AJH
LETTERS TO THE EDITOR
1992;5:930-
933
From December 4 to 6 , 1 9 9 1 , the National Heart, Lung, and Blood Institute (NHLBI) of the National Institutes of Health (Bethesda, MD) sponsored the first Conference on Human Gene Therapy to encourage researchers in disparate fields to consider the therapeutic potential of gene therapy. Despite the fact that this landmark conference was sponsored by NHLBI, the majority of the conference dealt with noncardiovascular diseases. The near absence at the first Conference on Human Gene Therapy of ideas relating to the treatment of the major cardiovascular diseases indicates that the time has come for cardiovascular scientists to increase their intellectual activities regarding gene therapy. The purpose of this letter is to stimulate increased activity in the area of gene therapy of cardiovascular diseases by presenting a speculative idea concerning gene therapy of hypertension that I call the "mutant angiotensinogen strategy for gene therapy of hypertension." Although this approach ultimately may not prove possible or practical, it is hoped that, at the very least, criticism of this proposal will provoke new and better ideas regarding gene therapy of hypertension. The mutant angiotensinogen strategy for gene therapy of hypertension is predicated on the premise that in many cases the renin-angiotensin system plays a pivotal role in the development and maintenance of hypertension, a premise supported by both clinical and experimental studies. For instance, angiotensin converting enzyme inhibitors are effective in approximately 50 to 7 5 % of hypertensive patients, and this fact has spurred intensive efforts to develop other approaches for interfering with the renin-angiotensin system such as renin inhibitors and nonpeptide angiotensin receptor antagonists. Although the exact prevalence of angiotensin-dependent hypertension in patients with essential hypertension is presently unknown, it no doubt will be determined with greater accuracy as highly specific and potent nonpeptide angiotensin receptor antagonists become widely employed in patients with essential hypertension. In this regard, chronic administration of nonpeptide, orally-active angiotensin receptor antagonists may afford a convenient and unambiguous way to assess whether or not a given patient has angiotensin-dependent hypertension. For those patients identified as having angiotensin-dependent hypertension, a permanent cure could be affected by gene therapy designed to inhibit the renin-angiotensin system. At least three different models for a mutant angiotensinogen strategy for gene therapy of hypertension can be conceived (Figure 1 ) . Delivery to cells of a properly © 1992 by the American Journal of Hypertension, Inc.
constructed gene coding for a mutant angiotensinogen would cause those cells to manufacture a mutant renin substrate (RSMUTANT)- This RSMUJAMT would be constitutively secreted by the same pathway that shuttles normal (wild type) angiotensinogen to the extracellular space. Provided RSMUTANT had a high affinity for renin, RSMUTANT would compete against angiotensinogen for the renin enzyme so that the production rate of angiotensin I (ANGI) from renin substrate would be reduced. Thus, with respect to normal renin substrate, the renin reaction would be inhibited. However, it is not sufficient that the renin reaction with respect to normal angiotensinogen be inhibited. To inactivate the renin-angiotensin system, it is also necessary for RSMUTANJ to be constructed such that no cleavage products (peptide molecules) with direct and/or indirect agonist activity result from the action of renin on RSMUTANT . This would be the case if: 1 ) the interaction of renin with RSMUTANT results in no product formation (model IA in Figure 1 ) ; 2 ) RSMUTANT is cleaved by renin, yet the product of that reaction is a mutant A N G I (ANGIMUTANT) that has little affinity for the angiotensin receptor per se and either is not a substrate for converting enzyme or, if it is, gives rise to a mutant angiotensin II (ANGIIMUTANT) or mutant angiotensin III (ANGIIIMUTANT) with little affinity for the angiotensin receptor (model IB in Figure 1 ) ; or 3 ) RSMUTANT is cleaved by renin, but the product of that reaction is an ANGIMUJANT that directly or indirectly (via conversion to ANGII/IIIMUTANT) antagonizes angiotensin^receptors (model II in Figure 1 ) . To summarize, model IA is characterized