[Long QT syndrome].

Romano-Ward syndrome, one of familial long QT syndromes, is an inherited disorder that causes sudden death from cardiac arrhythmias, specifically torsade de pointes and ventricular fibrillation. By linkage analyses, three LQT loci were previously mapped: LQT1 on chromosome 11p15.5, LQT2 on 7q35-36, LQT3 on 3p21-24. It was recently brought to light that LQT2 and LQT3 were caused by mutations of the gene encoding cardiac ion channels. Mutations in HERG on chromosome 7q35-36, encoding potassium channels (Ikr), cause LQT2, and block of Ikr is a known mechanism for drug-induced prolongation of cardiac action potentials, which provides a mechanistic link between LQT2 and certain forms of acquired LQT. Mutations in SCN5A on chromosome 3p21, encoding the human heart voltage-gated sodium-channel alpha-subunit, cause LQT3. Mutant channels show a sustained inward sodium current during membrane depolarization, which explains prolongation of cardiac action potentials.