In vivo selection of hematopoietic progenitor cells and temozolomide dose intensification in rhesus macaques through lentiviral transduction with a drug resistance gene

In vivo hematopoietic stem cell gene therapy ameliorates murine thalassemia intermedia

Current thalassemia gene therapy protocols require the collection of hematopoietic stem/progenitor cells (HSPCs), in vitro culture, lentivirus vector transduction, and retransplantation into myeloablated patients. Because of cost and technical complexity, it is unlikely that such protocols will be applicable in developing countries, where the greatest demand for a β-thalassemia therapy lies. We have developed a simple in vivo HSPC gene therapy approach that involves HSPC mobilization and an intravenous injection of integrating HDAd5/35++ vectors. Transduced HSPCs homed back to the bone marrow, where they persisted long-term. HDAd5/35++ vectors for in vivo gene therapy of thalassemia had a unique capsid that targeted primitive HSPCs through human CD46, a relatively safe SB100X transposase-based integration machinery, a micro-LCR-driven γ-globin gene, and an MGMT(P140K) system that allowed for increasing the therapeutic effect by short-term treatment with low-dose O6-benzylguanine plus bis-chloroethylnitrosourea. We showed in "healthy" human CD46-transgenic mice and in a mouse model of thalassemia intermedia that our in vivo approach resulted in stable γ-globin expression in the majority of circulating red blood cells. The high marking frequency was maintained in secondary recipients. In the thalassemia model, a near-complete phenotypic correction was achieved. The treatment was well tolerated. This cost-efficient and "portable" approach could permit a broader clinical application of thalassemia gene therapy.

Integrating HDAd5/35++ Vectors as a New Platform for HSC Gene Therapy of Hemoglobinopathies

We generated an integrating, CD46-targeted, helper-dependent adenovirus HDAd5/35++ vector system for hematopoietic stem cell (HSC) gene therapy. The ∼12-kb transgene cassette included a β-globin locus control region (LCR)/promoter driven human γ-globin gene and an elongation factor alpha-1 (EF1α)-mgmtP140K expression cassette, which allows for drug-controlled increase of γ-globin-expressing erythrocytes. We transduced bone marrow lineage-depleted cells from human CD46-transgenic mice and transplanted them into lethally irradiated recipients. The percentage of γ-globin-positive cells in peripheral blood erythrocytes in primary and secondary transplant recipients was stable and greater than 90%. The γ-globin level was 10%-20% of adult mouse globin. Transgene integration, mediated by a hyperactive Sleeping Beauty SB100x transposase, was random, without a preference for genes. A second set of studies was performed with peripheral blood CD34+ cells from mobilized donors. 10 weeks after transplantation of transduced cells, human cells were harvested from the bone marrow and differentiated ex vivo into erythroid cells. Erythroid cells expressed γ-globin at a level of 20% of adult α-globin. Our studies suggest that HDAd35++ vectors allow for efficient transduction of long-term repopulating HSCs and high-level, almost pancellular γ-globin expression in erythrocytes. Furthermore, our HDAd5/35++ vectors have a larger insert capacity and a safer integration pattern than currently used lentivirus vectors.

A Combined In Vivo HSC Transduction/Selection Approach Results in Efficient and Stable Gene Expression in Peripheral Blood Cells in Mice

We recently reported on an in vivo hematopoietic stem cell (HSC) gene therapy approach. It involves the subcutaneous injections of G-CSF/AMD3100 to mobilize HSCs from the bone marrow into the peripheral blood stream and the intravenous injection of an integrating helper-dependent adenovirus vector system. HSCs transduced in the periphery homed back to the bone marrow, where they persisted long-term. However, high transgene marking rates found in primitive bone marrow HSCs were not reflected in peripheral blood cells. Here, we tested small-molecule drugs to achieve selective mobilization and transduction of HSCs. We found more efficient GFP marking in bone marrow HSCs but no increased marking in the peripheral blood cells. We then used an in vivo HSC chemo-selection based on a mutant of the O⁶-methylguanine-DNA methyltransferase (mgmt^P140K) gene that confers resistance to O⁶-BG/BCNU and should give stably transduced HSCs a proliferation stimulus and allow for the selective survival and expansion of progeny cells. Short-term exposure of G-CSF/AMD3100-mobilized, in vivo-transduced mice to relatively low selection drug doses resulted in stable GFP expression in up to 80% of peripheral blood cells. Overall, the further improvement of our in vivo HSC transduction approach creates the basis for a simpler HSC gene therapy.

