{"gene":"CASQ2","run_date":"2026-06-09T22:57:17","timeline":{"discoveries":[{"year":2001,"finding":"A missense mutation (D307H) in CASQ2 converts a negatively charged aspartic acid to a positively charged histidine in a highly negatively charged domain, likely disrupting Ca2+ binding. CASQ2 protein serves as the major Ca2+ reservoir within the sarcoplasmic reticulum (SR) of cardiac myocytes and is part of a protein complex that contains the ryanodine receptor (RyR2).","method":"Direct sequencing of CASQ2 in affected Bedouin families; mutation segregation analysis; biochemical inference from domain charge properties","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutation identified with full segregation in seven families, biochemical inference from domain properties, but no direct in vitro Ca2+-binding assay in this paper","pmids":["11704930"],"is_preprint":false},{"year":2006,"finding":"Casq2 deletion in mice causes striking increases in SR volume and near absence of the Casq2-binding proteins triadin-1 and junctin, without upregulation of other Ca2+-binding proteins. Under catecholamine exposure, Casq2-null myocytes show increased diastolic SR Ca2+ leak and premature spontaneous SR Ca2+ releases, leading to triggered beats and ventricular arrhythmias in vivo.","method":"Casq2 knockout mouse model; Ca2+ imaging; immunoblotting for binding partners (triadin-1, junctin); in vivo ECG telemetry; SR volume measurement","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout with multiple orthogonal readouts (SR volume, protein levels, Ca2+ imaging, in vivo arrhythmia), replicated across labs","pmids":["16932808"],"is_preprint":false},{"year":2006,"finding":"CASQ2 mutations G112+5X (truncation) and L167H expressed in rat myocytes both decreased SR Ca2+-storing capacity and reduced Ca2+ transient amplitude and spontaneous Ca2+ spark amplitude. The truncated CASQ2(G112+5X) did not bind Ca2+, whereas CASQ2(L167H) had normal Ca2+-binding properties. CASQ2(G112+5X) expression led to delayed afterdepolarizations upon isoproterenol exposure.","method":"In vitro characterization of CASQ2 mutants expressed in rat cardiomyocytes; Ca2+ imaging (sparks and transients); Ca2+-binding assay","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — direct in vitro Ca2+-binding assay plus functional expression in myocytes with multiple Ca2+ readouts, single lab but multiple orthogonal methods","pmids":["16908766"],"is_preprint":false},{"year":2007,"finding":"CASQ2 D307H missense and CASQ2-null mutations in mice caused identical consequences: reduced CASQ2 protein, increased calreticulin and RyR2 expression (at post-transcriptional level), reduced total SR Ca2+, prolonged Ca2+ release, and delayed Ca2+ reuptake. Under stress, elevated cytosolic Ca2+ and frequent spontaneous SR Ca2+ release occurred. Mg2+ (a RyR2 inhibitor) normalized myocyte Ca2+ cycling and decreased CPVT in mutant mice, indicating that RyR2 dysfunction is central to CASQ2-deficiency pathophysiology.","method":"Knock-in (D307H) and null mouse models; immunoblotting; Ca2+ imaging; pharmacological rescue with Mg2+; in vivo ECG","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — two independent mouse models showing identical results, multiple orthogonal methods including pharmacological epistasis, replicated findings","pmids":["17607358"],"is_preprint":false},{"year":2008,"finding":"CASQ2(DEL/G112+5X) disrupts CASQ2 polymerization required for high-capacity Ca2+ binding. CASQ2(R33Q) compromises CASQ2's ability to control RyR2 channel activity through CASQ2-RyR2 interaction, markedly lowering the luminal [Ca2+] threshold for Ca2+ release termination. FRET assays showed CASQ2-CASQ2 variant interactions, and CASQ2 interaction with triadin was evaluated. Local Ca2+ release terminated at the same free luminal [Ca2+] in control, WT CASQ2-overexpressing, and CASQ2(DEL) cells, suggesting declining [Ca]SR signals RyR2 closure.","method":"Expression of CASQ2 mutants in canine ventricular myocytes; Ca2+ imaging; FRET-based CASQ2 polymerization assay; triadin interaction assay; measurement of luminal Ca2+ thresholds","journal":"Biophysical journal","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — FRET for protein-protein interactions, Ca2+ imaging with mechanistic dissection, multiple orthogonal methods in a single rigorous study","pmids":["18469084"],"is_preprint":false},{"year":2005,"finding":"ANKRD1 (CARP) specifically interacts with CASQ2 in cardiac tissue extracts from neonatal piglets. Pull-down, blot-overlay, and co-immunoprecipitation assays confirmed direct and specific interaction. Mapping identified five non-overlapping CASQ2-binding sequences on ANKRD1 and three ANKRD1-binding peptides in CASQ2. Both proteins are co-enriched in cardiac Purkinje cells.","method":"Pull-down from heart tissue extracts; blot-overlay; co-immunoprecipitation; peptide mapping; immunohistochemistry","journal":"Journal of molecular and cellular cardiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP and pull-down with peptide mapping in a single lab; localization by IHC adds supporting evidence","pmids":["15698842"],"is_preprint":false},{"year":2010,"finding":"The CASQ2 K206N mutation creates an additional N-glycosylation site, resulting in hyperglycosylation and higher molecular weight protein. The mutation reduced Ca2+ binding capacity, altered aggregation state, and resulted in lower SR Ca2+ load (impaired caffeine response). Maximal specific [3H]ryanodine binding was increased in K206N-expressing myocytes, suggesting increased RyR2 open state probability, and spontaneous SR Ca2+ release rate was higher under basal and beta-adrenergic stimulation conditions. Interaction with triadin was unchanged.","method":"Expression in eukaryotic cell lines and neonatal mouse myocytes; immunoblot (MW shift); Ca2+ binding assay; caffeine-induced Ca2+ transients; [3H]ryanodine binding; spontaneous Ca2+ release imaging","journal":"Journal of molecular and cellular cardiology","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — multiple orthogonal methods (Ca2+ binding, ryanodine binding, Ca2+ imaging, glycosylation assay) in a single focused study","pmids":["20302875"],"is_preprint":false},{"year":2010,"finding":"CASQ2 wild-type and CASQ2 mutants (R33Q, F189L, D307H) modulate hERG channel function in Xenopus oocytes, with CASQ2 mutants modulating hERG differently from wild-type. Free Ca2+ measurements showed altered Ca2+ buffer capacity in the mutants, paralleled by changes in dynamic behavior of CASQ2-mutants compared to wild-type.","method":"Two-electrode voltage clamp in Xenopus oocytes expressing CASQ2 variants with hERG; free Ca2+ measurements; molecular dynamics simulations","journal":"Cellular physiology and biochemistry","confidence":"Medium","confidence_rationale":"Tier 