BDNF and major depressive disorder: Epigenetic regulatory factors are involved in various pathways contributing to the pathogenesis of depression, including neurogenesis, synaptic plasticity, brain-derived neurotrophic factor expression, HPA axis regulation, neurotransmission, neuropeptide expression, neuroinflammation, and monoamine synthesis (Kocerha et al., 2015; Liu et al., 2020).