Isolation, molecular cloning and in vitro expression of rhesus monkey (Macaca mulatta) prominin-1.s1 complementary DNA encoding a potential hematopoietic stem cell antigen

Human prominin-1 (CD133 or AC133) is an important cell surface marker used to isolate primitive hematopoietic stem cells. The commercially available antibody to human prominin-1 does not recognize rhesus prominin-1. Therefore, we isolated, cloned and characterized the complementary DNA (cDNA) of rhesus prominin-1 gene and determined its coding potential. Following the nomenclature of prominin family of genes, we named this cDNA as rhesus prominin-1.s1. The amino acid sequence data of the putative rhesus prominin-1.s1 could be used in designing antigenic peptides to raise antibodies for use in isolation of pure populations of rhesus prominin-1(+) hematopoietic cells. To the best of our knowledge, there has been no previously published report about the isolation of a prominin-1 cDNA from rhesus monkey (Macaca mulatta).

Loss of P-glycoprotein expression in hematopoietic stem cells does not improve responses to imatinib in a murine model of chronic myelogenous leukemia

Selective inhibition of the BCR/ABL tyrosine kinase by imatinib has become a first-line therapy for chronic myelogenous leukemia (CML). However, BCR/ABL-positive progenitors often persist despite treatment, and relapse associated with resistance to imatinib has been described in many patients with advanced disease. Drug efflux by P-glycoprotein (P-gp), as well as point mutations in BCR/ABL oncoprotein, has been implicated in the mechanism of resistance to imatinib. In this study, we established a murine transplantation model of CML-like myeloproliferative disease using Mdr1a/1b-null mice and analyzed the effects of loss of P-gp on resistance to imatinib. We found that mice transplanted with Mdr1a/1b-null bone marrow (BM) that had been transduced with a BCR/ABL retroviral vector displayed similar responses to imatinib, compared with those transplanted with BCR/ABL-transduced wild-type BM. In the absence of P-gp, the incidence and latency of disease in secondary recipients was not changed in imatinib-treated mice, relative to wild-type controls. Furthermore, K562 cells engineered to overexpress P-gp remained sensitive to imatinib-induced growth inhibition and cell death. Together, our findings suggest that P-gp expression in hematopoietic stem cells does not significantly contribute to imatinib resistance in CML.

The ABC transporter Bcrp1/ABCG2 is expressed in a wide variety of stem cells and is a molecular determinant of the side-population phenotype

Stem cells from bone marrow, skeletal muscle and possibly other tissues can be identified by the 'side-population' (SP) phenotype. Although it has been assumed that expression of ABC transporters is responsible for this phenotype, the specific molecules involved have not been defined. Here we show that expression of the Bcrp1 (also known as Abcg2 murine/ABCG2 human) gene is a conserved feature of stem cells from a wide variety of sources. Bcrp1 mRNA was expressed at high levels in primitive murine hematopoietic stem cells, and was sharply downregulated with differentiation. Enforced expression of the ABCG2 cDNA directly conferred the SP phenotype to bone-marrow cells and caused a reduction in maturing progeny both in vitro and in transplantation-based assays. These results show that expression of the Bcrp1/ABCG2 gene is an important determinant of the SP phenotype, and that it might serve as a marker for stem cells from various sources.

The effect of multidrug-resistance 1 gene versus neo transduction on ex vivo and in vivo expansion of rhesus macaque hematopoietic repopulating cells

Transduction of murine stem cells with a multidrug-resistance 1 gene (MDR1) retrovirus results in dramatic ex vivo and in vivo expansion of repopulating cells accompanied by a myeloproliferative disorder. Given the use of MDR1-containing vectors in human trials, investigations have been extended to nonhuman primates. Peripheral blood stem cells from 2 rhesus monkeys were collected, CD34-enriched, split into 2 portions, and transduced with either MDR1 vectors or neo vectors and continued in culture for a total of 10 days before reinfusion. At engraftment, the copy number in granulocytes was extremely high from both MDR vectors and neo vectors, but the copy number fell to 0.01 to 0.05 for both. There were no perturbations of the leukocyte count or differential noted. After 3 cycles of stem cell factor/granulocyte colony-stimulating factor, there were no changes in the levels of MDR1 vector- or neo vector-containing cells. There was no evidence for expansion of MDR1 vector-transduced cells. Long-term engraftment with MDR1 vector- and neo vector-transduced cells occurred despite prolonged culture.

Fibronectin fragment CH-296 inhibits apoptosis and enhances ex vivo gene transfer by murine retrovirus and human lentivirus vectors independent of viral tropism in nonhuman primate CD34+ cells

The fibronectin fragment CH-296 improved gene transfer to cytokine-mobilized nonhuman primate CD34+ cells irrespective of tropism to the MoMLV, GaLV, and VSV-G envelope proteins using murine stem cell virus (MSCV) and human immunodeficiency virus-1 (HIV-1)-based retrovirus vectors. For the HIV-1 lentivirus vector, CH-296 enhanced gene transfer in the absence of added hematopoietic growth factors necessary for ex vivo stem cell expansion. In the presence of CH-296, apoptosis of CD34+ cells was inhibited, and in mobilized peripheral blood CD34+ cells, cell division was stimulated as measured by cell history/tracking experiments.

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.

