ALBERT

Description: Abstract 1511 Protein phosphatase 2A (PP2A) is a human tumor suppressor that inhibits cellular transformation by regulating the activity of several signaling proteins critical for malignant cell behavior. Our group has shown that PP2A activity is reduced in acute myeloid leukemia (AML), and that restoration of PP2A activity in AML cells blocks proliferation, induces caspase-dependent apoptosis, and affects AKT and ERK1/2 activity. It has been proposed that FTY720 suppress PP2A activity by directly interacting with PP2AA-C dimers, and possibly interfering with the SET-PP2A interplay. Reactivation of PP2A by FTY720 in vivo restricts leukemogenesis in different types of hematological malignancies; however, its antitumor effects in AML remain underexplored. Here, we developed lipid nanoparticles (LN) encapsulating FTY720, and evaluated its pharmacokinetic profile and biodistribution in mice, and its antileukemic effects in AML in vitro and in vivo. FTY720-LN show suitable size (around 210 nm) and encapsulation efficiency (nearly 75%) for the oral and intravenous administration of the compound. Blood concentration of FTY720 decreased rapidly 2 hours after the intravenous administration of FTY720-LN; however, orally administered LN provided both a sustained release of the drug over four days and a high distribution in tissues, significantly increasing the accumulation of FTY720 in spleen, compared to the administration of the same dose of FTY720 solution and intravenous administration of FTY720-LN. In vitro studies revealed that FTY720 and FTY720-LN restore PP2A phosphatase activity in AML cells, leading to cell growth arrest, induction of caspase-dependent apoptosis and decrease in phosphorylation of PP2A targets AKT and ERK. Moreover, in vivo daily intraperitoneal administration of FTY720 in RAG2−/− γc−/− mice bearing an AML xenograft tumor reduced significant tumor growth. Of note, oral administration of FTY720-LN every three days was as efficient in reducing AML xenograft tumor growth as a daily oral administration of FTY720. These results provide the first evidence for the potential use of lipid nanoparticles encapsulating FTY720 as a therapeutic agent in AML. Disclosures: No relevant conflicts of interest to declare.

Description: INTRODUCTION. The protein SET (I2PP2A), a potent protein phosphatase 2A (PP2A) inhibitor, has been implicated in many cell processes such as DNA replication, chromatin remodeling and gene transcription, differentiation, migration, and cell-cycle regulation. In fact, SET has been described as an oncogene that regulates important signaling pathways. Our group reported that PP2A inhibition is a common event in AML, and that SET is overexpressed in 28% of acute myeloid leukemia (AML) cases, where it is associated with short overall survival. Moreover, the anti-leukemic effects of the FTY720 and OP449 PP2A-activating drugs in AML cells depend on interaction/sequestration of SET. However, despite the importance of SET overexpression and its prognostic impact in both hematological and solid tumors, there are few data about the mechanisms involved in its regulation. AIM. To characterize the functional promoter region of the SET gene, and to identify transcription factors (TFs) involved in its regulation. RESULTS. Luciferase reporter assays with five truncatedconstructs allowed us to determine a 163bp-region as the minimal promoter region of SET that contains consensus sites for several TFs. Chromatin immunoprecipitation (ChIP) assays confirmed the binding of RUNX1, GATA2, MYC, and SP1. RUNX1 and GATA2 are two essential TFs in hematopoiesis, and localized on the SET promoter when the acetylation state of both histone H3 and H4 and the tri-methylation on H3K4 is high, confirming that they both could act as positive regulators of SET transcription. In silico analysis in large series of adult patient samples with de novo AML recently published by The Cancer Genome Atlas Research Network showed a significant positive correlation between SET and RUNX1 and GATA2 at mRNA level. Furthermore, knockdown of RUNX1 and/or GATA2 triggered SET downregulation, whereas only a simultaneous overexpression of these two TFs caused a significant up-regulation of SET. Interestingly, RUNX1 interacts with GATA2 in both HL-60 and HEL cell lines. Moreover, we found that SP1 is also part of this transcription complex. Altogether, these results show that RUNX1 and GATA2, together with SP1, regulate the transcription of the SET gene. CONCLUSIONS We have defined the minimal promoter region of the SET gene, and have demonstrated that RUNX1 and GATA2 regulate its expression in AML. Moreover, our functional studies demonstrate that RUNX1 and GATA2 form a complex with SP1 that activates the transcription of SET in AML cells. This study opens new directions to further understand the mechanisms of SET overexpressing leukemias. Disclosures No relevant conflicts of interest to declare.

