Comparison of promoter suppression in avian and murine retrovirus vectors

Therapeutic Angiogenesis by a "Dynamic Duo": Simultaneous Expression of HGF and VEGF165 by Novel Bicistronic Plasmid Restores Blood Flow in Ischemic Skeletal Muscle

Therapeutic angiogenesis is a promising strategy for relief of ischemic conditions, and gene delivery was used to stimulate blood vessels' formation and growth. We have previously shown that intramuscular injection of a mixture containing plasmids encoding vascular endothelial growth factor (VEGF)165 and hepatocyte growth factor (HGF) leads to restoration of blood flow in mouse ischemic limb, and efficacy of combined delivery was superior to each plasmid administered alone. In this work, we evaluated different approaches for co-expression of HGF and VEGF165 genes in a panel of candidate plasmid DNAs (pDNAs) with internal ribosome entry sites (IRESs), a bidirectional promoter or two independent promoters for each gene of interest. Studies in HEK293T culture showed that all plasmids provided synthesis of HGF and VEGF165 proteins and stimulated capillary formation by human umbilical vein endothelial cells (HUVEC), indicating the biological potency of expressed factors. Tests in skeletal muscle explants showed a dramatic difference and most plasmids failed to express HGF and VEGF165 in a significant quantity. However, a bicistronic plasmid with two independent promoters (cytomegalovirus (CMV) for HGF and chicken b-actin (CAG) for VEGF165) provided expression of both grow factors in skeletal muscle at an equimolar ratio. Efficacy tests of bicistronic plasmid were performed in a mouse model of hind limb ischemia. Intramuscular administration of plasmid induced significant restoration of perfusion compared to an empty vector and saline. These findings were supported by increased CD31+ capillary density in animals that received pHGF/VEGF. Overall, our study reports a first-in-class candidate gene therapy drug to deliver two pivotal angiogenic growth factors (HGF and VEGF165) with properties that provide basis for future development of treatment for an unmet medical need-peripheral artery disease and associated limb ischemia.

Production of functional human CuZn-SOD and EC-SOD in bitransgenic cloned goat milk

Human copper/zinc superoxide dismutase (CuZn-SOD) and extracellular superoxide dismutase (EC-SOD) are two superoxide dismutases that scavenge reactive oxygen species (ROS). Their biological role of eliminating oxidative stress caused by excessive ROS levels in living organisms has been utilized in medical treatment, preventing skin photoaging and food preservation. In this study, we employed two sequences that encode human CuZn-SOD and EC-SOD, along with goat beta-casein 5' and 3' regulatory elements, to construct mammary gland-specific expression vectors. Bitransgenic goats were generated using somatic cell nuclear transfer (SCNT), which employed co-transfection to generate bitransgenic goat fetal fibroblast cells as donor cells, and the expression of human CuZn-SOD and EC-SOD and their biological activities were assayed in the milk. PCR and Southern blot analysis confirmed that the cloned goat harbors both hCuZn-SOD and hEC-SOD transgenes. rhCuZn-SOD and rhEC-SOD were expressed in the mammary glands of bitransgenic goat, as determined by western blotting. The expression levels were 100.14 ± 5.09 mg/L for rhCuZn-SOD and 279.10 ± 5.38 mg/L for rhEC-SOD, as determined using ELISA. A total superoxide dismutase assay with WST-8 indicates that the biological activity of rhCuZn-SOD and rhEC-SOD in goat milk is 1451 ± 136 U/mL. The results indicate that two expression vectors can simultaneously transfect goat fetal fibroblast cells as donor cells to produce transgenic goats by SCNT, and the CuZn-SOD and EC-SOD proteins secreted in the mammary glands showed biological activity. The present study thus describes an initial step in the production of recombinant human SODs that may potentially be used for therapeutic purposes.

Tightly regulated and homogeneous transgene expression in human adipose-derived mesenchymal stem cells by lentivirus with tet-off system

Genetic modification of human adipose tissue–derived multilineage progenitor cells (hADMPCs) is highly valuable for their exploitation in therapeutic applications. Here, we have developed a novel single tet-off lentiviral vector platform. This vector combines (1) a modified tetracycline (tet)-response element composite promoter, (2) a multi-cistronic strategy to express an improved version of the tet-controlled transactivator and the blasticidin resistance gene under the control of a ubiquitous promoter, and (3) acceptor sites for easy recombination cloning of the gene of interest. In the present study, we used the cytomegalovirus (CMV) or the elongation factor 1 α (EF-1α) promoter as the ubiquitous promoter, and EGFP was introduced as the gene of interest. hADMPCs transduced with a lentiviral vector carrying either the CMV promoter or the EF-1α promoter were effectively selected by blasticidin without affecting their stem cell properties, and EGFP expression was strictly regulated by doxycycline (Dox) treatment in these cells. However, the single tet-off lentiviral vector carrying the EF-1α promoter provided more homogenous expression of EGFP in hADMPCs. Intriguingly, differentiated cells from these Dox-responsive cell lines constitutively expressed EGFP only in the absence of Dox. This single tet-off lentiviral vector thus provides an important tool for applied research on hADMPCs.