Overexpression of NAC1 confers drug resistance via HOXA9 in colorectal carcinoma cells

Colorectal carcinoma (CRC) is one of the most common types of malignancy worldwide. Recently, neoadjuvant chemotherapy has become an important treatment strategy for CRC. However, treatment frequently fails due to the development of chemoresistance, which is a major obstacle for positive prognosis. However, the underlying mechanisms of chemoresistance remain unclear. The present study assessed the functions of nucleus accumbens-associated protein 1 (NAC1), an important transcriptional regulator, in CRC progression. Reverse transcription-quantitative polymerase chain reaction, western blot analysis and immunohistochemistry were performed to detect the expression levels of NAC1. It was identified that NAC1 was significantly overexpressed in CRC compared with non-tumorous tissues, indicating an oncogenic role. Following this, gain and loss of function analyses were performed in vitro to further investigate the function of NAC1. Cell viability and caspase-3/7 activity assays were used to assess chemotherapy-induced apoptosis. These results indicated that overexpression of NAC1 in CRC cells increased resistance to chemotherapy and inhibited apoptosis. Additionally, RNA interference-mediated knockdown of NAC1 restored the chemosensitivity of CRC cells. Furthermore, mechanistic investigation revealed that NAC1 increased drug resistance via inducing homeobox A9 (HOXA9) expression, and that knockdown of HOXA9 abrogated NAC1-induced drug resistance. In conclusion, the results of the present study demonstrated that NAC1 may be a critical factor in the development of chemoresistance, offering a potential novel target for the treatment of CRC.

Gene Addition Strategies for β-Thalassemia and Sickle Cell Anemia

Beta-thalassemia and sickle cell anemia are two of the most common diseases related to the hemoglobin protein. In these diseases, the beta-globin gene is mutated, causing severe anemia and ineffective erythropoiesis. Patients can additionally present with a number of life-threatening co-morbidities, such as stroke or spontaneous fractures. Current treatment involves transfusion and iron chelation; allogeneic bone marrow transplant is the only curative option, but is limited by the availability of matching donors and graft-versus-host disease. As these two diseases are monogenic diseases, they make an attractive setting for gene therapy. Gene therapy aims to correct the mutated beta-globin gene or add back a functional copy of beta- or gamma-globin. Initial gene therapy work was done with oncoretroviral vectors, but has since shifted to lentiviral vectors. Currently, there are a few clinical trials underway to test the curative potential of some of these lentiviral vectors. This review will highlight the work done thus far, and present the challenges still facing gene therapy, such as genome toxicity concerns and achieving sufficient transgene expression to cure those with the most severe forms of thalassemia.

Towards in vivo amplification: Overcoming hurdles in the use of hematopoietic stem cells in transplantation and gene therapy

With the advent of safer and more efficient gene transfer methods, gene therapy has become a viable solution for many inherited and acquired disorders. Hematopoietic stem cells (HSCs) are a prime cell compartment for gene therapy aimed at correcting blood-based disorders, as well as those amenable to metabolic outcomes that can effect cross-correction. While some resounding clinical successes have recently been demonstrated, ample room remains to increase the therapeutic output from HSC-directed gene therapy. In vivo amplification of therapeutic cells is one avenue to achieve enhanced gene product delivery. To date, attempts have been made to provide HSCs with resistance to cytotoxic drugs, to include drug-inducible growth modules specific to HSCs, and to increase the engraftment potential of transduced HSCs. This review aims to summarize amplification strategies that have been developed and tested and to discuss their advantages along with barriers faced towards their clinical adaptation. In addition, next-generation strategies to circumvent current limitations of specific amplification schemas are discussed.

Foamy viral vector integration sites in SCID-repopulating cells after MGMTP140K-mediated in vivo selection

Foamy virus (FV) vectors are promising for hematopoietic stem cell (HSC) gene therapy but preclinical data on the clonal composition of FV vector-transduced human repopulating cells is needed. Human CD34(+) human cord blood cells were transduced with an FV vector encoding a methylguanine methyltransferase (MGMT)P140K transgene, transplanted into immunodeficient NOD/SCID IL2Rγ(null) mice, and selected in vivo for gene-modified cells. The retroviral insertion site profile of repopulating clones was examined using modified genomic sequencing PCR. We observed polyclonal repopulation with no evidence of clonal dominance even with the use of a strong internal spleen focus forming virus promoter known to be genotoxic. Our data supports the use of FV vectors with MGMTP140K for HSC gene therapy but also suggests additional safety features should be developed and evaluated.

In vivo enrichment of genetically manipulated platelets corrects the murine hemophilic phenotype and induces immune tolerance even using a low multiplicity of infection

BACKGROUND

Our previous studies have demonstrated that platelet-specific gene delivery to hematopoietic stem cells can induce sustained therapeutic levels of platelet factor VIII (FVIII) expression in mice with hemophilia A.

OBJECTIVE

In this study, we aimed to enhance platelet FVIII expression while minimizing potential toxicities.

