Isolated peptides and polynucleotides are provided that can be used to treat a subject that has a genetic predisposition to, or has been diagnosed as having, a cardiac disorder related to dysfunctional sodium channels. Also provided are methods of treatment, as well as methods of predicting the occurrence of a cardiac event or the severity or prognosis of cardiac symptoms, in a subject that has either been diagnosed as having, or is suspected of developing, a sodium channel related cardiac disorder, by testing the subject for the presence of H558R-polymorphism on the subject's SCN5A gene.