Gamma-globin Transcription Research Articles

New Concepts in Genome Regulation

Manipulating gene regulation to favor gamma-globin transcription over beta-globin transcription has been a goal of research in erythropoiesis for decades because of its relevance to amelioration of the pathophysiology of sickle cell disease and beta-thalassemia. A fundamental unanswered question in biology is how the unique pattern of gene expression, the transcriptome, of the many different individual mammalian cell types arises from the same genome blueprint and changes during development and differentiation. There is a growing appreciation that genome organization and the folding of chromosomes is a key determinant of gene transcription. Within this framework, enhancers function to increase the transcription of target genes over long linear distances. To accomplish this, enhancers engage in close physical contact with target promoters through chromosome folding, or looping. These long range interactions are orchestrated by cell type specific proteins and protein complexes that bind to enhancers and promoters and stabilize their interaction with each other. We have been studying LDB1, a member of an erythroid protein complex containing GATA1, TAL1 and LMO2. The LDB1complex activates erythroid genes through occupancy of virtually all erythroid enhancers. LDB1engages in homo- and heterotypic interactions with proteins occupying the promoters of erythroid genes to bring them into proximity with their enhancers. We find that enhancer long range looping activity can be redirected. Both targeting of the beta-globin locus control region to the gamma-globin gene in adult erythroid cells by the tethering of LDB1 or epigenetic unmasking of a silenced gamma-globin gene lead to increased locus control region (LCR)/gamma-globin contact frequency and reduced LCR/beta-globin contact. The outcome of these manipulations is robust, pan-cellular gamma-globin transcription activation with a concomitant reduction in beta-globin transcription. These examples suggest that chromosome looping can be considered a therapeutic target for gene activation or gene silencing to ameliorate genetic diseases such as sickle cell disease and beta-thalassemia. DisclosuresNo relevant conflicts of interest to declare.

Fetal Hemoglobin Expression Is Differentially Affected by Inhibition of the Proposed Dred Complex Constituents, LSD1 and DNMT1

