Tumor-specific gene expression in hepatic metastases by a replication-activated adenovirus vector

Clinical applications of tumor gene therapy require tumor-specific delivery or expression of therapeutic genes in order to maximize the oncolytic index and minimize side effects. This study demonstrates activation of transgene expression exclusively in hepatic metastases after systemic application of a modified first-generation (E1A/E1B-deleted) adenovirus vector (AdE1-) in mouse tumor models. The discrimination between tumors and normal liver tissue is based on selective DNA replication of AdE1- vectors in tumor cells. This new AdE1- based vector system uses homologous recombination between inverted repeats to mediate precise rearrangements within the viral genome. As a result of these rearrangements, a promoter is brought into conjunction with a reporter gene creating a functional expression cassette. Genomic rearrangements are dependent upon viral DNA replication, which in turn occurs specifically in tumor cells. In a mouse tumor model with liver metastases derived from human tumor cells, a single systemic administration of replication activated AdE1- vectors achieved transgene expression in every metastasis, whereas no extra-tumoral transgene induction was observed. Here we provide a new concept for tumor-specific gene expression that is also applicable for other conditionally replicating adenovirus vectors.     

Production of adenovirus vector for gene therapy

The field of gene therapy is rapidly expanding with a major focus on the treatment of cancer. Replication-defective adenoviruses are vectors of choice for delivering corrective genes into human cells. Major efforts are directed to design new generations of adenoviral vectors that feature reduced immunogenicity and improved targeting ability. However, the production of adenoviral vectors for gene therapy applications faces a number of challenges that limit the availability of high quality material at the early stages of research and development in the gene therapy field. Moreover, very few papers have been published on the subject and information on large-scale production methods are only available through specialized conference proceedings. This review outlines the problems associated with mass production of adenovirus vectors and describes research efforts by a number of groups who have contributed to optimize production methods. Better understanding of the adenovirus infection and replication kinetics as well as better understanding of complementing cell line physiology and metabolism greatly contributed to improving vector titers and volumetric productivity at higher cell densities. Also, the critical aspect of viral vector quantitation is discussed.     

Cauliflower mosaic virus as a gene expression vector for plants

It is possible to replace the CaMV (cauliflower mosaic virus) ORF (open reading frame) II with foreign sequences without interfering with virus viability. Such recom-binants can induce the synthesis of substantial amounts of a foreign protein in infected plants and confer new properties to these plants. However, so far only three genes have been successfully cloned and expressed in this way. The expression mechanism of CaMV demands precise replacement of ORF II and probably certain structural features of the viral 35S RNA, which should not be disturbed by inserted sequences. Since these features are largely unknown, it cannot at present be pre-dicted whether an insert will be tolerated. It is more likely that larger inserts will disturb the viral gene expression mechanism than smaller ones.     

Development and assessment of human adenovirus type 11 as a gene transfer vector

Adenovirus vectors based on human serotype 5 (Ad5) have successfully been used as gene transfer vectors in many gene therapy-based approaches to treat disease. Despite their widespread application, many potential therapeutic applications are limited by the widespread prevalence of vector-neutralizing antibodies within the human population and the inability of Ad5-based vectors to transduce important therapeutic target cell types. In an attempt to circumvent these problems, we have developed Ad vectors based on human Ad serotype 11 (Ad11), since the prevalence of neutralizing antibodies to Ad11 in humans is low. E1-deleted Ad11 vector genomes were generated by homologous recombination in 293 cells expressing the Ad11-E1B55K protein or by recombination in Escherichia coli. E1-deleted Ad11 genomes did not display transforming activity in rodent cells. Transduction of primary human CD34+ hematopoietic progenitor cells and immature dendritic cells was more efficient with Ad11 vectors than with Ad5 vectors. Thirty minutes after intravenous injection into mice that express one of the Ad11 receptors (CD46), we found, in a pattern and at a level comparable to what is found in humans, Ad11 vector genomes in all analyzed organs, with the highest amounts in liver, lung, kidney, and spleen. Neither Ad11 genomes nor Ad11 vector-mediated transgene expression were, however, detected at 72 h postinfusion. A large number of Ad11 particles were also found to be associated with circulating blood cells. We also discovered differences in in vitro transduction efficiencies and in vivo biodistributions between Ad11 vectors and chimeric Ad5 vectors possessing Ad11 fibers, indicating that Ad11 capsid proteins other than fibers influence viral infectivity and tropism. Overall, our study provides a basis for the application of Ad11 vectors for in vitro and in vivo gene transfer and for gaining an understanding of the factors that determine Ad tropism.     

