Download Epigenetic Regulation of the Glucocorticoid receptor in human brain

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
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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.
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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.
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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.
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In humans, the GR protein is encoded by NR3C1 gene which is located on chromosome 5.
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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.
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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
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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:
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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
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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
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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
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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
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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
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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
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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
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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
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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…
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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
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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
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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.
Minkel, Jr. "How Acquired Diseases Become Hereditary Illnesses: Scientific
American." Science News, Articles and Information | Scientific American.
Web. 18 Dec. 2011.
<http://www.scientificamerican.com/article.cfm?id=hereditaryacquisitions>.
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Cytokines & Cells Encyclopedia (Horst Ibelgaufts' COPE). Web. 18 Dec.
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