1
p.Y1449C SCN5A Mutation Associated with Overlap Disorder Comprising Conduction Disease, Brugada Syndrome, and Atrial Flutter SANDEEP S. HOTHI, M.A., Ph.D., M.B., B.Chir., M.R.C.P.,∗ ,†,‡ FARHANA ARA, M.B.B.S.,M.R.C.P.,‡,§ and JONATHAN TIMPERLEY, M.D., M.R.C.P.‡ From the ∗ Physiological Laboratory, University of Cambridge, Cambridge, UK; †Murray Edwards College, University of Cambridge, Cambridge, UK; ‡Heart Centre, Northampton General Hospital, Northampton, UK; and §Glenfield Hospital, Leicester, UK
p.Y1449C SCN5A Mutation in a Family with Overlap Disorder. Mutations in the SCN5A gene, which encodes the cardiac sodium channel, have been associated with cardiac arrhythmia syndromes and conduction disease. Specific SCN5A mutations had initially been considered to cause specific phenotypes. More recently, some SCN5A mutations have been associated with overlap syndromes, characterized by phenotypic heterogeneity within and between mutation carriers. Here we report and associate the presence of the p.Y1449C SCN5A mutation in a single family with a spectrum of cardiac phenotypes including conduction disease, Brugada syndrome and atrial arrhythmias, for the first time to our knowledge. (J Cardiovasc Electrophysiol, Vol. pp. 1-7) SCN5A, conduction disease, Brugada syndrome, atrial flutter, left bundle branch block, syncope Case A 32-year-old male rugby player developed left bundle branch block (LBBB) following administration of glycopyrrolate by the anesthetist prior to scheduled discectomy. A subsequent 12-lead electrocardiogram (ECG) at rest was consistent with a type-2 Brugada pattern with 1st degree heart block and left axis deviation (Fig. 1A). Echocardiography revealed a structurally normal heart and a 24-hour ECG did not reveal high degree atrioventricular (AV) block or arrhythmias. An ajmaline challenge resulted in broad LBBB and an increase in the P-R interval from 220 to 260 milliseconds (Fig. 1B). An electrophysiological study revealed a prolonged, resting His-Purkinje interval (76 milliseconds) and inducible polymorphic ventricular tachycardia (VT) at stage 12 of the Wellen’s protocol requiring external direct current cardioversion. A prophylactic implantable cardioverter defibrillator (ICD) and permanent pacemaker were offered but the patient declined. The proband’s 39-year-old sister was found to have a similar resting ECG (Fig. 2A) together with a history of recurrent syncope since childhood, associated with febrile illness. An ajmaline infusion resulted in a type-1 Brugada ECG pattern (Fig. 2B) which, together with her history of recurrent, unexplained syncope, fulfilled diagnostic criteria for Brugada syndrome (BrS).1 She was offered, and underwent, ICD implantation. The proband’s 14-year-old son was also admitted with recurrent syncope and atrial flutter with 2:1 AV conduction. A 9-year-old son had right bundle branch No disclosures. Address for correspondence: Sandeep S. Hothi, M.A., Ph.D., M.B., B.Chir., M.R.C.P., Murray Edwards College, University of Cambridge, Cambridge, CB3 0DF, UK. Fax: +44-122-376-3110; E-mail: [email protected] Manuscript received 6 July 2013; Revised manuscript received 16 May 2014; Accepted for publication 28 May 2014. doi: 10.1111/jce.12470
block but no cardiac symptoms. The proband’s mother had died aged 32 years, following a series of syncopal episodes complicated by febrile illness, ultimately resulting in a fatal head injury. Genetic analysis revealed a p.Y1449C mutation in the SCN5A gene in 7 related individuals comprising the proband, his 2 sons, the proband’s sister and her 3-yearold son (Fig. 3). Tissue obtained from the proband’s mother 17 years after death was not technically amenable to genetic analysis, while her asymptomatic sister was negative for the mutation and her asymptomatic brother declined testing. However, the proband’s asymptomatic, fit 60-year-old father and 1 of 2 paternal aunts carried the same p.Y1449C mutation (Fig. 3). The resting 12-lead ECG recorded from the proband’s father demonstrated sinus rhythm with first degree AV block, with a P-R interval of 230 milliseconds, a nonspecific intraventricular conduction delay with QRS duration 120 milliseconds, but no Brugada pattern. Following review by his local cardiologist, in the absence of symptoms of arrhythmia or conduction disease, he did not undergo further investigation. Discussion This is the first report, to our knowledge, that associates the p.Y1449C SCN5A mutation with conduction disease, BrS, and atrial arrhythmias. Our report adds to those of different BrS-associated SCN5A mutations with variable phenotypes, highlighting a need for an awareness of variable phenotypes with which such mutations may present. Indeed, in the original report by Brugada and Brugada, atrial fibrillation (AF) was reported in 2 of 8 BrS patients.2 BrS is characterized by ST segment elevation in the right precordial leads associated with an increased risk of polymorphic VT, ventricular fibrillation, syncope, and sudden cardiac death.2 It is inherited in an autosomal dominant manner with variable penetrance. Mutations have been described in 10 genes, most commonly in SCN5A, which encodes the α-subunit of the voltage-gated sodium channel.3,4 The p.Y1449C missense mutation results in replacement of tyrosine by cysteine,
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Figure 1. Proband’s resting electrocardiogram demonstrating a type-2 Brugada pattern, first degree heart block and left axis deviation (A). An ajmaline infusion resulted in left bundle branch block and increased the P-R interval (B).
