PO-629-02 NEW EVIDENCE TO CHALLENGE CLINGEN'S “DISPUTED EVIDENCE” DESIGNATION FOR AKAP9 AS A BONA FIDE SUSCEPTIBILITY GENE FOR CONGENITAL LONG QT SYNDROME

Abstract

A-kinase-anchoring protein-9 (AKAP9) mediates the phosphorylation state of the KCNQ1-encoded Kv7.1 potassium channel. Loss-of-function (LOF) variants in KCNQ1 cause type 1 long QT syndrome (LQT1). In 2007, we published data suggesting AKAP9-S1570L as a novel LQTS-causative variant and AKAP9 as a novel LQTS-susceptibility gene. However, in a recent re-appraisal of LQTS genes, the Clinical Genome Resource (ClinGen) demoted AKAP9 to a “disputed evidence” gene due to apparent lack of sufficient evidence to support causation of LQTS. To characterize the electrophysiology of a LQTS patient-derived induced pluripotent stem cell-cardiomyocyte (iPSC-CM) AKAP9-S1570L model and its corresponding CRISPR/Cas9 gene-edited/variant-corrected isogenic control (IC). iPSC-CMs were generated from an AKAP9-S1570L positive female diagnosed with LQTS at 14 years of age with a QTc > 480 ms, history of syncope, a LQT1-suggestive treadmill stress test, and a positive family history of QT-prolongation. Standard whole-cell patch clamp technique was performed to measure the action potential duration (APD) of both the AKAP9-S1570L and IC iPSC-CMs. To determine if KCNQ1 contributes to the APD prolongation of the variant iPSC-CMs, the KCNQ1 channel specific activator, ML277 was used in additional APD measurements. APD50 (457 ± 21) and APD90 (549 ± 22 ms) were significantly prolonged in the AKAP9-S1570L iPSC-CMs compared to the IC iPSC-CMs (APD50, 335 ± 15 ms; APD90, 413 ± 16 ms; p<0.05). Addition of ML277 resulted in a greater degree of APD shortening in AKAP9-S1570L iPSC-CMs (APD50 shortening, 349 ± 40 ms (p<0.05); APD90 shortening, 397 ± 42 ms (p<0.05) than in IC iPSC-CMs (APD50 shortening, 219 ± 31 ms; APD90 shortening, 242 ± 33 ms). This indicates KCNQ1 LOF may contribute to the mechanism underlying APD prolongation in the patient-specific AKAP9-S1570L iPSC-CMs, consistent with our previously published data. Using patient specific iPSC-CMs and isogenic controls, we provide new evidence bolstering AKAP9-S1570L as the pathogenic LQTS-causative variant responsible for this patient’s LQTS. Whether ClinGen deems this sufficient evidence to rescue AKAP9 from genetic purgatory and re-classify it as a sufficient evidence LQTS-susceptibility gene, remains to be seen.