Combinatorial incorporation of enhancer-blocking components of the chicken beta-globin 5'HS4 and human T-cell receptor alpha/delta BEAD-1 insulators in self-inactivating retroviral vectors reduces their genotoxic potential

Mechanisms and Delivery of tRNA Therapeutics

Transfer ribonucleic acid (tRNA) therapeutics will provide personalized and mutation specific medicines to treat human genetic diseases for which no cures currently exist. The tRNAs are a family of adaptor molecules that interpret the nucleic acid sequences in our genes into the amino acid sequences of proteins that dictate cell function. Humans encode more than 600 tRNA genes. Interestingly, even healthy individuals contain some mutant tRNAs that make mistakes. Missense suppressor tRNAs insert the wrong amino acid in proteins, and nonsense suppressor tRNAs read through premature stop signals to generate full length proteins. Mutations that underlie many human diseases, including neurodegenerative diseases, cancers, and diverse rare genetic disorders, result from missense or nonsense mutations. Thus, specific tRNA variants can be strategically deployed as therapeutic agents to correct genetic defects. We review the mechanisms of tRNA therapeutic activity, the nature of the therapeutic window for nonsense and missense suppression as well as wild-type tRNA supplementation. We discuss the challenges and promises of delivering tRNAs as synthetic RNAs or as gene therapies. Together, tRNA medicines will provide novel treatments for common and rare genetic diseases in humans.

Quantitative spatial and temporal assessment of regulatory element activity in zebrafish

Mutations or genetic variation in noncoding regions of the genome harbouring cis-regulatory elements (CREs), or enhancers, have been widely implicated in human disease and disease risk. However, our ability to assay the impact of these DNA sequence changes on enhancer activity is currently very limited because of the need to assay these elements in an appropriate biological context. Here, we describe a method for simultaneous quantitative assessment of the spatial and temporal activity of wild-type and disease-associated mutant human CRE alleles using live imaging in zebrafish embryonic development. We generated transgenic lines harbouring a dual-CRE dual-reporter cassette in a pre-defined neutral docking site in the zebrafish genome. The activity of each CRE allele is reported via expression of a specific fluorescent reporter, allowing simultaneous visualisation of where and when in development the wild-type allele is active and how this activity is altered by mutation.

Coordinated β-globin expression and α2-globin reduction in a multiplex lentiviral gene therapy vector for β-thalassemia

A primary challenge in lentiviral gene therapy of β-hemoglobinopathies is to maintain low vector copy numbers to avoid genotoxicity while being reliably therapeutic for all genotypes. We designed a high-titer lentiviral vector, LVβ-shα2, that allows coordinated expression of the therapeutic β^A-T87Q-globin gene and of an intron-embedded miR-30-based short hairpin RNA (shRNA) selectively targeting the α2-globin mRNA. Our approach was guided by the knowledge that moderate reduction of α-globin chain synthesis ameliorates disease severity in β-thalassemia. We demonstrate that LVβ-shα2 reduces α2-globin mRNA expression in erythroid cells while keeping α1-globin mRNA levels unchanged and β^A-T87Q-globin gene expression identical to the parent vector. Compared with the first β^A-T87Q-globin lentiviral vector that has received conditional marketing authorization, BB305, LVβ-shα2 shows 1.7-fold greater potency to improve α/β ratios. It may thus result in greater therapeutic efficacy and reliability for the most severe types of β-thalassemia and provide an improved benefit/risk ratio regardless of the β-thalassemia genotype.

Viral vector platforms within the gene therapy landscape

Throughout its 40-year history, the field of gene therapy has been marked by many transitions. It has seen great strides in combating human disease, has given hope to patients and families with limited treatment options, but has also been subject to many setbacks. Treatment of patients with this class of investigational drugs has resulted in severe adverse effects and, even in rare cases, death. At the heart of this dichotomous field are the viral-based vectors, the delivery vehicles that have allowed researchers and clinicians to develop powerful drug platforms, and have radically changed the face of medicine. Within the past 5 years, the gene therapy field has seen a wave of drugs based on viral vectors that have gained regulatory approval that come in a variety of designs and purposes. These modalities range from vector-based cancer therapies, to treating monogenic diseases with life-altering outcomes. At present, the three key vector strategies are based on adenoviruses, adeno-associated viruses, and lentiviruses. They have led the way in preclinical and clinical successes in the past two decades. However, despite these successes, many challenges still limit these approaches from attaining their full potential. To review the viral vector-based gene therapy landscape, we focus on these three highly regarded vector platforms and describe mechanisms of action and their roles in treating human disease.

A Small Key for a Heavy Door: Genetic Therapies for the Treatment of Hemoglobinopathies

Throughout the past decades, the search for a treatment for severe hemoglobinopathies has gained increased interest within the scientific community. The discovery that ɤ-globin expression from intact HBG alleles complements defective HBB alleles underlying β-thalassemia and sickle cell disease, has provided a promising opening for research directed at relieving ɤ-globin repression mechanisms and, thereby, improve clinical outcomes for patients. Various gene editing strategies aim to reverse the fetal-to-adult hemoglobin switch to up-regulate ɤ-globin expression through disabling either HBG repressor genes or repressor binding sites in the HBG promoter regions. In addition to these HBB mutation-independent strategies involving fetal hemoglobin (HbF) synthesis de-repression, the expanding genome editing toolkit is providing increased accuracy to HBB mutation-specific strategies encompassing adult hemoglobin (HbA) restoration for a personalized treatment of hemoglobinopathies. Moreover, besides genome editing, more conventional gene addition strategies continue under investigation to restore HbA expression. Together, this research makes hemoglobinopathies a fertile ground for testing various innovative genetic therapies with high translational potential. Indeed, the progressive understanding of the molecular clockwork underlying the hemoglobin switch together with the ongoing optimization of genome editing tools heightens the prospect for the development of effective and safe treatments for hemoglobinopathies. In this context, clinical genetics plays an equally crucial role by shedding light on the complexity of the disease and the role of ameliorating genetic modifiers. Here, we cover the most recent insights on the molecular mechanisms underlying hemoglobin biology and hemoglobinopathies while providing an overview of state-of-the-art gene editing platforms. Additionally, current genetic therapies under development, are equally discussed.

The synergistic effect of NELL1 and adipose-derived stem cells on promoting bone formation in osteogenesis imperfecta treatment

BACKGROUND

Osteogenesis imperfecta (OI) is a rare genetic disorder characterized by bone fragility and deformity. Mesenchymal stem cells (MSCs) infusion can improve bone performance mainly due to their differentiation into osteoblasts in OI therapy. The osteoinductive activity of NELL1 have benefited various bone defect and osteoporotic models by promoting bone formation. The present study investigated the efficacy of combined use of NELL1 and adipose-derived mesenchymal stem cells (ADSCs) in OI treatment.

