Infectious and selectable retrovirus containing an inducible rat growth hormone minigene

Use of a replication-defective vector to track cells initially infected by SIV in vivo: infected mononuclear cells rapidly appear in the draining lymph node after intradermal inoculation of rhesus monkeys

A better understanding of the mechanisms of HIV dissemination, a key step in pathogenesis, would be possible if the cellular pathways of viral dissemination could be followed in simian immunodeficiency virus (SIV)- inoculated monkeys or HIV-infected people. In an initial attempt to follow this process using a traceable virus infection, we inoculated rhesus monkeys intradermally (ID) or directly into lymph nodes with a replication-defective SIV-based vector expressing the enhanced jellyfish green fluorescent protein (EGFP), V1EGFP. EGFP expression was detected in mononuclear cells isolated from the sites of inoculation (skin and lymph node) at 5 and 16 hr after inoculation and then cultured in vitro for 6 days to allow maximum EGFP expression. Similarly, EGFP-expressing, SIV-infected cells could be detected at 16 hr postinfection in the lymph nodes that drained the sites of ID inoculation. Since V1EGFP is a replication-defective vector, the EGFP-expressing cells are the initial target cells infected by the virions in the original inoculum. The results of flow cytometric analysis were confirmed by a nested PCR assay to detect SIV DNA and hence infection of cells and reverse transcription. These experiments indicate that 16 hr after ID inoculation newly infected cells either remain in the skin at the site of inoculation or have migrated to the draining lymph node. The results in this SIV vector model probably reflect the short time (less than 16 hr) required for HIV to move from a site of epithelial penetration to the lymphoid tissues via lymphatic vessels.

Virus-based gene delivery systems

Within the past decade, gene therapy strategies have come to the forefront of novel therapeutics. Tremendous advances in vector technology along with deeper understandings of vector biology and the molecular mechanisms of disease have significantly advanced the field of human gene therapy. This manuscript will discuss the viral-based subset of current gene transfer vectors. In particular, the most established viral vectors to date, including parvovirus, adenovirus, retrovirus, lentivirus, and herpesvirus-based vectors, are described, as well as the current innovative improvements being made to each. From past experience, it has become evident that in addition to optimising the vectors in terms of transgene expression, minimising vector-related immunology, and vector production, methods of vector delivery resulting in optimum vector transduction of target cells need to be established. This review will also illustrate several current improved physical delivery systems for optimal vector administration.

Use of intron-disrupted polyadenylation sites to enhance expression and safety of retroviral vectors

Normal mRNA polyadenylation signals are composed of an AAUAAA motif and G/U box spaced 20 to 30 bp apart. If this spacing is increased further, then polyadenylation is disrupted. Previously it has been demonstrated that insertion of an intron will similarly disrupt this signal even though such introns are removed during a nuclear splicing reaction (X. Liu and J. Mertz, Nucleic Acids Res. 21:5256-5263, 1993). This observation has led to the suggestion that polyadenylation site selection is undertaken prior to intron excision. We now present results that both support and extend these observations and in doing so create a novel class of retroviral expression vector with improved qualities. We found that when an intron-disrupted polyadenylation signal is inserted within a retroviral expression vector, such a signal, although reformed in the producer cell, remains benign until transduction, where it is then preferentially used. Thus, we demonstrate that upon transduction these vectors now produce a majority of shortened subgenomic species and as a consequence have a reduced tendency for subsequent mobilization from transduced cells. In addition, we demonstrate that the use of this internal signal leads to enhanced expression from such vectors and that this is achieved without any loss in titer. Therefore, split polyadenylation signals confer enhanced performance and improved safety upon retroviral expression vectors into which they are inserted. Such split signals may prove useful for the future optimization of retroviral vectors in gene therapy.

Patterns of integration and expression of retroviral, non-replicative vectors in avian embryos: embryo developmental stage and virus subgroup envelope modulate tissue-tropism

