Germ-line transmission and developmental regulation of a 150-kb yeast artificial chromosome containing the human beta-globin locus in transgenic mice

Sequences in the (A)gamma-delta intergenic region are not required for stage-specific regulation of the human beta-globin gene locus

The human beta-globin locus has been extensively studied as a model of tissue and developmental stage-specific gene expression. Structural mapping of naturally occurring mutations, including transfection and transgenic studies, and the recent finding of intergenic transcripts have suggested that there are cis-acting sequence elements in the (A)gamma-delta intergenic region involved in regulating gamma- and beta-globin gene expression. To determine whether previously identified sequences in the (A)gamma-delta intergenic region are required for appropriate developmental expression of the human beta-globin gene cluster, transgenic mice were generated by transfer of yeast artificial chromosomes containing the entire human beta-globin locus. Three different deletions of the (A)gamma-delta intergenic region were introduced, including (i) deletion of the 750-bp (A)gamma 3' regulatory element ((A)gammae), (ii) deletion of 3.2 kb upstream of the delta-globin gene encompassing pyrimidine-rich sequences and the recently described intergenic transcript initiation site, and (iii) deletion of a 12.5-kb fragment encompassing most of the (A)gamma-delta globin intergenic region. Analysis of multiple transgenic lines carrying these deletion constructs demonstrated that the normal stage-specific sequential expression of the epsilon -, gamma-, and beta-globin genes was preserved, despite deletion of these putative regulatory sequences. These studies suggest that regulatory sequences required for activation and silencing of the human beta-globin gene family during ontogeny reside proximally to the genes and immediately 5' to the human gamma- and beta-globin genes.

LCR-regulated transgene expression levels depend on the Oct-1 site in the AT-rich region of beta -globin intron-2

Human beta-globin transgenes regulated by the locus control region (LCR) express at all integration sites in transgenic mice. For such LCR activity at ectopic sites, the 5'HS3 element requires the presence of the AT-rich region (ATR) in beta-globin intron-2. Here, we examine the dependence of 5'HS3 LCR activity on transcription factor binding sites in the ATR. In vitro DNaseI footprint analysis and electrophoretic mobility shift assays of the ATR identified an inverted double Gata-1 site composed of 2 noncanonical sequences (GATT and GATG) and an Oct-1 consensus site. Mutant Oct-1, Gata-1, or double mutant sites were created in the ATR of the BGT50 construct composed of a 5'HS3 beta/gamma-globin hybrid transgene. Transgenes with double mutant sites expressed at all sites of integration, but mean expression levels in transgenic mice were reduced from 64% per copy (BGT50) to 37% (P <.05). Mutation of the inverted double Gata-1 site had no effect at 61% per copy expression levels. In contrast, mutation of the Oct-1 site alone reduced per-copy expression levels to 31% (P <.05). We conclude that the ability of 5'HS3 to activate expression from all transgene integration sites is dependent on sequences in the ATR that are not bound at high affinity by transcription factors. In addition, the Oct-1 site in the ATR is required for high-level 5'HS3 beta/gamma-globin transgene expression and should be retained in LCRbeta-globin expression cassettes designed for gene therapy.

Promoters of the murine embryonic beta-like globin genes Ey and betah1 do not compete for interaction with the beta-globin locus control region

Mammalian beta-globin loci contain multiple beta-like genes that are expressed at different times during development. The murine beta-globin locus contains two genes expressed during the embryo stage, Ey and betah1, and two genes expressed at both the fetal and postnatal stages, beta-major and beta-minor. Studies of transgenic human beta-like globin loci in mice have suggested that expression of one gene at the locus will suppress expression of other genes at the locus. To test this hypothesis we produced mouse lines with deletions of either the Ey or betah1 promoter in the endogenous murine beta-globin locus. Promoter deletion eliminated expression of the mutant gene but did not affect expression of the remaining embryonic gene or the fetal-adult beta-globin genes on the mutant allele. These results demonstrate a lack of competitive effects between individual mouse embryonic beta-globin gene promoters and other genes in the locus. The implication of these findings for models of beta-globin gene expression are discussed.

Development of sensitive fluorescent assays for embryonic and fetal hemoglobin inducers using the human beta -globin locus in erythropoietic cells

Reactivation of fetal hemoglobin genes has been proposed as a potential therapeutic procedure in patients with beta-thalassemia, sickle cell disease, or other beta-hemoglobinopathies. In vitro model systems based on small plasmid globin gene constructs have previously been used in human and mouse erythroleukemic cell lines to study the molecular mechanisms regulating the expression of the fetal human globin genes and their reactivation by a variety of pharmacologic agents. These studies have led to great insights in globin gene regulation and the identification of a number of potential inducers of fetal hemoglobin. In this study we describe the development of enhanced green fluorescence protein (EGFP) reporter systems based on bacterial artificial chromosomes (EBACs) to monitor the activity of the epsilon-, (G)gamma-, (A)gamma-, delta-, and beta-globin genes in the beta-globin locus. Additionally, we demonstrate that transfection of erythroleukemia cells with our EBACs is greatly enhanced by expression of EBNA1, which also facilitates episomal maintenance of our constructs in human cells. Our studies in human cells have shown physiologically relevant differences in the expression of each of the globin genes and also demonstrate that hemin is a potent inducer of EGFP expression from EGFP-modified epsilon-, (G)gamma-, and (A)gamma-globin constructs. In contrast, the EGFP-modified delta- and beta-globin constructs consistently produced much lower levels of EGFP expression on hemin induction, mirroring the in vivo ontogeny. The EGFP-modified beta-globin eukaryotic BAC (EBAC) vector system can thus be used in erythroleukemia cells to evaluate induction of the epsilon- and gamma-globin genes from the intact human beta-globin locus.

The murine beta-globin locus control region regulates the rate of transcription but not the hyperacetylation of histones at the active genes

Locus control regions (LCRs) are defined by their ability to confer high-level tissue-specific expression to linked genes in transgenic assays. Previously, we reported that, at its native site, the murine beta-globin LCR is required for high-level beta-globin gene expression, but is not required to initiate an open chromatin conformation of the locus. To further investigate the mechanism of LCR-mediated transcriptional enhancement, we have analyzed allele-specific beta-globin expression and the pattern of histone acetylation in the presence and absence of the LCR. In single cells from mice heterozygous for a deletion of the LCR, beta-globin expression from the LCR-deleted allele is consistently low ( approximately 1-4% of wild type). Thus, the endogenous LCR enhances globin gene expression by increasing the rate of transcription from each linked allele rather than by increasing the probability of establishing transcription per se. Furthermore, in erythroid cells from mice homozygous for the highly expressing wild-type beta-globin locus, hyperacetylation of histones H3 and H4 is localized to the LCR and active genes. In mice homozygous for the LCR deletion reduced histone hyperacetylation is observed in LCR proximal sequences; however, deletion of the LCR has no effect on the localized hyperacetylation of the genes. Together, our results suggest that, in its native genomic context, the LCR follows the rate model of enhancer function, and that the developmentally specific hyperacetylation of the globin genes is independent of both the rate of transcription and the presence of the LCR.

