Characterization of the P140K, PVP(138-140)MLK, and G156A O6-methylguanine-DNA methyltransferase mutants: implications for drug resistance gene therapy

The Versatile Attributes of MGMT: Its Repair Mechanism, Crosstalk with Other DNA Repair Pathways, and Its Role in Cancer

O6-methylguanine-DNA methyltransferase (MGMT or AGT) is a DNA repair protein with the capability to remove alkyl groups from O6-AlkylG adducts. Moreover, MGMT plays a crucial role in repairing DNA damage induced by methylating agents like temozolomide and chloroethylating agents such as carmustine, and thereby contributes to chemotherapeutic resistance when these agents are used. This review delves into the structural roles and repair mechanisms of MGMT, with emphasis on the potential structural and functional roles of the N-terminal domain of MGMT. It also explores the development of cancer therapeutic strategies that target MGMT. Finally, it discusses the intriguing crosstalk between MGMT and other DNA repair pathways.

DNA Barcoding in Nonhuman Primates Reveals Important Limitations in Retrovirus Integration Site Analysis

In vivo tracking of retrovirus-tagged blood stem and progenitor cells is used to study hematopoiesis. Two techniques are used most frequently: sequencing the locus of retrovirus insertion, termed integration site analysis, or retrovirus DNA barcode sequencing. Of these, integration site analysis is currently the only available technique for monitoring clonal pools in patients treated with retrovirus-modified blood cells. A key question is how these two techniques compare in their ability to detect and quantify clonal contributions. In this study, we assessed both methods simultaneously in a clinically relevant nonhuman primate model of autologous, myeloablative transplantation. Our data demonstrate that both methods track abundant clones; however, DNA barcode sequencing is at least 5-fold more efficient than integration site analysis. Using computational simulation to identify the sources of low efficiency, we identify sampling depth as the major factor. We show that the sampling required for integration site analysis to achieve minimal coverage of the true clonal pool is likely prohibitive, especially in cases of low gene-modified cell engraftment. We also show that early subsampling of different blood cell lineages adds value to clone tracking information in terms of safety and hematopoietic biology. Our analysis demonstrates DNA barcode sequencing as a useful guide to maximize integration site analysis interpretation in gene therapy patients.

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.

(211)Astatine-Conjugated Monoclonal CD45 Antibody-Based Nonmyeloablative Conditioning for Stem Cell Gene Therapy

Most hematopoietic stem cell gene therapy studies require host conditioning to allow for efficient engraftment of gene-modified cells. Conditioning regimens with lower treatment-related toxicities are especially relevant for the treatment of nonmalignant blood disorders, such as hemoglobinopathies and immunodeficiencies, and for patients who are otherwise ineligible for conventional high-dose conditioning. Radioimmunotherapy, which employs an α- or a β-emitting radionuclide conjugated to a targeting antibody, is effective for delivering cytotoxic doses of radiation to a cell type of interest while minimizing off-target toxicity. Here, we demonstrate the feasibility of using a nonmyeloablative dose of a monoclonal anti-CD45 antibody conjugated to the α-emitter Astatine-211 ((211)At) to promote engraftment of an autologous gene-modified stem cell graft in the canine model. The doses used provided myelosuppression with rapid autologous recovery and minimal off-target toxicity. Engraftment levels were low in all dogs and reflected the low numbers of gene-modified cells infused. Our data suggest that a cell dose exceeding 1×10(6) cells/kg be used with nonmyeloablative doses of (211)At-anti-CD45 monoclonal antibodies for sustained engraftment in the dog model.

MGMT enrichment and second gene co-expression in hematopoietic progenitor cells using separate or dual-gene lentiviral vectors

The DNA repair gene O(6)-methylguanine-DNA methyltransferase (MGMT) allows efficient in vivo enrichment of transduced hematopoietic stem cells (HSC). Thus, linking this selection strategy to therapeutic gene expression offers the potential to reconstitute diseased hematopoietic tissue with gene-corrected cells. However, different dual-gene expression vector strategies are limited by poor expression of one or both transgenes. To evaluate different co-expression strategies in the context of MGMT-mediated HSC enrichment, we compared selection and expression efficacies in cells cotransduced with separate single-gene MGMT and GFP lentivectors to those obtained with dual-gene vectors employing either encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) or foot and mouth disease virus (FMDV) 2A elements for co-expression strategies. Each strategy was evaluated in vitro and in vivo using equivalent multiplicities of infection (MOI) to transduce 5-fluorouracil (5-FU) or Lin(-)Sca-1(+)c-kit(+) (LSK)-enriched murine bone marrow cells (BMCs). The highest dual-gene expression (MGMT(+)GFP(+)) percentages were obtained with the FMDV-2A dual-gene vector, but half of the resulting gene products existed as fusion proteins. Following selection, dual-gene expression percentages in single-gene vector cotransduced and dual-gene vector transduced populations were similar. Equivalent MGMT expression levels were obtained with each strategy, but GFP expression levels derived from the IRES dual-gene vector were significantly lower. In mice, vector-insertion averages were similar among cells enriched after dual-gene vectors and those cotransduced with single-gene vectors. These data demonstrate the limitations and advantages of each strategy in the context of MGMT-mediated selection, and may provide insights into vector design with respect to a particular therapeutic gene or hematologic defect.

Development of hematopoietic stem cell based gene therapy for HIV-1 infection: considerations for proof of concept studies and translation to standard medical practice

Over the past 15 years we have been investigating an alternative approach to treating HIV-1/AIDS, based on the creation of a disease-resistant immune system through transplantation of autologous, gene-modified (HIV-1-resistant) hematopoietic stem and progenitor cells (GM-HSPC). We propose that the expression of selected RNA-based HIV-1 inhibitors in the CD4+ cells derived from GM-HSPC will protect them from HIV-1 infection and results in a sufficient immune repertoire to control HIV-1 viremia resulting in a functional cure for HIV-1/AIDS. Additionally, it is possible that the subset of protected T cells will also be able to facilitate the immune-based elimination of latently infected cells if they can be activated to express viral antigens. Thus, a single dose of disease resistant GM-HSPC could provide an effective treatment for HIV-1+ patients who require (or desire) an alternative to lifelong antiretroviral chemotherapy. We describe herein the results from several pilot clinical studies in HIV-1 patients and our strategies to develop second generation vectors and clinical strategies for HIV-1+ patients with malignancy who require ablative chemotherapy as part of treatment and others without malignancy. The important issues related to stem cell source, patient selection, conditioning regimen and post-infusion correlative studies become increasingly complex and are discussed herein.

