Improved post-transcriptional processing of an MDR1 retrovirus elevates expression of multidrug resistance in primary human hematopoietic cells

Towards in vivo amplification: Overcoming hurdles in the use of hematopoietic stem cells in transplantation and gene therapy

With the advent of safer and more efficient gene transfer methods, gene therapy has become a viable solution for many inherited and acquired disorders. Hematopoietic stem cells (HSCs) are a prime cell compartment for gene therapy aimed at correcting blood-based disorders, as well as those amenable to metabolic outcomes that can effect cross-correction. While some resounding clinical successes have recently been demonstrated, ample room remains to increase the therapeutic output from HSC-directed gene therapy. In vivo amplification of therapeutic cells is one avenue to achieve enhanced gene product delivery. To date, attempts have been made to provide HSCs with resistance to cytotoxic drugs, to include drug-inducible growth modules specific to HSCs, and to increase the engraftment potential of transduced HSCs. This review aims to summarize amplification strategies that have been developed and tested and to discuss their advantages along with barriers faced towards their clinical adaptation. In addition, next-generation strategies to circumvent current limitations of specific amplification schemas are discussed.

Interference of Frizzled 1 (FZD1) reverses multidrug resistance in breast cancer cells through the Wnt/β-catenin pathway

Multidrug resistance (MDR) represents a major obstacle in the successful treatment of breast cancer. The MDR1 gene is a direct target of the Wnt/β-catenin signaling pathway, which controls tumor development. Overexpression of P-glycoprotein, encoded by the MDR1 gene, is one of the most common causes of MDR. We found that the Frizzled 1 (FZD1) protein, which is an essential component of the Wnt/β-catenin pathway, is overexpressed in the multidrug resistant breast cancer cell subline MCF-7/ADM, coincident with MDR1/P-gp. FZD1 silencing induced down-regulation of MDR1/P-gp, restored sensitivity to four chemotherapy drugs, and significantly decreased cytoplasmic and nuclear β-catenin levels. FZD1 appears to mediate multidrug resistance by regulating the Wnt/β-catenin pathway.

Current advances in retroviral gene therapy

There have been major changes since the incidents of leukemia development in X-SCID patients after the treatments using retroviral gene therapy. Due to the risk of oncogenesis caused by retroviral insertional activation of host genes, most of the efforts focused on the lentiviral therapies. However, a relative clonal dominance was detected in a patient with β-thalassemia Major, two years after the subject received genetically modified hematopoietic stem cells using lentiviral vectors. This disappointing result of the recent clinical trial using lentiviral vector tells us that the current and most advanced vector systems does not have enough safety. In this review, various safety features that have been tried for the retroviral gene therapy are introduced and the possible new ways of improvements are discussed. Additional feature of chromatin insulators, co-transduction of a suicidal gene under the control of an inducible promoter, conditional expression of the transgene only in appropriate target cells, targeted transduction, cell type-specific expression, targeted local administration, splitting of the viral genome, and site specific insertion of retroviral vector are discussed here.

Retrovirus-mediated multidrug resistance gene (MDR1) overexpression inhibits chemotherapy-induced toxicity of granulosa cells

OBJECTIVE

To protect granulosa cells from chemotherapy-induced toxicity by retrovirus-mediated multidrug resistance (MDR1) gene transfection.

DESIGN

Laboratory study.

SETTING

Academic research laboratory in a university hospital.

PATIENT(S)

None.

INTERVENTION(S)

KK15 immortalized murine granulosa cell line transiently transduced with sf91m3 retrovirus vector carrying MDR1 complementary DNA that encodes P-glycoprtoein (P-gp); transduced cells selected with colchicine and treated with doxorubicin or paclitaxel for 24-72 hours; expression and function of MDR1 and the messenger RNA (mRNA) expression of selected steroidogenesis enzymes evaluated by flow cytometry, cell viability assays, Western blot, and reverse-transcriptase polymerase chain reaction (RT-PCR).

MAIN OUTCOME MEASURE(S)

Viability of sf91m3-transduced KK15 cells after treatment with doxorubicin and paclitaxel.

