CBFbeta-SMMHC slows proliferation of primary murine and human myeloid progenitors

RUNX1 Upregulates CENPE to Promote Leukemic Cell Proliferation

RUNX1 is a Runt family transcription factor that plays a critical role in normal hematopoiesis, including the differentiation and proliferation of hematopoietic cells. RUNX1 mutations, including chromosomal translocations, cause abnormal cell differentiation, but the mutation alone is not sufficient to cause leukemia. In MLL-fusion-induced leukemia, dysregulated wild-type RUNX1 can promote leukemia survival. Nevertheless, the underlying mechanisms of dysregulated wild-type RUNX1 in leukemia development have not been fully elucidated. This study overexpressed and knocked down RUNX1 expression in THP-1 human leukemia cells and CD34⁺ hematopoietic stem/progenitor cells to investigate the biological functions affected by dysregulated RUNX1. Our data indicated RUNX1 facilitated proliferation to promote leukemia cell growth. Furthermore, we demonstrated that RUNX1 knockdown in leukemia cells drastically diminished colony-forming ability. Finally, the RUNX1-knocked down cell depletion phenotype could be rescued by overexpression of CENPE, a cell proliferation gene and a RUNX1 direct target gene. Our results indicate a possible mechanism involving the RUNX1-CENPE axis on promoting leukemic cell growth.

Preleukemia and Leukemia-Initiating Cell Activity in inv(16) Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a collection of hematologic malignancies with specific driver mutations that direct the pathology of the disease. The understanding of the origin and function of these mutations at early stages of transformation is critical to understand the etiology of the disease and for the design of effective therapies. The chromosome inversion inv(16) is thought to arise as a founding mutation in a hematopoietic stem cell (HSC) to produce preleukemic HSCs (preL-HSCs) with myeloid bias and differentiation block, and predisposed to AML. Studies in mice and human AML cells have established that inv(16) AML follows a clonal evolution model, in which preL-HSCs expressing the fusion protein CBFβ–SMMHC persist asymptomatic in the bone marrow. The emerging leukemia-initiating cells (LICs) are composed by the inv(16) and a heterogeneous set of mutations. In this review, we will discuss the current understanding of inv(16) preleukemia development, and the function of CBFβ–SMMHC related to preleukemia progression and LIC activity. We also discuss important open mechanistic questions in the etiology of inv(16) AML.

Targeting binding partners of the CBFβ-SMMHC fusion protein for the treatment of inversion 16 acute myeloid leukemia

Inversion of chromosome 16 (inv(16)) generates the CBFβ-SMMHC fusion protein and is found in nearly all patients with acute myeloid leukemia subtype M4 with Eosinophilia (M4Eo). Expression of CBFβ-SMMHC is causative for leukemia development, but the molecular mechanisms underlying its activity are unclear. Recently, there have been important advances in defining the role of CBFβ-SMMHC and its binding partners, the transcription factor RUNX1 and the histone deacetylase HDAC8. Importantly, initial trials demonstrate that small molecules targeting these binding partners are effective against CBFβ-SMMHC induced leukemia. This review will discuss recent advances in defining the mechanism of CBFβ-SMMHC activity, as well as efforts to develop new therapies for inv(16) AML.

Molecular Basis and Targeted Inhibition of CBFβ-SMMHC Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is characterized by recurrent chromosomal rearrangements that encode for fusion proteins which drive leukemia initiation and maintenance. The inv(16) (p13q22) rearrangement is a founding mutation and the associated CBFβ-SMMHC fusion protein is essential for the survival of inv(16) AML cells. This Chapter will discuss our understanding of the function of this fusion protein in disrupting hematopoietic homeostasis and creating pre-leukemic blasts, in its cooperation with other co-occurring mutations during leukemia initiation, and in leukemia maintenance. In addition, this chapter will discuss the current approaches used for the treatment of inv(16) AML and the recent development of AI-10-49, a selective targeted inhibitor of CBFβ-SMMHC/RUNX1 binding, the first candidate targeted therapy for inv(16) AML.

RUNX1/AML1 mutant collaborates with BMI1 overexpression in the development of human and murine myelodysplastic syndromes

BMI1 overexpression is one of the second hit partner genes of RUNX1 mutations that contribute to the development of MDSs.

