MeCP2 in neurons: closing in on the causes of Rett syndrome

Literature Cases Summarized Based on Their Polysomnographic Findings in Rett Syndrome

Rett syndrome (RTT) is a severe and rare neurodevelopmental disorder affecting mostly girls. In RTT, an impaired sleep pattern is a supportive criterion for the diagnosis, yet little is known regarding the sleep structure and sleep respiratory events. Aiming to delineate sleep by aggregating RTT case (series) data from published polysomnographic studies, seventy-four RTT cases were collected from eleven studies up until 6 February 2022 (PROSPERO: CRD 42020198099). We compared the polysomnographic data within RTT stratifications and to a typically developing population. MECP2 cases demonstrated shortened total sleep time (TST) with increased stage N3 and decreased REM sleep. In cases with CDKL5 mutations, TST was longer and they spent more time in stage N1 but less in stage N3 than those cases affected by MECP2 mutations and a typically developing population. Sleep-disordered breathing was confirmed by the abnormal apnea/hypopnea index of 11.92 ± 23.67/h TST in these aggregated cases. No association of sleep structure with chronological age was found. In RTT, the sleep macrostructure of MECP2 versus CDKL5 cases showed differences, particularly regarding sleep stage N3. A severe REM sleep propensity reduction was found. Aberrant sleep cycling, possibly characterized by a poor REM ‘on switch’ and preponderance in slow and high-voltage sleep, is proposed.

Rett Syndrome: A Timely Review From Recognition to Current Clinical Approaches and Clinical Study Updates

Since the discovery of the genetic basis of Rett syndrome in 1999, our understanding has grown considerably both in the scientific and the clinical realms. In the last two decades, we have learned about the far-reaching effects of the aberrant MeCP2 protein, the growing list of involved genetic factors, and the genotype-phenotype clinical expression of common MECP2 mutations. This knowledge has led to several basic science research and clinical trials, focusing specifically on emerging treatments of Rett syndrome. As the pathophysiology behind the disease is better understood, treatments aimed at specific molecular targets will become available for clinicians to improve the life of individuals with Rett syndrome.

Rett syndrome in adults with severe intellectual disability: Exploration of behavioral characteristics

Rett syndrome is a genetically linked form of autism spectrum disorder (ASD) accompanied by intellectual disability (ID). The disorder is also characterized by cardiorespiratory dysregulation, disturbance in muscle tone, reduced brain growth and scoliosis. Over 300 studies have been published on the disorder, most of which has focused on identification of causative factors, which appears to be the result of mutations of gene MECP2. Rarely have adults with Rett syndrome been studied, and behavioral characteristics in these individuals are largely unknown. The present study aimed to extend what little is known about behavioral characteristics of Rett syndrome in adults, with particular emphasis on social, communicative, and adaptive behavior. Rett syndrome adults with severe ID were matched to autistic adults with ID and ID only controls. The implications of these data for more fully describing and diagnosing the condition in adults are discussed.

P152R Mutation Within MeCP2 Can Cause Loss of DNA-Binding Selectivity

MeCP2 is a protein highly expressed in the brain that participates in the genetic expression and RNA splicing regulation. MeCP2 binds preferably to methylated DNA and other nuclear corepressors to alter chromatin. MECP2 gene mutations can cause rett syndrome (RTT), a severe neurological disorder that affects around one in ten thousand girls. In this paper, Molecular Dynamics (MD) simulations were performed to scrutinize how the MeCP2 P152R mutation influences the protein binding to DNA. Also, the Umbrella Sampling technique was used to obtain the potential mean forces (PMFs) of both wild-type and mutated MeCP2 Methyl-CpG-binding domain (MBD) binding to both non-methylated and methylated DNA. P152R is a common missense mutation in MBD associated with RTT; however, there are no studies that explain how it causes protein dysfunction. The results from this study hypothesize that P152R mutation leads to MBD binding more strongly to DNA, while selectively decreasing binding affinity to methylated DNA. These provide an explanation for previous not conclusive experimental results regarding the mechanism of how this mutation affects the binding of the protein to DNA, and subsequently its effects on RTT. Furthermore, the results of this research-in-progress can be used as the basis for further investigations into the molecular basis of RTT and to potentially reveal a target for therapy in the future.

