LQT3/6, DIGENIC, INCLUDED
LONG QT SYNDROME 2/3, DIGENIC, INCLUDED
LONG QT SYNDROME 3/6, DIGENIC, INCLUDED
LQT2/3, DIGENIC, INCLUDED
LQT3 LONG QT SYNDROME 3, ACQUIRED, SUSCEPTIBILITY TO, INCLUDED
LQT3
LQT3 is a sub-type of Romano-Ward syndrome, a variant of familial long QT syndrome and is associated with 2:1 atrioventricular block. Symptoms are most frequently observed during rest or at night. Typically, a late-appearing T wave is observed. It is characterized by mutations in the SCN5A gene (PMID:12574983; PMID:25274057). SCN5A is also associated with familial atrial fibrillation, 10 (ATFB10).
Congenital long QT syndrome is electrocardiographically characterized by a prolonged QT interval and polymorphic ventricular arrhythmias (torsade de pointes). These cardiac arrhythmias may result in recurrent syncopes, seizure, or sudden death (Jongbloed et al., 1999).
Wang et al. (1995) cited preliminary data suggesting that in chromosome 3-linked LQT the onset of the T wave is later and duration of the QT interval longer than in other forms. These data suggested to the authors ... Wang et al. (1995) cited preliminary data suggesting that in chromosome 3-linked LQT the onset of the T wave is later and duration of the QT interval longer than in other forms. These data suggested to the authors that chromosome 3-linked LQT is more severe than the other forms. See GENOTYPE/PHENOTYPE CORRELATIONS.
In a large collaborative study, Zareba et al. (1998) demonstrated that the genotype of the long QT syndrome influences the clinical course. The risk of cardiac events (syncope, aborted cardiac arrest, or sudden death) was significantly higher among ... In a large collaborative study, Zareba et al. (1998) demonstrated that the genotype of the long QT syndrome influences the clinical course. The risk of cardiac events (syncope, aborted cardiac arrest, or sudden death) was significantly higher among subjects with mutations at the LQT1 (192500) locus (KCNQ1; 607542) or the LQT2 (613688) locus (KCNH2; 152427) than among those with mutations at the LQT3 locus. Although the cumulative mortality was similar regardless of the genotype, the percentage of cardiac events that were lethal was significantly higher in families with mutations at the LQT3 locus. In this large study, 112 patients had mutations at the LQT1 locus, 72 at the LQT2 locus, and 62 at the LQT3 locus. Thus, paradoxically, cardiac events were less frequent in LQT3 but more likely to be lethal; the likelihood of dying during a cardiac event was 20% in families with an LQT3 mutation and 4% with either an LQT1 or an LQT2 mutation. Westenskow et al. (2004) analyzed the KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2 genes in 252 probands with long QT syndrome and identified 19 with biallelic mutations in LQTS genes, of whom 18 were either compound (monogenic) or double (digenic) heterozygotes and 1 was a homozygote. They also identified 1 patient who had triallelic digenic mutations (see 152427.0021). Compared with probands who had 1 or no identified mutation, probands with 2 mutations had longer QTc intervals (p less than 0.001) and were 3.5-fold more likely to undergo cardiac arrest (p less than 0.01). All 20 probands with 2 mutations had experienced cardiac events. Westenskow et al. (2004) concluded that biallelic mono- or digenic mutations (which the authors termed 'compound mutations') cause a severe phenotype and are relatively common in long QT syndrome. The authors noted that these findings support the concept of arrhythmia risk as a multi-hit process and suggested that genotype can be used to predict risk.
The voltage-gated cardiac sodium channel SCN5A is responsible for the initial upstroke of the action potential in the electrocardiogram. George et al. (1995) mapped SCN5A to 3p21 by fluorescence in situ hybridization, thus making it an important candidate ... The voltage-gated cardiac sodium channel SCN5A is responsible for the initial upstroke of the action potential in the electrocardiogram. George et al. (1995) mapped SCN5A to 3p21 by fluorescence in situ hybridization, thus making it an important candidate gene for LQT3. Missense mutations in the skeletal muscle sodium channel gene, SCN4A (603967), cause myotonia. Physiologic data show that these mutations affect sodium channel inactivation and lead to repetitive depolarizations, consistent with the myotonic phenotype. By analogy, similar mutations in the cardiac sodium channel gene might be expected to cause a phenotype like LQT. Indeed, Wang et al. (1995) found a mutation in the SCN5A gene in families with chromosome 3-linked LQT (see 600163.0001). Wang et al. (1995) identified SCN5A mutations in affected members of 4 additional families with chromosome 3-linked LQT. Two of the families had the same 9-bp deletion found earlier; the other families were found to have missense mutations affecting highly conserved amino acid residues (600163.0002 and 600163.0003). The location and character of the mutation suggested to the authors that this form of LQT results from a delay in cardiac sodium channel fast inactivation or altered voltage-dependence of inactivation. In an 8-generation Dutch family with a history of sudden death, some members of which demonstrated ECG features compatible with Brugada syndrome (601144) and QT prolongation characteristic of long QT syndrome-3, Bezzina et al. (1999) detected an aberrant conformer corresponding to a novel mutation in the C terminal of the SCN5A protein (1795insD; 600163.0013). This family demonstrated that long QT syndrome-3 and Brugada syndrome appear to lie on a spectrum of cardiac electrophysiologic pathology caused by SCN5A mutation. Veldkamp et al. (2003) studied the effect of the 1795insD mutation on sinoatrial (SA) pacemaking. Activity of 1795insD channels during SA node pacemaking was confirmed by action potential (AP) clamp experiments, and the previously characterized persistent inward current (I-pst) and