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J Am Coll Cardiol, 2007; 49:578-586, doi:10.1016/j.jacc.2006.09.044 (Published online 19 January 2007).
© 2007 by the American College of Cardiology Foundation
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CLINICAL RESEARCH: ATRIAL FIBRILLATION

Stretch-Sensitive KCNQ1 Mutation

A Link Between Genetic and Environmental Factors in the Pathogenesis of Atrial Fibrillation?

Robyn Otway, PhD*, Jamie I. Vandenberg, MB, BS, PhD{dagger},{ddagger}, Guanglan Guo, PhD*, Anthony Varghese, PhD§, M. Leticia Castro, BMedSc(Hons)*, Jian Liu, PhD*, JingTing Zhao, PhD{dagger}, Jane A. Bursill, BTC{dagger}, Ken R. Wyse, BSc{dagger}, Haley Crotty, B Biomed Sc*, Olivia Baddeley, MSc*, Bruce Walker, MB, BS, PhD||, Dennis Kuchar, MD, FACC||, Charles Thorburn, MB, ChB|| and Diane Fatkin, MD*,{ddagger},||,*

* Sr. Bernice Research Program in Inherited Heart Diseases, Darlinghurst, New South Wales, Australia
{dagger} Mark Cowley Lidwill Program in Cardiac Electrophysiology and Biophysics, Victor Chang Cardiac Research Institute, Darlinghurst, New South Wales, Australia
{ddagger} Faculties of Medicine and Life Sciences, University of New South Wales, Kensington, New South Wales, Australia
§ Department of Computer Science, University of Wisconsin at River Falls, River Falls, Wisconsin
|| Cardiology Department, St. Vincent’s Hospital, Darlinghurst, New South Wales, Australia

Manuscript received July 14, 2006; revised manuscript received September 5, 2006, accepted September 27, 2006.

* Reprint requests and correspondence: Dr. Diane Fatkin, Victor Chang Cardiac Research Institute, Level 6, 384 Victoria Street, Darlinghurst NSW 2010, Australia. (Email: d.fatkin{at}victorchang.unsw.edu.au).


   Abstract
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 Abstract
Methods
 Results
 Discussion
 Appendix
 References
 
OBJECTIVES: This study sought to evaluate mutations in genes encoding the slow component of the cardiac delayed rectifier K+ current (I Ks) channel in familial atrial fibrillation (AF).

BACKGROUND: Although AF can have a genetic etiology, links between inherited gene defects and acquired factors such as atrial stretch have not been explored.

METHODS: Mutation screening of the KCNQ1, KCNE1, KCNE2, and KCNE3 genes was performed in 50 families with AF. The effects of mutant protein on cardiac I Ks activation were evaluated using electrophysiological studies and human atrial action potential modeling.

RESULTS: One missense KCNQ1 mutation, R14C, was identified in 1 family with a high prevalence of hypertension. Atrial fibrillation was present only in older individuals who had developed atrial dilation and who were genotype positive. Patch-clamp studies of wild-type or R14C KCNQ1 expressed with KCNE1 in CHO cells showed no statistically significant differences between wild-type and mutant channel kinetics at baseline, or after activation of adenylate cyclase with forskolin. After exposure to hypotonic solution to elicit cell swelling/stretch, mutant channels showed a marked increase in current, a leftward shift in the voltage dependence of activation, altered channel kinetics, and shortening of the modeled atrial action potential duration.

CONCLUSIONS: These data suggest that the R14C KCNQ1 mutation alone is insufficient to cause AF. Rather, we suggest a model in which a "second hit", such as an environmental factor like hypertension, which promotes atrial stretch and thereby unmasks an inherited defect in ion channel kinetics (the "first hit"), is required for AF to be manifested. Such a model would also account for the age-related increase in AF development.

Abbreviations and Acronyms
  AF = atrial fibrillation
  CHO = Chinese hamster ovary
  DNA = deoxyribonucleic acid
  I Ks = slow component of the delayed rectifier K+ current
  PCR = polymerase chain reaction
  WT = wild-type

Atrial fibrillation (AF) is a cardiac arrhythmia characterized by rapid and irregular activation of the atrium. Loss of coordinated atrial contraction results in blood stasis and reduced ventricular filling. Consequently, AF is a major risk factor for thromboembolic stroke and heart failure. The prevalence of AF increases with age, ranging from <1% in young adults to nearly 10% of those older than 80 years of age (1). Although considerable advances have been made in pharmacologic and nonpharmacologic approaches to therapy, morbidity, and mortality associated with AF remain high (2).

