BDNF and Alzheimer disease: Post-hoc analyses showed significantly higher levels of BDNF exon I and IV promoters’ methylation in the MDD group than in the AD (exon I promoter: p = 0.0001, exon IV promoter: p < 0.0001) and control (exon I promoter: p = 0.0001, exon IV promoter: p < 0.0001) groups (Figure 4).