Exploring the association between serotonin transporter promoter region methylation levels and depressive symptoms: a systematic review and multi-level meta-analysis.

Depressive disorders result from complex interactions among genetic, epigenetic, and environmental factors. DNA methylation, a key epigenetic mechanism, is crucial in understanding depressive symptoms development. The serotonin transporter gene (5-HTT) and its polymorphisms, like 5-HTTLPR, have been extensively studied in relation to depression, yet conflicting findings regarding the association between 5-HTT promoter methylation and depressive symptoms persist, largely due to methodological differences. Thus, this systematic review and meta-analysis aims to assess (1) 5-HTT promoter methylation levels between depressed and non-depressed conditions and (2) the association between 5-HTT methylation and depressive symptoms severity. We searched PubMed, Google Scholar, and Web of Science from inception to January 15th, 2025 (PROSPERO: CRD42023355414) and performed two independent multi-level meta-analyses to answer our aims. Twenty-four trials were included in the systematic review. All reported effects carried potential for bias. The meta-analysis for depression occurrence (12 studies - 2028 subjects - 127 effects) indicated no significant effect (Hedges'g = 0.06) with moderate within- and low between-study heterogeneity. The depression severity analysis (14 studies - 2296 subjects - 116 effects) revealed a null effect size (Fisher's Z = 0.05), with no within- and moderate between-study heterogeneity. Asymmetry was detected for both meta-analyses. Moderator analyses demonstrated no significant effects of depression severity, methylation techniques, single-CpG sites, cell types assessed, age, and female percentage. This comprehensive review provides insights into the intricate interplay between 5-HTT promoter methylation and depressive symptoms. Furthermore, it offers well-considered recommendations for future research endeavors and delineates guidelines for reporting methylation research.