Functional and clinical characterization of a mutation in KCNJ2 associated with Andersen-Tawil syndrome.

BACKGROUND: Andersen-Tawil syndrome (ATS) is a rare inherited disorder, characterised by periodic paralysis, cardiac dysarrhythmias, and dysmorphic features, and is caused by mutations in the gene KCNJ2, which encodes the inward rectifier potassium channel, Kir2.1. This study sought to analyse KCNJ2 in patients with familial ATS and to determine the functional characteristics of the mutated gene. METHODS AND RESULTS: We screened a family with inherited ATS for the mutation in KCNJ2, using direct DNA sequencing. A missense mutation (T75R) of Kir2.1, located in the highly conserved cytoplasmic N-terminal domain, was identified in three affected members of this family. Using the Xenopus oocyte expression system and whole cell voltage clamp analyses, we found that the T75R mutant was non-functional and possessed a strong dominant negative effect when co-expressed with the same amount of wild type Kir2.1. Transgenic (Tg) mice expressing the mutated form of Kir2.1 in the heart had prolonged QTc intervals compared with mice expressing the wild type protein. Ventricular tachyarrhythmias were observed in 5 of 14 T75R-Tg mice compared with 1 of 7 Wt-Tg and none of 6 non-transgenic littermates. In three of five T75R-Tg mice with ventricular tachycardia, their ECG disclosed bidirectional tachycardia as in our proband. CONCLUSIONS: The in vitro studies revealed that the T75R mutant of Kir2.1 had a strong dominant negative effect in the Xenopus oocyte expression system. It still preserved the ability to co-assemble and traffic to the cell membrane in mammalian cells. For in vivo studies, the T75R-Tg mice had bidirectional ventricular tachycardia after induction and longer QT intervals.

Indications for mitral valve surgery.

The current indications for mitral valve surgery are summarized in this review. Remarkable advancements in surgical technique and in our ability to define the onset of ventricular dysfunction have led to increasingly liberal indications for mitral valve surgery. Mitral valve repair for mitral regurgitation (MR) might be employed in asymptomatic patients with normal left ventricular function if valve repair were almost certain while in patients with severe left ventricular dysfunction, where valve replacement with destruction of the valve apparatus is most unwise, successful repair can lead to an improved quality of life. However, much is left to be done: increased repair rate, better definition of the role of surgery in both ischemic and myopathic MR and the role of limited incision surgery are all on the exciting horizon of the future.

Progress in mitral and aortic regurgitation.

Over the past 15 years there has been rapid and dramatic change in the therapy for valvular heart disease. When mitral and aortic regurgitation are severe, they inevitably cause left ventricular damage, eventually resulting in death. However, when surgical correction of these lesions is timed appropriately, longevity can approach that of a normal population after surgery. As surgical techniques have improved, surgery is now indicated earlier in the course of these diseases. It is clear that some patients with mitral and aortic regurgitation require surgery even though they are entirely asymptomatic. However, it must be emphasized that mitral and aortic regurgitation are quite different from one another. These different lesions result in different loading conditions, different pathophysiologies, and have different means for surgical correction. All of these issues impact on the proper timing of surgery and are discussed.

Normal myocardial function in severe right ventricular volume overload hypertrophy.

Severe left ventricular volume overloading causes myocardial and cellular contractile dysfunction. Whether this is also true for severe right ventricular volume overloading was unknown. We therefore created severe tricuspid regurgitation percutaneously in seven dogs and then observed them for 3.5-4.0 yr. All five surviving operated dogs had severe tricuspid regurgitation and right heart failure, including massive ascites, but they did not have left heart failure. Right ventricular cardiocytes were isolated from these and from normal dogs, and sarcomere mechanics were assessed via laser diffraction. Right ventricular cardiocytes from the tricuspid regurgitation dogs were 20% longer than control cells, but neither the extent (0.171 +/- 0.005 microm) nor the velocity (2.92 +/- 0.12 microm/s) of sarcomere shortening differed from controls (0.179 +/- 0.005 microm and 3.09 +/- 0.11 microm/s, respectively). Thus, despite massive tricuspid regurgitation causing overt right heart failure, intrinsic right ventricular contractile function was normal. This finding for the severely volume-overloaded right ventricle stands in distinct contrast to our finding for the left ventricle severely volume overloaded by mitral regurgitation, wherein intrinsic contractile function is depressed.

The pathophysiology of mitral regurgitation.

The results of treatment of heart valve disease have improved steadily during the past 20 years. In aortic stenosis, although postoperative survival rates approximated those of age-matched controls, the outcome of surgery to treat ischemic and non-ischemic mitral regurgitation was grave. The reasons for this were two-fold: first, patients were referred for surgery late in the course of their disease, when irreversible left ventricular (LV) dysfunction prevented postoperative restoration of contractile function. Second, the value of the mitral valve apparatus in facilitating LV contraction was unrecognized, and this structure was often removed at surgery, in turn worsening pre-existent LV dysfunction. Consequently, patients with LV dysfunction due to mitral regurgitation underwent surgery that caused further damage to the left ventricle. Not surprisingly, postoperative LV function was poor, congestive heart failure persistent, and lifespan shortened. More recently, however, substantial insight has been gained into the value of the mitral valve apparatus, the causes of LV dysfunction in mitral regurgitation, and into the objective markers of LV function that permit the clinician to recommend surgery before muscle dysfunction has become severe and irreversible.

