A majority of mice show long-term expression of a human beta-globin gene after retrovirus transfer into hematopoietic stem cells

Novel therapies in β-thalassaemia

Beta-thalassaemia is one of the most significant haemoglobinopathies worldwide resulting in the synthesis of little or no β-globin chains. Without treatment, β-thalassaemia major is lethal within the first decade of life due to the complex pathophysiology, which leads to wide clinical manifestations. Current clinical management for these patients depends on repeated transfusions followed by iron-chelating therapy. Several novel approaches to correct the resulting α/β-globin chain imbalance, treat ineffective erythropoiesis and improve iron overload are currently being developed. Up to now, the only curative treatment for β-thalassemia is haematopoietic stem-cell transplantation, but this is a risky and costly procedure. Gene therapy, gene editing and base editing are emerging as a powerful approach to treat this disease. In β-thalassaemia, gene therapy involves the insertion of a vector containing the normal β-globin or γ-globin gene into haematopoietic stem cells to permanently produce normal red blood cells. Gene editing and base editing involves the use of zinc finger nucleases, transcription activator-like nucleases and clustered regularly interspaced short palindromic repeats/Cas9 to either correct the causative mutation or else insert a single nucleotide variant that will increase foetal haemoglobin. In this review, we will examine the current management strategies used to treat β-thalassaemia and focus on the novel therapies targeting ineffective erythropoiesis, improving iron overload and correction of the globin chain imbalance.

Gene therapy for sickle cell disease: An update

Sickle cell disease (SCD) is one of the most common life-threatening monogenic diseases affecting millions of people worldwide. Allogenic hematopietic stem cell transplantation is the only known cure for the disease with high success rates, but the limited availability of matched sibling donors and the high risk of transplantation-related side effects force the scientific community to envision additional therapies. Ex vivo gene therapy through globin gene addition has been investigated extensively and is currently being tested in clinical trials that have begun reporting encouraging data. Recent improvements in our understanding of the molecular pathways controlling mammalian erythropoiesis and globin switching offer new and exciting therapeutic options. Rapid and substantial advances in genome engineering tools, particularly CRISPR/Cas9, have raised the possibility of genetic correction in induced pluripotent stem cells as well as patient-derived hematopoietic stem and progenitor cells. However, these techniques are still in their infancy, and safety/efficacy issues remain that must be addressed before translating these promising techniques into clinical practice.

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.

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.

High-throughput genomic mapping of vector integration sites in gene therapy studies

Gene therapy has enormous potential to treat a variety of infectious and genetic diseases. To date hundreds of patients worldwide have received hematopoietic cell products that have been gene-modified with retrovirus vectors carrying therapeutic transgenes, and many patients have been cured or demonstrated disease stabilization as a result (Adair et al., Sci Transl Med 4:133ra57, 2012; Biffi et al., Science 341:1233158, 2013; Aiuti et al., Science 341:1233151, 2013; Fischer et al., Gene 525:170-173, 2013). Unfortunately, for some patients the provirus integration dysregulated the expression of nearby genes leading to clonal outgrowth and, in some cases, cancer. Thus, the unwanted side effect of insertional mutagenesis has become a major concern for retrovirus gene therapy. The careful study of retrovirus integration sites (RIS) and the contribution of individual gene-modified clones to hematopoietic repopulating cells is of crucial importance for all gene therapy studies. Supporting this, the US Food and Drug Administration (FDA) has mandated the careful monitoring of RIS in all clinical trials of gene therapy. An invaluable method was developed: linear amplification mediated-polymerase chain reaction (LAM-PCR) capable of analyzing in vitro and complex in vivo samples, capturing valuable genomic information directly flanking the site of provirus integration. Linking this method and similar methods to high-throughput sequencing has now made possible an unprecedented understanding of the integration profile of various retrovirus vectors, and allows for sensitive monitoring of their safety. It also allows for a detailed comparison of improved safety-enhanced gene therapy vectors. An important readout of safety is the relative contribution of individual gene-modified repopulating clones. One limitation of LAM-PCR is that the ability to capture the relative contribution of individual clones is compromised because of the initial linear PCR common to all current methods. Here, we describe an improved protocol developed for efficient capture, sequencing, and analysis of RIS that preserves gene-modified clonal contribution information. We also discuss methods to assess dominant/overrepresented gene-modified clones in preclinical and clinical models.

Combining gene therapy and fetal hemoglobin induction for treatment of β-thalassemia

β-thalassemias are caused by nearly 300 mutations of the β-globin gene, leading to a low or absent production of adult hemoglobin (HbA). Two major therapeutic approaches have recently been proposed: gene therapy and induction of fetal hemoglobin (HbF) with the objective of achieving clinically relevant levels of Hbs. The objective of this article is to describe the development of therapeutic strategies based on a combination of gene therapy and induction of HbFs. An increase of β-globin gene expression in β-thalassemia cells can be achieved by gene therapy, although de novo production of clinically relevant levels of adult Hb may be difficult to obtain. On the other hand, an increased production of HbF is beneficial in β-thalassemia. The combination of gene therapy and HbF induction appears to be a pertinent strategy to achieve clinically relevant results.

