Molecular analysis of the human beta-globin locus activation region

Generation of Transgenic Fluorescent Reporter Lines for Studying Hematopoietic Development in the Mouse

Hematopoiesis in the mouse and other mammals occurs in several waves and arises from distinct anatomic sites. Transgenic mice expressing fluorescent reporter proteins at various points in the hematopoietic hierarchy, from hematopoietic stem cell to more restricted progenitors to each of the final differentiated cell types, have provided valuable tools for tagging, tracking, and isolating these cells. In this chapter, we discuss general considerations in designing a transgene, survey available fluorescent probes, and describe methods for confirming and analyzing transgene expression in the hematopoietic tissues of the embryo, fetus, and postnatal/adult animal.

Transcriptional enhancers: from prediction to functional assessment on a genome-wide scale

Enhancers are cis-regulatory sequences located distally to target genes. These sequences consolidate developmental and environmental cues to coordinate gene expression in a tissue-specific manner. Enhancer function and tissue specificity depend on the expressed set of transcription factors, which recognize binding sites and recruit cofactors that regulate local chromatin organization and gene transcription. Unlike other genomic elements, enhancers are challenging to identify because they function independently of orientation, are often distant from their promoters, have poorly defined boundaries, and display no reading frame. In addition, there are no defined genetic or epigenetic features that are unambiguously associated with enhancer activity. Over recent years there have been developments in both empirical assays and computational methods for enhancer prediction. We review genome-wide tools, CRISPR advancements, and high-throughput screening approaches that have improved our ability to both observe and manipulate enhancers in vitro at the level of primary genetic sequences, chromatin states, and spatial interactions. We also highlight contemporary animal models and their importance to enhancer validation. Together, these experimental systems and techniques complement one another and broaden our understanding of enhancer function in development, evolution, and disease.

Improved Titer and Gene Transfer by Lentiviral Vectors Using Novel, Small β-Globin Locus Control Region Elements

β-globin lentiviral vectors (β-LV) have faced challenges in clinical translation for gene therapy of sickle cell disease (SCD) due to low titer and sub-optimal gene transfer to hematopoietic stem and progenitor cells (HSPCs). To overcome the challenge of preserving efficacious expression while increasing vector performance, we used published genomic and epigenomic data available through ENCODE to redefine enhancer element boundaries of the β-globin locus control region (LCR) to construct novel ENCODE core sequences. These novel LCR elements were used to design a β-LV of reduced proviral length, termed CoreGA-AS3-FB, produced at higher titers and possessing superior gene transfer to HSPCs when compared to the full-length parental β-LV at equal MOI. At low vector copy number, vectors containing the ENCODE core sequences were capable of reversing the sickle phenotype in a mouse model of SCD. These studies provide a β-LV that will be beneficial for gene therapy of SCD by significantly reducing the cost of vector production and extending the vector supply.

Sickle cells produce functional immune modulators and cytotoxics

Sickle erythrocytes' (SSRBCs) unique physical adaptation to hypoxic conditions renders them able to home to hypoxic tumor niches in vivo, shut down tumor blood flow and induce tumoricidal responses. SSRBCs are also useful vehicles for transport of encapsulated drugs and oncolytic virus into hypoxic tumors with enhanced anti-tumor effects. In search of additional modes for arming sickle cells with cytotoxics, we turned to a lentiviral β-globin vector with optimized Locus Control Region/β-globin coding region/promoter/enhancers. We partially replaced the β-globin coding region of this vector with genes encoding T cell cytolytics, perforin and granzyme or immune modulating superantigens SEG and SEI. These modified vectors efficiently transduced Sca⁺ ckit^- Lin^- hematopoietic stem cells (HSCs) from humanized sickle cell knockin mice. Irradiated mice reconstituted with these HSCs displayed robust expression of transgenic RNAs and proteins in host sickle cells that was sustained for more than 10 months. SSRBCs from reconstituted mice harboring SEG/SEI transgenes induced robust proliferation and a prototypical superantigen-induced cytokine reaction when exposed to human CD4+/CD8+ cells. The β-globin lentiviral vector therefore produces a high level of functional, erythroid-specific immune modulators and cytotoxics that circulate without toxicity. Coupled with their unique ability to target and occlude hypoxic tumor vessels these armed SSRBCs constitute a potentially useful tool for treatment of solid tumors.

Chromatin priming elements establish immunological memory in T cells without activating transcription: T cell memory is maintained by DNA elements which stably prime inducible genes without activating steady state transcription

We have identified a simple epigenetic mechanism underlying the establishment and maintenance of immunological memory in T cells. By studying the transcriptional regulation of inducible genes we found that a single cycle of activation of inducible factors is sufficient to initiate stable binding of pre-existing transcription factors to thousands of newly activated distal regulatory elements within inducible genes. These events lead to the creation of islands of active chromatin encompassing nearby enhancers, thereby supporting the accelerated activation of inducible genes, without changing steady state levels of transcription in memory T cells. These studies also highlighted the need for more sophisticated definitions of gene regulatory elements. The chromatin priming elements defined here are distinct from classical enhancers because they function by maintaining chromatin accessibility rather than directly activating transcription. We propose that these priming elements are members of a wider class of genomic elements that support correct developmentally regulated gene expression.

Gene Therapy of the β-Hemoglobinopathies by Lentiviral Transfer of the β(A(T87Q))-Globin Gene

β-globin gene disorders are the most prevalent inherited diseases worldwide and result from abnormal β-globin synthesis or structure. Novel therapeutic approaches are being developed in an effort to move beyond palliative management. Gene therapy, by ex vivo lentiviral transfer of a therapeutic β-globin gene derivative (β(AT87Q)-globin) to hematopoietic stem cells, driven by cis-regulatory elements that confer high, erythroid-specific expression, has been evaluated in human clinical trials over the past 8 years. β(AT87Q)-globin is used both as a strong inhibitor of HbS polymerization and as a biomarker. While long-term studies are underway in multiple centers in Europe and in the United States, proof-of-principle of efficacy and safety has already been obtained in multiple patients with β-thalassemia and sickle cell disease.

