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

Heterogeneity of fetal hemoglobin production in adult red blood cells

PURPOSE OF REVIEW

Small amounts of fetal hemoglobin can be expressed in a subset of adult red blood cells called F-cells. This review examines the potential mechanisms and clinical implications of the heterogeneity of fetal hemoglobin expression.

RECENT FINDINGS

Although the heterocellular nature of fetal hemoglobin expression in adult red blood cells has been noted for over 70 years, the molecular basis of this phenomenon has been unclear. Recent discoveries of novel regulators of fetal hemoglobin as well as technological advances have shed new light on these cells.

SUMMARY

Fetal hemoglobin reactivation in adult red blood cells through genetic or pharmacological approaches can involve both increasing the number of F-cells and cellular fetal hemoglobin content. New technologies enable the study and eventually the improvement of these parameters in patients with sickle cell disease and β-thalassemia.

The relation between mitogen activated protein kinase (MAPK) pathway and different genes expression in patients with beta Thalassemia

BACKGROUND

β-thalassemia is an inherited hemoglobinopathy resulting in quantitative changes in the β-globin chain. Understanding the molecular basis of that disorder requires studying the expression of genes controlling the pathways that affect the erythropoietic homeostasis especially the MAPK pathway. The MAPKs are a family of serine/threonine kinases that play an essential role in connecting cell-surface receptors to DNA in the nucleus of the cell.

AIM

to study the effect of expression of GNAI2, DUSP5 and ARRB1 genes on MAPK signaling pathway in pediatric patients with beta thalassemia.

METHODS

Forty children with beta thalassemia major (TM), forty children with beta thalassemia intermedia (TI) and forty age and gender matched healthy controls were enrolled in this study. Detection of GNAI2, DUSP5 and ARRB1 mRNA expression was done by real time polymerase chain reaction (RT-PCR).

RESULTS

revealed increased expression of ARRB1 (Arrestin Beta 1) gene, and decreased expression of both GNAI2 (Guanine nucleotide-binding protein G (i) subunit alpha-2) and DUSP5 (Dual specificity protein phosphatase 5) genes in both patient groups than control groups respectively.

CONCLUSIONS

Change in the rate of expression of ARRB1, GNAI2 and DUSP5 may have a role in the pathogenesis of abnormal hematopoiesis in cases of β thalassemia through affecting the MAPK pathway.

Control of fetal globin expression in man: new opportunities to challenge past discoveries

Decades-old findings supporting origin of F cells in adult life from adult-type progenitors and the in vitro and in vivo enhancement of fetal globin under stress conditions have been juxtaposed against recent mechanistic underpinnings. An updated molecular interrogation did not debunk prior conclusions on the origin of F cells. Although fetal globin reactivation by widely diverse approaches in vitro and in response to anemic stresses in vivo is a work in progress, accumulating evidence converges toward an integrated stress response pathway. The newly uncovered developmental regulators of globin gene switching not only have provided answers to the long-awaited quest of transregulation of switching, they are also reaching a clinical threshold. Although the effect of fetal globin silencers has been robustly validated in adult cells, the response of cells at earlier developmental stages has been unclear and inadequately studied.

Genetic variation of Krüppel-like factor 1 (KLF1) and fetal hemoglobin (HbF) levels in β0-thalassemia/HbE disease

Heterogeneity of HbF levels in β⁰-thalassemia/HbE disease has been reported to be associated with variations in clinical manifestations of the disease, and several genetic-modifying factors beyond the β-globin gene cluster have been identified as HbF regulators. Down-regulation or heterozygous mutations of Krüppel-like factor 1 (KLF1) is associated with elevated HbF levels in non-thalassemia subjects. This study confirms that experimental down-regulation of KLF1 in β⁰-thalassemia/HbE-derived erythroblasts significantly increases HbF production (up to 52.3 ± 2.4%), albeit with slightly delayed erythroid terminal differentiation. KLF1 exome sequencing of 130 Thai β⁰-thalassemia/HbE patients without co-inheritance of α-thalassemia found six patients with KLF1 heterozygous mutations including rs2072596 (p.F182L; n = 5) and rs745347362 (p.P284L; n = 1) missense mutations. However, while these patients had high HbF levels (38.1 ± 7.5%), they were all associated with a severe clinical phenotype. These results suggest that while reduction of KLF1 expression in β⁰-thalassemia/HbE erythroblasts can increase HbF levels, it is not sufficient to alleviate the clinical phenotype.