Microtubule depolymerization normalizes in vivo myocardial contractile function in dogs with pressure-overload left ventricular hypertrophy.

BACKGROUND: Because initially compensatory myocardial hypertrophy in response to pressure overloading may eventually decompensate to myocardial failure, mechanisms responsible for this transition have long been sought. One such mechanism established in vitro is densification of the cellular microtubule network, which imposes a viscous load that inhibits cardiocyte contraction. METHODS AND RESULTS: In the present study, we extended this in vitro finding to the in vivo level and tested the hypothesis that this cytoskeletal abnormality is important in the in vivo contractile dysfunction that occurs in experimental aortic stenosis in the adult dog. In 8 dogs in which gradual stenosis of the ascending aorta had caused severe left ventricular (LV) pressure overloading (gradient, 152+/-16 mm Hg) with contractile dysfunction, LV function was measured at baseline and 1 hour after the intravenous administration of colchicine. Cardiocytes obtained by biopsy before and after in vivo colchicine administration were examined in tandem. Microtubule depolymerization restored LV contractile function both in vivo and in vitro. CONCLUSIONS: These and additional corroborative data show that increased cardiocyte microtubule network density is an important mechanism for the ventricular contractile dysfunction that develops in large mammals with adult-onset pressure-overload-induced cardiac hypertrophy.

Bradycardia and the role of beta-blockade in the amelioration of left ventricular dysfunction.

BACKGROUND: It is clear that beta-blockers are effective for treatment of congestive heart failure, but their mechanism of action remains controversial. Hypothesized mechanisms include normalization of beta-receptor function and myocardial protection from the effects of catecholamines, possibly by the institution of bradycardia. We hypothesized that beta-blockade-induced bradycardia was an important mechanism by which these agents were effective for correction of LV dysfunction. METHODS AND RESULTS: In 2 groups of dogs with mitral regurgitation and LV dysfunction, beta-blockers were instituted. In 1 group that received beta-blockers and pacing (group beta+P), a pacemaker prevented the natural bradycardia that beta-blockers cause. In both groups, substantial LV dysfunction developed. Before beta-blockade, the end-systolic stiffness constant decreased from 3. 5+/-0.1 to 2.7+/-0.2 (P<0.01) at 3 months in group beta+P. A similar reduction occurred in the group that eventually received only beta-blockers (group betaB). In group betaB, end-systolic stiffness improved after 3 months of beta-blockade from 2.9+/-0.2 to 3.5+/-0.4 and was not different from baseline. However, in group beta+P, end-systolic stiffness failed to improve (2.7+/-0.2) after 3 months of mitral regurgitation, and was 2.9+/-0.2 at the end of the studies. The contractile function of cardiocytes isolated from the ventricles at the end of the studies confirmed these in vivo estimates of contractility. CONCLUSIONS: We conclude that institution of bradycardia is a major mechanism by which beta-blockers are effective for restoration of contractile function in a model of LV dysfunction.

Differential effects of pressure or volume overload on myocardial MMP levels and inhibitory control.

Left ventricular (LV) pressure (PO) or volume (VO) overload is accompanied by myocardial remodeling, but mechanisms that contribute to this progressive remodeling process remain unclear. The matrix metalloproteinases (MMPs) contribute to tissue remodeling in a number of disease states. This study tested the hypothesis that increased MMP expression and activity occur after the induction of an LV overload, which is accompanied by a loss of endogenous MMP inhibitory control. LV MMP zymographic activity and species abundance were measured in dogs under the following conditions: acute PO induced by ascending aortic balloon inflation (6 h, n = 9), prolonged PO by aortic banding (10 days, n = 5), acute VO through mitral regurgitation secondary to chordal rupture (6 h, n = 6), prolonged VO due to mitral regurgitation (14 days, n = 7), and sham controls (n = 11). MMP zymographic activity in the 92-kDa region, indicative of MMP-9 activity, increased over threefold in acute PO and VO and fell to control levels in prolonged PO and VO. The MMP-9 activity-to-abundance ratio increased by over fourfold with acute VO and twofold in acute PO, suggesting a loss of inhibitory control. Endogenous MMP inhibitor content was unchanged with either PO or VO. Interstitial collagenase (MMP-1) content decreased by 50% with acute VO but not with acute PO. Stromelysin (MMP-3) levels increased by 40% with acute VO and increased by 80% with prolonged PO. Although changes in LV myocardial MMP activity and inhibitory control occurred in both acute and prolonged PO and VO states, these changes were not identical. These results suggest that the type of overload stimulus may selectively influence myocardial MMP activity and expression, which in turn would affect the overall LV myocardial remodeling process in LV overload.

