© 2000 American Heart Association, Inc.
UltraRapid Communications |
Two Distinct Congenital Arrhythmias Evoked by a Multidysfunctional Na+ Channel
From the Experimental and Molecular Cardiology Group (M.W.V., C.B., A.B., A.A.M.W.) and Department of Clinical Genetics (C.B.), Academic Medical Center, Amsterdam, the Netherlands; Departments of Anesthesiology and Pharmacology (P.C.V., J.R.B.), Vanderbilt University School of Medicine, Nashville, Tenn.
Correspondence to Jeffrey R. Balser, MD, PhD, 23rd and Pierce Avenues, Room 560, MRB II, Vanderbilt University School of Medicine, Nashville, TN 37212. E-mail jeff.balser{at}mcmail.vanderbilt.edu
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Abstract |
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Abstract—The congenital long-QT syndrome (LQT3) and the Brugada syndrome are distinct, life-threatening rhythm disorders linked to autosomal dominant mutations in SCN5A, the gene encoding the human cardiac Na+ channel. It is believed that these two syndromes result from opposite molecular effects: LQT3 mutations induce a gain of function, whereas Brugada syndrome mutations reduce Na+ channel function. Paradoxically, an inherited C-terminal SCN5A mutation causes affected individuals to manifest electrocardiographic features of both syndromes: QT-interval prolongation (LQT3) at slow heart rates and distinctive ST-segment elevations (Brugada syndrome) with exercise. In the present study, we show that the insertion of the amino acid 1795insD has opposite effects on two distinct kinetic components of Na+ channel gating (fast and slow inactivation) that render unique, simultaneous effects on cardiac excitability. The mutation disrupts fast inactivation, causing sustained Na+ current throughout the action potential plateau and prolonging cardiac repolarization at slow heart rates. At the same time, 1795insD augments slow inactivation, delaying recovery of Na+ channel availability between stimuli and reducing the Na+ current at rapid heart rates. Our findings reveal a novel molecular mechanism for the Brugada syndrome and identify a new dual mechanism whereby single SCN5A mutations may evoke multiple cardiac arrhythmia syndromes by influencing diverse components of Na+ channel gating function. The full text of this article is available at http://www.circresaha.org.
Key Words: Na+ channel • inactivation • long-QT syndrome • Brugada syndrome
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Voltage-gated ion channels play a fundamental role in the emerging understanding of inherited disorders in cardiac rhythm. Mutations in SCN5A, the gene encoding the human cardiac Na+ channel,1 underlie two distinct syndromes of abnormal cardiac electrophysiology: the long-QT syndrome (LQT3, linked to chromosome 3)2 and Brugada syndrome.3 These disorders produce distinct ECG abnormalities, but both herald an exceptional risk for sudden death due to life-threatening cardiac arrhythmias. Patients afflicted with LQT3 exhibit a delay in cardiac repolarization manifest, as the name implies, as a prolonged QT interval on the ECG. The Brugada syndrome features a distinctive ST-segment elevation in ECG leads that reflect electrical forces in the right ventricle but is not associated with QT-interval prolongation.4 The peculiar ECG characteristics of these inherited syndromes are manifest in the more common life-threatening cardiac rhythm disorders arising from disease or drug intolerance5 and thereby serve as valuable models for understanding fundamental molecular mechanisms that underlie arrhythmias.
