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

Retroviral coexpression of a multidrug resistance gene (MDR1) and human alpha-galactosidase A for gene therapy of Fabry disease

Human alpha-galactosidase A (alpha-Gal A; EC.3.2.1.22) is a lysosomal exoglycosidase encoded by a gene on Xq22. Deficiencies of this enzyme result in Fabry disease, an X-chromosome-linked recessive disorder that leads to premature death in affected males. For treatment of genetic diseases, we have developed a retroviral vector system, pSXLC/pHa, that enables coexpression of drug-selectable markers with a second nonselectable gene as part of a bicistronic message using the promoter from the Harvey murine sarcoma virus and an internal ribosomal entry site (IRES) from encephalomyocarditis virus. Retroviral vectors based on this system that carry the human alpha-Gal A cDNA either upstream (pHa-alpha Gal-IRES-MDR) or downstream (pHa-MDR-IRES-alpha Gal) from the IRES relative to the drug-selectable MDR1 (P-glycoprotein) cDNA were constructed. Each of eight independent vincristine-resistant, pHa-alpha Gal-IRES-MDR-transfected clones and all four vincristine-resistant, pHa-alpha Gal-IRES-MDR retrovirus-transduced clones showed significantly higher activity of alpha-Gal A than the parental cells. More than 50% of the vincristine-resistant, pHa-MDR-IRES-alpha Gal-transfected clones and all four vincristine-resistant, pHa-MDR-IRES-alpha Gal retrovirus-transduced clones showed significantly higher activity of alpha-Gal A than the parental cells. In these bicistronic vectors, the cDNA whose translation was cap-dependent (upstream) was expressed at higher levels than when the same cDNA was translated in an IRES-dependent manner (downstream). These vectors may prove useful in the gene therapy of Fabry disease.

Treatment of experimental human mesothelioma using adenovirus transfer of the herpes simplex thymidine kinase gene

OBJECTIVE

The authors demonstrate the ability of an adenovirus vector expressing the herpes simplex thymidine kinase (HSVtk) gene to treat human malignant mesothelioma growing within the peritoneal cavity of severe combined immunodeficient (SCID) mice.

BACKGROUND DATA

Introduction of the HSVtk gene into tumor cells renders them sensitive to the antiviral drug ganciclovir (GCV). This approach has been used previously to treat experimental brain tumors. Although malignant mesothelioma is refractory to current therapies, its localized nature and the accessibility of the pleural space make it a potential target for a similar type of in vivo gene therapy using adenovirus.

METHODS

An adenovirus containing the HSVtk gene (Ad.RSVtk) was used to transduce mesothelioma cells in vitro. These cells were then injected into the flanks of SCID mice. Ad.RSVtk was also injected directly into the peritoneal cavity of SCID mice with established human mesothelioma tumors. Mice were subsequently treated for 7 days with GCV at a dose of 5 mg/kg.

RESULTS

Mesothelioma cells transduced in vitro with Ad.RSVtk formed nodules when injected in the subcutaneous tissue. These tumors could be eliminated by the administration of GCV, even when as few as 10% of cells were transduced to express HSVtk (bystander effect). Administration of Ad.RSVtk into the peritoneal space of animals with established multifocal human mesothelioma followed by GCV therapy resulted in the eradication of macroscopic tumor in 90% of animals and microscopic tumor in 80% of animals when evaluated after 30 days. The median survival of animals treated with Ad.RSVtk/GCV was significantly longer than that of control animals treated with similar protocols.

CONCLUSION

These results indicate that an adenoviral vector containing the HSVtk gene is effective in treating established malignant mesothelioma in an in vivo setting and raise the possibility of using adenovirus transfer of HSVtk for clinical trials in mesothelioma and other localized tumors.

