Cloning of a DNA fragment from the left-hand terminus of the adenovirus type 2 genome and its use in site-directed mutagenesis

FastAd: A versatile toolkit for rapid generation of single adenoviruses or diverse adenoviral vector libraries

Adenoviruses (Ads) are potent gene delivery vectors for in vitro and in vivo applications. However, current methods for their construction are time-consuming and inefficient, limiting their rapid production and utility in generating complex genetic libraries. Here, we introduce FastAd, a rapid and easy-to-use technology for inserting recombinant "donor" DNA directly into infectious "receiver" Ads in mammalian cells by the concerted action of two efficient recombinases: Cre and Bxb1. Subsequently, the resulting mixed recombinant Ad population is subjected to negative selections by flippase recombinase to remove viruses that missed the initial recombination. With this approach, recombinant Ad production time is reduced from 2 months to 10 days or less. FastAd can be applied for inserting complex genetic DNA libraries into Ad genomes, as demonstrated by the generation of barcode libraries with over 3 million unique clones from a T25 flask-scale transfection of 3 million cells. Furthermore, we leveraged FastAd to construct an Ad library containing a comprehensive genome-wide CRISPR-Cas9 guide RNA library and demonstrated its effectiveness in uncovering novel virus-host interactions. In summary, FastAd enables the rapid generation of single Ad vectors or complex genetic libraries, facilitating not only novel applications of Ad vectors but also research in foundamental virology.

Adenoviral Gene Therapy Vectors in Clinical Use-Basic Aspects with a Special Reference to Replication-Competent Adenovirus Formation and Its Impact on Clinical Safety

Adenoviral vectors are commonly used in clinical gene therapy. Apart from oncolytic adenoviruses, vector replication is highly undesired as it may pose a safety risk for the treated patient. Thus, careful monitoring for the formation of replication-competent adenoviruses (RCA) during vector manufacturing is required. To render adenoviruses replication deficient, their genomic E1 region is deleted. However, it has been known for a long time that during their propagation, some viruses will regain their replication capability by recombination in production cells, most commonly HEK293. Recently developed RCA assays have revealed that many clinical batches contain more RCA than previously assumed and allowed by regulatory authorities. The clinical significance of the higher RCA content has yet to be thoroughly evaluated. In this review, we summarize the biology of adenovirus vectors, their manufacturing methods, and the origins of RCA formed during HEK293-based vector production. Lastly, we share our experience using minimally RCA-positive serotype 5 adenoviral vectors based on observations from our clinical cardiovascular gene therapy studies.

Current Use of Adenovirus Vectors and Their Production Methods

Various adenovirus (AdV) vector systems have proven to be lucrative options for gene delivery. They can serve as potential vaccine candidates for prevention of several common infectious diseases and hold the promise for gene therapy, especially for cancer. Several AdV vector-based therapies are currently at various stages of clinical trials worldwide, which make an immense interest of both the clinicians and researchers. Since these vectors are easy to manipulate, have broad tropism, and have the capability to yield high titers, this delivery system has a wide range of applications for different clinical settings. This chapter emphasizes on some of the current usages of AdV vectors and their production methods.

Oncolytic Adenoviruses in Cancer Treatment

The therapeutic use of viruses against cancer has been revived during the last two decades. Oncolytic viruses replicate and spread inside tumors, amplifying their cytotoxicity and simultaneously reversing the tumor immune suppression. Among different viruses, recombinant adenoviruses designed to replicate selectively in tumor cells have been clinically tested by intratumoral or systemic administration. Limited efficacy has been associated to poor tumor targeting, intratumoral spread, and virocentric immune responses. A deeper understanding of these three barriers will be required to design more effective oncolytic adenoviruses that, alone or combined with chemotherapy or immunotherapy, may become tools for oncologists.

Altering the Ad5 packaging domain affects the maturation of the Ad particles

We have previously described a new family of mutant adenoviruses carrying different combinations of attB/attP sequences from bacteriophage PhiC31 flanking the Ad5 packaging domain. These novel helper viruses have a significantly delayed viral life cycle and a severe packaging impairment, regardless of the presence of PhiC31 recombinase. Their infectious viral titers are significantly lower (100–1000 fold) than those of control adenovirus at 36 hours post-infection, but allow for efficient packaging of helper-dependent adenovirus. In the present work, we have analyzed which steps of the adenovirus life cycle are altered in attB-helper adenoviruses and investigated whether these viruses can provide the necessary viral proteins in trans. The entry of attB-adenoviral genomes into the cell nucleus early at early timepoints post-infection was not impaired and viral protein expression levels were found to be similar to those of control adenovirus. However, electron microscopy and capsid protein composition analyses revealed that attB-adenoviruses remain at an intermediate state of maturation 36 hours post-infection in comparison to control adenovirus which were fully mature and infective at this time point. Therefore, an additional 20–24 hours were found to be required for the appearance of mature attB-adenovirus. Interestingly, attB-adenovirus assembly and infectivity was restored by inserting a second packaging signal close to the right-end ITR, thus discarding the possibility that the attB-adenovirus genome was retained in a nuclear compartment deleterious for virus assembly. The present study may have substantive implications for helper-dependent adenovirus technology since helper attB-adenovirus allows for preferential packaging of helper-dependent adenovirus genomes.

Production of adenovirus vectors and their use as a delivery system for influenza vaccines

IMPORTANCE OF THE FIELD

With the emergence of highly pathogenic avian influenza H5N1 viruses that have crossed species barriers and are responsible for lethal infections in humans in many countries, there is an urgent need for the development of effective vaccines which can be produced in large quantities at a short notice and confer broad protection against these H5N1 variants. In order to meet the potential global vaccine demand in a pandemic scenario, new vaccine-production strategies must be explored in addition to the currently used egg-based technology for seasonal influenza.

AREAS COVERED IN THIS REVIEW

Adenovirus (Ad) based influenza vaccines represent an attractive alternative/supplement to the currently licensed egg-based influenza vaccines. Ad-based vaccines are relatively inexpensive to manufacture, and their production process does not require either chicken eggs or labor-intensive and time-consuming processes necessitating enhanced biosafety facilities. Most importantly, in a pandemic situation, this vaccine strategy could offer a stockpiling option to reduce the response time before a strain-matched vaccine could be developed.

WHAT THE READER WILL GAIN

This review discusses Ad-vector technology and the current progress in the development of Ad-based influenza vaccines.

TAKE HOME MESSAGE

Ad vector-based influenza vaccines for pandemic preparedness are under development to meet global vaccine demand.

A simplified in vitro ligation approach to clone an E1B55k-deleted double-targeted conditionally-replicative adenovirus

BACKGROUND

Construction of conditionally-replicative Adenovirus (CRAd) is complex and time-consuming. While homologous recombination (HR) using a two-plasmid system in bacteria is commonly used to generate CRAds, alternative methods may be required when HR fails. Previously, in vitro ligation has been suggested to facilitate construction of E1/E3-deleted, replication-incompetent Ad vectors. However, in vitro ligation has only rarely been used to generate CRAds and may be a complex procedure for molecular biologists who are not experts in the field.