by the lack of product formation from the renin reaction with RSMUJANT · In models IB and II, product results from the renin reaction with RSMUTANTV however, in model IA the product has no biological activity and is not converted to any other peptide with biological activity, and in model II the product either blocks angiotensin receptors per se or is converted to other peptide(s) that block the angiotensin receptor. For all three models, amino acid substitution in RSMUJANT should not interfere with the ability of RSMUTANT bind with renin, ie, renin should have a low 1 ^ (high affinity) for RSMUTANT- ^ f / optimize renin inhibition, an RSMUTANT with increased affinity (relative to normal renin substrate) for renin would be desirable. In the case of model IA, RSMUJANT should bind to renin but should not serve as a substrate; therefore, the of renin for formation of ANGIMUTANT should be low. However, for models IB and II, renin should efficiently cleave RSMUTANT produce ANGIMUTANTt o
n
act
t o
t o
0895-7061/92/$5.00
Downloaded from https://academic.oup.com/ajh/article-abstract/5/12_Pt_1/930/176134 by Unitversity of Texas Libraries user on 18 November 2019
Gene Therapy for Hypertension
AJH-DECEMBER
MUTANT
1992-VOL
5, NO. 12, PART 1
ANGIOTENSINOGEN
STRATEGY
LETTERS TO THE EDITOR
FOR GENE THERAPY: MODELS IA & IB
RS,MUTANT
RS
Θ
RS
I 1 MODEL I IA
RENIN
MUTANT
MODEL
IB
t NO PRODUCT
ANGI
ANGI (LOW
CONVERTING ENZYME ANGII
ANG/RECEPTOR
L
M U T A N T
AFFINITY)
I I (or) • NO PRODUCT
RECEPTOR
ANGII/ΙΙΙμυΤΑΝΤ (LOW
AFFINITY)
EFFECT
FIGURE 1. Models of a mutant angiotensinogen gene therapy for hypertension. RS, renin substrate; ANGI, angiotensin I; ANGII, angio tensin II; RS , mutated renin substrate; ANGI , mutated angiotensin I; ANGII/IIImutant mutated angiotensin II and/or an giotensin III; ANGm, any mutated angiotensin peptide; dashed arrow at 1 indicates reduced interaction of renin substrate with renin; dashed anow at 2 indicates reduced inter action of angiotensin with receptor. MUTANT
MUTANT
MUTANT
ANGIOTENSINOGEN
STRA TEGY FOR GENE THERAPY: MODEL II
RS I
θ!
RENIN
ι t
ANGI
CONVERTING ENZYME ANGII 4 /
ANG/RECEPTOR
Λ
- •
RECEPTOR
EFFECT
ANG /RECEPTOR M
NO EFFECT
For model IB, neither ANGIMUTANT nor any "down stream peptides" that may be formed, such as ANGIIMUTANT or ANGIIIMUTANT/ should interact with the angiotensin subtype 1 ( A T ) receptor that mediates the known effects of A N G I I on the cardiovascular system. However, for model II, ANGIMUTANT or ANGII/IIIMUTANT should have a high affinity for the A T ! receptor, but low partial agonist activity. For both models IB and II, interaction of any mutant peptides with other biochemical systems would be undesirable. In particular, inhibition of converting enzyme by ANGIMUTANT would be disadvantageous, despite the potential for an additional antihypertensive mecha T
nism, because of the adverse effects associated with this biochemical action, such as chronic cough and angio neurotic edema (gene therapy must be extremely safe to be practical). Finally, regardless of the model for a mutant angiotensinogen strategy for gene therapy, RSMUTANT should not be antigenic and should not stimu late the production of neutralizing antibodies by the immune system that would bind to and inactivate RSMUTANT .
Model II has the advantage over models I A and I B that for any level of mutant gene expression, a greater inhibition of the renin-angiotensin system would be af fected since both the renin reaction and the angiotensin
Downloaded from https://academic.oup.com/ajh/article-abstract/5/12_Pt_1/930/176134 by Unitversity of Texas Libraries user on 18 November 2019
I I I I
931
932
LETTERS TO THE EDITOR
AJH-DECEMBER
1
8
1
5, NO. 12, PART 1
leap in antihypertensive therapy will be made not with the advent of new drugs, but rather with the introduction of highly safe and effective methods for permanently lowering arterial blood pressure. In this regard, gene therapy may be the technology of the future for the therapy of high blood pressure. EDWIN K. JACKSON From the Center for Clinical Pharmacology, Departments of Pharmacology and Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania. Address correspondence and reprint requests to Edwin K. Jackson, PhD, Associate Director, Center for Clinical Pharmacology, Scaife Hall, Room 623, Departments of Pharmacology and Medicine, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261. REFERENCES
1.