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.

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.

Comparative analysis of FV vectors with human α- or β-globin gene regulatory elements for the correction of β-thalassemia

β-Globin locus control region (LCR) sequences have been widely used for the regulated expression of the human β-globin gene in therapeutic viral vectors. In this study, we compare the expression of the human β-globin gene from either the HS2/HS3 β-globin LCR or the HS40 regulatory element from the α-globin locus in the context of foamy virus (FV) vectors for the genetic correction of β-thalassemia. Both regulatory elements expressed comparable levels of human β-globin in a murine erythroleukemic line, whereas in murine hematopoietic stem cells the HS40.β vector proved more efficient in β-globin expression and correction of the β-thalassemia phenotype. Following transplantation in the Hbb(th3/+) mouse model, the expression efficiency by the two vectors was similar, whereas the HS40.β vector achieved relatively more stable transgene expression. In addition, in an ex vivo assay using CD34+ cells from thalassemic patients, both vectors achieved significant human β-globin expression and restoration of the thalassemic phenotype as evidenced by enhanced erythropoiesis and decreased apoptosis. Our data suggest that FV vectors with the α-globin HS40 element can be used as alternative but equally efficient vehicles for human β-globin gene expression for the genetic correction of β-thalassemia.

Physiological and tissue-specific vectors for treatment of inherited diseases

After more than 1500 gene therapy clinical trials in the past two decades, the overall conclusion is that for gene therapy (GT) to be successful, the vector systems must still be improved in terms of delivery, expression and safety. The recent development of more efficient and stable vector systems has created great expectations for the future of GT. Impressive results were obtained in three primary immunodeficiencies and other inherited diseases such as congenital blindness, adrenoleukodystrophy or junctional epidermolysis bullosa. However, the development of leukemia in five children included in the GT clinical trials for X-linked severe combined immunodeficiency and the silencing of the therapeutic gene in the chronic granulomatous disease clearly showed the importance of improving safety and efficiency. In this review, we focus on the main strategies available to achieve physiological or tissue-specific expression of therapeutic transgenes and discuss the importance of controlling transgene expression to improve safety. We propose that tissue-specific and/or physiological viral vectors offer the best balance between efficiency and safety and will be the tools of choice for future clinical trials in GT of inherited diseases.

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.

Gene Therapy for beta-thalassemia

Gene transfer for beta-thalassemia requires gene transfer into hematopoietic stem cells using integrating vectors that direct regulated expression of beta globin at therapeutic levels. Among integrating vectors, oncoretroviral vectors carrying the human beta-globin gene and portions of the locus control region (LCR) have suffered from problems of vector instability, low titers and variable expression. In recent studies, human immunodeficiency virus-based lentiviral (LV) vectors were shown to stably transmit the human beta-globin gene and a large LCR element, resulting in correction of beta-thalassemia intermedia in mice. Several groups have since demonstrated correction of the mouse thalassemia intermedia phenotype, with variable levels of beta-globin expression. These levels of expression were insufficient to fully correct the anemia in thalassemia major mouse model. Insertion of a chicken hypersensitive site-4 chicken insulator element (cHS4) in self-inactivating (SIN) LV vectors resulted in higher and less variable expression of human beta-globin, similar to the observations with cHS4-containing retroviral vectors carrying the human gamma-globin gene. The levels of beta-globin expression achieved from insulated SIN-LV vectors were sufficient to phenotypically correct the thalassemia phenotype from 4 patients with human thalassemia major in vitro, and this correction persisted long term for up to 4 months, in xeno-transplanted mice in vivo. In summary, LV vectors have paved the way for clinical gene therapy trials for Cooley's anemia and other beta-globin disorders. SIN-LV vectors address several safety concerns of randomly integrating viral vectors by removing viral transcriptional elements and providing lineage-restricted expression. Flanking the proviral cassette with chromatin insulator elements, which additionally have enhancer-blocking properties, may further improve SIN-LV vector safety.

Protein/DNA polyplexes for gene therapy

The development of molecular conjugates as components of Protein/DNA polyplexes has resulted in the creation of a simple, non-virus vector for the targeted delivery of nucleic acids into specific cell types. This vector has many of the positive attributes of viruses, on without the limitations that continue to plague recombinant viruses. The simplicity of this vector allows for quick analysis of nucleic acids, expression vectors, and therapeutic genes in vitro and potentially in vivo, because the time that would be involved in the generation of recombinant viral vectors is not present. Essentially, the development of this delivery vector has resulted in the creation of a "synthetic virus" that has the capability of targeted delivery without the negative attributes of viruses.