We previously demonstrated that Avian Leukemia Viruses (ALV) carrying the v-myc gene specifically induce two types of tumors, cardiomyocytic tumors when the virus is injected before embryonic day 3 (E3), skin tumors when the virus is injected at E3 or E5. Aiming to elucidate the mechanisms which determine this time-dependent change in target, we infected chick and quail embryos at E3 and E5 with replication-deficient, lacZ gene-carrying, ALV-based viruses produced by a packaging cell line. Three constructs driven by 3 different Long Terminal Repeats (LTRs) were tested and yielded similar results. When the constructs were inoculated at E3 and the lacZ gene product revealed 5 days later, around 70% of the embryos carried lacZ+ clones in the heart, around 50% had positive clones in the skin anywhere on the body, while a few embryos displayed clones in internal organs (liver, stomach, lungs). Immunocytological identification of the heart cell type(s) expressing the virus revealed that the only cells infected were cardiomyocytes. When the constructs were inoculated at E5, no lacZ+ clones appeared in the heart but all were located in the cephalic skin. In order to examine the relationship between viral integration and expression, DNA of different organs or tissues from lacZ stained embryos was analyzed by PCR. A tight correlation between integration and expression in the heart and in the skin was revealed in most cases. In contrast, a significant PCR signal was often detected in the liver or the stomach despite weak or absent expression as revealed by lacZ+ clones. We then investigated the influence of envelope glycoprotein subgroups on the tropism of these constructs. The lacZ vector driven by RAV-2 LTRs was packaged as subgroups A, B or E viral particles. The A subgroup, used in the part of the study described above, infects both chick and quail while the B and E subgroups are specific for chick or quail respectively. These B and E subgroups induced lacZ+ clones in the heart (after E3 injection) while no clones or only a few were detected in the skin either after E3 or E5 injection. The following conclusions can be drawn: 1) cardiomyocytes are at E3 the major target for integration and expression of ALV-derived viruses in vivo; 2) targets change rapidly with embryonic age; and 3) tissue-specific infections depend on the envelope subgroup, thus presumably on the presence of the cognate receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term expression of a retrovirally introduced beta-galactosidase gene in rodent cells implanted in vivo using biodegradable polymer meshes

Grafts of various types of cells have been performed using bioresorbable polymer matrices. These synthetic fibers are degraded by hydrolysis into normal metabolic intermediates and induce a number of events that are conductive to healing and/or repair, the most important of which may be angiogenesis. The use of biodegradable meshes to deliver genetically altered cells was studied. A beta-galactosidase gene was inserted into Long-Evans rat bone marrow stromal (BMS) cells or fibroblasts derived from C57BL/6J mouse embryos using the retroviral vector LNL-SLX beta gal. Expression was monitored using X-gal staining. X-gal+ cells from monolayer cultures were seeded onto either polyglycolic acid (PGA) or polyglactin (PGL) biodegradable meshes and grown to confluence. Two types of grafts were performed: (1) embryonic C57BL/6J mouse fibroblasts (EMF) into either nude mice or adult C57BL/6J mice, and (2) Long-Evans rat BMS into Long-Evans rats. Beta-Galactosidase activity was found for up to 152 days for EMF in nude mice, 123 days for EMF in adult C57BL/6J mice, and 90 days for grafts of syngeneic BMS cells into Long-Evans rats. Noninfected cells grafted using the same methods did not stain with X-gal.

Appropriate in vivo expression of a muscle-specific promoter by using avian retroviral vectors for gene transfer [corrected]

The promoter regions of the chicken skeletal muscle alpha-actin (alpha sk-actin) and the cytoplasmic beta-actin genes were linked to the bacterial chloramphenicol acetyltransferase (CAT) gene. Replication-competent retroviral vectors were used to introduce these two actin/CAT cassettes into the chicken genome. Chickens infected with retroviruses containing the alpha sk-actin promoter expressed high levels of CAT activity in striated muscle (skeletal muscle and heart); much lower levels of CAT activity were produced in the other nonmuscle tissues. In contrast, chickens infected with retroviruses containing the beta-actin promoter linked to the CAT gene expressed low levels of CAT activity in many different tissue types and with no discernible tissue specificity. Data are presented to demonstrate that the high levels of CAT activity that were detected in the skeletal muscle of chickens infected with the retrovirus containing the alpha sk-actin promoter/CAT cassette were not due to preferential infectivity, integration, or replication of the retrovirus vector in the striated muscles of these animals.

Improvement of avian leukosis virus (ALV)-based retrovirus vectors by using different cis-acting sequences from ALVs

Production and expression of double-expression vectors which transduce both Neo(r) and lacZ genes and are based on the structure of avian leukosis virus were enhanced by using cis-acting sequences (long terminal repeats and noncoding sequences) from Rous-associated virus-1 and Rous-associated virus-2 rather than those of avian erythroblastosis virus previously used in our constructs. Polyclonal producer cells obtained after transfection of these vectors into the Isolde packaging cell line gave rise to titers as high as 3 x 10(5) lacZ CFU/ml, whereas it was possible to isolate clones of producer cells giving rise to titers of more than 10(6) resistance focus-forming units per ml.

A new vector using the human multidrug resistance gene as a selectable marker enables overexpression of foreign genes in eukaryotic cells

A new vector, pSK1.MDR, has been constructed for expressing nonselectable genes in eukaryotic cells. The vector uses the human multidrug resistance gene, MDR1, as a dominant selectable marker and contains an additional transcription unit plus a unique SalI cloning site for inserting nucleotide sequences to be expressed. To test this expression system, a cDNA (IL2R) for the 55-kDa interleukin-2 receptor was inserted into the SalI site, and the resulting plasmid was transfected into NIH3T3 cells. Cells which acquired the MDR1 gene were selected with colchicine, and cells with high levels of MDR1 expression were selected by growth in increasing concentrations of the drug. Drug resistant cells also expressed the cotransferred, nonselected IL2R gene, and its expression was increased to 740,000 receptors per cell by growing cells in high concentrations of colchicine. The MDR1 system represents a very efficient method for synthesizing large amounts of protein in a wide variety of eukaryotic cells.

