Introduction
Gene therapy offers promise to many patients suffering from cardiovascular diseases. Cardiovascular diseases are one of the major reasons for morbidity and mortality globally. Hence, there is always the urgency to obtain alternative solutions, pharmacological or otherwise (Ekhart & Koch; Frazier, Johnson & Sparks 315). In many clinical tests, it was found that gene therapy “using transgenes that enhance the contractile function of the heart have the potential to act as a therapeutic bridge before and during cardiac transplantation.” (Ekhart & Koch)
Current pharmacological interventions are not sufficient to ameliorate the suffering of people with heart ailments. Gene therapy can reduce the risk at “critical periods of cardiovascular disease processes.” (Ekhart & Koch) Furthermore, the nature of cardiovascular diseases was Mendelian and distinct mutations could be found in the deoxyribonucleic acid (DNA) sequence in a single gene. The disease is inheritable 50% of the time (Frazier, Johnson & Sparks 315). Genetic mutations further result in “congenital malformations of atrial and ventricular septation.” (316) Defects at the junction of the heart muscles cells developed into “myocardial dysfunction, or cardiomyopathy.” (315) Mutations involving ion channels (“ion channelopathy”) resulted in “primary arrhythmogenic disorders.” (315) Other manifestations of ion channels mutation include “LongQTsyndromes, Brugada syndrome (idiopathic ventricular fibrillation), and catecholaminergic ventricular tachycardia.” (315).
Aside from the therapeutic function of gene transfers, it also allows practitioners to examine the “disease-based mechanisms” of animal functions (Ekhart & Koch). Cardiovascular diseases under consideration for gene therapy include “therapeutic angiogenesis in ischaemic myocardium and limb muscles, treatment of hypertension, vascular bypass graft occlusion, and prevention of postangioplasty restenosis.” (Dishart et al) The treatment of cardiovascular diseases is dependent on advanced technologies, delivery mode, long-term achievement of “expression highly efficient, and targeted expression to relevant cells of the cardiovascular system” and the design of vector systems that assure safety for human application (Nabel). In particular, myocardial gene transfer not only promises therapeutic solutions but also provided a means to probe deeper into the causes of the disease (Donahue 332; Alexander et al 661).
Gene therapy has the potential to stimulate angiogenesis “to protect against reperfusion injury and hypoxic damage, to protect against late vein graft failure, to prevent thrombosis, and to treat hypertension and atherosclerosis.” (Nordlie et al 19) It also shows prospective options to mitigate “cellular manifestations” common in heart failures (19). Protecting the heart from further damage attributed to these failures is one of the features of gene therapy (19).
The main challenge of gene therapy remains the ethical considerations involved in extensively using gene therapy to address cardiovascular problems. The safety of the patients is of primary concern after a series of failed tests claimed several lives of participants or worsened their conditions.
Gene Delivery
Gene delivery vehicles include two strategies namely nonviral and viral vectors (Metcalfe et al 32). The simplest manner is to deliver the gene by injecting directly into a specific area with “naked” plasmid DNA (32). “Naked” plasmid DNA is effective for about a month but the disadvantages associated with this type of gene delivery include “inefficient delivery, limited time for transgene expression, high levels of vector breakdown in circulation, and lack of chromosomal integration.” (32) To overcome the limitation, “the Liposomes and hemagglutinating virus of Japan (HVJliposomes) have been developed to deliver “naked” DNA.
Viral vectors are becoming the preferred choice of gene delivery because it was designed as “replication-defective, while retaining their ability to infect target cells and transduce genetic material.” (32) Some of the viral vectors commonly used include “the adenovirus, adeno-associated virus (AAV), herpes simplex virus (HSV), and a family of retroviruses including the murine leukemia virus (MLV) and lenti-based HIV-1 viral vectors.” (32).
For cardiovascular diseases, the adenovirus vector delivery is more advantageous than other forms because it proved to be easy to manipulate, “generation of high titers, large insert capacity, infection of a wide variety of dividing and nondividing cells and tissue types, and extensive characterization.” (33) However, the technique has an inherent flaw. The inability of the viral vector to integrate with the host genome only created transient effects. The adenovirus vector would not work for patients that have developed immunity against the virus. Finally, the introduction of adenovirus could develop into toxicity and immunity levels among the patients (33).
Adeno-associated virus (AAV) appeared to be more effective because it could infect both dividing and non-dividing cells yet it is non-pathogenic (33). Metcalfe et al in Gene Therapy for Cardiovascular Disorders: Is There a Future? cited three ways to transcend the disadvantages of using this system before it could become an ideal vehicle for gene therapy.
1) increasing the genomic capacity to include various regulators and promoters for a transgene; 2) technical advances to allow the production of large-scale quantities of the virus, and 3) identifying the chromosomal integration site of the recombinant AAV (33).
