Expression Of Human Beta-globin Gene Research Articles

Specific Activation of Human beta-Globin Gene Expression by the Transcription Factor Ikaros.

The zinc finger transcription factor Ikaros is recognized as a key regulator of lymphocyte differentiation. Recently generated dominant negative mutants have hinted at a broader role in haematopoietic stem cell generation. Most recently, a mouse strain, IkarosPlastic, with a point mutation in Ikaros that disrupts DNA binding but preserves efficient assembly of Ikaros protein complexes, is embryonically lethal due to severe defects in erythrocyte differentiation (Papathanasiou P, et al,. Immunity, 2003). (1). These mice display normal murine globin gene expression in the fetal liver. However in humans the globin locus is under alternative regulatory control, particularly with respect to the fetal-to-adult globin switch. Thus, to determine if Ikaros plays a role in human globin switching we crossed the IkarosPlastic mice with mice transgenic for a YAC containing the entire human b-globin locus, which show human fetal to adult globin gene switching from E12 to E17. Embryos were harvested from E12.5 to E15.5 and globin expression was determined in the fetal liver by real-time PCR (relative to actin). At all time points human gamma-globin gene expression was not significantly altered by the presence of the IkarosPlastic mutatation (relative expression Ikaroswt/wt 1±0.11, IkarosPlastic/Plastic 0.82±0.12). In contrast, human beta-globin gene expression was significantly down-regulated in IkarosPlastic fetal livers (relative expression Ikaroswt/wt 1±0.14, IkarosPlastic/Plastic 0.18±0.07). Interestingly, neither murine a- or b-globin gene expression was significantly different to wild type mice, which suggests that the transcription factor Ikaros plays a specific role in the transcriptional activation of the human b-globin gene during development. The mechanism by which this occurs remains to be elucidated, however it is intriguing to consider that Ikaros may act as a potentiator of transcription for erythroid specific transcription factors such as EKLF. Experiments to address this will be presented.

Human Beta-Globin Expression during Erythroid Cells Development in Transgenic Mice Bearing Human Beta-Globin Gene with Mutated BP1 Binding Site.

Binding of beta protein 1 (BP1) to its site on the promoter of adult beta-globin gene has silencing effect on beta-globin transcription in vitro. To better understand the mechanism of the negative regulation of beta-globin expression by BP1 we have developed transgenic mice. Specifically, we introduced a mutated BP1 binding site into the promoter of beta-globin gene sequence of 35 kb cosmid construct. This construct containing the micro-LCR and other essential elements of human beta-globin gene cluster was microinjected into the single cell mouse embryos. To detect the differences in developmental regulation of the human beta-globin gene expression in the transgenic mice, we studied the yolk sac derived embryonic blood at embryonic day 10.5 (E10.5) and the fetal liver of mouse embryos at E13.5. In addition, we analyzed adult erythroid cells. To minimize experimental error, samples from individual animals of three transgenic lines were analyzed independently using real-time PCR assays. Levels of expression of murine alpha-globin mRNA were used as internal controls. The BP1 gene and its mouse analog Dlx4 belongs to the Distal-less family of homeobox genes, which are expressed during early development. We found that the mRNA levels of human beta-globin in transgenic mice containing mutated BP1 binding site were higher at all stages of erythroid cells development as compared with control transgenic mice bearing cosmid construct with wild type sequence of BP1 site. Particularly, we detected up to 20-fold increase in human beta-globin expression in embryonic blood at E10.5, 3-fold increase in fetal livers of transgenic mice at E13.5, and up to 1.4-fold increase in adult reticulocytes. We also found that increase in human beta-globin expression was correlated with expression pattern of murine Dlx4 which mRNA was predominantly expressed in embryonic blood at E10.5. Thus, our data indicate that transgenic mice bearing human beta-globin gene with mutated BP1 site have significantly higher human beta-globin transcripts levels in blood cells from primitive erythropoesis than control mice. These results may help develop the novel clinic approaches for the inhibition of the expression of abnormal beta-globin genes, such as sickle (hbs) and hbc.

