Activation of a diphtheria toxin A gene by expression of human immunodeficiency virus-1 Tat and Rev proteins in transfected cells

Tat-dependent conditionally replicating adenoviruses expressing diphtheria toxin A for specifically killing HIV-1-infected cells

HIV-1 infection remains a public health problem with no cure. Although antiretroviral therapy (ART) is effective for suppressing HIV-1 replication, it requires lifelong drug administration due to a stable reservoir of latent proviruses and may cause serious side effects and drive the emergence of drug-resistant HIV-1 variants. Gene therapy represents an alternative approach to overcome the limitations of conventional treatments against HIV-1 infection. In this study, we constructed and investigated the antiviral effects of an HIV-1 Tat-dependent conditionally replicating adenovirus, which selectively replicates and expresses the diphtheria toxin A chain (Tat-CRAds-DTA) in HIV-1-infected cells both in vitro and in vivo. We found that Tat-CRAds-DTA could specifically induce cell death and inhibit virus replication in HIV-1-infected cells mediated by adenovirus proliferation and DTA expression. A low titer of progeny Tat-CRAds-DTA was also detected in HIV-1-infected cells. In addition, Tat-CRAds-DTA showed no apparent cytotoxicity to HIV-1-negative cells and demonstrated significant therapeutic efficacy against HIV-1 infection in a humanized mouse model. The findings in this study highlight the potential of Tat-CRAds-DTA as a new gene therapy for the treatment of HIV-1 infection.

Suicide gene therapy in cancer and HIV-1 infection: An alternative to conventional treatments

Suicide Gene Therapy (SGT) aims to introduce a gene encoding either a toxin or an enzyme making the targeted cell more sensitive to chemotherapy. SGT represents an alternative approach to combat pathologies where conventional treatments fail such as pancreatic cancer or the high-grade glioblastoma which are still desperately lethal. We review the possibility to use SGT to treat these cancers which have shown promising results in vitro and in preclinical trials. However, SGT has so far failed in phase III clinical trials thus further improvements are awaited. We can now take advantages of the many advances made in SGT for treating cancer to combat other pathologies such as HIV-1 infection. In the review we also discuss the feasibility to add SGT to the therapeutic arsenal used to cure HIV-1-infected patients. Indeed, preliminary results suggest that both productive and latently infected cells are targeted by the SGT. In the last section, we address the limitations of this approach and how we might improve it.

Eradication of Human Immunodeficiency Virus Type-1 (HIV-1)-Infected Cells

Predictions made soon after the introduction of human immunodeficiency virus type-1 (HIV-1) protease inhibitors about potentially eradicating the cellular reservoirs of HIV-1 in infected individuals were too optimistic. The ability of the HIV-1 genome to remain in the chromosomes of resting CD4+ T cells and macrophages without being expressed (HIV-1 latency) has prompted studies to activate the cells in the hopes that the immune system can recognize and clear these cells. The absence of natural clearance of latently infected cells has led to the recognition that additional interventions are necessary. Here, we review the potential of utilizing suicide gene therapy to kill infected cells, excising the chromosome-integrated HIV-1 DNA, and targeting cytotoxic liposomes to latency-reversed HIV-1-infected cells.

Correct patterning of the primitive streak requires the anterior visceral endoderm

Anterior-posterior axis specification in the mouse requires signalling from a specialised extra-embryonic tissue called the anterior visceral endoderm (AVE). AVE precursors are induced at the distal tip of the embryo and move to the prospective anterior. Embryological and genetic analysis has demonstrated that the AVE is required for anterior patterning and for correctly positioning the site of primitive streak formation by inhibiting Nodal activity. We have carried out a genetic ablation of the Hex-expressing cells of the AVE (Hex-AVE) by knocking the Diphtheria toxin subunit A into the Hex locus in an inducible manner. Using this model we have identified that, in addition to its requirement in the anterior of the embryo, the Hex-AVE sub-population has a novel role between 5.5 and 6.5dpc in patterning the primitive streak. Embryos lacking the Hex-AVE display delayed initiation of primitive streak formation and miss-patterning of the anterior primitive streak. We demonstrate that in the absence of the Hex-AVE the restriction of Bmp2 expression to the proximal visceral endoderm is also defective and expression of Wnt3 and Nodal is not correctly restricted to the posterior epiblast. These results, coupled with the observation that reducing Nodal signalling in Hex-AVE ablated embryos increases the frequency of phenotypes observed, suggests that these primitive streak patterning defects are due to defective Nodal signalling. Together, our experiments demonstrate that the AVE is not only required for anterior patterning, but also that specific sub-populations of this tissue are required to pattern the posterior of the embryo.

Toxin-based therapeutic approaches

Protein toxins confer a defense against predation/grazing or a superior pathogenic competence upon the producing organism. Such toxins have been perfected through evolution in poisonous animals/plants and pathogenic bacteria. Over the past five decades, a lot of effort has been invested in studying their mechanism of action, the way they contribute to pathogenicity and in the development of antidotes that neutralize their action. In parallel, many research groups turned to explore the pharmaceutical potential of such toxins when they are used to efficiently impair essential cellular processes and/or damage the integrity of their target cells. The following review summarizes major advances in the field of toxin based therapeutics and offers a comprehensive description of the mode of action of each applied toxin.

Modular Cre/lox system and genetic therapeutics for colorectal cancer

The Cre/lox system is a powerful tool for targeting therapeutic effectors in a wide variety of human disorders. I review a Cre/lox Wnt-targeted system that has shown promise against Wnt-positive colorectal cancer cell lines. In addition to Wnt-specific targeting of cell death inducers, the modular nature of this gene therapy model system can be exploited by designing positive and negative feedback loops to either amplify or inhibit Wnt activity for experimental or therapeutic benefit. I discuss the structural components and performance parameters of the system, the implication of these findings with respect to cancer stem cells, as well as the general applicability of this system to any disorder characterized by differential gene expression. I also consider the issue of gene delivery as well as in vivo testing requirements necessary for the further characterization and development of this system.

