GATA-4 Incompletely Substitutes for GATA-1 in Promoting Both Primitive and Definitive Erythropoiesis in Vivo

Vertebrate GATA transcription factors have been classified into two subgroups; GATA-1, GATA-2, and GATA-3 are expressed in hematopoietic cells, whereas GATA-4, GATA-5, and GATA-6 are expressed in mesoendoderm-derived tissues. We previously discovered that expression of GATA-2 or GATA-3 under the transcriptional control for the Gata1 gene eliminates lethal anemia in Gata1 germ line mutant mice (Gata1.05/Y). Here, we show that the GATA-4 expression by the same regulatory cassette prolongs the life span of Gata1.05/Y embryos from embryonic day 12.5 to 15.5 but fails to abrogate its embryonic lethality. Gata1.05/Y mice bearing the GATA-4 transgene showed impaired maturation of both primitive and definitive erythroid cells and defective erythroid cell expansion in fetal liver. Moreover, the incidence of apoptosis was observed prominently in primitive erythroid cells. In contrast, a GATA-4-GATA-1 chimeric protein prepared by linking the N-terminal region of GATA-4 to the C-terminal region of GATA-1 significantly promoted the differentiation and survival of primitive erythroid cells, although this protein is still insufficient for rescuing Gata1.05/Y embryos from lethal anemia. These data thus show a functional incompatibility between hematopoietic and endodermal GATA factors in vivo and provide evidence indicating specific roles of the C-terminal region of GATA-1 in primitive erythropoiesis.

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Role of Atypical p50:p50:Bcl-3 NFκlB Complex in Differentiation of Erythroid Cells.

Blood ◽

10.1182/blood.v106.11.568.568 ◽

2005 ◽

Vol 106 (11) ◽

pp. 568-568 ◽

Cited By ~ 1

Author(s):

Stephen Sawyer ◽

Jingchun Chen

Keyword(s):

Dna Binding ◽

Transcriptional Control ◽

Biological Significance ◽

Erythroid Differentiation ◽

Erythroid Cell ◽

Erythroid Cells ◽

Necrosis Factor Alpha ◽

Erythroid Progenitor ◽

Tnf Α

Abstract We recently reported that mouse and human primary erythroid progenitor cells and erythroid cell lines synthesize and respond to Tumor Necrosis Factor-alpha (TNF-α). The nuclear transcriptional control complex, NFκlB is central in signaling downstream from TNF-α; so we began to study the function of NFκlB in erythroid cells. We made three very interesting initial findings: 1) first we found that NFκlB binding to DNA increased very slowly in HCD57 erythroid cells treated with erythropoietin (EPO, the hormone required for red blood cell development). An inhibitory effect of adding a neutralizing antibody to TNF-α on EPO-stimulated NFκlB DNA suggested this increase in NFκlB was due to TNF-α rather than direct EPO signaling. 2) We also found that NFκlB binding to DNA increased 10-fold or greater during erythroid differentiation. We found greatly increased NFκlB DNA binding in HCD57 cells that differentiated due to over-expression of JunB, F-MEL induced by DMSO, or human UT7-EPO or murine HCD57 cells induced to differentiate with hemin. 3) Surprisingly, we found that the NFκlB DNA binding complex in mouse primary erythroid cells and the erythroid cells lines tested was almost exclusively composed of the atypical p50/p50/Bcl3 NFκlB rather than the canonical p65/p50 or the non-canonical p65/p52 NFκlB. When we begin to study the biological significance of this atypical NFκlB in EPO-mediated erythroid differentiation in vivo using genetic tools, we found marked deficiencies in the development of erythroid cells in either the nfkb1−/− mice (p50−/−) or the bcl3 −/− mice. The nfkb1−/− mice were mildly anemic. The number of red blood cells in the circulation of these mice was statistically lower than in control mice. The number of CFU-e was also reduced in nfkb1−/− mice. Using the Ter-119 and CD71 staining method, we noted that proerythroblasts and immature erythroid cells increased and mature erythroblasts decreased in either non-anemic bone marrow or anemic spleens of nfkb1−/− mice. Forward scatter of Ter-119+ cells also showed an increased size of the average immature erythroid cell in the bone a marrow of nfkb1 −/− mice, suggesting a block in differentiation and continued cell cycling of the immature erythroblasts. Similar erythroid defects were observed in the spleens of anemic bcl-3−/− mice. nfkb1−/− mice and bcl-3−/− mice are also apparently unable to produce new reticulocytes as effectively as wild type mice after induction of anemia. Our working hypothesis is without expression of either p50 or Bcl-3 NFκlB proteins, immature erythroid cells continue to proliferate and ineffectively differentiate. In summary, the atypical p50/p50/Bcl-3 NFκlB complex appears necessary for maximal differentiation of immature erythroid cells.

