Protection and in vivo selection of hematopoietic stem cells using temozolomide, O6-benzylguanine, and an alkyltransferase-expressing retroviral vector

Inherent and toxicant-provoked reduction in DNA repair capacity: A key mechanism for personalized risk assessment, cancer prevention and intervention, and response to therapy

Genomic investigations reveal novel evidence which indicates that genetic predisposition and inherent drug response are key factors for development of cancer and for poor response to therapy. However, mechanisms for these outcomes and interactions with environmental factors have not been well-characterized. Therefore, cancer risk, prevention, intervention and prognosis determinations have still mainly been based on population, rather than on individualized, evaluations. The objective of this review was to demonstrate that a key mechanism which contributes to the determination is inherent and/or toxicant-provoked reduction in DNA repair capacity. In addition, functional and quantitative determination of DNA repair capacity on an individual basis would dramatically change the evaluation and management of health problems from a population to a personalized basis. In this review, justifications for the scenario were delineated. Topics to be presented include assays for detection of functional DNA repair deficiency, mechanisms for DNA repair defects, toxicant-perturbed DNA repair capacity, epigenetic mechanisms (methylation and miRNA expression) for alteration of DNA repair function, and bioinformatics approach to analyze large amount of genomic data. Information from these topics has recently been and will be used for better understanding of cancer causation and of response to therapeutic interventions. Consequently, innovative genomic- and mechanism-based evidence can be increasingly used to develop more precise cancer risk assessment, and target-specific and personalized medicine.

Efficient Enrichment of Gene-Modified Primary T Cells via CCR5-Targeted Integration of Mutant Dihydrofolate Reductase

Targeted gene therapy strategies utilizing homology-driven repair (HDR) allow for greater control over transgene integration site, copy number, and expression-significant advantages over traditional vector-mediated gene therapy with random genome integration. However, the relatively low efficiency of HDR-based strategies limits their clinical application. Here, we used HDR to knock in a mutant dihydrofolate reductase (mDHFR) selection gene at the gene-edited CCR5 locus in primary human CD4⁺ T cells and selected for mDHFR-modified cells in the presence of methotrexate (MTX). Cells were transfected with CCR5-megaTAL nuclease mRNA and transduced with adeno-associated virus containing an mDHFR donor template flanked by CCR5 homology arms, leading to up to 40% targeted gene insertion. Clinically relevant concentrations of MTX led to a greater than 5-fold enrichment for mDHFR-modified cells, which maintained a diverse TCR repertoire over the course of expansion and drug selection. Our results demonstrate that mDHFR/MTX-based selection can be used to enrich for gene-modified T cells ex vivo, paving the way for analogous approaches to increase the percentage of HIV-resistant, autologous CD4⁺ T cells infused into HIV⁺ patients, and/or for in vivo selection of gene-edited T cells for the treatment of cancer.

Clinical Trial of MGMT(P140K) Gene Therapy in the Treatment of Pediatric Patients with Brain Tumors

Gene transfer targeting hematopoietic stem cells (HSC) in children has shown sustained therapeutic benefit in the treatment of genetic diseases affecting the immune system, most notably in severe combined immunodeficiencies affecting T-cell function. The HSC compartment has also been successfully targeted using gene transfer in children with genetic diseases affecting the central nervous system, such as metachromatic leukodystrophy and adrenoleukodystrophy. HSCs are also a target for genetic modification in strategies aiming to confer drug resistance to chemotherapy agents so as to reduce off-target toxicity, and to allow for chemotherapy dose escalation with the possibility of enhanced therapeutic benefit. In a trial of this strategy in adult glioma patients, significant engraftment of gene-modified HSCs expressing a mutant of the DNA repair protein O6-methyl-guanine-methyl-transferase (MGMT(P140K)) showed potential in conferring drug resistance against the combined effect of O6-benzylguanine (O6BG)/temozolomide (TMZ) chemotherapy. The aim was to test the safety and feasibility of this approach in children with poor prognosis brain tumors. In this Phase I trial, seven patients received gene-modified HSC following myelo-suppressive conditioning, but with only transient low-level engraftment of MGMT(P140K) gene-modified cells detectable in four patients. All patients received O6BG/TMZ chemotherapy following infusion of gene-modified cells, with five patients eligible for chemotherapy dose escalation, though in the absence of demonstrable transgene-mediated chemoprotection. Since all gene-modified cell products met the criteria for release and assays for engraftment potential met expected outcome measures, inadequate cell dose, conditioning chemotherapy, and/or underlying bone-marrow function may have contributed to the lack of sustained engraftment of gene-modified cells. We were able to demonstrate safe conduct of a technically complex Phase I study encompassing manufacture of the gene therapy vector, genetically modified cells, and a drug product specifically for the trial in compliance with both local and national regulatory requirements.

Interleukin-13 receptor alpha 2-targeted glioblastoma immunotherapy

Glioblastoma (GBM) is the most lethal primary brain tumor, and despite several refinements in its multimodal management, generally has very poor prognosis. Targeted immunotherapy is an emerging field of research that shows great promise in the treatment of GBM. One of the most extensively studied targets is the interleukin-13 receptor alpha chain variant 2 (IL13Rα2). Its selective expression on GBM, discovered almost two decades ago, has been a target for therapy ever since. Immunotherapeutic strategies have been developed targeting IL13Rα2, including monoclonal antibodies as well as cell-based strategies such as IL13Rα2-pulsed dendritic cells and IL13Rα2-targeted chimeric antigen receptor-expressing T cells. Advanced therapeutic development has led to the completion of several clinical trials with promising outcomes. In this review, we will discuss the recent advances in the IL13Rα2-targeted immunotherapy and evaluate the most promising strategy for targeted GBM immunotherapy.

Lentiviral MGMT(P140K)-mediated in vivo selection employing a ubiquitous chromatin opening element (A2UCOE) linked to a cellular promoter

Notwithstanding recent successes, insertional mutagenesis as well as silencing and variegation of transgene expression still represent considerable obstacles to hematopoietic gene therapy. This also applies to O(6)-methylguanine DNA methyltransferase (MGMT)-mediated myeloprotection, a concept recently proven clinically effective in the context of glioblastoma therapy. To improve on this situation we here evaluate a SIN-lentiviral vector expressing the MGMT(P140K)-cDNA from a combined A2UCOE/PGK-promoter. In a murine in vivo chemoselection model the A2UCOE.PGK.MGMT construct allowed for significant myeloprotection as well as robust and stable selection of transgenic hematopoietic cells. In contrast, only transient enrichment and severe myelotoxicity was observed for a PGK.MGMT control vector. Selection of A2UCOE.PGK.MGMT-transduced myeloid and lymphoid mature and progenitor cells was demonstrated in the peripheral blood, bone marrow, spleen, and thymus. Unlike the PGK and SFFV promoters used as controls, the A2UCOE.PGK promoter allowed for sustained vector copy number-related transgene expression throughout the experiment indicating an increased resistance to silencing, which was further confirmed by CpG methylation studies of the PGK promoter. Thus, our data support a potential role of the A2UCOE.PGK.MGMT-vector in future MGMT-based myeloprotection and chemoselection strategies, and underlines the suitability of the A2UCOE element to stabilize lentiviral transgene expression in hematopoietic gene therapy.

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.

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.

