Calmodulinopathy: Functional Effects of CALM Mutations and Their Relationship With Clinical Phenotypes

Authors, Journal, Affiliations, Type, DOI

• Authors: Beatrice Badone, Carlotta Ronchi, Maria-Christina Kotta, Luca Sala, Alice Ghidoni, Lia Crotti†, Antonio Zaza†
• Journal: Frontiers in Cardiovascular Medicine, December 2018, Vol. 5, Article 176
• Affiliations: Department of Biotechnology and Bioscience, University of Milano-Bicocca; Center for Cardiac Arrhythmias of Genetic Origin and Laboratory of Cardiovascular Genetics, Istituto Auxologico Italiano, IRCCS, Milan, Italy
• Type: Review article
• DOI: https://doi.org/10.3389/fcvm.2018.00176

Overview

Calmodulinopathy is a recently described cardiac channelopathy caused by mutations in any of three CALM genes (CALM1, CALM2, CALM3) that encode identical calmodulin (CaM) protein. Despite a maximum mutant-to-wildtype ratio of 1:6 in heterozygous carriers, penetrance is remarkably high — a paradox explained by mutation-specific target interactions within the pre-bound CaM pool. Two principal arrhythmia phenotypes emerge: LQTS (from loss of Ca²⁺-dependent inactivation of Cav1.2, producing ICaL gain of function) and CPVT (from RyR2 destabilisation via altered CaM–RyR2 3D binding interface). The review covers CaM molecular biology, phenotype-mechanism linkage, hiPSC-CM validation, and therapeutic implications.

Keywords

Calmodulin mutations, ion channels, repolarization, Ca²⁺ handling, arrhythmia mechanisms

Key Takeaways

Calmodulin Biology and the Pre-Bound Pool

• CaM: 149 amino acids, 17 kDa; 100% sequence identity across vertebrates; encoded by CALM1/2/3 — all producing identical protein (triple genetic redundancy).
• Two lobes (N and C), each with 2 EF-hand Ca²⁺-binding domains (4 total). C-lobe has 10× higher Ca²⁺ affinity than N-lobe; C-lobe binding triggers cooperative increase in N-lobe affinity. At least 3 of 4 sites must be occupied ("holo-CaM") for target activation.
• In cardiomyocytes, 99% of total CaM is pre-bound to cellular proteins (~6 µM total; only ~50–100 nM free). Pre-bound pool is predominantly apo-CaM at diastolic Ca²⁺.
• Pre-association of CaM to channels at the preIQ region enables rapid CDI upon local Ca²⁺ rise — cytosolic bulk CaM cannot access the binding site fast enough during Ca²⁺ influx.
• Nine methionine residues per lobe provide conformational plasticity for diverse target binding.

CaM Modulation of Key Cardiac Channels

Cav1.2 (ICaL) — CDI and CDF

• Apo-CaM is constitutively pre-bound to the preIQ motif of Cav1.2 α1C subunit via its N-lobe.
• CDI: Upon Ca²⁺ entry through the open channel, C-lobe binds local Ca²⁺ → C-lobe affinity for the IQ motif increases → conformational change → inactivation particle occludes pore → CDI. This is the dominant mechanism of rapid ICaL decay during depolarisation.
• CDF: Operated by CaMKII phosphorylation of Cav1.2 (not direct CaM binding); two serine residues near EF-hand motif are the CaMKII substrates.

Kv7.1 (IKs)

• CaM pre-bound at helix A (C-lobe) at diastolic Ca²⁺. As Ca²⁺ rises: C-lobe dissociates from helix A → N-lobe reinforces binding at helix B → stabilises channel open state → IKs enhancement.
• PIP2 and CaM N-lobe compete at helix B — PLC-IP3 activation depletes PIP2 but may trigger IP3-Ca²⁺ release to activate CaM–IKs pathway as compensation.
• PIP2-dependent compensation may explain why LQTS calmodulin mutations that reduce C-lobe Ca²⁺ affinity do not detectably impair IKs.

Nav1.5 (INa)

• CaM binds IQ motif in a Ca²⁺-independent manner; stabilises Nav1.5 inactivated state.
• Loss of CaM binding → enhanced persistent INa (INaL/INaW). However, co-expression of WT + mutant CaM partially rescues — functional dominance for Nav1.5 is weaker than for Cav1.2.

RyR2 (SR Ca²⁺ Release Channel)

• CaM binds RyR2 with nanomolar Kd in both apo and holo forms → large pre-bound pool.
• Apo-CaM stabilises RyR2 closed state (Ca²⁺-independent) by allosterically reinforcing the "zipping" interaction between the N-terminal and central cytosolic RyR2 domains. This is the dominant CaM effect on RyR2 gating.
• Holo-CaM modulates RyR2 indirectly through CaMKII (Ser2814 phosphorylation → increased opening probability) — CaMKII is activated by Ca²⁺-CaM.
• Ca²⁺-independent apo-CaM inhibition explains why LQTS calmodulin mutations (reduced Ca²⁺ affinity) spare RyR2 — they disrupt Ca²⁺-dependent CDI at Cav1.2 without affecting apo-CaM–mediated RyR2 stabilisation.

