GATA1 in normal and malignant hematopoiesis

The negative impact of Wnt signaling on megakaryocyte and primitive erythroid progenitors derived from human embryonic stem cells

The Wnt gene family consists of structurally related genes encoding secreted signaling molecules that have been implicated in many developmental processes, including regulation of cell fate and patterning during embryogenesis. Previously, we found that Wnt signaling is required for primitive or yolk sac-derived-erythropoiesis using the murine embryonic stem cell (ESC) system. Here, we examine the effect of Wnt signaling on the formation of early hematopoietic progenitors derived from human ESCs. The first hematopoietic progenitor cells in the human ESC system express the pan-hematopoietic marker CD41 and the erythrocyte marker, glycophorin A or CD235. We have developed a novel serum-free, feeder-free, adherent differentiation system that can efficiently generate large numbers of CD41+CD235+ cells. We demonstrate that this cell population contains progenitors not just for primitive erythroid and megakaryocyte cells but for the myeloid lineage as well and term this population the primitive common myeloid progenitor (CMP). Treatment of mesoderm-specified cells with Wnt3a led to a loss of hematopoietic colony-forming ability while the inhibition of canonical Wnt signaling with DKK1 led to an increase in the number of primitive CMPs. Canonical Wnt signaling also inhibits the expansion and/or survival of primitive erythrocytes and megakaryocytes, but not myeloid cells, derived from this progenitor population. These findings are in contrast to the role of Wnt signaling during mouse ESC differentiation and demonstrate the importance of the human ESC system in studying species-specific differences in development.

PML4 facilitates erythroid differentiation by enhancing the transcriptional activity of GATA-1

Promyelocytic leukemia protein (PML) has been implicated as a participant in multiple cellular processes including senescence, apoptosis, proliferation, and differentiation. Studies of PML function in hematopoietic differentiation previously focused principally on its myeloid activities and also indicated that PML is involved in erythroid colony formation. However, the exact role that PML plays in erythropoiesis is essentially unknown. In this report, we found that PML4, a specific PML isoform expressed in erythroid cells, promotes endogenous erythroid genes expression in K562 and primary human erythroid cells. We show that the PML4 effect is GATA binding protein 1 (GATA-1) dependent using GATA-1 knockout/rescued G1E/G1E-ER4 cells. PML4, but not other detected PML isoforms, directly interacts with GATA-1 and can recruit it into PML nuclear bodies. Furthermore, PML4 facilitates GATA-1 trans-activation activity in an interaction-dependent manner. Finally, we present evidence that PML4 enhances GATA-1 occupancy within the globin gene cluster and stimulates cooperation between GATA-1 and its coactivator p300. These results demonstrate that PML4 is an important regulator of GATA-1 and participates in erythroid differention by enhancing GATA-1 trans-activation activity.

Transcriptional mechanisms underlying hemoglobin synthesis

The physiological switch in expression of the embryonic, fetal, and adult β-like globin genes has garnered enormous attention from investigators interested in transcriptional mechanisms and the molecular basis of hemoglobinopathies. These efforts have led to the discovery of cell type-specific transcription factors, unprecedented mechanisms of transcriptional coregulator function, genome biology principles, unique contributions of nuclear organization to transcription and cell function, and promising therapeutic targets. Given the vast literature accrued on this topic, this article will focus on the master regulator of erythroid cell development and function GATA-1, its associated proteins, and its frontline role in controlling hemoglobin synthesis. GATA-1 is a crucial regulator of genes encoding hemoglobin subunits and heme biosynthetic enzymes. GATA-1-dependent mechanisms constitute an essential regulatory core that nucleates additional mechanisms to achieve the physiological control of hemoglobin synthesis.

Proteomic analysis of murine bone marrow niche microenvironment identifies thioredoxin as a novel agent for radioprotection and for enhancing donor cell reconstitution

Hematopoiesis is regulated by the bone marrow (BM) niche microenvironment. We recently found that posttransplant administration of AMD3100 (a specific and reversible CXCR4 antagonist) enhanced donor cell engraftment and promoted recovery of all donor cell lineages in a congeneic mouse transplant model. We hypothesized that AMD3100 enhances donor cell reconstitution in part by modulating the levels and constitution of soluble factors in the niche microenvironment. In the current study, the effects of the BM extracellular fluid (supernatant) from AMD3100-treated transplant recipient mice on colony-forming units (CFUs) were examined. A semiquantitative, mass spectrometry-based proteomics approach was used to screen for differentially expressed proteins between the BM supernatants of PBS-treated transplant mice and AMD3100-treated transplant mice. A total of 178 proteins were identified in the BM supernatants. Thioredoxin was among the 32 proteins that displayed greater than a twofold increase in spectral counts in the BM supernatant of AMD3100-treated transplant mice. We found that thioredoxin increased CFUs in a dose-dependent manner. Thioredoxin improved hematopoiesis in irradiated mice and protected mice from radiation-related death. Furthermore, ex vivo exposure to thioredoxin for 24 hours enhanced the long-term repopulation of hematopoietic stem cells. Additionally, combined posttransplant administration of thioredoxin and AMD3100 improved hematologic recovery in primary and secondary transplant recipient mice. Our studies demonstrated that factors in the BM niche microenvironment play a critical role in hematopoiesis. Identifying these factors provides clues on potential novel targets that can be used to enhance hematologic recovery in hematopoietic stem cell transplan`tation.

Overexpression of GATA1 confers resistance to chemotherapy in acute megakaryocytic Leukemia

It has been previously shown that acute myeloid leukemia (AML) patients with higher levels of GATA1 expression have poorer outcomes. Furthermore, pediatric Down syndrome (DS) patients with acute megakaryocytic leukemia (AMKL), whose blast cells almost universally harbor somatic mutations in exon 2 of the transcription factor gene GATA1, demonstrate increased overall survival relative to non-DS pediatric patients, suggesting a potential role for GATA1 in chemotherapy response. In this study, we confirmed that amongst non-DS patients, GATA1 transcripts were significantly higher in AMKL blasts compared to blasts from other AML subgroups. Further, GATA1 transcript levels significantly correlated with transcript levels for the anti-apoptotic protein Bcl-xL in our patient cohort. ShRNA knockdown of GATA1 in the megakaryocytic cell line Meg-01 resulted in significantly increased cytarabine (ara-C) and daunorubicin anti-proliferative sensitivities and decreased Bcl-xL transcript and protein levels. Chromatin immunoprecipitation (ChIP) and reporter gene assays demonstrated that the Bcl-x gene (which transcribes the Bcl-xL transcripts) is a bona fide GATA1 target gene in AMKL cells. Treatment of the Meg-01 cells with the histone deacetylase inhibitor valproic acid resulted in down-regulation of both GATA1 and Bcl-xL and significantly enhanced ara-C sensitivity. Furthermore, additional GATA1 target genes were identified by oligonucleotide microarray and ChIP-on-Chip analyses. Our findings demonstrate a role for GATA1 in chemotherapy resistance in non-DS AMKL cells, and identified additional GATA1 target genes for future studies.

Blood group phenotypes resulting from mutations in erythroid transcription factors

PURPOSE OF REVIEW

This review describes the genetics of unusual blood group phenotypes, particularly those with altered expression of Lutheran antigens, and how this area of study has informed our understanding of erythropoiesis in general and haemoglobin switching in particular.

RECENT FINDINGS

Mutations in erythroid transcription factors GATA1 (GATA1 binding protein 1) and KLF1 (Kruppel-like factor 1) cause benign and disease phenotypes in humans [X-linked Lu(a-b-) phenotype, In(Lu) blood group phenotype, hereditary persistence of foetal haemoglobin, borderline HbA(2), and congenital dyserythropoietic anaemia (CDA)]. These studies explain the occurrence of rare blood group phenotypes with simultaneous altered expression of antigens from several blood group systems and illuminate the role of KLF1 in gamma and delta globin gene regulation.