METHODS AND RESULTS

A modified in vitro ligation approach was developed to construct a double-targeted, E1B55k-deleted CRAd. The method allowed the incorporation of a tumor-specific promoter, e.g. the heat-shock protein 70 (hsp70) promoter, upstream of E1a, deletion of the E1B55k gene, and HR-free cloning of the recombined E1Delta55k gene into the Ad genome. The genetic structure of the CRAd was confirmed using restriction analysis and PCR. The replication rate of the hsp70E1Delta55k CRAd was 1.5-2% of Ad without E1Delta55k deletion.

CONCLUSION

A 3-step cloning approach can generate a double-targeted, E1B55k-deleted CRAd using a straight-forward, modified in vitro ligation procedure.

Control of adenovirus packaging

The results of studies of Adenovirus have contributed to our basic understanding of the molecular biology of the cell. While a great body of knowledge has been developed concerning Ad gene expression, viral replication, and effects on the infected host, the molecular details of the assembly process of Adenovirus particles are largely unknown. In this article, we would like to propose a theoretical model for the packaging and assembly of Adenovirus and present an overview of the studies that have contributed to our present understanding. In particular, we will summarize the molecular details of the process for packaging of viral DNA into virus particles and highlight the events in packaging and assembly that require further study.

Possible mechanism of adenovirus generation from a cloned viral genome tagged with nucleotides at its ends

The entire cloned human adenovirus type 5 (Ad5) genome is known to be able to generate infectious virus after transfection into 293 cells when the both ends of the genome are exposed by digestion with appropriate restriction enzymes. However, when one or both ends of the genome are tagged with nucleotides and are not intact, whether the tagged end of the viral genome was remained tagged or corrected to be intact during the generation of viral clones has been unclear and, if such oligonucleotide removal occurs, how does the virus remove these tagged sequences and thereby restore its proper structure? Here, we show in our semi-quantitative study that the generation efficiency of virus clones decreases depending on the length of nucleotide tags at the both ends and that both the oligonucleotide tags were precisely removed during virus generation with restoration of the proper terminal sequences. Interestingly the viral genome of which one end was tagged, while the other was attached about 12-kb sequences, did generate intact viral clones at a reduced but significant efficiency. From these results, we here propose a possible mechanism whereby the terminal-protein-deoxycytidine complex enters from the enzyme-cleaved end and reaches deoxyguanine at the initiating position of DNA synthesis in vivo. A replication origin at one end, embedded deeply in double-stranded DNA, can be activated by two cycles of one-directional full-length DNA synthesis initiated by the other exposed replication origin about 30 kilobases away. We also describe new cassette cosmids which can use not only Pac I but also Bst BI for construction of an adenovirus vector, without reducing construction efficiency.

Cellular splicing and transcription regulatory protein p32 represses adenovirus major late transcription and causes hyperphosphorylation of RNA polymerase II

The cellular protein p32 is a multifunctional protein, which has been shown to interact with a large number of cellular and viral proteins and to regulate several important activities like transcription and RNA splicing. We have previously shown that p32 regulates RNA splicing by binding and inhibiting the essential SR protein ASF/SF2. To determine whether p32 also functions as a regulator of splicing in virus-infected cells, we constructed a recombinant adenovirus expressing p32 under the transcriptional control of an inducible promoter. Much to our surprise the results showed that p32 overexpression effectively blocked mRNA and protein expression from the adenovirus major late transcription unit (MLTU). Interestingly, the p32-mediated inhibition of MLTU transcription was accompanied by an approximately 4.5-fold increase in Ser 5 phosphorylation and an approximately 2-fold increase in Ser 2 phosphorylation of the carboxy-terminal domain (CTD). Further, in p32-overexpressing cells the efficiency of RNA polymerase elongation was reduced approximately twofold, resulting in a decrease in the number of polymerase molecules that reached the end of the major late L1 transcription unit. We further show that p32 stimulates CTD phosphorylation in vitro. The inhibitory effect of p32 on MLTU transcription appears to require the CAAT box element in the major late promoter, suggesting that p32 may become tethered to the MLTU via an interaction with the CAAT box binding transcription factor.

Functional interaction of the adenovirus IVa2 protein with adenovirus type 5 packaging sequences

Adenovirus type 5 (Ad5) DNA packaging is initiated in a polar fashion from the left end of the genome. The packaging process is dependent on the cis-acting packaging domain located between nucleotides 230 and 380. Seven AT-rich repeats that direct packaging have been identified within this domain. A1, A2, A5, and A6 are the most important repeats functionally and share a bipartite sequence motif. Several lines of evidence suggest that there is a limiting trans-acting factor(s) that plays a role in packaging. Both cellular and viral proteins that interact with adenovirus packaging elements in vitro have been identified. In this study, we characterized a group of recombinant viruses that carry site-specific point mutations within a minimal packaging domain. The mutants were analyzed for growth properties in vivo and for the ability to bind cellular and viral proteins in vitro. Our results are consistent with a requirement of the viral IVa2 protein for DNA packaging via a direct interaction with packaging sequences. Our results also indicate that higher-order IVa2-containing complexes that form on adjacent packaging repeats in vitro are the complexes required for the packaging activity of these sites in vivo. Chromatin immunoprecipitation was used to study proteins that bind directly to the packaging sequences. These results demonstrate site-specific interaction of the viral IVa2 and L1 52/55K proteins with the Ad5 packaging domain in vivo. These results confirm and extend those previously reported and provide a framework on which to model the adenovirus assembly process.

The alphavbeta5 integrin of hematopoietic and nonhematopoietic cells is a transduction receptor of RGD-4C fiber-modified adenoviruses

Epithelial and endothelial cells expressing the primary Coxsackie virus B adenovirus (Ad) receptor (CAR) and integrin coreceptors are natural targets of human Ad infections. The fiber knob of species A, C, D, E and F Ad serotypes binds CAR by mimicking the CAR-homodimer interface, and the penton base containing arginine-glycine-aspartate (RGD) motifs binds with low affinity to alphav integrins inducing cell activation. Here, we generated seven different genetically modified Ad vectors with RGD sequences inserted into the HI loop of fiber knob. All mutants bound and infected CAR and alphav integrin-positive epithelial cells with equal efficiencies. However, the Ads containing two additional cysteines, both N and C terminals of the RGD sequence (RGD-4C), were uniquely capable of transducing CAR-less hematopoietic and nonhematopoietic human tumor cell lines and primary melanoma cells. Both binding and transduction of RGD-4C Ad were blocked by soluble RGD peptides. Flow cytometry of cell surface integrins and virus binding to CAR-less cells in the presence of function-blocking anti-integrin antibodies indicated that the alphavbeta5 integrin, but not alphavbeta3, alphaIIbbeta3 or beta1,alpha5 or alpha6-containing integrins served as a functional transduction receptor of the RGD-4C Ads. However, in cells with low levels of alphavbeta5 integrin, the function-blocking anti-alphavbeta5 antibodies were not effective, unlike soluble RGD peptides. Collectively, our data demonstrate that the alphavbeta5 integrin is a functional transduction receptor of RGD-4C Ads in the absence of CAR, and that additional RGD receptors are targets of these viruses. The RGD-4C vectors further extend the tropism of Ads towards potential human therapies.