A versatile and reliable two-component system for tissue-specific gene induction in Arabidopsis

Developmental progression and differentiation of distinct cell types depend on the regulation of gene expression in space and time. Tools that allow spatial and temporal control of gene expression are crucial for the accurate elucidation of gene function. Most systems to manipulate gene expression allow control of only one factor, space or time, and currently available systems that control both temporal and spatial expression of genes have their limitations. We have developed a versatile two-component system that overcomes these limitations, providing reliable, conditional gene activation in restricted tissues or cell types. This system allows conditional tissue-specific ectopic gene expression and provides a tool for conditional cell type- or tissue-specific complementation of mutants. The chimeric transcription factor XVE, in conjunction with Gateway recombination cloning technology, was used to generate a tractable system that can efficiently and faithfully activate target genes in a variety of cell types. Six promoters/enhancers, each with different tissue specificities (including vascular tissue, trichomes, root, and reproductive cell types), were used in activation constructs to generate different expression patterns of XVE. Conditional transactivation of reporter genes was achieved in a predictable, tissue-specific pattern of expression, following the insertion of the activator or the responder T-DNA in a wide variety of positions in the genome. Expression patterns were faithfully replicated in independent transgenic plant lines. Results demonstrate that we can also induce mutant phenotypes using conditional ectopic gene expression. One of these mutant phenotypes could not have been identified using noninducible ectopic gene expression approaches.

In vivo genetic ablation by Cre-mediated expression of diphtheria toxin fragment A

We generated a ROSA26-eGFP-DTA mouse line by introducing an eGFP-DTA (enhanced green fluorescent protein -- diphtheria toxin fragment A) cassette into the ROSA26 locus by homologous recombination in ES cells. This mouse expresses eGFP ubiquitously, but DTA expression is prevented by the presence of eGFP, a Neo cassette, and a strong transcriptional stop sequence. Mice carrying this construct are normal and fertile, indicating the absence of DTA expression. However, upon Cre-mediated excision of the floxed region DTA expression is activated, resulting in the specific ablation of Cre-expressing cells. As an example of this approach, we ablated Nkx2.5 and Wnt1-expressing cells by using the Nkx2.5-Cre and Wnt1-Cre mouse lines, respectively. We observed loss of the precise tissues in which Nkx2.5 and Wnt1 are expressed. Apart from being a general GFP reporter, the ROSA26-GFP-DTA mouse line should provide a useful resource for genetic ablation of specific groups of cells.

Molecular strategies to inhibit HIV-1 replication

The human immunodeficiency virus type 1 (HIV-1) is the primary cause of the acquired immunodeficiency syndrome (AIDS), which is a slow, progressive and degenerative disease of the human immune system. The pathogenesis of HIV-1 is complex and characterized by the interplay of both viral and host factors. An intense global research effort into understanding the individual steps of the viral replication cycle and the dynamics during an infection has inspired researchers in the development of a wide spectrum of antiviral strategies. Practically every stage in the viral life cycle and every viral gene product is a potential target. In addition, several strategies are targeting host proteins that play an essential role in the viral life cycle. This review summarizes the main genetic approaches taken in such antiviral strategies.

Pharmacological and genetic modulation of Wnt-targeted Cre-Lox-mediated gene expression in colorectal cancer cells

Wnt-targeted gene therapy has been proposed as a treatment for human colorectal cancer (CRC). The Cre-Lox system consists of methodology for enhancing targeted expression from tissue-specific or cancer-specific promoters. We analyzed the efficiency of Wnt-specific promoters as drivers of the Cre-mediated activity of a luciferase reporter gene or cell death effector gene in CRC cell lines in the presence and absence of two modulators of Wnt activity, sodium butyrate and lithium chloride. Butyrate is present in the colonic lumen after digestion of fiber-rich foods, whereas the colonic lumen is readily accessible to lithium chloride. In both SW620 and HCT-116 CRC cells, a physiologically relevant concentration of butyrate upregulated reporter and effector activity and altered the Wnt-specific expression pattern. Lithium chloride markedly enhanced Cre-Lox-mediated Wnt-specific reporter expression only in APC wild-type CRC cells. Possibilities for genetic modulation of the proposed CRC therapy included Wnt-specific expression of a floxed Lef1-VP16 fusion that enhanced Wnt-specific cell death and of a floxed dominant-negative Tcf4 that specifically downregulated endogenous Wnt activity. These findings demonstrated that the Cre-Lox system, in combination with pharmacological and genetic modulators, represents effective methodology for enhancing Wnt-targeted gene therapy.

Genetic therapy for HIV/AIDS

Despite the tremendous success of highly active antiretroviral treatment (HAART) introduced nearly 8 years ago for the treatment of human immunodeficiency virus (HIV), innovative therapies, including gene transfer approaches, are still required for nearly half of the general patient population. A number of potential gene therapeutic targets for HIV have been identified and include both viral and cellular genes essential for viral replication. The diverse methods used to inhibit viral replication comprise RNA-based strategies such as ribozymes, RNA decoys, antisense messenger RNAs and small interfering RNA (siRNA) molecules. Other potential anti-HIV genes include dominant negative viral proteins, intracellular antibodies, intrakines and suicide genes, all of which have had a modicum of success in vitro. Cellular targets include CD4+ T cells, macrophages and their progenitors. The greatest gene transfer efficiency has been achieved using retroviral or, more recently, lentiviral vectors. A limited number of Phase I clinical trials suggest that the general method is safe. It is proposed that a national network for HIV gene therapy (similar to the AIDS Clinical Trial Groups) may be the best way to determine which approaches should proceed clinically.

Growth inhibition of cancer cells by co-transfection of diphtheria toxin A-chain gene plasmid with bovine leukemia virus-tax expression vector

We constructed a plasmid containing bovine leukemia virus (BLV)-tax gene driven by SR alpha promoter, designated as pME-BLVtax, to activate the promoter of the long terminal repeat (LTR) of BLV in various tumor cells. Activation of the promoter of BLV-LTR by pME-BLVtax was confirmed by luciferase assay. When the cells, such as COS-1, C8, and KU-1, were transfected with a plasmid pBLV-LUC1, which contained the luciferase gene under the control of BLV-LTR, and pME-BLVtax, luciferase was expressed in these cells, whereas no luciferase gene expression was observed when only pBLV-LUC1 was introduced into the cells. Activation of the BLV-LTR promoter was regulated by pME-BLVtax and 0.5 microg of pME-BLVtax was sufficient for the expression of the gene under the control of BLV-LTR. Furthermore, pME-BLVtax was used to direct the cell expression of the gene for diphtheria toxin A-chain under the control of BLV-LTR (pLTR-DT) to various tumor cell lines, KU-1, C8, COS-1, BL2M3, and HeLa cells. The transfection was carried out with cationic liposomes. In this experiment, co-transfection of pLTR-DT with pME-BLVtax exerted selective growth inhibitory effects on the tumor cell lines. Moreover, three co-introductions of pLTR-DT with pME-BLVtax into the cell lines resulted in significant inhibition of the cell growth. This result suggests that the delivery of the pLTR-DT and pME-BLVtax genes into tumor cells by the use of cationic liposomes may be potentially useful as a novel approach for the treatment of tumor cells.