2-3 / Weak — electrophysiology in oocyte system with Ca2+ measurements, single lab, heterologous expression system","pmids":["21063088"],"is_preprint":false},{"year":2011,"finding":"CASQ2(D307H) mutant protein expressed in CASQ2-null mice partially restored triadin-1 levels (which were decreased in null mice), but despite 2-fold expression relative to WT CASQ2, failed to increase SR Ca2+ load. CASQ2(D307H) myocytes showed slowed Ca2+ transient decay and exhibited spontaneous Ca2+ waves upon isoproterenol, similar to null myocytes, consistent with impaired Ca2+ buffering capacity and poor interaction with triadin-1 affecting RyR2 stability.","method":"Stable expression of D307H mutant in CASQ2-null mouse hearts; immunoblot for triadin-1; Ca2+ imaging (transients, waves); isoproterenol challenge; in vivo ECG","journal":"American journal of physiology. Heart and circulatory physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic rescue experiment with multiple Ca2+ and protein-level readouts in a single lab","pmids":["21984545"],"is_preprint":false},{"year":2012,"finding":"Crossbreeding CASQ2-null mice with mice overexpressing SERCA1a (skeletal SR Ca2+ATPase) caused early mortality, dilated cardiomyopathy, and increased apoptosis, with multiple periodic Ca2+ waves in diastole despite normal systolic Ca2+ transients. Similar results were obtained by crossing CASQ2-KO with phospholamban-KO mice. This genetic epistasis demonstrates that enhanced SR Ca2+ uptake combined with dysregulated RyR2s (from CASQ2 absence) causes sustained diastolic Ca2+ release leading to cardiomyopathy.","method":"Double knockout/overexpression mouse models; echocardiography; Ca2+ imaging in cardiomyocytes; apoptosis assays; muscle force measurements; genetic epistasis","journal":"Cardiovascular research","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis across two independent double-mutant strains with multiple orthogonal readouts (echocardiography, Ca2+ imaging, apoptosis, contractile function)","pmids":["23135969"],"is_preprint":false},{"year":2014,"finding":"AAV9-mediated delivery of wild-type CASQ2 into R33Q knock-in mice restored physiological expression and interaction of CASQ2, junctin, and triadin; rescued electrophysiological and ultrastructural abnormalities in calcium release units; and eliminated life-threatening arrhythmias for up to 1 year after a single injection.","method":"AAV9 gene transfer in CASQ2(R33Q/R33Q) knock-in mice; immunoblot/co-immunoprecipitation for CASQ2-junctin-triadin interactions; Ca2+ imaging; electron microscopy of calcium release units; in vivo arrhythmia monitoring","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 2 / Strong — gene replacement with rescue of molecular interactions and ultrastructure plus functional arrhythmia phenotype, multiple time points tested","pmids":["24888331"],"is_preprint":false},{"year":2015,"finding":"Double knockout (DKO) of both CASQ2 and HRC (histidine-rich Ca-binding protein) in mice alleviated catecholamine-dependent arrhythmia compared to CASQ2-KO alone, and reduced spontaneous Ca2+ waves and sparks. This indicates that CASQ2 and HRC modulate RyR2 in opposing ways: CASQ2 stabilizes RyR2 rendering it refractory during diastole, while HRC enhances RyR2 activity facilitating its recovery from refractoriness.","method":"Double knockout mouse model; in vivo arrhythmia telemetry; Ca2+ imaging (sparks, waves, transients); SR Ca2+ release restitution measurement; genetic epistasis","journal":"Cardiovascular research","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with double KO, multiple Ca2+ readouts and in vivo arrhythmia assessment revealing opposing functional roles","pmids":["26410369"],"is_preprint":false},{"year":2018,"finding":"CPVT phenotype in CASQ2-deficient mice is dependent on concurrent loss of Casq2 function in both the cardiac conduction system (CCS) and working cardiomyocytes. Restoration of Casq2 only in the CCS is sufficient to prevent CPVT. Resting heart rate depends on Casq2 expression only in the CCS and is also influenced by developmental history of Casq2 deficiency.","method":"Conditional cell-type-specific deletion and rescue mouse models (CCS-specific and working cardiomyocyte-specific); in vivo ECG telemetry; arrhythmia provocation","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional cell-type-specific genetic rescue experiments delineating tissue-autonomous roles of CASQ2, replicated across multiple conditional strains","pmids":["29452352"],"is_preprint":false},{"year":2018,"finding":"In CASQ2(R33Q/R33Q) atrial myocytes, reduced levels of the RyR2 stable subunits (Casq2, triadin, junctin) accompanied increased CaMKII expression, phospho-CaMKII, and CaMKII-mediated phospho-RyR2 (Ser2814), as well as increased SERCA and NCX1.1. CaMKII inhibition (KN93) reversed isoproterenol-enhanced Ca2+ sparks, Ca2+ waves, inward transient current (ITi), and membrane potential oscillations in R33Q atrial myocytes.","method":"CASQ2 R33Q knock-in mouse model; Ca2+ imaging (sparks, waves); patch-clamp; immunoblot for CaMKII, phospho-RyR2, SERCA, NCX1.1; pharmacological CaMKII inhibition with KN93","journal":"Frontiers in physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple biochemical and functional readouts in a single lab using pharmacological and knock-in mouse approaches","pmids":["30450052"],"is_preprint":false},{"year":2020,"finding":"Comparative analysis of R33Q and D307H CASQ2 knock-in mice revealed that despite similar clinical arrhythmia phenotypes, each point mutation causes distinct molecular mechanisms of CASQ2 degradation and evokes different specific adaptive cellular and molecular processes across four adaptive pathways.","method":"Comparative biochemical analysis of R33Q and D307H knock-in mouse hearts; protein quantification; analysis of multiple adaptive pathways","journal":"Journal of muscle research and cell motility","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — comparative analysis in two established knock-in models, single lab, limited methodological detail in abstract","pmids":["32902830"],"is_preprint":false},{"year":2016,"finding":"AAV9-mediated delivery of human wild-type CASQ2 into iPSC-derived cardiomyocytes from a patient carrying homozygous CASQ2-G112+5X restored physiological calsequestrin-2 protein expression, decreased delayed afterdepolarizations (DADs) upon adrenergic stimulation, re-established Ca2+ transient amplitude, and normalized Ca2+ spark density and duration.","method":"iPSC-derived cardiomyocytes from CPVT2 patient; AAV9 gene delivery; patch-clamp (DADs); Ca2+ imaging (transients, sparks); immunoblot","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — human patient-specific iPSC model with gene replacement rescue, multiple Ca2+ functional readouts, single lab","pmids":["27711080"],"is_preprint":false}],"current_model":"CASQ2 (cardiac calsequestrin 2) is the major Ca2+-buffering/storage protein in the junctional sarcoplasmic reticulum (SR) of cardiac myocytes, where it forms a regulatory complex with RyR2, triadin, and junctin; CASQ2 provides a releasable Ca2+ reservoir and directly controls luminal Ca2+-dependent RyR2 gating by stabilizing RyR2 in a refractory state during diastole, such that loss or mutation of CASQ2 reduces triadin/junctin levels, alters SR volume, elevates diastolic SR Ca2+ leak via dysregulated RyR2, and—exacerbated by catecholaminergic stimulation and CaMKII-mediated RyR2 phosphorylation—triggers spontaneous Ca2+ releases and delayed afterdepolarizations that underlie catecholaminergic polymorphic ventricular tachycardia (CPVT2)."