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UTX/KDM6A, a histone H3K27 demethylase and a key component of the COMPASS complex, is frequently lost or mutated in cancer; however, its tumor suppressor function remains largely uncharacterized in multiple myeloma (MM). Here, we show that the conditional deletion of the X-linked Utx in germinal center (GC) derived cells collaborates with the activating BrafV600E mutation and promotes induction of lethal GC/post-GC B cell malignancies with MM-like plasma cell neoplasms being the most frequent. Mice that developed MM-like neoplasms showed expansion of clonal plasma cells in the bone marrow and extramedullary organs, serum M proteins, and anemia. Add-back of either wild-type UTX or a series of mutants revealed that cIDR domain, that forms phase-separated liquid condensates, is largely responsible for the catalytic activity-independent tumor suppressor function of UTX in MM cells. Utx loss in concert with BrafV600E only slightly induced MM-like profiles of transcriptome, chromatin accessibility, and H3K27 acetylation, however, it allowed plasma cells to gradually undergo full transformation through activation of transcriptional networks specific to MM that induce high levels of Myc expression. Our results reveal a tumor suppressor function of UTX in MM and implicate its insufficiency in the transcriptional reprogramming of plasma cells in the pathogenesis of MM.

Supplementary Figure S6. Histopathology analysis of liver cancer.

Supplementary Materials and Methods Supplementary References Supplementary Tables Supplementary Figures and legends Supplementary Table S1 Combination index (CI) values. Supplementary Table S2 Significantly upregulated PRC2 gene sets in UNC1999-treated MM.1S cells (FDR q-value < 0.05) Supplementary Table S3 Significantly upregulated PRC2 gene sets in combination-treated MM.1S cells (FDR q-value < 0.05) Supplementary Table S4 Top 74 PRC2 genes that are significantly altered in UNC1999-treated MM.1S cells Supplementary Table S5 Significantly upregulated gene sets in RPMI8226 cells transduced with EZH2- overexpressing versus empty vectors. Supplementary Table S6 List of primers used for Quantitative RT-PCR Supplementary Table S7 List of primers used for ChIP-PCR Supplementary Figure S1. Pharmacological and lentiviral suppression of EZH2 and EZH1 in MM cell lines. Supplementary Figure S2. Proteasome inhibitors downregulate EZH2 in MM cell lines. Supplementary Figure S3. EZH2 suppression by bortezomib is mediated through E2F1. Supplementary Figure S4. MM patients with higher levels of EZH2 expression have poorer response to bortezomib. Supplementary Figure S5. EZH2 overexpression in MM cells confers resistance to bortezomib through upregulation of gene sets related to MYC, cell cycle and metabolism. Supplementary Figure S6. UNC1999 enhances the cytotoxicity induced by bortezomib in MM cell lines including resistant ones but not in PBMNCs from healthy donors. Supplementary Figure S7. The combination of UNC1999 and bortezomib induces significant apoptosis in vitro and is tolerated in vivo. Supplementary Figure S8. The expression of NR4As in MM. Supplementary Figure S9. Dual inhibition of EZH2 and EZH1, but not specific EZH2 inhibition, is critical for the synergy with bortezomib in MM cell lines. Supplementary Figure S10. UNC1999 exhibits synergy with bortezomib in DU145 prostate cancer cells.

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. Clin Cancer Res; 23(16); 4817–30. ©2017 AACR.

The identification of characteristic differences between cancer stem cells and their normal counterparts remains a key challenge for cancer treatment. Here, we investigated the role of immunoglobulin superfamily member 8 (Igsf8, also known as EWI-2, PGRL, and CD316) on normal and malignant hematopoietic stem cells, mainly using the conditional knockout model. Deletion of Igsf8 did not affect steady state hematopoiesis, but it led to a significant improvement of survival in mouse myeloid leukemia models. Deletion of Igsf8 significantly depletes leukemia stem cells (LSCs) through enhanced apoptosis and β-catenin degradation. At a molecular level, we found that activation of β-catenin in LSCs depends on Igsf8, which promotes the association of FZD4 with its co-receptor LRP6 in the presence of Igsf8. Similarly, IGSF8 inhibition blocks the colony-forming ability of LSCs and improves the survival of recipients in xenograft models of myeloid leukemia. Collectively, these data indicate strong genetic evidence identifying Igsf8 as a key regulator of myeloid leukemia and the possibility that targeting IGSF8 may serve as a new therapeutic approach against myeloid leukemia.

Insufficiency of polycomb repressive complex 2 (PRC2), which trimethylates histone H3 at lysine 27, is frequently found in primary myelofibrosis and promotes the development of JAK2V617F-induced myelofibrosis in mice by enhancing the production of dysplastic megakaryocytes. Polycomb group ring finger protein 1 (Pcgf1) is a component of PRC1.1, a non-canonical PRC1 that monoubiquitylates H2A at lysine 119 (H2AK119ub1). We herein investigated the impact of PRC1.1 insufficiency on myelofibrosis. The deletion of Pcgf1 in JAK2V617F mice strongly promoted the development of lethal myelofibrosis accompanied by a block in erythroid differentiation. Transcriptome and chromatin immunoprecipitation sequence analyses showed the de-repression of PRC1.1 target genes in Pcgf1-deficient JAK2V617F hematopoietic progenitors and revealed Hoxa cluster genes as direct targets. The deletion of Pcgf1 in JAK2V617F hematopoietic stem and progenitor cells (HSPCs), as well as the overexpression of Hoxa9, restored the attenuated proliferation of JAK2V617F progenitors. The overexpression of Hoxa9 also enhanced JAK2V617F-mediated myelofibrosis. The expression of PRC2 target genes identified in PRC2-insufficient JAK2V617F HSPCs was not largely altered in Pcgf1-deleted JAK2V617F HSPCs. The present results revealed a tumor suppressor function for PRC1.1 in myelofibrosis and suggest that PRC1.1 insufficiency has a different impact from that of PRC2 insufficiency on the pathogenesis of myelofibrosis.

