Sustained expression of human apolipoprotein A-I after adenoviral gene transfer in C57BL/6 mice: role of apolipoprotein A-I promoter, apolipoprotein A-I introns, and human apolipoprotein E enhancer

CMV/AAT promoter of MAR-based episomal vector enhanced transgene expression in human hepatic cells

We have previously developed a non-viral episomal vector based on matrix attachment region (MAR) that can facilitate plasmid replication episomally in mammal cells. In this study, we have focused on the development of an alternative tissue specific episomal vector by incorporating into cis-acting elements. We found that AAT promoter demonstrated the highest eGFP expression level in HepG2, Huh-7 and HL-7702 hepatic cells. Furthermore, hCMV enhancer when combined with AAT promoter significantly improved the eGFP expression level in the transfected HepG2 cells. The mean fluorescence intensity of eGFP in hCMV2 group was 1.33 fold, which was higher than that of the control (p < 0.01), followed by the hCMV1 group (1.21 fold). In addition, the percentages of eGFP-expressing cells in hCMV1 and hCMV2 groups were observed to be 49.3% and 57.2%, which were significantly higher than that of the enhancer-devoid control vector (44.3%) (p < 0.05). Moreover, the eGFP protein were up to 3.5 fold and 5.1 fold (p < 0.05), respectively. This observation could be related with the activities of some specific transcription factors (TFs) during the transcriptional process, such as SRF, REL and CREB1. The composite CMV/AAT promoter can be thus used for efficient transgene expression of MAR-based episomal vector in liver cells and as a potential gene transfer tools for the management of liver diseases.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03774-x.

Stent-based delivery of AAV2 vectors encoding oxidation-resistant apoA1

In-stent restenosis (ISR) complicates revascularization in the coronary and peripheral arteries. Apolipoprotein A1 (apoA1), the principal protein component of HDL possesses inherent anti-atherosclerotic and anti-restenotic properties. These beneficial traits are lost when wild type apoA1(WT) is subjected to oxidative modifications. We investigated whether local delivery of adeno-associated viral (AAV) vectors expressing oxidation-resistant apoA1(4WF) preserves apoA1 functionality. The efflux of 3H-cholesterol from macrophages to the media conditioned by endogenously produced apoA1(4WF) was 2.1-fold higher than for apoA1(WT) conditioned media in the presence of hypochlorous acid emulating conditions of oxidative stress. The proliferation of apoA1(WT)- and apoA1(4FW)-transduced rat aortic smooth muscle cells (SMC) was inhibited by 66% ± 10% and 65% ± 11%, respectively, in comparison with non-transduced SMC (p < 0.001). Conversely, the proliferation of apoA1(4FW)-transduced, but not apoA1(WT)-transduced rat blood outgrowth endothelial cells (BOEC) was increased 41% ± 5% (p < 0.001). Both apoA1 transduction conditions similarly inhibited basal and TNFα-induced reactive oxygen species in rat aortic endothelial cells (RAEC) and resulted in the reduced rat monocyte attachment to the TNFα-activated endothelium. AAV2-eGFP vectors immobilized reversibly on stainless steel mesh surfaces through the protein G/anti-AAV2 antibody coupling, efficiently transduced cells in culture modeling stent-based delivery. In vivo studies in normal pigs, deploying AAV2 gene delivery stents (GDS) preloaded with AAV2-eGFP in the coronary arteries demonstrated transduction of the stented arteries. However, implantation of GDS formulated with AAV2-apoA1(4WF) failed to prevent in-stent restenosis in the atherosclerotic vasculature of hypercholesterolemic diabetic pigs. It is concluded that stent delivery of AAV2-4WF while feasible, is not effective for mitigation of restenosis in the presence of severe atherosclerotic disease.

Sustained expression from DNA vectors

DNA vectors have the potential to become powerful medical tools for treatment of human disease. The human body has, however, developed a range of defensive strategies to detect and silence foreign or misplaced DNA, which is more typically encountered during infection or chromosomal damage. A clinically relevant human gene therapy vector must overcome or avoid these protections whilst delivering sustained levels of therapeutic gene product without compromising the vitality of the recipient host. Many non-viral DNA vectors trigger these defense mechanisms and are subsequently destroyed or rendered silent. Thus, without modification or considered design, the clinical utility of a typical DNA vector is fundamentally limited due to the transient nature of its transgene expression. The development of safe and persistently expressing DNA vectors is a crucial prerequisite for its successful clinical application and subsequently remains, therefore, one of the main strategic tasks of non-viral gene therapy research. In this chapter we will describe our current understanding of the mechanisms that can destroy or silence DNA vectors and discuss strategies, which have been utilized to improve their sustenance and the level and duration of their transgene expression.

