Abstract
<b><i>Introduction:</i></b> Long interspersed nuclear elements (LINEs) are endogenous retrotransposable elements. A few studies have linked the methylation pattern of LINE-1 to different mental disorders (e.g., post-traumatic stress disorder [PTSD], autism spectrum disorder [ASD], panic disorder [PD]). We sought to unify the existing knowledge in the field and provide a better understanding of the association between mental disorders and LINE-1 methylation. <b><i>Methods:</i></b> A systematic review was executed with 12 eligible articles according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. <b><i>Results:</i></b> For psychotic disorders, PTSD, ASD, and PD, lower LINE-1 methylation levels were detected, whereas for mood disorders, the findings are controversial. The studies were conducted with subjects aged 18–80 years. Peripheral blood samples were utilized in 7/12 articles. <b><i>Conclusion:</i></b> Although most studies have shown that LINE-1 hypomethylation was associated with mental disorders, there were still some divergences (i.e., hypermethylation associated with mental disorders). These studies suggest that LINE-1 methylation may be an important factor related to the development of mental disorders and highlight the need to better comprehend the biological mechanisms underlying the role of LINE-1 in mental disorders pathophysiology.