MECP2 mutation in male patients with non-specific X-linked mental retardation

State-of-the-art therapies for Rett syndrome

Rett syndrome (RTT) is an X-linked neurogenetic disorder caused by mutations of the MECP2 (methyl-CpG-binding protein 2) gene. Over two decades of work established MeCP2 as a protein with pivotal roles in the regulation of the epigenome, neuronal physiology, synaptic maintenance, and behaviour. Given the genetic aetiology of RTT and the proof of concept of its reversal in a mouse model, considerable efforts have been made to design therapeutic approaches to re-express MeCP2. By being at the forefront of the development of innovative gene therapies, research on RTT is of paramount importance for the treatment of monogenic neurological diseases. Here we discuss the recent advances and challenges of promising genetic strategies for the treatment of RTT including gene replacement therapies, gene/RNA editing strategies, and reactivation of the silenced X chromosome. WHAT THIS PAPER ADDS: Recent advances shed light on the promises of gene replacement therapy with new vectors designed to control the levels of MeCP2 expression. New developments in DNA/RNA editing approaches or reactivation of the silenced X chromosome open the possibility to re-express the native MeCP2 locus at endogenous levels. Current strategies still face limitations in transduction efficiency and future work is needed to improve brain delivery.

Early life stress exacerbates behavioural and neuronal alterations in adolescent male mice lacking methyl-CpG binding protein 2 (Mecp2)

The methyl-CpG binding protein 2 gene (MECP2) encodes an epigenetic transcriptional regulator implicated in neuronal plasticity. Loss-of-function mutations in this gene are the primary cause of Rett syndrome and, to a lesser degree, of other neurodevelopmental disorders. Recently, we demonstrated that both Mecp2 haploinsuficiency and mild early life stress decrease anxiety-like behaviours and neuronal activation in brain areas controlling these responses in adolescent female mice. Here, we extend this work to males by using Mecp2-null and wild type adolescent mice subjected to maternal separation and their non-stressed controls. We assessed their behavioural responses in a battery of anxiety-provoking tests. Upon exposure to an elevated plus maze in aversive conditions, we evaluated changes in c-FOS expression in stress- and anxiety-related brain regions. In addition, we assessed the impact of maternal separation in neuronal maturation using doublecortin and reelin as surrogate markers. Mutant males showed reduced motor abilities, increased activation of the olfactory bulbs, probably due to breathing abnormalities, and decreased activation of the paraventricular thalamic nucleus, when compared to wild type mice. In addition, maternal separation increased the number of immature doublecortin-like neurons found in Mecp2-null animals. Moreover, this work shows for the first time that reelin is decreased in the mutant animals at the olfactory tubercle, piriform cortex and hippocampal dentate gyrus, an effect also associated to maternal separation. Taken together, our results suggest that maternal separation exacerbates some phenotypical alterations associated with lack of MeCP2 in adolescent males.

Ending diagnostic odyssey using clinical whole-exome sequencing (CWES)

Objectives

Most rare diseases are genetic diseases. Due to the diversity of rare diseases and the high likelihood of patients with rare diseases to be undiagnosed or misdiagnosed, it is not unusual that these patients undergo a long diagnostic odyssey before they receive a definitive diagnosis. This situation presents a clear need to set up a dedicated clinical service to end the diagnostic odyssey of patients with rare diseases.

Methods

Therefore, in 2014, we started an Undiagnosed Diseases Program in Hong Kong with the aim of ending the diagnostic odyssey of patients and families with rare diseases by clinical whole-exome sequencing (CWES), who have not received a definitive diagnosis after extensive investigation.

Results

In this program, we have shown that genetic diseases diagnosed by CWES were different from that using traditional approaches indicating that CWES is an essential tool to diagnose rare diseases and ending diagnostic odysseys. In addition, we identified several novel genes responsible for monogenic diseases. These include the TOP2B gene for autism spectrum disorder, the DTYMK gene for severe cerebral atrophy, the KIF13A gene for a new mosaic ectodermal syndrome associated with hypomelanosis of Ito, and the CDC25B gene for a new syndrome of cardiomyopathy and endocrinopathy.

Conclusions

With the incorporation of CWES in an Undiagnosed Diseases Program, we have ended diagnostic odysseys of patients with rare diseases in Hong Kong in the past 7 years. In this program, we have shown that CWES is an essential tool to end diagnostic odysseys. With the declining cost of next-generation sequencers and reagents, CWES set-ups are now affordable for clinical laboratories. Indeed, owing to the increasing availability of CWES and treatment modalities for rare diseases, precedence can be given to both common and rare medical conditions.

MECP2-related conditions in males: A systematic literature review and 8 additional cases

OBJECTIVE

To present a cohort of 8 males and perform a systematic review of all published cases with a single copy of MECP2 carrying a pathogenic variant.

METHODS

We reviewed medical records of males with a single copy of MECP2 carrying a pathogenic variant. We searched in Medline (Pubmed) and Embase to collect all articles which included well-characterized males with a single copy of MECP2 carrying a pathogenic or likely pathogenic variant in MECP2 (1999-2020).

RESULTS

The literature search yielded a total of 3,185 publications, of which 58 were included in our systematic review. We were able to collect information on 27 published patients with severe neonatal encephalopathy, 47 individuals with isolated or familial mental retardation X-linked 13 (XLMR13), as well as 24 individuals with isolated or familial Pyramidal signs, parkinsonism, and macroorchidism (PPM-X). In our cohort, we met eight individuals aged 4 to 19-year-old at the last evaluation. Three MECP2-associated phenotypes were seen in male carriers of a single copy of the gene: severe neonatal encephalopathy (n = 5); X-linked intellectual deficiency 13 (n = 2); and pyramidal signs, parkinsonism, and macroorchidism (PPM-X) (n = 1). Two novel de novo variants [p.(Gly252Argfs∗7) and p.(Tyr132Cys)] were detected.

CONCLUSION

In males, the MECP2 pathogenic variants can be associated with different phenotypes, including neonatal severe encephalopathy, intellectual deficiency, or late-onset parkinsonism and spasticity. The typical RS phenotype is not expected in males, except in those with Klinefelter syndrome or somatic mosaicism for MECP2.

Report of a novel missense mutation in the MECP2 gene in a middle-aged man with intellectual disability syndrome

Exome sequencing revealed the cause of our 35-year-old male patient's progressive and severe intellectual and motor disability, namely a previously undescribed missense mutation of MECP2.

