Quantitative analysis of gene suppression in integrated retrovirus vectors

Modifying inter-cistronic sequence significantly enhances IRES dependent second gene expression in bicistronic vector: Construction of optimised cassette for gene therapy of familial hypercholesterolemia

Internal ribosome entry site (IRES) sequences have become a valuable tool in the construction of gene transfer and therapeutic vectors for multi-cistronic gene expression from a single mRNA transcript. The optimal conditions for effective use of this sequence to construct a functional expression vector are not precisely defined but it is generally assumed that the internal ribosome entry site dependent expression of the second gene in such as cassette is less efficient than the cap-dependent expression of the first gene. Mainly tailoring inter-cistronic sequence significantly enhances IRES dependent second gene expression in bicistronic vector further in construction of optimised cassette for gene therapy of familial hypercholesterolemia. We tailored the size of the inter-cistronic spacer sequence at the 5′ region of the internal ribosome entry site sequence using sequential deletions and demonstrated that the expression of the 3′ gene can be significantly increased to similar levels as the cap-dependent expression of the 5’ gene. Maximum expression efficiency of the downstream gene was obtained when the spacer is composed of 18–141 base pairs. In this case a single mRNA transcriptional unit containing both the first and the second Cistron was detected. Whilst constructs with spacer sequences of 216 bp or longer generate a single transcriptional unit containing only the first Cistron. This suggests that long spacers may affect transcription termination. When the spacer is 188 bp, both transcripts were produced simultaneously in most transfected cells, while a fraction of them expressed only the first but not the second gene. Expression analyses of vectors containing optimised cassettes clearly confirm that efficiency of gene transfer and biological activity of the expressed transgenic proteins in the transduced cells can be achieved. Furthermore, Computational analysis was carried out by molecular dynamics (MD) simulation to determine the most emerges as viable containing specific binding site and bridging of 5′ and 3′ ends involving direct RNA-RNA contacts and RNA-protein interactions. These results provide a mechanistic basis for translation stimulation and RNA resembling for the synergistic stimulation of cap-dependent translation.

A system for creating stable cell lines that express a gene of interest from a bidirectional and regulatable herpes simplex virus type 1 promoter

Expression systems used to study the biological function of a gene of interest can have limited utility due to three major factors: i) weak or heterogeneous gene expression; ii) poorly controlled gene expression; and iii) low efficiencies of stable integration and persistent expression. We envisioned that the ideal system should be tightly controlled and coupled with the ability to efficiently create and identify stable cell lines. Herein, we describe a system based upon a bidirectional Herpes simplex virus type 1 promoter that is naturally responsive to the VP16 transactivator and modified to permit tetracycline-regulated transcription on one side while maintaining constitutive activity on the other side. Incorporation of this element into the Sleeping Beauty transposon resulted in a novel bidirectional system with the capacity for high-efficiency stable integration. Using this system, we created stable cell lines in which expression of a gene of interest was tightly and uniformly controlled across a broad range of levels via a novel combination of doxycycline-sensitive de-repression and VP16-mediated sequence-specific induction. The unique characteristics of this system address major limitations of current methods and provide an excellent strategy to investigate the effects of gene dosing in mammalian models.

A bidirectional promoter architecture enhances lentiviral transgenesis in embryonic and extraembryonic stem cells

The two main challenges facing retroviral transgenesis are variable expression and epigenetic silencing. Although modern lentiviral vectors incorporate several elements to increase transgene expression and reduce position effect variegation and silencing, therapeutic research in stem cells, as well as production of transgenic animals, is still hampered by these two key problems. On the basis of recent studies demonstrating the chromatin insulating properties of divergent promoters, we sought to develop a bidirectional lentiviral vector with which to conduct RNA interference (RNAi)-based genetic screens in embryonic and extraembryonic stem cells. To this end, we designed and tested a series of synthetic bidirectional promoters, combining the mouse phosphoglycerate kinase 1 (Pgk1) promoter with other strong mammalian and viral promoters. Here, we demonstrate that a back-to-back configuration of the mouse Pgk1 and human eukaryotic translation elongation factor 1 alpha 1 promoters provided a substantive increase in both transgene expression and RNAi-based transcript depletion as compared with previous designs and other promoter combinations. Using this vector, we were able to achieve stable and robust depletion of a transfected luciferase reporter, as well as an endogenous non-coding RNA. The described constructs are an improved transgene delivery system capable of conducting RNAi screens in stem cells at single copy.

Retroviral vectors for gene therapy

Since their first clinical trial 20 years ago, retroviral (gretroviral and lentiviral) vectors have now been used in more than 350 gene-therapy studies. Retroviral vectors are particularly suited for gene-correction of cells due to long-term and stable expression of the transferred transgene(s), and also because little effort is required for their cloning and production. Several monogenic inherited diseases, mostly immunodeficiencies, can now be successfully treated. The occurrence of insertional mutagenesis in some studies allowed extensive analysis of integration profiles of retroviral vectors, as well as the design of lentiviral vectors with increased safety properties. These new-generation vectors will enable us to continue the successful story of gene therapy, and treat more patients and even more complex diseases.

An improved self-deleting retroviral vector derived from avian leukemia and sarcoma virus

We have previously developed a self-deleting avian leukosis and sarcoma virus (ALSV)- based retroviral vector carrying an additional attachment (att) sequence. Resulting proviruses underwent deletion of viral sequences and were flanked either by two LTRs (LTRs proviruses) or by the additional att sequence and the 3' LTR (att proviruses). Herein, we have tried to increase (1) the self-deleting properties of this vector, either by raising the selection pressure applied on target cells or by optimizing the size of the internal att sequence, (2) the titer of the vector by deleting or inverting some viral sequences. Moreover, a new type of provirus flanked by att sequences at each end was isolated. Finally, under specific conditions, 100% of proviruses had internal sequences deleted, and as many as 92-100% of proviruses were no longer mobilizable by a replication-competent virus. The inactivation procedure achieved here might improve the biosafety of retroviral vectors.

A novel self-deleting retroviral vector carrying an additional sequence recognized by the viral integrase (IN)

During retroviral integration, the viral integrase recognizes the attachment (att) sequence (formed by juxtaposition of two LTRs ends) as the substrate of integration. We have developed a self-deleting Avian Leukosis and Sarcoma Viruses (ALSVs)-based retroviral vector carrying an additional copy of the att sequence, between neo and puro genes. We observed that: (i) the resulting NP3Catt vector was produced at neo and puro titers respectively smaller and higher than that of the parental vector devoid of the att sequence; (ii) 61% of NP3Catt proviruses were flanked by LTRs; most of them were deleted of internal sequences, probably during the reverse transcription step; (iii) 31% of clones were deleted of the whole 5' part of their genome and were flanked, in 5', by the additional att sequence and, in 3', by an LTR. Integration of these last proviruses was often imprecise with respect to the viral ends. At total, 77% of proviruses had lost the packaging signal and were not mobilizable by a replication-competent virus and 92% had lost the selectable gene in a single round of replication. Although still to improve, the att vector could be considered as an interesting new safe retroviral vector for gene transfer experiments.

