Single Nucleotide Polymorphisms Of SCN5A Gene In South Chinese Han Population And Their Correlation With Arrhythmias

Abnormalities in the electophysiological activation of the myocardium are known as cardiac arrhythmias, a major cause of morbidity and mortality. Among individuals with the most dangerous forms of arrhythmia, such as 'torsades de pointes', ventricular tachycardia and ventricular fibrillation, sudden cardiac death is a frequent outcome. Arrhythmias lead to more than 350,000 sudden deaths per year in America. Because the heartbeat is so dependent on the proper movement of ions across thesurface membrane, disorders of ion channels——or 'channelopathies'——make up akey group of heart diseases. Channelopathies predispose individuals to disturbances of normal cardiac rhythm. Voltage-gated sodium channels are transmembrane proteins that produce the ionic current responsible for the rising phase of the cardiac action potential and play a fundamental role in the initaiation, propagation, and maintenance of normal cardiac rhythm. Inherited mutations in SCN5A, the geneencoding the pore-forming subunit of the cardiac Na+ channel, have been associated with distinct cardiac rhythm syndromes: the congenital long QT syndrome, Brugada syndrome, and isolated conduction disease. Electrophysiologic characterization of heterologously expressed mutant Na+ channels have revealed gating defects that, in many cases, can explain the distinct phenotype associated with the rhythm disorder. However, recent studies have revealed significant overlap between aberrant rhythm phenotypes, and single mutations have been identified that evoke multiple rhythm disorders with common gating lesions. These accumulating evidences suggest that 'multiple hits' affecting the interaction and integrity of multiple pathways might be responsible for the sodium channelopathies.Single nucleotide polymorphisms (SNPs) are common in the human genome, presenting in more than 1% population, and have been implicated in phenotypic variability in physiology, pharmacology and pathophysiology. Studies have linked gene polymorphisms to elevated risk for cystic fibrosis, Alzheimer disease, and even heart disease. In addition to their roles in diseases, polymorphisms are also thought to confer sensitivity to drug therapy, as well as proarrhythmic risk from drug therapy. Recently, a series of studies have show that polymorphisms in SCN5A also play an important role in the manifestation of disorders in cardiac excitability. This study was the first report of the polymorphisms of SCN5A gene in Chinese.Objective:To investigate the polymorphisms of SCN5 A gene in South Chinese Han population, and their correlation with arrhythmias.Methods:A cohort of 120 unrelated, healthy volunteers with Han nationality were accrued from several provinces in South China. Forty-eight Brugada syndrome patients including 36 unrelated probands and 12 sporadic ones were also recruited from hospitals in these same regions. The acquired arrhythmia case group consisted of 78 unrelated patients who were admitted to the affiliated hospital, Zhejiang University College of Medicine because of arrhythmia or because they were thought to be at risk for arrhythmia. All patients were from Han nationality. They underwent detailed clinical and cardiovascular examinations.Genomic DNA was isolated from each sample by means of standard procedures. The coding sequence and 3'- UTR, including the putative splicing sites in SCN5A gene were amplified by PCR and sequenced directly or after subcloning using ABI PRISM 377 DNA sequencer, to determine the genotyping. The sequencing reactions were performed by the dideoxynucleotide chain termination method with fluorescent dideoxynucleotides according to the BigDye Terminatory protocol. The deviations from Hardy-Weinberg equilibrium were tested by %2 analysis. Expectation maximization algorithm was used for haplotype frequencies analysis and pairwise linkage disequilibrium (LD) test. Heterozygosity and polymorphism information content of simple site and multi-sites were also calculated. The distributions of the genotype and allele frequencies were statistically compared between the case and control group using the Fisher's exact test. Odds ration (OR) and 95% confidence interval (95% CI) were calculated as the estimate of the relativerisk. Relation between the genotype and clinical manifestations characteristics of arrhythmias patients were calculated by Fish's exact test. Results:A total of 5 SNPs were identified in Chinese Han population, including 3 novel ones: G87A(A29A), 4245 + 82A>G and G6174A. The allele frequencies of each SNP in Chinese Han population were as follows: G87A(A29A) 27.5%, A1673G(H558R)10.4%, 4245+82A>G 32.8%, C5457T(D1819D)41.3% and G6174A 44.9%, respectively. S1102Y and other SNPs identified in other ethnic populations had not been detected in our study. Genotype distributions of all loci were in Hardy-Weinberg equilibrium both in healthy volunteers and cases. Pairwise linkage disequilibrium was observed between 4245 + 82A>G/C5457T, 4245 + 82A>G/G6174A and C5457T/G6174A ( D'>0.8 ) . 4245 + 82A>G/C5457T/G6174A composed a haplotype. We compare the distributions of SCN5A SNPs in different ethnic populations and found that C5457T allele frequency of Han population in South China was similar to that observed in Japanese, but higher than that in American. There was no significant difference in the allele frequency of A1673G within different ethical populations. The distributions of the identified SNPs between male group and female group are not significantly different.The allele G1673 was over-presented in Brugada syndrome patients compared with controls, while there was no significant difference of genotype frequency in this locus in the case-control study. The genotype and allele frequencies of 4 other identified SNPs between Brugada syndrome patients and healthy controls were similar.One novel mutation (4087insC) of SCN5A gene was found in one Brugada syndrome family by genotyping intragenic polymorphisms. It was the first mutation associated with Brugada syndrome found in the Chinese population. Pedigree of the family indicated an autosomal dominant mode of transmission. The polymorphism (A1673G) is also shown to be associated with acquired cardiac arrhythmia and might be involved in susceptibility to cardiac arrhythmia in the Chinese Han population. There was a significant increase in the allele frequency of G1673 in patients with acquired cardiac arrhythmia. The likelihood of displaying signs of arrhythmia in a G1673 carrier (G, A and G, G) yielded an odds ratio (OR) of 3.01 [95% CI: 1.6 to 15.7]. This odds ratio was not significantly altered after controlling for age and gender. There were no significant differences in distributions of 4 other SNPs between normal controls and arrhythmia patients. We analyzed the correlation between the genotypes and clinical features among acquired arrhythmia patients and found that the A6174 allele carrier (G6174A+A6174A) were more likely to have long QT interval than patients with G6174G genotype (OR = 5.3, 95% CI: 1.4-20.1).Conclusions:1. It is suggested that the SNPs distribution of SCN5A gene varies within different nationalities. The new SNPs 4245 + 82A>G and G6174A are Han nationalityspecific.2. The novel mutation 4087insC in SCN5A was the first mutation associated with Brugada syndrome in Chinese.3. The variant A1673G (H558R) has been found to show an association with both heritable Brugada syndrome and acquired cardiac arrhythmias. G1673 may be an elevated risk factor for onset of cardiac arrhythmia.4. G6174A may be related to QTc length in patients and represent risk factors for arrhythmias.