Gene Therapy to Control Hypertension: Current Studies and Future Perspectives

Hypertension is a complex pathophysiological state that leads to serious complications, including heart failure, coronary artery disease, and abnormal renal function. While traditional therapies can be effective in controlling the effects of hypertension, they offer no long-term cure and often lead to patient noncompliance, thereby diminishing their effectiveness. These reasons, coupled with the recent developments in gene transfer and somatic cell gene delivery, led researchers to explore alternative options that can produce longterm control of hypertension. Gene therapy offers the potential to yield lasting antihypertensive effects by influencing the genes associated with hypertension. In this review, we will discuss the merits of sense versus antisense strategies in controlling hypertension. We also discuss the advantages and disadvantages of both viral and nonviral vector types for the systemic delivery of genes for hypertension research. Results of our research group on the retrovirus-mediated delivery of the angiotensin type I receptor-antisense on the prevention of hypertension and related cardiovascular pathophysiology will be summarized. Finally, we discuss the future of this gene therapy approach in the reversal and long-term control of hypertension. KEY INDEXING TERMS: Antisense; Gene therapy; Hypertension; Spontaneously hypertensive rats (SHR); Viral vectors. [Am J Med Sci 2001;322(1):1–6. ] H is a complex pathophysiological state that primarily manifests as persistent high blood pressure and is associated with many complications in the cardiovascular system, such as heart failure, coronary artery disease, valvular heart disease, cardiac arrhythmia, cardiomyopathy, and abnormal renal function.1,2 Despite recent advances in pharmacotherapeutics that have resulted in the better management of this disease, hypertension and associated cardiovascular diseases remain at epidemic proportions and affect approximately 50 million Americans, by a recent American Heart Association estimate.3,4 Hypertension is a multifactorial disease, and many hormonal, metabolic, and genetic factors have been shown to be involved in the expression of its pathophysiology. The renin-angiotensin system (RAS) seems to be one of the key endocrine systems involved in the development, maintenance, and establishment of primary hypertension. This conclusion is supported by the fact that pharmacotherapies directed toward the inhibition of angiotensin II (Ang II) formation through the angiotensin-converting enzyme (ACE) inhibitors or actions of Ang II (AT1 receptor antagonists) are effective means to treat a majority of human hypertension cases.5–7 In addition, calcium channel blockers, adrenergic receptor blockers, and diuretics, individually or in combination, are current methods for the management of this disease.8 These pharmacotherapeutic strategies have been widely used to treat hypertension because of their many advantages, including reliability and relative affordability. In addition, because of the short duration of action, the therapy could be withdrawn quickly in case of adverse side effects.8 Although this strategy is quite successful in the limitation, management, and treatment of hypertension, it holds little or no promise for long-term control of this disease. Also, some of these therapies have significant side effects, which contribute to the third disadvantage of patient noncompliance.9,10 Problems have been associated with patients discontinuing the medication because they cannot discern the advantages of a medication whose outward side effects outweigh positive results. Finally, the pharmaceutical agents have limited potential in alleviating the end organ damage that may have already occurred before the diagnosis of hypertension. Because of these limitations in pharmacotherapy, coupled with the fact that the incidence of cardiovascular disease is on the rise, new and innovative approaches must be attempted for the treatment and cure for hypertension. From the Departments of Physiology (KWS, CHG, MKR) and Pharmacodynamics (MJK), Colleges of Medicine and Pharmacy and University of Florida McKnight Brain Institute, Gainesville, Florida. This work was supported by National Institutes of Health Grants HL52189 and HL56921 and by the American Heart Association, Florida Affiliate. Correspondence: Mohan K. Raizada, Ph.D., Professor, Department of Physiology, Box 100274, University of Florida College of Medicine, Gainesville, FL 32610. E-mail: mraizada@phys.med.ufl.edu. 1 THE AMERICAN JOURNAL OF THE MEDICAL SCIENCES Many investigators have begun to consider gene therapy a potentially exciting approach for the treatment of hypertension.9,11 They argue that gene therapy counters the disadvantages of traditional therapy by alleviating hypertension and its related pathophysiological effects on a long-term basis. In addition, long-term antihypertensive effects with only one or a few treatments