AbstractAbstract 3263 IntroductionSickle cell disease and beta thalassemia are disorders caused by mutations in adult hemoglobin (HbA) or defects in HbA expression. A potential therapeutic solution is reactivation of fetal hemoglobin (HbF) expression. Although HbF, comprising two alpha and two gamma globin chains, is the primary form of hemoglobin expressed in utero, gamma globin expression is silenced in adults. One proposed mechanism of gamma globin silencing involves binding of the direct repeat erythroid definitive (DRED) repressor complex to sequences in the gamma globin promoter. The DRED complex is reported to include the orphan nuclear hormone receptors TR2 and TR4, lysine specific demethylase (LSD1) and DNA methyltransferase (DNMT1). As both LSD1 and DNMT1 are epigenetic modifiers, gamma globin repression is proposed to be mediated by LSD1- and DNMT1-induced epigenetic changes. To investigate the role of DNMT1 and LSD1 in HbF silencing, HbF expression was evaluated in an erythroid differentiation model where hematopoietic progenitor cells were treated with either DNMT1 or LSD1 small molecule inhibitors or siRNA. MethodsHuman hematopoietic progenitor cells from healthy donors were induced to become erythroid using a two step protocol including erythropoietin, SCF, IL-3 and hydrocortisone for days 1–7 and erythropoietin and SCF for days 8–14. Cultures were treated with a range of concentrations of either tranylcypromine or S2101 (LSD1 inhibitors) or 5-azacytidine (DNMT1 inhibitor) and compared to HbF-inducing, positive control small molecules pomalidomide and lenalidomide. Cultures were also treated with LSD1 siRNAs and compared to controls. The effect of treatment on gamma, beta and alpha globin transcription was determined by qRT-PCR. The effect of treatment on HbA and HbF levels was determined by ELISA, HPLC, flow cytometry and imaging. Differentiation was characterized by morphology and flow-based detection of CD34 and glycophorin. Effects on viability were characterized by ViCell and flow cytometry. ResultsTreatment with a concentration range of 5-azacytidine increased the rate of red blood cell differentiation as measured by daily changes in CD34 and glycophorin and hemoglobinization. Quantitative ELISA demonstrated that HbF expression increased two-fold. In contrast, LSD1 inhibition reduced both the rate of proliferation and differentiation of erythroid progenitors. Consistent with impaired differentiation, both beta globin transcription and HbA expression were reduced by up to 84% (qRT-PCR) and 65% (quantitative ELISA), respectively. No increase in gamma globin transcription or HbF expression was observed in response to LSD1 inhibition. Control cultures differentiated as expected: after 14 days of treatment the majority of vehicle-, lenalidomide- or pomalidomide-treated cells were glycophorin-positive and enucleation was readily apparent. Both lenalidomide and pomalidomide treatment induced a two-fold increase in HbF expression, as previously reported. ConclusionsAlthough both LSD1 and DNMT1 are reported to be components of the DRED complex and are proposed to be jointly responsible for epigenetically modifying the gamma globin promoter to silence HbF expression, inhibition of the two proteins had different outcomes on HbF expression. DNMT1 inhibition upregulated HbF expression to a similar extent as pomalidomide (currently in Phase 1 clinical trials for HbF induction), whereas LSD1 inhibition impaired erythroid differentiation and hemoglobinization. These results suggest that the mechanism of gamma globin silencing and the proposed role of the DRED complex require further evaluation. Furthermore, this work also suggests that LSD1 inhibition is not a therapeutic strategy for HbF induction in patients with sickle cell disease or beta thalassemia. Disclosures:Arvedson:Amgen: Employment. Tran:Amgen: Employment. Ross:Amgen: Employment. Yoder:Amgen: Employment. Hertz:Amgen: Employment. Hale:Amgen: Employment. Eschelbach:Amgen: Employment. Dineen:Amgen: Employment. Matyas:Amgen: Employment. Hartley:Amgen: Employment. Morgenstern:Amgen: Employment. Winters:Amgen: Employment. Cindy:Amgen: Employment. Molineux:Amgen: Employment. Coxon:Amgen: Employment.

Mitosis Is Not Required for Coordinated Changes in Iron Metabolism and High-Level Globin Gene Transcription in Erythropoietin-Responsive CD34+ Cells.

Within the erythroid lineage, erythropoietin (EPO) responsiveness is manifested by cell division and growth as well as specific changes in heme and globin production that ultimately result in the production of erythrocytes. However, the nascent relationship between EPO-associated mitosis and globin gene regulation has not been fully defined. In this study, cultured adult human CD34+ cells from peripheral blood were used to investigate early cellular responses to erythropoietin in the context of mitosis. Matched cultures were performed in replicate using human cells from at least two healthy adults. To detect mitosis, one million cells were labeled with 2uM carboxyfluorescein diacetate succinamidyl ester (CFSE). The CFSE-labeled cells were then cultured in the presence [EPO(+)] or absence [EPO(−)] of 4U/mL EPO, and analyzed using flow cytometry. No cell divisions were detected in either condition during the first 24 hours in culture, and multiple cell divisions were noted on subsequent culture days only in the EPO(+) cultures. Remarkably, dual-staining with CFSE and CD71 revealed a small (<1%) population of the undivided cells at 24 hours with very high surface levels of transferrin receptor [CD71(++++)] exclusively in the EPO(+) cultures. Further analysis of those rare EPO-responsive, pre-mitotic cells revealed DNA synthesis and entry into the cell cycle in 62.5±4.7% compared with 1.5±1.8% among the cells with lower CD71 expression. None expressed glycophorin A. Based upon their distinct phenotype, single cells were sorted into 96-well plates, with sorting confirmation by quantitative RT-PCR of GAPDH mRNA (20 copies/cell detection limit). Next, gamma- and beta-globin transcripts were amplified for comparison (4 separate experiments). Among the pre-mitotic, CD71(++++) population, 20 of 182 total sorted cells (11%) lacked detectable levels of gamma- and beta-globin mRNA. Only 1 of 182 cells (0.5%) expressed gamma-globin mRNA and no detectable beta globin mRNA. 138 of 182 cells (76%) expressed only beta-globin mRNA, and 23 of 182 cells (13%) expressed both globin mRNAs. The median and mean levels of gamma-globin mRNA among the 24 gamma(+) cells were 92 and 245±1015 copies/cell respectively. In contrast, the median and mean levels of beta-globin mRNA among the 161 beta(+) cells were 1624 and 3999±5892 copies/cell respectively. By comparison, the CD34+ cells with low levels of surface CD71 in either EPO(+) or EPO(−) cultures demonstrated detectable levels of gamma globin mRNA in only 7 of 145 sorted cells (5%), and beta-globin mRNA in 85 of 145 cells (59%), with total (gamma+beta) globin mRNA under 200 copies/cell in >90% of those cells. These novel results suggest that the frequency and levels of gamma-globin transcripts are quite low at the earliest stages of an EPO response among adult human CD34+ cells. However, it is clearly demonstrated that the cells can increase their capacity to import iron and transcribe beta-globin mRNA at very high levels prior to their first EPO-dependent cell division.

