Use of Cre/loxP recombination to swap cell binding motifs on the adenoviral capsid protein IX

The genome position of a therapeutic transgene strongly influences the level of expression in an armed oncolytic human adenovirus vector

Efficacy of oncolytic, conditionally-replicating adenovirus (CRAd) vectors can be enhanced by "arming" the vector with therapeutic transgenes. We examined whether inclusion of an intact early region 3 (E3) and the reptilian reovirus fusogenic p14 fusion-associated small transmembrane (FAST) protein enhanced vector efficacy. The p14 FAST transgene was cloned between the fiber gene and E4 region, with an upstream splice acceptor for replication-dependent expression from the major late promoter. In A549 cells, this vector expressed p14 FAST protein at very low levels, and showed a poor ability to mediate cell-cell fusion, relative to a similar vector encoding p14 FAST within the E3 deletion. Although expression of E3 proteins from the CRAd increased plaque size, poor expression of p14 FAST protein compromised the fusogenic capacity of the vector. Thus, location of a therapeutic transgene within a CRAd can significantly impact expression of the transgene and is an important consideration in vector design.

Fusion of Large Polypeptides to Human Adenovirus Type 5 Capsid Protein IX Can Compromise Virion Stability and DNA Packaging Capacity

The human adenovirus (HAdV) protein IX (pIX) is a minor component of the capsid that acts in part to stabilize the hexon-hexon interactions within the mature capsid. Virions lacking pIX have a reduced DNA packaging capacity and exhibit thermal instability. More recently, pIX has been developed as a platform for presentation of large polypeptides, such as fluorescent proteins or large targeting ligands, on the viral capsid. It is not known whether such modifications affect the natural ability of pIX to stabilize the HAdV virion. In this study, we show that addition of large polypeptides to pIX does not alter the natural stability of virions containing sub-wild-type-sized genomes. However, similar virions containing wild-type-sized genomes tend to genetically rearrange, likely due to selective pressure caused by virion instability as a result of compromised pIX function.IMPORTANCE Human adenovirus capsid protein IX (pIX) is involved in stabilizing the virion but has also been developed as a platform for presentation of various polypeptides on the surface of the virion. Whether such modifications affect the ability of pIX to stabilize the virion is unknown. We show that addition of large polypeptides to pIX can reduce both the DNA packaging capacity and the heat stability of the virion, which provides important guidance for the design of pIX-modified vectors.

An Oncolytic Adenovirus Vector Expressing p14 FAST Protein Induces Widespread Syncytium Formation and Reduces Tumor Growth Rate In Vivo

Intratumoral injection of oncolytic viruses provides a direct means of tumor cell destruction for inoperable tumors. Unfortunately, oncolytic vectors based on human adenovirus (HAdV) typically do not spread efficiently throughout the tumor mass, reducing the efficacy of treatment. In this study, we explore the efficacy of a conditionally replicating HAdV vector expressing the p14 Fusion-Associated Small Transmembrane (FAST) protein (CRAdFAST) in both immunocompetent and immunodeficient mouse models of cancer. The p14 FAST protein mediates cell-cell fusion, which may enhance spread of the virus-mediated, tumor cell-killing effect. In the murine 4T1 model of cancer, treatment with CRAdFAST resulted in enhanced cell death compared to vector lacking the p14 FAST gene, but it did not reduce the tumor growth rate in vivo. In the human A549 lung adenocarcinoma model of cancer, CRAdFAST showed significantly improved oncolytic efficacy in vitro and in vivo. In an A549 xenograft tumor model in vivo, CRAdFAST induced tumor cell fusion, which led to the formation of large acellular regions within the tumor and significantly reduced the tumor growth rate compared to control vector. Our results indicate that expression of p14 FAST from an oncolytic HAdV can improve vector efficacy for the treatment of cancer.