A New Generation of Retroviral Producer Cells: Predictable and Stable Virus Production by Flp-Mediated Site-Specific Integration of Retroviral Vectors

We developed a new strategy that provides well-defined high-titer producer cells for recombinant retroviruses in a minimum amount of time. The strategy involves the targeted integration of the retroviral vector into a chromosomal locus with favorable properties. For proof of concept we established a novel HEK293-based retroviral producer cell line, called Flp293A, with a single-copy retroviral vector integrated at a selected chromosomal locus. The vector was flanked by noninteracting Flp-recombinase recognition sites and was exchanged for different retroviral vectors via Flp-mediated cassette exchange. All analyzed cell clones showed correct integration and identical titers for each of the vectors, confirming that the expression characteristics from the parental cell were preserved. Titers up to 2.5 x 10(7) infectious particles/10(6) cells were obtained. Also, high-titer producer cells for a therapeutic vector that encodes the 8.9-kb collagen VII cDNA in a marker-free cassette were obtained within 3 weeks without screening. Thus, we provide evidence that the precise integration of viral vectors into a favorable chromosomal locus leads to high and predictable virus production. This method is compatible with other retroviral vectors, including self-inactivating vectors and marker-free vectors. Further, it provides a tool for evaluation of different retroviral vector designs.

Overcoming promoter competition in packaging cells improves production of self-inactivating retroviral vectors

Retroviral vectors with self-inactivating (SIN) long-terminal repeats not only increase the autonomy of the internal promoter but may also reduce the risk of insertional upregulation of neighboring alleles. However, gammaretroviral as opposed to lentiviral packaging systems produce suboptimal SIN vector titers, a major limitation for their clinical use. Northern blot data revealed that low SIN titers were associated with abundant transcription of internal rather than full-length transcripts in transfected packaging cells. When using the promoter of Rous sarcoma virus or a tetracycline-inducible promoter to generate full-length transcripts, we obtained a strong enhancement in titer (up to 4 x 10(7) transducing units per ml of unconcentrated supernatant). Dual fluorescence vectors and Northern blots revealed that promoter competition is a rate-limiting step of SIN vector production. SIN vector stocks pseudotyped with RD114 envelope protein had high transduction efficiency in human and non-human primate cells. This study introduces a new generation of efficient gammaretroviral SIN vectors as a platform for further optimizations of retroviral vector performance.

Modeling a self-avoiding chromatin loop: relation to the packing problem, action-at-a-distance, and nuclear context

There is now convincing evidence that genomes are organized into loops, and that looping brings distant genes together so that they can bind to local concentrations of polymerases in "factories" or "hubs." As there remains no systematic analysis of how looping affects the probability that a gene can access binding sites in such factories/hubs, we used an algorithm that we devised and Monte Carlo methods to model a DNA or chromatin loop as a semiflexible (self-avoiding) tube attached to a sphere; we examine how loop thickness, rigidity, and contour length affect where particular segments of the loop lie relative to binding sites on the sphere. Results are compared with those obtained with the traditional model of an (infinitely thin) freely jointed chain. They provide insights into the packing problem (how long genomes are packed into small nuclei), and action-at-a-distance (how firing of one origin or gene can prevent firing of an adjacent one).

Selection with a regulated cell growth switch increases the likelihood of expression for a linked gamma-globin gene

Several lines of evidence indicate that in vivo drug selection can be used to overcome the low rates of gene transfer and engraftment encountered in many hematopoietic stem cell gene therapy settings. However, whether selection imposed on one transcription cassette effects the likelihood of expression from a second, independent transcription cassette within the same vector has been less well studied. In order to address this issue, we engineered an oncoretrovirus vector to express two separate transcription units: (i) a bicistronic cassette encoding both GFP and a pharmacologically regulated cell growth switch based on the thrombopoietin receptor Mpl; and (ii) a highly position-dependent second cassette encoding human gamma-globin. Studies in cell cultures and in mice transplanted with transduced marrow indicated that selective expansion increased by more than 9-fold the fraction of erythroid cells expressing the linked but separate expression cassette for gamma-globin. This increase was far greater then that observed for the bicistronic GFP gene, and cannot be explained by a simple increase in the fraction of cells containing provirus. These results suggest that selective expansion favors erythroid stem/progenitor cells with provirus integrated at chromosomal sites which are relatively resistant to silencing position effects.

Short interfering RNAs as a tool for cancer gene therapy

There are mainly two types of short RNAs that target complementary messengers in animals: small interfering RNAs and micro-RNAs. Both are produced by the cleavage of double-stranded RNA precursors by Dicer, a member of the Rnase III family of double-stranded specific endonucleases, and both guide the RNA-induced silencing complex to cleave specifically RNAs sharing sequence identity with them. In designing a particular RNA interference (RNAi), it is important to identify the sense/antisense combination that provides the most potent suppression of the target mRNA, and several rules have been established to give >90% gene expression inhibition. RNAi technology can be directed against cancer using a variety of strategies. These include the inhibition of overexpressed oncogenes, blocking cell division by interfering with cyclin E and related genes or promoting apoptosis by suppressing antiapoptotic genes. RNAi against multidrug resistance genes or chemoresistance targets may also provide useful cancer treatments. Studies investigating these approaches in preclinical models are also reviewed.

Development of a self-inactivating tet-on retroviral vector expressing bone morphogenetic protein 4 to achieve regulated bone formation

The aims of this study were to explore the possibility of improving the design of self-inactivating (SI) retroviral vectors and to develop an SI vector that would allow optimal tet-on-regulated therapeutic gene expression. To minimize any interference between the viral promoter and the inducible promoter, we deleted different regulatory elements in the 3'LTR and examined their effects on transgene expression in transfected or transduced cells. In transfected cells, such deletions reduced the transgene expression. The insertion of a polyadenylation sequence could not completely compensate for this effect. We observed three patterns of transgene expression in cells transduced with these tet-on retroviral vectors: (1) high levels of both basal and inducible expression, (2) low levels of both basal and inducible expression, and (3) low levels of basal and high levels of inducible expression. After using the optimal vector to transduce muscle-derived stem cells, we were able to regulate the strong in vitro expression of transgenes-including enhanced green fluorescent protein and bone morphogenetic protein 4-via the addition or withdrawal of doxycycline (Dox). Implantation of the transduced cells and subsequent Dox-dependent induction of gene expression resulted in bone formation in vivo. Thus, we have developed an optimal SI retroviral vector that maintains a high titer, efficiently transduces muscle-derived stem cells, and enables both high levels of inducible gene expression in vitro and robust regulated bone formation in vivo.