Erythroid-Specific Expression of LIN28A Is Sufficient for Robust Gamma-Globin Gene and Protein Expression in Adult Erythroblasts

Increasing fetal hemoglobin (HbF) levels in adult humans remains an active area in hematologic research. Here we explored erythroid-specific LIN28A expression for its effect in regulating gamma-globin gene expression and HbF levels in cultured adult erythroblasts. For this purpose, lentiviral transduction vectors were produced with LIN28A expression driven by erythroid-specific gene promoter regions of the human KLF1 or SPTA1 genes. Transgene expression of LIN28A with a linked puromycin resistance marker was restricted to the erythroid lineage as demonstrated by selective survival of erythroid colonies (greater than 95% of all colonies). Erythroblast LIN28A over-expression (LIN28A-OE) did not significantly affect proliferation or inhibit differentiation. Greater than 70% suppression of total let-7 microRNA levels was confirmed in LIN28A-OE cells. Increases in gamma-globin mRNA and protein expression with HbF levels reaching 30-40% were achieved. These data suggest that erythroblast targeting of LIN28A expression is sufficient for increasing fetal hemoglobin expression in adult human erythroblasts.

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.

A synthetic model of human beta-thalassemia erythropoiesis using CD34+ cells from healthy adult donors

Based upon the lack of clinical samples available for research in many laboratories worldwide, a significant gap exists between basic and clinical studies of beta-thalassemia major. To bridge this gap, we developed an artificially engineered model for human beta thalassemia by knocking down beta-globin gene and protein expression in cultured CD34+ cells obtained from healthy adults. Lentiviral-mediated transduction of beta-globin shRNA (beta-KD) caused imbalanced globin chain production. Beta-globin mRNA was reduced by 90% compared to controls, while alpha-globin mRNA levels were maintained. HPLC analyses revealed a 96% reduction in HbA with only a minor increase in HbF. During the terminal phases of differentiation (culture days 14–21), beta-KD cells demonstrated increased levels of insoluble alpha-globin, as well as activated caspase-3. The majority of the beta-KD cells underwent apoptosis around the polychromatophilic stage of maturation. GDF15, a marker of ineffective erythropoiesis in humans with thalassemia, was significantly increased in the culture supernatants from the beta-KD cells. Knockdown of beta-globin expression in cultured primary human erythroblasts provides a robust ex vivo model for beta-thalassemia.

Heterogeneity of F cells in β-thalassemia

BACKGROUND

Fetal hemoglobin (HbF), which is largely replaced after birth by the adult Hb, is concentrated in a few "F cells." Their number significantly increases in certain physiologic and clinical situations, including in β-thalassemia (β-thal). Their quantification is used to detect fetal-maternal hemorrhage (FMH), where fetal cells enter the maternal circulation. We were confronted with a pregnant woman with β-thal who was suspected to have FMH. To establish the usefulness of a flow cytometric procedure to differentiate between fetal cells and the maternal F cells, we screened adult β-thal patients.

STUDY DESIGN AND METHODS

Blood samples were simultaneously stained with fluorescent antibodies to HbF and to carbonic anhydrase (CA) isotype II, which is specific to adult red blood cells (RBCs).

RESULTS

A heterogeneous distribution of RBCs with respect to HbF and CA expression was observed: adult non-F cells (CA+HbF-) and F cells (CA+HbF+/HbF++) as well as F cells with characteristics of fetal cells (CA-HbF++).

CONCLUSIONS

The presence of CA-HbF++ RBCs in nonpregnant women, and even men, with thal indicates that the CA/HbF method is inappropriate for detection of FMH. The coexistence of F cells carrying fetal or adult markers suggests that they originate from two types of stem cell, adult and fetal, lineages. Normally, the fetal lineage is insignificant, but in β-thal, as HbF-containing RBCs have a selective advantage, the "fetal" lineage gains significance.

