X-chromosome inactivation patterns are unbalanced and affect the phenotypic outcome in a mouse model of rett syndrome

The Management of Bone Defects in Rett Syndrome

Rett syndrome (RS) is a rare neurodevelopmental disorder primarily caused by mutations in the X-linked methyl-CpG binding protein 2 (MECP2) gene, responsible for encoding MECP2 which plays a pivotal role in regulating gene expression. The neurological and non-neurological manifestations of RS vary widely in severity depending on the specific mutation type. Bone complications, mostly scoliosis but also osteoporosis, hip displacement, and a high rate of fractures, are among the most prevalent non-neurological comorbidities observed in girls with RS. Low bone mineral density (BMD) is primarily due to a slow rate of bone formation due to dysfunctional osteoblast activity. The use of anticonvulsants, immobilization, low physical activity, poor nutrition, and inadequate vitamin D intake all significantly hamper skeletal maturation and the accumulation of bone mass in RS girls, making them more susceptible to fragility fractures. In RS patients, the upper and lower limbs are the most common sites for fractures which are due to both a reduced BMD and a diminished bone size. This review summarizes the knowledge on risk factors for fragility fracture in patients with RS and proposes a potential diagnostic and therapeutic pathway to enhance low BMD and mitigate the risk of fragility fractures. In particular, this review focused on the importance of clinical and instrumental evaluation of bone status as a basis for adequate planning of nutritional, pharmacological, and surgical interventions to be undertaken. Additionally, the management of bone defects in individuals with RS should be customized to meet each person's specific needs, abilities, and general health.

Cerebellar impairments in genetic models of autism spectrum disorders: A neurobiological perspective

Functional and molecular alterations in the cerebellum are among the most widely recognised associates of autism spectrum disorders (ASD). As a critical computational hub of the brain, the cerebellum controls and coordinates a range of motor, affective and cognitive processes. Despite well-described circuits and integrative mechanisms, specific changes that underlie cerebellar impairments in ASD remain elusive. Studies in experimental animals have been critical in uncovering molecular pathology and neuro-behavioural correlates, providing a model for investigating complex disease conditions. Herein, we review commonalities and differences of the most extensively characterised genetic lines of ASD with reference to the cerebellum. We revisit structural, functional, and molecular alterations which may contribute to neurobehavioral phenotypes. The cross-model analysis of this study provides an integrated outlook on the role of cerebellar alterations in pathobiology of ASD that may benefit future translational research and development of therapies.

Distributed X chromosome inactivation in brain circuitry is associated with X-linked disease penetrance of behavior

The precise anatomical degree of brain X chromosome inactivation (XCI) that is sufficient to alter X-linked disorders in females is unclear. Here, we quantify whole-brain XCI at single-cell resolution to discover a prevalent activation ratio of maternal to paternal X at 60:40 across all divisions of the adult brain. This modest, non-random XCI influences X-linked disease penetrance: maternal transmission of the fragile X mental retardation 1 (Fmr1)-knockout (KO) allele confers 55% of total brain cells with mutant X-active, which is sufficient for behavioral penetrance, while 40% produced from paternal transmission is tolerated. Local XCI mosaicism within affected maternal Fmr1-KO mice further specifies sensorimotor versus social anxiety phenotypes depending on which distinct brain circuitry is most affected, with only a 50%-55% mutant X-active threshold determining penetrance. Thus, our results define a model of X-linked disease penetrance in females whereby distributed XCI among single cells populating brain circuitries can regulate the behavioral penetrance of an X-linked mutation.

Heterozygous Nexmif female mice demonstrate mosaic NEXMIF expression, autism-like behaviors, and abnormalities in dendritic arborization and synaptogenesis

Autism spectrum disorder (ASD) is a neurodevelopmental disorder with a strong genetic basis. ASDs are commonly characterized by impairments in language, restrictive and repetitive behaviors, and deficits in social interactions. Although ASD is a highly heterogeneous disease with many different genes implicated in its etiology, many ASD-associated genes converge on common cellular defects, such as aberrant neuronal morphology and synapse dysregulation. Our previous work revealed that, in mice, complete loss of the ASD-associated X-linked gene NEXMIF results in a reduction in dendritic complexity, a decrease in spine and synapse density, altered synaptic transmission, and ASD-like behaviors. Interestingly, human females of NEXMIF haploinsufficiency have recently been reported to demonstrate autistic features; however, the cellular and molecular basis for this haploinsufficiency-caused ASD remains unclear. Here we report that in the brains of Nexmif± female mice, NEXMIF shows a mosaic pattern in its expression in neurons. Heterozygous female mice demonstrate behavioral impairments similar to those of knockout male mice. In the mosaic mixture of neurons from Nexmif± mice, cells that lack NEXMIF have impairments in dendritic arborization and spine development. Remarkably, the NEXMIF-expressing neurons from Nexmif± mice also demonstrate similar defects in dendritic growth and spine formation. These findings establish a novel mouse model of NEXMIF haploinsufficiency and provide new insights into the pathogenesis of NEXMIF-dependent ASD.

Methyl-CpG-Binding Protein 2 Emerges as a Central Player in Multiple Sclerosis and Neuromyelitis Optica Spectrum Disorders

MECP2 and its product methyl-CpG binding protein 2 (MeCP2) are associated with multiple sclerosis (MS) and neuromyelitis optica spectrum disorders (NMOSD), which are inflammatory, autoimmune, and demyelinating disorders of the central nervous system (CNS). However, the mechanisms and pathways regulated by MeCP2 in immune activation in favor of MS and NMOSD are not fully understood. We summarize findings that use the binding properties of MeCP2 to identify its targets, particularly the genes recognized by MeCP2 and associated with several neurological disorders. MeCP2 regulates gene expression in neurons, immune cells and during development by modulating various mechanisms and pathways. Dysregulation of the MeCP2 signaling pathway has been associated with several disorders, including neurological and autoimmune diseases. A thorough understanding of the molecular mechanisms underlying MeCP2 function can provide new therapeutic strategies for these conditions. The nervous system is the primary system affected in MeCP2-associated disorders, and other systems may also contribute to MeCP2 action through its target genes. MeCP2 signaling pathways provide promise as potential therapeutic targets in progressive MS and NMOSD. MeCP2 not only increases susceptibility and induces anti-inflammatory responses in immune sites but also leads to a chronic increase in pro-inflammatory cytokines gene expression (IFN-γ, TNF-α, and IL-1β) and downregulates the genes involved in immune regulation (IL-10, FoxP3, and CX3CR1). MeCP2 may modulate similar mechanisms in different pathologies and suggest that treatments for MS and NMOSD disorders may be effective in treating related disorders. MeCP2 regulates gene expression in MS and NMOSD. However, dysregulation of the MeCP2 signaling pathway is implicated in these disorders. MeCP2 plays a role as a therapeutic target for MS and NMOSD and provides pathways and mechanisms that are modulated by MeCP2 in the regulation of gene expression.

Wild-type MECP2 expression coincides with age-dependent sensory phenotypes in a female mouse model for Rett syndrome

Rett syndrome is characterized by an early period of typical development and then, regression of learned motor and speech skills in girls. Loss of MECP2 protein is thought to cause Rett syndrome phenotypes. The specific underlying mechanisms from typical developmental trajectory to regression features throughout life are unclear. Lack of established timelines to study the molecular, cellular, and behavioral features of regression in female mouse models is a major contributing factor. Due to random X-chromosome inactivation, female patients with Rett syndrome and female mouse models for Rett syndrome (Mecp2Heterozygous , Het) express a functional copy of wild-type MECP2 protein in approximately half of all cells. As MECP2 expression is regulated during early postnatal development and experience, we characterized the expression of wild-type MECP2 in the primary somatosensory cortex of female Het mice. Here, we report increased MECP2 levels in non-parvalbumin-positive neurons of 6-week-old adolescent Het relative to age-matched wild-type controls, while also displaying typical levels of perineuronal net expression in the barrel field subregion of the primary somatosensory cortex, mild tactile sensory perception deficits, and efficient pup retrieval behavior. In contrast, 12-week-old adult Het express MECP2 at levels similar to age-matched wild-type mice, show increased perineuronal net expression in the cortex, and display significant tactile sensory perception deficits. Thus, we have identified a set of behavioral metrics and the cellular substrates to study regression during a specific time in the female Het mouse model, which coincides with changes in wild-type MECP2 expression. We speculate that the precocious increase in MECP2 expression within specific cell types of adolescent Het may provide compensatory benefits at the behavioral level, while the inability to further increase MECP2 levels leads to regressive behavioral phenotypes over time.

