Gene Therapy: A Double-Edged Modality with Few Propitious Targets against Cardiovascular Disorders like Heart Failure, Hypertension and Infarction
KHANNA A*, KHANNA R**
*Institute of Medical Technology, University of Tampere, Finland, E.U.
**Regea Institute, University of Tampere, Finland.
Dr. Anchit Khanna, M.B.B.S. M.S. (MEDICAL GENETICS). PhD Research Fellow, Institute Of Medical Technology, University of Tampere, Finland, E.U. Mob: +358-509342656; e-mail: Anchit.Khanna@uta.fi
Cardiovascular diseases like that of congestive heart failure, hypertension and infarction have reached epidemic proportions, and even though many novel pharmacological formulations and devices have improved survival, a real cure is yet to be found. After extensive research and trials (both preclinical and clinical), gene therapy is seen as an important upcoming tool against cardiovascular disorders. Advancements in the vector technology and in the molecular understanding of various diseases like that of heart failure, ischaemic heart diseases and even polygenic diseases like hypertension have opened doors to a new era of cure. With the improved understanding of the pharmacodynamics and the pharmacokinetics of gene transfer, there is a substantial growth being seen in the treatment of cardiovascular disorders using gene therapy with an increasing number of potential targets (genes), especially in the post-human genome era. Few potential targets have been identified for gene therapy from various molecular pathways, which along with the newly developing delivery systems will accelerate and strengthen the fight against heart failure and ischaemia (therapeutic angiogenesis), in which at present most of the clinical trials are going on. But at the same time, all the potential adverse effects and safety concerns arising with these new modalities should also be assessed before enforcement
: Gene therapy/transfer,genetic targets, delivery systems
cite this article :
KHANNA A,KHANNA R. GENE THERAPY: A DOUBLE-EDGED MODALITY WITH FEW PROPITIOUS TARGETS AGAINST CARDIOVASCULAR DISORDERS LIKE HEART FAILURE, HYPERTENSION AND INFARCTION. Journal of Clinical and Diagnostic Research [serial online] 2007 August [cited: 2013 May 18 ]; 1:312-324. Available from http://www.jcdr.net/back_issues.asp?issn=0973-709x&year=2007&month=August&volume=1&issue=4&page=312-324&id=104
Introduction Gene therapy is a therapeutic modality, which involves replacement of altered or non-functional gene with a healthy one. Indians have a genetic disadvantage when it comes to being affected by coronary disorders, because as an ethnic race, it tends to have a high lipoprotein(s) content compared to other races in the Asian region like that of Chinese(1). At present, the situation can be compared as a coronary epidemic, and the fact that in the past 30 years the average age of first heart attack in India decreased by 20 years (out of which men dominated the figures with over 50% of them being below 55 years and 25% being below 40 years) supports the statement (1). Heart being a localised organ makes it a potential target for surgical (or with the help of catheters) local in vivo gene delivery. The cardiomyocytes being post-mitotic cells require a prolonged and efficient gene expression, which is mainly met by vectors like adenoviruses, adeno-associated viruses and lentiviruses. The molecular pathways are again essential to identify the targets for the gene therapy for heart failure and also form the backbone for planning or bringing gene therapy into clinical practice.
Targets for Gene Therapy for Heart Failure Target 1 – calcium channels (levels of calcium in the body) Normally when an excitation impulse reaches the sarcoplasmic reticulum (SR) through the transverse (T) tubule, the voltage-gated calcium channels, longitudinal (L) type, open and allow small amount of calcium influx, which contributes to the calcium released by the SR through the Ryanodine receptors, which activates the myofilaments causing contraction of the myocardial muscle (Sliding Filament theory) (2). During relaxation, the SR re-accumulates the calcium back through the sarco-endoplasmic reticulum calcium ATPase pump (SERCA2a), and then this calcium is pumped out extracellularly by the sarcolemmal Na/Ca exchanger(Table/Fig 1). In humans ≈75% of the Calcium is removed by the SERCA2a, while the remaining removed by the Na/Ca exchanger (3). This action of SERCA2a is inhibited by phospholamban protein (unphosphorylated state), while the phosphorylated state (cAMP and Ca-Calmodulin–dependent protein kinase) reverses this inhibition(4)(Table/Fig 2). The architecture of a functional sarco-endoplasmic reticulum with the location of the key players is shown in (Table/Fig 1).
Target 2 – apoptosis In (Table/Fig 3), various signalling cascades are shown, which contribute to the mechanism of apoptosis (programmed cell death) of the cardiac myocytes, which in turn causes heart failure. These cascades include mitogen or stress-activated phospho kinases (SAPK) like p38, the p53 gene, and certain growth factors like insulin growth factor (IGF) 1, which inhibits other contributors like ischaemia and hypoxia.