The Impact of X-Chromosome Inactivation on Phenotypic Expression of X-Linked Neurodevelopmental Disorders

Nearly 20% of genes located on the X chromosome are associated with neurodevelopmental disorders (NDD) due to their expression and role in brain functioning. Given their location, several of these genes are either subject to or can escape X-chromosome inactivation (XCI). The degree to which genes are subject to XCI can influence the NDD phenotype between males and females. We provide a general review of X-linked NDD genes in the context of XCI and detailed discussion of the sex-based differences related to MECP2 and FMR1, two common X-linked causes of NDD that are subject to XCI. Understanding the effects of XCI on phenotypic expression of NDD genes may guide the development of stratification biomarkers in X-linked disorders.

X-Linked Parkinsonism: Phenotypic and Genetic Heterogeneity

X-linked parkinsonism encompasses rare heterogeneous disorders mainly inherited as a recessive trait, therefore being more prevalent in males. Recent developments have revealed a complex underlying panorama, including a spectrum of disorders in which parkinsonism is variably associated with additional neurological and non-neurological signs. In particular, a childhood-onset encephalopathy with epilepsy and/or cognitive disability is the most common feature. Their genetic basis is also heterogeneous, with many causative genes and different mutation types ranging from "classical" coding variants to intronic repeat expansions. In this review, we provide an updated overview of the phenotypic and genetic spectrum of the most relevant X-linked parkinsonian syndromes, namely X-linked dystonia-parkinsonism (XDP, Lubag disease), fragile X-associated tremor/ataxia syndrome (FXTAS), beta-propeller protein-associated neurodegeneration (BPAN, NBIA/PARK-WDR45), Fabry disease, Waisman syndrome, methyl CpG-binding protein 2 (MeCP2) spectrum disorder, phosphoglycerate kinase-1 deficiency syndrome (PGK1) and X-linked parkinsonism and spasticity (XPDS). All clinical and radiological features reported in the literature have been reviewed. Epilepsy occasionally represents the symptom of onset, predating parkinsonism even by a few years; action tremor is another common feature along with akinetic-rigid parkinsonism. A focus on the genetic background and its pathophysiological implications is provided. The pathogenesis of these disorders ranges from well-defined metabolic alterations (PGK1) to non-specific lysosomal dysfunctions (XPDS) and vesicular trafficking alterations (Waisman syndrome). However, in other cases it still remains poorly defined. Recognition of the phenotypic and genetic heterogeneity of X-linked parkinsonism has important implications for diagnosis, management, and genetic counseling. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

First description of an unusual novel double mutation in MECP2 co-occurring with the m.827A>G mutation in the MT-RNR1 gene associated with angelman-like syndrome

Mutations in Methyl-CpG-Binding protein 2 (MECP2), located on Xq28 and encoding a methyl CpG binding protein, are commonly related to Rett syndrome. However, MECP2 mutations have already been reported in patients with neurodevelopmental abnormalities such as X-linked mental retardation, severe neonatal encephalopathy and Angelman-like syndrome (AS-like). Accordingly, we report the clinical, molecular and bioinformatic analyses in a Tunisian patient with AS-like phenotype. In fact, the direct sequencing of MECP2 and cloning essay reveals the emergence of an unusual novel double mutation, including a de novo mutation c.397C > T (p.R133C) and an inherited one c.608C > T (p.T203 M) co-occurring in Trans. We also provide the molecular evidence of the c.608C > T transmission to the patient which was present in her father at somatic mosaicism state. To gain insight into the molecular basis of this disorder, we undertook, for the first time, a whole mitochondrial genome mutational analysis. Thus, the results showed the presence of several variations and a homoplasmic mutation m.827A > G in the MT-RNR1 gene, leading to the disruption of the 12S rRNA secondary structure. Our report is considered as the first to describe an unusual novel double mutation (c.397C > T in trans with c.608C > T) in MECP2 co-occurring with the mitochondrial m.827A > G mutation in the MT-RNR1 gene in a Tunisian patient with AS-like. Besides, our results highlight the importance of studying MECP2 and the significance of mDNA screening in AS-like disorder for a better understanding of its etiopathogenesis.

Early alterations in a mouse model of Rett syndrome: the GABA developmental shift is abolished at birth

Genetic mutations of the Methyl-CpG-binding protein-2 (MECP2) gene underlie Rett syndrome (RTT). Developmental processes are often considered to be irrelevant in RTT pathogenesis but neuronal activity at birth has not been recorded. We report that the GABA developmental shift at birth is abolished in CA3 pyramidal neurons of Mecp2^-/y mice and the glutamatergic/GABAergic postsynaptic currents (PSCs) ratio is increased. Two weeks later, GABA exerts strong excitatory actions, the glutamatergic/GABAergic PSCs ratio is enhanced, hyper-synchronized activity is present and metabotropic long-term depression (LTD) is impacted. One day before delivery, maternal administration of the NKCC1 chloride importer antagonist bumetanide restored these parameters but not respiratory or weight deficits, nor the onset of mortality. Results suggest that birth is a critical period in RTT with important alterations that can be attenuated by bumetanide raising the possibility of early treatment of the disorder.

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.

The array of clinical phenotypes of males with mutations in Methyl-CpG binding protein 2

Mutations in the X-linked gene MECP2 are associated with a severe neurodevelopmental disorder, Rett syndrome (RTT), primarily in girls. It had been suspected that mutations in Methyl-CpG-binding protein 2 (MECP2) led to embryonic lethality in males, however such males have been reported. To enhance understanding of the phenotypic spectrum present in these individuals, we identified 30 males with MECP2 mutations in the RTT Natural History Study databases. A wide phenotypic spectrum was observed, ranging from severe neonatal encephalopathy to cognitive impairment. Two males with a somatic mutation in MECP2 had classic RTT. Of the remaining 28 subjects, 16 had RTT-causing MECP2 mutations, 9 with mutations that are not seen in females with RTT but are likely pathogenic, and 3 with uncertain variants. Two subjects with RTT-causing mutations were previously diagnosed as having atypical RTT; however, careful review of the clinical history determined that an additional 12/28 subjects met criteria for atypical RTT, but with more severe clinical presentation and course, and less distinctive RTT features, than females with RTT, leading to the designation of a new diagnostic entity, male RTT encephalopathy. Increased awareness of the clinical spectrum and widespread comprehensive genomic testing in boys with neurodevelopmental problems will lead to improved identification.

