Epigenetic overlap in autism-spectrum neurodevelopmental disorders: MECP2 deficiency causes reduced expression of UBE3A and GABRB3

Ubiquitin ligase E6-AP and its role in human disease

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

The comorbidity of autism with the genomic disorders of chromosome 15q11.2-q13

A cluster of low copy repeats on the proximal long arm of chromosome 15 mediates various forms of stereotyped deletions and duplication events that cause a group of neurodevelopmental disorders that are associated with autism or autism spectrum disorders (ASD). The region is subject to genomic imprinting and the behavioral phenotypes associated with the chromosome 15q11.2-q13 disorders show a parent-of-origin specific effect that suggests that an increased copy number of maternally derived alleles contributes to autism susceptibility. Notably, nonimprinted, biallelically expressed genes within the interval also have been shown to be misexpressed in brains of patients with chromosome 15q11.2-q13 genomic disorders, indicating that they also likely play a role in the phenotypic outcome. This review provides an overview of the phenotypes of these disorders and their relationships with ASD and outlines the regional genes that may contribute to the autism susceptibility imparted by copy number variation of the region.

Genomic imprinting in the development and evolution of psychotic spectrum conditions

I review and evaluate genetic and genomic evidence salient to the hypothesis that the development and evolution of psychotic spectrum conditions have been mediated in part by alterations of imprinted genes expressed in the brain. Evidence from the genetics and genomics of schizophrenia, bipolar disorder, major depression, Prader-Willi syndrome, Klinefelter syndrome, and other neurogenetic conditions support the hypothesis that the etiologies of psychotic spectrum conditions commonly involve genetic and epigenetic imbalances in the effects of imprinted genes, with a bias towards increased relative effects from imprinted genes with maternal expression or other genes favouring maternal interests. By contrast, autistic spectrum conditions, including Kanner autism, Asperger syndrome, Rett syndrome, Turner syndrome, Angelman syndrome, and Beckwith-Wiedemann syndrome, commonly engender increased relative effects from paternally expressed imprinted genes, or reduced effects from genes favouring maternal interests. Imprinted-gene effects on the etiologies of autistic and psychotic spectrum conditions parallel the diametric effects of imprinted genes in placental and foetal development, in that psychotic spectrum conditions tend to be associated with undergrowth and relatively-slow brain development, whereas some autistic spectrum conditions involve brain and body overgrowth, especially in foetal development and early childhood. An important role for imprinted genes in the etiologies of psychotic and autistic spectrum conditions is consistent with neurodevelopmental models of these disorders, and with predictions from the conflict theory of genomic imprinting.

Genetic modifiers of MeCP2 function in Drosophila

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

Rett syndrome (RTT) is a progressive neurodevelopmental disorder that affects girls early in childhood and is caused by mutations in the MECP2 gene. Loss of MeCP2 function can also lead to clinically distinct conditions characterized by autism, learning disability, and mental retardation. Remarkably, increased levels of MeCP2 leads to related neurological disorders and mental retardation as well. These data emphasize the critical importance of regulating MeCP2 protein levels for normal post-natal development and function of the nervous system. MeCP2 is a protein that associates with chromatin and is thought to modulate gene expression. We have generated Drosophila that overexpress human MeCP2 to investigate the possibility that adjusting the activity of other genes may compensate for altered levels of MeCP2. In support of this hypothesis, we found a variety of modifier genes, including chromatin remodeling genes, that are able to ameliorate and/or aggravate the consequences of MeCP2 overexpression. These findings open the possibility of therapeutic avenues for RTT and related neuropsychiatric disorders by targeting proteins that are possibly easier to manipulate than MeCP2 itself.

Mechanisms of imprinting of the Prader-Willi/Angelman region

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

Gender influences monoallelic expression of ATP10A in human brain

Human chromosome 15q11-13 and the syntenic region of mouse chromosome 7 contain multiple imprinted genes necessary for proper neurodevelopment. Due to imprinting, paternal 15q11-13 deficiencies lead to Prader-Willi syndrome (PWS) while maternal 15q11-13 deficiencies cause Angelman syndrome (AS). The mechanisms involved in parental imprinting of this locus are conserved between human and mouse, yet inconsistencies exist in reports of imprinting of the maternally expressed gene Atp10a/ATP10A. Excess maternal 15q11-13 dosage often leads to autism-spectrum disorder therefore further investigation to characterize the true imprinting status of ATP10A in humans was warranted. In this study, we examined allelic expression of ATP10A transcript in 16 control brain samples, and found that 10/16 exhibited biallelic expression while only 6/16 showed monoallelic expression. Contrary to the expectation for a maternally expressed imprinted gene, quantitative RT-PCR revealed significantly reduced ATP10A transcript in Prader-Willi syndrome brains with two maternal chromosomes due to uniparental disomy (PWS UPD). Furthermore, a PWS UPD brain sample with monoallelic ATP10A expression demonstrated that monoallelic expression can be independent of imprinting. Investigation of factors that may influence allelic ATP10A expression status revealed that gender has a major affect, as females were significantly more likely to have monoallelic ATP10A expression than males. Regulatory sequences were also examined, and a promoter polymorphism that disrupts binding of the transcription factor Sp1 also potentially contributes to allelic expression differences in females. Our results show that monoallelic expression of human ATP10A is variable in the population and is influenced by both gender and common genetic variation.