Coordinate dual-gene transgenesis by lentiviral vectors carrying synthetic bidirectional promoters

Transferring multiple genes into the same cell allows for the combination of genetic correction, marking, selection and conditional elimination of transduced cells or the reconstitution of multisubunit components and synergistic pathways. However, this cannot be reliably accomplished by current gene transfer technologies. Based on the finding that some cellular promoters intrinsically promote divergent transcription, we have developed synthetic bidirectional promoters that mediate coordinate transcription of two mRNAs in a ubiquitous or a tissue-specific manner. Lentiviral vectors incorporating the new promoters enabled efficient dual gene transfer in several tissues in vivo after direct delivery or transgenesis, and in a human gene therapy model. Because divergent gene pairs, likely transcribed from shared promoters, are common in the genome, the synthetic promoters that we developed may mimic a well-represented feature of transcription. Vectors incorporating these promoters should increase the power of gene function studies and expand the reach and safety of gene therapy.

B-cell-specific transgene expression using a self-inactivating retroviral vector with human CD19 promoter and viral post-transcriptional regulatory element

Retroviral gene transfer resulting in transgene expression selectively restricted to specific cell lineages would be desirable for many gene therapeutic applications. Such transcriptional targeting of retroviruses can be accomplished by employing eukaryotic control elements in self-inactivating (SIN) retroviral vectors, but use of these vectors is complicated by an accompanying reduction in viral titers. To overcome this restriction and address the influence of the post-transcriptional regulatory element of the Woodchuck hepatitis virus (WPRE) on viral titers and transgene expression, we developed SIN-vectors with and without WPRE. Using the enhancer-promoter of the Spleen Focus Forming virus (SFFV) to direct eGFP expression to multiple hematopoietic lineages, we show that WPRE significantly (>10 x) increased viral titers (>10(6) per ml of unconcentrated supernatant) and transgene expression in NIH3T3 cells in vitro. Gene expression in vivo was significantly lowered in lymphoid cells, but not in myeloid cells when WPRE was present. Furthermore, the use of WPRE in combination with the B-cell lineage-specific CD19 promoter significantly increased viral titers and allowed targeting of transgene expression by SIN-vectors specifically to B cells throughout their development in primary and secondary lymphoid organs.

Modeling the dosage effect of oncogenes in leukemogenesis

PURPOSE OF REVIEW

This review discusses the dosage effects of some oncogenes in leukemogenesis and compares various methods that model human hematologic malignancies in mice by introducing genetic lesions in a cell type-specific, time-controlled, and dosage-relevant manner.

RECENT FINDINGS

Recent evidence indicates that optimal dosage of cancer-related gene products plays an important role in the induction of mouse tumors that recapitulate their human counterparts.

SUMMARY

The mouse is a very valuable model system for experimentally dissecting the in vivo pathogenesis of cancer, for identifying pharmacological targets of cancer and for evaluating cancer therapies. In modeling human cancer, it has been shown that both the timing of introducing/activating oncogenic mutation(s) and the cell types into which the genetic lesion(s) is targeted are critical for cancer development. Recent studies also showed that efficient induction of relevant human leukemia in mice by certain oncogenes, such as PML/RARalpha and TEL/ABL, only occurred when they were expressed at a low level or close to pathophysiologically relevant level. These studies stress the importance of studying oncoprotein function at pathophysiologically relevant expression levels. Conditional gene expression systems are powerful tools for developing mouse models for human cancer by introducing genetic lesions in a cell type-specific, time-controlled and dosage-relevant manner. The bone marrow retroviral transduction and transplantation system can also mimic the cell and temporally specific origin of hematological malignancies by targeting oncogenes into sorted hematopoietic cells. This versatile approach is particularly powerful in structure-function analysis of oncogenes in vivo. However, overexpression of a transgene driven by retroviral vectors may alter the biological outcomes of the transgene in vivo. My colleagues and I have shown that generating vectors with modulated transgene expression can overcome this limitation of the retroviral transduction system in modeling human cancer in mice. Conditional gene expression and the modified retroviral transduction systems will be complimentary in studying human cancers in mice.

Tight positive regulation of transgene expression by a single adenovirus vector containing the rtTA and tTS expression cassettes in separate genome regions

We previously developed single adenovirus (Ad) vectors that contained the components for a tetracycline-regulatable gene-expression system in the E1 and E3 deletion regions, and showed that the Ad vectors containing the tet-on system exhibit a much inferior regulation of transgene expression than those containing the tet-off system. In many cases, the tet-on system may be preferable because of its positive regulation of transgene expression. To this end, in the present study, by introducing the latest generation reverse tetracycline-responsive transcriptional activator (rtTA2s-M2 or rtTA2s-S2) and the tetracycline-controlled transcriptional silencer (tTS) into the original tet-on system, we constructed various modified Ad-mediated tet-on systems. Among them, the novel single Ad vector, which contained three heterologous gene-expression cassettes of the gene of interest, rtTA2s-S2, and tTS in the E1 deletion region, the E3 deletion region, and the region between E4 and 3'ITR, respectively, displayed vastly improved doxycycline-inducible gene expression in terms of low basal expression, high induced expression, and high responsiveness to doxycycline both in vitro and in vivo. These results also suggest that the low responsiveness to doxycycline may explain why the original tet-on system in the context of the Ad vector is not effective in vivo. This is the first report describing the cloning of three heterologous gene-expression cassettes into three separate regions of the E1/E3-deleted Ad genome. This improved Ad-mediated tet-on system might be useful for gene therapy and greatly facilitate the analyses of gene function.

Transduction and selection of human T cells with novel CD34/thymidine kinase chimeric suicide genes for the treatment of graft-versus-host disease

Clinical trials evaluating the herpes simplex virus thymidine kinase (HSV-tk)/ganciclovir (GCV) suicide gene therapy system for the control of graft-versus-host disease (GVHD) have been limited by low transduction efficiencies and inefficient selection procedures. In this study, we designed and evaluated a novel chimeric suicide gene consisting of the extracellular and transmembrane domains of human CD34 and full-length HSV-tk (DeltaCD34-tk). High-efficiency transfer of DeltaCD34-tk to primary human T cells was accomplished after a single exposure to VSV-G-pseudotyped, Moloney murine leukemia virus-based retrovirus 48 h after activation of human PBMCs with anti-CD3 and anti-CD28 antibodies immobilized on magnetic beads. Using an optimized 5-day transduction and selection procedure, transduction efficiencies averaged 71%, with isolation purities greater than 95% and yields exceeding 90%. The immunoselected T cells were selectively eliminated by GCV (IC(50) approximately 3 nM), maintained a normal subset composition, exhibited a polyclonal TCR Vbeta family repertoire, and contained 5 or 6 vector copies per transduced cell when optimally transduced. No increase in GCV sensitivity was observed upon incorporation of highly active mutant HSV-tk enzymes into the DeltaCD34-tk suicide gene. T cells modified with the DeltaCD34-tk gene using the optimized protocol should improve the overall efficacy of the HSV-tk/GCV suicide gene therapy method of GVHD control.

