Production of first generation adenovirus vectors: a review

Adenoviral vector platform for transduction of constitutive and regulated tricistronic or triple-transcript transgene expression in mammalian cells and microtissues

BACKGROUND

Adenoviral particles can efficiently transduce a broad spectrum of cell types, so they are widely used in basic research and clinical trials.

METHODS

We have developed a novel adenoviral vector platform for delivery of constitutive or streptogramin-inducible expression of up to three therapeutic transgenes into a variety of murine and human cell lines, primary cells and microtissues.

RESULTS

Coordinated expression of three independent transgenes in a compact genetic format was achieved by two different expression configurations: (i) The multicistronic expression format consisting of a single constitutive (simian virus 40 promoter, P(SV40); murine or human cytomegalovirus immediate-early promoter, P(mCMV), P(hCMV)) or regulated (streptogramin-inducible) promoters (P(PIR)ON2) driving the expression of a single multicistronic transcript of which the first cistron is translated in a cap-dependent manner and the two subsequent ones by internal ribosome entry site (IRES)-mediated translation initiation. (ii) The triple-transcript expression configuration, in which a combination of well-established (P(SV40), P(hCMV), P(mCMV)) and novel synthetic constitutive promoters (P(GTX)) control transcription of three expression units. The constitutive multigene expression design enabled coordinated high-level expression of the Bacillus stearothermophilus-derived secreted alpha-amylase (SAMY), the human vascular endothelial growth factor 121 (VEGF(121)) and the human placental secreted alkaline phosphatase (SEAP) in monolayer populations and microtissues of Chinese hamster ovary cells (CHO-K1), human fibrosarcoma cells (HT-1080), primary neonatal rat cardiomyocytes (NRCs) and primary human aortic fibroblasts (HAFs). Streptogramin-inducible tricistronic SAMY-VEGF(121)-SEAP expression provided excellent regulation performance-high-level induction in the presence of the streptogramin antibiotic pristinamycin I (PI), near-undetectable basal expression in the absence of PI, optimal adjustability and perfect reversibility-in all cell types, in particular in NRCs and NRC-derived myocardial microtissues.

CONCLUSIONS

Triple-transcript and tricistronic expression configurations conserve the DNA packaging capacity of the size-constrained viral transduction systems and enable coordinated and regulated expression of up to three therapeutic transgenes for concerted clinical interventions in future gene therapy scenarios.

Decreased Replication Ability of E1-Deleted Adenoviruses Correlates with Increased Brain Tumor Malignancy

E1 region replacement adenoviruses are replication defective and are propagated in cells providing adenovirus E1A and E1B proteins. Although they are being developed for antitumor therapies, the proliferative behaviors of these viruses in normal brain tissues or in brain tumors are unknown. To address this, freshly cultured cells from normal human brain and common brain tumors (astrocytomas and meningiomas) were infected using wild-type species C adenoviruses and adenoviruses missing E1A (H5dl312) or E1A plus E1B (H5dl434). Viral DNA replication, late viral protein expression, and production of infectious progeny were characterized. Wild-type adenoviruses grew efficiently in normal brain and brain tumor cells. In comparison, E1-deleted adenovirus DNA replication was delayed and lower in cells derived from normal brain tissues, meningiomas, and low-grade astrocytomas. However, in contrast, E1-deleted adenovirus DNA replication did not occur or was extremely low in cells derived from malignancy grade III and IV astrocytic tumors. Because wild-type adenoviruses infected and replicated in all cells, the malignancy grade-based differential E1-deleted adenovirus DNA replication was not explained by differential virus uptake. Infectious H5dl312 and H5dl434 production correlated with viral DNA replication. Compared with a 5-day average for wild-type infections, advanced cytopathology was noted approximately 4 weeks after H5dl312 or H5dl434 infection of meningioma, astrocytoma, and normal brain cells. Cytopathology was not observed after H5dl312 or H5dl434 infection of glioblastoma, anaplastic astrocytoma, and gliosarcoma cells. Because of this tumor grade-based differential growth, the E1-deleted adenoviruses may represent novel tools for studies of brain tumor malignancy.

