(Circulation. 1995;91:541-548.)
© 1995 American Heart Association, Inc.
Articles |
From the Cardiovascular Research Center and Department of Internal Medicine, University of Michigan (Ann Arbor).
Correspondence to Elizabeth G. Nabel, MD, University of Michigan, Cardiovascular Research Center, MSRB III, Rm 7301, 1150 W Medical Center Dr, Ann Arbor, MI 48109-0644.
Key Words: genes cardiovascular diseases myocardium
| Introduction |
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Advances in recombinant DNA technology, including gene transfer, have stimulated hope that this technology can be used to improve the practice of cardiovascular medicine. Applications of this technology that affect the clinical management of patients include the development of new therapeutic products, engineered by the overexpression of genes, such as recombinant tissue-type plasminogen activator. Recombinant DNA technology has also provided techniques that have been used to create animal models of cardiovascular diseases. These models permit definition of the role of specific gene products in the pathogenesis of cardiovascular diseases. Characterization of its molecular basis has led to a more precise definition of diseases and the potential for relevant clinical treatments. The development of molecular genetic interventions to treat cardiovascular diseases depends on technical advances in the development of methods of gene delivery; achievement of long-term, highly efficient, and targeted expression to relevant cells of the cardiovascular system; and design of vectors that are safe for long-term human administration.
| Methods of Gene Delivery |
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Vascular Cells
Retroviral Vectors
Because
vascular cells are accessible through the blood stream,
percutaneous, site-specific gene delivery was developed for local
arterial segments. Retroviral vectors were initially used in in vivo
vascular gene transfer studies.1 2 Interest in
retrovirus-mediated gene transfer was based on the use of these vectors
in gene transfer to other organ systems, including bone marrow stem
cells,3 liver,4 and the skin,5
where in some target cells, these vectors transduce a large proportion
of target cells. In the case of retroviral vectors, the viral vector
stably integrates into chromosomal DNA of the target cell, resulting in
potentially stable gene expression, although integration into
chromosomal DNA could potentially result in insertional
mutagenesis.6 These vectors are most appropriate for ex
vivo gene transfer for cardiovascular
disease,1 2 7 which
involves removal of the relevant target cells, ie, endothelial cells or
hepatocytes from the host, transfection of the cells in vitro, and
subsequent reintroduction of the modified cells into the animal or
patient. It is unknown whether stable high-level gene expression in
vivo will be achieved with retrovirally transduced cells.
Several features of retroviral gene transfer may limit its application to cardiovascular medicine, particularly with respect to direct in vivo gene therapy. Replication of target cells is necessary for proviral integration.8 Recent studies suggest that viral integration may depend on mitosis, not just DNA synthesis.9 Successful retrovirus-mediated gene transfer to vascular cells may require induction of proliferation in target endothelial or smooth muscle cells, at least for short periods of time. In uninjured arteries, endothelial and smooth muscle cell proliferation occurs slowly, and therefore retroviral integration would occur at a low frequency. After vascular injury or stimulation, such as with a balloon catheter, injured vascular cells may be transduced at higher rates. Previous studies have suggested that retroviral transduction of vascular cells can be used successfully in vivo. Limitations of this gene delivery vector include its low frequency of gene transfer and the relative lability of retroviral particles compared with other viruses. Retroviral particles are rapidly inactivated in vivo in primates, presumably by the presence of complement in serum.11 Because of the instability of retroviral particles and the inability of retroviruses to integrate in nonreplicating cells, retroviral vectors may have limited use for direct gene transfer in vivo, although there may be a role for these vectors for ex vivo gene transfer to vascular stents or prosthetic grafts.
Adenoviral Vectors
To
improve the frequency of direct gene transfer into arteries,
recent investigations have focused on adenoviral vectors. The
adenovirus genome is composed of linear, double-stranded DNA of
approximately 36 kb in length (for a review, see Reference 12). The
gene products are organized into early (E1-E4) and late (L1-L5)
regions, based on expression before or after initiation of DNA
replication. Expression of viral genes depends on cellular
transcription factors and expression of the adenoviral E1 region, which
encodes a transactivator of viral gene expression. The E3 region
encodes viral proteins that regulate immunosurveillance in vivo.