Nuclease-mediated gene editing by homologous recombination of the human globin locus

Tal-effector nucleases (TALENs) are engineered proteins that can stimulate precise genome editing through specific DNA double-strand breaks. Sickle cell disease and β-thalassemia are common genetic disorders caused by mutations in β-globin, and we engineered a pair of highly active TALENs that induce modification of 54% of human β-globin alleles near the site of the sickle mutation. These TALENS stimulate targeted integration of therapeutic, full-length beta-globin cDNA to the endogenous β-globin locus in 19% of cells prior to selection as quantified by single molecule real-time sequencing. We also developed highly active TALENs to human γ-globin, a pharmacologic target in sickle cell disease therapy. Using the β-globin and γ-globin TALENs, we generated cell lines that express GFP under the control of the endogenous β-globin promoter and tdTomato under the control of the endogenous γ-globin promoter. With these fluorescent reporter cell lines, we screened a library of small molecule compounds for their differential effect on the transcriptional activity of the endogenous β- and γ-globin genes and identified several that preferentially upregulate γ-globin expression.

Tumor-associated mutations in O⁶ -methylguanine DNA-methyltransferase (MGMT) reduce DNA repair functionality

MGMT is the primary vehicle for cellular removal of alkyl lesions from the O-6 position of guanine and the O-4 position of thymine. While key to the maintenance of genomic integrity, MGMT also removes damage induced by alkylating chemotherapies, inhibiting the efficacy of cancer treatment. Germline variants of human MGMT are well-characterized, but somatic variants found in tumors were, prior to this work, uncharacterized. We found that MGMT G132R, from a human esophageal tumor, and MGMT G156C, from a human colorectal cancer cell line, are unable to rescue methyltransferase-deficient Escherichia coli as well as wild type (WT) human MGMT after treatment with a methylating agent. Using pre-steady state kinetics, we biochemically characterized these variants as having a reduced rate constant. G132R binds DNA containing an O⁶ -methylguanine lesion half as tightly as WT MGMT, while G156C has a 40-fold decrease in binding affinity for the same damaged DNA versus WT. Mammalian cells expressing either G132R or G156C are more sensitive to methylating agents than mammalian cells expressing WT MGMT. G132R is slightly resistant to O⁶ -benzylguanine, an inhibitor of MGMT in clinical trials, while G156C is almost completely resistant to this inhibitor. The impared functionality of expressed variants G132R and G156C suggests that the presence of somatic variants of MGMT in a tumor could impact chemotherapeutic outcomes.

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.

[Cytotoxicity of lysomustine and its isomers, and their potential use for selection of cells]

N epsilon-Nitroso-N epsilon- [N'-(2-chloroethyl)carbamoyl]-L-lysine (I) and N epsilon- [N'-(2-chloroethyl)-N'-nitrosocarbamoyl]-L-lysine (II), the isomers being the constituents of antitumor agent Lysomustine, were obtained by RFHPLC. The study of cytotoxicity of the above compounds against K562 cells showed that the lesions induced by isomer (II) produce a significant cytotoxic effect but can be efficiently repaired by the action of MGMT (O6-methylaguanine DNA methyltransferase). Under similar conditions, the lesions induced by isomer (I) produce substantially smaller effect but are weakly if at all repairable by MGMT. The effects of a clinically approved agent Lysomustine, which is the mixture of isomers (I) and (II), are similar to those of isomer (II). The results obtained point to a different chemical nature of DNA lesions induced by two Lysomustine isomers. Our data indicate that Lysomustine and its isomer (II) can be used for in vitro selection of cells expressing MGMT.

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.

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.

Selection of genetically modified hematopoietic cells in vitro and in vivo using alkylating agent lysomustine

Efficient gene transfer into hematopoietic stem cells is vital for the success of gene therapy of hematopoietic and immune system disorders. An in vivo selection system based on a mutant form of the O(6)-methylguanine-DNA-methyltransferase gene (MGMTm) is considered one of the more promising strategies for expansion of hematopoietic cells transduced with viral vectors. Here we demonstrate that MGMTm-expressing cells can be efficiently selected using lysomustine, a nitrosourea derivative of lysine. K562 and murine bone marrow cells expressing MGMTm are protected from the cytotoxic action of lysomustine in vitro. We also show in a murine model that MGMTm-transduced hematopoietic cells can be expanded in vivo on transplantation into sublethally irradiated recipients followed by lysomustine treatment. These results indicate that lysomustine can be used as a potent novel chemoselection drug applicable for gene therapy of hematopoietic and immune system disorders.

Protection of stem cell-derived lymphocytes in a primate AIDS gene therapy model after in vivo selection

BACKGROUND

There is currently no effective AIDS vaccine, emphasizing the importance of developing alternative therapies. Recently, a patient was successfully transplanted with allogeneic, naturally resistant CCR5-negative (CCR5Delta32) cells, setting the stage for transplantation of naturally resistant, or genetically modified stem cells as a viable therapy for AIDS. Hematopoietic stem cell (HSC) gene therapy using vectors that express various anti-HIV transgenes has also been attempted in clinical trials, but inefficient gene transfer in these studies has severely limited the potential of this approach. Here we evaluated HSC gene transfer of an anti-HIV vector in the pigtailed macaque (Macaca nemestrina) model, which closely models human transplantation.

METHODS AND FINDINGS

We used lentiviral vectors that inhibited both HIV-1 and simian immunodeficiency virus (SIV)/HIV-1 (SHIV) chimera virus infection, and also expressed a P140K mutant methylguanine methyltransferase (MGMT) transgene to select gene-modified cells by adding chemotherapy drugs. Following transplantation and MGMT-mediated selection we demonstrated transgene expression in over 7% of stem-cell derived lymphocytes. The high marking levels allowed us to demonstrate protection from SHIV in lymphocytes derived from gene-modified macaque long-term repopulating cells that expressed an HIV-1 fusion inhibitor. We observed a statistically significant 4-fold increase of gene-modified cells after challenge of lymphocytes from one macaque that received stem cells transduced with an anti-HIV vector (p<0.02, Student's t-test), but not in lymphocytes from a macaque that received a control vector. We also established a competitive repopulation assay in a second macaque for preclinical testing of promising anti-HIV vectors. The vectors we used were HIV-based and thus efficiently transduce human cells, and the transgenes we used target HIV-1 genes that are also in SHIV, so our findings can be rapidly translated to the clinic.