Enhanced extinction of aversive memories in mice lacking SPARC-related protein containing immunoglobulin domains 1 (SPIG1/FSTL4)

Brain-derived neurotrophic factor (BDNF) plays an important role in synaptic plasticity related to learning and memory. We previously reported that SPARC-related protein containing immunoglobulin domains 1 (SPIG1, also known as Follistatin-like protein 4, FSTL4) binds to pro-BDNF and negatively regulates BDNF maturation; however, its neurological functions, particularly in learning and memory, have not yet been elucidated. We herein examined the electrophysiological and behavioral phenotypes of Spig1-knockout (Spig1-KO) mice. Adult Spig1-KO mice exhibited greater excitability and facilitated long-term potentiation (LTP) in the CA1 region of hippocampal slices than age- and sex-matched wild-type (WT) mice. Facilitated LTP was reduced to the level of WT by the bath application of an anti-BDNF antibody to hippocampal slices. A step-through inhibitory avoidance learning paradigm revealed that the extinction of aversive memories was significantly enhanced in adult Spig1-KO mice, while they showed the normal acquisition of aversive memories; besides, spatial reference memory formation was also normal in the standard Morris water maze task. An intracerebroventricular (icv) injection of anti-BDNF in the process of extinction learning transiently induced the recurrence of aversive memories in Spig1-KO mice, but exerted no effects in WT mice. These results indicate a critical role for SPIG1 in BDNF-mediated synaptic plasticity in extinction of inhibitory avoidance memory.

In vivo and in vitro sex differences in the dendritic morphology of developing murine hippocampal and cortical neurons

Altered dendritic morphology is common in neurodevelopmental disorders (NDDs), many of which show sex biases in prevalence, onset and/or severity. However, whether dendritic morphology varies as a function of sex in juvenile mice or primary neuronal cell cultures is largely unknown even though both are widely used models for studying NDDs. To address this gap, we quantified dendritic morphology in CA1 pyramidal hippocampal and adjacent somatosensory pyramidal cortical neurons from male and female postnatal day (P)28 C57BL/6J mice. As determined by Sholl analysis of Golgi-stained brain sections, dendritic arbors of male hippocampal neurons are more complex than females. Conversely, dendritic morphology of female cortical neurons is more complex than males. In primary neuron-glia co-cultures from P0 mouse hippocampi, male neurons have more complex dendritic arbors than female neurons. Sex differences are less pronounced in cortical cultures. In vitro sex differences in dendritic morphology are driven in part by estrogen-dependent mechanisms, as evidenced by decreased dendritic complexity in male hippocampal neurons cultured in phenol red-free media or in the presence of an estrogen receptor antagonist. Evidence that sex influences dendritic morphogenesis in two models of neurodevelopment in a region-specific manner has significant mechanistic implications regarding sex biases in NDDs.

Epigenetics, nutrition and mental health. Is there a relationship?

Many aspects of human development and disease are influenced by the interaction between genetic and environmental factors. Understanding how our genes respond to the environment is central to managing health and disease, and is one of the major contemporary challenges in human genetics. Various epigenetic processes affect chromosome structure and accessibility of deoxyribonucleic acid (DNA) to the enzymatic machinery that leads to expression of genes. One important epigenetic mechanism that appears to underlie the interaction between environmental factors, including diet, and our genome, is chemical modification of the DNA. The best understood of these modifications is methylation of cytosine residues in DNA. It is now recognized that the pattern of methylated cytosines throughout our genomes (the 'methylome') can change during development and in response to environmental cues, often with profound effects on gene expression. Many dietary constituents may indirectly influence genomic pathways that methylate DNA, and there is evidence for biochemical links between nutritional quality and mental health. Deficiency of both macro- and micronutrients has been associated with increased behavioural problems, and nutritional supplementation has proven efficacious in treatment of certain neuropsychiatric disorders. In this review we examine evidence from the fields of nutrition, developmental biology, and mental health that supports dietary impacts on epigenetic processes, particularly DNA methylation. We then consider whether such processes could underlie the demonstrated efficacy of dietary supplementation in treatment of mental disorders, and whether targeted manipulation of DNA methylation patterns using controlled dietary supplementation may be of wider clinical value.

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.