Mechanisms of robustness in gene regulatory networks involved in neural development

The functions of living organisms are affected by different kinds of perturbation, both internal and external, which in many cases have functional effects and phenotypic impact. The effects of these perturbations become particularly relevant for multicellular organisms with complex body patterns and cell type heterogeneity, where transcriptional programs controlled by gene regulatory networks determine, for example, the cell fate during embryonic development. Therefore, an essential aspect of development in these organisms is the ability to maintain the functionality of their genetic developmental programs even in the presence of genetic variation, changing environmental conditions and biochemical noise, a property commonly termed robustness. We discuss the implication of different molecular mechanisms of robustness involved in neurodevelopment, which is characterized by the interplay of many developmental programs at a molecular, cellular and systemic level. We specifically focus on processes affecting the function of gene regulatory networks, encompassing transcriptional regulatory elements and post-transcriptional processes such as miRNA-based regulation, but also higher order regulatory organization, such as gene network topology. We also present cases where impairment of robustness mechanisms can be associated with neurodevelopmental disorders, as well as reasons why understanding these mechanisms should represent an important part of the study of gene regulatory networks driving neural development.

Rett Syndrome and MECP2 Duplication Syndrome: Disorders of MeCP2 Dosage

Rett syndrome (RTT) is a neurodevelopmental disorder caused predominantly by loss-of-function mutations in the gene Methyl-CpG-binding protein 2 (MECP2), which encodes the MeCP2 protein. RTT is a MECP2-related disorder, along with MECP2 duplication syndrome (MDS), caused by gain-of-function duplications of MECP2. Nearly two decades of research have advanced our knowledge of MeCP2 function in health and disease. The following review will discuss MeCP2 protein function and its dysregulation in the MECP2-related disorders RTT and MDS. This will include a discussion of the genetic underpinnings of these disorders, specifically how sporadic X-chromosome mutations arise and manifest in specific populations. We will then review current diagnostic guidelines and clinical manifestations of RTT and MDS. Next, we will delve into MeCP2 biology, describing the dual landscapes of methylated DNA and its reader MeCP2 across the neuronal genome as well as the function of MeCP2 as a transcriptional modulator. Following this, we will outline common MECP2 mutations and genotype-phenotype correlations in both diseases, with particular focus on mutations associated with relatively mild disease in RTT. We will also summarize decades of disease modeling and resulting molecular, synaptic, and behavioral phenotypes associated with RTT and MDS. Finally, we list several therapeutics in the development pipeline for RTT and MDS and available evidence of their safety and efficacy.

Altered Bone Status in Rett Syndrome

Rett syndrome (RTT) is a monogenic neurodevelopmental disorder primarily caused by mutations in X-linked MECP2 gene, encoding for methyl-CpG binding protein 2 (MeCP2), a multifaceted modulator of gene expression and chromatin organization. Based on the type of mutation, RTT patients exhibit a broad spectrum of clinical phenotypes with various degrees of severity. In addition, as a complex multisystem disease, RTT shows several clinical manifestations ranging from neurological to non-neurological symptoms. The most common non-neurological comorbidities include, among others, orthopedic complications, mainly scoliosis but also early osteopenia/osteoporosis and a high frequency of fractures. A characteristic low bone mineral density dependent on a slow rate of bone formation due to dysfunctional osteoblast activity rather than an increase in bone resorption is at the root of these complications. Evidence from human and animal studies supports the idea that MECP2 mutation could be associated with altered epigenetic regulation of bone-related factors and signaling pathways, including SFRP4/WNT/β-catenin axis and RANKL/RANK/OPG system. More research is needed to better understand the role of MeCP2 in bone homeostasis. Indeed, uncovering the molecular mechanisms underlying RTT bone problems could reveal new potential pharmacological targets for the treatment of these complications that adversely affect the quality of life of RTT patients for whom the only therapeutic approaches currently available include bisphosphonates, dietary supplements, and physical activity.

Profiling beneficial and potential adverse effects of MeCP2 overexpression in a hypomorphic Rett syndrome mouse model

De novo loss-of-function mutations in methyl-CpG-binding protein 2 (MeCP2) lead to the neurodevelopmental disorder Rett syndrome (RTT). Despite promising results from strategies aimed at increasing MeCP2 levels, additional studies exploring how hypomorphic MeCP2 mutations impact the therapeutic window are needed. Here, we investigated the consequences of genetically introducing a wild-type MECP2 transgene in the Mecp2 R133C mouse model of RTT. The MECP2 transgene reversed the majority of RTT-like phenotypes exhibited by male and female Mecp2 R133C mice. However, three core symptom domains were adversely affected in female Mecp2^R133C/+ animals; these phenotypes resemble those observed in disease contexts of excess MeCP2. Parallel control experiments in Mecp2^Null/+ mice linked these adverse effects to the hypomorphic R133C mutation. Collectively, these data provide evidence regarding the safety and efficacy of genetically overexpressing functional MeCP2 in Mecp2 R133C mice and suggest that personalized approaches may warrant consideration for the clinical assessment of MeCP2-targeted therapies.

MeCP2 deficiency exacerbates the neuroinflammatory setting and autoreactive response during an autoimmune challenge

Rett syndrome is a severe and progressive neurological disorder linked to mutations in the MeCP2 gene. It has been suggested that immune alterations may play an active role in the generation and/or maintenance of RTT phenotypes. However, there is no clear consensus about which pathways are regulated in vivo by MeCP2 in the context of immune activation. In the present work we set to characterize the role of MeCP2 during the progression of Experimental Autoimmune Encephalomyelitis (EAE) using the MeCP2308/y mouse model (MUT), which represents a condition of "MeCP2 function deficiency". Our results showed that MeCP2 deficiency increased the susceptibility to develop EAE, along with a defective induction of anti-inflammatory responses and an exacerbated MOG-specific IFNγ expression in immune sites. In MUT-EAE spinal cord, we found a chronic increase in pro-inflammatory cytokines gene expression (IFNγ, TNFα and IL-1β) and downregulation of genes involved in immune regulation (IL-10, FoxP3 and CX3CR1). Moreover, our results indicate that MeCP2 acts intrinsically upon immune activation, affecting neuroimmune homeostasis by regulating the pro-inflammatory/anti-inflammatory balance in vivo. These results are relevant to identify the potential consequences of MeCP2 mutations on immune homeostasis and to explore novel therapeutic strategies for MeCP2-related disorders.

Sleep problems in Rett syndrome animal models: A systematic review

Due to the discovery of Rett Syndrome (RTT) genetic mutations, animal models have been developed. Sleep research in RTT animal models may unravel novel neural mechanisms for this severe neurodevelopmental heritable rare disease. In this systematic literature review we summarize the findings on sleep research of 13 studies in animal models of RTT. We found disturbed efficacy and continuity of sleep in all genetically mutated models of mice, cynomolgus monkeys, and Drosophila. Models presented highly fragmented sleep with distinct differences in 24-hr sleep/wake cyclicity and circadian arrhythmicity. Overall, animal models mimic sleep complaints reported in individuals with RTT. However, contrary to human studies, in mutant mice, attenuated sleep delta waves, and sleep apneas in non-rapid eye movement sleep were reported. Future studies may focus on sleep structure and EEG alterations, potential central mechanisms involved in sleep fragmentation and the occurrence of sleep apnea across different sleep stages. Given that locomotor dysfunction is characteristic of individuals with RTT, studies may consider to integrate its potential impact on the behavioral analysis of sleep.

Differential brain region-specific expression of MeCP2 and BDNF in Rett Syndrome patients: a distinct grey-white matter variation

INTRODUCTION AND OBJECTIVES

Rett Syndrome (RTT) is a neurodevelopmental disorder caused by Methyl CpG Binding Protein 2 (MECP2) gene mutations. Previous studies of MeCP2 in the human brain showed variable and inconsistent mosaic-pattern immunolabelling, which has been interpreted as a reflection of activation-state variability. We aimed to study post mortem MeCP2 and BDNF (MeCP2 target) degradation and brain region-specific detection in relation to RTT pathophysiology.

METHODS

We investigated MeCP2 and BDNF stabilities in non-RTT human brains by immunohistochemical labelling and compared them in three brain regions of RTT and controls.

