Polycomb group protein complexes: do different complexes regulate distinct target genes?

Epigenetic regulation of embryonic ectoderm development in stem cell differentiation and transformation during ontogenesis

Dynamic chromatin accessibility regulates stem cell fate determination and tissue homeostasis via controlling gene expression. As a histone-modifying enzyme that predominantly mediates methylation of lysine 27 in histone H3 (H3K27me1/2/3), Polycomb repressive complex 2 (PRC2) plays the canonical role in targeting developmental regulators during stem cell differentiation and transformation. Embryonic ectoderm development (EED), the core scaffold subunit of PRC2 and as an H3K27me3-recognizing protein, has been broadly implicated with PRC2 stabilization and allosterically stimulated PRC2. Accumulating evidences from experimental data indicate that EED-associating epigenetic modifications are indispensable for stem cell maintenance and differentiation into specific cell lineages. In this review, we discuss the most updated advances to summarize the structural architecture of EED and its contributions and underlying mechanisms to mediating lineage differentiation of different stem cells during epigenetic modification to expand our understanding of PRC2.

PCL2 regulates p53 stability and functions as a tumor suppressor in breast cancer

Polycomblike2 (PCL2) is a well-known component of polycomb repressive complex 2 (PRC2) and plays important roles in H3K27 methylation and homeotic gene silencing. However, the involvement of PCL2 in breast cancer development remains unclear. Here, we established PCL2 as a tumor suppressor gene in breast cancer. Expression level of PCL2 was significantly downregulated in breast cancer tissue samples observed at different TNM stages. Ectopic expression of PCL2 could significantly inhibit cell proliferation and promoted apoptosis. PCL2 also remarkably elevated levels of p53 and its targets by increasing p53 stability. Mechanistically, PCL2 protected p53 proteins from MDM2-mediated ubiquitination and degradation by sequestering MDM2 into the nucleolus. Overexpression of PCL2 also suppressed migration and invasion by inhibiting epithelial-mesenchymal transition. PCL2 expression was correlated with E-cadherin expression and was inversely correlated with vimentin expression. Furthermore, PCL2 knockdown could attenuate anti-tumor effect of MLN4924. Taken together, our findings indicated that PCL2 played a tumor suppressor role in development and progression of breast cancer and may be a prognostic and predictive marker for breast cancer.

The role of the polycomb repressive complex pathway in T and NK cell lymphoma: biological and prognostic implications

Polycomb repressive complex 2 (PRC2; formed by EZH2, SUZ12, and EED protein subunits) and PRC1 (BMI1 protein) induce gene silencing through histone modification, primarily H3K27me3, and deregulation of PRC pathways leads to tumorigenesis. In the present study, activation of PRC2, H3K27me3, and BMI1 was investigated by immunohistochemistry in 175 cases of T and natural killer (NK) cell lymphoma. Activation of PRC proteins was analyzed according to c-MYC activation, Epstein-Barr virus (EBV) infection, CD30 activation, and survival. Among all T and NK cell lymphomas, high expression rates of 54.7 % for EZH2, 33.3 % for SUZ12, 85.7 % for EED, 40.5 % for H3K27me3, and 30.9 % for BMI1 were discovered. Activation of PRC2, H3K27me3, and BMI1 showed positive correlations (P < 0.05). Activation of c-MYC was associated with activation of SUZ12 and triple coactivation of all PRC2 protein subunits (EZH2(high)/SUZ12(high)/EED(high)) (P < 0.05). In EBV-positive tumors, activation of EZH2 and H3K27me3 showed greater association (P < 0.05). H3K27me3 and BMI1 showed a negative association in tumors expressing CD30 (P < 0.05). With respect to survival, BMI1 activation was independently associated with poor prognosis in T and NK cell lymphomas (P = 0.002). In conclusion, T and NK cell lymphomas were associated with activation of PRC pathway markers, for which c-MYC activation and EBV infection could be suggested as possible causes. PRC pathway markers may be potential therapeutic targets and prognostic markers in T and NK cell lymphoma.

Role of epigenetic mechanisms in epithelial-to-mesenchymal transition of breast cancer cells

The epithelial-to-mesenchymal transition (EMT) is a crucial process during normal development that allows dynamic and reversible shifts between epithelial and mesenchymal cell states. Cancer cells take advantage of the complex, interrelated cellular networks that regulate EMT to promote their migratory and invasive capabilities. During the past few years, evidence has accumulated that indicates that genetic mutations and changes to epigenetic mechanisms are key drivers of EMT in cancer cells. Recent studies have begun to shed light on the epigenetic reprogramming in cancer cells that enables them to switch from a noninvasive form to an invasive, metastatic form. The authors review the current knowledge of alterations of epigenetic machinery, including DNA methylation, histone modifications, nucleosome remodeling and expression of microRNAs, associated with EMT and tumor progression of breast cancer cells. Last, existing and upcoming drug therapies targeting epigenetic regulators and their potential benefit for developing novel treatment strategies are discussed.

SAM domain polymerization links subnuclear clustering of PRC1 to gene silencing

The Polycomb-group (PcG) repressive complex-1 (PRC1) forms microscopically visible clusters in nuclei; however, the impact of this cluster formation on transcriptional regulation and the underlying mechanisms that regulate this process remain obscure. Here, we report that the sterile alpha motif (SAM) domain of a PRC1 core component Phc2 plays an essential role for PRC1 clustering through head-to-tail macromolecular polymerization, which is associated with stable target binding of PRC1/PRC2 and robust gene silencing activity. We propose a role for SAM domain polymerization in this repression by two distinct mechanisms: first, through capturing and/or retaining PRC1 at the PcG targets, and second, by strengthening the interactions between PRC1 and PRC2 to stabilize transcriptional repression. Our findings reveal a regulatory mechanism mediated by SAM domain polymerization for PcG-mediated repression of developmental loci that enables a robust yet reversible gene repression program during development.

Subnuclear distribution of SSX regulates its function

SSX, a family of genes clustered on the X chromosome, has been identified as a cancer-testis antigen and also forms a part of the SYT-SSX fusion gene found in synovial sarcoma, implying that it has an important role in tumorigenesis. However, knowledge of the molecular regulation of SSX is still limited. In this study, we demonstrate that SSX or its SYT fusion protein is distributed as nuclear speckles, in which it is co-localized with B cell-specific Moloney murine leukemia virus insertion site 1 (Bmi1), which is a core factor of polycomb repressor complex 1. The C-terminal residues of SSX are indispensable for the nuclear speckle distribution, while the N-terminal domain is necessary for the recruitment of Bmi1, indicating that intact SSX must be needed for interaction with Bmi1 both spatially and functionally. In addition, the N-terminus of SSX also proved to contain an intrinsic nucleolar localization signal, which mediates the nucleolar translocation of SSX in particular kinds of cell stress such as the oxidation of hydrogen peroxide or heat shock. This stress-induced translocation is reversible and accompanied by HSP 70 or p14ARF traffic, suggesting that SSX is a stress response gene. It is of note that nucleolar translocation of SSX can result in disassociation of SSX from Bmi1, with consequent down-regulation of Bmi1 activity. These novel findings regarding distinct domains of SSX and its interaction with Bmi1 may shed light on the mechanism by which synovial sarcoma develops and on the up-regulation of SSX in cancer cells.

