Mecp2-deficient Mice Research Articles

Intrinsic hippocampal network activity is altered in MeCP2-deficient mice

The mammalian medial temporal lobe is capable of generating synchronous rhythmic activities in isolation. For example, brain slices prepared from patients with temporal lobe epilepsy, as well as slices prepared from normal monkey hippocampus, exhibit spontaneous, IPSP-based rhythmic field potentials that have frequencies of between 0.5–3 Hz. These inhibitory rhythmic activities are mediated by GABA-A receptors, and culminate from the network activity of local GABAergic inhibitory interneurons. Analogous spontaneous population rhythmic activities of 0.5–4 Hz are also evident in the isolated rodent hippocampus. These IPSP-based population activities are referred to as spontaneous rhythmic field potentials (SRFPs), as they are readily detected with conventional extracellular recordings. Several lines of evidence suggest that the SRFPs are of physiological significance. The SRFPs spread from the hippocampus to subicular and entorhinal cortical areas, and their frequencies and regional spread are similar to hippocampal electroencephalographic irregular activities that occur in behaving animals during slow wave sleep and wake immobility. In addition, the SRFPs appear during the 2nd postnatal week and then persist in adulthood. Such development profile is in keeping with activity-dependent modifications of hippocampal networks. Moreover, field rhythms similar to SRFPs have been shown to regulate memory-related synaptic plasticity such as long-term potentiation. It remains to be explored whether SRFPs can be used as a neurophysiological marker for detecting disease-related alterations in hippocampal networks. Rett syndrome is a neurodevelopmental condition caused by loss of function mutations within the gene encoding methyl-CpG-binding protein 2 (MeCP2). While a subtle decrease in synaptic activity has been found in Mecp2-deficient mouse cortical and hippocampal neurons, it remains to be determined whether or how these changes affect the network activity of the Mecp2-deficient brain. To address this issue, we examined the SRFPs in conventional hippocampal slices via extracellular and whole-cell patch recordings. We found that although SRFPs were present in Mecp2-deficient slices, their frequency was significantly reduced. This reduction was not associated with significant alterations in the intracellular correlates of SRFPs, but was associated with diminished glutamate receptor-mediated excitatory activities in individual hippocampal CA3 neurons. The diminished excitatory drive appears to contribute to the slow SRFPs in the Mecp2-deficient hippocampus, as pharmacological attenuation of glutamate receptor activity was sufficient to induce similar slow SRFP activity in wild type slices. However, high frequency electrical stimulation of CA3 circuit in Mecp2-deficient slices did not reverse the slow SRFP phenotype, but rather induced excitatory, sharp wave like population events that were not observed in wild type slices. Taken together, our data indicate that the Mecp2-deficient hippocampus displays a reduction in basal glutamatergic activity in the CA3 recurrent network, that this reduced activity provides insufficient drive to the GABAergic inhibitory interneuronal network that establishes the normal SRFP frequency, but that there is a narrow window of tolerance for the Mecp2-deficient hippocampal network to excitatory stimulation.

Cerebellar gene expression profiles of mouse models for Rett syndrome reveal novel MeCP2 targets

