A signaling mechanism for growth-related expression of fetal hemoglobin

Absolute Reticulocyte Count Acts as a Surrogate for Fetal Hemoglobin in Infants and Children with Sickle Cell Anemia

Hemoglobin switching is largely complete in humans by six months of age. Among infants with sickle cell anemia (HbSS, SCA), reticulocytosis begins early in life as fetal hemoglobin (HbF) is replaced by sickle hemoglobin (HbS). The objective of this study was to determine if absolute reticulocyte count (ARC) is related to HbF levels in a cohort of pediatric SCA patients. A convenience sample of 106 children with SCA between the ages of 1 month and 20 years who were not receiving hydroxyurea or monthly blood transfusions were enrolled in this observational study. Hematologic data, including ARC and HbF levels, were measured at steady state. F-cells were enumerated by flow cytometry. Initial studies compared infants with ARC greater than or equal to 200 K/μL (ARC ≥ 200) based upon the previously reported utility of this threshold as a predictive marker for SCA severity. Mean HbF and F-cell levels were significantly lower in the ARC ≥ 200 group when compared to the ARC < 200 group. Both HbF and F-cell percentages were negatively correlated to ARC in infants and in children between the ages of 1 and 9 years. However, the inverse relationship was lost after the age of 10 years. Overall, decreased expression and distribution of HbF during childhood SCA is well-correlated with increased reticulocyte production and release into the peripheral blood. As such, these data further support the clinical use of reticulocyte enumeration as a disease severity biomarker for childhood sickle cell anemia.

A common signaling pathway is activated in erythroid cells expressing high levels of fetal hemoglobin: a potential role for cAMP-elevating agents in β-globin disorders

Background

Although erythroid cells prepared from fetal liver, cord blood, or blood from β-thalassemia patients are known to express fetal hemoglobin at high levels, the underlying mechanisms remain elusive. We previously showed that cyclic nucleotides such as cAMP and cGMP induce fetal hemoglobin expression in primary erythroid cells. Here we report that cAMP signaling contributes to high-level fetal hemoglobin expression in erythroid cells prepared from cord blood and β-thalassemia.

Methods

The status of the cAMP signaling pathway was investigated using primary erythroid cells prepared from cord blood and the mononuclear cells of patients with β-thalassemia; erythroid cells from adult bone marrow mononuclear cells served as the control.

Results

We found that intracellular cAMP levels were higher in erythroid cells from cord blood and β-thalassemia than from adult bone marrow. Protein kinase A activity levels and cAMP-response element binding protein phosphorylation were higher in erythroid cells from cord blood or β-thalassemia than in adult bone marrow progenitors. Mitogen-activated protein kinase pathways, which play a role in fetal hemoglobin expression, were not consistently activated in cord blood or β-thalassemia erythroid cells. When cAMP signaling was activated in adult erythroid cells, fetal hemoglobin was induced at high levels and associated with reduced expression of BCL11A, a silencer of the β-globin gene.

Conclusion

These results suggest that activated cAMP signaling may be a common mechanism among erythroid cells with high fetal hemoglobin levels, in part because of downregulation of BCL11A. Activation of the cAMP signaling pathway with cAMP-elevating agents may prove to be an important signaling mechanism to reactivate fetal hemoglobin expression in erythroid cells.

Erythropoietin-stimulated endothelial cells support erythroid cell differentiation of CD34(+) haematopoietic progenitors

BACKGROUND AND OBJECTIVES

Endothelial cells provide a unique medium for the proliferation and white lineage differentiation of haematopoietic progenitor cells (HPC). Whether this quality can be exploited to facilitate the differentiation of erythroid precursors is not yet known.

MATERIALS AND METHODS

Haematopoietic progenitor cells derived from cord blood were cultured for 3 weeks in erythropoietin-stimulated supernatants with (n = 6) or without cyclosporine A (CSA, n = 6). Cell count, phenotype and morphology were assessed on a weekly basis, and the haemoglobin content was analysed. These cultures were compared with erythroid differentiation induced by cytokines only (n = 6).

RESULTS

Endothelial supernatants combined with CSA led to equivalent numbers of CD71(+) erythroblasts after 1 week as cytokines only. The purity of glycophorin-positive, CD45-negative cells was higher in cells generated in endothelial supernatants than in cytokine-based media. Additional prostaglandin E2 induced a change from fetal to adult haemoglobin.

