NIDDK Workshop on AAV Vectors: Gene Transfer into Quiescent Cells

Phenotypic correction of a mouse model for primary hyperoxaluria with adeno-associated virus gene transfer

Primary hyperoxaluria type I (PH1) is an inborn error of metabolism caused by deficiency of the hepatic enzyme alanine-glyoxylate aminotransferase (AGXT or AGT) which leads to overproduction of oxalate by the liver and subsequent urolithiasis and renal failure. The current therapy largely depends on liver transplantation, which is associated with significant morbidity and mortality. To explore an alternative treatment, we used somatic gene transfer in a mouse genetic model for PH1 (Agxt1KO). Recombinant adeno-associated virus (AAV) vectors containing the human AGXT complementary DNA (cDNA) were pseudotyped with capsids from either serotype 8 or 5, and delivered to the livers of Agxt1KO mice via the tail vein. Both AAV8-AGXT and AAV5-AGXT vectors were able to reduce oxaluria to normal levels. In addition, treated mice showed blunted increase of oxaluria after challenge with ethylene glycol (EG), a glyoxylate precursor. In mice, AGT enzyme activity in whole liver extracts were restored to normal without hepatic toxicity nor immunogenicity for the 50 day follow-up. In summary, this study demonstrates the correction of primary hyperoxaluria in mice treated with either AAV5 or AAV8 vectors.

Quiescent subpopulations of human CD34-positive hematopoietic stem cells are preferred targets for stable recombinant adeno-associated virus type 2 transduction

We have previously demonstrated recombinant adeno-associated viral (rAAV) transduction of human CD34(+) hematopoietic stem cells (HSCs) capable of serial engraftment in vivo. Here we evaluated the capacity of rAAV2 to mediate gene transfer into nondividing, quiescent, primitive CD34(+) cells subdivided on the basis of metabolic, mitotic, and phenotypic properties. Results revealed that CD34(+)CD38() marrow cells are the most quiescent, exist primarily in G(0) at isolation and are the only population to remain nondividing during the entire exposure to free rAAV. Despite significant differences in the extended clonogenic capacities of CD34(+) subsets in stromal cultures, the frequency of rAAV marking of colonies derived from primitive progenitors was similar in all three populations, suggesting that both primitive and more differentiated progenitors were initially transduced at equal levels. After transduction, episomal and integrated rAAV genomes were detected in all CD34(+) subsets. However, the more quiescent cells displayed higher levels of integrated rAAV than did rapidly dividing cells. Importantly, stable long-term integration was observed only in the most primitive, quiescent CD34(+)CD38(-) subset, indicating that this HSC compartment comprises the preferred substrate for stable rAAV2 transduction. Previously described rate limitations to transgene expression were observed in transduced CD34(+) cells and could be overcome by tyrphostin pretreatment, which resulted in augmented second-strand synthesis. These results represent the first demonstration of rAAV-mediated gene transfer to primitive, quiescent human CD34(+)CD38(-) stem cells and reveal that nondividing CD34(+)CD38(-) HSCs are the optimal CD34(+) targets for rAAV transduction.

Gene therapy for inherited lung disorders: an insight into pulmonary defence

This review summarizes the latest developments in viral and nonviral gene delivery systems to the lung, and the problems that have to be overcome. Gene delivery has the potential to offer effective treatment to patients with life-threatening lung diseases such as cystic fibrosis and alpha(1)-antitrypsin deficiency, and could modify gene-environment relationships in asthma and other respiratory diseases. Phase I clinical trials conducted in the early 1990s showed that in principle gene transfer to the lung was safe. Although the preliminary results gave encouraging laboratory data, gene expression from viral or nonviral gene delivery systems was too inefficient or transient to offer clinical benefit. Initial optimism gave way to the realization that gene therapy to the lung was unlikely to be straightforward. The host innate and acquired immune system, which protects against infection from inhaled bacteria and viruses, represents a major barrier to successful gene transfer to the lung. A better understanding of the immunological barriers which exist in the lung may allow the development of pharmacological and/or immunological agents that modulate the host immune system to allow for a more continuous and regulated level of gene expression following gene transfer.

