Group differences in OXT methylation between patients with Major Depressive Disorder and healthy controls: A pre-registered replication study.

Depression is linked to stress which leaves traces in the epigenetic signature of genes. The oxytocin system is implicated in allostatic processes promoting adaption to environmental stressors. Interactions of the oxytocin system with the environment, e.g., methylation of the gene coding for oxytocin (OXT), are candidates for the investigation of the biological underpinnings of depression. Recently, we found hypomethylation of OXT in patients with Major Depressive Disorder (MDD) compared to healthy controls (HC). Since the replicability of findings is a key point of criticism in (epi‑)genetic research, we aimed to confirm our previous findings in a pre-registered study (data was stored in a database prior to pre-registration) within a new sample of n = 85 patients with MDD and n = 85 HC. We investigated OXT DNA-methylation in peripheral blood samples, stressful life events and depression severity. In accordance with our previous study, we found hypomethylation of OXT in patients with MDD compared to HC. Methylation was not associated with stressful life events. Patients reported significantly more stressful life events compared to HC. Our study revealed that hypomethylation of OXT can be demonstrated in a reproducible fashion and provides further evidence for the involvement of the oxytocin system in depression.