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Hematopoietic-specific Stat5-null mice display microcytic hypochromic anemia associated with reduced transferrin receptor gene expression

Blood ◽

10.1182/blood-2007-12-127480 ◽

2008 ◽

Vol 112 (5) ◽

pp. 2071-2080 ◽

Cited By ~ 72

Author(s):

Bing-Mei Zhu ◽

Sara K. McLaughlin ◽

Risu Na ◽

Jie Liu ◽

Yongzhi Cui ◽

...

Keyword(s):

Transferrin Receptor ◽

Iron Homeostasis ◽

Fetal Liver ◽

Receptor Gene ◽

Hypochromic Anemia ◽

Erythroid Cell ◽

Erythroid Cells ◽

Erythroid Progenitor ◽

Important Regulator

Abstract Iron is essential for all cells but is toxic in excess, so iron absorption and distribution are tightly regulated. Serum iron is bound to transferrin and enters erythroid cells primarily via receptor-mediated endocytosis of the transferrin receptor (Tfr1). Tfr1 is essential for developing erythrocytes and reduced Tfr1 expression is associated with anemia. The transcription factors STAT5A/B are activated by many cytokines, including erythropoietin. Stat5a/b−/− mice are severely anemic and die perinatally, but no link has been made to iron homeostasis. To study the function of STAT5A/B in vivo, we deleted the floxed Stat5a/b locus in hematopoietic cells with a Tie2-Cre transgene. These mice exhibited microcytic, hypochromic anemia, as did lethally irradiated mice that received a transplant of Stat5a/b−/− fetal liver cells. Flow cytometry and RNA analyses of erythroid cells from mutant mice revealed a 50% reduction in Tfr1 mRNA and protein. We detected STAT5A/B binding sites in the first intron of the Tfr1 gene and found that expression of constitutively active STAT5A in an erythroid cell line increased Tfr1 levels. Chromatin immunoprecipitation experiments confirmed the binding of STAT5A/B to these sites. We conclude that STAT5A/B is an important regulator of iron update in erythroid progenitor cells via its control of Tfr1 transcription.

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Fetal liver pro-B and pre-B lymphocyte clones: expression of lymphoid-specific genes, surface markers, growth requirements, colonization of the bone marrow, and generation of B lymphocytes in vivo and in vitro

Molecular and Cellular Biology ◽

10.1128/mcb.12.2.518-530.1992 ◽

1992 ◽

Vol 12 (2) ◽

pp. 518-530

Author(s):

R Palacios ◽

J Samaridis

Keyword(s):

B Cell ◽

B Lymphocytes ◽

Stromal Cells ◽

B Lymphocyte ◽

Germ Line ◽

Fetal Liver ◽

Rag 1 ◽

B Cell Precursors

We describe here the development and characterization of the FLS4.1 stromal line derived from 15-day fetal liver of BALB/c embryos and defined culture conditions that efficiently support the cloning and long-term growth of nontransformed B-220+ 14-day fetal liver cells at two stages of B-cell development, namely, pro-B lymphocytes (immunoglobulin [Ig] genes in germ line configuration) and pre-B cells (JH-rearranged genes with both light-chain Ig genes in the germ line state). All B-cell precursor clones require recombinant interleukin-7 (rIL-7) and FLS4.1 stromal cells for continuous growth in culture, but pro-B lymphocyte clones can also proliferate in rIL-3. None proliferate in rIL-1, rIL-2, rIL-4, rIL-5, rIL-6, or leukemia inhibitory factor. FLS4.1 stromal cells synthesize mRNA for Steel factor but not for IL-1 to IL-7; all pro-B and pre-B clones express c-Kit, the receptor for Steel factor, and a c-Kit-specific antibody inhibits the enhanced proliferative response of fetal liver B-220+ B-cell precursors supported by FLS4.1 stromal cells and exogenous rIL-7 but does not affect that promoted by rIL-7 alone. Northern (RNA) blot analysis of the expression of the MB-1, lambda 5, Vpre-B, c mu, RAG-1, and RAG-2 genes in pro-B and pre-B clones show that transcription of the MB-1 gene precedes IgH gene rearrangement and RNA synthesis from c mu, RAG-1, RAG-2, lambda 5, and Vpre-B genes. All clones at the pre-B-cell stage synthesize mRNA for c mu, RAG-1, and RAG-2 genes; transcription of the lambda 5 and Vpre-B genes seems to start after D-to-JH rearrangement in B-cell precursors, indicating that the proteins encoded by either gene are not required for B-cell progenitors to undergo D-to-JH gene rearrangement. These findings mark transcription of the MB-1 gene as one of the earliest molecular events in commitment to develop along the B-lymphocyte pathway. Indeed, both pro-B and pre-B clones can generate in vitro and in vivo B lymphocytes but not T lymphocytes; moreover, these clones do not express the CD3-gamma T-cell-specific gene, nor do they have rearranged gamma, delta, or beta T-cell antigen receptor genes.