The 5'HS4 core element of the human beta-globin locus control region is required for high-level globin gene expression in definitive but not in primitive erythropoiesis

To assess the contribution of DNase I-hypersensitive site 4 (HS4) of the beta-globin locus control region (LCR) to overall LCR function we deleted a 280 bp fragment encompassing the core element of 5'HS4 from a 248 kb beta-globin locus yeast artificial chromosome (beta-YAC) and analyzed globin gene expression during development in beta-YAC transgenic mice. Four transgenic lines were established; each contained at least one intact copy of the beta-globin locus. The deletion of the 5'HS4 core element had no effect on globin gene expression during embryonic erythropoiesis. In contrast, deletion of the 5'HS4 core resulted in a significant decrease of gamma and beta-globin gene expression during definitive erythropoiesis in the fetal liver and a decrease of beta-globin gene expression in adult blood. We conclude that the core element of 5'HS4 is required for globin gene expression only in definitive erythropoiesis. Absence of the core element of HS4 may limit the ability of the LCR to provide an open chromatin domain and/or enhance gamma and beta-globin gene expression in the adult erythroid cells.

Functional requirements for phenotypic correction of murine beta-thalassemia: implications for human gene therapy

As initial human gene therapy trials for beta-thalassemia are contemplated, 2 critical questions important to trial design and planning have emerged. First, what proportion of genetically corrected hematopoietic stem cells (HSCs) will be needed to achieve a therapeutic benefit? Second, what level of expression of a transferred globin gene will be required to improve beta-thalassemic erythropoiesis? These questions were directly addressed by means of a murine model of severe beta-thalassemia. Generation of beta-thalassemic mice chimeric for a minority proportion of genetically normal HSCs demonstrated that normal HSC chimerism levels as low as 10% to 20% resulted in significant increases in hemoglobin (Hb) level and diminished extramedullary erythropoiesis. A large majority of the peripheral red cells in these mice were derived from the small minority of normal HSCs. In a separate set of independent experiments, beta-thalassemic mice were bred with transgenic mice that expressed different levels of human globins. Human gamma-globin messenger RNA (mRNA) expression at 7% of the level of total endogenous alpha-globin mRNA in thalassemic erythroid cells resulted in improved red cell morphology, a greater than 2-g/dL increase in Hb, and diminished reticulocytosis and extramedullary erythropoiesis. Furthermore, gamma-globin mRNA expression at 13% resulted in a 3-g/dL increase in Hb and nearly complete correction of red cell morphology and other indices of inefficient erythropoiesis. These data indicate that a significant therapeutic benefit could be achieved with expression of a transferred globin gene at about 15% of the level of total alpha-globin mRNA in patients with severe beta-thalassemia in whom 20% of erythroid precursors express the vector genome.

Size matters: use of YACs, BACs and PACs in transgenic animals

In 1993, several groups, working independently, reported the successful generation of transgenic mice with yeast artificial chromosomes (YACs) using standard techniques. The transfer of these large fragments of cloned genomic DNA correlated with optimal expression levels of the transgenes, irrespective of their location in the host genome. Thereafter, other groups confirmed the advantages of YAC transgenesis and position-independent and copy number-dependent transgene expression were demonstrated in most cases. The transfer of YACs to the germ line of mice has become popular in many transgenic facilities to guarantee faithful expression of transgenes. This technique was rapidly exported to livestock and soon transgenic rabbits, pigs and other mammals were produced with YACs. Transgenic animals were also produced with bacterial or P1-derived artificial chromosomes (BACs/PACs) with similar success. The use of YACs, BACs and PACs in transgenesis has allowed the discovery of new genes by complementation of mutations, the identification of key regulatory sequences within genomic loci that are crucial for the proper expression of genes and the design of improved animal models of human genetic diseases. Transgenesis with artificial chromosomes has proven useful in a variety of biological, medical and biotechnological applications and is considered a major breakthrough in the generation of transgenic animals. In this report, we will review the recent history of YAC/BAC/PAC-transgenic animals indicating their benefits and the potential problems associated with them. In this new era of genomics, the generation and analysis of transgenic animals carrying artificial chromosome-type transgenes will be fundamental to functionally identify and understand the role of new genes, included within large pieces of genomes, by direct complementation of mutations or by observation of their phenotypic consequences.

Targeted disruption of the CP2 gene, a member of the NTF family of transcription factors

The NTF-like family of transcription factors have been implicated in developmental regulation in organisms as diverse as Drosophila and man. The two mammalian members of this family, CP2 (LBP-1c/LSF) and LBP-1a (NF2d9), are highly related proteins sharing an overall amino acid identity of 72%. CP2, the best characterized of these factors, is a ubiquitously expressed 66-kDa protein that binds the regulatory regions of many diverse genes. Consequently, a role for CP2 has been proposed in globin gene expression, T-cell responses to mitogenic stimulation, and several other cellular processes. To elucidate the in vivo role of CP2, we have generated mice nullizygous for the CP2 allele. These animals were born in a normal Mendelian distribution and displayed no defects in growth, behavior, fertility, or development. Specifically, no perturbation of hematopoietic differentiation, globin gene expression, or immunological responses to T- and B-cell mitogenic stimulation was observed. RNA and protein analysis confirmed that the nullizygous mice expressed no full-length or truncated version of CP2. Electrophoretic mobility shift assays with nuclear extracts from multiple tissues demonstrated loss of CP2 DNA binding activity in the -/- lines. However, a slower migrating complex that was ablated with antiserum to NF2d9, the murine homologue of LBP-1a, was observed with these extracts. Furthermore, we demonstrate that recombinant LBP-1a can bind to known CP2 consensus sites and form protein complexes with previously defined heteromeric partners of CP2. These results suggest that LBP-1a/NF2d9 may compensate for loss of CP2 expression in vivo and that further analysis of the role of the NTF family of proteins requires the targeting of the NF2d9 gene.

Erythroid Kruppel-like factor (EKLF) coordinates erythroid cell proliferation and hemoglobinization in cell lines derived from EKLF null mice

Erythroid Kruppel-like factor (EKLF) is a transcription factor of the C2H2 zinc-finger class that is essential for definitive erythropoiesis. We generated immortal erythroid cell lines from EKLF(-/-) fetal liver progenitor cells that harbor a single copy of the entire human beta-globin locus and then reintroduced EKLF as a tamoxifen-inducible, EKLF-mutant estrogen receptor (EKLF-ER) fusion protein. Addition of tamoxifen resulted in enhanced differentiation and hemoglobinization, coupled with reduced proliferation. Human beta-globin gene expression increased significantly, whereas gamma-globin transcripts remained elevated at levels close to endogenous mouse alpha-globin transcript levels. We conclude that EKLF plays a role in regulation of the cell cycle and hemoglobinization in addition to its role in beta-globin gene expression. The cell lines we used will facilitate structural and functional analyses of EKLF in these processes and provide useful tools for the elucidation of nonglobin EKLF target genes.

Proper Developmental Control of Human Globin Genes Reproduced by Transgenic Mice Containing a 160-kb BAC Carrying the Human β-Globin Locus

Four independent bacterial artificial chromosome (BAC) clones containing the human Beta-globin gene locus were obtained from a human genomic BAC library. A 160-kb clone (186D7) carrying the entire human Beta-globin locus including the Beta-globin gene family, locus control region (LCR), and 3' regulatory elements was used to transform mice. Four transgenic lines were generated by microinjecting the purified BAC DNA into the fertilized eggs. RNase protection analysis showed that the expression of human Beta-globin genes is tissue- and developmental stage-specific and the expression level is similar among the three independent transgenic lines which carry the entire human Beta-globin locus; however, no Beta-globin gene expression was detected in the transgenic mice lacking the LCR region. The results suggest that the transgenic mouse model system that we have produced and that uses BAC to study the complex human Beta-globin gene cluster is stable and reproducible. Our results also indicate that some newly characterized HSs upstream from the LCR appear not to play an important role in globin gene expression and switching, while the traditional LCR can ensure correct human Beta-globin gene expression in transgenic mice. The BAC-mediated transgenic system can be used for further studies to determine which kinds of cis-acting elements are included in regulating the developmental timing and the level of human Beta-globin gene expression.