RESULTS

In surgically excised samples of human hippocampus and cerebellum, MeCP2 was universally detected. There was no significantly obvious difference between males and females. However, post mortem delay in autopsy samples had substantial influence on MeCP2 detection. Immunohistochemistry studies in RTT patients showed lower MeCP2 detection in glial cells of the white matter. Glial fibrillary acidic protein (GFAP) expression was also reduced in RTT brain samples without obvious change in myelin basic protein (MBP). Neurons did not show any noticeable decrease in MeCP2 detection. BDNF immunohistochemical detection showed an astroglial/endothelial pattern without noticeable difference between RTT and controls.

CONCLUSIONS

Our findings indicate that MeCP2 protein is widely expressed in mature human brain cells at all ages. However, our data points towards a possible white matter abnormality in RTT and highlights the importance of studying human RTT brain tissues in parallel with research on animal and cell models of RTT.

Losing Dnmt3a dependent methylation in inhibitory neurons impairs neural function by a mechanism impacting Rett syndrome

Methylated cytosine is an effector of epigenetic gene regulation. In the brain, Dnmt3a is the sole 'writer' of atypical non-CpG methylation (mCH), and MeCP2 is the only known 'reader' for mCH. We asked if MeCP2 is the sole reader for Dnmt3a dependent methylation by comparing mice lacking either protein in GABAergic inhibitory neurons. Loss of either protein causes overlapping and distinct features from the behavioral to molecular level. Loss of Dnmt3a causes global loss of mCH and a subset of mCG sites resulting in more widespread transcriptional alterations and severe neurological dysfunction than MeCP2 loss. These data suggest that MeCP2 is responsible for reading only part of the Dnmt3a dependent methylation in the brain. Importantly, the impact of MeCP2 on genes differentially expressed in both models shows a strong dependence on mCH, but not Dnmt3a dependent mCG, consistent with mCH playing a central role in the pathogenesis of Rett Syndrome.

MeCP2 is involved in random mono-allelic expression for a subset of human autosomal genes

Widespread random monoallelic gene expression (RMAE) effects influence about 10% of human genes. However, the mechanisms by which RME of autosomal genes is established and those by which it is maintained both remain open questions. Because the choice of allelic expression is randomly performed cell-by-cell, the RMAE mechanism is not observable in non-clonal cell populations or in whole tissues. Several target genes of MeCP2, the gene involved in Rett syndrome (RTT), have been previously described as subject to RMAE, suggesting that MeCP2 may be involved in the establishment and/or maintenance of RME of autosomal genes. To improve our knowledge on this largely unknown phenomenon, and to study the role of MeCP2 in RMAE, we compared RMA gene expression profiles in clonal cell cultures expressing wild-type MeCP2 versus mutant MeCP2 from a RTT patient carrying a pathogenic non-sense variant. Our data clearly demonstrated that MeCP2 deficiency does not affect significantly allelic gene expression of X-linked genes, imprinted genes as well as the RMAE profile in the majority of genes. However, the functional deficiency in MeCP2 appeared to disrupt the mono-allelic or the bi-allelic expression of at least 49 genes allowing us to define a specific signature of MECP2 mutated clones.

Sex differences in Mecp2-mutant Rett syndrome model mice and the impact of cellular mosaicism in phenotype development

There is currently no effective treatment for Rett syndrome (RTT), a severe X-linked progressive neurodevelopmental disorder caused by mutations in the transcriptional regulator MECP2. Because MECP2 is subjected to X-inactivation, most affected individuals are female heterozygotes who display cellular mosaicism for normal and mutant MECP2. Males who are hemizygous for mutant MECP2 are more severely affected than heterozygous females and rarely survive. Mecp2 loss-of-function is less severe in mice, however, and male hemizygous null mice not only survive until adulthood, they have been the most commonly studied model system. Although heterozygous female mice better recapitulate human RTT, they have not been as thoroughly characterized. This is likely because of the added experimental challenges that they present, including delayed and more variable phenotypic progression and cellular mosaicism due to X-inactivation. In this review, we compare phenotypes of Mecp2 heterozygous female mice and male hemizygous null mouse models. Further, we discuss the complexities that arise from the many cell-type and tissue-type specific roles of MeCP2, as well as the combination of cell-autonomous and non-cell-autonomous disruptions that result from Mecp2 loss-of-function. This is of particular importance in the context of the female heterozygous brain, composed of a mixture of MeCP2+ and MeCP2- cells, the ratio of which can alter RTT phenotypes in the case of skewed X-inactivation. The goal of this review is to provide a clearer understanding of the pathophysiological differences between the mouse models, which is an essential consideration in the design of future pre-clinical studies.

The distinct methylation landscape of maturing neurons and its role in Rett syndrome pathogenesis

Rett syndrome (RTT) is one of the most common causes of intellectual and developmental disabilities in girls, and is caused by mutations in the gene encoding methyl-CpG binding protein 2 (MECP2). Here we will review our current understanding of RTT, the landscape of pathogenic mutations and function of MeCP2, and culminate with recent advances elucidating the distinct DNA methylation landscape in the brain that may explain why disease symptoms are delayed and selective to the nervous system.

MeCP2 Dysfunction in Rett Syndrome and Neuropsychiatric Disorders

Elucidating the functions of a particular gene is paramount to the understanding of how its dysfunction contributes to disease. This is especially important when the gene is implicated in multiple different disorders. One such gene is methyl-CpG-binding protein 2 (MECP2), which has been most prominently associated with the neurodevelopmental disorder Rett syndrome, as well as major neuropsychiatric disorders such as autism and schizophrenia. Being initially identified as a transcriptional regulator that modulates gene expression and subsequently also shown to be involved in other molecular events, dysfunction of the MeCP2 protein has the potential to affect many cellular processes. In this chapter, we will briefly review the functions of the MeCP2 protein and how its mutations are implicated in Rett syndrome and other neuropsychiatric disorders. We will further discuss about the mouse models that have been generated to specifically dissect the function of MeCP2 in different cell types and brain regions. It is envisioned that such thorough and targeted examination of MeCP2 functions can aid in enlightening the role that it plays in normal and dysfunctional physiological systems.

MeCP2 isoform e1 mutant mice recapitulate motor and metabolic phenotypes of Rett syndrome

Mutations in the X-linked gene MECP2 cause the majority of Rett syndrome (RTT) cases. Two differentially spliced isoforms of exons 1 and 2 (MeCP2-e1 and MeCP2-e2) contribute to the diverse functions of MeCP2, but only mutations in exon 1, not exon 2, are observed in RTT. We previously described an isoform-specific MeCP2-e1-deficient male mouse model of a human RTT mutation that lacks MeCP2-e1 while preserving expression of MeCP2-e2. However, RTT patients are heterozygous females that exhibit delayed and progressive symptom onset beginning in late infancy, including neurologic as well as metabolic, immune, respiratory and gastrointestinal phenotypes. Consequently, we conducted a longitudinal assessment of symptom development in MeCP2-e1 mutant females and males. A delayed and progressive onset of motor impairments was observed in both female and male MeCP2-e1 mutant mice, including hind limb clasping and motor deficits in gait and balance. Because these motor impairments were significantly impacted by age-dependent increases in body weight, we also investigated metabolic phenotypes at an early stage of disease progression. Both male and female MeCP2-e1 mutants exhibited significantly increased body fat compared to sex-matched wild-type littermates prior to weight differences. Mecp2e1-/y males exhibited significant metabolic phenotypes of hypoactivity, decreased energy expenditure, increased respiratory exchange ratio, but decreased food intake compared to wild-type. Untargeted analysis of lipid metabolites demonstrated a distinguishable profile in MeCP2-e1 female mutant liver characterized by increased triglycerides. Together, these results demonstrate that MeCP2-e1 mutation in mice of both sexes recapitulates early and progressive metabolic and motor phenotypes of human RTT.

Epigenetic and Cellular Diversity in the Brain through Allele-Specific Effects

The benefits of diploidy are considered to involve masking partially recessive mutations and increasing genetic diversity. Here, we review new studies showing evidence for diverse allele-specific expression and epigenetic states in mammalian brain cells, which suggest that diploidy expands the landscape of gene regulatory and expression programs in cells. Allele-specific expression has been thought to be restricted to a few specific classes of genes. However, new studies show novel genomic imprinting effects that are brain-region-, cell-type- and age-dependent. In addition, novel forms of random monoallelic expression that impact many autosomal genes have been described in vitro and in vivo. We discuss the implications for understanding the benefits of diploidy, and the mechanisms shaping brain development, function, and disease.