Locus control region elements HS2 and HS3 in combination with chromatin boundaries confer high‐level expression of a human β‐globin transgene in a centromeric region

Expression constructs are subject to position-effects in transgenic assays unless they harbour elements that protect them from negative or positive influences exerted by chromatin at the site of integration. Locus control regions (LCRs) and boundary elements are able to protect from position effects by preventing heterochromatization of linked genes. The LCR in the human beta-globin gene locus is located far upstream of the genes and composed of several erythroid specific DNase I hypersensitive (HS) sites. Previous studies demonstrated that the LCR HS sites act synergistically to confer position-independent and high-level globin gene expression at different integration sites in transgenic mice. Here we show that LCR HS sites 2 and 3, in combination with boundary elements derived from the chicken beta-globin gene locus, confer high-level human beta-globin gene expression in different chromosomal integration sites in transgenic mice. Moreover, we found that the construct is accessible to nucleases and highly expressed when integrated in a centromeric region. These results demonstrate that the combination of enhancer, chromatin opening and boundary activities can establish independent expression units when integrated into chromatin.

Analysis of adeno-associated virus-mediated ex vivo transferred human beta-globin gene in bone marrow engrafted mice.

Adeno-associated virus (AAV)-2 was developed as a useful vector for human gene therapy. In this report, we analyzed the integration and expression of AAV-mediated ex vivo transferred human beta-globin gene in bone marrow (BM) reconstituted mice. Recombinant AAV (rAAV) containing human beta-globin gene was packaged by infecting individual G418-resistant BHK-21 cell clones integrated with the plasmid AV53HS432Deltabeta2.0Neo with recombinant herpes simplex virus, which can express rep and cap genes of wild-type AAV. The titer of rAAV was determined using slot blot hybridization with a result of 10(13) virus particles/ml (genome copy number). Low-density mononuclear cells were isolated from fetal livers of embryos from pregnant C57BL/6 mice at 14-16 days of gestation and were infected with rAAV. The transduced hematopoietic cells were then reinfused into lethally irradiated C57BL/6 recipient mice via tail vein injection. To analyze the provirus in short-term and long-term BM reconstituted mice, PCR/Southern blot and RT-PCR were performed to identify the integrity of the provirus and to detect the expression of human beta-globin gene, respectively. Genomic DNA was extracted from spleen nodules of BM reconstituted mice 12 days after transplantation. Human beta-globin gene was detected in 1 out of 6 nodules using PCR combined with Southern blot. Human beta-globin gene was also detected in the BM and thymus of mouse Y6161, in the thymus and spleen of mouse Y6162 and in the BM of mice Y6211 and Y6212. RT-PCR revealed low levels of expression of human beta-globin gene in the BM of mouse Y6211. Our results suggested that the efficiency of AAV-mediated human beta-globin gene integration into hematopoietic stem/progenitor cells was very low. It is necessary to perform further research on AAV biology before applying gene therapy that requires integration of a foreign gene into host chromosomes.

Function of GATA transcription factors in hydroxyurea-induced HEL cells.

HEL cells, a human erythroleukemia cell line, mainly express the fetal (gamma) globin gene and trace amount of the embryonic (epsilon) globin gene, but not adult (beta) globin gene. Here we show that hydroxyurea (HU) can induce HEL cells to express adult (beta) globin gene and lead these cells to terminal differentiation. Results showed in Gel mobility shift assays that GATA factors could specifically bind to the regulatory elements of human beta-globin gene, including the proximal regulatory element (the beta-promoter) and the distal regulatory elements (the DNase I hypersensitive sites in the LCR, HS2-HS4 core sequences). However, the DNA binding patterns of GATA factors were quite different between HU-induced and uninduced HEL cells. Western-blot analysis of nuclear extracts from both the uninduced and HU-induced HEL cells revealed that the level of GATA-2 transcription factor decreased, whereas the level of GATA-1 transcription factor increased following the time of hydroxyurea induction. Furthermore, using RT-PCR analysis the expression of human beta-globin gene in HU-induced HEL cells could be blocked again when HEL cells were incubated in the presence of antisense oligonucleotides for hGATA-1, suggesting that the upregulation of hGATA-1 transcription factor might be critical for the expression of human beta-globin gene in HU-induced HEL cells.