Impact of temozolomide on immune response during malignant glioma chemotherapy

Malignant glioma, or glioblastoma, is the most common and lethal form of brain tumor with a median survival time of 15 months. The established therapeutic regimen includes a tripartite therapy of surgical resection followed by radiation and temozolomide (TMZ) chemotherapy, concurrently with radiation and then as an adjuvant. TMZ, a DNA alkylating agent, is the most successful antiglioma drug and has added several months to the life expectancy of malignant glioma patients. However, TMZ is also responsible for inducing lymphopenia and myelosuppression in malignant glioma patients undergoing chemotherapy. Although TMZ-induced lymphopenia has been attributed to facilitate antitumor vaccination studies by inducing passive immune response, in general lymphopenic conditions have been associated with poor immune surveillance leading to opportunistic infections in glioma patients, as well as disrupting active antiglioma immune response by depleting both T and NK cells. Deletion of O6-methylguanine-DNA-methyltransferase (MGMT) activity, a DNA repair enzyme, by temozolomide has been determined to be the cause of lymphopenia. Drug-resistant mutation of the MGMT protein has been shown to render chemoprotection against TMZ. The immune modulating role of TMZ during glioma chemotherapy and possible mechanisms to establish a strong TMZ-resistant immune response have been discussed.

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.

Cotransduction with MGMT and Ubiquitous or Erythroid-Specific GFP Lentiviruses Allows Enrichment of Dual-Positive Hematopoietic Progenitor Cells In Vivo

The P140K point mutant of MGMT allows robust hematopoietic stem cell (HSC) enrichment in vivo. Thus, dual-gene vectors that couple MGMT and therapeutic gene expression have allowed enrichment of gene-corrected HSCs in animal models. However, expression levels from dual-gene vectors are often reduced for one or both genes. Further, it may be desirable to express selection and therapeutic genes at distinct stages of cell differentiation. In this regard, we evaluated whether hematopoietic cells could be efficiently cotransduced using low MOIs of two separate single-gene lentiviruses, including MGMT for dual-positive cell enrichment. Cotransduction efficiencies were evaluated using a range of MGMT : GFP virus ratios, MOIs, and selection stringencies in vitro. Cotransduction was optimal when equal proportions of each virus were used, but low MGMT : GFP virus ratios resulted in the highest proportion of dual-positive cells after selection. This strategy was then evaluated in murine models for in vivo selection of HSCs cotransduced with a ubiquitous MGMT expression vector and an erythroid-specific GFP vector. Although the MGMT and GFP expression percentages were variable among engrafted recipients, drug selection enriched MGMT-positive leukocyte and GFP-positive erythroid cell populations. These data demonstrate cotransduction as a mean to rapidly enrich and evaluate therapeutic lentivectors in vivo.

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.

Cellular immunotherapy for high-grade glioma

The outcome for patients with the most common primary brain tumor, glioblastoma multiforme (GBM), remains poor. Several immunotherapeutic approaches are actively being pursued including antibodies and cell-based therapies. While the blood-brain barrier protects brain tumor cells from therapeutic antibodies, immune cells have the ability to traverse the blood-brain barrier and migrate into GBM tumors to exert their therapeutic function. Results of Phase I clinical studies with vaccines to induce GBM-specific T cells are encouraging and Phase II clinical trials are in progress. Nonvaccine-based cell therapy for GBM has been actively explored over the last four decades. Here we will review past clinical experience with adoptive cell therapies for GBM and summarize current strategies on how to improve these approaches.

Clonal inventory screens uncover monoclonality following serial transplantation of MGMT P140K-transduced stem cells and dose-intense chemotherapy

Gene transfer of mutant O(6)-methylguanine-DNA-methyltransferase (MGMT(P140K)) into hematopoietic stem cells (HSCs) protects hematopoiesis from alkylating agents and allows efficient in vivo selection of transduced HSCs. However, insertional mutagenesis, high regenerative stress associated with selection, and the genotoxic potential of alkylating drugs represent considerable risk factors for clinical applications of this approach. Therefore, we investigated the long-term effect of MGMT(P140K) gene transfer followed by repetitive, dose-intensive treatment with alkylating agents in a murine serial bone marrow transplant model and assessed clonality of hematopoiesis up to tertiary recipients. The substantial selection pressure resulted in almost completely transduced hematopoiesis in all cohorts. Ligation-mediated PCR and next-generation sequencing identified several repopulating clones carrying vector insertions in distinct genomic regions that were ∼ 9 kb of size (common integration sites). Beside polyclonal reconstitution in the majority of the mice, we also detected monoclonal or oligoclonal repopulation patterns with HSC clones showing vector insertions in the Usp10 or Tubb3 gene. Interestingly, neither Usp10, Tubb3, nor any of the genes located in common integration sites have been linked to clonal expansion in previous preclinical or clinical gene therapy trials. However, a considerable number of these genes are involved in DNA damage response and cell fate decision pathways following cytostatic drug application. Thus, in summary, our study advocates ligation-mediated PCR and next generation sequencing as an effective and reliable method to identify gene products associated with clonal survival in specific experimental settings such as chemoselection using alkylating agents.

Analysis of self-inactivating lentiviral vector integration sites and flanking gene expression in human peripheral blood progenitor cells after alkylator chemotherapy

Abstract Hematotoxicity is a major and frequently dose-limiting side effect of chemotherapy. Retroviral methylguanine-DNA-methyltransferase (MGMT; EC 2.1.1.63) gene transfer to primitive hematopoietic progenitor cells (CD34(+) cells) might allow the application of high-dose alkylator chemotherapy with almost mild to absent myelosuppression. Because gammaretroviral vector integration was found in association with malignant or increased proliferation, novel lentiviral vectors with self-inactivating (SIN) capacity might display a safer option for future gene transfer studies. We assessed the influence of chemoselection on integration patterns in 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)-treated and untreated human CD34(+) cells transduced with an SIN lentiviral vector carrying the MGMT(P140K) transgene, using ligation-mediated PCR (LM-PCR) and next-generation sequencing. In addition, for the first time, the local influence of the lentiviral provirus on the expression of hit and flanking genes in human CD34(+) cells was analyzed at a clonal level. For each colony, the integration site was detected (LM-PCR) and analyzed (QuickMap), and the expression of hit and flanking genes was measured (quantitative RT-PCR). Analyses of both treated and untreated CD34(+) cells revealed preferential integration into genes. Integration patterns in BCNU-treated cells showed mild, but not significant, differences compared with those found in untreated CD34(+) cells. Most importantly, when analyzing the local influence of the provirus, we saw no significant deregulation of the integration-flanking genes. These findings demonstrate that SIN vector-mediated gene transfer might display a feasible and possibly safe option for MGMT(P140K)-mediated chemoprotection of CD34(+) cells.

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.

Targeting O⁶-methylguanine-DNA methyltransferase with specific inhibitors as a strategy in cancer therapy

O (6)-methylguanine-DNA methyltransferase (MGMT) repairs the cancer chemotherapy-relevant DNA adducts, O (6)-methylguanine and O (6)-chloroethylguanine, induced by methylating and chloroethylating anticancer drugs, respectively. These adducts are cytotoxic, and given the overwhelming evidence that MGMT is a key factor in resistance, strategies for inactivating MGMT have been pursued. A number of drugs have been shown to inactivate MGMT in cells, human tumour models and cancer patients, and O (6)-benzylguanine and O (6)-[4-bromothenyl]guanine have been used in clinical trials. While these agents show no side effects per se, they also inactivate MGMT in normal tissues and hence exacerbate the toxic side effects of the alkylating drugs, requiring dose reduction. This might explain why, in any of the reported trials, the outcome has not been improved by their inclusion. It is, however, anticipated that, with the availability of tumour targeting strategies and hematopoetic stem cell protection, MGMT inactivators hold promise for enhancing the effectiveness of alkylating agent chemotherapy.