Generation of murine stromal cell lines supporting hematopoietic stem cell proliferation by use of recombinant retrovirus vectors encoding simian virus 40 large T antigen

The bone marrow is a complex microenvironment made up of multiple cell types which appears to play an important role in the maintenance of hematopoietic stem cell self-renewal and proliferation. We used murine long-term marrow cultures and a defective recombinant retrovirus vector containing the simian virus 40 large T antigen to immortalize marrow stromal cells which can support hematopoiesis in vitro for up to 5 weeks. Such cloned cell lines differentially supported stem cells which, when transplanted, allowed survival of lethally irradiated mice, formed hematopoietic spleen colonies in vivo, and stimulated lymphocyte proliferation in vitro. Molecular and functional analyses of these cell lines did not demonstrate the production of any growth factors known to support the proliferation of primitive hematopoietic stem cells. All cell lines examined produced macrophage colony-stimulating factor. The use of immortalizing retrovirus vectors may allow determination of unique cellular proteins important in hematopoietic stem cell proliferation by the systematic comparison of stromal cells derived from a variety of murine tissues.

Long-term expression of the human beta-globin gene after retroviral transfer into pluripotent hematopoietic stem cells of the mouse

We have studied the regulation of the human beta-globin gene after retroviral transfer into a variety of transformed and normal hematopoietic cells. After transfer into murine erythroleukemia cells (MEL) expression from the human beta-globin gene responds to inducers of erythroid maturation in parallel to the endogenous murine globin genes. After infection of human BFU-E, RNA expression from the virally-transferred beta-globin gene was measured at 2.5%-5% of the endogenous beta-globin level. The most improved globin vectors can transfer the human beta-globin gene into pluripotent hematopoietic stem cells in mouse bone marrow. Mice reconstituted with infected marrow show human beta-globin RNA and protein expression in peripheral blood cells for over 4 months. In these animals, both myeloid and lymphoid cells carry the integrated provirus at a level of about 1 copy per cell. In serial transplantation experiments, bone marrow from these animals is capable of repopulating secondary and tertiary recipient animals which go on to show long-term human beta-globin expression. Retroviral vectors thus provide a practical way to refine models of globin gene regulation through in vivo tests and to evaluate the feasibility of protocols for gene addition therapy.

Design of retrovirus vectors for transfer and expression of the human beta-globin gene

Regulated expression of the human beta-globin gene has been demonstrated in cultured murine erythroleukemia cells and in mice after retrovirus-mediated gene transfer. However, the low titer of recombinant viruses described to date results in relatively inefficient gene transfer, which limits their usefulness for animal studies and for potential gene therapy in humans for diseases involving defective beta-globin genes. We found regions that interfered with virus production within intron 2 of the beta-globin gene and on both sides of the gene. The flanking regions could be removed, but intron 2 was required for beta-globin expression. Inclusion of beta-globin introns necessitates an antisense orientation of the gene within the retrovirus vector. However, we found no effect of the antisense beta-globin transcription on virus production. A region downstream of the beta-globin gene that stimulates expression of the gene in transgenic mice was included in the viruses without detrimental effects on virus titer. Virus titers of over 10(6) CFU/ml were obtained with the final vector design, which retained the ability to direct regulated expression of human beta-globin in murine erythroleukemia cells. The vector also allowed transfer and expression of the human beta-globin gene in hematopoietic cells (CFU-S cells) in mice.

Retroviral-mediated gene transfer. Applications in neurobiology

There are now many examples of the successful expression of genes transduced by retroviruses in studies from outside the field of neuroscience. Retroviruses will undoubtedly also prove to be effective tools for neuro-scientists interested in expressing cloned neurotransmitter and receptor genes. There are also other less obvious applications of retroviruses, such as their insertional mutagenic effects, which may be useful in studies of the genetic factors and biochemical mechanisms involved in, for example, neurotoxicity. Strong cellular promoters have been identified by retroviral infection and subsequent rescue of the flanking genomic DNA. Retroviruses can be employed again to reintroduce these regulatory sequences back into cells. In this way the complexities of gene expression in the many subpopulations of neurons may be unraveled. Retroviruses can also serve as very useful genetic markers in studies of development and lineage relationships. Retroviruses may be used to efficiently transfer oncogenes into neuronal cells to create new cell lines. This application exploits one of the natural traits of retroviruses--oncogenesis--which led to their original discovery. Finally, there are neurotropic retroviruses that could serve as important vectors for delivering genes into neurons. Studying these retroviruses may lead to an understanding of how they cause neuropathologic changes in the CNS.

