HMGA2 Moderately Increases Fetal Hemoglobin Expression in Human Adult Erythroblasts

let-7 miRNAs repress HIC2 to regulate BCL11A transcription and hemoglobin switching

ABSTRACT

The switch from fetal hemoglobin (γ-globin, HBG) to adult hemoglobin (β-globin, HBB) gene transcription in erythroid cells serves as a paradigm for a complex and clinically relevant developmental gene regulatory program. We previously identified HIC2 as a regulator of the switch by inhibiting the transcription of BCL11A, a key repressor of HBG production. HIC2 is highly expressed in fetal cells, but the mechanism of its regulation is unclear. Here we report that HIC2 developmental expression is controlled by microRNAs (miRNAs), as loss of global miRNA biogenesis through DICER1 depletion leads to upregulation of HIC2 and HBG messenger RNA. We identified the adult-expressed let-7 miRNA family as a direct posttranscriptional regulator of HIC2. Ectopic expression of let-7 in fetal cells lowered HIC2 levels, whereas inhibition of let-7 in adult erythroblasts increased HIC2 production, culminating in decommissioning of a BCL11A erythroid enhancer and reduced BCL11A transcription. HIC2 depletion in let-7-inhibited cells restored BCL11A-mediated repression of HBG. Together, these data establish that fetal hemoglobin silencing in adult erythroid cells is under the control of a miRNA-mediated inhibitory pathway (let-7 ⊣ HIC2 ⊣ BCL11A ⊣ HBG).

Comparison of transcriptome profiles of nucleated red blood cells in cord blood between preterm and full-term neonates

BACKGROUND

The reactivation of fetal γ-globin expression is an effective strategy for ameliorating the clinical symptoms of β-hemoglobinopathies. However, the mechanism of globin switching, especially the roles of long non-coding RNAs (lncRNAs) in this process, remains elusive.

METHODS

We compared the in vivo transcriptome profiles of nucleated red blood cells (NRBCs) isolated from the umbilical cord blood of preterm and full-term newborns. We collected 75 umbilical cord blood samples and performed qPCR of the candidate genes.

RESULTS

In this study, we identified 7,166 differentially expressed protein-coding genes, 3,243 differentially expressed lncRNAs, and 79 differentially expressed microRNAs. Our data show that the Fanconi anemia pathway and the H19/let-7/LIN28B axis may be involved in γ- to β-globin gene switching. Moreover, we constructed the hub gene network of the differentially expressed transcription factors. Based on qPCR, we found that BCL11A was differentially expressed based on biological sex. We also confirmed that H19 is differentially expressed and established the H19-related network to reveal the potential regulatory mechanisms.

CONCLUSION

We present the profiles of the in vivo transcriptome differences of NRBCs between preterm and full-term neonates for the first time, and provide novel research targets for β-hemoglobinopathies.

Erythrocyte microRNAs: a tiny magic bullet with great potential for sickle cell disease therapy

Sickle cell disease (SCD) is a severe hereditary blood disorder caused by a mutation of the beta-globin gene, which results in a substantial reduction in life expectancy. Many studies are focused on various novel therapeutic strategies that include re-activation of the γ-globin gene. Among them, expression therapy caused by the fetal hemoglobin (HbF) at a later age is highly successful. The induction of HbF is one of the dominant genetic modulators of the hematological and clinical characteristics of SCD. In fact, HbF compensates for the abnormal beta chain and has an ameliorant effect on clinical complications. Erythropoiesis is a multi-step process that involves the proliferation and differentiation of a small population of hematopoietic stem cells and is affected by several factors, including signaling pathways, transcription factors, and small non-coding RNAs (miRNAs). miRNAs play a regulatory role through complex networks that control several epigenetic mechanisms as well as the post-transcriptional regulation of multiple genes. In this review, we briefly describe the current understanding of interactions between miRNAs, their molecular targets, and their regulatory effects in HbF induction in SCD.