Enhanced antitumor effects of a bicistronic adenovirus vector expressing both herpes simplex virus thymidine kinase and monocyte chemoattractant protein-1 against hepatocellular carcinoma

The efficacy of the suicide gene therapy using the herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) system for the treatment of cancer is limited because of the insufficient gene transfer and the low killing activity. To enhance the antitumor activity, we determined whether recombinant adenovirus vector (rAd)s expressing both HSV-tk and monocyte chemoattractant protein-1 (MCP-1) genes could potentiate the destruction of hepatocellular carcinoma (HCC). The rAd Ad-tk-MCP1 harboring HSV-tk and MCP-1 genes in sequence under the universal CAG promoter was constructed with a bicistronic unit including the encephalomyocarditis virus-internal ribosomal entry site. The levels of HSV-tk expression and GCV-sensitive tumoricidal activity of Ad-tk-MCP1 were comparable to those of rAd expressing HSV-tk alone. The growth of subcutaneous tumors in athymic nude mice was markedly suppressed when tumors were treated with Ad-tk-MCP1 as opposed to another bicistronic vector Ad-MCP1-tk, rAd expressing either HSV-tk or MCP-1, or both of these vectors. The antitumor effects of Ad-tkMCP1 may be dependent on the activation of macrophages, since the recruitment of macrophages was observed tumor necrosis factor-alpha production was enhanced in the tumor tissue. Furthermore, the enhanced antitumor effect was abolished by inactivating macrophages with carrageenan treatment. These results demonstrated that a bicistronic rAd harboring both suicide and chemokine genes in sequence exerted the enhanced, macrophage-dependent, antitumor effects in a model of HCC and support the use of this strategy for the treatment of HCC.

Hepatic fibrosis--role of hepatic stellate cell activation

Hepatic fibrosis is a reversible wound healing response characterized by accumulation of extracellular matrix (ECM), or "scar," that follows chronic but not self-limited liver disease. The ECM components in fibrotic liver are similar regardless of the underlying cause. Activation of hepatic stellate cells is the central event in hepatic fibrosis. These perisinusoidal cells orchestrate an array of changes including degradation of the normal ECM of liver, deposition of scar molecules, vascular and organ contraction, and release of cytokines. Not only is hepatic fibrosis reversible, but it is also increasingly clear that cirrhosis may be reversible as well. The exact stage at which fibrosis/cirrhosis becomes truly irreversible is not known. Antifibrotic therapies will soon be a clinical reality. Emerging therapies will be targeted to those patients with reversible disease. The paradigm of stellate cell activation provides an important framework for defining therapeutic targets.

High translation efficiency is mediated by the encephalomyocarditis virus internal ribosomal entry sites if the natural sequence surrounding the eleventh AUG is retained

Internal ribosomal entry sites (IRES) allow cap-independent translation and are sequences that are often used in gene therapy for strategies in which several genes need to be expressed. In this study, two different sequences of encephalomyocarditis virus (EMCV) IRES were compared for their translation efficiency in the context of retroviral vectors. When the sequence surrounding the 11th AUG of the IRES was conserved (IRESg), the translation efficiency was significantly higher than if the AUG of the downstream gene started with the 11th AUG of the IRES (IRESb). The translation efficiency with IRESg was influenced by the cell type and also by the nature of the transgene.

Expression from cell type-specific enhancer-modified retroviral vectors after transduction: influence of marker gene stability

The enhanced green fluorescent protein (EGFP) is increasingly used as a reporter gene in viral vectors for a number of applications. To establish a system to study the activity of cis-acting cellular regulatory sequences, we deleted the viral enhancer in EGFP-carrying retroviral vectors and replaced it with cell type-specific elements. In this study, we use this system to demonstrate the activity of the human CD2 lymphoid-specific and the Tie2 endothelial cell type-specific enhancers in cell lines and in primary cells transduced by retroviral vectors. Furthermore, we compare findings obtained with EGFP as the reporter gene to those obtained replacing EGFP with d2EGFP, an unstable variant of EGFP characterized by a much shorter half-life compared to EGFP, and by reduced accumulation in the cells. d2EGFP-carrying vectors were generated at titers which were not different from those generated by the corresponding vectors carrying EGFP. Moreover, the activity of a Moloney murine leukemia virus enhancer could be readily detected following transduction of target cells with either EGFP- or d2EGFP-carrying vectors. However, the activity of the relatively weak CD2 and Tie2 enhancers was exclusively detected using EGFP as the reporter gene. These findings indicate that enhancer replacement is a feasible and promising approach to address the function of cell type-specific regulatory elements in retroviral vectors carrying the EGFP gene.

High-level erythroid-specific gene expression in primary human and murine hematopoietic cells with self-inactivating lentiviral vectors

Use of oncoretroviral vectors in gene therapy for hemoglobinopathies has been impeded by low titer vectors, genetic instability, and poor expression. Fifteen self- inactivating (SIN) lentiviral vectors using 4 erythroid promoters in combination with 4 erythroid enhancers with or without the woodchuck hepatitis virus postregulatory element (WPRE) were generated using the enhanced green fluorescent protein as a reporter gene. Vectors with high erythroid-specific expression in cell lines were tested in primary human CD34(+) cells and in vivo in the murine bone marrow (BM) transplantation model. Vectors containing the ankyrin-1 promoter showed high-level expression and stable proviral transmission. Two vectors containing the ankyrin-1 promoter and 2 erythroid enhancers (HS-40 plus GATA-1 or HS-40 plus 5-aminolevulinate synthase intron 8 [I8] enhancers) and WPRE expressed at levels higher than the HS2/beta-promoter vector in bulk unilineage erythroid cultures and individual erythroid blast-forming units derived from human BM CD34(+) cells. Sca1(+)/lineage(-) Ly5.1 mouse hematopoietic cells, transduced with these 2 ankyrin-1 promoter vectors, were injected into lethally irradiated Ly5.2 recipients. Eleven weeks after transplantation, high-level expression was seen from both vectors in blood (63%-89% of red blood cells) and erythroid cells in BM (70%-86% engraftment), compared with negligible expression in myeloid and lymphoid lineages in blood, BM, spleen, and thymus (0%-4%). The I8/HS-40-containing vector encoding a hybrid human beta/gamma-globin gene led to 43% to 113% human gamma-globin expression/copy of the mouse alpha-globin gene. Thus, modular use of erythroid-specific enhancers/promoters and WPRE in SIN-lentiviral vectors led to identification of high-titer, stably transmitted vectors with high-level erythroid-specific expression for gene therapy of red cell diseases.

