Long-term expression of gamma-globin mRNA in mouse erythrocytes from retrovirus vectors containing the human gamma-globin gene fused to the ankyrin-1 promoter

The potential of gene therapy approaches for the treatment of hemoglobinopathies: achievements and challenges

Hemoglobinopathies, including β-thalassemia and sickle cell disease (SCD), are a heterogeneous group of commonly inherited disorders affecting the function or levels of hemoglobin. Disease phenotype can be severe with substantial morbidity and mortality. Bone marrow transplantation is curative, but limited to those patients with an appropriately matched donor. Genetic therapy, which utilizes a patient's own cells, is thus an attractive therapeutic option. Numerous therapies are currently in clinical trials or in development, including therapies utilizing gene replacement therapy using lentiviruses and the latest gene editing techniques. In addition, methods are being developed that may be able to expand gene therapies to those with poor access to medical care, potentially significantly decreasing the global burden of disease.

Gene therapy for hemoglobinopathies: progress and challenges

Hemoglobinopathies are genetic inherited conditions that originate from the lack or malfunction of the hemoglobin (Hb) protein. Sickle cell disease (SCD) and thalassemia are the most common forms of these conditions. The severe anemia combined with complications that arise in the most affected patients raises the necessity for a cure to restore hemoglobin function. The current routine therapies for these conditions, namely transfusion and iron chelation, have significantly improved the quality of life in patients over the years, but still fail to address the underlying cause of the diseases. A curative option, allogeneic bone marrow transplantation is available, but limited by the availability of suitable donors and graft-vs-host disease. Gene therapy offers an alternative approach to cure patients with hemoglobinopathies and aims at the direct recovery of the hemoglobin function via globin gene transfer. In the last 2 decades, gene transfer tools based on lentiviral vector development have been significantly improved and proven curative in several animal models for SCD and thalassemia. As a result, clinical trials are in progress and 1 patient has been successfully treated with this approach. However, there are still frontiers to explore that might improve this approach: the stoichiometry between the transgenic hemoglobin and endogenous hemoglobin with respect to the different globin genetic mutations; donor cell sourcing, such as the use of induced pluripotent stem cells (iPSCs); and the use of safer gene insertion methods to prevent oncogenesis. With this review we will provide insights about (1) the different lentiviral gene therapy approaches in mouse models and human cells; (2) current and planned clinical trials; (3) hurdles to overcome for clinical trials, such as myeloablation toxicity, insertional oncogenesis, and high vector expression; and (4) future perspectives for gene therapy, including safe harbors and iPSCs technology.

Development of gene therapy for thalassemia

Retroviral vector-mediated gene transfer into hematopoietic stem cells provides a potentially curative therapy for severe β-thalassemia. Lentiviral vectors based on human immunodeficiency virus have been developed for this purpose and have been shown to be effective in curing thalassemia in mouse models. One participant in an ongoing clinical trial has achieved transfusion independence after gene transfer into bone marrow stem cells owing, in part, to a genetically modified, dominant clone. Ongoing efforts are focused on improving the efficiency of lentiviral vector-mediated gene transfer into stem cells so that the curative potential of gene transfer can be consistently achieved.

A zinc-finger transcriptional activator designed to interact with the gamma-globin gene promoters enhances fetal hemoglobin production in primary human adult erythroblasts

Fetal hemoglobin (HbF) is a potent genetic modifier of the severity of beta-thalassemia and sickle cell anemia. We used an in vitro culture model of human erythropoiesis in which late-stage erythroblasts are derived directly from human CD34(+) hematopoietic cells to evaluate HbF production. This system recapitulates expression of globin genes according to the developmental stage of the originating cell source. When cytokine-mobilized peripheral blood CD34(+) cells from adults were cultured, background levels of HbF were 2% or less. Cultured cells were readily transduced with lentiviral vectors when exposed to vector particles between 48 and 72 hours. Among the genetic elements that may enhance fetal hemoglobin production is an artificial zinc-finger transcription factor, GG1-VP64, designed to interact with the proximal gamma-globin gene promoters. Our data show that lentiviral-mediated, enforced expression of GG1-VP64 under the control of relatively weak erythroid-specific promoters induced significant amounts of HbF (up to 20%) in erythroblasts derived from adult CD34(+) cells without altering their capacity for erythroid maturation and only modestly reducing the total numbers of cells that accumulate in culture after transduction. These observations demonstrate the potential for sequence-specific enhancement of HbF in patients with beta-thalassemia or sickle cell anemia.