SUMMARY

The study of rare blood group phenotypes is a potent tool for discovery of mutations in human genes. Elucidation of the molecular basis of the rare In(Lu) phenotype revealed the first mutations in human KLF1. Subsequently, numerous additional mutations have been described, one of which causes a rare form of CDA. Analysis of the X-linked Lu(a-b-) phenotype revealed a mutation in the C-terminal domain of human GATA1. The apparent sensitivity of the Lutheran glycoprotein to alterations in GATA1 and KLF1 activity suggest that it could be a useful biomarker of erythroid transcription factor mutation.

Establishment of a cell-type-specific genetic network by the mediator complex component Med1

The intense physiologic demand to generate vast numbers of red blood cells requires the establishment of a complex genetic network by the master regulatory transcription factor GATA-1 and its coregulators. This network dictates the genesis of enucleated erythrocytes by orchestrating the survival, proliferation, and differentiation of progenitor cells. In addition to the crucial GATA-1 coregulator Friend of GATA-1 (FOG-1), a component of the Mediator complex, Med1, facilitates GATA-1-dependent transcription at select target genes and controls erythropoiesis. It is not known to what extent Med1 contributes to GATA-1 function or whether Med1 controls a large or restricted cohort of genes that are not regulated by GATA-1. Using a genetic complementation assay in GATA-1-null erythroid cells, we demonstrate that Med1 and another Mediator component, Med25, regulate a restricted cohort of genes that are predominantly not controlled by GATA-1. Most of these genes were not regulated by Med1 in fibroblasts. Loss-of-function analyses with GATA-1-independent Med1 target genes indicate that Rrad, which encodes a small GTPase induced during human erythropoiesis, conferred erythroid cell survival. Thus, while Med1 is a context-dependent GATA-1 coregulator, it also exerts specialized functions in erythroid cells to control GATA-1-independent, cell-type-specific genes, which include candidate regulators of erythroid cell development and function.

GATA-1 genome-wide occupancy associates with distinct epigenetic profiles in mouse fetal liver erythropoiesis

We report the genomic occupancy profiles of the key hematopoietic transcription factor GATA-1 in pro-erythroblasts and mature erythroid cells fractionated from day E12.5 mouse fetal liver cells. Integration of GATA-1 occupancy profiles with available genome-wide transcription factor and epigenetic profiles assayed in fetal liver cells enabled as to evaluate GATA-1 involvement in modulating local chromatin structure of target genes during erythroid differentiation. Our results suggest that GATA-1 associates preferentially with changes of specific epigenetic modifications, such as H4K16, H3K27 acetylation and H3K4 di-methylation. Furthermore, we used random forest (RF) non-linear regression to predict changes in the expression levels of GATA-1 target genes based on the genomic features available for pro-erythroblasts and mature fetal liver-derived erythroid cells. Remarkably, our prediction model explained a high proportion of 62% of variation in gene expression. Hierarchical clustering of the proximity values calculated by the RF model produced a clear separation of upregulated versus downregulated genes and a further separation of downregulated genes in two distinct groups. Thus, our study of GATA-1 genome-wide occupancy profiles in mouse primary erythroid cells and their integration with global epigenetic marks reveals three clusters of GATA-1 gene targets that are associated with specific epigenetic signatures and functional characteristics.

Concise review: stem cell-derived erythrocytes as upcoming players in blood transfusion

Blood transfusions have become indispensable to treat the anemia associated with a variety of medical conditions ranging from genetic disorders and cancer to extensive surgical procedures. In developed countries, the blood supply is generally adequate. However, the projected decline in blood donor availability due to population ageing and the difficulty in finding rare blood types for alloimmunized patients indicate a need for alternative red blood cell (RBC) transfusion products. Increasing knowledge of processes that govern erythropoiesis has been translated into efficient procedures to produce RBC ex vivo using primary hematopoietic stem cells, embryonic stem cells, or induced pluripotent stem cells. Although in vitro-generated RBCs have recently entered clinical evaluation, several issues related to ex vivo RBC production are still under intense scrutiny: among those are the identification of stem cell sources more suitable for ex vivo RBC generation, the translation of RBC culture methods into clinical grade production processes, and the development of protocols to achieve maximal RBC quality, quantity, and maturation. Data on size, hemoglobin, and blood group antigen expression and phosphoproteomic profiling obtained on erythroid cells expanded ex vivo from a limited number of donors are presented as examples of the type of measurements that should be performed as part of the quality control to assess the suitability of these cells for transfusion. New technologies for ex vivo erythroid cell generation will hopefully provide alternative transfusion products to meet present and future clinical requirements.

Acetylation of histone deacetylase 1 regulates NuRD corepressor complex activity

BACKGROUND

HDAC1-containing NuRD complex is required for GATA-1-mediated repression and activation.

RESULTS

GATA-1 associated with acetylated HDAC1-containing NuRD complex, which has no deacetylase activity, for gene activation.

CONCLUSION

Acetylated HDAC1 converts NuRD complex from a repressor to an activator during GATA-1-directed erythroid differentiation program.

SIGNIFICANCE

HDAC1 acetylation may function as a master regulator for the activity of HDAC1 containing complexes. Histone deacetylases (HDACs) play important roles in regulating cell proliferation and differentiation. The HDAC1-containing NuRD complex is generally considered as a corepressor complex and is required for GATA-1-mediated repression. However, recent studies also show that the NuRD complex is involved in GATA-1-mediated gene activation. We tested whether the GATA-1-associated NuRD complex loses its deacetylase activity and commits the GATA-1 complex to become an activator during erythropoiesis. We found that GATA-1-associated deacetylase activity gradually decreased upon induction of erythroid differentiation. GATA-1-associated HDAC1 is increasingly acetylated after differentiation. It has been demonstrated earlier that acetylated HDAC1 has no deacetylase activity. Indeed, overexpression of an HDAC1 mutant, which mimics acetylated HDAC1, promotes GATA-1-mediated transcription and erythroid differentiation. Furthermore, during erythroid differentiation, acetylated HDAC1 recruitment is increased at GATA-1-activated genes, whereas it is significantly decreased at GATA-1-repressed genes. Interestingly, deacetylase activity is not required for Mi2 remodeling activity, suggesting that remodeling activity may be required for both activation and repression. Thus, our data suggest that NuRD can function as a coactivator or repressor and that acetylated HDAC1 converts the NuRD complex from a repressor to an activator during GATA-1-directed erythroid differentiation.

Megakaryocyte pathology and bone marrow fibrosis: the lysyl oxidase connection

Megakaryocytes (MKs), the platelet precursors, are capable of accumulating DNA greater than a diploid content as part of their cell cycle. MKs have been recognized as mediating fibrosis in a subset of hematologic malignancies, including acute megakaryoblastic leukemia and a subset of myeloproliferative neoplasms. The mechanisms responsible for fibrosis remain only partially understood. Past studies highlighted the role of growth factors in such pathologies, and recently, the protein lysyl oxidase (LOX) has been implicated in proliferation of MKs, ploidy and deposition of fibers. LOX was initially characterized as a protein responsible for the intermolecular cross-linking of elastin and collagen, and in recent years it has been identified as regulator of various pathologies, such as cancer and inflammation. Here, we review recent advances in the understanding of the contribution of MKs to the progression of myelofibrosis, highlighting the newly identified role of LOX.

