Posted in Cerebellum, Frontal cortex, NTRK2, tagged Depression, Development, DNA methylation, Epigenetics, Frontal lobe, Gene expression, Mental disorder, Mental health, Suicide on November 11, 2009|
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Few events are as hard to understand as the loss of a loved one to suicide – a fatal confluence of factors that are oft scrutinized – but whose analysis can provide little comfort to family and friends. To me, one frightening and vexing aspect of what is known about the biological roots of depression, anxiety, impulsivity and other mental traits and states associated with suicide, is the way in which early life (even prenatal) experience can influence events in later life. As covered in this blog here and here, there appear to be very early interactions between emotional experience in early life and the methylation of specific points in the genome. Such methylation – often referred to as epigenetic marks – can regulate the expression of genes that are important for synaptic plasticity and cognitive development.
The recent paper, “Alternative Splicing, Methylation State, and Expression Profile of Tropomyosin-Related Kinase B in the Frontal Cortex of Suicide Completers” is a recent example of a link between epigenetic marks and suicide. The team of Ernst et al., examined gene expression profiles from the frontal cortex and cerebellum of 28 males lost to suicide and 11 control, ethnically-matched control participants. Using a subject-by-subject comparison method described as “extreme value analysis” the team identified 2 Affymetrix probes: 221794_at and 221796_at – that are specific to NTRK2 (TRKB) gene – that showed significantly lower expression in several areas of the frontal cortex. The team also found that these probes were specific to exon 16 – which is expressed only in the TRKB.T1 isoform that is expressed only in astrocytes.
Further analysis showed that there were no genetic differences in the promoter region of this gene that would explain the expression differences, but, however, that there were 2 methylation sites (epigenetic differences) whose methylation status correlated with expression levels (P=0.01 and 0.004). As a control, the DNA-methylation at these sites was not correlated with TRKB.T1 expression when DNA and RNA was taken from the cerebellum (a control since the cerebellum is not thought to be directly involved in the regulation of mood).
In the case of TRKB.T1 expression, the team reports that more methylation at these 2 sites in the promoter region is associated with less TRKB.T1 expression in the frontal cortex. Where and when are these marks laid down? Are they reversible? How can we know or suspect what is happening to our epigenome (you can’t measure this by spitting into a cup as with current genome sequencing methods)? To me, the team has identified an important clue from which such follow-up questions can be addressed. Now that they have a biomarker, they can help us begin to better understand our complex and often difficult emotional lives within a broader biological context.
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In this podcast, Michael Corbin, founder of everyminute.org, shares some of his personal background, interests and efforts in the area of suicide prevention and mental health advocacy.
You can reach Michael via email or the website contact page.
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It is commonly known that some of us handle stress better than others. Some can calmly accept the dire economic news of an impending layoff while others may fret incessantly day-in-and-out and endure many a sleepless night. Why ? What are some of the brain systems that mediate the effects of accute and chronic stress ? What genetic and environmental differences might influence the development of these systems ?
In an ongoing set of experiments, Professor Michael Meaney’s laboratory has focused on the role of the glucocorticoid receptor (GR) and its role as a feedback modulator in the so-called hypothalamic-pituitary-adrenal (HPA) axis. A number of experiments have shown that upregulation of the GR is somewhat beneficial insofar as it dampens the deleterious rise of circulating corticosteroids during stress. Therefore, any mechanism that downregulates or blocks the expression of GR may make it harder for a person to cope with the typical physiologic responses (increases in corticosteroids) to stressful experiences (news of a layoff).
What Professor Meaney’s lab has shown so convincingly over the past several years is that individual differences in the reactivity of the HPA system are heavily influenced by maternal and early life experience. That is, offspring (often rat or mouse pups) who have attentive mothers who keep them warm and well groomed, have more responsive HPA systems that more readily dampen the deleterious rise of corticosteroids in response to steroids. In some cases, the level of maternal care is enough to modify the level of CpG methylation in the promoter region of the glucocorticoid receptor. This type of “epigenetic” form of gene regulation is a way in which the promoter region can be altered in a long-term manner given a particular early-life experience. Unfortunately, this type of epigenetic mark, can lead to life-long difficulty in managing stress.