Adeno-associated virus 2-mediated transduction and erythroid lineage-restricted long-term expression of the human beta-globin gene in hematopoietic cells from homozygous beta-thalassemic mice.

Adeno-associated virus 2 (AAV), a nonpathogenic human parvovirus, has gained attention as a potentially useful vector for human gene therapy. Here, we report successful AAV-mediated stable transduction and high-efficiency, long-term, erythroid lineage-restricted expression of a human beta-globin gene in primary murine hematopoietic stem cells in vivo. Bone marrow-derived primitive Sca-1(+), lin(-) hematopoietic stem cells from homozygous beta-thalassemic mice were transduced ex vivo with a recombinant AAV vector containing a normal human beta-globin gene followed by transplantation into low-dose-irradiated B6.c-kitW(41/41) anemic recipient mice. Six months posttransplantation, tail-vein blood samples were analyzed by PCR amplification to document the presence of the transduced human beta-globin gene sequences in the peripheral blood cells. Semiquantitative PCR analyses revealed that the transduced human beta-globin gene sequences were present at approximately 1 copy per cell. The efficiency of the human beta-globin gene expression was determined to be up to 35% compared with the murine endogenous beta-globin gene by semiquantitative RT-PCR analyses. Peripheral blood samples from several positive recipient mice obtained 10 months posttransplantation were fractionated to obtain enriched populations of granulocytes, lymphocytes, and erythroid cells. PCR analyses revealed the presence of the human beta-globin gene sequences in granulocytes and lymphocytes, indicating multilineage reconstitution. However, only the erythroid population was positive following RT-PCR analyses, suggesting lineage-restricted expression of the transduced human beta-globin gene. Southern blot analyses of total genomic DNA samples isolated from bone marrow cells from transplanted mice also documented proviral integration. These results provide further support for the potential use of recombinant AAV vectors in gene therapy of beta-thalassemia and sickle-cell disease.

The role of trans-acting factors and DNA-bending in the silencing of human beta-globin gene expression.

The molecular mechanisms which govern the develop-mental specificity of human beta-globin gene transcription have been studied in K562 cells, a human eyrthroleukemia line that expresses minimal beta-globin. Protein-binding analysis reveals that the 5' region contains three elements bound by trans-acting factors, beta-protein 1 (BP1) and beta-protein 2 (BP2). In vitro mutagenesis of each individual element in a beta-globin vector containing chloramphenicol acetyl-transferase (pCAT) followed by transient transfection into K562 cells increased levels of CAT activity 5. 5-fold higher than wild-type (wt) betaCAT, consistent with their silencing role. Mutagenesis of all three elements, however, resulted in activity significantly lower than wt betaCAT. BP1 and BP2 motifs have overlapping binding sites for high mobility group proteins (HMG1+2), DNA-bending factors, shown here to extrinsically bend the beta-globin promoter. Theoretically, mutations in all beta-protein binding sites could affect the binding of HMG1+2 sufficiently to impede DNA-protein and/or protein-protein interactions needed to facilitate constitutive gene expression. Placing two turns of DNA between BP1 and BP2 motifs also increased expression 3-fold, indicative of spatial constraints required for optimal silencing. However, insertion of the HMG1+2 DNA-bending motif (also equivalent to two turns) facilitates beta-silencing by re-establishment of BP1-BP2 proximity. Thus a combination of general DNA-bending and specific transcriptional factors appear to be involved in beta-globin silencing in the embryonic/fetal erythroid stage.

High-level transfer and long-term expression of the human beta-globin gene in a mouse transplant model.