Gene therapy: twenty-first century medicine

Broadly defined, the concept of gene therapy involves the transfer of genetic material into a cell, tissue, or whole organ, with the goal of curing a disease or at least improving the clinical status of a patient. A key factor in the success of gene therapy is the development of delivery systems that are capable of efficient gene transfer in a variety of tissues, without causing any associated pathogenic effects. Vectors based upon many different viral systems, including retroviruses, lentiviruses, adenoviruses, and adeno-associated viruses, currently offer the best choice for efficient gene delivery. Their performance and pathogenicity has been evaluated in animal models, and encouraging results form the basis for clinical trials to treat genetic disorders and acquired diseases. Despite some initial success in these trials, vector development remains a seminal concern for improved gene therapy technologies.

Seamless cloning and gene fusion

Gene fusion technology is a key tool in facilitating gene function studies. Hybrid molecules in which all the components are joined precisely, without the presence of intervening and unwanted extraneous sequences, enable accurate studies of molecules and the characterization of individual components. This article reviews situations in which seamlessly fused genes and proteins are required or desired and describes molecular approaches that are available for generating these hybrid molecules.

An adenovirus type 5 (Ad5) amplicon-based packaging cell line for production of high-capacity helper-independent deltaE1-E2-E3-E4 Ad5 vectors

Production of multiply deleted adenoviral (Ad) vectors with increased cloning capacity and reduced immunogenicity to adenovirus gene products requires the concomitant generation of efficient packaging cell lines. High expression levels of the complementing genes must be achieved in a coordinated fashion with viral replication. This is a particularly difficult task in light of the significant cytotoxicity displayed by adenoviral proteins. To this end, we developed a novel adenovirus-based amplicon with an Epstein-Barr virus origin of replication, Ad type 5 (Ad5) inverted terminal repeats, all Ad5 early region 2 (E2) genes, and the early region 4 (E4) open reading frame 6 (ORF6) under the control of a tetracycline-dependent promoter. The amplicon (pE2) was stably maintained in multiple copies in the nuclei of 293 cells stably expressing the Epstein-Barr virus nuclear antigen 1 (EBNA1) and allowed replication as a linear DNA upon induction of E2 and ORF6 gene expression. A stable cell line (2E2) was generated by introducing pE2 into 293EBNATet cells expressing the tetracycline-dependent transcriptional silencer and the reverse Tet transactivator (rtTA2). Upon induction with doxicycline, 2E2 cells produced higher levels of polymerase, precursor terminal protein (pTP), and DNA binding protein than noninduced 2E2 cells infected with first-generation Ad5 vector and supported efficient amplification of a multiply deleted Ad5 vector lacking E1, E2, E3, and E4 genes (Ad5DeltaE(1-4)). The high cloning capacity of Ad5DeltaE(1-4) (up to 12.6 kb) was exploited to construct a vector encoding the entire hepatitis C virus (HCV) polyprotein. Infection of HeLa cells by the resulting vector showed high levels of correctly processed HCV proteins.

Factors involved in the sensitivity of different hematopoietic cell lines to infection by subgroup C adenovirus: implication for gene therapy of human lymphocytic malignancies

Gene transfer approaches using viruses such human adenovirus (HAdV) may provide an alternative treatment for diseases involving hematopoietic cells. Better understanding of the cellular mechanisms by which the HAdV introduces DNA into these cells should help in vector design. We examined HAdV intracellular delivery in several cell lines including B and T lymphocytes. We demonstrated that HAdV resistance in most B lymphocytes is the result of moderate HAdV uptake. In contrast, high levels of coxsackie and HAdV receptor (hCAR) are expressed on the surface of HSB2 (T cells), allowing efficient binding and uptake but no transgene expression, probably because of deficient endosomolysis and subsequent exocytose. This work demonstrates the existence of hCAR-dependent and -independent endocytic route in hematopoietic cells. Moreover, it precises the intracellular barriers to be overcome by HAdV in such cells to be infectious and gives previous information's to design new vectors for gene transfer.

Suicide gene therapy with HSV-TK in pancreatic cancer has no effect in vivo in a mouse model

AIM

To study in vivo whether pancreatic cancer tumour growth and metastasis can be modified by a gene construct with HSV-TK suicide gene and IL2 co-expression.

METHODS

Seventy-eight female SCID mice were i.p. inoculated with retrovirally transduced or control MIA PaCa 2, CAPAN-1 and PANC-1 cell lines. The animals were then randomly selected for saline or ganciclovir (GCV) treatment from the second week, for a total of two weeks.

