Development of Novel Adenoviral Vectors to Overcome Challenges Observed With HAdV-5-based Constructs

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).

Development of novel vaccine vectors: Chimpanzee adenoviral vectors

Adenoviral vector has been employed as one of the most efficient means against infectious diseases and cancer. It can be genetically modified and armed with foreign antigens to elicit specific antibody responses and T cell responses in hosts as well as engineered to induce apoptosis in cancer cells. The chimpanzee adenovirus-based vector is one kind of novel vaccine carriers whose unique features and non-reactivity to pre-existing human adenovirus neutralizing antibodies makes it an outstanding candidate for vaccine research and development. Here, we review the different strategies for constructing chimpanzee adenoviral vectors and their applications in recent clinical trials and also discuss the oncolytic virotherapy and immunotherapy based on chimpanzee adenoviral vectors.

One-Vector System for Multiplexed CRISPR/Cas9 against Hepatitis B Virus cccDNA Utilizing High-Capacity Adenoviral Vectors

High-capacity adenoviral vectors (HCAdVs) devoid of all coding genes are powerful tools to deliver large DNA cargos into cells. Here HCAdVs were designed to deliver a multiplexed complete CRISPR/Cas9 nuclease system or a complete pair of transcription activator-like effector nucleases (TALENs) directed against the hepatitis B virus (HBV) genome. HBV, which remains a serious global health burden, forms covalently closed circular DNA (cccDNA) as a persistent DNA species in infected cells. This cccDNA promotes the chronic carrier status, and it represents a major hurdle in the treatment of chronic HBV infection. To date, only one study demonstrated viral delivery of a CRISPR/Cas9 system and a single guide RNA (gRNA) directed against HBV by adeno-associated viral (AAV) vectors. The advancement of this study is the co-delivery of multiple gRNA expression cassettes along with the Cas9 expression cassette in one HCAdV. Treatment of HBV infection models resulted in a significant reduction of HBV antigen production and the introduction of mutations into the HBV genome. In the transduction experiments, the HBV genome, including the HBV cccDNA, was degraded by the CRISPR/Cas9 system. In contrast, the combination of two parts of a TALEN pair in one vector could not be proven to yield an active system. In conclusion, we successfully delivered the CRISPR/Cas9 system containing three gRNAs using HCAdV, and we demonstrated its antiviral effect.

Lung macrophage scavenger receptor SR-A6 (MARCO) is an adenovirus type-specific virus entry receptor

Macrophages are a diverse group of phagocytic cells acting in host protection against stress, injury, and pathogens. Here, we show that the scavenger receptor SR-A6 is an entry receptor for human adenoviruses in murine alveolar macrophage-like MPI cells, and important for production of type I interferon. Scavenger receptors contribute to the clearance of endogenous proteins, lipoproteins and pathogens. Knockout of SR-A6 in MPI cells, anti-SR-A6 antibody or the soluble extracellular SR-A6 domain reduced adenovirus type-C5 (HAdV-C5) binding and transduction. Expression of murine SR-A6, and to a lower extent human SR-A6 boosted virion binding to human cells and transduction. Virion clustering by soluble SR-A6 and proximity localization with SR-A6 on MPI cells suggested direct adenovirus interaction with SR-A6. Deletion of the negatively charged hypervariable region 1 (HVR1) of hexon reduced HAdV-C5 binding and transduction, implying that the viral ligand for SR-A6 is hexon. SR-A6 facilitated macrophage entry of HAdV-B35 and HAdV-D26, two important vectors for transduction of hematopoietic cells and human vaccination. The study highlights the importance of scavenger receptors in innate immunity against human viruses.

Macrophages are a diverse group of phagocytic cells acting in host protection against stress, injury, and pathogens. They phenotypically and functionally adapt to their local environment, for example, peritoneal macrophages are distinct from brain-resident microglia, from liver-resident Kupffer cells or lung macrophages in the lung. Airway macrophages are among the first cells to encounter human respiratory viruses, such as adenoviruses. They release pro-inflammatory cytokines, kill pathogens, present antigens, and restore tissues. Yet, interactions of viruses with lung macrophages are poorly understood, and it is unclear, how they lead to infection or virus clearance. Here we identified the murine scavenger receptor SR-A6 as a receptor for a subset of human adenoviruses on alveolar macrophage-like cells, so-called MPI cells. Scavenger receptors comprise a large family of trans-membrane proteins, and contribute to the clearance of endogenous proteins, lipoproteins and pathogens. In a series of robust experimentation, we show that adenoviruses use SR-A6 as an entry receptor for infection of MPI cells, and production of type I interferon. MPI cells are non-transformed, self-renewing macrophages derived from fetal murine liver, and closely resemble adult alveolar macrophages. The results demonstrate that SR-A6 binds virions on the surface of alveolar macrophage-like cells, and leads to infection.

