Ube3a expression is not altered in Mecp2 mutant mice

MeCP2 ubiquitination and sumoylation, in search of a function†

MeCP2 (Methyl CpG binding protein 2) is an intrinsically disordered protein that binds to methylated genome regions. The protein is a critical transcriptional regulator of the brain, and its mutations account for 95% of Rett syndrome (RTT) cases. Early studies of this neurodevelopmental disorder revealed a close connection with dysregulations of the ubiquitin system (UbS), notably as related to UBE3A, a ubiquitin ligase involved in the proteasome-mediated degradation of proteins. MeCP2 undergoes numerous post-translational modifications (PTMs), including ubiquitination and sumoylation, which, in addition to the potential functional outcomes of their monomeric forms in gene regulation and synaptic plasticity, in their polymeric organization, these modifications play a critical role in proteasomal degradation. UbS-mediated proteasomal degradation is crucial in maintaining MeCP2 homeostasis for proper function and is involved in decreasing MeCP2 in some RTT-causing mutations. However, regardless of all these connections to UbS, the molecular details involved in the signaling of MeCP2 for its targeting by the ubiquitin-proteasome system (UPS) and the functional roles of monomeric MeCP2 ubiquitination and sumoylation remain largely unexplored and are the focus of this review.

Epigenetic regulation of neurotransmitter signaling in neurological disorders

Neurotransmission signaling is a highly conserved system attributed to various regulatory events. The excitatory and inhibitory neurotransmitter systems have been extensively studied, and their role in neuronal cell proliferation, synaptogenesis and dendrite formation in the adult brain is well established. Recent research has shown that epigenetic regulation plays a crucial role in mediating the expression of key genes associated with neurotransmitter pathways, including neurotransmitter receptor and transporter genes. The dysregulation of these genes has been linked to a range of neurological disorders such as attention-deficit/hyperactivity disorder, Parkinson's disease and schizophrenia. This article focuses on epigenetic regulatory mechanisms that control the expression of genes associated with four major chemical carriers in the brain: dopamine (DA), Gamma-aminobutyric acid (GABA), glutamate and serotonin. Additionally, we explore how aberrant epigenetic regulation of these genes can contribute to the pathogenesis of relevant neurological disorders. By targeting the epigenetic mechanisms that control neurotransmitter gene expression, there is a promising opportunity to advance the development of more effective treatments for neurological disorders with the potential to significantly improve the quality of life of individuals impacted by these conditions.

R306X Mutation in the MECP2 Gene Causes an Atypical Rett Syndrome in a Moroccan Patient: A Case Report

Rett syndrome (RTT) is a rare X-linked syndrome that predominantly affects girls. It is characterized by a severe and progressive neurodevelopmental disorder with neurological regression and autism spectrum features. The Rett syndrome is associated with a broad phenotypic spectrum. It ranges from a classical Rett syndrome defined by well-established criteria to atypical cases with symptoms similar to other syndromes, such as Angelman syndrome. The first case of a Moroccan female child carrying a R306X mutation in the MECP2 (Methyl-CpG-Binding Protein 2) gene, with an unusual manifestation of Rett syndrome, is presented here. She showed autistic regression, behavioral stagnation, epilepsy, unmotivated laughter, and craniofacial dysmorphia. Whole exome sequencing revealed a nonsense mutation (R306X), resulting in a truncated, nonfunctional MECP2 protein. The overlapping phenotypic spectrums between Rett and Angelman syndromes have been described, and an interaction between the MECP2 gene and the UBE3A (Ubiquitin Protein Ligase E3A) gene pathways is possible but has not yet been proven. An extensive genetic analysis is highly recommended in atypical cases to ensure an accurate diagnosis and to improve patient management and genetic counseling.

