Disruption of the Hbs1l-Myb locus causes hereditary persistence of fetal hemoglobin in a mouse model

Genetic Modifiers of Hemoglobin Expression from a Clinical Perspective in Hemoglobinopathy Patients with Beta Thalassemia and Sickle Cell Disease

Hemoglobinopathies, namely β-thalassemia and sickle cell disease (SCD), are hereditary diseases, characterized by molecular genetic aberrations in the beta chains of hemoglobin. These defects affect the normal production of hemoglobin with severe anemia due to less or no amount of beta globins in patients with β-thalassemia (quantitative disorder), while SCD is a serious disease in which a mutated form of hemoglobin distorts the red blood cells into a crescent shape at low oxygen levels (qualitative disorder). Despite the revolutionary progress in recent years with the approval of gene therapy and gene editing for specific patients, there is an unmet need for highlighting the mechanisms influencing hemoglobin production and for the development of novel drugs and targeted therapies. The identification of the transcription factors and other genetic modifiers of hemoglobin expression is of utmost importance for discovering novel therapeutic approaches for patients with hemoglobinopathies. The aim of this review is to describe these complex molecular mechanisms and pathways affecting hemoglobin expression and to highlight the relevant investigational approaches or pharmaceutical interventions focusing on restoring the hemoglobin normal function by linking the molecular background of the disease with the clinical perspective. All the associated drugs increasing the hemoglobin expression in patients with hemoglobinopathies, along with gene therapy and gene editing, are also discussed.

Fount, fate, features, and function of renal erythropoietin-producing cells

Renal erythropoietin (Epo)-producing (REP) cells represent a rare and incompletely understood cell type. REP cells are fibroblast-like cells located in close proximity to blood vessels and tubules of the corticomedullary border region. Epo mRNA in REP cells is produced in a pronounced "on-off" mode, showing transient transcriptional bursts upon exposure to hypoxia. In contrast to "ordinary" fibroblasts, REP cells do not proliferate ex vivo, cease to produce Epo, and lose their identity following immortalization and prolonged in vitro culture, consistent with the loss of Epo production following REP cell proliferation during tissue remodelling in chronic kidney disease. Because Epo protein is usually not detectable in kidney tissue, and Epo mRNA is only transiently induced under hypoxic conditions, transgenic mouse models have been developed to permanently label REP cell precursors, active Epo producers, and inactive descendants. Future single-cell analyses of the renal stromal compartment will identify novel characteristic markers of tagged REP cells, which will provide novel insights into the regulation of Epo expression in this unique cell type.

Abnormal regulation of microRNAs and related genes in pediatric β-thalassemia

Background

MicroRNAs (miRNAs) participate in the reactivation of γ‐globin expression in β‐thalassemia. However, the miRNA transcriptional profiles of pediatric β‐thalassemia remain unclear. Accordingly, in this study, we assessed miRNA expression in pediatric patients with β‐thalassemia.

Methods

Differentially expressed miRNAs in pediatric patients with β‐thalassemia were determined using microRNA sequencing.

Results

Hsa‐miR‐483‐3p, hsa‐let‐7f‐1‐3p, hsa‐let‐7a‐3p, hsa‐miR‐543, hsa‐miR‐433‐3p, hsa‐miR‐4435, hsa‐miR‐329‐3p, hsa‐miR‐92b‐5p, hsa‐miR‐6747‐3p and hsa‐miR‐495‐3p were significantly upregulated, whereas hsa‐miR‐4508, hsa‐miR‐20a‐5p, hsa‐let‐7b‐5p, hsa‐miR‐93‐5p, hsa‐let‐7i‐5p, hsa‐miR‐6501‐5p, hsa‐miR‐221‐3p, hsa‐let‐7g‐5p, hsa‐miR‐106a‐5p, and hsa‐miR‐17‐5p were significantly downregulated in pediatric patients with β‐thalassemia. After integrating our data with a previously published dataset, we found that hsa‐let‐7b‐5p and hsa‐let‐7i‐5p expression levels were also lower in adolescent or adult patients with β‐thalassemia. The predicted target genes of hsa‐let‐7b‐5p and hsa‐let‐7i‐5p were associated with the transforming growth factor β receptor, phosphatidylinositol 3‐kinase/AKT, FoxO, Hippo, and mitogen‐activated protein kinase signaling pathways. We also identified 12 target genes of hsa‐let‐7a‐3p and hsa‐let‐7f‐1‐3p and 21 target genes of hsa‐let‐7a‐3p and hsa‐let‐7f‐1‐3p, which were differentially expressed in patients with β‐thalassemia. Finally, we found that hsa‐miR‐190‐5p and hsa‐miR‐1278‐5p may regulate hemoglobin switching by modulation of the B‐cell lymphoma/leukemia 11A gene.

