RESEARCH

hERG Activators

The recent discovery of several structurally diverse hERG activators could be an immense breakthrough in terms of treating clinical conditions with hERG targets, as well as potentially increasing the safety of other drugs known to block hERG.  Recently five hERG activator compounds have been described: RPR260243 [Kang et al., 2005], NS1643 [Hansen et al., 2006a], NS3623 [Hansen et al., 2006b], PD‑118057 [Zhou et al., 2005], and mallotoxin [Zeng et al., 2006].  PD‑118057, NS3623 and RPR260243 have been shown to shorten both the ventricular AP duration and the QT interval.  RPR260243 and PD-118057 can reverse the action potential prolonging effects of dofetilide.  The mechanism of action of these channel activators is varied.  NS1643 and NS3623 primarily reduce the inactivation of hERG by shifting its voltage dependence rightward [Casis et al., 2006; Hansen et al., 2006b]; neither compound was designed to interact with the S5-Pore linker, and their sites of action with the hERG channel are as yet unknown.  Mallotoxin affects all three, strongly shifting the activation curve leftward, but also slowing deactivation and have minor effects on inactivation.  In addition it may be possible to modulate hERG activity with drugs acting on protein kinases, as hERG current can be reduced by protein kinase A (PKA) activity [Thomas et al., 1999; Kiehn et al., 1998], and further research has teased out an indirect role for protein kinase C (PKC) in hERG activity [Thomas et al., 2003].

Compounds Influencing hERG Intracellular Sorting

 One mechanism for the loss of normal channel function in some familial forms of LQTS is defective protein trafficking, which results in the failure of the hERG channel protein to reach the plasma membrane. E-4031, a hERG blocking drug, was found to correct the intracellular sorting of the mutant hERG-N470D protein [Zhou et al., 1999], although the E-4031 also blocked the channel.  Initially the success of rescue with pharmacological chaperones of hERG-G601S was thought to vary directly with the potency of blockade, and even the molecular determinants for the chaperones were found [Ficker et al., 2002].  However, thapsigargin was shown to rescue the surface expression of two different trafficking mutants of hERG without blocking the channel [Delisle et al., 2003], as was fexofenadine [Rajamani et al., 2002].  Hsp90 and Hsp70 are crucial for the maturation of the wild type hERG protein as well as for the retention of trafficking-deficient hERG mutants [Ficker et al., 2003].  In a recent study of ~25% of the known LQT2 (hERG-associated LQTS) missense mutations, 30 of 34 mutations caused trafficking defects, most of which could be pharmacologically corrected [Anderson et al., 2006].

hERG and Short QT Syndrome

Familial short QT syndrome (SQTS) was identified as a new clinical entity in 2000 [Gussak et al., 2000]; since then families with the condition have been identified in the USA, Italy, Spain, The Netherlands, Germany, France, Finland, Brazil, and Turkey (www.shortqt.org). The condition presents in patients who have shortened QTc intervals (typically of ~320 ms or less [Bjerregaard et al., 2006]), with episodes of atrial and/or ventricular arrhythmia reported, but in whom no evidence of structural heart disease has been identified [Gussak et al., 2000; Gaita et al., 2003]. SQTS is associated with both atrial and ventricular arrhythmias (including inducible ventricular fibrillation at electrophysiological study) and with sudden death.  Since 2004, it has become clear that SQTS is genetically heterogeneous, with mutations to three cardiac K+ ion channel genes identified in SQTS patients: KCNH2 for SQT1, KCNQ1 for SQT2, KCNJ2 for SQT3 [Brugada et al., 2004; McPate et al., 2006b; Bjerregaard et al., 2006].

The first form of SQTS (SQT1) to be identified (and the form that is most commonly seen in those patients for whom genotyping has been conducted) involves a mutation to hERG. The hERG mutation responsible for causing SQT1 is known as N588K (Figure 1), which is an asparagine to lysine mutation located on the extracellular S5-Pore linker of the channel [Brugada et al., 2004]; it has been proposed that inactivation results from the interaction of this part of the channel with residues on the outer pore domain [Clarke et al., 2006].  This mutation has been found in three independent families and has been described genetically as a hotspot for the disorder [Hong et al., 2005].  This mutated channel has attenuated inactivation (the inactivation is shifted rightward by 62 mV at 37ºC), which results in augmented hERG currents that repolarise the ventricular AP early [Cordeiro et al., 2005; McPate et al., 2005].

Figure 1. The Position of the N588K hERG Mutation.

Although the use of implantable cardioverter defibrillators (ICDs) can help protect against the fatal ventricular arrhythmias of SQTS, the tall T-waves characteristic of SQTS can lead to inappropriate shocks due to T-wave over-sensing [Schimpf et al., 2003]; thus, pharmacological adjunct therapies that restore normal QT intervals/T wave morphology would be beneficial. Initial efforts towards pharmacological restoration of QT intervals in SQT1 patients showed that the class III antiarrhythmic sotalol was ineffective at prolonging the QT interval in these patients, while the class I antiarrhythmic quinidine could prolong these shortened QT intervals [Gaita et al., 2004; Brugada et al., 2004].  The N588K mutation has been shown to attenuate significantly sotalol-induced block of hERG channels, whilst attenuating that of quinidine to a lesser extent.