HMGA Proteins in Hematological Malignancies

The high mobility group AT-Hook (HMGA) proteins are a family of nonhistone chromatin remodeling proteins known as "architectural transcriptional factors". By binding the minor groove of AT-rich DNA sequences, they interact with the transcription apparatus, altering the chromatin modeling and regulating gene expression by either enhancing or suppressing the binding of the more usual transcriptional activators and repressors, although they do not themselves have any transcriptional activity. Their involvement in both benign and malignant neoplasias is well-known and supported by a large volume of studies. In this review, we focus on the role of the HMGA proteins in hematological malignancies, exploring the mechanisms through which they enhance neoplastic transformation and how this knowledge could be exploited to devise tailored therapeutic strategies.

Emerging role of ZBTB7A as an oncogenic driver and transcriptional repressor

ZBTB7A is a member of the POK family of transcription factors that possesses a POZ-domain at the N-terminus and Krüppel-like zinc-finger at the c-terminus. ZBTB7A was initially isolated as a protein that binds to the inducer of the short transcript of HIV-1 virus TAT gene promoter. The protein forms a homodimer through protein-protein interaction via the N-terminus POZ-domains. ZBTB7A typically binds to the DNA elements through its zinc-finger domains and represses transcription both by modification of the chromatin organization and through the direct recruitment of transcription factors to gene regulatory regions. ZBTB7A is involved in several fundamental biological processes including cell proliferation, differentiation, and development. It also participates in hematopoiesis, adipogenesis, chondrogenesis, cellular metabolism and alternative splicing of BCLXL, DNA repair, development of oligodendrocytes, osteoclast and unfolded protein response. Aberrant ZBTB7A expression promotes oncogenic transformation and tumor progression, but also maintains a tumor suppressive role depending on the type and genetic context of cancer. In this comprehensive review we provide information about the structure, function, targets, and regulators of ZBTB7A and its role as an oncogenic driver and transcriptional repressor in various human diseases.

BCL11A Down-Regulation Induces γ-Globin in Human β-Thalassemia Major Erythroid Cells

Fetal hemoglobin (Hb F, α2γ2) is a potent genetic modifier of the severity of β-thalassemia (β-thal) and sickle cell anemia. Differences in the levels of HbF that persist into adulthood affect the severity of sickle cell disease and the β-thal syndromes. B-cell lymphoma 11 A (BCL11A) is a potent silencer of HbF. Here, we reactivated γ-globin expression by down-regulating BCL11A to alleviate anemia in the β-thal major (β-TM) patients. BCL11A were down-regulated by lentiviral RNAi (RNA interference) in the K562 cell line and an in vitro culture model of human erythropoiesis in which erythroblasts are derived from the normal donor mononuclear cells (MNC) or β-TM MNC. The expression of γ-globin were analyzed by qPCR (quantitative real-time polymerase chain reaction) and Western blot techniques. Our data showed that down-regulation of BCL11A induces γ-globin production in the K562 cell line and human erythrocytes from normal donors and β-TM donors, without altering erythroid maturation. This is the first report on γ-globin induction by down-regulation of BCL11A in human erythroblasts derived from β-TM.

Recent progress in understanding and manipulating haemoglobin switching for the haemoglobinopathies

The major β-haemoglobinopathies, sickle cell disease and β-thalassaemia, represent the most common monogenic disorders worldwide and a steadily increasing global disease burden. Allogeneic haematopoietic stem cell transplantation, the only curative therapy, is only applied to a small minority of patients. Common clinical management strategies act mainly downstream of the root causes of disease. The observation that elevated fetal haemoglobin expression ameliorates these disorders has motivated longstanding investigations into the mechanisms of haemoglobin switching. Landmark studies over the last decade have led to the identification of two potent transcriptional repressors of γ-globin, BCL11A and ZBTB7A. These regulators act with additional trans-acting epigenetic repressive complexes, lineage-defining factors and developmental programs to silence fetal haemoglobin by working on cis-acting sequences at the globin gene loci. Rapidly advancing genetic technology is enabling researchers to probe deeply the interplay between the molecular players required for γ-globin (HBG1/HBG2) silencing. Gene therapies may enable permanent cures with autologous modified haematopoietic stem cells that generate persistent fetal haemoglobin expression. Ultimately rational small molecule pharmacotherapies to reactivate HbF could extend benefits widely to patients.