Despite the medical and economic importance of this condition, very little is known about its underlying etiology. Atrial fibrillation occurs generally as a complication of cardiac and systemic diseases, such as hypertension, coronary artery disease, valvular heart disease, and cardiomyopathies. Increased atrial pressure and/or volume with subsequent chamber dilation and activation of stretch-sensitive signaling pathways has been thought to be integral to AF development. Once AF is established, continued electrical and structural remodeling within the atria promote arrhythmia maintenance (3,4). Atrial fibrillation can also occur in the absence of precipitating disorders or cardiac structural changes ("lone" AF), which suggests that primary electrophysiological defects could also be an important risk factor.

The role of inherited gene defects in the pathogenesis of AF has only recently begun to be appreciated (5,6). KCNQ1 was the first disease gene identified for familial AF, with a single missense mutation, S140G, found in 1 family (7). KCNQ1 encodes the pore-forming alpha subunit of the channel that conducts the slow component of the delayed rectifier K+ current (I Ks), which contributes to repolarization of the cardiac action potential. KCNQ1 can interact with alternative beta subunits, KCNE1, KCNE2, and KCNE3. Mutations in KCNQ1 and its binding partners have previously been associated with the long QT syndrome, a disorder complicated by ventricular tachyarrhythmias and sudden death (8). In contrast to the long-QT–causing KCNQ1 mutations that have loss-of-function effects, the S140G KCNQ1 mutation was shown to induce a gain of function of cardiac I Ks channels. The S140G variant is the only KCNQ1 mutation reported to date in adult-onset familial AF. Recently, a de novo KCNQ1 mutation was shown to cause AF in utero (9). A gain-of-function mutation, R27C, in the KCNE2 gene has been identified in 2 unrelated kindreds with AF (10).

To further define the relative importance of perturbation of cardiac I Ks channel function in the pathogenesis of familial AF, we performed genetic analyses of the KCNQ1, KCNE1, KCNE2, and KCNE3 genes in a cohort of 50 kindreds. Functional and computational modeling studies were performed to evaluate the consequences of mutant protein on I Ks channel activity.


   Methods
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 Abstract
 Methods
Results
 Discussion
 Appendix
 References
 
Clinical evaluation.   Informed, written consent was obtained for participation in a protocol approved by the institutional Human Research Ethics Committee. Study subjects were evaluated by history and physical examination, 12-lead electrocardiography, and transthoracic echocardiography. Familial AF was suspected when 2 or more first-degree family members presented with documented nonvalvular AF. The deoxyribonucleic acid (DNA) was isolated from peripheral blood samples using conventional techniques.

Mutation screening.   Protein-coding sequences of the KCNQ1, KCNE1, KCNE2, and KCNE3 genes were amplified by polymerase chain reaction (PCR) from genomic DNA. Amplimers were purified and sequenced using the Big Dye terminator (version 3.1, Applied Biosystems, Foster City, California) and were analyzed on an ABI PRISM 3700 DNA Analyzer (UNSW DNA Analysis Facility, Sydney, New South Wales, Australia). DNA sequence variants were confirmed by restriction-enzyme digestion in family members and in 100 normal control subjects.

Cell culture.   Chinese hamster ovary (CHO) cells were co-transfected with KCNQ1 (kindly donated by Prof. Jentsch, University of Hamburg, Hamburg, Germany) and KCNE1 in a ratio of 1:4 using lipofectamine (Invitrogen, Carlsbad, California). In experiments in which the A-kinase adaptor protein yotiao (kindly donated by Prof. Scott, Oregon Health Sciences University, Portland, Oregon) was included, the ratio of constructs was 1:1:4 (yotiao:KCNQ1:KCNE1). The KCNE1 was sub-cloned into the pTSV40 vector (Invitrogen), which contains eGFP under a separate promoter, and transfected cells were identified by detection of eGFP fluorescence (11). The R14C mutation was introduced into the KCNQ1 coding sequence using the megaprimer PCR method (12).

