Background The long QT syndrome is an inherited disorder with prolonged ventricular repolarization and a propensity to ventricular tachyarrhythmias and sudden arrhythmic death. Recent linkage studies have demonstrated three separate loci for this disorder on chromosomes 3, 7, and 11, and specific mutated genes for long QT syndrome have been identified on two of these chromosomes. We investigated ECG T-wave patterns (phenotypes) in members of families linked to three genetically distinct forms of the long QT syndrome.

Methods and Results Five quantitative ECG repolarization parameters, ie, four Bazett-corrected time intervals (QT_onset-c, QT_peak-c, QT_c, and T_duration-c, in milliseconds) and the absolute height of the T wave (T_amplitude, in millivolts), were measured in 153 members of six families with long QT syndrome linked to markers on chromosomes 3 (n=47), 7 (n=30), and 11 (n=76). Genotypic data were used to define each family member as being affected or unaffected with long QT syndrome. Affected members of all six families had longer QT intervals (QT_onset-c, QT_peak-c, or QT_c) than unaffected family members (P<.01). Each of the three long QT syndrome genotypes was associated with somewhat distinctive ECG repolarization features. Among affected individuals, the QT_onset-c was unusually prolonged in those individuals with mutations involving the cardiac sodium channel gene SCN5A on chromosome 3 (lead II QT_onset-c [mean±SD]: chromosome 3, 341±42 ms; chromosome 7, 290±56 ms; chromosome 11, 243±73 ms; P<.001); T_amplitude was generally quite small in the chromosome 7 genotype (lead II T_amplitude, mV: chromosome 3, 0.36±0.14; chromosome 7, 0.13±0.07; chromosome 11, 0.37±0.17; P<.001); and T_duration was particularly long in the chromosome 11 genotype (lead II T_duration-c: chromosome 3, 187±33 ms; chromosome 7, 191±51 ms; chromosome 11, 262±65 ms; P<.001). Similar ECG findings were observed in leads aVF and V₅. A considerable variability exists in the quantitative repolarization parameters associated with each genotype, with overlap in the T-wave patterns among the three genotypes.

Conclusions Three separate genetic loci for the long QT syndrome including mutations in two cardiac ionic channel genes were associated with different phenotypic T-wave patterns on the ECG. This study provides insight into the influence of genetic factors on ECG manifestations of ventricular repolarization.