DNA methylation and Rett syndrome

Developmental Disruption of Mef2c in Medial Ganglionic Eminence-Derived Cortical Inhibitory Interneurons Impairs Cellular and Circuit Function

BACKGROUND

MEF2C is strongly linked to various neurodevelopmental disorders including autism, intellectual disability, schizophrenia, and attention-deficit/hyperactivity disorder. Mice that constitutively lack 1 copy of Mef2c or selectively lack both copies of Mef2c in cortical excitatory neurons display a variety of behavioral phenotypes associated with neurodevelopmental disorders. The MEF2C protein is a transcription factor necessary for cellular development and synaptic modulation of excitatory neurons. MEF2C is also expressed in a subset of cortical GABAergic (gamma-aminobutyric acidergic) inhibitory neurons, but its function in those cell types remains largely unknown.

METHODS

Using conditional deletions of the Mef2c gene in mice, we investigated the role of MEF2C in parvalbumin-expressing interneurons (PV-INs), the largest subpopulation of cortical GABAergic cells, at 2 developmental time points. We performed slice electrophysiology, in vivo recordings, and behavior assays to test how embryonic and late postnatal loss of MEF2C from GABAergic INs impacts their survival and maturation and alters brain function and behavior.

RESULTS

Loss of MEF2C from PV-INs during embryonic, but not late postnatal, development resulted in reduced PV-IN number and failure of PV-INs to molecularly and synaptically mature. In association with these deficits, early loss of MEF2C in GABAergic INs led to abnormal cortical network activity, hyperactive and stereotypic behavior, and impaired cognitive and social behavior.

CONCLUSIONS

MEF2C expression is critical for the development of cortical GABAergic INs, particularly PV-INs. Embryonic loss of function of MEF2C mediates dysfunction of GABAergic INs, leading to altered in vivo patterns of cortical activity and behavioral phenotypes associated with neurodevelopmental disorders.

Developmental disruption of Mef2c in Medial Ganglionic Eminence-derived cortical inhibitory interneurons impairs cellular and circuit function

MEF2C is strongly linked to various neurodevelopmental disorders (NDDs) including autism, intellectual disability, schizophrenia, and attention-deficit/hyperactivity. Mice constitutively lacking one copy of Mef2c , or selectively lacking both copies of Mef2c in cortical excitatory neurons, display a variety of behavioral phenotypes associated with NDDs. The MEF2C protein is a transcription factor necessary for cellular development and synaptic modulation of excitatory neurons. MEF2C is also expressed in a subset of cortical GABAergic inhibitory neurons, but its function in those cell types remains largely unknown. Using conditional deletions of the Mef2c gene in mice, we investigated the role of MEF2C in Parvalbumin-expressing Interneurons (PV-INs), the largest subpopulation of cortical GABAergic cells, at two developmental timepoints. We performed slice electrophysiology, in vivo recordings, and behavior assays to test how embryonic and late postnatal loss of MEF2C from GABAergic interneurons impacts their survival and maturation, and alters brain function and behavior. We found that loss of MEF2C from PV-INs during embryonic, but not late postnatal, development resulted in reduced PV-IN number and failure of PV-INs to molecularly and synaptically mature. In association with these deficits, early loss of MEF2C in GABAergic interneurons lead to abnormal cortical network activity, hyperactive and stereotypic behavior, and impaired cognitive and social behavior. Our findings indicate that MEF2C expression is critical for the development of cortical GABAergic interneurons, particularly PV-INs. Embryonic loss of function of MEF2C mediates dysfunction of GABAergic interneurons, leading to altered in vivo patterns of cortical activity and behavioral phenotypes associated with neurodevelopmental disorders.

Mini-review of DNA Methylation Detection Techniques and Their Potential Applications in Disease Diagnosis, Prognosis, and Treatment

DNA methylation is the dominant epigenetic mechanism for regulating gene expression in mammals, playing crucial roles in development, differentiation, and tissue homeostasis. Aberrations in DNA methylation are closely associated with the potential onset of various diseases. Consequently, numerous DNA methylation detection techniques have been successively developed. These methods not only facilitate the exploration of disease mechanisms but also hold significant promise for the development of diagnostic and prognostic strategies. In this Perspective, we present a comprehensive overview of commonly employed DNA methylation detection techniques as well as biosensing based on their underlying analytical techniques. For its medical applications, we begin by examining the pathogenesis of different diseases and then proceed to discuss how relevant technologies are applied in the context of these specific medical conditions. Additionally, we briefly discuss the current limitations of these techniques and highlight future challenges in advancing methylation detection and analysis methodologies.

The role of DNA methylation in progression of neurological disorders and neurodegenerative diseases as well as the prospect of using DNA methylation inhibitors as therapeutic agents for such disorders

Genome-wide studies related to neurological disorders and neurodegenerative diseases have pointed to the role of epigenetic changes such as DNA methylation, histone modification, and noncoding RNAs. DNA methylation machinery controls the dynamic regulation of methylation patterns in discrete brain regions.

Objective

This review aims to describe the role of DNA methylation in inhibiting and progressing neurological and neurodegenerative disorders and therapeutic approaches.

Methods

A Systematic search of PubMed, Web of Science, and Cochrane Library was conducted for all qualified studies from 2000 to 2022.

Results

For the current need of time, we have focused on the DNA methylation role in neurological and neurodegenerative diseases and the expression of genes involved in neurodegeneration such as Alzheimer's, Depression, and Rett Syndrome. Finally, it appears that the various epigenetic changes do not occur separately and that DNA methylation and histone modification changes occur side by side and affect each other. We focused on the role of modification of DNA methylation in several genes associated with depression (NR3C1, NR3C2, CRHR1, SLC6A4, BDNF, and FKBP5), Rett syndrome (MECP2), Alzheimer's, depression (APP, BACE1, BIN1 or ANK1) and Parkinson's disease (SNCA), as well as the co-occurring modifications to histones and expression of non-coding RNAs. Understanding these epigenetic changes and their interactions will lead to better treatment strategies.

Conclusion

This review captures the state of understanding of the epigenetics of neurological and neurodegenerative diseases. With new epigenetic mechanisms and targets undoubtedly on the horizon, pharmacological modulation and regulation of epigenetic processes in the brain holds great promise for therapy.

