GATA-1-mediated proliferation arrest during erythroid maturation

GATA-1 forms distinct activating and repressive complexes in erythroid cells

GATA-1 is essential for the generation of the erythroid, megakaryocytic, eosinophilic and mast cell lineages. It acts as an activator and repressor of different target genes, for example, in erythroid cells it represses cell proliferation and early hematopoietic genes while activating erythroid genes, yet it is not clear how both of these functions are mediated. Using a biotinylation tagging/proteomics approach in erythroid cells, we describe distinct GATA-1 interactions with the essential hematopoietic factor Gfi-1b, the repressive MeCP1 complex and the chromatin remodeling ACF/WCRF complex, in addition to the known GATA-1/FOG-1 and GATA-1/TAL-1 complexes. Importantly, we show that FOG-1 mediates GATA-1 interactions with the MeCP1 complex, thus providing an explanation for the overlapping functions of these two factors in erythropoiesis. We also show that subsets of GATA-1 gene targets are bound in vivo by distinct complexes, thus linking specific GATA-1 partners to distinct aspects of its functions. Based on these findings, we suggest a model for the different roles of GATA-1 in erythroid differentiation.

FOG-1 recruits the NuRD repressor complex to mediate transcriptional repression by GATA-1

Transcription factor GATA-1 and its cofactor FOG-1 coordinate erythroid cell maturation by activating erythroid-specific genes and repressing genes associated with the undifferentiated state. Here we show that FOG-1 binds to the NuRD corepressor complex in vitro and in vivo. The interaction is mediated by a small conserved domain at the extreme N-terminus of FOG-1 that is necessary and sufficient for NuRD binding. This domain defines a novel repression module found in diverse transcriptional repressors. NuRD is present at GATA-1/FOG-1-repressed genes in erythroid cells in vivo. Point mutations near the N-terminus of FOG-1 that abrogate NuRD binding block gene repression by FOG-1. Finally, the ability of GATA-1 to repress transcription was impaired in erythroid cells expressing mutant forms of FOG-1 that are defective for NuRD binding. Together, these studies show that FOG-1 and likely other FOG-like proteins are corepressors that link GATA factors to histone deacetylation and nucleosome remodeling.

Developmental stage-selective effect of somatically mutated leukemogenic transcription factor GATA1

Acquired mutations in the hematopoietic transcription factor GATA binding protein-1 (GATA1) are found in megakaryoblasts from nearly all individuals with Down syndrome with transient myeloproliferative disorder (TMD, also called transient leukemia) and the related acute megakaryoblastic leukemia (DS-AMKL, also called DS-AML M7). These mutations lead to production of a variant GATA1 protein (GATA1s) that is truncated at its N terminus. To understand the biological properties of GATA1s and its relation to DS-AMKL and TMD, we used gene targeting to generate Gata1 alleles that express GATA1s in mice. We show that the dominant action of GATA1s leads to hyperproliferation of a unique, previously unrecognized yolk sac and fetal liver progenitor, which we propose accounts for the transient nature of TMD and the restriction of DS-AMKL to infants. Our observations raise the possibility that the target cells in other leukemias of infancy and early childhood are distinct from those in adult leukemias and underscore the interplay between specific oncoproteins and potential target cells.

Lyn deficiency reduces GATA-1, EKLF and STAT5, and induces extramedullary stress erythropoiesis

In vitro studies have implicated the Lyn tyrosine kinase in erythropoietin signaling. In this study, we show that J2E erythroid cells lacking Lyn have impaired signaling and reduced levels of transcription factors STAT5a, EKLF and GATA-1. Since mice lacking STAT5, EKLF or GATA-1 have red cell abnormalities, this study also examined the erythroid compartment of Lyn(-/-) mice. Significantly, STAT5, EKLF and GATA-1 levels were appreciably lower in Lyn(-/-) erythroblasts, and the phenotype of Lyn(-/-) animals was remarkably similar to GATA-1(low) animals. Although young adult Lyn-deficient mice had normal hematocrits, older mice developed anemia. Grossly enlarged erythroblasts and florid erythrophagocytosis were detected in the bone marrow of mice lacking Lyn. Markedly elevated erythroid progenitors and precursor levels were observed in the spleens, but not bone marrow, of Lyn(-/-) animals indicating that extramedullary erythropoiesis was occurring. These data indicate that Lyn(-/-) mice display extramedullary stress erythropoiesis to compensate for intrinsic and extrinsic erythroid defects.

