Exposure to a traumatic experience can lead to post-traumatic stress disorder (PTSD), an incapacitating disorder in susceptible persons with no reliable therapy. Particularly puzzling is understanding how transient exposure to trauma creates persistent long- term suffering from PTSD and why some people are susceptible to PTSD while others that were exposed to the same trauma remain resilient.
Epigenetic modifications are chemical marks on genes that program their activity. These marks are written into DNA during fetal development to correctly program how our genes function in different organs. However, research in the last two decades has suggested that these marks could also be modulated by experiences and exposures at different point of time in life.
Studies in humans have suggested that perhaps the initial trauma exposure results in “epigenetic alterations” that in turn mediate and embed the PTSD disorder. These ideas were based on analysis of blood DNA of humans with PTSD, but it was not known whether epigenetic changes play a causal role in the brain regions that are considered important for PTSD.
A team of scientists from Bar-Ilan University, led by Prof. Gal Yadid, of the Mina and Everard Goodman Faculty of Life Sciences and Gonda (Goldschmied) Multidisciplinary Brain Research Center, examined this question using an animal PTSD model and discovered a new way for treating PTSD that might be applied to humans.
Their findings were recently published in the Nature journal Molecular Psychiatry.
The researchers first mapped ‘epigenetic DNA methylation marks’ in a brain region which is important for PTSD. They found distinct epigenetic differences between animals that were exposed to trauma and were resilient, and those animals that were exposed to trauma and were susceptible and developed PTSD-like behavior.
The researchers found that an important ‘epigenetic’ enzyme that transfers methyl groups onto DNA, DNMT3A, is reduced in animals that are susceptible to PTSD.
The researchers also searched for groups of genes whose methylation is altered in the PTSD susceptible animals and found that one group of genes is controlled by the retinoic acid receptor which is activated by vitamin A.
Indeed, delivering DNMT3A or retinoic acid orphan receptor gene into the animal brains reverses the PTSD-like phenotypes, suggesting that these genes that are differentially methylated are responsible for PTSD behavior.
Injecting brains with genes is still not a feasible therapeutic option. Therefore, the authors tested whether nutritional supplements that mimic the activity of these genes could treat and reverse PTSD in susceptible animals.
Since DNMT3A increases DNA methylation, the researchers used a natural product that donates methyl groups S-adenosylmethionine (SAMe) and to activate the retinoic acid receptor they treated the animals with vitamin A. They found that combined treatment with the methyl donor SAM and retinoic acid reversed PTSD-like behaviors.
The data suggest a novel approach to treatment of PTSD, which involves combining two natural products that modulate the epigenome. Importantly, epigenetic treatments reverse the underlying causes of the disorder on DNA and thus serve as a “cure” rather than temporary relief of symptoms.
“Since these nutritional supplements are relatively nontoxic, they offer hope for a nontoxic treatment of PTSD that reverses the underlying genomic cause of the disease,” said Prof. Yadid.
Genetic research in PTSD
Posttraumatic stress disorder (PTSD) is a severe, chronic and debilitating trauma-related disorder that significantly impairs normal functioning and quality of life (DSM-5, American Psychiatric Association 2013). It is characterized by the presence of four symptom clusters: re-experiencing, avoidance, hyperarousal and negative alterations in cognition and mood (DSM-5, American Psychiatric Association 2013). The disorder occurs in about 7% of the general population (Kessler et al. 2005). The development of PTSD is associated with learned fear-conditioned responses, which serve as reminders of traumatic events, and which can persist for several years after traumatic exposure (Blechert et al. 2007; Orr et al. 2000).
Single nucleotide polymorphisms (SNPs) are one of the most commonly investigated polymorphisms in case–control candidate gene association studies of PTSD (Koenen 2007; Risch & Merikangas 1996). These studies rely on the selection of candidate genes based on the current knowledge regarding the neurobiology of the disorder. In PTSD research, such genes typically include those involved in hypothalamic–pituitary–adrenal (HPA) axis regulation, the noradrenergic system and limbic–frontal brain systems (especially genes that are involved in fear conditioning) (see the review by Cornelis et al. 2010 for more details).