EZH2 inhibition decreases p38 signaling and suppresses breast cancer motility and metastasis

EZH2 is a Polycomb group protein that exerts oncogenic functions in breast cancer, where its overexpression is associated with metastatic disease. While it reportedly acts a transcriptional repressor through trimethylation of histone H3 at lysine 27, EZH2 may exhibit context-dependent activating functions. Despite associations with worse outcome and metastasis in breast cancer, a functional role of EZH2 in breast cancer metastasis in vivo has not been demonstrated. Furthermore, whether EZH2 regulates cancer cell phenotype and motility are unknown. In this study, we discovered that knockdown of EZH2 induces a phenotypic reprogramming from mesenchymal to epithelial, reduces motility, and blocks invasion in breast cancer cell lines. In vivo, EZH2 downregulation in MDA-MB-231 cells decreases spontaneous metastasis to the lungs. We uncover an unexpected role of EZH2 in inducing the p38 mitogen-activated protein kinase signaling pathway, an important regulator of breast cancer invasion and metastasis. In breast cancer cells, EZH2 binds to phosphorylated p38 (p-p38) in association with other core members of the Polycomb repressive complex 2, EED, and SUZ12, and EZH2 overexpression leads to increased levels of p-p38 and of activated, downstream pathway proteins. The effect on p-p38 was confirmed in vivo, where it correlated with decreased spontaneous metastasis. In clinical specimens of matched primary and invasive breast carcinomas, we found that EZH2 expression was upregulated in 100 % of the metastases, and that EZH2 and p-p38 were coexpressed in 63 % of cases, consistent with the functional results. Together our findings reveal a new mechanism by which EZH2 functions in breast cancer, and provide direct evidence that EZH2 inhibition reduces breast cancer metastasis in vivo.

Bmi1 facilitates primitive endoderm formation by stabilizing Gata6 during early mouse development

The transcription factors Nanog and Gata6 are critical to specify the epiblast versus primitive endoderm (PrE) lineages. However, little is known about the mechanisms that regulate the protein stability and activity of these factors in the developing embryo. Here we uncover an early developmental function for the Polycomb group member Bmi1 in supporting PrE lineage formation through Gata6 protein stabilization. We show that Bmi1 is enriched in the extraembryonic (endoderm [XEN] and trophectodermal stem [TS]) compartment and repressed by Nanog in pluripotent embryonic stem (ES) cells. In vivo, Bmi1 overlaps with the nascent Gata6 and Nanog protein from the eight-cell stage onward before it preferentially cosegregates with Gata6 in PrE progenitors. Mechanistically, we demonstrate that Bmi1 interacts with Gata6 in a Ring finger-dependent manner to confer protection against Gata6 ubiquitination and proteasomal degradation. A direct role for Bmi1 in cell fate allocation is established by loss-of-function experiments in chimeric embryoid bodies. We thus propose a novel regulatory pathway by which Bmi1 action on Gata6 stability could alter the balance between Gata6 and Nanog protein levels to introduce a bias toward a PrE identity in a cell-autonomous manner.

Prognostic impact of cancer stem cell-related markers in non-small cell lung cancer patients treated with induction chemoradiotherapy

The expression of several cancer stem cell (CSC)-related markers has been confirmed in non-small cell lung cancer (NSCLC). The aim of this study was to clarify the clinical role of CSC-related markers in patients with NSCLC undergoing induction chemoradiotherapy (CRT). Fifty patients with clinically diagnosed N2 or N3 NSCLC who underwent induction CRT with docetaxel and cisplatin concurrently with thoracic radiation followed by surgery were examined in this study. The expressions of CSC related markers (CD133, ALDH1, ABCG2, and Bmi-1) were examined using immunohistochemical staining in surgically resected specimens. Among the 50 patients, 20 patients had no residual tumor cells in the resected specimen when examined pathologically; CSC-related marker expressions and their correlation to survival were evaluated in the other 30 patients. After a median follow-up period of 72 months, the 5-year overall survival rate of the patients with CD133-positive or ALDH1-positive specimens was significantly worse than that of the patients with both CD133-negative and ALDH1-negative expressions (44.9% vs. 90.0%, respectively; P = 0.042). In a multivariate analysis, CD133 and ALDH1 negativity (P = 0.047) and cN2-3 single station metastasis (P = 0.03) were significant independent prognostic factors for prolonged survival. The expressions of CSC-related markers after CRT were significantly correlated with a poor prognosis in patients with NSCLC. The development of therapeutic strategies including adjuvant therapy that take CSC-related marker positivity into consideration is likely to be a key factor in further improvements of the prognosis of patients undergoing trimodality therapy.

Histone methyltransferase EZH2 induces Akt-dependent genomic instability and BRCA1 inhibition in breast cancer

Increased levels of EZH2, a critical regulator of cellular memory, signal the presence of metastasis and poor outcome in breast cancer patients. High levels of EZH2 are associated with nuclear pleomorphism, lack of estrogen receptor expression, and decreased nuclear levels of BRCA1 tumor suppressor protein in invasive breast carcinomas. The mechanism by which EZH2 overexpression promotes the growth of poorly differentiated invasive carcinomas remains to be defined. Here, we show that EZH2 controls the intracellular localization of BRCA1 protein. Conditional doxycycline-induced upregulation of EZH2 in benign mammary epithelial cells results in nuclear export of BRCA1 protein, aberrant mitoses with extra centrosomes, and genomic instability. EZH2 inhibition in CAL51 breast cancer cells induces BRCA1 nuclear localization and rescues defects in ploidy and mitosis. Mechanistically, EZH2 overexpression is sufficient for activation of the phosphoinositide 3-kinase/Akt (PI3K/Akt) pathway specifically through activation of Akt isoform 1. EZH2-induced BRCA1 nuclear export, aneuploidy, and mitotic defects were prevented by treatment with the PI3K inhibitors LY294002 or wortmannin. Targeted inhibition of Akt-1, Akt-2, and Akt-3 isoforms revealed that the EZH2-induced phenotype requires specific activation of Akt-1. The relevance of our studies to human breast cancer is highlighted by the finding that high EZH2 protein levels are associated with upregulated expression of phospho-Akt-1 (Ser473) and decreased nuclear expression of phospho-BRCA1 (Ser1423) in 39% of invasive breast carcinomas. These results enable us to pinpoint one mechanism by which EZH2 regulates BRCA1 expression and genomic stability mediated by the PI3K/Akt-1 pathway.

