Roles of the Polycomb group proteins in stem cells and cancer

MicroRNAs Overcome Cell Fate Barrier by Reducing EZH2-Controlled REST Stability during Neuronal Conversion of Human Adult Fibroblasts

The ability to convert human somatic cells efficiently to neurons facilitates the utility of patient-derived neurons for studying neurological disorders. As such, ectopic expression of neuronal microRNAs (miRNAs), miR-9/9^∗ and miR-124 (miR-9/9^∗-124) in adult human fibroblasts has been found to evoke extensive reconfigurations of the chromatin and direct the fate conversion to neurons. However, how miR-9/9^∗-124 break the cell fate barrier to activate the neuronal program remains to be defined. Here, we identified an anti-neurogenic function of EZH2 in fibroblasts that acts outside its role as a subunit of Polycomb Repressive Complex 2 to directly methylate and stabilize REST, a transcriptional repressor of neuronal genes. During neuronal conversion, miR-9/9^∗-124 induced the repression of the EZH2-REST axis by downregulating USP14, accounting for the opening of chromatin regions harboring REST binding sites. Our findings underscore the interplay between miRNAs and protein stability cascade underlying the activation of neuronal program.

Role of EZH2 in cancer stem cells: from biological insight to a therapeutic target

Epigenetic modifications in cancer stem cells largely result in phenotypic and functional heterogeneity in many solid tumors. Increasing evidence indicates that enhancer of zeste homolog 2 (EZH2), the catalytic subunit of Polycomb repressor complex 2, is highly expressed in cancer stem cells of numerous malignant tumors and has a critical function in cancer stem cell expansion and maintenance. Here, we review up-to-date information regarding EZH2 expression patterns, functions, and molecular mechanisms in cancer stem cells in various malignant tumors and discuss the therapeutic potential of targeting EZH2 in tumors.

MUC1-C activates polycomb repressive complexes and downregulates tumor suppressor genes in human cancer cells

The PRC2 and PRC1 complexes are aberrantly expressed in human cancers and have been linked to decreases in patient survival. MUC1-C is an oncoprotein that is also overexpressed in diverse human cancers and is associated with a poor prognosis. Recent studies have supported a previously unreported function for MUC1-C in activating PRC2 and PRC1 in cancer cells. In the regulation of PRC2, MUC1-C (i) drives transcription of the EZH2 gene, (ii) binds directly to EZH2, and (iii) enhances occupancy of EZH2 on target gene promoters with an increase in H3K27 trimethylation. Regarding PRC1, which is recruited to PRC2 sites in the hierarchical model, MUC1-C induces BMI1 transcription, forms a complex with BMI1, and promotes H2A ubiquitylation. MUC1-C thereby contributes to the integration of PRC2 and PRC1-mediated repression of tumor suppressor genes, such as CDH1, CDKN2A, PTEN and BRCA1. Like PRC2 and PRC1, MUC1-C is associated with the epithelial-mesenchymal transition (EMT) program, cancer stem cell (CSC) state, and acquisition of anticancer drug resistance. In concert with these observations, targeting MUC1-C downregulates EZH2 and BMI1, inhibits EMT and the CSC state, and reverses drug resistance. These findings emphasize the significance of MUC1-C as a therapeutic target for inhibiting aberrant PRC function and reprogramming the epigenome in human cancers.

CBX7 is a glioma prognostic marker and induces G1/S arrest via the silencing of CCNE1

Chromobox homolog 7 (CBX7) cooperates with other polycomb group (PcG) proteins to maintain target genes in a silenced state. However, the precise role of CBX7 in tumor progression is still controversial. We found that the expression of CBX7 in four public databases was significantly lower in high grade glioma (HGG). The reduced expression of CBX7 correlated with poor outcome in HGG patients. Both KEGG and GO analyses indicated that genes that were negatively correlated to CBX7 were strongly associated with the cell cycle pathway. We observed that decreased CBX7 protein levels enhanced glioma cells proliferation, migration and invasion. Then, we verified that CBX7 overexpression arrested cells in the G₀/G₁ phase. Moreover, we demonstrated that the underlying mechanism involved in CBX7 induced repression of CCNE1 promoter requiring the recruitment of histone deacetylase 2 (HADC2). Finally, in vivo bioluminescence imaging and survival times of nude mice revealed that CBX7 behaved as a tumor suppressor in gliomas. In summary, our results validate the assumption that CBX7 is a tumor suppressor of gliomas. Moreover, CBX7 is a potential and novel prognostic biomarker in glioma patients. We also clarified that CBX7 silences CCNE1 via the combination of CCNE1 promoter and the recruitment of HDAC2.

The HDAC inhibitor SAHA regulates CBX2 stability via a SUMO-triggered ubiquitin-mediated pathway in leukemia

Polycomb group (PcG) proteins regulate transcription, playing a key role in stemness and differentiation. Deregulation of PcG members is known to be involved in cancer pathogenesis. Emerging evidence suggests that CBX2, a member of the PcG protein family, is overexpressed in several human tumors, correlating with lower overall survival. Unraveling the mechanisms regulating CBX2 expression may thus provide a promising new target for anticancer strategies. Here we show that the HDAC inhibitor SAHA regulates CBX2 stability via a SUMO-triggered ubiquitin-mediated pathway in leukemia. We identify CBX4 and RNF4 as the E3 SUMO and E3 ubiquitin ligase, respectively, and describe the specific molecular mechanism regulating CBX2 protein stability. Finally, we show that CBX2-depleted leukemic cells display impaired proliferation, underscoring its critical role in regulating leukemia cell tumorogenicity. Our results show that SAHA affects CBX2 stability, revealing a potential SAHA-mediated anti-leukemic activity though SUMO2/3 pathway.

Targeting EHMT2 reverses EGFR-TKI resistance in NSCLC by epigenetically regulating the PTEN/AKT signaling pathway

Epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) resistance is a major obstacle in the treatment of non-small cell lung cancer (NSCLC). Epigenetic alterations have been shown to be involved in NSCLC oncogenesis; however, their function in EGFR-TKI resistance remains uncharacterized. Here, we found that an EHMT2 inhibitor, UNC0638, can significantly inhibit cell growth and induce apoptosis in EGFR-TKI-resistant NSCLC cells. Additionally, we also found that EHMT2 expression and enzymatic activity levels were elevated in EGFR-TKI-resistant NSCLC cells. Moreover, we determined that genetic or pharmacological inhibition of EHMT2 expression enhanced TKI sensitivity and suppressed migration and tumor sphere formation in EGFR-TKI-resistant NSCLC cells. Further investigation revealed that EHMT2 contributed to PTEN transcriptional repression and thus facilitated AKT pathway activation. The negative relationship between EHMT2 and PTEN was confirmed by our clinical study. Furthermore, we determined that combination treatment with the EHMT2 inhibitor and Erlotinib resulted in enhanced antitumor effects in a preclinical EGFR-TKI-resistance model. We also found that high EHMT2 expression along with low PTEN expression can predict poor overall survival in patients with NSCLC. In summary, our findings showed that EHMT2 facilitated EGFR-TKI resistance by regulating the PTEN/AKT pathway in NSCLC cells, suggesting that EHMT2 may be a target in the clinical treatment of EGFR-TKI-resistant NSCLC.