affecting the S6 region of domain III, known to form the channel’s pore and selectivity filter.3 A retrospective analysis of 2,111 BrS patients identified 293 SCN5A mutations including one with the p.Y1449C mutation.3 Its incidence among conduction disease and atrial arrhythmias has not been determined. The mechanism of genetic transmission appears to have been from the proband’s father. However, the proband’s mother had a history of syncope with fever that raises the possibility of arrhythmias, possibly even BrS. Unfortunately, genetic analysis of her tissue was not possible. It is possible to speculate about a number of schemes. First, the p.Y1449C mutation may have been disease-causing, transmitted from the proband’s father alone, or possibly, from both the mother and father. Second, the p.Y1449C mutation may have been accompanied by a different mutation in the proband’s mother whether in SCN5A or elsewhere that could have modulated disease penetrance and phenotypic heterogeneity. Third, a mutation in a gene other than SCN5A might have been disease-causing, whether transmitted from the proband’s mother or father. However, the p.Y1449C mutation has previously been associated with BrS,3 consistent with a possible role.
The SCN5A gene is located on chromosome 3p21. It contains 28 exons and encodes the sodium channel protein consisting of 2,016 amino acids forming 4 homologous domains. Each domain is linked by cytoplasmic linkers and consists of 6 transmembrane spanning regions (S1–S6). The S5–S6 regions are joined by a P (pore) segment that lines the outer pore of channel. The p.Y1449C mutation identified in the family reported here is a missense mutation in exon 25 consisting of a change from adenine to guanine at nucleotide position 4,346. This missense mutation leads to the replacement of tyrosine by cysteine at position 1,449 in the transmembrane S5-S6 region of domain III that forms the channel’s pore region and selectivity filter.3 Retrospective analysis of 2,111 BrS patients identified 293 SCN5A mutations including one with the p.Y1449C mutation.3 Its incidence among conduction disease and atrial arrhythmias has not been determined. Insights from mammalian cell lines, computer modeling, and genetic mouse models5,6 have revealed different functional consequences of SCN5A mutations associated with BrS, conduction disease and long-QT 3 syndrome (LQT3) that include alterations, in positive or negative directions, in channel trafficking, altered expression, nonfunctional channels, altered voltage-, and time-dependence of channel
Hothi et al.
p.Y1449C SCN5A Mutation in a Family with Overlap Disorder
3
Figure 2. Electrocardiograms recorded from the proband’s sister showing a type-2 Brugada pattern at rest (A), and a type-1 Brugada pattern with infusion of ajmaline (B).
Figure 3. Family pedigree. Arrow represents proband; solid black symbols represent individuals with a p.Y1449C mutation in SCN5A and symptomatic, abnormal electrophysiology; asterisks (*) represent asymptomatic carriers; double asterisks (**) represent the p.Y1449C mutation and asymptomatic conduction disease.
gating processes.7,8 There may also be secondary changes in other proteins, such as, connexins, and levels of fibrosis as evidenced by the Scn5a+/− mouse.6 When considered according to the phase of the action potential, their actions occur within 2 broad intervals: effects upon phase 0, important for conduction velocity, or, effects upon the late sodium current during the plateau phase that may alter the action potential duration (APD).
