Mild Overexpression of Mecp2 in Mice Causes a Higher Susceptibility toward Seizures

Advances in AAV-mediated gene replacement therapy for pediatric monogenic neurological disorders

Pediatric monogenetic diseases encompass a spectrum of debilitating neurological disorders that affect infants and children, often resulting in profound cognitive and motor impairments. Gene replacement therapy holds immense promise in addressing the underlying genetic defects responsible for these conditions. Adeno-associated virus (AAV) vectors have emerged as a leading platform for delivering therapeutic genes due to their safety profile and ability to transduce various cell types, including neurons. This review highlights recent advancements in AAV-mediated gene replacement therapy for pediatric monogenetic diseases, focusing on key preclinical and clinical studies. We discuss various strategies to enhance transduction efficiency, target specificity, and safety. Furthermore, we explore challenges such as immune responses, along with innovative approaches to overcome these obstacles. Moreover, we examine the clinical outcomes and safety profiles of AAV-based gene therapies in pediatric patients, providing insights into the feasibility and efficacy of these interventions. Finally, we discuss future directions and potential avenues for further research to optimize the therapeutic potential of AAV-delivered gene replacement therapy for pediatric encephalopathies, ultimately aiming to improve the quality of life for affected individuals and their families.

Timing of MeCP2 Expression Determines Midbrain Dopamine Neuron Phenotype Specification

Midbrain dopamine (DA) neurons are associated with locomotor and psychiatric disorders. DA phenotype is specified in ancestral neural precursor cells (NPCs) and maintained throughout neuronal differentiation. Here we show that endogenous expression of MeCP2 coincides with DA phenotype specification in mouse mesencephalon, and premature expression of MeCP2 prevents in vitro cultured NPCs from acquiring DA phenotype through interfering NURR1 transactivation of DA phenotype genes. By contrast, ectopic MeCP2 expression does not disturb DA phenotype in the DA neurons. By analyzing the dynamic change of DNA methylation along DA neuronal differentiation at the promoter of DA phenotype gene tyrosine hydroxylase (Th), we show that Th expression is determined by TET1-mediated de-methylation of NURR1 binding sites within Th promoter. Chromatin immunoprecipitation assays demonstrate that premature MeCP2 dominates the DNA binding of the corresponding sites thereby blocking TET1 function in DA NPCs, whereas TET1-mediated de-methylation prevents excessive MeCP2 binding in DA neurons. The significance of temporal DNA methylation status is further confirmed by targeted methylation/demethylation experiments showing that targeted de-methylation in DA NPCs protects DA phenotype specification from ectopic MeCP2 expression, whereas targeted methylation disturbs phenotype maintenance in MeCP2-overexpressed DA neurons. These findings suggest the appropriate timing of MeCP2 expression as a novel determining factor for guiding NPCs into DA lineage.

Rett Syndrome and Fragile X Syndrome: Different Etiology With Common Molecular Dysfunctions

Rett syndrome (RTT) and Fragile X syndrome (FXS) are two monogenetic neurodevelopmental disorders with complex clinical presentations. RTT is caused by mutations in the Methyl-CpG binding protein 2 gene (MECP2) altering the function of its protein product MeCP2. MeCP2 modulates gene expression by binding methylated CpG dinucleotides, and by interacting with transcription factors. FXS is caused by the silencing of the FMR1 gene encoding the Fragile X Mental Retardation Protein (FMRP), a RNA binding protein involved in multiple steps of RNA metabolism, and modulating the translation of thousands of proteins including a large set of synaptic proteins. Despite differences in genetic etiology, there are overlapping features in RTT and FXS, possibly due to interactions between MeCP2 and FMRP, and to the regulation of pathways resulting in dysregulation of common molecular signaling. Furthermore, basic physiological mechanisms are regulated by these proteins and might concur to the pathophysiology of both syndromes. Considering that RTT and FXS are disorders affecting brain development, and that most of the common targets of MeCP2 and FMRP are involved in brain activity, we discuss the mechanisms of synaptic function and plasticity altered in RTT and FXS, and we consider the similarities and the differences between these two disorders.

Engineered microRNA-based regulatory element permits safe high-dose miniMECP2 gene therapy in Rett mice

MECP2 gene transfer has been shown to extend the survival of Mecp2-/y knockout (KO) mice modeling Rett syndrome (RTT), an X-linked neurodevelopmental disorder. However, controlling deleterious overexpression of MeCP2 remains the critical unmet obstacle towards a safe and effective gene therapy approach for RTT. A recently developed truncated miniMECP2 gene has also been shown to be therapeutic after AAV9-mediated gene transfer in KO neonates. We show that AAV9/miniMECP2 has a similar dose-dependent toxicity profile to that of a published second-generation AAV9/MECP2 vector after treatment in adolescent mice. To overcome that toxicity, we developed a risk-driven viral genome design strategy rooted in high-throughput profiling and genome mining to rationally develop a compact, synthetic miRNA target panel (miR-Responsive Auto-Regulatory Element, "miRARE") to minimize the possibility of miniMECP2 transgene overexpression in the context of RTT gene therapy. The goal of miRARE is to have a built-in inhibitory element responsive to MeCP2 overexpression. The data provided herein show that insertion of miRARE into the miniMECP2 gene expression cassette greatly improved the safety of miniMECP2 gene transfer without compromising efficacy. Importantly, this built-in regulation system does not require any additional exogenous drug application, and no miRNAs are expressed from the transgene cassette. Although broad applications of miRARE have yet to be determined, the design of miRARE suggests a potential use in gene therapy approaches for other dose-sensitive genes.

