Epigenetic and post-transcriptional dysregulation of gene expression in schizophrenia and related disease

Mapping miRNA Research in Schizophrenia: A Scientometric Review

Micro RNA (miRNA) research has great implications in uncovering the aetiology of neuropsychiatric conditions due to the role of miRNA in brain development and function. Schizophrenia, a complex yet devastating neuropsychiatric disorder, is one such condition that had been extensively studied in the realm of miRNA. Although a relatively new field of research, this area of study has progressed sufficiently to warrant dozens of reviews summarising findings from past to present. However, as a majority of reviews cannot encapsulate the full body of research, there is still a need to synthesise the diversity of publications made in this area in a systematic but easy-to-understand manner. Therefore, this study adopted bibliometrics and scientometrics, specifically document co-citation analysis (DCA), to review the literature on miRNAs in the context of schizophrenia over the course of history. From a literature search on Scopus, 992 papers were found and analysed with CiteSpace. DCA analysis generated a network of 13 major clusters with different thematic focuses within the subject area. Finally, these clusters are qualitatively discussed. miRNA research has branched into schizophrenia, among other medical and psychiatric conditions, due to previous findings in other forms of non-coding RNA. With the rise of big data, bioinformatics analyses are increasingly common in this field of research. The future of research is projected to rely more heavily on interdisciplinary collaboration. Additionally, it can be expected that there will be more translational studies focusing on the application of these findings to the development of effective treatments.

Insights into myelin dysfunction in schizophrenia and bipolar disorder

Schizophrenia and bipolar disorder are disabling psychiatric disorders with a worldwide prevalence of approximately 1%. Both disorders present chronic and deteriorating prognoses that impose a large burden, not only on patients but also on society and health systems. These mental illnesses share several clinical and neurobiological traits; of these traits, oligodendroglial dysfunction and alterations to white matter (WM) tracts could underlie the disconnection between brain regions related to their symptomatic domains. WM is mainly composed of heavily myelinated axons and glial cells. Myelin internodes are discrete axon-wrapping membrane sheaths formed by oligodendrocyte processes. Myelin ensheathment allows fast and efficient conduction of nerve impulses through the nodes of Ranvier, improving the overall function of neuronal circuits. Rapid and precisely synchronized nerve impulse conduction through fibers that connect distant brain structures is crucial for higher-level functions, such as cognition, memory, mood, and language. Several cellular and subcellular anomalies related to myelin and oligodendrocytes have been found in postmortem samples from patients with schizophrenia or bipolar disorder, and neuroimaging techniques have revealed consistent alterations at the macroscale connectomic level in both disorders. In this work, evidence regarding these multilevel alterations in oligodendrocytes and myelinated tracts is discussed, and the involvement of proteins in key functions of the oligodendroglial lineage, such as oligodendrogenesis and myelination, is highlighted. The molecular components of the axo-myelin unit could be important targets for novel therapeutic approaches to schizophrenia and bipolar disorder.

An Extracellular Perspective on CNS Maturation: Perineuronal Nets and the Control of Plasticity

During restricted time windows of postnatal life, called critical periods, neural circuits are highly plastic and are shaped by environmental stimuli. In several mammalian brain areas, from the cerebral cortex to the hippocampus and amygdala, the closure of the critical period is dependent on the formation of perineuronal nets. Perineuronal nets are a condensed form of an extracellular matrix, which surrounds the soma and proximal dendrites of subsets of neurons, enwrapping synaptic terminals. Experimentally disrupting perineuronal nets in adult animals induces the reactivation of critical period plasticity, pointing to a role of the perineuronal net as a molecular brake on plasticity as the critical period closes. Interestingly, in the adult brain, the expression of perineuronal nets is remarkably dynamic, changing its plasticity-associated conditions, including memory processes. In this review, we aimed to address how perineuronal nets contribute to the maturation of brain circuits and the regulation of adult brain plasticity and memory processes in physiological and pathological conditions.

miR-19b is elevated in peripheral blood of schizophrenic patients and attenuates proliferation of hippocampal neural progenitor cells