The GATA1s isoform is normally down-regulated during terminal haematopoietic differentiation and over-expression leads to failure to repress MYB, CCND2 and SKI during erythroid differentiation of K562 cells

Background

Although GATA1 is one of the most extensively studied haematopoietic transcription factors little is currently known about the physiological functions of its naturally occurring isoforms GATA1s and GATA1FL in humans—particularly whether the isoforms have distinct roles in different lineages and whether they have non-redundant roles in haematopoietic differentiation. As well as being of general interest to understanding of haematopoiesis, GATA1 isoform biology is important for children with Down syndrome associated acute megakaryoblastic leukaemia (DS-AMKL) where GATA1FL mutations are an essential driver for disease pathogenesis.

Methods

Human primary cells and cell lines were analyzed using GATA1 isoform specific PCR. K562 cells expressing GATA1s or GATA1FL transgenes were used to model the effects of the two isoforms on in vitro haematopoietic differentiation.

Results

We found no evidence for lineage specific use of GATA1 isoforms; however GATA1s transcripts, but not GATA1FL transcripts, are down-regulated during in vitro induction of terminal megakaryocytic and erythroid differentiation in the cell line K562. In addition, transgenic K562-GATA1s and K562-GATA1FL cells have distinct gene expression profiles both in steady state and during terminal erythroid differentiation, with GATA1s expression characterised by lack of repression of MYB, CCND2 and SKI.

Conclusions

These findings support the theory that the GATA1s isoform plays a role in the maintenance of proliferative multipotent megakaryocyte-erythroid precursor cells and must be down-regulated prior to terminal differentiation. In addition our data suggest that SKI may be a potential therapeutic target for the treatment of children with DS-AMKL.

Cofactor-mediated restriction of GATA-1 chromatin occupancy coordinates lineage-specific gene expression

GATA-1 and its cofactor FOG-1 are required for the differentiation of erythrocytes and megakaryocytes. In contrast, mast cell development requires GATA-1 and the absence of FOG-1. Through genome-wide comparison of the chromatin occupancy of GATA-1 and a naturally occurring mutant that cannot bind FOG-1 (GATA-1(V205G)), we reveal that FOG-1 intricately regulates the chromatin occupancy of GATA-1. We identified GATA1-selective and GATA-1(V205G)-selective binding sites and show that GATA-1, in the absence of FOG-1, occupies GATA-1(V205G)-selective sites, but not GATA1-selective sites. By integrating ChIP-seq and gene expression data, we discovered that GATA-1(V205G) binds and activates mast cell-specific genes via GATA-1(V205G)-selective sites. We further show that exogenous expression of FOG-1 in mast cells leads to displacement of GATA-1 from mast cell-specific genes and causes their downregulation. Together these findings establish a mechanism of gene regulation whereby a non-DNA binding cofactor directly modulates the occupancy of a transcription factor to control lineage specification.

What's in a name?

Mutations in numerous genes encoding ribosomal proteins (RPs) occur in 50%-70% of individuals with Diamond-Blackfan anemia (DBA), establishing the disease as a ribosomopathy. As described in this issue of JCI, Sankaran, Gazda, and colleagues used genome-wide exome sequencing to study DBA patients with no detectable mutations in RP genes. They identified two unrelated pedigrees in which the disease is associated with mutations in GATA1, which encodes an essential hematopoietic transcription factor with no known mechanistic links to ribosomes. These findings ignite an interesting and potentially emotional debate on how we define DBA and whether the term should be restricted to pure ribosomopathies. More generally, the work reflects the powerful knowledge and controversies arising from the deluge of data generated by new genetic technologies that are being used to analyze human diseases.

Signal transduction pathways, intrinsic regulators, and the control of cell fate choice

BACKGROUND

Information regarding changes in organismal status is transmitted to the stem cell regulatory machinery by a limited number of signal transduction pathways. Consequently, these pathways derive their functional specificity through interactions with stem cell intrinsic master regulators, notably transcription factors. Identifying the molecular underpinnings of these interactions is critical to understanding stem cell function.

SCOPE OF REVIEW

This review focuses on studies in Drosophila that identify the gene regulatory basis for interactions between three different signal transduction pathways and an intrinsic master transcriptional regulator in the context of hematopoietic stem-like cell fate choice. Specifically, the interface between the GATA:FOG regulatory complex and the JAK/STAT, BMP, and Hedgehog pathways is examined.

MAJOR CONCLUSIONS

The GATA:FOG complex coordinates information transmitted by at least three different signal transduction pathways as a means to control stem-like cell fate choice. This illustrates emerging principles concerning regulation of stem cell function and describes a gene regulatory link between changes in organismal status and stem cell response.

GENERAL SIGNIFICANCE

The Drosophila model system offers a powerful approach to identify the molecular basis of how stem cells receive, interpret, and then respond to changes in organismal status. This article is part of a Special Issue entitled: Biochemistry of Stem Cells.

Master regulatory GATA transcription factors: mechanistic principles and emerging links to hematologic malignancies

Numerous examples exist of how disrupting the actions of physiological regulators of blood cell development yields hematologic malignancies. The master regulator of hematopoietic stem/progenitor cells GATA-2 was cloned almost 20 years ago, and elegant genetic analyses demonstrated its essential function to promote hematopoiesis. While certain GATA-2 target genes are implicated in leukemogenesis, only recently have definitive insights emerged linking GATA-2 to human hematologic pathophysiologies. These pathophysiologies include myelodysplastic syndrome, acute myeloid leukemia and an immunodeficiency syndrome with complex phenotypes including leukemia. As GATA-2 has a pivotal role in the etiology of human cancer, it is instructive to consider mechanisms underlying normal GATA factor function/regulation and how dissecting such mechanisms may reveal unique opportunities for thwarting GATA-2-dependent processes in a therapeutic context. This article highlights GATA factor mechanistic principles, with a heavy emphasis on GATA-1 and GATA-2 functions in the hematopoietic system, and new links between GATA-2 dysregulation and human pathophysiologies.

Chromatin occupancy analysis reveals genome-wide GATA factor switching during hematopoiesis

There are many examples of transcription factor families whose members control gene expression profiles of diverse cell types. However, the mechanism by which closely related factors occupy distinct regulatory elements and impart lineage specificity is largely undefined. Here we demonstrate on a genome wide scale that the hematopoietic GATA factors GATA-1 and GATA-2 bind overlapping sets of genes, often at distinct sites, as a means to differentially regulate target gene expression and to regulate the balance between proliferation and differentiation. We also reveal that the GATA switch, which entails a chromatin occupancy exchange between GATA2 and GATA1 in the course of differentiation, operates on more than one-third of GATA1 bound genes. The switch is equally likely to lead to transcriptional activation or repression; and in general, GATA1 and GATA2 act oppositely on switch target genes. In addition, we show that genomic regions co-occupied by GATA2 and the ETS factor ETS1 are strongly enriched for regions marked by H3K4me3 and occupied by Pol II. Finally, by comparing GATA1 occupancy in erythroid cells and megakaryocytes, we find that the presence of ETS factor motifs is a major discriminator of megakaryocyte versus red cell specification.

