Molecular genetics of long QT syndrome.

In the long QT syndrome (LQT), individuals suffer from syncope, seizures and sudden death due to cardiac arrhythmias, specifically torsade de pointes and ventricular fibrillation. Many of these individuals also have prolongation of the QT interval on electrocardiograms, suggesting abnormal cardiac repolarization. To improve our understanding of the mechanisms underlying LQT and to facilitate presymptomatic diagnosis, we have begun to study families with autosomal dominant LQT. In 1991, we reported tight linkage between the LQT phenotype and the Harvey ras-1 gene (HRAS) in several families of Northern European descent. This discovery localized an LQT gene to chromosome 11p15.5 and made presymptomatic diagnosis in some families possible. In initial experiments, no recombination between HRAS and LQT was observed, making this protoncogene a candidate for LQT. This hypothesis was supported by physiologic data; other investigators had shown that ras proteins modulate cardiac potassium channels and an abnormality of potassium homeostasis could explain LQT. We eliminated HRAS as a candidate, however, by sequencing the coding region in 10 unrelated patients and finding no mutations. This indicated that the LQT locus was nearby, but not HRAS. Autosomal dominant LQT was previously thought to be genetically homogeneous and the first seven LQT families we studied were linked to 11p15.5. In 1992, however, several groups, including my laboratory, identified locus heterogeneity for LQT. Recently we identified a second LQT locus, LQT2, on chromosome 7q35-36. Because several families were unlinked, at least one more LQT locus exists. This degree of heterogeneity presents opportunities. It seems likely, for example, that proteins encoded by distinct LQT genes interact to modulate cardiac repolarization.(ABSTRACT TRUNCATED AT 250 WORDS)