Identification and characterization of interactions between the vertebrate polycomb-group protein BMI1 and human homologs of polyhomeotic

Critical Roles of Polycomb Repressive Complexes in Transcription and Cancer

Polycomp group (PcG) proteins are members of highly conserved multiprotein complexes, recognized as gene transcriptional repressors during development and shown to play a role in various physiological and pathological processes. PcG proteins consist of two Polycomb repressive complexes (PRCs) with different enzymatic activities: Polycomb repressive complexes 1 (PRC1), a ubiquitin ligase, and Polycomb repressive complexes 2 (PRC2), a histone methyltransferase. Traditionally, PRCs have been described to be associated with transcriptional repression of homeotic genes, as well as gene transcription activating effects. Particularly in cancer, PRCs have been found to misregulate gene expression, not only depending on the function of the whole PRCs, but also through their separate subunits. In this review, we focused especially on the recent findings in the transcriptional regulation of PRCs, the oncogenic and tumor-suppressive roles of PcG proteins, and the research progress of inhibitors targeting PRCs.

Mechanisms of Polycomb group protein function in cancer

Cancer arises from a multitude of disorders resulting in loss of differentiation and a stem cell-like phenotype characterized by uncontrolled growth. Polycomb Group (PcG) proteins are members of multiprotein complexes that are highly conserved throughout evolution. Historically, they have been described as essential for maintaining epigenetic cellular memory by locking homeotic genes in a transcriptionally repressed state. What was initially thought to be a function restricted to a few target genes, subsequently turned out to be of much broader relevance, since the main role of PcG complexes is to ensure a dynamically choregraphed spatio-temporal regulation of their numerous target genes during development. Their ability to modify chromatin landscapes and refine the expression of master genes controlling major switches in cellular decisions under physiological conditions is often misregulated in tumors. Surprisingly, their functional implication in the initiation and progression of cancer may be either dependent on Polycomb complexes, or specific for a subunit that acts independently of other PcG members. In this review, we describe how misregulated Polycomb proteins play a pleiotropic role in cancer by altering a broad spectrum of biological processes such as the proliferation-differentiation balance, metabolism and the immune response, all of which are crucial in tumor progression. We also illustrate how interfering with PcG functions can provide a powerful strategy to counter tumor progression.

Polycomb repressive complex 1: Regulators of neurogenesis from embryonic to adult stage

Development of vertebrate nervous system is a complex process which involves differential gene expression and disruptions in this process or in the mature brain, may lead to neurological disorders and diseases. Extensive work that spanned several decades using rodent models and recent work on stem cells have helped uncover the intricate process of neuronal differentiation and maturation. There are various morphological changes, genetic and epigenetic modifications which occur during normal mammalian neural development, one of the chromatin modifications that controls vital gene expression are the posttranslational modifications on histone proteins, that controls accessibility of translational machinery. Among the histone modifiers, polycomb group proteins (PcGs), such as Ezh2, Eed and Suz12 form large protein complexes-polycomb repressive complex 2 (PRC2); while Ring1b and Bmi1 proteins form core of PRC1 along with accessory proteins such as Cbx, Hph, Rybp and Pcgfs catalyse histone modifications such as H3K27me3 and H2AK119ub1. PRC1 proteins are known to play critical role in X chromosome inactivation in females but they also repress the expression of key developmental genes and tightly regulate the mammalian neuronal development. In this review we have discussed the signalling pathways, morphogens and nuclear factors that initiate, regulate and maintain cells of the nervous system. Further, we have extensively reviewed the recent literature on the role of Ring1b and Bmi1 in mammalian neuronal development and differentiation; as well as highlighted questions that are still unanswered.

The Mitotic Apparatus and Kinetochores in Microcephaly and Neurodevelopmental Diseases

Regulators of mitotic division, when dysfunctional or expressed in a deregulated manner (over- or underexpressed) in somatic cells, cause chromosome instability, which is a predisposing condition to cancer that is associated with unrestricted proliferation. Genes encoding mitotic regulators are growingly implicated in neurodevelopmental diseases. Here, we briefly summarize existing knowledge on how microcephaly-related mitotic genes operate in the control of chromosome segregation during mitosis in somatic cells, with a special focus on the role of kinetochore factors. Then, we review evidence implicating mitotic apparatus- and kinetochore-resident factors in the origin of congenital microcephaly. We discuss data emerging from these works, which suggest a critical role of correct mitotic division in controlling neuronal cell proliferation and shaping the architecture of the central nervous system.

Topology and enzymatic properties of a canonical Polycomb repressive complex 1 isoform

Polycomb repressive complex 1 (PRC1) catalyses monoubiquitination of histone H2A on Lys119, promoting gene silencing. Cells at different developmental stages and in different tissues express different PRC1 isoforms. The topology, subunit composition, structural architecture and molecular mechanism of most of these isoforms are still poorly characterized. Here, we have purified a PRC1 isoform comprising subunits RING1B, PCGF2, CBX2 and PHC2, two stable subcomplexes (RING1B‐PCGF2 and RING1B‐PHC2) and the catalytic subunit RING1B in isolation. By crosslinking mass spectrometry, we identified novel interactions between RING1B and the three non‐catalytic subunits. Biochemical, biophysical, and enzymatic data suggest that CBX2 and PHC2 play a structural role, whereas PCGF2 also modulates catalysis. Our data offer insights into the molecular architecture of PRC1 and its histone ubiquitination activity.