Further insight might be gained from a nearby mutation, G1406R, close to the p.Y1449C mutation presented in this report. G1406R defines a glycine to arginine change in the same transmembrane S5–S6 segment of domain III that is affected by p.Y1449C. It is associated with BrS and isolated cardiac conduction disease.9 It encodes a nonfunctional channel with normal trafficking but no detectable sodium current when assessed in COS-7 cells. Based upon such regional considerations, one might speculate that p.Y1449C encodes a nonfunctional channel. The phenotype that arises from a mutation depends on the specific channel gating processes that are affected, and the relative balance of such changes. However, while predictions can be made, it is not yet possible to predict the specific phenotype resulting from a mutation based upon its location alone. Indeed, there appear to be additional modifying factors that are poorly understood.9 Our report associates the p.Y1449C mutation with phenotypic heterogeneity comprising cardiac conduction disease, BrS and atrial flutter, and adds to other reports of SCN5A mutations associated with overlapping phenotypes (Table 1). LQT3 had originally been thought to arise from gain-of-function SCN5A mutations. In contrast, BrS had been thought to arise from loss-of-function SCN5A mutations. The question that follows concerns how a single mutation can
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TABLE 1 SCN5A Mutations Associated with Overlap Syndromes. For References see Online Supplementary File Clinical Phenotypes Brugada syndrome Conduction disease Brugada syndrome Conduction disease Sinus node disease Conduction disease Long QT 3 syndrome Brugada syndrome Conduction disease Long QT 3 syndrome Brugada syndrome Long QT 3 syndrome Sinus node disease Brugada syndrome Conduction disease Long QT 3 syndrome Sinus node disease Brugada syndrome Long QT 3 syndrome Conduction disease Dilated cardiomyopathy Long QT 3 syndrome Sinus node disease Long QT 3 syndrome Atrial flutter Brugada syndrome Conduction disease Atrial standstill Brugada syndrome Atrial flutter Sick sinus syndrome Conduction disease Ventricular tachycardia Conduction disease Sinus node disease Brugada syndrome Sinus node disease Brugada syndrome Paroxysmal complete AV block Sinus node disease Conduction disease Ventricular arrhythmia Conduction disease Dilated cardiomyopathy Long QT 3 syndrome Sinus node disease Atrial fibrillation Conduction disease Dilated cardiomyopathy Sinus node disease Brugada syndrome Atrial fibrillation Long QT 3 syndrome Atrial fibrillation Atrial standstill
Mutation E867X; F861fs951X; G1319V; G1406R; G752R; N406S; S1710L; S1812X; W1191X; W1440X delK1479; E1225K; E161K; G1262S
delKPQ1505; M1766L; P1332L; P2005A; T1620K V1763M; V1777M delK1500; I1350T; R1193Q
delF1617; E1784K
1795insD
P2006A; R1612P; V411M A1180V
E1784K R376H
R367H R121W
G1408R; R1623X; R1632H F1775Lfs*15; K1578fs/52; [S231CfsX251 (c.692–693delCA); T187I D356N p.I230T
delQKP1507–1509
D1275N
[S231CfsX251 (c.692–693delCA) Y1795C L212P
produce multiple phenotypes and do so with variable expression. Potential explanations include the phase of the action potential that is affected (early vs. late), the presence of more than one biophysical alteration in channel function from a single mutation, and modifying factors, whether clinical or genetic, that modulate biophysical function.7,8 At the biophysical level, a mutation might encode channels with heterogeneous, gain- and loss-of-function properties that are manifest simultaneously, or that are present to differing degrees under different conditions (reviewed by
Makita10 ). In such a scheme, cardiac conduction disease and BrS might arise from a positive shift in the voltagedependence of sodium channel activation. LQT3 might arise from an increase in late sodium current due to accelerated recovery from inactivation or a positive shift in the voltage dependence of inactivation. For example, the L1821fs/10 mutant channel exemplifies both a positive shift in voltage dependence of activation and an increased late sodium current.11 The R1193Q mutation results in an overlap of BrS, conduction disease and LQT3, in parallel with a negative shift of voltage dependence of inactivation that may explain the BrS phenotype and a persistent, inward sodium tail current that could underlie LQT3.12-14 Another mutation, K1500, results in channels bearing a negative shift of voltage dependence of inactivation (that may underlie the observed BrS phenotype) and a persistent, inward sodium tail current (that may underlie the observed LQT3 phenotype).15 The delF1617 mutation encodes channels with delayed inactivation (gain-of-function) and reduced peak current density with impaired recovery from inactivation (loss-of-function) in parallel with clinical phenotypes comprising LQT3 and BrS.16 Channel biophysics may also show heart-rate dependency that could lead to a rate-dependent phenotype. For instance, the Scn5a 1795insD heterozygote mouse shows prolonged repolarization at slow rates and reduced sodium channel availability at high rates.17 These findings were also reproduced in a computer model incorporating such heart rate dependency.18 Variation in the extent of expression may reflect clinical factors that include drug therapy, sex (the Brugada phenotype is more prevalent in males9 ), age, fever (relevant as the syncopal episodes suffered by the proband’s sister and mother were associated with fever), coexisting disease, autonomic tone and other, as yet unidentified factors.7,11,19 Genetic modifiers could include compound mutations, coexisting modifier alleles, SCN5A splice variants (Q1077del/Q1077), single nucleotide polymorphisms (SNPs) in SCN5A, and SNPs in non-SCN5A genes and unknown factors.7,11,19 Modifier genes might encode proteins affecting channel trafficking, surface expression, gating and kinetics, such as β-subunits, ankyrins, calmodulin, and caveolin-3. Supraventricular arrhythmias including AV-nodal reentry, AV-reentry, atrial tachycardia, AF, and atrial flutter have been reported in patients with BrS.20 At the cellular level, prolonged atrial action potentials might underlie atrial arrhythmias, while at the whole organ level, Holter data suggest that the arrhythmic trigger might comprise premature atrial beats.21 It has been speculated that AF could occur because atrial myocytes have reduced Na+ channel availability compared to ventricular cells due to a slightly more positive resting membrane potential in atrial cells.21 It has been suggested that sinus node dysfunction (SND) can exist in an overlap syndrome including BrS, due to a prolonged APD and slowed diastolic depolarization in SAN cells.22 Consistent with this scheme are findings from the E161 mutation, associated with bradycardia, SND, generalized conduction disease and BrS, that encodes channels with a positive shift in the voltage dependence of activation, reduced current density, and unchanged inactivation.23 Computer modeling of atrial effects demonstrated reduction in
Hothi et al.
