Adeno-associated virus helper activity of adenovirus DNA binding protein

A parvovirus isolated from royal python (Python regius) is a member of the genus Dependovirus

Parvoviruses were isolated from Python regius and Boa constrictor snakes and propagated in viper heart (VH-2) and iguana heart (IgH-2) cells. The full-length genome of a snake parvovirus was cloned and both strands were sequenced. The organization of the 4432-nt-long genome was found to be typical of parvoviruses. This genome was flanked by inverted terminal repeats (ITRs) of 154 nt, containing 122 nt terminal hairpins and contained two large open reading frames, encoding the non-structural and structural proteins. Genes of this new parvovirus were most similar to those from waterfowl parvoviruses and from adeno-associated viruses (AAVs), albeit to a relatively low degree and with some organizational differences. The structure of its ITRs also closely resembled those of AAVs. Based on these data, we propose to classify this virus, the first serpentine parvovirus to be identified, as serpentine adeno-associated virus (SAAV) in the genus Dependovirus.

Role of the adenovirus DNA-binding protein in in vitro adeno-associated virus DNA replication

A basic question in adeno-associated virus (AAV) biology has been whether adenovirus (Ad) infection provided any function which directly promoted replication of AAV DNA. Previously in vitro assays for AAV DNA replication, using linear duplex AAV DNA as the template, uninfected or Ad-infected HeLa cell extracts, and exogenous AAV Rep protein, demonstrated that Ad infection provides a direct helper effect for AAV DNA replication. It was shown that the nature of this helper effect was to increase the processivity of AAV DNA replication. Left unanswered was the question of whether this effect was the result of cellular factors whose activity was enhanced by Ad infection or was the result of direct participation of Ad proteins in AAV DNA replication. In this report, we show that in the in vitro assay, enhancement of processivity occurs with the addition of either the Ad DNA-binding protein (Ad-DBP) or the human single-stranded DNA-binding protein (replication protein A [RPA]). Clearly Ad-DBP is present after Ad infection but not before, whereas the cellular level of RPA is not apparently affected by Ad infection. However, we have not measured possible modifications of RPA which might occur after Ad infection and affect AAV DNA replication. When the substrate for replication was an AAV genome inserted into a plasmid vector, RPA was not an effective substitute for Ad-DBP. Extracts supplemented with Ad-DBP preferentially replicated AAV sequences rather than adjacent vector sequences; in contrast, extracts supplemented with RPA preferentially replicated vector sequences.

Defective viral vectors as agents for gene transfer in the nervous system

Viral vectors have attracted great interest as vehicles for gene therapy. Due to concerns regarding continued viral gene expression in several systems, new approaches have been sought for gene transfer in the nervous system. This article reviews the general concepts and basic biology of defective viral vectors. These are vectors which can package into a viral coat but contain no viral genes, thereby allowing efficient gene transfer in the absence of viral gene expression in target cells. The defective herpes simplex virus (HSV) vector has been applied to numerous interesting questions in neurobiology. The inability to completely eliminate helper viruses has raised concern regarding the application of this vector to human disease. The adeno-associated virus (AAV) vector has recently been introduced into the nervous system. This vector harbors no viral genes, however helper viruses can also be completely eliminated from the system. Although the smaller size may limit the range of applications for this vector, it has received great interest as a potential agent for gene therapy in the nervous system. Potential future directions are discussed as well.

Transduction with recombinant adeno-associated virus for gene therapy is limited by leading-strand synthesis

Adeno-associated virus is an integrating DNA parvovirus with the potential to be an important vehicle for somatic gene therapy. A potential barrier, however, is the low transduction efficiencies of recombinant adeno-associated virus (rAAV) vectors. We show in this report that adenovirus dramatically enhances rAAV transduction in vitro in a way that is dependent on expression of early region 1 and 4 (E1 and E4, respectively) genes and directly proportional to the appearance of double-stranded replicative forms of the rAAV genome. Expression of the open reading frame 6 protein from E4 in the absence of E1 accomplished a similar but attenuated effect. The helper activity of adenovirus E1 and E4 for rAAV gene transfer was similarly demonstrated in vivo by using murine models of liver- and lung-directed gene therapy. Our data indicate that conversion of a single-stranded rAAV genome to a duplex intermediate limits transduction and usefulness for gene therapy.