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.

cHS4 insulator-mediated alleviation of promoter interference during cell-based expression of tandemly associated transgenes

Expression of multiple transgenes in cells or whole organisms is a powerful tool for basic research of various biological functions and potentially for clinical applications such as gene therapy. As a model system for this purpose, multi-cDNA expression clones were constructed harboring two tandemly situated fluorescent protein cDNAs as reporter genes on a single plasmid. When 293 cells were transfected transiently, the downstream gene displayed significantly lower expression when compared with the upstream cDNA. Such transcriptional interference was markedly alleviated by inserting an insulator cassette of cHS4 elements derived from the chicken beta-globin locus at a site between two neighboring cDNAs. The introduction of cHS4 resulted in a drastic increase of the expression level of the downstream cDNA, ensuring comparable expression levels of the tandem transgenes. Using a chromatin immunoprecipitation assay, we demonstrated that CTCF and USF1 that recruit histone-modifying complexes are bound to the cHS4 region. Depletion of CTCF or USF1 by siRNA resulted in relief of the diminished effect. Our data thus indicate that CTCF and histone modifiers recruited by USF1 cooperatively mediate the suppression of transcriptional interference between apposed genes, presumably by facilitating active chromatin conformation over the transgenes.

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.

Development of 2A peptide-based strategies in the design of multicistronic vectors

As science progresses in its understanding of diseases and their treatment, advances have been made in the biotechnology used in disease therapy. Most gene therapy approaches utilise viral vectors to deliver genes of interest. However, multiple proteins are often involved in disease processes and there is often a need to efficiently deliver more than one gene. Researchers have employed several strategies to accomplish this goal. When designing vectors to express multiple genes, there are several factors that need to be taken into account, including cell type, the activity of the protein of interest and subcellular protein localisation. In most cases, it is ideal for each protein to be expressed at comparable levels, a leading issue with traditional strategies for multigene expression. This review describes some of the techniques that have been used to express multiple genes, and will focus on the use of 2A peptides or 2A peptide-like sequences in the design of multicistronic vectors that may alleviate some of these issues.

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.

Gene Therapy for beta-thalassemia

Gene transfer for beta-thalassemia requires gene transfer into hematopoietic stem cells using integrating vectors that direct regulated expression of beta globin at therapeutic levels. Among integrating vectors, oncoretroviral vectors carrying the human beta-globin gene and portions of the locus control region (LCR) have suffered from problems of vector instability, low titers and variable expression. In recent studies, human immunodeficiency virus-based lentiviral (LV) vectors were shown to stably transmit the human beta-globin gene and a large LCR element, resulting in correction of beta-thalassemia intermedia in mice. Several groups have since demonstrated correction of the mouse thalassemia intermedia phenotype, with variable levels of beta-globin expression. These levels of expression were insufficient to fully correct the anemia in thalassemia major mouse model. Insertion of a chicken hypersensitive site-4 chicken insulator element (cHS4) in self-inactivating (SIN) LV vectors resulted in higher and less variable expression of human beta-globin, similar to the observations with cHS4-containing retroviral vectors carrying the human gamma-globin gene. The levels of beta-globin expression achieved from insulated SIN-LV vectors were sufficient to phenotypically correct the thalassemia phenotype from 4 patients with human thalassemia major in vitro, and this correction persisted long term for up to 4 months, in xeno-transplanted mice in vivo. In summary, LV vectors have paved the way for clinical gene therapy trials for Cooley's anemia and other beta-globin disorders. SIN-LV vectors address several safety concerns of randomly integrating viral vectors by removing viral transcriptional elements and providing lineage-restricted expression. Flanking the proviral cassette with chromatin insulator elements, which additionally have enhancer-blocking properties, may further improve SIN-LV vector safety.

Mutational library analysis of selected amino acids in the receptor binding domain of envelope of Akv murine leukemia virus by conditionally replication competent bicistronic vectors

The envelope protein of retroviruses is responsible for viral entry into host cells. Here, we describe a mutational library approach to dissect functional domains of the envelope protein involving a retroviral vector, which expresses both the envelope protein of Akv murine leukemia virus (MLV) and the neomycin phosphotransferase II (Neo) selection marker from the same transcript. Envelope expression was achieved by inserting an internal ribosome entry site (IRES) between the neo and the env genes. We found the structure of the linker between the IRES element and env to be critical for sufficient envelope expression. This vector functions as a replication competent mini-virus in a culture of NIH 3T3 derived semi-packaging cells that express the viral Gag and Pol proteins. Titers comparable to those of wild type virus were achieved by this system. To test this vector system, we created a random mutational library of Arg 85 and Asp 86 in the first variable region of Akv envelope protein. Homologous amino acids to Asp 86 in Moloney and Friend murine leukemia viruses are thought to be directly involved in receptor binding. Subsequent selection of mutants capable of infecting murine NIH 3T3 cells indicated that the wild type aspartic acid or another hydrophilic residue at position 86 is an important determinant for envelope function.

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.

Insulators prevent transcriptional interference between two promoters in a double gene construct for transgenesis

In transgenesis, the expression of two transgenes is often subject to mutual interference by each of the two expression cassettes when they are driven by different transcriptional regulatory elements in a single construct. To study this problem, we constructed vectors consisting of two expression units, one contains a strong ubiquitous promoter and the other contains a tissue-specific transcriptional element. The expression pattern of each transgene was examined in transfected cell lines and also in transgenic mice. In both cases, two expression units in a single construct were expressed in an independent manner and were controlled by their respective regulatory element only if we placed insulators at both ends of one expression unit. These results indicate that usage of insulators is a valuable tool for transfection of double gene constructs in transgenesis.