Genetic and epigenetic defects in mental retardation

Mental retardation (MR) is a highly diverse group of cognitive disorders. The high incidence of MR, 2-3% in most populations, and the high burden for families and society makes this condition one of the major unsolved problems in modern medicine. Gene defects account for about half of all patients and more than 300 genes are known that, when mutated, lead to cognitive dysfunction. A strikingly high number of these MR genes encode regulators of chromatin structure and of chromatin-mediated transcription regulation. Prominent examples of these include the methyl CpG-binding protein MECP2, the H3K4 demethylase JARID1c and the H3K9 histone methyltransferase EHMT1. Moreover, several of these epigenetic MR proteins have been found to directly interact with one another or act in complexes that regulate the local chromatin structure at target genes that are key to normal neuronal activities. Thus, it appears that the function of individual MR genes converges to similar biological processes. More detailed knowledge about the altered DNA methylation and histone marks that are introduced by epigenetic gene mutations as well as more insight into neuronal genes whose expression is disrupted by this will provide a rationale for therapeutic strategies.

Mecp2 organizes juvenile social behavior in a sex-specific manner

Methyl-CpG-binding protein 2 (MeCP2) binds methylated DNA and recruits corepressor proteins to modify chromatin and alter gene transcription. Mutations of the MECP2 gene can cause Rett syndrome, whereas subtle reductions of MeCP2 expression may be associated with male-dominated social and neurodevelopmental disorders. We report that transiently decreased amygdala Mecp2 expression during a sensitive period of brain sexual differentiation disrupts the organization of sex differences in juvenile social play behavior. Interestingly, neonatal treatment with Mecp2 small interfering RNA within the developing amygdala reduced juvenile social play behavior in males but not females. Reduced Mecp2 expression did not change juvenile sociability or anxiety-like behavior, suggesting that this disruption is associated with subtle behavioral modification. This suggests that Mecp2 may have an overlooked role in the organization of sexually dimorphic behaviors and that male juvenile behavior is particularly sensitive to Mecp2 disruption during this period of development.

Microtransplantation of neurotransmitter receptors from postmortem autistic brains to Xenopus oocytes

Autism is a complex disorder that arises from the pervasive action of genetic and epigenetic factors that alter synaptic connectivity of the brain. Although GABA and glutamate receptors seem to be two of those factors, very little is known about the functional properties of the autistic receptors. Autistic tissue samples stored in brain banks usually have relatively long postmortem times, and it is highly desirable to know whether neurotransmitter receptors in such tissues are still functional. Here we demonstrate that native receptors microtransplanted from autistic brains, as well as de novo mRNA-expressed receptors, are still functional and susceptible to detailed electrophysiological characterization even after long postmortem intervals. The opportunity to study the properties of human receptors present in diseased brains not only opens new avenues toward understanding autism and other neurological disorders, but it also makes the microtransplantation method a useful translational system to evaluate and develop novel medicinal drugs.

Psychosis and autism as diametrical disorders of the social brain

Autistic-spectrum conditions and psychotic-spectrum conditions (mainly schizophrenia, bipolar disorder, and major depression) represent two major suites of disorders of human cognition, affect, and behavior that involve altered development and function of the social brain. We describe evidence that a large set of phenotypic traits exhibit diametrically opposite phenotypes in autistic-spectrum versus psychotic-spectrum conditions, with a focus on schizophrenia. This suite of traits is inter-correlated, in that autism involves a general pattern of constrained overgrowth, whereas schizophrenia involves undergrowth. These disorders also exhibit diametric patterns for traits related to social brain development, including aspects of gaze, agency, social cognition, local versus global processing, language, and behavior. Social cognition is thus underdeveloped in autistic-spectrum conditions and hyper-developed on the psychotic spectrum.;>We propose and evaluate a novel hypothesis that may help to explain these diametric phenotypes: that the development of these two sets of conditions is mediated in part by alterations of genomic imprinting. Evidence regarding the genetic, physiological, neurological, and psychological underpinnings of psychotic-spectrum conditions supports the hypothesis that the etiologies of these conditions involve biases towards increased relative effects from imprinted genes with maternal expression, which engender a general pattern of undergrowth. By contrast, autistic-spectrum conditions appear to involve increased relative bias towards effects of paternally expressed genes, which mediate overgrowth. This hypothesis provides a simple yet comprehensive theory, grounded in evolutionary biology and genetics, for understanding the causes and phenotypes of autistic-spectrum and psychotic-spectrum conditions.

The evolutionary social brain: From genes to psychiatric conditions

The commentaries on our target article, “Psychosis and Autism as Diametrical Disorders of the Social Brain,” reflect the multidisciplinary yet highly fragmented state of current studies of human social cognition. Progress in our understanding of the human social brain must come from studies that integrate across diverse analytic levels, using conceptual frameworks grounded in evolutionary biology.

The diagnosis of autism in a female: could it be Rett syndrome?

The overlap between autism and Rett syndrome clinical features has led to many cases of Rett syndrome being initially diagnosed with infantile autism or as having some autistic features. Both conditions seriously disrupt social and language development and are often accompanied by repetitive, nonpurposeful stereotypic hand movements. The aims of this study were to compare the early and subsequent clinical courses of female subjects with Rett syndrome categorised by whether or not a diagnosis of autism had been proposed before Rett syndrome had been diagnosed and compare the spectrum of methyl-CpG binding protein 2 (MECP2) mutations identified among the two groups. This study made use of a total of 313 cases recorded in two databases: the Australian Rett Syndrome Database (ARSD) and the International Rett Syndrome Phenotype Database (InterRett). Cases with an initial diagnosis of autism had significantly milder Rett syndrome symptoms and were more likely to remain ambulant, to have some functional hand use and not to have developed a scoliosis. Females with the p.R306C or p.T158M mutations in the MECP2 gene were more likely to have an initial diagnosis of autism, and the specific Rett syndrome symptoms were noted at a later age. We recommend that females who are initially considered to have autism be carefully monitored for the evolution of the signs and symptoms of Rett syndrome.

