Congenital long QT syndrome (LQTS) is a disease associated with delayed cardiac repolarization and prolonged QT intervals on the electrocardiogram, which can lead to ventricular arrhythmia with cardiac sudden death. One of the major forms of LQTS (LQT2) is caused by mutations in the human ether-a-go-go-related gene (HERG) that encodes the rapidly activating delayed rectifier potassium channel. To date, more than 100 HERG mutations have been identified in patients with LQTS. Our previous work has shown that a major mechanism for loss of HERG channel function in LQT2 is defective protein trafficking which results in failure of mutant channels to reach the cell surface. We also showed that high affinity HERG channel blockers can correct defective protein trafficking of some LQT2 mutants. The goals of this proposal are (1) to study the mechanisms of defective protein trafficking of LQT2 mutant channels, and (2) to determine how HERO channel blockers rescue trafficking defective LQT2 mutant channels. Our hypotheses are (1) LQT2 mutations cause misfolding or improper assembly of HERO protein which is recognized by quality control system leading to ER retention and degradation by the proteasome, and (2) drugs that bind to HERO channels with high affinity act as pharmacological chaperones to promote proper folding or assembly in a conformation that permits trafficking to the plasma membrane. We will test these hypotheses by four specific aims:
aim I to determine whether LQT2 mutations cause misfolding or improper assembly of mutant channels;
aim 2 to study the role of molecular chaperones in the ER retention of LQT2 mutant channels;
aim 3 to investigate the mechanisms by which LQT2 mutants are recognized and degraded by the proteasome;
and aim 4 to elucidate the mechanisms by which high affinity HERG channel blockers correct defective protein trafficking of LQT2 mutant channels. We will use a combination of biochemical, immunohistochemical and patch clamp techniques to study wild type HERG and LQT2 mutant channels expressed in transfected tissue culture cells and in cell-free systems. These studies will strengthen our knowledge of how misfolded and improperly assembled LQT2 mutant channels are recognized, retained and degraded by the ER quality control system and how HERG channel blockers modify these processes and rescue LQT2 mutant channels. Elucidating these mechanisms is an important step towards the development of pharmacological strategies for therapies of congenital LQTS.
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