MeCP2: multifaceted roles in gene regulation and neural development

IL-1β, the first piece to the puzzle of sepsis-related cognitive impairment?

Sepsis is a leading cause of death resulting from an uncontrolled inflammatory response to an infectious agent. Multiple organ injuries, including brain injuries, are common in sepsis. The underlying mechanism of sepsis-associated encephalopathy (SAE), which is associated with neuroinflammation, is not yet fully understood. Recent studies suggest that the release of interleukin-1β (IL-1β) following activation of microglial cells plays a crucial role in the development of long-lasting neuroinflammation after the initial sepsis episode. This review provides a comprehensive analysis of the recent literature on the molecular signaling pathways involved in microglial cell activation and interleukin-1β release. It also explores the physiological and pathophysiological role of IL-1β in cognitive function, with a particular focus on its contribution to long-lasting neuroinflammation after sepsis. The findings from this review may assist healthcare providers in developing novel interventions against SAE.

Purkinje-cell-specific MeCP2 deficiency leads to motor deficits and autistic-like behavior due to aberrations in PTP1B-TrkB-SK signaling

Patients with Rett syndrome suffer from a loss-of-function mutation of the Mecp2 gene, which results in various symptoms including autistic traits and motor deficits. Deletion of Mecp2 in the brain mimics part of these symptoms, but the specific function of methyl-CpG-binding protein 2 (MeCP2) in the cerebellum remains to be elucidated. Here, we demonstrate that Mecp2 deletion in Purkinje cells (PCs) reduces their intrinsic excitability through a signaling pathway comprising the small-conductance calcium-activated potassium channel PTP1B and TrkB, the receptor of brain-derived neurotrophic factor. Aberration of this cascade, in turn, leads to autistic-like behaviors as well as reduced vestibulocerebellar motor learning. Interestingly, increasing activity of TrkB in PCs is sufficient to rescue PC dysfunction and abnormal motor and non-motor behaviors caused by Mecp2 deficiency. Our findings highlight how PC dysfunction may contribute to Rett syndrome, providing insight into the underlying mechanism and paving the way for rational therapeutic designs.

MeCP2 ubiquitination and sumoylation, in search of a function†

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

The role of insulin/IGF1 signalling in neurodevelopmental and neuropsychiatric disorders - Evidence from human neuronal cell models

Insulin and insulin-like growth factor 1 (IGF1) signalling play a central role in the development and maintenance of neurons in the brain, and human neurodevelopmental as well as neuropsychiatric disorders have been linked to impaired insulin and IGF1 signalling. This review focuses on the impairments of the insulin and IGF1 signalling cascade in the context of neurodevelopmental and neuropsychiatric disorders, based on evidence from human neuronal cell models. Clear evidence was obtained for impaired insulin and IGF1 receptor downstream signalling in neurodevelopmental disorders, while the evidence for its role in neuropsychiatric disorders was less substantial. Human neuronal model systems can greatly add to our knowledge about insulin/IGF1 signalling in the brain, its role in restoring dendritic maturity, and complement results from clinical studies and animal models. Moreover, they represent a useful model for the development of new therapeutic strategies. Further research is needed to systematically investigate the exact role of the insulin/IGF1 signalling cascades in neurodevelopmental and neuropsychiatric disorders, and to elucidate the respective therapeutic implications.

The diverging role of CDC14B: from mitotic exit in yeast to cell fate control in humans

CDC14, originally identified as crucial mediator of mitotic exit in budding yeast, belongs to the family of dual-specificity phosphatases (DUSPs) that are present in most eukaryotes. Contradicting data have sparked a contentious discussion whether a cell cycle role is conserved in the human paralogs CDC14A and CDC14B but possibly masked due to redundancy. Subsequent studies on CDC14A and CDC14B double knockouts in human and mouse demonstrated that CDC14 activity is dispensable for mitotic progression in higher eukaryotes and instead suggested functional specialization. In this review, we provide a comprehensive overview of our current understanding of how faithful cell division is linked to phosphorylation and dephosphorylation and compare functional similarities and divergences between the mitotic phosphatases CDC14, PP2A, and PP1 from yeast and higher eukaryotes. Furthermore, we review the latest discoveries on CDC14B, which identify this nuclear phosphatase as a key regulator of gene expression and reveal its role in neuronal development. Finally, we discuss CDC14B functions in meiosis and possible implications in other developmental processes.

Maternal Diabetes Deregulates the Expression of Mecp2 via miR-26b-5p in Mouse Embryonic Neural Stem Cells

Maternal diabetes has been associated with a greater risk of neurodevelopmental disorders in offspring. It has been established that hyperglycemia alters the expression of genes and microRNAs (miRNAs) regulating the fate of neural stem cells (NSCs) during brain development. In this study, the expression of methyl-CpG-binding protein-2 (Mecp2), a global chromatin organizer and a crucial regulator of synaptic proteins, was analyzed in NSCs obtained from the forebrain of embryos of diabetic mice. Mecp2 was significantly downregulated in NSCs derived from embryos of diabetic mice when compared to controls. miRNA target prediction revealed that the miR-26 family could regulate the expression of Mecp2, and further validation confirmed that Mecp2 is a target of miR-26b-5p. Knockdown of Mecp2 or overexpression of miR-26b-5p altered the expression of tau protein and other synaptic proteins, suggesting that miR-26b-5p alters neurite outgrowth and synaptogenesis via Mecp2. This study revealed that maternal diabetes upregulates the expression of miR-26b-5p in NSCs, resulting in downregulation of its target, Mecp2, which in turn perturbs neurite outgrowth and expression of synaptic proteins. Overall, hyperglycemia dysregulates synaptogenesis that may manifest as neurodevelopmental disorders in offspring from diabetic pregnancy.

