Phenotype variability in patients carrying KCNJ2 mutations.

BACKGROUND: Mutations of KCNJ2, the gene encoding the human inward rectifier potassium channel Kir2.1, cause Andersen-Tawil syndrome (ATS), a disease exhibiting ventricular arrhythmia, periodic paralysis, and dysmorphic features. However, some KCNJ2 mutation carriers lack the ATS triad and sometimes share the phenotype of catecholaminergic polymorphic ventricular tachycardia (CPVT). We investigated clinical and biophysical characteristics of KCNJ2 mutation carriers with "atypical ATS." METHODS AND
RESULTS: Mutational analyses of KCNJ2 were performed in 57 unrelated probands showing typical (≥2 ATS features) and atypical (only 1 of the ATS features or CPVT) ATS. We identified 24 mutation carriers. Mutation-positive rates were 75% (15/20) in typical ATS, 71% (5/7) in cardiac phenotype alone, 100% (2/2) in periodic paralysis, and 7% (2/28) in CPVT. We divided all carriers (n=45, including family members) into 2 groups: typical ATS (A) (n=21, 47%) and atypical phenotype (B) (n=24, 53%). Patients in (A) had a longer QUc interval [(A): 695 ± 52 versus (B): 643 ± 35 ms] and higher U-wave amplitude (0.24 ± 0.07 versus 0.18 ± 0.08 mV). C-terminal mutations were more frequent in (A) (85% versus 38%, P<0.05). There were no significant differences in incidences of ventricular tachyarrhythmias. Functional analyses of 4 mutations found in (B) revealed that R82Q, R82W, and G144D exerted strong dominant negative suppression (current reduction by 95%, 97%, and 96%, respectively, versus WT at -50 mV) and T305S moderate suppression (reduction by 89%).
CONCLUSIONS: KCNJ2 gene screening in atypical ATS phenotypes is of clinical importance because more than half of mutation carriers express atypical phenotypes, despite their arrhythmia severity.