Predicting the long-QT genotype from clinical data: from sense to science

Association of the P441L KCNQ1 variant with severity of long QT syndrome and risk of cardiac events

Dysfunction of potassium voltage-gated channel subfamily Q member 1 (KCNQ1) is a primary cause of long QT syndrome type 1 (LQT1). Here, we report a missense mutation P441L in KCNQ1 C-terminus of a 37-year-old woman with severe LQT1 phenotype. Variant P441L transporting to the plasma membrane and interacting with KCNE1 were both markedly decreased, leading to potassium efflux disorder and eventually LQT1. Mutations between the C-terminal helix A and helix B of KCNQ1 have linked with low cardiac event risk, however, we firstly find variant P441L causing a severe LQT1 phenotype with a high risk of cardiac events.

A590T mutation in KCNQ1 C-terminal helix D decreases IKs channel trafficking and function but not Yotiao interaction

KCNQ1 encodes the α subunit of the voltage-gated channel that mediates the cardiac slow delayed rectifier K(+) current (IKs). Here, we report a KCNQ1 allele encoding an A590T mutation [KCNQ1(A590T)] found in a 39-year-old female with a mild QT prolongation. A590 is located in the C-terminal α helical region of KCNQ1 that mediates subunit tetramerization, membrane trafficking, and interaction with Yotiao. This interaction is known to be required for the proper modulation of IKs by cAMP. Since previous studies reported that mutations in the vicinity of A590 impair IKs channel surface expression and function, we examined whether and how the A590T mutation affects the IKs channel. Electrophysiological measurements in HEK-293T cells showed that the A590T mutation caused a reduction in IKs density and a right-shift of the current-voltage relation of channel activation. Immunocytochemical and immunoblot analyses showed the reduced cell surface expression of KCNQ1(A590T) subunit and its rescue by coexpression of the wild-type KCNQ1 [KCNQ1(WT)] subunit. Moreover, KCNQ1(A590T) subunit interacted with Yotiao and had a cAMP-responsiveness comparable to that of KCNQ1(WT) subunit. These findings indicate that the A590 of KCNQ1 subunit plays important roles in the maintenance of channel surface expression and function via a novel mechanism independent of interaction with Yotiao.

Exercise, heart and health

Regular physical activity provides a variety of health benefits, including improvement in cardiopulmonary or metabolic status, reduction of the risk of coronary artery disease or stroke, prevention of cancer, and decrease in total mortality. Exercise-related cardiac events are occasionally reported during highly competitive sports activity or vigorous exercises. However, the risk of sudden death is extremely low during vigorous exercise, and habitual vigorous exercise actually decreases the risk of sudden death during exercise. The cause of sudden death is ischemic in older subjects (≥35 years old), while cardiomyopathies or genetic ion channel diseases are important underlying pathology in younger (<35 years old) victims. The subgroup of patients who are particularly at higher risk of exercise-related sudden death may be identified in different ways, such as pre-participation history taking, physical examination and/or supplementary cardiac evaluation. Limitations exist because current diagnostic tools are not sufficient to predict a coronary artery plaque with potential risk of disruption and/or an acute thrombotic occlusion. Proper and cost-effective methods for identification of younger subjects with cardiac structural problems or genetic ion channel diseases are still controversial.

Spatiotemporal electrocardiographic characterization of ventricular depolarization and repolarization abnormalities in long QT syndrome

BACKGROUND AND PURPOSE

If only a standard electrocardiogram (ECG) is available, at least 25% of patients with long QT syndrome (LQTS) may be missed. Our goal is to quantify abnormal electrical activity and to develop an ECG decision rule for the patients with LQTS.

METHODS

One hundred forty-one subjects were included in this study (71 patients with LQTS and 70 healthy subjects). A 12-lead digital ECG was recorded for each subject and analyzed using the CAVIAR (comparative analysis of ECG-VCG and their interpretation with auto-reference to the patient) method.

RESULTS

A decision tree involving criteria based on 3 spatiotemporal ECG measurements-the QT interval and the maximum amplitude of the T wave, both corrected from heart rate, and the loss of planarity of the end of QRS-identified patients with LQTS from healthy subjects with a sensitivity of 89%, a specificity of 96%, and a total accuracy of 92%.

CONCLUSIONS

This study suggests that 3-dimensional ECG analysis may improve the detection of patients with LQTS.

Inherited Arrhythmia Syndromes

Inherited cardiac arrhythmia syndromes have received a lot of attention in recent years, particularly the molecular genetic basis, which has been unraveled to a great extent in the past years. Disease entities have been subdivided based on their causal gene defect, which, indeed, has been shown to impact on disease expression, clinical characteristics, prognosis and treatment. This particularly holds for the long QT syndrome. Studies in other, more recently described, disease entities, such as Brugada syndrome, catecholaminergic polymorphic ventricular arrhythmias and the short QT syndrome, are ongoing. For some of them the heterogenetic nature has just very recently been established. For these reasons, genetic testing has been introduced to clinical practice in several countries, which enables timely treatment of affected individuals and reassurance of those not inheriting the causal gene defect. Presymptomatic testing, however, is not without drawbacks. Psychosocial studies are needed in this field and should be promoted. It is likely that this development will further increase the knowledge of the (patho-) physiology of these disease entities, but also of more common arrhythmia syndromes.