Electroencephalographic (EEG) Biomarkers in Genetic Neurodevelopmental Disorders

Collectively, neurodevelopmental disorders are highly prevalent, but more than a third of neurodevelopmental disorders have an identifiable genetic etiology, each of which is individually rare. The genes associated with neurodevelopmental disorders are often involved in early brain development, neuronal signaling, or synaptic plasticity. Novel treatments for many genetic neurodevelopmental disorders are being developed, but disease-relevant clinical outcome assessments and biomarkers are limited. Electroencephalography (EEG) is a promising noninvasive potential biomarker of brain function. It has been used extensively in epileptic disorders, but its application in neurodevelopmental disorders needs further investigation. In this review, we explore the use of EEG in 3 of the most prevalent genetic neurodevelopmental disorders-Angelman syndrome, Rett syndrome, and fragile X syndrome. Quantitative analyses of EEGs, such as power spectral analysis or measures of connectivity, can quantify EEG signatures seen on qualitative review and potentially correlate with phenotypes. In both Angelman syndrome and Rett syndrome, increased delta power on spectral analysis has correlated with clinical markers of disease severity including developmental disability and seizure burden, whereas spectral power analysis on EEG in fragile X syndrome tends to demonstrate abnormalities in gamma power. Further studies are needed to establish reliable relationships between quantitative EEG biomarkers and clinical phenotypes in rare genetic neurodevelopmental disorders.

The Cell Adhesion Molecule L1 Interacts with Methyl CpG Binding Protein 2 via Its Intracellular Domain

Cell adhesion molecule L1 regulates multiple cell functions, and L1 deficiency is linked to several neural diseases. Recently, we have identified methyl CpG binding protein 2 (MeCP2) as a potential binding partner of the intracellular L1 domain. By ELISA we show here that L1's intracellular domain binds directly to MeCP2 via the sequence motif KDET. Proximity ligation assay with cultured cerebellar and cortical neurons suggests a close association between L1 and MeCP2 in nuclei of neurons. Immunoprecipitation using MeCP2 antibodies and nuclear mouse brain extracts indicates that MeCP2 interacts with an L1 fragment of ~55 kDa (L1-55). Proximity ligation assay indicates that metalloproteases, β-site of amyloid precursor protein cleaving enzyme (BACE1) and ɣ-secretase, are involved in the generation of L1-55. Reduction in MeCP2 expression by siRNA decreases L1-dependent neurite outgrowth from cultured cortical neurons as well as the migration of L1-expressing HEK293 cells. Moreover, L1 siRNA, MeCP2 siRNA, or a cell-penetrating KDET-containing L1 peptide leads to reduced levels of myocyte enhancer factor 2C (Mef2c) mRNA and protein in cortical neurons, suggesting that the MeCP2/L1 interaction regulates Mef2c expression. Altogether, the present findings indicate that the interaction of the novel fragment L1-55 with MeCP2 affects L1-dependent functions, such as neurite outgrowth and neuronal migration.

MeCP2 duplication causes hyperandrogenism by upregulating LHCGR and downregulating RORα

Duplication of MECP2 (methyl-CpG-binding protein 2) gene causes a serious neurological and developmental disorder called MECP2 duplication syndrome (MDS), which is usually found in males. A previous clinical study reported that MDS patient has precocious puberty with hyperandrogenism, suggesting increased MeCP2 may cause male hyperandrogenism. Here we use an MDS mouse model and confirm that MECP2 duplication significantly upregulates androgen levels. We show for the first time that MeCP2 is highly expressed in the Leydig cells of testis, where androgen is synthesized. Mechanistically, MECP2 duplication increases androgen synthesis and decreases androgen to estrogen conversion through either the upregulation of luteinizing hormone receptor (LHCGR) in testis, as a result of MeCP2 binds to G-quadruplex structure of Lhcgr promoter and recruits the transcription activator CREB1 or the downregulation of the expression of aromatase in testis by binding the CpG island of Rorα, an upstream regulator of aromatase. Taken together, we demonstrate that MeCP2 plays an important role in androgen synthesis, supporting a novel non-CNS function of MeCP2 in the process of sex hormone synthesis.

Molecular and clinical insights into complex genomic rearrangements related to MECP2 duplication syndrome

MECP2 duplication syndrome (MDS) is caused by copy number variation (CNV) spanning the MECP2 gene at Xq28 and is a major cause of intellectual disability (ID) in males. Herein, we describe two unrelated males harboring non-recurrent complex Xq28 rearrangements associated with MDS. Copy number gains were initially detected by quantitative real-time polymerase chain reaction and further delineated by high-resolution array comparative genomic hybridization, familial segregation, expression analysis and X-chromosome inactivation (XCI) evaluation in a carrier mother. SNVs within the rearrangements and/or fluorescent in situ hybridization (FISH) were used to assess the parental origin of the rearrangements. Patient 1 exhibited an intrachromosomal rearrangement, whose structure is consistent with a triplicated segment presumably embedded in an inverted orientation between two duplicated sequences (DUP-TRP/INV-DUP). The rearrangement was inherited from the carrier mother, who exhibits extreme XCI skewing and subtle psychiatric symptoms. Patient 2 presented a de novo (X;Y) unbalanced translocation resulting in duplication of Xq28 and deletion of Yp, originated in the paternal gametogenesis. Neurodevelopmental trajectory and non-neurological symptoms were consistent with previous reports, with the exception of cerebellar vermis hypoplasia in patient 2. Although both patients share the core MDS phenotype, patient 1 showed MECP2 transcript levels in blood similar to controls. Understanding the molecular mechanisms related to MDS is essential for designing targeted therapeutic strategies.

Transcriptomic and Epigenomic Landscape in Rett Syndrome

Rett syndrome (RTT) is an extremely invalidating, cureless, developmental disorder, and it is considered one of the leading causes of intellectual disability in female individuals. The vast majority of RTT cases are caused by de novo mutations in the X-linked Methyl-CpG binding protein 2 (MECP2) gene, which encodes a multifunctional reader of methylated DNA. MeCP2 is a master epigenetic modulator of gene expression, with a role in the organization of global chromatin architecture. Based on its interaction with multiple molecular partners and the diverse epigenetic scenario, MeCP2 triggers several downstream mechanisms, also influencing the epigenetic context, and thus leading to transcriptional activation or repression. In this frame, it is conceivable that defects in such a multifaceted factor as MeCP2 lead to large-scale alterations of the epigenome, ranging from an unbalanced deposition of epigenetic modifications to a transcriptional alteration of both protein-coding and non-coding genes, with critical consequences on multiple downstream biological processes. In this review, we provide an overview of the current knowledge concerning the transcriptomic and epigenomic alterations found in RTT patients and animal models.