Genetic therapy for beta-thalassemia: from the bench to the bedside

Beta-thalassemia is a genetic disorder with mutations in the β-globin gene that reduce or abolish β-globin protein production. Patients with β-thalassemia major (Cooley's anemia) become severely anemic by 6 to 18 months of age, and are transfusion dependent for life, while those with thalassemia intermedia, a less-severe form of thalassemia, are intermittently or rarely transfused. An allogeneically matched bone marrow transplant is curative, although it is restricted to those with matched donors. Gene therapy holds the promise of "fixing" one's own bone marrow cells by transferring the normal β-globin or γ-globin gene into hematopoietic stem cells (HSCs) to permanently produce normal red blood cells. Requirements for effective gene transfer for the treatment of β-thalassemia are regulated, erythroid-specific, consistent, and high-level β-globin or γ-globin expression. Gamma retroviral vectors have had great success with immune-deficiency disorders, but due to vector-associated limitations, they have limited utility in hemoglobinopathies. Lentivirus vectors, on the other hand, have now been shown in several studies to correct mouse and animal models of thalassemia. The immediate challenges of the field as it moves toward clinical trials are to optimize gene transfer and engraftment of a high proportion of genetically modified HSCs and to minimize the adverse consequences that can result from random integration of vectors into the genome by improving current vector design or developing novel vectors. This article discusses the current state of the art in gene therapy for β-thalassemia and some of the challenges it faces in human trials.

Gene therapy in thalassemia and hemoglobinopathies

Sickle cell disease (SCD) and ß-thalassemia represent the most common hemoglobinopathies caused, respectively, by the alteration of structural features or deficient production of the ß-chain of the Hb molecule. Other hemoglobinopathies are characterized by different mutations in the α- or ß-globin genes and are associated with anemia and might require periodic or chronic blood transfusions. Therefore, ß-thalassemia, SCD and other hemoglobinopathies are excellent candidates for genetic approaches since they are monogenic disorders and, potentially, could be cured by introducing or correcting a single gene into the hematopoietic compartment or a single stem cell. Initial attempts at gene transfer of these hemoglobinopathies have proved unsuccessful due to limitations of available gene transfer vectors. With the advent of lentiviral vectors many of the initial limitations have been overcame. New approaches have also focused on targeting the specific mutation in the ß-globin genes, correcting the DNA sequence or manipulating the fate of RNA translation and splicing to restore ß-globin chain synthesis. These techniques have the potential to correct the defect into hematopoietic stem cells or be utilized to modify stem cells generated from patients affected by these disorders. This review discusses gene therapy strategies for the hemoglobinopathies, including the use of lentiviral vectors, generation of induced pluripotent stem cells (iPS) cells, gene targeting, splice-switching and stop codon readthrough.

Hematopoietic stem cell gene transfer for the treatment of hemoglobin disorders

Hematopoietic stem cell (HSC)-targeted gene transfer is an attractive approach for the treatment of a number of hematopoietic disorders caused by single gene defects. Indeed, in a series of gene transfer trials for two different primary immunodeficiencies beginning early in this decade, outstanding success has been achieved. Despite generally low levels of engrafted, genetically modified HSCs, these trials were successful because of the marked selective advantage of gene-corrected lymphoid precursors that allowed reconstitution of the immune system. Unlike the immunodeficiencies, this robust level of in vivo selection is not available to hematopoietic repopulating cells or early progenitor cells following gene transfer of a therapeutic globin gene in the setting of beta-thalassemia and sickle cell disease. Both preclinical and clinical transplant studies involving bone marrow chimeras suggest that 20% or higher levels of engraftment of genetically modified HSCs will be needed for clinical success in the most severe of these disorders. Encouragingly, gene transfer levels in this range have recently been reported in a lentiviral vector gene transfer clinical trial for children with adrenoleukodystrophy. A clinical gene transfer trial for beta-thalassemia has begun in France, and one patient with transfusion-dependent HbE/beta-thalassemia has demonstrated a therapeutic effect after transplantation with autologous CD34(+) cells genetically modified with a beta-globin lentiviral vector. Here, the development and recent progress of gene therapy for the hemoglobin disorders is reviewed.

Correction of murine sickle cell disease using gamma-globin lentiviral vectors to mediate high-level expression of fetal hemoglobin

Increased levels of red cell fetal hemogloblin, whether due to hereditary persistence of expression or from induction with hydroxyurea therapy, effectively ameliorate sickle cell disease (SCD). Therefore, we developed erythroid-specific, gamma-globin lentiviral vectors for hematopoietic stem cell (HSC)-targeted gene therapy with the goal of permanently increasing fetal hemoglobin (HbF) production in sickle red cells. We evaluated two different gamma-globin lentiviral vectors for therapeutic efficacy in the BERK sickle cell mouse model. The first vector, V5, contained the gamma-globin gene driven by 3.1 kb of beta-globin regulatory sequences and a 130-bp beta-globin promoter. The second vector, V5m3, was identical except that the gamma-globin 3'-untranslated region (3'-UTR) was replaced with the beta-globin 3'-UTR. Adult erythroid cells have beta-globin mRNA 3'-UTR-binding proteins that enhance beta-globin mRNA stability and we postulated this design might enhance gamma-globin expression. Stem cell gene transfer was efficient and nearly all red cells in transplanted mice expressed human gamma-globin. Both vectors demonstrated efficacy in disease correction, with the V5m3 vector producing a higher level of gamma-globin mRNA which was associated with high-level correction of anemia and secondary organ pathology. These data support the rationale for a gene therapy approach to SCD by permanently enhancing HbF using a gamma-globin lentiviral vector.