Dual transgene expression by foamy virus vectors carrying an endogenous bidirectional promoter

Several gene therapy applications require the transfer and simultaneous expression of multiple genes in the same cell. In this study, we analyzed the potential for coordinated expression of an endogenous bidirectional promoter located on chromosome X, which controls the expression of the heterogeneous nuclear ribonucleoprotein H2 (HNRNPH2) and alpha-galactosidase (GLA) genes. The promoter was cloned in both transcriptional orientations in a foamy virus (FV) vector backbone, whereas the enhanced green fluorescent protein (EGFP) and low-affinity nerve growth factor receptor (DeltaLNGFR) reporter genes were cloned in the 5'-3' and 3'-5' transcriptional orientations, respectively. In all the cell lines tested, both vectors showed high levels of transgene coexpression that reached 76% of total positive cells (range from 76 to 18%). Comparison of EGFP and DeltaNGFR levels revealed that the side of the promoter that drives the expression of the HNRNPH2 gene in the genome was stronger and in accordance to its in situ activity. When tested with CD34(+) cells, transgene coexpression reached 35.3% of all positive cells in progenitor assays and 16.8% of all positive cells after transplantation in NOD/severe combined immunodeficient mice. In summary, we show that the endogenous promoter used in this study holds bidirectional activity in the context of FV vectors and can be used in gene therapy applications requiring synchronized expression of two genes.

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.

Suppression of HLA expression by lentivirus-mediated gene transfer of siRNA cassettes and in vivo chemoselection to enhance hematopoietic stem cell transplantation

Current approaches for hematopoietic stem cell (HSC) and organ transplantation are limited by donor and host-mediated immune responses to allo-antigens. Application of these therapies is limited by the toxicity of preparative and post-transplant immunosuppressive regimens and a shortage of appropriate HLA-matched donors. We have been exploring two complementary approaches for genetically modifying donor cells that achieve long-term suppression of cellular proteins that elicit host immune responses to mismatched donor antigens, and provide a selective advantage to genetically engineered donor cells after transplantation. The first approach is based on recent advances that make feasible targeted down-regulation of HLA expression. Suppression of HLA expression could help to overcome limitations imposed by extensive HLA polymorphisms that restrict the availability of suitable donors. Accordingly, we have recently investigated whether knockdown of HLA by RNA interference (RNAi) enables allogeneic cells to evade immune recognition. For efficient and stable delivery of short hairpin-type RNAi constructs (shRNA), we employed lentivirus-based gene transfer vectors that integrate into genomic DNA, thereby permanently modifying transduced donor cells. Lentivirus-mediated delivery of shRNA targeting pan-Class I and allele-specific HLA achieved efficient and dose-dependent reduction in surface expression of HLA in human cells, and enhanced resistance to allo-reactive T lymphocyte-mediated cytotoxicity, while avoiding non-MHC restricted killing. Complementary strategies for genetic engineering of HSC that would provide a selective advantage for transplanted donor cells and enable successful engraftment with less toxic preparative and immunosuppressive regimens would increase the numbers of individuals to whom HLA suppression therapy could be offered. Our second strategy is to provide a mechanism for in vivo selection of genetically modified HSC and other donor cells. We have uniquely combined transplantation during the neonatal period, when tolerance may be more readily achieved, with a positive selection strategy for in vivo amplification of drug-resistant donor HSC. This model system enables the evaluation of mechanisms of tolerance induction to neo-antigens, and allogeneic stem cells during immune ontogeny. HSC are transduced ex vivo by lentivirus-mediated gene transfer of P140K-O(6)-methylguanine-methyltransferase (MGMT(P140K)). The MGMT(P140K) DNA repair enzyme confers resistance to benzylguanine, an inhibitor of endogenous MGMT, and to chloroethylating agents such as BCNU. In vivo chemoselection enables enrichment of donor cells at the stem cell level. Using complementary approaches of in vivo chemoselection and RNAi-induced silencing of HLA expression may enable the generation of histocompatibility-enhanced, and eventually, perhaps "universally" compatible cellular grafts.

Methylguanine DNA methyltransferase-mediated drug resistance-based selective enrichment and engraftment of transplanted stem cells in skeletal muscle

Cell replacement therapy using stem cell transplantation holds much promise in the field of regenerative medicine. In the area of hematopoietic stem cell transplantation, O(6)-methylguanine-DNA methyltransferase MGMT (P140K) gene-mediated drug resistance-based in vivo enrichment strategy of donor stem cells has been shown to achieve up to 75%-100% donor cell engraftment in the host's hematopoietic stem cell compartment following repeated rounds of selection. This strategy, however, has not been applied in any other organ system. We tested the feasibility of using this MGMT (P140K)-mediated enrichment strategy for cell transplantation in skeletal muscles of mice. We demonstrate that muscle cells expressing an MGMT (P140K) drug resistance gene can be protected and selectively enriched in response to alkylating chemotherapy both in vitro and in vivo. Upon transplantation of MGMT (P140K)-expressing male CD34(+ve) donor stem cells isolated from regenerating skeletal muscle into injured female muscle treated with alkylating chemotherapy, donor cells showed enhanced engraftment in the recipient muscle 7 days following transplantation as examined by quantitative-polymerase chain reaction using Y-chromosome specific primers. Fluorescent in situ hybridization analysis using a Y-chromosome paint probe revealed donor-derived de novo muscle fiber formation in the recipient muscle 14 days following transplantation, with approximately 12.5% of total nuclei within the regenerated recipient muscle being of donor origin. Following engraftment, the chemo-protected donor CD34(+ve) cells induced substantial endogenous regeneration of the chemo-ablated host muscle that is otherwise unable to self-regenerate. We conclude that the MGMT (P140K)-mediated enrichment strategy can be successfully implemented in muscle.

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

Major limitations to gene therapy using HSCs are low gene transfer efficiency and the inability of most therapeutic genes to confer a selective advantage on the gene-corrected cells. One approach to enrich for gene-modified cells in vivo is to include in the retroviral vector a drug resistance gene, such as the P140K mutant of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT*). We transplanted 5 rhesus macaques with CD34+ cells transduced with lentiviral vectors encoding MGMT* and a fluorescent marker, with or without homeobox B4 (HOXB4), a potent stem cell self-renewal gene. Transgene expression and common integration sites in lymphoid and myeloid lineages several months after transplantation confirmed transduction of long-term repopulating HSCs. However, all animals showed only a transient increase in gene-marked lymphoid and myeloid cells after O6-benzylguanine (BG) and temozolomide (TMZ) administration. In 1 animal, cells transduced with MGMT* lentiviral vectors were protected and expanded after multiple courses of BG/TMZ, providing a substantial increase in the maximum tolerated dose of TMZ. Additional cycles of chemotherapy using 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) resulted in similar increases in gene marking levels, but caused high levels of nonhematopoietic toxicity. Inclusion of HOXB4 in the MGMT* vectors resulted in no substantial increase in gene marking or HSC amplification after chemotherapy treatment. Our data therefore suggest that lentivirally mediated gene transfer in transplanted HSCs can provide in vivo chemoprotection of progenitor cells, although selection of long-term repopulating HSCs was not seen.