RESULTS

Most inoculated mice developed tumour nodules and spleen metastases. The liver was colonized by control CAPAN-1 and MIA PaCa 2, but not by PANC-1. Tumours in transduced MIA PaCa 2 cell injected mice were smaller, and in transduced CAPAN-1 injected mice larger, than in control-inoculated mice. There were increased pancreatic and decreased spleen metastases from transduced CAPAN-1, and diminished liver involvement from transduced MIA PaCa 2. No differences were found between mice inoculated with transduced and control PANC-1 cell lines. GCV treatment had no effect on tumour's size or metastases.

CONCLUSIONS

The HSV-TK suicide gene does not confer GCV sensitivity to pancreatic cancer in this in vivo model. Different pancreatic cancer cell lines cause different growth and metastasis patterns after inoculation in SCID mice, possibly because of variations in their inherent characteristics. The different effects of our vector on cell growth and metastasis may be attributable to the effects of the immunostimulatory cytokine IL2.

Adenovirus vectors: biology, design, and production

The use of adenovirus as a gene transfer vehicle arose from early reports of recombinant viruses carrying heterologous DNA fragments. Adenovirus vectors offer many advantages for gene delivery: they are easy to propagate to high titers, they can infect most cell types regardless of their growth state, and in their most recent embodiments they can accommodate large DNA inserts. In this chapter, the development of adenovirus vectors is reviewed, from the use of so-called first-generation, E1-deleted viruses to the latest generation high-capacity, helper-dependent vectors. Examples of their use in the clinic are described, as are the current areas in which improvements to these vectors are being explored.

Generating MHC Class II+/Ii- phenotype after adenoviral delivery of both an expressible gene for MHC Class II inducer and an antisense Ii-RNA construct in tumor cells

Tumor cells engineered by gene transduction to be MHC Class II+/Ii- are novel APCs capable of presenting endogenous tumor antigen epitopes to activate T helper cells. The MHC Class II+/Ii- tumor cell phenotype is created by transfecting genes for either CIITA or IFN-gamma, and inhibiting induced Ii mRNA by an Ii reverse gene construct (Ii-RGC). Adenoviral vectors are preferred for the delivery of such genes because of high infection efficiency and ubiquity of the adenoviral receptor on many cell types and tumors. Here we show that at 5 MOI (multiplicity of infection), recombinant adenoviruses with CIITA or IFN-gamma genes converted virtually all MC-38 colon adenocarcinoma cells and Renca renal carcinoma cells in culture to MHC Class II+/Ii+ cells. A single recombinant adenovirus with both genes for IFN-gamma and Ii-RGC (rAV/IFN-gamma/Ii-RGC) efficiently induced the MHC Class II+/Ii- phenotype. Injection of tumor nodules with rAV/Ii-RGC and rAV/CIITA/IFN-gamma combined with a suboptimal dose of rAV/IL-2 induced a potent antitumor immune response. The methods are adaptable for producing enhanced genetic vaccines, attenuated virus vaccines (eg, vaccinia), and ex vivo cell-based vaccines (dendritic and tumor cells).

Selective gene expression using a DF3/MUC1 promoter in a human esophageal adenocarcinoma model

The efficacy of replication-deficient adenoviral vectors in gene therapy is confined to the number of tumor cells the vector infects. To focus and enhance the therapeutic efficacy, we employed a conditionally replication-competent adenoviral vector with a tissue-specific promoter, DF3/MUC1, in a human esophageal adenocarcinoma model. Our results demonstrate that Ad.DF3.E1A.CMV.TNF (Ad.DF3.TNF) specifically replicates in Bic-1 (DF3-producing cells) and mediates an enhanced biologic effect due to increased TNF-alpha in the same DF3-producing cells. We also show that the increased TNF-alpha interacts with ionizing radiation to produce greater tumor regression and a greater delay in tumor regrowth in Bic-1 (DF3-producing cells) compared to Seg-1 (DF3 non-producers). Tumor cell targeting using conditionally replication-competent adenoviral vectors with tumor-specific promoters to drive viral replication and deliver TNF-alpha provides a novel approach to enhancing tumor radiosensitivity.

