© 2000 American Heart Association, Inc.
Cellular Biology |
Abnormal Cardiac Na+ Channel Properties and QT Heart Rate Adaptation in Neonatal AnkyrinB Knockout Mice
From the Division of Cardiology (V.S.C., A.O.G.), Duke University Medical Center, and Department of Cell Biology (S.T., M.B., V.B.), Howard Hughes Medical Institute, Durham, NC.
Correspondence to A.O. Grant, Duke University Medical Center, Box 3504, Durham, NC 27710-3504. E-mail aog{at}carlin.mc.duke.edu
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Abstract—The cytoskeleton of the cardiomyocyte has been shown to modulate ion channel function. Cytoskeletal disruption in vitro alters Na+ channel kinetics, producing a late Na+ current that can prolong repolarization. This study describes the properties of the cardiac Na+ channel and cardiac repolarization in neonatal mice lacking ankyrinB, a cytoskeletal "adaptor" protein. Using whole-cell voltage clamp techniques, INa density was lower in ankyrinB(-/-) ventricular myocytes than in wild-type (WT) myocytes (-307±26 versus -444±39 pA/pF, P<0.01). AnkyrinB(-/-) myocytes exhibited a hyperpolarizing shift in activation and inactivation kinetics compared with WT. Slower recovery from inactivation contributed to the negative shift in steady-state inactivation in ankyrinB(-/-). Single Na+ channel mean open time was longer in ankyrinB(-/-) versus WT at test potentials (Vt) of -40 mV (1.0±0.1 versus 0.61±0.04 ms, P<0.05) and -50 mV (0.8±0.1 versus 0.39±0.05 ms, P<0.05). AnkyrinB(-/-) exhibited late single-channel openings at Vt -40 and -50 mV, which were not seen in WT. Late INa contributed to longer action potential durations measured at 90% repolarization (APD90) at 1 Hz stimulation in ankyrinB(-/-) compared with WT (354±26 versus 274±22 ms, P<0.05). From ECG recordings of neonatal mice, heart rates were slower in ankyrinB(-/-) than in WT (380±14 versus 434±13 bpm, P<0.01). Although the QT interval was similar in ankyrinB(-/-) and WT at physiological heart rates, QT-interval prolongation in response to heart rate deceleration was greater in ankyrinB(-/-). In conclusion, Na+ channels in ankyrinB(-/-) display reduced INa density and abnormal kinetics at the whole-cell and single-channel level that contribute to prolonged APD90 and abnormal QT-rate adaptation.
Key Words: Na+ channel • repolarization • transgenic mice • cytoskeleton • long-QT syndrome
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Introduction |
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Ankyrins are a family of cytoskeletal "adaptor" proteins that bind to integral membrane proteins, including ion channels such as the Cl–/HCO3– exchanger, Na+/K+-ATPase, Na+/Ca2+ exchanger, ryanodine receptor, inositol triphosphate receptor, and voltage-gated Na+ channel. Ankyrins play a key role in sustaining the structural integrity of the cell as well as localizing functionally distinct proteins to specific sites in the plasma membrane and intracellular membranes (for review, see Bennett and Gilligan1 ). In the heart, ankyrinB is present in (1) the sarcolemma in a costamere pattern,2 which represents sites of attachment of the sarcolemma to sarcomeres, and (2) the sarcoplasmic reticulum (SR).3 AnkyrinB is essential for correct intracellular assembly of Ca2+ homeostasis proteins to the SR of cardiomyocytes. We have previously described the phenotype of neonatal ankyrinB knockout (-/-) mice that lack ankyrinB. Four-day-old ankyrinB(-/-) ventricular myocytes exhibit abnormal cytosolic Ca2+ waves due to abnormal localization of ryanodine receptors, inositol triphosphate receptors, and SR-Ca2+ ATPase.3
The role of ankyrinB in sarcolemma ion channel function and regulation is unknown. The absence of ankyrinB in the costamere of ankyrinB(-/-) cardiomyocytes likely destabilizes the sarcolemma and the underlying cytoskeleton. Disruption of the cardiomyocyte cytoskeleton can modify the kinetics of the Na+,4 Ca2+,5 and KATP channel.6 Similarly, ion channel kinetics may be altered in ankyrinB(-/-) cardiomyocytes. With respect to the Na+ channel, Undrovinas et al4 have demonstrated that breakup of the actin-based cardiomyocyte cytoskeleton with cytochalasin D produced late Na+ current at hyperpolarizing test potentials. Late Na+ current has been shown to prolong cardiac repolarization in long-QT syndrome (LQTS) patients who carry the inactivation-deficient Na+ channel mutations.7 8 Thus, it is a plausible hypothesis that ankyrinB(-/-) cardiomyocytes will have altered Na+ channel kinetics that may influence cardiac repolarization. In contrast, ankyrinB(+/-) cardiomyocytes that have reduced cardiac ankyrinB levels when compared with wild-type (WT)9 may exhibit an intermediate Na+ channel phenotype.