Breathing abnormalities in animal models of Rett syndrome a female neurogenetic disorder

A characteristic feature of Rett syndrome (RTT) is abnormal breathing accompanied by several other neurological and cognitive disorders. Since RTT rodent models became available, studies have begun shedding insight into the breathing abnormalities at behavioral, cellular and molecular levels. Defects are found in several groups of brainstem neurons involved in respiratory control, and potential neural mechanisms have been suggested. The findings in animal models are helpful in therapeutic strategies for people with RTT with respect to lowering sudden and unexpected death, preventing secondary developmental consequences, and improving the quality of lives.

Silent angels the genetic and clinical aspects of Rett syndrome

Rett syndrome is a neurodevelopmental genetic disorder and, because of some behavioral characteristics, individuals affected by the disease are known as silent angels. Girls with Rett syndrome perform stereotyped movements, they have learning difficulties, their reaction time is prolonged, and they seem alienated in the environment. These children require constant pediatric, neurological and orthopedic care. In the treatment of Rett syndrome physical therapy, music therapy, hydrotherapy, hippotherapy, behavioral methods, speech therapy and diet, are also used. In turn, psychological therapy of the syndrome is based on the sensory integration method, using two or more senses simultaneously. In 80% of cases, the syndrome is related to mutations of the MECP2 gene, located on chromosome X. The pathogenesis of Rett syndrome is caused by the occurrence of a non-functional MeCP2 protein, which is a transcription factor of many genes, i.e. Bdnf, mef2c, Sgk1, Uqcrc1. Abnormal expression of these genes reveals a characteristic disease phenotype. Clinical symptoms relate mainly to the nervous, respiratory, skeletal and gastrointestinal systems. Currently causal treatment is not possible. However, researchers are developing methods by which, perhaps in the near future, it will be possible to eliminate the mutations in the MECP2 gene, and this will give a chance to the patient for normal functioning.

The paper presents the etiology and pathogenesis of the disease, genetic, clinical, pharmacological aspects and other forms of Rett syndrome treatment.

Effects of early-life exposure to THIP on phenotype development in a mouse model of Rett syndrome

Background

Rett syndrome (RTT) is a neurodevelopmental disorder caused mostly by disruptions in the MECP2 gene. MECP2-null mice show imbalances in neuronal excitability and synaptic communications. Several previous studies indicate that augmenting synaptic GABA receptors (GABA_ARs) can alleviate RTT-like symptoms in mice. In addition to the synaptic GABA_ARs, there is a group of GABA_ARs found outside the synaptic cleft with the capability to produce sustained inhibition, which may be potential therapeutic targets for the control of neuronal excitability in RTT.

Methods

Wild-type and MECP2-null mice were randomly divided into four groups, receiving the extrasynaptic GABA_AR agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP) and vehicle control, respectively. Low-dose THIP was administered to neonatal mice through lactation. RTT-like symptoms including lifespan, breathing, motor function, and social behaviors were studied when mice became mature. Changes in neuronal excitability and norepinephrine biosynthesis enzyme expression were studied in electrophysiology and molecular biology.

Results

With no evident sedation and other adverse side effects, early-life exposure to THIP extended the lifespan, alleviated breathing abnormalities, enhanced motor function, and improved social behaviors of MECP2-null mice. Such beneficial effects were associated with stabilization of locus coeruleus neuronal excitability and improvement of norepinephrine biosynthesis enzyme expression.

Conclusions

THIP treatment in early lives might be a therapeutic approach to RTT-like symptoms in MECP2-null mice and perhaps in people with RTT as well.

A review of Rett syndrome (RTT) with induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSCs) are pluripotent stem cells generated from somatic cells by the introduction of a combination of pluripotency-associated genes such as OCT4, SOX2, along with either KLF4 and c-MYC or NANOG and LIN28 via retroviral or lentiviral vectors. Most importantly, hiPSCs are similar to human embryonic stem cells (hESCs) functionally as they are pluripotent and can potentially differentiate into any desired cell type when provided with the appropriate cues, but do not have the ethical issues surrounding hESCs. For these reasons, hiPSCs have huge potential in translational medicine such as disease modeling, drug screening, and cellular therapy. Indeed, patient-specific hiPSCs have been generated for a multitude of diseases, including many with a neurological basis, in which disease phenotypes have been recapitulated in vitro and proof-of-principle drug screening has been performed. As the techniques for generating hiPSCs are refined and these cells become a more widely used tool for understanding brain development, the insights they produce must be understood in the context of the greater complexity of the human genome and the human brain. Disease models using iPS from Rett syndrome (RTT) patient's fibroblasts have opened up a new avenue of drug discovery for therapeutic treatment of RTT. The analysis of X chromosome inactivation (XCI) upon differentiation of RTT-hiPSCs into neurons will be critical to conclusively demonstrate the isolation of pre-XCI RTT-hiPSCs in comparison to post-XCI RTT-hiPSCs. The current review projects on iPSC studies in RTT as well as XCI in hiPSC were it suggests for screening new potential therapeutic targets for RTT in future for the benefit of RTT patients. In conclusion, patient-specific drug screening might be feasible and would be particularly helpful in disorders where patients frequently have to try multiple drugs before finding a regimen that works.

Defects in brainstem neurons associated with breathing and motor function in the Mecp2R168X/Y mouse model of Rett syndrome

Rett Syndrome (RTT) is an X-linked neurodevelopmental disorder caused mostly by disruption of the MECP2 gene. Among several RTT-like mouse models, one of them is a strain of mice that carries an R168X point mutation in Mecp2 and resembles one of the most common RTT-causing mutations in humans. Although several behavioral defects have previously been found in the Mecp2^R168X/Y mice, alterations in nerve cells remain unknown. Here we compare several behavioral and cellular outcomes between this Mecp2^R168X/Y model and a widely used Mecp2^Bird/Y mouse model. With lower body weight and shorter lifespan than their wild-type littermates, the Mecp2^R168X/Y mice showed impairments of breathing and motor function. Thus we studied brainstem CO₂-chemosensitive neurons and propriosensory cells that are associated with these two functions, respectively. Neurons in the locus coeruleus (LC) of both mutant strains showed defects in their intrinsic membrane properties, including changes in action potential morphology and excessive firing activity. Neurons in the mesencephalic trigeminal nucleus (Me5) of both strains displayed a higher firing response to depolarization than their wild-type littermates, likely attributable to a lower firing threshold. Because the increased excitability in LC and Me5 neurons tends to impact the excitation-inhibition balances in brainstem neuronal networks as well as their associated functions, it is likely that the defects in the intrinsic membrane properties of these brainstem neurons contribute to the breathing abnormalities and motor dysfunction. Furthermore, our results showing comparable phenotypical outcomes of Mecp2^R168X/Y mice with Mecp2^Bird/Y mice suggest that both strains are valid animal models for RTT research.

Apoptotic Activity of MeCP2 Is Enhanced by C-Terminal Truncating Mutations

Methyl-CpG binding protein 2 (MeCP2) is a widely abundant, multifunctional protein most highly expressed in post-mitotic neurons. Mutations causing Rett syndrome and related neurodevelopmental disorders have been identified along the entire MECP2 locus, but symptoms vary depending on mutation type and location. C-terminal mutations are prevalent, but little is known about the function of the MeCP2 C-terminus. We employ the genetic efficiency of Drosophila to provide evidence that expression of p.Arg294* (more commonly identified as R294X), a human MECP2 E2 mutant allele causing truncation of the C-terminal domains, promotes apoptosis of identified neurons in vivo. We confirm this novel finding in HEK293T cells and then use Drosophila to map the region critical for neuronal apoptosis to a small sequence at the end of the C-terminal domain. In vitro studies in mammalian systems previously indicated a role of the MeCP2 E2 isoform in apoptosis, which is facilitated by phosphorylation at serine 80 (S80) and decreased by interactions with the forkhead protein FoxG1. We confirm the roles of S80 phosphorylation and forkhead domain transcription factors in affecting MeCP2-induced apoptosis in Drosophila in vivo, thus indicating mechanistic conservation between flies and mammalian cells. Our findings are consistent with a model in which C- and N-terminal interactions are required for healthy function of MeCP2.