SUMMARYPolycomb repressive complex (PRC) 1 regulates stem cell fate by mediating mono-ubiquitination of histone H2A at lysine 119. While canonical PRC1 is critical for hematopoietic stem and progenitor cell (HSPC) maintenance, the role of non-canonical PRC1 in hematopoiesis remains elusive. PRC1.1, a non-canonical PRC1, consists of PCGF1, RING1B, KDM2B, and BCOR. We recently showed that PRC1.1 insufficiency induced by the loss of PCGF1 or BCOR causes myeloid-biased hematopoiesis and promotes transformation of hematopoietic cells in mice. Here we show that PRC1.1 serves as an epigenetic switch that coordinates homeostatic and emergency hematopoiesis. PRC1.1 maintains balanced output of steady-state hematopoiesis by restricting C/EBPα-dependent precocious myeloid differentiation of HSPCs and the HOXA9- and β-catenin-driven self-renewing network in myeloid progenitors. Upon regeneration, PRC1.1 is transiently inhibited to facilitate formation of granulocyte-macrophage progenitor (GMP) clusters, thereby promoting emergency myelopoiesis. Moreover, constitutive inactivation of PRC1.1 results in unchecked expansion of GMPs and eventual transformation. Collectively, our results define PRC1.1 as a novel critical regulator of emergency myelopoiesis, dysregulation of which leads to myeloid transformation.

Cancer cell heterogeneity is a major driver of therapy resistance. To characterize resistant cells and their vulnerabilities, we studied the PLZF-RARA variant of acute promyelocytic leukemia, resistant to retinoic acid (RA), using single-cell multiomics. We uncovered transcriptional and chromatin heterogeneity in leukemia cells. We identified a subset of cells resistant to RA with proliferation, DNA replication, and repair signatures that depend on a fine-tuned E2F transcriptional network targeting the epigenetic regulator enhancer of zeste homolog 2 (EZH2). Epigenomic and functional analyses validated the driver role of EZH2 in RA resistance. Targeting pan-EZH2 activities (canonical/noncanonical) was necessary to eliminate leukemia relapse-initiating cells, which underlies a dependency of resistant cells on an EZH2 noncanonical activity and the necessity to degrade EZH2 to overcome resistance. Our study provides critical insights into the mechanisms of RA resistance that allow us to eliminate treatment-resistant leukemia cells by targeting EZH2, thus highlighting a potential targeted therapy approach. Beyond RA resistance and acute promyelocytic leukemia context, our study also demonstrates the power of single-cell multiomics to identify, characterize, and clear therapy-resistant cells.

Dihydroorotate dehydrogenase (DHODH) catalyzes a rate-limiting step in de novo pyrimidine nucleotide synthesis. DHODH inhibition has recently been recognized as a potential new approach for treating acute myeloid leukemia (AML) by inducing differentiation. We investigated the efficacy of PTC299, a novel DHODH inhibitor, for myelodysplastic syndrome (MDS). PTC299 inhibited the proliferation of MDS cell lines, and this was rescued by exogenous uridine, which bypasses de novo pyrimidine synthesis. In contrast to AML cells, PTC299 was inefficient at inhibiting growth and inducing the differentiation of MDS cells, but synergized with hypomethylating agents, such as decitabine, to inhibit the growth of MDS cells. This synergistic effect was confirmed in primary MDS samples. As a single agent, PTC299 prolonged the survival of mice in xenograft models using MDS cell lines, and was more potent in combination with decitabine. Mechanistically, a treatment with PTC299 induced intra-S-phase arrest followed by apoptotic cell death. Of interest, PTC299 enhanced the incorporation of decitabine, an analog of cytidine, into DNA by inhibiting pyrimidine production, thereby enhancing the cytotoxic effects of decitabine. RNA-seq data revealed the marked downregulation of MYC target gene sets with PTC299 exposure. Transfection of MDS cell lines with MYC largely attenuated the growth inhibitory effects of PTC299, suggesting MYC as one of the major targets of PTC299. Our results indicate that the DHODH inhibitor PTC299 suppresses the growth of MDS cells and acts in a synergistic manner with decitabine. This combination therapy may be a new therapeutic option for the treatment of MDS.

Monomer tubulin polymerize into microtubules, which are highly dynamic and play a critical role in mitosis. Therefore, microtubule dynamics are an important target for anticancer drugs. The inhibition of tubulin polymerization or depolymerization was previously targeted and exhibited efficacy against solid tumors. The novel small molecule PTC596 directly binds tubulin, inhibits microtubule polymerization, downregulates MCL‐1, and induces p53‐independent apoptosis in acute myeloid leukemia cells. We herein investigated the efficacy of PTC‐028, a structural analog of PTC596, for myelodysplastic syndrome (MDS). PTC‐028 suppressed growth and induced apoptosis in MDS cell lines. The efficacy of PTC028 in primary MDS samples was confirmed using cell proliferation assays. PTC‐028 synergized with hypomethylating agents, such as decitabine and azacitidine, to inhibit growth and induce apoptosis in MDS cells. Mechanistically, a treatment with PTC‐028 induced G2/M arrest followed by apoptotic cell death. We also assessed the efficacy of PTC‐028 in a xenograft mouse model of MDS using the MDS cell line, MDS‐L, and the AkaBLI bioluminescence imaging system, which is composed of AkaLumine‐HCl and Akaluc. PTC‐028 prolonged the survival of mice in xenograft models. The present results suggest a chemotherapeutic strategy for MDS through the disruption of microtubule dynamics in combination with DNA hypomethylating agents., PTC‐028, a novel microtubule polymerization inhibitor, suppresses the growth of MDS cells. PTC‐028 synergizes with DNA hypomethylating agents to inhibit the growth of MDS cells. PTC‐028 prolongs the survival of mice in a xenograft MDS model.

