What are biological etiologies of MDD?

We have molecular susceptibility genes, protective genes and transcription factors (BDNF, CREB,PCK) that predispose cell growth, and environment will affect growth, neurotransmission and neural networks then early life adverse effects will cause behavioral cognitive and sensory motor disruptions. Study of influence of life stress on depression revieled that genotype alone is not sufficient to predict depression, 5-HT transporter gene needs envirnomental life stress to cause depression, life events alone can cause it and if combined with genetics (5-HT transporter) doubled the risk short allele (s allele) of 5-HT transporter is strongly associated with stress sensitivity increase (childhood maltreatment and specific medical conditions) We get glial hypoactivity and death