Yuki Kagiyama

Myelodysplastic syndromes are induced by histone methylation–altering ASXL1 mutations

Recurrent mutations in the gene encoding additional sex combs-like 1 (ASXL1) are found in various hematologic malignancies and associated with poor prognosis. In particular, ASXL1 mutations are common in patients with hematologic malignancies associated with myelodysplasia, including myelodysplastic syndromes (MDSs), and chronic myelomonocytic leukemia. Although loss-of-function ASXL1 mutations promote myeloid transformation, a large subset of ASXL1 mutations is thought to result in stable truncation of ASXL1. Here we demonstrate that C-terminal–truncating Asxl1 mutations (ASXL1-MTs) inhibited myeloid differentiation and induced MDS-like disease in mice. ASXL1-MT mice displayed features of human-associated MDS, including multi-lineage myelodysplasia, pancytopenia, and occasional progression to overt leukemia. ASXL1-MT resulted in derepression of homeobox A9 (Hoxa9) and microRNA-125a (miR-125a) expression through inhibition of polycomb repressive complex 2–mediated (PRC2-mediated) methylation of histone H3K27. miR-125a reduced expression of C-type lectin domain family 5, member a (Clec5a), which is involved in myeloid differentiation. In addition, HOXA9 expression was high in MDS patients with ASXL1-MT, while CLEC5A expression was generally low. Thus, ASXL1-MT–induced MDS-like disease in mice is associated with derepression of Hoxa9 and miR-125a and with Clec5a dysregulation. Our data provide evidence for an axis of MDS pathogenesis that implicates both ASXL1 mutations and miR-125a as therapeutic targets in MDS.

The molecular basis of myeloid malignancies

Myeloid malignancies consist of acute myeloid leukemia (AML), myelodysplastic syndromes (MDS) and myeloproliferative neoplasm (MPN). The latter two diseases have preleukemic features and frequently evolve to AML. As with solid tumors, multiple mutations are required for leukemogenesis. A decade ago, these gene alterations were subdivided into two categories: class I mutations stimulating cell growth or inhibiting apoptosis; and class II mutations that hamper differentiation of hematopoietic cells. In mouse models, class I mutations such as the Bcr-Abl fusion kinase induce MPN by themselves and some class II mutations such as Runx1 mutations induce MDS. Combinations of class I and class II mutations induce AML in a variety of mouse models. Thus, it was postulated that hematopoietic cells whose differentiation is blocked by class II mutations would autonomously proliferate with class I mutations leading to the development of leukemia. Recent progress in high-speed sequencing has enabled efficient identification of novel mutations in a variety of molecules including epigenetic factors, splicing factors, signaling molecules and proteins in the cohesin complex; most of these are not categorized as either class I or class II mutations. The functional consequences of these mutations are now being extensively investigated. In this article, we will review the molecular basis of hematological malignancies, focusing on mouse models and the interfaces between these models and clinical findings, and revisit the classical class I/II hypothesis.

Hes1 promotes blast crisis in chronic myelogenous leukemia through MMP-9 upregulation in leukemic cells

High levels of HES1 expression are frequently found in BCR-ABL(+) chronic myelogenous leukemia in blast crisis (CML-BC). In mouse bone marrow transplantation (BMT) models, co-expression of BCR-ABL and Hes1 induces CML-BC-like disease; however, the underlying mechanism remained elusive. Here, based on gene expression analysis, we show that MMP-9 is upregulated by Hes1 in common myeloid progenitors (CMPs). Analysis of promoter activity demonstrated that Hes1 upregulated MMP-9 by activating NF-κB. Analysis of 20 samples from CML-BC patients showed that MMP-9 was highly expressed in three, with two exhibiting high levels of HES1 expression. Interestingly, MMP-9 deficiency impaired the cobblestone area-forming ability of CMPs expressing BCR-ABL and Hes1 that were in conjunction with a stromal cell layer. In addition, CMPs expressing BCR-ABL and Hes1 secreted MMP-9, promoting the release of soluble Kit-ligand (sKitL) from stromal cells, thereby enhancing proliferation of the leukemic cells. In accordance, mice transplanted with CMPs expressing BCR-ABL and Hes1 exhibited high levels of sKitL as well as MMP-9 in the serum. Importantly, MMP-9 deficiency impaired the development of CML-BC-like disease induced by BCR-ABL and Hes1 in mouse BMT models. The present results suggest that Hes1 promotes the development of CML-BC, partly through MMP-9 upregulation in leukemic cells.

