ALBERT

Description: Key Points The shortest isoform of C/EBPβ, liver inhibitory protein (LIP), collaborates with Evi1 in leukemogenesis.

Description: Evi1 (ecotropic viral integration site 1) is essential for proliferation of hematopoietic stem cells and implicated in the development of myeloid disorders. Particularly, high Evi1 expression defines one of the largest clusters in acute myeloid leukemia and is significantly associated with extremely poor prognosis. However, mechanistic basis of Evi1-mediated leukemogenesis has not been fully elucidated. Here, we show that Evi1 directly represses phosphatase and tensin homologue deleted on chromosome 10 (PTEN) transcription in the murine bone marrow, which leads to activation of AKT/mammalian target of rapamycin (mTOR) signaling. In a murine bone marrow transplantation model, Evi1 leukemia showed modestly increased sensitivity to an mTOR inhibitor rapamycin. Furthermore, we found that Evi1 binds to several polycomb group proteins and recruits polycomb repressive complexes for PTEN down-regulation, which shows a novel epigenetic mechanism of AKT/mTOR activation in leukemia. Expression analyses and ChIPassays with human samples indicate that our findings in mice models are recapitulated in human leukemic cells. Dependence of Evi1-expressing leukemic cells on AKT/mTOR signaling provides the first example of targeted therapeutic modalities that suppress the leukemogenic activity of Evi1. The PTEN/AKT/mTOR signaling pathway and the Evi1-polycomb interaction can be promising therapeutic targets for leukemia with activated Evi1.

Description: Functional deregulation of transcription factors has been found in many types of tumors. Transcription factor AML1/RUNX1 is one of the most frequent targets of chromosomal abnormalities in human leukemia and altered function of AML1 is closely associated with malignant transformation of hematopoietic cells. However, the molecular basis and therapeutic targets of AML1-related leukemia are still elusive. Here, we explored immediate target pathways of AML1 by in vitro synchronous inactivation in hematopoietic cells. We found that AML1 inhibits NF-κB signaling through interaction with IκB kinase complex in the cytoplasm. Remarkably, AML1 mutants found in myeloid tumors lack the ability to inhibit NF-κB signaling, and human cases with AML1-related leukemia exhibits distinctly activated NF-κB signaling. Furthermore, inhibition of NF-κB signaling in leukemic cells with mutated AML1 efficiently blocks their growth and development of leukemia. These findings reveal a novel role for AML1 as a cytoplasmic attenuator of NF-κB signaling and indicate that NF-κB signaling is one of the promising therapeutic targets of hematologic malignancies with AML1 abnormality.

Description: Key Points Expression of ITGA2B (CD41) and MPL positively correlates with that of EVI1 in acute myeloid leukemia patients. Thrombopoietin/MPL signaling enhances growth and survival of CD41+ Evi1 leukemia cells with a high leukemia-initiating capacity.

