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      Biotin tagging of MeCP2 in mice reveals contextual insights into the Rett syndrome transcriptome

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          Abstract

          Mutations in MECP2 cause Rett syndrome (RTT), an X-linked neurological disorder characterized by regressive loss of neurodevelopmental milestones and acquired psychomotor deficits. However, the cellular heterogeneity of the brain impedes an understanding of how MECP2 mutations contribute to RTT. Here we developed a Cre-inducible method for cell type-specific biotin tagging of MeCP2 in mice. Combining this approach with an allelic series of knockin mice carrying frequent RTT mutations (T158M and R106W) enabled the selective profiling of RTT-associated nuclear transcriptomes in excitatory and inhibitory cortical neurons. We found that most gene expression changes are largely specific to each RTT mutation and cell type. Lowly expressed cell type-enriched genes are preferentially disrupted by MeCP2 mutations, with upregulated and downregulated genes reflecting distinct functional categories. Subcellular RNA analysis in MeCP2 mutant neurons further reveals reductions in the nascent transcription of long genes and uncovers widespread post-transcriptional compensation at the cellular level. Finally, we overcame X-linked cellular mosaicism in female RTT models and identified distinct gene expression changes between neighboring wild-type and mutant neurons, altogether providing contextual insights into RTT etiology that support personalized therapeutic interventions.

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          Most cited references48

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          Rett syndrome is caused by mutations in X-linked MECP2, encoding methyl-CpG-binding protein 2.

          Rett syndrome (RTT, MIM 312750) is a progressive neurodevelopmental disorder and one of the most common causes of mental retardation in females, with an incidence of 1 in 10,000-15,000 (ref. 2). Patients with classic RTT appear to develop normally until 6-18 months of age, then gradually lose speech and purposeful hand use, and develop microcephaly, seizures, autism, ataxia, intermittent hyperventilation and stereotypic hand movements. After initial regression, the condition stabilizes and patients usually survive into adulthood. As RTT occurs almost exclusively in females, it has been proposed that RTT is caused by an X-linked dominant mutation with lethality in hemizygous males. Previous exclusion mapping studies using RTT families mapped the locus to Xq28 (refs 6,9,10,11). Using a systematic gene screening approach, we have identified mutations in the gene (MECP2 ) encoding X-linked methyl-CpG-binding protein 2 (MeCP2) as the cause of some cases of RTT. MeCP2 selectively binds CpG dinucleotides in the mammalian genome and mediates transcriptional repression through interaction with histone deacetylase and the corepressor SIN3A (refs 12,13). In 5 of 21 sporadic patients, we found 3 de novo missense mutations in the region encoding the highly conserved methyl-binding domain (MBD) as well as a de novo frameshift and a de novo nonsense mutation, both of which disrupt the transcription repression domain (TRD). In two affected half-sisters of a RTT family, we found segregation of an additional missense mutation not detected in their obligate carrier mother. This suggests that the mother is a germline mosaic for this mutation. Our study reports the first disease-causing mutations in RTT and points to abnormal epigenetic regulation as the mechanism underlying the pathogenesis of RTT.
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            Methylated DNA and MeCP2 recruit histone deacetylase to repress transcription.

            CpG methylation in vertebrates correlates with alterations in chromatin structure and gene silencing. Differences in DNA-methylation status are associated with imprinting phenomena and carcinogenesis. In Xenopus laevis oocytes, DNA methylation dominantly silences transcription through the assembly of a repressive nucleosomal array. Methylated DNA assembled into chromatin binds the transcriptional repressor MeCP2 which cofractionates with Sin3 and histone deacetylase. Silencing conferred by MeCP2 and methylated DNA can be relieved by inhibition of histone deacetylase, facilitating the remodelling of chromatin and transcriptional activation. These results establish a direct causal relationship between DNA methylation-dependent transcriptional silencing and the modification of chromatin.
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              The story of Rett syndrome: from clinic to neurobiology.

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

                Journal
                9502015
                8791
                Nat Med
                Nat. Med.
                Nature medicine
                1078-8956
                1546-170X
                22 August 2017
                18 September 2017
                October 2017
                18 March 2018
                : 23
                : 10
                : 1203-1214
                Affiliations
                [1 ]Department of Genetics, University of Pennsylvania Perelman School of Medicine, Philadelphia, Pennsylvania 19104, USA
                [2 ]Department of Biological Sciences and Center for Systems Biology, The University of Texas at Dallas, Richardson, Texas 75080, USA
                Author notes
                [* ]To whom correspondence should be addressed: zhaolan@ 123456mail.med.upenn.edu
                [†]

                These authors contributed equally to this work.

                Article
                NIHMS901244
                10.1038/nm.4406
                5630512
                28920956
                4cd1b759-a602-4981-87eb-0382dd8f44c0

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