},"narrative":{"mechanistic_narrative":"CASQ2 is the major Ca2+-storage and -buffering protein of the junctional sarcoplasmic reticulum (SR) in cardiac myocytes, where it provides a releasable luminal Ca2+ reservoir and governs ryanodine-receptor (RyR2)-mediated Ca2+ release [PMID:11704930, PMID:16932808]. Its Ca2+-holding capacity depends on polymerization of CASQ2 monomers across a highly negatively charged domain; mutations that disrupt this charge or assembly (e.g. D307H, the G112+5X truncation, K206N hyperglycosylation) abolish or reduce Ca2+ binding and lower SR Ca2+ load [PMID:11704930, PMID:16908766, PMID:20302875]. Beyond passive buffering, CASQ2 acts as a luminal Ca2+ sensor that stabilizes RyR2 in a refractory, closed state during diastole: the R33Q mutation lowers the luminal Ca2+ threshold for terminating Ca2+ release without impairing Ca2+ binding, demonstrating a direct regulatory role at the CASQ2-RyR2 interface [PMID:18469084]. CASQ2 functions within a junctional complex with triadin-1 and junctin, whose levels collapse when CASQ2 is lost or mutated, and restoring CASQ2 re-establishes this complex and calcium-release-unit ultrastructure [PMID:16932808, PMID:21984545, PMID:24888331]. Loss or mutation of CASQ2 therefore enlarges SR volume, destabilizes RyR2, and—under catecholaminergic stress and CaMKII-mediated RyR2 phosphorylation—produces diastolic SR Ca2+ leak, spontaneous Ca2+ waves, and delayed afterdepolarizations that trigger arrhythmia [PMID:16932808, PMID:17607358, PMID:30450052]. CASQ2 mutations cause catecholaminergic polymorphic ventricular tachycardia, and the arrhythmic phenotype requires CASQ2 deficiency in the cardiac conduction system, with AAV9-mediated CASQ2 gene replacement rescuing the molecular complex and eliminating arrhythmia in mouse and patient iPSC-cardiomyocyte models [PMID:11704930, PMID:29452352, PMID:24888331, PMID:27711080].","teleology":[{"year":2001,"claim":"Establishing CASQ2 as a CPVT disease gene answered whether an SR Ca2+-storage protein, rather than RyR2 itself, could underlie inherited arrhythmia, implicating SR Ca2+ buffering in arrhythmogenesis.","evidence":"Direct sequencing and segregation of the D307H missense mutation in Bedouin families, with biochemical inference from charged-domain disruption","pmids":["11704930"],"confidence":"Medium","gaps":["No direct in vitro Ca2+-binding assay in this study","Mechanistic link between disrupted buffering and arrhythmia not yet demonstrated"]},{"year":2006,"claim":"A Casq2-null mouse revealed that CASQ2 loss reorganizes the junctional SR and destabilizes its protein partners, defining the molecular consequences of CASQ2 deficiency.","evidence":"Knockout mouse with SR volume measurement, immunoblot for triadin-1/junctin, Ca2+ imaging, and in vivo ECG telemetry","pmids":["16932808"],"confidence":"High","gaps":["Does not separate buffering loss from RyR2 regulatory effects","Tissue-specific contribution of CASQ2 loss not addressed"]},{"year":2006,"claim":"Comparing a non-Ca2+-binding truncation with a normally-binding point mutant began to dissociate CASQ2's Ca2+-buffering capacity from other functional defects driving arrhythmia.","evidence":"Expression of G112+5X and L167H mutants in rat cardiomyocytes with Ca2+-binding assay and spark/transient imaging","pmids":["16908766"],"confidence":"High","gaps":["Mechanism by which L167H impairs function despite normal Ca2+ binding unresolved","Heterologous rat myocyte context"]},{"year":2007,"claim":"Knock-in (D307H) and null models, plus pharmacological RyR2 inhibition, established that RyR2 dysfunction is the central downstream effector of CASQ2 deficiency.","evidence":"Two mouse models with immunoblotting, Ca2+ imaging, Mg2+ rescue, and in vivo ECG","pmids":["17607358"],"confidence":"High","gaps":["Post-transcriptional upregulation of RyR2/calreticulin mechanism not defined","Did not isolate luminal sensing from buffering"]},{"year":2008,"claim":"Dissecting R33Q versus polymerization-defective mutants distinguished CASQ2's RyR2 luminal-gating control from its high-capacity Ca2+ buffering, defining two separable functions.","evidence":"Mutant expression in canine myocytes with FRET polymerization assays, triadin interaction assays, and luminal Ca2+ threshold measurement","pmids":["18469084"],"confidence":"High","gaps":["Structural basis of CASQ2-RyR2 luminal coupling not resolved","FRET in heterologous expression"]},{"year":2010,"claim":"The K206N mutation linked an additional N-glycosylation event to altered CASQ2 aggregation, reduced SR load, and increased RyR2 open probability, broadening the mutational mechanisms beyond charge disruption.","evidence":"Expression in cell lines and neonatal mouse myocytes with glycosylation/MW immunoblot, Ca2+- and [3H]ryanodine-binding assays, and Ca2+ imaging","pmids":["20302875"],"confidence":"High","gaps":["Whether glycosylation drives mislocalization not established","Triadin interaction unchanged, leaving RyR2 effect mechanism partly open"]},{"year":2011,"claim":"Re-expressing D307H in null hearts showed that partial restoration of triadin-1 cannot rescue SR Ca2+ load, confirming impaired buffering as an intrinsic D307H defect.","evidence":"Stable D307H expression in CASQ2-null mouse hearts with triadin-1 immunoblot, Ca2+ imaging, and isoproterenol challenge","pmids":["21984545"],"confidence":"Medium","gaps":["Single lab rescue model","Relative contribution of buffering versus triadin binding to phenotype not quantified"]},{"year":2012,"claim":"Genetic epistasis with SERCA1a overexpression or phospholamban deletion demonstrated that enhanced SR Ca2+ uptake combined with dysregulated RyR2 produces sustained diastolic Ca2+ release and cardiomyopathy.","evidence":"Double mutant mouse strains with echocardiography, Ca2+ imaging, apoptosis assays, and contractile measurements","pmids":["23135969"],"confidence":"High","gaps":["Does not address whether human CPVT progresses to cardiomyopathy via this route"]},{"year":2015,"claim":"Double