Elsevier

Heart Rhythm

Volume 1, Issue 5, November 2004, Pages 600-607
Heart Rhythm

Spectrum and prevalence of cardiac sodium channel variants among black, white, Asian, and Hispanic individuals: Implications for arrhythmogenic susceptibility and Brugada/long QT syndrome genetic testing

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Objectives

The purpose of this study was to determine the prevalence and spectrum of nonsynonymous polymorphisms (amino acid variants) in the cardiac sodium channel among healthy subjects.

Background

Pathogenic mutations in the cardiac sodium channel gene, SCN5A, cause approximately 15 to 20% of Brugada syndrome (BrS1), 5 to 10% of long QT syndrome (LQT3), and 2 to 5% of sudden infant death syndrome.

Methods

Using single-stranded conformation polymorphism, denaturing high-performance liquid chromatography, and/or direct DNA sequencing, mutational analysis of the protein-encoding exons of SCN5A was performed on 829 unrelated, anonymous healthy subjects: 319 black, 295 white, 112 Asian, and 103 Hispanic.

Results

In addition to the four known common polymorphisms (R34C, H558R, S1103Y, and R1193Q), four relatively ethnic-specific polymorphisms were identified: R481W, S524Y, P1090L, and V1951L. Overall, 39 distinct missense variants (28 novel) were elucidated. Nineteen variants (49%) were found only in the black cohort. Only seven variants (18%) localized to transmembrane-spanning domains. Four variants (F1293S, R1512W, and V1951L cited previously as BrS1-causing mutations and S1787N previously published as a possible LQT3-causing mutation) were identified in this healthy cohort.

Conclusions

This study provides the first comprehensive determination of the prevalence and spectrum of cardiac sodium channel variants in healthy subjects from four distinct ethnic groups. This compendium of SCN5A variants is critical for proper interpretation of SCN5A genetic testing and provides an essential hit list of targets for future functional studies to determine whether or not any of these variants mediate genetic susceptibility for arrhythmias in the setting of either drugs or disease.

Introduction

More than 1000 individuals die suddenly each day in the United States due to ventricular tachycardia or ventricular fibrillation in the setting of myocardial infarction, acute myocardial ischemia, hypertrophic and dilated cardiomyopathy, medications, and cardiac channelopathies such as congenital long QT syndrome (LQTS) and Brugada syndrome.1, 2, 3 The cardiac channelopathies, particularly LQTS, have been suggested to be a “Rosetta stone” for studying the genetic basis of ventricular arrhythmogenesis and sudden death.4 Mutations involving the SCN5A gene, which encodes the cardiac sodium channel, are responsible for approximately 15 to 20% of Brugada syndrome (BrS1),5, 6 5 to 10% of LQTS (LQT3),7, 8 and 2 to 5% of sudden infant death syndrome.9, 10

Ackerman et al11 provided a compendium of potassium channel variants found among 744 healthy subjects from across four ethnic groups in the four potassium channel genes that account for approximately 60% of LQTS. We elucidated 49 distinct potassium channel variants and found that 8% of healthy blacks and nearly 5% of healthy whites were heterozygous for an uncommon missense variant (i.e., allelic frequency <0.5%). Except for two important functionally significant common polymorphisms, the prevalence, spectrum, ethnic specificity of SCN5A variants (alterations in primary amino acid sequence including nonsynonymous single nucleotide polymorphisms, insertions, and deletions), and their potential functional role in arrhythmogenesis among healthy subjects are unknown.

H558R is the most common sodium channel polymorphism. The functional phenotype associated with R558-SCN5A channels is indistinguishable from the wild-type H558-SCN5A channel.12 However, in the setting of a pathogenic mutation at residue 1766 (M1766L), expression studies in the context of H558 yielded a trafficking-defective BrS1-causing phenotype, whereas expression with R558 produced a gain-of-function LQT3-like channel, an example of intra-allelic complementation.13 More recently, in the setting of a ubiquitous alternatively spliced sodium channel containing Q1077, there is a marked reduction in sodium current when the transcript is expressed in the context of R558, providing in vitro evidence suggesting that the 5% of a population homozygous for R558 may have a “reduced depolarization capacity.”12

In addition, Splawski et al14 identified a predominantly black-specific common polymorphism in SCN5A (S1103Y, also annotated as Y1102) that was associated with arrhythmia risk, providing molecular proof of concept that common genetic determinants may mediating arrhythmia susceptibility.1, 15, 16, 17 In the present study, we address the lack of genetic variance knowledge in this critical cardiac sodium channel by defining the spectrum and frequency of variants in SCN5A in a large panel of healthy subjects from four major ethnic groups. This SCN5A compendium of nonsynonymous genetic variants provides a solid foundation for future functional, epidemiologic, and pharmacogenetic investigations of the role of sodium channel variants in susceptibility to cardiac arrhythmia.

Section snippets

Subjects

Genomic DNA from 829 apparently healthy anonymous subjects [319 African-Americans (blacks), 295 Caucasians of European ancestry (whites), 112 Asians, and 103 Hispanics] was obtained from either the Human Genetic Cell Repository sponsored by the National Institute of General Medical Sciences and the Coriell Institute for Medical Research (Camden, New Jersey, USA) or from volunteer blood donors. Volunteer donors were apparently healthy at time of collection and self-reported their ethnicity. To

Results

As anticipated, the four known common polymorphisms (R34C, H558R, S1103Y, and R1193Q) were represented in this study (Figure 1). R34C is a relatively black-specific variant with a minor allelic frequency of 9.2% (i.e., approximately 18% heterozygous for this polymorphism), although it was detected among Hispanics as well (minor allele frequency 1.7%, P < .0003). H558R was the only variant detected in all four ethnic groups, albeit with a much lower prevalence among Asians (9.2%, P < .002) than

Discussion

This study represents the first comprehensive mutational analysis of the cardiac sodium channel in nondisease subjects rather than cohorts of patients with suspected channelopathies such as BrS1 or LQT3. Akin to our previous work involving potassium channel variants in healthy subjects,11 we provide here a compendium of sodium channel variants that will facilitate future study of arrhythmia susceptibility. Our findings provide important insight and context for the proper interpretation of

Conclusion

This study is the first comprehensive determination of the frequency, spectrum, and ethnic specificity of cardiac sodium channel variants in apparently healthy individuals from across four different ethnic groups. As described, our results will help inform results of clinical genetic tests for BrS1 and LQT3 and will facilitate continued epidemiologic and functional studies of cardiac sodium channel variants. It is clear from our data that channel variants must be interpreted with consideration

Acknowledgments

Laboratory assistance of Geoff Joslyn, Kerry Berry, and Tim Andriese is gratefully acknowledged. We thank the sequencing and informatics efforts provided by Drs. Vincent Schulz, Janet Carr, and Carole Harris-Kerr at Genaissance Pharmaceuticals, Inc., New Haven, Connecticut.

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    M.J.A. was supported by a Mayo Foundation Clinic Research award, a Clinical Scientist Development Award from the Doris Duke Charitable Foundation, and a R01 grant from the National Institutes of Health (HD42569 to M.J.A., HL71092 to J.C.M.). M.T.K. and I.S. are supported by a Donald W. Reynolds Cardiovascular Clinical Research Center Grant. M.J.A. is a consultant for Genaissance Pharmaceuticals.

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