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The molecular causes of myeloproliferative neoplasms (MPNs) have not yet been fully elucidated. Approximately 7% to 8% of the patients carry predisposing genetic germline variants that lead to driver mutations, which enhance JAK-STAT signaling. To identify additional predisposing genetic germline variants, we performed whole-exome sequencing in 5 families, each with parent-child or sibling pairs affected by MPNs and carrying the somatic JAK2 V617F mutation. In 4 families, we detected rare germline variants in known tumor predisposition genes of the DNA repair pathway, including the highly penetrant BRCA1 and BRCA2 genes. The identification of an underlying hereditary tumor predisposition is of major relevance for the individual patients as well as for their families in the context of therapeutic options and preventive care. Two patients with essential thrombocythemia or polycythemia vera experienced progression to acute myeloid leukemia, which may suggest a high risk of leukemic transformation in these familial MPNs. Our study demonstrates the relevance of genetic germline diagnostics in elucidating the causes of MPNs and suggests novel therapeutic options (eg, PARP inhibitors) in MPNs. Furthermore, we uncover a broader tumor spectrum upon the detection of a germline mutation in genes of the DNA repair pathway., (© 2021 by The American Society of Hematology.)

We evaluated pazopanib (800 mg orally QD) in patients not eligible for intensive treatment with relapsed/refractory AML or at initial diagnosis. Patients receiving pazopanib for > 14 days were analyzed for safety, tolerability, and efficacy. Co-primary endpoints were cumulative response rate and reduction of bone marrow microvessel density. Twenty patients (median age 76 years, range 52-86) were treated. Fifteen had relapsed/refractory and five had newly diagnosed AML. Median ECOG performance status was 1 (range 1-3). Four patients had adverse, 15 intermediate, and 1 patient favorable cytogenetic/molecular risk (ELN 2010 criteria). The safety profile of pazopanib was as reported. The most common adverse events of any grade were gastrointestinal. Two patients achieved PR (blast reduction > 50%), 14 stable disease (SD), and 4 progressive disease. Median PFS was 65 days (95% CI 29-105). After the end of the study, 1 CRi and 1 CRp occurred on demethylating agents, and 1 CR upon alloSCT. In these patients, SD and improved general condition on pazopanib allowed therapy escalation. Median OS for the overall study population was 191 days (95% CI 87-435) and 1-year survival was 35%. There was no significant change in microvessel density. Clinical trial information: NCT01361334.

Dyskeratosis congenita (DKC) is a paradigmatic telomere disorder characterized by substantial and premature telomere shortening, bone marrow failure, and a dramatically increased risk of developing myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). DKC can occur as a late-onset, so-called cryptic form, with first manifestation in adults. Somatic MDS-related mutations are found in up to 35% of patients with acquired aplastic anemia (AA), especially in patients with short telomeres. The aim of our study was to investigate whether cryptic DKC is associated with an increased incidence of MDS-related somatic mutations, thereby linking the accelerated telomere shortening with the increased risk of MDS/AML. Samples from 15 adult patients (median age: 42 years, range: 23-60 years) with molecularly confirmed cryptic DKC were screened using next-generation gene panel sequencing to detect MDS-related somatic variants. Only one of the 15 patients (7%) demonstrated a clinically relevant MDS-related somatic variant. This incidence was dramatically lower than formerly described in acquired AA. Based on our data, we conclude that clonal evolution of subclones carrying MDS-related mutations is not the predominant mechanism for MDS/AML initiation in adult cryptic DKC patients.

