BDNF and major depressive disorder: The association of suicide with depression, particularly in major depressive disorder (MDD), can be explained by the synergic role of genetics, endogenous and exogenous stressors, epigenetics, the hypothalamic–pituitary–adrenal stress-response system, the involvement of the monoaminergic neurotransmitter systems, neuro-immunological biomarkers, the brain-derived neurotrophic factor, and other neuromodulators [58].