Potato virus X as a vector for gene expression in plants

The suitability of potato virus X (PVX) as a gene vector in plants was tested by analysis of two viral constructs. In the first, the GUS gene of Escherichia coli was substituted for the viral coat protein gene. In the second, GUS was added into the viral genome coupled to a duplicated copy of the viral promoter for the coat protein mRNA. The viral construct with the substituted coat protein gene accumulated poorly in inoculated protoplasts and failed to spread from the site of infection in plants. These results suggest a role for the viral coat protein in key stages of the viral infection cycle and show that gene replacement constructs are not suitable for the production of PVX-based gene vector. The construct with GUS coupled to the duplicated promoter for coat protein mRNA also accumulated less well in protoplasts than the unmodified PVX, but did infect systemically and directed high level synthesis of GUS in inoculated and systemically infected tissue. Although there was some genome instability in the PVX construct, much of the viral RNA in the systemically infected tissue had retained the foreign gene insertion, especially in infected Nicotiana clevelandii plants. These data point to a general utility of PVX as a vector for unregulated gene expression in plants.     

Potential of equine herpesvirus 1 as a vector for immunization

Key problems using viral vectors for vaccination and gene therapy are antivector immunity, low transduction efficiencies, acute toxicity, and limited capacity to package foreign genetic information. It could be demonstrated that animal and human cells were efficiently transduced with equine herpesvirus 1 (EHV-1) reconstituted from viral DNA maintained and manipulated in Escherichia coli. Between 13 and 23% of primary human CD3+, CD4+, CD8+, CD11b+, and CD19+ cells and more than 70% of CD4+ MT4 cells or various human tumor cell lines (MeWo, Huh7, HeLa, 293T, or H1299) could be transduced with one infectious unit of EHV-1 per cell. After intranasal instillation of EHV-1 into mice, efficient transgene expression in lungs was detectable. Successful immunization using EHV-1 was shown after delivery of the human immunodeficiency virus type 1 Pr55gag precursor by the induction of a Gag-specific CD8+ immune response in mice. Because EHV-1 was not neutralized by human sera containing high titers of antibodies directed against human herpesviruses 1 to 5, it is concluded that this animal herpesvirus has enormous potential as a vaccine vector, because it is able to efficiently transduce a variety of animal and human cells, has high DNA packaging capacity, and can conveniently be maintained and manipulated in prokaryotic cells.     

Development of a novel plasmid as a shuttle vector for heterologous gene expression in Mycoplasma yeatsii

A circular plasmid, pMyBK1, was detected in Mycoplasma yeatsii strain GIH(T). Analysis of the sequence of the 3432-bp replicon identified two predicted open reading frames (ORFs), one with sequence similarity to multiple plasmid mobilization proteins and one that matches only to hypothetical ORFs encoded by integrated chromosomal elements in the sequenced genomes of two Mycoplasma species. Shuttle vectors were constructed in Escherichia coli which could be introduced into M. yeatsii at high efficiency (10(4)-10(5) per μg DNA) by electroporation. Independent deletion analysis of the two ORFs disclosed that whereas mob was dispensable, orf2 was necessary for plasmid replication or maintenance. The absence of plasmid-encoded database matches for ORF2 indicates that pMyBK1 represents a novel plasmid family. One shuttle vector was used to demonstrate heterologous expression of the Mycoplasma fermentans malp gene and was stable during multiple passages. The host-plasmid system described has potential application for genetic manipulation in a genus for which few replicative vectors are available.     

Improved gene delivery into neuroglial cells using a fiber-modified adenovirus vector

One impediment to treating neuronal diseases is finding ways to introduce genes into specific neuroglial cell types. Here we describe the strategy for efficient gene delivery via transferrin receptor using an adenovirus bearing a peptide mimic for transferrin. The attachment of the peptide consisted of 12 amino acids on the C-terminus of adenovirus fiber protein significantly improved entry and expression of a beta-galactosidase transgene into neuroglial cells such as astrocytes, and Schwann cells. The entry of re-targeted viruses into cells depends on the attached peptide and the transferrin receptor. Furthermore, transferrin did not affect gene delivery by the engineered adenovirus, suggesting that the effectiveness of therapeutic agents targeted to the receptor would not be diminished by competition with the abundant endogenous transferrin present in the plasma. Therefore, such transduction systems hold promise for efficient delivering gene to neuroglial cells in gene therapy protocols.     