Binding of CCAAT displacement protein CDP to adenovirus packaging sequences

Adenovirus (Ad) type 5 DNA packaging is initiated in a polar fashion from the left end of the genome. The packaging process is dependent upon the cis-acting packaging domain located between nucleotides 194 and 380. Seven A/T-rich repeats have been identified within this domain that direct packaging. A1, A2, A5, and A6 are the most important repeats functionally and share a bipartite sequence motif. Several lines of evidence suggest that there is a limiting trans-acting factor(s) that plays a role in packaging. Two cellular activities that bind to minimal packaging domains in vitro have been previously identified. These binding activities are P complex, an uncharacterized protein(s), and chicken ovalbumin upstream promoter transcription factor (COUP-TF). In this work, we report that a third cellular protein, octamer-1 protein (Oct-1), binds to minimal packaging domains. In vitro binding analyses and in vivo packaging assays were used to examine the relevance of these DNA binding activities to Ad DNA packaging. The results of these experiments reveal that COUP-TF and Oct-1 binding does not play a functional role in Ad packaging, whereas P-complex binding directly correlates with packaging function. We demonstrate that P complex contains the cellular protein CCAAT displacement protein (CDP) and that full-length CDP is found in purified virus particles. In addition to cellular factors, previous evidence indicates that viral factors play a role in the initiation of viral DNA packaging. We propose that CDP, in conjunction with one or more viral proteins, binds to the packaging sequences of Ad to initiate the encapsidation process.

Minimal cis-acting elements required for adenovirus genome packaging

The design of drugs for treatment of virus infections and the exploitation of viruses as drugs for treatment of diseases could be made more successful by understanding the molecular mechanisms of virus-specific events. The process of assembly, and more specifically packaging of the genome into a capsid, is an obligatory step leading to future infections. To enhance our understanding of the molecular mechanism of packaging, it is necessary to characterize the viral components necessary for the event. In the case of adenovirus, sequences between nucleotides 200 and 400 at the left end of the genome are essential for packaging. This region contains a series of redundant bipartite sequences, termed A repeats, that function in packaging. Synthetic packaging sequences made of multimers of a single A repeat substitute for the authentic adenovirus packaging domain. A repeats are binding sites for the CCAAT displacement protein and the viral protein IVa2. Several lines of evidence implicate these proteins in the packaging process. It was not known, however, whether other cis-acting elements play a role in the packaging process as well. We utilized an in vivo approach to address the role of the inverted terminal repeats and the covalently linked terminal proteins in packaging of the adenovirus genome. Our results show that these elements are not necessary for efficient packaging of the viral genome. A significant implication of these results applicable to gene therapy vector design is that the linkage of the adenovirus packaging domain to heterologous DNA sequences should suffice for targeting to the viral capsid.

Nestin enhancer requirements for expression in normal and injured adult CNS

The nestin gene is expressed in many CNS stem/progenitor cells, both in the embryo and the adult, and nestin is used commonly as a marker for these cells. In this report we analyze nestin enhancer requirements in the adult CNS, using transgenic mice carrying reporter genes linked to three different nestin enhancer constructs: the genomic rat nestin gene and 5 kb of upstream nestin sequence (NesPlacZ/3), 636 bp of the rat nestin second intron (E/nestin:EGFP), and a corresponding 714 bp region from the human second intron (Nes714tk/lacZ). NesPlacZ/3 and E/nestin:EGFP mice showed reporter gene expression in stem cell-containing regions of brain and spinal cord during normal conditions. NesPlacZ/3 and E/nestin:EGFP mice showed increased expression in spinal cord after injury and NesPlacZ/3 mice displayed elevated expression in the periventricular area of the brain after injury, which was not the case for the E/nestin:EGFP mice. In contrast, no expression in adult CNS in vivo was seen in the Nes714tk/lacZ mice carrying the human enhancer, neither during normal conditions nor after injury. The Nes714 tk/lacZ mice, however, expressed the reporter gene in reactive astrocytes and CNS stem cells cultured ex vivo. Collectively, this suggests a species difference for the nestin enhancer function in adult CNS and that elements outside the second intron enhancer are required for the full injury response in vivo.

Adenovirus-based libraries: efficient generation of recombinant adenoviruses by positive selection with the adenovirus protease

Adenoviruses (Ad) deleted in the protease (PS) gene are capable of only one round of replication in non-complementing cells. This feature was exploited to develop a positive selection method for constructing adenoviral recombinants using ectopic expression of the PS gene in the E1 region. Very low levels of PS were sufficient to ensure the rescue of a PS-deleted Ad genome (Ad(Delta)PS), thereby eliminating deleterious effects PS over-expression might exert on cell or virus growth. In addition to the standard co-transfection method, an alternative protocol was developed in which the Ad5-(Delta)PS viral DNA was delivered by infection before subsequent transfection of 293 cells with the transfer vector. Under optimal conditions, at least one recombinant Ad per 10(3) cells was generated with 100% of the plaques being recombinant. Since the infection/transfection protocol is readily scalable, this represents the first method that allows for the easy construction of adenovirus vector (AdV) libraries with high diversities. This approach addresses in a novel way the bottleneck encountered when converting plasmid libraries, constructed in E. coli using a variety of well-established strategies, into corresponding AdV libraries. It maintains high diversity while generating recombinant viruses with 100% efficiency.

Rapid construction of adenoviral vectors by lambda phage genetics

Continued improvements of adenoviral vectors require the investigation of novel genome configurations. Since adenovirus can be generated directly by transfecting packaging cell lines with viral genomes isolated from plasmid DNA, it is possible to separate genome construction from virus production. In this way failure to generate a virus is not associated with an inability to generate the desired genome. We have developed a novel lambda-based system that allows rapid modification of the viral genome by double homologous recombination in Escherichia coli. The recombination reaction and newly generated genome may reside in a recombination-deficient bacterial host for enhanced plasmid stability. Furthermore, the process is independent of any restriction endonucleases. The strategy relies on four main steps: (i) homologous recombination between an adenovirus cosmid and a donor plasmid (the donor plasmid carries the desired modification[s] and flanking regions of homology to direct its recombination into the viral genome); (ii) in vivo packaging of the recombinant adenoviral cosmids during a productive lambda infection; (iii) transducing a recombination-deficient E. coli lambda lysogen with the generated lysate (the lysogen inhibits the helper phage used to package the recombinant andenoviral cosmid from productively infecting and destroying the host bacteria); (iv) effectively selecting for the desired double-recombinant cosmid. Approximately 10,000 double-recombinant cosmids are recovered per reaction with essentially all of them being the correct double-recombinant molecule. This system was used to generate quickly and efficiently adenoviral genomes deficient in the E1/E3 and E1/E3/E4 regions. The basis of this technology allows any region of the viral genome to be readily modified for investigation of novel configurations.