The objectives of this study were to compare (1) the properties of the Na+ channel in ankyrinB(-/-), ankyrinB-(+/-), and WT 1-day-old ventricular myocytes using whole-cell and single-channel voltage-clamp techniques and (2) the APD and QT interval of these mice using current clamp techniques and surface ECG recordings, respectively. It was necessary to study neonates because of the limited life span of the knockout mice related in part to abnormal nervous system development.9 One-day-old cardiomyocytes were used because their SR-Ca2+ protein apparatus was still not fully developed (S. Tuvia, V. Bennett, unpublished observations, 1999), which would minimize differences in Ca2+ homeostasis between ankyrinB(-/-) and WT myocytes.
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Materials and Methods |
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Knockout Mice
WT, ankyrinB(+/-), and ankyrinB(-/-) mice were obtained from a breeding colony using animals produced by homologous recombination as previously described.9 For each mouse, genotyping was determined using Southern blot analysis.9 Animal protocols conformed to institutional guidelines.
Single-Cell Studies
Single ventricular myocytes were isolated from
1-day-old mice according to the neonatal cardiomyocyte
isolation system (Worthington Biochemical Corp). Dispersed myocytes
were cultured in medium containing 10% equine serum, 5% FBS, 50
µg/mL gentamicin, and DMEM/F-12 (1:1). All
electrophysiological studies were performed
24 to 48 hours after isolation at room temperature (22°C to 25°C)
on quiescent elongated cells showing cross-striations using an EPC-7
amplifier (LIST). INa was recorded
using the whole-cell voltage clamp technique. Ventricular
myocytes were superfused with a modified Tyrode’s solution containing
(in mmol/L) NaCl 65, CsCl 69, CaCl2 1,
MgCl2 5, HEPES 5, and glucose 5 (pH 7.4 with
NaOH). ICa, L was blocked with 0.05
mmol/L CdCl2. Recording electrodes
contained (in mmol/L) CsCl 130, EGTA 5,
MgCl2 1, NaGTP 0.1, MgATP 5, and HEPES 10 (pH 7.2
with CsOH). Electrode resistance (Rp)
ranged from 0.7 to 1 M
. Capacitative transients were nulled by
analog compensation and whole-cell capacitance was estimated from the
applied compensation. Series resistance
(Rs) ranged from 3 to 4 M and was
compensated by 80% to 90%, leaving a voltage error of 
3 mV. To
avoid the contaminating effect of time-dependent changes in
Na+ channel kinetics,
INa recordings were made 5 minutes
after membrane rupture. Leak currents were typically <100 pA and were
not corrected.