Role of miR-132/methyl-CpG-binding protein 2 in the regulation of neural stem cell differentiation

Methyl-CpG-binding protein 2 (MeCP2) is a well-known transcription repressor, and mutations in MECP2 cause serious neurological disorders. Many studies have suggested that MeCP2 is involved in neural maturation only, and have not reported its role in neural stem cell differentiation. In the present study, we investigated this possible role of MeCP2 in neural stem cells. We used two different differentiation methods to explore how MeCP2 influences neural stem cell differentiation. When we transfected MeCP2-overexpressing lentivirus into neural stem cells, astrocytic differentiation was impaired. This impaired astrocytic differentiation occurred even in conditions of 20% fetal bovine serum, which favored astrocytic differentiation. In addition, miR-132 had the largest expression change after differentiation among several central nervous system related miRNAs. A luciferase assay confirmed that miR-132 directly targeted MeCP2, and that miR-132 was able to reduce MeCP2 expression at both the RNA and protein levels. The upregulation of miR-132 by miRNA mimics promoted astrocytic differentiation, which was fully recovered by MeCP2 overexpression. These results indicate that miR-132 regulates cell lineage differentiation by reducing MeCP2. The study was approved by the Ethics Committee of Shanghai Tenth People's Hospital of TongJi University, China (approval No. SHDSYY-2018-4748) on March 10, 2018.

Genome Editing for CNS Disorders

Central nervous system (CNS) disorders have a social and economic burden on modern societies, and the development of effective therapies is urgently required. Gene editing may prevent or cure a disease by inducing genetic changes at endogenous loci. Genome editing includes not only the insertion, deletion or replacement of nucleotides, but also the modulation of gene expression and epigenetic editing. Emerging technologies based on ZFs, TALEs, and CRISPR/Cas systems have extended the boundaries of genome manipulation and promoted genome editing approaches to the level of promising strategies for counteracting genetic diseases. The parallel development of efficient delivery systems has also increased our access to the CNS. In this review, we describe the various tools available for genome editing and summarize in vivo preclinical studies of CNS genome editing, whilst considering current limitations and alternative approaches to overcome some bottlenecks.

Functional Network Mapping Reveals State-Dependent Response to IGF1 Treatment in Rett Syndrome

Rett Syndrome (RTT) is a neurodevelopmental disorder associated with mutations in the gene MeCP2, which is involved in the development and function of cortical networks. The clinical presentation of RTT is generally severe and includes developmental regression and marked neurologic impairment. Insulin-Like growth factor 1 (IGF1) ameliorates RTT-relevant phenotypes in animal models and improves some clinical manifestations in early human trials. However, it remains unclear whether IGF1 treatment has an impact on cortical electrophysiology in line with MeCP2’s role in network formation, and whether these electrophysiological changes are related to clinical response. We performed clinical assessments and resting-state electroencephalogram (EEG) recordings in eighteen patients with classic RTT, nine of whom were treated with IGF1. Among the treated patients, we distinguished those who showed improvements after treatment (responders) from those who did not show any changes (nonresponders). Clinical assessments were carried out for all individuals with RTT at baseline and 12 months after treatment. Network measures were derived using statistical modelling techniques based on interelectrode coherence measures. We found significant interaction between treatment groups and timepoints, indicating an effect of IGF1 on network measures. We also found a significant effect of responder status and timepoint, indicating that these changes in network measures are associated with clinical response to treatment. Further, we found baseline variability in network characteristics, and a machine learning model using these measures applied to pretreatment data predicted treatment response with 100% accuracy (100% sensitivity and 100% specificity) in this small patient group. These results highlight the importance of network pathology in RTT, as well as providing preliminary evidence for the potential of network measures as tools for the characterisation of disease subtypes and as biomarkers for clinical trials.

The cell adhesion molecule L1 interacts with nuclear proteins via its intracellular domain

Proteolytic cleavage of the cell adhesion molecule L1 (L1) in brain tissue and in cultured cerebellar neurons results in the generation and nuclear import of a 30 kDa fragment comprising most of L1's C-terminal, intracellular domain. In search of molecules that interact with this domain, we performed affinity chromatography with the recombinant intracellular L1 domain and a nuclear extract from mouse brains, and identified potential nuclear L1 binding partners involved in transcriptional regulation, RNA processing and transport, DNA repair, chromatin remodeling, and nucleocytoplasmic transport. By co-immunoprecipitation and enzyme-linked immunosorbent assay using recombinant proteins, we verified the direct interaction between L1 and the nuclear binding partners non-POU domain containing octamer-binding protein and splicing factor proline/glutamine-rich. The proximity ligation assay confirmed this close interaction in cultures of cerebellar granule cells. Our findings suggest that L1 fragments regulate multiple nuclear functions in the nervous system. We discuss possible physiological and pathological roles of these interactions in regulation of chromatin structure, gene expression, RNA processing, and DNA repair.

MECP2 mutations affect ciliogenesis: a novel perspective for Rett syndrome and related disorders

Mutations in MECP2 cause several neurological disorders of which Rett syndrome (RTT) represents the best-defined condition. Although mainly working as a transcriptional repressor, MeCP2 is a multifunctional protein revealing several activities, the involvement of which in RTT remains obscure. Besides being mainly localized in the nucleus, MeCP2 associates with the centrosome, an organelle from which primary cilia originate. Primary cilia function as "sensory antennae" protruding from most cells, and a link between primary cilia and mental illness has recently been reported. We herein demonstrate that MeCP2 deficiency affects ciliogenesis in cultured cells, including neurons and RTT fibroblasts, and in the mouse brain. Consequently, the cilium-related Sonic Hedgehog pathway, which is essential for brain development and functioning, is impaired. Microtubule instability participates in these phenotypes that can be rescued by HDAC6 inhibition together with the recovery of RTT-related neuronal defects. Our data indicate defects of primary cilium as a novel pathogenic mechanism that by contributing to the clinical features of RTT might impact on proper cerebellum/brain development and functioning, thus providing a novel therapeutic target.