BackgroundMeCP2, methyl-CpG-binding protein 2, binds to methylated cytosines at CpG dinucleotides, as well as to unmethylated DNA, and affects chromatin condensation. MECP2 mutations in females lead to Rett syndrome, a neurological disorder characterized by developmental stagnation and regression, loss of purposeful hand movements and speech, stereotypic hand movements, deceleration of brain growth, autonomic dysfunction and seizures. Most mutations occur de novo during spermatogenesis. Located at Xq28, MECP2 is subject to X inactivation, and affected females are mosaic. Rare hemizygous males suffer from a severe congenital encephalopathy.MethodsTo identify the pathways mis-regulated by MeCP2 deficiency, microarray-based global gene expression studies were carried out in cerebellum of Mecp2 mutant mice. We compared transcript levels in mutant/wildtype male sibs of two different MeCP2-deficient mouse models at 2, 4 and 8 weeks of age. Increased transcript levels were evaluated by real-time quantitative RT-PCR. Chromatin immunoprecipitation assays were used to document in vivo MeCP2 binding to promoter regions of candidate target genes.ResultsOf several hundred genes with altered expression levels in the mutants, twice as many were increased than decreased, and only 27 were differentially expressed at more than one time point. The number of misregulated genes was 30% lower in mice with the exon 3 deletion (Mecp2tm1.1Jae) than in mice with the larger deletion (Mecp2tm1.1Bird). Between the mutants, few genes overlapped at each time point. Real-time quantitative RT-PCR assays validated increased transcript levels for four genes: Irak1, interleukin-1 receptor-associated kinase 1; Fxyd1, phospholemman, associated with Na, K-ATPase;Reln, encoding an extracellular signaling molecule essential for neuronal lamination and synaptic plasticity; and Gtl2/Meg3, an imprinted maternally expressed non-translated RNA that serves as a host gene for C/D box snoRNAs and microRNAs. Chromatin immunoprecipitation assays documented in vivo MeCP2 binding to promoter regions of Fxyd1, Reln, and Gtl2.ConclusionTranscriptional profiling of cerebellum failed to detect significant global changes in Mecp2-mutant mice. Increased transcript levels of Irak1, Fxyd1, Reln, and Gtl2 may contribute to the neuronal dysfunction in MeCP2-deficient mice and individuals with Rett syndrome. Our data provide testable hypotheses for future studies of the regulatory or signaling pathways that these genes act on.

Autonomic cardiovascular control in methyl-CpG-binding protein 2 (Mecp2) deficient mice

Methyl-CpG-binding protein 2 is a transcription factor that is involved in gene silencing. It is mutated in the majority of cases of Rett syndrome. This X-linked neurodevelopmental disorder is reported to involve abnormalities in autonomic cardiovascular regulation. As an initial step in understanding the basis for these abnormalities we have characterized autonomic cardiovascular function in Mecp2 deficient mice. Arterial pressure waves were recorded in freely moving animals using telemetry. Baseline blood pressure and pulse interval (PI) as well as indices of heart rate variability (HRV): standard deviation of PI (SDNN), range encompassing 90% of PIs (PI90) and standard deviation of adjacent PIs (SDSD) were similar in Mecp2 +/+ and Mecp2 +/− animals. Spectral analysis of mean arterial pressure (MAP) and PI in the frequency domain showed similar relative power in low frequency 1 (LF1, 08–0.4 Hz), low frequency 2 (LF2, 0.4–1.0 Hz), middle frequency (MF, 1–3 Hz) and high frequency (HF, 3.0–10.0 Hz) bands. Autonomic blockade with atropine or propranolol as well as elevation in ambient temperature to 32 °C resulted in changes in blood pressure, PI and HRV that did not differ between the strains. Atropine, propranolol and elevated temperature resulted in similar changes in both MAP and PI spectral power. Baroreceptor function was tested using intravenous injections of nitroprusside followed by phenylephrine. Maximum gain was not different. These results do reveal any disturbance of autonomic cardiovascular regulation in the Mecp2 deficient mouse genotype.

Mecp2 deficiency leads to delayed maturation and altered gene expression in hippocampal neurons

It is well known that Rett Syndrome, a severe postnatal childhood neurological disorder, is mostly caused by mutations in the MECP2 gene. However, how deficiencies in MeCP2 contribute to the neurological dysfunction of Rett Syndrome is not clear. We aimed to resolve the role of MeCP2 epigenetic regulation in postnatal brain development in an Mecp2-deficient mouse model. We found that, while Mecp2 was not critical for the production of immature neurons in the dentate gyrus (DG) of the hippocampus, the newly generated neurons exhibited pronounced deficits in neuronal maturation, including delayed transition into a more mature stage, altered expression of presynaptic proteins and reduced dendritic spine density. Furthermore, analysis of gene expression profiles of isolated DG granule neurons revealed abnormal expression levels of a number of genes previously shown to be important for synaptogenesis. Our studies suggest that MeCP2 plays a central role in neuronal maturation, which might be mediated through epigenetic control of expression pathways that are instrumental in both dendritic development and synaptogenesis.