Insertion of a normally functioning human beta-globin gene into the hematopoietic stem cells (HSC) of patients with beta-thalassemia may be an effective approach to the therapy of this disorder. Safe, efficient gene transfer and long-term, high-level expression of the transferred human beta-globin gene in animal models are prerequisites for HSC somatic gene therapy. We have recently shown for the first time that, using a modified beta-globin retroviral vector in a mouse transplant model, long-term, high-level expression of a transferred human beta-globin gene is possible. The human beta-globin gene continues to be detected up to eight months post-transplantation of beta-globin-transduced hematopoietic cells into lethally irradiated mice. The transferred human beta-globin gene is detected in three of five mice surviving long-term (> 4 months) transplanted with bone marrow cells transduced with high-titer virus. The unrearranged 5.1 kb human beta-globin gene-containing provirus is seen by Southern blotting in two of these mice. More importantly, long-term expression of the transferred gene is seen in two mice at levels of 5% and 20% that of endogenous murine beta-globin. We document stem cell transduction by showing continued high-level expression of the human beta-globin gene in secondarily transplanted recipient mice. These results provide evidence of HSC transduction with a human beta-globin gene in animals and demonstrate that retroviral-mediated unrearranged human beta-globin gene transfer leads to a high level of human beta-globin gene expression in the long term for the first time. A gene therapy strategy may be a feasible therapeutic approach to the beta-thalassemias if consistent human beta-globin gene transfer and expression into HSC can be achieved.

Globin Gene Switching: IN VIVO PROTEIN-DNA INTERACTIONS OF THE HUMAN β-GLOBIN LOCUS IN ERYTHROID CELLS EXPRESSING THE FETAL OR THE ADULT GLOBIN GENE PROGRAM

To characterize the protein-DNA interactions important for the developmental control of the human beta-globin locus, we analyzed by in vivo dimethyl sulfate footprinting erythroid cells expressing either the fetal or the adult globin developmental program. In the locus control region (LCR) of the beta-globin locus, in vivo footprints on NF-E2 (or AP-1) and GATA-1 motifs remained the same regardless of whether the fetal or the adult globin genes are expressed. In contrast, in vivo footprints on GT (CACCC) motifs differed between the cells expressing the fetal or the adult globin program. In promoter regions, the actively transcribed genes demonstrated extensive and consistent footprints over the canonical elements, such as CACCC and CCAAT motifs. The adult globin expressing cells displayed more extensive footprints than the fetal globin expressing cells in the 3' regulatory sequences of both the Agamma- and the beta-globin genes, suggesting a role of these 3' elements in beta-globin gene expression. Our results suggest that the bulk of protein-DNA interactions that underlies the developmental control of globin genes takes place in the gamma- and beta-globin gene promoters, and that GT motifs of the beta-globin locus LCR may play a role in the developmental regulation of human beta-globin gene expression, perhaps by increasing the probability of interaction of the LCR holocomplex with the fetal or the adult globin gene.

Role of Erythroid Kruppel-like Factor in Human γ- to β-Globin Gene Switching

Erythroid Kruppel-like factor (EKLF) is an erythroid-specific transcription factor that contains zinc finger domains similar to the Kruppel protein of Drosophila melanogaster. Previous studies demonstrated that EKLF binds to the CACCC box in the human beta-globin gene promoter and activates transcription. CACCC box mutations that cause severe beta-thalassemias in humans inhibit EKLF binding. Results described in this paper suggest that EKLF functions predominately in adult erythroid tissue. The EKLF gene is expressed at a 3-fold higher level in adult erythroid tissue than in fetal erythroid tissue, and the EKLF protein binds to the human beta-globin promoter 8-fold more efficiently than to the human gamma-globin promoter. Co-transfection experiments in the human fetal-like erythroleukemia cell line K562 demonstrate that over-expression of EKLF activates a beta-globin reporter construct 1000-fold; a linked gamma-globin reporter is activated only 3-fold. Mutation of the beta-globin CACCC box severely inhibits activation. These results demonstrate that EKLF is a developmental stage-enriched protein that preferentially activates human beta-globin gene expression. The data strongly suggest that EKLF is an important factor involved in human gamma- to beta-globin gene switching.

Multiple elements in human beta-globin locus control region 5' HS 2 are involved in enhancer activity and position-independent, transgene expression.

