MN1-TEL myeloid oncoprotein expressed in multipotent progenitors perturbs both myeloid and lymphoid growth and causes T-lymphoid tumors in mice

Extramedullary T-lymphoblastic Crisis in a Myelodysplastic/Myeloproliferative Neoplasm with a t(12;22)/MN1::ETV6 Translocation

Myelodysplastic/myeloproliferative neoplasms (MDS/MPN) are not a single disease, but rather a heterogenous group of entities which are increasingly subclassified according to recurrent genetic abnormalities. Chromosomal translocations involving meningioma 1 (MN1) and ETS variant 6 (ETV6) genes are extremely rare, but recurrent in myeloid neoplasms. We describe the case of a patient with a myelodysplastic/myeloproliferative neoplasm with neutrophilia, who developed an extramedullary T-lymphoblastic crisis with the t(12;22)(p13;q12) translocation as the only cytogenetic abnormality. This case shares several clinical and molecular features with myeloid/lymphoid neoplasms with eosinophilia. The treatment of this patient was challenging, as the disease proved to be highly refractory to chemotherapy, with allogenic stem cell transplantation as the only curative option. This clinical presentation has not been reported in association with these genetic alterations and supports the concept of a hematopoietic neoplasm originating in an early uncommitted precursor cell. Additionally, it stresses the importance of molecular characterization in the classification and prognostic stratification of these entities.

Novel strategy for rapid functional in vivo validation of oncogenic drivers in haematological malignancies

In cancer research, it remains challenging to functionally validate putative novel oncogenic drivers and to establish relevant preclinical models for evaluation of novel therapeutic strategies. Here, we describe an optimized and efficient pipeline for the generation of novel conditional overexpression mouse models in which putative oncogenes, along with an eGFP/Luciferase dual reporter, are expressed from the endogenous ROSA26 (R26) promoter. The efficiency of this approach was demonstrated by the generation and validation of novel R26 knock-in (KI) mice that allow conditional overexpression of Jarid2, Runx2, MN1 and a dominant negative allele of ETV6. As proof of concept, we confirm that MN1 overexpression in the hematopoietic lineage is sufficient to drive myeloid leukemia. In addition, we show that T-cell specific activation of MN1 in combination with loss of Pten increases tumour penetrance and stimulates the formation of Lyl1⁺ murine T-cell lymphoblastic leukemias or lymphomas (T-ALL/T-LBL). Finally, we demonstrate that these luciferase-positive murine AML and T-ALL/T-LBL cells are transplantable into immunocompromised mice allowing preclinical evaluation of novel anti-leukemic drugs in vivo.

High expression of meningioma 1 is correlated with reduced survival rates in colorectal cancer patients

The identification of prognostic markers for colorectal cancer (CRC) has important clinical implications. However, the association between meningioma 1 (MN1) expression and clinical outcomes of CRC has not been fully investigated. The aim of this study was to investigate the expression of MN1 in the clinical context of CRC. We first used immunohistochemistry (IHC) staining to examine and compare MN1 expression between multiple human cancer tissues and normal tissues. Initial screening revealed that the expression of MN1 proteins was significantly higher in tumor tissues of the breast, colon, and liver than in normal tissues. In further testing conducted on 59 paired CRC samples, we observed that the expression of MN1 in CRC tissue samples was significantly higher than in adjacent normal tissues. Moreover, high MN1 expression was not significantly associated with clinicopathological characteristics. Kaplan-Meier survival analysis revealed that high expression of MN1 mRNA or MN1 protein was significantly associated with poor CRC prognosis. Furthermore, univariate Cox analysis revealed that a high MN1 score was significantly associated with prognostic factors. Multivariate Cox analysis further indicated that gender, histologic grade, tumor-node-metastasis (TNM) stage, and a high MN1 score were independent factors of overall CRC survival rates. Finally, MN1 and PCNA protein levels were positively correlated, which suggests that MN1 may be involved in the cell proliferation process during CRC formation. Our results, which confirm those of other studies, indicate that (1) high levels of MN1 expression contribute to poor CRC prognosis and (2) MN1 can serve as a novel potential biomarker in predicting the prognosis of CRC patients.

