Unveiling the genetics of long QT syndrome: a local paediatric experience

Abstract

Long QT syndrome (LQTS) is the most common form of Sudden Arrhythmia Death Syndrome (SADS) with a prevalence of 1 in 2,500 individuals. Conventional diagnosis is based on case history, family history and ECG evaluation. Since 1996, thirteen LQTS genes have been discovered and current guidelines recommend that genetic testing should be incorporated as part of standard assessment in patients with suspected LQTS. The genotypes of our local cohort, is however, unknown. We recruited all the patients diagnosed with LQTS from the Department of Paediatric Cardiology of Queen Mary Hospital and offered genetic testing. Sequencing and MLPA were performed on 6 LQTS genes (LQT1-3, 5-7) by the Victorian Clinical Genetic Services to identify disease-causing mutations. A total of 19 patients were identified, 9 were male, with the QTc ranging from 460-619 ms. Mode of presentation included syncope (n=9), ventricular tachycardia (n=2), convulsion (n=1) and as incidental finding (n=7). Thirteen patients have been treated with b-blockers, one received mexiletine and ICD insertion, one was treated with a combination of mexiletine, propranolol and ICD insertion. Pathogenic mutations were identified in 9 patients (LQT1=3, LQT2=4, LQT3=1, LQT5=1), likely pathogenic mutations in 2 (LQT2), unclassified variants in 2, and no mutation in 6. Patients with pathogenic and likely pathogenic mutations had significantly longer mean QTc than those without such mutations (p=0.046). Three mutations, all in the LQT2 genes, represented novel mutations. All 3 patients with mutations in the porelooping forming domains of the KCNH2 (LQT2) channel had personal or family histories of malignant arrhythmia or sudden cardiac death compatible with previously reported genotype-phenotype correlation. The cost per positive genotype was approximately HK$47,000. Eight families involving 18 family members underwent cascade testing, and family mutations were identified in 10 individuals from 6 families. Autosomal dominant transmission was the likely mode of inheritance in these 6 families. Genetic testing is useful in diagnosis, prognostication and family screening in patients with LQTS, and should be offered to all patients with such condition. Future research direction would involve the use of next-generation sequencing techniques especially for mutation negative patients.