High-level erythroid expression of human alpha-globin genes in transgenic mice

Humanized Mouse Model of Cooley's Anemia

A novel humanized mouse model of Cooley's Anemia (CA) was generated by targeted gene replacement in embryonic stem (ES) cells. Because the mouse does not have a true fetal hemoglobin, a delayed switching human gamma to beta(0) globin gene cassette (gammabeta(0)) was inserted directly into the murine beta globin locus replacing both adult mouse beta globin genes. The inserted human beta(0) globin allele has a mutation in the splice donor site that produces the same aberrant transcripts in mice as described in human cells. No functional human beta globin polypeptide chains are produced. Heterozygous gammabeta(0) mice suffer from microcytic anemia. Unlike previously described animal models of beta thalassemia major, homozygous gammabeta(0) mice switch from mouse embryonic globin chains to human fetal gamma globin during fetal life. When bred with human alpha globin knockin mice, homozygous CA mice survive solely upon human fetal hemoglobin at birth. This preclinical animal model of CA can be utilized to study the regulation of globin gene expression, synthesis, and switching; the reactivation of human fetal globin gene expression; and the testing of genetic and cell-based therapies for the correction of thalassemia.

Deletion of the human beta-globin LCR 5'HS4 or 5'HS1 differentially affects beta-like globin gene expression in beta-YAC transgenic mice

A 213 kb human beta-globin locus yeast artificial chromosome (beta-YAC) was modified by homologous recombination to delete 2.9 kb of cross-species conserved sequence similarity encompassing the LCR 5' hypersensitive site (HS) 4 (Delta5'HS4 beta-YAC). In three transgenic mouse lines, completion of the gamma- to beta-globin switch during definitive erythropoiesis was delayed relative to wild-type beta-YAC mice. In addition, quantitative per-copy human beta-like globin mRNA levels were similar to wild-type beta-YAC transgenic lines, although beta-globin gene expression was slightly decreased in the day 12 fetal liver of Delta5'HS4 beta-YAC mice. A 0.8 kb 5'HS1 fragment was similarly deleted in the YAC. Three Delta5'HS1 beta-YAC transgenic lines were established. epsilon-globin gene expression was markedly reduced, approximately 16 fold, during primitive erythropoiesis compared to wild-type beta-YAC mice, but gamma-globin expression levels were unaffected. However, during the fetal stage of definitive erythropoiesis, gamma-globin gene expression was decreased approximately 4 fold at day 12 and approximately 5 fold at day 14. Temporal developmental expression profiles of the beta-like globin genes were unaffected by deletion of 5'HS1. Decreased expression of the epsilon- and gamma-globin genes is the first phenotype ascribed to a 5'HS1 mutation in the human beta-globin locus, suggesting that this HS does indeed have a role in LCR function beyond simply a combined synergism with the other LCR HSs.

Developmental regulation of the beta-globin gene locus

The beta-globin genes have become a classical model for studying regulation of gene expression. Wide-ranging studies have revealed multiple levels of epigenetic regulation that coordinately ensure a highly specialised, tissue- and stage-specific gene transcription pattern. Key players include cis-acting elements involved in establishing and maintaining specific chromatin conformations and histone modification patterns, elements engaged in the transcription process through long-range regulatory interactions, transacting general and tissue-specific factors. On a larger scale, molecular events occurring at the locus level take place in the context of a highly dynamic nucleus as part of the cellular epigenetic programme.

Correction of a mouse model of sickle cell disease: lentiviral/antisickling beta-globin gene transduction of unmobilized, purified hematopoietic stem cells

Although sickle cell anemia was the first hereditary disease to be understood at the molecular level, there is still no adequate long-term treatment. Allogeneic bone marrow transplantation is the only available cure, but this procedure is limited to a minority of patients with an available, histocompatible donor. Autologous transplantation of bone marrow stem cells that are transduced with a stably expressed, antisickling globin gene would benefit a majority of patients with sickle cell disease. Therefore, the development of a gene therapy protocol that corrects the disease in an animal model and is directly translatable to human patients is critical. A method is described in which unmobilized, highly purified bone marrow stem cells are transduced with a minimum amount of self-inactivating (SIN) lentiviral vector containing a potent antisickling beta-globin gene. These cells, which were transduced in the absence of cytokine stimulation, fully reconstitute irradiated recipients and correct the hemolytic anemia and organ pathology that characterize the disease in humans. The mean increase of hemoglobin concentration was 46 g/L (4.6 g/dL) and the average lentiviral copy number was 2.2; therefore, a 21-g/L /vector copy increase (2.1-g/dL) was achieved. This transduction protocol may be directly translatable to patients with sickle cell disease who cannot tolerate current bone marrow mobilization procedures and may not safely be exposed to large viral loads.

Autocrine stimulation by erythropoietin in transgenic mice results in erythroid proliferation without neoplastic transformation

Erythropoietin (Epo) autocrine stimulation has been implicated in erythroleukemia. To develop a model of Epo autocrine stimulation, we made transgenic mice using a construct that linked the human Epo gene to an erythroid-specific regulatory element, designated 5'HS-2, from the human beta-globin locus control region. We hypothesized that Epo gene expression would be targeted to erythroid cells in these mice, resulting in autocrine stimulation of erythroid progenitor cell growth in culture, and that chronic autocrine Epo stimulation would result in erythroleukemia. Transgenic mice containing intact copies of the 5'HS-2Epo construction had elevated hematocrits, reticulocyte counts and serum Epo levels and marked splenic enlargement. Analysis of RNA isolated from organs of transgenic mice revealed constitutive Epo mRNA expression primarily in spleen, blood and bone marrow. RNA samples from anemic transgenic mice revealed Epo gene induction only in the liver. Marrow derived from 5'HS-2Epo mice grew BFU-E in the absence of exogenous Epo. Despite observation of up to 2 years, no mouse developed erythroleukemia, demonstrating that Epo autocrine stimulation alone is insufficient for progression to malignancy. These studies show that 5'HS-2 can be used to target Epo gene expression to erythroid tissue. These mice could provide a model system for studying autocrine growth regulation.

Silencing of gene expression: implications for design of retrovirus vectors

Transcriptional silencing of retroviruses poses a major obstacle to their use as gene therapy vectors. Silencing is most pronounced in stem cells which are desirable targets for therapeutic gene delivery. Many vector designs combat silencing through cis-modifications of retroviral vector sequences. These designs include mutations of known retroviral silencer elements, addition of positive regulatory elements and insulator elements to protect the transgene from negative position effects. Similar strategies are being applied to lentiviral vectors that readily infect non-dividing quiescent stem cells. Collectively these cis-modifications have significantly improved vector design but optimal expression may require additional intervention to escape completely the trans-factors that scan for foreign DNA, establish silencing in stem cells and maintain silencing in their progeny. Cytosine methylation of CpG sites was proposed to cause retroviral silencing over 20 years ago. However, several studies provide evidence that retrovirus silencing acts through methylase-independent mechanisms. We propose an alternative silencing mechanism initiated by a speculative stem cell-specific "somno-complex". Further understanding of retroviral silencing mechanisms will facilitate better gene therapy vector design and raise new strategies to block transcriptional silencing in transduced stem cells.