Antisense oligonucleotide therapy in a humanized mouse model of MECP2 duplication syndrome

Many intellectual disability disorders are due to copy number variations, and, to date, there have been no treatment options tested for this class of diseases. MECP2 duplication syndrome (MDS) is one of the most common genomic rearrangements in males and results from duplications spanning the methyl-CpG binding protein 2 (MECP2) gene locus. We previously showed that antisense oligonucleotide (ASO) therapy can reduce MeCP2 protein amount in an MDS mouse model and reverse its disease features. This MDS mouse model, however, carried one transgenic human allele and one mouse allele, with the latter being protected from human-specific MECP2-ASO targeting. Because MeCP2 is a dosage-sensitive protein, the ASO must be titrated such that the amount of MeCP2 is not reduced too far, which would cause Rett syndrome. Therefore, we generated an "MECP2 humanized" MDS model that carries two human MECP2 alleles and no mouse endogenous allele. Intracerebroventricular injection of the MECP2-ASO efficiently down-regulated MeCP2 expression throughout the brain in these mice. Moreover, MECP2-ASO mitigated several behavioral deficits and restored expression of selected MeCP2-regulated genes in a dose-dependent manner without any toxicity. Central nervous system administration of MECP2-ASO is therefore well tolerated and beneficial in this mouse model and provides a translatable approach that could be feasible for treating MDS.

Role of DNA Methyl-CpG-Binding Protein MeCP2 in Rett Syndrome Pathobiology and Mechanism of Disease

Rett Syndrome (RTT) is a severe, rare, and progressive developmental disorder with patients displaying neurological regression and autism spectrum features. The affected individuals are primarily young females, and more than 95% of patients carry de novo mutation(s) in the Methyl-CpG-Binding Protein 2 (MECP2) gene. While the majority of RTT patients have MECP2 mutations (classical RTT), a small fraction of the patients (atypical RTT) may carry genetic mutations in other genes such as the cyclin-dependent kinase-like 5 (CDKL5) and FOXG1. Due to the neurological basis of RTT symptoms, MeCP2 function was originally studied in nerve cells (neurons). However, later research highlighted its importance in other cell types of the brain including glia. In this regard, scientists benefitted from modeling the disease using many different cellular systems and transgenic mice with loss- or gain-of-function mutations. Additionally, limited research in human postmortem brain tissues provided invaluable findings in RTT pathobiology and disease mechanism. MeCP2 expression in the brain is tightly regulated, and its altered expression leads to abnormal brain function, implicating MeCP2 in some cases of autism spectrum disorders. In certain disease conditions, MeCP2 homeostasis control is impaired, the regulation of which in rodents involves a regulatory microRNA (miR132) and brain-derived neurotrophic factor (BDNF). Here, we will provide an overview of recent advances in understanding the underlying mechanism of disease in RTT and the associated genetic mutations in the MECP2 gene along with the pathobiology of the disease, the role of the two most studied protein variants (MeCP2E1 and MeCP2E2 isoforms), and the regulatory mechanisms that control MeCP2 homeostasis network in the brain, including BDNF and miR132.

Interregulation between fragile X mental retardation protein and methyl CpG binding protein 2 in the mouse posterior cerebral cortex

Several X-linked neurodevelopmental disorders including Rett syndrome, induced by mutations in the MECP2 gene, and fragile X syndrome (FXS), caused by mutations in the FMR1 gene, share autism-related features. The mRNA coding for methyl CpG binding protein 2 (MeCP2) has previously been identified as a substrate for the mRNA-binding protein, fragile X mental retardation protein (FMRP), which is silenced in FXS. Here, we report a homeostatic relationship between these two key regulators of gene expression in mouse models of FXS (Fmr1 Knockout (KO)) and Rett syndrome (MeCP2 KO). We found that the level of MeCP2 protein in the cerebral cortex was elevated in Fmr1 KO mice, whereas MeCP2 KO mice displayed reduced levels of FMRP, implicating interplay between the activities of MeCP2 and FMRP. Indeed, knockdown of MeCP2 with short hairpin RNAs led to a reduction of FMRP in mouse Neuro2A and in human HEK-293 cells, suggesting a reciprocal coupling in the expression level of these two regulatory proteins. Intra-cerebroventricular injection of an adeno-associated viral vector coding for FMRP led to a concomitant reduction in MeCP2 expression in vivo and partially corrected locomotor hyperactivity. Additionally, the level of MeCP2 in the posterior cortex correlated with the severity of the hyperactive phenotype in Fmr1 KO mice. These results demonstrate that MeCP2 and FMRP operate within a previously undefined homeostatic relationship. Our findings also suggest that MeCP2 overexpression in Fmr1 KO mouse posterior cerebral cortex may contribute to the fragile X locomotor hyperactivity phenotype.