METHODS

A novel lentiviral vector (LV), which harbors dual genes, the FVIII gene driven by the αIIb promoter (2bF8) and a drug-resistance gene, the MGMT(P140K) cassette, was constructed. Platelet FVIII expression in mice with hemophilia A was introduced by transduction of hematopoietic stem cells and transplantation. The recipients were treated with O(6)-benzylguanine followed by 1,3-bis-2 chloroethyl-1-nitrosourea monthly three or four times. Animals were analyzed by using polymerase chain reaction (PCR), quantitative PCR, FVIII:C assays, and inhibitor assays. Phenotypic correction was assessed by tail clipping tests and rotational thromboelastometry analysis.

RESULTS

Even using a low multiplicity of infection of 1 and a non-myeloablative conditioning regimen, after in vivo selection, the levels of platelet FVIII expression in recipients increased to 4.33 ± 5.48 mU per 10(8) platelets (n = 16), which were 19.7-fold higher than the levels obtained from the recipients before treatment. Quantitative PCR results confirmed that 2bF8/MGMT-LV-transduced cells were effectively enriched after drug-selective treatment. Fifteen of 16 treated animals survived tail clipping. Blood loss and whole blood clotting time were normalized in the treated recipients. Notably, no anti-FVIII antibodies were detected in the treated animals even after recombinant human B-domain deleted FVIII challenge.

CONCLUSION

we have established an effective in vivo selective system that allows us to enrich 2bF8LV-transduced cells, enhancing platelet FVIII expression while reducing the potential toxicities associated with platelet gene therapy.

Genetic modification of mouse bone marrow by lentiviral vector-mediated delivery of hypoxanthine-Guanine phosphoribosyltransferase short hairpin RNA confers chemoprotection against 6-thioguanine cytotoxicity

We have recently developed a novel and highly efficient strategy that exclusively uses the purine analog 6-thioguanine (6TG) for both pretransplantation conditioning and post-transplantation chemoselection of hypoxanthine-guanine phosphoribosyltransferase (HPRT)-deficient bone marrow (BM). In a mouse BM transplantation model, combined 6TG preconditioning and in vivo chemoselection consistently achieved >95% engraftment of HPRT-deficient donor BM and long-term reconstitution of histologically and immunophenotypically normal hematopoiesis in both primary and secondary recipients, without significant toxicity and in the absence of any other cytotoxic conditioning regimen. To translate this strategy for combined 6TG conditioning and chemoselection into a clinically feasible approach, it is necessary to develop methods for genetic modification of normal hematopoietic stem cells (HSC) to render them HPRT-deficient and thus 6TG-resistant. Here we investigated a strategy to reduce HPRT expression and thereby confer protection against 6TG myelotoxicity to primary murine BM cells by RNA interference (RNAi). Accordingly, we constructed and validated a lentiviral gene transfer vector expressing short-hairpin RNA (shRNA) that targets the murine HPRT gene. Our results showed that lentiviral vector-mediated delivery of HPRT-targeted shRNA could achieve effective and long-term reduction of HPRT expression. Furthermore, in both an established murine cell line as well as in primary murine BM cells, lentiviral transduction with HPRT-targeted shRNA was associated with enhanced resistance to 6TG cytotoxicity in vitro. Hence this represents a translationally feasible method to genetically engineer HSC for implementation of 6TG-mediated preconditioning and in vivo chemoselection.

Efficiency and safety of O⁶-methylguanine DNA methyltransferase (MGMT(P140K))-mediated in vivo selection in a humanized mouse model

Efficient O⁶-methylguanine DNA methyltransferase (MGMT(P140K))-mediated myeloprotection and in vivo selection have been demonstrated in numerous animal models and most recently in a phase I clinical study in glioblastoma patients. However, this strategy may augment the genotoxic risk of integrating vectors because of chemotherapy-induced DNA damage and the proliferative stress exerted during the in vivo selection. Thus, to improve the safety of the procedure, we evaluated a self-inactivating lentiviral MGMT(P140K) vector for transduction of human cord blood-derived CD34⁺ cells followed by transplantation of the cells into NOD/LtSz-scid/Il2rγ⁻/⁻ mice. These experiments demonstrated significant and stable enrichment of MGMT(P140K) transgenic human cells in the murine peripheral blood and bone marrow. Clonal inventory analysis utilizing linear amplification-mediated polymerase chain reaction and high-throughput sequencing revealed a characteristic lentiviral integration profile. Among the bone marrow insertions retrieved, we observed considerable overlap to previous MGMT(P140K) preclinical models or the clinical study. However, no significant differences between our chemotherapy-treated and nontreated cohorts were observed. This also hold true when specific cancer gene databases and a functional annotation of hit genes by the Panther Database with respect to molecular function, biological process, or cellular component were assessed. Thus, in summary, our data demonstrate efficient and long-term in vivo selection without overt hematological abnormalities using the lentiviral MGMT(P140K) vector. Furthermore, the study introduces humanized mouse models as a novel tool for the pre-clinical assessment of human gene therapy related toxicity.