Behavioral Characterization of MeCP2 Dysfunction-Associated Rett Syndrome and Neuropsychiatric Disorders

The methyl-CpG-binding protein 2 (MECP2) gene has been implicated in multiple neuropsychiatric disorders such as autism and schizophrenia and, most notably, Rett syndrome (RTT). Mouse models of MeCP2 dysfunction that have been developed are thus important not only for examining the protein's contribution to RTT, but also for elucidating the etiologies of other MECP2-associated neuropsychiatric disorders. In this chapter, we present protocols for three behavioral assays for characterizing major functional domains of MeCP2 dysfunction-the open field test for measuring general locomotor activity and anxiety-like behavior, the three-chambered Crawley box test for assessing social preference and social novelty, and the rotarod assay for testing locomotor coordination. It is hoped that these information facilitate systematic characterization of mouse models that may aid in elucidating the role of MeCP2 in neurological disorders, as well as assessing the effects of putative mechanistic and therapeutic interventions.

Asperger syndrome and early-onset schizophrenia associated with a novel MECP2 deleterious missense variant

Methyl-CpG-binding protein 2 (MECP2) deleterious variants, which are responsible for Rett syndrome in girls, are involved in a wide spectrum of developmental disabilities in males. A neuropsychiatric phenotype without intellectual disability is uncommon in patients with MECP2 deleterious variants. We report on two dizygotic twins with an MECP2-related psychiatric disorder without intellectual disability. Neuropsychological and psychiatric phenotype assessments were performed, and a genetic analysis was carried out. Both patients fulfilled the Pervasive Developmental Disorder criteria on Autism Diagnostic Observation Schedule and Asperger syndrome criteria on Diagnostic and Statistical Manual of Mental Disorders, 4th ed. (DSM-IV). One patient developed early-onset schizophrenia (DSM-IV criteria) with two acute psychotic episodes, the latest one following corticosteroids and sodium valproate intake, with major hyperammonemia. A novel MECP2 gene transversion c.491 G>T [p.(Ser164Ile)] was found in both twins. Pathogenicity of this variant was considered on the basis of strong clinical and molecular data. The underlying molecular basis of neuropsychiatric disorders may have important consequences on genetic counseling and therapeutic strategies.

Familial cases and male cases with MECP2 mutations

This is the first report of Chinese familial cases with Rett syndrome (RTT) or X-linked mental retardation (XLMR). RTT is a neurodevelopmental disorder that almost exclusively affects females. Most RTT cases are sporadic. We have studied eight cases with MECP2 mutations in six Chinese families, including three females and five males with RTT or XLMR. All shared identical MECP2 mutations with their mothers. The three females fulfilled the diagnostic criteria for RTT, while the five males were XLMR. A random X-chromosome inactive (XCI) pattern was seen in all the three female patients and two mothers while a skewed XCI in the rest four mothers. The clinical manifestations and pathogenic gene spectrum between male and female patients were different. The different MECP2 mutations and different XCI pattern may be the determinants of the phenotypic heterogeneity between the family members.

From Function to Phenotype: Impaired DNA Binding and Clustering Correlates with Clinical Severity in Males with Missense Mutations in MECP2

Mutations in the MECP2 gene cause Rett syndrome (RTT). MeCP2 binds to chromocentric DNA through its methyl CpG-binding domain (MBD) to regulate gene expression. In heterozygous females the variable phenotypic severity is modulated by non-random X-inactivation, thus making genotype-phenotype comparisons unreliable. However, genotype-phenotype correlations in males with hemizygousMECP2 mutations can provide more accurate insights in to the true biological effect of specific mutations. Here, we compared chromatin organization and binding dynamics for twelve MeCP2 missense mutations (including two novel and the five most common MBD missense RTT mutations) and identifiedacorrelation with phenotype in hemizygous males. We observed impaired interaction of MeCP2-DNA for mutations around the MBD-DNA binding interface, and defective chromatin clustering for distal MBD mutations. Furthermore, binding and mobility dynamics show a gradient of impairment depending on the amino acid properties and tertiary structure within the MBD. Interestingly, a wide range of phenotypic/clinical severity, ranging from neonatal encephalopathy to mild psychiatric abnormalities were observed and all are consistent with our functional/molecular results. Overall, clinical severity showed a direct correlation with the functional impairment of MeCP2. These mechanistic and phenotypic correlations of MeCP2 mutations will enable improved and individualized diagnostics, and may lead to personalized therapeutic interventions.

Reduced neuronal size and mTOR pathway activity in the Mecp2 A140V Rett syndrome mouse model

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutation in the X-linked MECP2 gene, encoding methyl-CpG-binding protein 2. We have created a mouse model ( Mecp2 A140V “knock-in” mutant) expressing the recurrent human MECP2 A140V mutation linked to an X-linked mental retardation/Rett syndrome phenotype. Morphological analyses focused on quantifying soma and nucleus size were performed on primary hippocampus and cerebellum granule neuron (CGN) cultures from mutant ( Mecp2^A140V/y) and wild type ( Mecp2^+/y) male mice. Cultured hippocampus and cerebellar granule neurons from mutant animals were significantly smaller than neurons from wild type animals. We also examined soma size in hippocampus neurons from individual female transgenic mice that express both a mutant  (maternal allele) and a wild type Mecp2 gene linked to an eGFP transgene (paternal allele). In cultures from such doubly heterozygous female mice, the size of neurons expressing the mutant (A140V) allele also showed a significant reduction compared to neurons expressing wild type MeCP2, supporting a cell-autonomous role for MeCP2 in neuronal development. IGF-1 (insulin growth factor-1) treatment of neuronal cells from Mecp2 mutant mice rescued the soma size phenotype. We also found that Mecp2  mutation leads to down-regulation of the mTOR signaling pathway, known to be involved in neuronal size regulation. Our results suggest that i) reduced neuronal size is an important in vitro cellular phenotype of Mecp2 mutation in mice, and ii) MeCP2 might play a critical role in the maintenance of neuronal structure by modulation of the mTOR pathway. The definition of a quantifiable cellular phenotype supports using neuronal size as a biomarker in the development of a high-throughput, in vitro assay to screen for compounds that rescue small neuronal phenotype (“phenotypic assay”).

Expanding phenotype of p.Ala140Val mutation in MECP2 in a 4 generation family with X-linked intellectual disability and spasticity

Mutations in MECP2 (MIM #312750), located on Xq28 and encoding a methyl CpG binding protein, are classically associated with Rett syndrome in female patients, with a lethal effect in hemizygous males. However, MECP2 mutations have already been reported in surviving males with severe neonatal-onset encephalopathy, or with X-linked intellectual disability associated with psychosis, pyramidal signs, parkinsonian features and macro-orchidism (PPM-X syndrome; MIM3 #300055). Here we report on the identification of the p.Ala140Val mutation in the MECP2 gene in 4 males and 3 females of a large Caucasian family affected with X-linked intellectual disability. Females present with mild cognitive impairment and speech difficulties. Males have moderate intellectual disability, impaired language development, friendly behavior, slowly progressive spastic paraparesis and dystonic movements of the hands. Two of them show microcephaly. The p.Ala140Val mutation is recurrent, as it was already described in 4 families with X-linked mental retardation and in three sporadic male patients with intellectual disability. We further delineate the phenotype associated with the p.Ala140Val mutation, illustrating a variable expressivity even within a given family, and we compare our patients with previous reported cases in the literature.