Generation of murine stromal cell lines supporting hematopoietic stem cell proliferation by use of recombinant retrovirus vectors encoding simian virus 40 large T antigen

The bone marrow is a complex microenvironment made up of multiple cell types which appears to play an important role in the maintenance of hematopoietic stem cell self-renewal and proliferation. We used murine long-term marrow cultures and a defective recombinant retrovirus vector containing the simian virus 40 large T antigen to immortalize marrow stromal cells which can support hematopoiesis in vitro for up to 5 weeks. Such cloned cell lines differentially supported stem cells which, when transplanted, allowed survival of lethally irradiated mice, formed hematopoietic spleen colonies in vivo, and stimulated lymphocyte proliferation in vitro. Molecular and functional analyses of these cell lines did not demonstrate the production of any growth factors known to support the proliferation of primitive hematopoietic stem cells. All cell lines examined produced macrophage colony-stimulating factor. The use of immortalizing retrovirus vectors may allow determination of unique cellular proteins important in hematopoietic stem cell proliferation by the systematic comparison of stromal cells derived from a variety of murine tissues.

Expression of the human beta-globin gene after retroviral transfer into murine erythroleukemia cells and human BFU-E cells

Replication-defective amphotropic retrovirus vectors containing either the human beta-globin gene with introns or an intronless beta-globin minigene were constructed and used to study beta-globin expression following gene transfer into hematopoietic cells. The beta-globin genes were marked by introducing a 6-base-pair insertion into the region corresponding to the 5' untranslated region of the beta-globin mRNA to allow detection of RNA encoded by the new gene in human cells expressing normal human beta-globin RNA. Introduction of a virus containing the beta-globin gene with introns into murine erythroleukemia cells resulted in inducible expression of human beta-globin RNA and protein, while the viruses containing the minigene were inactive. The introduced human beta-globin gene was 6 to 110% as active as the endogenous mouse beta maj-globin genes in six randomly chosen cell clones. Introduction of the viruses into human BFU-E cells, followed by analysis of marked and unmarked globin RNAs in differentiated erythroid colonies, revealed that the introduced beta-globin gene was about 5% as active as the endogenous genes in these normal human erythroid cells and that again the minigene was inactive. These data are discussed in terms of the potential treatment of genetic disease by gene therapy.

Stably transmitted triple-promoter retroviral vectors and their use in transformation of primary mammalian cells

Retroviral vectors were constructed which coexpressed three inserted genes from independent transcriptional promoters in singly infected cells. Several such triple-promoter vectors were constructed with various combinations of oncogenes and selectable drug resistance genes. All expressed three mRNAs of the expected size in infected cells. One vector expressing the v-Ha-ras, v-myc, and neo genes was characterized in detail. This retrovirus did not undergo rearrangement during the process of infection, as judged by Southern analysis, and infection of primary rat embryo fibroblasts demonstrated that ras-myc-cotransformed cells could be selected in G418. This demonstration that retroviral vectors can be used to express three cistrons independently increases their value as gene transfer vehicles, particularly for studies involving oncogene cooperation in primary cells.

The spleen focus-forming virus (SFFV) envelope gene, when introduced into mice in the absence of other SFFV genes, induces acute erythroleukemia

Previous studies in our laboratory and others have been consistent with the hypothesis that the envelope (env) gene of the spleen focus-forming virus (SFFV) is the only gene essential for the induction of acute erythroleukemia. However, no studies have been carried out with the SFFV env gene in the complete absence of other SFFV sequences. To accomplish this goal, we isolated the sequences that encode the envelope glycoprotein, gp52, of SFFVA and expressed them in a Moloney murine leukemia virus-based double-expression vector containing the neomycin resistance gene. The method used to produce retrovirus stocks in tissue culture cells affected the expression of the gp52 gene in the vector and the subsequent pathogenicity of the vector in mice. Highly pathogenic virus stocks were obtained by cotransfection of vector and helper virus DNAs into fibroblasts, followed by virus replication and spread through the cell population. Mice infected with this stock developed a rapid erythroid disease that was indistinguishable from that induced by the entire SFFV genome, and the virus stock transformed erythroid cells in vitro. Spleen cells from the diseased mice expressed the SFFV env gene product but not the SFFV gag gene product. As expected, mice given the virus containing the SFFV env gene in the reverse orientation did not express the SFFV env gene product or develop erythroleukemia. This study, therefore, demonstrated (i) that double-expression retroviral vectors can be used under specific conditions to produce viruses expressing high levels of a particular gene and (ii) that incorporation of the SFFV env gene into such a vector in the absence of other SFFV sequences results in a retrovirus which is as pathogenic as the original SFFV.