The liver as a target organ for gene therapy: state of the art, challenges, and future perspectives

The liver is a target for gene therapy of inborn errors of metabolism, of hemophilia, and of acquired diseases such as liver cancer and hepatitis. The ideal gene transfer strategy should deliver the transgene DNA to parenchymal liver cells with accuracy and precision in the absence of side effects. Liver sinusoids are highly specialized capillaries with a particular endothelial lining: the endothelium contains open fenestrae, whereas a basal lamina is lacking. Fenestrae provide a direct access of gene transfer vectors to the space of Disse, in which numerous microvilli from parenchymal liver cells protrude. The small diameter of fenestrae in humans constitutes an anatomical barrier for most gene transfer vectors with the exception of adeno-associated viral (AAV) vectors. Recent studies have demonstrated the superiority of novel AAV serotypes for hepatocyte-directed gene transfer applications based on enhanced transduction, reduced prevalence of neutralizing antibodies, and diminished capsid immune responses. In a landmark clinical trial, hemophilia B was successfully treated with an AAV8 human factor IX expressing vector. Notwithstanding significant progress, clinical experience with these technologies remains very limited and many unanswered questions warrant further study. Therefore, the field should continue to progress as it has over the past decade, cautiously and diligently.

Regression and stabilization of advanced murine atherosclerotic lesions: a comparison of LDL lowering and HDL raising gene transfer strategies

Both reductions in atherogenic lipoproteins and increases in high-density lipoprotein (HDL) levels may affect atherosclerosis regression. Here, the relative potential of low-density lipoprotein (LDL) lowering and HDL raising gene transfer strategies to induce regression of complex murine atherosclerotic lesions was directly compared. Male C57BL/6 LDL receptor (LDLr)^−/− mice were fed an atherogenic diet (1.25% cholesterol and 10% coconut oil) to induce advanced atherosclerotic lesions. A baseline group was killed after 6 months and remaining mice were randomized into a control progression (Adnull or saline), an apolipoprotein (apo) A-I (AdA-I), an LDLr (AdLDLr), or a combined apo A-I/LDLr (AdA-I/AdLDLr) adenoviral gene transfer group and followed-up for another 12 weeks with continuation of the atherogenic diet. Gene transfer with AdLDLr decreased non-HDL cholesterol levels persistently by 95% (p < 0.001) compared with baseline. This drastic reduction of non-HDL cholesterol levels induced lesion regression by 28% (p < 0.001) in the aortic root and by 25% (p < 0.05) in the brachiocephalic artery at 12 weeks after transfer. Change in lesion size was accompanied by enhanced plaque stability, as evidenced by increased collagen content, reduced lesional macrophage content, a drastic reduction of necrotic core area, and decreased expression of inflammatory genes. Elevated HDL cholesterol following AdA-I transfer increased collagen content in lesions, but did not induce regression. Apo A-I gene transfer on top of AdLDLr transfer resulted in additive effects, particularly on inflammatory gene expression. In conclusion, drastic lipid lowering induced by a powerful gene transfer strategy leads to pronounced regression and stabilization of advanced murine atherosclerosis.

Construction of a novel expression cassette for increasing transgene expression in vivo in endothelial cells of large blood vessels

The success of gene therapy hinges on achievement of adequate transgene expression. To ensure high transgene expression, many gene-therapy vectors include highly active virus-derived transcriptional elements. Other vectors include tissue-specific eukaryotic transcriptional elements, intended to limit transgene expression to specific cell types, avoid toxicity and prevent immune responses. Unfortunately, tissue specificity is often accompanied by lower transgene expression. Here, we use eukaryotic (murine) transcriptional elements and a virus-derived posttranscriptional element to build cassettes designed to express a potentially therapeutic gene (interleukin (IL)-10) in large-vessel endothelial cells (ECs) at levels as high as obtained with the cytomegalovirus (CMV) immediate early promoter, while retaining EC specificity. The cassettes were tested by incorporation into helper-dependent adenoviral vectors, and transduction into bovine aortic EC in vitro and rabbit carotid EC in vivo. The murine endothelin-1 promoter showed EC specificity, but expressed only 3% as much IL-10 mRNA as CMV. Inclusion of precisely four copies of an EC-specific enhancer and a posttranscriptional regulatory element increased IL-10 expression to a level at or above the CMV promoter in vivo, while retaining--and possibly enhancing--EC specificity, as measured in vitro. The cassette reported here will likely be useful for maximizing transgene expression in large-vessel EC, while minimizing systemic effects.

Gene therapy for dyslipidemia: a review of gene replacement and gene inhibition strategies

Despite numerous technological and pharmacological advances and more detailed knowledge of molecular etiologies, cardiovascular diseases remain the leading cause of morbidity and mortality worldwide claiming over 17 million lives a year. Abnormalities in the synthesis, processing and catabolism of lipoprotein particles can result in severe hypercholesterolemia, hypertriglyceridemia or low HDL-C. Although a plethora of antidyslipidemic pharmacological agents are available, these drugs are relatively ineffective in many patients with Mendelian lipid disorders, indicating the need for new and more effective interventions. In vivo somatic gene therapy is one such intervention. This article summarizes current strategies being pursued for the development of clinical gene therapy for dyslipidemias that cannot effectively be treated with existing drugs.