p.Y1449C SCN5A Mutation in a Family with Overlap Disorder
SA node rate due to shallower diastolic depolarization rate and slower action potential upstroke velocity.23 10.
Conclusions This report identifies the p.Y1449C SCN5A mutation in association with a spectrum of cardiac phenotypes including conduction disease, BrS, and atrial arrhythmias. Together with other reports of overlap syndromes with different SCN5A mutations, we highlight the need for awareness of a variable genotype–phenotype correlation in SCN5A mutations. Future research should investigate the functional consequences of this and other BrS-associated mutations presenting with overlapping syndromes. Acknowledgment: The authors are grateful to Professor Christopher Huang, University of Cambridge, for reviewing the manuscript.
11.
12.
13. 14.
15.
References 1. Wilde AA, Antzelevitch C, Borggrefe M, Brugada J, Brugada R, Brugada P, Corrado D, Hauer RN, Kass RS, Nademanee K, Priori SG, Towbin JA; Study Group on the Molecular Basis of Arrhythmias of the European Society of Cardiology: Proposed diagnostic criteria for the Brugada syndrome. Eur Heart J 2002;23:1648-1654. 2. Brugada P, Brugada J: Right bundle branch block, persistent ST segment elevation and sudden cardiac death: A distinct clinical and electrocardiographic syndrome. A multicenter report. J Am Coll Cardiol 1992;20:1391-1396. 3. Kapplinger JD, Tester DJ, Alders M, Benito B, Berthet M, Brugada J, Brugada P, Fressart V, Guerchicoff A, Harris-Kerr C, Kamakura S, Kyndt F, Koopmann TT, Miyamoto Y, Pfeiffer R, Pollevick GD, Probst V, Zumhagen S, Vatta M, Towbin JA, Shimizu W, Schulze-Bahr E, Antzelevitch C, Salisbury BA, Guicheney P, Wilde AA, Brugada R, Schott JJ, Ackerman MJ: An international compendium of mutations in the SCN5A-encoded cardiac sodium channel in patients referred for Brugada syndrome genetic testing. Heart Rhythm 2010;7:33-46. 4. Berne P, Brugada J: Brugada syndrome 2012. Circ J 2012;76:15631571. 5. Nuyens D, Stengl M, Dugarmaa S, Rossenbacker T, Compernolle V, Rudy Y, Smits JF, Flameng W, Clancy CE, Moons L, Vos MA, Dewerchin M, Benndorf K, Collen D, Carmeliet E, Carmeliet P: Abrupt rate accelerations or premature beats cause life-threatening arrhythmias in mice with long-QT3 syndrome. Nat Med 2001;77:1021-1027. 6. Papadatos GA, Wallerstein PM, Head CE, Ratcliff R, Brady PA, Benndorf K, Saumarez RC, Trezise AE, Huang CL, Vandenberg JI, Colledge WH, Grace AA: Slowed conduction and ventricular tachycardia after targeted disruption of the cardiac sodium channel gene Scn5a. Proc Natl Acad Sci USA 2002;99:62106215. 7. Remme CA, Wilde AAM, Bezzina CR: Cardiac sodium channel overlap syndromes: Different faces of SCN5A mutations. Trends Cardiovasc Med 2008;183:78-87. 8. Remme CA, Wilde AAM: SCN5A overlap syndromes: No end to disease complexity? Europace 2008;10:1253-1255. 9. Kyndt F, Probst V, Potet F, Demolombe S, Chevallier JC, Baro I, Moisan JP, Boisseau P, Schott JJ, Escande D, Le Marec H: Novel
16.
17. 18. 19. 20.
21. 22. 23.
