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Hepatocellular carcinoma (HCC) is a leading cause of cancer mortality worldwide with no clinically confirmed oncogenic driver. Although preclinical studies implicate the FGF19 receptor FGFR4 in hepatocarcinogenesis, the dependence of human cancer on FGFR4 has not been demonstrated. Fisogatinib (BLU-554) is a potent and selective inhibitor of FGFR4 and demonstrates clinical benefit and tumor regression in patients with HCC with aberrant FGF19 expression. Mutations were identified in the gatekeeper and hinge-1 residues in the kinase domain of FGFR4 upon disease progression in 2 patients treated with fisogatinib, which were confirmed to mediate resistance in vitro and in vivo. A gatekeeper-agnostic, pan-FGFR inhibitor decreased HCC xenograft growth in the presence of these mutations, demonstrating continued FGF19–FGFR4 pathway dependence. These results validate FGFR4 as an oncogenic driver and warrant further therapeutic targeting of this kinase in the clinic. Significance: Our study is the first to demonstrate on-target FGFR4 kinase domain mutations as a mechanism of acquired clinical resistance to targeted therapy. This further establishes FGF19–FGFR4 pathway activation as an oncogenic driver. These findings support further investigation of fisogatinib in HCC and inform the profile of potential next-generation inhibitors. See related commentary by Subbiah and Pal, p. 1646. This article is highlighted in the In This Issue feature, p. 1631

Background: Cyclin dependent kinases (CDK) comprise a family of proteins which are activated upon cyclin binding to promote cell cycle progression. Historically, CDK family inhibitors have had limited utility in the clinic due to 1) toxicities associated with broad spectrum CDK inhibition; and 2) varied and unpredictable efficacy due to lack of a patient selection strategy. The recent approval of three selective CDK4/6 drugs (palbociclib, ribociclib and abemaciclib) has revived the pursuit of therapies against a related interphase kinase, CDK2. Cyclin E gene (CCNE) alterations are prevalent in cancers with high unmet medical need and are emerging as a potential mechanism of clinical resistance to targeted therapies (e.g. CDK4/6 therapies). Methods: Here we report preclinical validation leading to the development of a selective CDK2 inhibitor for the treatment of cancers harboring CCNE alterations. We highlight advanced, orally bioavailable compounds exhibiting single-digit low nanomolar biochemical and cellular potency (pRB), exquisite CDK family selectivity (CDK1, 4, 6, 7 and 9), and whole kinome selectivity. We have characterized the phenotypic and mechanistic consequences of targeting CDK2, employing both genetic and pharmacologic methods across cell lines and xenograft models (e.g. OVCAR-3, MKN-1, HCC1569). Results: In a panel of cell lines, CCNE1-amplified lines exhibited profound sensitivity to selective CDK2 inhibition by halting cells at the G1/S phase of the cell cycle. Upon further inspection, CDK2 inhibition induced multiple markers of senescence, e.g. SA-b-gal and IL-6, in CCNE1-amplified but not in CCNE non-amplified cell lines. Consistent with this, these compounds exhibit robust anti-tumor activity in multiple CCNE1-amplified xenograft models, which was sustained after removal of treatment with these compounds. Conclusions: These data provide a strong rationale for advancing this class of compounds toward clinical development in CCNE-altered cancers. Citation Format: Yoon J. Choi, Steve Wenglowsky, Victoria Brown, Neil Bifulco, Yeon S. Choi, Jian Guo, Megan Hatlen, Joseph Kim, Tim LaBranche, Riadh Lobbardi, Emanuele Perola, Emily Rozsahegyi, Michelle Maynard, Phil Ramsden, Grace Silva, Faith Stevison, Richard Vargas, Ruduan Wang, Doug Wilson, Rich Woessner, Dean Zhang, Rob Meissner, Klaus Hoeflich, Marion Dorsch. Development of a selective CDK2-E inhibitor in CCNE driven cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1279.

