Single Nucleotide Deletion Mutation of KCNH2 Gene is Responsible for LQT Syndrome in a 3-Generation Korean Family
The proband who was admitted at the St. Mary's Hospital on May 29th, 2001 was a 39-yr-old female patient referred for the evaluation of intermittent palpitation and dizziness. Clinical evaluation including QT interval assessment and T-wave morphology through ECG, 24 hr Holter monitoring, treadmill exercise test, and coronary angiography was performed. Pedigree analysis was also carried out. The QT intervals were measured by Ashman method and corrected for heart rate (QTc) (). All the family members were also evaluated by ECG.\nThe patient complained severe dizziness and syncope with palpitation when she was under the stress or emotionally upset. Twenty-four hour Holter monitoring showed a few premature atrial or ventricular complexes. During treadmill exercise test, horizontal ST depressions were appeared on leads II, III, avF, V4-V5 with chest pain. While we are taking coronary angiography, the patient complained palpitation, dizziness, and loss of consciousness for a few seconds. Then the ECG showed typical 'Torsades de Pointes' with T wave alternans (). She was diagnosed as long QT syndrome and prescribed propranolol 40 mg bid. After medication, the symptom improved and she was discharged with good condition.\nshows the pedigree of this LQTS family. Five family members (I2, II2, II4, II7, and III1) were diagnosed with LQTS using the diagnostic scoring system (). The rest 4 members showed normal ECG patterns (I1, II6, II8, and III2) with normal QTc intervals (<450 ms) and the representative ECG is shown (). With exception of subject II4, all affected members showed prolonged QTc intervals ranged from 454 to 527 ms and the proband's ECG is shown in . Subject II4 was diagnosed as LQTS based on the non-specific T-wave abnormality despite of shorter QTc interval than normal one (). The clinical characteristics are summarized in .\nGenomic DNA was extracted from the peripheral blood of the participants using a Blood extraction kit (Qiagen) following the manufacturer's protocol. Genomic DNA was used as a template for PCR amplification of the individual exons of the KCNQ1, KCNH2, and KCNE1 genes. Gene-specific primer sequences were designed using genomic sequences of each gene (Genbank accession numbers; NG_008935.1 for KCNQ1, NG_008916.1 for KCNH2, and NG_009091.1for KCNE1). PCR primer sequences and conditions are shown in the . The purified PCR product was directly sequenced using ABI Prism 3730XL DNA Analyzer (Applied Biosystems, Foster City, CA, USA) and the sequences analyzed using BLAST program (). The putative mutation was further investigated in the family members to determine its familial transmission. And the general Korean population was also screened for the presence of this mutation.\nThe sequencing of the entire coding regions as well as the splicing junctions of the KCNQ1, KCNH2, and KCNE1 genes of the proband revealed a typical chromatogram of mixed samples in the sequence read of an exon (). Subcloning of the PCR products followed by sequencing of individual clones identified a mutant allele with a single nucleotide C deletion in exon 3 of KCHN2 gene. This C deletion at the nucleotide position of 453 of the KCNH2 mRNA presumably resulted in frameshift of the coding region thus introducing a premature termination codon after 15 miscoded amino acids (151Pfs+15X) (). This mutation co-segregated with LQTS in this family, thus I2, II2, II4, II7 (proband), and III1 were all heterozygotes for delC453fs (151Pfs+15X). This mutation was not detected in 100 chromosomes in general Korean population.