Second generation knockout sickle mice: the effect of HbF

Sickle transgenic mice expressing exclusively human globins are desirable for studying pathophysiology and testing gene therapy strategies, but they must have significant pathology and show evidence of amelioration by antisickling hemoglobins. Mice were generated that expressed exclusively human sickle hemoglobin with 3 levels of HbF using their previously described sickle constructs (cointegrated human miniLCRalpha2 and miniLCRbeta(S) [PNAS 89:12150, 1992]), mouse alpha- and beta-globin-knockouts, and 3 different human gamma-transgenes. It was found that, at all 3 levels of HbF expression, these mice have balanced chain synthesis, nearly normal mean corpuscular hemoglobin, and, in some cases, F cells. Mice with the least adult HbF expression were the most severe. Progressive increase in HbF from less than 3% to 20% to 40% correlated with progressive increase in hematocrit (22% to 34% to 40%) and progressive decrease in reticulocyte count (from 60% to 30% to 13%). Urine concentrating ability was normalized at high HbF, and tissue damage detected by histopathology and organ weight were ameliorated by increased HbF. The gamma-transgene that produces intermediate levels of HbF was introduced into knockout sickle mice described by Pàszty and coworkers that express the miniLCRalpha1(G)gamma(A)gammadeltabeta(S) transgene and have fetal but not adult expression of HbF. It was found that the level of HbF required to ameliorate low hematocrit and normalize urine concentrating defect was different for the miniLCRalpha2beta(S) and miniLCRalpha1(G)gamma(A)gammadeltabeta(S) mice. We conclude that knockout mice with the miniLCRalpha2beta(S) transgene and postnatal expression of HbF have sufficiently faithful sickle pathology to serve as a platform for testing antisickling interventions.

A minimal c-fes cassette directs myeloid-specific expression in transgenic mice

The c-fes proto-oncogene encodes a 92-kd protein tyrosine kinase whose expression is restricted largely to myeloid and endothelial cells in adult mammals. A 13.2-kilobase (kb) humanc-fes genomic fragment was previously shown to containcis-acting element(s) sufficient for a locus control function in bone marrow macrophages. Locus control regions (LCRs) confer transgene expression in mice that is integration site independent, copy number dependent, and similar to endogenous murine messenger RNA levels. To identify sequences required for this LCR,c-fes transgenes were analyzed in mice. Myeloid-cell–specific, deoxyribonuclease-I–hypersensitive sites localized to the 3′ boundary of exon 1 and intron 3 are required to confer high-level transgene expression comparable to endogenous c-fes, independent of integration site. We define a minimal LCR element as DNA sequences (nucleotides +28 to +2523 relative to the transcription start site) located within intron 1 to intron 3 of the human locus. When this 2.5-kb DNA fragment was linked to a c-fes complementary DNA regulated by its own 446–base-pair promoter, integration-site–independent, copy-number–dependent transcription was observed in myeloid cells in transgenic mice. Furthermore, this 2.5-kb cassette directed expression of a heterologous gene (enhanced green fluorescent protein) exclusively in myeloid cells. The c-fes regulatory unit represents a novel reagent for targeting gene expression to macrophages and neutrophils in transgenic mice.

A minimal c-fes cassette directs myeloid-specific expression in transgenic mice

The c-fes proto-oncogene encodes a 92-kd protein tyrosine kinase whose expression is restricted largely to myeloid and endothelial cells in adult mammals. A 13.2-kilobase (kb) humanc-fes genomic fragment was previously shown to containcis-acting element(s) sufficient for a locus control function in bone marrow macrophages. Locus control regions (LCRs) confer transgene expression in mice that is integration site independent, copy number dependent, and similar to endogenous murine messenger RNA levels. To identify sequences required for this LCR,c-fes transgenes were analyzed in mice. Myeloid-cell–specific, deoxyribonuclease-I–hypersensitive sites localized to the 3′ boundary of exon 1 and intron 3 are required to confer high-level transgene expression comparable to endogenous c-fes, independent of integration site. We define a minimal LCR element as DNA sequences (nucleotides +28 to +2523 relative to the transcription start site) located within intron 1 to intron 3 of the human locus. When this 2.5-kb DNA fragment was linked to a c-fes complementary DNA regulated by its own 446–base-pair promoter, integration-site–independent, copy-number–dependent transcription was observed in myeloid cells in transgenic mice. Furthermore, this 2.5-kb cassette directed expression of a heterologous gene (enhanced green fluorescent protein) exclusively in myeloid cells. The c-fes regulatory unit represents a novel reagent for targeting gene expression to macrophages and neutrophils in transgenic mice.

Human gamma-globin gene promoter element regulates human beta-globin gene developmental specificity

The persistence of fetal hemoglobin in many patients with deletion type beta thalassemias and the expression patterns of human globin genes in transgenic mice suggest that gamma- to beta-globin gene switching results primarily from competition of gamma- and beta-globin genes for interaction with the beta-globin locus control region (LCR). To define regulatory sequences that are essential for the competitive advantage of the gamma gene at early developmental stages, stable transgenic mouse lines were produced with LCR gamma-beta constructs containing deletions of gamma 5'-flanking DNA. All constructs contained the full 22 kb LCR, a 4.1 kb beta-globin gene and a gamma-globin gene with 1348, 383, 202, 130, 72 or 52 bp of 5'-flanking sequence. Primer extension analysis of yolk sac, fetal liver and blood RNA from these lines demonstrated that a region between -202 and -130 of the human gamma-globin gene promoter was required to suppress beta-globin gene expression at early developmental stages. Four transcription factor binding sites within this region [GATA(p), Oct1, GATA(d) and CACCC] were mutated independently in LCR gamma-beta constructs and transgenic mouse lines were produced. Only the gamma CACCC box mutation resulted in high levels of beta-globin gene expression in early embryos. These results demonstrate that the CACCC box of the human gamma-globin gene plays a critical role in human beta-globin gene developmental specificity. The data also suggest that gamma CACCC box binding factors mediate LCR-gamma interactions which normally enhance gamma-globin and suppress beta-globin gene expression in fetal erythroid cells.