Current and future alternative therapies for beta-thalassemia major

Beta-thalassemia is a group of frequent genetic disorders resulting in the synthesis of little or no β-globin chains. Novel approaches are being developed to correct the resulting α/β-globin chain imbalance, in an effort to move beyond the palliative management of this disease and the complications of its treatment (e.g. life-long red blood cell transfusion, iron chelation, splenectomy), which impose high costs on healthcare systems. Three approaches are envisaged: fetal globin gene reactivation by pharmacological compounds injected into patients throughout their lives, allogeneic hematopoietic stem cell transplantation (HSCT), and gene therapy. HSCT is currently the only treatment shown to provide an effective, definitive cure for β-thalassemia. However, this procedure remains risky and histocompatible donors are identified for only a small fraction of patients. New pharmacological compounds are being tested, but none has yet made it into common clinical practice for the treatment of beta-thalassemia major. Gene therapy is in the experimental phase. It is emerging as a powerful approach without the immunological complications of HSCT, but with other possible drawbacks. Rapid progress is being made in this field, and long-term efficacy and safety studies are underway.

Pharmacological Induction of Human Fetal Globin Gene in Hydroxyurea-Resistant Primary Adult Erythroid Cells

Pharmacological induction of the fetal γ globin gene and the consequent formation of HbF (α2/γ2) in adult erythroid cells are one feasible therapeutic strategy for sickle cell disease (SCD) and severe β-thalassemias. Hydroxyurea (HU) is the current drug of choice for SCD, but serious side effects limit its clinical use. Moreover, 30 to 50% of patients are irresponsive to HU treatment. We have used high-throughput screening to identify benzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one and its derivatives (compounds I to VI) as potent γ globin inducers. Of the compounds, I to V exert superior γ globin induction and have better therapeutic potential than HU, likely because of their activation of the p38 mitogen-activated protein kinase (MAPK) signaling pathway and modulation of expression levels and/or chromosome binding of γ globin gene regulators, including BCL11A, and chromatin structure over the γ globin promoter. Unlike sodium butyrate (NaB), the global levels of acetylated histones H3 and H4 are not changed by compound II treatment. Remarkably, compound II induces the γ globin gene in HU-resistant primary human adult erythroid cells, the p38 signaling pathway of which appears to be irresponsive to HU and NaB as well as compound II. This study provides a new framework for the development of new and superior compounds for treating SCD and severe β-thalassemias.

Current Status and Developments in Gene Therapy for Thalassemia and Sickle Cell Disease

β-thalassemias and sickle cell anemia (SCA) are the most common monogenic diseases worldwide for which curative treatments remain a desired goal. Allogeneic hematopoietic stem cell transplantation (allo-HCT), - the only curative treatment currently available for hemoglobinopaties-, has a narrow application window whereas it incurs several immunological risks. Gene therapy (GT), that is the autologous transplantation of genetically modified hematopoietic stem cells (CD34+), represents a promising new therapeutic strategy which is anticipated to reestablish effective hemoglobin production and render patients transfusion- and drug- independent without the immunological complications that normally accompany allo-HCT. Prior to the application of GT for hemoglobinopathies in the clinic, many years of extensive preclinical research were spent for the optimization of the gene transfer tools and conditions. To date, three GT clinical trials for β-thalassemia and sickle cell disease (SCD) have been conducted or are in progress and 3 cases of transfusion independence in thalassemic β0/βΕ patients have been reported. In the present review, the prerequisites for successful implementation of GT, the tough pathway of GT for hemoglobinopathies towards the clinic and the knowledge gained from the first clinical trials as well as the remaining questions and challenges, will be discussed. Overall, after decades of research including achievements but pitfalls as well, the path to GT of human patients with hemoglobinopathies is currently open and highly promising...

Generation of transgenic mouse fluorescent reporter lines for studying hematopoietic development

During the development of the hematopoietic system, at least eight distinct lineages are generated in the mouse embryo. Transgenic mice expressing fluorescent proteins at various points in the hematopoietic hierarchy, from hematopoietic stem cell to multipotent progenitors to each of the final differentiated cell types, have provided valuable tools for tagging, tracking, and isolating these cells. In this chapter, we discuss general considerations in designing a transgene and survey available fluorescent probes and methods for confirming and analyzing transgene expression in the hematopoietic systems of the embryo, fetus, and postnatal/adult animal.

Analysis of primitive erythroid cell proliferation and enucleation using a cyan fluorescent reporter in transgenic mice

Primitive erythropoiesis is a vital process for mammalian embryonic development. Here we report the generation and characterization of a new transgenic mouse line that expresses a histone H2B-CFP fusion protein in the nuclei of primitive erythroid cells. We demonstrate the potential of this ε-globin-histone H2B-CFP line for multicolor imaging and flow cytometry analysis. The ε-globin-H2B-CFP line was used to analyze the cell cycle distribution and proliferation of CFP-expressing primitive erythroblasts from E8.5-E13.5. We also evaluated phagocytosis of extruded CFP-positive nuclei by macrophages in fetal liver and placenta. The ε-globin-H2B-CFP transgenic mouse line adds to the available tools for studying the development of the primitive erythroid lineage.

Identification of functional nucleotide and haplotype variants in the promoter of the CEBPE gene

The current study examined the promoter activity of an association signal in a 5'-upstream region of the gene encoding CCAAT/enhancer binding protein epsilon (CEBPE) identified from a recent genome-wide association study (GWAS) for complex acute lymphoblastic leukemia (ALL). This follow-up study first compared the activity of reporter constructs with three haplotypes estimated with the rs2239633 and its proximity nucleotide variants in strong linkage. The most frequent haplotype was CTTTTGT (H1), and the second most frequent haplotype consisted of entirely opposite alleles to H1 (TCGCACC, H2). Their luciferase activity revealed the strongest expression with H2 and the weakest with H1. Subsequent analysis revealed that different luciferase activity was found by the single-nucleotide substitution at rs2239632 and rs2239633 (P<0.05). Especially, the difference in luciferase activity between two alleles of rs2239632 corresponded to the difference between H1 and H2. We concluded that rs2239632 could regulate the expression of the CEBPE gene. We suggest a hypothesis that its risk allele (G) might increase the gene product and lead to leukemogenesis. As a result, a person with the allele or the corresponding haplotype might have increased susceptibility to ALL. Further research is warranted to investigate this hypothesis and the underlying mechanisms.

LCR 5' hypersensitive site specificity for globin gene activation within the active chromatin hub

The DNaseI hypersensitive sites (HSs) of the human β-globin locus control region (LCR) may function as part of an LCR holocomplex within a larger active chromatin hub (ACH). Differential activation of the globin genes during development may be controlled in part by preferential interaction of each gene with specific individual HSs during globin gene switching, a change in conformation of the LCR holocomplex, or both. To distinguish between these possibilities, human β-globin locus yeast artificial chromosome (β-YAC) lines were produced in which the ε-globin gene was replaced with a second marked β-globin gene (β^m), coupled to an intact LCR, a 5′HS3 complete deletion (5′ΔHS3) or a 5′HS3 core deletion (5′ΔHS3c). The 5′ΔHS3c mice expressed β^m-globin throughout development; γ-globin was co-expressed in the embryonic yolk sac, but not in the fetal liver; and wild-type β-globin was co-expressed in adult mice. Although the 5′HS3 core was not required for β^m-globin expression, previous work showed that the 5′HS3 core is necessary for ε-globin expression during embryonic erythropoiesis. A similar phenotype was observed in 5′HS complete deletion mice, except β^m-globin expression was higher during primitive erythropoiesis and γ-globin expression continued into fetal definitive erythropoiesis. These data support a site specificity model of LCR HS-globin gene interaction.