Induction of human fetal globin gene expression by a novel erythroid factor, NF-E4

The stage selector protein (SSP) is a heteromeric complex involved in preferential expression of the human gamma-globin genes in fetal-erythroid cells. We have previously identified the ubiquitous transcription factor CP2 as a component of this complex. Using the protein dimerization domain of CP2 in a yeast two-hybrid screen, we have cloned a novel gene, NF-E4, encoding the tissue-restricted component of the SSP. NF-E4 and CP2 coimmunoprecipitate from extract derived from a fetal-erythroid cell line, and antiserum to NF-E4 ablates binding of the SSP to the gamma promoter. NF-E4 is expressed in fetal liver, cord blood, and bone marrow and in the K562 and HEL cell lines, which constitutively express the fetal globin genes. Enforced expression of NF-E4 in K562 cells and primary erythroid progenitors induces endogenous fetal globin gene expression, suggesting a possible strategy for therapeutic intervention in the hemoglobinopathies.

Generation of transgenic rats with YACs and BACs: preparation procedures and integrity of microinjected DNA

The aim of the present study was to investigate differences in the methods for preparing a large DNA fragment to be used for making transgenic rats from the standpoint of transgenic production efficiency and integrity of the introduced gene. In yeast artificial chromosome (YAC) transgenesis, three methods for preparing DNA for microinjection were compared: amplification of YAC in yeast (AMP), amplification of YAC in yeast and removal of the amplification element (AMP/RE), and no amplification of the YAC in yeast (AMP-). Production efficiency per microinjected ovum with DNA by the AMP method was four times higher than that by the AMP/RE and AMP-. Based on these results, we favor the AMP method in spite of the thymidine kinase gene-induced male sterility. In bacterial artificial chromosome (BAC) transgenesis, linear DNA fragments for microinjection prepared by three kinds of purification procedures were compared: Not I digestion and CsCl gradient ultra-centrifugation (Prep. 1), CsCl gradient ultra-centrifugation, Not I digestion, gel electrophoresis, and beta-agarase digestion (Prep. 2), and CsCl gradient ultra-centrifugation, Not I digestion, pulse field gel electrophoresis, and beta-agarase digestion (Prep. 3). Although the efficiency of producing transgenic rats was similar with all these three DNA preparations, integration of the intact DNA fragment only occurred with the Prep. 3 procedure. We therefore favor the Prep. 3 method for preparing BAC DNA fragments. These results indicate that the method used to prepare a large DNA fragment such as YAC and BAC DNAs is important in order to produce transgenic rats with an intact transgene.

Beta-globin gene switching and DNase I sensitivity of the endogenous beta-globin locus in mice do not require the locus control region

We have generated mice with a targeted deletion of the beta-globin locus control region (LCR). Mice homozygous for the deletion die early in embryogenesis but can be rescued with a YAC containing the human beta-globin locus. After germline passage, deletion of the LCR leads to a severe reduction in expression of all mouse beta-like globin genes, but no alteration in the developmental specificity of expression. Furthermore, a DNase I-sensitive "open" chromatin conformation of the locus is established and maintained. Thus, the dominant role of the LCR in the native locus is to confer high-level transcription, and elements elsewhere in the locus are sufficient to establish and maintain an open conformation and to confer developmentally regulated globin gene expression.

Fetal gene transfer by transuterine injection of cationic liposome-DNA complexes

In utero injection of cationic liposome-DNA complexes (CLDCs) containing chloramphenicol acetyltransferase, beta-galactosidase (beta-gal), or human granulocyte colony-stimulating factor (hG-CSF) expression plasmids produced high-level gene expression in fetal rats. Tissues adjacent to the injection site exhibited the highest levels of gene expression. Chloramphenicol acetyltransferase expression persisted for at least 14 days and was reexpressed following postnatal reinjection of CLDCs. Intraperitoneal administration of the hG-CSF gene produced high serum hG-CSF levels. X-gal staining demonstrated widespread beta-gal expression in multiple fetal tissues and cell types. No toxic or inflammatory responses were observed, nor was there evidence of fetal-maternal or maternal-fetal gene transfer, suggesting that CLDCs may provide a useful alternative to viral vectors for in utero gene transfer.

Transgenic Analysis of a 100-kb Human β-Globin Cluster–Containing DNA Fragment Propagated as a Bacterial Artificial Chromosome

To date, the normal transcriptional regulation of the human β-globin gene cluster has been recapitulated most accurately in transgenic mice that carry large yeast artificial chromosome (YAC) or ligated cosmid constructs. However, these large transgenes still exhibit variegated expression levels, perhaps because they tend to rearrange upon integration, or because the cloning vectors remain attached to the globin inserts. To try to circumvent these potential problems, we investigated the transgenic properties of a 100-kb DNA fragment containing the entire human β-globin cluster propagated in a bacterial artificial chromosome (BAC). We created 9 independent mouse lines, each carrying 1 to 6 copies of the human β-globin cluster without the attached BAC vector. Five of the lines carry unrearranged copies of the cluster. Reverse-transcriptase polymerase chain reaction (RT-PCR) analysis of adult F1 mice showed that 2 lines express human β globin at levels approximately equivalent to the endogenous mouse β-major genes. One line expresses no human β globin, while the remaining 6 lines show intermediate expression levels. Complete γ→β-globin gene switching occurs, but is slightly delayed with respect to the endogenous mouse embryonic→adult switch. Since these data are similar to what has been obtained using globin YACs or ligated cosmids, we conclude that (1) globin transgenes propagated in BACs are no less likely to rearrange than their cosmid or YAC counterparts, and (2) the retention of YAC vector sequences in a transgene probably has no significant impact on globin expression when using constructs of this size.

Transgenic Analysis of a 100-kb Human β-Globin Cluster–Containing DNA Fragment Propagated as a Bacterial Artificial Chromosome

To date, the normal transcriptional regulation of the human β-globin gene cluster has been recapitulated most accurately in transgenic mice that carry large yeast artificial chromosome (YAC) or ligated cosmid constructs. However, these large transgenes still exhibit variegated expression levels, perhaps because they tend to rearrange upon integration, or because the cloning vectors remain attached to the globin inserts. To try to circumvent these potential problems, we investigated the transgenic properties of a 100-kb DNA fragment containing the entire human β-globin cluster propagated in a bacterial artificial chromosome (BAC). We created 9 independent mouse lines, each carrying 1 to 6 copies of the human β-globin cluster without the attached BAC vector. Five of the lines carry unrearranged copies of the cluster. Reverse-transcriptase polymerase chain reaction (RT-PCR) analysis of adult F1 mice showed that 2 lines express human β globin at levels approximately equivalent to the endogenous mouse β-major genes. One line expresses no human β globin, while the remaining 6 lines show intermediate expression levels. Complete γ→β-globin gene switching occurs, but is slightly delayed with respect to the endogenous mouse embryonic→adult switch. Since these data are similar to what has been obtained using globin YACs or ligated cosmids, we conclude that (1) globin transgenes propagated in BACs are no less likely to rearrange than their cosmid or YAC counterparts, and (2) the retention of YAC vector sequences in a transgene probably has no significant impact on globin expression when using constructs of this size.