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.

Effects of homology length in the repeat region on minus-strand DNA transfer and retroviral replication

Homology between the two repeat (R) regions in the retroviral genome mediates minus-strand DNA transfer during reverse transcription. We sought to define the effects of R homology lengths on minus-strand DNA transfer. We generated five murine leukemia virus (MLV)-based vectors that contained identical sequences but different lengths of the 3' R (3, 6, 12, 24 and 69 nucleotides [nt]); 69 nt is the full-length MLV R. After one round of replication, viral titers from the vector with a full-length downstream R were compared with viral titers generated from the other four vectors with reduced R lengths. Viral titers generated from vectors with R lengths reduced to one-third (24 nt) or one-sixth (12 nt) that of the wild type were not significantly affected; however, viral titers generated from vectors with only 3- or 6-nt homology in the R region were significantly lower. Because expression and packaging of the RNA were similar among all the vectors, the differences in the viral titers most likely reflected the impact of the homology lengths on the efficiency of minus-strand DNA transfer. The molecular nature of minus-strand DNA transfer was characterized in 63 proviruses. Precise R-to-R transfer was observed in most proviruses generated from vectors with 12-, 24-, or 69-nt homology in R, whereas aberrant transfers were predominantly used to generate proviruses from vectors with 3- or 6-nt homology. Reverse transcription using RNA transcribed from an upstream promoter, termed read-in RNA transcripts, resulted in most of the aberrant transfers. These data demonstrate that minus-strand DNA transfer is homology driven and a minimum homology length is required for accurate and efficient minus-strand DNA transfer.

Fusagene vectors: a novel strategy for the expression of multiple genes from a single cistron

Transduction of cells with multiple genes, allowing their stable and co-ordinated expression, is difficult with the available methodologies. A method has been developed for expression of multiple gene products, as fusion proteins, from a single cistron. The encoded proteins are post-synthetically cleaved and processed into each of their constituent proteins as individual, biologically active factors. Specifically, linkers encoding cleavage sites for the Golgi expressed endoprotease, furin, have been incorporated between in-frame cDNA sequences encoding different secreted or membrane bound proteins. With this strategy we have developed expression vectors encoding multiple proteins (IL-2 and B7.1, IL-4 and B7.1, IL-4 and IL-2, IL-12 p40 and p35, and IL-12 p40, p35 and IL-2 ). Transduction and analysis of over 100 individual clones, derived from murine and human tumour cell lines, demonstrate the efficient expression and biological activity of each of the encoded proteins. Fusagene vectors enable the co-ordinated expression of multiple gene products from a single, monocistronic, expression cassette.

Glucocorticoid-inducible retrovector for regulated transgene expression in genetically engineered bone marrow stromal cells

Transplantable bone marrow stromal cells can be utilized for cell therapy of mesenchymal disorders. They can also be genetically engineered to express synthetic transgenes and subsequently serve as a platform for systemic delivery of therapeutic proteins in vivo. Inducible production of therapeutic proteins would markedly enhance the usefulness of stromal cells for cell therapy applications. We determined whether synthetic corticosteroid hormones can be used to tightly control transgene expression via the glucocorticoid response pathway in primary bone marrow stromal cells. This regulatory mechanism does not require the presence of potentially immunogenic prokaryotic or chimeric "Trans-activators." Further, synthetic corticosteroids are pharmaceutical agents that can be readily used in vivo. We designed a self-inactivating retroviral vector in which expression of the green fluorescent protein (GFP) reporter is controlled by a minimal synthetic promoter composed of five tandem glucocorticoid response elements upstream of a TATAA box. Vesicular stomatitis virus G-pseudotyped retroparticles were synthesized and utilized to transduce cultured cell lines and primary rat bone marrow stromal cells. We have shown that primary rat bone marrow stromal cells could be efficiently engineered with our GRE-containing retrovector, basal reporter expression was low in the absence of exogenous synthetic corticosteroids, and GFP expression was dexamethasone inducible and reversible. To summarize, this strategy allows dexamethasone-induced, "on-demand" transgene expression from transplantable genetically engineered bone marrow stromal cells.

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.

Viral vectors for gene transfer: a review of their use in the treatment of human diseases

The efficient delivery of therapeutic genes and appropriate gene expression are the crucial issues for clinically relevant gene therapy. Viruses are naturally evolved vehicles which efficiently transfer their genes into host cells. This ability made them desirable for engineering virus vector systems for the delivery of therapeutic genes. The viral vectors recently in laboratory and clinical use are based on RNA and DNA viruses processing very different genomic structures and host ranges. Particular viruses have been selected as gene delivery vehicles because of their capacities to carry foreign genes and their ability to efficiently deliver these genes associated with efficient gene expression. These are the major reasons why viral vectors derived from retroviruses, adenovirus, adeno-associated virus, herpesvirus and poxvirus are employed in more than 70% of clinical gene therapy trials worldwide. Among these vector systems, retrovirus vectors represent the most prominent delivery system, since these vectors have high gene transfer efficiency and mediate high expression of therapeutic genes. Members of the DNA virus family such as adenovirus-, adeno-associated virus or herpesvirus have also become attractive for efficient gene delivery as reflected by the fast growing number of clinical trials using these vectors. The first clinical trials were designed to test the feasibility and safety of viral vectors. Numerous viral vector systems have been developed for ex vivo and in vivo applications. More recently, increasing efforts have been made to improve infectivity, viral targeting, cell type specific expression and the duration of expression. These features are essential for higher efficacy and safety of RNA- and DNA-virus vectors. From the beginning of development and utilisation of viral vectors it was apparent that they harbour risks such as toxicities, immunoresponses towards viral antigens or potential viral recombination, which limit their clinical use. However, many achievements have been made in vector safety, the retargeting of virus vectors and improving the expression properties by refining vector design and virus production. This review addresses important issues of the current status of viral vector design and discusses their key features as delivery systems in gene therapy of human inherited and acquired diseases at the level of laboratory developments and of clinical applications.