Importance of gene dosage in controlling dendritic arbor formation during development

Proper dendrite morphology is crucial for normal nervous system functioning. While a number of genes have been implicated in dendrite morphogenesis in both invertebrates and mammals, it remains unclear how developing dendrites respond to changes in gene dosage and what type of patterns their responses may follow. To understand this, I review here evidence from the recent literature, focusing on the genetic studies performed in the Drosophila larval dendritic arborization class IV neuron, an excellent cell type to understand dendrite morphogenesis. I summarize how class IV arbors change morphology in response to developmental fluctuations in the expression levels of 47 genes, studied by means of genetic manipulations such as loss-of-function and gain-of-function, and for which sufficient information is available. I find that arbors can respond to changing gene dosage in several distinct ways, each characterized by a singular dose-response curve. Interestingly, in 72% of cases arbors are sensitive, and thus adjust their morphology, in response to both decreases and increases in the expression of a given gene, indicating that dendrite morphogenesis is a process particularly sensitive to gene dosage. By summarizing the parallels between Drosophila and mammals, I show that many Drosophila dendrite morphogenesis genes have orthologs in mammals, and that some of these are associated with mammalian dendrite outgrowth and human neurodevelopmental disorders. One notable disease-related molecule is kinase Dyrk1A, thought to be a causative factor in Down syndrome. Both increases and decreases in Dyrk1A gene dosage lead to impaired dendrite morphogenesis, which may contribute to Down syndrome pathoetiology.

How big is the myelinating orchestra? Cellular diversity within the oligodendrocyte lineage: facts and hypotheses

Since monumental studies from scientists like His, Ramón y Cajal, Lorente de Nó and many others have put down roots for modern neuroscience, the scientific community has spent a considerable amount of time, and money, investigating any possible aspect of the evolution, development and function of neurons. Today, the complexity and diversity of myriads of neuronal populations, and their progenitors, is still focus of extensive studies in hundreds of laboratories around the world. However, our prevalent neuron-centric perspective has dampened the efforts in understanding glial cells, even though their active participation in the brain physiology and pathophysiology has been increasingly recognized over the years. Among all glial cells of the central nervous system (CNS), oligodendrocytes (OLs) are a particularly specialized type of cells that provide fundamental support to neuronal activity by producing the myelin sheath. Despite their functional relevance, the developmental mechanisms regulating the generation of OLs are still poorly understood. In particular, it is still not known whether these cells share the same degree of heterogeneity of their neuronal companions and whether multiple subtypes exist within the lineage. Here, we will review and discuss current knowledge about OL development and function in the brain and spinal cord. We will try to address some specific questions: do multiple OL subtypes exist in the CNS? What is the evidence for their existence and those against them? What are the functional features that define an oligodendrocyte? We will end our journey by reviewing recent advances in human pluripotent stem cell differentiation towards OLs. This exciting field is still at its earliest days, but it is quickly evolving with improved protocols to generate functional OLs from different spatial origins. As stem cells constitute now an unprecedented source of human OLs, we believe that they will become an increasingly valuable tool for deciphering the complexity of human OL identity.

[Epigenetic regulation in neuronal differentiation and brain function]

DNA methylation compacts chromatin structure and represses gene transcription. It is important for numerous cellular processes, including embryonic development, X-chromosome inactivation, suppression of transposable elements, and cellular differentiation. In addition, environmental cues, including drugs, pollutants, trauma or early-life social environment, alter DNA methylation patterns in different organs. For instance, studies have unravelled a complex and dynamic interplay between environment, DNA methylation and neuron function during development and in the adult. This crosstalk is hypothesized as an essential molecular event underlying the effects of long-term memory, drug addiction, and several psychotic and behavioural disorders. In this review, we give a summary of this exciting field of research and highlight the molecular functions of DNA methylation and of proteins interacting with methylated DNA.