The beta-globin LCR is not necessary for an open chromatin structure or developmentally regulated transcription of the native mouse beta-globin locus

The murine beta-globin locus control region (LCR) was deleted from its native chromosomal location. The approximately 25 kb deletion eliminates all sequences and structures homologous to those defined as the human LCR. In differentiated ES cells and erythroleukemia cells containing the LCR-deleted chromosome, DNasel sensitivity of the beta-globin domain is established and maintained, developmental regulation of the locus is intact, and beta-like globin RNA levels are reduced 5%-25% of normal. Thus, in the native murine beta-globin locus, the LCR is necessary for normal levels of transcription, but other elements are sufficient to establish the open chromatin structure, transcription, and developmental specificity of the locus. These findings suggest a contributory rather than dominant function for the LCR in its native location.

CpG islands from the alpha-globin gene cluster increase gene expression in an integration-dependent manner

In contrast to other globin genes, the human and rabbit alpha-globin genes are expressed in transfected erythroid and nonerythroid cells in the absence of an enhancer. This enhancer-independent expression of the alpha-globin gene requires extensive sequences not only from the 5' flanking sequence but also from the intragenic region. However, the features of these internal sequences that are responsible for their positive effect are unclear. We tested several possible determinants of this activity. One possibility is that a previously identified array of discrete binding sites for known and potential regulatory proteins within the alpha-globin gene comprise an intragenic enhancer specific for the alpha-globin promoter, but directed rearrangements of the sequences show that this is not the case. Alternatively, the promoter may extend into the gene, with the function of the discrete binding sites being dependent on maintenance of their proper positions and orientations relative to the 5' flanking sequence. However, the positive effects observed in gene fusions do not localize to a discrete region of the alpha-globin gene and the results of internal deletions and point mutations argue against a required role of the targeted discrete binding sites. A third possibility is that the CpG island, which includes both the 5' flanking and intragenic regions associated with the positive activity, may itself have a more general effect on expression in transfected cells. Indeed, we show that the size of the CpG island in constructs correlates with the level of gene expression. Furthermore, the alpha-globin promoter is more active in the context of a previously inactive CpG island than in an A+T-rich context, showing that the CpG island provides an environment more permissive for expression. These effects are seen only after integration, suggesting a possible mechanism at the level of chromatin structure.

Recent advances in transgenic technology

Techniques that allow modification of the mammalian genome have made a considerable contribution to many areas of biological science. Despite these achievements, challenges remain in two principal areas of transgenic technology, namely gene regulation and efficient transgenic livestock production. Obtaining reliable and sophisticated expression that rivals that of endogenous genes is frequently problematic. Transgenic science has played an important part in increasing understanding of the complex processes that underlie gene regulation, and this in turn has assisted in the design of transgene constructs expressed in a tightly regulated and faithful manner. The production of transgenic livestock is an inefficient process compared to that of laboratory models, and the lack of totipotential embryonic stem (ES) cell lines in farm animal species hampers the development of this area of work. This article highlights recent progress in efficient trans gene expression systems, and the current efforts being made to find alternative means of generating transgenic livestock.

Reactivation of silenced, virally transduced genes by inhibitors of histone deacetylase

Retroviral and adeno-associated viral sequences can dramatically silence transgene expression in mice. We now report that this repression also occurs in stably infected HeLa cells when the cells are grown without selection. Expression of a transduced lacZ gene (rAAV/CMVlacZ) is silenced in greater than 90% of cells after 60 days in culture. Surprisingly, high-level expression can be reactivated by treating the cells with sodium butyrate or trichostatin A but not with 5-azacytidine. When cell clones with integrated copies of rAAV/CMVlacZ were isolated, lacZ expression was silenced in 80% of the clones; however, lacZ expression was reactivated in all of the silenced clones by treatment with butyrate or trichostatin A. The two drugs also reactivated a silenced globin gene construct (rAAV/HS2alphabetaAS3) in stably infected K562 cells. Trichostatin A is a specific inhibitor of histone deacetylase; therefore, we propose that hyperacetylation of histones after drug treatment changes the structure of chromatin on integrated viral sequences and relieves repression of transduced genes. The reactivation of silenced, transduced genes has implications for gene therapy. Efficient viral gene transfer followed by drug treatment to relieve suppression may provide a powerful combination for treatment of various genetic and infectious diseases.

A globin enhancer acts by increasing the proportion of erythrocytes expressing a linked transgene

Enhancer elements have been shown to affect the probability of a gene establishing an active transcriptional state and suppress the silencing of reporter genes in cell lines, but their effect in transgenic mice has been obscured by the use of assays that do not assess expression on a cell-by-cell basis. We have examined the effect of a globin enhancer on the variegation of lacZ expression in erythrocytes of transgenic mice. Mice carrying lacZ driven by the alpha-globin promoter exhibit beta-galactosidase (beta-Gal) expression in only a very small proportion of embryonic erythrocytes. When the transgenic construct also contains the (alphaHS-40 enhancer, which controls expression of the alpha-globin gene, expression is seen in a high proportion of embryonic erythrocytes, although there are variations between transgenic lines which can be attributed to different sites of integration. Analysis of beta-Gal expression levels suggests that expressing cells in lines carrying only the alpha-globin promoter express as much beta-Gal as those in which the transgene also contains alphaHS-40. A marked decline in transgene expression occurs as mice age, which is mainly due to a decrease in the proportion of cells expressing the transgene. Thus, a globin enhancer can act to suppress variegation of a linked transgene; this result is consistent with a model in which enhancers act to establish and maintain an active domain without directly affecting the transcriptional rate.

Properties of the mouse alpha-globin HS-26: relationship to HS-40, the major enhancer of human alpha-globin gene expression

HS-26, the mouse homologue of HS-40, is the major regulatory element of the mouse alpha-globin gene locus. Like HS-40, HS-26 is located within an intron of a house-keeping gene; comparison of the nucleotide sequences of HS-26 and HS-40 reveals conservation of the sequences and positions of several DNA binding motifs in the 5' regions of both elements (3 GATA, 2 NFE-2, and 1 CACCC sites) and the absence in HS-26 of three CACCC sites and one GATA site that are present in the 3' region of HS-40, suggesting that the two elements might not be identical. We report here that when HS-26 is linked to a 1.5 kb Pstl human alpha-globin gene fragment, it has a weak enhancer activity in induced MEL cells and in transgenic embryos, and it does not have any detectable activity in adult transgenic mice. This suggests that HS-26 does not have Locus Control Region (LCR) activity but can act as an enhancer during the embryonic life when integrated at a permissive locus. To further test the importance of HS-26 at its natural locus, we have generated embryonic stem cells and chimeric animals in which 350 bp containing HS-26 have been replaced by a neomycin resistance gene by homologous recombination. The sizes of the chimeras' red cells were then estimated by measuring forward scattering on a FacsScan apparatus in hypotonic conditions. This revealed that a fraction of the chimeric animals' red cells were smaller than normal mouse red cells and were very similar to cells from mice heterozygous for alpha-thalassemia. Density gradient analysis also suggested the presence of thalassemic cells. These results indicated that despite its lack of LCR activity, HS-26 is important for the regulation of the mouse alpha-globin gene locus.