Gene therapy using hematopoietic stem cells: Sisyphus approaches the crest

Gene transfer targeting cells of the blood and immune system was one of the first areas of investigation in the field of gene therapy. Despite the encouraging results achieved in early studies using murine bone marrow, the task of gene transfer into human hematopoietic stem cells proved to be far more difficult. As a result, progress has been disappointingly slow and initial clinical trials generally failed to achieve significant levels of gene marking. The continued application of new advances in vectorology and hematopoietic stem cell biology has now led to improvements in preclinical models that are being translated into clinical trials. The progress and remaining problems are discussed in this review article.

Progress with retroviral gene vectors

Retroviral vectors have become a standard tool for gene transfer technology. Compared with other gene transfer systems, retroviral vectors have several advantages, including their ability to transduce a variety of cell types, to integrate efficiently into the genomic DNA of the recipient cells and to express the transduced gene at high levels. The relatively well understood biology of retroviruses has made possible the development of packaging cell lines which provide in trans all the viral proteins required for viral particle formation. The design of different types of packaging cells has evolved to reduce the possibility of helper virus production. The host range of retroviruses has been expanded by pseudotyping the vectors with heterologous viral glycoproteins and receptor-specific ligands. The development of lentivirus vectors has allowed efficient gene transfer to quiescent cells. This review describes different strategies adopted for developing vectors to be used in gene therapy applications.

Endothelial cell-specific transcriptional targeting from a hybrid long terminal repeat retrovirus vector containing human prepro-endothelin-1 promoter sequences

For many applications, specificity of gene expression by recombinant retroviral vectors is necessary. We wished to obtain transcriptional targeting in endothelial cells as part of an antivasculature approach to cancer treatment and have achieved specificity by using the promoter for human prepro-endothelin-1. In particular, we have inserted this heterologous promoter within the 3' long terminal repeat (LTR), replacing all viral upstream transcriptional regulatory sequences, to generate a hybrid LTR with precise fusion at the TATA box for initiation of transcription at the viral start site. Reverse transcription and integration resulted in duplication of this hybrid promoter in the 5' LTR of the provirus for transcription of the internal transgene. An important feature of our vectors is the absence of a selectable marker gene or additional promoters to avoid potential complications of silencing or interference and because selection will be inappropriate for clinical application. This vector design showed endothelial cell specificity of beta-galactosidase expression when tested on a panel of human cell lines and primary breast microvascular endothelial cells, matching the specificity of expression of the endogenous promoter. Such simplified vectors exhibiting transcriptional specificity are likely to be useful for the development of a gene therapy approach to targeting tumor vasculature.

Identification in the rat prolactin gene of sequences homologous to the distal promoter of the human prolactin gene

The goal of the present study was to test the hypothesis that a second, more distal, promoter exists upstream from the rat prolactin (PRL) gene (rPrl) that is homologous to that identified upstream from the human prolactin gene (hPrl). The nucleotide sequence of the rat genome extending from 7.4 to 2.5 kb upstream from the proximal rPrl promoter was determined, revealing significant sequence homology to the hPrl distal promoter and its 5'-flanking domain. Using reverse transcription-polymerase chain reaction, novel rPrl transcripts that originate upstream from the proximal rPrl promoter were detected in rat uterus, spleen, pons medulla, anterior pituitary and the GH4C1 pituitary tumor cell line. Further characterization of these novel 5'-extended rPrl transcripts from GH4C1 cells indicated that they were full length, polyadenylated and properly spliced. However, data from primer extension (5'-RACE) experiments strongly suggested that the 5'-extended rPrl transcripts originate, not at the distal promoter-like motif, but at scattered sites located 60-153 bp upstream from the proximal promoter. Therefore, it appears improbable that the rat sequences homologous to the hPrl distal promoter comprise a functional promoter.

Nonreciprocal pseudotyping: murine leukemia virus proteins cannot efficiently package spleen necrosis virus-based vector RNA

It has been documented that spleen necrosis virus (SNV) can package murine leukemia virus (MLV) RNA efficiently and propagate MLV vectors to the same titers as it propagates SNV-based vectors. Although the SNV packaging signal (E) and MLV packaging signal (Psi) have little sequence homology, similar double-hairpin RNA structures were predicted and supported by experimental evidence. To test whether SNV RNA can be packaged by MLV proteins, we modified an SNV vector to be expressed in an MLV-based murine helper cell line. Surprisingly, we found that MLV proteins could not support the replication of SNV vectors. The decrease in titer was approximately 2,000- to 20,000-fold in one round of retroviral replication. RNA analysis revealed that SNV RNA was not efficiently packaged by MLV proteins. RNA hybridization of the cellular and viral RNAs indicated that SNV RNA was packaged at least 25-fold less efficiently than MLV RNA, which was the sensitivity limit of the hybridization assay. The contrast between the MLV and SNV packaging specificity is striking. SNV proteins can recognize both SNV E and MLV Psi, but MLV can recognize only MLV Psi. This is the first demonstration of two retroviruses with nonreciprocal packaging specificities.

