Background: Bradyarrhythmia is a common clinical manifestation. Although the majority of cases are acquired, genetic analysis of families with bradyarrhythmia has identified a growing number of causative gene mutations. Because the only ultimate treatment for symptomatic bradyarrhythmia has been invasive surgical implantation of a pacemaker, the discovery of novel therapeutic molecular targets is necessary to improve prognosis and quality of life. Methods: We investigated a family containing 7 individuals with autosomal dominant bradyarrhythmias of sinus node dysfunction, atrial fibrillation with slow ventricular response, and atrioventricular block. To identify the causative mutation, we conducted the family-based whole exome sequencing and genome-wide linkage analysis. We characterized the mutation-related mechanisms based on the pathophysiology in vitro. After generating a transgenic animal model to confirm the human phenotypes of bradyarrhythmia, we also evaluated the efficacy of a newly identified molecular-targeted compound to upregulate heart rate in bradyarrhythmias by using the animal model. Results: We identified one heterozygous mutation, KCNJ3 c.247A>C, p.N83H, as a novel cause of hereditary bradyarrhythmias in this family. KCNJ3 encodes the inwardly rectifying potassium channel Kir3.1, which combines with Kir3.4 (encoded by KCNJ5 ) to form the acetylcholine-activated potassium channel ( I KACh channel) with specific expression in the atrium. An additional study using a genome cohort of 2185 patients with sporadic atrial fibrillation revealed another 5 rare mutations in KCNJ3 and KCNJ5 , suggesting the relevance of both genes to these arrhythmias. Cellular electrophysiological studies revealed that the KCNJ3 p.N83H mutation caused a gain of I KACh channel function by increasing the basal current, even in the absence of m 2 muscarinic receptor stimulation. We generated transgenic zebrafish expressing mutant human KCNJ3 in the atrium specifically. It is interesting to note that the selective I KACh channel blocker NIP-151 repressed the increased current and improved bradyarrhythmia phenotypes in the mutant zebrafish. Conclusions: The I KACh channel is associated with the pathophysiology of bradyarrhythmia and atrial fibrillation, and the mutant I KACh channel ( KCNJ3 p.N83H) can be effectively inhibited by NIP-151, a selective I KACh channel blocker. Thus, the I KACh channel might be considered to be a suitable pharmacological target for patients who have bradyarrhythmia with a gain-of-function mutation in the I KACh channel.

Dilated cardiomyopathy (DCM) is a major cause of heart failure, characterized by ventricular dilatation and systolic dysfunction. Familial DCM is reportedly caused by mutations in more than 50 genes, requiring precise disease stratification based on genetic information. However, the underlying genetic causes of 60 to 80% of familial DCM cases remain unknown. Here, we identified that homozygous truncating mutations in the gene encoding Bcl-2–associated athanogene (BAG) co-chaperone 5 ( BAG5 ) caused inherited DCM in five patients among four unrelated families with complete penetrance. BAG5 acts as a nucleotide exchange factor for heat shock cognate 71 kDa protein (HSC70), promoting adenosine diphosphate release and activating HSC70-mediated protein folding. Bag5 mutant knock-in mice exhibited ventricular dilatation, arrhythmogenicity, and poor prognosis under catecholamine stimulation, recapitulating the human DCM phenotype, and administration of an adeno-associated virus 9 vector carrying the wild-type BAG5 gene could fully ameliorate these DCM phenotypes. Immunocytochemical analysis revealed that BAG5 localized to junctional membrane complexes (JMCs), critical microdomains for calcium handling. Bag5 -mutant mouse cardiomyocytes exhibited decreased abundance of functional JMC proteins under catecholamine stimulation, disrupted JMC structure, and calcium handling abnormalities. We also identified heterozygous truncating mutations in three patients with tachycardia-induced cardiomyopathy, a reversible DCM subtype associated with abnormal calcium homeostasis. Our study suggests that loss-of-function mutations in BAG5 can cause DCM, that BAG5 may be a target for genetic testing in cases of DCM, and that gene therapy may potentially be a treatment for this disease.

This is an open access article under the terms of the Creat ive Commo ns Attri bution-NonCo mmercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

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