Role of mecp2 in experience-dependent epigenetic programming

Mutations in the X-linked gene MECP2, the founding member of a family of proteins recognizing and binding to methylated DNA, are the genetic cause of a devastating neurodevelopmental disorder in humans, called Rett syndrome. Available evidence suggests that MECP2 protein has a critical role in activity-dependent neuronal plasticity and transcription during brain development. Moreover, recent studies in mice show that various posttranslational modifications, notably phosphorylation, regulate Mecp2’s functions in learning and memory, drug addiction, depression-like behavior, and the response to antidepressant treatment. The hypothalamic-pituitary-adrenal (HPA) axis drives the stress response and its deregulation increases the risk for a variety of mental disorders. Early-life stress (ELS) typically results in sustained HPA-axis deregulation and is a major risk factor for stress related diseases, in particular major depression. Interestingly, Mecp2 protein has been shown to contribute to ELS-dependent epigenetic programming of Crh, Avp, and Pomc, all of these genes enhance HPA-axis activity. Hereby ELS regulates Mecp2 phosphorylation, DNA binding, and transcriptional activities in a tissue-specific and temporospatial manner. Overall, these findings suggest MECP2 proteins are so far underestimated and have a more dynamic role in the mediation of the gene-environment dialog and epigenetic programming of the neuroendocrine stress system in health and disease.

E6AP in the brain: one protein, dual function, multiple diseases

E6-Associated Protein (E6AP), the founding member of the HECT (Homologus to E6AP C terminus) family of ubiquitin ligases, has been gaining increased attention from the scientific community. In addition to its ubiquitin ligase function, our laboratory has also identified steroid hormone receptor transcriptional coactivation as yet another essential function of this protein. Furthermore, it has been established that E6AP has a role in numerous diseases including cancers and neurological syndromes. In this review, we delineate genetic and biochemical knowledge of E6AP and we focus on its role in the pathobiology of neuro-developmental and neuro-aging diseases; bringing to light important gaps of knowledge related to the involvement of its well-studied ligase function versus the much less studied nuclear receptor transcriptional coactivation function in the pathogenesis of these diseases. Tackling these gaps of knowledge could reveal novel possible neuro-pathobiological mechanisms and present crucial information for the design of effective treatment modalities for devastating CNS diseases.

The role of MeCP2 in the brain

Methyl-CpG binding protein 2 (MeCP2) was first identified in 1992 as a protein that binds specifically to methylated DNA. Mutations in the MECP2 gene were later found to be the cause of an autism spectrum disorder, Rett syndrome. Despite almost 20 years of research into the molecular mechanisms of MeCP2 function, many questions are yet to be answered conclusively. This review considers several key questions and attempts to evaluate the current state of evidence. For example, is MeCP2 just a methyl-CpG binding protein? Is it a multifunctional protein or primarily a transcriptional repressor? We also consider whether MeCP2, as a chromosome-binding protein, acts at specific sites within the genome or more globally, and in which cell types it is functionally important. Finally, we consider two alternative views of MeCP2 in the brain: as a regulator of brain development or as a factor that helps maintain neuronal/glial function.

ATRX partners with cohesin and MeCP2 and contributes to developmental silencing of imprinted genes in the brain

Human developmental disorders caused by chromatin dysfunction often display overlapping clinical manifestations, such as cognitive deficits, but the underlying molecular links are poorly defined. Here, we show that ATRX, MeCP2, and cohesin, chromatin regulators implicated in ATR-X, RTT, and CdLS syndromes, respectively, interact in the brain and colocalize at the H19 imprinting control region (ICR) with preferential binding on the maternal allele. Importantly, we show that ATRX loss of function alters enrichment of cohesin, CTCF, and histone modifications at the H19 ICR, without affecting DNA methylation on the paternal allele. ATRX also affects cohesin, CTCF, and MeCP2 occupancy within the Gtl2/Dlk1 imprinted domain. Finally, we show that loss of ATRX interferes with the postnatal silencing of the maternal H19 gene along with a larger network of imprinted genes. We propose that ATRX, cohesin, and MeCP2 cooperate to silence a subset of imprinted genes in the postnatal mouse brain.

