a battery of neuropsychological tests. Multivariate analyses were conducted on measures of verbal learning and memory (e. g., learning a list of words and recalling them 30 min later), visual memory (e.g., reproducing a previously copied figure from memory), and attention span (e.g., repeating long lists of digits), after adjustments for age, and estimated IQ. Results indicated that performance on learning and memory tasks was significantly poorer in adults having any APOE epsilon 4 allele, relative to adults with APOE epsilon 2 and epsilon 3 genotypes (P <.01). Attention span did not differ by genotype. These findings, the first in a sample of middle-aged adults, suggest that the APOE polymorphism is a marker for age-related decline in memory (detectable prior to overt, clinical manifestations of memory loss), and/or a marker for individual differences in memory ability across the life span. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:707-711, 2000.     

Depression, APOE genotype and subjective memory impairment: a cross-sectional study in an African-Caribbean population

BACKGROUND: Subjective memory impairment (SMI) is common in older populations but its aetiology and clinical significance is uncertain. Depression has been reported to be strongly associated with SMI. Associations with objective cognitive impairment are less clear cut. Other factors suggested to be associated with SMI include poor physical health and the apolipoprotein E (APOE) epsilon4 allele. Studies of SMI have been predominantly confined to white Caucasian populations. METHOD: A community study was carried out in a UK African-Caribbean population aged 55-75, sampled from primary care lists. Twenty-three per cent were classified with SMI. Depression was defined using the 10-item Geriatric Depression Scale. Other aetiological factors investigated were education, objective cognitive function, APOE genotype, disablement and vascular disease/risk. The principal analysis was restricted to 243 participants scoring > 20 on the Mini-Mental State Examination (85%). A second analysis included all 290 participants. RESULTS: Depression, self-reported physical impairment and APOE epsilon4 were associated with SMI. The association between SMI and physical impairment was not explained by depression, vascular disease/risk, or disability/handicap. The association between epsilon4 and SMI increased as MMSE scores decreased and was particularly strong in those with depression. The epsilon4 allele was present in 69% (95% CI 41-89%) of those with depression and SMI compared with 28% (20-36%) of those with neither. CONCLUSIONS: Depression may not be a sufficient explanation for subjective memory complaints. Memory complaints in the presence of depression are associated with high prevalence of epsilon4 and therefore, presumably, a raised risk of subsequent dementia.     

Apolipoprotein E polymorphisms and spontaneous pregnancy loss in patients with endometriosis

Endometriosis affects >10% of women during their reproductive years, many of whom report high rates of spontaneous pregnancy loss (SPL). We examined whether gene polymorphisms in apolipoprotein E (APOE), which is involved in lipoprotein metabolism, are associated with endometriosis and/or endometriosis-associated infertility. We conducted a cross-sectional genetic association study of women surgically confirmed to have endometriosis (n = 345) and no surgical evidence of the disease (n = 266). Genotyping of APOE polymorphism (ε2, ε3, ε4) was conducted by polymerase chain reaction-restriction fragment length polymorphism followed by visualization of specific patterns by gel electrophoresis. Statistical significance of differences in genotype and allelic frequencies was assessed using Pearson's χ(2) test and Risk analysis. Overall, we found no association between APOE genotype and diagnosis of endometriosis. However, patients with endometriosis who reported at least one SPL were three times more likely to be ε2 carriers and 2-fold less likely to be ε4 carriers. Compared with ε3 carriers, patients with endometriosis who were ε2 carriers and had at least one live birth reported four times the rate of SPL, while ε4 carriers were <0.4-fold less likely to report an SPL. Our data suggest that there may be an association between APOE allelic frequency and SPL in patients with endometriosis, which appears to be independent of mechanisms associated with infertility, an intriguing observation that deserves further investigation.     

Salivary cortisol, APOE-ε4 allele and cognitive decline in a prospective study of older persons

Objective

We determined whether salivary cortisol levels were associated with cognitive decline at follow-up in older persons and whether this association was modified by the APOE-ε4 allele.

Methods

Within the Longitudinal Aging Study Amsterdam (LASA), a population-based prospective cohort study, 911 persons (74.5 ± 7.2 years, 46.4% male) collected salivary cortisol in the morning and late in the evening. At baseline and after 4 years of follow-up, global cognitive functioning, verbal memory performance, and processing speed were assessed. The longitudinal associations between cortisol measures and cognitive decline were estimated using linear mixed models, adjusted for potential confounders and the modifying role of the APOE-ε4 allele was examined.

