Molecular Genetic Studies Of Brugada Syndrome

Background Genetic defect of SCN5A, the gene encoding human cardiac sodium channel protein a unit, is responsible for Brugada syndrome, a disease associated with ventricular fibrillation and sudden death. Currently dozens of mutation had been identified in SCN5A, part of which had been proved can cause abnormal electrophysiological property of cardiac sodium channel. In China, there are increasing case reports of Brugada syndrome, but no directed evidence of association between genetic defect and this disease had been found out. Objective We aim to study the molecular genetic character of Chinese Brugada syndrome. Mutation screening of SCN5A was performed in five patients diagnosed as Brugada syndrome and in their family members. We sought to determinate whether genetic defect of SCN5A play a role in Brugada syndrome of Chinese and to further look insight into the possible molecular genetic mechanism underlining the pathophysiology of the disease. Method Genomic DNA was extracted from peripheral blood of all five patients and their family members by standard procedure. 41 pairs of PCR primers was designed to amplify all 28 exons of SCN5A gene, then SSCP electrophoresis was followed to determinate the abnormal conformer of PCR products. If aberrant conformer was seen in the patient but not in more than 200 healthy unrelated individuals, mutation was suspected and further confirmation by DNA sequencing and analysis would be followed. Result All five patients had experienced episodes of syncope or aborted sudden death, ECG at rest showed coved type or saddleback type ST segment elevation in more than one precordial leads of V1,V2 and V3, with J wave > 2mm. PCR-SSCP analysis identified in a patient(code J3001) an abnormal conformer in exon 8 of SCN5A gene, but this abnormal was not exist in more than 200 healthy controls(400 alleles). Subsequently DNA sequencingdisclosed a G-C substitution in the third nucleotide of codon 317 of SCN5A,causing a transition of Lysine by Asparagine in the pore-linking region of domain I (D I ) of sodium channel. No SCN5A mutation was identified in four other patients. Genetic investigation of J3001 family revealed that there were 11 of totally 21 members carried this mutation, among them 6 individuals had clinical symptom or idiopathic "Brugada ECG pattern", giving the penetrance of 64% in this family.Conclusion A novel SCN5A mutation (K317N) of Brugada syndrome had been identified. It is the first time that the genetic defect is proved to be related to Chinese Brugada syndrome family. We believe this mutation can deform the sodium channel inflow tract and cause reduced sodium ion inflow during the early stage of action potential and induce abnormal electrophysiological property of cardiac myocyte across the ventricular wall, which underlined the ST segment elevation and phase 2 reentry mechanism leading to malignant ventricular arrhythmia. Family genetic investigation suggested that the penetrance of the disease is not high, and genetic heterogeneity might exist, SCN5A is not the only gene that is responsible.