Mutations in FMS-like tyrosine kinase 3 (FLT3), such as internal tandem duplications (ITDs), can be found in up to 23% of patients with acute myeloid leukemia (AML) and confer a poor prognosis. Current treatment options for FLT3(ITD)-positive AMLs include genotoxic therapy and FLT3 inhibitors (FLT3i's), which are rarely curative. PARP1 inhibitors (PARP1i's) have been successfully applied to induce synthetic lethality in tumors harboring BRCA1/2 mutations and displaying homologous recombination (HR) deficiency. We show here that inhibition of FLT3(ITD) activity by the FLT3i AC220 caused downregulation of DNA repair proteins BRCA1, BRCA2, PALB2, RAD51, and LIG4, resulting in inhibition of 2 major DNA double-strand break (DSB) repair pathways, HR, and nonhomologous end-joining. PARP1i, olaparib, and BMN673 caused accumulation of lethal DSBs and cell death in AC220-treated FLT3(ITD)-positive leukemia cells, thus mimicking synthetic lethality. Moreover, the combination of FLT3i and PARP1i eliminated FLT3(ITD)-positive quiescent and proliferating leukemia stem cells, as well as leukemic progenitors, from human and mouse leukemia samples. Notably, the combination of AC220 and BMN673 significantly delayed disease onset and effectively reduced leukemia-initiating cells in an FLT3(ITD)-positive primary AML xenograft mouse model. In conclusion, we postulate that FLT3i-induced deficiencies in DSB repair pathways sensitize FLT3(ITD)-positive AML cells to synthetic lethality triggered by PARP1i's. Therefore, FLT3(ITD) could be used as a precision medicine marker for identifying AML patients that may benefit from a therapeutic regimen combining FLT3 and PARP1i's., (© 2018 by The American Society of Hematology.)

The quest for novel therapeutic targets in acute myeloid leukemia (AML) is still ongoing. One of such targets, cyclin A1, was shown to be overexpressed in AML including AML stem cells. However, the function of cyclin A1 in AML is largely unknown, and the data on its impact on patients' survival remain controversial. Therefore, we developed a transgenic mouse model of stem cell-directed inducible cyclin A1 overexpression and crossed these mice with PML-RARα-knockin mice, which develop an AML M3-like phenotype. To observe the effects of cyclin A1 loss-of-function, we also crossed PML-RARα-knockin mice to cyclin A1-knockout mice. Neither overexpression nor loss of cyclin A1 significantly altered leukemogenesis in PML-RARα-knockin mice. These findings imply that upregulation of cyclin A1 is not essential for leukemogenesis. Our data suggest that cyclin A1 does not represent a suitable target for AML therapy.

The adapter protein metastasis suppressor 1 (MTSS1) is implicated as a tumor suppressor or tumor promoter, depending on the type of solid cancer. Here, we identified Mtss1 expression to be increased in AML subsets with favorable outcome, while suppressed in high risk AML patients. High expression of MTSS1 predicted better clinical outcome of patients with normal-karyotype AML. Mechanistically, MTSS1 expression was negatively regulated by FLT3-ITD signaling but enhanced by the AML1-ETO fusion protein. DNMT3B, a negative regulator of MTSS1, showed strong binding to the MTSS1 promoter in PML-RARA positive but not AML1-ETO positive cells, suggesting that AML1-ETO leads to derepression of MTSS1. Pharmacological treatment of AML cell lines carrying the FLT3-ITD mutation with the specific FLT3 inhibitor PKC-412 caused upregulation of MTSS1. Moreover, treatment of acute promyelocytic cells (APL) with all-trans retinoic acid (ATRA) increased MTSS1 mRNA levels. Taken together, our findings suggest that MTSS1 might have a context-dependent function and could act as a tumor suppressor, which is pharmacologically targetable in AML patients.

Few studies have evaluated granulocyte colony-stimulating factor (G-CSF) priming in elderly patients with intensively treated acute myeloid leukemia (AML), and no data are available for genetically defined AML subgroups. We provide long-term results (median follow-up 7.6 years) of a randomized trial in which 183 patients (median age 67 years) received G-CSF prior to (G-CSF priming) or after two cycles of induction chemotherapy. CR rates with G-CSF priming and G-CSF post-chemotherapy were comparable (57 vs. 67 %, p = 0.153), with overall survival (OS) probabilities of 14 vs. 17 % at 10 years. Induction mortality was significantly higher with G-CSF priming (23 vs. 10 %, p = 0.015), primarily in normal karyotype (NK) AML. In this subgroup, a trend for better relapse-free survival (RFS) was observed with G-CSF priming (44 vs. 22 % at 10 years, p = 0.074) but did not translate into an OS benefit. G-CSF priming had no impact on AML with FLT3-ITD and NPM mutations and did not improve outcome in patients with adverse cytogenetics. In a landmark analysis, late consolidation with autologous stem cell transplantation or a second consolidation cycle significantly improved RFS compared with one consolidation cycle (21.0 vs. 12.8 months, p = 0.046). Future studies on G-CSF priming should be restricted to NK AML and used only in post-remission therapy.