Erythroid Kruppel like factor: from fishing expedition to gourmet meal

Erythroid Kruppel like factor (EKLF) is the founding member of a family of transcription factors which are defined by the presence of three C-terminal C2H2-type zinc fingers. Since its discovery 6 years ago, the study of EKLF has been intense. In this review I will revisit the discovery of EKLF, and highlight recent advances in our understanding of how it interacts with other proteins to regulate erythroid gene transcription. The current knowledge of the biological role/s of EKLF in erythroid cell differentiation and globin gene switching are summarized.

The polyoma virus enhancer cannot substitute for DNase I core hypersensitive sites 2-4 in the human beta-globin LCR

The polyoma virus enhancer (PyE) is capable of conferring integration position-independent expression to linked genes in stably transfected erythroid cells after joining to DNase I hypersensitive site (HS) 5 of the human beta-globin locus control region (LCR). In attempting to separate the chromatin opening activity of the LCR from its enhancer activity and to investigate contributions of the individual HS core elements to LCR function, the human beta-globin LCR HS2, HS3 and HS4 core elements were replaced with the PyE within the context of a yeast artificial chromosome (YAC) bearing the whole locus. We show here that, in contrast to its function in cultured cells, the PyE is unable to replace HS core element function in vivo. We found that the PyE substitution mutant LCR is unable to provide either chromatin opening or transcriptional potentiating activity at any erythroid developmental stage in transgenic mice. These data provide direct evidence that the human beta-globin LCR core elements specify unique functions that cannot be replaced by a ubiquitous enhancer activity.

Effects of altered gene order or orientation of the locus control region on human beta-globin gene expression in mice

The five human beta-type-globin genes, epsilon, Ggamma, Agamma, delta and beta, are close together and are regulated by a locus control region (LCR) located at the 5' end of the locus. Here we investigate the functional consequences of this organization with respect to temporal regulation of the individual genes, by using recombination techniques to invert the order of either the genes or the LCR in vivo. Our analysis of transgenic mice bearing either normal or mutant transgenes leads to two new observations. First, the position of the epsilon-globin gene next to the LCR is mandatory for its expression during the yolk-sac stage of erythropoiesis. Second, LCR activity is orientation dependent, and so the LCR does not act as a simple enhancer to stimulate transcription of the globin genes. Thus, in the absence of any change in transgene integration position, transgene copy number, trans-acting factors or other resident genetic information, simple inversion of the human genes or the LCR fundamentally alters the transcription of beta-type globin genes.

Correct Function of the Locus Control Region May Require Passage Through a Nonerythroid Cellular Environment

The function of the β-globin locus control region (LCR) has been studied both in cell lines and in transgenic mice. We have previously shown that when a 248-kb β-locus YAC was first microinjected into L-cells and then transferred into MEL cells by fusion, the YAC loci of the LxMEL hybrids displayed normal expression and developmental regulation.To test whether direct transfer of a β-globin locus (β-YAC) into MEL cells could be used for studies of the function of the LCR, a 155-kb β-YAC that encompasses the entire β-globin locus was used. This YAC was retrofitted with a PGK-neo selectable marker and with two I-PpoI sites at the vector arm-cloned insert junctions, allowing detection of the intact globin loci on a single I-PpoI fragment by pulsed field gel electrophoresis (PFGE). ThePpo-155 β-YAC was used to directly lipofect MEL 585 cells. In 7 β-YAC MEL clones with at least one intact copy of the YAC, the levels of total human globin mRNA (ie, ɛ + γ + β) per copy of integrated β-YAC varied more than 97-fold between clones. These results indicated that globin gene expression was strongly influenced by the position of integration of the β-YAC into the MEL cell genome and suggested that the LCR cannot function properly when the locus is directly transferred into an erythroid cell environment as naked β-YAC DNA. To test whether passage of the β-YAC through L-cells before transfer into MEL cells was the reason for the previously observed correct developmental regulation of human globin genes in the LxMEL hybrid cells, we transfected the YAC into L-cells by lipofection. Three clones carried the intact 144-kb I-PpoI fragment and transcribed the human globin genes with a fetal-like pattern. Subsequent transfer of the YAC of these L(β-YAC) clones into MEL cells by fusion resulted in LxMEL hybrids that synthesized human globin mRNA. The variation in human β-globin mRNA (ie, ɛ + γ + β) levels between hybrids was 2.5-fold, indicating that globin gene expression was independent of position of integration of the transgene, as expected for normal LCR function. The correct function of the LCR when the YAC is first transferred into the L-cell environment raises the possibility that normal activation of the LCR requires interaction with the transcriptional environment of an uncommitted, nonerythroid cell. We propose that the activation of the LCR may represent a multistep process initiated by the binding of ubiquitous transcription factors early during the differentiation of hematopoietic stem cells and completed with the binding of erythroid type of factors in the committed erythroid progenitors.

Correct Function of the Locus Control Region May Require Passage Through a Nonerythroid Cellular Environment

The function of the β-globin locus control region (LCR) has been studied both in cell lines and in transgenic mice. We have previously shown that when a 248-kb β-locus YAC was first microinjected into L-cells and then transferred into MEL cells by fusion, the YAC loci of the LxMEL hybrids displayed normal expression and developmental regulation.To test whether direct transfer of a β-globin locus (β-YAC) into MEL cells could be used for studies of the function of the LCR, a 155-kb β-YAC that encompasses the entire β-globin locus was used. This YAC was retrofitted with a PGK-neo selectable marker and with two I-PpoI sites at the vector arm-cloned insert junctions, allowing detection of the intact globin loci on a single I-PpoI fragment by pulsed field gel electrophoresis (PFGE). ThePpo-155 β-YAC was used to directly lipofect MEL 585 cells. In 7 β-YAC MEL clones with at least one intact copy of the YAC, the levels of total human globin mRNA (ie, ɛ + γ + β) per copy of integrated β-YAC varied more than 97-fold between clones. These results indicated that globin gene expression was strongly influenced by the position of integration of the β-YAC into the MEL cell genome and suggested that the LCR cannot function properly when the locus is directly transferred into an erythroid cell environment as naked β-YAC DNA. To test whether passage of the β-YAC through L-cells before transfer into MEL cells was the reason for the previously observed correct developmental regulation of human globin genes in the LxMEL hybrid cells, we transfected the YAC into L-cells by lipofection. Three clones carried the intact 144-kb I-PpoI fragment and transcribed the human globin genes with a fetal-like pattern. Subsequent transfer of the YAC of these L(β-YAC) clones into MEL cells by fusion resulted in LxMEL hybrids that synthesized human globin mRNA. The variation in human β-globin mRNA (ie, ɛ + γ + β) levels between hybrids was 2.5-fold, indicating that globin gene expression was independent of position of integration of the transgene, as expected for normal LCR function. The correct function of the LCR when the YAC is first transferred into the L-cell environment raises the possibility that normal activation of the LCR requires interaction with the transcriptional environment of an uncommitted, nonerythroid cell. We propose that the activation of the LCR may represent a multistep process initiated by the binding of ubiquitous transcription factors early during the differentiation of hematopoietic stem cells and completed with the binding of erythroid type of factors in the committed erythroid progenitors.