Vascular dysfunction in a mouse model of Rett syndrome and effects of curcumin treatment

Mutations in the coding sequence of the X-linked gene MeCP2 (Methyl CpG–binding protein) are present in around 80% of patients with Rett Syndrome, a common cause of intellectual disability in female and to date without any effective pharmacological treatment. A relevant, and so far unexplored feature of RTT patients, is a marked reduction in peripheral circulation. To investigate the relationship between loss of MeCP2 and this clinical aspect, we used the MeCP2 null mouse model B6.129SF1-MeCP2tm1Jae for functional and pharmacological studies. Functional experiments were performed on isolated resistance mesenteric vessels, mounted on a pressurized myograph. Vessels from female MeCP2^+/− mice show a reduced endothelium-dependent relaxation, due to a reduced Nitric Oxide (NO) availability secondary to an increased Reactive Oxygen Species (ROS) generation. Such functional aspects are associated with an intravascular increase in superoxide anion production, and a decreased vascular eNOS expression. These alterations are reversed by curcumin administration (5% (w/w) dietary curcumin for 21 days), which restores endothelial NO availability, decreases intravascular ROS production and normalizes vascular eNOS gene expression. In conclusion our findings highlight alterations in the vascular/endothelial system in the absence of a correct function of MeCP2, and uncover related cellular/molecular mechanisms that are rescued by an anti-oxidant treatment.

X inactivation in females with X-linked Charcot-Marie-Tooth disease

X-linked Charcot–Marie–Tooth disease (CMT1X) is the second most common inherited neuropathy, caused by mutations in gap junction beta-1 (GJB1). Males have a uniformly moderately severe phenotype while females have a variable phenotype, suggested to be due to X inactivation. We aimed to assess X inactivation pattern in females with CMT1X and correlate this with phenotype using the CMT examination score to determine whether the X inactivation pattern accounted for the variable phenotype in females with CMT1X. We determined X inactivation pattern in 67 females with CMT1X and 24 controls using the androgen receptor assay. We were able to determine which X chromosome carried the GJB1 mutation in 30 females. There was no difference in X inactivation pattern between patients and controls. In addition, there was no correlation between X inactivation pattern in blood and phenotype. A possible explanation for these findings is that the X inactivation pattern in Schwann cells rather than in blood may explain the variable phenotype in females with CMT1X.

Epigenetic regulation of nervous system development by DNA methylation and histone deacetylation

Alterations in the epigenetic modulation of gene expression have been implicated in several developmental disorders, cancer, and recently, in a variety of mental retardation and complex psychiatric disorders. A great deal of effort is now being focused on why the nervous system may be susceptible to shifts in activity of epigenetic modifiers. The answer may simply be that the mammalian nervous system must first produce the most complex degree of developmental patterning in biology and hardwire cells functionally in place postnatally, while still allowing for significant plasticity in order for the brain to respond to a rapidly changing environment. DNA methylation and histone deacetylation are two major epigenetic modifications that contribute to the stability of gene expression states. Perturbing DNA methylation, or disrupting the downstream response to DNA methylation - methyl-CpG-binding domain proteins (MBDs) and histone deacetylases (HDACs) - by genetic or pharmacological means, has revealed a critical requirement for epigenetic regulation in brain development, learning, and mature nervous system stability, and has identified the first distinct gene sets that are epigenetically regulated within the nervous system. Epigenetically modifying chromatin structure in response to different stimuli appears to be an ideal mechanism to generate continuous cellular diversity and coordinate shifts in gene expression at successive stages of brain development - all the way from deciding which kind of a neuron to generate, through to how many synapses a neuron can support. Here, we review the evidence supporting a role for DNA methylation and histone deacetylation in nervous system development and mature function, and present a basis from which to understand how the clinical use of HDAC inhibitors may impact nervous system function.

Bdnf overexpression in hippocampal neurons prevents dendritic atrophy caused by Rett-associated MECP2 mutations

The expression of the methylated DNA-binding protein MeCP2 increases during neuronal development, which suggests that this epigenetic factor is crucial for neuronal terminal differentiation. We evaluated dendritic and axonal development in embryonic day-18 hippocampal neurons in culture by measuring total length and counting branch point numbers at 4 days in vitro, well before synapse formation. Pyramidal neurons transfected with a plasmid encoding a small hairpin RNA (shRNA) to knockdown endogenous Mecp2 had shorter dendrites than control untransfected neurons, without detectable changes in axonal morphology. On the other hand, overexpression of wildtype (wt) human MECP2 increased dendritic branching, in addition to axonal branching and length. Consistent with reduced neuronal growth and complexity in Rett syndrome (RTT) brains, overexpression of human MECP2 carrying missense mutations common in RTT individuals (R106W or T158M) reduced dendritic and axonal length. One of the targets of MeCP2 transcriptional control is the Bdnf gene. Indeed, endogenous Mecp2 knockdown increased the intracellular levels of BDNF protein compared to untransfected neurons, suggesting that MeCP2 represses Bdnf transcription. Surprisingly, overexpression of wt MECP2 also increased BDNF levels, while overexpression of RTT-associated MECP2 mutants failed to affect BDNF levels. The extracellular BDNF scavenger TrkB-Fc prevented dendritic overgrowth in wt MECP2-overexpressing neurons, while overexpression of the Bdnf gene reverted the dendritic atrophy caused by Mecp2-knockdown. However, this effect was only partial, since Bdnf increased dendritic length only to control levels in mutant MECP2-overexpressing neurons, but not as much as in Bdnf-transfected cells. Our results demonstrate that MeCP2 plays varied roles in dendritic and axonal development during neuronal terminal differentiation, and that some of these effects are mediated by autocrine actions of BDNF.

