#genetics #calcium-handling #channelopathies #CPVT #gene-therapy

CASQ2 (Calsequestrin-2)

Details

CASQ2 encodes calsequestrin-2 (CSQ2), the principal luminal Ca²⁺ buffer of the junctional sarcoplasmic reticulum (SR). It is a member of the quaternary calcium release unit — the macromolecular complex comprising RYR2 (ryanodine receptor), CASQ2, triadin (TRDN), and junctin (ASPH) — that governs SR Ca²⁺ release during excitation-contraction coupling. CASQ2 performs two distinct roles: it buffers free luminal Ca²⁺ (increasing SR storage capacity) and it tonically inhibits RYR2 in the closed state by signalling through triadin and junctin. Loss-of-function of CASQ2 disrupts both functions simultaneously, producing a more severe and earlier-onset CPVT phenotype than RYR2 mutations alone. CASQ2 mutations cause CPVT2, accounting for approximately 2–5% of all CPVT cases and representing the most clinically advanced gene therapy target in inherited arrhythmias.

Key Facts

Gene and Protein Biology

CASQ2 protein forms polymers at the lumenal face of the junctional SR terminal cisternae; polymerisation is Ca²⁺-dependent and necessary for both Ca²⁺ buffering and RYR2 inhibitory signalling. (sources/channelopathies-jaha-2025, rating: high)
The inhibitory connection to RYR2 is indirect: CASQ2 binds triadin (TRDN) and junctin (ASPH), which physically link CASQ2 to the cytoplasmic face of RYR2. Reduced CASQ2 → reduced triadin-mediated inhibitory tone on RYR2 → lower threshold for spontaneous Ca²⁺ release. (sources/channelopathies-jaha-2025, sources/genetics-va-fcvm-2022, rating: high/medium)
CASQ2 expression is epigenetically regulated as part of the broader Ca²⁺-handling protein network; CASQ2 is among the key targets identified in arrhythmia epigenetics frameworks. (sources/genetics-va-fcvm-2022)

CPVT2 — Clinical Phenotype

Epidemiology: CASQ2 loss-of-function causes CPVT2, accounting for 2–5% of CPVT cases — the second most common CPVT gene after RYR2. (sources/channelopathies-jaha-2025, sources/arrhythmia-genetics-mgenetik-2025, rating: high)
Inheritance: Primarily autosomal recessive — biallelic (homozygous or compound heterozygous) loss-of-function variants are required for full phenotypic expression.
Penetrance: Biallelic CASQ2 pathogenic variants confer 100% penetrance — the highest among CPVT-associated genes. Compare: RYR2 (CPVT1) 75–80%. (sources/cpvt-jcm-2024, rating: medium)
Phenotypic severity: Earlier onset, greater clinical severity, and greater beta-blocker resistance than CPVT1. Without calsequestrin buffering, SR Ca²⁺ mishandling occurs at lower Ca²⁺ loads and is less suppressible by sympatholysis alone. (sources/channelopathies-jaha-2025, sources/arrhythmia-genetics-mgenetik-2025)
Novel neurological manifestations: Recently identified in association with CASQ2 variants — extends the phenotypic spectrum beyond arrhythmia. (sources/cpvt-jcm-2024)
Clinical presentation: Identical to CPVT1 in character — adrenergically triggered bidirectional ventricular tachycardia progressing to polymorphic VT/VF at higher exercise loads. Syncope and SCD in childhood/adolescence during exertion or emotional stress.

Pathomechanism

CASQ2 LOF pathway: Misfolding, impaired Ca²⁺-dependent polymerisation, or protein absence → (1) reduced luminal Ca²⁺ buffering capacity → SR overloaded at lower Ca²⁺ content, and (2) loss of triadin/junctin-mediated RYR2 inhibitory signal → RYR2 opens spontaneously during diastole → Ca²⁺ leak → NCX extrudes Ca²⁺ generating an inward current (INCX) → delayed afterdepolarisation (DAD) → triggered action potential → bidirectional VT. (sources/channelopathies-jaha-2025, sources/membrane-potential-physrev-2021, rating: very high)
The two-hit mechanism (impaired buffering + impaired inhibitory signalling) explains why biallelic CASQ2 produces a more severe phenotype than heterozygous RYR2 gain-of-function; the threshold for spontaneous release is lower and the SR cannot increase its Ca²⁺ load as a compensatory reservoir.