Description: Abstract 1962 Poster Board I-985 Introduction: AML1/Runx1 is one of the most frequent targets of chromosomal abnormalities in human leukemia. Functional impairment of AML1 caused by point mutation is also reported in patients with leukemia or myelodysplastic syndrome (MDS). However, molecular basis for leukemogenesis caused by functional impairment of AML1 is still elusive. In this study, we clarified the deregulated signaling pathway induced by loss of AML1. Results: To find the direct target of AML1, we compared gene expression profile between AML1-conditionally deleted and normal KSL cells using Cre-ER system. Gene set enrichment analysis (GSEA) using molecular signature database (MSigDB) clarified enhanced expression of NF-kB target genes in AML1 deficient cells. In addition, NF-kB inhibitor attenuated the enhanced colony forming activity of bone marrow cells from AML1 conditional knockout (cKO) mice. These data indicate the aberrant activation of NF-kB signaling pathway in stem/progenitor cells of AML1 deficient mice. NF-kB is a transcription factor which is involved in many physiological phenomena including proliferation, survival, and inflammation. Because deregulated activation of NF-kB signaling has been reported to be responsible for many types of tumors including hematological malignancies, we assumed that lack of AML1-mediated suppression of NF-kB signaling lead to malignant transformation of hematopoietic cells. p65, one of the major components of NF-kB stays in cytoplasm with IkB in a steady state. Once receiving stimulating signals from cell surface receptors such as TNF-a receptor, IkB is phosphorylated by IKK complex and subsequently degraded through the ubiquitin-proteasome pathway, resulting in nuclear translocation of p65 and transactivation of NF-kB target genes. First, we found that AML1 inhibits nuclear translocation of p65 and that nuclear localization of p65 is enhanced in AML1 deficient cells, which is cancelled by NF-kB inhibitors. In addition, AML1 inhibited p65 phosphorylation at serine 536, which is important for its activation. We found that AML1 physically interacts with IKK complex and thus suppresses its kinase activity, which accounts for a mechanistic basis for inhibition of NF-kB signaling by AML1. Suppression of IKK kinase activity by AML1 results in inhibition of both nuclear translocation of p65 and activation of NF-kB target genes. Next, we examined how leukemia-related AML1 mutants affect NF-kB signaling. Remarkably, AML1 D171N mutant found in MDS neither inhibited nuclear translocation of p65 nor attenuated the kinase activity of IKK complex. Similar results were obtained with AML1/ETO generated in leukemia with t(8;21). Mouse bone marrow cells immortalized by AML1/ETO showed enhanced nuclear localization of p65 compared with those immortalized by MLL/ENL, another leukemia-related fusion protein. Indeed, AML1/ETO immortalized cells are more sensitive to NF-kB inhibitor-mediated growth suppression, indicating a critical role of NF-kB signaling in transformation by AML1/ETO. To verify the activation of NF-kB signaling by AML1/ETO in human hematopoietic cells, we analyzed the gene expression data reported by Valk et al. in silico. We found that NF-kB signaling is distinctly activated in AML1-related leukemia patients. These results suggest that aberrant activation of NF-kB signaling induced by functional impairment of AML1 may contribute to the development of leukemia via proliferation signals. Conclusions: We found that AML1 is a cytoplasmic attenuator of NF-kB signaling pathway. Functional impairment of AML1 caused by genetic disruption results in distinct activation of NF-kB signaling by altering IKK kinetic activity. This aberrant activation may play a central role in pathogenesis of AML1-related leukemia and MDS. Therefore, NF-kB signaling is one of the attractive candidates for molecular targeted therapy against AML1-related hematological disorders. Disclosures: No relevant conflicts of interest to declare.

Description: To understand myelodysplastic syndromes (MDS) at the molecular level, we used a mouse bone marrow transplant (BMT) model. Several leukemogenic fusion proteins were demonstrated to induce leukemia in mouse BMT models. However, analysis of molecular basis of MDS has been hampered by the lack of mouse MDS models. Mutations of a transcription factor AML-1/Runx-1 were identified in 15–40% of MDS-RAEB and MDS/AML and 5–10% of de novo AML. To test transforming abilities of the mutant AML-1, we transduced AML-1 with different mutations (#5 and #27) into mouse bone marrow cells using our highly efficient retrovirus-mediated gene transfer, transplanted the transduced cells to irradiated mice, and found that most mice developed leukemia within 4–13 months after the transplant (table1 and figure1). The leukemic mice exhibited marked hepato-splenomegaly, and morphological abnormalities of myeloid and erythroid lineages were frequently observed. Some mice developed to leukemia after a long latency with MDS-like symptoms, either with pancytopenia or leukocytosis with anemia, thus mimicking the human disease. To clarify the differences between the leukemic mice with early onsets and those that developed leukemia after a long period of MDS-like symptoms in the mutant AML-1-induced mouse diseases, the integration sites of the transduced retroviruses were investigated using a polymerase chain reaction (PCR)-based technique bubble PCR. Our preliminary results suggested that the integration sites of the retroviruses harboring the mutant AML-1 contribute to the early onset of leukemia. In conclusion, we have developed a useful mouse in vivo model of MDS/AML that should help understand the molecular basis of MDS and progression of MDS to leukemia. penetrance and latency mutation number position form penetrance latency #5 D171N point mutation in RHD 69.6 % 4–13 months #27 S291fsX300 truncation type 100 % 4–9 months Figure Figure