Manipulations of MeCP2 in glutamatergic neurons highlight their contributions to Rett and other neurological disorders

Many postnatal onset neurological disorders such as autism spectrum disorders (ASDs) and intellectual disability are thought to arise largely from disruption of excitatory/inhibitory homeostasis. Although mouse models of Rett syndrome (RTT), a postnatal neurological disorder caused by loss-of-function mutations in MECP2, display impaired excitatory neurotransmission, the RTT phenotype can be largely reproduced in mice simply by removing MeCP2 from inhibitory GABAergic neurons. To determine what role excitatory signaling impairment might play in RTT pathogenesis, we generated conditional mouse models with Mecp2 either removed from or expressed solely in glutamatergic neurons. MeCP2 deficiency in glutamatergic neurons leads to early lethality, obesity, tremor, altered anxiety-like behaviors, and impaired acoustic startle response, which is distinct from the phenotype of mice lacking MeCP2 only in inhibitory neurons. These findings reveal a role for excitatory signaling impairment in specific neurobehavioral abnormalities shared by RTT and other postnatal neurological disorders.

Restoration of Mecp2 expression in GABAergic neurons is sufficient to rescue multiple disease features in a mouse model of Rett syndrome

The postnatal neurodevelopmental disorder Rett syndrome, caused by mutations in MECP2, produces a diverse array of symptoms, including loss of language, motor, and social skills and the development of hand stereotypies, anxiety, tremor, ataxia, respiratory dysrhythmias, and seizures. Surprisingly, despite the diversity of these features, we have found that deleting Mecp2 only from GABAergic inhibitory neurons in mice replicates most of this phenotype. Here we show that genetically restoring Mecp2 expression only in GABAergic neurons of male Mecp2 null mice enhanced inhibitory signaling, extended lifespan, and rescued ataxia, apraxia, and social abnormalities but did not rescue tremor or anxiety. Female Mecp2^+/- mice showed a less dramatic but still substantial rescue. These findings highlight the critical regulatory role of GABAergic neurons in certain behaviors and suggest that modulating the excitatory/inhibitory balance through GABAergic neurons could prove a viable therapeutic option in Rett syndrome.

DOI:http://dx.doi.org/10.7554/eLife.14198.001

Rett syndrome is a childhood brain disorder that mainly affects girls and causes symptoms including anxiety, tremors, uncoordinated movements and breathing difficulties. Rett syndrome is caused by mutations in a gene called MECP2, which is found on the X chromosome. Males with MECP2 mutations are rare but have more severe symptoms and die young. Many researchers who study Rett syndrome use mice as a model of the disorder. In particular, male mice with the mouse equivalent of the human MECP2 gene switched off in every cell in the body (also known as Mecp2-null mice) show many of the features of Rett syndrome and die at a young age.

The MECP2 gene is important for healthy brain activity. The brain contains two major types of neurons: excitatory neurons, which encourage other neurons to be active; and inhibitory neurons, which stop or dampen the activity of other neurons. In 2010, researchers reported that mice lacking Mecp2 in only their inhibitory neurons develop most of the same problems as those mice with no Mecp2 at all. This discovery led Ure et al. – including a researcher involved in the 2010 study – to ask if activating Mecp2 in the same neurons in otherwise Mecp2-null mice was enough to prevent some of their Rett syndrome-like symptoms.

The experiments showed that male mice that only have Mecp2 activated in their inhibitory neurons lived several months longer than male Mecp2-null mice. These male “rescue mice” also moved normally and had a normal body weight, though they still experienced anxiety, tremors and breathing difficulties. Female mice represent a better model of human Rett syndrome patients, and Ure et al. found that female rescue mice showed smaller improvements than the males.

These data suggest that when a brain is missing Mecp2 everywhere, as in male Mecp2-null mice, turning on Mecp2 in inhibitory neurons can make the brain network nearly normal and prevent most Rett-syndrome-like symptoms. However, the brains of female rescue mice contain both normal cells and cells with mutated Mecp2. This mixture of normal and abnormal cells appears to cause abnormalities that cannot be overcome by rescuing just the activity of the inhibitory neurons. These findings also highlight the importance of doing future studies in female mice to better understand the development of Rett syndrome.

The next challenge is to test different ways of activating the inhibitory neurons in the female mouse brain, for example by using drugs that target these neurons. It is hoped these methods will help researchers to refine a path toward potential new treatments for Rett syndrome patients. Finally, in a related study, Meng et al. asked how deleting or activating Mecp2 only in the excitatory neurons of mice affected Rett-syndrome-like symptoms.

DOI:http://dx.doi.org/10.7554/eLife.14198.002

Characterization of Rett Syndrome-like phenotypes in Mecp2-knockout rats

Background

Rett Syndrome (RTT) is a neurodevelopmental disease caused by the disruption of the MECP2 gene. Several mouse models of RTT have been developed with Mecp2 disruptions. Although the mouse models are widely used in RTT research, results obtained need to be validated in other species. Therefore, we performed these studies to characterize phenotypes of a novel Mecp2^−/Y rat model and compared them with the Mecp2^tm1.1Bird mouse model of RTT.

Methods

RTT-like phenotypes were systematically studied and compared between Mecp2^−/Y rats and Mecp2^−/Y mice. In-cage conditions of the rats were monitored. Grip strength and spontaneous locomotion were used to evaluate the motor function. Three-chamber test was performed to show autism-type behaviors. Breathing activity was recorded with the plethysmograph. Individual neurons in the locus coeruleus (LC) were studied in the whole-cell current clamp. The lifespan of the rats was determined with their survival time.

Results

Mecp2^−/Y rats displayed growth retardation, malocclusion, and lack of movements, while hindlimb clasping was not seen. They had weaker forelimb grip strength and a lower rate of locomotion than the WT littermates. Defects in social interaction with other rats were obvious. Breathing frequency variation and apnea in the null rats were significantly higher than in the WT. LC neurons in the null rats showed excessive firing activity. A half of the null rats died in 2 months. Most of the RTT-like symptoms were comparable to those seen in Mecp2^−/Y mice, while some appeared more or less severe. The findings that most RTT-like symptoms exist in the rat model with moderate variations and differences from the mouse models support the usefulness of both Mecp2^−/Y rodent models.

Conclusions

The novel Mecp2^−/Y rat model recapitulated numerous RTT-like symptoms as Mecp2^−/Y mouse models did, which makes it a valuable alternative model in the RTT studies when the body size matters.

MeCP2 co-ordinates liver lipid metabolism with the NCoR1/HDAC3 corepressor complex

Rett syndrome (RTT; OMIM 312750), a progressive neurological disorder, is caused by mutations in methyl-CpG-binding protein 2 (MECP2; OMIM 300005), a ubiquitously expressed factor. A genetic suppressor screen designed to identify therapeutic targets surprisingly revealed that downregulation of the cholesterol biosynthesis pathway improves neurological phenotypes in Mecp2 mutant mice. Here, we show that MeCP2 plays a direct role in regulating lipid metabolism. Mecp2 deletion in mice results in a host of severe metabolic defects caused by lipid accumulation, including insulin resistance, fatty liver, perturbed energy utilization, and adipose inflammation by macrophage infiltration. We show that MeCP2 regulates lipid homeostasis by anchoring the repressor complex containing NCoR1 and HDAC3 to its lipogenesis targets in hepatocytes. Consistently, we find that liver targeted deletion of Mecp2 causes fatty liver disease and dyslipidemia similar to HDAC3 liver-specific deletion. These findings position MeCP2 as a novel component in metabolic homeostasis. Rett syndrome patients also show signs of peripheral dyslipidemia; thus, together these data suggest that RTT should be classified as a neurological disorder with systemic metabolic components. We previously showed that treatment of Mecp2 mice with statin drugs alleviated motor symptoms and improved health and longevity. Lipid metabolism is a highly treatable target; therefore, our results shed light on new metabolic pathways for treatment of Rett syndrome.

X inactivation and reactivation in X-linked diseases

X chromosome inactivation (XCI) is the phenomenon by which mammals compensate for dosage of X-linked genes in females (XX) versus males (XY). XCI patterns can be random or show extreme skewing, and can modify the mode of inheritance of X-driven phenotypes, which contributes to the variability of human pathologies. Recent findings have shown reversibility of the XCI process, which has opened new avenues in the approaches used for the treatment of X-linked diseases.