Translational mechanisms accelerate the rate of protein synthesis during canine pressure-overload hypertrophy.

This study examined how translational mechanisms regulate the rate of cardiac protein synthesis during canine pressure overload in vivo. Acute aortic stenosis (AS) was produced by inflating a balloon catheter in the ascending aorta for 6 h; sustained AS was created by controlled banding of the ascending aorta. AS caused significant hypertrophy as reflected by increased left ventricular (LV) mass after 5 and 10 days. To monitor LV protein synthesis in vivo, myosin heavy chain (MHC) synthesis was measured by continuous infusion of radiolabeled leucine. Acute AS accelerated the rate of myosin synthesis without a corresponding increase in ribosomal RNA, indicating an increase in translational efficiency. Total MHC synthesis (mg MHC/LV per day) was significantly increased at 5 and 10 days of sustained AS. Total MHC degradation was not significantly altered at 5 days of AS but increased at 10 days of AS in concordance with a new steady state with respect to growth. Translational capacity (mg total RNA/LV) was significantly increased after 5 and 10 days of AS and was preceded by an increase in the rate of ribosome formation. MHC mRNA levels remained unchanged during AS. These findings demonstrate that cardiac protein synthesis is accelerated in response to pressure overload by an initial increase in translational efficiency, followed by an adaptive increase in translational capacity during sustained hypertrophic growth.

Hypertrophic response to hemodynamic overload: role of load vs. renin-angiotensin system activation.

Myocardial hypertrophy is one of the basic mechanisms by which the heart compensates for hemodynamic overload. The mechanisms by which hemodynamic overload is transduced by the cardiac muscle cell and translated into cardiac hypertrophy are not completely understood. Candidates include activation of the renin-angiotensin system (RAS) and angiotensin II receptor (AT1) stimulation. In this study, we tested the hypothesis that load, independent of the RAS, is sufficient to stimulate cardiac growth. Four groups of cats were studied: 14 normal controls, 20 pulmonary artery-banded (PAB) cats, 7 PAB cats in whom the AT1 was concomitantly and continuously blocked with losartan, and 8 PAB cats in whom the angiotensin-converting enzyme (ACE) was concomitantly and continuously blocked with captopril. Losartan cats had at least a one-log order increase in the ED50 of the blood pressure response to angiotensin II infusion. Right ventricular (RV) hypertrophy was assessed using the RV mass-to-body weight ratio and ventricular cardiocyte size. RV hemodynamic overload was assessed by measuring RV systolic and diastolic pressures. Neither the extent of RV pressure overload nor RV hypertrophy that resulted from PAB was affected by AT1 blockade with losartan or ACE inhibition with captopril. RV systolic pressure was increased from 21 +/- 3 mmHg in normals to 68 +/- 4 mmHg in PAB, 65 +/- 5 mmHg in PAB plus losartan and 62 +/- 3 mmHg in PAB plus captopril. RV-to-body weight ratio increased from 0.52 +/- 0.04 g/kg in normals to 1.11 +/- 0.06 g/kg in PAB, 1.06 +/- 0.06 g/kg in PAB plus losartan and 1.06 +/- 0.06 g/kg in PAB plus captopril. Thus 1) pharmacological modulation of the RAS with losartan and captopril did not change the extent of the hemodynamic overload or the hypertrophic response induced by PAB; 2) neither RAS activation nor angiotensin II receptor stimulation is an obligatory and necessary component of the signaling pathway that acts as an intermediary coupling load to the hypertrophic response; and 3) load, independent of the RAS, is capable of stimulating cardiac growth.

Mechanisms of cardiac hypertrophy in canine volume overload.

This study tested whether the modest hypertrophy that develops in dogs in response to mitral regurgitation is due to a relatively small change in the rate of protein synthesis or, alternatively, is due to a decreased rate of protein degradation. After 3 mo of severe experimental mitral regurgitation, the left ventricular (LV) mass-to-body weight ratio increased by 23% compared with baseline values. This increase in LV mass occurred with a small, but not statistically significant, increase in the fractional rate of myosin heavy chain (MHC) synthesis (Ks), as measured using continuous infusion with [3H]leucine in dogs at 2 wk, 4 wk, and 3 mo after creation of severe mitral regurgitation. Translational efficiency was unaffected by mitral regurgitation as measured by the distribution of MHC mRNA in polysome gradients. Furthermore, there was no detectable increase in translational capacity as measured by either total RNA content or the rate of ribosome formation. These data indicate that translational mechanisms that accelerate the rate of cardiac protein synthesis are not responsive to the stimulus of mitral regurgitation. Most of the growth after mitral regurgitation was accounted for by a decrease in the fractional rate of protein degradation, calculated by subtracting fractional rates of protein accumulation at each time point from the corresponding Ks values. We conclude that 1) volume overload produced by severe mitral regurgitation does not trigger substantial increases in the rate of protein synthesis and 2) the modest increase in LV mass results primarily from a decrease in the rate of protein degradation.