Congenital long QT syndromes: clinical features, molecular genetics and genetic testing

Congenital long QT syndrome (LQTS) is a primary electrical disease characterized by a prolonged QT interval in the surface electrocardiogram and increased predisposition to a typical polymorphic ventricular tachycardia, termed Torsade de Pointes. Most patients with LQTS are asymptomatic and are diagnosed incidentally based on an electrocardiogram. Symptomatic patients may suffer from severe cardiac events, such as syncope and/or sudden cardiac death. Autosomal dominant forms are caused by heterozygous mutations in genes encoding the components of the ion channels. The autosomal recessive form with congenital deafness is also known as Jervell and Lang-Nielsen syndrome. It is caused by homozygous mutations or certain compound heterozygous mutations. Depending on the genetic defects, there are differences in the age of onset, severity of symptoms, and number of cardiac events and event triggers. With advances in gene technology, it is now feasible to perform genetic testing for LQTS, especially for those with family history. Identification of the mutation will lead to better management of symptoms and more targeted treatment, depending on the underlying genetic defect, resulting in a reduction of mortality and cardiac events.

The heterogeneous spectrum of the long QT syndrome

The long QT syndrome affects predominantly younger people who demonstrate structurally normal hearts. The underlying defect in the long QT syndrome seems to be genetic mutations in the cardiac ionic channels responsible for generating action potentials. Genetic linkage mapping has identified six genes (designated LQT1-6) associated with the Romano-Ward syndrome; two of these genes (LQT1, LQT5) are associated with the Jervell and Lange-Nielsen syndrome. All of these genes encode potassium channels with the exception of LQT3, which encodes a sodium channel. Mutations affecting these channels will lead to a derangement in ionic flows across the cytoplasmic membranes of cardiac cells, thereby leading to prolongation of the cardiac action potential and lengthening of the QT interval on the surface electrocardiogram. Long QT syndrome is a cause of death in young, otherwise healthy individuals. The heterogeneity of the long QT syndrome also makes prognosis and risk stratification difficult. In patients with long QT syndrome genotypes 1 and 2, as well as during slower heart rates, men exhibited shorter mean QTc interval durations than did women; thus, women possess a predilection for developing torsades de pointes. In female probands with the congenital long QT syndrome, the postpartum period appears to confer a significant risk for experiencing a cardiac event. The study determined that certain combinations, such as exhibiting a QTc of 500ms or more, along with the presence of LQT1, LQT2, and LQT3 (with male gender), conferred a 50% or greater risk of a first cardiac event. Based on the observation that physical exertion and emotional stress are significant triggers for cardiac events in the setting of congenital long QT syndrome (specifically the LQT1 and LQT2 genotypes), avoidance of competitive sports seems to be a prudent lifestyle modification. This heterogeneity stems from the presence of different mutations in the genes that encode cardiac ion channels. The triggering events, prognosis, and risk stratification of the patient with long QT syndrome appear to be influenced by the underlying genotype. The primary treatment of congenital long QT syndrome, i.e., beta-blockade therapy with internal cardioverter defibrillator therapy, appears to be useful in a subset of patients.

Cardiac retention of [11C]HED in genotyped long QT patients: a potential amplifier role for severity of the disease

Although mutations in cardiac sodium and potassium channel genes are associated with congenital long QT syndrome (LQTS), a "modifier" role of the sympathetic nervous system was proposed to explain the distinct severity of the disease. We evaluated cardiac sympathetic innervation using [11C]hydroxyephedrine ([11C]HED) and positron emission tomography (PET) in genotyped LQTS patients. H215O and [11C]HED PET studies were performed in 11 patients (5 symptomatic) and 8 controls. Perfusion and [11C]HED images were depicted as 36-sector polar maps. Sectorial values of perfusion (H2O%), absolute (HEDRet) and relative retention (HED%Ret) of [11C]HED, and the ratio of HED%Ret to H2O% (HED%Ret/H2O%) were calculated. Normal databases were obtained from controls. Sectorial values below 2SD database values were defined as "outside sectors." Controls and patients showed similar sectorial perfusion. Sectorial HEDRet did not differ between groups, but means of HED%Ret were lower in three sectors for patients (P < 0.05). Three sectors from 3 controls had HED%Ret below 2SD, whereas 36 sectors in 9 patients were outside sectors (P < 0.01). In patients, average HED%Ret/H2O% was lower in 9 sectors (P < 0.05 vs. controls); 2 outside sectors were found in controls, but 43 outside sectors were found in patients (P < 0.01), 77% of them in the 5 symptomatic patients. Heterogeneous [11C]HED retention was localized in the septal, anterior, and lateral walls. Most LQTS patients showed a localized and decreased pattern of [11C]HED retention. The larger number of heterogeneous sectors in symptomatic patients suggests that sympathetic function could play an amplifier role for severity of the disease.