Tropism and transduction of oncolytic adenovirus 5 vectors in cancer therapy: Focus on fiber chimerism and mosaicism, hexon and pIX

The cellular internalization (infection of cells) of adenovirus 5 (Ad5) is mediated by the initial attachment of the globular knob domain of the capsid fiber protein to the cell surface coxsackievirus and adenovirus receptor (CAR), then followed by the interaction of the virus penton base proteins with cellular integrins. In tumors, there is a substantial intra- and intertumoral variability in CAR expression. The CAR-negative cells generally exhibit very low infectability. Since the fiber knob is a primary mediator of Ad5 binding to the cell surface, improved infectivity of Ad5-based vectors as oncolytic agents may be achieved via genetic modifications of this domain. The strategies to modify or broaden tropism and increase transduction efficiency of Ad5-based vectors include: 1) an incorporation of a targeting peptide into the fiber knob domain (the HI loop and/or C-terminus); 2) fiber knob serotype switching, or pseudotyping, by constructing chimeric fibers consisting of the knob domain derived from an alternate serotype (e.g., Ad5/3 or Ad5/35 chimeras), which binds to receptor(s) other than CAR (e.g., desmoglein 2/DSG2 and/or CD46); 3) "fiber complex mosaicism", an approach of combining serotype chimerism with peptide ligand(s) incorporation (e.g., Ad5/3-RGD); 4) "dual fiber mosaicism" by expressing two separate fibers with distinct receptor-binding capabilities on the same viral particle (e.g., Ad5-5/3 or Ad5-5/σ1); 5) fiber xenotyping by replacing the knob and shaft domains of wild-type Ad5 fiber protein with fibritin trimerization domain of T4 bacteriophage or σ1 attachment protein of reovirus. Other genetic approaches to increase the CAR-independent transduction efficiency include insertion of a targeting peptide into the hypervariable region of the capsid protein hexon or fusion to the C-terminus of pIX. Finally, we consider a yet unsolved molecular mechanism of liver targeting by Ad5-based vectors (CAR-, integrin-, fiber shaft KKTK motif-, and hepatic heparan sulfate glycosaminoglycans-independent, but fiber-, hexon- and blood factor X-dependent).

Human adenovirus type 5 vectors deleted of early region 1 (E1) undergo limited expression of early replicative E2 proteins and DNA replication in non-permissive cells

Adenovirus (Ad) vectors deleted of the early region 1 (E1) are widely used for transgene delivery in preclinical and clinical gene therapy studies. Although proteins encoded within the E1 region are required for efficient virus replication, previous studies have suggested that certain viral or cellular proteins can functionally compensate for E1, leading to expression of the early region 2 (E2)-encoded replicative proteins and subsequent virus replication. We have generated a series of E1-encoding and E1-deficient Ad vectors containing a FLAG-epitope tag on each of the E2-encoded proteins: DNA-binding protein (DBP), terminal protein (TP) and DNA polymerase (Pol). Using these constructs, we show that for the replication-competent virus, the expression level of each E2-encoded protein declines with increasing distance from the E2 promoter, with E2A-encoded DBP expression being ~800-fold higher than E2B-encoded TP. Pol was expressed at extremely low levels in infected cells, and immunoprecipitation from cell lysates was required prior to its detection by immunoblot. We further show that DBP was expressed 200- to 400-fold less efficiently from an E1-deficient virus compared to a replication-competent virus in A549 and HepG2 cells, which was accompanied by a very small increase in genome copy number. For the E1-deficient virus, late gene expression (a marker of virus replication) was only observed at very high multiplicities of infection. These data show that E1-deleted Ad gives rise to limited expression of the E2-encoded genes and replication in infected cells, but highlight the importance of considering viral dose-dependent effects in gene therapy studies.

Adenoviral Vectors Armed with Cell Fusion-Inducing Proteins as Anti-Cancer Agents

Cancer is a devastating disease that affects millions of patients every year, and causes an enormous economic burden on the health care system and emotional burden on affected families. The first line of defense against solid tumors is usually extraction of the tumor, when possible, by surgical methods. In cases where solid tumors can not be safely removed, chemotherapy is often the first line of treatment. As metastatic cancers often become vigorously resistant to treatments, the development of novel, more potent and selective anti-cancer strategies is of great importance. Adenovirus (Ad) is the most commonly used virus in cancer clinical trials, however, regardless of the nature of the Ad-based therapeutic, complete responses to treatment remain rare. A number of pre-clinical studies have shown that, for all vector systems, viral spread throughout the tumor mass can be a major limiting factor for complete tumor elimination. By expressing exogenous cell-fusion proteins, many groups have shown improved spread of Ad-based vectors. This review summarizes the research done to examine the potency of Ad vectors expressing fusogenic proteins as anti-cancer therapeutics.

