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Genetic responses to environmental stress underlying major depressive disorder Aleix Arnau Soler Doctor of Philosophy The University of Edinburgh 2019 Declaration I hereby declare that this thesis has been composed by myself and that the work presented within has not been submitted for any other degree or professional qualification. I confirm that the work submitted is my own, except where work which has formed part of jointly-authored publications has been included. My contribution and those of the other authors to this work are indicated below. I confirm that appropriate credit has been given within this thesis where reference has been made to the work of others. I composed this thesis under guidance of Dr. Pippa Thomson. Chapter 2 has been published in PLOS ONE and is attached in the Appendix A, chapter 4 and chapter 5 are published in Translational Psychiatry and are attached in the Appendix C and D, and I expect to submit chapter 6 as a manuscript for publication. Therefore, the use of “we” is retained in these chapters. Only minor changes have been applied in comparison to the submitted manuscripts to keep a uniform format along the entire body of the thesis. Co-authors to these studies are indicated below. Co-authors contributed with the collection, access and preliminary curation of some raw data and/or providing critical revisions. Dr. Pippa Thomson also contributed to the initial draft of the introduction in chapter 2 and with supervision of all chapters. I confirm that I performed all the analyses and wrote the draft manuscripts myself. Dr. Pippa Thomson, Dr. Caroline Hayward (my 2nd supervisor) and Dr. Mark Adams co-author chapters 2, 4, 5 and 6. Prof. Andrew McIntosh co-author chapters 4, 5 and 6 and was the Chair of my thesis committee. Dr. Toni-Kim Clarke, Dr. Donald MacIntyre, Mr. Keith Milburn and Dr. Lauren Navrady co-author chapters 4 and 5. MSc Erin Macdonald-Dunlop co-author chapter 5. Generation Scotland and the Major Depressive Disorder Working Group of the Psychiatric Genomics Consortium co-author chapters 2, 4, 5 and 6. Aleix Arnau Soler 10th January 2019 i Abstract Major depressive disorder (MDD) is a common psychiatric disorder and a leading cause of disability worldwide. Such illness is the result of a complex interplay between genetic susceptibility and environmental risk factors. Adverse life events are experienced before the onset of depressive episodes in most patients, with robust evidence for the role of stressful life events (SLE) as a main trigger of depressive symptoms. However, not all individuals develop depression after episodes of stress. Thus, an individual’s sensitivity to stress is an important predictor of stress response that may mediate the association between stress and depression. A deeper understanding of the genetic mechanisms underlying stress-sensitivity and stress response is, therefore, crucial to a better understanding of MDD and thus to improve treatments for both depressive symptoms and other stress-related conditions. This PhD thesis uses empirical data from white Caucasian population-based samples. By incorporating in new hypothesis-free genome-wide association studies and polygenic approaches quantitative measures of recent SLE and neuroticism---a personality trait though to mediate or moderate the effects of adversity on depression risk---, this PhD thesis identifies the genetic influences to a proxy for sensitivity to environmental stress and genotype-by- environment interaction (GxE) effects underlying depressive symptoms. Following an introductory chapter, chapter 2 conceptualizes a proxy for our sensitivity to negative outcomes by modelling the interaction between genetic variants and MDD status on neuroticism score through a genome-wide interaction study. This chapter seeks to identify genetic variants contributing to a potential endophenotype mediating the associations between stress and depression, and examines whether genetic effects on such proxy for stress sensitivity partially explains the genetic contributions to liability not attributable to additive main effects. The strongest signals came from genetic variants associated with the glucocorticoid receptor function. Therefore, iii Chapter 3 assesses the enrichment of the genetic contributions to liability of MDD within three glucocorticoid-related gene sets: one gene set reflecting “up-stream” cortisol signalling genes and two gene sets reflecting “down- stream” cortisol response genes. Chapter 4 empirically tests and assesses the diathesis-stress theory for depression; using polygenic risk scores weighted by the additive effects of MDD derived from the Psychiatric Genetic Consortium MDD genome-wide association study and self-reported measures on recent SLE. This chapter provides evidence for the presence of GxE effects between stress and common genetic variants on risk of depressive symptoms. The empirical support for this theory validates other GxE approaches applying a genome-wide approach to investigate the causative effect of stress in the development of depressive symptoms. Thus, chapter 5 presents findings from genome-wide by environment interaction studies in two cohorts that seek to identify common variants displaying an increased risk of liability to depressive symptoms in response to SLE. Whether inclusion of GxE effects improves the prediction of liability to MDD over that explained by genetic additive main effects alone is also tested. Furthermore, two potential forms of gene-environment interplay (i.e. GxE and gene-environment correlation) and their biological interpretation are extensively discussed. Stress contributes to many human conditions. Therefore, the GxE effects are also used to predict other stress-related physical and mental conditions. This chapter reports evidence of a potential shared aetiology between depression and other traits, such as schizotypal personality or heart disease, due to genetic mechanism underlying the effects of SLE. Finally, chapter 6 brings back the diathesis-stress model investigated in chapter 4. This chapter incorporates into the diathesis framework the genetics effects for stress sensitivity and stress response estimated in chapters 2 and chapter 5, respectively, and assess their relevance to the diathesis-stress theory. Genetic differences between women and men in stress response underlying the aetiology of depression are also discussed. iv Genetics plays a significant role in the effects of stress. The findings presented in this thesis emphasize the relevance of genetic effects for stress sensitivity and stress response in depression and health in general. Overall, this thesis presents a range of original studies in order to advance our understanding of the genetic response to stress, comprehensively discussing the limitations and pitfalls of this research area, and provides a basis for future lines of research on gene-environment interplay in psychiatry. v Lay Summary Depression is a common mental disorder that impacts on individuals, families and society. Genetic and environmental factors combine to increase the risk of suffering from depression. Indeed, before developing symptoms, most patients are affected by adverse or stressful environments. However, not all individuals that are exposed to adverse circumstances get depressed, suggesting differences in vulnerability to stress. One can think about this sensitivity towards the effects of stress as a behavioural or personality trait that mediates the association between stress and depression. Hence, several genes may influence such trait. A genetic component is behind the differences in how people respond to the effects of stress. Some genes modulate the effects of negative environments incrementing the risk of illness. This is known as gene-by-environment interaction effect. In this PhD thesis, the theory that genetic variation and stressful life events combine to increase the liability to depression is investigated. Multiple methods are explored in order to identify the nature of the genetic variants underlying both sensitivity and response to the effects of stress, as well as their importance in predicting depressive symptoms, in white European population. Genetic response to stress plays a key role on the onset and development of depression. Identifying genetic mechanisms underpinning the causative effects of stress may help to advance our understanding, not only of depression, but also of other disease of which stress is a main trigger, and thus, improve current treatments. This PhD thesis encompasses several original studies on this area of research seeking along the entire genome those genes contributing to the responses to environmental