knockout of CASQ2 and HRC revealed opposing modulation of RyR2—CASQ2 stabilizing the refractory state while HRC promotes recovery—refining the model of luminal control of RyR2.","evidence":"CASQ2/HRC double knockout mice with arrhythmia telemetry, Ca2+ imaging, and SR Ca2+ release restitution measurement","pmids":["26410369"],"confidence":"High","gaps":["Molecular basis of CASQ2 versus HRC opposing effects on RyR2 not structurally defined"]},{"year":2018,"claim":"Cell-type-specific deletion and rescue established that CASQ2 function in the cardiac conduction system is necessary and sufficient for the CPVT phenotype, identifying tissue-autonomous roles.","evidence":"Conditional CCS-specific and cardiomyocyte-specific deletion/rescue mice with in vivo ECG and arrhythmia provocation","pmids":["29452352"],"confidence":"High","gaps":["Cellular basis of conduction-system-specific susceptibility unresolved"]},{"year":2018,"claim":"Atrial myocyte analysis connected CASQ2 deficiency to CaMKII activation and CaMKII-mediated RyR2 Ser2814 phosphorylation as a driver of triggered activity.","evidence":"R33Q knock-in mice with Ca2+ imaging, patch-clamp, immunoblot for CaMKII/phospho-RyR2, and KN93 inhibition","pmids":["30450052"],"confidence":"Medium","gaps":["Whether CaMKII activation is cause or consequence of altered Ca2+ handling not resolved","Single lab"]},{"year":2020,"claim":"Comparing R33Q and D307H knock-ins showed that distinct mutations trigger distinct degradation routes and adaptive pathways despite convergent arrhythmia phenotypes.","evidence":"Comparative biochemical analysis of two knock-in mouse hearts with protein quantification across adaptive pathways","pmids":["32902830"],"confidence":"Medium","gaps":["Limited methodological detail","Functional significance of distinct adaptive pathways not tested"]},{"year":2016,"claim":"AAV9 CASQ2 gene replacement in patient iPSC-cardiomyocytes and in R33Q knock-in mice established that restoring CASQ2 rescues the junctional complex, ultrastructure, and arrhythmia, providing therapeutic proof of concept.","evidence":"iPSC-derived cardiomyocytes and R33Q mice with AAV9 delivery, co-IP for CASQ2-junctin-triadin, Ca2+ imaging, electron microscopy, patch-clamp, and arrhythmia monitoring","pmids":["27711080","24888331"],"confidence":"Medium","gaps":["Durability and dose requirements in human heart not established","Recessive versus dominant mutation responses may differ"]},{"year":null,"claim":"The atomic structure of the CASQ2-RyR2-triadin-junctin junctional complex and the precise mechanism by which declining luminal Ca2+ is transduced to RyR2 closure remain undefined.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No high-resolution structure of the CASQ2-RyR2 interface in the corpus","Mechanism of luminal Ca2+-dependent RyR2 refractoriness not structurally resolved","ANKRD1-CASQ2 interaction function not characterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0,2,6]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[4,11]},{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[4]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[0,1]}],"pathway":[{"term_id":"R-HSA-397014","term_label":"Muscle contraction","supporting_discovery_ids":[1,3]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,2]}],"complexes":["RyR2-triadin-junctin junctional SR complex","calcium release unit"],"partners":["RYR2","TRDN","ASPH","ANKRD1","HRC","CASQ2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O14958","full_name":"Calsequestrin-2","aliases":["Calsequestrin, cardiac muscle isoform"],"length_aa":399,"mass_kda":46.4,"function":"Calsequestrin is a high-capacity, moderate affinity, calcium-binding protein and thus acts as an internal calcium store in muscle. Calcium ions are bound by clusters of acidic residues at the protein surface, especially at the interface between subunits. Can bind around 60 Ca(2+) ions. Regulates the release of lumenal Ca(2+) via the calcium release channel RYR2; this plays an important role in triggering muscle contraction. Plays a role in excitation-contraction coupling in the heart and in regulating the rate of heart beats","subcellular_location":"Sarcoplasmic reticulum lumen","url":"https://www.uniprot.org/uniprotkb/O14958/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CASQ2","classification":"Not Classified","n_dependent_lines":7,"n_total_lines":1208,"dependency_fraction":0.005794701986754967},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CASQ2","total_profiled":1310},"omim":[{"mim_id":"617242","title":"TRANS-2,3-ENOYL-CoA REDUCTASE-LIKE PROTEIN; TECRL","url":"https://www.omim.org/entry/617242"},{"mim_id":"615441","title":"CARDIAC ARRHYTHMIA SYNDROME, WITH OR WITHOUT SKELETAL MUSCLE WEAKNESS; CARDAR","url":"https://www.omim.org/entry/615441"},{"mim_id":"614916","title":"VENTRICULAR TACHYCARDIA, CATECHOLAMINERGIC POLYMORPHIC, 4; CPVT4","url":"https://www.omim.org/entry/614916"},{"mim_id":"614021","title":"VENTRICULAR TACHYCARDIA, CATECHOLAMINERGIC POLYMORPHIC, 3; CPVT3","url":"https://www.omim.org/entry/614021"},{"mim_id":"611938","title":"VENTRICULAR TACHYCARDIA, CATECHOLAMINERGIC POLYMORPHIC, 2; CPVT2","url":"https://www.omim.org/entry/611938"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"heart muscle","ntpm":989.2}],"url":"https://www.proteinatlas.org/search/CASQ2"},"hgnc":{"alias_symbol":["PDIB2"],"prev_symbol":[]},"alphafold":{"accession":"O14958","domains":[{"cath_id":"3.40.30.10","chopping":"36-143","consensus_level":"high","plddt":95.1005,"start":36,"end":143},{"cath_id":"3.40.30.10","chopping":"147-245","consensus_level":"high","plddt":97.1525,"start":147,"end":245},{"cath_id":"3.40.30.10","chopping":"250-368","consensus_level":"high","plddt":95.2728,"start":250,"end":368}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O14958","model_url":"https://alphafold.ebi.ac.uk/files/AF-O14958-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O14958-F1-predicted_aligned_error_v6.png","plddt_mean":90.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CASQ2","jax_strain_url":"https://www.jax.org/strain/search?query=CASQ2"},"sequence":{"accession":"O14958","fasta_url":"https://rest.uniprot.org/uniprotkb/O14958.