Methylation as a key regulator of Tau aggregation and neuronal health in Alzheimer's disease

Neurodegenerative diseases like Alzheimer's, Parkinson's and Huntington's disease involves abnormal aggregation and accumulation of toxic proteins aggregates. Post-translational modifications (PTMs) of the causative proteins play an important role in the etiology of disease as they could either slow down or accelerate the disease progression. Alzheimer disease is associated with the aggregation and accumulation of two major protein aggregates-intracellular neurofibrillary tangles made up of microtubule-associated protein Tau and extracellular Amyloid-β plaques. Post-translational modifications are important for the regulation of Tau`s function but an imbalance in PTMs may lead to abnormal Tau function and aggregation. Tau methylation is one of the important PTM of Tau in its physiological state. However, the methylation signature on Tau lysine changes once it acquires pathological aggregated form. Tau methylation can compete with other PTMs such as acetylation and ubiquitination. The state of PTM at these sites determines the fate of Tau protein in terms of its function and stability. The global methylation in neurons, microglia and astrocytes are involved in multiple cellular functions involving their role in epigenetic regulation of gene expression via DNA methylation. Here, we have discussed the effect of methylation on Tau function in a site-specific manner and their cross-talk with other lysine modifications. We have also elaborated the role of methylation in epigenetic aspects and neurodegenerative conditions associated with the imbalance in methylation metabolism affecting global methylation state of cells. Video abstract.

MeCP2: The Genetic Driver of Rett Syndrome Epigenetics

Mutations in methyl CpG binding protein 2 (MeCP2) are the major cause of Rett syndrome (RTT), a rare neurodevelopmental disorder with a notable period of developmental regression following apparently normal initial development. Such MeCP2 alterations often result in changes to DNA binding and chromatin clustering ability, and in the stability of this protein. Among other functions, MeCP2 binds to methylated genomic DNA, which represents an important epigenetic mark with broad physiological implications, including neuronal development. In this review, we will summarize the genetic foundations behind RTT, and the variable degrees of protein stability exhibited by MeCP2 and its mutated versions. Also, past and emerging relationships that MeCP2 has with mRNA splicing, miRNA processing, and other non-coding RNAs (ncRNA) will be explored, and we suggest that these molecules could be missing links in understanding the epigenetic consequences incurred from genetic ablation of this important chromatin modifier. Importantly, although MeCP2 is highly expressed in the brain, where it has been most extensively studied, the role of this protein and its alterations in other tissues cannot be ignored and will also be discussed. Finally, the additional complexity to RTT pathology introduced by structural and functional implications of the two MeCP2 isoforms (MeCP2-E1 and MeCP2-E2) will be described. Epigenetic therapeutics are gaining clinical popularity, yet treatment for Rett syndrome is more complicated than would be anticipated for a purely epigenetic disorder, which should be taken into account in future clinical contexts.

From 1957 to Nowadays: A Brief History of Epigenetics

Due to the spectacular number of studies focusing on epigenetics in the last few decades, and particularly for the last few years, the availability of a chronology of epigenetics appears essential. Indeed, our review places epigenetic events and the identification of the main epigenetic writers, readers and erasers on a historic scale. This review helps to understand the increasing knowledge in molecular and cellular biology, the development of new biochemical techniques and advances in epigenetics and, more importantly, the roles played by epigenetics in many physiological and pathological situations.

MeCP2 deficiency promotes cell reprogramming by stimulating IGF1/AKT/mTOR signaling and activating ribosomal protein-mediated cell cycle gene translation

The generation of induced pluripotent stem cells (iPSCs) offers a great opportunity in research and regenerative medicine. The current poor efficiency and incomplete mechanistic understanding of the reprogramming process hamper the clinical application of iPSCs. MeCP2 connects histone modification and DNA methylation, which are key changes of somatic cell reprogramming. However, the role of MeCP2 in cell reprogramming has not been examined. In this study, we found that MeCP2 deficiency enhanced reprogramming efficiency and stimulated cell proliferation through regulating cell cycle protein expression in the early stage of reprogramming. MeCP2 deficiency enhanced the expression of ribosomal protein genes, thereby enhancing reprogramming efficiency through promoting the translation of cell cycle genes. In the end, MeCP2 deficiency stimulated IGF1/AKT/mTOR signaling and activated ribosomal protein gene expression. Taken together, our data indicate that MeCP2 deficiency promoted cell reprogramming through stimulating IGF1/AKT/mTOR signaling and activating ribosomal protein-mediated cell cycle gene translation in the early stage of reprogramming.

The role of MeCP2 in learning and memory

Gene transcription is a crucial step in the sequence of molecular, synaptic, cellular, and systems mechanisms underlying learning and memory. Here, we review the experimental evidence demonstrating that alterations in the levels and functionality of the methylated DNA-binding transcriptional regulator MeCP2 are implicated in the learning and memory deficits present in mouse models of Rett syndrome and MECP2 duplication syndrome. The significant impact that MeCP2 has on gene transcription through a variety of mechanisms, combined with well-defined models of learning and memory, make MeCP2 an excellent candidate to exemplify the role of gene transcription in learning and memory. Together, these studies have strengthened the concept that precise control of activity-dependent gene transcription is a fundamental mechanism that ensures long-term adaptive behaviors necessary for the survival of individuals interacting with their congeners in an ever-changing environment.

Double-blind, randomized, placebo-controlled study of trofinetide in pediatric Rett syndrome

OBJECTIVE

To determine safety, tolerability, and pharmacokinetics of trofinetide and evaluate its efficacy in female children/adolescents with Rett syndrome (RTT), a debilitating neurodevelopmental condition for which no pharmacotherapies directed at core features are available.

METHODS

This was a phase 2, multicenter, double-blind, placebo-controlled, parallel-group study, in which safety/tolerability, pharmacokinetics, and clinical response to trofinetide were characterized in 82 children/adolescents with RTT, aged 5 to 15 years. Sixty-two participants were randomized 1:1:1:1 to receive placebo twice a day (bid) for 14 days, followed by placebo, 50, 100, or 200 mg/kg bid of trofinetide for 42 days. Following blinded safety data review, 20 additional participants were randomized 1:1 to the 200 mg/kg or placebo bid groups. Safety assessments included adverse events, clinical laboratory tests, physical examinations, and concomitant medications. Clinician- and caregiver-based efficacy measurements assessed clinically relevant, phenotypic dimensions of impairment of RTT.

RESULTS

All dose levels were well tolerated and generally safe. Trofinetide at 200 mg/kg bid showed statistically significant and clinically relevant improvements relative to placebo on the Rett Syndrome Behaviour Questionnaire, RTT-Clinician Domain Specific Concerns-Visual Analog Scale, and Clinical Global Impression Scale-Improvement. Exploratory analyses suggested that observed changes correlated with trofinetide exposure.