The size of endothelial fenestrae in human liver sinusoids: implications for hepatocyte-directed gene transfer

Fenestrae allow the passage of gene transfer vectors from the sinusoidal lumen to the surface of hepatocytes. We have previously shown that the diameter of fenestrae correlates with species and strain differences of transgene expression following intravenous adenoviral transfer. In the current study, we demonstrate that the diameter of fenestrae in humans without liver pathology is 107+/-1.5 nm. This is similar to the previously reported diameter in New Zealand White (NZW) rabbits (103+/-1.3 nm) and is significantly smaller than in C57BL/6 mice (141+/-5.4 nm) and Sprague-Dawley rats (161+/-2.7 nm). We show that the diameter of fenestrae in one male NZW rabbit and its offspring characterized by a more than 50-fold increase of transgene expression after adenoviral gene transfer is significantly (113+/-1.5 nm; P<0.001) larger than in control NZW rabbits. In vitro filtration experiments using polycarbonate filters with increasing pore sizes demonstrate that a relatively small increment of the diameter of pores potently enhances passage of adenoviral vectors, consistent with in vivo data. In conclusion, the small diameter of fenestrae in humans is likely to be a major obstacle for hepatocyte transduction by adenoviral vectors.

Direct comparison of hepatocyte-specific expression cassettes following adenoviral and nonviral hydrodynamic gene transfer

Hepatocytes are a key target for treatment of inborn errors of metabolism, dyslipidemia and coagulation disorders. The development of potent expression cassettes is a critical target to improve the therapeutic index of gene transfer vectors. Here we evaluated 22 hepatocyte-specific expression cassettes containing a human apo A-I transgene following hydrodynamic transfer of plasmids or adenoviral transfer with E1E3E4-deleted vectors in C57BL/6 mice. The DC172 promoter consisting of a 890 bp human alpha(1)-antitrypsin promoter and two copies of the 160 bp alpha(1)-microglobulin enhancer results in superior expression levels compared to constructs containing the 1.5 kb human alpha(1)-antitrypsin promoter, the 790 bp synthetic liver-specific promoter or the DC190 promoter containing a 520 bp human albumin promoter and two copies of the 99 bp prothrombin enhancer. The most potent expression cassette consists of the DC172 promoter upstream of the transgene and two copies of the hepatic control region-1. Minicircles containing this expression cassette induce persistent physiological human apo A-I or human factor IX levels after hydrodynamic transfer. In conclusion, in this comparative study of 22 hepatocyte-specific expression cassettes, the DC172 promoter in combination with two copies of the hepatic control region-1 induces the highest expression levels following hydrodynamic and adenoviral transfer.

In vitro and in vivo analysis of expression cassettes designed for vascular gene transfer

Increasing the level and duration of transgene expression and restricting expression to vascular cells are important goals for clinically useful gene therapy vectors. We evaluated several promoters, enhancers and introns in endothelial, smooth muscle and liver cells in tissue culture and in vivo, comparing local delivery to the carotid artery with intravenous delivery to the liver. A 1800-bp fragment of the oxidized LDL receptor (LOX-1) promoter showed highest in vivo activity in the carotid artery, achieving 39% the activity of the reference cytomegalovirus promoter, with 188-fold greater specificity for carotid artery over liver. An enhancer from the Tie2 gene in combination with the intracellular adhesion molecule-2 promoter improved endothelial specificity of plasmid vectors, increased the expression from adenoviral vectors in cultured endothelial cells and doubled the specificity for carotid artery over liver in vivo. Adding a short intron to expression cassettes increased expression in both endothelial and smooth muscle cells in vitro; however, the eNOS enhancer failed to consistently increase the expression or endothelial specificity of the vector. In conclusion, elements from the LOX-1 promoter and Tie2 enhancer together with an intron can be used to improve vectors for vascular gene transfer.

An efficient and safe herpes simplex virus type 1 amplicon vector for transcriptionally targeted therapy of human hepatocellular carcinomas

Our previous studies have shown that transgene expression could be targeted to proliferating cells when cell cycle transcriptional regulatory elements were incorporated into herpes simplex virus type 1 (HSV-1) amplicon backbone vectors. In the study reported here, we further demonstrated the transcriptional activation of transgene expression in association with the onset of cellular proliferation using the mouse partial hepatectomy model. Moreover, transcriptional regulation could be rendered specific to human hepatocellular carcinoma (HCC) cells by inserting the chimeric gene Gal4/NF-YA under the regulation of the HCC-specific hybrid promoter. The hybrid promoter, which consists of four copies of the apolipoprotein E (ApoE) enhancer element inserted upstream of the human alpha1-antitrypsin(hAAT) promoter, induced an higher level of transcription than other liver-specific promoters such as alpha-fetoprotein (AFP) and albumin (Alb) promoter. As a consequence, the enhancement of tissue-specific expression in the context of Gal4/NF-YA fusion proteins enabled the monitoring of transgene expression using a bioluminescence imaging system. Furthermore, these vectors have been shown to be non-toxic and exhibited potent infectivity for proliferating primary HCC cells and HCC cell lines. Together, these results demonstrated that the new hybrid vectors could provide options for the design of safe and efficient systemic gene therapeutic strategies for human HCC.

