In addition to inducing IKs dysfunction through dominant
In addition to inducing IKs dysfunction through dominant-negative loss-of-function effects and defective channel trafficking, mutations in KCNQ1 suppress IKs channel function by reducing the channel affinity of interacting proteins [68,69]. Phosphatidylinositol-4,5-bisphosphate (PIP2) is a cofactor necessary for the activity of KCNQ1 s6 kinase [32,68,69]. It has been shown that intracellular PIP2 regulates KCNQ1 channel activity in such a way that PIP2 stabilizes the open state of the channels, which leads to an increased current amplitude, slowed deactivation kinetics, and a shift in the activation curve toward negative potentials. Park et al. showed that mutations in the S4 domain (R243H) and COOH terminus (R539W and R555C) increased the rate of dissociation of PIP2 from the KCNQ1 channel, which decreased the number of open-state channels in the membrane . Coyan et al. confirmed that R243H and R555C mutations cause an acceleration of KCNQ1 current rundown when membrane PIP2 levels are decreasing. By observing the interaction of the KCNQ1 R539W mutant with cholesterol, this group further suggested that the channel-cholesterol interaction might overcome the channel-PIP2 interaction and stabilize the channel open-state .
Regulation by PKA It is well known that cardiac events in LQT1 syndrome patients are more frequently triggered by adrenergic stimuli (e.g., physical or emotional stress) than those in other forms of LQTS. A clinical study of 371 LQT1 patients found that cardiac events were most common during exercise (62%) and emotional arousal (26%), while occasional during sleep or rest (3%) and from other triggers (9%) . Approximately 35–36% of genotype-confirmed LQT1 patients have a normal QTc range without any clinical symptoms at rest [47,48], but lethal arrhythmias can occur in these apparently healthy silent mutation carriers without any premonitory sign, especially during adrenergic stimuli [49–51]. Recently, a heterozygous missense KCNQ1 mutation G269S was identified in 11 patients from four unrelated families. Most of the 11 patients had normal to borderline QTc intervals at rest, but had a significant QTc prolongation after exercise (Fig. 2). One family member had died suddenly and another one experienced syncope while dancing. Functional characterization of the IKs channel reconstituted with G269S in mammalian cells showed that the mutation modestly affected IKs, but severely blunted the increase in IKs after treatment with isoproterenol, pharmacological activators of PKA (Fig. 3), or in the PKA phosphomimetic mutation KCNQ1-S27D (Fig. 4), which mimics PKA-mediated phosphorylation of IKs channels. These findings provide important insight into the molecular mechanisms underlying adrenergic-induced LQTS and may explain why patients with silent mutations exhibit an excessive prolongation of QT intervals during exercise. The results also suggest that beta-blocker therapy may have a beneficial effect in these patients. In human ventricular myocytes, the IKs (outward current), rapid component of delayed rectifier K+ current IKr (outward current), and L-type Ca2+ current ICa,L (inward current) play a dominant role in the repolarization of APs and are the most important determinants of APD. Under physiological conditions, IKr and ICa,L, but not IKs, normally play a crucial role in controlling the ventricular AP at rest . Therefore, KCNQ1 mutations (e.g., G269S) that cause a mild-to-moderate functional defect in IKs might ordinarily have little effect on the ventricular AP, which may explain why some KCNQ1 mutation carriers have normal to borderline QTc intervals with no or mild clinical symptoms at rest. In addition, the reason why individuals carrying a KCNQ1 mutation display a silent phenotype at rest may also be due to the “repolarization reserve” mechanism . On the other hand, IKs plays a major role in regulating the ventricular AP after adrenergic stimuli (that upregulates IKs through cAMP-dependent PKA pathway) to prevent excessive ventricular APD or QT prolongation due to an ICa,L increase [15, 72]. It is possible that the slow deactivation kinetics of IKs also contribute to the current upregulation through adrenergic stimuli. Due to the incomplete deactivation of IKs, there is residual activation at the onset of the succeeding AP that accumulates at fast rates, thus increasing the probability of the channel being in an open state .