Role of HDAC5 and SIRT2 in depression and clinical efficacy of antidepressants

Personalised medicine proposes the use of peripheral biomarkers of major depression (MD) to optimize the treatment and medical care to each person from the first day. The epigenetic hypothesis of depression proposes that MD is linked to remodelation of chromatin leading to neuronal plasticity failure in limbic brain regions. Among epigenetic mechanisms, histone posttranslational modifications such as histone acetylation could influence neuroplasticity. Among the members of the histone deacetylase (HDAC) superfamily, our laboratory has identified two HDAC enzymes, HDAC5 and SIRT2, as oppositely regulated by stress and antidepressant drug treatment in the brain of animal models. Importantly, both HDAC5 and SIRT2 exert a repressing action on the expression of BDNF, a gene with a leading role in neuroplasticity and that is downregulated in MD. In line with preclinical studies, peripheral blood mononuclear cells or prefrontal cortex tissue from depressed patients show an increase of HDAC5 and SIRT2. The general aim of this thesis is to explore in monocytes or T-cells whether HDAC5 and SIRT2, as well as the expression of genes linked to their function could be epigenetic peripheral biomarkers of MD and/or antidepressant therapy. A team of psychiatrists from different hospitals and health centres from Pamplona (Navarra, Spain) recruited 56 MD patients scoring for their Montgomery-Åsberg scale (MADRS&#8805;20) and age and sex matched healthy controls (MADRS<7). Blood (20 mL, heparine tubes) was obtained from each participant and monocytes (classic, intermediate and non-classic) and T-cells (CD3+) were isolated by fluorescence activated cell sorter. HDAC5 and SIRT2 immunofluorescences were carried out and their distribution between cytoplasm and nucleus was calculated by confocal microscopy. In addition, classic monocytes and T-cells were incubated in vitro with the &#61537; adrenoceptor agonist naphazoline and phosphorylation of HDAC5 at serine 498 was measured by immunofluorescence. Gene expression studies by qPCR included HDAC5 and SIRT2 as well as other genes linked to their function related to brain plasticity (BDNF), and other functions such as inflammation (KLF2, eNOS). In T-cells, the expression of some regulatory markers was also studied (FOXP3, PD-1, CTLA-4). Subsequently, full-length RNAseq of classic monocytes was applied to a selected sample of 9 MD patients and 9 healthy controls. Both in monocytes and T-cells of MD patients HDAC5 mRNA expression was upregulated and HDAC5 cytoplasm/nucleus ratio was decreased. Moreover, a negative correlation between MADRS score and HDAC5 cytoplasm/nucleus ratio was observed. In addition, SIRT2 cytoplasm/nucleus ratio was decreased in responder MD patients. Thus, in MD patients, these enzymes might accumulate in the nucleus of these cells. In T-cells of MD patients, BDNF and KLF2 mRNA expression were downregulated matching with the expected repressing action of nuclear HDAC5 and SIRT2. In monocytes, ADRB2 mRNA, gene encoding &#946;2 adrenoceptor, was upregulated Interestingly, a significant negative correlation between ADRB2 mRNA expression and HDAC5 cytoplasm/nucleus ratio in monocytes of MD patients. Longitudinal studies showed a persistent BDNF mRNA downregulation in monocytes of non-responder MD patients. Finally, RNA sequencing of monocytes from highly TRD patients revealed alterations in biological functions that suggest an inflammatory profile in these cells. In this study, we have provided evidence of the existence of peripheral epigenetic biomarkers associated to MD and that might influence the activity of immune system. Moreover, BDNF expression in T-cells appears for the first time as a peripheral biomarker of MD. Future studies should explore whether restoring these biomarkers up to healthy values would contribute to clinical response in MD patients.