CONCLUSION

These results, together with those from a previous adolescent/adult trial, indicate trofinetide's potential for treating core RTT symptoms and support further trials.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that for children/adolescents with RTT, trofinetide was safe, well-tolerated, and demonstrated improvement over placebo at 200 mg/kg bid in functionally important dimensions of RTT.

Epilepsy as a Network Disorder (2): What can we learn from other network disorders such as dementia and schizophrenia, and what are the implications for translational research?

There is common agreement that many disorders of the central nervous system are 'complex', that is, there are many potential factors that influence the development of the disease, underlying mechanisms, and successful treatment. Most of these disorders, unfortunately, have no cure at the present time, and therapeutic strategies often have debilitating side effects. Interestingly, some of the 'complexities' of one disorder are found in another, and the similarities are often network defects. It seems likely that more discussions of these commonalities could advance our understanding and, therefore, have clinical implications or translational impact. With this in mind, the Fourth International Halifax Epilepsy Conference and Retreat was held as described in the prior paper, and this companion paper focuses on the second half of the meeting. Leaders in various subspecialties of epilepsy research were asked to address aging and dementia or psychosis in people with epilepsy (PWE). Commonalities between autism, depression, aging and dementia, psychosis, and epilepsy were the focus of the presentations and discussion. In the last session, additional experts commented on new conceptualization of translational epilepsy research efforts. Here, the presentations are reviewed, and salient points are highlighted.

A Double-Blind, Randomized, Placebo-Controlled Clinical Study of Trofinetide in the Treatment of Rett Syndrome

BACKGROUND

This study aimed to determine the safety and tolerability of trofinetide and to evaluate efficacy measures in adolescent and adult females with Rett syndrome, a serious and debilitating neurodevelopmental condition for which no therapies are available for its core features.

METHODS

This was an exploratory, phase 2, multicenter, double-blind, placebo-controlled, dose-escalation study of the safety and tolerability of trofinetide in 56 adolescent and adult females with Rett syndrome. Subjects were randomly assigned in a 2:1 ratio to 35 mg/kg twice daily of trofinetide or placebo for 14 days; 35 mg/kg twice daily or placebo for 28 days; or 70 mg/kg twice daily or placebo for 28 days. Safety assessments included adverse events, clinical laboratory tests, vital signs, electrocardiograms, physical examinations, and concomitant medications. Efficacy measurements were categorized into four efficacy domains, which related to clinically relevant, phenotypic dimensions of impairment associated with Rett syndrome.

RESULTS

Both 35 mg/kg and 70 mg/kg dose levels of trofinetide were well tolerated and generally safe. Trofinetide at 70 mg/kg demonstrated efficacy compared with placebo based on prespecified criteria.

CONCLUSION

Trofinetide was well tolerated in adolescent and adult females with Rett syndrome. Although this study had a relatively short duration in a small number of subjects with an advanced stage of disease, consistent efficacy trends at the higher dose were observed in several outcome measures that assess important dimensions of Rett syndrome. These results represented clinically meaningful improvement from the perspective of the clinicians as well as the caregivers.

Gamma-Normal-Gamma Mixture Model for Detecting Differentially Methylated Loci in Three Breast Cancer Cell Lines

With state-of-the-art microarray technologies now available for whole genome CpG island (CGI) methylation profiling, there is a need to develop statistical models that are specifically geared toward the analysis of such data. In this article, we propose a Gamma-Normal-Gamma (GNG) mixture model for describing three groups of CGI loci: hypomethylated, undifferentiated, and hypermethylated, from a single methylation microarray. This model was applied to study the methylation signatures of three breast cancer cell lines: MCF7, T47D, and MDAMB361. Biologically interesting and interpretable results are obtained, which highlights the heterogeneity nature of the three cell lines. This underlies the premise for the need of analyzing each of the microarray slides individually as opposed to pooling them together for a single analysis. Our comparisons with the fitted densities from the Normal-Uniform (NU) mixture model in the literature proposed for gene expression analysis show an improved goodness of fit of the GNG model over the NU model. Although the GNG model was proposed in the context of single-slide methylation analysis, it can be readily adapted to analyze multi-slide methylation data as well as other types of microarray data.

Binding of MBD proteins to DNA blocks Tet1 function thereby modulating transcriptional noise

Aberrant DNA methylation is a hallmark of various human disorders, indicating that the spatial and temporal regulation of methylation readers and modifiers is imperative for development and differentiation. In particular, the cross-regulation between 5-methylcytosine binders (MBD) and modifiers (Tet) has not been investigated. Here, we show that binding of Mecp2 and Mbd2 to DNA protects 5-methylcytosine from Tet1-mediated oxidation. The mechanism is not based on competition for 5-methylcytosine binding but on Mecp2 and Mbd2 directly restricting Tet1 access to DNA. We demonstrate that the efficiency of this process depends on the number of bound MBDs per DNA molecule. Accordingly, we find 5-hydroxymethylcytosine enriched at heterochromatin of Mecp2-deficient neurons of a mouse model for Rett syndrome and Tet1-induced reexpression of silenced major satellite repeats. These data unveil fundamental regulatory mechanisms of Tet enzymes and their potential pathophysiological role in Rett syndrome. Importantly, it suggests that Mecp2 and Mbd2 have an essential physiological role as guardians of the epigenome.

Epigenetics and cardiovascular risk in childhood

Cardiovascular disease (CVD) can arise at the early stages of development and growth. Genetic and environmental factors may interact resulting in epigenetic modifications with abnormal phenotypic expression of genetic information without any change in the nucleotide sequence of DNA. Maternal dietary imbalance, inadequate to meet the nutritional needs of the fetus can lead to intrauterine growth retardation, decreased gestational age, low birth weight, excessive post-natal growth and metabolic alterations, with subsequent appearance of CVD risk factors. Fetal exposure to high cholesterol, diabetes and maternal obesity is associated with increased risk and progression of atherosclerosis. Maternal smoking during pregnancy and exposure to various environmental pollutants induce epigenetic alterations of gene expression relevant to the onset or progression of CVD. In children with hypercholesterolemia and/or obesity, oxidative stress activates platelets and monocytes, which release proinflammatory and proatherogenic substances, inducing endothelial dysfunction, decreased Doppler flow-mediated dilation and increased carotid intima-media thickness. Primary prevention of atherosclerosis should be implemented early. It is necessary to identify, through screening, high-risk apparently healthy children and take care of them enforcing healthy lifestyle (mainly consisting of Mediterranean diet and physical activity), prescribing nutraceuticals and eventual medications, if required by a high-risk profile. The key issue is the restoration of endothelial function in the reversible stage of atherosclerosis. Epigenetics may provide new markers for an early identification of children at risk and thereby develop innovative therapies and specific nutritional interventions in critical times.

