TPH2 and major depressive disorder: Numerous studies have found that several candidate genetic polymorphisms, including serotonin transporter-lined polymorphic region (5-HTTLPR), brain-derived neurotrophic factor (BDNF) Val66Met, tryptophan hydroxylase-2 (TPH-2), catechol-o-methyltransferase (COMT) Val158Met, serotonin receptor 1A gene (HTR1A), and monoamine oxidase A gene (MAOA) modulate morphologic brain changes in MDD patients [6, 7].