Glioma pathogenesis‑related protein 1 (GliPR1) is a pleiotropic protein involved in cell proliferation, tumor growth and apoptosis. The aim of the present study was to further characterize GliPR1 in regard to its subcellular localization and its overall effect on cellular gene expression. Knockdown of GliPR1 and Affymetrix microarray mRNA expression analysis revealed 262 GliPR1‑dependent differentially expressed genes, of which 40 were induced and 222 were suppressed. Differentially expressed genes were overrepresented in five Gene Ontology categories: G protein signaling pathways, regulation of cyclin‑dependent protein kinase activity, ER to Golgi vesicle-mediated transport, axon guidance and dephosphorylation. GliPR1-EGFP fusion protein co‑localized with the endoplasmic reticulum (ER) or with cytoplasmic vesicles as demonstrated by confocal microscopy. GliPR1 expression was found to be significantly increased in acute myeloid leukemia (AML) bone marrow samples, while markedly reduced in acute lymphoblastic leukemia, unchanged in myelodysplastic syndrome and slightly decreased in chronic lymphocytic leukemia as well as in chronic myelocytic leukemia (CML) when compared to normal samples. GliPR1 was localized and involved in the ER secretory protein pathway. GliPR1 affects G protein signaling and cell cycle regulation. Based on the observed overexpression in AML samples, GliPR1 should be further explored as a potential target for AML.

The gene product of the zinc finger CCCH-type containing 15 (ZC3H15) gene, an immediate early erythropoietin response gene (synonymous: LEREPO4), was further characterized. ZC3H15 was expressed ubiquitously in all human tissues tested by northern blotting and showed mainly a diffuse cytoplasmic distribution by immune fluorescence microscopy and western blotting of subcellular protein fractions. The expression of ZC3H15 was downregulated effectively in HeLa cells to ≤13% of the control by transfection of specific small interfering RNA (siRNA). Subsequent Affymetrix microarray analysis revealed 202 differentially expressed genes including 114 induced (≥3-fold) genes and 88 suppressed (≤0.3-fold) genes. The gene ontology (GO) categories containing an over-representation of differentially expressed genes comprised cell growth, transcription, cell adhesion, regulation of NF-κB, regulation of MAPK, cell cycle arrest and immune response. ZC3H15 interacted with the signaling adapter protein tumor necrosis factor receptor associated factor 2 (TRAF-2) as shown by co-immunoprecipitation. ZC3H15 expression was found to be significantly increased in acute myeloid leukemia (AML) samples compared to MDS, CML, ALL and normal bone marrow samples using the Leukemia Gene Atlas (LGA) database. Based on these data, it is hypothesized that ZC3H15 may interact with TRAF-2 functionally within the NF-κB pathway, and may be explored as a potential target in AML.

Introduction: Older patients with acute myeloid leukemia (AML) experience short survival despite intensive chemotherapy. Azacitidine has promising activity in patients with low proliferating AML. The aim of this dose-finding part of this trial was to evaluate feasibility and safety of azacitidine combined with a cytarabine- and daunorubicin-based chemotherapy in older patients with AML., Trial Design: Prospective, randomised, open, phase II trial with parallel group design and fixed sample size., Patients and Methods: Patients aged 61 years or older, with untreated acute myeloid leukemia with a leukocyte count of <20,000/µl at the time of study entry and adequate organ function were eligible. Patients were randomised to receive azacitidine either 37.5 (dose level 1) or 75 mg/sqm (dose level 2) for five days before each cycle of induction (7+3 cytarabine plus daunorubicine) and consolidation (intermediate-dose cytarabine) therapy. Dose-limiting toxicity was the primary endpoint., Results: Six patients each were randomised into each dose level and evaluable for analysis. No dose-limiting toxicity occurred in either dose level. Nine serious adverse events occurred in five patients (three in the 37.5 mg, two in the 75 mg arm) with two fatal outcomes. Two patients at the 37.5 mg/sqm dose level and four patients at the 75 mg/sqm level achieved a complete remission after induction therapy. Median overall survival was 266 days and median event-free survival 215 days after a median follow up of 616 days., Conclusions: The combination of azacitidine 75 mg/sqm with standard induction therapy is feasible in older patients with AML and was selected as an investigational arm in the randomised controlled part of this phase-II study, which is currently halted due to an increased cardiac toxicity observed in the experimental arm., Trial Registration: This trial is registered at clinical trials.gov (identifier: NCT00915252).

