The electrophysiologic effects of KCNQ1 extend beyond expression of IKs: evidence from genetic and pharmacologic block.

AIMS: While variants in KCNQ1 are the commonest cause of the congenital long QT syndrome, we and others find only a small IKs in cardiomyocytes from human-induced pluripotent stem cells (iPSC-CMs) or human ventricular myocytes. METHODS AND RESULTS: We studied population control iPSC-CMs and iPSC-CMs from a patient with Jervell and Lange-Nielsen (JLN) syndrome due to compound heterozygous loss-of-function (LOF) KCNQ1 variants. We compared the effects of pharmacologic IKs block to those of genetic KCNQ1 ablation, using JLN cells, cells homozygous for the KCNQ1 LOF allele G643S, or siRNAs reducing KCNQ1 expression. We also studied the effects of two blockers of IKr, the other major cardiac repolarizing current, in the setting of pharmacologic or genetic ablation of KCNQ1: moxifloxacin, associated with a very low risk of drug-induced long QT, and dofetilide, a high-risk drug. In control cells, a small IKs was readily recorded but the pharmacologic IKs block produced no change in action potential duration at 90% repolarization (APD90). In contrast, in cells with genetic ablation of KCNQ1 (JLN), baseline APD90 was markedly prolonged compared with control cells (469 ± 20 vs. 310 ± 16 ms). JLN cells displayed increased sensitivity to acute IKr block: the concentration (µM) of moxifloxacin required to prolong APD90 100 msec was 237.4 [median, interquartile range (IQR) 100.6-391.6, n = 7] in population cells vs. 23.7 (17.3-28.7, n = 11) in JLN cells. In control cells, chronic moxifloxacin exposure (300 µM) mildly prolonged APD90 (10%) and increased IKs, while chronic exposure to dofetilide (5 nM) produced greater prolongation (67%) and no increase in IKs. However, in the siRNA-treated cells, moxifloxacin did not increase IKs and markedly prolonged APD90. CONCLUSION: Our data strongly suggest that KCNQ1 expression modulates baseline cardiac repolarization, and the response to IKr block, through mechanisms beyond simply generating IKs.

Generation of the human induced pluripotent stem cell (hiPSC) line PSMi002-A from a patient affected by the Jervell and Lange-Nielsen syndrome and carrier of two compound heterozygous mutations on the KCNQ1 gene.

We report the generation of human induced pluripotent stem cells (hiPSCs) from dermal fibroblasts of a female patient carrier of the two compound heterozygous mutations c.568 C>T p.R190W (maternal allele), and c.1781 G>A p.R594Q (paternal allele) on the KCNQ1 gene, causing Jervell and Lange-Nielsen Syndrome (JLNS). To obtain hiPSCs, we used the classical approach of the four retroviruses each encoding for a reprogramming factor OCT4, SOX2, KLF4, cMYC. The obtained hiPSC clones display pluripotent stem cell characteristics, and differentiate into spontaneously beating cardiomyocytes (hiPSC-CMs).

Jervell and Lange-Nielsen syndrome with homozygous missense mutation of the KCNQ1 gene.

Jervell and Lange-Nielsen syndrome (JLNS) is an autosomal recessive cardioauditory ion channel disorder characterized by congenital bilateral sensorineural deafness and long QT interval. JLNS is a ventricular repolarization abnormality and is caused by mutations in the KCNQ1 or KCNE1 gene. It has a high mortality rate in childhood due to ventricular tachyarrhythmias, episodes of torsade de pointes which may cause syncope or sudden cardiac death. Here, we present a 4.5-year-old female patient who had a history of syncope and congenital sensorineural deafness. She had a cochlear implant operation at 15 months of age and received an implantable cardioverter defibrillator (ICD) at 3 years of age because of recurrent syncope attacks. Five months after cochlear implant placement, she could say her first words and is now able to speak. With ß-blocker therapy and ICD, she has remained syncope-free for a year. On the current admission, the family visited the genetics department to learn about the possibility of prenatal diagnosis of sensorineural deafness, as the mother was 9 weeks pregnant. A diagnosis of JLNS was established for the first time, and a homozygous missense mutation in the KCNQ1 gene (c.128 G>A, p.R243H) was detected. Heterozygous mutations of KCNQ1 were identified in both parents, thereby allowing future prenatal diagnoses. The family obtained prenatal diagnosis for the current pregnancy, and fetal KCNQ1 analysis revealed the same homozygous mutation. The pregnancy was terminated at the 12th week of gestation. The case presented here is the third molecularly confirmed Turkish JLNS case; it emphasizes the importance of timely genetic diagnosis, which allows appropriate genetic counseling and prenatal diagnosis, as well as proper management of the condition.

Jervell and Lange-Nielsen syndrome: homozygous missense mutation of KCNQ1 in a Turkish family.

Long QT syndrome is one of the most common cardiac ion channel diseases, but its morbidity and mortality rate can be lessened with an early diagnosis and proper treatment. This cardiac ventricular repolarization abnormality is characterized by a prolonged QT interval and a propensity for ventricular tachycardia (VT) of the torsades de pointes type. The long QT syndrome represents a high risk for presyncope, syncope, cardiac arrest, and sudden death. Jervell and Lange-Nielsen syndrome (JLNS) is a recessively inherited form of long QT syndrome characterized by profound sensorineural deafness and prolongation of the QT interval. Findings have shown that JLNS occurs due to homozygous and compound heterozygous pathogenic variants in KCNQ1 or KCNE1. A 3.5-year-old girl presented to the hospital with recurrent syncope, seizures, and congenital sensorineural deafness. Her electrocardiogram showed a markedly prolonged QT interval, and she had a diagnosis of JLNS. The sequence analysis of the proband showed the presence of a pathogenic homozygous missense variant (c.728G>A, p.Arg243His). Heterozygous mutations of KCNQ1 were identified in her mother, father, and sister, demonstrating true homozygosity. Even with high-dose beta-blocker therapy, the patient had two VT attacks, so an implantable cardioverter defibrillator was fitted. The authors suggest early genetic diagnosis for proper management of the disease in the proband and genetic counseling for both the proband and the girl's extended family.

Computational model of vectorial potassium transport by cochlear marginal cells and vestibular dark cells.

Cochlear marginal cells and vestibular dark cells transport potassium into the inner ear endolymph, a potassium-rich fluid, the homeostasis of which is essential for hearing and balance. We have formulated an integrated mathematical model of ion transport across these epithelia that incorporates the biophysical properties of the major ion transporters and channels located in the apical and basolateral membranes of the constituent cells. The model is constructed for both open- and short-circuit situations to test the extremes of functional capacity of the epithelium and predicts the steady-state voltages, ion concentrations, and transepithelial currents as a function of various transporter and channel densities. We validate the model by establishing that the cells are capable of vectorial ion transport consistent with several experimental measurements. The model indicates that cochlear marginal cells do not make a significant direct contribution to the endocochlear potential and illustrates how changes to the activity of specific transport proteins lead to reduced K(+) flux across the marginal and dark cell layers. In particular, we investigate the mechanisms of loop diuretic ototoxicity and diseases with hearing loss in which K(+) and Cl(-) transport are compromised, such as Jervell and Lange-Nielsen syndrome and Bartter syndrome, type IV, respectively. Such simulations demonstrate the utility of compartmental modeling in investigating the role of ion homeostasis in inner ear physiology and pathology.