Tough decoy targeting of predominant let-7 miRNA species in adult human hematopoietic cells

Background

In humans, the heterochronic cascade composed of the RNA-binding protein LIN28 and its major target, the let-7 family of microRNAs (miRNAs), is highly regulated during human erythroid ontogeny. Additionally, down-regulation of the let-7 miRNAs in cultured adult CD34(+) cells or the over-expression of LIN28 in cultured erythrocytes from pediatric patients with HbSS genotype causes increased levels of fetal hemoglobin (HbF) in the range of 19–40% of the total. Therefore, we hypothesized that focused targeting of individual let-7 miRNA family members would exhibit regulatory effect on HbF expression in human adult erythroblasts.

Methods

The expression levels of mature let-7 family members were measured by RT-qPCR in purified cell populations sorted from peripheral blood. To study the effects of let-7 miRNAs upon globin expression, a lentiviral construct that incorporated the tough decoy (TuD) design to target let-7a or let-7b was compared with empty vector controls. Transductions were performed in CD34(+) cells from adult healthy volunteers cultivated ex vivo in erythropoietin-supplemented serum-free media for 21 days. Downstream analyses included RT-qPCR, Western blot and HPLC for the characterization of adult and fetal hemoglobins.

Results

The expression of individual let-7 miRNA family members in adult peripheral blood cell populations demonstrated that let-7a and let-7b miRNAs are expressed at much higher levels than the other let-7 family members in purified adult human blood cell subsets with expression being predominantly in reticulocytes. Therefore, we focused this study upon the targeted inhibition of let-7a and let-7b with the TuD design to explore its effects upon developmentally-timed erythroid genes. Let-7a-TuD transductions significantly increased gamma-globin mRNA expression and HbF to an average of 38%. Let-7a-TuD also significantly decreased the mRNA expression of some ontogeny-regulated erythroid genes, namely CA1 and GCNT2. In addition, the erythroid-related transcription factors BCL11A and HMGA2 were down- and up-regulated, respectively, by let-7a-TuD, while ZBTB7A, KLF1 and SOX6 remained unchanged.

Conclusions

Overall, our data demonstrate that let-7 miRNAs are differentially expressed in human hematopoietic cells, and that targeted inhibition of the highly-expressed species of this family is sufficient for developmentally-specific changes in gamma-globin expression and HbF levels.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-017-1273-x) contains supplementary material, which is available to authorized users.

IGF2BP1 overexpression causes fetal-like hemoglobin expression patterns in cultured human adult erythroblasts

Here we investigated in primary human erythroid tissues a downstream element of the heterochronic let-7 miRNA pathway, the insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1), for its potential to affect the hemoglobin profiles in human erythroblasts. Comparison of adult bone marrow to fetal liver lysates demonstrated developmental silencing in IGF2BP1. Erythroid-specific overexpression of IGF2BP1 caused a nearly complete and pancellular reversal of the adult pattern of hemoglobin expression toward a more fetal-like phenotype. The reprogramming of hemoglobin expression was achieved at the transcriptional level by increased gamma-globin combined with decreased beta-globin transcripts resulting in gamma-globin rising to 90% of total beta-like mRNA. Delta-globin mRNA was reduced to barely detectable levels. Alpha-globin levels were not significantly changed. Fetal hemoglobin achieved levels of 68.6 ± 3.9% in the IGF2BP1 overexpression samples compared with 5.0 ± 1.8% in donor matched transduction controls. In part, these changes were mediated by reduced protein expression of the transcription factor BCL11A. mRNA stability and polysome studies suggest IGF2BP1 mediates posttranscriptional loss of BCL11A. These results suggest a mechanism for chronoregulation of fetal and adult hemoglobin expression in humans.