Description: Abstract 2402 Poster Board II-379 The MYB proto-oncogene encodes a nuclear transcription factor with an essential role in proliferation, lineage commitment, and differentiation of hematopoietic progenitor cells. Proper levels of MYB are known to be important during hematopoietic cell development, and the Myb gene is a frequent target of retroviral insertions in myeloid, B- and T-cell leukemias in the mouse. Overexpression of MYB in T-acute lymphoblastic leukemia (T-ALL) causes a differentiation block of the T cells, and it has been shown that NOTCH1 mutation and MYB duplication cooperate in the pathogenesis of T-ALL. Our aim was to study the role of MYB in the pathogenesis of acute myeloid leukemia (AML), and to investigate its potential as a target for therapy. We functionally characterized MYB in 15 AML cell lines. Twelve of the 15 cell lines tested had MYB overexpression. Knockdown of MYB by siRNA in these cell lines caused decreased cell viability and proliferation, and reduced the clonogenic capacity, that could be explained in some cell lines by changes on the stage of cell differentiation. These results show that MYB overexpression is involved in the pathogenesis of AML. Moreover, knockdown of MYB in combination with common AML treatments (Idarubicin, Cytarabine and Sorafenib) had a strong synergistic effect on proliferation and viability of cells, suggesting that MYB could be a new target for therapy in AML. These observations prompted us to quantify MYB expression in a cohort of 159 patients with AML at diagnosis. We detected MYB overexpression in 14.5% (23/159) patients, with a higher prevalence within the intermediate prognosis group (17/83, 20.5%), particularly in patients with normal karyotype (NK) (14/62, 22.6%). Interestingly, 33% of patients without FLT-3 ITD and NPM1 mutations had MYB overexpression. To study the prognosis impact of MYB overexpression in AML, we performed a survival analysis in a preliminary series of 100 AML patients at diagnosis. As expected, significant differences in OS according to age, complete remission and cytogenetic prognostic group were found (p

Description: Acute myeloid leukemias (AMLs) result from multiple genetic alterations in hematopoietic stem cells. We describe a novel t(12;18)(p13;q12) involving ETV6 in a patient with AML. The translocation resulted in overexpression of SETBP1 (18q12), located close to the breakpoint. Overexpression of SETBP1 through retroviral insertion has been reported to confer growth advantage in hematopoietic progenitor cells. We show that SETBP1 overexpression protects SET from protease cleavage, increasing the amount of full-length SET protein and leading to the formation of a SETBP1–SET-PP2A complex that results in PP2A inhibition, promoting proliferation of the leukemic cells. The prevalence of SETBP1 overexpression in AML at diagnosis (n = 192) was 27.6% and was associated with unfavorable cytogenetic prognostic group, monosomy 7, and EVI1 overexpression (P 〈 .01). Patients with SETBP1 overexpression had a significantly shorter overall survival, and the prognosis impact was remarkably poor in patients older than 60 years in both overall survival (P = .015) and event-free survival (P = .015). In summary, our data show a novel leukemogenic mechanism through SETBP1 overexpression; moreover, multivariate analysis confirms the negative prognostic impact of SETBP1 overexpression in AML, especially in elderly patients, where it could be used as a predictive factor in any future clinical trials with PP2A activators.