An Engineered Virus Library as a Resource for the Spectrum-wide Exploration of Virus and Vector Diversity

Adenoviruses (Ads) are large human-pathogenic double-stranded DNA (dsDNA) viruses presenting an enormous natural diversity associated with a broad variety of diseases. However, only a small fraction of adenoviruses has been explored in basic virology and biomedical research, highlighting the need to develop robust and adaptable methodologies and resources. We developed a method for high-throughput direct cloning and engineering of adenoviral genomes from different sources utilizing advanced linear-linear homologous recombination (LLHR) and linear-circular homologous recombination (LCHR). We describe 34 cloned adenoviral genomes originating from clinical samples, which were characterized by next-generation sequencing (NGS). We anticipate that this recombineering strategy and the engineered adenovirus library will provide an approach to study basic and clinical virology. High-throughput screening (HTS) of the reporter-tagged Ad library in a panel of cell lines including osteosarcoma disease-specific cell lines revealed alternative virus types with enhanced transduction and oncolysis efficiencies. This highlights the usefulness of this resource.

Rapid Cloning of Novel Rhesus Adenoviral Vaccine Vectors

Human and chimpanzee adenovirus vectors are being developed to circumvent preexisting antibodies against common adenovirus vectors such as Ad5. However, baseline immunity to these vectors still exists in human populations. Traditional cloning of new adenovirus vaccine vectors is a long and cumbersome process that takes 2 months or more and that requires rare unique restriction enzyme sites. Here we describe a novel, restriction enzyme-independent method for rapid cloning of new adenovirus vaccine vectors that reduces the total cloning procedure to 1 week. We developed 14 novel adenovirus vectors from rhesus monkeys that can be grown to high titers and that are immunogenic in mice. All vectors grouped with the unusual adenovirus species G and show extremely low seroprevalence in humans. Rapid cloning of novel adenovirus vectors is a promising approach for the development of new vector platforms. Rhesus adenovirus vectors may prove useful for clinical development.IMPORTANCE To overcome baseline immunity to human and chimpanzee adenovirus vectors, we developed 14 novel adenovirus vectors from rhesus monkeys. These vectors are immunogenic in mice and show extremely low seroprevalence in humans. Rhesus adenovirus vectors may prove useful for clinical development.

Adenoviral vector with shield and adapter increases tumor specificity and escapes liver and immune control

Most systemic viral gene therapies have been limited by sequestration and degradation of virions, innate and adaptive immunity, and silencing of therapeutic genes within the target cells. Here we engineer a high-affinity protein coat, shielding the most commonly used vector in clinical gene therapy, human adenovirus type 5. Using electron microscopy and crystallography we demonstrate a massive coverage of the virion surface through the hexon-shielding scFv fragment, trimerized to exploit the hexon symmetry and gain avidity. The shield reduces virion clearance in the liver. When the shielded particles are equipped with adaptor proteins, the virions deliver their payload genes into human cancer cells expressing HER2 or EGFR. The combination of shield and adapter also increases viral gene delivery to xenografted tumors in vivo, reduces liver off-targeting and immune neutralization. Our study highlights the power of protein engineering for viral vectors overcoming the challenges of local and systemic viral gene therapies.

Viral gene therapy can be limited by the efficacy of virion sequestration, immune responses and the silencing of genetic payloads. Here the authors engineer an advenovirus protein coat which shields the virion from the immune system while targeting cancer cells.