Patch clamping.   Cells were superfused with normal Tyrode solution (in mM): 129 NaCl, 5 Na pyruvate, 5 Na acetate, 4 KCl, 1 MgCl2, 1.8 CaCl2, 11.1 glucose, 5 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES) (titrated to pH 7.4 with NaOH) at 37°C. Forskolin was added to the superfusate at a concentration of 1 µM. Cell swelling was induced by reducing the NaCl in the superfusate from 135 mM to 100 mM (osmolality reduced from ~280 mOsm to ~215 mOsm). The patch pipette solution contained (in mM): 30 KCl, 110 potassium aspartate, 1 MgCl2, 5 magnesium adenosine triphosphate, 10 ethylene glycol tetraacetic acid, 5 HEPES (pH 7.4 with potassium hydroxide). The liquid:liquid junction potential, –15 mV, was corrected for in all experiments. Recordings were made using an Axopatch 200A or Axopatch 200B amplifier (Axon Instruments, Foster City, California). Capacitance current transients were electronically subtracted, and series resistance compensation was at least 80%. Residual uncompensated series resistance was typically ~1 M{Omega}. Experiments in which currents resulted in voltage errors of >5 mV because of residual series resistance were excluded from the analysis. Current signals were filtered at 2 kHz and sampled at 5 kHz. Acquisition and analysis of data were performed using pClamp9 software (Axon Instruments, Union City, California). All summary data are expressed as mean ± SEM. Statistical comparisons were performed using analysis of variance or Student t test.

Atrial cardiac modeling.   Two human atrial cardiomyocyte models (13,14) were modified to include I Ks (15). In addition, the model of Courtemanche et al. (14) was modified to stabilize the long-term behavior (16). The parameters of the I Ks current were determined using experimental data obtained in CHO cells transfected with WT and mutant KCNQ1-KCNE1 (). Each cell model was paced at 1, 2, and 4 Hz using 1-ms current pulses for 3,602 s to reach a steady state. A differential-algebraic numerical integration scheme (17) was implemented using C++, Windows (Microsoft, Redmond, Washington) and Mac OS X (Apple, Cupertino, California) software.


   Results
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 Abstract
 Methods
 Results
Discussion
 Appendix
 References
 
Mutation screening.   Mutation screening of the coding sequences of the KCNQ1, KCNE1, KCNE2, and KCNE3 genes was performed in 50 familial AF probands using DNA sequence analysis. One missense mutation in the KCNQ1 gene was identified in the proband of Family D.R. (Fig. 1). This sequence variant, 40C to T, in exon 1A of the KCNQ1 gene, alters the amino acid at codon 14 from arginine to cysteine (R14C). The R14C variant results in loss of an Afe1 site and was independently confirmed in the proband and family members by restriction enzyme digestion. The sequence change was not found in 200 control chromosomes and had not been reported in a compendium of KCNQ1 variants in 744 healthy individuals (18). No mutations were identified in the KCNE1, KCNE2, or KCNE3 genes.


Figure 1
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  		Figure 1 Pedigree of Family D.R.

Solid symbols denote presence of hypertension (left) and/or atrial fibrillation (AF) (right), and striped symbols denote individuals with uncertain phenotype. Male subjects are shown as squares and female subjects as circles. The family proband (arrow) and the presence (+) or absence (–) of the R14C KCNQ1 mutation are indicated.

 
Family phenotype evaluation.   The proband in Family D.R., II-4, had been diagnosed with AF at the age of 59 years (Table 1). A detailed family history showed a high prevalence of hypertension (Fig. 1, Table 1, []). Of the 10 individuals with hypertension, 6 were positive and 4 were negative for the R14C variant. Atrial fibrillation was present only in those individuals who were genotype positive and who had atrial dilation. Evaluation of electrocardiographic recordings showed that corrected QT intervals were normal for all family members studied, with the exception of 1 individual who had a left bundle branch block.


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  		Table 1. Clinical Phenotype of Living Genotype-Positive Individuals in Family D.R.
 
Functional studies of the R14C KCNQ1 variant.   Expression of wild-type (WT) I Ks (WT KCNQ1 + KCNE1) or R14C I Ks (R14C KCNQ1 + KCNE1) channels in CHO cells resulted in slowly activating currents characteristic of cardiac I Ks channels (Fig. 2A). The peak current density measured at the end of the 4-s pulse was similar for the 2 channels (e.g., 165 ± 17 pA/pF at +60 mV for WT I Ks compared with 172 ± 21 pA/pF for R14C I Ks). The voltage for half-activation for WT I Ks channels, 28 ± 1.4 mV (n = 13), also was not statistically different from that for R14C I Ks channels, 28 ± 1.9 mV (n = 12) (Fig. 2A). Similar results were obtained in Xenopus oocytes (data not shown). Rates of activation, estimated by fitting a single exponential function to the second half of the currents recorded during depolarization steps to voltages in the range +20 to +60 mV, were slightly slower in R14C I Ks compared with WT I Ks channels (Fig. 2B), however, these differences were not statistically significant. Similarly, rates of deactivation of R14C I Ks channels were slightly slower than WT I Ks channels, although the differences were not statistically significant (Fig. 2C). Overall, very little difference was found between WT and R14C I Ks channels under baseline conditions.