Gene expression regulation and domain function of hematopoietic GATA factors

The hierarchical gene regulatory network in hematopoiesis is highly complex, making elucidation of the processes of specification and differentiation of hematopoietic cells a challenging task. Recent discoveries have divulged the GATA factors as central to the genetic control of hematopoiesis. In particular, hematopoietic development is subject to extensive and precise regulation of GATA-1 and GATA-2 at the molecular level. We wish to emphasize the regulatory relationships between GATA-1 and GATA-2 implicated in cell development. An advanced experimental genetic approach has provided evidence that abnormalities in this network may result in a variety of blood disorders. The most striking new finding is the novel pathogenesis arising from GATA-1 dysfunction that leads to leukemia.

Stage-specific repression by the EKLF transcriptional activator

Dynamic changes in transcription factor function can be mediated by switching its interaction with coactivators and corepressors. Erythroid Kruppel-like factor (EKLF) is an erythroid cell-specific transcription factor that plays a critical role in beta-globin gene activation via its interactions with CBP/p300 and SWI/SNF proteins. Unexpectedly, it also interacts with Sin3A and histone deacetylase 1 (HDAC1) corepressors via its zinc finger domain. We now find that selected point mutants can uncouple activation and repression and that an intact finger structure is not required for interactions with Sin3A/HDAC1 or for transrepression. Most intriguingly, EKLF repression exhibits stage specificity, with reversible EKLF-Sin3A interactions playing a key role in this process. Finally, we have located a key lysine residue that is both a substrate for CBP acetylation and required for Sin3A interaction. These data suggest a model whereby the stage of the erythroid cell alters the acetylation status of EKLF and plays a critical role in directing its coactivator-corepressor interactions and downstream transcriptional effects.

Origins of leukaemia in children with Down syndrome

Transient megakaryoblastic leukaemia is found in 10% of newborns with Down syndrome, characterized by constitutional trisomy 21. Although in most cases the leukaemic cells disappear spontaneously after the first months of life, irreversible acute megakaryoblastic leukaemia develops in 20% of these individuals within 4 years. The leukaemic cells typically harbour somatic mutations of the gene encoding GATA1, an essential transcriptional regulator of normal megakaryocytic differentiation. Leukaemia that specifically arises in the context of constitutional trisomy 21 and somatic GATA1 mutations is a unique biological model of the incremental process of leukaemic transformation.

Leukemogenesis caused by incapacitated GATA-1 function

GATA-1 is essential for the development of erythroid and megakaryocytic lineages. We found that GATA-1 gene knockdown female (GATA-1.05/X) mice frequently develop a hematopoietic disorder resembling myelodysplastic syndrome that is characterized by the accumulation of progenitors expressing low levels of GATA-1. In this study, we demonstrate that GATA-1.05/X mice suffer from two distinct types of acute leukemia, an early-onset c-Kit-positive nonlymphoid leukemia and a late-onset B-lymphocytic leukemia. Since GATA-1 is an X chromosome gene, two types of hematopoietic cells reside within heterozygous GATA-1 knockdown mice, bearing either an active wild-type GATA-1 allele or an active mutant GATA-1.05 allele. In the hematopoietic progenitors with the latter allele, low-level GATA-1 expression is sufficient to support survival and proliferation but not differentiation, leading to the accumulation of progenitors that are easily targeted by oncogenic stimuli. Since such leukemia has not been observed in GATA-1-null/X mutant mice, we conclude that the residual GATA-1 activity in the knockdown mice contributes to the development of the malignancy. This de novo model recapitulates the acute crisis found in preleukemic conditions in humans.

