• Philip S Insel, Oskar Hansson, and Niklas Mattsson-Carlgren.
• Clinical Memory Research Unit, Faculty of Medicine, Lund University, Lund, Sweden.
• JAMA Neurol. 2021 Feb 1; 78 (2): 229-235.

ImportanceAlthough the most common recent approach in Alzheimer disease drug discovery is to directly target the β-amyloid (Aβ) pathway, the high prevalence of apolipoprotein E ε4 (APOE ε4) in Alzheimer disease and the ease of identifying ε4 carriers make the APOE genotype and its corresponding protein (apoE) an appealing therapeutic target to slow Aβ accumulation.ObjectiveTo determine whether the ε2 allele is protective against Aβ accumulation in the presence of the ε4 allele and evaluate how age and the APOE genotype are associated with emerging Aβ accumulation and cognitive dysfunction.Design, Setting, And ParticipantsThis cross-sectional study used screening data from the Anti-Amyloid Treatment in Asymptomatic Alzheimer Disease Study (A4 Study) collected from April 2014 to December 2017 and analyzed from November 2019 to July 2020. Of the 6943 participants who were a part of the multicenter clinical trial screening visit, 4432 were adults without cognitive impairment aged 65 to 85 years who completed a fluorine 18-labeled (18F)-florbetapir positron emission tomography scan, had APOE genotype information, and had a Clinical Dementia Rating of 0. Participants who were taking a prescription Alzheimer medication or had a current serious or unstable illness that could interfere with the study were excluded.Main Outcomes And MeasuresAβ pathology, measured by 18F-florbetapir positron emission tomography and cognition, measured by the Preclinical Alzheimer Cognitive Composite.ResultsA total of 4432 participants were included (mean [SD] age, 71.3 [4.7] years; 2634 women [59.4%]), with a mean (SD) of 16.6 (2.8) years of education and 1512 (34.1%) with a positive Aβ level. APOE ε2 was associated with a reduction in both the overall (standardized uptake value ratio [SUVR], ε24, 1.11 [95% CI, 1.08-1.14]; ε34, 1.18 [95% CI, 1.17-1.19]) and the age-dependent level of Aβ in the presence of ε4, with Aβ levels in the APOE ε24 group (n = 115; ε24, 0.005 SUVR increase per year of age) increasing at less than half the rate with respect to increasing age compared with the APOE ε34 group (n = 1295; 0.012 SUVR increase per year of age; P = .04). The association between Aβ and decreasing Preclinical Alzheimer Cognitive Composite scores did not differ by APOE genotype, and the reduced performance on the Preclinical Alzheimer Cognitive Composite in APOE ε4 carriers compared with noncarriers was completely mediated by Aβ (unadjusted difference in composite scores between ε4 carriers and noncarriers = -0.084, P = .005; after adjusting for 18F-florbetapir = -0.006, P = .85; after adjusting for 18F-florbetapir and cardiovascular scores = -0.009, P = .78).Conclusions And RelevanceThese findings suggest that the protective outcome of carrying an ε2 allele in the presence of an ε4 allele against Aβ accumulation is important for potential treatments that attempt to biochemically mimic the function of the ε2 allele in order to facilitate Aβ clearance in ε4 carriers. Such a treatment strategy is appealing, as ε4 carriers make up approximately two-thirds of patients with Alzheimer disease dementia. This strategy could represent an early treatment option, as many ε4 carriers begin to accumulate Aβ in early middle age.

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