Figure 2
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  		Figure 2 Baseline Characteristics of WT and R14C I Ks Channels

(A) Typical examples of currents recorded from wild-type (WT) (KCNQ1 + KCNE1) and R14C (KCNQ1-R14C + KCNE1) I Ks channels in Chinese hamster ovary (CHO) cells during 4-s depolarization steps from a holding potential of –80 mV to test potentials in the range –50 to +70 mV followed by repolarization to –60 mV (voltage protocol shown in inset). Scale bars are 0.5 s and 50 pA/pF. Horizontal dashed line indicates zero current level. Solid and dashed arrows indicate points where currents were measured for current-voltage relationships (upper panel to right) and tail currents for isochronal activation curves (lower panel on right), respectively. Tail current data were normalized to the maximum current value (Imax) and fitted with a Boltzmann function: I/Imax = [1 + exp((V0.5 – Vt)/k)]–1, where V0.5 is the half-activation voltage, Vt is the test potential, and k is the slope factor (values for V0.5 and k are summarized in []). (B) Typical examples of currents recorded during 4-s depolarization steps to voltages in the range 0 to +60 mV (voltage protocol shown in inset). Scale bars as for panel A. I Ks channels undergo transitions through multiple pre-open closed states before finally opening, as indicated by the sigmoidal shape of activation time courses. Rates of activation were estimated by fitting an exponential function (dashed lines) to the second half of the activation time course. Lower panel summarizes the time constants for activation at voltages in the range +20 to +60 mV for WT (red) and R14C (blue) I Ks channels. (C) Typical examples of tail currents recorded at voltages in the range –60 mV to –120 mV (voltage protocol shown in inset). Rates of deactivation were obtained by fitting a single exponential to tail current recordings. Lower panel summarizes time constants for deactivation in the range –120 mV to –60 mV for WT (red) and R14C (blue) I Ks channels.

 
To determine whether there might be differences in R14C I Ks channel responses to protein kinase A signaling pathways, CHO cells were transfected with WT or R14C KCNQ1, KCNE1, and yotiao. Activation of adenylate cyclase with 1 µM forskolin resulted in a gradual increase in current amplitude over a period of ~5 min, which was similar for WT and R14C I Ks channels (Figs. 3A and 3B). Forskolin caused a small increase in maximum current density for both WT and R14C I Ks channels (Fig. 3C) and a small but significant leftward shift in the voltage dependence of activation (Fig. 3D). There was, however, no statistically significant difference between forskolin-stimulated R14C and forskolin-stimulated WT I Ks channels. Forskolin resulted in similar changes in the rates of activation and deactivation for both WT and R14C I Ks channels (Figs. 3E and 3F).


Figure 3
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  		Figure 3 Effects of Fsk on WT and R14C I Ks Channels

(A) Forskolin (Fsk), 1 µM, resulted in an increase in current amplitude (green) for both wild-type (WT) and R14C I Ks channels. (B) The Fsk resulted in an ~60% increase in current recorded at the end of a 1-s depolarization pulse to +20 mV in both WT (red) and R14C (blue) I Ks channels. (C) The Fsk caused a small increase in current density measured after 4-s depolarization steps (protocol as shown in Figure 2A) for both WT and R14C IKs channels. (D) The Fsk caused a leftward shift in the V0.5 of the voltage-dependence of activation for WT (red dashed line = control, red boxes = + Fsk) and R14C (blue dashed line = control, blue boxes = + Fsk) I Ks channels. Both shifts were statistically significant (p < 0.05). The Fsk caused an acceleration of the rates of activation (E) and a deceleration of the rates of deactivation (F) for both WT and R14C I Ks channels. Symbols in panels E and F are the same as for panel D.

 
To evaluate R14C I Ks channel responses to stretch, transfected CHO cells were exposed to hypotonic solution to elicit cell swelling (19). An increase in current (Figs. 4A, 4B, and 4C), leftward shift in the voltage dependence of activation (Fig. 4D), acceleration of activation, and slowing of deactivation (Figs. 4E and 4F) were observed. These effects were all more marked in R14C I Ks channels than in WT I Ks channels. For example, the V0.5 of the voltage dependence of activation was shifted to a greater extent in R14C I Ks channels (23 ± 2.1 mV, n = 6) than in WT I Ks channels (13 ± 1.9 mV, n = 6, p < 0.05). Values for V0.5 and k for all experiments are summarized in (). The slowing of deactivation at –60 mV was also more marked for R14C I Ks channels (e.g., {tau}deact,–60 mV increase from 531 ± 41 ms to 937 ± 65 ms, n = 6) compared with WT ({tau}deact,–60 mV increase from 453 ± 42 ms to 640 ± 60 ms, n = 6) (Fig. 4F). The latter finding is likely to be of particular significance at faster heart rates because slower deactivation will result in accumulation of current during subsequent action potentials.