Genome-wide association studies (GWAS) represent an alternative, more robust and hypothesis-neutral approach that can be applied to case–control studies. In GWAS, SNPs (frequencies of SNPs) across the entire genome of cases are compared to controls (Hirschhorn & Daly 2005). However, to date, very few GWAS have been conducted in anxiety disorders and in PTSD in particular. In a PTSD GWAS by Logue et al. (2013) the sample comprised trauma-exposed Caucasian (non-Hispanic) military veterans and their intimate partners.
Although several SNPs were found to be associated with PTSD, only one withstood correction for multiple testing; rs8042149, which is located in the retinoid-related orphan receptor alpha gene (RORA) was significantly associated with a lifetime diagnosis of PTSD. Recently, Xie et al. (2013) conducted a GWAS in a sample of European Americans and African Americans in order to find novel common risk alleles for PTSD. They identified a SNP on chromosome 7p12, rs406001, which exceeded genome-wide significance.
Furthermore, a SNP that maps to the first intron of the Tolloid-Like 1 gene (TLL1) also showed strong evidence of association but did not reach genome-wide significance. However, further analysis of two SNPs in the first intron of TLL1, rs6812849 and rs7691872, in 2000 European Americans replicated the association findings from the GWAS.
As PTSD, by definition, requires exposure to a traumatic event and only a subset of individuals develop PTSD after trauma, studies of gene–environment (G × E) interactions might be better suited to elucidate the genetic underpinnings of the disorder. These studies have provided evidence that PTSD is influenced by interactive effects from both environmental and genetic factors (for more information, refer to the review article by Mehta & Binder 2012).
Transcriptional perturbations in PTSD
An alternative approach to understanding the genetic underpinnings of complex disorders, such as PTSD, includes gene expression profiling studies. Distinct differences in gene expression patterns between PTSD-affected and unaffected individuals have been observed in genes involved in the HPA axis, immune function and genes that transcribe neural and endocrine proteins (Segman et al. 2005; Uddin et al. 2010; Weaver et al. 2002; Yehuda et al. 2010; Zieker et al. 2007). Identification of differentially expressed genes involved in the aetiology of PTSD could aid in the identification of pathways involved in the development of the disorder. In addition, factors that contribute to altered gene expression patterns hold promising clues to the complex biological underpinnings of PTSD. A number of genes have been reported to be differentially expressed in either human or animal PTSD models (reviewed by Skelton et al. 2012).
Regulatory gene regions, such as epigenetic elements, have recently received attention as major contributors to phenotypic diversity and disease, especially in complex disorders. Several researchers have hypothesized that epigenetic perturbations, such as DNA methylation, may facilitate the process whereby life experiences alter gene expression patterns (Fraga et al. 2005). Epigenetics provides a link between the environment and the transcriptome – the effect of environmental influences, such as maternal separation and childhood trauma, on DNA methylation and subsequent gene expression profiles – has been well reported in the literature (Binder et al. 2008; Champagne 2008; Franklin et al. 2010; Koenen & Uddin 2010). Epigenetic modifications may explain the interindividual variation in disease susceptibility as well as the long-lasting effects elicited by trauma exposure (Yehuda & Bierer 2009). This review will focus on the main findings of DNA methylation studies in PTSD and how this may shape the development of new treatment strategies.
Epigenetics is the study of mitotically and/or meiotically heritable changes in gene function that are not attributable to DNA sequence changes (Russo et al. 1996). These epigenetic changes are heritable and potentially reversible (Jaenisch & Bird 2003), and provide an additional layer of transcriptional control that may mediate the interaction between genetic predisposition, changes in neural functioning and environmental factors (Bjornsson et al. 2004). Such epigenetic mechanisms include DNA methylation, posttranslational modifications of histone proteins (acetylation, methylation, phosphorylation, ubiquitination and sumoylation) and non-coding RNA-mediated alterations (such as microRNAs and small interfering RNAs).
