Association of Rare APOE Missense Variants V236E and R251G With Risk of Alzheimer Disease

Importance

TheAPOEε2 andAPOEε4 alleles are the strongest protective and risk-increasing, respectively, genetic variants for late-onset Alzheimer disease (AD). However, the mechanisms linkingAPOEto AD—particularly the apoE protein’s role in AD pathogenesis and how this is affected byAPOEvariants—remain poorly understood. Identifying missense variants in addition toAPOEε2 andAPOEε4 could provide critical new insights, but given the low frequency of additional missense variants, AD genetic cohorts have previously been too small to interrogate this question robustly.

Objective

To determine whether rare missense variants onAPOEare associated with AD risk.

Design, Setting, and Participants

Association with case-control status was tested in a sequenced discovery sample (stage 1) and followed up in several microarray imputed cohorts as well as the UK Biobank whole-exome sequencing resource using a proxy-AD phenotype (stages 2 and 3). This study combined case-control, family-based, population-based, and longitudinal AD-related cohorts that recruited referred and volunteer participants. Stage 1 included 37 409 nonunique participants of European or admixed European ancestry, with 11 868 individuals with AD and 11 934 controls passing analysis inclusion criteria. In stages 2 and 3, 475 473 participants were considered across 8 cohorts, of which 84 513 individuals with AD and proxy-AD and 328 372 controls passed inclusion criteria. Selection criteria were cohort specific, and this study was performed a posteriori on individuals who were genotyped. Among the available genotypes, 76 195 were excluded. All data were retrieved between September 2015 and November 2021 and analyzed between April and November 2021.

Main Outcomes and Measures

In primary analyses, the AD risk associated with each missense variant was estimated, as appropriate, with either linear mixed-model regression or logistic regression. In secondary analyses, associations were estimated with age at onset using linear mixed-model regression and risk of conversion to AD using competing-risk regression.

Results

A total of 544 384 participants were analyzed in the primary case-control analysis; 312 476 (57.4%) were female, and the mean (SD; range) age was 64.9 (15.2; 40-110) years. Two missense variants were associated with a 2-fold to 3-fold decreased AD risk:APOEε4 (R251G) (odds ratio, 0.44; 95% CI, 0.33-0.59;P = 4.7 × 10⁻⁸) andAPOEε3 (V236E) (odds ratio, 0.37; 95% CI, 0.25-0.56;P = 1.9 × 10⁻⁶). Additionally, the cumulative incidence of AD in carriers of these variants was found to grow more slowly with age compared with noncarriers.

Conclusions and Relevance

In this genetic association study, a novel variant associated with AD was identified: R251G always coinherited with ε4 on theAPOEgene, which mitigates the ε4-associated AD risk. The protective effect of the V236E variant, which is always coinherited with ε3 on theAPOEgene, was also confirmed. The location of these variants confirms that the carboxyl-terminal portion of apoE plays an important role in AD pathogenesis. The large risk reductions reported here suggest that protein chemistry and functional assays of these variants should be pursued, as they have the potential to guide drug development targetingAPOE.