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.

Background-Long-QT Syndrome (LQTS) is a cardiovascular disorder characterized by prolongation of the QT interval on ECG and presence of syncope, seizures, and sudden death. Five genes have been implicated in Romano-Ward syndrome, the autosomal dominant form of LQTS: KVLQT1, HERG, SCN5A, KCNE1, and KCNE2. Mutations in KVLQT1 and KCNE1 also cause the Jervell and Lange-Nielsen syndrome, a form of LQTS associated with deafness, a phenotypic abnormality inherited in an autosomal recessive fashion. Methods and Results-We used mutational analyses to screen a pool of 262 unrelated individuals with LQTS for mutations in the 5 defined genes. We identified 134 mutations in addition to the 43 that we previously reported. Eighty of the mutations were novel. The total number of mutations in this population is now 177 (68% of individuals). Conclusions-KVLQT1 (42%) and HERG (45%) accounted for 87% of identified mutations, and SCN5A (8%), KCNE1 (3%), and KCNE2 (2%) accounted for the other 13%. Missense mutations were most common (72%), followed by frameshift mutations (10%), in-frame deletions, and nonsense and splice-site mutations (5% to 7% each). Most mutations resided in intracellular (52%) and transmembrane (30%) domains; 12% were found in pore and 6% in extracellular segments. In most cases (78%), a mutation was found in a single family or an individual.

Life-threatening arrhythmias in LQTS patients tend to occur under specific circumstances in a gene-specific manner. These data allow new insights into the mechanisms that relate the electrophysiological consequences of mutations on specific genes to clinical manifestations and offer the possibility of complementing traditional therapy with gene-specific approaches.

ConclusionsSinus rhythm can be maintained long term in the majority of patients with chronic atrial fibrillation by means of circumferential pulmonary-vein ablation independently of the effects of antiarrhythmic-drug therapy, cardioversion, or both. The maintenance of sinus rhythm is associated with a significant decrease in both the severity of symptoms and the left atrial diameter.

Ion-channel beta-subunits are ancillary proteins that co-assemble with alpha-subunits to modulate the gating kinetics and enhance stability of multimeric channel complexes. Despite their functional importance, dysfunction of potassium-channel beta-subunits has not been associated with disease. Recent physiological studies suggest that KCNE1 encodes beta-subunits (hminK) that co-assemble with KvLQT1 alpha-subunits to form the slowly activating delayed rectifier K+ (IKs) channel. Because KVLQT1 mutations cause arrhythmia susceptibility in the long QT syndrome (LQT), we hypothesized that mutations in KCNE1 also cause this disorder. Here, we define KCNE1 missense mutations in affected members of two LQT families. Both mutations (S74L, D76N) reduced IKs by shifting the voltage dependence of activation and accelerating channel deactivation. D76N hminK also had a strong dominant-negative effect. The functional consequences of these mutations would be delayed cardiac repolarization and an increased risk of arrhythmia. This is the first description of KCNE1 as an LQT gene and confirms that hminK is an integral protein of the IKs channel.

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Background-The role of androgens in producing cardiac hypertrophy by direct action on cardiac myocytes is uncertain.Accordingly, we tested the hypothesis that cardiac myocytes in adult men and women express an androgen receptor gene and that myocytes respond to androgens by a hypertrophic response. Methods and Results-We used reverse transcription-polymerase chain reaction methods to demonstrate androgen receptor transcripts in multiple tissues and [ 3 H]phenylalanine incorporation and atrial natriuretic peptide secretion as markers of hypertrophy in cultured rat myocytes. Messenger RNA encoding androgen receptors was detected in myocytes of male and female adult rats, neonatal rat myocytes, rat heart, dog heart, and infant and adult human heart. Both testosterone and dihydrotestosterone produced a robust receptor-specific hypertrophic response in myocytes, determined by indices of protein synthesis and atrial natriuretic peptide secretion. Conclusions-Androgen receptors are present in cardiac myocytes from multiple species, including normal men and women, in a context that permits androgens to modulate the cardiac phenotype and produce hypertrophy by direct, receptor-specific mechanisms. There are clinical implications for therapeutic or illicit use of androgens in humans.(Circulation. 1998;98:256-261.)

Background-Congenital long-QT syndrome (LQTS) is caused by mutations of genes encoding the slow component of the delayed rectifier current (LQT1, LQT5), the rapid component of the delayed rectifier current (LQT2, LQT6), or the Na ϩ current (LQT3), resulting in ST-T-wave abnormalities on the ECG. This study evaluated the spectrum of ST-T-wave patterns and repolarization parameters by genotype and determined whether genotype could be identified by ECG. Methods and Results-ECGs of 284 gene carriers were studied to determine ST-T-wave patterns, and repolarization parameters were quantified. Genotypes were identified by individual ECG versus family-grouped ECG analysis in separate studies using ECGs of 146 gene carriers from 29 families and 233 members of 127 families undergoing molecular genotyping, respectively. Ten typical ST-T patterns (4 LQT1, 4 LQT2, and 2 LQT3) were present in 88% of LQT1 and LQT2 carriers and in 65% of LQT3 carriers. Repolarization parameters also differed by genotype. A combination of quantified repolarization parameters identified genotype with sensitivity/specificity of 85%/70% for LQT1, 83%/94% for LQT2, and 47%/63% for LQT3. Typical patterns in family-grouped ECGs best identified the genotype, being correct in 56 of 56 (21 LQT1, 33 LQT2, and 2 LQT3) families with mutation results. Conclusions-Typical ST-T-wave patterns are present in the majority of genotyped LQTS patients and can be used to identify LQT1, LQT2, and possibly LQT3 genotypes. Family-grouped ECG analysis improves genotype identification accuracy. This approach can simplify genetic screening by targeting the gene for initial study. The multiple ST-T patterns in each genotype raise questions regarding the pathophysiology and regulation of repolarization in LQTS.