Increased risk for distant metastasis in patients with familial early-stage breast cancer and high EZH2 expression

The identification of women with early-stage breast cancer who will develop distant metastasis may improve clinical management. The transcriptional regulator Enhancer of Zeste-2 (EZH2) is overexpressed in invasive breast carcinoma compared with benign breast tissues, with maximal expression in breast cancer metastasis. In this article, our purpose was to investigate the performance of EZH2 protein detection as a predictor of metastasis in women with early-stage breast cancer, which is unknown. We developed a cohort of 480 women with stage I-IIA breast cancer diagnosed between 1996 and 2002 and recorded detailed sociodemographic, clinical, and pathological information. Tumors were histologically characterized and arrayed in tissue microarrays containing 1,443 samples. The nuclear EZH2 expression was investigated by immunohistochemistry and was scored as 1-2 (negative and weak) or 3-4 (moderate and strong) using a validated scoring schema. Scores 1-2 were considered low EZH2; scores 3-4 were considered high EZH2. In this study, we found that after a median follow up of 9 years (range 0.04-14.5 years) 46 of 480 patients (9.6%) developed distant metastasis. High EZH2 was associated with larger size, high histological grade, negative hormone receptors, and first degree family history of breast and/or ovarian carcinoma. While EZH2 could not predict survival in the entire cohort, high EZH2 was a predictor of disease-specific survival in patients with early-stage disease and first degree family history (log rank P value 0.05). Importantly, in this group of patients, high EZH2 was an independent predictor of distant metastasis up to 15 years after primary carcinoma diagnosis (hazard ratio 6.58, 95% CI: 1.40-30.89, P = 0.016) providing survival information above and beyond currently used prognosticators. In conclusion, EZH2 may be a useful biomarker of long-term metastatic risk in women with familial early-stage breast cancer, and warrant further validation studies.

STAT6-mediated displacement of polycomb by trithorax complex establishes long-term maintenance of GATA3 expression in T helper type 2 cells

Polycomb group (PcG) and trithorax group (TrxG) complexes exert opposing effects on the maintenance of the transcriptional status of the developmentally regulated Hox genes. In this study, we show that activation of STAT6 induces displacement of the PcG complex by the TrxG complex at the upstream region of the gene encoding GATA3, a transcription factor essential for T helper type 2 (Th2) cell differentiation. Once Th2 cells differentiate, TrxG complex associated with the TrxG component Menin binds to the whole GATA3 gene locus, and this binding is required for the long-term maintenance of expression of GATA3 and Th2 cytokine. Thus, STAT6-mediated displacement of PcG by the TrxG complex establishes subsequent STAT6-independent maintenance of GATA3 expression in Th2 cells via the recruitment of the Menin-TrxG complex.

Polycomb group gene product Ring1B regulates Th2-driven airway inflammation through the inhibition of Bim-mediated apoptosis of effector Th2 cells in the lung

Polycomb group (PcG) gene products regulate the maintenance of homeobox gene expression in Drosophila and vertebrates. In the immune system, PcG molecules control cell cycle progression of thymocytes, Th2 cell differentiation, and the generation of memory CD4 T cells. In this paper, we extended the study of PcG molecules to the regulation of in vivo Th2 responses, especially allergic airway inflammation, by using conditional Ring1B-deficient mice with a CD4 T cell-specific deletion of the Ring1B gene (Ring1B(-/-) mice). In Ring1B(-/-) mice, CD4 T cell development appeared to be normal, whereas the differentiation of Th2 cells but not Th1 cells was moderately impaired. In an Ag-induced Th2-driven allergic airway inflammation model, eosinophilic inflammation was attenuated in Ring1B(-/-) mice. Interestingly, Ring1B(-/-) effector Th2 cells were highly susceptible to apoptosis in comparison with wild-type effector Th2 cells in vivo and in vitro. The in vitro experiments revealed that the expression of Bim was increased at both the transcriptional and protein levels in Ring1B(-/-) effector Th2 cells, and the enhanced apoptosis in Ring1B(-/-) Th2 cells was rescued by the knockdown of Bim but not the other proapoptotic genes, such as Perp, Noxa, or Bax. The enhanced apoptosis detected in the transferred Ring1B(-/-) Th2 cells in the lung of the recipient mice was also rescued by knockdown of Bim. Therefore, these results indicate that Ring1B plays an important role in Th2-driven allergic airway inflammation through the control of Bim-dependent apoptosis of effector Th2 cells in vivo.

Critical role of the Polycomb and Trithorax complexes in the maintenance of CD4 T cell memory

The maintenance of memory CD4 T cells is crucial for the establishment of immunological memory. The Polycomb (PcG) and Trithorax group (TrxG) genes control key developmental regulators such as the homeobox genes, and these two antagonize each other in the same developmental processes. Recently, PcG gene Bmi1 has been found to control memory Th1/Th2 cell survival and TrxG gene MLL is to control the maintenance of memory Th2 cell function selectively. Therefore, in memory CD4 T cells, PcG and TrxG genes appear to control distinct processes in a distinct manner, which indicates a novel regulatory feature of the PcG/TrxG genes.

Downregulation of EZH2 decreases growth of estrogen receptor-negative invasive breast carcinoma and requires BRCA1

Increased levels of enhancer of zeste homolog 2 (EZH2), a critical regulator of cellular memory, are associated with negative estrogen receptor (ER) expression and disease progression in breast cancer. High levels of EZH2 signal the presence of metastasis and poor outcome in breast cancer patients. To test the hypothesis that deregulation of EZH2 contributes to ER-negative breast cancer progression, EZH2 expression was inhibited in ER-negative breast cancer cells MDA-MB-231 and CAL51 using a lentivirus system. EZH2 knockdown decreased proliferation and delayed the G(2)/M cell-cycle transition, although not affecting apoptosis. In vivo, EZH2 downregulation significantly decreased breast xenograft growth and improved survival. EZH2 knockdown upregulated BRCA1 protein. Of note, BRCA1 knockdown was sufficient to rescue the effects of EZH2 downregulation on proliferation, G(2)/M arrest, and on the levels of hyperphosphorylated mitotic Cdc25C and Cyclin B1 proteins, crucial for entry into mitosis. Invasive ER-negative breast carcinomas show significant overexpression of EZH2 and downregulation of BRCA1 proteins. Taken together, we show that EZH2 is important in ER-negative breast cancer growth in vivo and in vitro, and that BRCA1 is required for the proliferative effects of EZH2. Blockade of EZH2 may provide a prime target to prevent and/or halt ER-negative breast cancer progression.

Repression of E-cadherin by the polycomb group protein EZH2 in cancer

Enhancer of zeste homolog 2 (EZH2) is a critical component of the polycomb-repressive complex 2 (PRC2), which is involved in gene silencing and histone H3 lysine 27 methylation. EZH2 has a master regulatory function in controlling such processes as stem cell differentiation, cell proliferation, early embryogenesis and X chromosome inactivation. Although benign epithelial cells express very low levels of EZH2, increased levels of EZH2 have been observed in aggressive solid tumors such as those of the prostate, breast and bladder. The mechanism by which EZH2 mediates tumor aggressiveness is unclear. Here, we demonstrate that EZH2 mediates transcriptional silencing of the tumor suppressor gene E-cadherin by trimethylation of H3 lysine 27. Histone deacetylase inhibitors can prevent EZH2-mediated repression of E-cadherin and attenuate cell invasion, suggesting a possible mechanism that may be useful for the development of therapeutic treatments. Taken together, these observations provide a novel mechanism of E-cadherin regulation and establish a functional link between dysregulation of EZH2 and repression of E-cadherin during cancer progression.