Jackson EK: In vivo evaluation of the antagonist activity of angiotensin I analogs. J Pharmacol Exp Ther 1992;260: 223-231.
A Note About Khoury et al's "The Early Morning Rise in Blood Pressure Is Mainly Related to Ambulation" Khoury et al have provided more information regarding how ambulation triggers some of the changes associated with the circadian pattern of blood pressure. This is complementary to the work of Parker et al, showing that ischemic events occur not when patients awaken, but when they assume the upright posture. There are many possibilities, as Khoury points out, that may trigger cardiovascular events: increases in catecholamines, rises in blood pressure, and increases in platelet aggregation. While a sudden rise in blood pressure might predispose one to plaque fracture, I think it may be too early to suggest a causal relationship. The ISAM study, when retrospectively analyzed, showed protection in patients treated with ^-blockers as opposed to those treated with calcium blockers. While we all appreciate the problems associated with retrospective analysis, these data might suggest that blood pressure per se is not the determining factor. In a prospective study, Deedwania showed that compared to placebo, /?-blockade was much more effective than calcium blockade in decreasing episodes of morning silent ischemia. Until future, well-designed, prospective studies comparing /?-blockers with calcium antagonists are performed, I feel that it may be premature to link the control
of blood pressure in the morning with protection against cardiovascular events.
1
2
3
PHILIP ALTUS From the Department of Internal Medicine, University of South Florida College of Medicine, Tampa, Florida. Address correspondence and reprint requests to Philip Altus, MD, Associate Chairman, Department of Internal Medicine, University of South Florida College of Medicine, 4 Columbia Drive, Tampa, FL 33606. REFERENCES
1.
Khoury AF, Sunderajan P, Kaplan NM: The early morning rise in blood pressure is related mainly to ambulation. Am J Hypertens 1992;5:339-344.
2.
Parker JD, Jimenez AH, Farrell B, et al: The morning increase in ambulatory ischemia is dependent on time of onset of physical activity. Circulation 1990;82:111-391.
3.
Willich SN, Linderer T, Wegscheider K, et al: Increased morning incidence of myocardial infarction in the ISAM Study: absence with prior ^-adrenergic blockade. Circulation 1989;80:853-858.
4.
Deedwania PC, Carbajal EV, Nelson JR, et al. Anti-ischemic effects of atenolol versus nifedipine in patients with coronary artery disease and ambulatory silent ischemia. J Am Coll Cardiol 1991;17:963-969.
4
Sodium Intake and Salt Sensitive Hypertension A recent paper by Alexiewicz et al reported that in salt sensitive hypertensive patients intracellular calcium was elevated on a high salt diet and was normalized either by a low salt diet or by nifedipine. The article claims that in salt-resistant hypertension intracellular 1
calcium was normal on high or low salt intake, with or without nifedipine. The article, therefore, suggests that elevation of intracellular calcium on a high salt diet and its normalization by low salt or nifedipine are characteristic of salt sensitive hypertension.