RESULT(S)

The sf91m3-transduced KK15 demonstrated high expression of biologically active MDR1, as shown by flow cytometry analysis and immunoblotting using P-gp monoclonal antibody and Rhodamine 123 efflux assays. The sf91m3-transduced KK15 exhibited statistically significant resistance to toxicity of 10 μM paclitaxel. The MDR1-transduced KK15 cells were also protected from doxorubicin toxicity (10 nM to 2.5 μM), as shown by cell viability assay. Both flow cytometry and cell viability assays showed that the protection of KK15 from doxorubicin toxicity was lost at 5 μM of doxorubicin; equivalent to 500 times LD50. The sf91m3-transduced KK15 showed normal mRNA expression of a panel of selected steroidogenesis enzymes.

CONCLUSION(S)

Retroviral gene delivery of human MDR1 inhibited chemotherapy-induced granulosa cell toxicity and offered chemoprotection in an in vitro model.

Chemoprotection of human hematopoietic stem cells by simultaneous lentiviral overexpression of multidrug resistance 1 and O(6)-methylguanine-DNA methyltransferase(P140K)

Myelotoxicity is a dose-limiting effect of many chemotherapeutic regimens. Thus, there is great interest in protecting human hematopoietic stem cells by the transfer of drug resistance genes. The main focus of this study was the simultaneous overexpression of multidrug resistance 1 (MDR1) and the O(6)-benzylguanine (O(6)-BG)-resistant mutant MGMT(P140K) (O(6)-methylguanine-DNA methyltransferase) with a bicistronic lentiviral vector (HR'SIN-MDR1-IRES-MGMT(P140K)), with regard to the capability to convey chemoprotection in the leukemia cell line, HL60, and human hematopoietic stem cells (CD34(+)). Combination therapy with O(6)-BG/1-(2-chloroethyl)-3-(4-amino-2-methylpyrimidine-5-yl)methyl-1-nitrosourea) (ACNU) plus paclitaxel showed a significant survival advantage of HL60 cells transduced with this combination vector. In CD34(+) cells, monotherapy with O(6)-BG/temozolomide (TMZ) resulted in an increased percentage of MGMT-positive cells (vs untreated cells) after transduction with HR'SIN-MDR1-IRES-MGMT(P140K) (28.3%). For combination therapy with O(6)-BG/temozolomide plus paclitaxel the increase was higher with the combination vector (52.8%) than with a vector expressing MGMT(P140K) solely (29.1%). With regard to MDR1-positive cells the protective effect of the combination vector (88.5%) was comparable to the single vector HR'SIN-MDR1 (90.0%) for monotherapy with paclitaxel and superior for combination therapy with O(6)-BG/temozolomide plus paclitaxel (84.6 vs 69.7%). In conclusion, the combination vector presents simultaneous protective effects of two drug-resistance genes, offering an opportunity to increase the cancer therapeutic index.

Chemoprotection by transfer of resistance genes

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

MDR1 gene transfer using a lentiviral SIN vector confers radioprotection to human CD34+ hematopoietic progenitor cells

Tumor radiotherapy with large-field irradiation results in an increase in apoptosis of the radiosensitive hematopoietic stem cells (CD34(+)). The aim of this study was to demonstrate the radioprotective potential of MDR1 overexpression in human CD34(+) cells using a lentiviral self-inactivating vector. Transduced human undifferentiated CD34(+) cells were irradiated with 0-8 Gy and held in liquid culture under myeloid-specific maturation conditions. After 12 days, MDR1 expression was determined by the rhodamine efflux assay. The proportion of MDR1-positive cells in cells from four human donors increased with increasing radiation dose (up to a 14-fold increase at 8 Gy). Determination of expression of myeloid-specific surface marker proteins revealed that myeloid differentiation was not affected by transduction and MDR1 overexpression. Irradiation after myeloid differentiation also led to an increase of MDR1-positive cells with escalating radiation doses (e.g. 12.5-16% from 0-8 Gy). Most importantly, fractionated irradiation (3 x 2 Gy; 24-h intervals) of MDR1-transduced CD34(+) cells resulted in an increase in MDR1-positive cells (e.g. 3-8% from 0-3 x 2 Gy). Our results clearly support a radioprotective effect of lentiviral MDR1 overexpression in human CD34(+) cells. Thus enhancing repopulation by surviving stem cells may increase the radiation tolerance of the hematopoietic system, which will contribute to widening the therapeutic index in radiotherapy.