Approaches to gene therapy for human immunodeficiency virus infection

Much progress has been made in developing new and more efficient treatments for human immunodeficiency virus (HIV) infection, the cause of acquired immunodeficiency syndrome (AIDS). However, the scope of the HIV epidemic and the limitations of existing treatments necessitate the continued development of novel treatment strategies. Gene therapy is one such forward-looking strategy. Gene therapy approaches for HIV infection include efforts to interfere with viral replication directly by engineering HIV-resistant cells or indirectly by eliminating infected cells from the body, primarily by eliciting a therapeutic immune response to destroy HIV-infected cells. Although the prospect of gene therapy as a routine treatment for HIV infection remains distant, continuous progress is being made, which should also have implications for gene therapy strategies for a variety of other diseases. This article reviews some of the strategies for investigating the feasibility of gene transfer for the treatment of HIV infection.

Selective transduction of HIV-1-infected cells by the combination of HIV and MMLV vectors

Human immunodeficiency virus 1 (HIV-1)-infected cells are important targets of gene therapy for acquired immune deficiency syndrome. We have developed a novel strategy for targeted gene transfer into HIV-1-infected cells based on 2-step gene transfer. The first step involves the stable introduction of the HIV vector containing the ecotropic Moloney murine leukemia virus (MMLV) receptor gene (EcoRec) into human CD4+ T cells as a molecular switch. Because the HIV-long terminal repeat (HIV-LTR) is Tat inducible, it is expected that EcoRec is expressed only after HIV-1 infection. Northern blot analysis and a retrovirus binding assay confirmed that the HIV-LTR of the integrated vector was silent in transduced cells but strongly transactivated in HIV-1 infection. High levels of EcoRec expression were observed only in HIV-1-infected cells. These cells became highly susceptible to ecotropic MMLV infection and, therefore, in the second step, HIV-1-infected cells were selectively transduced with ecotropic MMLV vectors. More than 70% of HIV-1-infected cells were transduced by this strategy. These findings indicate that this 2-step method can be used for selective and stable gene transfer into HIV-1-infected cells.

Increased transgene expression by the mouse tyrosinase enhancer is restricted to neural crest-derived pigment cells

In this study, we have addressed the impact of the mouse tyrosinase enhancer on regulated expression from the mouse tyrosinase promoter during embryonic development. Stable and transient transgenic experiments using the reporter gene lacZ reveal that (1) expression is detected in neural crest-derived melanoblasts from E11.5 onward, (2) the enhancer does not increase transgenic expression in optic cup-derived pigment cells of the retinal pigment epithelium (RPE), and (3) expression in the telencephalon is not any longer detected. The importance of the enhancer for expression in pigment cells of the eye was further investigated in adult mice using an attenuated diphtheria toxin A gene. This demonstrated that in presence of the enhancer the transgene expression is specifically targeted to neural crest-derived melanocytes of the choroid and not, or slightly, to the RPE. This suggests that tyrosinase is differentially regulated in the two pigment cell lineages, and that this promoter can be used to target expression preferentially to the neural crest-derived melanocyte lineage.

Lentivirus vector mobilization and spread by human immunodeficiency virus

The rapid advancement of lentivirus-based gene transfer systems and their demonstrated utility in a variety of in vitro and in vivo settings has heightened the need for assays to evaluate the safety of these vectors prior to human clinical trials. Two major concerns relating to the use of lentivirus-based vectors in a clinical setting are the presence of contaminating replication-competent retroviruses in vector preparations and the efficiency of vector mobilization and spread by wild-type helper virus (rescue). This article describes an in vitro system to study the rescue of lentivirus-based vectors by wild-type HIV. We show that lentivirus-based vectors can be readily rescued from T cell lines and to a lesser extent from primary human lymphocytes by wildtype HIV, resulting in the spread of mobilized vector particles to previously untransduced cells. Furthermore, we show that vector mobilization can be prevented by antiretroviral drugs such as AZT. In contrast to recently published reports by Bukovsky et al. and An et al., the lentivirus vectors used in these studies had little or no effect on the replication and spread of HIV in transduced cells [Bukovsky et al. (1999). J. Virol. 73, 7087-7092; An et al. (1999). J. Virol. 73, 7671-7677]. Whereas vector spread is a significant concern for most gene therapy applications, in the context of gene therapy for HIV infection it may have beneficial effects.

The transcriptional inhibitors, actinomycin D and alpha-amanitin, activate the HIV-1 promoter and favor phosphorylation of the RNA polymerase II C-terminal domain

Actinomycin D and alpha-amanitin are commonly used to inhibit transcription. Unexpectedly, however, the transcription of the human immunodeficiency virus (HIV-1) long terminal repeats (LTR) is shown to be activated at the level of elongation, in human and murine cells exposed to these drugs, whereas the Rous sarcoma virus LTR, the human cytomegalovirus immediate early gene (CMV), and the HSP70 promoters are repressed. Activation of the HIV LTR is independent of the NFkappaB and TAR sequences and coincides with an enhanced average phosphorylation of the C-terminal domain (CTD) from the largest subunit of RNA polymerase II. Both the HIV-1 LTR activation and the bulk CTD phosphorylation enhancement are prevented by several CTD kinase inhibitors, including 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole. The efficacies of the various compounds to block CTD phosphorylation and transcription in vivo correlate with their capacities to inhibit the CDK9/PITALRE kinase in vitro. Hence, the positive transcription elongation factor, P-TEFb, is likely to contribute to the average CTD phosphorylation in vivo and to the activation of the HIV-1 LTR induced by actinomycin D.