Effect of AGM and fetal liver-derived stromal cell lines on globin expression in adult baboon (P. anubis) bone marrow-derived erythroid progenitors

This study was performed to investigate the hypothesis that the erythroid micro-environment plays a role in regulation of globin gene expression during adult erythroid differentiation. Adult baboon bone marrow and human cord blood CD34+ progenitors were grown in methylcellulose, liquid media, and in co-culture with stromal cell lines derived from different developmental stages in identical media supporting erythroid differentiation to examine the effect of the micro-environment on globin gene expression. Adult progenitors express high levels of γ-globin in liquid and methylcellulose media but low, physiological levels in stromal cell co-cultures. In contrast, γ-globin expression remained high in cord blood progenitors in stromal cell line co-cultures. Differences in γ-globin gene expression between adult progenitors in stromal cell line co-cultures and liquid media required cell-cell contact and were associated with differences in rate of differentiation and γ-globin promoter DNA methylation. We conclude that γ-globin expression in adult-derived erythroid cells can be influenced by the micro-environment, suggesting new potential targets for HbF induction.

Involvement of phosphatases in proliferation, maturation, and hemoglobinization of developing erythroid cells

Production of RBCs is triggered by the action of erythropoietin (Epo) through its binding to surface receptors (Epo-R) on erythroid precursors in the bone marrow. The intensity and the duration of the Epo signal are regulated by several factors, including the balance between the activities of kinesase and phosphatases. The Epo signal determines the proliferation and maturation of the precursors into hemoglobin (Hb)-containing RBCs. The activity of various protein tyrosine phosphatases, including those involved in the Epo pathway, can be inhibited by sodium orthovanadate (Na₃VO₄, vanadate). Adding vanadate to cultured erythroid precursors of normal donors and patients with β-thalassemia enhanced cell proliferation and arrested maturation. This was associated with an increased production of fetal hemoglobin (HbF). Increased HbF in patients with β-hemoglobinopathies (β-thalassemia and sickle cell disease) ameliorates the clinical symptoms of the disease. These results raise the possibility that specific and nontoxic inhibitors of phosphatases may be considered as a therapeutic modality for elevating HbF in patients with β-hemoglobinopathies as well as for intensifying the Epo response in other forms of anemia.

Fetal hemoglobin in sickle cell anemia

Fetal hemoglobin (HbF) is the major genetic modulator of the hematologic and clinical features of sickle cell disease, an effect mediated by its exclusion from the sickle hemoglobin polymer. Fetal hemoglobin genes are genetically regulated, and the level of HbF and its distribution among sickle erythrocytes is highly variable. Some patients with sickle cell disease have exceptionally high levels of HbF that are associated with the Senegal and Saudi-Indian haplotype of the HBB-like gene cluster; some patients with different haplotypes can have similarly high HbF. In these patients, high HbF is associated with generally milder but not asymptomatic disease. Studying these persons might provide additional insights into HbF gene regulation. HbF appears to benefit some complications of disease more than others. This might be related to the premature destruction of erythrocytes that do not contain HbF, even though the total HbF concentration is high. Recent insights into HbF regulation have spurred new efforts to induce high HbF levels in sickle cell disease beyond those achievable with the current limited repertory of HbF inducers.

Update on fetal hemoglobin gene regulation in hemoglobinopathies

PURPOSE OF REVIEW

The developmental switch from fetal to adult hemoglobin has long fascinated biologists and attracted hematologists given its importance for patients with hemoglobin disorders. New discoveries have reinvigorated the field of globin gene regulation. These results hold promise for improved treatment of the major hemoglobinopathies.

RECENT FINDINGS

Both genome-wide association studies and traditional linkage studies have identified several genetic loci involved in silencing fetal hemoglobin. BCL11A is a potent silencer of fetal hemoglobin in both mouse and humans. It controls the beta-globin gene cluster in concert with other factors. KLF1, a vital erythroid transcription factor, activates BCL11A and assists in coordinating the switch from fetal to adult hemoglobin. A regulatory network of cell-intrinsic and cell-extrinsic factors maintains the epigenetic homeostasis of the beta-globin cluster and accounts for the precise lineage-specific and developmental stage-specific regulation of the globin genes.