Microdeletion syndromes disclose replication timing alterations of genes unrelated to the missing DNA

Background

The temporal order of allelic replication is interrelated to the epigenomic profile. A significant epigenetic marker is the asynchronous replication of monoallelically-expressed genes versus the synchronous replication of biallelically-expressed genes. The present study sought to determine whether a microdeletion in the genome affects epigenetic profiles of genes unrelated to the missing segment. In order to test this hypothesis, we checked the replication patterns of two genes – SNRPN, a normally monoallelically expressed gene (assigned to 15q11.13), and the RB1, an archetypic biallelically expressed gene (assigned to 13.q14) in the genomes of patients carrying the 22q11.2 deletion (DiGeorge/Velocardiofacial syndrome) and those carrying the 7q11.23 deletion (Williams syndrome).

Results

The allelic replication timing was determined by fluorescence in situ hybridization (FISH) technology performed on peripheral blood cells. As expected, in the cells of normal subjects the frequency of cells showing asynchronous replication for SNRPN was significantly (P < 10^-12) higher than the corresponding value for RB1. In contrast, cells of the deletion-carrying patients exhibited a reversal in this replication pattern: there was a significantly lower frequency of cells engaging in asynchronous replication for SNRPN than for RB1 (P < 10^-4 and P < 10^-3 for DiGeorge/Velocardiofacial and Williams syndromes, respectively). Accordingly, the significantly lower frequency of cells showing asynchronous replication for SNRPN than for RB1 is a new epigenetic marker distinguishing these deletion syndrome genotypes from normal ones.

Conclusion

In cell samples of each deletion-carrying individual, an aberrant, reversed pattern of replication is delineated, namely, where a monoallelic gene replicates more synchronously than a biallelic gene. This inverted pattern, which appears to be non-deletion-specific, clearly distinguishes cells of deletion-carriers from normal ones. As such, it offers a potential epigenetic marker for suspecting a hidden microdeletion that is too small to be detected by conventional karyotyping methods.

Temporal and epigenetic regulation of neurodevelopmental plasticity

The anticipated therapeutic uses of neural stem cells depend on their ability to retain a certain level of developmental plasticity. In particular, cells must respond to developmental manipulations designed to specify precise neural fates. Studies in vivo and in vitro have shown that the developmental potential of neural progenitor cells changes and becomes progressively restricted with time. For in vitro cultured neural progenitors, it is those derived from embryonic stem cells that exhibit the greatest developmental potential. It is clear that both extrinsic and intrinsic mechanisms determine the developmental potential of neural progenitors and that epigenetic, or chromatin structural, changes regulate and coordinate hierarchical changes in fate-determining gene expression. Here, we review the temporal changes in developmental plasticity of neural progenitor cells and discuss the epigenetic mechanisms that underpin these changes. We propose that understanding the processes of epigenetic programming within the neural lineage is likely to lead to the development of more rationale strategies for cell reprogramming that may be used to expand the developmental potential of otherwise restricted progenitor populations.