CONCLUSION

For the generation of erythroblasts from HPC, endothelial supernatants are a simple and cost-effective alternative to culture conditions based on cytokines.

Fetal haemopoiesis marking low-grade urinary bladder cancer

BACKGROUND

The immunohistochemical features of fetal haemoglobin cells and their distribution patterns in solid tumours, such as colorectal cancer and blastomas, suggest that fetal haemopoiesis may take place in these tumour tissues. These locally highly concentrated fetal haemoglobin (HbF) cells may promote tumour growth by providing a more efficient oxygen supply.

METHODS AND RESULTS

Biomarkers of HbF were checked in transitional cell carcinoma (TCC) of the urinary bladder, assessing this as a new parameter for disease management. Fetal haemoglobin was immunohistochemically examined in tumours from 60 patients with TCC of the bladder. Fetal haemoglobin erythrocytes and erythroblasts were mainly clonally distributed in proliferating blood vessels and not mixed with normal haemoglobin erythrocytes. The proportion of such HbF blood vessels could reach more than half of the total number of vessels. There were often many HbF erythroblasts distributed in one-cell or two-cell capillaries and present as 5-15% of cells in multi-cell vessels. This suggests a local proliferation of HbF-cell progenitors. Fetal haemoglobin cells were prominently marking lower grades of tumours, as 76% (n=21) of the patients with G1pTa were HbF+, whereas only 6.7% (n=30) of the patients with G3pT1-pT2a were HbF+.

CONCLUSION

Our results suggest that HbF, besides being a potential new marker for early tumour detection, might be an essential factor of early tumour development, as in fetal life. Inhibiting HbF upregulation may provide a therapeutic target for the inhibition of tumour growth.

The proinflammatory cytokine GM-CSF downregulates fetal hemoglobin expression by attenuating the cAMP-dependent pathway in sickle cell disease

Although reduction in leukocyte counts following hydroxyurea therapy in sickle cell disease (SCD) predicts fetal hemoglobin (HbF) response, the underlying mechanism remains unknown. We previously reported that leukocyte counts are regulated by granulocyte-macrophage colony-stimulating factor (GM-CSF) in SCD patients. Here we examined the roles of GM-CSF in the regulation of HbF expression in SCD. Upon the analysis of retrospective data in 372 patients, HbF levels were inversely correlated with leukocyte counts and GM-CSF levels in SCD patients without hydroxyurea therapy, while HbF increments after hydroxyurea therapy correlated with a reduction in leukocyte counts, suggesting a negative effect of GM-CSF on HbF expression. Consistently, in vitro studies using primary erythroblasts showed that the addition of GM-CSF to erythroid cells decreased HbF expression. We next examined the intracellular signaling pathway through which GM-CSF reduced HbF expression. Treatment of erythroid cells with GM-CSF resulted in the reduction of intracellular cAMP levels and abrogated phosphorylation of cAMP response-element-binding-protein, suggesting attenuation of the cAMP-dependent pathway, while the phosphorylation levels of mitogen-activated protein kinases were not affected. This is compatible with our studies showing a role for the cAMP-dependent pathway in HbF expression. Together, these results demonstrate that GM-CSF plays a role in regulating both leukocyte count and HbF expression in SCD. Reduction in GM-CSF levels upon hydroxyurea therapy may be critical for efficient HbF induction. The results showing the involvement of GM-CSF in HbF expression may suggest possible mechanisms for hydroxyurea resistance in SCD.

Proof of principle for transfusion of in vitro-generated red blood cells

In vitro RBC production from stem cells could represent an alternative to classic transfusion products. Until now the clinical feasibility of this concept has not been demonstrated. We addressed the question of the capacity of cultured RBCs (cRBCs) to survive in humans. By using a culture protocol permitting erythroid differentiation from peripheral CD34(+) HSC, we generated a homogeneous population of cRBC functional in terms of their deformability, enzyme content, capacity of their hemoglobin to fix/release oxygen, and expression of blood group antigens. We then demonstrated in the nonobese diabetes/severe combined immunodeficiency mouse that cRBC encountered in vivo the conditions necessary for their complete maturation. These data provided the rationale for injecting into one human a homogeneous sample of 10(10) cRBCs generated under good manufacturing practice conditions and labeled with (51)Cr. The level of these cells in the circulation 26 days after injection was between 41% and 63%, which compares favorably with the reported half-life of 28 ± 2 days for native RBCs. Their survival in vivo testifies globally to their quality and functionality. These data establish the proof of principle for transfusion of in vitro-generated RBCs and path the way toward new developments in transfusion medicine. This study is registered at http://www.clinicaltrials.gov as NCT0929266.

