EPIGENETIC REGULATION OF THE GLUCOCORTICOID RECEPTOR IN HUMAN BRAIN ASSOCIATES WITH CHILDHOOD ABUSE PATRICK O MCGOWAN, AYA SASAKI, ANA C D’ALESSIO, SERGIY DYMOV, BENOIT LABONTÉ, MOSHE SZYF, GUSTABO TURECKI & MICHAEL J MEANEY Vocabulary Epigenetics: the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA nucleotide sequence. DNA methylation and histone deacetylation are two processes which can cause these heritable changes. DNA Methylation: a biochemical process that involves the addition of a methyl group to the 5 position of the cytosine pyrimidine ring or the number 6 nitrogen of the adenine purine ring. This modification serves to suppress gene expression, and can be inherited through cell division. DNA methylation is a crucial part of normal organismal development, because it stably alters the gene expression pattern in cells such that cells can "remember where they have been” and can decrease gene expression. Glucocorticoid Receptor NR3C1: the receptor to which cortisol and other glucocorticoids bind. Its primary mechanism of action is the regulation of gene transcription. The activated complex up-regulates the expression of anti-inflammatory proteins in the nucleus or represses the expression of pro-inflammatory proteins in the cytosol. In humans, the GR protein is encoded by NR3C1 gene which is located on chromosome 5. Hypothalamic-Pituitary-Adrenal (HPA) Axis: a complex set of direct influences and feedback interactions among the hypothalamus, the pituitary gland, and the adrenal glands. Interactions between these organs constitutes a major part of the neuroendocrine system that controls reactions to stress and regulates many body processes. Nerve growth factor-inducible A (NGFI-A): NGFI genes are early response genes, and other signals that initiate growth and differentiation of cells. NGFIs are a family of related proteins that function as transcription factors. Abstract Past studies have shown that maternal care influences hypothalamicpituitary-adrenal (HPA) function in rat through epigenetic programming of glucocorticoid receptor expression. In humans, childhood abuse has also been seen to alter HPA stress responses and increase suicide risk. This lab examined the epigenetic differences in a neuron-specific glucocorticoid receptor (NR3C1) promoter between postmortem hippocampus obtained from suicide victims with a history of childhood abuse, victims with no childhood abuse, and controls. Major Findings of This Study: Decreased levels of glucocorticoid receptor mRNA mRNA transcripts bearing the glucocorticoid receptor 1F splice variant Increased cytosine methylation of an NR3C1 promoter Abused suicide victims showed decreased NGFI-A transcription factor binding and NGFI-A-inducible gene transcription These findings translate previous results from rat to humans and suggest a common effect of parental care on epigenetic regulation of hippocampal glucocorticoid receptor expression. Background- In Rats Maternal behavior alters the development of HPA responses to stress in the rat through tissue-specific effects on gene transcription, including forebrain glucocorticoid receptor expression, the activation of which inhibits HPA activity through negative feedback inhibition. The selective knockdown of glucocorticoid receptor expression in the corticolimbic system in rodents is associated with increased HPA activity under stressful conditions. Glucocorticoid receptor overexpression in associated with a damped HPA stress response. The transcription factor NGFI-A regulates the expression of NR3C1 promoter in the rat, and effect that is inhibited by DNA methylation. The effects of maternal care on hippocampal glucocorticoid receptor expression and, thus, HPA responses to stress, in the adult rodent are associated with an epigenetic modification of a neuron-specific exon 17 glucocorticoid receptor (NR3C1) promoter. BIG PICTURE: The rat homolog of 1F NR3C1 promoter is differently methylated as a function of variation in maternal care. This methylation is a highly stable epigenetic mark, which changes the patterns of the regulation of gene expression. GOAL: Try to translate these findings to humans Background- In Humans Familial function and childhood adversity are linked to altered HPA stress responses in humans, which are linked to psychopathology and, in some cases, suicide. Environmental events that associate with decreased hippocampal glucocorticoid receptor expression and increased HPA activity enhance the risk of suicide. Methods The researchers examined the expression of total glucocorticoid receptor and glucocorticoid receptor 1F using quantitative reverse transcription PCR (qRTPCR) with RNA extracted from hippocampal tissue of suicide completers with and without a history of childhood abuse and from controls. Hypotheses The researchers hypothesized that suicide victims would show decreased expression both of glucocorticoid receptor and glucocorticoid receptor 1F, compared with control subjects. They also hypothesized that DNA methylation regulates the expression of the NR3C1 promoter through alterations in transcription factor binding. Results Expression of total glucocorticoid receptor mRNA was significantly reduced in suicide victims with a history of childhood abuse There was a significant effect on the expression of transcripts containing the exon 1F NR3C1 promoter Glucocorticoid receptor 1F expression was significantly lower in samples from suicide victims with a history of childhood abuse No difference between nonabused victims and controls Exon 17 of NR3C1 is similar to the rat exon 17, which reveals a maternal effect on cytosine methylation and expression Data Another Question The researchers also questioned whether, as in the rat, NGFI-A could regulate gene transcription through the NR3C1 promoter and whether this effect might be influenced by the methylation status of the promoter. Results The results found indicate that methylation reduces the effect of NGFI-A induction of gene expression through the NR3C1 promoter. The decreased glucocorticoid receptor transcription observed in suicide victims with a history of childhood abuse was associated with differences in methylation levels occurring only at specific sites in the exon 1F NR3C1 promoter. They also found an effect of methylation status on transcription factor-induced gene expression from the NR3C1 promoter. There was also a significant interaction between methylation status and the NGFI-A expression NGFI-A binding and transcriptional activation is reduced through the exon 1F NR3C1 promoter. Discussion Changes in glucocorticoid receptor expression are linked with a developmental history of family adversity, in this case a history of child abuse, then with suicide completion. The findings were consistent with those from studies conducted with rodent and nonhuman primates showing that persistent disruptions of mother-infant interactions are associated with increased hypothalamic corticotrophin-releasing hormone expression and increased HPA responses to stress. The results show that the methylation of exon 1F NR3C1 promoter amongst victims with a history of childhood abuse is statistically significant, and therefore, that the environments in which the subjects were raised affected their genome and their body’s chemistry Discussion Continued… Their data revealed increased site-specific methylation in the exon 1F NR3C1 promoter in suicide victims with a history of childhood abuse, and suggest that there is a relationship between cytosine methylation, transcription factor binding, and gene expression. These findings suggest that the transmission of vulnerability for depression could occur from parent to offspring through the epigenetic modification of genomic regions that are implicated in the regulation of stress responses. Further Studies Since one interpretation for these findings is that childhood adversity could be altering the development of systems that serve to regulated stress responses, eg. hippocampal glucocorticoid receptor expression, and enhance the effect of stress, a challenge for the future is to understand how epigenetic variation might, for example, explain the developmental origins of vulnerability for chronic illnesses. Bibliography McGowan, Patrick O., Aya Sasaki, Ana C. D'Alessio, Sergiy Dymov, Benoit Labonté, Moshe Szyf, Gustavo Turecki, and Michael J. Meaney. "Epigenetic Regulation of the Glucocorticoid Receptor in Human Brain Associates with Childhood Abuse." Nature- Neuroscience (2009). Print. 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