The long QT syndrome: ion channel diseases of the heart

Single-channel characteristics of wild-type IKs channels and channels formed with two minK mutants that cause long QT syndrome

IKs channels are voltage dependent and K+ selective. They influence cardiac action potential duration through their contribution to myocyte repolarization. Assembled from minK and KvLQT1 subunits, IKs channels are notable for a heteromeric ion conduction pathway in which both subunit types contribute to pore formation. This study was undertaken to assess the effects of minK on pore function. We first characterized the properties of wild-type human IKs channels and channels formed only of KvLQT1 subunits. Channels were expressed in Xenopus laevis oocytes or Chinese hamster ovary cells and currents recorded in excised membrane patches or whole-cell mode. Unitary conductance estimates were dependent on bandwidth due to rapid channel "flicker." At 25 kHz in symmetrical 100-mM KCl, the single-channel conductance of IKs channels was approximately 16 pS (corresponding to approximately 0.8 pA at 50 mV) as judged by noise-variance analysis; this was fourfold greater than the estimated conductance of homomeric KvLQT1 channels. Mutant IKs channels formed with D76N and S74L minK subunits are associated with long QT syndrome. When compared with wild type, mutant channels showed lower unitary currents and diminished open probabilities with only minor changes in ion permeabilities. Apparently, the mutations altered single-channel currents at a site in the pore distinct from the ion selectivity apparatus. Patients carrying these mutant minK genes are expected to manifest decreased K+ flux through IKs channels due to lowered single-channel conductance and altered gating.

A novel mutation in KVLQT1 is the molecular basis of inherited long QT syndrome in a near-drowning patient's family

After identifying a 10-year-old boy with inherited long QT syndrome (LQTS) after a near-drowning that required defibrillation from torsades de pointes, evaluation of first degree relatives revealed a four-generation kindred comprising 26 individuals with four additional symptomatic and eight asymptomatic members harboring an abnormally prolonged QTc (defined as > or =0.46 s1/2). We set out to determine the molecular basis of their LQTS. The inherited LQTS represents a collection of genetically distinct ion channelopathies with over 40 mutations in four fundamental cardiac ion channels identified. Molecular studies, including linkage analysis and identification of the disease-associated mutation, were performed on genomic DNA isolated from peripheral blood samples from 29 available family members. Genetic linkage analysis excluded the regions for LQT2, LQT3, and LQT5. However, the chromosome 11p15.5 region (LQT1) showed evidence of linkage with a maximum lod score of 3.36. Examination of the KVLQT1 gene revealed a novel 3-bp deletion resulting in an in-frame deltaF339 (phenylalanine) deletion in the proband. This deltaF339 mutation was confirmed in nine additional family members who shared both an assigned affected phenotype and the disease-associated linked haplotype. Importantly, three asymptomatic family members, with a tentative clinical diagnosis based on their QTc, did not have this mutation and could be reclassified as unaffected. It is noteworthy that the proband's ECG suggested the sodium channel-based LQT3 genotype. These findings show the potential importance of establishing a molecular diagnosis rather than initiating genotype-specific interventions based upon inspection of a patient's ECG.