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O14958/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O14958"}},"corpus_meta":[{"pmid":"11704930","id":"PMC_11704930","title":"A missense mutation in a highly conserved region of CASQ2 is associated with autosomal recessive catecholamine-induced polymorphic ventricular tachycardia in Bedouin families from Israel.","date":"2001","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/11704930","citation_count":495,"is_preprint":false},{"pmid":"16932808","id":"PMC_16932808","title":"Casq2 deletion causes sarcoplasmic reticulum volume increase, premature Ca2+ release, and catecholaminergic polymorphic ventricular tachycardia.","date":"2006","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/16932808","citation_count":383,"is_preprint":false},{"pmid":"16908766","id":"PMC_16908766","title":"Clinical phenotype and functional characterization of CASQ2 mutations associated with catecholaminergic polymorphic ventricular tachycardia.","date":"2006","source":"Circulation","url":"https://pubmed.ncbi.nlm.nih.gov/16908766","citation_count":164,"is_preprint":false},{"pmid":"17607358","id":"PMC_17607358","title":"Calsequestrin 2 (CASQ2) mutations increase expression of calreticulin and ryanodine receptors, causing catecholaminergic polymorphic ventricular tachycardia.","date":"2007","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/17607358","citation_count":150,"is_preprint":false},{"pmid":"24888331","id":"PMC_24888331","title":"Single delivery of an adeno-associated viral construct to transfer the CASQ2 gene to knock-in mice affected by catecholaminergic polymorphic ventricular tachycardia is able to cure the disease from birth to advanced age.","date":"2014","source":"Circulation","url":"https://pubmed.ncbi.nlm.nih.gov/24888331","citation_count":87,"is_preprint":false},{"pmid":"18469084","id":"PMC_18469084","title":"Modulation of SR Ca release by luminal Ca and calsequestrin in cardiac myocytes: effects of CASQ2 mutations linked to sudden cardiac death.","date":"2008","source":"Biophysical journal","url":"https://pubmed.ncbi.nlm.nih.gov/18469084","citation_count":82,"is_preprint":false},{"pmid":"12732448","id":"PMC_12732448","title":"A missense mutation in the CASQ2 gene is associated with autosomal-recessive catecholamine-induced polymorphic ventricular tachycardia.","date":"2003","source":"Trends in cardiovascular medicine","url":"https://pubmed.ncbi.nlm.nih.gov/12732448","citation_count":50,"is_preprint":false},{"pmid":"27711080","id":"PMC_27711080","title":"Adeno-associated virus-mediated CASQ2 delivery rescues phenotypic alterations in a patient-specific model of recessive catecholaminergic polymorphic ventricular tachycardia.","date":"2016","source":"Cell death & disease","url":"https://pubmed.ncbi.nlm.nih.gov/27711080","citation_count":50,"is_preprint":false},{"pmid":"21685173","id":"PMC_21685173","title":"Common variants in CASQ2, GPD1L, and NOS1AP are significantly associated with risk of sudden death in patients with coronary artery disease.","date":"2011","source":"Circulation. 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CASQ2 protein serves as the major Ca2+ reservoir within the sarcoplasmic reticulum (SR) of cardiac myocytes and is part of a protein complex that contains the ryanodine receptor (RyR2).\",\n      \"method\": \"Direct sequencing of CASQ2 in affected Bedouin families; mutation segregation analysis; biochemical inference from domain charge properties\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutation identified with full segregation in seven families, biochemical inference from domain properties, but no direct in vitro Ca2+-binding assay in this paper\",\n      \"pmids\": [\"11704930\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Casq2 deletion in mice causes striking increases in SR volume and near absence of the Casq2-binding proteins triadin-1 and junctin, without upregulation of other Ca2+-binding proteins. Under catecholamine exposure, Casq2-null myocytes show increased diastolic SR Ca2+ leak and premature spontaneous SR Ca2+ releases, leading to triggered beats and ventricular arrhythmias in vivo.\",\n      \"method\": \"Casq2 knockout mouse model; Ca2+ imaging; immunoblotting for binding partners (triadin-1, junctin); in vivo ECG telemetry; SR volume measurement\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout with multiple orthogonal readouts (SR volume, protein levels, Ca2+ imaging, in vivo arrhythmia), replicated across labs\",\n      \"pmids\": [\"16932808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"CASQ2 mutations G112+5X (truncation) and L167H expressed in rat myocytes both decreased SR Ca2+-storing capacity and reduced Ca2+ transient amplitude and spontaneous Ca2+ spark amplitude. The truncated CASQ2(G112+5X) did not bind Ca2+, whereas CASQ2(L167H) had normal Ca2+-binding properties. CASQ2(G112+5X) expression led to delayed afterdepolarizations upon isoproterenol exposure.\",\n      \"method\": \"In vitro characterization of CASQ2 mutants expressed in rat cardiomyocytes; Ca2+ imaging (sparks and transients); Ca2+-binding assay\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — direct in vitro Ca2+-binding assay plus functional expression in myocytes with multiple Ca2+ readouts, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"16908766\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"CASQ2 D307H missense and CASQ2-null mutations in mice caused identical consequences: reduced CASQ2 protein, increased calreticulin and RyR2 expression (at post-transcriptional level), reduced total SR Ca2+, prolonged Ca2+ release, and delayed Ca2+ reuptake. Under stress, elevated cytosolic Ca2+ and frequent spontaneous SR Ca2+ release occurred. Mg2+ (a RyR2 inhibitor) normalized myocyte Ca2+ cycling and decreased CPVT in mutant mice, indicating that RyR2 dysfunction is central to CASQ2-deficiency pathophysiology.\",\n      \"method\": \"Knock-in (D307H) and null mouse models; immunoblotting; Ca2+ imaging; pharmacological rescue with Mg2+; in vivo ECG\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two independent mouse models showing identical results, multiple orthogonal methods including pharmacological epistasis, replicated findings\",\n      \"pmids\": [\"17607358\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"CASQ2(DEL/G112+5X) disrupts CASQ2 polymerization required for high-capacity Ca2+ binding. CASQ2(R33Q) compromises CASQ2's ability to control RyR2 channel activity through CASQ2-RyR2 interaction, markedly lowering the luminal [Ca2+] threshold for Ca2+ release termination. FRET assays showed CASQ2-CASQ2 variant interactions, and CASQ2 interaction with triadin was evaluated. Local Ca2+ release terminated at the same free luminal [Ca2+] in control, WT CASQ2-overexpressing, and CASQ2(DEL) cells, suggesting declining [Ca]SR signals RyR2 closure.\",\n      \"method\": \"Expression of CASQ2 mutants in canine ventricular myocytes; Ca2+ imaging; FRET-based CASQ2 polymerization assay; triadin interaction assay; measurement of luminal Ca2+ thresholds\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — FRET for protein-protein interactions, Ca2+ imaging with mechanistic dissection, multiple orthogonal methods in a single rigorous study\",\n      \"pmids\": [\"18469084\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"ANKRD1 (CARP) specifically interacts with CASQ2 in cardiac tissue extracts from neonatal piglets. Pull-down, blot-overlay, and co-immunoprecipitation assays confirmed direct and specific interaction. Mapping identified five non-overlapping CASQ2-binding sequences on ANKRD1 and three ANKRD1-binding peptides in CASQ2. Both proteins are co-enriched in cardiac Purkinje cells.