CONCLUSIONS

Here we demonstrate the ability to select protected HSC-derived lymphocytes in vivo in a clinically relevant nonhuman primate model of HIV/SHIV infection. This approach can now be evaluated in human clinical trials in AIDS lymphoma patients. In this patient setting, chemotherapy would not only kill malignant cells, but would also increase the number of MGMTP140K-expressing HIV-resistant cells. This approach should allow for high levels of HIV-protected cells in AIDS patients to evaluate AIDS gene therapy.

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.

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.

Chemoprotection by transfer of resistance genes

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.

Antiviral gene therapy

This chapter describes the major gene therapeutic approaches for viral infections. The vast majority of published approaches target severe chronic viral infections such as hepatitis B or C and HIV infection. Two basic gene therapy strategies are introduced here. The first involves the expression of a protein or an RNA that inhibits viral replication by targeting crucial steps of the viral life cycle or by interfering with a cellular factor required for virus replication. The major limitation of this approach is that primary levels of gene modification have generally not been sufficient to reduce the availability of target cells permissive for virus replication to a level that significantly decreases overall viral load. Thus, investigators have banked on the expectation that gene-protected cells have a sufficient selective advantage to accumulate and gain prevalence over time, a prediction that so far could not be confirmed in clinical trials. In vivo levels of gene modification can be improved, however, by introducing an additional selectable marker. In addition, a secreted antiviral gene product that exerts a bystander effect could significantly reduce overall virus replication despite relatively low levels of gene modification. In addition to these direct antiviral approaches, several strategies have been developed that employ or aim to enhance host immune responses. The innate immune response has been enhanced, for example, by the in vivo expression of interferons. Alternatively, T cells can be grafted with recombinant receptors to boost adaptive virus-specific immunity. These approaches are especially promising for chronic virus infection, where natural immune responses are evidently not sufficient to effectively control virus replication.

Murine retroviral but not human cellular promoters induce in vivo erythroid-specific deregulation that can be partially prevented by insulators

We are developing lentiviral vectors for gene therapy of red blood cell disorders that co-express a transgene in an erythroid-specific manner and the O(6)-methylguanine-DNA-methyltransferase (MGMT) selective gene in a constitutive way. We report that transduction of murine hematopoietic stem cells (HSCs) with a human phosphoglycerate kinase promoter-based vector at low multiplicity of infection (MOI) does not result in a selective in vivo expansion in the presence of alkylating agents. In contrast, by replacing this cellular promoter with the powerful retroviral-derived myeloproliferative sarcoma virus enhancer, negative control region-deleted, dl587rev primer-binding site substituted promoter, the vector allowed efficient chemoprotection of transduced HSCs at low MOI. However, this promoter interacted with the erythroid HS40/ankyrin enhancer/promoter driving green fluorescent protein, leading to an unexpected loss of erythroid specificity. A partial restoration of tissue-specific expression was obtained by interposition of insulator sequences between the expression units. Alternatively, we found that the strong human cellular elongation factor1-alpha promoter allows similar chemoprotection but without any deregulation of the erythroid-specific promoter in the absence of insulators. These data demonstrate that the level of in vivo deregulation induced by a promoter is not correlated with its transcriptional activity.

Lentiviral vector conferring resistance to mycophenolate mofetil and sensitivity to ganciclovir for in vivo T-cell selection

Several clinical studies of gene-modified T cells have shown limited in vivo function of the cells, immunogenicity of the transgene, and lack of a selective advantage for gene-modified T cells. To address these problems, we developed a lentiviral vector (LV) that provides a selectable, proliferative advantage and potentially decreases immunogenicity for transduced T cells. The bicistronic vector expressed two genes linked with an internal ribosomal entry site. One gene is a variant of the inosine monophosphate dehydrogenase 2, inosine monophosphate dehydrogenase (IMPDH(IY)), conferring resistance to the immunosuppressive drug mycophenolate mofetil (MMF). The other is a suicide gene, herpes simplex virus thymidine kinase (HSV-TK), rendering proliferating cells sensitive to ablation with ganciclovir, fused to the selectable transmembrane marker DeltaCD34 (DeltaCD34/TK). Cells transduced with LV-DeltaCD34/TK.IMPDH(IY) were efficiently enriched by immunomagnetic selection for CD34, proliferated in 0.5-5 microM MMF, and were killed by 0.5-25 microg ml(-1) ganciclovir. We demonstrate efficient selection and killing of gene-modified cells and suggest LV-DeltaCD34/TK.IMPDH(IY)-transduced T cells could be used to facilitate allogeneic hematopoietic cell engraftment. The expression of IMPDH(IY) would allow in vivo selection with MMF, and DeltaCD34/TK expression would allow rapid and safe elimination of transduced T cells if graft-versus-host disease developed.

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.

Stable differentiation and clonality of murine long-term hematopoiesis after extended reduced-intensity selection for MGMT P140K transgene expression

Efficient in vivo selection increases survival of gene-corrected hematopoietic stem cells (HSCs) and protects hematopoiesis, even if initial gene transfer efficiency is low. Moreover, selection of a limited number of transduced HSCs lowers the number of cell clones at risk of gene activation by insertional mutagenesis. However, a limited clonal repertoire greatly increases the proliferation stress of each individual clone. Therefore, understanding the impact of in vivo selection on proliferation and lineage differentiation of stem-cell clones is essential for its clinical use. We established minimal cell and drug dosage requirements for selection of P140K mutant O6-methylguanine-DNA-methyltransferase (MGMT P140K)-expressing HSCs and monitored their differentiation potential and clonality under long-term selective stress. Up to 17 administrations of O6-benzylguanine (O6-BG) and 1,3-bis(2-chloroethyl)-1-nitroso-urea (BCNU) did not impair long-term differentiation and proliferation of MGMT P140K-expressing stem-cell clones in mice that underwent serial transplantation and did not lead to clonal exhaustion. Interestingly, not all gene-modified hematopoietic repopulating cell clones were efficiently selectable. Our studies demonstrate that the normal function of murine hematopoietic stem and progenitor cells is not compromised by reduced-intensity long-term in vivo selection, thus underscoring the potential value of MGMT P140K selection for clinical gene therapy.

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.