LQTS Phenotype

• Mechanism: C-lobe Ca²⁺ affinity reduction → pre-bound CaM fails to sense Ca²⁺ at Cav1.2 mouth → CDI abolished → ICaL gain of function → APD prolongation → QT prolongation → EADs → TdP/VF
• High penetrance: A WT/mutant ratio of 7 (heterozygous) is sufficient to impair CDI — because mutant CaMs compete for the pre-bound pool at Cav1.2 IQ motifs.
• Key mutations (C-lobe EF III/IV residues): CALM1-p.D130G (also CALM3), CALM2-p.D96V, CALM1-p.F142L, CALM2-p.D130V, CALM1-p.E141G, CALM2-p.D132H, CALM1-p.D132V
• Mixed mutations also causing QT prolongation: CALM2-p.D132E, CALM2-p.Q136P (EF III/IV; reduced Ca²⁺ affinity; also associated with CPVT-type Ca²⁺ instability)
• hiPSC-CM validation (CALM1-p.F142L):
  • CDI severely impaired → APD prolonged, rate-adaptive APD shortening absent → loss of 1:1 response to fast pacing in large proportion of cells
  • Ca²⁺ transient amplitude increased (matched increase in Ca²⁺ influx); SR Ca²⁺ content normal; no spontaneous Ca²⁺ release events — confirms CDI loss, not SR instability, as primary mechanism
  • IKs unaffected; INaW reduced unexpectedly; CaMKII preserved/slightly enhanced (secondary to increased Ca²⁺ transients)
  • APD abnormalities reversed by ICaL blockade (confirming ICaL as causal)
• Treatment implication: Verapamil (ICaL blocker) shortens QT in hiPSC-CMs. Selective sustained ICaL blockade proposed as ideal (analogous to mexiletine for INaL in LQT3 — not yet clinically available).

CPVT Phenotype

• Mutations: CALM1-p.N98S, CALM1-p.N54I, CALM3-p.A103V
• Distinguishing from LQTS mutations: Smaller reduction in C-lobe Ca²⁺ affinity; CDI relatively preserved (confirmed for CALM3-p.A103V). These mutations do not primarily disrupt Ca²⁺ binding at Cav1.2.
• Mechanism: Mutant CaM alters the 3D CaM–RyR2 binding interface → destabilises RyR2 closed state → spontaneous Ca²⁺ release waves → NCX-mediated transient inward current (ITI) → DADs → bidirectional VT
• Unresolved: Whether mutant CaM affinity for RyR2 is increased or decreased is contradicted across studies. CALM3-p.A103V strongly increases Ca²⁺ release events with normal RyR2 affinity — suggesting conformational binding interface changes (not affinity per se) are the key mechanism.
• Treatment: Flecainide and carvedilol proposed by analogy to RYR2-CPVT. No calmodulinopathy-specific CPVT trial data as of 2018.

Mixed and IVF Phenotypes

• Some mutations (CALM2-p.D132E, CALM2-p.Q136P) produce QT prolongation + CPVT-like Ca²⁺ instability — likely ICaL CDI impairment plus secondary SR instability when homeostatic compensation is insufficient.
• CALM2-p.N98S and CALM1-p.N98S (same amino acid, different gene, identical protein product) → LQTS vs. CPVT phenotype in different reports — gene-independent phenotype confirmed; modifier factors remain unknown.
• CALM1-p.F90L → IVF only (mild QT prolongation during exercise recovery); impairs C-lobe conformational stability, CaM–RyR2 interaction, and SK channel function.

Negative Dominance — Summary

• For targets requiring pre-bound apo-CaM + Ca²⁺-C-lobe signalling (Cav1.2, IKs): strong negative dominance in LQTS mutations (reduced Ca²⁺ affinity of mutant is sufficient to impair CDI even at 1:7 ratio)
• For CaMKII (requires holo-CaM): not dominant — mutant CaM with reduced Ca²⁺ affinity forms fewer holo-CaM copies → WT CaM remains the dominant activator
• For RyR2 (apo-CaM–dependent inhibition): LQTS mutations spare; CPVT mutations disrupt via a Ca²⁺-affinity-independent conformational mechanism

Limitations of the Document

• 2018 review — no clinical trial data for any treatment strategy in calmodulinopathy; disease entity was newly described
• Patient cohorts extremely small worldwide at time of publication; mostly mechanistic/cellular data
• hiPSC-CM models lack T-tubule structure — CPVT studies require Ca²⁺ spark dynamics that these models cannot fully replicate; better suited for LQTS (APD/CDI)
• Mechanism of RyR2 destabilisation in CPVT calmodulinopathy unresolved — conflicting data from different labs

Key Concepts Mentioned

• concepts/Calmodulinopathy — primary topic of this review
• concepts/Cardiac-Action-Potential — CDI mechanism, APD prolongation
• concepts/iPSC-Derived-Cardiomyocytes — hiPSC-CM validation of CALM1-p.F142L
• concepts/Electrical-Remodeling — CaM as broad regulator of cardiac ion channel function

Key Entities Mentioned

• entities/Long-QT-Syndrome — LQTS phenotype of calmodulinopathy; CDI mechanism
• entities/CPVT — CPVT phenotype; calmodulinopathy CPVT subtype
• entities/RYR2 — CaM pre-bound regulation of RyR2 closed state; CPVT mutations disrupt via 3D interface
• entities/KCNQ1 — IKs modulation by CaM; spared in LQTS calmodulinopathy (PIP2 compensation hypothesised)

Wiki Pages Updated

• wiki/sources/calm-fcvm-2018.md — created
• wiki/concepts/Calmodulinopathy.md — created
• wiki/entities/CPVT.md — added Calmodulinopathy CPVT subtype to Pathophysiology section
• wiki/entities/Long-QT-Syndrome.md — expanded Calmodulinopathy LQTS section with CDI mechanism detail
• wiki/entities/RYR2.md — added CaM–RyR2 regulatory mechanism subsection