This is concordant with previous data showing that the molecular determinants of hERG blockade for some low affinity blockers (e.g. [Milnes et al., 2003]) may differ from those associated class III antiarrhythmic agents [Mitcheson et al., 2000; Lees-Miller et al., 2000]. Moreover, when comparing those class I to class III antiarrhythmics that have been tested for hERG blockade, the class I antiarrhythmics quinidine [Lees-Miller et al., 2000], disopyramide [Paul et al., 2001] and propafenone [Witchel et al., 2004] can block inactivation-attenuated hERG channels, whereas all of the class III antiarrhythmics had their hERG blockade dramatically attenuated in channels that had reduced inactivation.  Recently McPate et al. suggested that hERG block by the class I antiarrhythmic disopyramide is only slightly attenuated by the N588K mutation, suggesting that it may be worth investigating disopyramide as a clinical adjunct for SQT1 [McPate et al., 2006a].

With SQTS now recognised as a distinct clinical condition, further incidences of individuals with this condition continue to be reported [Hong et al., 2005] (see also: www.shortqt.org), raising the possibility that in time it may be as common as hereditary long QT syndrome and could provide an explanation for incidences of sudden cardiac death [Gussak and Bjerregaard, 2005].

 

References

Anderson CL, Delisle BP, Anson BD, Kilby JA, Will ML, Tester DJ, Gong Q, Zhou Z, Ackerman MJ, January CT (2006). Most LQT2 mutations reduce Kv11.1 (hERG) current by a class 2 (trafficking-deficient) mechanism. Circulation 113:365-373.
Bjerregaard P, Jahangir A, Gussak I (2006). Targeted therapy for short QT syndrome. Expert Opin. Ther. Targets 10:393-400.
Brugada R, Hong K, Dumaine R, Cordeiro J, Gaita F, Borggrefe M, Menendez TM, Brugada J, Pollevick GD, Wolpert C, Burashnikov E, Matsuo K, Wu YS, Guerchicoff A, Bianchi F, Giustetto C, Schimpf R, Brugada P, Antzelevitch C (2004). Sudden death associated with short-QT syndrome linked to mutations in HERG. Circulation 109:30-35.
Casis O, Olesen SP, Sanguinetti MC (2006). Mechanism of action of a novel human ether-a-go-go-related gene channel activator. Mol. Pharmacol. 69:658-665.
Clarke CE, Hill AP, Zhao J, Kondo M, Subbiah RN, Campbell TJ, Vandenberg JI (2006). Effect of S5P a-helix charge mutants on inactivation of hERG K+ channels. J. Physiol. 573 (Pt. 2):291-304.
Cordeiro JM, Brugada R, Wu YS, Hong K, Dumaine R (2005). Modulation of IKr inactivation by mutation N588K in KCNH2: a link to arrhythmogenesis in short QT syndrome. Cardiovasc. Res. 67:498-509.
Delisle BP, Anderson CL, Balijepalli RC, Anson BD, Kamp TJ, January CT (2003). Thapsigargin selectively rescues the trafficking-defective LQT2 channels G601S and F805C. J. Biol. Chem. 278:35749-54357.
Ficker E, Dennis AT, Wang L, Brown AM (2003). Role of the cytosolic chaperones Hsp70 and Hsp90 in maturation of the cardiac potassium channel HERG. Circ. Res. 92:e87-e100.
Ficker E, Obejero-Paz CA, Zhao S, Brown AM (2002). The binding site for channel blockers that rescue misprocessed human long QT syndrome type 2 ether-a-gogo-related gene (HERG) mutations. J. Biol. Chem. 277:4989-4998.
Gaita F, Giustetto C, Bianchi F, Schimpf R, Haissaguerre M, Calo L, Brugada R, Antzelevitch C, Borggrefe M, Wolpert C (2004). Short QT syndrome: pharmacological treatment. J. Am. Coll. Cardiol. 43:1494-1499.
Gaita F, Giustetto C, Bianchi F, Wolpert C, Schimpf R, Riccardi R, Grossi S, Richiardi E, Borggrefe M (2003). Short QT Syndrome: a familial cause of sudden death. Circulation 108:965-970.
Gussak I, Bjerregaard P (2005). Short QT syndrome -- 5 years of progress. J. Electrocardiol. 38:375-377.
Gussak I, Brugada P, Brugada J, Wright RS, Kopecky SL, Chaitman BR, Bjerregaard P (2000). Idiopathic short QT interval: a new clinical syndrome? Cardiology 94:99-102.
Hansen RS, Diness TG, Christ T, Demnitz J, Ravens U, Olesen SP, Grunnet M (2006a). Activation of human ether-a-go-go-related gene potassium channels by the diphenylurea 1,3-bis-(2-hydroxy-5-trifluoromethyl-phenyl)-urea (NS1643). Mol. Pharmacol. 69:266-277.