Epigenetic remodelling has been found to be a crucial component of the neuronal changes that underlie learning and memory processes (Bredy et al. 2007; Chwang et al. 2006; Miller & Sweatt 2007). It has been postulated that epigenetic factors play an important role in the regulation of activity-dependent neuronal gene expression (Chen et al. 2003; Martinowich et al. 2003). Epigenetic regulation may be particularly important in shaping the effect of early environment on the development of dysfunctional fear extinction given that epigenetic regulation of gene expression may underlie neural plasticity in the event of early-life adversity. For example, early life experience in the form of maternal care has been shown to result in stable epigenetic markings that contribute to an anxiety-like phenotype in adult rats (Weaver et al. 2004, 2005, 2006). These results have recently been extrapolated to human subjects (McGowan et al. 2009).
DNA methylation (5mC)
In mammals, DNA methylation occurs mainly at the C-5 position of cytosine residues within CpG dinucleotides (Fig. 1). Globally, about 70–80% of all CpG dinucleotides in the human genome are methylated (Ehrlich et al. 1982); however, numerous temporal and spatial variations are evident, especially during early development (Reik & Walter 2001). DNA methylation regulates developmental genes and is vital for genomic imprinting.
During specific stages of mammalian development CpG methylation undergoes dramatic global changes. New methylation patterns are acquired during early development; primordial germ cells are characterized by genome-wide removal of DNA methylation marks and, following fertilization, the sperm-derived genome is stripped of DNA methylation (Sasaki & Matsui 2008). DNA methylation patterns are maintained after cell division and are consequently passed from parent to daughter cells (Taylor & Jones 1985; Turner 2002; Razin 1998). Dysregulation of methylation can lead to aberrant transcriptional control and subsequent alterations in gene expression (Yehuda & LeDoux 2007). Another essential role of DNA methylation is the repression of retrotransposons and other foreign elements (Sasaki & Matsui 2008).
The process of DNA methylation is strongly dependent on DNA methyltransferases (DNMTs), namely DNMT1 and de novo DNMT enzymes, DNMT3A and DNMT3B (essential for DNA methylation patterns in early development). DNMT1 acts as a maintenance DNMT which, in turn, acts on hemimethylated CpG sites (Turek-Plewa & Jagodzinski 2005), whereas DNMT3A and 3B are responsible for de novo DNA methylation by acting on hemimethylated and unmethylated CpG sites (Xie et al. 1999). DNMT1 and DNMT3A are abundant in the mature brain (Feng et al. 2010), whereas DNMT3B and DNMT3L are almost undetectable in the mature brain. DNMT3L is an accessory protein; it is catalytically inactive and is required to stimulate the DNA methylation activity of DNMT3A and 3B in embryonic stem (ES) cells (Turek-Plewa & Jagodzinski 2005). De novo methylation in cells that express DNMT3L requires a tetrameric complex of two DNMT3A2 and DNMT3L molecules as well as the nucleosome.
The nucleosome forms the fundamental unit of eukaryotic chromatin and consists of DNA wound around eight histone protein cores (McGhee & Felsenfeld 1980). Active transcription start sites (TSSs) lack nucleosomes and, as a result, do not contain this substrate for de novo methylation (Ooi et al. 2007). A family of methyl CpG-binding domain (MBD) proteins [including methyl CpG-binding protein 2 (MeCP2) and MBD 1–4 (MBD1-4)] interpret DNA methylation by interacting with histone deacetylases (HDACs) and DNMTs to induce gene silencing.
In addition, the binding of these proteins to methylated DNA seems to be important in maintaining the DNA methylation status because site-specific demethylation is associated with the dissociation of this complex (specifically MeCP2) (Chen et al. 2003; Martinowich et al. 2003; Murgatroyd et al. 2009). The process of active demethylation requires a mechanism that involves cell division or DNA repair and the removal of the base rather than the methyl group directly from the 5mC unit (Bhutani et al. 2010; Popp et al. 2010). Recent studies indicate the involvement of enzymes such as ten-eleven translocation (TET) methylcytosine dioxygenases, thymine DNA glycosylase and activation-induced cytidine deaminase in active and passive demethylation as well as in gene activation (Bhutani et al. 2010; Inoue & Zhang 2011; Iqbal et al. 2011).