Partial rescue of GATA-3 by yeast artificial chromosome transgenes

GATA-3 is essential for murine embryonic development, but elucidating the genetic controls over the complex temporal and tissue-specific transcriptional regulatory pattern of this transcription factor gene has been problematic. Here we report the isolation and characterization of two yeast artificial chromosomes (YACs) bearing the murine GATA-3 gene. Ordered deletions of both YACs show that they define a 1-megabase pair contig spanning the GATA-3 locus. We found that a 120-kb YAC transgene, including 35 kb of 5' as well as 60 kb of 3' flanking sequence, confers normal GATA-3 expression at sites not revealed previously through analysis of plasmid transgenic lines. However, even this 120-kb YAC does not contain sufficient information to recapitulate the complete GATA-3 expression program during embryogenesis. While not complete in its regulatory capacity, the YAC transgene is nonetheless able to complement several homozygous GATA-3 mutant phenotypes and thereby prolong embryonic life.

Transgenic knockout mice exclusively expressing human hemoglobin S after transfer of a 240-kb betas-globin yeast artificial chromosome: A mouse model of sickle cell anemia

Sickle cell anemia (SCA) and thalassemia are among the most common genetic diseases worldwide. Current approaches to the development of murine models of SCA involve the elimination of functional murine alpha- and beta-globin genes and substitution with human alpha and betas transgenes. Recently, two groups have produced mice that exclusively express human HbS. The transgenic lines used in these studies were produced by coinjection of human alpha-, gamma-, and beta-globin constructs. Thus, all of the transgenes are integrated at a single chromosomal site. Studies in transgenic mice have demonstrated that the normal gene order and spatial organization of the members of the human beta-globin gene family are required for appropriate developmental and stage-restricted expression of the genes. As the cis-acting sequences that participate in activation and silencing of the gamma- and beta-globin genes are not fully defined, murine models that preserve the normal structure of the locus are likely to have significant advantages for validating future therapies for SCA. To produce a model of SCA that recapitulates not only the phenotype, but also the genotype of patients with SCA, we have generated mice that exclusively express HbS after transfer of a 240-kb betas yeast artificial chromosome. These mice have hemolytic anemia, 10% irreversibly sickled cells in their peripheral blood, reticulocytosis, and other phenotypic features of SCA.

Substitution of the human beta-spectrin promoter for the human agamma-globin promoter prevents silencing of a linked human beta-globin gene in transgenic mice

During development, changes occur in both the sites of erythropoiesis and the globin genes expressed at each developmental stage. Previous work has shown that high-level expression of human beta-like globin genes in transgenic mice requires the presence of the locus control region (LCR). Models of hemoglobin switching propose that the LCR and/or stage-specific elements interact with globin gene sequences to activate specific genes in erythroid cells. To test these models, we generated transgenic mice which contain the human Agamma-globin gene linked to a 576-bp fragment containing the human beta-spectrin promoter. In these mice, the beta-spectrin Agamma-globin (betasp/Agamma) transgene was expressed at high levels in erythroid cells throughout development. Transgenic mice containing a 40-kb cosmid construct with the micro-LCR, betasp/Agamma-, psibeta-, delta-, and beta-globin genes showed no developmental switching and expressed both human gamma- and beta-globin mRNAs in erythroid cells throughout development. Mice containing control cosmids with the Agamma-globin gene promoter showed developmental switching and expressed Agamma-globin mRNA in yolk sac and fetal liver erythroid cells and beta-globin mRNA in fetal liver and adult erythroid cells. Our results suggest that replacement of the gamma-globin promoter with the beta-spectrin promoter allows the expression of the beta-globin gene. We conclude that the gamma-globin promoter is necessary and sufficient to suppress the expression of the beta-globin gene in yolk sac erythroid cells.

The A gamma-globin 3' element provides no unique function(s) for human beta-globin locus gene regulation

The human beta-globin locus is activated transcriptionally by a complex series of events that culminate in appropriate temporal and tissue-specific control over five separate genes during embryonic and early postnatal development. One cis-regulatory element in the locus, originally identified as an enhancer 3' to the Agamma-globin gene, more recently has been suggested to harbor alternative or additional properties, including stage-specific silencer, insulator, nuclear matrix, or chromosome scaffold attachment activities. We have re-evaluated the activity during erythropoiesis that is conferred by this element by deleting it from a yeast artificial chromosome (YAC) containing the entire human beta-globin locus and then assaying for the expression of each gene at each developmental stage after incorporation of the mutant YAC into the mouse germline. The data show that loss of the Agamma-globin 3' element confers no phenotype in six independent lines of intact YAC mutant transgenic mice, thus demonstrating (minimally) that any activities attributable to this element are fully compensated by other DNA sequences within the beta-globin locus.

Developmental specificity of the interaction between the locus control region and embryonic or fetal globin genes in transgenic mice with an HS3 core deletion

The human beta-globin locus control region (LCR) consists of five erythroid-lineage-specific DNase I-hypersensitive sites (HSs) and is required for activation of the beta-globin locus chromatin domain and globin gene expression. Each DNase I-HS of the LCR consists of a highly conserved core element and flanking sequences. To analyze the functional role of the core elements of the HSs, we deleted a 234-bp fragment encompassing the core of HS3 (HS3c) from a beta-globin locus residing on a 248-kb beta-locus yeast artificial chromosome and analyzed its function in F2 progeny of transgenic mice. Human epsilon-globin gene expression was absent at day 10 and severely reduced in the day 12 embryonic erythropoiesis of mice lacking HS3c. In contrast, gamma-globin gene expression was normal in embryonic erythropoiesis but it was absent in definitive erythropoiesis in the fetal liver. These results indicate that the core element of HS3 is necessary for epsilon-globin gene transcription in embryonic cells and for gamma-globin gene transcription in definitive cells. Normal gamma-globin gene expression in embryonic cells and the absence of gamma-globin gene expression in definitive cells show that different HSs interact with gamma-globin gene promoters in these two stages of development. Such results provide direct evidence for developmental stage specificity of the interactions between the core elements of HSs and the promoters of the globin genes.

beta-YAC transgenic mice for studying LCR function

We have developed methods to produce transgenic mice using yeast artificial chromosomes (YACs) and have applied these methods to the analysis of globin gene regulation using 248 kb beta-globin locus YACs (beta-YACs). The advantages of YAC transgenics are: 1) developmental regulation can be studied in the context of the whole locus, 2) mutations may be readily introduced into the YAC, and 3) the effect of these mutations on gene expression can be analyzed. Mice containing the wild-type beta-YAC show proper regulation of globin gene expression during development. Transgenics carrying a beta-YAC bearing a -117 A gamma mutation showed the anticipated phenotype of Greek HPFH, demonstrating that mutant beta-YACs can be used to generate mice that recreate human globin developmental mutants. Transgenic mice with YACs have also been used to examine the function of the LCR. Transgenic mice were generated with a beta-YAC containing a deletion of LCR DNAse I-hypersensitive site 3 (5'HS3). Our results suggest that: 1) the LCR contains functionally redundant elements, 2) the formation of a LCR complex does not require all of the HSs, 3) the individual HSs may modulate the interaction of the LCR with specific globin genes during development, and 4) that most of the HS activity is confined to the core region.