Na+ channels initiate excitability in most
cardiac cells, opening abruptly to produce an inward depolarizing ionic
current (INa). Once activated, the
channels rapidly inactivate, extinguishing
INa within 10 ms. Nonetheless, the myocyte
remains depolarized for several hundred milliseconds, exhibiting the
signature plateau that distinguishes the cardiac action potential. LQT3
mutations produce gain-of-function defects by slightly disrupting
Na+ channel inactivation thereby causing a small
but persistent INa during the action
potential plateau.6 7 8 These mutations reside mainly
at positions in or near the cytoplasmic linker between domains III and
IV or the charged S4 segment in domain IV (Figure 1A
), regions that critically influence the rapid inactivation of
Na+ channels.9 10 In contrast, the
SCN5A mutations linked to the Brugada syndrome (Figure 1A) reside at diverse loci that have not been
consistently tied to a particular gating process. However,
Brugada syndrome mutations have consistently produced loss of
function.11 Although some Brugada-linked mutations
encode Na+ channels that are entirely
nonfunctional,3 others may elicit a functional deficit by
further accelerating the rate of rapid inactivation.12
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The opposing nature of the functional effects of LQT3 and Brugada
syndrome mutations on inactivation gating suggests that the two
disorders should not coexist in the same patient. Surprisingly, an
inherited SCN5A mutation was recently described in which
affected individuals exhibited the electrocardiographic manifestations
of both syndromes: QT-interval prolongation and distinctive ST-segment
elevations.13 In the present study, we identify
the molecular mechanism whereby this single C-terminal aspartic acid
insertion (1795insD) evokes both arrhythmia syndromes (Figure 1A
). The mutation disrupts the most rapid component of
inactivation (fast inactivation), causing a plateau of persistent
INa during sustained depolarization,
prolonging the QT interval at slow heart rates. At the same time,
1795insD induces depolarized Na+ channels to
undergo excessive slow inactivation during the sustained depolarization
period intrinsic to the cardiac action potential. This abnormal
behavior reduces Na+ channel availability
primarily at rapid heart rates and underlies the pronounced ST-segment
elevation seen in carriers during exercise. By conferring this unique
dual effect on Na+ channel gating function, this
single mutation provokes two distinct cardiac arrhythmia
syndromes.
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Materials and Methods |
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Site-directed mutagenesis was performed on SCN5A cDNA cloned in pSP64T, as previously described.13 The 1795insD cDNA was then subcloned into the HindIII–XbaI sites of the expression vector pCGI (kindly provided by David Johns and Eduardo Marbán, Johns Hopkins University, Baltimore, Md) for bicistronic expression of the channel protein and GFP reporter in HEK 293 cells. Cells were cotransfected with an equimolar ratio of Na+ channel human ß1 subunit (provided by A.L. George, Vanderbilt University). Whole-cell Na+ currents were recorded at 21°C within 1 to 3 days from HEK cells exhibiting green fluorescence using pipette resistances of 1 to 3 M
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(Axopatch 200B, Axon Instruments). The pipette solution contained
(in mmol/L) NaF 10, CsF 110, CsCl 20, EGTA 10, and HEPES 10 (pH
7.35 with NaOH). The bath solution contained (in mmol/L) NaCl 145,
KCl 4, CaCl2 1.8, MgCl2 1,
HEPES 10, and glucose 10 (pH 7.35). Currents were sampled at 20 kHz
(Digidata 1200 A/D board, Axon Instruments) and low pass–filtered at 5
kHz. The data were acquired using pClamp 8.0 (Axon Instruments) and
analyzed using Clampfit (Axon Instruments). Voltage-clamp
protocols are described with each figure (provided as insets). The
results are expressed as mean±SEM, and statistical comparisons were
made using one-way ANOVA (Microcal Origin), with P<0.05
indicating significance. Multiexponential functions were fitted to the
data using nonlinear least-squares methods (Microcal Origin). |
Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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We first examined the phenotypic effect of 1795insD on INa measured during a sustained depolarization. Figure 1B
shows wild-type
and 1795insD INa and reveals a persistent
plateau of noninactivating current in the mutant
that is nearly absent in wild-type cells. This current plateau is
sensitive to 30 µmol/L tetrodotoxin (Figure 1B, inset),
indicating that the current is produced by the transfected
Na+ channels. In cells expressing 1795insD, the
INa plateau measured at the end of a 200-ms
depolarization was 1.41±0.2% of the peak inward
INa measured at the onset of stimulation,
significantly greater than the plateau measured in cells expressing
wild-type channels (0.41±0.1%, P<0.01 versus 1795insD).
This degree of destabilization of fast inactivation is typical of
reports for other LQT3 mutations (0.5% to
2%).7 8 14 The rate of
INa decay was dominated (
90%) by a fast
component (
1=0.79±0.02 ms) that was not
slowed by the mutation (1=0.81±0.03, see
Figure 1B legend), similar to most other long-QT
mutations.6 7 14 A second, longer exponential
component (2, see Figure 1B legend)
described the terminal phase of the current decay, and this component
was slowed from 6.1±0.7 to 14.4±1.3 ms by the mutation
(P<0.001).