METHODS

Lentiviral vector carrying mouse Nell1 gene was constructed and lentivirus were used to infect ADSCs. The osteogenic capacity of MC3T3-E1 and ADSCs stimulated by recombinant mouse NELL1 protein (rmNELL1) and Nell1 gene genetically modified ADSCs (lenti-Nell1-ADSCs) were estimated by real-time quantitative PCR. Thirty adult male OI type I mice with single Col1a1 gene knockout were randomly divided into five groups and received intravenously injected PBS, rmNELL1 (1.25 mg/Kg), ADSCs (2 × 10⁵ cells per mice), rmNELL1 (1.25 mg/Kg) combined with ADSCs (2 × 10⁵ cells per mice), or lenti-Nell1-ADSCs (2 × 10⁵ cells per mice) respectively. Six wildtype (WT) mice served as positive control. Bone formation was examined after 4 weeks using micro-CT, histological and immunohistochemical methods.

RESULTS

Three osteoblast related genes of MC3T3-E1 and ADSCs were significantly up-regulated by rmNELL1 in vitro. Lenti-Nell1-ADSCs showed greatly enhanced osteogenic differentiation capacity. The infused lenti-Nell1-ADSCs could migrate to femur and differentiate into ALPL-positive cells. Systemic administration of rmNELL1 combined with ADSCs or lenti-Nell1-ADSCs markedly improved the femoral microstructure and promoted bone formation through increasing the ALPL and osteocalcin (OCN) expression, much better than mice that received single rmNELL1 or ADSCs. And Nell1 gene engineered ADSCs achieved slightly better outcomes than that of combinative use of rmNELL1 and ADSCs.

CONCLUSIONS

NELL1 and ADSCs exhibited synergistic effect on stimulating bone formation of OI mice, which might provide an alternative strategy in OI treatment. Compared with dose escalation or multiple administration of rmNELL1, lentivirus-mediated long term expression of NELL1 might be more feasible and convenient. However, further studies are needed to confirm the safety and optimize the therapeutic regime.

Gene Therapy for Hemoglobinopathies

Gene therapy for β-thalassemia and sickle-cell disease is based on transplantation of genetically corrected, autologous hematopoietic stem cells. Preclinical and clinical studies have shown the safety and efficacy of this therapeutic approach, currently based on lentiviral vectors to transfer a β-globin gene under the transcriptional control of regulatory elements of the β-globin locus. Nevertheless, a number of factors are still limiting its efficacy, such as limited stem-cell dose and quality, suboptimal gene transfer efficiency and gene expression levels, and toxicity of myeloablative regimens. In addition, the cost and complexity of the current vector and cell manufacturing clearly limits its application to patients living in less favored countries, where hemoglobinopathies may reach endemic proportions. Gene-editing technology may provide a therapeutic alternative overcoming some of these limitations, though proving its safety and efficacy will most likely require extensive clinical investigation.

Improved biosafety of a lentiviral vector by reducing cellular gene activation

BACKGROUND

Lentiviral vectors (LVs) have enhancer activity and/or transcriptional read-through (EATRT) properties that can lead to the activation of adjacent genes. Consequently, patients may be at increased risk for adverse effects if such vectors are used clinically.

METHODS

In the present study, we assessed the abilities of different "pro-LV"-like constructs with respect to decreasing its EATRT, including the "pro-LV" vector bearing a chimeric ΔLTR of the human foamy virus R-U5 region replaced by that of an LV (HF).

RESULTS

By analyzing the EATRT of "pro-LV" constructs transfected in 293T cells, we observed that the inclusion of the first 400 bp of the chicken β-globin locus HS4 insulator core sequence oriented in the reverse direction (C-) combined with two copies of the simian virus 40 upstream-sequence element (U) at the ΔU3 of ΔLTR region of "pro-LV" tended to shield the adjacent genomic sequences, such that the EATRT rate was lower than when either of the C- or U was included in the "pro-LV". Moreover, upon transduction, the pro-HF appears to reduce the EATRT rate in the chromosomes of 293T (by 80%) and human peripheral blood mononuclear cells (PBMCs) (by 75%) compared to when pro-LV C-U was included (with a 60% and 89% reduction in 293T and PBMCs, respectively). The HF construct had a significant reduction of viral biological titer compared tiowhen the pro-LV C-U was used in 293T cells.

CONCLUSIONS

The results of the present study provide an important basis for the clinical applicability of LVs in gene and cell therapy.

Engineering Globin Gene Expression

Hemoglobinopathies, including sickle cell disease and thalassemia, are among the most common inherited genetic diseases worldwide. Due to the relative ease of isolating and genetically modifying hematopoietic stem and progenitor cells, recent gene editing and gene therapy strategies have progressed to clinical trials with promising outcomes; however, challenges remain and necessitate the continued exploration of new gene engineering and cell transplantation protocols. Current gene engineering strategies aim at reactivating the expression of the fetal γ-globin genes in adult erythroid cells. The γ-globin proteins exhibit anti-sickling properties and can functionally replace adult β-globin. Here, we describe and compare the current genetic engineering procedures that may develop into safe and efficient therapies for hemoglobinopathies in the near future.

Promise of gene therapy to treat sickle cell disease

INTRODUCTION

Sickle cell anemia (SCA) is a hereditary blood disease caused by a single-gene mutation that affects millions of individuals world-wide. In this review, we focus on techniques to treat SCA by ex vivo genetic manipulation of hematopoietic stem/progenitor cells (HSPC), emphasizing replacement gene therapy and gene editing.

AREAS COVERED

Viral transduction of an anti-sickling β-like globin gene has been tested in pre-clinical and early-phase clinical studies, and shows promising preliminary results. Targeted editing of endogenous genes by site-directed nucleases has been developed more recently, and several approaches also are nearing clinical translation.

EXPERT OPINION

The indications and timing of gene therapy for SCA in lieu of supportive care treatment and allogeneic hematopoietic cell transplantation are still undefined. In addition, ensuring access to the treatment where the disease is endemic will present important challenges that must be addressed. Nonetheless, gene therapy and gene editing techniques have transformative potential as a universal curative option in SCA.

Hematopoietic Stem Cell Gene Therapy: Progress and Lessons Learned

The use of allogeneic hematopoietic stem cells (HSCs) to treat genetic blood cell diseases has become a clinical standard but is limited by the availability of suitable matched donors and potential immunologic complications. Gene therapy using autologous HSCs should avoid these limitations and thus may be safer. Progressive improvements in techniques for genetic correction of HSCs, by either vector gene addition or gene editing, are facilitating successful treatments for an increasing number of diseases. We highlight the progress, successes, and remaining challenges toward the development of HSC gene therapies and discuss lessons they provide for the development of future clinical stem cell therapies.

