C/EBPδ and C/EBPγ bind the CCAAT-box in the human β-globin promoter and modulate the activity of the CACC-box binding protein, EKLF

Homozygous mild beta-thalassaemia promoter transversion -71 C>T HBB:c.-121 C>T

Beta-thalassaemia is one of the most common genetic disorders worldwide, which is caused by absent or decreased synthesis of beta-globin chain subunits. Beta-thalassaemias are diverse groups of disease with a wide spectrum of clinical phenotypes. The clinical phenotypes can include asymptomatic forms of beta-thalassaemia minor, intermediate and severe transfusion dependent beta-thalassaemia major. Clinical severity varies depending on the underlying β globin gene mutation. There are a number of mild β-thalassaemia gene defects that could be referred as a 'silent carrier'. Identifying the underlying molecular defect is essential to predict phenotype severity for optimal management, tailored treatment and improved quality of life.We report the first identification of a homozygous point mutation located within the promoter region of the β-globin gene at position -71 (C>T). The patient was a female child, who was referred to our clinic after she was found to have hypochromic microcytic anaemia with low haemoglobin (Hb) (67 g/L) and an Hb A2 level at the upper limit of the normal value (3.7%). This observation is a new example of homozygous mild β-thalassaemia with a borderline Hb A2 level, and illustrates a potential source of pitfall in the diagnosis of β-thalassaemia disease.

CCAAT enhancer binding protein gamma (C/EBP-γ): An understudied transcription factor

The CCAAT enhancer binding protein (C/EBP) family of transcription factors are important transcriptional mediators of a wide range of physiologic processes. C/EBP-γ is the shortest C/EBP protein and lacks a canonical activation domain for the recruitment of transcriptional machinery. Despite its ubiquitous expression and ability to dimerize with other C/EBP proteins, C/EBP-γ has been studied far less than other C/EBP proteins, and, to our knowledge, no review of its functions has been written. This review seeks to integrate the current knowledge about C/EBP-γ and its physiologic roles, especially in cell proliferation, the integrated stress response, oncogenesis, hematopoietic and nervous system development, and metabolism, as well as to identify areas for future research.

The Effect of HBB: c.-121C>T Variant [-71 (C>T)] on the β-Globin Promoter: Case Series Study

One of the effective strategies in controlling thalassemia is recognition of carriers, followed by prenatal diagnosis (PND) to prevent the occurrence of new cases. There are some rare mutations and variants, for which there are not enough evidences of their effects, and can lead to misdiagnosis and even cause confusion in decision about termination of pregnancy. That is why it is very critical to know the effect of each mutation on the β chain gene. The variant of HBB: c.-121C>T [-71 (C>T)] located in the CAAT box of the promoter region, is a rare mutation. We report seven patients in Hormozagn Province, Iran, who were referred to the PND Center of Hormozgan University of Medical Science (HUMS), Bandar Abbas, Iran during 10 years (2010-2020). Briefly, this mutation causes minor changes in blood indices [mean corpuscular volume (MCV): 75.0 ± 4.0 fL; mean corpuscular hemoglobin (MCH): 25.8 ± 2.5 pg; Hb A₂: 3.4 ± 0.5%] showed anemia with a trait milder than minor β-thalassemia (β-thal). Though the existence of α mutations (deletions/point mutations) along with HBB: c.-121C>T can change blood indices due to the changes in α/β ratio. The phenotype of β-thal intermedia (β-TI) was observed in one case, who was a compound heterozygosity for codon 15 (G>A)/-71(C>T) (HBB: c.48G>A/HBB: c.-121C>T. The analysis of transcription level by real-time polymerase chain reaction (real-time PCR) confirmed that this allele induces a mild β⁺ phenotype due to a decrease in the transcription level.

The regulation of human globin promoters by CCAAT box elements and the recruitment of NF-Y

CCAAT boxes are motifs found within the proximal promoter of many genes, including the human globin genes. The highly conserved nature of CCAAT box motifs within the promoter region of both α-like and β-like globin genes emphasises the functional importance of the CCAAT sequence in globin gene regulation. Mutations within the β-globin CCAAT box result in β-thalassaemia, while mutations within the distal γ-globin CCAAT box cause the Hereditary Persistence of Foetal Haemoglobin, a benign condition which results in continued γ-globin expression during adult life. Understanding the transcriptional regulation of the globin genes is of particular interest, as reactivating the foetal γ-globin gene alleviates the symptoms of β-thalassaemia and sickle cell anaemia. NF-Y is considered to be the primary activating transcription factor which binds to globin CCAAT box motifs. Here we review recruitment of NF-Y to globin CCAAT boxes and the role NF-Y plays in regulating globin gene expression. This article is part of a Special Issue entitled: Nuclear Factor Y in Development and Disease, edited by Prof. Roberto Mantovani.