Species differences in transgene DNA uptake in hepatocytes after adenoviral transfer correlate with the size of endothelial fenestrae

Sinusoidal fenestrae may restrict the transport of gene transfer vectors according to their size. Using Vitrobot technology and cryo-electron microscopy, we show that the diameter of human adenoviral serotype 5 vectors is 93 nm with protruding fibers of 30 nm. Thus, a diameter of fenestrae of 150 nm or more is likely to be sufficient for passage of vectors from the sinusoidal lumen to the space of Disse and subsequent uptake of vectors in hepatocytes. The average diameter of fenestrae in New Zealand White rabbits (103+/-1.3 nm) was 1.4-fold (P<0.0001) lower than in C57BL/6 mice (141+/-5.4 nm). The percentage of sinusoidal fenestrae with a diameter larger than 150 nm was 10-fold (P<0.01) lower in rabbits (3.2+/-0.24%) than in C57BL/6 mice (32+/-5%), and this resulted in 8.8-fold (P=0.01) lower transgene DNA levels in hepatocytes in rabbits after adenoviral transfer. Injection of N-acetylcysteine combined with transient liver ischemia preceding intraportal transfer in rabbits increased the percentage of sinusoidal fenestrae above 150 nm 2.0-fold (P<0.001) and increased transgene DNA levels in hepatocytes 6.6-fold (P<0.05). In conclusion, species differences in transgene DNA uptake in hepatocytes after adenoviral transfer correlate with the diameter of fenestrae.

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.

Elimination of innate immune responses and liver inflammation by PEGylation of adenoviral vectors and methylprednisolone

Improvement of the therapeutic index of adenoviral gene transfer requires the development of strategies to abrogate adenoviral capsid-induced inflammation and cytokine production. The effect of monomethoxypolyethylene glycol (MPEG) conjugation to adenoviral vectors and of methylprednisolone (MP) on innate immunity, liver inflammation, and thrombocyte counts was evaluated after transfer of 1011 particles of E1/E3/E4- deleted adenoviral vector expressing human apolipoprotein A-I (apoA-I). Gene transfer with unPEGylated vectors induced peak interleukin-6 (IL-6) plasma levels that were 66-fold above baseline levels in C57BL/6 mice. PEGylation combined with 4 mg of MP 6 hr before and at the time of gene transfer suppressed IL-6 plasma levels to baseline values at all time points. This combination resulted in 24-, 28-, 5.9-, 42-, 26-, and 2.5- fold reduced mRNA expression in the liver of monocyte chemoattractant protein-1, macrophage inflammatory protein-2, interferon-inducible protein-10, macrophage inflammatory protein-1 beta, lipopolysaccharide-induced CXC chemokine, and keratinocyte-derived chemokine, respectively; abrogated neutrophil infiltration in the liver; and reduced alanine aminotransferase levels. PEGylation reduced vector uptake in the spleen and in nonparenchymal liver cells. PEGylation also inhibited the development of thrombocytopenia. In conclusion, PEGylation of adenoviral vectors combined with MP administration improves the therapeutic index of adenoviral gene transfer.

AAV serotype-dependent apolipoprotein A-I Milano gene expression

Recent evidence from a double-blind, randomized study showed that treatment with apolipoprotein A-I Milano (ApoA-I Milano) in a complex with phospholipids produced significant regression of the coronary atheroma burden in patients with acute coronary syndromes. We previously showed similar regression of atherosclerosis in an animal model. Here, we examined a viral vector-based gene delivery system as a basis for ApoA-I Milano gene therapy. Comparing levels of expression using combinations of the cytomegalovirus (CMV) promoter in a recombinant serotype 2 adeno-associated virus (rAAV2) linked to ApoA-I Milano or the enhanced green fluorescent protein (EGFP) genes, we found that a promoter construct of two CMV core promoters sharing a CMV enhancer was more active than other combinations or a single CMV promoter. In vivo assessment of this optimal CMV construct using rAAV2 virus particles for intravenous (IV) or intramuscular (IM) routes of delivery produced high circulating levels of ApoA-I Milano protein for extended periods (up to 220 ng/ml at 22 weeks p.i.) by IV delivery while the IM route resulted in a relatively short period of very low-level ApoA-I Milano expression. Since there was no difference in the immune response between the two routes of delivery, we reasoned that tissue tropism might be responsible for this differential gene expression. To explore this possibility, we investigated the effect of different AAV serotypes on ApoA-I Milano gene expression in vivo. It found that rAAV1-mediated expression of ApoA-I Milano was approximately 15- and 9-fold higher than rAAV2 and rAAV5, respectively when IM injection routes were compared while all three AAV serotypes produced substantial levels of ApoA-I Milano expression from IV injection. These studies demonstrate that by modifying the promoter and serotype, increases in the efficiency of AAV-directed transgene expression could be achieved and support the potential of AAV-mediated gene therapy.

Versatility of gene therapy vectors through viruses

Several viruses have been engineered for gene therapy applications, and the specific properties of each viral vector have been exploited to target a variety of inherited and acquired diseases. Preclinical and clinical studies demonstrated that viral vectors are highly versatile tools capable of efficient transfer of foreign genetic information into almost all cell types and tissues. Gene therapy applications depend on vector characteristics, such as host range, cell- or tissue-specific targeting, genome integration, efficiency and duration of transgene expression, packaging capacity, and suitability for scale-up production. This review discusses the advances in the development of viral vectors, with particular emphasis on how knowledge of virus biology has been exploited to design a variety of vectors with improved safety characteristics and efficiency, potentially suitable for a large number of gene therapy applications.

