Abstract 1062: Kcnh2 Mutations Cause Silent Ventricle in Zebrafish Null Model
Numerous congenital and acquired cardiac diseases are characterized by arrhythmia and sudden cardiac death, including autosomal-dominant long QT syndrome 2 (LQT2). LQT2 is caused by defects in potassium channel Kcnh2 (hERG). Translucent zebrafish embryos are useful for studying cardiac function during early development and modeling human disease. In a forward genetic screen, we identified two alleles - s213 and s290 - of a mutation that causes a silent ventricle phenotype at 48 hours post fertilization (hpf). Mutants were distinguishable as early as 33 hpf by abnormal heart tube contraction. Pericardial edema and loss of circulation result in death by 10 dpf. The gene affected by these mutations was mapped to kcnh2 on chromosome 3. Two nanograms of a morpholino against this target recapitulated the silent ventricle phenotype. Along with electrophysiological data, the morpholino data suggest that 2:1 heart block represents partial kcnh2 function while silent ventricle is the null phenotype. Sequencing revealed an I462R mutation in s213 and a M554K mutation in s290, in the S3 and S5 transmembrane domains, respectively. Heterozygotes survive, but like human kcnh2 heterozygotes, they show increased sensitivity to Kcnh2 blockers such as terfenadine. Normal-appearing embryos from an incross were treated with terfenadine, sorted by whether they developed 2:1 heartblock, and genotyped afterwards. For each allele (N=18), heartblock correlated with the heterozygous genotype with 100% sensitivity and specificity. Finally, mutants were crossed to a transgenic line expressing a fluorescent calcium indicator protein, which reveals fluctuating intracellular calcium concentrations as a proxy for cardiac conduction. Both alleles appeared to have a coordinated wave of conduction through the atrium but no ventricular conduction wave. The zebrafish LQT2 model reported here includes two autosomal-dominant null alleles with characterized electrophysiology. This model will facilitate the study of LQT2 pathogenesis, especially of embryonic lethal forms not reported in humans to date, and of the role of Ikr in the developing heart.