SUMMARY

With an improved understanding of pathways involved in the switch from fetal to adult hemoglobin, new targets have emerged for the treatment of the common hemoglobin disorders, sickle cell anemia and beta-thalassemia.

siDNMT1 increases γ-globin expression in chemical inducer of dimerization (CID)-dependent mouse βYAC bone marrow cells and in baboon erythroid progenitor cell cultures

OBJECTIVE

These studies were performed to test the hypothesis that DNMT1 is required for maintenance of DNA methylation and repression of the γ-globin gene in adult-stage erythroid cells.

MATERIALS AND METHODS

DNMT1 levels were reduced by nucleofection of small interfering RNA targeting DNMT1 in chemical inducer of dimerization-dependent multipotential mouse bone marrow cells containing the human β-globin gene locus in the context of a yeast artificial chromosome and in primary cultures of erythroid progenitor cells derived from CD34(+) baboon bone marrow cells. The effect of reduced DNMT1 levels on globin gene expression was measured by real-time polymerase chain reaction and the effect on globin chain synthesis in primary erythroid progenitor cell cultures was determined by biosynthetic radiolabeling of globin chains followed by high-performance liquid chromatography analysis. The effect on DNA methylation was determined by bisulfite sequence analysis.

RESULTS

Reduced DNMT1 levels in cells treated with siDNMT1 were associated with increased expression of γ-globin messenger RNA, an increased γ/γ+β chain ratio in cultured erythroid progenitors, and decreased DNA methylation of the γ-globin promoter. Similar effects were observed in cells treated with decitabine, a pharmacological inhibitor of DNA methyltransferase inhibitor.

CONCLUSIONS

DNMT1 is required to maintain DNA methylation of the γ-globin gene promoter and repress γ-globin gene expression in adult-stage erythroid cells.

Identification of a PRMT5-dependent repressor complex linked to silencing of human fetal globin gene expression

Defining the molecular mechanisms underpinning fetal (gamma) globin gene silencing may provide strategies for reactivation of gamma-gene expression, a major therapeutic objective in patients with beta-thalassemia and sickle cell disease (SCD). We have previously demonstrated that symmetric methylation of histone H4 Arginine 3 (H4R3me2s) by the protein arginine methyltransferase PRMT5 is required for recruitment of the DNA methyltransferase DNMT3A to the gamma-promoter, and subsequent DNA methylation and gene silencing. Here we show in an erythroid cell line, and in primary adult erythroid progenitors that PRMT5 induces additional repressive epigenetic marks at the gamma-promoter through the assembly of a multiprotein repressor complex containing the histone modifying enzymes SUV4-20h1, casein kinase 2alpha (CK2alpha), and components of the nucleosome remodeling and histone deacetylation complex. Expression of a mutant form of PRMT5 lacking methyltransferase activity or shRNA-mediated knockdown of SUV4-20h1 resulted in loss of complex binding to the gamma-promoter, reversal of both histone and DNA repressive epigenetic marks, and increased gamma-gene expression. The repressive H4K20me3 mark induced by SUV4-20h1 is enriched on the gamma-promoter in erythroid progenitors from adult bone marrow compared with cord blood, suggesting developmental specificity. These studies define coordinated epigenetic events linked to fetal globin gene silencing, and provide potential therapeutic targets for the treatment of beta-thalassemia and SCD.