The Ongoing Challenge of Hematopoietic Stem Cell-Based Gene Therapy for β-Thalassemia

β-thalassemia is characterized by reduced or absence of β-globin production, resulting in anemia. Current therapies include blood transfusion combined with iron chelation. BM transplantation, although curative, is restricted by the matched donor limitation. Gene therapy, on the other hand, is promising, and its success lies primarily on designing efficient globin vectors that can effectively and stably transduce HSCs. The major breakthrough in β-thalassemia gene therapy occurred a decade ago with the development of globin LVs. Since then, researchers focused on designing efficient and safe vectors, which can successfully deliver the therapeutic transgene, demonstrating no insertional mutagenesis. Furthermore, as human HSCs have intrinsic barriers to HIV-1 infection, attention is drawn towards their ex vivo manipulation, aiming to achieve higher yield of genetically modified HSCs. This paper presents the current status of gene therapy for β-thalassemia, its success and limitations, and the novel promising strategies available involving the therapeutic role of HSCs.

Prevention of transcriptional silencing by a replicator-binding complex consisting of SWI/SNF, MeCP1, and hnRNP C1/C2

Transcriptional silencing selectively impedes gene expression. Silencing is often accompanied by replication delay and can be prevented by replicator sequences. Here we report a replicator-binding protein complex involved in the prevention of transcriptional silencing. The protein complex interacts with an essential asymmetric region within the human β-globin Rep-P replicator and includes hnRNP C1/C2, SWI/SNF complex, and MeCP1, which are members of the locus control region (LCR)-associated remodeling complex (LARC). Interaction between LARC and Rep-P prevented transcriptional silencing and replication delay. Transgenes that did not contain the asymmetric LARC-binding region of Rep-P replicated late and exhibited stable silencing that could not be affected by a DNA methylation inhibitor. In contrast, transgenes that contain a mutation of the asymmetric region of Rep-P that could not bind LARC exhibited a silent state that could transiently be reactivated by DNA demethylation. The effect of DNA demethylation was transient, and prolonged exposure to a methylation inhibitor induced distinct, stable, methylation-independent silencing. These observations suggest that the interaction of LARC complex with replicators plays a role in preventing gene silencing and provides support for a novel, epigenetic mechanism of resistance to methylation inhibitors.

Transcriptional regulation of fetal to adult hemoglobin switching: new therapeutic opportunities

In humans, embryonic, fetal, and adult hemoglobins are sequentially expressed in developing erythroblasts during ontogeny. For the past 40 years, this process has been the subject of intensive study because of its value to enlighten the biology of developmental gene regulation and because fetal hemoglobin can significantly ameliorate the clinical manifestations of both sickle cell disease and β-thalassemia. Understanding the normal process of loss of fetal globin expression and activation of adult globin expression could potentially lead to new therapeutic approaches for these hemoglobin disorders. Herein, we briefly review the history of the study of hemoglobin switching and then focus on recent discoveries in the field that now make new therapeutic approaches seem feasible in the future. Erythroid-specific knockdown of fetal gene repressors or enforced expression of fetal gene activators may provide clinically applicable approaches for genetic treatment of hemoglobin disorders that would benefit from increased fetal hemoglobin levels.

HbS-Savaria: the anti-polymerization effect of a single mutation in human alpha-chains

Recombinant alpha-Savaria globin (alpha(S49R)) was assembled with beta(S) chains by the alloplex intermediate pathway to generate tetrameric rHbS-Sarvaria (alpha (2) (S49R) beta (2) (E6V) ) that exhibited normal O(2) affinity and co-operatively at pH 7.4. Allosteric effectors, 2,3-DPG, L35, and NaCl increased O(2) affinity by 15%. Bohr effects were similar for rHbS-Savaria and HbS (0.38 +/- 0.025 vs. 0.46 +/- 0.03, respectively). The C(SAT) of HbS increased from 16.7 +/- 0.8 to 27.0 +/- 1.0 g/dL. Co-polymerization demonstrated inhibition predominantly by the Cis-dimer. Molecular modeling indicated that the positive charge at alpha-49 generated a strong anion-binding site and reduced flexibility of the CD-region by restricting movement in the E and F helices. The molecular distance between Arg-49 and Asn-78 in the neighboring double strand decreased, and electrostatic repulsion between the inter-double strands increased, resulting in inhibition of polymerization. The Savaria mutation may be useful for the design of super-inhibitory alpha-chains and gene therapy of sickle cell anemia.

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.

Measuring the rates of transcriptional elongation in the female Drosophila melanogaster germ line by nuclear run-on

We adapted the nuclear run-on method to measure changes in the rate of RNA polymerase II (pol II) transcription of repetitive elements and transposons in the female germ line of Drosophila melanogaster. Our data indicate that as little as an approximately 1.5-fold change in the rate of transcription can be detected by this method. Our nuclear run-on protocol likely measures changes in transcriptional elongation, because rates of transcription decline with time, consistent with a low rate of pol II re-initiation in the isolated nuclei. Surprisingly, we find that the retrotransposon gypsy and the repetitive sequence mst40 are silenced posttranscriptionally in fly ovaries.

Pair-wise interactions of polymerization inhibitory contact site mutations of hemoglobin-S

The linkage of pair-wise interactions of contact site mutations of HbS has been studied using Le Lamentin [His-20 (alpha)-->Gln], Hoshida [Glu-43 (beta)-->Gln] and alpha(2)beta (2) (T87Q) mutations as the prototype of three distinct classes of contact sites of deoxy HbS fiber. Binary mixture experiments established that beta(A)-chain with the Thr-87 (beta)-->Gln mutation is as potent as the gamma-chain of HbF (alpha(2)gamma(2)) in inhibiting polymerization. On combining the influence of Le Lamentin mutation with that of beta (2) (T87Q) mutations; the net influence is only partial additivity. On the other hand, in binary mixture studies, combined influence of Hoshida mutation with that of beta (2) (T87Q) mutations is synergistic. Besides, a significant level of synergistic complementation is also seen when the Le Lamentin and Hoshida mutations are combined in HbS (symmetrical tetramers). Le Lamentin and Hoshida mutation introduced into the cis-dimer of the asymmetric hybrid tetramer completely neutralizes the Val-6 (beta) dependent polymerization. Accordingly, we propose that combining the perturbation of intra-double strand contact site with that of an inter-double strand contact site exhibit synergy when they are present in two different chains of the alphabeta dimer. A comparison of the present results with that of the earlier studies suggest that when the two contact site perturbations are from the same sub-unit of the alphabeta dimer only partial additivity is observed. The map of interaction linkage of the contact site mutations exposes new strategies in the design of novel anti-sickling Hbs for the gene therapy of sickle cell disease.

