Background: This study aims to investigate if DNA methylation of the apolipoprotein E (APOE) locus affects the risks of dementia, Alzheimers disease (AD) or cardiovascular disease (CVD).

Methods: DNA methylation across the APOE gene has previously been categorized into three distinct regions: a hypermethylated region in the promoter, a hypomethylated region in the first two introns and exons and a hypermethylated region in the 3'exon that also harbours the APOE epsilon 2 and epsilon 4 alleles. DNA methylation levels in leukocytes were measured using the Illumina 450K array in 447 Swedish twins (mean age 78.1 years). We used logistic regression to investigate whether methylation levels in those regions affect the odds of disease.

Results: We found that methylation levels in the promoter region were associated with dementia and AD after adjusting for sex, age at blood draw, education, smoking and relatedness among twins [odds ratio (OR) 1.32 per standard deviation increase in methylation levels, 95% confidence interval (CI) 1.08-1.62 for dementia; OR 1.38, 95% CI 1.07-1.78 for AD). We did not detect any difference in methylation levels between CVD cases and controls. Results were similar when comparing within discordant twin pairs, and did not differ as a function of APOE genotype.

Conclusions: We found that higher DNA methylation levels in the promoter region of APOE increase the odds of dementia and AD, but not CVD. The effect was independent of APOE genotype, indicating that allelic variation and methylation variation in APOE may act independently to increase the risk of dementia.