Treatment with desipramine improves breathing and survival in a mouse model for Rett syndrome

Rett syndrome (RS) is a severe X-linked neurological disorder in which most patients have mutations in the methyl-CpG binding protein2 (MECP2) gene. No effective treatment exists. We previously showed that the Mecp2-deficient mice, a mouse model of RS, have highly variable respiratory rhythm and frequent apneas due to reduced norepinephrine (NE) content, and a drastic decrease of tyrosine hydroxylase (TH)-expressing neurons in the medulla. We showed here that treating these mice with desipramine (DMI), which specifically inhibits NE reuptake, significantly improved their respiratory rhythm during several weeks. In addition, the treatment significantly extended their lifespan. At the cellular level, we showed that the reduced number of TH-expressing neurons before treatment in the mutant animals was not due to apoptosis. Conversely, we found that DMI treatment increased the number of TH-expressing neurons in the mutant brainstem to reach wild-type levels. We showed that this increase was not due to cellular proliferation. We propose that the Mecp2-deficient TH-expressing neurons lose their ability to synthesize TH at some point during their postnatal development. Our results suggest that a pharmacological stimulation of the noradrenergic system could be a promising approach for the treatment of the respiratory dysfunction which causes a significant proportion of death in RS patients.

Differential distribution of the MeCP2 splice variants in the postnatal mouse brain

Mutations in the gene encoding methyl CpG binding protein 2 (MeCP2) are the primary cause of the neurodevelopmental disorder Rett syndrome (RTT). Mecp2-deficient mice develop a neurological phenotype that recapitulates many of the symptoms of RTT, including postnatal onset of the neurological deficits. MeCP2 has two isoforms, MeCP2e1 and MeCP2e2, with distinct amino termini, which are generated by alternative splicing. We examined the distribution of the Mecp2 splice variants in the postnatal mouse brain by in situ hybridization and found regional and age-related differences in transcript abundance. In newborn mice, signals for total Mecp2 and the Mecp2e2 transcripts were widely distributed, with overlapping expression patterns throughout the brain. Expression of the Mecp2e2 splice variant became largely restricted to nuclei within the dorsal thalamus (DT) and cortical layer V in juvenile animals, a pattern that was maintained into adulthood. In contrast, the total Mecp2 riboprobe only weakly labeled the DT and cortical layer V in juvenile and adult animals, although it heavily labeled surrounding brain regions, suggesting that Mecp2e1 is the predominant transcript outside the thalamus. Quantitative real-time PCR was used to measure Mecp2e1 and Mecp2e2 abundance in the diencephalon of adult mice, demonstrating significantly more Mecp2e2 in the DT than in the hypothalamus, which is in agreement with the Mecp2e2 in situ hybridization. The differential distribution of the Mecp2e1 and Mecp2e2 transcripts indicates regional and developmental regulation of Mecp2 splicing in the postnatal mouse brain.

Effect of inspired oxygen concentration on periodic breathing in methyl‐CpG‐binding protein 2 (Mecp2) deficient mice

Rett syndrome (RTT) is characterized by periodic breathing (PB). The goal of this study was to determine if moderate hyperoxia or hypoxia affected the incidence and/or characteristics of PB in Mecp2+/− mice. Individual mice were exposed for 30 min to air, 40% or 12% oxygen. Respiratory pattern was monitored by telemetric measurement of pleural pressure. PB was defined as 3 or more periods of 3 to 20 incrementing then decrementing amplitude breaths separated by apneas. Hyperoxia increased incidence of PB in Mecp2+/− mice from 115.3 ± 27.5 to 219 ± 58.9 s and number of PB episodes from 8.1 ± 1.6 to 12.3 ± 2.4. Episode duration was the same. 40% oxygen increased A/V from 0.44 ± 0.04 to 0.58 ± 0.05. Hypoxia did not significantly affect the incidence (71.3 ± 31 vs 115.3 ± 27.5 sec), number of episodes (4.6 ± 1.6 vs 8.1 ± 1.6), period length nor A/V in Mecp2+/− mice. The duration of PB episodes was decreased in 12 % oxygen (10.2 ± 1.6 vs 14.1 ± 1.5 sec). This paradoxical effect of inspired oxygen concentration suggests that the increased loop gain in Mecp2+/− mice is not due to an augmented hypoxic drive from peripheral chemoreceptors. Rather the results suggest that this mouse model of RTT has an unbalanced contribution from excitatory and inhibitory inputs to inspiratory central neurons such that hyperoxic modulation of the inhibitory influences exacerbates their breathing instability. Support: FY06-314 March of Dimes

