APOE and Alzheimer disease: A major genetic risk factor for sporadic AD is the ε4 allele of the APOE gene encoding for apolipoprotein (Apo)E4 in comparison to the other APOE isoforms, ε2 and ε3 (Corder et al., 1993; Strittmatter et al., 1993; Zhang and Hong, 2015).