Abnormal transmethylation/transsulfuration metabolism and DNA hypomethylation among parents of children with autism

An integrated metabolic profile reflects the combined influence of genetic, epigenetic, and environmental factors that affect the candidate pathway of interest. Recent evidence suggests that some autistic children may have reduced detoxification capacity and may be under chronic oxidative stress. Based on reports of abnormal methionine and glutathione metabolism in autistic children, it was of interest to examine the same metabolic profile in the parents. The results indicated that parents share similar metabolic deficits in methylation capacity and glutathione-dependent antioxidant/detoxification capacity observed in many autistic children. Studies are underway to determine whether the abnormal profile in parents reflects linked genetic polymorphisms in these pathways or whether it simply reflects the chronic stress of coping with an autistic child.

Advances in autism genetics: on the threshold of a new neurobiology

Autism is a heterogeneous syndrome defined by impairments in three core domains: social interaction, language and range of interests. Recent work has led to the identification of several autism susceptibility genes and an increased appreciation of the contribution of de novo and inherited copy number variation. Promising strategies are also being applied to identify common genetic risk variants. Systems biology approaches, including array-based expression profiling, are poised to provide additional insights into this group of disorders, in which heterogeneity, both genetic and phenotypic, is emerging as a dominant theme.

Immunologic and neurodevelopmental susceptibilities of autism

Symposium 5 focused on research approaches that are aimed at understanding common patterns of immunological and neurological dysfunction contributing to neurodevelopmental disorders such as autism and ADHD. The session focused on genetic, epigenetic, and environmental factors that might act in concert to influence autism risk, severity and co-morbidities, and immunological and neurobiological targets as etiologic contributors. The immune system of children at risk of autism may be therefore especially susceptible to psychological stressors, exposure to chemical triggers, and infectious agents. Identifying early biomarkers of risk provides tangible approaches toward designing studies in animals and humans that yield a better understanding of environmental risk factors, and can help identify rational intervention strategies to mitigate these risks.

MeCP2-dependent repression of an imprinted miR-184 released by depolarization

Both fragile X syndrome and Rett syndrome are commonly associated with autism spectrum disorders and involve defects in synaptic plasticity. MicroRNA is implicated in synaptic plasticity because fragile X mental retardation protein was recently linked to the microRNA pathway. DNA methylation is also involved in synaptic plasticity since methyl CpG-binding protein 2 (MeCP2) is mutated in patients with Rett syndrome. Here we report that expression of miR-184, a brain-specific microRNA repressed by the binding of MeCP2 to its promoter, is upregulated by the release of MeCP2 after depolarization. The restricted release of MeCP2 from the paternal allele results in paternal allele-specific expression of miR-184. Our finding provides a clue to the link between the microRNA and DNA methylation pathways.

Dynamic changes in Histone H3 lysine 9 acetylation localization patterns during neuronal maturation require MeCP2

Mutations within the gene encoding methyl CpG binding protein 2 (MECP2) cause the autism-spectrum neurodevelopmental disorder Rett Syndrome (RTT). MECP2 recruits histone deacetylase to methylated DNA and acts as a long-range regulator of methylated genes. Despite ubiquitous MECP2 expression, the phenotype of RTT and the Mecp2-deficient mouse is largely restricted to the postnatal brain. Since Mecp2-deficient mice have a defect in neuronal maturation, we sought to understand how MECP2/Mecp2 mutations globally affect histone modifications during postnatal brain development by an immunofluorescence approach. Using an antibody specific to acetylated histone H3 lysine 9 (H3K9ac), a bright punctate nuclear staining pattern was observed as MECP2 expression increased in early postnatal neuronal nuclei. As neurons matured in juvenile and adult brain samples, the intensity of H3K9ac staining was reduced. Mecp2-deficient mouse and RTT cerebral neurons lacked this developmental reduction in H3K9ac staining compared to age-matched controls, resulting in a significant increase in neuronal nuclei with bright H3K9ac punctate staining. In contrast, trimethylated histone H3 lysine 9 (H3K9me3) localized to heterochromatin independent of MeCP2, but showed significantly reduced levels in Mecp2 deficient mouse and RTT brain. Autism brain with reduced MECP2 expression displayed similar histone H3 alterations as RTT brain. These observations suggest that MeCP2 regulates global histone modifications during a critical postnatal stage of neuronal maturation. These results have implications for understanding the molecular pathogenesis of RTT and autism in which MECP2 mutation or deficiency corresponds with arrested neurodevelopment.

Reduced MeCP2 expression is frequent in autism frontal cortex and correlates with aberrant MECP2 promoter methylation