CACCC and GATA-1 sequences make the constitutively expressed alpha-globin gene erythroid-responsive in mouse erythroleukemia cells

Although the human alpha-globin and beta-globin genes are co-regulated in adult life, they achieve the same end by very different mechanisms. For example, a transfected beta-globin gene is expressed in an inducible manner in mouse erythroleukemia (MEL) cells while a transfected alpha-globin gene is constitutively expressed at a high level in induced and uninduced MEL cells. Interestingly, when the alpha-globin gene is transferred into MEL cells as part of human chromosome 16, it is appropriately expressed in an inducible manner. We explored the basis for the lack of erythroid-responsiveness of the proximal regulatory elements of the human alpha-globin gene. Since the alpha-globin gene is the only functional human globin gene that lacks CACCC and GATA-1 motifs, we asked whether their addition to the alpha-globin promoter would make the gene erythroid-responsive in MEL cells. The addition of each of these binding sites to the alpha-globin promoter separately did not result in inducibility in MEL cells. However, when both sites were added together, the alpha-globin gene became inducible in MEL cells. This suggests that erythroid non-responsiveness of the alpha-globin gene results from the lack of erythroid binding sites and is not necessarily a function of the constitutively active, GC rich promoter.

Generation of a high-titer retroviral vector capable of expressing high levels of the human beta-globin gene

Retrovirus-mediated gene transfer into hematopoietic cells may provide a means of treating both inherited and acquired diseases involving hematopoietic cells. Implementation of this approach for disorders resulting from mutations affecting the beta-globin gene (e.g., beta-thalassemia and sickle cell anemia), however, has been hampered by the inability to generate recombinant viruses able to efficiently and faithfully transmit the necessary sequences for appropriate gene expression. We have addressed this problem by carefully examining the interactions between retroviral and beta-globin gene sequences which affect vector transmission, stability, and expression. First, we examined the transmission properties of a large number of different recombinant proviral genomes which vary both in the precise nature of vector, beta-globin structural gene, and locus control region (LCR) core sequences incorporated and in the placement and orientation of those sequences. Through this analysis, we identified one specific vector, termed M beta 6L, which carries both the human beta-globin gene and core elements HS2, HS3, and HS4 from the LCR and faithfully transmits recombinant proviral sequences to cells with titers greater than 10(6) per ml. Populations of murine erythroleukemia (MEL) cells transduced by this virus expressed levels of human beta-globin transcript which, on a per gene copy basis, were 78% of the levels detected in an MEL-derived cell line, Hu11, which carries human chromosome 11, the site of the beta-globin locus. Analysis of individual transduced MEL cell clones, however, indicated that, while expression was detected in every clone tested (n = 17), the levels of human beta-globin treatment varied between 4% and 146% of the levels in Hu11. This clonal variation in expression levels suggests that small beta-globin LCR sequences may not provide for as strict chromosomal position-independent expression of beta-globin as previously suspected, at least in the context of retrovirus-mediated gene transfer.

Production of unmodified human adult hemoglobin in Escherichia coli

We have constructed a plasmid (pHE2) in which the synthetic human alpha- and beta-globin genes and the methionine aminopeptidase (Met-AP) gene from Escherichia coli are coexpressed under the control of separate tac promoters. The Hbs were expressed in E. coli JM109 and purified by fast protein liquid chromatography, producing two major components, a and b. Electrospray mass spectrometry shows that at least 98% and about 90% of the expressed alpha and beta chains of component a, respectively, have the expected masses. The remaining 10% of the beta chain in component a corresponds in mass to the beta chain plus methionine. In component b, both alpha and beta chains have the correct masses without detectable N-terminal methionine (< 2%). These results have been confirmed by Edman degradation studies of the amino-terminal sequences of the alpha and beta chains of these two recombinant Hb (rHb) samples. rHbs from components a and b exhibit visible optical spectra identical to that of human normal adult Hb (Hb A). Component a and Hb A have very similar oxygen-binding properties, but component b shows somewhat altered oxygen binding, especially at low pH values. 1H-NMR spectra of component a and Hb A are essentially identical, whereas those of component b exhibit altered ring current-shifted and hyperfine-shifted proton resonances, indicating altered heme conformation in the beta chain. These altered resonance patterns can be changed to those of Hb A by converting component b to the ferric state and then to the deoxy state and finally back to either the carbonmonoxy or oxy form. Thus, our E. coli expression system produces native, unmodified Hb A in high yield and can be used to produce desired mutant Hbs.

Rhabdomyosarcoma arising in transgenic mice harboring the beta-globin locus control region fused with simian virus 40 large T antigen gene

The beta-globin locus control region (LCR) confers a high level of erythroid-specific and copy-number-dependent expression to human globin genes in transgenic mice. Simian virus 40 T (tumor) antigen (Tag) with its own natural enhancer causes choroid plexus tumors in mice. We investigated the effect of the LCR on Tag gene expression, reasoning that mice harboring a LCR-Tag fusion gene might develop hematopoietic malignancies. To test this hypothesis we introduced an enhancerless Tag gene downstream of a LCR cassette into the germ lines of mice. The phenotypes of the transgenic mice depended on the copy number of the transgene. While mice with 1-2 copies matured normally, those with 3-7 copies developed rhabdomyosarcomas in different anatomic sites at high frequency and showed hyperplasia of the pancreatic islet cells which progressed to pancreatic islet tumors. In addition, the mice bearing 7 copies of the transgene had hypoglycemia and were stunted in growth. Mice with more than 10 copies were markedly stunted in growth and died within 2-4 weeks. Tag expression was detected at high levels in the mouse tumors but not in any other tissues, including the hematopoietic cells.