Amelioration of murine beta-thalassemia through drug selection of hematopoietic stem cells transduced with a lentiviral vector encoding both gamma-globin and the MGMT drug-resistance gene

Correction of murine models of beta-thalassemia has been achieved through high-level globin lentiviral vector gene transfer into mouse hematopoietic stem cells (HSCs). However, transduction of human HSCs is less robust and may be inadequate to achieve therapeutic levels of genetically modified erythroid cells. We therefore developed a double gene lentiviral vector encoding both human gamma-globin under the transcriptional control of erythroid regulatory elements and methylguanine methyltransferase (MGMT), driven by a constitutive cellular promoter. MGMT expression provides cellular resistance to alkylator drugs, which can be administered to kill residual untransduced, diseased HSCs, whereas transduced cells are protected. Mice transplanted with beta-thalassemic HSCs transduced with a gamma-globin/MGMT vector initially had subtherapeutic levels of red cells expressing gamma-globin. To enrich gamma-globin-expressing cells, transplanted mice were treated with the alkylator agent 1,3-bis-chloroethyl-1-nitrosourea. This resulted in significant increases in the number of gamma-globin-expressing red cells and the amount of fetal hemoglobin, leading to resolution of anemia. Selection of transduced HSCs was also obtained when cells were drug-treated before transplantation. Mice that received these cells demonstrated reconstitution with therapeutic levels of gamma-globin-expressing cells. These data suggest that MGMT-based drug selection holds promise as a modality to improve gene therapy for beta-thalassemia.

Long-term polyclonal and multilineage engraftment of methylguanine methyltransferase P140K gene-modified dog hematopoietic cells in primary and secondary recipients

Overexpression of methylguanine methyltransferase P140K (MGMTP140K) has been successfully used for in vivo selection and chemoprotection in mouse and large animal studies, and has promise for autologous and allogeneic gene therapy. We examined the long-term safety of MGMTP140K selection in a clinically relevant dog model. Based on the association of provirus integration and proto-oncogene activation leading to leukemia in the X-linked immunodeficiency trial, we focused our analysis on the distribution of retrovirus integration sites (RIS) relative to proto-oncogene transcription start sites (TSS). We analyzed RIS near proto-oncogene TSS before (n = 157) and after (n = 129) chemotherapy in dogs that received MGMTP140K gene-modified cells and identified no overall increase of RIS near proto-oncogene TSS after chemotherapy. We also wanted to determine whether in vivo selected cells retained fundamental characteristics of hematopoietic stem cells. To that end, we performed secondary transplantation of MGMTP140K gene-modified cells after in vivo selection in dog leukocyte antigen (DLA)-matched dogs. Gene-modified cells achieved multilineage repopulation, and we identified the same gene-modified clone in both dogs more than 800 and 900 days after transplantation. These data suggest that MGMTP140K selection is well tolerated and should allow clinically for selection of gene-corrected cells in genetic or infectious diseases or chemoprotection for treatment of malignancy.

Characterization of a side population of astrocytoma cells in response to temozolomide

OBJECT

Cancer progenitor-like cells isolated by Hoechst 33342 dye efflux (termed the "side population" [SP]) have been studied in a variety of cancers, including malignant brain tumors. In this study, the authors investigate the nature of the SP phenotype in 2 glioma cell lines, U87MG and T98G, and their response to temozolomide. The roles of several adenosine triphosphate-binding cassette (ABC) multidrug transporters expressed by SP cells, in particular ABCG2, are also examined.

METHODS

Using fluorescence-activated cell sorting, the cells were separated into SP and non-SP fractions and analyzed for progenitor cell-like properties with immunofluorescence staining, quantitative real-time polymerase chain reaction, and their ability to reform glioma mass in an immune-compromised mouse. The response of the SP cells to temozolomide was investigated at the cellular and molecular levels. Small interfering RNA knockdown was used to examine the specific role of the ABCG2 transporter, and the cells' tumorigenic potential was measured using the soft agar clonogenic assay.

RESULTS

Side population cells are characterized by the presence of progenitor cell-like properties: increased expression of nestin, musashi-1, and ABCG2 were observed. In addition, only SP cells were able to reconstitute cellular heterogeneity; these cells were also more invasive than the non-SP cells, and possessed tumorigenic capacity. Temozolomide treatment increased the number of SP cells, and this corresponded to more progenitor-like cells, concurrent with elevated expression of several ABC transporters.

CONCLUSIONS

Knockdown of ABCG2 transporters did not abrogate the SP cell response to temozolomide. Upregulation of several other ABC drug transporter genes is proposed to account for this chemoresistance.

Reciprocal relationship between O6-methylguanine-DNA methyltransferase P140K expression level and chemoprotection of hematopoietic stem cells

Retroviral-mediated delivery of the P140K mutant O(6)-methylguanine-DNA methyltransferase (MGMT(P140K)) into hematopoietic stem cells (HSC) has been proposed as a means to protect against dose-limiting myelosuppressive toxicity ensuing from chemotherapy combining O(6)-alkylating agents (e.g., temozolomide) with pseudosubstrate inhibitors (such as O(6)-benzylguanine) of endogenous MGMT. Because detoxification of O(6)-alkylguanine adducts by MGMT is stoichiometric, it has been suggested that higher levels of MGMT will afford better protection to gene-modified HSC. However, accomplishing this goal would potentially be in conflict with current efforts in the gene therapy field, which aim to incorporate weaker enhancer elements to avoid insertional mutagenesis. Using a panel of self-inactivating gamma-retroviral vectors that express a range of MGMT(P140K) activity, we show that MGMT(P140K) expression by weaker cellular promoter/enhancers is sufficient for in vivo protection/selection following treatment with O(6)-benzylguanine/temozolomide. Conversely, the highest level of MGMT(P140K) activity did not promote efficient in vivo protection despite mediating detoxification of O(6)-alkylguanine adducts. Moreover, very high expression of MGMT(P140K) was associated with a competitive repopulation defect in HSC. Mechanistically, we show a defect in cellular proliferation associated with elevated expression of MGMT(P140K), but not wild-type MGMT. This proliferation defect correlated with increased localization of MGMT(P140K) to the nucleus/chromatin. These data show that very high expression of MGMT(P140K) has a deleterious effect on cellular proliferation, engraftment, and chemoprotection. These studies have direct translational relevance to ongoing clinical gene therapy studies using MGMT(P140K), whereas the novel mechanistic findings are relevant to the basic understanding of DNA repair by MGMT.

Co-expression of MGMT(P140K) and alpha-L-iduronidase in primary hepatocytes from mucopolysaccharidosis type I mice enables efficient selection with metabolic correction

BACKGROUND

Systemic in vivo gene therapy has resulted in widespread correction in animal models when treated at birth. However, limited improvement was observed in postnatally treated animals with mainly targeting to the liver and bone marrow. It has been shown that an O(6)-methylguanine-DNA-methyltransferase variant (MGMT(P140K)) mediated in vivo selection of transduced hematopoietic stem cells (HSC) in animals.

METHODS

We investigated the feasibility of MGMT(P140K)-mediated selection in primary hepatocytes from a mouse model of mucopolysaccharidosis type I (MPS I) in vitro using lentiviral vectors.

RESULTS

We found that multiple cycles of O(6)-benzylguanine (BG)/1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) treatment at a dosage effective for ex vivo HSC selection led to a two-fold increase of MGMT-expressing primary hepatocytes under culture conditions with minimum cell expansion. This enrichment level was comparable to that obtained after selection at a hepatic maximal tolerated dose of BCNU. Similar levels of increase were observed regardless of initial transduction frequency, or the position of MGMT (upstream or downstream of internal ribosome entry site) in the vector constructs. In addition, we found that elongation factor 1alpha promoter was superior to the long-terminal repeat promoter from spleen focus-forming virus with regard to transgene expression in primary hepatocytes. Moreover, the levels of therapeutic transgene expression in transduced, enzyme-deficient hepatocytes directly correlated with the doses of BCNU, leading to metabolic correction in transduced hepatocytes and metabolic cross-correction in neighbouring non-transduced MPS I cells.