MicroRNAs (miRNAs) have been investigated in neurodevelopmental and psychiatric disorders including schizophrenia (SZ). Previous studies showed miRNAs dysregulation in postmortem brain tissues and peripheral blood of SZ patients. These suggest that miRNAs may play a role in the pathophysiology of SZ and be a potential biomarker of SZ. Previous studies also showed that miRNAs regulated neurogenesis and that neurogenesis was involved in the pathophysiology of SZ. In addition, a recent study showed that miR-19a and 19b, enriched in neural progenitor cells (NPC) in adult hippocampus, were increased in human NPC derived from induced pluripotent stem cell derived from SZ patients. However, it remains unclear whether the levels of miR-19a and 19b are altered in peripheral blood of SZ patients and how miR-19a and 19b affects neurogenesis. To elucidate them, first we examined the levels of miR-19a and 19b in peripheral blood of SZ patients with quantitative RT-PCR and showed that the level of miR-19b, but not miR-19a, was significantly higher (miR-19a: p = 0.5733, miR-19b: p = 0.0038) in peripheral blood of SZ patients (N = 22) than that of healthy controls (N = 19). Next, we examined the involvement of miR-19b in proliferation and survival of mouse neonatal mice hippocampus-derived NPC with BrdU assay and TUNEL assay. The silencing of miR-19b significantly increased proliferation (N = 5, p = 0.0139), but not survival (N = 5, p = 0.9571), of neonatal mice hippocampus-derived NPC. These results suggest that the level of miR-19b in peripheral blood is a potential biomarker of schizophrenia and that the higher level of miR-19b may increase the vulnerability of SZ via attenuating proliferation of hippocampal NPC.

Effects of Antipsychotic Drugs on the Epigenetic Modification of Brain-Derived Neurotrophic Factor Gene Expression in the Hippocampi of Chronic Restraint Stress Rats

Recent studies have shown that antipsychotic drugs have epigenetic effects. However, the effects of antipsychotic drugs on histone modification remain unclear. Therefore, we investigated the effects of antipsychotic drugs on the epigenetic modification of the BDNF gene in the rat hippocampus. Rats were subjected to chronic restraint stress (6 h/d for 21 d) and then were administered with either olanzapine (2 mg/kg) or haloperidol (1 mg/kg). The levels of histone H3 acetylation and MeCP2 binding at BDNF promoter IV were assessed with chromatin immunoprecipitation assays. The mRNA levels of total BDNF with exon IV, HDAC5, DNMT1, and DNMT3a were assessed with a quantitative RT-PCR procedure. Chronic restraint stress resulted in the downregulation of total and exon IV BDNF mRNA levels and a decrease in histone H3 acetylation and an increase in MeCP2 binding at BDNF promoter IV. Furthermore, there were robust increases in the expression of HDAC5 and DNMTs. Olanzapine administration largely prevented these changes. The administration of haloperidol had no effect. These findings suggest that the antipsychotic drug olanzapine induced histone modification of BDNF gene expression in the hippocampus and that these epigenetic alterations may represent one of the mechanisms underlying the actions of antipsychotic drugs.

Alleviative effects of fluoxetine on depressive-like behaviors by epigenetic regulation of BDNF gene transcription in mouse model of post-stroke depression

Fluoxetine, one of the selective serotonin reuptake inhibitor (SSRI) antidepressants, has been thought to be effective for treating post-stroke depression (PSD). Recent work has shown that fluoxetine may exert an antidepressive effect through increasing the level of brain-derived neurotrophic factor (BDNF), but the underlying mechanism still remains unclear. In the present study, we successfully established the PSD model using male C57BL/6 J mice by photothrombosis of the left anterior cortex combined with isolatied-housing conditions. In the process, we confirmed that fluoxetine could improve the depression-like behaviors of PSD mice and upregulate the expression of BDNF in the hippocampus. However, depletion of BDNF by transfecting lentivirus-derived shBDNF in hippocampus suppressed the effect of fluoxetine. Furthermore, we demonstrated the epigenetic mechanisms involved in regulation of BDNF expression induced by fluoxetine. We found a statistically significant increase in DNA methylation at specific CpG sites (loci 2) of Bdnf promoter IV in the hippocampus of PSD mice. We also found that fluoxetine treatment could disassociate the MeCP2-CREB-Bdnf promoter IV complex via phosphorylation of MeCP2 at Ser421 by Protein Kinase A (PKA). Our research highlighted the importance of fluoxetine in regulating BDNF expression which could represent a potential strategy for preventing PSD.