Development of gene therapy for blood disorders: an update

This review addresses the current status of gene therapy for immunodeficiencies, chronic granulomatous disease, suicide gene therapy for graft-versus-host disease, viral infections, malignant hematologic disorders, hemophilia, and the hemoglobin disorders. New developments in vector design have fostered improved expression as well as enhanced safety, particularly of integrating retroviral vectors. Several immunodeficiencies have been treated successfully by stem cell-targeted, retroviral-mediated gene transfer with reconstitution of the immune system following infusion of the transduced cells. In a trial for hemophilia B, long-term expression of human FIX has been observed following adeno-associated viral vector-mediated gene transfer into the liver. This approach should be successful in treating any disorder in which liver production of a specific protein is therapeutic.

Which drug or drug delivery system can change clinical practice for brain tumor therapy?

The prognosis and treatment outcome for primary brain tumors have remained unchanged despite advances in anticancer drug discovery and development. In clinical trials, the majority of promising experimental agents for brain tumors have had limited impact on survival or time to recurrence. These disappointing results are partially explained by the inadequacy of effective drug delivery to the CNS. The impediments posed by the various specialized physiological barriers and active efflux mechanisms lead to drug failure because of inability to reach the desired target at a sufficient concentration. This perspective reviews the leading strategies that aim to improve drug delivery to brain tumors and their likelihood to change clinical practice. The English literature was searched for defined search items. Strategies that use systemic delivery and those that use local delivery are critically reviewed. In addition, challenges posed for drug delivery by combined treatment with anti-angiogenic therapy are outlined. To impact clinical practice and to achieve more than just a limited local control, new drugs and delivery systems must adhere to basic clinical expectations. These include, in addition to an antitumor effect, a verified favorable adverse effects profile, easy introduction into clinical practice, feasibility of repeated or continuous administration, and compatibility of the drug or delivery system with any tumor size and brain location.

Efficient selection of genetically modified human T cells using methotrexate-resistant human dihydrofolate reductase

Genetic modification of human T cells to express transgene-encoded polypeptides, such as tumor targeting chimeric antigen receptors, is an emerging therapeutic modality showing promise in clinical trials. The development of simple and efficient techniques for purifying transgene⁺ T cells is needed to facilitate the derivation of cell products with uniform potency and purity. Unlike selection platforms that utilize physical methods (immunomagnetic or sorting) that are technically cumbersome and limited by the expense and availability of clinical-grade components, we focused on designing a selection system based on the pharmaceutical drug methotrexate (MTX), a potent allosteric inhibitor of human dihydrofolate reductase (DHFR). Here, we describe the development of SIN lentiviral vectors that direct the coordinated expression of a CD19-specific CAR, the human EGFRt tracking/suicide construct, and a methotrexate-resistant human DHFR mutein (huDHFR^FS; L22F, F31S). Our results demonstrate that huDHFR^FS co-expression renders lentivirally transduced primary human CD45RO⁺CD62L⁺ central memory T cells resistant to lymphotoxic concentrations of MTX up to 0.1 µM. Our modular cDNA design insures that selected MTX-resistant T cells co-express functionally relevant levels of the CD19-specific CAR and EGFRt. This selection system based on huDHFR^FS and MTX has considerable potential utility in the manufacturing of clinical-grade T cell products.

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.

Stem cell selection in vivo using foamy vectors cures canine pyruvate kinase deficiency

Background

Hematopoietic stem cell (HSC) gene therapy has cured immunodeficiencies including X-linked severe combined immunodeficiency (SCID-X1) and adenine deaminase deficiency (ADA). For these immunodeficiencies corrected cells have a selective advantage in vivo, and low numbers of gene-modified cells are sufficient to provide therapeutic benefit. Strategies to efficiently transduce and/or expand long-term repopulating cells in vivo are needed for treatment of diseases that require higher levels of corrected cells, such as hemoglobinopathies. Here we expanded corrected stem cells in vivo in a canine model of a severe erythroid disease, pyruvate kinase deficiency.

Methodology/Principal Findings

We used a foamy virus (FV) vector expressing the P140K mutant of methylguanine methyltransferase (MGMTP140K) for in vivo expansion of corrected hematopoietic repopulating cells. FV vectors are attractive gene transfer vectors for hematopoietic stem cell gene therapy since they efficiently transduce repopulating cells and may be safer than more commonly used gammaretroviral vectors. Following transplantation with HSCs transduced ex vivo using a tri-cistronic FV vector that expressed EGFP, R-type pyruvate kinase, and MGMTP140K, we were able to increase marking from approximately 3.5% to 33% in myeloid long-term repopulating cells resulting in a functional cure.

