The role of the -50 region of the human gamma-globin gene in switching

Klf1 affects DNase II-alpha expression in the central macrophage of a fetal liver erythroblastic island: a non-cell-autonomous role in definitive erythropoiesis

A key regulatory gene in definitive erythropoiesis is the erythroid Kruppel-like factor (Eklf or Klf1). Klf1 knockout (KO) mice die in utero due to severe anemia, while residual circulating red blood cells retain their nuclei. Dnase2a is another critical gene in definitive erythropoiesis. Dnase2a KO mice are also affected by severe anemia and die in utero. DNase II-alpha is expressed in the central macrophage of erythroblastic islands (CMEIs) of murine fetal liver. Its main role is to digest the DNA of the extruded nuclei of red blood cells during maturation. Circulating erythrocytes retain their nuclei in Dnase2a KO mice. Here, we show that Klf1 is expressed in CMEIs and that it binds and activates the promoter of Dnase2a. We further show that Dnase2a is severely downregulated in the Klf1 KO fetal liver. We propose that this downregulation of Dnase2a in the CMEI contributes to the Klf1 KO phenotype by a non-cell-autonomous mechanism.

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.

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.

Functional interaction of CP2 with GATA-1 in the regulation of erythroid promoters

We observed that binding sites for the ubiquitously expressed transcription factor CP2 were present in regulatory regions of multiple erythroid genes. In these regions, the CP2 binding site was adjacent to a site for the erythroid factor GATA-1. Using three such regulatory regions (from genes encoding the transcription factors GATA-1, EKLF, and p45 NF-E2), we demonstrated the functional importance of the adjacent CP2/GATA-1 sites. In particular, CP2 binds to the GATA-1 HS2 enhancer, generating a ternary complex with GATA-1 and DNA. Mutations in the CP2 consensus greatly impaired HS2 activity in transient transfection assays with K562 cells. Similar results were obtained by transfection of EKLF and p45 NF-E2 mutant constructs. Chromatin immunoprecipitation with K562 cells showed that CP2 binds in vivo to all three regulatory elements and that both GATA-1 and CP2 were present on the same GATA-1 and EKLF regulatory elements. Adjacent CP2/GATA-1 sites may represent a novel module for erythroid expression of a number of genes. Additionally, coimmunoprecipitation and glutathione S-transferase pull-down experiments demonstrated a physical interaction between GATA-1 and CP2. This may contribute to the functional cooperation between these factors and provide an explanation for the important role of ubiquitous CP2 in the regulation of erythroid genes.

β-Minor globin gene expression is preferentially reduced in EKLF Knock-Out mice

The CACCC box is duplicated in the beta-globin gene promoter of humans and other mammals. While the function of the proximal element as a binding site for EKLF has already been well established, the role of the distal element remains unclear. Mice present two adult beta-globin genes, beta-major and beta-minor, bearing a single CACCC box, the consensus sequence of which is identical to that of the proximal or distal human element, respectively. In the present study we analyzed the mRNA expression of beta-minor and beta-major in EKLF Knock-Out (KO) mice in comparison to wild-type (wt) littermates. The murine early fetal liver up to day 13/14 post coitum (pc) expresses mainly beta-minor globin chains. Nevertheless, expression of the beta-minor globin gene in EKLF KO mice has not been assessed to date. We provide evidence that expression of the beta-minor globin gene is dependent upon EKLF and is more affected by EKLF deprivation than the beta-major gene. The results obtained support a general role of EKLF in beta-globin gene activation and are in agreement with models involving an advantage of the LCR proximal respect to distal gene.

delta-Globin gene structure and expression in the K562 cell line

The delta-globin gene produces the delta chain of Hb A2 which represents less than 3% of the hemoglobin (Hb) in normal individuals. The delta-globin gene is also expressed in the human erythroleukemia cell line K562. The expression of the delta-globin gene in this cell line is unexpected since K562 shows an embryonic-fetal globin gene expression pattern with no expression of the adult beta-globin gene. delta-Globin gene activation has been proposed as a potential therapeutic tool for the cure of delta-thalassemia (thal). In order to shed some light on the delta-globin gene activation in K562 the present study has: (1) determined the complete nucleotide sequence of the delta- and beta-globin genes; (2) assessed, by reverse transcription-polymerase chain reaction (RT-PCR), the relative delta- and beta-globin mRNA level; and (3) analyzed the exact level of the endogenous expression delta-globin gene by S1 mapping. No sequence variations were identified in the (delta- and beta-globin genes when compared to the normal sequences. delta-Globin mRNA represent more than 95% of the total delta + beta-mRNA content. The level of expression of the delta-globin gene is 12.3% (+/- 1.2) compared to the endogenous alpha-globin gene. These results indicate that the high expression of the delta-globin gene in K562 is most likely due to the transacting environment. Therefore, the presence and/or absence of specific transacting factors are able to specifically activate the human delta-globin gene. The level of expression of the delta-globin gene in this cell line suggests that it could be of relevance to identify the transacting factor(s) responsible for this selective activation in order to better understand the molecular mechanisms undergoing gene activation.