Self-selection by genetically modified committed lymphocyte precursors reverses the phenotype of JAK3-deficient mice without myeloablation

Janus kinase 3 (JAK3) is an essential component of cytokine receptor signal transduction pathways required for normal lymphocyte development and function. JAK3 deficiency in both mice and humans results in severe combined immunodeficiency (SCID) and increased susceptibility to opportunistic infections. We have previously shown that JAK3 gene transfer into irradiated recipients could restore immune function. However, since this toxic conditioning would be undesirable for infants in a clinical application, we have tested whether immune function could be restored in nonmyeloablated JAK3-deficient (-/-) mice. Murine JAK3 retroviral vectors were transduced into hematopoietic stem cells from the livers of newborn JAK3(-/-) mice. These cells were then injected intraperitoneally into nonirradiated JAK3(-/-) neonates. Transduced cells were detectable in these mice at time points 4 to 6 months after injection and resulted in significant correction of T and B lymphocyte numbers and circulating immunoglobulin (Ig) levels. After immune challenge with a dose of influenza A virus that was lethal to nonmanipulated JAK3(-/-) mice, mice injected with transduced cells showed development of circulating virus-specific IgG and enhanced survival. This work shows that the large selective advantage for JAK3-corrected lymphoid cells may be sufficient to overcome the need for myeloablative conditioning in JAK3 gene therapy protocols.

Retroviral vectors containing a variant dihydrofolate reductase gene for drug protection and in vivo selection of hematopoietic cells

Transfer of drug resistance genes to hematopoietic cells is being studied as a means to protect against the myelosuppression associated with cancer chemotherapy and as a strategy for the in vivo selection and amplification of genetically modified cells. The goal of this study was to test if retroviral-mediated gene transfer of a dihydrofolate reductase (DHFR) variant (L22Y) could be used for in vivo selection of transduced myeloid cells and to determine what proportion of transduced cells was required for protection from myelosuppression. Based on previous work suggesting that selection with antifolates may also require inhibition of nucleoside transport mechanisms, mice transplanted with DHFR-transduced bone marrow cells were treated with trimetrexate and the nucleoside transport inhibitor prodrug nitrobenzylmercaptopurine riboside phosphate. In vivo selection of transduced myeloid progenitors was seen in the bone marrow and in circulating mature peripheral blood cells following drug treatment. These results show that the novel combination of the L22Y-DHFR cDNA, trimetrexate and nitrobenzylmercaptopurine riboside phosphate can be used to select for transduced myeloid cells, and that this approach warrants further study in large animal models. A bicistronic vector containing a human CD24 reporter gene was used to determine the number of modified cells needed for chemoprotection. Partial protection from neutropenia was seen when greater than 10% of myeloid cells expressed the vector, and high levels of protection were obtained when the proportion exceeded 30%. These results suggest that gene transfer may be useful for myeloprotection in certain pediatric cancers, but that more efficient gene transfer will be required to apply this approach to adult cancer patients.

Inactivation of aldophosphamide by human aldehyde dehydrogenase isozyme 3

Tumors resistant to chemotherapeutic oxazaphosphorines such as cyclophosphamide often overexpress aldehyde dehydrogenase (ALDH), some isozymes of which catalyze the oxidization of aldophosphamide, an intermediate of cyclophosphamide activation, with formation of inert carboxyphosphamide. Since resistance to oxazaphosphorines can be produced in mammalian cells by transfecting them with the gene for human ALDH isozyme 3 (hALDH3), it seems possible that patients receiving therapy for solid tumors with cyclophosphamide might be protected from myelosuppression by their prior transplantation with autologous bone marrow that has been transduced with a retroviral vector causing overexpression of hALDH3. We investigated whether retroviral introduction of hALDH3 into a human leukemia cell line confers resistance to oxazaphosphorines. This was examined in the polyclonal transduced population, that is, without selecting out high expression clones. hALDH3 activity was 0.016 IU/mg protein in the transduced cells (compared with 2x10(-5) IU/mg in untransduced cells), but there was no detectable resistance to aldophosphamide-generating compounds (mafosfamide or 4-hydroperoxycyclophosphamide). The lack of protection was due, in part, to low catalytic activity of hALDH3 towards aldophosphamide, since, with NAD as cofactor, the catalytic efficiency of homogeneous, recombinant hALDH3 for aldophosphamide oxidation was shown to be about seven times lower than that of recombinant hALDH1. The two polymorphic forms of hALDH3 had identical kinetics with either benzaldehyde or aldophosphamide as substrate. Results of initial velocity measurements were consistent with an ordered sequential mechanism for ALDH1 but not for hALDH3; a kinetic mechanism for the latter is proposed, and the corresponding rate equation is presented.

Enforced P-glycoprotein pump function in murine bone marrow cells results in expansion of side population stem cells in vitro and repopulating cells in vivo

The human multidrug resistance-1 (MDR1) gene product, P-glycoprotein (P-gp), is well known for its ability to confer drug resistance; however, recent evidence suggests that P-gp expression can have more general effects on cellular development. In support of this idea, it was previously shown that retroviral-mediated MDR1 expression in murine bone marrow cells resulted in the expansion of stem cells in culture and in the development of a myeloproliferative syndrome in transplanted mice. It is now reported that MDR1-mediated stem cell expansion is associated with an increase in side population (SP) stem cells, defined by Hoechst dye staining. Transduction of murine bone marrow cells with an MDR1 retroviral vector resulted in an almost 2 log increase in SP cell numbers over 12 days in culture, whereas there was a rapid loss of SP cells from control cultures. Stem cell amplification was not limited to ex vivo expansion cultures but was also evident when MDR1-transduced cells were directly transplanted into irradiated mice. In these cases, stem cell expansion was associated with relatively high vector copy numbers in stem cell clones. As previously reported, some cases were associated with a characteristic myeloproliferative syndrome. A functionally inactive MDR1 mutant cDNA was used to show that P-gp pump function was required both for amplification of phenotypically defined SP cells and functionally defined repopulating cells. These studies further support the concept that ABC transporter function can have important effects on hematopoietic stem cell development.