GATA1 in normal and malignant hematopoiesis

In the late 1980s, several research groups independently discovered the founding member of the GATA family of transcription factors, GATA-1. Each group had evidence that GATA-1 played an important role in erythroid gene expression, but little did they know that it would turn out to be a key regulator of development of not only red blood cells, but of several other hematopoietic cell types as well. Furthermore, few would have guessed that missense mutations in GATA1 would cause inherited blood disorders, while acquired mutations would be found associated with essentially all cases of acute megakaryoblastic leukemia (AMKL) in children with Down syndrome (DS). With respect to the latter disorder, the presence of a GATA1 mutation is now arguably the defining feature of this leukemia. In this review, I will summarize our current knowledge of the role of GATA-1 in normal development, and discuss how mutations in GATA1 lead to abnormal and malignant hematopoiesis.

Gfi-1B plays a critical role in terminal differentiation of normal and transformed erythroid progenitor cells

Growth factor independence-1B (Gfi-1B) is a transcription factor with a highly conserved transcriptional repressor snail-Gfi-1 (SNAG) domain and 6 zinc-finger domains at the N- and C-terminus, respectively. Disruption of the Gfi-1B gene is lethal in the embryo with failure to produce definitive enucleated erythrocytes. In this study, we analyzed the role of Gfi-1B in human erythropoiesis. We observed an increase of Gfi-1B expression during erythroid maturation of human primary progenitor cells. We studied the consequences of variations in Gfi-1B expression in 2 transformed cell lines (K562 and UT7 cells), as well as in primary CD36(+)/GPA(-) progenitors. A knock-down of Gfi-1B delayed the terminal differentiation of K562 and primary cells. Forced expression of Gfi-1B in UT7 and K562 cells led to an arrest of proliferation and an induction of erythroid differentiation. Enforced expression of Gfi-1B in primary cells at the colony-forming units-erythroid (CFU-E) stage led to a partial glycophorin A (GPA) induction after erythropoietin (EPO) withdrawal but failed to protect cells from apoptosis. Deletion of the SNAG repressor domain abolished Gfi-1B-induced erythroid maturation, strongly suggesting that Gfi-1B acts in the late stage of erythroid differentiation as a transcriptional repressor.

Dynamic GATA factor interplay at a multicomponent regulatory region of the GATA-2 locus

Given the simplicity of the DNA sequence that mediates binding of GATA transcription factors, GATA motifs reside throughout chromosomal DNA. However, chromatin immunoprecipitation analysis has revealed that GATA-1 discriminates exquisitely among these sites. GATA-2 selectively occupies the -2.8-kilobase (kb) region of the GATA-2 locus in the active state despite there being numerous GATA motifs throughout the locus. The GATA-1-mediated displacement of GATA-2 is tightly coupled to repression of GATA-2 transcription. We have used high resolution chromatin immunoprecipitation to show that GATA-1 and GATA-2 occupy two additional regions, -3.9 and -1.8 kb of the GATA-2 locus. GATA-1 and GATA-2 had distinct preferences for occupancy at these regions, with GATA-1 and GATA-2 occupancy highest at the -3.9- and -1.8-kb regions, respectively. Activation of an estrogen receptor fusion to GATA-1 (ER-GATA-1) induced similar kinetics of ER-GATA-1 occupancy and GATA-2 displacement at the sites. In the transcriptionally active state, DNase I hypersensitive sites (HSs) were detected at the -3.9- and -1.8-kb regions, with a weak HS at the -2.8-kb region. Whereas ER-GATA-1-instigated repression abolished the -1.8-kb HS, the -3.9-kb HS persisted in the repressed state. Transient transfection analysis provided evidence that the -3.9-kb region functions distinctly from the -2.8- and -1.8-kb regions. We propose that GATA-2 transcription is regulated via the collective actions of complexes assembled at the -2.8- and -1.8-kb regions, which share similar properties, and through a qualitatively distinct activity of the -3.9-kb complex.

