Project Summary Childhood adversity (e.g., abuse, poverty) is a potent risk factor for depression, increasing lifetime risk of this common and burdensome disorder by at least two-fold. While the association between adversity and depression risk is well documented, the biological mechanisms explaining this relationship are poorly understood. In this proposal, we will address this gap by testing the central hypothesis that vulnerability to adolescent- and young adult-onset depression arises, in part, via the effects of adversity-induced epigenetic changes during an early sensitive period that occurs in the first five years of life. Sensitive periods are life stages when the brain is highly plastic and experience (e.g., adversity) can impart enduring effects. This hypothesis will be prospectively tested across three aims – in a discovery and replication approach – using data from two large birth cohorts: (1) the Avon Longitudinal Study of Parents and Children and (2) Generation R. In Aim 1, we will investigate the extent to which the developmental timing of exposure to adversity predicts blood DNA methylation (DNAm). We will use an innovative two- stage structured lifecourse statistical modeling approach to investigate the role of repeated exposure to seven distinct types of adversities during early life (up to age 7) on DNAm in middle childhood (age 7). For each type of adversity, we will investigate the following theoretical models to determine which one or more are best supported by the data: (1) a sensitive period model, in which the effect of presence or absence of exposure to adversity on DNAm depends on the time period of the exposure; (2) an accumulation model, in which the effect of exposure to adversity on DNAm increases with the number of occasions exposed, regardless of timing; and (3) a recency model, in which the effect of exposure to adversity on DNAm is stronger for more proximal events. In Aim 2, we will use regression and causal inference-based mediation approaches and Mendelian randomization to determine the degree to which age 7 DNAm changes predict adolescent-onset depression and mediate the effect of adversity on adolescent depression. In Aim 3, we will determine the short- vs. longer-term effects of DNAm on risk for young-adult depression by examining: (a) the persistence of DNAm profiles from age 7 to age 17; and (b) the relative contribution of early vs. adolescent adversity on age 17 DNAm and risk for young-adult onset depression. Throughout, we will control for genetic factors shown to explain variability in DNAm and depression. This research will identify molecular biomarkers of exposure to adversity and risk for depression and determine the age stages when adversity is most likely to affect this biomarker. These findings will inform our understanding of the high-risk/high-reward stages of development when adversity is most harmful and when public health investments could be most efficacious in preventing depression.