Promoter suppression in cultured mammalian cells can be blocked by the chicken beta-globin chromatin insulator 5'HS4 and matrix/scaffold attachment regions

Studies have indicated that two transcriptionally active units can repress one another when they lie adjacent in head-to-tail tandem on a chromosome. Repression of a downstream (3') unit by an upstream (5') unit is known as "transcriptional interference", whereas repression of a 5' unit by a 3' unit is termed "promoter suppression". These two processes can occur between head-to-tail tandem copies of a transgene, or between transgenes and adjacent chromosomal genes. Interference can be blocked by inserting a transcription terminator between adjacent units. Here, we report that "promoter suppression" could be blocked by the insulator 5' DNaseI hypersensitive site 4, or matrix/scaffold attachment regions (MAR/SARs), when these elements were interposed between adjacent units. Because intergenic spacers of many repeated eukaryotic genes contain MAR/SARs and insulators, our observations suggest that these elements have the ability to segregate repeated genes into domains that act independently of one another. Our observations also suggest strategies to design transgenes that can act as autonomous units of expression.

Evaluation of retroviral vector design in defined chromosomal loci by Flp-mediated cassette replacement

Successful retroviral vector construction is still empirical. Test systems for vector efficiency are based on statistical comparison of numerous infectants with single proviral integrates, since their expression depends on the chromosomal surroundings. More reliable data would be obtained if different vector constructs were studied in an identical chromosomal context. Here, we demonstrate the use of a new method, in which chromosomal sites are provirally tagged in such a way that they can be targeted with other expression cassettes. The original tagging integrate is replaced in one step by the targeting element. This permits a reliable comparison of different retroviral vector configurations, eliminating the influence of neighboring chromosomal elements. We compared different retroviral vector types for coexpression of two genes: a vector containing an internal promoter and a vector with an internal ribosome entry site (IRES) element. In contrast to bicistronic retroviral vectors, dual-promoter proviruses exhibited rapid inactivation of the long terminal repeat (LTR)-driven gene expression. Targeted exchange of the dual-promoter provirus with a bicistronic retroviral cassette resulted in gain of expression stability. The reverse experiment confirmed this promoter interaction phenomenon since initial expression stability from a single-promoter bicistronic provirus was lost by targeted exchange with a dual-promoter cassette. In addition, targeting exchange of the dual-promoter provirus, replacing the LTR with an artificial (Tet) promoter restored expression stability. These observations, valid for various integration sites, prove the strong interaction between the LTR and the internal promoter. Our results have implications for retroviral vector design and suggest that retroviral coexpression of two genes is more predictable in the bicistronic configuration.

Retroviral vector design studies toward hematopoietic stem cell gene therapy for mucopolysaccharidosis type I

To optimize a gene transfer system for hematopoietic stem cell gene therapy of patients with mucopolysaccharidosis (MPS) type I, 10 retroviral vectors were constructed to express the human alpha-L-iduronidase (IDUA) cDNA. These vectors were designed to evaluate the potential effects of specific promoters, the addition of selectable markers, and the use of multiple promoters versus an internal ribosome entry site for expression of IDUA and selectable maker genes. The effect of vector design was investigated in primary patient fibroblasts (F(MPS)) or murine fibroblast cell lines; while overall comparison of transgene expression was determined in patients' peripheral blood lymphocytes (PBL(MPS)) and CD34+ progenitors (PBPC(MPS)). We observed that the human PGK promoter introduced the highest IDUA activity per 1% relative transgene frequency in F(MPS). Use of the same promoter to separately regulate both the therapeutic gene and a drug-resistance gene resulted in decreased expression of the unselected gene. Co-selection using bicistronic vectors not only increased the number of transductants, but also elevated transgene expression under selective pressure in transgene-positive progenitors. Bicistronic vector LP1CD overcame down-regulation and practically introduced the highest IDUA level in unselected PBL(MPS) and an intermediate level in PBPC(MPS). These studies provide a better understanding of factors contributing to efficient gene expression in hematopoietic cells.

New vectors for gene therapy

Retrovirus-based vectors are presently the most efficient gene transfer vehicles for introducing genes into human hematopoietic stem and progenitor cells. However, their use for gene therapy is still problematic. A major obstacle is viral sequences such as the tRNA primer binding site or the dimerization and encapsidation signals that are not required for the expression of the therapeutic gene. These sequences can recombine with endogenous and/or exogenous retroviruses to generate new forms of unpredictable retroviruses. Moreover, these sequences are the targets for transcriptional repressors which inhibit the expression of the transduced genes. Therefore we have developed a new generation of retrovirus vectors which self-delete upon integration. The vectors are based on the natural life cycle of retroviruses, involving duplication of the terminal control regions U5 and U3 to generate long terminal repeats, and on the ability of the P1-phage site-specific recombinase (Cre) to excise any sequences positioned between two target sequences (loxP). Thus, while inserting a therapy gene into the genome, the vectors simultaneously excise most proviral sequences that are not required for gene expression.

Tricistronic and tetracistronic retroviral vectors for gene transfer

We have combined the picornavirus foot-and-mouth disease virus (FMDV) 2A sequence and the internal ribosome entry sites (IRESes) from encephalomyocarditis virus (ECMV) and avian reticuloendotheliosis virus type A (REV-A) to construct tricistronic and tetracistronic vectors. All the polycistronic constructs show high titers and expression of the genes inserted. Clones have been obtained in which cells simultaneously express the three or four genes carried by the polycistronic vectors.

Inclusion of the hepatic locus control region, an intron, and untranslated region increases and stabilizes hepatic factor IX gene expression in vivo but not in vitro

We systematically compared human factor IX gene expression from a variety of plasmids containing different cis-regulatory sequences after transfection into different hepatocyte cell lines, or in vivo, after their injection into the livers of mice. Although there was a 1.5- to 2.0-fold variation in gene expression from cultured cells, a 65-fold variation was observed in the in vivo studies. We found that a plasmid containing the apolipoprotein E locus control region (HCR), human alpha1-antitrypsin (hAAT) promoter, hFIX minigene (hFIXmg) sequence including a portion of the first intron (intron A), 3'-untranslated region (3'-UTR), and a bovine growth hormone polyadenylation signal (bpA) produced the highest serum level of human factor IX, reaching 18 microg/ml (normal = 5 microg/ml) 1 day after injection. Although most of the plasmid DNAs resulted in transient gene expression, inclusion of an intron, a polyadenylation signal from either the 1.7-kb 3'-UTR or the 0.3-kb bpA, and the HCR resulted in persistent and therapeutic levels of hFIX gene expression, ranging from 0.5 to 2 microg/ml (10 to 40% of normal) for 225 days (length of experiment). These data underscore the importance of cis sequences for enhancing in vivo hepatic gene expression and reemphasize the lack of correlation of gene expression in tissue culture and in vivo studies.

IRES-dependent second gene expression is significantly lower than cap-dependent first gene expression in a bicistronic vector

The internal ribosome entry site (IRES) has been widely used to coexpress heterologous gene products by a message from a single promoter. However, little is known about the efficiency of IRES-dependent second gene expression in comparison with that of first gene expression. This study was undertaken to characterize the relative expression of IRES-dependent second gene in a bicistronic vector, which was derived from the 5' untranslated regions of the encephalomyocarditis virus (EMCV). IRES-dependent second gene expression was compared with cap-dependent first gene expression in several cultured cell lines and in mouse liver in vivo. The expression of the IRES-dependent second gene ranged from 6 to 100% (in most cases between 20 and 50%) that of the first gene. Second gene expression in a plasmid without the IRES was 0.1-0.8% (with some exceptions) that of the first gene. These findings have important implications for the use of IRES, i.e., care should be taken regarding the decreased capacity of IRES-dependent downstream gene expression as well as in determining which gene should be positioned as the first or second gene in a bicistronic vector.