EZH1 and EZH2 are enzymatic components of polycomb repressive complex (PRC) 2, which catalyzes histone H3K27 tri-methylation (H3K27me3) to repress the transcription of PRC2 target genes. We previously reported that the hematopoietic cell-specific Ezh2 deletion (Ezh2Δ/Δ) induced a myelodysplastic syndrome (MDS)-like disease in mice. We herein demonstrated that severe PRC2 insufficiency induced by the deletion of one allele Ezh1 in Ezh2-deficient mice (Ezh1+/-Ezh2Δ/Δ) caused advanced dyserythropoiesis accompanied by a differentiation block and enhanced apoptosis in erythroblasts. p53, which is activated by impaired ribosome biogenesis in del(5q) MDS, was specifically activated in erythroblasts, but not in hematopoietic stem or progenitor cells in Ezh1+/-Ezh2Δ/Δ mice. Cdkn2a, a major PRC2 target encoding p19Arf, which activates p53 by inhibiting MDM2 E3 ubiquitin ligase, was de-repressed in Ezh1+/-Ezh2Δ/Δ erythroblasts. The deletion of Cdkn2a as well as p53 rescued dyserythropoiesis in Ezh1+/-Ezh2Δ/Δ mice, indicating that PRC2 insufficiency caused p53-dependent dyserythropoiesis via the de-repression of Cdkn2a. Since PRC2 insufficiency is often involved in the pathogenesis of MDS, the present results suggest that p53-dependent dyserythropoiesis manifests in MDS in the setting of PRC2 insufficiency.

Loss-of-function mutations in ten-eleven translocation-2 (TET2) are recurrent events in acute myeloid leukemia (AML) as well as in preleukemic hematopoietic stem cells (HSCs) of age-related clonal hematopoiesis. TET3 mutations are infrequent in AML, but the level of TET3 expression in HSCs has been found to decline with age. We examined the impact of gradual decrease of TET function in AML development by generating mice with Tet deficiency at various degrees. Tet2f/f and Tet3f/f mice were crossed with mice expressing Mx1-Cre to generate Tet2f/wtTet3f/fMx-Cre+ (T2ΔT3), Tet2f/fTet3f/wtMx-Cre+ (ΔT2T3), and Tet2f/fTet3f/fMx-Cre+ (ΔT2ΔT3) mice. All ΔT2ΔT3 mice died of aggressive AML at a median survival of 10.7 weeks. By comparison, T2ΔT3 and ΔT2T3 mice developed AML at longer latencies, with a median survival of ∼27 weeks. Remarkably, all 9 T2ΔT3 and 8 ΔT2T3 mice with AML showed inactivation of the remaining nontargeted Tet2 or Tet3 allele, respectively, owing to exonic loss in either gene or stop-gain mutations in Tet3. Recurrent mutations other than Tet3 were not noted in any mice by whole-exome sequencing. Spontaneous inactivation of residual Tet2 or Tet3 alleles is a recurrent genetic event during the development of AML with Tet insufficiency.

Polycomb repressive complex 2 (PRC2) components, EZH2 and its homolog EZH1, and PI3K/Akt signaling pathway are focal points as therapeutic targets for multiple myeloma. However, the exact crosstalk between their downstream targets remains unclear. We herein elucidated some epigenetic interactions following Akt inhibition and demonstrated the efficacy of the combined inhibition of Akt and PRC2. We found that TAS‐117, a potent and selective Akt inhibitor, downregulated EZH2 expression at the mRNA and protein levels via interference with the Rb‐E2F pathway, while EZH1 was compensatively upregulated to maintain H3K27me3 modifications. Consistent with these results, the dual EZH2/EZH1 inhibitor, UNC1999, but not the selective EZH2 inhibitor, GSK126, synergistically enhanced TAS‐117‐induced cytotoxicity and provoked myeloma cell apoptosis. RNA‐seq analysis revealed the activation of the FOXO signaling pathway after TAS‐117 treatment. FOXO3/4 mRNA and their downstream targets were upregulated with the enhanced nuclear localization of FOXO3 protein after TAS‐117 treatment. ChIP assays confirmed the direct binding of FOXO3 to EZH1 promoter, which was enhanced by TAS‐117 treatment. Moreover, FOXO3 knockdown repressed EZH1 expression. Collectively, the present results reveal some molecular interactions between Akt signaling and epigenetic modulators, which emphasize the benefits of targeting PRC2 full activity and the Akt pathway as a therapeutic option for multiple myeloma., In this manuscript, we elucidate some epigenetic interactions following Akt inhibition and demonstrate the efficacy of the combined inhibition of Akt and PRC2. We found that TAS‐117, a potent and selective Akt inhibitor, differently controls PRC2 components, EZH2 and EZH1, in multiple myeloma. While EZH2 was downregulated, through Rb‐E2F stabilization, EZH1 was compensatively upregulated, to maintain H3K27me3 modifications. ChIP assays confirmed the direct binding of FOXO3 to EZH1 promoter, which was enhanced by TAS‐117 treatment. Moreover, the combination of TAS‐117 and the dual EZH1/2 inhibitor, UNC1999, but not the selective EZH2 inhibitor, GSK126, has a synergistic cytotoxic effect against multiple myeloma cell lines.

KDM2B together with RING1B, PCGF1, and BCOR or BCORL1 comprise polycomb repressive complex 1.1 (PRC1.1), a noncanonical PRC1 that catalyzes H2AK119ub1. It binds to nonmethylated CpG islands through its zinc finger-CxxC DNA binding domain and recruits the complex to target gene loci. Recent studies identified the loss of function mutations in the PRC1.1 gene, BCOR and BCORL1 in human T-cell acute lymphoblastic leukemia (T-ALL). We previously reported that Bcor insufficiency induces T-ALL in mice, supporting a tumor suppressor role for BCOR. However, the function of BCOR responsible for tumor suppression, either its corepressor function for BCL6 or that as a component of PRC1.1, remains unclear. We herein examined mice specifically lacking the zinc finger-CxxC domain of KDM2B in hematopoietic cells. Similar to Bcor-deficient mice, Kdm2b-deficient mice developed lethal T-ALL mostly in a NOTCH1-dependent manner. A chromatin immunoprecipitation sequence analysis of thymocytes revealed the binding of KDM2B at promoter regions, at which BCOR and EZH2 colocalized. KDM2B target genes markedly overlapped with those of NOTCH1 in human T-ALL cells, suggesting that noncanonical PRC1.1 antagonizes NOTCH1-mediated gene activation. KDM2B target genes were expressed at higher levels than the others and were marked with high levels of H2AK119ub1 and H3K4me3, but low levels of H3K27me3, suggesting that KDM2B target genes are transcriptionally active or primed for activation. These results indicate that PRC1.1 plays a key role in restricting excessive transcriptional activation by active NOTCH1, thereby acting as a tumor suppressor in the initiation of T-cell leukemogenesis.