In vivo selection of transduced hematopoietic stem cells and little evidence of their conversion into hepatocytes in vivo

BACKGROUND/AIMS

FMEV-type retroviral vector provides high transgene expression in hepatocytes and hematopoietic stem cells (HSCs). Here, we examined whether these vectors could provide a sufficient drug-resistance gene expression in HSCs and whether transduced HSCs could differentiate into hepatocytes in vivo.

METHODS

The CD45(+)/Lin(-) cells were transduced in vitro by FMEV-type vectors containing human O(6)-methylguanine-DNA methyltransferase (MGMT)/reporter genes and transferred into recipient mice. After the treatment with temozolomide and O(6)-benzylguanine (TMZ/BG) in vivo, we analyzed the transgene expression in peripheral blood cells by flow-cytometry. Immunohistochemistry was performed on the liver slices in partial hepatectomized recipient mice.

RESULTS

After TMZ/BG treatment, transduced host cells were enriched in recipient mice. In the liver, we observed the efficient transgene expression in many small cells along sinusoids. However, only few large cells in hepatic lobules expressed albumin. They also expressed both a transgene and a recipient marker gene, suggesting the fusion of donor HSCs with recipient hepatocytes.

CONCLUSIONS

This vector expressed a drug-resistance gene in HSCs highly enough to protect them from the drugs. But, the conversion of HSCs into hepatocytes in vivo might be a rare event in this model.

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.

Inactivation of O6-alkylguanine DNA alkyltransferase as a means to enhance chemotherapy

DNA adducts at the O6-position of guanine are a result of the carcinogenic, mutagenic and cytotoxic actions of methylating and chloroethylating agents. The presence of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) renders cells resistant to the biological effects induced by agents that attack at this position. O6-Benzylguanine (O6-BG) is a low molecular weight substrate of AGT and therefore, results in sensitizing cells and tumors to alkylating agent-induced cytotoxicity and antitumor activity. Presently, chemotherapy regimens of O6-BG in combination with BCNU, temozolomide and Gliadel are in clinical development. Other ongoing clinical trials include expression of mutant AGT proteins that confer resistance to O6-BG in bone marrow stem cells, in an effort to reduce the potential enhanced toxicity and mutagenicity of alkylating agents in the bone marrow. O6-BG has also been found to enhance the cytotoxicity of agents that do not form adducts at the O6-position of DNA, including platinating agents. O6-BG's mechanism of action with these agents is not fully understood; however, it is independent of AGT activity or AGT inactivation. A better understanding of the effects of this agent will contribute to its clinical usefulness and the design of better analogs to further improve cancer chemotherapy.

Differential competitive resistance to methylating versus chloroethylating agents among five O6-alkylguanine DNA alkyltransferases in human hematopoietic cells

P140K-MGMT and G156A-MGMT genes encode two O(6)-benzylguanine-resistant O(6)-alkylguanine DNA alkyltransferase proteins that confer a high degree of O(6)-benzylguanine and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or O(6)-benzylguanine and temozolomide resistance to primary hematopoietic cells. In this study, we directly compared these and three other O(6)-benzylguanine-resistant MGMT genes for their ability to protect the human erythroleukemia cell line, K562, using a direct competitive selection strategy to identify the mutation that conferred the greatest degree of protection from O(6)-benzylguanine and either BCNU or temozolomide. MFG retroviral vector plasmids for each of these mutants [G156A-MGMT (ED(50) for O(6)-benzylguanine, 60 micromol/L); and P140K-MGMT, MGMT-2 (S152H, A154G, Y158H, G160S, L162V), MGMT-3 (C150Y, A154G, Y158F, L162P, K165R), and MGMT-5 (N157T, Y158H, A170S; ED(50) for benzylguanine, >1,000 micromol/L)] were mixed, and the virus produced from Phoenix cells was transduced into K562 cells. Stringent selection used high doses of O(6)-benzylguanine (800 micromol/L) and temozolomide (1,000 micromol/L) or BCNU (20 micromol/L) administered twice, and following regrowth, surviving clones were isolated, and the MGMT transgene was sequenced. None of the mutants was lost during selection. Using temozolomide, the enrichment factor was greatest for P140K-MGMT (1.7-fold). Using BCNU selection, the greatest enrichment was observed with MGMT-2 (1.5-fold). G156A-MGMT, which is the least O(6)-benzylguanine-resistant MGMT gene of the mutants tested, was not lost during selection but was selected against. The optimal mutant MGMT useful as a drug resistance gene may depend on whether a methylating or chloroethylating agent is used for drug selection.

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.

Hematopoietic stem cell gene therapy with drug resistance genes: an update

Transfer of drug resistance genes into hematopoietic stem cells (HSCs) has promise for the treatment of a variety of inherited, that is, X-linked severe combined immune deficiency, adenosine deaminase deficiency, thalassemia, and acquired disorders, that is, breast cancer, lymphomas, brain tumors, and testicular cancer. Drug resistance genes are transferred into HSCs either for providing myeloprotection against chemotherapy-induced myelosuppression or for selecting HSCs that are concomitantly transduced with another gene for correction of an inherited disorder. In this review, we describe ongoing experimental approaches, observations from clinical trials, and safety concerns related to the drug resistance gene transfer.

Equal potency of gammaretroviral and lentiviral SIN vectors for expression of O6-methylguanine-DNA methyltransferase in hematopoietic cells

Severe adverse events related to insertional mutagenesis have reinforced interest in self-inactivating (SIN) retroviral vectors lacking enhancer-promoter sequences in the long terminal repeats (LTRs). Here, we have compared the potency of gammaretroviral and lentiviral vectors expressing the P140K mutant of O(6)-methylguanine-DNA methyltransferase (MGMT). MGMT-P140K is a clinically relevant selection marker that mediates a strong survival advantage in hematopoietic cells exposed to alkylating agents. We designed gammaretroviral and lentiviral vectors that contained identical enhancer-promoter sequences located either in the LTR or downstream of the packaging region, for internal initiation of transcription from SIN backbones. Gammaretroviral vectors with intact LTRs containing enhancer-promoter sequences showed both higher titers and higher expression levels than the lentiviral counterparts, likely a result of suboptimal RNA processing of the lentiviral leader region. In the SIN context, gammaretroviral and lentiviral vectors with comparable internal cassettes had similar expression properties. Interestingly, gammaretroviral SIN vectors pseudotyped with RD114/TR had a higher transduction efficiency on proliferating human CD34(+) cells than lentiviral counterparts. These results encourage further investigations into the formation of retroviral hybrid vectors that combine the desired properties of high efficiency and increased biosafety.