Effective gene therapy of mice with congenital erythropoietic porphyria is facilitated by a survival advantage of corrected erythroid cells

Achieving long-term expression of a therapeutic gene in a given hematopoietic lineage remains an important goal of gene therapy. Congenital erythropoietic porphyria (CEP) is a severe autosomal-recessive disorder characterized by a deficiency in uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthetic pathway. We used a recently obtained murine model to check the feasibility of gene therapy in this disease. Lentivirus-mediated transfer of the human UROS cDNA into hematopoietic stem cells (HSCs) from Uros(mut248) mice resulted in a complete and long-term enzymatic, metabolic, and phenotypic correction of the disease, favored by a survival advantage of corrected red blood cells. These results demonstrate that the cure of this mouse model of CEP at a moderate transduction level supports the proof of concept of a gene therapy in this disease by transplantation of genetically modified hematopoietic stem cells.

Modification of globin gene expression by RNA targeting strategies

OBJECTIVE

Sickle cell anemia is a genetic blood disease resulting from production of mutant beta-globin (beta(S)) and has severe clinical consequences. It is known that a higher cellular gamma-globin level, e.g., higher ratio of cellular gamma-globin to beta(S)-globin (gamma/beta(S) ratio), inhibits sickle hemoglobin (HbS) polymerization tendency. Hence, therapeutic treatment of sickle cell anemia has been focused on introducing gamma-globin gene into red blood cells to increase the cellular gamma/beta(S) ratio. Here, we have introduced ribozymes and small interfering RNAs (siRNAs) against beta(S)-globin mRNA into blood cells as a means to increase the gamma/beta(S) ratio.

MATERIALS AND METHODS

Single and multiribozymes against beta(S)-globin mRNA have been tested in vitro and in human erythroleukemia K562beta(S) cells that stably express exogenous beta(S)-globin gene. Primary human hematopoietic progenitor cells were also transfected with multiribozyme and the gamma/(gamma + beta) ratio determined and compared with cells transfected with long hairpin beta-globin cDNA and synthetic siRNA genes.

RESULTS

We have found that the multiribozyme zb21A containing two ribozyme units effectively reduces beta(S)-globin mRNA both in vitro and in K562beta(S) cells. The gamma-globin mRNA to beta(S)-globin mRNA ratio in the multiribozyme transfected cells is about a factor of 2 more than that in the control cells. We have also found that the gamma/(gamma + beta) ratio in the transfected hematopoietic progenitor cells is increased by more than twofold in cells treated with multiribozyme zb21A or siRNA ib5.

CONCLUSION

Our results suggest that introducing multiribozymes or siRNAs into red blood cells is comparable in their effectiveness to increase the ratio of cellular gamma-globin mRNA to beta- or beta(S)-globin mRNA, providing possible strategies to increase the effectiveness of gamma-globin gene transfer as gene therapy for treatment of patients with sickle cell anemia.

Erythroid-specific expression of beta-globin by the sleeping beauty transposon for Sickle cell disease

Sickle cell disease (SCD) results predominately from a single monogenic mutation that affects thousands of individuals worldwide. Gene therapy approaches have focused on using viral vectors to transfer wild-type beta- or gamma-globin transgenes into hematopoietic stem cells for long-term expression of the recombinant globins. In this study, we investigated the use of a novel nonviral vector system, the Sleeping Beauty (SB) transposon (Tn) to insert a wild-type beta-globin expression cassette into the human genome for sustained expression of beta-globin. We initially constructed a beta-globin expression vector composed of the hybrid cytomegalovirus (CMV) enhancer chicken beta-actin promoter (CAGGS) and full-length beta-globin cDNA, as well as truncated forms lacking either the 3' or 3' and 5' untranslated regions (UTRs), to optimize expression of beta-globin. Beta-globin with its 5' UTR was efficiently expressed from its cDNA in K-562 cells induced with hemin. However, expression was constitutive and not erythroid-specific. We then constructed cis SB-Tn-beta-globin plasmids using a minimal beta-globin gene driven by hybrid promoter IHK (human ALAS2 intron 8 erythroid-specific enhancer, HS40 core element from human alphaLCR, ankyrin-1 promoter), IHbetap (human ALAS2 intron 8 erythroid-specific enhancer, HS40 core element from human alphaLCR, beta-globin promoter), or HS3betap (HS3 core element from human betaLCR, beta-globin promoter) to establish erythroid-specific expression of beta-globin. Stable genomic insertion of the minimal gene and expression of the beta-globin transgene for >5 months at a level comparable to that of the endogenous gamma-globin gene were achieved using a SB-Tn beta-globin cis construct. Interestingly, erythroid-specific expression of beta-globin driven by IHK was regulated primarily at the translational level, in contrast to post-transcriptional regulation in non-erythroid cells. The SB-Tn system is a promising nonviral vector for efficient genomic insertion conferring stable, persistent erythroid-specific expression of beta-globin.