Functional regulation of pre-B-cell leukemia homeobox interacting protein 1 (PBXIP1/HPIP) in erythroid differentiation

Pre-B-cell leukemia homeobox interacting protein 1 or human PBX1 interacting protein (PBXIP1/HPIP) is a co-repressor of pre-B-cell leukemia homeobox 1 (PBX1) and is also known to regulate estrogen receptor functions by associating with the microtubule network. Despite its initial discovery in the context of hematopoietic cells, little is yet known about the role of HPIP in hematopoiesis. Here, we show that lentivirus-mediated overexpression of HPIP in human CD34(+) cells enhances hematopoietic colony formation in vitro, whereas HPIP knockdown leads to a reduction in the number of such colonies. Interestingly, erythroid colony number was significantly higher in HPIP-overexpressing cells. In addition, forced expression of HPIP in K562 cells, a multipotent erythro-megakaryoblastic leukemia cell line, led to an induction of erythroid differentiation. HPIP overexpression in both CD34(+) and K562 cells was associated with increased activation of the PI3K/AKT pathway, and corresponding treatment with a PI3K-specific inhibitor, LY-294002, caused a reduction in clonogenic progenitor number in HPIP-expressing CD34(+) cells and decreased K562 cell differentiation. Combined, these findings point to an important role of the PI3K/AKT pathway in mediating HPIP-induced effects on the growth and differentiation of hematopoietic cells. Interestingly, HPIP gene expression was found to be induced in K562 cells in response to erythroid differentiation signals such as DMSO and erythropoietin. The erythroid lineage-specific transcription factor GATA1 binds to the HPIP promoter and activates HPIP gene transcription in a CCCTC-binding factor (CTCF)-dependent manner. Co-immunoprecipitation and co-localization experiments revealed the association of CTCF with GATA1 indicating the recruitment of CTCF/GATA1 transcription factor complex onto the HPIP promoter. Together, this study provides evidence that HPIP is a target of GATA1 and CTCF in erythroid cells and plays an important role in erythroid differentiation by modulating the PI3K/AKT pathway.

Expression and relevance of TRPS-1: a new GATA transcription factor in breast cancer

GATA transcription factor family members have been found to play a critical role in the differentiation of many tissue types. For example, GATA-3 has been found to be highly correlated with estrogen receptor α (ER) expression and is emerging as one of the "master regulators" in breast ductal epithelial cell differentiation. Recently, we discovered another GATA family member highly prevalent in breast cancer called the trichorhinophalangeal syndrome-1 gene (TRPS-1). Using a quantitative immunohistochemistry (qIHC) approach, we found that TRPS-1 was significantly correlated with ER, PR, GATA-3, as well as HER2 expression. However, TRPS-1 was also found to be expressed in a high proportion of ER(-) ductal epithelial breast cancers (BCs), indicating that it may act as a ductal epithelial cell-specific transcription factor regulating cell fate at some point in the epithelial cell differentiation pathway. In keeping with this hypothesis, we found that TRPS-1 protein expression in BC above a certain threshold using qIHC correlated with markedly improved overall survival. Cox proportional hazards analysis found that both TRPS-1 and ER expression above critical threshold equally predicted for improved survival. Thus, TRPS-1 may be a powerful new positive prognostic marker in BC, and further IHC studies, as well as examination of its molecular function in ductal epithelial cell differentiation in the breast, are warranted. In this regard, data on the role of TRPS-1 in the differentiation of cells from mesenchymal precursors in other tissues, such as kidney metanephric mesenchymal cells, columnar chondrocytes, and osteoblasts, in mouse models may be useful. Indeed, these studies have found that TRPS-1 is a critical regulator of mesenchymal-to-epithelial cell transition. In the mammary gland, the restricted expression of TRPS-1 in human, mouse, and rat ductal epithelial cells suggests that it may also play a similar role during ductal luminal progenitor/stem cell differentiation. We present a model of TRPS-1 action in which it may act upstream of GATA-3 and ER on an earlier ductal epithelial progenitor cell or mammary stem cell during mammary gland development and also helps prevent reversion of ER(+) BC cells back into mesenchymal-like cells. This model predicts that BCs with low or no TRPS-1 expression may inherently be much less differentiated and more aggressive tumors with less favorable prognosis.

A unique role of GATA1s in Down syndrome acute megakaryocytic leukemia biology and therapy

BACKGROUND

Acute megakaryocytic leukemia (AMkL) in Down syndrome (DS) children is uniformly associated with somatic GATA1 mutations, which result in the synthesis of a shorter protein (GATA1s) with altered transactivation activity compared to the wild-type GATA1. It is not fully established whether leukemogenesis and therapeutic responses in DS AMkL patients are due to loss of the wild-type GATA1 or due to a unique function of GATA1s.

METHODOLOGY

Stable clones of CMK cells with decreased GATA1s or Bcl-2 levels were generated by using GATA1- or BCL-2-specific lentivirus shRNAs. In vitro ara-C, daunorubicin, and VP-16 cytotoxicities of the shRNA stable clones were determined by using the Cell Titer-blue reagent. Apoptosis and cell cycle distribution were determined by flow cytometry analysis. Changes in gene transcript levels were determined by gene expression microarray and/or real-time RT-PCR. Changes in protein levels were measured by Western blotting. In vivo binding of GATA1s to IL1A promoter was determined by chromatin immunoprecipitation assays.

RESULTS

Lentivirus shRNA knockdown of the GATA1 gene in the DS AMkL cell line, CMK (harbors a mutated GATA1 gene and only expresses GATA1s), resulting in lower GATA1s protein levels, promoted cell differentiation towards the megakaryocytic lineage and repressed cell proliferation. Increased basal apoptosis and sensitivities to ara-C, daunorubicin, and VP-16 accompanied by down-regulated Bcl-2 were also detected in the CMK GATA1 shRNA knockdown clones. Essentially the same results were obtained when Bcl-2 was knocked down with lentivirus shRNA in CMK cells. Besides Bcl-2, down-regulation of GATA1s also resulted in altered expression of genes (e.g., IL1A, PF4, and TUBB1) related to cell death, proliferation, and differentiation.

CONCLUSION

Our results suggest that GATA1s may facilitate leukemogenesis and potentially impact therapeutic responses in DS AMkL by promoting proliferation and survival, and by repressing megakaryocytic lineage differentiation, potentially by regulating expression of Bcl-2 protein and other relevant genes.

GATA Transcription Factors and Cancer

It has been almost a quarter century since it was first appreciated that a class of oncogenes contained in rapidly transforming avian retroviruses encoded DNA-binding transcription factors. As with other oncogenes, genetic recombination with the viral genome led to their overexpression or functional alteration. In the years that followed, alterations of numerous transcription factors were shown to be causatively involved in various cancers in human patients and model organisms. Depending on their normal cellular functions, these factors were subsequently categorized as proto-oncogenes or tumor suppressor genes. This review focuses on the role of GATA transcription factors in carcinogenesis. GATA factors are zinc finger DNA binding proteins that control the development of diverse tissues by activating or repressing transcription. GATA factors thus coordinate cellular maturation with proliferation arrest and cell survival. Therefore, a role of this family of genes in human cancers is not surprising. Prominent examples include structural mutations in GATA1 that are found in almost all megakaryoblastic leukemias in patients with Down syndrome; loss of GATA3 expression in aggressive, dedifferentiated breast cancers; and silencing of GATA4 and GATA5 expression in colorectal and lung cancers. Here, we discuss possible mechanisms of carcinogenesis vis-à-vis the normal functions of GATA factors as they pertain to human patients and mouse models of cancer.

Normal and malignant megakaryopoiesis

Megakaryopoiesis is the process by which bone marrow progenitor cells develop into mature megakaryocytes (MKs), which in turn produce platelets required for normal haemostasis. Over the past decade, molecular mechanisms that contribute to MK development and differentiation have begun to be elucidated. In this review, we provide an overview of megakaryopoiesis and summarise the latest developments in this field. Specially, we focus on polyploidisation, a unique form of the cell cycle that allows MKs to increase their DNA content, and the genes that regulate this process. In addition, because MKs have an important role in the pathogenesis of acute megakaryocytic leukaemia and a subset of myeloproliferative neoplasms, including essential thrombocythemia and primary myelofibrosis, we discuss the biology and genetics of these disorders. We anticipate that an increased understanding of normal MK differentiation will provide new insights into novel therapeutic approaches that will directly benefit patients.