Nuclear condensates of the Polycomb protein chromobox 2 (CBX2) assemble through phase separation

Polycomb group (PcG) proteins repress master regulators of development and differentiation through organization of chromatin structure. Mutation and dysregulation of PcG genes cause developmental defects and cancer. PcG proteins form condensates in the cell nucleus, and these condensates are the physical sites of PcG-targeted gene silencing via formation of facultative heterochromatin. However, the physiochemical principles underlying the formation of PcG condensates remain unknown, and their determination could shed light on how these condensates compact chromatin. Using fluorescence live-cell imaging, we observed that the Polycomb repressive complex 1 (PRC1) protein chromobox 2 (CBX2), a member of the CBX protein family, undergoes phase separation to form condensates and that the CBX2 condensates exhibit liquid-like properties. Using site-directed mutagenesis, we demonstrated that the conserved residues of CBX2 within the intrinsically disordered region (IDR), which is the region for compaction of chromatin in vitro, promote the condensate formation both in vitro and in vivo We showed that the CBX2 condensates concentrate DNA and nucleosomes. Using genetic engineering, we report that trimethylation of Lys-27 at histone H3 (H3K27me3), a marker of heterochromatin formation produced by PRC2, had minimal effects on the CBX2 condensate formation. We further demonstrated that the CBX2 condensate formation does not require CBX2-PRC1 subunits; however, the condensate formation of CBX2-PRC1 subunits depends on CBX2, suggesting a mechanism underlying the assembly of CBX2-PRC1 condensates. In summary, our results reveal that PcG condensates assemble through liquid-liquid phase separation (LLPS) and suggest that phase-separated condensates can organize PcG-bound chromatin.

BMI1 regulates PRC1 architecture and activity through homo- and hetero-oligomerization

BMI1 is a core component of the polycomb repressive complex 1 (PRC1) and emerging data support a role of BMI1 in cancer. The central domain of BMI1 is involved in protein-protein interactions and is essential for its oncogenic activity. Here, we present the structure of BMI1 bound to the polyhomeotic protein PHC2 illustrating that the central domain of BMI1 adopts an ubiquitin-like (UBL) fold and binds PHC2 in a β-hairpin conformation. Unexpectedly, we find that the UBL domain is involved in homo-oligomerization of BMI1. We demonstrate that both the interaction of BMI1 with polyhomeotic proteins and homo-oligomerization via UBL domain are necessary for H2A ubiquitination activity of PRC1 and for clonogenic potential of U2OS cells. Here, we also emphasize need for joint application of NMR spectroscopy and X-ray crystallography to determine the overall structure of the BMI1-PHC2 complex.

Microarray characterization of gene expression changes in blood during acute ethanol exposure

Background

As part of the civil aviation safety program to define the adverse effects of ethanol on flying performance, we performed a DNA microarray analysis of human whole blood samples from a five-time point study of subjects administered ethanol orally, followed by breathalyzer analysis, to monitor blood alcohol concentration (BAC) to discover significant gene expression changes in response to the ethanol exposure.

Methods

Subjects were administered either orange juice or orange juice with ethanol. Blood samples were taken based on BAC and total RNA was isolated from PaxGene™ blood tubes. The amplified cDNA was used in microarray and quantitative real-time polymerase chain reaction (RT-qPCR) analyses to evaluate differential gene expression. Microarray data was analyzed in a pipeline fashion to summarize and normalize and the results evaluated for relative expression across time points with multiple methods. Candidate genes showing distinctive expression patterns in response to ethanol were clustered by pattern and further analyzed for related function, pathway membership and common transcription factor binding within and across clusters. RT-qPCR was used with representative genes to confirm relative transcript levels across time to those detected in microarrays.

Results

Microarray analysis of samples representing 0%, 0.04%, 0.08%, return to 0.04%, and 0.02% wt/vol BAC showed that changes in gene expression could be detected across the time course. The expression changes were verified by qRT-PCR.

The candidate genes of interest (GOI) identified from the microarray analysis and clustered by expression pattern across the five BAC points showed seven coordinately expressed groups. Analysis showed function-based networks, shared transcription factor binding sites and signaling pathways for members of the clusters. These include hematological functions, innate immunity and inflammation functions, metabolic functions expected of ethanol metabolism, and pancreatic and hepatic function. Five of the seven clusters showed links to the p38 MAPK pathway.

Conclusions

The results of this study provide a first look at changing gene expression patterns in human blood during an acute rise in blood ethanol concentration and its depletion because of metabolism and excretion, and demonstrate that it is possible to detect changes in gene expression using total RNA isolated from whole blood. The analysis approach for this study serves as a workflow to investigate the biology linked to expression changes across a time course and from these changes, to identify target genes that could serve as biomarkers linked to pilot performance.

Structure of the polycomb group protein PCGF1 in complex with BCOR reveals basis for binding selectivity of PCGF homologs

Polycomb-group RING finger homologs (PCGF1, PCGF2, PCGF3, PCGF4, PCGF5, and PCGF6) are critical components in the assembly of distinct Polycomb repression complex 1 (PRC1)-related complexes. Here, we identify a protein interaction domain in BCL6 corepressor, BCOR, which binds the RING finger- and WD40-associated ubiquitin-like (RAWUL) domain of PCGF1 (NSPC1) and PCGF3 but not of PCGF2 (MEL18) or PCGF4 (BMI1). Because of the selective binding, we have named this domain PCGF Ub-like fold discriminator (PUFD). The structure of BCOR PUFD bound to PCGF1 reveals that (1) PUFD binds to the same surfaces as observed for a different Polycomb group RAWUL domain and (2) the ability of PUFD to discriminate among RAWULs stems from the identity of specific residues within these interaction surfaces. These data show the molecular basis for determining the binding preference for a PCGF homolog, which ultimately helps determine the identity of the larger PRC1-like assembly.

The polycomb complex PRC1: composition and function in plants

Polycomb group (PcG) proteins are crucial epigenetic regulators conferring transcriptional memory to cell lineages. They assemble into multi-protein complexes, e.g., Polycomb Repressive Complex 1 and 2 (PRC1, PRC2), which are thought to act in a sequential manner to stably maintain gene repression. PRC2 induces histone H3 lysine 27 (H3K27) trimethylation (H3K27me3), which is subsequently read by PRC1 that further catalyzes H2A monoubiquitination (H2Aub1), creating a transcriptional silent chromatin conformation. PRC2 components are conserved in plants and have been extensively characterized in Arabidopsis. In contrast, PRC1 composition and function are more diverged between animals and plants. Only more recently, PRC1 existence in plants has been documented. Here we review the aspects of plant specific and conserved PRC1 and highlight critical roles of PRC1 components in seed embryonic trait determinacy, shoot stem cell fate determinacy, and flower development in Arabidopsis.

