ALBERT

Description: Nrf2 plays critical roles in animals’ defense against electrophiles and oxidative stress by orchestrating the induction of cytoprotective genes. We previously isolated the zebrafish mutant it768, which displays up-regulated expression of Nrf2 target genes in an uninduced state. In this paper, we determine that the gene responsible for it768 was the zebrafish homolog of phosphomannomutase 2 (Pmm2), which is a key enzyme in the initial steps of N-glycosylation, and its mutation in humans leads to PMM2-CDG (congenital disorders of glycosylation), the most frequent type of CDG. The pmm2it768 larvae exhibited mild defects in N-glycosylation, indicating that the pmm2it768 mutation is a hypomorph, as in human PMM2-CDG patients. A gene expression analysis showed that pmm2it768 larvae display up-regulation of endoplasmic reticulum (ER) stress, suggesting that the activation of Nrf2 was induced by the ER stress. Indeed, the treatment with the ER stress-inducing compounds up-regulated the gstp1 expression in an Nrf2-dependent manner. Furthermore, the up-regulation of gstp1 by the pmm2 inactivation was diminished by knocking down or out double-stranded RNA-activated protein kinase (PKR)-like ER kinase (PERK), one of the main ER stress sensors, suggesting that Nrf2 was activated in response to the ER stress via the PERK pathway. ER stress-induced activation of Nrf2 was reported previously, but the results have been controversial. Our present study clearly demonstrated that ER stress can indeed activate Nrf2 and this regulation is evolutionarily conserved among vertebrates. Moreover, ER stress induced in pmm2it768 mutants was ameliorated by the treatment of the Nrf2-activator sulforaphane, indicating that Nrf2 plays significant roles in the reduction of ER stress.

Description: PI3K/Akt pathway is constitutively activated in multiple myeloma (MM). A plethora of studies extensively investigated Akt inhibitors, alone or in combination; however, the outcomes in hematological malignancies were largely unsatisfactory, emphasizing the need for critical preclinical evaluations. Polycomb repressive complex 2 (PRC2) components, EZH2 and its related homolog EZH1, induce H3K27me3 to silence the transcription of target genes. Recent studies ensured that EZH2 inhibition alone is not sufficient to completely disrupt the oncogenic functions of PRC2. With the importance of PRC2 as a therapeutic target in MM, we aimed to investigate the mechanisms by which Akt inhibition may impact PRC2 function, and test whether targeting both EZH2 and EZH1 together with Akt inhibition is a promising treatment strategy for MM. We herein evaluated the cytotoxic effect of TAS-117, a potent and selective non-competitive Akt inhibitor, against different MM cell lines and found that responsive cell lines tended to have significant levels of activated Akt, coupled with low/deleted PTEN. Then, we examined signaling-epigenetic crosstalk on EZH2 level. TAS-117 significantly down-regulated EZH2 mRNA and protein in dose- and time-dependent manners, while H3K27me3 levels were rather maintained or elevated, suggesting compensation by EZH1. As EZH2 is a direct target for E2F1, we focused on Rb-E2F pathway as a regulatory mechanism for EZH2. TAS-117 induced marked down-regulation of E2F1 and E2F2. Moreover, TAS-117 induced the up-regulation of CDKN1B, in addition to the inactivation of cyclins and cyclin dependent kinases, hence, hypo-phosphorylated Rb, thereby stabilizing Rb-E2F1 complex and diminishing free E2F1 available for binding to its target genes, including EZH2 promoter. This prompted us to examine the impact of TAS-117 combination with either dual EZH2/1 inhibitor, UNC1999, or selective EZH2 inhibitor, GSK126. In agreement, UNC1999, but not GSK126, synergistically enhanced TAS-117-induced cytotoxicity, confirmed by combination index calculation, and provoked MM cell apoptosis. As we observed an increase in H3K27me3 levels after TAS-117 treatment, we hypothesized that EZH1 function was augmented. Consistently, we found that EZH1 was markedly up-regulated after TAS-117 treatment in dose- and time-dependent manners. Importantly, EZH1 knockdown significantly enhanced the sensitivity of myeloma cells to TAS-117-induced cytotoxicity. To clarify the molecular mechanisms underlying EZH1 up-regulation, we performed RNA-seq followed by KEGG pathway analysis for up-regulated genes in TAS-117-treated group. We focused on FOXO pathway enrichment as it is a crucial target in MM treatment using Akt inhibitors. We then focused on FOXO3 as it was the main FOXO family gene expressed in MM cells according to our RNA-seq data. We examined the nuclear localization of FOXO3 following TAS-117 treatment. We found that TAS-117 significantly enhanced the nuclear accumulation of FOXO3, as depicted by both the immunostaining images and the digital calculations of the nuclear subset of FOXO3. Murine Ezh1 promoter was shown to be bound by Foxo transcription factors (TFs) in neuronal progenitors, T-regulatory cells, CD8+ cells, and pre-B cells. More than 80% of FOXO3-binding sites share the common binding motif, GTAAACAA, which was found both in human EZH1 (+48 from the TSS) and mouse Ezh1 (+77 from the TSS) promoter regions. So, we hypothesized that FOXO3 may be a regulatory partner for human EZH1 gene in myeloma cells in response to TAS-117 treatment. To this end, we performed ChIP-qPCR analysis for TAS-117-treated and -untreated cells. TAS-117 promoted the binding of FOXO3 to EZH1 promoter, in addition to one of the canonical FOXO3 targets, BIM promoter. To further confirm our results, we expressed shRNA against FOXO3 (shFOXO3) in MM cells which, interestingly, induced the down-regulation of EZH1 mRNA. In conclusion, the present results defined novel signaling-epigenetic crosstalk between PI3K/Akt pathway and PRC2 components, EZH2 and EZH1, and demonstrated that Akt inhibition can differently modulates EZH2 and EZH1 levels via Akt downstream effectors, E2F1 and FOXO3, respectively. Therefore, targeting both EZH2 and EZH1 in addition to Akt inhibition may be a promising rationale to eradicate MM, leading to significant advances in treatment. Disclosures No relevant conflicts of interest to declare.