The human beta-globin Locus Control Region (LCR) has two important activities. First, the LCR opens a 200 kb chromosomal domain containing the human epsilon-, gamma- and beta-globin genes and, secondly, these sequences function as a powerful enhancer of epsilon-, gamma- and beta-globin gene expression. Erythroid-specific, DNase I hypersensitive sites (HS) mark sequences that are critical for LCR activity. Previous experiments demonstrated that a 1.9 kb fragment containing the 5' HS 2 site confers position-independent expression in transgenic mice and enhances human beta-globin gene expression 100-fold. Further analysis of this region demonstrates that multiple sequences are required for maximal enhancer activity; deletion of SP1, NF-E2, GATA-1 or USF binding sites significantly decrease beta-globin gene expression. In contrast, no single site is required for position-independent transgene expression; all mice with site-specific mutations in 5' HS 2 express human beta-globin mRNA regardless of the site of transgene integration. Apparently, multiple combinations of protein binding sites in 5' HS 2 are sufficient to prevent chromosomal position effects that inhibit transgene expression.

Long-term high-level expression of human beta-globin occurs following transplantation of transgenic marrow into irradiated mice.

When the human beta-globin gene is transferred into the bone marrow cells of live mice, its expression is very low. To investigate the reason for this, we transferred the bone marrow of transgenic mice containing and expressing the human beta-globin into irradiated recipients. We demonstrate that long-term high level expression of the human beta-globin gene can be maintained in the marrow and blood of irradiated recipients following transplantation. Although expression decreased over time in most animals because of host marrow reconstitution, the ratio of human beta-globin transgene expression to endogenous mouse beta-globin gene expression in donor-derived erythroid cells remained constant over time. We conclude that there is no inherent limitation to efficient expression of an exogenous human beta-globin gene in mouse bone marrow cells following marrow transplantation.

A 36-base-pair core sequence of locus control region enhances retrovirally transferred human beta-globin gene expression.

The locus control region of the human beta-globin cluster consists of four major DNase I hypersensitive sites (HS). When linked to globin genes, the locus control region confers a high level of erythroid-specific expression of these genes in transgenic mice or transfected erythroid cell lines. We have examined the effect of one of these sites, HS2, on human beta-globin gene expression in a murine erythroleukemia cell line (MEL) after retrovirus-mediated gene transfer. We incorporated a 732- or 412-base-pair (bp) segment of HS2 in the retroviral construct carrying the human beta-globin gene. These fragments rendered the viruses unstable as the human beta-globin gene was rearranged or deleted in all the packaging cell lines examined. On the other hand, when a 36-bp fragment containing the NFE-2/AP-1 binding consensus in this region was inserted into the retroviral construct, we recovered 6 stable packaging cell lines of 12 examined, similar in percentage to the construct with the beta-globin gene alone. The virus titers of the packaging cell lines from these two constructs were similar. We infected MEL cells with viruses produced from three packaging cell lines of each of the two constructs and measured the ratio of human beta-globin to mouse alpha-globin mRNA after hexamethylenebisacetamide induction. The overall level of expression increased 2-fold from 6.0% to 12.7% with the addition of this 36-bp enhancer.

Human beta-globin locus control region: analysis of the 5' DNase I hypersensitive site HS 2 in transgenic mice.

The human beta-globin locus control region (LCR) is essential for high-level expression of human epsilon-, gamma-, and beta-globin genes. Developmentally stable DNase I hypersensitive sites (designated HS) mark sequences within this region that are important for LCR activity. A 1.9-kilobase (kb) fragment containing the 5' HS 2 site enhances human beta-globin gene expression 100-fold in transgenic mice and also confers position-independent expression. To further define important sequences within this region, deletion mutations of the 1.9-kb fragment were introduced upstream of the human beta-globin gene, and the constructs were tested for activity in transgenic mice. Although enhancer activity was gradually lost with deletions of both 5' and 3' sequences, a 373-base-pair (bp) fragment retained the ability to confer relative position-independent expression. Three prominent DNase I footprints were observed in this region with extracts from the human erythroleukemia cell line K-562, one of which contained duplicated binding sites for transcription factor AP-1 (activator protein 1). When the 1.9-kb fragment containing an 18-bp deletion of the AP-1 binding sites was tested in transgenic mice, enhancer activity decreased 20-fold but position-independent expression was retained.

Human beta-globin gene expression in transgenic mice is enhanced by a distant DNase I hypersensitive site.