Single-channel recordings were performed in the cell-attached
configuration using 10-M microelectrodes. Cells were depolarized to
0 mV in a high-K+ bath containing (in
mmol/L) potassium aspartate 130, KCl 10, NaCl 10,
MgCl2 5, EGTA 0.05, and glucose 5 (pH 7.4 with
KOH). The recording pipette solution contained (in mmol/L)
NaCl 140, KCl 5, MgCl2 2.5,
CaCl2 0.02, HEPES 5, and glucose 5 (pH 7.4 with
NaOH). Currents were filtered at 2.5 kHz and sampled at 20 kHz.
Action potentials were recorded in the current-clamp configuration.
Myocytes were bathed in Tyrode’s solution containing (in mmol/L)
NaCl 140, KC1 4, CaCl2 2,
MgCl2 2, HEPES 5, and glucose 5 (pH 7.4 with
NaOH). The recording electrode (Rp
2 to 5 M) was filled with an internal solution containing (in
mmol/L) KCl 140, NaCl 10, EGTA 2, MgCl2 1, NaGTP
0.1, MgATP 5, and HEPES 10 (pH 7.2 with KOH). Cells were current
clamped (WT, 13±3 pA; ankyrinB[+/-], 19±3
pA; ankyrinB[-/-], 16±3 pA; P=NS)
to a resting membrane potential of -80 mV. Action potentials were then
elicited by 2-ms depolarizing pulses of twice-diastolic
threshold at 1 Hz. APD was measured at 90% repolarization
(APD90).
Electrocardiography
Surface ECGs were recorded from 1-day-old mice. Conscious
mice were held in the recumbent position with thin adhesive strips on a
heating pad at 35°C. Dry adhesive electrode strips were wrapped
around each of their 4 limbs, and ECG leads were connected to an
amplifier (Propac) (0.5 to 40 Hz). Lead I consistently resolved
ECG waveforms well and was digitized at 500 Hz using a 16-bit
analog/digital converter. The recording protocol
consisted of a baseline ECG at 35°C. Subsequent ECGs were
recorded at 5-minute intervals after transferring the mouse to a
room temperature environment that induced heart rate deceleration. To
minimize the noise in these low-amplitude ECG waveforms, a
signal-averaged ECG (SAECG) was derived from 10 consecutive,
well-defined complexes. The onset of the QRS complex was manually
selected as the fiducial point for ensemble averaging using custom
software (Testpoint, CEC). The QT interval was measured manually from
each SAECG from the beginning of QRS complex to the end of the T wave,
defined as the point of return to the isoelectric baseline.
Data Analysis
Voltage-clamp whole-cell and single-channel data were
compiled and analyzed using custom software written in C
language as previously described.8 Only whole-cell
experiments demonstrating adequate voltage control were
analyzed as defined by Whalley et al.10
INa density was determined by dividing
current amplitude by cell capacitance. For ECG analysis, the
rate-corrected QT interval using Bazett’s formula is inaccurate at the
rapid heart rates characteristic of neonatal mice.11
Therefore, for each mouse, QT intervals were plotted with their
corresponding R-R intervals. A semilogarithmic transformation of this
plot was best fitted with linear least-squares regression
analysis [QT=m(log R-R)+b]. The
regression coefficients (m and b) were used to
calculate the QT interval corresponding to a predetermined R-R interval
of 150 ms for each mouse. Continuous variables were compared
between WT and ankyrinB(+/-), as well as WT and
ankyrinB(-/-) using the unpaired Student
t test. Comparisons of continuous variables within
groups was done with the paired Student t test. Data are
expressed as mean±SEM. All tests were 2-sided, and differences were
considered significant at P<0.05.
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Results |
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Na+ Channel Studies
Whole-cell capacitance was comparable for WT (18±2 pF, n=18), ankyrinB(+/-) (19±2 pF, n=15), and ankyrinB(-/-) (16±1 pF, n=18). Peak INa density occurred at a test potential (Vt) of
-20 mV and was similar in WT
(444±39 pA/pF) and ankyrinB(+/-) (347±35
pA/pF). However, in ankyrinB(-/-),
INa density was less than in WT (-307±26
pA/pF, P<0.01) (Figure 1B
). To evaluate the basis for the
reduced INa density in
ankyrinB(-/-), cell-attached single-channel
recordings were performed serially at
Vt of -20, -40, and -50 mV.
