The Synovial Sarcoma-Associated SYT-SSX2 Oncogene Antagonizes the Polycomb Complex Protein Bmi1 Roy Barco, Christina B. Garcia, Josiane E. Eid* Department of Cancer Biology, Vanderbilt University Medical Center, Nashville, Tennessee, United States of America Abstract This study demonstrates deregulation of polycomb activity by the synovial sarcoma-associated SYT-SSX2 oncogene, also known as SS18-SSX2. Synovial sarcoma is a soft tissue cancer associated with a recurrent t(X:18) translocation event that generates one of two fusion proteins, SYT-SSX1 or SYT-SSX2. The role of the translocation products in this disease is poorly understood. We present evidence that the SYT-SSX2 fusion protein interacts with the polycomb repressive complex and modulates its gene silencing activity. SYT-SSX2 causes destabilization of the polycomb subunit Bmi1, resulting in impairment of polycomb-associated histone H2A ubiquitination and reactivation of polycomb target genes. Silencing by polycomb complexes plays a vital role in numerous physiological processes. In recent years, numerous reports have implicated gain of polycomb silencing function in several cancers. This study provides evidence that, in the appropriate context, expression of the SYT-SSX2 oncogene leads to loss of polycomb function. It challenges the notion that cancer is solely associated with an increase in polycomb function and suggests that any imbalance in polycomb activity could drive the cell toward oncogenesis. These findings provide a mechanism by which the SYT-SSX2 chimera may contribute to synovial sarcoma pathogenesis. Citation: Barco R, Garcia CB, Eid JE (2009) The Synovial Sarcoma-Associated SYT-SSX2 Oncogene Antagonizes the Polycomb Complex Protein Bmi1. PLoS ONE 4(4): e5060. doi:10.1371/journal.pone.0005060 Editor: Nils Cordes, Dresden University of Technology, Germany Received November 14, 2008; Accepted March 6, 2009; Published April 1, 2009 Copyright: 2009 Barco et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by Grant RO1CA106481, by the Public Health Service award T32 GM07347 from the National Institute of General Medical Studies for the Vanderbilt Medical-Scientist Training Program, and by the Vanderbilt Ingram Cancer Center Core Grant P30 CA068485. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: josiane.eid@vanderbilt.edu Introduction The manipulation of chromatin organization is at the heart of a plethora of biological processes. Many proteins which modify the structure of chromatin during normal cellular events are often deregulated in disease processes, including cancer. One group of proteins involved in the stable and dynamic regulation of chromatin heritable over successive cellular divisions is the polycomb group (PcG) family of complexes [1]. PcG complexes are implicated in the repression of gene transcription through exquisite modulation of chromatin structure. They were originally identified in Drosophila as repressors of Hox genes for posterior body segmentation [2]. polycomb complexes are classically divided into Polycomb Repressive Complex 1 (PRC1) and Polycomb Repres- sive Complex 2 (PRC2) [1]. The PRC2 complex is comprised of the subunits EZH2, EED and SUZ12. PRC2 promotes gene silencing by associating with histone deacetylases and DNA- methyltransferase 1 [3,4], and through methylation of histone H3 at lysine 27 (K27) [5,6]. Methyl-H3K27 in turn serves as a binding platform for PRC1, composed mostly of PC2, HPH, Bmi1 and Ring1A/B. PRC1 compacts chromatin and hinders its accessibil- ity to transcriptional activators [7]. Moreover, the complex between subunits Bmi1 and Ring1B (an E3 ligase) is implicated in the monoubiquitination of histone H2A at lysine 119 (K119) [8], a modification frequently detected in the promoters of polycomb target genes. Polycomb silencing plays an important role in cell fate determination, self-renewal in both embryonic and adult stem cells, as well as X-chromosome inactivation [1]. These processes result from polycomb suppression of key developmental pathways, including Wnt and Notch [9–11]. Recent evidence implicates deregulation of polycomb function in cancer promotion. A number of reports have described an increase in the expression of polycomb complex proteins in various malignancies; overexpression of Bmi1 in medulloblastoma [12], EZH2 in advanced prostate cancer [13] and SUZ12 in colon cancer [14]. These increases in protein level are thought to result in the aberrant silencing of tumor suppressor genes that would normally prevent carcinogenesis [15]. Conversely, several lines of evidence suggest that a decrease in polycomb function can favor tumor formation in the appropriate context. Downregulation of polycomb complex proteins has been described in certain tumors; these include reduced levels of Ring1B in germ cell tumors [16] and a decrease in Mel18 (polycomb RING Finger) in some breast cancers [17]. Additionally, many cancers, including leukemias and colon cancers, are associated with reactivation of Hox genes, the classical targets of polycomb silencing [18,19]. More directly, one study determined that expression of a loss-of-function mutant of the PRC1 component PC2 resulted in the cellular transformation of fibroblasts [20]. Recently, a report demonstrated a reduction in histone H2A monoubiquitination- a polycomb-induced post- translational modification- in prostate cancer tumor samples [21]. Much like gain-of-polycomb function effects, these studies imply that perturbation of normal polycomb-mediated transcrip- tion repression is involved in some aspect of tumorigenesis. However, it remains to be seen whether any known oncogene can directly influence polycomb silencing in a negative fashion. PLoS ONE | www.plosone.org 1 April 2009 | Volume 4 | Issue 4 | e5060