Fine structure of the "PcG body" in human U-2 OS cells established by correlative light-electron microscopy

Polycomb group (PcG) proteins of the Polycomb repressive complex 1 (PRC1) are found to be diffusely distributed in nuclei of cells from various species. However they can also be localized in intensely fluorescent foci, whether imaged using GFP fusions to proteins of PRC1 complex, or by conventional immunofluorescence microscopy. Such foci are termed PcG bodies, and are believed to be situated in the nuclear intechromatin compartment. However, an ultrastructural description of the PcG body has not been reported to date. To establish the ultrastructure of PcG bodies in human U-2 OS cells stably expressing recombinant polycomb BMI1-GFP protein, we used correlative light-electron microscopy (CLEM) implemented with high-pressure freezing, cryosubstitution and on-section labeling of BMI1 protein with immunogold. This approach allowed us to clearly identify fluorescent PcG bodies, not as distinct nuclear bodies, but as nuclear domains enriched in separated heterochromatin fascicles. Importantly, high-pressure freezing and cryosubstitution allowed for a high and clear-cut immunogold BMI1 labeling of heterochromatin structures throughout the nucleus. The density of immunogold labeled BMI1 in the heterochromatin fascicles corresponding to fluorescent "PcG bodies" did not differ from the density of labeling of heterochromatin fascicles outside of the "PcG bodies". Accordingly, an appearance of the fluorescent "PcG bodies" seems to reflect a local accumulation of the labeled heterochromatin structures in the investigated cells. The results of this study should allow expansion of the knowledge about the biological relevance of the "PcG bodies" in human cells.

Compaction of chromatin by diverse Polycomb group proteins requires localized regions of high charge

Polycomb group (PcG) proteins are required for the epigenetic maintenance of developmental genes in a silent state. Proteins in the Polycomb-repressive complex 1 (PRC1) class of the PcG are conserved from flies to humans and inhibit transcription. One hypothesis for PRC1 mechanism is that it compacts chromatin, based in part on electron microscopy experiments demonstrating that Drosophila PRC1 compacts nucleosomal arrays. We show that this function is conserved between Drosophila and mouse PRC1 complexes and requires a region with an overrepresentation of basic amino acids. While the active region is found in the Posterior Sex Combs (PSC) subunit in Drosophila, it is unexpectedly found in a different PRC1 subunit, a Polycomb homolog called M33, in mice. We provide experimental support for the general importance of a charged region by predicting the compacting capability of PcG proteins from species other than Drosophila and mice and by testing several of these proteins using solution assays and microscopy. We infer that the ability of PcG proteins to compact chromatin in vitro can be predicted by the presence of domains of high positive charge and that PRC1 components from a variety of species conserve this highly charged region. This supports the hypothesis that compaction is a key aspect of PcG function.

Expression and clinicopathological significance of Mel-18 and Bmi-1 mRNA in gastric carcinoma

Background

The Polycomb group (PcG) genes are a class of regulators responsible for maintaining homeotic gene expression throughout cell division. PcG expression is deregulated in some types of human cancer. Both Bmi-1 and Mel-18 are of the key PcG proteins. We investigate the expression and clinicopathological roles of Mel-18 and Bmi-1 mRNA in gastric cancer.

Methods

The expression of Mel-18 and Bmi-1 in a series of 71 gastric cancer tissues and paired normal mucosal tissues distant from the tumorous lesion was assayed by quantitative real time RT-PCR. The correlation between Mel-18 and Bmi-1 mRNA expression, and between Mel-18 or Bmi-1 mRNA level and clinicopathological characteristics were analyzed.

Results

Expression of Mel-18 and Bmi-1 genes was variably detected, but overexpression of Bmi-1 mRNA and decreased expression of Mel-18 mRNA were the most frequent alteration. In addition, the expression of Bmi-1 and Mel-18 mRNA inversely correlates in gastric tumors. Moreover, a significant positive correlation between Bmi-1 overexpression and tumor size, depth of invasion, or lymph node metastasis, and a significant negative correlation between Mel-18 low-expression with lymph node metastasis or the clinical stage were observed.

Conclusion

Our data suggest that Mel-18 and Bmi-1 may play crucial but opposite roles in gastric cancer. Decreased Mel-18 and increased Bmi-1 mRNA expression was associated with the carcinogenesis and progression of gastric cancer. It is possible to list Bmi-1 and Mel-18 as biomarkers for predicting the prognosis of gastric cancer.

Polycomb group molecule PHC3 regulates polycomb complex composition and prognosis of osteosarcoma

Polyhomeotic homolog 3 (PHC 3) is a member of the human polycomb complex and has been regarded as a candidate tumor suppressor of osteosarcoma. In the present paper, we performed a mutation survey and PHC3 expression analysis by quantitative real-time PCR using 10 osteosarcoma cell lines and 42 primary osteosarcoma samples. Relative PHC3 expression values of clinical samples were analyzed with clinical outcomes, and it was suggested that lower PHC3-expressing patients had significantly worse overall survival. Relative PHC3 values of clinical samples were less than those of normal bone tissues, whereas they were greater than those of cell lines. By denaturing high performance liquid chromatography analysis and direct sequencing, we found a PHC3 missense mutation in U2OS cells, which resulted in arginine56 to proline substitution. The same point mutation existed in four of 42 primary osteosarcoma samples. Regarding functional analysis, PHC3 expression significantly suppressed the colony formation of tumor cells. Intriguingly, polycomb repressive complex 1 members, Bmi1 and Ring1b proteins, were reduced in PHC3-expressing osteosarcoma cells. Deletion mutant PHC3 expression suggested that the carboxyl terminus of PHC3 has a role in suppression; the above-mentioned point mutation of PHC3 also lost inhibitory activities. Conversely, Bmi1 expression reduced PHC3 at the mRNA level and induced the proliferation of osteosarcoma cells. Taken together, we confirmed the role of PHC3 as a tumor suppressor in osteosarcoma cells and found that PHC3-dependent tumor suppression may be caused by modification of the composition of polycomb repressive complex 1 in cancer cells.

