Analysis of the mechanism of action of non-deletion hereditary persistence of fetal hemoglobin mutants in transgenic mice

Pharmacological and molecular approaches for the treatment of β-hemoglobin disorders

β-hemoglobin disorders, such as β-thalassemia and sickle cell anemia are among the most prevalent inherited genetic disorders worldwide. These disorders are caused by mutations in the gene encoding hemoglobin-β (HBB), a vital protein found in red blood cells (RBCs) that carries oxygen from lungs to all parts of the human body. As a consequence, there has been an enduring interest in this field in formulating therapeutic strategies for the treatment of these diseases. Currently, there is no cure available for hemoglobin disorders, although, some patients have been treated with bone marrow transplantation, whose scope is limited because of the difficulty in finding a histocompatible donor and also due to transplant-associated clinical complications that can arise during the treatment. On account of these constraints, reactivation of fetal hemoglobin (HbF) synthesis holds immense promise and is a viable strategy to alleviate the symptoms of β-hemoglobin disorders. Development of new genomic tools has led to the identification of important natural genetic modifiers of hemoglobin switching which include BCL11A, KLF1, HBSIL-MYB, LRF, LSD1, LDB1, histone deacetylases 1 and 2 (HDAC1 and HDAC2). miRNAs are also promising therapeutic targets for development of more effective strategies for the induction of HbF production. Many new small molecule pharmacological inducers of HbF production are already under pre-clinical and clinical development. Furthermore, recent advancements in gene and cell therapy includes targeted genome editing and iPS cell technologies, both of which utilizes a patient's own cells, are emerging as extremely promising approaches for significantly reducing the burden of β-hemoglobin disorders.

Fetal hemoglobin regulation in β-thalassemia: heterogeneity, modifiers and therapeutic approaches

INTRODUCTION

Stress erythropoiesis induces fetal hemoglobin (HbF) expression in β-thalassemias, however the level of expression is highly variable. The last decade has seen dramatic advances in our understanding of the molecular regulators of HbF production and the genetic factors associated with HbF levels, leading to the promise of new methods of the clinical induction of HbF. Areas covered: This article will review the heterogeneity and genetic modifiers of HbF and HbF induction therapy in β-thalassemia. Expert commentary: One promising curative β-thalassemia therapy is to induce HbF synthesis in β-thalassemic erythrocytes to therapeutic levels before clinical symptom occurs. Further understanding of HbF level variation and regulation is needed in order to predict the response from HbF-inducing approaches.

The New -474(C→T) Substitution Discovered in the HBG2 Promoter of a Sardinian δβ-Thalassemia Carrier

During a screening for hemoglobinopathies, we found a carrier of the Sardinian δβ-thalassemia condition. The proband's hematology and hemoglobin (Hb) profile agreed with those of the other carriers previously identified during our diagnostic program except for the fetal Hb (HbF) composition, which consisted of both α2Aγ2 and α2Gγ2 instead of nearly 100% α2Aγ2. In order to explain the unusual γ-chain ratio, sequencing of the Gγ promoter was carried out and revealed two nucleotide substitutions in cis: C→T at position -474 and A→G at position -309 from the Cap site. The latter had previously been observed in subjects with raised HbF levels, although it has not yet been evaluated at functional level. We used the luciferase assay to determine whether the two mutations modify the transcriptional activity of the Gγ promoter. Results indicated that the observed in vivo Gγ-globin production cannot be translated into increased in vitro promoter function, suggesting that the assessed mutations cannot be considered as functional single nucleotide polymorphisms per se; instead, a more complex regulatory mechanism might be involved.

Locus-specific proteomics by TChP: targeted chromatin purification

Here, we show that transcription factors bound to regulatory sequences can be identified by purifying these unique sequences directly from mammalian cells in vivo. Using targeted chromatin purification (TChP), a double-pull-down strategy with a tetracycline-sensitive "hook" bound to a specific promoter, we identify transcription factors bound to the repressed γ-globin gene-associated regulatory regions. After validation of the binding, we show that, in human primary erythroid cells, knockdown of a number of these transcription factors induces γ-globin gene expression. Reactivation of γ-globin gene expression ameliorates the symptoms of β-thalassemia and sickle cell disease, and these factors provide potential targets for the development of therapeutics for treating these patients.

