QT Syndrome Mutation Research Articles

Heart rate bin method for identifying repolarization changes in LQT1 and LQT2 patients

www.elsevier.com/locate/jelectrocard Heart rate bin method for identifying repolarization changes in LQT1 and LQT2 patients Martino Vaglio, Jean-Philippe Couderc, Scott McNitt, Xiaojuan Xia, Wojciech Zareba, Arthur J. Moss (Heart Research Follow-up Program, Cardiology, Department, University of Rochester Medical Center, Rochester, NY 14642, USA) The clinical course and the precipitating risk factors in the congenital long QT syndrome are genotype-specific. Among long QT syndrome mutations, KvLQT1 (LQT1) and HERG (LQT2) mutations have an increased risk of recurrent cardiac events and their early diagnosis and treatment is crucial to reduce the risk of mortality. Twelvelead electrocardiogram Holters were recorded in 49 LQT1 and 25 LQT2 carriers. The cardiac beats were clustered based on heart rate (HR) and scalar and vectorial repolarization parameters were compared for similar HR. 0022-0736/$ – see front matter D 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jelectrocard.2006.05.019 The QT, QTpeak and the Tpeak to Tend intervals, T-wave magnitude as well as T-loop morphology were automatically quantified using custom-made algorithms. T-wave magnitude and Tpeak to Tend interval from lead V5 were the most discriminant parameters. The predictive model using these scalar parameters provided 87% sensitivity and 96% specificity for discriminating LQT1 and LQT2 carriers. A model including 3 parameters based on the vectorial T-loop reached 96% of sensitivity and 96% of specificity. T-wave morphology (mainly T-wave magnitude) was found as HR-independent in LQT2 carriers, whereas it was HR-dependent in LQT1 carriers. In conclusion, automatic algorithm quantifying T-wave morphology discriminates LQT1 and LQT2 carriers with high accuracy. T-wave morphology presents distinct heart-rate dynamics by genotype.

Temporal repolarization lability differences among genotyped patients with the long QT syndrome

The investigators sought to test whether certain long QT syndrome (LQTS) mutations are associated with increased repolarization lability and whether repolarization lability (quantified by the QT variability index [QTVI]) is increased in patients with LQTS compared with controls. In 32 genotyped patients with LQTS type 1 (LQT1), 32 genotyped patients with LQTS type 2 (LQT2), and 32 controls, the QTVI was increased in patients with LQT2 (-0.973 +/- 0.394, p = 0.01 vs controls) and in patients with LQT1 with mutations other than KCNQ1-FIN (-0.942 +/- 0.264, p = 0.04 vs controls) but was similar between the KCNQ1-FIN group and controls.

Cardiac IKr Channels Minimally Comprise hERG 1a and 1b Subunits

Previous studies suggest native cardiac IKr channels are composed of alpha subunits encoded solely by the 1a transcript of the ERG1 gene. Using isoform-specific ERG1 antibodies, we have new evidence that subunits encoded by an alternate transcript, ERG1b, are also expressed in rat, canine, and human heart. The ERG1a and -1b subunits associate in vivo where they localize to the T tubules of ventricular myocytes. These data indicate native ventricular IKr channels are heteromers containing two alpha subunit types, ERG1a and -1b. The hERG1b-specific exon thus represents a novel target to screen for mutations causing type 2 long QT syndrome. These findings also suggest phenotypic analyses of existing type 2 long QT syndrome mutations, especially those exclusive to the hERG1a amino terminus, should be carried out in systems expressing both subunits.

Mechanisms by which SCN5A mutation N1325S causes cardiac arrhythmias and sudden death in vivo.

Mutations in the cardiac sodium channel gene SCN5A are responsible for type-3 long QT disease (LQT3). The genesis of cardiac arrhythmias in LQT3 is multifaceted, and the aim of this study was to further explore mechanisms by which SCN5A mutations lead to arrhythmogenesis in vivo. We engineered selective cardiac expression of a long QT syndrome (LQTS) mutation (N1325S) in human SCN5A and generated a transgenic mouse model, TGM(NS31). Conscious and unrestrained TGM(NS31)L12 mice demonstrated a significant prolongation of the QT-interval and a high incidence of spontaneous polymorphic ventricular tachycardia (VT) and fibrillation (VF), often resulting in sudden cardiac death (n=52:156). Arrhythmias were suppressed by mexiletine, a sodium channel blocker for the late persistent sodium current. Action potentials (APs) from TGM(NS31)L12 ventricular myocytes exhibited early afterdepolarizations and longer 90% AP durations (APD90=69 +/- 5.9 ms) than control (APD90=46.7 +/- 4.8 ms). Voltage-clamp experiments in transgenic myocytes revealed a peak inward sodium current (INa) followed by a slow recovery from inactivation. After mexiletine application, transgenic ventricular APDs were shortened, and recovery from inactivation of INa was enhanced. These suggest that the N1325S transgene is responsible for the abnormal signals present in transgenic cells as well as the genesis of lethal arrhythmias in mice. Interestingly, transgenic but not wild-type myocytes displayed longer APDs with a shortening of CLs. Our findings show that a new model for LQTS has been established, and we report on an arrhythmogenic mechanism that, unlike other SCN5A mutations, results in poor restitution of APD with increasing rate as a possible substrate for arrhythmogenesis.