The Role of p22 NF-E4 in Human Globin Gene Switching

The human stage selector protein, a complex containing the ubiquitous transcription factor CP2 and the erythroid-specific factor p22 NF-E4, facilitates the interaction of the gamma-globin genes with the locus control region in fetal erythroid cells. Enforced expression of p22 NF-E4 in K562 cells and human cord blood progenitors increases fetal globin gene expression, and in progenitors, reduces beta-globin expression. To examine the role of NF-E4 in an in vivo model of hemoglobin switching, we enforced the expression of p22 NF-E4 in transgenic mice carrying the human beta-globin locus yeast artificial chromosome. Although murine erythropoiesis and globin gene expression is unaffected in these mice, the expression profile of the human globin genes is altered. All three transgenic lines displayed an increased gamma:beta-globin ratio in E12.5-14.5 fetal liver, resulting in a delay in the fetal/adult switch. At E12.5, this is primarily due to a reduction of beta-gene expression, whereas at E14.5, the increased gamma:beta ratio is due to enhanced gamma-gene expression. Despite this, the switch in globin subtype is fully completed in the adult bone marrow. These findings indicate that p22 NF-E4 is capable of influencing human globin gene expression in vivo but is incapable of overriding the intrinsic mechanisms governing gamma-gene silencing in this context.

Transcription through intergenic chromosomal memory elements of the Drosophila bithorax complex correlates with an epigenetic switch

The proteins of the trithorax and Polycomb groups maintain the differential expression pattern of homeotic genes established by the early embryonic patterning system during development. These proteins generate stable and heritable chromatin structures by acting via particular chromosomal memory elements. We established a transgenic assay system showing that the Polycomb group response elements bxd and Mcp confer epigenetic inheritance throughout development. With previously published data for the Fab7 cellular memory module, we confirmed the cellular memory function of Polycomb group response elements. In Drosophila melanogaster, several of these memory elements are located in the large intergenic regulatory regions of the homeotic bithorax complex. Using a transgene assay, we showed that transcription through a memory element correlated with the relief of silencing imposed by the Polycomb group proteins and established an epigenetically heritable active chromatin mode. A memory element remodeled by the process of transcription was able to maintain active expression of a reporter gene throughout development. Thus, transcription appears to reset and change epigenetic marks at chromosomal memory elements regulated by the Polycomb and trithorax proteins. Interestingly, in the bithorax complex of D. melanogaster, the segment-specific expression of noncoding intergenic transcripts during embryogenesis seems to fulfill this switching role for memory elements regulating the homeotic genes.

Regulation of the globin genes

The a- and b-globin gene clusters are subject to several levels of regulation. They are expressed exclusively in the erythroid cells, only during defined periods of development and in a perfectly tuned way, assuring, at any stage of ontogeny, a correct balance in the availability of a- and b-globin chains for hemoglobin assembling. Such a tight control is dependent on regulatory regions of DNA located either in proximity or at great distances from the globin genes in a region characterized by the presence of several DNAse I hypersensitive sites and known as the Locus Control Region. All these sequences exert stimulatory, inhibitory or more complex activities by interacting with transcription factors that bridge these regions of DNA to the RNA polymerase machinery. Many of these factors have now been cloned and the corresponding mouse genes inactivated, shading new light on the metabolic pathways they control. It is increasingly recognized that such factors are organized into hierarchies according to the number of genes and circuits they regulate. Some genes such as GATA-1 and 2 are master regulators that act on large numbers of genes at early stage of differentiation whereas others, like EKLF, stand on the lowest step and control only single or limited number of genes at late stages of differentiation. We will review recent data gathered from expression studies in cell cultures, in transgenic or K.O. murine models as well as from a clinical settings. We will also discuss the development of novel theories on the regulation of the a- and b-globin genes and clusters.