Conclusion

The results of the study show that several microRNAs are dysregulated in pediatric β‐thalassemia. Further, the results also indicate toward a critical role of let7 miRNAs in the pathogenesis of pediatric β‐thalassemia, which needs to be investigated further.

GATA zinc finger domain-containing protein 2A (GATAD2A) deficiency reactivates fetal haemoglobin in patients with β-thalassaemia through impaired formation of methyl-binding domain protein 2 (MBD2)-containing nucleosome remodelling and deacetylation (NuRD) complex

Reactivation of fetal haemoglobin (HbF) expression is an effective way to treat β-thalassaemia and sickle cell anaemia. In the present study, we identified a novel GATA zinc finger domain-containing protein 2A (GATAD2A) mutation, which contributed to the elevation of HbF and ameliorated clinical severity in a patient with β-thalassaemia, by targeted next-generation sequencing. Knockout of GATAD2A led to a significant induction of HbF in both human umbilical cord blood-derived erythroid progenitor-2 (HUDEP-2) and human cluster of differentiation (CD)34⁺ cells with a detectable impact on erythroid differentiation. Furthermore, heterozygous knockout of GATAD2A impaired recruitment of chromodomain helicase DNA-binding protein 4 (CHD4) to the methyl-binding domain protein 2 (MBD2)-containing nucleosome remodelling and deacetylation (NuRD) complex. Our present data suggest that mutations causing the haploinsufficiency of GATAD2A might contribute to amelioration of clinical severity in patients with β-thalassaemia.

Physiological and Aberrant γ-Globin Transcription During Development

The expression of the fetal Gγ- and Aγ-globin genes in normal development is confined to the fetal period, where two γ-globin chains assemble with two α-globin chains to form α₂γ₂ tetramers (HbF). HbF sustains oxygen delivery to tissues until birth, when β-globin replaces γ-globin, leading to the formation of α₂β₂ tetramers (HbA). However, in different benign and pathological conditions, HbF is expressed in adult cells, as it happens in the hereditary persistence of fetal hemoglobin, in anemias and in some leukemias. The molecular basis of γ-globin differential expression in the fetus and of its inappropriate activation in adult cells is largely unknown, although in recent years, a few transcription factors involved in this process have been identified. The recent discovery that fetal cells can persist to adulthood and contribute to disease raises the possibility that postnatal γ-globin expression could, in some cases, represent the signature of the fetal cellular origin.

Long noncoding RNA HBBP1 enhances γ-globin expression through the ETS transcription factor ELK1

β-Thalassemia is an autosomal recessive genetic disease caused by defects in the production of adult hemoglobin (HbA, α₂β₂), which leads to an imbalance between α- and non-α-globin chains. Reactivation of γ-globin expression is an effective strategy to treat β-thalassemia patients. Previously, it was demonstrated that hemoglobin subunit beta pseudogene 1 (HBBP1) is associated with elevated fetal hemoglobin (HbF, α₂γ₂) in β-thalassemia patients. However, the mechanism underlying HBBP1-mediated HbF production is unknown. In this study, using bioinformatics analysis, we found that HBBP1 is involved in γ-globin production, and then preliminarily confirmed this finding in K562 cells. When HBBP1 was overexpressed, γ-globin expression was increased at the transcript and protein levels in HUDEP-2 cells. Next, we found that ETS transcription factor ELK1 (ELK1) binds to the HBBP1 proximal promoter and significantly promotes its activity. Moreover, the synthesis of γ-globin was enhanced when ELK1 was overexpressed in HUDEP-2 cells. Surprisingly, ELK1 also directly bound to and activated the γ-globin proximal promoter. Furthermore, we found that HBBP1 and ELK1 can interact with each other in HUDEP-2 cells. Collectively, these findings suggest that HBBP1 can induce γ-globin by enhancing ELK1 expression, providing some clues for γ-globin reactivation in β-thalassemia.