Hb Lansing (HBA2: c.264C > G) and a new β promoter transversion [-52 (G > T)]: an attempt to define the phenotype of two mutations found in the Omani population

We report two examples showing how problematic it can be to define the phenotype of new or rare globin genes mutations. We describe two mutations observed for the first time in the Omani population: the first was found in the consanguineous parents of a deceased newborn with hepatomegaly, cardiomegaly and severe hemolytic anemia, putatively homozygous for the rare Hb Lansing (HBA2: c.264C > G) variant. The second is a novel β-globin gene promoter mutation [-52 (G > T)] observed in four independent patients. Two with borderline/elevated Hb A2, α-thalassemia (α-thal) and hypochromic red cell indices, and two heterozygotes for Hb S (HBB: c.20A > T), α-thal and with Hb A/Hb S ratios possibly indicating a very mild β(+)-thalassemia (β(+)-thal) mutation.

The β-globin promoter -71 C>T mutation is a β+ thalassemic allele

A novel β-globin gene promoter (-71 C>T) nucleotide change was recently posted to the HbVar database (ID 2701) without precision on phenotype and ethnicity. We found the same change in compound heterozygosity with Hb S [β6(A3)Glu>Val] in an Omani family with almost equal expression of Hb A and Hb S. This suggested that the -71 C to T mutation may be a mild β-thalassemic allele. Subsequent search found three other independent cases with the same atypical Hb A:Hb S ratio, further confirming the mild thalassemic feature of this mutation. In addition, molecular screening of a set of subjects (with only Hb A) with borderline Hb A(2) or MCV values revealed the presence of -71 C>T change in heterozygous state, altogether assigning the mutation as a mild β(+) thalassemic allele. In a region such as Oman, where several genetic conditions of the red blood cell coexist (α- and β-thalassemia, Hb S, Hb D, Hb E) in significant frequencies, it is crucial to decipher the molecular basis of these atypical forms of β(+) thalassemias, especially in a genetic counseling setting.

NFI-A directs the fate of hematopoietic progenitors to the erythroid or granulocytic lineage and controls beta-globin and G-CSF receptor expression

It is generally conceded that selective combinations of transcription factors determine hematopoietic lineage commitment and differentiation. Here we show that in normal human hematopoiesis the transcription factor nuclear factor I-A (NFI-A) exhibits a marked lineage-specific expression pattern: it is upmodulated in the erythroid (E) lineage while fully suppressed in the granulopoietic (G) series. In unilineage E culture of hematopoietic progenitor cells (HPCs), NFI-A overexpression or knockdown accelerates or blocks erythropoiesis, respectively: notably, NFI-A overexpression restores E differentiation in the presence of low or minimal erythropoietin stimulus. Conversely, NFI-A ectopic expression in unilineage G culture induces a sharp inhibition of granulopoiesis. Finally, in bilineage E + G culture, NFI-A overexpression or suppression drives HPCs into the E or G differentiation pathways, respectively. These NFI-A actions are mediated, at least in part, by a dual and opposite transcriptional action: direct binding and activation or repression of the promoters of the beta-globin and G-CSF receptor gene, respectively. Altogether, these results indicate that, in early hematopoiesis, the NFI-A expression level acts as a novel factor channeling HPCs into either the E or G lineage.

EKLF/KLF1 controls cell cycle entry via direct regulation of E2f2

Differentiation of erythroid cells requires precise control over the cell cycle to regulate the balance between cell proliferation and differentiation. The zinc finger transcription factor, erythroid Krüppel-like factor (EKLF/KLF1), is essential for proper erythroid cell differentiation and regulates many erythroid genes. Here we show that loss of EKLF leads to aberrant entry into S-phase of the cell cycle during both primitive and definitive erythropoiesis. This cell cycle defect was associated with a significant reduction in the expression levels of E2f2 and E2f4, key factors necessary for the induction of S-phase gene expression and erythropoiesis. We found and validated novel intronic enhancers in both the E2f2 and E2f4 genes, which contain conserved CACC, GATA, and E-BOX elements. The E2f2 enhancer was occupied by EKLF in vivo. Furthermore, we were able to partially restore cell cycle dynamics in EKLF(-/-) fetal liver upon additional genetic depletion of Rb, establishing a genetic causal link between reduced E2f2 and the EKLF cell cycle defect. Finally, we propose direct regulation of the E2f2 enhancer is a generic mechanism by which many KLFs regulate proliferation and differentiation.

