APOE and major depressive disorder: The main risk factor is considered to be age [1,9,12], but sporadic AD is a complex disorder and other risk factors have been identified, such as the female sex, traumatic brain injury, depression, environmental pollution, physical inactivity, social isolation, low academic level, metabolic syndrome [9,13] and genetic susceptibility, mainly mutations in the ε4 allele of apolipoprotein E (APOE, 19q13.32) [1,9], considering a heritability of up to 60–80% [14].