Long Qt Syndrome 5

A number sign (#) is used with this entry because of evidence that long QT syndrome-5 (LQT5) is caused by heterozygous mutation in the KCNE1 gene (176261) on chromosome 21q22.

Digenic inheritance has also been reported; see MOLECULAR GENETICS.

Description

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 syncope, seizure, or sudden death (Jongbloed et al., 1999).

For a discussion of genetic heterogeneity of long QT syndrome, see LQT1 (192500).

Molecular Genetics

In affected members of 2 families with long QT syndrome, Splawski et al. (1997) identified heterozygosity for different missense mutations in the KCNE1 gene (176261.0003-176261.0004).

Splawski et al. (2000) screened 262 unrelated individuals with LQT syndrome for mutations in the 5 defined genes (KCNQ1; KCNH2; SCN5A, 600163; KCNE1; and KCNE2 603796) and identified mutations in 177 individuals (68%). KCNQ1 and KCNH2 accounted for 87% of mutations (42% and 45%, respectively), and SCN5A, KCNE1, and KCNE2 for the remaining 13% (8%, 3%, and 2%, respectively).

Acquired Long QT Syndrome

Paulussen et al. (2004) screened 5 congenital long QT syndrome-associated genes (KCNQ1, KCNH2, SCN5A, KNCE1, and KCNE2) in 32 individuals with drug-induced long QT syndrome and identified 3 heterozygous mutations in 4 patients that were not found in 32 healthy controls (see, e.g., 176261.0005).

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). In 272 (50%) patients, they identified 211 different pathogenic mutations, including 88 in KCNQ1, 89 in KCNH2, 32 in SCN5A, and 1 each in KCNE1 and KCNE2. 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). Millat et al. (2006) suggested that because double heterozygosity appears to be more common than expected, molecular diagnosis should be performed on all LQTS-related genes, even after a single mutation has been identified.

Genotype/Phenotype Correlations

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 176261.0005). 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.