NF-Y recruits both transcription activator and repressor to modulate tissue- and developmental stage-specific expression of human γ-globin gene

The human embryonic, fetal and adult β-like globin genes provide a paradigm for tissue- and developmental stage-specific gene regulation. The fetal γ-globin gene is expressed in fetal erythroid cells but is repressed in adult erythroid cells. The molecular mechanism underlying this transcriptional switch during erythroid development is not completely understood. Here, we used a combination of in vitro and in vivo assays to dissect the molecular assemblies of the active and the repressed proximal γ-globin promoter complexes in K562 human erythroleukemia cell line and primary human fetal and adult erythroid cells. We found that the proximal γ-globin promoter complex is assembled by a developmentally regulated, general transcription activator NF-Y bound strongly at the tandem CCAAT motifs near the TATA box. NF-Y recruits to neighboring DNA motifs the developmentally regulated, erythroid transcription activator GATA-2 and general repressor BCL11A, which in turn recruit erythroid repressor GATA-1 and general repressor COUP-TFII to form respectively the NF-Y/GATA-2 transcription activator hub and the BCL11A/COUP-TFII/GATA-1 transcription repressor hub. Both the activator and the repressor hubs are present in both the active and the repressed γ-globin promoter complexes in fetal and adult erythroid cells. Through changes in their levels and respective interactions with the co-activators and co-repressors during erythroid development, the activator and the repressor hubs modulate erythroid- and developmental stage-specific transcription of γ-globin gene.

Silencing of Agamma-globin gene expression during adult definitive erythropoiesis mediated by GATA-1-FOG-1-Mi2 complex binding at the -566 GATA site

Autonomous silencing of gamma-globin transcription is an important developmental regulatory mechanism controlling globin gene switching. An adult stage-specific silencer of the (A)gamma-globin gene was identified between -730 and -378 relative to the mRNA start site. A marked copy of the (A)gamma-globin gene inserted between locus control region 5' DNase I-hypersensitive site 1 and the epsilon-globin gene was transcriptionally silenced in adult beta-globin locus yeast artificial chromosome (beta-YAC) transgenic mice, but deletion of the 352-bp region restored expression. This fragment reduced reporter gene expression in K562 cells, and GATA-1 was shown to bind within this sequence at the -566 GATA site. Further, the Mi2 protein, a component of the NuRD complex, was observed in erythroid cells with low gamma-globin levels, whereas only a weak signal was detected when gamma-globin was expressed. Chromatin immunoprecipitation of fetal liver tissue from beta-YAC transgenic mice demonstrated that GATA-1, FOG-1, and Mi2 were recruited to the (A)gamma-globin -566 or (G)gamma-globin -567 GATA site when gamma-globin expression was low (day 18) but not when gamma-globin was expressed (day 12). These data suggest that during definitive erythropoiesis, gamma-globin gene expression is silenced, in part, by binding a protein complex containing GATA-1, FOG-1, and Mi2 at the -566/-567 GATA sites of the proximal gamma-globin promoters.

Embryonic and fetal beta-globin gene repression by the orphan nuclear receptors, TR2 and TR4

The TR2 and TR4 orphan nuclear receptors comprise the DNA-binding core of direct repeat erythroid definitive, a protein complex that binds to direct repeat elements in the embryonic and fetal beta-type globin gene promoters. Silencing of both the embryonic and fetal beta-type globin genes is delayed in definitive erythroid cells of Tr2 and Tr4 null mutant mice, whereas in transgenic mice that express dominant-negative TR4 (dnTR4), human embryonic epsilon-globin is activated in primitive and definitive erythroid cells. In contrast, human fetal gamma-globin is activated by dnTR4 only in definitive, but not in primitive, erythroid cells, implicating TR2/TR4 as a stage-selective repressor. Forced expression of wild-type TR2 and TR4 leads to precocious repression of epsilon-globin, but in contrast to induction of gamma-globin in definitive erythroid cells. These temporally specific, gene-selective alterations in epsilon- and gamma-globin gene expression by gain and loss of TR2/TR4 function provide the first genetic evidence for a role for these nuclear receptors in sequential, gene-autonomous silencing of the epsilon- and gamma-globin genes during development, and suggest that their differential utilization controls stage-specific repression of the human epsilon- and gamma-globin genes.

Purification and identification of proteins that bind to the hereditary persistence of fetal hemoglobin -198 mutation in the gamma-globin gene promoter

Expression of the gamma-globin gene is silenced in adult humans. However, certain point mutations in the gamma-globin gene promoter are capable of maintaining expression of this gene during adult erythropoiesis, a condition called non-deletion hereditary persistence of fetal hemoglobin (HPFH). Among these, the British form of HPFH carrying a T-->C point mutation at position -198 of the Agamma-globin gene promoter results in 4-10% fetal hemoglobin in heterozygotes. In this study, we used nuclear extracts from murine erythroleukemia cells to purify a protein complex that binds the HPFH -198 gamma-globin gene promoter. Members of this protein complex were identified by mass spectrometry and include DNMT1, the transcriptional coactivator p52, the protein SNEV, and RAP74 (the largest subunit of the general transcription factor IIF). Sp1, which was previously considered responsible for HPFH -198 gamma-globin gene activation, was not identified. The potential role of these proteins in the reactivation and/or maintenance of gamma-globin gene expression in the adult transcriptional environment is discussed.