Vaccine vectors based on Adenovirus 19a/64 exhibit broad cellular tropism and potently restimulate HCMV-specific T cell responses ex vivo

Human Cytomegalovirus (HCMV) remains a major health burden and the development of a vaccine is a global priority. We developed new viral vectors delivering the T cell immunogens IE-1 and pp65 based on Adenovirus 19a/64 (Ad19a/64), a member of subgroup D. In this ex vivo study, the novel vectors were compared side by side to Ad5 or modified Vaccinia Ankara (MVA) strains expressing the same transgenes. We found that unlike Ad5, Ad19a/64 vectors readily transduce a broad panel of immune cells, including monocytes, T cells, NK cells and monocyte-derived dendritic cells (moDCs). Both Ad19a/64- and MVA-transduced moDCs efficiently restimulated IE-1 or pp65-specific T cells but MVA induced a higher amount of cytotoxicity in this cell type. Ad5 and Ad19 induced upregulation of CD86 and HLA-DR in moDCs whereas expression of CD80 and CD83 was largely unaltered. By contrast, MVA transduction led to downregulation of all markers. Taken together, our data demonstrate that Ad19a/64 is a promising vector for the delivery of HCMV immunogens since it transduces dendritic cells with an efficiency that is comparable to MVA, but cytotoxicity and interference with dendritic cell maturation are less pronounced.

Generation and characterization of a novel candidate gene therapy and vaccination vector based on human species D adenovirus type 56

The vectorization of rare human adenovirus (HAdV) types will widen our knowledge of this family and their interaction with cells, tissues and organs. In this study we focus on HAdV-56, a member of human Ad species D, and create ease-of-use cloning systems to generate recombinant HAdV-56 vectors carrying foreign genes. We present in vitro transduction profiles for HAdV-56 in direct comparison to the most commonly used HAdV-5-based vector. In vivo characterizations demonstrate that when it is delivered intravenously (i.v.) HAdV-56 mainly targets the spleen and, to a lesser extent, the lungs, whilst largely bypassing liver transduction in mice. HAdV-56 triggered robust inflammatory and cellular immune responses, with higher induction of IFNγ, TNFα, IL5, IL6, IP10, MCP1 and MIG1 compared to HAdV-5 following i.v. administration. We also investigated its potential as a vaccine vector candidate by performing prime immunizations in mice with HAdV-56 encoding luciferase (HAdV-56-Luc). Direct comparisons were made to HAdV-26, a highly potent human vaccine vector currently in phase II clinical trials. HAdV-56-Luc induced luciferase 'antigen'-specific IFNγ-producing cells and anti-HAdV-56 neutralizing antibodies in Balb/c mice, demonstrating a near identical profile to that of HAdV-26. Taken together, the data presented provides further insight into human Ad receptor/co-receptor usage, and the first report on HAdV-56 vectors and their potential for gene therapy and vaccine applications.

Advancements in the design and scalable production of viral gene transfer vectors

The last 10 years have seen a rapid expansion in the use of viral gene transfer vectors, with approved therapies and late stage clinical trials underway for the treatment of genetic disorders, and multiple forms of cancer, as well as prevention of infectious diseases through vaccination. With this increased interest and widespread adoption of viral vectors by clinicians and biopharmaceutical industries, there is an imperative to engineer safer and more efficacious vectors, and develop robust, scalable and cost-effective production platforms for industrialization. This review will focus on major innovations in viral vector design and production systems for three of the most widely used viral vectors: Adenovirus, Adeno-Associated Virus, and Lentivirus.

Chimpanzee adenoviral vectors as vaccines - challenges to move the technology into the fast lane

INTRODUCTION

In recent years, replication-defective chimpanzee-derived adenoviruses have been extensively evaluated as genetic vaccines. These vectors share desirable properties with human adenoviruses like the broad tissue tropism and the ease of large-scale manufacturing. Additionally, chimpanzee adenoviruses have the advantage to overcome the negative impact of pre-existing anti-human adenovirus immunity. Areas covered: Here the authors review current pre-clinical research and clinical trials that utilize chimpanzee-derived adenoviral vectors as vaccines. A wealth of studies are ongoing to evaluate different vector backbones and administration routes with the aim of improving immune responses. The challenges associated with the identification of an optimal chimpanzee vector and immunization strategies for different immunological outcomes will be discussed. Expert commentary: The demonstration that chimpanzee adenoviruses can be safely used in humans has paved the way to the use of a whole new array of vectors of different serotypes. However, so far no predictive signature of vector immunity in humans has been identified. The high magnitude of T cell responses elicited by chimpanzee adenoviruses has allowed dissecting the qualitative aspects that may be important for protective immunity. Ultimately, only the results from the most clinically advanced products will help establish the efficacy of the vaccine vector platform in the field of disease prevention.