Several lines of evidence suggest that erythroid-specific DNase I hypersensitive sites (HS) located far upstream of the human beta-globin gene are important in regulating beta-globin gene expression. We used the polymerase chain reaction technique to amplify and clone an 882-base-pair DNA fragment spanning one of these HS, designated HSII, which is located 54 kilobases upstream of the beta-globin gene. The cloned HSII fragment was linked to a human beta-globin gene in either the genomic (HSII-beta) or antigenomic (HSII-beta) orientation. These two constructs and a beta-globin gene alone (beta) were injected into fertilized mouse eggs, and expression was analyzed in liver and brain from day-16 transgenic fetuses. Five of 7 beta-transgenic fetuses expressed human beta-globin mRNA, but the level of expression per gene copy was low, ranging from 0.93 to 22.4% of mouse alpha-globin mRNA (average 9.9%). In contrast, 11 of 12 HSII-beta transgenic fetuses expressed beta-globin mRNA at levels per gene copy ranging from 31.3 to 336.6% of mouse alpha-globin mRNA (average 139.5%). Only three fetuses containing intact copies of the HSII-beta construct were produced. Two of three expressed human beta-globin mRNA at levels per gene copy of 179.2 and 387.1%. Expression of human beta-globin mRNA was tissue-specific in all three types of transgenic fetuses. These studies demonstrate that a small DNA fragment containing a single erythroid-specific HS can stimulate high-level human beta-globin gene expression in transgenic mice.

Gene transfer. A potential approach to gene therapy for sickle cell disease.

The current status of gene transfer experiments indicates that it is possible to provide (1) safe and efficient retroviral packaging lines for gene transfer; and (2) vectors containing the human beta-globin genes and selectable marker genes which can be transmitted into erythroid cells and are appropriately expressed. In animal autologous bone marrow transplantation experiments, stable and high-level expression of retroviral vectors containing human beta-globin genes has not yet been achieved. New retroviral vectors are being tested that contain different components of the retrovirus as well as the newly described enhancer elements 5' to the epsilon gene and surrounding the beta-globin gene. The long-term goal of human gene therapy for sickle cell disease then consists of constructing optimally safe and efficient retroviral packaging lines as well as retroviral vectors containing the human beta-globin gene and selectable markers such as the neoR gene. One would then remove bone marrow cells from patients with sickle cell disease, transfer the retroviral vectors into the bone marrow cells, and subject the cells to G418 selection in vitro. Next, one would ablate the host bone marrow and autotransplant the manipulated bone marrow bearing the retroviral vector. Finally, one would analyze the reconstituted bone marrow for human beta-globin gene expression. These experiments must await the demonstration of safe and efficient gene transfer in animals, and particularly experiments must be done in monkeys prior to the use of these approaches in humans. Alternative approaches to gene therapy include direct correction of the defect in the beta-globin gene by site-specific recombination of the defective gene with incoming normal gene sequences. This technology is not yet achievable, although preliminary experiments have been performed in which gene correction at a low frequency has been obtained. The obvious advantage of this approach is that the native human beta gene would be reconstituted. Site-specific recombination and addition of normal beta genes to the human genome represent exciting and feasible approaches to human gene therapy using the extraordinary resources of modern molecular and cellular biology. Success in treating disorders of human hemoglobin only awaits additional technical advances for increasing the efficiency of gene transfer and the level of gene expression.

A single erythroid-specific DNase I super-hypersensitive site activates high levels of human beta-globin gene expression in transgenic mice.

Erythroid-specific DNase I super-hypersensitive (HS) sites that are normally located far upstream of the human beta-globin locus were inserted immediately upstream of a 4.1-kb fragment containing the human beta-globin gene. These constructs (HS beta) and a construct containing the beta-globin gene alone (beta) were microinjected into fertilized mouse eggs, and expression was analyzed in erythroid fetal liver and brain of day-16 embryos that developed. Only 7 of 23 animals that contained the beta gene alone expressed human beta-globin mRNA in erythroid tissue, and the average level of expression per gene copy was 0.3% of endogenous mouse beta-globin mRNA. In contrast, 50 of 51 transgenic mice that contained various HS beta constructs expressed the transgene specifically in erythroid tissue. The average level of expression per gene copy for constructs containing all five upstream HS sites was 109% of endogenous mouse beta-globin mRNA. Constructs that contained a single super-hypersensitive site (HS II beta) expressed 40% as much human beta-globin as mouse beta-globin mRNA per gene copy. These results demonstrate that the HS VI site, normally located downstream of the human beta-globin locus, is not required for high-level expression. Furthermore, the results demonstrate that high levels of human beta-globin gene expression can be obtained in transgenic mice even when a relatively small fragment of DNA (1.9 kb) containing erythroid-specific super-hypersensitive site II (HS II) is inserted upstream of the human beta-globin gene.

