Loss-of-function mutations in ANK2 can result in a broad spectrum of clinical cardiac phenotypes. Carriers of some mutations display QT interval prolongation, stress- and/or exercise-induced polymorphic ventricular arrhythmia, syncope, and sudden cardiac death. Patients with other variants show clinical phenotypes, sometimes mild, extending beyond LQTS, leading to the label 'ankyrin-B syndrome.' These phenotypes include bradycardia, sinus arrhythmia, delayed conduction/conduction block, idiopathic ventricular fibrillation, and catecholaminergic polymorphic ventricular tachycardia. [From OMIM]

LQT4 and Ankyrin-B syndrome both follow an autosomal dominant pattern of inheritance and are caused by heterozygous mutations in the ANK2 gene.  This gene encodes Ankyrin-2, a protein involved in the localization and stabilization of ion channels to their membranes in cardiomyocyte cells.  Ankyrin-2 is thus essential for the regulation of heart rate.  

At least ten different heterozygous mutations in the ANK2 gene have been implicated in Long QT syndrome 4 and Ankyrin-B syndrome.  These are all missense mutations that result in a loss of ANK2 protein function.