The role of stat1b in zebrafish hematopoiesis

STAT1 mediates response to interferons and regulates immunity, cell proliferation, apoptosis, and sensitivity of Fanconi Anemia cells to apoptosis after interferon signaling; the roles of STAT1 in embryos, however, are not understood. To explore embryonic functions of STAT1, we investigated stat1b, an unstudied zebrafish co-ortholog of human STAT1. Zebrafish stat1a encodes all five domains of the human STAT1-alpha splice form but, like the human STAT1-beta splice variant, stat1b lacks a complete transactivation domain; thus, two unlinked zebrafish paralogs encode protein forms translated from two splice variants of a single human gene, as expected by sub-functionalization after genome duplication. Phylogenetic and conserved synteny studies showed that stat1b and stat1a arose as duplicates in the teleost genome duplication (TGD) and clarified the evolutionary origin of STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B and STAT6 by tandem and genome duplication. RT-PCR revealed maternal expression of stat1a and stat1b. In situ hybridization detected stat1b but not stat1a expression in embryonic hematopoietic tissues. Morpholino knockdown of stat1b, but not stat1a, decreased expression of the myeloid and granulocyte markers spi and mpo and increased expression of the hematopoietic progenitor marker scl, the erythrocyte marker gata1, and hemoglobin. These results suggest that zebrafish Stat1b promotes myeloid development at the expense of erythroid development.

Activator protein-2α functions as a master regulator of multiple transcription factors in the mouse liver

AIM

  Activator protein 2α (AP-2α) belongs to the AP-2 family of transcription factors that are involved in the regulation of cell proliferation, differentiation, apoptosis and carcinogenesis and has been suggested to function as a tumor suppressor in many cancers. However, the physiological role of AP-2α in hepatocytes is unknown. The present study is to characterize the expression and function of AP-2α in the liver of conscience mouse.

METHODS

  Exogenous AP-2α was overexpressed in the mouse liver by in vivo gene delivery and changes in transcription factor expression were identified by using protein-DNA arrays and immunoblotting.

RESULTS

  Western blotting and protein/DNA arrays showed that AP-2α is expressed in the nuclei of mouse hepatocytes. Overexpression of AP-2αin vivo significantly suppressed transcription factors AP-1, CREB and c-Myc, and markedly increased CBF, c-Myb, NF-1, Pax-5, RXR, Smad3/4, TR(DR-4), USF-1 and GATA. Among all GATA proteins, only GATA-4 level was dramatically elevated and there was a concomitant loss of phospho-GATA-4. Corresponding changes were detected in upstream kinases Akt, GSK-3β and PKA, which regulates the phosphorylation status and stability of GATA-4 protein.

CONCLUSIONS

  AP-2α is expressed in mouse hepatocytes and it acts as a master regulator of numerous transcription factors in the liver.

c-Myb and GATA-1 alternate dominant roles during megakaryocyte differentiation

BACKGROUND

Transcription factors are essential for blood cell formation. Mice expressing low levels of c-Myb (c-Myb(low)) have an increased number of bone marrow megakaryocytes (MKs) and corresponding thrombocytosis. In contrast, mice engineered to express low levels of GATA-1 (GATA-1(low)) in the megakaryocytic lineage exhibit aberrant megakaryocytopoiesis with hyperproliferation of progenitors and defective terminal differentiation leading to thrombocytopenia. These seemingly opposite roles may affect platelet turnover and thus be of clinical relevance.

OBJECTIVE

To determine how these two transcription factors act together to control megakaryocytopoiesis and platelet formation.

METHODS

We used a combination of cellular and molecular in vitro assays to examine the ability of bone marrow cells from mice expressing low levels of both c-Myb and GATA-1 (referred to as double(low)) to produce MKs and platelets.

RESULTS

Double(low) cells, or those with low GATA-1 levels in which c-Myb is conditionally deleted, lack the hyperproliferative capacity of GATA-1(low) cells, allowing the cells to proceed towards more committed MKs that are, however, impaired in their capacity to produce fully differentiated cells, as confirmed by the abundance of morphologically aberrant cells that lack the ability to form proplatelets.

CONCLUSION

c-Myb and GATA-1 act in concert to achieve correct megakaryocytic differentiation. GATA-1 regulates both the proliferation of megakaryocytic progenitors and their terminal maturation. c-Myb also acts at the level of the progenitor by influencing its commitment to differentiation, but in contrast to GATA-1 it does not have any effect on the process of terminal differentiation.

Stat3 and CCAAT/enhancer binding protein beta (C/EBP-beta) regulate Jab1/CSN5 expression in mammary carcinoma cells

Introduction

The c-Jun coactivator, Jun activation-domain binding protein 1 (Jab1) also known as the fifth component of the COP9 signalosome complex (CSN5), is a novel candidate oncogene whose aberrant expression contributes to the progression of breast carcinoma and other human cancers. The mechanism of Jab1 gene expression and its deregulation in cancer cells remains to be identified. We therefore investigated the transcriptional regulatory mechanisms of Jab1 expression in human breast carcinoma cells.

Methods

To identify potential regulators of Jab1 transcription, we cloned the 5' upstream region of the human Jab1 gene and mapped its transcriptional start site. We identified binding sequences for the CCAAT/enhancer binding protein (C/EBP) and GATA, as well as a signal transducer and activator of transcription-3 (Stat3) consensus sequence overlapping the C/EBP site, using 5'- deletion analysis and a gene reporter assay. Mutational analysis of these binding sites was performed to confirm their roles in promoting Jab1 transcription in breast cancer cells. We further confirmed these binding sites using electrophoretic mobility shift assays (EMSAs) and chromatin immunoprecipitation (ChIP) assays. We also analyzed whether the siRNA-mediated inactivation of Stat3 and Src could reduce Jab1-promoter activity and whether interleukine-6 (IL-6) could mediate increased Jab1 expression through Stat3 signaling.

Results

We identified binding sequences for C/EBP, GATA, as well as a Stat3 consensus sequence overlapping the C/EBP site in the promoter region of Jab1. C/EBP-beta2 is a potential transcriptional activator of Jab1 and mutation of the C/EBP/Stat3 binding site significantly reduced Jab1-promoter activity. In addition, inhibiting Stat3 significantly reduced Jab1-promoter activation. EMSA and ChIP assays confirmed that C/EBP, GATA1 and Stat3 bind to Jab1 promoter in breast carcinoma cells. We also found that Src, an activator of Stat3, is involved in Jab1-promoter activation. siRNA knockdown of Src reduced the Jab1-promoter activity, similar to the results seen when Stat3 was inhibited in breast carcinoma cells. Interestingly, reactivation of Stat3 in normal mammary epithelial cells (MCF-10A, MCF-10F) is sufficient to reactivate Jab1 expression. Treatment with the cytokine IL-6 resulted in increased Jab1 expression that was blocked by inhibition of Stat3.

Conclusions

These findings reveal a novel mechanism of Jab1 gene regulation and provide functional and mechanistic links between the Src/Stat3 and IL-6/Stat3 signaling axes that are involved in the activation of Jab1 transcription and regulation of this novel oncogenic protein.