CONCLUSIONS

These results demonstrate that MGMT(P140K) expression confers successful protection/selection in primary hepatocytes, and provide 'proof of concept' to the prospect of MGMT(P140K)-mediated co-selection for hepatocytes and HSC using BG/BCNU treatment.

Imaging Stem Cell Implant for Cellular-Based Therapies

Stem cell–based cellular therapy represents a promising outlook for regenerative medicine. Imaging techniques provide a means for noninvasive, repeated, and quantitative tracking of stem cell implant or transplant. From initial deposition to the survival, migration and differentiation of the transplant/implanted stem cells, imaging allows monitoring of the infused cells in the same live object over time. The current review briefly summarizes and compares existing imaging methods for cell labeling and imaging in animal models. Several studies performed by our group using different imaging techniques are described, with further discussion on the issues with these current imaging approaches and potential directions for future development in stem cell imaging.

Rapid lentiviral transduction preserves the engraftment potential of Fanca(-/-) hematopoietic stem cells

Fanconi anemia (FA) is a rare recessive syndrome, characterized by congenital anomalies, bone marrow failure, and predisposition to cancer. Two earlier clinical trials utilizing gamma-retroviral vectors for the transduction of autologous FA hematopoietic stem cells (HSCs) required extensive in vitro manipulation and failed to achieve detectable long-term engraftment of transduced HSCs. As a strategy for minimizing ex vivo manipulation, we investigated the use of a "rapid" lentiviral transduction protocol in a murine Fanca(-/-) model. Importantly, while this and most murine models of FA fail to completely mimic the human hematopoietic phenotype, we observed a high incidence of HSC transplant engraftment failure and low donor chimerism after conventional transduction (CT) of Fanca(-/-) donor cells. In contrast, rapid transduction (RT) of Fanca(-/-) HSCs preserved engraftment to the level achieved in wild-type cells, resulting in long-term multilineage engraftment of gene-modified cells. We also demonstrate the correction of the characteristic hypersensitivity of FA cells against the cross-linking agent mitomycin C (MMC), and provide evidence for the advantage of using pharmacoselection as a means of further increasing gene-modified cells after RT. Collectively, these data support the use of rapid lentiviral transduction for gene therapy in FA.

Gene therapy of beta(c)-deficient pulmonary alveolar proteinosis (beta(c)-PAP): studies in a murine in vivo model

Pulmonary alveolar proteinosis (PAP) due to deficiency of the common beta-chain (beta(c)) of the interleukin-3 (IL-3)/IL-5/granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors is a rare monogeneic disease characterized by functional insufficiency of pulmonary macrophages. Hematopoietic stem cell gene therapy for restoring expression of beta(c)-protein in the hematopoietic system may offer a curative approach. Toward this end, we generated a retroviral construct expressing the murine beta(c) (mbeta(c)) gene and conducted investigations in a murine model of beta(c)-deficient PAP. Functional correction of mbeta(c) activity in mbeta(c)(-/-) bone marrow (BM) cells was demonstrated by restoration of in vitro colony formation in response to GM-CSF. In addition, in a murine in vivo model of mbeta(c)-deficient PAP mbeta(c) gene transfer to hematopoietic stem cells not only restored the GM-CSF-sensitivity of hematopoietic progenitor cells but also, within a period of 12 weeks, almost completely reversed the morphologic features of surfactant accumulation. These results were obtained despite modest transduction levels (10-20%) and, in comparison to wild-type mice, clearly reduced beta(c) expression levels were detected in hematopoietic cells. Therefore, our data demonstrating genetic and functional correction of mbeta(c)(-/-) deficiency in vitro as well as in a murine in vivo model of PAP strongly suggest gene therapy as a potential new treatment modality in beta(c)-deficient PAP.

Bone marrow-derived cells exhibiting lung epithelial cell characteristics are enriched in vivo using methylguanine DNA methyltransferase-mediated drug resistance

Previous studies have suggested that donor bone marrow-derived cells can differentiate into lung epithelial cells at low frequency. We investigated whether we could enrich the number of donor-derived hematopoietic cells that have type II pneumocyte characteristics by overexpression of the drug resistance gene methylguanine DNA methyltransferase (MGMT). MGMT encodes O(6)-alkylguanine DNA alkyltransferase (AGT), a drug resistance protein for DNA damage induced by N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU), and the mutant P140K MGMT confers resistance to BCNU and the AGT inactivator O(6)-benzylguanine (BG). For this study, we used two MGMT selection models: one in which donor cells had a strong selection advantage because the recipient lung lacked MGMT expression, and another in which drug resistance was conferred by gene transfer of P140K MGMT. In both models, we saw an increase in the total number of donor-derived cells in the lung after BCNU treatment. Analysis of single-cell suspensions from 28 mice showed donor-derived cells with characteristics of type II pneumocytes, determined by surfactant protein C (SP-C) expression. Furthermore, an increase in the percentage of donor-derived SP-C cells was noted after BCNU or BG and BCNU treatment. This study demonstrates that bone marrow cells expressing MGMT can engraft in the lung and convert into cells expressing the type II pneumocyte protein SP-C. Furthermore, these cells can be enriched in response to alkylating agent-mediated lung injury. These results suggest that expression of MGMT could enhance the capacity of bone marrow-derived cells to repopulate lung epithelium, and when used in combination with a gene of interest, MGMT could have therapeutic applications.

Transduction of murine hematopoietic stem cells and in vivo selection of gene-modified cells

Hematopoietic stem cells (HSCs) were among the first targets of genetic manipulation for the purpose of treating human diseases. The translational aspects of the first human clinical trials were based on results obtained using the mouse as an experimental model. Murine studies have shown that the major limitations of HSC gene therapy are similar to those encountered when using non-hematopoietic cells as targets and include (1) an inability to genetically modify sufficient numbers of target cells, (2) the loss of transgene function over time, and (3) potential complications due to vector integration. With continued improvements in transduction protocols, murine HSC transduction and transplantation are now routine with transduction efficiencies >50% easily achievable and even >90% feasible. However, attaining high-level engraftment of gene-modified cells after transplantation is still problematic. Basic transduction conditions entail cytokine stimulation of HSC populations, such as stem cell antigen-1 positive (Sca-1(+)) cells isolated from bone marrow, in serum-free media followed by multiple additions of recombinant retrovirus. Analysis of peripheral blood 12 weeks post transplantation of transduced cells into lethally irradiated recipients shows genetic marking in all hematopoietic lineages. Transduction of HSCs is then confirmed by transplanting bone marrow cells harvested from primary transplant recipients into lethally irradiated secondary recipients. Analysis of these mice shows that recombinant retroviruses transduce murine HSCs efficiently and stably and that the genetically modified cells are capable of completely repopulating the hematopoietic system.