Epigenomic Hard Drive Imprinting: A Hidden Code Beyond the Biological Death of Cancer Patients

Several genetic and epigenetic theories have been suggested to explain the intricacies of life and death. However, several questions remain unsettled regarding cellular death events, particularly of living tissue in the case of cancer patients, such as the fate and adaptation of cancer cells after biological death. It is possible that cancer cells can display the intent to communicate with the external environment after biological death by means of molecular, genetic, and epigenetic pathways. Whether these cancer cells contain special information in the form of coding that may help them survive beyond the biological death of cancer patients is unknown. To understand these queries in the cancer field, we hypothesize the epigenomic hard drive (EHD) as a cellular component to record and store global epigenetic events in cancerous and non-cancerous tissues of cancer patients. This mini-review presents the novel concept of EHD that is reinforced with the existing knowledge of genetic and epigenetic events in cancer. Further, we summarize the EHD understanding that may impart much potential and interest for basic and clinical scientists to unravel mechanisms of carcinogenesis, therapeutic markers, and differential drug responses.

The histone methyltransferase EZH2 as a novel prosurvival factor in clinically aggressive chronic lymphocytic leukemia

The histone methyltransferase EZH2 induces gene repression through trimethylation of histone H3 at lysine 27 (H3K27me3). EZH2 overexpression has been reported in many types of cancer and associated with poor prognosis. Here we investigated the expression and functionality of EZH2 in chronic lymphocytic leukemia (CLL). Aggressive cases with unmutated IGHV genes (U-CLL) displayed significantly higher EZH2 expression compared to indolent CLL cases with mutated IGHV genes (M-CLL); furthermore, in U-CLL EZH2 expression was upregulated with disease progression. Within U-CLL, EZH2high cases harbored significantly fewer (p = 0.033) TP53 gene abnormalities compared to EZH2low cases. EZH2high cases displayed high H3K27me3 levels and increased viability suggesting that EZH2 is functional and likely confers a survival advantage to CLL cells. This argument was further supported by siRNA-mediated downmodulation of EZH2 which resulted in increased apoptosis. Notably, at the intraclonal level, cell proliferation was significantly associated with EZH2 expression. Treatment of primary CLL cells with EZH2 inhibitors induced downregulation of H3K27me3 levels leading to increased cell apoptosis. In conclusion, EZH2 is overexpressed in adverse-prognosis CLL and associated with increased cell survival and proliferation. Pharmacologic inhibition of EZH2 catalytic activity promotes apoptosis, highlighting EZH2 as a novel potential therapeutic target for specific subgroups of patients with CLL.

Epigallocatechin-3-gallate targets cancer stem-like cells and enhances 5-fluorouracil chemosensitivity in colorectal cancer

Resistance to cytotoxic chemotherapy is a major cause of mortality in colorectal cancer (CRC) patients. A small subset of cancer cells, termed “cancer stem cells” (CSCs), are believed to be key contributors of chemoresistance and tumor recurrence. Recently, epigallocatechin-3-gallate (EGCG), an active catechin present in green tea, has been shown to suppress CSC growth in various cancers, but whether it can specifically target CSCs and subsequently sensitize chemoresistant CRC cells to standard of care chemotherapeutic treatments remains unknown. Herein, we investigated the chemosensitizing effects of EGCG in 5-fluorouracil (5FU)-resistant (5FUR) CRC cells and spheroid-derived CSCs (SDCSCs), and interrogated the underlying molecular mechanisms responsible for its chemopreventive activity. EGCG enhanced 5FU-induced cytotoxicity and inhibited proliferation in 5FUR cell lines through enhancement of apoptosis and cell cycle arrest. The 5FUR cells showed higher spheroid forming capacity compared to parental cells, indicating higher CSC population. EGCG treatment in these cells resulted in suppression of SDCSC formation and enhanced 5FU sensitivity to SDCSCs. Furthermore, EGCG suppressed Notch1, Bmi1, Suz12, and Ezh2, and upregulated self-renewal suppressive-miRNAs, miR-34a, miR-145, and miR-200c, which are some of the key pathways targeted in 5FUR CRC cells. These findings were validated in vivo, wherein EGCG treatment resulted in inhibited tumor growth in a SDCSC xenograft model. Collectively our data provide novel and previously unrecognized evidence for EGCG-induced sensitization to 5FU through targeting of CSCs in CRC. Our data highlight that in addition to its chemopreventive ability, EGCG may serve as an adjunctive treatment to conventional chemotherapeutic drugs in CRC patients.

The E3 Ligase RING1 Targets p53 for Degradation and Promotes Cancer Cell Proliferation and Survival

As a component of the transcriptional repression complex 1 (PRC1), the ring finger protein RING1 participates in the epigenetic regulation in cancer. However, the contributions of RING1 to cancer etiology or development are unknown. In this study, we report that RING1 is a critical negative regulator of p53 homeostasis in human hepatocellular and colorectal carcinomas. RING1 acts as an E3 ubiquitin (Ub) ligase to directly interact with and ubiquitinate p53, resulting in its proteasome-dependent degradation. The RING domain of RING1 was required for its E3 Ub ligase activity. RING1 depletion inhibited the proliferation and survival of the p53 wild-type cancer cells by inducing cell-cycle arrest, apoptosis, and senescence, with only modest effects on p53-deficient cells. Its growth inhibitory effect was partially rescued by p53 silencing, suggesting an important role for the RING1-p53 complex in human cancer. In clinical specimens of hepatocellular carcinoma, RING1 upregulation was evident in association with poor clinical outcomes. Collectively, our results elucidate a novel PRC1-independent function of RING1 and provide a mechanistic rationale for its candidacy as a new prognostic marker and/or therapeutic target in human cancer.Significance: These results elucidate a novel PRC1-independent function of RING1 and provide a mechanistic rationale for its candidacy as a new prognostic marker and/or therapeutic target in human cancer. Cancer Res; 78(2); 359-71. ©2017 AACR.