Current Strategies in Diagnosis of Inherited Storage Pool Defects

Inherited platelet defects lead to bleeding symptoms of varying severity. Typically, easy bruising, petechiae, epistaxis, and mucocutaneous bleeding are observed in affected patients. The platelet defects are classified into disorders affecting either platelet surface receptors or intracellular organelles of platelets. The latter are represented by platelet storage pool diseases (SPD) which share a defect of platelet granules. Platelet α-granules, δ-granules, or both may be affected resulting in the clinical picture of α-SPD (e.g. Gray platelet syndrome, Quebec platelet disorder, arthrogryposis, renal dysfunction, and cholestasis syndrome), δ-SPD (e.g. Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, Griscelli syndrome), or αδ-SPD (e.g. X-linked thrombocytopenia, Wiskott-Aldrich syndrome). Diagnosis of SPD is very extensive and requires platelet aggregation and flow cytometry analyses with interpretation from a specialist. Many of these disorders share common treatments, however, efficacy can vary between different patients. Therapy regiments with tranexamic acid, DDAVP, activated FVIIa, and platelet transfusions have been published. Stem cell or bone marrow transplantations are preserved for severe defects. Here, we describe the pathophysiology, clinical manifestations, and diagnosis of the major human SPDs.

Differential coregulator requirements for function of the hematopoietic transcription factor GATA-1 at endogenous loci

The critical regulator of hematopoiesis GATA-1 recruits diverse coregulators to chromatin, which mediate transcriptional activation and repression. These coregulators include the cell-type-specific multi-zinc finger protein Friend of GATA-1 (FOG-1), the histone acetyltransferase CREB binding protein (CBP), and the key component of the Mediator complex Med1. While FOG-1 is an established GATA-1 coregulator, the importance of interactions between GATA-1 and other coregulators is poorly understood. Furthermore, whether GATA-1 utilizes multiple coregulators at all loci, or if certain coregulators are dedicated to specific loci is unknown. We compared the capacity of GATA-1 to recruit and utilize FOG-1 and Med1 at activated and repressed target genes. Similar to FOG-1, GATA-1 recruited Med1 to activated genes, and the kinetics of FOG-1 and Med1 recruitment were similar. GATA-1 recruited Med1 in Fog1(-/-) cells, indicating that GATA-1-mediated Med1 recruitment is FOG-1-independent. In contrast to FOG-1, GATA-1 evicted Med1 during transcriptional repression. Whereas knocking-down FOG-1 had catastrophic effects on GATA-1-mediated activation and repression, knocking-down Med1 modestly impaired GATA-1 activity only at select loci. These results illustrate both similarities and differences between GATA-1-mediated recruitment of FOG-1 and Med1 to chromatin, with a fundamental difference being the quantitatively greater requirement for FOG-1.

Functional differences between myeloid leukemia-initiating and transient leukemia cells in Down's syndrome

Children with constitutional trisomy 21 or Down's syndrome (DS) are predisposed to develop myeloid leukemia (ML) at a young age. DS-ML is frequently preceded by transient leukemia (TL), a spontaneously resolving accumulation of blasts during the newborn period. Somatic mutations of GATA1 in the blasts of TL and DS-ML likely function as an initiating event. We hypothesized that the phenotypic difference between TL and DS-ML is due to a divergent functional repertoire of the leukemia-initiating cells. Using an NOD/SCID model, we found that cells initiating DS-ML engrafted, disseminated to distant bone marrow sites, and propagated the leukemic clone in secondary recipients. In contrast, TL cells lacked the ability to expand and to migrate, but were able to persist in the recipient bone marrow. We found some evidence of genomic progression with 1 of 9 DS-ML samples and none of 11 TL samples harboring a mutation of N-RAS. The findings of this pilot study provide evidence for the functional impact of second events underlying the transformation of TL into DS-ML and a needed experimental tool for the functional testing of these promoting events.

TMEM8 - a non-globin gene entrapped in the globin web

For more than 30 years it was believed that globin gene domains included only genes encoding globin chains. Here we show that in chickens, the domain of α-globin genes also harbor the non-globin gene TMEM8. It was relocated to the vicinity of the α-globin cluster due to inversion of an ∼170-kb genomic fragment. Although in humans TMEM8 is preferentially expressed in resting T-lymphocytes, in chickens it acquired an erythroid-specific expression profile and is upregulated upon terminal differentiation of erythroblasts. This correlates with the presence of erythroid-specific regulatory elements in the body of chicken TMEM8, which interact with regulatory elements of the α-globin genes. Surprisingly, TMEM8 is not simply recruited to the α-globin gene domain active chromatin hub. An alternative chromatin hub is assembled, which includes some of the regulatory elements essential for the activation of globin gene expression. These regulatory elements should thus shuttle between two different chromatin hubs.

Discovering hematopoietic mechanisms through genome-wide analysis of GATA factor chromatin occupancy

GATA factors interact with simple DNA motifs (WGATAR) to regulate critical processes, including hematopoiesis, but very few WGATAR motifs are occupied in genomes. Given the rudimentary knowledge of mechanisms underlying this restriction and how GATA factors establish genetic networks, we used ChIP-seq to define GATA-1 and GATA-2 occupancy genome-wide in erythroid cells. Coupled with genetic complementation analysis and transcriptional profiling, these studies revealed a rich collection of targets containing a characteristic binding motif of greater complexity than WGATAR. GATA factors occupied loci encoding multiple components of the Scl/TAL1 complex, a master regulator of hematopoiesis and leukemogenic target. Mechanistic analyses provided evidence for crossregulatory and autoregulatory interactions among components of this complex, including GATA-2 induction of the hematopoietic corepressor ETO-2 and an ETO-2-negative autoregulatory loop. These results establish fundamental principles underlying GATA factor mechanisms in chromatin and illustrate a complex network of considerable importance for the control of hematopoiesis.

NuRD mediates activating and repressive functions of GATA-1 and FOG-1 during blood development

GATA transcription factors interact with FOG proteins to regulate tissue development by activating and repressing transcription. FOG-1 (ZFPM1), a co-factor for the haematopoietic factor GATA-1, binds to the NuRD co-repressor complex through a conserved N-terminal motif. Surprisingly, we detected NuRD components at both repressed and active GATA-1/FOG-1 target genes in vivo. In addition, while NuRD is required for transcriptional repression in certain contexts, we show a direct requirement of NuRD also for FOG-1-dependent transcriptional activation. Mice in which the FOG-1/NuRD interaction is disrupted display defects similar to germline mutations in the Gata1 and Fog1 genes, including anaemia and macrothrombocytopaenia. Gene expression analysis in primary mutant erythroid cells and megakaryocytes (MKs) revealed an essential function for NuRD during both the repression and activation of select GATA-1/FOG-1 target genes. These results show that NuRD is a critical co-factor for FOG-1 and underscore the versatile use of NuRD by lineage-specific transcription factors to activate and repress gene transcription in the appropriate cellular and genetic context.

Down syndrome and malignancies: a unique clinical relationship: a paper from the 2008 william beaumont hospital symposium on molecular pathology

The patterns of malignancies in Down syndrome (DS) are unique and highlight the relationship between chromosome 21 and cancer. DS children have a approximately 10- to 20-fold higher risk for developing acute lymphoblastic leukemia and acute myeloid leukemia (AML), as compared with non-DS children, although they do not have a uniformly increased risk of developing solid tumors. DS children with acute lymphoblastic leukemia frequently experience higher levels of treatment-related toxicity and inferior event-free survival rates, as compared with non-DS children. DS children also develop AML with unique features and have a 500-fold increased risk of developing the AML subtype, acute megakaryocytic leukemia (AMkL; M7). Nearly 10% of DS newborns are diagnosed with a variant of AMkL, the transient myeloproliferative disorder, which can resolve spontaneously without treatment; event-free survival rates for DS patients with AMkL ranges from 80% to 100%, in comparison with <30% for non-DS children with AMkL. In addition, somatic mutations of the GATA1 gene have been detected in nearly all DS TMD and AMkL cases and not in leukemia cases in non-DS children. GATA1 mutations are key factors linked to both leukemogenesis and the high cure rates of DS AMkL patients. Identifying the mechanisms that account for the high event-free survival rates of DS AMkL patients may ultimately improve AML treatment as well. Examining leukemogenesis in DS children may identify factors linked to the general development of childhood leukemia and lead to potential new therapeutic strategies to fight this disease.