Description: Recurrent inactivating mutations have been identified in the X-linked plant homeodomain finger protein 6 (PHF6) gene, encoding a chromatin-binding transcriptional regulator protein, in various hematological malignancies. However, the role of PHF6 in normal hematopoiesis and its tumor-suppressor function remain largely unknown. We herein generated mice carrying a floxed Phf6 allele and inactivated Phf6 in hematopoietic cells at various developmental stages. The Phf6 deletion in embryos augmented the capacity of hematopoietic stem cells (HSCs) to proliferate in cultures and reconstitute hematopoiesis in recipient mice. The Phf6 deletion in neonates and adults revealed that cycling HSCs readily acquired an advantage in competitive repopulation upon the Phf6 deletion, whereas dormant HSCs only did so after serial transplantations. Phf6-deficient HSCs maintained an enhanced repopulating capacity during serial transplantations; however, they did not induce any hematological malignancies. Mechanistically, Phf6 directly and indirectly activated downstream effectors in tumor necrosis factor α (TNFα) signaling. The Phf6 deletion repressed the expression of a set of genes associated with TNFα signaling, thereby conferring resistance against the TNFα-mediated growth inhibition on HSCs. Collectively, these results not only define Phf6 as a novel negative regulator of HSC self-renewal, implicating inactivating PHF6 mutations in the pathogenesis of hematological malignancies, but also indicate that a Phf6 deficiency alone is not sufficient to induce hematopoietic transformation.

Description: Polycomb repressive complex (PRC) resides in two major complexes PRC1 and PRC2. They cooperate with each other to coordinate proper developmental process by silencing target genes; PRC1 posits H2AK119ub1 and PRC2 catalyzes trimethylation of H3K27 (H3K27me3). The PRC1 component BMI1/PCGF4 has long been recognized to be essential in the maintenance of normal and malignant hematopoietic stem cells (HSCs). Recently, diversity of PRC1 has been noticed and PRC1 is now classified into six alternative complexes depending on PCGF proteins. In embolic stem cells, PRC1 which contains PCGF1 (PCGF1-PRC1) has been demonstrated to serve upstream of the BMI1/PCGF4-PRC1. However, the impact of BCOR, which is a component of PCGF1-PRC1 on hematopoiesis is clearly different from BMI1/PCGF4; the mice deficient for BCOR exhibited normal HSC activities and BCOR rather prevented leukemic transformation of HSCs, suggesting the previously unappreciated gene control mechanisms of PCGF1-PRC1. To tackle this issue, we focused on the roles of PCGF1 in hematopoiesis. Loss of Pcgf1 in hematopoietic stem cells led to severe reduction of B lineage cells with an expansion of myeloid progenitors due to defects in lymphoid-primed multipotent progenitor (LMPP) cells. To explore the molecular mechanisms, we have established Id3-overexpressing hematopoietic progenitor cells (IdHPs) which correspond to LMPP-like cells (Ikawa et al. 2015) from bone marrow of ERT2-Cre Pcgf1 flox mice. The ChIP-seq analysis of normal IdHPs identified 1274 genes whose promoters were associated with PCGF1 peaks and 37% of them exhibited enrichment of H3K27me3 and binding of SUZ12 (PRC2). Deletion of Pcgf1 destabilized H3K27me3 levels, resulting in re-activation of genes associated with PCGF1 and SUZ12 peaks, whereas the chromatin occupancy of SUZ12 was not affected. Intriguingly, proteomic analysis demonstrated that PCGF1 interacts with key factors responsible for the organization of nucleosomes and PCGF1 loss triggered a decline of nucleosome-densities in promoters of genes occupied by PCGF1 and SUZ12 peaks. Since enzymatic activity of PRC2 is dependent on nucleosome-densities, PCGF1 is likely to regulate the susceptibility of H3K27me3 by PRC2 through determination of the nucleosome-densities. Furthermore, genes which were downregulated by PCGF1-nucleosome-H3K27me3 axis entailed many myeloid-related genes and knock down of one of those myeloid genes partially restored the B cell differentiation potential of Pcgf1-KO hematopoietic stem/progenitor cells (HSPCs), supporting the biological significance of the PCGF1-nucleosome-H3K27me3 axis. Collectively, these results indicate PCGF1 determines cellular fate of HSPCs through stabilization of nucleosomal organization. Disclosures No relevant conflicts of interest to declare.