Multiple displacement amplification enables large-scale clonal analysis following retroviral gene therapy

Analysis of the fate of retrovirally transduced cells after transplantation is often hampered by the scarcity of available DNA. We evaluated a promising method for whole-genome amplification, called multiple displacement amplification (MDA), with respect to even and accurate representation of retrovirally transduced genomic DNA. We proved that MDA is a suitable method to subsequently quantify engraftment efficiencies by quantitative real-time PCR by analyzing retrovirally transduced DNA in a background of untransduced DNA and retroviral integrations found in primary material from a retroviral transplantation model. The portion of these retroviral integrations in the amplified samples was 1.02-fold (range 0.2, to 2.1-fold) the portion determined in the original genomic DNA. Integration site analysis by ligation-mediated PCR (LM-PCR) is essential for the detection of retroviral integrations. The combination of MDA and LM-PCR showed an increase in the sensitivity of integration site analysis, as a specific integration site could be detected in a background of untransduced DNA, while the transduced DNA made up only 0.001%. These results show for the first time that MDA enables large-scale sensitive detection and reliable quantification of retrovirally transduced human genomic DNA and therefore facilitates follow-up analysis in gene therapy studies even from the smallest amounts of starting material.

Retroviral MDR1 gene transfer into marrow-engrafting human peripheral blood progenitor cells results in preferential transgene expression in the immature myeloid compartment rather than in mature myeloid progeny in vivo

BACKGROUND

The objective of multidrug resistance-1 (MDR1) gene therapy is protection of the myeloid cell lineage. It is therefore important to examine the effect of retroviral transduction on myeloid maturation. Transfer of the human MDR1 gene can confer resistance to a variety of cytostatic drugs. For a safe application in humans it is paramount to follow-up the development of transduced cells.

METHODS

We transduced human mobilized peripheral blood progenitor cells (PBPC) with a viral vector containing the human MDR1 cDNA and transplanted the transduced cells into non-obese diabetic severe combined immunodeficient (NOD/SCID) mice. The progeny of the transduced cells was analyzed in detail by flow cytometry.

RESULTS

A detailed analysis by four-color flow cytometry showed that MDR1 transgene-expressing CD33+ myeloid cells were preferentially negative for the maturation-associated myeloid markers CD11b and CD10, while the untransduced CD33+ myeloid cells expressed significantly higher proportions of these Ag (P<0.01 each). There was no difference in the expression of B- or T-lymphoid Ag among the MDR1-transduced and untransduced lymphoid cells.

DISCUSSION

These data indicate that retroviral MDR1 gene transfer results in preferential P-glycoprotein expression in myeloid progenitor cells, which is the target cell population for myelotoxicity of cytostatic drugs.

Gene transfer of cytidine deaminase protects myelopoiesis from cytidine analogs in an in vivo murine transplant model

Hematopoietic stem cell gene transfer of the drug-resistance gene cytidine deaminase (CDD) protecting cells from the cytotoxic cytidine analogs cytarabine and gemcitabine was investigated in a murine transplant model. Following transplantation of CDD-transduced cells and cytarabine application (500 mg/kg; days 1-4; intraperitoneally) significant myeloprotection was demonstrated with nadir counts of peripheral blood granulocytes and thrombocytes of 2.9 ± 0.6/nL versus 0.7 ± 0.1/nL (P < .001) and 509 ± 147/nL versus 80 ± 9/nL (P = .008), respectively (CDD versus control). Protection also was observed from otherwise lethal gemcitabine treatment (250 mg/kg; days 1-3). Stable levels of gene-marked cells in primary and secondary recipients were demonstrated for up to 9 months, and whereas CDD overexpression clearly reduced B- and T-lymphocyte numbers, no major toxicity was observed in the myeloid compartment. Despite the profound myeloprotective properties, however, CDD overexpression did not allow for pharmacologic enrichment of transduced hematopoiesis in our model. Thus, in summary, our data establish CDD as a drug-resistance gene highly suitable for myeloprotective purposes, which, given the lack of selection observed in our hands, might best be used in combination with selectable drugresistance genes such as MGMT (P140K) or MDR1.