\",\n      \"method\": \"Pull-down from heart tissue extracts; blot-overlay; co-immunoprecipitation; peptide mapping; immunohistochemistry\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP and pull-down with peptide mapping in a single lab; localization by IHC adds supporting evidence\",\n      \"pmids\": [\"15698842\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The CASQ2 K206N mutation creates an additional N-glycosylation site, resulting in hyperglycosylation and higher molecular weight protein. The mutation reduced Ca2+ binding capacity, altered aggregation state, and resulted in lower SR Ca2+ load (impaired caffeine response). Maximal specific [3H]ryanodine binding was increased in K206N-expressing myocytes, suggesting increased RyR2 open state probability, and spontaneous SR Ca2+ release rate was higher under basal and beta-adrenergic stimulation conditions. Interaction with triadin was unchanged.\",\n      \"method\": \"Expression in eukaryotic cell lines and neonatal mouse myocytes; immunoblot (MW shift); Ca2+ binding assay; caffeine-induced Ca2+ transients; [3H]ryanodine binding; spontaneous Ca2+ release imaging\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — multiple orthogonal methods (Ca2+ binding, ryanodine binding, Ca2+ imaging, glycosylation assay) in a single focused study\",\n      \"pmids\": [\"20302875\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CASQ2 wild-type and CASQ2 mutants (R33Q, F189L, D307H) modulate hERG channel function in Xenopus oocytes, with CASQ2 mutants modulating hERG differently from wild-type. Free Ca2+ measurements showed altered Ca2+ buffer capacity in the mutants, paralleled by changes in dynamic behavior of CASQ2-mutants compared to wild-type.\",\n      \"method\": \"Two-electrode voltage clamp in Xenopus oocytes expressing CASQ2 variants with hERG; free Ca2+ measurements; molecular dynamics simulations\",\n      \"journal\": \"Cellular physiology and biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Weak — electrophysiology in oocyte system with Ca2+ measurements, single lab, heterologous expression system\",\n      \"pmids\": [\"21063088\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"CASQ2(D307H) mutant protein expressed in CASQ2-null mice partially restored triadin-1 levels (which were decreased in null mice), but despite 2-fold expression relative to WT CASQ2, failed to increase SR Ca2+ load. CASQ2(D307H) myocytes showed slowed Ca2+ transient decay and exhibited spontaneous Ca2+ waves upon isoproterenol, similar to null myocytes, consistent with impaired Ca2+ buffering capacity and poor interaction with triadin-1 affecting RyR2 stability.\",\n      \"method\": \"Stable expression of D307H mutant in CASQ2-null mouse hearts; immunoblot for triadin-1; Ca2+ imaging (transients, waves); isoproterenol challenge; in vivo ECG\",\n      \"journal\": \"American journal of physiology. Heart and circulatory physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic rescue experiment with multiple Ca2+ and protein-level readouts in a single lab\",\n      \"pmids\": [\"21984545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Crossbreeding CASQ2-null mice with mice overexpressing SERCA1a (skeletal SR Ca2+ATPase) caused early mortality, dilated cardiomyopathy, and increased apoptosis, with multiple periodic Ca2+ waves in diastole despite normal systolic Ca2+ transients. Similar results were obtained by crossing CASQ2-KO with phospholamban-KO mice. This genetic epistasis demonstrates that enhanced SR Ca2+ uptake combined with dysregulated RyR2s (from CASQ2 absence) causes sustained diastolic Ca2+ release leading to cardiomyopathy.\",\n      \"method\": \"Double knockout/overexpression mouse models; echocardiography; Ca2+ imaging in cardiomyocytes; apoptosis assays; muscle force measurements; genetic epistasis\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis across two independent double-mutant strains with multiple orthogonal readouts (echocardiography, Ca2+ imaging, apoptosis, contractile function)\",\n      \"pmids\": [\"23135969\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"AAV9-mediated delivery of wild-type CASQ2 into R33Q knock-in mice restored physiological expression and interaction of CASQ2, junctin, and triadin; rescued electrophysiological and ultrastructural abnormalities in calcium release units; and eliminated life-threatening arrhythmias for up to 1 year after a single injection.\",\n      \"method\": \"AAV9 gene transfer in CASQ2(R33Q/R33Q) knock-in mice; immunoblot/co-immunoprecipitation for CASQ2-junctin-triadin interactions; Ca2+ imaging; electron microscopy of calcium release units; in vivo arrhythmia monitoring\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — gene replacement with rescue of molecular interactions and ultrastructure plus functional arrhythmia phenotype, multiple time points tested\",\n      \"pmids\": [\"24888331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Double knockout (DKO) of both CASQ2 and HRC (histidine-rich Ca-binding protein) in mice alleviated catecholamine-dependent arrhythmia compared to CASQ2-KO alone, and reduced spontaneous Ca2+ waves and sparks. This indicates that CASQ2 and HRC modulate RyR2 in opposing ways: CASQ2 stabilizes RyR2 rendering it refractory during diastole, while HRC enhances RyR2 activity facilitating its recovery from refractoriness.\",\n      \"method\": \"Double knockout mouse model; in vivo arrhythmia telemetry; Ca2+ imaging (sparks, waves, transients); SR Ca2+ release restitution measurement; genetic epistasis\",\n      \"journal\": \"Cardiovascular research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with double KO, multiple Ca2+ readouts and in vivo arrhythmia assessment revealing opposing functional roles\",\n      \"pmids\": [\"26410369\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"CPVT phenotype in CASQ2-deficient mice is dependent on concurrent loss of Casq2 function in both the cardiac conduction system (CCS) and working cardiomyocytes. Restoration of Casq2 only in the CCS is sufficient to prevent CPVT. Resting heart rate depends on Casq2 expression only in the CCS and is also influenced by developmental history of Casq2 deficiency.