Functional study of transcription factor KLF11 by targeted gene inactivation

Sp1/Krüppel-like factor (KLF) family of transcription factors regulates diverse biological processes including cell growth, differentiation, and development through modulation of gene expression. This family of factors regulates transcription positively and negatively by binding to the GC and GT/CACCC boxes in the promoter through their highly conserved three zinc finger domains. Although the molecular mechanism of gene regulation by this family of proteins has been well studied, their exact role in growth and development in vivo remains largely unknown. KLF11 has been implicated in the regulation of cell growth and gene expression. To determine the physiological function of KLF11, we generated KLF11-null mice by gene-targeting technology. Homologous KLF11(-/-) mice were bred normally and were fertile. Hematopoiesis at all stages of development was normal in the KLF11(-/-) mice. There was no effect on globin gene expression. These mice lived as long as the wild-type mice without evident pathological defects. Thus, despite its cell growth inhibition and transcriptional regulation functions observed when transiently or stably expressed in cultured cells in vitro, the results from genetic knockout suggest that KLF11 is not absolutely required for hematopoiesis, growth, and development.

Repression of human gamma-globin gene expression by a short isoform of the NF-E4 protein is associated with loss of NF-E2 and RNA polymerase II recruitment to the promoter

Binding of the stage selector protein (SSP) to the stage selector element (SSE) in the human gamma-globin promoter contributes to the preferential expression of the gamma-gene in fetal erythroid cells. The SSP contains the transcription factor CP2 and an erythroid-specific partner, NF-E4. The NF-E4 gene encodes a 22-kDa polypeptide employing a non-AUG initiation codon. Antisera specific to NF-E4 detects this species and an additional 14 kDa protein, which initiates from an internal methionine. Enforced expression of p14 NF-E4 in the K562 fetal/erythroid cell line, and in primary erythroid cord blood progenitors, results in repression of gamma-gene expression. Biochemical studies reveal that p14 NF-E4 interacts with CP2, resulting in diminished association of CP2 with the SSE in chromatin immunoprecipitation assays. p45 NF-E2 recruitment to the gamma-promoter is also lost, resulting in a reduction in RNA polymerase II and TBP binding and a fall in promoter transcriptional activity. This effect is specific, as enforced expression of a mutant form of p14 NF-E4, which fails to interact with CP2, also fails to repress gamma-gene expression in K562 cells. These findings provide one potential mechanism that could contribute to the autonomous silencing of the human gamma-genes in adult erythroid cells.

Transcriptional regulation of FKLF-2 (KLF13) gene in erythroid cells

FKLF-2 (KLF13) was cloned from fetal globin-expressing tissues and has been shown to be abundantly expressed in erythroid cells. In this study we examined the transcriptional regulation of the KLF13 gene. A 5.5 kb 5' flanking region cloned from mouse erythroleukemia (MEL) cell genomic DNA showed that major cis regulatory activities exist in the 550 bp sequence to the unique transcription start site, and that the promoter is more active in K562 cells than in COS-7 cells. The promoter was trans-activated by co-expressed GATA-1 through the sequence containing two CCAAT motifs, suggesting that GATA-1 is involved in the abundant expression of KLF13 mRNA in the erythroid tissue. Dual action, i.e. activating effect in COS-7 and repressive effect in K562 cell, was observed on its own promoter, suggesting a feedback mechanism for the transcriptional control of the KLF13 gene in the erythroid environment. These findings provide an insight on the mechanism of inducible mRNA expression of the KLF13 gene in erythroid cells.