Conservation and diversification of polycomb repressive complex 2 (PRC2) proteins in the green lineage

The polycomb group (PcG) proteins are key epigenetic regulators of gene expression in animals and plants. They act in multiprotein complexes, of which the best characterized is the polycomb repressive complex 2 (PRC2), which catalyses the trimethylation of histone H3 at lysine 27 (H3K27me3) at chromatin targets. In Arabidopsis thaliana, PRC2 proteins are involved in the regulation of diverse developmental processes, including cell fate determination, vegetative growth and development, flowering time control and embryogenesis. Here, we systematically analysed the evolutionary conservation and diversification of PRC2 components in lower and higher plants. We searched for and identified PRC2 homologues from the sequenced genomes of several green lineage species, from the unicellular green alga Ostreococcus lucimarinus to more complicated angiosperms. We found that some PRC2 core components, e.g. E(z), ESC/FIE and MSI/p55, are ancient and have multiplied coincidently with multicellular evolution. For one component, some members are newly formed, especially in the Cruciferae. During evolution, higher plants underwent copy number multiplication of various PRC2 components, which occurred independently for each component, without any obvious co-amplification of PRC2 members. Among the amplified members, usually one was well-conserved and the others were more diversified. Gene amplification occurred at different times for different PcG members during green lineage evolution. Certain PRC2 core components or members of them were highly conserved. Our study provides an insight into the evolutionary conservation and diversification of PcG proteins and may guide future functional characterization of these important epigenetic regulators in plants other than Arabidopsis.

Xenobiotic-induced activation of human aryl hydrocarbon receptor target genes in Drosophila is mediated by the epigenetic chromatin modifiers

Aryl hydrocarbon receptor (AHR) is the key transcription factor that controls animal development and various adaptive processes. The AHR’s target genes are involved in biodegradation of endogenous and exogenous toxins, regulation of immune response, organogenesis, and neurogenesis. Ligand binding is important for the activation of the AHR signaling pathway. Invertebrate AHR homologs are activated by endogenous ligands whereas vertebrate AHR can be activated by both endogenous and exogenous ligands (xenobiotics). Several studies using mammalian cultured cells have demonstrated that transcription of the AHR target genes can be activated by exogenous AHR ligands, but little is known about the effects of AHR in a living organism. Here, we examined the effects of human AHR and its ligands using transgenic Drosophila lines with an inducible human AhR gene. We found that exogenous AHR ligands can increase as well as decrease the transcription levels of the AHR target genes, including genes that control proliferation, motility, polarization, and programmed cell death. This suggests that AHR activation may affect the expression of gene networks that could be critical for cancer progression and metastasis. Importantly, we found that AHR target genes are also controlled by the enzymes that modify chromatin structure, in particular components of the epigenetic Polycomb Repressive complexes 1 and 2. Since exogenous AHR ligands (alternatively – xenobiotics) and small molecule inhibitors of epigenetic modifiers are often used as pharmaceutical anticancer drugs, our findings may have significant implications in designing new combinations of therapeutic treatments for oncological diseases.

RNA polymerase II primes Polycomb-repressed developmental genes throughout terminal neuronal differentiation

Polycomb repression in mouse embryonic stem cells (ESCs) is tightly associated with promoter co‐occupancy of RNA polymerase II (RNAPII) which is thought to prime genes for activation during early development. However, it is unknown whether RNAPII poising is a general feature of Polycomb repression, or is lost during differentiation. Here, we map the genome‐wide occupancy of RNAPII and Polycomb from pluripotent ESCs to non‐dividing functional dopaminergic neurons. We find that poised RNAPII complexes are ubiquitously present at Polycomb‐repressed genes at all stages of neuronal differentiation. We observe both loss and acquisition of RNAPII and Polycomb at specific groups of genes reflecting their silencing or activation. Strikingly, RNAPII remains poised at transcription factor genes which are silenced in neurons through Polycomb repression, and have major roles in specifying other, non‐neuronal lineages. We conclude that RNAPII poising is intrinsically associated with Polycomb repression throughout differentiation. Our work suggests that the tight interplay between RNAPII poising and Polycomb repression not only instructs promoter state transitions, but also may enable promoter plasticity in differentiated cells.

CBX6 overexpression contributes to tumor progression and is predictive of a poor prognosis in hepatocellular carcinoma

Aberrant chromobox (CBX) family protein expression has been reported in a variety of human malignancies. However, the role of CBX6 in hepatocellular carcinoma (HCC) progression and patient prognosis remains unknown. In this study, we found that CBX6 was frequently up-regulated in HCC clinical samples and HCC cell lines and that CBX6 expression was significantly correlated with larger tumor sizes (≥ 5 cm, p = 0.011) and multiple tumors (n ≥ 2, p = 0.018). Survival analyses indicated that patients with higher CBX6 expression levels had significantly shorter recurrence-free survival (RFS) and overall survival (OS) than patients with lower CBX6 expression levels, and multivariate analyses confirmed that increased CBX6 expression was an independent unfavorable prognostic factor for HCC patients. Functional study demonstrated that CBX6 profoundly promoted HCC cell growth both in vitro and in vivo, and mechanistic investigation revealed that the S100A9/NF-κB/MAPK pathway was essential for mediating CBX6 function. In conclusion, our results represent the first evidence that CBX6 contributes to tumor progression and indicate that the protein may serve as a novel prognostic biomarker for HCC and as a therapeutic target in the treatment of the disease.

Deregulated expression of microRNA-200b/c and SUZ12, a Polycomb repressive complex 2 subunit, in chemoresistant colorectal cancer cells

In colorectal cancer, chemotherapy and/or radiotherapy can lead to the formation of resistant cells that become metastatic through Epithelial-Mesenchymal Transition (EMT). Invasive and metastatic characteristics of carcinoma cells in primary tumors are mediated by EMT. During EMT, the primary tumor cells lose cell-cell adhesion, have increased intercellular separation, and gain an elongated shape with pseudopodia. There is also dysregulation of Polycomb group proteins (such as BMI1, SUZ12, and EZH2), and changes in the expression of microRNA-200 (miR-200) family. In this study, we developed a chemoresistant colorectal cancer cell line (DLD-1-OxR) by exposing DLD-1 colorectal cancer cells to increasing concentrations of oxaliplatin (a chemotherapy drug used for colorectal cancer), and tested for EMT characteristics. We found that DLD-1-OxR exhibited EMT characteristics by morphologic, biochemical and molecular markers. SUZ12, a Polycomb repressive complex 2 subunit, was upregulated in DLD-1-OxR. The miRNA-200 family members that target SUZ12 were downregulated. Drug resistance is an impediment to chemotherapy and understanding the molecular mechanisms of chemoresistance can lead to its reversal and improvement of chemotherapy outcomes.