MeCP2 in cholinergic interneurons of nucleus accumbens regulates fear learning

Methyl-CpG-binding protein 2 (MeCP2) encoded by the MECP2 gene is a transcriptional regulator whose mutations cause Rett syndrome (RTT). Mecp2-deficient mice show fear regulation impairment; however, the cellular and molecular mechanisms underlying this abnormal behavior are largely uncharacterized. Here, we showed that Mecp2 gene deficiency in cholinergic interneurons of the nucleus accumbens (NAc) dramatically impaired fear learning. We further found that spontaneous activity of cholinergic interneurons in Mecp2-deficient mice decreased, mediated by enhanced inhibitory transmission via α2-containing GABA_A receptors. With MeCP2 restoration, opto- and chemo-genetic activation, and RNA interference in ChAT-expressing interneurons of the NAc, impaired fear retrieval was rescued. Taken together, these results reveal a previously unknown role of MeCP2 in NAc cholinergic interneurons in fear regulation, suggesting that modulation of neurons in the NAc may ameliorate fear-related disorders.

miRNA regulation of social and anxiety-related behaviour

Neuropsychiatric disorders, including autism spectrum disorders (ASD) and anxiety disorders are characterized by a complex range of symptoms, including social behaviour and cognitive deficits, depression and repetitive behaviours. Although the mechanisms driving pathophysiology are complex and remain largely unknown, advances in the understanding of gene association and gene networks are providing significant clues to their aetiology. In recent years, small noncoding RNA molecules known as microRNA (miRNA) have emerged as a new gene regulatory layer in the pathophysiology of mental illness. These small RNAs can bind to the 3'-UTR of mRNA thereby negatively regulating gene expression at the post-transcriptional level. Their ability to regulate hundreds of target mRNAs simultaneously predestines them to control the activity of entire cellular pathways, with obvious implications for the regulation of complex processes such as animal behaviour. There is growing evidence to suggest that numerous miRNAs are dysregulated in pathophysiology of neuropsychiatric disorders, and there is strong genetic support for the association of miRNA genes and their targets with several of these conditions. This review attempts to cover the most relevant microRNAs for which an important contribution to the control of social and anxiety-related behaviour has been demonstrated by functional studies in animal models. In addition, it provides an overview of recent expression profiling and genetic association studies in human patient-derived samples in an attempt to highlight the most promising candidates for biomarker discovery and therapeutic intervention.

Epigenetics and epilepsy prevention: The therapeutic potential of adenosine and metabolic therapies

Prevention of epilepsy and its progression remains the most urgent need for epilepsy research and therapy development. Novel conceptual advances are required to meaningfully address this fundamental challenge. Maladaptive epigenetic changes, which include methylation of DNA and acetylation of histones - among other mechanisms, are now well recognized to play a functional role in the development of epilepsy and its progression. The methylation hypothesis of epileptogenesis suggests that changes in DNA methylation are implicated in the progression of the disease. In this context, global DNA hypermethylation is particularly associated with chronic epilepsy. Likewise, acetylation changes of histones have been linked to epilepsy development. Clinical as well as experimental evidence demonstrate that epilepsy and its progression can be prevented by metabolic and biochemical manipulations that target previously unrecognized epigenetic functions contributing to epilepsy development and maintenance of the epileptic state. This review will discuss epigenetic mechanisms implicated in epilepsy development as well as metabolic and biochemical interactions thought to drive epileptogenesis. Therefore, metabolic and biochemical mechanisms are identified as novel targets for epilepsy prevention. We will specifically discuss adenosine biochemistry as a novel therapeutic strategy to reconstruct the DNA methylome as antiepileptogenic strategy as well as metabolic mediators, such as beta-hydroxybutyrate, which affect histone acetylation. Finally, metabolic dietary interventions (such as the ketogenic diet) which have the unique potential to prevent epileptogenesis through recently identified epigenetic mechanisms will be reviewed. This article is part of the special issue entitled 'New Epilepsy Therapies for the 21st Century - From Antiseizure Drugs to Prevention, Modification and Cure of Epilepsy'.

Reversal of Social Recognition Deficit in Adult Mice with MECP2 Duplication via Normalization of MeCP2 in the Medial Prefrontal Cortex

Methyl-CpG binding protein 2 (MeCP2) is a basic nuclear protein involved in the regulation of gene expression and microRNA processing. Duplication of MECP2-containing genomic segments causes MECP2 duplication syndrome, a severe neurodevelopmental disorder characterized by intellectual disability, motor dysfunction, heightened anxiety, epilepsy, autistic phenotypes, and early death. Reversal of the abnormal phenotypes in adult mice with MECP2 duplication (MECP2-TG) by normalizing the MeCP2 levels across the whole brain has been demonstrated. However, whether different brain areas or neural circuits contribute to different aspects of the behavioral deficits is still unknown. Here, we found that MECP2-TG mice showed a significant social recognition deficit, and were prone to display aversive-like behaviors, including heightened anxiety-like behaviors and a fear generalization phenotype. In addition, reduced locomotor activity was observed in MECP2-TG mice. However, appetitive behaviors and learning and memory were comparable in MECP2-TG and wild-type mice. Functional magnetic resonance imaging illustrated that the differences between MECP2-TG and wild-type mice were mainly concentrated in brain areas regulating emotion and social behaviors. We used the CRISPR-Cas9 method to restore normal MeCP2 levels in the medial prefrontal cortex (mPFC) and bed nuclei of the stria terminalis (BST) of adult MECP2-TG mice, and found that normalization of MeCP2 levels in the mPFC but not in the BST reversed the social recognition deficit. These data indicate that the mPFC is responsible for the social recognition deficit in the transgenic mice, and provide new insight into potential therapies for MECP2 duplication syndrome.

Genetic Causes and Modifiers of Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is one of the most prevalent neurodevelopmental disorders, affecting an estimated 1 in 59 children. ASD is highly genetically heterogeneous and may be caused by both inheritable and de novo gene variations. In the past decade, hundreds of genes have been identified that contribute to the serious deficits in communication, social cognition, and behavior that patients often experience. However, these only account for 10-20% of ASD cases, and patients with similar pathogenic variants may be diagnosed on very different levels of the spectrum. In this review, we will describe the genetic landscape of ASD and discuss how genetic modifiers such as copy number variation, single nucleotide polymorphisms, and epigenetic alterations likely play a key role in modulating the phenotypic spectrum of ASD patients. We also consider how genetic modifiers can alter convergent signaling pathways and lead to impaired neural circuitry formation. Lastly, we review sex-linked modifiers and clinical implications. Further understanding of these mechanisms is crucial for both comprehending ASD and for developing novel therapies.

