Novel CALM3 Variant Causing Calmodulinopathy With Variable Expressivity in a 4-Generation Family

Background: CaM (calmodulin), encoded by 3 separate genes (CALM1, CALM2, and CALM3), is a multifunctional Ca²⁺-binding protein involved in many signal transduction events including ion channel regulation. CaM variants may present with early-onset long QT syndrome (LQTS), catecholaminergic polymorphic ventricular tachycardia, or sudden cardiac death. Most reported variants occurred de novo. We identified a novel CALM3 variant, p.Asn138Lys (N138K), in a 4-generation family segregating with LQTS. The aim of this study was to elucidate its pathogenicity and to compare it with that of p.D130G-CaM - a variant associated with a severe LQTS phenotype. Methods: We performed whole exome sequencing for a large, 4-generation family affected by LQTS. To assess the effect of the detected CALM3 variant, the intrinsic Ca²⁺-binding affinity was measured by stoichiometric Ca²⁺titrations and equilibrium titrations. L-type Ca²⁺and slow delayed rectifier potassium currents (I_CaLand I_Ks) were recorded by whole-cell patch-clamp. Cav1.2 and Kv7.1 membrane expression were determined by optical fluorescence assays. Results: We identified 14 p.N138K-CaM carriers in a family where 2 sudden deaths occurred in children. Several members were only mildly affected compared with CaM-LQTS patients to date described in literature. The intrinsic Ca²⁺-binding affinity of the CaM C-terminal domain was 10-fold lower for p.N138K-CaM compared with wild-type-CaM. I_CaLinactivation was slowed in cells expressing p.N138K-CaM but less than in p.D130G-CaM cells. Unexpectedly, a larger I_Kscurrent density was observed in cells expressing p.N138K-CaM, but not for p.D130G-CaM, compared with wild-type-CaM. Conclusions: The p.N138K CALM3 variant impairs Ca²⁺-binding affinity of CaM and I_CaLinactivation but potentiates I_Ks. The variably expressed phenotype of this variant compared with previously published de novo LQTS-CaM variants is likely explained by a milder impairment of I_CaLinactivation combined with I_Ksaugmentation.