Familial Sinus Node Disease Caused by a Gain of GIRK (G-Protein Activated Inwardly Rectifying K+ Channel) Channel Function.

BACKGROUND: Inherited forms of sinus node dysfunction (SND) clinically include bradycardia, sinus arrest, and chronotropic incompetence and may serve as disease models to understand sinus node physiology and impulse generation. Recently, a gain-of-function mutation in the G-protein gene GNB2 led to enhanced activation of the GIRK (G-protein activated inwardly rectifying K+ channel). Thus, human cardiac GIRK channels are important for heart rate regulation and subsequently, genes encoding their subunits Kir3.1 and Kir3.4 ( KCNJ3 and KCNJ5) are potential candidates for inherited SND in human. METHODS: We performed a combined approach of targeted sequencing of KCNJ3 and KCNJ5 in 52 patients with idiopathic SND and subsequent whole exome sequencing of additional family members in a genetically affected patient. A putative novel disease-associated gene variant was functionally analyzed by voltage-clamp experiments using various heterologous cell expression systems (Xenopus oocytes, CHO cells, and rat atrial cardiomyocytes). RESULTS: In a 3-generation family with SND we identified a novel variant in KCNJ5 which leads to an amino acid substitution (p.Trp101Cys) in the first transmembrane domain of the Kir3.4 subunit of the cardiac GIRK channel. The identified variant cosegregated with the disease in the family and was absent in the Exome Variant Server and Exome Aggregation Consortium databases. Expression of mutant Kir3.4 (±native Kir3.1) in different heterologous cell expression systems resulted in increased GIRK currents ( IK,ACh) and a reduced inward rectification which was not compensated by intracellular spermidine. Moreover, in silico modeling of heterotetrameric mutant GIRK channels indicates a structurally altered binding site for spermine. CONCLUSIONS: For the first time, an inherited gain-of-function mutation in the human GIRK3.4 causes familial human SND. The increased activity of GIRK channels is likely to lead to a sustained hyperpolarization of pacemaker cells and thereby reduces heart rate. Modulation of human GIRK channels may pave a way for further treatment of cardiac pacemaking.

A Mutation in the G-Protein Gene GNB2 Causes Familial Sinus Node and Atrioventricular Conduction Dysfunction.

RATIONALE: Familial sinus node and atrioventricular conduction dysfunction is a rare disorder that leads to paroxysmal dizziness, fatigue, and syncope because of a temporarily or permanently reduced heart rate. To date, only a few genes for familial sinus and atrioventricular conduction dysfunction are known, and the majority of cases remain pathogenically unresolved. OBJECTIVE: We aim to identify the disease gene in a large 3-generation family (n=25) with autosomal dominant sinus node dysfunction (SND) and atrioventricular block (AVB) and to characterize the mutation-related pathomechanisms in familial SND+AVB. METHODS AND RESULTS: Genome-wide linkage analysis mapped the SND+AVB disease locus to chromosome 7q21.1-q31.1 (2-point logarithm of the odds score: 4.64; θ=0); in this region, targeted exome sequencing identified a novel heterozygous mutation (p.Arg52Leu) in the GNB2 gene that strictly cosegregated with the SND+AVB phenotype. GNB2 encodes the ß2 subunit (Gß2) of the heterotrimeric G-protein complex that is being released from G-protein-coupled receptors on vagal stimulation. In 2 heterologous expression systems (HEK-293T cells and Xenopus laevis oocytes), an enhanced activation of the G-protein-activated K+ channel (GIRK; Kir3.1/Kir3.4) was shown when mutant Gß2 was coexpressed with Gγ2; this was in contrast to coexpression of mutant Gß2-Gγ2 with other cardiac ion channels (HCN4, HCN2, and Cav1.2). Molecular dynamics simulations suggested a reduced binding property of mutant Gß2 to cardiac GIRK channels when compared with native Gß2. CONCLUSIONS: A GNB2 gene mutation is associated with familial SND+AVB and leads to a sustained activation of cardiac GIRK channels, which is likely to hyperpolarize the myocellular membrane potential and thus reduces their spontaneous activity. Our findings describe for the first time a role of a mutant G-protein in the nonsyndromic pacemaker disease because of GIRK channel activation.