HSV1 VP1-2 deubiquitinates STING to block type I interferon expression and promote brain infection

Herpes simplex virus (HSV) is the main cause of viral encephalitis in the Western world, and the type I interferon (IFN) system is important for antiviral control in the brain. Here, we have compared Ifnb induction in mixed murine brain cell cultures by a panel of HSV1 mutants, each devoid of one mechanism to counteract the IFN-stimulating cGAS-STING pathway. We found that a mutant lacking the deubiquitinase (DUB) activity of the VP1-2 protein induced particularly strong expression of Ifnb and IFN-stimulated genes. HSV1 ΔDUB also induced elevated IFN expression in murine and human microglia and exhibited reduced viral replication in the brain. This was associated with increased ubiquitination of STING and elevated phosphorylation of STING, TBK1, and IRF3. VP1-2 associated directly with STING, leading to its deubiquitination. Recruitment of VP1-2 to STING was dependent on K150 of STING, which was ubiquitinated by TRIM32. Thus, the DUB activity of HSV1 VP1-2 is a major viral immune-evasion mechanism in the brain.

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

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

Treating Rett syndrome: from mouse models to human therapies

Rare diseases are very difficult to study mechanistically and to develop therapies for because of the scarcity of patients. Here, the rare neuro-metabolic disorder Rett syndrome (RTT) is discussed as a prototype for precision medicine, demonstrating how mouse models have led to an understanding of the development of symptoms. RTT is caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2). Mecp2-mutant mice are being used in preclinical studies that target the MECP2 gene directly, or its downstream pathways. Importantly, this work may improve the health of RTT patients. Clinical presentation may vary widely among individuals based on their mutation, but also because of the degree of X chromosome inactivation and the presence of modifier genes. Because it is a complex disorder involving many organ systems, it is likely that recovery of RTT patients will involve a combination of treatments. Precision medicine is warranted to provide the best efficacy to individually treat RTT patients.

Disturbed redox homeostasis and oxidative stress: Potential players in the developmental regression in Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder affecting mostly girls. A seemingly normal initial development is followed by developmental stagnation and regression, leading to severe mental impairment with autistic features, motor dysfunction, irregular breathing and epilepsy. Currently, a cure does not exist. Due to the close association of RTT with mitochondrial alterations, cellular redox-impairment and oxidative stress, compounds stabilizing mitochondrial function, cellular redox-homeostasis, and oxidant detoxification are increasingly considered as treatment concepts. Indeed, antioxidants and free-radical scavengers ameliorate certain aspects of the complex and severe clinical presentation of RTT. To further evaluate these strategies, reliable biosensors are needed to quantify redox-conditions in brain and peripheral organs of mouse models or in patient-derived cells. Genetically-encoded redox-sensors meet these requirements. Expressed in transgenic mouse-models such as our unique Rett-redox indicator mice, they will report for any cell type desired the severity of oxidant stress throughout the various disease stages of RTT. Furthermore, these sensors will be crucial to evaluate in vitro and in vivo the outcome of mitochondria- and redox-balance targeted treatments.

Rescue of hyperexcitability in hippocampal CA1 neurons from Mecp2 (-/y) mouse through surface potential neutralization

Hyperventilation is a known feature of Rett syndrome (RTT). However, how hyperventilation is related to other RTT symptoms such as hyperexcitability is unknown. Intense breathing during hyperventilation induces hypocapnia and culminates in respiratory alkalosis. Alkalinization of extracellular milieu can trigger epilepsy in patients who already have neuronal hyperexcitability. By combining patch-clamp electrophysiology and quantitative glutamate imaging, we compared excitability of CA1 neurons of WT and Mecp2 (-/y) mice, and analyzed the biophysical properties of subthreshold membrane channels. The results show that Mecp2 (-/y) CA1 neurons are hyperexcitable in normal pH (7.4) and are increasingly vulnerable to alkaline extracellular pH (8.4), during which their excitability increased further. Under normal pH conditions, an abnormal negative shift in the voltage-dependencies of HCN (hyperpolarization-activated cyclic nucleotide-gated) and calcium channels in the CA1 neurons of Mecp2 (-/y) mice was observed. Alkaline pH also enhanced excitability in wild-type (WT) CA1 neurons through modulation of the voltage dependencies of HCN- and calcium channels. Additionally alkaline pH augmented spontaneous glutamate release and burst firing in WT CA1 neurons. Conversely, acidic pH (6.4) and 8 mM Mg2+ exerted the opposite effect, and diminished hyperexcitability in Mecp2 (-/y) CA1 neurons. We propose that the observed effects of pH and Mg2+ are mediated by changes in the neuronal membrane surface potential, which consecutively modulates the gating of HCN and calcium channels. The results provide insight to pivotal cellular mechanisms that can regulate neuronal excitability and help to devise treatment strategies for hyperexcitability induced symptoms of Rett syndrome.

