Epigenetic Risk Factors for Suicide in Comorbid Depression and Alcoholism

The goal of this project is to identify epigenetic risk factors associated with suicide in comorbid alcohol use disorder (AUD) and depressive disorder (DEP; including major depressive disorder (MDD), dysthymia, or depression-not otherwise specified (NOS)), a comorbidity featuring a worse course of illness and greater suicidality than either disorder alone. We will assess DNA methylation and related gene expression in non-neuronal nuclei in the orbitofrontal cortex (OFC) to examine the association of suicide with comorbid DEP/AUD vs. non-suicide with DEP/AUD, as compared to suicide with DEP vs. non-suicide with DEP. 
In molecular and cellular studies of suicide and MDD, AUD is often an important contributor yet it is routinely excluded as a confounding factor. In human brain there is evidence for altered genome-wide DNA methylation in either MDD or AUD, but this has not been studied in suicide with comorbid DEP/AUD. A significant gap exists in understanding the pathophysiology of suicide in this comorbidity and is a barrier to the development of more effective treatments.

Hypothesis: Suicide in comorbid DEP/AUD is associated with unique alterations in DNA methylation and related gene expression in non-neuronal nuclei in the OFC vs. non-suicide with comorbid DEP/AUD, and compared to suicide with DEP vs. non-suicide with DEP.   

Aim 1: Assess differential DNA methylation of non-neuronal gene promoter regions in OFC of suicides with comorbid DEP/AUD vs. non-suicides with comorbid DEP/AUD, as compared to suicides with DEP vs. non-suicides with DEP. Psychiatrically-normal subjects also will be examined. It is predicted that in suicides with DEP/AUD, there will be unique changes in methylation of non-neuronal DNA promoters vs. non-suicides with DEP/AUD, as compared to suicides with DEP vs. non-suicides with DEP. 

Aim 2: Based on differential DNA methylation of gene promoters, targeted RNA-sequencing will be used to assess expression of these genes in non-neuronal nuclei in OFC in the cohorts in Aim 1. It is predicted that there will be uniquely greater expression of genes with reduced promoter methylation, and uniquely diminished expression of genes with enhanced promoter methylation in suicides with DEP/AUD vs. the other cohorts. 

Translational Impact: Results from this study in brain should demonstrate unique changes in DNA methylation and related gene expression in suicides with comorbid DEP/AUD, and identify potential sites for which therapeutics may be designed. While there may be only modest correlations between markers in blood vs. brain, it may be more likely that a combination of brain imaging plus biochemical and genomic biomarkers will provide a convergence of findings as potential predictors of suicide attempt.

Next: In an NIH proposal, we will assess DNA methylation in isolated nuclei from neurons, astrocytes, and microglia. We will also use tissue blocks from the same subjects for western blotting, immunohistochemistry, and in situ hybridization to identify the particular cell populations and cortical laminae in which altered gene expression and DNA methylation occur in suicides with DEP/AUD. We will also examine DNA methylation, histone modifications and related gene expression in these subjects in other parts of neurocircuits disturbed in AUD and depression.