Gene therapy for chronic granulomatous disease

Patients with chronic granulomatous disease (CGD) cannot generate reactive oxygen metabolites, and suffer from severe recurrent infections and dysregulated inflammation. Haematopoietic stem cell transplantation is the only established option for definitive cure for patients with a suitable donor and is indicated when conventional prophylaxis and therapy with antimicrobial medication fail. Gene therapy has the potential to cure CGD, and several clinical trials have been conducted since 1997. Whereas initial studies resulted in low and short-term engraftment of CGD-corrected cells, recent trials demonstrated clinical benefit when engraftment was enhanced by busulfan conditioning prior to infusion of gene-corrected cells. However, the progress in gene therapy has been hampered by the appearance of insertional mutagenesis causing leukaemia in a trial for patients with X-linked severe combined immunodeficiency and by the emergence of dominant clones in a recent trial for the X-linked form of CGD. These findings stimulated the development of modified vector systems that demonstrate reduced genotoxicity in vitro and in animal models. New gene therapy protocols that allow efficient gene transfer and durable expression but limit the risk for insertional mutagenesis are envisioned to become an important therapeutic option for patients with CGD.

Live and let die: in vivo selection of gene-modified hematopoietic stem cells via MGMT-mediated chemoprotection

Gene transfer into hematopoietic stem cells (HSC) provides a potential means of correcting monogenic defects and altering drug sensitivity of normal bone marrow to cytotoxic agents. These applications have significant therapeutic potential but the translation of successful murine studies into human therapies has been hindered by low gene transfer in large animals (including humans), and recent serious side effects in a human immunodeficiency trial related to insertional mutagenesis. The latter trial, along with other subsequent trials, while bringing into focus the potential risks of integrating vector systems, also clearly demonstrate the potential usefulness of in vivo selection as it relates to inefficient stem cell transduction. Developing from initial studies by our group and other investigators in which drug resistance was utilized to demonstrate the feasibility of using gene transfer to effect protection from myelotoxicity of chemotherapeutic agents, expression of mutant forms of O(6)-methyguanine-DNA-methytransferase (MGMT) coupled with the simultaneous use of pharmacologic inhibitors and chemotherapeutic agents has been shown to provide a powerful method to select HSC in vivo. While stem and progenitor cell protection and resulting selection in vivo has potential applications for the treatment of selected cancers (allowing dose escalation) and for correction of monogenic disease (allowing an iatrogenic survival advantage of transduced cells in vivo), such an in vivo selection may have untoward effects on stem cell behavior. These deleterious effects may include stem cell exhaustion; lineage skewing; accumulation of genotoxic lesions; and clonal dominance driven towards a pro-leukemic phenotype. Knowledge of the likelihood of such deleterious events occurring as well as their potential implications will be critical to future clinical applications and may also enhance our understanding of both normal stem cell behavior and the evolution of hematopoietic malignancies.

Gene therapy with drug resistance genes

A major side effect of cancer chemotherapy is myelosuppression. Expression of drug-resistance genes in hematopoietic stem cells (HSC) using gene transfer methodologies holds the promise of overcoming marrow toxicity in cancer chemotherapy. Adequate protection of marrow cells in cancer patients from myelotoxicity in this way would permit the use of escalating doses of chemotherapy for eradicating residual disease. A second use of drug-resistance genes is for coexpression with a therapeutic gene in HSCs to provide a selection advantage to gene-modified cells. In this review, we discuss several drug resistance genes, which are well suited for in vivo selection as well as other newer candidate genes with potential for use in this manner.

O6-methylguanine-DNA-methyltransferase (MGMT) gene therapy targeting haematopoietic stem cells: studies addressing safety issues

As haematopoietic stem cell gene therapy utilizing O(6)-methylguanine-DNA-methyltransferase has reached the clinical stage, safety-related questions become increasingly important. These issues concern insertional mutagenesis of viral vectors, the acute toxicity of pre-transplant conditioning protocols and in vivo selection regimens as well as potential genotoxic side effects of the alkylating drugs administered in this context. To address these questions, we have investigated toxicity-reduced conditioning regimens combining low-dose alkylator application with sublethal irradiation and have analysed their influence on engraftment and subsequent selectability of transduced haematopoietic stem cells. In addition, a strategy to monitor the acute and long-term genotoxic effects of drugs with high guanine-O(6) alkylating potential, such as chloroethylnitrosoureas or temozolomide is introduced. For this purpose, assays were implemented which allow an assessment of the generation and fate of primary drug-induced adducts as well as their long-term effect on chromosomal integrity at the single cell level.

Vector design for expression of O6-methylguanine-DNA methyltransferase in hematopoietic cells

Enhancing DNA repair activity of hematopoietic cells by stably integrating gene vectors that express O(6)-methylguanine-DNA-methyltransferase (MGMT) is of major interest for innovative approaches in tumor chemotherapy and for the control of hematopoietic chimerism in the treatment of multiple other acquired or inherited disorders. Crucial determinants of this selection principle are the stringency of treatment with O(6)-alkylating agents and the level of transgenic MGMT expression. Attempts to generate clinically useful MGMT vectors focus on the design of potent expression cassettes, an important component of which is formed by enhancer sequences that are active in primitive as well as more differentiated hematopoietic cells. However, recent studies have revealed that vectors harboring strong enhancer sequences are more likely to induce adverse events related to insertional mutagenesis. Safety-improved vectors that maintain high levels of MGMT expression may be constructed based on the following principles: choice of enhancer-promoter sequences with relatively mild long-distance effects despite a high transcription rate, improved RNA processing (export, stability and translation), and protein design. The need for optimizing MGMT protein design is supported by recent observations suggesting that the P140K mutant of MGMT, developed to be resistant to inhibitors such as O(6)-benzylguanine, may confer a selective disadvantage when expressed at high levels. Here, we provide a review of the literature exploring MGMT expression vectors for bone marrow chemoprotection, and describe experimental evidence suggesting that high expression of MGMT P140K induces a selective disadvantage in the absence of alkylating agents. We conclude that the appropriate design of expression vectors and MGMT protein features will be crucial for the long-term prospects of this promising selection principle.

The Genetic Engineering of Hematopoietic Stem Cells: the Rise of Lentiviral Vectors, the Conundrum of the LTR, and the Promise of Lineage-restricted Vectors

Recent studies on the integration patterns of different categories of retroviral vectors, the genotoxicity of long-terminal repeats (LTRs) and other genetic elements, the rise of lentiviral technology and the emergence of regulated vector systems providing tissue-restricted transgene expression and RNA interference, are profoundly changing the landscape of stem cell-based therapies. New developments in vector design and an increasing understanding of the mechanisms underlying insertional oncogenesis are ushering in a new phase in hematopoietic stem cell (HSC) engineering, thus bringing the hitherto exclusive reliance on LTR-driven, gamma-retroviral vectors to an end. Based on their ability to transduce non-dividing cells and their genomic stability, lentiviral vectors offer new prospects for the manipulation of HSCs. Tissue-specific vectors, as exemplified by globin vectors, not only provide therapeutic efficacy, but may also enhance safety, insofar that they restrict transgene expression in stem cells, progenitor cells and blood cells in all but the transcriptionally targeted lineage. This review provides a survey of these advances as well as several remaining challenges, focusing in particular on the importance of achieving adequate levels of protein expression from a limited number of vector copies per cell-ideally one to two.

Reliable Generation of Stable High Titer Producer Cell Lines for Gene Therapy

OBJECTIVE

Retroviral vectors represent one of the most robust technologies for in vivo expression of heterologous gene sequences and are still the most commonly used vectors in clinical gene therapy trials. The production of high titer retroviral preparations, however, can be a problematic procedure for certain constructs.

METHODS

GALV- or RD114-pseudotyped retroviral particles carrying selectable fluorescence markers or drug resistance genes, such as the green fluorescent protein (GFP) or the O(6)-methylguanine-DNA-methyltransferase (MGMT) mutants, were used to stably transduce Phoenix-(FNX-)eco cells. Thereafter, a polyclonal population of producer cells was generated by enriching cells with high marker gene expression. In addition, single producer clones were selected by limiting dilution.