A zinc-finger transcriptional activator designed to interact with the gamma-globin gene promoters enhances fetal hemoglobin production in primary human adult erythroblasts

Fetal hemoglobin (HbF) is a potent genetic modifier of the severity of beta-thalassemia and sickle cell anemia. We used an in vitro culture model of human erythropoiesis in which late-stage erythroblasts are derived directly from human CD34(+) hematopoietic cells to evaluate HbF production. This system recapitulates expression of globin genes according to the developmental stage of the originating cell source. When cytokine-mobilized peripheral blood CD34(+) cells from adults were cultured, background levels of HbF were 2% or less. Cultured cells were readily transduced with lentiviral vectors when exposed to vector particles between 48 and 72 hours. Among the genetic elements that may enhance fetal hemoglobin production is an artificial zinc-finger transcription factor, GG1-VP64, designed to interact with the proximal gamma-globin gene promoters. Our data show that lentiviral-mediated, enforced expression of GG1-VP64 under the control of relatively weak erythroid-specific promoters induced significant amounts of HbF (up to 20%) in erythroblasts derived from adult CD34(+) cells without altering their capacity for erythroid maturation and only modestly reducing the total numbers of cells that accumulate in culture after transduction. These observations demonstrate the potential for sequence-specific enhancement of HbF in patients with beta-thalassemia or sickle cell anemia.

The assessment of human erythroid output in NOD/SCID mice reconstituted with human hematopoietic stem cells

The third-generation NOD/LtSz-scid/IL2Rγ(null) (NOD/SCID IL2Rγ(null)) mouse represents a significantly improved xenograft model allowing high levels of human leukocyte engraftment over extended follow up. One remaining limitation of this mouse model, however, is the low level of circulating human erythrocytes. We established a practical ex vivo erythroid culture system of xenograft marrow progenitors to enrich for human erythroid progeny. At various time points after transplant, erythroid cells were easily assayed after 17 days of ex vivo culture of xenograft marrow, with nearly all nucleated cells of human origin and approximately 60% human GPA or CD71 positive. We then transplanted cord blood CD34(+) cells marked with a lentiviral vector encoding green fluorescent protein (GFP). Three months later, ex vivo culture of xenograft marrow progenitors showed 41.3% of the cultured erythroid cells were positive for GFP and human CD71, and 56.2% were positive for GFP and human GPA, similar to that of circulating leukocytes at the same time point. Next, G-CSF mobilized peripheral blood CD34(+) cells from a sickle cell trait subject were infused in this mouse model to determine if the hemoglobin pattern could be modeled. CD34(+) cells from the sickle cell trait subject engrafted equally compared to CD34(+) cells from normal subjects, establishing the sickle cell trait phenotype. Lastly, a comparison of adult-derived peripheral blood CD34(+) cells and cord blood-derived CD34(+) cells xenografted mice was made, and long term follow-up demonstrated a recapitulation of the fetal to adult hemoglobin switch. This approach should prove a useful tool for testing strategies for genetic manipulation of erythroid progeny and the study of hemoglobin switching.

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.

Transcriptional activation of the gamma-globin gene in baboons treated with decitabine and in cultured erythroid progenitor cells involves different mechanisms

OBJECTIVE

To investigate the mechanism(s) responsible for increased gamma-globin expression in vivo in decitabine-treated baboons and in vitro in cultured erythroid progenitor cells (EPC) from adult baboon bone marrow (BM).

MATERIALS AND METHODS

Fetal liver, adult BM erythroid cells pre- and post-decitabine, and cultured EPCs were analyzed for distribution of RNA polymerase II, histone acetylation, and histone H3 (lys4) trimethyl throughout the gamma-globin gene complex by chromatin immunoprecipitation. DNA methylation of the gamma-globin promoter was determined by bisulfite sequencing. Expression of the baboon Igamma- and Vgamma-globin chains was determined by high performance liquid chromatography (HPLC). Expression of BCL11A, a recently identified repressor of gamma-globin expression, was analyzed by Western blot.

RESULTS

Increased gamma-globin expression in decitabine-treated baboons and cultured EPC correlated with increased levels of RNA polymerase II, histone acetylation, and histone H3 (lys4) trimethyl associated with the gamma-globin gene consistent with a transcriptional activation mechanism. Cultured EPC expressed the Igamma- and Vgamma-globin chains in a pattern characteristic of fetal development. The level of DNA methylation of the gamma-globin gene promoter in EPC cultures was similar to BM erythroid cells from normal adult baboons. Different BCL11A isoforms were observed in BM erythroid cells and cultured EPC.