Overexpression ofMDR1Using a Retroviral Vector Differentially Regulates Genes Involved in Detoxification and Apoptosis and Confers Radioprotection

Overexpression of P-glycoprotein (P-gp), the product of the MDR1 (multidrug resistance 1) gene, might complement chemotherapy and radiotherapy in the treatment of tumors. However, for safety and mechanistic reasons, it is important to know whether MDR1 overexpression influences the expression of other genes. Therefore, we analyzed differential gene expression in cells of the human lymphoblast cell line TK6 retrovirally transduced with MDR1 using the GeneChip Human Genome U133 Plus2.0 (Affymetrix). Sixty-one annotated genes showed a significant change in expression (P < 10(-4)) in MDR1-overexpressing cells compared to untransduced cells and cells transduced with a control virus expressing the neomycin phosphotransferase gene. Several genes coding for proteins involved in detoxification and exocytosis showed approximately 1.4- 4-fold increases in transcript levels (e.g. ALDH1A, UNC13). Additionally, pro-apoptosis genes were down-regulated (e.g. twofold for CASP1, 2.5-fold for NALP7) with concomitant increased expression of the potential anti-apoptosis gene AKT3. In functional assays the influence of MDR1 overexpression on apoptosis signaling was further corroborated by showing reduced rates of apoptosis in response to irradiation in TK6 cells transduced with MDR1. In conclusion, the resistant phenotype of MDR1-mediated P-gp-overexpressing cells is associated with differential expression of genes coding for metabolic and apoptosis-related proteins. These results have important implications for understanding the mechanisms by which MDR1 gene therapy can protect normal tissues from radiation- or chemotherapy-induced damage during tumor treatment.

Resistance to cytarabine and gemcitabine and in vitro selection of transduced cells after retroviral expression of cytidine deaminase in human hematopoietic progenitor cells

Overexpression of the detoxifying enzyme cytidine deaminase (CDD) renders normal and leukemic hematopoietic cells resistant to cytarabine (1-beta-D-arabinofuranosylcytosine), and studies on murine cells have suggested transgenic CDD overexpression as a way to reduce the substantial myelotoxicity induced by the deoxycytidine analogs cytarabine and gemcitabine (2',2'-difluorodeoxycytidine). We now have investigated CDD (over-)expression in the human hematopoietic system. Retroviral gene transfer significantly increased the resistance of CDD-transduced cord blood and peripheral blood-derived progenitor cells for doses ranging from 20-100 nM cytarabine and 8-10 nM gemcitabine. Protection was observed for progenitors of erythroid as well as myeloid differentiation, though the degree of protection varied for individual drugs. In addition, significant selection of CDD-transduced cells was obtained after a 4-day culture in 30-100 nM cytarabine. Thus, our data demonstrate that overexpression of CDD cDNA results in significant protection of human progenitors from cytarabine- as well as gemcitabine-induced toxicity, and allows in vitro selection of transduced cells. This strongly argues for a potential therapeutic role of CDD gene transfer in conjunction with dose-intensive cytarabine- or gemcitabine-containing chemotherapy regimen.

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

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

A novel gene transfer strategy that combines promoter and transgene activities for improved tumor cell inhibition

Typically, gene transfer strategies utilize a promoter/transgene arrangement that treat these elements independently and do not offer any interplay between them. Our goal was to establish a promoter/transgene combination that would result in improvement in both expression and therapeutic effect by utilizing the transcriptional properties of p53 to drive its own expression as well as act as a tumor suppressor. The pCL retroviral system was modified in the U3 region of the 3' LTR by the addition of a p53-responsive sequence (the PG element), creating the pCLPG system. Upon reverse transcription, the 5' LTR is converted, as shown here, to a p53-dependent promoter. We also show, using a temperature-sensitive model, that the pCLPG system could be driven by p53 encoded within the virus construct and expression was modulated depending on the p53 phenotype, demonstrating a regulatory feedback loop. Moreover, the pCLPG system was shown to express the transgene at a higher level and to inhibit tumor cell proliferation more robustly than the original pCL system. This novel system employs the transgene to serve two purposes, drive viral expression and inhibit tumor cell proliferation. The pCLPG vectors represent a new gene transfer strategy of synergizing the promoter and transgene activities.

