Development of a modified selective amplifier gene for hematopoietic stem cell gene therapy

Ras-mediated up-regulation of survivin expression in cytokine-dependent murine pro-B lymphocytic cells

Survivin, a member of the inhibitor of apoptosis protein (IAP) family, has been widely studied because of its aberrant expression in human cancer. Survivin has multiple functions, including cell-cycle regulation at mitosis, inhibition of apoptosis and caspase-independent cytoprotection. Clinical studies have shown that survivin is associated with resistance to treatment and its expression is linked to poor prognosis. Recent studies indicated that Ras pathways up-regulate survivin expression in hematopoietic cells. Here we analyzed downstream pathways of Ras in interleukin-3 (IL-3)-dependent Baf-3 murine-derived pro-B lymphocytic cells that express constitutively active Ras mutants, using signaling pathway-specific inhibitors. Both mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3-K) pathways are involved in the induction of survivin. Downstream of PI3-K, the signaling pathway is composed of two kinases, Akt and mammalian target of rapamycin (mTOR) pathways. In the downstream targets of PI3-K, mTOR but not Akt is responsible for survivin expression. Using a counterflow centrifugal elutriator, we observed G2/M phase-dominant survivin expression in Baf-3 cells. Interestingly, constitutively active Ras mutants also induced survivin in a cell cycle-dependent manner. Reporter assays of the survivin gene promoter revealed a transcriptional regulatory cis-acting region that is responsible for Ras signaling, indicating that Ras increases the transcription of the survivin gene through specific enhancer elements. These data illustrate the pathways regulating survivin expression by Ras. Ras activates the MAPK, PI3-K and mTOR pathways, and these signals enhance survivin transcription. Our data will provide the new information about mechanisms of survivin expression by Ras-signalling pathways.

[Development and application of gene therapy technologies]

The success of hematopoietic stem cell gene therapy for X-linked severe combined immunodeficiency (X-SCID) was a major breakthrough in the field of gene therapy. However, two patients treated with this gene therapy developed leukemia at a later time, and retroviral vector-mediated gene transfer was considered to trigger leukemogenesis; i.e. insertional mutagenesis caused activation of LMO 2 gene, which was one step toward leukemia development. To cope with this serious problem, basic studies are required to improve the safety of retroviral vectors and to develop the method for site-specific integration of transgenes. In addition, we have to develop technologies such as selective amplifier genes (SAGs), the system for selective expansion of transduced cells, in order to obtain therapeutic efficacy of hematopoietic stem cell gene therapy in many other disorders. Moreover, clinical applications of AAV vector are promising from the standpoint of safety issue, because this vector is derived from non-pathogenic virus. AAV vector is appropriate for gene transfer into neurons, muscles, and hepatocytes. For example, gene therapy for Parkinson's disease is investigated using AAV vectors. Genetic manipulation is also one of the indispensable technologies in the field of regeneration medicine, and further promotion of basic research is important.

Expansion of genetically corrected neutrophils in chronic granulomatous disease mice by cotransferring a therapeutic gene and a selective amplifier gene

Hematopoietic stem cell gene therapy has not provided clinical success in disorders such as chronic granulomatous disease (CGD), where genetically corrected cells do not show a selective advantage in vivo. To facilitate selective expansion of transduced cells, we have developed a fusion receptor system that confers drug-induced proliferation. Here, a 'selective amplifier gene (SAG)' encodes a chimeric receptor (GcRER) that generates a mitotic signal in response to estrogen. We evaluated the in vivo efficacy of SAG-mediated cell expansion in a mouse disease model of X-linked CGD (X-CGD) that is deficient in the NADPH oxidase gp91phox subunit. Bone marrow cells from X-CGD mice were transduced with a bicistronic retrovirus encoding GcRER and gp91phox, and transplanted to lethally irradiated X-CGD recipients. Estrogen was administered to a cohort of the transplants, and neutrophil superoxide production was monitored. A significant increase in oxidase-positive cells was observed in the estrogen-treated mice, and repeated estrogen administration maintained the elevation of transduced cells for 20 weeks. In addition, oxidase-positive neutrophils were increased in the X-CGD transplants given the first estrogen even at 9 months post-transplantation. These results showed that the SAG system would enhance the therapeutic effects by boosting genetically modified, functionally corrected cells in vivo.