Maturation and enucleation of primitive erythroblasts during mouse embryogenesis is accompanied by changes in cell-surface antigen expression

Primitive erythroblasts (EryPs) are the first hematopoietic cell type to form during mammalian embryogenesis and emerge within the blood islands of the yolk sac. Large, nucleated EryPs begin to circulate around midgestation, when connections between yolk sac and embryonic vasculature mature. Two to 3 days later, small cells of the definitive erythroid lineage (EryD) begin to differentiate within the fetal liver and rapidly outnumber EryPs in the circulation. The development and maturation of EryPs remain poorly defined. Our analysis of embryonic blood at different stages reveals a stepwise developmental progression within the EryP lineage from E9.5 to E12.5. Thereafter, EryDs are also present in the bloodstream, and the 2 lineages are not easily distinguished. We have generated a transgenic mouse line in which the human epsilon-globin gene promoter drives expression of green fluorescent protein exclusively within the EryP lineage. Here, we have used this line to characterize changes in cell morphology and surface-marker expression as EryPs mature and to track EryP numbers and enucleation throughout gestation. This study identifies previously unrecognized synchronous developmental stages leading to the maturation of EryPs in the mouse embryo. Unexpectedly, we find that EryPs are a stable cell population that persists through the end of gestation.

Remembering the cell fate during cellular differentiation

Higher eukaryote contains several hundreds of different cell types, each with a distinctive set of property defined by a unique gene expression pattern, even though every cell (with minor exception) shares the common genome. During cellular differentiation, the committed gene expression pattern is set up and propagated through numerous cell divisions. Therefore, cells must have evolved some elegant and inherent mechanisms to remember their expression states for the requirement of the stability of differentiation and development. Here we speculate a hypothetically cellular memory mechanism. In this hypothesis, the cell-cell variation during cellular differentiation may result from the inherent stochastic gene expression. The evolution of histone and distant regulatory sequences change the parameters of expression stochasticity. S-phase-dependent gene activation and epigenetic marks on chromatin provide means to discriminate transcriptionally active and repressive states. Eventually, mitotic memory mechanisms have been developed through which these expression states are transmitted through numerous cell divisions.

Investigations of a human embryonic globin gene silencing element using YAC transgenic mice

A silencing element has been previously located upstream of the human epsilon-globin gene promoter using transient assays and transgenic mice carrying plasmid constructs in which the element has been deleted or its transcriptional motifs have been mutated. To investigate whether this element functions in the context of the whole beta-globin locus, we analyzed epsilon-globin gene expression in transgenic mice carrying a deletion of the silencing element in the context of a 213-kilobase human beta-globin yeast artificial chromosome (beta-YAC). epsilon-Globin gene expression was measured during embryonic and fetal development and in adult mice. epsilon-mRNA levels in embryonic cells in Day 12 blood were as high as those measured in wild-type beta-YAC controls, indicating that the deletion does not affect epsilon gene promoter function. epsilon-Globin gene expression was confined to the embryonic cells, indicating that deletion of this silencing element did not affect epsilon-globin developmental expression in the context of the beta-YAC. These results suggest that in the context of the whole beta-globin locus, other proximal and upstream epsilon gene promoter elements as well as competition by the downstream globin genes contribute to the silencing of the epsilon-globin gene in the cells of definitive erythropoiesis.

Use of the hereditary persistence of fetal hemoglobin 2 enhancer to increase the expression of oncoretrovirus vectors for human gamma-globin

The development of oncoretrovirus vectors for human gamma-globin has been hampered by problems of low expression and gene silencing. In order to address these problems, we investigated an enhancer element identified from individuals with deletional hereditary persistence of fetal hemoglobin 2 (HPFH2), a genetic condition characterized by elevated levels of gamma-globin in adults. Plasmid transfection studies in erythroid MEL (murine erythroleukemia) cells demonstrated the HPFH2 element could function synergistically with the beta-globin locus control region to enhance the expression of an Agamma-globin gene with a truncated -382 bp promoter. A series of oncoretrovirus vectors were subsequently generated that contain an expression cassette for Agamma-globin linked to various combinations of the HPFH2 enhancer, the alpha-globin HS40 enhancer, and several versions of the promoter from Agamma-globin or beta-globin. Expression analysis in transduced MEL cell clones revealed very high levels of promoter-autonomous silencing that was at least partially abrogated by the HPFH2 enhancer. The vector containing a combination of a -201 bp Agamma-globin gene promoter with the Greek HPFH -117 point mutation and both the HPFH2 and HS40 enhancers exhibited no signs of vector silencing and was expressed at 248+/-99% per copy of mouse alpha-globin (62% of total alpha-globin). This represents a significant improvement over previously reported oncoretrovirus vectors for Agamma-globin, and demonstrates the capacity of the HPFH2 enhancer to abrogate sequence-autonomous silencing of the Agamma-globin promoter in the context of a gene transfer vector.

Sox6 directly silences epsilon globin expression in definitive erythropoiesis

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

Allele-specific transcription of fetal genes in primary erythroid cell cultures from Lepore and δβ° thalassemia patients

OBJECTIVE

Autonomous gene silencing and gene competition by globin promoters for locus control region (LCR) function have been proposed as mechanisms in developmental regulation of beta-like genes. deltabeta degrees thalassemias are syndromes presenting an increased production of fetal hemoglobin in adult life; the majority of them are due to various deletions in beta-globin gene cluster. We studied samples from double heterozygotes for beta-thalassemia and for Lepore or Sicilian deltabeta degrees deletions, both lacking beta-promoter sequence. Our goal was to address the question of whether the allele carrying the deltabeta degrees deletion is responsible for high level of fetal hemoglobin (HbF) production.