It has been hypothesized that DNA methylation and histone deacetylation may function along a common pathway to induce transcriptional repression (Cameron et al. 1999; Jones et al. 1998; Nan et al. 1998). Proteins that contain MBDs recognize methylated DNA and recruit an HDAC complex to remodel the chromatin (Jones et al. 1998; Nan et al. 1998; Zhang et al. 1999). The association between DNA methylation and histone deacetylation was shown to be more direct than originally anticipated, when results from Fuks et al. (2000) indicated that DNMT1 was directly associated with HDAC activity in vivo. Results showed that HDAC1 has the ability to bind DNMT1 and to purify methyltransferase activity from nuclear extracts. Furthermore, a transcriptional repression domain in DNMT1, which functions partly by recruiting HDAC activity, was identified in this study (Fuks et al. 2000). The authors suggested that DNMT1-mediated DNA methylation may generate or depend on a transformed chromatin state through HDAC activity.
Methylation, in close proximity to the TSS, prevents transcription factors and RNA polymerase from accessing the DNA and results in silencing of the gene (Fig. 1). In addition to gene silencing, these methyl groups also attract other protein complexes that promote histone deacetylation, further inhibiting gene expression (Strathdee & Brown 2002; Turner 2002). The bond between the methyl group and the cytosine nucleotide is very strong, resulting in stable, yet potentially reversible, changes in gene expression.
It has been well established that transcription cannot be initiated at methylated CpG islands (CGIs) of TSSs after the DNA has been assembled into nucleosomes (Hashimshony et al. 2003; Kass et al. 1997; Venolia & Gartler 1983). The question of which comes first, silencing or methylation, has been the subject of much discussion. In 1987, Lock et al. showed that methylation of the hypoxanthine phosphoribosyltransferase (Hprt) gene (on the inactive X chromosome) occurred only after inactivation of the chromosome. Consequently, it was postulated that methylation serves as a lock that reinforces a previously silenced state of X-linked genes (Lock et al. 1987). However, results from a study that investigated the role of DNMT3A in haematopoietic stem cell differentiation have raised questions about the universality of the long-term locking model (Challen et al. 2012).
The aforementioned study indicated that methylase was vital for differentiation of a short-lived cell type. It is likely that DNA methylation instructs rather than reinforces gene silencing and that there is a general mechanism whereby silencing precedes methylation, although more data are required to confirm this. The process of DNA methylation is, therefore, more complex than was initially thought and requires in-depth research to address a number of unanswered questions.
It is also important to note that the position of methylation affects gene expression. Methylation in the TSS prevents initiation of transcription (as discussed above), whereas methylation in the gene body does not necessarily block transcription, and may even stimulate transcription elongation. It has been suggested that gene body methylation may play a role in splicing (Moarefi and Chedin 2011). Gene body methylation is a feature of transcribed genes (Wolf et al. 1984); the majority of gene bodies contain a limited number of CpG dinucleotides, numerous repetitive and transposable elements, and they are extensively methylated.
One of the main causes of C → T transition mutations is CpG methylation in gene exons, which could result in disease-causing mutations in the germline and cancer-causing mutations in somatic cells (Jones 2012; Rideout et al. 1990). A ‘methylation paradox’ thus exists, whereby promoter methylation is inversely correlated with gene expression, and gene body methylation is positively correlated with gene expression (Jones 1999). Thus, initiation of transcription, and not transcription elongation, appears to be sensitive to DNA methylation silencing in mammals. The presence of a 5mC does not, of itself, elicit a transcriptional effect; this effect is elicited by the interpretation of the 5mC in a particular genomic and cellular context (Jones 2012).
As most genes have at least two TSSs, it has also been suggested that methylation could help regulate the process of alternative promoter usage (Maunakea et al. 2010). CpG-rich sequences are abundant in the genome and are referred to as CGIs, most often situated in promoter regions. These CGIs are usually protected from methylation (Yehuda & LeDoux 2007). A fraction of these CGIs, present in certain tissues during ageing (Issa 2000) or in abnormal cells (such as cancer cells) (Baylin & Herman 2000), are susceptible to progressive methylation.
