Toxicological comparison of E2a-deleted and first-generation adenoviral vectors expressing alpha1-antitrypsin after systemic delivery

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.

Adenovirus as a Vector and Oncolytic Virus

Adenoviral vectors, both oncolytic viruses and gene delivery vectors, are among the earliest approved and commercialised vectors for gene therapy. Adenoviruses have high cytotoxicity and immunogenicity. Therefore, lentiviruses or adeno-associated viruses as viral vectors and herpes simplex virus as an oncolytic virus have recently drawn attention. Thus, adenoviral vectors are often considered relatively obsolete. However, their high cargo limit and transduction efficiency are significant advantages over newer viral vectors. This review provides an overview of the new-generation adenoviral vectors. In addition, we describe the modification of the fiber knob region that enhances affinity of adenoviral vectors for cancer cells and the utilisation of cancer-cell-specific promoters to suppress expression of unwanted transgenes in non-malignant tissues.

Untangling the Intricacies of Infection, Thrombosis, Vaccination, and Antiphospholipid Antibodies for COVID-19

Advanced SARS-CoV-2 infections not uncommonly associate with the occurrence of silent or manifest thrombotic events which may be found as focal or systemic disease. Given the potential complexity of COVID-19 illnesses, a multifactorial causation is likely, but several studies have focused on infection-induced coagulopathy. Procoagulant states are commonly found in association with the finding of antiphospholipid antibodies. The correlation of the latter with thrombosis and/or clinical severity remains controversial. Although measures of antiphospholipid antibodies most commonly include assessments for lupus anticoagulant, anticardiolipin, and anti-ß2-glycoprotein-I antibodies, lesser common antibodies have been detected, and there remains speculation that other yet undiscovered autoimmune thrombotic events may yet be found. The recent discovery of post-vaccination thromboses associated with platelet factor 4 antibody has created another level of concern. The pathogenesis of antiphospholipid antibodies and their role in COVID-19-related thrombosis deserves further attention. The multifactorial nature of thrombosis associated with both infection and vaccination should continue to be studied as new events unfold. Even if a cause-and-effect relationship is variable at best, such dedicated research is likely to generate other valuable insights that are applicable to medicine generally.

Helper virus-free gutless adenovirus (HF-GLAd): a new platform for gene therapy

Gene therapy is emerging as a treatment option for inherited genetic diseases. The success of this treatment approach greatly depends upon gene delivery vectors. Researchers have attempted to harness the potential of viral vectors for gene therapy applications over many decades. Among the viral vectors available, gutless adenovirus (GLAd) has been recognized as one of the most promising vectors for in vivo gene delivery. GLAd is constructed by deleting all the viral genes from an adenovirus. Owing to this structural feature, the production of GLAd requires a helper that supplies viral proteins in trans. Conventionally, the helper is an adenovirus. Although the helper adenovirus efficiently provides helper functions, it remains as an unavoidable contaminant and also generates replication-competent adenovirus (RCA) during the production of GLAd. These two undesirable contaminants have raised safety concerns and hindered the clinical applications of GLAd. Recently, we developed helper virus-free gutless adenovirus (HF-GLAd), a new version of GLAd, which is produced by a helper plasmid instead of a helper adenovirus. Utilization of this helper plasmid eliminated the helper adenovirus and RCA contamination in the production of GLAd. HF-GLAd, devoid of helper adenovirus and RCA contaminants, will facilitate its clinical applications. In this review, we discuss the characteristics of adenoviruses, the evolution and production of adenoviral vectors, and the unique features of HF-GLAd as a new platform for gene therapy. Furthermore, we highlight the potential applications of HF-GLAd as a gene delivery vector for the treatment of various inherited genetic diseases.

Helper virus-free gutless adenovirus (HF-GLAd): a new platform for gene therapy

Gene therapy is emerging as a treatment option for inherited genetic diseases. The success of this treatment approach greatly depends upon gene delivery vectors. Researchers have attempted to harness the potential of viral vectors for gene therapy applications over many decades. Among the viral vectors available, gutless adenovirus (GLAd) has been recognized as one of the most promising vectors for in vivo gene delivery. GLAd is constructed by deleting all the viral genes from an adenovirus. Owing to this structural feature, the production of GLAd requires a helper that supplies viral proteins in trans. Conventionally, the helper is an adenovirus. Although the helper adenovirus efficiently provides helper functions, it remains as an unavoidable contaminant and also generates replicationcompetent adenovirus (RCA) during the production of GLAd. These two undesirable contaminants have raised safety concerns and hindered the clinical applications of GLAd. Recently, we developed helper virus-free gutless adenovirus (HF-GLAd), a new version of GLAd, which is produced by a helper plasmid instead of a helper adenovirus. Utilization of this helper plasmid eliminated the helper adenovirus and RCA contamination in the production of GLAd. HF-GLAd, devoid of helper adenovirus and RCA contaminants, will facilitate its clinical applications. In this review, we discuss the characteristics of adenoviruses, the evolution and production of adenoviral vectors, and the unique features of HF-GLAd as a new platform for gene therapy. Furthermore, we highlight the potential applications of HF-GLAd as a gene delivery vector for the treatment of various inherited genetic diseases. [BMB Reports 2020; 53(11): 565-575].

Gene Therapy in Retinal Dystrophies

Inherited retinal dystrophies (IRDs) are a group of clinically and genetically heterogeneous degenerative disorders. To date, mutations have been associated with IRDs in over 270 disease genes, but molecular diagnosis still remains elusive in about a third of cases. The methodologic developments in genome sequencing techniques that we have witnessed in this last decade have represented a turning point not only in diagnosis and prognosis but, above all, in the identification of new therapeutic perspectives. The discovery of new disease genes and pathogenetic mechanisms underlying IRDs has laid the groundwork for gene therapy approaches. Several clinical trials are ongoing, and the recent approval of Luxturna, the first gene therapy product for Leber congenital amaurosis, marks the beginning of a new era. Due to its anatomical and functional characteristics, the retina is the organ of choice for gene therapy, although there are quite a few difficulties in the translational approaches from preclinical models to humans. In the first part of this review, an overview of the current knowledge on methodological issues and future perspectives of gene therapy applied to IRDs is discussed; in the second part, the state of the art of clinical trials on the gene therapy approach in IRDs is illustrated.