Adenoviruses have a lytic life cycle, characterized by attachment to an
adenoviral glycoprotein receptor on mammalian cells and cell entry by
receptor-mediated endocytosis. Adenoviruses escape degradation in
lysosomes due to adenoviral capsid proteins, and viral DNA is
transported to the nucleus. In the nucleus, adenoviral genome persists
in an unintegrated form. During the lytic infection, viral genome
replicates to several thousand copies per cell.
Adenovirus serotypes 2 (Ad-2) and 5 (Ad-5) have been developed as viral vectors for gene transfer (for a review, see Reference 13). These vectors are engineered to be replication incompetent by deleting the E1A and E1B genes from the viral genome. Vectors are produced by homologous recombination in 293 cells or any cell line that contains an integrated copy of the adenoviral E1 gene. A foreign cDNA with eukaryotic regulatory sequences is introduced into a bacterial plasmid containing a region of the left adenoviral genome that is deleted of the E1 gene. This plasmid is cotransfected into 293 cells with an incomplete adenoviral genome. Homologous recombination between the two DNAs generates a recombinant genome in which the E1 gene is replaced by the foreign DNA. Viral stock is propagated in 293 cells to high titer, approximately 1010 to 1012 particles per milliliter.
Adenoviruses effectively infect mammalian cells, including nondividing cells in vitro and in vivo. Physiological levels of recombinant proteins have been secreted into the circulation after adenoviral infection of skeletal muscle.14 The virus particle is relatively stable and amenable to purification and concentration at a high titer. Integration of adenoviral DNA sequences into chromosomal DNA of the target cell occurs at a low frequency, and adenoviral DNA is maintained in an extrachromosomal form. Extrachromosomal replication of the vector reduces the likelihood of mutation by random integration and dysregulation of cellular genes.
Despite these advantages, there are
limitations to current, or
"first-generation," adenoviral vectors. In most models, gene
expression is transient after adenoviral infection, generally less than
3 weeks, and inflammation is observed in organs expressing the
transgene.15 16 17 18 Gene
expression in vascular cells is
transient as well, usually persisting for only several
weeks.19 20 21 22 23
Although transient gene expression may be well
suited to vascular therapies requiring expression of a gene product
over a short period of time, development of an immune response to
adenoviral proteins is a major limitation to the use of these vectors.
Recent studies in genetically defined strains of mice have demonstrated
that viral proteins, expressed from the E1-deleted adenoviral genome,
are presented as foreign antigens and lead to the generation of
cytolytic T lymphocytes that destroy adenovirus-infected
cells.24 Insertion of a temperature-sensitive mutation
within the E2A region of E1-deleted adenoviral vectors results in lack
of expression of late viral gene products at nonpermissive
temperatures, resulting in prolonged gene expression (
70 days) and
blunted cytolytic T-cell infiltration in mouse liver.25 In
arterial gene transfer studies using first-generation adenoviral
vectors, mononuclear cell infiltrates have been occasionally observed
in the adventitia of peripheral26 and
pulmonary23 arteries of pigs, but medial and intimal
inflammation, necrosis, and aneurysm formation have not been observed.
Further studies are required to examine the use of first-generation
adenoviral vectors for vascular gene transfer studies. It is likely
that further modifications in these vectors, including deletions in the
E2 and E4 regions and modifications in the E3 region, will diminish
host immune responses.
Adenovirus-Augmented,
Receptor-Mediated Gene Delivery
Viral vector conjugate systems may
have application in vascular
gene therapy and include adenovirus-augmented, receptor-mediated gene
delivery. This vector uses inactivated adenovirus complexed to a
receptor ligand to facilitate entry of DNA to a cell.27
This vector consists of two components. DNA condenses with polylysine,
which in turn is bound to inactivated virus.28 29 The
virus is coupled to a ligand such as transferrin. The transferrin
ligand binds to a transferrin receptor in a cell, and the transferrin
viral polylysine DNA complex enters the cell by receptor-mediated
endocytosis. The inactivated adenovirus functions to disrupt lysosomes
in the host cell, reducing DNA degradation and releasing DNA into the
cytoplasm.30 The use of these vectors for vascular gene
transfer is being investigated.
