Radically truncated MeCP2 rescues Rett syndrome-like neurological defects

Abstract

Heterozygous mutations in the X-linked MECP2 gene cause the profound neurological disorder Rett syndrome (RTT)1. MeCP2 protein is an epigenetic reader whose binding to chromatin primarily depends on 5-methylcytosine (mC)2,3. Functionally, MeCP2 has been implicated in several cellular processes based on its reported interaction with >40 binding partners4, including transcriptional co-repressors (e.g. the NCoR/SMRT complex5), transcriptional activators6, RNA7, chromatin remodellers8,9, microRNA-processing proteins10 and splicing factors11. Accordingly, MeCP2 has been cast as a multi-functional hub that integrates diverse processes that are essential in mature neurons12. At odds with the concept of broad functionality, missense mutations that cause RTT are concentrated in two discrete clusters coinciding with interaction sites for partner macromolecules: the Methyl-CpG Binding Domain (MBD)13 and the NCoR/SMRT Interaction Domain (NID)5. Here, we test the hypothesis that the single dominant function of MeCP2 is to physically connect DNA with the NCoR/SMRT complex, by removing almost all amino acid sequences except the MBD and NID. We find that mice expressing truncated MeCP2 lacking both the N- and C-terminal regions (approximately half of the native protein) are phenotypically near-normal; and those expressing a minimal MeCP2 additionally lacking a central domain survive for over one year with only mild symptoms. This minimal protein is able to prevent or reverse neurological symptoms when introduced into MeCP2-deficient mice by genetic activation or virus-mediated delivery to the brain. Thus, despite evolutionary conservation of the entire MeCP2 protein sequence, the DNA and co-repressor binding domains alone are sufficient to avoid RTT-like defects and may therefore have therapeutic utility.

Neuronal non-CG methylation is an essential target for MeCP2 function

“…The key function of MeCP2 is to form a bridge between methylated DNA and the NCOR1/2 corepressor complexes, dependent on the MBD and NID (NCOR1/2 Interaction Domain), respectively (Guy et al, 2018;Kruusvee et al, 2017;Lyst et al, 2013;Tillotson et al, 2017).…”

MeCP2 nuclear dynamics in live neurons results from low and high affinity chromatin interactions

“…Several functional domains have been identified previously in MeCP2 and shown to play important roles in vivo and in vitro (Baker et al, 2013;Ghosh et al, 2010b;Heckman et al, 2014;Tillotson et al, 2017). Here we have examined the contributions of these domains to the dynamic behavior of single MeCP2 molecules in live granule cell nuclei, revealing several interesting features of MeCP2 that have not been reported previously.…”

Partial rescue of neuronal genes deregulated in Cornelia de Lange Syndrome by cohesin

“…Our data raise the possibility that aspects of the CdLS phenotype may be reversible by postnatal intervention. Precedent for such rescue comes from pioneering studies in mouse models of Rett syndrome, where postnatal correction of Mecp2 deficiency ameliorates deregulated gene expression and behavioural defects 22,23 . As the Rett protein MECP2, cohesin exerts its effects on transcription by interfacing with chromatin marking systems 44 , and the dynamics of cohesin on chromatin is controlled in part by acetylation and deacetylation of its SMC subunits 45 .…”

“…Postnatal intervention can alleviate transcriptional changes and abnormal behaviours in mouse models of Rett Syndrome 22,23 . In light of these results, we asked whether gene expression changes induced by acute cohesin depletion in Rad21 Tev/Tev neurons can be reversed when cohesin levels are restored.…”

“…However, these studies have been unable to discern whether these changes are the result of cohesin deficiency during development, or reflect an ongoing requirement for cohesin in mature neurons. This is of interest, as mouse models of neurodevelopmental syndromes such as Rett Syndrome can benefit from postnatal interventions that correct aberrant gene expression and animal behaviour 22,23 . Although aberrant gene expression is a likely cause of neuronal dysfunction in CdLS, studies of gene expression in CdLS patient cells have to date been limited to induced pluripotent stem cells (iPSCs) 24 , cardiac 24 and lymphoblastoid cells 25 .…”