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.

Summary Polycomb repressive complex (PRC) 2 represses transcription through histone H3K27 trimethylation (H3K27me3). We previously reported that the hematopoietic-cell-specific deletion of Ezh2, encoding a PRC2 enzyme, induced myelodysplastic syndrome (MDS) in mice, whereas the concurrent Ezh1 deletion depleted hematopoietic stem and progenitor cells (HSPCs). We herein demonstrated that mice with only one Ezh1 allele (Ezh1+/-Ezh2Δ/Δ) maintained HSPCs. A chromatin immunopreciptation sequence analysis revealed that residual PRC2 preferentially targeted genes with high levels of H3K27me3 and H2AK119 monoubiquitination (H2AK119ub1) in HSPCs (designated as Ezh1 core target genes), which were mostly developmental regulators, and maintained H3K27me3 levels in Ezh1+/-Ezh2Δ/Δ HSPCs. Even upon the complete depletion of Ezh1 and Ezh2, H2AK119ub1 levels were largely retained, and only a minimal number of Ezh1 core targets were de-repressed. These results indicate that genes marked with high levels of H3K27me3 and H2AK119ub1 are the core targets of polycomb complexes in HSPCs as well as MDS stem cells., Graphical Abstract, Highlights • One allele of Ezh1 is enough to maintain self-renewing HSCs and MDS stem cells • Ezh1 core targets are marked with high levels of H3K27me3 and H2AK119ub1 in HSPCs • Ezh1 core targets are mostly bivalent developmental regulators and critical for HSCs, Biological Sciences; Genetics; Molecular Biology; Immunology; Cell Biology; Developmental Biology

Hematopoietic stem cells (HSCs) exhibit considerable cell-intrinsic changes with age. Here, we present an integrated analysis of transcriptome and chromatin accessibility of aged HSCs and downstream progenitors. Alterations in chromatin accessibility preferentially take place in HSCs with aging, which gradually resolve with differentiation. Differentially open accessible regions (open DARs) in aged HSCs are enriched for enhancers and show enrichment of binding motifs of the STAT, ATF, and CNC family transcription factors that are activated in response to external stresses. Genes linked to open DARs show significantly higher levels of basal expression and their expression reaches significantly higher peaks after cytokine stimulation in aged HSCs than in young HSCs, suggesting that open DARs contribute to augmented transcriptional responses under stress conditions. However, a short-term stress challenge that mimics infection is not sufficient to induce persistent chromatin accessibility changes in young HSCs. These results indicate that the ongoing and/or history of exposure to external stresses may be epigenetically inscribed in HSCs to augment their responses to external stimuli.

Insufficiency of polycomb repressive complex 2 (PRC2), which trimethylates histone H3 at lysine 27, is frequently found in primary myelofibrosis and promotes the development of JAK2V617F-induced myelofibrosis in mice by enhancing the production of dysplastic megakaryocytes. Polycomb group ring finger protein 1 (Pcgf1) is a component of PRC1.1, a non-canonical PRC1 that monoubiquitylates H2A at lysine 119 (H2AK119ub1). We herein investigated the impact of PRC1.1 insufficiency on myelofibrosis. The deletion of Pcgf1 in JAK2V617F mice strongly promoted the development of lethal myelofibrosis accompanied by a block in erythroid differentiation. Transcriptome and chromatin immunoprecipitation sequence analyses showed the de-repression of PRC1.1 target genes in Pcgf1-deficient JAK2V617F hematopoietic progenitors and revealed Hoxa cluster genes as direct targets. The deletion of Pcgf1 in JAK2V617F hematopoietic stem and progenitor cells (HSPCs), as well as the overexpression of Hoxa9, restored the attenuated proliferation of JAK2V617F progenitors. The overexpression of Hoxa9 also enhanced JAK2V617F-mediated myelofibrosis. The expression of PRC2 target genes identified in PRC2-insufficient JAK2V617F HSPCs was not largely altered in Pcgf1-deleted JAK2V617F HSPCs. The present results revealed a tumor suppressor function for PRC1.1 in myelofibrosis and suggest that PRC1.1 insufficiency has a different impact from that of PRC2 insufficiency on the pathogenesis of myelofibrosis.

Kuribayashi et al. demonstrate that the engraftment of aged hematopoietic stem cells into intact young niche largely rejuvenates their transcriptional profile but does not restore their function, highlighting a key role for age-associated cell-intrinsic defects in HSC aging., Hematopoietic stem cells (HSCs) exhibit functional alterations, such as reduced regenerative capacity and myeloid-biased differentiation, with age. The HSC niche, which is essential for the maintenance of HSCs, also undergoes marked changes with aging. However, it has been technically challenging to directly evaluate the contribution of niche aging to age-associated HSC alterations without niche-damaging myeloablation in HSC transplantation assays. We herein transplanted an excess of aged HSCs into young mice without preconditioning. Although aged HSCs successfully engrafted in the intact young bone marrow niche, they poorly regenerated downstream progenitors and exhibited persistent myeloid-biased differentiation, resulting in no significant functional rejuvenation. Transcriptome and methylome analyses revealed that the young niche largely restored the transcriptional profile of aged HSCs, but not their DNA methylation profiles. Therefore, the restoration of the young niche is insufficient for rejuvenating HSC functions, highlighting a key role for age-associated cell-intrinsic defects in HSC aging., Graphical Abstract