Cationic Liposomes
Most nonviral methods of gene transfer rely on normal mechanisms
used by cells for the uptake and cellular transport of macromolecules.
These methods rely on receptor-mediated endocytotic pathways or fusion
of cell membranes. One example is cationic liposomes, which are
positively charged artificial lipid vesicles that incorporate
negatively charged DNA and deliver nucleic acid to cells through fusion
with cell membranes or receptor-mediated endocytosis.31
Plasmid DNA is released in the cytoplasm and transported to the nucleus
where it is maintained in an unintegrated form. Cationic liposomes
interact spontaneously and rapidly with polyanions, such as DNA and
mRNA, to form liposome complexes. Cationic liposome reagents used in
vascular gene transfer studies include DOTMA/DOPE
(Lipofectin),32 DC-cholesterol,32 DOSPA/DOPE
(Lipofectamine),23 and DMRIE/DOPE.33
Expression of recombinant genes in vivo after liposomal transfection
has been reported in rats,34 rabbits,35
dogs,36 37 and
pigs38 39 40 41 . Cationic
liposomes
produce more efficient gene delivery compared with neutrally charged or
anionic liposomes,42 but current formulations, including
DOSPA/DOPE, are less efficient than adenoviral vectors.23
Further modifications in plasmids and chemical formulations of
liposomes appear to have promise in improving transfection
efficiency.43 Cationic liposomes have a favorable safety
profile for in vivo administration.32 44 Liposome
vectors
contain no viral sequences, and there are no cDNA size constraints in
vector construction. In addition, this vector is straightforward to
prepare for clinical use.44 Cell division is not required
for liposome transfection, although the efficiency appears to be
increased in proliferating cells.45
HVJ
Liposome Conjugates
Recent studies suggest that complexing inactivated
HVJ with
liposomes improves transfection efficiencies of vascular smooth muscle
cells in in vitro and in vivo models of vascular injury, including the
injured rat carotid artery.34 These vectors have also been
successfully used for hepatic46 and renal47
in vivo gene transfer. It is likely that further modifications to
vectors used for vascular gene transfer will include components of
viral and nonviral vectors that optimize delivery, improve gene
expression, and minimize toxic side effects.48
Polymers
Additional strategies for the local
delivery of therapeutic agents
include impregnating oligonucleotides into polymer gels and applying
the polymer to the external surfaces of
arteries.49 50
Although pharmacokinetics of oligonucleotide delivery and retention
have not been precisely defined, the data suggest that there is
sufficient retention of oligonucleotide to inhibit
c-myb49 and PCNA50 RNA expression
within 24 hours after balloon injury and inhibit intimal thickening
after 2 weeks. Plasmid DNA51 and adenoviral
vectors52 have been applied directly to polyethylene
balloons coated with a hydrogel polymer. Although there is some loss of
plasmid DNA from the balloon during transit through the circulation,
DNA is distributed transmurally after inflation of the balloon.
Modifications in polymers to provide slow release of therapeutic agents
hold promise for site-specific delivery of oligonucleotides and vectors
to arterial segments.
Myocyte Gene Transfer
Dissection of molecular mechanisms
governing myocardial
differentiation has been performed in neonatal cardiac myocytes in
culture, in part because these cells are relatively amenable to gene
transfer with plasmid-based
transfections.53 54 55 56 57
Recombinant gene expression in adult myocardium in vivo requires an
expression vector with high-level activity in adult cardiac myocytes
and a method for introducing this vector into myocardial cells. Because
cardiac myocytes are terminally differentiated cells, they require a
vector that is not dependent on cell replication for delivery and
expression. The analysis of foreign genes within intact adult
myocardium has been performed by direct injection of plasmid
DNA.58 59 Although direct injection of genes is a
simple
procedure and permits examination of the behavior of genes in vivo,
this technique is limited by transfection of a small number of cells
within several millimeters around the injection site.60
Expression of recombinant genes is temporally limited as well, with
expression peaking within several weeks and declining rapidly
thereafter.58 60 Episomal persistence of the
introduced
DNA and the postmitotic state of adult cardiac myocytes, which prevent
integration of genes into chromosomes, limit stability of the
transgene.