Description: Abstract 1233 GATA2 encodes a transcription factor with essential functions in hematopoiesis. Somatic mutations of GATA2 have been reported in patients with chronic myeloid leukemia (CML) with blast crisis, and in bi-CEBPA-positive acute myeloid leukemia (AML); moreover, our group and others have shown that overexpression of this gene is a recurrent event associated with poor prognosis in AML. Several recent studies report mutations in the GATA2 gene in three different familial syndromes characterized by predisposition to myelodysplastic syndrome (MDS) and AML. Despite some differences, these mutations are very similar, and in some cases identical. This implies that individual mutations, although located in similar regions, may differentially affect GATA2 function. Therefore, additional research is required to explain why similar mutations lead to different syndromes (Hyde and Liu, 2011). On the other hand, it has been extensively studied in murine models that GATA2 activates its own transcription by binding to regions located at −2.8 and −1.8 kb from the transcription start site (TSS). We hypothesized that these reported GATA2 mutations could alter the GATA2 autoregulatory loop, affecting the transcription of GATA2. With this aim, we first aligned the murine and human GATA2 promoters in search for homologous GATA2 binding sites. Regions containing the cis-regulatory elements located at −2.8 and −1.8 kb from IS exon TSS in the murine promoter were highly homologous to two regions in the human promoter, with two putative GATA2 binding sites located at −3.4 and −2.4 kb from IS TSS, respectively. ChIP-qPCR assays showed that GATA2 binds to these sites in the human GATA2 promoter. To assess the ability of both wild-type and GATA2 mutations to regulate its own transcription, we transfected these GATA2 gene variants along with different GATA2 promoter constructs into HEK293T cells, and performed luciferase reporter assays. Wild-type GATA2 activated its transcription through the −2.4 kb site; however, it was not able to activate the full length promoter construct containing both the −3.4 and −2.4 sites. CEBPA binding sites near the −3.4 site could explain these results, since it has been reported that expression of GATA2 is transcriptionally repressed by CEBPA in a DNA binding-dependent manner. The T354M mutant activated GATA2 transcription in a similar manner than the GATA2 wild-type, raising the question about the complex function of T354M. On the contrary, del355T was totally incapable of sustain any activation of GATA2. Finally, the L359V mutation, present in 10% of CML cases with blast crisis, was able to activate the GATA2 promoter, even the full length promoter construct that contains both −3.4 and −2.4 sites, supporting that L359V is a gain-of-function mutation. In summary, GATA2 mutations had different effects on the GATA2 promoter that could affect the dose of GATA2. Expression of GATA2 is critical at various stages of hematopoiesis and since it in part determines the fate of distinct myeloid lineages, this could alter normal hematopoiesis. Moreover, as happened with GATA2, mutant GATA2 proteins could affect the expression of other targets of GATA2, as SCL, BMP4, PU.1, WT1 and others. Studies to further clarify these questions are in progress. Disclosures: No relevant conflicts of interest to declare.

Description: The ETV6 gene (first identified as TEL) is a frequent target of chromosomal translocations in both myeloid and lymphoid leukemias. At present, more than 40 distinct translocations have been cytogenetically described, of which 13 have now also been characterized at the molecular level. These studies revealed the generation of in-frame fusion genes between different domains of ETV6 and partner genes encoding either kinases or transcription factors. However, in a number of cases—including a t(6;12)(q23;p13), the recurrent t(5;12)(q31;p13), and some cases of the t(4;12)(q11-q12;p13) described in this work—functionally significant fusions could not be identified, raising the question as to what leukemogenic mechanism is implicated in these cases. To investigate this, we have evaluated the genomic regions at 4q11-q12 and 5q31, telomeric to the breakpoints of the t(4;12)(q11-q12;p13) and t(5;12)(q31;p13). The homeobox geneGSH2 at 4q11-q12 and the IL-3/CSF2locus at 5q31 were found to be located close to the respective breakpoints. In addition, GSH2 and IL-3 were found to be ectopically expressed in the leukemic cells, suggesting that expression of GSH2 and IL-3 was deregulated by the translocation. Our results indicate that, besides the generation of fusion transcripts, deregulation of the expression of oncogenes could be a variant leukemogenic mechanism for translocations involving the 5′ end of ETV6, especially for those translocations lacking functionally significant fusion transcripts.