SIAH-1 interacts with mammalian polyhomeotic homologues HPH2 and affects its stability via the ubiquitin-proteasome pathway

Polycomb Group (PcG) genes encode proteins that form large multimeric and chromatin-associated complexes implicated in the stable repression of developmentally essential genes. HPH2, the Homo sapiens polyhomeotic homologue 2, functions as one of the subunits of PcG complex 1. In our study, SIAH-1, an E3 ligase, could directly associate with HPH2 both in vitro and in vivo. Both the cysteine-rich region of SIAH-1 and the PxVxAxP motif of HPH2 were essential for the interaction. HPH2 was co-localized with SIAH-1 in nuclei. Furthermore, SIAH-1 was able to facilitate the ubiquitination and degradation of HPH2 via ubiquitin-proteasome pathway in vivo. The ubiquitination activity was severely impaired in the SIAH-1 mutant that either lost E3 ligase activity or had weakened binding ability with HPH2, strongly suggesting that SIAH-1 was the direct E3 ligase of HPH2. Thus, our results propose a novel role of SIAH-1 in regulating the expression level of HPH2 through the ubiquitin-proteasome pathway.

Reciprocal expression of Bmi1 and Mel-18 is associated with functioning of primitive hematopoietic cells

OBJECTIVE

The Polycomb-group (PcG) genes regulate global gene expression in many biological processes, including hematopoiesis, by manipulating specific target genes. It is known that various PcG genes regulate self-renewal of hematopoietic stem cells (HSCs). Here we have shown that the reciprocal expression of PcG proteins regulates self-renewal and differentiation of HSCs.

METHODS

We used murine and human bone marrow cells and evaluated the reciprocal expression of PcG proteins on the basis of their respective intranuclear distributions. PcG-gene expression in HSCs was knocked down by small interfering RNAs. The function of each gene in HSCs was analyzed in vitro and in vivo.

RESULTS

Cells were either Bmi1-positive or Mel-18-positive. The Bmi1-positive cells contained very little amounts of Mel-18 and vice versa. The bmi1-knockdown marrow cells did not show HSC function, while the mel-18-knockdown marrow cells showed increased stem cell function. Results of the analysis on human cells were similar to those observed in case of murine cells. In a clinical investigation, transplantation using sources with a low Bmi1 to Mel-18 ratio was associated with early hematopoietic recovery.

CONCLUSION

Reciprocal expression of Bmi1 and Mel-18 regulated HSC function. Here, we observed that expression of the PcG genes-bmi1 and mel-18-is correlated with self-renewal and differentiation of HSCs. Thus, it was suggested that the balance between Bmi1 and Mel-18 regulates self-renewal of HSCs.

MIG-32 and SPAT-3A are PRC1 homologs that control neuronal migration in Caenorhabditis elegans

The Polycomb repression complex 2 (PRC2) methylates histone H3 lysine 27 at target genes to modify gene expression, and this mark is recognized by PRC1, which ubiquitylates histone H2A. In Caenorhabditis elegans, a complex of the MES-2, MES-3 and MES-6 proteins is functionally analogous to the PRC2 complex, but the functional analog of PRC1, and indeed whether C. elegans has such a complex, has been unclear. We describe here that MIG-32 and SPAT-3A are functional analogs of PRC1 in C. elegans, where they are required for neuronal migrations and during vulval development. mig-32 and spat-3 mutants are defective in H2A ubiquitylation, and have nervous system defects that partially overlap with those of mes mutants. However, unlike the mes mutants, mig-32 and spat-3 mutants are fertile, suggesting that PRC1 function is not absolutely required in the germline for essential functions of PRC2.

Loss of BMI-1 expression is associated with clinical progress of malignant melanoma

BMI-1 is a member of the Polycomb group of genes (PcGs) and is involved in embryonic gene regulation and maintenance of adult stem cells. It has been suggested that BMI-1 protein is important in cell cycle regulation, since both p16/INK4a and p14/ARF are downstream BMI-1 targets. BMI-1 has been implicated in the development and progression of several malignancies, but its role in melanocytic tumors of the skin is unknown. In the present study, using immunohistochemistry on 178 benign and malignant melanocytic lesions and two different antibodies, BMI-1 expression was reduced in melanomas compared with benign nevi. In established melanomas, loss of BMI-1 expression was associated with features of aggressive tumors, such as increased tumor cell proliferation, presence of necrosis and increased expression of both N-cadherin and beta3-integrin, indicating a more invasive and mesenchymal phenotype. Low BMI-1 expression was associated with low p14 and CDK4 but not with p16 expression. Low levels of BMI-1 expression were also significantly associated with decreased patient survival.

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.

Juxtaposed Polycomb complexes co-regulate vertebral identity

Best known as epigenetic repressors of developmental Hox gene transcription, Polycomb complexes alter chromatin structure by means of post-translational modification of histone tails. Depending on the cellular context, Polycomb complexes of diverse composition and function exhibit cooperative interaction or hierarchical interdependency at target loci. The present study interrogated the genetic, biochemical and molecular interaction of BMI1 and EED, pivotal constituents of heterologous Polycomb complexes, in the regulation of vertebral identity during mouse development. Despite a significant overlap in dosage-sensitive homeotic phenotypes and co-repression of a similar set of Hox genes, genetic analysis implicated eed and Bmi1 in parallel pathways, which converge at the level of Hox gene regulation. Whereas EED and BMI1 formed separate biochemical entities with EzH2 and Ring1B, respectively, in mid-gestation embryos, YY1 engaged in both Polycomb complexes. Strikingly, methylated lysine 27 of histone H3 (H3-K27), a mediator of Polycomb complex recruitment to target genes, stably associated with the EED complex during the maintenance phase of Hox gene repression. Juxtaposed EED and BMI1 complexes, along with YY1 and methylated H3-K27, were detected in upstream regulatory regions of Hoxc8 and Hoxa5. The combined data suggest a model wherein epigenetic and genetic elements cooperatively recruit and retain juxtaposed Polycomb complexes in mammalian Hox gene clusters toward co-regulation of vertebral identity.