Angelman syndrome: current understanding and research prospects

Angelman syndrome is a neurogenetic disorder characterized by developmental delay, severe intellectual disability, absent speech, exuberant behavior with happy demeanor, motor impairment, and epilepsy, due to deficient UBE3A gene expression that may be caused by various abnormalities of chromosome 15. Recent findings in animal models demonstrated altered dendritic spine formation as well as both synaptic [including gamma-aminobutyric acid (GABA)(A) and N-methyl-D-aspartate (NMDA) transmission] and nonsynaptic (including gap junction) influences in various brain regions, including hippocampus and cerebellar cortex. Reversal of selected abnormalities in rescue genetically engineered animal models is encouraging, although it should not be misinterpreted as promising "cure" for affected patients. Much research is still required to fully understand the functional links between lack of UBE3A expression and clinical manifestations of Angelman syndrome. Studies of regulation of UBE3A expression, including imprinting-related methylation, may point to possibilities of therapeutic upregulation. Understanding relevant roles of the gene product might lead to targeted intervention. Further documentation of brain network dynamics, with particular emphasis on hippocampus, thalamocortical, and cerebellar networks is needed, including in a developmental perspective. There is also a need for further clinical research for improving management of problems such as epilepsy, behavior, communication, learning, motor impairment, and sleep disturbances.

Ube3a is required for experience-dependent maturation of the neocortex

Experience-dependent maturation of neocortical circuits is required for normal sensory and cognitive abilities, which are distorted in neurodevelopmental disorders. We tested whether experience-dependent neocortical modifications require Ube3a, an E3 ubiquitin ligase whose dysregulation has been implicated in autism and Angelman syndrome. Using visual cortex as a model, we found that experience-dependent maturation of excitatory cortical circuits was severely impaired in Angelman syndrome model mice deficient in Ube3a. This developmental defect was associated with profound impairments in neocortical plasticity. Normal plasticity was preserved under conditions of sensory deprivation, but was rapidly lost by sensory experiences. The loss of neocortical plasticity is reversible, as late-onset visual deprivation restored normal synaptic plasticity. Furthermore, Ube3a-deficient mice lacked ocular dominance plasticity in vivo when challenged with monocular deprivation. We conclude that Ube3a is necessary for maintaining plasticity during experience-dependent neocortical development and suggest that the loss of neocortical plasticity contributes to deficits associated with Angelman syndrome.

Mouse models of Rett syndrome: from behavioural phenotyping to preclinical evaluation of new therapeutic approaches

Rett syndrome (RTT) is a neurodevelopmental disorder, primarily affecting girls. RTT causes severe cognitive, social, motor and physiological impairments and no cure currently exists. The discovery of a monogenic origin for RTT and the subsequent generation of RTT mouse models provided a major breakthrough for RTT research. Although the characterization of these mutant mice is far from complete, they recapitulate several RTT symptoms. This review provides an overview of the behavioural domains so far investigated in these models, including the very few mouse data concerning the developmental course of RTT. Both clinical and animal studies support the presence of early defects and highlight the importance of probing the presymptomatic phase for both the precocious identification of biomarkers and the early assessment of potential therapies. Preclinical evaluations of pharmacological and nonpharmacological interventions so far carried out are also illustrated. In addition, genetic manipulations are reported that demonstrate rescue from the damage caused by the absence of the methyl-CpG-binding protein 2 (MeCP2) gene even at a mature stage. Given the rare occurrence of RTT cases, transnational collaborative networks are expected to provide a deeper understanding of aetiopathology and the development of new therapeutic approaches.

