Alterations of the AML1 transcription factor in human leukemia

Cohesin Components Stag1 and Stag2 Differentially Influence Haematopoietic Mesoderm Development in Zebrafish Embryos

Cohesin is a multiprotein complex made up of core subunits Smc1, Smc3, and Rad21, and either Stag1 or Stag2. Normal haematopoietic development relies on crucial functions of cohesin in cell division and regulation of gene expression via three-dimensional chromatin organization. Cohesin subunit STAG2 is frequently mutated in myeloid malignancies, but the individual contributions of Stag variants to haematopoiesis or malignancy are not fully understood. Zebrafish have four Stag paralogues (Stag1a, Stag1b, Stag2a, and Stag2b), allowing detailed genetic dissection of the contribution of Stag1-cohesin and Stag2-cohesin to development. Here we characterize for the first time the expression patterns and functions of zebrafish stag genes during embryogenesis. Using loss-of-function CRISPR-Cas9 zebrafish mutants, we show that stag1a and stag2b contribute to primitive embryonic haematopoiesis. Both stag1a and stag2b mutants present with erythropenia by 24 h post-fertilization. Homozygous loss of either paralogue alters the number of haematopoietic/vascular progenitors in the lateral plate mesoderm. The lateral plate mesoderm zone of scl-positive cells is expanded in stag1a mutants with concomitant loss of kidney progenitors, and the number of spi1-positive cells are increased, consistent with skewing toward primitive myelopoiesis. In contrast, stag2b mutants have reduced haematopoietic/vascular mesoderm and downregulation of primitive erythropoiesis. Our results suggest that Stag1 and Stag2 proteins cooperate to balance the production of primitive haematopoietic/vascular progenitors from mesoderm.

Inhibition of the RUNX1-CBFβ transcription factor complex compromises mammary epithelial cell identity: a phenotype potentially stabilized by mitotic gene bookmarking

RUNX1 has recently been shown to play an important role in determination of mammary epithelial cell identity. However, mechanisms by which loss of the RUNX1 transcription factor in mammary epithelial cells leads to epithelial-to-mesenchymal transition (EMT) are not known. Here, we report that interaction between RUNX1 and its heterodimeric partner CBFβ is essential for sustaining mammary epithelial cell identity. Disruption of RUNX1-CBFβ interaction, DNA binding, and association with mitotic chromosomes alters cell morphology, global protein synthesis, and phenotype-related gene expression. During interphase, RUNX1 is organized as punctate, predominantly nuclear, foci that are dynamically redistributed during mitosis, with a subset localized to mitotic chromosomes. Genome-wide RUNX1 occupancy profiles for asynchronous, mitotically enriched, and early G1 breast epithelial cells reveal RUNX1 associates with RNA Pol II-transcribed protein coding and long non-coding RNA genes and RNA Pol I-transcribed ribosomal genes critical for mammary epithelial proliferation, growth, and phenotype maintenance. A subset of these genes remains occupied by the protein during the mitosis to G1 transition. Together, these findings establish that the RUNX1-CBFβ complex is required for maintenance of the normal mammary epithelial phenotype and its disruption leads to EMT. Importantly, our results suggest, for the first time, that RUNX1 mitotic bookmarking of a subset of epithelial-related genes may be an important epigenetic mechanism that contributes to stabilization of the mammary epithelial cell identity.

The Ins and Outs of Autophagy and Metabolism in Hematopoietic and Leukemic Stem Cells: Food for Thought

Discovered over fifty years ago, autophagy is a double-edged blade. On one hand, it regulates cellular energy sources by "cannibalization" of its own cellular components, feeding on proteins and other unused cytoplasmic factors. On the other, it is a recycling process that removes dangerous waste from the cytoplasm keeping the cell clean and healthy. Failure of the autophagic machinery is translated in dysfunction of the immune response, in aging, and in the progression of pathologies such as Parkinson disease, diabetes, and cancer. Further investigation identified autophagy with a protective role in specific types of cancer, whereas in other cases it can promote tumorigenesis. Evidence shows that treatment with chemotherapeutics can upregulate autophagy in order to maintain a stable intracellular environment promoting drug resistance and cell survival. Leukemia, a blood derived cancer, represents one of the malignancies in which autophagy is responsible for drug treatment failure. Inhibition of autophagy is becoming a strategic target for leukemic stem cell (LSC) eradication. Interestingly, the latest findings demonstrate that LSCs show higher levels of mitochondrial metabolism compared to normal stem cells. With this review, we aim to explore the links between autophagy and metabolism in the hematopoietic system, with special focus on primitive LSCs.

The effects of proliferation and DNA damage on hematopoietic stem cell function determine aging

In most of the mammalian tissues, homeostasis as well as injury repair depend upon a small number of resident adult stem cells. The decline in tissue/organ function in aged organisms has been directly linked with poorly functioning stem cells. Altered function of hematopoietic stem cells (HSCs) is at the center of an aging hematopoietic system, a tissue with high cellular turnover. Poorly engrafting, myeloid-biased HSCs with higher levels of DNA damage accumulation are the hallmark features of an aged hematopoietic system. These cells show a higher proliferation rate than their younger counterparts. It was proposed that quiescence of these cells over long period of time leads to accumulation of DNA damage, eventually resulting in poor function/pathological conditions in hematopoietic system. However, various mouse models with premature aging phenotype also show highly proliferative HSCs. This review examines the evidence that links proliferation of HSCs with aging, which leads to functional changes in the hematopoietic system. Developmental Dynamics 245:739-750, 2016. © 2016 Wiley Periodicals, Inc.

Two novel RUNX1 mutations in a patient with congenital thrombocytopenia that evolved into a high grade myelodysplastic syndrome

Here we report two new RUNX1 mutations in one patient with congenital thrombocytopenia that transformed into a high grade myelodysplastic syndrome with myelomonocytic features. The first mutation was a nucleotide base substitution from guanine to adenine within exon 8, resulting in a nonsense mutation in the DNA-binding inhibitory domain of the Runx1 protein. This nonsense mutation is suspected a de novo germline mutation since both parents are negative for the mutation. The second mutation identified was an in-frame six nucleotide base pair insertion in exon 5 of the RUNX1 gene, which is predicted to result in an insertion in the DNA-binding runt homology domain (RHD). This mutation is believed to be a somatic mutation as it was mosaic before allogeneic hematopoietic cell transplantation and disappeared after transplant. As no other genetic mutation was found using genetic screening, it is speculated that the combined effect of these two RUNX1 mutations may have exerted a stronger dominant negative effect than either RUNX1 mutation alone, thus leading to a myeloid malignancy.