Regulatory functions and pathological relevance of the MECP2 3'UTR in the central nervous system

Methyl-CpG-binding protein 2 (MeCP2), encoded by the gene MECP2, is a transcriptional regulator and chromatin-remodeling protein, which is ubiquitously expressed and plays an essential role in the development and maintenance of the central nervous system (CNS). Highly enriched in post-migratory neurons, MeCP2 is needed for neuronal maturation, including dendritic arborization and the development of synapses. Loss-of-function mutations in MECP2 cause Rett syndrome (RTT), a debilitating neurodevelopmental disorder characterized by a phase of normal development, followed by the progressive loss of milestones and cognitive disability. While a great deal has been discovered about the structure, function, and regulation of MeCP2 in the time since its discovery as the genetic cause of RTT, including its involvement in a number of RTT-related syndromes that have come to be known as MeCP2-spectrum disorders, much about this multifunctional protein remains enigmatic. One unequivocal fact that has become apparent is the importance of maintaining MeCP2 protein levels within a narrow range, the limits of which may depend upon the cell type and developmental time point. As such, MeCP2 is amenable to complex, multifactorial regulation. Here, we summarize the role of the MECP2 3' untranslated region (UTR) in the regulation of MeCP2 protein levels and how mutations in this region contribute to autism and other non-RTT neuropsychiatric disorders.

miR-199a Links MeCP2 with mTOR Signaling and Its Dysregulation Leads to Rett Syndrome Phenotypes

Rett syndrome (RTT) is a neurodevelopmental disorder caused by MECP2 mutations. Although emerging evidence suggests that MeCP2 deficiency is associated with dysregulation of mechanistic target of rapamycin (mTOR), which functions as a hub for various signaling pathways, the mechanism underlying this association and the molecular pathophysiology of RTT remain elusive. We show here that MeCP2 promotes the posttranscriptional processing of particular microRNAs (miRNAs) as a component of the microprocessor Drosha complex. Among the MeCP2-regulated miRNAs, we found that miR-199a positively controls mTOR signaling by targeting inhibitors for mTOR signaling. miR-199a and its targets have opposite effects on mTOR activity, ameliorating and inducing RTT neuronal phenotypes, respectively. Furthermore, genetic deletion of miR-199a-2 led to a reduction of mTOR activity in the brain and recapitulated numerous RTT phenotypes in mice. Together, these findings establish miR-199a as a critical downstream target of MeCP2 in RTT pathogenesis by linking MeCP2 with mTOR signaling.

NUDT21-spanning CNVs lead to neuropsychiatric disease and altered MeCP2 abundance via alternative polyadenylation

The brain is sensitive to the dose of MeCP2 such that small fluctuations in protein quantity lead to neuropsychiatric disease. Despite the importance of MeCP2 levels to brain function, little is known about its regulation. In this study, we report eleven individuals with neuropsychiatric disease and copy-number variations spanning NUDT21, which encodes a subunit of pre-mRNA cleavage factor Im. Investigations of MECP2 mRNA and protein abundance in patient-derived lymphoblastoid cells from one NUDT21 deletion and three duplication cases show that NUDT21 regulates MeCP2 protein quantity. Elevated NUDT21 increases usage of the distal polyadenylation site in the MECP2 3′ UTR, resulting in an enrichment of inefficiently translated long mRNA isoforms. Furthermore, normalization of NUDT21 via siRNA-mediated knockdown in duplication patient lymphoblasts restores MeCP2 to normal levels. Ultimately, we identify NUDT21 as a novel candidate for intellectual disability and neuropsychiatric disease, and elucidate a mechanism of pathogenesis by MeCP2 dysregulation via altered alternative polyadenylation.

DOI:http://dx.doi.org/10.7554/eLife.10782.001

The X-chromosome carries a number of genes that are involved in a child's intellectual development. One of these genes encodes a protein called MeCP2, which is important for brain function after birth. Mutations in the MECP2 gene cause a disorder known as Rett syndrome. At around 18 months of age, affected children begin to lose the cognitive and motor skills that they had previously acquired. Individuals with extra copies of this gene also show cognitive impairments. For both diseases, individuals with levels of the MeCP2 protein that are the most different from those found in healthy individuals also show the most severe symptoms.

To produce the protein that is encoded by a particular gene, enzymes inside the cell must first make a copy of that gene using a molecule called messenger ribonucleic acid (or mRNA). This mRNA is then used as a template to assemble the protein itself. In the case of MECP2, two different mRNA templates are produced: a long version and a short version. A gene called NUDT21 makes a protein that regulates whether the long or short version of MECP2 mRNA is made.

Gennarino, Alcott et al. have now discovered that people with too many, or too few, copies of the NUDT21 gene have intellectual disabilities and altered levels of MeCP2 protein. Specifically, individuals with extra copies of NUDT21—and thus higher levels of the corresponding protein—produce more of the long MECP2 mRNA. The production of proteins from this long mRNA is less efficient than from the short mRNA; therefore, these individuals have lower levels of MeCP2 protein. The opposite is true for individuals who lack a copy of the NUDT21 gene.

To confirm these data, Gennarino, Alcott et al. grew cells in the laboratory from patients with extra copies of the NUDT21 gene and found that reducing the production of its protein returned the levels of the MeCP2 protein back to normal. These findings show that alterations in the NUDT21 gene cause changes in the level of MeCP2 protein in cells and leads to neuropsychiatric diseases.

DOI:http://dx.doi.org/10.7554/eLife.10782.002

Rett syndrome: disruption of epigenetic control of postnatal neurological functions

Loss-of-function mutations in the X-linked gene Methyl-CpG-binding protein 2 (MECP2) cause a devastating pediatric neurological disorder called Rett syndrome. In males, these mutations typically result in severe neonatal encephalopathy and early lethality. On the other hand, owing to expression of the normal allele in ∼50% of cells, females do not suffer encephalopathy but instead develop Rett syndrome. Typically females with Rett syndrome exhibit a delayed onset of neurologic dysfunction that manifests around the child's first birthday and progresses over the next few years. Features of this disorder include loss of acquired language and motor skills, intellectual impairment and hand stereotypies. The developmental regression observed in patients with Rett syndrome arises from altered neuronal function and is not the result of neurodegeneration. Maintenance of an appropriate level of MeCP2 appears integral to the function of healthy neurons as patients with increased levels of MeCP2, owing to duplication of the Xq28 region encompassing the MECP2 locus, also present with intellectual disability and progressive neurologic symptoms. Despite major efforts over the past two decades to elucidate the molecular functions of MeCP2, the mechanisms underlying the delayed appearance of symptoms remain unclear. In this review, we will highlight recent findings that have expanded our knowledge of MeCP2's functions, and we will discuss how epigenetic regulation, chromatin organization and circuit dynamics may contribute to the postnatal onset of Rett syndrome.