Background: XIAP (X-linked inhibitor of apoptosis protein) is an inhibitor of caspases 3 and 9 that is overexpressed in acute myeloid leukemia (AML) and may contribute to chemoresistance. We report an open-label randomized phase II trial of reinduction chemotherapy with and without the XIAP antisense oligonucleotide AEG35156 in patients with AML who did not achieve remission with initial induction chemotherapy., Methods: Twenty-seven patients with AML who were refractory to initial induction chemotherapy were randomized and treated with AEG35156 (650 mg) in combination with high-dose cytarabine and idarubicin. Thirteen patients were randomized and treated with high-dose cytarabine and idarubicin alone. The rates of response and toxicity were determined., Results: Of the 27 patients assigned to AEG35156 in combination with high-dose cytarabine and idarubicin, 3 died during reinduction chemotherapy, 5 achieved complete remission (CR), and 6 achieved CR with incomplete platelet count recovery (CRp), for an overall response rate of 41%. Of the 13 patients assigned to the control arm of the study, none died during reinduction, 6 achieved CR, and 3 achieved CRp, for an overall response rate of 69%. The differences in response rates between patients in the AEG35156 and control arms were not statistically different (P = 0.18 by the χ(2) test)., Conclusions: The addition of AEG35156 to reinduction chemotherapy was well tolerated but did not improve rates of remission. Therefore alternative therapeutic strategies should be explored in patients with AML refractory to induction chemotherapy., (Copyright © 2011 Elsevier Inc. All rights reserved.)

The cell cycle is driven by the kinase activity of cyclin·cyclin-dependent kinase (CDK) complexes, which is negatively regulated by CDK inhibitor proteins. Recently, we identified INCA1 as an interaction partner and a substrate of cyclin A1 in complex with CDK2. On a functional level, we identified a novel cyclin-binding site in the INCA1 protein. INCA1 inhibited CDK2 activity and cell proliferation. The inhibitory effects depended on the cyclin-interacting domain. Mitogenic and oncogenic signals suppressed INCA1 expression, whereas it was induced by cell cycle arrest. We established a deletional mouse model that showed increased CDK2 activity in spleen with altered spleen architecture in Inca1(-/-) mice. Inca1(-/-) embryonic fibroblasts showed an increase in the fraction of S-phase cells. Furthermore, blasts from acute lymphoid leukemia and acute myeloid leukemia patients expressed significantly reduced INCA1 levels highlighting its relevance for growth control in vivo. Taken together, this study identifies a novel CDK inhibitor with reduced expression in acute myeloid and lymphoid leukemia. The molecular events that control the cell cycle occur in a sequential process to ensure a tight regulation, which is important for the survival of a cell and includes the detection and repair of genetic damage and the prevention of uncontrolled cell division.