The bone marrow (BM) microenvironment, known as the BM niche, regulates hematopoiesis but is also affected by interactions with hematopoietic cells. Recent evidence indicates that extracellular matrix components are involved in these interactions. Chondroitin sulfate (CS), a glycosaminoglycan, is a major component of the extracellular matrix; however, it is not known whether CS has a physiological role in hematopoiesis. Here, we analyzed the functions of CS in hematopoietic and niche cells. CSGalNAcT1, which encodes CS N-acetylgalactosaminyltransferase-1 (T1), a key enzyme in CS biosynthesis, was highly expressed in hematopoietic stem and progenitor cells (HSPCs) and endothelial cells (ECs), but not in mesenchymal stromal cells (MSCs) in BM. In T1 knockout (T1KO) mice, a greater number of HSPCs existed compared with the wild-type (WT), but HSPCs from T1KO mice showed significantly impaired repopulation in WT recipient mice on serial transplantation. RNA sequence analysis revealed the activation of IFN-α/β signaling and endoplasmic reticulum stress in T1KO HSPCs. In contrast, the number of WT HSPCs repopulated in T1KO recipient mice was larger than that in WT recipient mice after serial transplantation, indicating that the T1KO niche supports repopulation of HSPCs better than the WT niche. There was no obvious difference in the distribution of vasculature and MSCs between WT and T1KO BM, suggesting that CS loss alters vascular niche functions without affecting its structure. Our results revealed distinct roles of CS in hematopoietic cells and BM niche, indicating that crosstalk between these components is important to maintain homeostasis in BM.

The novel small molecule PTC596 inhibits microtubule polymerization and its clinical development has been initiated for some solid cancers. We herein investigated the preclinical efficacy of PTC596 alone and in combination with proteasome inhibitors in the treatment of multiple myeloma (MM). PTC596 inhibited the proliferation of MM cell lines as well as primary MM samples in vitro, and this was confirmed with MM cell lines in vivo. PTC596 synergized with bortezomib or carfilzomib to inhibit the growth of MM cells in vitro. The combination treatment of PTC596 with bortezomib exerted synergistic effects in a xenograft model of human MM cell lines in immunodeficient mice and exhibited acceptable tolerability. Mechanistically, treatment with PTC596 induced cell cycle arrest at G2/M phase followed by apoptotic cell death, associated with the inhibition of microtubule polymerization. RNA sequence analysis also revealed that PTC596 and the combination with bortezomib affected the cell cycle and apoptosis in MM cells. Importantly, endoplasmic reticulum stress induced by bortezomib was enhanced by PTC596, providing an underlying mechanism of action of the combination therapy. Our results indicate that PTC596 alone and in combination with proteasome inhibition are potential novel therapeutic options to improve outcomes in patients with MM.

EZH1 and EZH2 are enzymatic components of polycomb repressive complex (PRC) 2, which catalyzes histone H3K27 tri-methylation (H3K27me3) to repress the transcription of PRC2 target genes. We previously reported that the hematopoietic cell-specific Ezh2 deletion (Ezh2

Early T cell precursor acute lymphoblastic leukemia (ETP-ALL) is a new pathological entity with poor outcomes in T cell ALL (T-ALL) that is characterized by a high incidence of loss-of-function mutations in polycomb repressive complex 2 (PRC2) genes. We generated a mouse model of ETP-ALL by deleting Ezh2, one of the PRC2 genes, in p53-null hematopoietic cells. The loss of Ezh2 in p53-null hematopoietic cells impeded the differentiation of ETPs and eventually induced ETP-ALL–like disease in mice, indicating that PRC2 functions as a bona fide tumor suppressor in ETPs. A large portion of PRC2 target genes acquired DNA hypermethylation of their promoters following reductions in H3K27me3 levels upon the loss of Ezh2, which included pivotal T cell differentiation–regulating genes. The reactivation of a set of regulators by a DNA-demethylating agent, but not the transduction of single regulator genes, effectively induced the differentiation of ETP-ALL cells. Thus, PRC2 protects key T cell developmental regulators from DNA hypermethylation in order to keep them primed for activation upon subsequent differentiation phases, while its insufficiency predisposes ETPs to leukemic transformation. These results revealed a previously unrecognized epigenetic switch in response to PRC2 dysfunction and provide the basis for specific rational epigenetic therapy for ETP-ALL with PRC2 insufficiency.