Downloaded from https://academic.oup.com/ajh/article-abstract/5/12_Pt_1/930/176134 by Unitversity of Texas Libraries user on 18 November 2019
receptor would be blocked. Its disadvantage is that it will be quite difficult to construct a RSMUTANT with all of the required properties. However, in a recent study, three ANGI analogs were identified that in vivo blocked the angiotensin receptor with a potency similar to Sar Ile -ANGII, but did not affect converting enzyme. One of these ANGI analogs has low partial agonist activity in vivo and its corresponding tetradecapeptide analog is a reasonably good substrate for purified renin (unpublished data). Thus our preliminary studies indicate that it may indeed be possible to construct a RSMUTANT that is processed by renin such that an ANGIMUTANJ is formed that both directly and indirectly (via ANGII/IIIMUTANT) blocks ATj receptors without excessive partial agonist activity and without inhibiting converting enzyme. Many antihypertensive drugs are now available for the safe and effective treatment of essential hypertension in most patients. Therefore, the next great quantum
1992-VOL
LETTERS TO THE EDITOR
1992;5:930-
933
From December 4 to 6 , 1 9 9 1 , the National Heart, Lung, and Blood Institute (NHLBI) of the National Institutes of Health (Bethesda, MD) sponsored the first Conference on Human Gene Therapy to encourage researchers in disparate fields to consider the therapeutic potential of gene therapy. Despite the fact that this landmark conference was sponsored by NHLBI, the majority of the conference dealt with noncardiovascular diseases. The near absence at the first Conference on Human Gene Therapy of ideas relating to the treatment of the major cardiovascular diseases indicates that the time has come for cardiovascular scientists to increase their intellectual activities regarding gene therapy. The purpose of this letter is to stimulate increased activity in the area of gene therapy of cardiovascular diseases by presenting a speculative idea concerning gene therapy of hypertension that I call the "mutant angiotensinogen strategy for gene therapy of hypertension." Although this approach ultimately may not prove possible or practical, it is hoped that, at the very least, criticism of this proposal will provoke new and better ideas regarding gene therapy of hypertension. The mutant angiotensinogen strategy for gene therapy of hypertension is predicated on the premise that in many cases the renin-angiotensin system plays a pivotal role in the development and maintenance of hypertension, a premise supported by both clinical and experimental studies. For instance, angiotensin converting enzyme inhibitors are effective in approximately 50 to 7 5 % of hypertensive patients, and this fact has spurred intensive efforts to develop other approaches for interfering with the renin-angiotensin system such as renin inhibitors and nonpeptide angiotensin receptor antagonists. Although the exact prevalence of angiotensin-dependent hypertension in patients with essential hypertension is presently unknown, it no doubt will be determined with greater accuracy as highly specific and potent nonpeptide angiotensin receptor antagonists become widely employed in patients with essential hypertension. In this regard, chronic administration of nonpeptide, orally-active angiotensin receptor antagonists may afford a convenient and unambiguous way to assess whether or not a given patient has angiotensin-dependent hypertension. For those patients identified as having angiotensin-dependent hypertension, a permanent cure could be affected by gene therapy designed to inhibit the renin-angiotensin system. At least three different models for a mutant angiotensinogen strategy for gene therapy of hypertension can be conceived (Figure 1 ) . Delivery to cells of a properly © 1992 by the American Journal of Hypertension, Inc.
constructed gene coding for a mutant angiotensinogen would cause those cells to manufacture a mutant renin substrate (RSMUTANT)- This RSMUJAMT would be constitutively secreted by the same pathway that shuttles normal (wild type) angiotensinogen to the extracellular space. Provided RSMUTANT had a high affinity for renin, RSMUTANT would compete against angiotensinogen for the renin enzyme so that the production rate of angiotensin I (ANGI) from renin substrate would be reduced. Thus, with respect to normal renin substrate, the renin reaction would be inhibited. However, it is not sufficient that the renin reaction with respect to normal angiotensinogen be inhibited. To inactivate the renin-angiotensin system, it is also necessary for RSMUTANJ to be constructed such that no cleavage products (peptide molecules) with direct and/or indirect agonist activity result from the action of renin on RSMUTANT . This would be the case if: 1 ) the interaction of renin with RSMUTANT results in no product formation (model IA in Figure 1 ) ; 2 ) RSMUTANT is cleaved by renin, yet the product of that reaction is a