Protection of hematopoietic stem cells from chemotherapy-induced toxicity by multidrug-resistance 1 gene transfer

An increased chemotherapeutic dose intensity is believed to translate into higher survival rates among cancer patients. Pancytopenia is the dose-limiting toxic result of most anticancer agents. Overexpression of the human multidrug resistance 1 (MDR1) gene in transgenic animals resulted in complete myeloprotection against high doses of cytostatic drugs. Stem cell research, vector development, and experimental pharmacology are uniting their efforts in an attempt to achieve a similar effect in human hematopoietic stem cells. This article gives an overview of the crucial steps involved, from retroviral vector design and optimization of viral titers to vector uptake, gene integration, and expression. The authors' own results are presented with special regard in vitro and in vivo assays for the detection of hematopoietic stem cell transduction.

A conditional self-inactivating retrovirus vector that uses a tetracycline-responsive expression system

We developed a novel conditional self-inactivating (C-SIN) vector, TL-SN, by replacement of the enhancer-promoter of the 3' long terminal repeat of Moloney murine leukemia virus with a synthetic tetracycline operator-cytomegalovirus promoter (tetP) from the tetracycline-responsive expression system (TRES). The other component of the TRES, a chimeric transactivator (tTA), was stably incorporated into PA317 amphotropic packaging cells, thus generating the packaging cell line PA317-tTA. C-SIN amphotropic G418-resistant virus particles were generated with a titer of 2 x 10(5) CFU/ml within 2 days of transinfection of PA317-tTA cells with TL-SN ecotropic virus particles. This titer was approximately 2 log units higher than that obtained by transinfection of parental PA317 cells and was due to the high level of viral transcripts originating from the tetP promoter at the 5' end of the transduced vector in the presence of tTA. Our C-SIN vector has the potential for use in human gene therapy since it incorporates the advantages of previous SIN vectors in having weak tetP promoter activity (in the absence of tTA in target cells) while at the same time achieving high viral titers with PA317-tTA packaging cells.

Transcriptional Silencing of Retroviral Vectors

Although retroviral vector systems have been found to efficiently transduce a variety of cell types in vitro, the use of vectors based on murine leukemia virus in preclinical models of somatic gene therapy has led to the identification of transcriptional silencing in vivo as an important problem. Extinction of long-term vector expression has been observed after implantation of transduced hematopoietic cells as well as fibroblasts, myoblasts and hepatocytes. Here we review the influence of vector structure, integration site and cell type on transcriptional silencing. While down-regulation of proviral transcription is known from a number of cellular and animal models, major insight has been gained from studies in the germ line and embryonal cells of the mouse. Key elements for the transfer and expression of retroviral vectors, such as the viral transcriptional enhancer and the binding site for the tRNA primer for reverse transcription may have a major influence on transcriptional silencing. Alterations of these elements of the vector backbone as well as the use of internal promoter elements from housekeeping genes may contribute to reduce transcriptional silencing. The use of cell culture and animal models in the testing and improvement of vector design is discussed. Copyright 1996 S. Karger AG, Basel

The simian foamy virus type 1 transcriptional transactivator (Tas) binds and activates an enhancer element in the gag gene

Simian and human foamy viruses (SFV and HFV) encode a transcriptional transactivator, Tas, which governs the levels of viral transcripts initiated by both the promoter in the long terminal repeat (LTR) and the internal promoter (IP) located within the env gene of these viruses. Tas-responsive target elements,(TRE) LTR in the LTR and (TRE) IP in the env gene, are located 5' of the TATA box in both viral promoters and function as orientation- and position-independent enhancers. We have identified a strong Tas-responsive element, designated TRE (GP), near the 3' end of the gag gene and preceding the pol gene of SFV-1. In transient-expression assays with plasmids containing reporter genes, a 59-bp DNA fragment containing TRE (GP) (nucleotides 2224 to 2282) functioned as an enhancer element, dependent on Tas, in several cell types and in the context of a heterologous basal promoter. DNase footprinting revealed that the fusion protein glutathione S-transferase-Tas, purified from genetically engineered bacteria, interacts with about 40 hp (nucleotides 2237 to 2279) in the TRE (GP). A low degree of sequence homology was noted between TRE (GP) and TRE (IP). In virus-infected cells, novel transcripts with 5' ends immediately upstream from the reverse transcriptase translation frame (nucleotides 2611 to 5778) were identified. Upstream of the start site for these transcripts is a TATA box (nucleotides 2575 to 2579), which was required for transcription in transient-expression assays. Although a spliced mRNA initiated in the viral LTR is implicated in the synthesis of the HFV Pol polyprotein which encodes protease, reverse transcriptase, and integrase, it is possible that SFV-1 contains a promoter within the pol gene for initiating a reverse transcriptase transcript. Taken together, these studies define a novel Tas-responsive enhancer element, which binds the viral transactivator, and a potential promoter within the pol gene.

