9/11 TWIST #3: I1333V abolishes the Nav1.5 response to cAMP! cAMP normally produces Nav1.5 upregulation, due to trafficking of reserve channels or loss of slow inactivation. We don't know whether mutant channels do not get upregulated, or if they are "pre-upregulated". Either way, interesting stuff!

7/11 So if I1333V does not affect AP morphology, how does it cause arrhythmia? The model also predicted ⬆️ Na influx at AP onset and ⬆️ Ca import by the Na/Ca exchanger (NCX). We propose that this causes ⬆️Ca release and then Ca overload, similar to canonical CPVT mutations.

6/11 AP-clamp experiments also show insignificant increase in late current. And the "ToR-ORd" model predicts no AP prolongation. The "window current" increases only towards negative voltages. Increased Nav1.5 availability only happens at the very end of the AP, where it can have no effect.

5/11 TWIST #2: in Nav1.5, opening is also augmented, but fast inactivation is left intact. We can learn much from variant effects on paralogues, but not everything: experiments are still essential! This likely explains why LQT is only a minor effect in the patients. We looked into AP effects next.

4/11 The mutation (I1333V) ablates just one methyl group in a protein of over 2000 residues! But its position is conserved in all Na-channel isoforms. When in 💪Nav1.4, this mutation causes #paralysis; in 🧠Nav1.6, #epilepsy. In both cases, opening was augmented while fast inactivation was impaired.

2/11 We discovered 19 people with familial exercise-induced arrhythmia. This pointed to #CPVT, caused by mutations in Ca-handling genes. Except their Ca-genes were OK! Tragically, this led to misdiagnosis & preventable deaths. TWIST #1: all had a mutation in SCN5A (Nav1.5), the 🫀 sodium channel.