Adenovirus vector designed for expression of toxic proteins

To construct recombinant adenoviruses expressing biologically active proteins may be impossible, or result in a significant reduction in virus yield, if the protein expressed has an inhibitory effect on virus replication or cellular growth. To overcome this problem, we previously designed adenovirus vectors expressing foreign proteins from inducible promoters. However, during our work with a replication-deficient virus expressing the ASF/SF2 splicing factor from a progesterone antagonist-inducible gene cassette, we discovered that ASF/SF2 was expressed at a significant level in the 293 producer cell line, even in the absence of inducer. 293 cells code for adenovirus E1A and E1B proteins and thus support the growth of E1-deficient adenoviruses. Here we show that this background ASF/SF2 expression results from a low level of E1A-mediated transactivation of the basal promoter driving transgene expression. To overcome the problem of leaky expression, we reconstructed a novel gene cassette that combines an inducible promoter and a Lac repressor protein-based block to reduce transcriptional elongation. We show that this novel vector system dramatically reduced background transgene expression and therefore should be useful for the rescue and propagation of high-titer stocks of recombinant adenoviruses expressing toxic proteins.

Differentiation of embryonic mesenchymal cells to odontoblast-like cells by overexpression of dentin matrix protein 1

Cells of the craniofacial skeleton are derived from a common mesenchymal progenitor. The regulatory factors that control their differentiation into various cell lineages are unknown. To investigate the biological function of dentin matrix protein 1 (DMP1), an extracellular matrix gene involved in calcified tissue formation, stable transgenic cell lines and adenovirally infected cells overexpressing DMP1 were generated. The findings in this paper demonstrate that overexpression of DMP1 in pluripotent and mesenchyme-derived cells such as C3H10T1/2, MC3T3-E1, and RPC-C2A can induce these cells to differentiate and form functional odontoblast-like cells. Functional differentiation of odontoblasts requires unique sets of genes being turned on and off in a growth- and differentiation-specific manner. The genes studied include transcription factors like core binding factor 1 (Cbfa1), bone morphogenetic protein 2 (BMP2), and BMP4; early markers for extracellular matrix deposition like alkaline phosphatase (ALP), osteopontin, osteonectin, and osteocalcin; and late markers like DMP2 and dentin sialoprotein (DSP) that are expressed by terminally differentiated odontoblasts and are responsible for the formation of tissue-specific dentin matrix. However, this differentiation pathway was limited to mesenchyme-derived cells only. Other cell lines tested by the adenoviral expression system failed to express odontoblast-phenotypic specific genes. An in vitro mineralized nodule formation assay demonstrated that overexpressed cells could differentiate and form a mineralized matrix. Furthermore, we also demonstrate that phosphorylation of Cbfa1 (osteoblast-specific transcription factor) was not required for the expression of odontoblast-specific genes, indicating the involvement of other unidentified odontoblast-specific transcription factors or coactivators. Cell lines that differentiate into odontoblast-like cells are useful tools for studying the mechanism involved in the terminal differentiation process of these postmitotic cells.

Adeno-associated virus (AAV) Rep protein enhances the generation of a recombinant mini-adenovirus (Ad) utilizing an Ad/AAV hybrid virus

Mini-adenoviruses (mAd) deleted of all viral coding regions represent an emerging approach for transgene expression. We have exploited the unique features of the adeno-associated virus (AAV) terminal repeats within the context of an adenovirus-adeno-associated hybrid virus (Ad/AAV) as a strategy for rapid and efficient generation of mAd. Excision and generation of mAd from the parental Ad/AAV hybrid vector was achieved in 293 cells through recombination but without selection for mAd production. Analysis of mAd isolated from 293 cells indicated that mAd DNA exists as monomer and dimer forms within the recombinant viral capsid. Formation of recombinant mAd was significantly increased using an AAV Rep78- or Rep68-expressing cell line through Rep-mediated excision utilizing the AAV terminal repeat sequences present in the Ad/AAV hybrid virus genome. The mAd viruses were infectious and able to transfer functional gene to A549 and HeLa cells. This approach is rapid and efficient, thereby providing a simplified methodology for generating mAd with functional transducing capabilities.

Production of first generation adenovirus vectors: a review

In the past decade, adenovirus vectors have generated tremendous interest, especially in gene therapy applications. In the so-called 'first generation' adenovirus vectors, the transgenes are inserted in place of the E1 region, or less often the E3 region. Although second-generation and helper-dependent adenovirus vectors will probably prevail in the future in applications that require long-term gene expression, first generation adenovirus vectors will remain very useful in other settings, such as cancer and vaccination, or simply to transfect cell lines that are refractory to other transfection methods. Until a few years ago, the construction of first generation adenovirus vectors was a labor-intensive and time-consuming process. More than 20 methods have appeared that facilitate their construction and are reviewed below.

Overexpression of essential splicing factor ASF/SF2 blocks the temporal shift in adenovirus pre-mRNA splicing and reduces virus progeny formation

Expression of cytoplasmic mRNA from most adenovirus transcription units is subjected to a temporal regulation at the level of alternative pre-mRNA splicing. The general tendency is that splice site selection changes from proximal to distal late after infection. Interestingly, ASF/SF2, which is a prototypical member of the SR family of splicing factors, has the opposite effect on splice site selection, inducing an increase in proximal splice site usage. We have previously shown that SR proteins late during an adenovirus infection become partially inactivated as splicing regulatory proteins. A prediction from these results is that overexpression of an SR protein, such as ASF/SF2, during virus growth will interfere with virus replication by disturbing the balance of functional and nonfunctional ASF/SF2 in the infected cell. To test this hypothesis, we reconstructed a recombinant adenovirus expressing ASF/SF2 under the transcriptional control of a regulated promoter. The results show that, as predicted, induction of ASF/SF2 during lytic virus growth prevents the early to late shift in mRNA expression from both early (E1A and E1B) and late (L1) transcription units. Furthermore, ASF/SF2 overexpression blocks viral DNA replication and reduces selectively cytoplasmic accumulation of major late mRNA, resulting in a lower virus yield. Collectively, our results provide additional support for the hypothesis that viral control of SR protein function is important for the proper expression of viral proteins during lytic virus growth.

The E4-6/7 protein functionally compensates for the loss of E1A expression in adenovirus infection

The E1A gene products are required and sufficient for activation of adenovirus gene expression in cultured cells. The E4-6/7 gene product induces the binding of the cellular transcription factor E2F to the viral E2a promoter region. The induction of E2F binding to the E2a promoter in vitro is directly correlated with transcriptional activation of the E2a promoter in vivo. The E2 region encodes the viral replication proteins, yet adenoviruses lacking E4-6/7 function demonstrate no defective phenotype in infected cells. Here we show that the E4-6/7 protein can functionally compensate for E1A expression in virus infection. In the absence of the E1A gene products, expression of the E4-6/7 protein is sufficient to displace retinoblastoma protein family members from E2Fs, activate expression of early region 2 via induction of E2F DNA binding to the E2a promoter region, and significantly enhance replication of an E1A-defective adenovirus. These results have implications in the regulation of viral gene expression and for the development of recombinant adenovirus vectors.