Clinical and genetic Rett syndrome variants are defined by stable electrophysiological profiles

BACKGROUND

Rett Syndrome (RTT) is a complex neurodevelopmental disorder, frequently associated with epilepsy. Despite increasing recognition of the clinical heterogeneity of RTT and its variants (e.g Classical, Hanefeld and PSV(Preserved Speech Variant)), the link between causative mutations and observed clinical phenotypes remains unclear. Quantitative analysis of electroencephalogram (EEG) recordings may further elucidate important differences between the different clinical and genetic forms of RTT.

METHODS

Using a large cohort (n = 42) of RTT patients, we analysed the electrophysiological profiles of RTT variants (genetic and clinical) in addition to epilepsy status (no epilepsy/treatment-responsive epilepsy/treatment-resistant epilepsy). The distribution of spectral power and inter-electrode coherence measures were derived from continuous resting-state EEG recordings.

RESULTS

RTT genetic variants (MeCP2/CDLK5) were characterised by significant differences in network architecture on comparing first principal components of inter-electrode coherence across all frequency bands (p < 0.0001). Greater coherence in occipital and temporal pairs were seen in MeCP2 vs CDLK5 variants, the main drivers in between group differences. Similarly, clinical phenotypes (Classical RTT/Hanefeld/PSV) demonstrated significant differences in network architecture (p < 0.0001). Right tempero-parietal connectivity was found to differ between groups (p = 0.04), with greatest coherence in the Classical RTT phenotype. PSV demonstrated a significant difference in left-sided parieto-occipital coherence (p = 0.026). Whilst overall power decreased over time, there were no difference in asymmetry and inter-electrode coherence profiles over time. There was a significant difference in asymmetry in the overall power spectra between epilepsy groups (p = 0.04) in addition to occipital asymmetry across all frequency bands. Significant differences in network architecture were also seen across epilepsy groups (p = 0.044).

CONCLUSIONS

Genetic and clinical variants of RTT are characterised by discrete patterns of inter-electrode coherence and network architecture which remain stable over time. Further, hemispheric distribution of spectral power and measures of network dysfunction are associated with epilepsy status and treatment responsiveness. These findings support the role of discrete EEG profiles as non-invasive biomarkers in RTT and its genetic/clinical variants.

Alterations in the MicroRNA of the Blood of Autism Spectrum Disorder Patients: Effects on Epigenetic Regulation and Potential Biomarkers

Aims: Autism spectrum disorder (ASD) refers to a group of heterogeneous brain-based neurodevelopmental disorders with different levels of symptom severity. Given the challenges, the clinical diagnosis of ASD is based on information gained from interviews with patients’ parents. The heterogeneous pathogenesis of this disorder appears to be driven by genetic and environmental interactions, which also plays a vital role in predisposing individuals to ASD with different commitment levels. In recent years, it has been proposed that epigenetic modifications directly contribute to the pathogenesis of several neurodevelopmental disorders, such as ASD. The microRNAs (miRNAs) comprises a species of short noncoding RNA that regulate gene expression post-transcriptionally and have an essential functional role in the brain, particularly in neuronal plasticity and neuronal development, and could be involved in ASD pathophysiology. The aim of this study is to evaluate the expression of blood miRNA in correlation with clinical findings in patients with ASD, and to find possible biomarkers for the disorder. Results: From a total of 26 miRNA studied, seven were significantly altered in ASD patients, when compared to the control group: miR34c-5p, miR92a-2-5p, miR-145-5p and miR199a-5p were up-regulated and miR27a-3p, miR19-b-1-5p and miR193a-5p were down-regulated in ASD patients. Discussion: The main targets of these miRNAs are involved in immunological developmental, immune response and protein synthesis at transcriptional and translational levels. The up-regulation of both miR-199a-5p and miR92a-2a and down-regulation of miR-193a and miR-27a was observed in AD patients, and may in turn affect the SIRT1, HDAC2, and PI3K/Akt-TSC:mTOR signaling pathways. Furthermore, MeCP2 is a target of miR-199a-5p, and is involved in Rett Syndrome (RTT), which possibly explains the autistic phenotype in male patients with this syndrome.

The Communication Between the Immune and Nervous Systems: The Role of IL-1β in Synaptopathies

In the last 15 years, groundbreaking genetic progress has underlined a convergence onto coherent synaptic pathways for most psychiatric and neurodevelopmental disorders, which are now collectively called “synaptopathies.” However, the modest size of inheritance detected so far indicates a multifactorial etiology for these disorders, underlining the key contribution of environmental effects to them. Inflammation is known to influence the risk and/or severity of a variety of synaptopathies. In particular, pro-inflammatory cytokines, produced and released in the brain by activated astrocytes and microglia, may play a pivotal role in these pathologies. Although the link between immune system activation and defects in cognitive processes is nowadays clearly established, the knowledge of the molecular mechanisms by which inflammatory mediators specifically hit synaptic components implicated in synaptopathies is still in its infancy. This review summarizes recent evidence showing that the pro-inflammatory cytokine interleukin-1β (IL-1β) specifically targets synaptopathy molecular substrate, leading to memory defects and pathological processes. In particular, we describe three specific pathways through which IL-1β affects (1) synaptic maintenance/dendritic complexity, (2) spine morphology, and (3) the excitatory/inhibitory balance. We coin the term immune synaptopathies to identify this class of diseases.

Tyr120Asp mutation alters domain flexibility and dynamics of MeCP2 DNA binding domain leading to impaired DNA interaction: Atomistic characterization of a Rett syndrome causing mutation

Mutations in the X-linked MECP2 gene represent the main origin of Rett syndrome, causing a profound intellectual disability in females. MeCP2 is an epigenetic transcriptional regulator containing two main functional domains: a methyl-CpG binding domain (MBD) and a transcription repression domain (TRD). Over 600 pathogenic mutations were reported to affect the whole protein; almost half of missense mutations affect the MBD. Understanding the impact of these mutations on the MBD structure and interaction with DNA will foster the comprehension of their pathogenicity and possibly genotype/phenotype correlation studies. Herein, we use molecular dynamics simulations to obtain a detailed view of the dynamics of WT and mutated MBD in the presence and absence of DNA. The pathogenic mutation Y120D is used as paradigm for our studies. Further, since the Y120 residue was previously found to be a phosphorylation site, we characterize the dynamic profile of the MBD also in the presence of Y120 phosphorylation (pY120). We found that addition of a phosphate group to Y120 or mutation in aspartic acid affect domain mobility that samples an alternative conformational space with respect to the WT, leading to impaired ability to interact with DNA. Experimental assays showing a significant reduction in the binding affinity between the mutated MBD and the DNA confirmed our predictions.

