Human papilloma virus E6 and E7 proteins support DNA replication of adenoviruses deleted for the E1A and E1B genes

The action of transforming proteins from small DNA tumor viruses seems to be remarkably similar between different viruses, as they all use pRb and p53 pathways as cellular targets. This leads to deregulation of host cell cycling, which in turn creates an environment favorable for viral replication. Based on this, we hypothesized that regulatory proteins from human papillomaviruses (HPVs) can functionally trans-complement viral DNA replication of adenoviruses deleted for the E1A and E1B genes (AdE1-). To test this, we constructed AdE1- vectors expressing the human papilloma virus 16 (HPV-16) proteins E6 and E7. Expression of both E6 and E7 from these vectors partially complemented adenoviral DNA replication activity in vitro, in SK-Hep1 cells and primary human astrocytes, as well as in vivo in mouse liver. AdE1- vectors expressing E6 and E7 also increased hepatocellular DNA synthesis in vivo. Efficient AdE1- DNA replication was detected in HPV-associated cervical carcinoma cells but not in primary human cells. Linking the expression of regulatory oncoviral proteins to DNA replication of E1-mutant adenoviruses may provide a rationale for antitumor strategies.

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

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

DNA replication of first-generation adenovirus vectors in tumor cells

A major role of the early gene 1A and 1B products (E1A and E1B) in adenovirus infection is to create a cellular environment appropriate for viral DNA replication. This is, in part, achieved by inactivation of tumor suppressor gene products such as pRb or p53. The functions of these same cellular proteins are also frequently lost in tumor cells. Therefore, we hypothesized that tumor cell lines with deregulated p53 and/or pRb pathways might support replication of E1A/E1B-deleted, first-generation adenovirus vectors (AdE1(-)). Here, we analyzed the impact of virus uptake, cell cycling, and the status of cell cycle regulators on AdE1(-) DNA synthesis. Cellular internalization of AdE1(-) vectors varied significantly among different tumor cell lines, whereas nuclear import of incoming viral DNA appeared to be less variable. Replication assays performed under equalized infection conditions demonstrated that all analyzed tumor cell lines supported AdE1(-) synthesis to varying degrees. There was no obvious correlation between the efficiency of viral DNA replication and the status of p53, pRb, and p16. However, the amount of virus attached and internalized changed with the cell cycle, affecting the intracellular concentration of viral DNA and thereby the replication efficacy. Furthermore, infection with AdE1 - vectors caused a partial G(2)/M arrest or delay in cell cycle progression, which became more pronounced in consecutive cell cycles. Correspondingly, vector DNA replication was found to be enhanced in cells artificially arrested in G(2)/M. Our findings suggest that cell cycling and thus passing through G(2)/M supports AdE1(-) DNA replication in the absence of E1A/E1B. This has potential implications for the use of first-generation adenovirus vectors in tumor gene therapy.

Insulation from viral transcriptional regulatory elements improves inducible transgene expression from adenovirus vectors in vitro and in vivo

Recombinant adenoviruses (Ad) are attractive vectors for gene transfer in vitro and in vivo. However, the widely used E1-deleted vectors as well as newer generation vectors contain viral sequences, including transcriptional elements for viral gene expression. These viral regulatory elements can interfere with heterologous promoters used to drive transgene expression and may impair tissue-specific or inducible transgene expression. This study demonstrates that the activity of a metal-inducible promoter is affected by Ad sequences both upstream and downstream of the transgene cassette in both orientations. Interference with expression from the heterologous promoter was particularly strong by viral regulatory elements located within Ad sequences nucleotides 1-341. This region is present in all recombinant Ad vectors, including helper-dependent vectors. An insulator element derived from the chicken gamma-globin locus (HS-4) was employed to shield the inducible promoter from viral enhancers as tested after gene transfer with first-generation Ad vectors in vitro and in vivo. Optimal shielding was obtained when the transgene expression cassette was flanked on both sides by HS-4 elements, except for when the HS-4 element was placed in 3'-->5' orientation in front of the promoter. The insulators reduced basal expression to barely detectable levels in the non-induced stage, and allowed for induction factors of approximately 40 and approximately 230 in vitro and in vivo, respectively. Induction ratios from Ad vectors without insulators were approximately 40-fold lower in vitro and approximately 15-fold lower in vivo. This study proves the potential of insulators to improve inducible or tissue-specific gene expression from adenovirus vectors, which is important for studying gene functions as well as for gene therapy approaches. Furthermore, our data show that insulators exert enhancer-blocking effects in episomal DNA.