Successful treatment of murine beta-thalassemia intermedia by transfer of the human beta-globin gene

The beta-thalassemias are caused by more than 200 mutations that reduce or abolish beta-globin production. The severity of the resulting anemia can lead to lifelong transfusion dependency. A genetic treatment based on globin gene transfer would require that transgene expression be erythroid specific, elevated, and sustained over time. We report here that long-term synthesis of chimeric hemoglobin (mualpha(2):hubeta(A)(2)) could be achieved in mice with beta-thalassemia intermedia following engraftment with bone marrow cells transduced with a lentiviral vector encoding the human beta-globin gene. In the absence of any posttransduction selection, the treated chimeras exhibit durably increased hemoglobin levels without diminution over 40 weeks. Ineffective erythropoiesis and extramedullary hematopoiesis (EMH) regress, as reflected by normalization of spleen size, architecture, hematopoietic colony formation, and disappearance of liver EMH. These findings establish that a sustained increase of 3 to 4 g/dL hemoglobin is sufficient to correct ineffective erythropoiesis. Hepatic iron accumulation is markedly decreased in 1-year-old chimeras, indicating persistent protection from secondary organ damage. These results demonstrate for the first time that viral-mediated globin gene transfer in hematopoietic stem cells effectively treats a severe hemoglobin disorder.

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.

Toward gene therapy for disorders of globin synthesis

Inherited disorders of hemoglobin remain desirable targets for genetically based therapies. That stem cell replacement reverses the phenotype of both thalassemia and sickle cell anemia has been well established through allogeneic bone marrow transplantation studies, yet significant toxicities and finite donor availability limit this approach to a minority of affected individuals. Genetically based strategies that have as their goal addition of a normal copy of the human beta-globin gene along with key regulatory sequences to autologous hematopoietic stem cells represent a viable alternative to allogeneic transplantation, but this approach has been impeded by formidable obstacles over the last decade. Large animal models have become the standard for the development of clinically relevant gene addition strategies, and significant progress in the techniques used to deliver potentially therapeutic genes has been achieved. The clinical application of such strategies may be close at hand, at least for disorders in which modest level, constitutive expression is sufficient to correct the phenotype. For the thalassemias and hemoglobinopathies, complex, regulated, lineage specific expression of the beta-globin gene at relatively high levels will be required. The discovery of the beta-globin locus control region renewed interest in the thalassemias and sickle cell anemia as targets for gene transfer, but difficulties in attaining high-titer vectors along with a tendency toward rearrangement when segments of the locus control region (LCR) were incorporated into retroviral vectors stalled further progress. Recent advances in vector construction have circumvented this problem and others limiting both gene transfer efficiency and regulation of transgene expression, offering new hope for clinical application.

Development of a stable retrovirus vector capable of long-term expression of gamma-globin mRNA in mouse erythrocytes

Gene therapy for patients with hemoglobin disorders such has been hampered by the inability of retrovirus vectors to transfer globin genes and the locus control region (LCR) into hematopoietic stem cells without rearrangement. In addition, the expression from intact globin gene vectors has been variable in red blood cells as a result of position effects and retrovirus silencing. We hypothesized that by substituting the globin gene promoter for the promoter of another gene expressed in red blood cells, we could generate stable retrovirus vectors that would express globin at sufficient levels to treat hemoglobinopathies. Transgenic mice containing the human ankyrin (Ank) gene promoter fused to the human gamma-globin gene showed position-independent, copy number-dependent expression of a linked gamma-globin mRNA. We generated a "double-copy" Ank/A gamma-globin retrovirus vector that transferred two copies of the Ank/A gamma-globin gene into target cells. Stable gene transfer was observed in primary primary mouse progenitor cells and long-term repopulating hematopoietic stem cells. Expression of Ank/A gamma-globin mRNA in mature red blood cells was approximately 8% of the level of mouse alpha-globin mRNA. We conclude that this novel retrovirus vector may be valuable for treating a variety of hemoglobinopathies by gene therapy if the level of expression can be further increased.

The future of gene therapy for stroke

New diagnostic and treatment strategies are being developed for stroke. Gene therapy has several potential advantages over classical pharmacologic therapy. Direct administration of DNA into the brain offers the advantage of producing high concentrations of therapeutic agents in a relatively localized environment. Gene transfer also provides longer duration of effect than traditional drug therapy. Recent studies indicate that gene transfer can produce functional proteins in brain parenchyma and cerebral blood vessels after stroke. In animal models, gene transfer may reduce effects of cerebral ischemia or subarachnoid hemorrhage. This review summarizes some current methods of gene transfer to the brain and recent progress that may lead to gene therapy for stroke.