Puerarin Up-regulates Methyl-CpG Binding Protein 2 Phosphorylation in Hippocampus of Vascular Dementia Rats

OBJECTIVE

To observe the effect of puerarin on methyl-CpG binding protein 2 (MeCP2) phosphorylation (pMeCP2) in the hippocampus of a rat model of vascular dementia (VD).

METHODS

Thirty-six healthy Sprague-Dawley rats were randomly assigned to the sham-operated group, dementia group and puerarintreated group using a random number table (n=12 per group). The modifified permanent bilateral common carotid artery occlusion method was used to establish the VD model. The sham-operated and dementia groups were given 2 mL/d of saline, while the puerarin-treated group was given 100 mg/(kg•d) of puerarin for 17 days. The learning and memory abilities were evaluated by the Morris water maze test. Hematoxylin-eosin staining, immunohistochemical (IHC) staining and Western blot analysis were carried out to observe changes in neuron morphology and in level of pMeCP2 in the hippocampus, respectively.

RESULTS

The morphologies of rat hippocampal neurons in the puerarintreated group were markedly improved compared with the dementia group. The escape latency of the dementia group was significantly longer than the sham-operated group (P<0.05), while the puerarin-treated group was obviously shorter than the dementia group (P<0.05). Cross-platform times of the dementia group were signifificantly decreased compared with the sham-operated group (P<0.05), while the puerarin-treated group was obviously increased compared with the dementia group (P<0.05). IHC staining showed no significant difference in the number of MeCP2 positive cells among 3 groups (P>0.05). The number of pMeCP2 positive cells in the CA1 region of hippocampus in the dementia group was signifificantly increased compared with the sham-operated group, and the puerarin-treated group was signifificantly increased compared with the dementia group (both P<0.05). Western blot analysis showed no signifificant difference of MeCP2 expression among 3 groups (P>0.05). The expression of pMeCP2 in the dementia group was signifificantly increased compared with the sham-operated group, while it in the puerarin-treated group was signifificantly increased compared with the dementia group (P<0.05).

CONCLUSION

Puerarin could play a role in the protection of nerve cells through up-regulating pMeCP2 in the hippocampus, improving neuron morphologies, and enhancing learning and memory ablities in a rat model of VD.

Motor, Somatosensory, Viscerosensory and Metabolic Impairments in a Heterozygous Female Rat Model of Rett Syndrome

Rett Syndrome (RTT), an autism-related disorder caused by mutation of the X-linked Methyl CpG-binding Protein 2 (MECP2) gene, is characterized by severe cognitive and intellectual deficits. While cognitive deficits are well-documented in humans and rodent models, impairments of sensory, motor and metabolic functions also occur but remain poorly understood. To better understand non-cognitive deficits in RTT, we studied female rats heterozygous for Mecp2 mutation (Mecp2^−/x); unlike commonly used male Mecp2^−/y rodent models, this more closely approximates human RTT where males rarely survive. Mecp2^−/x rats showed rapid, progressive decline of motor coordination through six months of age as assessed by rotarod performance, accompanied by deficits in gait and posture. Mecp2^−/x rats were hyper-responsive to noxious pressure and cold, but showed visceral hyposensitivity when tested by colorectal distension. Mecp2^−/x rats ate less, drank more, and had more body fat resulting in increased weight gain. Our findings reveal an array of progressive non-cognitive deficits in this rat model that are likely to contribute to the compromised quality of life that characterizes RTT.

Identification of autism-related MECP2 mutations by whole-exome sequencing and functional validation

BACKGROUND

Methyl-CpG-binding protein-2 (MeCP2) is a critical regulator for neural development. Either loss- or gain-of-function leads to severe neurodevelopmental disorders, such as Rett syndrome (RTT) and autism spectrum disorder (ASD). We set out to screen for MECP2 mutations in patients of ASD and determine whether these autism-related mutations may compromise the proper function of MeCP2.

METHODS

Whole-exome sequencing was performed to screen MECP2 and other ASD candidate genes for 120 patients diagnosed with ASD. The parents of patients who were identified with MECP2 mutation were selected for further Sanger sequencing. Each patient accomplished the case report form including general information and clinical scales applied to assess their clinical features. Mouse cortical neurons and HEK-293 cells were cultured and transfected with MeCP2 wild-type (WT) or mutant to examine the function of autism-associated MeCP2 mutants. HEK-293 cells were used to examine the expression of MeCP2 mutant constructs with Western blot. Mouse cortical neurons were used to analyze neurites and axon outgrowth by immunofluorescence experiments.

RESULTS

We identified three missense mutations of MECP2 from three autism patients by whole-exome sequencing: p.P152L (c.455C>T), p.P376S (c.1162C>T), and p.R294X (c.880C>T). Among these mutations, p.P152L and p.R294X were de novo mutations, whereas p.P376S was inherited maternally. The diagnosis of RTT was excluded in all three autism patients. Abnormalities of dendritic and axonal growth were found after autism-related MeCP2 mutants were expressed in mouse cortical neurons; suggesting that autism-related MECP2 mutations impair the proper development of neurons.

CONCLUSIONS

Our study identified genetic mutations of the MECP2 gene in autism patients, which were previously considered to be associated primarily with RTT. This finding suggests that loss-of-function mutations of MECP2 may also lead to autism spectrum disorders.

Identification of autism-related MECP2 mutations by whole-exome sequencing and functional validation

Background

Methyl-CpG-binding protein-2 (MeCP2) is a critical regulator for neural development. Either loss- or gain-of-function leads to severe neurodevelopmental disorders, such as Rett syndrome (RTT) and autism spectrum disorder (ASD). We set out to screen for MECP2 mutations in patients of ASD and determine whether these autism-related mutations may compromise the proper function of MeCP2.