Influence of loxP insertion upstream of the cis-acting packaging domain on adenovirus packaging efficiency

First-generation AdV enables efficient gene transduction, although its immunogenicity is an important problem in vivo. Helper-dependent AdV (HD-AdV) is one possible solution to this problem. The construction of HD-AdV requires a helper virus, in which the viral packaging domain is flanked by two inserted loxP to hamper its packaging in Cre-expressing 293 cells. Here, we constructed 19L viruses containing loxP at 191 nt from the left end of the genome upstream of the packaging domain, 15L viruses bearing loxP at 143 nt, and a control ΔL virus lacking loxP at these positions. The 19L position is used worldwide, and the 15L position has been reported to result in a lower titer than that of 19L. When the titers were compared for six pairs of 19L and 15L AdV, the 19L AdV produced titers similar to, or sometimes lower than, the 15L and ΔL AdV, unlike the results of previous reports. We next chose one pair of 15L and 19L AdV that produced titers similar to that of ΔL and a competitor AdV lacking loxP for use in a competition assay. When a small amount of the competitor AdV was co-infected, both the 15L and the 19L AdV, but not ΔL, gradually became minority components during subsequent viral passages. Therefore, the loxP insertions at 143 nt and 191 nt decreased the viral packaging efficiency.

Helper-Dependent Adenoviral Vectors

Helper-dependent adenoviral vectors are devoid of all viral coding sequences, possess a large cloning capacity, and can efficiently transduce a wide variety of cell types from various species independent of the cell cycle to mediate long-term transgene expression without chronic toxicity. These non-integrating vectors hold tremendous potential for a variety of gene transfer and gene therapy applications. Here, we review the production technologies, applications, obstacles to clinical translation and their potential resolutions, and the future challenges and unanswered questions regarding this promising gene transfer technology.

Gene therapy with helper-dependent adenoviral vectors: current advances and future perspectives

Recombinant Adenoviral vectors represent one of the best gene transfer platforms due to their ability to efficiently transduce a wide range of quiescent and proliferating cell types from various tissues and species. The activation of an adaptive immune response against the transduced cells is one of the major drawbacks of first generation Adenovirus vectors and has been overcome by the latest generation of recombinant Adenovirus, the Helper-Dependent Adenoviral (HDAd) vectors. HDAds have innovative features including the complete absence of viral coding sequences and the ability to mediate high level transgene expression with negligible chronic toxicity. This review summarizes the many aspects of HDAd biology and structure with a major focus on in vivo gene therapy application and with an emphasis on the unsolved issues that these vectors still presents toward clinical application.

Adenoviral producer cells

Adenovirus (Ad) vectors, in particular those of the serotype 5, are highly attractive for a wide range of gene therapy, vaccine and virotherapy applications (as discussed in further detail in this issue). Wild type Ad5 virus can replicate in numerous tissue types but to use Ad vectors for therapeutic purposes the viral genome requires modification. In particular, if the viral genome is modified in such a way that the viral life cycle is interfered with, a specific producer cell line is required to provide trans-complementation to overcome the modification and allow viral production. This can occur in two ways; use of a producer cell line that contains specific adenoviral sequences incorporated into the cell genome to trans-complement, or use of a producer cell line that naturally complements for the modified Ad vector genome. This review concentrates on producer cell lines that complement non-replicating adenoviral vectors, starting with the historical HEK293 cell line developed in 1977 for first generation Ad vectors. In addition the problem of replication-competent adenovirus (RCA) contamination in viral preparations from HEK293 cells is addressed leading to the development of alternate cell lines. Furthermore novel cell lines for more complex Ad vectors and alternate serotype Ad vectors are discussed.

Efficient, specific and targeted delivery of genes to the lung

Many inherited and acquired pulmonary disorders without satisfactory therapies may be amenable to gene therapy. Despite numerous advances, efficient delivery and expression of the therapeutic transgene at physiological levels for phenotypic correction of disease has proved elusive. This article focuses on various strategies aimed at achieving targeted delivery to the lungs. Both physical methods and biological targeting have been successfully applied in various gene delivery systems. Targeting of different cell types has been achieved by pseudotyping of viral vectors with capsids from different serotypes and modification of nonviral vectors with targeting ligands. Both classes of vectors are discussed with respect to their gene delivery and expression efficiencies, longevity of expression and immunogenicity. Moreover, gene therapy clinical trials for different lung diseases are discussed.

Efficient, long-term hepatic gene transfer using clinically relevant HDAd doses by balloon occlusion catheter delivery in nonhuman primates

Helper-dependent adenoviral vectors (HDAd) are devoid of all viral coding sequences and are thus an improvement over early generation Ad because they can provide long-term transgene expression in vivo without chronic toxicity. However, high vector doses are required to achieve efficient hepatic transduction by systemic intravenous injection, and this unfortunately results in dose-dependent acute toxicity. To overcome this important obstacle, we have developed a minimally invasive method to preferentially deliver HDAd into the liver of nonhuman primates. Briefly, a balloon occlusion catheter was percutaneously positioned in the inferior vena cava to occlude hepatic venous outflow. HDAd was injected directly into the occluded liver via a percutaneously placed hepatic artery catheter. Compared to systemic vector injection, this approach resulted in substantially higher hepatic transduction efficiency using clinically relevant low vector doses and was accompanied by mild-to-moderate acute but transient toxicities. Transgene expression was sustained for up to 964 days. These results suggest that our minimally invasive method of delivery can significantly improve the vector's therapeutic index and may be a first step toward clinical application of HDAd for liver-directed gene therapy.

Serum-free transient protein production system based on adenoviral vector and PER.C6 technology: high yield and preserved bioactivity

Stable E1 transformed cells, like PER.C6, are able to grow at scale and to high cell densities. E1-deleted adenoviruses replicate to high titer in PER.C6 cells whereas subsequent deletion of E2A from the vector results in absence of replication in PER.C6 cells and drastically lowers the expression of adenovirus proteins in such cells. We therefore considered the use of an DeltaE1/DeltaE2 type 5 vector (Ad5) to deliver genes to PER.C6 cells growing in suspension with the aim to achieve high protein yield. To evaluate the utility of this system we constructed DeltaE1/DeltaE2 vector carrying different classes of protein, that is, the gene coding for spike protein derived from the Coronavirus causing severe acute respiratory syndrome (SARS-CoV), a gene coding for the SARS-CoV receptor or the genes coding for an antibody shown to bind and neutralize SARS-CoV (SARS-AB). The DeltaE1/DeltaE2A-vector backbones were rescued on a PER.C6 cell line engineered to constitutively over express the Ad5 E2A protein. Exposure of PER.C6 cells to low amounts (30 vp/cell) of DeltaE1/DeltaE2 vectors resulted in highly efficient (>80%) transduction of PER.C6 cells growing in suspension. The efficient cell transduction resulted in high protein yield (up to 60 picogram/cell/day) in a 4 day batch production protocol. FACS and ELISA assays demonstrated the biological activity of the transiently produced proteins. We therefore conclude that DeltaE1/DeltaE2 vectors in combination with the PER.C6 technology may provide a viable answer to the increasing demand for high quality, high yield recombinant protein.