The Drosophila homologue of the Angelman syndrome ubiquitin ligase regulates the formation of terminal dendritic branches

Angelman syndrome is a severe neurodevelopmental disorder mostly caused by loss-of-function mutations in the maternal allele of UBE3A, a gene that encodes an E3 ubiquitin ligase. Drosophila UBE3A (dUBE3A) is highly homologous to human UBE3A (hUBE3A) at the amino acid sequence level, suggesting their functional conservation. We generated dUBE3A-null mutant fly lines and found that dUBE3A is not essential for viability. However, loss of dUBE3A activity reduced dendritic branching of sensory neurons in the peripheral nervous system and slowed the growth of terminal dendritic fine processes. Several lines of evidence indicated that dUBE3A regulates dendritic morphogenesis in a cell autonomous manner. Moreover, overexpression of dUBE3A also decreased dendritic branching, suggesting that the proper level of dUBE3A is critically important for the normal dendritic patterning. These findings suggest that dendritic pathology may contribute to neurological deficits in patients with Angelman syndrome.

MECP2 genomic structure and function: insights from ENCODE

MECP2, a relatively small gene located in the human X chromosome, was initially described with three exons transcribing RNA from which the protein MeCP2 was translated. It is now known to have four exons from which two isoforms are translated; however, there is also evidence of additional functional genomic structures within MECP2, including exons potentially transcribing non-coding RNAs. Accompanying the recognition of a higher level of intricacy within MECP2 has been a recent surge of knowledge about the structure and function of human genes more generally, to the extent that the definition of a gene is being revisited. It is timely now to review the published and novel functional elements within MECP2, which is proving to have a complexity far greater than was previously thought.

Ubiquitin ligase E6-AP and its role in human disease

The ubiquitin ligase E6-AP (E6-associated protein) represents a prime example for the notion that deregulated modification of proteins with ubiquitin contributes to the development of human disease: loss of E6-AP function by mutation is responsible for the development of AS (Angelman syndrome), a neurological disorder, and unscheduled activation of E6-AP by complex formation with the E6 oncoprotein of HPVs (human papillomaviruses) contributes to cervical carcinogenesis. However, while there is a considerable amount of data concerning the oncogenic properties of the E6–E6-AP complex, only little is known about the function(s) of E6-AP in neurons. This is mainly due to the fact that although some E6-AP substrates have been identified, it is at present unclear whether deregulated modification/degradation of these proteins is involved in the pathogenesis of AS. Similarly, the cellular pathways involving E6-AP remain enigmatic. To obtain insights into the physiological functions of E6-AP, we are currently employing several strategies, including quantitative affinity proteomics and RNA interference approaches. The results obtained will eventually allow the introduction of E6-AP into functional protein networks and so reveal potential targets for molecular approaches in the treatment of E6-AP-associated diseases.

Genetic modifiers of MeCP2 function in Drosophila

The levels of methyl-CpG-binding protein 2 (MeCP2) are critical for normal post-natal development and function of the nervous system. Loss of function of MeCP2, a transcriptional regulator involved in chromatin remodeling, causes classic Rett syndrome (RTT) as well as other related conditions characterized by autism, learning disabilities, or mental retardation. Increased dosage of MeCP2 also leads to clinically similar neurological disorders and mental retardation. To identify molecular mechanisms capable of compensating for altered MeCP2 levels, we generated transgenic Drosophila overexpressing human MeCP2. We find that MeCP2 associates with chromatin and is phosphorylated at serine 423 in Drosophila, as is found in mammals. MeCP2 overexpression leads to anatomical (i.e., disorganized eyes, ectopic wing veins) and behavioral (i.e., motor dysfunction) abnormalities. We used a candidate gene approach to identify genes that are able to compensate for abnormal phenotypes caused by MeCP2 increased activity. These genetic modifiers include other chromatin remodeling genes (Additional sex combs, corto, osa, Sex combs on midleg, and trithorax), the kinase tricornered, the UBE3A target pebble, and Drosophila homologues of the MeCP2 physical interactors Sin3a, REST, and N-CoR. These findings demonstrate that anatomical and behavioral phenotypes caused by MeCP2 activity can be ameliorated by altering other factors that might be more amenable to manipulation than MeCP2 itself.