Differentiation of multipotent neural stem cells derived from Rett syndrome patients is biased toward the astrocytic lineage

Background

Rett syndrome (RTT) is one of the most prevalent neurodevelopmental disorders in females, caused by de novo mutations in the X-linked methyl CpG-binding protein 2 gene, MECP2. Although abnormal regulation of neuronal genes due to mutant MeCP2 is thought to induce autistic behavior and impaired development in RTT patients, precise cellular mechanisms underlying the aberrant neural progression remain unclear.

Results

Two sets of isogenic pairs of either wild-type or mutant MECP2-expressing human induced pluripotent stem cell (hiPSC) lines were generated from a single pair of 10-year-old RTT-monozygotic (MZ) female twins. Mutant MeCP2-expressing hiPSC lines did not express detectable MeCP2 protein during any stage of differentiation. The lack of MeCP2 reflected altered gene expression patterns in differentiated neural cells rather than in undifferentiated hiPSCs, as assessed by microarray analysis. Furthermore, MeCP2 deficiency in the neural cell lineage increased astrocyte-specific differentiation from multipotent neural stem cells. Additionally, chromatin immunoprecipitation (ChIP) and bisulfite sequencing assays indicated that anomalous glial fibrillary acidic protein gene (GFAP) expression in the MeCP2-negative, differentiated neural cells resulted from the absence of MeCP2 binding to the GFAP gene.

Conclusions

An isogenic RTT-hiPSC model demonstrated that MeCP2 participates in the differentiation of neural cells. Moreover, MeCP2 deficiency triggers perturbation of astrocytic gene expression, yielding accelerated astrocyte formation from RTT-hiPSC-derived neural stem cells. These findings are likely to shed new light on astrocytic abnormalities in RTT, and suggest that astrocytes, which are required for neuronal homeostasis and function, might be a new target of RTT therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0121-2) contains supplementary material, which is available to authorized users.

Genotype-specific effects of Mecp2 loss-of-function on morphology of Layer V pyramidal neurons in heterozygous female Rett syndrome model mice

Rett syndrome (RTT) is a progressive neurological disorder primarily caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2). The heterozygous female brain consists of mosaic of neurons containing both wild-type MeCP2 (MeCP2+) and mutant MeCP2 (MeCP2-). Three-dimensional morphological analysis was performed on individually genotyped layer V pyramidal neurons in the primary motor cortex of heterozygous (Mecp2(+/-) ) and wild-type (Mecp2(+/+) ) female mice ( > 6 mo.) from the Mecp2(tm1.1Jae) line. Comparing basal dendrite morphology, soma and nuclear size of MeCP2+ to MeCP2- neurons reveals a significant cell autonomous, genotype specific effect of Mecp2. MeCP2- neurons have 15% less total basal dendritic length, predominantly in the region 70-130 μm from the cell body and on average three fewer branch points, specifically loss in the second and third branch orders. Soma and nuclear areas of neurons of mice were analyzed across a range of ages (5-21 mo.) and X-chromosome inactivation (XCI) ratios (12-56%). On average, MeCP2- somata and nuclei were 15 and 13% smaller than MeCP2+ neurons respectively. In most respects branching morphology of neurons in wild-type brains (MeCP2 WT) was not distinguishable from MeCP2+ but somata and nuclei of MeCP2 WT neurons were larger than those of MeCP2+ neurons. These data reveal cell autonomous effects of Mecp2 mutation on dendritic morphology, but also suggest non-cell autonomous effects with respect to cell size. MeCP2+ and MeCP2- neuron sizes were not correlated with age, but were correlated with XCI ratio. Unexpectedly the MeCP2- neurons were smallest in brains where the XCI ratio was highly skewed toward MeCP2+, i.e., wild-type. This raises the possibility of cell non-autonomous effects that act through mechanisms other than globally secreted factors; perhaps competition for synaptic connections influences cell size and morphology in the genotypically mosaic brain of RTT model mice.

Characterization of X chromosome inactivation using integrated analysis of whole-exome and mRNA sequencing

In females, X chromosome inactivation (XCI) is an epigenetic, gene dosage compensatory mechanism by inactivation of one copy of X in cells. Random XCI of one of the parental chromosomes results in an approximately equal proportion of cells expressing alleles from either the maternally or paternally inherited active X, and is defined by the XCI ratio. Skewed XCI ratio is suggestive of non-random inactivation, which can play an important role in X-linked genetic conditions. Current methods rely on indirect, semi-quantitative DNA methylation-based assay to estimate XCI ratio. Here we report a direct approach to estimate XCI ratio by integrated, family-trio based whole-exome and mRNA sequencing using phase-by-transmission of alleles coupled with allele-specific expression analysis. We applied this method to in silico data and to a clinical patient with mild cognitive impairment but no clear diagnosis or understanding molecular mechanism underlying the phenotype. Simulation showed that phased and unphased heterozygous allele expression can be used to estimate XCI ratio. Segregation analysis of the patient's exome uncovered a de novo, interstitial, 1.7 Mb deletion on Xp22.31 that originated on the paternally inherited X and previously been associated with heterogeneous, neurological phenotype. Phased, allelic expression data suggested an 83∶20 moderately skewed XCI that favored the expression of the maternally inherited, cytogenetically normal X and suggested that the deleterious affect of the de novo event on the paternal copy may be offset by skewed XCI that favors expression of the wild-type X. This study shows the utility of integrated sequencing approach in XCI ratio estimation.

Aberrant redox homoeostasis and mitochondrial dysfunction in Rett syndrome

RTT (Rett syndrome) is a severe progressive neurodevelopmental disorder with a monogenetic cause, but complex and multifaceted clinical appearance. Compelling evidence suggests that mitochondrial alterations and aberrant redox homoeostasis result in oxidative challenge. Yet, compared with other severe neuropathologies, RTT is not associated with marked neurodegeneration, but rather a chemical imbalance and miscommunication of neuronal elements. Different pharmacotherapies mediate partial improvement of conditions in RTT, and also antioxidants or compounds improving mitochondrial function may be of potential merit. In the present paper, we summarize findings from patients and transgenic mice that point towards the nature of RTT as a mitochondrial disease. Also, open questions are addressed that require clarification to fully understand and successfully target the associated cellular redox imbalance.

Investigation of Rett syndrome using pluripotent stem cells

Rett syndrome (RTT) is one of most prevalent female neurodevelopmental disorders. De novo mutations in X-linked MECP2 are mostly responsible for RTT. Since the identification of MeCP2 as the underlying cause of RTT, murine models have contributed to understanding the pathophysiology of RTT and function of MeCP2. Reprogramming is a procedure to produce induced pluripotent stem cells (iPSCs) by overexpression of four transcription factors. iPSCs obtain similar features as embryonic stem cells and are capable of self-renewing and differentiating into cells of all three layers. iPSCs have been utilized in modeling human diseases in vitro. Neurons differentiated from RTT-iPSCs showed the recapitulation of RTT phenotypes. Despite the early success, genetic and epigenetic instability upon reprogramming and ensuing maintenance of iPSCs raise concerns in using RTT-iPSCs as an accurate in vitro model. Here, we update the current iPSC-based RTT modeling, and its concerns and challenges.

MeCP2-Related Diseases and Animal Models

The role of epigenetics in human disease has become an area of increased research interest. Collaborative efforts from scientists and clinicians have led to a better understanding of the molecular mechanisms by which epigenetic regulation is involved in the pathogenesis of many human diseases. Several neurological and non-neurological disorders are associated with mutations in genes that encode for epigenetic factors. One of the most studied proteins that impacts human disease and is associated with deregulation of epigenetic processes is Methyl CpG binding protein 2 (MeCP2). MeCP2 is an epigenetic regulator that modulates gene expression by translating epigenetic DNA methylation marks into appropriate cellular responses. In order to highlight the importance of epigenetics to development and disease, we will discuss how MeCP2 emerges as a key epigenetic player in human neurodevelopmental, neurological, and non-neurological disorders. We will review our current knowledge on MeCP2-related diseases, including Rett Syndrome, Angelman Syndrome, Fetal Alcohol Spectrum Disorder, Hirschsprung disease, and Cancer. Additionally, we will briefly discuss about the existing MeCP2 animal models that have been generated for a better understanding of how MeCP2 impacts certain human diseases.

Autosomal monoallelic expression: genetics of epigenetic diversity?