Erythroid Kruppel-like factor (EKLF) is recruited to the gamma-globin gene promoter as a co-activator and is required for gamma-globin gene induction by short-chain fatty acid derivatives

OBJECTIVES

The erythroid Kruppel-like factor (EKLF) is an essential transcription factor for beta-type globin gene switching, and specifically activates transcription of the adult beta-globin gene promoter. We sought to determine if EKLF is also required for activation of the gamma-globin gene by short-chain fatty acid (SCFA) derivatives, which are now entering clinical trials.

METHODS

The functional and physical interaction of EKLF and co-regulatory molecules with the endogenous human globin gene promoters was studied in primary human erythroid progenitors and cell lines, using chromatin immunoprecipitation (ChIP) assays and genetic manipulation of the levels of EKLF and co-regulators.

RESULTS AND CONCLUSIONS

Knockdown of EKLF prevents SCFA-induced expression of the gamma-globin promoter in a stably expressed microLCRbeta(pr)R(luc) (A)gamma(pr)F(luc) cassette, and prevents induction of the endogenous gamma-globin gene in primary human erythroid progenitors. EKLF is actively recruited to endogenous gamma-globin gene promoters after exposure of primary human erythroid progenitors, and murine hematopoietic cell lines, to SCFA derivatives. The core ATPase BRG1 subunit of the human SWI/WNF complex, a ubiquitous multimeric complex that regulates gene expression by remodeling nucleosomal structure, is also required for gamma-globin gene induction by SCFA derivatives. BRG1 is actively recruited to the endogenous gamma-globin promoter of primary human erythroid progenitors by exposure to SCFA derivatives, and this recruitment is dependent upon the presence of EKLF. These findings demonstrate that EKLF, and the co-activator BRG1, previously demonstrated to be required for definitive or adult erythropoietic patterns of globin gene expression, are co-opted by SCFA derivatives to activate the fetal globin genes.

Activation of Eklf expression during hematopoiesis by Gata2 and Smad5 prior to erythroid commitment

The hierarchical progression of stem and progenitor cells to their more-committed progeny is mediated through cell-to-cell signaling pathways and intracellular transcription factor activity. However, the mechanisms that govern the genetic networks underlying lineage fate decisions and differentiation programs remain poorly understood. Here we show how integration of Bmp4 signaling and Gata factor activity controls the progression of hematopoiesis, as exemplified by the regulation of Eklf during establishment of the erythroid lineage. Utilizing transgenic reporter assays in differentiating mouse embryonic stem cells as well as in the murine fetal liver, we demonstrate that Eklf expression is initiated prior to erythroid commitment during hematopoiesis. Applying phylogenetic footprinting and in vivo binding studies in combination with newly developed loss-of-function technology in embryoid bodies, we find that Gata2 and Smad5 cooperate to induce Eklf in a progenitor population, followed by a switch to Gata1-controlled regulation of Eklf transcription upon erythroid commitment. This stage- and lineage-dependent control of Eklf expression defines a novel role for Eklf as a regulator of lineage fate decisions during hematopoiesis.

A global role for zebrafish klf4 in embryonic erythropoiesis

There are two waves of erythropoiesis, known as primitive and definitive waves in mammals and lower vertebrates including zebrafish. The founding member of the Kruppel-like factor (KLF) family of CACCC-box binding proteins, EKLF/Klf1, is essential for definitive erythropoiesis in mammals but only plays a minor role in primitive erythropoiesis. Morpholino knockdown experiments have shown a role for zebrafish klf4 in primitive erythropoiesis and hatching gland formation. In order to generate a global understanding of how klf4 might influence gene expression and differentiation, we have performed expression profiling of klf4 morphants, and then performed validation of many putative target genes by qRT-PCR and whole mount in situ hybridization. We found a critical role for klf4 in embryonic globin, heme synthesis and hatching gland gene expression. In contrast, there was an increase in expression of definitive hematopoietic specific genes such as larval globin genes, runx1 and c-myb from 24 hpf, suggesting a selective role for klf4 in primitive rather than definitive erythropoiesis. In addition, we show klf4 preferentially binds CACCC box elements in the primitive zebrafish beta-like globin gene promoters. These results have global implications for primitive erythroid gene regulation by KLF-CACCC box interactions.