Electrophysiological phenotypes of MeCP2 A140V mutant mouse model

AIMS

MeCP2 gene mutations are associated with Rett syndrome and X-linked mental retardation (XLMR), diseases characterized by abnormal brain development and function. Recently, we created a novel MeCP2 A140V mutation mouse model that exhibited abnormalities of cell packing density and dendritic branching consistent with that seen in Rett syndrome patients as well as other MeCP2 mutant mouse models. Therefore, we hypothesized that some deficits of neuronal and synaptic functions might also be present in the A140V mutant model.

METHODS

Here, we tested our hypothesis in hippocampal slices using electrophysiological recordings.

RESULTS

We found that in young A140V mutant mice (3- to 4-week-old), hippocampal CA1 pyramidal neurons exhibited more positive resting membrane potential, increased action potential (AP) firing frequency induced by injection of depolarizing current, wider AP duration, and smaller after hyperpolarization potential compared to neurons prepared from age-matched wild-type mice, suggesting a neuronal hyperexcitation. At the synaptic level, A140V mutant neurons exhibited a reduced frequency of spontaneous IPSCs (inhibitory postsynaptic potentials) and an enhanced probability of evoked glutamate release, both suggesting neuronal hyperexcitation. However, hippocampal CA1 long-term potentiation was not significantly different between A140V and WT mice. In adult mice (11- to 13-month-old), in addition to neuronal hyperexcitation, we also found significant deficits of both short-term and long-term potentiation of CA3-CA1 synapses in A140V mice compared to WT mice.

CONCLUSIONS

These results clearly illustrate the age-dependent abnormalities of neuronal and synaptic function in the MeCP2 A140V mutant mouse model, which provides new insights into the understanding of the pathogenesis of Rett syndrome.

Clinical impacts of genomic copy number gains at Xq28

Duplications of the Xq28 region are the most frequent chromosomal aberrations observed in patients with intellectual disability (ID), especially in males. These duplications occur by variable mechanisms, including interstitial duplications mediated by segmental duplications in this region and terminal duplications (functional disomy) derived from translocation with other chromosomes. The most commonly duplicated region includes methyl CpG-binding protein 2 gene (MECP2), which has a minimal duplicated size of 0.2 Mb. Patients with MECP2 duplications show severe ID, intractable seizures and recurrent infections. Duplications in the telomeric neighboring regions, which include GDP dissociation inhibitor 1 gene (GDI1) and ras-associated protein RAB39B gene (RAB39B), are independently associated with ID, and many segmental duplications located in this region could mediate these frequently observed interstitial duplications. In addition, large duplications, including MECP2 and GDI1, induce hypoplasia of the corpus callosum. Abnormalities observed in the white matter, revealed by brain magnetic resonance imaging, are a common finding in patients with MECP2 duplications. As primary sequence analysis cannot be used to determine the region responsible for chromosomal duplication syndrome, finding this region relies on the collection of genotype-phenotype data from patients.

Practice parameter for the assessment and treatment of children and adolescents with autism spectrum disorder

Autism spectrum disorder is characterized by patterns of delay and deviance in the development of social, communicative, and cognitive skills that arise in the first years of life. Although frequently associated with intellectual disability, this condition is distinctive in its course, impact, and treatment. Autism spectrum disorder has a wide range of syndrome expression and its management presents particular challenges for clinicians. Individuals with an autism spectrum disorder can present for clinical care at any point in development. The multiple developmental and behavioral problems associated with this condition necessitate multidisciplinary care, coordination of services, and advocacy for individuals and their families. Early, sustained intervention and the use of multiple treatment modalities are indicated.

Clinical characterization and identification of duplication breakpoints in a Japanese family with Xq28 duplication syndrome including MECP2

Xq28 duplication syndrome including MECP2 is a neurodevelopmental disorder characterized by axial hypotonia at infancy, severe intellectual disability, developmental delay, mild characteristic facial appearance, epilepsy, regression, and recurrent infections in males. We identified a Japanese family of Xq28 duplications, in which the patients presented with cerebellar ataxia, severe constipation, and small feet, in addition to the common clinical features. The 488-kb duplication spanned from L1CAM to EMD and contained 17 genes, two pseudo genes, and three microRNA-coding genes. FISH and nucleotide sequence analyses demonstrated that the duplication was tandem and in a forward orientation, and the duplication breakpoints were located in AluSc at the EMD side, with a 32-bp deletion, and LTR50 at the L1CAM side, with "tc" and "gc" microhomologies at the duplication breakpoints, respectively. The duplicated segment was completely segregated from the grandmother to the patients. These results suggest that the duplication was generated by fork-stalling and template-switching at the AluSc and LTR50 sites. This is the first report to determine the size and nucleotide sequences of the duplicated segments at Xq28 of three generations of a family and provides the genotype-phenotype correlation of the patients harboring the specific duplicated segment.

Adolescent onset cognitive regression and neuropsychiatric symptoms associated with the A140V MECP2 mutation

The phenotype attributed to MECP2 mutations continues to expand. In addition to classic and variant Rett syndrome, phenotypes include non-specific intellectual disability and autism spectrum disorder in females, and fatal neonatal encephalopathy in males. One particular phenotype of parkinsonism, pyramidal signs, and neuropsychiatric symptoms (PPM-X) has been described only in males. We report on the first female with the A140V MECP2 mutation presenting with late onset cognitive regression, pyramidal symptoms, parkinsonism, and bipolar symptoms. This finding emphasizes the need to consider MECP2 sequencing in females with non-classic Rett phenotypes, particularly those with intellectual disability and neuropsychiatric features.

The relationship of Rett syndrome and MECP2 disorders to autism

Rett syndrome (RTT, MIM#312750) is a neurodevelopmental disorder that is classified as an autism spectrum disorder. Clinically, RTT is characterized by psychomotor regression with loss of volitional hand use and spoken language, the development of repetitive hand stereotypies, and gait impairment. The majority of people with RTT have mutations in Methyl-CpG-binding Protein 2 (MECP2), a transcriptional regulator. Interestingly, alterations in the function of the protein product produced by MECP2, MeCP2, have been identified in a number of other clinical conditions. The many clinical features found in RTT and the various clinical problems that result from alteration in MeCP2 function have led to the belief that understanding RTT will provide insight into a number of other neurological disorders. Excitingly, RTT is reversible in a mouse model, providing inspiration and hope that such a goal may be achieved for RTT and potentially for many neurodevelopmental disorders.