Epigenetic regulation and chromatin remodeling in learning and memory

Understanding the underlying mechanisms of memory formation and maintenance has been a major goal in the field of neuroscience. Memory formation and maintenance are tightly controlled complex processes. Among the various processes occurring at different levels, gene expression regulation is especially crucial for proper memory processing, as some genes need to be activated while some genes must be suppressed. Epigenetic regulation of the genome involves processes such as DNA methylation and histone post-translational modifications. These processes edit genomic properties or the interactions between the genome and histone cores. They then induce structural changes in the chromatin and lead to transcriptional changes of different genes. Recent studies have focused on the concept of chromatin remodeling, which consists of 3D structural changes in chromatin in relation to gene regulation, and is an important process in learning and memory. In this review, we will introduce three major epigenetic processes involved in memory regulation: DNA methylation, histone methylation and histone acetylation. We will also discuss general mechanisms of long-term memory storage and relate the epigenetic control of learning and memory to chromatin remodeling. Finally, we will discuss how epigenetic mechanisms can contribute to the pathologies of neurological disorders and cause memory-related symptoms.

Insulin-like growth factors and their potential role in cardiac epigenetics

Cardiovascular disease (CVD) constitutes a major public health threat worldwide, accounting for 17.3 million deaths annually. Heart disease and stroke account for the majority of healthcare costs in the developed world. While much has been accomplished in understanding the pathophysiology, molecular biology and genetics underlying the diagnosis and treatment of CVD, we know less about the role of epigenetics and their molecular determinants. The impact of environmental changes and epigenetics in CVD is now emerging as critically important in understanding the origin of disease and the development of new therapeutic approaches to prevention and treatment. This review focuses on the emerging role of epigenetics mediated by insulin like-growth factors-I and -II in major CVDs such as heart failure, cardiac hypertrophy and diabetes.

Impairments in dendrite morphogenesis as etiology for neurodevelopmental disorders and implications for therapeutic treatments

Dendrite morphology is pivotal for neural circuitry functioning. While the causative relationship between small-scale dendrite morphological abnormalities (shape, density of dendritic spines) and neurodevelopmental disorders is well established, such relationship remains elusive for larger-scale dendrite morphological impairments (size, shape, branching pattern of dendritic trees). Here, we summarize published data on dendrite morphological irregularities in human patients and animal models for neurodevelopmental disorders, with focus on autism and schizophrenia. We next discuss high-risk genes for these disorders and their role in dendrite morphogenesis. We finally overview recent developments in therapeutic attempts and we discuss how they relate to dendrite morphology. We find that both autism and schizophrenia are accompanied by dendritic arbor morphological irregularities, and that majority of their high-risk genes regulate dendrite morphogenesis. Thus, we present a compelling argument that, along with smaller-scale morphological impairments in dendrites (spines and synapse), irregularities in larger-scale dendrite morphology (arbor shape, size) may be an important part of neurodevelopmental disorders' etiology. We suggest that this should not be ignored when developing future therapeutic treatments.

Epigenetic Pattern on the Human Y Chromosome Is Evolutionarily Conserved

DNA methylation plays an important role for mammalian development. However, it is unclear whether the DNA methylation pattern is evolutionarily conserved. The Y chromosome serves as a powerful tool for the study of human evolution because it is transferred between males. In this study, based on deep-rooted pedigrees and the latest Y chromosome phylogenetic tree, we performed epigenetic pattern analysis of the Y chromosome from 72 donors. By comparing their respective DNA methylation level, we found that the DNA methylation pattern on the Y chromosome was stable among family members and haplogroups. Interestingly, two haplogroup-specific methylation sites were found, which were both genotype-dependent. Moreover, the African and Asian samples also had similar DNA methylation pattern with a remote divergence time. Our findings indicated that the DNA methylation pattern on the Y chromosome was conservative during human male history.

Epigenetic mechanisms: A possible link between autism spectrum disorders and fetal alcohol spectrum disorders

The etiology of autism spectrum disorders (ASDs) still remains unclear and seems to involve a considerable overlap between polygenic, epigenetic and environmental factors. We have summarized the current understanding of the interplay between gene expression dysregulation via epigenetic modifications and the potential epigenetic impact of environmental factors in neurodevelopmental deficits. Furthermore, we discuss the scientific controversies of the relationship between prenatal exposure to alcohol and alcohol-induced epigenetic dysregulations, and gene expression alterations which are associated with disrupted neural plasticity and causal pathways for ASDs. The review of the literature suggests that a better understanding of developmental epigenetics should contribute to furthering our comprehension of the etiology and pathogenesis of ASDs and fetal alcohol spectrum disorders.

Mecp2 regulates neural cell differentiation by suppressing the Id1 to Her2 axis in zebrafish

Rett syndrome (RTT) is a progressive neurological disorder caused by mutations in the X-linked protein methyl-CpG-binding protein 2 (MeCP2). The endogenous function of MeCP2 during neural differentiation is still unclear. Here, we report that mecp2 is required for brain development in zebrafish. Mecp2 was broadly expressed initially in embryos and enriched later in the brain. Either morpholino knockdown or genetic depletion of mecp2 inhibited neuronal differentiation, whereas its overexpression promoted neuronal differentiation, suggesting an essential role of mecp2 in directing neural precursors into differentiated neurons. Mechanistically, her2 (the zebrafish ortholog of mammalian Hes5) was upregulated in mecp2 morphants in an Id1-dependent manner. Moreover, knockdown of either her2 or id1 fully rescued neuronal differentiation in mecp2 morphants. These results suggest that Mecp2 plays an important role in neural cell development by suppressing the Id1–Her2 axis, and provide new evidence that embryonic neural defects contribute to the later motor and cognitive dysfunctions in RTT.