In vitro mutations in dihydrofolate reductase that confer resistance to methotrexate: potential for clinical application

Mammalian cells cultured in the presence of the chemotherapeutic agent, methotrexate, develop resistance to this drug. Sometimes this is due to mutations in the gene for dihydrofolate reductase, the primary target of methotrexate. However, it has not been possible to link such polymorphism to resistance of neoplastic disease to therapy with methotrexate. Nevertheless, interest in this possibility lead to the introduction of many mutations into the cDNA for human DHFR by mutagenesis. Most of the corresponding enzyme variants have been expressed in Escherichia coli and characterized. Many mutations in codons for hydrophobic residues at the active site greatly decrease inhibition by methotrexate, and by the related substrate analogue, trimetrexate, while allowing the retention of considerable catalytic efficiency. Introduction of some of these mutants into mammalian cells by retroviral transfer provides substantial protection from toxic effects of the inhibitors, and has promise for the myeloprotection of patients receiving therapy with methotrexate or trimetrexate. Another potential use is in therapy for inherited disorders of hematopoiesis, where genetic modification of enough cells is a perennial problem. After transplantation of bone marrow that has been transduced with a bicistronic vector encoding both the mutant DHFR and a therapeutic gene, subsequent administration of methotrexate or trimetrexate should permit selection and enrichment of genetically modified hematopoietic cells.

Retroviral transfer of the hENT2 nucleoside transporter cDNA confers broad-spectrum antifolate resistance in murine bone marrow cells

Antifolate drugs such as methotrexate are commonly used in cancer chemotherapy. It may be possible to increase the antitumor activity of antifolates by the coadministration of drugs that inhibit nucleoside transport, thereby blocking the capacity of tumor cells to salvage nucleotide precursors. An important limitation of this approach is severe myelosuppression caused by many of these drug combinations. For this reason, we have developed a gene therapy strategy to protect bone marrow cells against combined treatment with antifolates and nitrobenzylmercaptopurine riboside (NBMPR), a potent inhibitor of the es nucleoside transporter. A retroviral vector (MeiIRG) was constructed that expressed the NBMPR-insensitive ei transporter, hypothesizing that transduced bone marrow cells would survive drug treatment because of the preservation of nucleoside salvage pathways. In vitro clonogenic assays confirmed that the MeiIRG vector did protect myeloid progenitors against the toxic effects of 3 different antifolates when each was combined with NBMPR. On testing this system in vivo, decreased myelosuppression was observed in mice transplanted with MeiIRG-transduced bone marrow cells and subsequently treated with trimetrexate and NBMPR-P. In these mice, significant increases were noted in absolute neutrophil count nadirs, reticulocyte indices, and the numbers of myeloid progenitors in the bone marrow. Furthermore, a survival advantage was associated with transfer of the MeiIRG vector, indicating that significant dose intensification was possible with this approach. In summary, the MeiIRG vector can decrease the toxicity associated with the combined use of antifolates and NBMPR-P and thereby may provide a strategy for simultaneously sensitizing tumor cells while protecting hematopoietic cells.

High levels of lymphoid expression of enhanced green fluorescent protein in nonhuman primates transplanted with cytokine-mobilized peripheral blood CD34(+) cells

We have used a murine retrovirus vector containing an enhanced green fluorescent protein complimentary DNA (EGFP cDNA) to dynamically follow vector-expressing cells in the peripheral blood (PB) of transplanted rhesus macaques. Cytokine mobilized CD34(+) cells were transduced with an amphotropic vector that expressed EGFP and a dihydrofolate reductase cDNA under control of the murine stem cell virus promoter. The transduction protocol used the CH-296 recombinant human fibronectin fragment and relatively high concentrations of the flt-3 ligand and stem cell factor. Following transplantation of the transduced cells, up to 55% EGFP-expressing granulocytes were obtained in the peripheral circulation during the early posttransplant period. This level of myeloid marking, however, decreased to 0.1% or lower within 2 weeks. In contrast, EGFP expression in PB lymphocytes rose from 2%-5% shortly following transplantation to 10% or greater by week 5. After 10 weeks, the level of expression in PB lymphocytes continued to remain at 3%-5% as measured by both flow cytometry and Southern blot analysis, and EGFP expression was observed in CD4(+), CD8(+), CD20(+), and CD16/56(+) lymphocyte subsets. EGFP expression was only transiently detected in red blood cells and platelets soon after transplantation. Such sustained levels of lymphocyte marking may be therapeutic in a number of human gene therapy applications that require targeting of the lymphoid compartment. The transient appearance of EGFP(+) myeloid cells suggests that transduction of a lineage-restricted myeloid progenitor capable of short-term engraftment was obtained with this protocol. (Blood. 2000;95:445-452)

Effects of retroviral-mediated MDR1 expression on hematopoietic stem cell self-renewal and differentiation in culture

Ex vivo expansion of hematopoietic stem cells would be useful for bone marrow transplantation and gene therapy applications. Toward this goal, we have investigated whether retrovirally-transduced murine stem cells could be expanded in culture with hematopoietic cytokines. Bone marrow cells were transduced with retroviral vectors expressing either the human multidrug resistance 1 gene (HaMDR1), a variant of human dihydrofolate reductase (HaDHFR), or both MDR1 and DHFR in an internal ribosomal entry site (IRES)-containing bicistronic vector (HaMID). Cells were then expanded for 15 days in cultures stimulated with interleukin (IL)-3, IL-6, and stem cell factor. When very low marrow volumes were injected into lethally irradiated recipient mice, long-term reconstitution with 100% donor cells was seen in all mice injected with HaMDR1- or HaMID-transduced cells. By contrast, engraftment with HaDHFR- or mock-transduced cells ranged from partial to undetectable despite injection of significantly larger marrow volumes. In addition, mice transplanted with expanded HaMDR1- or HaMID-transduced stem cells developed a myeloproliferative disorder that was characterized by an increase in abnormal peripheral blood leukocytes. These results show that MDR1-transduced stem cells can be expanded in vitro with hematopoietic cytokines, but indicate that an increased stem cell division frequency can lead to stem cell damage.