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Pattern of pyruvate kinase isozymes in erythroleukemia cell lines and in normal human erythroblasts

Blood ◽

10.1182/blood.v64.4.930.bloodjournal644930 ◽

1984 ◽

Vol 64 (4) ◽

pp. 930-936 ◽

Cited By ~ 1

Author(s):

I Max-Audit ◽

U Testa ◽

D Kechemir ◽

M Titeux ◽

W Vainchenker ◽

...

Keyword(s):

Pyruvate Kinase ◽

Cell Lines ◽

Fetal Liver ◽

Erythroid Cells ◽

Low Concentrations ◽

Erythroleukemia Cell ◽

Erythroleukemic Cell ◽

High Level

To further investigate the erythroid nature of the two human erythroleukemia cell lines, K562 and HEL-60, and to define the ontogeny of pyruvate kinase (PK) isozymes (R, M2) in developing human erythroid cells, we have studied the isozymic alterations, if any, during differentiation of these cell lines in vitro and normoblasts isolated from fetal liver in vivo. PK activity of erythroleukemic cell lines was intermediate between that observed in leukocytes and in fetal liver erythroblasts. These cell lines contained a high level of M2-PK, but R- PK was always present, albeit at low concentrations, in all the clones or subclones we studied. Erythroblasts from fetal liver were separated according to density on a Stractan gradient. R-PK levels were nearly constant in the different fractions, whereas M2-PK levels markedly decreased as the erythroblasts became mature and almost completely disappeared in late erythroid cells. Thus, these results clearly demonstrate the erythroid origin of these cell lines.

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Pattern of pyruvate kinase isozymes in erythroleukemia cell lines and in normal human erythroblasts

Blood ◽

10.1182/blood.v64.4.930.930 ◽

1984 ◽

Vol 64 (4) ◽

pp. 930-936 ◽

Cited By ~ 17

Author(s):

I Max-Audit ◽

U Testa ◽

D Kechemir ◽

M Titeux ◽

W Vainchenker ◽

...

Keyword(s):

Pyruvate Kinase ◽

Cell Lines ◽

Fetal Liver ◽

Erythroid Cells ◽

Low Concentrations ◽

Erythroleukemia Cell ◽

Erythroleukemic Cell ◽

High Level

Abstract To further investigate the erythroid nature of the two human erythroleukemia cell lines, K562 and HEL-60, and to define the ontogeny of pyruvate kinase (PK) isozymes (R, M2) in developing human erythroid cells, we have studied the isozymic alterations, if any, during differentiation of these cell lines in vitro and normoblasts isolated from fetal liver in vivo. PK activity of erythroleukemic cell lines was intermediate between that observed in leukocytes and in fetal liver erythroblasts. These cell lines contained a high level of M2-PK, but R- PK was always present, albeit at low concentrations, in all the clones or subclones we studied. Erythroblasts from fetal liver were separated according to density on a Stractan gradient. R-PK levels were nearly constant in the different fractions, whereas M2-PK levels markedly decreased as the erythroblasts became mature and almost completely disappeared in late erythroid cells. Thus, these results clearly demonstrate the erythroid origin of these cell lines.