Mutations in MECP2, encoding methyl CpG binding protein 2 (MeCP2), cause most cases of Rett syndrome (RTT), an X-linked neurodevelopmental disorder. Both RTT and autism are "pervasive developmental disorders" and share a loss of social, cognitive and language skills and a gain in repetitive stereotyped behavior, following apparently normal perinatal development. Although MECP2 coding mutations are a rare cause of autism, MeCP2 expression defects were previously found in autism brain. To further study the role of MeCP2 in autism spectrum disorders (ASDs), we determined the frequency of MeCP2 expression defects in brain samples from autism and other ASDs. We also tested the hypotheses that MECP2 promoter mutations or aberrant promoter methylation correlate with reduced expression in cases of idiopathic autism. MeCP2 immunofluorescence in autism and other neurodevelopmental disorders was quantified by laser scanning cytometry and compared with control postmortem cerebral cortex samples on a large tissue microarray. A significant reduction in MeCP2 expression compared to age-matched controls was found in 11/14 autism (79%), 9/9 RTT (100%), 4/4 Angelman syndrome (100%), 3/4 Prader-Willi syndrome (75%), 3/5 Down syndrome (60%), and 2/2 attention deficit hyperactivity disorder (100%) frontal cortex samples. One autism female was heterozygous for a rare MECP2 promoter variant that correlated with reduced MeCP2 expression. A more frequent occurrence was significantly increased MECP2 promoter methylation in autism male frontal cortex compared to controls. Furthermore, percent promoter methylation of MECP2 significantly correlated with reduced MeCP2 protein expression. These results suggest that both genetic and epigenetic defects lead to reduced MeCP2 expression and may be important in the complex etiology of autism.

The story of Rett syndrome: from clinic to neurobiology

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

The changing impact of genes and environment on brain development during childhood and adolescence: initial findings from a neuroimaging study of pediatric twins

Human brain development is created through continuing complex interactions of genetic and environmental influences. The challenge of linking specific genetic or environmental risk factors to typical or atypical behaviors has led to interest in using brain structural features as an intermediate phenotype. Twin studies in adults have found that many aspects of brain anatomy are highly heritable, demonstrating that genetic factors provide a significant contribution to variation in brain structures. Less is known about the relative impact of genes and environment while the brain is actively developing. We summarize results from the ongoing National Institute of Mental Health child and adolescent twin study that suggest that heritability of different brain areas changes over the course of development in a regionally specific fashion. Areas associated with more complex reasoning abilities become increasingly heritable with maturation. The potential mechanisms by which gene-environment interactions may affect heritability values during development is discussed.

Advances in behavioral genetics: mouse models of autism

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

Human disorders caused by the disruption of the regulation of excitatory neurotransmission

The nicotinic acetylcholine receptors (nAChRs) are members of the large family of ligand-gated ion channels, and are constituted by the assembly of five subunits arranged pseudosymmetrically around the central axis that forms a cation-selective ion pore. They are widely distributed in both the nervous system and non-neuronal tissues, and can be activated by endogenous agonists such as acetylcholine or exogenous ligands such as nicotine. Mutations in neuronal nAChRs are found in a rare form of familial nocturnal frontal lobe epilepsy (ADNFLE), while mutations in the neuromuscular subtype of the nAChR are responsible for either congenital myasthenia syndromes (adult subtype of neuromuscular nAChR) or a form of arthrogryposis multiplex congenita type Escobar (fetal subtype of neuromuscular nAChR).

The role of the neurobiologist in redefining the diagnosis of autism

Until recently, autism, along with the other developmental disabilities, was largely ignored by the medical and research community. At this early point in our understanding of the syndrome, neurobiologists and especially those who work with human brain tissue have a great deal to offer. A thorough understanding of the clinically defined syndrome is essential. Along with the other psychiatric diseases listed in the Diagnostic and Statistical Manual of Mental Disorders (DSM), autism is defined by gross behavioral macros that, in all probability, are only indirectly related to basic biological systems. The diagnostic schema is not etiologically based. The diagnostic triad of symptoms that defines autism--impaired communication, impaired social interaction, and restricted and repetitive interests and activities--has been found to be present in the general population with no clear demarcation between pathological severity and being a common trait. In addition, the three basic symptoms of autism appear not to associate highly, thus leaving undetermined the validity of studying autism in its currently defined triad of symptoms. It is proposed that a close working relationship between neurobiologists and clinicians is necessary in order to identify etiologically based diagnostic schemas that would complement, rather than replace, the clinical diagnosis.

Integrated epigenomic analyses of neuronal MeCP2 reveal a role for long-range interaction with active genes

Mutations in MECP2 cause the autism-spectrum disorder Rett syndrome. MeCP2 is predicted to bind to methylated promoters and silence transcription. However, the first large-scale mapping of neuronal MeCP2-binding sites on 26.3 Mb of imprinted and nonimprinted loci revealed that 59% of MeCP2-binding sites are outside of genes and that only 6% are in CpG islands. Integrated genome-wide promoter analysis of MeCP2 binding, CpG methylation, and gene expression revealed that 63% of MeCP2-bound promoters are actively expressed and that only 6% are highly methylated. These results indicate that the primary function of MeCP2 is not the silencing of methylated promoters.

Early defects of GABAergic synapses in the brain stem of a MeCP2 mouse model of Rett syndrome

Rett syndrome is a neurodevelopmental disorder caused by mutations in the transcriptional repressor methyl-CpG-binding protein 2 (MeCP2) and represents the leading genetic cause for mental retardation in girls. MeCP2-mutant mice have been generated to study the molecular mechanisms of the disease. It was suggested that an imbalance between excitatory and inhibitory neurotransmission is responsible for the behavioral abnormalities, although it remained largely unclear which synaptic components are affected and how cellular impairments relate to the time course of the disease. Here, we report that MeCP2 KO mice present an imbalance between inhibitory and excitatory synaptic transmission in the ventrolateral medulla already at postnatal day 7. Focusing on the inhibitory synaptic transmission we show that GABAergic, but not glycinergic, synaptic transmission is strongly depressed in MeCP2 KO mice. These alterations are presumably due to both decreased presynaptic gamma-aminobutyric acid (GABA) release with reduced levels of the vesicular inhibitory transmitter transporter and reduced levels of postsynaptic GABA(A)-receptor subunits alpha2 and alpha4. Our data indicate that in the MeCP2 -/y mice specific synaptic molecules and signaling pathways are impaired in the brain stem during early postnatal development. These observations mandate the search for more refined diagnostic tools and may provide a rationale for the timing of future therapeutic interventions in Rett patients.