Targeted RNases: a feasibility study for use in HIV gene therapy

A targeted RNase would be ideal for gene therapy of several acquired and inherited disorders. Such an RNase may be engineered to contain a ribonucleolytic domain and a specific target RNA binding domain. To demonstrate the feasibility of this approach, an RNase targeted against human immunodeficiency virus (HIV) RNA--Tev-RNase T1--was designed and tested for its use in HIV-1 gene therapy. A human CD4+ T lymphoid (MT4) cell line and human peripheral blood lymphocytes (PBLs) were transduced with retroviral vectors lacking or expressing the tevT1 gene. Expression of enzymatically functional Tev-RNase T1 protein and its lack of toxicity was demonstrated in stable MT4 transductants. Compared with control cells lacking this protein, both transduced MT4 cells and PBLs expressing Tev-RNase T1 delayed HIV-1 replication. Tev-RNase T1 was shown to act after integration, since HIV-1 proviral DNA could be detected, but the amount of HIV-1 RNA produced in MT4 cells and PBLs was significantly decreased. This study demonstrates the feasibility of a targeted RNase strategy for therapeutic use.

The human immunodeficiency virus-1 Tat protein increases cell proliferation, alters sensitivity to zinc chelator-induced apoptosis, and changes Sp1 DNA binding in HeLa cells

The HIV-1 transcriptional regulatory protein Tat is a pleiotropic factor that represses expression of the human Mn-superoxide dismutase. Tat increases oxidative stress, as shown by decreased glutathione and NADPH levels. These redox changes enhance proliferation and apoptosis and alter the activity of zinc thiolate-containing proteins such as Sp1. Cells stably producing the Tat protein have an increased proliferation rate, which can be inhibited by pretreatment with the antioxidant mercaptopropionylglycine. Conversely, cells exposed to low concentrations of the oxidant paraquat are stimulated to divide. Intermediate and higher paraquat levels result in increased apoptosis or necrosis, respectively, suggesting that the physiological end point depends on the dose of oxidant used. Furthermore, treatment with the zinc chelator (N,N,N', N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) sensitizes HeLa-tat cells to apoptosis. In these cells, binding of the zinc-containing factor Sp1 to its DNA sequence is higher than in parental cells. Normal DNA binding is partially restored by pretreatment with a compound that mimics superoxide dismutase activity. Interestingly, Sp1-DNA interactions decrease more rapidly in the HeLa-tat cells after TPEN treatment. HeLa cell extracts incubated in the presence of purified Tat protein have increased Sp1 binding, consistent with the results observed in Tat-transfected cells. These results suggest that the Tat protein, via direct or indirect mechanisms, increases proliferation, sensitizes cells to apoptosis, and changes the conformation of Sp1, affecting its ability to bind to its cognate DNA sequence and to retain its zinc.

Liposome-mediated delivery of antiviral agents to human immunodeficiency virus-infected cells

Intracellular delivery of novel macromolecular drugs against human immunodeficiency virus type-1 (HIV-1), including antisense oligodeoxynucleotides, ribozymes and therapeutic genes, may be achieved by encapsulation in or association with certain types of liposomes. Liposomes may also protect these drugs against nucleases. Low-molecular-weight, charged antiviral drugs may also be delivered more efficiently via liposomes. Liposomes were targeted to HIV-1-infected cells via covalently coupled soluble CD4. An HIV-1 protease inhibitor encapsulated in conventional negatively charged multilamellar liposomes was about 10-fold more effective and had a lower EC90 than the free drug in inhibiting HIV-1 production in human monocyte-derived macrophages. The drug encapsulated in sterically stabilized liposomes was as effective as the free drug. The EC50 of the reverse transcriptase inhibitor 9-(2-phosphonylmethoxyethyl)adenine (PMEA) was reduced by an order of magnitude when delivered to HIV-1-infected macrophages in pH-sensitive liposomes. A 15-mer antisense oligodeoxynucleotide against the Rev response element was ineffective in free form against HIV-1 replication in macrophages, while delivery of the oligonucleotide in pH-sensitive liposomes inhibited virus replication. The oligodeoxynucleotide encapsulated in sterically stabilized pH-sensitive liposomes with prolonged circulation in vivo, which were recently developed in the laboratories of the authors, was also highly effective. A ribozyme complementary to HIV-1 5'-LTR delivered in pH-sensitive liposomes inhibited virus production by 90%, while the free ribozyme caused only a slight inhibition. Cationic liposome-mediated co-transfection of the HIV-regulated diphtheria toxin A fragment gene and a proviral HIV clone into HeLa cells completely inhibited virus production, while the frame-shifted mutant gene was ineffective. Co-transfection of the proviral genome and a gene encoding a Rev-binding aptamer into HeLa cells via transferrin-associated cationic liposomes inhibited virus production. These studies indicate that liposomes can be used to facilitate the intracellular delivery of certain anti-HIV agents and to enhance their therapeutic effects. These properties may be particularly advantageous in the development of novel macromolecular drugs, which may be necessary because of the emergence of virus strains resistant to the currently available drugs.

Targeted expression of a toxin gene to D1 dopamine receptor neurons by cre-mediated site-specific recombination

Idiopathic Parkinson's disease involves the loss of midbrain dopaminergic neurons, resulting in the presynaptic breakdown of dopaminergic transmission in the striatum. Huntington's disease and some neurodegenerative diseases with Parkinsonian features have postsynaptic defects caused by striatal cell death. Mice were generated in which an attenuated form of the diphtheria toxin gene (tox-176) was expressed exclusively in D1 dopamine receptor (D1R)-positive cells with the aim of determining the effect of this mutation on development of the basal ganglia and on the locomotor phenotype. Transgenic mice expressing Cre, a site-specific DNA recombinase, were crossed with a second line in which a transcriptionally silenced tox-176 gene was inserted into the D1R gene locus by homologous recombination. Young doubly transgenic mutant mice expressing the tox-176 gene displayed bradykinesia, dystonia, and had falls caused by myoclonic jerks. The mutant brain had evidence of apoptosis and reactive gliosis and, consistent with the D1R expression pattern, the striatum was reduced in volume, and the Islands of Calleja were absent. In contrast, the cortex was of normal thickness. D1Rs were not detectable in mutants by in situ hybridization or ligand autoradiography, whereas D2 dopamine receptor (D2R) mRNA and protein was present in the striatum. In addition, substance P and dynorphin, neuropeptides known to be expressed in D1R-positive striatonigral projection neurons were not detectable. Enkephalin, a marker found in D2-positive striatopallidal projection neurons was expressed in the mutant brain. The mutant represents a novel neurodegenerative disease model with a dramatic extrapyramidal phenotype.