Association Between Genetic Polymorphisms and Hb F Levels in Heterozygous β-Thalassemia 3.5 kb Deletions

Single nucleotide polymorphisms (SNPs) in several genetic modifying factors have been related to Hb F levels, including ^Gγ XmnI polymorphism, B-cell lymphoma/leukemia 11 A (BCL11A), HBS1L-MYB intergenic polymorphism (HMIP) and a mutation in the Krüppel-like factor 1 (KLF1). This study aimed to determine whether genetic variability of these modifying factors affects Hb F levels in heterozygous β-thalassemia (β-thal) 3.5 kb deletion (NC_000011.10: g.5224302-5227791del13490bp). A total of 111 β-thal 3.5 kb deletion carriers with Hb F levels ranging from 0.9 to 18.4% was recruited for this study. Genotyping of SNPs including HBG2 rs7482144, HMIP rs4895441 and rs9399137, BCL11A rs4671393 and KLF1 rs2072596 was identified. Multiple regression analyses showed that only two SNPs (HMIP rs4895441 and rs9399137) influenced Hb F levels. Interestingly, a combination of these two SNPs was associated with higher Hb F levels. Our study is the first to demonstrate that the rs4895441, rs9399137 of HMIP are associated with elevated Hb F levels in the heterozygous β-thal 3.5 kb deletion.

Small molecule therapeutics to treat the β-globinopathies

PURPOSE OF REVIEW

The current review focuses on recent insights into the development of small molecule therapeutics to treat the β-globinopathies.

RECENT FINDINGS

Recent studies of fetal γ-globin gene regulation reveal multiple insights into how γ-globin gene reactivation may lead to novel treatment for β-globinopathies.

SUMMARY

We summarize current information regarding the binding of transcription factors that appear to be impeded or augmented by different hereditary persistence of fetal hemoglobin (HPFH) mutations. As transcription factors have historically proven to be difficult to target for therapeutic purposes, we next address the contributions of protein complexes associated with these HPFH mutation-affected transcription factors with the aim of defining proteins that might provide additional targets for chemical molecules to inactivate the corepressors. Among the enzymes associated with the transcription factor complexes, a group of corepressors with currently available inhibitors were initially thought to be good candidates for potential therapeutic purposes. We discuss possibilities for pharmacological inhibition of these corepressor enzymes that might significantly reactivate fetal γ-globin gene expression. Finally, we summarize the current clinical trial data regarding the inhibition of select corepressor proteins for the treatment of sickle cell disease and β-thalassemia.

Surface plasmon resonance based analysis of the binding of LYAR protein to the rs368698783 (G>A) polymorphic Aγ-globin gene sequences mutated in β-thalassemia

Recent studies have identified and characterized a novel putative transcriptional repressor site in a 5' untranslated region of the Aγ-globin gene that interacts with the Ly-1 antibody reactive clone (LYAR) protein. LYAR binds the 5'-GGTTAT-3' site of the Aγ-globin gene, and this molecular interaction causes repression of gene transcription. In β-thalassemia patients, a polymorphism has been demonstrated (the rs368698783 G>A polymorphism) within the 5'-GGTTAT-3' LYAR-binding site of the Aγ-globin gene. The major results gathered from surface plasmon resonance based biospecific interaction analysis (SPR-BIA) studies (using crude nuclear extracts, LYAR-enriched lysates, and recombinant LYAR) support the concept that the rs368698783 G>A polymorphism of the Aγ-globin gene attenuates the efficiency of LYAR binding to the LYAR-binding site. This conclusion was fully confirmed by a molecular docking analysis. This might lead to a very important difference in erythroid cells from β-thalassemia patients in respect to basal and induced levels of production of fetal hemoglobin. The novelty of the reported SPR-BIA method is that it allows the characterization and validation of the altered binding of a key nuclear factor (LYAR) to mutated LYAR-binding sites. These results, in addition to theoretical implications, should be considered of interest in applied pharmacology studies as a basis for the screening of drugs able to inhibit LYAR-DNA interactions. This might lead to the identification of molecules facilitating induced increase of γ-globin gene expression and fetal hemoglobin production in erythroid cells, which is associated with possible reduction of the clinical severity of the β-thalassemia phenotype. Graphical abstract.

Study on the Role of Polymorphisms of the SOX-6 and MYB Genes and Fetal Hemoglobin Levels in Sicilian Patients with β-Thalassemia and Sickle Cell Disease

The hemoglobinopathies, as β-thalassemia (β-thal) and sickle cell disease, are the most common hereditary hemolytic anemias. The increase of fetal hemoglobin (Hb F) levels can ameliorate the symptoms of hemoglobinopathies. There are several transcription factors such as MYB and SOX-6, which are involved in the regulation of Hb F. There are not enough studies investigating the association between single nucleotide polymorphisms (SNPs) of the SOX-6 and MYB genes and the variation of Hb F levels in patients affected by sickle cell disease and β-thal. We therefore decided to analyze the role of four missense variants of MYB and SOX-6 genes in the regulation of Hb F levels. In order to do so, we examinated 30 Sicilian patients affected by sickle cell disease and β-thal, to understand if these variants could also have an influence in our populations. Comparing two groups of patients with low and high levels of Hb F, we found no significant differences in the genetic distribution and allelic frequency of MYB and SOX-6 gene polymorphisms. We also created and compared a 'high producer' and 'low producer' genotype with different genes achieving the same result of no significant difference. Our results may be due either to the fact that the association between these genes and the regulation of Hb F levels are influenced by environmental history and population genetics, or to the small number of samples being analyzed.