Neuronal expression of chimeric genes in transgenic mice

Gene expression may occur in unexpected ectopic sites when diverse genetic elements are juxtaposed as chimeric genes in transgenic mice. To determine the specific contribution of the promoter and reporter gene in ectopic expression, we have analyzed the expression of 14 different fusion genes in transgenic mice. Chimeric genes containing the mouse metallothionein-I promoter linked to either the rat or human growth hormone gene or the calcitonin/CGRP gene are expressed in a very similar pattern of neuronal regions. This ectopic expression is not a unique feature of the metallothionein promoter, since transferring the human growth hormone gene to four other heterologous promoters resulted in varying degrees of ectopic expression in overlapping subsets of cortical and hypothalamic neurons. The novel pattern of ectopic expression suggests that these otherwise unrelated neurons share a common developmental regulatory machinery for activation of gene transcription.

Expression of the human beta-globin gene after retroviral transfer into murine erythroleukemia cells and human BFU-E cells

Replication-defective amphotropic retrovirus vectors containing either the human beta-globin gene with introns or an intronless beta-globin minigene were constructed and used to study beta-globin expression following gene transfer into hematopoietic cells. The beta-globin genes were marked by introducing a 6-base-pair insertion into the region corresponding to the 5' untranslated region of the beta-globin mRNA to allow detection of RNA encoded by the new gene in human cells expressing normal human beta-globin RNA. Introduction of a virus containing the beta-globin gene with introns into murine erythroleukemia cells resulted in inducible expression of human beta-globin RNA and protein, while the viruses containing the minigene were inactive. The introduced human beta-globin gene was 6 to 110% as active as the endogenous mouse beta maj-globin genes in six randomly chosen cell clones. Introduction of the viruses into human BFU-E cells, followed by analysis of marked and unmarked globin RNAs in differentiated erythroid colonies, revealed that the introduced beta-globin gene was about 5% as active as the endogenous genes in these normal human erythroid cells and that again the minigene was inactive. These data are discussed in terms of the potential treatment of genetic disease by gene therapy.

Stably transmitted triple-promoter retroviral vectors and their use in transformation of primary mammalian cells

Retroviral vectors were constructed which coexpressed three inserted genes from independent transcriptional promoters in singly infected cells. Several such triple-promoter vectors were constructed with various combinations of oncogenes and selectable drug resistance genes. All expressed three mRNAs of the expected size in infected cells. One vector expressing the v-Ha-ras, v-myc, and neo genes was characterized in detail. This retrovirus did not undergo rearrangement during the process of infection, as judged by Southern analysis, and infection of primary rat embryo fibroblasts demonstrated that ras-myc-cotransformed cells could be selected in G418. This demonstration that retroviral vectors can be used to express three cistrons independently increases their value as gene transfer vehicles, particularly for studies involving oncogene cooperation in primary cells.

Construction and characterization of a retroviral vector demonstrating efficient expression of cloned cDNA sequences

We describe the construction and properties of a retroviral expression vector, designated pMV-7, designed to transfer unselected cDNAs and produce their encoded proteins in recipient cells. The vector is flanked by the long terminal repeats (LTRs) of the Moloney murine sarcoma virus (MSV) and contains the selectable drug resistance gene neo under the regulation of the herpes simplex virus (HSV) thymidine kinase (tk) promoter. Unique Eco RI and Hind III sites facilitate the introduction of sequences whose transcription is regulated by the 5' LTR. We have inserted cDNAs encoding: (i) the human lymphocyte antigen T4, (ii) the human lymphocyte antigen T8, and (iii) the murine hypoxanthine-guanine phosphoribosyl transferase (HPRT), into the pMV-7 vector. These constructions were used to transduce recipient cells to the neo+ phenotype. In each case, functional assays demonstrated that 65-92% of the neo+ clones produced the appropriate protein encoded by its corresponding cDNA. These clones were characterized further by analyzing the expression of vector-regulated transcripts. The neo+T4+ clones expressed a single full-length LTR-to-LTR transcript as detected by a T4 probe. The neo+T8+ clones, however, expressed both a full-length LTR-to-LTR transcript and an additional smaller transcript as detected by a T8 probe. This smaller transcript probably resulted from the utilization of cryptic signals which control 3' RNA processing. Furthermore, all of the neo+ clones expressed a transcript that initiated from the tk promoter, contained the neo gene, and used polyadenylation signals provided by the 3' LTR. Thus, the pMV-7 vector is capable of high-efficiency transfer and high-frequency expression of the cDNA-encoded protein.

