Objective: Congenital long QT syndrome (LQTS) is an inherited disorder of cardiac repolarization characterized by prolongation of QT interval and polymorphic ventricular tachycardia torsade de pointes (TdP) in the electrocardiogram (ECG). Clinical symptoms include recurrent syncope, seizure or even sudden death. It is caused by mutations of at least seven genes, six of them encoding ion channels that determine the duration of ventricular action potentials. One of these genes, KCNQ1, encodes an alpha-subunit of cardiac slowly activated delayed rectifier potassium channel. Patients carrying mutations of KCNQ1 are named as LQT1, which accounts for 42% of patients with LQTS. This study sought to analyze the clinical data of Chinese with LQTS and to screen for the mutations of KCNQ1.
Methods: The universally accepted phenotypic criteria of LQTS was used for identification of probands. There were six families with LQTS. They were enrolled in this study. Clinical and ECG data of each family member were recorded. Genomic DNA was prepared from peripheral blood lymphocytes. Polymerase chain reaction-single strand conformation polymorphism analysis was used to screen for mutations throughout the whole coding region of KCNQ1 and DNA sequencing was performed to determine the exact mutation site.
Results: There were totally 13 patients in the six LQTS families. Five were male and eight female. One suffered from sudden death, 10 had syncope and 2 were asymptomatic. Eleven of the 13 patients had ECG data. Their QT and QTc (mean +/- SD) were (0.460 +/- 0.058) s and (0.516 +/- 0.058) s, respectively. TdP was observed in 3 patients (27%) during the syncope attack. By PCR-SSCP analysis, two novel KCNQ1 deletion mutations 356-357 Delta QQ and 626-631 Delta GSGGPP were identified in 7 patients of 2 families. None of 50 normal individuals carried these mutations, indicating these two mutations were likely to cause the disease. In addition, P448R was found in one affected and some unaffected members in other two families and in 7 of 50 (14%) normal individuals, indicating that this might be a polymorphism. All the three mutations located in C-terminal domain of KCNQ1 protein.
Conclusions: Two novel deletion mutations and one novel polymorphism of KCNQ1 gene were identified among 6 Chinese families with LQTS.