PATIENTS AND METHODS

We analyzed the globin gene transcription in human erythroid cell cultures from peripheral blood stem cells, using primary transcript in situ hybridization. We performed primary erythroid cultures from patients with the following genotypes: Lepore/beta degrees 39, Sicilian deltabeta degrees /beta degrees 39, and, as controls, two thalassemia patients with nondeletional mutations (IVS1,6/IVS1,6; IVS1,6/beta degrees 39), and one normal individual.

RESULTS

The cells where it is possible to unambiguously assign gamma genes transcription in cis with the deletion (gamma:beta) are strongly represented with respect to the nine other combinations of gamma and beta hybridization signals. These cells are at least nine times more represented than those expressing the gamma allele in trans to the deletion.

CONCLUSION

The allele-specific transcription of fetal genes in cis with the deletion is favored in both deletional genotypes. The absence of the adult promoter may influence LCR recruitment by fetal promoter, supporting the hypothesis that competition mechanism and gene silencing can coexist in regulating human globin gene transcription.

Transcriptional potentials of the beta-like globin genes at different developmental stages in transgenic mice and hemoglobin switching

Developmental-stage-specific regulation and physiological levels of expression of the globin genes can be recaptured in transgenic mice carrying a YAC/BAC- or cosmid-based construct. By contrast, proper developmental regulation and high-level expression cannot be achieved coordinately in transgenic mice carrying a more manipulated construct, such as a plasmid-based globin gene construct. These differences provide us an opportunity to define the requirements for a developmentally regulated, high-level expression of the globin genes in vivo. To achieve this, as a first step, we studied maximum transcriptional potentials of the beta-globin genes at various stages of development. microLCR-enhanced expression of the epsilon-, gamma-, and beta-globin genes driven by their minimal promoters was estimated and compared with that in betaYAC transgenic mice. Quantitative measurements of steady state mRNA levels of the epsilon-, gamma-, and beta-globin genes showed that the microLCR was able to enhance expression of each beta-like globin gene to levels similar to those in the betaYAC mice. Moreover, transcriptional potentials of each globin gene were unchanged during the entire course of development. These observations indicate that the highest level of expression of the globin genes can be achieved in both embryonic and definitive erythropoiesis regardless of developmental specificity of the genes. This finding implies that transcription suppression is the major mechanism of the developmental specificity of the expression of the beta-like globin genes.

Differential large-scale chromatin compaction and intranuclear positioning of transcribed versus non-transcribed transgene arrays containing beta-globin regulatory sequences

Previous work has demonstrated a more decondensed large-scale chromatin structure and a more internal nuclear position for gene-rich versus gene-poor chromosome regions. Here, we show that large-scale chromatin opening and changes in intranuclear positioning of chromosome regions can be induced by normal levels of endogenous transcription factors acting on mammalian regulatory sequences. We transfected mouse erythroleukemia cells with a 15 kbp plasmid containing a lac operator repeat plus beta-globin regulatory sequences driving a beta-galactosidase reporter gene. After green-fluorescent-protein/lac-repressor fusion-protein binding or after fluorescence in situ hybridization, the volume and location of the transgene array signal were measured. With both detection methods, we found that the volume was severalfold larger when transcription was on. While silent transgene arrays were located close to the nuclear membrane, we observed a significantly more internal position for the transcriptionally active state. Our results indicate that both large-scale chromatin decondensation and changes in nuclear positioning as observed for large, complex gene-rich chromosome regions can be reproduced by endogenous regulatory sequences acting within simple repetitive transgene arrays.

Differences of globin transgene expression in stably transfected cell lines and transgenic mice

Previous studies demonstrated that DNase I hypersensitive site -40 (HS-40) of the alpha-globin locus is capable of greatly enhancing expression of a hybrid beta/gamma-globin transcriptional unit in plasmid-transfected murine erythroleukemia (MEL) cells. However, as reported here, this same gamma-globin gene expression cassette was only transcribed at trace amounts in erythroid cells of transgenic mice. This lack of expression was not directly attributable to the beta/gamma-globin transcriptional unit, since this same unit linked to a composite beta-globin locus control region was expressed at high levels in transgenic mice. This lack of expression was also not directly attributable to chromosomal position effects, since addition of chromatin insulators failed to increase the frequency of expression. DNase I hypersensitivity and chromatin immunoprecipitation assays demonstrated that the lack of expression was correlated with a closed chromatin structure. We hypothesize that transgenes undergo dynamic changes in chromatin conformation following chromosomal integration and that the discrepant results reported here can be attributed to the relatively high level of chromatin remodeling that occurs in the transgenic mouse model, coupled with the relative inability of the HS-40 element to maintain an open chromatin state under such conditions.

Expression of phenylalanine hydroxylase (PAH) in erythrogenic bone marrow does not correct hyperphenylalaninemia in Pah(enu2) mice

BACKGROUND

Treatment of many inherited liver enzyme deficiencies requires the removal of toxic intermediate metabolites from the blood of affected individuals. We propose that circulating toxins can be adequately cleared and disease phenotype influenced by enzyme expressed in tissues other than the liver, such as bone marrow. Our specific hypothesis was that phenylalanine hydroxylase (PAH) expressed in bone marrow would lower blood phenylalanine levels in hyperphenylalaninemic Pah(enu2) mice, a model of human phenylketonuria (PKU).

METHODS

Germline-modified marrow PAH-expressing mice were developed using a transgene that contained the mouse liver PAH cDNA under the transcriptional control of a human beta-globin promoter. Marrow PAH-expressing mice were bred to Pah(enu2) mice to generate progeny that lacked liver PAH activity but expressed PAH in bone marrow.

RESULTS

Marrow PAH expression did not affect the health, function, or reproductive capacity of transgenic animals. Hyperphenylalaninemia persisted in transgenic Pah(enu2) homozygous mice despite PAH activity in marrow lysates, and was not altered following supplementation with tetrahydrobiopterin (BH(4)), a required cofactor for PAH. PAH activity measured in intact marrow cells was significantly lower than in marrow lysates; no such difference was measured in isolated hepatocytes vs. liver homogenate.

CONCLUSIONS

Marrow PAH expression did not correct hyperphenylalaninemia in Pah(enu2) mice. Phenylalanine clearance may have been limited by the natural perfusion rate of the marrow compartment, by insufficient PAH expression in marrow, or by other cellular factors affecting phenylalanine metabolism in intact marrow cells. Differences in PAH activity measured in intact marrow cells vs. cell lysates suggest that hepatocytes and PAH-expressing marrow cells are fundamentally different in their ability to metabolize phenylalanine. The efficacy of bone-marrow-directed gene therapy as a metabolic sink in the treatment of phenylketonuria may be limited, although further experiments with greater marrow PAH expression levels will be necessary to definitively prove this conclusion.