High expression of human beta S- and alpha-globins in transgenic mice: hemoglobin composition and hematological consequences

A line of transgenic mice (alpha H beta S-11; where alpha H is human alpha-globin) was created in which the human beta S and human alpha 2 globin genes, each linked to the beta-globin locus control region, were cointegrated into the mouse genome. On a normal genetic background, the transgenic mice produced 36% human beta S-globin chains with an alpha H/beta S ratio of 1.3. Higher levels of beta S were achieved by breeding the transgenic mice with mutant mice carrying a mouse beta major-globin gene deletion. Mice heterozygous for the beta major deletion (alpha H beta S[beta MD]; MD, mouse deletion) had 54% beta S with an alpha H/beta S ratio of 1.0; mice homozygous for the beta major deletion (alpha H beta S[beta MDD]) had 72.5% beta S and an alpha H/beta S ratio of 0.73. Because mouse alpha chains inhibit hemoglobin (Hb) S polymerization, we bred the mice to heterozygosity for a mouse alpha-globin deletion. These mice (alpha H beta S[alpha MD beta MDD]) had an increased alpha H/beta S ratio of 0.89 but expressed 65% beta S. Expression of the human genes cured the thalassemic phenotype associated with the murine beta major deletion. Transgenic alpha H beta S[beta MDD] mice had normal hematocrit and Hb and somewhat elevated reticulocytes (6% vs. 3% for control), whereas the mice carrying the alpha-globin deletion (alpha H beta S[alpha MD beta MDD]) had a normal hematocrit and Hb and more elevated reticulocytes (10.3 +/- 7.6% vs. 3.4 +/- 1.0%). Expression of the transgene restored a normal distribution of erythrocyte densities when compared to thalassemic mice; however, the average mean corpuscular Hb concentration of alpha H beta S[beta MDD] mice increased to 35.7 g/dl (vs. control 33.7 g/dl) whereas that of alpha H beta S[alpha MD beta MDD] mice was further elevated to 36.3 g/dl. The intrinsic oxygen affinity was increased in transgenic mouse erythrocytes at 280 milliosmolal, and the PO2 at midsaturation of alpha H beta S[alpha MD beta MDD] erythrocytes was higher than that of alpha H beta S[beta MDD] cells (37.4 +/- 2 vs. 33.5 +/- 1 mmHg). The higher values of the mean corpuscular Hb concentration and intrinsic PO2 at midsaturation, which favor in vivo sickling, may explain the slightly more severe hematological picture in alpha H beta S[alpha MD beta MDD] mice. We conclude that the transgenic mouse with high Hb S expression does not exhibit adult anemia but does have abnormal hematological features: increased erythrocyte density, high oxygen affinity, and reticulocytosis with increased stress reticulocytes.

Activation of the beta-globin locus control region precedes commitment to the erythroid lineage

The beta-globin locus control region (LCR) is characterized by erythroid-specific DNase I hypersensitive sites and is involved in the chromatin organization, transcriptional potentiation, developmental regulation, and replication timing of the entire beta-globin gene cluster. When and how the LCR is first activated during erythropoiesis is not known. Here we analyze the chromatin structure of the LCR during early hematopoietic differentiation using nontransformed, multipotential, growth factor-dependent, murine hematopoietic progenitor cells. We show that LCR hypersensitive sites characteristic of erythroid cells are present in three independent multilineage progenitors [FDCP (factor-dependent cell, Paterson)-mix A4, B6SUtA, and LyD9] under conditions of self-renewal. Induction of differentiation down a nonerythroid pathway causes a progressive loss of hypersensitivity in the LCR. These results show that the beta-globin LCR is in an active chromatin configuration prior to erythroid commitment and indicate a significant role for selective gene repression in lineage specification.

The mouse beta-globin locus control region: hypersensitive sites 3 and 4

The human beta-globin LCR plays a key role in the transcriptional regulation of the beta-globin locus and comprises four erythroid specific DNase I hypersensitive sites, designated 5'HS1-4. We have now isolated genomic clones containing 5'HS3 and 5'HS4 of the mouse beta-globin LCR. 5'HS3 and 5'HS4 are located 15 kb and 22 kb upstream of the mouse epsilon y-globin gene, respectively. Sequence analysis of murine 5'HS3 and 5'HS4 reveals a significant degree of sequence conservation with their human homologues, including the presence of recognition sites for functionally relevant transcription factors. 5'HS3 and 5'HS4 regions were found to form hypersensitive sites in nuclei from murine erythroid cells, but not in nuclei from a variety of nonerythroid haematopoietic cell lines. Analysis of different mouse strains revealed the existence of a polymorphism that alters the spacing between 5'HS3 and 5'HS4. Taken together, our results emphasize the extent of evolutionary conservation and complexity of mammalian beta-globin LCRs. Finally, the cloning of mouse 5'HS3 and 5'HS4 will facilitate the molecular analysis of LCR function in the mouse model.

Position-independent expression of the ovine beta-lactoglobulin gene in transgenic mice

The major milk whey protein of sheep, beta-lactoglobulin (BLG), is expressed specifically in the mammary gland in a developmentally regulated pattern. To identify the cis-acting DNA regions involved in the regulation of BLG expression, resected gene constructs were analysed in transgenic mice. BLG transgenes which contain at least the proximal 406 bp of the 5' flanking region were expressed in all mice analysed, at levels related to transgene copy number, and thus were expressed in a position-independent manner. Expression was restricted to the mammary gland, except in a few lines where low-level expression was also detected in the salivary gland. In these mice, BLG transgenes were expressed during pregnancy and lactation in the appropriate temporal pattern. Further resection of the 5' proximal region to -146 bp resulted in a dramatically reduced frequency of expression, without affecting tissue specificity, while a construct which retained only 79 bp of 5' flanking region was not expressed. Chromatin analysis of isolated sheep nuclei showed that the promoter resides within a DNAaseI-hypersensitive region in the mammary gland but not in the liver. A BLG transgene displayed a similar tissue-specific pattern of DNAaseI hypersensitivity in mice. These data demonstrate an essential role of the proximal DNAaseI-hypersensitive sequences for position-independent expression of the BLG gene.

The LCR-like alpha-globin positive regulatory element functions as an enhancer in transiently transfected cells during erythroid differentiation

A positive regulatory element (PRE) similar to the locus control region (LCR) of the human beta-globin gene cluster has recently been identified 40 kb upstream of the human zeta-globin mRNA cap site (Higgs D.R. W.G. Wood, A.P. Jarman, J. Sharpe, J. Lida, I.M. Pretorius, and H. Ayyub. 1990). We investigated the influence of the alpha PRE on human alpha-globin promoter activity in transiently transfected cells. The introduction of the alpha PRE into alpha-globin promoter/CAT expression constructs increased alpha-globin promoter activity by 15-30 fold in a human erythroid cell line (Putko) as well as in mouse erythroleukemia cells (MELCs) induced with hexamethylene bisacetamide (HMBA). When these constructs were introduced into uninduced MELCs or HeLa cells, only a 2-3 fold increase in alpha-globin promoter activity was observed. Deletion of 600 bp of alpha-globin 5' flanking sequences containing six putative SP1-binding sites had no significant effect on levels of alpha-globin promoter enhancement by the alpha PRE. We further demonstrated that the alpha PRE and HS2 of the beta-LCR could similarly enhance transcriptional activity of the SV40 early promoter in HMBA induced MELCs. Finally, we showed that alpha-globin promoter activity in the presence of the alpha PRE increased with continued HMBA exposure and was coincident with transcriptional activation of endogenous globin genes.