Stem cell marker expression in small cell lung carcinoma and developing lung tissue

Histopathologic and clinical findings suggest that small cell lung cancer is derived from a multipotent proximal airway epithelial cell. In order to investigate the histogenetic origin of small cell lung cancer, we compared stem cell marker expression in human fetal lung tissue, human adult bronchial tissue, and a cohort of 64 small cell lung cancers. Supporting derivation of a multipotent precursor cell, 87.5% (56/64) of small cell lung cancers showed a dot-like expression of podocalyxin-like protein 1 (PODXL-1), a marker of embryonic and hematopoetic stem cells. Of small cell lung cancers, 98.4% (63/64) ubiquitously expressed Bmi-1, a key player in self-renewal of stem cells. Oct4 and AP2gamma were not expressed. Although podocalyxin-like protein 1 did not correlate with p53 or Wilms tumor suppressor 1, known regulators of podocalyxin-like protein 1, we could demonstrate demethylated CpG islands in the podocalyxin-like protein 1 promoter in small cell lung cancer, indicating epigenetic regulation. During fetal lung development and within adult bronchial mucosa, Bmi-1 was expressed ubiquitously. In contrast, podocalyxin-like protein 1 was detected in few stromal cells during the pseudoglandular phase (n = 7) and, importantly, in clustered epithelial cells within proximal bronchi and the trachea during the canalicular phase (n = 10). Interestingly, podocalyxin-like protein 1 was not expressed in normal or metaplastic adult bronchial epithelium (n = 36) but was expressed in sparse epithelial cells in half of the cases of normal tumor adjacent bronchial mucosa (20/40). Taken together, we show that small cell lung cancers and clustered epithelial cells in developing proximal bronchi share the expression of stem cell markers, suggesting a possible histogenetic link.

RAWUL: a new ubiquitin-like domain in PRC1 ring finger proteins that unveils putative plant and worm PRC1 orthologs

BACKGROUND

Polycomb group (PcG) proteins are a set of chromatin-modifying proteins that play a key role in epigenetic gene regulation. The PcG proteins form large multiprotein complexes with different activities. The two best-characterized PcG complexes are the PcG repressive complex 1 (PRC1) and 2 (PRC2) that respectively possess histone 2A lysine 119 E3 ubiquitin ligase and histone 3 lysine 27 methyltransferase activities. While PRC2-like complexes are conserved throughout the eukaryotic kingdoms, PRC1-like complexes have only been described in Drosophila and vertebrates. Since both complexes are required for the gene silencing mechanism in Drosophila and vertebrates, how PRC1 function is realized in organisms that apparently lack PRC1 such as plants, is so far unknown. In vertebrates, PRC1 includes three proteins, Ring1B, Ring1A, and Bmi-1 that form an E3 ubiquitin ligase complex. These PRC1 proteins have an N-terminally located Ring finger domain associated to a poorly characterized conserved C-terminal region.

RESULTS

We obtained statistically significant evidences of sequence similarity between the C-terminal region of the PRC1 Ring finger proteins and the ubiquitin (Ubq)-like family proteins, thus defining a new Ubq-like domain, the RAWUL domain. In addition, our analysis revealed the existence of plant and worm proteins that display the conserved combination of a Ring finger domain at the N-terminus and a RAWUL domain at the C-terminus.

CONCLUSION

Analysis of the conserved domain architecture among PRC1 Ring finger proteins revealed the existence of long sought PRC1 protein orthologs in these organisms, suggesting the functional conservation of PRC1 throughout higher eukaryotes.

The pleiohomeotic gene is required for maintaining expression of genes functioning in ventral appendage formation in Drosophila melanogaster

Polycomb group (PcG) proteins are negative regulators that maintain the expression of homeotic genes and affect cell proliferation. Pleiohomeotic (Pho) is a unique PcG member with a DNA-binding zinc finger motif and was proposed to recruit other PcG proteins to form a complex. The pho null mutants exhibited several mutant phenotypes such as the transformation of antennae to mesothoracic legs. We examined the effects of pho on the identification of ventral appendages and proximo-distal axis formation during postembryogenesis. In the antennal disc of the pho mutant, Antennapedia (Antp), which is a selector gene in determining leg identity, was ectopically expressed. The homothorax (hth), dachshund (dac) and Distal-less (Dll) genes involved in proximo-distal axis formation were also abnormally expressed in both the antennal and leg discs of the pho mutant. The engrailed (en) gene, which affects the formation of the anterior-posterior axis, was also misexpressed in the anterior compartment of antennal and leg discs. These mutant phenotypes were enhanced in the mutant background of Posterior sex combs (Psc) and pleiohomeotic-like (phol), which are another PcG genes. These results suggest that pho functions in maintaining expression of genes involved in the formation of ventral appendages and the proximo-distal axis.

Bmi1 regulates memory CD4 T cell survival via repression of the Noxa gene

The maintenance of memory T cells is central to the establishment of immunological memory, although molecular details of the process are poorly understood. In the absence of the polycomb group (PcG) gene Bmi1, the number of memory CD4⁺ T helper (Th)1/Th2 cells was reduced significantly. Enhanced cell death of Bmi1^−/− memory Th2 cells was observed both in vivo and in vitro. Among various proapoptotic genes that are regulated by Bmi1, the expression of proapoptotic BH3-only protein Noxa was increased in Bmi1^−/− effector Th1/Th2 cells. The generation of memory Th2 cells was restored by the deletion of Noxa, but not by Ink4a and Arf. Direct binding of Bmi1 to the Noxa gene locus was accompanied by histone H3-K27 methylation. The recruitment of other PcG gene products and Dnmt1 to the Noxa gene was highly dependent on the expression of Bmi1. In addition, Bmi1 was required for DNA CpG methylation of the Noxa gene. Moreover, memory Th2-dependent airway inflammation was attenuated substantially in the absence of Bmi1. Thus, Bmi1 controls memory CD4⁺ Th1/Th2 cell survival and function through the direct repression of the Noxa gene.

Prognostic value of Bmi-1 oncoprotein expression in NSCLC patients: a tissue microarray study

PURPOSE

Bmi-1 is a Polycomb group member which participates in many physiological processes as well as in a wide spectrum of cancers. The aim of this study was to investigate Bmi-1 expression in non-small cell lung cancer (NSCLC) in respect to clinicopathological features and therapeutic outcomes.

METHODS

Immunohistochemical staining for Bmi-1 was performed on tissue microarrays (TMAs) constructed from 179 formalin-fixed and paraffin-embedded NSCLC samples (106 squamous, 58 adeno-, and 15 large cell carcinomas). Data were subject to statistical analysis by SPSS.

RESULTS

Overall evaluation of all tumor cases showed that 20 (11.43%) were negative, 37 (21.14%) showed weak, 65 (37.14%) moderate and 57 (32.57%) strong nuclear positivity for Bmi-1. Statistical analysis of our data revealed that the expression of Bmi-1 was significantly higher in stage III (P = 10(-6)) and stage IV (P = 10(-5)) tumors compared to stages I and II tumors. The administration of adjuvant chemotherapy significantly increased DFS at stage I and II patients who did not express Bmi-1 when compared to their Bmi-1 positive counterparts (P = 0.05).

CONCLUSIONS

Our results suggest that Bmi-1 is significantly associated with progression of NSCLC and might serve as a prognostic marker of adverse disease outcome.