mutant A N G I (ANGIMUTANT) that has little affinity for the angiotensin receptor per se and either is not a substrate for converting enzyme or, if it is, gives rise to a mutant angiotensin II (ANGIIMUTANT) or mutant angiotensin III (ANGIIIMUTANT) with little affinity for the angiotensin receptor (model IB in Figure 1 ) ; or 3 ) RSMUTANT is cleaved by renin, but the product of that reaction is an ANGIMUJANT that directly or indirectly (via conversion to ANGII/IIIMUTANT) antagonizes angiotensin^receptors (model II in Figure 1 ) . To summarize, model IA is characterized by the lack of product formation from the renin reaction with RSMUJANT · In models IB and II, product results from the renin reaction with RSMUTANTV however, in model IA the product has no biological activity and is not converted to any other peptide with biological activity, and in model II the product either blocks angiotensin receptors per se or is converted to other peptide(s) that block the angiotensin receptor. For all three models, amino acid substitution in RSMUJANT should not interfere with the ability of RSMUTANT bind with renin, ie, renin should have a low 1 ^ (high affinity) for RSMUTANT- ^ f / optimize renin inhibition, an RSMUTANT with increased affinity (relative to normal renin substrate) for renin would be desirable. In the case of model IA, RSMUJANT should bind to renin but should not serve as a substrate; therefore, the of renin for formation of ANGIMUTANT should be low. However, for models IB and II, renin should efficiently cleave RSMUTANT produce ANGIMUTANTt o
n
act
t o
t o
0895-7061/92/$5.00
Downloaded from https://academic.oup.com/ajh/article-abstract/5/12_Pt_1/930/176134 by Unitversity of Texas Libraries user on 18 November 2019
Gene Therapy for Hypertension
AJH-DECEMBER
MUTANT
1992-VOL
5, NO. 12, PART 1
ANGIOTENSINOGEN
STRATEGY
LETTERS TO THE EDITOR
FOR GENE THERAPY: MODELS IA & IB
RS,MUTANT
RS
Θ
RS
I 1 MODEL I IA
RENIN
MUTANT
MODEL
IB
t NO PRODUCT
ANGI
ANGI (LOW
CONVERTING ENZYME ANGII
ANG/RECEPTOR
L
M U T A N T
AFFINITY)
I I (or) • NO PRODUCT
RECEPTOR
ANGII/ΙΙΙμυΤΑΝΤ (LOW
AFFINITY)
EFFECT
FIGURE 1. Models of a mutant angiotensinogen gene therapy for hypertension. RS, renin substrate; ANGI, angiotensin I; ANGII, angio tensin II; RS , mutated renin substrate; ANGI , mutated angiotensin I; ANGII/IIImutant mutated angiotensin II and/or an giotensin III; ANGm, any mutated angiotensin peptide; dashed arrow at 1 indicates reduced interaction of renin substrate with renin; dashed anow at 2 indicates reduced inter action of angiotensin with receptor. MUTANT
MUTANT
MUTANT
ANGIOTENSINOGEN
STRA TEGY FOR GENE THERAPY: MODEL II
RS I
θ!
RENIN
ι t
ANGI
CONVERTING ENZYME ANGII 4 /
ANG/RECEPTOR
Λ
- •
RECEPTOR
EFFECT
ANG /RECEPTOR M
NO EFFECT
For model IB, neither ANGIMUTANT nor any "down stream peptides" that may be formed, such as ANGIIMUTANT or ANGIIIMUTANT/ should interact with the angiotensin subtype 1 ( A T ) receptor that mediates the known effects of A N G I I on the cardiovascular system. However, for model II, ANGIMUTANT or ANGII/IIIMUTANT should have a high affinity for the A T ! receptor, but low partial agonist activity. For both models IB and II, interaction of any mutant peptides with other biochemical systems would be undesirable. In particular, inhibition of converting enzyme by ANGIMUTANT would be disadvantageous, despite the potential for an additional antihypertensive mecha T
nism, because of the adverse effects associated with this biochemical action, such as chronic cough and angio neurotic edema (gene therapy must be extremely safe to be practical). Finally, regardless of the model for a mutant angiotensinogen strategy for gene therapy, RSMUTANT should not be antigenic and should not stimu late the production of neutralizing antibodies by the immune system that would bind to and inactivate RSMUTANT .
Model II has the advantage over models I A and I B that for any level of mutant gene expression, a greater inhibition of the renin-angiotensin system would be af fected since both the renin reaction and the angiotensin
Downloaded from https://academic.oup.com/ajh/article-abstract/5/12_Pt_1/930/176134 by Unitversity of Texas Libraries user on 18 November 2019
I I I I
931
932
LETTERS TO THE EDITOR
AJH-DECEMBER
1
8
1
5, NO. 12, PART 1
leap in antihypertensive therapy will be made not with the advent of new drugs, but rather with the introduction of highly safe and effective methods for permanently lowering arterial blood pressure. In this regard, gene therapy may be the technology of the future for the therapy of high blood pressure. EDWIN K. JACKSON From the Center for Clinical Pharmacology, Departments of Pharmacology and Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania. Address correspondence and reprint requests to Edwin K. Jackson, PhD, Associate Director, Center for Clinical Pharmacology, Scaife Hall, Room 623, Departments of Pharmacology and Medicine, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261. REFERENCES
1.