High-efficiency retroviral gene transfer into murine high-proliferative-potential cells cycle-activated by cytosine arabinoside

We investigated cytosine arabinoside (Ara-C) as a potential agent for in vivo cycle activation of hematopoietic progenitors for the purpose of retroviral-mediated gene transfer. C57Bl mice were treated intraperitoneally with one of three regimens of Ara-C: a single 1,750 mg/kg dose (regimen 1), a 1,750 mg/kg dose on day 0, and a 1,500 mg/kg dose on day 2 (LD50) (regimen 2), or a 1,750 mg/kg dose on day 0 and a 1,500 mg/kg dose on day 3 (regimen 3). The high-proliferative-potential cells (HPPC)/10(5) cells were 47.0 +/- 7.5 pretreatment. The post-treatment HPPC cloning efficiencies were 40.6 +/- 3.4, 83.6 +/- 6.1, and 20.4 +/- 3.2 HPPC/10(5) cells on days 1, 2, and 4, respectively, with regimen 1; 60.0 +/- 7.9, 194.0 +/- 9.6, and 103.0 +/- 11.0 HPPC/10(5) cells 1, 2, and 4 days after the second Ara-C dose, respectively, with regimen 2; and 266 +/- 13.4, 132 +/- 23.9, and 118.0 +/- 5.7/10(5) cells 1, 2, and 4 days after the second Ara-C dose, respectively, with regimen 3. The transduction efficiency of HPPC from untreated animals with N2 viral supernatant was 4.9 +/- 5.8%.(ABSTRACT TRUNCATED AT 250 WORDS)

Basic principles associated with gene therapy of cancer

Advances in gene delivery systems have made possible the development of strategies to eradicate cancers via genetic manipulation. Although the strategy of 'gene therapy' remains in its infancy, experimental tumour models have produced encouraging results and have demonstrated that tumour growth or development can be altered by genetic manipulations. Investigators are hopeful that current and future human trials will confirm the role of these modalities in cancer treatment. This review focuses on several aspects of gene therapy that provide clinicians with a framework to understand the rationale and basic principles underlying current gene therapy protocols being conducted for cancer treatment. The relative merits of different gene delivery systems and the mechanisms underlying clinical gene therapy strategies are reviewed. In addition, we discuss the relevance of these new techniques to the oncologic surgeon.

Gene therapy on renal-cell carcinoma: magic bullet or tragic insanity?

Correction of the aberrant genetic code as a means of rational therapy has been a challenge since the first discoveries of an abnormal genetic link to expression of certain disorders. Our growing understanding of the molecular basis of cancer has also led us into a new era in cancer therapy. The possibility of gene therapy represents one of the biggest potential returns on the investment in molecular biology research over the past several years. As a massive gene therapy attack mounts against many forms of malignancy employing various techniques, strategies, and concepts, there appears to be reason to be optimistic, with expectations thus far decidedly outweighing results. Scientists and clinicians have joined together to target directly the molecular basis of tumorigenesis through the restoration of tumor-suppressor gene function or inhibition of oncogene expression. In addition, scientists mapping the human genome have supplied us with a number of genes that can be used to destroy cancer cells selectively [e.g., the herpes simplex-thymidine kinase (HS-tk) gene], induce a potent antitumor immune response (e.g., interleukin 2), and afford protection to normal tissues from the toxic effects of standard chemotherapy [e.g., multidrug resistance gene type 1 (mdr 1)]. These new anticancer tools provide new opportunities for more specific tumor cell destruction in vivo without the common regional and systemic side effects related to conventional forms of chemotherapy, immunotherapy, radiation, and surgery. Hence, over the next 5-10 years, gene therapy is likely to become a realistic treatment option for certain cancers.(ABSTRACT TRUNCATED AT 250 WORDS)

Mammalian glutathione S-transferase: regulation of an enzyme system to achieve chemotherapeutic efficacy

