Background: A-kinase-anchoring protein 9 (AKAP9) mediates the phosphorylation state of the slowly activating delayed rectifier I_Ks potassium channel. In 2007, our heterologous expression data supported AKAP9-S1570L as a long QT syndrome (LQTS)-causative mutation and AKAP9 as a LQTS-susceptibility gene. However, AKAP9 has been demoted recently by the Clinical Genome Resource (ClinGen) group to a “limited evidence” gene for LQTS.
Objective: To characterize a patient-derived induced pluripotent stem cell-derived cardiomyocyte (iPSC-CM) AKAP9-S1570L model and its “gene-corrected” isogenic control.
Methods: AKAP9-S1570L iPSC-CMs were generated from a 44-year-old female (QTc > 485 ms) with a history of syncope, high burden of premature ventricular contractions, and mild left ventricular enlargement. An isogenic iPSC “gene-corrected” control line was created using CRISPR/Cas9. Patch-clamp technique was used to measure action potential duration (APD) at 1 Hz pacing rate before/after IKs activator (ML277), isoproterenol (iso), and membrane permeable cAMP plus okadaic acid (cAMP+OA).
Results: APD50 and APD90 were prolonged significantly by 36.2% and 32.8% respectively from 335.4±14.8 ms and 413.3±16.1 ms (n=37, control) to 456.8±21.2 ms and 548.7±22.2 ms (n=27, AKAP9-S1570L, p<0.05 vs. control). 0.5 µM ML277 dramatically shortened APD90 by 72.9% and 63.3% respectively in both AKAP9-S1570L (n=10) and control iPSC-CMs (n=11, p<0.05 vs. before ML277). 100 nM Iso and cAMP+OA (50 µM+0.2 µM) also profoundly shortened APD90 by 44.9% and 57.5% (AKAP9-S1570L, n=8-9, p<0.05 vs. before drugs), and 32.9% and 29.2% (control, n=9-11, p<0.05 vs. before drugs).
Conclusion: Using a patient’s variant-specific iPSC-CMs and her isogenic control lines, we demonstrate that the AKAP9-S1570L is the patient’s self-sufficient monogenetic substrate for her LQTS phenotype. However, AKAP9-S1570L iPSC-CMs were more sensitive than isogenic control iPSC-CMs to IKs activator, Iso and cAMP signal pathway regarding their APD shortening effects. In contrast to previous heterologous, non-cardiac expression studies focused only on the I_Ks potassium channel, the APD-prolonging effect of AKAP9-S1570L appears to be mediated through other ion channel final common pathways.