BACKGROUND: Long QT Syndrome type 2 (LQT2) is caused by mutations in the KCNH2 gene that encodes for the α-subunit (hERG) of the ion channel conducting the rapid delayed rectifier potassium current (IKr). We have previously identified a disease causing mutation (IVS9-28A/G) in the branch point of the splicing of KCNH2 intron 9. However, the mechanism through which this mutation causes the disease is unknown. METHODS AND RESULTS: We generated human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from fibroblasts of two IVS9-28A/G mutation carriers. IVS9-28A/G iPSC-CMs showed prolonged repolarization time, mimicking what observed at the ECG level in the same patients. The expression of the full-length ERG1a isoform resulted reduced, whereas the C-terminally truncated ERG1aUSO isoform was upregulated in mutant iPSC-CMs, with consequent alteration of the physiological ERG1aUSO/ERG1a ratio. Importantly, we observed an impairment of hERG trafficking to the cell membrane. The severity of the alterations in hERG expression and trafficking correlated with the clinical severity of the disease in the two patients under study. Finally, we were able to revert the trafficking defect and reduce the repolarization duration in LQT2 iPSC-CMs using the proteasome inhibitor ALLN. CONCLUSION: Our results highlight the key role of the KCNH2 intron 9 branch point in the regulation of KCNH2 isoform expression and hERG channel function, and allow to categorize the IVS9-28A/G mutation as LQT2 class 2 mutation. These findings may result in a more personalized clinical management of IVS9-28A/G mutation carriers..
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