Results

Lower morning cortisol levels, higher evening cortisol levels, and flattened diurnal variability of cortisol levels were associated with increased risk for memory decline in APOE-ε4 carriers but not in non-carriers.

Conclusion

Our findings suggest that in this older non-demented population APOE-ε4 carriers may be more vulnerable to the potential detrimental effect of hypothalamic-pituitary-adrenal axis dysfunction on verbal memory performance.     

Apolipoprotein E polymorphism and its relation to plasma lipids in coronary heart disease

Background : The present investigation is aimed at examining the Apolipoprotein E (APOE) genotypic influence on coronary heart disease (CHD) risk in northwest India (Punjab), where this disease is emerging as a major threat to public-health care system. Materials and Methods: The present study comprised of angiographically diagnosed coronary heart disease patients (n = 193) and controls (n = 150) of Punjab. Genetic polymorphism of APOE gene was investigated by polymerase chain reaction (PCR), and its association with lipid levels was evaluated. Results : The allele frequencies of epsilon2, epsilon3, and epsilon4 were 0.054, 0.795, 0.151; and 0.077, 0.856, 0.067 in patients and controls respectively. The bearers of E3/E4 genotype had threefold higher propensity of developing CHD in this population (OR, 3.04; CI, 1.55-6.25; P P P Conclusions : A significant association (P = 0.016) of epsilon4 allele, especially E3/E4 genotype, with CHD was observed, along with HDL-C and LDL-C concentrations, in the population of northwest India.     

Accelerated age-related cortical thinning in healthy carriers of apolipoprotein E epsilon 4

Effects of APOE genotype on age-related slopes of cortical thinning was estimated by measuring the thickness of the cerebral cortex on a point-by-point basis across the cortical mantle in 96 healthy non-demented volunteers aged 48-75 years. Fifty nine were APOE epsilon 4- (no epsilon 4 allele) and 37 were epsilon 4+ (1 or 2 epsilon 4 alleles). The genotype groups had similar age, sex and IQ. Two T(1)-weighted MP-RAGE sequences were averaged for each participant to yield images with high signal-to-noise ratio, and quantified using semi-automated analysis tools. epsilon 4 carriers had thicker cortex than non-carriers in several frontal and temporal areas in both hemispheres, but showed a steeper age-related decline in adjacent areas. Upon comparison of the epsilon 4-specific age-related thinning with previously published patterns of thinning in normal aging and Alzheimer's disease (AD), we conclude that APOE epsilon 4 may function to accelerate thinning in areas found to decline in aging (medial prefrontal and pericentral cortex), but also to initiate thinning in areas associated with AD and amyloid-beta aggregation (occipitotemporal and basal temporal cortex).     

Evolution of human apolipoprotein E (APOE) isoforms: Gene structure,protein function and interaction with dietary factors

Apolipoprotein E (APOE) is a member of the vertebrate protein family of exchangeable apolipoproteins that is characterized by amphipathic α-helices encoded by multiple nucleotide tandem repeats. Its equivalent in flying insects − apolipophorin-III − shares structural and functional commonalities with APOE, suggesting the possibility of an evolutionary relationship between the proteins. In contrast to all other known species, human APOE is functionally polymorphic and possesses three major allelic variants (ε4, ε3 and ε2). The present review examines the current knowledge on APOE gene structure, phylogeny and APOE protein topology as well as its human isoforms. The ε4 allele is associated with an increased age-related disease risk but is also the ancestral form. Despite increased mortality in the elderly, ε4 has not become extinct and is the second-most common allele worldwide after ε3. APOE ε4, moreover, shows a non-random geographical distribution, and similarly, the ε2 allele is not homogenously distributed among ethnic populations. This likely suggests the existence of selective forces that are driving the evolution of human APOE isoforms, which may include differential interactions with dietary factors. To that effect, micronutrients such as vitamin D and carotenoids or dietary macronutrient composition are elucidated with respect to APOE evolution.     

The frequencies of apolipoprotein E genotypes in health and disease in the Croatian population--an overview of expectations and real results

The aim of the overview is to show the distribution of common apolipoprotein E (APOE) genotypes in the Croatian population, and to test whether it could serve as a new molecular biomarker in some clinical entities. The study included the following groups: patients with angiographically confirmed coronary artery disease, myocardial infarction, Alzheimer's dementia, vascular dementia, hyperlipidemias, diabetes mellitus, pancreatitis, and healthy subjects. Group comparisons of different clinical entities and control group were performed using Pearson's Chi2-test. There was no difference in APOE genotype frequencies between coronary artery disease neither myocardial infarction and control group. The ApoE genotype frequencies in patients with Alzheimer's disease were significantly different from those in the control group. APOE-4 allele tends to be a risk factor for the development of Alzheimer's disease. The frequencies were only marginally different in vascular dementia. Patients with hypercholesterolemia, those with inherited familial hypercholesterolemia, children with diabetes mellitus, and patients with pancreatitis of different etiology showed distributions of APOE genotypes that differed from the control group. It is concluded that the frequencies of APOE genotypes yielded no statistically significant result to confirm the association between APOE genotypes and any specific disease with the exception of Alzheimer's disease; APO-epsilon4 allell has become one of the important biomarkers in diagnosis of Alzheimer's dementia.     