ClinGen Gene-Disease Validity (2026)

CASQ2 AR → CPVT2: Definitive (curated 01/20/2021). Biallelic loss-of-function is definitively established as the mechanism of CPVT2. (sources/clingen-summary-2026-05-09, rating: high)
CASQ2 AD → CPVT2: Moderate only (01/20/2021). Autosomal dominant CASQ2-CPVT is supported by limited evidence — a de novo or heterozygous pathogenic CASQ2 variant alone does not confirm CPVT2 diagnosis at the same confidence level. Interpret AD CASQ2 variants cautiously. (sources/clingen-summary-2026-05-09)
CASQ2 → HCM: Disputed (05/10/2022). ClinGen GCEP disputes causality between CASQ2 and HCM; variants reported in HCM context should not be classified P/LP for HCM without extraordinary evidence. (sources/clingen-summary-2026-05-09)
See concepts/ClinGen-Gene-Disease-Validity for full framework.

Genetic Testing

CASQ2 is included in the ACMG Secondary Findings v3.1 list as a clinically actionable gene (CPVT panel: RYR2, CASQ2, TRDN, TECRL, CALM1). (sources/incident-gene-aha-2023, sources/consumer-genetictest-aha-2025, rating: high)
AHA 2020 genetic testing recommendation: CASQ2 testing recommended in all CPVT probands; cascade genetic testing in first-degree relatives once a P/LP variant is identified. (sources/genetic-test-aha-2020, rating: high)
In the Giudicessi diagnostic scorecard for CPVT pre-test probability, a pathogenic/likely pathogenic result in RYR2, CASQ2, TRDN, or CALM1-3 = +4 points (increases CPVT probability); a VUS = 0; a negative result in these genes = −1 point. (sources/cpvt-jcm-2024)

Relationship to TRDN and TECRL

TRDN (triadin): Anchors CASQ2 to the junctional SR membrane and directly connects it to RYR2. TRDN LOF → reduced RYR2, CASQ2, and junctin protein levels simultaneously → defective excitation-contraction coupling → CPVT phenotype (CPVT5, AR, ClinGen Definitive). (sources/genetics-va-fcvm-2022, rating: medium)
TECRL: Encodes an ER-localised oxidoreductase; homozygous pathogenic TECRL variants reduce both RYR2 and CASQ2 protein levels → aberrant Ca²⁺ handling. Produces a distinctive overlap phenotype of CPVT and LQTS (adrenergically triggered bidirectional VT + QT prolongation) — phenotypically distinct from CPVT1 and CPVT2. (sources/genetics-va-fcvm-2022, sources/channelopathies-jaha-2025)
The TRDN-CASQ2 interdependence was confirmed preclinically: individual AAV delivery of either CASQ2 or triadin alone eliminates arrhythmia in CASQ2-deficient models. (sources/aav-gene-therapy-arrhythmia-hr-2024, rating: high)

Gene Therapy — Most Advanced CPVT Target

CASQ2 is the most clinically advanced target for inherited arrhythmia gene therapy due to the small size of the CASQ2 coding sequence (fits within AAV packaging limits) and its clear mechanistic role.