In Vivo Silencing of the Human γ-Globin Gene in Murine Erythroid Cells Following Retroviral Transduction

Increased expression of fetal hemoglobin can ameliorate the clinical severity of sickle cell disease. Whereas temporary induction of fetal hemoglobin can be achieved by pharmacologic therapy, gene transfer resulting in high-level expression of the fetal gamma-globin gene may provide a permanent cure for sickle cell disease. We had previously developed a high-titer, genetically stable retroviral vector in which the human gamma-globin gene was linked to HS-40, the major regulatory element of the human alpha-globin gene cluster. Based on experience in transgenic mice, the truncated promoter of the gamma-globin gene of this vector should be active in adult erythroid cells. Our earlier studies demonstrated that this retroviral vector can give rise to high-level expression of the human gamma-globin gene in murine erythroleukemia (MEL) cells. We have now utilized this vector to transduce murine bone marrow cells that were transplanted into W/W(v) recipient mice. Analysis of transduction of murine BFU-e's in vitro and peripheral blood cells from transplanted mice in vivo demonstrated efficient transfer of the human gamma-globin gene. However, in contrast to the high level of expression of the human gamma-globin gene of this vector in MEL cells, the gene was completely silent in vivo in all transplanted mice. These observations confirm that all the necessary regulatory elements responsible for the developmental stage-specific expression of the human gamma-globin gene reside in its proximal sequences. They also emphasize the differences between gene regulation in MEL cells, transgenic mice, and retroviral gene transfer vectors. For this form of globin gene therapy to succeed, the proximal regulatory elements of the human gamma-globin gene may have to be replaced with different regulatory elements that allow the expression of the gamma-globin coding sequences in adult red cells in vivo.

Issues in the manufacture and transplantation of genetically modified hematopoietic stem cells

The advent of safe and practical means to correct, enhance or protect blood cells at the genetic level offers tantalizing therapeutic perspectives. At present, gene delivery using a replication-defective retrovirus is the most efficient method to stably transduce hematopoietic cells. The successful adaptation of retroviral infection to hematopoietic stem cells requires optimized transduction conditions that maximize gene transfer while preserving the cells' potential for engraftment and longterm hematopoiesis. The successful establishment of effective transduction protocols hinges on retrovirus biology as well as stem cell and transplantation biology. Interestingly, the genetic approach could permit novel strategies to promote host repopulation by transplanted stem cells. However, regulated and predictable expression of any transgene integrated at random chromosomal locations cannot be taken for granted. Investigation of the control of transgene expression and prevention of vector silencing will become increasingly important.

Delivering Erythropoietin through Genetically Engineered Cells

Abstract. Erythropoietin (Epo) is a glycoprotein hormone produced by genetic engineering. Many pathologic conditions could benefit from its administration, such as chronic renal failure or hemoglobinopathies. Epo secretion from genetically modified tissued could be proposed to patients only if the protocol is low cost and low risk. For that purpose, retroviral vectors and adeno-associated vectors expressing the Epo cDNA were developed. Gene transfer was performed into skeletal muscles. To avoid polycythemia, a tetracycline-regulated system was used to control the levels of protein secretion in vivo. β-thalassemias are among diseases that could benefit from an Epo gene transfer. β-thalassemias are attributable to deficient synthesis of β-globin and accumulation of unpaired α-chains. Stimulation of fetal globin synthesis is one strategy to correct the globin chain imbalance. There is evidence that Epo could play this role. In a mouse model of β-thalassemia, an adeno-associated vector expressing the Epo cDNA was injected intramuscularly. Epo was secreted continuously during at least 1 yr. Erythropoiesis was improved in those mice by increasing the synthesis of fetal hemoglobin.

Inactivation of a GFP retrovirus occurs at multiple levels in long-term repopulating stem cells and their differentiated progeny

Hematopoietic stem cell gene therapy holds promise for the treatment of many hematologic disorders. One major variable that has limited the overall success of gene therapy to date is the lack of sustained gene expression from viral vectors in transduced stem cell populations. To understand the basis for reduced gene expression at a single-cell level, we have used a murine retroviral vector, MFG, that expresses the green fluorescent protein (GFP) to transduce purified populations of long-term self-renewing hematopoietic stem cells (LT-HSC) isolated using the fluorescence-activated cell sorter. Limiting dilution reconstitution of lethally irradiated recipient mice with 100% transduced, GFP+ LT-HSC showed that silencing of gene expression occurred rapidly in most integration events at the LT-HSC level, irrespective of the initial levels of GFP expression. When inactivation occurred at the LT-HSC level, there was no GFP expression in any hematopoietic lineage clonally derived from silenced LT-HSC. Inactivation downstream of LT-HSC that stably expressed GFPin long-term reconstituted animals was restricted primarily to lymphoid cells. These observations suggest at least 2 distinct mechanisms of silencing retrovirally expressed genes in hematopoietic cells.

Inactivation of a GFP retrovirus occurs at multiple levels in long-term repopulating stem cells and their differentiated progeny

Hematopoietic stem cell gene therapy holds promise for the treatment of many hematologic disorders. One major variable that has limited the overall success of gene therapy to date is the lack of sustained gene expression from viral vectors in transduced stem cell populations. To understand the basis for reduced gene expression at a single-cell level, we have used a murine retroviral vector, MFG, that expresses the green fluorescent protein (GFP) to transduce purified populations of long-term self-renewing hematopoietic stem cells (LT-HSC) isolated using the fluorescence-activated cell sorter. Limiting dilution reconstitution of lethally irradiated recipient mice with 100% transduced, GFP+ LT-HSC showed that silencing of gene expression occurred rapidly in most integration events at the LT-HSC level, irrespective of the initial levels of GFP expression. When inactivation occurred at the LT-HSC level, there was no GFP expression in any hematopoietic lineage clonally derived from silenced LT-HSC. Inactivation downstream of LT-HSC that stably expressed GFPin long-term reconstituted animals was restricted primarily to lymphoid cells. These observations suggest at least 2 distinct mechanisms of silencing retrovirally expressed genes in hematopoietic cells.