Elevated expression of a pharmacologic Polycomb signature predicts poor prognosis in gastric and breast cancer

AIM

Polycomb Group complexes are epigenetic repressors that silence tumor suppressive genes. Studies demonstrated that pharmacologic inhibition of Polycomb Group complexes with 3-deazaneplanocin A (DZNeP) induces cancer cell death by re-expressing silenced genes. Here we evaluate the prognostic significance of DZNeP target genes in gastric and breast cancer. Patients & methods/materials: The prognostic impact of a DZNeP-regulated gene signature was investigated using the KM Plotter and cBio Portal resources containing microarray data from tumor tissue.

RESULTS

We report that elevated expression of DZNeP targets is associated with poor clinical outcome in gastric and breast cancer. In gastric cancer, elevated expression of DZNeP signature is inversely correlated with decreased overall survival. In breast cancer, DZNeP signature predicted poor prognosis in HER2+ tumors but not in HER2^- neoplasms.

CONCLUSION

These findings demonstrate that DZNeP target genes are not predictive of better but rather of poor clinical outcome in gastric and breast cancer.

Breast cancer stem cells as last soldiers eluding therapeutic burn: A hard nut to crack

Cancer stem cells (CSCs) are found in many cancer types, including breast carcinoma. Breast cancer stem cells (BCSCs) are considered as seed of cancer formation and they are associated with metastasis and genotoxic drug resistance. Several studies highlighted the presence of BCSCs in tumor microenvironment and they are accentuated with several carcinoma events including metastasis and resistance to genotoxic drugs and they also rebound after genotoxic burn. Stemness properties of a small population of cells in carcinoma have provided clues regarding the role of tumor microenvironment in tumor pathophysiology. Hence, insights in cancer stem cell biology with respect to molecular signaling, genetics and epigenetic behavior of CSCs have been used to modulate tumor drug resistance due to genotoxic drugs and signaling protein inhibitors. This review summarizes major scientific breakthroughs in understanding the contribution of BCSCs towards tumor's capability to endure destruction inflicted by molecular as well as genotoxic drugs.

Targeting MUC1-C suppresses polycomb repressive complex 1 in multiple myeloma

The polycomb repressive complex 1 (PRC1) includes the BMI1, RING1 and RING2 proteins. BMI1 is required for survival of multiple myeloma (MM) cells. The MUC1-C oncoprotein is aberrantly expressed by MM cells, activates MYC and is also necessary for MM cell survival. The present studies show that targeting MUC1-C with (i) stable and inducible silencing and CRISPR/Cas9 editing and (ii) the pharmacologic inhibitor GO-203, which blocks MUC1-C function, downregulates BMI1, RING1 and RING2 expression. The results demonstrate that MUC1-C drives BMI1 transcription by a MYC-dependent mechanism. MUC1-C thus promotes MYC occupancy on the BMI1 promoter and thereby activates BMI1 expression. We also show that the MUC1-C→MYC pathway induces RING2 expression. Moreover, in contrast to BMI1 and RING2, we found that MUC1-C drives RING1 by an NF-κB p65-dependent mechanism. Targeting MUC1-C and thereby the suppression of these key PRC1 proteins was associated with downregulation of the PRC1 E3 ligase activity as evidenced by decreases in ubiquitylation of histone H2A. Targeting MUC1-C also resulted in activation of the PRC1-repressed tumor suppressor genes, PTEN, CDNK2A and BIM. These findings identify a heretofore unrecognized role for MUC1-C in the epigenetic regulation of MM cells.

Intrinsically disordered chromatin protein NUPR1 binds to the C-terminal region of Polycomb RING1B

Intrinsically disordered proteins (IDPs) are ubiquitous in eukaryotes, and they are often associated with diseases in humans. The protein NUPR1 is a multifunctional IDP involved in chromatin remodeling and in the development and progression of pancreatic cancer; however, the details of such functions are unknown. Polycomb proteins are involved in specific transcriptional cascades and gene silencing. One of the proteins of the Polycomb complex is the Ring finger protein 1 (RING1). RING1 is related to aggressive tumor features in multiple cancer types. In this work we characterized the interaction between NUPR1 and the paralogue RING1B in vitro, in silico, and in cellulo. The interaction occurred through the C-terminal region of RING1B (C-RING1B), with an affinity in the low micromolar range (∼10 μM). The binding region of NUPR1, mapped by NMR, was a hydrophobic polypeptide patch at the 30s region of its sequence, as pinpointed by computational results and site-directed mutagenesis at Ala33. The association between C-RING1B and wild-type NUPR1 also occurred in cellulo as tested by protein ligation assays; this interaction is inhibited by trifluoperazine, a drug known to hamper binding of wild-type NUPR1 with other proteins. Furthermore, the Thr68Gln and Ala33Gln/Thr68Gln mutants had a reduction in the binding toward C-RING1B as shown by in vitro, in silico, and in cellulo studies. This is an example of a well-folded partner of NUPR1, because its other interacting proteins are also unfolded. We hypothesize that NUPR1 plays an active role in chromatin remodeling and carcinogenesis, together with Polycomb proteins.

Structure of the PRC2 complex and application to drug discovery

The polycomb repressive complexes 2 (PRC2) complex catalyzes tri-methylation of histone H3 lysine 27 (H3K27), a repressive chromatin marker associated with gene silencing. Overexpression and mutations of PRC2 are found in a wide variety of cancers, making the catalytic activity of PRC2 an important target of cancer therapy. This review highlights recent structural breakthroughs of the human PRC2 complex bound to the H3K27 peptide and a small molecule inhibitor, which provide critically needed insight into PRC2-targeted drug discovery.

