Edinburgh Research Explorer The molecular basis of MeCP2 function in the brain Citation for published version: Tillotson, R & Bird, A 2020, 'The molecular basis of MeCP2 function in the brain', Journal of Molecular Biology, vol. 432, no. 6. https://doi.org/10.1016/j.jmb.2019.10.004 Digital Object Identifier (DOI): 10.1016/j.jmb.2019.10.004 Link: Link to publication record in Edinburgh Research Explorer Document Version: Publisher's PDF, also known as Version of record Published In: Journal of Molecular Biology General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. 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Oct. 2021 Review The Molecular Basis of MeCP2 Function in the Brain Rebekah Tillotson 1,2 and Adrian Bird 3 1 - Genetics and Genome Biology Program, The Hospital for Sick Children, The Peter Gilgan Centre for Research and Learning, Toronto, ON M5G 0A4, Canada 2 - Medical Research Council (MRC) Molecular Haematology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, John Radcliffe Hospital, Headington, Oxford, OX3 9DS, UK 3 - Wellcome Centre for Cell Biology, University of Edinburgh, The Michael Swann Building, King's Buildings, Max Born Crescent, Edinburgh, EH9 3BF, UK Correspondence to Adrian Bird: [email protected] https://doi.org/10.1016/j.jmb.2019.10.004 Edited by Tuncay Baubec Abstract MeCP2 is a reader of the DNA methylome that occupies a large proportion of the genome due to its high abundance and the frequency of its target sites. It has been the subject of extensive study because of its link with ‘MECP2-related disorders’, of which Rett syndrome is the most prevalent. This review integrates evidence from patient mutation data with results of experimental studies using mouse models, cell lines and in vitro systems to critically evaluate our understanding of MeCP2 protein function. Recent evidence challenges the idea that MeCP2 is a multifunctional hub that integrates diverse processes to underpin neuronal function, suggesting instead that its primary role is to recruit the NCoR1/2 co-repressor complex to methylated sites in the genome, leading to dampening of gene expression. © 2019 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Introduction Next-generation sequencing technologies have revealed where MeCP2 binds in the genome and Numerous chromatin-associated proteins and how it interprets the DNA methylome to influence noncoding RNAs work together to establish cell gene expression. Finally, structural studies have type-specific epigenetic states that regulate gene defined in molecular detail the interactions between expression. These factors are vital for normal MeCP2 and its key binding partners. Together, the mammalian development, with deletion of individual findings provide a coherent picture of MeCP2 members often resulting in lethality in mutant mice function as an essential reader of the DNA methy- [1]. Additionally, epigenetic factors often have links lome in the brain that optimises neuronal transcrip- to neurological disease, caused by mutations tion programmes. affecting dosage such as haploinsufficiency, locus duplications or hypomorphic alleles [2]. Here, we discuss the role of MeCP2, a reader of the DNA MECP2-Related Disorders and Mouse methylome, encoded by the X-linked MECP2 gene. Models The gene has been implicated in several ‘MECP2- related disorders’ [3] prompting numerous studies of Loss of function mutations in the MECP2 gene in MeCP2 protein function. We discuss a spectrum of hemizygous male patients cause neonatal encepha- evidence that sheds light on the molecular mechan- lopathy, which is usually fatal before the age of 2 isms involved, including clinical genetic investiga- years [4]. The same mutations cause the severe tions of genotype-phenotype correlations and neurological disorder Rett syndrome (RTT) in mouse models of the resulting human conditions. heterozygous females [5]. RTT occurs in 1 in 0022-2836/© 2019 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Journal of Molecular Biology (2020) 432, 1602e1623 MeCP2 function in Brain 1603 10e15,000 live female births [6], making it one of the fied 10 other proteins that contain this domain: most common causes of monogenic intellectual MBD1-4, MBD5 (alternative names TAM1, disability in females. The condition is characterised KIAA1461), MBD6 (alternative names TAM2, by 6e18 months of normal development before KIAA1887), BAZ2A (alternative name TIP5), cognitive regression. Affected individuals lose BAZ2B, SETDB1 (alternative names ESET, learned speech and purposeful hand movements, KMT1E) and SETDB2 (alternative name CLLD8) develop stereotypies such as ‘hand-wringing’ and [25e32]. Of these, only MeCP2, MBD1, MBD2 and acquire motor deficiencies