Adult chicken alpha-globin gene expression in transfected QT6 quail cells: evidence for a negative regulatory element in the alpha D gene region

The chicken adult alpha-globin genes, alpha A and alpha D, are closely linked in chromosomal DNA and are coordinately expressed in vivo in an approximate 3:1 ratio, respectively. When subcloned DNAs containing one or the other gene are stably transfected into QT6 quail fibroblasts, the alpha A-globin gene is expressed at measurable RNA levels, but the alpha D gene is not. The alpha A gene expression can be considerably increased by the presence of a linked Rous sarcoma virus long terminal repeat enhancer, but that of the alpha D gene remains undetectable. Transfection with subclones containing both genes, either in cis or in trans, leads to considerably reduced alpha A RNA levels and still no observable alpha D gene expression. Transfection with deleted subclones suggests that maximal expression levels in this system require the alpha A-globin gene promoter, as opposed to that of the alpha D gene, but that such expression is greatly reduced by one or more DNA sequences which lie approximately 2,000 base pairs upstream of the alpha A gene, within the body of the alpha D gene.

Characterization of the major regulatory element upstream of the human alpha-globin gene cluster

The major positive regulatory activity of the human alpha-globin gene complex has been localized to an element associated with a strong erythroid-specific DNase I hypersensitive site (HS -40) located 40 kb upstream of the zeta 2-globin mRNA cap site. Footprint and gel shift analyses of the element have demonstrated the presence of four binding sites for the nuclear factor GATA-1 and two sites corresponding to the AP-1 consensus binding sequence. This region resembles one of the major elements of the beta-globin locus control region in its constitution and characteristics; this together with evidence from expression studies suggests that HS -40 is a primary element controlling alpha-globin gene expression.

Restriction of neuroblastoma to the prostate gland in transgenic mice

Male transgenic mice that carry a construct containing 5'-flanking sequences of the gp91-phox gene linked to the early region of the simian virus 40 (SV40) genome reproducibly develop tumors arising from the prostate gland. As gp91-phox is expressed exclusively in terminally differentiating hematopoietic cells of the myelomonocytic lineage, the induction of tumors arising from the prostate gland was unexpected. These lesions appear to be due to a novel transcription signal that was generated during the construction of the transgene. Surprisingly, the histopathological and biochemical properties of the tumor are diagnostic of neuroblastoma rather than of adenocarcinoma of the prostate gland. Tumors produce SV40 T antigen and isoforms of neural cell adhesion molecule characteristic of neuronal cells, and they occur in a testosterone-independent manner. Microscopic examination of prostate glands from young transgenic mice reveals the presence of small lesions arising outside of the prostate gland epithelium, which is consistent with the diagnosis of neuroblastoma and further distinguishes this tumor from prostatic adenocarcinoma. Prostate gland tumors occur in all male animals of susceptible lines carrying the gp91-phox promoter/SV40 early-region transgene. However, variability in the time at which gross tumors appear and the presence of cells expressing T antigen prior to tumorigenesis suggest that somatic events in addition to T-antigen production are required for the development of a malignancy. The extraordinary restriction of the site of tumorigenesis in these animals indicates the presence in the prostate gland of a novel, tissue-specific neuroectodermal cell of origin. These transgenic animals provide a model system for the study of neuroectodermal malignancies.

Restriction of neuroblastoma to the prostate gland in transgenic mice

Male transgenic mice that carry a construct containing 5'-flanking sequences of the gp91-phox gene linked to the early region of the simian virus 40 (SV40) genome reproducibly develop tumors arising from the prostate gland. As gp91-phox is expressed exclusively in terminally differentiating hematopoietic cells of the myelomonocytic lineage, the induction of tumors arising from the prostate gland was unexpected. These lesions appear to be due to a novel transcription signal that was generated during the construction of the transgene. Surprisingly, the histopathological and biochemical properties of the tumor are diagnostic of neuroblastoma rather than of adenocarcinoma of the prostate gland. Tumors produce SV40 T antigen and isoforms of neural cell adhesion molecule characteristic of neuronal cells, and they occur in a testosterone-independent manner. Microscopic examination of prostate glands from young transgenic mice reveals the presence of small lesions arising outside of the prostate gland epithelium, which is consistent with the diagnosis of neuroblastoma and further distinguishes this tumor from prostatic adenocarcinoma. Prostate gland tumors occur in all male animals of susceptible lines carrying the gp91-phox promoter/SV40 early-region transgene. However, variability in the time at which gross tumors appear and the presence of cells expressing T antigen prior to tumorigenesis suggest that somatic events in addition to T-antigen production are required for the development of a malignancy. The extraordinary restriction of the site of tumorigenesis in these animals indicates the presence in the prostate gland of a novel, tissue-specific neuroectodermal cell of origin. These transgenic animals provide a model system for the study of neuroectodermal malignancies.

Characterization of the major regulatory element upstream of the human alpha-globin gene cluster

The major positive regulatory activity of the human alpha-globin gene complex has been localized to an element associated with a strong erythroid-specific DNase I hypersensitive site (HS -40) located 40 kb upstream of the zeta 2-globin mRNA cap site. Footprint and gel shift analyses of the element have demonstrated the presence of four binding sites for the nuclear factor GATA-1 and two sites corresponding to the AP-1 consensus binding sequence. This region resembles one of the major elements of the beta-globin locus control region in its constitution and characteristics; this together with evidence from expression studies suggests that HS -40 is a primary element controlling alpha-globin gene expression.

The mitochondrial uncoupling protein gene in brown fat: correlation between DNase I hypersensitivity and expression in transgenic mice

The mitochondrial uncoupling protein gene is rapidly induced in mouse brown fat following cold exposure. To identify cis-regulatory elements, approximately 50 kb of chromatin surrounding the uncoupling protein gene was examined for its hypersensitivity to DNase I. Seven DNase I-hypersensitive sites were identified in the 5'-flanking DNA, and one site was identified in the 3'-flanking DNA. Transgenic mice with an uncoupling protein minigene were generated by microinjection of fertilized eggs with a transgene containing 3 kb of 5'-flanking DNA and 0.3 kb of 3'-flanking DNA. Expression of the transgene is restricted to brown fat and is cold inducible. Four additional transgenic lines were generated with a second transgene containing a 1.8-kb deletion in the 5'-flanking DNA, and expression of this minigene is absent in all tissues analyzed. A DNase I-hypersensitive site located in the 1.8-kb deletion contains a cyclic AMP response element that binds a brown fat tumor enriched nuclear factor. On the basis of these observations, we propose that a cis-acting regulatory sequence between -3 and -1.2 kb of the 5'-flanking region, possibly at a DNase I-hypersensitive site, is required for controlling uncoupling protein expression in vivo.

