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Background: Familial atrial septal defect (ASD) has previously been attributed primarily to mutations in cardiac transcription factors. Here, we report a large, multi-generational family (78 members) with ASD combined with a late-onset dilated cardiomyopathy and further characterize the consequences of mutant α-actin., Methods: We combined a genome-wide linkage analysis with cell biology, microscopy, and molecular biology tools to characterize a novel ACTC1 (cardiac α-actin) mutation identified in association with ASD and late-onset dilated cardiomyopathy in a large, multi-generational family., Results: Using a genome-wide linkage analysis, the ASD disease locus was mapped to chromosome 15q14 harboring the ACTC1 gene. In 15 affected family members, a heterozygous, nonsynonymous, and fully penetrant mutation (p. Gly247Asp) was identified in exon 5 of ACTC1 that was absent in all healthy family members (n=63). In silico tools predicted deleterious consequences of this variant that was found absent in control databases. Ultrastructural analysis of myocardial tissue of one of the mutation carriers showed sarcomeric disarray, myofibrillar degeneration, and increased apoptosis, while cardiac proteomics revealed a significant increase in extracellular matrix proteins. Consistently, structural defects and increased apoptosis were also observed in neonatal rat ventricular cardiomyocytes overexpressing the mutant, but not native human ACTC1. Molecular dynamics studies and additional mechanistic analyses in cardiomyocytes confirmed actin polymerization/turnover defects, thereby affecting contractility., Conclusions: A combined phenotype of ASD and late-onset heart failure was caused by a heterozygous, nonsynonymous ACTC1 mutation. Mechanistically, we found a shared molecular mechanism of defective actin signaling and polymerization in both cardiac development and contractile function. Detection of ACTC1 mutations in patients with ASD may thus have further clinical implications with regard to monitoring for (late-onset) dilated cardiomyopathy.

Background: Arrhythmogenic right ventricular cardiomyopathy (ARVC) is associated with pathogenic/likely pathogenic (P/LP) variants in genes encoding the cardiac desmosomal proteins. Origin of these variants, including de novo mutation rate and extent of founder versus recurrent variants has implications for variant adjudication and clinical care, yet this has never been systematically investigated., Methods: We identified arrhythmogenic right ventricular cardiomyopathy probands who met 2010 Task Force Criteria and had undergone genotyping that included sequencing of the desmosomal genes (PKP2, DSP, DSG2, DSC2, and JUP) from 3 arrhythmogenic right ventricular cardiomyopathy registries in America and Europe. We classified the desmosomal variants, defined the contribution of unique versus nonunique (ie, not family-specific) P/LP variants, and identified the frequency and characteristics of de novo variants. Next, we haplotyped nonunique variants to determine how often they likely represent a single mutation event in a common ancestor (implied by shared haplotypes) versus multiple mutation events at the same genetic location., Results: Of 501 arrhythmogenic right ventricular cardiomyopathy probands, 322 (64.3%) carried 327 desmosomal P/LP variants. Most variants (n=247, 75.6%, in 245 patients) were identified in more than one proband and, therefore, considered nonunique. For 212/327 variants (64.8%) genetic cascade screening was performed extensively enough to identify the parental origin of the P/LP variant. Only 3 variants were de novo, 2 of which were whole gene deletions. For 24 nonunique P/LP PKP2 variants, haplotyping was conducted in 183 available families. For all 24 variants, multiple seemingly unrelated families sharing identical haplotypes were identified, suggesting that these variants originate from common founders., Conclusions: Most desmosomal P/LP variants are inherited, nonunique, and originate from ancient founders. Two of 3 de novo variants were large deletions. These observations inform genetic testing, cascade screening, and variant adjudication.

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 ( I K,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.