When on-target resistance mutations occur in response to potent and selective therapeutics, the target is often considered a validated driver of disease. Hepatocellular carcinoma (HCC) is a leading cause of cancer mortality worldwide whose drivers remain unvalidated because approved therapies utilize multi-kinase inhibitors or immune modulation. In this study, we validate for the first time a driver of disease in the FGF19 expressing subset of HCC through the identification of Fisogatinib resistance mutations in FGFR4. Fisogatinib (formerly known as BLU-554), an investigational agent, was developed to specifically inhibit the tyrosine kinase activity of FGFR4. Approximately one third of HCC patients harbor aberrant expression of fibroblast growth factor 19 (FGF19), a ligand hypothesized to drive paracrine FGFR4 activation and enhance mitogenic signaling in HCC cells. The identification of FGF19 amplifications in a subset of HCC patients through genomic profiling is further suggestive of the involvement of the FGF19/FGFR4 signaling axis in oncogenesis. The first-in-human, phase I study of Fisogatinib in patients with advanced HCC (NCT02508467) suggests that expression of FGF19 as assessed by immunohistochemistry is responsible for oncogenesis and predictive of response to treatment. We identified FGFR4 gatekeeper (V550) and hinge-1 (C552) mutations upon disease progression in two patients treated with Fisogatinib. These resistance mutations had been predicted by our structure-based studies and were also identified through in vitro and in vivo Fisogatinib resistance screens. We confirm by liquid chromatography tandem mass-spectrometry that Fisogatinib binding to FGFR4 is prevented when either the gatekeeper or the hinge-1 mutation is present and demonstrate the capability of these mutations to mediate resistance to Fisogatinib through genetic engineering. Using a gatekeeper-agnostic pan-FGFR inhibitor we show, in preclinical models, continued FGF19/FGFR4 pathway dependence. These results validate FGF19/FGFR4 as an oncogenic driver and inform the profile of potential next-generation inhibitors designed to facilitate improved patient outcomes. Citation Format: Megan A Hatlen, Oleg Schmidt-Kittler, Cori-Ann Sherwin, Emily Rozsahegyi, Nooreen Rubin, Michael Sheets, Joseph L Kim, Chandra Miduturu, Neil Bifulco, Natasja Brooijmans, Hongliang Shi, Timothy Guzi, Andy Boral, Christoph Lengauer, Marion Dorsch, Richard D Kim, Yoon-Koo Kang, Beni B Wolf, Klaus P Hoeflich. Identification of resistance mechanisms to FGFR4 targeted therapy in hepatocellular carcinoma [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A118. doi:10.1158/1535-7163.TARG-19-A118

Specific combinations of acute myeloid leukemia (AML) disease alleles, including FLT3 and TET2 mutations, confer distinct biologic features and adverse outcome. We generated mice with mutations in Tet2 and Flt3, which resulted in fully penetrant, lethal AML. Multipotent Tet2(-/-);Flt3(ITD) progenitors (LSK CD48(+)CD150(-)) propagate disease in secondary recipients and were refractory to standard AML chemotherapy and FLT3-targeted therapy. Flt3(ITD) mutations and Tet2 loss cooperatively remodeled DNA methylation and gene expression to an extent not seen with either mutant allele alone, including at the Gata2 locus. Re-expression of Gata2 induced differentiation in AML stem cells and attenuated leukemogenesis. TET2 and FLT3 mutations cooperatively induce AML, with a defined leukemia stem cell population characterized by site-specific changes in DNA methylation and gene expression.

Multiple myeloma is a plasma cell neoplasm with an extremely variable clinical course. Animal models are needed to better understand its pathophysiology and for preclinical testing of potential therapeutic agents. Hematopoietic cells expressing the hypermorphic Rad50s allele show hematopoietic failure, which can be mitigated by the lack of a transcription factor, Mef/Elf4. However, we find that 70% of Mef−/−Rad50s/s mice die from multiple myeloma or other plasma cell neoplasms. These mice initially show an abnormal plasma cell proliferation and monoclonal protein production, and then develop anemia and a decreased bone mineral density. Tumor cells can be serially transplanted and according to array CGH and whole exome sequencing, the pathogenesis of plasma cell neoplasms in these mice is not linked to activation of a specific oncogene, or inactivation of a specific tumor suppressor. This model recapitulates the systemic manifestations of human plasma cell neoplasms, and implicates cooperativity between the Rad50s and Mef/Elf4 pathways in initiating myelomagenic mutations that promote plasma cell transformation.