Epigenetic Regulation of Glutamic Acid Decarboxylase 67 in a Hippocampal Circuit

GABAergic dysfunction in hippocampus, a key feature of schizophrenia (SZ), may contribute to cognitive impairment in this disorder. In stratum oriens (SO) of sector CA3/2 of the human hippocampus, a network of genes involved in the regulation of glutamic acid decarboxylase GAD67 has been identified. Several of the genes in this network including epigenetic factors histone deacetylase 1 (HDAC1) and death-associated protein 6 (DAXX), the GABAergic enzyme GAD65 as well as the kainate receptor (KAR) subunits GluR6 and 7 show significant changes in expression in this area in SZ. We have tested whether HDAC1 and DAXX regulate GAD67, GAD65, or GluR in the intact rodent hippocampus. Stereotaxic injections of lentiviral vectors bearing shRNAi sequences for HDAC1 and DAXX were delivered into the SO of CA3/2, followed by laser microdissection of individual transduced GABA neurons. Quantitative PCR (QPCR) analyses demonstrated that inhibition of HDAC1 and DAXX increased expression of GAD67, GAD65, and GluR6 mRNA. Inhibition of DAXX, but not HDAC1 resulted in a significant increase in GluR7 mRNA. Our data support the hypothesis that HDAC1 and DAXX play a central role in coordinating the expression of genes in the GAD67 regulatory pathway in the SO of CA3/2.

Theranostic Biomarkers for Schizophrenia

Schizophrenia is a highly heritable, chronic, severe, disabling neurodevelopmental brain disorder with a heterogeneous genetic and neurobiological background, which is still poorly understood. To allow better diagnostic procedures and therapeutic strategies in schizophrenia patients, use of easy accessible biomarkers is suggested. The most frequently used biomarkers in schizophrenia are those associated with the neuroimmune and neuroendocrine system, metabolism, different neurotransmitter systems and neurotrophic factors. However, there are still no validated and reliable biomarkers in clinical use for schizophrenia. This review will address potential biomarkers in schizophrenia. It will discuss biomarkers in schizophrenia and propose the use of specific blood-based panels that will include a set of markers associated with immune processes, metabolic disorders, and neuroendocrine/neurotrophin/neurotransmitter alterations. The combination of different markers, or complex multi-marker panels, might help in the discrimination of patients with different underlying pathologies and in the better classification of the more homogenous groups. Therefore, the development of the diagnostic, prognostic and theranostic biomarkers is an urgent and an unmet need in psychiatry, with the aim of improving diagnosis, therapy monitoring, prediction of treatment outcome and focus on the personal medicine approach in order to improve the quality of life in patients with schizophrenia and decrease health costs worldwide.

Metabotropic Glutamate 2/3 Receptors and Epigenetic Modifications in Psychotic Disorders: A Review

Schizophrenia and Bipolar Disorder are chronic psychiatric disorders, both considered as “major psychosis”; they are thought to share some pathogenetic factors involving a dysfunctional gene x environment interaction. Alterations in the glutamatergic transmission have been suggested to be involved in the pathogenesis of psychosis. Our group developed an epigenetic model of schizophrenia originated by Prenatal Restraint Stress (PRS) paradigm in mice. PRS mice developed some behavioral alterations observed in schizophrenic patients and classic animal models of schizophrenia, i.e. deficits in social interaction, locomotor activity and prepulse inhibition. They also showed specific changes in promoter DNA methylation activity of genes related to schizophrenia such as reelin, BDNF and GAD67, and altered expression and function of mGlu2/3 receptors in the frontal cortex. Interestingly, behavioral and molecular alterations were reversed by treatment with mGlu2/3 agonists. Based on these findings, we speculate that pharmacological modulation of these receptors could have a great impact on early phase treatment of psychosis together with the possibility to modulate specific epigenetic key protein involved in the development of psychosis.

In this review, we will discuss in more details the specific features of the PRS mice as a suitable epigenetic model for major psychosis. We will then focus on key proteins of chromatin remodeling machinery as potential target for new pharmacological treatment through the activation of metabotropic glutamate receptors.

Increased cortical expression of the zinc transporter SLC39A12 suggests a breakdown in zinc cellular homeostasis as part of the pathophysiology of schizophrenia

Our expression microarray studies showed messenger RNA (mRNA) for solute carrier family 39 (zinc transporter), member 12 (SLC39A12) was higher in dorsolateral prefrontal cortex from subjects with schizophrenia (Sz) in comparison with controls. To better understand the significance of these data we ascertained whether SLC39A12 mRNA was altered in a number of cortical regions (Brodmann’s area (BA) 8, 9, 44) from subjects with Sz, in BA 9 from subjects with mood disorders and in rats treated with antipsychotic drugs. In addition, we determined whether inducing the expression of SLC39A12 resulted in an increased cellular zinc uptake. SLC39A12 variant 1 and 2 mRNA was measured using quantitative PCR. Zinc uptake was measured in CHO cells transfected with human SLC39A12 variant 1 and 2. In Sz, compared with controls, SLC39A12 variant 1 and 2 mRNA was higher in all cortical regions studied. The were no differences in levels of mRNA for either variant of SLC39A12 in BA 9 from subjects with mood disorders and levels of mRNA for Slc39a12 was not different in the cortex of rats treated with antipsychotic drugs. Finally, expressing both variants in CHO-K1 cells was associated with an increase in radioactive zinc uptake. As increased levels of murine Slc39a12 mRNA has been shown to correlate with increasing cellular zinc uptake, our data would be consistent with the possibility of a dysregulated zinc homeostasis in the cortex of subjects with schizophrenia due to altered expression of SLC39A12.