Conclusions/Significance

Here we describe in one affected dog a functional cure for a severe erythroid disease using stem cell selection in vivo. In addition to providing a potential cure for patients with pyruvate kinase deficiency, in vivo selection using foamy vectors with MGMTP140K has broad potential for several hematopoietic diseases including hemoglobinopathies.

Chemoselection of allogeneic HSC after murine neonatal transplantation without myeloablation or post-transplant immunosuppression

The feasibility of allogeneic transplantation, without myeloablation or post-transplant immunosuppression, was tested using in vivo chemoselection of allogeneic hematopoietic stem cells (HSCs) after transduction with a novel tricistronic lentiviral vector (MGMT(P140K)-2A-GFP-IRES-TK (MAGIT)). This vector contains P140K-O(6)-methylguanine-methyltransferase (MGMT(P140K)), HSV-thymidine kinase (TK(HSV)), and enhanced green fluorescent protein (eGFP) enabling (i) in vivo chemoselection of HSC by conferring resistance to benzylguanine (BG), an inhibitor of endogenous MGMT, and to chloroethylating agents such as 1,3-bis(2-chloroethyl)nitrosourea (BCNU) and, (ii) depletion of proliferating cells such as malignant clones or transduced donor T cells mediating graft versus host disease (GVHD), by expression of the suicide gene TK(HSV) and Ganciclovir (GCV) administration. Non-myeloablative transplantation of transduced, syngeneic, lineage-depleted (Lin(-)) BM in neonates resulted in 0.67% GFP(+) mononuclear cells in peripheral blood. BG/BCNU chemoselection, 4 and 8 weeks post-transplant, produced 50-fold donor cell enrichment. Transplantation and chemoselection of major histocompatibility complex (MHC)-mismatched MAGIT-transduced Lin(-) BM also produced similar expansion for >40 weeks. The efficacy of this allotransplant approach was validated in Hbb(th3) heterozygous mice by correction of β-thalassemia intermedia, without toxicity or GVHD. Negative selection, by administration of GCV resulted in donor cell depletion without graft ablation, as re-expansion of donor cells was achieved with BG/BCNU treatment. These studies show promise for developing non-ablative allotransplant approaches using in vivo positive/negative selection.

Treatment of a solid tumor using engineered drug-resistant immunocompetent cells and cytotoxic chemotherapy

Multimodal therapy approaches, such as combining chemotherapy agents with cellular immunotherapy, suffers from potential drug-mediated toxicity to immune effector cells. Overcoming such toxic effects of anticancer cellular products is a potential critical barrier to the development of combined therapeutic approaches. We are evaluating an anticancer strategy that focuses on overcoming such a barrier by genetically engineering drug-resistant variants of immunocompetent cells, thereby allowing for the coadministration of cellular therapy with cytotoxic chemotherapy, a method we refer to as drug-resistant immunotherapy (DRI). The strategy relies on the use of cDNA sequences that confer drug resistance and recombinant lentiviral vectors to transfer nucleic acid sequences into immunocompetent cells. In the present study, we evaluated a DRI-based strategy that incorporates the immunocompetent cell line NK-92, which has intrinsic antitumor properties, genetically engineered to be resistant to both temozolomide and trimetrexate. These immune effector cells efficiently lysed neuroblastoma cell lines, which we show are also sensitive to both chemotherapy agents. The antitumor efficacy of the DRI strategy was demonstrated in vivo, whereby neuroblastoma-bearing NOD/SCID/γ-chain knockout (NSG) mice treated with dual drug-resistant NK-92 cell therapy followed by dual cytotoxic chemotherapy showed tumor regression and significantly enhanced survival compared with animals receiving either nonengineered cell-based therapy and chemotherapy, immunotherapy alone, or chemotherapy alone. These data show there is a benefit to using drug-resistant cellular therapy when combined with cytotoxic chemotherapy approaches.

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.

Contributions of gene marking to cell and gene therapies

The first human genetic modification studies used replication-incompetent integrating vector vectors to introduce marker genes into T lymphocytes and subsequently into hematopoietic stem cells. Such studies have provided numerous insights into the biology of hematopoiesis and immune reconstitution and contributed to clinical development of gene and cell therapies. Tracking of hematopoietic reconstitution and analysis of the origin of residual malignant disease after hematopoietic transplantation has been possible via gene marking. Introduction of selectable marker genes has enabled preselection of specific T-cell populations for tumor and viral immunotherapy and reduced the threat of graft-versus-host disease, improving the survival of patients after allogeneic marrow transplantation. Marking studies in humans, murine xenografts, and large animals have helped optimize conditions for gene transfer into CD34(+) hematopoietic progenitors, contributing to the achievement of gene transfer efficiencies sufficient for clinical benefit in several serious genetic diseases such as X-linked severe combined immunodeficiency and adrenoleukodystrophy. When adverse events linked to insertional mutagenesis arose in clinical gene therapy trials for inherited immunodeficiencies, additional animal studies using gene-marking vectors have greatly increased our understanding of genotoxicity. The knowledge gained from these studies is being translated into new vector designs and clinical protocols, which we hope will continue to improve the efficiency, effectiveness and safety of these promising therapeutic approaches.