Lipid emulsions potently increase transgene expression in hepatocytes after adenoviral transfer

Elimination of Kupffer cells by cytotoxic clodronate liposomes increases transgene expression in the liver after adenoviral transfer. Here, we demonstrate that empty l-alpha-phosphatidylcholine liposomes block uptake of vectors in the reticuloendothelial cells of the liver and increase human apolipoprotein (apo) A-I (approved gene symbol apo A-I) expression in C57BL/6 (1.3-fold) and Balb/c mice (3.1-fold) to the same extent as clodronate liposomes (1.5- and 3.4-fold, respectively). A similar elevation of human apo A-I levels was induced by the lipid emulsion Intralipid (1.5- and 2.8-fold in C57BL/6 and Balb/c mice, respectively). Not only Kupffer cells but also hepatic sinusoidal endothelial cells (HSEC) constitute the reticuloendothelial cells of the liver. The uptake of adenoviral vectors 1 h after transfer in C57BL/6 mice was 2.9-fold lower in Kupffer cells than in HSEC. In contrast, Kupffer cell uptake in Balb/c mice was 2.6-fold higher than in HSEC. Vector uptake in reticuloendothelial cells of the liver was reduced and transgene expression was increased in splenectomized and Rag2-deficient Balb/c mice but not in splenectomized and Rag1-deficient C57BL/6 mice. In conclusion, lipid emulsions for parenteral clinical use block uptake of adenoviral vectors by the reticuloendothelial cells of the liver and potently increase transgene expression.

The size of sinusoidal fenestrae is a critical determinant of hepatocyte transduction after adenoviral gene transfer

The hepatotropism and intrahepatic distribution of adenoviral vectors may be species dependent. Hepatocyte transduction was evaluated in three rabbit strains after transfer with E1E3E4-deleted adenoviral vectors containing a hepatocyte specific alpha1-antitrypsin promoter-driven expression cassette (AdAT4). Intravenous administration of 4 x 10(12) particles/kg of AdAT4 induced human apo A-I levels above 40 mg/dl in Dutch Belt, but below 1 mg/dl in New Zealand White and Fauve de Bourgogne rabbits. Diameters of sinusoidal fenestrae were significantly (P=0.0014) larger in Dutch Belt (124+/-3.4 nm) than in New Zealand White (108+/-1.3 nm) and Fauve de Bourgogne (105+/-2.6 nm) rabbits, suggesting that a smaller size constitutes a barrier for hepatocyte transduction. Indeed, intraportal transfer preceded by intraportal injection of sodium decanoate, which increases the diameter of sinusoidal fenestrae to 123+/-3.4 nm (P<0.01) in New Zealand White rabbits, increased human apo A-I levels 32- and 120-fold in New Zealand White and Fauve de Bourgogne rabbits, respectively, but did not affect expression in Dutch Belt rabbits. In conclusion, size of sinusoidal fenestrae appears to be a critical determinant of hepatocyte transduction after adenoviral transfer.

Gene therapy progress and prospects: adenoviral vectors

In September 1999, the perceptions of the use of adenoviral (Ad) vectors for gene therapy were altered when a patient exposed via the hepatic artery to a high dose of adenoviral vector succumbed to the toxicity related to vector administration. Appropriately, concerns were raised about continued use of the Ad vector system and, importantly, there were increased efforts to more fully understand the toxicity. Today it is recognized that there is no ideal vector system, and that while Ad vectors are not suitable for all applications, the significant advantages over other vector systems including efficient transduction of a variety of cell types, both quiescent and dividing, make it optimal for certain applications. These include protocols where high levels of short-term expression are sufficient to provide a therapeutic benefit. Potential target applications include therapeutic angiogenesis, administration into immune-privileged sites such as the CNS, or treatments where the adjuvant effect of adenovirus can be of benefit such as cancer vaccines. Broader applicability of Ad vectors will require resolution of toxicity issues. This review will therefore focus on studies conducted over the last 2 years that have advanced our understanding of the toxicity associated with Ad vectors, studies that have employed methods to reduce toxicity and improvements in Ad vectors themselves that will reduce toxicity by one of several mechanisms. These mechanisms include retargeting vector to the tissue of interest, minimizing or eliminating viral gene expression that is thought to result in loss of transduced cells, or by methods that seek to reduce the vector dose required for therapeutic benefit. An area where there remains significant room for improvement is when readministration of vector is required because transgene expression has decreased to background levels.

In vivo ligand-inducible regulation of gene expression in a gutless adenoviral vector system

Transcriptional regulation that is rapid, reversible, and repeatedly inducible would greatly enhance the safety and efficacy of many gene therapy strategies. We developed a chimeric ligand-inducible regulation system based on the human estrogen receptor. This system has two components, the responsive promoter driving expression of the transgene of interest, and the ligand-inducible chimeric transcription factor. The transcription factor is composed of a novel DNA binding domain and a modified estrogen receptor ligand-binding domain. A point mutation in the ligand-binding domain significantly reduces estrogen binding while allowing binding of the estrogen antagonist, tamoxifen. We used a gutless adenoviral vector system and incorporated both components into two separate vectors. A single gutless vector encoding both system components was also generated. The tamoxifen-mediated induciblity of transgene expression of the gutless vector system was compared in vitro and in vivo with the analogous components incorporated into early generation, E1/E2a/E3-deficient adenoviral vectors. In normal mice, both the gutless vector and early generation systems displayed inducibility in the presence of tamoxifen. Importantly, the gutless vector system was inducible to extremely high levels, at least four times over a 2-month period. In contrast, the early generation vector system was inducible only once. Furthermore, the early generation system displayed significant toxicity, as evidenced by extremely high liver enzyme levels, abnormal liver pathology, and rapid loss of vector DNA from the liver, while the gutless vector system displayed minimal toxicity. These data directly demonstrate the improved in vivo function of the tamoxifen-inducible transcriptional regulation system in the context of the gutless adenoviral vectors.

