Article summary with Effects of the social environment and stress on glucocorticoid receptor gene methylation: A systematic review by Turecki & Meaney - 2016

Introduction 

The early-life social environment can induce stable changes that influence neurodevelopment and mental health, meaning that they have a persistent impact on gene expression and behavior through epigenetic mechanisms. The hypothalamus-pituitary-adrenal (HPA) axis is sensitive to changes in the early-life environment that associate with DNA methylation of neuro-specific exon 17 of the glucocorticoid receptor (GR). 

DNA methylation is a process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts to repress gene transcription.

In rats, the offspring of mothers with an increased frequency of pup licking/grooming (LG) show increased GR expression, greater negative feedback regulation over hypothalamic corticotropin releasing factor (CRF), and more modest responses to stress compared with offspring of low LG mothers. Variations in maternal LG are linked to an epigenetic modification of a neuron-specific exon 17 GR promoter such that increased maternal LG associates with decreased methylation of the exon 17 promoter and increased hippocampal GR expression. 

In humans, evidence for a long-term effect of early-life adversity (ELA) on the epigenetic state of the human genome was observed while investigating the methylation state of the GR gene in the hippocampus of individuals who committed suicide and had a history of child abuse. ELA in humans reprograms the DNA methylation patterns of the GR gene exon 1F (GR1F) expression in the hippocampus of suicide completers with a history of child abuse compared to non-abused suicide completers and healthy control subjects. Another study has found that children born in mothers with depression, irrespective of SSRI use, exhibited higher GR1F methylation levels. 

The article summarizes 40 articles that either used humans or mammals like mice or rats to find evidence for their hypotheses.

Early life adversity 

Experimental groups and GR region Studied

In animal models, ELA was characterized by either the use of maternal care or maternal separation models. All of the animal studies examined exon 17 when studying the GR region. 

In human studies, ELA was defined as exposure to traumatic events in childhood, including emotional, physical, or sexual abuse, neglect, early parental death, and other traumatic events. The majority of human studies examined exon 1F, one examined exon 1D, while 2 also included exon 1B and 1C and 3 studies examined exon 1H, when studying the GR region.

Effect on Methylation

The ELA group contains results from 10 studies for each animal and human studies. 

7/10 animal studies reported a significant increase in methylation in the exon 17 promoter due to ELA, while 3 studies reported no such change. 9/10 human studies reported increased promoter methylation with ELA.  

Parental Stress

Experimental groups and GR region Studied

One animal study used a variety of non-pain-inducing, non-habituating, stressors over a course of 7 days to stress mouse dams. The study examined exon 17 when studying the GR region. 

Human studies examined the methylation status of children of women who had experienced anxiety and mood disorders (2 studies), pregnancy-related anxiety (1 study), violence (1 study), or war stress (2 studies) during pregnancy. Another study examined the effects of parental stress on GR gene methylation in adolescence, and an additional study examined the methylation pattern of individuals whose parents had experienced war but not during pregnancy. 7/8 studies examined exon 1F, while one study examined exons 1B and 1D, when studying the GR region. 

Effect on Methylation

All 8 studies investigating exon 1F, respectively 17 for animals, reported an increased methylation of exon 1F/17 in offspring of parentally stressed individuals. Of these, 2 studies also reported decreased methylation at specific CpG sites and in particular conditions, such as that the stressor during pregnancy only existed during the first or second trimester of the pregnancy. The one study that was investigating exon 1B and 1D reported a decrease in methylation of exon 1B and an increase in methylation of exon 1D in children of women experiencing fear of delivery in all trimesters or fear of the integrity of the baby in the first trimester. 

Psychological stress/ Psychopathology 

Experimental groups and GR region Studied

3 animal studies used acute or chronic stress models to assess the impact of social stressors on GR gene methylation. All of which examined exon 17 region.

8 human studies examined the effects of psychopathologies on human GR gene methylation. Specifically, one study involved response to stress, a second BPD, a third bulimia nervosa, another two PTSD and an additional two depression. Of which 6/8 examined exon 1F and in addition 3 studies examined exon 1B, and 2 studies examined 1J and 1E. 