Altered regulation of beta-like globin genes by a redesigned erythroid transcription factor

OBJECTIVE

Targeted regulation of beta-like globin genes was studied using designer zinc finger transcription factors containing the DNA binding domain of the red cell specific transcription factor erythroid Kruppel-like factor (EKLF) fused to repression domains.

METHODS

Globin gene expression was analyzed after introduction of the modified transcription factors into cell lines, embryonic stem cells and transgenic mice.

RESULTS

As would be predicted, when introduced transiently into cells these transcription factors were effective in repressing the adult beta-globin promoter CACCC element, which is the natural target for EKLF. In murine erythroleukemia cells repression of the adult beta-globin gene was accompanied by a reactivation of the endogenous embryonic betaH1-globin gene. Studies in differentiated embryonic stem cells and transgenic mice confirmed the reactivation of embryonic gene expression during development.

CONCLUSION

Our studies support a competition model for beta-globin gene expression and underscore the importance of EKLF in the embryonic/fetal-to-adult globin switch. They also demonstrate the feasibility of designer zinc finger transcription factors in the study of transcriptional control mechanisms at the beta-globin locus and as potential gene therapy agents for sickle cell disease and related hemoglobinopathies.

Recent advances in globin gene transfer for the treatment of beta-thalassemia and sickle cell anemia

PURPOSE OF REVIEW

The beta-thalassemias and sickle cell anemia are severe congenital anemias for which there is presently no curative therapy other than allogeneic hematopoietic stem cell transplantation. This therapeutic option, however, is not available to most patients due to the lack of an HLA-matched bone marrow donor. The transfer of a regulated globin gene in autologous hematopoietic stem cells is therefore a highly attractive alternative treatment. This strategy, simple in principle, raises major challenges in terms of controlling transgene expression, which ideally should be erythroid specific, differentiation and stage restricted, elevated, position independent, and sustained over time.

RECENT FINDINGS

Using lentiviral vectors, May et al. demonstrated that an optimized combination of proximal and distal transcriptional control elements permits lineage-specific and elevated beta-globin expression in vivo, resulting in therapeutic hemoglobin production and correction of anemia in beta-thalassemic mice. Several groups have extended these findings to various models of beta-thalassemia and sickle cell disease. While the addition of the wild-type beta-globin gene is naturally suited for treating beta-thalassemia, several alternatives have been proposed for the treatment of sickle cell disease, using either gamma or mutant beta-globin gene addition, trans-splicing or RNA interference.

SUMMARY

These recent advances bode well for the clinical investigation of stem cell-based gene therapy in the severe hemoglobinopathies.

Progress Toward the Genetic Treatment of the β-Thalassemias

The beta-thalassemias are congenital anemias that are caused by mutations that reduce or abolish expression of the beta-globin gene. They can be cured by allogeneic hematopoietic stem cell (HSC) transplantation, but this therapeutic option is not available to most patients. The transfer of a regulated beta-globin gene in autologous HSCs is a highly attractive alternative treatment. This strategy, which is simple in principle, raises major challenges in terms of controlling expression of the globin transgene, which ideally should be erythroid specific, differentiation- and stage-restricted, elevated, position independent, and sustained over time. Using lentiviral vectors, May et al. demonstrated in 2000 that an optimized combination of proximal and distal transcriptional control elements permits lineage-specific and elevated beta-globin expression, resulting in therapeutic hemoglobin production and correction of anemia in beta-thalassemic mice. Several groups have by now replicated and extended these findings to various mouse models of severe hemoglobinopathies, thus fueling enthusiasm for a potential treatment of beta-thalassemia based on globin gene transfer. Current investigation focuses on safety issues and the need for improved vector production methodologies. The safe implementation of stem cell-based gene therapy requires the prevention of the formation of replication-competent viral genomes and minimization of the risk of insertional oncogenesis. Importantly, globin vectors, in which transcriptional activity is highly restricted, have a lesser risk of activating oncogenes in hematopoietic progenitors than non-tissue-specific vectors, by virtue of their late-stage erythroid specificity. As such, they provide a general paradigm for improving vector safety in stem cell-based gene therapy.

