Blood epigenome-wide association studies of suicide attempt in adults with bipolar disorder

Abstract

Suicide attempt (SA) risk is elevated in individuals with bipolar disorder (BD), and DNA methylation patterns may serve as possible biomarkers of SA. We conducted epigenome-wide association studies (EWAS) of blood DNA methylation associated with BD and SA. DNA methylation was measured at > 700,000 positions in a discovery cohort of n = 84 adults with BD with a history of SA (BD/SA), n = 79 adults with BD without history of SA (BD/non-SA), and n = 76 non-psychiatric controls (CON). EWAS revealed six differentially methylated positions (DMPs) and seven differentially methylated regions (DMRs) between BD/SA and BD/non-SA, with multiple immune-related genes implicated. There were no epigenome-wide significant differences when BD/SA and BD/non-SA were each compared to CON, and patterns suggested that epigenetics differentiating BD/SA from BD/non-SA do not differentiate BD/non-SA from CON. Weighted gene co-methylation network analysis and trait enrichment analysis of the BD/SA vs. BD/non-SA contrast further corroborated immune system involvement, while gene ontology analysis implicated calcium signalling. In an independent replication cohort of n = 48 BD/SA and n = 47 BD/non-SA, fold-changes at the discovery cohort's significant sites showed moderate correlation across cohorts and agreement on direction. In both cohorts, classification accuracy for SA history among individuals with BD was highest when methylation at the significant CpG sites as well as information from clinical interviews were combined, with an AUC of 88.8% (CI = 83.8-93.8%) and 82.1% (CI = 73.6-90.5%) for the combined epigenetic-clinical predictor in the discovery and replication cohorts, respectively. Our results provide novel insight to the role of immune system functioning in SA and BD and also suggest that integrating information from multiple levels of analysis holds promise to improve risk assessment for SA in adults with BD.

Keywords: biomarker; bipolar disorder; blood; epigenetics; methylation; suicide attempt.

Figure 1.

Results from the discovery cohort epigenome-wide association study of BD/SA (n=79) vs. BD/non-SA (n=83), demonstrated as A) Manhattan plot of significance by chromosomal location and B) Volcano plot of significance against log2 fold change, where positive and negative log2 fold changes indicate hypermethylation and hypomethylation in the BD/SA group, respectively. In both plots, the horizontal dotted line demarcates the FDR adjusted q < 0.05 threshold, and FDR-significant CpG sites are labelled based on their annotated genes. BD - bipolar disorder; FDR - false discovery rate; SA - suicide attempt.

Figure 2.

Gene ontology and trait enrichment results for the discovery cohort BD/SA vs. BD/non-SA EWAS using the DMPs at nominal p < 0.001. A shows the top ten gene ontology pathways ordered by −log(p value), with the number of genes assigned to each pathway represented by the size of the point. B shows the top ten traits ordered by −log(p value), with the odds ratio for each trait represented by the size of the point. BD - bipolar disorder; DMP – differentially methylated position; EWAS - epigenome-wide association study; SA - suicide attempt.

Figure 3.

Receiver operating curves (ROC) for generalised linear models predicting SA history among individuals with BD, based on beta-values of the six FDR-significant discovery cohort DMPs (“DMPs only”), six clinical correlates of SA history in mood disorder (“Clinical only”), and the beta-values and clinical correlates combined (“DMPs+Clinical combined”). AUC of the ROC are presented with 95% confidence intervals. In A, ROC are presented for prediction of SA history in the discovery cohort (n=162). In B, ROC are presented for prediction of SA history in the replication cohort (n=95). AUC - area under the curve; DMP - differentially methylated positions; FDR - false discovery rate; SA - suicide attempt.