Some limitations of in vivo plasmid DNA injection have been addressed by adenoviral vectors. Adenoviruses effectively infect nonreplicating mammalian cells, including skeletal and cardiac myotubes. These viruses are grown and purified in high titer. These properties result in highly efficient gene transfer into adult cardiac myocytes in vitro61 62 and in vivo.22 62 63 Quantitative comparisons of chloramphenicol acetyltransferase (CAT) activity resulting from injection of a CAT plasmid or an adenoviral vector encoding CAT revealed that the amount of CAT activity resulting from adenovirus infection was 10- to 100-fold higher compared with plasmid DNA.62 Similar findings were observed comparing adenoviral vectors and plasmids encoding lacZ.22 63 Although adenoviral vectors produce efficient gene transfer into the myocardium, expression is transient, peaking at 1 to 2 weeks. Acute inflammatory responses have been observed in hearts injected with adenovirus,63 although inflammation along the injection path has also been noted after injection of plasmid DNA.58 64 Further investigations will identify factors that account for the transient nature of gene expression and will characterize potential proinflammatory effects of this vector.
| Animal Models of Gene Transfer |
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The feasibility of direct gene transfer to arteries in vivo was demonstrated using viral (retrovirus) and nonviral (liposomes) vectors in several animal species, including pigs,38 39 rabbits,35 and dogs.36 37 These studies reported a low efficiency of gene transfer, generally 1% or less of vascular cells in vivo. More recent studies have suggested that the efficiency of gene transfer into arteries can be improved with adenoviral vectors; increased expression of reporter genes has been reported in sheep,19 rat,20 21 rabbit,22 52 and pig23 26 vessels. Endothelial cells of normal arteries and endothelial and smooth muscle cells in injured arteries have been transduced at efficiencies approximately 10- to 100-fold higher than reported for retroviral and liposome vectors. A major limitation to adenoviral gene transfer in the vasculature has been transient expression; in most studies, expression of reporter gene has been observed for 7 to 14 days and is diminished or lost by 28 days. Lack of persistence of gene expression may result from cytolytic responses directed against infected cells. Transient gene expression, however, may be desirable for vascular diseases, like restenosis after angioplasty, which are characterized by cellular proliferation peaking in the first several weeks after arterial injury.26 65 66 67
Several observations concerning the delivery of recombinant genes and patterns of gene expression can be drawn from these studies. Infusion of vector into normal arteries with an intact endothelium results in transfection of intimal cells (primarily endothelial cells).19 22 23 52 Injury to the vessel and/or application of pressure to the vector infusate results in delivery of DNA transmurally and gene expression in the media.21 26 52 65 Several catheters have been used in gene transfer studies, including double-balloon catheters, porous balloon catheters, and hydrogel catheters, and the patterns of gene expression within an artery may differ depending on the design of the catheter, animal species, and type of artery transduced.51 52 68
Direct gene transfer has been used to create somatic transgene models to define gene function in arteries. In this system, genes can be expressed within arterial segments, and their biological function can be investigated. This approach has proved useful for investigation of genes whose direct in vivo effects have been difficult to analyze. For example, transfection of a recombinant angiotensin-converting enzyme gene into rat arteries using HVJ liposomes promotes angiotensin IImediated vascular hypertrophy.69 After transfer of a recombinant endothelial celltype nitric oxide (NO) synthase (ec-NOS) gene into balloon-injured rat carotid arteries, NO production was associated with a reduction in intimal thickening.70 Gene transfer approaches have also proved useful in the analysis of atrial natriuretic peptide,71 type 2 angiotensin II receptor,72 and VCAM-1.73
Growth factors and cytokines stimulate vascular cell proliferation and vessel formation in vivo. Although the genes encoding many factors have been cloned and their mechanism of action defined in vitro, definition of their role in vivo has been more difficult to analyze. Several recombinant growth factor genes, including platelet-derived growth factorB (PDGF-B), a secreted form of acidic fibroblast growth factor (FGF-1), and an active form of transforming growth factorß1 (TGF-ß1) have been expressed by direct gene transfer in porcine arteries, and the function of these gene products has been analyzed. Expression of a PDGF-B gene in porcine arteries stimulated intimal hyperplasia characterized by smooth muscle cell proliferation.40 Synthesis and secretion of FGF-1 were associated with expansion of the intima as well as intimal angiogenesis.41 Arteries transfected with a TGF-ß1 gene demonstrated increased procollagen synthesis in the intima and media as early as 4 days after gene transfer compared with control arteries transfected with a reporter gene.74 Although these recombinant genes stimulate vascular cell proliferation in vivo, they exert otherwise distinct effects on smooth muscle cell proliferation, angiogenesis, and extracellular matrix formation. These studies suggest that intimal thickening may represent a common response to gene expression of multiple growth factors, which in turn exert different effects on vessel repair.