Retrovirus-mediated gene transfer of human adenosine deaminase: expression of functional enzyme in murine hematopoietic stem cells in vivo

Simplified Moloney murine leukemia virus-based recombinant retrovirus vectors have been constructed which transduce human adenosine deaminase (ADA) cDNA. ADA transcription is under the control of the constitutive promoter for the human X chromosome phosphoglycerate kinase (pgk) gene. In these simplified vectors, dominant selectable markers are not included and selection is dependent on overproduction of functional ADA enzyme. Primary murine hematopoietic cells were infected with helper-free recombinant ADA virus generated from Psi-2 packaging cells. Protein analysis revealed that human ADA enzyme was expressed in progenitor-derived hematopoietic colonies in vitro and CFU-S-derived spleen colonies in vivo. Enzyme expression was dependent on transcription from the pgk promoter. ADA expression in primary murine hematopoietic cells directed by the internal promoter was not adversely affected by the presence of the Moloney virus long terminal repeat enhancer sequence. Use of these vectors allows systematic evaluation of the effects of specific sequences in recombinant retrovirus vectors on expression in primary murine hematopoietic cells in vivo.

Developmentally regulated expression of a truncated myosin light-chain 1F/3F gene

Fast skeletal muscle myosin light-chain I (MLC1f) and myosin light-chain 3 (MLC3f) mRNAs are both derived from a single rat MLC1/3f gene. MLC1f mRNA begins at the first exon of the gene, while MLC3f mRNA begins with exon 2, 10 kilobases downstream. Both mRNAs require alternate splicing of internal exons for accurate expression. We showed that a truncated rat MLC1f/3f gene lacking exon 1 and the first 6.3 kilobases of the intron separating exons 1 and 2 produced rat MLC3f mRNA in a developmentally regulated manner after introduction into myogenic mouse cells, thus demonstrating in vivo the presence of a functional promoter associated with exon 2. Correctly spliced mRNA was produced after transfer of this truncated gene into both myogenic and nonmyogenic cells, indicating that the pattern of splicing of this complex transcript was due to a structural features of the RNA and was independent of cell type.

Transcription elements and factors of RNA polymerase B promoters of higher eukaryotes

The promoter for eukaryotic genes transcribed by RNA polymerase B can be divided into the TATA box (located at -30) and startsite (+1), the upstream element (situated between -40 and about -110), and the enhancer (no fixed position relative to the startsite). Trans-acting factors, which bind to these elements, have been identified and at least partially purified. The role of the TATA box is to bind factors which focus the transcription machinery to initiate at the startsite. The upstream element and the enhancer somehow modulate this interaction, possibly through direct protein-protein interactions. Another class of transcription factors, typified by viral proteins such as the adenovirus EIA products, do not appear to require binding to a particular DNA sequence to regulate transcription. The latest findings in these various subjects are discussed.

Developmentally regulated expression of a truncated myosin light-chain 1F/3F gene

Fast skeletal muscle myosin light-chain I (MLC1f) and myosin light-chain 3 (MLC3f) mRNAs are both derived from a single rat MLC1/3f gene. MLC1f mRNA begins at the first exon of the gene, while MLC3f mRNA begins with exon 2, 10 kilobases downstream. Both mRNAs require alternate splicing of internal exons for accurate expression. We showed that a truncated rat MLC1f/3f gene lacking exon 1 and the first 6.3 kilobases of the intron separating exons 1 and 2 produced rat MLC3f mRNA in a developmentally regulated manner after introduction into myogenic mouse cells, thus demonstrating in vivo the presence of a functional promoter associated with exon 2. Correctly spliced mRNA was produced after transfer of this truncated gene into both myogenic and nonmyogenic cells, indicating that the pattern of splicing of this complex transcript was due to a structural features of the RNA and was independent of cell type.

Retrovirus-mediated gene transfer of human adenosine deaminase: expression of functional enzyme in murine hematopoietic stem cells in vivo

Simplified Moloney murine leukemia virus-based recombinant retrovirus vectors have been constructed which transduce human adenosine deaminase (ADA) cDNA. ADA transcription is under the control of the constitutive promoter for the human X chromosome phosphoglycerate kinase (pgk) gene. In these simplified vectors, dominant selectable markers are not included and selection is dependent on overproduction of functional ADA enzyme. Primary murine hematopoietic cells were infected with helper-free recombinant ADA virus generated from Psi-2 packaging cells. Protein analysis revealed that human ADA enzyme was expressed in progenitor-derived hematopoietic colonies in vitro and CFU-S-derived spleen colonies in vivo. Enzyme expression was dependent on transcription from the pgk promoter. ADA expression in primary murine hematopoietic cells directed by the internal promoter was not adversely affected by the presence of the Moloney virus long terminal repeat enhancer sequence. Use of these vectors allows systematic evaluation of the effects of specific sequences in recombinant retrovirus vectors on expression in primary murine hematopoietic cells in vivo.

