epigenetics of anxiety and stress–related disorders epigenetics of anxiety and stress–related disorders epigenetics of anxiety and stress–related disorders is the field studying the relationship between epigenetic modifications of genes and anxiety and stress-related disorders, including mental health disorders such as generalized anxiety disorder (gad), post-traumatic stress disorder, obsessive-compulsive disorder (ocd), and more. epigenetic modifications play a role in the development and heritability of these disorders and related symptoms. for example, regulation of the hypothalamus-pituitary-adrenal axis by glucocorticoids plays a major role in stress response and is known to be epigenetically regulated. as of 2015 most work has been done in animal models in laboratories, and little work has been done in humans; the work is not yet applicable to clinical psychiatry. epigenetic writers, erasers, and readers epigenetic changes are performed by enzymes known as writers, which can add epigenetic modifications, erasers, which erase epigenetic modifications, and readers, which can recognize epigenetic modifications and cause a downstream effect. stress-induced modifications of these writers, erasers, and readers result in important epigenetic modifications such as dna methylation and acetylation. dna methylation during dna methylation, cytosine is methylated. dna methylation is a type of epigenetic modification in which methyl groups are added to cytosines of dna. dna methylation is an important regulator of gene expression and is usually associated with gene repression. mecp2 laboratory studies have found that early life stress in rodents can cause phosphorylation of methyl cpg binding protein 2 (mecp2), a protein that preferentially binds cpgs and is most often associated with suppression of gene expression. stress-dependent phosphorylation of mecp2 causes mecp2 to dissociate from the promoter region of a gene called arginine vasopressin (avp), causing avp to become demethylated and upregulated. this may be significant because arginine vasopressin is known to regulate mood and cognitive behavior. additionally, arginine vasopressin upregulates corticotropin-releasing hormone (crh), which is a hormone important for stress response. thus, stress-induced upregulation of avp due to demethylation might alter mood, behavior, and stress responses. demethylation of this locus can be explained by reduced binding of dna methyl transferases (dnmt), an enzyme that adds methyl groups to dna, to this locus. mecp2 is known to have interactions with several other enzymes that modify chromatin (for example, hdac-containing complexes and co-repressors) and in turn regulate activity of genes that modulate stress response either by increasing or decreasing stress tolerance. for example, epigenetic upregulation of genes that increase stress response may cause decreased stress tolerance in an organism. these interactions are dependent on the phosphorylation status of mecp2, which as previously mentioned, can be altered by stress. dnmt1 dna methyltransferase 1 (dnmt1) belongs to a family of proteins known as dna methyltransferases, which are enzymes that add methyl groups to dna. dnmt1 is specifically involved in maintaining dna methylation; hence it is also known as the maintenance methylase dnmt1. dnmt1 aids in regulation of gene expression by methylating promoter regions of genes, causing transcriptional repression of these genes. dnmt1 is transcriptionally repressed under stress-mimicking exposure both in vitro and in vivo using a mouse model. accordingly, transcriptional repression of dnmt1 in response to long-term stress-mimicking exposure causes decreased dna methylation, which is a marker of gene activation. in particular, there is decreased methylation of a gene called fkbp5, which plays a role in stress response as a glucocorticoid-responsive gene. thus, chronic stress may cause demethylation and hyperactivation of a stress-related gene, causing increased stress response. additionally, dnmt1 gene locus has increased methylation in individuals who were exposed to trauma and developed post-traumatic stress disorder (ptsd). increased methylation of dnmt1 did not occur in trauma-exposed individuals who did not develop ptsd. this may indicate an epigenetic phenotype that can differentiate ptsd-susceptible and ptsd-resilient individuals after exposure to trauma. transcription factors transcription factors are proteins that bind dna and modulate the transcription of genes into rna such as mrna, trna, rrna, and more; thus they are essential components of gene activation. stress and trauma can affect expression of transcription factors, which in turn alter dna methylation patterns. for example, transcription factor nerve