Targeted mutational analysis of ankyrin-B in 541 consecutive, unrelated patients referred for long QT syndrome genetic testing and 200 healthy subjects

Abstract

Mutations in ANK2-encoded ankyrin-B underlie long QT syndrome type 4 (LQT4) and various other dysrhythmia phenotypes. The purpose of this study was to determine the prevalence and spectrum of ankyrin-B mutations in a large cohort of unrelated patients referred for LQTS genetic testing and among healthy control subjects. Between August 1997 and July 2004, 541 consecutive, unrelated patients (358 females, average age at diagnosis 24 years, average QTc 482 ms) were referred to Mayo Clinic's Sudden Death Genomics Laboratory for comprehensive mutational analysis of the five cardiac channel genes implicated in LQTS: KCNQ1 (LQT1), KCNH2 (LQT2), SCN5A (LQT3), KCNE1 (LQT5), and KCNE2 (LQT6). Based on this prior analysis, 269 of 541 cases lacked an identifiable mutation (genotype negative). In this study, targeted mutational analysis of 10 ANK2 exons (36,37,39-46) encoding the critical C-terminal regulatory domain or implicated previously as hosting pathogenic mutations was performed on genomic DNA from 541 patients and 200 control subjects using polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing. Overall, 14 distinct nonsynonymous variants (10 novel) were observed in 9 (3.3%) of 269 genotype-negative LQTS patients, 5 (1.8%) of 272 genotype-positive LQTS cases, 4 (4%) of 100 white controls, and 9 (9%) of 100 black controls. Four variants found in controls (L1622I, T1626N, R1788W, and E1813K) were implicated previously as LQT4-associated mutations and displayed functional perturbations in vitro. All genotype-negative LQTS cases hosting ANK2 variants had been diagnosed as "atypical" or "borderline" cases, most presenting with normal QTc, nonexertional syncope, U waves, and/or sinus bradycardia. Nonsynonymous ankyrin-B variants were detected in nearly 3% of unrelated LQTS patients and nearly 7% of healthy control subjects. Genotype-negative LQTS patients with a single ANK2 variant displayed nonexertional syncope, U waves, sinus bradycardia, and extracardiac findings. Whether the identification of previously reported functionally significant variants residing in 2% of apparently healthy subjects suggests proarrhythmic potential or potential misclassification warrants further scrutiny.