Taking these findings together suggests that DNA methylation at the SLC6A4 promoter is not limited to peripheral tissues and is paralleled in the brain, where it may be one of the physiological mechanisms underlying how early stress could translate into altered brain development. Latrunculin A Hippocampal changes may be particularly relevant, since this brain region is densely innervated with 5-HT and is highly involved in stress regulation. The aim of the present study was to test the hypothesis that DNA methylation in specific CpG sites at the SLC6A4 promoter in peripheral cells is L-371,257 associated with childhood trauma, depressive symptomatology, and smaller hippocampal volume. To this end, we assessed DNA methylation at the SLC6A4 promoter in whole blood DNA in depressed patients and in ageand sex-matched healthy controls, all of whom previously underwent high-resolution structural Magnetic Resonance Imaging to measure hippocampal volume. Secondary aims of the study were to examine the association between demographic and clinical characteristics of depression and between DNA methylation at the SLC6A4 promoter. We focused on DNA methylation in a region of the SLC6A4 promoter and associated specific CpG sites that were previously shown to be most strongly associated with in vivo measures of 5-HT synthesis. Childhood abuse was previously found to be associated with altered levels of peripheral methylation states in SLC6A4 promoter regions later in life. The present study confirmed these findings and showed that the results were most pronounced in those CpG sites specifically associated with brain 5-HT synthesis. Among the selected variables of investigation, lower hippocampal volume was most strongly associated with SLC6A4 regulatory region methylation state across CpG sites, as well as in the specific, a priori-selected CpG sites. Interestingly, this was the case for the whole hippocampus and also for CA1 as well as the hippocampal subfields gyrus dentate and CA2/3. The relatively strong association between peripheral methylation of SLC6A4 regulatory region and hippocampal volumes expands on our previous study showing that childhood adversity interacts with the polymorphism in the promoter region of the SLC6A4 gene on hippocampal volumes. It suggests that SLC6A4 methylation may be an underlying physiological mechanism of how gene and environment interact to affect hippocampal development.