The glutathione S-transferases are a family of Phase II detoxication enzymes that catalyze the conjugation of glutathione to a large variety of electrophilic compounds. In the 1990s, there have been many advances regarding the function of these enzymes in protecting a cell from the toxic effects of these electrophiles. The complexity of this enzyme family has been realized and much work has been performed to identify the specific roles played by individual isozymes in resistance to a variety of agents. Likewise, the determination of the crystal structure of these enzymes has allowed the identification of specific amino acid residues that are involved in the catalysis of important reactions. The important role that these enzymes play in carcinogenesis and in drug resistance has warranted an attempt to bring together these different subfields of glutathione S-transferase biology to investigate possible ways that this system could be regulated in therapeutically useful ways. In this report, we have reviewed the recent advances and ways in which this knowledge could be utilized in the advancement of the treatment of cancer.

Gene-modified cells for the treatment of cancer

Gene therapy involves the insertion of a gene into an organism to treat a disease. Since its early development in the 1970s, gene therapy has expanded rapidly both in terms of the methods available and the number of candidate diseases for treatment. This report reviews gene therapy for cancer, including methodology, pre-clinical studies and experimental clinical trials.

Human somatic gene therapy: progress and problems

Whilst the potential of gene therapy is considerable, current applications have been restricted by the limitations of available vectors. As yet, no vector is able to produce the desired safe, targeted and efficient transfer of genetic material with regulation of the new gene in the targeted cell. Notwithstanding these limitations, more than 65 clinical gene transfer protocols have been approved in the US. The majority of these are open to patients with malignant disease, in whom the risk:benefit ratio is most appropriate. Current progress and problems in gene transfer are illustrated by reference to gene transfer into haemopoietic stem cells (HSC), an area that has attracted particular attention, both because of the logistic advantages of these cells and because of the wide range of pathologies that may be corrected in the HSC itself or in its progeny. Because of the low efficiency of transfer into HSC, initial studies have involved transfer of marker genes to determine the origin of relapse after autologous bone marrow transplantation and to learn more about the conditions that enhance gene transfer and expression in haemopoietic tissue. Information gained from these studies is already guiding the practice of autologous and allogeneic marrow transplantation and has contributed to the development of gene therapy protocols for the treatment of malignant disease, immune deficiency syndromes and lysosomal storage disorders. Over the next decade, as the technology of gene transfer advances, many further clinical applications of the approach will become evident.

Autologous bone marrow transplantation

Autologous bone marrow transplantation has become a very popular and successful treatment for many patients with lymphomas and other malignancies. The current indications, pretreatment regimes, and laboratory manipulations are discussed as well as the application of gene transfer to eliminate selected genetic diseases and detect disease relapse.

Gene transfer into hematopoietic progenitor and stem cells: progress and problems

Gene transfer to hematopoietic cells for the purpose of "gene therapy" is a new and rapidly developing field with clinical trials in progress. A fundamental goal of research in this field is the incorporation of exogenous genes into the chromosomes of the most primitive hematopoietic progenitor cells--stem cells. Recombinantly engineered retroviral vectors are the best characterized and are currently the only vector type in clinical trials directed at the hematopoietic system. High efficiency gene transfer and expression in murine stem cells and their progeny is now routine, but in larger animal models such as dogs or primates and preliminary clinical trials, gene transfer has been less successful. Problems such as retroviral efficiency, gene expression, insertional mutagenesis and helper virus contamination are being addressed. A promising new vector, the adeno-associated virus (AAV), has shown promise and may allow production of high titer, stable, recombinant virions without helper contamination and with potentially better safety parameters. However, the technology for AAV gene transfer is currently underdeveloped, and issues related to the reproducible production of vectors must be addressed. Other non-viral vector systems are being explored, but little data are available on applications to hematopoietic cells. Better preclinical models are needed to study gene targeting and expression in human cells. An overview of recombinant retroviral and adeno-associated viral vector production, preclinical data and preliminary clinical data will be given, and problems needing to be addressed at all stages of development before broad clinical utility can be achieved will be discussed.