Curative approaches for sickle cell disease: A review of allogeneic and autologous strategies

Despite sickle cell disease (SCD) first being reported >100years ago and molecularly characterized >50years ago, patients continue to experience severe morbidity and early mortality. Although there have been substantial clinical advances with immunizations, penicillin prophylaxis, hydroxyurea treatment, and transfusion therapy, the only cure that can be offered is hematopoietic stem cell transplantation (HSCT). In this work, we summarize the various allogeneic curative approaches reported to date and discuss open and upcoming clinical research protocols. Then we consider gene therapy and gene editing strategies that may enable cure based on autologous HSCs.

Preclinical studies for a phase 1 clinical trial of autologous hematopoietic stem cell gene therapy for sickle cell disease

BACKGROUND AIMS

Gene therapy by autologous hematopoietic stem cell transplantation (HSCT) represents a new approach to treat sickle cell disease (SCD). Optimization of the manufacture, characterization and testing of the transduced hematopoietic stem cell final cell product (FCP), as well as an in depth in vivo toxicology study, are critical for advancing this approach to clinical trials.

METHODS

Data are shown to evaluate and establish the feasibility of isolating, transducing with the Lenti/β^AS3-FB vector and cryopreserving CD34⁺ cells from human bone marrow (BM) at clinical scale. In vitro and in vivo characterization of the FCP was performed, showing that all the release criteria were successfully met. In vivo toxicology studies were conducted to evaluate potential toxicity of the Lenti/β^AS3-FB LV in the context of a murine BM transplant.

RESULTS

Primary and secondary transplantation did not reveal any toxicity from the lentiviral vector. Additionally, vector integration site analysis of murine and human BM cells did not show any clonal skewing caused by insertion of the Lenti/β^AS3-FB vector in cells from primary and secondary transplanted mice.

CONCLUSIONS

We present here a complete protocol, thoroughly optimized to manufacture, characterize and establish safety of a FCP for gene therapy of SCD.

Evidence for the in vivo safety of insulated foamy viral vectors

Retroviral vector mediated stem cell gene therapy is a promising approach for the treatment of hematopoietic disorders. However, genotoxic side effects from integrated vector proviruses are a significant concern for the use of retroviral vectors in the clinic. Insulated foamy viral (FV) vectors are potentially safer retroviral vectors for hematopoietic stem cell gene therapy. We evaluated two newly identified human insulators, A1 and A2 for use in FV vectors. These insulators had moderate insulating capacity and higher titers than previously developed insulated FV vectors. The A1 insulated FV vector was chosen for comparison with the previously described 650cHS4 insulated FV vector in human cord blood CD34⁺ repopulating cells in an immunodeficient mouse model. To maximize the effects of the insulators on the safety of FV vectors, FV vectors containing a highly genotoxic spleen focus forming virus (SFFV) promoter was used to elicit differences in genotoxicity. In vivo, the A1 insulated FV vector showed an approximate 50% reduction in clonal dominance compared to either the 650cHS4 insulated or control FV vectors, although the transduction efficiency of the A1 insulated vector was higher. This data suggests that the A1 insulated FV vector is promising for future pre-clinical and clinical studies.

Retroviral vectors and transposons for stable gene therapy: advances, current challenges and perspectives

Gene therapy protocols require robust and long-term gene expression. For two decades, retrovirus family vectors have offered several attractive properties as stable gene-delivery vehicles. These vectors represent a technology with widespread use in basic biology and translational studies that require persistent gene expression for treatment of several monogenic diseases. Immunogenicity and insertional mutagenesis represent the main obstacles to a wider clinical use of these vectors. Efficient and safe non-viral vectors are emerging as a promising alternative and facilitate clinical gene therapy studies. Here, we present an updated review for beginners and expert readers on retro and lentiviruses and the latest generation of transposon vectors (sleeping beauty and piggyBac) used in stable gene transfer and gene therapy clinical trials. We discuss the potential advantages and disadvantages of these systems such as cellular responses (immunogenicity or genome modification of the target cell) following exogenous DNA integration. Additionally, we discuss potential implications of these genome modification tools in gene therapy and other basic and applied science contexts.

Insulated Foamy Viral Vectors

Retroviral vector-mediated gene therapy is promising, but genotoxicity has limited its use in the clinic. Genotoxicity is highly dependent on the retroviral vector used, and foamy viral (FV) vectors appear relatively safe. However, internal promoters may still potentially activate nearby genes. We developed insulated FV vectors, using four previously described insulators: a version of the well-studied chicken hypersensitivity site 4 insulator (650cHS4), two synthetic CCCTC-binding factor (CTCF)-based insulators, and an insulator based on the CCAAT box-binding transcription factor/nuclear factor I (7xCTF/NF1). We directly compared these insulators for enhancer-blocking activity, effect on FV vector titer, and fidelity of transfer to both proviral long terminal repeats. The synthetic CTCF-based insulators had the strongest insulating activity, but reduced titers significantly. The 7xCTF/NF1 insulator did not reduce titers but had weak insulating activity. The 650cHS4-insulated FV vector was identified as the overall most promising vector. Uninsulated and 650cHS4-insulated FV vectors were both significantly less genotoxic than gammaretroviral vectors. Integration sites were evaluated in cord blood CD34(+) cells and the 650cHS4-insulated FV vector had fewer hotspots compared with an uninsulated FV vector. These data suggest that insulated FV vectors are promising for hematopoietic stem cell gene therapy.

Genetic treatment of a molecular disorder: gene therapy approaches to sickle cell disease

Effective medical management for sickle cell disease (SCD) remains elusive. As a prevalent and severe monogenic disorder, SCD has been long considered a logical candidate for gene therapy. Significant progress has been made in moving toward this goal. These efforts have provided substantial insight into the natural regulation of the globin genes and illuminated challenges for genetic manipulation of the hematopoietic system. The initial γ-retroviral vectors, next-generation lentiviral vectors, and novel genome engineering and gene regulation approaches each share the goal of preventing erythrocyte sickling. After years of preclinical studies, several clinical trials for SCD gene therapies are now open. This review focuses on progress made toward achieving gene therapy, the current state of the field, consideration of factors that may determine clinical success, and prospects for future development.

Gene therapy for hemoglobin disorders - a mini-review

Gene therapy by either gene insertion or editing is an exciting curative therapeutic option for monogenic hemoglobin disorders like sickle cell disease and β-thalassemia. The safety and efficacy of gene transfer techniques has markedly improved with the use of lentivirus vectors. The clinical translation of this technology has met with good success, although key limitations include number of engraftable transduced hematopoietic stem cells and adequate transgene expression that results in complete correction of β0 thalassemia major. This highlights the need to identify and address factors that might be contributing to the in-vivo survival of the transduced hematopoietic stem cells or find means to improve expression from current vectors. In this review, we briefly discuss the gene therapy strategies specific to hemoglobinopathies, the success of the preclinical models and the current status of gene therapy clinical trials.