Target 3 – β2 adrenergic receptors There is quite a difference in the consequences of β2 adrenergic receptor (AR) stimulation and the β1AR formas, the basis of it being used as a target against failing heart (Table/Fig 4). Stimulation of the β1AR has an apoptotic arrythmogenic potential, whereas the β2AR signalling pathway is devoid of these negative effects (2). Therefore, using gene transfer for β2AR and βARK inhibitor will restore the cAMP levels, resulting in increased functionality of the cardiomyocytes.
Experimental data and the information received from various ongoing clinical trials on gene therapy for cardiovascular disorders (many more in addition to the ones discussed in this dissertation) have been very encouraging, but again the searches for an ideal vector system, and even for the right gene, are still few of the major obstacles in our quest for a safe and efficient therapeutic modality against these disorders. For complex disorders like that of hypertension, more than one gene needs to be targeted and so what we can call a ‘therapeutic gene cocktail’ (20) should serve the purpose. The clinical trials for vascular gene transfer have not only shown it to be safe but also with lot of therapeutic potential when delivered intravascularly (20). But for future clinical trials, we must first test and find an efficient vector and study model (keeping in mind the good manufacturing practices). Then after the protocol approval and toxicological testing, we should bring it in for clinical trials and try in patients and human tissues. From 1989 (the first gene transfer) till present date, the technology has seen rapid progress in terms of yielding novel vectors and also the progress in clinical trials. Up to 2004, there have been 76 clinical trials for cardiovascular gene therapy, out of which 64 were for therapeutic angiogenesis (mainly involving FGF and VEGF genes) (21). At present, majority of the new trials in this field are in Phase I still, but with the development of specialised gene therapy units like that of University Of Pittsburgh (U.S.A.) and the A.I. Virtanen Institute for Molecular Sciences (Finland, Europe), to name a few, trials have entered phase II and some even phase III stages. The rationale at present against gene therapy as a therapeutic modality, especially after few setbacks like the famous Jesse Gelsinger death (which was attributed to the overdosage of drug), can be summarised by the joint statement issued by the American and European Gene Therapy Societies in response to an article in Nature, “The field of gene therapy is working to develop new and better methods to treat a variety of severe disorders, including genetic diseases such as hemophilia and SCID, and also cancer and AIDS. The clear-cut therapeutic benefits seen in recent clinical trials of gene therapy for XSCID and ADA-deficient SCID warrant judicious consideration of the benefits and risks of this approach compared to imperfect alternatives, such as haplo-identical hematopoietic stem cell transplantation.”
GLOSSARY Angiogenesis - Formation of a new blood vessel AT1R/ATR - Angiotensin II type 1 receptor ATP - Adenosine Tri-phosphate Arrhythmia - Loss of normal rhythm ANP - Atrial natriuretic peptide Apoptosis - Programmed cell death β1ARK - β1 adrenergic receptor kinase Cardiomyocytes - Cells of the heart muscle cAMP - cyclic adenosine mono-phosphate Epicardium - Layer covering the heart ECM - Extra-cellular matrix FGF - Fibroblast growth factor Glioma - Tumour of the glia (cells of the central nervous system) Hyperplasia - Increase in number of cells Hypernephroma - Tumour of the kidney Ischaemia - Lack of blood supply Intraluminal - Inside the lumen LDL - Low-density lipids Mitogen - Substance that stimulates mitosis Myofilament - Muscle filament Neoplasms - Cancers or tumours ODN - Oligodeoxynucleotides Sarcoplasmic reticulum - Cell organelle that controls the calcium levels Transgene - The gene used in the gene transfer TGF - Transforming growth factor Therapeutic gene - Gene that is used for the purpose of treating the disorder VCAM - Vascular cell adhesion molecule VEGF - Vascular endothelial growth factor Vector - Carrier of the protein/DNA (deoxyribonucleic acid)
A special thanks to Dr Douglas Wilcox, Honorary Consultant in Medical Genetics and Director of Education Yorkhill NHS Trust Glasgow, UK, for his guidance, and Dr. Andrew Baker , British Heart Foundation, Glasgow, UK. for his critical comments when the article was prepared as a part of my Masters thesis at University Of Glasgow, Scotland, UK. Also a special vote of thanks to my parents ( Anil Khanna and Usha Khanna) , Dr. Rishiv Jain, Brij Mohan Ajmani, and Aarohi Jain for inspiration and support.
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