Virus-specific immunity after gene therapy in a murine model of severe combined immunodeficiency

Human severe combined immunodeficiency (SCID) can be caused by defects in Janus kinase 3 (JAK3)-dependent cytokine signaling pathways. As a result, patients are at high risk of life-threatening infection. A JAK3 -/- SCID mouse model for the human disease has been used to test whether transplant with retrovirally transduced bone marrow (BM) cells (JAK3 BMT) could restore immunity to an influenza A virus. The immune responses also were compared directly with those for mice transplanted with wild-type BM (+/+ BMT). After infection, approximately 90% of the JAK3 BMT or +/+ BMT mice survived, whereas all of the JAK3 -/- mice died within 29 days. Normal levels of influenza-specific IgG were present in plasma from JAK3 BMT mice at 14 days after respiratory challenge, indicating restoration of B cell function. Influenza-specific CD4(+) and CD8(+) T cells were detected in the spleen and lymph nodes, and virus-specific CD8(+) effectors localized to the lungs of the JAK3 BMT mice. The kinetics of the specific host response correlated with complete clearance of the virus within 2 weeks of the initial exposure. By contrast, the JAK3 -/- mice did not show any evidence of viral immunity and were unable to control this viral pneumonia. Retroviral-mediated JAK3 gene transfer thus restores diverse aspects of cellular and humoral immunity and has obvious potential for human autologous BMT.

In vivo selection of retrovirally transduced hematopoietic stem cells

One of the main impediments to effective gene therapy of blood disorders is the resistance of human hematopoietic stem cells to stable genetic modification. We show here that a small minority of retrovirally transduced stem cells can be selectively enriched in vivo, which might be a way to circumvent this obstacle. We constructed two retroviral vectors containing an antifolate-resistant dihydrofolate reductase cDNA transcriptionally linked to a reporter gene. Mice were transplanted with transduced bone marrow cells and then treated with an antifolate-based regimen that kills unmodified stem cells. Drug treatment significantly increased the percentage of vector-expressing peripheral blood erythrocytes, platelets, granulocytes, and T and B lymphocytes. Secondary transplant experiments demonstrated that selection occurred at the level of hematopoietic stem cells. This system for in vivo stem-cell selection provides a means to increase the number of genetically modified cells after transplant, and may circumvent an substantial obstacle to successful gene therapy for human blood diseases.

Transduction of murine bone marrow cells with an MDR1 vector enables ex vivo stem cell expansion, but these expanded grafts cause a myeloproliferative syndrome in transplanted mice

Attempts to expand repopulating hematopoietic cells ex vivo have yielded only modest amplification in stem cell numbers. We now report that expression of an exogenous human multi-drug resistance 1 (MDR1) gene enables dramatic ex vivo stem cell expansion in the presence of early acting hematopoietic cytokines. Bone marrow cells were transduced with retroviral vectors expressing either the MDR1 gene or a variant of human dihydrofolate reductase (DHFR), and then expanded for 12 days in the presence of interleukin-3 (IL-3), IL-6, and stem cell factor. When these cells were injected into nonirradiated mice, high levels of long-term engraftment were only seen with MDR1-transduced grafts. To verify that expansion of MDR1-transduced repopulating cells had occurred, competitive repopulation assays were performed using MDR1 expanded grafts. These experiments showed progressive expansion of MDR1-transduced repopulating cells over the expansion period, with a 13-fold overall increase in stem cells after 12 days. In all of the experiments, mice transplanted with expanded MDR1-transduced stem cells developed a myeloproliferative disorder characterized by high peripheral white blood cell counts and splenomegaly. These results show that MDR1-transduced stem cells can be expanded in vitro using hematopoietic cytokines without any drug selection, but enforced stem cell self-renewal divisions can have adverse consequences.

Protection of CCRF-CEM human lymphoid cells from antifolates by retroviral gene transfer of variants of murine dihydrofolate reductase

Expression of certain variants of dihydrofolate reductase (DHFR) in mammalian cells protects them from methotrexate. Retroviral transfer of the gene for such a variant DHFR into hematopoietic cells might permit selection of modified cells in vivo by antifolate administration or alleviate antifolate-induced myelosuppression in patients receiving antifolate therapy. We examined protection of cells of the human lymphoblastoid line, CCRF-CEM, transduced with variants of mouse DHFR. In transduced cells selected with G418 but not with antifolate, the variant that had arginine substituted for leucine 22 did not protect against either methotrexate or trimetrexate; however, four other variants did offer protection, with the best having leucine 22 changed to tyrosine. Polyclonal cultures transduced with the different variants express DHFR at about the same level, but clones within each polyclonal population differ in DHFR expression levels and in resistance. These differences in expression were shown to reflect different integration sites for proviral DNA. Exposure to trimetrexate selects highly resistant clones, with high expression due to both high copy number and integration sites that are favorable for expression. Differences in the resistance of cultures expressing different variants at the same level are due to differences in the catalytic activity of the expressed DHFR, its affinity for antifolates, and its stability.

