Human MLL-AF9 Overexpression Induces Aberrant Hematopoietic Expansion in Zebrafish.
ABSTRACT: The 11q23 of the mixed lineage leukemia 1 (MLL1) gene plays a crucial role in early embryonic development and hematopoiesis. The MLL-AF9 fusion gene, resulting from chromosomal translocation, often leads to acute myeloid leukemia with poor prognosis. Here, we generated a zebrafish model expressing the human MLL-AF9 fusion gene. Microinjection of human MLL-AF9 mRNA into zebrafish embryos resulted in enhanced hematopoiesis and the activation of downstream genes such as meis1 and hox cluster genes. Embryonic MLL-AF9 expression upregulated HSPC and myeloid lineage markers. Doxorubicin and MI-2 (a menin inhibitor) treatments significantly restored normal hematopoiesis in MLL-AF9-expressing animals. This study provides insight into the role of MLL-AF9 in zebrafish hematopoiesis and establishes a robust and efficient in vivo model for high-throughput drug screening.
Project description:The proto-oncogene EVI1 (ecotropic viral integration site-1), located on chromosome band 3q26, is aberrantly expressed in human acute myeloid leukemia (AML) with 3q26 rearrangements. In the current study, we showed, in a large AML cohort carrying 11q23 translocations, that ? 43% of all mixed lineage leukemia (MLL)-rearranged leukemias are EVI1(pos). High EVI1 expression occurs in AMLs expressing the MLL-AF6, -AF9, -AF10, -ENL, or -ELL fusion genes. In addition, we present evidence that EVI1(pos) MLL-rearranged AMLs differ molecularly, morphologically, and immunophenotypically from EVI1(neg) MLL-rearranged leukemias. In mouse bone marrow cells transduced with MLL-AF9, we show that MLL-AF9 fusion protein maintains Evi1 expression on transformation of Evi1(pos) HSCs. MLL-AF9 does not activate Evi1 expression in MLL-AF9-transformed granulocyte macrophage progenitors (GMPs) that were initially Evi1(neg). Moreover, shRNA-mediated knockdown of Evi1 in an Evi1(pos) MLL-AF9 mouse model inhibits leukemia growth both in vitro and in vivo, suggesting that Evi1 provides a growth-promoting signal. Using the Evi1(pos) MLL-AF9 mouse leukemia model, we demonstrate increased sensitivity to chemotherapeutic agents on reduction of Evi1 expression. We conclude that EVI1 is a critical player in tumor growth in a subset of MLL-rearranged AMLs.
Project description:Mixed lineage leukemia (MLL) fusion proteins cause oncogenic transformation of hematopoietic cells by constitutive recruitment of elongation factors to HOX promoters, resulting in overexpression of target genes. The structural basis of transactivation by MLL fusion partners remains undetermined. We show that the ANC1 homology domain (AHD) of AF9, one of the most common MLL translocation partners, is intrinsically disordered and recruits multiple transcription factors through coupled folding and binding. We determined the structure of the AF9 AHD in complex with the elongation factor AF4 and show that aliphatic residues, which are conserved in each of the AF9 binding partners, form an integral part of the hydrophobic core of the complex. Nuclear magnetic resonance relaxation measurements show that AF9 retains significant dynamic behavior which may facilitate exchange between disordered partners. We propose that AF9 functions as a signaling hub that regulates transcription through dynamic recruitment of cofactors in normal hematopoiesis and in acute leukemia.
Project description:Patients with a t(9;11) translocation (MLL-AF9) develop acute myeloid leukemia (AML), and while in mice the expression of this fusion oncogene also results in the development of myeloid leukemia, it is with long latency. To identify mutations that cooperate with Mll-AF9, we infected neonatal wild-type (WT) or Mll-AF9 mice with a murine leukemia virus (MuLV). MuLV-infected Mll-AF9 mice succumbed to disease significantly faster than controls presenting predominantly with myeloid leukemia while infected WT animals developed predominantly lymphoid leukemia. We identified 88 candidate cancer genes near common sites of proviral insertion. Analysis of transcript levels revealed significantly elevated expression of Mn1, and a trend toward increased expression of Bcl11a and Fosb in Mll-AF9 murine leukemia samples with proviral insertions proximal to these genes. Accordingly, FOSB and BCL11A were also overexpressed in human AML harboring MLL gene translocations. FOSB was revealed to be essential for growth in mouse and human myeloid leukemia cells using shRNA lentiviral vectors in vitro. Importantly, MN1 cooperated with Mll-AF9 in leukemogenesis in an in vivo BM viral transduction and transplantation assay. Together, our data identified genes that define transcription factor networks and important genetic pathways acting during progression of leukemia induced by MLL fusion oncogenes.