Gestational stress induces depressive-like and anxiety-like phenotypes through epigenetic regulation of BDNF expression in offspring hippocampus

Exposure to stressful life events during pregnancy exerts profound effects on neurodevelopment and increases the risk for several neurodevelopmental disorders including major depression. The mechanisms underlying the consequences of gestational stress are complex and remain to be elucidated. This study investigated the effects of gestational stress on depressive-like behavior and epigenetic modifications in young adult offspring. Gestational stress was induced by a combination of restraint and 24-hour light disturbance to pregnant dams throughout gestation. Depressive-like and anxiety-like behaviors of young adult offspring were examined. The expression and promoter methylation of brain derived neurotrophic factor (BDNF) were measured using RT-qPCR, Western blot, methylated DNA immunoprecipitation (MeDIP) and chromatin immunoprecipitation (ChIP). In addition, the expressions of histone deacetylases (HDACs) and acetylated histone H3 lysine 14 (AcH3K14) were also analyzed. Our results show that offspring from gestational stress dams exhibited depressive-like and anxiety-like behaviors. Biochemically, stress-offspring showed decreased expression of BDNF, increased expression of DNMT1, HDAC1, and HDAC2, and decreased expression of AcH3K14 in the hippocampus as compared to non-stress offspring. Data from MeDIP and ChIP assays revealed an increased methylation as well as decreased binding of AcH3K14 on specific BDNF promoters. Pearson analyses indicated that epigenetic changes induced by gestational stress were correlated with depressive-like and anxiety-like behaviors. These data suggest that gestational stress may be a suitable model for understanding the behavioral and molecular epigenetic changes observed in patients with depression.

DNA Methylation of BDNF Gene in Schizophrenia

Background

Although genetic factors are risk factors for schizophrenia, some environmental factors are thought to be required for the manifestation of disease. Epigenetic mechanisms regulate gene functions without causing a change in the nucleotide sequence of DNA. Brain-derived neurotrophic factor (BDNF) is a neurotrophin that regulates synaptic transmission and plasticity. It has been suggested that BDNF may play a role in the pathophysiology of schizophrenia. It is established that methylation status of the BDNF gene is associated with fear learning, memory, and stressful social interactions. In this study, we aimed to investigate the DNA methylation status of BDNF gene in patients with schizophrenia.

Material/Methods

The study included 49 patients (33 male and 16 female) with schizophrenia and 65 unrelated healthy controls (46 male and 19 female). Determination of methylation pattern of CpG islands was based on the principle that bisulfite treatment of DNA results in conversion of unmethylated cytosine residues into uracil, whereas methylated cytosine residues remain unmodified. Methylation-specific PCR was performed with primers specific for either methylated or unmethylated DNA.

Results

There was no significant difference in methylated or un-methylated status for BDNF promoters between schizophrenia patients and controls. The mean duration of illness was significantly lower in the hemi-methylated group compared to the non-methylated group for BDNF gene CpG island-1 in schizophrenia patients.

Conclusions

Although there were no differences in BDNF gene methylation status between schizophrenia patients and healthy controls, there was an association between duration of illness and DNA methylation.

Toward dissecting the etiology of schizophrenia: HDAC1 and DAXX regulate GAD67 expression in an in vitro hippocampal GABA neuron model