would essentially eliminate the problem of patient compliance. However, several issues need to be addressed if gene therapy strategy is to be successful in the future. First, what genes must one target to produce the most effective antihypertensive response? Second, what is the most ideal vector that could be used to deliver the gene in vivo? Finally, one must compare the ethics of gene therapy strategies with the traditional pharmacotherapy. In this review, we present strategies taken by many investigators to target various genes, summarize developments in antisense (AS) targeting of the RAS, present our observations, and provide future perspectives in the use of gene therapy for hypertension. Sense versus Antisense Gene Therapy Gene therapy for hypertension can be approached by either overexpressing a vasodilator-relevant gene (sense approach) or suppressing the expression of a vasoconstrictor-relevant gene (AS approach). Chao and her collaborators12–17 have taken the former approach to overexpress genes involved in vasodilation, such as kallikrein and atrial natriuretic peptide in adult spontaneously hypertensive rats (SHR). A single systemic injection of sense DNA through a viral vector or as naked DNA resulted in a significant reduction in high blood pressure for up to 12 weeks. In addition, certain hypertension-related pathophysiologies diminished in the major target organs (heart, kidney, and blood vessels). Thus, the sense approach shows exciting promise in controlling hypertension; however, although the sense gene therapy approach reversed hypertension, the reversal was for limited time. In addition, there is no comparable pharmacotherapy currently available to assess the efficacy and efficiency of such a therapy. Antisense gene therapy is a second approach that holds promise in eliminating and reversing hypertension. The principle behind AS treatment is to block the assembly of a specific protein by having AS RNA hybridize with the active messenger RNA (mRNA) and thereby block the translation of the mRNA.10,18 Many research groups, including our own, have focused on the AS approach to target the RAS for the following 3 reasons: (1) the RAS is widely distributed, and its role in the normal control of cardiovascular system is well understood; (2) impaired activity of the RAS in hypertension is well documented; and (3) traditional pharmacological agents that inhibit the RAS are proven to exert antihypertensive effects.8 Phillips and others15,19 demonstrated an antihypertensive effect using AS treatment to angiotensinogen and Ang II type 1 receptor AS (AT1RAS). After injecting these AS oligonucleotides either centrally or peripherally, the blood pressure decreased in the SHR. This effect lasted for days with the injection of naked DNA and for weeks with adeno-associated viral (AAV) vector-mediated delivery. However, no data regarding changes in the pathophysiology associated with hypertension were measured. Our research group expanded on these initial findings to determine if long-term antihypertensive effects could be induced with the use of a retroviral vector gene delivery system. Our observations demonstrate that a single intracardiac injection of a retroviral vector containing AT1R-AS or ACE-AS into the blood resulted in prevention of high blood pressure for at least 210 days. Attenuation of both vascular and cardiac pathophysiologies associated with hypertension included arterial wall thickness, cardiac hypertrophy, and perivascular and myocardial fibrosis, as well as prevention in alterations of vascular reactivity, endothelial dysfunction, renal damage, and ion channel dysfunction in renal arterioles.8,20,21 Further experiments demonstrated that when these AT1R-AS-treated rats were bred, the offspring had significantly lower blood pressure than did the hypertensive rats, indicating life-long control of hypertension.22 With evidence to suggest that both sense and AS strategies are conceptually sound and potentially useful, the most important issue concerns the means by which the transgene is delivered systemically so as to achieve a high degree of transduction efficiency. Which vector is most efficient as well as practical for such an antihypertensive therapy? We consider the advantages and disadvantages to both viral and nonviral methods of delivery in the following section (Table 1). Viral versus Nonviral Antisense Delivery Vehicles Before the formation of inhibitory hybrids between AS and endogenous mRNA, in terms of AS or expression of a functional gene for the sense strands, the transgene must first be efficiently delivered to the host cells. Thus, choice of delivery vehicles is extremely important. They are broadly divided into viral and nonviral vectors. Viral vectors are further divided into subtypes including retroviruses, adenoviruses, AAV, and lentiviruses. Nonviral vectors that have been used to deliver genes for hypertension and coronary artery disease include both naked DNA and liposomes. Although each has advantages