Jackson EK: In vivo evaluation of the antagonist activity of angiotensin I analogs. J Pharmacol Exp Ther 1992;260: 223-231.
A Note About Khoury et al's "The Early Morning Rise in Blood Pressure Is Mainly Related to Ambulation" Khoury et al have provided more information regarding how ambulation triggers some of the changes associated with the circadian pattern of blood pressure. This is complementary to the work of Parker et al, showing that ischemic events occur not when patients awaken, but when they assume the upright posture. There are many possibilities, as Khoury points out, that may trigger cardiovascular events: increases in catecholamines, rises in blood pressure, and increases in platelet aggregation. While a sudden rise in blood pressure might predispose one to plaque fracture, I think it may be too early to suggest a causal relationship. The ISAM study, when retrospectively analyzed, showed protection in patients treated with ^-blockers as opposed to those treated with calcium blockers. While we all appreciate the problems associated with retrospective analysis, these data might suggest that blood pressure per se is not the determining factor. In a prospective study, Deedwania showed that compared to placebo, /?-blockade was much more effective than calcium blockade in decreasing episodes of morning silent ischemia. Until future, well-designed, prospective studies comparing /?-blockers with calcium antagonists are performed, I feel that it may be premature to link the control
of blood pressure in the morning with protection against cardiovascular events.
1
2
3
PHILIP ALTUS From the Department of Internal Medicine, University of South Florida College of Medicine, Tampa, Florida. Address correspondence and reprint requests to Philip Altus, MD, Associate Chairman, Department of Internal Medicine, University of South Florida College of Medicine, 4 Columbia Drive, Tampa, FL 33606. REFERENCES
1.
Khoury AF, Sunderajan P, Kaplan NM: The early morning rise in blood pressure is related mainly to ambulation. Am J Hypertens 1992;5:339-344.
2.
Parker JD, Jimenez AH, Farrell B, et al: The morning increase in ambulatory ischemia is dependent on time of onset of physical activity. Circulation 1990;82:111-391.
3.
Willich SN, Linderer T, Wegscheider K, et al: Increased morning incidence of myocardial infarction in the ISAM Study: absence with prior ^-adrenergic blockade. Circulation 1989;80:853-858.
4.
Deedwania PC, Carbajal EV, Nelson JR, et al. Anti-ischemic effects of atenolol versus nifedipine in patients with coronary artery disease and ambulatory silent ischemia. J Am Coll Cardiol 1991;17:963-969.
4
Sodium Intake and Salt Sensitive Hypertension A recent paper by Alexiewicz et al reported that in salt sensitive hypertensive patients intracellular calcium was elevated on a high salt diet and was normalized either by a low salt diet or by nifedipine. The article claims that in salt-resistant hypertension intracellular 1
calcium was normal on high or low salt intake, with or without nifedipine. The article, therefore, suggests that elevation of intracellular calcium on a high salt diet and its normalization by low salt or nifedipine are characteristic of salt sensitive hypertension.
Downloaded from https://academic.oup.com/ajh/article-abstract/5/12_Pt_1/930/176134 by Unitversity of Texas Libraries user on 18 November 2019
receptor would be blocked. Its disadvantage is that it will be quite difficult to construct a RSMUTANT with all of the required properties. However, in a recent study, three ANGI analogs were identified that in vivo blocked the angiotensin receptor with a potency similar to Sar Ile -ANGII, but did not affect converting enzyme. One of these ANGI analogs has low partial agonist activity in vivo and its corresponding tetradecapeptide analog is a reasonably good substrate for purified renin (unpublished data). Thus our preliminary studies indicate that it may indeed be possible to construct a RSMUTANT that is processed by renin such that an ANGIMUTANJ is formed that both directly and indirectly (via ANGII/IIIMUTANT) blocks ATj receptors without excessive partial agonist activity and without inhibiting converting enzyme. Many antihypertensive drugs are now available for the safe and effective treatment of essential hypertension in most patients. Therefore, the next great quantum
1992-VOL