Restoration of lymphocyte function in Janus kinase 3-deficient mice by retroviral-mediated gene transfer

Janus kinase-3 (JAK3) deficiency has recently been identified as a cause of severe combined immunodeficiency (SCID) in humans. We used a mouse model of Jak3-deficient SCID to test a gene therapy approach for treatment of this disease. Transfer of a Jak3 retroviral vector to repopulating hematopoietic stem cells resulted in increased numbers of T and B lymphocytes, reversal of hypogammaglobulinemia, restoration of T-cell activation upon stimulation with mitogens, and development of an antigen-specific immune response after immunization. Analysis for vector copy number in lymphoid and myeloid populations showed a large in vivo selective advantage for Jak3-expressing lymphoid cells. These results show that gene replacement is a feasible treatment strategy for this disease and that naturally occurring in vivo selection of corrected cells is an important advantage of this approach.

Comparison of the protection of cells from antifolates by transduced human dihydrofolate reductase mutants

Retroviral transduction of antifolate-resistant variants of human dihydrofolate reductase (hDHFR) into cells can increase their resistance to the cytotoxic effects of these drugs. We evaluated the ability of wild-type hDHFR and 20 mutant enzymes (13 with single-amino acid substitutions, 7 with two substitutions) to prevent growth inhibition in antifolate-treated CCRF-CEM cells. The wild-type enzyme and all of the variants significantly protected transduced cells from trimetrexate (TMTX)-induced growth inhibition. However, only half of the variants conferred more protection than does the wild-type enzyme. For the variants tested, the observed protective effect was higher for TMTX than for methotrexate (< or =7.5-fold increased resistance), piritrexim (< or =16-fold), and edatrexate (negligible). Transduction of the variants L22Y-F31S and L22Y-F31R led to the greatest protection against TMTX (approximately 200-fold). Protection from loss of cell viability was similar to protection from growth inhibition. The protection associated with a particular mutant hDHFR did not result from the level of expression: Efficient protection resulted from low affinity of the variant for antifolates, reasonable catalytic activity, and good thermal stability. Clones isolated from a polyclonal population of transduced cells varied by as much as 30-fold in their resistance to TMTX, the resistance differences depending on hDHFR expression levels.

Sensitization of hematopoietic stem and progenitor cells to trimetrexate using nucleoside transport inhibitors

Antifolates such as methotrexate (MTX) and trimetrexate (TMTX) are widely used in the treatment of cancer and nonmalignant disorders. Transient, yet sometimes severe myelosuppression is an important limitation to the use of these drugs. It has previously been shown that clonogenic myeloid progenitors and colony-forming units-spleen are resistant to antifolates, suggesting that myelotoxicity occurs late in hematopoietic development. The goal of this study was to define the mechanisms by which primitive hematopoietic cells resist the toxic effects of antifolate drugs. To test the hypothesis that myeloid progenitors may salvage extracellular nucleotide precursors to resist TMTX toxicity, a defined liquid culture system was developed to measure TMTX toxicity in expanding progenitor populations. These in vitro experiments showed that both human and murine progenitors can resist TMTX toxicity by importing thymidine and hypoxanthine from the serum. As predicted from these findings, several drugs that block thymidine transport sensitized progenitors to TMTX in vitro, although to differing degrees. These nucleoside transport inhibitors were used to test whether progenitors and hematopoietic stem cells (HSCs) could be sensitized to TMTX in vivo. Treatment of mice with TMTX and nitrobenzylmercaptopurineriboside phosphate (NBMPR-P), a potent transport inhibitor, caused significant depletions of clonogenic progenitors within the bone marrow (20-fold) and spleen (6-fold). Furthermore, NBMPR-P administration dramatically sensitized HSCs to TMTX, with dual-treated mice showing a greater than 90% reduction in bone marrow repopulating activity. These studies demonstrate that both myeloid progenitor cells and HSCs resist TMTX by using nucleotide salvage mechanisms and that these pathways can be pharmacologically blocked in vivo using nucleoside transport inhibitors. These results have important implications regarding the use of transport inhibitors for cancer therapy and for using variants of dihydrofolate reductase for in vivo selection of genetically modified HSCs.

A bicistronic retroviral vector for protecting hematopoietic cells against antifolates and P-glycoprotein effluxed drugs

Chemoresistance gene transfer is an experimental method to protect hematopoietic cells from the toxicity of anticancer drugs. Because multiple drugs are usually given together in cancer therapy, this strategy will ultimately require vectors expressing multiple chemoresistance genes. For this reason, we designed a bicistronic retroviral vector (HaMID) containing a modified human multidrug resistance-1 cDNA and a mutant human dihydrofolate reductase cDNA bearing a leucine to tyrosine substitution at codon 22 (L22Y). To determine if this vector would confer dual drug resistance to hematopoietic cells, recombinant retrovirus was used to transduce the human CEM T lymphoblastic cell line as well as primary murine myeloid progenitors. Growth suppression assays, using polyclonal transduced CEM cells, demonstrated increased resistance to taxol (13-fold), trimetrexate (8.9-fold), vinblastine (5.6-fold), methotrexate (2.5-fold), and etoposide (1.5-fold) when used as single agents. HaMID-transduced cells also grew at a logarithmic rate in the simultaneous presence of 25 nM taxol and 100 nM trimetrexate while control cells were entirely growth inhibited by this drug combination. Similarly, HaMID-transduced murine myeloid progenitors acquired increased resistance to taxol (2.9-fold) and trimetrexate (140-fold), and were able to form colonies in the simultaneous presence of both drugs. Our results suggest that retroviral transfer of HaMID into primary hematopoietic cells should reduce the myelosuppression associated with the combined use of antifolates and P-glycoprotein-effluxed drugs.