Rotenone Stereospecifically Increases (S)-2-Hydroxyglutarate in SH-SY5Y Neuronal Cells

The α-ketoglutarate metabolite, 2-hydroxyglutarate (2-HG), has emerged as an important mediator in a subset of cancers and rare inherited inborn errors of metabolism. Because of potential enantiospecific metabolism, chiral analysis is essential for determining the biochemical impacts of altered 2-HG metabolism. We have developed a novel application of chiral liquid chromatography-electron capture/atmospheric pressure chemical ionization/mass spectrometry, which allows for the quantification of both (R)-2-HG (D-2-HG) and (S)-2-HG (L-2-HG) in human cell lines. This method avoids the need for chiral derivatization, which could potentially distort enantiomer ratios through racemization during the derivatization process. The study revealed that the pesticide rotenone (100 nM), a mitochondrial complex I inhibitor, caused a significant almost 3-fold increase in the levels of (S)-2-HG, (91.7 ± 7.5 ng/10(6) cells) when compared with the levels of (R)-2-HG (24.1 ± 1.2 ng/10(6) cells) in the SH-SY5Y neuronal cells, a widely used model of human neurons. Stable isotope tracers and isotopologue analysis revealed that the increased (S)-2-HG was derived primarily from l-glutamine. Accumulation of highly toxic (S)-2-HG occurs in the brains of subjects with reduced L-2-HG dehydrogenase activity that results from mutations in the L2HGDH gene. This suggests that the observed stereospecific increase of (S)-2-HG in neuronal cells is due to rotenone-mediated inhibition of L-2-HG dehydrogenase but not D-2-HG dehydrogenase. The high sensitivity chiral analytical methodology that has been developed in the present study can also be employed for analyzing other disruptions to 2-HG formation and metabolism such as those resulting from mutations in the isocitrate dehydrogenase gene.

Epileptic encephalopathies: new genes and new pathways

Epileptic encephalopathies represent a group of devastating epileptic disorders that occur early in life and are often characterized by pharmaco-resistant epilepsy, persistent severe electroencephalographic abnormalities, and cognitive dysfunction or decline. Next generation sequencing technologies have increased the speed of gene discovery tremendously. Whereas ion channel genes were long considered to be the only significant group of genes implicated in the genetic epilepsies, a growing number of non-ion-channel genes are now being identified. As a subgroup of the genetically mediated epilepsies, epileptic encephalopathies are complex and heterogeneous disorders, making diagnosis and treatment decisions difficult. Recent exome sequencing data suggest that mutations causing epileptic encephalopathies are often sporadic, typically resulting from de novo dominant mutations in a single autosomal gene, although inherited autosomal recessive and X-linked forms also exist. In this review we provide a summary of the key features of several early- and mid-childhood onset epileptic encephalopathies including Ohtahara syndrome, Dravet syndrome, Infantile spasms and Lennox Gastaut syndrome. We review the recent next generation sequencing findings that may impact treatment choices. We also describe the use of conventional and newer anti-epileptic and hormonal medications in the various syndromes based on their genetic profile. At a biological level, developments in cellular reprogramming and genome editing represent a new direction in modeling these pediatric epilepsies and could be used in the development of novel and repurposed therapies.

VPA alleviates neurological deficits and restores gene expression in a mouse model of Rett syndrome

Rett syndrome (RTT) is a devastating neurodevelopmental disorder that occurs once in every 10,000–15,000 live female births. Despite intensive research, no effective cure is yet available. Valproic acid (VPA) has been used widely to treat mood disorder, epilepsy, and a growing number of other disorders. In limited clinical studies, VPA has also been used to control seizure in RTT patients with promising albeit somewhat unclear efficacy. In this study we tested the effect of VPA on the neurological symptoms of RTT and discovered that short-term VPA treatment during the symptomatic period could reduce neurological symptoms in RTT mice. We found that VPA restores the expression of a subset of genes in RTT mouse brains, and these genes clustered in neurological disease and developmental disorder networks. Our data suggest that VPA could be used as a drug to alleviate RTT symptoms.

Stress and anxiety across the lifespan: structural plasticity and epigenetic regulation

The brain is the central organ of the body's response to and perception of stress. Both the juvenile and the adult brain show a significant capacity for lasting physiological, structural and behavioral plasticity as a consequence of stress exposure. The hypothesis that epigenetic mechanisms might lie behind the lasting effects of stress upon the brain has proven a fruitful one. In this review, we examine the growing literature showing that stress has a direct impact on epigenetic marks at all life history stages thus far examined and how, in turn, epigenetic mechanisms play a role in altering stress responsiveness, anxiety and brain plasticity across the lifespan and beyond to succeeding generations. In addition, we will examine our own recent findings that stress interacts with the epigenome to regulate the expression of transposable elements in a regionally specific fashion, a finding with significant implications for a portion of the genome which is tenfold larger than that occupied by the genes themselves.

Promoter-like sequences regulating transcriptional activity in neurexin and neuroligin genes

Synapse function requires the cell-adhesion molecules neurexins (Nrxn) and neuroligins (Nlgn). Although these molecules are essential for neurotransmission and prefer distinct isoform combinations for interaction, little is known about their transcriptional regulation. Here, we started to explore this important aspect because expression of Nrxn1-3 and Nlgn1-3 genes is altered in mice lacking the transcriptional regulator methyl-CpG-binding protein2 (MeCP2). Since MeCP2 can bind to methylated CpG-dinucleotides and Nrxn/Nlgn contain CpG-islands, we tested genomic sequences for transcriptional activity in reporter gene assays. We found that their influence on transcription are differentially activating or inhibiting. As we observed an activity difference between heterologous and neuronal cell lines for distinct Nrxn1 and Nlgn2 sequences, we dissected their putative promoter regions. In both genes, we identify regions in exon1 that can induce transcription, in addition to the alternative transcriptional start points in exon2. While the 5′-regions of Nrxn1 and Nlgn2 contain two CpG-rich elements that show distinct methylation frequency and binding to MeCP2, other regions may act independently of this transcriptional regulator. These data provide first insights into regulatory sequences of Nrxn and Nlgn genes that may represent an important aspect of their function at synapses in health and disease.