MYB - A regulatory factor in hematopoiesis

MYB is a transcription factor which was identified in birds as a viral oncogene (v-MYB). Its cellular counterpart was subsequently isolated as c-MYB which has three functional domains - DNA binding domain, transactivation domain and negative regulatory domain. c-MYB is essential for survival, and deletion of both alleles of the gene results in embryonic death. It is highly expressed in hematopoietic cells, thymus and neural tissue, and required for T and B lymphocyte development and erythroid maturation. Additionally, aberrant MYB expression has been found in numerous solid cancer cells and human leukemia. Recent studies have also implicated c-MYB in the regulation of expression of fetal hemoglobin which is highly beneficial to the β-hemoglobinopathies (beta thalassemia and sickle cell disease). These findings suggest that MYB could be a potential therapeutic target in leukemia, and possibly also a target for therapeutic increase of fetal hemoglobin in the β-hemoglobinopathies.

Impact of Genetic Determinants of HbA1c on Type 2 Diabetes Risk and Diagnosis

PURPOSE OF REVIEW

Glycated hemoglobin (A1c) is used to diagnose type 2 diabetes and monitor glycemic control. Specific genetic variants interfere with A1c and effects/frequencies of some variants vary by ancestry. In this review, we summarize findings from large trans-ethnic meta-analyses of genome-wide association studies (GWAS) of A1c and describe some variants influencing erythrocyte biology and interfering with A1c.

RECENT FINDINGS

Recent GWAS meta-analyses have revealed 60 loci associated with A1c in multi-ethnic populations. The main A1c genetic driver in African Americans is rs1050828 (G6PD). Some identified loci are located in/near genes known as monogenic causes of erythrocytic disorders (ANK1, SPTA1) or iron disorders (TMPRSS6, HFE). Uncommon genetic variants (not revealed by GWAS) that are known to cause hemoglobinopathies may also influence A1C levels, partly by interfering with laboratory assays. Specific genetic variants that have a large impact on A1c levels may influence clinical practice, especially in individuals of African descent. Efforts to reveal novel A1c loci should focus on increasing representation of GWAS in non-European ancestries, and on using better genome-wide coverage of uncommon variants that are specific to each population.

Ribosomopathies: There's strength in numbers

Ribosomopathies are a group of human disorders most commonly caused by ribosomal protein haploinsufficiency or defects in ribosome biogenesis. These conditions manifest themselves as physiological defects in specific cell and tissue types. We review current molecular models to explain ribosomopathies and attempt to reconcile the tissue specificity of these disorders with the ubiquitous requirement for ribosomes in all cells. Ribosomopathies as a group are diverse in their origins and clinical manifestations; we use the well-described Diamond-Blackfan anemia (DBA) as a specific example to highlight some common features. We discuss ribosome homeostasis as an overarching principle that governs the sensitivity of specific cells and tissue types to ribosomal protein mutations. Mathematical models and experimental insights rationalize how even subtle shifts in the availability of ribosomes, such as those created by ribosome haploinsufficiency, can drive messenger RNA-specific effects on protein expression. We discuss recently identified roles played by ribosome rescue and recycling factors in regulating ribosome homeostasis.

A long noncoding RNA from the HBS1L-MYB intergenic region on chr6q23 regulates human fetal hemoglobin expression

The HBS1L-MYB intergenic region (chr6q23) regulates erythroid cell proliferation, maturation, and fetal hemoglobin (HbF) expression. An enhancer element within this locus, highlighted by a 3-bp deletion polymorphism (rs66650371), is known to interact with the promoter of the neighboring gene, MYB, to increase its expression, thereby regulating HbF production. RNA polymerase II binding and a 50-bp transcript from this enhancer region reported in ENCODE datasets suggested the presence of a long noncoding RNA (lncRNA). We characterized a novel 1283bp transcript (HMI-LNCRNA; chr6:135,096,362-135,097,644; hg38) that was transcribed from the enhancer region of MYB. Within erythroid cells, HMI-LNCRNA was almost exclusively present in nucleus, and was much less abundant than the mRNA for MYB. HMI-LNCRNA expression was significantly higher in erythroblasts derived from cultured adult peripheral blood CD34+ cells which expressed more HBB, compared to erythroblasts from cultured cord blood CD34+ cells which expressed much more HBG. Down-regulation of HMI-LNCRNA in HUDEP-2 cells, which expressed mostly HBB, significantly upregulated HBG expression both at the mRNA (200-fold) and protein levels, and promoted erythroid maturation. No change was found in the expression of BCL11A and other key transcription factors known to modulate HBG expression. HMI-LNCRNA plays an important role in regulating HBG expression, and its downregulation can result in a significant increase in HbF. HMI-LNCRNA might be a potential therapeutic target for HbF induction treatment in sickle cell disease and β-thalassemia.