Runx1 loss minimally impacts long-term hematopoietic stem cells

RUNX1 encodes a DNA binding subunit of the core-binding transcription factors and is frequently mutated in acute leukemia, therapy-related leukemia, myelodysplastic syndrome, and chronic myelomonocytic leukemia. Mutations in RUNX1 are thought to confer upon hematopoietic stem cells (HSCs) a pre-leukemic state, but the fundamental properties of Runx1 deficient pre-leukemic HSCs are not well defined. Here we show that Runx1 deficiency decreases both apoptosis and proliferation, but only minimally impacts the frequency of long term repopulating HSCs (LT-HSCs). It has been variously reported that Runx1 loss increases LT-HSC numbers, decreases LT-HSC numbers, or causes age-related HSC exhaustion. We attempt to resolve these discrepancies by showing that Runx1 deficiency alters the expression of several key HSC markers, and that the number of functional LT-HSCs varies depending on the criteria used to score them. Finally, we identify genes and pathways, including the cell cycle and p53 pathways that are dysregulated in Runx1 deficient HSCs.

Phosphorylation of RUNX1 by cyclin-dependent kinase reduces direct interaction with HDAC1 and HDAC3

RUNX1 regulates formation of the definitive hematopoietic stem cell and its subsequent lineage maturation, and mutations of RUNX1 contribute to leukemic transformation. Phosphorylation of Ser-48, Ser-303, and Ser-424 by cyclin-dependent kinases (cdks) increases RUNX1 trans-activation activity without perturbing p300 interaction. We now find that endogenous RUNX1 interacts with endogenous HDAC1 or HDAC3. Mutation of the three RUNX1 serines to aspartic acid reduces co-immunoprecipitation with HDAC1 or HDAC3 when expressed in 293T cells; mutation of these three serines to alanine increases HDAC interaction, and mutation of each serine individually to aspartic acid also reduces these interactions. GST-RUNX1 isolated from bacterial extracts bound in vitro translated HDAC1 or HDAC3, and these interactions were weakened by mutation of Ser-48, Ser-303, and Ser-424 to aspartic acid. The ability of RUNX1 phosphorylation and not only serine to aspartic acid conversion to reduce HDAC1 binding was demonstrated using wild-type GST-RUNX1 phosphorylated in vitro using cdk1/cyclinB and by exposure of 293T cells transduced with RUNX1 and HDAC1 to roscovitine, a cdk inhibitor. Finally, RUNX1 or RUNX1(tripleD), in which Ser-48, Ser-303, and Ser-424 are mutated to aspartic acid, stimulated proliferation of transduced, lineage-negative murine marrow progenitors more potently than did RUNX1(tripleA), in which these serines are mutated to alanine, suggesting that stimulation of RUNX1 trans-activation by cdk-mediated reduction in HDAC interaction increases marrow progenitor cell proliferation.

Cell cycle and developmental control of hematopoiesis by Runx1

Runx1 binds DNA in cooperation with CBFbeta to activate or repress transcription, dependent upon cellular context and interaction with a variety of co-activators and co-repressors. Runx1 is required for emergence of adult hematopoietic stem cells (HSC) during embryonic development and for lymphoid, myeloid, and megakaryocyte lineage maturation from HSC in adult marrow. Runx1 levels vary during the cell cycle, and Runx1 regulates G1 to S cell cycle progression. Both Cdk and ERK phosphorylate Runx1 to influence its interaction with co-repressors, and the Wnt effector LEF-1/TCF also modulates Runx1 activities. These links likely allow cytokines and signals from adjacent cells to influence HSC proliferation versus quiescence and the rate of progenitor expansion, in response to developmental or environmental demands. J. Cell. Physiol. 219: 520-524, 2009. (c) 2009 Wiley-Liss, Inc.

Cyclin-dependent kinase phosphorylation of RUNX1/AML1 on 3 sites increases transactivation potency and stimulates cell proliferation

RUNX1/AML1 regulates lineage-specific genes during hematopoiesis and stimulates G1 cell-cycle progression. Within RUNX1, S48, S303, and S424 fit the cyclin-dependent kinase (cdk) phosphorylation consensus, (S/T)PX(R/K). Phosphorylation of RUNX1 by cdks on serine 303 was shown to mediate destabilization of RUNX1 in G2/M. We now use an in vitro kinase assay, phosphopeptide-specific antiserum, and the cdk inhibitor roscovitine to demonstrate that S48 and S424 are also phosphorylated by cdk1 or cdk6 in hematopoietic cells. S48 phosphorylation of RUNX1 paralleled total RUNX1 levels during cell-cycle progression, S303 was more effectively phosphorylated in G2/M, and S424 in G1. Single, double, and triple mutation of the cdk sites to the partially phosphomimetic aspartic acid mildly reduced DNA affinity while progressively increasing transactivation of a model reporter. Mutation to alanine increased DNA affinity, suggesting that in other gene or cellular contexts phosphorylation of RUNX1 by cdks may reduce transactivation. The tripleD RUNX1 mutant rescued Ba/F3 cells from inhibition of proliferation by CBFbeta-SMMHC more effectively than the tripleA mutant. Together these findings indicate that cdk phosphorylation of RUNX1 potentially couples stem/progenitor proliferation and lineage progression.