Their recent paper, “Epigenetic regulation of the glucocorticoid receptor in human brain associates with child abuse” [doi 10.1038/nn.2270] explores the extent of CpG methylation in post-mortem tissue (hippocampus) from 24 individuals who tragically passed away in completion of suicide. The research team compared the levels of methylation (via bisulfite mapping) in the GR promoter region and found that there was significantly more methylation in (n=12) individuals who had a recorded history of childhood abuse (sexual contact, severe physical abuse and/or severe neglect) as compared to (n=12) individuals with no history of abuse (their CpG levels were not distinguishable from control tissue). Thus (as confirmed by qRT-PCR) it seems as if epigenetic marks were visible in the genomes of hippocampal cell nuclei – which may have very well been written during early childhood trauma – and may have exacerbated the difficulties these individuals may have had in coping with psychosocial stress.
Further studies conducted by the team evaluate the possibility that the sites of abuse-induced-CpG methylation have the effect of blocking the binding of the EGR1 transcription factor which provides an additional mechanistic part in a larger complex of proteins that transduce the effects of experience into long-lasting behavioral predispositions.
For more on the exciting rise of epigenetics and its role in nature-meets-nuture and cognitive development click here.
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Image via Wikipedia The “Central Dogma” of molecular biology rightfully points out a somewhat one-way transfer of information from DNA to RNA to protein. This mechanism has obvious implications for evolution insofar as you are issued a newly shuffled genome at birth and must make the best of it – no cheating allowed by receiving the acquired levels of fitness of your parents – since these cannot be transmitted via the bare thread of DNA. This being the case, however, it is, of course, fun to encounter ways in which mother nature skirts the rules. The term ‘mother‘ is particularly apt to the work of Michael Meaney and colleagues in Montreal, who have for many years been teasing apart mechanisms underlying maternal care in mammals. It seems that when a female rat has been deprived of good mothering (copious licking & grooming are the traits of a good rat mom) they, themselves, also demonstrate poor mothering skills (sadly, daughters DO grow up to be their mothers). The Meany group provide a great review of the mechanisms of this seeming example of “inheritance of acquired characteristics” in their review, “Epigenetic Programming of Phenotypic Variations in Reproductive Strategies in the Rat Through Maternal Care” [DOI: 10.1111/j.1365-2826.2008.01725.x]. Apparently, this mode of inheritance is dependent on the early development of neuro-endocrine circuits that regulate emotional responsivity and are dependent on early, neonatal environmental stimulation (licking & grooming activate these developing circuits) – and is not dependent on the sperm/egg-bourne passage of a particular stretch of DNA. Interestingly though, the team demonstrates that genomic CpG hypermethylation of the estrogen receptor might serve as a mechanism to maintain the effect – at the level of the genome – of the mother’s poor parenting. Mom’s who were poorly cared for as infants may have a hypermethylated estrogen receptor and therefore are more likely to demonstrate poor parenting in adulthood as a result of the maintainence of this methylated (transcriptionally repressed) estrogen receptor. More interestingly, the team has recently begun to investigate this mechanism in humans, and reported that in post-mortem analysis of hippocampal tissue from individuals who experienced early life neglect and, tragically, suffered death from suicide, that there seems to be a similar type of hypermethylation. Their PLoS ONE article, “Promoter-Wide Hypermethylation of the Ribosomal RNA Gene Promoter in the Suicide Brain” [DOI: 10.1371/journal.pone.0002085] provides an analysis of promoters of rRNA genes in the hippocampus – a brain region whose development and structure is negatively affected by environmental stress and neglect. This is a line of research that is interesting on many levels – from Lamarck to Freud. As a parent, the work shows how important it is to understand & appreciate the role of parenting and social welfare in mental health.
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Posted in 5HTT, AMPA receptor, Glutamate, GRIA3, GRIK2, tagged Antidepressant, Depression, Major depressive disorder, SSRI, Suicide on September 1, 2007|
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Image via Wikipedia The recent paper, “Genetic Markers of Suicidal Ideation Emerging During Citalopram Treatment of Major Depression” finds that among 68 candidate genes, markers for 2 AMPA-type glutamate receptors (rs4825476, rs2518224: GRIA3 and GRIK2) show significant association in 120 individuals who experienced suicidal ideation in a large medication trial for major depressive disorder. Many families with loved ones suffering from depression remain wary and confused about a possible causal relationship between selective serotonin reuptake inhibitor (SSRI) antidepressants and suicide. A current FDA-mandated black box warning advises youths on the potential risks. This recent genetic study seems to provide a meaningful step forward in better understanding the mechanism of shifts in mood and cognition that occur in some individuals. But like many brain research studies though, shining a tiny ray of light on a puzzle suddenly illuminates massive complexities, previously unseen. A great deal of research shows that SSRI exposure leads to long lasting changes in AMPA receptor expression, localization and function, – but it’s unclear where a specific link between this and changes in mood and cognition will be drawn.
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