Single-channel current was plotted against
Vt, and conductance was determined from the
linear regression slope. Single-channel conductance (g) was
similar in WT (11.6±0.1 pS, n=7) and
ankyrinB(-/-) (10.9±0.1 pS, n=8).
Single-channel open probability (Po) at
Vt of -20 mV, determined according to
Chandra et al,8 was also equivalent between
ankyrinB(-/-) (0.66±0.03) and WT (0.61±0.1).
Because whole-cell current is proportional to g,
Po, and channel number, the lower current
density in ankyrinB(-/-)
cardiomyocytes was the result of fewer functional
Na+ channels.
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The whole-cell voltage dependence of activation and inactivation
are summarized in the Table. No differences in
the kinetics of activation or inactivation were apparent between WT and
ankyrinB(+/-). When compared with WT,
ankyrinB(-/-) had a hyperpolarizing shift in
V0.5 for activation (-49.2±0.9 [n=18]
versus -46.3±0.9 mV [n=18], P<0.05) (Figure 1C
).
The V0.5 for inactivation was also more
negative in ankyrinB(-/-) (-79.5±1.4 mV,
n=11) than in WT (-74.6±1.6 mV (n=12), P<0.05) (Figure 2). No accompanying differences in slope
factors were found. To determine the basis for the inactivation shift
in ankyrinB(-/-), recovery from inactivation
kinetics was assessed using a 2-pulse protocol with varying recovery
intervals between the pulses at a holding potential
(Vh) of -100 mV (Figure 3). At -100 mV, the rate of
development of inactivation approximates 0, which permits recovery
rates from inactivation to be accurately determined. Recovery kinetics
were fit with a biexponential function, giving a slower rate of
recovery from inactivation in ankyrinB(-/-)
when compared with WT (Table), which accounted for the
hyperpolarizing shift in V0.5 for
inactivation in ankyrinB(-/-).
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To resolve the macroscopic kinetic differences between WT and
ankyrinB(-/-), cell-attached single-channel
studies were performed. For ankyrinB(-/-),
patches with only 1 to 2 channels were readily obtained at a
Vh of -100 mV. In contrast, WT patches
frequently had multiple channels at Vh of
-100 mV and required more depolarized Vh
to resolve single-channel events. This behavior is consistent
with our findings of reduced INa density in
ankyrinB(-/-) as a result of lower channel
number. Representative single-channel
recordings from 10 consecutive depolarizing steps at -20,
-40, and -50 mV are shown in Figure 4
for WT (2-channel patch) and ankyrinB(-/-)
(1-channel patch). Despite fewer channels in the
ankyrinB(-/-) patch, the ensemble average
currents at Vt of -40 and -50 mV were
larger in ankyrinB(-/-), suggesting a
hyperpolarizing shift in activation, which is consistent with
the whole-cell studies. In ankyrinB(-/-) and WT
patches, mean open times were voltage dependent and were 2-fold longer
in ankyrinB(-/-) at
Vt of -40 and -50 mV (Figure 5A). Open-time histograms were fit with a
monoexponential function, suggesting a single open
state.12 As expected from a Poisson process, the time
constants for channel closing (
closing)
derived from these histograms and the mean open times were the same
(Figure 5B). In ankyrinB(-/-), more
frequent reopenings were seen at Vt of -40
and -50 mV when compared with WT. To characterize these reopening and
mean closing intervals, closed-time histograms at
Vt of -40 and -50 mV were constructed. In
WT, a biexponential function was fit to the closing events of 6
patches (slow=4.0±0.5 ms,
fast=0.31±0.05 ms) at -40 mV. Closed-time
histograms could not be fit at Vt of -50
mV because of infrequent reopenings. For
ankyrinB(-/-), among the 3 patches fit with a
biexponential function, opening rates were faster than for WT
(slow=1.8±0.9 ms, P<0.05;
fast= 0.12±0.03 ms, P<0.05) at
-40 mV (Figure 6). The remaining 5
patches were fit with a monoexponential function and
were therefore not comparable with WT. At -50 mV, frequent reopenings
in ankyrinB(-/-) permitted closed-time
histograms to be constructed in 6 patches, 3 of which were fit with a
biexponential function (slow=7.2±1.5 ms,
fast= 0.26±0.05 ms).