RESULTS

Retroviral titers were increased 1-2 logs by enriching for a polyclonal population of producer cells, and selection of single producer clones allowed another 1- to 2-log increase in titers. Using this method, reproducibly high titer viral preparations allowing efficient transduction of hematopoietic stem cells were generated.

CONCLUSION

A reliable and time-effective method to generate stable high titer producer cells based on the FNX-cell line for problematic retroviral vector constructs is described.

Ex Vivo Selection and Expansion of Cells Based on Expression of a Mutated Inosine Monophosphate Dehydrogenase 2 after HIV Vector Transduction: Effects on Lymphocytes, Monocytes, and CD34+ Stem Cells

Hematopoietic progenitor cells (HPCs) represent an ideal target for gene therapy treatment of human immunodeficiency virus (HIV) infection. However, gene delivery into quiescent HPCs by retroviral or lentiviral vectors remains relatively poor. We evaluated a selection scheme based on the expression of a variant of inosine monophosphate dehydrogenase 2 (IMPDH2), the rate-limiting enzyme in the de novo purine biosynthesis pathway. As lymphocytes depend more than other cell types on de novo synthesis of purines, IMPDH inhibitors such as mycophenolic acid (MPA) can selectively expand lymphocytes overexpressing the enzymes. We used HIV vectors to deliver an IMPDH variant into T cells and HPCs. We showed that the transduced T cells became resistant to MPA selection. By expressing a short hairpin RNA gene targeted to the HIV gag transcript, the MPA-selected T cells became resistant to HIV-1 infection. Monocyte/macrophages derived from the transduced HPCs differentiated normally and exhibited normal function as measured by B7 up-regulation and phagocytosis when stimulated. Our results suggest that this system may be applicable as a selection strategy to enrich transduced T lymphocytes and mononuclear cells in vivo for HIV gene therapy.

Unique risk factors for insertional mutagenesis in a mouse model of XSCID gene therapy

Although gene therapy can cure patients with severe combined immunodeficiency (SCID) syndromes, the clinical occurrence of T cell malignancies due to insertional mutagenesis has raised concerns about the safety of gene therapy. Several key questions have remained unanswered: (i) are there unique risk factors for X-linked SCID (XSCID) gene therapy that increase the risk of insertional mutagenesis; (ii) what other genetic lesions may contribute to transformation; and (iii) what systems can be used to test different vectors for their relative safety? To address these questions, we have developed an XSCID mouse model in which both the Arf tumor-suppressor gene and the gammac gene were ablated. Gene therapy in this animal model recapitulates the high incidence of integration-dependent, T cell tumors that was seen in the clinical trial. Ligation-mediated PCR analysis showed integration sites near or within established protooncogenes (Chd9, Slamf6, Tde1, Camk2b, and Ly6e), demonstrating that T cell transformation was associated with targeting of oncogene loci; however, no integrations within the Lmo2 locus were identified. The X-SCID background in transplanted cells was required for high rate transformation and was associated with expansion of primitive hematopoietic cells that may serve tumor precursors. This model should be useful for testing safety-modified vectors and for further exploring the risk factors leading to insertional mutagenesis in gene therapy trials.

Stringent Regulation of DNA Repair During Human Hematopoietic Differentiation: A Gene Expression and Functional Analysis

For the lymphohematopoietic system, maturation-dependent alterations in DNA repair function have been demonstrated. Because little information is available on the regulatory mechanisms underlying these changes, we have correlated the expression of DNA damage response genes and the functional repair capacity of cells at distinct stages of human hematopoietic differentiation. Comparing fractions of mature (CD34-), progenitor (CD34+ 38+), and stem cells (CD34+ 38-) isolated from umbilical cord blood, we observed: 1) stringently regulated differentiation-dependent shifts in both the cellular processing of DNA lesions and the expression profiles of related genes and 2) considerable interindividual variability of DNA repair at transcriptional and functional levels. The respective repair phenotype was found to be constitutively regulated and not dominated by adaptive response to acute DNA damage. During blood cell development, the removal of DNA adducts, the resealing of repair gaps, the resistance to DNA-reactive drugs clearly increased in stem or mature compared with progenitor cells of the same individual. On the other hand, the vast majority of differentially expressed repair genes was consistently upregulated in the progenitor fraction. A positive correlation of repair function and transcript levels was found for a small number of genes such as RAD23 or ATM, which may serve as key regulators for DNA damage processing via specific pathways. These data indicate that the organism might aim to protect the small number of valuable slow dividing stem cells by extensive DNA repair, whereas fast-proliferating progenitor cells, once damaged, are rather eliminated by apoptosis.

In vivo effects of myeloablative alkylator therapy on survival and differentiation of MGMTP140K-transduced human G-CSF-mobilized peripheral blood cells

High-intensity alkylator-based chemotherapy is required to eradicate tumors expressing high levels of O6-methylguanine DNA methyltransferase (MGMT). This treatment, however, can lead to life-threatening myelosuppression. We investigated a gene therapy strategy to protect human granulocyte colony-stimulating factor-mobilized peripheral blood CD34+ cells (MPB) from a high-intensity alkylator-based regimen. We transduced MPB with an oncoretroviral vector that coexpresses MGMT(P140K) and the enhanced green fluorescent protein (EGFP) (n = 5 donors). At 4 weeks posttransplantation into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, cohorts were not treated or were treated with low- or high-intensity alkylating chemotherapy. In the high-intensity-treated cohort, it was necessary to infuse NOD/SCID bone marrow (BM) to alleviate hematopoietic toxicity. At 8 weeks posttreatment, human CD45+ cells in the BM of mice treated with either regimen were EGFP+ and contained MGMT-specific DNA repair activity. In cohorts receiving low-intensity therapy, both primitive and mature hematopoietic cells were present in the BM. Although B-lymphoid and myeloid cells were resistant to in vivo drug treatment in cohorts that received high-intensity therapy, no human CD34+ cells or B-cell precursors were detected. These data suggest that improved strategies to optimize repair of DNA damage in primitive human hematopoietic cells are needed when using high-intensity anti-cancer therapy.

Characterisation of a P140K mutantO6-methylguanine-DNA-methyltransferase (MGMT)-expressing transgenic mouse line with drug-selectable bone marrow

BACKGROUND

Gene transfer of the P140K mutant of O6-methylguanine-DNA-methyltransferase (MGMT(P140K)) into hematopoietic stem cells (HSC) provides a mechanism for drug resistance and the selective expansion of gene-modified cells in vivo. Possible clinical applications for this strategy include chemoprotection to allow dose escalation of alkylating chemotherapy, or combining MGMT(P140K) expression with a therapeutic gene in the treatment of genetic diseases. Our aim is to use MGMT(P140K)-driven in vivo selection to develop allogeneic micro-transplantation protocols that rely on post-engraftment selection to overcome the requirement for highly toxic pre-transplant conditioning, and to establish and maintain predictable levels of donor/recipient chimerism.

METHODS

Using stably transfected murine embryonic stem (ES) cells, we have generated a C57BL/6 transgenic mouse line with expression of MGMT(P140K) within the hematopoietic compartment for use as a standard source of donor HSC in such models. Functional characterisation of transgene expression was carried out in chemotherapy-treated transgenic mice and in allogeneic recipients of transgenic HSC.