Self-inactivating retroviral vectors with improved RNA processing

Three RNA features have been identified that elevate retroviral transgene expression: an intron in the 5' untranslated region (5'UTR), the absence of aberrant translational start codons and the presence of the post-transcriptional regulatory element (PRE) of the woodchuck hepatitis virus in the 3'UTR. To include such elements into self-inactivating (SIN) vectors with potentially improved safety, we excised the strong retroviral promoter from the U3 region of the 3' long terminal repeat (LTR) and inserted it either downstream or upstream of the retroviral RNA packaging signal (Psi). The latter concept is new and allows the use of an intron in the 5'UTR, taking advantage of retroviral splice sites surrounding Psi. Three LTR and four SIN vectors were compared to address the impact of RNA elements on titer, splice regulation and transgene expression. Although titers of SIN vectors were about 20-fold lower than those of their LTR counterparts, inclusion of the PRE allowed production of more than 10(6) infectious units per ml without further vector optimizations. In comparison with state-of-the-art LTR vectors, the intron-containing SIN vectors showed greatly improved splicing. With regard to transgene expression, the intron-containing SIN vectors largely matched or even exceeded the LTR counterparts in all cell types investigated (embryonic carcinoma cells, fibroblasts, primary T cells and hematopoietic progenitor cells).

Enhanced in vivo selection of bone marrow cells by retroviral-mediated coexpression of mutant O6-methylguanine-DNA-methyltransferase and HOXB4

To attain therapeutic levels of gene-modified hematopoietic stem cells, it may be necessary in the majority of disorders to provide an in vivo selective advantage that facilitates the expansion of their numbers. A popular strategy to achieve in vivo selection has been to employ drug selection while coexpressing a transgene that conveys chemoresistance, such as O6-methylguanine-DNA-methyltransferase (MGMT). An alternate approach is to confer an enhanced proliferative potential upon gene-modified hematopoietic stem cells through the delivery of the homeobox transcription factor HOXB4. By developing a novel tricistronic retroviral vector, we have facilitated the simultaneous coexpression of a mutant version of MGMT and HOXB4 in retrovirally transduced bone marrow. Using an in vivo competitive repopulation assay, we demonstrate that primary bone marrow cells containing this construct show enhanced reconstitution following transplant and improved selection subsequent to chemotherapeutic challenge in comparison to cells expressing either HOXB4 or MGMT alone. This selection advantage was evident even when HOXB4/MGMT-coexpressing cells were infused along with a large excess of unmodified cells. We propose that this selection cassette may facilitate the in vivo expansion of gene-modified hematopoietic stem cells at a level in excess of previous strategies.

Predictable and efficient retroviral gene transfer into murine bone marrow repopulating cells using a defined vector dose

OBJECTIVE

Current protocols of retroviral gene transfer into murine hematopoietic stem cells (HSC) result in variable gene transfer efficiency and involve various procedures that are not clinically applicable. We developed and evaluated a reliable transduction protocol that is more related to clinical methods.

MATERIALS AND METHODS

HSC were enriched from steady-state bone marrow by magnetic cell sorting (lineage depletion) and cultured in defined serum-free medium containing an improved growth factor cocktail (Flt3-ligand, stem cell factor, interleukin-3, interleukin-11). Cell-free ecotropic retroviral vector particles, generated by transient transfection of human 293T-based packaging cells, were preloaded at defined titers on CH296-coated tissue culture plates, thus largely avoiding serum contamination. These conditions were evaluated in 17 experiments involving 29 transduction cultures and 185 recipient mice.

RESULTS

After two rounds of infection, the gene marking rates in cultured mononuclear cells and stem/progenitor cells (Lin(-)c-Kit(+)) were 15 to 85% (53.7%+/-21.7%, n=23) and 30 to 95% (69.8%+/-20.4%, n=17), respectively. Even after one round of infection, gene transfer was efficient (31.2%+/-15.1%, n=12). Using identical conditions, gene transfer rates were highly reproducible. Average transgene expression in reconstituted animals correlated well with pretransplant data. Using a moderate multiplicity of infection, the majority of transduced cells carried less than three transgene copies. In addition, coinfection was possible to establish two different vectors in single cells.