Project description:Oncogenic fusion proteins are capable of initiating tumorigenesis, but the role of their wild-type counterparts in this process is poorly understood. The mixed lineage leukemia (MLL) gene undergoes chromosomal translocations, resulting in the formation of oncogenic MLL fusion proteins (MLL-FPs). Here, we show that menin recruits both wild-type MLL and oncogenic MLL-AF9 fusion protein to the loci of HOX genes to activate their transcription. Wild-type MLL not only catalyzes histone methylation at key target genes but also controls distinct MLL-AF9-induced histone methylation. Notably, the wild-type Mll allele is required for MLL-AF9-induced leukemogenesis and maintenance of MLL-AF9-transformed cells. These findings suggest an essential cooperation between an oncogene and its wild-type counterpart in MLL-AF9-induced leukemogenesis.
Project description:Faithful modeling of mixed-lineage leukemia in murine cells has been difficult to achieve. We show that expression of MLL-AF9 in human CD34+ cells induces acute myeloid, lymphoid, or mixed-lineage leukemia in immunodeficient mice. Some leukemia stem cells (LSC) were multipotent and could be lineage directed by altering either the growth factors or the recipient strain of mouse, highlighting the importance of microenvironmental cues. Other LSC were strictly lineage committed, demonstrating the heterogeneity of the stem cell compartment in MLL disease. Targeting the Rac signaling pathway by pharmacologic or genetic means resulted in rapid and specific apoptosis of MLL-AF9 cells, suggesting that the Rac signaling pathway may be a valid therapeutic target in MLL-rearranged AML.
Project description:Rearrangement of the mixed lineage leukemia (MLL; also known as lysine methyltransferase 2A) gene is a recurrent genomic aberration in acute myeloid leukemia (AML). MLLT3, super elongation complex subunit (AF9) is one of the most common MLL fusion partners in AML. The present study aimed to explore the aberrant expression of genes associated with the MLL?AF9 translocation and identified potential new targets for the therapy of AML with MLL?AF9 translocation. The transcriptomic and epigenetic datasets were downloaded from National Center of Biotechnology Information Gene Expression Omnibus (GEO) database. Differentially expressed genes were obtained from two independent datasets (GSE68643 and GSE73457). Gene Ontology biological process and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis was performed using the Database for Annotation, Visualization and Integrated Discovery. MLL?AF9?associated chromatin immunoprecipitation sequencing (ChIP?Seq) data was analyzed and identified binding sites for MLL?AF9 and wild type MLL (MLL WT). The ChIP?Seq of histone modification data was downloaded from the GEO database, including histone 3 lysine 4 trimethylation (H3K4me3), histone 3 lysine 79 dimethylation (H3K79me2) and histone 3 lysine 27 acetylation (H3K27ac), was used for comparing histone modification marks between the MLL?AF9 leukemia cells and normal hematopoietic cells at MLL?AF9 and MLL WT binding sites. The differentially expressed genes with the same trend in H3K79me2, H3K27ac and H3K4me3 alteration were identified as potential MLL?AF9 direct target genes. Upon validation using RNA?Seq data from the Therapeutically Applicable Research to Generate Effective Treatments AML project, eight potential direct target genes of MLL?AF9 were identified and further confirmed in MLL?AF9 mouse model using reverse transcription?quantitative polymerase chain reaction. These genes may have a critical role in AML with MLL?AF9 translocation.