Effect on Methylation 

In the animal models, 2/3 studies reported increased exon 17 methylation in stressed animals. 

In human studies, results were more varied. One study reported increased methylation of exon 1 F, while 3 studies reported no change and 2 reported decreased methylation. Among the other exon variants examined, there was no consensus on the effect of psychological stress/psychopathology on methylation status. 

Conclusion

This article reviewed a set of 40 studies that research GR gene methylation in relation to various psychological stressors, including parental stress, ELA in social environments in animals, ELA in humans, and psychological stress or psychopathology in adults, by, in most of the studies, investigating the GR exon variant 1F or 17 for animals. 

The majority of the studies concerning ELA and in utero adversity reported increased methylation at the respective exon. In particular negative early-life social environment were associated with greater exon 1F methylation in the large majority of the studies. These findings show a compelling consensus across studies and methodologies (eg. Tissue samples from different locations were used in different studies, such as brain or blood tissue) Furthermore, childhood maltreatment associates with increased exon 1F methylation and the promoter methylation status was closely correlated with both the frequency and severity of maltreatment. One study reported that increased methylation of the exon 1F NR3C1 gene promoter in leukocytes is associated with the disruption of normal parent-offspring interaction or maltreatment, and linked an attenuated cortisol response. Another study supported this claim and noted that parental loss was also associated with increased methylation on the same gene promoter. 

Recent evidence suggests that epigenetic plasticity is sustained in the brain throughout adulthood, potentially as a mechanism to cope with the evolving demands of the environment, yet, there are clear moments during development when plasticity is heightened and these may be more strongly associated with the establishment of life-long epigenetic modifications. 

Studies with adults revealed that childhood maltreatment is associated with an increased HPA response to stress. Subsequent statistical analyses revealed that childhood abuse was the strongest predictor of adrenocorticotropic hormone (ACTH) responsiveness, followed by the frequency of abusive events, adulthood traumas, and depression. An interaction term of childhood and adulthood trauma proved to be the best predictor for ACTH responses, suggesting that a history of childhood abuse is related to increased stress reactivity, which is further enhanced when additional trauma occurs in adulthood. 

Cerebrospinal fluid CRF concentrations were correlated with the severity and duration of physical and sexual abuse, and high CRF may arise due to GR downregulation and impaired negative feedback inhibition. Some subjects having experienced childhood abuse exhibit a decreased cortisol production, which may reflect an adaptation to chronically stressful situations, whereas elevated cortisol production may prime individuals to react to unpredictable stressors, and these situations may both constitute ELA. Currently, it is difficult to draw conclusions  on the overall impact of GR methylation variations on basal and reactive cortisol levels, as the majority of studies investigating GR promoter methylation did not measure cortisol levels. 

These findings suggest that childhood adversity stably influences HPA responses to stress, which is consistent with the idea that childhood maltreatment sensitizes neural and endocrine responses to stress, thus establishing a vulnerability for mood disorders. 

Recent studies suggest that epigenetic programming of HPA function occurs at multiple levels of the HPA axis in addition to effect on hippocampal GR expression. 

Environmental conditions that increase the frequency of LG in the rat are associated with decreased paraventricular CRF expression. Augmented maternal care in return, reduced the number of excitatory synapses onto CRF neurons. Decreased CRF, through disruption of maternal care in mice, showed enhanced glutamatergic transmission to hypothalamic CRF neurons in offspring. Moreover, prolonged periods of maternal separation alter the methylation state of the promoter for the Avp gene, increasing hypothalamic arginine vasopressin synthesis and HPA responses to stress. These findings reveal that the quality of postnatal maternal care in rodents epigenetically programs gene expression at multiple levels of the HPA axis to regulate both basal and stress induced activity. 

A remarkable feature of all the findings is the coordinated epigenetic effects on multiple genes, in multiple tissues, that collectively serve to increase HPA responsivity in stress responses in early social adversity.