Generation of bivalent chromatin domains during cell fate decisions

Background

In self-renewing, pluripotent cells, bivalent chromatin modification is thought to silence (H3K27me3) lineage control genes while 'poising' (H3K4me3) them for subsequent activation during differentiation, implying an important role for epigenetic modification in directing cell fate decisions. However, rather than representing an equivalently balanced epigenetic mark, the patterns and levels of histone modifications at bivalent genes can vary widely and the criteria for identifying this chromatin signature are poorly defined.

Results

Here, we initially show how chromatin status alters during lineage commitment and differentiation at a single well characterised bivalent locus. In addition we have determined how chromatin modifications at this locus change with gene expression in both ensemble and single cell analyses. We also show, on a global scale, how mRNA expression may be reflected in the ratio of H3K4me3/H3K27me3.

Conclusions

While truly 'poised' bivalently modified genes may exist, the original hypothesis that all bivalent genes are epigenetically premarked for subsequent expression might be oversimplistic. In fact, from the data presented in the present work, it is equally possible that many genes that appear to be bivalent in pluripotent and multipotent cells may simply be stochastically expressed at low levels in the process of multilineage priming. Although both situations could be considered to be forms of 'poising', the underlying mechanisms and the associated implications are clearly different.

Chicken HS4 insulators have minimal barrier function among progeny of human hematopoietic cells transduced with an HIV1-based lentiviral vector

Position effects limit the curative potential of gene transfer strategies for the hemoglobinopathies by inducing clonal variability of transgene expression. We evaluated the mitigating effects of the chicken hypersensitivity site 4 (HS4) insulator among lentiviral vector-transduced human hematopoietic cells. We constructed various lentiviral vectors using a green fluorescent protein (GFP) reporter under the control of a reverse-oriented murine stem cell virus (MSCV)-long-term repeat (LTR) promoter or a reverse-oriented β-globin expression cassette. A full-length HS4, a tandem HS4 core, and a single core insulator were inserted into the 3' LTR in both forward and reverse orientation. All but the reverse single core insulator significantly decreased titers. All reduced %GFP without increasing mean fluorescence intensity (MFI) among erythroid progeny of transduced human CD34(+) cells. A lower coefficient of variation (CV) was observed only among progeny of the full-length vector-transduced cells, yet a fivefold reduction in transduction efficiency was observed. In xenografted mice, the single core insulator decreased both the %GFP and the MFI at 4 and 8 weeks after transplantation with no difference in CVs. These data demonstrate that the inclusion of HS4 insulator elements lowers viral titers, reduces efficiency of transduction, and produces minimal effects on transgene expression among human hematopoietic cells in vitro and in vivo.

Mechanism of human Hb switching: a possible role of the kit receptor/miR 221-222 complex

BACKGROUND

The human hemoglobin switch (HbF-->HbA) takes place in the peri/post-natal period. In adult life, however, the residual HbF (<1%) may be partially reactivated by chemical inducers and/or cytokines such as the kit ligand (KL). MicroRNAs (miRs) play a pivotal role in normal hematopoiesis: downmodulation of miR-221/222 stimulates human erythropoietic proliferation through upmodulation of the kit receptor.

DESIGN AND METHODS

We have explored the possible role of kit/KL in perinatal Hb switching by evaluating: i) the expression levels of both kit and kit ligand on CD34(+) cells and in plasma isolated from pre-, mid- and full-term cord blood samples; ii) the reactivation of HbF synthesis in KL-treated unilineage erythroid cell cultures; iii) the functional role of miR-221/222 in HbF production.

RESULTS

In perinatal life, kit expression showed a gradual decline directly correlated to the decrease of HbF (from 80-90% to <30%). Moreover, in full-term cord blood erythroid cultures, kit ligand induced a marked increase of HbF (up to 80%) specifically abrogated by addition of the kit inhibitor imatinib, thus reversing the Hb switch. MiR-221/222 expression exhibited rising levels during peri/post-natal development. In functional studies, overexpression of these miRs in cord blood progenitors caused a remarkable decrease in kit expression, erythroblast proliferation and HbF content, whereas their suppression induced opposite effects.