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Channel reopenings in ankyrinB(-/-) were
observed late into the depolarizing test pulse at
Vt of -40 and -50 mV. During whole-cell
INa relaxation or decay
(INa-relax), channel reopenings have been
described at these hyperpolarizing test
potentials.12 13 To identify late channel reopenings
beyond the time course of INa-relax,
channel openings were detected after a time interval (T) corresponding
to 5 times the time constant for INa-relax
(relax). relax was
derived from a monoexponential fit of
INa-relax and was longer in WT than
ankyrinB(-/-) at Vt
of -40 (4.4±0.4 [n=18] versus 3.3±0.2 ms [n=18],
P<0.05) and -50 mV (10±1 versus 7.4±0.6 ms,
P<0.05). relax for WT was
therefore also applied to ankyrinB(-/-) in
calculating T. Late channel openings were defined as single-channel
openings after 25 ms at Vt of -40 mV and
after 50 ms at Vt of -50 mV for each
100-ms test depolarization. The integrated current from each
depolarization was divided by the single-channel current amplitude and
the number of channels in the patch to give the single-channel open
probability (Po). A diary of late events
was generated by plotting Po versus trace
number for 200 successive depolarizations. In all 7 WT patches, late
reopenings were infrequent and occurred at widely spaced intervals as
small spikes in the Po diary. In contrast,
3 of the 8 ankyrinB(-/-) patches showed
significantly more late activity that was distributed throughout the
Po diary at Vt
of -40 and -50 mV (Figure 7).
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Action Potential Measurements
To determine the effect of late INa on
ventricular repolarization, action potentials were
recorded from single myocytes (Figure 8). At a stimulation frequency of 1 Hz,
APD90s were comparable in WT (274±22 ms, n=9)
and ankyrinB(+/-) (283±14 ms, n=9). However,
ankyrinB(-/-) had longer
APD90 than WT (354±26 ms [n=8],
P<0.05).
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ECG Characteristics
Standard bipolar lead I was recorded from 1-day-old mice.
Their body weights differed slightly, as follows: WT, 1.53±0.16 g
(n=14); ankyrinB(+/-), 1.33±0.15 g (n=8,
P=0.05 versus WT); and
ankyrinB(-/-), 1.26±0.03 g (n=12,
P<0.01 versus WT). Three
ankyrinB(-/-) mice were excluded because of
marked heart rate variability at 35°C, which made QT-interval
measurements unreliable. No such heart rate variability was observed in
WT or ankyrinB(+/-) mice.
Representative SAECG traces are shown in Figure 9. The T-wave morphology was similar in
the 3 genotypes and characterized by a monophasic, upright T
wave. Baseline heart rate at 35°C was similar between WT (434±13
bpm) and ankyrinB(+/-) (424±8 bpm) mice but was
lower in ankyrinB(-/-) than WT (380±14 bpm,
P<0.01) mice. The average decline in heart rate at room
temperature over a 20-minute recording period was similar
among WT (194±12 bpm), ankyrinB(+/-) (185±8
bpm), and ankyrinB(-/-) (172±14 bpm) mice. The
QT interval at an R-R interval of 150 ms, which corresponded to a
physiological heart rate of 400 bpm, was comparable
in WT (83±2 ms), ankyrinB(+/-) (86±1 ms), and
ankyrinB(-/-) (81±5 ms) mice. The slope of the
QT–log R-R linear relationship was also similar between WT (150±7)
and ankyrinB(+/-) (167±18) mice. However,
ankyrinB(-/-) mice showed greater QT
prolongation in response to R-R interval lengthening, as suggested by a
larger regression slope when compared with WT (202±22,
P<0.05). Over a 150-ms change in cycle length from 150 to
300 ms, the QT interval increased by 61±7 ms in
ankyrinB(-/-) compared with 45±2 ms in WT
(P<0.05) mice.