CONCLUSION

The protocol described here achieves efficient retroviral transduction of murine bone marrow repopulating cells with a defined gene dosage, largely avoiding procedures that decrease stem cell output and repopulating capacity. This protocol may help to improve the predictive value of preclinical efficiency/toxicity studies for gene therapeutic interventions and basic research.

Prospects and RISC score of viral gene therapy for sarcoma

Soft tissue sarcomas are a challenge for medical oncology and gene therapy. Protective and sensitising approaches that target normal and malignant tissue, respectively, both have their role for opening the therapeutic window. Recent data show that an intensive maintenance chemotherapy significantly reduces metastatic spread and improves disease-free survival in selected patient groups. However, delays of treatment due to cytopenia are frequent. Cytostatic drug resistance gene transfer to haematopoietic progenitor cells using retroviral vectors may allow further improvement of therapy results. In recent years, retroviral vector design, transduction techniques and engraftment capability of transduced cells have been optimised. Safety considerations of retroviral gene transfer have attracted public attention and can be addressed by analysis of genomic vector integration sites. A data bank project, 'retroviral insertion estimate of chromosomal integration' (RISC), containing > 200 integration sequences, has been set up by the authors' group to recognise critical genomic regions and genes involved with possible transforming capacity. Monitoring these parameters will allow the selection of the most suitable vectors for clinical application. Sarcoma cells seem to be highly susceptible to a variety of vectors, such as recombinant adeno-associated virus-2 (rAAV-2) vectors, adenoviral vectors or oncolytic herpes simplex viruses. Results from the first clinical trials with adenoviral vectors encoding for cytokines are promising. The other systems await further development towards clinical applications. Perspectives for further research are discussed in this review.

Hematoprotection by transfer of drug-resistance genes

Myelosuppression represents a major side effect of cytotoxic anti-cancer agents. Infection due to granulocytopenia and the risk of bleeding due to thrombocytopenia compromise the potential of curative and palliative chemotherapy. Considering the many chemotherapeutic agents for which drug resistance genes have been described, and the recent improvements in vector and transduction technology, it seems conceivable that drug resistance gene transfer into a patient's autologous hematopoietic stem or progenitor cells will be able to reduce or abolish chemotherapy-induced myelosuppression.

Gene therapy: theoretical and bioethical concepts

Gene therapy holds great promise. Somatic gene therapy has the potential to treat a wide range of disorders, including inherited conditions, cancers, and infectious diseases. Early progress has already been made in the treatment of a range of disorders. Ethical issues surrounding somatic gene therapy are primarily those concerned with safety. Germline gene therapy is theoretically possible but raises serious ethical concerns concerning future generations.

Impact of splice-site mutations of the human MDR1 cDNA on its stability and expression following retroviral gene transfer

The multidrug resistance 1 (MDR1) gene transfer to hematopoietic cells for protection against cytotoxic drugs has received considerable attention in gene therapy. However, ectopic expression of MDR1 from retroviral vectors has been hampered by its genetic instability resulting from cryptic splice sites within the cDNA. We have evaluated the efficiency of retroviral MDR1 vectors with introduced mutations of the MDR1 cryptic splice donor (cSD) located at nucleotide +339 and of the cryptic splice acceptor (cSA) at nucleotide +2319 of the cDNA. Sequence alterations of the cSD reduced the expression of MDR1 P-glycoprotein (P-gp), even when generated as silent mutations. A silent mutation of the cSA reduced the splicing activity shifting the splice acceptor site one base downstream; however, it significantly improved the expression of P-gp. The incidence of wild-type MDR1 pregenome splicing was markedly reduced when vectors were produced in human 293 packaging cells as opposed to murine PG13 and GP+envAm12. We conclude that complete splice correction of MDR1 in retroviral vectors may only be achieved with extensive alterations of the cDNA or neighboring vector sequences and that the splicing is significantly influenced by the choice of the packaging cells.