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SCN5A mutation leading either to isolated cardiac conduction defect or Brugada syndrome in a large French family. Circulation 2001;104:3081-3086. Makita N: phenotypic overlap of cardiac sodium channelopathiesIndividal-specific or mutation specific? Circ J 2009;73:810-817. Tan BH, Iturralde-Torres P, Medeiros-Domingo A, Nava S, Tester DJ, Valdivia CR, Tusi´e-Luna T, Ackerman MJ, Makielski JC: A novel C-terminal truncation SCN5A mutation from a patient with sick sinus syndrome, conduction disorder and ventricular tachycardia. Cardiovasc Res 2008;76:409-417. Wang Q, Chen S, Chen Q, Wan X, Shen J, Hoeltge GA, Timur AA, Keating MT, Kirsch GE: The common SCN5A mutation R1193Q causes LQTS-type electrophysiological alterations of the cardiac sodium channel. J Med Genet 2004;41:e66-e66. Huang H, Zhao J, Barrane F, Champagne J, Chahine M: Nav 1.5 / R1193Q polymorphism is associated with both. Can J Cardiol 2006;22:309-313. Sun A, Xu L, Wang S, Wang K, Huang W, Wang Y, Zou Y, Ge J: SCN5A R1193Q polymorphism associated with progressive cardiac conduction defects and long QT syndrome in a Chinese family. J Med Genet 2008;45:127-128. Grant AO, Carboni MP, Neplioueva V, Starmer CF, Memmi M, Napolitano C, Priori S: Long QT syndrome, Brugada syndrome, and conduction system disease are linked to a single sodium channel mutation. J Clin Invest 2002;110:1201-1209. Benson DW, Wang DW, Dyment M, Knilans TK, Fish FA, Strieper MJ, Rhodes TH, George AL Jr: Congenital sick sinus syndrome caused by recessive mutations in the cardiac sodium channel gene (SCN5A). J Clin Invest 2003;112:1019-1028. Veldkamp MW, Viswanathan PC, Bezzina C, Baartscheer A, Wilde AAM, Balser JR: Two distinct congenital arrhythmias evoked by a multidysfunctional Na+ channel. Circ Res 2000;86:e91-e97. Clancy CE, Rudy Y: Na+ channel mutation that causes both brugada and long-QT syndrome phenotypes: A simulation study of mechanism. Circulation 2002;105:1208-1213. Scicluna B, Wilde A, Bezzina C: The primary arrhythmia syndromes: Same mutation, different manifestations. Are we starting to understand why? J Cardiovasc Electrophysiol 2008;19:445-452. Schimpf R, Giustetto C, Eckardt L, Veltmann C, Wolpert C, Gaita F, Breithardt G, Borggrefe M: Prevalence of supraventricular tachyarrhythmias in a cohort of 115 patients with Brugada syndrome. Ann. Noninvasive Electrocardiol 2008;13:266-269. Johnson F, Antzelevitch C: Atrial fibrillation and Brugada syndrome. J Am Coll Cardiol 2009;51:1149-1153. Sumiyoshi M, Nakazato Y, Tokano T, Yasuda M, Mineda Y, Nakata Y, Daida H: Sinus node dysfunction concomitant with Brugada syndrome. Circ J 2005;69:946-950. Smits JP, Koopmann TT, Wilders R, Veldkamp MW, Opthof T, Bhuiyan ZA, Mannens MM, Balser JR, Tan HL, Bezzina CR, Wilde AA: A mutation in the human cardiac sodium channel (E161K) contributes to sick sinus syndrome, conduction disease and Brugada syndrome in two families. J Mol Cell Cardiol 2005;38:969981.
Supporting Information Additional supporting information may be found in the online version of this article at the publisher’s website: Table S1. SCN5A Mutations Associated with Overlap Syndromes.
p.Y1449C SCN5A Mutation Associated with Overlap Disorder Comprising Conduction Disease, Brugada Syndrome, and Atrial Flutter SANDEEP S. HOTHI, M.A., Ph.D., M.B., B.Chir., M.R.C.P.,∗ ,†,‡ FARHANA ARA, M.B.B.S.,M.R.C.P.,‡,§ and JONATHAN TIMPERLEY, M.D., M.R.C.P.‡ From the ∗ Physiological Laboratory, University of Cambridge, Cambridge, UK; †Murray Edwards College, University of Cambridge, Cambridge, UK; ‡Heart Centre, Northampton General Hospital, Northampton, UK; and §Glenfield Hospital, Leicester, UK
p.Y1449C SCN5A Mutation in a Family with Overlap Disorder. Mutations in the SCN5A gene, which encodes the cardiac sodium channel, have been associated with cardiac arrhythmia syndromes and conduction disease. Specific SCN5A mutations had initially been considered to cause specific phenotypes. More recently, some SCN5A mutations have been associated with overlap syndromes, characterized by phenotypic heterogeneity within and between mutation carriers. Here we report and associate the presence of the p.Y1449C SCN5A mutation in a single family with a spectrum of cardiac phenotypes including conduction disease, Brugada syndrome and atrial arrhythmias, for the first time to our knowledge. (J Cardiovasc Electrophysiol, Vol. pp. 1-7) SCN5A, conduction disease, Brugada syndrome, atrial flutter, left bundle branch block, syncope Case A 32-year-old male rugby player developed left bundle branch block (LBBB) following administration of glycopyrrolate by the anesthetist prior to scheduled discectomy. A subsequent 12-lead electrocardiogram (ECG) at rest was consistent with a type-2 Brugada pattern with 1st degree heart block and left axis deviation (Fig. 1A). Echocardiography revealed a structurally normal heart and a 24-hour ECG did not reveal high degree atrioventricular (AV) block or arrhythmias. An ajmaline