In mammals, relative expression of the two parental alleles of many genes is controlled by one of three major epigenetic phenomena: X chromosome inactivation, imprinting, and mitotically stable autosomal monoallelic expression (MAE). MAE affects a large fraction of human autosomal genes and introduces enormous epigenetic heterogeneity in otherwise similar cell populations. Despite its prevalence, many functional and mechanistic aspects of MAE biology remain unknown. Several lines of evidence imply that MAE establishment and maintenance are controlled by a variety of genetic elements. Based on known genomic features regulating X-inactivation and imprinting, we outline likely features of MAE-controlling elements. We also assess implications of MAE for genotype-phenotype relationship, with a focus on haploinsufficiency.

Long-term home cage activity scans reveal lowered exploratory behaviour in symptomatic female Rett mice

Numerous experimental models have been developed to reiterate endophenotypes of Rett syndrome, a neurodevelopmental disorder with a multitude of motor, cognitive and vegetative symptoms. Here, female Mecp2(Stop) mice [1] were characterised at mild symptomatic conditions in tests for anxiety (open field, elevated plus maze) and home cage observation systems for food intake, locomotor activity and circadian rhythms. Aged 8-9 months, Mecp2(Stop) mice presented with heightened body weight, lower overall activity in the open field, but no anxiety phenotype. Although home cage activity scans conducted in two different observation systems, PhenoMaster and PhenoTyper, confirmed normal circadian activity, they revealed severely compromised habituation to a novel environment in all parameters registered including those derived from a non-linear decay model such as initial exploration maximum, decay half-life of activity and span, as well as plateau. Furthermore, overall activity was significantly reduced in nocturnal periods due to reductions in both fast ambulatory movements, but also a slow lingering. In contrast, light-period activity profiles during which the amount of sleep was highest remained normal in Mecp2(Stop) mice. These data confirm the slow and progressive development of Rett-like symptoms in female Mecp2(Stop) mice resulting in a prominent reduction of overall locomotor activity, while circadian rhythms are maintained. Alterations in the time-course of habituation may indicate deficiencies in cognitive processing.

X Chromosome-Inactivation Patterns in 31 Individuals with PHACE Syndrome

Segmental hemangiomas of the head and neck can be associated with multiple congenital anomalies in the disorder known as PHACE syndrome (OMIM 606519) (posterior fossa malformations, hemangioma, arterial anomalies, cardiac defects, and eye anomalies). All reported cases of PHACE syndrome to date have been sporadic, and the genetic basis of this disorder has not yet been established. PHACE syndrome has a striking female predominance which has raised the question of X-linked inheritance. In this study, the X chromosome-inactivation (XCI) patterns of 31 females with PHACE syndrome and their mothers were analyzed using blood-derived DNA and X-chromosome locus methylation assay. This study was performed to test the hypothesis that some cases of PHACE syndrome are due to X-linked inheritance and favorable skewing in the mothers may protect against a severe phenotype, but the clinical phenotype may be unmasked in daughters with a random pattern of X-inactivation. XCI analysis was informative in 27/31 mothers. Our results identified skewed XCI in 5 of 27 (19%) informative mothers, which is not statistically significant with a p value of 0.41. None of the mothers reported significant medical problems, although a full PHACE work-up has not been performed in these individuals. Skewed XCI in the mothers of children with PHACE was identified in only a minority of cases. Based on these results, genetic heterogeneity is likely in PHACE syndrome, although it is possible a subset of cases are caused by a mutation in an X-linked gene.

Female Mecp2(+/-) mice display robust behavioral deficits on two different genetic backgrounds providing a framework for pre-clinical studies

Rett syndrome (RTT) is an X-linked neurological disorder caused by mutations in the gene encoding the transcriptional modulator methyl-CpG-binding protein 2 (MeCP2). Typical RTT primarily affects girls and is characterized by a brief period of apparently normal development followed by the loss of purposeful hand skills and language, the onset of anxiety, hand stereotypies, autistic features, seizures and autonomic dysfunction. Mecp2 mouse models have extensively been studied to demonstrate the functional link between MeCP2 dysfunction and RTT pathogenesis. However, the majority of studies have focused primarily on the molecular and behavioral consequences of the complete absence of MeCP2 in male mice. Studies of female Mecp2(+/-) mice have been limited because of potential phenotypic variability due to X chromosome inactivation effects. To determine whether reproducible and reliable phenotypes can be detected Mecp2(+/-) mice, we analyzed Mecp2(+/-) mice of two different F1 hybrid isogenic backgrounds and at young and old ages using several neurobehavioral and physiological assays. Here, we report a multitude of phenotypes in female Mecp2(+/-) mice, some presenting as early as 5 weeks of life. We demonstrate that Mecp2(+/-) mice recapitulate several aspects of typical RTT and show that mosaic expression of MeCP2 does not preclude the use of female mice in behavioral and molecular studies. Importantly, we uncover several behavioral abnormalities that are present in two genetic backgrounds and report on phenotypes that are unique to one background. These findings provide a framework for pre-clinical studies aimed at improving the constellation of phenotypes in a mouse model of RTT.

Early intervention with psychostimulants or antidepressants to increase methyl-CpG-binding protein 2 (MeCP2) expressions: a potential therapy for Rett syndrome

Rett syndrome (RTT) is a severe X-linked postnatal neurodevelopmental disorder. The syndrome is caused primarily by mutations in the methyl CpG binding protein 2 (MeCP2) gene on Xq28. Most individuals with RTT are female, and female RTT is normally heterozygous for mutations in MeCP2. Patients with RTT display a normal period of development prior to the onset of symptoms, at which point they undergo a period of regression. Currently, no effective medication is available for this disorder, although animal studies have suggested that RTT symptoms are potentially reversible. For females with RTT, the severity of symptoms and progression of the disease varies a great deal, despite its homogenous genetic origin. These differences could be attributed to differences in the mutation points of MeCP2 and the skew caused by X-chromosome inactivation. Thus, the increased expression in the normal MeCP2 gene could decrease the severity of the disease. Based on findings from studies on animals indicating that fluoxetine (an antidepressant) and cocaine (a psychostimulant) can increase MeCP2 expression in the brain, it is suggested that early intervention with antidepressants or psychostimulants could increase the normal MeCP2 expression in females with RTT, who are normally heterozygous. This therapeutic hypothesis could be tested in an RTT animal model. Following the identification of the antidepressants or psychostimulants with the greatest influence on MeCP2 expression, a combination of early detection of the disorder with early intervention may result in improved therapeutic outcomes. Furthermore, a trial investigating the effects of antidepressants or psychostimulants on MeCP2 expression in lymphocyte culture from patients with RTT is suggested for clinical therapeutic prediction.

Methyl CpG-binding protein isoform MeCP2_e2 is dispensable for Rett syndrome phenotypes but essential for embryo viability and placenta development

Methyl CpG-binding protein 2 gene (MeCP2) mutations are implicated in Rett syndrome (RTT), one of the common causes of female mental retardation. Two MeCP2 isoforms have been reported: MeCP2_e2 (splicing of all four exons) and MeCP2_e1 (alternative splicing of exons 1, 3, and 4). Their relative expression levels vary among tissues, with MeCP2_e1 being more dominant in adult brain, whereas MeCP2_e2 is expressed more abundantly in placenta, liver, and skeletal muscle. In this study, we performed specific disruption of the MeCP2_e2-defining exon 2 using the Cre-loxP system and examined the consequences of selective loss of MeCP2_e2 function in vivo. We performed behavior evaluation, gene expression analysis, using RT-PCR and real-time quantitative PCR, and histological analysis. We demonstrate that selective deletion of MeCP2_e2 does not result in RTT-associated neurological phenotypes but confers a survival disadvantage to embryos carrying a MeCP2_e2 null allele of maternal origin. In addition, we reveal a specific requirement for MeCP2_e2 function in extraembryonic tissue, where selective loss of MeCP2_e2 results in placenta defects and up-regulation of peg-1, as determined by the parental origin of the mutant allele. Taken together, our findings suggest a novel role for MeCP2 in normal placenta development and illustrate how paternal X chromosome inactivation in extraembryonic tissues confers a survival disadvantage for carriers of a mutant maternal MeCP2_e2 allele. Moreover, our findings provide an explanation for the absence of reports on MeCP2_e2-specific exon 2 mutations in RTT. MeCP2_e2 mutations in humans may result in a phenotype that evades a diagnosis of RTT.