Schizophrenia (SZ) is associated with GABA neuron dysfunction in the hippocampus, particularly the stratum oriens of sector CA3/2. A gene expression profile analysis of human postmortem hippocampal tissue followed by a network association analysis had shown a number of genes differentially regulated in SZ, including the epigenetic factors HDAC1 and DAXX. To characterize the contribution of these factors to the developmental perturbation hypothesized to underlie SZ, lentiviral vectors carrying short hairpin RNA interference (shRNAi) for HDAC1 and DAXX were used. In the hippocampal GABA neuron culture model, HiB5, transduction with HDAC1 shRNAi showed a 40% inhibition of HDAC1 mRNA and a 60% inhibition of HDAC1 protein. GAD67, a enzyme associated with GABA synthesis, was increased twofold (mRNA); the protein showed a 35% increase. The expression of DAXX, a co-repressor of HDAC1, was not influenced by HDAC1 inhibition. Transduction of HiB5 cells with DAXX shRNAi resulted in a 30% inhibition of DAXX mRNA that translated into a 90% inhibition of DAXX protein. GAD1 mRNA was upregulated fourfold, while its protein increased by ~30%. HDAC1 expression was not altered by inhibition of DAXX. However, a physical interaction between HDAC1 and DAXX was demonstrated by co-immunoprecipitation. Inhibition of HDAC1 or DAXX increased expression of egr-1, transcription factor that had previously been shown to regulate the GAD67 promoter. Our in vitro results point to a key role of both HDAC1 and DAXX in the regulation of GAD67 in GABAergic HiB5 cells, strongly suggesting that these epigenetic/transcription factors contribute to mechanisms underlying GABA cell dysfunction in SZ.

Insights into the origin of DNA methylation differences between monozygotic twins discordant for schizophrenia

Background

DNA methylation differences between monozygotic twins discordant for schizophrenia have been previously reported. However, the origin of methylation differences between monozygotic twins discordant for schizophrenia is not clear. The findings here argue that all DNA methylation differences may not necessarily represent the cause of the disease; rather some may result from the effect of antipsychotics.

Methods

Methylation differences in rat brain regions and also in two pairs of unrelated monozygotic twins discordant for schizophrenia have been studied using genome-wide DNA methylation arrays at Arraystar Inc. (Rockville, Maryland, USA). The identified gene promoters showing significant alterations to DNA methylation were then further characterized using ingenuity pathway analysis (Ingenuity System Inc, CA, USA).

Results

Pathway analysis of the most significant gene promoter hyper/hypomethylation revealed a significant enrichment of DNA methylation changes in biological networks and pathways directly relevant to neural development and psychiatric disorders. These included HIPPO signaling (p = 3.93E-03) and MAPK signaling (p = 4.27E-03) pathways involving hypermethylated genes in schizophrenia-affected patients as compared to their unaffected co-twins. Also, a number of significant pathways and networks involving genes with hypomethylated gene promoters have been identified. These included CREB signaling in neurons (p = 1.53E-02), Dopamine-DARPP32 feedback in cAMP signaling (p = 7.43E-03) and Ephrin receptors (p = 1.13E-02). Further, there was significant enrichment for pathways involved in nervous system development and function (p = 1.71E-03-4.28E-02).

Conclusion

The findings highlight the significance of antipsychotic drugs on DNA methylation in schizophrenia patients. The unique pathways affected by DNA methylation in the two pairs of monozygotic twins suggest that patient-specific pathways are responsible for the disease; suggesting that patient-specific treatment strategies may be necessary in treating the disorder. The study reflects the need for developing personalized medicine approaches that take into consideration epigenetic variations between patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s40303-015-0013-5) contains supplementary material, which is available to authorized users.

Epigenetic mechanisms in schizophrenia

Epigenetic modifications, including DNA methylation, histone modifications, and non-coding RNAs, have been implicated in a number of complex diseases. Schizophrenia and other major psychiatric and neurodevelopmental disorders are associated with abnormalities in multiple epigenetic mechanisms, resulting in altered gene expression during development and adulthood. Polymorphisms and copy number variants in schizophrenia risk genes contribute to the high heritability of the disease, but environmental factors that lead to epigenetic modifications may either reduce or exacerbate the expression of molecular and behavioral phenotypes associated with schizophrenia and related disorders. In the present paper, we will review the current understanding of molecular dysregulation in schizophrenia, including disruption of the dopamine, NMDA, and GABA signaling pathways, and discuss the role of epigenetic factors underlying disease pathology.

DNA methylation differences in monozygotic twin pairs discordant for schizophrenia identifies psychosis related genes and networks

Background

Despite their singular origin, monozygotic twin pairs often display discordance for complex disorders including schizophrenia. It is a common (1%) and often familial disease with a discordance rate of ~50% in monozygotic twins. This high discordance is often explained by the role of yet unknown environmental, random, and epigenetic factors. The involvement of DNA methylation in this disease appears logical, but remains to be established.

Methods

We have used blood DNA from two pairs of monozygotic twins discordant for schizophrenia and their parents in order to assess genome-wide methylation using a NimbleGen Methylation Promoter Microarray.