Multiple Testing in the Context of Gene Discovery in Sickle Cell Disease Using Genome-Wide Association Studies

The issue of multiple testing, also termed multiplicity, is ubiquitous in studies where multiple hypotheses are tested simultaneously. Genome-wide association study (GWAS), a type of genetic association study that has gained popularity in the past decade, is most susceptible to the issue of multiple testing. Different methodologies have been employed to address the issue of multiple testing in GWAS. The purpose of the review is to examine the methodologies employed in dealing with multiple testing in the context of gene discovery using GWAS in sickle cell disease complications.

Molecular basis of β thalassemia and potential therapeutic targets

The remarkable phenotypic diversity of β thalassemia that range from severe anemia and transfusion-dependency, to a clinically asymptomatic state exemplifies how a spectrum of disease severity can be generated in single gene disorders. While the genetic basis for β thalassemia, and how severity of the anemia could be modified at different levels of its pathophysiology have been well documented, therapy remains largely supportive with bone marrow transplant being the only cure. Identification of the genetic variants modifying fetal hemoglobin (HbF) production in combination with α globin genotype provide some prediction of disease severity for β thalassemia but generation of a personalized genetic risk score to inform prognosis and guide management requires a larger panel of genetic modifiers yet to be discovered. Nonetheless, genetic studies have been successful in characterizing the key variants and pathways involved in HbF regulation, providing new therapeutic targets for HbF reactivation. BCL11A has been established as a quantitative repressor, and progress has been made in manipulating its expression using genomic and gene-editing approaches for therapeutic benefits. Recent discoveries and understanding in the mechanisms associated with ineffective and abnormal erythropoiesis have also provided additional therapeutic targets, a couple of which are currently being tested in clinical trials.

Genetic Variants at BCL11A and HBS1L-MYB loci Influence Hb F Levels in Chinese Zhuang β-Thalassemia Intermedia Patients

Increased Hb F levels can ameliorate the symptoms of β-thalassemia (β-thal). Due to the genetic heterogenicity of β-thal, the relationship between genetic variants in modifier genes and Hb F level has been studied in different populations. The Chinese Zhuang has the second largest population in China and has 6.78% prevalence of β-thal. However, the effects of these single nucleotide polymorphism (SNP) variants on the Hb F levels of β-thal intermedia (β-TI) patients in this population have not been reported. To explore the association between modifier loci (β-globin gene cluster, HBS1L-MYB intergenic region and BCL11A) and Hb F levels in Chinese Zhuang β-TI patients, 96 unrelated β-TI patients (50 males and 46 females) with different Hb F levels were recruited and genotyped by mass spectrometry. A total of 13 SNPs were confirmed to be in a significant relationship with Hb F levels in this population. Of these, high-risk genotypes of six Hb F-associated SNPs, rs9376090, rs7776054, rs9399137, rs9389268, rs9402685 in the HBS1L-MYB intergenic region and rs189984760 in the BCL11A locus, showed association with high Hb F levels, especially for SNPs in linkage disequilibrium. One novel Hb F-associated SNP, rs189984760, was identified in our study. Our findings will be of valuable reference for correlation between modifier genes and Hb F in Chinese Zhuang populations and may lead to better understand the modifying mechanisms for β-thal.

Genetic Basis and Genetic Modifiers of β-Thalassemia and Sickle Cell Disease

β-thalassemia and sickle cell disease (SCD) are prototypical Mendelian single gene disorders, both caused by mutations affecting the adult β-globin gene. Despite the apparent genetic simplicity, both disorders display a remarkable spectrum of phenotypic severity and share two major genetic modifiers-α-globin genotype and innate ability to produce fetal hemoglobin (HbF, α₂γ₂).This article provides an overview of the genetic basis for SCD and β-thalassemia, and genetic modifiers identified through phenotype correlation studies. Identification of the genetic variants modifying HbF production in combination with α-globin genotype provide some prediction of disease severity for β-thalassemia and SCD but generation of a personalized genetic risk score to inform prognosis and guide management requires a larger panel of genetic modifiers yet to be discovered.Nonetheless, genetic studies have been successful in characterizing some of the key variants and pathways involved in HbF regulation, providing new therapeutic targets for HbF reactivation.