Adenovirus containing a deletion of the early region 2A gene allows growth of adeno-associated virus with decreased efficiency

Efficient growth of adeno-associated virus (AAV) requires helper functions provided by a coinfecting adenovirus or herpesvirus. Earlier studies using adenoviruses having temperature-sensitive lesions in the early region 2A gene (E2A) produced contradictory evidence regarding the role of the E2A 72-kDa DNA-binding protein (DBP) in allowing efficient AAV growth. These disparate results may reflect varying levels of residual function in the temperature-sensitive DBP. We examined this issue using an adenovirus type 5 mutant (Add/802) that fails to produce any detectable DBP or any fragment of it. Our experiments show that AAV can carry out a full growth cycle in the complete absence of DBP. However, AAV DNA replication and rep and capsid protein synthesis were reduced several fold and the yield of infectious AAV was reduced by an order of magnitude. This appears to reflect mainly decreased post-transcriptional expression of AAV rep and capsid protein genes.

Colocalization of adeno-associated virus Rep and capsid proteins in the nuclei of infected cells

The mechanism of adeno-associated virus (AAV) DNA replication was characterized both genetically and biochemically. In this study, we used monoclonal and polyclonal antibodies to examine the AAV p5 (Rep78 and Rep68) and p19 (Rep52 and Rep40) proteins in infected cells. By overexpressing a truncated Rep78 protein in Escherichia coli, we obtained monoclonal antibody anti-78/68, which is specific for the p5 Rep proteins, and monoclonal antibody anti-52/40, which recognized both the p5 and p19 Rep proteins. In single-fluorochrome indirect immunofluorescence labeling experiments, the viral Rep proteins were localized in distinct intranuclear foci. Analysis of AAV proteins by double-fluorochrome indirect immunofluorescence experiments demonstrated that (i) all four AAV Rep proteins occupied the same intranuclear compartments and (ii) the Rep and capsid proteins colocalized in the nuclei of infected cells. These results suggest that replication centers similar to those established by other viruses exist for AAV. These reagents should provide a useful tool for further delineation of the mechanism of AAV replication in vitro.

A subset of herpes simplex virus replication genes provides helper functions for productive adeno-associated virus replication

Herpesviruses are helper viruses for productive adeno-associated virus (AAV) replication. To analyze the herpes simplex virus type 1 (HSV-1) functions mediating helper activity, we coinfected HeLa cells with AAV type 2 (AAV-2) and different HSV-1 mutants defective in individual HSV replication genes. AAV replication was fully accomplished in the absence of HSV DNA replication and thus did not require expression of late HSV genes. In addition, HSV mutants lacking either the origin-binding protein or the functional DNA polymerase fully maintained the capacity to replicate AAV. Cotransfection of the cloned, replication-competent AAV-2 genome together with the seven HSV replication genes (UL5, UL8, UL9, UL29, UL30, UL42, and UL52) led to productive AAV replication. Cotransfections with different combinations of these genes demonstrated that a subset of four of them, coding for the HSV helicase-primase complex (UL5, UL8, UL52) and the major DNA-binding protein (UL29), was already sufficient to mediate the helper effect. Thus, the HSV helper activity for productive AAV replication seems to consist of DNA replication functions. This appears to be different from the helper effect provided by adenovirus, which predominantly modulates AAV gene regulation.