Effect of inspired oxygen on periodic breathing in methy-CpG-binding protein 2 (Mecp2) deficient mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (Mecp2) that encodes a DNA binding protein involved in gene silencing. Periodic breathing (Cheyne-Stokes respiration) is commonly seen in RTT. Freely moving mice were studied with continuous recording of pleural pressure by telemetry. Episodes of periodic breathing in heterozygous Mecp2 deficient (Mecp2(+/-)) female mice (9.4 +/- 2.2 h(-1)) exceeded those in wild-type (Mecp2(+/+)) animals (2.5 +/- 0.4 h(-1)) (P = 0.010). Exposing Mecp2(+/-) animals to 40% oxygen increased the amount of periodic breathing from 118 +/- 25 s/30 min in air to 242 +/- 57 s/30 min (P = 0.001), and 12% oxygen tended to decrease it (67 +/- 29 s/30 min, P = 0.14). Relative hyperoxia and hypoxia did not affect the incidence of periodic breathing in Mecp2(+/+) animals. The ventilation/apnea ratio (V/A) was less at all levels of oxygen in heterozygous Mecp2(+/-) females compare with wild type (P = 0.003 to P < 0.001), indicating that their loop gain is larger. V/A in Mecp2(+/-) fell from 2.42 +/- 0.18 in normoxia to 1.82 +/- 0.17 in hyperoxia (P = 0.05) indicating an increase in loop gain with increased oxygen. Hyperoxia did not affect V/A in Mecp2(+/+) mice (3.73 +/- 0.28 vs. 3.5 +/- 0.28). These results show that periodic breathing in this mouse model of RTT is not dependent on enhanced peripheral chemoreceptor oxygen sensitivity. Rather, the breathing instability is of central origin.

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

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

How environmental and genetic factors combine to cause autism: A redox/methylation hypothesis

Recently higher rates of autism diagnosis suggest involvement of environmental factors in causing this developmental disorder, in concert with genetic risk factors. Autistic children exhibit evidence of oxidative stress and impaired methylation, which may reflect effects of toxic exposure on sulfur metabolism. We review the metabolic relationship between oxidative stress and methylation, with particular emphasis on adaptive responses that limit activity of cobalamin and folate-dependent methionine synthase. Methionine synthase activity is required for dopamine-stimulated phospholipid methylation, a unique membrane-delimited signaling process mediated by the D4 dopamine receptor that promotes neuronal synchronization and attention, and synchrony is impaired in autism. Genetic polymorphisms adversely affecting sulfur metabolism, methylation, detoxification, dopamine signaling and the formation of neuronal networks occur more frequently in autistic subjects. On the basis of these observations, a "redox/methylation hypothesis of autism" is described, in which oxidative stress, initiated by environment factors in genetically vulnerable individuals, leads to impaired methylation and neurological deficits secondary to reductions in the capacity for synchronizing neural networks.

The overlapping spectrum of rett and angelman syndromes: a clinical review

Rett and Angelman syndromes comprise part of the spectrum of neurologic disorders associated with autism. Their clinical presentations overlap, with both presenting in later infancy with global developmental delays, severe speech and communication impairments, progressive microcephaly, seizures, autistic behaviors, and characteristic albeit different movement disorders and stereotypic hand movements. Although other features can help differentiate these disorders, significant phenotypic overlap and variation in severity sometimes cloud the underlying diagnosis. Rett syndrome is caused by a mutation in the MECP2 gene located on Xq28, whereas Angelman syndrome results from the loss of UBE3A function on chromosomal region 15q11-q13 related to a variety of molecular genetic mechanisms. Recent advances have uncovered interactions between these and other genes that affect the function and structure of neurons in the brain. The reversal of symptoms of Rett syndrome in a mature mouse model suggests the possibility for treatment of these and perhaps other autism-related disorders in the future.

Imprinting status of paternally imprinted DLX5 gene in Japanese patients with Rett syndrome

Rett syndrome (RTT) is an X-linked severe neurodevelopmental disorder mostly affecting female and is mainly caused by mutations of methyl-CpG-binding protein 2 gene (MECP2). MECP2, which has a crucial role for transcriptional repression and chromatin remodeling, consists of methyl-CpG binding domain (MBD) and transcriptional repression domain (TRD). Paternally imprinted distal-less homeobox gene 5 (DLX5), that has an important role for the development of gamma-aminobutyric acid (GABA)-ergic neurons, was identified as a target of MECP2 recently. We selected the 12 samples from the 40 RTT lymphoblast cell lines by a mononucleotide repeat polymorphism within the 3'UTR of DLX5. In 12 samples, 5 and 6 samples have the mutations located in MBD and TRD, respectively. No expression and 25-75% expression of the mutated MECP2 allele were detected in 4 samples with MBD mutation and 4 samples with TRD mutation. In this study, the expression of mutated MECP2 alleles was low especially in the samples with the MBD mutation suggesting the biased frequency of the cells during the culture. However, a sample with high expression of mutated MECP2 in TRD mutation showed bialleic expression of DLX5 suggesting loss of imprinting.