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.

Dissection of the Major Hematopoietic Quantitative Trait Locus in Chromosome 6q23.3 Identifies miR-3662 as a Player in Hematopoiesis and Acute Myeloid Leukemia

Chromosomal aberrations and multiple genome-wide association studies (GWAS) have established a major hematopoietic quantitative trait locus in chromosome 6q23.3. The locus comprises an active enhancer region, in which some of the associated SNPs alter transcription factor binding. We now identify miR-3662 as a new functional driver contributing to the associated phenotypes. The GWAS SNPs are strongly associated with higher miR-3662 expression. Genome editing of rs66650371, a three-base-pair deletion, suggests a functional link between the SNP genotype and the abundance of miR-3662. Increasing miR-3662's abundance increases colony formation in hematopoietic progenitor cells, particularly the erythroid lineage. In contrast, miR-3662 is not expressed in acute myeloid leukemia cells, and its overexpression has potent antileukemic effects in vitro and in vivo Mechanistically, miR-3662 directly targets NF-κB-mediated transcription. Thus, miR-3662 is a new player of the hematopoietic 6q23.3 locus.

SIGNIFICANCE

The characterization of miR-3662 has identified a new actor in the prominent hematopoietic quantitative trait locus in chromosome 6q23.3. The mechanistic insights into miR-3662's function may reveal novel or only partially known pathways for normal and malignant hematopoietic cell proliferation. Cancer Discov; 6(9); 1036-51. ©2016 AACR.This article is highlighted in the In This Issue feature, p. 932.

Hydroxyurea down-regulates BCL11A, KLF-1 and MYB through miRNA-mediated actions to induce γ-globin expression: implications for new therapeutic approaches of sickle cell disease

Background

The major therapeutic benefit of hydroxyurea, the only FDA-approved pharmacologic treatment for sickle cell disease (SCD), is directly related to fetal hemoglobin (HbF) production that leads to significant reduction of morbidity and mortality. However, potential adverse effects such as infertility, susceptibility to infections, or teratogenic effect have been subject of concerns. Therefore, understanding HU molecular mechanisms of action, could lead to alternative therapeutic agents to increase HbF with less toxicity. This paper investigated whether HU-induced HbF could operate through post-transcriptional miRNAs regulation of BCL11A, KLF-1 and MYB, potent negative regulators of HbF. Both ex vivo differentiated primary erythroid cells from seven unrelated individuals, and K562 cells were treated with hydroxyurea (100 μM) and changes in BCL11A, KLF-1, GATA-1, MYB, β- and γ-globin gene expression were investigated. To explore potential mechanisms of post-transcriptional regulation, changes in expression of seven targeted miRNAs, previously associated with basal γ-globin expression were examined using miScript primer assays. In addition, K562 cells were transfected with miScript miRNA inhibitors/anti-miRNAs followed by Western Blot analysis to assess the effect on HbF protein levels. Direct interaction between miRNAs and the MYB 3′-untranslated region (UTR) was also investigated by a dual-luciferase reporter assays.

Results

Down-regulation of BCL11A and MYB was associated with a sevenfold increase in γ-globin expression in both primary and K562 cells (p < 0.003). Similarly, KLF-1 was down-regulated in both cell models, corresponding to the repressed expression of BCL11A and β-globin gene (p < 0.04). HU induced differential expression of all miRNAs in both cell models, particularly miR-15a, miR-16, miR-26b and miR-151-3p. An HU-induced miRNAs-mediated mechanism of HbF regulation was illustrated with the inhibition of miR-26b and -151-3p resulting in reduced HbF protein levels. There was direct interaction between miR-26b with the MYB 3′-untranslated region (UTR).

Conclusions

These experiments have shown the association between critical regulators of γ-globin expression (MYB, BCL11A and KLF-1) and specific miRNAs; in response to HU, and demonstrated a mechanism of HbF production through HU-induced miRNAs inhibition of MYB. The role of miRNAs-mediated post-transcriptional regulation of HbF provides potential targets for new treatments of SCD that may minimize alterations to the cellular transcriptome.

Electronic supplementary material

The online version of this article (doi:10.1186/s40169-016-0092-7) contains supplementary material, which is available to authorized users.