Novel MeCP2 isoform-specific antibody reveals the endogenous MeCP2E1 expression in murine brain, primary neurons and astrocytes

Rett Syndrome (RTT) is a severe neurological disorder in young females, and is caused by mutations in the X-linked MECP2 gene. MECP2/Mecp2 gene encodes for two protein isoforms; MeCP2E1 and MeCP2E2 that are identical except for the N-terminus region of the protein. In brain, MECP2E1 transcripts are 10X higher, and MeCP2E1 is suggested to be the relevant isoform for RTT. However, due to the unavailability of MeCP2 isoform-specific antibodies, the endogenous expression pattern of MeCP2E1 is unknown. To gain insight into the expression of MeCP2E1 in brain, we have developed an anti-MeCP2E1 antibody and validated its specificity in cells exogenously expressing individual MeCP2 isoforms. This antibody does not show any cross-reactivity with MeCP2E2 and detects endogenous MeCP2E1 in mice brain, with no signal in Mecp2^tm1.1Bird y/− null mice. Additionally, we show the endogenous MeCP2E1 expression throughout different brain regions in adult mice, and demonstrate its highest expression in the brain cortex. Our results also indicate that MeCP2E1 is highly expressed in primary neurons, as compared to primary astrocytes. This is the first report of the endogenous MeCP2E1 expression at the protein levels, providing novel avenues for understanding different aspects of MeCP2 function.

MeCP2 as a genome-wide modulator: the renewal of an old story

Since the discovery of MeCP2, its functions have attracted the interest of generations of molecular biologists. Its function as a transducer of DNA methylation, the major post-biosynthetic modification found throughout genomes, and its association with the neurodevelopmental disease Rett syndrome highlight its central role as a transcriptional regulator, and, at the same time, poses puzzling questions concerning its roles in physiology and pathology. The classical model of the MeCP2 function predicts its role in gene-specific repression through the binding of methylated DNA, via its interaction with the histone deacetylases and co-repressor complexes. This view has been questioned and, intriguingly, new roles for MeCP2 as a splicing modulator and as a transcriptional activator have been proposed. Recent data have demonstrated that MeCP2 is extremely abundant in the neurons, where it reaches the level of histone H1; it is widely distributed, tracking the methylated CpGs, and regulates repetitive elements expression. The role of MeCP2 in maintaining the global chromatin structure is further sustained by its involvement in other biologically relevant phenomena, such as the Line-1 repetitive sequences retrotransposition and the pericentromeric heterochromatin clustering during cellular differentiation. These new concepts renew the old view suggesting a role for DNA methylation in transcriptional noise reduction, pointing to a key role for MeCP2 in the modulation of the genome architecture.

Algorithmic approach for methyl-CpG binding protein 2 (MECP2) gene testing in patients with neurodevelopmental disabilities

Methyl-CpG binding protein 2 gene (MECP2) testing is indicated for patients with numerous clinical presentations, including Rett syndrome (classic and atypical), unexplained neonatal encephalopathy, Angelman syndrome, nonspecific mental retardation, autism (females), and an X-linked family history of developmental delay. Because of this complexity, a gender-specific approach for comprehensive MECP2 gene testing is described. Briefly, sequencing of exons 1 to 4 of MECP2 is recommended for patients with a Rett syndrome phenotype, unexplained neonatal encephalopathy, an Angelman syndrome phenotype (with negative 15q11-13 analysis), nonspecific mental retardation, or autism (females). Additional testing for large-scale MECP2 deletions is recommended for patients with Rett syndrome or Angelman syndrome phenotypes (with negative 15q11-13 analysis) following negative sequencing. Alternatively, testing for large-scale MECP2 duplications is recommended for males presenting with mental retardation, an X-linked family history of developmental delay, and a significant proportion of previously described clinical features (particularly a history of recurrent respiratory infections).

Adult Phenotypes in Angelman- and Rett-Like Syndromes

BACKGROUND: Angelman- and Rett-like syndromes share a range of clinical characteristics, including intellectual disability (ID) with or without regression, epilepsy, infantile encephalopathy, postnatal microcephaly, features of autism spectrum disorder, and variable other neurological symptoms. The phenotypic spectrum generally has been well studied in children; however, evolution of the phenotypic spectrum into adulthood has been documented less extensively. To obtain more insight into natural course and prognosis of these syndromes with respect to developmental, medical, and socio-behavioral outcomes, we studied the phenotypes of 9 adult patients who were recently diagnosed with 6 different Angelman- and Rett-like syndromes. METHODS: All these patients were ascertained during an ongoing cohort study involving a systematic clinical genetic diagnostic evaluation of over 250, mainly adult patients with ID of unknown etiology. RESULTS: We describe the evolution of the phenotype in adults with EHMT1, TCF4, MECP2, CDKL5, and SCN1A mutations and 22qter deletions and also provide an overview of previously published adult cases with similar diagnoses. CONCLUSION: These data are highly valuable in adequate management and follow-up of patients with Angelman- and Rett-like syndromes and accurate counseling of their family members. Furthermore, they will contribute to recognition of these syndromes in previously undiagnosed adult patients.

Mecp2 truncation in male mice promotes affiliative social behavior

Mouse models of Rett syndrome, with targeted mutations in the Mecp2 gene, show a high degree of phenotypic consistency with the clinical syndrome. In addition to severe and age-specific regression in motor and cognitive abilities, a variety of studies have demonstrated that Mecp2 mutant mice display impaired social behavior. Conversely, other studies indicate complex enhancements of social behavior in Mecp2 mutant mice. Since social behavior is a complicated accumulation of constructs, we performed a series of classic and refined social behavior tasks and revealed a relatively consistent pattern of enhanced pro-social behavior in hypomorphic Mecp2 (308/Y) mutant mice. Analyses of repetitive motor acts, and cognitive stereotypy did not reveal any profound differences due to genotype. Taken together, these results suggest that the mutations associated with Rett syndrome are not necessarily associated with autism-relevant social impairment in mice. However, this gene may be a valuable candidate for revealing basic mechanisms of affiliative behavior.