Conditional transformation of mouse liver epithelial cells. An in vitro model for analysis of genetic events in hepatocarcinogenesis

Primary rodent and human hepatocytes have a very limited lifespan in culture and are not readily applicable to transformation studies in vitro. To facilitate the investigation of early genetic events involved in hepatocarcinogenesis, we examined a transformation assay system utilizing conditionally immortalized mouse liver epithelial cells as an alternative to primary hepatocytes. By infecting primary mouse hepatocytes with a recombinant retrovirus carrying a temperature-sensitive simian virus 40 large T antigen gene, two mouse liver epithelial cell lines, CHST8 and CHST10-2.1, were established. Because of the heat-labile nature of the large T antigen, the cell lines proliferated rapidly at 33 degrees C, but were growth-arrested at 39 degrees C. Because activated c-H-ras and c-myc oncogenes are frequently found to be involved in mouse hepatocarcinogenesis in vivo, we assessed whether those oncogenes can complement the immortalizing function of the large T antigen at the nonpermissive temperature. When CHST8 cells were doubly transfected with activated c-H-ras and c-myc at 33 degrees C, they exhibited clonal growth ability even after shifting the temperature to 39 degrees C. However, neither c-H-ras nor c-myc alone allowed growth at 39 degrees C. On the other hand, c-H-ras alone was sufficient for overcoming the growth defect of CHST10-2.1 cells at 39 degrees C, whereas c-myc alone was again ineffective. Northern blot studies revealed that endogenous c-myc expression was significantly downregulated in the parental CHST8 cells after a temperature shift from 33 to 39 degrees C. In contrast, in the parental CHST10-2.1 cells, appreciable c-myc expression was observed at both temperatures. These results indicate that c-H-ras and c-myc can cooperate in complementing the ability of the temperature-sensitive large T antigen to immortalize mouse liver cells at the nonpermissive temperature. In addition, the mutant c-H-ras, but not c-myc, cooperated with the functional T antigen at 33 degrees C to allow growth in soft agarose of the CHST8 and CHST10-2.1 cell lines. However, cell lines carrying mutant c-H-ras and overexpressing c-myc were unable to grow in soft agarose at 39 degrees C. Thus, the two cellular oncogenes were insufficient for full transformation of the liver epithelial cells. The present in vitro model should be useful for investigating molecular events involved in both early and late stages of hepatocarcinogenesis.

Rapid identification and isolation of transcriptionally active regions from mouse genomes

We report here the design, construction and testing of a self-inactivating (Sin) retrovirus promoter-trap vector suitable for identifying and isolating transcriptionally active regions from the mouse genome. When this vector, which contains the bacterial aph gene as its reporter, is integrated into a site downstream from an active host cell promoter, it expresses aph, whose product, aminoglycoside phosphotransferase, produces resistance to the antibiotic G418 in mammalian cells. The construct also contains a native aph promoter which functions in bacteria, but not in mouse cells, to express kanamycin (Km) resistance, plus an adjacent pBR322-derived replication origin. Thus, mammalian DNA segments containing actively transcribed regions flanking aph can be quickly isolated by restriction endonuclease treatment of total DNA from provirus-containing mouse cells, followed by self-ligation, transformation and Km selection of plasmids carried by bacteria transformed with this DNA. We tested this Sin retrovirus promoter-trap system by isolating eight DNA segments upstream to the provirus integration sites in the genome of virus-infected mouse F9 cells. We found that the Sin retrovirus vector produces a high yield of infectious virus particles carrying aph, and that the isolated genomic DNA fragments of F9 cells are transcriptionally active.

Antisense suppression of skeletal muscle myosin light chain-1 biosynthesis impairs myofibrillogenesis in cultured myotubes

Although the alkali or essential light chains of skeletal muscle myosin are not required for actin-activated myosin ATPase activity, these myosin subunits are necessary for force transmission with in vitro actin motility assays and are believed to stabilize the alpha-helical neck region of myosin subfragment-1. To probe the functions of the essential light chains during myofibril assembly, we used recombinant DNA procedures to deplete this light chain in cultured muscle. Retroviral expression vectors were constructed which encoded the exon-1 sequence of the myosin light chain-1 gene in antisense orientation. These vectors were applied to myogenic cells from embryonic chick and quail pectoralis muscle. Colonies expressing antisense RNA were selected in growth medium containing the neomycin analogue G-418, plus 5-bromo-2'-deoxyuridine (BrdU) and triggered to differentiate by removal of the latter. Expression of antisense myosin light chain-1 mRNA impaired muscle development. In the antisense cultures there were more mononucleated cells, fewer and smaller myotubes which had poorly developed myofibrils and high levels of diffusely staining myosin heavy chain, not apparent in control myotubes. Protein synthesis in the myotube cultures was analyzed by 35S-methionine labelling and two-dimensional gel electrophoresis. Except for a suppression of approximately 80% of myosin light chain-1f synthesis, the overall pattern of protein synthesis was not altered significantly. These studies suggest that retardation of myosin light chain-1f accumulation inhibits or delays myofibrillogenesis.