Figure 4
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  		Figure 4 Effects of Cell Swelling on WT and R14C I Ks Channels

(A) Hypotonic solution resulted in an increase in current amplitude (green) for both wild-type (WT) and R14C I Ks channels. (B) After 5-min exposure to hypotonic solution, currents recorded at the end of a 1-s depolarization pulse to +20 mV increased to a greater extent in R14C (blue, 215 ± 34%) compared with WT (red, 96 ± 34%) I Ks channels (WT vs. R14C, p < 0.05). (C) Hypotonic solution caused a greater increase in current density at all voltages for R14C compared with WT I Ks channels. (D) Hypotonic solution caused a greater leftward shift in the V0.5 of the voltage dependence of activation for R14C than for WT I Ks channels (values for V0.5 and k obtained from Boltzmann fits to the steady-state activation curves are summarized in ). Hypotonic solution caused an acceleration of the rates of activation (E) and deceleration of the rates of deactivation (F) for both WT and R14C I Ks channels. However, the changes in values for {tau}deact were more marked for R14C than for WT I Ks channels. Symbols in panels D to F are the same as in Figure 3.

 
Effects of R14C KCNQ1 on atrial electrophysiology.   To determine whether the kinetic differences and changes in whole cell conductance observed with cell swelling for WT and R14C I Ks channels would alter atrial electrophysiology, we evaluated 2 human atrial cardiomyocyte models (13,14) that were modified to incorporate an I Ks model (15). The R14C KCNQ1 mutation alone had a minimal effect on atrial action potential characteristics (data not shown). Cell swelling caused a greater reduction in the action potential duration for R14C I Ks channels compared with WT I Ks channels with both models (Fig. 5, Table 2), although the difference between WT and mutant channels was more marked in the Nygren model.


Figure 5
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  		Figure 5 Modeled Effects of R14C I Ks on Atrial Action Potentials

Modeled atrial action potentials using the (i) Nygren (13) and (ii) Courtemanche (14) human atrial action potential models. Results shown were obtained after stimulation for 6 min at 1 Hz (A), 2 Hz (B), and 4 Hz (C). Control data for WT and R14C were indistinguishable (black). Cell swelling caused more action potential shortening in models with R14C (blue) compared with WT I Ks channels. The heterozygous phenotype (WT/R14C, dashed blue) was intermediate between the WT and R14C phenotypes but is only shown for the Nygren model, panel (i), where the differences can be seen clearly.

 

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  		Table 2. Model-Derived Values for APD90 for the Nygren and Courtemanche Human Atrial Action Potential Models
 

   Discussion
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 Abstract
 Methods
 Results
 Discussion
Appendix
 References
 
Here we report a novel stretch-responsive KCNQ1 mutation in a kindred with late-onset familial AF. Functional studies indicate that the R14C variant does not alter baseline I Ks current but does have a gain-of-function effect with cell swelling. Our findings suggest a "two-hit" model in which genetically predisposed individuals (the first "hit") require a second key factor, such as atrial dilation (the second "hit") for AF to be manifested.

Although the I Ks channel is thought to have a major role in repolarization of the human ventricle, its physiological significance in the atrium is unclear because it provides only a small contribution to atrial repolarization under resting conditions (20). The KCNQ1 protein comprises 6 membrane-spanning motifs (S1-S6) with cytoplasmic N- and C-terminal domains. The majority (>90%) of long-QT–causing KCNQ1 mutations reported to date have been located in the transmembrane and C-terminal domains (21). The R14C variant is located within the short cytoplasmic N-terminus of the KCNQ1 protein. This region has previously been implicated in cardiomyocyte responses to sympathetic stimulation. Specifically, cyclic adenosine monophosphate–mediated regulation of cardiac I Ks channel function, which is dependent on a macromolecular complex, comprised of protein kinase A, protein phosphatase 1, and the A-kinase adaptor protein yotiao (22,23), results in phosphorylation of Ser27 in the N-terminal domain of KCNQ1. We found, however, that activation of adenylate cyclase with forskolin had similar effects on WT and R14C I Ks channels. The N-terminal domain of KCNQ1 is also involved in cellular responses to mechanical stretch. Cell membrane stretch can activate mechanosensitive ion channels by direct effects on membrane curvature or thickness, or by changes in cytoskeletal-membrane protein interactions. The KCNQ1 N-terminal domain interacts with the actin cytoskeleton and is thought to act as a sensor of cellular stretch (24). The I Ks channel activation in cardiac myocytes has been shown to be stretch responsive, whether stretch is elicited by positive pressure or cell swelling (25,26). Our data confirm the stretch sensitivity of normal I Ks channels and show enhanced effects in R14C mutant channels.