PD-L1 promotes OCT4 and Nanog expression in breast cancer stem cells by sustaining PI3K/AKT pathway activation

The expression of PD‐L1 in breast cancer is associated with estrogen receptor negativity, chemoresistance and epithelial‐to‐mesenchymal transition (EMT), all of which are common features of a highly tumorigenic subpopulation of cancer cells termed cancer stem cells (CSCs). Hitherto, the expression and intrinsic role of PD‐L1 in the dynamics of breast CSCs has not been investigated. To address this issue, we used transcriptomic datasets, proteomics and several in vitro and in vivo assays. Expression profiling of a large breast cancer dataset (530 patients) showed statistically significant correlation (p < 0.0001, r = 0.36) between PD‐L1 expression and stemness score of breast cancer. Specific knockdown of PD‐L1 using ShRNA revealed its critical role in the expression of the embryonic stem cell transcriptional factors: OCT‐4A, Nanog and the stemness factor, BMI1. Conversely, these factors could be induced upon PD‐L1 ectopic expression in cells that are normally PD‐L1 negative. Global proteomic analysis hinted for the central role of AKT in the biology of PD‐L1 expressing cells. Indeed, PD‐L1 positive effect on OCT‐4A and Nanog was dependent on AKT activation. Most importantly, downregulation of PD‐L1 compromised the self‐renewal capability of breast CSCs in vitro and in vivo as shown by tumorsphere formation assay and extreme limiting dilution assay, respectively. This study demonstrates a novel role for PD‐L1 in sustaining stemness of breast cancer cells and identifies the subpopulation and its associated molecular pathways that would be targeted upon anti‐PD‐L1 therapy.

What's new?

Cancer cells that express the T‐cell inhibitory molecule programmed death‐ligand 1 (PD‐L1) readily escape immune attack. In addition, PD‐L1 expression contributes to chemoresistance and is associated with epithelial‐to‐mesenchymal transition, a process that generates cancer stem cells (CSCs). This study shows that in breast cancer, PD‐L1 expression further plays a direct part in maintaining CSC stemness. In breast cancer cells, PD‐L1 expression sustained stemness factors OCT‐4A and Nanog, via a PI3K/AKT‐dependent pathway, and promoted expression of the stemness controlling factor BMI1, independent of PI3K/AKT. Targeting PD‐L1 could help advance breast cancer therapy, owing to impacts on the pool of breast CSCs.

Cellular reprogramming technology for dissecting cancer epigenome in vivo

Decades of studies have shown that epigenetic alterations play a significant role on cancer development both in vitro and in vivo. However, considering that many cancers harbor mutations at epigenetic modifier genes and that transcription factor-mediated gene regulations are tightly coupled with epigenetic modifications, the majority of epigenetic alterations in cancers could be the consequence of the dysfunction or dysregulation of epigenetic modifiers caused by genetic abnormalities. Therefore, it remains unclear whether bona fide epigenetic abnormalities have causal roles on cancer development. Reprogramming technologies enable us to actively alter epigenetic regulations while preserving genomic information. Taking advantage, recent studies have provided in vivo evidence for the significant impact of epigenetic abnormalities on the initiation, maintenance and progression of cancer cells.

Essential Role for Polycomb Group Protein Pcgf6 in Embryonic Stem Cell Maintenance and a Noncanonical Polycomb Repressive Complex 1 (PRC1) Integrity

The Polycomb group (PcG) proteins have an important role in controlling the expression of key genes implicated in embryonic development, differentiation, and decision of cell fates. Emerging evidence suggests that Polycomb repressive complexes 1 (PRC1) is defined by the six Polycomb group RING finger protein (Pcgf) paralogs, and Pcgf proteins can assemble into noncanonical PRC1 complexes. However, little is known about the precise mechanisms of differently composed noncanonical PRC1 in the maintenance of the pluripotent cell state. Here we disrupt the Pcgf genes in mouse embryonic stem cells by CRISPR-Cas9 and find Pcgf6 null embryonic stem cells display severe defects in self-renewal and differentiation. Furthermore, Pcgf6 regulates genes mostly involved in differentiation and spermatogenesis by assembling a noncanonical PRC1 complex PRC1.6. Notably, Pcgf6 deletion causes a dramatic decrease in PRC1.6 binding to target genes and no loss of H2AK119ub1. Thus, Pcgf6 is essential for recruitment of PRC1.6 to chromatin. Our results reveal a previously uncharacterized, H2AK119ub1-independent chromatin assembly associated with PRC1.6 complex.

Compositional and functional diversity of canonical PRC1 complexes in mammals

The compositional complexity of Polycomb Repressive Complex 1 (PRC1) increased dramatically during vertebrate evolution. What is considered the "canonical" PRC1 complex consists of four subunits originally identified as regulators of body segmentation in Drosophila. In mammals, each of these four canonical subunits consists of two to six paralogs that associate in a combinatorial manner to produce over a hundred possible distinct PRC1 complexes with unknown function. Genetic studies have begun to define the phenotypic roles for different PRC1 paralogs; however, relating these phenotypes to unique biochemical and transcriptional function for the different paralogs has been challenging. In this review, we attempt to address how the compositional diversity of canonical PRC1 complexes relates to unique roles for individual PRC1 paralogs in transcriptional regulation. This review focuses primarily on PRC1 complex composition, genome targeting, and biochemical function.

New Cross-Talk Layer between Ultraconserved Non-Coding RNAs, MicroRNAs and Polycomb Protein YY1 in Bladder Cancer

MicroRNAs (miRNAs) are highly conserved elements in mammals, and exert key regulatory functions. Growing evidence shows that miRNAs can interact with another class of non-coding RNAs, so-called transcribed ultraconserved regions (T-UCRs), which take part in transcriptional, post-transcriptional and epigenetic regulation processes. We report here the interaction of miRNAs and T-UCRs as a network modulating the availability of these non-coding RNAs in bladder cancer cells. In our cell system, antagomiR-596 increased the expression of T-UCR 201+. Moreover, T-UCR 8+ silencing increased miR-596 expression, which in turn reduced total T-UCR 283+, showing that the perturbation of one element in this network changes the expression of other interactors. In addition, we identify the polycomb protein Yin Yang 1 (YY1) as mediator of binding between miR-596 and T-UCR 8+. These new findings describe for the first time a network between T-UCRs, miRNAs and YY1 protein, highlighting the existence of an additional layer of gene expression regulation.