Inducible expression of herpes simplex virus thymidine kinase from a bicistronic HIV1 vector

The possibility of protecting human CD4+ lymphocytes from human immunodeficiency virus type 1 (HIV1) infection, through a suicide mechanism elicited by the HIV1 transcription apparatus itself, offers a potentially useful approach for gene therapy of the acquired immunodeficiency syndrome. A replication-defective lentiviral HIV1 vector (HYIRES-TK) was designed to carry both the hygromycin (Hy) phosphotransferase gene for positive selection and the thymidine kinase (TK) gene of herpes simplex virus driven by the viral long terminal repeat (LTR). The internal ribosome entry site (IRES) from encephalomyocarditis virus was placed between the two genes for their efficient simultaneous translation. Transient expression of active TK into transfected COS-1 cells was shown to be induced by Tat and Rev over a detectable basal level. By providing the missing viral proteins in trans, recombinant viruses were generated and used to transduce Jurkat cells. The Hy-resistant population of cells was sensitive to ganciclovir (GCV) and acyclovir (ACV), a result consistent with a basal level of TK expression. Cocultivation of transduced cells with cells chronically infected with HIV in the presence of 10 microM ACV, a concentration non-toxic for the uninfected cells, resulted in increased killing of cells transduced with the HY-IRES-TK vector. These data indicate that two genes can be expressed from the viral LTR in the context of an HIV1 vector, with the aid of an IRES sequence. The expression is inducible by the HIV proteins Tat and Rev and it is possible to specifically kill infected cells with subtoxic concentrations of drug. To decrease the sensitivity of the transduced cells towards GCV, a variant vector expressing a truncated TK was constructed. The truncated version was expressed at levels similar to those of wild-type TK but induced sensitivity towards GCV in transduced cells that was intermediate between that of untransduced cells and of cells expressing wild-type TK.

Antitumor effect of diphtheria toxin A-chain gene-containing cationic liposomes conjugated with monoclonal antibody directed to tumor-associated antigen of bovine leukemia cells

Monoclonal antibody c143 against tumor-associated antigen (TAA) expressed on bovine leukemia cells was conjugated to cationic liposomes carrying a plasmid pLTR-DT which contained a gene for diphtheria toxin A-chain (DT-A) under the control of the long terminal repeat (LTR) of bovine leukemia virus (BLV) in the multicloning site of pUC-18. The specificity and antitumor effects of the conjugates were examined in vitro and in vivo using TAA-positive bovine B-cell lymphoma line as the target tumor. In vitro studies with the TAA-positive cell line indicated that luciferase gene-containing cationic liposomes associated with the c143 anti-TAA monoclonal antibody caused about 2-fold increase in luciferase activity compared with cationic liposomes having no antibody, and also that the c143-conjugated cationic liposomes containing pLTR-DT exerted selective growth-inhibitory effects on the TAA-positive B-cell line. Three injections of pLTR-DT-containing cationic liposomes coupled with c143 into tumor-bearing nude mice resulted in significant inhibition of the tumor growth. The antitumor potency of the c143-conjugated cationic liposomes containing pLTR-DT was far greater than that of normal mouse IgG-coupled cationic liposomes containing pLTR-DT as assessed in terms of tumor size. These results suggest that cationic liposomes bearing c143 are an efficient transfection reagent for BLV-infected B-cells lymphoma cells, and that the delivery of the pLTR-DT gene into BLV-infected B-cells by the use of such liposomes may become a useful technique for gene therapy of bovine leukosis.

Diphtheria toxin A gene-mediated HIV-1 protection of cord blood-derived T cells in the SCID-hu mouse model

The reconstitutive potential of CD34+-derived cord blood (CB) cells, transduced with a regulated diphtheria toxin A (DT-A) chain gene, was examined in SCID-hu mice harboring a conjoint organ composed of human thymus and liver (thy/liv). The DT-A-transduced cells, injected directly into the thy/liv organ, showed the same engraftment potential as control CB cells transduced with the non-DT-A parental vector. CB cells, distinguishable from the thy/liv cells by the HLA marker B7, were preferentially maintained in ex vivo culture. In the thy/liv organ, the engrafted CB cells represented >80% of the total cells. A majority of cells (>70%) in the thy/liv organ were also CD4+CD8+, as would be expected of maturing thymocytes. The incidence of double-positive cells was highest at 44 days (compared with 30 days and 80 days) after injection of CB cells. This suggested that a minimum time was required to achieve optimal proliferation of cells in the thy/liv organ but that, at later times, all of the early cells had matured. Thus, the population used for engraftment contained early cells but not self-renewing cells. The double-positive cells matured rapidly into single-positive cells (either CD4+ or CD8+) when placed in ex vivo culture. Marked cells (neo+) could readily be detected in the thy/liv-derived cells. The cells transduced with DT-A showed long-term protection in ex vivo culture against HIV T lymphotropic isolate NL4-3. This study shows that DT-A-transduced cells had no apparent disadvantage in engraftment of the thy/liv organ and did not have any toxic effects in vivo. Such cells were protected against HIV infection even when challenged more than 2 months after transduction and after a 44-day engraftment period in the thy/liv mice. These data support the feasibility of toxin gene therapy as a strategy for HIV infection.