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

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

Mechanisms of disease: neurogenetics of MeCP2 deficiency

Rett syndrome (RTT) is unique among genetic, chromosomal and other developmental disorders because of its extreme female gender bias, early normal development, and subsequent developmental regression with loss of motor and language skills. RTT is caused by heterozygosity for mutations in the X-linked gene MECP2, which encodes methyl-CpG binding protein 2. MeCP2 is a multifunctional protein that can act as an architectural chromatin-binding protein, a function that is unrelated to its ability to bind methyl-CpG and to attract chromatin modification complexes. Inactivating mutations that cause RTT in females are not prenatally lethal in males, but lead to profound congenital encephalopathy. Molecular diagnoses of RTT, through demonstration of a MECP2 mutation, made at an early stage of the disorder, usually confirm the sporadic nature and very low recurrence risk of the condition. A positive DNA test result, however, also predicts the inevitable clinical course, given the lack of effective intervention. Initial hypotheses indicating that the MeCP2 protein acts as a genome-wide transcriptional repressor were not confirmed by global gene expression studies in various tissues of individuals with RTT and mouse models of MeCP2 deficiency. Rather, recent evidence points to low-magnitude effects of a small number of genes--including the brain--derived neurotrophic factor pathway and glucocorticoid response genes-that might affect formation and maturation of synapses or synaptic function in postmitotic neurons.

A role for transcriptional repressor methyl-CpG-binding protein 2 and plasticity-related gene serum- and glucocorticoid-inducible kinase 1 in the induction of inflammatory pain states

Activity-dependent changes in neurons of the rat superficial dorsal horn are crucial for the induction and maintenance of neuropathic and inflammatory pain states. To identify the molecular mechanisms underlying this sensitization of superficial dorsal horn neurons, we undertook a genome-wide microarray profiling of dorsal horn gene transcripts at various times after induction of peripheral inflammation of the rat ankle joint. At early time points, upregulation of gene expression dominated, but by 7 d, downregulation was predominant. Two to 24 h after inflammation, we identified a small number of highly upregulated transcripts previously shown to be repressed by the Methyl-CpG-binding protein 2 (MeCP2), including serum- and glucocorticoid-inducible kinase (SGK1) and FK 506 binding protein 5, genes known to be important in experience-dependent plasticity. A decrease in expression of SIN3A, a corepressor in the MeCP2 silencing complex, was also found after inflammation. Phosphorylation of MeCP2 regulates activity-dependent gene transcription, and crucially we found that MeCP2 was phosphorylated in lamina I projection neurons 1 h after induction of peripheral inflammation. Lamina I projection neurons have been shown to be essential for the development of most pain states. SGK1 protein was also localized, in part, to lamina I projection neurons, and its expression in the superficial dorsal horn increased after inflammation. Furthermore, antisense knock-down of SGK1 delayed the onset of inflammatory hyperalgesia by 24 h at least. Our results uncover an unexpected complexity in the regulation of gene expression, including the modulation of transcriptional repression, that accompanies development and maintenance of an inflammatory pain state.

Rett syndrome

Rett syndrome (RS) is an X-linked neurodevelopmental disorder and the second most common cause of genetic mental retardation in females. Different mutations in MECP2 are found in up to 95% of typical cases of RS. This mainly neuronal expressed gene functions as a major transcription repressor. Extensive studies on girls who have RS and mouse models are aimed at finding main gene targets for MeCP2 protein and defining neuropathologic changes caused by its defects. Studies comparing autistic features in RS with idiopathic autism and mentally retarded patients are presented. Decreased dendritic arborization is common to RS and autism, leading to further research on similarities in pathogenesis, including MeCP2 protein levels in autistic brains and MeCP2 effects on genes connected to autism, like DLX5 and genes on 15q11-13 region. This area also is involved in Angelman syndrome, which has many similarities to RS. Despite these connections, MECP2 mutations in nonspecific autistic and mentally retarded populations are rare.

The proneural determinant MASH1 regulates forebrain Dlx1/2expression through the I12b intergenic enhancer

Establishment of neuronal networks is an extremely complex process involving the interaction of a diversity of neuronal cells. During mammalian development, these highly organized networks are formed through the differentiation of multipotent neuronal progenitors into multiple neuronal cell lineages. In the developing forebrain of mammals, the combined function of the Dlx1, Dlx2, Dlx5 and Dlx6 homeobox genes is necessary for the differentiation of the GABAergic interneurons born in the ventricular and subventricular zones of the ventral telencephalon, as well as for the migration of these neurons to the hippocampus, cerebral cortex and olfactory bulbs. The 437 bp I12b enhancer sequence in the intergenic region of the Dlx1/2 bigene cluster is involved in the forebrain regulation of Dlx1/2. Using DNase I footprinting, we identified six regions of I12b potentially bound by transcription factors. Mutagenesis of each binding site affected the expression of reporter constructs in transgenic mice. However,the effects of impairing protein-DNA interactions were not uniform across the forebrain Dlx1/2 expression domains, suggesting that distinct regulatory interactions are taking place in the different populations of neuronal precursors. Analyses of protein-DNA interactions provide evidence of a direct role for MASH1 in Dlx1/2 regulation in the forebrain. DLX proteins play a crucial role in the maintenance of their own expression, as shown by transgenic and co-transfection experiments. These studies suggest that the seemingly continuous domains of Dlx gene expression in the telencephalon and diencephalon are in fact the combination of distinct cell populations within which different genetic regulatory interactions take place.