Phylogenetic analysis and structural predictions of human adenovirus penton proteins as a basis for tissue-specific adenovirus vector design

The penton base is a major capsid protein of human adenoviruses (HAdV) which forms the vertices of the capsid and interacts with hexon and fiber protein. Two hypervariable loops of the penton are exposed on the capsid surface. Sequences of these and 300 adjacent amino acid residues of all 51 HAdV and closely related simian adenoviruses were studied. Adjacent sequences and predicted overall secondary structure were conserved. Phylogenetic analysis revealed clustering corresponding to the HAdV species and recombination events in the origin of HAdV prototypes. All HAdV except serotypes 40 and 41 of species F exhibited an integrin binding RGD motif in the second loop. The lengths of the loops (HVR1 and RGD loops) varied significantly between HAdV species with the longest RGD loop observed in species C and the longest HVR1 in species B. Long loops may permit the insertion of motifs that modify tissue tropism. Genetic analysis of HAdV prime strain p17'H30, a neutralization variant of HAdV-D17, indicated the significance of nonhexon neutralization epitopes for HAdV immune escape. Fourteen highly conserved motifs of the penton base were analyzed by site-directed mutagenesis of HAdV-D8 and tested for sustained induction of early cytopathic effects. Thus, three new motifs essential for penton base function were identified additionally to the RGD site, which interacts with a secondary cellular receptor responsible for internalization. Therefore, our penton primary structure data and secondary structure modeling in combination with the recently published fiber knob sequences may permit the rational design of tissue-specific adenoviral vectors.     

The improvement of adenovirus vector production by increased expression of coxsackie adenovirus receptor

Adenoviruses are promising vectors for gene therapy. The production of adenoviral vectors (AdV), however, is limited by the cell density effect, namely when cell infection is performed at above 10⁶ cells/ml, a drop in cell-specific adenovirus productivity occurs. Our results also show that the coxsackie adenovirus receptor (CAR) plays an important role in AdV production. CAR expression of infected cells varied with culture time and the cell-specific AdV productivity dropped rapidly along with decreased CAR expression. Furthermore, CAR expression of cells was maintained at a high level by replacing the medium or supplementing it with trichostatin A, which could improve the cell-specific productivity. Thus, a higher CAR expression level at infection time could improve cell-specific AdV productivity at high cell densities.     

Tissue-specific, tumor-selective, replication-competent adenovirus vector for cancer gene therapy

We have previously described two replication-competent adenovirus vectors, named KD1 and KD3, for potential use in cancer gene therapy. KD1 and KD3 have two small deletions in the E1A gene that restrict efficient replication of these vectors to human cancer cell lines. These vectors also have increased capacity to lyse cells and spread from cell to cell because they overexpress the adenovirus death protein, an adenovirus protein required for efficient cell lysis and release of adenovirus from the cell. We now describe a new vector, named KD1-SPB, which is the KD1 vector with the E4 promoter replaced by the promoter for surfactant protein B (SPB). SPB promoter activity is restricted in the adult to type II alveolar epithelial cells and bronchial epithelial cells. Because KD1-SPB has the E1A mutations, it should replicate within and destroy only alveolar and bronchial cancer cells. We show that KD1-SPB replicates, lyses cells, and spreads from cell to cell as well as does KD1 in H441 cells, a human cancer cell line where the SPB promoter is active. KD1-SPB replicates, lyses cells, and spreads only poorly in Hep3B liver cancer cells. Replication was determined by expression of the E4ORF3 protein, viral DNA accumulation, fiber synthesis, and virus yield. Cell lysis and vector spread were measured by lactate dehydrogenase release and a "vector spread" assay. In addition to Hep3B cells, KD1-SPB also did not express E4ORF3 in HT29.14S (colon), HeLa (cervix), KB (nasopharynx), or LNCaP (prostate) cancer cell lines, in which the SPB promoter is not expected to be active. Following injection into H441 or Hep3B tumors growing in nude mice, KD1-SPB caused a three- to fourfold suppression of growth of H441 tumors, similar to that seen with KD1. KD1-SPB had only a minimal effect on the growth of Hep3B tumors, whereas KD1 again caused a three- to fourfold suppression. These results establish that the adenovirus E4 promoter can be replaced by a tissue-specific promoter in a replication-competent vector. The vector has three engineered safety features: the tissue-specific promoter, the mutations in E1A that preclude efficient replication in nondividing cells, and a deletion of the E3 genes which shield the virus from attack by the immune system. KD1-SPB may have use in treating human lung cancers in which the SPB promoter is active.     

