Increased Expression of PcG Protein YY1 Negatively Regulates B Cell Development while Allowing Accumulation of Myeloid Cells and LT-HSC Cells Xuan Pan1, Morgan Jones2, Jie Jiang 1 , Kristina Zaprazna1, Duonan Yu 3 , Warren Pear4, Ivan Maillard2, 1 Michael L. Atchison * 1 Department of Animal Biology, School of Veterinary, University of Pennsylvania, Medicine, Philadelphia, Pennsylvania, United States of America, 2 Center for Stem Cell Biology, Life Sciences Institute, Departments of Medicine & Cell and Developmental Biology, University of Michigan, Ann Arbor, Michigan, United States of America, 3 Division of Hematology, The Children’s Hospital of Philadelphia, Philadelphia, Pennsylvania, United States of America, 4 Department of Pathology and Laboratory Medicine, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America Abstract Ying Yang 1 (YY1) is a multifunctional Polycomb Group (PcG) transcription factor that binds to multiple enhancer binding sites in the immunoglobulin (Ig) loci and plays vital roles in early B cell development. PcG proteins have important functions in hematopoietic stem cell renewal and YY1 is the only mammalian PcG protein with DNA binding specificity. Conditional knock-out of YY1 in the mouse B cell lineage results in arrest at the pro-B cell stage, and dosage effects have been observed at various YY1 expression levels. To investigate the impact of elevated YY1 expression on hematopoetic development, we utilized a mouse in vivo bone marrow reconstitution system. We found that mouse bone marrow cells expressing elevated levels of YY1 exhibited a selective disadvantage as they progressed from hematopoietic stem/progenitor cells to pro-B, pre- B, immature B and re-circulating B cell stages, but no disadvantage of YY1 over-expression was observed in myeloid lineage cells. Furthermore, mouse bone marrow cells expressing elevated levels of YY1 displayed enrichment for cells with surface markers characteristic of long-term hematopoietic stem cells (HSC). YY1 expression induced apoptosis in mouse B cell lines in vitro, and resulted in down-regulated expression of anti-apoptotic genes Bcl-xl and NFkB2, while no impact was observed in a mouse myeloid line. B cell apoptosis and LT-HSC enrichment induced by YY1 suggest that novel strategies to induce YY1 expression could have beneficial effects in the treatment of B lineage malignancies while preserving normal HSCs. Citation: Pan X, Jones M, Jiang J, Zaprazna K, Yu D, et al. (2012) Increased Expression of PcG Protein YY1 Negatively Regulates B Cell Development while Allowing Accumulation of Myeloid Cells and LT-HSC Cells. PLoS ONE 7(1): e30656. doi:10.1371/journal.pone.0030656 Editor: Maurizio Pesce, Centro Cardiologico Monzino, Italy Received May 20, 2011; Accepted December 22, 2011; Published January 23, 2012 Copyright: 2012 Pan 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 National Institutes of Health Grants GM071830 and AI079002 to MA. 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: atchison@vet.upenn.edu Introduction Yin Yang 1 (YY1) is a ubiquitous and multifunctional zinc- finger transcription factor that mediates multiple diverse functions. YY1 can act as a transcriptional activator, repressor or initiator protein depending upon DNA binding site context or cell type [1,2,3]. Homozygous disruption of the yy1 gene in mice results in peri-implantation lethality. Heterozygous knock-out mice show growth retardation and some neurological defects [4]. Reduction of YY1 levels impairs embryonic growth and viability in a dose- dependent manner [5]. There is a tight correlation between YY1 dosage and cell proliferation, with deletion of the yy1 gene resulting in cytokinesis failure and cell cycle arrest [5]. YY1 is also implicated in lineage differentiation and cell growth control [6,7,8], as well as in oncogenesis and other diseases such as dystrophic muscle disease [6,9]. YY1 can bind the retinoblastoma (Rb) protein to accelerate cell cycle progression to the S phase [8,10], can activate c-myc P1 promoter activity in Burkitt’s lymphoma [11], and can enhance murine double minute 2 (mdm2)-mediated p53 inactivation thus potentiating cellular proliferation and tumorigenesis [12]. On the contrary, in human basal cell carcinoma YY1 shows repressive activity at the GST locus and may prevent tumor progression caused by the GSTM3 genotype [13]. Indeed, Lichy et al found a marked decrease in YY1 binding in malignant HeLa/fibroblast somatic cell hybrids when compared to non-tumor cells, [14] while Austen and colleagues showed that YY1 is a negative regulator of cell growth via potent inhibition of c-myc transforming activity and possible involvement in tumor suppression [15]. These diverse YY1 functions probably result from its ability to interact with numerous proteins and complexes. YY1 is the only known mammalian Polycomb Group (PcG) protein with DNA binding specificity. We found that YY1 is functionally similar to the apparently orthologous Drosophila PcG protein, Pleiohomeotic (PHO) [16]. YY1 can repress transcription in a PcG-dependent fashion, can recruit PcG proteins to DNA, can correct phenotypic defects in PHO mutant flies, and can control genes needed for development and differentiation [17,18,19,20]. In mammals, PcG proteins are implicated in Homeobox (Hox) gene regulation and stable silencing of specific sets of genes through chromatin modifications. PcG proteins are also involved in maintenance of embryonic and adult stem cells. PLoS ONE | www.plosone.org 1 January 2012 | Volume 7 | Issue 1 | e30656