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α-Fetoprotein production and erythropoiesis in cell cultures of human fetal liver

Canadian Journal of Biochemistry ◽

10.1139/o77-081 ◽

1977 ◽

Vol 55 (5) ◽

pp. 571-575 ◽

Cited By ~ 1

Author(s):

L. F. Congote ◽

F. Bruno ◽

S. Solomon

Keyword(s):

Cell Function ◽

Fetal Liver ◽

Calf Serum ◽

Liver Cells ◽

Velocity Sedimentation ◽

Erythroid Cell ◽

Erythroid Cells ◽

Human Fetal Liver ◽

Fetal Liver Cells ◽

Human Fetal Liver Cells

α-Fetoprotein and the synthesis of heme associated with hemoglobin were measured simultaneously in short-term cultures of human fetal liver cells to correlate the relationship of α-fetoprotein to erythroid cell function. Both synthetic processes decreased exponentially during the first 5 days of culture. The use of media supplemented with different batches of fetal calf serum and porcine portal vein serum indicated that the optimal conditions for the production of α-fetoprotein were different from those required for the synthesis of heme associated with hemoglobin. Moreover, the α-fetoprotein-producing cells could be separated from erythroid cells after velocity sedimentation in Ficoll gradients. Although it is well known that erythropoiesis and α-fetoprotein production occur simultaneously during ontogenesis, α-fetoprotein itself (0.01–100 μg/ml) did not stimulate heme synthesis in liver erythroid cells. Erythropoietin did not stimulate α-fetoprotein production. It is concluded that there is no cause–effect relationship between α-fetoprotein production and erythroid cell function in human fetal liver cells and that the two processes occur independently in different cell types.

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The SCL Transcription Factor Supports Erythroid Cell Survival and Differentiation Downstream of the Erythropoietin Receptor.

Blood ◽

10.1182/blood.v104.11.818.818 ◽

2004 ◽

Vol 104 (11) ◽

pp. 818-818

Author(s):

Rachid Lahlil ◽

Richard Martin ◽

Peter D. Aplan ◽

C. Glenn Begley ◽

Jacqueline E. Damen ◽

...

Keyword(s):

Transcription Factor ◽

Cell Survival ◽

Cell Fate ◽

Genetic Interaction ◽

Fetal Liver ◽

Erythropoietin Receptor ◽

Erythroid Cell ◽

Loss Of Function

Abstract Erythroid cell development critically depends on the SCL/Tal1 transcription factor and on erythropoietin signalling. In the present study, we have taken several approaches to show that the two genes operate within the same pathway to consolidate the erythroid lineage. Signaling through the erythropoietin receptor (EpoR) upregulates SCL protein levels in a clonal cell line (TF-1) in vitro, and in murine fetal liver cells in vivo, when Epor−/− cells were compared to those of wild type littermates at E12.5. In addition, we provide functional evidence for a linear pathway from EpoR to SCL that regulates erythropoiesis. Interfering with SCL induction or SCL function prevents the anti-apoptotic effect of Epo in TF-1 cells and conversely, ectopic SCL expression is sufficient to substitute for Epo to transiently maintain cell survival. In vivo, SCL gain of function complements the cellular defects in Epor−/− embryos to support cell survival and maturation during primitive and definitive erythropoiesis, as assessed by cellular and histological analyses of Epor−/− SCLtg embryos. Moreover, several erythroid specific genes that are decreased in Epor−/− embryos are rescued by the SCL transgene including glycophorinA, bH1 and bmaj globin, providing molecular confirmation of the functional and genetic interaction between Epor and SCL. Conversely, erythropoiesis becomes deficient in compound Epor+/−SCL+/− heterozygote mice, indicating that the genetic interaction between EpoR and SCL is synthetic. Finally, using EpoR mutants that harbour well defined signalling deficiencies, combined with gain and loss of function approaches for specific kinases, we identify MAPK as the major signal transduction pathway downstream of EpoR that upregulates SCL function, necessary for erythroid cell survival and differentiation. Taken together, our observations are consistent with the view that cytokines can influence cell fate by altering the dosage of lineage transcriptional regulators.

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Emp Null Mice Are Non-Viable and Exhibit Erythroid Differentiation Defect.

Blood ◽

10.1182/blood.v106.11.806.806 ◽

2005 ◽

Vol 106 (11) ◽

pp. 806-806 ◽

Cited By ~ 1

Author(s):

Shivani Soni ◽

Shashi Bala ◽

Babette Gwynn ◽

Kenneth E. Sahr ◽

Luanne L. Peters ◽

...