EZH2 deletion in early mesenchyme compromises postnatal bone microarchitecture and structural integrity and accelerates remodeling

In this study, we examined the functional importance of EZH2 during skeletal development and homeostasis using the conditional deletion of Ezh2 (Ezh2^fl/fl ) in early mesenchyme with the use of a Prrx-1-cre driver mouse (Ezh2^+/+). Heterozygous (Ezh2^+/-) newborn and 4-wk-old mice exhibited increased skeletal size, growth plate size, and weight when compared to the wild-type control (Ezh2^+/+), whereas homozygous deletion of Ezh2 (Ezh2^-/-) resulted in skeletal deformities and reduced skeletal size, growth plate size, and weight in newborn and 4-wk-old mice. Ezh2^-/- mice exhibited enhanced trabecular patterning. Osteogenic cortical and trabecular bone formation was enhanced in Ezh2^+/- and Ezh2^-/- animals. Ezh2^+/- and Ezh2^-/- mice displayed thinner cortical bone and decreased mechanical strength compared to the wild-type control. Differences in cortical bone thickness were attributed to an increased number of osteoclasts, corresponding with elevated levels of the bone turnover markers cross-linked C-telopeptide-1 and tartrate-resistant acid phosphatase, detected within serum. Moreover, Ezh2^+/- mice displayed increased osteoclastogenic potential coinciding with an upregulation of Rankl and M-csf expression by mesenchymal stem cells (MSCs). MSCs isolated from Ezh2^+/- mice also exhibited increased trilineage potential compared with wild-type bone marrow stromal/stem cells (BMSCs). Gene expression studies confirmed the upregulation of known Ezh2 target genes in Ezh2^-/- bone tissue, many of which are involved in Wnt/BMP signaling as promoters of osteogenesis and inhibitors of adipogenesis. In summary, EZH2 appears to be an important orchestrator of skeletal development, postnatal bone remodelling and BMSC fate determination in vitro and in vivo-Hemming, S., Cakouros, D., Codrington, J., Vandyke, K., Arthur, A., Zannettino, A., Gronthos, S. EZH2 deletion in early mesenchyme compromises postnatal bone microarchitecture and structural integrity and accelerates remodeling.

MUC1-C activates BMI1 in human cancer cells

BMI1 is a component of the PRC1 complex that is overexpressed in breast and other cancers, and promotes self-renewal of cancer stem-like cells. The oncogenic mucin 1 (MUC1) C-terminal (MUC1-C) subunit is similarly overexpressed in human carcinoma cells and has been linked to their self-renewal. There is no known relationship between MUC1-C and BMI1 in cancer. The present studies demonstrate that MUC1-C drives BMI1 transcription by a MYC-dependent mechanism in breast and other cancer cells. In addition, we show that MUC1-C blocks miR-200c-mediated downregulation of BMI1 expression. The functional significance of this MUC1-C→BMI1 pathway is supported by the demonstration that targeting MUC1-C suppresses BMI1-induced ubiquitylation of H2A and thereby derepresses homeobox HOXC5 and HOXC13 gene expression. Notably, our results further show that MUC1-C binds directly to BMI1 and promotes occupancy of BMI1 on the CDKN2A promoter. In concert with BMI1-induced repression of the p16^INK4a tumor suppressor, we found that targeting MUC1-C is associated with induction of p16^INK4a expression. In support of these results, analysis of three gene expresssion datasets demonstrated highly significant correlations between MUC1-C and BMI1 in breast cancers. These findings uncover a previously unrecognized role for MUC1-C in driving BMI1 expression and in directly interacting with this stem cell factor, linking MUC1-C with function of the PRC1 in epigenetic gene silencing.

Correlation between CBX8 protein and tumors

Chromobox protein homolog 8 (CBX8), the core component of the polycomb group (PcG) protein family PRC1 complex, plays an important role in cell proliferation, senescence, maintenance of stem cell self-renewal and/or relapse, and the occurrence of tumors. Recently, CBX8 was found to be overexpressed in a variety of malignant tumors and closely related to the progression and prognosis of tumors. This paper reviews the current progress in research of CBX8 in tumors.

Epigenetic and genetic changes in soft tissue sarcomas: a review

Soft tissue sarcomas are a versatile group of tumors with a proposed origin from mesenchymal stem cells. During recent years, the molecular biologic mechanisms behind the histogenesis of these tumors have become clearer. In addition to translocations and other genomic changes, epigenetic mechanisms have been shown to be greatly involved in the histogenesis of sarcomas as well as other cancers. Even though the molecular mechanisms behind sarcomas appear to be more complex than previously expected, epigenetic mechanisms bring new opportunities and means for the treatment of these complex diseases.

A Polycomb-mir200 loop regulates clinical outcome in bladder cancer

Bladder cancer (BC) is a highly prevalent disease, ranking fifth in the most common cancers worldwide. Various miRNAs have recently emerged as potential prognostic biomarkers in cancer. The miR-200 family, which repressed the epithelial-to-mesenchymal transition (EMT), is repressed in multiple advanced cancers. However, its expression and function in BC is still poorly understood. Here we show that miR-200 family displays increased expression, probably due to the activation of specific oncogenic signaling pathways, and reduced promoter methylation, in BC compared to normal bladder samples. Furthermore, we show that the expression of these miRNAs is decreased in high grade and stage tumors, and the down-regulation is associated with patient's poor clinical outcome. Our data indicate that the miR-200 family plays distinct roles in Non-Muscle (NMIBC) and Muscle-Invasive BC (MIBC). In MIBC, miR-200 expression post transcriptionally regulates EMT-promoting transcription factors ZEB1 and ZEB2, whereas suppresses BMI1 expression in NMIBC. Interestingly, we show that increased EZH2 and/or BMI1 expression repress the expression of miR-200 family members. Collectively, these findings support a model of BC progression through a coordinated action between the Polycomb Repression Complex (PRC) members repressing the miR-200 expression, which ultimately favors invasive BC development. Since pharmacological inhibition of EZH2 in BC cell lines lead to increased miR-200 expression, our findings may support new therapeutic strategies for BC clinical management.

ChIP-seq Data Processing for PcG Proteins and Associated Histone Modifications

Chromatin Immunoprecipitation followed by massively parallel DNA sequencing (ChIP-sequencing) has emerged as an essential technique to study the genome-wide location of DNA- or chromatin-associated proteins, such as the Polycomb group (PcG) proteins. After being generated by the sequencer, raw ChIP-seq sequence reads need to be processed by a data analysis pipeline. Here we describe the computational steps required to process PcG ChIP-seq data, including alignment, peak calling, and downstream analysis.

Honokiol inhibits bladder tumor growth by suppressing EZH2/miR-143 axis

The oncoprotein EZH2, as a histone H3K27 methyltransferase, is frequently overexpressed in various cancer types. However, the mechanisms underlying its role in urinary bladder cancer (UBC) cells have not yet fully understood. Herein, we reported that honokiol, a biologically active biphenolic compound isolated from the Magnolia officinalis inhibited human UBC cell proliferation, survival, cancer stemness, migration, and invasion, through downregulation of EZH2 expression level, along with the reductions of MMP9, CD44, Sox2 and the induction of tumor suppressor miR-143. Either EZH2 overexpression or miR-143 inhibition could partially reverse honokiol-induced cell growth arrest and impaired clonogenicity. Importantly, it was first revealed that EZH2 could directly bind to the transcriptional regulatory region of miR-143 and repress its expression. Furthermore, honokiol treatment on T24 tumor xenografts confirmed its anticancer effects in vivo, including suppression tumor growth and tumor stemness, accompanied by the dysregulation of EZH2 and miR-143 expressions. Our data suggest a promising therapeutic option to develop drugs targeting EZH2/miR-143 axis, such as honokiol, for bladder cancer treatment.