The mitochondrial uncoupling protein gene in brown fat: correlation between DNase I hypersensitivity and expression in transgenic mice

The mitochondrial uncoupling protein gene is rapidly induced in mouse brown fat following cold exposure. To identify cis-regulatory elements, approximately 50 kb of chromatin surrounding the uncoupling protein gene was examined for its hypersensitivity to DNase I. Seven DNase I-hypersensitive sites were identified in the 5'-flanking DNA, and one site was identified in the 3'-flanking DNA. Transgenic mice with an uncoupling protein minigene were generated by microinjection of fertilized eggs with a transgene containing 3 kb of 5'-flanking DNA and 0.3 kb of 3'-flanking DNA. Expression of the transgene is restricted to brown fat and is cold inducible. Four additional transgenic lines were generated with a second transgene containing a 1.8-kb deletion in the 5'-flanking DNA, and expression of this minigene is absent in all tissues analyzed. A DNase I-hypersensitive site located in the 1.8-kb deletion contains a cyclic AMP response element that binds a brown fat tumor enriched nuclear factor. On the basis of these observations, we propose that a cis-acting regulatory sequence between -3 and -1.2 kb of the 5'-flanking region, possibly at a DNase I-hypersensitive site, is required for controlling uncoupling protein expression in vivo.

Human alpha-globin genes demonstrate autonomous developmental regulation in transgenic mice

Recent studies have demonstrated that transcriptional activation of the human adult beta-globin transgene in mice by coinsertion of the beta-globin cluster locus control region (beta-LCR) results in loss of its adult restricted pattern of expression. Normal developmental control is reestablished by coinsertion of the fetal gamma-globin transgene in cis to the adult beta-globin gene. To test the generality of this interdependence of two globin genes for their proper developmental control, we generated transgenic mice in which the human adult alpha-globin genes are transcriptionally activated by the beta-LCR either alone or in cis to their corresponding embryonic zeta-globin gene. In both cases, the human globin transgenes were expressed at the appropriate developmental period. In contrast to the beta-globin gene, developmental control of the human adult alpha-globin transgenes appears to be autonomous and maintained even when activated by an adjacent locus control region.

Human alpha-globin genes demonstrate autonomous developmental regulation in transgenic mice

Recent studies have demonstrated that transcriptional activation of the human adult beta-globin transgene in mice by coinsertion of the beta-globin cluster locus control region (beta-LCR) results in loss of its adult restricted pattern of expression. Normal developmental control is reestablished by coinsertion of the fetal gamma-globin transgene in cis to the adult beta-globin gene. To test the generality of this interdependence of two globin genes for their proper developmental control, we generated transgenic mice in which the human adult alpha-globin genes are transcriptionally activated by the beta-LCR either alone or in cis to their corresponding embryonic zeta-globin gene. In both cases, the human globin transgenes were expressed at the appropriate developmental period. In contrast to the beta-globin gene, developmental control of the human adult alpha-globin transgenes appears to be autonomous and maintained even when activated by an adjacent locus control region.

The proximal element of the beta globin locus control region is not functionally required in vivo

In addition to local sequence elements the regulation of the high-level, development- and tissue-specific expression of the human beta globin gene cluster appears to require distant regulatory sequences which have been termed locus control region. In the chromatin of erythroid cells the locus control region is characterized by four DNaseI hypersensitive sites that are located 6-18 kb 5' of the epsilon globin gene. The definition of the sequences minimally required for locus control region activity is likely to further the understanding of its physiology and will be of interest for the development of somatic gene therapy strategies of the hemoglobinopathies. We present here the analysis of a family with a 3,030-bp deletion of sequences upstream of the epsilon globin gene including the most 3' locus control region element and cosegregating beta(0) thalassemia. The deletion is linked in cis to a structurally and functionally normal beta globin gene. The proximal element of the locus control region does not therefore appear to be necessary for beta globin gene activity in vivo.

Regulated expression of human alpha- and beta-globin genes in transient heterokaryons

We have examined the expression of human alpha- and beta-like globin genes in transient heterokaryons formed by fusion of human nonerythroid cells with terminally differentiating mouse erythroleukemia (MEL) cells or with a MEL cell variant (GM979) in which the endogenous mouse embryonic beta-globin genes are activated. In both the parental MEL cells and the heterokaryons, the alpha-globin genes were activated at least 12 h earlier than the embryonic, fetal, and adult beta-globin genes. These results suggest that kinetic differences in the activation of alpha- and beta-like globin genes are not simply the result of different rates of accumulation of erythroid-specific regulatory factors but may reflect differences in the mechanisms governing the transcriptional activation of these genes during erythroid cell differentiation. In mouse GM979 x human nonerythroid heterokaryons, the human embryonic beta-globin gene was activated, consistent with our previous demonstration that erythroid cells contain stage-specific trans-acting regulators of globin gene expression. Moreover, a dramatic increase in the ratio of human fetal to adult beta-globin transcription was observed compared with that seen in MEL-human nonerythroid hybrids. This ratio change may reflect competition between the fetal and adult beta-globin genes for productive interactions with erythroid cell-specific regulatory elements. Finally, we demonstrate that the behavior of naturally occurring mutations that lead to aberrant hemoglobin switching in humans also leads to aberrant expression in transient heterokaryons. Therefore, erythroid cells must contain trans-acting factors that interact with mutated regulatory elements to induce high-level expression of the human fetal globin genes.

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.

Regulated expression of human alpha- and beta-globin genes in transient heterokaryons

We have examined the expression of human alpha- and beta-like globin genes in transient heterokaryons formed by fusion of human nonerythroid cells with terminally differentiating mouse erythroleukemia (MEL) cells or with a MEL cell variant (GM979) in which the endogenous mouse embryonic beta-globin genes are activated. In both the parental MEL cells and the heterokaryons, the alpha-globin genes were activated at least 12 h earlier than the embryonic, fetal, and adult beta-globin genes. These results suggest that kinetic differences in the activation of alpha- and beta-like globin genes are not simply the result of different rates of accumulation of erythroid-specific regulatory factors but may reflect differences in the mechanisms governing the transcriptional activation of these genes during erythroid cell differentiation. In mouse GM979 x human nonerythroid heterokaryons, the human embryonic beta-globin gene was activated, consistent with our previous demonstration that erythroid cells contain stage-specific trans-acting regulators of globin gene expression. Moreover, a dramatic increase in the ratio of human fetal to adult beta-globin transcription was observed compared with that seen in MEL-human nonerythroid hybrids. This ratio change may reflect competition between the fetal and adult beta-globin genes for productive interactions with erythroid cell-specific regulatory elements. Finally, we demonstrate that the behavior of naturally occurring mutations that lead to aberrant hemoglobin switching in humans also leads to aberrant expression in transient heterokaryons. Therefore, erythroid cells must contain trans-acting factors that interact with mutated regulatory elements to induce high-level expression of the human fetal globin genes.