DNA methylation regulates tissue-specific expression of Shank3

Tissue-specific gene expression can be controlled by epigenetic modifications such as DNA methylation. SHANK3, together with its homologues SHANK1 and SHANK2, has a central functional and structural role in excitatory synapses and is involved in the human chromosome 22q13 deletion syndrome. In this report, we show by DNA methylation analysis in lymphocytes, brain cortex, cerebellum and heart that the three SHANK genes possess several methylated CpG boxes, but only SHANK3 CpG islands are highly methylated in tissues where protein expression is low or absent and unmethylated where expression is present. SHANK3 protein expression is significantly reduced in hippocampal neurons after treatment with methionine, while HeLa cells become able to express SHANK3 after treatment with 5-Aza-2'-deoxycytidine. Altogether, these data suggest the existence of a specific epigenetic control mechanism regulating SHANK3, but not SHANK1 and SHANK2, expression.

Evaluation of autism traits in Angelman syndrome: a resource to unfold autism genes

Linkage and cytogenetics studies have found the Angelman syndrome (AS) chromosomal region to be of relevance to autism disorder (AD) or autism spectrum disorder (ASD). Autism is considered part of the behavioural phenotype in AS based on formal autism assessments (autism diagnostic interview-revised [ADI-R] and autism diagnostic observation schedule [ADOS]), which have mainly addressed the deleted AS group. We explored 23 AS patients including all genetic subtypes and made a co-morbid diagnosis of AD/ASD in 14/23 (61%), which does not include 4 cases classified within the broader autism spectrum disorder (bASD). Deletions accounted for the main fraction (35%), ubiquitin-protein ligase E3A (UBE3A) mutation represented 13%, imprinting defects and uniparental disomy 9 and 4%, respectively. UBE3A mutations due to lack of the homologous to the E6-associated protein carboxyl terminus domain (n = 3) were associated with the ASD, while more distal mutations (n = 3) seem to escape from a co-morbid diagnosis of autism/autism spectrum. Differences in severity of autistic features were seen across subtypes of AS, with some behavioural features being unique to AS and some representing all forms of developmental disability. Autism signs (poor/lack of eye contact, showing, spontaneous initiation of joint attention, social quality of overtures [ADOS algorithm items for Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV)/International Statistical Classification of Diseases and Related Health Problems-10 (ICD-10) autism diagnosis belonging to the reciprocal social interaction domain]) discriminating all the co-morbid AS categories from non-autistic AS belonged to the social interaction domain. Impairments in the communication domain (gestures, pointing, use of another's body, frequency of vocalisation towards others [ADOS algorithm items for DSM-IV/ICD-10 autism diagnosis belonging to the communication domain]) justified classification of co-morbid AD/ASD vs the classification of less affected bASD. Evaluation of the behaviour domain suggested that repetitive sensory and motor behaviours correlate with a low developmental profile rather than being specific to autism.

Autistic-like symptomatology in Prader-Willi syndrome: a review of recent findings

Prader-Willi syndrome (PWS) is caused by either the structural loss of material or the absence of gene expression from the paternally inherited copy of chromosome 15 in the q11-q13 region. In addition to a well-described behavioral phenotype that includes hyperphagia, obsessive-compulsive symptoms, disruptive behavior, and an increased risk for mood disorders, recent evidence also suggests that some individuals with PWS have repetitive behavior and social deficits reminiscent of autism spectrum disorders. In particular, it appears as if those with maternal uniparental disomy (UPD) as the cause of PWS are at greater risk for autistic symptomatology than those with paternal deletions of 15q11-q13. These findings are particularly intriguing in light of data implicating maternal duplications and triplications of the same chromosomal interval in idiopathic autism, as well as evidence that functional alterations of genes in this region are associated with social deficits found in a variety of neurodevelopmental disorders. This paper will review the recent evidence for phenotypic similarities between autism and PWS and the risk of symptomatology for the UPD subtype.

Methyl CpG-binding protein 2 (a mutation of which causes Rett syndrome) directly regulates insulin-like growth factor binding protein 3 in mouse and human brains

Rett syndrome (RTT) is a major neurodevelopmental disorder, characterized by mental retardation and autistic behavior. Mutation of the MeCP2 gene, encoding methyl CpG-binding protein 2, causes the disease. The pathomechanism by which MeCP2 dysfunction leads to the RTT phenotype has not been elucidated. We found that MeCP2 directly regulates expression of insulin-like growth factor binding protein 3 (IGFBP3) gene in human and mouse brains. A chromatin immunoprecipitation assay showed that the IGFBP3 promoter contained an MeCP2 binding site. IGFBP3 overexpression was observed in the brains of mecp2-null mice and human RTT patients using real-time quantitative polymerase chain reaction and Western blot analyses. Moreover, mecp2-null mice showed a widely distributed and increased number of IGFBP3-positive cells in the cerebral cortex, whereas wild-type mice at the same age showed fewer IGFBP3-positive cells. These results suggest that IGFBP3 is a downstream gene regulated by MeCP2 and that the previously reported BDNF and DLX5 genes and MeCP2 may contribute directly to the transcriptional expression of IGFBP3 in the brain. Interestingly, the pathologic features of mecp2-null mice have some similarities to those of IGFBP3-transgenic mice, which show a reduction of early postnatal growth. IGFBP3 overexpression due to lack of MeCP2 may lead to delayed brain maturation.

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

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

Covalent Modification of DNA Regulates Memory Formation

DNA methylation is a covalent chemical modification of DNA catalyzed by DNA methyltransferases (DNMTs). DNA methylation is associated with transcriptional silencing and has been studied extensively as a lifelong molecular information storage mechanism put in place during development. Here we report that DNMT gene expression is upregulated in the adult rat hippocampus following contextual fear conditioning and that DNMT inhibition blocks memory formation. In addition, fear conditioning is associated with rapid methylation and transcriptional silencing of the memory suppressor gene PP1 and demethylation and transcriptional activation of the synaptic plasticity gene reelin, indicating both methyltransferase and demethylase activity during consolidation. DNMT inhibition prevents the PP1 methylation increase, resulting in aberrant transcription of the gene during the memory-consolidation period. These results demonstrate that DNA methylation is dynamically regulated in the adult nervous system and that this cellular mechanism is a crucial step in memory formation.