Gene therapy for gliomas: molecular targets, adenoviral vectors, and oncolytic adenoviruses

Currently, most of the approved clinical gene therapy protocols involve cancer patients and several of the therapies are designed to treat brain tumors. Two factors promoting the use of gene therapy for gliomas are the failure and toxicity of conventional therapies and the identification of the genetic abnormalities that contribute to the malignancy of gliomas. During the malignant progression of astrocitic tumors several tumor suppressor genes are inactivated, and numerous growth factors and oncogenes are overexpressed progressively. Thus, theoretically, brain tumors could be treated by targeting their fundamental molecular defects, provided the gene-drug can be delivered to a sufficient number of malignant cells. However, gene therapy strategies have not been abundantly successful clinically, in part because the delivery systems are still imperfect. In the first part of this brief review we will discuss the most common targets for gene therapy in brain tumors. In the second part, we will review the evolution of adenoviruses as gene vehicles. In addition, we will examine the role of recombinant mutant oncolytic adenoviruses as anticancer tools. From the results to date it is clear that gene therapy strategies for brain tumors are quite promising but more critical research is required, mainly in the vector field, if the strategies are to achieve their true potential in ameliorating patients with gliomas.

Adenovirus-mediated tissue-targeted expression of the HSVtk gene for the treatment of breast cancer

In an effort to develop a genetic therapy for the treatment of breast cancer, we constructed adenoviral vectors containing either the beta-galactosidase (beta-gal) reporter gene or the herpes simplex thymidine kinase (HSVtk) suicide gene driven by breast tissue-specific promoters. We utilized upstream regulatory sequences from either the human alpha-lactalbumin (hALA) gene, or the ovine beta-lactoglobulin (oBLG) gene in these vector constructs to target expression of heterologous genes transcriptionally to breast cancer cells both in vitro and in vivo. Data derived from breast tissue-specific reporter vectors in vitro demonstrate that expression from the hALA and oBLG promoters are indeed specific for breast cells (T47D, MCF-7, ZR75-1) when compared with non-breast cells (U2OS, HeLa). Moreover, these vectors displayed tumor cell specificity when compared with the normal MCF-10A breast cell line. These vectors also displayed breast tissue specificity when injected systemically (i.v.) into lactating Balb/c mice, which suggests that these promoters maintain their tissue-specific expression pattern within the context of the adenoviral genome in vivo. Tumors, derived from T47D human breast cancer cells, were established in nude mice and injected with either the tissue-specific reporter or suicide vectors. Results from tumors injected (i.t.) with reporter adenoviruses demonstrate that these promoters are active in T47D cells when grown as established tumors and we observed a marked regression of tumors injected with suicide vectors and treated systemically with gancyclovir (150 mg/kg/day) when compared with control animals. Moreover, mouse survival was prolonged after 35 days in mice undergoing therapy with the suicide vectors in conjunction with gancyclovir when compared with the control animals. These data suggest that the transcriptionally targeted hALA or oBLG driven expression of the HSVtk gene may be a feasible therapy for the treatment of human breast cancer.

Recombinant adenovirus is an appropriate vector for endocytotic protein trafficking studies in cultured neurons

Endocytosis of full-length beta-amyloid precursor protein (APP) from the plasma membrane contributes to beta-amyloid peptide (Abeta) secretion, and, hence, potentially contributes to the molecular pathogenesis of Alzheimer's disease. We recently have demonstrated that central neuronal APP is endocytosed in a common vesicular compartment with recycling synaptic vesicle integral membrane proteins, but is then sorted away from synaptic vesicles for retrograde transport to the neuronal soma. For this report, we explore whether recombinant adenovirus can be used to modulate APP expression in cultured central neurons to study APP processing by the endocytotic pathway in these cells. Using a replication-deficient recombinant adenovirus that expresses a lacZ reporter (Ad5/CMV-lacZ), we demonstrate high efficiency of transfection (30-35%) at low viral titer (10-20 MOI), with no significant neuronal toxicity or cytoarchitectural change. In addition, we demonstrate that infection with the control virus does not result in re-direction of endogenous neuronal APP from usual endocytotic pathways. We have prepared, using the same genomic background as the control virus, an adenoviral vector that expresses the neuronal isoform of human APP (Ad5/CMV-APP). Infection with Ad5/CMV-APP at 10-20 MOI results in significantly increased immunoreactivity for endocytosed APP with preservation of usual endocytotic trafficking. These results demonstrate that recombinant adenovirus at low titer is an appropriate and effective vector for protein trafficking/processing studies in cultured central neurons.

Efficient inhibition of intimal hyperplasia by adenovirus-mediated inducible nitric oxide synthase gene transfer to rats and pigs in vivo

BACKGROUND

Inadequate nitric oxide (NO) availability may underlie vascular smooth muscle overgrowth that contributes to vascular occlusive diseases including atherosclerosis and restenosis. NO possesses a number of properties that should inhibit this hyperplastic healing response, such as promoting reendothelialization, preventing platelet and leukocyte adherence, and inhibiting cellular proliferation.

STUDY DESIGN

We proposed that shortterm but sustained increases in NO synthesis achieved with inducible NO synthase (iNOS) gene transfer at sites of vascular injury would prevent intimal hyperplasia. We constructed an adenoviral vector, AdiNOS, carrying the human iNOS cDNA and used it to express iNOS at sites of arterial injury in vivo.

RESULTS

AdiNOS-treated cultured vascular smooth muscle cells produced up to 100-fold more NO than control cells. In vivo iNOS gene transfer, using low concentrations of AdiNOS (2 x 10(6) plaque forming units [PFU]/rat) to injured rat carotid arteries, resulted in a near complete (>95%) reduction in neointima formation even when followed longterm out to 6 weeks post-injury. This protective effect was reversed by the continuous administration of an iNOS selective inhibitor L-N6-(1-iminoethyl)-lysine. However, iNOS gene transfer did not lead to regression of preestablished neointimal lesions. In an animal model more relevant to human vascular healing, iNOS gene transfer (5 x 10(8) PFU/pig) to injured porcine iliac arteries in vivo was also efficacious, reducing intimal hyperplasia by 51.8%.

CONCLUSIONS

These results indicate that shortterm overexpression of the iNOS gene initiated at the time of vascular injury is an effective method of locally increasing NO levels to prevent intimal hyperplasia.