Project description:The 'Yamanaka factors' (Oct4, Sox2, Klf4 and c-Myc) are able to generate induced pluripotent stem (iPS) cells from different cell types. However, to what degree primary malignant cells can be reprogrammed into a pluripotent state has not been vigorously assessed. We established an acute myeloid leukemia (AML) model by overexpressing the human mixed-lineage leukemia-AF9 (MLL-AF9) fusion gene in mouse hematopoietic cells that carry Yamanaka factors under the control of doxycycline (Dox). On addition of Dox to the culture, the transplantable leukemia cells were efficiently converted into iPS cells that could form teratomas and produce chimeras. Interestingly, most chimeric mice spontaneously developed the same type of AML. Moreover, both iPS reprogramming and leukemia reinitiation paths could descend from the same leukemia-initiating cell. RNA-seq analysis showed reversible global gene expression patterns between these interchangeable leukemia and iPS cells on activation or reactivation of MLL-AF9, suggesting a sufficient epigenetic force in driving the leukemogenic process. This study represents an important step for further defining the potential interplay between oncogenic molecules and reprogramming factors during MLL leukemogenesis. More importantly, our reprogramming approach may be expanded to characterize a range of hematopoietic malignancies in order to develop new strategies for clinical diagnosis and treatment.
Project description:Chromosome rearrangements involving the mixed-lineage leukemia gene (MLL) create MLL-fusion proteins, which could drive both acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). The lineage decision of MLL-fusion leukemia is influenced by the fusion partner and microenvironment. To investigate the interplay of fusion proteins and microenvironment in lineage choice, we transplanted human hematopoietic stem and progenitor cells (HSPCs) expressing MLL-AF9 or MLL-Af4 into immunodeficient NSGS mice, which strongly promote myeloid development. Cells expressing MLL-AF9 efficiently developed AML in NSGS mice. In contrast, MLL-Af4 cells, which were fully oncogenic under lymphoid conditions present in NSG mice, displayed compromised transformation capacity in a myeloid microenvironment. MLL-Af4 activated a self-renewal program in a lineage-dependent manner, showing the leukemogenic activity of MLL-Af4 was interlinked with lymphoid lineage commitment. The C-terminal homology domain (CHD) of Af4 was sufficient to confer this linkage. Although the MLL-CHD fusion protein failed to immortalize HSPCs in myeloid conditions in vitro, it could successfully induce ALL in NSG mice. Our data suggest that defective self-renewal ability and leukemogenesis of MLL-Af4 myeloid cells could contribute to the strong B-cell ALL association of MLL-AF4 leukemia observed in the clinic.
Project description:Chromosomal translocations involving the mixed lineage leukemia (MLL) gene lead to the development of acute leukemias. Constitutive HOX gene activation by MLL fusion proteins is required for MLL-mediated leukemogenesis; however, the underlying mechanisms remain elusive. Here, we show that chromobox homolog 8 (CBX8), a Polycomb Group protein that interacts with MLL-AF9 and TIP60, is required for MLL-AF9-induced transcriptional activation and leukemogenesis. Conversely, both CBX8 ablation and specific disruption of the CBX8 interaction by point mutations in MLL-AF9 abrogate HOX gene upregulation and abolish MLL-AF9 leukemic transformation. Surprisingly, Cbx8-deficient mice are viable and display no apparent hematopoietic defects. Together, our findings demonstrate that CBX8 plays an essential role in MLL-AF9 transcriptional regulation and leukemogenesis.
Project description:Chromosomal translocations of the mixed lineage leukemia (MLL) gene are a common cause of acute leukemias. The oncogenic function of MLL fusion proteins is, in part, mediated through aberrant activation of Hoxa genes and Meis1, among others. Here we demonstrate using a tamoxifen-inducible Cre-mediated loss of function mouse model that DOT1L, an H3K79 methyltransferase, is required for both initiation and maintenance of MLL-AF9-induced leukemogenesis in vitro and in vivo. Through gene expression and chromatin immunoprecipitation analysis we demonstrate that mistargeting of DOT1L, subsequent H3K79 methylation, and up-regulation of Hoxa and Meis1 genes underlie the molecular mechanism of how DOT1L contributes to MLL-AF9-mediated leukemogenesis. Our study not only provides the first in vivo evidence for the function of DOT1L in leukemia, but also reveals the molecular mechanism for DOT1L in MLL-AF9 mediated leukemia. Thus, DOT1L may serve as a potential therapeutic target for the treatment of leukemia caused by MLL translocations.