In mammals, the glucocorticoid (GC) content of CGIs is roughly 65% compared to 40% for the entire genome (Suzuki & Bird 2008). CpG island shores and shelves are regions outside CGIs. Shores are 0–2000 bp outside CGIs, whereas CpG shelves flank CpG shores and are 2000–4000 bp adjacent to CGIs (Pastor et al. 2011). Methylation mostly occurs a short distance from the CGIs at the CGI shores.
Although gene promoters contain many CGIs, CGIs also exist within the gene bodies and within gene deserts (long stretches of the genome devoid of protein-coding genes) (Jones 1999; Venter et al. 2001). In the human brain up to 34% of all intragenic CGIs are methylated (Maunakea et al. 2010); however, the exact function of CGI methylation at these intragenic locations remains to be fully elucidated. One hypothesis is that these regions may represent ‘orphan promoters’ that have escaped methylation in the germline, thus maintaining their high CpG density.
It is therefore plausible that they play a functional role during development (Illingworth et al. 2010). The function of gene body methylation outside CGIs was initially assumed to be a mechanism for silencing repetitive DNA elements, such as retroviruses, LINE1 and Alu elements (Yoder et al. 1997). Whole-genome studies have recently revealed possible alternative functions for DNA methylation in gene bodies. For example, exons show a higher level of methylation than introns and changes in the degree of methylation occur at exon–intron boundaries, suggesting a role for methylation in regulating splicing (Laurent et al. 2010).
Initially, it was believed that cytosine methylation in mammalian DNA was limited to both strands of the symmetrical CpG sequence; however, research has shown that sequences other than CpG may also be methylated (Grafstrom et al. 1985; Ramsahoye et al. 2000; Salomon & Kaye 1970). Approximately 25% of all the ES cell methylation is in a non-CpG context (Lister et al. 2009). In addition to human and mouse ES cells and human induced pluripotent stem (iPS) cells, non-CpG methylation has also been observed in mouse brain and mouse germinal vesicle oocytes, human somatic tissue and brain tissue (Kobayashi et al. 2012; Shirane et al. 2013; Stadler et al. 2011; Xie et al. 2012).
In human ES cells and mouse brain, CA methylation sites are most common, while lower levels of methylation are present in the CT and CC sites (Laurent et al. 2010; Lister et al. 2009; Xie et al. 2012). In ES cells non-CpG methylation is enriched in gene bodies and mostly absent in protein-binding sites and enhancers. Following induced differentiation of the ES cells, non-CpG methylation disappears and in iPS cells, non-CpG methylation is restored (Lister et al. 2009).
These findings suggest that different methylation mechanisms may be used by ES cells to control gene regulation. Recently, non-CpG methylation was also found to be present in male germ cells among B1 retrotransposon sequences scattered in the mouse genome (Ichiyanagi et al. 2013). Accumulating levels are evident in mitotically arrested foetal prospermatogonia, with the highest levels of non-CpG methylation reached by the time of birth, occurring in a DNMT3L-dependent manner. CpA is the most common form of non-CpG methylation site in male germ cells (Ichiyanagi et al. 2013). Although DNMT3A is mainly a CpG methylase, it is also capable of inducing methylation at CpA and at CpT sites (Lister et al. 2009; Ramsahoye et al. 2000).
In addition to epigenetic mechanisms themselves, the various enzymes that regulate these mechanisms have also been linked to memory formation (Day & Sweatt 2010). One such example is the regulation of active DNA demethylation, with focus on the Gadd45 (growth arrest and DNA-damage-inducible, beta) family (Leach et al. 2012; Sultan et al. 2012). Gadd45b in particular has been found to be involved in activity-dependent demethylation in the adult central nervous system (CNS). The deletion of GADD45B (GADD45B−/−) (the gene that encodes the growth arrest and DNA-damage-inducible, beta protein) leads to the abolishment of neuronal activity-induced DNA demethylation in the adult mouse dentate gyrus at specific genomic loci, including the promoters of the brain-derived neurotrophic factor (BDNF) gene and fibroblast growth factor 1 (FGF1). This reduces activity-induced adult hippocampal neurogenesis (Ma et al. 2008). In addition, studies have shown that pharmacological inhibition of changes in DNA methylation also affects synaptic plasticity, learning and memory (Day & Sweatt 2010).