Direct binding of pRb/E2F-2 to GATA-1 regulates maturation and terminal cell division during erythropoiesis

Cell differentiation is often coupled with cell cycle arrest. Here, we show that direct binding of the erythroid transcription factor GATA-1 to the retinoblastoma protein and the pRb/E2F transcription factor complex is critical for red blood cell formation.

How cell proliferation subsides as cells terminally differentiate remains largely enigmatic, although this phenomenon is central to the existence of multicellular organisms. Here, we show that GATA-1, the master transcription factor of erythropoiesis, forms a tricomplex with the retinoblastoma protein (pRb) and E2F-2. This interaction requires a LXCXE motif that is evolutionary conserved among GATA-1 orthologs yet absent from the other GATA family members. GATA-1/pRb/E2F-2 complex formation stalls cell proliferation and steers erythroid precursors towards terminal differentiation. This process can be disrupted in vitro by FOG-1, which displaces pRb/E2F-2 from GATA-1. A GATA-1 mutant unable to bind pRb fails to inhibit cell proliferation and results in mouse embryonic lethality by anemia. These findings clarify the previously suspected cell-autonomous role of pRb during erythropoiesis and may provide a unifying molecular mechanism for several mouse phenotypes and human diseases associated with GATA-1 mutations.

Red blood cell production, or erythropoiesis, proceeds by a tight coupling of proliferation and differentiation. The earliest erythroid progenitor identifiable possesses remnant stem cell characteristics as it both self-renews and differentiates. Each progenitor gives rise to more than 10,000 cells, including secondary progenitors. Yet, during the next stage of differentiation, much of this renewal capability is lost, and terminal erythroid differentiation progresses in a stepwise manner through several stages separated by a single mitosis. The transcription factor GATA-1 is essential for erythroid differentiation because it induces the expression of all the known erythroid-specific genes. Here, we show that GATA-1 directly interacts with proteins that are central to the process of cell division: the retinoblastoma protein pRb and the transcription factor E2F. Specifically, E2F becomes inactivate after engaging in a GATA-1/pRb/E2F tricomplex. Another erythroid transcription factor, termed FOG-1, is able to displace pRb/E2F from this complex in vitro upon binding to GATA-1. We hypothesize that the liberated pRb/E2F can then be the target of subsequent regulation to ultimately release free E2F, which triggers cell division. The physiological role of this new pathway is evidenced by transgenic mouse experiments with GATA-1 mutants unable to bind pRb/E2F, which result in embryonic lethality by anemia.

Graded repression of PU.1/Sfpi1 gene transcription by GATA factors regulates hematopoietic cell fate

GATA-1 and PU.1 are essential hematopoietic transcription factors that control erythromegakaryocytic and myelolymphoid differentiation, respectively. These proteins antagonize each other through direct physical interaction to repress alternate lineage programs. We used immortalized Gata1(-) erythromegakaryocytic progenitor cells to study how PU.1/Sfpi1 expression is regulated by GATA-1 and GATA-2, a related factor that is normally expressed at earlier stages of hematopoiesis. Both GATA factors bind the PU.1/Sfpi1 gene at 2 highly conserved regions. In the absence of GATA-1, GATA-2 binding is associated with an undifferentiated state, intermediate level PU.1/Sfpi1 expression, and low-level expression of its downstream myeloid target genes. Restoration of GATA-1 function induces erythromegakaryocytic differentiation. Concomitantly, GATA-1 replaces GATA-2 at the PU.1/Sfpi1 locus and PU.1/Sfpi1 expression is extinguished. In contrast, when GATA-1 is not present, shRNA knockdown of GATA-2 increases PU.1/Sfpi1 expression by 3-fold and reprograms the cells to become macrophages. Our findings indicate that GATA factors act sequentially to regulate lineage determination during hematopoiesis, in part by exerting variable repressive effects at the PU.1/Sfpi1 locus.

A s-myly route toward lymphoid differentiation

In this issue of Immunity, Ng et al. (2009) show that lymphoid-lineage priming occurs in hematopoietic stem cells and is dependent on the Ikaros transcription factor, as is repression of self-renewal genes during lymphoid differentiation.

GATA-1 associates with and inhibits p53

In addition to orchestrating the expression of all erythroid-specific genes, GATA-1 controls the growth, differentiation, and survival of the erythroid lineage through the regulation of genes that manipulate the cell cycle and apoptosis. The stages of mammalian erythropoiesis include global gene inactivation, nuclear condensation, and enucleation to yield circulating erythrocytes, and some of the genes whose expression are altered by GATA-1 during this process are members of the p53 pathway. In this study, we demonstrate a specific in vitro interaction between the transactivation domain of p53 (p53TAD) and a segment of the GATA-1 DNA-binding domain that includes the carboxyl-terminal zinc-finger domain. We also show by immunoprecipitation that the native GATA-1 and p53 interact in erythroid cells and that activation of p53-responsive promoters in an erythroid cell line can be inhibited by the overexpression of GATA-1. Mutational analysis reveals that GATA-1 inhibition of p53 minimally requires the segment of the GATA-1 DNA-binding domain that interacts with p53TAD. This inhibition is reciprocal, as the activation of a GATA-1-responsive promoter can be inhibited by p53. Based on these findings, we conclude that inhibition of the p53 pathway by GATA-1 may be essential for erythroid cell development and survival.

Stem cells of GATA1-related leukemia undergo pernicious changes after 5-fluorouracil treatment

OBJECTIVE

Transcription factor GATA1 plays a critical role in erythropoiesis through the integrated regulation of cell proliferation, differentiation, and apoptosis. In Gata1.05 gene knockdown mice, Gata1 expression deteriorates to 5% of wild-type allelic expression, a level insufficient for supporting normal erythropoiesis and one that leads to accumulation of erythroid progenitors that are readily transformed into erythroblastic leukemia. Serial engraftment of leukemic cells into primary or subsequent nude mice reconstituted complete leukemic phenotype in recipient. To delineate characteristics of leukemic stem cells (LSCs), we analyzed LSCs of Gata1.05 leukemia, which have a potential to reestablish leukemia in mice.

MATERIALS AND METHODS

Leukemic cells isolated from the first recipient mice of Gata1.05 leukemia cells were divided into two fractions using Hoechst dye. Fractionated cells were transplanted into second recipient, or analyzed gene expression profiles and cell-cycle status. Consequences of 5-fluorouracil (5-FU) treatment on leukemic cells in vivo were studied.

RESULTS

LSCs were enriched in the Hoechst dye-excluded side population (SP), and leukemic cells in the SP population (LSP cells) were morphologically and immunophenotypically indistinguishable from other leukemic cells. However, expression of hematopoietic stem cell (HSC)-related genes was upregulated in the LSP cells. In cell-cycle analyses, LSP cells were quiescent like HSCs, but reentry into the cell cycle was stimulated by 5-FU treatment. Nonetheless, 5-FU treatment established a point of newly adjusted equilibrium in the LSP cells and the cells never recovered to their previous quiescent state.

