Sodium channel β1 subunit mutations associated with Brugada syndrome and cardiac conduction disease in humans

Abstract

Brugada syndrome is a genetic disease associated with sudden cardiac death that is characterized by ventricular fibrillation and right precordial ST segment elevation on ECG. Loss-of-function mutations in SCN5A, which encodes the predominant cardiac sodium channel alpha subunit NaV1.5, can cause Brugada syndrome and cardiac conduction disease. However, SCN5A mutations are not detected in the majority of patients with these syndromes, suggesting that other genes can cause or modify presentation of these disorders. Here, we investigated SCN1B, which encodes the function-modifying sodium channel beta1 subunit, in 282 probands with Brugada syndrome and in 44 patients with conduction disease, none of whom had SCN5A mutations. We identified 3 mutations segregating with arrhythmia in 3 kindreds. Two of these mutations were located in a newly described alternately processed transcript, beta1B. Both the canonical and alternately processed transcripts were expressed in the human heart and were expressed to a greater degree in Purkinje fibers than in heart muscle, consistent with the clinical presentation of conduction disease. Sodium current was lower when NaV1.5 was coexpressed with mutant beta1 or beta1B subunits than when it was coexpressed with WT subunits. These findings implicate SCN1B as a disease gene for human arrhythmia susceptibility.

Figure 1. Structure of β1 and β1B subunits.

(A) Genomic structure of SCN1B. (B) Extension of exon 3 (c.208–458) into intron 3 creates a novel 3′ end of the transcript (exon 3A, c.208–978) and generates an alternate transcript encoding β1B. The gray region indicates the unique sequence of exon 3A. (C) Predicted topology of β1 and β1B. The β1B protein has unique juxtamembrane, transmembrane, and intracellular domains. The arrow indicates the initial amino acid of the β1B-specific segment. Circles indicate the locations of the mutations.

Figure 2. SCN1B mutations found in patients with Brugada syndrome and conduction disease.

(A) Pedigrees and phenotypes of the families affected by Brugada syndrome and/or conduction disease. Individuals carrying the mutation are indicated (+). Individuals who tested negative for the mutation are indicated (–). Individuals I-1 from family 1; and I-1, I-2, and II-3 from family 2 did not undergo genetic testing. Arrows indicate probands. (B) The c.259G→C mutation in SCN1B resulting in p.Glu87Gln found in family 1. (C) Alignment of β1 across species showing the high conservation of Glu87. (D) The c.536G→A (middle) and c.537G→A (right) mutations in exon 3A of β1B, both resulting in p.Trp179X found in families 2 and 3, respectively. (E) Twelve-lead ECG from the proband of family 2 (II-4). The arrowheads indicate ST-segment elevation typical of Brugada syndrome.

Figure 3. Expression profile of β1 and β1B transcripts in nondiseased human ventricular tissue as determined by quantitative real-time PCR.

Relative expression levels of the β1 and β1B subunits are presented, normalized to those of HPRT1 in LV (circles), RV (squares), and Purkinje fibers (triangles). Tissues for each group were collected from 6 human donors (nondiseased hearts, n = 6). Data points indicate the average of 2 measurements in each tissue sample. Larger symbols and error bars indicate median ± median absolute deviation for all samples.

Figure 4. Electrophysiological characteristics of the p. Trp179X β1B mutant.

(A) Representative traces of sodium current demonstrating an increase in sodium current with WT but not mutant subunit. (B) Sodium current density at –30 mV for Na_V1.5 alone (n = 29), Na_V1.5 coexpressed with WT β1B (n = 28), Na_V1.5 coexpressed with p.Trp179X β1B (n = 18), Na_V1.5 coexpressed with WT β1B plus p.Trp179X β1B (1 μg for each; n = 14), and Na_V1.5 coexpressed with WT β1B plus p.Trp179X β1B (0.5 μg for each; n = 10). (C) Voltage dependence of activation and inactivation. Filled circles, open circles, and squares indicate Na_V1.5 alone, Na_V1.5 coexpressed with WT β1B, and Na_V1.5 coexpressed with p.Trp179X β1B, respectively. The pulse protocol used to study the voltage dependence of inactivation is shown in the inset. (D) Recovery from inactivation. Biophysical properties are provided in Table 1.

Figure 5. Electrophysiological characteristics of the p.Glu87Gln mutant.

(A) Representative traces of sodium current. (B) Current density at –30 mV for Na_V1.5 alone (n = 13), Na_V1.5 coexpressed with WT β1 (n = 17), Na_V1.5 coexpressed with p.Glu87Gln β1 (n = 18), and Na_V1.5 coexpressed with WT β1 plus p.Glu87Gln β1 (n = 15). (C) Voltage dependence of activation and inactivation. Filled circles, open circles, and squares indicate Na_V1.5 alone, Na_V1.5 coexpressed with WT β1, and Na_V1.5 coexpressed with p.Glu87Gln β1, respectively. (D) Recovery from inactivation. Biophysical properties are provided in Table 2.