Retrovirus-mediated transfer and long-term expression of HIV type 1 tat gene in murine hematopoietic tissues

Replication of the human immunodeficiency virus (HIV) is regulated tightly by the tat and rev genes. The tat gene of HIV is a potent trans-activator of virus gene expression. trans-Activation is mediated through the tat-responsive element (TAR). Tat also has been shown to affect transcription of cellular genes and to trans-activate other viral promoters. In transgenic animals, tat expression in skin was implicated in the development of lesions resembling Kaposi's sarcoma (KS). More recently, evidence has been presented that suggests that Tat might play a role in the maintenance of KS cells. To study the possible role(s) of Tat in pathogenesis and disease progression, we have developed a retroviral vector for the transfer of tat into murine bone marrow cells. We used this transduced bone marrow to repopulate recipient animals, which expressed the tat gene in peripheral blood 6 months after transplantation as determined by PCR amplification of first-strand cDNA. Analysis of the hematopoietic tissues of mice 6 months posttransplantation indicated persistence of the tat gene and its expression in thymus, lymph nodes, spleen, bone marrow, and peripheral blood. Although tat expression was sustained in all hematopoietic tissues, no gross abnormalities were observed. The presence of tat in all hematopoietic tissues strongly suggests transduction of stem or multipotential progenitor cells.

CD34+ selected cells in clinical transplantation

The CD34 antigen is expressed by early hematopoietic stem cells and progenitors and is detected on the surface of approximately 1% of bone marrow mononuclear cells [1-3]. Several monoclonal antibody-based methods have been developed to isolate these cells from clinical samples of bone marrow or peripheral blood based on their expression of this antigen, utilizing either biotin-avidin affinity, panning or immunomagnetic beads. Roughly 50% of CD34+ cells, with 20-90% purity, are recovered from clinical samples using these methods. Several clinical trials have demonstrated hematopoietic recovery using CD34+ selected cells to support high dose therapy. CD34+ cells may be useful in several areas of clinical stem cell transplantation, including purging of tumor cells, T cell depletion, stem cell expansion and gene therapy. This paper reviews the current methods for purification of CD34+ cells from clinical samples and discusses potential uses of these cells in transplantation.

Purified GPI-anchored CD4DAF as a receptor for HIV-mediated gene transfer

CD4 is the major cellular receptor for the human immunodeficiency virus (HIV). A hybrid gene encoding the extracellular domains of CD4, linked to the sequence encoding the membrane attachment region of the glycosylphosphatidylinositol (GPI)-anchored protein decay accelerating factor (DAF) was stably transfected into HeLa cells. The resultant cell line (T4HD) expressed GPI-anchored CD4DAF at high levels and was susceptible to gene transfer with a recombinant HIV vector. In an effort to expand the spectrum of cells susceptible to HIV gene transfer, CD4DAF was released from the surface of the T4HD cell line by detergent lysis, purified by immunoaffinity chromatography, and reincorporated into native HeLa cells. Incorporation occurred via the GPI anchor as evidenced by cleavage with phosphatidylinositol-specific phospholipase C. More than 95% of the CD4DAF-treated HeLa cells were CD4-positive by flow cytometry, and kinetic analysis demonstrated that over 75% of the fusion protein remained anchored to the cell membrane after 90 min at 37 degrees C. The purified protein retained its ability to bind the envelope protein of HIV. When incorporated, it bound fluorescein isothiocyanate (FITC)-conjugated gp120, and in its soluble form blocked transduction of CD4-positive cells incubated with an HIV-derived vector containing the Neo gene. In contrast to the T4HD cells, exposure of CD4DAF-treated cells to the Neo HIV vector yielded only transient neomycin-resistant colonies. These results suggest that endogenous synthesis of the CD4 molecule may be necessary for successful HIV genomic integration.