Prognostic Impact of Blood MN1 Copy Numbers Before Allogeneic Stem Cell Transplantation in Patients With Acute Myeloid Leukemia

High expression of the leukemia-associated gene meningioma-1 (MN1) is frequently found at diagnosis of acute myeloid leukemia (AML) and associates with adverse outcomes. The presence of measurable residual disease (MRD) in complete remission (CR) indicates high risk of relapse and worse outcome in AML patients. However, the prognostic impact of MN1 expression levels as MRD marker has not been evaluated. Digital droplet polymerase chain reaction (ddPCR) is a novel technique allowing sensitive and specific absolute gene expression quantification. We retrospectively analyzed 124 AML patients who received allogeneic hematopoietic stem cell transplantation (HSCT) in CR or CR with incomplete peripheral recovery. Absolute MN1 copy numbers in peripheral blood were assessed prior to HSCT (median 7; range 0-29 days) using ddPCR. High pre-HSCT MN1/Abelson murine leukemia viral oncogene homolog 1 gene (ABL1) copy numbers associated with a higher cumulative incidence of relapse after HSCT and-in relapsing patients-shorter time to relapse. In multivariable analysis, high pre-HSCT MN1/ABL1 copy numbers remained an independent prognosticator for relapse after HSCT. Patients with the highest pre-HSCT MN1/ABL1 copy numbers also had the highest risk of relapse. MN1 copy number assessment also added prognostic information to nucleophosmin 1 gene (NPM1) mutation- and brain and acute leukemia, cytoplasmic (BAALC) and Wilm's tumor gene 1 (WT1) expression-based MRD evaluation. Our study demonstrates the feasibility of the novel ddPCR technique for MN1/ABL1 copy number assessment as a marker for MRD. Evaluation of MN1/ABL1 copy numbers allows the identification of patients at high risk of relapse, independently of other diagnostic risk factors and MRD markers.

Myeloid neoplasms with t(12;22)(p13;q12)/MN1-EVT6: a systematic review of 12 cases

t(12;22)(p13;q12) is a rare but recurrent chromosomal abnormality involving the ETS transcription factor ETV6 and meningioma 1 (MN1) genes. In this study, we analyzed the clinical, cytogenetic, and molecular features of five new patients with the t(12;22)/MN1-EVT6 who presented with acute myeloid leukemia or chronic myelomonocytic leukemia. We subsequently reviewed the literature and identified seven additional cases reported with t(12;22)/MN1-EVT6. Our data suggest that neoplasms carrying the t(12;22)/MN1-ETV6, although rare, can commonly present as myeloid neoplasms at the initial diagnosis, including acute myeloid leukemia (n = 8), myelodysplastic syndrome (n = 2), and myelodysplastic/myeloproliferative neoplasms (n = 2). There were five men and seven women with a median age of 43 years (range, 15-63 years) at initial diagnosis. Cytogenetics revealed t(12;22) as the sole abnormality in five patients, with the remaining seven patients harboring additional chromosomal aberrations. Of the five patients who received known therapy regimens, all of them had poor response to the idarubicin/mitoxantrone + cytarabine regimen. Of the seven patients with follow-up information, six patients died with a median overall survival time of only 5 months (range, 1-12 months) after the emergence of t(12;22). In summary, patients with t(12;22) are frequently associated with myeloid neoplasms, poor response to chemotherapy, and inferior outcome.

Cell fate decisions in malignant hematopoiesis: leukemia phenotype is determined by distinct functional domains of the MN1 oncogene

Extensive molecular profiling of leukemias and preleukemic diseases has revealed that distinct clinical entities, like acute myeloid (AML) and T-lymphoblastic leukemia (T-ALL), share similar pathogenetic mutations. It is not well understood how the cell of origin, accompanying mutations, extracellular signals or structural differences in a mutated gene determine the phenotypic identity of leukemias. We dissected the functional aspects of different protein regions of the MN1 oncogene and their effect on the leukemic phenotype, building on the ability of MN1 to induce leukemia without accompanying mutations. We found that the most C-terminal region of MN1 was required to block myeloid differentiation at an early stage, and deletion of an extended C-terminal region resulted in loss of myeloid identity and cell differentiation along the T-cell lineage in vivo. Megakaryocytic/erythroid lineage differentiation was blocked by the N-terminal region. In addition, the N-terminus was required for proliferation and leukemogenesis in vitro and in vivo through upregulation of HoxA9, HoxA10 and Meis2. Our results provide evidence that a single oncogene can modulate cellular identity of leukemic cells based on its active gene regions. It is therefore likely that different mutations in the same oncogene may impact cell fate decisions and phenotypic appearance of malignant diseases.