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We report two new RUNX1 mutations in a patient with thrombocytopenia and MDS.

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We demonstrate that a second hit to RUNX1 results in transformed MDS.

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Allogeneic transplant was successfully used to treat double RUNX1 mutant MDS.

Combining the differentiating effect of panobinostat with the apoptotic effect of arsenic trioxide leads to significant survival benefit in a model of t(8;21) acute myeloid leukemia

Background

One of the most frequently found abnormalities in acute myeloid leukemia (AML) is the t(8;21)(q22;q22) translocation, which is seen in around 15% of patients. This translocation results in the production of the AML1/ETO (A/E) fusion protein and commonly involves cooperating activating mutations of RAS. AE9a encodes a C-terminally truncated A/E protein of 575 amino acids that retains the ability to recruit histone deacetylases (HDACs). Expression of AE9a leads to rapid development of leukemia in experimental mouse systems. We have recently shown that treatment of mice bearing A/E9a;Nras^G12D tumors with the histone deacetylase inhibitor (HDACi) panobinostat leads to degradation of the A/E9a fusion protein, cell cycle arrest, differentiation of AML blasts into mature granulocytes and prolonged survival. Herein, we sought to enhance this therapeutic effect.

Findings

Combined treatment of mice bearing A/E9a;Nras^G12D leukemias with panobinostat and arsenic trioxide (ATO) resulted in a significant survival advantage compared to mice treated with either agent alone. Moreover, some of the mice treated with the panobinostat/ATO combination showed complete tumor responses and remained in remission for over 220 days. Panobinostat caused differentiation of A/E9a;Nras^G12D cells while ATO induced apoptosis of the leukemic cells, an effect that was enhanced following co-treatment with panobinostat.

Conclusions

Our results indicate that leukemic blast differentiation mediated by panobinostat combined with induction of apoptosis by ATO could be therapeutically beneficial and should be considered for patients with t(8;21) AML.

Electronic supplementary material

The online version of this article (doi:10.1186/s13148-014-0034-4) contains supplementary material, which is available to authorized users.

Drosophila as a model for the two myeloid blood cell systems in vertebrates

Fish, mice, and humans rely on two coexisting myeloid blood cell systems. One is sustained by hematopoietic progenitor cells, which reside in specialized microenvironments (niches) in hematopoietic organs and give rise to cells of the monocyte lineage. The other system corresponds to the independent lineage of self-renewing tissue macrophages, which colonize organs during embryonic development and are maintained during later life by proliferation in local tissue microenvironments. However, little is known about the nature of these microenvironments and their regulation. Moreover, many vertebrate tissues contain a mix of both tissue-resident and monocyte-derived macrophages, posing a challenge to the study of lineage-specific regulatory mechanisms and function. This review highlights how research in the simple model organism Drosophila melanogaster can address many of these outstanding questions in the field. Drawing parallels between hematopoiesis in Drosophila and vertebrates, we illustrate the evolutionary conservation of the two myeloid systems across animal phyla. Much like vertebrates, Drosophila possesses a lineage of self-renewing tissue-resident macrophages, which we refer to as tissue hemocytes, as well as a "definitive" lineage of macrophages that derive from hematopoiesis in the progenitor-based lymph gland. We summarize key findings from Drosophila hematopoiesis that illustrate how local microenvironments, systemic signals, immune challenges, and nervous inputs regulate adaptive responses of tissue-resident macrophages and progenitor-based hematopoiesis to maximize fitness of the animal.

Transcription of the AML1/ETO chimera is guided by the P2 promoter of the AML1 gene in the Kasumi-1 cell line

Chromosomal translocation t (8;21)(q22;22) is one of the most frequent cytogenetic abnormalities found in acute myeloid leukaemia (AML). It generates the AML1/ETO fusion gene, which itself supports human haematopoietic stem cell self-renewal. However, the mechanism guiding transcription of this chimeric gene remains unclear. In our work, we attempted to shed light on this essential issue. We investigated the promoter from which transcription of the AML1/ETO gene is initiated and defined the three-dimensional structure of the whole rearranged locus.

Epigenetic mechanisms in leukemia

Focal organization of regulatory machinery within the interphase nucleus is linked to biological responsiveness and perturbed in cancer. Lineage determinant Runx proteins organize and assemble multi-protein complexes at sites of transcription within the nucleus and regulate both RNA polymerase II- and I-mediated gene expression. In addition, Runx proteins epigenetically control lineage determining transcriptional programs including: 1) architectural organization of macromolecular complexes in interphase, 2) regulation of gene expression through bookmarking during mitosis, and 3) microRNA-mediated translational control in the interphase nucleus. These mechanisms are compromised with the onset and progression of cancer. For example, the oncogenic AML1-ETO protein, which results from a chromosomal translocation between chromosomes 8 and 21, is expressed in nearly 25% of all acute myelogenous leukemias, disrupts Runx1 subnuclear localization during interphase and compromises transcriptional regulation. Epigenetically, the leukemic protein redirects the Runx1 DNA binding domain to leukemia-specific nuclear microenvironments, modifies regulatory protein accessibility to Runx1 target genes by imprinting repressive chromatin marks, and deregulates the microRNA (miR) profile of diseased myeloid cells. Consequently, the entire Runx1-dependent transcriptional program of myeloid cells is deregulated leading to onset and progression of acute myeloid leukemia and maintenance of leukemic phenotype. We discuss the potential of modified epigenetic landscape of leukemic cells as a viable therapeutic target.

Hereditary leukemia due to rare RUNX1c splice variant (L472X) presents with eczematous phenotype

Deleterious mutations in the RUNX1 gene cause hereditary leukemia due to a rare syndrome called Familial platelet Disorder with Associated Myeloid Malignancy (FPDMM). We describe the characteristics of a family with FPDMM due to a novel RUNX1 mutation (L472X), located in the most 3-prime end of the gene reported to date. Our 36-year old proband presented with incidentally detected thrombocytopenia and a family history suggestive of FPDMM. Contrary to previously described families, affected members of our kindred express an eczematous phenotype, reportedly most severe in members who develop leukemia. Pedigree analysis shows that the L472X mutation tracks with thrombocytopenia, acute leukemia, and eczema. The L472X mutation produces a stably expressed RUNX1 protein product with a corresponding decrease in wild type RUNX1 expression. Our data supports the inclusion of eczema in the FPDMM phenotype and suggests the possibility that the RUNX1 L472X mutant causes the type of dominant negative affect that is associated with an elevated risk of leukemia in FPDMM families.