Gene Therapy and Cardiovascular Diseases
Gene therapy in cardiovascular diseases is gaining ground because of some positive clinical findings. Principally, gene therapy delivers “beneficial gene(s), either systemically or directly into the tissue. Successful delivery thereby changes the expression levels of that gene to correct the altered expression in the pathophysiological state.” (Metcalfe et al 32) Gene therapy is applicable in the majority of cardiovascular problems and the potentials for success is outlined in Mandi J. McKay and Mohamed A. Gaballa’s article, Gene Transfer Therapy in Vascular Diseases. The manner of delivery using gene therapy vectors gave rise to some concerns regarding the safety of using viral materials to treat human diseases. However, “a variety of cardiovascular cell types, including vascular endothelial cells and vascular smooth muscle cells, have been used to treat vascular diseases.” (246) Genetically modified endothelial cells producing tissue plasminogen activator indicated positive results when applied to “reduce the thrombotic potential of grafts and native injured coronary arteries.” (247, Morishita et al 369).
In the treatment of atherosclerosis, a potential candidate for gene therapy is the use of nitric oxide synthase (NOS) (247). Clinical studies found that synthesis. Gene delivery of the “inducible NOS (iNOS) cDNA to sites of vascular injury in animal models dramatically reduced smooth muscle proliferation and intimal hyperplasia.” (247) Another vascular gene transfer was found to yield positive results is the use of metalloproteinase (MMP) (247). Inhibition of MMP has been found to decrease the incidence of “atheroma size [by 32%], as well as a decrease in macrophage infiltrate.” (247).
The more complicated process of restenosis required multiple introductions of genes to address the problem. Some of the genes used in the clinical experiments include “tumor suppressor genes, such as p53, have been investigated as a possible mechanism to inhibit neointimal formation after vascular injury.” (248) The introduction of Kallikerin tissue “cleaves kininogen into vasoactive kinin peptides. These peptides are known to cause a series of reactions, including the release of bradykinin.” (250).
For thrombosis, clinical studies found that Adenoviral-mediated transfer of the thrombomodulin gene into laboratory New Zealand rabbits “after balloon-induced vascular injury to the femoral artery resulted in a 39% decrease in intima to media (I/M) ratio compared with controls at 28 days.” (252) Early thrombus formations in the injured area was markedly suppressed (252).
Challenges of Gene Therapy
The prospect of using genetic discovery in clinical practice could prove to be challenging when it will test the ability of clinicians to interpret genetic research (Frazier, Johnson & Sparks 318). For genetic tests to be useful, they must be “analytic and diagnostically valid.” (318) Genetic testing in clinical settings is quite expensive and care should be taken when “generalizing, extrapolating, anticipating, or making predictions about the future of an individual-based phenotypic feature of biological relatives.” (318) Attempting to identify specific genetic characteristics and basis proved to be frustrating because of the accompanying environmental factors such as “diet, inactivity, and cigarette smoking contribute to difficulties in distinguishing the mechanisms for disease manifestation (i.e., monogenic, polygenic, or multifactorial)” and “ambiguous or endemic phenotypes (i.e., obesity, hypertension, and diabetes).”(318).
Risks associated with vector insertion oncogenesis had manifested in “two cases of leukaemia in X-SCID children” (Nathwani et al 77). After the gene transfer, it was observed that in these two cases, the children had “poor reconstitution of natural killer (NK) cells after gene transfer, which probably compromised immune surveillance.” (77).
Prospects of Gene Therapy
J. Kevin Donahue in Gene Therapy for Cardiac Arrhythmias: A Dream Soon to Come True? wrote about the promise of gene therapy applicable for treating Arrhythmias. The best way to implement global ventricular gene transfer was by way of intracoronary perfusion (557). The author also mentioned another viable method of gene therapy delivery for cardiac arrhythmias that recent clinical studies implied positive results. Some of them include gene transfer using AAV vectors, long QT therapy strategy, and post-infarct VT strategy (557-558).
Recent developments also combined the use of efficacious therapeutic genes and modified delivery systems showed potentials for success in clinical vascular gene therapy applications (Baker 543). The development of disease-selective gene therapeutics would be in the right direction for finding safer methods of using gene therapies in the future.
Morishita, Aoki & Ogihara, in their paper, Does Gene Therapy Become Pharmacotherapy? stressed several points regarding the potentials of gene therapy for cardiovascular diseases. There is a need to develop safer gene delivery procedures considering previous tragedies involved in clinical experiments. Strategies specific to disease targets and cell-specific targeting strategies should be studied further. In addition, regulation of these methodologies is a requisite for safer gene therapy strategies (310-311).
Cited Works
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Dishart, Kate L. et al, “Gene Therapy for Cardiovascular Disease,” Journal of Biomedicine and Biotechnology,.2003(2) (2003), 138–148.
Donahue, J. Kevin, “Gene Therapy for Cardiac Arrythmias,” Annals New York Academy of Sciences, 1015(2004), 332-337.
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Nordlie, Margaret A. et al, “Molecular Aspects of Ischemic Heart Disease: Ischemia/Reperfusion–Induced Genetic Changes and Potential Applications of Gene and RNA Interference Therapy,” Journal of Cardiovascular Pharmacology and Therapeutics, 11(1)(2006),17–30.