Molecular mechanisms of ETS transcription factor-mediated tumorigenesis

The E26 transformation-specific (ETS) family of transcription factors is critical for development, differentiation, proliferation and also has a role in apoptosis and tissue remodeling. Changes in expression of ETS proteins therefore have a significant impact on normal physiology of the cell. Transcriptional consequences of ETS protein deregulation by overexpression, gene fusion, and modulation by RAS/MAPK signaling are linked to alterations in normal cell functions, and lead to unlimited increased proliferation, sustained angiogenesis, invasion and metastasis. Existing data show that ETS proteins control pathways in epithelial cells as well as stromal compartments, and the crosstalk between the two is essential for normal development and cancer. In this review, we have focused on ETS factors with a known contribution in cancer development. Instead of focusing on a prototype, we address cancer associated ETS proteins and have highlighted the diverse mechanisms by which they affect carcinogenesis. Finally, we discuss strategies for ETS factor targeting as a potential means for cancer therapeutics.

The leukemia-associated fusion protein MN1-TEL blocks TEL-specific recognition sequences

The leukemia-associated fusion protein MN1-TEL combines the transcription-activating domains of MN1 with the DNA-binding domain of the transcriptional repressor TEL. Quantitative photobleaching experiments revealed that ∼20% of GFP-tagged MN1 and TEL is transiently immobilised, likely due to indirect or direct DNA binding, since transcription inhibition abolished immobilisation. Interestingly, ∼50% of the MN1-TEL fusion protein was immobile with much longer binding times than unfused MN1 and TEL. MN1-TEL immobilisation was not observed when the TEL DNA-binding domain was disrupted, suggesting that MN1-TEL stably occupies TEL recognition sequences, preventing binding of factors required for proper transcription regulation, which may contribute to leukemogenesis.

Functional characterization of high levels of meningioma 1 as collaborating oncogene in acute leukemia

Retroviral expression of leukemogenic oncogenes in the murine hematopoietic system is essential but not sufficient to induce acute leukemia. Proviral integration-mediated elevated expression of the meningioma 1 (MN1) oncogene suggested MN1 acting as cooperating event in mixed-lineage leukemia 1 (MLL) and eleven nineteen leukemia (ENL)-induced murine leukemia. Indeed, co-expression of MN1 with MLL-ENL enhanced transformation in vivo, and resulted in a significantly reduced latency for induction of an aggressive acute leukemia when compared with MN1 or MLL-ENL alone. In addition, co-expression of MN1 increased the granulocyte macrophage progenitor cell population with leukemia-initiating properties as shown in secondary transplantation experiments. Gene expression profiling experiments identified putative downstream MN1 targets, of which FMS-like tyrosine kinase 3 (FLT3) and CD34 were upregulated in both MN1-overexpressing murine leukemias and in pediatric acute leukemias with high MN1 levels. Interestingly, small interfering RNA (siRNA)-mediated MN1 knockdown resulted in cell cycle arrest and impaired clonogenic growth of human leukemia cell lines with high MN1 levels. Our work shows for the first time that high MN1 levels are important for the growth of leukemic cells, and that increased MN1 expression can synergize with MLL-ENL and probably other transforming fusion genes in leukemia induction through a distinct gene expression program that is able to expand the leukemia-initiating cell population.

Meningioma 1 is required for appropriate osteoblast proliferation, motility, differentiation, and function

The vitamin D endocrine system is essential for calcium and phosphate homeostasis and skeletal mineralization. The 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) hormone binds to the vitamin D receptor (VDR) to regulate gene expression. These gene products in turn mediate the actions of 1,25(OH)(2)D(3) in mineral-regulating target cells such as the osteoblast. We showed previously that meningioma 1 (MN1) is a novel target of 1,25(OH)(2)D(3) in MG-63 osteoblastic cells and that it is a coactivator for VDR-mediated transcription (Sutton, A. L., Zhang, X., Ellison, T. I., and MacDonald, P. N. (2005) Mol. Endocrinol. 19, 2234-2244). However, the functional significance of MN1 in osteoblastic cell biology is largely unknown. Here, we demonstrate that MN1 expression is increased dramatically during differentiation of primary osteoblastic cells. Using calvarial osteoblasts derived from wild-type and MN1 knock-out mice, we provide data supporting an essential role of MN1 in maintaining appropriate osteoblast proliferation, differentiation, and function. MN1 knock-out osteoblasts displayed altered morphology, decreased growth rate, impaired motility, and attenuated 1,25(OH)(2)D(3)/VDR-mediated transcription as well as reduced alkaline phosphatase activity and mineralized nodule formation. MN1 null osteoblasts were also impaired in supporting osteoclastogenesis in co-culture studies presumably because of marked reduction in the RANKL:OPG ratio in the MN1 null cells. Mechanistic studies supported a transcriptional role for MN1 in controlling RANKL gene expression through activation of the RANKL promoter. Cumulatively, these studies indicate an important role for MN1 in maintaining the appropriate maturation and function of calvarial osteoblasts.