The human SWI/SNF complex associates with RUNX1 to control transcription of hematopoietic target genes

The acute myeloid leukemia 1 (AML1, RUNX1) transcription factor is a key regulator of hematopoietic differentiation that forms multi-protein complexes with co-regulatory proteins. These complexes are assembled at target gene promoters in nuclear microenvironments to mediate phenotypic gene expression and chromatin-related epigenetic modifications. Here, immunofluorescence microscopy and biochemical assays are used to show that RUNX1 associates with the human ATP-dependent SWI/SNF chromatin remodeling complex. The SWI/SNF subunits BRG1 and INI1 bind in vivo to RUNX1 target gene promoters (e.g., GMCSF, IL3, MCSF-R, MIP, and p21). These interactions correlate with histone modifications characteristic of active chromatin, including acetylated H4 and dimethylated H3 lysine 4. Downregulation of RUNX1 by RNA interference diminishes the binding of BRG1 and INI1 at selected target genes. Taken together, our findings indicate that RUNX1 interacts with the human SWI/SNF complex to control hematopoietic-specific gene expression.

Definitive hematopoiesis requires Runx1 C-terminal-mediated subnuclear targeting and transactivation

Runx1 is a key hematopoietic transcription factor required for definitive hematopoiesis and is a frequent target of leukemia-related chromosomal translocations. The resulting fusion proteins, while retaining DNA binding activity, display loss of subnuclear targeting and associated transactivation functions encoded by the C-terminus of the protein. To define the precise contribution of the Runx1 C-terminus in development and leukemia, we created a knock-in mouse with a C-terminal truncation by introducing a single nucleic acid substitution in the native Runx1 locus. This mutation (Runx1(Q307X)) models genetic lesions observed in patients with leukemia and myeloproliferative disorders. The Runx1(Q307X) homozygous mouse exhibits embryonic lethality at E12.5 due to central nervous system hemorrhages and a complete lack of hematopoietic stem cell function. While able to bind DNA, Runx1(Q307X) is unable to activate target genes, resulting in deregulation of various hematopoietic markers. Thus, we demonstrate that the subnuclear targeting and transcriptional regulatory activities of the Runx1 C-terminus are critical for hematopoietic development. We propose that compromising the C-terminal functions of Runx1 is a common mechanism for the pathological consequences of a variety of somatic mutations and Runx1-related leukemic fusion proteins observed in human patients.

Restoration of Runx1 expression in the Tie2 cell compartment rescues definitive hematopoietic stem cells and extends life of Runx1 knockout animals until birth

Mice deficient in the runt homology domain transcription factor Runx1/AML1 fail to generate functional clonogenic hematopoietic cells and die in utero by embryonic day 12.5. We previously generated Runx1 reversible knockout mice, in which the Runx1 locus can be restored by Cre-mediated recombination. We show here that selective restoration of the Runx1 locus in the Tie2 cell compartment rescues clonogenic hematopoietic progenitors in early Runx1-null embryos and rescues lymphoid and myeloid lineages during fetal development. Furthermore, fetal liver cells isolated from reactivated Runx1 embryos are capable of long-term multilineage lymphomyeloid reconstitution of adult irradiated recipients, demonstrating the rescue of definitive hematopoietic stem cells. However, this rescue of the definitive hematopoietic hierarchy is not sufficient to rescue the viability of animals beyond birth, pointing to an essential role for Runx1 in other vital developmental processes.

Monocytic leukemia zinc finger protein is essential for the development of long-term reconstituting hematopoietic stem cells

Monocytic leukemia zinc finger protein (MOZ), a transcriptional coactivator and member of the MYST family of histone acetyltransferases, is the target of recurrent translocations in acute myeloid leukemia. Since genes associated with translocations in leukemia are typically important regulators of blood formation, we investigated if Moz has a role in normal hematopoiesis. We generated mice carrying a mutation in the Moz gene. Homozygous Moz mutant mice died at birth. Moz mutant fetal liver hematopoietic cells were incapable of contributing to the hematopoietic system of recipients after transplantation. We observed profound defects in the stem cell compartment of Moz-deficient mice. Progenitors of all lineages were reduced in number. However, blood cell lineage commitment was unaffected. Together, these results show that Moz is essential for a fundamental property of hematopoietic stem cells, the ability to reconstitute the hematopoietic system of a recipient after transplantation and that Moz is specifically required in the stem cell compartment.

Normal and transforming functions of RUNX1: a perspective

Converging studies from many investigators indicate that RUNX1 has a critical role in the correct maintenance of essential cellular functions during embryonic development and after birth. The discovery that this gene is also frequently mutated in human leukemia has increased the interest in the role that RUNX1 plays in both normal and transforming pathways. Here, we provide an overview of the many roles of RUNX1 in hematopoietic self-renewal and differentiation and summarize the information that is currently available on the many mechanisms of RUNX1 deregulation in human leukemia.

Luteinizing hormone-induced RUNX1 regulates the expression of genes in granulosa cells of rat periovulatory follicles

The LH surge induces specific transcription factors that regulate the expression of a myriad of genes in periovulatory follicles to bring about ovulation and luteinization. The present study determined 1) the localization of RUNX1, a nuclear transcription factor, 2) regulation of Runx1 mRNA expression, and 3) its potential function in rat ovaries. Up-regulation of mRNA and protein for RUNX1 is detected in preovulatory follicles after human chorionic gonadotropin (hCG) injection in gonadotropin-treated immature rats as well as after the LH surge in cycling animals by in situ hybridization and immunohistochemical and Western blot analyses. The regulation of Runx1 mRNA expression was investigated in vitro using granulosa cells from rat preovulatory ovaries. Treatments with hCG, forskolin, or phorbol 12 myristate 13-acetate stimulated Runx1 mRNA expression. The effects of hCG were reduced by inhibitors of protein kinase A, MAPK kinase, or p38 kinase, indicating that Runx1 expression is regulated by the LH-initiated activation of these signaling mediators. In addition, hCG-induced Runx1 mRNA expression was inhibited by a progesterone receptor antagonist and an epidermal growth factor receptor tyrosine kinase inhibitor, whereas amphiregulin stimulated Runx1 mRNA expression, demonstrating that the expression is mediated by the activation of the progesterone receptor and epidermal growth factor receptor. Finally, knockdown of Runx1 mRNA by small interfering RNA decreased progesterone secretion and reduced levels of mRNA for Cyp11a1, Hapln1, Mt1a, and Rgc32. The hormonally regulated expression of Runx1 in periovulatory follicles, its involvement in progesterone production, and regulation of preovulatory gene expression suggest important roles of RUNX1 in the periovulatory process.