Neuronal morphology in MeCP2 mouse models is intrinsically variable and depends on age, cell type, and Mecp2 mutation

Rett Syndrome (RTT), a progressive neurological disorder characterized by developmental regression and loss of motor and language skills, is caused by mutations in the X-linked gene encoding methyl-CpG binding protein 2 (MECP2). Neurostructural phenotypes including decreased neuronal size, dendritic complexity, and spine density have been reported in postmortem RTT brain tissue and in Mecp2 animal models. How these changes in neuronal morphology are related to RTT-like phenotype and MeCP2 function, and the extent to which restoration of neuronal morphology can be used as a cellular readout in therapeutic studies, however, remain unclear. Here, we systematically examined neuronal morphology in vivo across three Mecp2 mouse models representing Mecp2 loss-of-function, partial loss-of-function, and gain-of-function mutations, at developmental time points corresponding to early- and late-symptomatic RTT-like behavioral phenotypes. We found that in Mecp2 loss-of-function mouse models, dendritic complexity is reduced in a mild, age-dependent, and brain region-specific manner, whereas soma size is reduced consistently throughout development. Neither phenotype, however, is altered in Mecp2 gain-of-function mice. Our results suggest that, in the cell types we examined, the use of dendritic morphology as a cellular readout of RTT phenotype and therapeutic efficacy should be cautioned, as it is intrinsically variable. In contrast, soma size may be a robust and reliable marker for evaluation of MeCP2 function in Mecp2 loss-of-function studies.

DNA co-methylation analysis suggests novel functional associations between gene pairs in breast cancer samples

Localized promoter hypermethylation and overall DNA hypomethylation have been associated with the presence of tumor in humans. Yet, despite the large amount of recently produced epigenetic data, there is still a lack of understanding on how several genes behave in tumor cells with respect to their epigenetic alterations such as DNA methylation. Here we performed a novel type of analysis that measures the correlation of DNA methylation levels between two genes across many samples. We linked this so-called co-methylation to the genomic distance of these genes, their functional similarity and their expression levels. Co-methylation analysis of more than 300 breast cancer samples from the TCGA portal yielded 187 pairs of genes showing Pearson correlation coefficients |r| ≥ 0.75. These pairs were formed by 133 genes. Less than half of these pairs are located on the same chromosome. For these, we found that the level of co-methylation is weakly anti-correlated with genomic distance (r = -0.29). Linking co-methylation with the functional similarity of genes showed that genes with r ≥ 0.8 tend to have similar molecular function and to be involved in the same biological process as described in the Gene Ontology project. Clustering of highly co-methylated genes identified four enriched KEGG pathways. Hence we have introduced co-methylation as a new indicator to discover functional associations between gene pairs in breast cancer and furthermore to discover new candidate genes that should be inspected more closely in the context of the studied disease.

The impact of MeCP2 loss- or gain-of-function on synaptic plasticity

Methyl-CpG-binding protein 2 (MeCP2) is a transcriptional regulator of gene expression that is an important epigenetic factor in the maintenance and development of the central nervous system. The neurodevelopmental disorders Rett syndrome and MECP2 duplication syndrome arise from loss-of-function and gain-of-function alterations in MeCP2 expression, respectively. Several animal models have been developed to recapitulate the symptoms of Rett syndrome and MECP2 duplication syndrome. Cell morphology, neurotransmission, and cellular processes that support learning and memory are compromised as a result of MeCP2 loss- or gain-of-function. Interestingly, loss-of-MeCP2 function and MeCP2 overexpression trigger diametrically opposite changes in synaptic transmission. These findings indicate that the precise regulation of MeCP2 expression is a key requirement for the maintenance of synaptic and neuronal homeostasis and underscore its importance in central nervous system function. This review highlights the functional role of MeCP2 in the brain as a regulator of synaptic and neuronal plasticity as well as its etiological role in the development of Rett syndrome and MECP2 duplication syndrome.

Impact of an exercise intervention on DNA methylation in skeletal muscle from first-degree relatives of patients with type 2 diabetes

To identify epigenetic patterns, which may predispose to type 2 diabetes (T2D) due to a family history (FH) of the disease, we analyzed DNA methylation genome-wide in skeletal muscle from individuals with (FH(+)) or without (FH(-)) an FH of T2D. We found differential DNA methylation of genes in biological pathways including mitogen-activated protein kinase (MAPK), insulin, and calcium signaling (P ≤ 0.007) and of individual genes with known function in muscle, including MAPK1, MYO18B, HOXC6, and the AMP-activated protein kinase subunit PRKAB1 in skeletal muscle of FH(+) compared with FH(-) men. We further validated our findings from FH(+) men in monozygotic twin pairs discordant for T2D, and 40% of 65 analyzed genes exhibited differential DNA methylation in muscle of both FH(+) men and diabetic twins. We further examined if a 6-month exercise intervention modifies the genome-wide DNA methylation pattern in skeletal muscle of the FH(+) and FH(-) individuals. DNA methylation of genes in retinol metabolism and calcium signaling pathways (P < 3 × 10(-6)) and with known functions in muscle and T2D including MEF2A, RUNX1, NDUFC2, and THADA decreased after exercise. Methylation of these human promoter regions suppressed reporter gene expression in vitro. In addition, both expression and methylation of several genes, i.e., ADIPOR1, BDKRB2, and TRIB1, changed after exercise. These findings provide new insights into how genetic background and environment can alter the human epigenome.

Possible involvement of histone acetylation in the development of emotional resistance to stress stimuli in mice

Recent reports have implied that aberrant biochemical processes in the brain frequently accompany subtle shifts in the cellular epigenetic profile that might underlie the pathogenic progression of psychiatric disorders. Furthermore, certain antidepressants or mood stabilizers have been reported to have the ability to modulate epigenetic parameters. We previously reported that pretreatment of mice with 5-HT(1A) receptor agonists 24 h before testing suppressed the decrease in emotional behaviors induced by exposure to acute restraint stress. Based on this finding, the aim of the present study was to examine the association between the development of emotional resistance to stress stimuli and the modulation of an epigenetic parameter, particularly histone acetylation. We found that acetylated histone H3 was increased in the hippocampus of mice that had developed resistance to emotional stress by pretreatment with flesinoxan (1 mg/kg, i.p.) 24 h before testing. On the other hand, pretreatment with benzodiazepine anxiolytic diazepam (1 mg/kg, i.p.) did not have similar effects. Interestingly, similar to flesinoxan, the histone deacetylase inhibitor trichostatin A also protected against the emotional changes induced by acute restraint stress, as well as histone H3 acetylation. The present findings suggest that the epigenetic mechanisms of gene regulation may play an important role in the development of emotional resistance to stress stimuli.