MeCP2: structure and functionThis paper is one of a selection of papers published in a Special Issue entitled 31st Annual International Asilomar Chromatin and Chromosomes Conference, and has undergone the Journal’s usual peer review process

Despite a vast body of literature linking chromatin structure to regulation of gene expression, the role of architectural proteins in higher order chromatin transitions required for transcription activation and repression has remained an under-studied field. To demonstrate the current knowledge of the role of such proteins, we have focused our attention on the methylated DNA binding and chromatin-associated protein MeCP2. Structural studies using chromatin assembled in vitro have revealed that MeCP2 can associate with nucleosomes in an N-terminus dependent manner and efficiently condense nucleosome arrays. The present review attempts to match MeCP2 structural domains, or lack thereof, and specific chromatin features needed for the proper recruitment of MeCP2 to its multiple functions as either activator or repressor. We specifically focused on MeCP2’s role in Rett syndrome, a neurological disorder associated with specific MeCP2 mutations.

Identification and characterization of novel sequence variations in MECP2 gene in Rett syndrome patients

Rett syndrome (RS) is a neurodevelopmental disorder caused by mutations in MECP2 gene. Exons 2, 3, and 4, in addition to intronic and 3'UTR adjacent regions, were sequenced in 80 patients with RS. Twenty-nine sequence variations were detected in 49 patients, 34 (69.4%) patients with the classic form of RS, and 15 (30.6%) patients with atypical forms of RS. Thirteen of the 29 detected mutations represent novel sequence variations. Missense mutation T158M was the most commonly observed mutation, detected in nine patients (11.2%). Six hotspot pathogenic mutations (R133C, T158M, R168X, R255X, R270X, and R294X) were responsible for the phenotype in 26/80 patients (32.5%).

The genetic basis of non-syndromic intellectual disability: a review

Intellectual disability (ID), also referred to as mental retardation (MR), is frequently the result of genetic mutation. Where ID is present together with additional clinical symptoms or physical anomalies, there is often sufficient information available for the diagnosing physician to identify a known syndrome, which may then educe the identification of the causative defect. However, where co-morbid features are absent, narrowing down a specific gene can only be done by ‘brute force’ using the latest molecular genetic techniques. Here we attempt to provide a systematic review of genetic causes of cases of ID where no other symptoms or co-morbid features are present, or non-syndromic ID. We attempt to summarize commonalities between the genes and the molecular pathways of their encoded proteins. Since ID is a common feature of autism, and conversely autistic features are frequently present in individuals with ID, we also look at possible overlaps in genetic etiology with non-syndromic ID.

Analysis of the parental origin of de novo MECP2 mutations and X chromosome inactivation in 24 sporadic patients with Rett syndrome in China

Rett syndrome is an X-linked neurodevelopmental disorder that predominantly affects females. It is caused by mutations in methyl-CpG-binding protein 2 gene. Due to the sex-limited expression, it has been suggested that de novo X-linked mutations may exclusively occur in male germ cells and thus only females are affected. In this study, the authors have analyzed the parental origin of mutations and the X-chromosome inactivation status in 24 sporadic patients with identified methyl-CpG-binding protein 2 gene mutations. The results showed that 22 of 24 patients have a paternal origin. Only 2 patients have a maternal origin. Except for 2 cases which were homozygotic at the androgen receptor gene locus, of the remaining 22 cases, 16 cases have a random X-chromosome inactivation pattern; the other 6 cases have a skewed X-chromosome inactivation and they favor expression of the wild allele. The relationship between X-chromosome inactivation and phenotype may need more cases to explore.

Autism-lessons from the X chromosome

Recognized cases of autism spectrum disorders are on the rise. It is unclear whether this increase is attributable to secular trends in biological susceptibility, or to a change in diagnostic practices and recognition. One hint concerning etiological influences is the universally reported male excess (in the range of 4:1 to 10:1). Evidence suggests that genetic influences from the X chromosome play a crucial role in engendering this male vulnerability. In this review, we discuss three categories of genetic disease that highlight the importance of X-linked genes in the manifestation of an autistic phenotype: aneuploides (Turner syndrome and Klinefelter syndrome), trinucleotide expansions (Fragile X syndrome) and nucleotide mutations (Rett Syndrome, Neuroligins 3 & 4, and SLC6A8). The lessons from these diseases include an understanding of autistic features as a broad phenotype rather than as a single clinical entity, the role of multiple genes either alone or in concert with the manifestation of autistic features, and the role of epigenetic factors such as imprinting and X-inactivation in the expression of disease severity. Better understanding of the clinical phenotypes of social cognition and the molecular neurogenetics of X-linked gene disorders will certainly provide additional tools for understanding autism in the years to come.

Phenotypic and genotypic variability in four males with MECP2 gene sequence aberrations including a novel deletion

The MECP2 gene mutations cause Rett syndrome (RTT) (OMIM: 312750), an X-linked dominant disorder primarily affecting girls. Until RTT was considered lethal in males, although now approximately 60 cases have been reported. Males with MECP2 mutations present with a broad spectrum of phenotypes ranging from neonatal encephalopathy to nonsyndromic mental retardation (MR). Four boys (aged, 3-11 y) were evaluated for MR. Patient 1 had autistic features. Patients 2 and 3 were brothers both presenting with psychomotor delay. Patient 4 showed dysmorphic features and behavioral problems reminiscent of FXS. All patients had a normal 46, XY karyotype and three were tested for FXS with negative results. MECP2 gene analysis of exons 3 and 4 was performed using methods based on the PCR, including Enzymatic Cleavage Mismatched Analysis (ECMA) and direct sequencing. Patient 1 presented somatic mosaicism for the classic RTT p.R106W mutation and patient 4 carried the p.T203M polymorphism. Analysis of the mothers in both cases revealed normal DNA sequences. Patients 2 and 3 had a novel deletion (c.1140del86) inherited from their unaffected mother. MECP2 gene mutations may be considered a rare cause of MR in males although great phenotypic variation hinders genotype-phenotype correlation.

Semi-dominant X-chromosome linked learning disability with progressive ataxia, spasticity and dystonia associated with the novel MECP2 variant p.V122A: akin to the new MECP2 duplication syndrome?

A novel X-chromosome linked phenotype is reported. Three affected males had learning disability in early childhood and subsequently developed progressive ataxia, dystonia, and spasticity with death at ages 9, 14 and 19 years. Two female obligate carriers had learning difficulties with psychosis in one case. A third, possible carrier had learning difficulties and epilepsy. A family study indicates that this inherited syndrome is most likely due to an unreported MECP2 variant, p.V122A, located in the methyl binding domain of the MECP2 protein. The clinical features are similar to those present in the newly reported MECP2 duplication syndrome. Non-progressive neuropsychiatric symptoms in female relatives of a male child with learning disability, ataxia and progressive spasticity may constitute a clue to inherited, MECP2 pathogenesis.

