Beyond the blueprint: decoding calmodulinopathy-a case report showcasing the utility of multifaceted treatments.

Background: Calmodulinopathies are adrenergically-induced life-threatening arrhythmias. Available therapies are disquietingly insufficient, especially for CALM-LQTS (calmodulinopathy-associated long QT syndrome). This case report illustrates a novel mutation in CALM-LQTS and its response to multimodality treatment strategies.

Case summary: The proband was the first child born to a nonconsanguineous Indian couple, a 26-year-old woman and a 30-year-old man. The child was delivered prematurely, and at birth, a functional 2:1 atrioventricular block was noted with sinus bradycardia with a corrected QT by Bazzet's of 716 ms. Clinical exome sequencing of the proband revealed a novel missense variant c.287A>G in exon 5 of the CALM3 gene in a heterozygous state, resulting in an Asp96Gly change. The OMIM phenotype associated with it is long QT syndrome 16 (#618782). Despite receiving a dose of 4.5 mg/kg/day of propranolol, the child still had a persistent long QTc. Mexiletine was started at the trial dose of 1.5 mg/kg/day, and after 1 h, QTc was reduced to 507 ms from 560 ms. After a left-cardiac sympathectomy, he remains asymptomatic after 1.3 years of follow-up with a QTc value of 490 ms.

Discussion: CALM3 pathogenic variants are gain-of-function variants mainly affecting amino acids residing in the Ca2-binding loops. Earlier data suggested the role of the Na_v1.5 channel in leading to persistent Na⁺ leaks resulting in LQTS. However, they only focused on LQTS-CALM1 and CALM2 models and did not include CALM3-related genes. Despite similarities, the precise impact of CaM on Na_v1.5 channels still needs to be defined as Ca_V1.2. The exact role of mexiletine is not fully understood.