Childhood maltreatment is a major global public health problem that increases the risk of depressive symptoms later in life. However, important gaps remain in our understanding of how such experiences "get under the skin". Emerging evidence suggests that early adversity contributes to epigenetic modifications of key regulatory elements within the hypothalamic-pituitary-adrenal (HPA) axis, potentially increasing vulnerability to psychopathology. In addition, rumination---the tendency to repetitively dwell on one's problems, concerns, and distressing feelings---represents a key cognitive mechanism. Crucially, biological embedding and cognitive processes do not operate in isolation. According to transdiagnostic models of psychopathology, distal environmental risks may heighten rumination by undermining adaptive stress responses. Moreover, because specific subtypes of childhood maltreatment may have distinct epigenetic signatures and differential contributions to depression severity, these mechanisms may vary across maltreatment subtypes. Accordingly, this study examined whether methylation changes in the promoter region of the NR3C1 gene and rumination explain the longitudinal association between childhood maltreatment and adult depressive symptoms, and whether maltreatment subtypes differ in their predictive validity.

A total of 593 Chinese young adults (Mage = 19.47 ± 0.69 years; 49.2% female; 98.6% Han Chinese) were recruited from Shandong, China, and followed for six months. Depressive symptoms were assessed at both time points using self-report questionnaires, and childhood maltreatment was assessed retrospectively. Genomic DNA was extracted from buccal swab samples and used for DNA methylation analyses. We tested a series of serial mediation models in which childhood maltreatment predicted depressive symptoms in young adulthood through DNA methylation in the NR3C1 promoter region and rumination in sequence. We also examined whether different childhood maltreatment subtypes showed differential predictive validity.

Results showed that DNA methylation of NR3C1 and rumination mediated the longitudinal association between childhood emotional neglect and depressive symptoms in young adulthood. Specifically, emotional neglect was associated with higher methylation levels in the NR3C1 promoter region, which in turn predicted greater rumination and higher depressive symptoms. In addition, all maltreatment subtypes were associated with depressive symptoms via elevated rumination. However, the serial mediation pathway was not observed for childhood emotional abuse, physical abuse, physical neglect, or total maltreatment. Moreover, NR3C1 promoter methylation did not significantly mediate the associations between any childhood maltreatment subtype and depressive symptoms in young adulthood.

These findings highlight the importance of epigenetic processes and cognitive vulnerability in understanding how childhood adversity increases risk for depressive symptoms across the lifespan. Notably, this study provides further support for transdiagnostic models of psychopathology, which propose that distal environmental risks can shape individuals' stress responses, giving rise to proximal cognitive risk factors that, in turn, precipitate psychopathological symptoms. In addition, the results suggest that distinguishing among subtypes of childhood maltreatment may help clarify the pathways through which maltreatment contributes to depressive symptoms. Specifically, different maltreatment subtypes may exhibit distinct epigenetic signatures, while also sharing common mechanisms---such as rumination in the present study---in the development of depressive symptoms.

By identifying epigenetic (NR3C1 methylation) and cognitive (rumination) vulnerability markers, as well as their roles in the development of depressive symptoms, this study has important implications for prevention and intervention. First, these vulnerability markers at the molecular and cognitive levels may facilitate early screening to identify individuals at high risk for depressive disorders. Second, this study calls for early interventions tailored to specific maltreatment subtypes. Prioritizing early family-based programs, parenting support, and cognitive strategies to reduce rumination---and tailoring these approaches to different maltreatment experiences and pathways---may interrupt risk transmission, curb symptom escalation, and ultimately reduce the long-term burden of depression.