GATA-1 and NF-Y cooperate to mediate erythroid-specific transcription of Gfi-1B gene

Expression of Gfi (growth factor-independence)-1B, a Gfi-1-related transcriptional repressor, is restricted to erythroid lineage cells and is essential for erythropoiesis. We have determined the transcription start site of the human Gfi-1B gene and located its first non-coding exon approximately 7.82 kb upstream of the first coding exon. The genomic sequence preceding this first non-coding exon has been identified to be its erythroid-specific promoter region in K562 cells. Using gel-shift and chromatin immunoprecipitation (ChIP) assays, we have demonstrated that NF-Y and GATA-1 directly participate in transcriptional activation of the Gfi-1B gene in K562 cells. Ectopic expression of GATA-1 markedly stimulates the activity of the Gfi-1B promoter in a non-erythroid cell line U937. Interestingly, our results have indicated that this GATA-1-mediated trans-activation is dependent on NF-Y binding to the CCAAT site. Here we conclude that functional cooperation between GATA-1 and NF-Y contributes to erythroid-specific transcriptional activation of Gfi-1B promoter.

Myeloid Leukemia Factor 1 inhibits erythropoietin-induced differentiation, cell cycle exit and p27Kip1 accumulation

Myeloid leukemia factor 1 (MLF1) is a novel oncoprotein involved in translocations associated with acute myeloid leukemia (AML), especially erythroleukemias. In this study, we demonstrate that ectopic expression of Mlf1 prevented J2E erythroleukemic cells from undergoing biological and morphological maturation in response to erythropoietin (Epo). We show that Mlf1 inhibited Epo-induced cell cycle exit and suppressed a rise in the cell cycle inhibitor p27(Kip1). Unlike differentiating J2E cells, Mlf1-expressing cells did not downregulate Cul1 and Skp2, components of the ubiquitin E3 ligase complex SCF(Skp2) involved in the proteasomal degradation of p27(Kip1). In contrast, Mlf1 did not interfere with increases in p27(Kip1) and terminal differentiation initiated by thyroid hormone withdrawal from erythroid cells, or cytokine-stimulated maturation of myeloid cells. These data demonstrate that Mlf1 interferes with an Epo-responsive pathway involving p27(Kip1) accumulation, which inhibits cell cycle arrest essential for erythroid terminal differentiation.

Microarray transcript profiling distinguishes the transient from the acute type of megakaryoblastic leukaemia (M7) in Down's syndrome, revealing PRAME as a specific discriminating marker

Transient myeloproliferative disorder (TMD) is a unique, spontaneously regressing neoplasia specific to Down's syndrome (DS), affecting up to 10% of DS neonates. In 20-30% of cases, it reoccurs as progressive acute megakaryoblastic leukaemia (AMKL) at 2-4 years of age. The TMD and AMKL blasts are morphologically and immuno-phenotypically identical, and have the same acquired mutations in GATA1. We performed transcript profiling of nine TMD patients comparing them with seven AMKL patients using Affymetrix HG-U133A microarrays. Similar overall transcript profiles were observed between the two conditions, which were only separable by supervised clustering. Taqman analysis on 10 TMD and 10 AMKL RNA samples verified the expression of selected differing genes, with statistical significance (P < 0.05) by Student's t-test. The Taqman differences were also reproduced on TMD and AMKL blasts sorted by a fluorescence-activated cell sorter. Among the significant differences, CDKN2C, the effector of GATA1-mediated cell cycle arrest, was increased in AMKL but not TMD, despite the similar level of GATA1. In contrast, MYCN (neuroblastoma-derived oncogene) was expressed in TMD at a significantly greater level than in AMKL. MYCN has not previously been described in leukaemogenesis. Finally, the tumour antigen PRAME was identified as a specific marker for AMKL blasts, with no expression in TMD. This study provides markers discriminating TMD from AMKL-M7 in DS. These markers have the potential as predictive, diagnostic and therapeutic targets. In addition, the study provides further clues into the pathomechanisms discerning self-regressive from the progressive phenotype.