Sustained expression of human apolipoprotein A-I after adenoviral gene transfer in C57BL/6 mice: role of apolipoprotein A-I promoter, apolipoprotein A-I introns, and human apolipoprotein E enhancer

Elevation of HDL cholesterol, after adenoviral apolipoprotein A-I (apo A-I) gene transfer, may delay or revert ischemic cardiovascular disease, provided transgene expression is persistent. Previously, we observed transient human apo A-I expression after adenoviral gene transfer with a cytomegalovirus (CMV)-driven construct containing the human apo A-I cDNA. Therefore, the effects of promoters (CMV or 256 base pairs of the human apo A-I promoter), introns of the human apo A-I gene, and the liver-specific human apolipoprotein E (apo E) enhancer on adenovirus-mediated human apo A-I expression were evaluated in C57BL/6 mice. In the presence of the CMV promoter, human apo A-I introns prolonged expression above 20 mg/dl from 14 to 35 days. Addition of one, two, or four copies of the human apo E enhancer in these constructs resulted in a copy-dependent but transient increase in expression for 14 days. The apo A-I promoter induced 3.2-fold lower peak levels of human apo A-I than did the CMV promoter, but insertion of four apo E enhancers in the apo A-I promoter-driven construct resulted in human apo A-I levels above 20 mg/dl for 6 months. The decline between day 6 and day 35 of human apo A-I expression driven by the CMV promoter was due to (1) a 2.5-fold decline in transgene DNA levels that is not observed with apo A-I promoter-driven constructs, and (2) CMV promoter attenuation as evidenced by a 7.6-fold decline in the human apo A-I mRNA/human apo A-I DNA copy number ratio between day 6 and day 35. Hepatotoxicity, as evidenced by up to 10-fold higher serum levels of transaminases on day 6 after gene transfer with CMV promoter-driven constructs than with apo A-I promoter-driven constructs, probably caused the accelerated decline of transgene DNA. In conclusion, gene transfer with an adenovirus comprising the 256-bp apo A-I promoter, the genomic apo A-I DNA, and four apo E enhancers, all of human origin, is associated with low hepatotoxicity and with the absence of promoter shutoff resulting in human apo A-I expression above 20 mg/dl for up to 6 months.

Variegation of retroviral vector gene expression in myeloid cells

We have comparatively evaluated the efficiency of a series of retroviral vectors transducing the gp91-phox gene, whose defects are responsible for impaired production of superoxide anion (O2-) by phagocytic cells and lead to the X-linked form of chronic granulomatous disease (X-CGD). These vectors included four constructs based on the MoMuLV backbone and expressing gp91-phox from the viral long terminal repeat (LTR) or from internal promoters, and one construct based on the myelotropic FMEV vector. Expression of the therapeutic gene from the MoMuLV LTR was unsatisfactory after transduction of the PLB985 X-CGD knockout cell line and of primary CD34+ hematopoietic progenitors from X-CGD patients. The presence of either constitutive or inducible internal promoters did not result in important improvements in the efficiency of O2- production and lowered the titers of the viral preparations. In contrast, sustained levels of superoxide generation were obtained upon transduction with the FMEV vector. To analyze the efficiency of transgene expression at the single cell level, over 150 cellular clones were generated from bulk cultures of PLB985 X-CGD cells transduced with this vector, each one representative of an individual transduction event. These clones revealed a markedly heterogeneous pattern of gp91-phox expression, ranging from complete silencing to full restoration of superoxide production. Within each clone, expression of the therapeutic gene correlated with the number of expressing cells rather than with the average levels of expression from each cell, indicating that at the single cell level, the proviral promoter is regulated by a binary, on/off mechanism. Moreover, both transduced bulk and clonal cell populations displayed a tendency to a progressive extinction of expression over time, with a mechanism involving LTR methylation. The design of novel retroviral vectors escaping silencing is highly desirable for efficient gene therapy.

A general approach to the non-invasive imaging of transgenes using cis-linked herpes simplex virus thymidine kinase

Non-invasive imaging of gene expression opens new prospects for the study of transgenic animals and the implementation of genetically based therapies in patients. We have sought to establish a general paradigm to enable whole body non-invasive imaging of any transgene. We show that the expression and imaging of HSV1-tk (a marker gene) can be used to monitor the expression of the LacZ gene (a second gene) under the transcriptional control of a single promoter within a bicistronic unit that includes a type II internal ribosomal entry site. In cells bearing a single copy of the vector, the expression of the two genes is proportional and constant, both in vitro and in vivo. We demonstrate that non-invasive imaging of HSV1-tk gene accurately reflects the topology and activity of the other cis-linked transgene.

Lentivirus-mediated Bcl-2 expression in betaTC-tet cells improves resistance to hypoxia and cytokine-induced apoptosis while preserving in vitro and in vivo control of insulin secretion

betaTC-tet cells are conditionally immortalized pancreatic beta cells which can confer long-term correction of hyperglycemia when transplanted in syngeneic streptozocin diabetic mice. The use of these cells for control of type I diabetes in humans will require their encapsulation and transplantation in non-native sites where relative hypoxia and cytokines may threaten their survival. In this study we genetically engineered betaTC-tet cells with the anti-apoptotic gene Bcl-2 using new lentiviral vectors and showed that it protected this cell line against apoptosis induced by hypoxia, staurosporine and a mixture of cytokines (IL-1beta, IFN-gamma and TNF-alpha). We further demonstrated that Bcl-2 expression permitted growth at higher cell density and with shorter doubling time. Expression of Bcl-2, however, did not inter- fere either with the intrinsic mechanism of growth arrest present in the betaTC-tet cells or with their normal glucose dose-dependent insulin secretory activity. Furthermore, Bcl-2 expressing betaTC-tet cells retained their capacity to secrete insulin under mild hypoxia. Finally, transplantation of these cells under the kidney capsule of streptozocin diabetic C3H mice corrected hyperglycemia for several months. These results demonstrate that the murine betaTC-tet cell line can be genetically modified to improve its resistance against different stress-induced apoptosis while preserving its normal physiological function. These modified cells represent an improved source for cell transplantation therapy of type I diabetes.