Replication-attenuated Human Adenoviral Type 4 vectors elicit capsid dependent enhanced innate immune responses that are partially dependent upon interactions with the complement system

Human Adenovirus Type 4 (HAdV-4) is responsible for epidemic outbreaks of Acute Respiratory Disease (especially in military recruits), and is known to cause significant morbidity with several reported cases of mortality. However, we do not understand why this serotype causes such high morbidity, and have little insight into the immunobiology of HAdV-4 infections. We have now developed a replication attenuated HAdV-4 vector system, and through it, demonstrate that HAdV-4 virions have enhanced infectivity of certain cell types and reveal aspects of the serotype-specific heightened innate immunogenicity of infectious HAdV-4 capsids both in vitro and in vivo. We further found that elements of this serotype-specific immunogenicity were dependent upon interactions with the complement system. These findings provide insights into the mechanisms possibly underlying the known morbidity accompanying wild-type HAdV-4 infections as well as highlight important considerations when considering development of alternative serotype vectors.

Expression of pIX Gene Induced by Transgene Promoter: Possible Cause of Host Immune Response in First-Generation Adenoviral Vectors

First-generation (FG) adenoviral vectors (AdVs) have been widely used not only for gene therapy but also for basic studies. Because vectors of this type lack the E1A gene that is essential for the expression of other viral genes, their expression levels in target cells have been considered low. However, we found that the viral pIX gene, located immediately downstream of the inserted expression unit of the transgene, was significantly coexpressed with the transgene in cells infected with FG AdV. Whereas CAG and SRalpha promoters activated the pIX promoter considerably through their enhancer effects, the EF1alpha promoter hardly did. Moreover, when the expression unit was inserted in the rightward orientation, not only the pIX protein but also a fusion protein consisting of the N-terminal part of transgene product and pIX were sometimes coexpressed with the transgene product through an aberrant splicing mechanism. In in vivo experiments, a LacZ-expressing AdV bearing the CAG promoter caused an elevation of alanine aminotransferase, but an AdV bearing the EF1alpha promoter produced no detectable levels. Whereas the FG AdV expressing human growth hormone under the control of the CAG promoter maintained a high hormone level for less than 1 month, the FG AdV under the control of the EF1alpha promoter maintained a high level for at least 6 months. These results suggest that pIX coexpression may be one of the main causes of AdV-induced immune responses, and that the EF1alpha promoter is probably valuable for the long-term expression of FG AdV. Thus, the in vivo utility of FG AdV should be reevaluated.

Gene therapy used for tissue engineering applications

This review highlights the advances at the interface between tissue engineering and gene therapy. There are a large number of reports on gene therapy in tissue engineering, and these cover a huge range of different engineered tissues, different vectors, scaffolds and methodology. The review considers separately in-vitro and in-vivo gene transfer methods. The in-vivo gene transfer method is described first, using either viral or non-viral vectors to repair various tissues with and without the use of scaffolds. The use of a scaffold can overcome some of the challenges associated with delivery by direct injection. The ex-vivo method is described in the second half of the review. Attempts have been made to use this therapy for bone, cartilage, wound, urothelial, nerve tissue regeneration and for treating diabetes using viral or non-viral vectors. Again porous polymers can be used as scaffolds for cell transplantation. There are as yet few comparisons between these many different variables to show which is the best for any particular application. With few exceptions, all of the results were positive in showing some gene expression and some consequent effect on tissue growth and remodelling. Some of the principal advantages and disadvantages of various methods are discussed.

Viral Vector–mediated and Cell-based Therapies for Treatment of Cystic Fibrosis

Gene and cell-based therapies are considered to be potentially powerful new approaches for the management of cystic fibrosis (CF) lung disease. Despite tremendous efforts that have been made, especially in studies to understand the obstacles to gene delivery, major challenges to the application of these approaches remain to be solved. This article will review the advancements made and challenges remaining in the development of viral vector-mediated and cell-based approaches to treat patients with CF.

Toll-like Receptor 9 Triggers an Innate Immune Response to Helper-dependent Adenoviral Vectors

A major obstacle to the clinical application of systemic adenoviral gene replacement therapy is the host innate immune response. Although recent studies have attempted to characterize the cellular basis for this response to systemically administered helper-dependent adenoviral vector (HD-Ad), the underlying molecular components of the innate immune repertoire required to recognize the viral vector have yet to be identified. Here, we show that primary macrophages can sense HD-Ad vectors via the Toll-like Receptor 9 (TLR9) and respond by increasing pro-inflammatory cytokine secretion. Moreover, TLR9 sensing is involved in the rapid innate immune response to HD-Ad in vivo. TLR9 deficiency attenuates the innate immune response to HD-Ad, whereas TLR9 blockade reduces the acute inflammatory response after intravenous injection of the vector. Moreover, HD-Ad upregulates TLR9 gene expression independent of TLR9 function, suggesting that additional innate signaling pathways work cooperatively with TLR9. The identification of the components of the innate immune response to adenovirus will facilitate the development of combinatorial therapy directed at increasing the maximal tolerated dose of systemically delivered adenoviral vectors.

Sustained phenotypic correction in a mouse model of hypoalphalipoproteinemia with a helper-dependent adenovirus vector

We examined the efficacy and host response to the adenovirus (Ad)-mediated delivery of human apolipoprotein A-I (APOA1) gene to the liver of APOA1(-/-) mice. Administration of a first-generation vector (FGAd-AI) resulted in a transient appearance of APOA1 in plasma and induced an anti-APOA1 antibody titer, whereas treatment with a helper-dependent vector (HDAd-AI) resulted in sustained APOA1 expression without inducing an antibody titer. With these results, we studied the effects of FGAd vectors on APOAI expression by HDAd-AI vector. Co-treatment with an FGAd vector inhibited HDAd-AI- mediated APOA1 expression independent of transgene cassettes, but only FGAd-AI induced a humoral response. Furthermore, APOA1 mRNA levels in mice co-treated with FGAd vectors were much lower than those expected from the vector copy number, suggesting that DNA of FGAd vectors interferes with the HDAd-AI vector's APOA1 promoter. A single treatment with an HDAd-AI vector produced a supraphysiological plasma APOA1 level that gradually declined to about half the normal human level over the course of 2 years, associated with a plasma cholesterol level that is persistently higher than that in controls. This investigation provides the proof of principle that liver-directed HDAd gene delivery is effective for the long-term phenotypic correction of monogenic hypoalphalipoproteinemia.