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.

Polycomb silencers control cell fate, development and cancer

Polycomb group (PcG) proteins are epigenetic gene silencers that are implicated in neoplastic development. Their oncogenic function might be associated with their well-established role in the maintenance of embryonic and adult stem cells. In this review, we discuss new insights into the possible mechanisms by which PcGs regulate cellular identity, and speculate how these functions might be relevant during tumorigenesis.

PHC3, a component of the hPRC-H complex, associates with E2F6 during G0 and is lost in osteosarcoma tumors

Polyhomeotic-like 3 (PHC3) is a ubiquitously expressed member of the polycomb gene family and part of the human polycomb complex hPRC-H. We found that in normal cells PHC3 associated with both hPRC-H complex components and with the transcription factor E2F6. In differentiating and confluent cells, PHC3 and E2F6 showed nuclear colocalization in a punctate pattern that resembled the binding of polycomb bodies to heterochromatin. This punctate pattern was not seen in proliferating cells suggesting that PHC3 may be part of an E2F6-polycomb complex that has been shown to occupy and silence target promoters in G(0). Previous loss of heterozygosity (LoH) analyses had shown that the region containing PHC3 underwent frequent LoH in primary human osteosarcoma tumors. When we examined normal bone and human osteosarcoma tumors, we found loss of PHC3 expression in 36 of 56 osteosarcoma tumors. Sequence analysis revealed that PHC3 was mutated in nine of 15 primary osteosarcoma tumors. These findings suggest that loss of PHC3 may favor tumorigenesis by potentially disrupting the ability of cells to remain in G(0).

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.

Mammalian polyhomeotic homologues Phc2 and Phc1 act in synergy to mediate polycomb repression of Hox genes

The Polycomb group (PcG) gene products form multimeric protein complexes and contribute to anterior-posterior (A-P) specification via the transcriptional regulation of Hox cluster genes. The Drosophila polyhomeotic genes and their mammalian orthologues, Phc1, Phc2, and Phc3, encode nuclear proteins that are constituents of evolutionarily conserved protein complexes designated class II PcG complexes. In this study, we describe the generation and phenotypes of Phc2-deficient mice. We show posterior transformations of the axial skeleton and premature senescence of mouse embryonic fibroblasts associated with derepression of Hox cluster genes and Cdkn2a genes, respectively. Synergistic actions of a Phc2 mutation with Phc1 and Rnf110 mutations during A-P specification, coimmunoprecipitation of their products from embryonic extracts, and chromatin immunoprecipitation by anti-Phc2 monoclonal antibodies suggest that Hox repression by Phc2 is mediated through the class II PcG complexes, probably via direct binding to the Hox locus. The genetic interactions further reveal the functional overlap between Phc2 and Phc1 and a strict dose-dependent requirement during A-P specification and embryonic survival. Functional redundancy between Phc2 and Phc1 leads us to hypothesize that the overall level of polyhomeotic orthologues in nuclei is a parameter that is critical in enabling the class II PcG complexes to exert their molecular functions.

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.

The many faces of SAM

Protein-protein interactions are essential for the assembly, regulation, and localization of functional protein complexes in the cell. SAM domains are among the most abundant protein-protein interaction motifs in organisms from yeast to humans. Although SAM domains adopt similar folds, they are remarkably versatile in their binding properties. Some identical SAM domains can interact with each other to form homodimers or polymers. In other cases, SAM domains can bind to other related SAM domains, to non-SAM domain-containing proteins, and even to RNA. Such versatility earns them functional roles in myriad biological processes, from signal transduction to transcriptional and translational regulation. In this review, we describe the structural basis of SAM domain interactions and highlight their roles in the scaffolding of protein complexes in normal and pathological processes.

Increased expression of the EZH2 polycomb group gene in BMI-1-positive neoplastic cells during bronchial carcinogenesis

Polycomb group (PcG) genes are responsible for maintenance of cellular identity and contribute to regulation of the cell cycle. Recent studies have identified several PcG genes as oncogenes, and a role for PcG proteins in human oncogenesis is suspected. We investigated the expression of BMI-1 and EZH2 PcG oncogenes in human bronchial squamous cell carcinomas (SCCs) and bronchial premalignant precursor lesions (PLs). Whereas normal bronchial epithelium was associated with widespread expression of BMI-1 in resting EZH2-negative cells, neoplastic cells in lung carcinomas displayed altered expression of both BMI-1 and EZH2. Two patterns of abnormal PcG expression were observed: increased expression of BMI-1 in dividing neoplastic cells of PLs and SCCs, and enhanced expression of EZH2 and Ki-67 in BMI-1-positive cells according to severity of the histopathologic stage. We propose that altered expression of BMI-1 and EZH2 is an early event that precedes high rates of proliferation in lung cancer. Because PcG complexes are normally involved in the maintenance of cell characteristics, abnormal PcG expression may contribute to loss of cell identity.

Polycomb group gene mel-18 regulates early T progenitor expansion by maintaining the expression of Hes-1, a target of the Notch pathway

Polycomb group (PcG) proteins play a role in the maintenance of cellular identity throughout many rounds of cell division through the regulation of gene expression. In this report we demonstrate that the loss of the PcG gene mel-18 impairs the expansion of the most immature T progenitor cells at a stage before the rearrangement of the TCR beta-chain gene in vivo and in vitro. This impairment of these T progenitors appears to be associated with increased susceptibility to cell death. We also show that the expression of Hes-1, one of the target genes of the Notch signaling pathway, is drastically down-regulated in early T progenitors isolated from mel-18(-/-) mice. In addition, mel-18(-/-) T precursors could not maintain the Hes-1 expression induced by Delta-like-1 in monolayer culture. Collectively, these data indicate that mel-18 contributes to the maintenance of the active state of the Hes-1 gene as a cellular memory system, thereby supporting the expansion of early T progenitors.