Expression of the fusogenic p14 FAST protein from a replication-defective adenovirus vector does not provide a therapeutic benefit in an immunocompetent mouse model of cancer

When injected directly into a tumor mass, adenovirus (Ad) vectors only transduce cells immediately along the injection tract. Expression of fusogenic proteins from the Ad vector can lead to syncytium formation, which efficiently spreads the therapeutic effect. Fusogenic proteins can also cause cancer cell death directly, and enhance the release of exosome-like particles containing tumor-associated antigens, which boosts the anti-tumor immune response. In this study, we have examined whether delivery of an early region 1 (E1)-deleted, replication-defective Ad vector encoding the reptilian reovirus p14 fusion-associated small transmembrane (FAST) protein can provide therapeutic efficacy in an immunocompetent mouse tumor model. A high multiplicity of infection of AdFAST is required to induce cell fusion in mouse mammary carcinoma 4T1 cells in vitro, and FAST protein expression caused a modest reduction in cell membrane integrity and metabolic activity compared with cells infected with a control vector. Cells expressing FAST protein released significantly higher quantities of exosomes. In immunocompetent Balb/C mice harboring subcutaneous 4T1 tumors, AdFAST did not induce detectable cancer cell fusion, promote tumor regression or prolong mouse survival compared with untreated mice. This study suggests that in the context of the 4T1 model, Ad-mediated FAST protein expression did not elicit a therapeutic effect.

A reduction in the human adenovirus virion size through use of a shortened fibre protein does not enhance muscle transduction following systemic or localised delivery in mice

We have investigated whether reducing the overall size of adenovirus (Ad), through use of a vector containing a shortened fibre, leads to enhanced distribution and dissemination of the vector. Intravenous or intraperitoneal injection of Ad5SlacZ (12 nm fibre versus the normal Ad5 37 nm fibre) or Ad5SpKlacZ (shortened fibre with polylysine motif in the H-I loop of fibre knob domain) led to similar levels of lacZ expression compared to Ad5LlacZ (native Ad5 fibre) in the liver of treated animals, but did not enhance extravasation into the tibialis anterior muscle. Direct injection of the short-fibre vectors into the tibialis anterior muscle did not result in enhanced spread of the vector through muscle tissue, and led to only sporadic transgene expression in the spinal cord, suggesting that modifying the fibre length or redirecting viral infection to a more common cell surface receptor does not enhance motor neuron uptake or retrograde transport.

Supraphysiological expression of survival motor neuron protein from an adenovirus vector does not adversely affect cell function

Spinal muscular atrophy (SMA) is the most common inherited neurodegenerative disease that leads to infant mortality. It is caused by mutations in the survival motor neuron (SMN) protein resulting in death of alpha motor neurons. Increasing evidence suggests that several other tissues are also affected in SMA, including skeletal and cardiac muscle, liver, and pancreas, indicating that systemic delivery of therapeutics may be necessary for true disease correction. Due to the natural biodistribution of therapeutics, a level of SMN several-fold above physiological levels can be achieved in some tissues. In this study, we address whether supraphysiological levels of SMN adversely affects cell function. Infection of a variety of cell types with an adenovirus (Ad) vector encoding SMN leads to very high expression, but the resulting protein correctly localizes within the cell, and associates with normal cellular partners. Although SMN affects transcription of certain target genes and can alter the splicing pattern of others, we did not observe any difference in select target gene splicing or expression in cells overexpressing SMN. However, normal human fibroblasts treated with Ad-SMN showed a slight reduction in growth rate, suggesting that certain cell types may be differently impacted by high levels of SMN.