CONCLUSION

Based on this observation, distinct self-renewal regulatory mechanisms in LSCs may be considered as one of the causes of worsening of the features of leukemia after injury and relapse.

Leukemia-related transcription factor TEL/ETV6 expands erythroid precursors and stimulates hemoglobin synthesis

TEL/ETV6 located at chromosome 12p13 encodes a member of the E26 transformation-specific family of transcription factors. TEL is known to be rearranged in a variety of leukemias and solid tumors resulting in the formation of oncogenic chimeric protein. Tel is essential for maintaining hematopoietic stem cells in the bone marrow. To understand the role of TEL in erythropoiesis, we generated transgenic mice expressing human TEL under the control of Gata1 promoter that is activated during the course of the erythroid-lineage differentiation (GATA1-TEL transgenic mice). Although GATA1-TEL transgenic mice appeared healthy up to 18 months of age, the level of hemoglobin was higher in transgenic mice compared to non-transgenic littermates. In addition, CD71+/TER119+ and c-kit+/CD41+ populations proliferated with a higher frequency in transgenic mice when bone marrow cells were cultured in the presence of erythropoietin and thrombopoietin, respectively. In transgenic mice, enhanced expression of Alas-e and beta-major globin genes was observed in erythroid-committed cells. When embryonic stem cells expressing human TEL under the same Gata1 promoter were differentiated into hematopoietic cells, immature erythroid precursor increased better compared to controls as judged from the numbers of burst-forming unit of erythrocytes. Our findings suggest some roles of TEL in expanding erythroid precursors and accumulating hemoglobin.

SCL and associated proteins distinguish active from repressive GATA transcription factor complexes

GATA-1 controls hematopoietic development by activating and repressing gene transcription, yet the in vivo mechanisms that specify these opposite activities are unknown. By examining the composition of GATA-1-associated protein complexes in a conditional erythroid rescue system as well as through the use of tiling arrays we detected the SCL/TAL1, LMO2, Ldb1, E2A complex at all positively acting GATA-1-bound elements examined. Similarly, the SCL complex is present at all activating GATA elements in megakaryocytes and mast cells. In striking contrast, at sites where GATA-1 functions as a repressor, the SCL complex is depleted. A DNA-binding defective form of SCL maintains association with a subset of active GATA elements indicating that GATA-1 is a key determinant for SCL recruitment. Knockdown of LMO2 selectively impairs activation but not repression by GATA-1. ETO-2, an SCL-associated protein with the potential for transcription repression, is also absent from GATA-1-repressed genes but, unlike SCL, fails to accumulate at GATA-1-activated genes. Together, these studies identify the SCL complex as a critical and consistent determinant of positive GATA-1 activity in multiple GATA-1-regulated hematopoietic cell lineages.

The key role of stem cell factor/KIT signaling in the proliferation of blast cells from Down syndrome-related leukemia

Transient leukemia (TL) has been observed in approximately 10% of newborn infants with Down syndrome (DS). Although treatment with cytarabine is effective in high-risk TL cases, approximately 20% of severe patients still suffer early death. In this study, we demonstrate abundant KIT expression in all 13 patients with GATA1 mutations, although no significant difference in expression levels was observed between TL and acute myeloid leukemia. Stem cell factor (SCF) stimulated the proliferation of the TL cells from five patients and treatment with the tyrosine kinase inhibitor imatinib suppressed the proliferation effectively in vitro. To investigate the signal cascade, we established the first SCF-dependent, DS-related acute megakaryoblastic leukemia cell line, KPAM1. Withdrawal of SCF or treatment with imatinib induced apoptosis of KPAM1 cells. SCF activated the RAS/MAPK and PI3K/AKT pathways, followed by downregulation of the pro-apoptotic factor BIM and upregulation of the anti-apoptotic factor MCL1. Although we found novel missense mutations of KIT in 2 of 14 TL patients, neither mutation led to KIT activation and neither reduced the cytotoxic effects of imatinib. These results suggest the essential role of SCF/KIT signaling in the proliferation of DS-related leukemia and the possibility of therapeutic benefits of imatinib for TL patients.

Human phenotypes associated with GATA-1 mutations

GATA-1 is one of the six members of the GATA gene family, a group of related transcription factors discovered in the 1980s. In the past few decades, the crucial role of GATA-1 in normal human hematopoiesis has been delineated. As would be expected, mutations in GATA-1 have subsequently been found to have important clinical significance, and are directly linked to deregulated formation of certain blood cell lineages. This paper reviews the functional consequences of GATA-1 mutations by linking specific errors in the gene, or its downstream protein products, to documented human diseases. These five human diseases are: X-linked thrombocytopenia (XLT), X-linked thrombocytopenia with thalassemia (XLTT), congenital erythropoietic porphyria (CEP), transient myeloproliferative disorder (TMD) and acute megarakaryoblastic leukemia (AMKL) associated with Trisomy 21, and, lastly, a particular subtype of anemia associated with the production of GATA-1s, a shortened, mutant isoform of the wild-type GATA-1. The different phenotypic expressions associated with GATA-1 mutations illustrate the integral function of the transcription factor in overall body homeostasis. Furthermore, these direct genotype-phenotype correlations reinforce the importance of unraveling the human genome, as such connections may lead to important therapeutic or preventive therapies.

Trisomy 21 enhances human fetal erythro-megakaryocytic development

Children with Down syndrome exhibit 2 related hematopoietic diseases: transient myeloproliferative disorder (TMD) and acute megakaryoblastic leukemia (AMKL). Both exhibit clonal expansion of blasts with biphenotypic erythroid and megakaryocytic features and contain somatic GATA1 mutations. While altered GATA1 inhibits erythro-megakaryocytic development, less is known about how trisomy 21 impacts blood formation, particularly in the human fetus where TMD and AMKL originate. We used in vitro and mouse transplantation assays to study hematopoiesis in trisomy 21 fetal livers with normal GATA1 alleles. Remarkably, trisomy 21 progenitors exhibited enhanced production of erythroid and megakaryocytic cells that proliferated excessively. Our findings indicate that trisomy 21 itself is associated with cell-autonomous expansion of erythro-megakaryocytic progenitors. This may predispose to TMD and AMKL by increasing the pool of cells susceptible to malignant transformation through acquired mutations in GATA1 and other cooperating genes.

Molecular hallmarks of endogenous chromatin complexes containing master regulators of hematopoiesis

Combinatorial interactions among trans-acting factors establish transcriptional circuits that orchestrate cellular differentiation, survival, and development. Unlike circuits instigated by individual factors, efforts to identify gene ensembles controlled by multiple factors simultaneously are in their infancy. A paradigm has emerged in which the important regulators of hematopoiesis GATA-1 and GATA-2 function combinatorially with Scl/TAL1, another key regulator of hematopoiesis. The underlying mechanism appears to involve preferential assembly of a multimeric complex on a composite DNA element containing WGATAR and E-box motifs. Based on this paradigm, one would predict that GATA-2 and Scl/TAL1 would commonly co-occupy such composite elements in cells. However, chromosome-wide analyses indicated that the vast majority of conserved composite elements were occupied by neither GATA-2 nor Scl/TAL1. Intriguingly, the highly restricted set of GATA-2-occupied composite elements had characteristic molecular hallmarks, specifically Scl/TAL1 occupancy, a specific epigenetic signature, specific neighboring cis elements, and preferential enhancer activity in GATA-2-expressing cells. Genes near the GATA-2-Scl/TAL1-occupied composite elements were regulated by GATA-2 or GATA-1, and therefore these fundamental studies on combinatorial transcriptional mechanisms were also leveraged to discover novel GATA factor-mediated cell regulatory pathways.