Selection against expression of the Escherichia coli gene gpt in hprt+ mouse teratocarcinoma and hybrid cells

Site-specific and directional gene replacement mediated by Cre recombinase

A novel method for the site-specific introduction of genes into eukaryotic cells using the prokaryotic Cre-LoxP recombination system is presented. Cre recombinase catalyzes recombination between two LoxP sites or between two mutant LoxP 511 sites. However, recombination is not catalyzed between a LoxP and a LoxP 511 site. We now demonstrate that it is possible to catalyze accurate exchange between two DNA segments each flanked by a LoxP and a LoxP 511 site. In the example presented, expression of the Cre recombinase resulted in the replacement of a murine IgA constant region gene with a LoxP site at the 5' end and a LoxP 511 site at the 3' end by a human IgA constant region gene flanked by the same wild type and mutant LoxP sites. This method provides a novel approach for the site-specific substitution of specific genes.

Construction of a selectable nef-defective live-attenuated human immunodeficiency virus expressing Escherichia coli gpt gene

We have developed a replication-competent human immunodeficiency virus (HIV) carrying a selective marker that can be used in vivo. This recombinant virus (Z6 Delta nef gpt) was generated by replacing the 5' half of the HIV nef gene with the Escherichia coli guanine phosphoribosyl transferase gene (gpt). This new vector can express the gpt product on infection and works as a positive selective marker for mycophenolic acid (MPA) resistance, a potent immunosuppressive drug used in organ rejection therapy. Conversely, gpt expression also served as a negative selectable marker, since its intracellular expression induces host-cell susceptibility to 6-thioxantine (6-TX), a nucleotide analog that is toxic to the infected cell under these conditions. In this manner, we could suppress the recombinant virus replication through 6-TX selection in both transformed cells and primary human peripheral blood mononuclear cells (PBMCs), suggesting the vector's potential as a model for a new live-attenuated vaccine approach against HIV.

In situ use of suicide genes for therapy of brain tumours

Suicide gene therapy represents a new therapeutic approach to the treatment of patients with otherwise incurable malignant brain tumours. This strategy involves the introduction of a gene that renders the tumour cell susceptible to an otherwise nontoxic prodrug. The most often used genetic prodrug activation system is the herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) paradigm. An important aspect of this system is the 'bystander effect', the extension of cytotoxic effects to untransduced cells. For gene delivery, retroviral, adenoviral vectors and HSV-1 mutants have been used. Clinical studies have revealed that the HSV-tk/GCV approach is safe, but also that responses are observed only in very small brain tumours, indicating insufficient vector distribution and very low transduction efficiency with replication-deficient vector systems. To improve treatment efficacy, the use of replication-competent oncolytic vectors in combination with new or improved prodrug-suicide gene systems as a part of a multimodal approach is warranted. In the context of replication-competent vectors, suicide genes might also be used as fail-safe genes in the case of runaway infection.

A new set of positive/negative selectable markers for mammalian cells

Five new positive and negative selectable markers were created for use in mammalian cells. Their negative selectabilities are based on the Thymidine kinase (Tk) gene of Herpes Simplex virus (HSV) or the Cytidine deaminase (codA) gene of E. coli. The markers can be selected positively by their ability to induce either Hygromycin (Hyg), neomycin (neo), puromycin (PAC) or Blasticidin S (BlaS) resistance. With these markers, two complete sets of markergenes are available that induce independent negative selectable phenotypes.

New positive/negative selectable markers for mammalian cells on the basis of Blasticidin deaminase-thymidine kinase fusions

Two positive and negative selectable markers were created for use in mammalian cells. They are based on two genes for the resistance to Blasticidin S (BlaS) and on the thymidine kinase (Tk) gene of herpes simplex virus (HSV). The markers can be selected positively by their ability to induce BlaS resistance and negatively on the induced sensitivity towards gancyclovir (GANC). Both constructs are also expressed in Escherichia coli and transfer BlaS resistance to this organism as well, making these markers very suitable for the construction of shuttle vectors.