RESULTS

Expression of the transgene provided chemoprotection and allowed in vivo selection of MGMT(P140K)-expressing cells in transgenic mice after exposure to O6-benzylguanine (BG) and N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU). In an allogeneic transplant experiment in which transgenic HSC were engrafted into 129 strain recipients following low intensity conditioning (Busulfan, anti-CD8, anti-CD40Ligand), MGMT(P140K)-expressing cells could be selected using chemotherapy.

CONCLUSIONS

This MGMT(P140K) transgenic mouse line provides a useful source of drug-selectable donor cells for the development of non-myeloablative allogeneic transplant models in which variation in transplant conditioning elements can be investigated independently of gene transfer efficiency.

Selection with a regulated cell growth switch increases the likelihood of expression for a linked gamma-globin gene

Several lines of evidence indicate that in vivo drug selection can be used to overcome the low rates of gene transfer and engraftment encountered in many hematopoietic stem cell gene therapy settings. However, whether selection imposed on one transcription cassette effects the likelihood of expression from a second, independent transcription cassette within the same vector has been less well studied. In order to address this issue, we engineered an oncoretrovirus vector to express two separate transcription units: (i) a bicistronic cassette encoding both GFP and a pharmacologically regulated cell growth switch based on the thrombopoietin receptor Mpl; and (ii) a highly position-dependent second cassette encoding human gamma-globin. Studies in cell cultures and in mice transplanted with transduced marrow indicated that selective expansion increased by more than 9-fold the fraction of erythroid cells expressing the linked but separate expression cassette for gamma-globin. This increase was far greater then that observed for the bicistronic GFP gene, and cannot be explained by a simple increase in the fraction of cells containing provirus. These results suggest that selective expansion favors erythroid stem/progenitor cells with provirus integrated at chromosomal sites which are relatively resistant to silencing position effects.

Current status of gene therapy strategies to treat HIV/AIDS

Progress in developing effective gene transfer approaches to treat HIV-1 infection has been steady. Many different transgenes have been reported to inhibit HIV-1 in vitro. However, effective translation of such results to clinical practice, or even to animal models of AIDS, has been challenging. Among the reasons for this failure are uncertainty as to the most effective cell population(s) to target, the diffuseness of these target cells in the body, and ineffective or insufficiently durable gene delivery. Better understanding of the HIV-1 replicative cycle, host factors involved in HIV-1 infection, vector biology and application, transgene technology, animal models, and clinical study design have all contributed vastly to planning current and future strategies for application of gene therapeutic approaches to the treatment of AIDS. This review focuses on the newest developments in these areas and provides a strong basis for renewed optimism that gene therapy will have an important role to play in treating people infected with HIV-1.

Competitive engraftment of hematopoietic stem cells genetically modified with a truncated erythropoietin receptor

Transplantation of genetically modified hematopoietic stem cells (HSCs) has therapeutic potential for a variety of blood genetic disorders. Engraftment of HSCs, however, requires toxic myeloablative treatments, which render this approach questionable for non-life-threatening disorders. A potential alternative is the use of transgenes, which allows positive selection of HSCs in vivo. We used retroviral vectors to express a truncated derivative of the erythropoietin receptor (tEpoR) in murine and human hematopoietic cells. Murine HSCs expressing tEpoR at different levels (1500 to 13,000 receptors/cell) acquire a competitive repopulation capacity in vivo upon transplantation into fully or partially myeloablated co-isogenic mouse recipients. Long-term analysis of transplanted mice showed that expression of tEpoR at paraphysiological levels (approximately 1500 receptors/cell) has no effect on steady-state hematopoiesis and induces no further expansion of transduced cells after the engraftment period. Human cord blood-derived CD34+ stem/progenitor cells transduced with a lentiviral vector expressing tEpoR expand their clonogenic capacity in vitro, and significantly increase their marrow repopulation capacity upon xenotransplantation into sublethally irradiated NOD-SCID mice, with no alteration in their phenotype, survival, and differentiation properties. These data indicate that expression of tEpoR is an effective strategy to promote selective engraftment of genetically modified HSCs upon transplantation in both myeloablative and nonmyeloablative conditions, without the use of toxic drugs for selection.

Tolerance by Selective In Vivo Expansion of Foreign Major Histocompatibility Complex-Transduced Autologous Bone Marrow1

BACKGROUND

Application of gene therapy to induce antigen-specific immune tolerance could be important for transplantation or treatment of autoimmune diseases. Hematopoietic stem cell-based gene therapy has been hampered by relatively weak gene expression in vivo and loss of transduced cells over time. Selective expansion of transduced hematopoietic stem cells has been accomplished by incorporating the dihydrofolate reductase (DHFR) gene into the gene transfer vector.

METHODS

To assess whether this strategy could be applied to transplantation, we constructed a retroviral vector plasmid (KA274) containing the cDNA encoding human leukocyte antigen (HLA)-A2.1 and a tyr22 mutant DHFR and generated vesicular stomatitis virus-G-pseudotyped recombinant retrovirus by transfection into 293GPG cells. Bone marrow cells from C57BL/6 mice were infected with KA274 at a multiplicity of infection of 100, and transplanted into lethally irradiated syngeneic mice.

RESULTS

After transplantation with transduced bone marrow, the proportion of peripheral blood cells expressing HLA-A2 ranged from 3.2% to 38% and increased 2- to 4.9-fold after selection for DHFR-expressing cells using trimetrexate and nitrobenzylmercaptpurine riboside 5' monophosphate. HLA-A2 expression remained above pretreatment levels throughout the study. Cytotoxic spleen cells from reconstituted mice lysed third-party HLA-B7-expressing targets but were unable to lyse HLA-A2-expressing targets. All KA274 reconstituted C57BL/6 mice accepted skin grafts from HLA-A2.1 transgenic mice for more than 245 days but rejected third-party Balb/c skin grafts in 12 days.

CONCLUSION

Long-term transgene expression and immunologic tolerance to retrovirus-encoded HLA-A2, equivalent to that obtained by donor bone marrow transplantation, was accomplished, and selective expansion of transduced bone marrow cells was induced using DHFR as a selectable marker.

Therapeutic expression of the platelet-specific integrin, alphaIIbbeta3, in a murine model for Glanzmann thrombasthenia

Integrins mediate the adhesion of cells to each other and to the extracellular matrix during development, immunity, metastasis, thrombosis, and wound healing. Molecular defects in either the alpha- or beta-subunit can disrupt integrin synthesis, assembly, and/or binding to adhesive ligands. This is exemplified by the bleeding disorder, Glanzmann thrombasthenia (GT), where abnormalities of the platelet-specific integrin, alphaIIbbeta3, prevent platelet aggregation following vascular injury. We previously used a retrovirus vector containing a cDNA cassette encoding human integrin beta3 to restore integrin alphaIIbbeta3 on the surface of megakaryocytes derived from peripheral blood stem cells of GT patients. In the present study, bone marrow from beta3-deficient (beta3-/-) mice was transduced with the ITGbeta3-cassette to investigate whether the platelet progeny could establish hemostasis in vivo. A lentivirus transfer vector equipped with the human ITGA2B gene promoter confined transgene expression to the platelet lineage. Human beta3 formed a stable complex with murine alphaIIb, effectively restoring platelet function. Mice expressing significant levels of alphaIIbbeta3 on circulating platelets exhibited improved bleeding times. Intravenous immunoglobulin effectively diminished platelet clearance in animals that developed an antibody response to alphaIIbbeta3. These results indicate the feasibility of targeting platelets with genetic therapies for better management of patients with inherited bleeding disorders.