P-glycoprotein-mediated multidrug resistance in normal and neoplastic hematopoietic cells

The multidrug transporter, P-glycoprotein (P-gp), is expressed by CD34-positive bone marrow cells, which include hematopoietic stem cells, and in other cells in the bone marrow and peripheral blood, including some lymphoid cells. Multidrug resistance mediated by P-gp appears to be a major impediment to successful treatment of acute myeloid leukemias and multiple myelomas. However, the impact of P-gp expression on prognosis has to be confirmed in several other hematopoietic neoplasms. The role of P-gp in normal and malignant hematopoiesis and clinical attempts to circumvent multidrug resistance in hematopoietic malignancies are reviewed. The recent transduction of the MDR1 gene into murine hematopoietic cells, which protects them from toxic effects of chemotherapy, suggests that MDR1 gene therapy may help prevent myelosuppression following chemotherapy.

Retrovirus-mediated transfer of the multidrug resistance gene into human haemopoietic progenitor cells

We report the utilization of cord blood (CB) or bone marrow (BM) derived low density or purified CD34+ cells as a target for human multidrug resistance (MDR1) gene transfer. Cells were cocultivated for 48 h with an irradiated MDR1 retroviral producer line. Since some degree of MDR1 gene expression has been reported to occur in haemopoietic progenitor cells and in peripheral blood cells, efficiency of MDR1 gene transfer was assessed by: (1) Drug selection and culture in presence of 50 ng/ml doxorubicin, 10 ng/ml colchicine and 0.85 micrograms/ml taxol. In uninfected control, 1-2% of CFU-GM and CFU-GEMM were found to be drug-resistant, while 14-31% of original clonogenic activity was found after 2 weeks of culture of transduced cells. Efficiency of MDR1 transfer was significantly enhanced by prestimulation with cytokines, and found to be significantly superior in CB-derived compared to BM-derived progenitors. (2) Analysis of MDR1 gene expression by evaluating MDR1 mRNA through polymerase chain reaction. MDR1 expression was very low in cultures of uninfected controls, whereas, after drug selection, MDR1 mRNA levels in transduced cells was as high as in the MDR1 retroviral producer line (positive controls). (3) Flow cytometric analysis of the expression of CD34 and P-glycoprotein, the product of the MDR1 gene. After MDR1 transduction and 2 weeks of culture, membrane expression of P-glycoprotein was found on 17-25% of viable CD34+ cells. (4) Cytochemical localization by APAAP staining of P-glycoprotein. No specific localization was found in untransduced controls, whereas transduced and cultured CB-cells expressed P-glycoprotein on plasma and nuclei membrane. In conclusion, MDR1 gene transfer into CB- and BM-derived progenitor cells seems a feasible and attractive approach to generate a drug-resistant haemopoiesis.

Retrovirus-mediated gene transfer in cancer therapy

Retroviral vectors are one of the most promising systems for the transfer and the expression of therapeutic genes in human gene therapy protocols. This review will focus both on the advantages and intricacies of retroviral vectors themselves as well as on the application of these vector systems in experimental and clinical cancer therapy protocols. Therefore, the retrovirus life cycle and the general features of retroviral vectors, including possible targeting strategies with retroviral vectors, are overviewed. These topics are followed by the presentation of genes with emphasis on their potential as tools in somatic cell cancer therapy (cytokines, lymphokines, colony-stimulating growth factors, suppressor genes, antisense oncogenes, suicide genes). Finally, a prospect on the application of retroviral vectors will be described.