Engraftment of gene-marked hematopoietic progenitors in myeloma patients after transplant of autologous long-term marrow cultures

We conducted a phase I hematopoietic stem cell (HSC) gene-marking trial in patients undergoing autologous blood or marrow stem cell transplant for the treatment of multiple myeloma. Between 500 and 1000 ml of bone marrow was harvested from each of 14 myeloma patients and 1 syngeneic donor. A mean of 3.3x10(9) cells per patient were plated in 20 to 50 long-term marrow culture (LTMC) flasks and maintained for 3 weeks. LTMCs were exposed on days 8 and 15 to clinical-grade neo(r)-containing retrovirus supernatant (G1Na). A mean of 8.23x10(8) day-21 LTMC cells containing 5.2x10(4) gene-marked granulocyte-macrophage progenitor cells (CFU-GM) were infused along with an unmanipulated peripheral blood stem cell graft into each patient after myeloablative therapy. Proviral DNA was detected in 71% of 68 tested blood and bone marrow samples and 150 of 2936 (5.1%) CFU-GM derived from patient bone marrow samples after transplant. The proportion of proviral DNA-positive CFU-GM declined from a mean of 9.8% at 3 months to a mean of 2.3% at 24 months postinfusion. Southern blots of 26 marrow and blood samples were negative. Semiquantitative PCR analysis indicated that gene transfer was achieved in 0.01-1% of total bone marrow and blood mononuclear cells (MNCs). Proviral DNA was also observed in EBV-transformed B lymphocytes, in CD34+ -enriched bone marrow cells, and in CFUs derived from the latter progenitors. Gene-modified cells were detected by PCR in peripheral blood and bone marrow for 24 months after infusion of LTMC cells. Sensitivity and specificity of the PCR assays were independently validated in four laboratories. Our data confirm that HSCs may be successfully transduced in stromal based culture systems. The major obstacle to therapeutic application of this approach remains the overall low level of genetically modified cells among the total hematopoietic cell pool in vivo.

Slow and Steady Wins The Race? Progress in the Development of Vectors for Gene Therapy of β-Thalassemia and Sickle Cell Disease

The cloning of the human β-globin genes more than 20 years ago led to predictions that β-thalassemia and sickle cell disease would be among the first monogenic diseases to be successfully treated by gene replacement therapy. However, despite the worldwide enrollment of more than 3,000 patients in approved gene transfer protocols, none have involved therapy for these diseases. This has been due to several technical hurdles that need to be overcome before gene replacement therapy for β-thalassemia and sickle cell disease can become practical. These problems include inefficient transduction of hematopoietic stem cells and an inability to achieve consistent, long-term, high-level expression of transferred β-like globin genes with current gene transfer vectors. In this review we highlight the relationships between understanding the fundamental mechanisms of β-globin gene locus function and basic vector biology and the development of strategies for β-globin gene replacement therapy. Despite slow initial progress in this field, recent advances in a variety of critical areas provide hope that clinical trials may not be far away.

Long-term genetic modification of rhesus monkey hematopoietic cells following transplantation of adenoassociated virus vector-transduced CD34+ cells

We have explored the potential of recombinant adenoassociated virus (AAV) vectors for gene transfer of the human beta-globin gene and the genetic modification of primate pluripotent hematopoietic stem cells (P-PHSCs). Transduction of P-PHSCs was tested in a preclinical bone marrow transplantation model in rhesus monkeys. CD34+ cells were transduced ex vivo and autologously transplanted without prior selection into irradiated rhesus monkeys. Vector-transduced peripheral blood mononuclear cells and granulocytes were present in the circulation for more than 15 months after transplantation. Approximately 1 in 10(5) cells in the circulation was vector modified. The vector was detected in the bone marrow, in granulocytes, and in purified populations of B and T cells, thus demonstrating multilineage repopulation by vector-transduced stem cells. Comparison of transduction protocols suggested that short-term culture of P-PHSCs enhances transduction and subsequent repopulation by rAAV-transduced cells. These results demonstrate that rAAV vectors can be used for the permanent genetic modification of a rhesus monkey hematopoietic system in the absence of selective pressure.

Viral and nonviral gene delivery vectors for cancer gene therapy

The development of vectors that are capable of efficient gene delivery is crucial to the success of gene therapy. We have developed both recombinant viral and nonviral vectors with the goal of correcting genetic abnormalities in cancer cells that are responsible for malignant transformation. Infection of cancer cells by recombinant adenovirus (Adv) indicates that the level of transduction is variable and dependent on the virus-to-cell ratio. Infection of cells with Adv/p53 resulted in levels of tumor suppressor p53 gene expression that could mediate tumor cell growth suppression and apoptosis, both in vitro and in vivo. The treatment of cancer cells with cisplatin prior to Adv transduction resulted in a higher level of therapeutic gene expression. Epidermal growth factor (EGF)/DNA complexes targeted to cancer cells overexpressing the EGF receptor resulted in efficient transduction of several lung cancer cell lines in vitro. As a result, these vectors provide improved methods with which to treat cancer in the clinical setting with gene therapy.