Nonetheless, these gain-of-function effects would not explain the
coexistent ST-segment changes in 1795insD carriers. The
effect of 1795insD on Na+ channel gating over a
range of membrane potentials is examined in Figure 2A, which plots the voltage dependence of
activation (squares) and inactivation (circles) for wild-type and
mutant channels. The solid lines show Boltzmann curves fit to the data
(see Figure 2A legend for parameters). Over a range
of depolarized membrane potentials (clamp protocols, Figure 2A, inset), activation was not changed by the mutation. In contrast, over a
range of holding potentials, inactivation of the mutant channel was
altered: V1/2 was shifted negative by nearly 10
mV, from -88.5±1.61 to -98.2±2.1 mV (P=0.003).
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, 
) and
inactivation (
, •) for wild-type (filled symbols) and 1795insD
channels (open symbols). The voltage-clamp protocols are shown, and the
data were fitted to a Boltzmann function
(y=[1+exp{(V-V1/2)/k}]-1),
where V1/2 is the half-maximal voltage and
k is the slope factor. For activation, wild-type (n=8)
V1/2=-37.3±2.3 mV and
k=6.9±0.64. For 1795insD (n=10),
V1/2=-38.3±2.05 mV (P=0.76
vs wild type) and k=7.9±0.58 (P=0.23 vs
wild type). For inactivation, wild-type (n=7)
V1/2=-88.5±1.61 mV and
k=6.2±0.36. For 1795insD (n=8),
V1/2=-98.2±2.1 mV (P=0.003
vs wild type) and k=6.8±0.24 (P=0.15 vs
wild type). The mutation caused a negative shift in the voltage
dependence of inactivation but did not alter activation. B, Separation
of steady-state inactivation into fast (
A negative shift in the voltage dependence of inactivation implies that
the mutation stabilized inactivation, an observation in apparent
conflict with the results in Figure 1B, which shows that at
least a portion of the mutant channels exhibit impaired inactivation
properties. We therefore considered the possibility that distinct
kinetic components of inactivation gating, fast and slow inactivation,
were affected differently by the mutation. The voltage dependence of
fast and slow inactivation was examined individually using the
protocols shown in Figure 2B (inset). To assess fast
inactivation, brief test pulses (10 ms) were used to avoid slow
inactivation. Because channels may undergo both fast and slow
inactivation during a sustained depolarization, slow inactivation was
assessed using longer test pulses, but a brief recovery period at -120
mV was inserted to allow recovery from only fast inactivation. Figure 2B (squares) shows that nearly all of the wild-type and mutant
channels were fast-inactivated (unavailable) at
membrane potentials positive to -40 mV (voltages near the action
potential plateau), whereas only a portion of the channels
slow-inactivated over the same membrane potential range
(Figure 2B, circles). Moreover, nearly 50% of the mutant
channels (open circles) were induced to slow-inactivate at
membrane potentials positive to -40 mV versus only 20% of the
wild-type channels (filled circles). In addition,
V1/2 for slow inactivation of the mutant channels
shifted to more negative membrane potentials than wild type
(-101.1±2.2 versus -80.3±5.4 mV, P=0.003), suggesting
that the mutant enhanced slow-inactivated state
stability.
The V1/2 for fast inactivation was also shifted
negatively by the mutation (-75.1±2.6 versus -55.7±1.6 mV,
P<0.001, Figure 2B), but the magnitude of the shift
was distorted by the much slower rate of fast inactivation at more
hyperpolarized potentials. In additional experiments that examined
INa availability after varying
depolarization periods at -100 mV, the time constant for development
of fast inactivation was 35 ms for wild type but only 15 ms for
1795insD (data not shown). Hence, the measured
INa availability after 10 ms (Figure 2B) underestimates the full extent of fast inactivation at these
hyperpolarized membrane potentials. This discrepancy is greater for
wild type than for 1795insD, explaining the rather marked shift between
the 10-ms wild-type and mutant voltage-dependent availability curves,
as well as the apparent change in the slope (Figure 2B). At the
same time, prolonging the depolarization period to 20 to 30 ms would
partly engage the slow inactivation process (see Figure 3A, 70 ms) and if used in Figure 2B would tend to overestimate the extent of fast inactivation.