Fine Tuning of Globin Gene Expression by DNA Methylation

Expression patterns in the globin gene cluster are subject to developmental regulation in vivo. While the γA and γG genes are expressed in fetal liver, both are silenced in adult erythrocytes. In order to decipher the role of DNA methylation in this process, we generated a YAC transgenic mouse system that allowed us to control γA methylation during development. DNA methylation causes a 20-fold repression of γA both in non-erythroid and adult erythroid cells. In erythroid cells this modification works as a dominant mechanism to repress γ gene expression, probably through changes in histone acetylation that prevent the binding of erythroid transcription factors to the promoter. These studies demonstrate that DNA methylation serves as an elegant in vivo fine-tuning device for selecting appropriate genes in the globin locus. In addition, our findings provide a mechanism for understanding the high levels of γ-globin transcription seen in patients with Hereditary Persistence of Fetal Hemoglobin, and help explain why 5azaC and butyrate compounds stimulate γ-globin expression in patients with β-hemoglobinopathies.

A sustained and pancellular reversal of gamma-globin gene silencing in adult human erythroid precursor cells

AbstractWe systematically compared cytokine-mediated increases or decreases in proliferation with globin gene and protein expression in adult human erythroblasts. Despite their opposite effects on growth, stem cell factor (SCF) and transforming growth factorbeta (TGF-B) had synergistic effects with respect to fetal hemoglobin (HbF): average HbF/HbF + adult hemoglobin (HbA) ratio in erythropoietin (EPO) = 1.4 ± 1.0%; EPO + TGF-B = 10.8 ± 1.9%; EPO + SCF = 19.1 ± 6.2%; and EPO + SCF + TGF-B (EST) = 39.3 ± 6.3%. Polymerase chain reaction (PCR) revealed significant increases in gamma-globin transcripts that were balanced by reduced beta-globin transcripts. Single-cell quantitative PCR demonstrated a complete reversal of gamma-globin gene silencing with detectable gamma-globin mRNA in more than 95% of the cells. Immunostaining with HbF antibodies also showed a pancellular distribution in EST (96.2 ± 0.01% HbF positive) compared with a heterocellular distribution in EPO (42.9 ± 0.01% HbF positive). As shown here for the first time, a robust and pancellular reversal of gamma-globin gene silencing among hemoglobinized erythroblasts from adult humans may be achieved in the absence of hereditary mutation or direct genomic manipulation. (Blood. 2005;105:387-393)

Prospective identification of erythroid elements in cultured peripheral blood

We have developed a prospective approach to identify the generation of erythroid cells derived from cultured peripheral blood mononuclear cells (PBMC) by monitoring the expression of the cell surface protein CD48. Unpurified populations of PBMC obtained from the buffy coats of normal volunteers were grown in suspension culture in the absence or presence of erythropoietin. A profile of surface CD48 expression permitted a flow cytometric identification of erythropoietin responsive populations at various stages of their maturation. In the absence of erythropoietin (EPO) supplemented media, the CD48 − cells represented <5% of the total population of PBMC remaining in culture. In cultures supplemented with 1 U/mL EPO, the mean percentage of CD48 − cells increased to 34.7 + 14.9% ( p < 0.01) after 14 days in culture. Coordinated CD34 and CD71 (transferrin receptor) expression, morphology, gamma-globin transcription, and colony formation in methylcelluose were observed during the 14-day culture period. Flow cytometric monitoring of bulk cultured PBMC provides a simple and reliable means for the prospective or real-time study of human erythropoiesis.