Adult hematopoietic stem cells (HSCs) reside and are protected in a unique bone marrow (BM) microenvironment, termed the HSC niche, which consists mainly of vascular endothelial cells (EC) and EC-associated mesenchymal stromal cells (MSC). Myeloablative stresses, such as ionizing radiation (IR) and chemotherapy, induce not only depletion of hematopoietic cells but also disruption of HSC niche components, as exemplified by dilation and leakiness of BM vasculature and depletion and dysfunction of BM MSCs. These structural and functional changes in the HSC niche restrain efficient hematopoietic recovery, which often compromises the efficacy of HSC transplantation (HSCT) and chemotherapy. YAP/TAZ are the two transcriptional coactivators normally repressed by LATS kinases downstream of the Hippo pathway. Although cumulative evidence has established a critical role of YAP/TAZ activation in tissue regeneration of various solid organs, their role in BM regeneration remains poorly understood. Our quantitive RT-PCR revealed that YAP/TAZ are abundantly expressed in steady-state mouse ECs (CD45 -Ter119 -CD31 +Sca1 +CD105 hi) and MSCs (CD45 -Ter119 -CD31 -PDGFRα +CD51 +LepR +) but scarcely in hematopoietic cells including HSCs (Lin -cKit +Sca1 +Flk2 -CD150 +CD48 -CD34 lo), which was confirmed by reanalysis of the published single cell RNA-seq datasets (GSE128423). Immunofluorescent imaging of BM sections revealed that YAP/TAZ are distributed mainly in the cytoplasm of ECs but evenly in the cytoplasm and nuclei of MSCs, indicating their differential basal activity in these two HSC niche components. Kinetic transcriptome analysis revealed that YAP/TAZ activity is transiently activated in ECs at 24 hours and returns to a basal repressive state by day 3 after sublethal IR. This transient activation of endothelial YAP/TAZ was critical for vascular integrity, as conditional deletion of YAP/TAZ in ECs (Cdh5-Cre ERT2Yap1 f/fTaz f/f) caused 100% lethality of mice within 10 days following sublethal IR. In sharp contrast, the kinetic expression analysis of a YAP/TAZ target gene CTGF indicated their transient inhibition in MSCs after sublethal IR, and the conditional YAP/TAZ deletion in BM MSCs (Ebf3-Cre ERT2Yap1 f/fTaz f/f) led to their reduced colony forming ability when assessed by colony forming unit fibroblast (CFU-F) assay. Recently, we discovered a novel and potent LATS inhibitor GA-003 that selectively induces mouse and human YAP/TAZ activation in vitro (IC 50 against LATS1 = 1.06 ± 0.08 nM). To analyze the effect of pharmacological YAP/TAZ activation on BM regeneration in vivo, we treated mice with intraperitoneal injection of GA-003 (50 mg/kg per day, for 8 days) following sublethal IR. Remarkably, we observed an accelerated recovery of hematopoiesis, with the absolute numbers of BM cellularity, GMP (Lin -cKit +Sca1 -FcγR +CD34 +) and HSC EPCR (Lin -cKit +Sca1 +CD150 +EPCR +) on day 14 increased by 3.50-fold (p=0.0002), 6.49-fold (p=0.0022) and 11.41-fold (p=0.022), respectively in the GA-003-treated group compared to vehicle-treated group. In addition, GA-003 also promoted hematopoietic recovery after 5-FU injection (150 mg/kg) and HSCT. Nonetheless, consistent with the scarce expression of YAP/TAZ in hematopoietic stem and progenitor cells (HSPC), in vitro GA-003 treatment did not enhance HSPC growth, suggesting niche-mediated effects by GA-003. Indeed, in vitro tube formation assay indicated accelerated angiogenesis by GA-003-treated human umbilical vein ECs, and CFU-F assays revealed significant enhancement of colony formation by mouse BM-derived MSCs upon GA-003 treatment. To reveal the effect of GA-003 on the HSC niche components in vivo, we performed whole BM immunofluorescent imaging at various time points following sublethal IR and GA-003 treatment. We observed alleviated vascular dilation and leakiness and earlier restoration of vascular damage in GA-003-treated group compared to vehicle-treated group, which was associated with increased VE-Cadherin expression in ECs. These results suggest that reinforcing YAP/TAZ activity upon myelosuppression promotes HSC niche integrity and recovery and accelerates hematopoietic regeneration. Taken together, our results establish YAP/TAZ as novel regulators of HSC niche and highlight YAP/TAZ as promising therapeutic targets to boost hematopoietic recovery after myeloablative interventions such as chemotherapy and HSCT. Disclosures Aihara: Nissan Chemical Corporation: Current Employment. Iwawaki: Nissan Chemical Corporation: Current Employment. Nishino: Nissan Chemical Corporation: Current Employment. Iwama: Nissan Chemical Corporation: Research Funding.

During aging, hematopoietic stem cells (HSCs) alter quantitatively as well as qualitatively due to accumulating damages induced by intrinsic and extrinsic stresses. Functional decline of HSCs causes deregulated hematopoiesis resulting in anemia, immune dysfunction, and increased risk of hematologic malignancies. The absolute number of HSCs in aged mice evidently increases compared to young mice. In addition, aged HSCs show abnormal hematopoiesis such as myeloid-biased differentiation accompanied by low production of lymphocytes. However, the molecular mechanisms underlying age-associated changes in hematopoiesis remain largely unknown. In this study, we performed single-cell RNA sequence analysis (scRNA-seq) of HSCs from young (10-week-old), middle-aged (12-month-old) and aged (20-month-old) mice to gain insight into the dynamics of HSC aging. scRNA-seq analysis revealed three major clusters (A, B, C) and three minor clusters (D, E, F) in young HSCs. Of interest, aged HSCs also showed similar cluster formation. One of the minor clusters was characterized by gene signature associated with inflammatory response and significantly increased with age, while the remaining two minor clusters, which showed cell cycle gene signature, did not change in proportion. One of the major clusters, Cluster C, which showed the strongest expression of HSC-specific gene sets compared with other clusters, moderately increased with age. Our scRNA-seq analysis confirmed upregulation of age-related genes previously reported, such as Selp, Mt1, and Vwf, in a considerable portion of aged HSCs. von Willebrand factor (Vwf) encodes a blood glycoprotein produced by megakaryocytes and endothelial cells. Several groups have reported that Vwf-expressing HSCs show myeloid/platelet-biased differentiation (Joana C et al., Nature 2013; Sandra P et al., Developmental Cell 2018). Importantly, our scRNA-seq data identified that Clusterin (Clu) is rarely expressed in young HSCs and is dramatically upregulated in aged HSCs. Clu expression was predominantly increased in one of the major clusters, Cluster C. Clusterin encodes a secreted chaperone involved in clearance of cellular debris and regulation of apoptosis. We hypothesized that Clu would be useful as a marker of a unique subpopulation of aged HSCs, and thus conducted further analysis by using Clu reporter mice with Clu BAC clone, in which an EGFP reporter gene was inserted at the initiating ATG codon of the Clu gene so that EGFP expression is driven by the regulatory sequences of the BAC gene. Clu/GFP was preferentially expressed in HSCs and at lower frequencies in MPP1 than HSCs in Lineage -Sca-1 +c-Kit + (LSK) cell fraction. Clu-positive HSCs expressed high levels of CD150 and were detected in 10% and 60% of HSCs in 10-week-old young mice and 8-month-old middle-aged mice, respectively, indicating that Clu-positive HSCs increase with aging. We next assessed the function of Clu-positive HSCs in young mice. Clu-positive young HSCs established significantly lower chimerism than Clu-negative young HSCs and preferentially differentiated into myeloid cells in competitive transplantation assays. RNA-seq analysis of Clu-positive and Clu-negative HSCs from young mice confirmed that Clu-positive HSCs show the gene signature of myeloid-biased HSCs. Characterization of Clu-positive and negative subpopulations in aged HSCs is currently underway. These results suggest that Clu-positive HSCs represent myeloid-biased HSCs which expand with aging, thus Clu expression serves as a novel marker to monitor the alterations in HSC heterogeneity with aging. Disclosures Iwama: Nissan Chemical Corporation: Research Funding.