Retroviral-mediated transfer of the green fluorescent protein gene into murine hematopoietic cells facilitates scoring and selection of transduced progenitors in vitro and identification of genetically modified cells in vivo

We have investigated the utility of the green fluorescent protein (GFP) to serve as a marker to assess retroviral gene transfer into hematopoietic cells and as a tool to identify and enrich for cells expressing high levels of the vector-encoded transcript. GFP, by virtue of a naturally occurring chromophore encoded in its primary sequence, displays autonomous fluorescence, thus eliminating the need for antibody or cytochemical staining to detect its expression. A bicistronic murine stem cell virus (MSCV)-based retroviral vector was constructed containing the GFP cDNA and a mutant, human dihydrofolate reductase gene. High-titer, ecotropic retroviral producer cells free of replication competent virus were generated and used to transduce murine bone marrow cells by cocultivation. Within 24 hours after completion of the transduction procedure, a high proportion (40% to 70%) of the marrow cells were intensely fluorescent compared to mock-transduced cells or cells transduced with a control retrovirus. Erythroid and myeloid hematopoietic colonies derived from GFP-transduced marrow were easily scored for retroviral gene transfer by direct in situ fluorescence microscopy. Clonogenic progenitors expressing increased levels of antifolate drug resistance could be enriched from the GFP-transduced marrow population by fluorescence activated cell sorting of cells expressing high levels of GFP. In vivo, splenic hematopoietic colonies and peripheral blood cells from animals transplanted with GFP-transduced marrow displayed intense fluorescence. These results show that GFP is an excellent marker for scoring and tracking gene-modified hematopoietic cells and for allowing rapid selection and enrichment of transduced cells expressing high levels of the transgene.

Coding region-specific destabilization of mRNA transcripts attenuates expression from retroviral vectors containing class 1 aldehyde dehydrogenase cDNAs

Class 1 aldehyde dehydrogenases (ALDH-1) function as drug resistance gene products by catalyzing the irreversible conversion of aldophosphamide, an active metabolite of cyclophosphamide, to an inert compound. Because the dose-limiting toxicity of cyclophosphamide is myelosuppression, retrovirus-mediated transfer of ALDH-1 to bone marrow cells has been proposed as a protective strategy. Here we show that expression of ALDH-1 vectors was problematic due to low levels of ALDH-1 mRNA accumulation. A number of vectors containing several different ALDH-1 cDNAs were introduced into a variety of different cell lines either by transfection or transduction. Detectable ALDH-1 protein and enzyme activity was only seen in one transfected cell clone. Cells transduced with ALDH-1 retroviral vectors had no detectable protein expression and very low levels of ALDH-1 mRNA. Analogous vectors containing other drug resistance cDNAs led to much higher levels of steady-state mRNA. The mRNA half-life from ALDH-1 vectors was less than 2 hr suggesting that vector-derived mRNAs were destabilized by ALDH-1 coding sequences. These results suggest that methods which increase the stability of ALDH-1 mRNAs will be important for increased drug resistance in retrovirally transduced hematopoietic cells.

A gene transfer strategy for making bone marrow cells resistant to trimetrexate

Trimetrexate (TMTX) is an anticancer drug with potential advantages over the more commonly used antifolate, methotrexate (MTX); however, its use has been limited by severe myelosuppression. Retroviral vectors containing mutant dihydrofolate reductase (DHFR) genes have been used to protect bone marrow cells from MTX, suggesting a similar approach could be used for TMTX. We first screened six variants of human DHFR to determine which allowed maximal TMTX resistance in fibroblasts. A variant enzyme containing a Leu-to-Tyr mutation in the 22nd codon (L22Y) was best, allowing a 100-fold increase in resistance over controls. Murine hematopoietic progenitor cells transduced with an L22Y-containing retroviral vector also showed high-level TMTX resistance in vitro. Mice reconstituted with L22Y-transduced bone marrow cells were challenged with a 5-day course of TMTX to determine whether hematopoiesis could be protected in vivo. Transfer of the L22Y vector resulted in consistent protection from TMTX-induced neutropenia and reticulocytopenia at levels that correlated with the proviral copy number in circulating leukocytes. We conclude that the L22Y vector is highly effective in protecting hematopoiesis from TMTX toxicity and may provide a means for increasing the therapeutic utility of TMTX in certain cancers.

Mutations in the gene for human dihydrofolate reductase: an unlikely cause of clinical relapse in pediatric leukemia after therapy with methotrexate

Resistance to methotrexate (MTX) in some sublines of mammalian cells is reported to be due to one of the following amino acid substitutions in dihydrofolate reductase (DHFR) that lower inhibition by MTX: Gly15 to Trp, Leu22 to Arg or Phe or Phe31 to Trp or Ser. We have produced variants of human DHFR (hDHFR) with these substitutions by directed mutagenesis. Recombinant hDHFR variants expressed in Escherichia coli have greatly decreased inhibition by MTX, but decreased catalytic efficiency, and in one case decreased stability. When a retroviral vector encoding wild-type (wt) hDHFR or one of these variants was introduced into murine fibroblasts or bone marrow progenitors, modest protection from MTX was conferred, even by wt. Relapsed pediatric patients with acute lymphoblastic leukemia who have received multiple courses of high-dose MTX seem most likely to develop such MTX resistance. cDNA was reverse transcribed from blast mRNA from 17 of these patients. However, upon amplification and sequencing of DHFR cDNA, no resistance mutation was found. The explanation for this probably lies in the need for considerable gene amplification to offset lowered catalytic efficiency, and the need for two-base changes for most substitutions, both of which are probably infrequent events.