AAV as a viral vector for human gene therapy

Investigation of the adeno-associated virus (AAV) life cycle has enabled the establishment of methodology and identification of critical cis-acting sequences required for recombinant AAV production. Vectors derived from the defective human parvovirus (AAV) have been used for successful gene transfer and expression in many diverse mammalian cell types, such as erythroid, airway epithelium, and neuronal cells. One of the crucial steps in the continued case of AAV as a vector is the development of packaging systems that will allow efficient encapsidation of foreign genes into AAV virions. For this reason, the focus of this article will be generation of recombinant AAV vectors.     

pHUSH: a single vector system for conditional gene expression

Background

Conditional expression vectors have become a valuable research tool to avoid artefacts that may result from traditional gene expression studies. However, most systems require multiple plasmids that must be independently engineered into the target system, resulting in experimental delay and an increased potential for selection of a cell subpopulation that differs significantly from the parental line. We have therefore developed pHUSH, an inducible expression system that allows regulated expression of shRNA, miRNA or cDNA cassettes on a single viral vector.

Results

Both Pol II and Pol III promoters have been successfully combined with a second expression cassette containing a codon-optimized tetracycline repressor and selectable marker. We provide examples of how pHUSH has been successfully employed to study the function of target genes in a number of cell types within in vitro and in vivo assays, including conditional gene knockdown in a murine model of brain cancer.

Conclusion

We have successfully developed and employed a single vector system that enables Doxycycline regulated RNAi or transgene expression in a variety of in vitro and in vivo model systems. These studies demonstrate the broad application potential of pHUSH for conditional genetic engineering in mammalian cells.     

Site-specific recombination mediated by an adenovirus vector expressing the Cre recombinase protein: a molecular switch for control of gene expression.

We have constructed replication-defective human adenovirus (Ad) type 5 vectors containing the gene for the Cre recombinase from bacteriophage P1 under control of the human cytomegalovirus immediate-early promoter (AdCre). Expression of the protein was detected in replication-permissive (293) and in nonpermissive (MRC5) cell lines, and its biochemical activity was demonstrated in a cell-free recombination assay using a plasmid containing two loxP sites. To study Cre-mediated recombination in an intracellular system, we constructed an Ad vector (AdMA19) containing the luciferase cDNA under control of the human cytomegalovirus promoter but separated from it by an extraneous spacer sequence flanked by loxP sites which blocked luciferase expression. Upon coinfection of 293 or MRC5 cells with AdMA19 and AdCre, luciferase expression was specifically induced by Cre-mediated excision of the intervening sequence. The use of Ad vectors combined with the Cre-loxP system for regulation of gene expression and other possible applications is discussed.     

Correction of a deletion mutant by gene targeting with an adenovirus vector.

The usefulness of adenovirus type 5 as a vector for homologous recombination was examined in CHO cells by using the adenine phosphoribosyltransferase (aprt) gene. Infection of a hemizygous CHO APRT- cell line containing a 3-bp deletion in exon 5 of the aprt gene with a recombinant adenovirus containing the wild-type gene resulted in restoration of the APRT+ phenotype at a frequency of 10(-5) to 10(-6) per infected cell. A relatively high frequency (approximately 6 to 20%) of the transductants appears to result from a homologous recombination event. The mutation on the chromosomal aprt gene is corrected in the homologous recombinants, and APRT expression is restored to a normal hemizygous level. Neither adenovirus nor exogenous promoter sequences are detected in the homologous recombinants. The remaining transductants result from random integration of the aprt gene with the adenovirus sequence. A number of adenovirus vectors containing different promoter sequences linked to the hamster aprt gene were constructed. A possible role for the promoter region in the homologous recombination event was indicated by the lack of homologous recombination in constructs lacking an active promoter.