Immunoglobulin superfamily member 8 (IGSF8, also known as EWI-2, PGRL, and CD316), is a cell surface protein containing 4 immunoglobulin domains. IGSF8 directly binds to the tetraspanin molecules, CD9 and CD81, and modulates cell adhesion, migration, and growth. Previous studies demonstrated that IGSF8 was associated with prognosis and metastasis in several solid tumors. However, the role of IGSF8 in normal hematopoiesis and myeloid leukemia is still unclear. First, we examined the expression levels of Igsf8 in various hematopoietic fraction of wild-type murine bone marrow cells, and found that Igsf8 is expressed in all hematopoietic lineages. To investigate hematopoietic functions of Igsf8, we generated hematopoietic cells specific Igsf8 deleted mice (Igsf8fl/fl; Vav-Cre) and tamoxifen induced Igsf8 deleted mice (Igsf8fl/fl; Rosa26-CreERT). Igsf8fl/fl, Vav-Cre (denoted as Igsf8-/-) mice represented normal maturation. Deletion of Igsf8 did not significantly affect adult hematopoiesis in peripheral blood and bone marrow. Igsf8-/- long-term hematopoietic stem cells (LT-HSCs: CD34- Flk2- c-Kit+ Sca-1+ Lineage- cells) reduced colony forming ability in vitro, and serial competitive transplantation assay showed comparable donor chimerism by 3 months, but led to decrease Igsf8-/- donor chimerism at 4 months and those after second transplantation in vivo. These results suggest that Igsf8 does not affect the adult hematopoiesis, but it can affect their proliferative and reconstitutive capacity of HSCs. To investigate the effects of Igsf8 on myeloid leukemia, we generated MLL-AF9 and NRASG12V-driven acute myelogenous leukemia (AML), or BCR-ABL and NUP98-HOXA9-driven blast crisis of chronic myelogenous leukemia (CML-BC) mice models. Igsf8-/- led to a dramatic reduction in the number of leukemic colonies formed in vitro (Figure 1A). Igsf8-/- leukemia mice showed significantly longer survival in vivo (Figure 1B). This effect was also observed by eliminating Igsf8 expression after leukemia establishment using conditionally deletion. Igsf8-/- AML cells showed decreased S phase fraction. Igsf8-/- leukemia stem cells (LSCs: c-Kit+ Lineage- cells) triggered an increment of the apoptosis, which contribute to significantly lower proportion of LSCs in spleen of Igsf8-/- leukemic mice. Given that Igsf8-/- did not affect homing ability of leukemia cells, these results indicate that Igsf8 is required for propagation of myeloid leukemia and maintenance of LSC. To understand the Igsf8-mediated regulation of myeloid leukemia, we conducted RNA sequencing analysis of LT-HSCs, and LSCs of AML and CML-BC. Gene set enrichment analysis exhibited increase apoptosis related genes and decrease Wnt/β-catenin related genes in Igsf8-/- leukemic cells, but not in LT-HSCs (Figure 1C). Increment of pro-apoptosis genes, and decrement of anti-apoptosis genes and Wnt/β-catenin target genes in Igsf8-/- AML stem cells were validated in quantitative polymerase chain reaction analysis. Further, expression levels of β-catenin protein in Igsf8-/- leukemic cells were significantly lower compared to Igsf8+/+ leukemic cells, but not in normal hematopoietic stem and progenitor cells (Figure 1D). These results suggest that Igsf8 might be critical for myeloid leukemia maintenance via Wnt/β-catenin signaling pathway. Then, we investigated the effects of IGSF8 on human myeloid leukemia. We confirmed IGSF8 expression in several human myeloid leukemia cell line and primary patient-derived leukemia cells. Knockdown of IGSF8 by small hairpin RNA in myeloid leukemia cell lines (THP-1, MV4-11, SKM-1, and K562) and primary patient-derived AML cells exhibited reduced numbers of colony forming cells in vitro. Knockdown of IGSF8 also caused decrease expression of β-CATENIN in AML cell lines. These results indicate that IGSF8 is also required for propagation of human myeloid leukemia cells. Taken together, our present study reveals that Igsf8 is indispensable for myeloid leukemia, but not adult hematopoiesis, suggesting that IGSF8 inhibition should be considered for targeting myeloid leukemia. Disclosures Jimbo: Japan Society for the Promotion of Science: Research Funding. Konuma:SGH Foundation: Research Funding; The Japanese Society of Hematology: Research Funding; Institute for Frontier Life and Medical Sciences, Kyoto University: Research Funding. Ito:Institute for Frontier Life and Medical Sciences, Kyoto University: Research Funding.

Obesity increases the risk of cancers, including hepatocellular carcinomas (HCC). However, the precise molecular mechanisms through which obesity promotes HCC development are still unclear. Recent studies have shown that gut microbiota may influence liver diseases by transferring its metabolites and components. Here, we show that the hepatic translocation of obesity-induced lipoteichoic acid (LTA), a Gram-positive gut microbial component, promotes HCC development by creating a tumor-promoting microenvironment. LTA enhances the senescence-associated secretory phenotype (SASP) of hepatic stellate cells (HSC) collaboratively with an obesity-induced gut microbial metabolite, deoxycholic acid, to upregulate the expression of SASP factors and COX2 through Toll-like receptor 2. Interestingly, COX2-mediated prostaglandin E2 (PGE2) production suppresses the antitumor immunity through a PTGER4 receptor, thereby contributing to HCC progression. Moreover, COX2 overexpression and excess PGE2 production were detected in HSCs in human HCCs with noncirrhotic, nonalcoholic steatohepatitis (NASH), indicating that a similar mechanism could function in humans. Significance: We showed the importance of the gut–liver axis in obesity-associated HCC. The gut microbiota–driven COX2 pathway produced the lipid mediator PGE2 in senescent HSCs in the tumor microenvironment, which plays a pivotal role in suppressing antitumor immunity, suggesting that PGE2 and its receptor may be novel therapeutic targets for noncirrhotic NASH-associated HCC. Cancer Discov; 7(5); 522–38. ©2017 AACR. This article is highlighted in the In This Issue feature, p. 443

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.