Gene therapy in pediatric oncology

An increased understanding of the molecular mechanisms of cancer and the ability to introduce exogenous genes into mammalian cells has led to the development of oncologic treatment strategies based upon gene transfer. Preclinical animal models have suggested a variety of approaches which are now being tested in pediatric trials. Studies using marker genes to trace cell origin have already generated important information regarding autologous bone marrow transplantation for pediatric cancers. A variety of therapeutic genes are also being clinically tested. Trials are underway to determine if introduction of immunostimulatory genes into cancer cells can be used to enhance host antitumor immunity. Treatment of primary brain tumors with insertion of drug sensitization genes is a promising new therapy that is also being clinically evaluated. Other strategies such as insertion of drug resistance genes into hematopoietic cells, anti-oncogene therapy, and tumor suppressor gene replacement are being tested in adults and may find use in pediatric cancer treatment. Although gene transfer offers promising new approaches for the therapy of pediatric cancer, many technical problems remain which limit efficacy and widespread use. Further basic research in the molecular biology of cancer and in vector development will be required to realize the full potential of gene therapy strategies.

Retroviral transfer of a bacterial alkyltransferase gene into murine bone marrow protects against chloroethylnitrosourea cytotoxicity

The chloroethylnitrosoureas (CENUs) are important antineoplastic drugs for which clinical utility has been restricted by the development of severe delayed myelosuppression in most patients. To investigate the potential of DNA repair proteins to reduce bone marrow sensitivity to the CENUs, we transferred the Escherichia coli ada gene, which encodes a Mr 39,000 O6-alkylguanine-DNA alkyltransferase (ATase), into murine bone marrow cells by the use of a high-titer ecotropic retrovirus. The ada-encoded ATase is resistant to O6-benzylguanine (O6-BG), a potent inhibitor of the mammalian ATases, thus affording the bone marrow an additional level of protection against CENUs. In methylcellulose cultures, ada-infected hematopoietic progenitor cells were twice as resistant as uninfected cells to the toxic effects of 1, 3-bis(2-chloroethyl)-1-nitrosourea (BCNU) following treatment with O6-BG. Although showing no obvious protective effects against leukopenia, overexpression of the bacterial ATase activity reduced the severity of anemia and thrombocytopenia in mice treated with O6-BG and BCNU. These effects, which were maximal at a BCNU dose of 12.5 mg/kg, were associated with improved survival when BCNU was given at this dose. At lower BCNU doses cytotoxicity was limited in both transduced and control mice, and at higher doses the protective effect was saturated due to cytotoxicity. These results suggest that ada gene therapy may be a feasible approach to amelioration of delayed myelosuppression following O6-BG plus CENU combination chemotherapy.

Expression of retroviral vectors containing the human multidrug resistance 1 cDNA in hematopoietic cells of transplanted mice

Transfer of the human multidrug resistance 1 (MDR1) gene to hematopoietic stem cells offers an approach to overcome the myelosuppression caused by a number of antineoplastic drugs. This study was designed to determine the effect of MDR1 gene transfer on overall P-glycoprotein (P-gp) expression in murine hematopoietic cells. Mice were transplanted with bone marrow cells infected with either of two different MDR1 retroviral vectors. A reverse-transcriptase polymerase chain reaction-based assay was used to quantify expression levels of both endogenous and vector-derived P-gp encoding transcripts in hematopoietic cells of transplanted mice. Expression of both a truncated and full-length MDR1 mRNA species was noted in bone marrow and spleen colony cells. The truncated message resulted from cryptic mRNA splice sites within the MDR1 cDNA and was detected with both vectors. Full-length message levels exceeded those from the endogenous genes in all but one case and roughly approximated that seen in the modestly drug-resistant cell line SW620. We conclude that transfer of MDR1 retroviral vectors resulted in a significant increase in P-gp expression in most cases; however, aberrant splicing of MDR1 transcripts can result in reduced expression of vector-derived P-gp.

Pluripotent hematopoietic stem cells contain high levels of mRNA for c-kit, GATA-2, p45 NF-E2, and c-myb and low levels or no mRNA for c-fms and the receptors for granulocyte colony-stimulating factor and interleukins 5 and 7

Pluripotent hematopoietic stem cells (PHSCs) were highly enriched from mouse bone marrow by counterflow centrifugal elutriation, lineage subtraction, and fluorescence-activated cell sorting based on high c-kit receptor expression (c-kitBR). We used reverse transcriptase polymerase chain reaction to assay the c-kitBR subset and the subsets expressing low (c-kitDULL) and no (c-kitNEG) c-kit receptor for expression of mRNA encoding hematopoietic growth factor receptors and transcription factors. The c-kitBR cells had approximately 3.5-fold more c-kit mRNA than unfractionated bone marrow cells. The c-kitDULL cells had 47-58% of the c-kit mRNA found in c-kitBR cells and the c-kitNEG cells had 4-9% of the c-kit mRNA present in c-kitBR cells. By comparing mRNA levels in c-kitBR cells (enriched for PHSCs) with those of unfractionated bone marrow, we demonstrated that c-kitBR cells contained low or undetectable levels of mRNA for c-fms, granulocyte colony-stimulating factor receptor, interleukin 5 receptor (IL-5R), and IL-7R. These same cells had moderate levels of mRNA for erythropoietin receptor, IL-3R subunits IL-3R alpha (SUT-1), AIC-2A, and AIC-2B, IL-6R and its partner gp-130, and the transcription factor GATA-1 and high levels of mRNA for transcription factors GATA-2, p45 NF-E2, and c-myb. We conclude from these findings that PHSCs are programmed to interact with stem cell factor, IL-3, and IL-6 but not with granulocyte or macrophage colony-stimulating factor. These findings also indicate that GATA-2, p45 NF-E2, and c-myb activities may be involved in PHSC maintenance or proliferation.