Adeno-associated virus DNA replication is induced by genes that are essential for HSV-1 DNA synthesis

Adeno-associated virus (AAV) DNA replication is not detectable unless cells are coinfected with a helper adenovirus (Ad) or herpesvirus or unless AAV infection is carried out in certain established cell lines that have been treated with various metabolic inhibitors or uv irradiation. In helper-dependent infections, it has been shown that AAV DNA synthesis depends on one or more early Ad genes, whereas little is known concerning any herpesvirus gene that promotes AAV DNA synthesis. In this study we tested the ability of four cloned Xbal fragments of herpes simplex virus type 1 (HSV-1) DNA to induce AAV DNA synthesis in Vero cells. Cotransfections, which were carried out with pAV1 (an infectious AAV2 plasmid), revealed that AAV DNA synthesis could be optimally induced by three of these clones (C,D, and F) plus a clone of the HSV-1 ICP4 (IE 175) gene. ICP4, an immediate early gene, was presumably required to activate expression of other HSV genes. To help identify the additionally needed HSV genes, we tested Xbal C,D, and F subclones that contain genes previously found necessary for origin-dependent HSV DNA synthesis and found that at least five of these genes (UL 5, 8, 9, 29, and 30) contributed to the induction of AAV DNA synthesis. In contrast to their absolute requirement for HSV DNA synthesis, none of these genes were strictly necessary for AAV DNA replication. Because they are all known to specify proteins that are directly involved in HSV DNA synthesis, our results suggest that some or all of their products also may directly participate in the replication of AAV DNA.

Parvovirus replication

The members of the family Parvoviridae are among the smallest of the DNA viruses, with a linear single-stranded genome of about 5 kilobases. Currently the family is divided into three genera, two of which contain viruses of vertebrates and a third containing insect viruses. This review concentrates on the vertebrate viruses, with emphasis on recent advances in our insights into the molecular biology of viral replication. Traditionally the vertebrate viruses have been distinguished by the presence or absence of a requirement for a coinfection with a helper virus before productive infection can occur, hence the notion that the dependoviruses (adeno-associated viruses [AAV]) are defective. Recent data would suggest that not only is there a great deal of structural and genetic organizational similarity between the two types of vertebrate viruses, but also there is significant similarity in the molecular biology of productive replication. What differs is the physiological condition of the host cell that renders it permissive. Healthy dividing cells are permissive for productive replication by autonomous parvoviruses; such cells result in latent infection by dependoviruses. For a cell to become permissive for productive AAV replication, it must have been exposed to toxic conditions which activate a latent AAV genome. Such conditions can be caused by helper-virus infection or exposure to physical (UV light) or chemical (some carcinogens) agents. In this paper the molecular biology of replication is reviewed, with special emphasis on the role of the host and the consequences of viral infection for the host.

Propagation of human parvovirus B19 in primary culture of erythroid lineage cells derived from fetal liver

Erythroid lineage cells derived from fetal liver were demonstrated to be target cells for human parvovirus B19 infection. B19 virus antigen-positive serum was inoculated into primary cultures containing erythroid lineage cells enriched from fetal liver. The B19 virus antigen was detected on about 5% of cells in the culture by immunofluorescence staining, and the stained cells were identified as erythroid lineage cells by double staining with anti-B19 virus-positive serum and anti-erythroid lineage monoclonal antibody. The immunofluorescence staining study also revealed that the B19 virus antigen localized in the nucleus and the periphery of cytoplasm. We also detected B19 virus DNA, which was generated by replication in the infected cells, not only in the cells but also in the culture supernatants, in which the amount of B19 DNA increased depending on the period of culture, indicating that the cells infected with B19 virus produced B19 virus and released it into the medium. The ability of B19 virus released into the medium to infect fetal erythroid lineage cells was demonstrated quantitatively. Because of the absence of any cytopathic effect of B19 virus during culture periods of at least 15 days, this culture system should be useful in the study of B19 virus replication and in vitro generation of B19 virus. In addition, the present study may contribute to a better understanding of the pathogenesis of hydrops fetalis, which is probably associated with B19 virus infection during pregnancy.