Genotype-phenotype relationship in Brugada syndrome: electrocardiographic features differentiate SCN5A-related patients from non-SCN5A-related patients

OBJECTIVES

We have tested whether a genotype-phenotype relationship exists in Brugada syndrome (BS) by trying to distinguish BS patients with (carriers) and those without (non-carriers) a mutation in the gene encoding the cardiac sodium channel (SCN5A) using clinical parameters.

BACKGROUND

Brugada syndrome is an inherited cardiac disease characterized by a varying degree of ST-segment elevation in the right precordial leads and (non)specific conduction disorders. In a minority of patients, SCN5A mutations can be found. Genetic heterogeneity has been demonstrated, but other causally related genes await identification. If a genotype-phenotype relationship exists, this might facilitate screening.

METHODS

In a multi-center study, we have collected data on demographics, clinical history, family history, electrocardiogram (ECG) parameters, His to ventricle interval (HV), and ECG parameters after pharmacologic challenge with I(Na) blocking drugs for BS patients with (n = 23), or those without (n = 54), an identified SCN5A mutation.

RESULTS

No differences were found in demographics, clinical history, or family history. Carriers had a significantly longer PQ interval on the baseline ECG and a significantly longer HV time. A PQ interval of > or =210 ms and an HV interval > or =60 ms seem to be predictive for the presence of an SCN5A mutation. After I(Na) blocking drugs, carriers had significantly longer PQ and QRS intervals and more increase in QRS duration.

CONCLUSIONS

We observed significantly longer conduction intervals on baseline ECG in patients with established SCN5A mutations (PQ and HV interval and, upon class I drugs, more QRS increase). These results concur with the observed loss of function of mutated BS-related sodium channels. Brugada syndrome patients with, and those without, an SCN5A mutation can be differentiated by phenotypical differences.

Differentiation between LQT1 and LQT2 patients and unaffected subjects using 24-hour electrocardiographic recordings

This study assesses the use of 24-hour ambulatory electrocardiographic recordings in distinguishing patients with long-QT1 syndrome (LQT1) from those with LQT2, and for distinguishing affected from unaffected patients. The diagnoses of the congenital LQT syndrome and its most common types LQT1 and LQT2 are made difficult because of the limitations of the electrocardiogram as a diagnostic tool. With an automated computerized program, Holter recordings from 15 LQT1 and 15 LQT2 patients and 43 healthy subjects (training set) were reviewed to select the best criteria using QT duration and rate dependence as well as the difference between QT end and QT apex to separate the 3 groups. Fixed criteria were then applied in blinded fashion to separate a different group of 32 genotyped patients and 16 unaffected subjects (test set). In the training set, the RR interval (100 ms), a slope value for median QT/RR curves of -0.016 separated 25 of 30 (83%) and a minimal QT end - QT apex value of 80 ms, separated 26 of 30 (87%) LQT1 patients from LQT2 patients. When all selected criteria were applied to differentiate LQT1 from LQT2 versus unaffected genotypes in the test set, 38 of 48 cases (79%) were correctly identified, whereas using the electrocardiogram alone, 60% of patients were correctly classified into 3 genotypes (p = 0.03). Combining measures for QT duration, rate dependence, and QT end - QT apex interval, derived from Holter recordings, complements the clinical differentiation between LQT1 versus LQT2 patients and between affected and unaffected persons for genotype screening purposes.

Effect of age and overweight on the QT interval and the prevalence of long QT syndrome in children

The change in QT interval with age during childhood of normal children and children with long QT syndrome (LQTS) and the effects of body mass index on the QT interval have not been studied in detail. The prevalence of LQTS in children is not well known. We measured 3 consecutive QT and RR intervals in 4,655 children. Their electrocardiograms along with their height and weight were recorded when they were in the first grade in 1994 and again when they were in the seventh grade in 2000. The QT interval was corrected by Bazett's formula. The longer corrected QT intervals in female subjects than male subjects start at elementary school age, earlier than previously reported. Overweight did not have an impact on the uncorrected or corrected QT interval. None of the 4 children diagnosed with LQTS in the seventh grade had characteristic electrocardiographic findings of LQTS in the first grade. All 4 are nonfamilial cases. The prevalence of LQTS in children was found to be 1 of 1,164. These data suggest that abnormal electrocardiographic phenotypes in children with nonfamilial LQTS may appear during the elementary school year. The longer QT intervals in female subjects than male subjects start at the same period. No correlation was found between obesity and length of the QT interval. Finally, the prevalence of LQTS in children is greater than previously suspected.