A novel potassium channel gene has been cloned, characterized, and associated with cardiac arrhythmia. The gene encodes MinK-related peptide 1 (MiRP1), a small integral membrane subunit that assembles with HERG, a pore-forming protein, to alter its function. Unlike channels formed only with HERG, mixed complexes resemble native cardiac IKr channels in their gating, unitary conductance, regulation by potassium, and distinctive biphasic inhibition by the class III antiarrhythmic E-4031. Three missense mutations associated with long QT syndrome and ventricular fibrillation are identified in the gene for MiRP1. Mutants form channels that open slowly and close rapidly, thereby diminishing potassium currents. One variant, associated with clarithromycin-induced arrhythmia, increases channel blockade by the antibiotic. A mechanism for acquired arrhythmia is revealed: genetically based reduction in potassium currents that remains clinically silent until combined with additional stressors.

Drug-induced long QT syndrome (LQTS) is a prevalent disorder of uncertain etiology that predisposes to sudden death. KCNE2 encodes MinK-related peptide 1 (MiRP1), a subunit of the cardiac potassium channel IKr that has been associated previously with inherited LQTS. Here, we examine KCNE2 in 98 patients with drug-induced LQTS, identifying three individuals with sporadic mutations and a patient with sulfamethoxazole-associated LQTS who carried a single-nucleotide polymorphism (SNP) found in Ϸ1.6% of the general population. While mutant channels showed diminished potassium flux at baseline and wild-type drug sensitivity, channels with the SNP were normal at baseline but inhibited by sulfamethoxazole at therapeutic levels that did not affect wild-type channels. We conclude that allelic variants of MiRP1 contribute to a significant fraction of cases of drug-induced LQTS through multiple mechanisms and that common sequence variations that increase the risk of life-threatening drug reactions can be clinically silent before drug exposure.MiRP1 ͉ LQTS ͉ SNP ͉ Bactrim ͉ sulfamethoxazole I nherited long QT syndrome (LQTS) is an uncommon cardiac arrhythmia that predisposes to torsades de pointes (TdP), ventricular fibrillation, and sudden death (1-4). The molecular basis for LQTS is known: delayed repolarization of the myocardium prolongs the cardiac action potential increasing the QT interval measured on the surface electrocardiogram. Mutations in five ion channel genes cause the majority of cases of inherited LQTS. LQTS mutations in SCN5A increase activity of the sodium channel that depolarizes the myocardium to initiate the cardiac action potential (5); LQTS mutations in KCNE1 or KvLQT1 (encoding subunits of I Ks channels) and KCNE2 or HERG (encoding subunits of I Kr channels) diminish potassium fluxes that repolarize the heart to end each beat (6-9).Acquired LQTS is a common disorder caused by drugs and metabolic abnormalities. The risk for acquired LQTS increases when factors that decrease potassium flux act concurrently to impair the ability of the myocardium to repolarize. Wellrecognized conditions that diminish ''repolarization reserve'' include female gender, hypokalemia, and drugs that inhibit cardiac potassium channels (10). Several lines of evidence suggest that patients with drug-induced LQTS have an underlying predisposition to dysrrhythmia. The QT interval measured before drug exposure tends to be longer in patients who later develop drug-induced LQTS than in individuals who receive the same agent safely (11, 12). Moreover, sporadic mutations have been identified in patients with drug-induced TdP (9,13,14). Thus, we demonstrated previously that patients with ''acquired'' LQTS can have a genetic predisposition to arrhythmia because of mutation in the MinK-related peptide 1 (MiRP1) subunit of their I Kr potassium channels (9). In that study, a woman with clarithromycin-induced TdP was found to carry a sporadic missense mutation in KCNE2; channels formed with the altered subunit (Q9E-MiRP1) were abnormal at bas...

The cyclic nucleotide-gated channel from rod photoreceptors is composed of two distinct subunits (alpha and beta). The properties of the alpha subunit, which can form functional channels by itself, are modified by coexpression with a homologous polypeptide, designated the beta subunit. However, the alpha subunit from rod photoreceptor membranes copurifies with a 240 kDa protein that is significantly larger than this putative beta subunit. We now demonstrate by peptide sequencing and by cloning and functional expression of cDNA that the 240 kDa protein represents the complete beta subunit with an unusual bipartite structure. The N-terminal part is essentially identical to a glutamic acid-rich protein (GARP), whereas the C-terminal part is highly homologous to the previously cloned human "beta subunit." Expression of the complete beta subunit in HEK 293 cells results in a polypeptide with the same apparent molecular weight as the 240 kDa protein of the native rod channel. Coexpression of the alpha subunit with the full-length beta subunit yields hetero-oligomeric channels with properties characteristic of the native channel.