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

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

Interaction between chromatin proteins MECP2 and ATRX is disrupted by mutations that cause inherited mental retardation

Mutations in the human methyl-CpG-binding protein gene MECP2 cause the neurological disorder Rett syndrome and some cases of X-linked mental retardation (XLMR). We report that MeCP2 interacts with ATRX, a SWI2/SNF2 DNA helicase/ATPase that is mutated in ATRX syndrome (alpha-thalassemia/mental retardation, X-linked). MeCP2 can recruit the helicase domain of ATRX to heterochromatic foci in living mouse cells in a DNA methylation-dependent manner. Also, ATRX localization is disrupted in neurons of Mecp2-null mice. Point mutations within the methylated DNA-binding domain of MeCP2 that cause Rett syndrome or X-linked mental retardation inhibit its interaction with ATRX in vitro and its localization in vivo without affecting methyl-CpG binding. We propose that disruption of the MeCP2-ATRX interaction leads to pathological changes that contribute to mental retardation.

Partial rescue of MeCP2 deficiency by postnatal activation of MeCP2

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

Breathing dysfunctions associated with impaired control of postinspiratory activity in Mecp2-/y knockout mice

Rett syndrome (RTT) is an inborn neurodevelopmental disorder caused by mutations in the X-linked methyl-CpG binding protein 2 gene (MECP2). Besides mental retardation, most patients suffer from potentially life-threatening breathing arrhythmia. To study its pathophysiology, we performed comparative analyses of the breathing phenotype of Mecp2-/y knockout (KO) and C57BL/6J wild-type mice using the perfused working heart-brainstem preparation (WHBP). We simultaneously recorded phrenic and efferent vagal nerve activities to analyse the motor pattern of respiration, discriminating between inspiration, postinspiration and late expiration. Our results revealed respiratory disturbances in KO preparations that were similar to those reported from in vivo measurements in KO mice and also to those seen in RTT patients. The main finding was a highly variable postinspiratory activity in KO mice that correlated closely with breathing arrhythmias leading to repetitive apnoeas even under undisturbed control conditions. Analysis of the pontine and peripheral sensory regulation of postinspiratory activity in KO preparations revealed: (i) prolonged apnoeas associated with enhanced postinspiratory activity after glutamate-induced activation of the pontine Kölliker-Fuse nucleus; and (ii) prolonged apnoeas and lack of reflex desensitization in response to repetitive vagal stimulations. We conclude that impaired network and sensory mediated synaptic control of postinspiration induces severe breathing dysfunctions in Mecp2-/y KO preparations. As postinspiration is particularly important for the control of laryngeal adductors, the finding might explain the upper airway-related clinical problems of patients with RTT such as apnoeas, loss of speech and weak coordination of breathing and swallowing.

Power calculations for likelihood ratio tests for offspring genotype risks, maternal effects, and parent-of-origin (POO) effects in the presence of missing parental genotypes when unaffected siblings are available

Genotype-based likelihood-ratio tests (LRT) of association that examine maternal and parent-of-origin effects have been previously developed in the framework of log-linear and conditional logistic regression models. In the situation where parental genotypes are missing, the expectation-maximization (EM) algorithm has been incorporated in the log-linear approach to allow incomplete triads to contribute to the LRT. We present an extension to this model which we call the Combined_LRT that incorporates additional information from the genotypes of unaffected siblings to improve assignment of incompletely typed families to mating type categories, thereby improving inference of missing parental data. Using simulations involving a realistic array of family structures, we demonstrate the validity of the Combined_LRT under the null hypothesis of no association and provide power comparisons under varying levels of missing data and using sibling genotype data. We demonstrate the improved power of the Combined_LRT compared with the family-based association test (FBAT), another widely used association test. Lastly, we apply the Combined_LRT to a candidate gene analysis in Autism families, some of which have missing parental genotypes. We conclude that the proposed log-linear model will be an important tool for future candidate gene studies, for many complex diseases where unaffected siblings can often be ascertained and where epigenetic factors such as imprinting may play a role in disease etiology.

Genomic imprinting and the social brain

Genomic imprinting refers to the parent-of-origin-specific epigenetic marking of a number of genes. This epigenetic mark leads to a bias in expression between maternally and paternally inherited imprinted genes, that in some cases results in monoallelic expression from one parental allele. Genomic imprinting is often thought to have evolved as a consequence of the intragenomic conflict between the parental alleles that occurs whenever there is an asymmetry of relatedness. The two main examples of asymmetry of relatedness are when there is partiality of parental investment in offspring (as is the case for placental mammals, where there is also the possibility of extended postnatal care by one parent), and in social groups where there is a sex-biased dispersal. From this evolutionary starting point, it is predicted that, at the behavioural level, imprinted genes will influence what can broadly be termed bonding and social behaviour. We examine the animal and human literature for examples of imprinted genes mediating these behaviours, and divide them into two general classes. Firstly, mother-offspring interactions (suckling, attachment and maternal behaviours) that are predicted to occur when partiality in parental investment in early postnatal offspring occurs; and secondly, adult social interactions, when there is an asymmetry of relatedness in social groups. Finally, we return to the evolutionary theory and examine whether there is a pattern of behavioural functions mediated by imprinted genes emerging from the limited data, and also whether any tangible predictions can be made with regards to the direction of action of genes of maternal or paternal origin.