challenge resulted in broad LBBB and an increase in the P-R interval from 220 to 260 milliseconds (Fig. 1B). An electrophysiological study revealed a prolonged, resting His-Purkinje interval (76 milliseconds) and inducible polymorphic ventricular tachycardia (VT) at stage 12 of the Wellen’s protocol requiring external direct current cardioversion. A prophylactic implantable cardioverter defibrillator (ICD) and permanent pacemaker were offered but the patient declined. The proband’s 39-year-old sister was found to have a similar resting ECG (Fig. 2A) together with a history of recurrent syncope since childhood, associated with febrile illness. An ajmaline infusion resulted in a type-1 Brugada ECG pattern (Fig. 2B) which, together with her history of recurrent, unexplained syncope, fulfilled diagnostic criteria for Brugada syndrome (BrS).1 She was offered, and underwent, ICD implantation. The proband’s 14-year-old son was also admitted with recurrent syncope and atrial flutter with 2:1 AV conduction. A 9-year-old son had right bundle branch No disclosures. Address for correspondence: Sandeep S. Hothi, M.A., Ph.D., M.B., B.Chir., M.R.C.P., Murray Edwards College, University of Cambridge, Cambridge, CB3 0DF, UK. Fax: +44-122-376-3110; E-mail: [email protected] Manuscript received 6 July 2013; Revised manuscript received 16 May 2014; Accepted for publication 28 May 2014. doi: 10.1111/jce.12470
block but no cardiac symptoms. The proband’s mother had died aged 32 years, following a series of syncopal episodes complicated by febrile illness, ultimately resulting in a fatal head injury. Genetic analysis revealed a p.Y1449C mutation in the SCN5A gene in 7 related individuals comprising the proband, his 2 sons, the proband’s sister and her 3-yearold son (Fig. 3). Tissue obtained from the proband’s mother 17 years after death was not technically amenable to genetic analysis, while her asymptomatic sister was negative for the mutation and her asymptomatic brother declined testing. However, the proband’s asymptomatic, fit 60-year-old father and 1 of 2 paternal aunts carried the same p.Y1449C mutation (Fig. 3). The resting 12-lead ECG recorded from the proband’s father demonstrated sinus rhythm with first degree AV block, with a P-R interval of 230 milliseconds, a nonspecific intraventricular conduction delay with QRS duration 120 milliseconds, but no Brugada pattern. Following review by his local cardiologist, in the absence of symptoms of arrhythmia or conduction disease, he did not undergo further investigation. Discussion This is the first report, to our knowledge, that associates the p.Y1449C SCN5A mutation with conduction disease, BrS, and atrial arrhythmias. Our report adds to those of different BrS-associated SCN5A mutations with variable phenotypes, highlighting a need for an awareness of variable phenotypes with which such mutations may present. Indeed, in the original report by Brugada and Brugada, atrial fibrillation (AF) was reported in 2 of 8 BrS patients.2 BrS is characterized by ST segment elevation in the right precordial leads associated with an increased risk of polymorphic VT, ventricular fibrillation, syncope, and sudden cardiac death.2 It is inherited in an autosomal dominant manner with variable penetrance. Mutations have been described in 10 genes, most commonly in SCN5A, which encodes the α-subunit of the voltage-gated sodium channel.3,4 The p.Y1449C missense mutation results in replacement of tyrosine by cysteine,
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Figure 1. Proband’s resting electrocardiogram demonstrating a type-2 Brugada pattern, first degree heart block and left axis deviation (A). An ajmaline infusion resulted in left bundle branch block and increased the P-R interval (B).
affecting the S6 region of domain III, known to form the channel’s pore and selectivity filter.3 A retrospective analysis of 2,111 BrS patients identified 293 SCN5A mutations including one with the p.Y1449C mutation.3 Its incidence among conduction disease and atrial arrhythmias has not been determined. The mechanism of genetic transmission appears to have been from the proband’s father. However, the proband’s mother had a history of syncope with fever that raises the possibility of arrhythmias, possibly even BrS. Unfortunately, genetic analysis of her tissue was not possible. It is possible to speculate about a number of schemes. First, the p.Y1449C mutation may have been disease-causing, transmitted from the proband’s father alone, or possibly, from both the mother and father. Second, the p.Y1449C mutation may have been accompanied by a different mutation in the proband’s mother whether in SCN5A or elsewhere that could have modulated disease penetrance and phenotypic heterogeneity. Third, a mutation in a gene other than SCN5A might have been disease-causing, whether transmitted from the proband’s mother or father. However, the p.Y1449C mutation has previously been associated with BrS,3 consistent with a possible role.