A dinucleotide deletion in the ankyrin promoter alters gene expression, transcription initiation and TFIID complex formation in hereditary spherocytosis

Ankyrin defects are the most common cause of hereditary spherocytosis (HS). In some HS patients, mutations in the ankyrin promoter have been hypothesized to lead to decreased ankyrin mRNA synthesis. The ankyrin erythroid promoter is a member of the most common class of mammalian promoters which lack conserved TATA, initiator or other promoter cis elements and have high G+C content, functional Sp1 binding sites and multiple transcription initiation sites. We identified a novel ankyrin gene promoter mutation, a TG deletion adjacent to a transcription initiation site, in a patient with ankyrin-linked HS and analyzed its effects on ankyrin expression. In vitro, the mutant promoter directed decreased levels of gene expression, altered transcription initiation site utilization and exhibited defective binding of TATA-binding protein (TBP) and TFIID complex formation. In a transgenic mouse model, the mutant ankyrin promoter led to abnormalities in gene expression, including decreased expression of a reporter gene and altered transcription initiation site utilization. These data indicate that the mutation alters ankyrin gene transcription and contributes to the HS phenotype by decreasing ankyrin gene synthesis via disruption of TFIID complex interactions with the ankyrin core promoter. These studies support the model that in promoters that lack conserved cis elements, the TFIID complex directs preinitiation complex formation at specific sites in core promoter DNA and provide the first evidence that disruption of TBP binding and TFIID complex formation in this type of promoter leads to alterations in start site utilization, decreased gene expression and a disease phenotype in vivo.

Gene therapy for the hemoglobin disorders

The hemoglobin disorders of beta-thalassemia and sickle cell disease together constitute the most prevalent group of human monogenic diseases. Although curative allogeneic stem cell transplantation therapy and palliative therapies have been developed for these disorders, the majority of patients still suffer significant morbidity and early mortality. The development of therapeutic approaches based on genetic manipulation of autologous stem cells therefore remains an attractive alternative. In the past 4 years, significant advances have been made toward this goal using lentiviral vectors to obtain high-level expression of complex globin gene cassettes. Therapeutic correction in murine models of both beta-thalassemia and sickle cell anemia has been achieved using this approach. These advances, coupled with progress in the ability to achieve in vivo selection of genetically modified cells, can now be evaluated in the well-developed nonhuman primate autologous transplant model. The goal in these studies is to provide preclinical safety and efficacy data prior to human clinical trials in order to maximize the likelihood of success in the context of an acceptable risk to benefit ratio. Here we review progress in each of these areas.

Effect of zeta-globin substitution on the O2-transport properties of Hb S in vitro and in vivo

Hemoglobin zeta(2)beta(2)(S) is generated by substituting embryonic zeta-globin subunits for the normal alpha-globin components of Hb S (alpha(2)beta(2)(S)). This novel hemoglobin has recently been shown to inhibit polymerization of Hb S in vitro and to normalize the pathological phenotype of mouse models of sickle cell disease in vivo. Despite its promise as a therapeutic tool in human disease, however, the basic O(2)-transport properties of Hb zeta(2)beta(2)(S) have not yet been described. Using human hemoglobins purified from complex transgenic-knockout mice, we show that Hb zeta(2)beta(2)(S) exhibits an O(2) affinity as well as a Hill coefficient, Bohr response, and allosteric properties in vitro that are suboptimally suited for physiological O(2) transport in vivo. These data are substantiated by in situ analyses demonstrating an increase in the O(2) affinity of intact erythrocytes from mice that express Hb zeta(2)beta(2)(S). Surprisingly, though, co-expression of Hb zeta(2)beta(2)(S) leads to a substantial improvement in the tissue oxygenation of mice that model sickle cell disease. These analyses suggest that, in the context of sickle cell disease, the beneficial antisickling effects of Hb zeta(2)beta(2)(S) outweigh its O(2)-transport liabilities. The potential structural bases for the antisickling properties of Hb zeta(2)beta(2)(S) are discussed in the context of these new observations.

Extended beta-globin locus control region elements promote consistent therapeutic expression of a gamma-globin lentiviral vector in murine beta-thalassemia

Since increased fetal hemoglobin diminishes the severity of beta-thalassemia and sickle cell anemia, a strategy using autologous, stem cell-targeted gene transfer of a gamma-globin gene may be therapeutically useful. We previously found that a gamma-globin lentiviral vector utilizing the beta-globin promoter and elements from the beta-globin locus control region (LCR) totaling 1.7 kb could correct murine beta-thalassemia. However, therapeutic consistency was compromised by chromosomal position effects on vector expression. In contrast, we show here that the majority of animals that received transplants of beta-thalassemic stem cells transduced with a new vector containing 3.2 kb of LCR sequences expressed high levels of fetal hemoglobin (17%-33%), with an average vector copy number of 1.3. This led to a mean 26 g/L (2.6 g/dL) increase in hemoglobin concentration and enhanced amelioration of other hematologic parameters. Analysis of clonal erythroid cells of secondary spleen colonies from mice that underwent transplantation demonstrated an increased resistance of the larger LCR vector to stable and variegating position effects. This trend was also observed for vector insertion sites located inside genes, where vector expression was often compromised, in contrast to intergenic sites, where higher levels of expression were observed. These data emphasize the importance of overcoming detrimental position effects for consistent therapeutic globin vector expression.