Epidemiologic studies have shown that age, hypertension, and left atrial dilation (a marker of atrial stretch) are independent risk factors for AF (1,3). Hypertension can cause left atrial dilation as a result of elevated left ventricular filling pressures and impaired diastolic function. Progressive atrial dilation predisposes to AF. Once AF is established, further increases in atrial size may occur as a consequence of structural remodeling of the atrial wall. Therefore, one possible interpretation of the findings in Family D.R. is that R14C KCNQ1 predisposes to hypertension rather than AF, and that the occurrence of AF in affected family members is predominantly related to patient age and the sequelae of hypertension. It is notable, however, that 4 of the 10 family members with hypertension did not carry the R14C variant, and that 2 of the 5 family members with hypertension who were age >80 years did not carry the R14C variant and did not have AF. None of the genotype-positive individuals with hypertension who were age <80 years had either atrial dilation or AF. A parsimonious explanation for these data is that the combination of R14C KCNQ1, together with atrial dilation, associated with hypertension and increasing age, promotes AF development in this family.

KCNQ1 is not only present in the main body of the left atrium, but is also highly expressed in the pulmonary veins (27). Recent studies have shown that AF is frequently initiated by focal ectopic activity in the vicinity of the pulmonary veins (28). Although the mechanisms by which pulmonary veins contribute to arrhythmia development are not well understood, dilation of the pulmonary venous ostia has been implicated. Hypertension and persistent AF have both been associated with increased ostial pulmonary vein diameter (29). It is intriguing to speculate that stretch-induced augmentation of pulmonary venous I Ks channel activation could be involved specifically in the initiation and/or maintenance of AF in the family investigated here. Ion channel defects that shorten action potential duration might be expected to be associated not only with AF, but also with a short QT interval. The presence of normal QT intervals in genotype-positive individuals in Family D.R. is entirely consistent with our finding that R14C KCNQ1 does not alter baseline I Ks channel function, and can most readily be explained by a differential extent of atrial and ventricular chamber stretch.

The low prevalence of KCNQ1 mutations in the present study (1 of 50 families) is similar to findings in 2 other cohorts of individuals with AF. Chen et al. (7) identified a KCNQ1 mutation in 1 of 7 families with no mutations identified in 19 sporadic cases, and Ellinor et al. (30) reported no mutations in a series of 141 unrelated subjects with lone AF. Although KCNQ1 gene mutations are not a common cause of familial AF, identification of these mutations is useful because there are potentially major implications for the management of genotype-positive family members. Although drug therapy with selective I Ks-blocking agents (31) may prove to be useful in some families with gain-of-function KCNQ1 mutations, our findings suggest that in others, aggressive treatment of hypertension and maintenance of normal left atrial size may be relatively more important, particularly in young individuals, in whom AF may be potentially preventable.

There are 2 main limitations of this study. The small number of affected individuals limits the definitive conclusions that can be drawn, and further studies in larger patient populations are required. Cell swelling is frequently used as a surrogate experimental model for cell stretch because the basic physical principle involved in both perturbations is distortion of the surface membrane and its intrinsic integral membrane proteins. The effects of acute cell swelling induced by hypotonic solution do not, however, fully reproduce those of chronic atrial dilation. Despite these limitations, we suggest that our data provide proof of principle for significant interactions between genetic and environmental factors in the pathogenesis of AF. Our findings have a number of potentially important implications for both experimental and clinical studies. For example, a modifying effect of environmental factors needs to be taken into account when performing in vitro studies of putative AF-causing mutations, and might explain the apparently negative functional assays of a recently described KCNE3 sequence variant (32). More generally, in populations of individuals with conditions such as hypertension that predispose to atrial dilation, identification of genetic risk factors may select a subgroup with an increased propensity to develop AF, in whom close medical surveillance and monitoring of atrial size are warranted. Further investigation of the roles of inherited gene defects and environmental factors in the development of AF may have major ramifications for the clinical management not only of the relatively rare familial syndrome, but also of the more commonly occurring acquired disorder. (5)


   Appendix
Top
 Abstract
 Methods
 Results
 Discussion
 Appendix
References
 
For supplementary Tables 1 and 2, and the equations and constants for I Ks, please see the online version of this article.