Ube3a expression is not altered in Mecp2 mutant mice

Rett syndrome (RTT) is a neurodevelopmental disorder characterized by cognitive regression, loss of purposeful hand movements and speech, stereotypies, ataxia, seizures, mental retardation and acquired microcephaly. Mutations in MECP2, encoding methyl-CpG-binding protein 2, are responsible for approximately 90% of classic RTT cases. RTT displays phenotypic overlap with Angelman syndrome, a disorder caused by loss of expression of the imprinted gene UBE3A. MeCP2 binds to methylated DNA and may alter the expression of imprinted genes, thereby suggesting a mechanistic link between the two disorders. Here, we tested the hypothesis that MeCP2 deficiency affects expression of Ube3a in mouse models of RTT. As Ube3a is only imprinted in brain, we evaluated Ube3a expression in brains of 15 different litters of neonatal or 8-week-old male Mecp2 mutant mice by real-time quantitative RT-PCR and western blot analysis. We found no significant differences between Mecp2(tm1.1Bird/Y) or Mecp2(tm1.1Jae/Y) mutants and their wild-type male siblings that served as negative controls. In positive control mice carrying a maternally inherited Ube3a deletion, Ube3a sense transcript and protein levels were drastically reduced. Our data contrast with two recent reports of substantially decreased Ube3a expression in brain tissues of MeCP2-deficient mice. We, therefore, challenge the conclusion that decreased UBE3A/Ube3a expression contributes to the pathophysiology of RTT.

Inhibitors of differentiation (ID1, ID2, ID3 and ID4) genes are neuronal targets of MeCP2 that are elevated in Rett syndrome

Rett syndrome (RTT) is an X-linked dominant neurodevelopmental disorder caused by mutations in MECP2, encoding methyl-CpG-binding protein 2. MeCP2 is a transcriptional repressor elevated in mature neurons and is predicted to be required for neuronal maturation by regulating multiple target genes. Identifying primary gene targets in either Mecp2-deficient mice or human RTT brain has proven to be difficult, perhaps because of the transient requirement for MeCP2 during neuronal maturation. In order to experimentally control the timing of MeCP2 expression and deficiency during neuronal maturation, human SH-SY5Y cells undergoing mature neuronal differentiation were transfected with methylated MeCP2 oligonucleotide decoy to disrupt the binding of MeCP2 to endogenous targets. Genome-wide expression microarray analysis identified all four known members of the inhibitors of differentiation or inhibitors of DNA-binding (ID1, ID2, ID3 and ID4) subfamily of helix-loop-helix genes as novel neuronal targets of MeCP2. Chromatin immunoprecipitation analysis confirmed binding of MeCP2 near or within the promoters of ID1, ID2 and ID3, and quantitative RT-PCR confirmed increased expression of all four Id genes in Mecp2-deficient mouse brain. All four ID proteins were significantly increased in Mecp2-deficient mouse and human RTT brain using immunofluorescence and laser scanning cytometric analyses. Because of their involvement in cell differentiation and neural development, ID genes are ideal primary targets for MeCP2 regulation of neuronal maturation that may explain the molecular pathogenesis of RTT.