New selective amplifier genes containing c-Mpl for hematopoietic cell expansion

We previously developed "selective amplifier genes (SAGs)" which confer a growth advantage to transduced cells. The SAG is a chimeric gene encoding the G-CSF receptor (GCR) and the estrogen or tamoxifen (Tm) receptor and is able to expand transduced hematopoietic cells by treatment with estrogen or Tm. In the current study, we examined the in vitro efficacy of modified SAGs containing the thrombopoietin (TPO) receptor (c-Mpl) gene instead of GCR as a more potent signal generator. In addition, we constructed various mutant Mpl-type SAGs to abolish the responsiveness to endogenous TPO while retaining Tm-dependency. When Ba/F3 cells were retrovirally transduced with the Mpl-type SAGs, the cells showed Tm- and TPO-dependent growth even without IL-3. The Mpl-type SAGs induced more potent proliferation of Ba/F3 and cynomolgus CD34(+) cells than the GCR-type SAG. One mutant Mpl-type SAG (Delta GCRMplTmR) successfully lost the responsiveness to TPO without affecting the Tm-dependence.

Side effects of retroviral gene transfer into hematopoietic stem cells

Recent conceptual and technical improvements have resulted in clinically meaningful levels of gene transfer into repopulating hematopoietic stem cells. At the same time, evidence is accumulating that gene therapy may induce several kinds of unexpected side effects, based on preclinical and clinical data. To assess the therapeutic potential of genetic interventions in hematopoietic cells, it will be important to derive a classification of side effects, to obtain insights into their underlying mechanisms, and to use rigorous statistical approaches in comparing data. We here review side effects related to target cell manipulation; vector production; transgene insertion and expression; selection procedures for transgenic cells; and immune surveillance. We also address some inherent differences between hematopoiesis in the most commonly used animal model, the laboratory mouse, and in humans. It is our intention to emphasize the need for a critical and hypothesis-driven analysis of "transgene toxicology," in order to improve safety, efficiency, and prognosis for the yet small but expanding group of patients that could benefit from gene therapy.

In vivo expansion of transduced murine hematopoietic cells with a selective amplifier gene

BACKGROUND

Hematopoietic stem-cell-directed gene transfer has achieved limited success in transducing clinically relevant levels of target cells. The expansion of gene-modified cells is one way to circumvent the problem of inefficient transduction with current vectors. To this end, we have developed 'selective amplifier genes' (SAGs) that encode chimeric proteins that are a fusion of granulocyte colony-stimulating factor receptor and the steroid-binding domain. Prototype SAGs conferred estrogen-responsive growth on murine hematopoietic progenitors.

METHODS

We constructed a retroviral vector coexpressing an SAG for 4-hydroxytamoxifen (Tm)-specific proliferation and the enhanced green fluorescent protein (EGFP). Murine bone marrow cells were transduced with this vector and transplanted into myeloablated mice. Subsequently, recipients were challenged with Tm, and EGFP(+) cells were enumerated.

RESULTS

The challenge induced a significant increase in EGFP(+) leukocytes (21 +/- 4% to 27 +/- 5%), while EGFP(+) cells decreased in untreated animals (21 +/- 5% to 10 +/- 3%). Three months later, bone marrow cells were transplanted from the unchallenged mice to secondary hosts. Again the administration of Tm resulted in an increase of EGFP(+) cells (16 +/- 4% to 35 +/- 3%), contrasting to a decrease in controls (22 +/- 4% to 12 +/- 4%), and the difference was significant for more than 3 months. A detailed study of lineage showed a preferential expansion of EGFP(+) cells in granulocytes and monocytes following Tm administration.

CONCLUSIONS

Long-term repopulating cells were transduced with the SAG, and the transduced granulocyte/monocyte precursors were most likely to be expandable in vivo upon Tm stimulation.

Selective expansion of transduced cells for hematopoietic stem cell gene therapy

Although gene transfer into hematopoietic stem cells holds a considerable therapeutic potential, clinical trials targeting this cell compartment have achieved limited success. Poor transduction efficiency with gene transfer vectors used in human studies has hindered delivering therapeutic genes to clinically relevant numbers of target cells. One way to overcome the low-efficiency problem is by selecting or expanding the number of genetically modified cells to a suprathreshold level to achieve clinical efficacy. This approach may be further classified into 2 categories: one is to transfer a drug resistance gene and eliminate unmodified cells with cytotoxic drugs, and the other is to confer a direct growth advantage on target cells. This review aims at an overview of recent advances involving these strategies, with some details of "selective amplifier genes," a novel system that we have developed for specific expansion of genetically modified hematopoietic cells.