Multifunctional reversible knockout/reporter system enabling fully functional reconstitution of the AML1/Runx1 locus and rescue of hematopoiesis

Mice deficient in the runt homology domain transcription factor Runx1 die of severe anemia in utero by embryonic day (E)12.5. A reactivatable Runx1 knockout embryonic stem cell (ESC) and mouse systems were generated by the targeted insertion of a loxP-flanked multipartite gene stop/trap cassette designed to simultaneously ablate the expression of Runx1 and report on the activity of its promoters. The cassette's in-frame LacZ reporter enabled activities of the proximal and the distal promoters to be differentially monitored. Although Runx1-null ESCs were capable of primitive erythroid differentiation in vitro, their capacity to generate granulocyte/macrophage or mixed myelo-erythroid embryoid bodies was lost. Cre-mediated reactivation restored Runx1 structural integrity and rescued the hematopoietic differentiation potential of ESCs. Mice with the reactivated allele survived, showed no hematopoietic deficit, and expressed all major splice isoforms of Runx1 appropriately. This multipurpose mouse model will be useful for the analysis of the critical Runx1-dependent check-point(s) in hematopoietic development.

The Leukemia-associated ETO homologues are differently expressed during hematopoietic differentiation

The Eight twenty-one (ETO) homologues are nuclear repressor proteins including ETO, myeloid-transforming gene-related protein 1 (MTGR1), and myeloid-transforming gene chromosome 16 (MTG16). ETO and MTG16 are both part of fusion proteins resulting from chromosomal translocations associated with acute myeloid leukemia. Expression of these chimeras results in a differentiation block that contributes to the onset of leukemia. In order to elucidate the relation between the ETO homologues and hematopoietic differentiation, we determined the expression of the homologues during differentiation of leukemic and normal hematopoietic cells. Our results showed MTGR1 and MTG16 to be ubiquitously expressed in leukemic cell lines, whereas expression of ETO was observed only in an erythroleukemic cell line. The MTGR1 and MTG16 proteins decreased during all trans-retinoic acid-, but not vitamin D(3)-induced differentiation of leukemic cells. The reduction seemed to reflect a decrease in transcript levels as well as in protein stability. MTGR1 transcripts were ubiquitously expressed in human bone marrow cells. The MTG16 transcripts of CD34(+) progenitor cells were rapidly downregulated by cytokine-induced differentiation into myeloid or erythroid lineages. ETO transcripts, present at very low abundance in CD34(+) progenitor cells, were transiently upregulated during erythroid differentiation. In conclusion, the differential expression of the ETO homologues suggests that they may have a potential role in hematopoietic differentiation.

AML1-ETO fusion protein up-regulates TRKA mRNA expression in human CD34+ cells, allowing nerve growth factor-induced expansion

The AML1-ETO fusion protein, generated by the t(8;21) in acute myeloid leukemia (AML), exerts dominant-negative functions and a variety of gains of function, including a positive effect on the growth of primary human CD34+ hematopoietic stem/progenitor cells. We now show that AML1-ETO expression up-regulates the level of TRKA mRNA and protein in these cells and that AML1-ETO-expressing CD34+ hematopoietic cells grown in the presence of five early-acting hematopoietic cytokines further proliferate in response to nerve growth factor (NGF). These cells also show a unique response to NGF and IL-3; namely, they expand in liquid culture. To determine the biological relevance of our findings, we analyzed 262 primary AML patient samples using real-time RT-PCR and found that t(8;21)-positive AML samples express significantly higher levels of TRKA mRNA than other subtypes of AML. NGF, which is normally expressed by bone marrow stromal cells, could provide important proliferative or survival signals to AML1-ETO-expressing leukemic or preleukemic cells, and the NGF/TRKA signaling pathway may be a suitable target for therapeutic approaches to AML.

The polymorphisms of Tim-1 promoter region are associated with rheumatoid arthritis in a Korean population

It has been determined that the family of T-cell immunoglobulin domain and mucin domain (TIM) proteins is expressed on T cells. A member of the TIM family, TIM-1, is considered to be a membrane protein associated with the development of Th2-biased immune responses and selectively expressed on Th2 cells. We previously showed that the exon 4 variations of Tim-1 are associated with susceptibility to allergic diseases, as well as autoimmune diseases such as rheumatoid arthritis (RA). In this study, we assessed the association between genotype and allele frequencies of the Tim-1 gene promoter region, in both RA patients and the controls without RA, using polymerase chain reaction-restriction fragment length polymorphism and single-base extension methods. We further investigated the relationships among the genotypes of each polymorphism and C-reactive protein or rheumatoid factor levels in RA patients. The genotype and allele frequencies of the -1637A>G polymorphism in RA patients are significantly different from those in the non-RA controls (P=0.0004 and P=0.001, respectively). Our results strongly suggest that polymorphism in the Tim-1 promoter region might be associated with susceptibility to RA.