Melatonin inhibits tumorigenicity of glioblastoma stem-like cells via the AKT-EZH2-STAT3 signaling axis

Glioblastoma stem-like cells (GSCs) displaying self-renewing and tumor-propagating capacity play a particularly important role in maintaining tumor growth, therapeutic resistance, and tumor recurrence. Therefore, new therapeutic strategies focusing on impairing GSC maintenance are urgently needed. Here, we used GSCs isolated from surgical specimens from patients with glioblastoma multiforme (GBM) to study the roles and underlying mechanisms associated with melatonin in GSC biology. The results showed that melatonin directly targeted glioma tumor cells by altering GSC biology and inhibiting GSC proliferation. Additionally, melatonin altered profile of transcription factors to inhibit tumor initiation and propagation. Furthermore, EZH2 S21 phosphorylation and EZH2-STAT3 interaction in GSCs were impaired following melatonin treatment. These results suggested that melatonin attenuated multiple key signals involved in GSC self-renewal and survival, and further supported melatonin as a promising GBM therapeutic.

Structural aspects of small-molecule inhibition of methyllysine reader proteins

Methyl reader proteins recognize and bind to post-translationally methylated residues. They execute the commands issued by protein methyltransferases and play functional roles in diverse cellular processes including gene regulation, development and oncogenesis. Efforts to inhibit these proteins are relatively new. Only a small number of methyl reader proteins belonging to the chromodomain, malignant brain tumor domain, plant homeodomain finger and Tudor domain families have been targeted by chemical inhibitors. This review summarizes inhibitors that have been reported to date, and provides a perspective for future progress. Structural determinants for methyl reader inhibition will be presented, along with an analysis of the molecular interactions that control potency and selectivity for inhibitors of each family.

Cbx7 is epigenetically silenced in glioblastoma and inhibits cell migration by targeting YAP/TAZ-dependent transcription

Glioblastomas (GBM) are the most malignant form of astrocytomas which are difficult to treat and portend a grave clinical course and poor prognosis. In this study, we identified Chromobox homolog 7 (Cbx7), a member of Polycomb Repressive Complex 1 (PRC1), as a downregulated gene in GBM owing to its promoter hypermethylation. Bisulphite sequencing and methylation inhibitor treatment established the hypermethylation of Cbx7 in GBM. Exogenous overexpression of Cbx7 induced cell death, inhibited cell proliferation, colony formation and migration/invasion of the glioma cells. GSEA of Cbx7 regulated genes identified Cbx7 as a repressor of transcription co-activators YAP/TAZ, the inhibitory targets of the Hippo signalling pathway. In good correlation, the exogenous expression of Cbx7 repressed the YAP/TAZ-dependent transcription and downregulated CTGF, a bonafide YAP/TAZ target. We also observed reduced levels of phospho-JNK in Cbx7 expressing cells. Additionally, CTGF silencing and pharmacological inhibition of JNK also inhibited glioma cell migration. Further, Cbx7 failed to inhibit cell migration significantly in the presence of exogenously overexpressed CTGF or constitutively active JNK. Thus, our study identifies Cbx7 as an inhibitor of glioma cell migration through its inhibitory effect on YAP/TAZ-CTGF-JNK signalling axis and underscores the importance of epigenetic inactivation of Cbx7 in gliomagenesis.

Targeting Enhancer of Zeste Homolog 2 as a promising strategy for cancer treatment

Polycomb group proteins represent a global silencing system involved in development regulation. In specific, they regulate the transition from proliferation to differentiation, contributing to stem-cell maintenance and inhibiting an inappropriate activation of differentiation programs. Enhancer of Zeste Homolog 2 (EZH2) is the catalytic subunit of Polycomb repressive complex 2, which induces transcriptional inhibition through the tri-methylation of histone H3, an epigenetic change associated with gene silencing. EZH2 expression is high in precursor cells while its level decreases in differentiated cells. EZH2 is upregulated in various cancers with high levels associated with metastatic cancer and poor prognosis. Indeed, aberrant expression of EZH2 causes the inhibition of several tumor suppressors and differentiation genes, resulting in an uncontrolled proliferation and tumor formation. This editorial explores the role of Polycomb repressive complex 2 in cancer, focusing in particular on EZH2. The canonical function of EZH2 in gene silencing, the non-canonical activities as the methylation of other proteins and the role in gene transcriptional activation, were summarized. Moreover, mutations of EZH2, responsible for an increased methyltransferase activity in cancer, were recapitulated. Finally, various drugs able to inhibit EZH2 with different mechanism were described, specifically underscoring the effects in several cancers, in order to clarify the role of EZH2 and understand if EZH2 blockade could be a new strategy for developing specific therapies or a way to increase sensitivity of cancer cells to standard therapies.

Tipping the lysine methylation balance in disease

Genomic instability is a major contributing factor to the development and onset of diseases such as cancer. Emerging evidence has demonstrated that maintaining the proper balance of histone lysine methylation is critical to preserve genomic integrity. Genome-wide association studies, gene sequencing, and genome-wide mapping approaches have helped identify mutations, copy number changes, and aberrant gene regulation of lysine methyltransferases (KMTs) and demethylases (KDMs) associated with cancer and cognitive disorders. Structural analysis of KMTs and KDMs has demonstrated the drugability of these enzymes and has led to the discovery of small molecule inhibitors. Use of these inhibitors has allowed better understanding of the biochemical properties of KMTs and KDMs and demonstrated potential for therapeutic use. This review will highlight the methyl modifications, KMTs and KDMs associated with cancer and neurological disorders and how KMT and KDM and the potential for treatment of these conditions with small molecule inhibitors.