In vivo selective expansion of gene-modified hematopoietic cells in a nonhuman primate model

A major problem limiting hematopoietic stem cell (HSC) gene therapy is the low efficiency of gene transfer into human HSCs using retroviral vectors. Strategies, which would allow in vivo expansion of gene-modified hematopoietic cells, could circumvent the problem. To this end, we developed a selective amplifier gene (SAG) consisting of a chimeric gene composed of the granulocyte colony-stimulating factor (G-CSF) receptor gene and the estrogen receptor gene hormone-binding domain. We have previously demonstrated that primary bone marrow progenitor cells transduced with the SAG could be expanded in response to estrogen in vitro. In the present study, we evaluated the efficacy of the SAG in the setting of a clinically applicable cynomolgus monkey transplantation protocol. Cynomolgus bone marrow CD34(+) cells were transduced with retroviral vectors encoding the SAG and reinfused into each myeloablated monkey. Three of the six monkeys that received SAG transduced HSCs showed an increase in the levels of circulating progeny containing the provirus in vivo following administration of estrogen or tamoxifen without any serious adverse effects. In one monkey examined in detail, transduced hematopoietic progenitor cells were increased by several-fold (from 5% to 30%). Retroviral integration site analysis revealed that this observed increase was polyclonal and no outgrowth of a dominant single clonal population was observed. These results demonstrate that the inclusion of our SAG in the retroviral construct allows selective in vivo expansion of genetically modified cells by a non-toxic hormone treatment.

Comparison of bone marrow cells harvested from various bones of cynomolgus monkeys at various ages by perfusion or aspiration methods: a preclinical study for human BMT

Using cynomolgus monkeys, we have previously established a new method for harvesting bone marrow cells (BMCs) with minimal contamination of the BMCs with T cells from the peripheral blood. We originally conducted this new "perfusion method" in the long bones (the humerus, femur, and tibia) of cynomolgus monkeys. Here, we apply the perfusion method to obtain BMCs from the ilium of cynomolgus monkeys, since BMCs are usually collected from the ilium by the conventional aspiration method in humans. The perfusion method consists of two approaches: transverse iliac perfusion and longitudinal iliac perfusion. BMCs harvested by the perfusion method from the long bones and ilium were compared with those collected from the ilium by the aspiration method. The contamination of BMCs with peripheral blood, determined by the frequencies of CD4+ and CD8+ T cells, was significantly lower in BMCs obtained from the ilium or long bones by the perfusion method (CD4+ plus CD8+ T cells <4%) than in those obtained by the iliac aspiration method (CD4+ plus CD8+ T cells >20%). However, the numbers of immature myeloid cells, such as myeloblasts, promyelocytes, myelocytes, and metamyelocytes, were higher in BMCs obtained by the iliac perfusion method than in those obtained by the iliac aspiration method. The assays for in vitro colony-forming unit in culture revealed that progenitor activity was significantly higher in BMCs obtained by the perfusion method than in those obtained by the aspiration method. These findings suggest that the contamination of BMCs with peripheral blood is much less when using the perfusion method than when using the aspiration method. To determine the best site for harvesting BMCs by the perfusion method, age-dependent changes in BMCs harvested by the perfusion method from the long bones and ilium were examined. The numbers of BMCs varied in the long bones (humerus > femur > tibia) and showed age-dependent decreases, whereas they remained similar in the ilium of cynomolgus monkeys from 3 years to 6 years of age. However, in cynomolgus monkeys, BMC harvesting by the perfusion method from the ilium (but not from the long bones) is found to involve the risk of fat emboli, particularly when the BMCs are quickly perfused under high pressure. These findings suggest, even in humans, that the perfusion method is better than the aspiration method, and that the best site for collection of BMCs is the humerus.

Hematopoietic stem cell gene therapy

Gene therapy applications that target hematopoietic stem cells (HSCs) offer great potential for the treatment of hematologic disease. Despite this promise, clinical success has been limited by poor rates of gene transfer, poor engraftment of modified cells, and poor levels of gene expression. We describe here the basic approach used for HSC gene therapy, briefly review some of the seminal clinical trials in the field, and describe several recent advances directed toward overcoming these limitations.