Mitochondrial targeting of human O6-methylguanine DNA methyltransferase protects against cell killing by chemotherapeutic alkylating agents

DNA repair capacity of eukaryotic cells has been studied extensively in recent years. Mammalian cells have been engineered to overexpress recombinant nuclear DNA repair proteins from ectopic genes to assess the impact of increased DNA repair capacity on genome stability. This approach has been used in this study to specifically target O(6)-methylguanine DNA methyltransferase (MGMT) to the mitochondria and examine its impact on cell survival after exposure to DNA alkylating agents. Survival of human hematopoietic cell lines and primary hematopoietic CD34(+) committed progenitor cells was monitored because the baseline repair capacity for alkylation-induced DNA damage is typically low due to insufficient expression of MGMT. Increased DNA repair capacity was observed when K562 cells were transfected with nuclear-targeted MGMT (nucl-MGMT) or mitochondrial-targeted MGMT (mito-MGMT). Furthermore, overexpression of mito-MGMT provided greater resistance to cell killing by 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) than overexpression of nucl-MGMT. Simultaneous overexpression of mito-MGMT and nucl-MGMT did not enhance the resistance provided by mito-MGMT alone. Overexpression of either mito-MGMT or nucl-MGMT also conferred a similar level of resistance to methyl methanesulfonate (MMS) and temozolomide (TMZ) but simultaneous overexpression in both cellular compartments was neither additive nor synergistic. When human CD34(+) cells were infected with oncoretroviral vectors that targeted O(6)-benzylguanine (6BG)-resistant MGMT (MGMT(P140K)) to the nucleus or the mitochondria, committed progenitors derived from infected cells were resistant to 6BG/BCNU or 6BG/TMZ. These studies indicate that mitochondrial or nuclear targeting of MGMT protects hematopoietic cells against cell killing by BCNU, TMZ, and MMS, which is consistent with the possibility that mitochondrial DNA damage and nuclear DNA damage contribute equally to alkylating agent-induced cell killing during chemotherapy.

Lentivirus-mediated expression of mutant MGMTP140K protects human CD34+ cells against the combined toxicity of O6-benzylguanine and 1,3-bis(2-chloroethyl)-nitrosourea or temozolomide

Lentiviral vectors are capable of efficiently transducing nondividing and slowly dividing cells, including hematopoietic stem cells, resulting in stable integration and sustained transgene expression. We constructed human immunodeficiency virus type 1-based self-inactivating lentiviral vectors to express either wild-type or an O6-benzylguanine (O6-beG)-resistant mutant form of the human O6-alkylguanine-DNA methyltransferase (MGMT; DNA-O6-methylguanine:[protein]-L-cysteine S-methyltransferase, EC 2.1.1.63) and transduced K562 and granulocyte colony-stimulating factor-mobilized human peripheral blood CD34+ cells. After transduction, K562 cells expressed high levels of MGMT as determined by Western blot, immunocytochemistry, and biochemical assay. A colony-forming survival assay showed significant protection against O6-beG plus 1,3-bis(2-chloroethyl)-nitrosourea (BCNU) or temozolomide (TMZ) toxicity. Similarly, a single transduction of CD34+ cells resulted in a 13- to 14-fold increase in the level of MGMT expression. In comparison with non-transduced cells, mutant MGMTP140K-transduced CD34+ cells showed significant resistance against the combined toxicity of O6-beG with either TMZ or BCNU: there was an approximately 9-fold increase in the survival of colony-forming cells as indicated by the IC50 values after O6-beG plus TMZ treatment and an approximately 5-fold increase in the case of O6-beG plus BCNU treatment. These results show that lentivirus-mediated expression of MGMTP140K can efficiently protect the hematopoietic compartment against the combined toxicity of O6-beG plus TMZ or BCNU.

Hematopoietic stem cell gene therapy of murine protoporphyria by methylguanine-DNA-methyltransferase-mediated in vivo drug selection

Erythropoietic protoporphyria (EPP) is an inherited defect of the ferrochelatase (FECH) gene characterized by the accumulation of toxic protoporphyrin in the liver and bone marrow resulting in severe skin photosensitivity. We previously described successful gene therapy of an animal model of the disease with erythroid-specific lentiviral vectors in the absence of preselection of corrected cells. However, the high-level of gene transfer obtained in mice is not translatable to large animal models and humans if there is no selective advantage for genetically modified hematopoietic stem cells (HSCs) in vivo. We used bicistronic SIN-lentiviral vectors coexpressing EGFP or FECH and the G156A-mutated O6-methylguanine-DNA-methyltransferase (MGMT) gene, which allowed efficient in vivo selection of transduced HSCs after O6-benzylguanine and BCNU treatment. We demonstrate for the first time that the correction and in vivo expansion of deficient transduced HSC population can be obtained by this dual gene therapy, resulting in a progressive increase of normal RBCs in EPP mice and a complete correction of skin photosensitivity. Finally, we developed a novel bipromoter SIN-lentiviral vector with a constitutive expression of MGMT gene to allow the selection of HSCs and with an erythroid-specific expression of the FECH therapeutic gene.

Limited lentiviral transgene expression with increasing copy number in an MGMT selection model: lack of copy number selection by drug treatment

Retroviral vector integration into the human genome carries increased risk of oncogenesis with increasing integrations. To boost transgene expression for gene therapy, multiple integrations are often sought. We studied the relationship between the number of vector integrations and transgene expression and the effect that drug selection in an MGMT-selection model would have on vector copy number. K562 cells were transduced using a lentiviral vector and a library of clones was generated. Median proviral copy number was 4 and a positive correlation with transgene expression was observed. Transgene expression increased at a linear rate between 1 and 4 vector copies/cell, but was unpredictable at >4 integrations/cell. When lentivirus MGMT(P140K)-transduced K562 cells were treated with O(6)-benzylguanine (BG)/BCNU, there was no selection for increased median copy number in colony-forming units, despite strong selection pressure and an increase in transgene expression and activity. These data show a direct and linear correlation between MGMT(P140K) transgene expression and vector copy number. Strong BG/BCNU selective pressure does not result in preferential survival of high-copy-number clones but does select for strong transgene expression. Thus drug selection would not be expected to increase the risk of oncogenesis due to exaggerated selection in favor of high-copy-number vector integration.