T to C Substitution at –175 or –173 of the γ-Globin Promoter Affects GATA-1 and Oct-1 Binding in Vitro Differently but Can Independently Reproduce the Hereditary Persistence of Fetal Hemoglobin Phenotype in Transgenic Mice

The T to C substitution at position -175 of the gamma-globin gene has been identified in some individuals with non-deletion hereditary persistence of fetal hemoglobin (HPFH). In this study, the HPFH phenotype was reestablished in transgenic mice carrying the mu'LCRAgamma(-175)psibetadeltabeta construct, which contained a 3.1-kb mu'LCR cassette linked to a 29-kb fragment from the Agamma-to beta-globin gene with the natural chromosome arrangement but with the -175 mutation, which provided evidence for this single mutation as the cause of this form of HPFH. The HPFH phenotype was also reproduced in transgenic mice carrying the mu'LCRAgamma(-173)psibetadeltabeta construct, in which the -175 T to C Agamma gene was substituted with the -173 T to C Agamma gene. In vitro experiments proved that the -175 mutation significantly reduced binding of Oct-1 but not GATA-1, whereas the -173 mutation dramatically decreased binding of GATA-1 but not Oct-1. These results suggest that abrogation of either GATA-1 or Oct-1 binding to this promoter region may result in the HPFH phenotype. An in vivo footprinting assay revealed that either the -175 mutation or the -173 mutation significantly decreased overall protein binding to this promoter region in adult erythrocytes of transgenic mice. We hypothesize that a multiprotein complex containing GATA-1, Oct-1, and other protein factors may contribute to the formation of a repressive chromatin structure that silences gamma-globin gene expression in normal adult erythrocytes. Both the -173 and -175 T to C substitutions may disrupt the complex assembly and result in the reactivation of the gamma-globin gene in adult erythrocytes.

The minimal promoter plays a major role in silencing of the galago gamma-globin gene in adult erythropoiesis

The human gamma-globin gene and its orthologous galago gamma-globin gene evolved from an ancestral epsilon-globin gene. In galago, expression of the gamma-gene remained restricted to the embryonic stage of development, whereas in humans, expression of the gamma-gene was recruited to the fetal stage. To localize the cis-elements responsible for this developmentally distinct regulation, we studied the expression patterns of the human gamma-gene driven by either the human or the galago gamma-promoters in transgenic mice. gamma-gene transcription driven by either promoter reached similar levels in embryonic erythropoiesis. In adult erythropoiesis the gamma-gene was silenced when controlled by the galago gamma-promoter, but it was expressed at a high level when it was linked to the human gamma-promoter. By a series of gamma-promoter truncations the sequences required for the down-regulation of the galago gamma-globin gene were localized to the minimal promoter. Furthermore, by interchanging the TATA, CCAAT, and CACCC elements between the human and galago minimal promoters we found that whereas each box made a developmentally distinctive contribution to gamma-globin gene expression, the CACCC box was largely responsible for the down-regulation of the gamma-gene in adult erythropoiesis.

Analysis of the mechanism of action of the Brazilian type (A γ −195 C → G) of hereditary persistence of fetal hemoglobin

We report an in vitro expression study of the Agamma-globin gene promoter containing the Agamma-195 C --> G mutation that causes the Brazilian type of hereditary persistence of fetal hemoglobin (HPFH). To demonstrate that this mutation results in increased promoter strength, we evaluated the mutant promoter linked to the hypersensitive site-2 of the locus control region with the luciferase reporter gene system and examined protein interactions by eletrophoretic mobility shift assay. The transient expression was studied in three cell lines: K562, HEL and 293, and indicated increased promoter activity of the promoter containing the Brazilian mutation in all cell lines. The protein-DNA interaction showed that, in contrast to the Agamma-198 T --> C mutation which has increased affinity for the Sp1 protein and creates a motif that behaves like a novel CACCC box in the gamma promoter, the Brazilian HPFH mutation decreases the affinity at the Sp1 protein and does not act as a CACCC motif. These results suggest that this mutation may act to increase the Agamma-globin chain production. In addition, the mechanism by which this increased production occurs is different to that of the -198 mutation. Other proteins may be involved in the overexpression of the gamma-globin chain and/or may be dependent upon the DNA structure.

Hemoglobin switching and modulation: genes, cells, and signals

A detailed understanding of hemoglobin production in erythroid cells is of fundamental clinical importance for the treatment of hemoglobinopathies. Several hundred scientific reports and dozens of reviews describe this intriguing topic of research. Early studies demonstrated the temporal nature of a hemoglobin-switching phenomenon during development in the circulating erythrocytes of humans. The focus then shifted from descriptive to experimental analyses and model systems in an effort to define the switching mechanisms. The application of molecular biology in those experimental models has been a primary focus for the last two decades. Today, advances in the fields of stem cell biology and signal transduction are being integrated with those genetic studies. Genomic and proteomic approaches are also being developed to provide a more robust description of the biologic variables involved. This review highlights recent advances in erythroid genetics and cellular biology with an emphasis upon the modulation of fetal hemoglobin expression during the maturation of adult human erythrocytes.