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

Preclinical IV busulfan dose-finding study to induce reversible myeloablation in a non-human primate model

In this study we utilized a large animal model to identify a dose of intravenous busulfan that can cause reversible myelosuppression. Nine baboons (Papio anubis) were treated with IV busulfan at 6.4 (Group A), 8 (Group B), or 9.6 mg/kg (Group C). Peripheral blood counts were measured up to 90 days after treatment and serial bone marrow samples were obtained to analyze CD34+ cell content and colony forming units. Overall, the highest grade of peripheral blood cytopenia was observed 15 days after treatment in all three groups (n = 3/group). In particular, we observed a notable reduction of neutrophil and platelet counts in the blood and the number of marrow CD34+ cells and colony forming units. In contrast, the effect of busulfan on hemoglobin levels was mild. Baboons who received the highest dose of busulfan showed only a 25-35% recovery of marrow CD34+ cells and colony forming units after 90 days of busulfan administration. However, all three groups of animals showed a full recovery of peripheral blood counts and normal marrow cellularity and tri-lineage hematopoiesis after treatment. Notably, all three doses of busulfan were tolerated well without significant extra-medullary toxicity. These results validate the hierarchy of blood cells likely targeted by busulfan, and based on these findings, clinical trials using myelotoxic but not myeloablative doses of intravenous busulfan will be designed for patients with myeloid malignancies.

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.

Humanized bone marrow mouse model as a preclinical tool to assess therapy-mediated hematotoxicity

PURPOSE

Preclinical in vivo studies can help guide the selection of agents and regimens for clinical testing. However, one of the challenges in screening anticancer therapies is the assessment of off-target human toxicity. There is a need for in vivo models that can simulate efficacy and toxicities of promising therapeutic regimens. For example, hematopoietic cells of human origin are particularly sensitive to a variety of chemotherapeutic regimens, but in vivo models to assess potential toxicities have not been developed. In this study, a xenograft model containing humanized bone marrow is utilized as an in vivo assay to monitor hematotoxicity.

EXPERIMENTAL DESIGN

A proof-of-concept, temozolomide-based regimen was developed that inhibits tumor xenograft growth. This regimen was selected for testing because it has been previously shown to cause myelosuppression in mice and humans. The dose-intensive regimen was administered to NOD.Cg-Prkdc(scid)IL2rg(tm1Wjl)/Sz (NOD/SCID/γchain(null)), reconstituted with human hematopoietic cells, and the impact of treatment on human hematopoiesis was evaluated.

RESULTS

The dose-intensive regimen resulted in significant decreases in growth of human glioblastoma xenografts. When this regimen was administered to mice containing humanized bone marrow, flow cytometric analyses indicated that the human bone marrow cells were significantly more sensitive to treatment than the murine bone marrow cells and that the regimen was highly toxic to human-derived hematopoietic cells of all lineages (progenitor, lymphoid, and myeloid).

CONCLUSIONS

The humanized bone marrow xenograft model described has the potential to be used as a platform for monitoring the impact of anticancer therapies on human hematopoiesis and could lead to subsequent refinement of therapies prior to clinical evaluation.

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.

In vivo selection of human embryonic stem cell-derived cells expressing methotrexate-resistant dihydrofolate reductase

Human embryonic stem cells (hESCs) provide a novel source of hematopoietic and other cell populations suitable for gene therapy applications. Preclinical studies to evaluate engraftment of hESC-derived hematopoietic cells transplanted into immunodeficient mice demonstrate only limited repopulation. Expression of a drug resistance gene, such as Tyr22-dihydrofolate reductase (Tyr22-DHFR), coupled to methotrexate (MTX) chemotherapy has the potential to selectively increase engraftment of gene-modified hESC-derived cells in mouse xenografts. Here, we describe the generation of Tyr22-DHFR – GFP expressing hESCs that maintain pluripotency, produce teratomas and can differentiate into MTXr-hemato-endothelial cells. We demonstrate that MTX administered to nonobese diabetic/severe combined immunodeficient/IL-2Rγc^null (NSG) mice after injection of Tyr22-DHFR-derived cells significantly increases human CD34⁺ and CD45⁺ cell engraftment in the bone marrow (BM) and peripheral blood of transplanted MTX-treated mice. These results demonstrate that MTX treatment supports selective, long-term engraftment of Tyr22-DHFR-cells in vivo, and provides a novel approach for combined human cell and gene therapy.

In vivo selection of hematopoietic stem cells transduced at a low multiplicity-of-infection with a foamy viral MGMT(P140K) vector

OBJECTIVE

Using a clinically relevant transduction strategy, we investigated to what extent hematopoietic stem cells in lineage-negative bone marrow (Lin(neg) BM) could be genetically modified with an foamy virus (FV) vector that expresses the DNA repair protein, O(6)-methylguanine DNA methyltransferase (MGMT(P140K)) and selected in vivo with submyeloablative or myeloablative alkylator therapy.

MATERIALS AND METHODS

Lin(neg) BM was transduced at a low multiplicity-of-infection with the FV vector, MD9-P140K, which coexpresses MGMT(P140K) and the enhanced green fluorescent protein, transplanted into C57BL/6 mice, and mice treated with submyeloablative or myeloablative alkylator therapy. The BM was analyzed for the presence of in vivo selected, MD9-P140K-transduced cells at 6 months post-transplantation and subsequently transplanted into secondary recipient animals.

RESULTS

Following submyeloablative therapy, 55% of the mice expressed MGMT(P140K) in the BM. Proviral integration was observed in approximately 50% of committed BM-derived progenitors and analysis of proviral insertion sites indicated up to two integrations per transduced progenitor colony. Transduced BM cells selected with submyeloablative therapy reconstituted secondary recipient mice for up to 6 months post-transplantation. In contrast, after delivery of myeloablative therapy to primary recipient mice, only 25% survived. Hematopoietic stem cells were transduced because BM cells from the surviving animals reconstituted secondary recipients with MGMT(P140K)-positive cells for 5 to 6 months.

CONCLUSIONS

In vivo selection of MD9-P140K-transduced BM cells was more efficient following submyeloablative than myeloablative therapy. These data indicate that a critical number of transduced stem cells must be present to produce sufficient numbers of genetically modified progeny to protect against acute toxicity associated with myeloablative therapy.

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.

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.

In vitro erythropoiesis from bone marrow-derived progenitors provides a physiological assay for toxic and mutagenic compounds

The goal of this study was to create an in vitro cell culture system that captures essential features of the in vivo erythroid micronucleus (MN) genotoxicity assay, thus enabling increased throughput and controlled studies of the hematopoietic DNA damage response. We show that adult bone marrow (BM) cultures respond to erythropoietin, the principal hormone that stimulates erythropoiesis, with physiological erythropoietic proliferation, differentiation, and enucleation. We then show that this in vitro erythropoietic system clearly signals exposure to genotoxicants through erythroid MN formation. Furthermore, we determined that DNA repair-deficient (MGMT(-/-)) BM displayed sensitivity to genotoxic exposure in vivo compared with WT BM and that this phenotypic response was reflected in erythropoietic cultures. These findings suggest that this in vitro erythroid MN assay is capable of screening for genotoxicity on BM in a physiologically reflective manner. Finally, responses to genotoxicants during erythroid differentiation varied with exposure time, demonstrating that this system can be used to study the effect of DNA damage at specific developmental stages.

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.