Update on gene therapy for hereditary hematological disorders

The past 3 years have been characterized by a number of impressive advances as well as setbacks in gene therapy for genetic disease. Children with X-linked severe combined immunodeficiency disorder (SCID-X1) have shown almost complete reconstitution of their immune system after receiving retrovirally transduced autologous CD34+ hematopoietic stem cells (HSCs). However, two of 11 treated patients subsequently developed a leukemia-like disease probablydue to the undesired activation of an oncogene. Gene transfer to HSCs resulted in substantial correction of immune function and multi-lineage engraftment in two patients with adenosine deaminase (ADA)-SCID. Several Phase I clinical trials for treatment of hemophilia A and B have been initiated or completed. Partial correction of hemophilia A, albeit transient, has been reported by ex vivo gene transfer to autologous fibroblasts. Intramuscular injection of adeno-associated viral (AAV) vector to patients with severe hemophilia B resulted in evidence of Factor IX gene transfer to skeletal muscle and a separate trial based on hepatic infusion of AAV vector is ongoing. Sustained therapeutic levels of coagulation factor expression have been achieved in preclinical models using retroviral, lentiviral, AAV and high capacity adenoviral vectors. Efficient lentiviral gene transfer to HSC in murine models of beta-thalassemia and sickle cell disease demonstrated sustained phenotypic correction.

Deletion of a Negatively Acting Sequence in a Chimeric GATA-1 Enhancer-Long Terminal Repeat Greatly Increases Retrovirally Mediated Erythroid Expression

The locus control region of the beta-globin gene cluster has been used previously to direct erythroid expression of globin genes from retroviral vectors for the purpose of gene therapy. Short erythroid regulatory elements represent a potentially valuable alternative to the locus control region. Among them, the GATA-1 enhancer HS2 was used to replace the retroviral enhancer within the 3'-long terminal repeat (LTR) of the retroviral vector SFCM, converting it into an erythroid-specific regulatory element. In this work, we have functionally studied an additional GATA-1 enhancer, HS1. HS1 participates in the transcriptional autoregulation of GATA-1 through an essential GATA-binding site that is footprinted in vivo. In this work we identified within HS1 a new in vivo footprinted region, and we showed that this sequence indeed binds a nuclear protein in vitro. Addition of HS1 to HS2 within the LTR of SFCM significantly improves the expression of a reporter gene. The deletion of the newly identified footprinted sequence in the retroviral construct further increases expression up to a level almost equal to that of the wild type retroviral LTR, without loss of erythroid specificity, suggesting that this sequence may act as a negative regulatory element. An improved vector backbone, MDeltaN, allows even better expression from the new GATA cassette. These results suggest that substantial improvement of overall expression can be achieved by the combination of multiple changes in both regulatory elements and vectors.

The degree of phenotypic correction of murine beta -thalassemia intermedia following lentiviral-mediated transfer of a human gamma-globin gene is influenced by chromosomal position effects and vector copy number

Increased fetal hemoglobin (HbF) levels diminish the clinical severity of beta-thalassemia and sickle cell anemia. A treatment strategy using autologous stem cell-targeted gene transfer of a gamma-globin gene may therefore have therapeutic potential. We evaluated oncoretroviral- and lentiviral-based gamma-globin vectors for expression in transduced erythroid cell lines. Compared with gamma-globin, oncoretroviral vectors containing either a beta-spectrin or beta-globin promoter and the alpha-globin HS40 element, a gamma-globin lentiviral vector utilizing the beta-globin promoter and elements from the beta-globin locus control region demonstrated a higher probability of expression. This lentiviral vector design was evaluated in lethally irradiated mice that received transplants of transduced bone marrow cells. Long-term, stable erythroid expression of human gamma-globin was observed with levels of vector-encoded gamma-globin mRNA ranging from 9% to 19% of total murine alpha-globin mRNA. The therapeutic efficacy of the vector was subsequently evaluated in a murine model of beta-thalassemia intermedia. The majority of mice that underwent transplantation expressed significant levels of chimeric m(alpha)(2)h(gamma)(2) molecules (termed HbF), the amount of which correlated with the degree of phenotypic improvement. A group of animals with a mean HbF level of 21% displayed a 2.5 g/dL (25 g/L) improvement in Hb concentration and normalization of erythrocyte morphology relative to control animals. gamma-Globin expression and phenotypic improvement was variably lower in other animals due to differences in vector copy number and chromosomal position effects. These data establish the potential of using a gamma-globin lentiviral vector for gene therapy of beta-thalassemia.