The complete genome sequence of human adenovirus 84, a highly recombinant new Human mastadenovirus D type with a unique fiber gene

A novel human adenovirus was isolated from a pediatric case of acute respiratory disease in Panama City, Panama in 2011. The clinical isolate was initially identified as an intertypic recombinant based on hexon and fiber gene sequencing. Based on the analysis of its complete genome sequence, the novel complex recombinant Human mastadenovirus D (HAdV-D) strain was classified into a new HAdV type: HAdV-84, and it was designated Adenovirus D human/PAN/P309886/2011/84[P43H17F84]. HAdV-D types possess usually an ocular or gastrointestinal tropism, and respiratory association is scarcely reported. The virus has a novel fiber type, most closely related to, but still clearly distant from that of HAdV-36. The predicted fiber is hypothesised to bind sialic acid with lower affinity compared to HAdV-37. Bioinformatic analysis of the complete genomic sequence of HAdV-84 revealed multiple homologous recombination events and provided deeper insight into HAdV evolution.

Getting genetic access to natural adenovirus genomes to explore vector diversity

Recombinant vectors based on the human adenovirus type 5 (HAdV5) have been developed and extensively used in preclinical and clinical studies for over 30 years. However, certain restrictions of HAdV5-based vectors have limited their clinical applications because they are rather inefficient in specifically transducing cells of therapeutic interest that lack the coxsackievirus and adenovirus receptor (CAR). Moreover, enhanced vector-associated toxicity and widespread preexisting immunity have been shown to significantly hamper the effectiveness of HAdV-5-mediated gene transfer. However, evolution of adenoviruses in the natural host is driving the generation of novel types with altered virulence, enhanced transmission, and altered tissue tropism. As a consequence, an increasing number of alternative adenovirus types were identified, which may represent a valuable resource for the development of novel vector types. Thus, researchers are focusing on the other naturally occurring adenovirus types, which are structurally similar but functionally different from HAdV5. To this end, several strategies have been devised for getting genetic access to adenovirus genomes, resulting in a new panel of adenoviral vectors. Importantly, these vectors were shown to have a host range different from HAdV5 and to escape the anti-HAdV5 immune response, thus underlining the great potential of this approach. In summary, this review provides a state-of-the-art overview of one essential step in adenoviral vector development.

Phylogenomic analysis of the complete sequence of a gastroenteritis-associated cetacean adenovirus (bottlenose dolphin adenovirus 1) reveals a high degree of genetic divergence

Adenoviruses are common pathogens in vertebrates, infecting a wide range of hosts, but only having rarely been detected and correlated with disease in cetaceans. This article describes the first complete genomic sequence of a cetacean adenovirus, bottlenose dolphin adenovirus 1 (BdAdV-1), detected in captive bottlenose dolphin population (Tursiops truncatus) suffering from self-limiting gastroenteritis. The complete genome sequence of BdAdV-1 was recovered from data generated by high-throughput sequencing and validated by Sanger sequencing. The genome is 34,080bp long and has 220 nucleotides long inverted terminal repeats. A total of 29 coding sequences were identified, 26 of which were functionally annotated. Among the unusual features of this genome is a remarkably long 4380bp E3 ORF1, that displays no sequence homology with the corresponding E3 regions of other adenoviruses. In addition, the fiber protein only has 26% identity with fiber proteins described in other adenoviruses. Three hypothetical proteins were predicted. The phylogenetic analysis indicates that the closest known relative to BdAdV-1 is an adenovirus detected in bottlenose dolphin (KR024710), with an amino acid sequence identity between 36 and 79% depending on the protein. Based on the phylogenic analysis, the BdAdV-1 appears to have co-evolved with its host. The results indicate that BdAdV-1 belongs to the Mastadenovirus genus of the Adenoviridae family, however, it is clearly different from other adenoviruses, especially in the 3'-end of the viral genome. The high degree of sequence divergence suggests that BdAdV-1 should be considered as a novel species in the Mastadenovirus genus. The study also demonstrates the usefulness of high-throughput sequencing to obtain full-length genomes of genetically divergent viruses.