Two research groups have investigated the effects of deletion of the GADD45B gene on fear conditioning and memory. Both studies found that GADD45B transcription is regulated in an experience-dependent manner and suggested its involvement in regulating memory capacity (Leach et al. 2012; Sultan et al. 2012). However, conflicting data have emerged from these two studies with regard to the involvement of GADD45B in fear conditioning. Sultan et al. (2012) observed enhanced contextual fear conditioning in GADD45B−/−, whereas Leach et al. (2012) observed a deficit in contextual fear conditioning.
Although there is no clear explanation for these contradictory findings, Sultan et al. (2012) hypothesized that a loss of such a potent epigenomic regulator could be sensitive to the background genome where strain differences may have arisen during backcrossing. Different training facilities or housing environments could have augmented background genome or epigenome differences in the mutant mice (Crews 2011). Another factor that could have contributed to the discrepant results is the difference in training paradigms; Leach et al. (2012) utilized a foreground training paradigm, whereas Sultan et al. (2012) used background training for contextual memory assessment. Irrespective of these differences, both studies have emphasized the importance of epigenetic DNA modification mechanisms in the adult nervous system.
They showed that the transcription of GADD45B is regulated by experience and that GADD45B may play an important role in long-term hippocampus-dependent memory. However, it is not only a methyl group that occurs on the C-5 position of cytosine residues but also 5-hydroxymethylcytosine (5hmC), and although these two groups are very similar, they may have distinct effects on gene expression.
DNA methylation and personalized medicine in PTSD
Since the advent of the phrase ‘personalized medicine’ there have been high expectations that patient-specific pharmacogenetic data will improve treatment outcomes in neuropsychiatric disorders. However, owing to the complexity of transcriptional regulation and the influence of environmental factors and the epigenome, simple translation of individual genetic information into personalized treatment has not proved to be enough. How could pharmacogenetics explain the fact that monozygotic twins, who are both treated for major depression with the same drug, exhibit different clinical responses? Why do some patients who suffer from recurrent major depression not show an efficacious response to a drug as they did during a previous episode? The answers might lie in epigenetics seeing that the dynamic nature of DNA methylation patterns and histone acetylation provide plausible explanations for some of these puzzling pharmacogenetic questions (Holsboer 2008).
DNA and histone methylation in the brain can be compromised when neurons are starved of methyl donors. The depletion of methyl donors may be due to inherited metabolism errors, folate deficiency or methyl donor deficiencies of folate, l-methylfolate and S-adenosylmethionine (SAM) in the diet or depletion due to pregnancy, gastrointestinal disease, smoking, alcohol or drug addiction (Stahl 2007, 2010). The depletion of methyl donors can occur to such an extent that it induces elevated homocysteine levels, psychosis and developmental delay (Freeman et al. 1975; Regland et al. 1994). Where hypomethylation causes susceptibility to a disease, methyl donors or drugs that target methyl metabolism may have utility as therapeutic agents.
Another possible epigenetic biomarker is the enzyme enolase phosphatase. Different isoforms of this enzyme were found to be present in high-anxiety compared to low-anxiety mice. Differences in enolase phosphatase protein levels are attributable to SNPs that alter the amino acid sequence (Weaver et al. 2004). Enolase phosphatase is involved in the methionine recovery pathway that reconstitutes methionine after its conversion into SAM (a methyl donor). Variations in the methionine/SAM ratio may affect DNA methylation levels. Indeed, methionine infusion was found to reverse the effect of poor maternal care on DNA methylation profiles (detected in the rats during adulthood) as well as behavioural responses to stress (Kagan et al. 1990); SAM has been reported to have antidepressant effects (Kagan et al. 1990). More research is required, however, to determine whether a mutation in the enolase phosphatase gene is a potential biomarker of treatment response to SAM (Holsboer 2008).
Epigenetic markings, such as DNA methylation, may also be transmitted transgenerationally (Mill & Petronis 2007); thus, the level of gene expression, as well as its timing and location, could be heritable and subsequently influence an individual’s phenotype, disease susceptibility and drug response. Epigenetic therapies have enabled researchers to correct some of these aberrant expression profiles (Simonini et al. 2006; Tremolizzo et al. 2002).