Upregulation of CD200R1 in lineage-negative leukemic cells is characteristic of AML1-ETO-positive leukemia in mice

Activating mutations of c-Kit are frequently found in acute myeloid leukemia (AML) patients harboring t(8;21) chromosomal translocation generating a fusion protein AML1-ETO. Here we show that an active mutant of c-Kit cooperates with AML1-ETO to induce AML in mouse bone marrow transplantation models. Leukemic cells expressing AML1-ETO with c-KitD814V were serially transplantable. Transplantation experiments indicated that lineage−c-Kit+Sca-1+ (KSL) leukemic cells, but not lineage+ leukemic cells, were enriched for leukemia stem cells (LSCs). Comparison of gene expression profiles between KSL leukemic and normal cells delineated that CD200R1 was highly expressed in KSL leukemic cells as compared with KSL normal cells. Upregulation of CD200R1 was verified in lineage− leukemic cells, but not in lineage+ leukemic cells. CD200R1 expression in the lineage− leukemic cells was not correlated with the frequency of LSCs, indicating that CD200R1 is not a useful marker for LSCs in these models. Interestingly, CD200R1 was upregulated in KSL cells transduced with AML1-ETO, but not with other leukemogenic mutants, including c-KitD814V, AML1D171N, and AML1S291fsX300. Consistently, upregulation of CD200R1 in lineage− leukemic cells was observed only in the BM of mice suffering from AML1-ETO-positive leukemia. In conclusion, AML1-ETO upregulated CD200R1 in lineage− cells, which was characteristic of AML1-ETO-positive leukemia in mice.

Fyn is not essential for Bcr-Abl-induced leukemogenesis in mouse bone marrow transplantation models

The Bcr-Abl oncogene causes human Philadelphia chromosome-positive (Ph+) leukemias, including B-cell acute lymphoblastic leukemia (B-ALL) and chronic myeloid leukemia (CML) with chronic phase (CML-CP) to blast crisis (CML-BC). Previous studies have demonstrated that Src family kinases are required for the induction of B-ALL, but not for CML, which is induced by Bcr-Abl in mice. In contrast, it has been reported that Fyn is up-regulated in human CML-BC compared with CML-CP, implicating Fyn in the blast crisis transition. Here, we aimed to delineate the exact role of Fyn in the induction/progression of Ph+ leukemias. We found that Fyn is expressed in mouse hematopoietic cells at varying stages of development, including c-kit+Sca-1+Lin− cells. Notably, Fyn is highly expressed in some of human lymphomas, but not in human Ph+ leukemias including CML-BC. In mouse bone marrow transplantation models, mice transplanted with wild-type or Fyn-deficient bone marrow cells transduced with Bcr-Abl showed no differences in the development of B-ALL or CML-like diseases. Similarly, Fyn deficiency failed to impact the development of myeloid CML-BC induced by Bcr-Abl and Hes1. Elevated expression of Fyn was not found in mouse samples of Bcr-Abl-mediated CML- and CML-BC-like diseases. Thus, Fyn is not required for the pathogenesis of Bcr-Abl-mediated leukemias.

Novel working hypothesis for pathogenesis of hematological malignancies: combination of mutations-induced cellular phenotypes determines the disease (cMIP-DD).