Generation and characterization of rat and mouse monoclonal antibodies specific for MeCP2 and their use in X-inactivation studies

Methyl CpG binding protein 2 (MeCP2) binds DNA, and has a preference for methylated CpGs and, hence, in cells, it accumulates in heterochromatin. Even though it is expressed ubiquitously MeCP2 is particularly important during neuronal maturation. This is underscored by the fact that in Rett syndrome, a neurological disease, 80% of patients carry a mutation in the MECP2 gene. Since the MECP2 gene lies on the X chromosome and is subjected to X chromosome inactivation, affected patients are usually chimeric for wild type and mutant MeCP2. Here, we present the generation and characterization of the first rat monoclonal MeCP2 specific antibodies as well as mouse monoclonal antibodies and a rabbit polyclonal antibody. We demonstrate that our antibodies are suitable for immunoblotting, (chromatin) immunoprecipitation and immunofluorescence of endogenous and ectopically expressed MeCP2. Epitope mapping revealed that most of the MeCP2 monoclonal antibodies recognize the C-terminal domain and one the N-terminal domain of MeCP2. Using slot blot analysis, we determined a high sensitivity of all antibodies, detecting amounts as low as 1 ng of MeCP2 protein. Moreover, the antibodies recognize MeCP2 from different species, including human, mouse, rat and pig. Lastly, we have validated their use by analyzing and quantifying X chromosome inactivation skewing using brain tissue of MeCP2 heterozygous null female mice. The new MeCP2 specific monoclonal antibodies described here perform well in a large variety of immunological applications making them a very valuable set of tools for studies of MeCP2 pathophysiology in situ and in vitro.

MeCP2 and Rett syndrome: reversibility and potential avenues for therapy

Mutations in the X-linked gene MECP2 (methyl CpG-binding protein 2) are the primary cause of the neurodevelopmental disorder RTT (Rett syndrome), and are also implicated in other neurological conditions. The expression product of this gene, MeCP2, is a widely expressed nuclear protein, especially abundant in mature neurons of the CNS (central nervous system). The major recognized consequences of MECP2 mutation occur in the CNS, but there is growing awareness of peripheral effects contributing to the full RTT phenotype. MeCP2 is classically considered to act as a DNA methylation-dependent transcriptional repressor, but may have additional roles in regulating gene expression and chromatin structure. Knocking out Mecp2 function in mice recapitulates many of the overt neurological features seen in RTT patients, and the characteristic postnatally delayed onset of symptoms is accompanied by aberrant neuronal morphology and deficits in synaptic physiology. Evidence that reactivation of endogenous Mecp2 in mutant mice, even at adult stages, can reverse aspects of RTT-like pathology and result in apparently functionally mature neurons has provided renewed hope for patients, but has also provoked discussion about traditional boundaries between neurodevelopmental disorders and those involving dysfunction at later stages. In the present paper we review the neurobiology of MeCP2 and consider the various genetic (including gene therapy), pharmacological and environmental interventions that have been, and could be, developed to attempt phenotypic rescue in RTT. Such approaches are already providing valuable insights into the potential tractability of RTT and related conditions, and are useful pointers for the development of future therapeutic strategies.

Temporal and regional alterations in NMDA receptor expression in Mecp2-null mice

Our previous postmortem study of girls with Rett Syndrome (RTT), a development disorder caused by MECP2 mutations, found increases in the density of N-Methyl-D-aspartate (NMDA) receptors in the prefrontal cortex of 2-8-year-old girls, whereas girls older than 10 years had reductions in NMDA receptors compared with age-matched controls (Blue et al., Ann Neurol 1999b;45:541-545). Using [(3)H]-CGP to label NMDA-type glutamate receptors in 2- and 7-week old wild-type (WT), Mecp2-null, and Mecp2-heterozygous (HET) mice (Bird model), we found that frontal areas of the brain also exhibited a bimodal pattern in NMDA expression, with increased densities of NMDA receptors in Mecp2-null mice at 2 weeks of age but decreased densities at 7 weeks of age. Visual cortex showed a similar pattern, while other cortical regions only exhibited changes in NMDA receptor densities at 2 weeks (retrosplenial granular) or 7 weeks (somatosensory). In thalamus of null mice, NMDA receptors were increased at 2 and 7 weeks. No significant differences in density were found between HET and WT mice at both ages. Western blots for NMDAR1 expression in frontal brain showed higher levels of expression in Mecp2-null mice at 2 weeks of age but not at 1 or 7 weeks of age. Our mouse data support the notion that deficient MeCP2 function is the primary cause of the NMDA receptor changes we observed in RTT. Furthermore, the findings of regional and temporal differences in NMDA expression illustrate the importance of age and brain region in evaluating different genotypes of mice.

Mutations in ANKRD11 cause KBG syndrome, characterized by intellectual disability, skeletal malformations, and macrodontia

KBG syndrome is characterized by intellectual disability associated with macrodontia of the upper central incisors as well as distinct craniofacial findings, short stature, and skeletal anomalies. Although believed to be genetic in origin, the specific underlying defect is unknown. Through whole-exome sequencing, we identified deleterious heterozygous mutations in ANKRD11 encoding ankyrin repeat domain 11, also known as ankyrin repeat-containing cofactor 1. A splice-site mutation, c.7570-1G>C (p.Glu2524_Lys2525del), cosegregated with the disease in a family with three affected members, whereas in a simplex case a de novo truncating mutation, c.2305delT (p.Ser769GlnfsX8), was detected. Sanger sequencing revealed additional de novo truncating ANKRD11 mutations in three other simplex cases. ANKRD11 is known to interact with nuclear receptor complexes to modify transcriptional activation. We demonstrated that ANKRD11 localizes mainly to the nuclei of neurons and accumulates in discrete inclusions when neurons are depolarized, suggesting that it plays a role in neural plasticity. Our results demonstrate that mutations in ANKRD11 cause KBG syndrome and outline a fundamental role of ANKRD11 in craniofacial, dental, skeletal, and central nervous system development and function.

Neuronal maturation defect in induced pluripotent stem cells from patients with Rett syndrome

Rett syndrome (RTT) is one of the most prevalent female neurodevelopmental disorders that cause severe mental retardation. Mutations in methyl CpG binding protein 2 (MeCP2) are mainly responsible for RTT. Patients with classical RTT exhibit normal development until age 6-18 mo, at which point they become symptomatic and display loss of language and motor skills, purposeful hand movements, and normal head growth. Murine genetic models and postmortem human brains have been used to study the disease and enable the molecular dissection of RTT. In this work, we applied a recently developed reprogramming approach to generate a novel in vitro human RTT model. Induced pluripotent stem cells (iPSCs) were derived from RTT fibroblasts by overexpressing the reprogramming factors OCT4, SOX2, KLF4, and MYC. Intriguingly, whereas some iPSCs maintained X chromosome inactivation, in others the X chromosome was reactivated. Thus, iPSCs were isolated that retained a single active X chromosome expressing either mutant or WT MeCP2, as well as iPSCs with reactivated X chromosomes expressing both mutant and WT MeCP2. When these cells underwent neuronal differentiation, the mutant monoallelic or biallelelic RTT-iPSCs displayed a defect in neuronal maturation consistent with RTT phenotypes. Our in vitro model of RTT is an important tool allowing the further investigation of the pathophysiology of RTT and the development of the curative therapeutics.

MeCP2 is critical within HoxB1-derived tissues of mice for normal lifespan

Rett syndrome is a neurodevelopmental disorder caused by mutations in methyl-CpG-binding protein 2 (MECP2), a transcriptional regulator. In addition to cognitive, communication, and motor problems, affected individuals have abnormalities in autonomic function and respiratory control that may contribute to premature lethality. Mice lacking Mecp2 die early and recapitulate the autonomic and respiratory phenotypes seen in humans. The association of autonomic and respiratory deficits with premature death suggests that Mecp2 is critical within autonomic and respiratory control centers for survival. To test this, we compared the autonomic and respiratory phenotypes of mice with a null allele of Mecp2 to mice with Mecp2 removed from their brainstem and spinal cord. We found that MeCP2 is necessary within the brainstem and spinal cord for normal lifespan, normal control of heart rate, and respiratory response to hypoxia. Restoration of MeCP2 in a subset of the cells in this same region is sufficient to rescue abnormal heart rate and abnormal respiratory response to hypoxia. Furthermore, restoring MeCP2 function in neural centers critical for autonomic and respiratory function alleviates the lethality associated with loss of MeCP2 function, supporting the notion of targeted therapy toward treating Rett syndrome.