Murine erythroleukemia cell line GM979 contains factors that can activate silent chromosomal human gamma-globin genes

The state of the art of fetal hemoglobin-inducing agents

INTRODUCTION

Sickle cell anemia (SCA) is a hematological genetic disorder caused by a mutation in the gene of the β-globin. Pharmacological treatments will continue to be an important approach, including the strategy to induce fetal hemoglobin (HbF).

AREAS COVERED

Here, we analyzed the articles described in the literature regarding the drug discovery of HbF inducers. The main approaches for such strategy will be discussed, highlighting those most promising.

EXPERT OPINION

The comprehension of the mechanisms involved in the β-globin regulation is the main key to design new drugs to induce HbF. Among the strategies, gamma-globin regulation by epigenetic enzymes seems to be a promising approach to be pursued, although the comprehension of the selectivity role for those new drugs is crucial to reduce adverse effects. The low druggability of transcription factors and their vital role in embryonic human development are critical points that should be taken in account for drug design. The guanylate cyclase and the NO/cGMP signaling pathway seem to be promising not only for HbF induction, but also for the protective effects in the cardiovascular system. The association of drugs acting through different mechanisms to induce HbF seems to be promising for the discovery of new drugs.

Sox6 directly silences epsilon globin expression in definitive erythropoiesis

Sox6 is a member of the Sox transcription factor family that is defined by the conserved high mobility group (HMG) DNA binding domain, first described in the testis determining gene, Sry. Previous studies have suggested that Sox6 plays a role in the development of the central nervous system, cartilage, and muscle. In the Sox6-deficient mouse, p100H, epsilony globin is persistently expressed, and increased numbers of nucleated red cells are present in the fetal circulation. Transfection assays in GM979 (erythroleukemic) cells define a 36-base pair region of the epsilony proximal promoter that is critical for Sox6 mediated repression. Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assays demonstrate that Sox6 acts as a repressor by directly binding to the epsilony promoter. The normal expression of Sox6 in wild-type fetal liver and the ectopic expression of epsilony in p100H homozygous fetal liver demonstrate that Sox6 functions in definitive erythropoiesis. The present study shows that Sox6 is required for silencing of epsilony globin in definitive erythropoiesis and suggests a role for Sox6 in erythroid cell maturation. Thus, Sox6 regulation of epsilony globin might provide a novel therapeutical target in the treatment of hemoglobinopathies such as sickle cell anemia and thalassemia.

Spontaneous switch from Agamma- to beta-globin promoter activity in a stable transfected dual reporter vector

Here it is analyzed the expression of a mini locus dual reporter construct composed by a micro-LCR and by the promoters for (A)gamma- and beta-globin gene, each one linked to a different Luciferase, in stably transfected GM979 cells for as long as 1-2 years from transfection. The transfected GM979 cells rapidly (within 1 month) evolved into a stable population which expresses constant levels of reporters for more than a year of continuous bulk culture. No silencing of the inserted construct was observed over time. In contrast, after 1 month, the reporter activity (both from (A)gamma- and beta-promoter) expressed per cell increased over time. The analysis of the Luciferase contained in single cell clones indicated that the higher reporter activity was due to increased gene expression per cell rather than to clonal selection of the most expressing clones. Since the activity driven by the beta-promoter increased 10-fold more than that driven by the (A)gamma one, the ratio between (A)gamma-driven/((A)gamma-driven + beta-driven) reporter activity in the cells decreased after 1 month and became similar to the gamma/(gamma + beta) globin mRNA ratio expressed by adult erythroid cells. Moreover, although both cells from early and late bulk culture responded to incubation with butyric acid, a known inducer of fetal globin gene expression, by increasing the reporter activity driven by the (A)gamma-promoter, only cells from late bulk culture decreased, as normal primary erythroblasts do, the activity of the reporter driven by the beta-promoter. These results suggest that the rapid changes in activity driven by the (A)gamma- and beta-globin promoters occurring during the first month after transfection may represent a novel in vitro model to study epigenetic regulation of the (A)gamma- and beta-promoter during the fetal to adult hemoglobin switch and confirm GM979 cells stably transfected with the dual reporter construct as a reliable assay for automated screening of chemical inducers of fetal globin gene activation.