CONCLUSIONS

Our studies indicate that human perinatal Hb switching is under control of the kit receptor/miR 221-222 complex. We do not exclude, however, that other mechanisms (i.e. glucocorticoids and the HbF inhibitor BCL11A) may also contribute to the peri/post-natal Hb switch.

Role of stem cell factor in the reactivation of human fetal hemoglobin

In humans the switch from fetal to adult hemoglobin (HbF → HbA) takes place in the perinatal and postnatal period, determining the progressive replacement of HbF with HbA synthesis (i.e., the relative HbF content in red blood cells decreases from 80–90% to <1%). In spite of more than twenty years of intensive investigations on this classic model, the molecular mechanisms regulating the Hb switching, as well as HbF synthesis in adults, has been only in part elucidated. In adult life, the residual HbF, restricted to F cell compartment, may be reactivated up to 10–20% of total Hb synthesis in various conditions associated with “stress erythropoiesis”: this reactivation represented until now an interesting model of partial Hb switch reverse with important therapeutic implications in patients with hemoglobinopathies, and particularly in β-thalassemia. In vitro and in vivo models have led to the identification of several chemical compounds able to reactivate HbF synthesis in adult erythroid cells. Although the impact of these HbF inducers, including hypomethylating agents, histone deacetylase inhibitors and hydroxyurea, was clear on the natural history of sickle cell anemia, the benefit on the clinical course of β-thalassemia was only limited: particularly, the toxicity and the modest increase in γ-globin reactivation indicated the need for improved agents able to induce higher levels of HbF. In the present review we describe the biologic properties of Stem Cell Factor (SCF), a cytokine sustaining the survival and proliferation of erythroid cells, that at pharmacological doses acts as a potent stimulator of HbF synthesis in adult erythroid cells.

Development of a human immunodeficiency virus type 1-based lentiviral vector that allows efficient transduction of both human and rhesus blood cells

Human immunodeficiency virus type 1 (HIV-1) vectors transduce rhesus blood cells poorly due to a species-specific block by TRIM5alpha and APOBEC3G, which target HIV-1 capsid and viral infectivity factor (Vif), respectively. We sought to develop a lentiviral vector capable of transducing both human and rhesus blood cells by combining components of both HIV-1 and simian immunodeficiency virus (SIV), including SIV capsid (sCA) and SIV Vif. A chimeric HIV-1 vector including sCA (chiHIV) was superior to the conventional SIV in transducing a human blood cell line and superior to the conventional HIV-1 vector in transducing a rhesus blood cell line. Among human CD34(+) hematopoietic stem cells (HSCs), the chiHIV and HIV-1 vectors showed similar transduction efficiencies; in rhesus CD34(+) HSCs, the chiHIV vector yielded superior transduction rates. In in vivo competitive repopulation experiments with two rhesus macaques, the chiHIV vector demonstrated superior marking levels over the conventional HIV-1 vector in all blood lineages (first rhesus, 15 to 30% versus 1 to 5%; second rhesus, 7 to 15% versus 0.5 to 2%, respectively) 3 to 7 months postinfusion. In summary, we have developed an HIV-1-based lentiviral vector system that should allow comprehensive preclinical testing of HIV-1-based therapeutic vectors in the rhesus macaque model with eventual clinical application.

SCF induces gamma-globin gene expression by regulating downstream transcription factor COUP-TFII

Increased fetal hemoglobin expression in adulthood is associated with acute stress erythropoiesis. However, the mechanisms underlying gamma-globin induction during the rapid expansion of adult erythroid progenitor cells have not been fully elucidated. Here, we examined COUP-TFII as a potential repressor of gamma-globin gene after stem cell factor (SCF) stimulation in cultured human adult erythroid progenitor cells. We found that COUP-TFII expression is suppressed by SCF through phosphorylation of serine/threonine phosphatase (PP2A) and correlated well with fetal hemoglobin induction. Furthermore, down-regulation of COUP-TFII expression with small interfering RNA (siRNA) significantly increases the gamma-globin expression during the erythroid maturation. Moreover, SCF-increased expression of NF-YA associated with redox regulator Ref-1 and cellular reducing condition enhances the effect of SCF on gamma-globin expression. Activation of Erk1/2 plays a critical role in SCF modulation of downstream transcriptional factor COUP-TFII, which is involved in the regulation of gamma-globin gene induction. Our data show that SCF stimulates Erk1/2 MAPK signaling pathway, which regulates the downstream repressor COUP-TFII by inhibiting serine/threonine phosphatase 2A activity, and that decreased COUP-TFII expression resulted in gamma-globin reactivation in adult erythropoiesis. These observations provide insight into the molecular pathways that regulate gamma-globin augmentation during stress erythropoiesis.