Regression of experimental brain tumors with 6-thioxanthine and Escherichia coli gpt gene therapy

The identification of transgenes with antitumor activity is critical to the development of gene therapy of cancer. Retrovirus-mediated transfer of the Escherichia coli gpt gene into rat C6 glioma cells without subsequent selection still inhibited the proliferation of this mixed polyclonal population upon addition of the prodrug, 6-thioxanthine, with an ID50 of 4.1 microM, whereas parental C6 cells were not affected at a concentration of 500 microM. In a time-course assay, effects of the prodrug on the mixed polyclonal cell proliferation required at least 10 days of exposure. In mixed co-cultures, a bystander effect was not present over the first 4 days of prodrug exposure, but required trypsinization of the co-cultures and replating at lower densities. This "modified" bystander assay thus revealed a 50% decrease in C6 cell proliferation, even when the initial ratio of gpt-expressing to parental C6 cells was as low as 1:19. In a nude mouse model of subcutaneous tumors, co-grafts of C6 glioma and gpt-retrovirus producer cells displayed retarded growth upon exposure to 6-thioxanthine (6-TX). In a nude mouse model of intracerebral tumors, grafting of the gpt-retrovirus producer cells leads to an 80% reduction in intracerebral tumor volumes after 6-TX treatment. This reduction results in a 28% increase in the mean time of survival of animals that harbor intracerebral tumors (p < 0.0005). These antitumor effects indicate that the gpt/6-TX enzyme/prodrug pair is a promising alternative to the thymidine kinase gene and ganciclovir combination in the gene therapy of cancer.

Gene therapy for brain tumors

Gene therapy has opened new doors for treatment of neoplastic diseases. This new approach seems very attractive, especially for glioblastomas, since treatment of these brain tumors has failed using conventional therapy regimens. Many different modes of gene therapy for brain tumors have been tested in culture and in vivo. Many of these approaches are based on previously established anti-neoplastic principles, like prodrug activating enzymes, inhibition of tumor neovascularization, and enhancement of the normally weak anti-tumor immune response. Delivery of genes to tumor cells has been mediated by a number of viral and synthetic vectors. The most widely used paradigm is based on the activation of ganciclovir to a cytotoxic compound by a viral enzyme, thymidine kinase, which is expressed by tumor cells, after the gene has been introduced by a retroviral vector. This paradigm has proven to be a potent therapy with minimal side effects in several rodent brain tumor models, and has proceeded to phase 1 clinical trials. In this review, current gene therapy strategies and vector systems for treatment of brain tumors will be described and discussed in light of further developments needed to make this new treatment modality clinically efficacious.

Novel use of a selectable fusion gene as an "in-out" marker for studying genetic loss in mammalian cells

Recent demonstrations of loss of heterozygosity in a wide variety of human cancers suggest that large multilocus genetic deletions (presumably including tumor suppressor genes) constitute a major class of genetic alteration in human carcinogenesis. Here we show that a bifunctional fusion gene (Hytk), suitable for both positive and negative selection, is an effective marker for studying genetic loss in mammalian cells with minimal interference from point-mutational changes. Studies with a transgenic V79 cell line in which a single functional copy of Hytk was stably inserted into the genome in a retroviral vector showed that loss of the marker (and presumably flanking cellular genetic material) could be induced efficiently by ionizing radiation (gamma-rays and fast neutrons) but only weakly by the powerful point-mutagen benzo[a]pyrene diol epoxide. In a first application of the system, we provide evidence that radiation-induced loss can occur through an indirect mechanism after a high-frequency event. Collectively, our results suggest that the Hytk marker should be a valuable tool for studying genome position effects on the tolerance of genetic loss in cultured human cells that represent different stages in clonal evolution and tumor progression.