negative shift were implemented into SA node (AP) models. The -10 mV shift decreased the sinus rate by decreasing the diastolic depolarization rate, whereas the I-pst decreased the sinus rate by AP prolongation, despite a concomitant increase in the diastolic depolarization rate. In combination, a moderate I-pst (1 to 2%) and the shift reduced the sinus rate by about 10%. Veldkamp et al. (2003) concluded that sodium channel mutations displaying an I-pst or a negative shift in inactivation may account for the bradycardia seen in LQT3 patients, whereas SA node pauses or arrest may result from failure of SA node cells to repolarize under conditions of extra net inward current. In a patient with long QT syndrome-3, Rivolta et al. (2001) identified a tyr1795-to-cys mutation in the SCN5A gene (Y1795C; 600163.0029). In a patient with Brugada syndrome, they identified a different mutation at the same codon (Y1795H; 600163.0030). They concluded that these findings provided further evidence of the close interrelationship between Brugada syndrome and long QT syndrome-3 at the molecular level. Clancy et al. (2002) performed detailed kinetic analyses of the Y1795C mutant described by Rivolta et al. (2001). Theoretical entry and exit rates from the bursting mode of gating were derived from single channels. Computational analysis suggested that the amount of time mutant channels spend bursting (burst mode dwell time) is primarily responsible for rate-dependent changes in single-channel bursting and macroscopic inward sodium channel (I-sus), hence delaying repolarization and prolonging the QT interval. This prediction was experimentally confirmed by analysis of delta-KPQ mutant channels (600163.0001) for which the burst mode exit rate (determined by the burst mode dwell time) was found to be very similar to the derived rate for Y1795C channels. These results provided an explanation of the molecular mechanism for bradycardia-induced QT prolongation in patients carrying LQT3 mutations. In a male infant diagnosed with ventricular arrhythmias and cardiac decompensation in utero at 28 weeks' gestation and with LQT3 at birth, Miller et al. (2004) identified heterozygosity for a mutation in the SCN5A gene (R1623Q; 600163.0007). The patient required cardiac transplantation at 5 months of age for control of ventricular tachycardia. The mother had no ECG abnormalities, but a previous and a subsequent pregnancy had both ended in stillbirth at 7 months. Initial studies detected no genetic abnormality, but a sensitive restriction enzyme-based assay revealed a small percentage (8 to 10%) of cells harboring the mutation in the mother's blood, skin, and buccal mucosa; the mutation was also identified in cord blood from the third fetus. Miller et al. (2004) concluded that recurrent late-term fetal loss or sudden infant death can result from unsuspected parental mosaicism for LQT-associated mutations. Tester et al. (2005) analyzed 5 LQTS-associated cardiac channel genes in 541 consecutive unrelated patients with LQT syndrome (average QTc, 482 ms). In 272 (50%) patients, they identified 211 different pathogenic mutations, including 88 in KCNQ1 (607542), 89 in KCNH2 (152427), 32 in SCN5A, and 1 each in KCNE1 (176261) and KCNE2 (603796). Mutations considered pathogenic were absent in more than 1,400 reference alleles. Among the mutation-positive patients, 29 (11%) had 2 LQTS-causing mutations, of which 16 (8%) were in 2 different LQTS genes (biallelic digenic). Tester et al. (2005) noted that patients with multiple mutations were younger at diagnosis, but they did not discern any genotype/phenotype correlations associated with location or type of mutation. In 44 unrelated patients with LQT syndrome, Millat et al. (2006) used DHLP chromatography to analyze the KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2 genes for mutations and SNPs. Most of the patients (84%) showed a complex molecular pattern, with an identified mutation associated with 1 or more SNPs located in several LQTS genes; 4 of the patients also had a second mutation in a different LQTS gene (biallelic digenic inheritance; see, e.g., 600163.0007 and 600163.0035). Makita et al. (2008) genotyped 66 members of 44 LQT3 families of multiple ethnicities and identified the E1784K mutation in the SCN5A gene (600163.0008) in 41 individuals from 15 (34%) of the kindreds; the diagnoses in these individuals included LQT3 syndrome, Brugada syndrome (601144), and/or sinus node dysfunction (see 608567). In vitro functional characterization of E1784K compared to properties reported for other LQT3 variants suggested that a negative shift of steady-state Na channel inactivation and enhanced tonic block in response to Na channel blockers represent common biophysical mechanisms underlying the phenotypic overlap of LQT3 and Brugada syndromes, and further indicated that class IC drugs should be avoided in patients with Na channels displaying these behaviors. - Digenic Inheritance Tester et al. (2005) analyzed 5 LQTS-associated cardiac channel genes in 541 consecutive unrelated patients with LQT syndrome (average QTc, 482 ms). Among the mutation-positive patients, 29 (11%) had 2 LQTS-causing mutations, of which 16 (8%) were in 2 different LQTS genes (biallelic digenic). Tester et al. (2005) noted that patients with multiple mutations were younger at diagnosis, but they did not discern any genotype/phenotype correlations associated with location or type of mutation. In 44 unrelated patients with LQT syndrome, Millat et al. (2006) used DHLP chromatography to analyze the KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2 genes for mutations and SNPs. Most of the patients (84%) showed a complex molecular pattern, with an identified mutation associated with 1 or more SNPs located in several LQTS genes; 4 of the patients also had a second mutation in a different LQTS gene (biallelic digenic inheritance; see, e.g., 600163.0007 and 600163.0035).