Results

The genome-wide results show that differentially methylated regions (DMRs) exist between members representing discordant monozygotic twins. Some DMRs are shared with parent(s) and others appear to be de novo. We found twenty-seven genes affected by DMR changes that were shared in the affected member of two discordant monozygotic pairs from unrelated families. Interestingly, the genes affected by pair specific DMRs share specific networks. Specifically, this study has identified two networks; “cell death and survival” and a “cellular movement and immune cell trafficking”. These two networks and the genes affected have been previously implicated in the aetiology of schizophrenia.

Conclusions

The results are compatible with the suggestion that DNA methylation may contribute to the discordance of monozygotic twins for schizophrenia. Also, this may be accomplished by the direct effect of gene specific methylation changes on specific biological networks rather than individual genes. It supports the extensive genetic, epigenetic and phenotypic heterogeneity implicated in schizophrenia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12920-015-0093-1) contains supplementary material, which is available to authorized users.

Brain-derived neurotrophic factor epigenetic modifications associated with schizophrenia-like phenotype induced by prenatal stress in mice

BACKGROUND

Prenatal stress (PRS) is considered a risk factor for several neurodevelopmental disorders including schizophrenia (SZ). An animal model involving restraint stress of pregnant mice suggests that PRS induces epigenetic changes in specific GABAergic and glutamatergic genes likely to be implicated in SZ, including the gene for brain-derived neurotrophic factor (BDNF).

METHODS

Studying adult offspring of pregnant mice subjected to PRS, we explored the long-term effects of PRS on behavior and on the expression of key chromatin remodeling factors including DNA methyltransferase 1, ten-eleven-translocation hydroxylases, methyl CpG binding protein 2, histone deacetylases, and histone methyltransferases and demethylase in the frontal cortex and hippocampus. We also measured the expression of BDNF.

RESULTS

Adult PRS offspring demonstrate behavioral abnormalities suggestive of SZ and molecular changes similar to changes seen in postmortem brains of patients with SZ. This includes a significant increase in DNA methyltransferase 1 and ten-eleven-translocation hydroxylase 1 in the frontal cortex and hippocampus but not in cerebellum; no changes in histone deacetylases, histone methyltransferases and demethylases, or methyl CpG binding protein 2, and a significant decrease in Bdnf messenger RNA variants. The decrease of the corresponding Bdnf transcript level was accompanied by an enrichment of 5-methylcytosine and 5-hydroxymethylcytosine at Bdnf gene regulatory regions. In addition, the expression of Bdnf transcripts (IV and IX) correlated positively with social approach in both PRS mice and nonstressed mice.

CONCLUSIONS

Because patients with psychosis and PRS mice show similar epigenetic signature, PRS mice may be a suitable model for understanding the behavioral and molecular epigenetic changes observed in patients with SZ.

Perinatal programming of neurodevelopment: epigenetic mechanisms and the prenatal shaping of the brain

The recent years have witnessed an exponential growth in the knowledge of epigenetic mechanisms, and piling evidence now links DNA methylation and histone modifications with a wide range of physiological processes from embryonic development to memory formation and behavior. Not surprisingly, deregulation of epigenetic modifications is associated with human diseases as well.An important feature of epigenetics is the ability of transducing environmental input into biological signaling, mainly by modulation of the transcriptome in response to a particular scenario. This characteristic generates developmental plasticity and allows the manifestation of a variety of phenotypes from the same genome.The early-life years represent a period of particular susceptibility to epigenetic alteration, as active changes in DNA methylation and histone marks are occurring as part of developmental programs and in response to environmental cues, which notably include psychosocial stimulation and maternal behavior. Memory formation and storage, response to stress in adult life, behavior, and manifestation of neurodegenerative conditions can all be imprinted in the organism by epigenetic modifications that contribute to shape the brain during prenatal or early postnatal life. Moreover, if these epigenetic alterations are preserved in the germ line, changes induced in one generation are likely inherited by future offspring. Programming by transgenerational inheritance thus represents a central mechanism by which environmental conditions may influence disease risk across multiple generations.As novel techniques emerge and as genome-wide profiling of disease-associated methylomes is achieved, epigenetic marks open a new source for biomarker discovery.