Sexual dimorphism of AMBRA1-related autistic features in human and mouse

Ambra1 is linked to autophagy and neurodevelopment. Heterozygous Ambra1 deficiency induces autism-like behavior in a sexually dimorphic manner. Extraordinarily, autistic features are seen in female mice only, combined with stronger Ambra1 protein reduction in brain compared to males. However, significance of AMBRA1 for autistic phenotypes in humans and, apart from behavior, for other autism-typical features, namely early brain enlargement or increased seizure propensity, has remained unexplored. Here we show in two independent human samples that a single normal AMBRA1 genotype, the intronic SNP rs3802890-AA, is associated with autistic features in women, who also display lower AMBRA1 mRNA expression in peripheral blood mononuclear cells relative to female GG carriers. Located within a non-coding RNA, likely relevant for mRNA and protein interaction, rs3802890 (A versus G allele) may affect its stability through modification of folding, as predicted by in silico analysis. Searching for further autism-relevant characteristics in Ambra1^+/- mice, we observe reduced interest of female but not male mutants regarding pheromone signals of the respective other gender in the social intellicage set-up. Moreover, altered pentylentetrazol-induced seizure propensity, an in vivo readout of neuronal excitation-inhibition dysbalance, becomes obvious exclusively in female mutants. Magnetic resonance imaging reveals mild prepubertal brain enlargement in both genders, uncoupling enhanced brain dimensions from the primarily female expression of all other autistic phenotypes investigated here. These data support a role of AMBRA1/Ambra1 partial loss-of-function genotypes for female autistic traits. Moreover, they suggest Ambra1 heterozygous mice as a novel multifaceted and construct-valid genetic mouse model for female autism.

Improved MECP2 Gene Therapy Extends the Survival of MeCP2-Null Mice without Apparent Toxicity after Intracisternal Delivery

Intravenous administration of adeno-associated virus serotype 9 (AAV9)/hMECP2 has been shown to extend the lifespan of Mecp2-/y mice, but this delivery route induces liver toxicity in wild-type (WT) mice. To reduce peripheral transgene expression, we explored the safety and efficacy of AAV9/hMECP2 injected into the cisterna magna (ICM). AAV9/hMECP2 (1 × 1012 viral genomes [vg]; ICM) extended Mecp2-/y survival but aggravated hindlimb clasping and abnormal gait phenotypes. In WT mice, 1 × 1012 vg of AAV9/hMECP2 induced clasping and abnormal gait. A lower dose mitigated these adverse phenotypes but failed to extend survival of Mecp2-/y mice. Thus, ICM delivery of this vector is impractical as a treatment for Rett syndrome (RTT). To improve the safety of MeCP2 gene therapy, the gene expression cassette was modified to include more endogenous regulatory elements believed to modulate MeCP2 expression in vivo. In Mecp2-/y mice, ICM injection of the modified vector extended lifespan and was well tolerated by the liver but did not rescue RTT behavioral phenotypes. In WT mice, these same doses of the modified vector had no adverse effects on survival or neurological phenotypes. In summary, we identified limitations of the original vector and demonstrated that an improved vector design extends Mecp2-/y survival, without apparent toxicity.

The presence of two rare genomic syndromes, 1q21 deletion and Xq28 duplication, segregating independently in a family with intellectual disability

Background

1q21 microdeletion syndrome is a rare contiguous gene deletion disorder with de novo or autosomal dominant inheritance patterns and its phenotypic features include intellectual disability, distinctive facial dysmorphism, microcephaly, cardiac abnormalities, and cataracts. MECP2 duplication syndrome is an X-linked recessive neurodevelopmental disorder characterized by intellectual disability, global developmental delay, and other neurological complications including late-onset seizures. Previously, these two different genetic syndromes have not been reported segregating independently in a same family.

Case presentation

Here we describe two siblings carrying either a chromosome 1q21 microdeletion or a chromosome Xq28 duplication. Using a comparative genomic hybridization (CGH) array, we identified a 1.24 Mb heterozygous deletion at 1q21 resulting in the loss of 9 genes in a girl with learning disability, hypothyroidism, short stature, sensory integration disorder, and soft dysmorphic features including cupped ears and a unilateral ear pit. We also characterized a 508 kb Xq28 duplication encompassing MECP2 in her younger brother with hypotonia, poor speech, cognitive and motor impairment. The parental CGH and quantitative PCR (qPCR) analyses revealed that the 1q21 deletion in the elder sister is de novo, but the Xq28 duplication in the younger brother was originally inherited from the maternal grandmother through the mother, both of whom are asymptomatic carriers. RT-qPCR assays revealed that the affected brother has almost double the amount of MECP2 mRNA expression compared to other family members of both genders including maternal grandmother and mother who have the same Xq28 duplication with no phenotype. This suggests the X chromosome with an Xq28 duplication in the carrier females is preferentially silenced.

Conclusion

From our understanding, this would be the first report showing the independent segregation of two genetically unrelated syndromes, 1q21 microdeletion and Xq28 duplication, in a same family, especially in siblings. Although these two chromosomal abnormalities share some similar phenotypes such as intellectual disability, mild dysmorphic features, and cardiac abnormalities, the presence of two unrelated and rare syndromes in siblings is very unusual. Therefore, further comprehensive investigations in similar cases are required for future studies.

Loss and Gain of MeCP2 Cause Similar Hippocampal Circuit Dysfunction that Is Rescued by Deep Brain Stimulation in a Rett Syndrome Mouse Model

Loss- and gain-of-function mutations in methyl-CpG-binding protein 2 (MECP2) underlie two distinct neurological syndromes with strikingly similar features, but the synaptic and circuit-level changes mediating these shared features are undefined. Here we report three novel signs of neural circuit dysfunction in three mouse models of MECP2 disorders (constitutive Mecp2 null, mosaic Mecp2(+/-), and MECP2 duplication): abnormally elevated synchrony in the firing activity of hippocampal CA1 pyramidal neurons, an impaired homeostatic response to perturbations of excitatory-inhibitory balance, and decreased excitatory synaptic response in inhibitory neurons. Conditional mutagenesis studies revealed that MeCP2 dysfunction in excitatory neurons mediated elevated synchrony at baseline, while MeCP2 dysfunction in inhibitory neurons increased susceptibility to hypersynchronization in response to perturbations. Chronic forniceal deep brain stimulation (DBS), recently shown to rescue hippocampus-dependent learning and memory in Mecp2(+/-) (Rett) mice, also rescued all three features of hippocampal circuit dysfunction in these mice.