The chromosomal translocations found in acute myelogenous leukemia (AML) generate oncogenic fusion transcription factors with aberrant transcriptional regulatory properties. Although therapeutic targeting of most leukemia fusion proteins remains elusive, the posttranslational modifications that control their function could be targetable. We found that AML1-ETO, the fusion protein generated by the t(8;21) translocation, is acetylated by the transcriptional coactivator p300 in leukemia cells isolated from t(8;21) AML patients, and that this acetylation is essential for its self-renewal-promoting effects in human cord blood CD34(+) cells and its leukemogenicity in mouse models. Inhibition of p300 abrogates the acetylation of AML1-ETO and impairs its ability to promote leukemic transformation. Thus, lysine acetyltransferases represent a potential therapeutic target in AML.

SummaryThe JAK2V617F constitutively activated tyrosine kinase is found in most patients with myeloproliferative neoplasms. While examining the interaction between JAK2 and PRMT5, an arginine methyltransferase originally identified as JAK-binding protein 1, we found that JAK2V617F (and JAK2K539L) bound PRMT5 more strongly than did wild-type JAK2. These oncogenic kinases also acquired the ability to phosphorylate PRMT5, greatly impairing its ability to methylate its histone substrates, and representing a specific gain-of-function that allows them to regulate chromatin modifications. We readily detected PRMT5 phosphorylation in JAK2V617F-positive patient samples, and when we knocked down PRMT5 in human CD34+ cells using shRNA, we observed increased colony formation and erythroid differentiation. These results indicate that phosphorylation of PRMT5 contributes to the mutant JAK2-induced myeloproliferative phenotype.

Inhibitor of DNA binding 1 (Id1) functions as an E protein inhibitor, and overexpression of Id1 is seen in acute myeloid leukemia (AML) patients. To define the effects of Id1 on leukemogenesis, we expressed MLL-AF9 in fetal liver (FL) cells or bone marrow (BM) cells isolated from wild-type, Id1(-/-), p21(-/-), or Id1(-/-)p21(-/-) mice, and transplanted them into syngeneic recipient mice. We found that although mice receiving MLL-AF9-transduced FL or BM cells develop AML, loss of Id1 significantly prolonged the median survival of mice receiving FL cells but accelerated leukemogenesis in recipients of BM cells. Deletion of Cdkn1a (p21), an Id1 target gene, can rescue the effect of Id1 loss in both models, suggesting that Cdkn1a is a critical target of Id1 in leukemogenesis. It has been suggested that the FL transplant model mimics human fetal-origin (infant) MLL fusion protein (FP)-driven leukemia, whereas the BM transplantation model resembles postnatal MLL leukemia; in fact, the analysis of clinical samples from patients with MLL-FP(+) leukemia showed that Id1 expression is elevated in the former and reduced in the latter type of MLL-FP(+) AML. Our findings suggest that Id1 could be a potential therapeutic target for infant MLL-AF9-driven leukemia.

Hatlen et al. provide an integrative analysis of the mutational landscape of mouse and human AML and identify functionally relevant cooperation between AML1-ETO and PTPN11 D61Y. Based on these findings, they generate a novel mouse model of t(8;21)+ AML., t(8;21) is one of the most frequent chromosomal abnormalities observed in acute myeloid leukemia (AML). However, expression of AML1-ETO is not sufficient to induce transformation in vivo. Consistent with this observation, patients with this translocation harbor additional genetic abnormalities, suggesting a requirement for cooperating mutations. To better define the genetic landscape in AML and distinguish driver from passenger mutations, we compared the mutational profiles of AML1-ETO–driven mouse models of leukemia with the mutational profiles of human AML patients. We identified TET2 and PTPN11 mutations in both mouse and human AML and then demonstrated the ability of Tet2 loss and PTPN11 D61Y to initiate leukemogenesis in concert with expression of AML1-ETO in vivo. This integrative genetic profiling approach allowed us to accurately predict cooperating events in t(8;21)+ AML in a robust and unbiased manner, while also revealing functional convergence in mouse and human AML.