Proteomic analysis of EZH2 downstream target proteins in hepatocellular carcinoma

Enhancer of zeste homolog 2 (EZH2) is suggested to be a potential therapeutic target and a diagnostic marker for cancer. Our previous study also showed the critical role of EZH2 in hepatocellular carcinoma (HCC) tumorigenesis. The present study is aimed at revealing the comprehensive downstream pathways of EZH2 by functional proteomic profiling. Lentivirus mediated RNA interference (RNAi) was employed to knockdown EZH2 in HCC cells. The 2-DE was employed to compare the expression profile difference between parental and EZH2-knockdown HCC cells. In total, 28 spots were differentially expressed during EZH2 inhibition. Among all, 18 proteins were identified by PMF with MALDI-TOF MS. Western blotting further validated upregulation of 60S acidic ribosomal protein P0 (L10E), and downregulation of two proteins with EZH2 inhibition: stathmin1 and probable protein disulfide isomerase (PDI) ER-60 precursor (ERp57). Moreover, L10E was downregulated with overexpression of EZH2 in hepatocytes, and L10E reversed the effect of EZH2 on cell proliferation, suggesting it a downstream target of EZH2. The comprehensive and comparative analyses of proteins associated with EZH2 could further our understanding on the downstream signal cascade of EZH2 leading to tumorigenesis.

The (epi)genetics of human synovial sarcoma

Human synovial sarcomas are aggressive soft tissue tumors with relatively high rates of recurrences and metastases. They display a variable response to common treatment protocols such as radiation and chemotherapy. For the development of novel diagnostic, prognostic, and therapeutic approaches, detailed information on the molecular mechanisms underlying the development of these tumors is of imperative importance. Fusion of the SS18 and (one of the) SSX genes is a molecular hallmark of human synovial sarcomas. The SS18 and SSX genes encode nuclear proteins that exhibit opposite transcription regulatory activities, likely through epigenetic mechanisms. The SS18 protein functions as a transcriptional coactivator and interacts directly with members of the epigenetic chromatin remodeling and modification machineries. In contrast, the SSX proteins function as transcriptional corepressors and are associated with several Polycomb group proteins. Since the domains involved in these apparently opposite transcription regulatory activities are retained in the SS18-SSX fusion proteins, we hypothesize that these fusion proteins function as "activator-repressors" of transcription. The implications of this model for human synovial sarcoma development and future treatment are discussed.

Human SFMBT is a transcriptional repressor protein that selectively binds the N-terminal tail of histone H3

Human SFMBT (hSFMBT) is postulated to be a Polycomb (PcG) protein. Similar to other PcG proteins, we found that hSFMBT displays robust transcriptional repressor activity. In addition, hSFMBT localized to the nucleus where it strongly associates with chromatin by directly and selectively binding the N-terminal tail of histone H3. Importantly, we discovered that the four tandem MBT repeats of hSFMBT were sufficient for nuclear matrix-association, N-terminal tail H3 binding, and required for transcriptional repression. These findings indicate that the tandem MBT repeats form a functional structure required for biological activity of hSFMBT and predict similar properties for other MBT domain-containing proteins.

Regulation of Th2 cell development by Polycomb group gene bmi-1 through the stabilization of GATA3

The Polycomb group (PcG) gene products regulate the maintenance of the homeobox gene expression in Drosophila and vertebrates and also the cell cycle progression in thymocytes and Th2 cell differentiation in mature T cells. We herein studied the role of PcG gene bmi-1 product in Th1/Th2 cell differentiation and found that Bmi-1 facilitates Th2 cell differentiation in a Ring finger-dependent manner. Biochemical studies indicate that Bmi-1 interacts with GATA3 in T cells, which is dependent on the Ring finger of Bmi-1. The overexpression of Bmi-1 resulted in a decreased ubiquitination and an increased protein stability of GATA3. In bmi-1-deficient Th cells, the levels of Th2 cell differentiation decreased as the degradation and ubiquitination on GATA3 increased. Therefore, Bmi-1 plays a crucial role in the control of Th2 cell differentiation in a Ring finger-dependent manner by regulating GATA3 protein stability.

Biology of Polycomb and Trithorax Group Proteins

Cellular phenotypes can be ascribed to different patterns of gene expression. Epigenetic mechanisms control the generation of different phenotypes from the same genotype. Thus differentiation is basically a process driven by changes in gene activity during development, often in response to transient factors or environmental stimuli. To keep the specific characteristics of cell types, tissue-specific gene expression patterns must be transmitted stably from one cell to the daughter cells, also in the absence of the early-acting determination factors. This heritability of patterns of active and inactive genes is enabled by epigenetic mechanisms that create a layer of information on top of the DNA sequence that ensures mitotic and sometimes also meiotic transmission of expression patterns. The proteins of the Polycomb and Trithorax group comprise such a cellular memory mechanism that preserves gene expression patterns through many rounds of cell division. This review provides an overview of the genetics and molecular biology of these maintenance proteins, concentrating mainly on mechanisms of Polycomb group-mediated repression.

Transcriptional regulation by C-terminal binding proteins

C-terminal binding protein family members function predominantly as transcriptional corepressors in association with sequence specific DNA-binding transcriptional repressors. The vertebrates have two CtBP genes while the invertebrates contain a single gene. Genetic studies indicate that the CtBP genes play pivotal roles in animal development. The vertebrate C-terminal binding proteins (CtBP1 and CtBP2) are highly related and are functionally redundant for certain developmental processes and non-redundant for others. The animal C-terminal binding proteins exhibit structural and functional similarity to d-isomer-specific 2-hydroxy acid dehydrogenases (D2-HDH). They function as dimers, recruiting transcriptional regulators through two protein-binding interfaces in each monomer. The corepressor complex of CtBP1 contains enzymatic constituents that mediate coordinated histone modification by deacetylation and methylation of histone H3-Lysine 9 and demethylation of histone H3-Lysine 4. CtBP also recruits the small ubiquitin-related modifier (SUMO) conjugating E2 enzyme UBC9 and a SUMO E3 ligase (HPC2), suggesting that CtBP-mediated transcriptional regulation may also involve SUMOylation of transcription factors. In addition to gene-specific transcriptional repression, CtBP1 appears to antagonize the activity of the global transcriptional coactivators, p300/CBP. Genetic evidence also suggests that the fly CtBP (dCtBP) and the vertebrate CtBP2 might activate transcription in a context-dependent manner. The transcriptional regulatory activity of CtBP is modulated by the nuclear NADH/NAD+ ratio and hence appears to be influenced by the metabolic status of the cell. The nuclear dinucleotide ratio may differentially influence the repression activities of factors that recruit CtBP through PLDLS-like motifs and those through non-PLDLS-motifs.

Bmi-1 is induced by the Epstein-Barr virus oncogene LMP1 and regulates the expression of viral target genes in Hodgkin lymphoma cells

Polycomb group (PcG) proteins are chromatin modifiers that are necessary for the maintenance and renewal of embryonic and adult stem cells. However, overexpression of the PcG protein, Bmi-1, causes lymphoma in transgenic mice. We show that Bmi-1 is up-regulated in Hodgkin lymphoma (HL) cells by the Epstein-Barr virus (EBV) oncogene latent membrane protein-1 (LMP1) and that this up-regulation is mediated by NF-κB signaling. We also show that Bmi-1 is up-regulated by NF-κB in EBV-negative HL cells. Down-regulation of LMP1 and Bmi-1 decreased the survival of HL cells, suggesting that Bmi-1 may mediate the prosurvival effects of LMP1-induced NF-κB signaling in HL cells. Transcriptional targets of Bmi-1 were identified after its knockdown in an HL cell line. We show here that Bmi-1 and LMP1 down-regulate the ataxia telangiectasia–mutated (ATM) tumor suppressor and conclude that Bmi-1 contributes to LMP1-induced oncogenesis in HL.