Efficient expression of drug-selectable genes in retroviral vectors under control of an internal ribosome entry site

We describe a new retroviral vector system pSXLC/pHa that utilizes a putative internal ribosome entry site (IRES) from encephalomyocarditis virus downstream from a multicloning site to co-express drug-selectable markers with a second non-selectable cDNA in a eukaryotic expression vector. The positive drug-selectable marker, MDR1, and the positive-negative marker, herpes simplex virus thymidine kinase (HSV-TK), were successfully introduced and expressed in the pSXLC/pHa system. The pSXLC-MDR and pSXLC-TK vectors contain the drug-selectable genes under translational control of the IRES and multiple cloning sites upstream for insertion of second cDNAs which can be co-expressed in this system. The inserts of these pSXLC plasmids were designed for easy transfer to the pHa retrovirus vector which has a strong promoter from Harvey murine sarcoma virus. The IRES-MDR-carrying retroviral vector, pHa-MCS-IRES-MDR, conferred resistance to vincristine and adriamycin. The IRES-TK-containing vector, pHa-MCS-IRES-TK conferred HAT-resistance in TK-deficient cells and the transfectants showed hypersensitivity to ganciclovir. These "flexible" vectors should be useful for co-expression of genes for selectable gene transfer and for positive-negative (suicide) selections in vitro and in vivo.

Gene therapy utilizing drug resistance genes: a review

The generation of drug resistant bone marrow may facilitate the development of aggressive chemotherapeutic regimens that might otherwise be lethal due to marrow toxicity. With the availability of technology that permits in vitro manipulation of human marrow and peripheral blood stem cells, it is now possible to introduce genes that confer drug resistance to these hematopoietic progenitors. Animal models and in vitro work with human progenitors using drug resistance genes are reviewed.

Gene transfer of drug resistance genes. Implications for cancer therapy

Two general approaches to the gene therapy of cancer have been proposed: (1) strategies that use exogenous genes to modify cancer cells so that they are less malignant or more susceptible to host defenses or to killing by exogenous agents; and (2) approaches that modify host cells so that they are more effective in eliminating cancer cells or more resistant to agents that are used to treat cancer. In both cases, the development of vectors that encode in vivo selectable phenotypes, such as drug resistance, would be extremely valuable because of the inherent inefficiency of gene transfer and the potential of such vectors to protect normal tissues against toxic agents. To allow the selection of cells in vivo that have been transduced with vectors for gene therapy, we have utilized the human multidrug resistance (MDR1) gene. The product of this gene is a 170,000-dalton glycoprotein known as P-glycoprotein, which acts as an energy-dependent efflux pump for a great many cytotoxic anticancer drugs, including doxorubicin, daunorubicin, etoposide, teniposide, actinomycin D, and taxol. Vectors encoding an MDR1 cDNA are able to transduce many cell types, including bone marrow cells, with high efficiency to allow selection of drug resistance in vitro and in vivo in mouse models. Thus, it should be possible to protect the bone marrow of patients undergoing intensive chemotherapy by transduction of their bone marrow with MDR1 vectors. Furthermore, the ability to select for the presence of the MDR1 cDNA in vivo means that it can be used to introduce otherwise nonselectable genes into the bone marrow for therapy of cancer and other diseases.

Gene therapy for cancer

Initiation of clinical trials of gene therapies for cancer has been made possible by two major technological advances: the ability to clone genes that constitute the genetic basis of carcinogenesis or that have therapeutic potential, and the development of an increasing number of gene transfer methods. As a result, 30 experimental trials of gene therapy for the treatment of human cancer have been approved in the United States of America. Here, we discuss the current status of gene therapy for cancer together with future directions for its development.

Drug-selected coexpression of human glucocerebrosidase and P-glycoprotein using a bicistronic vector

Bicistronic cassettes under control of a single promoter have recently been suggested as useful tools for coordinate expression of two different foreign proteins in mammalian cells. Using the long 5' untranslated region of encephalomyocarditis virus as translational enhancer of the second gene, a bicistronic unit composed of cDNA for human P-glycoprotein [the product of the multidrug resistance gene, MDR1 (also called PGY1)] as selectable marker and cDNA for human glucocerebrosidase (GC; EC 3.2.1.45) (a membrane-associated lysosomal hydrolase) was constructed. NIH 3T3 cells transfected with a Harvey murine sarcoma virus retroviral vector carrying this bicistronic cassette (pHaMCG) express active P-glycoprotein and GC and expression of both proteins augments coordinately with selection for increased colchicine resistance. Percoll gradient analysis of homogenates showed that GC was targeted to the lysosomal fraction. The ability to select for expression of GC with natural product drugs after introduction of the pHaMCG retroviral vector may be useful in gene therapy strategies for Gaucher disease.