Short-chain fatty acid-mediated effects on erythropoiesis in primary definitive erythroid cells

Short-chain fatty acids (SCFAs; butyrate and propionate) up-regulate embryonic/fetal globin gene expression through unclear mechanisms. In a murine model of definitive erythropoiesis, SCFAs increased embryonic beta-type globin gene expression in primary erythroid fetal liver cells (eFLCs) after 72 hours in culture, from 1.7% (+/- 1.2%) of total beta-globin gene expression at day 0 to 4.9% (+/- 2.2%) in propionate and 5.4% (+/- 3.4%) in butyrate; this effect was greater in butyrate plus insulin/erythropoietin (BIE), at 19.5% (+/- 8.3%) compared with 0.1% (+/- 0.1%) in ins/EPO alone (P < .05). Fetal gamma-globin gene expression was increased in human transgene-containing eFLCs, to 35.9% (+/- 7.0%) in BIE compared with 4.4% (+/- 4.2%) in ins/EPO only (P < .05). Embryonic globin gene expression was detectable in 11 of 15 single eFLCs treated with BIE, but in0 of 15 ins/EPO-only treated cells. Butyrate-treated [65.5% (+/- 9.9%)] and 77.5% (+/- 4.0%) propionate-treated eFLCs were highly differentiated in culture, compared with 21.5% (+/- 3.5%) in ins/EPO (P < .005). Importantly, signaling intermediaries, previously implicated in induced embryonic/fetal globin gene expression (STAT5, p42/44, and p38), were not differentially activated by SCFAs in eFLCs; but increased bulk histone (H3) acetylation was seen in SCFA-treated eFLCs. SCFAs induce embryonic globin gene expression in eFLCS, which are a useful short-term and physiologic primary cell model of embryonic/fetal globin gene induction during definitive erythropoiesis.

Hemoglobin research and the origins of molecular medicine

Much of our understanding of human physiology, and of many aspects of pathology, has its antecedents in laboratory and clinical studies of hemoglobin. Over the last century, knowledge of the genetics, functions, and diseases of the hemoglobin proteins has been refined to the molecular level by analyses of their crystallographic structures and by cloning and sequencing of their genes and surrounding DNA. In the last few decades, research has opened up new paradigms for hemoglobin related to processes such as its role in the transport of nitric oxide and the complex developmental control of the alpha-like and beta-like globin gene clusters. It is noteworthy that this recent work has had implications for understanding and treating the prevalent diseases of hemoglobin, especially the use of hydroxyurea to elevate fetal hemoglobin in sickle cell disease. It is likely that current research will also have significant clinical implications, as well as lessons for other aspects of molecular medicine, the origin of which can be largely traced to this research tradition.

Fetal hemoglobin chemical inducers for treatment of hemoglobinopathies

The switch from fetal ((G)gamma and (A)gamma) to adult (beta and delta) globin gene expression occurs at birth, leading to the gradual replacement of HbF with HbA. Genetic regulation of this switch has been studied for decades, and the molecular mechanisms underlying this developmental change in gene expression have been in part elucidated. The understanding of the developmental regulation of gamma-globin gene expression was paralleled by the identification of a series of chemical compounds able to reactivate HbF synthesis in vitro and in vivo in adult erythroid cells. Reactivation of HbF expression is an important therapeutic option in patients with hemoglobin disorders, such as sickle cell anemia and beta-thalassemia. These HbF inducers can be grouped in several classes based on their chemical structures and mechanisms of action. Clinical studies with some of these agents have shown that they were effective, in a part of patients, in ameliorating the clinical condition. The increase in HbF in response to these drugs varies among patients with beta-thalassemia and sickle cell disease due to individual genetic determinants.