including gait ataxia [3]. MBD4 have been shown to bind DNA in vitro in a Symptoms can also include microcephaly [7], methylation-specific manner [23,26,32,33]. Specifi- respiratory problems [8] and seizures [9]. Disease city for the same binding sites in DNA raises the progression plateaus and although patients show possibility of competition between these proteins, slow regression in later life, 70% reach the age of 50 though analysis of their temporal-spatial expression [10]. Other MECP2 mutations that may retain more patterns reveals differences between cell types. protein function have been implicated in milder MeCP2 is expressed in all tissues but reaches psychiatric disorders, such as autism and schizo- near-histone abundance in neurons (~16 Â 106 phrenia in both males and females [http://mecp2. molecules per nucleus) [34]. Its levels increase chw.edu.au/]. The importance of correct MeCP2 during embryonic and postnatal development, pla- dosage is evidenced by MECP2 duplication syn- teauing at 10 years in humans [35] and 5 weeks in drome, which doubles MeCP2 levels and mostly mice [34]. MBD1 is expressed during neurogenesis affects males as the locus containing MECP2 is but is then downregulated [36]. MBD2 and MBD4 are almost always copied within the X chromosome. more widely expressed across somatic tissues, and This condition can run in families, with carrier MBD4 is the only family member detected in females developing no or mild symptoms because embryonic stem cells [36,37]. Whereas complete of extreme (>90%) skewing of X chromosome deletion of Mecp2 in mice leads to severe neurolo- inactivation, thereby silencing the duplicated locus gical symptoms and death around 9 weeks of age [11]. Affected boys display some symptoms that [13,14], knocking out the other members results in overlap with RTT including intellectual disability, minimal phenotypes [36,38e43]. Even though loss of impaired language, gait abnormalities and seizures. MBD1 causes decreased neurogenesis, the animals Individuals with this condition suffer from recurrent have a normal lifespan and only mild behavioural infections because of immunological dysfunction, defects [38,39]. These results suggest that MeCP2 is often leading to death at around 25 years of age the MBD family member with the greatest role in [11,12]. Both RTT and MECP2 duplication syndrome interpreting the DNA methylome in the brain. have been extensively modelled in mice, which MeCP2 protein levels in human and mouse tissues display overt neurological defects as well as correlate poorly with transcription of the MECP2/ phenotypes that can be assessed by behavioural Mecp2 gene [35]. This may be explained by the testing (Table 1)[13e17]. Such studies have activity of several posttranscriptional regulatory established that MeCP2 is required for maintenance mechanisms, including alternative splicing, use of of neuronal function since deletion of the gene in different polyadenylation sites and posttranslational adult mice causes RTT-like defects [18,19]. Excit- modification. The gene spans four exons, which are ingly, reactivation of Mecp2 in symptomatic MeCP2- transcribed and spliced to form two isoforms, e1 and deficient mice leads to phenotypic reversal, indicat- e2, where only e2 includes exon 2 (Fig. 1)[44,45]. ing that development in the absence of MeCP2 Isoform e1 is the ancestral form, conserved across causes little or no lasting damage [20,21]. Similarly, vertebrates, whereas isoform e2 is only present in the behavioural phenotypes of mice overexpressing mammals. The two isoforms are very similar, MeCP2 can be rescued by genetic deletion or differing only at the extreme N-terminus, and most antisense oligonucleotideemediated downregula- evidence indicates that they are functionally inter- tion of one copy [22]. These findings provide hope changeable [46]. Importantly, e2 mRNA is translated that both disorders will be curable. at a much lower efficiency because of the presence of an upstream ATG [44], so the great majority of MeCP2 protein in the brain is derived from e1. The MeCP2 is an Essential Reader of DNA gene contains four alternative polyadenylation sites, 0 Methylation in the Brain producing 3 UTRs ranging from 0.1 to 8.6 kb, although only the longest and shortest mRNAs are MeCP2 was initially discovered over quarter of a detected in neurons. The use of different polyade- century ago because of its ability to bind DNA in a nylation sites determines whether they contain methylation-specific manner [23]. The ~90 amino binding sites for proteins and miRNAs that regulate acid region responsible for binding was called the mRNA stability and translation [47e49]. MeCP2 methyl-CpG binding domain (MBD) [24].