Cell signaling pathways involved in drug-mediated fetal hemoglobin induction: Strategies to treat sickle cell disease

The developmental regulation of globin gene expression has shaped research efforts to establish therapeutic modalities for individuals affected with sickle cell disease and β-thalassemia. Fetal hemoglobin has been shown to block sickle hemoglobin S polymerization to improve symptoms of sickle cell disease; moreover, fetal hemoglobin functions to replace inadequate hemoglobin A synthesis in β-thalassemia thus serving as an effective therapeutic target. In the perinatal period, fetal hemoglobin is synthesized at high levels followed by a decline to adult levels by one year of age. It is known that naturally occurring mutations in the γ-globin gene promoters and distant cis-acting transcription factors produce persistent fetal hemoglobin synthesis after birth to ameliorate clinical symptoms. Major repressor proteins that silence γ-globin during development have been targeted for gene therapy in β-hemoglobinopathies patients. In parallel effort, several classes of pharmacological agents that induce fetal hemoglobin expression through molecular and cell signaling mechanisms have been identified. Herein, we reviewed the progress made in the discovery of signaling molecules targeted by pharmacologic agents that enhance γ-globin expression and have the potential for future drug development to treat the β-hemoglobinopathies.

An international effort to cure a global health problem: A report on the 19th Hemoglobin Switching Conference

Every 2 years since 1978, an international group of scientists, physicians, and other researchers meet to discuss the latest developments in the underlying etiology, mechanisms of action, and developmental acquisition of cellular and systemic defects exhibited and elicited by the most common inherited human disorders, the hemoglobinopathies. The 19th Hemoglobin Switching Conference, held in September 2014 at St. John's College in Oxford, once again exceeded all expectations by describing cutting edge research in cellular, molecular, developmental, and genomic advances focused on these diseases. The conference comprised about 60 short talks over 3 days by leading investigators in the field. This meeting report describes the highlights of the conference.

Variation in Gamma-Globin Expression before and after Induction with Hydroxyurea Associated with BCL11A, KLF1 and TAL1

The molecular mechanisms governing γ-globin expression in a subset of fetal hemoglobin (α2γ2: HbF) expressing red blood cells (F-cells) and the mechanisms underlying the variability of response to hydroxyurea induced γ-globin expression in the treatment of sickle cell disease are not completely understood. Here we analyzed intra-person clonal populations of basophilic erythroblasts (baso-Es) derived from bone marrow common myeloid progenitors in serum free cultures and report the level of fetal hemoglobin production in F-cells negatively correlates with expression of BCL11A, KLF1 and TAL1. We then examined the effects of hydroxyurea on these three transcription factors and conclude that a successful induction of γ-globin includes a reduction in BCL11A, KLF1 and TAL1 expression. These data suggests that expression changes in this transcription factor network modulate γ-globin expression in F-cells during steady state erythropoiesis and after induction with hydroxyurea.

Fetal globin gene repressors as drug targets for molecular therapies to treat the β-globinopathies

The human β-globin locus is comprised of embryonic, fetal, and adult globin genes that are expressed in a developmental stage-specific manner. Mutations in the globin locus give rise to the β-globinopathies, β-thalassemia and sickle cell disease, which begin to manifest symptoms around the time of birth. Although the fetal globin genes are autonomously silenced in adult-stage erythroid cells, mutations lying both within and outside the locus lead to natural variations in the level of fetal globin gene expression, and some of these significantly ameliorate the clinical symptoms of the β-globinopathies. Multiple reports have now identified several transcription factors that are involved in fetal globin gene repression in definitive (adult)-stage erythroid cells (the TR2/TR4 heterodimer, MYB, KLFs, BCL11A, and SOX6). To carry out their repression functions, chromatin-modifying enzymes (such as DNA methyltransferase, histone deacetylases, and lysine-specific histone demethylase 1) are additionally involved as a consequence of forming large macromolecular complexes with the DNA-binding subunits of these cellular machines. This review focuses on the molecular mechanisms underlying fetal globin gene silencing and possible near-future molecularly targeted therapies for treating the β-globinopathies.

Diagnosis and prevention of thalassemia

Thalassemia is the most common monogenic inherited disease worldwide and it affects most countries to various extents. This review summarizes the current approaches to phenotypic and genotypic diagnosis of thalassemia in clinical practice. Prevention strategies that encompass carrier screening, genetic counseling and prenatal diagnosis are discussed. The importance of public education and an awareness of a changing perception regarding this group of diseases are emphasized. It also addresses the impact of the rapidly increasing knowledge in disease severity modification by hemoglobin F (Hb F).