Folate-responsive neurologic diseases

Folate is a water-soluble vitamin of the B complex group, and is required for optimal health, growth, and development. In humans, it cannot be synthesized de novo. As a cofactor or coenzyme, folate plays key biological roles in a variety of physiologic processes: maintenance and repair of the genome, regulation of gene expression, amino-acid metabolism, neurotransmitter synthesis, and the formation of myelin. Dietary folates must undergo multiple, tightly regulated absorption and metabolic processes before their cellular utilization occurs. Clinical conditions associated with abnormal body folate status are very diverse. They range from genetic syndromes defined prior to conception, to malformations that develop during embryogenesis (neural tube defects), to disorders that are postnatally acquired and progressive (e.g., cerebral folate deficiency, or folinic acid-responsive seizures). Central nervous system folate deficiency or impaired availability can occur in the settings of normal or decreased systemic folate levels. Because the majority of patients respond to treatment with folinic acid, pediatric neurologists should remain vigilant to the possibility of deficiencies of folate in patients with unexplained neurologic disorders. The deleterious outcomes of untreated patients underscore the importance of making an early diagnosis.

The mental retardation gene CC2D1A/Freud-1 encodes a long isoform that binds conserved DNA elements to repress gene transcription

The CC2D1A/Freud-1 gene has recently been linked to non-syndromic mental retardation and a short isoform of mouse Five prime REpressor Under Dual repression binding protein 1 (Freud-1) can repress the serotonin-1A (5-HT1A) receptor gene in rodent cells. In this study, we addressed the expression, localization and regulation of the human 5-HT1A receptor gene by a long isoform of human Freud-1 protein (Freud-1L). We show that human CC2D1A/Freud-1 RNA is expressed in brain and peripheral tissues and encodes short and long isoforms, which differ by an upstream in-frame translational start site. Whereas previous studies identified the short isoform of Freud-1 as the predominant isoform in rodent cells, we demonstrate that the long isoform is more abundant in human cells, especially in the nuclear fraction. The nuclear localization of Freud-1L was enriched upon inhibition of chromosome region maintenance 1/exportin 1-dependent nuclear export, indicating a dynamic regulation of Freud-1 nuclear localization. Consistent with a functional role in the nucleus, human Freud-1L bound specifically to its dual repressor element in the 5-HT1A receptor gene in vitro and repressed transcription from these sites. Importantly, chromatin immunoprecipitation using antibodies specific for human Freud-1L demonstrated that it is bound to the dual repressor element in chromatin, indicating a functional role in regulating the basal expression of the 5-HT1A receptor gene. Taken together, these results indicate that both the short and long isoforms of Freud-1 are expressed, although Freud-1L is the major isoform that regulates the human 5-HT1A receptor gene. Disruption of transcriptional regulation by mutation of Freud-1 may play a role in abnormal brain function leading to mental retardation.

Differential Repression by Freud-1/CC2D1A at a Polymorphic Site in the Dopamine-D2 Receptor Gene

Freud-1/CC2D1A is a transcriptional repressor of the serotonin-1A receptor gene and was recently genetically linked to non-syndromic mental retardation. To identify new Freud-1 gene targets, data base mining for Freud-1 recognition sequences was done. A highly homologous intronic element (D2-DRE) was identified in the human dopamine-D2 receptor (DRD2) gene, and the role of Freud-1 in regulating the gene at this site was assessed. Recombinant Freud-1 bound specifically to the D2-DRE, and a major protein-D2-DRE complex was identified in nuclear extracts that was supershifted using Freud-1-specific antibodies. Endogenous Freud-1 binding to the D2-DRE in cells was detected using chromatin immunoprecipitation. The D2-DRE conferred strong repressor activity in transcriptional reporter assays that was dependent on the Freud-1 recognition sequence. In three different human cell lines, the level of Freud-1 protein was inversely related to DRD2 expression. Knockdown of endogenous Freud-1 using small interfering RNA resulted in an up-regulation of DRD2 RNA and binding sites, demonstrating a crucial role for Freud-1 in DRD2 regulation. A previously uncharacterized single nucleotide A/G polymorphism (rs2734836) was located adjacent to the D2-DRE and conferred allele-specific Freud-1 binding and repression, with the major G-allele having reduced activity. These studies demonstrate a key role for Freud-1 to regulate DRD2 expression and provide the first mechanistic insights into its transcriptional regulation. Allele-specific regulation of DRD2 expression by Freud-1 may possibly associate with psychiatric disorders or mental retardation.