CONCLUSION

The mechanism responsible for increased gamma-globin expression in cultured EPC was unexpectedly not associated with increased DNA hypomethylation of the gamma-globin gene promoter compared to normal BM erythroid cells, in contrast to BM erythroid cells of decitabine-treated baboons. Rather, increased fetal hemoglobin in EPC cultures was associated with a fetal Igamma/Vgamma chain ratio and a difference in the size of the BCL11A protein compared to normal BM erythroid cells.

Cytokine-mediated increases in fetal hemoglobin are associated with globin gene histone modification and transcription factor reprogramming

Therapeutic regulation of globin genes is a primary goal of translational research aimed toward hemoglobinopathies. Signal transduction was used to identify chromatin modifications and transcription factor expression patterns that are associated with globin gene regulation. Histone modification and transcriptome profiling were performed using adult primary CD34(+) cells cultured with cytokine combinations that produced low versus high levels of gamma-globin mRNA and fetal hemoglobin (HbF). Embryonic, fetal, and adult globin transcript and protein expression patterns were determined for comparison. Chromatin immunoprecipitation assays revealed RNA polymerase II occupancy and histone tail modifications consistent with transcriptional activation only in the high-HbF culture condition. Transcriptome profiling studies demonstrated reproducible changes in expression of nuclear transcription factors associated with high HbF. Among the 13 genes that demonstrated differential transcript levels, 8 demonstrated nuclear protein expression levels that were significantly changed by cytokine signal transduction. Five of the 8 genes are recognized regulators of erythropoiesis or globin genes (MAFF, ID2, HHEX, SOX6, and EGR1). Thus, cytokine-mediated signal transduction in adult erythroid cells causes significant changes in the pattern of globin gene and protein expression that are associated with distinct histone modifications as well as nuclear reprogramming of erythroid transcription factors.

Sustained and full fetal hemoglobin production after failure of bone marrow transplant in a patient homozygous for beta 0-thalassemia: a clinical remission despite genetic disease and transplant rejection

An adult patient affected by beta(0)-thalassemia major underwent allogeneic bone marrow transplant (BMT) from a matched related donor. Forty days after transplant, allogeneic engraftment failure and autologous beta(0)-thalassemic bone marrow recovery were documented. Red blood cell transfusions were required until 118 days post-transplant. Thereafter, the haemoglobin (Hb) levels stabilized over 11.8 gr/dl throughout the ongoing 34-month follow-up, abolishing the need for transfusion support. The Hb electrophoresis showed 100% Hb Fetal (HbF). This unexplained case suggests full HbF production may occur in an adult patient with beta(0)-thalassemia major.

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.

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.

A cell stress signaling model of fetal hemoglobin induction: what doesn't kill red blood cells may make them stronger

A major goal of hemoglobinopathy research is to develop treatments that correct the underlying molecular defects responsible for sickle cell disease and beta-thalassemia. One approach to achieving this goal is the pharmacologic induction of fetal hemoglobin (HbF). This strategy is capable of inhibiting the polymerization of sickle hemoglobin and correcting the globin chain imbalance of beta-thalassemia. Despite this promise, none of the currently available HbF-inducing agents exhibit the combination of efficacy, safety, and convenience of use that would make them applicable to most patients. The recent success of targeted drug therapies for malignant diseases suggests that this approach could be effective for developing optimal HbF-inducing agents. A first step in applying this approach is the identification of specific molecular targets. However, while >70 HbF-inducing agents have been described, neither molecular mechanisms nor target molecules have been definitively verified for any of these compounds. To help focus investigation in this area, we have reviewed known HbF-inducing agents and their proposed mechanisms of action. We find that in many cases, current models inadequately explain key experimental results. By integrating features of the erythropoietic stress model of HbF induction with data from recent intracellular signaling experiments, we have developed a new model that has the potential to explain several findings that are inconsistent with previous models and to unify most HbF-inducing agents under a common mechanism: cell stress signaling. If correct, this or related models could lead to new opportunities for development of targeted therapies for the beta-hemoglobinopathies.

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.