Inhibition of human immunodeficiency virus replication by the herpes simplex virus virion host shutoff protein

The herpes simplex virus (HSV) virion host shutoff gene (vhs) encodes a protein which nonspecifically accelerates the degradation of mRNA molecules, leading to inhibition of protein synthesis. This ability to inhibit a critical cellular function suggested that vhs could be used as a suicide gene in certain gene therapy applications. To investigate whether vhs might be useful for treatment of AIDS, we tested the ability of both HSV type 1 (HSV-1) and HSV-2 vhs to inhibit replication of human immunodeficiency virus (HIV). Replication of HIV was substantially inhibited when an infectious HIV proviral clone was cotransfected into HeLa cells together with vhs under the control of the cytomegalovirus (CMV) immediate-early promoter. HSV-2 vhs was more active than HSV-1 vhs in these experiments, consistent with previously published studies on these genes. Since expression of vhs from the CMV promoter is essentially unregulated, we also tested the ability of vhs expressed from the HIV long terminal repeat (LTR) promoter to inhibit HIV replication. Wild-type HSV-1 vhs inhibited HIV replication more than 44,000-fold in comparison to a mutant vhs gene encoding a nonfunctional form of the Vhs protein. Production of Vhs in transfected cells was verified by Western blot assays. A larger amount of Vhs was observed in cells transfected with plasmids expressing vhs from the HIV LTR than from the CMV promoter, consistent with the greater inhibition of HIV replication observed with these constructs. Mutant forms of Vhs were expressed at higher levels than wild-type Vhs, most likely due to the ability of wild-type Vhs to degrade its own mRNA. The strong inhibitory activity of the vhs gene and its unique biological properties make vhs an interesting candidate for use as a suicide gene for HIV gene therapy.

Cationic liposome-mediated expression of HIV-regulated luciferase and diphtheria toxin a genes in HeLa cells infected with or expressing HIV

HIV-regulated expression of the diphtheria toxin A fragment gene (HIV-DT-A) is a potential gene therapy approach to AIDS. Since cationic liposomes are safe and non-immunogenic for in vivo gene delivery, we examined whether LipofectAMINE or DMRIE reagent could mediate the transfection of HIV-DT-A (pTHA43) or the HIV-regulated luciferase gene (pLUCA43) into HIV-infected or uninfected HeLa cells. pLUCA43 was expressed at a 10(3)-fold higher level in HeLa/LAV cells than in uninfected HeLa cells, while the extent of expression of RSV-regulated luciferase was the same in both cell lines. Co-transfection of HeLa cells with pTHA43 and the proviral HIV clone, HXB deltaBgl, resulted in complete inhibition of virus production. In contrast, the delivery of HIV-DT-A to chronically infected HeLa/LAV or HeLa/IIIB cells, or to HeLa CD4+ cells before infection, did not have a specific effect on virus production, since treatment of cells with control plasmids also reduced virus production. This reduction could be ascribed to cytotoxicity of the reagents. The efficiency of transfection, as measured by the percentage of cells expressing beta-gal, was approximately 5%. Thus, cationic liposome-mediated transfection was too inefficient to inhibit virus production when the DT-A was delivered by cationic liposomes to chronically- or de novo- infected cells. However, when both the virus and DT-A genes were delivered into the same cells by cationic liposomes, DT-A was very effective at inhibiting virus production. Our results indicate that the successful use of cationic liposomes for gene therapy will require the improvement of their transfection efficiency.

Molecular biology of human immunodeficiency virus type-1

Tremendous progress has been made in our understanding of the multiplication and pathogenesis of the human immunodeficiency virus, the causative agent of acquired immunodeficiency syndrome (AIDS). To block virus multiplication several targets in the life cycle of the virus have already been identified for which antiviral drugs can be developed and gene therapy can be envisaged as a possible treatment or cure of AIDS. The combination of several therapies might be needed for effective treatment. Prevention of HIV infections through effective vaccines still awaits novel, unconventional strategies.

HIV-regulated diphtheria toxin A chain gene confers long-term protection against HIV type 1 infection in the human promonocytic cell line U937

Gene therapy approaches have recently been investigated for the treatment of acquired immunodeficiency syndrome (AIDS), both in preclinical and clinical studies, because more traditional antiviral agents have proven to be of limited effectiveness. We have previously shown that long-term protection against both laboratory and clinical isolates of human immunodeficiency virus type 1 (HIV-1) was conferred by HIV-regulated diphtheria toxin A (DT-A) chain in a human T cell line. Because the monocyte/macrophage cell is an important reservoir for HIV-1 in infected individuals, we sought here to determine whether HIV-regulated DT-A would also be effective in the promonocytic cell line U937. We report here that long-term protection, conferred by HIV-regulated DT-A, was observed in U937 cells, but that protection was dependent on the stock of HIV IIIB used for challenge. HIV production was measured by p24 assays, polymerase chain reaction (PCR) for HIV vif, gag, and reverse transcriptase (RT) sequences, and cocultivation with peripheral blood mononuclear cells (PBMCs). Complete protection was seen in DT-A-transduced cells with a stock of IIIB propagated on H9 cells and titered on peripheral blood mononuclear cells (PBMCs), while protection in these same cells with a second stock of IIIB, propagated and titered on H9 cells, was only partial and dose dependent.

Prostate tissue specificity of the prostate-specific antigen promoter isolated from a patient with prostate cancer

We have cloned and characterized a 620-bp fragment of DNA that flanks 5' of the prostate-specific antigen (PSA) gene from a prostate cancer patient. Using DNA transfection, the efficacy of this putative promoter in regulating gene expression was quantitated in several prostate and nonprostate tissue cell lines. Our results demonstrated that the 620-dp DNA fragment actively drives gene expression in LNCaP, a PSA-producing prostate tumor cell line. No promoter activity was detected in the non-PSA-producing prostate tumor lines, DU145 and PC-3, nor in a renal (R11) or breast (MCF-7) cancer cell line. Furthermore, the promoter activity could be regulated in vitro by androgen stimulation. Dihydrotestosterone (DHT) concentrations between 3 and 30 nM induced the highest promoter activity in the transfected LNCaP cells, which parallels the expression profile of the androgen receptor in LNCaP cells. In addition, our PSA promoter exhibited competitive inhibition of the endogenous genomic PSA promoter in transfected LNCaP cells, suggesting that prostate cell-specific DNA-binding proteins are required to activate the PSA promoter. increased its potency four- to five-fold while retaining tissue specificity. Our data suggest that a strong tissue-specific negative regulatory element capable of overriding the nonspecific CMV promoter is present in the PSA promoter and confers its tissue specificity. The use of a highly specific promoter-driven gene vector will allow selective expression of therapeutic genes within PSA-producing prostate cancer cells, providing a unique strategy for prostate cancer gene therapy.