Cortical network dysfunction caused by a subtle defect of myelination

Subtle white matter abnormalities have emerged as a hallmark of brain alterations in magnetic resonance imaging or upon autopsy of mentally ill subjects. However, it is unknown whether such reduction of white matter and myelin contributes to any disease‐relevant phenotype or simply constitutes an epiphenomenon, possibly even treatment‐related. Here, we have re‐analyzed Mbp heterozygous mice, the unaffected parental strain of shiverer, a classical neurological mutant. Between 2 and 20 months of age, Mbp^+/‐ versus Mbp^+/+ littermates were deeply phenotyped by combining extensive behavioral/cognitive testing with MRI, 1H‐MR spectroscopy, electron microscopy, and molecular techniques. Surprisingly, Mbp‐dependent myelination was significantly reduced in the prefrontal cortex. We also noticed a mild but progressive hypomyelination of the prefrontal corpus callosum and low‐grade inflammation. While most behavioral functions were preserved, Mbp^+/‐ mice exhibited defects of sensorimotor gating, as evidenced by reduced prepulse‐inhibition, and a late‐onset catatonia phenotype. Thus, subtle but primary abnormalities of CNS myelin can be the cause of a persistent cortical network dysfunction including catatonia, features typical of neuropsychiatric conditions. GLIA 2016;64:2025–2040

MeCP2 deficiency results in robust Rett-like behavioural and motor deficits in male and female rats

Since the identification of MECP2 as the causative gene in the majority of Rett Syndrome (RTT) cases, transgenic mouse models have played a critical role in our understanding of this disease. The use of additional mammalian RTT models offers the promise of further elucidating critical early mechanisms of disease as well as providing new avenues for translational studies. We have identified significant abnormalities in growth as well as motor and behavioural function in a novel zinc-finger nuclease model of RTT utilizing both male and female rats throughout development. Male rats lacking MeCP2 (Mecp2^ZFN/y) were noticeably symptomatic as early as postnatal day 21, with most dying by postnatal day 55, while females lacking one copy of Mecp2 (Mecp2^ZFN/+) displayed a more protracted disease course. Brain weights of Mecp2^ZFN/y and Mecp2^ZFN/+rats were significantly reduced by postnatal day 14 and 21, respectively. Early motor and breathing abnormalities were apparent in Mecp2^ZFN/y rats, whereas Mecp2^ZFN/+rats displayed functional irregularities later in development. The large size of this species will provide profound advantages in the identification of early disease mechanisms and the development of appropriately timed therapeutics. The current study establishes a foundational basis for the continued utilization of this rat model in future RTT research.

Brain phosphorylation of MeCP2 at serine 164 is developmentally regulated and globally alters its chromatin association

MeCP2 is a transcriptional regulator whose functional alterations are responsible for several autism spectrum and mental disorders. Post-translational modifications (PTMs), and particularly differential phosphorylation, modulate MeCP2 function in response to diverse stimuli. Understanding the detailed role of MeCP2 phosphorylation is thus instrumental to ascertain how MeCP2 integrates the environmental signals and directs its adaptive transcriptional responses. The evolutionarily conserved serine 164 (S164) was found phosphorylated in rodent brain but its functional role has remained uncharacterized. We show here that phosphorylation of S164 in brain is dynamically regulated during neuronal maturation. S164 phosphorylation highly impairs MeCP2 binding to DNA in vitro and largely affects its nucleosome binding and chromatin affinity in vivo. Strikingly, the chromatin-binding properties of the global MeCP2 appear also extensively altered during the course of brain maturation. Functional assays reveal that proper temporal regulation of S164 phosphorylation controls the ability of MeCP2 to regulate neuronal morphology. Altogether, our results support the hypothesis of a complex PTM-mediated functional regulation of MeCP2 potentially involving a still poorly characterized epigenetic code. Furthermore, they demonstrate the relevance of the Intervening Domain of MeCP2 for binding to DNA.

Fast cerebellar reflex circuitry requires synaptic vesicle priming by munc13-3

Munc13-3 is a member of the Munc13 family of synaptic vesicle priming proteins and mainly expressed in cerebellar neurons. Munc13-3 null mutant (Munc13-3^−/−) mice show decreased synaptic release probability at parallel fiber to Purkinje cell, granule cell to Golgi cell, and granule cell to basket cell synapses and exhibit a motor learning deficit at highest rotarod speeds. Since we detected Munc13-3 immunoreactivity in the dentate gyrus, as reported here for the first time, and current studies indicated a crucial role for the cerebellum in hippocampus-dependent spatial memory, we systematically investigated Munc13-3^−/− mice versus wild-type littermates of both genders with respect to hippocampus-related cognition and a range of basic behaviors, including tests for anxiety, sensory functions, motor performance and balance, sensorimotor gating, social interaction and competence, and repetitive and compulsive behaviors. Neither basic behavior nor hippocampus-dependent cognitive performance, evaluated by Morris water maze, hole board working and reference memory, IntelliCage-based place learning including multiple reversals, and fear conditioning, showed any difference between genotypes. However, consistent with a disturbed cerebellar reflex circuitry, a reliable reduction in the acoustic startle response in both male and female Munc13-3^−/− mice was found. To conclude, complete deletion of Munc13-3 leads to a robust decrease in the acoustic startle response. This readout of a fast cerebellar reflex circuitry obviously requires synaptic vesicle priming by Munc13-3 for full functionality, in contrast to other behavioral or cognitive features, where a nearly perfect compensation of Munc13-3 deficiency by related synaptic proteins has to be assumed.