Differential expression of polycomb repression complex 1 (PRC1) members in the developing mouse brain reveals multiple complexes

Polycomb group (PcG) genes are regulators of body segmentation and cell growth, therefore being important players during development. PcG proteins form large complexes (PRC) that fulfil mostly repressive regulative functions on homeotic gene expression. Although expression of PcG genes in the brain has been noticed, the involvement of PcG genes in the processes of brain development is not understood. In this study, we analysed the expression patterns of PRC1 complex members to reveal PcG proteins that might be relevant for mouse brain development. Using in situ hybridisation, we show PRC1 activity in proliferative progenitor cells during neurogenesis, but also in maturated neuronal structures. PRC1 complex compositions vary in a spatial and temporal controlled manner during mouse brain development, providing cellular tools to act in different developmental contexts of cell proliferation, cell fate determination, and differentiation.

The Synovial Sarcoma–Associated SS18-SSX2 Fusion Protein Induces Epigenetic Gene (De)Regulation

Fusion of the SS18 and either one of the SSX genes is a hallmark of human synovial sarcoma. The SS18 and SSX genes encode nuclear proteins that exhibit opposite transcriptional activities. The SS18 protein functions as a transcriptional coactivator and is associated with the SWI/SNF complex, whereas the SSX proteins function as transcriptional corepressors and are associated with the polycomb complex. The domains involved in these opposite transcriptional activities are retained in the SS18-SSX fusion proteins. Here, we set out to determine the direct transcriptional consequences of conditional SS18-SSX2 fusion protein expression using complementary DNA microarray-based profiling. By doing so, we identified several clusters of SS18-SSX2-responsive genes, including a group of genes involved in cholesterol synthesis, which is a general characteristic of malignancy. In addition, we identified a group of SS18-SSX2-responsive genes known to be specifically deregulated in primary synovial sarcomas, including IGF2 and CD44. Furthermore, we observed an uncoupling of EGR1, JUNB, and WNT signaling in response to SS18-SSX2 expression, suggesting that the SWI/SNF-associated coactivation functions of the SS18 moiety are impaired. Finally, we found that SS18-SSX2 expression affects histone modifications in the CD44 and IGF2 promoters and DNA methylation levels in the IGF2 imprinting control region. Together, we conclude that the SS18-SSX2 fusion protein may act as a so-called transcriptional "activator-repressor," which induces downstream target gene deregulation through epigenetic mechanisms. Our results may have implications for both the development and clinical management of synovial sarcomas.

Transient requirements of YY1 expression for PcG transcriptional repression and phenotypic rescue

A hallmark of PcG transcriptional repression is stability of the repressed state, although the mechanism of this stability is unclear. The mammalian transcription factor YY1 can function as a PcG protein in Drosophila resulting in transcriptional repression and correction of phenotypic defects caused by mutation of its homologue, Pleiohomeotic (PHO). To discern the temporal requirements of YY1 expression for these functions, and to study its mechanism of stable repression in vivo, we used a Drosophila larval wing imaginal disc transcriptional repression system. We found that YY1 was needed transiently at day 3 or later of embryonic development to stably repress a reporter transgene at day 8 in wing imaginal discs. Stable transcriptional repression did not correlate with maintenance of YY1 or Polycomb DNA binding, but did correlate with persistence of histone H3 methylation on lysine 27. We also found that YY1 expressed during the first 7 days of development was sufficient for rescue of fly development (a 14 day process) in pho mutant flies. Therefore, YY1 was transiently required for correction of fly defects and was dispensable past the pharate adult stage. Possible mechanisms of repression by YY1 are discussed.

The Drosophila RYBP gene functions as a Polycomb-dependent transcriptional repressor

The Polycomb and trithorax groups of genes control the maintenance of homeotic gene expression in a variety of organisms. A putative participant in the regulation of this process is the murine RYBP (Ring and YY1 Binding Protein) gene. Sequence comparison between different species has identified the homologous gene in Drosophila, the dRYBP gene. We have investigated whether dRYBP participates in the mechanisms of silencing of homeotic genes expression. We first studied its expression by RNA in situ hybridisation and detected dRYBP expression ubiquitously and throughout development. Moreover, we generated a polyclonal anti-dRYBP antibody that recognises the dRYBP protein. dRYBP protein is nuclear and expressed maternally and ubiquitously throughout development. To study the transcriptional activity of dRYBP, we generated a fusion protein containing the entire dRYBP protein and the GAL4 DNA binding domain. This fusion protein functions, in vivo, as a transcriptional repressor throughout development. Importantly, this repression is dependent on the function of the Polycomb group genes. Furthermore, using the GAL4/UAS system, we have over expressed dRYBP in the haltere and the wing imaginal discs. In the haltere discs, high levels of dRYBP repress the expression of the homeotic Ultrabithorax gene. This repression is Polycomb dependent. In the wing discs, dRYBP over expression produces a variety of phenotypes suggesting the overall miss regulation of the many putative genes affected by high levels of dRYBP. Taking together, our results indicate that dRYBP is able to interact with PcG proteins to repress transcription suggesting that the dRYBP gene might belong to the Polycomb group of genes in Drosophila.

Increased expression of the polycomb group gene, EZH2, in transitional cell carcinoma of the bladder

PURPOSE

The Polycomb group gene, EZH2, functions as a transcriptional repressor involved in gene silencing. Amplification of EZH2 has been reported in several malignancies, including prostate, breast, and lymphoma. We evaluated EZH2 mRNA and protein expression in bladder specimens from patients and the EZH2 mRNA expression in five bladder cancer cell lines.

EXPERIMENTAL DESIGN

EZH2 mRNA expression was assessed by reverse transcription-PCR (RT-PCR) in 38 bladder tissue specimens. We also evaluated 39 bladder cancer specimens for EZH2 protein expression using immunohistochemistry with affinity-purified antibodies to human EZH2. In addition, five human bladder cancer cell lines were analyzed by RT-PCR for EZH2 mRNA expression.

RESULTS

Five of 14 (36%) nontumor bladder specimens versus 21 of 24 (88%) bladder tumors showed EZH2 mRNA expression (P=0.003). All of the invasive tumors (10 of 10) had detectable EZH2 mRNA expression, compared with 11 of 14 (79%) superficial tumors. In addition, EZH2 mRNA expression was noted in 100% (16 of 16) of high-grade bladder tumors versus 50% (4 of 8) of low-grade tumors (P=0.01). EZH2 protein expression, meanwhile, was increased in neoplastic tissue compared with nontumor urothelium (78% versus 69% of nuclei, P<0.005). There were no differences in EZH2 protein levels between superficial and invasive tumors. High-grade tumors had increased EZH2 staining compared with normal urothelium (78% versus 68%, P<0.005), whereas low-grade lesions did not. Four of five human bladder cancer cell lines expressed high levels of EZH2, whereas only low levels were detected in one cell line.

CONCLUSIONS

We report a significant increase in EZH2 expression in transitional cell carcinoma of the bladder compared with normal urothelium. These data suggest that similar to other human malignancies, increased EZH2 expression correlates with oncogenesis of the bladder.