Control of globin gene expression during development and erythroid differentiation

Extensive studies during the last 30 years have led to considerable understanding of cellular and molecular control of hemoglobin switching. Cell biology studies in the 1970s defined the control of globin genes during erythroid differentiation and led to development of therapies for sickle cell disease. Molecular investigations of the last 20 years have delineated the two basic mechanisms that control globin gene activity during development--autonomous silencing and gene competition. Studies of hemoglobin switching have provided major insights on the control of gene loci by remote regulatory elements. Research in this field has an impact on understanding regulatory mechanisms in general and is of particular importance for eventual development of molecular cures for sickle cell disease and beta thalassemia.

Induction of human gamma globin gene expression by histone deacetylase inhibitors

We investigated the induction of human gamma globin gene activity by 3 classes of histone deacetylase inhibitors: amide analogues of trichostatin A, hydroxamic acid analogues of trapoxin, and scriptaid and its analogues. The screening consisted of measuring the effects of these compounds on gamma and beta human gene promoter activity by using cultures of GM979 cells stably transfected with a construct containing a gamma promoter linked to firefly luciferase and a beta promoter linked to renilla luciferase. Compounds belonging to all 3 classes induced gamma gene promoter activity in the screening assay in low micromolar concentrations. Histone deacetylase (HDAC) inhibitors increased acetylation of histone H4 and induced the expression of endogenous murine embryonic genes. They also increased the levels of gamma mRNA and the frequency of fetal hemoglobin-containing erythroblasts in erythroid burst-forming unit (BFUe) cultures from healthy adult individuals. Compounds that displayed very similar degrees of inhibition of the HDAC activity in an HDAC enzymatic assay differed strikingly on their effects on gamma gene promoter activity, raising the possibility of selectivity of HDACs that interact with the gamma globin gene chromatin.

Long-distance control of origin choice and replication timing in the human beta-globin locus are independent of the locus control region

DNA replication in the human beta-globin locus is subject to long-distance regulation. In murine and human erythroid cells, the human locus replicates in early S phase from a bidirectional origin located near the beta-globin gene. This Hispanic thalassemia deletion removes regulatory sequences located over 52 kb from the origin, resulting in replication of the locus from a different origin, a shift in replication timing to late S phase, adoption of a closed chromatin conformation, and silencing of globin gene expression in murine erythroid cells. The sequences deleted include nuclease-hypersensitive sites 2 to 5 (5'HS2-5) of the locus control region (LCR) plus an additional 27-kb upstream region. We tested a targeted deletion of 5'HS2-5 in the normal chromosomal context of the human beta-globin locus to determine the role of these elements in replication origin choice and replication timing. We demonstrate that the 5'HS2-5-deleted locus initiates replication at the appropriate origin and with normal timing in murine erythroid cells, and therefore we conclude that 5'HS2-5 in the classically defined LCR do not control replication in the human beta-globin locus. Recent studies also show that targeted deletion of 5'HS2-5 results in a locus that lacks globin gene expression yet retains an open chromatin conformation. Thus, the replication timing of the locus is closely correlated with nuclease sensitivity but not globin gene expression.

Coinduction of Embryonic and Adult-Type Globin mRNAs by Sodium Butyrate and Trichostatin A in Two Murine Interleukin-3–Dependent Bone Marrow–Derived Cell Lines

Using an RNase protection assay, globin mRNA species expressed in clones derived from Ba/F3 and B6SUtA cells transfected with the erythropoietin receptor (EpoR) and selected with erythropoietin (Epo) were compared with globin mRNA species induced in corresponding parental cells by sodium butyrate (SB) and trichostatin A (TSA). βMajor/βminor- and -1/-2–globin mRNAs were the major species, with trace amounts of ɛ-globin mRNA, formed in Epo-stimulated EpoR+ Ba/F3 clones, whereas SB and TSA allowed expression of all species of globin mRNAs, ie, ɛ, βh1, βmajor/βminor, ζ, and -1/-2, in parental Ba/F3 cells. In contrast, ɛ- and -1/-2–globin mRNAs were the major species present in Epo-stimulated EpoR+ B6SUtA clones, whereas SB and TSA activated ɛ-, βh1-, βS/βT-, and -1/-2–globin genes in parental B6SUtA cells; ζ-globin mRNA was not detected in SB- and TSA-treated B6SUtA cells. Because TSA is a specific inhibitor of histone deacetylase, the mimicry of action exhibited by SB and TSA suggests that the effects of SB are mediated through its ability to inhibit histone deacetylase and that histone deacetylase is an integral part of the repression of globin genes in these interleukin-3–dependent cells. Efficient coinduction of embryonic and adult types of globin mRNA in bone marrow cell lines derived from adult mice indicates that adult hematopoietic precursors possess an embryonic nature. These cell lines are useful models to study the mechanism(s) of developmental globin gene switching.