Separate Respiratory Phenotypes in Methyl-CpG-Binding Protein 2 (Mecp2) Deficient Mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2) that encodes a DNA binding protein involved in gene silencing. Selective deletion of Mecp2 in post-mitotic neurons in mice results in a Rett-like phenotype characterized by disturbances in motor activity and body weight, suggesting that these symptoms are exclusively caused by neuronal deficiency. Included in the RTT phenotype are episodes of respiratory depression that follow hyperventilation. Here we show that the respiratory phenotype depends on the organ distribution of Mecp2 deficiency. Both female mice heterozygous for a null mutation in Mecp2 (Mecp2+/-) and those with selective deletion of the protein in neurons (Mecp2+/nestin-Cre lox), showed an initial response to hypoxia that exceeded that in wild type (WT). However, marked respiratory depression following hypoxic hyperventilation was only seen in Mecp2+/- animals. Addition of carbon dioxide to the hypoxic exposure eliminated the respiratory depression. Tidal volume and lung volume were larger in Mecp2+/- and respiratory depression was directly related to tidal volume. Taken together these results indicate that the depression is due to hypocapnia. Respiratory depression in this mouse model of Rett Syndrome is seen in with ubiquitous deficiency in Mecp2 but not when it is confined to neurons.

Autonomic cardiovascular regulation in methyl‐CpG‐binding protein 2 (Mecp2) deficient mice

The transcription regulating protein Mecp2, which is involved in gene silencing, is deficient in Rett syndrome. There are suggestions that the autonomic nervous system is affected in this X-linked neurodevelopmental disorder. Blood pressure waveforms were sampled by telemetry in mice at 6–8 months. Heart rate variability (HRV) was evaluated from the standard deviation of pulse interval (PI), the range that encompassed 90% of PI and the low frequency to high frequency ratio from spectral analysis of PI in the frequency domain. Atropine produced a greater increase in heart rate in Mecp2+/− (576±22 to 674±10 bpm; ± SEM) compared to wild type (WT) (631±22 to 673±13) (P = 0.038) Vagal blockade resulted in ~ 50% decrease in indices of HRV in both strains. Heart rate decreased in WT from 596±24 to 506±13 following propranolol, which was not different from Mecp2+/−(620±22 to 504±25). There were no significant changes in mean arterial pressure (MAP) after propranolol. HRV changes were the same in both strains Elevating ambient temperature to 32° C resulted in a fall in heart rate (609±25 to 488±46) and MAP (105±3 to 91±3 mmHg) in WT that was not different from Mecp2+/−(617±30 to 473±22) (119±3 to 99±6). The indices for HRV were increased (~35%) in both strains with increased ambient temperature. The autonomic control of cardiovascular regulation in this mouse model of Rett syndrome is similar to wild type. (Supported by HD044453 from NIH)

Breeding and maintenance of an Mecp2-deficient mouse model of Rett syndrome

In this report, we present a retrospective assessment of our experiences in maintaining a colony of Mecp2-deficient mice, and present conditions that we have found to foster breeding success of this mutant mouse strain. Data from our colony of mutant Mecp2 mice show that the Mecp2-null allele is under-represented in the weaned litter population, that litters born to heterozygous Mecp2-null females are consistently smaller in number than wild-type, and that fewer litters from heterozygous Mecp2-null females survive to weaning age than wild-type. Our results also reveal that overall litter viability is significantly higher in heterozygous Mecp2-null females that frequently breed, and that the addition of sunflower seeds to the cages of expecting dams improves the overall breeding success of these mice. Taken together, these data highlight the breeding tendencies for this mutant mouse strain, and from these data, we suggest strategies to maximise their breeding efficiency.

Dynamic Changes in Histone H3 Lysine 9 Acetylation Localization Patterns During Neuronal Maturation Require MeCP2

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

The Molecular Pathology of Rett Syndrome: Synopsis and Update

Genetic mutations of the X-linked gene MECP2, encoding methyl-CpG-binding protein 2, cause Rett syndrome (RTT) and other neurological disorders. It is increasingly recognized that MECP2 is a multifunctional protein, with at least four different functional domains: (1) a methyl-CpG-binding domain; (2) an arginine-glycine repeat RNA-binding domain; (3) a transcriptional repression domain; and (4) an RNA splicing factor binding region (WW group II binding domain). There is evidence that MECP2 is important for large-scale reorganization of pericentromeric heterochromatin during differentiation. Studies in MECP2-deficient mouse brain have identified a diverse set of genes with altered levels of mRNA expression or splicing. It is still unclear how altered MECP2 function ultimately results in neuronal disease after a period of grossly normal development. However, mounting evidence suggests that neuronal health and development depend on precise regulation of MECP2 expression. In genetically engineered mice, both increased and decreased levels of MECP2 result in a neurological phenotype. Furthermore, it was recently discovered that MECP2 gene duplications underlie a small number of atypical Rett cases and mental retardation syndromes. The finding that MECP2 levels are tightly regulated in neurons has important implications for the design of gene replacement or reactivation strategies for treatment of RTT, because affected individuals typically are somatic mosaics with one set of cells expressing a mutated MECP2 from the affected X, and another set expressing normal MECP2 from the unaffected X. Further studies are necessary to elucidate the molecular pathology of both loss-of-function and gain-of-function mutations in MECP2.