Mechanisms of imprinting of the Prader-Willi/Angelman region

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are two distinct neurodevelopmental disorders, each caused by several genetic and epigenetic mechanisms involving the proximal long arm of chromosome 15. Lack of a functional paternal copy of 15q11-q13 causes PWS; lack of a functional maternal copy of UBE3A, a gene within 15q11-q13, causes AS. This region of chromosome 15 contains a number of imprinted genes that are coordinately regulated by an imprinting center (PWS/AS-IC) that contains two functional elements, the PWS-SRO and the AS-SRO. A chromosome lacking the PWS-SRO has the maternal state of gene activity and epigenetic modification after either maternal or paternal transmission; a chromosome lacking the AS-SRO but containing the PWS-SRO has the paternal state of gene activity and epigenetic modification after either maternal or paternal transmission. The maternal state of chromosome 15q11-q13 is associated with methylation of the PWS-SRO, while the paternal state is associated with lack of methylation of the PWS-SRO. Although most models of PWS/AS region imprinting assume that the PWS-SRO is methylated during oogenesis and that this methylation of the maternal PWS-SRO is maintained after fertilization, several lines of evidence suggest that the maternal PWS-SRO is in fact not methylated until after fertilization. Imprinting defects affecting the PWS/AS region can arise from failure to demethylate the PWS-SRO in the male germ line, from failure to methylate the maternal PWS-SRO, or from failure to maintain PWS-SRO methylation after fertilization.

SnoRNA Snord116 (Pwcr1/MBII-85) deletion causes growth deficiency and hyperphagia in mice

Prader-Willi syndrome (PWS) is the leading genetic cause of obesity. After initial severe hypotonia, PWS children become hyperphagic and morbidly obese, if intake is not restricted. Short stature with abnormal growth hormone secretion, hypogonadism, cognitive impairment, anxiety and behavior problems are other features. PWS is caused by lack of expression of imprinted genes in a ∼4 mb region of chromosome band 15q11.2. Our previous translocation studies predicted a major role for the C/D box small nucleolar RNA cluster SNORD116 (PWCR1/HBII-85) in PWS. To test this hypothesis, we created a ∼150 kb deletion of the >40 copies of Snord116 (Pwcr1/MBII-85) in C57BL/6 mice. Snord116del mice with paternally derived deletion lack expression of this snoRNA. They have early-onset postnatal growth deficiency, but normal fertility and lifespan. While pituitary structure and somatotrophs are normal, liver Igf1 mRNA is decreased. In cognitive and behavior tests, Snord116del mice are deficient in motor learning and have increased anxiety. Around three months of age, they develop hyperphagia, but stay lean on regular and high-fat diet. On reduced caloric intake, Snord116del mice maintain their weight better than wild-type littermates, excluding increased energy requirement as a cause of hyperphagia. Normal compensatory feeding after fasting, and ability to maintain body temperature in the cold indicate normal energy homeostasis regulation. Metabolic chamber studies reveal that Snord116del mice maintain energy homeostasis by altered fuel usage. Prolonged mealtime and increased circulating ghrelin indicate a defect in meal termination mechanism. Snord116del mice, the first snoRNA deletion animal model, reveal a novel role for a non-coding RNA in growth and feeding regulation.

The story of Rett syndrome: from clinic to neurobiology

The postnatal neurodevelopmental disorder Rett syndrome (RTT) is caused by mutations in the gene encoding methyl-CpG binding protein 2 (MeCP2), a transcriptional repressor involved in chromatin remodeling and the modulation of RNA splicing. MECP2 aberrations result in a constellation of neuropsychiatric abnormalities, whereby both loss of function and gain in MECP2 dosage lead to similar neurological phenotypes. Recent studies demonstrate disease reversibility in RTT mouse models, suggesting that the neurological defects in MECP2 disorders are not permanent. To investigate the potential for restoring neuronal function in RTT patients, it is essential to identify MeCP2 targets or modifiers of the phenotype that can be therapeutically modulated. Moreover, deciphering the molecular underpinnings of RTT is likely to contribute to the understanding of the pathogenesis of a broader class of neuropsychiatric disorders.

