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Functional characterization of EZH2β reveals the increased complexity of EZH2 isoforms involved in the regulation of mammalian gene expression

Adrienne Grzenda1, Gwen Lomberk1, Phyllis Svingen1, Angela Mathison1, Ezequiel Calvo2, Juan Iovanna3, Yuning Xiong1, William Faubion1 and Raul Urrutia1345*

Author Affiliations

1 Laboratory of Epigenetics and Chromatin Dynamics, Mayo Clinic, Rochester, MN, 55905, USA

2 Molecular Endocrinology and Oncology Research Center, CHUL Research Center, Quebec, Canada

3 INSERM U.624, Stress Cellulaire, 163 Avenue de Luminy, Case 915, Parc Scientifique et Technologique de Luminy, Marseille Cedex 9, 13288, France

4 Translational Epigenomics Program, Center for Individualized Medicine (CIM), Mayo Clinic, Rochester, MN, 55905, USA

5 Departments of Medicine, Physiology and Biochemistry, Mayo Clinic, Rochester, MN, 55905, USA

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Epigenetics & Chromatin 2013, 6:3  doi:10.1186/1756-8935-6-3

Published: 28 February 2013

Abstract

Background

Histone methyltransferase enhancer of zeste homologue 2 (EZH2) forms an obligate repressive complex with suppressor of zeste 12 and embryonic ectoderm development, which is thought, along with EZH1, to be primarily responsible for mediating Polycomb-dependent gene silencing. Polycomb-mediated repression influences gene expression across the entire gamut of biological processes, including development, differentiation and cellular proliferation. Deregulation of EZH2 expression is implicated in numerous complex human diseases. To date, most EZH2-mediated function has been primarily ascribed to a single protein product of the EZH2 locus.

Results

We report that the EZH2 locus undergoes alternative splicing to yield at least two structurally and functionally distinct EZH2 methyltransferases. The longest protein encoded by this locus is the conventional enzyme, which we refer to as EZH2α, whereas EZH2β, characterized here, represents a novel isoform. We find that EZH2β localizes to the cell nucleus, complexes with embryonic ectoderm development and suppressor of zeste 12, trimethylates histone 3 at lysine 27, and mediates silencing of target promoters. At the cell biological level, we find that increased EZH2β induces cell proliferation, demonstrating that this protein is functional in the regulation of processes previously attributed to EZH2α. Biochemically, through the use of genome-wide expression profiling, we demonstrate that EZH2β governs a pattern of gene repression that is often ontologically redundant from that of EZH2α, but also divergent for a wide variety of specific target genes.

Conclusions

Combined, these results demonstrate that an expanded repertoire of EZH2 writers can modulate histone code instruction during histone 3 lysine 27-mediated gene silencing. These data support the notion that the regulation of EZH2-mediated gene silencing is more complex than previously anticipated and should guide the design and interpretation of future studies aimed at understanding the biochemical and biological roles of this important family of epigenomic regulators.

Keywords:
Chromatin; Enhancer of zeste homologue 2; Epigenetics; EZH2; Histone methyltransferase; Polycomb; Polycomb repressive complex 2; PRC2