Mediation of the APOE associations with Alzheimer's and coronary heart diseases through body mass index and lipids

Causal mediation analysis of the neuroprotection of APOE2 through lipid pathways

Background

Recent studies have revealed a strong association between the e2 allele of the Apolipoprotein E (APOE2) gene and lipid metabolites. In addition, APOE2 carriers appear to be protected from cognitive decline and Alzheimer's disease. This correlation supports the hypothesis that lipids may mediate the protective effect of APOE2 on cognitive function, thereby providing potential targets for therapeutic intervention.

Methods

We conducted a causal mediation analysis to estimate both the direct effect of APOE2 and its indirect effect through 19 lipid species on cognitive function, using metrics from the digital Clock Drawing Test (CDT) in 1291 Long Life Family Study (LLFS) participants. The CDT metrics included think-time, ink-time, and their sum as total-time to complete the test.

Results

Compared to carriers of the common APOE3, APOE2 carriers completed the CDT significantly faster. Two lipids showed protective mediation when elevated in the blood, resulting in shorter CDT think-time (CE 18:3), ink-time (TG 56:5), and total completion time (CE 18:3 and TG 56:5). Elevated TG 56:4, in contrast, showed deleterious mediation resulting in increased ink-time. The combined indirect effect through all lipids significantly mediated 23.1% of the total effect of APOE2 on total-time, reducing it by 0.92s (95% CI: 0.17, 2.00). Additionally, the sum of total indirect effect from all lipids also mediated 27.3% of the total effect on think-time, reducing it by 0.75s, and 13.6% of the total effect on ink-time, reducing it by 0.17s, though these reductions were statistically insignificant. Sensitivity analysis yielded consistent results of the combined indirect effects and total effects and identified additional significant lipid pathways (CE 22:6, TG 51:3, and TG 54:2).

Conclusions

We found that the combined indirect effect through all lipids could mediate 10%-27% of the total direct effect of APOE2 on CDT times. We identified both protective and deleterious lipids, providing insights for new therapeutics targeting those lipids to modulate the protective effects of APOE2 on cognition.

SR-B1-/-ApoE-R61h/h Mice Mimic Human Coronary Heart Disease

Cardiovascular diseases are the leading cause of death in the modern world. Atherosclerosis underlies the majority of these pathologies and may result in sudden life-threatening events such as myocardial infarction or stroke. Current concepts consider a rupture (resp. erosion) of "unstable/vulnerable" atherosclerotic plaques as a primary cause leading to thrombus formation and subsequent occlusion of the artery lumen finally triggering an acute clinical event. We and others described SR-B1-/-ApoE-R61h/h mice mimicking clinical coronary heart disease in all major aspects: from coronary atherosclerosis through vulnerable plaque ruptures leading to thrombus formation/coronary artery occlusion, finally resulting in myocardial infarction/ischemia. SR-B1-/-ApoE-R61h/h mouse provides a valuable model to study vulnerable/occlusive plaques, to evaluate bioactive compounds as well as new anti-inflammatory and "anti-rupture" drugs, and to test new technologies in experimental cardiovascular medicine. This review summarizes and discuss our knowledge about SR-B1-/-ApoE-R61h/h mouse model based on recent publications and experimental observations from the lab.

Pleiotropic Associations with Alzheimer's Disease and Physical Activity: Sex Differences and the Effects of Environment

Physical activity (PA) is a modifiable factor in mitigating/preventing Alzheimer's disease (AD). It is crucial to identify the conditions under which PA's effects on AD risk would be beneficial. This study aims to gain insights into pleiotropic predisposition to AD and PA within and across sexes and environmental effects. We performed a genome-wide association study (GWAS) of pleiotropic AD-PA associations in individuals (65 years and older) of European ancestry in a US sample (14,628 individuals), for men and women separately and combined, and contrasted them with the UK biobank (204,789 individuals) to elucidate the effects of the environment. Fisher's method and Wald's test were used for estimating the significance of pleiotropic associations and differences between the samples. We identified genetic markers in 60 loci with significant pleiotropic associations. Of them, 91.7% of loci exhibited antagonistic relationships characterized by a misalignment of the signs of the associations of the same alleles with AD and PA and a correlation between these phenotypes. Only 16.7% of associations were replicated in the UKB. Phosphorylation and the regulation of transcription were identified as more pronounced biological mechanisms of AD-PA pleiotropy in females and males, respectively. Our results demonstrate the intrinsic heterogeneity of AD-PA pleiotropy and suggest that PA should be used as an intervention against AD with caution, after identifying groups of individuals and combinations of gene-environment interactions with beneficial effects.