MAPKAP Kinase 3pK Phosphorylates and Regulates Chromatin Association of the Polycomb Group Protein Bmi1

Polycomb group (PcG) proteins form chromatin-associated, transcriptionally repressive complexes, which are critically involved in the control of cell proliferation and differentiation. Although the mechanisms involved in PcG-mediated repression are beginning to unravel, little is known about the regulation of PcG function. We showed previously that PcG complexes are phosphorylated in vivo, which regulates their association with chromatin. The nature of the responsible PcG kinases remained unknown. Here we present the novel finding that the PcG protein Bmi1 is phosphorylated by 3pK (MAPKAP kinase 3), a convergence point downstream of activated ERK and p38 signaling pathways and implicated in differentiation and developmental processes. We identified 3pK as an interaction partner of PcG proteins, in vitro and in vivo, by yeast two-hybrid interaction and co-immunoprecipitation, respectively. Activation or overexpression of 3pK resulted in phosphorylation of Bmi1 and other PcG members and their dissociation from chromatin. Phosphorylation and subsequent chromatin dissociation of PcG complexes were expected to result in de-repression of targets. One such reported Bmi1 target is the Cdkn2a/INK4A locus. Cells overexpressing 3pK showed PcG complex/chromatin dissociation and concomitant de-repression of p14(ARF), which was encoded by the Cdkn2a/INK4A locus. Thus, 3pK is a candidate regulator of phosphorylation-dependent PcG/chromatin interaction. We speculate that phosphorylation may not only affect chromatin association but, in addition, the function of individual complex members. Our findings linked for the first time MAPK signaling pathways to the Polycomb transcriptional memory system. This suggests a novel mechanism by which a silenced gene status can be modulated and implicates PcG-mediated repression as a dynamically controlled process.

Maintenance of gene expression patterns

In development, cells pass on established gene expression patterns to daughter cells over multiple rounds of cell division. The cellular memory of the gene expression state is termed maintenance, and the proteins required for this process are termed maintenance proteins. The best characterized are proteins of the Polycomb and trithorax Groups that are required for silencing and maintenance of activation of target loci, respectively. These proteins act through DNA elements termed maintenance elements. Here, we re-examine the genetics and molecular biology of maintenance proteins. We discuss molecular models for the maintenance of activation and silencing, and the establishment of epigenetic marks, and suggest that maintenance proteins may play a role in propagating the mark through DNA synthesis.

Developmentally regulated alterations in Polycomb repressive complex 1 proteins on the inactive X chromosome

Polycomb group (PcG) proteins belonging to the polycomb (Pc) repressive complexes 1 and 2 (PRC1 and PRC2) maintain homeotic gene silencing. In Drosophila, PRC2 methylates histone H3 on lysine 27, and this epigenetic mark facilitates recruitment of PRC1. Mouse PRC2 (mPRC2) has been implicated in X inactivation, as mPRC2 proteins transiently accumulate on the inactive X chromosome (Xi) at the onset of X inactivation to methylate histone H3 lysine 27 (H3-K27). In this study, we demonstrate that mPRC1 proteins localize to the Xi, and that different mPRC1 proteins accumulate on the Xi during initiation and maintenance of X inactivation in embryonic cells. The Xi accumulation of mPRC1 proteins requires Xist RNA and is not solely regulated by the presence of H3-K27 methylation, as not all cells that exhibit this epigenetic mark on the Xi show Xi enrichment of mPRC1 proteins. Our results implicate mPRC1 in X inactivation and suggest that the regulated assembly of PcG protein complexes on the Xi contributes to this multistep process.

A novel repressive E2F6 complex containing the polycomb group protein, EPC1, that interacts with EZH2 in a proliferation-specific manner

The transcriptional repressor E2F6 has been identified as a component of two distinct polycomb group protein (PcG)-containing complexes, suggesting a mechanism for the recruitment of repressive complexes to target sequences in DNA. Whereas one complex is involved in the repression of classic E2F target genes in G0, a role for E2F6 within the cell cycle has yet to be defined. We searched for novel E2F6-binding proteins using a yeast two-hybrid screen and identified the PcG protein, EPC1. We showed that, both in vitro and in vivo, E2F6, DP1, and EPC1 form a stable core complex with repressive activity. Furthermore, we identified the proliferation-specific PcG, EZH2, as an EPC1-interacting protein. Using affinity purification, we showed that E2F6, DP1, EPC1, EZH2, and Sin3B co-elute, suggesting the identification of a novel E2F6 complex that exists in vivo in both normal and transformed human cell lines. EZH2 is required for cellular proliferation and consistent with this, EZH2 elutes with the E2F6-EPC1 complex only in proliferating cells. Thus we have identified a novel E2F6-PcG complex (E2F6-EPC1) that interacts with EZH2 and may regulate genes required for cell cycle progression.

Histone hypomethylation is an indicator of epigenetic plasticity in quiescent lymphocytes

Post-translational modifications of histone amino termini are thought to convey epigenetic information that extends the coding potential of DNA. In particular, histone lysine methylation has been implicated in conveying transcriptional memory and maintaining lineage fidelity. Here an analysis of histone lysine methylation in quiescent (G(0)) and cycling lymphocytes showed that methylation of histone H3 at lysines 4 (H3K4), 9 (H3K9), 27 (H3K27) and histone H4 at lysine 20 is markedly reduced in resting B lymphocytes as compared with cycling cells. Quiescent B cells also lacked heterochromatin-associated HP1beta and Ikaros at pericentric chromatin and expressed low levels of Ezh2 and ESET histone methyl transferases (HMTases). Nuclei from resting B or T cells were approximately three times more efficiently reprogrammed in nuclear transfer assays than cells in which HMTase expression, histone methylation and HP1beta binding had been restored following mitotic stimulation. These results showing local and global changes in histone lysine methylation levels in vivo demonstrate that constitutive heterochromatin organization is modified in resting lymphocytes and suggest that histone hypomethylation is a useful indicator of epigenetic plasticity.