Nuclear-localizing O6-benzylguanine-resistant GFP-MGMT fusion protein as a novel in vivo selection marker

OBJECTIVE

We characterized a novel in vivo selectable fusion protein, green fluorescence protein-O6-benzylguanine (BG)-resistant O6-methylguanine-methyltransferase (GFP-MGMT* [*refers to mutant MGMT]) used to delineate optimum selection regimens for transduced hematopoietic stem cells (HSC) ex vivo and in vivo.

MATERIALS AND METHODS

We transduced human or mouse HSC with retrovirus vector encoding GFP-MGMT* where BG-resistant forms of human P140K-hMGMT* and mouse P144K-mMGMT* were studied. We evaluated selection of transduced HSC ex vivo and in vivo using either BG/1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or BG/temozolomide (TMZ) combinations, evaluating transduction marking by flow cytometry and real-time TaqMan PCR.

RESULTS

GFP-MGMT* transduction confers nuclear-localized GFP fluorescence and BG resistance. Optimum selection ex vivo of GFP-MGMT*-transduced HSC occurred with BG (2.5-10 microM)/BCNU (5-10 microM) or TMZ (100-200 microM), which increases marking while preserving maximum viable transduced cells. Starting at low levels (0.1%) or high levels (>30%) of in vivo bone marrow gene making in mice, in vivo selection with BG/BCNU (20/6 mg/kg) (weeks 4 and 5) or BG/TMZ (20/60 mg/kg) (daily x 5 at week 4) increased bone marrow marking to 8.58% +/- 3.52% or 82.0% +/- 3.4% GFP+ cells, respectively, in the low- or high-level initial marking mice.

CONCLUSIONS

GFP-MGMT* is an informative tool to explore optimization of in vivo selection regimens using BG/BCNU or BG/TMZ to increase gene marking of HSC. Both timing and dosing of selection regimens and the starting level of marking may all be important to the level of selective increase of in vivo marking achieved.

In vivo selection for human and murine hematopoietic cells transduced with a therapeutic MGMT lentiviral vector that inhibits HIV replication

We have developed an HIV-based lentiviral vector, VRX496, which efficiently transduces human CD34+ progenitors and CD4+ T lymphocytes. VRX496 contains an antisense sequence against the HIV envelope and is currently being evaluated for safety in a clinical trial for treatment of HIV. Selective outgrowth of transduced hematopoietic cells in vivo is anticipated to increase the therapeutic efficacy of this treatment by maximizing the persistence of virus-resistant cells in the body. Although HIV resistance is selective, additional selection may aid in treatment efficacy due to the vast quantity of target cells. Therefore, we engineered VRX496 to express the P140K MGMT gene to drive potent drug-mediated in vivo selection for transduced hematopoietic long-term repopulating cells. Suboptimally transduced T cell cultures treated with O6-benzylguanine and BCNU were selected from 3 to 100%, and after selection cultures did not support HIV replication. Primary CD34+ progenitors derived from G-CSF-mobilized peripheral blood were transduced at 27 to 35% efficiency. Approximate sixfold selection was observed for transduced CD34+ progenitors, colony-forming units, and long-term culture-initiating cells. Multilineage in vivo selection was demonstrated for transduced murine hematopoietic cells in human CD34(+)-derived hematopoietic cells in NOD-SCID mice. These results establish efficient ex vivo and in vivo selection for hematopoietic cells transduced with lentiviral vectors and support the potential therapeutic benefit of this strategy in human gene therapy.

A bicistronic SIN-lentiviral vector containing G156A MGMT allows selection and metabolic correction of hematopoietic protoporphyric cell lines

BACKGROUND

Erythropoietic protoporphyria (EPP) is an inherited disease characterised by a ferrochelatase (FECH) deficiency, the latest enzyme of the heme biosynthetic pathway, leading to the accumulation of toxic protoporphyrin in the liver, bone marrow and spleen. We have previously shown that a successful gene therapy of a murine model of the disease was possible with lentiviral vectors even in the absence of preselection of corrected cells, but lethal irradiation of the recipient was necessary to obtain an efficient bone marrow engraftment. To overcome a preconditioning regimen, a selective growth advantage has to be conferred to the corrected cells.

METHODS

We have developed a novel bicistronic lentiviral vector that contains the human alkylating drug resistance mutant O(6)-methylguanine DNA methyltransferase (MGMT G156A) and FECH cDNAs. We tested their capacity to protect hematopoietic cell lines efficiently from alkylating drug toxicity and correct enzymatic deficiency.

RESULTS

EPP lymphoblastoid (LB) cell lines, K562 and cord-blood-derived CD34(+) cells were transduced at a low multiplicity of infection (MOI) with the bicistronic constructs. Resistance to O(6)-benzylguanine (BG)/N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU) was clearly shown in transduced cells, leading to the survival and expansion of provirus-containing cells. Corrected EPP LB cells were selectively amplified, leading to complete restoration of enzymatic activity and the absence of protoporphyrin accumulation.

CONCLUSIONS

This study demonstrates that a lentiviral vector including therapeutic and G156A MGMT genes followed by BG/BCNU exposure can lead to a full metabolic correction of deficient cells. This vector might form the basis of new EPP mouse gene therapy protocols without a preconditioning regimen followed by in vivo selection of corrected hematopoietic stem cells.

Myeloablation is not required to select and maintain expression of the drug-resistance gene, mutant MGMT, in primary and secondary recipients

Gene transduction of hematopoietic progenitors capable of reconstituting both primary and secondary recipients is an important milestone in preclinical development of gene therapy. Myeloablation conditioning prior to infusion of transduced stem cells causes significant host morbidity. In contrast, drug-resistance gene transfer utilizes judicious in vivo selection of transduced stem cells over time, reaching only the level of transduction and expression required. The O(6)-benzylguanine (BG)-resistant mutant O(6)-methylguanine-DNA methyltransferase (MGMT) gene is a potent selection gene for transduced cells. Using two different mutant MGMTs, G156A and P140K, that vary in BG resistance by a factor of 1:20, we asked whether long-term repopulating and secondary mouse-repopulating cells could be transduced, transplanted, and selected for in the nonmyeloablated recipient and whether the mutant MGMT would continue to be expressed in secondary recipient repopulating cells. We found that under stringent drug-selection competition, cells expressing the more BG-resistant variant, P140K-MGMT, were enriched over G156A-MGMT-expressing progenitors. In addition, the MFG retroviral vector transmitted the mutant MGMT gene to long-term repopulating cells that, after selective enrichment in the nonmyeloablated primary recipient, repopulated secondary mice and continued to express the transgene. Thus, MFG mutant MGMT vectors transduce repopulating hematopoietic stem cells that may be used both for chemotherapeutic drug resistance and to enrich for second therapeutic genes.