Defining the success of cardiac gene therapy: how can nuclear imaging contribute?

Gene therapy is a promising modality for the treatment of various cardiovascular diseases such as ischaemia, heart failure, restenosis after revascularisation, hypertension and hyperlipidaemia. An increasing number of approaches are moving from experimental and preclinical validation to clinical application, and several multi-centre trials are currently underway. Despite the rapid progress in cardiac gene therapy, many basic tools and principles remain under development. Questions with regard to the optimal method for gene delivery in a given situation remain open, as do questions concerning therapeutic efficacy and the time course and magnitude of gene expression in target and remote areas. Nuclear imaging provides valuable tools to address these open issues non-invasively. Functional effects of molecular therapy at the tissue level can be identified using tracers of blood flow, metabolism, innervation or cell death. The use of reporter genes and radiolabelled reporter probes allows for non-invasive assessment of location, magnitude and persistence of transgene expression in the heart and the whole body. Co-expression of a reporter gene will allow for indirect imaging of the expression of a therapeutic gene of choice, and linkage of measures of transgene expression to downstream functional effects will enhance the understanding of basic mechanisms of cardiac gene therapy. Hence, nuclear imaging offers great potential to facilitate and refine the determination of therapeutic effects in preclinical and clinical cardiovascular gene therapy.

Increased low-density lipoprotein oxidation and impaired high-density lipoprotein antioxidant defense are associated with increased macrophage homing and atherosclerosis in dyslipidemic obese mice: LCAT gene transfer decreases atherosclerosis

BACKGROUND

Obesity-associated dyslipidemia in humans is associated with increased low-density lipoprotein (LDL) oxidation. Mice with combined leptin and LDL receptor deficiency are obese and show severe dyslipidemia and insulin resistance. We investigated the association between oxidation of apolipoprotein B-containing lipoproteins, high-density lipoprotein (HDL) antioxidant defense, and atherosclerosis in these mice.

METHODS AND RESULTS

LDL receptor knockout (LDLR-/-), leptin-deficient (ob/ob), double-mutant (LDLR-/-;ob/ob), and C57BL6 mice were fed standard chow. Double-mutant mice had higher levels of non-HDL (P<0.001) and HDL (P<0.01) cholesterol and of triglycerides (P<0.001). They also had higher oxidative stress, evidenced by higher titers of autoantibodies against malondialdehyde-modified LDL (P<0.001). C57BL6 and ob/ob mice had no detectable lesions. Lesions covered 20% of total area of the thoracic abdominal aorta in double-mutant mice compared with 3.5% in LDLR-/- mice (P<0.01). Higher macrophage homing and accumulation of oxidized apolipoprotein B-100-containing lipoproteins were associated with larger plaque volumes in the aortic root of double-mutant mice (P<0.01). The activity of the HDL-associated antioxidant enzymes paraoxonase and lecithin:cholesterol acyltransferase (LCAT) (ANOVA; P<0.0001 for both) was lower in double-mutant mice. Adenovirus-mediated LCAT gene transfer in double-mutant mice increased plasma LCAT activity by 64% (P<0.01) and reduced the titer of autoantibodies by 40% (P<0.01) and plaque volume in the aortic root by 42% (P<0.05) at 6 weeks.

CONCLUSIONS

Dyslipidemia and insulin resistance in obese LDL receptor-deficient mice are associated with increased oxidative stress and impaired HDL-associated antioxidant defense, evidenced by decreased paraoxonase and LCAT activity. Transient LCAT overexpression was associated with a reduction of oxidative stress and atherosclerosis.

Humoral immune response in mice against a circulating antigen induced by adenoviral transfer is strictly dependent on expression in antigen-presenting cells

Adenoviral transfer of human apo A-I in Balb/c mice induces a strong humoral immune response against the transgene product when expression is driven from the ubiquitously active CMV promoter but induces no immune response when driven by the hepatocyte-specific 256-base pair apo A-I promoter. Here the hypothesis was tested, which is that the humoral immune response against the circulating transgene product correlates with its expression in antigen-presenting cells. No humoral immune response was observed after adenoviral transfer of vectors with human apo A-I expression driven by the hepatocyte-specific apo C-II or 1.5-kilobase (kb) human alpha(1)-antitrypsin promoter, but antibodies were induced after transfer with vectors driven by the ubiquitously active U1b promoter and the murine MHCII E beta promoter. A strict correlation was observed between antigen expression in the spleen and the occurrence of an immune response. Coinjection of the 1.5-kb human alpha(1)-antitrypsin and the murine MHCII E beta promoter-driven vectors resulted in a very short-lived humoral immune response against human apo A-I, suggesting that the time course of human apo A-I expression is a critical determinant of the development of tolerance for human apo A-I. High titers of antibodies against human apo A-I after subcutaneous gene transfer with the MHCII E beta promoter-driven vector underscore the potential of this promoter for vaccination purposes. In conclusion, humoral immune response in mice against a circulating antigen induced by adenoviral transfer is strictly dependent on expression in antigen-presenting cells.