Development of an Ad7 cosmid system and generation of an Ad7deltaE1deltaE3HIV(MN) env/rev recombinant virus

A strategy to circumvent immune responses to adenovirus (Ad) resulting from natural infection or repeated vector administrations involves sequential use of vectors from different Ad serotypes. To further develop an Ad-HIV recombinant AIDS vaccine approach, a replication-defective recombinant Ad from a non-subgroup C virus was required. Using a cosmid system, we generated an Ad7deltaE1deltaE3HIV(MN) env/rev recombinant virus and compared expression of the inserted HIV genes with a similarly constructed replication-competent Ad7deltaE3HIV(MN)env/rev recombinant. Ad7deltaE1deltaE3HIV(MN)env/rev expressed both HIV env and rev gene products. The envelope protein was correctly processed and functional, mediating syncytia formation of Ad7deltaE1deltaE3HIV(MN) env/rev-infected cells and CD4(+) T lymphocytes. Ad7deltaE1deltaE3HIV(MN)env/rev could be amplified on 293-ORF6 cells, containing the E4 ORF6 gene, shown earlier to support production of an Ad7 vector lacking the E1a gene. The utility of this cell line is now extended to the production of replication-defective Ad7 recombinants lacking E1a, E1b, and protein IX genes. Sequential immunizations with Ad-HIV recombinants based in different Ad serotypes have been shown to effectively elicit both humoral and cellular HIV-specific immune responses. The recombinant Ad7deltaE1deltaE3HIV(MN)env/rev will be useful in such AIDS vaccine strategies. Further, these studies have created new cosmid vectors that can be applied to generation of single- or double-deleted Ad7 recombinants with foreign genes inserted into the E1 and/or E3 regions.

Gene therapy for pituitary tumors: from preclinical models to clinical implementation

Gene therapy, which entails the use of nucleic acids as drugs, is a new approach to treat disease. Gene therapy has been successfully implemented in several preclinical animal models, including several paradigms of experimental pituitary tumors. In spite of these successes, several critical issues need to be addressed before gene therapy can become a clinical reality for the treatment of pituitary tumors. These include the development of safer and more effective gene delivery vectors, the uncovering of novel therapeutic targets, the development of molecular switches which will allow turning therapeutic transgene expression "on" and "off" as and when it is needed, and the ability to scale up the vector preparations devoid of any putative contaminants. There are still many basic science developments that must take place in order to allow this new therapeutic technology to make its way successfully into the clinical arena to treat pituitary disease. We envisage these developments taking place within the next five years, gene therapy for pituitary tumors will then form part of the armamentarium available to better treat and manage pituitary tumors.

Production of adenovirus vector for gene therapy

The field of gene therapy is rapidly expanding with a major focus on the treatment of cancer. Replication-defective adenoviruses are vectors of choice for delivering corrective genes into human cells. Major efforts are directed to design new generations of adenoviral vectors that feature reduced immunogenicity and improved targeting ability. However, the production of adenoviral vectors for gene therapy applications faces a number of challenges that limit the availability of high quality material at the early stages of research and development in the gene therapy field. Moreover, very few papers have been published on the subject and information on large-scale production methods are only available through specialized conference proceedings. This review outlines the problems associated with mass production of adenovirus vectors and describes research efforts by a number of groups who have contributed to optimize production methods. Better understanding of the adenovirus infection and replication kinetics as well as better understanding of complementing cell line physiology and metabolism greatly contributed to improving vector titers and volumetric productivity at higher cell densities. Also, the critical aspect of viral vector quantitation is discussed.

Recombinant adenovirus-mediated cytotoxic gene therapy of lymphoproliferative disorders: is CAR important for the vector to ride?

The literature has seen an incredible booming of publications related to the use of recombinant adenoviruses as therapeutic tools for lymphoproliferative disorders over the last decade. Several approaches of adenovirus-mediated gene expression have been used to transfect cell lines that are derived from lymphoid tumors and would have otherwise been refractory to other transfection methods. The identification of high-affinity receptor for human adenoviruses serotype 2 and 5, the coxsackie-adenovirus receptor (CAR), has raised the question about its relevance for the efficacy of recombinant adenovirus-mediated gene therapy. We review published studies that have analyzed the use of recombinant adenovirus vectors expressing cytotoxic genes for gene therapy in lymphomas, chronic lymphocytic leukemia and multiple myeloma. For simplicity, we group all these diseases under the term lymphoproliferative disorders. We analyze the use of recombinant adenovirus-mediated cytotoxicity by assessing the importance of the biochemical and intrinsic signaling pathways interacting with the products of the exogenous viral-mediated expression. Ultimately, we discuss studies that have been finalized to by-pass the limitations of the biodistribution of CAR by modifying or targeting adenovirus to other membrane proteins in cells derived from lymphoproliferative disorders.