Adenovirus: from foe to friend

Human adenoviruses (HAdVs) can cause mild respiratory, gastrointestinal, urogenital and ocular disease. Knowledge about HAdVs has been expanding for more than five decades putting them amongst the most-studied viruses. This continued interest stems, to a great extent, from the fact that these double-stranded DNA viruses have proven to be a versatile tool to probe the basic phenomena of eukaryotic cells. HAdV research has led to the discovery of, for instance, RNA splicing and greatly contributed to our knowledge of processes as fundamental as replication, transcription and translation. Moreover, the transformation of rodent cells by HAdVs has provided a system to unravel the molecular pathways that control cell proliferation. As a result, the genetic organisation of these agents is known in great detail allowing the straightforward manipulation of their genomes. In addition, the virus itself became renowned for its ability to produce large amounts of progeny and to efficiently infect mammalian cells regardless of their cell cycle status. These features contributed to the broad use of recombinant HAdVs as gene carriers particularly in in vivo settings where the vast majority of target cells are post-mitotic. The most advanced type of HAdV vectors can accommodate up to 37 kb of foreign DNA and are devoid of viral genes. With the aid of these high-capacity HAdV vectors large physiologically responsive transcriptional elements and/or genes can be efficiently introduced into target cells while minimising adaptive immune responses against the transduced cells. This article provides information on HAdV especially on the aspects pertinent to the design, production and performance of its recombinant forms. The development and characteristics of the main HAdV-based vector types are also briefly reviewed.

Modulation of TNFalpha, a determinant of acute toxicity associated with systemic delivery of first-generation and helper-dependent adenoviral vectors

Understanding the determinants of the host innate immune response to systemic administration of adenoviral (Ad) vectors is critical for clinical gene therapy. Acute toxicity occurs within minutes to hours after vector administration and is characterized by activation of innate immune responses. Our data indicate that in mice, indicators of vector toxicity include elevations of cytokine levels, liver transaminase levels and thrombocytopenia. To discern potential targets for blunting this host response, we evaluated genetic factors in the host response to systemically administered first-generation Ad vectors (FGV) and helper-dependent Ad vectors (HDV) containing beta-galactosidase expression cassettes. A preliminary screen for modulation of vector-induced thrombocytopenia revealed no role for interferon-gamma, mast cells or perforin. However, vector-induced thrombocytopenia and interleukin 6 (IL-6) expression are less evident in tumor necrosis factor alpha (TNFalpha)-deficient mice. Moreover, we also demonstrated that TNFalpha blockade via antibody or huTNFR:Fc pretreatment attenuates both thrombocytopenia (>40% increase in platelet count) and IL-6 expression (>80% reduction) without affecting interleukin 12 , liver enzymes, hematological indices or vector transduction in a murine model. Our data indicate that the use of HDV, in combination with clinically approved TNFalpha immunomodulation, may represent an approach for improving the therapeutic index of Ad gene therapy for human clinical trials.

Sustained phenotypic correction of canine hemophilia B after systemic administration of helper-dependent adenoviral vector

We have evaluated the potential of liver-directed, helper-dependent adenoviral (HDAd) vector-mediated gene therapy in the hemophilia B dog. Two dogs were injected intravenously with HDAd (3 x 10(12) VP/kg) bearing a liver-restricted canine coagulation factor IX (FIX) expression cassette. After injection, the whole blood clotting time for both dogs declined from >60 min to </=20 min for at least 604 and 446 days, respectively. Peak FIX activities of 34.1 and 129.2% were detected at 12x14 days and then slowly declined to 2 to 5% by 120 days and stabilized at these therapeutic levels for at least 418 and 257 days. For one dog, a peak FIX level of 500 ng/ml was achieved and stabilized at >170 ng/ml for at least 256 days. For the other dog, a peak FIX level of 1258 ng/ml was achieved and stabilized at >400 ng/ml for at least 213 days. Inhibitor formation was not evident in either animal. Importantly, whereas untreated hemophilia B dogs suffer five or six spontaneous bleeds per year, the treated dogs suffered no such bleeds postinjection. Significantly, this study is the first to demonstrate long-term phenotypic correction of a genetic disorder in a large animal with HDAd. Although no evidence of chronic toxicity was observed in either animal, systemic vector administration at 3 x 10(12) VP/kg was accompanied by acute, albeit transient and variable laboratory abnormalities (alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, creatine phosphokinase, and platelet counts). The results of this study highlight both the potential benefit and the risk associated with systemic intravascular delivery of high-dose HDAd for liver-directed gene therapy.

Adenovirus binding to cultured synoviocytes triggers signaling through MAPK pathways and induces expression of cyclooxygenase-2

BACKGROUND

Recombinant adenovirus can be administered in vivo to achieve transduction of a number of cell types including human synoviocytes. Immunogenicity of adenoviruses has limited their utility as vectors for gene delivery; however, specific mechanisms underlying the acute inflammatory response to adenovirus are not well understood. Activation of a number of signal transduction pathways occurs rapidly upon adenovirus binding to cell-surface receptors. We investigated stimulated expression of mitogen-activated protein kinases (MAPKs), cyclooxygenase-2 (COX-2) and prostaglandin E(2) (PGE(2)) in human primary synovial fibroblasts to adenovirus expressing the E. coli beta-galactosidase gene.

METHODS

Cultured rheumatoid synoviocytes were exposed to transduction-competent Ad/RSVlacZ recombinant adenovirus or transduction-incompetent (psoralen/UV-irradiated) Ad/RSVlacZ. The effects on COX-2 expression, PGE(2) levels and MAPK signaling in synoviocytes were assessed using a combination of reverse-transcription polymerase chain reaction amplification and immunoblotting.

RESULTS

Adenovirus treatment of synoviocytes increased levels of COX-2 mRNA and protein as well as PGE(2). Psoralen-treated transcriptionally inactive adenovirus was equivalent to untreated adenovirus for early COX-2 induction suggesting that viral genes were not required. Adenovirus treatment stimulated phosphorylation of ERK-1/-2, p38 MAPK, and JNK. Inhibition of the ERK and p38 MAPK pathways inhibited COX-2 expression and PGE(2) production.

