Depression and the serotonin transporter 5-HTTLPR polymorphism: a review and a hypothesis concerning gene-environment interaction.

Studies of the interaction of the serotonin transporter genotype and environment upon adult depression (G x E) have suggested a role for both childhood maltreatment and stressful life events. This paper deals with two main issues. First, do both contribute? Evidence that G x E with childhood maltreatment plays a role is much stronger than that for G x E with life events occurring close to onset, although that for G x E with life events occurring over a 5-year period before the presence of the recorded depression is stronger. However, non-genetic research shows that life events occurring so long before onset as 5 years have little or no relationship with adult depression once childhood maltreatment is taken into account, suggesting they serve as a marker for childhood maltreatment rather than making a direct contribution to G x E. Second, genetic research has dealt only with the presence of depression and taking account of course may radically change ideas about the point at which G x E occurs. Two findings from non-genetic research concerning childhood maltreatment are relevant. Childhood maltreatment is associated with a particularly high risk of an adult onset of depression taking a chronic course (i.e. lasting 12 months or more). Moreover such maltreatment makes a substantial direct contribution - i.e. its link with course is independent of all other childhood and adult risk factors. This is consistent with early changes in brain function associated with the polymorphism in the context of childhood maltreatment explaining the link of such maltreatment with adult chronic episodes. It also follows that restricting analysis to such episodes would increase current estimates of G x E.