Extracellular Vesicles and MicroRNAs: Their Role in Tumorigenicity and Therapy for Brain Tumors

MicroRNAs are small non-coding RNAs which mediate post-transcriptional gene regulation. Recently, microRNAs have also been found to be localized to the extracellular space, often encapsulated in secreted extracellular vesicles (EVs). This tandem of EVs and tissue-specific expressed/secreted microRNAs that can be taken up by neighboring or distant recipient cells, leading to changes in gene expression-suggests a cell-specialized role in physiological and pathological conditions. The complexity of solid tumors and their distinct pathophysiology relies on interactive communications between the various cell types in the neoplasm (tumor, endothelial, or macrophages, for instance). Understanding how such EV/microRNA-mediated communication occurs may actually lead to avenues for therapeutic exploitation and/or intervention, particularly for the most formidable cancers, such as those in the brain. In this review, the role of microRNAs/EVs in brain tumors will be discussed with emphasis on how these molecules could be utilized for tumor therapy.

The many facets of MLL1 regulation

In the last 20 years, we have witnessed an exponential number of evidences linking the human mixed lineage leukemia-1 (MLL1) gene to several acute and myelogenous leukemias. MLL1 is one of the founding members of the SET1 family of lysine methyltransferases and is key for the proper control of developmentally regulated gene expression. MLL1 is a structurally complex protein composed of several functional domains. These domains play pivotal roles for the recruitment of regulatory proteins. These MLL1 regulatory proteins (MRPs) dynamically interact with MLL1 and consequently control gene expression. In this review, we summarize recent structural and functional studies of MRPs and discuss emergent structural paradigms for the control of MLL1 activity.

Genome-wide transcriptional profiling analysis reveals annexin A6 as a novel EZH2 target gene involving gastric cellular proliferation

A histone methyltransferase enhancer of zeste homologue 2 (EZH2) catalyzes trimethylation at histone H3 lysine27 (H3K27me3) and is frequently dysregulated in a wide range of human cancers. EZH2-mediated gene silencing contributes to carcinogenesis and regulates stem cell maintenance and differentiation; however, the underlining mechanisms remain to be completely understood. Here, we found that downregulation of EZH2 by RNA interference (RNAi) in gastric cancer cells suppresses cell growth, migration, invasion, and induces cell cycle arrest. Transcriptome analysis identified 1223 EZH2 responsive genes upon EZH2 knockdown. These genes are involved in the biological processes of cell cycle, proliferation and metastasis. Particularly, we found that annexin A6 (ANXA6) is a new target of EZH2 and is repressed in gastric cancer cells. Restoration of ANXA6 expression inhibits gastric cellular proliferation. We further demonstrated that EZH2-mediated H3K27me3, rather than promoter DNA methylation, is primarily responsible for ANXA6 inhibition. Taken together, our results provide a framework for understanding EZH2 biology and reveal ANXA6 as a new EZH2 target involving gastric cellular proliferation.

Chromobox Homolog 4 is Positively Correlated to Tumor Growth, Survival and Activation of HIF-1α Signaling in Human Osteosarcoma under Normoxic Condition

Objectives: The clinical significance and tumorigenesis of Chromobox homolog 4 (CBX4) have been reported in hepatocellular carcinoma. The purpose of this study is to confirm the expression, elucidate the biological function and investigate the potential mechanism of CBX4 in osteosarcoma (OS). Methods: The expression of CBX4 in OS samples and cell lines was measured by RT-PCR and western blot test. Cell cycle, CCK8 and colony-forming assays were used to detect changes of cells growth. Cell apoptosis assay was used to measure cell survival capacity. Trans-well assay was used to test the activities of migration and invasion. The expression of genes regulated by CBX4 was detected by qRT-PCT test. Results: The expression of CBX4 was up-regulated in multiple OS cell lines and clinical samples. Overexpression of CBX4 was correlated with advanced clinical stage, high degree of malignancy and low tumor necrosis rate. Moreover, knockdown of CBX4 resulted in significant inhibition of cell growth and cell survival in OS cells under normoxic condition. In addition, we found that knockdown of CBX4 lead to down-regulating of HIF-1α-targeted genes without changing HIF-1α expression itself. Conclusion: Taken together, CBX4 is up-regulated and has a pro-tumor effect in OS with an activation of HIF-1α signaling pathway under normoxic condition. Therefore, targeting CBX4 may provide a new therapeutic method for OS.

Chemical Inhibitors of Epigenetic Methyllysine Reader Proteins

Protein methylation is a common post-translational modification with diverse biological functions. Methyllysine reader proteins are increasingly a focus of epigenetics research and play important roles in regulating many cellular processes. These reader proteins are vital players in development, cell cycle regulation, stress responses, oncogenesis, and other disease pathways. The recent emergence of a small number of chemical inhibitors for methyllysine reader proteins supports the viability of these proteins as targets for drug development. This article introduces the biochemistry and biology of methyllysine reader proteins, provides an overview of functions for those families of readers that have been targeted to date (MBT, PHD, tudor, and chromodomains), and reviews the development of synthetic agents that directly block their methyllysine reading functions.

Heterochromatin components in germline stem cell maintenance

Stem cell maintenance requires expression of genes essential for stemness and repression of differentiation genes. How this is achieved remains incompletely understood. Here we investigate the requirement for central components of heterochromatin, Heterochromatin Protein 1 (HP1) and the histone H3 lys9 methyltransferase Su(var)3-9, in the Drosophila male germline stem cell (GSC) self-renewal, a paradigm for studying adult stem cell behavior. We found that mutations or RNAi knock down of HP1 or Su(var)3-9 cause loss of GSCs, accompanied by defects in cell division or survival and premature expression of the differentiation gene bag of marbles (bam). Conversely, over-expressing HP1 increases GSC number in wildtype flies and, strikingly, restores fertility to the sterile hopscotch (hop) mutant flies that lack niche signals. These results suggest that the central components of heterochromatin play roles including repressing differentiation genes in Drosophila male GSC maintenance.

ZRF1: a novel epigenetic regulator of stem cell identity and cancer

The Zuotin-related factor 1, ZRF1, has recently been identified as an epigenetic regulator of gene transcription in stem cells and cancer. During differentiation of human teratocarcinoma cells, ZRF1 promotes transcriptional induction of developmental genes that are repressed by Polycomb complexes. Importantly, ZRF1 has recently been shown to be required for both neural differentiation of embryonic stem cells (ESCs) and for maintenance of neural progenitor cell (NPC) identity. Moreover, a dual role has now emerged for ZRF1 in cancer: on the one hand, ZRF1 plays a crucial role in oncogene-induced senescence (OIS) by activating the INK4/ARF locus, thus working as a tumor suppressor; on the other hand, ZRF1 promotes leukemogenesis in acute myeloid leukemia (AML) in a Polycomb-independent fashion. Therefore, increasing evidence points to ZRF1 as a novel target for therapy of neurodegenerative diseases and cancer.