Current developments in the design of onco-retrovirus and lentivirus vector systems for hematopoietic cell gene therapy

Over the past dozen years, the majority of clinical gene therapy trials for inherited genetic diseases and cancer therapy have been performed using murine onco-retrovirus as the gene delivery vector. The earliest systems used were relatively inefficient in both the rates of transduction and expression of the transgene. Formidable obstacles inherent in the cell biology and/or the immunology of the target cell systems limited the efficacy of gene therapy for many target diseases. Development of novel retrovirus gene transfer systems that are in progress have begun to overcome these obstacles. Evidence of this progress is the recent successful functional correction of the immune T and B lymphocyte deficiency in patients with X-linked severe combined immunodeficiency (X-SCID) and adenosine deaminase (ADA)-deficient SCID following onco-retrovirus vector ex vivo transduction of autologous marrow stem cells [Science 296 (2002) 2410; Science 288 (2000) 669; N. Engl. J. Med. 346 (2002) 1185]. These achievements of prolonged clinical benefit from gene therapy were tempered by the finding of insertional mutageneses in two of the treated X-SCID patients [N. Engl. J. Med. 348 (2003) 255].

Hematoprotection and enrichment of transduced cells in vivo after gene transfer of MGMT(P140K) into hematopoietic stem cells

The overexpression of mutant forms of O(6)-methylguanine-DNA-methyltransferase (MGMT), resistant to the MGMT inhibitor O(6)-benzylguanine (BG), protects hematopoietic cells from the toxicity of combined BG plus O(6)-alkylating agent chemotherapy. To evaluate the feasibility of this approach for clinically relevant O(6)-alkylating agents, combined therapy with BG and two chloroethylnitrosourea-type drugs, ACNU or BCNU, or the triazene derivative temozolomide (TMZ) was investigated in a murine bone marrow transplant model allowing transgenic expression of the highly BG-resistant MGMT(P140K) mutant. Whereas 20/20 control animals transplanted with nontransduced cells died of progressive myelosuppression during therapy, nearly all animals transplanted with MGMT(P140K)-transduced cells survived treatment with BG/ACNU (12/15), BG/TMZ (10/10), or BG/BCNU (5/5). In surviving animals, hematological parameters improved during chemotherapy and pretreatment levels were reestablished during or shortly after therapy. All animals showed enrichment of transgenic granulocytes (range: 15- to 101-fold) and lymphocytes (range: 16- to 55-fold) in peripheral blood, bone marrow, and spleen. No significant differences were observed between individual treatment groups. Serial transplants demonstrated protection in secondary recipients and confirmed the transduction of transplantable stem cells. Thus, these data demonstrate efficient protection from hematotoxicity and substantial enrichment of transgenic cells following MGMT(P140K) gene transfer and treatment with different O(6)-alkylating drugs.

Gene therapy for pancreatic cancer

Gene transfer technology has the potential to revolutionize cancer treatment. Developments in molecular biology, genetics, genomics, stem cell technology, virology, bioengineering, and immunology are accelerating the pace of innovation and movement from the laboratory bench to the clinical arena. Pancreatic adenocarcinoma, with its particularly poor prognosis and lack of effective traditional therapy for most patients, is an area where gene transfer and immunotherapy have a maximal opportunity to demonstrate efficacy. In this review, we have discussed current preclinical and clinical investigation of gene transfer technology for pancreatic cancer. We have emphasized that the many strategies under investigation for cancer gene therapy can be classified into two major categories. The first category of therapies rely on the transduction of cells other than tumor cells, or the limited transduction of tumor tissue. These therapies, which do not require efficient gene transfer, generally lead to systemic biological effects (e.g., systemic antitumor immunity, inhibition of tumor angiogenesis, etc) and therefore the effects of limited gene transfer are biologically "amplified." The second category of gene transfer strategies requires the delivery of therapeutic genetic material to all or most tumor cells. While these elegant approaches are based on state-of-the-art advances in our understanding of the molecular biology of cancer, they suffer from the current inadequacies of gene transfer technology. At least in the short term, it is very likely that success in pancreatic cancer gene therapy will involve therapies that require only the limited transduction of cells. The time-worn surgical maxim, "Do what's easy first," certainly applies here.

Interaction of mammalian O(6)-alkylguanine-DNA alkyltransferases with O(6)-benzylguanine

Human O(6)-alkylguanine-DNA alkyltransferase (hAGT) activity is a major factor in providing resistance to cancer chemotherapeutic alkylating agents. Inactivation of hAGT by O(6)-benzylguanine (BG) is a promising strategy for overcoming this resistance. Previous studies, which have focused on the region encompassed by residues Pro138 to Gly173, have identified more than 100 individual mutations located at 23 discrete sites at which alterations can render AGT less sensitive to BG. We have now extended the examination of possible sites in hAGT at which alterations might lead to BG resistance to include the residues from Val130 to Asn137, which also make up part of the binding pocket into which BG is postulated to fit. A further 21 mutations located at positions Gly132, Met134, Arg135, and Gly136 were found to lower sensitivity to BG. Mutants R135L, R135Y, and G136P were the most strikingly resistant, with a 50-fold increase in the amount of BG needed to obtain 50% inactivation. These results therefore increase the number of sites at which BG resistance can occur in response to a single amino acid change to 27. Although mammalian AGTs are very similar in amino acid sequence, mouse AGT (mAGT) is significantly less sensitive to BG than rat AGT (rAGT) or hAGT. Construction of chimeric proteins in which portions came from the rAGT and the mAGT indicated that the difference in inactivation resided solely in the amino acids located in the sequence from residues 150 to 188. Individual mutations of the three residues where rAGT and mAGT differ in this region showed that the principal reason for the reduced ability of the mAGT to react with BG was the presence of a histidine residue at position 161, which is occupied by asparagine in rAGT and hAGT. These experiments indicate that many minor changes in amino acids forming all parts of the nucleoside binding pocket of AGT can alter its ability to react with BG and that the possibility that polymorphisms or variants may occur reducing the effectiveness of combination therapy with BG and alkylating agents must be considered.

