Toxicity of overexpressed MeCP2 is independent of HDAC3 activity.

Martha V Koerner,Mitchell A Lazar,Jim Selfridge,Jacky Guy,Alastair Kerr,Rebekah Tillotson,Dina De Sousa,Zheng Sun,Matthew J Lyst,Adrian Bird,Laura Fitzpatrick

doi:10.1101/gad.320325.118

Abstract

Duplication of the X-linked MECP2 gene causes a severe neurological syndrome whose molecular basis is poorly understood. To determine the contribution of known functional domains to overexpression toxicity, we engineered a mouse model that expresses wild-type or mutated MeCP2 from the Mapt (Tau) locus in addition to the endogenous protein. Animals that expressed approximately four times the wild-type level of MeCP2 failed to survive to weaning. Strikingly, a single amino acid substitution that prevents MeCP2 from binding to the TBL1X(R1) subunit of nuclear receptor corepressor 1/2 (NCoR1/2) complexes, when expressed at equivalent high levels, was phenotypically indistinguishable from wild type, suggesting that excessive corepressor recruitment underlies toxicity. In contrast, mutations affecting the DNA-binding domain were toxic when overexpressed. As the NCoR1/2 corepressors are thought to act through histone deacetylation by histone deacetylase 3 (HDAC3), we asked whether mutations in NCoR1 and NCoR2 that drastically reduced their ability to activate this enzyme would relieve the MeCP2 overexpression phenotype. Surprisingly, severity was unaffected, indicating that the catalytic activity of HDAC3 is not the mediator of toxicity. Our findings shed light on the molecular mechanisms underlying MECP2 duplication syndrome and call for a re-evaluation of the precise biological role played by corepressor recruitment.

Highlights

Mutations involving the X-linked Methyl-CpG-binding protein 2 (MECP2) gene cause an array of human neurological disorders
As the NID interacts with a shared subunit of the nuclear receptor corepressor 1/2 (NCoR1/2) corepressors, it seemed plausible that overrecruitment might cause excessive repression mediated by a catalytically active component of these complexes: histone deacetylase 3 (HDAC3) (Guenther et al 2001)
In order to create a model of MECP2 duplication syndrome, we chose to replace the endogenous the autosomal Mapt (Tau)-coding region with an Mecp2 cDNA using constructs similar to those described previously (Fig. 1A; Luikenhuis et al 2004)

Summary

Introduction

Mutations involving the X-linked MECP2 gene cause an array of human neurological disorders. GENES & DEVELOPMENT 32:1514–1524 Published by Cold Spring Harbor Laboratory Press; ISSN 0890-9369/18; www.genesdev.org that interfere with either NID or MBD function failed to register deleterious phenotypes, suggesting that these two domains must be intact for overexpression to be detrimental (Heckman et al 2014) To test this with increased stringency, we used overexpression of the coding region of MeCP2 from the autosomal Mapt (Tau) locus, which achieves higher MeCP2 abundance and leads to a severe phenotype. As the NID interacts with a shared subunit of the NCoR1/2 corepressors, it seemed plausible that overrecruitment might cause excessive repression mediated by a catalytically active component of these complexes: histone deacetylase 3 (HDAC3) (Guenther et al 2001) To test this hypothesis genetically, we asked whether toxicity associated with overexpression of MeCP2 in mice could be ameliorated by mutation of the deacetylase activation domains (DADs) of both NCoR1 and NCoR2 (You et al 2013). While underlining the importance of the NID as the primary underlying cause of MECP2 duplication syndrome, the findings raise questions about downstream mechanisms that mediate this effect

Methods

Results

Conclusion