Hence, the data in Figure 2B do not estimate steady-state fast
inactivation. Rather, the data provide a snapshot that compares the
extent of fast inactivation for the wild-type and mutant channels at 10
ms and suggest that fast inactivation of 1795insD over a range of
hyperpolarized membrane potentials was hastened.
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This enhancement in inactivation at membrane potentials negative to the
channel-opening threshold (-50 mV) would suggest that fast
inactivation from closed states may have been augmented by the
mutation. At the same time, Figure 1B shows clearly that the
fast inactivation of open channels at -20 mV was partly disabled. In
fact, the analysis of Figure 2B is insensitive to
effects on fast inactivation of open channels, because only peak
INa after a period of depolarization is
plotted. It is noteworthy that the destabilizing effect on fast
inactivation of the open channel would predominate at the depolarized
membrane potentials associated with the action potential plateau.
Analogous opposing effects on fast inactivation from open and closed
states were recently described for skeletal muscle
Na+ channel mutations linked to paramyotonia
congenita.15
To evaluate the kinetic features of enhanced 1795insD slow
inactivation, we used a two-pulse protocol (Figure 3A). The
duration of the first depolarization (P1) was varied, and the extent of
slow inactivation was indicated by the fractional reduction in peak
INa during the P2 pulse relative to that
recorded in the P1 pulse. A 10-ms repolarization to -120 mV was
interposed between P1 and P2 to allow recovery from fast inactivation.
Figure 3A shows that with increasing P1 duration, the current
elicited by P2 decreases as a result of slow inactivation. The solid
line shows a single exponential function
(y=Ae-t/)
fitted to the data. The amplitude (A) of the slowly
inactivating component was significantly greater in the mutant
(0.19±0.01) than wild type (0.06±0.01, P<0.001), a result
consistent with Figure 2B. The time constant () was
<200 ms in both cases (see Figure 3A legend),
consistent with the intermediate component of inactivation
induced by relatively short depolarizations (hundreds of milliseconds),
termed "IM"16 17 18 to
distinguish it from the classic slow-inactivated state
induced by much longer periods of depolarization.19
Recovery from inactivation was also evaluated with a paired-pulse
protocol (Figure 3B) by depolarizing the cells for 1 second (P1)
and varying the hyperpolarization interval before
the P2 pulse. Both wild-type and mutant data followed a biphasic time
course, reflecting components of recovery attributable to fast and slow
inactivation, and were therefore fitted with a biexponential function
(solid lines). The time constants for recovery from fast and slow
inactivation did not differ significantly for the mutant and wild-type
channels (see Figure 3B legend). The time constants for recovery
from slow inactivation approached 100 ms and were similar to values
reported previously for recovery from the
IM state.17 The amplitude
of the slow recovery component was greater for the mutant (0.2±0.01)
than the wild-type channel (0.13±0.01, P<0.001),
consistent with increased entry into the
slow-inactivated state during the preceding 1-second
depolarizing (P1) pulse, as demonstrated in Figure 3A.
The 1795insD mutation substantially increases the likelihood that
channels will slow-inactivate during a depolarization
lasting 100 to 200 ms (Figure 3A), well within the period of a
cardiac action potential. By increasing the fraction of
slow-inactivated channels, the mutation slows the overall
rate of recovery of availability after a sustained depolarization
(Figure 3B). These findings suggest that 1795insD should reduce
Na+ channel availability to a greater extent at
rapid stimulation rates or during a premature contraction, where the
hyperpolarized diastolic intervals between pulses are
brief. To examine this prediction, we repeatedly depolarized wild-type
or mutant channels for 0.5 seconds (0 mV) at cycle lengths of either
0.52 or 2.5 seconds (Figure 4A). These
values were selected on the basis of the observed differences in
wild-type and mutant recovery kinetics (Figure 3B) and also lie
within clinically observed cycle lengths and diastolic
intervals20 (Figure 4E). Wild-type and mutant
INa elicited during the 1st, 2nd, and the
20th depolarizations are shown. At the slower cycle lengths (Figure 4A, top), the magnitude of INa
remained constant throughout the train of stimuli for both channels.