Progress in gene therapy for primary immunodeficiencies using lentiviral vectors

PURPOSE OF REVIEW

This review gives an overview over the most recent progress in the field of lentiviral gene therapy for primary immunodeficiencies (PIDs). The history and state-of-the-art of lentiviral vector development are summarized and the recent advancements for a number of selected diseases are reviewed in detail. Past retroviral vector trials for these diseases, the most recent improvements of lentiviral vector platforms and their application in preclinical development as well as ongoing clinical trials are discussed.

RECENT FINDINGS

Main focus is on the preclinical studies and clinical trials for the treatment of Wiskott-Aldrich syndrome, chronic granulomatous disease, adenosine deaminase deficient severe combined immunodeficiency (ADA-SCID) and X-linked severe combined immunodeficiency with lentiviral gene therapy.

SUMMARY

Gene therapy for PIDs is an effective treatment, providing potential long-term clinical benefit for affected patients. Substantial progress has been made to make lentiviral gene therapy platforms available for a number of rare genetic diseases. Although many ongoing gene therapy trials are based on ex-vivo approaches with autologous hematopoietic stem cells, other approaches such as in-vivo gene therapy or gene-repair platforms might provide further advancement for certain PIDs.

Potentially therapeutic levels of anti-sickling globin gene expression following lentivirus-mediated gene transfer in sickle cell disease bone marrow CD34+ cells

Sickle cell disease (SCD) can be cured by allogeneic hematopoietic stem cell transplant. However, this is only possible when a matched donor is available, making the development of gene therapy using autologous hematopoietic stem cells a highly desirable alternative. We used a culture model of human erythropoiesis to directly compare two insulated, self-inactivating, and erythroid-specific lentiviral vectors, encoding for γ-globin (V5m3-400) or a modified β-globin (βAS3-FB) for production of antisickling hemoglobin (Hb) and correction of red cell deformability after deoxygenation. Bone marrow CD34+ cells from three SCD patients were transduced using V5m3-400 or βAS3-FB and compared with mock-transduced SCD or healthy donor CD34+ cells. Lentiviral transduction did not impair cell growth or differentiation, as gauged by proliferation and acquisition of erythroid markers. Vector copy number averaged approximately one copy per cell, and corrective globin mRNA levels were increased more than sevenfold over mock-transduced controls. Erythroblasts derived from healthy donor and mock-transduced SCD cells produced a low level of fetal Hb that was increased to 23.6 ± 4.1% per vector copy for cells transduced with V5m3-400. Equivalent levels of modified normal adult Hb of 17.6 ± 3.8% per vector copy were detected for SCD cells transduced with βAS3-FB. These levels of antisickling Hb production were sufficient to reduce sickling of terminal-stage red blood cells upon deoxygenation. We concluded that the achieved levels of fetal Hb and modified normal adult Hb would likely prove therapeutic to SCD patients who lack matched donors.

The human ankyrin 1 promoter insulator sustains gene expression in a β-globin lentiviral vector in hematopoietic stem cells

Lentiviral vectors designed for the treatment of the hemoglobinopathies require the inclusion of regulatory and strong enhancer elements to achieve sufficient expression of the β-globin transgene. Despite the inclusion of these elements, the efficacy of these vectors may be limited by transgene silencing due to the genomic environment surrounding the integration site. Barrier insulators can be used to give more consistent expression and resist silencing even with lower vector copies. Here, the barrier activity of an insulator element from the human ankyrin-1 gene was analyzed in a lentiviral vector carrying an antisickling human β-globin gene. Inclusion of a single copy of the Ankyrin insulator did not affect viral titer, and improved the consistency of expression from the vector in murine erythroleukemia cells. The presence of the Ankyrin insulator element did not change transgene expression in human hematopoietic cells in short-term erythroid culture or in vivo in primary murine transplants. However, analysis in secondary recipients showed that the lentiviral vector with the Ankyrin element preserved transgene expression, whereas expression from the vector lacking the Ankyrin insulator decreased in secondary recipients. These studies demonstrate that the Ankyrin insulator may improve long-term β-globin expression in hematopoietic stem cells for gene therapy of hemoglobinopathies.

Design and Potential of Non-Integrating Lentiviral Vectors

Lentiviral vectors have demonstrated promising results in clinical trials that target cells of the hematopoietic system. For these applications, they are the vectors of choice since they provide stable integration into cells that will undergo extensive expansion in vivo. Unfortunately, integration can have unintended consequences including dysregulated cell growth. Therefore, lentiviral vectors that do not integrate are predicted to have a safer profile compared to integrating vectors and should be considered for applications where transient expression is required or for sustained episomal expression such as in quiescent cells. In this review, the system for generating lentiviral vectors will be described and used to illustrate how alterations in the viral integrase or vector Long Terminal Repeats have been used to generate vectors that lack the ability to integrate. In addition to their safety advantages, these non-integrating lentiviral vectors can be used when persistent expression would have adverse consequences. Vectors are currently in development for use in vaccinations, cancer therapy, site-directed gene insertions, gene disruption strategies, and cell reprogramming. Preclinical work will be described that illustrates the potential of this unique vector system in human gene therapy.

Uncovering and dissecting the genotoxicity of self-inactivating lentiviral vectors in vivo

Self-inactivating (SIN) lentiviral vectors (LV) have an excellent therapeutic potential as demonstrated in preclinical studies and clinical trials. However, weaker mechanisms of insertional mutagenesis could still pose a significant risk in clinical applications. Taking advantage of novel in vivo genotoxicity assays, we tested a battery of LV constructs, including some with clinically relevant designs, and found that oncogene activation by promoter insertion is the most powerful mechanism of early vector-induced oncogenesis. SIN LVs disabled in their capacity to activate oncogenes by promoter insertion were less genotoxic and induced tumors by enhancer-mediated activation of oncogenes with efficiency that was proportional to the strength of the promoter used. On the other hand, when enhancer activity was reduced by using moderate promoters, oncogenesis by inactivation of tumor suppressor gene was revealed. This mechanism becomes predominant when the enhancer activity of the internal promoter is shielded by the presence of a synthetic chromatin insulator cassette. Our data provide both mechanistic insights and quantitative readouts of vector-mediated genotoxicity, allowing a relative ranking of different vectors according to these features, and inform current and future choices of vector design with increasing biosafety.