Vectored immunoprophylaxis: an emerging adjunct to traditional vaccination

The successful development of effective vaccines has been elusive for many of the world's most important infectious diseases. Additionally, much of the population, such as the aged or immunocompromised, are unable to mount an effective immunologic response for existing vaccines. Vectored Immunoprophylaxis (VIP) is a novel approach designed to address these challenges. Rather than utilizing an antigen to trigger a response from the host's immune system as is normally done with traditional vaccines, VIP genetically engineers the production of tailored antibodies from non-hematopoietic cells, bypassing the humoral immune system. Direct administration of genes encoding for neutralizing antibodies has proven to be effective in both preventing and treating several infectious diseases in animal models. While, a significant amount of work has focused on HIV, including an ongoing clinical trial, the approach has also been shown to be effective for malaria, dengue, hepatitis C, influenza, and more. In addition to presenting itself as a potentially efficient approach to solving long-standing vaccine challenges, the approach may be the best, if not only, method to vaccinate immunocompromised individuals. Many issues still need to be addressed, including which tissue(s) makes the most suitable platform, which vector(s) are most efficient at transducing the platform tissue used to secrete the antibodies, and what are the long-term effects of such a treatment. Here we provide a brief overview of this approach, and its potential application in treating some of the world's most intractable infectious diseases.

Viral Vectors, Engineered Cells and the CRISPR Revolution

Over the past few decades the ability to edit human cells has revolutionized modern biology and medicine. With advances in genome editing methodologies, gene delivery and cell-based therapeutics targeted at treatment of genetic disease have become a reality that will become more and more essential in clinical practice. Modifying specific mutations in eukaryotic cells using CRISPR-Cas systems derived from prokaryotic immune systems has allowed for precision in correcting various disease mutations. Furthermore, delivery of genetic payloads by employing viral tropism has become a crucial and effective mechanism for delivering genes and gene editing systems into cells. Lastly, cells modified ex vivo have tremendous potential and have shown effective in studying and treating a myriad of diseases. This chapter seeks to highlight and review important progress in the realm of the editing of human cells using CRISPR-Cas systems, the use of viruses as vectors for gene therapy, and the application of engineered cells to study and treat disease.

Using the E4orf6-Based E3 Ubiquitin Ligase as a Tool To Analyze the Evolution of Adenoviruses

E4orf6 proteins from all human adenoviruses form Cullin-based ubiquitin ligase complexes that, in association with E1B55K, target cellular proteins for degradation. While most are assembled with Cul5, a few utilize Cul2. BC-box motifs enable all these E4orf6 proteins to assemble ligase complexes with Elongins B and C. We also identified a Cul2-box motif used for Cul2 selection in all Cul2-based complexes. With this information, we set out to determine if other adenoviruses also possess the ability to form the ligase complex and, if so, to predict their Cullin usage. Here we report that all adenoviruses known to encode an E4orf6-like protein (mastadenoviruses and atadenoviruses) maintain the potential to form the ligase complex. We could accurately predict Cullin usage for E4orf6 products of mastadenoviruses and all but one atadenovirus. Interestingly, in nonhuman primate adenoviruses, we found a clear segregation of Cullin binding, with Cul5 utilized by viruses infecting great apes and Cul2 by Old/New World monkey viruses, suggesting that a switch from Cul2 to Cul5 binding occurred during the period when great apes diverged from monkeys. Based on the analysis of Cullin selection, we also suggest that the majority of human adenoviruses, which exhibit a broader tropism for the eye and the respiratory tract, exhibit Cul5 specificity and resemble viruses infecting great apes, whereas those that infect the gastrointestinal tract may have originated from monkey viruses that share Cul2 specificity. Finally, aviadenoviruses also appear to contain E4orf6 genes that encode proteins with a conserved XCXC motif followed by, in most cases, a BC-box motif.

