Most people carry a genetic variant that scientists have long dismissed as harmless. That assumption, according to a sweeping new analysis, may have been a colossal miscalculation.
Researchers from University College London and the University of Eastern Finland now estimate that between 72% and 93% of Alzheimer’s disease cases would never occur without the influence of two common versions of the APOE gene. The finding repositions a molecule that has been studied for three decades from important risk factor to something closer to a master switch for the disease.
The team reached this conclusion after analyzing genetic and clinical data from more than 450,000 people across four major international studies, including UK Biobank, FinnGen, a large U.S. amyloid imaging trial, and a dataset where Alzheimer’s was confirmed through brain tissue examination after death. In that last group, where diagnostic certainty was highest, the proportion of disease attributable to APOE reached 92.7%.
The Quiet Contributor
APOE comes in three main variants. The ε4 version has been recognized as dangerous since the 1990s, dramatically increasing Alzheimer’s risk for those who carry it. The rare ε2 variant offers protection. But ε3, carried by roughly 95% of people worldwide, has typically been treated as neutral.
That neutrality appears to be an illusion. By using individuals with two copies of ε2 as a low-risk baseline, something only possible with datasets this large, the researchers demonstrated that ε3 substantially raises Alzheimer’s risk compared to ε2. The variant isn’t a bystander. It’s a quiet contributor whose population-wide impact dwarfs that of its more notorious sibling simply because so many people carry it.
The study also found that approximately 45% of all-cause dementia cases are tied to these same genetic variants. No other known genetic marker for Alzheimer’s, or even for heart disease, carries such a massive attributable fraction of total disease burden.
Curiously, the protein APOE produces helps transport cholesterol in the brain. Why evolution preserved variants that increase dementia risk remains unclear.
What This Means for Treatment
The researchers are careful to note that genetics isn’t destiny. Even among people carrying two copies of ε4, the highest-risk combination, lifetime Alzheimer’s risk stays below 70%. Smoking, high cholesterol, social isolation, and other factors still influence whether the disease takes hold. The gene creates vulnerability. Other forces exploit it.
Still, the central implication is difficult to ignore. Despite decades of focus on amyloid plaques and tau tangles, very few therapies in clinical trials directly target APOE or its downstream biology. The study’s authors argue this represents a missed opportunity.
“We have long underestimated how much the APOE gene contributes to the burden of Alzheimer’s disease,” Dylan Williams of University College London explains. “Much disease would not occur without the additional impact of the common ε3 allele.”
Recent advances in gene therapy and protein-targeting drugs make APOE a realistic target. Researchers are already exploring treatments that mimic the protective effects of ε2 or neutralize the risks posed by ε3 and ε4. If such interventions prove effective, the data suggests they could potentially eliminate the majority of Alzheimer’s cases before they ever begin.
The results were strikingly consistent across all four datasets despite differences in how Alzheimer’s and dementia were defined. That consistency, the researchers note, strengthens confidence that APOE’s role isn’t an artifact of any single study’s methodology. The gene’s influence appears fundamental to the disease itself.
npj Dementia: 10.1038/s44400-025-00045-9
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