Abstract BCOR, encoding BCL-6 corepressor (BCOR), is X-linked and targeted by somatic mutations in various hematological malignancies including myelodysplastic syndrome (MDS). We previously reported that mice lacking Bcor exon 4 (BcorΔE4/y) in the hematopoietic compartment developed NOTCH-dependent acute T-cell lymphoblastic leukemia (T-ALL). Here, we analyzed mice lacking Bcor exons 9 and 10 (BcorΔE9-10/y), which express a carboxyl-terminal truncated BCOR that fails to interact with core effector components of polycomb repressive complex 1.1. BcorΔE9-10/y mice developed lethal T-ALL in a similar manner to BcorΔE4/y mice, whereas BcorΔE9-10/y hematopoietic cells showed a growth advantage in the myeloid compartment that was further enhanced by the concurrent deletion of Tet2. Tet2Δ/ΔBcorΔE9-10/y mice developed lethal MDS with progressive anemia and leukocytopenia, inefficient hematopoiesis, and the morphological dysplasia of blood cells. Tet2Δ/ΔBcorΔE9-10/y MDS cells reproduced MDS or evolved into lethal MDS/myeloproliferative neoplasms in secondary recipients. Transcriptional profiling revealed the derepression of myeloid regulator genes of the Cebp family and Hoxa cluster genes in BcorΔE9-10/y progenitor cells and the activation of p53 target genes specifically in MDS erythroblasts where massive apoptosis occurred. Our results reveal a tumor suppressor function of BCOR in myeloid malignancies and highlight the impact of Bcor insufficiency on the initiation and progression of MDS.

Purpose EZH2 and EZH1, the catalytic components of polycomb repressive complex 2 (PRC2), trigger trimethylation of H3K27 (H3K27me3) to repress the transcription of target genes and are implicated in the pathogenesis of various cancers including multiple myeloma and prostate cancer. Here, we investigated the preclinical effects of UNC1999, a dual inhibitor of EZH2 and EZH1, in combination with proteasome inhibitors on multiple myeloma and prostate cancer. Experimental Design In vitro and in vivo efficacy of UNC1999 and the combination with proteasome inhibitors was evaluated in multiple myeloma cell lines, primary patient cells, and in a xenograft model. RNA-seq and ChIP-seq were performed to uncover the targets of UNC1999 in multiple myeloma. The efficacy of the combination therapy was validated in prostate cancer cell lines. Results Proteasome inhibitors repressed EZH2 transcription via abrogation of the RB-E2F pathway, thereby sensitizing EZH2-dependent multiple myeloma cells to EZH1 inhibition by UNC1999. Correspondingly, combination of proteasome inhibitors with UNC1999, but not with an EZH2-specific inhibitor, induced synergistic antimyeloma activity in vitro. Bortezomib combined with UNC1999 remarkably inhibited the growth of myeloma cells in vivo. Comprehensive analyses revealed several direct targets of UNC1999 including the tumor suppressor gene NR4A1. Derepression of NR4A1 by UNC1999 resulted in suppression of MYC, which was enhanced by the combination with bortezomib, suggesting the cooperative blockade of PRC2 function. Notably, this combination also exhibited strong synergy in prostate cancer cells. Conclusions Our results identify dual inhibition of EZH2 and EZH1 together with proteasome inhibition as a promising epigenetics-based therapy for PRC2-dependent cancers.

Key Points Setdb1, an H3K9 histone methyltransferase, is essential for the maintenance of HSPCs. Setdb1 restricts the activation of nonhematopoietic genes, such as gluconeogenic pathway genes, to maintain HSPCs.

Metformin has been widely used as an oral drug for diabetes mellitus for approximately 60 years. Interestingly, recent reports showed that metformin exhibited an anti-tumor action in a wide range of malignancies including hepatocellular carcinoma (HCC). In the present study, we investigated its impact on tumor-initiating HCC cells. Metformin suppressed cell growth and induced apoptosis in a dose-dependent manner. Flow cytometric analysis showed that metformin treatment markedly reduced the number of tumor-initiating epithelial cell adhesion molecule (EpCAM)+ HCC cells. Non-adherent sphere formation assays of EpCAM+ cells showed that metformin impaired not only their sphere-forming ability, but also their self-renewal capability. Consistent with this, immunostaining of spheres revealed that metformin significantly decreased the number of component cells positive for hepatic stem cell markers such as EpCAM and α-fetoprotein. In a xenograft transplantation model using non-obese diabetic/severe combined immunodeficient mice, metformin and/or sorafenib treatment suppressed the growth of tumors derived from transplanted HCC cells. Notably, the administration of metformin but not sorafenib decreased the number of EpCAM+ cells and impaired their self-renewal capability. As reported, metformin activated AMP-activated protein kinase (AMPK) through phosphorylation; however its inhibitory effect on the mammalian target of rapamycin (mTOR) pathway did not necessarily correlate with its anti-tumor activity toward EpCAM+ tumor-initiating HCC cells. These results indicate that metformin is a promising therapeutic agent for the elimination of tumor-initiating HCC cells and suggest as-yet-unknown functions other than its inhibitory effect on the AMPK/mTOR pathway.

Histone H3 lysine 9 trimethylation (H3K9me3) is associated with transcriptional repression and regulated by histone lysine methyltransferases such as SUV39H1 and ESET. However, the functional roles of these enzymes in hepatocellular carcinoma (HCC) remain uncertain. In this study, we conducted loss-of-function assays for HCC cells. SUV39H1 knockdown but not ESET knockdown reduced H3K9me3 levels and impaired HCC cell growth and sphere formation. The pharmacological inhibition of SUV39H1 by chaetocin resulted in cell growth inhibition and inducing cellular apoptosis in culture and xenograft subcutaneous tumors. Real-time polymerase chain reaction analysis indicated high levels of SUV39H1 expression in 24 of 42 (57.1%) HCC surgical samples compared with corresponding nontumor tissues. Immunohistochemistry identified high levels of H3K9me3 and ESET proteins in 23 (54.8%) and 29 (69.0%) tumor tissues, respectively. However, these proteins' expressions were only observed in biliary epithelial cells and periportal hepatocytes of nontumor tissues. Expression levels of SUV39H1 but not those of ESET were significantly correlated with H3K9me3 levels. The cumulative HCC recurrence rate was significantly higher for patients with elevated SUV39H1 expression and H3K9me3 levels. In conclusion, our data indicate that elevated SUV39H1 expression and high levels of H3K9me3 have important roles in HCC development and progression. Therefore, the pharmacological inhibition of SUV39H1 may be a promising therapeutic approach for HCC treatment.Epigenetic mechanisms, including DNA methylation, histone modification and noncoding RNAs, are closely associated with transcriptional regulation in eukaryotic cells.