Persistent hepatic expression of human apo A-I after transfer with a helper-virus independent adenoviral vector

Gene transfer with 'gutted' vectors is associated with persistent transgene expression and absence of hepatotoxicity, but the requirement of helper viruses hampers efficient production and leads to contamination of viral batches with these helper-viruses. In the present study, gene transfer with a helper-virus independent E(1)/E(3)/E(4)-deleted adenoviral vector induced persistent expression of human apo A-I (200 +/- 16 mg/dl at day 35, 190 +/- 15 mg/dl at 4 months, 170 +/- 16 mg/dl at 6 months) and stable transgene DNA levels (3.5 +/- 0.60 at day 35, 3.3 +/- 0.39 at 4 months, 3.1 +/- 0.47 mg/dl at 6 months) in C57BL/6 mice in the absence of significant toxicity. The vector contained the 1.5 kb human alpha(1)-antitrypsin promoter in front of the genomic human apo A-I sequence and four copies of the human apo E enhancer (hAAT.gA-I.4xapoE) and was deleted in E(1), E(3) and E(4). Reintroduction of E(4) ORF 3 and E(4) ORF 4 in the viral backbone caused a more than four-fold decline of transgene DNA between day 35 and 4 months after transfer both in wild-type and in C57BL/6 SCID and C57BL/6 Rag-1(-/-) mice, indicating that the effect of E(4) ORF 3 and E(4) ORF 4 is independent of a cellular immune response against viral epitopes. Co-injection of an E(1)-deleted vector containing no expression cassette and the E(1)/E(3)/E(4)-deleted vector containing the hAAT.gA-I.4xapoE expression cassette indicated that E(4) gene products destabilize transgene DNA in trans. Gene transfer with an E(1)/E(3)/E(4)-deleted vector containing only E(4) ORF 3 and the hAAT.gA-I.4xapoE expression cassette was associated with transgene DNA decline, but not with hepatotoxicity, indicating that transgene DNA persistence and hepatotoxicity are dissociated processes. After transfer with E(1)/E(3)/E(4)-deleted vectors containing expression cassettes with a different promoter or a different position of the apo E enhancers, transgene DNA levels were less stable than after transfer with the vector containing hAAT.gA-I.4xapoE, indicating that the expression cassette is an important determinant of episomal stability. In conclusion, gene transfer with an E(1)/E(3)/E(4)-deleted vector containing the hAAT.gA-I.4xapoE expression cassette induces persistent expression of human apo A-I in the absence of hepatotoxicity. Transgene DNA turnover is independent of an adaptive cellular immune response against viral epitopes and of hepatotoxicity. E(1)/E(3)/E(4)-deleted vectors containing transgenes under control of the hAAT promoter in combination with four copies of the human apo E enhancer may be suitable for hepatocyte-specific overexpression of transgenes after gene transfer. doi:10.1038/sj.gt.3301824

Effect of promoters and enhancers on expression, transgene DNA persistence, and hepatotoxicity after adenoviral gene transfer of human apolipoprotein A-I

Liver-directed gene transfer offers new perspectives for the treatment of inherited metabolic diseases and disorders of lipoprotein metabolism. Potent expression cassettes for transgenes in the liver may optimize gene transfer efficiency and improve the therapeutic index of gene transfer vectors. An E(1)-deleted adenovirus comprising the hepatocyte specific 256-base pair (bp) human apolipoprotein A-I (apo A-I) promoter, the genomic human apo A-I DNA, and four human apo E enhancers (AdA-I.gA-I.4xapoE) was associated with low hepatotoxicity, high transgene DNA persistence and absence of promoter shut-off, resulting in human apo A-I plasma levels above 100 mg/dl for 35 days in C57BL/6 mice. In the present investigation, the human apo A-I promoter was compared to the murine small nuclear RNA U1b, the human apolipoprotein C-II (apo C-II), and the human alpha(1) antitrypsin (hAAT) promoters and the effect of copy number and position of liver-specific human apo E enhancers in 16 adenoviral constructs was evaluated. The vector containing the 1.5-kb hAAT instead of the apo A-I promoter (AdhAAT.gA-I.4xapoE) induced 3.7-fold (p < 0.01) more human apo A-I reaching plasma levels above 300 mg/dl for 35 days. The composition of the expression cassette was a major determinant of human apo A-I transgene DNA copy number at day 35. Hepatotoxicity after adenoviral gene transfer was dependent on the promoter and the number of enhancers, and was higher with the enhancers in a 5' position. The combination of the hAAT promoter and four copies of the human apo E enhancer appears to be the expression cassette of choice for hepatocyte-specific overexpression of transgenes after gene transfer.