The AML1-ETO fusion transcription factor is generated by the t(8;21) translocation, which is present in approximately 4%-12% of adult and 12%-30% of pediatric acute myeloid leukemia (AML) patients. Both human and mouse models of AML have demonstrated that AML1-ETO is insufficient for leukemogenesis in the absence of secondary events. In this review, we discuss the pathogenetic insights that have been gained from identifying the various events that can cooperate with AML1-ETO to induce AML in vivo. We also discuss potential therapeutic strategies for t(8;21) positive AML that involve targeting the fusion protein itself, the proteins that bind to it, or the genes that it regulates. Recently published studies suggest that a targeted therapy for t(8;21) positive AML is feasible and may be coming sometime soon.

t(8;21) is the most frequent chromosomal abnormality in acute myeloid leukemia (AML), occurring in 4-12% of adult and 12-30% of pediatric patients. This translocation fuses the N-terminus of AML1 to nearly the entire coding region of ETO, resulting in expression of the fusion protein AML1-ETO. Observations that mice expressing AML1-ETO develop AML only if treated with mutagenic agents have suggested that AML1-ETO requires cooperating disease alleles for leukemogenesis. Consistent with this, t(8;21)+ AML patients harbor multiple genetic abnormalities. Recent exome/genome sequencing studies have expanded the number of known mutations in t(8;21)+ AML patients; however, efforts to distinguish driver from passenger mutations have yielded few cooperative events and the requirements for AML1-ETO leukemogenesis remain largely unknown. To better define the genetic landscape in AML and distinguish driver from passenger mutations, we compared the mutational profiles of two specific AML1-ETO driven mouse models of leukemia to the mutational profiles of human AML patients. We found that the mouse models of AML1-ETO driven AML were phenotypically similar in terms of their extensive latency, myeloid progenitor immunophenotype, and the acquired secondary disease alleles. The first model relies upon the expression of AML1-ETO in transplanted p21 null cells, while the second model relies upon the expression of AML1-ETO9a, a splice variant of AML1-ETO, in transplanted wild type cells. p21 is neither disrupted, nor methylated in t(8;21)+ AML. Because loss of p21 prevents the repair of damaged DNA, leukemogenesis may occur in this model once a cooperating disease allele has been naturally acquired in an AML1-ETO positive hematopoietic progenitor. AML1-ETO9a itself deregulates the expression of several DNA repair genes, suggesting that AML1-ETO9a could similarly facilitate the acquisition of a cooperating disease allele. When we compared the mutational landscape of these murine leukemias to AML patients, we found that the murine leukemias enrich for disease alleles present in human AML (hypergeometric p ≤ 4.26x10-20) and that there is a significant tendency for disease alleles mutated in both species to possess mutations in the same protein domain (hypergeometric p ≤ 4.23x10-3). Furthermore, domains mutated in both species were affected by recurrent mutations (Spearman correlation of domain p-values r = 0.53, p ≤ 2.73x10-8). While the frequency with which various protein classes were affected by mutations was significantly different in MLL-AF9 and AML1-ETO/AML1-ETO9a positive murine AML compared to MLL-fusion and t(8;21)+ positive human AML (p = 0.049), the protein classes targeted in AML1-ETO/AML1-ETO9a murine AML vs. human t(8;21)+ AML were not significantly different (p = 0.327). To identify disease alleles capable of cooperating with AML1-ETO, we determined that of the 424 genes mutated in both species, 38 of those genes were significantly mutated in human AML (Genome MuSiC SMG FDR ≤ 30%). These 38 genes represented 45 mouse orthologues, 38 of which were significantly mutated in AML1-ETO driven murine leukemias (FDR ≤ 10%). These 38 orthologues corresponded to 32 human orthologues, 3 of which were annotated in COSMIC as cancer-related genes: TET2, PTPN11, and THRAP3. Using retroviral transduction and transplantation experiments, we demonstrated that the expression of AML1-ETO in transplanted Tet2 null cells or PTPN11 D61Y cells was sufficient for leukemogenesis. At euthanasia, mice exhibited leukocytosis, anemia, thrombocytopenia, splenomegaly, and an expansion in the myeloid progenitor compartment. Our identification of Tet2 loss as a cooperating allele implicates mutations in epigenetic regulators as potential driving events in t(8;21)+ AML, while the discovery of PTPN11 D61Y solidifies the role of constitutive MAPK signaling in t(8;21)+ AML. This integrative genetic profiling approach allowed us to accurately predict cooperating events in t(8;21)+ AML in a robust and unbiased manner, while also revealing functional convergence in mouse and human AML. Collectively, these findings illustrate the power of integrating murine and human genomic profiling to identify functionally relevant disease alleles in AML. Disclosures Levine: CTI BioPharma: Membership on an entity's Board of Directors or advisory committees; Loxo Oncology: Membership on an entity's Board of Directors or advisory committees; Foundation Medicine: Consultancy.