Lentiviral transduction of P140K MGMT into human CD34(+) hematopoietic progenitors at low multiplicity of infection confers significant resistance to BG/BCNU and allows selection in vitro

Lentiviral vectors may improve hematopoietic stem cell (HSC) gene transfer because of their enhanced ability to transduce nondividing cells. However, many studies report efficient transduction only at high multiplicities of infection (MOI). This study reports efficient transduction of human CD34(+) cells with a drug resistance gene allowing post-transduction selection using lentivirus under low-MOI conditions that did not require cytokine stimulation or viral concentration. We used the P140K methylguanine-DNA-methyltransferase mutant (P140K MGMT) as the gene insert into a second-generation lentiviral backbone and triple-plasmid transfection to generate vesicular stomatitis virus (VSV)-G protein-pseudotyped virus. The P140K MGMT gene product, O(6)-alkylguanine-DNA-alkyltransferase (AGT), provides protection from the therapeutic drug combination of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and the wild-type AGT inhibitor O(6)-benzylguanine (BG). Low-speed spinoculation enhanced transduction more than addition of Polybrene or multiple virus exposures. Addition of cytokines was not required. Low-MOI transduction (< or =1) of human CD34(+) and CD34(+) lin(-) cells with P140K MGMT lentivirus resulted in an average 41% and 89% gene transfer rate as assessed by PCR, respectively, and concordant AGT expression that conferred substantial clonogenic survival advantage after BG/BCNU treatment. During in vitro drug selection, 87% of surviving CD34(+) cell-derived colony-forming units (CFU) were transduced. This work shows the potential utility of lentiviral vectors for drug resistance gene transfer to HSCs for the purpose of in vivo selection and marrow protection. Because drug selection will enrich for transduced progenitors, high MOI can be avoided, improving the safety profile of lentiviral gene transfer.

Monosaccharide-linked inhibitors of O(6)-methylguanine-DNA methyltransferase (MGMT): synthesis, molecular modeling, and structure-activity relationships

A series of potential inhibitors of the human DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) were synthesized, characterized in detail by NMR, and tested for their ability to deplete MGMT activity in vitro. The new compounds, omega-[O(6)-R-guan-9-yl]-(CH(2))(n)-beta-d-glucosides with R = benzyl or 4-bromothenyl and omega = n = 2, 4,. 12, were compared with the established inhibitors O(6)-benzylguanine (O(6)-BG), 8-aza-O(6)-benzylguanine (8-aza-BG), and O(6)-(4-bromothenyl)guanine (4-BTG), which exhibit in an in vitro assay IC(50) values of 0.62, 0.038, and 0.009 microM, respectively. Potential advantages of the glucosides are improved water solubility and selective uptake in tumor cells. The 4-BTG glucosides with n = 2, 4, 6 show moderate inhibition with an IC(50) of ca. 0.5 microM, while glucosides derived from BG and 8-aza-BG showed significantly poorer inhibition compared to the parent compounds. The 4-BTG glucosides with n = 8, 10, 12 were effective inhibitors with IC(50) values of ca. 0.03 microM. To understand this behavior, extensive molecular modeling studies were performed using the published crystal structure of MGMT (PDB entry: ). The inhibitor molecules were docked into the BG binding pocket, and molecular dynamics simulations with explicit water molecules were carried out. Stabilization energies for the interactions of specific regions of the inhibitor and individual amino acid residues were calculated. The alkyl spacer is located in a cleft along helix 6 of MGMT. With increasing spacer length there is increasing interaction with several amino acid residues which play an important role in the proposed nucleotide flipping mechanism required for DNA repair.

Protection of hematopoietic cells from O(6)-alkylation damage by O(6)-methylguanine DNA methyltransferase gene transfer: studies with different O(6)-alkylating agents and retroviral backbones

Overexpression of O(6)-methylguanine DNA methyltransferase (MGMT) can protect hematopoietic cells from O(6)-alkylation damage. To identify possible clinical applications of this technology we compared the effect of MGMT gene transfer on the hematotoxicity induced by different O(6)-alkylating agents in clinical use: the chloroethylnitrosoureas ACNU, BCNU, CCNU and the tetrazine derivative temozolomide. In addition, various retroviral vectors expressing the MGMT-cDNA were investigated to identify optimal viral backbones for hematoprotection by MGMT expression. Protection from ACNU, BCNU, CCNU or temozolomide toxicity was evaluated utilizing a Moloney murine leukemia virus-based retroviral vector (N2/Zip-PGK-MGMT) to transduce primary murine bone marrow cells. Increased resistance in murine colony-forming units (CFU) was demonstrated for all four drugs. In comparison to mock-transduced controls, after transduction with N2/Zip-PGK-MGMT the IC50 for CFU increased on average 4.7-fold for ACNU, 2.5-fold for BCNU, 6.3-fold for CCNU and 1.5-fold for temozolomide. To study the effect of the retroviral backbone on hematoprotection various vectors expressing the human MGMT-cDNA from a murine embryonic sarcoma virus LTR (MSCV-MGMT) or a hybrid spleen focus-forming/murine embryonic sarcoma virus LTR (SF1-MGMT) were compared with the N2/Zip-PGK-MGMT vector. While all vectors increased resistance of transduced human CFU to ACNU, the SF1-MGMT construct was most efficient especially at high ACNU concentrations (8-12 microg/ml). Similar results were obtained for protection of murine high-proliferative-potential colony-forming cells. These data may help to optimize treatment design and retroviral constructs in future clinical studies aiming at hematoprotection by MGMT gene transfer.

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

Transfer of drug resistance genes to hematopoietic stem cells offers the potential to protect cancer patients from drug-induced myelosuppression and to increase the number of gene-modified cells by in vivo selection. In this study, a retroviral vector expressing both a P140K variant of human O6-methylguanine-DNA methyltransferase (MGMT) and an EGFP reporter gene was evaluated for stem cell protection in a murine transplant model. Mice transplanted with vector-transduced cells showed significant resistance to the myelosuppressive effects of temozolomide (TMZ), an orally administered DNA-methylating drug, and O6-benzylguanine (BG), a drug that depletes cells of wild-type MGMT activity. Following drug treatment, increases in EGFP(+) peripheral blood cells were seen in all peripheral blood lineages, and secondary transplant experiments proved that selection had occurred at the stem cell level. In a second set of experiments in which transduced cells were diluted with unmarked cells, efficient stem cell selection was noted together with progressive marrow protection with repeated treatment courses. Altogether, these results show that P140K MGMT gene transfer can protect stem cells against the toxic effects of TMZ and BG and that this vector/drug system may be useful for clinical myeloprotection and for in vivo selection of transduced stem cells.