Description: INTRODUCTION Inappropriate recruitment of functional Polycomb-Group proteins (PcG) may trigger epigenetic unbalance at very specific genomic loci that substantially contribute to the pathogenesis of Acute Myeloid Leukemia (AML). This concept was first described in Acute Promyelocytic Leukemia (APL) in which PcG proteins were abnormally addressed due to the expression of X-RARA fusion proteins and were involved in the treatment response of the disease. For instance, in the context of APL with t(11;17)(q23;q21) translocation, the resulted oncogenic fusion protein PLZF/RARA leads to abnormal recruitment of PcG at the promoters of genes involved in acid-trans-retinoic acid (ATRA) response (Boukarabila et al.). As a consequence of that and compared to other APL subtypes (e-g: PML/RARA), APL with PLZF/RARA are insensitive to ATRA. In the recent years, a repertoire of cis-regulatory enhancer elements has been dissected to reveal important insight about leukemia onset and define new subsets of the disease with different treatment responses (Bhagwat et al). As we previously reported that PLZF displayed epigenetic specificity on enhancers (Poplineau et al.) we questioned the role of PLZF/RARA on these regulatory regions during APL onset. METHODS We performed in vivo comparative epigenomic profiling (H3K27ac, H3K4me1, H3K27me3 and H3K4me3 ChIPseq) between normal myeloid progenitors (granulocyte-monocyte progenitors purified from wild-type mice) and PLZF/RARA transformed mouse progenitors (late promyelocytes purified from mice developing APL). To question the role of PcG in APL onset, we used retroviral overexpression of PLZF/RARA and transduced Lineage negative cells from a conditional KO EZH2 mouse model. Transformation was tested by replating assay and cells were characterized by FACS and morphology analyses. We also performed EZH2 pharmacological inhibition using GSK126 and UNC1999 on a human cell line expressing the fusion protein PLZF/RARA. We analyzed the impact of this inhibition on their transcriptomic signature (RNAseq) and their proliferative capacity. RESULTS Upon PLZF/RARA expression and APL progression, specific cis-regulatory enhancer elements were targeted by the H3K27me3 PcG repressive mark. This gain in poised enhancer regions, upon PLZF/RARA expression reflected a reoriented PcG activity, from enhancers regulating developmental processes to those regulating stress and immune responses. To demonstrate the importance of this H3K27me3 switch for APL progression, we investigated the effect of EZH2 loss during PLZF/RARA transformation. Using a conditional KO EZH2 mouse model, we demonstrated that PLZF/RARA required EZH2 activity to efficiently transform progenitors since EZH2 loss promoted differentiation that altered the replating capacity of the PLZF/RARA expressing cells. In addition, EZH2 inhibition by GSK126 revealed some interesting benefits since it sensitized PLZF/RARA transformed progenitors to ATRA treatment. Moreover, inhibition of EZH2 with GSK126 or UNC1999 induced a decrease in the proliferation advantage of a human PLZF/RARA-inducible cell line. This was linked to a change of its transcriptomic signature towards an expression pattern closer to the one observed in the parent cell line. CONCLUSION Taken together, our data showed that PLZF/RARA modifies H3K27me3 profiles at enhancer regions and requires EZH2 activity for APL onset. Finally, our results suggest that EZH2 inhibition could be a new promising therapeutic approach for retinoic-acid resistant APL. Disclosures No relevant conflicts of interest to declare.