The shift in sites of hematopoiesis during embryonic development leads to primitive hematopoiesis within the fetal liver at E12.5, which generates primarily erythrocytes. Definitive hematopoiesis, which occurs within the bone marrow, follows at birth. During the erythroid differentiation of fetal hematopoiesis, progenitor cell maturation is accompanied by global DNA demethylation, a process necessary for fetal liver erythrocyte formation and accompanied by diminishing expression of the de novo DNA methyltransferases, Dnmt3a and Dnmt3b. In our current study of hematopoietic cell specific Prmt5 knockout mice, we have identified Prmt5 as a master regulator of erythropoiesis; the cell-specific deletion of Prmt5 in fetal liver cells is embryonic lethal as Prmt5-null embryos have severe anemia and increased expression of Dnmt3a and Dnmt3b proteins. RNA-seq and pathway analysis studies revealed profound defects in several critical pathways that regulate normal hematopoiesis, including the tumor suppressor p53 pathway. Methyl-seq studies are currently being conducted to determine the effects of the enforced expression of key DNA methyltransferases on global DNA methylation and gene expression in erythroid progenitors and how it leads to a block in erythrocyte maturation. To decipher the extent of Dnmt3a or p53's involvement in the observed phenotypes, we have generated double knockout mouse models that are being analyzed. Mechanistically, p53 has been shown to be directly methylated by Prmt5, a modification that affects its tumor suppressor activity. Here we have found that Dnmt3a is also a substrate of Prmt5 and the effects of the di methylation of Dnmt3a on its function are currently under investigation. Thus, we have uncovered a potential functional interaction between DNA methylation and protein arginine methylation triggered by Prmt5 that regulates primitive erythropoiesis. Disclosures Levine: Foundation Medicine: Consultancy; CTI BioPharma: Membership on an entity's Board of Directors or advisory committees; Loxo Oncology: Membership on an entity's Board of Directors or advisory committees.

In this report we review the current knowledge of the interaction of RUNX1(AML1) with serine/threonine kinases, lysine and arginine methyltransferases, lysine acetyltransferases, and histone deacetylases. We also discuss the effect of RUNX1-ETO fusion gene on DNA methylation. RUNX1 post-transcriptional modification can affect its role in influencing differentiation and self-renewal of hematopoietic cells. The goal of these studies is to develop targets for improved leukemia therapy.

Abstract 2822 The JAK2 V617F mutation is the most prevalent genetic abnormality in Philadelphia chromosome negative myeloproliferative neoplasia. We recently discovered that JAK2 V617F phosphorylates and inactivates PRMT5, a type II arginine methyltransferase that promotes transcriptional repression. To evaluate the PRMT5 dependent and independent effects of JAK2 V617F, we performed microarray analysis of human erythroleukemia cells (homozygous for JAK2 V617F) treated with a JAK2 inhibitor or transduced with PRMT5 shRNA. We discovered 5 autophagy-related genes (ATG3, ATG7, ATG2B, ATG5, and ATG16L1) that were positively regulated by JAK2 V617F, but negatively regulated by PRMT5. PRMT5 was bound to the promoters of these genes and binding was enhanced when PRMT5 was re-activated by a JAK2 inhibitor. Sequence analysis of ATG promoters identified at least one potential GATA-1 binding site in each ATG gene. Yet, knock-down of GATA-1 decreased the expression of ATG3 (by approximately 50%), but not the other ATG genes. ChIP analysis identified binding of both GATA-1 and PRMT5 to the ATG3 promoter, which suggests that ATG3 is transcriptionally induced by GATA-1 when PRMT5 is inactivated by JAK2 V617F. But when PRMT5 was active, it was bound to the GATA-1 promoter and ATG3 expression was downregulated. As JAK2 V617F (but not wild type) induces ATG3 expression and autophagy in AML cells and ATG3 levels are elevated in CD34+ cells from polycythemia vera patients, we conclude that JAK2 V617F promotes autophagy by inactivating PRMT5 and promoting GATA-1-dependent transcription of ATG3. Targeting autophagy with JAK2 inhibitors or other agents may be efficacious in the treatment of myeloproliferative neoplasia. Disclosures: No relevant conflicts of interest to declare.