Cellular pathophysiology of portal hypertension and prospects for management with gene therapy

In summary, regulation of sinusoidal blood flow in normal and injured liver involves structural, cellular, and humoral components. Available data suggest that stellate cells, resident perisinusoidal mesenchymal cells with a histologic orientation in the sinusoid analogous to [figure: see text] vasoregulatory pericytes, modulate sinusoidal blood flow. This regulation by stellate cells is most evident in the context of liver injury but may apply also to the normal liver. The endothelin and NO systems are important in modulating stellate cell contractility, and their degree of equilibrium is significant in determining the level of local intrahepatic resistance, especially in the injured liver. Manipulation of either or both of these systems is feasible and effective in experimental models. Such findings have obvious clinical implications and are expected to set the [figure: see text] stage for novel gene therapy approaches for treatment of patients with portal hypertension.

AAV vectors: is clinical success on the horizon?

Potential applications and impact of the adeno-associated virus (AAV) as a gene transfer vector have expanded rapidly in the last decade. Recent advances in the production of high-titer purified rAAV vector stocks have made the transition to human clinical trials a reality in the last moments of the millenium. Production improvements will be complemented in the coming years with understanding of and innovations in the targeting and packaging of rAAV, the design of transgene cassettes, and the host immune response to the vectors. These expected areas of progress are discussed, with special attention to clinical applications for which rAAV vectors may help close the gap towards successful gene therapy. Gene Therapy (2000) 7, 24-30.

Immune responses to adenovirus and adeno-associated virus in humans

Vectors based on human adenovirus (Ad) and adeno-associated virus (AAV) are being evaluated for human gene therapy. The response of the host to the vector, in terms of antigen-specific immunity, will play a substantial role in clinical outcome. We have surveyed cohorts of normal subjects and cystic fibrosis patients for pre-existing immunity to these viruses, caused by naturally acquired infections. A number of humoral and cellular assays to adenovirus serotype 5 (Ad5) and adeno-associated virus serotype 2 (AAV2) were performed from serum and peripheral blood mononuclear cells. Virtually all subjects had Ig to Ad5 although only 55% of these antibodies neutralized virus (NAB). Approximately two of three patients demonstrated CD4+ T cells that proliferated to Ad antigens of which most were of the TH1 subset, based on cytokine secretion. A substantially different pattern of immune responses was observed to AAV2. Although virtually all patients had Ig to AAV2, most of these antibodies were not neutralizing (32% NAB) and only 5% of patients had peripheral blood lymphocytes that proliferated in response to AAV2 antigens. These studies demonstrate marked heterogeneity in pre-existing immunity to Ad5 and AAV2 in human populations. The impact of these findings on outcome following gene therapy will require further study.

Adenoassociated virus-mediated transfer of a functional water channel into salivary epithelial cells in vitro and in vivo

Aquaporin 1 (AQP1) is the archetypal member of a family of integral membrane proteins that function as water channels. Previously we have shown that this protein can be expressed transiently from a recombinant adenovirus (AdhAQP1) in vitro in different epithelial cell lines, and in vivo in rat submandibular glands. In the present study we have constructed a recombinant adenoassociated virus (rAAV) containing the human aquaporin 1 gene (rAAVhAQP1). rAAVhAQP1 was produced at relatively high titers. Approximately 10(11)-10(12) particles/ml and approximately 10(8)-10(9) transducing units/ml. We show that the rAAVhAQP1 can transduce in vitro four epithelial cell lines of different origins, at a level sufficient to detect the recombinant hAQP1 protein by either Western blot or confocal microscopic analysis. The recombinant hAQP1 was correctly targeted to the plasma membranes in all cell lines. Function of the recombinant hAQP1 was measured as fluid flow, in response to an osmotic gradient, across a monolayer of transduced epithelial cells. The data show that even at a low level of transduction, typically approximately 10% of the cells in the monolayer, transepithelial fluid movement is enhanced about threefold above basal levels. In addition, we report that rAAVhAQP1 can transduce epithelial cells in the salivary glands and liver of mice in vivo. These results suggest that rAAVs may be useful gene transfer vectors to direct the production of functional transgenes in salivary epithelial cell types.