HBS1L-MYB intergenic variants modulate fetal hemoglobin via long-range MYB enhancers

Genetic studies have identified common variants within the intergenic region (HBS1L-MYB) between GTP-binding elongation factor HBS1L and myeloblastosis oncogene MYB on chromosome 6q that are associated with elevated fetal hemoglobin (HbF) levels and alterations of other clinically important human erythroid traits. It is unclear how these noncoding sequence variants affect multiple erythrocyte characteristics. Here, we determined that several HBS1L-MYB intergenic variants affect regulatory elements that are occupied by key erythroid transcription factors within this region. These elements interact with MYB, a critical regulator of erythroid development and HbF levels. We found that several HBS1L-MYB intergenic variants reduce transcription factor binding, affecting long-range interactions with MYB and MYB expression levels. These data provide a functional explanation for the genetic association of HBS1L-MYB intergenic polymorphisms with human erythroid traits and HbF levels. Our results further designate MYB as a target for therapeutic induction of HbF to ameliorate sickle cell and β-thalassemia disease severity.

Genotyping of BCL11A and HBS1L-MYB SNPs associated with fetal haemoglobin levels: a SNaPshot minisequencing approach

BACKGROUND

B-thalassaemia and sickle cell disease (SCD) are two of the most common monogenic diseases that are found in many populations worldwide. In both disorders the clinical severity is highly variable, with the persistence of fetal haemoglobin (HbF) being one of the major ameliorating factors. HbF levels are affected by, amongst other factors, single nucleotide polymorphisms (SNPs) at the BCL11A gene and the HBS1L-MYB intergenic region, which are located outside the β-globin locus. For this reason, we developed two multiplex assays that allow the genotyping of SNPs at these two genomic regions which have been shown to be associated with variable HbF levels in different populations.

RESULTS

Two multiplex assays based on the SNaPshot minisequencing approach were developed. The two assays can be used to simultaneous genotype twelve SNPs at the BCL11A gene and sixteen SNPs at HBS1L-MYB intergenic region which were shown to modify HbF levels. The different genotypes can be determined based on the position and the fluorescent colour of the peaks in a single electropherogram. DNA sequencing and restriction fragment length polymorphism (PCR-RFLP) assays were used to verify genotyping results obtained by SNaPshot minisequencing.

CONCLUSIONS

In summary, we propose two multiplex assays based on the SNaPshot minisequencing approach for the simultaneous identification of SNPs located at the BCL11A gene and HBS1L-MYB intergenic region which have an effect on HbF levels. The assays can be easily applied for accurate, time and cost efficient genotyping of the selected SNPs in various populations.

Transcriptional regulators Myb and BCL11A interplay with DNA methyltransferase 1 in developmental silencing of embryonic and fetal β-like globin genes

The clinical symptoms of hemoglobin disorders such as β-thalassemia and sickle cell anemia are significantly ameliorated by the persistent expression of γ-globin after birth. This knowledge has driven the discovery of important regulators that silence γ-globin postnatally. Improved understanding of the γ- to β-globin switching mechanism holds the key to devising targeted therapies for β-hemoglobinopathies. To further investigate this mechanism, we used the murine erythroleukemic (MEL) cell line containing an intact 183-kb human β-globin locus, in which the (G)γ- and β-globin genes are replaced by DsRed and eGFP fluorescent reporters, respectively. Following RNA interference (RNAi)-mediated knockdown of two key transcriptional regulators, Myb and BCL11A, we observed a derepression of γ-globin, measured by DsRed fluorescence and qRT-PCR (P<0.001). Interestingly, double knockdown of Myb and DNA methyltransferase 1 (DNMT1) resulted in a robust induction of ε-globin, (up to 20% of total β-like globin species) compared to single knockdowns (P<0.001). Conversely, double knockdowns of BCL11A and DNMT1 enhanced γ-globin expression (up to 90% of total β-like globin species) compared to single knockdowns (P<0.001). Moreover, following RNAi treatment, expression of human β-like globin genes mirrored the expression levels of their endogenous murine counterparts. These results demonstrate that Myb and BCL11A cooperate with DNMT1 to achieve developmental repression of embryonic and fetal β-like globin genes in the adult erythroid environment.

Genetic association studies in β-hemoglobinopathies

Characterization of the molecular basis of the β-thalassemias and sickle cell disease (SCD) clearly showed that individuals with the same β-globin genotypes can have extremely diverse clinical severity. Two key modifiers, an innate ability to produce fetal hemoglobin and coinheritance of α-thalassemia, both derived from family and population studies, affect the pathophysiology of both disorders at the primary level. In the past 2 decades, scientific research had applied genetic approaches to identify additional genetic modifiers. The review summarizes recent genetic studies and key genetic modifiers identified and traces the story of fetal hemoglobin genetics, which has led to an emerging network of globin gene regulation. The discoveries have provided insights on new targets for therapeutic intervention and raise possibilities of developing fetal hemoglobin predictive diagnostics for predicting disease severity in the newborn and for integration into prenatal diagnosis to better inform genetic counseling.