Abstract
Introduction
Epigenetics can serve as a marker of susceptibility to many known psychiatric diseases. DNA methylation patterns of multiple genes have been studied in both civilian populations and military personnel with post-traumatic stress disorder (PTSD). Many of these genes serve various functions that span the hypothalamic-pituitary-adrenal axis, immune system, and central nervous system (CNS) growth factors and neurotransmission. It is thought that the methylation levels of such genes may be able to identify individuals who are at higher risk of developing PTSD. Our study seeks to establish whether previously reported PTSD genes possess a particular methylation pattern that is predictive of PTSD in active duty military members with combat exposure.
Materials and Methods
This is an institutional review board (IRB)-approved, cross-sectional, case control, gene-environment interaction study. About 170 active military members with and without PTSD were recruited. Patients with a history of structural brain damage, traumatic brain injury (TBI) resulting in loss of consciousness, predeployment diagnosis of PTSD or anxiety disorder, and predeployment prescription of an antidepressant or psychoactive medication were excluded. Validated measures of childhood trauma and adversity (adverse childhood experience [ACE] score), PTSD symptoms (PTSD check-list military version [PCL-M]), and combat exposure scales (CES) were measured via validated questionnaires for all subjects. After extracting DNA from peripheral blood provided by the 170 subjects, we determined methylation percentages, via pyrosequencing assays, for nine target areas within the following seven genes: BDNF, NR3C1, MAN2C1, TLR8, SLC6A4, IL-18, and SKA2. These genes are commonly reported in the literature as being highly correlated with PTSD and early-life traumatic experiences.
Methylation levels were measured as a percentage at specific sites within the previously mentioned genes. Data were examined with SPSS v 22.0 Statistics and JMP v13.1 software using a general linear model for methylation × trauma (CES scores) split by diagnosis of PTSD or not, methylation versus childhood trauma (ACE scores), and methylation versus PTSD severity (PCL-M score). Two-way ANOVA was performed to control for antidepressant use. A two-tailed Student t-test was performed for PTSD analyses and was correlated with PTSD diagnosis, demographic information as well as ACE score, PCL-M score, and CES scores.
Results
Differentially methylated sites that were highly associated with PTSD diagnosis were found in three of seven candidate genes: BDNF, NR3C1, and MAN2C1. When compared to controls, patients with PTSD diagnosis had significantly lower levels of methylation, even after controlling for antidepressant use. PCL-M, ACE, and CES scores were significantly associated with PTSD diagnosis.
Conclusion
Our study suggests that methylation of key genes involved in synaptic plasticity and the hypothalamic-pituitary-adrenal axis is associated with lower levels of methylation in military PTSD subjects exposed to combat when compared to their non-PTSD counterparts. Strengths of this study include controlling for antidepressant use and excluding TBI patients. Similar studies in an active duty population of this size are scarce. What is not clear is whether methylation changes are driving PTSD symptomology or whether they are merely a marker of disease. Future areas of research include prospective studies that measure methylation pre- and postcombat exposure in the same individual.
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