growth-induced protein a (ngfi-a, also called nab1) is up-regulated in response to high maternal care in rodents, and down-regulated in response to low maternal care (a form of early life stress). decreased ngfi-a due to low maternal care increases methylation of a glucocorticoid receptor promoter in rats. glucocorticoid is known to play a role in downregulating stress response; therefore, downregulation of glucocorticoid receptor by methylation causes increased sensitivity to stress. histone acetylation during histone acetylation, lysines are acetylated. histone acetylation and deacetylation is a type of epigenetic modification in which acetyl groups are added to lysine on histone tails. histone acetylation, performed by enzymes known as histone acetyltransferases (hats), removes the positive charge from lysine and results in gene activation by weakening the histone's interaction with negatively-charged dna. in contrast, histone deacetylation performed by histone deacetylases (hdacs) results in gene deactivation. hdac transcriptional activity and expression of hdacs is altered in response to early life stress. for animals exposed to early life stress, hdac expression tends to be lower when they are young and higher when they are older. this suggests an age-dependent effect of early life stress on hdac expression. these hdacs may result in deacetylation and thus activation of genes that upregulate stress response and decrease stress tolerance. transgenerational epigenetic influences genome-wide association studies have shown that psychiatric disorders are partly heritable; however, heritability cannot be fully explained by classical mendelian genetics. epigenetics has been postulated to play a role. this is because there is strong evidence of transgenerational epigenetic effects in general. for example, one study found transmission of dna methylation patterns from fathers to offspring during spermatogenesis. more specifically to mental illnesses, several studies have shown that traits of psychiatric illnesses (such as traits of ptsd and other anxiety disorders) can be transmitted epigenetically. parental exposure to various stimuli, both positive and negative, can cause these transgenerational epigenetic and behavioral effects. parental exposure to trauma and stress trauma and stress experienced by a parent can cause epigenetic changes to its offspring. this has been observed both in population and experimental studies. holocaust an epidemiological study investigating behavioral, physiological, and molecular changes in the children of holocaust survivors found epigenetic modifications of a glucocorticoid receptor gene, nr3c1. this is significant because glucocorticoid is a regulator of the hypothalamus-pituitary-adrenal axis (hpa) and is known to affect stress response. these stress-related epigenetic changes were accompanied by other characteristics that indicated higher stress and anxiety in these offspring, including increased symptoms of ptsd, greater risk of anxiety, and higher levels of the stress hormone cortisol. experimental evidence the effect of parental exposure to stress has been tested experimentally as well. for example, male mice who were put under early life stress through poor maternal care—a scenario analogous to human childhood trauma—passed on epigenetic changes that resulted in behavioral changes in offspring. offspring experienced altered dna methylation of stress-response genes such as cb1 and crf2 in the cortex, as well as epigenetic alterations in transcriptional regulation gene mecp2. offspring were also more sensitive to stress, which is in accordance with the altered epigenetic profile. these changes persisted for up to three generations. in another example, male mice were socially isolated as a form of stress. offspring of these mice had increased anxiety in response to stressful conditions, increased stress hormone levels, dysregulation of the hpa axis which plays a key role in stress response, and several other characteristics that indicated increased sensitivity to stress. inheritance of small-noncoding rna studies have found that early life stress induced through poor maternal care alters sperm epigenome in male mice. in particular, expression patterns of small-noncoding rnas (sncrnas) are altered in the sperm, as well as in stress-related regions of the brain. offspring of these mice exhibited the same sncrna expression changes in the brain, but not in the sperm. these changes were coupled with behavioral changes in the offspring that were comparable to behavior of the stressed fathers, especially in terms of stress response. additionally, when the sncrnas in the fathers' sperm were isolated and injected into fertilized eggs, the resulting offspring inherited the