Methylation-independent silencing of the tumor suppressor INK4b (p15) by CBFbeta-SMMHC in acute myelogenous leukemia with inv(16)

The tumor suppressor gene INK4b (p15) is silenced by CpG island hypermethylation in most acute myelogenous leukemias (AML), and this epigenetic phenomenon can be reversed by treatment with hypomethylating agents. Thus far, it was not investigated whether INK4b is hypermethylated in all cytogenetic subtypes of AML. A comparison of levels of INK4b methylation in AML with the three most common cytogenetic alterations, inv(16), t(8;21), and t(15;17), revealed a strikingly low level of methylation in all leukemias with inv(16) compared with the other types. Surprisingly, the expression level of INK4b in inv(16)+ AML samples was low and comparable with that of the other subtypes. An investigation into an alternative mechanism of INK4b silencing determined that the loss of INK4b expression was caused by inv(16)-encoded core binding factor beta-smooth muscle myosin heavy chain (CBFbeta-SMMHC). The silencing was manifested in an inability to activate the normal expression of INK4b RNA as shown in vitamin D3-treated U937 cells expressing CBFbeta-SMMHC. CBFbeta-SMMHC was shown to displace RUNX1 from a newly determined CBF site in the promoter of INK4b. Importantly, this study (a) establishes that the gene encoding the tumor suppressor p15(INK4b) is a target of CBFbeta-SMMHC, a finding relevant to the leukemogenesis process, and (b) indicates that, in patients with inv(16)-containing AML, reexpression from the INK4b locus in the leukemia would not be predicted to occur using hypomethylating drugs.

Identification of a region on the outer surface of the CBFbeta-SMMHC myeloid oncoprotein assembly competence domain critical for multimerization

In the core binding factor (CBF)beta-smooth muscle myosin heavy chain (SMMHC) acute myeloid leukemia (AML) oncoprotein, CBFbeta lies N-terminal to the alpha-helical rod domain of SMMHC. Deletion of the SMMHC assembly competence domain (ACD), conserved among skeletal, smooth and nonmuscle myosins, prevents multimerization, inhibition of CBF and inhibition of cell proliferation. To define the amino acids critical for ACD function, three outer surface residues of ACD helices A-D, the subsequent helices E-H or the more N-terminal X or Z helices were now mutated. Variants were assessed for multimerization in low ionic strength in vitro and for nuclear localization as a measure of in vivo multimerization. Mutation of individual helices C-H reduced multimerization, with alteration of the outer surface of helices D or E having the greatest effect. The ability of these SMMHC variants to slow murine myeloid progenitor proliferation largely paralleled their effects on multimerization. Divergence at the boundaries of the ACD may reflect quantitative differences between in vitro and in vivo filament assembly. Each helix mutant retained the ability to bind the mSin3A corepressor. Agents interacting with the outer surface of the CBFbeta-SMMHC ACD that prevent multimerization may be effective as novel therapeutics in AML.

Human CD34+ cells expressing the inv(16) fusion protein exhibit a myelomonocytic phenotype with greatly enhanced proliferative ability

The t(16:16) and inv(16) are associated with FAB M4Eo myeloid leukemias and result in fusion of the CBFB gene to the MYH11 gene (encoding smooth muscle myosin heavy chain [SMMHC]). Knockout of CBFbeta causes embryonic lethality due to lack of definitive hematopoiesis. Although knock-in of CBFB-MYH11 is not sufficient to cause disease, expression increases the incidence of leukemia when combined with cooperating events. Although mouse models are valuable tools in the study of leukemogenesis, little is known about the contribution of CBFbeta-SMMHC to human hematopoietic stem and progenitor cell self-renewal. We introduced the CBFbeta-MYH11 cDNA into human CD34+ cells via retroviral transduction. Transduced cells displayed an initial repression of progenitor activity but eventually dominated the culture, resulting in the proliferation of clonal populations for up to 7 months. Long-term cultures displayed a myelomonocytic morphology while retaining multilineage progenitor activity and engraftment in NOD/SCID-B2M-/- mice. Progenitor cells from long-term cultures showed altered expression of genes defining inv(16) identified in microarray studies of human patient samples. This system will be useful in examining the effects of CBFbeta-SMMHC on gene expression in the human preleukemic cell, in characterizing the effect of this oncogene on human stem cell biology, and in defining its contribution to the development of leukemia.