Description: Abstract 1295 Ecotropic viral integration site 1 (Evi1) is a transcription factor which is highly expressed in hematopoietic stem cells and crucial for their self-renewal capacity. Aberrant expression of Evi1 is observed in several types of acute myeloid leukemia (AML), of which the prognosis is generally poor, mainly due to therapeutic resistance. Effective Evi1-targeted therapies, however, have not yet been developed. Therefore, cell surface molecules or their downstream signaling pathways specific to Evi1 high-expressing leukemia cells would be good candidates for novel targeted therapy. Here in this study, we first revealed by gene expression data of AML patients that the expression of several megakaryocytic differentiation markers including CD41, CD61 and thrombopoietin (Thpo) receptor, Mpl was positively correlated to that of Evi1. To validate the association between these surface markers and Evi1, murine c-kit(+) hematopoietic progenitors were transduced with Evi1 or other several leukemia oncogenes and serially replated in methylcellulose semisolid medium. FACS analysis showed that the CD41(+) fraction emerged after third or fourth replating in Evi1-transduced progenitors, but not in progenitors immortalized by MLL-ENL or AML1-ETO. Similarly, the CD41(+) fraction was clearly observed in the bone marrow and spleen mononuclear cells of Evi1-induced leukemia mice. These results indicated the association between Evi1 and CD41 in AML. We next investigated the functional significance of the CD41(+) population in the Evi1-induced leukemia mouse model. The CD41(+) leukemia cells were morphologically more immature than the CD41(-) cells. Interestingly, the CD41(+) cells were highly positive for c-kit, and showed an enhanced colony-forming capacity in semisolid medium than the CD41(-) cells. Moreover, the CD41(+) cells developed AML with shorter latency than the CD41(-) cells in a murine bone marrow transplantation model (p

Description: Myelodysplastic syndrome (MDS) is a hematopoietic stem-cell disorder characterized by trilineage dysplasia and susceptibility to acute myelogenous leukemia (AML). Analysis of molecular basis of MDS has been hampered by the heterogeneity of the disease. Recently, mutations of the transcription factor AML1/RUNX1 have been identified in 15% to 40% of MDS–refractory anemia with excess of blasts (RAEB) and MDS/AML. We performed mouse bone marrow transplantation (BMT) using bone marrow cells transduced with the AML1 mutants. Most mice developed MDS and MDS/AML-like symptoms within 4 to 13 months after BMT. Interestingly, among integration sites identified, Evi1 seemed to collaborate with an AML1 mutant harboring a point mutation in the Runt homology domain (D171N) to induce MDS/AML with an identical phenotype characterized by marked hepatosplenomegaly, myeloid dysplasia, leukocytosis, and biphenotypic surface markers. Collaboration between AML1-D171N and Evi1 was confirmed by a BMT model where coexpression of AML1-D171N and Evi1 induced acute leukemia of the same phenotype with much shorter latencies. On the other hand, a C-terminal truncated AML1 mutant (S291fsX300) induced pancytopenia with erythroid dysplasia in transplanted mice, followed by progression to MDS-RAEB or MDS/AML. Thus, we have developed a useful mouse model of MDS/AML that should help in the understanding of the molecular basis of MDS and the progression of MDS to overt leukemia.