Improved self-inactivating retroviral vectors derived from spleen necrosis virus

Self-inactivating (SIN) retroviral vectors contain a deletion spanning most of the right long terminal repeat's (LTR's) U3 region. Reverse transcription copies this deletion to both LTRs. As a result, there is no transcription from the 5' LTR, preventing further replication. Many previously developed SIN vectors, however, had reduced titers or were genetically unstable. Earlier, we reported that certain SIN vectors derived from spleen necrosis virus (SNV) experienced reconstitution of the U3-deleted LTR at high frequencies. This reconstitution occurred on the DNA level and appeared to be dependent on defined vector sequences. To study this phenomenon in more detail, we developed an almost completely U3-free retroviral vector. The promoter and enhancer of the left LTR were replaced with those of the cytomegalovirus immediate-early genes. This promoter swap did not impair the level of transcription or alter its start site. Our data indicate that SNV contains a strong initiator which resembles that of human immunodeficiency virus. We show that the vectors replicate with efficiencies similar to those of vectors possessing two wild-type LTRs. U3-deleted vectors carrying the hygromycin B phosphotransferase gene did not observably undergo LTR reconstitution, even when replicated in helper cells containing SNV-LTR sequences. However, vectors carrying the neomycin resistance gene did undergo LTR reconstitution with the use of homologous helper cell LTR sequences as template. This supports our earlier finding that sequences within the neomycin resistance gene can trigger recombination.

Retrovirus-mediated gene transfer in cancer therapy

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

Comparison of 5' and 3' long terminal repeat promoter function in human immunodeficiency virus

The architecture of a retroviral genome presents some unusual features for transcriptional regulation because of duplication of the transcriptional control sequences in the 5' and 3' long terminal repeats (LTRs). We have studied the transcriptional activity of the 5' and 3' LTRs of human immunodeficiency virus type 1 (HIV-1) vectors. Using full-length HIV molecular clones, we demonstrate that both LTRs function as Tat-inducible promoters. However, the absolute levels of transcription were found to be much higher for the 5' LTR than for the 3' LTR promoter. When transcription was assayed for an integrated HIV-1 provirus, we also found that the upstream 5' LTR element was the major transcriptional promoter. 3' LTR transcription, however, can be triggered by inactivation of the 5' LTR promoter. Likewise, 5' LTR transcription is induced in constructs lacking a functional 3' LTR promoter. This phenomenon of promoter suppression may have important implications for the design of HIV-based retrovirus vectors.

Gene ecology: a cis-acting gene-to-gene interaction due to the spatial arrangement of genes in chromosomes affects neighbouring transfected c-H-ras expression

A cis-acting interference between gene activities, which occurs when two genes lie on the same DNA strand and have an intergenic distance less than a defined length, was previously deduced when chromosomal organizations of various higher eukaryote nuclear genes in clusters were compared. In order to investigate such an interference due to arrangement of genes along chromosomes, we have isolated a few cell lines which possessed (i) human mutated c-H-ras fused with the mouse mammary tumour virus long terminal repeat and (ii) the E. coli xanthine-guanine phosphoribosyltransferase (gpt) gene with the SV40 promoter, on the same or on different DNA strands, separated by a short intergenic distance or unlinked. Since the cancerous phenotype of a cell can be readily identified due to c-H-ras expression, we examined in these cell lines whether continuous c-H-ras expression, induced by dexamethasone, is disturbed through a cis-acting gene-to-gene interaction when the expression of the neighbouring gpt gene is enforced and as a result, the cancerous state of a cell is converted to the 'normal' state. The enforced expression of the neighbouring gpt gene was shown to alter c-H-ras expression, and thus reversible conversion of a cell between cancerous and normal states occurred only when the cell possessed an optimum number of the gene pair, in which both c-H-ras and the gpt gene were on the same DNA strand. This implies that the spatial arrangement of genes in chromosomes plays an important role in the regulation of gene expression in a cluster.

Targeting of retroviral vectors for gene therapy

Retroviral vectors are one of the most promising vehicles for the delivery of therapeutic genes in human gene therapy protocols. Retroviral-mediated gene transfer currently being used in human clinical trials is based upon ex vivo transduction of target cells. The ability to target the delivery and expression of therapeutic genes in vivo using retroviral vectors is a prerequisite for widespread and routine use in the clinic and will be of great importance for the safe and successful treatment of certain genetic disorders as well as tumors and viral infections. A number of approaches have been taken to develop retroviral vectors that are able to target particular cell types both at the level of the transduction event and at the level of expression. Using various combinations of the restrictive features reviewed in this article, it should be possible to achieve definitive targeting of genes transduced by retroviral vectors.