Cellular components interact with adenovirus type 5 minimal DNA packaging domains

Adenovirus type 5 DNA packaging is initiated from the left end of the viral genome and depends on the presence of a cis-acting packaging domain located between nucleotides 194 and 380. Multiple redundant packaging elements (termed A repeats I through VII [AI through AVII]) are contained within this domain and display differential abilities to support DNA packaging in vivo. The functionally most important repeats, AI, AII, AV, and AVI, follow a bipartite consensus motif exhibiting AT-rich and CG-rich core sequences. Results from previous mutational analyses defined a fragment containing AV, AVI, and AVII as a minimal packaging domain in vivo, which supports a functional independence of the respective cis-acting sequences. Here we describe multimeric versions of individual packaging elements as minimal packaging domains that can confer viability and packaging activity to viruses carrying gross truncations within their left end. These mutant viruses directly rate the functional role that different packaging elements play relative to each other. The A repeats are likely to be binding sites for limiting, trans-acting packaging factors of cellular and/or viral origin. We report here the characterization of two cellular binding activities interacting with all of the minimal packaging domains in vitro, an unknown binding activity termed P-complex, and the transcription factor chicken ovalbumin upstream promoter transcription factor. The binding of both activities is dependent on the integrity of the AT-rich, but not the CG-rich, consensus half site. In the case of P-complex, binding affinity for different minimal packaging domains in vitro correlates well with their abilities to support DNA packaging in vivo. Interestingly, P-complex interacts not only with packaging elements but also with the left terminus of the viral genome, the core origin of replication. Our data implicate cellular factors as components of the viral packaging machinery. The dual binding specificity of P-complex for packaging and replication sequences may further suggest a direct involvement of left-end replication sequences in viral DNA encapsidation.

Inducible nitric oxide synthase suppresses the development of allograft arteriosclerosis

In cardiac transplantation, chronic rejection takes the form of an occlusive vasculopathy. The mechanism underlying this disorder remains unclear. The purpose of this study was to investigate the role nitric oxide (NO) may play in the development of allograft arteriosclerosis. Rat aortic allografts from ACI donors to Wistar Furth recipients with a strong genetic disparity in both major and minor histocompatibility antigens were used for transplantation. Allografts collected at 28 d were found to have significant increases in both inducible NO synthase (iNOS) mRNA and protein as well as in intimal thickness when compared with isografts. Inhibiting NO production with an iNOS inhibitor increased the intimal thickening by 57.2%, indicating that NO suppresses the development of allograft arteriosclerosis. Next, we evaluated the effect of cyclosporine (CsA) on iNOS expression and allograft arteriosclerosis. CsA (10 mg/kg/d) suppressed the expression of iNOS in response to balloon-induced aortic injury. Similarly, CsA inhibited iNOS expression in the aortic allografts, associated with a 65% increase in intimal thickening. Finally, we investigated the effect of adenoviral-mediated iNOS gene transfer on allograft arteriosclerosis. Transduction with iNOS using an adenoviral vector suppressed completely the development of allograft arteriosclerosis in both untreated recipients and recipients treated with CsA. These results suggest that the early immune-mediated upregulation in iNOS expression partially protects aortic allografts from the development of allograft arteriosclerosis, and that iNOS gene transfer strategies may prove useful in preventing the development of this otherwise untreatable disease process.

Bipartite structure and functional independence of adenovirus type 5 packaging elements

Selectivity and polarity of adenovirus type 5 DNA packaging are believed to be directed by an interaction of putative packaging factors with the cis-acting adenovirus packaging domain located within the genomic left end (nucleotides 194 to 380). In previous studies, this packaging domain was mutationally dissected into at least seven functional elements called A repeats. These elements, albeit redundant in function, exhibit differences in the ability to support viral packaging, with elements I, II, V, and VI as the most critical repeats. Viral packaging was shown to be sensitive to spatial changes between individual A repeats. To study the importance of spatial constraints in more detail, we performed site-directed mutagenesis of the 21-bp linker regions separating A repeats I and II, as well as A repeats V and VI. The results of our mutational analysis reveal previously unrecognized sequences that are critical for DNA encapsidation in vivo. On the basis of these results, we present a more complex consensus motif for the adenovirus packaging elements which is bipartite in structure. DNA encapsidation is compromised by changes in spacing between the two conserved parts of the consensus motif, rather than between different A repeats. Genetic evidence implicating packaging elements as independent units in viral DNA packaging is derived from the selection of revertants from a packaging-deficient adenovirus: multimerization of packaging repeats is sufficient for the evolution of packaging-competent viruses. Finally, we identify minimally sized segments of the adenovirus packaging domain that can confer viability and packaging activity to viruses carrying gross truncations within their left-end sequences. Coinfection experiments using the revertant as well as the minimal-packaging-domain mutant viruses strengthen existing arguments for the involvement of limiting, trans-acting components in viral DNA packaging.

Sequence-mediated regulation of adenovirus gene expression by repression of mRNA accumulation

Gene expression in complex transcription units can be regulated at virtually every step in the production of mature cytoplasmic mRNA, including transcription initiation, elongation, termination, pre-mRNA processing, nucleus-to-cytoplasm mRNA transport, and alterations in mRNA stability. We have been characterizing alternative poly(A) site usage in the adenovirus major late transcription unit (MLTU) as a model for regulation at the level of pre-mRNA 3'-end processing. The MLTU contains five polyadenylation sites (L1 through L5). The promoter proximal site (L1) functions as the dominant poly(A) site during the early stage of adenovirus infection and in plasmid transfections when multiple poly(A) sites are present at the 3' end of a reporter plasmid. In contrast, stable mRNA processed at all five poly(A) sites is found during the late stage of adenovirus infection, after viral DNA replication has begun. Despite its dominance during early infection, L1 is a comparatively poor substrate for 3'-end RNA processing both in vivo and in vitro. In this study we have investigated the basis for the early L1 dominance. We have found that mRNA containing an unprocessed L1 poly(A) site is compromised in its ability to enter the steady-state pool of stable mRNA. This inhibition, which affects either the nuclear stability or nucleus-to-cytoplasm transport of the pre-mRNA, requires a cis-acting sequence located upstream of the L1 poly(A) site.

Construction of adenovirus vectors through Cre-lox recombination

Two barriers prevent adenovirus-based vectors from having wide application. One is the difficulty of making new adenoviruses, and the second is the strong immunological reaction to viral proteins. Here we describe uses of Cre-lox recombination to overcome these problems. First, we demonstrate a simple method for constructing E1-substituted adenoviruses. Second, we demonstrate a method to construct adenovirus vectors carrying recombinant genes in place of all of the viral genes, so-called gutless adenovirus vectors. The pivotal feature in each method is the use of a negatively selected adenovirus named psi5. We engineered a cis-acting selection into psi5 by flanking its packaging site with loxP sites. When psi5 was grown in cells making a high level of Cre recombinase, the packaging site was deleted by recombination and the yield of psi5 was reduced to 5% of the wild-type level. To make a new E1-substituted virus, we used psi5 as a donor virus and recombined it with a shuttle vector via a loxP site. The resulting recombinant virus has a single loxP site next to the packaging site and therefore outgrows psi5 in the presence of Cre recombinase. To make a gutless virus, we used psi5 as a helper virus. The only viral sequences included in the gutless vector are those needed in cis for its replication and packaging. We found that a loxP site next to the packaging site of the gutless virus was necessary to neutralize homologous recombination between psi5 and the gutless viruses within their packaging domains.

