DNA methylation of OXT, SLC6A4, NR3C1 genes fails to predict depression treatment response, highlighting blood cell confounders

The search for reliable biomarkers in Major Depressive Disorder (MDD) remains a critical challenge, with current diagnostic and treatment selection methods often relying on subjective assessments. DNA methylation, a key epigenetic mechanism, has been proposed as a potential biomarker, particularly for genes involved in stress response and neurotransmission like the glucocorticoid receptor (NR3C1), serotonin transporter (SLC6A4), and oxytocin (OXT). However, inconsistencies in replication and methodological concerns, especially regarding cellular heterogeneity in peripheral blood, have hampered progress. This study addresses the gap by rigorously investigating the longitudinal association of these gene methylations with treatment response, while carefully controlling for confounding factors.

Researchers conducted a matched case-control study, enrolling N=66 patients with depression and N=66 healthy controls. Peripheral blood samples were collected at baseline and after two weeks of inpatient treatment. The study investigated DNA methylation levels of the NR3C1, SLC6A4, and OXT genes. They tested for group differences, convergence of methylation over time, associations with changes in depression severity, and relations to mRNA abundance. Crucially, they adjusted for potential confounders, including blood cell composition (neutrophil and lymphocyte abundance), to enhance the rigor of their epigenetic analysis.

At baseline, OXT methylation was significantly lower in patients with depression. However, this initial effect disappeared entirely when adjusting for neutrophil and lymphocyte abundance, underscoring the profound impact of cellular composition on methylation measurements. Despite a significant decrease in depression severity observed over the two weeks of inpatient treatment, changes in DNA methylation in none of the candidate genes (NR3C1, SLC6A4, OXT) were associated with the treatment response. This represents a significant null finding for these commonly studied epigenetic markers. In healthy controls, two OXT CpG sites showed negative associations with mRNA abundance, suggesting a functional link, but these associations were notably absent in patients.

The study found no association between changes in DNA methylation of NR3C1, SLC6A4, or OXT genes and treatment response in patients with depression, despite significant clinical improvement.