Gene transfer into human hemopoietic progenitor cells

Considerable progress is being made in the transfer of genetic material to hematopoietic stem cells. In this chapter we describe how gene transfer is being used to: mark marrow and peripheral blood progenitor cells prior to autologous transplantation, to track their fate on reinfusion and to detect contaminating tumorigenic cells; modulate immunocyte function--important in immunologic disorders and perhaps in cancer therapy; generate tumor vaccines from tumor cells isolated from marrow; correct single gene defects--the 'classical' concept of gene therapy; and finally to modify the drug sensitivity of progenitor cells--enabling them to resist the suppressive effects of cytotoxic drugs during cancer therapy and perhaps providing a mechanism for in vivo selection of gene modified cells.

Retroviral mediated transfer of the human multidrug resistance gene (MDR-1) into hematopoietic stem cells during autologous transplantation after intensive chemotherapy for metastatic breast cancer

Patients with metastatic breast cancer will receive 4-5 cycles of induction chemotherapy on one of the ongoing Medicine Branch protocols. Patients achieving at least a partial response, and who do not have evidence of bone marrow involvement and who do not have metastatic bone disease, will undergo PBSC and bone marrow harvest when hematologic recovery has occurred. Patients who have not achieved a PR, but who are responding to therapy, may be treated with additional cycles of therapy in an attempt to achieve a PR. Such patients will be eligible for transplant if a PR is obtained. 70% of the bone marrow and PBSC will be cryopreserved. The CD34+ subpopulation from the remaining 30% of the bone marrow and PBSC harvest will be obtained using an anti-CD34+ antibody and immunoabsorption column. The bone marrow and peripheral blood CD34 cells will be transduced with a retroviral vector expressing the human MDR-1 cDNA. Patients with positive bone scans or histologic evidence of bone marrow involvement will be excluded from the gene transfer component of the protocol. The MDR-1 transduced CD34 cells will be reinfused along with the non-transduced bone marrow and PBSC into patients following high dose ICE chemotherapy. Serial peripheral blood and bone marrow samples will be obtained to study hematopoietic reconstitution with MDR-1 transduced cells. Patients with residual or progressive disease after ABMT will be treated with taxol or vinblastine. In these relapsed patients, peripheral blood and bone marrow samples will be obtained to study whether chemotherapy amplifies the proportion of hematopoietic cells containing the MDR-1 provirus. We will monitor the nadir blood counts of each patient receiving salvage chemotherapy for evidence of myeloprotection and correlate this data with changes in the mean proviral copy number. Sites of relapsed tumor will be biopsied to test for the presence of the MDR-1 provirus.

Selection of drug-resistant bone marrow cells in vivo after retroviral transfer of human MDR1

Experiments were performed to determine if retroviral-mediated transfer of the human multidrug resistance 1 gene (MDR1) into murine bone marrow cells would confer drug resistance to the cells and whether the MDR1 gene could be used as a dominant selectable marker in vivo. When mice transplanted with bone marrow cells containing a transferred MDR1 gene were treated with the cytotoxic drug taxol, a substantial enrichment for transduced bone marrow cells was observed. This demonstration of positive selection establishes the ability to amplify clones of transduced hematopoietic cells in vivo and suggests possible applications in human therapy.

Inducibility of the HS II enhancer depends on binding of an erythroid specific nuclear protein

An erythroid specific, inducible enhancer associated with hypersensitive site II (HS II) plays a central role in the function of the human beta globin dominant control region. The HS II enhancer consists of tandem AP-1 binding sites and has been shown to bind members of the ubiquitous jun and fos families of proteins. The same sites are now shown to bind the erythroid specific protein, NF-E2. Inducibility of the HS II enhancer depends on NF-E2 binding, even in the presence of another hypersensitive site. Further, increased activity of the enhancer in induced K562 cells correlates with the presence of NF-E2, which appears to be present in a modified form. NF-E2 is distinct from some enhancer binding proteins in K562 nuclear extracts, in that it does not contain Fos or Fra-1 protein. Thus, binding by NF-E2 may be the mechanism, whereby tandem AP-1 binding sites confer erythroid specificity on the HS II enhancer.

Tandem AP-1-binding sites within the human beta-globin dominant control region function as an inducible enhancer in erythroid cells

A powerful enhancer has been mapped to an 18-bp DNA segment located 11 kb 5' to the human epsilon-globin gene within the dominant control or locus-activating region. This enhancer is inducible in K562 human erythroleukemia cells, increasing linked gamma-globin promoter/luciferase gene expression to 170-fold over an enhancerless construct. The enhancer consists of tandem AP-1-binding sites, phased 10 bp apart, which are both required for full activity. DNA-protein binding assays with nuclear extracts from induced cells demonstrate a high molecular weight complex on the enhancer. The formation of this complex also requires both AP-1 sites and correlates with maximal enhancer activity. Induction of the enhancer may have a role in the increase in globin gene transcription that characterizes erythroid maturation. Enhancer activity appears to be mediated by the binding of a complex of proteins from the jun and fos families to tandem AP-1 consensus sequences.