Inhibition of erythroblast growth and fetal hemoglobin production by ribofuranose-substituted adenosine derivatives

In vivo, inhibition of fetal hemoglobin (HbF) expression in humans around the time of birth causes the clinical manifestation of sickle cell and beta-thalassemia syndromes. Inhibition of HbF among cultured cells was recently described by the adenosine derivative molecule named SQ22536. Here, a primary cell culture model was utilized to further explore the inhibition of HbF by adenosine derivative molecules. SQ22536 demonstrated down-regulation of growth and HbF expression among erythroblasts cultured from fetal and adult human blood. The effects upon HbF were noted in a majority of cells, and quantitative PCR analysis demonstrated a transcriptional mechanism. Screening assays demonstrated that two additional molecules named 5'-deoxy adenosine and 2',3'-dideoxy adenosine had effects on HbF comparable to SQ22536. Other adenosine derivative molecules, adenosine receptor binding ligands, and cAMP-signaling regulators failed to inhibit HbF in matched cultures. These results suggest that structurally related ribofuranose-substituted adenosine analogues act through an unknown mechanism to inhibit HbF expression in fetal and adult human erythroblasts.

Hypoxia alters progression of the erythroid program

Hypoxia can induce erythropoiesis through regulated increase of erythropoietin (Epo) production. We investigated the direct influence of oxygen tension (pO(2)) in the physiologic range (2-8%) on erythroid progenitor cell differentiation using cultures of adult human hematopoietic progenitor cells exposed to decreasing (20% to 2%) pO(2) and independent of variation in Epo levels. Decreases in hemoglobin (Hb)-containing cells were observed at the end of the culture period with decreasing pO(2). This is due, in part, to a reduction in cell growth and, at 2% O(2), a marked increase in cell toxicity. Analysis of the kinetics of cell differentiation showed an increase in the proportion of cells with glycophorin-A expression and Hb accumulation at physiologic pO(2). Cells were characterized by an early induction of gamma-globin expression and a delay and reduction in peak levels of beta-globin expression. Overall, fetal Hb and gamma-globin expression were increased at physiologic pO(2), but these increases were reduced at 2% O(2) as cultures become cytotoxic. At reduced pO(2), induction of Epo-receptor (Epo-R) by Epo was decreased and delayed, analogous to the delay in beta-globin induction. The oxygen-dependent reduction of Epo-R can account for the associated cytotoxicity at 2% O(2). Epo induction of erythroid transcription factors, EKLF, GATA-1, and SCL/Tal-1, was also delayed and decreased at reduced pO(2), consistent with lower levels of Epo-R and resultant Epo signaling. These changes in Epo-R and globin gene expression raise the possibility that the early increase of gamma-globin is a consequence of reduced Epo signaling and a delay in induction of erythroid transcription factors.

Effective erythropoiesis and HbF reactivation induced by kit ligand in beta-thalassemia

In human beta-thalassemia, the imbalance between alpha- and non-alpha-globin chains causes ineffective erythropoiesis, hemolysis, and anemia: this condition is effectively treated by an enhanced level of fetal hemoglobin (HbF). In spite of extensive studies on pharmacologic induction of HbF synthesis, clinical trials based on HbF reactivation in beta-thalassemia produced inconsistent results. Here, we investigated the in vitro response of beta-thalassemic erythroid progenitors to kit ligand (KL) in terms of HbF reactivation, stimulation of effective erythropoiesis, and inhibition of apoptosis. In unilineage erythroid cultures of 20 patients with intermedia or major beta-thalassemia, addition of KL, alone or combined with dexamethasone (Dex), remarkably stimulated cell proliferation (3-4 logs more than control cultures), while decreasing the percentage of apoptotic and dyserythropoietic cells (<5%). More important, in both thalassemic groups, addition of KL or KL plus Dex induced a marked increase of gamma-globin synthesis, thus reaching HbF levels 3-fold higher than in con-trol cultures (eg, from 27% to 75% or 81%, respectively, in beta-thalassemia major). These studies indicate that in beta-thalassemia, KL, alone or combined with Dex, induces an expansion of effective erythropoiesis and the reactivation of gamma-globin genes up to fetal levels and may hence be considered as a potential therapeutic agent for this disease.

Expression of the gamma-globin gene is sustained by the cAMP-dependent pathway in beta-thalassaemia