Retroviral vectors aimed at the gene therapy of human beta-globin gene disorders

We are focusing on the development of complex retroviral vectors containing human beta-globin gene and beta-LCR for the gene therapy of sickle cell disease and beta-thalassemias. First generation vectors containing mutated splice-sites to insure stability of proviral transfer enabled long-term reconstitution in 10/12 transplanted mice for a least 8 months with high expression levels in 2 out of 3 mice analyzed (5% and 20% murine beta). Transfer and expression were also achieved in secondary recipients (range: 3-11% murine beta). Position independent expression was not observed. In an effort to increase the efficiency of gene transfer and obtain complete reconstitution of recipient mice with exclusively transduced cells while enriching for proviral integration into active chromatin regions, we have incorporated a cassette expressing CD24 or the green fluorescent protein (GFP). Stable transfer to murine bone marrow cells allowed efficient FACS-sorting of pure populations of transduced cells. A family of vectors based on these principles and containing segments of gamma- or delta-globin genes were also designed for systematic analysis of their anti-sickling properties.

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

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

A positive effect of p21cip1/waf1 in the colony formation from murine myeloid progenitor cells as assessed by retroviral-mediated gene transfer

p21cip1/waf1 is a cyclin dependent kinase inhibitor. We have previously reported stimulation of p21cip1/waf1 by steel factor and GM-CSF in a factor dependent cell line and of p21cip1/waf1 involvement in hematopoiesis in vivo in p21cip1/waf1 gene knockout (-/-) mice. To further assess a role for increased p21cip1/waf1 in hematopoietic progenitor cells, we developed the retroviral vector L(p21cip1)SN to transcriptionally regulate p21cip1/waf1 from the Mo-MLV LTR. L(p21cip1)SN and the control vector LXSN were used to transduce murine bone marrow progenitor cells from p21cip1/waf1 (-/-) and littermate control (+/+) mice, as well as from other mouse strains. Hematopoietic colony formation by transduced cells was assessed in semi-solid culture medium with multiple growth factors. Myeloid colony formation by bone marrow cells from p21cip1/waf1 (-/-) mice was significantly lower than that by (+/+) mouse cells. Transduction of cells with LXSN had no effect on colony formation: however, (-/-) cells transduced with L(p21cip1)SN formed significantly greater numbers of colonies than either LXSN-transduced (-/-) or (+/-) cells. Moreover, L(p21cip1)SN-transduced (+/+) cells formed significantly more colonies than LXSN-transduced (+/+) cells. Increased cloning efficiency of progenitors from normal strains of mice induced by L(p21cip1)SN compared to LXSN-transduced cells was seen whether unseparated or highly purified populations of Sca1+ Lin- marrow cells were used. Gene transfer of L(p21cip1)SN increased the size and number of cells per colony, as well as the number of colonies compared to LXSN gene transfer. No colonies grew from non-transduced, LXSN-, or L(p21cip1)SN-transduced cells when no growth factors were added to the cultures. These results document the positive effect of p21cip1/waf1 in the proliferation and/or differentiation of the murine myeloid progenitor cells that lead to colony formation.

Current status of stem cell therapy and prospects for gene therapy for the disorders of globin synthesis

Sickle cell anaemia and beta-thalassaemia are today curable through the use of stem cell transplantation. Nevertheless, the disadvantages inherent in stem cell transplantation underscore the need for better therapies. A recent finding of potentially major importance is that complete eradication of host haematopoiesis is not an absolute requirement for achieving therapeutic effects in thalassaemia and sickle cell anaemia. Future stem cell transplantation protocols will use less toxic conditioning regimens in an effort to achieve a state of stable mixed chimerism between donor and host haematopoietic elements. An improved understanding of globin gene regulation and stem cell biology will allow for the first gene therapy trials for sickle cell anaemia and beta-thalassaemia in the relatively near future. Initial gene therapy protocols will emphasize safety, are likely to target progenitor cells, and will involve repeated cycles of mobilization, transduction and reinfusion, with little or no conditioning. These first generation gene therapy trials are unlikely to confer major therapeutic benefits, but will provide the foundation upon which subsequent, more effective protocols will be based.