CONCLUSIONS

Taken together, these data demonstrate that a MAPK-dependent increase in COX-2 results in local prostaglandin production. These findings have clinical implications for use of adenovirus as vectors for in vivo gene delivery.

Characterization of Hemodynamic Events Following Intravascular Infusion of Recombinant Adenovirus Reveals Possible Solutions for Mitigating Cardiovascular Responses

Intravascular administration of recombinant adenovirus (rAd) in cancer patients has been well tolerated. However, dose-limiting hemodynamic responses associated with suppression of cardiac output have been observed at doses of 7.5 x 10(13) particles. While analysis of hemodynamic responses induced by small-molecule pharmaceuticals is well established, little is known about the cardiovascular effects of rAd. Telemetric cardiovascular (CV) monitoring in mice was utilized to measure hemodynamic events following intravascular rAd administration. Electrocardiogram analysis revealed a block in the SA node 3-4 min postinfusion, resulting in secondary pacemaking initiated at the AV node. This was associated with acute bradycardia, reduced blood pressure, and hypothermia followed by gradual recovery. Adenovirus-primed murine sera with high neutralizing antibody (nAb) titers could inhibit CV responses, whereas human sera with equivalent nAb titers induced by natural infection were, surprisingly, not inhibitory. Interestingly, repeat dosing within 2-4 h of the primary injection resulted in desensitization, resembling tachyphylaxis, for subsequent CV responses. Last, depletion of Kupffer cells prior to rAd infusion precluded induction of CV responses. These inhibitory effects suggest that rAd interactions with certain cells of the reticular endothelial system are associated with induction of CV responses. Significantly, these studies may provide insight into management of acute adverse effects following rAd systemic delivery, enabling a broadening of therapeutic index.

Approaches to improving the kinetics of adenovirus-delivered genes and gene products

Adenovirus (Ad) vectors have been expected to play a great role in gene therapy because of their extremely high transduction efficiency and wide tropism. However, due to the intrinsic deficiency of their immunogenic toxicities, Ad vectors are rapidly cleared from the host, transgene expression is transient, and readministration of the same serotype Ad vectors is problematic. As a result, Ad vectors are continually undergoing refinement to realize their potential for gene therapy application. Even after 1999, when a patient fatally succumbed to the toxicity associated with Ad vector administration at a University of Pennsylvania (U.S.) experimental clinic, enthusiasm of gene therapists for Ad vectors has not waned. With great efforts from various research groups, significant advances have been achieved through comprehensive approaches to improving the kinetics of Ad vector-delivered genes and gene products.

Lifelong elimination of hyperbilirubinemia in the Gunn rat with a single injection of helper-dependent adenoviral vector

Crigler-Najjar syndrome is a recessively inherited disorder characterized by severe unconjugated hyperbilirubinemia caused by a deficiency of uridine diphospho-glucuronosyl transferase 1A1. Current therapy relies on phototherapy to prevent kernicterus, but liver transplantation presently is the only permanent cure. Gene therapy is a potential alternative, and recent work has shown that helper-dependent adenoviral (HD-Ad) vectors, devoid of all viral coding sequences, induce prolonged transgene expression and exhibit significantly less chronic toxicity than early-generation Ad vectors. We used a HD-Ad vector to achieve liver-restricted expression of human uridine diphospho-glucuronosyl transferase 1A1 in the Gunn rat, a model of the human disorder. Total plasma bilirubin levels were reduced from >5.0 mg/dl to <<1.4 mg/dl for >2 yr after a single i.v. administration of vector expressing the therapeutic transgene at a dose of 3 x 10(12) viral particles per kg. HPLC analysis of bile from treated rats showed the presence of bilirubin glucuronides at normal WT levels >2 yr after one injection of vector, and i.v. injection of bilirubins IIIalpha and XIIIalpha in the same animals revealed excess bilirubin-conjugating capacity. There was no significant elevation of liver enzymes (alanine aminotransferase) and only transient, moderate thrombocytopenia after injection of the vector. A clinically significant reduction in serum bilirubin was observed with a dose as low as 6 x 10(11) viral particles per kg. We conclude that complete, long-term correction of hyperbilirubinemia in the Gunn rat model of Crigler-Najjar syndrome can be achieved with one injection of HD-Ad vector and negligible chronic toxicity.

Adenoviruses with an RGD-4C modification of the fiber knob elicit a neutralizing antibody response but continue to allow enhanced gene delivery

OBJECTIVE

The purpose of this study was to investigate the effect of preexisting neutralizing antibody (NAbs) in naive mice and the effect of induced NAbs in mice immunized with either an RGD or nonmodified Ad5 vector on the transduction efficiency of adenoviral vectors.

METHODS

BALB/c mice were immunized with Ad5LucRGD, with the unmodified Ad5Luc1, or with Opti-MEM intraperitoneally (ip) from one to three times. Sera were collected on day 27 and serially diluted to block Ad5Luc1 or Ad5LucRGD prior to infection of SKOV3.ip1 human ovarian carcinoma cells with these same vectors. Forty-eight hours post Ad infection, a luciferase assay was performed to determine the titer of NAbs.

RESULTS

Luciferase assay data showed that the gene transfer efficacy of Ad5LucRGD was 1.56-fold higher than Ad5Luc1 in the presence of serum from naive mice. In the presence of serum from Ad5Luc1-challenged mice, the transduction efficiency of Ad5LucRGD was 3.27-fold higher (single challenge) and 4.2-fold higher (triple challenge) than Ad5Luc1. In the presence of serum from Ad5LucRGD-challenged mice, the transduction efficiency of Ad5LucRGD was 2.24-fold higher (single challenge) and 2.53-fold higher (triple challenge) than Ad5Luc1.

CONCLUSION

The RGD-modified human Ad vectors appear to be less recognizable than unmodified Ad to preexisting NAbs in mouse models. RGD-modified Ad vectors also appear to elicit a relatively lower level of NAbs that may also contribute to the higher gene transduction efficiency of these modified vectors. Therefore, RGD-modified Ad vectors may be reagents of clinical utility in the context of preformed anti-Ad immunity and in the setting of repetitive dosing.