Reduced beta-globin gene expression in adult mice containing deletions of locus control region 5' HS-2 or 5' HS-3

To gain insights into the functions of individual DNA'se hypersensitive sites within the beta globin locus control region (LCR), we deleted the endogenous 5' HS-2 and HS-3 regions from the mouse germline using homologous recombination techniques. We demonstrated that the deletion of either murine 5' HS-2 or 5' HS-3 reduced the expression of the embryonic epsilon y and beta h1 globin genes minimally in yolk sac-derived erythrocytes, but that both knockouts reduced the output of the adult beta (beta-Major + beta-Minor) globin genes by approximately 30% in adult erythrocytes. When the selectable marker PGK-Neo cassette was retained within either the HS-2 or HS-3 region, a much more severe reduction in globin gene expression was observed at all developmental stages. PGK-Neo was shown to be expressed in an erythroid-specific fashion when it was retained in the HS-3 position. These results show that neither 5' HS-2 nor HS-3 is required for the activity of embryonic globin genes, nor are these sites required for correct developmental switching. However, each site is required for approximately 30% of the total LCR activity associated with adult beta-globin gene expression in adult red blood cells. Each site therefore contains some non-redundant information that contributes to adult globin gene function.

Position-independent expression of a human nerve growth factor-luciferase reporter gene cloned on a yeast artificial chromosome vector

Two yeast artificial chromosomes containing the entire human nerve growth factor gene were isolated and mapped. By homologous recombination a luciferase gene was precisely engineered into the coding portion of the NGF gene and a neomycin selection marker was placed adjacent to one of the YAC telomeres. Expression of the YAC-based NGF reporter gene and a plasmid-based NGF reporter gene were compared with the regulation of endogenous mouse NGF protein in mouse L929 fibroblasts. In contrast to the plasmid-based reporter gene, expression and regulation of the YAC-based reporter gene was independent of the site of integration of the transgene. Basic fibroblast growth factor and okadaic acid stimulated expression of the YAC transgene, whereas transforming growth factor-beta and dexamethasone inhibited it. Although cyclic AMP strongly stimulated production of the endogenous mouse NGF, no effect was seen on the human NGF reporter genes. Downregulation of the secretion of endogenous mouse NGF already occurred at an EC50 of 1-2 nM dexamethasone, but downregulation of the expression of NGF reporter genes occurred only at EC50 of 10 nM. This higher concentration was also required for upregulation of luciferase genes driven by the dexamethasone-inducible promoter of the mouse mammary tumor virus in L929 fibroblasts.

Exploring gene function: use of yeast artificial chromosome transgenesis

Transgenesis is a very powerful tool in functional analysis of proteins and control of gene expression. One of the main drawbacks has been the low levels of expression, lack of tissue specificity, and inappropriate expression frequently observed for transgenes made with small plasmid-based constructs. The use of much larger DNA fragments cloned in yeast artificial clones (YACs), bacterial artificial clones, or P1-based artificial clones has been found to give much better levels of expression, generally very close to that of an endogenous gene, and tissue-specific expression matching that of the endogenous gene. In addition, the large DNA can easily be subtly modified by homologous recombination. This article describes the background and methods of YAC transgenesis.

The Human β Globin Locus Introduced by YAC Transfer Exhibits a Specific and Reproducible Pattern of Developmental Regulation in Transgenic Mice

The human β globin locus spans an 80-kb chromosomal region encompassing both the five expressed globin genes and the cis-acting elements that direct their stage-specific expression during ontogeny. Sequences proximal to the genes and in the locus control region, 60 kb upstream of the adult β globin gene, are required for developmental regulation. Transgenic studies have shown that altering the structural organization of the locus disrupts the normal pattern of globin gene regulation. Procedures for introducing yeast artificial chromosomes (YACs) containing large genetic loci now make it possible to define the sequences required for stage-restricted gene expression in constructs that preserve the integrity of the β globin locus. We demonstrate that independent YAC transgenic lines exhibit remarkably similar patterns of globin gene expression during development. The switch from γ to β globin predominant expression occurs between day 11.5 and 12.5 of gestation, with no more than twofold differences in human β globin mRNA levels between lines. Human β globin mRNA levels were twofold to fourfold lower than that of mouse βmaj, revealing potentially significant differences in the regulatory sequences of the two loci. These findings provide an important basis for studying regulatory elements within the β globin locus.

The Human β Globin Locus Introduced by YAC Transfer Exhibits a Specific and Reproducible Pattern of Developmental Regulation in Transgenic Mice

The human β globin locus spans an 80-kb chromosomal region encompassing both the five expressed globin genes and the cis-acting elements that direct their stage-specific expression during ontogeny. Sequences proximal to the genes and in the locus control region, 60 kb upstream of the adult β globin gene, are required for developmental regulation. Transgenic studies have shown that altering the structural organization of the locus disrupts the normal pattern of globin gene regulation. Procedures for introducing yeast artificial chromosomes (YACs) containing large genetic loci now make it possible to define the sequences required for stage-restricted gene expression in constructs that preserve the integrity of the β globin locus. We demonstrate that independent YAC transgenic lines exhibit remarkably similar patterns of globin gene expression during development. The switch from γ to β globin predominant expression occurs between day 11.5 and 12.5 of gestation, with no more than twofold differences in human β globin mRNA levels between lines. Human β globin mRNA levels were twofold to fourfold lower than that of mouse βmaj, revealing potentially significant differences in the regulatory sequences of the two loci. These findings provide an important basis for studying regulatory elements within the β globin locus.

Knockout-transgenic mouse model of sickle cell disease

When transgenic mice that expressed human sickle hemoglobin were mated with mice having knockout mutations of the mouse alpha- and beta-globin genes, animals were produced that synthesized only human hemoglobin in adult red blood cells. Similar to many human patients with sickle cell disease, the mice developed a severe hemolytic anemia and extensive organ pathology. Numerous sickled erythrocytes were observed in peripheral blood. Although chronically anemic, most animals survived for 2 to 9 months and were fertile. Drug and genetic therapies can now be tested in this mouse model of sickle cell disease.

Two vectors for the insertion of mammalian selectable genes into yeast artificial chromosome cloned DNA

The introduction of cloned DNA into mammalian cells allows functional testing of genes contained on the fragments. In many cases, the exogenous DNA introduced into mammalian cells requires selectable genes that mark the presence of input DNA. Two new vectors, carrying mammalian selectable markers encoding for either neomycin-resistance (neo) or histidinol-resistance (hol), have been constructed for targeted integration to specific single-copy sites within yeast artificial chromosome (YAC) insert DNA. The integration cassettes comprise a single selectable yeast gene adjacent to a mammalian selectable gene, either LEU2 with neo or HIS3 with hol. Modification of the YAC occurs in yeast by transfection with linear DNA containing YAC-specific, unique, recombinogenic ends, thereby ensuring co-integration of the markers. Analysis of modified YACs confirms that both vectors correctly integrate into the targeted unique sites. The precise localization of selectable marker genes in the cloned DNA ensures the integrity of the genomic fragments during functional testing. Placement of mammalian selectable markers within the YAC insert DNA should allow for YAC-based gene targeting experiments in a variety of mammalian cell lines.

Position-independent and high-level expression of human alpha-lactalbumin in the milk of transgenic rats carrying a 210-kb YAC DNA

The level of expression of transgenes in transgenic animals varies among lines, and is often much lower than that of endogenous genes (position effects). In order to surmount position effects and establish a more efficient production system of transgenic animals producing pharmaceutical proteins in their milk, transgenic rats carrying 210-kb YAC DNA containing the human alpha-lactalbumin gene were produced. Three transgenic lines transmitted the transgene to the next generation. They had one copy of the alpha-lactalbumin gene and secreted human alpha-lactalbumin in their milk at concentrations of 2.0-4.3 mg/ml. No position effect was seen. The transgene was expressed specifically in the mammary gland of the transgenic rats. The 210-kb region is thought to contain all the DNA elements required for proper expression of the human alpha-lactalbumin gene. The YAC carrying the human alpha-lactalbumin gene is a potential vector for the expression of foreign genes in the mammary gland.