AAV9-CASQ2 gene replacement (Denegri 2012): Single injection abolished adrenergically induced VT in CASQ2 knockout mice; effect sustained ≥3 months. (sources/gene-therapy-arrhythmia-2025, sources/cpvt-jcm-2024, rating: high)
AAV9-CASQ2 in knock-in model (Denegri 2014): Single injection in CASQ2-R33Q knock-in mice → >85% arrhythmia reduction sustained ≥1 year — demonstrating durable single-administration effect. (sources/gene-therapy-arrhythmia-2025, sources/aav-gene-therapy-arrhythmia-hr-2024)
CASQ2 overexpression for CPVT1: Exogenous CASQ2 delivery suppressed arrhythmias in RYR2-mediated CPVT1 mouse models (Santiago Castillo 2023). This pathway-directed strategy bypasses the fundamental obstacle that RYR2 (~15,000 nt) far exceeds AAV packaging capacity (~4.7 kb) — making CASQ2 overexpression a viable mechanism-based alternative for CPVT1. (sources/gene-therapy-arrhythmia-2025, sources/AAV-Gene-Delivery)
Allele-specific siRNA silencing (CASQ2-R33Q): ~60% mRNA and protein reduction; prevented arrhythmias in preclinical models (Bongianino 2017). Limitation: Required 3 injections every 2 weeks to maintain effect — not a durable single-dose therapy. (sources/gene-therapy-arrhythmia-2025, sources/Gene-Silencing-Therapy)
Engineered calmodulin delivery via AAV9: Modified calmodulin protein increased RYR2 refractoriness and prevented arrhythmic events in CASQ2-R33Q model. Mutation-agnostic approach but carries inherent proarrhythmic risk given that calmodulin gene mutations themselves cause severe channelopathies. (sources/gene-therapy-arrhythmia-2025, sources/aav-gene-therapy-arrhythmia-hr-2024)
CaMKII inhibitory peptide (AIP) via AAV9 (Bezzerides 2019): Mutation-agnostic pathway modulation; suppressed Ca²⁺ dysregulation and arrhythmias in CASQ2 KO mice. (sources/gene-therapy-arrhythmia-2025)
Triadin gene replacement: Also individually sufficient to eliminate arrhythmia in CASQ2-deficient models — consistent with TRDN-CASQ2 interdependence. (sources/aav-gene-therapy-arrhythmia-hr-2024)
See concepts/AAV-Gene-Delivery for packaging constraints and clinical translation challenges.

Contradictions / Open Questions

CASQ2 AD evidence — moderate, not definitive: ClinGen rates AD CASQ2 as "Moderate" evidence for CPVT (not Definitive). A heterozygous CASQ2 variant on a panel should not trigger management decisions equivalent to biallelic CASQ2 disease. The clinical significance of heterozygous CASQ2 variants remains an active area — single-copy LOF may confer mild phenotype or serve as a susceptibility allele, but full CPVT2 typically requires biallelic mutations. (sources/clingen-summary-2026-05-09, rating: high)
CASQ2 disputed for HCM: CASQ2 was previously reported in HCM cohorts, but ClinGen GCEP disputed this association in 2022. CASQ2 variants found incidentally in HCM patients should not be attributed as causal without extraordinary functional evidence. (sources/clingen-summary-2026-05-09)
CASQ2 overexpression as CPVT1 therapy — stoichiometry concern: Exogenous CASQ2 delivery in CPVT1 is conceptually compelling as it bypasses the unfixable RYR2 packaging problem. However, the therapeutic Ca²⁺-buffering and RYR2-inhibitory effects require appropriate stoichiometry with triadin and junctin — CASQ2 overexpression relative to the endogenous triadin/junctin levels could produce aberrant signalling. All data are preclinical; no human dosing or safety data exist. (sources/gene-therapy-arrhythmia-2025)
siRNA durability gap: Allele-specific siRNA for CASQ2-R33Q required repeated injections every 2 weeks, versus single-injection AAV9-CASQ2 replacement sustaining effect ≥1 year. Head-to-head durability comparison between siRNA and AAV replacement has not been performed; neither has human data. (sources/gene-therapy-arrhythmia-2025)
Engineered calmodulin safety — proarrhythmic risk: Modified calmodulin delivery reduces RYR2-driven Ca²⁺ release and was arrhythmia-suppressive in CASQ2 models. However, calmodulin mutations (CALM1/2/3) are among the most severe channelopathies — any modification that alters CaM function at Cav1.2 or other targets risks introducing a new arrhythmogenic mechanism. CaM-based strategies require comprehensive safety evaluation before clinical translation. (sources/gene-therapy-arrhythmia-2025, sources/aav-gene-therapy-arrhythmia-hr-2024)

Connections

Related to entities/CPVT
Related to entities/RYR2
Related to concepts/AAV-Gene-Delivery
Related to concepts/Gene-Silencing-Therapy
Related to concepts/CRISPR-Cas9-in-Channelopathies
Related to concepts/ClinGen-Gene-Disease-Validity
Related to concepts/Haploinsufficiency
Related to concepts/Incidental-Cardiovascular-Variants
Related to concepts/Epigenetics-Cardiac-Arrhythmia
Related to concepts/Calmodulinopathy