Keyword(s):

Fetal Liver ◽

Direct Sequencing ◽

Embryonic Stem ◽

Gene Trap ◽

Physical Contact ◽

Erythroid Cells ◽

Wild Type ◽

Definitive Erythropoiesis

Abstract Emp, erythroblast macrophage protein, was originally detected in erythroblasts and macrophages, which form erythroblastic islands during erythropoiesis in the human bone marrow. The physical contact between erythroblasts and macrophages was suggested to promote the terminal maturation of erythroblasts, leading to their enucleation in vitro. To evaluate the function of Emp in vivo, we employed gene targeting studies to develop an Emp(−/−) mouse model. Mouse embryonic stem cells containing a gene-trap insertion in Emp were obtained from BayGenomics. Insertion of the gene-trap vector into Emp was verified by direct sequencing of cDNA obtained by 5′RACE. Chimeric mice generated by blastocyst microinjection were intercrossed, and the offspring were genotyped by PCR and Southern hybridization. The Emp (+/−) mice were healthy and fertile. However, no live Emp (−/−) mice were found among the progeny of the Emp (+/−) intercrosses. Analysis of timed pregnancies revealed that Emp (−/−) embryos were present at a frequency roughly consistent with Mendelian inheritance throughout the embryonal stages. Homozygous Emp (−/−) embryos were small and pale compared to their littermates, and they survived embryonic development but died at birth. To determine the effect, if any, of Emp gene deletion on definitive hematopoiesis, livers of +/+, +/−, and −/− embryos at E15.5 were examined after H&E and Giemsa staining of paraffin-embedded serial sections, and cytospins. We found few mature erythroid cells in the sinusoids of homozygotes, in contrast to those of either wild-type or heterozygotes, where abundant enucleated red blood cells were observed. Although nucleated erythrocytes were found in both wild-type and mutant embryos, their relative proportions were very different: the less mature forms (proerythroblasts) predominated in the −/− embryos whereas the more mature forms (polychromatophilic/orthochromatic and enucleated erythrocytes) were most common in +/+ and +/− embryos. Furthermore, erythroblastic islands consisting of a central macrophage surrounded by developing erythroblasts were seen in the cytospin preparations of wild-type and heterozygote livers but not in those of homozygous null livers. Since fetal liver macrophages (FLMs) are indispensable for definitive erythropoiesis, we investigated the effect that Emp’s absence might have on development of FLMs. The E15.5 fetal liver sections were stained with the macrophage-specific F4/80 antigen. Numerous F4/80-positive macrophages were present throughout the liver of normal embryos whereas, the number was substantially reduced in Emp (−/−) liver. In summary, in the absence of Emp, FLMs are significantly reduced and terminal maturation of erythroid cells is negatively affected. Thus, the availability of Emp(−/−) embryos will provide a unique experimental model to study the function of macrophages in definitive erythropoiesis.

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Klf1 Regulatory Networks in Primary Erythroid Cells.

Blood ◽

10.1182/blood.v114.22.1462.1462 ◽

2009 ◽

Vol 114 (22) ◽

pp. 1462-1462

Author(s):

Michael Tallack ◽

Thomas Whitington ◽

Brooke Gardiner ◽

Eleanor Wainwright ◽

Janelle Keys ◽

...

Keyword(s):

Regulatory Networks ◽

Conflicts Of Interest ◽

Fetal Liver ◽

Es Cells ◽

Erythroid Differentiation ◽

Erythroid Cell ◽

Gene Promoters ◽

Erythroid Cells ◽

Red Cell Membrane ◽

Sequencing Platform

Abstract Abstract 1462 Poster Board I-485 Klf1/Eklf regulates a diverse suite of genes to direct erythroid cell differentiation from bi-potent progenitors. To determine the local cis-regulatory contexts and transcription factor networks in which Klf1 works, we performed Klf1 ChIP-seq using the SOLiD deep sequencing platform. We mapped more than 10 million unique 35mer tags and found ∼1500 sites in the genome of primary fetal liver erythroid cells are occupied by endogenous Klf1. Many reside within well characterised erythroid gene promoters (e.g. b-globin) or enhancers (e.g. E2f2 intron 1), but some are >100kb from any known gene. We tested a number of Klf1 bound promoter and intragenic sites for activity in erythroid cell lines and zebrafish. Our data suggests Klf1 directly regulates most aspects of terminal erythroid differentiation including synthesis of the hemoglobin tetramer, construction of a deformable red cell membrane and cytoskeleton, bimodal regulation of proliferation, and co-ordination of anti-apoptosis and enucleation pathways. Additionally, we suggest new mechanisms for Klf1 co-operation with other transcription factors such as those of the gata, ets and myb families based on over-representation and spatial constraints of their binding motifs in the vicinity of Klf1-bound promoters and enhancers. Finally, we have identified a group of ∼100 Klf1-occupied sites in fetal liver which overlap with Klf4-occupied sites in ES cells defined by Klf4 ChIP-seq. These sites are associated with genes controlling the cell cycle and proliferation and are Klf4-dependent in skin, gut and ES cells, suggesting a global paradigm for Klfs as regulators of differentiation in many, if not all, cell types. Disclosures No relevant conflicts of interest to declare.