Regulation of the myeloid-cell-expressed human gp91-phox gene as studied by transfer of yeast artificial chromosome clones into embryonic stem cells: suppression of a variegated cellular pattern of expression requires a full complement of distant cis elements

Identifying the full repertoire of cis elements required for gene expression in mammalian cells (or animals) is challenging, given the moderate sizes of many loci. To study how the human gp91-phox gene is expressed specifically in myeloid hematopoietic cells, we introduced yeast artificial chromosome (YAC) clones and derivatives generated in yeast into mouse embryonic stem cells competent to differentiate to myeloid cells in vitro or into mouse chimeras. Fully appropriate regulation was recapitulated with a 130-kb YAC containing 60 and 30 kb of 5' and 3' flanking sequences, respectively. Immunodetection of human gp91-phox protein revealed uniform expression in individual myeloid cells. The removal of upstream sequences led to decreased overall expression which reflected largely a variegated pattern of expression, such that cells were either "on" or "off," rather than pancellular loss of expression. The proportion of clones displaying marked variegation increased with progressive deletion. DNase I mapping of chromatin identified two hypersensitive clusters, consistent with the presence of multiple regulatory elements. Our findings point to cooperative interactions of complex regulatory elements and suggest that the presence of an incomplete set of elements reduces the probability that an open chromatin domain (or active transcriptional complex) may form or be maintained in the face of repressive influences of neighboring chromatin.

High levels of human gamma-globin gene expression in adult mice carrying a transgene of deletion-type hereditary persistence of fetal hemoglobin

Persistent expression of the gamma-globin genes in adults with deletion types of hereditary persistence of fetal hemoglobin (HPFH) is thought to be mediated by enhancer-like effects of DNA sequences at the 3' breakpoints of the deletions. A transgenic mouse model of deletion-type HPFH was generated by using a DNA fragment containing both human gamma-globin genes and HPFH-2 breakpoint DNA sequences linked to the core sequences of the locus control region (LCR) of the human beta-globin gene cluster. Analysis of gamma-globin expression in six HPFH transgenic lines demonstrated persistence of gamma-globin mRNA and peptides in erythrocytes of adult HPFH transgenic mice. Analysis of the hemoglobin phenotype of adult HPFH transgenic animals by isoelectric focusing showed the presence of hybrid mouse alpha2-human gamma2 tetramers as well as human gamma4 homotetramers (hemoglobin Bart's). In contrast, correct developmental regulation of the gamma-globin genes with essentially absent gamma-globin gene expression in adult erythroid cells was observed in two control non-HPFH transgenic lines, consistent with autonomous silencing of normal human gamma-globin expression in adult transgenic mice. Interestingly, marked preferential overexpression of the LCR-distal (A)gamma-globin gene but not of the LCR-proximal (G)gamma-globin gene was observed at all developmental stages in erythroid cells of HPFH-2 transgenic mice. These findings were also associated with the formation of a DNase I-hypersensitive site in the HPFH-2 breakpoint DNA of transgenic murine erythroid cells, as occurs in normal human erythroid cells in vivo. These results indicate that breakpoint DNA sequences in deletion-type HPFH-2 can modify the developmentally regulated expression of the gamma-globin genes.

Evaluation of beta-globin gene therapy constructs in single copy transgenic mice

Effective gene therapy constructs based on retrovirus or adeno-associated virus vectors will require regulatory elements that direct expression of genes transduced at single copy. Most beta-globin constructs designed for therapy of beta-thalassemias are regulated by the 5'HS2 component of the locus control region (LCR). Here we show that a human beta-globin gene flanked by two small 5'HS2 core elements or flanked by a 5'HS3 (footprints 1-3) core and a 5'HS2 core are not reproducibly expressed in single copy transgenic mice. In addition, low copy transgene concatamers that contain only dimer 5'HS2 cores fail to express, whereas those that contain monomer 5'HS2 cores express at 14% per copy. These data suggest that spacing between HS cores is crucial for LCR activity. We therefore constructed a novel 3.0 kb LCR cassette in which the 5'HS2, 5'HS3 and 5'HS4 cores are each separated by approximately 700 bp. When linked to the 815 bp beta-globin promoter this LCR directs 45% levels of expression from four independent single copy transgenes. However, the 3.0 kb LCR linked to the 265 bp promoter expresses variable levels, averaging 18%, from three single copy transgenes. Our findings suggest that sequences in the distal promoter play a role in single copy transgene activation and that larger LCR and promoter elements are most suitable for gene therapy applications.

Globin Gene Switching in Transgenic Mice Carrying HS2-Globin Gene Constructs

We have examined the pattern of human globin gene switching in transgenic mice containing three different γ and β gene constructs (HS2GγAγδβ, HS2Aγβneo, and HS2Aγenβ) and compared the results with previously described transgenics (HS2Aγβ, HS2GγAγ-117δβ, and LCRεGγAγδβ). Developmental regulation was observed in all cases with identical patterns in lines bearing the same construct. Three different patterns of switching were observed: LCRεGγAγδβ and HS2Aγβneo mice switched rapidly, HS2GγAγδβ and HS2GγAγ-117δβ at an intermediate rate, and HS2Aγβ and HS2Aγenβ mice showed delayed switching, with a plateau in late fetal-early neonatal life and readily detectable levels of γ mRNA in adults. No difference was observed in the time of switching of the HS2GγAγδβ mice compared with those with the Aγ-117 hereditary persistence of fetal hemoglobin mutation, but adult levels of γ mRNA were significantly higher (≈5%) in lines carrying the mutation than in those without (≈1%). Reversion to the rapid switch of the LCRεGγAγδβ mice was observed in three lines with the HS2Aγβ neo construct in which expression of the tk-neo gene was approximately equal to that of the globin genes. The inclusion of the Aγ enhancer in HS2Aγβ mice did not alter the pattern of switching, or reduce the relatively high levels of γ mRNA in these lines. However, unlike other HS2 mice, the combination of HS2 and the Aγ enhancer resulted in copy number-dependent expression in HS2Aγenβ lines, with intrauterine death at ≈12.5 days gestation at high copy numbers. These results demonstrate that numerous elements throughout the β globin gene cluster interact to produce the correct pattern of developmental regulation of these genes. Furthermore, extinction of γ gene expression in adult life is not completely autonomous and is incomplete when HS2 is the only LCR element present.

Mutation of gene-proximal regulatory elements disrupts human epsilon-, gamma-, and beta-globin expression in yeast artificial chromosome transgenic mice

Previous studies have defined transcriptional control elements, in addition to the promoters, that both lie near individual human beta-globin locus genes and have been implicated in their differential stage-specific regulation during development (i.e., are believed to directly participate in hemoglobin switching). We have reinvestigated the activities during erythropoiesis that might be conferred by two of the more intensively analyzed of these elements, the epsilon-globin gene 5' silencer and the beta-globin gene 3' enhancer, by deleting them from a yeast artificial chromosome that spans the human beta-globin locus, and then analyzing transgenic mice for expression of all of the human genes. These studies show that sequences within the epsilon-globin "silencer" are not only required for silencing but are also required for activation of epsilon-globin transcription; furthermore, deletion of the silencer simultaneously reduced gamma-globin transcription during the yolk sac stage of erythroid development. Analysis of the adult beta-globin gene 3' enhancer deletion showed that its deletion affects only that gene.