Differential DNA methylation in purified human blood cells: implications for cell lineage and studies on disease susceptibility

Methylation of cytosines at CpG sites is a common epigenetic DNA modification that can be measured by a large number of methods, now even in a genome-wide manner for hundreds of thousands of sites. The application of DNA methylation analysis is becoming widely popular in complex disorders, for example, to understand part of the “missing heritability”. The DNA samples most readily available for methylation studies are derived from whole blood. However, blood consists of many functionally and developmentally distinct cell populations in varying proportions. We studied whether such variation might affect the interpretation of methylation studies based on whole blood DNA. We found in healthy male blood donors there is important variation in the methylation profiles of whole blood, mononuclear cells, granulocytes, and cells from seven selected purified lineages. CpG methylation between mononuclear cells and granulocytes differed for 22% of the 8252 probes covering the selected 343 genes implicated in immune-related disorders by genome-wide association studies, and at least one probe was differentially methylated for 85% of the genes, indicating that whole blood methylation results might be unintelligible. For individual genes, even if the overall methylation patterns might appear similar, a few CpG sites in the regulatory regions may have opposite methylation patterns (i.e., hypo/hyper) in the main blood cell types. We conclude that interpretation of whole blood methylation profiles should be performed with great caution and for any differences implicated in a disorder, the differences resulting from varying proportions of white blood cell types should be considered.

SIRT1-mediated deacetylation of MeCP2 contributes to BDNF expression

Methyl-CpG binding protein 2 (MeCP2) binds methylated cytosines at CpG sites on DNA and it is thought to function as a critical epigenetic regulator. Mutations in the MeCP2 gene have been associated to Rett syndrome, a human neurodevelopmental disorder. Here we show that MeCP2 is acetylated by p300 and that SIRT1 mediates its deacetylation. SIRT1, the mammalian homologue of Sir2 in yeast, is a nicotinamide-adenine dinucleotide (NAD(+))-dependent histone deacetylase that belongs to the family of HDAC class III sirtuins. Importantly, SIRT1 has been shown to play a critical role in synaptic plasticity and memory formation. This study reveals a functional interplay between two critical epigenetic regulators, MeCP2 and SIRT1, which controls MeCP2 binding activity to the brain-derived neurotrophic factor (BDNF) promoter in a specific region of the brain.

Epigenetic effects of stress and corticosteroids in the brain

Stress is a common life event with potentially long lasting effects on health and behavior. Stress, and the corticosteroid hormones that mediate many of its effects, are well known for their ability to alter brain function and plasticity. While genetic susceptibility may influence the impact of stress on the brain, it does not provide us with a complete understanding of the capacity of stress to produce long lasting perturbations on the brain and behavior. The growing science of epigenetics, however, shows great promise of deepening our understanding of the persistent impacts of stress and corticosteroids on health and disease. Epigenetics, broadly defined, refers to influences on phenotype operating above the level of the genetic code itself. At the molecular level, epigenetic events belong to three major classes: DNA methylation, covalent histone modification and non-coding RNA. This review will examine the bi-directional interactions between stress and corticosteroids and epigenetic mechanisms in the brain and how the novel insights, gleaned from recent research in neuro-epigenetics, change our understanding of mammalian brain function and human disease states.

Gene clusters, molecular evolution and disease: a speculation

Traditionally eukaryotic genes are considered independently expressed under the control of their promoters and cis-regulatory domains. However, recent studies in worms, flies, mice and humans have shown that genes co-habiting a chromatin domain or “genomic neighborhood” are frequently co-expressed. Often these co-expressed genes neither constitute part of an operon nor function within the same biological pathway. The mechanisms underlying the partitioning of the genome into transcriptional genomic neighborhoods are poorly defined. However, cross-species analyses find that the linkage among the co-expressed genes of these clusters is significantly conserved and that the expression patterns of genes within clusters have coevolved with the clusters. Such selection could be mediated by chromatin interactions with the nuclear matrix and long-range remodeling of chromatin structure. In the context of human disease, we propose that dysregulation of gene expression across genomic neighborhoods will cause highly pleiotropic diseases. Candidate genomic neighborhood diseases include the nuclear laminopathies, chromosomal translocations and genomic instability disorders, imprinting disorders of errant insulator function, syndromes from impaired cohesin complex assembly, as well as diseases of global covalent histone modifications and DNA methylation. The alteration of transcriptional genomic neighborhoods provides an exciting and novel model for studying epigenetic alterations as quantitative traits in complex common human diseases.

Animal models of epigenetic regulation in neuropsychiatric disorders

Epigenetics describes the phenomenon of heritable changes in gene regulation that are governed by non-Mendelian processes, primarily through biochemical modifications to chromatin structure that occur during cell development and differentiation. Numerous lines of evidence link abnormal levels of chromatin modifications (either to DNA, histones, or both) in patients with a wide variety of diseases including cancer, psychiatry, neurodegeneration, metabolic and inflammatory disorders. Drugs that target the proteins controlling chromatin modifications can modulate the expression of clusters of genes, potentially offering higher therapeutic efficacy than classical agents with single target pharmacologies that are susceptible to biochemical pathway degeneracy. Here, we summarize recent research linking epigenetic dysregulation with diseases in neurosciences, the application of relevant animal models, and the potential for small molecule modulator development to facilitate target discovery, validation and translation into clinical treatments.

Epigenetic regulatory mechanisms associated with infertility

Infertility is a complex human condition and is known to be caused by numerous factors including genetic alterations and abnormalities. Increasing evidence from studies has associated perturbed epigenetic mechanisms with spermatogenesis and infertility. However, there has been no consensus on whether one or a collective of these altered states is responsible for the onset of infertility. Epigenetic alterations involve changes in factors that regulate gene expression without altering the physical sequence of DNA. Understanding these altered epigenetic states at the genomic level along with higher order organisation of chromatin in genes associated with infertility and pericentromeric regions of chromosomes, particularly 9 and Y, could further identify causes of idiopathic infertility. Determining the association between DNA methylation, chromatin state, and noncoding RNAs with the phenotype could further determine what possible mechanisms are involved. This paper reviews certain mechanisms of epigenetic regulation with particular emphasis on their possible role in infertility.