Another approach to investigating the pathogenesis of vascular cell proliferation in vivo is to examine gene products that inhibit cell proliferation. Local delivery of an antiproliferative agent during the peak of smooth muscle cell proliferation or extracellular matrix synthesis after balloon injury might limit expansion of the intima. Several approaches have been explored in this setting, including recombinant chimeric toxins,75 76 77 antisense oligonucleotide strategies,49 50 78 79 80 81 82 and gene transfer.26 65 66 67
One approach to the selective elimination of dividing cells is to express a herpes virus thymidine kinase (HSV-tk) gene in smooth muscle cells after balloon injury. Thymidine kinase, when expressed in transduced cells, converts ganciclovir, a nucleoside analogue, into an active toxic form, and subsequent incorporation of phosphorylated ganciclovir into cellular DNA induces chain termination in dividing cells, causing cell death.83 84 A bystander effect, demonstrated in smooth muscle and endothelial cells, confers susceptibility to ganciclovir in neighboring dividing cells, leading to inhibition of cell growth in nontransduced neighboring cells as well.85 Adenoviral vectors encoding a HSV-tk gene or no cDNA insert were introduced into porcine arteries immediately after balloon injury, and a course of ganciclovir or saline was initiated. Three weeks after balloon injury and adenoviral infection, a significant reduction in intima-to-media area ratios (54% to 59%) was observed.26 A reduction in intimal BrdC (5-bromo-deoxycytosine) incorporation of 40% was observed in HSV-tk ganciclovir-treated animals compared with HSV-tk saline-treated animals 7 days after gene transfer, indicating that inhibition of smooth muscle cell proliferation contributed to this effect. A significant reduction in intima-to-media area ratios in the HSV-tk ganciclovir-treated animals was observed 6 weeks after treatment, suggesting that the decrease in intimal hyperplasia was stable. In addition, no major systemic toxicities were observed associated with adenoviral infection and ganciclovir treatment. These data suggest that expression of an enzyme that catalyzes the formation of a cytotoxic drug locally within an artery may limit smooth muscle cell proliferation after balloon injury.
In balloon-injured rat carotid arteries, introduction of adenoviral vectors encoding HSV-tk immediately after balloon injury66 or 7 days later67 and treatment with ganciclovir also result in significant reductions in intima-to-media area ratios. Reendothelialization was present in rat and porcine arteries infected with HSV-tk adenoviral vectors and treated with ganciclovir, and significant toxicities were not observed in treated arteries or systemic organs. Additional approaches to limiting smooth muscle cell proliferation after vascular injury include targeting of nuclear cell cycle regulatory pathways, including the retinoblastoma gene product (Rb). Studies in injured rat carotid and porcine femoral artery models suggest that expression of a nonphosphorylatable, constitutively active form of Rb after adenoviral infection limits intimal smooth muscle proliferation for at least 3 weeks after vascular injury.65
Myocardial Gene Transfer
Recent exciting developments hold
promise for transduction of
adult myocytes in vivo. Initial studies demonstrated the feasibility of
expression of reporter genes in
rat58 59 60 86 and
canine87 myocardium by direct injection of plasmid DNA,
but these studies were limited by low efficiencies that hindered
investigations of gene expression in myocytes. The observation that
adenoviruses infect nondividing cells has heightened interest in these
vectors for gene transfer to adult myocardium. Indeed, recent studies
have demonstrated higher levels of gene expression in rat myocardium
after direct injection of adenovirus vectors compared with injection of
plasmid DNA alone.62 63 Adult myocardium in vivo has
also
been transduced by intravascular administration of adenoviral vectors
encoding reporter genes.22 Gene expression was observed in
both the coronary vasculature and the adjacent myocardium. Levels of
gene expression in the myocardium were 10- to 50-fold higher compared
with direct DNA plasmid injection. Although adenoviral vectors provide
efficient gene transfer, gene expression in the myocardium is
transient. In most studies, reporter gene expression peaked at 1 week,
diminished at 2 weeks, and was present at low levels after 1 month
in adult myocytes.22 62 63 The mechanisms
for loss of gene
expression, including immune responses, are not completely understood.