Epigenetics and Epilepsy

Epigenetic processes in the brain involve the transfer of information arising from short-lived cellular signals and changes in neuronal activity into lasting effects on gene expression. Key molecular mediators of epigenetics include methylation of DNA, histone modifications, and noncoding RNAs. Emerging findings in animal models and human brain tissue reveal that epilepsy and epileptogenesis are associated with changes to each of these contributors to the epigenome. Understanding and influencing the molecular mechanisms controlling epigenetic change could open new avenues for treatment. DNA methylation, particularly hypermethylation, has been found to increase within gene body regions and interference with DNA methylation in epilepsy can change gene expression profiles and influence epileptogenesis. Posttranscriptional modification of histones, including transient as well as sustained changes to phosphorylation and acetylation, have been reported, which appear to influence gene expression. Finally, roles have emerged for noncoding RNAs in brain excitability and seizure thresholds, including microRNA and long noncoding RNA. Together, research supports strong effects of epigenetics influencing gene expression in epilepsy, suggesting future therapeutic approaches to manipulate epigenetic processes to treat or prevent epilepsy.

Association of Tag SNPs and Rare CNVs of the MIR155HG/miR-155 Gene with Epilepsy in the Chinese Han Population

BACKGROUND

miR-155 likely acts as an important modulator in the inflammatory mechanism of epilepsy, and this study investigated its association with epilepsy from the perspective of molecular genetics.

METHODS

This study enrolled 249 epileptic patients and 289 healthy individuals of the Chinese Han population; 4 tag single-nucleotide polymorphisms (SNPs: rs969885, rs12483428, rs987195, and rs4817027) of the MIR155HG/miR-155 gene were selected, and their association with epilepsy was investigated. Additionally, this study determined the copy numbers of the MIR155HG/miR-155 gene.

RESULTS

The TCA haplotype (rs12483428-rs987195-rs4817027) and the AA genotype at rs4817027 conferred higher vulnerability to epilepsy in males. Stratification by age of onset revealed that the CC haplotype (rs969885-rs987195) was a genetic susceptibility factor for early-onset epilepsy. Further stratification by drug-resistant status indicated the CC haplotype (rs969885-rs987195) and the AA genotype at rs4817027 were genetic susceptibility factors for drug-resistant epilepsy (DRE) but the CG haplotype (rs987195-rs969885) was a genetically protective factor against DRE. Besides, 3 epileptic patients with copy number variants of the MIR155HG/miR-155 gene were observed.

CONCLUSIONS

This study first demonstrates the association of MIR155HG/miR-155 tag SNPs with epilepsy and shows that rare CNVs were found exclusively in epileptic patients, clarifying the genetic role of miR-155 in epilepsy.

Ethanol deregulates Mecp2/MeCP2 in differentiating neural stem cells via interplay between 5-methylcytosine and 5-hydroxymethylcytosine at the Mecp2 regulatory elements

Methyl CpG Binding Protein 2 (MeCP2) is an important epigenetic factor in the brain. MeCP2 expression is affected by different environmental insults including alcohol exposure. Accumulating evidence supports the role of aberrant MeCP2 expression in ethanol exposure-induced neurological symptoms. However, the underlying molecular mechanisms of ethanol-induced MeCP2 deregulation remain elusive. To study the effect of ethanol on Mecp2/MeCP2 expression during neurodifferentiation, we established an in vitro model of ethanol exposure, using differentiating embryonic brain-derived neural stem cells (NSC). Previously, we demonstrated the impact of DNA methylation at the Mecp2 regulatory elements (REs) on Mecp2/MeCP2 expression in vitro and in vivo. Here, we studied whether altered DNA methylation at these REs is associated with the Mecp2/MeCP2 misexpression induced by ethanol. Binge-like and continuous ethanol exposure upregulated Mecp2/MeCP2, while ethanol withdrawal downregulated its expression. DNA methylation analysis by methylated DNA immunoprecipitation indicated that increased 5-hydroxymethylcytosine (5hmC) and decreased 5-methylcytosine (5mC) enrichment at specific REs were associated with upregulated Mecp2/MeCP2 following continuous ethanol exposure. The reduced Mecp2/MeCP2 expression upon ethanol withdrawal was associated with reduced 5hmC and increased 5mC enrichment at these REs. Moreover, ethanol altered global DNA methylation (5mC and 5hmC). Under the tested conditions, ethanol had minimal effects on NSC cell fate commitment, but caused changes in neuronal morphology and glial cell size. Taken together, our data represent an epigenetic mechanism for ethanol-mediated misexpression of Mecp2/MeCP2 in differentiating embryonic brain cells. We also show the potential role of DNA methylation and MeCP2 in alcohol-related neurological disorders, specifically Fetal Alcohol Spectrum Disorders.