   Acknowledgments
 
The authors thank all of the participating family members, Jasmine Hessell and Louise Lynagh for assistance with clinical data collection, Robert Graham for critical review of the manuscript, and the following physicians for referring study probands: Ruth Arnold, Richard Cranswick, Deborah Hayes, Peter Hayes, Christopher Hayward, Anne Keogh, Jim Leitch, Alex Levendel, Peter Macdonald, Matthew Pincus, David Ross, Jonathan Silberberg, Paul West, and Thomas Yeoh.


   Footnotes
 
Supported by the Sylvia and Charles Viertel Charitable Foundation, Melbourne, Victoria, Australia; the National Health and Medical Research Council, Canberra, Australian Capital Territory, Australia; and St. Vincent’s Clinic Foundation, Sydney, New South Wales, Australia. Drs. Otway and Vandenberg contributed equally to this work.


   References
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 Abstract
 Methods
 Results
 Discussion
 Appendix
 References
 

  1. Kannel WB, Wolf PA, Benjamin EJ, Levy D. Prevalence, incidence, prognosis and predisposing conditions for atrial fibrillation: population-based estimates Am J Cardiol 1998;82:2N-9N.[CrossRef][ISI][Medline]
  2. Benjamin EJ, Wolf PA, D’Agostino RB, Silbershatz H, Kannel WB, Levy D. Impact of atrial fibrillation on the risk of deathThe Framingham Heart Study. Circulation 1998;98:946-952.
  3. Wijffels MC, Kirchhof CJ, Dorland R, Allessie MA. Atrial fibrillation begets atrial fibrillationA study in awake chronically instrumented goats. Circulation 1995;92:1954-1968.
  4. Nattel S. New ideas about atrial fibrillation 50 years on Nature 2002;415:219-226.[CrossRef][Medline]
  5. Darbar D, Herron KJ, Ballew JD, et al. Familial atrial fibrillation is a genetically heterogeneous disorder J Am Coll Cardiol 2003;41:2185-2192.[Abstract/Free Full Text]
  6. Fox CS, Parise H, D’Agostino RB, et al. Parental atrial fibrillation as a risk factor for atrial fibrillation in offspring JAMA 2004;291:2851-2855.[Abstract/Free Full Text]
  7. Chen YH, Xu SJ, Bendahhou S, et al. KCNQ1 gain-of-function mutation in familial atrial fibrillation Science 2003;299:251-254.[Abstract/Free Full Text]
  8. Keating MT, Sanguinetti MC. Molecular and cellular mechanisms of cardiac arrhythmias Cell 2001;104:569-580.[CrossRef][ISI][Medline]
  9. Hong K, Piper DR, Diaz-Valdecantos A, et al. De novo KCNQ1 mutation responsible for atrial fibrillation and short QT syndrome in utero Cardiovasc Res 2005;68:433-440.[Abstract/Free Full Text]
  10. Yang Y, Xia M, Jin Q, et al. Identification of a KCNE2 gain-of-function mutation in patients with familial atrial fibrillation Am J Hum Genet 2004;75:899-905.[CrossRef][ISI][Medline]
  11. Lu Y, Mahaut-Smith MP, Varghese A, Huang CL, Kemp PR, Vandenberg JI. Effects of premature stimulation on HERG K+ channels J Physiol 2001;537:843-851.[Abstract/Free Full Text]
  12. Clarke CE, Hill AP, Zhao J, et al. Effect of S5P {alpha}-helix charge mutants on inactivation of hERG K+ channels J Physiol 2006;573:291-304.[Abstract/Free Full Text]
  13. Nygren A, Fiset C, Firek L, et al. Mathematical model of an adult human atrial cell: the role of K+ currents in repolarization Circ Res 1998;82:63-81.[Abstract/Free Full Text]
  14. Courtemanche M, Ramirez RJ, Nattel S. Ionic mechanisms underlying human atrial action potential properties: insights from a mathematical model Am J Physiol 1998;275:H301-H321.
  15. Terrenoire C, Clancy CE, Cormier JW, Sampson KJ, Kass RS. Autonomic control of cardiac action potentials: role of potassium channel kinetics in response to sympathetic stimulation Circ Res 2005;96:e25-e34.[Abstract/Free Full Text]