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Control of Hemoglobin Switching by BCL11A.

Blood ◽

10.1182/blood.v114.22.5.5 ◽

2009 ◽

Vol 114 (22) ◽

pp. 5-5

Author(s):

Jian Xu ◽

Vijay G. Sankaran ◽

Yuko Fujiwara ◽

Stuart H. Orkin

Keyword(s):

Gene Silencing ◽

Conflicts Of Interest ◽

Fetal Liver ◽

Association Studies ◽

Globin Gene ◽

Clinical Severity ◽

Genome Wide Association Studies ◽

Erythroid Cells ◽

B Locus

Abstract Abstract 5 All vertebrates switch expression of globin chains during development. In humans b-like globins switch from embryonic to fetal to adult, whereas in the mouse a single switch from embryonic to adult occurs. The switch from human fetal (g) to adult (b) expression is especially critical in the b-hemoglobin disorders, such as sickle cell anemia and the b-thalassemias. Delay of the switch or reactivation of the fetal gene in the adult stage greatly ameliorates clinical severity. Despite intensive molecular studies of the human b-globin cluster over more than two decades, the proteins regulating the switch, and the mechanisms controlling the process, have been largely elusive. Recently, genome-wide association studies identified genetic variation at a chromosome 2 locus that correlates with the level of HbF in different populations. The most highly associated single nucleotide polymorphisms (SNPs) reside in an intron of the BCL11A gene, which encodes a zinc-finger repressor protein. Previously we showed that shRNA-mediated ex vivo knockdown of BCL11A in cultured human CD34-derived erythroid precursors leads to robust HbF expression, consistent with a role for BCL11A in maintaining g-genes in a silenced state in adult cells. To address in vivo roles of BCL11A either in development or in globin gene silencing in an intact individual, we have employed stringent genetic tests of function in mice that carry a complete human b-globin gene cluster as a yeast artificial chromosome transgene (b-locus mice). Knockout of BCL11A in mice leads to failure to silence the endogenous b-like embryonic genes in adult erythroid cells of the fetal liver (>2500-fold derepression). The ratio of human g to b globin RNA in the fetal liver of BCL11A knockout mice is inverted compared to controls, such that g constitutes >90% of the b-like human expression at embryonic day (E)14.5 and >75% at E18.5. These quantitatively striking findings indicate that BCL11A controls developmental silencing of g-globin gene expression. To address by formal genetics the contribution of BCL11A to g silencing in adult animals we have employed conditional inactivation of BCL11A through hematopoietic- and erythroid-specific Cre-alleles. These experiments reveal that BCL11A is also required in vivo for g-gene silencing in adults. We observed that human g-globin expression is persistently derepressed >2000-fold (as compared to littermate controls) in bone marrow erythroblasts of 15-20 week old b-locus mice upon erythroid-specific deletion of BCL11A. Taken together, these findings establish BCL11A as the first genetically validated transcriptional regulator of both developmental control of globin switching and silencing of g-globin expression in adults. The recognition of these roles for BCL11A now permits focused mechanistic studies of the switch. In human erythroid cells, BCL11A physically interacts with at least two corepressor complexes, Mi-2/NuRD and LSD1/CoREST, as well as the erythroid transcription factor GATA-1 and the HMG-box protein SOX6. Rather than binding to the promoters of the g- or b-globin genes as do these latter factors, BCL11A protein occupies the upstream locus control and g-d-intergenic regions of the b-globin cluster (as determined by high resolution ChIP-Chip analysis), suggesting that BCL11A mediates long-range interactions and/or reconfigures the locus during different stages. An in-depth mechanistic understanding of globin switching offers the prospect for design of target-based activation of HbF in adult erythroid cells of patients with hemoglobin disorders. Disclosures: No relevant conflicts of interest to declare.

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