The SCN5A gene is located on chromosome 3p21. It contains 28 exons and encodes the sodium channel protein consisting of 2,016 amino acids forming 4 homologous domains. Each domain is linked by cytoplasmic linkers and consists of 6 transmembrane spanning regions (S1–S6). The S5–S6 regions are joined by a P (pore) segment that lines the outer pore of channel. The p.Y1449C mutation identified in the family reported here is a missense mutation in exon 25 consisting of a change from adenine to guanine at nucleotide position 4,346. This missense mutation leads to the replacement of tyrosine by cysteine at position 1,449 in the transmembrane S5-S6 region of domain III that forms the channel’s pore region and selectivity filter.3 Retrospective analysis of 2,111 BrS patients identified 293 SCN5A mutations including one with the p.Y1449C mutation.3 Its incidence among conduction disease and atrial arrhythmias has not been determined. Insights from mammalian cell lines, computer modeling, and genetic mouse models5,6 have revealed different functional consequences of SCN5A mutations associated with BrS, conduction disease and long-QT 3 syndrome (LQT3) that include alterations, in positive or negative directions, in channel trafficking, altered expression, nonfunctional channels, altered voltage-, and time-dependence of channel
Hothi et al.
p.Y1449C SCN5A Mutation in a Family with Overlap Disorder
3
Figure 2. Electrocardiograms recorded from the proband’s sister showing a type-2 Brugada pattern at rest (A), and a type-1 Brugada pattern with infusion of ajmaline (B).
Figure 3. Family pedigree. Arrow represents proband; solid black symbols represent individuals with a p.Y1449C mutation in SCN5A and symptomatic, abnormal electrophysiology; asterisks (*) represent asymptomatic carriers; double asterisks (**) represent the p.Y1449C mutation and asymptomatic conduction disease.
gating processes.7,8 There may also be secondary changes in other proteins, such as, connexins, and levels of fibrosis as evidenced by the Scn5a+/− mouse.6 When considered according to the phase of the action potential, their actions occur within 2 broad intervals: effects upon phase 0, important for conduction velocity, or, effects upon the late sodium current during the plateau phase that may alter the action potential duration (APD).
Further insight might be gained from a nearby mutation, G1406R, close to the p.Y1449C mutation presented in this report. G1406R defines a glycine to arginine change in the same transmembrane S5–S6 segment of domain III that is affected by p.Y1449C. It is associated with BrS and isolated cardiac conduction disease.9 It encodes a nonfunctional channel with normal trafficking but no detectable sodium current when assessed in COS-7 cells. Based upon such regional considerations, one might speculate that p.Y1449C encodes a nonfunctional channel. The phenotype that arises from a mutation depends on the specific channel gating processes that are affected, and the relative balance of such changes. However, while predictions can be made, it is not yet possible to predict the specific phenotype resulting from a mutation based upon its location alone. Indeed, there appear to be additional modifying factors that are poorly understood.9 Our report associates the p.Y1449C mutation with phenotypic heterogeneity comprising cardiac conduction disease, BrS and atrial flutter, and adds to other reports of SCN5A mutations associated with overlapping phenotypes (Table 1). LQT3 had originally been thought to arise from gain-of-function SCN5A mutations. In contrast, BrS had been thought to arise from loss-of-function SCN5A mutations. The question that follows concerns how a single mutation can
4
Journal of Cardiovascular Electrophysiology
Vol. No.
