BDNF and major depressive disorder: Several studies have reported that higher levels of DNA methylation in the BDNF gene are related to bipolar disorder, schizophrenia, borderline personality disorder, and depression (Perroud et al., 2008, 2013; Keller et al., 2010; Fuchikami et al., 2011; D’Addario et al., 2012; Kordi-Tamandani et al., 2012; Ikegame et al., 2013; Ferrer et al., 2019).