Study On The Molecular Genetic Correlation Between The Third Degree Of Atrioventricular Block And Sodium Channel Gene SCN5A

BackgroundAtrioventricular block(AVB) is a disorder of the atrioventricular conduction. The third-degree atrioventricular block, also referred to as complete heart block is a disorder where there is complete dissociation of the atrial and ventricular activity due to the absence of conduction through the atrioventricular node or His-Purkinje system.With the development of genetics and molecular biology, the relationship between the pathogeny of AVB and genetic mutations attracted broad attention in the world in recent years. Recent studies have shown that gene variations of ion channels, the gap junction proteins, transient receptor potential, transcription factor and Metabolic regulation factor are closely associated with AVB. The SCN5 A gene encodes a sodium channel that is principally expressed in heart and is responsible for the generation of the rapid upstroke of the myocardial action potential. Recent studies show SCN5 A mutation is not only associated with SCN5 A inherited and isolated AVB, but associated with the-third AVB combination of structural heart disease. Objective:To investigate the correlation between third-degree atrioventricular block(AVB) and the mutation of cardiac sodium channel encoding gene SCN5 A in Chinese population. Methods:We collected the clinical data and blood samples of patients with third-degree AVB in our hospital. The genomic DNA were extracted, and all exons and exon–intron boundaries of SCN5 A gene were screened by direct DNA sequencing method. Genomic DNA of 200 healthy ethnically matched individuals was used as controls to distinguish the type of genetic variations. Using site-directed mutagenesis system, we constructed the mutated SCN5A-A1428 S and SCN5A-S593 G, then transfected the mutant and wild type into HEK293 cells after verifying the sequence of mutant-plasmid. By using whole cell patch clamp technique to analyze the parameters of sodium current comparing wild type with mutant SCN5 A channel proteins, and to discuss the relationship between genotype and phenotype. Results:35 Patients with third-degree AVB were eligible in this study. Two heterozygosis missense mutation A1428 S and S593 G of SCN5 A gene was identified: the nucleotide changed from G to T in the locus 4282(c.4282G>T) lead to the 1428 th amino acid change from alanine to serine; and the nucleotide changed from A to G in the locus 4282(c.1971A>G) lead to the 1971 th amino acid change from serine to glycine. And we also identified one synonymous mutation I1603I(c.5003C>T).Additionally, five single nucleotide polymorphisms(SNPs) of A29 A, H558 R, I603 I, R1192 Q, D1818 D in SCN5 A gene were detected in other patients. The Patch clamp results suggest mutants(A1428S, S593G) expression loss of function. A1428 S mutant channel produced significant reduction of peak INa,peak current is reduced about 60%, inducing a depolarization shift of the I-V relationship curve and voltage dependence of SSA of INa. S593 G mutant channel produced significant reduction of peak INa,peak current is reduced about 30%, inducing a depolarization shift of the I-V relationship curve and voltage dependence of SSA of INa. Conclusions:1.We firstly revealed the relationship between AVB in Chinese patients with SCN5 A gene variations, and detect three SCN5 A mutations: A1428 S, S593 G, I1603 I.2. SCN5A-A1428 S, S593 G mutations show a significant loss of Nav1.5 channel function, decrease sodium current density and intensity, slower conduction.3. Loss of function of SCN5 A mutations are strongly linked to the-third AVB.