Epigenetic changes play a crucial role in leukemogenesis. HDACs are frequently recruited to target gene promoters by balanced translocation derived oncogenic fusion proteins. As important epigenetic effector mechanisms, histone deacetylases (HDAC) have emerged as potential therapeutic targets. However, the patterns of HDAC1 localization and the role of HDACs in leukemia pathogenesis remain to be elucidated. Using ChIP-Chip analyses we analyzed HDAC1 deposition patterns at more than 10,000 gene promoters in a large cohort of leukemia patients and CD34+ controls. HDAC1 binding was significantly increased in AML blasts compared to CD34+ progenitor cells at 130 gene promoters whereas decreased binding was observed at 66 gene promoters. Distinct HDAC1 binding patterns occurred in AML subtypes with balanced translocations t(15;17), t(8;21) and inv(16). In addition, a more generalized signature was established, that revealed an AML specific pattern of HDAC1 distribution. Many of the HDAC1-binding altered promoters regulate genes involved in hematopoiesis, transcriptional regulation and signal transduction. HDAC1 binding patterns were associated with patients' event free survival. This is the first study to determine HDAC1 modification patterns in a large number of AML and ALL specimens. Our findings suggest that dyslocalization of HDAC1 is a common feature in AML. Importantly, HDAC1 modifications possess prognostic power for patient survival. Our findings suggest that altered HDAC1 localization is an explanation for the observed benefit of HDAC inhibitors in AML therapy., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

The most frequent translocation t(8;21) in acute myeloid leukemia (AML) generates the chimeric AML1/ETO protein, which blocks differentiation and induces self-renewal in hematopoietic progenitor cells. The underlying mechanisms mediating AML1/ETO-induced self-renewal are largely unknown. Using expression microarray analysis, we identified the Groucho-related amino-terminal enhancer of split (AES) as a consistently up-regulated AML1/ETO target. Elevated levels of AES mRNA and protein were confirmed in AML1/ETO-expressing leukemia cells, as well as in other AML specimens. High expression of AES mRNA or protein was associated with improved survival of AML patients, even in the absence of t(8;21). On a functional level, knockdown of AES by RNAi in AML1/ETO-expressing cell lines inhibited colony formation. Similarly, self-renewal induced by AML1/ETO in primary murine progenitors was inhibited when AES was decreased or absent. High levels of AES expression enhanced formation of immature colonies, serial replating capacity of primary cells, and colony formation in colony-forming unit-spleen assays. These findings establish AES as a novel AML1/ETO-induced target gene that plays an important role in the self-renewal phenotype of t(8;21)-positive AML.

Background: About 50% of patients (age ≥60 years) who have acute myeloid leukaemia and are otherwise medically healthy (ie, able to undergo intensive chemotherapy) achieve a complete remission (CR) after intensive chemotherapy, but with a substantially increased risk of early death (ED) compared with younger patients. We verified the association of standard clinical and laboratory variables with CR and ED and developed a web-based application for risk assessment of intensive chemotherapy in these patients., Methods: Multivariate regression analysis was used to develop risk scores with or without knowledge of the cytogenetic and molecular risk profiles for a cohort of 1406 patients (aged ≥60 years) with acute myeloid leukaemia, but otherwise medically healthy, who were treated with two courses of intensive induction chemotherapy (tioguanine, standard-dose cytarabine, and daunorubicin followed by high-dose cytarabine and mitoxantrone; or with high-dose cytarabine and mitoxantrone in the first and second induction courses) in the German Acute Myeloid Leukaemia Cooperative Group 1999 study. Risk prediction was validated in an independent cohort of 801 patients (aged >60 years) with acute myeloid leukaemia who were given two courses of cytarabine and daunorubicin in the Acute Myeloid Leukaemia 1996 study., Findings: Body temperature, age, de-novo leukaemia versus leukaemia secondary to cytotoxic treatment or an antecedent haematological disease, haemoglobin, platelet count, fibrinogen, and serum concentration of lactate dehydrogenase were significantly associated with CR or ED. The probability of CR with knowledge of cytogenetic and molecular risk (score 1) was from 12% to 91%, and without knowledge (score 2) from 21% to 80%. The predicted risk of ED was from 6% to 69% for score 1 and from 7% to 63% for score 2. The predictive power of the risk scores was confirmed in the independent patient cohort (CR score 1, from 10% to 91%; CR score 2, from 16% to 80%; ED score 1, from 6% to 69%; and ED score 2, from 7% to 61%)., Interpretation: The scores for acute myeloid leukaemia can be used to predict the probability of CR and the risk of ED in older patients with acute myeloid leukaemia, but otherwise medically healthy, for whom intensive induction chemotherapy is planned. This information can help physicians with difficult decisions for treatment of these patients., Funding: Deutsche Krebshilfe and Deutsche Forschungsgemeinschaft., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Acute myeloid leukemia (AML) is commonly associated with alterations in transcription factors because of altered expression or gene mutations. These changes might induce leukemia-specific patterns of histone modifications. We used chromatin-immunoprecipitation on microarray to analyze histone 3 lysine 9 trimethylation (H3K9me3) patterns in primary AML (n = 108), acute lymphoid leukemia (n = 28), CD34(+) cells (n = 21) and white blood cells (n = 15) specimens. Hundreds of promoter regions in AML showed significant alterations in H3K9me3 levels. H3K9me3 deregulation in AML occurred preferentially as a decrease in H3K9me3 levels at core promoter regions. The altered genomic regions showed an overrepresentation of cis-binding sites for ETS and cyclic adenosine monophosphate response elements (CREs) for transcription factors of the CREB/CREM/ATF1 family. The decrease in H3K9me3 levels at CREs was associated with increased CRE-driven promoter activity in AML blasts in vivo. AML-specific H3K9me3 patterns were not associated with known cytogenetic abnormalities. But a signature derived from H3K9me3 patterns predicted event-free survival in AML patients. When the H3K9me3 signature was combined with established clinical prognostic markers, it outperformed prognosis prediction based on clinical parameters alone. These findings demonstrate widespread changes of H3K9me3 levels at gene promoters in AML. Signatures of histone modification patterns are associated with patient prognosis in AML.