IMPORTANCE

Two early adenoviral proteins, E4orf6 and E1B55K, form a ubiquitin ligase complex with cellular proteins to ubiquitinate specific substrates, leading to their degradation by the proteasome. In studies with representatives of each human adenovirus species, we (and others) previously discovered that some viruses use Cul2 to form the complex, while others use Cul5. In the present study, we expanded our analyses to all sequenced adenoviruses and found that E4orf6 genes from all mast- and atadenoviruses encode proteins containing the motifs necessary to form the ligase complex. We found a clear separation in Cullin specificity between adenoviruses of great apes and Old/New World monkeys, lending support for a monkey origin for human viruses of the Human mastadenovirus A, F, and G species. We also identified previously unrecognized E4orf6 genes in the aviadenoviruses that encode proteins containing motifs permitting formation of the ubiquitin ligase.

Oncolytic adenovirus-mediated therapy for prostate cancer

Prostate cancer is a leading cause of cancer-related death and morbidity in men in the Western world. Tumor progression is dependent on functioning androgen receptor signaling, and initial administration of antiandrogens and hormone therapy (androgen-deprivation therapy) prevent growth and spread. Tumors frequently develop escape mechanisms to androgen-deprivation therapy and progress to castration-resistant late-stage metastatic disease that, in turn, inevitably leads to resistance to all current therapeutics, including chemotherapy. In spite of the recent development of more effective inhibitors of androgen–androgen receptor signaling such as enzalutamide and abiraterone, patient survival benefits are still limited. Oncolytic adenoviruses have proven efficacy in prostate cancer cells and cause regression of tumors in preclinical models of numerous drug-resistant cancers. Data from clinical trials demonstrate that adenoviral mutants have limited toxicity to normal tissues and are safe when administered to patients with various solid cancers, including prostate cancer. While efficacy in response to adenovirus administration alone is marginal, findings from early-phase trials targeting local-ized and metastatic prostate cancer suggest improved efficacy in combination with cytotoxic drugs and radiation therapy. Here, we review recent progress in the development of multimodal oncolytic adenoviruses as biological therapeutics to improve on tumor elimination in prostate cancer patients. These optimized mutants target cancer cells by several mechanisms including viral lysis and by expression of cytotoxic transgenes and immune-stimulatory factors that activate the host immune system to destroy both infected and noninfected prostate cancer cells. Additional modifications of the viral capsid proteins may support future systemic delivery of oncolytic adenoviruses.

Crystal structure of raptor adenovirus 1 fibre head and role of the beta-hairpin in siadenovirus fibre head domains

BACKGROUND

Most adenoviruses recognize their host cells via an interaction of their fibre head domains with a primary receptor. The structural framework of adenovirus fibre heads is conserved between the different adenovirus genera for which crystal structures have been determined (Mastadenovirus, Aviadenovirus, Atadenovirus and Siadenovirus), but genus-specific differences have also been observed. The only known siadenovirus fibre head structure, that of turkey adenovirus 3 (TAdV-3), revealed a twisted beta-sandwich resembling the reovirus fibre head architecture more than that of other adenovirus fibre heads, plus a unique beta-hairpin embracing a neighbouring monomer. The TAdV-3 fibre head was shown to bind sialyllactose.

METHODS

Raptor adenovirus 1 (RAdV-1) fibre head was expressed, crystallized and its structure was solved and refined at 1.5 Å resolution. The structure could be solved by molecular replacement using the TAdV-3 fibre head structure as a search model, despite them sharing a sequence identity of only 19 %. Versions of both the RAdV-1 and TAdV-3 fibre heads with their beta-hairpin arm deleted were prepared and their stabilities were compared with the non-mutated proteins by a thermal unfolding assay.

RESULTS

The structure of the RAdV-1 fibre head contains the same twisted ABCJ-GHID beta-sandwich and beta-hairpin arm as the TAdV-3 fibre head. However, while the predicted electro-potential surface charge of the TAdV-3 fibre head is mainly positive, the RAdV-1 fibre head shows positively and negatively charged patches and does not appear to bind sialyllactose. Deletion of the beta-hairpin arm does not affect the structure of the raptor adenovirus 1 fibre head and only affects the stability of the RAdV-1 and TAdV-3 fibre heads slightly.

CONCLUSIONS

The high-resolution structure of RAdV-1 fibre head is the second known structure of a siadenovirus fibre head domain. The structure shows that the siadenovirus fibre head structure is conserved, but differences in the predicted surface charge suggest that RAdV-1 uses a different natural receptor for cell attachment than TAdV-3. Deletion of the beta-hairpin arm shows little impact on the structure and stability of the siadenovirus fibre heads.