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

Autism is a common neurodevelopmental disorder of complex genetic etiology. Rett syndrome, an X-linked dominant disorder caused by MECP2 mutations, and Angelman syndrome, an imprinted disorder caused by maternal 15q11-q13 or UBE3A deficiency, have phenotypic and genetic overlap with autism. MECP2 encodes methyl-CpG-binding protein 2 that acts as a transcriptional repressor for methylated gene constructs but is surprisingly not required for maintaining imprinted gene expression. Here, we test the hypothesis that MECP2 deficiency may affect the level of expression of UBE3A and neighboring autism candidate gene GABRB3 without necessarily affecting imprinted expression. Multiple quantitative methods were used including automated quantitation of immunofluorescence and in situ hybridization by laser scanning cytometry on tissue microarrays, immunoblot and TaqMan PCR. The results demonstrated significant defects in UBE3A/E6AP expression in two different Mecp2 deficient mouse strains and human Rett, Angelman and autism brains compared with controls. Although no difference was observed in the allelic expression of several imprinted transcripts in Mecp2-null brain, Ube3a sense expression was significantly reduced, consistent with the decrease in protein. A non-imprinted gene from 15q11-q13, GABRB3, encoding the beta3 subunit of the GABAA receptor, also showed significantly reduced expression in multiple Rett, Angelman and autism brain samples, and Mecp2 deficient mice by quantitative immunoblot. These results suggest an overlapping pathway of gene dysregulation within 15q11-q13 in Rett, Angelman and autism and implicate MeCP2 in the regulation of UBE3A and GABRB3 expressions in the postnatal mammalian brain.

Temporal and regional differences in the olfactory proteome as a consequence of MeCP2 deficiency.

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the gene encoding MeCP2. By binding to methylated CpG dinucleotide promoter regions, MeCP2 acts as a transcriptional repressor, predicting that its absence might result in widespread aberrant gene transcription, leading to the RTT phenotype. Considering this potentially broad action of MeCP2 on expression and the complexity of the brain, especially during development, we approached the consequences of MeCP2 deficiency in a mouse model by using a temporal and regional proteomic strategy. We used the olfactory system (olfactory epithelium and bulb) because its attributes make it an excellent developmental model system. We find evidence of temporal and regional proteomic pattern differences between WT and MeCP2-deficient mice. It was possible to segregate these changes in protein expression into five biological function groups: cytoskeleton arrangement, chromatin modeling, energy metabolism, cell signaling, and neuroprotection. By combining the proteomic results with the RNA levels of the identified proteins, we show that protein expression changes are the consequence of differences in mRNA level or posttranslational modifications. We conclude that brain regions and ages must be carefully considered when investigating MeCP2 deficiency, and that not only transcription should be taken into account as a source for these changes, but posttranslational protein modifications as well.

Diseases of the mind and brain: Rett's syndrome, part II.