Bone marrow extracellular matrix molecules improve gene transfer into human hematopoietic cells via retroviral vectors

Direct contact between hematopoietic cells and viral packaging cell lines or other sources of stroma has been shown to increase the efficiency of retroviral-mediated gene transfer into these target cells compared with infection with viral supernatant. We have investigated the role of defined bone marrow extracellular matrix molecules (ECM) in this phenomenon. Here we report that infection of cells adhering to the carboxy-terminal 30/35-kD fragment of the fibronectin molecule (30/35 FN), which contains the alternatively spliced CS-1 cell adhesion domain, significantly increases gene transfer into hematopoietic cells. Two retroviral vectors differing in recombinant viral titer were used. Gene transfer into committed progenitor cells and long-term culture-initiating cells, an in vitro assay for human stem cells, was significantly increased when the cells were infected while adherent to 30/35 FN-coated plates compared with cells infected on BSA-coated control plates or plates coated with other bone marrow ECM molecules. Although gene transfer into committed progenitor cells and to a lesser degree into long-term culture-initiating cells was increased on intact fibronectin as well, increased gene transfer efficiency into hematopoietic cells on 30/35 FN was dependent on CS-1 sequence since infection on a similar FN fragment lacking CS-1 (42 FN) was suboptimal. 30/35 FN has previously been shown by our laboratory and other investigators to mediate adhesion of primitive murine and human hematopoietic stem cells to the hematopoietic microenvironment. Additional studies showed that neither soluble 30/35 FN nor nonspecific binding of hematopoietic cells to poly-L-lysine-coated plates had any appreciable effect on the infection efficiency of these cells. Our findings indicate that hematopoietic stem cell adhesion to specific ECM molecules alters retroviral infection efficiency. These findings should aid in the design of gene transfer protocols using hematopoietic progenitor and stem cells for somatic gene therapy.

Circumvention of chemotherapy-induced myelosuppression by transfer of the mdr1 gene

Drug-induced myelosuppression is a frequent reason for curtailing chemotherapy in cancer patients. 'Rescue' of myelosuppressed patients with autologous marrow transplants is reasonably advanced and permits an increase in the dose of anticancer drugs. Despite this improvement, patients often relapse with drug resistance disease. The human multidrug resistance (mdr1) gene might make it possible to render hemopoietic stem cells resistant to anticancer drugs after transfer of this gene. By introducing resistant stem cells into patients it might be possible to treat these patients repeatedly with otherwise ablative therapy. This review explores the feasibility of mdr1 gene therapy.

Current trends and future directions in the genetic therapy of human neoplastic disease

All of these initiatives are exciting and may provide therapy that is not as destructive to normal tissues as most existing modalities of therapy, because the therapy is directed to particular molecular defects in the tumor cells. Possibly, this therapy can be individualized for the defects present within the cancer cell that result in disease. Such therapy could reduce treatment costs, because it will be less toxic and, therefore, more cost-effective. We are making the transition to therapy directed to molecular targets and specific for each patient. The potential rewards of this new direction are great in terms of improvement of the therapeutic outcome and reduction of toxicity and cost. Let us hope that the systematic study of these principles of therapy, which is now underway in many medical centers, will alter the unfavorable natural history that characterizes many of the most commonly encountered neoplastic diseases in humans.