Adeno-associated virus vector-mediated transgene integration into neurons and other nondividing cell targets

The site-specific integration of wild-type adeno-associated virus (wtAAV) into the human genome is a very attractive feature for the development of AAV-based gene therapy vectors. However, knowledge about integration of wtAAV, as well as currently configured recombinant AAV (rAAV) vectors, is limited. By using a modified Alu-PCR technique to amplify and sequence the vector-cellular junctions, we provide the first direct evidence both in vitro and in vivo of rAAV-mediated transgene integration in several types of nondividing cells, including neurons. This novel technique will be highly useful for further delineating the mechanisms underlying AAV-mediated integration, including issues of frequency, site preference, and DNA rearrangement in human as well as animal cells. Results from these studies should be beneficial for the development of the next generation of gene delivery vectors.

Gene therapy for prostate cancer

Prostate cancer remains one of the most significant challenges in clinical oncology, yet present therapies provide incomplete treatment in many cases. Innovative and practical gene therapy-based approaches will prove invaluable in filling the gaps that now exist in the treatment of localized and distant disease. Although multiple potential strategies have been developed, early clinical trials in prostate cancer gene therapy are now in the phase I/II stage of development. Novel preclinical and early clinical data should be considered optimistically, yet cautiously, as this field emerges from its infancy.

Membrane-associated heparan sulfate proteoglycan is a receptor for adeno-associated virus type 2 virions

The human parvovirus adeno-associated virus (AAV) infects a broad range of cell types, including human, nonhuman primate, canine, murine, and avian. Although little is known about the initial events of virus infection, AAV is currently being developed as a vector for human gene therapy. Using defined mutant CHO cell lines and standard biochemical assays, we demonstrate that heparan sulfate proteoglycans mediate both AAV attachment to and infection of target cells. Competition experiments using heparin, a soluble receptor analog, demonstrated dose-dependent inhibition of AAV attachment and infection. Enzymatic removal of heparan but not chondroitin sulfate moieties from the cell surface greatly reduced AAV attachment and infectivity. Finally, mutant cell lines that do not produce heparan sulfate proteoglycans were significantly impaired for both AAV binding and infection. This is the first report that proteoglycan has a role in cellular attachment of a parvovirus. Together, these results demonstrate that membrane-associated heparan sulfate proteoglycan serves as the viral receptor for AAV type 2, and provide an explanation for the broad host range of AAV. Identification of heparan sulfate proteoglycan as a viral receptor should facilitate development of new reagents for virus purification and provide critical information on the use of AAV as a gene therapy vector.

In vivo gene delivery to the liver using reconstituted chylomicron remnants as a novel nonviral vector

Lipoproteins are emulsion particles that consist of lipids and apolipoproteins. Their natural function is to transport lipids and/or cholesterol to different tissues. We have taken advantage of the hydrophobic interior of these natural emulsions to solubilize DNA. Negatively charged DNA was first complexed with cationic lipids containing a quaternary amine head group. The resulting hydrophobic complex was extracted by chloroform and then incorporated into reconstituted chylomicron remnant particles ( approximately 100 nm in diameter) with an efficiency approximately 65%. When injected into the portal vein of mice, there were approximately 5 ng of a transgene product (luciferase) produced per mg of liver protein per 100 microg injected DNA. This level of transgene expression was approximately 100-fold higher than that of mice injected with naked DNA. However, such a high expression was not found after tail vein injection. Histochemical examination revealed that a large number of parenchymal cells and other types of cells in the liver expressed the transgene. Gene expression in the liver increased with increasing injected dose, and was nearly saturated with 50 microg DNA. At this dose, the expression was kept at high level in the liver for 2 days and then gradually reduced and almost disappeared by 7 days. However, by additional injection at day 7, gene expression in the liver was completely restored. By injection of plasmid DNA encoding human alpha1-antitrypsin, significant concentrations of hAAT were detected in the serum of injected animals. This is the first nonviral vector that resembles a natural lipoprotein carrier.