Integrating adenovirus-adeno-associated virus hybrid vectors devoid of all viral genes

Recently, we demonstrated that inverted repeat sequences inserted into first-generation adenovirus (Ad) vector genomes mediate precise genomic rearrangements resulting in vector genomes devoid of all viral genes that are efficiently packaged into functional Ad capsids. As a specific application of this finding, we generated adenovirus-adeno-associated virus (AAV) hybrid vectors, first-generation Ad vectors containing AAV inverted terminal repeat sequences (ITRs) flanking a reporter gene cassette inserted into the E1 region. We hypothesized that the AAV ITRs present within the hybrid vector genome could mediate the formation of rearranged vector genomes (DeltaAd.AAV) and stimulate transgene integration. We demonstrate here that DeltaAd.AAV vectors are efficiently generated as by-products of first-generation adenovirus-AAV vector amplification. DeltaAd.AAV genomes contain only the transgene flanked by AAV ITRs, Ad packaging signals, and Ad ITRs. DeltaAd.AAV vectors can be produced at a high titer and purity. In vitro transduction properties of these deleted hybrid vectors were evaluated in direct comparison with first-generation Ad and recombinant AAV vectors (rAAVs). The DeltaAd.AAV hybrid vector stably transduced cultured cells with efficiencies comparable to rAAV. Since cells transduced with DeltaAd.AAV did not express cytotoxic viral proteins, hybrid viruses could be applied at very high multiplicities of infection to increase transduction rates. Southern analysis and pulsed-field gel electrophoresis suggested that DeltaAd.AAV integrated randomly as head-to-tail tandems into the host cell genome. The presence of two intact AAV ITRs was crucial for the production of hybrid vectors and for transgene integration. DeltaAd.AAV vectors, which are straightforward in their production, represent a promising tool for stable gene transfer in vitro and in vivo.

Generation of adenovirus vectors devoid of all viral genes by recombination between inverted repeats

Direct or inverse repeated sequences are important functional features of prokaryotic and eukaryotic genomes. Considering the unique mechanism, involving single-stranded genomic intermediates, by which adenovirus (Ad) replicates its genome, we investigated whether repetitive homologous sequences inserted into E1-deleted adenoviral vectors would affect replication of viral DNA. In these studies we found that inverted repeats (IRs) inserted into the E1 region could mediate predictable genomic rearrangements, resulting in vector genomes devoid of all viral genes. These genomes (termed DeltaAd.IR) contained only the transgene cassette flanked on both sides by precisely duplicated IRs, Ad packaging signals, and Ad inverted terminal repeat sequences. Generation of DeltaAd.IR genomes could also be achieved by coinfecting two viruses, each providing one inverse homology element. The formation of DeltaAd.IR genomes required Ad DNA replication and appeared to involve recombination between the homologous inverted sequences. The formation of DeltaAd. IR genomes did not depend on the sequence within or adjacent to the inverted repeat elements. The small DeltaAd.IR vector genomes were efficiently packaged into functional Ad particles. All functions for DeltaAd.IR replication and packaging were provided by the full-length genome amplified in the same cell. DeltaAd.IR vectors were produced at a yield of approximately 10(4) particles per cell, which could be separated from virions with full-length genomes based on their lighter buoyant density. DeltaAd.IR vectors infected cultured cells with the same efficiency as first-generation vectors; however, transgene expression was only transient due to the instability of deleted genomes within transduced cells. The finding that IRs present within Ad vector genomes can mediate precise genetic rearrangements has important implications for the development of new vectors for gene therapy approaches.

Nonphysiological overexpression of low-density lipoprotein receptors causes pathological intracellular lipid accumulation and the formation of cholesterol and cholesteryl ester crystals in vitro

Recent therapeutic strategies for the treatment of familial hypercholesterolemia have been based on liver-directed gene transfer of a functional low-density lipoprotein (LDL) receptor cDNA under control of viral or strong housekeeping promoters. Strong viral promoters including cytomegalovirus, Rous sarcoma virus, and simian virus 40 promoters are commonly employed to reach significant physiological effects. These promoters mediate constitutive and nonphysiological overexpression in every transduced cell, while the endogenous LDL receptor expression is controlled by a complex feedback mechanism based on intracellular cholesterol concentration. To investigate intracellular consequences of persistent LDL receptor overexpression we constructed a recombinant adenovirus encoding the human LDL receptor under control of the Rous sarcoma virus promoter. The metabolic and morphological effects of LDL receptor expression were characterized by uptake experiments with human hepatoma cells using fluorescent and radiolabeled LDL. We observed that large amounts of LDL accumulate within LDL receptor transduced cells, which eventually lead to massive intracellular lipid deposition. Kinetic experiments with LDL-supplemented medium resulted in numerous crystal shaped structures in the cytosol of transduced cells as visualized by digital interference contrast optic within 60 min after LDL supplementation. Thin layer chromatography analyses of cellular lipids suggested these crystalline structures to be dependent on intracellular cholesterol and cholesterol ester levels. Mock-infected cells showed neither cholesterol lipid accumulation nor crystal formation. In conclusion, our data demonstrate that nonphysiological overexpression of the LDL receptor can cause massive lipid accumulation, which cannot be compensated by the hepatoma cell metabolism. This phenomenon may result in negative selection of LDL receptor overexpressing cells in vitro and in vivo.