An alternative to the magic bullet paradigm for specific cancer therapy

To improve on current cancer therapies, which attack cells on the basis of their proliferative tendencies, much effort has been devoted to a search for properties of tumor cells that are tumor-specific rather than proliferation specific. Evidence from molecular genetic studies suggests, however, that most tumors may lack such properties. An alternative approach to therapy is described that is based on a property known to characterize the majority of human tumors; viz., a monoclonal origin. The strategy requires the prophylactic induction in tissues of mosaicism for genes dictating sensitivity or resistance to chemotherapeutic agents, and exploits the observation that any clone of cells arising in a mosaic tissue must inevitably differ from some other cells in the mosaic. Recent advances in genetic technology imply that the strategy is likely to be testable soon in animals, and that it may significantly improve the results of cancer therapy when a technology safe and efficient enough for its human implementation becomes available.

Regulated expression of a complete human beta-globin gene encoded by a transmissible retrovirus vector

We introduced a human beta-globin gene into murine erythroleukemia (MEL) cells by infection with recombinant retroviruses containing the complete genomic globin sequence. The beta-globin gene was correctly regulated during differentiation, steady-state mRNA levels being induced 5- to 30-fold after treatment of the cells with the chemical inducer dimethyl sulfoxide. Studies using vectors which yield integrated proviruses lacking transcriptional enhancer sequences indicated that neither retroviral transcription nor the retroviral enhancer sequences themselves had any obvious effect on expression of the globin gene. Viral RNA expression also appeared inducible, being considerably depressed in uninduced MEL cells but approaching normal wild-type levels after dimethyl sulfoxide treatment. We provide data which suggest that the control point for both repression and subsequent activation of virus expression in MEL cells lies in the viral enhancer element.

Construction of a defective retrovirus containing the human hypoxanthine phosphoribosyltransferase cDNA and its expression in cultured cells and mouse bone marrow

Defective ecotropic and amphotropic retroviral vectors containing the cDNA for human hypoxanthine phosphoribosyltransferase (HPRT) were developed for efficient gene transfer and high-level cellular expression of HPRT. Helper cell clones which produced a high viral titer were generated by a simplified method which minimizes cell culture. We used the pZIP-NeoSV(X) vector containing a human hprt cDNA. Viral titers (1 X 10(3) to 5 X 10(4)/ml) of defective SVX HPRT B, a vector containing both the hprt and neo genes, were increased 3- to 10-fold by cocultivation of the ecotropic psi 2 and amphotropic PA-12 helper cells. Higher viral titers (8 X 10(5) to 7.5 X 10(6] were obtained when nonproducer NIH 3T3 cells or psi 2 cells carrying a single copy of SVX HPRT B were either transfected or infected by Moloney leukemia virus. The SVX HPRT B defective virus partially corrected the HPRT deficiency (4 to 56% of normal) of cultured rodent and human Lesch-Nyhan cells. However, instability of HPRT expression was detected in several infected clones. In these unstable variants, both retention and loss of the SVX HPRT B sequences were observed. In the former category, cells which became HPRT- (6-thioguanine resistant [6TGr]) also became G418s, indicative of a cis-acting down regulation of expression. Both hypoxanthine-aminopterin-thymidine resistance (HATr) and G418r could be regained by counterselection in hypoxanthine-aminopterin-thymidine. In vitro mouse bone marrow experiments indicated low-level expression of the neo gene in in vitro CFU assays. Individual CFU were isolated and pooled, and the human hprt gene was shown to be expressed. These studies demonstrated the applicability of vectors like SVX HPRT B for high-titer production of defective retroviruses required for hematopoietic gene transfer and expression.

Retroviral-mediated transfer of genomic globin genes leads to regulated production of RNA and protein

A high-titer amphotropic retroviral vector containing the neomycin resistance gene and a hybrid gamma-beta genomic human globin gene has been constructed. Mouse erythroleukemia cells infected with this virus were found to contain the full transcriptional unit of the transferred human globin gene by Southern blot analysis. These cells contain normally initiated, spliced, and terminated human globin mRNA. The human globin mRNA level increased 5- to 10-fold upon induction of the mouse erythroleukemia cells. Human globin chains were produced but only in a fraction of the cells as detected by immunofluorescent staining. A similar retrovirus containing a human beta-globin gene was used to transduce mouse erythroleukemia cells resulting in much higher levels of human globin synthesis than detected in mouse erythroleukemia cells transduced with the gamma-beta globin virus.