Polycomb group gene mel-18 modulates the self-renewal activity and cell cycle status of hematopoietic stem cells

OBJECTIVE

Mel-18 is a member of the mammalian Polycomb group (PcG) genes. This family of genes regulates global gene expression in many biologic processes, including hematopoiesis and anterior-posterior axis formation by manipulating specific target genes, including members of the Hox family. Here, we demonstrate that mel-18 negatively regulates the self-renewal activity of hematopoietic stem cells (HSCs).

MATERIALS AND METHODS

Long-term reconstitution activity was evaluated by competitive repopulating unit (CRU) and mean activity of the stem cells (MAS) assays in vivo in bone marrow cells (BMCs) derived from mel-18(-/-) and mel-18 tg mice. The expression levels of mel-18 and Hoxb4 were measured by quantitative real-time reverse transcription polymerase chain reaction.

RESULTS

The Hoxb4 gene was highly expressed in HSCs derived from mel-18(-/-) mice. The observed CRUs were 3.21, 4.77, 3.32, and 1.64 CRU per 10(5) BMCs in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. MAS was 0.58, 0.18, 0.41, and 5.89 in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. The percentage in G0 phase HSCs (lin(-)flk2(-)c-Kit(+)Sca1+ cells) was increased in mel-18(-/-) mice and decreased in mel-18 tg mice.

CONCLUSION

Loss or knockdown of mel-18 leads to the expression of Hoxb4, an increase in the proportion of HSCs in G0 phase, and the subsequent promotion of HSC self-renewal. These findings will enable us to develop new approaches for controlling HSC activity for hematopoietic transplantations based on ex vivo expansion of HSCs.

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.

Unique polycomb gene expression pattern in Hodgkin's lymphoma and Hodgkin's lymphoma-derived cell lines

Human Polycomb-group (PcG) genes play a crucial role in the regulation of embryonic development and regulation of the cell cycle and hematopoiesis. PcG genes encode proteins that form two distinct PcG complexes, involved in maintenance of cell identity and gene silencing patterns. We recently showed that expression of the BMI-1 and EZH2 PcG genes is separated during normal B-cell development in germinal centers, whereas Hodgkin/Reed-Sternberg (H/RS) cells co-express BMI-1 and EZH2. In the current study, we used immunohistochemistry and immunofluorescence to determine whether the binding partners of these PcG proteins are also present in H/RS cells and H/RS-derived cell lines. PcG expression profiles were analyzed in combination with expression of the cell cycle inhibitor p16INK4a, because experimental model systems indicate that p16 is a downstream target of Bmi-1. We found that H/RS cells and HL-derived cell lines co-express all core proteins of the two known PcG complexes, including BMI-1, MEL-18, RING1, HPH1, HPC1, and -2, EED, EZH2, YY1, and the HPC2 binding partner, CtBP. Expression of HPC1 has not been found in normal mature B cells and other malignant lymphomas of B-cell origin, suggesting that the PcG expression profile of H/RS is unique. In contrast to Bmi-1 transgenic mice where p16INK4a is down-regulated, 27 of 52 BMI-1POS cases of HL revealed strong nuclear expression of p16INK4a. We propose that abnormal expression of BMI-1 and its binding partners in H/RS cells contributes to development of HL. However, abnormal expression of BMI-1 in HL is not necessarily associated with down-regulation of p16INK4a.

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.

Proteomics analysis of the centromere complex from HeLa interphase cells: UV-damaged DNA binding protein 1 (DDB-1) is a component of the CEN-complex, while BMI-1 is transiently co-localized with the centromeric region in interphase

CENP-A, a centromere-specific histone H3, is conserved throughout eukaryotes, and formation of CENP-A chromatin defines the active centromere region. Here, we report the isolation of CENP-A chromatin from HeLa interphase nuclei by chromatin immunoprecipitation using anti-CENP-A monoclonal antibody, and systematic identification of its components by mass spectrometric analyses. The isolated chromatin contained CENP-B, CENP-C, CENP-H, CENP-I/hMis 6 and hMis 12 as well as CENP-A, suggesting that the isolated chromatin may represent the centromere complex (CEN-complex). Mass spectrometric analyses of the CEN-complex identified approximately 40 proteins, including the previously reported centromere proteins and the proteins of unknown function. In addition, we unexpectedly identified a series of proteins previously reported to be related to functions other than chromosome segregation, such as uvDDB-1, XAP8, hSNF2H, FACTp180, FACTp80/SSRP1, polycomb group proteins (BMI-1, RING1, RNF2, HPC3 and PHP2), KNL5 and racGAP. We found that uvDDB-1 was actually localized to the centromeric region throughout cell cycle, while BMI-1 was transiently co-localized with the centromeres in interphase. These results give us new insights into the architecture, dynamics and function of centromeric chromatin in interphase nuclei, which might reflect regulation of cell proliferation and differentiation.