Adeno-cosmid cloning vectors for regulated gene expression

BACKGROUND

Adenovectors are widely used for efficient delivery of genes into a variety of cell types and organisms. However, the construction of the desired vector/genes combination, especially if it involves the cloning of several gene cassettes, can be laborious due to the large size of these vectors. New methods are needed to simplify the construction of complex combinations of gene cassettes into adenovectors.

METHODS

Using simple cloning techniques and exploiting the lambda-phage packaging system, we devised efficient methods for the 'selection' of the desired vector constructs. Thus we generated a series of cosmids containing the adeno helper dependent (HD) backbone in which we inserted cis- and trans-acting tetracycline (tet) elements for the regulation of any gene of interest. One of these cosmids has been used to produce an HD adenovirus carrying a tetracycline-regulated gene expressing beta-galactosidase.

RESULTS

We have demonstrated that the adeno-cosmid system allows rapid and efficient cloning of genes of interest in helper dependent vectors, and described a prototype 'ready-to-use' vector in which any gene of interest can be easily expressed under the control of the tet system. The HD viruses produced with this novel methodology can be grown at high titers, can be easily separated from the helper adenovirus, and allow delivery and regulated gene expression in a variety of tissues.

CONCLUSIONS

Exploiting the lambda-packaging system, complex adeno constructs can be generated with a simple and reproducible protocol, which allows selection of the desired size construct, counterselecting for the frequently observed intramolecular recombinations and deletions.

High in vivo production of a model monoclonal antibody on adenoviral gene transfer

The therapeutic potential of monoclonal antibodies (MAbs) for treating a variety of severe or life-threatening diseases is high. Although intravenous infusion appears to be the simplest and most obvious mode of administration, it is not applicable in many long-term treatments. It might, however, be advantageously replaced by gene/cell therapies, rendering treatments cost-effective and eliminating the short- and long-term side effects associated with injection of massive doses of antibodies. Grafting of ex vivo genetically modified cells of various types has already been used for in vivo production and systemic delivery of MAbs in mice. However, although sustained for long periods of time, serum levels of ectopic MAbs were low. We show here that in vivo administration to mice of a first-generation adenoviral vector expressing a model MAb also permits achievement of the same goal, but with 100 to 200 times better efficiency that in any other case of gene transfer described thus far. We also investigated for possible anti-idiotypic response against the ectopic MAb. None was detected in the animals expressing the lowest levels of ectopic MAb production; a response was detected among the highest producers. In the latter case, however, the response was low and could not exert any significant neutralizing activity. In conclusion, our work indicates that high levels of circulating ectopic MAb can be obtained on direct in vivo gene transfer without inducing an anti-idiotypic response sufficiently robust to exert a neutralizing effect. This observation is encouraging in the perspective of clinical applications of this technology.

Retrovirus-mediated herpes simplex virus thymidine kinase gene transfer in pancreatic cancer cell lines: an incomplete antitumor effect

INTRODUCTION

The transfer of drug-susceptible (suicide) genes to tumor cells by retroviral or adenoviral vectors is a novel approach to the treatment of human tumors.

AIMS

To ascertain the antitumor effect of retroviral transduction of the pancreatic cancer cell lines MIA PaCa 2, CAPAN-1, PANC1, and PSN1 with the herpes simplex virus thymidine kinase (HSV-TK) gene.

METHODOLOGY

The vector carried a neoselectable marker gene, the human interleukin-2 gene, an internal ribosome entry coding site, and the region coding HSV-TK.

RESULTS

Twenty micromoles or less of ganciclovir did not modify nontransduced TK- cell growth, whereas > or =100 micromol completely inhibited TK- cell growth, indicating that this dosage is cytotoxic per se. The 4 TK- and the 4 transduced cell lines were treated daily with 0.001, 0.01, 0.1, 1, 10, and 20 micromol of ganciclovir for 13 days. CAPAN-1 cell growth was completely inhibited by 0.1 micromol of ganciclovir; higher doses were required to kill PANC1 (10 micromol) and PSN1 (20 micromol). MIA PaCa 2 cell growth decreased following a 20-micromol ganciclovir dosing. The bystander effect was great in the CAPAN-1 cell line and moderate in PANC1; no bystander effect was recorded in MIA PaCa 2 and PSN1 cell lines.