Advances in behavioral genetics: mouse models of autism

Autism is a neurodevelopmental syndrome with markedly high heritability. The diagnostic indicators of autism are core behavioral symptoms, rather than definitive neuropathological markers. Etiology is thought to involve complex, multigenic interactions and possible environmental contributions. In this review, we focus on genetic pathways with multiple members represented in autism candidate gene lists. Many of these pathways can also be impinged upon by environmental risk factors associated with the disorder. The mouse model system provides a method to experimentally manipulate candidate genes for autism susceptibility, and to use environmental challenges to drive aberrant gene expression and cell pathology early in development. Mouse models for fragile X syndrome, Rett syndrome and other disorders associated with autistic-like behavior have elucidated neuropathology that might underlie the autism phenotype, including abnormalities in synaptic plasticity. Mouse models have also been used to investigate the effects of alterations in signaling pathways on neuronal migration, neurotransmission and brain anatomy, relevant to findings in autistic populations. Advances have included the evaluation of mouse models with behavioral assays designed to reflect disease symptoms, including impaired social interaction, communication deficits and repetitive behaviors, and the symptom onset during the neonatal period. Research focusing on the effect of gene-by-gene interactions or genetic susceptibility to detrimental environmental challenges may further understanding of the complex etiology for autism.

Ube3a mRNA and protein expression are not decreased in Mecp2R168X mutant mice

Mutations in the transcriptional repressor methyl CpG binding protein 2 (MeCP2) are responsible for most cases of Rett Syndrome (RS), a severe neurodevelopmental disorder characterized by developmental regression, minimal speech, seizures, postnatal microcephaly and hand stereotypies. Absence of the maternal copy of ubiquitin protein ligase 3A (UBE3A) results in Angelman syndrome, also a severe developmental disorder that shares some clinical features with RS. As MeCP2 regulates gene expression, this has led to the hypothesis that MeCP2 may regulate UBE3A expression; however, there are conflicting reports regarding the expression of Ube3a in MeCP2 null mutant mice. We have generated a novel MeCP2 mutant knock-in mouse with the mutation R168X, one of the most common mutations in patients with RS. These mice show features similar to RS, including hypoactivity, forelimb stereotypies, breathing irregularities, weight changes, hind limb atrophy, and scoliosis. The male mice experience early death. Analysis of Ube3a mRNA and protein levels in the Mecp2(R168X) male mice showed no significant difference in expression compared to their wild type littermates.

DLX5 and DLX6 expression is biallelic and not modulated by MeCP2 deficiency

Mutations in MECP2 and Mecp2 (encoding methyl-CpG binding protein 2 [MeCP2]) cause distinct neurological phenotypes in humans and mice, respectively, but the molecular pathology is unclear. Recent literature claimed that the developmental homeobox gene DLX5 is imprinted and that its imprinting status is modulated by MeCP2, leading to biallelic expression in Rett syndrome and twofold overexpression of Dlx5 and Dlx6 in Mecp2-null mice. The conclusion that DLX5 is a direct target of MeCP2 has implications for research on the molecular bases of Rett syndrome, autism, and genomic imprinting. Attempting to replicate the reported data, we evaluated allele-specific expression of DLX5 and DLX6 in mouse x human somatic cell hybrids, lymphoblastoid cell lines, and frontal cortex from controls and individuals with MECP2 mutations. We identified novel single-nucleotide polymorphisms in DLX5 and DLX6, enabling the first imprinting studies of DLX6. We found that DLX5 and DLX6 are biallelically expressed in somatic cell hybrids and in human cell lines and brain, with no differences between affected and control samples. We also determined expression levels of Dlx5 and Dlx6 in forebrain from seven male Mecp2-mutant mice and eight wild-type littermates by real-time quantitative reverse-transcriptase polymerase chain reaction assays. Expression of Dlx5 and Dlx6, as well as of the imprinted gene Peg3, in mouse forebrain was highly variable, with no consistent differences between Mecp2-null mutants and controls. We conclude that DLX5 and DLX6 are not imprinted in humans and are not likely to be direct targets of MeCP2 modulation. In contrast, the imprinting status of PEG3 and PEG10 is maintained in MeCP2-deficient tissues. Our results confirm that MeCP2 plays no role in the maintenance of genomic imprinting and add PEG3 and PEG10 to the list of studied imprinted genes.