Erythroid Kruppel-like Factor Regulates the G1 Cyclin Dependent Kinase Inhibitor p18INK4c

Erythroid Kruppel-like factor (EKLF, KLF1) is an essential erythroid cell specific C(2)H(2) zinc finger transcription factor that binds CACC box elements in promoters and distant regulatory elements to activate transcription. Forced expression of EKLF arrests cell division. The cyclin dependent kinase (Cdk) inhibitor p18(INK4c) was identified as a potential novel EKLF target gene from an expression profiling study. The p18(INK4c) protein functions as an inhibitor of Cdk4 and Cdk6 activity during early G1 phase of the cell cycle, thus acting as a physiological brake on cell division. We confirmed p18(INK4c) is downregulated in EKLF null mice by real-time PCR and Western blotting, and identified three closely associated and highly conserved EKLF binding sites (CCNCNCCCN) approximately 1 kb upstream of the p18(INK4c) transcriptional start site. We showed that EKLF binds to one of these elements by gel shift assay and demonstrated this site is capable of driving EKLF dependent transcription. We also determined by chromatin immunoprecipitation (ChIP) that this region of the p18(INK4c) promoter is bound by EKLF in erythroid cells. Thus, EKLF is a direct regulator of p18(INK4c) gene expression, and much of EKLF's role in driving erythroid cell differentiation may occur via p18(INK4c).

Genomic organisation and regulation of murine alpha haemoglobin stabilising protein by erythroid Kruppel-like factor

Alpha haemoglobin stabilising protein (AHSP) binds free alpha-globin chains and plays an important role in the protection of red cells, particularly during beta-thalassaemia. Murine ASHP was discovered as a GATA-1 target gene and human AHSP is directly regulated by GATA-1. More recently, AHSP was rediscovered as a highly erythroid Kruppel-like factor (EKLF) -dependent transcript. We have determined the organisation of the murine AHSP gene and compared it to orthologs. There are two CACC box elements in the proximal promoter. The proximal element is absolutely conserved, but does not bind EKLF as it is not a canonical binding site. In rodents, the distal element contains a 3 bp insertion that disrupts the typical EKLF binding consensus region. Nevertheless, EKLF binds this atypical site by gel mobility shift assay, specifically occupies the AHSP promoter in vivo in a chromatin immunoprecipitation assay, and transactivates AHSP through this CACC site in promoter-reporter assays. Our results suggest EKLF can occupy CACC elements in vivo that are not predictable from the consensus binding site inferred from structural studies. We also propose that absence of AHSP in EKLF-null red cells exacerbates the toxicity of free alpha-globin chains, which exist because of the defect in beta-globin gene activation.

A global role for EKLF in definitive and primitive erythropoiesis

Erythroid Kruppel-like factor (EKLF, KLF1) plays an important role in definitive erythropoiesis and beta-globin gene regulation but failure to rectify lethal fetal anemia upon correction of globin chain imbalance suggested additional critical EKLF target genes. We employed expression profiling of EKLF-null fetal liver and EKLF-null erythroid cell lines containing an inducible EKLF-estrogen receptor (EKLF-ER) fusion construct to search for such targets. An overlapping list of EKLF-regulated genes from the 2 systems included alpha-hemoglobin stabilizing protein (AHSP), cytoskeletal proteins, hemesynthesis enzymes, transcription factors, and blood group antigens. One EKLF target gene, dematin, which encodes an erythrocyte cytoskeletal protein (band 4.9), contains several phylogenetically conserved consensus CACC motifs predicted to bind EKLF. Chromatin immunoprecipitation demonstrated in vivo EKLF occupancy at these sites and promoter reporter assays showed that EKLF activates gene transcription through these DNA elements. Furthermore, investigation of EKLF target genes in the yolk sac led to the discovery of unexpected additional defects in the embryonic red cell membrane and cytoskeleton. In short, EKLF regulates global erythroid gene expression that is critical for the development of primitive and definitive red cells.