Gene therapy for sarcoma

Soft tissue sarcomas are mesenchymal tumors which respond poorly to systemic therapy. Recent studies suggest a higher response rate with an increased doxorubicin dosage. However, this was parallel with a profound hematotoxicity in 75% of patients. Transfer of the human multidrug resistance 1 (MDR1) gene to normal hematopoietic stem cells and transplantation may significantly reduce the hematotoxicity of anthracyclin-based chemotherapy. To test this concept of supportive gene therapy in advance of a clinical study, we transduced mobilized peripheral blood progenitor cells (PBPC) with the retroviral vector SF91m3 containing the human MDR1 gene, transplanted these cells to immune-deficient mice, allowed 6 weeks for engraftment to occur and treated the animals with MDR1-based chemotherapy. In the MDR1-transduced group the human leukocytes were significantly protected from the toxicity of chemotherapy (p < 0.05). While the gene transfer rate was in the range of 10% and thus comparable to recent clinical trials, the gene expression was 59% of transduced cells and thus significantly higher than previously reported for less-advanced vectors. On the other hand, ifosfamide, a drug which has been used successfully for stem cell mobilization, is active in soft tissue sarcoma. Due to these favorable characteristics sarcoma is an attractive target to test the efficacy of MDR1 gene therapy in a clinical setting. Gene therapeutic strategies may also be used to directly target sarcoma cells, e.g. by transfer of suicide genes. We found that adenoassociated virus 2 (AAV-2) vectors efficiently transduce human HS-1 and HT1080 sarcoma cells (>90%) while other tumor cell lines and primary human PBPC were less susceptible. The thymidine kinase (TK) suicide gene was cloned into an AAV-2 vector and a complete kill of TK-transduced HS-1 and HT1080 cells was observed following exposure to aciclovir or ganciclovir (GCV), while >90% of mock-transduced HS-1 cells survived at these dosages. Transplantation of those sarcoma cells to nonobese diabetic (NOD)/LtSz-severe-combined immunodeficient (scid)/scid (NOD/SCID) mice resulted in a survival of >5 months in the AAV-TK-transduced/GCV-treated group, while the mice in the mock-transduced/GCV-treated group had died after 3 weeks. These data show that soft tissue sarcomas are a particularly suitable model system for the development and clinical testing of new gene therapeutic concepts.

Drug-selected co-expression of P-glycoprotein and gp91 in vivo from an MDR1-bicistronic retrovirus vector Ha-MDR-IRES-gp91

BACKGROUND

Retroviral transduction of human hematopoietic stem cells is an attractive strategy in gene therapy; however, transduction efficiency and duration of transgene expression may not be satisfactory in current protocols. Co-expression of a human multidrug resistance gene (MDR1) with a therapeutic gene affords selectable growth advantage to genetically modified cells.

METHODS

A bicistronic retrovirus vector, Ha-MDR-IRES-gp91, was constructed for the co-expression of MDR1 and gp91, a gene responsible for X-linked chronic granulomatous disease (X-CGD). Drug-selected co-expression of P-glycoprotein and gp91 was evaluated in transduced cells.

RESULTS

Epstein-Barr virus-transformed B cells from X-CGD patients transduced with Ha-MDR-IRES-gp91 co-expressed human P-glycoprotein and gp91, and acquired superoxide-generating activity. Human CD34-positive cells from an X-CGD patient were transduced with Ha-MDR-IRES-gp91 and subsequently treated with 2 ng/ml vincristine. After 13 days, 20% of Ha-MDR-IRES-gp91-transduced cells were P-glycoprotein- and gp91-positive by FACS analysis. The superoxide-generating activity of the transduced population was 27% of that of normal cells. Mice transplanted with Ha-MDR-IRES-gp91-transduced bone marrow cells showed co-expression of P-glycoprotein and gp91 in peripheral blood mononuclear cells. By administering paclitaxel, the proportions of P-glycoprotein- and gp91-positive cells were increased in all the four mice examined. When mice transplanted with Ha-MDR-IRES-gp91-transduced cells were repeatedly administered paclitaxel, the ratios of P-glycoprotein- and gp91-positive cells were maintained for over 1 year.

CONCLUSIONS

These results suggest that MDR1-bicistronic vectors may be useful to select the transduced hematopoietic cells in vivo. This may lead to the sustained expression of transgenes in the blood cells of patients treated with stem cell gene therapy.

Transfer of drug resistance genes in hematopoietic progenitors for chemoprotection: is it still an option?