However, at short cycle lengths, both wild-type and mutant
INa decreased during successive stimuli
(Figure 4A, bottom). This effect was more pronounced for the
mutant, and the marked difference from wild type was apparent by the
second beat. Data for a number of cells at both stimulation rates are
summarized in Figure 4B. By the 20th stimulus at the rapid rate,
1795insD INa was decreased by 74±5%
(n=5), whereas wild-type INa was decreased
by only 51±4% (n=6, P=0.006).
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The observed time-dependent effect of stimulus rate on
INa availability (Figures 4A and 4B)
was manifest in the ECG ST-segment changes of family members carrying
the 1795insD mutation. Figure 4C shows the ECG tracing from an
affected patient during rest (top) and with exercise (bottom). Although
the ST-segment elevation at rest was moderate, it was profoundly
increased during exercise. Figure 4D (top) shows that exercise
nearly tripled the ST-segment elevation, from 1.2 to 3.3 mm,
and this trend mirrored the change in heart rate (Figure 4D, bottom). Conversely, Figure 4E plots the QT interval measured in
the same patient (and a control patient) at various heart rates during
the exercise test. The QT interval of the 1795insD carrier was markedly
prolonged, but only at slow heart rates; at an R-R interval of 1150 ms,
the QT interval was 540 ms (504 ms corrected for heart rate, see Figure 4E legend). Consistent with this clinical effect, we
observed a significant reduction in the maintained,
noninactivating inward current when 1795insD
channels were depolarized at high frequencies that did not allow full
recovery from slow inactivation between pulses. For paired 500-ms
depolarizations to -20 mV separated by a 50-ms interval at -120 mV,
the second pulse-peak INa was 90.5±1.4%
of the first pulse, but the maintained current of the second pulse was
only 48.3±8.5% of the first pulse (n=5, P=0.0012, data not
shown). Hence, the mutation prolongs cardiac repolarization with a rate
dependence opposite to that of ST-segment elevation but
consistent with the classic long-QT phenotype.
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Discussion |
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Fast-inactivated Na+ channels normally remain quiescent during the action potential plateau and rapidly recover from inactivation during the hyperpolarized diastolic interval between stimuli. However, during sustained depolarization, Na+ channels may enter stable, nonconducting conformational states that require a prolonged period of hyperpolarization to recover (slow inactivation). The classic slow-inactivated state has recovery time constants >1 second and is induced by much longer depolarizations (
60 seconds)19 than considered in the
present study. Although cardiac Na+ channels
are less prone to occupy this ultrastable state than their skeletal
muscle counterparts,21 the cardiac channels do exhibit a
more prominent intermediate kinetic component of slow inactivation
(IM) that is induced by shorter
depolarization periods,18 well within the length of
the cardiac action potential. Our results (Figures 2 and 3) show that 1795insD channels have an even greater propensity
to slow-inactivate over depolarization periods typical for
the cardiac action potential, consistent with a functional
effect on IM. This facilitated slow
inactivation reduced the availability of Na+
channels at rapid stimulation rates (Figures 4A and 4B).
Conversely, at slow stimulation rates where the recovery interval is
prolonged, slow-inactivated channels can recover fully, so
peak INa was not affected by the mutation
(Figures 4A and 4B).
The anticipated proarrhythmic manifestation of the small, persistent
inward Na+ current caused by disrupted fast
inactivation (Figure 1B) is prolonged action potential duration.
This delay in myocyte repolarization, apparent on the ECG as
QT-interval prolongation, was recently validated in an elegant
quantitative model that incorporated the LQT3 fast inactivation gating
defect.22 In that study, the maintained current and other
LQT3 gating characteristics were reproduced by incorporating a
low-frequency gating mode. This gating mode permitted bursts of
openings throughout the action potential plateau due to a destabilized
fast inactivation process. The bursting mode did not allow slow
inactivation, in contrast to the higher frequency background gating
mode that did allow channels to both fast- and
slow-inactivate. Our finding that high-frequency
depolarization suppressed maintained current even more than peak
INa may be explained by this model, because
frequent depolarization may pull the gating equilibrium toward the
background mode, permitting slow inactivation but preventing bursting.