Epigenetic approaches to psychiatric disorders

Psychiatric diseases place a tremendous burden on affected individuals, their caregivers, and the health care system. Although evidence exists for a strong inherited component to many of these conditions, dedicated efforts to identify DNA sequence-based causes have not been exceptionally productive, and very few pharmacologic treatment options are clinically available. Many features of psychiatric diseases are consistent with an epigenetic dysregulation, such as discordance of monozygotic twins, late age of onset, parent-of-origin and sex effects, and fluctuating disease course. In recent years, experimental technologies have significantly advanced, permitting indepth studies of the epigenome and its role in maintenance of normal genomic functions, as well as disease etiopathogenesis. Here, we present an epigenetic explanation for many characteristics of psychiatric disease, review the current literature on the epigenetic mechanisms involved in major psychosis, Alzheimer's disease, and autism spectrum disorders, and describe some future directions in the field of psychiatric epigenomics.

Reversibility of functional deficits in experimental models of Rett syndrome

Mutations in the X-linked MECP2 gene are the primary cause of the severe autism spectrum disorder RTT (Rett syndrome). Deletion of Mecp2 in mice recapitulates many of the overt neurological features seen in humans, and the delayed onset of symptoms is accompanied by deficits in neuronal morphology and synaptic physiology. Recent evidence suggests that reactivation of endogenous Mecp2 in young and adult mice can reverse aspects of RTT-like pathology. In the current perspective, we discuss these findings as well as other genetic, pharmacological and environmental interventions that attempt phenotypic rescue in RTT. We believe these studies provide valuable insights into the tractability of RTT and related conditions and are useful pointers for the development of future therapeutic strategies.

The role of DNA methylation in the central nervous system and neuropsychiatric disorders

DNA methylation is an epigenetic mechanism in which the methyl group is covalently coupled to the C5 position of the cytosine residue of CpG dinucleotides. DNA methylation generally leads to gene silencing and is catalyzed by a group of enzymes known as DNA methyltransferases (Dnmt). During development, the epigenome undergoes waves of demethylation and methylation changes. As a result, there are cell type/tissue-specific DNA methylation patterns. Since DNA methylation changes only happen during DNA replication to maintain methylation patterns on hemimethylated DNA or establish new methylation, Dnmt expression generally decreases greatly after cell division. However, significant levels of Dnmts were noticed specifically in postmitotic neurons, suggesting a functional importance of Dnmt in the nervous system. Accumulating evidence showed that DNA methylation correlates with certain neuropsychiatric disorders such as schizophrenia, Rett syndrome, and ICF syndrome. Studies of methyl-CpG-binding proteins, Dnmt inhibitors, and Dnmt knockout mice also explored the key role of DNA methylation in neural development, plasticity, learning, and memory. Though an enzyme exhibiting DNA demethylation capability in vertebrates still remains to be identified, DNA methylation status in the CNS appeared to be reversible at certain genomic loci. This supports a maintenance role of Dnmt to prevent active demethylation in postmitotic neurons. Taken together, DNA methylation provides an epigenetic mechanism of gene regulation in neural development, function, and disorders.

MeCP2 functions largely cell-autonomously, but also non-cell-autonomously, in neuronal maturation and dendritic arborization of cortical pyramidal neurons

Rett syndrome is a human neurodevelopmental disorder presenting almost exclusively in female infants; it is the second most common cause of mental retardation in girls, after Down's syndrome. The identification in 1999 that mutation of the methyl-CpG-binding protein 2 (MECP2) gene on the X chromosome causes Rett syndrome has led to a rapid increase in understanding of the neurobiological basis of the disorder. However, much about the functional role of MeCP2, and the cellular phenotype of both patients with Rett syndrome and mutant Mecp2 mouse models, remains unclear. Building on prior work in which we demonstrated that cortical layer 2/3 pyramidal neurons (primarily interhemispheric "callosal projection neurons" (CPN)) have reduced dendritic complexity and smaller somata in Mecp2-null mice, here we investigate whether Mecp2 loss-of-function affects neuronal maturation cell-autonomously and/or non-cell-autonomously by creating physical chimeras. We transplanted Mecp2-null or wild-type (wt) E17-18 cortical neuroblasts and immature neurons from mice constitutively expressing enhanced green fluorescent protein (eGFP) into wt P2-3 mouse cortices to generate chimeric cortices. Mecp2-null layer 2/3 pyramidal neurons in both Mecp2-null and wt neonatal cortices exhibit equivalent reduction in dendritic complexity, and are smaller than transplanted wt neurons, independent of recipient environment. These results indicate that the phenotype of Mecp2-null pyramidal neurons results largely from cell-autonomous mechanisms, with additional non-cell-autonomous effects. Dysregulation of MeCP2 target genes in individual neuronal populations such as CPN is likely centrally involved in Rett syndrome pathogenesis. Our results indicating MeCP2 function in the centrally affected projection neuron population of CPN themselves provide a foundation and motivation for identification of transcriptionally regulated MeCP2 target genes in developing CPN.

Pharmacology of epigenetics in brain disorders

Epigenetics is a rapidly growing field and holds great promise for a range of human diseases, including brain disorders such as Rett syndrome, anxiety and depressive disorders, schizophrenia, Alzheimer disease and Huntington disease. This review is concerned with the pharmacology of epigenetics to treat disorders of the epigenome whether induced developmentally or manifested/acquired later in life. In particular, we will focus on brain disorders and their treatment by drugs that modify the epigenome. While the use of DNA methyl transferase inhibitors and histone deacetylase inhibitors in in vitro and in vivo models have demonstrated improvements in disease-related deficits, clinical trials in humans have been less promising. We will address recent advances in our understanding of the complexity of the epigenome with its many molecular players, and discuss evidence for a compromised epigenome in the context of an ageing or diseased brain. We will also draw on examples of species differences that may exist between humans and model systems, emphasizing the need for more robust pre-clinical testing. Finally, we will discuss fundamental issues to be considered in study design when targeting the epigenome.

Non-cell autonomous influence of MeCP2-deficient glia on neuronal dendritic morphology

The neurodevelopmental disorder Rett syndrome (RTT) is caused by sporadic mutations in the transcriptional factor methyl-CpG-binding protein 2 (MeCP2). Although it is thought that the primary cause of RTT is cell autonomous, resulting from a lack of functional MeCP2 in neurons, whether non-cell autonomous factors contribute to the disease is unknown. We found that the loss of MeCP2 occurs not only in neurons but also in glial cells of RTT brains. Using an in vitro co-culture system, we found that mutant astrocytes from a RTT mouse model, and their conditioned medium, failed to support normal dendritic morphology of either wild-type or mutant hippocampal neurons. Our studies suggest that astrocytes in the RTT brain carrying MeCP2 mutations have a non-cell autonomous effect on neuronal properties, probably as a result of aberrant secretion of soluble factor(s).