Conservation and expression of an alternative 3' exon of Runx2 encoding a novel proline-rich C-terminal domain

The Runx2 (Cbfa1, Aml3, PEBP2alphaA) gene plays an essential role in bone development and is one of a three-member family of closely related genes that encode the alpha-chain DNA binding components of the heterodimeric core binding factor complex. While all three mammalian Runx genes share a complex dual promoter structure (P1, P2) and display alternative splicing, a distinctive feature of Runx2 is the potential to encode larger isoforms in which the C-terminal domain encoded by the standard 3' terminal exon (exon 6) is replaced by an extended 200-201 amino acid C-terminal sequence including an extensive proline-rich domain and a C-terminal amphipathic helix. We report that the novel exon that gives rise to these variants (exon 6.1) is located over 100 kb downstream of exon 6 in the mouse, rat and human genomes. Exon 6.1 spans a CpG-rich island, and human/rodent conservation is evident through the coding sequence and the 3' untranslated region (UTR). Reverse transcriptase polymerase chain reaction (RT-PCR) and blot hybridisation analyses reveal that exon 6.1 is utilised at low levels in all mouse tissues and cell lines that express Runx2, regardless of which promoter is active, giving Runx2 the potential to encode more than 12 distinct isoforms. RT-PCR analysis of human RUNX2 exon 6.1 expression shows that utilisation of this exon is also conserved. In vitro transcription/translation of cDNAs encoding several exon 6.1 isoforms reveals that the novel Runx proteins are able to bind specifically to canonical Runx DNA target sequences. Antibodies raised to the unique C-terminal domain were shown to be reactive by immunoprecipitation and immunoblot assay, and were used in confocal immunofluorescence microscopy to reveal low level cytoplasmic staining in osteosarcoma and lymphoma cells that express high levels of Runx2 mRNA. However, reactive protein could not be detected in immunoblots of extracts from either cell type, suggesting that these proteins are unstable in lymphoid and osteosarcoma cells. In conclusion, the conservation and widespread utilisation of Runx2 exon 6.1 suggest that its encoded isoforms play an as yet undetermined role in mammalian development.

Haploinsufficiency of AML1 results in a decrease in the number of LTR-HSCs while simultaneously inducing an increase in more mature progenitors

The AML1/CBFbeta transcriptional complex is essential for the formation of definitive hematopoietic stem cells (HSCs). Moreover, development of the hematopoietic system is exquisitely sensitive to the level of this complex. To investigate the effect of AML1 dosage on adult hematopoiesis, we compared the hematopoietic systems of AML1+/- and AML1+/+ mice. Surprisingly, loss of a single AML1 allele resulted in a 50% reduction in long-term repopulating hematopoietic stem cells (LTR-HSCs). This decrease did not, however, extend to the next level of hematopoietic differentiation. Instead, AML1+/- mice had an increase in multilineage progenitors, an expansion that resulted in enhanced engraftment following transplantation. The expanded pool of AML1+/- progenitors remained responsive to homeostatic mechanisms and thus the number of mature cells in most lineages remained within normal limits. Two notable exceptions were a decrease in CD4(+) T cells, leading to an inversion of the CD4(+) to CD8(+) T-cell ratio and a decrease in circulating platelets. These data demonstrate a dosage-dependent role for AML1/CBFbeta in regulating the quantity of HSCs and their downstream committed progenitors, as well as a more restricted role in T cells and platelets. The latter defect mimics one of the key abnormalities in human patients with the familial platelet disorder resulting from AML1 haploinsufficiency.

The Runx genes: lineage-specific oncogenes and tumor suppressors

The Runx genes present a challenge to the simple binary classification of cancer genes as oncogenes or tumor suppressors. There is evidence that loss of function of two of the three mammalian Runx genes promotes cancer, but in a highly lineage-restricted manner. In human leukemias, the RUNX1 gene is involved in various chromosomal translocation events that create oncogenic fusion proteins, at least some of which appear to function as dominant-negative inhibitors of the normal gene product. Paradoxically, evidence is mounting that structurally intact Runx genes are also oncogenic when overexpressed. All the three murine genes act as targets for transcriptional activation by retroviral insertional mutagenesis, and the oncogenic potential of Runx2 has been confirmed in transgenic mice. Moreover, the RUNX1 gene is often amplified or overexpressed in cases of acute leukemia. The state of progress in elucidating the oncogenic roles of the Runx genes is the subject of this review, and we draw together recent observations in a tentative model for the effects of Runx deregulation on hematopoietic cell differentiation. We suggest that lineage-specific factors determine the sensitivity to the oncogenic effects of loss or overexpression of Runx factors.

Lymphoid Enhancer Factor-1 Links Two Hereditary Leukemia Syndromes through Core-binding Factor α Regulation of ELA2

Two hereditary human leukemia syndromes are severe congenital neutropenia (SCN), caused by mutations in the gene ELA2, encoding the protease neutrophil elastase, and familial platelet disorder with acute myelogenous leukemia (AML), caused by mutations in the gene AML1, encoding the transcription factor core-binding factor alpha (CBFalpha). In mice, CBFalpha regulates the expression of ELA2, suggesting a common link for both diseases. However, gene-targeted mouse models have failed to reproduce either human disease, thus prohibiting further in vivo studies in mice. Here we investigate CBFalpha regulation of the human ELA2 promoter, taking advantage of bone marrow obtained from patients with either illness. In particular, we have identified novel ELA2 promoter substitutions (-199 C to A) within a potential motif for lymphoid enhancer factor-1 (LEF-1), a transcriptional mediator of Wnt/beta-catenin signaling, in SCN patients. The LEF-1 motif lies adjacent to a potential CBFalpha binding site that is in a different position in human compared with mouse ELA2. We find that LEF-1 and CBFalpha co-activate ELA2 expression. In vitro, the high mobility group domain of LEF-1 interacts with the runt DNA binding and proline-, serine-, threonine-rich activation domains of CBFalpha. ELA2 transcript levels are up-regulated in bone marrow of an SCN patient with the -199 C to A substitution. Conversely, a mutation of the CBFalpha activation domain, found in a patient with familial platelet disorder with AML, fails to stimulate the ELA2 promoter in vitro, and bone marrow correspondingly demonstrates reduced ELA2 transcript. Observations in these complementary patients indicate that LEF-1 cooperates with CBFalpha to activate ELA2 in vivo and also suggest the possibility that up-regulating promoter mutations can contribute to SCN. Two hereditary AML predisposition syndromes may therefore intersect via LEF-1, potentially linking them to more generalized cancer mechanisms.