Retroviral transduction of murine and human hematopoietic progenitors and stem cells

Genetic modification of cells using retroviral vectors is the method of choice when the cell population is difficult to transfect and/or requires persistent transgene expression in progeny cells. There are innumerable potential applications for these procedures in laboratory research and clinical therapeutic interventions. One paradigmatic example is the genetic modification of hematopoietic stem and progenitor cells (HSPCs). These are rare nucleated cells which reside in a specialized microenvironment within the bone marrow, and have the potential to self-renew and/or differentiate into all hematopoietic lineages. Due to their enormous regenerative capacity in steady state or under stress conditions these cells are routinely used in allogeneic bone marrow transplantation to reconstitute the hematopoietic system in patients with metabolic, inflammatory, malignant, and other hematologic disorders. For patients lacking a matched bone marrow donor, gene therapy of autologous hematopoietic stem cells has proven to be an alternative as highlighted recently by several successful gene therapy trials. Genetic modification of HSPCs using retrovirus vectors requires ex vivo manipulation to efficiently introduce the new genetic material into cells (transduction). Optimal culture conditions are essential to facilitate this process while preserving the stemness of the cells. The most frequently used retroviral vector systems for the genetic modifications of HSPCs are derived either from Moloney murine leukemia-virus (Mo-MLV) or the human immunodeficiency virus-1 (HIV-1) and are generally termed according to their genus gamma-retroviral (γ-RV) or lentiviral vectors (LV), respectively. This chapter describes in a step-by-step fashion some techniques used to produce research grade vector supernatants and to obtain purified murine or human hematopoietic stem cells for transduction, as well as follow-up methods for analysis of transduced cell populations.

Identification and characterization of enhancer-blocking insulators to reduce retroviral vector genotoxicity

The chromatin insulator cHS4 can reduce silencing chromosomal position effects and genotoxicity associated with integrating viral vectors. However, the fully active version of this element can also reduce vector titers and is only partially effective. In order to identify alternatives to cHS4, we developed a functional lentiviral vector-based reporter screen for enhancer-blocking insulators. Using this system, we screened candidate sequences that were initially identified by chromatin profiling for binding by CTCF and for DNase hypersensitivity. All 12 analyzed candidates blocked enhancer-promoter activity. The enhancer-blocking activity of the top two candidates was confirmed in two complementary plasmid-based assays. Studies in a gammaretroviral reporter vector indicated these two candidates have little to no effect on vector titers, and do not diminish vector expression in primary mouse bone marrow cultures. Subsequent assessment in a mouse in vivo tumor formation model demonstrated that both candidates reduced the rate of gammaretroviral vector-mediated genotoxicity as effectively as the cHS4 insulator. In summary, we have developed a novel lentiviral vector-based method of screening candidate elements for insulator activity, and have used this method to identify two new insulator elements capable of improving the safety of retroviral vectors without diminishing vector titers or expression. These findings expand the limited arsenal of insulators functionally validated to reduce the rate of retroviral vector-mediated genotoxicity.

Biosafety features of lentiviral vectors

Over the past decades, lentiviral vectors have evolved as a benchmark tool for stable gene transfer into cells with a high replicative potential. Their relatively flexible genome and ability to transduce many forms of nondividing cells, combined with the potential for cell-specific pseudotyping, provides a rich resource for numerous applications in experimental platforms and therapeutic settings. Here, we give an overview of important biosafety features of lentiviral vectors, with detailed discussion of (i) the principles of the lentiviral split-genome design used for the construction of packaging cells; (ii) the relevance of modifications introduced into the lentiviral long terminal repeat (deletion of enhancer/promoter sequences and introduction of insulators); (iii) the basic features of mRNA processing, including the Rev/Rev-responsive element (RRE) interaction and the modifications of the 3' untranslated region of lentiviral vectors with various post-transcriptional regulatory elements affecting transcriptional termination, polyadenylation, and differentiation-specific degradation of mRNA; and (iv) the characteristic integration pattern with the associated risk of transcriptional interference with cellular genes. We conclude with considerations regarding the importance of cell targeting via envelope modifications. Along this course, we address canonical biosafety issues encountered with any type of viral vector: the risks of shedding, mobilization, germline transmission, immunogenicity, and insertional mutagenesis.

A self-deletion lentiviral vector to reduce the risk of replication-competent virus formation

BACKGROUND

Major improvements have been made progressively on human immunodeficiency virus (HIV)-1 based lentiviral vectors to minimize the probability of replication-competent lentivirus formation. This includes the deletion of U3 promoter and the use of packaging cells, which has increased their potential for use in gene therapy and other in vivo applications. However, the risk of forming replication-competent lentiviruses remains.

METHODS

We investigated the use of Cre-loxP mediation with the insertion of the transgene-expressing cassette in ΔU3 to remove additional parts of the HIV-1 backbone upon cre expression, after integration. This, leads to deletion of the packaging signal, primer binding site and Rev response element, including cre itself.

RESULTS

This approach left a split truncated form of long terminal repeat flanked by a loxP and a transgene-expressing cassette in the genome, which made replication-competent lentivirus formation almost impossible. This self-deletion vector could stably express transgenes both in cell lines and transgenic mice with only modest losses of viral titer. The maximum size of the inserts was approximately 3 kb, which was sufficient for most transgenic applications. Moreover, the addition of some enhancer blocking agents downstream of the transgene could reduce the probability of transcriptional read-through in transfected 293T cells.

CONCLUSIONS

Our approach could improve the biosafety of lentiviral vectors, thus improving their potential application for use in clinical trials and other in vivo applications.

β-globin gene transfer to human bone marrow for sickle cell disease

Autologous hematopoietic stem cell gene therapy is an approach to treating sickle cell disease (SCD) patients that may result in lower morbidity than allogeneic transplantation. We examined the potential of a lentiviral vector (LV) (CCL-βAS3-FB) encoding a human hemoglobin (HBB) gene engineered to impede sickle hemoglobin polymerization (HBBAS3) to transduce human BM CD34+ cells from SCD donors and prevent sickling of red blood cells produced by in vitro differentiation. The CCL-βAS3-FB LV transduced BM CD34+ cells from either healthy or SCD donors at similar levels, based on quantitative PCR and colony-forming unit progenitor analysis. Consistent expression of HBBAS3 mRNA and HbAS3 protein compromised a fourth of the total β-globin-like transcripts and hemoglobin (Hb) tetramers. Upon deoxygenation, a lower percentage of HBBAS3-transduced red blood cells exhibited sickling compared with mock-transduced cells from sickle donors. Transduced BM CD34+ cells were transplanted into immunodeficient mice, and the human cells recovered after 2-3 months were cultured for erythroid differentiation, which showed levels of HBBAS3 mRNA similar to those seen in the CD34+ cells that were directly differentiated in vitro. These results demonstrate that the CCL-βAS3-FB LV is capable of efficient transfer and consistent expression of an effective anti-sickling β-globin gene in human SCD BM CD34+ progenitor cells, improving physiologic parameters of the resulting red blood cells.

