APOE and dementia - resequencing and genotyping in 105,597 individuals

Abstract

Introduction: The mechanism behind the strong association between the ɛ2/ɛ3/ɛ4 apolipoprotein E gene (APOE) polymorphism and Alzheimer's disease is not well-characterized. Because low plasma levels of apoE associate with risk of dementia, genetic variants altering apoE levels in general may also associate with dementia.

Methods: The APOE gene was sequenced in 10,369 individuals, and nine amino acid-changing variants with frequencies ≥2/10,000 were further genotyped in 95,228 individuals. Plasma apoE levels were measured directly.

Results: Risk of all dementia and Alzheimer's disease (AD) increased with decreasing genetically determined apoE levels (P = 5 × 10^-4 and P = 1 × 10^-4 after APOE ɛ2/ɛ3/ɛ4 adjustment). Hazard ratios (95% confidence intervals) for all dementia and AD were 2.76 (1.39 to 5.47) and 4.92 (2.36 to 10.29) for the group with the genetically lowest apoE versus ɛ33.

Discussion: We found that genetically low apoE levels increase and genetically high levels decrease risk, beyond ɛ2/ɛ3/ɛ4. This underscores that dementia risk more likely relates to variants affecting levels of apoE.

Keywords: APOE; Alzheimer's disease; apolipoprotein E; dementia; genetics; rare variation.

FIGURE 1

Population‐based resequencing of the translated region of the apolipoprotein E gene (APOE) in the Copenhagen City Heart Study followed by genotyping in the Copenhagen General Population Study. Gene screening and genotyping are described in the Methods Section. The dark gray regions indicate the translated regions of the APOE gene. Of the 27 rare variants found by resequencing, nine amino acid–changing variants with frequencies ≥2/10,000 are indicated in green. Amino acid–changing variants with frequencies <2/10,000 are indicated in black, and synonymous variants are indicated in light gray. In addition, the APOE ε2/ε3/ε4 polymorphism is indicated in purple. ApoE consists of two domains connected by a “hinge region” and forms a tetrameric α‐helical bundle, and in the presence of lipids, the bundle “opens” to expose the hydrophobic cores, making the protein available to interact with lipids. ²⁸ , ⁴² The amino‐terminal domain (amino acids ≈19 to 209) contains the receptor binding functions (amino acids ≈148 to 168). This is preceded by a posttranslationally removed 18‐residue signal peptide (amino acids 1 to 18). ⁴⁵ The carboxyl‐terminal domain (amino acids ≈243 to 317) contains the lipid‐binding functions. APOE, apolipoprotein E gene; NA, rs number not available

FIGURE 2

Multifactorially adjusted hazard ratios for all dementia and Alzheimer's disease according to plasma levels of apolipoprotein E (apoE) and weighted allele score in individuals in the general population. Solid lines are multifactorially adjusted hazard ratios, whereas dashed lines indicate 95% confidence intervals derived from restricted cubic splines with three knots, with the reference defined as the mean plasma level of apoE (4.3 mg/dL) or ε33 (weighted allele score = 0). For the observational estimates, follow‐up began at study entry and only if plasma apoE measurements were available. Graphs are truncated at 15 mg/dL for plasma levels of apoE and at −1.5 and 3.3 for the weighted allele score due to statistically unstable estimates outside these values, thus including 103,682 and 105,578 individuals from the Copenhagen City Heart Study and the Copenhagen General Population Study for these observational and genetic analyses. Cox regression models were adjusted for age (time scale), sex, body mass index, smoking, hypertension, diabetes, lipid‐lowering therapy, alcohol consumption, physical inactivity, postmenopausal status, hormonal replacement therapy, and education (left and middle columns), and further for APOE ε2/ε3/ε4 genotype (right column). CI, 95% confidence interval

FIGURE 3

Risk of all dementia and Alzheimer's disease as a function of apolipoprotein E (apoE)‐weighted allele score in five groups. Geometric mean ± standard errors of the mean are given for plasma apolipoprotein E (left column). Cox regression models were adjusted for age (time scale), sex, body mass index, smoking, hypertension, diabetes, lipid‐lowering therapy, alcohol consumption, physical inactivity, postmenopausal status, hormonal replacement therapy, and education (middle column), and further for APOE ε2/ε3/ε4 genotype (right column). ; CI, 95% confidence interval

FIGURE 4

Risk of all dementia and Alzheimer's disease as a function of the weighted allele score stratified according to apolipoprotein E gene (APOE) ε2/ε3/ε4 status. The weighted allele score was divided in 13 groups according to APOE ε2/ε3/ε4 genotype (clean ε22, ε32, ε42, ε33, ε43 and ε44 genotypes without rare variation). Geometric mean ± standard errors of the mean are given for plasma apoE (left column). Cox regression models were adjusted for age (time scale), sex, body mass index, smoking, hypertension, diabetes, lipid‐lowering therapy, alcohol consumption, physical inactivity, postmenopausal status, hormonal replacement therapy, and education (middle and right column). P for trends were from 3.32‐4.47 to 3.30 (ε22) to 1.67‐2.82 to 1.65 (ε32) to 1.23‐1.34 to 1.20 (ε42) to 0.02‐1.10 to 0 (ε33) to −0.41‐(−0.13) to −0.45 (ε43) to −0.88‐(−0.55) to −0.89 (ε44) to −1.80‐(−1.00). ApoE, apolipoprotein E; CI, 95% confidence interval

FIGURE 5

Risk for all dementia and Alzheimer's disease for a 1 mg/dL decrease in plasma apolipoprotein E (apoE) levels. Cox regression models were adjusted for age (time scale), sex, body mass index, smoking, hypertension, diabetes, lipid‐lowering therapy, alcohol consumption, physical inactivity, postmenopausal status, hormonal replacement therapy, and education (left column), and further for APOE ε2/ε3/ε4 genotype (right column). For the observational estimates, follow‐up began at study entry and only if plasma apoE measurements were available, thus including 103,744 individuals in these analyses. The first weighted allele score, ε2/ε3/ε4 + rare variants, is the score described in the Methods section (n = 105,597). For the second score, ε2/ε3/ε4 alone, the score was obtained similarly, but in a model only including the ε4 and the ε2 alleles, and not the nine rare variants (n = 105,597). The third score, ε2/ε3/ε4 + promoter variants, was obtained like the main genetic score, but included weights from ε4, ε2, and three common promoter variants (rs449647, rs769446, and rs405509) (n = 74,940). The estimates for rare variants alone and for promotor variants alone were for the same genetic scores for ε33 individuals only (n = 58,737 and 41,789). CI, 95% confidence interval