Interactions between the leukaemia-associated ETO homologues of nuclear repressor proteins

The eight-twenty-one (ETO) homologues, represented by ETO, myeloid transforming gene-related protein 1 (MTGR1) and myeloid transforming gene chromosome 16 (MTG16), are nuclear repressor proteins. ETO is part of the fusion protein acute myeloid leukaemia (AML)1-ETO, resulting from the translocation (8;21). Similarly, MTG16 is disrupted to become part of AML1/MTG16 in t(16;21). The aberrant expression of these chimeras could affect interplay between ETO homologues and contribute to the leukaemogenic process. We investigated possible interactions between the ETO homologues. Ectopic co-expression in COS-cells resulted in heterodimerisation of the various ETO homologues suggesting that they may co-operate. Similarly, the chimeric oncoprotein AML1-ETO interacted with both MTGR1 and MTG16. However, results from cell lines endogenously expressing more than one ETO homologue did not demonstrate co-precipitation. Results from IP-Western and size determination by gel filtration of deletion mutants expressed in COS-cells, indicated an important role of the HHR domain for oligomerisation. A role was also suggested for the Nervy domain in the homologue interactions. Our results suggest that ETO homologues can interact with each other as well as with AML1-ETO, although it is unclear as to what extent these interactions occur in vivo.

Runx3 is required for hematopoietic development in zebrafish

We cloned zebrafish runx3/aml2/cbfa3 and examined its expression and function during embryogenesis. In the developing embryo, runx3 is dynamically expressed in hematopoietic, neuronal, and cartilaginous tissues. Hematopoietic expression of runx3 commences late in embryogenesis in the ventral tail intermediate cell mass and later colocalizes with spi1 and lyz in circulating blood cells. In the cloche mutant, hematopoietic expression was absent, suggesting that Runx3 functions downstream of cloche in a hematopoietic pathway. Neuronal tissues expressing runx3 include the trigeminal ganglia and Rohon-Beard neurons. Runx3 appears to contribute to normal development of primitive and definitive hematopoietic cells. When Runx3 function was compromised using morpholino oligonucleotides, a reduction in the number of mature blood cells was observed. Furthermore, Runx3 depletion decreased runx1 expression in the ventral wall of the dorsal aorta and reduced the number of spi1- and lyz-containing blood cells. Conversely, ubiquitous overexpression of runx3 led to an increase in primitive blood cell numbers, together with an increase in runx1-expressing cells in the ventral wall of the dorsal aorta. We propose a role for Runx3 in the regulation of blood cell numbers.

Normal and leukemic hematopoiesis: are leukemias a stem cell disorder or a reacquisition of stem cell characteristics?

Leukemia can be viewed as a newly formed, abnormal hematopoietic tissue initiated by a few leukemic stem cells (LSCs) that undergo an aberrant and poorly regulated process of organogenesis analogous to that of normal hematopoietic stem cells. A hallmark of all cancers is the capacity for unlimited self-renewal, which is also a defining characteristic of normal stem cells. Given this shared attribute, it has been proposed that leukemias may be initiated by transforming events that take place in hematopoietic stem cells. Alternatively, leukemias may also arise from more committed progenitors caused by mutations and/or selective expression of genes that enhance their otherwise limited self-renewal capabilities. Identifying the LSCs for each type of leukemia is a current challenge and a critical step in understanding their respective biologies and may provide key insights into more effective treatments. Moreover, LSC identification and purification will provide a powerful diagnostic, prognostic, and therapeutic tool in the clinic.

The Runx1 transcription factor inhibits the differentiation of naive CD4+ T cells into the Th2 lineage by repressing GATA3 expression

Differentiation of naive CD4+ T cells into helper T (Th) cells is controlled by a combination of several transcriptional factors. In this study, we examined the functional role of the Runx1 transcription factor in Th cell differentiation. Naive T cells from transgenic mice expressing a dominant interfering form of Runx1 exhibited enhanced interleukin 4 production and efficient Th2 differentiation. In contrast, transduction of Runx1 into wild-type T cells caused a complete attenuation of Th2 differentiation and was accompanied by the cessation of GATA3 expression. Furthermore, endogenous expression of Runx1 in naive T cells declined after T cell receptor stimulation, at the same time that expression of GATA3 increased. We conclude that Runx1 plays a novel role as a negative regulator of GATA3 expression, thereby inhibiting the Th2 cell differentiation.

Transcriptional regulation of the human MIP-1alpha promoter by RUNX1 and MOZ

The transcription factor RUNX1 (AML-1, PEBP2alphaB and CBFA2) is essential for definitive haematopoiesis, and chromosomal translocations involving the RUNX1 gene are frequently found in acute leukaemias. The gene encoding the histone acetyltransferase MOZ is also rearranged in some acute leukaemias, resulting in the expression of MOZ fusion proteins. MOZ has recently been shown to interact directly with RUNX1, indicating that MOZ fusion proteins act by deregulating RUNX1 function. Macrophage inflammatory protein-1alpha (MIP-1alpha) is a proinflammatory cytokine that also inhibits proliferation of haematopoietic stem cells. Amongst the conserved sequence elements in the human MIP-1alpha promoter are two consensus RUNX sites. We have investigated the role of these RUNX sites in the regulation of the MIP-1alpha promoter by PMA/PHA stimulation in Jurkat T-cells. RUNX1 can specifically bind to both RUNX sites in vitro and chromatin immunoprecipitation assays demonstrated that endogenous RUNX1 is constitutively bound to the endogenous MIP-1alpha promoter. Mutation of the RUNX sites demonstrated that the proximal RUNX site is essential for PMA/PHA-stimulated activation of the MIP-1alpha promoter. Activation of the promoter can also be inhibited by heterologous expression of the repressor protein AML-1/ETO. We further demonstrate that MOZ can activate the MIP-1alpha promoter and that this activation is largely dependent upon the proximal RUNX site. Moreover, we show that co-expression of MOZ and RUNX1 can synergistically activate the MIP-1alpha promoter. The regulation of MIP-1alpha expression by RUNX1/MOZ is discussed in the context of MIP-1alpha's role as an inhibitor of haematopoietic stem cell proliferation and its potential importance in leukaemias associated with RUNX1 or MOZ chromosomal rearrangements.