Auditory rehabilitation of older people from the general population--the Leiden 85-plus study.

BACKGROUND: Very few older people with severe hearing loss use hearing aids to reduce the negative consequences of reduced hearing in daily functioning. AIM: Assessment of a screening test and a standardised auditory rehabilitation programme for older people from the general population with untreated severe hearing loss. DESIGN OF STUDY: Intervention study and qualitative exploration. SETTING: Leiden 85-Plus Study, a prospective population-based study of 85-year-old inhabitants of Leiden, the Netherlands. METHOD: Hearing loss was measured by pure-tone audiometry in 454 subjects aged 85 years. Subjects with hearing loss above 35 dB at 1, 2, and 4 kHz who did not use hearing aids were invited to participate in a standardised programme for auditory rehabilitation. In-depth interviews were held with participants to explore arguments for participating in this programme. RESULTS: Of the 367 participants with severe hearing loss (prevalence = 81%), 66% (241/367) did not use a hearing aid. Three out of four of these participants (n = 185) declined participation in the auditory rehabilitation programme. The most common reason given for not participating was the subjects' feeling that their current hearing loss did not warrant the use of a hearing aid. Subjects who participated in the programme were found to suffer from more severe hearing loss and experienced more hearing disability. Those who did not participate in the programme felt they could cope with their disabilities and considered a hearing aid unnecessary. CONCLUSION: Untreated hearing loss is prevalent among older people from the general population. The majority of older people decline auditory rehabilitation. For these people the use of a hearing aid is not perceived as necessary in order to function on a daily basis. Older people who have expected benefits from a hearing aid have already obtained them, marginalising the benefits of a rehabilitation (and screening) programme.     

Reduced plasticity and mild cognitive impairment-like deficits after entorhinal lesions in hAPP/APOE4 mice

Mild cognitive impairment (MCI) is a clinical condition that often precedes Alzheimer disease (AD). Compared with apolipoprotein E-ε3 (APOE3), the apolipoprotein E-ε4 (APOE4) allele is associated with an increased risk of developing MCI and spatial navigation impairments. In MCI, the entorhinal cortex (EC), which is the main innervation source of the dentate gyrus, displays partial neuronal loss. We show that bilateral partial EC lesions lead to marked spatial memory deficits and reduced synaptic density in the dentate gyrus of APOE4 mice compared with APOE3 mice. Genotype and lesion status did not affect the performance in non-navigational tasks. Thus, partial EC lesions in APOE4 mice were sufficient to induce severe spatial memory impairments and synaptic loss in the dentate gyrus. In addition, lesioned APOE4 mice showed no evidence of reactional increase in cholinergic terminals density as opposed to APOE3 mice, suggesting that APOE4 interferes with the ability of the cholinergic system to respond to EC input loss. These findings provide a possible mechanism underlying the aggravating effect of APOE4 on the cognitive outcome of MCI patients.     

P3 from auditory stimuli in healthy elderly subjects: hearing threshold and tone stimulus frequency.

The P3 event-related potential (ERP) was elicited by auditory stimuli under two conditions: (1) 250 Hz standard, 500 Hz target; and, (2) 1000 Hz standard, 2000 Hz target. A group with normal hearing (n = 10), defined as subjects with less than a 16 dB loss at each tone frequency was compared to a group with hearing impairment (n = 8), defined as subjects with a loss of 16 dB or more at each tone frequency. The hearing impaired group showed significant P3 latency prolongations compared to those with normal hearing under the 2000 Hz, but not the 500 Hz target, even though subject age was used as a covariate. Under both conditions, hearing impaired subjects showed reduced P3 amplitudes compared to those with normal hearing. These findings are discussed in relation to the sensitivity of ERP assessment procedures in older adults.     