Characterization of a third generation lentiviral vector pseudotyped with Nipah virus envelope proteins for endothelial cell transduction

Lentiviruses are becoming progressively more popular as gene therapy vectors due to their ability to integrate into quiescent cells and recent clinical trial successes. Directing these vectors to specific cell types and limiting off-target transduction in vivo remains a challenge. Replacing the viral envelope proteins responsible for cellular binding, or pseudotyping, remains a common method to improve lentiviral targeting. Here, we describe the development of a high titer, 3^rd generation lentiviral vector pseudotyped with Nipah virus fusion protein (NiV-F) and attachment protein (NiV-G). Critical to high titers was truncation of the cytoplasmic domains of both NiV-F and NiV-G. As known targets of wild-type Nipah virus, primary endothelial cells are shown to be effectively transduced by the Nipah pseudotype. In contrast, human CD34+ hematopoietic progenitors were not significantly transduced. Additionally, the Nipah pseudotype has increased stability in human serum compared to VSV pseudotyped lentivirus. These findings suggest that the use of Nipah virus envelope proteins in 3^rd generation lentiviral vectors would be a valuable tool for gene delivery targeted to endothelial cells.

The DN2 Myeloid-T (DN2mt) Progenitor is a Target Cell for Leukemic Transformation by the TLX1 Oncogene

INTRODUCTION

Inappropriate activation of the TLX1 (T-cell leukemia homeobox 1) gene by chromosomal translocation is a recurrent event in human T-cell Acute Lymphoblastic Leukemia (T-ALL). Ectopic expression of TLX1 in murine bone marrow progenitor cells using a conventional retroviral vector efficiently yields immortalized cell lines and induces T-ALL-like tumors in mice after long latency.

METHODS

To eliminate a potential contribution of retroviral insertional mutagenesis to TLX1 immortalizing and transforming function, we incorporated the TLX1 gene into an insulated self-inactivating retroviral vector.

RESULTS

Retrovirally transduced TLX1-expressing murine bone marrow progenitor cells had a growth/survival advantage and readily gave rise to immortalized cell lines. Extensive characterization of 15 newly established cell lines failed to reveal a common retroviral integration site. This comprehensive analysis greatly extends our previous study involving a limited number of cell lines, providing additional support for the view that constitutive TLX1 expression is sufficient to initiate the series of events culminating in hematopoietic progenitor cell immortalization. When TLX1-immortalized cells were co-cultured on OP9-DL1 monolayers under conditions permissive for T-cell differentiation, a latent T-lineage potential was revealed. However, the cells were unable to transit the DN2 myeloid-T (DN2mt)-DN2 T-lineage determined (DN2t) commitment step. The differentiation block coincided with failure to upregulate the zinc finger transcription factor gene Bcl11b, the human ortholog of which was shown to be a direct transcriptional target of TLX1 downregulated in the TLX1+ T-ALL cell line ALL-SIL. Other studies have described the ability of TLX1 to promote bypass of mitotic checkpoint arrest, leading to aneuploidy. We likewise found that diploid TLX1-expressing DN2mt cells treated with the mitotic inhibitor paclitaxel bypassed the mitotic checkpoint and displayed chromosomal instability. This was associated with elevated expression of TLX1 transcriptional targets involved in DNA replication and mitosis, including Ccna2 (cyclin A2), Ccnb1 (cyclin B1), Ccnb2 (cyclin B2) and Top2a (topoisomerase IIα). Notably, enforced expression of BCL11B in ALL-SIL T-ALL cells conferred resistance to the topoisomerase IIα poison etoposide.

CONCLUSION

Taken together with previous findings, the data reinforce a mechanism of TLX1 oncogenic activity linked to chromosomal instability resulting from dysregulated expression of target genes involved in mitotic processes. We speculate that repression of BCL11B expression may provide part of the explanation for the observation that aneuploid DNA content in TLX1+ leukemic T cells does not necessarily portend an unfavorable prognosis. This TLX1 hematopoietic progenitor cell immortalization/T-cell differentiation assay should help further our understanding of the mechanisms of TLX1-mediated evolution to malignancy and has the potential to be a useful predictor of disease response to novel therapeutic agents in TLX1+ T-ALL.

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.

Thymidine kinase suicide gene-mediated ganciclovir ablation of autologous gene-modified rhesus hematopoiesis

Despite the genotoxic complications encountered in clinical gene therapy trials for primary immunodeficiency diseases targeting hematopoietic cells with integrating vectors; this strategy holds promise for the cure of several monogenic blood, metabolic and neurodegenerative diseases. In this study, we asked whether the inclusion of a suicide gene in a standard retrovirus vector would allow elimination of vector-containing stem and progenitor cells and their progeny in vivo following transplantation, using our rhesus macaque transplantation model. Following stable engraftment with autologous CD34(+) cells transduced with a retrovirus vector encoding a highly sensitive modified Herpes simplex virus thymidine kinase SR39, the administration of the antiviral prodrug ganciclovir (GCV) was effective in completely eliminating vector-containing cells in all hematopoietic lineages in vivo. The sustained absence of vector-containing cells over time, without additional GCV administration, suggests that the ablation of TkSR39 GCV-sensitive cells occurred in the most primitive hematopoietic long-term repopulating stem or progenitor cell compartment. These results are a proof-of-concept that the inclusion of a suicide gene in integrating vectors, in addition to a therapeutic gene, can provide a mechanism for later elimination of vector-containing cells, thereby increasing the safety of gene transfer.

A tale of two SCIDs

Hematopoietic stem cell (HSC) transplantation may be curative for severe combined immunodeficiency (SCID). However, for a majority of infants with SCID a suitable donor is not available, and even with a matched donor, allogeneic HSC transplantation itself carries potential complications such as graft-versus-host disease as well as side effects from myelosuppressive chemotherapy. In the past decade, substantial advances have been made in the transplantation of gene-modified autologous HSCs, especially for two forms of SCID: X-linked SCID (SCID-X1) and adenosine deaminase (ADA)-deficient SCID. Two new reports in this issue of Science Translational Medicine add to the accumulating findings from gene therapy trials in Italy, France, and the United States that show clinical benefits of this alternative treatment.

Factor VIII delivered by haematopoietic stem cell-derived B cells corrects the phenotype of haemophilia A mice

The main impediments to clinical application of haematopoietic stem cell (HSC) gene therapy for treatment of haemophilia A are the bone marrow transplant-related risks and the potential for insertional mutagenesis caused by retroviral vectors. To circumvent these limitations, we have adapted a non-myeloablative conditioning regimen and directed factor VIII (FVIII) protein synthesis to B lineage cells using an insulated lentiviral vector containing an immunoglobulin heavy chain enhancer-promoter. Transplantation of lentiviral vector-modified HSCs resulted in therapeutic levels of FVIII in the circulation of all transplanted mice for the duration of the study (six months). Immunostaining of spleen cells showed that the majority of FVIII was synthesised by B220+ B cells and CD138+ plasma cells. Subsequent challenge with recombinant FVIII elicited at most a minor anti-FVIII antibody response, demonstrating induction of immune hyporesponsiveness. All transplant recipients exhibited clot formation and survived tail clipping, indicating correction of their haemophilic phenotype. Therapeutic levels of FVIII could be transferred to secondary recipients by bone marrow transplantation, confirming gene transfer into long-term repopulating HSCs. Moreover, short-term therapeutic FVIII levels could also be achieved in secondary recipients by adoptive transfer of HSC-derived splenic B cells. Our findings support pursuit of B cell-directed protein delivery as a potential clinical approach to treat haemophilia A and other disorders correctable by systemically distributed proteins.