A Novel Function of Stat1α and Stat3 Proteins in Erythropoietin-Induced Erythroid Differentiation of a Human Leukemia Cell Line

We recently determined that erythropoietin (EPO) activates 3 members of the signal transducer and activator of transcription (STAT) family, Stat1alpha, Stat3, and Stat5, in the human EPO-dependent cell lines, UT-7 and UT-7/EPO (Kirito et al, J Biol Chem 272:16507, 1997). In addition, we have shown that Stat1alpha, but not Stat3, is involved in EPO-induced cellular proliferation. In this study, we examined the roles of Stat1alpha and Stat3 in EPO-induced erythroid differentiation. UT-7/GM was used as a model system, because this cell line can differentiate into erythroid-lineage cells with EPO treatment (Komatsu et al, Blood 89:4021, 1997). We found that EPO did not activate Stat1alpha or Stat3 in UT-7/GM cells. Transfection experiments showed that both Stat1alpha and Stat3 inhibited the induction by EPO of gamma-globin and erythroid-specific 5-aminolevulinate synthetase transcripts, resulting in a reduction of the percentage of hemoglobin-positive cells. Dominant negative forms of Stat1alpha or Stat3 promoted the EPO-induced erythroid differentiation of UT-7/GM cells, even in the presence of granulocyte-macrophage colony-stimulating factor, although this cytokine never induced erythroid differentiation of the parent UT-7/GM cells with or without EPO. A cell cycle analysis showed that the constitutive activation of Stat1alpha, but not Stat3, shortened the period of G0/G1 prolongation caused by EPO stimulation. Taken together, our data suggest that Stat1alpha and Stat3 act as negative regulators in EPO-induced erythroid differentiation. Specifically, Stat1alpha may activate a cell cycle-associated gene(s), leading to the entry of cells into the cell cycle.

Hemoglobin switching protocols in thalassemia. Experience with sodium phenylbutyrate and hydroxyurea.

Homozygous beta thalassemia affects thousands of people around the world. Current management of this condition includes regular transfusion of red cells, which leads to transfusional iron overload requiring chelation therapy: increasing hemoglobin levels while decreasing or eliminating iron overload is therefore a major therapeutic goal in the treatment of thalassemia. Bone marrow transplantation may achieve this goal, but it is not an option for most patients. This study reports on efforts to increase gamma-globin transcription and HbF production using sodium phenylbutyrate (SPB) and hydroxyurea (HU). It was found that 36% (4/11) of all patients or 50% (4/8) of non-transfused patients responded to SPB (increase in Hb levels of 1 g/dL). A positive correlation between baseline serum erythropoietin level and likelihood of response to SPB was observed. Since HU may also increase HbF production, evaluation of combination therapy with these drugs is underway and preliminary results are reported.

Silencing of human fetal globin expression is impaired in the absence of the adult beta-globin gene activator protein EKLF.

Globin genes are subject to tissue-specific and developmental stage-specific regulation. A switch from human fetal (gamma)-to adult (beta)-globin expression occurs within erythroid precursor cells of the adult lineage. Previously we and others showed by targeted gene disruption that the zinc finger gene, erythroid Krüppel-like factor (EKLF), is required for expression of the beta-globin gene in mice, presumably through interaction with a high-affinity binding site in the proximal promoter. To examine the role of EKLF in the developmental regulation of the human gamma-globin gene we interbred EKLF heterozygotes (+/-) with mice harboring a human beta-globin yeast artificial chromosome transgene. We find that in the absence of EKLF, while human beta-globin expression is dramatically reduced, gamma-globin transcripts are elevated approximately 5-fold. Impaired silencing of gamma-globin expression identifies EKLF as the first transcription factor participating quantitatively in the gamma-globin to beta-globin switch. Our findings are compatible with a competitive model of switching in which EKLF mediates an adult stage-specific interaction between the beta-globin gene promoter and the locus control region that excludes the gamma-globin gene.