BVL-1-like VL30 promoter sustains long-term expression in erythroid progenitor cells

Congenital blood disorders are common and yet clinically challenging globin disorders. Gene therapy continues to serve as a potential therapeutic method to treat these disorders. While tremendous advances have been made in vivo, gene delivery protocols and vector prototypes still require optimization. Alternative cis-acting promoter elements derived from VL30 retroelements have been effective in expressing tissue-specific transgene expression in vivo in nonerythroid cells. VL30 promoter elements were isolated from ELM-I-1 erythroid progenitor cells upon erythropoietin (epo) treatment. These promoters were inserted into a VL30-derived expression vector and reintroduced into the ELM-I-1 cells. beta-Galactosidase reporter gene activity from the ELM 5 clone, a BVL-1-like VL30 promoter, was capable of expressing sustained levels of the transgene expression over a 16-week assay period. These findings delineate the potential utility of these retroelement promoters as transcriptionally active, erythroid-specific, long terminal repeat (LTR) components for current globin vector constructs.

Regulation of erythrocyte membrane protein gene expression

Proteins of the erythrocyte membrane have served as the prototypes of homologous families of multifunctional proteins in erythroid and nonerythroid cells. These proteins demonstrate many different cell type, tissue-specific, and developmental stage-specific functions. This complex pattern of functional diversity appears to have evolved from the cell type, tissue-specific, developmentally regulated expression of multiple protein isoforms. Isoform diversity arises from different gene products from related genes; from differential, alternate splicing of the same gene product; from the use of tissue-specific promoters; and from alternate polyadenylation. The identification and characterization of the regulatory elements that control erythrocyte membrane protein gene expression have important implications for several biologic processes. These include disease pathogenesis, membrane assembly, hematopoiesis, gene regulation, and direction of other erythroid-specific genes in transgenic mouse and gene therapy applications.

Hematopoietic stem cell gene therapy for inherited bone marrow disorders: past accomplishments and continued challenges

From the time that the genes encoding the defective proteins were cloned for a number of inherited diseases, it became a goal to correct those conditions by restoring the normal gene and thereby, its product. For the inherited disorders affecting the blood and its progenitor cells, the hematopoietic stem cells were the ideal target cells for gene transfer, because the normal gene would then be transferred to all of the progeny cells, theoretically for the lifetime of the recipient. However, the tasks of isolating the hematopoietic stem cells, introducing the new genes in such a manner as to preserve engraftment of the manipulated cells, and achieving long-term gene expression, have not been straightforward in the clinical trial setting, although there has been moderate success for cells in vitro, and in murine studies. With the report of clinical efficacy of gene transfer in children with X-linked severe combined immunodeficiency disease, the dream of clinical gene transfer to hematopoietic cells has become a reality. But there are still significant impediments remaining for a number of diseases. The innovations of introduction of synthetic receptors that confer growth advantage, the use of lentiviral vectors with increased stem cell transduction efficiency, and the addition of modified promoter/enhancer sequences to augment and preserve gene expression may bring wider success to gene therapy clinical trials for bone marrow disorders in the near future.

Development of virus vectors for gene therapy of beta chain hemoglobinopathies: flanking with a chromatin insulator reduces gamma-globin gene silencing in vivo

We have previously described the development of oncoretrovirus vectors for human gamma-globin using a truncated beta-globin promoter, modified gamma-globin cassette, and alpha-globin enhancer. However, one of these vectors is genetically unstable, and both vectors exhibit variable expression patterns in cultured cells, common characteristics of oncoretrovirus vectors for globin genes. To address these problems, we identified and removed the vector sequences responsible for genetic instability and flanked the resultant vector with the chicken beta-globin HS4 chromatin insulator to protect expression from chromosomal position effects. After determining that flanking with the cHS4 element allowed higher, more uniform levels of gamma-globin expression in MEL cell lines, we tested these vectors using a mouse bone marrow transduction and transplantation model. When present, the gamma-globin cassettes from the uninsulated vectors were expressed in only 2% to 5% of red blood cells (RBCs) long term, indicating they are highly sensitive to epigenetic silencing. In contrast, when present the gamma-globin cassette from the insulated vector was expressed in 49% +/- 20% of RBCs long term. RNase protection analysis indicated that the insulated gamma-globin cassette was expressed at 23% +/- 16% per copy of mouse alpha-globin in transduced RBCs. These results demonstrate that flanking a globin vector with the cHS4 insulator increases the likelihood of expression nearly 10-fold, which in turn allows for gamma-globin expression approaching the therapeutic range for sickle cell anemia and beta thalassemia.