For numerous malignancies a relationship between the intensity of antineoplastic chemotherapy and tumor response has been demonstrated. Myelotoxicity is the main cause of chemotherapy-associated morbidity and of treatment delays. The concept of myeloprotective cytostatic drug resistance gene transfer to normal hematopoietic stem cells (HSC) therefore sparks great enthusiasm. While initial studies using murine retroviral vectors on murine HSC showed that the concept works, a number of clinical studies in the last decade were not informative because of limitations in transduction efficiency and transgene expression.Furthermore, possible side effects such as unforeseen transgene activity and vector integration-based leukemogenesis have been reported. Among others, these developments raised some scepticism against the feasibility of myeloprotective gene transfer. Recently, considerable improvements have been achieved in vector design, HSC manipulation, selection protocols and risk assessment methods which are discussed in detail here. Based on these experimental studies successful clinical trials can now be anticipated.

Development and optimization of a real-time quantitative PCR-based method for the titration of AAV-2 vector stocks

Despite the clinical application of adeno-associated virus (AAV) gene therapy, the titration of viral stocks has not yet been standardized. This complicates the comparison of viral stocks between laboratories. Functional titering of AAV is time-consuming, requires the manipulation of hazardous material, and often has a high degree of variability. We established an optimized real-time quantitative polymerase chain reaction (RQ-PCR) titration assay to determine viral titers and compared it with a functional green fluorescent protein (GFP)-based titration method. With a combination of improved lysis procedures and RQ-PCR protocols we could decrease the intraexperimental coefficient of variation (CV) from 0.24 +/- 0.03 to 0.042 +/- 0.004 and the interexperimental CV from 0.34 +/- 0.06 to 0.093 +/- 0.028 following functional and RQPCR-based titration, respectively. This low variability conforms to even the strictest quality standards required, for example, in clinical laboratories. The highly standardized titration by RQPCR described here will be especially advantageous for groups working on AAV-based gene therapy in a good manufacturing practice setting.

Genetic protection of repopulating hematopoietic cells with an improved MDR1-retrovirus allows administration of intensified chemotherapy following stem cell transplantation in mice

This study was undertaken to analyze the hematotoxicity of paclitaxel (Taxol) and to test whether transduction of repopulating hematopoietic cells with a retroviral vector (SF1m) expressing the human multidrug resistance 1 gene (MDR1) would permit dose intensification following bone marrow transplantation (BMT). While the regimen chosen (8 x 20 mg/kg i.p. within 12 days) produced a non-lethal, reversible hematotoxicity in mice with steady-state hematopoiesis, only 35.3% (6/17) of control mice survived when treated starting 14 days post BMT. In contrast, 83.3% (15/18) of mice transplanted with SF1m-transduced cells survived, owing to a significant protection against severe acute myelotoxicity (as determined by neutrophil counts, white and red blood cell counts and values for hemoglobin and hematocrit). After recovery from chemotherapy, an increase of myeloid cells that were resistant to colchicine and effluxed the fluorochrome Rhodamine 123 was observed in SF1m-mice, but not in controls. These results reveal that the lethal, dose-limiting hematotoxicity of an intensified post-transplantation chemotherapy with paclitaxel can be prevented by retroviral transfer of the MDR1 gene to a minor proportion of repopulating cells. Our mouse model, mimicking clinically achievable gene transfer rates, thus suggests that bone marrow chemoprotection may widen the therapeutic window and permit an earlier onset of post-transplantation chemotherapy.

Multidrug resistance 1 gene transfer can confer chemoprotection to human peripheral blood progenitor cells engrafted in immunodeficient mice

Myelosuppression is the main side effect of cancer chemotherapy. An improved rate of retroviral vector-mediated gene transfer to hematopoietic stem cells, shown in more recent clinical trials, has created the basis to test the concept of myeloprotective gene therapy. We transplanted clinical-scale human peripheral blood progenitor cell grafts (n = 2) transduced with retroviral vector SF91m3, which contains the human multidrug resistance 1 gene (MDR1), into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. Engrafted mice of one cohort were protected from paclitaxel toxicity (p < 0.05) and we noted a similar trend in the second cohort. In paclitaxel-treated mice that had received gene-transduced cells we found a significant increase in gene marking (p < 0.05 - p < 0.01) or P-glycoprotein expression (p < 0.01) compared with their chemotherapy-naive counterparts. This is the first report showing that cytostatic drug resistance gene therapy can mediate chemoprotection of human clinically relevant stem cell populations with marrow engraftment potential.