Transcription factor OTF-1 interacts with two distinct DNA elements in the A.gamma.-globin gene promoter

A DNA region (site II) in the promoter of the human A gamma-globin gene (-182 to -168) is involved in transcriptional regulation. At least two nuclear proteins bind to this region: the erythroid-specific factor NF-E1/GF-1 and another factor present in many cell lines. In the present study, we demonstrate that the ubiquitous factor binding to site II has immunological identity with the octamer transcription factor OTF-1, which has been implicated in the regulation of expression of genes such as histone H2b and small nuclear RNA. In addition, we show that OTF-1 binds to site I (-291 to -267), a purine-rich region upstream of site II. Interestingly, OTF-1 binds to sites I and II with equal affinity. This was unexpected since the 14 bp site I binding site AAGAATAAATTAGA (-291 to -278), determined by methylation interference, does not show obvious similarities to the canonical octamer binding site for OTF-1 in site II (ATGCAAAT). Interaction of OTF-1 with functionally active binding sites in the gamma-globin promoter suggests that this factor has a role in gamma-globin transcription.

The beta globin 3' enhancer element confers regulated expression on the human gamma globin gene in the human embryonic-fetal erythroleukemia cell line K562

We have constructed fusion genes comprised of gamma and beta globin elements and globin sequences linked to neomycin resistance (neoR) genes to define the cis acting sequences responsible for developmental stage-specific expression and induction of fetal globin genes in embryonic-fetal erythroleukemia K562 cells. The results indicate that the gamma promoter is required for proper initiation of transcription. However, the accumulation of gamma globin transcripts in response to hemin induction requires the additional presence of either gamma intervening sequence 2 or the 3′ enhancer element of the beta globin gene. Thus, the gamma promoter may provide the elements for developmental stage-specific gene expression during fetal life. By contrast, the beta 3′ enhancer is erythroid-specific but not developmental stage- or gene-specific.

The beta globin 3' enhancer element confers regulated expression on the human gamma globin gene in the human embryonic-fetal erythroleukemia cell line K562

We have constructed fusion genes comprised of gamma and beta globin elements and globin sequences linked to neomycin resistance (neoR) genes to define the cis acting sequences responsible for developmental stage-specific expression and induction of fetal globin genes in embryonic-fetal erythroleukemia K562 cells. The results indicate that the gamma promoter is required for proper initiation of transcription. However, the accumulation of gamma globin transcripts in response to hemin induction requires the additional presence of either gamma intervening sequence 2 or the 3' enhancer element of the beta globin gene. Thus, the gamma promoter may provide the elements for developmental stage-specific gene expression during fetal life. By contrast, the beta 3' enhancer is erythroid-specific but not developmental stage- or gene-specific.

Protein - DNA interactions upstream from the Human A gamma globin gene

We have used DNAase I footprinting and the gel mobility shift assay to study proteins which bind to promoter elements located between -140 and -382 upstream of the human A gamma globin gene. Footprints are found with both erythroid and nonerythroid nuclear extracts at three sites: from -294 to -264, -242 to -227, and -189 to -172 from the transcription initiation site. An erythroid-specific footprint is identified from -194 to -189. We demonstrate that two known transcription factors, the ubiquitous octamer-binding protein OTF-1 and the erythroid regulatory factor NFE-1, bind to the -194 to -172 region and that their footprints overlap. Binding of OTF-1 to this region is reduced by a mutation at -175 associated with a form of non-deletion hereditary persistence of fetal hemoglobin. We conclude that OTF-1 may compete with NFE-1 for the -175 binding site, possibly functioning as a repressor of gamma globin transcription.