Human Calmodulin Mutations

Abstract

Fluxes of calcium (Ca²⁺) across cell membranes enable fast cellular responses. Calmodulin (CaM) senses local changes in Ca²⁺ concentration and relays the information to numerous interaction partners. The critical role of accurate Ca²⁺ signaling on cellular function is underscored by the fact that there are three independent CaM genes (CALM1-3) in the human genome. All three genes are functional and encode the exact same CaM protein. Moreover, CaM has a completely conserved amino acid sequence across all vertebrates. Given this degree of conservation, it was long thought that mutations in CaM were incompatible with life. It was therefore a big surprise when the first CaM mutations in humans were identified six years ago. Today, more than a dozen human CaM missense mutations have been described, all found in patients with severe cardiac arrhythmias. Biochemical studies have demonstrated differential effects on Ca²⁺ binding affinities for these CaM variants. Moreover, CaM regulation of central cardiac ion channels is impaired, including the voltage-gated Ca²⁺ channel, Ca_V1.2, and the sarcoplasmic reticulum Ca²⁺ release channel, ryanodine receptor isoform 2, RyR2. Currently, no non-cardiac phenotypes have been described for CaM variant carriers. However, sequencing of large human cohorts reveals a cumulative frequency of additional rare CaM mutations that raise the possibility of CaM variants not exclusively causing severe cardiac arrhythmias. Here, we provide an overview of the identified CaM variants and their known consequences for target regulation and cardiac disease phenotype. We discuss experimental data, patient genotypes and phenotypes as well as which questions remain open to understand this complexity.

Keywords: CALM1; CALM2; CALM3; CPVT; LQTS; calmodulin; calmodulinopathy; cardiac arrhythmia.

FIGURE 1

Human calmodulin mutations. (A) The calmodulin (CaM) protein has two lobes connected by a flexible α-helical linker. Each lobe has two EF-hands, which each can coordinate a Ca²⁺ ion (gray spheres), generating four Ca²⁺ binding sites. Residues highlighted in color and stick representation are mutated in cardiac arrhythmia patients, where they are associated with CPVT, LQTS, IVF, or a combination of CPVT and LQTS as indicated. The structure was visualized in PyMOL (PDB: 1CLL) (Chattopadhyaya et al., 1992). (B) CaM residues shown in dark gray stick representation are found mutated in the GnomAD database. GnomAD includes 277,364 alleles from 138,632 individuals (Lek et al., 2016). (C) Overview of mutations in each of the three human CALM genes, displayed as the translated protein with the initiator-Met as residue 1. Residues highlighted in brown participate in Ca²⁺ coordination. Published arrhythmogenic human variants are indicated by circles above the sequences, with colors indicating the disease phenotype. Dark gray circles below the sequence are individual allele variations observed in GnomeAD without a known phenotype. Amino acid variants are shown by letters, a slash (deletion), or a cross (frame-shift or premature stop codon). CPVT, catecholaminergic polymorphic ventricular tachycardia; LQTS, long QT syndrome; IVF, idiopathic ventricular fibrillation.