Long-Term Transfer and Expression of the Human β-Globin Gene in a Mouse Transplant Model

Somatic gene therapy of hemoglobinopathies depends initially on the demonstration of safe, efficient gene transfer and long-term, high-level expression of the transferred human β-globin gene in animal models. We have used a β-globin gene/β-locus control region retroviral vector containing several modifications to optimize gene transfer and expression in a mouse transplant model. In this report we show that transplantation of β-globin–transduced hematopoietic cells into lethally irradiated mice leads to the continued presence of the gene up to 8 months posttransplantation. The transferred human β-globin gene is detected in 3 of 5 mice surviving long term (>4 months) transplanted with bone marrow cells transduced with high-titer virus. Southern blotting confirms the presence of the unrearranged 5.1-kb human β-globin gene-containing provirus in 2 of these mice. In addition, long-term expression of the transferred gene is seen in 2 mice at levels of 5% and 20% that of endogenous murine β-globin at 6 and 8 months posttransplantation. We further document stem cell transduction by the successful transfer and high-level expression of the human β-globin gene from mice transduced 9 months earlier into irradiated secondary recipient mice. These results demonstrate high-level, long-term somatic human β-globin gene transfer into the hematopoietic stem cells of an animal for the first time, and suggest the potential feasibility of a retroviral gene therapy approach to sickle cell disease and the β thalassemias.

Retroviral transduction of murine epidermal stem cells demonstrates clonal units of epidermal structure

It has been suggested that the number and position of epidermal stem cells are related to the units of columnar structure in the upper epidermal strata and that the cells of each unit are derived from a single stem cell. Studies of cell lineage in developing tissues have been facilitated by the use of retroviral transduction to provide inherited expression of a histochemically demonstrable foreign gene product. To provide direct evidence about the clonal nature of epidermal units, murine epidermal keratinocytes were transduced with a replication-deficient retroviral vector carrying the beta-galactosidase gene. Subepidermal injection of virus in vivo led to infrequent transduction with only transient presence of beta-gal-staining keratinocytes within the epidermis. Transduction of keratinocytes in vitro and transplantation back to in vivo sites permitted demonstration of the transduced gene in clusters of cells within the reformed epidermis throughout a 12-wk period. The epidermis redeveloped an ordered columnar structure with restriction of transduced cells to individual columnar units. This clonal appearance is compatible with derivation of each epidermal unit from a single stem cell but is not compatible with a random pattern of cell proliferation. Transduced epidermal sheets that were recombined with oral mucosal connective tissue also redeveloped normal columnar structure with restriction of beta-gal staining to individual columnar units. These data suggest that the establishment of an epidermal stem cell pattern related to units of structure is an intrinsic property of the epithelium and is not dependent on regionally-specific connective tissue influences.

Expression of the gibbon ape leukemia virus receptor-1 in rhesus macaque tissues

A cDNA fragment specific for the rhesus macaque gibbon ape leukemia virus (GaLV) receptor (Glvr-1) was isolated by polymerase chain reaction (PCR) amplification and oligonucleotide primers specific for human Glvr-1 and a cDNA library derived from rhesus macaque brain. Sequence analysis of the fragment revealed the polypeptide domain necessary for infection by GaLV. This fragment was utilized to elucidate expression levels of Glvr-1 RNA in rhesus macaque tissues. By Northern blot analysis, Glvr-1 RNA is most abundantly expressed in the thymus and bone marrow, with detectable levels also in the brain and testes of juvenile male macaques. In the developing 70-days gestation fetus, Glvr-1 expression was observed predominately in the liver and spleen. Although additional studies are required, these studies support the notion that cell types involved in hematopoiesis express Glvr-1.

Systemic delivery of human growth hormone or human factor IX in dogs by reintroduced genetically modified autologous bone marrow stromal cells

Canine bone marrow stromal cells were expanded to numbers in excess of 10(9) cells from the initial 10-20 ml of marrow aspirates and transfected to express high levels of human growth hormone (hGH) in vitro. Ex vivo-modified marrow stromal cells were used in a gene therapy model system for the systemic delivery of transgene products in dogs. Adherent bone marrow stromal cell cultures, established and expanded from iliac crest marrow aspirates from each of 8 dogs, were transfected with a hGH gene plasmid expression vector and shown to express from 0.54-3.84 micrograms/10(6) cells per 24 hr hGH in vitro. The transfected plasmid vector does not possess a eukaryotic origin of replication nor does it possess sequences required for efficient integration into the host cell genome. As such, expression was expected to be transient. Transfected cells were autologously reintroduced into each dog by either infusion into a foreleg vein or directly into iliac crest marrow. In two cases, the stromal cells were cryopreserved following transfection, and subsequently thawed and infused. In one case, the expanded stromal cells were first cryopreserved, and then thawed, recultured, transfected, and infused. Reintroduced cell numbers ranged from 2.2 x 10(7) to 2.6 x 10(9), with total hGH expression capacities ranging from 62 to 1,400 micrograms/24 hr. Plasma of each of the dogs contained detectable hGH for a mean of 3.1 days (SD +/- 0.8 day) ranging from 2 to 5 days following reinfusion of cells. Peak plasma levels ranged from 0.10 to 1.76 ng/ml. Similar hGH expression values, based upon total expression capacity of the cells infused and dog body weight, were obtained for all dogs. Vector-modified stromal cells were detectable, by polymerase chain reaction (PCR) analysis, in the peripheral circulation following reinfusion in all 4 dogs analyzed. In 3 of the dogs, modified stromal cells were detected for 8.5-15 weeks. In addition, modified stromal cells were detected in iliac crest marrow of 2 dogs for 9 and 13 weeks, respectively, following reinfusion. In another experiment, cultured bone marrow stromal cells were transfected with a human factor IX (hFIX) plasmid vector. Modified cells (5.57 x 10(8)), with a total hFIX expression capacity of 281 micrograms/24 hr, were reinfused, resulting in detectable hFIX in plasma continuously for 9 days with a peak level of 8 ng/ml on day 1. These results demonstrate that the ex vivo bone marrow stromal cell system is a potentially powerful method by which to deliver secreted transgene product to the systemic circulation of large animals.