Retrovirus-mediated gene transfer to cystic fibrosis airway epithelial cells: effect of selectable marker sequences on long-term expression

Retrovirus-mediated gene transfer offers the potential for stable long-term expression of transduced genes in host cells subsequent to integration of vector DNA into the host genome. Using a murine amphotropic retrovirus vector containing an interleukin-2 receptor (IL-2R) gene as a reporter and a neomycin phosphotransferase (neor) gene as a dominant selectable marker, we measured the efficiency of retrovirus-mediated gene transfer and the stability of transduced gene expression in a cystic fibrosis tracheal epithelial cell line (CFT1). The use of the IL-2R cell surface marker as a reporter of infection permitted both quantitation of vector gene expression and flow cytometric sorting of cells transduced with the vector. In initial studies, the optimal conditions for retrovirus-mediated gene transfer were determined. The presence of a polycation was required for optimal transduction efficiency. The efficiency of infection of CFT1 cells was increased by repetitive exposure to virus such that it was possible to transduce approximately 80% of the cells following three successive daily exposures. The long-term stability of expression of the non-selected IL-2R gene was also evaluated. A slow decline in the percentage of cells expressing IL-2R was seen with cells that were maintained under constant selection pressure for expression of the neor gene, which was expressed from an internal promoter. Similar results were obtained when cultures were selected initially for neor gene expression and maintained without selection thereafter. In contrast, stable expression was observed in CFT1 cells for at least one year following multiple infections in the absence of G418 selection. In conclusion, (i) transduction of foreign genes into human airway epithelial cells using an amphotropic retrovirus vector can be highly efficient in the presence of appropriate polycations and multiple exposures; and (ii) stable expression of a non-selected gene in these epithelial cells is better maintained without selection.

Recent advances in retrovirus vector technology

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

An improved retroviral vector for assaying promoter activity. Analysis of promoter interference in pIP211 vector

We recently developed a novel promoter assay system using a retroviral vector (pIP200 series). Transcription from the internal promoter, which had been inserted for the promoter assay, was shown to be interfered with by transcription from the upstream long terminal repeat (LTR). Here we report a new high-titer 'self-inactivating' vector, in which transcription interference was virtually eliminated. This new vector was constructed by introducing only a very minor mutation into the 'TATA box' in the 3'-LTR. This mutation was successfully transferred to the 5'-LTR after reverse transcription, yielding a provirus incapable of transcribing viral RNA. The viral titer was not reduced by the mutation, permitting general application of this virus.

Transplantable myeloproliferative disease induced in mice by an interleukin 6 retrovirus

Lethally irradiated mice transplanted with bone marrow cells infected with a novel recombinant retrovirus (murine stem cell virus-interleukin 6 [MSCV-IL-6]) bearing a mouse IL-6 gene developed a fatal myeloproliferative disease within 4 wk of engraftment. The hematologic manifestations of the syndrome included elevated peripheral leukocyte counts (up to 430 x 10(3) cells/mm3) with a predominance of neutrophilic granulocytes, microcytic anemia, and thrombocytosis or thrombocytopenia. The mice showed extensive neutrophil infiltration of the lungs, liver, and occasionally lymph nodes, plus splenomegaly resulting from enhanced splenic myelopoiesis (30-60-fold increase in progenitor numbers). Despite the chronic stimulation of neutrophil excess by IL-6, bone marrow from affected mice was capable of repopulating the hematopoietic tissues (bone marrow and spleen) of lethally irradiated hosts during repeated serial transplantation. In the longest documented case, the progeny of a single MSCV-IL-6-marked cell transferred the myeloproliferative disease to two secondary, four tertiary, and two quaternary recipients (the clone endured for a total of 72 wk). These results, demonstrating considerable proliferative longevity of the IL-6-producing cells, support an in vivo role of IL-6 in the maintenance of hematopoietic precursors. Dysregulated IL-6 production also had significant systemic effects. The mice displayed increased mesangial cell proliferation in the kidney, frequent liver abnormalities, and marked alterations in plasma protein levels. Unlike previous studies where constitutive expression of exogenous IL-6 genes resulted in lymphoproliferative disorders characterized by massive plasmacytosis, minimal plasma cell expansion occurred in the MSCV-IL-6 mice during the observation period. Potential explanations for the differences in disease phenotypes observed in the present and previous studies are different cell types expressing the exogenous IL-6 genes, higher sustained circulating levels of IL-6 achieved using the MSCV-IL-6 retroviral delivery system, and/or the premature death (3-15 wk after transplantation) of the MSCV-IL-6 mice before the onset of plasmacytosis. This animal model should prove useful for further investigation of the function of IL-6 in normal and abnormal hematopoiesis and in inflammatory responses.

Transcriptional interaction between retroviral long terminal repeats (LTRs): mechanism of 5' LTR suppression and 3' LTR promoter activation of c-myc in avian B-cell lymphomas

Chicken syncytial viruses induce bursal lymphomas by integrating into the c-myc locus and activating myc expression by 3' long terminal repeat (LTR) promoter insertion. In contrast to wild-type proviruses, in which transcription initiates predominantly in the 5'LTR, these myc-associated proviruses exhibit a predominance of transcription from the 3' LTR and little transcription from the 5' LTR. Most of these proviruses contain deletions within the 5' end of their genome that spare the 5' LTR. We report the identification of a 0.3-kb viral leader sequence that modulates 5' and 3' LTR transcriptional activities. In the presence of this sequence, transcription from the 5' LTR predominates, but in its absence, the 3' LTR promoter becomes activated, resulting in a high level of myc expression. This viral sequence does not behave like a classical enhancer; it activates transcription only when located downstream from the promoter and in the sense orientation. In this regard, it resembles the recently described human immunodeficiency virus RNA enhancer. This study suggests that retroviruses contain internal sequences which directionally activate the 5' LTR promoter to facilitate transcription of the viral genome and that deletion of these sequences is one step in the activation of the 3' LTR of myc-associated proviruses in avian bursal lymphomas.