Novel mutations in domain I of SCN5A cause Brugada syndrome

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Abstract

Brugada syndrome, an autosomal dominantly inherited form of ventricular fibrillation characterized by ST-segment elevation in leads V1–V3 and right bundle-branch block on surface electrocardiogram, is caused by mutations in the cardiac sodium channel gene SCN5A. Patients with Brugada syndrome were studied using single-strand conformation polymorphism analysis, denaturing high-performance liquid chromatography, and DNA sequencing of SCN5A. Mutations were identified in SCN5A in two families and one sporadic case. In one family, a missense mutation leading to a glycine to valine sustitution (G351V) in the pore region between the DIS5 and DIS6 transmembrane segments was detected. Biophysical analysis demonstrated that this mutation caused significant current reduction. In the other family, a 20-bp deletion of the exon 5 splice acceptor site was identified; as exon 5 encodes part of the intracellular loop between DIS2 and DIS3, this portion of the channel is disrupted. In the sporadic patient, a missense mutation resulting in the substitution of lysine by glutamic acid (K126E) in the intracellular loop at the boundary with DIS1 was identified. These three new SCN5A mutations in Brugada syndrome patients are all located within domain I of SCN5A, a region not previously considered important in the development of ventricular arrhythmias.

Introduction

Brugada syndrome, a form of idiopathic ventricular fibrillation first described as a clinical entity in 1992 [1], is characterized by persistent ST-segment elevation in the right precordial leads and, in some patients, a right bundle-branch block (RBBB) pattern. These patients have a propensity for developing life-threatening ventricular fibrillation, particularly during sleep [1], [2], [3], [4], [5].

The incidence of Brugada syndrome reportedly ranges between 5 and 66 per 10,000 in regions where the disease is endemic, but is not well defined in the United States and many other regions of the world. In Southeast Asia, however, it is the leading cause of death of young adults [6], exceeded only by deaths from vehicular accidents.

The Brugada syndrome was first linked by our group to mutations in SCN5A, the gene encoding for the α-subunit of the cardiac sodium channel, in 1998 [5]. This gene has also been shown to cause a form of dominantly inherited long QT syndrome, LQT3 [7], [8], [9], as well as conduction system disease [10], [11] and sudden infant death syndrome (SIDS) [12]. The mutations in SCN5A in patients with Brugada syndrome result in different electrophysiologic abnormalities than those known to cause the long QT syndrome (LQTS) [7], [8], [9] or conduction system disease [10], [11]. In our original report, we identified three mutations, including [1] a missense mutation (T1620M) affecting the extracellular loop between DIVS3 and DIVS4 of the sodium channel, [2] a single nucleotide deletion which results in an in-frame stop codon that eliminates DIIIS6–DIVS6, and [3] a two nucleotide insertion which disrupts the splice-donor sequence of intron 7 of SCN5A, affecting the intracellular loop between DIS2 and DIS3 [5]. The T1620M SCN5A mutant was shown to recover more rapidly than wild-type channels [13], in contradistinction to SCN5A mutations responsible for LQT3, which typically cause a persistent noninactivating sodium current as the major phenotypic change [7], [8], [9]. This effect of T1620M is temperature dependent [13]. Other mutations in SCN5A have been shown to reduce the contribution of INa by shifting the voltage dependence of activation and inactivation of the channel [14], [15], [16], [17], [18], [19]. In one family, the combination of clinical features of both Brugada syndrome and LQT3 has been reported while many patients with conduction system disease associated with Brugada syndrome are also described [15], [16]. For these reasons, the specific electrophysiologic features involved in the development of the Brugada syndrome phenotype have been controversial.

In order to further evaluate the molecular and biophysical mechanisms of disease, we pursued the analysis of families and sporadic cases with Brugada syndrome. In this report we describe three new mutations in a novel region of the SCN5A sodium channel in familial and sporadic cases of Brugada syndrome and the resultant electrophysiologic abnormality in one family.

Section snippets

Clinical evaluation

Brugada syndrome kindreds were recruited from medical clinics in North America, Germany, Spain, and Belgium. The phenotype of each family member was characterized according to previously described criteria [1], including previous history of sudden death in probands or in a family member, no demonstrable structural heart defects or long QT syndrome by invasive and noninvasive procedures, and an ECG pattern of ST-segment elevation in leads V1–V3 at baseline or after intravenous ajmaline [20].

Mutation analysis

Mutations in the sodium channel gene SCN5A were identified in two families (M007 and M066) and one sporadic case (M011), while a polymorphism was identified in three sporadic cases.

Family M007. An abnormal conformer was detected by SSCP in exon 9 of SCNSA of the proband (Fig. 1A). The abnormal conformer was subsequently detected by both SSCP and DHPLC in each of the three affected family members, but not in either of the unaffected individuals (Figs. 1A and B). DNA sequence analysis (Fig. 1C)

Discussion

Previous studies from our group [5] and others [14], [15], [16], [17], [18], [19], [27] have demonstrated that Brugada syndrome results from mutations in the cardiac sodium channel gene SCN5A. In this report, we describe novel mutations in the region of SCN5A encoding domain I, including two missense mutations (G351V, K126E) and a 20-bp deletion. The two missense mutations occur within the pore of domain I, the first such mutations in this portion of the channel. In the third case, a 20-bp

Acknowledgements

The authors are grateful for the administrative support of Melba Koegele. This work was funded by Grant R01 HL62570 from the National Institutes of Health, National Heart, Lung, and Blood Institutes (J.A.T.), the Texas Children's Hospital Foundation Chair in Pediatric Cardiovascular Research (J.A.T.), and the Abercrombie Pediatric Cardiology Fund of Texas Children's Hospital (J.A.T.).

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