Enhanced anxiety and stress-induced corticosterone release are associated with increased Crh expression in a mouse model of Rett syndrome

Rett syndrome (RTT), a postnatal neurodevelopmental disorder, is caused by mutations in the methyl-CpG-binding protein 2 (MECP2) gene. Children with RTT display cognitive and motor abnormalities as well as autistic features. We studied mice bearing a truncated Mecp2 allele (Mecp2(308/Y) mice) and found evidence of increased anxiety-like behavior and an abnormal stress response as evidenced by elevated serum corticosterone levels. We found increased corticotropin-releasing hormone (Crh) gene expression in the paraventricular nucleus of the hypothalamus, the central amygdala, and the bed nucleus of the stria terminalis. Finally, we discovered that MeCP2 binds the Crh promoter, which is enriched for methylated CpG dinucleotides. In contrast, the MeCP2(308) protein was not detected at the Crh promoter. This study identifies Crh as a target of MeCP2 and implicates Crh overexpression in the development of specific features of the Mecp2(308/Y) mouse, thereby providing opportunities for clinical investigation and therapeutic intervention in RTT.

Genetics and pathophysiology of mental retardation

Mental retardation (MR) is defined as an overall intelligence quotient lower than 70, associated with functional deficit in adaptive behavior, such as daily-living skills, social skills and communication. Affecting 1-3% of the population and resulting from extraordinary heterogeneous environmental, chromosomal and monogenic causes, MR represents one of the most difficult challenges faced today by clinician and geneticists. Detailed analysis of the Online Mendelian Inheritance in Man database and literature searches revealed more than a thousand entries for MR, and more than 290 genes involved in clinical phenotypes or syndromes, metabolic or neurological disorders characterized by MR. We estimate that many more MR genes remain to be identified. The purpose of this review is to provide an overview on the remarkable progress achieved over the last decade in delineating genetic causes of MR, and to highlight the emerging biological and cellular processes and pathways underlying pathogeneses of human cognitive disorders.

The human reelin gene: Transcription factors (+), repressors (−) and the methylation switch (+/−) in schizophrenia

A recent report suggests that the down-regulation of reelin and glutamic acid decarboxylase (GAD(67)) mRNAs represents 2 of the more consistent findings thus far described in post-mortem material from schizophrenia (SZ) patients [reviewed in. Neurochemical markers for schizophrenia, bipolar disorder amd major depression in postmortem brains. Biol Psychiatry 57, 252-260]. To study mechanisms responsible for this down-regulation, we have analyzed the promoter of the human reelin gene. Collectively, our studies suggest that SZ is characterized by a gamma-amino butyric acid (GABA)-ergic neuron pathology presumably mediated by promoter hypermethylation facilitated by the over-expression of the methylating enzyme DNA methyltransferase (Dnmt) 1. Using transient expression assays, promoter deletions and co-transfection assays with various transcription factors, we have shown a clear synergistic action that is a critical component of the mechanism of the trans-activation process. Equally important is the observation that the reelin promoter is more heavily methylated in brain regions in patients diagnosed with SZ as compared to non-psychiatric control subjects [Grayson, D. R., Jia, X., Chen, Y., Sharma, R. P., Mitchell, C. P., & Guidotti, A., et al. (2005). Reelin promoter hypermethylation in schizophrenia. Proc Natl Acad Sci U S A 102, 9341-9346]. The combination of studies in cell lines and in animal models of SZ, coupled with data obtained from post-mortem human material provides compelling evidence that aberrant methylation may be part of a core dysfunction in this psychiatric disease. More interestingly, the hypermethylation concept provides a coherent mechanism that establishes a plausible link between the epigenetic misregulation of multiple genes that are affected in SZ and that collectively contribute to the associated symptomatology.

Chromatin architecture

A complete understanding of the structure-function relationships of chromatin requires extending primarily one dimensional information, obtained from molecular genetic techniques and based on the underlying linear DNA sequence, to the three dimensional conformation. Recent progress in this endeavor has included the examination of fully defined nucleosomes and nucleosomal arrays assembled in vitro using X-ray diffraction, NMR spectroscopy, electron microscopy and atomic force microscopy. These studies have provided valuable insights into the structural roles of histone variants, the impact of histone mutations and the compaction of nucleosomal arrays. In addition, the diverse structural consequences of the binding of specific chromatin 'architectural' proteins are becoming apparent. These approaches provide an essential basis for understanding the conformation of the 'epigenome'.