CONCLUSION

Gene therapy with HSV-TK for pancreatic cancer seems effective in only a limited number of tumor-derived cell lines, and this limits its application in vivo.

Gene transfer to salivary glands

This article provides a review of the application of gene transfer technology to studies of salivary glands. Salivary glands provide an uncommon target site for gene transfer but offer many experimental situations likely of interest to the cell biologist. The reader is provided with a concise overview of salivary biology, along with a general discussion of the strategies available for gene transfer to any tissue. In particular, adenoviral vectors have been useful for proof of concept studies with salivary glands. Several examples are given, using adenoviral-mediated gene transfer, for addressing both biological and clinical questions. Additionally, benefits and shortcomings affecting the utility of this technology are discussed.

Development and characterization of bovine x human hybrid cell lines that efficiently support the replication of both wild-type bovine and human adenoviruses and those with E1 deleted

The 293 cell line that was generated by transforming human embryonic kidney cells with human adenovirus type 5 (HAV5) early region 1 (E1) sequences is an excellent host for generating and growing HAV5 recombinants with E1 deleted, but it does not support the replication of bovine adenovirus type 3 (BAV3). Madin-Darby bovine kidney (MDBK), an established bovine cell line, is an excellent host for growing and plaquing BAV3. For the purpose of combining the unique characteristics of these two cell lines (293 and MDBK), we generated a number of bovine x human hybrid (BHH) cell lines. Comparison of three BHH hybrid clones-BHH3, BHH8, and BHH2C-with 293-Puro (puromycin-resistant 293 cells) and MDBK-Neo (G418-resistant MDBK cells) cell lines for total cellular DNA content, species-specific surface markers, isoenzyme analysis, and karyotyping indicate that they are hybrid in nature. BHH clones constitutively expressed the E1 proteins (E1A, E1B-21kDa, and E1B-55kDa) of HAV5 and efficiently supported the replication of both wild-type and replication-incompetent bovine or human adenoviruses. Transient gene expression experiments with a plasmid encoding the bacterial beta-galactosidase gene demonstrated that BHH cell hybrids seem to have better transfection efficiencies than either of the parental cell lines. These cell lines will be useful for isolating and growing replication-competent human or bovine adenovirus recombinants with E1 deleted and for the study of cellular or viral factors important for viral replication. The development of somatic cell hybrids appears to be a simple way of combining some of the desirable characteristics present separately in two parental cell lines.

A rapid and easy method for production and selection of recombinant adenovirus genomes

Adenoviruses are used widely as vectors for gene therapy. Due to the large size of their genome there is a low frequency of unique restriction sites and many techniques have been described to construct recombinant viruses. Whatever the considered technique, the Escherichia coli strain BJ5183 is used to obtain recombinant adenovirus genomes in a plasmid, or to construct defective viral backbones which will be used to produce infectious viral particles by homologous recombination in HEK293 cells. Unfortunately BJ5183 bacteria do not produce a sufficient amount of plasmid DNA to allow for restriction analysis. Plasmids have to be transferred into another strain to detect the expected construction. It is reported now that the common E. coli strain, Top10F' can be used for the construction of recombinant adenovirus genomes. A plasmid carrying a kanamycin resistance gene and containing the two ends of the adenovirus genome was used. It permits modification by classical molecular biology techniques or homologous recombination at both ends of the genome. The remainder of the genome is introduced by homologous recombination in Top10F'. Several homologous recombination steps were successfully performed without the steps of extraction and introduction of plasmid DNA in another strain to check the plasmids obtained.

Gene therapy for cardiovascular disorders: is there a future?

Incidence of cardiovascular disease has reached epidemic proportions in spite of recent advances in improving the efficacy of pharmacotherapeutics. This has led many to conclude that drug therapy has reached a plateau in its effectiveness. As a result, our efforts have been diverted to explore the use of gene transfer approaches for long-term control of these pathophysiological conditions. The purpose of this review is to present various approaches that are being undertaken to provide "proof of principle" for gene therapy for cardiovascular diseases. Finally, we will discuss the future of gene therapy and other new technologies that may further advance this field of therapeutics.