Escherichia coli gpt as a positive and negative selectable marker in embryonal stem cells

Transfection of HPRT- L fibroblasts with a plasmid containing two linked selectable markers genes, gpt and neo, regulated by the same eukaryotic control elements, yielded a 6-fold higher transfection frequency on selection for neo than for gpt. Transfection of HPRT- embryonal stem (ES) cells with the same plasmid yielded high levels of transfectants when selected for neo expression with G418, but a level of transfection greater than two orders of magnitude lower was observed when HAT supplemented medium was used to select for gpt expression. Selection for gpt expression in ES cells with medium containing mycophenolic acid and xanthine gave slightly higher frequencies of transfection, but still considerably lower than that for neo selection. In addition, mycophenolic acid exhibited a general cytotoxicity to ES cells with the window between toxicity of this compound to gpt- ES cells and gpt+ ES cells being very narrow. Cells selected with mycophenolic acid and xanthine for expression of gpt remained sensitive to HAT selection. Expression of gpt in a representative ES cell line, selected on mycophenolic acid and xanthine, was verified by Northern analysis and sensitivity to 6-thioguanine. While the level of mRNA expression in this ES cell line was insufficient to support growth via purine salvage when exposed to HAT medium, identical levels of gpt expression in HPRT- L cells, as judged by Northern analysis, allowed for normal growth in HAT medium. This suggests that ES cells place a greater demand on purine nucleotide biosynthesis than L cells. These results are discussed in terms of the use of gpt as a positive and negative selectable marker for gene targeting via homologous recombination in ES cells.

Retrovirally transduced Escherichia coli gpt genes combine selectability with chemosensitivity capable of mediating tumor eradication

"Suicide genes" encoding exceptional sensitivity to chemotherapeutic agents can potentially improve the selectivity of cancer therapy. As a component of a retroviral gene therapy vector, a suicide gene might also improve the safety of gene therapy by permitting the subsequent ablation of transduced cells exhibiting neoplastic or other aberrant behavior. An extra gene, however, cannot easily be added to vectors already carrying a therapeutic gene and a selectable drug resistance marker without compromising gene expression. To circumvent this obstacle, we have investigated a retrovirally transduced Escherichia coli gpt gene on the basis of evidence that it might subserve a dual sensitivity/resistance function. A gpt vector was used to transduce the gene into murine K3T3 sarcoma cells. In vitro, gpt-positive K3T3 clones could be selected on the basis of resistance to a regimen containing mycophenolic acid and xanthine; the same clones were 18 to 86 times as sensitive to 6-thioxanthine (6TX) as their gpt-negative counterparts. In mice, systemic 6TX therapy induced durable regressions in 19/20 gpt-positive K3T3 sarcomas without affecting gpt-negative tumors. These results indicate that selectability and in vivo chemosensitivity can be expressed in the same cell population from a single retrovirally transduced gene and imply the additional possibility of fusing the gpt gene with a therapeutic gene to create vectors expressing three important functions from a single gene.

Positive and negative regulation of a transfected chimeric tyrosine aminotransferase gene: effect of copy number

To define a selective system for the study of rat tyrosine aminotransferase (TAT; EC 2.6.1.5) gene expression, we have introduced into cultured cells the selectable bacterial gene gpt linked to TAT gene flanking sequences. After integration in host cell DNA, the chimeric gene exhibits the same pattern of regulation as the TAT gene. In hepatoma cells, its expression is induced after glucocorticoid hormone treatment and repressed after fusion with fibroblasts. In fibroblasts, the chimeric gene is not expressed. The correct pattern of regulation is lost when the number of integrated copies is high. At copy number above 10, the transfected gene responds poorly to glucocorticoids in hepatoma cells. At copy number above 50, the gene is expressed in fibroblasts. Another gene present in the same construction and controlled by the SV40 early promoter and enhancer is positively regulated by glucocorticoids in hepatoma cells but not after fusion with fibroblasts. These data indicate that in hybrid cells, both TAT promoter and glucocorticoid-responsive elements are negatively regulated.