Genetic aspects in acquired long QT syndrome ? a piece in the puzzle

Abstract

Torsade de pointes (TdP) is a serious complication which is induced by a large variety of cardiovascular and non-cardiovascular drugs. Many clinical conditions and risk factors for the occurrence of TdP during administration of drugs with a proarrhythmic potential have been identified. All such drugs have in common that they reversibly alter myocardial repolarization due to the prolongation of the action potential (acquired QT interval prolongation) which is per se not arrhythmogenic. TdP is initiated (acquired long QT (LQT) syndrome) only when a threshold level is reached leading to early after-depolarizations (EADs) and triggered beats together with a marked dispersion in recovery of excitability. The underlying mechanisms of TdP are not yet satisfactorily elucidated but, in general, alterations in cardiac ion currents which tune the normal action potential play a major role in arrhythmogenesis. Following recent advances in molecular biology and genetics, it has become clear that in some clinical instances (e.g. congestive heart failure or cardiac hypertrophy) ion channel genes become less expressed (down-regulated) and the consequent reduced ion currents (e.g. IKr) are likely to cause prolonged myocardial repolarization. In this setting, the use of drugs with action potential-prolonging properties could possibly be harmful and could not be compensated by the normal cardiac ‘repolarization reserve’. In congenital LQT syndrome some of the same ion channel components were found to be genetically altered, suggesting that either quantitative or qualitative changes of ion currents may be involved in ventricular arrhythmogenesis through similar mechanisms (‘final common pathway’). A variable clinical expressivity and, especially, an incomplete penetrance has been found in patients carrying the same LQT genotype, even when near-relatives, which raises the question on the frequency and importance of ‘silent’ (i.e. minor functional) ion channel gene mutations that may become functionally significant in presence of action potential prolonging drugs and other coexisting factors. The observation of adverse drug reactions in apparently healthy (‘normal heart’) individuals is suggestive of a genetic susceptibility for ‘acquired’ arrhythmias. This report reviews and summarizes the recent knowledge on unapparent ion channel gene mutations and preliminary concepts about ‘acquired’ arrhythmias.