Gene transfer to the myocardium has proven to be a useful tool in
understanding cardiac gene regulation in vivo. For example,
transcriptional elements regulating basal and thyroid
hormoneresponsive cardiac
-myosin heavy chain (
-MHC) gene
expression in adult rat hearts in vivo have been
studied.59 Sequences upstream of the rat
-MHC gene
linked to a luciferase reporter were injected into adult rat hearts,
and thyroid hormone responsiveness was evaluated. The thyroid
hormoneresponsive element was necessary, but not sufficient, to
confer positive and negative regulation of thyroid hormone. Direct
injection of constructs into the myocardium is a model system for
investigating DNA elements and regulatory pathways that control gene
expression and growth in the heart.
An additional promising area is the direct injection of adenoviral vectors into skeletal muscle for production of secreted proteins. Myoblasts, transduced by retroviral vectors expressing human growth hormone, injected into skeletal muscle produced physiological levels of human growth hormone in the serum.88 89 Recent studies suggest that physiological levels of recombinant erythropoietin are secreted into the circulation after intramuscular injection of adenovirus into skeletal muscle of neonatal mice or adult SCID mice.14 Neonatal and adult SCID mice injected once with 107 to 109 plaque-forming units demonstrated significant dose-dependent elevations in serum human erythropoietin levels and increased hematocrit levels that were stable over the 4-month time course of the experiments, and no evidence of a localized inflammatory response or systemic infection was present. Intramuscular injection of adenoviral vectors may be useful for the treatment of inherited disorders of deficient serum proteins.
Lipoprotein Metabolism
Direct gene transfer has been a
promising tool for investigations
of hepatic regulation of lipoprotein metabolism. Initial approaches
used ex vivo transduction of hepatocytes to express an LDL receptor
(LDLR). Hypercholesterolemia in the Watanabe rabbit was reversed after
infusion of LDLR-expressing hepatocytes into the
liver.90 91 This approach has been used in a human
gene
therapy trial of familial hypercholesterolemia.7 Recently,
adenoviral vectors have been used to reconstitute LDLR function in
Watanabe rabbits92 and in homozygous mice lacking LDLRs
produced by homologous recombination.93 In this latter
study, adenoviral vectors encoding a human LDL cDNA were injected
intravenously into the tail vein of mice, and the hypercholesterolemic
effects of the LDLR deficiency were reversed at 4 days. Adenoviral
targeting to hepatocytes has also been demonstrated in healthy mice,
where transient gene expression was observed.17
Hepatic chylomicron remnant uptake has been studied by adenovirus-mediated gene transfer. The hypothesis that LDLR-related protein (LRP) mediates the uptake of dietary lipoprotein into hepatocytes in concert with LDLR was tested by transferring a dominant negative regulator of LRP function into livers of homozygous mice lacking LDLR.94 Inhibition of LRP was associated with accumulation of chylomicron remnants in these mice, suggesting a role for LRP and LDLR in chylomicron remnant clearance. Intravenous injection of an adenoviral vector encoding an apolipoprotein A-I gene results in transient hepatic production of HDL and total cholesterol.95 These studies demonstrate the usefulness of genetically engineered animal models for the study of lipoprotein metabolism.
| Clinical Trials |
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In summary, the field of cardiovascular gene transfer has progressed considerably in the past 5 years. Gene transfer, alone or in combination with other genetic technologies, is proving to be useful in creating complex animal models of cardiovascular diseases in which the in vivo function of gene products can be investigated. These technologies also show promise for the treatment of human cardiovascular diseases, despite remaining technical issues. The field of cardiovascular gene transfer holds tremendous promise and excitement for basic and clinical scientists to work together to explore the possibilities of molecular genetic technologies for the treatment of cardiovascular diseases.
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