Methods

Whole-exome sequencing was performed to screen MECP2 and other ASD candidate genes for 120 patients diagnosed with ASD. The parents of patients who were identified with MECP2 mutation were selected for further Sanger sequencing. Each patient accomplished the case report form including general information and clinical scales applied to assess their clinical features. Mouse cortical neurons and HEK-293 cells were cultured and transfected with MeCP2 wild-type (WT) or mutant to examine the function of autism-associated MeCP2 mutants. HEK-293 cells were used to examine the expression of MeCP2 mutant constructs with Western blot. Mouse cortical neurons were used to analyze neurites and axon outgrowth by immunofluorescence experiments.

Results

We identified three missense mutations of MECP2 from three autism patients by whole-exome sequencing: p.P152L (c.455C>T), p.P376S (c.1162C>T), and p.R294X (c.880C>T). Among these mutations, p.P152L and p.R294X were de novo mutations, whereas p.P376S was inherited maternally. The diagnosis of RTT was excluded in all three autism patients. Abnormalities of dendritic and axonal growth were found after autism-related MeCP2 mutants were expressed in mouse cortical neurons; suggesting that autism-related MECP2 mutations impair the proper development of neurons.

Conclusions

Our study identified genetic mutations of the MECP2 gene in autism patients, which were previously considered to be associated primarily with RTT. This finding suggests that loss-of-function mutations of MECP2 may also lead to autism spectrum disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s13229-017-0157-5) contains supplementary material, which is available to authorized users.

Prenatal exposure to valproic acid increases miR-132 levels in the mouse embryonic brain

Background

MicroRNAs, small non-coding RNAs, are highly expressed in the mammalian brain, and the dysregulation of microRNA levels may be involved in neurodevelopmental disorders such as autism spectrum disorder (ASD). In the present study, we examined whether prenatal valproic acid (VPA) exposure affects levels of microRNAs, especially the brain specific and enriched microRNAs, in the mouse embryonic brain.

Results

Prenatal exposure to VPA at E12.5 immediately increased miR-132 levels, but not miR-9 or miR-124 levels, in the male embryonic brain. Prenatal exposure to VPA at E12.5 also increased miR-132 levels in the female embryonic brain. We further found that the prenatal exposure to VPA at E12.5 increased mRNA levels of Arc, c-Fos and brain-derived neurotrophic factor in both male and female embryonic brains, prior to miR-132 expression. In contrast, prenatal exposure to VPA at E14.5 did not affect miR-132 levels in either male or female embryonic brain. The prenatal VPA exposure at E12.5 also decreased mRNA levels of methyl-CpG-binding protein 2 and Rho GTPase-activating protein p250GAP, both of which are molecular targets of miR-132. Furthermore, RNA sequence analysis revealed that prenatal VPA exposure caused changes in several microRNA levels other than miR-132 in the embryonic whole brain.

Conclusions

These findings suggest that the alterations in neuronal activity-dependent microRNAs levels, including an increased level of miR-132, in the embryonic period, at least in part, underlie the ASD-like behaviors and cortical pathology produced by prenatal VPA exposure.

MicroRNAs as biomarkers for psychiatric disorders with a focus on autism spectrum disorder: Current progress in genetic association studies, expression profiling, and translational research

MicroRNAs (miRNAs) are a group of small noncoding RNA molecules, 18-25 nucleotides in length, which can negatively regulate gene expression at the post-transcriptional level by binding to messenger RNAs. About half of all identified miRNAs in humans are expressed in the brain and display regulatory functions important for many biological processes related to the development of the central nervous system (CNS). Disruptions in miRNA biogenesis and miRNA-target interaction have been related to CNS diseases, including psychiatric disorders. In this review, we focus on the role of miRNAs in autism spectrum disorder (ASD) and summarize recent findings about ASD-associated genetic variants in miRNA genes, in miRNA biogenesis genes, and miRNA targets. We discuss deregulation of miRNA expression in ASD and functional validation of ASD-related miRNAs in animal models. Including miRNAs in studies of ASD will contribute to our understanding of its etiology and pathogenesis and facilitate the discrimination between different disease subgroups. Autism Res 2017. © 2017 International Society for Autism Research, Wiley Periodicals, Inc. Autism Res 2017, 10: 1184-1203. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.

Expression of Phospho-MeCP2s in the Developing Rat Brain and Function of Postnatal MeCP2 in Cerebellar Neural Cell Development

Abnormal expression and dysfunction of methyl-CpG binding protein 2 (MeCP2) cause Rett syndrome (RTT). The diverse phosphorylation modifications modulate MeCP2 function in neural cells. Using western blot and immunohistochemistry, we examined the expression patterns of MeCP2 and three phospho-MeCP2s (pMeCP2s) in the developing rat brain. The expression of MeCP2 and phospho-S80 (pS80) MeCP2 increased while pS421 MeCP2 and pS292 MeCP2 decreased with brain maturation. In contrast to the nuclear localization of MeCP2 and pS80 MeCP2, pS421 MeCP2 and pS292 MeCP2 were mainly expressed in the cytoplasmic compartment. Apart from their distribution in neurons, they were also detected at a low level in astrocytes. Postnatally-initiated MeCP2 deficiency affected cerebellar neural cell development, as determined by the abnormal expression of GFAP, DCX, Tuj1, MAP-2, and calbindin-D28k. Together, these results demonstrate that MeCP2 and diverse pMeCP2s have distinct features of spatio-temporal expression in the rat brain, and that the precise levels of MeCP2 in the postnatal period are vital to cerebellar neural cell development.

Therapeutic Potential of Transcranial Focused Ultrasound for Rett Syndrome

Rett syndrome (RTT) is a severe neurodevelopmental disorder occurring almost exclusively in females and is caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2) in the majority of cases. MeCP2 is essential for the normal function of nerve cells, including neuronal development, maturation, and synaptic activity. RTT is characterized by normal early development followed by autistic-like features, slowed brain and head growth, gait abnormalities, seizures, breathing irregularities, and cognitive disabilities. Medical management in RTT remains supportive and symptomatic. Brain-derived neurotrophic factor (BDNF) has been implicated in the pathophysiology of RTT. Recent studies have shown a phenotypic reversal by increasing BDNF expression in a RTT mouse model. Thus, manipulation of BDNF expression/signaling in the brain could be therapeutic for this disease. Transcranial focused ultrasound for (tFUS) can noninvasively focally modulate human cortical function, stimulate neurogenesis, and increase BDNF in animal studies. Consequently, tFUS may be of therapeutic potential for Rett syndrome. Further evaluation of the therapeutic effects of tFUS in Mecp2 deficient animal models is needed before clinical trials can begin.