How are APOE4, changes in body weight, and longevity related? Insights from a causal mediation analysis

The ε4 allele of the APOE gene (APOE4) is known for its negative association with human longevity; however, the mechanism is unclear. APOE4 is also linked to changes in body weight, and the latter changes were associated with survival in some studies. Here, we explore the role of aging changes in weight in the connection between APOE4 and longevity using the causal mediation analysis (CMA) approach to uncover the mechanisms of genetic associations. Using the Health and Retirement Study (HRS) data, we tested a hypothesis of whether the association of APOE4 with reduced survival to age 85+ is mediated by key characteristics of age trajectories of weight, such as the age at reaching peak values and the slope of the decline in weight afterward. Mediation effects were evaluated by the total effect (TE), natural indirect effect, and percentage mediated. The controlled direct effect and natural direct effect are also reported. The CMA results suggest that APOE4 carriers have 19%-22% (TE p = 0.020-0.039) lower chances of surviving to age 85 and beyond, in part, because they reach peak values of weight at younger ages, and their weight declines faster afterward compared to non-carriers. This finding is in line with the idea that the detrimental effect of APOE4 on longevity is, in part, related to the accelerated physical aging of ε4 carriers.

Association between APOE-ε4 allele and cognitive function is mediated by Alzheimer's disease pathology: a population-based autopsy study in an admixed sample

BACKGROUND

Apolipoprotein E ε4 allele (APOE-ε4) is the main genetic risk factor for late-onset Alzheimer's disease (AD) and may impact cognitive function also via other neuropathological lesions. However, there is limited evidence available from diverse populations, as APOE associations with dementia seem to differ by race. Therefore, we aimed to evaluate the pathways linking APOE-ε4 to cognitive abilities through AD and non-AD neuropathology in an autopsy study with an admixed sample.

METHODS

Neuropathological lesions were evaluated following international criteria using immunohistochemistry. Participants were classified into APOE-ε4 carriers (at least one ε4 allele) and non-carriers. Cognitive abilities were evaluated by the Clinical Dementia Rating Scale sum of boxes. Mediation analyses were conducted to assess the indirect association of APOE-ε4 with cognition through AD-pathology, lacunar infarcts, hyaline arteriosclerosis, cerebral amyloid angiopathy (CAA), Lewy body disease (LBD), and TAR DNA-binding protein 43 (TDP-43).

RESULTS

We included 648 participants (mean age 75 ± 12 years old, mean education 4.4 ± 3.7 years, 52% women, 69% White, and 28% APOE-ε4 carriers). The association between APOE-ε4 and cognitive abilities was mediated by neurofibrillary tangles (β = 0.88, 95% CI = 0.45; 1.38, p < 0.001) and neuritic plaques (β = 1.36, 95% CI = 0.86; 1.96, p < 0.001). Lacunar infarcts, hyaline arteriosclerosis, CAA, LBD, and TDP-43 were not mediators in the pathway from APOE-ε4 to cognition.

CONCLUSION

The association between APOE-ε4 and cognitive abilities was partially mediated by AD-pathology. On the other hand, cerebrovascular lesions and other neurodegenerative diseases did not mediate the association between APOE-ε4 and cognition.

Exome-Wide Association Study Identified Clusters of Pleiotropic Genetic Associations with Alzheimer's Disease and Thirteen Cardiovascular Traits

Alzheimer's disease (AD) and cardiovascular traits might share underlying causes. We sought to identify clusters of cardiovascular traits that share genetic factors with AD. We conducted a univariate exome-wide association study and pair-wise pleiotropic analysis focused on AD and 16 cardiovascular traits-6 diseases and 10 cardio-metabolic risk factors-for 188,260 UK biobank participants. Our analysis pinpointed nine genetic markers in the APOE gene region and four loci mapped to the CDK11, OBP2B, TPM1, and SMARCA4 genes, which demonstrated associations with AD at p ≤ 5 × 10-4 and pleiotropic associations at p ≤ 5 × 10-8. Using hierarchical cluster analysis, we grouped the phenotypes from these pleiotropic associations into seven clusters. Lipids were divided into three clusters: low-density lipoprotein and total cholesterol, high-density lipoprotein cholesterol, and triglycerides. This split might differentiate the lipid-related mechanisms of AD. The clustering of body mass index (BMI) with weight but not height indicates that weight defines BMI-AD pleiotropy. The remaining two clusters included (i) coronary heart disease and myocardial infarction; and (ii) hypertension, diabetes mellitus (DM), systolic and diastolic blood pressure. We found that all AD protective alleles were associated with larger weight and higher DM risk. Three of the four (75%) clusters of traits, which were significantly correlated with AD, demonstrated antagonistic genetic heterogeneity, characterized by different directions of the genetic associations and trait correlations. Our findings suggest that shared genetic factors between AD and cardiovascular traits mostly affect them in an antagonistic manner.

