A Tissue Comparison of DNA Methylation of the Glucocorticoid Receptor Gene (Nr3c1) in European Starlings

Abstract Negative feedback of the vertebrate stress response via the hypothalamic–pituitary–adrenal (HPA) axis is regulated by glucocorticoid receptors in the brain. Epigenetic modification of the glucocorticoid receptor gene (Nr3c1), including DNA methylation of the promoter region, can influence expression of these receptors, impacting behavior, physiology, and fitness. However, we still know little about the long-term effects of these modifications on fitness. To better understand these fitness effects, we must first develop a non-lethal method to assess DNA methylation in the brain that allows for multiple measurements throughout an organism’s lifetime. In this study, we aimed to determine if blood is a viable biomarker for Nr3c1 DNA methylation in two brain regions (hippocampus and hypothalamus) in adult European starlings (Sturnus vulgaris). We found that DNA methylation of CpG sites in the complete Nr3c1 putative promoter varied among tissue types and was lowest in blood. Although we identified a similar cluster of correlated Nr3c1 putative promoter CpG sites within each tissue, this cluster did not show any correlation in DNA methylation among tissues. Additional studies should consider the role of the developmental environment in producing epigenetic modifications in different tissues.