Description: Background The SET oncoprotein, an inhibitor of the protein phosphatase 2A (PP2A), is overexpressed in leukemia cells, preventing PP2A from performing its regulatory role in deactivating signaling proteins by dephosphorylation. Restoration of PP2A activity in both chronic myeloid leukemia (CML) and acute myeloid leukemia (AML) cells to normal levels through shRNA-mediated knockdown of SET results in reduced leukemogenesis. Given the central role of PP2A and SET in regulating various kinase-dependent and -independent downstream signaling pathways, we evaluated the efficacy of SET antagonism in CML and AML cell lines as well as primary patient cells using OP449, a novel, specific, cell-penetrating SET antagonist. Results Treatment of human and murine CML cells with OP449 resulted in dose-dependent increase in PP2A activity and selective inhibition of cell growth (IC50: 0.60 to 1.11 μM), while parental Ba/F3 cells exhibited no measurable cytotoxicity. OP449-mediated decrease in the viability of leukemia cells was significantly rescued by co-treatment with okadaic acid, a PP2A inhibitor, confirming efficacy is mediated through PP2A activation. OP449 was also 3 to 8-fold more potent than FTY720 (a known activator of PP2A) and induced dephosphorylation/degradation of BCR-ABL1, AKT, and STAT5. Importantly, OP449 demonstrated activity against the ABL1 tyrosine kinase inhibitor-resistant BCR-ABL1T315I mutant and the BCR-ABL1E255V/T315I compound mutant (IC50: 1.62 and 1.97 μM, respectively). Consistent with cell line findings, OP449 also inhibited growth of primary cells from CML blastic phase patients harboring either wildtype BCR-ABL1 or BCR-ABL1T315I while normal CD34+ cells exhibited minimal effect. Further, treatment of CML cell lines and primary CD34+ CML cells with OP449 in combination with the ABL1 tyrosine kinase inhibitors showed significantly increased cytotoxicity as compared to each compound alone. For example, treatment of primary CD34+ CML cells with 2.5 μM OP449 or 200 nM nilotinib alone each resulted in a 50% reduction in colony formation, while combination of OP449 and nilotinib at these concentrations reduced colony formation by approximately 87%, suggesting synergistic reduction of clonogenicity (combination index: 0.195). Similar to our findings in CML cells, OP449 increased PP2A activity and suppressed growth in a dose-dependent manner in AML cell lines and primary patient samples harboring various different genetic lesions including FLT3-ITD, CSF1R overexpression, NRASQ61L, and JAK3A572V. Additionally, synergistic inhibition of these cells was observed when OP449 was combined with relevant tyrosine kinase inhibitors and chemotherapy. For example, treatment of MOLM-14 cells (FLT3-ITD) with 2.5 μM OP449 or 1 nM AC220 alone reduced cell viability by 58% and 75%, respectively; combined treatment reduced cell growth 96% (combination index: 0.723). Similarly, treatment of HL-60 cells (NRASQ61L) with 1 μM OP449 or 250 nM cytarabine alone reduced cell viability by 40% and 60%, respectively, whereas combined treatment led to a 94% reduction in viability (combination index: 0.630). Mechanistically, AML patient samples showed significantly increased SET expression compared to normal CD34+ cells, and treatment of AML cells with OP449 reduced phosphorylation of downstream ERK, STAT5, AKT and S6 ribosomal protein signaling. Finally, to evaluate OP449 antitumor efficacy in vivo, we tested OP449 (5 mg/kg intraperitoneally every 3 days) in xenograft mice bearing human HL-60 cell derived tumors. OP449 significantly inhibited tumor growth measured over time and resulted in a 〉2-fold reduction in tumor burden at the end of the experiment compared to vehicle-treated controls (Day 18: 1.14±0.06 g vs. 0.45±0.08 g, respectively; p

Description: The EVI1 gene (3q26) codes for a transcription factor with important roles both in normal development and in leukemogenesis. EVI1 overexpression (OE) through either 3q26 rearrangements or other unknown mechanisms is associated with a particularly unfavorable prognosis in AML. High expression of the splice form EVI1-1D has been reported to be an independent negative prognostic indicator of survival in AML, irrespective of the presence of 3q26 rearrangements. Recently, two studies have provided new information on the prognostic significance of EVI1 splice variants in AML with and without 3q26 lesions, demonstrating that the OE of the EVI1 5′-end variants correlated with significantly reduced event free survival and disease free survival (DFS). However, these results require further validation in other cohorts. Our aim was to validate in a new cohort of 490 patients with AML the prevalence and prognostic significance of EVI1 OE, and to further define the AML subgroup of patients in which it could be useful to distinguish the EVI1+ cases for diagnostic purposes. We first analyzed all EVI1 variants reported in 16 myeloid cell lines and in a series of patients, and confirmed that the EVI1+ cases overexpressed several EVI1 transcripts. In our series, EVI1+ cases always expressed the EVI1-1C transcript. Therefore, we decide to perform the analysis of EVI1- 1C and EVI1-1D in our cohort of 490 well genetically characterized AML patients. All patients were treated with schedules based on anthracycline and cytarabine, as induction therapy. High dose cytarabine, and autologous or allogenic stem cell transplantation when indicated, were used as consolidation therapy. EVI1 was highly expressed in 19% of patients. High EVI1-1D predicted a distinctly worse DFS in the total patient cohort (p=0.0153) and in patients