Mutation of a transcriptional motif of a distant regulatory element reduces the expression of embryonic and fetal globin genes

High-level beta-globin gene expression is dependent on the presence of the locus control region (LCR), a powerful regulatory element physically characterized by five DNase I-hypersensitive sites (HS), designated HS1-HS5. Of these, HS3 contains seven GT motifs that are essential for its activity. One of the motifs, GT6, has been shown by in vivo footprinting to display the largest difference in signal between fetal and adult globin expressing cells. We assessed the contribution of GT6 on the downstream globin gene expression by mutating this motif in a 248 kb beta-globin locus yeast artificial chromosome and measuring the activity of beta-globin genes in GT6m beta-YAC transgenic mice. Seven transgenic lines were established, three of which contained at least one intact copy of the beta-globin locus and were further investigated. The mutation of the GT6 motif reduced the expression of epsilon- and gamma-globin genes during embryonic erythropoiesis. During definitive erythropoiesis, gamma-globin gene expression was significantly reduced while beta-globin gene expression was virtually indistinguishable from wild-type controls. We conclude that the GT6 motif of hypersensitive site 3 of the LCR is required for normal epsilon- and gamma-globin gene expression during embryonic erythropoiesis and for gamma-globin gene expression during definitive erythropoiesis in the fetal liver. Our results provide evidence that mutations of single transcriptional motifs of distant regulatory elements can have profound effects on gene expression.

Molecular aspects of embryonic hemoglobin function

In order to provide the appropriate level of oxygen transport to respiring tissues, we need to produce a molecular oxygen transporting system to supplement oxygen diffusion and solubility. This supplementation is provided by hemoglobin. The role of hemoglobin in providing oxygen transport from lung to tissues in the adult is well-documented and functional characteristics of the fetal hemoglobin, which provide placental oxygen exchange, are also well understood. However the characteristics of the three embryonic hemoglobins, which provide oxygen transport during the first three months of gestation, are not well recognized. This review seeks to describe the state of our understanding of the temporal control of the expression of these proteins and the oxygen binding characteristics of the individual protein molecules. The modulation of the oxygen binding properties of these proteins, by the various allosteric effectors, is described and the structural origins of these characteristics are probed.

Evidence that DNase I hypersensitive site 5 of the human beta-globin locus control region functions as a chromosomal insulator in transgenic mice

We have previously reported that DNase I hypersensitive site 5 (5'HS5) of the human beta-globin locus control region functions as a chromatin insulator in stable transfection assays. In this report we show that a 3.2 kb DNA fragment containing the entire 5'HS5 region can protect a position-sensitive (A)gamma-globin gene against position effects in transgenic mice. Bracketing is required for function of 5'HS5 as an insulator. The 5'HS5 insulator operates in adult as well as in embryonic murine erythroid cells. The insulator has no significant stimulatory effects of its own. These results indicate that 5'HS5 can function as a chromatin insulator in vivo.

Functional and binding studies of HS3.2 of the beta-globin locus control region

The distal locus control region (LCR) is required for high-level expression of the complex of genes (HBBC) encoding the beta-like globins of mammals in erythroid cells. Several major DNase hypersensitive sites (HSs 1-5) mark the LCR. Sequence conservation and direct experimental evidence have implicated sequences within and between the HS cores in function of the LCR. In this report we confirm the mapping of a minor HS between HS3 and HS4, called HS3.2, and show that sequences including it increase the number of random integration sites at which a drug resistance gene is expressed. We also show that nuclear proteins including GATA1 and Oct1 bind specifically to sequences within HS3.2. However, the protein Pbx1, whose binding site is the best match to one highly conserved sequence, does not bind strongly. GATA1 and Oct1 also bind in the HS cores of the LCR and to promoters in HBBC. Their binding to this minor HS suggests that they may be used in assembly of a large complex containing multiple regulatory sequences.

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.

In vivo formation of a human beta-globin locus control region core element requires binding sites for multiple factors including GATA-1, NF-E2, erythroid Kruppel-like factor, and Sp1

The active elements of the beta-globin locus control region (LCR) are located within domains of unique chromatin structure. These nuclease hypersensitive sites (HSs) are characterized by high DNase I sensitivity, erythroid specificity, similar nucleosomal structure, and evolutionarily conserved clusters of cis-acting elements that are required for the formation and function of the core elements. To determine the requirements for HS core formation in the setting of nuclear chromatin, we constructed a series of artificial HS cores containing binding sites for GATA-1, NF-E2, and Sp1. In contrast to the results of previous in vitro experiments, we found that when constructs were stably integrated in mouse erythroleukemia cells the binding sites for NF-E2, GATA-1, or Sp1 alone or in any combination were unable to form core HS structures. We subsequently identified two new cis-acting elements from the LCR HS4 core that, when combined with the NF-E2, Sp1, and tandem inverted GATA elements, result in core structure formation. Both new cis-acting elements bind Sp1, and one binds erythroid Kruppel-like factor (EKLF). We conclude that in vivo beta-globin LCR HS core formation is more complex than previously thought and that several factors are required for this process to occur.

Gene transfer into nonhuman primate hematopoietic stem cells: implications for gene therapy

Hematopoietic stem cells (HSCs) are desirable targets for gene therapy because of their self-renewal and multilineage differentiation abilities. Retroviral vectors are extensively used for HSC gene therapy. However, the initial human trials of HSC gene marking and therapy showed that the gene transfer efficiency into human HSCs with retroviral vectors was very low in contrast to the much higher efficiency observed in murine experiments. The more quiescent nature of human HSCs and the lower density of retroviral receptors on them hindered the efficient gene transfer with retroviral vectors. Since nonhuman primates have marked similarity to humans in all aspects including the HSC biology, their models are considered to be important to evaluate and improve gene transfer into human HSCs. Using these models, clinically relevant levels (around 10% or even more) of gene-modified cells in peripheral blood have recently been achieved after gene transfer into HSCs and their autologous transplantation. This has been made possible by improving ex vivo transduction conditions such as introduction of Flt-3 ligand and specific fibronectin fragment (CH-296) into ex vivo culture during transduction, and the use of retroviral vectors pseudotyped with the gibbon ape leukemia virus or feline endogenous retrovirus envelope. Other strategies including the use of lentiviral vectors and in vivo selective expansion of gene-modified cells with the drug resistance gene or selective amplifier gene (also designated the molecular growth switch) are now being tested to further increase the fraction of gene-modified cells using nonhuman primate models. In addition to the high gene transfer efficiency, high-level and long-term expression of transgenes in human HSCs and their progeny is also required for effective HSC gene therapy. For this purpose, other backbones of retroviral vectors such as the murine stem cell virus and cis-DNA elements, such as the ss-globin locus control region and the chromatin insulator, also need to be tested in nonhuman primate models. Nonhuman primate studies will continue to provide an important framework for human HSC gene therapy. Well-designed nonhuman primate studies will also offer unique insights into the HSCs, immune system, and transplantation biology characteristic of large animals.