Immune responses against adenoviral vectors and their transgene products: a review of strategies for evasion

Human adenoviruses have been adopted as attractive vectors for in vivo gene therapy since they have a well-characterized genomic organization, can be grown to high titres and efficiently transduce a wide spectrum of dividing and non-dividing cells. However, the first-generation of adenoviral (Ad) vectors yielded only transient expression of the transgene in most immunocompetent mice. This constituted a major limitation of this early vector type. In contrast, persistent transgene expression can be established in immunodeficient mice. This suggests that the immunogenicity of adenoviral vectors limits the effective period of adenovirus-based gene therapy. Much effort has been put in devising strategies to circumvent the limitations imposed onto gene therapy by the immune system. Improvements in vector design have significantly improved the performance of the adenovirus vectors. Based on these results it is reasonable to anticipate that new modifications of the vectors will overcome some of the immunological barriers and will further expand the applicability of adenovirus-derived vectors.

Regulatory Aspects in the Development of Gene Therapies

Preclinical therapeutics development research is directed toward fulfilling two overlapping sets of goals. A set of scientific goals includes defining the best molecule or biologic construct for the task at hand, and proving the case for its development. The second set of goals addresses regulatory requirements necessary to introduce the agent into human subjects. In the case of “small molecule” drugs, in most cases the identity of the molecule and appropriate safety studies are straightforward. In contrast, the development of biologic agents, including gene therapies discussed here, presents distinct challenges. The nature of the “drug” may be an organism subject to mutation or selection of variants through recombination. Its properties may vary depending on the scale and method of its preparation, purification, and storage. How to test adequately for its safety prior to first introduction in humans may not be straightforward owing to intrinsic differences in response to the agent expected in humans as compared to animals.

Helper-Dependent Adenoviral Vectors for Gene Therapy

Helper-dependent adenoviral vectors possess a number of characteristics that make them attractive gene therapy vectors. These vectors are completely devoid of viral coding sequences and are able to mediate high-efficiency transduction in vivo to direct sustain high-level transgene expression with negligible chronic toxicity. This review focuses on advances in helper-dependent adenoviral vector technology, selected examples of in vivo studies of particular interest, and the issue of vector-mediated acute toxicity.

Vascular administration of adenoviral vector soaked in absorbable gelatin sponge particles (GSP) prolongs the transgene expression in hepatocytes

Transcatheter hepatic arterial chemoembolization using emulsions composed of anticancer agents and gelatin sponges (GS) has been an efficient and safe palliative treatment for inoperable hepatocellular carcinoma (HCC). We employed catheter-mediated left hepatic arterial embolization (CHAE) to increase transduction efficiency of adenoviral vector in canine hepatocytes. The emulsion was prepared by mixing pieces of GSP and adenoviral vectors expressing recombinant beta-galactosidase (Ad.LacZ) or human hepatocyte growth factor (Ad.hHGF). After the left hepatic artery was catheterized under angiography, CHAE with Ad.LacZ or Ad.hHGF was performed. Livers were removed and stained for LacZ activity on day 7. The expression pattern of LacZ staining was either scarce or patchy around the central hilum of the hepatic artery, or was homogeneously distributed in whole lobes, depending on whether large or small pieces of GSP were used. Hematological and serum biochemical changes during CHAE exhibited only a few effects. The chronological measurement of serum HGF concentration showed that the duration of transgene expression was greater after CHAE with Ad.hHGF. A similar pattern of transgene expression was observed in a rat model after hepatic arterial embolization with differential doses of Ad.hHGF soaked in GSP. These results suggest that hepatic arterial embolization by transcatheter mediated infusion with a mixture of adenovirus-GSP could be used for human HCC.

Liver toxicities typically induced by first-generation adenoviral vectors can be reduced by use of E1, E2b-deleted adenoviral vectors

Adenoviral vectors from which the E1 region has been deleted ([E1(-)] Ad) are known to induce strong immune responses after systemic delivery. In this study we have evaluated liver toxicities in mice after intravenous injection with high doses of [E1(-)] or modified [E1(-), E2b(-)] Ad vectors (both expressing the bacterial beta-galactosidase [lacZ] marker gene) in C57BL/6, BALB/c, and SCID mice. Our data demonstrate a marked reduction in maximal liver toxicities and pathologies (typically noted at 21 days postinjection) with the use of the [E1(-), E2b(-)] modified vector in all strains of mice tested. Our data also demonstrated that despite the use of the [E1(-), E2b(-)] Ad vector, significant liver toxicities were still observed. To address this issue and the fact that the lacZ gene was perceived as a foreign antigen in the immune-competent C57BL/6 and BALB/c mice, we similarly injected mice tolerant of LacZ (lacZ-TG). In contrast to our studies in C57BL/6 and BALB/c mice, LacZ-TG mice exhibited virtually no evidence of hepatotoxicity after intravenous injection with the [E1(-), E2b(-)] vector, in contrast to use of the [E1(-)] Ad vector. Our results demonstrate that the [E1(-), E2b(-)] Ad vector class can reduce liver toxicities typically ascribed to Ad vector-mediated gene transfer after transfer of a highly immunogenic or foreign gene, whereas transfer of a transgene that is perceived as nonforeign by the host can be delivered with virtually no evidence of toxicity. On the basis of a careful review of the literature, these improvements in vector safety rival those noted with other, more significantly modified Ad vectors described to date.

In vivo gene therapy for diabetes mellitus

Gene therapy has been hyped as a possible 'cure' for diabetes mellitus in the near future ever since insulin was first cloned and expressed in cultured cells in the late 1970s. In the past decade, however, the bar for gene therapy for diabetes has been raised because of recent advances in the clinical management of diabetes. Although current treatment modalities fall far short of a cure, they produce greatly improved, if imperfect, glycemic control. In this context, we review the latest advances in in vivo gene therapy and conclude that the most widely applied strategy of insulin gene transfer does not measure up to the existing treatment options, whereas the recently proved concept of induced islet neogenesis has the potential of bettering the currently available therapy. Much work remains to be done, however, before this regimen can be taken from the bench to the bedside.

Helper-dependent adenoviral vector-mediated long-term expression of human apolipoprotein A-I reduces atherosclerosis in apo E-deficient mice

Apolipoprotein A-I (APOA-I) is the major protein component of high-density lipoproteins (HDL). It has been shown that over-expression of human APOA-I increases HDL cholesterol and decreases atherosclerosis. We constructed a helper-dependent adenoviral (HD-Ad) vector that contains the entire human APOA-I gene (hgAI). Intravenous delivery of 1x10(13) viral particles/kg of this vector was followed by high levels of human APOA-I expression (up to 200 mg/dl) in the absence of detectable hepatic toxicity. We treated apo E-deficient mice with the hgAI vector and fed them either with a high-fat diet or with regular chow. As a control, two groups of mice were treated with PBS. The apo E-deficient mice treated with the hgAI vector showed supraphysiological levels of expression of human APOA-I at week 4 and high levels of HDL cholesterol compared to the control groups. Analysis of aortic atherosclerotic lesions 20 weeks after treatment, showed a significant reduction of lesion size in the treated mice with both diets. In conclusion, liver-directed gene transfer of human APOA-I using a HD-Ad vector resulted in a reduction of the development of atherosclerosis with the absence of significant toxicity.