Skewed X inactivation in healthy individuals and in different diseases

In female mammalian cells, one of the two X chromosomes is inactivated in early embryonic life. Females are mosaics for two cell populations, one with the maternal and one with the paternal X as the active chromosome. Skewed X inactivation is arbitrarily defined, often as a pattern where 80% or more of the cells show a preferential inactivation of one X chromosome. Inactivation is presumed to be permanent for all descendants of a cell; however, after about 55 years of age, the frequency of skewed X inactivation in peripheral blood cells increases, probably through selection. Unfavourable skewing of X inactivation, where the X chromosome carrying a mutant allele is the predominantly active X, has been found in affected female carriers of several X-linked disorders; however, for many X-linked disorders, a consistent relationship between the pattern of X inactivation and clinical phenotype has been difficult to demonstrate. One reason for this may be that peripheral blood cells are not a representative or relevant tissue in many disorders. In some severe X-linked disorders, post-inactivation selection takes place against the X chromosome carrying the mutant allele, leading to a completely skewed X-inactivation pattern. Skewed X inactivation has also been reported in young females with breast cancer, and may indicate an effect of X-linked genes on the development of this condition.

CONCLUSION

The process of X inactivation and the resultant degree of skewing is clearly important for the expression of genetic diseases. It is also important to consider, however, that under normal conditions the frequency of skewed X inactivation increases with age in peripheral blood cells. Analysis of the expression of a large proportion of the genes on the X chromosome has revealed that X-chromosome inactivation is more heterogeneous than previously thought.

Organization of interphase chromatin

The organization of interphase chromatin spans many topics, ranging in scale from the molecular level to the whole nucleus, and its study requires a concomitant range of experimental approaches. In this review, we examine these approaches, the results they have generated, and the interfaces between them. The greatest challenge appears to be the integration of information on whole nuclei obtained by light microscopy with data on nucleosome-nucleosome interactions and chromatin higher-order structures, obtained in vitro using biophysical characterization, atomic force microscopy, and electron microscopy. We consider strategies that may assist in the integration process, and we review emerging technologies that promise to reduce the "resolution gap."

Altered gene silencing and human diseases

Epigenetic regulation of gene expression is mediated through several mechanisms, including modifications in DNA methylation, covalent modifications of core nucleosomal histones, rearrangement of histones and RNA interference. It is now clear that deregulation of epigenetic mechanisms cooperates with genetic alterations in the development and progression of several Mendelian disorders. Here, we summarize the recent findings that highlight how certain inherited diseases, such as Rett syndrome, Immunodeficiency-centromeric instability-facial anomalies syndrome, and facioscapulohumeral muscular dystrophy, result from altered gene silencing.

Structure and ESCRT-III protein interactions of the MIT domain of human VPS4A

The VPS4 AAA ATPases function both in endosomal vesicle formation and in the budding of many enveloped RNA viruses, including HIV-1. VPS4 proteins act by binding and catalyzing release of the membrane-associated ESCRT-III protein lattice, thereby allowing multiple rounds of protein sorting and vesicle formation. Here, we report the solution structure of the N-terminal VPS4A microtubule interacting and transport (MIT) domain and demonstrate that the VPS4A MIT domain binds the C-terminal half of the ESCRT-III protein, CHMP1B (Kd = 20 +/- 13 microM). The MIT domain forms an asymmetric three-helix bundle that resembles the first three helices in a tetratricopeptide repeat (TPR) motif. Unusual interhelical interactions are mediated by a series of conserved aromatic residues that form coiled-coil interactions between the second two helices and also pack against the conserved alanines that interdigitate between the first two helices. Mutational analyses revealed that a conserved leucine residue (Leu-64) on the third helix that would normally bind the fourth helix in an extended TPR is used to bind CHMP1B, raising the possibility that ESCRT-III proteins may bind by completing the TPR motif.

Developmental Neurobiology and Clinical Disorders: Lost in Translation?

Advances in defining mechanisms of cortical development have been paralleled in recent years by an intense interest in translating these findings into greater insight of both childhood- and adult-onset cognitive and mental health disorders of developmental etiology. Successful integration of basic and clinical findings have been applied to monogenic disorders. The greater challenge lies in studying cortical development in the context of gene x environment interactions, which underlie the pathogenesis of the most common neurodevelopmental disorders. This can occur through an improved delineation of pathophysiological characteristics unique to specific complex disorders and the application of this information to the refinement of the most relevant model systems.