Longitudinal change in cognitive performance among individuals with mild cognitive impairment

The authors used mixed-effects growth models to examine longitudinal change in neuropsychological performance over a 4-year period among 197 individuals who were either normal or had mild cognitive impairment (MCI) at baseline. At follow-up, the participants were divided into 4 groups: (a) controls: participants who were normal at both baseline and follow-up (n = 33), (b) stables: participants with MCI whose Clinical Dementia Rating-Sum of Boxes (CDR-SB) score did not differ between the first and last evaluations (n = 22), (c) decliners: participants with MCI whose CDR-SB score declined between the first and last evaluations (n = 95), and (d) converters: participants who received a clinical diagnosis of Alzheimer's disease during the follow-up period (n = 47). Only the Episodic Memory factor showed a significantly greater rate of decline over the follow-up period among the converters. Two other factors were significantly lower in converters at baseline in comparison with other groups (the executive function factor and the general knowledge factor), but the rate of decline over time did not differ. Individuals with an APOE epsilon4 allele scored lower on the episodic memory and executive function factors at baseline.     

Apolipoprotein E, cardiovascular disease and cognitive function in aging women

The apolipoprotein E (APOE) e4 allele increases risk of Alzheimer disease and cardiovascular diseases. We examined APOE genotypes alone or combined with cardiovascular conditions in relation to cognitive function in 4227 Nurses' Health Study participants, 70-80 years old. From 1995 to 2001, and again 2 years later, participants received telephone cognitive assessments of general cognition, category fluency, verbal memory and working memory. In biennial questionnaires since 1976, participants have provided extensive health information including cardiovascular conditions. Compared with women with the e3/3 genotype, e4 carriers performed worse at baseline across all tests (mean global composite score was lower by 0.10 points (95% confidence interval=-0.15, -0.05)) and declined more (mean change in global score was -0.07 points (95% CI=-0.12, -0.03)), with a strong allele dose-response trend (P-trend=0.0003). Among participants 75+ years, e2 carriers performed best. Women with an e4 allele and cardiovascular conditions such as transient ischemic attack or untreated hypertension had the worst cognition. Thus, APOE genotypes strongly influenced cognitive function and decline; prevention of cardiovascular disease may limit these effects.     

Apolipoprotein E and α-1-antichymotrypsin allele polymorphism in sporadic and familial Alzheimer's disease

Alzheimer disease (AD) patients with both sporadic and familial forms of AD and non-demented controls were genotyped for common polymorphisms in the signal peptide for α-1-antichymotrypsin (ACT) gene and in two different regions of apolipoprotein E (APOE) gene. The ACT TT genotype was over-represented (P=0.025) in patients with early onset of sporadic AD. In this patient's group ACT TT genotype conferred a significant crude odds ratio for the disease (OR=2.09; 95% CI=1.09–4.00, P=0.025). After adjustment for the APOE ε4 and APOE −491 genotypes, logistic regression analysis confirmed that the ACT TT genotype resulted independently associated with early onset AD (adjusted OR=2.56; 85% C.I.=1.3–5.2, P=0.009). The frequency of APOE ε4 allele was increased in AD, as expected (OR=5.92, 95% CI=3.60–9.70, P=0.0001). On the contrary, the APOE −491 A/T genotypes were not associated with AD. No preferential association of the APOE ε4 allele or APOE −491 A/T genotypes with ACT A/T alleles was observed in AD. Present findings indicated that subjects with ACT TT genotype had an increased risk of developing AD and suggested that this genotype influenced the risk of an early onset of the disease by affecting the production of ACT molecules.     

Association of RFC1 A80G and MTHFR C677T polymorphisms with Alzheimer's disease

We examined polymorphisms in reduced folate carrier gene (RFC1) and methylenetetrahydrofolate reductase gene (MTHFR) for association with sporadic AD (SAD) in Chinese population. Significant associations of RFC1 A80G G allele and GG genotype with SAD (p=0.008, OR=1.312, 95%CI=1.072-1.605, and p=0.042, OR=1.383, 95%CI=1.012-1.890) were found. Further stratification of total samples by APOE epsilon4 carrier status, age/age at onset and gender revealed that RFC1 A80G G allele was an APOE epsilon4-independent risk factor for late-onset AD, and it might increase the risk of AD in females. No significant associations of MTHFR C677T allele and genotype with AD were observed in total samples, but significant associations of T allele and TT genotype with AD (p=0.031, OR=1.586, 95%CI=1.042-2.414, and p=0.028, OR=2.250, 95%CI=1.074-4.712) were identified in APOE epsilon4 carrier subgroup, suggesting that MTHFR 677 T allele and APOE epsilon4 allele may synergistically act to increase AD risk. No significant effect of RFC1 G80A and MTHFR C677T polymorphisms on plasma folate and homocysteine levels was detected.     

