Basic Science and Pathogenesis

Apolipoprotein E plays a critical role in lipid metabolism and is associated with cardiovascular disease, cognitive decline, and Alzheimer's disease. While the APOE ε4 allele is a well-established risk factor for these conditions, the APOE ε2 allele is considered protective, though its homozygosity increases cardiovascular risk.

This study investigates whether APOE ε2 homozygous carriers with African local ancestry are at differential risk for cognitive conditions compared to ε2 heterozygous carriers and other APOE genotypes. Given the unique genetic diversity of the Brazilian population, we specifically examined the role of African local ancestry in APOE ε2 homozygotes, as this group may exhibit distinct risk profiles due to population-specific genetic and environmental factors. We genotyped 1,589 individuals from a population-based cohort in São Paulo, Brazil, derived from a brain bank at the University of São Paulo. Surprisingly, we observed that the frequency of the rare homozygous ε2/ε2 genotype was significantly higher in the brain bank sample, suggesting that it may confer an increased risk for carriers.

To assess deviations in genotype frequencies, we performed Hardy-Weinberg equilibrium tests between the genotypes 2/2, 2/3, 2/4, 3/3, 3/4, and 4/4. The ε2/ε2 genotype showed significant Hardy-Weinberg disequilibrium (X² (1; 0.05) = 3.841 > 1.601), indicating potential selective pressures or founder effects. Using microarray data from a subgroup of 720 individuals, we explored the role of local ancestry (African, European, or Native American) in modulating the risk associated with APOE ε2 homozygosity. Local ancestry inference (LAI) revealed a Hardy-Weinberg disequilibrium among homozygote African ancestry ε2/ε2 carriers, with 6 out of 9 individuals, showing significant deviation (X² (3; 0.05) = 7.815 > 7.441). These findings suggest that APOE ε2 homozygosity, particularly in carriers of African local ancestry, may confer differential risks for cognitive conditions.

Our results highlight the importance of considering local ancestry in genetic studies of complex diseases, especially in admixed populations like Brazil. This study provides new insights into the role of APOE ε2 homozygosity and local ancestry in modulating disease risk, with potential implications for personalized medicine and public health strategies.