Transcriptional coactivator Cited2 induces Bmi1 and Mel18 and controls fibroblast proliferation via Ink4a/ARF

Cited2 (CBP/p300 interacting transactivator with ED-rich tail 2) is required for embryonic development, coactivation of transcription factor AP-2, and inhibition of hypoxia-inducible factor 1 transactivation. Cited2 is induced by multiple growth factors and cytokines and oncogenically transforms cells. Here, we show that the proliferation of Cited2(-/-) mouse embryonic fibroblasts ceases prematurely. This is associated with a reduction in growth fraction, senescent cellular morphology, and increased expression of the cell proliferation inhibitors p16(INK4a), p19(ARF), and p15(INK4b). Deletion of INK4a/ARF (encoding p16(INK4a) and p19(ARF)) completely rescued the defective proliferation of Cited2(-/-) fibroblasts. However, the deletion of INK4a/ARF did not rescue the embryonic malformations observed in Cited2(-/-) mice, indicating that INK4a/ARF-independent pathways are likely to be involved here. We found that Cited2(-/-) fibroblasts had reduced expression of the polycomb-group genes Bmi1 and Mel18, which function as INK4a/ARF and Hox repressors. Complementation with CITED2-expressing retrovirus enhanced proliferation, induced Bmi1/Mel18 expression, and decreased INK4a/ARF expression. Bmi1- and Mel18-expressing retroviruses enhanced the proliferation of Cited2(-/-) fibroblasts, indicating that they function downstream of Cited2. Our results provide genetic evidence that Cited2 controls the expression of INK4a/ARF and fibroblast proliferation, at least in part via the polycomb-group genes Bmi1 and Mel18.

Dissociation of mammalian Polycomb-group proteins, Ring1B and Rae28/Ph1, from the chromatin correlates with configuration changes of the chromatin in mitotic and meiotic prophase

The Polycomb group (PcG) gene products form complexes that regulate chromatin configuration to mediate cellular memory to postmitotic somatic cells and postmeiotic oocytes in Drosophila melanogaster. Structural and functional similarities of PcG proteins between invertebrates and vertebrates suggest mammalian PcG proteins may be involved to imprint transcriptional status at various loci into postmitotic and postmeiotic daughter cells. To address molecular mechanisms underlying PcG-mediated cellular memory, it might be a prerequisite to understand subcellular localization of PcG proteins during mitosis and meiosis. In this study, we analyzed subcellular localization of Rae28/Ph1 and Ring1B by using newly generated monoclonal antibodies in mitotic somatic cells and meiotic mouse oocytes. Results suggest that Rae28/Ph1 and Ring1B dissociate from the chromatin upon its condensation in mitotic prophase in the U2-OS human osteosarcoma cell line. During maturation of oocytes, significant alterations of Rae28/Ph1 and Ring1B localization are concordant with configuration changes of the chromatin at the germinal vesicle stage of meiotic prophase. Importantly, dissociation of Rae28/Ph1 and Ring1B from the chromatin temporally correlates with transcriptional arrest both in mitosis and meiosis. Present and previous observations suggest molecular mechanisms required for mitotic regulation of RNA polymerase II could be involved in dissociation of PcG proteins.

Polycomb group genes as epigenetic regulators of normal and leukemic hemopoiesis

Epigenetic modification of chromatin structure underlies the differentiation of pluripotent hemopoietic stem cells (HSCs) into their committed/differentiated progeny. Compelling evidence indicates that Polycomb group (PcG) genes play a key role in normal and leukemic hemopoiesis through epigenetic regulation of HSC self-renewal/proliferation and commitment. The PcG proteins are constituents of evolutionary highly conserved molecular pathways regulating cell fate in several other tissues through diverse mechanisms, including 1) regulation of self-renewal/proliferation, 2) regulation of senescence/immortalization, 3) interaction with the initiation transcription machinery, 4) interaction with chromatin-condensation proteins, 5) modification of histones, 6) inactivation of paternal X chromosome, and 7) regulation of cell death. It is therefore not surprising that PcG genes lead to pleiotropic phenotypes when mutated and have been associated with malignancies in several systems in both mice and humans. Although much remains to be learned regarding the PcG mechanism(s) of action, advances in identifying the functional domains and enzymatic activities of these multimeric protein complexes have provided insights into how PcG proteins accomplish such processes. Some of the new insights into a role for the PcG cellular memory system in regulating normal and leukemic hemopoiesis are reviewed here, with special emphasis on their potential involvement in epigenetic regulation of gene expression through modification of chromatin structure.

The polycomb protein Pc2 is a SUMO E3

Polycomb group (PcG) proteins form large multimeric complexes (PcG bodies) which are involved in the stable repression of gene expression. The human PcG protein, Pc2, has been shown to recruit the transcriptional corepressor, CtBP, to PcG bodies. We show that CtBP is sumoylated at a single lysine. In vitro, CtBP sumoylation minimally requires the SUMO E1 and E2 (Ubc9) and SUMO-1. However, Pc2 dramatically enhances CtBP sumoylation. In vivo, this is likely due to the ability of Pc2 to recruit both CtBP and Ubc9 to PcG bodies, thereby bringing together substrate and E2, and stimulating the transfer of SUMO to CtBP. These results demonstrate that Pc2 is a SUMO E3, and suggest that PcG bodies may be sumoylation centers.

Rnf2 (Ring1b) deficiency causes gastrulation arrest and cell cycle inhibition

The highly homologous Rnf2 (Ring1b) and Ring1 (Ring1a) proteins were identified as in vivo interactors of the Polycomb Group (PcG) protein Bmi1. Functional ablation of Rnf2 results in gastrulation arrest, in contrast to relatively mild phenotypes in most other PcG gene null mutants belonging to the same functional group, among which is Ring1. Developmental defects occur in both embryonic and extraembryonic tissues during gastrulation. The early lethal phenotype is reminiscent of that of the PcG-gene knockouts Eed and Ezh2, which belong to a separate functional PcG group and PcG protein complex. This finding indicates that these biochemically distinct PcG complexes are both required during early mouse development. In contrast to the strong skeletal transformation in Ring1 hemizygous mice, hemizygocity for Rnf2 does not affect vertebral identity. However, it does aggravate the cerebellar phenotype in a Bmi1 null-mutant background. Together, these results suggest that Rnf2 or Ring1-containing PcG complexes have minimal functional redundancy in specific tissues, despite overlap in expression patterns. We show that the early developmental arrest in Rnf2-null embryos is partially bypassed by genetic inactivation of the Cdkn2a (Ink4aARF) locus. Importantly, this finding implicates Polycomb-mediated repression of the Cdkn2a locus in early murine development.