Neither DNA hypomethylation nor changes in the kinetics of erythroid differentiation explain 5-azacytidine's ability to induce human fetal hemoglobin

5-azacytidine (5-Aza) is a potent inducer of fetal hemoglobin (HbF) in people with beta-thalassemia and sickle cell disease. Two models have been proposed to explain this activity. The first is based on the drug's ability to inhibit global DNA methylation, including the fetal globin genes, resulting in their activation. The second is based on 5-Aza's cytotoxicity and observations that HbF production is enhanced during marrow recovery. We tested these models using human primary cells in an in vitro erythroid differentiation system. We found that doses of 5-Aza that produce near maximal induction of gamma-globin mRNA and HbF do not alter cell growth, differentiation kinetics, or cell cycle, but do cause a localized demethylation of the gamma promoter. However, when we reduced gamma promoter methylation to levels equivalent to those seen with 5-Aza or to the lower levels seen in primary fetal erythroid cells using DNMT1 siRNA and shRNA, we observed no induction of gamma-globin mRNA or HbF. These results suggest that 5-Aza induction of HbF is not the result of global DNA demethylation or of changes in differentiation kinetics, but involves an alternative, previously unrecognized mechanism. Other results suggest that posttranscriptional regulation plays an important role in the 5-Aza response.

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.

HIF prolyl hydroxylase inhibition results in endogenous erythropoietin induction, erythrocytosis, and modest fetal hemoglobin expression in rhesus macaques

The hypoxia-inducible factor (HIF) pathway is crucial in mitigating the deleterious effects of oxygen deprivation. HIF-alpha is an essential component of the oxygen-sensing mechanisms and under normoxic conditions is targeted for degradation via hydroxylation by HIF-prolyl hydroxylases. Several HIF-prolyl hydroxylase inhibitors (PHIs) induced erythropoietin (epo) expression in vitro and in mice, with peak epo expression ranging from 5.6- to 207-fold above control animals. Furthermore, several PHIs induced fetal hemoglobin (HbF) expression in primary human erythroid cells in vitro, as determined by flow cytometry. One PHI, FG-2216, was further tested in a nonhuman primate model without and with chronic phlebotomy. FG-2216 was orally bioavailable and induced significant and reversible Epo induction in vivo (82- to 309-fold at 60 mg/kg). Chronic oral dosing in male rhesus macaques was well tolerated, significantly increased erythropoiesis, and prevented anemia induced by weekly phlebotomy. Furthermore, modest increases in HbF-containing red cells and reticulocytes were demonstrated by flow cytometry, though significant increases in HbF were not demonstrated by high-pressure liquid chromatography (HPLC). HIF PHIs represent a novel class of molecules with broad potential clinical application for congenital and acquired anemias.

Cl-IB-MECA inhibits human erythropoiesis

Candidate drugs are being sought for the suppression of human erythropoiesis. Cl-IB-MECA [2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide] is a derivative of adenosine that inhibits the growth of leukaemic cell lines. To determine the effects of Cl-IB-MECA upon erythropoiesis, studies were performed by using an ex vivo culture system of primary human CD34+ cells. Cl-IB-MECA suppressed erythroblast growth and maturation at doses >/=50 mumol/l through a mechanism of cell cycle inhibition and accumulation of cells in the G1/G0 phase. These findings demonstrate that Cl-IB-MECA inhibits human erythropoiesis, and suggest that further consideration of this drug is warranted for patients with erythrocytosis or polycythemia syndromes.

Molecular signature of CD34+ hematopoietic stem and progenitor cells of patients with CML in chronic phase