Efficient synthesis of adeno-associated virus structural proteins requires both adenovirus DNA binding protein and VA I RNA

We have shown previously that replication of defective parvoviruses [adeno-associated viruses (AAV)] requires several early adenovirus (Ad) gene products [J. E. Janik, M. M. Huston, and J. A. Rose (1981) Proc. Natl. Acad. Sci. USA 78, 1925-1929]. To examine their possible roles in the transcription and translation of AAV mRNA, 293-31 cells, a human embryonic kidney cell line that constitutively expresses the Ad early region IA and IB gene products, were transfected with a pBR325 plasmid (pLH1) that contains a duplex AAV2 DNA segment (0.03-0.97 map units) which encompasses the promoters and coding sequences necessary for expression of all AAV polypeptides. When cells were transfected with pLH1 alone, both spliced and unspliced AAV-specific cytoplasmic RNAs accumulated. These transcripts were capable of directing synthesis of the three AAV capsid polypeptides in vitro, whereas in vivo synthesis of AAV protein was not detected by immunofluorescence or immunoprecipitation. When cells were cotransfected with pLH1 and intact Ad DNA, the level of cytoplasmic AAV RNA was enhanced and AAV protein was synthesized in vivo. Additional experiments demonstrated that in vivo AAV protein synthesis also could be induced when pLH1 was cotransfected with plasmids that contain the Ad DNA-binding protein (pDBP) and VA I RNA (p2BalM) genes; however, a low level of in vivo AAV capsid protein was occasionally detected in cotransfections with pLH1 and a plasmid that contains both VA I and VA II RNA coding sequences (p2SalC). Cotransfection of pLH1 and pDBP or pLH1 and p2SalC showed complex alterations in the steady-state patterns of AAV cytoplasmic transcripts. In both cases, increased levels of transcripts, particularly the 2.3-kb spliced species, were detected in comparison to levels seen in cells transfected with pLH1 alone. Despite these increases, however, there was little, if any, induction of AAV protein synthesis unless both the DNA-binding protein (DBP) and VA I RNA coding sequences were present in cotransfection with pLH1. We conclude that, in 293-31 cells, the Ad VA I RNA and DBP gene products regulate AAV capsid protein synthesis at least at two levels: (i) by increasing the steady-state levels of structural protein transcripts in the cytoplasm, especially the spliced species, and (ii) by enhancing the translation of these messages.

The adeno-associated virus rep gene inhibits replication of an adeno-associated virus/simian virus 40 hybrid genome in cos-7 cells

A hybrid adeno-associated virus (AAV)/simian virus 40 (SV40) genome is described. In this construct SV40 regulatory sequences, including the early promoter/enhancers and origin of DNA replication, were substituted for the AAV p5 promoter, which normally controls expression of the AAV rep gene. The hybrid genome was phenotypically indistinguishable from wild-type AAV in human cells in the presence or absence of helper virus. Upon transfection into cos-7 cells, which constitutively produced the SV40 tumor antigen, the genome replicated as a plasmid when the SV40 origin was used, although with a low efficiency compared with that of a non-AAV/SV40 replicon. The low level of replication was due to an inhibitory effect of an AAV rep gene product and was specific for replicons containing AAV sequences. Target AAV sequences required for inhibition by rep appeared to reside in the terminal repetitions since deletion of these sequences allowed efficient replication in the presence of the rep gene. The possible role for negative autoregulation of AAV DNA replication in latent infection and helper-dependent replication by AAV is discussed.