Mortality and apolipoprotein E in African-American,and White elders: an attempted replication

We have tried, with only partial success, to confirm findings in a recently reported study in this journal on the relationship of APOE genotype to mortality in community representative Hispanics (n = 659), Whites (n = 272), and African-Americans (n = 450), aged 65 and over, living in Northern Manhattan, New York. That study found that using proportional hazards models adjusted for sex and lipid levels, Hispanics and Whites with the E2/E3 genotype, but not African-Americans, had the lowest mortality risk. Those under age 75 had risks comparable to those over age 75, suggesting minimal survivor bias. Nearly 50% of the mortality risk associated with the APOE genotype appeared to act through heart disease, diabetes, and stroke. The current study of African-Americans (n = 1,083) and Whites (n = 915) aged 71 and over living in the more rural Southeastern US, found no protective effect of the E2/E3 genotype for either African-Americans or Whites. Among younger Whites (age 71-75), point estimates suggested that the E2/E3 genotype might be protective, but at a nonsignificant level; self-reported African-American race, but not genotype, was a risk factor for mortality in this age group. Neither lipid level nor health condition attenuated the effect of APOE genotype. Differences in findings may reflect issues of sampling, age, the relative distribution of the APOE alleles, or some other factor. Until such time as studies use truly representative samples and include younger ages, findings in this area must be treated with caution.     

Type 2 diabetes mellitus and its renal complications in relation to apolipoprotein E gene polymorphism

The apolipoprotein E (APOE) epsilon2 allele is reported to be associated with greater risk of renal impairment in type 2 diabetes. Relationships among APOE polymorphisms, renal impairment, and biochemical parameters were explored. A prospective study of 405 consenting Chinese type 2 diabetic patients [mean age +/- standard deviation (SD): 59.2 +/- 10.3 years] without advanced complications at entry was conducted. APOE genotyping and measurement of plasma biomarkers of oxidative stress and antioxidants were performed at entry. HbA1C, plasma glucose, lipids, creatinine, urine albumin/creatinine, and blood pressure were measured at entry and at up to 4 years of follow-up. APOE allelic frequencies were in Hardy-Weinberg equilibrium. Odds ratios of albuminuria at entry and/or during follow-up for different APOE groups were not significantly different. The non-epsilon2 (epsilon3/3, epsilon3/4, epsilon4/4) group had significantly greater plasma ascorbate (51.6 +/- 20.1 mumol/L) than the epsilon2 (epsilon2/2, epsilon2/3) group (44.5 +/- 16.2 mumol/L, P = 0.021), but higher plasma ascorbate levels did not seem to decrease the risk of renal impairment in the non-epsilon2 group. Baseline plasma lipid-standardized alpha-tocopherol levels were least in epsilon2 subjects with persistent albuminuria (3.6 +/- 1.1 mumol/mmol of total cholesterol plus triglycerides, P = 0.008) compared with epsilon2 subjects who had no albuminuria at entry or during follow-up (4.5 +/- 0.8 mumol/mmol of total cholesterol plus triglycerides). The APOE epsilon2 allele does not seem to be associated with increased risk of renal impairment in Chinese type 2 diabetic patients. Plasma lipid-standardized alpha-tocopherol may play a role in determining risk of renal dysfunction in type 2 diabetes.     

Apolipoprotein E genotype influences spatial distribution of cerebral microbleeds

In cerebral amyloid angiopathy patients, microbleeds often cluster, mostly occipital, and are associated with apolipoprotein E (APOE) genotype. Microbleeds also frequently occur in the asymptomatic, general population. In this population, we investigated spatial distribution of microbleeds and whether this is influenced by APOE genotype. In 292 persons with microbleeds, we labeled microbleeds on baseline and follow-up magnetic resonance images. We calculated distance between incident and prevalent microbleeds within and between persons and performed lobar segmentation on the magnetic resonance images. Subsequently, we investigated proximity and lobar distribution in strata of APOE genotype. Microbleeds occurred closer within persons than between persons (−42.2 mm, 95% confidence interval, −44.6 to −39.9; p < 0.001). Microbleeds within APOE ε2 and ε4 carriers occurred closer than those in persons with ε3ε3 genotype (−11.9 mm, 95% confidence interval, −24.4 to 0.6; p = 0.06). Persons with ε2 and ε4 alleles had a larger proportion of microbleeds in the occipital lobe than persons with ε3ε3 genotype. Similar to cerebral amyloid angiopathy patients, microbleeds in the general population cluster and the distribution is affected by APOE genotype.