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At physiological heart rates, 2 ankyrinB(-/-) mice exhibited intermittent failure of AV conduction, resulting in P waves without corresponding QRS complexes, which resolved at lower heart rates. Heart block was not apparent in any WT or ankyrinB(+/-) mouse. No ventricular arrhythmias were observed in the 3 genotypes.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In the present study, neonatal ankyrinB(-/-) ventricular myocytes exhibited several abnormalities in the Na+ channel compared with WT, including the following: (1) reduced INa density due to fewer functional Na+ channels; (2) hyperpolarizing shift in voltage-dependent activation and inactivation, the latter due to slower recovery from inactivation; and (3) greater late INa due to longer channel openings and late reopenings. Greater late INa may contribute to longer APD90 in ankyrinB(-/-) cardiomyocytes. Although prolonged cardiomyocyte repolarization is not associated with QT-interval lengthening at physiological heart rates, QT-rate adaptation is abnormal in ankyrinB(-/-) mice when compared with WT, resulting in greater QT prolongation with heart rate deceleration. In contrast, ankyrinB(+/-) mice have similar Na+ channel properties, APD90, and QT adaptation to WT, despite reduced cardiac ankyrinB levels.9
The phenotypic abnormalities of neonatal ankyrinB(-/-) mice have been well described and include hypoplasia of select regions of the nervous system9 and a skeletal muscle myopathy.3 These derangements may contribute to the reduced life span of ankyrinB(-/-) mice, with more than half of the animals dying on the first postnatal day. Although knockout mice have smaller hearts, cardiomyopathy is not evident. In contrast, ankyrinB(+/-) mice are similar to WT with respect to body weight, heart size, and survival (S. Tuvia, V. Bennett, unpublished observations, 1999). In the present study, 1-day-old mice were used to permit comparisons among the 3 genotypes. The neonatal period is characterized by cardiac Na+ channel maturation due to postnatal sympathetic innervation of the heart.14 In addition, developmental changes in potassium currents contribute to shorter APD in the adult when compared with the neonatal mouse.15 Thus, the differences in INa and QT intervals between neonatal ankyrinB(-/-) and WT mice may not be generalizable to the adult phenotypes. Despite this limitation, the present study is the first to characterize the effect of ankyrinB deficiency on the cardiac Na+ channel and ventricular repolarization in vivo.
Basis for Abnormal Na+ Channel Properties
Modulation of Na+ channel function by the
cytoskeleton has been previously described. In particular, disruption
of the actin-based cytoskeleton in ventricular myocytes
with cytochalasin D alters INa and
Na+ channel kinetics. Whole-cell
INa and Na+ channel
conductance is reduced without a change in single-channel conductance.
In addition, late Na+ channel burst openings
develop at hyperpolarizing test potentials.4 Greater
Na+ channel activation has also been induced in
epithelial16 and leukemic cells17 after actin
depolymerization with cytochalasin D or
gelsolin.
AnkyrinB localizes to the sarcolemma in a costamere pattern.3 The lack of ankyrinB at these membrane sites may cause incorrect organization of the costamere and the cytoskeletal elements known to bind to it, such as spectrin, dystrophin, and vinculin. Because costameres attach the sarcolemma to the sarcomeres,2 ankyrinB deficiency may disrupt folding of the sarcolemma during myocyte contraction. In view of this, we speculate that ankyrinB(-/-) cardiomyocytes have abnormal arrangement of their cytoskeleton, which may contribute to fewer functional Na+ channels with altered kinetics.