Sequences flanking hypersensitive sites of the beta-globin locus control region are required for synergistic enhancement

The major distal regulatory sequence for the beta-globin gene locus, the locus control region (LCR), is composed of multiple hypersensitive sites (HSs). Different models for LCR function postulate that the HSs act either independently or synergistically. To test these possibilities, we have constructed a series of expression cassettes in which the gene encoding the enhanced green fluorescent protein (EGFP) is under the control of DNA fragments containing single and multiple HSs of the LCR. LCR DNA fragments containing only the minimal region needed for position-independent expression (HS cores) or containing cores plus flanking sequences (HS units) were compared to ascertain whether conserved sequences between the HS cores contributed to enhancement. Expression of these constructs was measured after targeted integration into three defined loci in murine erythroleukemia cells using recombinase-mediated cassette exchange. At all three marked loci, synergistic enhancement of expression was observed in cassettes containing a combination of HS2, HS3, and HS4 units. In contrast, HS2, HS3, and HS4 cores (without flanking sequences) give an activity equivalent to the sum of the activities of the individual HS cores. These data suggest a model in which an HS core plus flanking regions, bound by specific proteins, forms a structure needed for interaction with other HS units to confer strong enhancement by the LCR. The three targeted integration sites differ substantially in their permissivity for expression, but even the largest LCR construct tested could not overcome these position effects to confer equal expression at all three sites.

A negative cis-element regulates the level of enhancement by hypersensitive site 2 of the beta-globin locus control region

The core of DNase hypersensitive site (HS) 2 from the beta-globin locus control region is a potent enhancer of globin gene expression. Although it has been considered to contain only positive cis-regulatory sequences, our study of the enhancement conferred by segments of HS2 in erythroid cells reveals a novel negative element. Individual cis-regulatory elements from HS2 such as E boxes or Maf-response elements produced as great or greater enhancement than the intact core in mouse erythroleukemia (MEL) cells, indicating the presence of negative elements within HS2. A deletion series through HS2 revealed negative elements at the 5' and 3' ends of the core. Analysis of constructs with and without the 5' negative element showed that the effect is exerted on the promoters of globin genes expressed at embryonic, fetal, or adult stages. The negative effect was observed in bipotential human cells (K562 and human erythroleukemia (HEL) cells), proerythroblastic mouse (MEL) cells, and normal adult human erythroid cells. The novel negative element also functions after stable integration into MEL chromosomes. Smaller deletions at the 5' end of the HS2 core map the negative element within a 20-base pair region containing two conserved sequences.

Primary T lymphocytes as targets for gene therapy

Peripheral blood T lymphocytes have been considered an attractive target for gene therapy applications. They can be easily harvested and readily expanded ex vivo. The transduction efficiency of primary human lymphocytes with standard retroviral vectors approaches 50% or more using optimized methods of gene transfer. Other methods of gene transfer, including adenoviral, adeno-associated viral, and lentiviral vectors, or nonviral techniques, have also been used for gene transfer into primary lymphocytes. Despite encouraging results in vitro, human clinical trials using retroviral vectors to transduce primary lymphocytes have been hindered by low expression levels of transgenes and immune responses against transgene products. Strategies to overcome these problems need to be developed.

A (GATA)(7) motif located in the 5' boundary area of the human beta-globin locus control region exhibits silencer activity in erythroid cells

A 40-bp DNA, consisting of seven tandem GATA repeats, is located near the HS5 site in the 5' boundary area of the locus control region (LCR) of human beta-globin gene. This (GATA)(7) motif, named 5a, exhibits silencer activity in erythroid cells. In transfected, recombinant plasmids containing the chloramphenicol acetyltransferase (CAT) reporter gene, 5a repressed the activity of the cis-linked housekeeping phosphoglycerate kinase (pgk) promoter; 5a also repressed the activity of the cis-linked HS2 enhancer regardless of whether the CAT gene was driven by the pgk or the epsilon-globin promoter. Repression by 5a was most severe when 5a was spliced upstream of HS2 at a distance of less than 200 bases from the HS2 enhancer core. The silencer activity of 5a was independent of whether the component GATA motifs were in head to tail orientation as in the wild type 5a or in head to head or tail to tail orientation as in a mutant 5a. Band shift experiments show that the GATA-1 protein binds to both 5a and the mutant 5a and forms a large protein complex. Together, the results suggest that GATA-1 bound at 5a is a strong, proximal repressor of HS2 enhancer activity.

Independent formation of DnaseI hypersensitive sites in the murine β-globin locus control region

Mammalian β-globin loci are composed of multiple orthologous genes whose expression is erythroid specific and developmentally regulated. The expression of these genes both from the endogenous locus and from transgenes is strongly influenced by a linked 15-kilobase region of clustered DNaseI hypersensitive sites (HSs) known as the locus control region (LCR). The LCR encompasses 5 major HSs, each of which is highly homologous among humans, mice, and other mammals. To analyze the function of individual HSs in the endogenous murine β-globin LCR, we have used homologous recombination in embryonic stem cells to produce 5 mouse lines, each of which is deficient for 1 of these major HSs. In this report, we demonstrate that deletion of the conserved region of 5′HS 1, 2, 3, 4, or 5/6 abolishes HS formation at the deletion site but has no influence on the formation of the remaining HSs in the LCR. Therefore, in the endogenous murine locus, there is no dominant or initiating site whose formation must precede the formation of the other HSs. This is consistent with the idea that HSs form autonomously. We discuss the implications of these findings for current models of β-globin regulation.

Independent formation of DnaseI hypersensitive sites in the murine β-globin locus control region

Mammalian β-globin loci are composed of multiple orthologous genes whose expression is erythroid specific and developmentally regulated. The expression of these genes both from the endogenous locus and from transgenes is strongly influenced by a linked 15-kilobase region of clustered DNaseI hypersensitive sites (HSs) known as the locus control region (LCR). The LCR encompasses 5 major HSs, each of which is highly homologous among humans, mice, and other mammals. To analyze the function of individual HSs in the endogenous murine β-globin LCR, we have used homologous recombination in embryonic stem cells to produce 5 mouse lines, each of which is deficient for 1 of these major HSs. In this report, we demonstrate that deletion of the conserved region of 5′HS 1, 2, 3, 4, or 5/6 abolishes HS formation at the deletion site but has no influence on the formation of the remaining HSs in the LCR. Therefore, in the endogenous murine locus, there is no dominant or initiating site whose formation must precede the formation of the other HSs. This is consistent with the idea that HSs form autonomously. We discuss the implications of these findings for current models of β-globin regulation.