Most of the epigenetic therapies target DNA methylation and histone deacetylation enzymes and several drugs (mainly developed to treat cancer) have been tested in clinical trials (Szyf 2009). Some DNMT inhibitors have been approved for clinical therapy [such as azacytidine (AZA) and decitabine (DAC)], others are in phase 1 (e.g. 5-fluoro-2′-deoxycytidine) or still in preclinical development (e.g. zebularine) (Amatori et al. 2010).
A more thorough understanding of the genes and epigenetic events associated with a specific disease is a necessary step in pursuing targeted approaches (Graff & Mansuy 2009). The role of epigenetics in antidepressant response was demonstrated in a study that found that in chronically stressed mice, there was a decrease in the activity of the HDAC5 enzyme, leading to the removal of acetyl groups from histones and subsequently inhibited gene activity (Renthal et al. 2007). Upon chronic administration of an antidepressant, this effect on HDAC5 was effectively reversed (Renthal et al. 2007).
Furthermore, increased stress-induced depression-like behaviour was evident in HDAC5-knockout mice. In a similar fashion, stress-induced downregulation of BDNF expression was associated with increased methylation of the BDNF promoter. Antidepressants were able to reverse this effect and activate BDNF gene expression by increasing histone acetylation at the BDNF promoter (Tsankova et al. 2006). Subsequently, one of the strategies to achieve demethylation in the brain involves the use of HDACis.
Indeed, in animal models pharmacological treatment with HDACi effectively reversed hypermethylation of RELN (in the context of schizophrenia) (Simonini et al. 2006; Tremolizzo et al. 2002). In addition, treatment with valproate (Dong et al. 2007), TSA (Weaver et al. 2004) and a benzamide HDACi, N-(2-aminophenyl)-4-[N-(pyridin-3-ylmethoxycarbonyl) aminomethyl] benzamide derivative (MS-275) (Simonini et al. 2006) has all been shown to effectively induce demethylation in the brain. Development of DNMT antagonists using nanotechnology could enable the development of DNA methylation inhibitors that are effective in postmitotic tissues, such as the brain, and provide exciting new directions in psychiatric drug development (Szyf 2009).
An alternate approach to generic epigenetic inhibitors is the development of drugs designed for gene-specific epigenetic targeting. With regard to DNA methylation, this has recently been achieved both in vitro and in vivo in a study that used specific zinc finger peptides that confer de novo methylation to specific loci (Smith & Ford 2007). However, these therapies are not without counter-implications or side effects. Azanucleosides, such as AZA and DAC, function by silencing DNMT and are one of the only demethylating strategies approved for clinical therapy.
One of the side effects is that AZA and DAC could be incorporated directly into centromeric DNA sequences, which could lead to decondensation of the heterochromatin and altered centromeric structure, ultimately resulting in destabilization of the genome and impaired kinetochore formation. Consequently, the whole mitotic process could malfunction (Amatori et al. 2010).
Furthermore, AZA is not very stable, has quite a short half-life of 1.5 ± 2.3 h and is dependent on the cell cycle for its activity; thus, prolonged administration schedules are required. Decitabine, however, is more stable, has a half-life of 20 ± 5 h in aqueous solutions (Momparler 2005; Rudek et al. 2005) and effectively incorporates into DNA, making it more effective than AZA in inducing DNA demethylation. Moreover, it can be administered at lower doses (Appleton et al. 2007; Kantarjian et al. 2006, 2007; Kornblith et al. 2002; Silverman et al. 2002, 2006).
reference link: https://onlinelibrary.wiley.com/doi/full/10.1111/gbb.12102
“Reduction of DNMT3a and RORA in the nucleus accumbens plays a causal role in post-traumatic stress disorder-like behavior: reversal by combinatorial epigenetic therapy” by Gal Warhaftig, Noa Zifman, Chaya Mushka Sokolik, Renaud Massart, Orshay Gabay, Daniel Sapozhnikov, Farida Vaisheva, Yehuda Lictenstein, Noa Confortti, Hadas Ahdoot, Avi Jacob, Tzofnat Bareli, Moshe Szyf & Gal Yadid. Molecular Psychiatry