International Cohort of Neonatal Timothy Syndrome.

INTRODUCTION: Timothy syndrome (TS) is an extremely rare, multisystem disorder classically associated with long QT, syndactyly, ventricular arrhythmias, and hypoglycaemia. A neonatal diagnosis allows maximal medical and device therapy to be implemented to avoid malignant arrhythmias and sudden cardiac death. METHODS: This was a retrospective case series study of type I TS (TS1) patients using data from the Timothy Syndrome Foundation's international registry, encompassing patients with a genetic diagnosis (CACNA1C variant G406R in exon 8A) recruited over a 28-year period. RESULTS: Forty-four cases of TS1 were included (26 male; 60%). Mean gestational age (GA) was 35.6 weeks (range 28 weeks - term), with 43% of patients born less than 37 weeks GA. In TS1 patients presenting with foetal bradycardia, mean GA was significantly lower (34.2 weeks, p < 0.05). Foetal bradycardia secondary to atrioventricular block was present in 20 patients (45%), resulting in premature delivery in 14 patients (32%). Fifteen patients (34%) were diagnosed with TS1 as neonates. Long QT at birth helped secure a diagnosis in 25 patients (57%). Syndactyly was seen in most patients (n = 40, 91%). Twenty patients died, with an average age of death of 2.3 years (range 1 month-6 years). Of the 7 patients who died before the first year of life (16%), the average age of death was 2.5 months. CONCLUSION: TS is associated with high early mortality. TS should be considered in paediatric patients presenting with long QT and syndactyly. Recognition of TS in the neonatal period allows for early intervention to prevent life-threatening arrhythmias.

Hyperinsulinemic Hypoglycemia Associated with a CaV1.2 Variant with Mixed Gain- and Loss-of-Function Effects.

The voltage-dependent L-type calcium channel isoform CaV1.2 is critically involved in many physiological processes, e.g., in cardiac action potential formation, electromechanical coupling and regulation of insulin secretion by beta cells. Gain-of-function mutations in the calcium voltage-gated channel subunit alpha 1 C (CACNA1C) gene, encoding the CaV1.2 α1-subunit, cause Timothy syndrome (TS), a multisystemic disorder that includes autism spectrum disorders and long QT (LQT) syndrome. Strikingly, TS patients frequently suffer from hypoglycemia of yet unproven origin. Using next-generation sequencing, we identified a novel heterozygous CACNA1C mutation in a patient with congenital hyperinsulinism (CHI) and associated hypoglycemic episodes. We characterized the electrophysiological phenotype of the mutated channel using voltage-clamp recordings and in silico action potential modeling experiments. The identified CaV1.2L566P mutation causes a mixed electrophysiological phenotype of gain- and loss-of-function effects. In silico action potential modeling supports that this mixed electrophysiological phenotype leads to a tissue-specific impact on beta cells compared to cardiomyocytes. Thus, CACNA1C variants may be associated with non-syndromic hyperinsulinemic hypoglycemia without long-QT syndrome, explained by very specific electrophysiological properties of the mutated channel. We discuss different biochemical characteristics and clinical impacts of hypoglycemia in the context of CACNA1C variants and show that these may be associated with significant morbidity for Timothy Syndrome patients. Our findings underline that the potential of hypoglycemia warrants careful attention in patients with CACNA1C variants, and such variants should be included in the differential diagnosis of non-syndromic congenital hyperinsulinism.

An intronic mutation causes long QT syndrome.

OBJECTIVES: The purpose of this research was to determine whether an intronic variant (T1945+6C) in KCNH2 is a disease-causing mutation, and if expanded phenotyping criteria produce improved identification of long QT syndrome (LQTS) patients. BACKGROUND: Long QT syndrome is usually caused by mutations in conserved coding regions or invariant splice sites, yet no mutation is found in 30% to 50% of families. In one such family, we identified an intronic variant in KCNH2. Long QT syndrome diagnosis is hindered by reduced penetrance, as the long QT phenotype is absent on baseline electrocardiogram (ECG) in about 30%. METHODS: Fifty-two family members were phenotyped by baseline QTc, QTc maximum on serial ECGs (Ser QTc-max), and on exercise ECGs (Ex QTc-max) and by T-wave patterns. Linkage analysis tested association of the intronic change with phenotype. The consequences of T1945+6C on splicing was studied using a minigene system and on function by heterologous expression. RESULTS: Expanded phenotype/pedigree criteria identified 23 affected and 29 unaffected. Affected versus unaffected had baseline QTc 484 +/- 48 ms versus 422 +/- 20 ms, Ser QTc-max 508 +/- 48 ms versus 448 +/- 10 ms, Ex QTc-max 513 +/- 54 ms versus 444 +/- 11 ms, and LQT2 T waves in 87% versus 0%. Linkage analysis demonstrated a logarithm of odds score of 10.22. Splicing assay showed T1945+6C caused downstream intron retention. Complementary deoxyribonucleic acid with retained intron 7 failed to produce functional channels. CONCLUSIONS: T1945+6C is a disease-causing mutation. It alters KCNH2 splicing and cosegregates with the LQT2 phenotype. Expanded ECG criteria plus pedigree analysis provided accurate clinical diagnosis of all carriers including those with reduced penetrance. Intronic mutations may be responsible for LQTS in some families with otherwise negative mutation screening.