Retroviral-mediated transfer and expression of human beta-globin genes in cultured murine and human erythroid cells.

We have cloned human beta-globin DNA sequences from a genomic library prepared from DNA isolated from the human leukemia cell line K562 and have used the retroviral vector pZip-NeoSV(X)1 to introduce a 3.0-kilobase segment encompassing the globin gene into mouse erythroleukemia cells. Whereas the endogenous K562 beta-globin gene is repressed in K562 cells, when introduced into mouse erythroleukemia cells by retroviral-mediated gene transfer, the beta-globin gene from K562 cells was transcribed and induced 5-20-fold after treatment of the cells with dimethyl sulfoxide. The transcripts were correctly initiated, and expression and regulation of the K562 gene were identical to the expression of a normal human beta-globin gene transferred into mouse erythroleukemia cells in the same way. We have also introduced the normal human beta-globin gene into K562 cells using the same retrovirus vector. SP6 analysis of the RNA isolated from the transduced cells showed that the normal beta-globin gene was transcribed at a moderately high level, before or after treatment with hemin. Based on these data, we suggest that the lack of expression of the endogenous beta-globin gene in K562 cells does not result from an alteration in the gene itself and may not result from a lack of factor(s) necessary for beta-globin gene transcription. Retroviral-mediated transfer of the human beta-globin gene may, however, uniquely influence expression of the gene in K562 cells.

Human β-Globin Gene Expression after Gene Transfer Using Retroviral Vectors

A retroviral vector containing a 4.4-kb Pst I human beta S-globin gene and a neomycin resistance gene was used to infect NIH-3T3 and mouse erythroleukemia cells (MELC). In MELC, human beta-globin mRNA transcripts are transcribed and properly initiated and spliced. In some cases, there is an appropriate increase in beta-globin mRNA on addition of dimethylsulfoxide (DMSO), an inducer of hemoglobin synthesis and erythroid differentiation in these cells. When NIH-3T3 cells are infected with the same retroviral vector, there is less globin mRNA accumulation and no evidence for appropriate regulation. Human beta-globin gene expression in MELC clones induced with DMSO is 2-3% that of endogenous mouse beta-globin gene expression. These results indicate that retroviral vectors can be used to transfer and appropriately express human beta-globin genes in erythroid cells.

Two 3' sequences direct adult erythroid-specific expression of human beta-globin genes in transgenic mice.

Previous experiments have demonstrated that the human beta-globin gene is correctly regulated in transgenic mice. The beta-globin gene is not expressed in yolk sac-derived erythroid cells in early embryonic development but is expressed concomitantly with the adult mouse beta-globin genes in 14- to 16-day fetal liver and adult reticulocytes. In an attempt to localize sequences that direct erythroid-specific expression, fragments of the human beta-globin gene were inserted in the opposite orientation 200 base pairs (bp) upstream of an intact human A gamma marker gene, which is not expressed on its own in mouse fetal liver. In the experiments reported here, two beta-globin 3' sequences activated the marker gene specifically in fetal liver. One sequence is located in a 250-bp Pst I fragment 550-800 bp downstream from the poly(A) site; the other is located near an EcoRI site in the third exon. These two sequences are active individually, and their combined effect is greater than their effects alone. beta-Globin 5' sequences from -815 to -50 were also analyzed for activity in this assay. The 5' sequences did not activate the marker gene when tested alone but did stimulate expression that was already directed to adult erythroid tissue by the two 3' sequences. These results suggest that three separate sequences are involved in human beta-globin gene regulation. The two 3' sequences act as adult erythroid enhancers and the 5' sequence stimulates expression that is already determined to be erythroid specific.