Abstract 794 Background: The cytoplasmic, non-receptor JAK2 tyrosine kinase is mutated at amino acid residue 617 (from valine to phenylalanine) in most patients with myeloproliferative neoplasms (MPNs), resulting in a constitutively activated kinase that phosphorylates STAT proteins in the absence of upstream signals. Overexpression of JAK2V617F leads to cytokine-independent growth of Ba/F3 cells and the JAK2V617F transgenic and knockin mice develop a disease phenotype resembling human polycythemia vera. Results: We hypothesized that the JAK2V617F occurs so consistently in MPNs because it gains some functional property. The type II arginine methyltransferase PRMT5 was initially identified because of its interaction with JAK2 in a yeast two hybrid screen. We examined the interaction between JAK2 and PRMT5 and found that JAK2V617F and JAK2K539L (another active JAK2 kinase) bound PRMT5 more strongly than did wild-type JAK2. PRMT5 mediates the symmetrical dimethylation of arginine residues within histones H2A and H4 and methylates other cellular proteins as well, such as p53. The oncogenic forms of JAK2 acquire the ability to phosphorylate PRMT5, which greatly impaired its methyltransferase activity. We have shown the in vivo importance of this post-translation modification as treating JAK2V617F-positive cells (but not the wild-type JAK2-harboring cells) with different JAK2 inhibitors significantly increased histone arginine methylation levels. To define the effect of inhibiting PRMT5 activity on hematopoiesis, we knocked down PRMT5 in human cord blood derived CD34+ cells using shRNA and observed increased colony formation and erythroid differentiation; In contrast, PRMT5 overexpression in these cells led to reduced colony formation and inhibition of erythroid differentiation. Furthermore, overexpression of PRMT5, especially a phosphorylation site mutant form of PRMT5 (PRMT5M6), diminishes the proliferative and erythroid generating capacity of JAK2V617F+ CD34+ cells isolated from MPN patients to a greater degree than normal cord blood CD34+ cells. Importantly, we found marked increase in PRMT5 phosphorylation in JAK2V617F-positive MPN patents relative to normal cord blood CD34+ cells, suggesting that this phosphorylation is important for the myeloproliferation phenotype. Conclusion: we show that the oncogenic mutant forms of JAK2 kinase, such as JAK2V617F and JAK2K539L, are not simply constitutively active forms of wild-type JAK2, rather they have specific gains-of-function that allow them to phosphorylate PRMT5 and down-regulate its enzymatic activity. Inhibition of PRMT5 contributes to the myeloproliferation and erythroid differentiation promoting effects of JAK2V617F. This gain-of –function mutation results in cross-talk between oncogenic kinases and histone arginine methylation. Taken together, we demonstrate a novel link between the mutant JAK2 kinases and PRMT5 methyltransferase activity, which contributes to MPN pathogenesis. Further insights about the shared gene expression profile of JAK2 inhibition vs. PRMT5 knockdown will be presented to understand the basics for the behavior change in hematopoietic stem/progenitor cells brought about by these two interventions. Disclosures: No relevant conflicts of interest to declare.

Abstract 1588 Transcription factors and histones are similarly modified through acetylation, phosphorylation, ubiquitination and methylation, which impact on the transcriptional regulation of gene expression and various biological processes in normal and malignant hematopoiesis. The t(8;21) associated AML1-ETO fusion protein is found in 40% of the FAB M2 subtype of acute myeloid leukemia, but how the post-translational modification of AML1-ETO affects its leukemogenicity is largely unknown. Here we show that AML1-ETO directly interacts with the lysine acetyltransferase, p300, via the region containing NHR1 domain and that p300 can acetylate two lysine residues in AML1-ETO and AML1-ETO (exon 9a) in human and mouse leukemia cells. To understand the biological effects of AML1-ETO acetylation, we used human CD34+ cord blood cells as a preleukemia model. The maintenance of CD34+ cells by the acetylation defective form of AML1-ETO was 5 fold less than with AML1-ETO (p Disclosures: No relevant conflicts of interest to declare.