Mph2 expression in germinal zones of the mouse brain

The telencephalon develops from a seemingly homogeneous population of precursor cells that generates neurons with divergent properties and cell fates. An important question concerns the genetic control of cell states belonging to different generations. Mph2 was identified by serial analysis of gene expression as a differentially expressed gene during cortical development. Belonging to the Polycomb group, Mph2 may be involved in transcriptional repression of cell states from one generation to the next. Here, we use in situ hybridization to determine the spatial localization of Mph2 expression in the developing nervous system. Mph2 expression evident in proliferating precursor cell populations in the developing mouse brain such as the ventricular zones of the cortex and ganglionic eminences as well as in mature structures such as the granule cell layer of the adult dentate gyrus and cerebellum indicates a role for Mph2 in both the developing and adult brain.

Clinicopathological significance of EZH2 mRNA expression in patients with hepatocellular carcinoma

Enhancer of zeste homologue 2 (EZH2), a member of the polycomb group protein family, plays a crucial role in the regulation of embryonic development and has been associated with the regulation of the cell cycle. Recently, several studies have shown that EZH2 is highly expressed in aggressive tumours, including human breast cancer, prostate cancer, and lymphomas. We thus analysed EZH2 expression using real-time reverse transcription-polymerase chain reaction, and correlated its expression status with various clinicopathological parameters in 66 patients with hepatocellular carcinoma (HCC). We found high expression of EZH2 in human liver cancer cell lines. Furthermore, EZH2 gene-expression levels in tumour tissue specimens (0.34+/-0.52) were significantly higher (P<0.0001) than those in the corresponding nontumour tissue specimens (0.07+/-0.09). The incidence of cancer cell invasion into the portal vein was significantly higher (P<0.001) in the high EZH2 expression group (26 of the 33, 79%) than in the low expression group (13 of the 33, 39%). However, there was no significant difference in the disease-free survival rate between the two groups. The findings of this study indicate that EZH2 mRNA expression was upregulated in human HCC and may play an important role in tumour progression, especially by facilitating portal vein invasion.

Impaired maturation of myeloid progenitors in mice lacking novel Polycomb group protein MBT-1

Polycomb group (PcG) proteins participate in DNA-binding complexes with gene-repressing activity, many of which have been highlighted for their involvement in hematopoiesis. We have identified a putative PcG protein, termed MBT-1, that is associated with Rnf2, an in vivo interactor of PcG proteins. MBT-1 structurally resembles the H-L(3)MBT protein, whose deletion is predicted to be responsible for myeloid hematopoietic malignancies. The human MBT-1 gene is located on chromosome 6q23, a region frequently deleted in leukemia cells, and shows a transient expression spike in response to maturation-inducing stimuli in myeloid leukemia cells. MBT-1(-/-) myeloid progenitor cells exhibit a maturational deficiency but maintain normal proliferative activities. This results in the accumulation of immature myeloid progenitors and hence, a marked decrease of mature myeloid blood cells, causing the MBT-1(-/-) mice to die of anemia during a late embryonic stage. Together, we conclude that MBT-1 specifically regulates the maturational advancement of myeloid progenitor cells during transitions between two developmental stages. We also show that MBT-1 appears to influence myelopoiesis by transiently enhancing p57(KIP2) expression levels.

Polycomb homologs are involved in teratogenicity of valproic acid in mice

BACKGROUND

Valproic acid (VPA) is widely used to treat epilepsy and bipolar disorder and is also a potent teratogen, but its teratogenic mechanisms are unknown. We have attempted to describe a fundamental role of the Polycomb group (Pc-G) in VPA-induced transformations of the axial skeleton.

METHODS

Pregnant NMRI mice were given a single subcutaneous injection of vehicle or VPA (800 mg/kg) on gestation day (GD) 8. The expression of genes encoding Polycomb and trithorax groups was measured by quantitative real-time RT-PCR using total RNA isolated from the embryos exposed to vehicle or VPA for 1, 3, and 6 hr. In addition, the use of two less teratogenic antiepileptic chemicals valpromide (VPD) and valnoctamide (VCD) provide reliable evidence to support the relationship between VPA teratogenicity and the Polycomb group.

RESULTS

At a teratogenic level, VPA inhibits the expression of the Polycomb group genes, including Eed, Ezh2, Zfp144, Bmi1, Cbx2, Rnf2, and YY1 in the mouse embryos. In contrast, neither VPD nor VCD have significant effects on the expression of those genes affected by VPA. The trithorax group (trx-G) gene MLL, which is known to be required to maintain homeobox gene expression such as the Polycomb gene, is not affected by a teratogenic dose of VPA.

CONCLUSIONS

We propose that, during embryonic development, VPA may affect the gene silencing pathway mediated by the Polycomb group complex. The epigenetic mechanism of VPA teratogenicity on anteroposterior patterning is suspected.

Mammalian polycomb-mediated repression of Hox genes requires the essential spliceosomal protein Sf3b1

Polycomb group (PcG) proteins are responsible for the stable repression of homeotic (Hox) genes by forming multimeric protein complexes. We show (1) physical interaction between components of the U2 small nuclear ribonucleoprotein particle (U2 snRNP), including Sf3b1 and PcG proteins Zfp144 and Rnf2; and (2) that Sf3b1 heterozygous mice exhibit skeletal transformations concomitant with ectopic Hox expressions. These alterations are enhanced by Zfp144 mutation but repressed by Mll mutation (a trithorax-group gene). Importantly, the levels of Sf3b1 in PcG complexes were decreased in Sf3b1-heterozygous embryos. These findings suggest that Sf3b1-PcG protein interaction is essential for true PcG-mediated repression of Hox genes.

Signaling pathways in Th2 development

In order for an immune response to be successful, it must be of the appropriate type and magnitude. Intracellular residing pathogens require a cell-mediated immune response, whereas extracellular pathogens evoke a humoral immune response. T-helper (Th) cells orchestrate the immune response and are divided into two subsets, Th1 and Th2 cells. Here, we discuss the mechanisms of Th2 development with a focus on signal transduction pathways that influence Th2 differentiation.

Loss of Estrogen Receptor Signaling Triggers Epigenetic Silencing of Downstream Targets in Breast Cancer

Alterations in histones, chromatin-related proteins, and DNA methylation contribute to transcriptional silencing in cancer, but the sequence of these molecular events is not well understood. Here we demonstrate that on disruption of estrogen receptor (ER) α signaling by small interfering RNA, polycomb repressors and histone deacetylases are recruited to initiate stable repression of the progesterone receptor (PR) gene, a known ERα target, in breast cancer cells. The event is accompanied by acquired DNA methylation of the PR promoter, leaving a stable mark that can be inherited by cancer cell progeny. Reestablishing ERα signaling alone was not sufficient to reactivate the PR gene; reactivation of the PR gene also requires DNA demethylation. Methylation microarray analysis further showed that progressive DNA methylation occurs in multiple ERα targets in breast cancer genomes. The results imply, for the first time, the significance of epigenetic regulation on ERα target genes, providing new direction for research in this classical signaling pathway.