High mutation rate of a spleen necrosis virus-based retrovirus vector

Spleen necrosis virus (SNV) is an avian retrovirus that efficiently infects some mammalian cells (e.g., dog and rat cells). We constructed an SNV-based vector, which contains less than 1 kilobase (kb) of the retrovirus sequence, and a number of derivatives containing selectable markers. We obtained high-titer virus stocks, over 10(6) transforming units per ml, with a vector whose genomic RNA consists of 1,850 bases (full-length SNV RNA is 7.7 kb). We also studied two vectors that both carry two genes which should be expressed from a single promoter, one gene from unspliced mRNA and the other gene from spliced mRNA. In one vector, both genes were efficiently expressed as expected. However, in the other vector, expression of the gene 3' to the splice acceptor was inhibited. When we selected for expression of the 3' gene is this latter case, we found that the resistant cells contained mutant proviruses in which the 3' gene could be expressed. Furthermore, we found that mutations were generated during a single round of virus replication (provirus to provirus) at a rate of approximately 0.5% mutations per cycle.

Regulated expression of a complete human beta-globin gene encoded by a transmissible retrovirus vector

We introduced a human beta-globin gene into murine erythroleukemia (MEL) cells by infection with recombinant retroviruses containing the complete genomic globin sequence. The beta-globin gene was correctly regulated during differentiation, steady-state mRNA levels being induced 5- to 30-fold after treatment of the cells with the chemical inducer dimethyl sulfoxide. Studies using vectors which yield integrated proviruses lacking transcriptional enhancer sequences indicated that neither retroviral transcription nor the retroviral enhancer sequences themselves had any obvious effect on expression of the globin gene. Viral RNA expression also appeared inducible, being considerably depressed in uninduced MEL cells but approaching normal wild-type levels after dimethyl sulfoxide treatment. We provide data which suggest that the control point for both repression and subsequent activation of virus expression in MEL cells lies in the viral enhancer element.

Construction of a defective retrovirus containing the human hypoxanthine phosphoribosyltransferase cDNA and its expression in cultured cells and mouse bone marrow

Defective ecotropic and amphotropic retroviral vectors containing the cDNA for human hypoxanthine phosphoribosyltransferase (HPRT) were developed for efficient gene transfer and high-level cellular expression of HPRT. Helper cell clones which produced a high viral titer were generated by a simplified method which minimizes cell culture. We used the pZIP-NeoSV(X) vector containing a human hprt cDNA. Viral titers (1 X 10(3) to 5 X 10(4)/ml) of defective SVX HPRT B, a vector containing both the hprt and neo genes, were increased 3- to 10-fold by cocultivation of the ecotropic psi 2 and amphotropic PA-12 helper cells. Higher viral titers (8 X 10(5) to 7.5 X 10(6] were obtained when nonproducer NIH 3T3 cells or psi 2 cells carrying a single copy of SVX HPRT B were either transfected or infected by Moloney leukemia virus. The SVX HPRT B defective virus partially corrected the HPRT deficiency (4 to 56% of normal) of cultured rodent and human Lesch-Nyhan cells. However, instability of HPRT expression was detected in several infected clones. In these unstable variants, both retention and loss of the SVX HPRT B sequences were observed. In the former category, cells which became HPRT- (6-thioguanine resistant [6TGr]) also became G418s, indicative of a cis-acting down regulation of expression. Both hypoxanthine-aminopterin-thymidine resistance (HATr) and G418r could be regained by counterselection in hypoxanthine-aminopterin-thymidine. In vitro mouse bone marrow experiments indicated low-level expression of the neo gene in in vitro CFU assays. Individual CFU were isolated and pooled, and the human hprt gene was shown to be expressed. These studies demonstrated the applicability of vectors like SVX HPRT B for high-titer production of defective retroviruses required for hematopoietic gene transfer and expression.

Approach to the molecular mechanisms of the modulation of growth hormone gene expression by glucocorticoid and thyroid hormones

Glucocorticoid and thyroid hormones modulate the expression of the growth hormone gene. To investigate this control mechanism, we have determined whether this gene contains sites that bind the human glucocorticoid and thyroid hormone receptors in vitro. To do so, we have designed a novel assay for studying binding of the purified glucocorticoid receptor to cloned fragments of the human growth hormone (hGH) gene, and have adapted a DNA-competition assay for the thyroid receptor in nuclear cell extracts. Two glucocorticoid receptor binding regions were found in the hGH gene, one of high affinity in a fragment of the gene containing the first intron, and one of low affinity located within a 290 bp-fragment of 5'-flanking DNA. In contrast, the thyroid receptor bound with high affinity to the 5'-flanking fragment. Homologous binding regions for the two types of receptor were found in the human placental lactogen (chronic somatomammotropin) gene. DNA binding of the two receptor types appeared to depend on the presence of the hormone, yet antagonist-bound glucocorticoid receptor was still capable of interacting specifically with DNA. There was no evidence for synergism or antagonism of the two receptor types in binding to their respective sites on the hGH gene. The data also make it unlikely that the thyroid receptor negatively controls gene transcription and that the stimulatory effect of thyroid hormone results from a derepression mechanism.