Endocrine aspects of cancer gene therapy

The field of cancer gene therapy is in continuous expansion, and technology is quickly moving ahead as far as gene targeting and regulation of gene expression are concerned. This review focuses on the endocrine aspects of gene therapy, including the possibility to exploit hormone and hormone receptor functions for regulating therapeutic gene expression, the use of endocrine-specific genes as new therapeutic tools, the effects of viral vector delivery and transgene expression on the endocrine system, and the endocrine response to viral vector delivery. Present ethical concerns of gene therapy and the risk of germ cell transduction are also discussed, along with potential lines of innovation to improve cell and gene targeting.

Development and characterization of novel empty adenovirus capsids and their impact on cellular gene expression

Adenovirus (Ad) has been extensively studied as a eukaryotic viral vector. As these vectors have evolved from first-generation vectors to vectors that contain either very few or no viral genes ("gutless" Ad), significant reductions in the host innate immune response upon infection have been observed. Regardless of these vector improvements an unknown amount of toxicity has been associated with the virion structural proteins. Here we demonstrate the ability to generate high particle numbers (10(11) to 10(12)) of Ad empty virions based on a modification of Cre/lox gutless Ad vectors. Using a battery of analyses (electron microscopy, atomic force microscopy, confocal images, and competition assays) we characterized this reagent and determined that it (i) makes intact virion particles, (ii) competes for receptor binding with wild-type Ad, and (iii) enters the cell proficiently, demonstrating an ability to carry out essential steps of viral entry. To further study the biological impact of these Ad empty virions on infected cells, we carried out DNA microarray analysis. Compared to that for recombinant Ad, the number of mRNAs modulated upon infection was significantly reduced but the expression signatures were similar. This reagent provides a valuable tool for studies of Ad in that researchers can examine the effect of infection in the presence of the virion capsid alone.

Strategies for reversing drug resistance

Drug resistance, intrinsic or acquired, is a problem for all chemotherapeutic agents. In this review, we examine numerous strategies that have been tested or proposed to reverse drug resistance. Included among these strategies are approaches targeting the apoptosis pathway. Although the process of apoptosis is complex, it provides several potential sites for therapeutic intervention. A variety of targets and approaches are being pursued, including the suppression of proteins inhibiting apoptosis using antisense oligonucleotides (ASOs), and small molecules targeted at proteins that modulate apoptosis. An alternate strategy is based on numerous studies that have documented methylation of critical regions in the genome in human cancers. Consequently, efforts have been directed at re-expressing genes, including genes that affect drug sensitivity, using 5-azacytidine and 2'-deoxy-5-azacytidine (DAC, decitabine) as demethylating agents. While this strategy may be effective as a single modality, success will most likely be achieved if it is used to modulate gene expression in combination with other modalities such as chemotherapy. At a more basic level, attempts have been made to modulate glutathione (GSH) levels. Owing to its reactivity and high intracellular concentrations, GSH has been implicated in resistance to several chemotherapeutic agents. Several approaches designed to deplete intracellular GSH levels have been pursued including the use of buthionine-(S,R)-sulfoxime (BSO), a potent and specific inhibitor of gamma-glutamyl cysteine synthetase (gamma-GCS), the rate-limiting step in the synthesis of GSH, a hammerhead ribozyme against gamma-GCS mRNA to downregulate specifically its levels and targeting cJun expression to reduce GSH levels. Alternate strategies have targeted p53. The frequent occurrence of p53 mutations in human cancer has led to the development of numerous approaches to restore wild-type (wt) p53. The goals of these interventions are to either revert the malignant phenotype or enhance drug sensitivity. The approach most extensively investigated has utilized one of several viral vectors. An alternate approach, the use of small molecules to restore wt function to mutant p53, remains an option. Finally, the conceptually simplest mechanism of resistance is one that reduces intracellular drug accumulation. Such reduction can be effected by a variety of drug efflux pumps, of which the most widely studied is P-glycoprotein (Pgp). The first strategy utilized to inhibit Pgp function relied on the identification of non-chemotherapeutic agents as competitors. Other approaches have included the use of hammerhead ribozymes against the MDR-1 gene and MDR-1-targeted ASOs. Although modulation of drug resistance has not yet been proven to be an effective clinical tool, we have learned an enormous amount about drug resistance. Should we succeed, these pioneering basic and clinical studies will have paved the road for future developments.

Reduced inflammation and improved airway expression using helper-dependent adenoviral vectors with a K18 promoter

Efforts have been made to deliver transgenes to the airway epithelia of laboratory animals and humans to develop gene therapy for cystic fibrosis. These investigations have been disappointing due to combinations of transient and low-level gene expression, acute toxicity, and inflammation. We have developed new helper-dependent adenoviral vectors to deliver an epithelial cell-specific keratin 18 expression cassette driving the beta-galactosidase (beta-gal) or human alpha-fetoprotein (AFP) reporter genes. Following intranasal administration to mice, we found that the reporter genes were widely expressed in airway epithelial and submucosal cells, and secreted human AFP was also detectable in serum. In contrast to a first-generation adenoviral vector, inflammation was negligible at doses providing efficient transduction, and expression lasted longer than typically reported-up to 28 days with beta-gal and up to 15 weeks with human AFP. These results suggest that delivery to the airway of helper-dependent adenoviral vectors utilizing a tissue-specific promoter could be a significant advance in the development of gene therapy for cystic fibrosis.