The Mn1 transcription factor acts upstream of Tbx22 and preferentially regulates posterior palate growth in mice

The mammalian secondary palate exhibits morphological, pathological and molecular heterogeneity along the anteroposterior axis. Although the cell proliferation rates are similar in the anterior and posterior regions during palatal outgrowth, previous studies have identified several signaling pathways and transcription factors that specifically regulate the growth of the anterior palate. By contrast, no factor has been shown to preferentially regulate posterior palatal growth. Here, we show that mice lacking the transcription factor Mn1 have defects in posterior but not anterior palatal growth. We show that Mn1 mRNA exhibits differential expression along the anteroposterior axis of the developing secondary palate, with preferential expression in the middle and posterior regions during palatal outgrowth. Extensive analyses of palatal gene expression in wild-type and Mn1(-/-) mutant mice identified Tbx22, the mouse homolog of the human X-linked cleft palate gene, as a putative downstream target of Mn1 transcriptional activation. Tbx22 exhibits a similar pattern of expression with that of Mn1 along the anteroposterior axis of the developing palatal shelves and its expression is specifically downregulated in Mn1(-/-) mutants. Moreover, we show that Mn1 activated reporter gene expression driven by either the human or mouse Tbx22 gene promoters in co-transfected NIH3T3 cells. Overexpression of Mn1 in NIH3T3 cells also increased endogenous Tbx22 mRNA expression in a dose-dependent manner. These data indicate that Mn1 and Tbx22 function in a novel molecular pathway regulating mammalian palate development.

Gene expression with prognostic implications in cytogenetically normal acute myeloid leukemia

Acute myeloid leukemia (AML) represents a heterogeneous group of leukemia entities that differ with regard to biology, clinical course, and prognosis. Over the past decades, it has been shown that most AML cases exhibit chromosomal aberrations, gene mutations, and disordered gene expression that alter normal gene function, thereby contributing to leukemic transformation. Especially, in cytogenetically normal AML (CN-AML) molecular genetic and gene expression analyses are becoming of increasing importance. In addition to the impact of gene mutations, including the MLL, FLT3, CEBPA, or NPM1 genes in CN-AML, recent analyses have provided evidence that altered gene expression might not only be of biological but also of prognostic relevance in CN-AML patients. Quantitative reverse-transcriptase polymerase chain reaction (Q-RT-PCR) and recent advances in genome-wide DNA microarray-based gene expression profiling (GEP) represent powerful tools for the systematic exploration of the molecular variation underlying the biologic and clinical heterogeneity of CN-AML. Ultimately, a better understanding of gene expression alterations and hence the molecular basis of the disease will contribute to a refined leukemia classification, which will include both previously known CN-AML subgroups and novel classes defined by distinct gene expression clusters with prognostic significance.

Review: genetic models of acute myeloid leukaemia

The use of genetically engineered mice (GEM) have been critical in understanding disease states such as cancer, and none more so than acute myelogenous leukaemia (AML), a disease characterized by over 100 distinct chromosomal translocations. A substantial proportion of cases exhibiting recurrent reciprocal translocations at diagnosis, such as t(8;21) or t(15;17) have been exhaustively studied and are currently employed in clinical diagnosis. However, a definitive conclusion regarding the leukaemogenic potential of defined transgenes for this disease remains elusive. While it is increasingly apparent that a number of cooperating mutations are necessary to develop a leukaemic phenotype, the number of models reflecting these synergisms remains few. Furthermore, little emphasis has been paid to the effect of chromosomal translocations other than recurrent genetic abnormalities, with no models reflecting the multiple abnormalities observed in high-risk cases of AML accounting for 8-10% of adult AML. Here we review the differing technologies employed in generation of GEM of AML. We discuss the relevance of GEM AML from embryonic stem cell-mediated (for example retinoic acid receptor-alpha fusions and AML1/ETO) models; through to the valuable retroviral-mediated gene transfer models. The latter have been used to great effect in defining the transforming properties of chromosomal translocation products such as MLL (found in 5-6% of all AML cases) and NUP98 (denoting poor prognosis in therapy-related disease) and particularly when co-transduced with bad prognostic factors such as Flt3 mutations. Finally, we comment on the emergence of newer transduction technologies, which can regulate the level of expression to defined cell lineages in both primary murine and human xenografts, and discuss how combining multiple genetic modalities, more relevant models of this complex disease are being generated.