Regulated expression of globin chains and the erythroid transcription factor GATA-1 during erythropoiesis in the developing mouse

Erythropoiesis in vertebrates is characterized by sequential changes in erythropoietic site, erythroblast morphology, and hemoglobin synthesis. We have examined the expression of globin chains and the major erythroid transcription factor GATA-1 (previously known as GF-1/NF-E1/Eryf 1) from days 7.5 to 17.5 of mouse development. mRNAs for embryonic (epsilon y2, beta H1, and zeta) and adult (alpha and beta) globin chains were quantitated by RNase protection assays. Switching of globins within the alpha-globin cluster (alpha and zeta) was not strictly coordinated with that within the beta-globin cluster (epsilon y2, beta H1, and beta). Regulation of globin switches during development was primarily transcriptional. Of particular note, we found two developmental switches (beta H1 to epsilon y2 and epsilon y2 to beta) in the mouse, more analogous than previously thought to shifts found in human development. The erythroid transcription factor GATA-1, believed to be a principal regulator of genes expressed in erythroid cells, first appeared in the embryo in yolk sac at the time of blood island formation and remained at a low level during embryonic erythropoiesis (8 to 11 days) relative to that found later in fetal liver (12 to 15 days). The rise in GATA-1 mRNA in fetal liver paralleled and preceded the rapid accumulation of adult beta-globin RNA. RNase protection assays and a GATA-1-specific peptide antiserum were used to establish that a single GATA-1 polypeptide is expressed throughout mouse development. Overall, these findings suggest that the levels of this erythroid transcription factor during development may contribute to the differential gene activation characteristic of definitive versus primitive erythropoiesis.

Developmental regulation of the human zeta globin gene in transgenic mice

We have characterized the expression of the human zeta (zeta) gene, which encodes an embryonic alpha-like globin, in transgenic mice. We find that a 777 base pair fragment spanning erythroid specific hypersensitive site II (HSII) from the distal 5. region of the human beta globin gene cluster potentiates expression of the zeta globin gene. In the absence of the HSII fragment, no zeta expression is observed. Expression of the human zeta gene in mice parallels expression of a murine embryonic alpha-like globin gene (x). Thus, expression of the human zeta gene in mice requires linkage to an erythroid-specific enhancer sequence, but the presence of the enhancer does not affect the developmental regulation of the transgene. Our results indicate that the factors involved in switching from embryonic to adult alpha globin gene expression during development are evolutionarily conserved, and suggest that the transgenic mouse is an in vivo system in which the requirements for the developmental switch in alpha globin gene expression can be analyzed in detail.

Retroviral vectors for the beta-globin gene that demonstrate improved titer and expression

To study the feasibility of a therapy for thalassemia based on addition of a correctly functioning globin gene to bone marrow stem cells, we have developed retroviral vectors that can transfer the human beta-globin gene into pluripotent hematopoietic stem cells of the mouse. Mice reconstituted with virus-infected bone marrow cells showed long-term tissue-specific expression of human beta-globin RNA and protein. Recently, we have redesigned the retroviral vector to improve the efficiency of stem cell infection and to raise the level of globin expression obtained from the virally transduced gene. Removal of a portion of the second intron of the beta-globin gene resulted in the accumulation of a higher level of full-length viral RNA in retrovirus packaging cell lines, and these cell lines produced beta-globin virus particles at substantially higher titers. Addition of fragments from the locus activation region (LAR) of the beta-like globin gene cluster to the retroviral vectors increased beta-globin expression in infected murine erythroleukemia (MEL) cells. Fragments from the -18 and -10.9 kbp DNase I-hypersensitive sites of the LAR increased human beta-globin RNA levels to 35% and 132% of the endogenous mouse beta maj-globin RNA level, respectively. Increased expression was also found for neomycin phosphotransferase RNA, which was transcribed from the retroviral long terminal repeat (LTR), showing that the LAR fragments also activated expression from a nearby heterologous promoter. These results are discussed in the context of the efficacy and safety of gene therapy for chronic anemia in humans.

Regulated expression of globin chains and the erythroid transcription factor GATA-1 during erythropoiesis in the developing mouse

Erythropoiesis in vertebrates is characterized by sequential changes in erythropoietic site, erythroblast morphology, and hemoglobin synthesis. We have examined the expression of globin chains and the major erythroid transcription factor GATA-1 (previously known as GF-1/NF-E1/Eryf 1) from days 7.5 to 17.5 of mouse development. mRNAs for embryonic (epsilon y2, beta H1, and zeta) and adult (alpha and beta) globin chains were quantitated by RNase protection assays. Switching of globins within the alpha-globin cluster (alpha and zeta) was not strictly coordinated with that within the beta-globin cluster (epsilon y2, beta H1, and beta). Regulation of globin switches during development was primarily transcriptional. Of particular note, we found two developmental switches (beta H1 to epsilon y2 and epsilon y2 to beta) in the mouse, more analogous than previously thought to shifts found in human development. The erythroid transcription factor GATA-1, believed to be a principal regulator of genes expressed in erythroid cells, first appeared in the embryo in yolk sac at the time of blood island formation and remained at a low level during embryonic erythropoiesis (8 to 11 days) relative to that found later in fetal liver (12 to 15 days). The rise in GATA-1 mRNA in fetal liver paralleled and preceded the rapid accumulation of adult beta-globin RNA. RNase protection assays and a GATA-1-specific peptide antiserum were used to establish that a single GATA-1 polypeptide is expressed throughout mouse development. Overall, these findings suggest that the levels of this erythroid transcription factor during development may contribute to the differential gene activation characteristic of definitive versus primitive erythropoiesis.

Autonomous developmental control of human embryonic globin gene switching in transgenic mice

The mechanisms by which expression of the beta-like globin genes are developmentally regulated are under intense investigation. The temporal control of human embryonic (epsilon) globin expression was analyzed. A 3.7-kilobase (kb) fragment that contained the entire human epsilon-globin gene was linked to a 2.5-kb cassette of the locus control region (LCR), and the developmental time of expression of this construct was studied in transgenic mice. The human epsilon-globin transgene was expressed in yolk sac-derived primitive erythroid cells, but not in fetal liver or bone marrow-derived definitive erythroid cells. The absence of epsilon gene expression in definitive erythroid cells suggests that the developmental regulation of the epsilon-globin gene depends only on the presence of the LCR and the epsilon-globin gene itself (that is, an autonomous negative control mechanism). The autonomy of epsilon-globin gene developmental control distinguishes it from the competitive mechanism of regulation of gamma and beta-globin genes, and therefore, suggests that at least two distinct mechanisms function in human hemoglobin switching.