Trafficking Deficient KV11.1 (HERG) Mutations Linked to Long QT Syndrome Localize to Different Endoplasmic Reticulum Subcompartments

Type 2 Long QT syndrome (LQT2) is commonly caused by trafficking-deficient Kv11.1 missense mutations. The goal of this study is to determine the subcellular localization of different LQT2 mutations expressed in human inducible pluripotent stem cell derived cardiomyocytes (iPSC-CMs). iPSC-CMs transfected with the LQT2 mutation F805C showed an anti-Kv11.1 staining pattern that primarily consisted of several intracellular circles that overlapped with the endoplasmic reticulum (ER) associated degradation marker derlin-1, but not the rough ER marker calnexin, the transitional ER marker BAP31, the COPII vesicle marker Sec31, or the ER Golgi intermediate marker ERGIC-53. Incubating cells in the proteasome inhibitor bortezomib (10μM for 4 hrs) altered the F805C staining pattern from several circles to a largely diffuse intracellular pattern. We next studied the trafficking-deficient LQT2 mutation G601S, which, unlike F805C, has a trafficking-deficient phenotype that is corrected by culturing cells in drugs that bind to Kv11.1 and block IKr (e.g. the class III anti-arrhythmic E-4031). iPSC-CMs transfected with G601S showed a diffuse intracellular anti-Kv11.1 staining pattern that colocalized with BAP31, but not calnexin, Sec31, derlin-1 or ERGIC-53. Culturing cells in E-4031 (10μM for 24 hrs) decreased G601S colocalization with BAP31. We conclude that F805C and G601S expressed in iPSC-CMs localize to different ER subcompartments: F805C localizes to a proteasome-sensitive ER associated degradation subcompartment, whereas G601S is sequestered in the transitional ER subcompartment with BAP31.