Recent progress in high-speed sequencing technology has revealed that tumors harbor novel mutations in a variety of genes including those for molecules involved in epigenetics and splicing, some of which were not categorized to previously thought malignancy-related genes. However, despite thorough identification of mutations in solid tumors and hematological malignancies, how these mutations induce cell transformation still remains elusive. In addition, each tumor usually contains multiple mutations or sometimes consists of multiple clones, which makes functional analysis difficult. Fifteen years ago, it was proposed that combination of two types of mutations induce acute leukemia; Class I mutations induce cell growth or inhibit apoptosis while class II mutations block differentiation, co-operating in inducing acute leukemia. This notion has been proven using a variety of mouse models, however most of recently found mutations are not typical class I/II mutations. Although some novel mutations have been found to functionally work as class I or II mutation in leukemogenesis, the classical class I/II theory seems to be too simple to explain the whole story. We here overview the molecular basis of hematological malignancies based on clinical and experimental results, and propose a new working hypothesis for leukemogenesis.

Abstract 4672: Novel epigenetic approach to relapsed mantle cell lymphoma based on dual inhibition of EZH1/EZH2

Mantle cell lymphoma (MCL) is a well-defined and aggressive type of B cell non-Hodgkin’s lymphoma that is genetically characterized by the t(11;14)(q13;q32) chromosomal translocation, which results in constitutive overexpression of CYCLIN D1. Although newly developed drugs such as ibrutinib show promising clinical outcomes, relapsed MCL often acquires drug resistance, which is a critical obstacle to treatment. Alternative approaches to overcoming the drug resistance of relapsed MCL are urgently needed. PRC1 and 2 are important epigenetic regulators that maintain the stemness of embryonic and hematopoietic stem cells. EZH1 and 2 are catalytic components of PRC2, which trimethylates histone H3 at lysine 27 (H3K27) to repress transcription of target genes. Mutation and overexpression of EZH1/2 are associated with cancers, including hematopoietic malignancies. Here, we used a novel dual inhibitor of EZH1/2 to show that inhibiting EZH1/2 is a promising therapeutic strategy for MCL. First, we developed a xenograft (PDX) mouse model using cells from a heavily pretreated and relapsed MCL patient, and then orally administered an inhibitor of EZH1/2, called OR-S1. OR-S1 strongly impaired proliferation of the patient-derived tumors and did not cause any serious side effects. Additionally, an in vitro assay using MCL cell lines showed that OR-S1 inhibited the growth of MCL cells, and that the effect was much more significant than that using the single EZH2 inhibitor (GSK126). The IC50 of OR-S1 was about one tenth that of GSK126. These results strongly suggest that dual inhibition of EZH1/2 could be a promising therapeutic strategy for relapsed MCL. Next, to investigate the effect induced by dual inhibition of EZH1/2, we conducted further analyses of the MCL cell lines. Cells exposed to OR-S1 showed cell cycle arrest (G1 arrest) along with a dose-dependent reduction in phospho-Rb and cell differentiation, coupled with increased cell surface expression of hCD138. We then used RNA-seq analysis of MCL cell lines to compare OR-S1-treated cells with vehicle-treated cells and found that cell cycle-related signaling was significantly affected and that a cyclin-dependent kinase inhibitor, CDKN1C (TP57), was one of the genes most markedly upregulated by OR-S1. ChIP qPCR of MCL cell lines showed that the CDKN1C locus was strongly marked by H3K27 trimethylation, and that OR-S1 induced a significant reduction in the level of this histone marker. Furthermore, administration of OR-S1 alone to PDX mice induced increased expression of CDKN1C (as in the in vitro assay). Thus, dual inhibition of EZH1/2 in MCL induces expression of CDKN1C, which in turn causes cell cycle arrest and reduced growth of MCL. Taken together, these results strongly suggested that dual inhibition of EZH1 and EZH2 is a promising therapeutic strategy for MCL, illustrating the potential of novel epigenetic approaches to overcoming drug resistance of relapsed MCL. Citation Format: Shuhei Fujita, Yuki Kagiyama, Daisuke Honma, Nobuaki Adachi, Kazushi Araki, Issay Kitabayashi. Novel epigenetic approach to relapsed mantle cell lymphoma based on dual inhibition of EZH1/EZH2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4672. doi:10.1158/1538-7445.AM2017-4672