Characterization of a novel Long QT syndrome mutation G52R-KCNE1 in a Chinese family

To identify the underlying genetic basis of a Chinese pedigree with Long QT syndrome, the causally related genes were screened in a family and the functional consequence of the identified gene mutation was evaluated in vitro. Mutations in the five defined Long QT syndrome related genes were screened with polymerase chain reaction and single-strand conformation polymorphism methods and direct sequencing. The electrophysiological properties of the identified mutation were characterized in the Xenopus oocyte heterologous expression system. A novel missense mutation, G to A at position 154 in the KCNE1 gene was identified in a Chinese Long QT syndrome family, which leads to an amino acid substitution of arginine (R) for glycine (G) at position 52 (G52R-KCNE1). Of 26 family members (one DNA was not available), seven were mutation carriers and two of them with normal electrocardiogram. Compared with wild-type KCNE1/KCNQ1 channels, coexpression of G52R-KCNE1 with KCNQ1 in Xenopus oocytes did not amplify the KCNQ1 current amplitudes and slightly changed the activation kinetics of the KCNQ1 channels. Coexpression of KCNQ1 together with wild type KCNE1 and G52R-KCNE1 reduced the wild-type I(ks) current amplitude by 50%, whereas other biophysical properties of the I(ks) were not altered. Our findings indicate that glycine52 in the transmembrane domain is critical for KCNE1 function. The mutant G52R-KCNE1 has a dominant negative effect on I(ks) current, which reduces the I(ks) current amplitude and leads to a prolongation of the cardiac action potential. This could underlie the molecular mechanism of ventricular arrhythmias and sudden death in those patients.

Molecular screening of selected long QT syndrome (LQTS) mutations in 165 consecutive bodies found in water.

The association of the long QT-syndrome (LQTS) with single accidental drowning or near-drowning cases has been recently emphasised, but no data on the prevalence of LQTS among drowning victims are currently available. In this study, we have retrospectively screened specific founder mutations in KCNQ1 (KVLQT1) and KCNH2 (HERG) genes in 165 consecutive bodies found in water in Finland. We found a KCNH2-Fin mutation in a 44-year-old woman whose death was classified as suicidal drowning, whereas no other carriers of the two LQTS founder mutations were identified among the remaining 164 victims. This study provides the first estimate of the minimum prevalence of LQTS (0.61%, CI(95): 0.02-3.33) in such a setting and demonstrates the value of genetic analysis of LQTS in putative drownings. The detection of a LQTS founder mutation in a body found in water is a relatively rare event based on our study sample. This finding is, however, of utmost medico-legal importance, since it broadens the spectrum of potential causes and manners of death.

Interaction with GM130 during HERG Ion Channel Trafficking: DISRUPTION BY TYPE 2 CONGENITAL LONG QT SYNDROME MUTATIONS

Many mutations in the Human Ether-à-go-go-Related Gene (HERG) cause type 2 congenital long QT syndrome (LQT2) by disrupting trafficking of the HERG-encoded potassium channel. Beyond observations that some mutations trap channels in the endoplasmic reticulum, little is known about how trafficking fails. Even less is known about what checkpoints are encountered in normal trafficking. To identify protein partners encountered as HERG channels are transported among subcellular compartments, we screened a human heart library with the C terminus of HERG using yeast two-hybrid technology. Among the proteins isolated was GM130, a Golgi-associated protein involved in vesicular transport. The interaction mapped to two non-contiguous regions of HERG and to a region just upstream of the GRASP-65 interaction domain of GM130. GM130 did not interact with the N or C terminus of either KvLQT1 or Shaker channels. LQT2-causing mutations in the HERG C terminus selectively disrupted interactions with GM130 but not Tara, another HERG-interacting protein. Native GM130 and stably expressed HERG were co-immunoprecipitated from HEK-293 cells using GM130 antibodies. In rat cardiac myocytes and HEK-293 cells, confocal immunocytochemistry showed co-localization of GM130 and HERG to the Golgi apparatus. Overexpression of GM130 suppressed HERG current amplitude in Xenopus oocytes, as if by providing an excess of substrate at the Golgi checkpoint. These findings indicate that GM130 plays a previously undefined role in cargo transport. We propose that the cytoplasmic C terminus of HERG participates in the tethering or possibly targeting of HERG-containing vesicles within the Golgi via its interaction with GM130.

A founder mutation of the potassium channel KCNQ1 in long QT syndrome: Implications for estimation of disease prevalence and molecular diagnostics

OBJECTIVESWe took advantage of the genetic isolate of Finns to characterize a common long QT syndrome (LQTS) mutation, and to estimate the prevalence of LQTS.BACKGROUNDThe LQTS is caused by mutations in different ion channel genes, which vary in their molecular nature from family to family.METHODSThe potassium channel gene KCNQ1 was sequenced in two unrelated Finnish patients with Jervell and Lange-Nielsen syndrome (JLNS), followed by genotyping of 114 LQTS probands and their available family members. The functional properties of the mutation were studied using a whole-cell patch-clamp technique.RESULTSWe identified a novel missense mutation (G589D or KCNQ1-Fin) in the C-terminus of the KCNQ1 subunit. The voltage threshold of activation for the KCNQ1-Fin channel was markedly increased compared to the wild-type channel. This mutation was present in homozygous form in two siblings with JLNS, and in heterozygous form in 34 of 114 probands with Romano-Ward syndrome (RWS) and 282 family members. The mean (± SD) rate-corrected QT intervals of the heterozygous subjects (n = 316) and noncarriers (n = 423) were 460 ± 40 ms and 410 ± 20 ms (p < 0.001), respectively.CONCLUSIONSA single missense mutation of the KCNQ1 gene accounts for 30% of Finnish cases with LQTS, and it may be associated with both the RWS and JLNS phenotypes of the syndrome. The relative enrichment of this mutation most likely represents a founder gene effect. These circumstances provide an excellent opportunity to examine how genetic and nongenetic factors modify the LQTS phenotype.