Residual expression of reporter genes in constructs mimicking HIV genome organization

Plasmids were constructed whereby the expression of a reporter gene, either the cDNA corresponding to the secreted form of human alkaline phosphatase (SEAP) or the herpes simplex virus type 1 (HSV1) thymidine kinase (tk) gene, was rendered dependent upon the expression of the human immunodeficiency virus type 1 (HIV1) tat and rev proteins. The SEAP or tk genes were placed between HIV1 splice donor and acceptor sites. One SEAP construct carried a series of alternating splice donor and acceptor sites. In all cases, the rev response element mapped within an intron. Despite such mimicry of the HIV1 genome, residual expression of the reporter gene in the absence of tat and rev was observed. These results, as well as non-specific T-cell recruitment, suggest limits to the specificity of using HIV-activated toxic gene expression to kill HIV-infected cells.

Inhibition of human immunodeficiency virus expression by sense transcripts encoding the retroviral leader RNA

Towards gene therapy for the treatment of human immunodeficiency virus type 1 (HIV-1) infections, we tested the potency of several antiviral constructs in transient HIV-1 production assays. Whereas little effect was obtained with antisense- and TAR decoy-constructs, we measured efficient inhibition of HIV-1 mRNA translation and virion production in the presence of HIV-1 leader-containing transcripts. The infectivity of these virions was also reduced by this sense inhibitor RNA. These results suggest that leader-encoded functions, like the dimer-linkage structure, can be used to specifically inhibit HIV expression in trans.

Suppression of single and double nonsense mutations introduced into the diphtheria toxin A-chain gene: a potential binary system for toxin gene therapy

We have previously shown that ablation of specific cells can be achieved through the transcriptionally regulated expression of the diphtheria toxin A-chain (DT-A) gene in both cell culture and transgenic mice. Such targeted toxin gene expression provides a novel approach to cancer and acquired immunodeficiency syndrome (AIDS) therapy. The use of mutants of DT-A with attenuated toxicity may allow targeting of cells for which only moderately selective gene regulatory elements are available. Alternatively, conditional mutants might be used to target cells in which conditions can be established for suppression of the mutation. We have investigated the effects of mutating selected serine codons to amber (TAG) nonsense codons in the DT-A coding sequence. In transient transfection of HeLa cells, DT-A activity was markedly reduced by the introduction of a single amber codon and was virtually eliminated by two amber mutations. Cotransfection of a serine inserting suppressor tRNA expression plasmid substantially restored DT-A expression from both single and double amber mutants. Expression of the same suppressor tRNA also suppressed a previously described amber mutation at the tyrosine codon 28 in DT-A. Thus, nonsense suppression can be used to control the expression of DT-A in mammalian cells, potentially allowing binary control over the targeting of tissues for selective ablation.

Further evaluation of soluble CD4 as an anti-HIV type 1 gene therapy: demonstration of protection of primary human peripheral blood lymphocytes from infection by HIV type 1

We previously reported on the construction of retroviral vectors that produce a secreted form of the HIV-1 receptor, T cell antigen CD4 (Morgan et al., AIDS Res Hum Retroviruses 1990;6:183-191). In this article we test the ability of these sCD4-expressing retroviral vectors to protect human T-cell lines or primary T cells from HIV-1 infection. To demonstrate that protection from HIV-1 infection is mediated by the soluble nature of this protein, two coculture protection experiments were conducted. In these experiments, sCD4-expressing retroviral vectors were used to engineer mouse NIH 3T3 cells. In one coculture experiment the human SupT1 cell line was added directly to the culture of sCD4-producing NIH 3T3 cells, and in another experiment the two cell types were separated physically by a semipermeable membrane. In both coculture configurations, the T cell line was protected from HIV-1 challenge as measured by syncytium formation and indirect immunofluorescent assays. In addition, the SupT1 line was directly engineered with sCD4-expressing retroviral vectors and shown to be protected from HIV-1 challenge. As a prelude to further preclinical studies, we tested the ability of retroviral vectors to transduce primary human peripheral blood lymphocytes (PBLs). Conditions used to stimulate T cell growth resulted in significant shifts in the CD4/CD8 cell in favor of CD8 cells. Retroviral-mediated gene transfer under these conditions resulted in low levels of gene transfer (< 5%).(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term inhibition of clinical and laboratory human immunodeficiency virus strains in human T-cell lines containing an HIV-regulated diphtheria toxin A chain gene

The human immunodeficiency virus (HIV) causes persistent infection of T cells. Chemotherapy for infection in humans may slow HIV-related disease progression, but it does not eradicate virus. Thus, other treatment modalities are warranted. We have previously demonstrated that the human T cell line H9, ordinarily permissive for HIV infection, may be protected against infection with the LAI strain of HIV by intracellular immunization with the gene encoding diphtheria toxin A chain (DT-A) under the control of HIV Tat and Rev. Cloned cells were protected for up to 6 days in vitro. We now report protection against the LAI laboratory isolate for up to 59 days, and against clinical HIV strains of differing phenotypic properties and cell tropisms for up to 59 days. In some cases, protection was complete in that no residual HIV was detected by HIV p24 antigen production, co-culture with parental H9 cells, or the polymerase chain reaction (PCR). CD4+ surface expression of DT-A transduced cloned H9 cells was similar to parental H9 in most cases. These results suggest that toxin gene therapy for HIV infection may ultimately be feasible.

The development and testing of retroviral vectors expressing trans-dominant mutants of HIV-1 proteins to confer anti-HIV-1 resistance

Trans-dominant mutants of human immunodeficiency virus type 1 (HIV-1) Tat and Rev are attractive candidates for use in gene therapy in the treatment of HIV-1 infections because both are essential for viral replication. Retroviral vectors were constructed to allow either Tat-inducible or Tat- and Rev-inducible expression of trans-dominant mutants of Tat and Rev. These vectors were used to infect a human CD4+ lymphocyte-derived cell line, MT4. To determine the efficacy of various Tat and Rev mutants in inhibiting HIV-1 multiplication, MT4 cells containing mutant-expressing constructs were infected with HIV-1, and the amount of HIV-1 released in the culture medium was measured for up to 30 days. A high level of resistance was observed in cells expressing the double tat/rev mutant in a Tat-inducible manner.