The emerging role of viral vectors as vehicles for DMD gene editing

Duchenne muscular dystrophy (DMD) is a genetic disorder caused by mutations in the dystrophin-encoding DMD gene. The DMD gene, spanning over 2.4 megabases along the short arm of the X chromosome (Xp21.2), is the largest genetic locus known in the human genome. The size of DMD, combined with the complexity of the DMD phenotype and the extent of the affected tissues, begs for the development of novel, ideally complementary, therapeutic approaches. Genome editing based on the delivery of sequence-specific programmable nucleases into dystrophin-defective cells has recently enriched the portfolio of potential therapies under investigation. Experiments involving different programmable nuclease platforms and target cell types have established that the application of genome-editing principles to the targeted manipulation of defective DMD loci can result in the rescue of dystrophin protein synthesis in gene-edited cells. Looking towards translation into the clinic, these proof-of-principle experiments have been swiftly followed by the conversion of well-established viral vector systems into delivery agents for DMD editing. These gene-editing tools consist of zinc-finger nucleases (ZFNs), engineered homing endoculeases (HEs), transcription activator-like effector nucleases (TALENs), and RNA-guided nucleases (RGNs) based on clustered, regularly interspaced, short palindromic repeats (CRISPR)-Cas9 systems. Here, we succinctly review these fast-paced developments and technologies, highlighting their relative merits and potential bottlenecks, when used as part of in vivo and ex vivo gene-editing strategies.

Genome analysis of four Old World monkey adenoviruses supports the proposed species classification of primate adenoviruses and reveals signs of possible homologous recombination

Within the family Adenoviridae, presently Simian mastadenovirus A is the single species approved officially for monkey adenoviruses (AdVs), whilst the establishment of six further species (Simian mastadenovirus B to Simian mastadenovirus G) has been proposed in the last few years. We examined the genetic content and phylogenetic relationships of four Old World monkey (OWM) AdV types [namely simian AdV (SAdV)-8, -11, -16 and -19] for which it had been proposed that they should be classified into different AdV species: SAdV-11 to Human mastadenovirus G, and the other three viruses into three novel species. By full genome sequencing, we identified gene contents characteristic for the genus Mastadenovirus. Among the 36 ORFs, 2 genes of different lengths, predicted to encode the adenoviral cellular attachment protein (the fibre), were found. The E3 regions contained six genes, present in every OWM AdV, but lacked the E3 19K gene, which has seemingly appeared only in the ape (hominid) AdV lineages during evolution. For the first time in SAdVs, two other exons belonging to the gene of the so-called U exon protein were also predicted. Phylogenetic calculations, based on the fibre-1 and the major capsid protein, the hexon, implied that recombination events might have happened between different AdV species. Phylogeny inference, based on the viral DNA-dependent DNA polymerase and the penton base protein, further supported the species classification proposed earlier.

Initial Characterization of Integrase-Defective Lentiviral Vectors for Pancreatic Cancer Gene Therapy

The vast majority (85%) of pancreatic ductal adenocarcinomas (PDACs) are discovered at too of a late stage to allow curative surgery. In addition, PDAC is highly resistant to conventional methods of chemotherapy and radiotherapy, which only offer a marginal clinical benefit. Consequently, the prognosis of this cancer is devastating, with a 5-year survival rate of less than 5%. In this dismal context, we recently demonstrated that PDAC gene therapy using nonviral vectors is safe and feasible, with early signs of efficacy in selected patients. Our next step is to transfer to the clinic HIV-1-based lentiviral vectors (LVs) that outshine other therapeutic vectors to treat experimental models of PDAC. However, a primary safety issue presented by LVs that may delay their use in patients is the risk of oncogenesis after vector integration in the host's cell DNA. Thus, we developed a novel anticancerous approach based on integrase-defective lentiviral vectors (IDLVs) and demonstrated that IDLVs can be successfully engineered to transiently deliver therapeutic genes to inhibit pancreatic cancer cells proliferation. This work stems for the use of therapeutic IDLVs for the management of PDAC, in forthcoming early phase gene therapy clinical trial for this disease with no cure.