Background: Myelodysplastic Syndrome (MDS) is a clonal bone marrow disorder characterized by ineffective and clonal hematopoiesis accompanied by morphological dysplasia and variable cytopenia. There are few treatment options for MDS, and allogenic hematopoietic stem cell transplantation is the only curative option. Tubulin belongs to protein superfamily of globular proteins. Monomeric tubulin can polymerize into microtubules, which play an important role in the attachment and segregation of chromosomes in various phases of cell division. Therefore, the targeting of microtubules represents a therapeutic strategy against both solid and hematological cancers. The first approved tubulin binding agent by the FDA was vincristine, which has been clinically used to treat multiple types of cancers, particularly hematological malignancies. Over the past few decades, additional tubulin binding agents have been developed and received FDA approval, mostly for applications to cancer therapy. These agents have been classified by their binding sites on tubulin, which influences their roles in the inhibition or stabilization of polymerized microtubules. We herein investigated the efficacy of PTC-028, a novel microtubule polymerization inhibitor for MDS. Method: Anti-MDS efficacy of PTC-028 was studied using human MDS cell lines and primary MDS cells in vitro. The efficacy of PTC-028 was also assessed in a xenograft mouse model of MDS using an MDS cell lines. PTC-028 was synthesized at PTC Therapeutics Inc. Mechanistic studies were conducted via flow cytometry and RNA sequencing. Results: A previous study reported that PTC596 suppressed cell proliferation and induced apoptosis in AML cell lines. Since MDS is regarded as a pre-leukemic stage, we examined the effects of PTC-028, another novel microtubule polymerization inhibitor, on MDS cells. PTC-028 induced a dose-dependent inhibition of cell proliferation on MDS cell lines, such as MDS-L and SKM-1 cells. Caspase 3/7 activity was also significantly induced in MDS-L and SKM-1 cells in the presence of PTC-028, suggesting the induction of apoptotic cell death by PTC-028. We then isolated CD34+ cells from primary MDS BM samples and investigated the efficacy of PTC-028 on primary MDS cells. The efficacy of PTC-028 in CD34+ MDS cells was also confirmed by cell proliferation assays. To enhance the therapeutic benefit of PTC-028 on MDS cells, we investigated synergism between PTC-028 and DNA hypomethylating agents. We treated MDS-L and SKM-1 cells with PTC-028 in combination with DNA hypomethylating agents. After 3 days of culture, cell growth was analyzed by MTS and Annexin V assays. PTC-028 synergized with hypomethylating agents, such as decitabine and azacitidine, to inhibit the growth and induce apoptosis of MDS cells. We assessed the efficacy of PTC-028 in a xenograft mouse model of MDS using an MDS cell line. Recipient mice were treated with PTC-028 for 7 weeks. PTC-028 significantly inhibited the growth of MDS-L cells and prolonged the overall survival of recipient mice. Furthermore, a significant synergistic effect was observed between PTC-028 and decitabine. Mice that received combination therapy showed moderate weight loss 11 days after the initiation of the treatment, but subsequently recovered. Mechanistically, a treatment with PTC-028 induced G2/M arrest followed by apoptotic cell death. We then investigated the effect of PTC-028 on the levels of soluble (unpolymerized) versus polymerized tubulin in MDS-L cells. Cells were treated with PTC-028 and paclitaxel for 4 hours, and cell lysates were then separated into soluble and polymerized fractions by centrifugation. The visualization of tubulin fractions by Western blotting demonstrated that the PTC-028 treatment for 4 hours resulted in the near-complete loss of polymerized microtubules. In contrast, polymerized microtubules increased in cells treated with paclitaxel, which stabilizes microtubules against depolymerization. These result indicates that PTC-028 also acts as a microtubule polymerization inhibitor. Conclusion: Our data reveal a possible chemotherapeutic strategy for PTC-028 and PTC-596 in MDS by disruption of microtubule dynamics as a single agent and in combination with hypomethylating agents. The present study provides a preclinical framework for the clinical evaluation of this promising therapeutic approach to improve outcomes in MDS patients. Disclosures Lennox: PTC therapeutics: Current Employment. Sheedy:PTC therapeutics: Current Employment. Weetall:PTC Therapeutic: Current Employment.

Background Myelodysplastic syndrome (MDS) is a clonal bone marrow disorder characterized by ineffective and clonal hematopoiesis accompanied by morphological dysplasia and variable cytopenia. Although DNA methyltransferase inhibitors have been used as chemotherapeutic agents for high-risk MDS, the disease become uncontrolled in many cases. Dihydroorotate dehydrogenase (DHODH) catalyzes a rate-limiting step in de novo pyrimidine nucleotide synthesis. DHODH inhibition has recently been recognized as a potential new approach for treating acute myeloid leukemia (AML) by inducing differentiation. We herein investigated the efficacy of PTC299, a novel DHODH inhibitor, for MDS. Methods Anti-MDS efficacy of PTC299 was studied using human MDS cell lines and primary MDS cells in vitro. In addition, we assessed the efficacy of PTC299 in xenograft mouse models of MDS by AkaBLI system, which is composed of AkaLumine-HCl and Akaluc. AkaBLI system has improved performance by a factor of 100 to 1,000 over the conventional bioluminescence imaging system composed of D-luciferin and firefly luciferase. Mechanistic studies were conducted via LC-MS/MS, cell cycle analysis, RNA-seq and RT-qPCR. Results In contrast to AML cells, PTC299 was inefficient at inhibiting growth and inducing the differentiation of MDS cells, but synergized with hypomethylating agents to inhibit the growth of MDS cells. As a single agent, PTC299 prolonged the survival of mice in xenograft models using MDS cell lines, and was more potent in combination with decitabine. Of interest, PTC299 enhanced the incorporation of decitabine, an analog of cytidine, into DNA by inhibiting pyrimidine production, thereby enhancing the cytotoxic effects of decitabine. Conclusions Our results indicate that the DHODH inhibitor PTC299 suppresses the growth of MDS cells and acts in a synergistic manner with decitabine.