Silencing of human fetal globin expression is impaired in the absence of the adult beta-globin gene activator protein EKLF

Globin genes are subject to tissue-specific and developmental stage-specific regulation. A switch from human fetal (gamma)-to adult (beta)-globin expression occurs within erythroid precursor cells of the adult lineage. Previously we and others showed by targeted gene disruption that the zinc finger gene, erythroid Krüppel-like factor (EKLF), is required for expression of the beta-globin gene in mice, presumably through interaction with a high-affinity binding site in the proximal promoter. To examine the role of EKLF in the developmental regulation of the human gamma-globin gene we interbred EKLF heterozygotes (+/-) with mice harboring a human beta-globin yeast artificial chromosome transgene. We find that in the absence of EKLF, while human beta-globin expression is dramatically reduced, gamma-globin transcripts are elevated approximately 5-fold. Impaired silencing of gamma-globin expression identifies EKLF as the first transcription factor participating quantitatively in the gamma-globin to beta-globin switch. Our findings are compatible with a competitive model of switching in which EKLF mediates an adult stage-specific interaction between the beta-globin gene promoter and the locus control region that excludes the gamma-globin gene.

Inappropriate splicing of a chimeric gene containing a large internal exon results in exon skipping in transgenic mice

We generated transgenic mice containing a chimeric construct consisting of the alpha-cardiac myosin heavy chain (alpha cMHC) promoter and the human renin (hRen) gene in order to target hRen synthesis specifically to the heart. The construct consisted of three segments: (i) an alpha cMHC DNA segment including 4.5 kb of 5' flanking DNA and an additional 1.1 kb of genomic DNA encompassing exons I-III (non-coding) and the first two introns; (ii) a partial hRen cDNA consisting of exons I-VI; and (iii) a hRen genomic segment containing exons VII through IX, their intervening introns, and 400 bp of 3' flanking DNA. This results in the formation of a 909 bp internal fusion exon consisting of alpha cMHC, polylinker, and hRen sequences. Despite the presence of splice acceptor and donor sites bracketing this exon, transcription of this transgene resulted in a major alternatively spliced mRNA lacking the exon and therefore a majority of the hRen coding sequence. Cloning and sequencing of RT-PCR products from several heart samples from two independent transgenic lines confirmed accurate and faithful splicing of alpha cMHC exon II to hRen exon VII thus bypassing the internal fusion exon. All other exons (alpha cMHC exons I and II and hRen exons VII, VIII and IX) were appropriately spliced. These results are consistent with the hypothesis on exon definition which states that internal exons have a size limitation. Moreover, the results demonstrate that transgenes present in the genome at independent insertion sites and in either a single copy or multiple copies can be subject to exon skipping. The implications for transgene design will be discussed.

Effect of deletion of 5'HS3 or 5'HS2 of the human beta-globin locus control region on the developmental regulation of globin gene expression in beta-globin locus yeast artificial chromosome transgenic mice

To analyze the function of the 5' DNase I hypersensitive sites (HSs) of the locus control region (LCR) on beta-like globin gene expression, a 2.3-kb deletion of 5'HS3 or a 1.9-kb deletion of 5'HS2 was recombined into a beta-globin locus yeast artificial chromosome, and transgenic mice were produced. Deletion of 5'HS3 resulted in a significant decrease of epsilon-globin gene expression and an increase of gamma-globin gene expression in embryonic cells. Deletion of 5'HS2 resulted in only a small decrease in expression of epsilon-, gamma-, and beta-globin mRNA at all stages of development. Neither deletion affected the temporal pattern of globin gene switching. These results suggest that the LCR contains functionally redundant elements and that LCR complex formation does not require the presence of all DNase I hypersensitive sites. The phenotype of the 5'HS3 deletion suggests that individual HSs may influence the interaction of the LCR with specific globin gene promoters during the course of ontogeny.

Analysis of mice containing a targeted deletion of beta-globin locus control region 5' hypersensitive site 3

To examine the function of murine beta-globin locus region (LCR) 5' hypersensitive site 3 (HS3) in its native chromosomal context, we deleted this site from the mouse germ line by using homologous recombination techniques. Previous experiments with human 5' HS3 in transgenic models suggested that this site independently contains at least 50% of total LCR activity and that it interacts preferentially with the human gamma-globin genes in embryonic erythroid cells. However, in this study, we demonstrate that deletion of murine 5' HS3 reduces expression of the linked embryonic epsilon y- and beta H 1-globin genes only minimally in yolk sac-derived erythroid cells and reduces output of the linked adult beta (beta major plus beta minor) globin genes by approximately 30% in adult erythrocytes. When the selectable marker PGK-neo cassette was left within the HS3 region of the LCR, a much more severe phenotype was observed at all developmental stages, suggesting that PGK-neo interferes with LCR activity when it is retained within the LCR. Collectively, these results suggest that murine 5' HS3 is not required for globin gene switching; importantly, however, it is required for approximately 30% of the total LCR activity associated with adult beta-globin gene expression in adult erythrocytes.

YAC transgenesis in farm animals: rescue of albinism in rabbits

The generation of transgenic mice with mammalian genes cloned in yeast artificial chromosomes (YACs) has generated great interest in the field of gene transfer into livestock. Many of the problems associated with standard transgenesis-such as lack of crucial regulator elements and position effects related to the integration site, which lead to variation in expression levels irrespective of the dose of the transgene-have been practically overcome. The large size of YAC-derived gene constructs (in excess of 1 Mb) facilitates the presence and transfer of all elements required for the faithful regulation of a gene. With the experiments discussed in this report, we have addressed the possibility of applying the obvious advantages of YAC transgenesis to farm animals. We have generated transgenic rabbits carrying a 250 kb YAC covering the mouse tyrosinase gene by pronuclear microinjection, and thus rescued the albino phenotype of the transgenic individuals. To date, this is the first demonstration of a successful transfer of large genetic units into the germ line of farm animals. This development might improve the occurrence of transgene expression at physiological levels and specific sites in livestock. YAC transgenesis therefore will be applied in genetic engineering, for example, in the production of pharmacologically interesting proteins encoded by large gene units and generating transgenic donors for xenotransplantation.

Partial repression of human gamma-globin genes by LCR element HS3 when linked to beta-globin genes and LCR element HS2 in MEL cells

Clues for overcoming fetal (gamma-) globin gene repression in adult human erythroid cells may come from understanding why repression of isolated gamma-globin genes has not previously been achieved in the adult erythroid environment of mouse erythroleukemia cells (MEL). Repression of human gamma-globin genes has been demonstrated in MEL cells when transferred as part of the entire beta-globin gene cluster packaged in chromatin. Major differences in these approaches are prior packaging into chromatin and the presence of additional sequences, notably from the locus control region (LCR). In this report we focus on the contribution to gamma-globin gene repression that multiple elements of the LCR may have. We first show preferential activation of beta-globin genes over gamma-globin genes in MEL cells when linked to each other and to LCR sequences containing the core elements of DNase I hypersensitive sites 4, 3, and 2. Removal of the HS4 element had no effect, however, removal of the 225 bp HS3 core element resulted in a five-fold increase in gamma-globin gene expression. The enhancer 3' to the A gamma-globin gene also had no apparent effect on gamma-globin gene expression. These results provide first evidence of gamma-globin gene repression involving the core region of HS3 in the presence of the core region of HS2 and a beta-globin gene. A mechanism for repression involving sequestration of the gamma-promoter away from the strong enhancer activity of HS2 is proposed.