Epigenetic consequences of childhood maltreatment on genes associated with stress response, oxytocinergic system, and biological aging : an intergenerational approach

Abstract

The consequences of childhood maltreatment (CM) extend into adulthood, manifesting as risk factors associated with impaired physical and psychological health, and even with premature biological aging. These consequences might be biologically transmitted to the next generation via DNA methylation, a pivotal epigenetic mechanism involved in dynamic systemic adaptation of the gene transcription to cope with environmental challenges. The general aim of this thesis is to investigate, in mother-infant dyads, the associations of CM with changes of the DNA methylation levels and whether they are biologically transmitted to the next generation. In particular, the first aim of this thesis is to investigate the impact of CM on the regulation of DNA methylation of key genes involved in stress response (i.e. FKBP5, NR3C1, and CRHR1), using isolated peripheral blood mononuclear as a cellular model to study epigenetics in the context of CM. The second aim of this thesis is to explore the effects of CM on the epigenetic regulation of the oxytocingeric system, a physiological system that is involved in the modulation of mother-infant bonding and emotion regulation. The third aim is to determine whether CM accelerates biological aging in terms of DNA methylation of ELOVL2, a recently identified biomarker for age. The last and most important aim of this thesis is to examine the possible intergenerational effects of maternal CM on infant DNA methylation patterns shortly after parturition. Epigenetic alterations in the context of childhood, adolescence, and adulthood is a growing field of study. Study I provides the literature basis for the hypotheses of this dissertation: a review of the evidence for somatic and psychiatric consequences associated with lifetime trauma, as well as the potential mechanisms for their intergenerational transmission. In order to investigate the impact of CM on DNA methylation of genes involved in the stress response, Study II investigates the levels of DNA methylation in immune cells collected from 117 women with varying CM severity in three selected genes involved in the physiological stress response: FK506 binding protein 51 (FKBP5), glucocorticoid receptor (NR3C1), and corticotropin-releasing hormone receptor 1 (CRHR1), additionally accounting for the role of the FKBP5 rs1360780 genotype. Study II reveals a relationship of CM and the epigenetic regulation of the targeted genes. Following a dose-response relationship, the overall levels of DNA methylation within FKBP5 and CRHR1 genes decreased with increasing CM severity. The overall methylation levels of NR3C1 were increased in women with mild to severe CM experiences when compared to women with no CM. The FKBP5 rs1360780 genotype moderated the association between CM and FKBP5 methylation levels in a gene-by-environment (G × E) interaction. The oxytocinergic system interplays with the immune response to adapt to environmental stress and has emerged as one buffering factor of the long-term effects of CM on physical and mental health. To explore the effects of CM in the molecular regulation of the oxytocinergic system, Study III investigates the associations between CM and OXTR methylation as well as OXTR genetic expression in the same cohort as Study II. Overall OXTR methylation levels and CM were not associated, but there were site-specific effects of CM in DNA methylation of four CpG units. Moreover, the OXTR rs53576 and OXT rs2740210 polymorphisms modulated OXTR mean DNA methylation levels in interaction with the severity of CM experiences, illustrating a G × E interaction. Based on the observations of premature onset of age-associated diseases in individuals with CM, CM has furthermore been hypothesized to be one factor contributing to accelerated aging. For a more comprehensive understanding of whether CM contributes to accelerated epigenetic aging, Study IV explores the DNA methylation of ELOVL2, a recently identified epigenetic biomarker for estimating the biological age of human cells. In line with previous work, ELOVL2 methylation was strongly correlated with the chronological age of the 117 analysed women. Morevoer, Study IV shows for the first time that CM is not associated with accelerated biological aging in terms of ELOVL2 methylation. This study also describes explorative analyses of DNA methylation in an extended region of the ELOVL2 gene and showes preliminary evidence for an association between the CM severity and DNA methylation of the exon 1 of ELOVL2. Finally, in order to explore a potential biological transmission of the observed CM-related changes in DNA methylation from CM-exposed mothers to their children, the DNA methylation of FKBP5, CRHR1, NR3C1 (Study II), OXTR (Study III), and ELOVL2 (Study IV) were further assessed in immune cells isolated from cord blood samples. There were positive associations of OXTR methylation levels between mothers and their infants, but only in dyads with non-CM mothers. However, there were no main effects of maternal CM on the DNA methylation levels of any of the targeted genes in the infants at the time of birth, suggesting that the epigenetic consequences of CM are not directly inherited by the offspring. In sum, this thesis identifies effects of CM on the epigenetic regulation of pivotal genes for stress response and emotion regulation in combination with genetic factors, and also shows that CM does not accelerate biological aging in terms of ELOVL2 DNA methylation in CM-affected individuals. This thesis shows first-hand evidence in humans that, in immune cells, the CM-associated epigenetic changes on the targeted genes are not biologically transmitted to the next generation – which is encouraging for the offspring of mothers with CM experiences, and contributes to the understanding of pathways that underlie the biological consequences of CM. These results could be related to the increased risk for physical illness in CM-affected individuals, and set a potential starting point for the development of new treatment approaches.