Neurokinin-B transcription in erythroid cells: direct activation by the hematopoietic transcription factor GATA-1

The GATA family of transcription factors establishes genetic networks that control developmental processes including hematopoiesis, vasculogenesis, and cardiogenesis. We found that GATA-1 strongly activates transcription of the Tac-2 gene, which encodes proneurokinin-B, a precursor of neurokinin-B (NK-B). Neurokinins function through G protein-coupled transmembrane receptors to mediate diverse physiological responses including pain perception and the control of vascular tone. Whereas an elevated level of NK-B was implicated in pregnancy-associated pre-eclampsia (Page, N. M., Woods, R. J., Gardiner, S. M., Lomthaisong, K., Gladwell, R. T., Butlin, D. J., Manyonda, I. T., and Lowry, P. J. (2000) Nature 405, 797-800), the regulation of NK-B synthesis and function are poorly understood. Tac-2 was expressed in normal murine erythroid cells and was induced upon ex vivo erythropoiesis. An estrogen receptor fusion to GATA-1 (ER-GATA-1) and endogenous GATA-1 both occupied a region of Tac-2 intron-7, which contains two conserved GATA motifs. Genetic complementation analysis in GATA-1-null G1E cells revealed that endogenous GATA-2 occupied the same region of intron-7, and expression of ER-GATA-1 displaced GATA-2 and activated Tac-2 transcription. Erythroid cells did not express neurokinin receptors, whereas aortic and yolk sac endothelial cells differentially expressed neurokinin receptor subtypes. Since NK-B induced cAMP accumulation in yolk sac endothelial cells, these results suggest a new mode of vascular regulation in which GATA-1 controls NK-B synthesis in erythroid cells.

Context-dependent regulation of GATA-1 by friend of GATA-1

The transcription factor GATA-1 and its cofactor, friend of GATA-1 (FOG-1), are essential for normal erythroid development. FOG-1 physically interacts with GATA-1 to augment or inhibit its activity. The mechanisms by which FOG-1 regulates GATA-1 function are unknown. By using an assay that is based on the phenotypic rescue of a GATA-1-null erythroid cell line, we found that a conditional form of GATA-1 (GATA-1-ER) strongly induced histone acetylation at the beta-major globin promoter in vivo, consistent with previous results. In contrast, GATA-1 bearing a point mutation that impairs FOG-1 binding [GATA-1(V205M)-ER] failed to induce high levels of histone acetylation at this site. However, at DNase I-hypersensitive site (HS)3 of the beta-globin locus control region, GATA-1-induced histone acetylation was FOG-1-independent. Because the V205M mutation does not disrupt GATA-1 binding to DNA templates in vitro, we were surprised to find that in vivo GATA-1(V205M)-ER fails to bind the beta-globin promoter. However, at HS3, DNA binding by GATA-1 was FOG-1-independent, thus correlating histone acetylation with GATA-1 occupancy. Examination of additional GATA-1-dependent regulatory elements showed that the interaction with FOG-1 is required for GATA-1 occupancy at select sites, such as HS2, but is dispensable at others, including the FOG-1-independent GATA-1 target gene EKLF. Remarkably, at the GATA-2 gene, which is repressed by GATA-1, interaction with FOG-1 was dispensable for GATA-1 occupancy and was required for transcriptional inhibition and histone deacetylation. These results indicate that FOG-1 employs distinct mechanisms when cooperating with GATA-1 during transcriptional activation and repression.