Age is a strong adverse prognostic factor in acute myeloid leukemia. Little is known about the biology of acute myeloid leukemia in elderly patients. The aim of this study was to identify genes with age-dependent changes of expression in leukemic blasts and their relevance for the patient prognosis. Gene expression profiling was carried out by mRNA microarray analysis from blasts of 67 adult acute myeloid leukemia patients of different age (range, 17-80 years). Among the genes that correlated with age, PRPF4 and SMC1A were selected for protein expression studies on a tissue array containing bone marrow histologies of 135 patients with newly diagnosed AML of different ages. A significant correlation between mRNA expression levels and patient age was shown by 131 genes. Increasing age was associated with significantly decreased mRNA levels of SMC1A. On the protein level, expression of SMC1A was low or absent in 74 out of 116 acute myeloid leukemia specimens. Importantly, patients with low protein expression levels of SMC1A experienced significantly shortened event free (2.6 months versus 10.3 months, p=0.003) and overall survival (10.4 months versus 22.6 months, p=0.015). The SMC1A protein expression level remained a significant prognostic factor for event free survival (p=0.014) with a borderline significance for overall survival (p=0.066) in a multivariate analysis. SMC1A protein expression might play a role in the determination of the prognosis and might have possible implications in therapy decision in patients with acute myeloid leukemia.

Although the potential role of Pim2 as a cooperative oncogene has been well described in lymphoma, its role in leukemia has remained largely unexplored. Here we show that high expression of Pim2 is observed in patients with acute promyelocytic leukemia (APL). To further characterize the cooperative role of Pim2 with promyelocytic leukemia/retinoic acid receptor alpha (PML/RARalpha), we used a well-established PML-RARalpha (PRalpha) mouse model. Pim2 coexpression in PRalpha-positive hematopoietic progenitor cells (HPCs) induces leukemia in recipient mice after a short latency. Pim2-PRalpha cells were able to repopulate mice in serial transplantations and to induce disease in all recipients. Neither Pim2 nor PRalpha alone was sufficient to induce leukemia upon transplantation in this model. The disease induced by Pim2 overexpression in PRalpha cells contained a slightly higher fraction of immature myeloid cells, compared with the previously described APL disease induced by PRalpha. However, it also clearly resembled an APL-like phenotype and showed signs of differentiation upon all-trans retinoic acid (ATRA) treatment in vitro. These results support the hypothesis that Pim2, which is also a known target of Flt3-ITD (another gene that cooperates with PML-RARalpha), cooperates with PRalpha to induce APL-like disease.