Schizophrenia as a disorder of molecular pathways

Over the last decade, transcriptome studies of postmortem tissue from subjects with schizophrenia revealed that synaptic, mitochondrial, immune system, gamma-aminobutyric acidergic, and oligodendrocytic changes are all integral parts of the disease process. The combined genetic and transcriptomic studies argue that the molecular underpinnings of the disease are even more varied than the symptomatic diversity of schizophrenia. Ultimately, to decipher the pathophysiology of human disorders in general, we will need to understand the function of hundreds of genes and regulatory elements in our genome and the consequences of their overexpression and reduced expression in a developmental context. Furthermore, integration of knowledge from various data sources remains a monumental challenge that has to be systematically addressed in the upcoming decades. In the end, our success in interpreting the molecular changes in schizophrenia will depend on our ability to understand the biology using innovative ideas and cannot depend on the hope of developing novel, more powerful technologies.

Neuroprogression in schizophrenia: Pathways underpinning clinical staging and therapeutic corollaries

OBJECTIVE

Whilst dopaminergic dysfunction remains a necessary component involved in the pathogenesis of schizophrenia, our current pharmacological armoury of dopamine antagonists does little to control the negative symptoms of schizophrenia. This suggests other pathological processes must be implicated. This paper aims to elaborate on such theories.

METHODS

Data for this review were sourced from the electronic database PUBMED, and was not limited by language or date of publication.

RESULTS

It has been suggested that multiple 'hits' may be required to unveil the clinical syndrome in susceptible individuals. Such hits potentially first occur in utero, leading to neuronal disruption, epigenetic changes and the establishment of an abnormal inflammatory response. The development of schizophrenia may therefore potentially be viewed as a neuroprogressive response to these early stressors, driven on by changes in tryptophan catabolite (TRYCAT) metabolism, reactive oxygen species handling and N-methyl d-aspartate (NMDA) circuitry. Given the potential for such progression over time, it is prudent to explore the new treatment strategies which may be implemented before such changes become established.

CONCLUSIONS

Outside of the dopaminergic model, the potential pathogenesis of schizophrenia has yet to be fully elucidated, but common themes include neuropil shrinkage, the development of abnormal neuronal circuitry, and a chronic inflammatory state which further disrupts neuronal function. Whilst some early non-dopaminergic treatments show promise, none have yet to be fully studied in appropriately structured randomized controlled trials and they remain little more than potential attractive targets.

Reduced striatal adenosine A2A receptor levels define a molecular subgroup in schizophrenia

Schizophrenia (SZ) is a mental disorder of unknown origin. Some scientific evidence seems to indicate that SZ is not a single disease entity, since there are patient groups with clear symptomatic, course and biomarker differences. SZ is characterized by a hyperdopaminergic state related to high dopamine D2 receptor activity. It has also been proposed that there is a hypoadenosynergic state. Adenosine is a nucleoside widely distributed in the organism with neuromodulative and neuroprotective activity in the central nervous system. In the brain, the most abundant adenosine receptors are A1R and A2AR. In the present report, we characterize the presence of both receptors in human postmortem putamens of patients suffering SZ with real time TaqMan PCR, western blotting and radioligand binding assay. We show that A1R levels remain unchanged with respect to age-matched controls, whereas nearly fifty percent of patients have reduced A2AR, at the transcriptional and translational levels. Moreover, we describe how DNA methylation plays a role in the pathological A2AR levels with the bisulfite-sequencing technique. In fact, an increase in 5-methylcytosine percentage in the 5' UTR region of ADORA2A was found in those SZ patients with reduced A2AR levels. Interestingly, there was a relationship between the A2A/β-actin ratio and motor disturbances as assessed with some items of the PANSS, AIMS and SAS scales. Therefore, there may be a subgroup of SZ patients with reduced striatal A2AR levels accompanied by an altered motor phenotype.

Epigenetics of schizophrenia: an open and shut case

During the last decade and a half, there has been an explosion of data regarding epigenetic changes in schizophrenia. Most initial studies have suggested that schizophrenia is characterized by an overly restrictive chromatin state based on increases in transcription silencing histone modifications and DNA methylation at schizophrenia candidate gene promoters and increases in the expression of enzymes that catalyze their formation. However, recent studies indicate that the pathology is more complex. This complexity may greatly impact pharmacological approaches directed at targeting epigenetic abnormalities in schizophrenia. The current review explores epigenetic studies of schizophrenia and what this can tell us about the underlying pathophysiology. We hypothesize based on recent studies that it is also plausible that drugs that further restrict chromatin may be efficacious.