MN1 overexpression induces acute myeloid leukemia in mice and predicts ATRA resistance in patients with AML

Overexpression of wild-type MN1 is a negative prognostic factor in patients with acute myeloid leukemia (AML) with normal cytogenetics. We evaluated whether MN1 plays a functional role in leukemogenesis. We demonstrate using retroviral gene transfer and bone marrow (BM) transplantation that MN1 overexpression rapidly induces lethal AML in mice. Insertional mutagenesis and chromosomal instability were ruled out as secondary aberrations. MN1 increased resistance to all-trans retinoic acid (ATRA)–induced cell-cycle arrest and differentiation by more than 3000-fold in vitro. The differentiation block could be released by fusion of a transcriptional activator (VP16) to MN1 without affecting the ability to immortalize BM cells, suggesting that MN1 blocks differentiation by transcriptional repression. We then evaluated whether MN1 expression levels in patients with AML (excluding M3-AML) correlated with resistance to ATRA treatment in elderly patients uniformly treated within treatment protocol AMLHD98-B. Strikingly, patients with low MN1 expression who received ATRA had a significantly prolonged event-free (P = .008) and overall (P = .04) survival compared with patients with either low MN1 expression and no ATRA, or high MN1 expression with or without ATRA. MN1 is a unique oncogene in hematopoiesis that both promotes proliferation/self-renewal and blocks differentiation, and may become useful as a predictive marker in AML treatment.

Platelet-Derived Growth Factor Receptor Regulates Myeloid and Monocytic Differentiation of HL-60 Cells

Here, we show that the platelet-derived growth factor receptor (PDGFR) regulates myeloid and monocytic differentiation of HL-60 myeloblastic leukemia cells in response to retinoic acid (RA) and vitamin D3 (D3), respectively. Both RA and D3 decreased the expression of PDGFR-alpha and PDGFR-beta throughout differentiation. When cells were treated with the PDGFR inhibitor AG1296 in addition to RA or D3, signs of terminal differentiation such as inducible oxidative metabolism and cell substrate adhesion were enhanced. These changes were accompanied by an increased extracellular signal-regulated kinase 1/2 activation. AG1296 also resulted in elevated expression of differentiation markers CD11b and CD66c when administered with RA or D3. Interestingly, other markers did not follow the same pattern. Cells receiving AG1296 in addition to RA or D3 showed decreased G1-G0 arrest and CD14, CD38, and CD89 expression. We thus provide evidence that certain sets of differentiation markers can be enhanced, whereas others can be inhibited by the PDGFR pathway. In addition, we found calcium levels to be decreased by RA and D3 but increased when AG1296 was given in addition to RA or D3, suggesting that calcium levels decrease during myeloid or monocytic differentiation, and elevated calcium levels can disturb the expression of certain differentiation markers.

MN1, a novel player in human AML

The transcriptional coactivator MN1 has been identified as a gene overexpressed in certain types of human acute myeloid leukemia. Upregulation is invariantly associated with inv(16) AML but is also found in other AML subtypes. Overexpression of this gene is also associated with a worse prognosis and a shorter survival in AML patients with a normal karyotype. In this short review, I will discuss the role of MN1 in myeloid leukemia.

MN1 overexpression is an important step in the development of inv(16) AML

The gene encoding the transcriptional co-activator MN1 is the target of the reciprocal chromosome translocation (12;22)(p13;q12) in some patients with acute myeloid leukemia (AML). In addition, expression array analysis showed that MN1 was overexpressed in AML specified by inv(16), in some AML overexpressing ecotropic viral integration 1 site (EVI1) and in some AML without karyotypic abnormalities. Here we describe that mice receiving transplants of bone marrow (BM) overexpressing MN1 rapidly developed myeloproliferative disease (MPD). This BM also generated myeloid cell lines in culture. By mimicking the situation in human inv(16) AML, forced coexpression of MN1 and Cbfbeta-SMMHC rapidly caused AML in mice. These findings identify MN1 as a highly effective hematopoietic oncogene and suggest that MN1 overexpression is an important cooperative event in human inv(16) AML.