Globin gene transfer for the treatment of severe hemoglobinopathies: a paradigm for stem cell-based gene therapy

The prospect of treating blood disorders with genetically modified stem cells is highly promising. This therapeutic approach, however, raises a number of fundamental biological questions, spanning several research fields. Further investigation is required to better understand how to isolate and efficiently transduce hematopoietic stem cells (HSCs), while preserving optimal homing and self-renewing properties; how to design safe vectors permitting controlled expression of the transgene products; and how to promote host repopulation by engrafted HSCs. This article addresses basic issues in stem cell-based gene therapy from the perspective of regulating transgene expression, taking globin gene transfer for the treatment of severe hemoglobinopathies as a paradigm.

Correction of sickle cell disease in transgenic mouse models by gene therapy

Sickle cell disease (SCD) is caused by a single point mutation in the human betaA globin gene that results in the formation of an abnormal hemoglobin [HbS (alpha2betaS2)]. We designed a betaA globin gene variant that prevents HbS polymerization and introduced it into a lentiviral vector we optimized for transfer to hematopoietic stem cells and gene expression in the adult red blood cell lineage. Long-term expression (up to 10 months) was achieved, without preselection, in all transplanted mice with erythroid-specific accumulation of the antisickling protein in up to 52% of total hemoglobin and 99% of circulating red blood cells. In two mouse SCD models, Berkeley and SAD, inhibition of red blood cell dehydration and sickling was achieved with correction of hematological parameters, splenomegaly, and prevention of the characteristic urine concentration defect.

High-level erythroid-specific gene expression in primary human and murine hematopoietic cells with self-inactivating lentiviral vectors

Use of oncoretroviral vectors in gene therapy for hemoglobinopathies has been impeded by low titer vectors, genetic instability, and poor expression. Fifteen self- inactivating (SIN) lentiviral vectors using 4 erythroid promoters in combination with 4 erythroid enhancers with or without the woodchuck hepatitis virus postregulatory element (WPRE) were generated using the enhanced green fluorescent protein as a reporter gene. Vectors with high erythroid-specific expression in cell lines were tested in primary human CD34(+) cells and in vivo in the murine bone marrow (BM) transplantation model. Vectors containing the ankyrin-1 promoter showed high-level expression and stable proviral transmission. Two vectors containing the ankyrin-1 promoter and 2 erythroid enhancers (HS-40 plus GATA-1 or HS-40 plus 5-aminolevulinate synthase intron 8 [I8] enhancers) and WPRE expressed at levels higher than the HS2/beta-promoter vector in bulk unilineage erythroid cultures and individual erythroid blast-forming units derived from human BM CD34(+) cells. Sca1(+)/lineage(-) Ly5.1 mouse hematopoietic cells, transduced with these 2 ankyrin-1 promoter vectors, were injected into lethally irradiated Ly5.2 recipients. Eleven weeks after transplantation, high-level expression was seen from both vectors in blood (63%-89% of red blood cells) and erythroid cells in BM (70%-86% engraftment), compared with negligible expression in myeloid and lymphoid lineages in blood, BM, spleen, and thymus (0%-4%). The I8/HS-40-containing vector encoding a hybrid human beta/gamma-globin gene led to 43% to 113% human gamma-globin expression/copy of the mouse alpha-globin gene. Thus, modular use of erythroid-specific enhancers/promoters and WPRE in SIN-lentiviral vectors led to identification of high-titer, stably transmitted vectors with high-level erythroid-specific expression for gene therapy of red cell diseases.

Gene therapy of a mouse model of protoporphyria with a self-inactivating erythroid-specific lentiviral vector without preselection

Successful treatment of blood disorders by gene therapy has several complications, one of which is the frequent lack of selective advantage of genetically corrected cells. Erythropoietic protoporphyria (EPP), caused by a ferrochelatase deficiency, is a good model of hematological genetic disorders with a lack of spontaneous in vivo selection. This disease is characterized by accumulation of protoporphyrin in red blood cells, bone marrow, and other organs, resulting in severe skin photosensitivity. Here we develop a self-inactivating lentiviral vector containing human ferrochelatase cDNA driven by the human ankyrin-1/beta-globin HS-40 chimeric erythroid promoter/enhancer. We collected bone marrow cells from EPP male donor mice for lentiviral transduction and injected them into lethally irradiated female EPP recipient mice. We observed a high transduction efficiency of hematopoietic stem cells resulting in effective gene therapy of primary and secondary recipient EPP mice without any selectable system. Skin photosensitivity was corrected for all secondary engrafted mice and was associated with specific ferrochelatase expression in the erythroid lineage. An erythroid-specific expression was sufficient to reverse most of the clinical and biological manifestations of the disease. This improvement in the efficiency of gene transfer with lentiviruses may contribute to the development of successful clinical protocols for erythropoietic diseases.

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.