Characterization of adeno-associated virus rep proteins in human cells by antibodies raised against rep expressed in Escherichia coli

The rep gene of the defective human parvovirus, adeno-associated virus, (AAV) mediates several trans-acting functions important to virus replication, transcription, and gene expression. At least four overlapping polypeptides are expressed from the rep gene. We have constructed a prokaryotic vector which expressed in Escherichia coli a region of AAV comprising 93% of the largest AAV rep protein. The protein expressed in E. coli, rep 78.93, was used to raise specific antibodies in rabbits. These antibodies were capable of detecting all four AAV rep proteins in human cells transfected with AAV-containing plasmids as well as new species of 47 and 35 kDa in molecular weight. These new rep proteins originate from the transcription promoter at map unit 19 in the AAV genome and may indicate use of alternate AUG codons or protein modification. The antibodies also recognized novel forms of the rep proteins expressed from mutant AAV genomes. Immunofluorescence analysis of AAV-infected human cells revealed that the rep proteins are localized primarily in the nucleus of the infected cell and have a distribution different from that of AAV capsid protein. These results demonstrate that antisera raised against an AAV rep protein synthesized in E. coli are capable of detecting wild-type AAV rep proteins in virus-infected mammalian cells. These specific antibodies should facilitate further characterization of the functionally pleiotropic viral rep proteins.

Immunological analysis of 140-kDa adenovirus-encoded DNA polymerase in adenovirus type 2-infected HeLa cells using antibodies raised against the protein expressed in Escherichia coli

The E2B region of adenovirus genome contains a long open reading frame (ORF) extending from 24 to 14.2 map units which encodes most of the 140-kDa DNA polymerase. It was cloned at the polylinker region of pUC18 vector with Escherichia coli JM109 as the host. A clone was serendipitously isolated that expressed in E. coli a protein of approximately 120 kDa in size at high levels. DNA sequence analysis of this clone showed the presence of an in-frame fusion of a region, encoding 13 amino acids located upstream, to the first ATG of the ORF. Polyclonal antibodies raised against this protein purified from E. coli were used for immunological analysis. The antibodies were able to detect a 140- and a 66-kDa polypeptide from the adenovirus type 2-infected HeLa cells on Western blots. In addition, the antibodies showed evidence of cross-reactivity with partially purified DNA polymerase alpha from uninfected HeLa cells. The subcellular localization of the viral polymerase in the infected HeLa cells by using indirect immunofluorescence showed that the viral protein is associated with globular structures in the nucleus. The replicating viral DNA and the polymerase were colocalized in these globular sites. Furthermore, HeLa cells infected with Ad5ts149, a temperature-sensitive mutant defective in DNA replication, showed the presence of these globular sites only at the permissive temperature, suggesting that these sites are probably involved in viral DNA replication.

Functional analysis of the adenovirus type 5 DNA-binding protein: site-directed mutants which are defective for adeno-associated virus helper activity

We generated four point mutations in the DNA-binding protein (DBP) gene of adenovirus type 5 by oligonucleotide-directed site-specific mutagenesis. The sites mutated were in the three conserved regions (CR; amino acids 178-186 [CR1], 322-330 [CR2], and 464-475 [CR3]) identified previously by comparative sequence analysis (G. R. Kitchingman, Virology 146:90-101, 1985). The mutations resulted in changes in amino acids 181 (Trp to Leu), 323 (Arg to Leu), 324 (Trp to Leu), and 469 (Phe to Ile). The mutated DBP genes were put under the control of the simian virus 40 early promoter and analyzed by transfection for their ability to help adeno-associated virus replicate its DNA in COS-1 monkey cells. Mutations in the aromatic amino acids 324 and 469 reduced the amount of AAV DNA replication approximately 10-fold, while the mutation in Arg 323 produced a reduction of approximately fourfold. The Trp-to-Leu mutation in amino acid 181 had no effect on AAV DNA replication. The decreased helper activity of the 323, 324, and 469 mutations was not caused by any effect of the mutation on the stability of the DBP. These results suggest that CR2 and CR3 are involved in AAV helper activity, specifically in AAV DNA replication. The relevance of these findings to the identification of residues important for the functions of DBP in adenovirus infection is discussed.

