Mutations in the delayed rectifier K+ channel subunit KvLQT1 have been identified as responsible for both Romano-Ward (RW) and Jervell and Lange-Nielsen (JLN) inherited long QT syndromes. We report the molecular cloning of a human KvLQT1 isoform that is expressed in several human tissues including heart. Expression studies revealed that the association of KvLQT1 with another subunit, IsK, reconstitutes a channel responsible for the IKs current involved in ventricular myocyte repolarization. Six RW and two JLN mutated KvLQT1 subunits were produced and co-expressed with IsK in COS cells. All the mutants, except R555C, fail to produce functional homomeric channels and reduce the K+ current when co-expressed with the wild-type subunit. Thus, in both syndromes, the main effect of the mutations is a dominant-negative suppression of KvLQT1 function. The JLN mutations have a smaller dominant-negative effect, in agreement with the fact that the disease is recessive. The R555C subunit forms a functional channel when expressed with IsK, but with altered gating properties. The voltage dependence of the activation is strongly shifted to more positive values, and deactivation kinetics are accelerated. This finding indicates the functional importance of a small positively charged cytoplasmic region of the KvLQT structure where two RW and one JLN mutations have been found to take place.

Benign familial neonatal convulsions, an autosomal dominant epilepsy of newborns, are linked to mutations affecting two six-transmembrane potassium channels, KCNQ2 and KCNQ3. We isolated four splice variants of KCNQ2 in human brain. Two forms generate, after transient expression in COS cells, a potassium-selective current similar to the KCNQ1 current. L-735,821, a benzodiazepine molecule which inhibits the KCNQ1 channel activity (EC SH = 0.08 W WM), also blocks KCNQ2 currents (EC SH = 1.5 W WM). Using in situ hybridization, KCNQ2 and KCNQ3 have been localized within the central nervous system, in which they are expressed in the same areas, mainly in the hippocampus, the neocortex and the cerebellar cortex. During brain development, KCNQ3 is expressed later than KCNQ2.z 1998 Federation of European Biochemical Societies.

Abstract-The Jervell and Lange-Nielsen (JLN) syndrome affects the human cardioauditory system, associating a profound bilateral deafness with an abnormally long QT interval on the ECG. It results from mutations in KVLQT1 and ISK genes that encode the 2 subunits forming the K ϩ channel responsible for the cardiac and inner ear slowly activating component of the delayed rectifier K ϩ current (I Ks ). A JLN mouse model that presents typical inner ear defects has been created by knocking out the isk gene (iskϪ/Ϫ). This study specifically reports on the cardiac phenotype counterpart, determined in the whole animal and at mRNAs and cellular levels. Surface ECG recordings of iskϪ/Ϫ mice showed a longer QT interval at slow heart rates, a paradoxical shorter QT interval at fast heart rates, and an overall exacerbated QT-heart rate adaptation compared with wild-type (WT) mice. A 300-ms increase in the heart rate cycle length induces a 309Ϯ21% increase in the QT duration of the WT mice versus a 500Ϯ50% in iskϪ/Ϫ mice (PϽ0.001). It is concluded that the isk gene product and/or I Ks , when present, blunts the QT adaptation to heart rate variations and that steeper QT-RR relationships reflect a greater susceptibility to arrhythmias in patients lacking I Ks . (Circ Res. 1998;83:95-102.)