Suicidal behaviour is a polygenic and multifactorial disorder affecting people all around the world. It is influenced by multiple factors, which can be tied together by epigenetics. Although comprehensive amount of knowledge on suicidal behaviour has been gathered, complete mechanism and factors leading to suicidal behaviour are yet to be determined. However, based on molecular studies, we can conclude that there is an important biological component to suicidal behaviour, and DNA methylation could play an important role.
For the purpose of this work we formed two study groups. First included 25 male suicide victims, who died by hanging. The second group was a control group, including 28 males who died from sudden cardiac arrest. Using advances of the next-generation sequencing technology (NGS), we first analysed genome-wide DNA methylation on a smaller subset. DNA methylation was examined in hippocampus and in a cortical region, Brodmann area (BA) 9. Next, we interrogated DNA methylation of selected candidate genes in additional brain regions (beside hippocampus also amygdala, insula and BA46) and blood using the complete set of subjects. We wanted to confirm the results obtained using NGS with an additional method. Candidate genes for which we observed an altered rate of methylation between the two study groups were further examined by analysing their expression by qPCR for both the first and second part of the doctoral thesis.
In our study we determined multiple differences in DNA methylation state between suicide victims and the control group. Genome-wide DNA methylation revealed increased hypomethylation in suicide victims for both brain regions. We found a large number of differentially methylated cytosines with the difference of the percentage of methylation between the two groups greater than 25 % and with the q-value below 0.01. Gene ontology analysis showed enrichment for genes associated with cell structural integrity and nervous system regulation. In BA9 expression of two genes was elevated in suicide victims (ZNF714 and NRIP3). In the second part of our work, we performed DNA methylation analysis for candidate genes, and we detected differences in DNA methylation for all studied brain regions and blood. Although we observed differences within all candidate genes, some differences in DNA methylation were low (less than one percent between studied groups). Gene expression between suicide victims and control group differed statistically in the hippocampus (ZNF714, NR3C1, SLC6A4, and HTR1A) and blood (NRIP3).
The results of our analyses provide new insights into the methylation-altered state in suicidal behaviour and suggest the possibility of changes in brain plasticity. We identified new potential candidate genes and strengthen the association with suicidal behaviour in already known candidate genes. The results therefore support the role of the biological component in suicidal behaviour and contribute to its understanding in a highly suicidal Slovenian population.