Epigenetics in the human brain

Many cellular constituents in the human brain permanently exit from the cell cycle during pre- or early postnatal development, but little is known about epigenetic regulation of neuronal and glial epigenomes during maturation and aging, including changes in mood and psychosis spectrum disorders and other cognitive or emotional disease. Here, we summarize the current knowledge base as it pertains to genome organization in the human brain, including the regulation of DNA cytosine methylation and hydroxymethylation, and a subset of (altogether >100) residue-specific histone modifications associated with gene expression, and silencing and various other functional chromatin states. We propose that high-resolution mapping of epigenetic markings in postmortem brain tissue or neural cultures derived from induced pluripotent cells (iPS), in conjunction with transcriptome profiling and whole-genome sequencing, will increasingly be used to define the molecular pathology of specific cases diagnosed with depression, schizophrenia, autism, or other major psychiatric disease. We predict that these highly integrative explorations of genome organization and function will provide an important alternative to conventional approaches in human brain studies, which mainly are aimed at uncovering group effects by diagnosis but generally face limitations because of cohort size.

An epigenetic framework for neurodevelopmental disorders: from pathogenesis to potential therapy

Neurodevelopmental disorders (NDDs) are characterized by aberrant and delayed early-life development of the brain, leading to deficits in language, cognition, motor behaviour and other functional domains, often accompanied by somatic symptoms. Environmental factors like perinatal infection, malnutrition and trauma can increase the risk of the heterogeneous, multifactorial and polygenic disorders, autism and schizophrenia. Conversely, discrete genetic anomalies are involved in Down, Rett and Fragile X syndromes, tuberous sclerosis and neurofibromatosis, the less familiar Phelan-McDermid, Sotos, Kleefstra, Coffin-Lowry and "ATRX" syndromes, and the disorders of imprinting, Angelman and Prader-Willi syndromes. NDDs have been termed "synaptopathies" in reference to structural and functional disturbance of synaptic plasticity, several involve abnormal Ras-Kinase signalling ("rasopathies"), and many are characterized by disrupted cerebral connectivity and an imbalance between excitatory and inhibitory transmission. However, at a different level of integration, NDDs are accompanied by aberrant "epigenetic" regulation of processes critical for normal and orderly development of the brain. Epigenetics refers to potentially-heritable (by mitosis and/or meiosis) mechanisms controlling gene expression without changes in DNA sequence. In certain NDDs, prototypical epigenetic processes of DNA methylation and covalent histone marking are impacted. Conversely, others involve anomalies in chromatin-modelling, mRNA splicing/editing, mRNA translation, ribosome biogenesis and/or the regulatory actions of small nucleolar RNAs and micro-RNAs. Since epigenetic mechanisms are modifiable, this raises the hope of novel therapy, though questions remain concerning efficacy and safety. The above issues are critically surveyed in this review, which advocates a broad-based epigenetic framework for understanding and ultimately treating a diverse assemblage of NDDs ("epigenopathies") lying at the interface of genetic, developmental and environmental processes. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Upregulation of TET1 and downregulation of APOBEC3A and APOBEC3C in the parietal cortex of psychotic patients

Increasing evidence suggests that epigenetic dysfunction may account for the alteration of gene transcription present in neuropsychiatric disorders such as schizophrenia (SZ), bipolar disorder (BP) and autism. Here, we studied the expression of the ten-eleven translocation (TET) gene family and activation-induced deaminase/apolipoprotein B mRNA-editing enzymes (AID/APOBEC) in the inferior parietal lobule (IPL) (BA39-40) and the cerebellum of psychotic (PSY) patients, depressed (DEP) patients and nonpsychiatric (CTR) subjects obtained from the Stanley Foundation Neuropathology Consortium Medical Research Institute. These two sets of enzymes have a critical role in the active DNA demethylation pathway. The results show that TET1, but not TET2 and TET3, mRNA and protein expression was increased (two- to threefold) in the IPL of the PSY patients compared with the CTR subjects. TET1 mRNA showed no change in the cerebellum. Consistent with the increase of TET1, the level of 5-hydroxymethylcytosine (5hmC) was elevated in the IPL of PSY patients but not in the other groups. Moreover, higher 5hmC levels were detected at the glutamic acid decarboxylase67 (GAD67) promoter only in the PSY group. This increase was inversely related to the decrease of GAD67 mRNA expression. Of 11 DNA deaminases measured, APOBEC3A mRNA was significantly decreased in the PSY and DEP patients, while APOBEC3C was decreased only in PSY patients. The other APOBEC mRNA studied failed to change. Increased TET1 and decreased APOBEC3A and APOBEC3C found in this study highlight the possible role of altered DNA demethylation mechanisms in the pathophysiology of psychosis.