Back to table of contents Previous article Next article Images in NeuroscienceFull AccessDiseases of the Mind and BrainRett’s Syndrome, Part IISchahram Akbarian, M.D., Ph.D., Schahram AkbarianSearch for more papers by this author, M.D., Ph.D., Worcester, Mass.Published Online:1 Aug 2002https://doi.org/10.1176/appi.ajp.159.8.1294AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InEmail Rett’s syndrome is an illness expressed predominantly in female infants that is caused by mutations in a gene encoding methyl-CpG binding protein 2 (MECP2). This protein binds to methylated DNA and may function as a transcriptional repressor by associating with the DNA chromatin-remodeling complexes. The resultant infant syndrome begins in seemingly normal 6- to 12-month-old babies and causes profound mental retardation and premature death across a wide age range (12–40 years). The mental retardation occurs without significant neurodegeneration but rather with a reduction in neuronal size and loss of neuropil throughout gray matter areas, in conjunction with a decrease in white matter. For this illness, only limited information can be collected by studying patients or tissue samples; this information is essential for developing a full understanding of the pathophysiologic processes caused by the gene mutation. Such a mechanistic understanding must be available to develop a rational treatment for these affected infants. However, since Rett’s syndrome is a genetic illness, and we know the specific genetic defect, the syndrome can be reproduced in animals and studied in these models.Laboratory animals can be created with the same loss of MECP2 function as seen in Rett’s syndrome infants. Scientists have created several lines of mouse mutants in which various parts of the MECP2 gene are deleted in the brain. These mice provide a representative model of the infant condition: they are born neurologically normal and fertile but then develop abnormal behaviors around 4–5 weeks of age; they then regress and die by 8–10 weeks of age. Reduced brain size as well as a reduction in neuronal size and in neuropil are seen in many brain areas, including the neocortex and parts of the limbic system. These MECP2 mutant mice will be directly used in research to develop treatments for Rett’s syndrome.Address reprint requests to Dr. Tamminga, Maryland Psychiatric Research Center, University of Maryland, P.O. Box 21247, Baltimore, MD 21228; [email protected] (e-mail). Images A–D are courtesy of Dr. T.L. Kemper and Dr. M.L. Baumann. Images E and F are courtesy of Dr. Akbarian. Rett's SyndromeMyelin stains of coronal sections through the parietal and temporal lobes taken from a healthy subject (A) and a Rett’s syndrome patient (B) illustrate the reduction of both gray and white matter seen in Rett’s syndrome. Low-power Nissl-stained sections of the hippocampus at the hilus of dentate gyrus level in a healthy subject (C) and a Rett’s syndrome patient (D) show the reduced neuronal size associated with Rett’s syndrome. Last, high-power Nissl-stained sections from the CA1/CA2 border region of the hippocampus taken from a normal mouse (E) and an MECP2-deficient mouse (F) reveal shrunken, dystrophic, and densely packed neurons in the latter. FiguresReferencesCited byDetailsCited byNone Volume 159Issue 8 August 2002Pages 1294-1294 Metrics PDF download History Published online 1 August 2002 Published in print 1 August 2002

Deficiency of methyl-CpG binding protein-2 in CNS neurons results in a Rett-like phenotype in mice.

Mecp2 is an X-linked gene encoding a nuclear protein that binds specifically to methylated DNA (ref. 1) and functions as a general transcriptional repressor by associating with chromatin-remodeling complexes. Mecp2 is expressed at high levels in the postnatal brain, indicating that methylation-dependent regulation of gene expression may have a crucial role in the mammalian central nervous system. Consistent with this notion is the recent demonstration that MECP2 mutations cause Rett syndrome (RTT, MIM 312750), a childhood neurological disorder that represents one of the most common causes of mental retardation in females. Here we show that Mecp2-deficient mice exhibit phenotypes that resemble some of the symptoms of RTT patients. Mecp2-null mice were normal until 5 weeks of age, when they began to develop disease, leading to death between 6 and 12 weeks. Mutant brains showed substantial reduction in both weight and neuronal cell size, but no obvious structural defects or signs of neurodegeneration. Brain-specific deletion of Mecp2 at embryonic day (E) 12 resulted in a phenotype identical to that of the null mutation, indicating that the phenotype is caused by Mecp2 deficiency in the CNS rather than in peripheral tissues. Deletion of Mecp2 in postnatal CNS neurons led to a similar neuronal phenotype, although at a later age. Our results indicate that the role of Mecp2 is not restricted to the immature brain, but becomes critical in mature neurons. Mecp2 deficiency in these neurons is sufficient to cause neuronal dysfunction with symptomatic manifestation similar to Rett syndrome.