In this study, we provide a molecular signature of highly enriched CD34+ cells from bone marrow of untreated patients with chronic myelogenous leukemia (CML) in chronic phase in comparison with normal CD34+ cells using microarrays covering 8746 genes. Expression data reflected several BCR-ABL-induced effects in primary CML progenitors, such as transcriptional activation of the classical mitogen-activated protein kinase pathway and the phosphoinositide-3 kinase/AKT pathway as well as downregulation of the proapoptotic gene IRF8. Moreover, novel transcriptional changes in comparison with normal CD34+ cells were identified. These include upregulation of genes involved in the transforming growth factorbeta pathway, fetal hemoglobin genes, leptin receptor, sorcin, tissue inhibitor of metalloproteinase 1, the neuroepithelial cell transforming gene 1 and downregulation of selenoprotein P. Additionally, genes associated with early hematopoietic stem cells (HSC) and leukemogenesis such as HoxA9 and MEIS1 were transcriptionally activated. Differential expression of differentiation-associated genes suggested an altered composition of the CD34+ cell population in CML. This was confirmed by subset analyses of chronic phase CML CD34+ cells showing an increase of the proportion of megakaryocyte-erythroid progenitors, whereas the proportion of HSC and granulocyte-macrophage progenitors was decreased in CML. In conclusion, our results give novel insights into the biology of CML and could provide the basis for identification of new therapeutic targets.

Understanding mechanisms of gamma-globin gene regulation to develop strategies for pharmacological fetal hemoglobin induction

The developmental regulation of gamma-globin gene expression has shaped research efforts to establish therapeutic modalities for individuals affected with sickle cell disease (SCD). Fetal hemoglobin (Hb F) synthesis is high at birth, followed by a decline to adult levels by 10 months of age. The expression of gamma-globin is controlled by a developmentally regulated transcriptional program that is recapitulated during normal erythropoiesis in the adult bone marrow. It is known that naturally occurring mutations in the gamma-gene promoters cause persistent Hb F synthesis after birth, which ameliorates symptoms in SCD by inhibiting hemoglobin S polymerization and vaso-occlusion. Several pharmacological agents have been identified over the past 2 decades that reactivate gamma-gene transcription through different cellular systems. We will review the progress made in our understanding of molecular mechanisms that control gamma-globin expression and insights gained from Hb F-inducing agents that act through signal transduction pathways.

cMYB is involved in the regulation of fetal hemoglobin production in adults

A quantitative trait locus (QTL) controlling HbF levels has previously been mapped to chromosome 6q23 in an Asian-Indian kindred with beta thalassemia and heterocellular hereditary persistence of fetal hemoglobin (HPFH). Five protein-coding genes, ALDH8A1, HBS1L, cMYB, AHI1, and PDE7B reside in this 1.5-megabase (Mb) candidate interval of 6q23. To direct sequencing efforts we compared the expression profiles of these 5 genes between 12 individuals with elevated and 14 individuals with normal HbF levels during adult erythropoiesis by real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). Two genes, cMYB and HBS1L, demonstrated simultaneous transcriptional down-regulation in individuals with elevated HbF levels. Transfection of K562 cells encoding human cDNA of cMYB and HBS1L genes showed that, although overexpression of ectopic cMYB inhibited gamma-globin gene expression, overexpression of HBS1L had no effect. Low levels of cMYB were associated with low cell expansions, accelerated erythroid maturation, and higher number of macrophages in erythroid cell culture. These observations suggest that differences in the intrinsic levels of cMYB may account for some of the variation in adult HbF levels. The possible mechanism of cMYB influencing gamma- to beta-globin switching is discussed.

Lineage-specific activators affect beta-globin locus chromatin in multipotent hematopoietic progenitors

During development, the regulated expression of tissue-specific genes can be preceded by their potentiation, that is, by chromatin activation in progenitor cells. For example, the human beta-like globin genes are potentiated in a gene- and developmental-specific manner in hematopoietic progenitors. Developmental regulation of human beta-gene expression in erythroid cells is mostly determined by transcriptional activators; however, it is not clear how gene-specific potentiation is set in hematopoietic progenitors. Using human and transgenic multipotent hematopoietic progenitors, we demonstrate that human beta-globin locus activation is characterized by TBP, NF-E2, CBP and BRG1 recruitment at both the Locus Control Region and human beta-gene promoter. Our results further indicate that in hematopoietic progenitors, EKLF influences chromatin organization at the human beta-globin locus and is instrumental for human beta-gene potentiation. Thus, we show that lineage-specific transcriptional activators expressed at basal levels in progenitor cells can participate in gene potentiation.

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.