Na v 1.5, the pore-forming ␣ subunit of the cardiac voltagegated Na ؉ channel complex, is required for the initiation and propagation of the cardiac action potential. Mutations in Na v 1.5 cause cardiac arrhythmias and sudden death. The cardiac Na ؉ channel functions as a protein complex; however, its complete components remain to be fully elucidated. A yeast two-hybrid screen identified a new candidate Na v 1.5-interacting protein, ␣B-crystallin. GST pull-down, co-immunoprecipitation, and immunostaining analyses validated the interaction between Na v 1.5 and ␣B-crystallin. Whole-cell patch clamping showed that overexpression of ␣B-crystallin significantly increased peak sodium current (I Na ) density, and the underlying molecular mechanism is the increased cell surface expression level of Na v 1.5 via reduced internalization of cell surface Na v 1.5 and ubiquitination of Na v 1.5. Knock-out of ␣B-crystallin expression significantly decreased the cell surface expression level of Na v 1.5. Co-immunoprecipitation analysis showed that ␣B-crystallin interacted with Nedd4-2; however, a catalytically inactive Nedd4-2-C801S mutant impaired the interaction and abolished the up-regulation of I Na by ␣B-crystallin. Na v 1.5 mutation V1980A at the interaction site for Nedd4-2 eliminated the effect of ␣B-crystallin on reduction of Na v 1.5 ubiquitination and increases of I Na density. Two disease-causing mutations in ␣B-crystallin, R109H and R151X (nonsense mutation), eliminated the effect of ␣B-crystallin on I Na . This study identifies ␣B-crystallin as a new binding partner for Na v 1.5. ␣B-Crystallin interacts with Na v 1.5 and increases I Na by modulating the expression level and internalization of cell surface Na v 1.5 and ubiquitination of Na v 1.5, which requires the protein-protein interactions between ␣B-crystallin and Na v 1.5 and between ␣B-crystallin and functionally active Nedd4-2.Na v 1.5 is the pore-forming ␣ subunit of the major cardiac voltage-gated Na ϩ channel complex. It generates the sodium current (I Na ) 4 that plays an essential role in the initiation and propagation of the cardiac action potential (1-3). Mutations in the SCN5A gene (encoding Na v 1.5) cause several inherited arrhythmias, including atrial fibrillation, Brugada syndrome, long QT syndrome, progressive cardiac conduction defect disease, sick sinus syndrome, and dilated cardiomyopathy (4). Na v 1.5 exists in vivo in a multiprotein complex, which interacts with the actin cytoskeleton and the extracellular matrix to provide an important functional link between channel complexes, cardiac structure, and electrical functioning (5, 6). Several proteins have been reported to bind to Na v 1.5 (5-7). We have previously reported a small protein, MOG1, with a function in nucleocytoplasmic protein transport that interacts directly with Na v 1.5, promotes trafficking of Na v 1.5 to the cell surface, and increases peak I Na density (4, 6). Specifically, MOG1 facilitates export of Na v 1.5 from the endoplasmic reticulum as well as targeting of Na v 1.5...

Background Genomic variants identified by genome-wide association studies (GWAS) explain <20% of heritability of coronary artery disease (CAD), thus many risk variants remain missing for CAD. Identification of new variants may unravel new biological pathways and genetic mechanisms for CAD. To identify new variants associated with CAD, we developed a candidate pathway-based GWAS by integrating expression quantitative loci (eQTL) analysis and mining of GWAS data with variants in a candidate pathway. Methods and Results Mining of GWAS data was performed to analyze variants in 32 complement system genes for positive association with CAD. Functional variants in genes showing positive association were then identified by searching existing expression quantitative loci databases and validated by RT-PCR. A follow-up case control design was then used to determine whether the functional variants are associated with CAD in two independent GeneID Chinese populations. Candidate pathway-based GWAS identified positive association between variants in C3AR1 and C6 and CAD. Two functional variants, rs7842 in C3AR1 and rs4400166 in C6, were found to be associated with expression levels of C3AR1 and C6, respectively. Significant association was identified between rs7842 and CAD (P=3.99×10−6, OR=1.47) and between rs4400166 and CAD (P=9.30×10−3, OR=1.24) in the validation cohort. The significant findings were confirmed in the replication cohort (P=1.53×10−5, OR=1.37 for rs7842; P=8.41×10−3, OR=1.21 for rs4400166. Conclusions Integration of GWAS with biological pathways and eQTL is effective in identifying new risk variants for CAD. Functional variants increasing C3AR1 and C6 expression were shown to confer significant risk of CAD for the first time.