Electroporation enhances c-myc antisense oligodeoxynucleotide efficacy

Obtaining high transfection efficiencies and achieving appropriate intracellular concentrations and localization are two of the most important barriers to the implementation of gene targeted therapy. The efficiency of endogenous uptake of oligodeoxynucleotides (ODNs) varies from cell type to cell type and may be a limiting factor of antisense efficacy. The use of electroporation to obtain high intracellular concentrations of a synthetic ODN in essentially 100% of viable cells is described. It is also shown that the transfected ODNs initially localize to the nucleus and remain there for at least 48 hours. The cellular trafficking of electroporated ODNs is shown to be an energy dependent process. Targeting of the c-myc proto-oncogene of U937 cells by electroporation of phosphorothioate-modified ODNs results in rapid and specific suppression of this gene at ODN concentrations much lower than would otherwise be required. This technique appears to be applicable to a variety of cell types and may represent a powerful new investigate tool as well as a promising approach to the ex vivo treatment of hematologic disorders.

The basic science of gene therapy

The development over the past decade of methods for delivering genes to mammalian cells has stimulated great interest in the possibility of treating human disease by gene-based therapies. However, despite substantial progress, a number of key technical issues need to be resolved before gene therapy can be safely and effectively applied in the clinic. Future technological developments, particularly in the areas of gene delivery and cell transplantation, will be critical for the successful practice of gene therapy.

Enhanced expression of alpha-type DNA polymerase genes reduces AZT cytotoxicity in hamster tr5 cells

To study the mechanism of azidothymidine (AZT) cytotoxicity, human DNA was transfected to a variant of Chinese hamster V79 fibroblasts, the tr5 line. This cell line was used for this study for its elevated sensitivity to 5 microM AZT. Primary and secondary transfectants of tr5 cells using total human DNA and pSV2neo plasmid were selected by sequential incubations in AZT (20-50 microM), G418 (400 micrograms/ml active dose), and medium containing hypoxanthine, aminopterin, and thymidine (HAT). One DNA Alu fragment was detected in transfectants using primer TC-65, specific for human Alu sequences in the polymerase chain reaction (PCR). Moreover, cDNA of Chinese hamster alpha-type DNA polymerases was detected in transfectants by reverse transcriptase PCR (RT-PCR) using specific oligo-primer from a DNA polymerase-alpha cDNA sequence and in elevated annealing temperatures. In untransfected tr5 cells, neither of these sequences was detected. The data suggested that the genetic basis for AZT sensitivity may be related to the expression of alpha-type DNA polymerase, and the result indicated that AZT cytotoxicity could be reversed by transfection of appropriate human DNA into tr5 cells. This animal cell model has applications for studies of AZT metabolism and the isolation of the human gene that modulates AZT cytotoxicity.

Recent advances in retrovirus vector technology

Retroviral vectors are widely used for the study of retroviral replication and to introduce DNA into somatic cells. An exciting new approach in retroviral vector technology is the use of internal ribosome entry sites from picornaviruses to provide stable expression of multiple genes. In addition, strategies are being developed that target the expression of retroviral vectors to specific cell populations.

Gene therapy of cancer

Retroviral-mediated gene transfer has permitted the development of clinical protocols for the study and treatment of cancer. These protocols can be divided into gene-labeling and gene therapy proposals. Labeling studies include the tracking of tumor infiltrating lymphocytes (TIL) following the administration of those cells, and the detection, at the time of relapse, of tumor cells from transplanted autologous bone marrow. Most gene therapy protocols are designed to induce an immune attack against the tumor by inserting genes into tumor cells themselves. Although uncertainty about the safety of the procedure still exists, gene therapy of cancer holds much promise as an effective treatment modality.

Retroviral vectors for persistent expression in vivo

Retroviral vectors provide a safe and efficient method of introducing genes of therapeutic interest into dividing cells. The principle limitation of these vectors in the past has been poor gene expression in vivo. This problem has been overcome recently through the use of tissue-specific enhancers in commonly used retroviral vectors. In this review we discuss both the relevant biology and some of the practical applications of retroviral vectors in gene therapy.