The present study found that the cyclic adenosine monophosphate (cAMP)-dependent pathway efficiently induced gamma-globin expression in adult erythroblasts, and this pathway plays a role in gamma-globin gene (HBG) expression in beta-thalassaemia. Expression of HBG mRNA increased to about 46% of non-HBA mRNA in adult erythroblasts treated with forskolin, while a cyclic guanosine monophosphate (cGMP) analogue induced HBG mRNA to levels <20% of non-HBA mRNA. In patients with beta-thalassaemia intermedia, cAMP levels were elevated in both red blood cells and nucleated erythroblasts but no consistent elevation was found with cGMP levels. The transcription factor cAMP response element binding protein (CREB) was phosphorylated in nucleated erythroblasts and its phosphorylation levels correlated with HBG mRNA levels of the patients. Other signalling molecules, such as mitogen-activated protein kinases and signal transducers and activators of transcription proteins, were phosphorylated at variable levels and showed no correlations with the HBG mRNA levels. Plasma levels of cytokines, such as erythropoietin, stem cell factor and transforming growth factor-beta were increased in patients, and these cytokines induced both HBG mRNA expression and CREB phosphorylation. These results demonstrate that the cAMP-dependent pathway, the activity of which is augmented by multiple cytokines, plays a role in regulating HBG expression in beta-thalassaemia.

Role of cyclic nucleotides in fetal hemoglobin induction in cultured CD34+ cells

OBJECTIVE

In vivo, several drugs have been shown to increase fetal hemoglobin (HbF), including 5-azacytidine (AZA), sodium butyrate (SB), and hydroxyurea (HU). Studies in K562 cells suggest that cyclic guanosine monophosphate (cGMP) is required for HbF induction; however, the role of cyclic nucleotides in HbF induction in primary erythroid cultures has not been established.

METHODS

CD34-selected peripheral blood monocytes cultured in a semi-solid serum-free system that mimics in vivo F-cell production are utilized to explore the role of cyclic adenosine monophosphate (cAMP) and cGMP in HbF induction in response to HU, AZA, and SB.

RESULTS

In serum-free CD34 cultures, HU, SB, and AZA all markedly stimulate FNRBC production up to 30-fold, associated with induction of gamma-globin mRNA and total HbF protein. Guanylate cyclase inhibition results in only minimal blunting of HbF induction by each agent. In contrast, adenylate cyclase inhibition markedly reduces HU, SB, and AZA-mediated FNRBC induction and gamma-globin mRNA induction. The adenylate cyclase activator forskolin modestly induces FNRBC production and augments the action of standard induction agents. HU, AZA, and SB, however, fail to significantly stimulate adenylate cyclase themselves.

CONCLUSIONS

In human CD34(+) cultures, cAMP production is required for full induction of HbF by HU, SB, and AZA, while perturbation of cGMP production has only minimal effects. These findings are in marked contrast to data in K562 cells where cGMP production is critical for HbF induction while cAMP stimulation blunts HbF response, and suggest that these agents may share a common induction pathway.

Fetal hemoglobin silencing in humans

Interruption of the normal fetal-to-adult transition of hemoglobin expression should largely ameliorate sickle cell and beta-thalassemia syndromes. Achievement of this clinical goal requires a robust understanding of gamma-globin gene and protein silencing during human development. For this purpose, age-related changes in globin phenotypes of circulating human erythroid cells were examined from 5 umbilical cords, 99 infants, and 5 adult donors. Unexpectedly, an average of 95% of the cord blood erythrocytes and reticulocytes expressed HbA and the adult beta-globin gene, as well as HbF and the gamma-globin genes. The distribution of hemoglobin and globin gene expression then changed abruptly due to the expansion of cells lacking HbF or gamma-globin mRNA (silenced cells). In adult reticulocytes, less than 5% expressed gamma-globin mRNA. These data are consistent with a "switching" model in humans that initially results largely from gamma- and beta-globin gene coexpression and competition during fetal development. In contrast, early postnatal life is marked by the rapid accumulation of cells that possess undetectable gamma-globin mRNA and HbF. The silencing phenomenon is mediated by a mechanism of cellular replacement. This novel silencing pattern may be important for the development of HbF-enhancing therapies.

cAMP differentially regulates gamma-globin gene expression in erythroleukemic cells and primary erythroblasts through c-Myb expression