A gene transfer vector-cell line system for complete functional complementation of adenovirus early regions E1 and E4

The improvements to adenovirus necessary for an optimal gene transfer vector include the removal of virus gene expression in transduced cells, increased transgene capacity, complete replication incompetence, and elimination of replication-competent virus that can be produced during the growth of first-generation adenovirus vectors. To achieve these aims, we have developed a vector-cell line system for complete functional complementation of both adenovirus early region 1 (E1) and E4. A library of cell lines that efficiently complement both E1 and E4 was constructed by transforming 293 cells with an inducible E4-ORF6 expression cassette. These 293-ORF6 cell lines were used to construct and propagate viruses with E1 and E4 deleted. While the construction and propagation of AdRSV beta gal.11 (an E1-/E4- vector engineered to contain a deletion of the entire E4 coding region) were possible in 293-ORF6 cells, the yield of purified virus was depressed approximately 30-fold compared with that of E1- vectors. The debilitation in AdRSV beta gal.11 vector growth was found to correlate with reduced fiber protein and mRNA accumulation. AdCFTR.11A, a modified E1-/E4- vector with a spacer sequence placed between late region 5 and the right inverted terminal repeat, efficiently expressed fiber and grew with the same kinetic profile and virus yield as did E1- vectors. Moreover, purified AdCFTR.11A yields were equivalent to E1- vector levels. Since no overlapping sequences exist in the E4 regions of E1-/E4- vectors and 293-ORF6 cell lines, replication-competent virus cannot be generated by homologous recombination. In addition, these second-generation E1-/E4- vectors have increased transgene capacity and have been rendered virus replication incompetent outside of the new complementing cell lines.

Absence of an essential regulatory influence of the adenovirus E1B 19-kilodalton protein on viral growth and early gene expression in human diploid WI38, HeLa, and A549 cells

Mutations in the gene encoding the adenovirus (Ad) early region 1B 19-kDa protein (the 19K gene) result in multiple phenotypic effects upon infection of permissive human cells. It has been reported, for example, that Ad type 2 (Ad2) and Ad5 with mutations in the 19K gene (19K-defective mutants) have a marked growth advantage compared with wild-type virus in human diploid WI38 cells (E. White, B. Faha, and B. Stillman, Mol. Cell. Biol. 6:3763-3773, 1986), and it was proposed that this host range phenotype stems from the large increase in viral early gene expression reported to occur in the mutant-infected cells. These observations gave rise to the hypothesis that the 19-kDa protein (the 19K protein) normally functions as a negative regulator of Ad early gene expression and growth. We have tested this hypothesis and find that Ad5 and Ad12 wild-type viruses grow as efficiently as their respective 19K-defective mutants, in1 and dl337 and pm700 and in700, in WI38 and other human cell types. Neither the accumulation of E1A cytoplasmic mRNAs nor the synthesis of E1A and other viral early proteins in these cells is altered as a result of these mutations in the 19K gene, and we conclude that the 19K protein does not play an essential role in regulating viral early gene expression or viral growth in human cells.

The NFIII/OCT-1 binding site stimulates adenovirus DNA replication in vivo and is functionally redundant with adjacent sequences

The inverted terminal repeat (ITR) of adenovirus type 5 (Ad5) is 103 bp in length and contains the origin of DNA replication. Cellular transcription factors NFI/CTF and NFIII/OCT-1 bind to sites within the ITR and participate in the initiation of viral DNA replication in vitro. The ITR also contains multiple copies of two conserved sequence motifs that bind the cellular transcription factors SP1 and ATF. We have analyzed a series of viruses that carry deletions at the left terminus of Ad5. A virus carrying a deletion of the NFIII/OCT-1, SP1, and ATF sites within the ITR (mutant dl309-44/107) was wild type for virus growth. However, the deletion of these elements in addition to sequences immediately flanking the ITR (mutant dl309-44/195) resulted in a virus that grew poorly. The analysis of growth parameters of these and other mutants demonstrate that the NFIII/OCT-1 and adjacent SP1 sites augment the accumulation of viral DNA following infection. The function of these elements was most evident in coinfections with a wild-type virus, suggesting that these sites enhance the ability of a limiting trans-acting factor(s), that stimulates viral DNA replication, to interact with the ITR. The results of these analyses indicate functional redundancy between different transcription elements at the left terminus of the Ad5 genome and demonstrate that the NFIII/OCT-1 site and adjacent SP1 site, previously thought to be nonessential for adenovirus growth, play a role in viral DNA replication in vivo.

The E1B 19-kilodalton protein is not essential for transformation of rodent cells in vitro by adenovirus type 5

The newly constructed adenovirus type 5 mutant in1 carries a single AT base pair insertion immediately after nucleotide position 1715 in the E1B gene sequence which destroys the proximal AUG normally present in E1B messages and prevents production of intact E1B 19-kDa protein in infected cells. We have used in1, variants of in1 containing mutant alleles of viral genes known to enhance transformation frequency, and adenovirus type 5 mutant dl337 (S. Pilder, J. Logan, and T. Shenk, J. Virol. 52:664-671, 1984), in which the sequence between nucleotides 1770 and 1916 within the 19-kDa reading frame is deleted, to test the generally accepted hypothesis that this E1B protein is essential for the transformation of rodent cells and maintenance of the transformed phenotype. We find that these mutants transform rat embryo cells, rat kidney and mouse kidney primary cells, and cells of the 3Y1 rat line with decreased frequencies only when virus is added to these various cells at high input multiplicities of infection. In contrast, when lower doses of virus are used, the mutants transform with wild-type frequencies. Cells infected with higher doses of mutant virus show increased levels of DNA degradation and cell killing compared with those of cells infected with the same levels of wild-type virus, and these effects most likely contribute to the decreased transformation frequencies observed. On the basis of these results and the results of phenotypic analyses of numerous transformants, we propose that the E1B 19-kDa protein is not required for induction and/or maintenance of transformed-cell characteristics in rodent cells infected with adenovirus type 5.

Identification of specific adenovirus E1A N-terminal residues critical to the binding of cellular proteins and to the control of cell growth

Adenovirus early region 1A (E1A) oncogene-encoded sequences essential for transformation- and cell growth-regulating activities are localized at the N terminus and in regions of highly conserved amino acid sequence designated conserved regions 1 and 2. These regions interact to form the binding sites for two classes of cellular proteins: those, such as the retinoblastoma gene product, whose association with the E1A products is specifically dependent on region 2, and another class which so far is known to include only a large cellular DNA-binding protein, p300, whose association with the E1A products is specifically dependent on the N-terminal region. Association between the E1A products and either class of cellular proteins can be disrupted by mutations in conserved region 1. While region 2 has been studied intensively, very little is known so far concerning the nature of the essential residues in the N-terminal region, or about the manner in which conserved region 1 participates in the binding of two distinct sets of cellular proteins. A combination of site-directed point mutagenesis and monoclonal antibody competition experiments reported here suggests that p300 binding is dependent on specific, conserved residues in the N terminus, including positively charged residues at positions 2 and 3 of the E1A proteins, and that p300 and pRB bind to distinct, nonoverlapping subregions within conserved region 1. The availability of precise point mutations disrupting p300 binding supports previous data linking p300 with cell cycle control and enhancer function.

cis and trans requirements for the selective packaging of adenovirus type 5 DNA

Polar packaging of adenovirus DNA into virions is dependent on the presence of cis-acting sequences at the left end of the viral genome. Our previous analyses demonstrated that the adenovirus type 5 (Ad5) packaging domain (nucleotides 194 to 358) is composed of at least five elements that are functionally redundant. A repeated sequence, termed the A repeat, was associated with packaging function. Here we report a more detailed analysis of the requirements for the selective packaging of Ad5 DNA. By introducing site-directed point mutations into specific A repeat sequences, we demonstrate that the A repeats represent cis-acting functional components of the packaging signal. Additional elements, located outside the originally defined packaging domain boundaries and that resemble the A repeat consensus sequence, also are capable of promoting the packaging of viral DNA. The cis-acting components of the packaging signal appear to be subject to certain spatial constraints for function, possibly reflecting a necessity for the coordinate binding of packaging proteins to these sites. In agreement with this idea, we present evidence that the interaction of a limiting trans-acting factor(s) with the packaging domain in vivo is required for efficient encapsidation of the Ad5 genome.