The Rep78 gene product of adeno-associated virus (AAV) self-associates to form a hexameric complex in the presence of AAV ori sequences

The Rep78 and Rep68 proteins of adeno-associated virus (AAV) are replication initiator proteins that bind the viral replicative-form origin of replication, nick the origin in a site- and strand-specific fashion, and mediate vectorial unwinding of the DNA duplex via an ATP-dependent helicase activity, thus initiating a strand displacement mechanism of viral DNA replication. Genetic and biochemical studies have identified Rep mutants that demonstrate a trans-dominant negative phenotype in vitro and in vivo, suggesting the possibility that multimerization of Rep is essential for certain replicative functions. In this study, we have investigated the ability of the largest of the Rep proteins, Rep78, to self-associate in vitro and in vivo. Self-association of Rep78 in vivo was demonstrated through the use of a mammalian two-hybrid system. Rep-Rep protein interaction was confirmed in vitro through coimmunoprecipitation experiments with a bacterially expressed maltose-binding protein-Rep78 fusion protein in combination with [35S]methionine-labeled Rep78 synthesized in a coupled in vitro transcription-translation system. Mapping studies with N- and C-terminal truncation mutant forms of Rep indicate that amino acid sequences required for maximal self-association occur between residues 164 and 484. Site-directed mutagenesis identified two essential motifs within this 321-amino-acid region: (i) a putative alpha-helix bearing a 3,4-hydrophobic heptad repeat reminiscent of those found in coiled-coil domains and (ii) a previously recognized nucleoside triphosphate-binding motif. Deletion of either of these regions from the full-length polypeptide resulted in severe impairment of Rep-Rep interaction. In addition, gel filtration chromatography and protein cross-linking experiments indicated that Rep78 forms a hexameric complex in the presence of AAV ori sequences.

HSV/AAV hybrid amplicon vectors extend transgene expression in human glioma cells

Novel hybrid vectors, which incorporate critical elements of both herpes simplex virus type 1 (HSV-1) amplicon vectors and adeno-associated virus (AAV) vectors, are able to sustain transgene expression in dividing glioma cells for over 2 weeks. These vectors combine the high infectibility and large transgene capacity of HSV-1 vectors with the potential for episomal amplification and chromosomal integration of AAV vectors. The hybrid vectors contain the HSV-1 origin of DNA replication, oriS, and the DNA cleavage/packaging signal, pac, which allow amplicon replication and packaging in HSV-1 virions. The lacZ reporter gene under control of the CMV IE1 promoter is flanked by AAV inverted terminal repeat (ITR) sequences, which facilitate replication and genomic integration of this cassette in the host cell nucleus. Constructs were generated with or without the AAV rep gene (rep+ and rep-) to assess its importance in extending transgene expression. Expression of Rep proteins was confirmed by Western blot analysis. An HSV-1 amplicon construct containing the reporter gene, but no AAV sequences, was used as a control. Constructs were packaged into HSV-1 virions with or without helper virus and these vector stocks were used to infect human U87 glioma cells in culture. The hybrid vectors supported transgene retention and expression for over 2 weeks, whereas the control amplicon vector lost the transgene after 10 days. Expression was somewhat longer for the rep+ as compared to the rep- hybrid vectors. Toxicity due to the HSV-1 helper virus was eliminated using helper virus-free amplicon vector stocks. Transgene constructs could also be packaged in AAV virions, using AAV and adenovirus or HSV-1 helper functions. These HSV/AAV hybrid vectors should allow long-term, nontoxic gene delivery of DNA constructs to both dividing and nondividing cells.