Structural analysis and mapping of DNase I hypersensitivity of HS5 of the beta-globin locus control region

The beta-globin locus control region (LCR) is a cis regulatory element that is located in the 5' part of the locus and confers high-level erythroid lineage-specific and position-independent expression of the globin genes. The LCR is composed of five DNase I hypersensitive sites (HSs), four of which are formed in erythroid cells. The function of the 5'-most site, HS5, remains unknown. To gain insights into its function, mouse HS5 was cloned and sequenced. Comparison of the HS5 sequences of mouse, human, and galago revealed two extensively conserved regions, designated HS5A and HS5B. DNase I hypersensitivity mapping revealed that two hypersensitive sites are located within the HS5A region (designated HS5A(major) and HS5A(minor)), and two are located within the HS5B region (HS5B(major), HS5B(minor)). The positions of each of these HSs colocalize with either GATA-1 or Ap1/NF-E2 motifs, suggesting that these protein binding sites are implicated in the formation of HS5. Gel retardation assays indicated that the Ap1/NF-E2 motifs identified in murine HS5A and HS5B interact with NF-E2 or similar proteins. Studies of primary murine cells showed that HS5 is formed in all hemopoietic tissues tested (fetal liver, adult thymus, and spleen), indicating that this HS is not erythroid lineage specific. HS5 was detected in murine brain but not in murine kidney or adult liver, suggesting that this site is not ubiquitous. The presence of GATA-1 and NF-E2 motifs (which are common features of the DNase I hypersensitive sites of the LCR) suggests that the HS5 is organized in a manner similar to that of the other HSs. Taken together, our results suggest that HS5 is an inherent component of the beta-globin locus control region.

Development of Viral Vectors for Gene Therapy of β-Chain Hemoglobinopathies: Optimization of a γ-Globin Gene Expression Cassette

Progress toward gene therapy of β-chain hemoglobinopathies has been limited in part by poor expression of globin genes in virus vectors. To derive an optimal expression cassette, we systematically analyzed the sequence requirements and relative strengths of theAγ- and β-globin promoters, the activities of various erythroid-specific enhancers, and the importance of flanking and intronic sequences. Expression was analyzed by RNase protection after stable plasmid transfection of the murine erythroleukemia cell line, MEL585. Promoter truncation studies showed that theAγ-globin promoter could be deleted to −159 without affecting expression, while deleting the β-globin promoter to −127 actually increased expression compared with longer fragments. Expression from the optimal β-globin gene promoter was consistently higher than that from the optimal Aγ-globin promoter, regardless of the enhancer used. Enhancers tested included a 2.5-kb composite of the β-globin locus control region (termed a μLCR), a combination of the HS2 and HS3 core elements of the LCR, and the HS-40 core element of the -globin locus. All three enhancers increased expression from the β-globin gene to roughly the same extent, while the HS-40 element was notably less effective with theAγ-globin gene. However, the HS-40 element was able to efficiently enhance expression of a Aγ-globin gene linked to the β-globin promoter. Inclusion of extended 3′ sequences from either the β-globin or the Aγ-globin genes had no significant effect on expression. A 714-bp internal deletion ofAγ-globin intron 2 unexpectedly increased expression more than twofold. With the combination of a −127 β-globin promoter, anAγ-globin gene with the internal deletion of intron 2, and a single copy of the HS-40 enhancer, γ-globin expression averaged 166% of murine -globin mRNA per copy in six pools and 105% in nine clones. When placed in a retrovirus vector, this cassette was also expressed at high levels in MEL585 cells (averaging 75% of murine -globin mRNA per copy) without reducing virus titers. However, recombined provirus or aberrant splicing was observed in 5 of 12 clones, indicating a significant degree of genetic instability. Taken together, these data demonstrate the development of an optimal expression cassette for γ-globin capable of efficient expression in a retrovirus vector and form the basis for further refinement of vectors containing this cassette.

Development of Viral Vectors for Gene Therapy of β-Chain Hemoglobinopathies: Optimization of a γ-Globin Gene Expression Cassette

Progress toward gene therapy of β-chain hemoglobinopathies has been limited in part by poor expression of globin genes in virus vectors. To derive an optimal expression cassette, we systematically analyzed the sequence requirements and relative strengths of theAγ- and β-globin promoters, the activities of various erythroid-specific enhancers, and the importance of flanking and intronic sequences. Expression was analyzed by RNase protection after stable plasmid transfection of the murine erythroleukemia cell line, MEL585. Promoter truncation studies showed that theAγ-globin promoter could be deleted to −159 without affecting expression, while deleting the β-globin promoter to −127 actually increased expression compared with longer fragments. Expression from the optimal β-globin gene promoter was consistently higher than that from the optimal Aγ-globin promoter, regardless of the enhancer used. Enhancers tested included a 2.5-kb composite of the β-globin locus control region (termed a μLCR), a combination of the HS2 and HS3 core elements of the LCR, and the HS-40 core element of the -globin locus. All three enhancers increased expression from the β-globin gene to roughly the same extent, while the HS-40 element was notably less effective with theAγ-globin gene. However, the HS-40 element was able to efficiently enhance expression of a Aγ-globin gene linked to the β-globin promoter. Inclusion of extended 3′ sequences from either the β-globin or the Aγ-globin genes had no significant effect on expression. A 714-bp internal deletion ofAγ-globin intron 2 unexpectedly increased expression more than twofold. With the combination of a −127 β-globin promoter, anAγ-globin gene with the internal deletion of intron 2, and a single copy of the HS-40 enhancer, γ-globin expression averaged 166% of murine -globin mRNA per copy in six pools and 105% in nine clones. When placed in a retrovirus vector, this cassette was also expressed at high levels in MEL585 cells (averaging 75% of murine -globin mRNA per copy) without reducing virus titers. However, recombined provirus or aberrant splicing was observed in 5 of 12 clones, indicating a significant degree of genetic instability. Taken together, these data demonstrate the development of an optimal expression cassette for γ-globin capable of efficient expression in a retrovirus vector and form the basis for further refinement of vectors containing this cassette.