Ezh2 reduces the ability of HDAC1-dependent pRb2/p130 transcriptional repression of cyclin A

The polycomb group (PcG) proteins are known to be involved in maintaining the silenced state of several developmentally regulated genes. Enhancer of zeste homolog 2 (Ezh2), a member of this large protein family, has also been shown to be deregulated in different tumor types and its role, both as a potential primary effector and as a mediator of tumorigenesis, has become a subject of increased interest. We observed that Ezh2 binds to pRb2/p130, a member of the retinoblastoma family; as such, we were led to consider the possible ability of Ezh2 to modulate cell cycle progression. Both Ezh2 and pRb2/p130 repress gene expression by recruiting histone deacetylase (HDAC1), which decreases DNA accessibility for activating transcription factors. Additionally, we observed that Ezh2 interacts with the C-terminal region of pRb2/p130, essential for interaction with HDAC1. We show that Ezh2 is able to reverse pRb2/p130-HDAC1-mediated repression of the cyclin A promoter. This indicates a functional role of this complex in regulating cyclin A expression, known to be crucial in mediating cell cycle advancement. We also detected a significant decrease in the retention of HDAC1 activity associated with pRb2/p130 when Ezh2 was overexpressed. Finally, electromobility shift assays (EMSA) demonstrated that overexpression of Ezh2 caused the abrogation of the pRb2/p130-HDAC1 complex on the cyclin A promoter. These data, taken together, suggest that Ezh2 competes with HDAC1 in binding to pRb2/p130, disrupting their occupancy on the cyclin A promoter. In this study, we propose a new mechanism for the functional inactivation of pRb2/p130 that ultimately contributes to cell cycle progression and malignant transformation.

Poorly differentiated breast carcinoma is associated with increased expression of the human polycomb group EZH2 gene

Polycomb group (PcG) genes contribute to the maintenance of cell identity, cell cycle regulation, and oncogenesis. We describe the expression of five PcG genes (BMI-1, RING1, HPC1, HPC2, and EZH2) innormal breast tissues, invasive breast carcinomas, and their precursors. Members of the HPC-HPH/PRC1 PcG complex, including BMI-1, RING1, HPC1, and HPC2, were detected in normal resting and cycling breast cells. The EED-EZH/PRC2 PcG complex protein EZH2 was only found in rare cycling cells, whereas normal resting breast cells were negative for EZH2. PcG gene expression patterns in ductal hyperplasia (DH), well-differentiated ductal carcinoma in situ (DCIS), and well-differentiated invasive carcinomas closely resembled the pattern in healthy cells. However, poorly differentiated DCIS and invasive carcinomas frequently expressed EZH2 in combination with HPC-HPH/PRC1 proteins. Most BMI-1/EZH2 double-positive cells in poorly differentiated DCIS were resting. Poorly differentiated invasive carcinoma displayed an enhanced rate of cell division within BMI-1/EZH2 double-positive cells. We propose that the enhanced expression of EZH2 in BMI-1(+) cells contributes to the loss of cell identity in poorly differentiated breast carcinomas, and that increased EZH2 expression precedes high frequencies of proliferation. These observations suggest that deregulated expression of EZH2 is associated with loss of differentiation and development of poorly differentiated breast cancer in humans.

Propagation of silencing; recruitment and repression of naive chromatin in trans by polycomb repressed chromatin

The Polycomb group (PcG) proteins maintain stable and heritable repression of homeotic genes. Typically, Polycomb response elements (PRE) that direct PcG repression are located at great distances (10s of kb) from the promoters of PcG-repressed genes, and it is not known how these PREs can communicate with promoters over such distances. Using Class II mouse PRC core complexes (mPCCs) assembled from recombinant subunits, we investigated how PcG complexes might bridge distant chromosomal regions. Like native and recombinant Drosophila Class II complexes, mPCC represses chromatin remodeling and transcription. Interestingly, mPCC bound to one polynucleosome template can recruit a second template from solution and renders it refractory to transcription and chromatin remodeling. A Drosophila PRC core complex (dPCC) also is able to recruit a second template. Posterior sex combs (PSC), a subunit of dPCC, inhibits chromatin remodeling and transcription efficiently but requires assembly with dRING1 to recruit chromatin. Thus, repression and template bridging require different subunits of PcG complexes, suggesting that long-range effects may be mechanistically distinct from repression.

Site-specific expression of polycomb-group genes encoding the HPC-HPH/PRC1 complex in clinically defined primary nodal and cutaneous large B-cell lymphomas

Polycomb-group (PcG) genes preserve cell identity by gene silencing, and contribute to regulation of lymphopoiesis and malignant transformation. We show that primary nodal large B-cell lymphomas (LBCLs), and secondary cutaneous deposits from such lymphomas, abnormally express the BMI-1, RING1, and HPH1 PcG genes in cycling neoplastic cells. By contrast, tumor cells in primary cutaneous LBCLs lacked BMI-1 expression, whereas RING1 was variably detected. Lack of BMI-1 expression was characteristic for primary cutaneous LBCLs, because other primary extranodal LBCLs originating from brain, testes, and stomach were BMI-1-positive. Expression of HPH1 was rarely detected in primary cutaneous LBCLs of the head or trunk and abundant in primary cutaneous LBCLs of the legs, which fits well with its earlier recognition as a distinct clinical pathological entity with different clinical behavior. We conclude that clinically defined subclasses of primary LBCLs display site-specific abnormal expression patterns of PcG genes of the HPC-HPH/PRC1 PcG complex. Some of these patterns (such as the expression profile of BMI-1) may be diagnostically relevant. We propose that distinct expression profiles of PcG genes results in abnormal formation of HPC-HPH/PRC1 PcG complexes, and that this contributes to lymphomagenesis and different clinical behavior of clinically defined LBCLs.

Proliferation-linked expression of the novel murine gene m4mbt encoding a nuclear zinc finger protein with four mbt domains

Here we describe the cloning and characterization of a novel murine gene named m4mbt that encodes a homolog of the lethal (3) malignant brain tumor (l(3)mbt) and Scm proteins. It is localized on mouse chromosome 15E2 and is organized into 17 exons. As demonstrated by Northern blot analysis, m4mbt mRNA is expressed in virtually all tested tissues and cell lines with the exception of stomach and muscle. The m4mbt transcript was most abundant in the testes. m4mbt expression was shown to initiate early during mouse embryonal development (before day 7) and continue until adulthood. The expression of m4mbt mRNA also appears to correlate with cellular proliferation, since we observed down-regulation of m4mbt expression during terminal monocytic differentiation and in contact-inhibited fibroblasts. Computer analysis of the amino acid (aa) sequence revealed that the M4mbt protein comprises an amino-terminally located atypical C2C2 zinc finger and four centrally located mbt repeats. Mbt repeats are also found in proteins of the Polycomb group (PcG) that associate with heterochromatin and function as long-term repressors of transcription. Using Western blot analysis and confocal fluorescent microscopy, we demonstrated that the M4mbt protein is localized in the nucleus. Since M4mbt has structural domains similar to chromatin-associated proteins, its expression is associated with proliferation, and it has a nuclear localization, it may have a regulatory role related to proliferation and/or differentiation.

Self-renewal of hematopoietic and leukemic stem cells: a central role for the Polycomb-group gene Bmi-1

Self-renewal of hematopoietic stem cells is vital for the sustained daily production of blood cells. Two recent studies have shown that the Polycomb-group gene Bmi-1 is indispensable for regulation of self-renewal by normal and leukemic stem cells. This identifies Polycomb-group genes as potential targets for therapeutic intervention in leukemia, and possibly other forms of cancer.