Simian virus 40 small-t does not transactivate RNA polymerase II promoters in virus infections

Transcriptional stimulatory properties of virus-encoded transactivators appear to be critical for viral gene expression and may be linked to cellular transformation in certain cases. Recently, the simian virus 40 (SV40) 17-kDa small-t antigen was shown to stimulate transcription of polymerase II and III genes in transient transfection assays. In experiments performed in our laboratory, two of the polymerase II promoters of the adenovirus genome, namely, the EII-early and EIII promoters, were transactivation, we examined the transient transfection assays. To further elucidate the mechanism of this transactivation, we examined the ability of small-t to transactivate the adenovirus type 5 EII-early and EIII promoters in CV-1 cells under conditions in which the small-t gene or the reporter genes were introduced into the cells through transfection and other routes. In one approach, we used established CV-1 cell lines which constitutively express the small-t gene, and study of the EII-early promoter was afforded by infection of an EIA-negative adenovirus type 5 variant. For the second approach, a recombinant adenovirus was constructed in which small-t was expressed from a replication origin-negative SV40 early promoter in the EIA region of an adenovirus vector (Ad-SV-t). The effect of small-t on adenovirus EII-early and EIII promoter expression was studied in coinfection or single-infection experiments. In both cases, transcription of the adenovirus early promoters was not stimulated by small-t. These and other results indicate that transactivation of polymerase II promoters by small-t occurs only when the target gene is in a transiently transfected state. Thus, small-t-mediated transactivation of polymerase II promoters is dependent on the type of assay system used and may be mechanistically different from that of the widely studied EIA.

Redundant elements in the adenovirus type 5 inverted terminal repeat promote bidirectional transcription in vitro and are important for virus growth in vivo

The adenovirus inverted terminal repeat (ITR) contains a number of cis-acting elements that are involved in the initiation of viral DNA replication, as well as multiple binding motifs for the cellular transcription factors SP1 and ATF. In this study, we utilized a Hela cell transcription extract to demonstrate that the adenovirus type 5 ITR promotes bidirectional transcription in vitro. Primer extension analyses demonstrated that the ITR directed transcription at initiation sites both within the terminal repeat and at fixed distances outside of the ITR. The ITR also strongly stimulated transcription at the early region 1A (E1A) initiation site when it was situated immediately upstream of the E1A TATA box region. Deletion and point mutational analyses demonstrated that two distinct cis-acting elements were involved in these ITR-dependent transcriptional activities in vitro. Cellular transcription factors SP1 and ATF were previously shown to bind to these two regions. Analysis of viral mutants in vivo demonstrated that the NFIII/OCT-1 binding site and a conserved ATF motif were important for efficient viral growth. Regulatory elements in the ITR flanking region were found to functionally substitute for these sites.

Cooperative binding of EF-1A to the E1A enhancer region mediates synergistic effects on E1A transcription during adenovirus infection

A cellular nuclear factor, EF-1A, binds to a sequence motif which is repeated in the adenovirus type 5 E1A transcriptional control region. Previous genetic analyses demonstrated that two of these binding sites are predominant functional elements of the E1A enhancer region in vivo. In this report, we demonstrate that the cooperative binding of EF-1A to neighboring sites in the E1A enhancer region results in a synergistic activation of E1A transcription in infected cells.

Linker mutation scanning of the genes encoding the adenovirus type 5 terminal protein precursor and DNA polymerase

The replication of adenovirus DNA requires, in addition to several host factors, three virus-encoded proteins: a DNA binding protein, the precursor of the terminal protein (pTP), and a DNA polymerase (Ad pol). Ad pol and pTP form a tight complex that is necessary for the initiation step in DNA replication. To perform mutation scanning of the adenovirus type 5 pTP and Ad pol a series of in-frame linker insertions of a 12-mer oligonucleotide d(CCCATCGATGGG) were introduced into cloned viral DNA fragments containing coding sequences of these proteins. The insertions are located at recognition sites for several blunt end-cutting restriction endonucleases. Forty different sites were mutagenized and the mutated genes were transferred to a plasmid that contains the left 42% of the adenovirus genome. They were rebuilt into the viral genome by means of in vivo recombination between plasmid DNA and digested adenovirus DNA-TP complex. The resulting viral genomes were tested for viability and rescued virus was analyzed for the presence of the inserted linker oligonucleotide. This procedure resulted in recovery of a number of viable virus mutants with insertions in the pTP or Ad pol genes, all of which are phenotypically silent. The other mutations did not allow virus production. The positions of these apparent lethal codon insertion mutations were useful to identify regions of functional importance in both proteins. It can be concluded that the precursor-specific region of pTP plays an important role in virus multiplication.

Enteric adenovirus type 40: expression of E1B mRNA and proteins in permissive and nonpermissive cells

The enteric adenovirus type 40 (strain Dugan) grows well in tissue culture only when the E1B 55K protein of Ad5 or Ad12 is supplied in trans, either constitutively expressed in an established cell line or by coinfection with an appropriate helper virus (V. Mautner, N. Mackay, and V. Steinthorsdottir, 1989, Virology 171, 619-622). The synthesis of Ad40 E1B mRNAs and proteins has been examined under permissive and nonpermissive conditions: At late times postinfection in permissive cells, E1B-specific mRNA species of 22 and 13-14 S are made, as well as 15 and 9 S messages for the late IVa2 and ppIX proteins. None of these are detected before the onset of DNA replication and none of them accumulate in the presence of a cytosine arabinoside block to DNA replication. The failure to detect cytoplasmic mRNAs as early times cannot be attributed to a failure of mRNA transport from the nucleus as there is no accumulation of nuclear E1 RNA. In nonpermissive Hela cells only traces of E1B- and ppIX-specific mRNAs are detectable, at very late times postinfection. Antibodies raised to synthetic oligopeptides corresponding to the N- and C-terminal domains of the putative E1B 19K and 55K proteins show a high titer against the cognate peptide by ELISA, but only the E1B 19K C-terminus-specific sera have detected a unique polypeptide in Ad40-infected cells, at late times postinfection. There is no shut-off of host protein synthesis in permissive cells, despite the expression of Ad2 55K protein.