  16. Kneller J, Ramirez RJ, Chartier D, Courtemanche M, Nattel S. Time-dependent transients in an ionically based mathematical model of the canine atrial action potential Am J Physiol Heart Circ Physiol 2002;282:H1437-H1451.[Abstract/Free Full Text]
  17. Varghese A, Sell G. A conservation principle and its effect on the formulation of Na-Ca exchanger current in cardiac cells J Theor Biol 1997;189:33-40.[CrossRef][ISI][Medline]
  18. Ackerman MJ, Tester DJ, Jones GS, Will ML, Burrow CR, Curran ME. Ethnic differences in cardiac potassium channel variants: implications for genetic susceptibility to sudden cardiac death and genetic testing for congenital long QT syndrome Mayo Clin Proc 2003;78:1479-1487.[ISI][Medline]
  19. Rees SA, Vandenberg JI, Wright AR, Yoshida A, Powell T. Cell swelling has differential effects on the rapid and slow components of delayed rectifier potassium current in guinea pig cardiac myocytes J Gen Physiol 1995;106:1151-1170.[Abstract/Free Full Text]
  20. Wang Z, Fermini B, Nattel S. Rapid and slow components of delayed rectifier current in human atrial myocytes Cardiovasc Res 1994;28:1540-1546.[ISI][Medline]
  21. Tester DJ, Will ML, Haglund CM, Ackerman MJ. Compendium of cardiac channel mutations in 541 consecutive unrelated patients referred for long QT syndrome genetic testing Heart Rhythm 2005;2:507-517.[CrossRef][ISI][Medline]
  22. Marx SO, Kurokawa J, Reiken S, et al. Requirement of a macromolecular signaling complex for ß adrenergic receptor modulation of the KCNQ1-KCNE1 potassium channel Science 2002;295:496-499.[Abstract/Free Full Text]
  23. Kurokawa J, Motoike HK, Rao J, Kass RS. Regulatory actions of the A-kinase anchoring protein yotiao on a heart potassium channel downstream of PKA phosphorylation Proc Natl Acad Sci U S A 2004;101:16374-16378.[Abstract/Free Full Text]
  24. Grunnet M, Jespersen T, MacAulay N, et al. KCNQ1 channels sense small changes in cell volume J Physiol 2003;549:419-427.[Abstract/Free Full Text]
  25. Sasaki N, Mitsuiye T, Wang Z, Noma A. Increase of the delayed rectifier K+ and Na+-K+ pump currents by hypotonic solutions in guinea pig cardiac myocytes Circ Res 1994;75:887-895.[Abstract/Free Full Text]
  26. Wang Z, Mitsuiye T, Noma A. Cell distension-induced increase of the delayed rectifier K+ current in guinea pig ventricular myocytes Circ Res 1995;78:466-474.
  27. Melnyk P, Ehrlich JR, Pourrier M, Villeneuve L, Cha TJ, Nattel S. Comparison of ion channel distribution and expression in cardiomyocytes of canine pulmonary veins versus left atrium Cardiovasc Res 2005;65:104-116.[Abstract/Free Full Text]
  28. Yamane T, Shah DC, Jais P, et al. Dilation as a marker of pulmonary veins initiating atrial fibrillation J Interv Card Electrophysiol 2002;6:245-249.[CrossRef][ISI][Medline]
  29. Herweg B, Sichrovsly T, Polosajian L, Rozenshtein A, Steinberg JS. Hypertension and hypertensive heart disease are associated with increased ostial pulmonary vein diameter J Cardiovasc Electrophysiol 2005;16:2-5.[CrossRef][ISI][Medline]
  30. Ellinor PT, Moore RK, Patton KK, Ruskin JN, Mollak MR, MacRae CA. Mutations in the long QT gene, KCNQ1, are an uncommon cause of atrial fibrillation Heart 2004;90:1487-1488.[Free Full Text]
  31. Bauer A, Koch M, Kraft P, et al. The new selective IKs-blocking agent HMR 1556 restores sinus rhythm and prevents heart failure in pigs with persistent atrial fibrillation Basic Res Cardiol 2005;100:270-278.[CrossRef][ISI][Medline]
  32. Zhang D, Liang B, Lin J, Liu B, Zhou Q, Yang Y. KCNE3 R53H substitution in familial atrial fibrillation Chin Med J 2005;118:1735-1738.



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