Mild expression differences of MECP2 influencing aggressive social behavior

The X-chromosomal MECP2/Mecp2 gene encodes methyl-CpG-binding protein 2, a transcriptional activator and repressor regulating many other genes. We discovered in male FVB/N mice that mild (∼50%) transgenic overexpression of Mecp2 enhances aggression. Surprisingly, when the same transgene was expressed in C57BL/6N mice, transgenics showed reduced aggression and social interaction. This suggests that Mecp2 modulates aggressive social behavior. To test this hypothesis in humans, we performed a phenotype-based genetic association study (PGAS) in >1000 schizophrenic individuals. We found MECP2 SNPs rs2239464 (G/A) and rs2734647 (C/T; 3′UTR) associated with aggression, with the G and C carriers, respectively, being more aggressive. This finding was replicated in an independent schizophrenia cohort. Allele-specific MECP2mRNA expression differs in peripheral blood mononuclear cells by ∼50% (rs2734647: C > T). Notably, the brain-expressed, species-conserved miR-511 binds to MECP2 3′UTR only in T carriers, thereby suppressing gene expression. To conclude, subtle MECP2/Mecp2 expression alterations impact aggression. While the mouse data provides evidence of an interaction between genetic background and mild Mecp2 overexpression, the human data convey means by which genetic variation affects MECP2 expression and behavior.

Using sex differences in the developing brain to identify nodes of influence for seizure susceptibility and epileptogenesis

Sexual differentiation of the developing brain organizes the neural architecture differently between males and females, and the main influence on this process is exposure to gonadal steroids during sensitive periods of prenatal and early postnatal development. Many molecular and cellular processes are influenced by steroid hormones in the developing brain, including gene expression, cell birth and death, neurite outgrowth and synaptogenesis, and synaptic activity. Perturbations in these processes can alter neuronal excitability and circuit activity, leading to increased seizure susceptibility and the promotion of pathological processes that constitute epileptogenesis. In this review, we will provide a general overview of sex differences in the early developing brain that may be relevant for altered seizure susceptibility in early life, focusing on limbic areas of the brain. Sex differences that have the potential to alter the progress of epileptogenesis are evident at molecular and cellular levels in the developing brain, and include differences in neuronal excitability, response to environmental insult, and epigenetic control of gene expression. Knowing how these processes differ between the sexes can help us understand fundamental mechanisms underlying gender differences in seizure susceptibility and epileptogenesis.

Late onset epileptic spasms is frequent in MECP2 gene duplication: electroclinical features and long-term follow-up of 8 epilepsy patients

Mutation of the X-linked methyl CpG binding protein 2 (MECP2) has been first identified as the cause of Rett syndrome. More recently, MECP2 gene duplication syndrome has been identified in males. The MECP2 duplication syndrome is characterized by severe mental retardation, infantile hypotonia, progressive spasticity and recurrent infections. Epileptic seizures are inconstant but poorly described. The aim of the study is to describe the electroclinical features of epilepsy in MECP2 duplication patients in order to refine the epilepsy phenotype and its evolution.

METHODS

We conducted a retrospective study in four child neurology departments in France. Eight boys with a MECP2 gene duplication and epilepsy were retrospectively studied. We evaluated both clinical and electroencephalographic data before seizure onset, at seizure onset and during the follow-up.

RESULTS

The patients started seizures at the median age of 6 years (range: 2.5-17 years). Half exhibits late onset epileptic spasms while the other exhibit either focal epilepsy or unclassified generalized epilepsy. Before seizure onset, EEGs were abnormal in all patients showing a slowing of the background or a normal background with fast activities, while EEG performed in epileptic patients, showed a slowing of the background in 6/8 and localized slow or sharp waves in 7/8. Most patients (6/8) have evolved to drug resistant epilepsy.

CONCLUSION

Although late onset epileptic spasms are common in patients with MECP2 duplication, no specific electroclinical phenotype emerges, probably due to genetic heterogeneity of the syndrome. Further studies are needed to individualize specific epileptic subtype in larger cohort of patients.

Heterozygous ambra1 deficiency in mice: a genetic trait with autism-like behavior restricted to the female gender

Autism-spectrum disorders (ASD) are heterogeneous, highly heritable neurodevelopmental conditions affecting around 0.5% of the population across cultures, with a male/female ratio of approximately 4:1. Phenotypically, ASD are characterized by social interaction and communication deficits, restricted interests, repetitive behaviors, and reduced cognitive flexibility. Identified causes converge at the level of the synapse, ranging from mutation of synaptic genes to quantitative alterations in synaptic protein expression, e.g., through compromised transcriptional or translational control. We wondered whether reduced turnover and degradation of synapses, due to deregulated autophagy, would lead to similar phenotypical consequences. Ambra1, strongly expressed in cortex, hippocampus, and striatum, is a positive regulator of Beclin1, a principal player in autophagosome formation. While homozygosity of the Ambra1 null mutation causes embryonic lethality, heterozygous mice with reduced Ambra1 expression are viable, reproduce normally, and lack any immediately obvious phenotype. Surprisingly, comprehensive behavioral characterization of these mice revealed an autism-like phenotype in Ambra1 (+/-) females only, including compromised communication and social interactions, a tendency of enhanced stereotypies/repetitive behaviors, and impaired cognitive flexibility. Reduced ultrasound communication was found in adults as well as pups, which achieved otherwise normal neurodevelopmental milestones. These features were all absent in male Ambra1 (+/-) mice. As a first hint explaining this gender difference, we found a much stronger reduction of Ambra1 protein in the cortex of Ambra1 (+/-) females compared to males. To conclude, Ambra1 deficiency can induce an autism-like phenotype. The restriction to the female gender of autism-generation by a defined genetic trait is unique thus far and warrants further investigation.