Tat protein of human immunodeficiency virus type 1 represses expression of manganese superoxide dismutase in HeLa cells

Using a HeLa cell line stably transfected with the tat gene from human immunodeficiency virus type 1, we have found that the expression of the regulatory Tat protein suppresses the expression of cellular Mn-containing superoxide dismutase (Mn-SOD). This enzyme is one of the cell's primary defenses against oxygen-derived free radicals and is vital for maintaining a healthy balance between oxidants and antioxidants. The parental HeLa cells expressed nearly equivalent amounts of Cu,Zn- and Mn-SOD isozymes. Those cells expressing the Tat protein, however, contained 52% less Mn-SOD activity than parental cells, whereas that of the Cu,Zn enzyme was essentially unchanged. The steady-state levels of Mn-SOD-specific RNAs were also lower in the HeLa-tat cell line than in the parental line. No difference was seen in the steady-state levels of Cu,Zn-SOD-specific RNAs. In addition to the decreased Mn-SOD-activity, HeLa-tat cell showed evidence of increased oxidative stress. Carbonyl proteins were markedly higher, and total cellular sulfhydryl content decreased in cell extracts at a faster rate, probably reflecting ongoing lipid peroxidation. HeLa and HeLa-tat extracts were incubated with radiolabeled Mn-SOD transcripts, and the reaction products were subjected to UV crosslinking, digestion with ribonuclease A, and electrophoretic analysis. The results suggest a direct interaction between Tat protein and Mn-SOD gene transcripts.

Positive-negative selection gene targeting with the diphtheria toxin A-chain gene in mouse embryonic stem cells

The diphtheria toxin A-chain gene was used in a positive-negative selection gene targeting vector to alter the CD4 gene which is transcriptionally silent in mouse embryonic stem cells. Expression of the toxin gene was driven by a constitutively active enhancer, yet the targeting construct exhibited only minimal transient toxicity while enriching for targeted clones 9- to 29-fold. Germline transmission of the stem cell-derived genome was obtained. These data suggest the usefulness of this diphtheria toxin A-chain cassette in replacement-type positive-negative selection vectors. Its potential for novel applications, particularly in the enrichment for 'hit-and-run' insertion-type vectors, is discussed.

Effects of enhancer mutations on the expression of human immunodeficiency virus 1-regulated luciferase and diphtheria toxin A chain genes in transfected cells

This study explores human immunodeficiency virus 1 (HIV-1)-regulated diphtheria toxin A (DT-A) gene expression as a means of eradicating HIV-infected cells. Previously, we constructed luciferase and DT-A plasmids, containing cis-acting Tat and Rev responsive elements, which showed low basal expression and required both Tat and Rev for maximal expression. Cell lines which had stably integrated the DT-A constructs were resistant to HIV production. To reduce toxicity due to basal expression, this study investigates the effect of mutations in the HIV enhancer on expression of luciferase and DT-A plasmids. Some mutations were found to substantially reduce basal expression while still allowing for trans-activation. Such mutations, in combination with attenuated versions of DT-A, may make regulated toxin gene expression feasible as a therapy for AIDS.

A novel transgenic mouse model for the in vivo evaluation of anti-human immunodeficiency virus type 1 drugs

We have developed a binary transgenic mouse system that allows easy in vivo evaluation of new anti-human immunodeficiency virus type 1 (HIV-1) drugs or therapies specifically designed to target the viral transactivator protein (TAT) or long terminal repeat (LTR) functions. This approach consists of a simple genetic cross between an "activator" transgenic mouse expressing the HIV-1-tat gene exclusively to T lymphocytes and a "target" transgenic mouse bearing a silent reporter gene whose expression is under the control of the HIV-1-LTR. As expected, most of the target transgenic animals did not express the reporter gene; on the contrary, all the double-transgenic mice bearing both the activator and target transgenes strongly expressed the TAT-induced reporter gene. The choice of a secreted human alpha 1-antitrypsin variant (alpha 1-AT) as reporter gene readily permits in a single animal the quantitative determination of the plasma level of alpha 1-AT protein before and after anti-LTR or anti-TAT treatments. Such mice may be valuable as new laboratory models for the in vivo evaluation of agents with potential anti-HIV-1 activity.

A brief history of gene therapy

The concepts of gene therapy arose initially during the 1960s and early 1970s whilst the development of genetically marked cells lines and the clarification of mechanisms of cell transformation by the papaovaviruses polyoma and SV40 was in progress. With the arrival of recombinant DNA techniques, cloned genes became available and were used to demonstrate that foreign genes could indeed correct genetic defects and disease phenotypes in mammalian cells in vitro. Efficient retroviral vectors and other gene transfer methods have permitted convincing demonstrations of efficient phenotype correction in vitro and in vivo, now making gene therapy a broadly accepted approach to therapy and justifying clinically applied studies with human patients.

Human immunodeficiency virus vectors for inducible expression of foreign genes

Tat-dependent expression of an endogenous lethal or deleterious foreign gene might be useful for abrogating the production of human immunodeficiency virus (HIV) from cells. This type of HIV-induced cellular killing, as well as other approaches to gene therapy for HIV infection, would be facilitated by simple HIV vectors that express introduced genes in a Tat-inducible manner. As part of studies to examine the feasibility of this concept, we constructed HIV-1 vectors that express the hygromycin B phosphotransferase gene (Hygr) in a Tat-dependent manner. Comparison of the efficiency of propagation of each vector indicates that sequences extending into the gag open reading frame are necessary in cis for efficient vector propagation. Southern blot analysis of genomic DNA isolated from vector-infected cells demonstrated that the vectors were capable of being propagated as expected without gross rearrangements or deletions. A fragment of the influenza A virus hemagglutinin (H5 HA) gene, capable of eliciting antibody and cytotoxic T-cell responses, was used as a marker for further characterization of the vector system. A Tat-dependent vector conferring the H5 HA+ phenotype was assayed by indirect immunofluorescence, and cells which contained but did not express the H5 HA gene were isolated. The activation of H5 HA expression following HIV infection of Tat- cells that stably contained but did not express the H5 HA construct was determined to be an efficient process.