Description: Polycomb group (PcG) proteins are epigenetic regulators crucial for the maintenance and differentiation of stem cells. PcG proteins form in the nucleus two kinds of complexes, PRC (polycomb repressive complex) 1 containing Pcgf family proteins and PRC2. PRC1 ubiquitylates histone H2A at lysine 119 and PRC2 trimethylates lysine 27 of histone H3. Bcor (BCL6 corepressor) together with Pcgf1 has recently been demonstrated to form a non-canonical PRC1 distinct from canonical PRC1 containing Cbx and Bmi1/Pcgf4. Of note, recurrent somatic mutations have been identified in BCOR in myeloid malignancies such as acute myeloid leukemia and myelodysplastic syndrome. However, little is known about its role in hematopoiesis and hematological malignancies. In this study, we investigated the role of Bcor in hematopoiesis using Bcor conditional knockout mice. We first examined basal levels of Bcor expression in BM hematopoietic cell populations by real-time qPCR and found that Bcor is ubiquitously expressed with the highest expression in Flk2-CD34+c-Kit+Sca-1+Lineage- ST-HSCs. We transplanted total bone marrow (BM) cells (CD45.2+) from Bcor floxed male mice (Bcorf/Y ; Bcor is located at X chromosome) crossed with Rosa::Cre-ERT mice into lethally irradiated recipient mice with the equal numbers of BM cells from CD45.1+ congenic WT mice and deleted Bcor by inducing nuclear translocation of Cre by tamoxifen injection at 4 weeks post-transplantation. Chimerism of Bcor-/Y cells in the peripheral blood gradually decreased in all lineages by 6 months after tamxifen injection. BM analysis also showed decreased chimerism of Bcor-/Y cells in Lineage-Sca-1+c-Kit+ (LSK) HSCs/MPPs and myeloid progenitors at 7 months after tamoxifen injection. In non-competitive repopulating assays, recipient mice repopulated with Bcor-/Y BM cells showed leukopenia mainly due to impaired B lymphopoiesis. BM analysis showed CD34-LSK HSC number was significantly decreased in recipients repopulated with Bcor-/Y BM cells compared to the controls. However, no mice showed any signs of myeloid malignancies. These data suggested that Bcor is essential for the maintenance of repopulating capacity of HSCs. We then examined the effects of deletion of Bcor on HSC growth in vitro. Bcor-/Y CD34-LSK cells were cultured for 14 days. Deletion of Bcor profoundly impaired the growth of CD34-LSK cells in the presence of SCF, TPO and IL3 but not significantly in the presence of SCF and TPO only. This finding suggests that Bcor is critical for IL3-responsive proliferation and differentiation. To understand molecular mechanisms underlying defective hematopoiesis in the absence of Bcor, we performed western blot analysis in LK cells and gene expression analysis in LSK cells. Western blot analysis revealed no significant difference in the levels of global H3K27me3 and H3K36me2 between Bcor-/Y LK cells and control. Unexpectedly, however, the level of global H2A119ub1 was not reduced at all in Bcor-/Y LK cells compared to control. Gene expression analysis revealed a trend of de-repression of PRC2 genes, but major target genes of canonical PRC1, such as cyclin-dependent kinase inhibitor genes were not altered in expression in Bcor-/Y LSK cells. These results indicate that Bcor regulates hematopoietic cells in a way different from canonical PRC1. Taken together, Bcor plays an important role in normal hematopoiesis, but its simple loss is not sufficient to induce myeloid malignancies within a short latency. To identify the target genes of Bcor in more detail, we are currently working on RNA sequencing and ChIP-sequencing of histone modifications in Bcor-/Y stem/progenitor cells. Disclosures No relevant conflicts of interest to declare.

Description: To search for genes that promote hematopoietic development from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs), we overexpressed several known hematopoietic regulator genes in hESC/iPSC-derived CD34+CD43− endothelial cells (ECs) enriched in hemogenic endothelium (HE). Among the genes tested, only Sox17, a gene encoding a transcription factor of the SOX family, promoted cell growth and supported expansion of CD34+CD43+CD45−/low cells expressing the HE marker VE-cadherin. SOX17 was expressed at high levels in CD34+CD43− ECs compared with low levels in CD34+CD43+CD45− pre-hematopoietic progenitor cells (pre-HPCs) and CD34+CD43+CD45+ HPCs. Sox17-overexpressing cells formed semiadherent cell aggregates and generated few hematopoietic progenies. However, they retained hemogenic potential and gave rise to hematopoietic progenies on inactivation of Sox17. Global gene-expression analyses revealed that the CD34+CD43+CD45−/low cells expanded on overexpression of Sox17 are HE-like cells developmentally placed between ECs and pre-HPCs. Sox17 overexpression also reprogrammed both pre-HPCs and HPCs into HE-like cells. Genome-wide mapping of Sox17-binding sites revealed that Sox17 activates the transcription of key regulator genes for vasculogenesis, hematopoiesis, and erythrocyte differentiation directly. Depletion of SOX17 in CD34+CD43− ECs severely compromised their hemogenic activity. These findings suggest that SOX17 plays a key role in priming hemogenic potential in ECs, thereby regulating hematopoietic development from hESCs/iPSCs.