Coinduction of Embryonic and Adult-Type Globin mRNAs by Sodium Butyrate and Trichostatin A in Two Murine Interleukin-3–Dependent Bone Marrow–Derived Cell Lines

Using an RNase protection assay, globin mRNA species expressed in clones derived from Ba/F3 and B6SUtA cells transfected with the erythropoietin receptor (EpoR) and selected with erythropoietin (Epo) were compared with globin mRNA species induced in corresponding parental cells by sodium butyrate (SB) and trichostatin A (TSA). βMajor/βminor- and -1/-2–globin mRNAs were the major species, with trace amounts of ɛ-globin mRNA, formed in Epo-stimulated EpoR+ Ba/F3 clones, whereas SB and TSA allowed expression of all species of globin mRNAs, ie, ɛ, βh1, βmajor/βminor, ζ, and -1/-2, in parental Ba/F3 cells. In contrast, ɛ- and -1/-2–globin mRNAs were the major species present in Epo-stimulated EpoR+ B6SUtA clones, whereas SB and TSA activated ɛ-, βh1-, βS/βT-, and -1/-2–globin genes in parental B6SUtA cells; ζ-globin mRNA was not detected in SB- and TSA-treated B6SUtA cells. Because TSA is a specific inhibitor of histone deacetylase, the mimicry of action exhibited by SB and TSA suggests that the effects of SB are mediated through its ability to inhibit histone deacetylase and that histone deacetylase is an integral part of the repression of globin genes in these interleukin-3–dependent cells. Efficient coinduction of embryonic and adult types of globin mRNA in bone marrow cell lines derived from adult mice indicates that adult hematopoietic precursors possess an embryonic nature. These cell lines are useful models to study the mechanism(s) of developmental globin gene switching.

The locus control region is necessary for gene expression in the human beta-globin locus but not the maintenance of an open chromatin structure in erythroid cells

Studies in many systems have led to the model that the human beta-globin locus control region (LCR) regulates the transcription, chromatin structure, and replication properties of the beta-globin locus. However the precise mechanisms of this regulation are unknown. We have developed strategies to use homologous recombination in a tissue culture system to examine how the LCR regulates the locus in its natural chromosomal environment. Our results show that when the functional components of the LCR, as defined by transfection and transgenic studies, are deleted from the endogenous beta-globin locus in an erythroid background, transcription of all beta-globin genes is abolished in every cell. However, formation of the remaining hypersensitive site(s) of the LCR and the presence of a DNase I-sensitive structure of the beta-globin locus are not affected by the deletion. In contrast, deletion of 5'HS5 of the LCR, which has been suggested to serve as an insulator, has only a minor effect on beta-globin transcription and does not influence the chromatin structure of the locus. These results show that the LCR as currently defined is not necessary to keep the locus in an "open" conformation in erythroid cells and that even in an erythroid environment an open locus is not sufficient to permit transcription of the beta-like globin genes.

Developmental control of epsilon- and gamma-globin genes

In the last few years there have been considerable advances in the understanding of the molecular control of globin genes during development. Several insights have been obtained with studies using transgenic mice. The 5' to 3' order of the genes in the beta locus, the proximity of the genes to the locus control region and the availability of transcriptional factors have been implicated in the developmental activation of globin genes. Globin genes are turned off by two general mechanisms, autonomous gene silencing involving sequences located in the proximal and distal promoters and competition between genes for interaction with the locus control region. The current understanding of the control of embryonic (epsilon) and fetal (gamma) globin genes is reviewed.

Use of yeast artificial chromosomes (YACs) for studying control of gene expression: correct regulation of the genes of a human beta-globin locus YAC following transfer to mouse erythroleukemia cell lines

We demonstrate that transfer of a yeast artificial chromosome (YAC) containing 230 kb of the human beta-globin locus into mouse erythroleukemia cells by fusion results in correct developmental regulation of the human beta-like globin genes. Additionally, we show that early after hybrid formation, human embryonic epsilon- and fetal gamma-globin genes are coexpressed with the adult beta gene but that after 10-20 weeks in culture, globin gene expression switches to predominantly adult. Thus, in contrast to shorter gene constructs, the globin genes of the beta-globin locus YAC are regulated like the chromosomal globin genes. These results indicate that transfer of YACs into established cell lines can be used for the analysis of the developmental control of multigenic and developmentally regulated human loci.