The frequency-dependent effects of membrane potential on slow
inactivation and maintained current in disease-linked
Na+ channel mutants deserve further
investigation.
Less is known about the mechanisms linking Na+ channel gating to the ECG manifestations of the Brugada syndrome. However, repolarization in the epicardial cell layer is known to be more sensitive to untoward suppression of peak INa (eg, by ischemia or drugs) due to a more prominent repolarizing K+ current in this layer (Ito).23 24 It is therefore postulated that mutations that reduce peak INa could selectively hasten epicardial repolarization, creating a transmural gradient for repolarization in the right ventricular outflow tract, consistent with the observed right precordial ST-segment changes in Brugada syndrome.25
Studies of 1795insD with whole-cell recordings from Xenopus oocytes did not identify a persistent inward current, nor did they reveal slow gating defects that could explain the dual clinical phenotype.13 Using expressed Na+ channels in cultured mammalian cells to allow improved voltage-clamp conditions, we show that 1795insD enhances Na+ channel slow inactivation and thereby reduces peak INa predominantly at rapid stimulation rates. This rate-dependent effect on INa availability mimics the exercise-dependent ST-segment changes in the affected carrier, suggesting that augmented slow inactivation is the molecular mechanism underlying the observed ECG changes during exercise. At the same time, we detected a plateau of persistent Na+ current during sustained depolarization, indicative of impaired fast inactivation. This gating effect readily explains the QT prolongation observed at slow heart rates, typical for the LQT3 phenotype.22
These results suggest that patients with both LQT3 and Brugada syndrome
would benefit from heart rate control, and, indeed, pacemaker therapy
has proved to be beneficial in this particular family.26
In the on-demand mode (VVIR or even AAIR), pacing only occurs when the
intrinsic heart rate drops (generally below 50 bpm). As such, higher
pacing rates are not induced, and the problem of pacing-induced
ST-segment elevation has not occurred. Given the detrimental effect of
exercise on the ST segments of 1795insD carriers (Figure 4D),
ß-adrenergic stimulation may also exacerbate the ST-segment elevation
of 1795insD carriers, although this may be less predictable. Studies of
patients with other Brugada mutations have found that unopposed
ß-adrenergic stimulation may improve right precordial ST-segment
elevation, presumably by increasing the L-type
Ca2+ current in a manner that restores the
epicardial action potential dome and thereby reducing electrical
heterogeneity.25
The fact that a single mutation in the C-terminus alters both the fast and slow components of inactivation suggests these kinetically distinct gating processes may interact functionally and even share structural characteristics.21 27 28 29 Although previous studies have shown that a nearby mutation in the carboxy terminus (E1784K) disrupts fast inactivation and provokes the long-QT syndrome,8 our findings suggest that the C-terminus may have a mechanistic role in the intermediate (IM) component of slow inactivation. Recent studies have postulated a mechanistic link between the IM gating component and use-dependent local anesthetic action.30 31 Hence, an increased propensity for Na+ channels to enter the IM state may not only explain the heart rate-dependent ECG phenotype (ST elevation) but also could underlie the unusual sensitivity of Brugada syndrome patients to potent Na+ channel blocking agents.4 32 Our findings reveal a novel molecular mechanism for the Brugada ECG phenotype and clarify how SCN5A mutations at a single locus may engender more than one arrhythmia syndrome by influencing kinetically distinct components of Na+ channel gating function.
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Acknowledgments |
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Financial support for this study was provided by NHS 95014 from the Dutch Heart Foundation, project 27 of the Netherlands Interuniversity Cardiology Institute (A.A.M.W.), and National Institutes of Health grant R01 GM56307 (J.R.B.).
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Footnotes |
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1 Both authors contributed equally to this study.
Received March 30, 2000; accepted April 18, 2000.
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