EDITORIAL COMMENTARY
Brugada syndrome: Lots of questions, some answers Dan M. Roden, MD From the Departments of Medicine and Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee. The initial reports of the Brugada syndrome emphasized the unusual ECG phenotype, the structurally normal heart, the high propensity to sudden death due to ventricular fibrillation, and the fact that the phenotype appeared in multiple family members.1–3 One feature recognized early on was provocation of the distinctive ECG phenotype by challenge with sodium channel-blocking drugs. This finding naturally led to the idea that abnormal function of the cardiac sodium channel gene, encoded by SCN5A, might cause the disease. That hypothesis received its initial validation in 1998, with the report of SCN5A mutations, each leading to predicted decreased sodium current, in individuals affected with the Brugada syndrome.4 The field has come a long way since these initial exciting discoveries, and, as usual, the accumulation of new knowledge has answered some questions but has raised others. For example, although the finding of SCN5A “loss-offunction” mutations in individual patients with Brugada syndrome makes eminent pathophysiologic sense, the parallel genetic evidence—segregation of the Brugada phenotype with a mutation in a large kindred— did not arrive until later. When it did, the initial family studied was also one of the first to display the mixed phenotype of Brugada syndrome, sinus node dysfunction, and long QT syndrome type 3 (LQT3).5 We now know that this phenomenon is fairly common and that the mutant channels display both reduced peak current (characteristic of Brugada syndrome) and unstable fast inactivation (signature of LQT3) in vitro.6,7 We still do not know why some individuals display either or both phenotypes.
How many sodium channel mutations in Brugada syndrome? Genetic testing for Brugada syndrome by screening for rare DNA variants (mutations) that disrupt the SCN5A amino acid sequence has been available in research settings for a decade and now is available commercially. In this issue of Heart Rhythm, Kapplinger et al8 present the results of such testing in nine high-volume centers. The results of their
Address reprint requests and correspondence: Dr. Dan M. Roden, Departments of Medicine and Pharmacology, Vanderbilt University School of Medicine, 1285 Medical Research Building IV, Nashville, Tennessee 37232-0575. E-mail address:
[email protected].
study reinforce the findings of smaller reports and raise an interesting and important new hypothesis for further testing. Screening SCN5A in 2,111 probands identified rare nonsynonymous variants (i.e., those that change the amino acid sequence) in 21%, a number that is in line with earlier reports. Before making the assumption that each of these variants is disease-causing, it is important to ask whether any are seen in “normal” individuals. We are familiar with the idea that common genetic variants (polymorphisms) can be nonsynonymous, and that they may alter the function of encoded proteins and SCN5A is no exception. For example, 8% of African-Americans carry a common variant that results in substitution of a tyrosine (Y) for a serine (S) at position 1103. S1103Y has been associated with increased susceptibility to a range of arrhythmias, ranging from druginduced long QT syndrome to sudden death in adults and infants.9 –12 Although most of the variants found in the Brugada probands in the present report were “private” (seen in only one individual), some were identified in multiple subjects, including E1784K found in 14 probands. We previously reported that E1784K confers a mixed clinical phenotype (largely LQT3) and is detected in subjects of multiple ancestries, effectively ruling out a “founder” effect.7
Establishing cause and effect Our very rapidly expanding ability to screen individual genes and indeed very large segments of human genomes has highlighted the fact that each of us harbors thousands of rare nonsynonymous DNA variants.13 Work from the Ackerman lab previously reported that such variants in SCN5A were detected in 39 of 829 seemingly normal individuals, and, as is now well recognized, these vary by ethnicity.14 The present report includes a reference population of 1,300 normal subjects of varying ethnicity, and rare nonsynonymous SCN5A variants were identified in 42 (3.2%). This finding throws a bit of a monkey wrench into interpreting the results of genetic testing. If many normal individuals have SCN5A mutations, what can we make of a positive genetic screening test? Ackerman and colleagues previously noted that the vast majority of mutations identified in Brugada syndrome probands are located in transmembranespanning segments of the proteins,14 whereas variants identified in control subjects tend to be in other regions, notably interdomain linkers, and the present findings reinforce that conclusion. Nevertheless, the separation is not complete.
