hERG

hERG (the human Ether-à-go-go-Related Gene) is a gene (KCNH2) that codes for a protein known as K_v11.1, the alpha

hERG has also been associated with modulating the functions of some cells of the nervous system[8]^[9] and with establishing and maintaining cancer-like features in leukemic cells.^[10]

Function

hERG forms the major portion of one of the ion channel proteins (the 'rapid' delayed rectifier current (I_Kr)) that conducts potassium (K⁺) ions out of the muscle cells of the heart (

A detailed atomic structure for hERG based on X-ray crystallography is not yet available, but structures have recently been solved by electron microscopy.^[20] In the laboratory the heterologously expressed hERG potassium channel comprises 4 identical alpha subunits, which form the channel's pore through the plasma membrane. Each hERG subunit consists of 6 transmembrane alpha helices, numbered S1-S6, a pore helix situated between S5 and S6, and cytoplasmically located N- and C-termini. The S4 helix contains a positively charged arginine or lysine amino acid residue at every 3rd position and is thought to act as a voltage-sensitive sensor, which allows the channel to respond to voltage changes by changing conformations between conducting and non-conducting states (called 'gating'). Between the S5 and S6 helices, there is an extracellular loop (known as 'the turret') and 'the pore loop', which begins and ends extracellularly but loops into the plasma membrane; the pore loop for each of the hERG subunits in one channel faces into the ion-conducting pore and is adjacent to the corresponding loops of the 3 other subunits, and together they form the selectivity filter region of the channel pore. The selectivity sequence, SVGFG, is very similar to that contained in bacterial KcsA channels.^[7] Although a full crystal structure for hERG is not yet available, a structure has been found for the cytoplasmic N-terminus, which was shown to contain a PAS domain (aminoacid 26–135) that slows the rate of deactivation.^[21]

Genetics

Loss-of-function mutations in this channel may lead to

This channel is also sensitive to drug binding, as well as decreased extracellular potassium levels, both of which can result in decreased channel function and drug-induced (acquired) long QT syndrome. Among the drugs that can cause QT prolongation, the more common ones include antiarrhythmics (especially Class 1A and Class III), anti-psychotic agents, and certain antibiotics (including quinolones and macrolides).^[7]

Although there exist other potential targets for cardiac adverse effects, the vast majority of drugs associated with acquired QT prolongation are known to interact with the hERG potassium channel. One of the main reasons for this phenomenon is the larger inner vestibule of the hERG channel, thus providing more space for many different drug classes to bind and block this potassium channel.^[23]

hERG containing channels are blocked by amiodarone, and it does prolong the QT interval, but its multiple other antiarrhythmic effects prevent this from causing torsades de pointes.^[24]

Thioridazine causes peculiarly severe QTc prolongation by blocking hERG and was withdrawn by the manufacturer for this reason.

Drug development considerations

Due to the documented potential of QT-interval-prolonging drugs, the United States Food and Drug Administration issued recommendations for the establishment of a cardiac safety profile during pre-clinical drug development: ICH S7B.

Naming

The hERG gene was first named and described in a paper by Jeff Warmke and Barry Ganetzky, then both at the University of Wisconsin–Madison.^[16] The hERG gene is the human homolog of the Ether-à-go-go gene found in the Drosophila fly; Ether-à-go-go was named in the 1960s by William D. Kaplan and William E. Trout, III, while at the City of Hope Hospital in Duarte, California. When flies with mutations in the Ether-à-go-go gene are anaesthetised with ether, their legs start to shake, like the dancing at the then popular Whisky a Go Go nightclub in West Hollywood, California.^[26]

Interactions

HERG has been shown to

• Voltage-gated potassium channel

References