A new method for bone marrow cell harvesting

To minimize contamination of bone marrow cells (BMCs) with T cells from the peripheral blood, a new "perfusion method" for collecting BMCs is proposed using cynomolgus monkeys. Two BM puncture needles are inserted into a long bone such as the humerus, femur, or tibia. One needle is connected to an extension tube and the end of the tube is inserted into a culture flask to collect the BM fluid. The other needle is connected to a syringe containing 30 ml of phosphate-buffered saline. The solution is pushed gently from the syringe into the medullary cavity, and the medium containing the BM fluid is collected into the culture flask. There is significantly less contamination with peripheral blood, determined from the frequencies of CD4(+) and CD8(+) T cells, when using this method (<6%) than when using the conventional method (>20%) consisting of multiple BM aspirations from the iliac crest. Furthermore, the number and progenitor activities of the cells harvested using this "perfusion method" are greater than those harvested using the conventional aspiration method. This perfusion method was carried out 42 times using 15 cynomolgus monkeys, and no complications such as pulmonary infarction or paralysis were observed. These findings suggest that the "perfusion method" is safe and simple and would be of great advantage in obtaining pure BMCs, resulting in a less frequent occurrence of acute graft-versus-host-disease in allogeneic BM transplantation.

Clinical Gene Therapy in Hematology: Past and Future

Gene transfer into hematopoietic cells using viral vectors has focused mostly on lymphocytes and hematopoietic stem cells (HSCs). HSCs have been considered particularly important as target cells because of their pluripotency and ability to reconstitute hematopoiesis after myeloablation and transplantation. HSCs are believed to have the ability to live a long time, perhaps a lifetime, in the recipient following bone marrow transplantation. Genetic correction of HSCs can therefore potentially last a lifetime and permanently cure hematologic disorders in which genetic deficiencies cause the pathology. Oncoretroviral vectors have been the main vectors used for HSCs because of their ability to integrate into the chromosomes of their target cells. Gene-transfer efficiency of murine HSCs is high using oncoretroviral vectors. In contrast, gene-transfer efficiency using the same viral vectors to transduce human HSCs or HSCs from large animals has been much lower. Although these difficulties may have several causes, the main reason for the low efficiency of human HSC transduction with oncoretroviral vectors is probably because of the nondividing nature of HSCs. Murine HSCs can be easily stimulated to divide in culture, whereas it is more problematic to stimulate human HSCs to divide rapidly in vitro. Because oncoretroviral vectors require dividing target cells for successful nuclear import of the preintegration complex and subsequent integration of the provirus, only the dividing fraction of the target cells can be transduced. This review focuses on gene transfer into human hematopoietic cells, particularly human HSCs. We review the clinical studies that have been reported, including the recent successful gene therapy for X-linked severe combined immunodeficiency. We discuss how the gene-transfer efficiency of human HSCs can be improved using oncoretroviral and lentiviral vectors.

Long-term tracking of murine hematopoietic cells transduced with a bicistronic retrovirus containing CD24 and EGFP genes

Hematopoietic stem cells (HSCs) are attractive targets for gene therapy, but current gene transfer methodologies are inadequate for efficient HSC transduction and perpetual transgene expression. To improve gene transfer vectors and transduction protocols, it is vital to establish a system to evaluate transgene expression and the long-term behavior of transduced cells in vivo. For this purpose, we constructed a bicistronic retrovirus encoding the human CD24 (as the first cistron) and the enhanced green fluorescent protein (EGFP; as the second cistron). Murine bone marrow cells were transduced with this vector and the transgene expression was monitored along with hematopoietic reconstitution. Stable expression of CD24 and EGFP was demonstrated in the long-term repopulating cells for at least 6 months, and multi-parameter flow cytometry illustrated expression of both markers in all the lymphohematopoietic lineages examined (B and T lymphoid, erythroid and myeloid). Sustained expression was also shown in the secondary transplants for 6 months, suggesting that self-renewing HSCs were transduced by this vector. Overall, EGFP-tagged bicistronic retroviruses would provide powerful tools for detailed in vivo analysis of transduced hematopoietic cells, such as transgene expression in conjunction with lineage differentiation. Gene Therapy (2000) 7, 1193-1199.