Genome-wide redistribution of MeCP2 in dorsal root ganglia after peripheral nerve injury

Background

Methyl-CpG-binding protein 2 (MeCP2), a protein with affinity for methylated cytosines, is crucial for neuronal development and function. MeCP2 regulates gene expression through activation, repression and chromatin remodeling. Mutations in MeCP2 cause Rett syndrome, and these patients display impaired nociception. We observed an increase in MeCP2 expression in mouse dorsal root ganglia (DRG) after peripheral nerve injury. The functional implication of increased MeCP2 is largely unknown. To identify regions of the genome bound by MeCP2 in the DRG and the changes induced by nerve injury, a chromatin immunoprecipitation of MeCP2 followed by sequencing (ChIP-seq) was performed 4 weeks after spared nerve injury (SNI).

Results

While the number of binding sites across the genome remained similar in the SNI model and sham control, SNI induced the redistribution of MeCP2 to transcriptionally relevant regions. To determine how differential binding of MeCP2 can affect gene expression in the DRG, we investigated mmu-miR-126, a microRNA locus that had enriched MeCP2 binding in the SNI model. Enriched MeCP2 binding to miR-126 locus after nerve injury repressed miR-126 expression, and this was not mediated by alterations in methylation pattern at the miR-126 locus. Downregulation of miR-126 resulted in the upregulation of its two target genes Dnmt1 and Vegfa in Neuro 2A cells and in SNI model compared to control. These target genes were significantly downregulated in Mecp2-null mice compared to wild-type littermates, indicating a regulatory role for MeCP2 in activating Dnmt1 and Vegfa expression. Intrathecal delivery of miR-126 was not sufficient to reverse nerve injury-induced mechanical and thermal hypersensitivity, but decreased Dnmt1 and Vegfa expression in the DRG.

Conclusions

Our study shows a regulatory role for MeCP2 in that changes in global redistribution can result in direct and indirect modulation of gene expression in the DRG. Alterations in genome-wide binding of MeCP2 therefore provide a molecular basis for a better understanding of epigenetic regulation-induced molecular changes underlying nerve injury.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-016-0073-5) contains supplementary material, which is available to authorized users.

Specificity of antinuclear autoantibodies recognizing the dense fine speckled nuclear pattern: Preferential targeting of DFS70/LEDGFp75 over its interacting partner MeCP2

Human antinuclear autoantibodies (ANAs) targeting the dense fine speckled (DFS) nuclear protein DFS70, commonly known as lens epithelium derived growth factor p75 (LEDGFp75), present a clinical puzzle since their significance remains elusive. While their frequencies are low in ANA-positive autoimmune rheumatic diseases, they are relatively elevated in clinical laboratory referrals, diverse inflammatory conditions, and 'apparently' healthy individuals. We reported previously that DFS70/LEDGFp75 is an autoantigen in prostate cancer that closely interacts with another 70kD DFS nuclear protein, methyl CpG binding protein 2 (MeCP2). This led us to investigate if anti-DFS sera exclusively target DFS70/LEDGFp75 or also recognize MeCP2. Using several complementary autoantibody detection platforms and cellular/molecular approaches we evaluated 65 human sera producing anti-DFS autoantibodies. Our results show that these antibodies are highly specific for DFS70/LEDGFp75 and do not target MeCP2. Establishing the specificity of anti-DFS autoantibodies has implications for increasing our understanding of their biological significance and clinical utility.

Chromatin remodeling gene EZH2 involved in the genetic etiology of autism in Chinese Han population

Autism spectrum disorder (ASD) is a group of severe neurodevelopmental disorders. Epigenetic factors play a critical role in the etiology of ASD. Enhancer of zest homolog 2 (EZH2), which encodes a histone methyltransferase, plays an important role in the process of chromatin remodeling during neurodevelopment. Further, EZH2 is located in chromosome 7q35-36, which is one of the linkage regions for autism. However, the genetic relationship between autism and EZH2 remains unclear. To investigate the association between EZH2 and autism in Chinese Han population, we performed a family-based association study between autism and three tagged single nucleotide polymorphisms (SNPs) that covered 95.4% of the whole region of EZH2. In the discovery cohort of 239 trios, two SNPs (rs740949 and rs6464926) showed a significant association with autism. To decrease false positive results, we expanded the sample size to 427 trios. A SNP (rs6464926) was significantly associated with autism even after Bonferroni correction (p=0.008). Haplotype G-T (rs740949 and rs6464926) was a risk factor for autism (Z=2.655, p=0.008, Global p=0.024). In silico function prediction for SNPs indicated that these two SNPs might be regulatory SNPs. Expression pattern of EZH2 showed that it is highly expressed in human embryonic brains. In conclusion, our findings demonstrate that EZH2 might contribute to the genetic etiology of autism in Chinese Han population.

Characterizing autism spectrum disorders by key biochemical pathways

The genetic and phenotypic heterogeneity of autism spectrum disorders (ASD) presents a substantial challenge for diagnosis, classification, research, and treatment. Investigations into the underlying molecular etiology of ASD have often yielded mixed and at times opposing findings. Defining the molecular and biochemical underpinnings of heterogeneity in ASD is crucial to our understanding of the pathophysiological development of the disorder, and has the potential to assist in diagnosis and the rational design of clinical trials. In this review, we propose that genetically diverse forms of ASD may be usefully parsed into entities resulting from converse patterns of growth regulation at the molecular level, which lead to the correlates of general synaptic and neural overgrowth or undergrowth. Abnormal brain growth during development is a characteristic feature that has been observed both in children with autism and in mouse models of autism. We review evidence from syndromic and non-syndromic ASD to suggest that entities currently classified as autism may fundamentally differ by underlying pro- or anti-growth abnormalities in key biochemical pathways, giving rise to either excessive or reduced synaptic connectivity in affected brain regions. We posit that this classification strategy has the potential not only to aid research efforts, but also to ultimately facilitate early diagnosis and direct appropriate therapeutic interventions.