\",\n      \"method\": \"Conditional cell-type-specific deletion and rescue mouse models (CCS-specific and working cardiomyocyte-specific); in vivo ECG telemetry; arrhythmia provocation\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional cell-type-specific genetic rescue experiments delineating tissue-autonomous roles of CASQ2, replicated across multiple conditional strains\",\n      \"pmids\": [\"29452352\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In CASQ2(R33Q/R33Q) atrial myocytes, reduced levels of the RyR2 stable subunits (Casq2, triadin, junctin) accompanied increased CaMKII expression, phospho-CaMKII, and CaMKII-mediated phospho-RyR2 (Ser2814), as well as increased SERCA and NCX1.1. CaMKII inhibition (KN93) reversed isoproterenol-enhanced Ca2+ sparks, Ca2+ waves, inward transient current (ITi), and membrane potential oscillations in R33Q atrial myocytes.\",\n      \"method\": \"CASQ2 R33Q knock-in mouse model; Ca2+ imaging (sparks, waves); patch-clamp; immunoblot for CaMKII, phospho-RyR2, SERCA, NCX1.1; pharmacological CaMKII inhibition with KN93\",\n      \"journal\": \"Frontiers in physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple biochemical and functional readouts in a single lab using pharmacological and knock-in mouse approaches\",\n      \"pmids\": [\"30450052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Comparative analysis of R33Q and D307H CASQ2 knock-in mice revealed that despite similar clinical arrhythmia phenotypes, each point mutation causes distinct molecular mechanisms of CASQ2 degradation and evokes different specific adaptive cellular and molecular processes across four adaptive pathways.\",\n      \"method\": \"Comparative biochemical analysis of R33Q and D307H knock-in mouse hearts; protein quantification; analysis of multiple adaptive pathways\",\n      \"journal\": \"Journal of muscle research and cell motility\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — comparative analysis in two established knock-in models, single lab, limited methodological detail in abstract\",\n      \"pmids\": [\"32902830\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"AAV9-mediated delivery of human wild-type CASQ2 into iPSC-derived cardiomyocytes from a patient carrying homozygous CASQ2-G112+5X restored physiological calsequestrin-2 protein expression, decreased delayed afterdepolarizations (DADs) upon adrenergic stimulation, re-established Ca2+ transient amplitude, and normalized Ca2+ spark density and duration.\",\n      \"method\": \"iPSC-derived cardiomyocytes from CPVT2 patient; AAV9 gene delivery; patch-clamp (DADs); Ca2+ imaging (transients, sparks); immunoblot\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — human patient-specific iPSC model with gene replacement rescue, multiple Ca2+ functional readouts, single lab\",\n      \"pmids\": [\"27711080\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CASQ2 (cardiac calsequestrin 2) is the major Ca2+-buffering/storage protein in the junctional sarcoplasmic reticulum (SR) of cardiac myocytes, where it forms a regulatory complex with RyR2, triadin, and junctin; CASQ2 provides a releasable Ca2+ reservoir and directly controls luminal Ca2+-dependent RyR2 gating by stabilizing RyR2 in a refractory state during diastole, such that loss or mutation of CASQ2 reduces triadin/junctin levels, alters SR volume, elevates diastolic SR Ca2+ leak via dysregulated RyR2, and—exacerbated by catecholaminergic stimulation and CaMKII-mediated RyR2 phosphorylation—triggers spontaneous Ca2+ releases and delayed afterdepolarizations that underlie catecholaminergic polymorphic ventricular tachycardia (CPVT2).\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CASQ2 is the major Ca2+-storage and -buffering protein of the junctional sarcoplasmic reticulum (SR) in cardiac myocytes, where it provides a releasable luminal Ca2+ reservoir and governs ryanodine-receptor (RyR2)-mediated Ca2+ release [#0, #1]. Its Ca2+-holding capacity depends on polymerization of CASQ2 monomers across a highly negatively charged domain; mutations that disrupt this charge or assembly (e.g. D307H, the G112+5X truncation, K206N hyperglycosylation) abolish or reduce Ca2+ binding and lower SR Ca2+ load [#0, #2, #6]. Beyond passive buffering, CASQ2 acts as a luminal Ca2+ sensor that stabilizes RyR2 in a refractory, closed state during diastole: the R33Q mutation lowers the luminal Ca2+ threshold for terminating Ca2+ release without impairing Ca2+ binding, demonstrating a direct regulatory role at the CASQ2-RyR2 interface [#4]. CASQ2 functions within a junctional complex with triadin-1 and junctin, whose levels collapse when CASQ2 is lost or mutated, and restoring CASQ2 re-establishes this complex and calcium-release-unit ultrastructure [#1, #8, #10]. Loss or mutation of CASQ2 therefore enlarges SR volume, destabilizes RyR2, and—under catecholaminergic stress and CaMKII-mediated RyR2 phosphorylation—produces diastolic SR Ca2+ leak, spontaneous Ca2+ waves, and delayed afterdepolarizations that trigger arrhythmia [#1, #3, #13]. CASQ2 mutations cause catecholaminergic polymorphic ventricular tachycardia, and the arrhythmic phenotype requires CASQ2 deficiency in the cardiac conduction system, with AAV9-mediated CASQ2 gene replacement rescuing the molecular complex and eliminating arrhythmia in mouse and patient iPSC-cardiomyocyte models [#0, #12, #10, #15].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Establishing CASQ2 as a CPVT disease gene answered whether an SR Ca2+-storage protein, rather than RyR2 itself, could underlie inherited arrhythmia, implicating SR Ca2+ buffering in arrhythmogenesis.\",\n      \"evidence\": \"Direct sequencing and segregation of the D307H missense mutation in Bedouin families, with biochemical inference from charged-domain disruption\",\n      \"pmids\": [\"11704930\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct in vitro Ca2+-binding assay in this study\", \"Mechanistic link between disrupted buffering and arrhythmia not yet demonstrated\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"A Casq2-null mouse revealed that CASQ2 loss reorganizes the junctional SR and destabilizes its protein partners, defining the molecular consequences of CASQ2 deficiency.\",\n      \"evidence\": \"Knockout mouse with SR volume measurement, immunoblot for triadin-1/junctin, Ca2+ imaging, and in vivo ECG telemetry\",\n      \"pmids\": [\"16932808\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not separate buffering loss from RyR2 regulatory effects\", \"Tissue-specific contribution of CASQ2 loss not addressed\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Comparing a non-Ca2+-binding truncation with a normally-binding point mutant began to dissociate CASQ2's Ca2+-buffering capacity from other functional defects driving arrhythmia.