Congratulations! Also, great still! I think that's my favorite Black Books episode :-)

11/11 Congrats to S. Pozzi for spectacular COVG experiments & her 1st 1st-author paper! Our gratitude goes to patients & their doctors, our clinical collaborators at Sahlgrenska for reaching out & doing clinical characterization, and team-HPLarsson for pharmacology expts. And thank you for reading ❤️

10/11 So, just like MK Shyamalan stories, we end ours with an open question on the mechanism of arrhythmogenesis. But unlike MKS, we are following up! We are doing more molecular studies (VCF, slow inactivation) and we expect mutagenized iPSCs any day now. We are also procuring patient stem cells.

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!

8/11 One more piece for this puzzle: the arrhythmias are exercise induced. During exercise, the heart is under adrenergic stimulation. When we tried to mimic how Nav1.5 responds to this condition (by perfusing membrane-permeable cAMP), we found that... 🥁🥁🥁

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.

3/11 SCN5A can cause several 🫀disorders
-LQT: underrepresented & should improve with exercise
-BrS: not evident
-MEPPC: causes cardiomyopathy (not evident)
Ie patients had the "wrong variant" for their disease, or the "wrong disease" for their variant. Could this be a new type of cardiac arrhythmia?

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.