Transcriptional Targeting of Retroviral Vectors to the Erythroblastic Progeny of Transduced Hematopoietic Stem Cells

Targeted expression to specific tissues or cell lineages is a necessary feature of a gene therapy vector for many clinical applications, such as correction of hemoglobinopathies or thalassemias by transplantation of genetically modified hematopoietic stem cells. We developed retroviral vectors in which the constitutive viral enhancer in the U3 region of the 3′ LTR is replaced by an autoregulatory enhancer of the erythroid-specific GATA-1 transcription factor gene. The replaced enhancer is propagated to the 5′ LTR upon integration into the target cell genome. The modified vectors were used to transduce human hematopoietic cell lines, cord blood-derived CD34+ stem/progenitor cells, and murine bone marrow repopulating stem cells. The expression of appropriate reporter genes (▵LNGFR, EGFP) was analyzed in the differentiated progeny of transduced stem cells in vitro, in liquid culture as well as in clonogenic assay, and in vivo, after bone marrow transplantation in lethally irradiated mice. The GATA-1 autoregulatory enhancer effectively restricts the expression of the LTR-driven proviral transcription unit to the erythroblastic progeny of both human progenitors and mouse-repopulating stem cells. Packaging of viral particles, integration into the target genome, and stability of the integrated provirus are not affected by the LTR modification. Enhancer replacement is therefore an effective strategy to target expression of a retroviral transgene to a specific progeny of transduced hematopoietic stem cells.

Second gene expression in bicistronic constructs using short synthetic intercistrons and viral IRES sequences

In this study, we describe the efficiency of second gene translation in bicistronic constructs containing either a short (36bp) synthetic intercistron or known internal ribosomal entry sites (IRES). Experiments were performed using two different gene combinations: Herpes simplex virus-thymidine kinase (HSV-TK) and neomycine (NEO) or human glucocerebrosidase (hGC) and a methotrexate (MTX) resistant mutant dihydrofolate reductase (DHFR). We demonstrate that upon transfection, second gene translation is efficient using either an IRES or a 36-bp intercistron. Infection with retrovirus carrying the TK and NEO genes linked via a 36-bp intercistron resulted in both G418R (NEO expression) and gancyclovir (GCV) sensitivity (TK expression), indicating that both genes were expressed and thus that the genomic DNA and RNA of this bicistronic construct were intact. Likewise, retrovirus carrying the hGC and mutant DHFR gene separated by a short intercistron was harvested from MTXR murine PsiCRE cells. However, infection of PA317 cells with this virus supernatant did not result in the presence of hGC enzyme activity in these murine cells. Proviral DNA and RNA analyses indicated that the hGC coding region was lost from the original construct in the infected PA317 cells. In contrast, retrovirus carrying the hGC and DHFR cDNAs was linked via an IRES functioned as expected. Based on these results, we conclude that the efficiency of second gene translation using short synthetic intercistrons might prove useful in bicistronic constructs, depending on the gene combination used.

Retroviral transfer of herpes simplex virus-thymidine kinase and beta-galactosidase genes into U937 cells with bicistronic vector

In this study we describe a new retroviral vector utilizing an internal ribosome entry site (IRES) from encephalomyocarditis virus to co-express two genes. One is the herpes simplex virus type 1 thymidine kinase gene (HSV-TK) which induces sensitivity to ganciclovir, and the second is the bacterial beta-galactosidase gene (LacZ) which was revealed by an histochemical staining with 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal). We engineered the U937 human cell line to co-express both genes and monitored transduced cells using X-Gal staining. Several transduced clones were selected. The clones exhibiting X-Gal positive cells were sensitive to ganciclovir treatment (1 microgram/ml) while X-Gal negative clones were not. Monoclonal cell lines showed a single copy of the provirus integrated in their genome with the TK-IRES-LacZ sequence stably inserted in all clones. The band distribution pattern of the proviral DNA differed only at the long terminal repeat (LTR) level. Northern blot analysis of an X-Gal positive/ganciclovir sensitive clone showed an mRNA band of 6 kb with both LacZ and TK probes. An X-Gal negative/ganciclovir resistant clone was negative with both probes. This report shows: (1) a therapeutic gene can be linked to a marker gene by an IRES element achieving equivalent expression of both proteins; (2) the co-expression of a marker gene makes fluorescein-di-beta-D-galactopyranoside staining possible, and consequently separation of cells expressing the LacZ gene by fluorescence activated cell sorting. Thus the cells expressing the HSV-Tk gene are enriched; (3) the use of a marker gene such as LacZ could open up interesting perspectives in gene therapy protocols because of the opportunity to monitor the transduced cells using a simple cytochemical stain.

Long-term control of erythropoietin secretion by doxycycline in mice transplanted with engineered primary myoblasts

We investigated tetracycline regulation of gene expression in an experimental model relevant to gene therapy. Mouse primary myogenic cells were engineered for doxycycline-inducible and skeletal muscle-specific expression of the mouse erythropoietin (Epo) cDNA by using two retrovirus vectors. Gene expression increased 200 fold in response to both myogenic cell differentiation and doxycycline stimulation. After transplantation of transduced cells into mouse skeletal muscles, Epo secretion could be iteratively switched on and off over a five-month period, depending on the presence or the absence of doxycycline in the drinking water. We conclude that tetracycline regulation provides a suitable control system for adjusting the delivery of therapeutic proteins from engineered tissues over long periods of time.

Modulation of erythropoietin delivery from engineered muscles in mice

In most relevant diseases, the permanent systemic delivery of a therapeutic protein from engineered cells might be proposed only if secretion levels can be regulated. The tetracycline resistance operon of Escherichia coli provides a transcriptional regulatory system effective in mammalian cells, which could be used for that purpose. A chimeric transactivator (tTA) consisting of the tetracycline repressor fused to the transactivation domain of the herpes simplex virus VP16 protein stimulates transcription by binding a minimal cytomegalovirus (CMV) promoter containing repeats of the tetracycline operator (tetO-CMV). Binding is abolished by tetracycline, thus impairing promoter activation. We have transduced C2.7 myoblasts with two U3-deleted retroviral vectors containing these regulatory elements. The tetP-Epo vector expressed the murine erythropoietin (Epo) cDNA under the control of the tetO-CMV promoter. The D-De-tTA vector expressed tTA under the control of the muscle-specific human desmin enhancer-promoter. Northern blot analysis showed background Epo mRNA expression in myoblasts. Myotubes differentiation induced tTA expression, leading to a 28-fold increase of Epo mRNAs, which was suppressed by tetracycline. Basal Epo secretion in myoblasts increased 23- to 41-fold during the formation of multinucleated myotubes, and turned back close to myoblast level when tetracycline was added. Myoblasts transduced with both vectors and treated with mitomycin with the aim to prevent tumor formation were engrafted in skeletal muscles of syngeneic C3H mice. Hematocrit levels were significantly higher in animals bearing cells transduced with both vectors than in control animals grafted with cells transduced with the Epo vector only. This difference was abolished when tetracycline was given to mice. These data indicate that the tetracycline regulatory elements can modulate transcription in the context of retroviral vector genomes, and that this system can be used to control the in vivo delivery of a therapeutic protein from genetically modified muscles.