Hansen RS, Diness TG, Christ T, Wettwer E, Ravens U, Olesen SP, Grunnet M (2006b). Biophysical Characterization of the New Human Ether-A-Go-Go-Related Gene Channel Opener NS3623 [N-(4-Bromo-2-(1H-tetrazol-5-yl)-phenyl)-N'-(3'-trifluoromethylphenyl)urea ]. Mol. Pharmacol. 70:1319-1329.
Hong K, Bjerregaard P, Gussak I, Brugada R (2005). Short QT syndrome and atrial fibrillation caused by mutation in KCNH2. J. Cardiovasc. Electrophysiol. 16:394-396.
Kang J, Chen XL, Wang H, Ji J, Cheng H, Incardona J, Reynolds W, Viviani F, Tabart M, Rampe D (2005). Discovery of a small molecule activator of the human ether-a-go-go-related gene (HERG) cardiac K+ channel. Mol. Pharmacol. 67:827-836.
Kiehn J, Karle C, Thomas D, Yao X, Brachmann J, Kubler W (1998). HERG potassium channel activation is shifted by phorbol esters via protein kinase A-dependent pathways. J. Biol. Chem. 273:25285-25291.
Lees-Miller JP, Duan Y, Teng GQ, Duff HJ (2000). Molecular determinant of high-affinity dofetilide binding to HERG1 expressed in Xenopus oocytes: involvement of S6 sites. Mol. Pharmacol. 57:367-374.
McPate MJ, Duncan RS, Milnes JT, Witchel HJ, Hancox JC (2005). The N588K-HERG K+ channel mutation in the 'short QT syndrome': mechanism of gain-in-function determined at 37 ºC. Biochem. Biophys. Res. Commun. 334:441-449.
McPate MJ, Duncan RS, Witchel HJ, Hancox JC (2006a). Disopyramide is an effective inhibitor of mutant HERG K(+) channels involved in variant 1 short QT syndrome. J. Mol. Cell Cardiol. 41:563-566.
McPate MJ, Witchel HJ, Hancox JC (2006b). Short QT syndrome. Future Cardiology 2:293-301.
Milnes JT, Crociani O, Arcangeli A, Hancox JC, Witchel HJ (2003). Blockade of HERG potassium currents by fluvoxamine: incomplete attenuation by S6 mutations at F656 or Y652. Br. J. Pharmacol. 139:887-898.
Mitcheson JS, Chen J, Lin M, Culberson C, Sanguinetti MC (2000). A structural basis for drug-induced long QT syndrome. Proc. Natl. Acad. Sci. U. S. A. 97:12329-12333.
Paul A, Witchel HJ, Hancox JC (2001). Inhibition of HERG potassium channel current by the class 1a antiarrhythmic agent disopyramide. Biochem. Biophys. Res. Commun. 280:1243-1250.
Rajamani S, Anderson CL, Anson BD, January CT (2002). Pharmacological rescue of human K(+) channel long-QT2 mutations: human ether-a-go-go-related gene rescue without block. Circulation 105:2830-2835.
Schimpf R, Wolpert C, Bianchi F, Giustetto C, Gaita F, Bauersfeld U, Borggrefe M (2003). Congenital short QT syndrome and implantable cardioverter defibrillator treatment: inherent risk for inappropriate shock delivery. J Cardiovasc. Electrophysiol. 14:1273-1277.
Thomas D, Zhang W, Karle CA, Kathofer S, Schols W, Kubler W, Kiehn J (1999). Deletion of protein kinase A phosphorylation sites in the HERG potassium channel inhibits activation shift by protein kinase A. J. Biol. Chem. 274:27457-27462.
Thomas D, Zhang W, Wu K, Wimmer AB, Gut B, Wendt-Nordahl G, Kathofer S, Kreye VA, Katus HA, Schoels W, Kiehn J, Karle CA (2003). Regulation of HERG potassium channel activation by protein kinase C independent of direct phosphorylation of the channel protein. Cardiovasc. Res. 59:14-26.
Witchel HJ, Dempsey CE, Sessions RB, Perry M, Milnes JT, Hancox JC, Mitcheson JS (2004). The low potency, voltage-dependent HERG blocker propafenone - molecular determinants and drug trapping. Mol. Pharmacol. 66:1201-1212.
Zeng H, Lozinskaya IM, Lin Z, Willette RN, Brooks DP, Xu X (2006). Mallotoxin is a Novel Human Ether-a-go-go-Related Gene (hERG) Potassium Channel Activator. J. Pharmacol. Exp. Ther. 319:957-962.
Zhou J, Augelli-Szafran CE, Bradley JA, Chen X, Koci BJ, Volberg WA, Sun Z, Cordes JS (2005). Novel potent human ether-a-go-go-related gene (hERG) potassium channel enhancers and their in vitro antiarrhythmic activity. Mol. Pharmacol. 68:876-884.
Zhou Z, Gong Q, January CT (1999). Correction of defective protein trafficking of a mutant HERG potassium channel in human long QT syndrome. Pharmacological and temperature effects. J. Biol. Chem. 274:31123-31126.