Our previous studies demonstrated roles of cyclic nucleotides in gamma-globin gene expression. We recently found that, upon activation of the cAMP pathway, expression of the gamma-globin gene is inhibited in K562 cells but induced in adult erythroblasts. Here we show that c-Myb, a proto-oncogene product that plays a role in cell growth and differentiation, is involved in the cAMP-mediated differential regulation of gamma-globin gene expression in K562 cells and primary erythroblasts. Our studies found that c-Myb is expressed at a high level in K562 cells compared to primary erythroblasts, and that c-Myb expression is further increased following the treatment with forskolin, an adenylate cyclase activator. The induction of gamma-globin gene expression was also inhibited in K562 cells by raising the levels of c-Myb expression. Importantly, forskolin-induced erythroid differentiation in K562 cells, as determined by the expression of glycophorins and CD71, suggesting that high-level expression of c-Myb may not be sufficient to inhibit the differentiation of erythroid cells. In contrast, c-Myb was not expressed in adult erythroblasts treated with forskolin and primary erythroblasts may lack the c-Myb-mediated inhibitory mechanism for gamma-globin gene expression. Together, these results show that the cAMP pathway blocks gamma-globin gene expression in K562 cells by increasing c-Myb expression and c-Myb plays a role in defining the mode of response of the gamma-globin gene to fetal hemoglobin inducers in erythroid cells.

Development of fetal haemoglobin-blood cells (F cells) within colorectal tumour tissues

AIM

To evaluate the sources of fetal haemoglobin (HbF) as an indicator in cancer. An immunohistochemical study was carried out on some of the most common kinds of cancer. All of these cancers had serologically high levels of HbF as evaluated previously.

METHODS

Immunoaffinity-purified anti-HbF was immunohistochemically used to study F cell distribution in the following cancers: colorectal adenocarcinoma, urinary bladder transitional cell carcinoma, brain tumours, lung carcinoma, breast adenocarcinoma, leukaemia, Burkitt's lymphoma and endometrial carcinoma.

RESULTS

In colorectal adenocarcinoma, HbF-containing red blood cells (FRBC) were present within thin-walled vessels or were disposed in dense clusters within the tumour. Some of these cells were nucleated or binucleated HbF-erythroblasts or HbF-normoblasts (FNBS). In two cases, high levels of mitoses within HbF-erythroblasts were observed. In half of the cases with transitional cell carcinoma of the urinary bladder, regional intratumoral blood vessels were found to contain 5-50% FRBC. In the other tumours examined, F cells were not observed. FRBCs, however, were occasionally observed in the regional lymph nodes of some of these cancers.

CONCLUSIONS

The evaluation of HbF as a potential plasma marker is suggested by the high concentration of FRBCs in colorectal tumours. The apparent development of FRBCs in colorectal tumour tissues is an interesting observation, as these cells were previously thought to develop in medullary or lymphoid tissues. It is thus suggested that the colonic microenvironment may stimulate extramedullary fetal-type haematopoiesis.

Regulation of human fetal hemoglobin: new players, new complexities

The human globin genes are among the most extensively characterized in the human genome, yet the details of the molecular events regulating normal human hemoglobin switching and the potential reactivation of fetal hemoglobin in adult hematopoietic cells remain elusive. Recent discoveries demonstrate physical interactions between the beta locus control region and the downstream structural gamma- and beta-globin genes, and with transcription factors and chromatin remodeling complexes. These interactions all play roles in globin gene expression and globin switching at the human beta-globin locus. If the molecular events in hemoglobin switching were better understood and fetal hemoglobin could be more fully reactivated in adult cells, the insights obtained might lead to new approaches to the therapy of sickle cell disease and beta thalassemia by identifying specific new targets for molecular therapies.

Fetal globin induction--can it cure beta thalassemia?

The beta thalassemias are one of a few medical conditions in which reactivation of a gene product that is expressed during fetal life can functionally replace a deficiency of essential proteins expressed at a later developmental stage. The fetal globin genes are present and normally integrated in hematopoietic stem cells, and at least one fetal gene appears accessible for reactivation, particularly in beta degrees thalassemia. However, rapid cellular apoptosis from alpha globin chain precipitation, and relatively low levels of endogenous erythropoietin (EPO) in some beta(+) thalassemia patients contribute to the anemia in beta thalassemia syndromes. In clinical trials, three classes of therapeutics have demonstrated proof-of-principle of this approach by raising total hemoglobin levels by 1-4 g/dL above baseline in thalassemia patients: EPO preparations, short chain fatty acid derivatives (SCFADs), and chemotherapeutic agents. Although thalassemic erythrocytes survive only for a few days, the magnitude of these responses is similar to those induced by rhu-EPO in anemic conditions of normal erythrocyte survival. New oral therapeutic candidates, which stimulate both fetal globin gene expression and erythropoiesis, and combinations of therapeutics with complementary molecular actions now make this gene-reactivation approach feasible to produce transfusion independence in many patients. Development of the candidate therapeutics is hindered largely by costs of drug development for an orphan patient population.

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

We 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 factor beta (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.