The expression cassette determines the functional activity of ribozymes in mammalian cells by controlling their intracellular localization

In order to better understand the influence of RNA transcript context on RNA localization and catalytic RNA efficacy in vivo, we have constructed and characterized several expression cassettes useful for transcribing short RNAs with well defined 5' and 3' appended flanking sequences. These cassettes contain promoter sequences from the human U1 snRNA, U6 snRNA, or tRNA Meti genes, fused to various processing/stabilizing sequences. The levels of expression and the sub-cellular localization of the resulting RNAs were determined and compared with those obtained from Pol II promoters normally linked to mRNA production, which include a cap and polyadenylation signal. The tRNA, Ul, and U6 transcripts were nuclear in localization and expressed at the highest levels, while the standard Pol II promoted transcripts were cytoplasmic and present at lower levels. The ability of these cassettes to confer ribozyme activity in vivo was tested with two assays. First, an SIV-growth hormone reporter gene was transiently transfected into human embryonic kidney cells expressing an anti-SIV ribozyme. Second, cultured T lymphocytes expressing an anti-HIV ribozyme were challenged with HIV. In both cases, we found that the ribozymes were effective only when expressed as capped, polyadenylated RNAs transcribed from Pol II cassettes that generate a cytoplasmically localized ribozyme that facilitates co-localization with its target. We also show that the inability of the other cassettes to support ribozyme-mediated inhibitory activity against their cytoplasmic target is very likely due to the resulting nuclear localization of these ribozymes. These studies demonstrate that the ribozyme expression cassette determines its intracellular localization and, hence, its corresponding functional activity.

In vivo drug-selectable genes: a new concept in gene therapy

Chemoresistance genes, initially considered to be a major impediment to the successful treatment of cancer, may become useful tools for gene therapy of cancer and of genetically determined disorders. Various target cells are rendered resistant to anticancer drugs by transfer of chemoresistance genes encoding P-glycoprotein, the multidrug resistance-associated protein-transporter, dihydrofolate reductase, glutathione-S-transferase, O6-alkylguanine DNA alkyltransferase, or aldehyde reductase. These genes can be used for selection in vivo because of the pharmacology and pharmacokinetics of their substrates. In contrast, several other selectable marker genes conferring resistance to substrates like neomycin or hygromycin can only be utilized in tissue culture. Possible applications for chemoresistance genes include protection of bone marrow and other organs from adverse effects caused by the toxicity of chemotherapy. Strategies have also been developed to introduce and overexpress nonselectable genes in target cells by cotransduction with chemoresistance genes. Thereby expression of both transgenes can be increased following selection with drugs. Moreover, treatment with chemotherapeutic agents should restore transgene expression when or if expression levels decrease after several weeks or months. This approach may improve the efficacy of somatic gene therapy of hematopoietic disorders which is hampered by low or unstable gene expression in progenitor cells. In this article we review preclinical studies in tissue culture and animal models, and ongoing clinical trials on transfer of chemoresistance genes to hematopoietic precursor cells of cancer patients.

Complete restoration of glucocerebrosidase deficiency in Gaucher fibroblasts using a bicistronic MDR retrovirus and a new selection strategy

Retrovirus-mediated gene transfer is currently the most common method for the application of genetic therapy to cancer and many inherited and acquired disorders. Here we report the generation of an amphotropic producer cell line (CA2) that synthesizes viral particles carrying a bicistronic cassette in which the selectable MDR1 cDNA encoding P-glycoprotein (P-gp) a multidrug efflux pump, and the human glucocerebrosidase (GC) gene are transcriptionally fused. Transduction of human Gaucher fibroblasts with this recombinant virus allowed coordinate expression of P-gp and GC. Treatment of the transduced fibroblasts with various cytotoxic substrates of P-gp selected for cells with increased expression of GC, which paralleled the stringency of drug selection. Thus, selection of the genetically modified Gaucher fibroblasts in 1 microgram/ml colchicine raised their GC activity levels from nearly undetectable to those present in WI-38 normal human fibroblasts, correcting the enzyme deficiency present in Gaucher cells. Moreover, by simultaneously inhibiting the P-gp pump, it was possible to use much lower concentrations of colchicine to select for high-level expression of MDR1 and GC. Thus, selection with colchicine at 5 ng/ml in combination with the P-gp inhibitors verapamil or PSC 833 produced a complete correction of the GC deficiency in the CA2-transduced fibroblasts. These combination regimens, already in clinical use for the treatment of multidrug-resistant malignancies, may prove useful in gene therapy trials when utilized for high level selection of a nonselectable gene such as glucocerebrosidase when transcriptionally fused to the MDR1 gene.

Novel retroviral vector transferring a suicide gene and a selectable marker gene with enhanced gene expression by using a tetracycline-responsive expression system

A retroviral vector for the enhanced expression of the herpes simplex virus thymidine kinase (HSV tk) gene was developed by using a tetracycline-responsive expression system (TRES). The two components of the TRES, the chimeric transactivator (tTA) and the corresponding tTA-binding cis element (tetO), were both incorporated into a retroviral vector and resulted in high levels of tk gene expression from tetO in target cells. Amphotropic virus supernatants from stable producer cells, generated by the retroviral vector containing the TRES, gave titers of 10(4) to 10(5) G418-resistant CFU/ml on murine NIH 3T3 cells. The retroviral vector (G1tTA-[tetOTkINa]R), in which tetO was used in the opposite orientation relative to viral transcription, was capable of transducing tk and neo genes into murine NIH 3T3 cells to yield a high level of tk gene expression. TK enzyme activity in NIH 3T3 cells transduced by this vector was 417-fold higher than in control cells. This increased TK activity was returned to basal levels in the presence of tetracycline. The level of tk gene expression driven by tetO from G1tTA-[tetOTkINa]R vector in NIH 3T3 cells was fourfold higher at both the mRNA level and the TK enzyme level than that produced by the long terminal repeat of G1Tk1SvNa, the vector being used in the ongoing brain tumor gene therapy trial. Retroviral vectors containing the TRES may be useful therefore in achieving higher levels of tk gene expression, which should facilitate gene therapy approaches in the treatment of cancer.

Self-deleting retrovirus vectors for gene therapy

A new generation of retrovirus vectors for gene therapy has been developed. The vectors have the ability to excise themselves after inserting a gene into the genome, thereby avoiding problems encountered with conventional retrovirus vectors, such as recombination with helper viruses or transcriptional repression of transduced genes. The strategy exploited (i) the natural life cycle of retroviruses, involving duplication of terminal control regions U5 and U3 to generate long terminal repeats (LTRs) and (ii) the ability of the P1 phage site-specific recombinase (Cre) to excise any sequences positioned between two loxP target sequences from the mammalian genome. Thus, an independently expressed selectable marker gene flanked by a loxP target sequence was cloned into the U3 region of a Moloney murine leukemia virus vector. A separate cassette expressing the Cre recombinase was inserted between the LTRs into the body of the virus. LTR-mediated duplication placed vector sequences, including Cre, between loxP sites in the integrated provirus. This enabled Cre to excise from the provirus most of the viral and nonviral sequences unrelated to transcription of the U3 gene.