Cerebellar gene expression profiles of mouse models for Rett syndrome reveal novel MeCP2 targets

Background

MeCP2, methyl-CpG-binding protein 2, binds to methylated cytosines at CpG dinucleotides, as well as to unmethylated DNA, and affects chromatin condensation. MECP2 mutations in females lead to Rett syndrome, a neurological disorder characterized by developmental stagnation and regression, loss of purposeful hand movements and speech, stereotypic hand movements, deceleration of brain growth, autonomic dysfunction and seizures. Most mutations occur de novo during spermatogenesis. Located at Xq28, MECP2 is subject to X inactivation, and affected females are mosaic. Rare hemizygous males suffer from a severe congenital encephalopathy.

Methods

To identify the pathways mis-regulated by MeCP2 deficiency, microarray-based global gene expression studies were carried out in cerebellum of Mecp2 mutant mice. We compared transcript levels in mutant/wildtype male sibs of two different MeCP2-deficient mouse models at 2, 4 and 8 weeks of age. Increased transcript levels were evaluated by real-time quantitative RT-PCR. Chromatin immunoprecipitation assays were used to document in vivo MeCP2 binding to promoter regions of candidate target genes.

Results

Of several hundred genes with altered expression levels in the mutants, twice as many were increased than decreased, and only 27 were differentially expressed at more than one time point. The number of misregulated genes was 30% lower in mice with the exon 3 deletion (Mecp2^tm1.1Jae) than in mice with the larger deletion (Mecp2^tm1.1Bird). Between the mutants, few genes overlapped at each time point. Real-time quantitative RT-PCR assays validated increased transcript levels for four genes: Irak1, interleukin-1 receptor-associated kinase 1; Fxyd1, phospholemman, associated with Na, K-ATPase;Reln, encoding an extracellular signaling molecule essential for neuronal lamination and synaptic plasticity; and Gtl2/Meg3, an imprinted maternally expressed non-translated RNA that serves as a host gene for C/D box snoRNAs and microRNAs. Chromatin immunoprecipitation assays documented in vivo MeCP2 binding to promoter regions of Fxyd1, Reln, and Gtl2.

Conclusion

Transcriptional profiling of cerebellum failed to detect significant global changes in Mecp2-mutant mice. Increased transcript levels of Irak1, Fxyd1, Reln, and Gtl2 may contribute to the neuronal dysfunction in MeCP2-deficient mice and individuals with Rett syndrome. Our data provide testable hypotheses for future studies of the regulatory or signaling pathways that these genes act on.