Targeted disruption of a highly conserved distal enhancer reduces expression of the PU.1 transcription factor by 80% and leads to acute myeloid leukemia (AML) with frequent cytogenetic aberrations in mice. Here we identify a SNP within this element in humans that is more frequent in AML with a complex karyotype, leads to decreased enhancer activity, and reduces PU.1 expression in myeloid progenitors in a development-dependent manner. This SNP inhibits binding of the chromatin-remodeling transcriptional regulator special AT-rich sequence binding protein 1 (SATB1). Overexpression of SATB1 increased PU.1 expression, and siRNA inhibition of SATB1 downregulated PU.1 expression. Targeted disruption of the distal enhancer led to a loss of regulation of PU.1 by SATB1. Interestingly, disruption of SATB1 in mice led to a selective decrease of PU.1 RNA in specific progenitor types (granulocyte-macrophage and megakaryocyte-erythrocyte progenitors) and a similar effect was observed in AML samples harboring this SNP. Thus we have identified a SNP within a distal enhancer that is associated with a subtype of leukemia and exerts a deleterious effect through remote transcriptional dysregulation in specific progenitor subtypes.

In acute myeloid leukemia (AML), mutational activation of the receptor tyrosine kinase (RTK) Flt3 is frequently involved in leukemic transformation. However, little is known about a possible role of highly expressed wild-type Flt3 in AML. The proto-oncogene c-Cbl is an important regulator of RTK signaling, acting through its ubiquitin ligase activity and as a platform for several signaling adaptor molecules. Here, we analyzed the role of c-Cbl in Flt3 signal transduction and myeloid transformation. C-Cbl physically interacted with Flt3 and was tyrosine phosphorylated in the presence of Flt3-ligand (FL). Overexpression of a dominant-negative form of c-Cbl (Cbl-70Z) inhibited FL-induced Flt3 ubiquitylation and internalization, indicating involvement of c-Cbl in Flt3 signaling. DNA sequencing of AML bone marrow revealed a case with a c-Cbl point mutation (Cbl-R420Q). Cbl-R420Q inhibited Flt3 internalization and ubiquitylation. Coexpression of Cbl-R420Q or Cbl-70Z with Flt3 induced cytokine-independent growth and survival of 32Dcl3 cells in the absence of FL. Also, the mutant Cbl proteins altered the amplitude and duration of Flt3-dependent signaling events. Our results indicate an important role of Cbl proteins in Flt3 signal modulation. Also, the data suggest a novel mechanism of leukemic transformation in AML by mutational inactivation of negative RTK regulators.

Aberrant DNA methylation is the most frequent molecular alteration in acute myeloid leukemia (AML). To identify methylation-silenced genes in AML, we performed microarray analyses in U937 cells exposed to the demethylating agent 5-aza-deoxy-cytidine. Overall, 274 transcripts were significantly induced. Interestingly, C/EBPdelta expression was significantly induced (more than 10-fold) by demethylation whereas expression of all other C/EBP family members remained unchanged. The C/EBPdelta promoter was strongly methylated in different leukemic cell lines and showed signs of a repressed chromatin state. Analyses of the promoter regions of the entire C/EBP family (alpha, beta, gamma, delta, epsilon, zeta) in bone marrow samples from AML patients (n = 80) and controls (n = 15) by mass spectrometry revealed that C/EBPdelta is the most commonly hypermethylated C/EBP gene in AML. Hypermethylation occurred in more than 35% of AML patients at primary diagnosis. A significant correlation (P = .016) was observed between hypermethylation of the C/EBPdelta promoter and low expression of C/EBPdelta in AML patients. C/EBPdelta promoter activity was strongly repressed by methylation in vitro, and transcriptional repression partially depended on MeCP2 activity. C/EBPdelta exhibited growth-inhibitory properties in primary progenitor cells as well as in Flt3-ITD-transformed cells. Taken together, C/EBPdelta is a novel tumor suppressor gene in AML that is silenced by promoter methylation.