Project Summary Major depression (MD) is highly prevalent, ranking second in the global burden of disease, with the overall lifetime risk estimated to be 16.2% in the general population.1 MD also is associated with increased mortality, particularly suicide.2 Amongst adolescents, MD is associated with the greatest level of impairment of all psychiatric conditions, with 16% of females and 12% of males endorsing at least one major depressive episode (MDE) by age 183; an early age of onset confers increased risk for future impairment.4 Given the collective influence of genetic factors and environmental events on MD risk liability and progression, epigenetic mechanisms are promising candidates for MD research. Epigenetic mechanisms are biological processes that influence genomic health and regulation without changing the DNA sequence. Small noncoding RNAs are the most diverse, numerous, and dynamic class of epigenetic mechanisms. They perform a large number of regulatory and functional roles, including intercellular signaling. For psychiatric research, microRNAs (miRNAs) are an excellent candidate for identifying biological pathways associated with MD and risk.5-9 Interindividual differences in miRNA profiles have been associated with sex-based differences in pathophysiology, medication response in bipolar disorder,10,11 current depressive symptom severity in MD cases, and MD case status. Importantly, miRNAs from brain cell lineages can be accessed in extracellular vesicles (ECVs) in peripheral blood plasma. ECVs easily cross the blood brain barrier9 and differential cargo analysis of neurally-derived ECVs from peripheral blood is possible.12 Mounting evidence underscores the potential for peripheral blood ECVs to map disease trajectories of central nervous system cell type and to provide a snapshot of brain biological processes that may be salient to MD pathophysiology. We propose to leverage existing samples to determine ECV miRNA profiles to investigate MD pathophysiology in a sample of young people during a period of peak MD incidence12-15. Specifically, this proposal will build on the Adolescent and Young Adult Twin Study (NTotal=860 twins; R01MH101518) to improve our understanding of miRNA cargo, particularly those deriving from neurons, to add to the current understanding of the pathophysiology of early-onset MD. Twin pairs in the parent R01 completed a broad battery of measures assessing psychiatric history, risk factors associated with MD, life stress and adversities, environmental factors (e.g., parenting), laboratory challenges (e.g., Trier Social Stress Test), and they provided blood from which plasma was separated. This R21 proposal will select 284 plasma samples collected from a subset of monozygotic and dizygotic twins (ages 15-22; ~65% female) from the parent R01 to address critical basic science questions about the nature of circulating miRNAs in young people and their relationship to MD that onsets early in life. Data generated in th...