Variables affecting in vivo performance of high-capacity adenovirus vectors

In high-capacity adenovirus (HC-Ad) vectors the size and/or composition of the vector genome influences vector stability during production and the expression profile following gene transfer. Typically, an HC-Ad vector will contain both a gene or an expression cassette and stuffer DNA that is required to balance the final vector genome to a size of between 27 and 36 kb. To gain an improved understanding of factors that may influence gene expression from HC-Ad vectors, we have generated a series of vectors that carry different combinations of human alpha-1 antitrypsin (hAAT) expression constructs and stuffer DNAs. Expression in vitro did not predict in vivo performance: all vectors expressed hAAT at similar levels when tested in cell culture. Hepatic expression was evaluated following in vivo gene transfer in C57BL/6J mice. hAAT levels obtained from genomic DNA were significantly higher than levels achieved with small cDNA expression cassettes. Expression was independent of the orientation and only marginally influenced by the location of the expression cassette within the vector genome. The use of lambda stuffer DNA resulted in low-level but stable expression for at least 3 months when higher doses were applied. A potential matrix attachment region element was identified within the hAAT gene and caused a 10-fold increase in expression when introduced in an HC-Ad vector genome carrying a phosphoglycerate kinase (pgk) hAAT cDNA construct. We also illustrate the influence of the promoter on anti-hAAT antibody formation in C57BL/6J mice: a human cytomegalovirus but not a pgk promoter resulted in an anti-hAAT antibody response. Thus, the overall design of HC-Ad vectors may significantly influence amounts and duration of gene expression at different levels.

Adenovirus-Mediated Gene Transfer of Human Platelet-Activating Factor–Acetylhydrolase Prevents Injury-Induced Neointima Formation and Reduces Spontaneous Atherosclerosis in Apolipoprotein E–Deficient Mice

Background —Atherosclerosis is characterized by an early inflammatory response involving proinflammatory mediators such as platelet-activating factor (PAF)-like phospholipids, which are inactivated by PAF-acetylhydrolase (PAF-AH). The effect of adenovirus-mediated expression of PAF-AH on injury-induced neointima formation and spontaneous atherosclerosis was studied in apolipoprotein E–deficient mice.

Methods and Results —Intravenous administration of an adenovirus (5×10 8 plaque-forming units) directing liver-specific expression of human PAF-AH resulted in a 3.5-fold increase of plasma PAF-AH activity at day 7 ( P <0.001); this was associated with a 2.4- and 2.3-fold decrease in malondialdehyde-modified LDL autoantibodies and the lysophosphatidylcholine/phosphatidylcholine ratio, respectively ( P <0.001 for both). Non-HDL and HDL cholesterol levels in PAF-AH –treated mice were similar to those of control virus-treated mice. Seven days after virus injection, endothelial denudation of the common left carotid artery was induced with a guidewire. Neointima formation was assessed 18 days later. PAF-AH gene transfer reduced oxidized lipoproteins by 82% ( P <0.001), macrophages by 69% ( P =0.006), and smooth muscle cells by 84% ( P =0.002) in the arterial wall. This resulted in a 77% reduction ( P <0.001) of neointimal area. Six weeks after adenovirus-mediated gene transfer, spontaneous atherosclerotic lesions in the aortic root were analyzed. PAF-AH gene transfer reduced atherosclerotic lesions by 42% ( P =0.02) in male mice, whereas a nonsignificant 14% reduction was observed in female mice. Basal and PAF-AH activity after gene transfer were higher in male mice than in female mice ( P =0.01 and P =0.04, respectively).

Conclusions —Gene transfer of PAF-AH inhibited injury-induced neointima formation and spontaneous atherosclerosis in apolipoprotein E–deficient mice. Our data indicate that PAF-AH, by reducing oxidized lipoprotein accumulation, is a potent protective enzyme against atherosclerosis.

Sustained expression of human apo A-I following adenoviral gene transfer in mice

Elevation of HDL cholesterol, following adenoviral apolipoprotein A-I (apo A-I) gene transfer, may delay or revert ischemic cardiovascular disease, provided transgene expression is persistent. The choice of promoter may have significant impact on persistence of transgene expression. Human apo A-I expression was compared after adenoviral gene transfer with a cytomegalovirus promoter (CMV) driven construct (AdCMV/A-I.gA-I) and with a construct (AdA-I.gA-I.4xapoE) containing the endogenous 256 bp apo A-I promoter (A-I), the genomic human apo A-I DNA (gA-I) and 4 human apo E enhancers (4xapoE) in three different mouse strains: C57BL/6, Balb/c and Fvb. After gene transfer with 5 x 10(8) p.f.u. of AdCMV/A-I.gA-I, human apo A-I expression was observed for 35 days in C57BL/6 mice, but declined below 1 mg/dl within 14 days both in Balb/c and Fvb mice, due to a strong humoral immune response against human apo A-I. In contrast, after transfer with AdA-I.gA-I.4xapoE, human apo A-I expression persisted for 6 months in all three strains and no antibodies against human apo A-I occurred in Fvb or Balb/c mice. Human apo A-I transgene DNA level 35 days after transfer with AdA-I.gA-I.4xapoE was 4.6- to 5.5-fold higher than with AdCMV/A-I.gA-I. CMV promoter attenuation occurred in all three strains, but promoter attenuation was not observed in any strain after transfer with AdA-I.gA-I.4xapoE. In conclusion, gene transfer with AdA-I.gA-I.4xapoE is associated with absence of an immune response against human apo A-I, improved transgene DNA persistence and absence of promoter shut-off, resulting in human apo A-I expression for up to 6 months in three different mouse strains. Possibly, the absence of human apo A-I expression in antigen-presenting cells with the liver-specific apo A-I promoter containing construct abrogated the immune response against human apo A-I in Balb/c and Fvb mice.