stress behavior of the father. this suggests that stress-induced modifications of sncrnas in sperm can cause inheritance of stress phenotype independent of the father's dna. exercise just as parental stress can alter epigenetics of offspring, parental exposure to positive environmental factors cause epigenetic modifications as well. for example, male mice that participated in voluntary physical exercise resulted in offspring that had reduced fear memory and anxiety-like behavior in response to stress. this behavioral change likely occurred due to expressions of small non-coding rnas that were altered in sperm cells of the fathers. stress effect reversal additionally, exposing fathers to enriching environments can reverse the effect of early life stress on their offspring. when early life stress is followed by environmental enrichment, anxiety-like behavior in offspring is prevented. similar studies have been conducted in humans and suggest that dna methylation plays a role. post-traumatic stress disorder (ptsd) post-traumatic stress disorder (ptsd) is a stress-related mental health disorder that emerges in response to traumatic or highly stressful experiences. it is believed that ptsd develops as a result of an interaction between these traumatic experiences and genetic factors. the signs and symptoms of ptsd can include avoidance behaviors, invasive thoughts, and significant alterations in normal behavior and thinking. there is evidence suggesting ptsd formation is associated with epigenetic changes such as dna methylation and acetylation of histone proteins. increased dna methylation has been found to regulate the induction of fear conditioning behaviors associated with ptsd triggers. histone modifications, like acetylation and deacetylation, play an important role in the development of ptsd, which is related to fear memory from traumatic events. the dsm-5 asserts that ptsd manifests differently in children over six years old than in adults. specifically that their flashbacks and/or intrusive memories may be explained by recreating their traumatic event(s) through their play. they may also experience reoccurring nightmares that are indirectly related to the event. additionally, there is a separate criteria altogether for ptsd in children under six years old. epigenetic modifications dna methylation through a number of human studies, ptsd is known to affect dna methylation of cpg islands in several genes involved in numerous activities, including stress responses and neurotransmitter activity. cpgs are used to describe cytosine-guanine adjacent nucleotides within the same strand of dna. cpg islands are defined by computer algorithms as being made up of at least 60% cpgs and being anywhere between 200 and 3000 base pairs in size. the methylation of these cpg islands can cause histone modifications which can lead to the condensation of chromatin which can ultimately alter gene expression. dnmt enzyme dna methyltransferase, dnmt, is an enzyme responsible for increased methylation of dna. it has been found that dnmt and its associated increased methylation can regulate risk for memory consolidation and fear conditioning. tet enzyme the removal of methyl groups from cytosine is initiated by a tet enzyme. tet is an enzyme known to oxidize 5-methylcytosine (5mc) to 5-hydroxymethylcytosine (5hmc) within the genome. this reaction initiates active dna demethylation to ultimately alter gene expression. it has been found that the tet enzyme exists as two isoforms which are differentially regulated and expressed across brain regions. the regulation of these isoforms can affect synaptic connections and ultimately memory formation. the manipulation of the tet enzymes' expression levels has become a potential source of interest for ptsd medication. the table below identifies differentially methylated regions (dmrs) across the genome which undergo ptsd-induced epigenetic changes which alter gene expression. human studies supporting the role of dna methylation in ptsd genetic loci finding(s) slc6a4 following trauma exposure, low methylation levels of slc6a4 increases risk of ptsd; high methylation levels decreases risk of ptsd man2c1 higher man2c1 methylation is correlated to greater risk of ptsd in individuals exposed to traumatic events tpr, clec9a, apc5, anxa2, tlr8 ptsd is associated with increased global methylation of these genes adcyap1r1 higher methylation is associated with ptsd symptoms in individuals exposed to trauma line-1, alu higher methylation of these loci is observed in postdeployed veterans who developed ptsd compared to those who do not develop ptsd slc6a3 high slc6a3 promoter methylation, combined with a nine-repeat allele of slc6a3, is correlated to higher ptsd risk igf2, h19, il8, il16, il18 higher