NeuroD-betacellulin gene therapy induces islet neogenesis in the liver and reverses diabetes in mice

To explore induced islet neogenesis in the liver as a strategy for the treatment of diabetes, we used helper-dependent adenovirus (HDAD) to deliver the pancreatic duodenal homeobox-1 gene (Ipf1; also known as Pdx-1) to streptozotocin (STZ)-treated diabetic mice. HDAD is relatively nontoxic as it is devoid of genes encoding viral protein. Mice treated with HDAD-Ipf1 developed fulminant hepatitis, however, because of the exocrine-differentiating activity of Ipf1. The diabetes of STZ mice was partially reversed by HDAD-mediated transfer of NeuroD (Neurod), a factor downstream of Ipf1, and completely reversed by a combination of Neurod and betacellulin (Btc), without producing hepatitis. Treated mice were healthy and normoglycemic for the duration of the experiment (>120 d). We detected in the liver insulin and other islet-specific transcripts, including proinsulin-processing enzymes, beta-cell-specific glucokinase and sulfonylurea receptor. Immunocytochemistry detected the presence of insulin, glucagon, pancreatic polypeptide and somatostatin-producing cells organized into islet clusters; immuno-electron microscopy showed typical insulin-containing granules. Our data suggest that Neurod-Btc gene therapy is a promising regimen to induce islet neogenesis for the treatment of insulin-dependent diabetes.

Pilot trial of intravenous infusion of a replication-selective adenovirus (ONYX-015) in combination with chemotherapy or IL-2 treatment in refractory cancer patients

ONYX-015 is an adenovirus that selectively replicates in p53 dysfunctional or mutated malignant cells. We performed a pilot trial to determine the safety and feasibility of treatment with ONYX-015 delivered intravenously in patients with advanced malignancy. One cohort of five patients received ONYX-015 once a week for 6 weeks at a dose of 2 x 10(12) particles per infusion in combination with weekly infusions of irinotecan (CPT11, 125 mg per week) and 5-fluorouracil (5FU, 500 mg per week). A second cohort of five patients received the combination of ONYX-015 at a dose of 2 x 10(11) particles per week for 6 weeks in combination with interleukin 2 (IL 2, 1.1 x 10(6) units daily via subcutaneous injection for 5 days each week for 4 weeks). Toxicity attributable to ONYX-015 was limited to transient fever. All patients demonstrated elevations in neutralizing antibody titers within 4 weeks of the infusion of ONYX-015. Serum levels of IL-6, IL-10, tumor necrosis factor-alpha, and interferon-gamma increased within 6 hours of viral infusion, suggesting immune activation. This response was more pronounced in the cohort of patients who received 2 x 10(12) particles per infusion. Two patients demonstrated uptake of viral particles in malignant tissue by quantitative PCR. Electron microscopy confirmed selective cytoplasmic viral particles within malignant cells but not within adjacent normal tissue in a third patient. In conclusion ONYX-015 can be administered safely in combination with CPT11, 5FU or low-dose IL 2 and is able to access malignant tissue following intravenous infusion. Further investigation of ONYX-015, possibly with agents that may modulate replication activity, or duration of virus survival, is indicated.

Progress and problems with the use of viral vectors for gene therapy

Gene therapy has a history of controversy. Encouraging results are starting to emerge from the clinic, but questions are still being asked about the safety of this new molecular medicine. With the development of a leukaemia-like syndrome in two of the small number of patients that have been cured of a disease by gene therapy, it is timely to contemplate how far this technology has come, and how far it still has to go.

Intratracheal adenovirus-mediated gene transfer is optimal in experimental lung transplantation

OBJECTIVE

Gene transfer to experimental lung grafts has been shown to reduce ischemia-reperfusion injury and acute rejection. The optimal delivery route should produce high lung expression with no inflammation and minimal systemic expression. The goal of this study was to determine the optimal gene transfer route for use in experimental lung transplantation.

METHODS

F344 rats were injected with 2.9 x 10(10) plaque-forming units of adenovirus vector encoding beta-galactosidase through intratracheal, intravenous, intraperitoneal, or intramuscular delivery routes and killed 48 hours later. Gene expression was measured by means of enzyme-linked immunosorbent assay.

RESULTS

Intratracheal delivery produces significantly greater gene expression in the lung (75,350 +/- 47,288 pg/100 microg of protein, P <.001 vs intravenous, intraperitoneal, and intramuscular routes) and minimal systemic expression (nonsignificant in serum, kidney, liver, spleen, and muscle vs that seen in control animals, P =.016 for heart). Immunohistochemistry staining showed beta-galactosidase expression in the bronchial epithelium of lungs transfected through the intratracheal route with mild inflammation.

CONCLUSIONS

Intratracheal gene transfer provides significant expression in the lung with mild to no inflammation and minimal systemic expression. This delivery strategy has tremendous potential in experimental lung transplant models to reduce ischemia-reperfusion injury and acute allograft rejection and should be investigated further.

Urea-cycle disorders as a paradigm for inborn errors of hepatocyte metabolism

Urea-cycle disorders (UCDs) are a group of inborn errors of hepatocyte metabolism that are caused by the loss of enzymes involved in the process of transferring nitrogen from ammonia to urea, via the urea cycle (UC). Recent genetic analyses of inherited disorders that present with hyperammonemia demonstrate the function of cellular transporters that regulate the availability of UC intermediates. The regulation of UC intermediates, such as arginine, could have far reaching implications on nitric-oxide synthesis and vascular tone. Hence, each UCD and UC-related disorder constitutes a unique gene-nutrient interaction that is crucial for postnatal homeostasis. Recent advances in the diagnosis and management of UCDs include the application of in vivo metabolic-flux measurements. Cumulative morbidity is still high despite dietary and pharmacological therapies and, hence, both cell and gene therapies are being pursued as possible long-term corrective treatments. Although gene-replacement therapy has suffered recent clinical setbacks, new vector developments offer hope for the treatment of cell-autonomous defects of hepatocyte metabolism.

PEGylation of E1-deleted adenovirus vectors allows significant gene expression on readministration to liver

Systemic administration of adenoviral vectors leads to activation of innate and antigen-specific immunity. In an attempt to diminish T and B cell-specific immune responses to E1-deleted adenoviral vectors, capsid proteins were modified with various activated monomethoxypolyethylene glycols (MPEGs). The impact of this modification was studied in a murine model of liver-directed gene transfer in which an E1-deleted adenovirus expressing the lacZ gene was given intravenously. The efficiency of vector transduction of hepatocytes in vivo was not compromised by any of the polymer chemistries. PEGylation of the virus, however, diminished the activation of cytotoxic T lymphocytes and helper T cells of the type 1 subset (Th1 cells) against native viral antigens; neutralizing antibodies to native virus were also diminished. PEGylation prolonged transgene expression and allowed partial readministration with native virus or with a virus PEGylated with a heterologous chemical moiety. Apparently, modification of the capsid leads to a shift in antigenic epitopes because vector readministration was not possible when the immunizing vector had been modified by the same PEGylation chemistry used to modify the second vector. In light of these results, the concept of improving the performance of adenoviral vectors through modification of the capsid with PEG shows promise.