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doi:10.1016/j.hrthm.2009.10.016
48 Thus, it is conceivable that some of the mutations identified in the probands are, in fact, innocent bystander variants, and conversely that rare variants identified in some of the controls may actually connote an increased susceptibility to arrhythmias. These may include not only Brugada syndrome, conduction system disease, or LQT3 but also other common findings, such as atrial fibrillation and heart failure.15,16 In addition, the present report does not include information on clinical course, so we are not helped with common clinical problems such as stratifying risk. One approach to this problem would be to test the function of each of these hundreds mutations in in vitro settings. However, even in vitro studies can be deceptive. Variants that do not seem to do much to alter function when studied under physiologic conditions in Xenopus oocytes or in Chinese hamster ovary (CHO) cells may display striking abnormalities of function when those conditions are altered. For example, S1103Y displays minimal changes in gating compared to wild-type channels under conditions of normal of pH9 but exhibits striking gating changes with extracellular acidification.11 Similarly, the D1275N variant produces very minor changes in function when expressed in Xenopus oocytes,17 but mice bearing this mutation display striking reduction in sodium current.18 The solution to this problem, which will only get bigger with time, is not obvious. Perhaps efforts in advanced informatics and structural biology eventually will allow in silico prediction of the functional effects of individual mutations.
What causes Brugada syndrome in the remaining 80%? A very vexing issue highlighted by the present report is that, after more than a decade of work by many laboratories worldwide, we are missing an explanation for the Brugada syndrome in approximately 80% of probands. In a handful of individual SCN5A mutation-negative families, mutations have been identified in other logical candidate disease genes, such as SCN5A associated proteins, and calcium channel genes.19,20 In one large family, mutation in a new gene (GPD1L) was linked to Brugada syndrome21 and initially was implicated as a modulator of SCN5A trafficking to the cell surface. More recently, however, exploration of the role of GPD1L has identified a prominent role of modulation of sodium current by NADH-modulated cellular metabolism.22 A recent report of large kindreds with the Brugada syndrome suggests one way of thinking about the missing 80%.23 Five extended kindreds were identified in which a sodium channel mutation was identified in the proband but other family members displayed the Brugada syndrome ECG without the proband’s mutation. Does this mean that the whole association between mutant sodium channels and the Brugada syndrome is simply a house of cards? That seems unlikely given the strength of the clinical, genetic, and molecular data. Rather, it seems far more likely that the Brugada syndrome represents the clinical culmination of
Heart Rhythm, Vol 7, No 1, January 2010 multiple molecular lesions, possibly converging on reduced sodium current as a final common manifestation.
Multiple lesions may be required to disrupt function in complex systems We already recognize such multiple lesions: genetically reduced sodium current, sodium channel-blocking drugs, and variable regulation of sodium current by cellular metabolism. What other lesions might contribute? One especially intriguing possibility is that developmental variability in the distal end of the primitive heart tube, which ultimately constitutes the right ventricular outflow tract, may play a role.24 This is in keeping with an emerging story that susceptibility to atrial fibrillation may include a developmental component.25,26 Taken together, these new findings are consistent with the general view that normal cardiac electrophysiology represents the complex interplay among multiple, often redundant, components. As a consequence, multiple lesions, be they rare mutations, common polymorphisms, drug exposure, or other factors that we do not completely understand, generate the arrhythmia-prone substrate: altered “reserve” as a generic mechanism contributing to arrhythmia susceptibility. The opening of cardiac sodium channels initiates the cardiac cycle. It is a stunning testament to the stability of the amazingly complex ballet of channels and other proteins underlying the action potential that the vast majority of heartbeats in patients with SCN5A mutations are normal. The catalog of sodium channel variants seen across healthy populations and in those with Brugada syndrome is therefore an especially useful starting point for further understanding of the mechanisms conferring this resiliency.
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Editorial Commentary
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49 20. Antzelevitch C, Pollevick GD, Cordeiro JM, et al. Loss-of-function mutations in the cardiac calcium channel underlie a new clinical entity characterized by ST-segment elevation, short QT intervals, and sudden cardiac death. Circulation 2007;115:442– 449. 21. London B, Michalec M, Mehdi H, et al. Mutation in glycerol-3-phosphate dehydrogenase 1 like gene (GPD1-L) decreases cardiac Na⫹ current and causes inherited arrhythmias. Circulation 2007;116:2260 –2268. 22. Liu M, Sanyal S, Gao G, et al. Cardiac Na⫹ current regulation by pyridine nucleotides. Circ Res 2009;105:737–745. 23. Probst V, Wilde AAM, Barc J, et al. SCN5A mutations and the role of genetic background in the pathophysiology of Brugada syndrome. Circ Cardiovasc Genet 2009;(In press). 24. Boukens BJ, Christoffels VM, Coronel R, Moorman AF. Developmental basis for electrophysiological heterogeneity in the ventricular and outflow tract myocardium as a substrate for life-threatening ventricular arrhythmias. Circ Res 2009;104:19 –31. 25. Mommersteeg MTM, Brown NA, Prall OWJ, et al. Pitx2c and Nkx2-5 Are required for the formation and identity of the pulmonary myocardium. Circ Res 2007;101:902–909. 26. Gudbjartsson DF, Arnar DO, Helgadottir A, et al. Variants conferring risk of atrial fibrillation on chromosome 4q25. Nature 2007;448:353–357.