Ex vivo gene transfer and correction for cell-based therapies

Cell-based therapies are fast-growing forms of personalized medicine that make use of the steady advances in stem cell manipulation and gene transfer technologies. In this Review, I highlight the latest developments and the crucial challenges for this field, with an emphasis on haematopoietic stem cell gene therapy, which is taken as a representative example given its advanced clinical translation. New technologies for gene correction and targeted integration promise to overcome some of the main hurdles that have long prevented progress in this field. As these approaches marry with our growing capacity for genetic reprogramming of mammalian cells, they may fulfil the promise of safe and effective therapies for currently untreatable diseases.

Imaging stem cell-derived persistent foci after in vivo selection of lentiviral MGMT-P140K transduced murine bone marrow cells

The engraftment of hematopoietic stem cells (HSCs) after drug resistance gene transfer and drug selection may recapitulate stress response hematopoiesis, but the processes remain elusive. Homing, trafficking, and localization of transduced cells and the impact of insertion site on focal expansion have not been well characterized. With the goal of optimizing and understanding these processes under conditions of low multiplicity of infection (MOI) lentiviral gene transfer, we used drug resistance gene O(6)-methylguanine-DNA methyltransferase (MGMT)-P140K and in vivo selection to enrich for transduced and transgene-expressing HSCs. To systemically monitor homing, trafficking, and expansion after transplantation and drug selection over time, we linked MGMT-P140K to the firefly luciferase gene in lentiviral self-inactivating vectors. Periodic bioluminescence imaging (BLI) of transplanted recipients was followed for up to 9 months after both primary and secondary transplantation. Initial dispersion and widespread early homing and engraftment were transient, followed by detection of persistent and discrete foci at stable tissue sites after in vivo drug selection. From these studies, we concluded that drug resistance gene transfer followed by early or late drug selection can result in stable gene expression and cell expansion in persistent foci of transduced bone marrow cells that often remain in fixed sites for extended periods of time.

Differential Secondary Reconstitution of In Vivo-Selected Human SCID-Repopulating Cells in NOD/SCID versus NOD/SCID/γ chain Mice

Humanized bone-marrow xenograft models that can monitor the long-term impact of gene-therapy strategies will help facilitate evaluation of clinical utility. The ability of the murine bone-marrow microenvironment in NOD/SCID versus NOD/SCID/γ chain^null mice to support long-term engraftment of MGMT^P140K-transduced human-hematopoietic cells following alkylator-mediated in vivo selection was investigated. Mice were transplanted with MGMT^P140K-transduced CD34⁺ cells and transduced cells selected in vivo. At 4 months after transplantation, levels of human-cell engraftment, and MGMT^P140K-transduced cells in the bone marrow of NOD/SCID versus NSG mice varied slightly in vehicle- and drug-treated mice. In secondary transplants, although equal numbers of MGMT^P140K-transduced human cells were transplanted, engraftment was significantly higher in NOD/SCID/γ chain^null mice compared to NOD/SCID mice at 2 months after transplantation. These data indicate that reconstitution of NOD/SCID/γ chain^null mice with human-hematopoietic cells represents a more promising model in which to test for genotoxicity and efficacy of strategies that focus on manipulation of long-term repopulating cells of human origin.

Repetitive busulfan administration after hematopoietic stem cell gene therapy associated with a dominant HDAC7 clone in a nonhuman primate

The risk of genotoxicity of retroviral vector-delivered gene therapy targeting hematopoietic stem cells (HSCs) has been highlighted by the development of clonal dominance and malignancies in human and animal gene therapy trials. Large-animal models have proven invaluable to test the safety of retroviral vectors, but the detection of clonal dominance may require years of follow-up. We hypothesized that hematopoietic stress may accelerate the proliferation and therefore the detection of abnormal clones in these models. We administered four monthly busulfan (Bu) infusions to induce hematopoietic stress in a healthy rhesus macaque previously transplanted with CD34+ cells transduced with retroviral vectors carrying a simple marker gene. Busulfan administration resulted in significant cytopenias with each cycle, and prolonged pancytopenia after the final cycle with eventual recovery. Before busulfan treatment there was highly polyclonal marking in all lineages. After Bu administration clonal diversity was markedly decreased in all lineages. Unexpectedly, we found no evidence of selection of the MDS1/EVI1 clones present before Bu administration, but a clone with a vector integration in intron 1 of the histone deacetylase-7 (HDAC7) gene became dominant in granulocytes over time after Bu administration. The overall marking level in the animal was increased significantly after Bu treatment and coincident with expansion of the HDAC7 clone, suggesting an in vivo advantage for this clone under stress. HDAC7 expression was upregulated in marrow progenitors containing the vector. Almost 5 years after Bu administration, the animal developed progressive cytopenias, and at autopsy the marrow showed complete lack of neutrophil or platelet maturation, with a new population of approximately 20% undifferentiated blasts. These data suggest that chemotherapeutic stress may accelerate vector-related clonal dominance, even in the absence of drug resistance genes expressed by the vector. This model may both accelerate the detection of abnormal clones to facilitate analysis of genotoxicity for human gene therapy, and help assess the safety of administering myelotoxic chemotherapeutic agents in patients previously engrafted with vector-containing cells.