TABLE 1 SCN5A Mutations Associated with Overlap Syndromes. For References see Online Supplementary File Clinical Phenotypes Brugada syndrome Conduction disease Brugada syndrome Conduction disease Sinus node disease Conduction disease Long QT 3 syndrome Brugada syndrome Conduction disease Long QT 3 syndrome Brugada syndrome Long QT 3 syndrome Sinus node disease Brugada syndrome Conduction disease Long QT 3 syndrome Sinus node disease Brugada syndrome Long QT 3 syndrome Conduction disease Dilated cardiomyopathy Long QT 3 syndrome Sinus node disease Long QT 3 syndrome Atrial flutter Brugada syndrome Conduction disease Atrial standstill Brugada syndrome Atrial flutter Sick sinus syndrome Conduction disease Ventricular tachycardia Conduction disease Sinus node disease Brugada syndrome Sinus node disease Brugada syndrome Paroxysmal complete AV block Sinus node disease Conduction disease Ventricular arrhythmia Conduction disease Dilated cardiomyopathy Long QT 3 syndrome Sinus node disease Atrial fibrillation Conduction disease Dilated cardiomyopathy Sinus node disease Brugada syndrome Atrial fibrillation Long QT 3 syndrome Atrial fibrillation Atrial standstill
Mutation E867X; F861fs951X; G1319V; G1406R; G752R; N406S; S1710L; S1812X; W1191X; W1440X delK1479; E1225K; E161K; G1262S
delKPQ1505; M1766L; P1332L; P2005A; T1620K V1763M; V1777M delK1500; I1350T; R1193Q
delF1617; E1784K
1795insD
P2006A; R1612P; V411M A1180V
E1784K R376H
R367H R121W
G1408R; R1623X; R1632H F1775Lfs*15; K1578fs/52; [S231CfsX251 (c.692–693delCA); T187I D356N p.I230T
delQKP1507–1509
D1275N
[S231CfsX251 (c.692–693delCA) Y1795C L212P
produce multiple phenotypes and do so with variable expression. Potential explanations include the phase of the action potential that is affected (early vs. late), the presence of more than one biophysical alteration in channel function from a single mutation, and modifying factors, whether clinical or genetic, that modulate biophysical function.7,8 At the biophysical level, a mutation might encode channels with heterogeneous, gain- and loss-of-function properties that are manifest simultaneously, or that are present to differing degrees under different conditions (reviewed by
Makita10 ). In such a scheme, cardiac conduction disease and BrS might arise from a positive shift in the voltagedependence of sodium channel activation. LQT3 might arise from an increase in late sodium current due to accelerated recovery from inactivation or a positive shift in the voltage dependence of inactivation. For example, the L1821fs/10 mutant channel exemplifies both a positive shift in voltage dependence of activation and an increased late sodium current.11 The R1193Q mutation results in an overlap of BrS, conduction disease and LQT3, in parallel with a negative shift of voltage dependence of inactivation that may explain the BrS phenotype and a persistent, inward sodium tail current that could underlie LQT3.12-14 Another mutation, K1500, results in channels bearing a negative shift of voltage dependence of inactivation (that may underlie the observed BrS phenotype) and a persistent, inward sodium tail current (that may underlie the observed LQT3 phenotype).15 The delF1617 mutation encodes channels with delayed inactivation (gain-of-function) and reduced peak current density with impaired recovery from inactivation (loss-of-function) in parallel with clinical phenotypes comprising LQT3 and BrS.16 Channel biophysics may also show heart-rate dependency that could lead to a rate-dependent phenotype. For instance, the Scn5a 1795insD heterozygote mouse shows prolonged repolarization at slow rates and reduced sodium channel availability at high rates.17 These findings were also reproduced in a computer model incorporating such heart rate dependency.18 Variation in the extent of expression may reflect clinical factors that include drug therapy, sex (the Brugada phenotype is more prevalent in males9 ), age, fever (relevant as the syncopal episodes suffered by the proband’s sister and mother were associated with fever), coexisting disease, autonomic tone and other, as yet unidentified factors.7,11,19 Genetic modifiers could include compound mutations, coexisting modifier alleles, SCN5A splice variants (Q1077del/Q1077), single nucleotide polymorphisms (SNPs) in SCN5A, and SNPs in non-SCN5A genes and unknown factors.7,11,19 Modifier genes might encode proteins affecting channel trafficking, surface expression, gating and kinetics, such as β-subunits, ankyrins, calmodulin, and caveolin-3. Supraventricular arrhythmias including AV-nodal reentry, AV-reentry, atrial tachycardia, AF, and atrial flutter have been reported in patients with BrS.20 At the cellular level, prolonged atrial action potentials might underlie atrial arrhythmias, while at the whole organ level, Holter data suggest that the arrhythmic trigger might comprise premature atrial beats.21 It has been speculated that AF could occur because atrial myocytes have reduced Na+ channel availability compared to ventricular cells due to a slightly more positive resting membrane potential in atrial cells.21 It has been suggested that sinus node dysfunction (SND) can exist in an overlap syndrome including BrS, due to a prolonged APD and slowed diastolic depolarization in SAN cells.22 Consistent with this scheme are findings from the E161 mutation, associated with bradycardia, SND, generalized conduction disease and BrS, that encodes channels with a positive shift in the voltage dependence of activation, reduced current density, and unchanged inactivation.23 Computer modeling of atrial effects demonstrated reduction in
Hothi et al.
p.Y1449C SCN5A Mutation in a Family with Overlap Disorder
SA node rate due to shallower diastolic depolarization rate and slower action potential upstroke velocity.23 10.
Conclusions This report identifies the p.Y1449C SCN5A mutation in association with a spectrum of cardiac phenotypes including conduction disease, BrS, and atrial arrhythmias. Together with other reports of overlap syndromes with different SCN5A mutations, we highlight the need for awareness of a variable genotype–phenotype correlation in SCN5A mutations. Future research should investigate the functional consequences of this and other BrS-associated mutations presenting with overlapping syndromes. Acknowledgment: The authors are grateful to Professor Christopher Huang, University of Cambridge, for reviewing the manuscript.
11.
12.
13. 14.
15.
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Supporting Information Additional supporting information may be found in the online version of this article at the publisher’s website: Table S1. SCN5A Mutations Associated with Overlap Syndromes.