15q11-13 GABAA receptor genes are normally biallelically expressed in brain yet are subject to epigenetic dysregulation in autism-spectrum disorders

Human chromosome 15q11-13 is a complex locus containing imprinted genes as well as a cluster of three GABA(A) receptor subunit (GABR) genes-GABRB3, GABRA5 and GABRG3. Deletion or duplication of 15q11-13 GABR genes occurs in multiple human neurodevelopmental disorders including Prader-Willi syndrome (PWS), Angelman syndrome (AS) and autism. GABRB3 protein expression is also reduced in Rett syndrome (RTT), caused by mutations in MECP2 on Xq28. Although Gabrb3 is biallelically expressed in mouse brain, conflicting data exist regarding the imprinting status of the 15q11-13 GABR genes in humans. Using coding single nucleotide polymorphisms we show that all three GABR genes are biallelically expressed in 21 control brain samples, demonstrating that these genes are not imprinted in normal human cortex. Interestingly, four of eight autism and one of five RTT brain samples showed monoallelic or highly skewed allelic expression of one or more GABR gene, suggesting that epigenetic dysregulation of these genes is common to both disorders. Quantitative real-time RT-PCR analysis of PWS and AS samples with paternal and maternal 15q11-13 deletions revealed a paternal expression bias of GABRB3, while RTT brain samples showed a significant reduction in GABRB3 and UBE3A. Chromatin immunoprecipitation and bisulfite sequencing in SH-SY5Y neuroblastoma cells demonstrated that MeCP2 binds to methylated CpG sites within GABRB3. Our previous studies demonstrated that homologous 15q11-13 pairing in neurons was dependent on MeCP2 and was disrupted in RTT and autism cortex. Combined, these results suggest that MeCP2 acts as a chromatin organizer for optimal expression of both alleles of GABRB3 in neurons.

MeCP2 in Rett syndrome: transcriptional repressor or chromatin architectural protein?

Rett syndrome is a progressive neurological disorder caused by mutations in the methyl-DNA binding protein MeCP2. The longstanding model depicting MeCP2 as a transcriptional repressor predicts that the Rett syndrome phenotype probably results from misregulation of MeCP2 target genes. Somewhat unexpectedly, the identification of such targets has proven challenging. The recent identification of two MeCP2 targets, BDNF and DLX5, are suggestive of two very different roles for this protein--one as a classical repressor protein, and the other as a mediator of a complex, specialized chromatin structure.

Partial rescue of MeCP2 deficiency by postnatal activation of MeCP2

In humans, mutations in the X-linked MECP2 gene, are the cause of Rett syndrome (RTT), a neurodevelopmental disorder that affects mainly girls. MeCP2 binds to methylated CpGs and is thought to act as a transcriptional repressor. In male mice, deletion or targeted mutation of Mecp2 leads to lethality and causes a neuronal phenotype. Selective mutation of Mecp2 in postnatal neurons results in a similar, although delayed, phenotype, suggesting that the symptoms are caused by MeCP2 deficiency in postmitotic neurons. In agreement with this idea, expression of a Mecp2 transgene in postmitotic neurons of Mecp2-null mutant mice resulted in the phenotypical rescue of the symptoms. To assess whether postnatal activation of MeCP2 in mutant animals could also affect the progression of the disorder, we constructed a conditionally active Mecp2 "rescue transgene" that was activated between P0 and P30. The Mecp2 transgene was under the control of the CAGGS promoter and was activated by using brain specific Cre-mediated recombination. Our results indicate that postnatal, neuron-specific activation of MeCP2 as late as 2-4 weeks of age significantly prolonged the lifespan of mutant animals and delayed the onset of neurologic symptoms.

The Odyssey of MeCP2 and parental imprinting

DNA methylation in mammals has long been implicated in the epigenetic mechanism of parental imprinting, in which selective expression of one allele of specific genes is based on parental origin. Methyl CpG binding protein 2 (MeCP2) selectively binds to methylated DNA and mutations in the MECP2 cause the autism-spectrum neurodevelopmental disorder Rett syndrome. This review outlines the emerging story of how MeCP2 has been implicated in the regulation of specific imprinted genes and loci, including UBE3A and DLX5. The story of MeCP2 and parental imprinting has unfolded with some interesting but unexpected twists, revealing new insights on the function of MeCP2 in the process.