Retroviral stem cell gene therapy

Long-term in vivo gene transfer studies in mice have shown that recombinant murine retroviruses are able to infect murine hemopoietic stem cells with high efficiency. Taken together the results indicated that the proviral structure was present at high frequency in circulating hemopoietic cells resulting in significant expression levels. Because of the success of these murine studies, it was believed that gene therapy would soon be applicable to treat a wide variety of congenital or acquired human diseases associated with the hemopoietic system. However, results from gene transfer studies in nonhuman primates and first human clinical trails have indicated that murine retrovirus infection of primate hemopoietic stem cells is inefficient. Although there are essential differences between the murine and primate gene therapy studies with respect to the recombinant viruses and transduction protocols used, these differences cannot solely account for the differences observed in infection efficiency. Therefore, in recent years effort has been spent on the identification of factors limiting retroviral transduction of primate hemopoietic stem cells. Increasing knowledge concerning hemopoiesis and retroviral infection has helped in identifying a number of limiting factors. Novel transduction strategies and tools have been generated which attempt to circumvent these limiting factors. These factors as well as the strategies that showed increased retroviral infection of primate hemopoietic stem cells will be discussed.

Transcriptional Silencing of Retroviral Vectors

Although retroviral vector systems have been found to efficiently transduce a variety of cell types in vitro, the use of vectors based on murine leukemia virus in preclinical models of somatic gene therapy has led to the identification of transcriptional silencing in vivo as an important problem. Extinction of long-term vector expression has been observed after implantation of transduced hematopoietic cells as well as fibroblasts, myoblasts and hepatocytes. Here we review the influence of vector structure, integration site and cell type on transcriptional silencing. While down-regulation of proviral transcription is known from a number of cellular and animal models, major insight has been gained from studies in the germ line and embryonal cells of the mouse. Key elements for the transfer and expression of retroviral vectors, such as the viral transcriptional enhancer and the binding site for the tRNA primer for reverse transcription may have a major influence on transcriptional silencing. Alterations of these elements of the vector backbone as well as the use of internal promoter elements from housekeeping genes may contribute to reduce transcriptional silencing. The use of cell culture and animal models in the testing and improvement of vector design is discussed. Copyright 1996 S. Karger AG, Basel

Long-term persistence of canine hematopoietic cells genetically marked by retrovirus vectors

In 1991 we reported gene transduction into autologous long-term repopulating marrow cells in dogs using amphotropic helper-free retrovirus vectors containing the bacterial neomycin phosphotransferase gene (neo) and the human adenosine deaminase gene (ADA). Two of the dogs are still alive and healthy now more than 5 years after transplantation of transduced autologous marrow cells. In one of the surviving dogs, polymerase chain reaction (PCR) analysis showed the neo and ADA genes to be present in peripheral blood granulocytes and lymphocytes up to the present time. The estimated percentage of neo-positive cells ranged from < 0.001% to 0.1%. ADA mRNA expression was detected by reverse transcriptase PCR (RT-PCR) in granulocytes 63 months after transplantation. The other surviving dog failed to show either persistence or expression of the transduced genes after 50 months. Three additional dogs have been transplanted according to the same transduction protocols and with the same retrovirus vectors, and persistence of the transduced neo gene has been documented in peripheral blood myeloid and lymphoid cells along with G418-resistant colony-forming unit-granulocyte/macrophage (CFU-GM) for now more than 2 years. These findings represent the longest follow-up of retrovirus-mediated gene transduction in any animal species. Long-term transduction efficiency, though, has remained low and will need to be improved for therapeutic application to be possible.

The use of umbilical cord blood as a cellular source for correction of genetic diseases affecting the hematopoietic system

Human umbilical cord blood contains abundant primitive and committed hematopoietic progenitors and has been used as an alternative source of reconstituting hematopoietic stem cells. Recent advances in the understanding of molecular aspects of multiple diseases and improvements in technology associated with prenatal diagnosis now allow the in utero identification of many genetic diseases affecting the hematopoietic system. Advances in technology raise the potential for genetic correction and subsequent transplantation of autologous cord and placental blood hematopoietic stem cells into affected patients prior to expression of the disease phenotype. This review will summarize the recent data on advances in prenatal diagnosis, characterization of the biology of cord blood stem cells, and efforts at developing methods for genetic transduction of cord blood hematopoietic stem/progenitor cells.