AML-1A and AML-1B regulation of MIP-1alpha expression in multiple myeloma

Macrophage inflammatory protein-1alpha (MIP-1alpha) is produced in high concentration by multiple myeloma (MM) cells in about 70% of patients, and MIP-1alpha levels correlate with their disease activity. Patients who have high levels of MIP-1alpha have a poor prognosis. Furthermore, blocking MIP-1alpha expression in an in vivo model of human MM profoundly decreases both tumor burden and bone destruction, suggesting that MIP-1alpha is an important mediator of MM bone disease. Therefore, to analyze the regulation of MIP-1alpha production in MM, we cloned the human MIP-1alpha promoter and characterized the transcription factor (TF) motifs that control MIP-1alpha expression in MM cells. The proximal region of MIP-1alpha promoter was composed of 2 sets of identical transcription regulatory regions consisting of GATA-2(+) AML-1(+) C/EBPalpha motifs. Since 2 alternatively spliced variants of the acute myeloid leukemia-1 (AML-1) class of TFs can bind the AML-1 region, AML-1A and AML-1B, the relationship between the expression levels of AML-1A or AML-1B in MM cells and their capacity to express MIP-1alpha was examined. AML-1A mRNA was relatively overexpressed compared with AML-1B in MM cell lines that produced high levels of MIP-1alpha (> 1 ng/mL per 10(6) cells per 72 hours), but AML-1A was not increased in MM cell lines that expressed less than 200 pg/mL MIP-1alpha. More importantly, the ratio of AML-1A to AML-1B mRNA levels was also increased in 3 of 3 highly purified myeloma cells from patients with MM who expressed increased amounts of MIP-1alpha. The ratio of AML-1A to AML-1B mRNA in patients with MM was 8-fold higher than in healthy controls. Transduction of AML-1B into the MM-derived MM.1S and ARH-77 cells totally blocked MIP-1alpha production, while AML-1A did not further increase the already high levels of MIP-1alpha produced by these cells. Taken together, these data demonstrate that in patients with MM who produce increased concentrations of MIP-1alpha, the relative level of AML-1B is significantly decreased compared with healthy controls. The data suggest that strategies that enhance AML-1B expression or decrease AML-1A in MM cells may be beneficial therapeutically.

A Serrate-expressing signaling center controls Drosophila hematopoiesis

The differentiation of Drosophila blood cells relies on a functional hierarchy between the GATA protein, Serpent (Srp), and multiple lineage-specific transcription factors, such as the AML1-like protein, Lozenge (Lz). Two major branches of Drosophila hematopoiesis give rise to plasmatocytes/macrophages and crystal cells. Serrate signaling through the Notch pathway is critical in the regulation of Lz expression and the specification of crystal cell precursors, thus providing a key distinction between the two lineages. The expression of Serrate marks a discrete cluster of cells in the lymph gland, a signaling center, with functional similarities to stromal signaling in mammalian hematopoiesis.

The AML1-ETO fusion gene promotes extensive self-renewal of human primary erythroid cells

The t(8;21) translocation, which encodes the AML1-ETO fusion protein (now known as RUNX1-CBF2T1), is one of the most frequent translocations in acute myeloid leukemia, although its role in leukemogenesis is unclear. Here, we report that exogenous expression of AML1-ETO in human CD34(+) cells severely disrupts normal erythropoiesis, resulting in virtual abrogation of erythroid colony formation. In contrast, in bulk liquid culture of purified erythroid cells, we found that while AML1-ETO initially inhibited proliferation during early (erythropoietin [EPO]-independent) erythropoiesis, growth inhibition gave way to a sustained EPO-independent expansion of early erythroid cells that continued for more than 60 days, whereas control cultures became growth arrested after 10 to 13 days (at the EPO-dependent stage of development). Phenotypic analysis showed that although these cells were CD13(-) and CD34(-), unlike control cultures, these cells failed to up-regulate CD36 or to down-regulate CD33, suggesting that expression of AML1-ETO suppressed the differentiation of these cells and allowed extensive self-renewal to occur. In the early stages of this expansion, addition of EPO was able to promote both phenotypic (CD36(+), CD33(-), glycophorin A(+)) and morphologic differentiation of these cells, almost as effectively as in control cultures. However, with extended culture, cells expressing AML1-ETO became refractory to addition of this cytokine, suggesting that a block in differentiation had been established. These data demonstrate the capacity of AML1-ETO to promote the self-renewal of human hematopoietic cells and therefore support a causal role for t(8;21) translocations in leukemogenesis.

Multiple subnuclear targeting signals of the leukemia-related AML1/ETO and ETO repressor proteins

Leukemic disease can be linked to aberrant gene expression. This often is the result of molecular alterations in transcription factors that lead to their misrouting within the nucleus. The acute myelogenous leukemia-related fusion protein AML1ETO is a striking example. It originates from a gene rearrangement t(8;21) that fuses the N-terminal part of the key hematopoietic regulatory factor AML1 (RUNX1) to the ETO (MTG8) repressor protein. AML1ETO lacks the intranuclear targeting signal of the wild-type AML1 and is directed by the ETO component to alternate nuclear matrix-associated sites. To understand this aberrant subnuclear trafficking of AML1ETO, we created a series of mutations in the ETO protein. These were characterized biochemically by immunoblotting and in situ by immunofluorescence microscopy. We identified two independent subnuclear targeting signals in the N- and C-terminal regions of ETO that together direct ETO to the same binding sites occupied by AML1ETO. However, each segment alone is targeted to a different intranuclear location. The N-terminal segment contains a nuclear localization signal and the conserved hydrophobic heptad repeat domain responsible for protein dimerization and interaction with the mSin3A transcriptional repressor. The C-terminal segment spans the nervy domain and the zinc finger region, which together support interactions with the corepressors N-CoR and HDACs. Our findings provide a molecular basis for aberrant subnuclear targeting of the AML1ETO protein, which is a principal defect in t(8;21)-related acute myelogenous leukemia.

Genomic characterization of the RUNX2 gene of Fugu rubripes

A 105 kb Fugu rubripes genomic region containing the RUNX2 ortholog (frunx2) was sequenced and analysed. Spanning 32 kb, frunx2 is seven times smaller than its human orthologue (223 kb). By comparison of Fugu and human genomic environment a stretch of conserved synteny, comprising the neighbouring genes on both sides, was identified. Except one exon that is alternatively spliced in human RUNX2, all other seven exons could be identified in frunx2. The predicted protein sequence of frunx2 shows a high degree of sequence conservation compared with RUNX2 (83% identity). Like all human paralogues, frunx2 possesses two promoter regions separated by a large intron. Both promoter regions are conserved between the two species and contain several RUNX binding sites pointing to a self-regulatory function. Three further conserved non-coding regions were identified possibly functioning as enhancer elements for tissue-specific expression of RUNX2.