Abnormalities of cell packing density and dendritic complexity in the MeCP2 A140V mouse model of Rett syndrome/X-linked mental retardation

Background

Rett syndrome (RTT), a common cause of mental retardation in girls, is associated with mutations in the MECP2 gene. Most human cases of MECP2 mutation in girls result in classical or variant forms of RTT. When these same mutations occur in males, they often present as severe neonatal encephalopathy. However, some MECP2 mutations can also lead to diseases characterized as mental retardation syndromes, particularly in boys. One of these mutations, A140V, is a common, recurring missense mutation accounting for about 0.6% of all MeCP2 mutations and ranking 21^st by frequency. It has been described in familial X-linked mental retardation (XLMR), PPM- X syndrome (Parkinsonism, Pyramidal signs, Macroorchidism, X-linked mental retardation) and in other neuropsychiatric syndromes. Interestingly, this mutation has been reported to preserve the methyl-CpG binding function of the MeCP2 protein while compromising its ability to bind to the mental retardation associated protein ATRX.

Results

We report the construction and initial characterization of a mouse model expressing the A140V MeCP2 mutation. These initial descriptive studies in male hemizygous mice have revealed brain abnormalities seen in both RTT and mental retardation. The abnormalities found include increases in cell packing density in the brain and a significant reduction in the complexity of neuronal dendritic branching. In contrast to some MeCP2 mutation mouse models, the A140V mouse has an apparently normal lifespan and normal weight gain patterns with no obvious seizures, tremors, breathing difficulties or kyphosis.

Conclusion

We have identified various neurological abnormalities in this mouse model of Rett syndrome/X-linked mental retardation which may help to elucidate the manner in which MECP2 mutations cause neuronal changes resulting in mental retardation without the confounding effects of seizures, chronic hypoventilation, or other Rett syndrome associated symptoms.

Autism and other neuropsychiatric symptoms are prevalent in individuals with MeCP2 duplication syndrome

OBJECTIVE

There have been no objective assessments to determine whether boys with MECP2 duplication have autism or whether female carriers manifest phenotypes. This study characterizes the clinical and neuropsychiatric phenotypes of affected boys and carrier females.

METHODS

Eight families (9 males and 9 females) with MECP2 duplication participated. A detailed history, physical examination, electroencephalogram, developmental evaluation, Autism Diagnostic Observation Schedule, and Autism Diagnostic Interview-Revised were performed for each boy. Carrier females completed the Symptom Checklist-90-R, Wechsler Abbreviated Scale of Intelligence, Broad Autism Phenotype Questionnaire, and detailed medical and mental health histories. Size and gene content of each duplication were determined by array comparative genome hybridization. X-chromosome inactivation patterns were analyzed using leukocyte DNA. MECP2 and IRAK1 RNA levels were quantified from lymphoblast cell lines, and western blots were performed to assess MeCP2 protein levels.

RESULTS

All of the boys demonstrated mental retardation and autism. Poor expressive language, gaze avoidance, repetitive behaviors, anxiety, and atypical socialization were prevalent. Female carriers had psychiatric symptoms, including generalized anxiety, depression, and compulsions that preceded the birth of their children. The majority exhibited features of the broad autism phenotype and had higher nonverbal compared to verbal reasoning skills.

INTERPRETATION

Autism is a defining feature of the MECP2 duplication syndrome in boys. Females manifest phenotypes despite 100% skewing of X-inactivation and normal MECP2 RNA levels in peripheral blood. Analysis of the duplication size, MECP2 and IRAK1 RNA levels, and MeCP2 protein levels revealed that most of the traits in affected boys are likely due to the genomic region spanning of MECP2 and IRAK1. The phenotypes observed in carrier females may be secondary to tissue-specific dosage alterations and require further study. Ann Neurol 2009;66:771-782.

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

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

The genetic landscape of intellectual disability arising from chromosome X

X-linked mental retardation (XLMR) or intellectual disability (ID) is a common, clinically complex and genetically heterogeneous disease arising from many mutations along the X chromosome. It affects between 1/600-1/1000 males and a substantial number of females. Research during the past decade has identified >90 different XLMR genes, affecting a wide range of cellular processes. Many more genes remain uncharacterized, especially for the non-syndromic XLMR forms. Currently, approximately 11% of X-chromosome genes are implicated in XLMR; however, apart from a few notable exceptions, most contribute individually to <0.1% of the total landscape, which arguably remains only about half complete. There remain many hills to climb and valleys to cross before the ID landscape is fully triangulated.

The robotic mouse: understanding the role of AF4, a cofactor of transcriptional elongation and chromatin remodelling, in purkinje cell function

Neurological disorders represent a large share of the disease burden worldwide, and the incidence of age-related forms will continue to rise with life expectancy. Gene targeting has been and will remain a valuable approach to the generation of clinically relevant mouse models from which to elucidate the underlying molecular basis. However, as the aetiology of the majority of these conditions is still unknown, a reverse approach based on large-scale random chemical mutagenesis is now being used in an attempt to identify new genes and associated signalling pathways that control neuronal cell death and survival. Here, we review the characterisation of a novel model of autosomal dominant cerebellar ataxia which shows general growth retardation and develops adult-onset region-specific Purkinje cell loss as well as cataracts and defects in early T-cell maturation. We have previously established that the mutated protein Af4, which is a member of the AF4/LAF4/FMR2 (ALF) family of transcription cofactors frequently translocated in childhood leukaemia, undergoes slower proteasomal turnover through the ubiquitin pathway and abnormally accumulates in Purkinje cells of the cerebellum. We have also shown that Af4 functions as part of a large multiprotein complex that stimulates RNA polymerase II elongation and mediates chromatin remodelling during transcription. With the forthcoming identification of the gene targets that trigger Purkinje cell death in the robotic cerebellum, and the functional conservation among the ALF proteins, the robotic mouse promises to deliver important insights into the pathogenesis of human ataxia, but also of mental retardation to which FMR2 and LAF4 have been linked.

A MECP2 mutation in a highly conserved aminoacid causing mental retardation in a male

MeCP2 is a protein that functions as a key factor in epigenetic transcriptional regulation. Mutations in MECP2 gene have been reported as being the major cause of Rett syndrome. These mutations may also cause a wide spectrum of neurological disorders in males. Here, we report the identification of the mutation p.P405L in a 19-year-old Brazilian male with mental retardation. This variant is localized in a highly conserved aminoacid from the carboxy terminal domain and may affect the protein function. Segregation analysis on the patient's mother revealed that this is a de novo mutation and it was not identified in the control sample. The programs SIFT, PolyPhen and A-GVGD considered that the p.P405L may be damaging. Despite the high frequency of non pathogenic variants that have been identified in this gene, our data lead us to consider the p.P405L a disease-causing mutation.