APOE and major depressive disorder: SMC is associated with several biological and environmental markers including depression (Geerlings et al., 1999; Stewart et al., 2001; Minett et al., 2005; Jessen et al., 2014b), old age (Geerlings et al., 1999; Jessen et al., 2014b), sex (Gillett et al., 2003; Tomita et al., 2014), education (van Oijen et al., 2007; Alagoa Joao et al., 2016), vascular risk (Paradise et al., 2011; Sajjad et al., 2015), and apolipoprotein E (APOE) ε4 (Stewart et al., 2001; Rowe et al., 2010; Jessen et al., 2014b).