Identification of two nuclear subclasses of the adenovirus type 5-encoded DNA-binding protein

The synthesis, accumulation, and subcellular distribution of the adenovirus serotype 5 DNA-binding protein (DBP) has been examined during the infectious cycle in HeLa cells. With the onset of viral DNA replication and entry into the late phase, two nuclear subclasses of DBP are distinguishable by immunofluorescence microscopy and can be separately isolated by in situ cell fractionation. The first subclass, represented by diffuse-staining DBP, is released by the addition of 1% Nonidet P-40-150 mM NaCl. The second subclass of DBP, which is sequestered into intranuclear globular structures, requires a high ionic strength (2 M NaCl) for extraction and appears to be associated with centers of active viral DNA replication. This association is based on the observations that: DBP within the globules and viral DNA, as detected by in situ hybridization, form identical structures that colocalize within the nuclei of infected cells, the formation of DBP globular structures requires the onset and continuation of viral DNA replication, and once formed, the globular structures can be perturbed by modulating viral DNA synthesis.

Human cytomegalovirus completely helps adeno-associated virus replication

Coinfection of adeno-associated virus (AAV) with human cytomegalovirus (HCMV) strain Towne in human embryonic fibroblasts resulted in accumulation of AAV capsid antigen and production of infectious AAV with a lag of 24 hr compared to AAV replication in AAV-adenovirus coinfections. In contrast to previous observation, these findings demonstrated that HCMV is a competent helper virus for the complete replication of AAV. In addition, HCMV and AAV were synergistic in their cytopathic effects on cells, suggesting the possibility that AAV may play a role in the pathogenicity of HCMV infections.

Direct demonstration of the human parvovirus in erythroid progenitor cells infected in vitro

The human parvovirus (HPV), the cause of transient aplastic crisis of hereditary hemolytic anemia, has been shown to be cytotoxic for erythroid progenitor cells and its presence in these cells demonstrated by morphologic techniques. A relatively pure population of progenitors, isolated by removal of immature erythroid bursts from primary culture, was the target of the virus infection. Infected cells failed to proliferate in secondary culture. Using a monoclonal antibody to HPV, specific fluorescence was demonstrated in a minority of cells 24-48 h after infection with virus. Infected cells examined by electron microscopy showed marked toxic ultrastructural alterations and parvovirus-like particles in crystalline arrays in the nucleus.

The function(s) provided by the adenovirus-specified, DNA-binding protein required for viral late gene expression is independent of the role of the protein in viral DNA replication

The adenovirus type 2 (Ad2) host range mutant Ad2hr400 grows efficiently in cultured monkey cells at 37 degrees C, but is cold sensitive for plaque formation and late gene expression at 32.5 degrees C. After nitrous acid mutagenesis of an Ad2hr400 stock, cold-resistant variants were selected in CV1 monkey cells at 32.5 degrees C. One such variant, Ad2ts400, was also temperature sensitive (ts) for growth in both CV1 and HeLa cells. Marker rescue analysis has been used to show that the two phenotypes, cold resistant and temperature sensitive, are due to two independent mutations, each of which resides in a different segment of the gene encoding the 72-kilodalton DNA binding protein (DBP). The cold-resistant mutation (map coordinates 63.6 to 66) is a host range alteration that enhances the ability of the virus to express late genes and grow productively in monkey cells at 32.5 degrees C. The temperature-sensitive mutation is in the same complementation group and maps to the same segment of the DBP gene (map coordinates 61.3 to 63.6) as the well-characterized DBP mutant Ad5ts125. Like Ad5ts125, Ad2ts400 is unable to replicate viral DNA or to properly shut off early mRNA expression at the nonpermissive temperature. Two sets of experiments with Ad2ts400 suggest that DBP contains separate functional domains. First, when CV1 cells are coinfected at the nonpermissive temperature with Ad2 plus Ad2ts400 (Ad2 allows DNA replication and entry into, but not completion of, the late phase of infection), normal late gene expression and productive growth occur. Second, temperature shift experiments show that, although DNA replication is severely restricted at the nonpermissive temperature in ts400-infected monkey cells, late gene expression occurs normally. These results indicate that the DBP activity required for normal late gene expression in monkey cells is functional even when the DBP's DNA replication activity is disrupted.