Atrial fibrillation (AF) affects 33.5 million individuals worldwide. It accounts for 15% of strokes and increases risk of heart failure and sudden death. The voltage-gated cardiac sodium channel complex is responsible for the generation and conduction of the cardiac action potential, and composed of the main pore-forming α-subunit Nav1.5 (encoded by the SCN5A gene) and one or more auxiliary β-subunits, including Navβ1 to Navβ4 encoded by SCN1B to SCN4B, respectively. We and others identified loss-of-function mutations in SCN1B and SCN2B and dominant-negative mutations in SCN3B in patients with AF. Three missense variants in SCN4B were identified in sporadic AF patients and small nuclear families; however, the association between SCN4B variants and AF remains to be further defined. In this study, we performed mutational analysis in SCN4B using a panel of 477 AF patients, and identified one nonsynonymous genomic variant p.Gly8Ser in four patients. To assess the association between the p.Gly8Ser variant and AF, we carried out case-control association studies with two independent populations (944 AF patients vs. 9,81 non-AF controls in the first discovery population and 732 cases and 1,291 controls in the second replication population). Significant association was identified in the two independent populations and in the combined population (p = 4.16 × 10−4, odds ratio [OR] = 3.14) between p.Gly8Ser and common AF as well as lone AF (p = 0.018, OR = 2.85). These data suggest that rare variant p.Gly8Ser of SCN4B confers a significant risk of AF, and SCN4B is a candidate susceptibility gene for AF.

These data suggest that SAR1A and SAR1B are the critical regulators of trafficking of Na1.5. Moreover, SAR1A and SAR1B interact with MOG1, and are required for MOG1-mediated cell surface expression and function of Na1.5.

Aim: Loss-of-function KCNMA1 variants cause Liang-Wang syndrome (MIM #618729), a newly identified multiple malformation syndrome with a broad spectrum of developmental and neurological phenotypes. However, the full spectrum of clinical features and underlying pathogenic mechanisms need full elucidation.Methods: Exome sequencing was used to identify pathogenic variants. Patchclamp recordings were performed to access the effects of KCNMA1 variants on BK channels. Total and membrane protein expression levels of BK channels were characterized using Western blotting. Results: We report identification and functional characterization of two new de novo loss-of-function KCNMA1 variants p.(A172T) and p.(A314T) with characteristics of Liang-Wang syndrome. Variant p.(A172T) is associated with developmental delay, cognitive impairment and ataxia. Mechanistically, p.(A172T) abolishes BK potassium current, inhibits Mg 2+ -dependent gating, but shifts conductance-voltage (G-V) curves to more positive potentials when complexed with WT channels. Variant p.(A314T) is associated with developmental delay, intellectual disability, cognitive impairment, mild ataxia and generalized epilepsy; suppresses BK current amplitude; and shifts G-V curves to more positive potentials when expressed with WT channels. In addition, two new patients with previously reported gain-of-function variants p.(N536H) and p.(N995S) are found to show epilepsy and paroxysmal dyskinesia as reported previously, but also exhibit additional symptoms of cognitive impairment and dysmorphic features. Furthermore, variants p.(A314T) and p.(N536H) reduced total and membrane levels of BK proteins. Conclusion: Our findings identified two new loss-of-function mutations of KCNMA1 associated with Liang-Wang syndrome, expanded the spectrum of clinical features associated with gain-of-function KCNMA1 variants and emphasized the overlapping features shared by gain-of-function and loss-of-function mutations.