High meningioma 1 (MN1) expression as a predictor for poor outcome in acute myeloid leukemia with normal cytogenetics

The translocation t(12;22) involves MN1 and TEL and is rarely found in acute myeloid leukemia (AML). Recently, it has been shown in a mouse model that the fusion protein MN1-TEL can promote growth of primitive hematopoietic progenitor cells (HPCs) and, in cooperation with HOXA9, induce AML. We quantified MN1 expression by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) in 142 adult patients with AML with normal cytogenetics treated uniformly in trial AML-SHG 01/99. AML samples were dichotomized at the median MN1 expression. High MN1 expression was significantly correlated with unmutated NPM1 (P < .001), poor response to the first course of induction treatment (P = .02), a higher relapse rate (P = .03), and shorter relapse-free (P = .002) and overall survivals (P = .03). In multivariate analysis, MN1 expression was an independent prognostic marker (P = .02) in addition to age and Eastern Cooperative Oncology Group (ECOG) performance status. Excluding patients with NPM1(mutated)/FLT3ITD(negative), high MN1 expression was associated with shorter relapse-free survival (P = .057). MN1 was highly expressed in some patients with acute lymphoblastic but not chronic lymphocytic or myeloid leukemia. MN1 was highly expressed in HPCs compared with differentiated cells and was down-regulated during in vitro differentiation of CD34(+) cells, suggesting a functional role in HPCs. In conclusion, our data suggest MN1 overexpression as a new prognostic marker in AML with normal cytogenetics.

MN1-TEL, the product of the t(12;22) in human myeloid leukemia, immortalizes murine myeloid cells and causes myeloid malignancy in mice

MN1-TEL is the product of the recurrent t(12;22)(p12;q11) associated with human myeloid malignancies. MN1-TEL functions as an activated transcription factor, exhibiting weak transforming activity in NIH3T3 fibroblasts that depends on the presence of a functional TEL DNA-binding domain, the N-terminal transactivating sequences of MN1 and C-terminal sequences of MN1. We determined the transforming activity of MN1-TEL in mouse bone marrow (BM) by using retroviral transfer. MN1-TEL-transduced BM showed increased self-renewal capacity of primitive progenitors in vitro, and prolonged in vitro culture of MN1-TEL-expressing BM produced immortalized myeloid, interleukin (IL)-3/stem cell factor-dependent cell lines with a primitive morphology. Transplantation of such cell lines into lethally irradiated mice rescued them from irradiation-induced death and resulted in the contribution of MN1-TEL-expressing cells to all hematopoietic lineages, underscoring the primitive nature of these cells and their capacity to differentiate in vivo. Three months after transplantation, all mice succumbed to promonocytic leukemia. Transplantation of freshly MN1-TEL-transduced BM into lethally irradiated mice also caused acute myeloid leukemia within 3 months of transplantation. We infer that MN1-TEL is a hematopoietic oncogene that stimulates the growth of hematopoietic cells, but depends on secondary mutations to cause leukemia in mice.

Conditional MN1-TEL knock-in mice develop acute myeloid leukemia in conjunction with overexpression of HOXA9

The chromosomal translocation t(12; 22)(p13;q11) in human myeloid leukemia generates an MN1-TEL (meningioma 1-translocation-ETS-leukemia) fusion oncoprotein. This protein consists of N-terminal MN1 sequences, a transcriptional coactivator fused to C-terminal TEL sequences, an ETS (E26 transformation-specific) transcription factor. Enforced expression of MN1-TEL in multipotent hematopoietic progenitors in knock-in mice perturbed growth and differentiation of myeloid as well as lymphoid cells. Depending on obligatory secondary mutations, these mice developed T-cell lympholeukemia. Here we addressed the role of MN1-TEL in myeloid leukemogenesis using the same mouse model. Expression of MN1-TEL enhanced the growth of myeloid progenitors in an interleukin 3/stem cell factor (IL-3/SCF)-dependent manner in vitro whereas 10% of MN1-TEL-expressing mice developed altered myelopoiesis with severe anemia after long latency. Coexpression of MN1-TEL and IL-3, but not SCF, rapidly caused a fatal myeloproliferative disease rather than acute myeloid leukemia (AML). Because MN1-TEL+ AML patient cells overexpress HOXA9 (homeobox A9), we tested the effect of coexpression of MN1-TEL and HOXA9 in mice and found that 90% of MN1-TEL+/HOXA9+ mice developed AML much more rapidly than control HOXA9+ mice. Thus, the leukemogenic effect of MN1-TEL in our knock-in mice is pleiotropic, and the type of secondary mutation determines disease outcome.