The use of chromatin insulators to improve the expression and safety of integrating gene transfer vectors

The therapeutic application of recombinant retroviruses and other integrating gene transfer vectors has been limited by problems of vector expression and vector-mediated genotoxicity. These problems arise in large part from the interactions between vector sequences and the genomic environment surrounding sites of integration. Strides have been made in overcoming both of these problems through the modification of deleterious vector sequences, the inclusion of better enhancers and promoters, and the use of alternative virus systems. However, these modifications often add other restrictions on vector design, which in turn can further limit therapeutic applications. As an alternative, several groups have been investigating a class of DNA regulatory elements known as chromatin insulators. These elements provide a means of blocking the interaction between an integrating vector and the target cell genome in a manner that is independent of the vector transgene, regulatory elements, or virus of origin. This review outlines the background, rationale, and evidence for using chromatin insulators to improve the expression and safety of gene transfer vectors. Also reviewed are topological factors that constrain the use of insulators in integrating gene transfer vectors, alternative sources of insulators, and the role of chromatin insulators as one of several components for optimal vector design.

Development of murine leukemia virus-based retroviral vectors with a minimum possibility of cis-activation

The possibility of insertional mutagenesis in retroviral gene therapy can be reduced by using a vector lacking the enhancer sequence present in the U3 of the long-terminal repeats. However, such vectors suffer from many pitfalls. We attempted to improve a murine leukemia virus-based retroviral vector containing the enhancer-free U3, first by making it easier to construct a producer line and then by introducing the cellular RPL10 promoter as an internal promoter. The reverse orientation of the expression cassette of the transgene was found to give higher transducing titer and higher-level gene expression. The deletion analysis revealed that the 54-bp-long sequence of U3 (34 and 20 bp present at 5' and 3' extreme ends, respectively) was sufficient for the functions of retroviral vectors. The data from the in vitro cell culture assay indicated that the final construct, ROK, containing all these features, had little cis-activation activity, even if it was placed right upstream from the RNA start site of the neighboring gene. Our data suggested that the newly developed vector might provide increased safety, while still producing high viral titer from a stable producer line and high-level gene expression in various target cells including human CD34(+) stem cells.

Gene therapy for β-thalassaemia: the continuing challenge

The β-thalassaemias are inherited anaemias that form the most common class of monogenic disorders in the world. Treatment options are limited, with allogeneic haematopoietic stem cell transplantation offering the only hope for lifelong cure. However, this option is not available for many patients as a result of either the lack of compatible donors or the increased risk of transplant-related mortality in subjects with organ damage resulting from accumulated iron. The paucity of alternative treatments for patients that fall into either of these categories has led to the development of a revolutionary treatment strategy based on gene therapy. This approach involves replacing allogeneic stem cell transplantation with the transfer of normal globin genes into patient-derived, autologous haematopoietic stem cells. This highly attractive strategy offers several advantages, including bypassing the need for allogeneic donors and the immunosuppression required to achieve engraftment of the transplanted cells and to eliminate the risk of donor-related graft-versus-host disease. This review discusses the many advances that have been made towards this endeavour as well as the hurdles that must still be overcome before gene therapy for β-thalassaemia, as well as many other gene therapy applications, can be widely applied in the clinic.

Scavenger receptors and their potential as therapeutic targets in the treatment of cardiovascular disease

Scavenger receptors act as membrane-bound and soluble proteins that bind to macromolecular complexes and pathogens. This diverse supergroup of proteins mediates binding to modified lipoprotein particles which regulate the initiation and progression of atherosclerotic plaques. In vascular tissues, scavenger receptors are implicated in regulating intracellular signaling, lipid accumulation, foam cell development, and cellular apoptosis or necrosis linked to the pathophysiology of atherosclerosis. One approach is using gene therapy to modulate scavenger receptor function in atherosclerosis. Ectopic expression of membrane-bound scavenger receptors using viral vectors can modify lipid profiles and reduce the incidence of atherosclerosis. Alternatively, expression of soluble scavenger receptors can also block plaque initiation and progression. Inhibition of scavenger receptor expression using a combined gene therapy and RNA interference strategy also holds promise for long-term therapy. Here we review our current understanding of the gene delivery by viral vectors to cells and tissues in gene therapy strategies and its application to the modulation of scavenger receptor function in atherosclerosis.

A self-inactivating lentiviral vector for SCID-X1 gene therapy that does not activate LMO2 expression in human T cells

To develop safer and more effective vectors for gene therapy of X-linked severe combined immunodeficiency (SCID-X1), we have evaluated new self-inactivating lentiviral vectors based on the HIV virus. The CL20i4-hgamma(c)-Revgen vector contains the entire human common gamma chain (gamma(c)) genomic sequence driven by the gamma(c) promoter. The CL20i4-EF1alpha-hgamma(c)OPT vector uses a promoter fragment from the eukaryotic elongation factor alpha (EF1alpha) gene to express a codon-optimized human gamma(c) cDNA. Both vectors contain a 400-bp insulator fragment from the chicken beta-globin locus within the self-inactivating long-terminal repeat. Transduction of bone marrow cells using either of these vectors restored T, B, and natural killer lymphocyte development and function in a mouse SCID-X1 transplantation model. Transduction of human CD34(+) bone marrow cells from SCID-X1 patients with either vector restored T-cell development in an in vitro assay. In safety studies using a Jurkat LMO2 activation assay, only the CL20i4-EF1alpha-hgamma(c)OPT vector lacked the ability to transactivate LMO2 protein expression, whereas the CL20i4-hgamma(c)-Revgen vector significantly activated LMO2 protein expression. In addition, the CL20i4-EF1alpha-hgamma(c)OPT vector has not caused any tumors in transplanted mice. We conclude that the CL20i4-EF1alpha-hgamma(c)OPT vector may be suitable for testing in a clinical trial based on these preclinical demonstrations of efficacy and safety.