Description: Abstract 1958 Poster Board I-981 Background: Evi1 gene is located on chromosome 3q26 and aberrantly expressed in acute myeloid leukemia (AML) patients with or without 3q26 abnormalities, and inappropriate expression of Evi1 associates with poor prognosis. Evi-1 is originally identified in a common integration site of murine leukemia retrovirus and enhanced expression of Evi1 by retrovirus integration is thought to be responsible for leukemogenesis in mouse models. However, retroviral expression of marker genes such as GFP has not induced leukemia even if some clones possessing integration at Evi1 site have been identified. These data indicate that Evi1 requires cooperative factors to induce progressive leukemia, whereas overexpression of Evi1 is enough to lead to clonal expansion of hematopoietic cells. Therefore, identifying genes collaborating with Evi1 is one of the key issues of understanding Evi1-related leukemogenesis. Recently, we demonstrated that a point mutation of the transcription factor AML1 (AML1-D171N) can induce myelodysplastic syndrome (MDS) that progresses to AML in association with overexpression of Evi1 through a mouse bone marrow transplantation model. In that work, we analyzed mice transplanted with BM cells transduced Evi1 alone as control and surprisingly confirmed that all of the mice developed leukemia within 6-11 months after the transplant. In this report, we will describe interesting findings in the novel mouse model of Evi1-induced leukemia. Result: C57BL6/Ly-5.1 murine BM cells infected with retroviruses harboring Evi1 were transplanted into irradiated syngeneic Ly-5.2 mice. The mice looked fine until 5 months, but GFP-positive-Evi1 expressing cells were gradually increased in the peripheral blood (PB), and then the mice died at 6-11 months after the transplantation. The mice showed dysplastic features in myeloid and erythroid cells, increase of blasts in the PB and the BM, hepatosplenomegaly, slight anemia, and some of the mice showed severe leukocytosis. The mice were thought to die of multiple organ failure due to invasion of leukemic cells not due to anemia. The phenotype is different from that of the mouse BMT model expressing Evi1 by retrovirus reported by another group, in which the mice died about 10 months with severe peripheral cytopenia and finally the disease did not progress to AML. Therefore, we considered that Evi1 might have collaborated with unknown genes near retrovirus integration sites in our case and analyzed integration sites by the bubble PCR method. Interestingly, frequent integration at 3' side of C/EBPb gene was found in six mice out of eight mice transplanted with Evi1-transduced BM cells. The integrations were located at 62.5-86.7kb downstream of C/EBPb gene. Next, we examined the expression level of C/EBPb, Tmem189, and Ptpn1, all of which are located near the integration site, and confirmed that C/EBPb showed elevated expression although neither Tmem189 nor Ptpn1 did. We also identified Bcas1, Rps6ka1, and Rapgef4 genes at the retroviral integration site in the other two mice without integration near C/EBPb. Discussion: C/EBPb, also known as NF-IL6, is a transcription factor that specifically binds to an IL1-responsive element in the IL-6 gene and has a role in regulation not only for the IL-6 gene but also for several cytokine genes such as TNF, IL-8, and G-CSF. The hematopoietic progenitor cells of C/EBPb-deficient mice have been reported to respond imperfectly to GM-CSF and G-CSF. Furthermore, C/EBPb is a downstream target of the Ras-Raf pathway. The locus of C/EBPb gene has been reported as a common integration site in the Retrovirus Tagged Cancer Gene Database (RTCGD), which is a database of retroviral insertional mutagenesis in mouse tumors. AKxD mice, Cdkn2a-KO mice, NUP98/HOXD13 transgenic mice, and MYC/Runx2 transgenic mice were reported to develop myeloid or lymphoid leukemia by retroviral insertion into 3' side of C/EBPb gene. In this study, we identified frequent integration at 3' side of C/EBPb gene in Evi1-transduced leukemic cells, whereas we have not identified this locus in AML1-mutants-transduced leukemic cells. Based on these findings and our results, C/EBPb is supposed to be a candidate gene to collaborate with Evi1 in leukemogenesis. Conclusion: We identified involvement of C/EBPb in Evi1-induced leukemogenesis. The novel mouse model that we generated in this study could help understanding the molecular basis of Evi1-related leukemia. Disclosures: No relevant conflicts of interest to declare.