Understanding Alzheimer's disease in the context of aging: Findings from applications of stochastic process models to the Health and Retirement Study

There is growing literature on applications of biodemographic models, including stochastic process models (SPM), to studying regularities of age dynamics of biological variables in relation to aging and disease development. Alzheimer's disease (AD) is especially good candidate for SPM applications because age is a major risk factor for this heterogeneous complex trait. However, such applications are largely lacking. This paper starts filling this gap and applies SPM to data on onset of AD and longitudinal trajectories of body mass index (BMI) constructed from the Health and Retirement Study surveys and Medicare-linked data. We found that APOE e4 carriers are less robust to deviations of trajectories of BMI from the optimal levels compared to non-carriers. We also observed age-related decline in adaptive response (resilience) related to deviations of BMI from optimal levels as well as APOE- and age-dependence in other components related to variability of BMI around the mean allostatic values and accumulation of allostatic load. SPM applications thus allow revealing novel connections between age, genetic factors and longitudinal trajectories of risk factors in the context of AD and aging creating new opportunities for understanding AD development, forecasting trends in AD incidence and prevalence in populations, and studying disparities in those.

The Joint Effect of Body Mass Index and Serum Lipid Levels on Incident Dementia among Community-Dwelling Older Adults

OBJECTIVES

This study aimed to explore the joint effect of body mass index (BMI) and serum lipids levels on incident dementia.

METHODS

We prospectively followed up with 1,627 dementia-free community residents aged ≥60 for 5.7 years on average. At baseline, weight, and height were measured, and total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C) were detected in serum. Demographic characteristics were collected through questionnaires. Dementia was based on consensus diagnosis of neurologists and neuropsychologists using DSM-IV criteria. Additive Cox proportional model was used to assess the exposure-response relationship between BMI and serum lipid levels and dementia risk. Interactions and further classifications of BMI and serum lipid levels were further presented by bivariate surface models and decision-tree models.

RESULTS

The joint effects of TC with BMI, TG with BMI, and LDL-C with BMI on the risk of incident dementia shared a similar pattern, different from their independent exposure-response curves. The joint effect of HDL-C with BMI showed an S-surface but without statistical significance. Participants with TC<5.4 mmol/L and BMI<21 kg/m2 (Hazard Ratio(HR) 1.93, 95% Confidence Interval (CI) 1.05-3.53), TC<5.4 mmol/L and BMI≥21 kg/m2 (HR 1.73, 95% CI 1.09-2.72), and TC≥5.4 mmol/L and BMI<21 kg/m2 (HR 4.02, 95% CI 2.10-7.71) were identified to have the increased risk of incident dementia compared to those with TC≥5.4 mmol/L and BMI≥21 kg/m2. Participants with TG<1.7 mmol/L and BMI<21 kg/m2 had an increased risk of incident dementia compared to those with TG≥1.7 mmol/L and BMI≥21 kg/m2 (HR 1.98, 95%CI 1.17-3.3). Participants with LDL-C≥3.3 mmol/L and BMI<21 kg/m2 were identified to have an increased risk of incident dementia compared to those with LDL-C≥3.3 mmol/L and BMI≥21 kg/m2 (HR 3.33, 95%CI 1.64-6.78).

CONCLUSIONS

Our study showed that low BMI combined with low or high levels of serum lipids may increase the risk of dementia among older adults. This finding suggests the potential impacts of these two metabolic indexes on the risk of dementia.

APOE alleles modulate associations of plasma metabolites with variants from multiple genes on chromosome 19q13.3

The APOE ε2, ε3, and ε4 alleles differentially impact various complex diseases and traits. We examined whether these alleles modulated associations of 94 single-nucleotide polymorphisms (SNPs) harbored by 26 genes in 19q13.3 region with 217 plasma metabolites using Framingham Heart Study data. The analyses were performed in the E2 (ε2ε2 or ε2ε3 genotype), E3 (ε3ε3 genotype), and E4 (ε3ε4 or ε4ε4 genotype) groups separately. We identified 31, 17, and 22 polymorphism-metabolite associations in the E2, E3, and E4 groups, respectively, at a false discovery rate P FDR < 0.05. These entailed 51 and 19 associations with 20 lipid and 12 polar analytes. Contrasting the effect sizes between the analyzed groups showed 20 associations with group-specific effects at Bonferroni-adjusted P < 7.14E-04. Three associations with glutamic acid or dimethylglycine had significantly larger effects in the E2 than E3 group and 12 associations with triacylglycerol 56:5, lysophosphatidylethanolamines 16:0, 18:0, 20:4, or phosphatidylcholine 38:6 had significantly larger effects in the E2 than E4 group. Two associations with isocitrate or propionate and three associations with phosphatidylcholines 32:0, 32:1, or 34:0 had significantly larger effects in the E4 than E3 group. Nine of 70 SNP-metabolite associations identified in either E2, E3, or E4 groups attained P FDR < 0.05 in the pooled sample of these groups. However, none of them were among the 20 group-specific associations. Consistent with the evolutionary history of the APOE alleles, plasma metabolites showed higher APOE-cluster-related variations in the E4 than E2 and E3 groups. Pathway enrichment mainly highlighted lipids and amino acids metabolism and citrate cycle, which can be differentially impacted by the APOE alleles. These novel findings expand insights into the genetic heterogeneity of plasma metabolites and highlight the importance of the APOE-allele-stratified genetic analyses of the APOE-related diseases and traits.