Expression of a conditional AML1-ETO oncogene bypasses embryonic lethality and establishes a murine model of human t(8;21) acute myeloid leukemia

The AML1/CBFbeta transcription factor complex, a frequent target of chromosomal translocations in leukemia, is essential for the generation of definitive hematopoietic stem cells. Paradoxically, expression of the acute myeloid leukemia-associated AML1-ETO fusion protein in mice results not in leukemia, but in embryonic lethality due to an absence of normal hematopoiesis. To bypass the embryonic lethality, we generated a mouse strain with a conditional AML1-ETO knockin allele that contains a loxP bracketed transcriptional stop cassette 5' to the AML1-ETO fusion site. Activation of this allele in vivo by Cre-mediated recombination resulted in an enhanced replating efficiency of myeloid progenitors, but it did not block their differentiation, nor was it sufficient to induce leukemia. However, induction of cooperating mutations resulted in the development of an acute myeloid disease that mimicked many of the features of human AML1-ETO-expressing leukemia.

The AML1-ETO fusion protein promotes the expansion of human hematopoietic stem cells

The acute myelogenous leukemia-1 (AML1)-ETO fusion protein is generated by the t(8;21), which is found in 40% of AMLs of the French-American-British M2 subtype. AML1-ETO interferes with the function of the AML1 (RUNX1, CBFA2) transcription factor in a dominant-negative fashion and represses transcription by binding its consensus DNA-binding site and via protein-protein interactions with other transcription factors. AML1 activity is critical for the development of definitive hematopoiesis, and haploinsufficiency of AML1 has been linked to a propensity to develop AML. Murine experiments suggest that AML1-ETO expression may not be sufficient for leukemogenesis; however, like the BCR-ABL isoforms, the cellular background in which these fusion proteins are expressed may be critical to the phenotype observed. Retroviral gene transfer was used to examine the effect of AML1-ETO on the in vitro behavior of human hematopoietic stem and progenitor cells. Following transduction of CD34(+) cells, stem and progenitor cells were quantified in clonogenic assays, cytokine-driven expansion cultures, and long-term stromal cocultures. Expression of AML1-ETO inhibited colony formation by committed progenitors, but enhanced the growth of stem cells (cobblestone area-forming cells), resulting in a profound survival advantage of transduced over nontransduced cells. AML1-ETO-expressing cells retained progenitor activity and continued to express CD34 throughout the 5-week long-term culture. Thus, AML1-ETO enhances the self-renewal of pluripotent stem cells, the physiological target of many acute myeloid leukemias.

Overexpression of AML1 transcription factor drives thymocytes into the CD8 single-positive lineage

To understand the gene regulation involved in the development of single-positive (SP) thymocytes, we generated transgenic mice in which the AML1 transcription factor is overexpressed. In these mice the number of CD8 SP thymocytes was greatly increased, and this continued to be true even when MHC class I was absent. This promotion to the CD8 SP lineage was not, however, observed when both class I and class II were absent. Furthermore, even thymocytes carrying MHC class II-restricted TCR differentiated into the CD8 SP lineage when AML1 was overexpressed. The selected CD8 SP cells were, however, unable to mature, as judged by the expression level of heat-stable Ag. Thus, overexpression of AML1 is able to skew class II-restricted thymocytes into the CD8 SP lineage, but not to drive the maturation of resulting selected CD8 SP cells.

AML1-ETO expression is directly involved in the development of acute myeloid leukemia in the presence of additional mutations

The t(8;21) is one of the most frequent chromosomal abnormalities associated with acute myeloid leukemia (AML). The translocation, which involves the AML1 gene on chromosome 21 and the ETO gene on chromosome 8, generates an AML1-ETO fusion transcription factor. To examine the effect of the AML1-ETO fusion protein on leukemogenesis, we made transgenic mice in which expression of AML1-ETO is under the control of the human MRP8 promoter (hMRP8-AML1-ETO). AML1-ETO is specifically expressed in myeloid cells, including common myeloid progenitors of hMRP8-AML1-ETO transgenic mice. The transgenic mice were healthy during their life spans, suggesting that AML1-ETO alone is not sufficient for leukemogenesis. However, after treatment of newborn hMRP8-AML1-ETO transgenic mice and their wild-type littermates with a strong DNA-alkylating mutagen, N-ethyl-N-nitrosourea, 55% of transgenic mice developed AML and the other 45% of transgenic mice and all of the wild-type littermates developed acute T lymphoblastic leukemia. Our results provide direct evidence that AML1-ETO is critical for causing myeloid leukemia, but one or more additional mutations are required for leukemogenesis. The hMRP8-AML1-ETO-transgenic mice provide an excellent model that can be used to isolate additional genetic events and to further understand the molecular pathogenesis of AML1-ETO-related leukemia.

Function of the inv(16) fusion gene CBFB-MYH11

Inv(16)(p13q22) is associated with acute myeloid leukemia subtype M4Eo, which is characterized by the presence of myelomonocytic blasts and atypical eosinophils. This chromosomal rearrangement results in the fusion of CBFB and MYH11 genes. Mouse models indicate that the fusion gene, Cbfb-MYH11, inhibits differentiation of hematopoietic cells. Although expression of Cbfb-MYH11 is not sufficient for leukemogenesis, a combination of Cbfb-MYH11 and additional mutations can lead specifically to the development of myeloid leukemia. Normally, CBFbeta interacts with CBFalpha to form a transcriptionally active nuclear complex. In vitro studies indicate that expression of CBFB-MYH11 leads to sequestration of CBFalpha2 in the cytoplasm. It also has been shown to inhibit CBF-mediated transactivation, slow cell cycle progression, delay the apoptotic response to DNA damaging agents, and protect CBFalpha2 from degradation. The importance of these functions in vivo remains to be determined.

Regulation of hemangioblast development

The in vitro differentiation of embryonic stem (ES) cells provides a powerful approach for studying the earliest events involved in the commitment of the hematopoietic and endothelial lineages. Using this model system, we have identified a precursor with the potential to generate both primitive and definitive hematopoietic cells as well as cells with endothelial characteristics. The developmental potential of this precursor suggests that it represents the in vitro equivalent of the hemangioblast, a common stem cell for both lineages. ES cells deficient for the transcription factor scl/tal-1 are unable to generate hemangioblasts, while those deficient for Runx1 generate reduced numbers of these precursors. These findings indicate that both genes play pivotal roles at the earliest stages of hematopoietic and endothelial development. In addition, they highlight the strength of this model system in studying the function of genes in embryonic development.