\",\n      \"evidence\": \"Expression of G112+5X and L167H mutants in rat cardiomyocytes with Ca2+-binding assay and spark/transient imaging\",\n      \"pmids\": [\"16908766\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which L167H impairs function despite normal Ca2+ binding unresolved\", \"Heterologous rat myocyte context\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Knock-in (D307H) and null models, plus pharmacological RyR2 inhibition, established that RyR2 dysfunction is the central downstream effector of CASQ2 deficiency.\",\n      \"evidence\": \"Two mouse models with immunoblotting, Ca2+ imaging, Mg2+ rescue, and in vivo ECG\",\n      \"pmids\": [\"17607358\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Post-transcriptional upregulation of RyR2/calreticulin mechanism not defined\", \"Did not isolate luminal sensing from buffering\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Dissecting R33Q versus polymerization-defective mutants distinguished CASQ2's RyR2 luminal-gating control from its high-capacity Ca2+ buffering, defining two separable functions.\",\n      \"evidence\": \"Mutant expression in canine myocytes with FRET polymerization assays, triadin interaction assays, and luminal Ca2+ threshold measurement\",\n      \"pmids\": [\"18469084\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of CASQ2-RyR2 luminal coupling not resolved\", \"FRET in heterologous expression\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The K206N mutation linked an additional N-glycosylation event to altered CASQ2 aggregation, reduced SR load, and increased RyR2 open probability, broadening the mutational mechanisms beyond charge disruption.\",\n      \"evidence\": \"Expression in cell lines and neonatal mouse myocytes with glycosylation/MW immunoblot, Ca2+- and [3H]ryanodine-binding assays, and Ca2+ imaging\",\n      \"pmids\": [\"20302875\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether glycosylation drives mislocalization not established\", \"Triadin interaction unchanged, leaving RyR2 effect mechanism partly open\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Re-expressing D307H in null hearts showed that partial restoration of triadin-1 cannot rescue SR Ca2+ load, confirming impaired buffering as an intrinsic D307H defect.\",\n      \"evidence\": \"Stable D307H expression in CASQ2-null mouse hearts with triadin-1 immunoblot, Ca2+ imaging, and isoproterenol challenge\",\n      \"pmids\": [\"21984545\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab rescue model\", \"Relative contribution of buffering versus triadin binding to phenotype not quantified\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Genetic epistasis with SERCA1a overexpression or phospholamban deletion demonstrated that enhanced SR Ca2+ uptake combined with dysregulated RyR2 produces sustained diastolic Ca2+ release and cardiomyopathy.\",\n      \"evidence\": \"Double mutant mouse strains with echocardiography, Ca2+ imaging, apoptosis assays, and contractile measurements\",\n      \"pmids\": [\"23135969\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not address whether human CPVT progresses to cardiomyopathy via this route\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Double knockout of CASQ2 and HRC revealed opposing modulation of RyR2—CASQ2 stabilizing the refractory state while HRC promotes recovery—refining the model of luminal control of RyR2.\",\n      \"evidence\": \"CASQ2/HRC double knockout mice with arrhythmia telemetry, Ca2+ imaging, and SR Ca2+ release restitution measurement\",\n      \"pmids\": [\"26410369\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of CASQ2 versus HRC opposing effects on RyR2 not structurally defined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Cell-type-specific deletion and rescue established that CASQ2 function in the cardiac conduction system is necessary and sufficient for the CPVT phenotype, identifying tissue-autonomous roles.\",\n      \"evidence\": \"Conditional CCS-specific and cardiomyocyte-specific deletion/rescue mice with in vivo ECG and arrhythmia provocation\",\n      \"pmids\": [\"29452352\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cellular basis of conduction-system-specific susceptibility unresolved\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Atrial myocyte analysis connected CASQ2 deficiency to CaMKII activation and CaMKII-mediated RyR2 Ser2814 phosphorylation as a driver of triggered activity.\",\n      \"evidence\": \"R33Q knock-in mice with Ca2+ imaging, patch-clamp, immunoblot for CaMKII/phospho-RyR2, and KN93 inhibition\",\n      \"pmids\": [\"30450052\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether CaMKII activation is cause or consequence of altered Ca2+ handling not resolved\", \"Single lab\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Comparing R33Q and D307H knock-ins showed that distinct mutations trigger distinct degradation routes and adaptive pathways despite convergent arrhythmia phenotypes.\",\n      \"evidence\": \"Comparative biochemical analysis of two knock-in mouse hearts with protein quantification across adaptive pathways\",\n      \"pmids\": [\"32902830\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Limited methodological detail\", \"Functional significance of distinct adaptive pathways not tested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"AAV9 CASQ2 gene replacement in patient iPSC-cardiomyocytes and in R33Q knock-in mice established that restoring CASQ2 rescues the junctional complex, ultrastructure, and arrhythmia, providing therapeutic proof of concept.\",\n      \"evidence\": \"iPSC-derived cardiomyocytes and R33Q mice with AAV9 delivery, co-IP for CASQ2-junctin-triadin, Ca2+ imaging, electron microscopy, patch-clamp, and arrhythmia monitoring\",\n      \"pmids\": [\"27711080\", \"24888331\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Durability and dose requirements in human heart not established\", \"Recessive versus dominant mutation responses may differ\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The atomic structure of the CASQ2-RyR2-triadin-junctin junctional complex and the precise mechanism by which declining luminal Ca2+ is transduced to RyR2 closure remain undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No high-resolution structure of the CASQ2-RyR2 interface in the corpus\", \"Mechanism of luminal Ca2+-dependent RyR2 refractoriness not structurally resolved\", \"ANKRD1-CASQ2 interaction function not characterized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0, 2, 6]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [4, 11]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-397014\", \"supporting_discovery_ids\": [1, 3]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"complexes\": [\"RyR2-triadin-junctin junctional SR complex\", \"calcium release unit\"],\n    \"partners\": [\"RYR2\", \"TRDN\", \"ASPH\", \"ANKRD1\", \"HRC\", \"CASQ2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}