Hematopoietic immortalizing function of the NKL-subclass homeobox gene TLX1

Translocations resulting in ectopic expression of the TLX1 homeobox gene (previously known as HOX11) are recurrent events in human T-cell acute lymphoblastic leukemia (T-ALL). Transduction of primary murine hematopoietic stem/progenitor cells with retroviral vectors expressing TLX1 readily yields immortalized hematopoietic progenitor cell lines. Understanding the processes involved in TLX1-mediated cellular immortalization should yield insights into the growth and differentiation pathways altered by TLX1 during the development of T-ALL. In recent clinical gene therapy trials, hematopoietic clonal dominance or T-ALL-like diseases have occurred as a direct consequence of insertional activation of the EVI1, PRDM16 or LMO2 proto-oncogenes by the retroviral vectors used to deliver the therapeutic genes. Additionally, the generation of murine hematopoietic progenitor cell lines due to retroviral integrations into Evi1 or Prdm16 has also been recently reported. Here, we determined by linker-mediated nested polymerase chain reaction the integration sites in eight TLX1-immortalized hematopoietic cell lines. Notably, no common integration site was observed among the cell lines. Moreover, no insertions into the Evi1 or Prdm16 genes were identified although insertion near Lmo2 was observed in one instance. However, neither Lmo2 nor any of the other genes examined surrounding the integration sites showed differential vector-influenced expression compared to the cell lines lacking such insertions. While we cannot exclude the possibility that insertional side effects transiently provided a selective growth/survival advantage to the hematopoietic progenitor populations, our results unequivocally rule out insertions into Evi1 and Prdm16 as being integral to the TLX1-initiated immortalization process.

Strategies to insulate lentiviral vector-expressed transgenes

Lentiviruses are capable of infecting many cells irrespective of their cycling status, stably inserting DNA copies of the viral RNA genomes into host chromosomes. This property has led to the development of lentiviral vectors for high-efficiency gene transfer to a wide variety of cell types, from slowly proliferating hematopoietic stem cells to terminally differentiated neurons. Regardless of their advantage over gammaretroviral vectors, which can only introduce transgenes into target cells that are actively dividing, lentiviral vectors are still susceptible to chromosomal position effects that result in transgene silencing or variegated expression. In this chapter, various genetic regulatory elements are described that can be incorporated within lentiviral vector backbones to minimize the influences of neighboring chromatin on single-copy transgene expression. The modifications include utilization of strong internal enhancer-promoter sequences, addition of scaffold/matrix attachment regions, and flanking the transcriptional unit with chromatin domain insulators. Protocols are provided to evaluate the performance as well as the relative biosafety of lentiviral vectors containing these elements.

Development of gene therapy: potential in severe combined immunodeficiency due to adenosine deaminase deficiency

The history of stem cell gene therapy is strongly linked to the development of gene therapy for severe combined immunodeficiencies (SCID) and especially adenosine deaminase (ADA)-deficient SCID. Here we discuss the developments achieved in over two decades of clinical and laboratory research that led to the establishment of a protocol for the autologous transplant of retroviral vector-mediated gene-modified hematopoietic stem cells, which has proved to be both successful and, to date, safe. Patients in trials in three different countries have shown long-term immunological and metabolic correction. Nevertheless, improvements to the safety profile of viral vectors are underway and will undoubtedly reinforce the position of stem cell gene therapy as a treatment option for ADA-SCID.

Insertional transformation of hematopoietic cells by self-inactivating lentiviral and gammaretroviral vectors

Gene transfer vectors may cause clonal imbalance and even malignant cell transformation by insertional upregulation of proto-oncogenes. Lentiviral vectors (LV) with their preferred integration in transcribed genes are considered less genotoxic than gammaretroviral vectors (GV) with their preference for integration next to transcriptional start sites and regulatory gene regions. Using a sensitive cell culture assay and a series of self-inactivating (SIN) vectors, we found that the lentiviral insertion pattern was approximately threefold less likely than the gammaretroviral to trigger transformation of primary hematopoietic cells. However, lentivirally induced mutants also showed robust replating, in line with the selection for common insertion sites (CIS) in the first intron of the Evi1 proto-oncogene. This potent proto-oncogene thus represents a CIS for both GV and LV, despite major differences in their integration mechanisms. Altering the vectors' enhancer-promoter elements had a greater effect on safety than the retroviral insertion pattern. Clinical grade LV expressing the Wiskott-Aldrich syndrome (WAS) protein under control of its own promoter had no transforming potential. Mechanistic studies support the conclusion that enhancer-mediated gene activation is the major cause for insertional transformation of hematopoietic cells, opening rational strategies for risk prevention.

Correction of murine hemophilia A following nonmyeloablative transplantation of hematopoietic stem cells engineered to encode an enhanced human factor VIII variant using a safety-augmented retroviral vector

Insertional mutagenesis by retroviral vectors is a major impediment to the clinical application of hematopoietic stem cell gene transfer for the treatment of hematologic disorders. We recently developed an insulated self-inactivating gammaretroviral vector, RMSinOFB, which uses a novel enhancer-blocking element that significantly decreases genotoxicity of retroviral integration. In this study, we used the RMSinOFB vector to evaluate the efficacy of a newly bioengineered factor VIII (fVIII) variant (efVIII)--containing a combination of A1 domain point mutations (L303E/F309S) and an extended partial B domain for improved secretion plus A2 domain mutations (R484A/R489A/P492A) for reduced immunogenicity--toward successful treatment of murine hemophilia A. In cell lines, efVIII was secreted at up to 6-fold higher levels than an L303E/F309S A1 domain-only fVIII variant (sfVIIIDeltaB). Most important, when compared with a conventional gammaretroviral vector expressing sfVIIIDeltaB, lower doses of RMSin-efVIII-OFB-transduced hematopoietic stem cells were needed to generate comparable curative fVIII levels in hemophilia A BALB/c mice after reduced-intensity total body irradiation or nonmyeloablative chemotherapy conditioning regimens. These data suggest that the safety-augmented RMSin-efVIII-OFB platform represents an encouraging step in the development of a clinically appropriate gene addition therapy for hemophilia A.

Preventing and exploiting the oncogenic potential of integrating gene vectors

Gene therapy requires efficient gene delivery to cure or prevent disease by modifying the genome of somatic cells. However, gene vectors, which insert themselves into the host genome in order to achieve persistent protein expression, can trigger oncogenesis by upregulating cellular protooncogenes. This adverse event, known as insertional mutagenesis, has become a major hurdle in the field. Vectors developed on the basis of lentiviruses are considered to be less genotoxic than the hitherto used gamma-retroviral vectors. For their report in this issue of the JCI, Montini et al. utilized a tumor-prone mouse model to identify the genetic determinants of insertional mutagenesis (see the related article beginning on page 964). They report that the lentiviral integration pattern and additional improvements in vector design reduce the genotoxic risk. These findings will inform future vector design with the goal of limiting genotoxicity for gene therapy or increasing genotoxicity for protooncogene discovery.