Efficient and stable MGMT-mediated selection of long-term repopulating stem cells in nonhuman primates

HSC transplantation using genetically modified autologous cells is a promising therapeutic strategy for various genetic diseases, cancer, and HIV. However, for many of these conditions, the current efficiency of gene transfer to HSCs is not sufficient for clinical use. The ability to increase the percentage of gene-modified cells following transplantation is critical to overcoming this obstacle. In vivo selection with mutant methylguanine methyltransferase (MGMTP140K) has been proposed to overcome low gene transfer efficiency to HSCs. Previous studies have shown efficient in vivo selection in mice and dogs but only transient selection in primates. Here, we report efficient and stable MGMTP140K-mediated multilineage selection in both macaque and baboon nonhuman primate models. Treatment consisting of both O6-benzylguanine (O6BG) and N,N'-bis(2-chloroethyl)-N-nitroso-urea (BCNU) stably increased the percentage of transgene-expressing cells from a range of initial levels of engrafted genetically modified cells, with the longest follow-up after drug treatment occurring over 2.2 years. Drug treatment was well tolerated, and selection occurred in myeloid, lymphoid, and erythroid cells as well as platelets. Retrovirus integration site analysis before and after drug treatments confirmed the presence of multiple clones. These nonhuman primate studies closely model a clinical setting and should have broad applications for HSC gene therapy targeting human diseases of malignant, genetic, and infectious nature, including HIV.

Modified multipotent stromal cells with epidermal growth factor restore vasculogenesis and blood flow in ischemic hind-limb of type II diabetic mice

Diabetes is increasing in the world and causes severe cardiovascular complications. Diabetes-induced limb ischemia leads to foot amputation and therapeutic remedies are urgently needed. Here we report that local injection of mesenchymal stem cells (MSCs) prestimulated with epidermal growth factor (EGF) restored blood flow and vasculogenesis in the ischemic hind-limb of type II diabetic (db(-)/db(-)) mice. Bone marrow cells from db(-)/db(-) mice are altered as evidenced by increased oxidative stress and reduced Akt and adhesion molecules when compared with control (db(-)/db(+)). Femoral artery ligation-induced ischemia was performed in the hind-limb of db(-)/db(-) and db(-)/db(+) mice for 28 days. Enhanced green fluorescent protein (EGFP)-MSCs stimulated+/-exogenous EGF for 24 h were injected locally into the ischemic muscle. Blood flow measured with MoorLDI-Laser and microangiography assessed with X-ray showed 100% recovery in db(-)/db(+) compared to 50% recovery in db(-)/db(-) mice. Interestingly, db(-)/db(-) mice had 60 and 96% blood flow recovery and 61 and 98% of vasculogenesis when treated with MSCs alone or MSCs modified with EGF, respectively. Western blot analysis of hind-limb muscles revealed an increase in Akt and vascular endothelial growth factor receptor phosphorylation and hypoxia-inducible factor) expression in db(-)/db(-) mice injected with MSCs or MSCs+EGF compared to db(-)/db(-) mice. Fluorescent microscopic images show that EGFP-MSCs differentiate into new microvessels. Adhesion and migration of MSCs on cultured endothelial cells were ICAM1-, VCAM1- and Akt-dependent mechanism and elevated when MSCs were prestimulated with EGF compared with nonstimulated MSCs. Our novel study data provide evidence that in type II diabetes, stimulated MSCs with EGF enhance the recovery of blood flow and angiogenesis.