Rett syndrome and MeCP2

Rett syndrome (RTT) is a severe and progressive neurological disorder, which mainly affects young females. Mutations of the methyl-CpG binding protein 2 (MECP2) gene are the most prevalent cause of classical RTT cases. MECP2 mutations or altered expression are also associated with a spectrum of neurodevelopmental disorders such as autism spectrum disorders with recent links to fetal alcohol spectrum disorders. Collectively, MeCP2 relation to these neurodevelopmental disorders highlights the importance of understanding the molecular mechanisms by which MeCP2 impacts brain development, mental conditions, and compromised brain function. Since MECP2 mutations were discovered to be the primary cause of RTT, a significant progress has been made in the MeCP2 research, with respect to the expression, function and regulation of MeCP2 in the brain and its contribution in RTT pathogenesis. To date, there have been intensive efforts in designing effective therapeutic strategies for RTT benefiting from mouse models and cells collected from RTT patients. Despite significant progress in MeCP2 research over the last few decades, there is still a knowledge gap between the in vitro and in vivo research findings and translating these findings into effective therapeutic interventions in human RTT patients. In this review, we will provide a synopsis of Rett syndrome as a severe neurological disorder and will discuss the role of MeCP2 in RTT pathophysiology.

Mouse models of neurodevelopmental disease of the basal ganglia and associated circuits

This chapter focuses on neurodevelopmental diseases that are tightly linked to abnormal function of the striatum and connected structures. We begin with an overview of three representative diseases in which striatal dysfunction plays a key role--Tourette syndrome and obsessive-compulsive disorder, Rett's syndrome, and primary dystonia. These diseases highlight distinct etiologies that disrupt striatal integrity and function during development, and showcase the varied clinical manifestations of striatal dysfunction. We then review striatal organization and function, including evidence for striatal roles in online motor control/action selection, reinforcement learning, habit formation, and action sequencing. A key barrier to progress has been the relative lack of animal models of these diseases, though recently there has been considerable progress. We review these efforts, including their relative merits providing insight into disease pathogenesis, disease symptomatology, and basal ganglia function.

Decitabine alters the expression of Mecp2 isoforms via dynamic DNA methylation at the Mecp2 regulatory elements in neural stem cells

Background

Aberrant MeCP2 expression in brain is associated with neurodevelopmental disorders including autism. In the brain of stressed mouse and autistic human patients, reduced MeCP2 expression is correlated with Mecp2/MECP2 promoter hypermethylation. Altered expression of MeCP2 isoforms (MeCP2E1 and MeCP2E2) is associated with neurological disorders, highlighting the importance of proper regulation of both isoforms. While known regulatory elements (REs) within the MECP2/Mecp2 promoter and intron 1 are involved in MECP2/Mecp2 regulation, Mecp2 isoform-specific regulatory mechanisms are unknown. We hypothesized that DNA methylation at these REs may impact the expression of Mecp2 isoforms.

Methods

We used a previously characterized in vitro differentiating neural stem cell (NSC) system to investigate the interplay between Mecp2 isoform-specific expression and DNA methylation at the Mecp2 REs. We studied altered expression of Mecp2 isoforms, affected by global DNA demethylation and remethylation, induced by exposure and withdrawal of decitabine (5-Aza-2′-deoxycytidine). Further, we performed correlation analysis between DNA methylation at the Mecp2 REs and the expression of Mecp2 isoforms after decitabine exposure and withdrawal.

Results

At different stages of NSC differentiation, Mecp2 isoforms showed reciprocal expression patterns associated with minor, but significant changes in DNA methylation at the Mecp2 REs. Decitabine treatment induced Mecp2e1/MeCP2E1 (but not Mecp2e2) expression at day (D) 2, associated with DNA demethylation at the Mecp2 REs. In contrast, decitabine withdrawal downregulated both Mecp2 isoforms to different extents at D8, without affecting DNA methylation at the Mecp2 REs. NSC cell fate commitment was minimally affected by decitabine under tested conditions. Expression of both isoforms negatively correlated with methylation at specific regions of the Mecp2 promoter, both at D2 and D8. The correlation between intron 1 methylation and Mecp2e1 (but not Mecp2e2) varied depending on the stage of NSC differentiation (D2: negative; D8: positive).

Conclusions

Our results show the correlation between the expression of Mecp2 isoforms and DNA methylation in differentiating NSC, providing insights on the potential role of DNA methylation at the Mecp2 REs in Mecp2 isoform-specific expression. The ability of decitabine to induce Mecp2e1/MeCP2E1, but not Mecp2e2 suggests differential sensitivity of Mecp2 isoforms to decitabine and is important for future drug therapies for autism.