Late INa and Cardiac
Repolarization
Late INa can prolong cardiac
repolarization, as is evident from the inactivation-deficient
Na+ channels responsible for
LQT3.7 8 Unlike the LQT3-mutant
Na+ channel, which undergoes late reopenings at
plateau potentials of -20 mV, the late Na+
channel reopenings in ankyrinB(-/-) is seen
only at more hyperpolarized potentials of -40 and -50 mV. At these
potentials, no significant inward current normally competes with the
delayed and inward rectifying currents. Thus, late
INa may still prolong phase 3 of the action
potential in ankyrinB(-/-). However,
differences in other ionic currents may also play a role in lengthening
cardiac repolarization in ankyrinB(-/-).
Despite longer APD90 at 1 Hz stimulation in ankyrinB(-/-) cardiomyocytes, their QT interval at physiological heart rates is similar to that of WT. The discrepancy in APD90 and QT-interval response may be due to differences in cycle length. Cardiac repolarization time is dependent on cycle length, which will influence the contribution of abnormal repolarizing currents to the action potential. In ankyrinB(-/-), late INa may have less influence on APD and QT interval at physiological heart rates at which potassium repolarizing currents such as IKs accumulate. With less IKs at lower heart rates, late INa may predominate enough to prolong APD and the QT interval in the knockout mouse. This hypothesis is supported by longer QT intervals in ankyrinB(-/-) at lower heart rates when compared with WT.
Clinical Correlation With LQTS
LQTS is a genetically heterogeneous condition
characterized by QT-interval prolongation due to well-defined mutations
in the K channel18 (LQT1, LQT2, and LQT5) or
Na+ channel7 (LQT3). In contrast to
LQT1, LQT2, LQT3, and LQT5, LQT4 is characterized by marked QT-interval
prolongation and sinus bradycardia.19 The genetic defect
in LQT4 has not yet been defined, although it has been localized to
4q25 to 27. AnkyrinB(-/-) mice display several
ECG features of LQTS, including bradycardia,20 incomplete
AV block,21 and abnormal QT-rate
adaptation,22 the latter due in part to their late
INa. The basis for the slower heart rates
in ankyrinB(-/-), as well as functional
incomplete heart block in a few knockout mice, is not clear but may
relate to abnormalities in other ionic currents such as pacemaker
currents and ICa. The gene for
ankyrinB (ANK2) has been localized to 4q25 to 27,
which is also shared with LQT4. We therefore speculate that
ankyrinB deficiency may contribute to LQTS, in
particular LQT4. In previously described transgenic murine models of
LQTS involving loss of potassium channel function, several similarities
are seen with ankyrinB(-/-) mice, including APD
prolongation,23 24 25 normal QT intervals at
physiological heart rates,23 24 and
abnormal QT-rate adaptation.23
Unlike ankyrinB(-/-), the neonatal ankyrinB(+/-) mice have less cardiac ankyrinB deficiency and do not share the same ECG abnormalities. In contrast, adult LQT4 patients manifest the disease as heterozygote gene carriers as a result of autosomal dominant inheritance. It is possible that developmental maturation of cardiac ion channels, including the Na+ channel, may unmask repolarization abnormalities in adult ankyrinB(+/-) that are not apparent in the neonatal mice. It will be of interest to examine adult ankyrinB(+/-) mice for bradycardia and repolarization abnormalities of LQT4.
In conclusion, ankyrinB deficiency alters Na+ channel function and prolongs cardiac repolarization in neonatal ankyrinB(-/-) mice. A plausible mechanism for these changes may relate to cardiomyocyte cytoskeletal disruption. Future genetic testing of LQT4 patients for ANK2 mutations is warranted to further define the genetic basis for this condition.
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Acknowledgments |
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This study was supported in part by NIH Grant HL-32708 (to A.O.G.). V.S.C. is a research fellow of the Heart and Stroke Foundation of Canada.
Received September 27, 1999; accepted December 15, 1999.
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