Large animal models of hematopoietic stem cell gene therapy

Large animal models have been instrumental in advancing hematopoietic stem cell (HSC) gene therapy. Here we review the advantages of large animal models, their contributions to the field of HSC gene therapy and recent progress in this field. Several properties of human HSCs including their purification, their cell-cycle characteristics, their response to cytokines and the proliferative demands placed on them after transplantation are more similar in large animal models than in mice. Progress in the development and use of retroviral vectors and ex vivo transduction protocols over the last decade has led to efficient gene transfer in both dogs and nonhuman primates. Importantly, the approaches developed in these models have translated well to the clinic. Large animals continue to be useful to evaluate the efficacy and safety of gene therapy, and dogs with hematopoietic diseases have now been cured by HSC gene therapy. Nonhuman primates allow evaluation of aspects of transplantation as well as disease-specific approaches such as AIDS (acquired immunodeficiency syndrome) gene therapy that can not be modeled well in the dog. Finally, large animal models have been used to evaluate the genotoxicity of viral vectors by comparing integration sites in hematopoietic repopulating cells and monitoring clonality after transplantation.

Feline leukemia virus integrase and capsid packaging functions do not change the insertion profile of standard Moloney retroviral vectors

Adverse events linked to perturbations of cellular genes by vector insertion reported in gene therapy trials and animal models have prompted attempts to better understand the mechanisms directing viral vector integration. The integration profiles of vectors based on MLV, ASLV, SIV, and HIV have all been shown to be non-random, and novel vectors with a safer integration pattern have been sought. Recently we developed a producer cell line called CatPac that packages standard MoMLV vectors with FeLV gag, pol and env gene products. We now report the integration profile of this vector, asking if the FeLV integrase and capsid proteins could modify the MoMLV integration profile, potentially resulting in a less genotoxic pattern. We transduced rhesus macaque CD34+ hematopoietic progenitor cells with CatPac or standard MoMLV vectors, and determined their integration profile by LAM-PCR. We obtained 184 and 175 unique integration sites (IS) respectively for CatPac and standard MoMLV vectors, and these were compared to 10 000 in silico-generated random IS. The integration profile for CatPac vector was similar to MoMLV and equally non-random, with a propensity for integration near transcription start sites and in highly dense gene regions. We found an IS for CatPac vector localized 715 nucleotides upstream of LMO-2, the gene involved in the ALL developed by X-SCID patients treated via gene therapy using MoMLV vectors. In conclusion, we found that replacement of MoMLV env, gag, and pol gene products with FeLV did not alter the basic integration profile. Thus there appears to be no safety advantage for this packaging system. However, considering the stability and efficacy of CatPac vectors, further development is warranted, utilizing potentially safer vector backbones, for instance those with a SIN configuration.

MGMT promoter methylation in malignant gliomas: ready for personalized medicine?

The DNA repair enzyme O(6)-methylguanine-DNA methyltransferase (MGMT) antagonizes the genotoxic effects of alkylating agents. MGMT promoter methylation is the key mechanism of MGMT gene silencing and predicts a favorable outcome in patients with glioblastoma who are exposed to alkylating agent chemotherapy. This biomarker is on the verge of entering clinical decision-making and is currently used to stratify or even select glioblastoma patients for clinical trials. In other subtypes of glioma, such as anaplastic gliomas, the relevance of MGMT promoter methylation might extend beyond the prediction of chemosensitivity, and could reflect a distinct molecular profile. Here, we review the most commonly used assays for evaluation of MGMT status, outline the prerequisites for standardized tests, and evaluate reasons for difficulties in reproducibility. We critically discuss the prognostic and predictive value of MGMT silencing, reviewing trials in which patients with different types of glioma were treated with various chemotherapy schedules, either up-front or at recurrence. Standardization of MGMT testing requires comparison of different technologies across laboratories and prospectively validated cut-off values for prognostic or predictive effects. Moreover, future clinical trials will need to determine, for each subtype of glioma, the degree to which MGMT promoter methylation is predictive or prognostic, and whether testing should become routine clinical practice.

Engineered drug-resistant immunocompetent cells enhance tumor cell killing during a chemotherapy challenge

Establishment of immunocompetent cell mediated anti-tumor immunity is often mitigated by the myelosuppressive effects during administration of chemotherapy. We hypothesized that protecting these immune cells from drug induced toxicities may allow for the combined administration of immunotherapy and chemotherapy. Using a SIV-based lentiviral gene transfer system we delivered the drug-resistant variant P140KMGMT into the immunocompetent cell lines NK-92 and TALL-104, and the myelogenous leukemia cell line, K562, which is a target for both NK-92 and TALL-104 cells. Genetically engineered immunocompetent cells developed significant resistance to temozolomide compared to non-modified cells, and genetic modification of these cells did not affect their ability to kill K562 cells. We then evaluated the effectiveness of drug-resistant immunocompetent cell mediated killing of tumor cells in the presence and absence of chemotherapy. During a chemotherapy challenge the cytotoxic activity of non-modified immunocompetent cells was dramatically impaired. However, when combined with chemotherapy, genetically-modified immune cells retained their cytotoxic activities and efficiently killed non-modified target cells. These results show that engineering immunocompetent cells to withstand chemotherapy challenges can enhance tumor cell killing when chemotherapy is applied in conjunction with cell-based immunotherapy.