A cardiac arrhythmia syndrome caused by loss of ankyrin-B function.

220-kDa ankyrin-B is required for coordinated assembly of Na/Ca exchanger, Na/K ATPase, and inositol trisphosphate (InsP(3)) receptor at transverse-tubule/sarcoplasmic reticulum sites in cardiomyocytes. A loss-of-function mutation of ankyrin-B identified in an extended kindred causes a dominantly inherited cardiac arrhythmia, initially described as type 4 long QT syndrome. Here we report the identification of eight unrelated probands harboring ankyrin-B loss-of-function mutations, including four previously undescribed mutations, whose clinical features distinguish the cardiac phenotype associated with loss of ankyrin-B activity from classic long QT syndromes. Humans with ankyrin-B mutations display varying degrees of cardiac dysfunction including bradycardia, sinus arrhythmia, idiopathic ventricular fibrillation, catecholaminergic polymorphic ventricular tachycardia, and risk of sudden death. However, a prolonged rate-corrected QT interval was not a consistent feature, indicating that ankyrin-B dysfunction represents a clinical entity distinct from classic long QT syndromes. The mutations are localized in the ankyrin-B regulatory domain, which distinguishes function of ankyrin-B from ankyrin-G in cardiomyocytes. All mutations abolish ability of ankyrin-B to restore abnormal Ca(2+) dynamics and abnormal localization and expression of Na/Ca exchanger, Na/K ATPase, and InsP(3)R in ankyrin-B(+/-) cardiomyocytes. This study, considered together with the first description of ankyrin-B mutation associated with cardiac dysfunction, supports a previously undescribed paradigm for human disease due to abnormal coordination of multiple functionally related ion channels and transporters, in this case the Na/K ATPase, Na/Ca exchanger, and InsP(3) receptor.

Compound mutations: a common cause of severe long-QT syndrome.

BACKGROUND: Long QT syndrome (LQTS) predisposes affected individuals to sudden death from cardiac arrhythmias. Although most LQTS individuals do not have cardiac events, significant phenotypic variability exists within families. Probands can be very symptomatic. The mechanism of this phenotypic variability is not understood. METHODS AND RESULTS: Genetic analyses of KVLQT1, HERG, KCNE1, KCNE2, and SCN5A detected compound mutations in 20 of 252 LQTS probands (7.9%). Carriers of 2 mutations had longer QTc intervals (527+/-54 versus 489+/-44 ms; P<0.001); all had experienced cardiac events (20 of 20 [100%] versus 128 of 178 [72%]; P<0.01) and were 3.5-fold more likely to have cardiac arrest (9 of 16 [56%] versus 45 of 167 [27%]; P<0.01; OR, 3.5; 95% CI, 1.2 to 9.9) compared with probands with 1 or no identified mutation. Two-microelectrode voltage clamp of Xenopus oocytes was used to characterize the properties of variant slow delayed rectifier potassium (I(Ks)) channels identified in 7 of the probands. When wild-type and variant subunits were coexpressed in appropriate ratios to mimic the genotype of the proband, the reduction in I(Ks) density was equivalent to the additive effects of the single mutations. CONCLUSIONS: LQTS-associated compound mutations cause a severe phenotype and are more common than expected. Individuals with compound mutations need to be identified, and their management should be tailored to their increased risk for arrhythmias.

Modulating effects of age and gender on the clinical course of long QT syndrome by genotype.

OBJECTIVES: We aimed to determine whether long QT syndrome (LQTS) genotype has a differential effect on clinical course of disease in male and female children and adults after adjustment for QTc duration. BACKGROUND: Genotype influences clinical course of the LQTS; however, data on the effect of age and gender on this association are limited. METHODS: The LQTS genotype, QTc duration, and follow-up were determined in 243 cases of LQTS caused by the KCNQ1 potassium channel gene mutations (LQT1), 209 cases of LQTS caused by the HERG potassium channel gene mutations (LQT2), and 81 cases of LQTS caused by the SCN5A sodium channel gene mutation (LQT3) gene carriers. The probability of cardiac events (syncope, aborted cardiac arrest, or sudden death) was analyzed by genotype, gender, and age (children < or = 15 years and adults 16 to 40 years). In addition, the risk of sudden death and lethality of cardiac events were evaluated in 1,075 LQT1, 976 LQT2, and 324 LQT3 family members from families with known genotype. RESULTS: During childhood, the risk of cardiac events was significantly higher in LQT1 males than in LQT1 females (hazard ratio [HR] = 1.72), whereas there was no significant gender-related difference in the risk of cardiac events among LQT2 and LQT3 carriers. During adulthood, LQT2 females (HR = 3.71) and LQT1 females (HR = 3.35) had a significantly higher risk of cardiac events than respective males. The lethality of cardiac events was highest in LQT3 males and females (19% and 18%), and higher in LQT1 and LQT2 males (5% and 6%) than in LQT1 and LQT2 females (2% for both). CONCLUSIONS; Age and gender have different, genotype-specific modulating effects on the probability of cardiac events and electrocardiographic presentation in LQT1 and LQT2 patients.