{"gene":"POPDC2","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":2012,"finding":"The conserved Popeye domain of POPDC2 functions as a high-affinity cAMP-binding site; POPDC2 interacts with the potassium channel TREK-1, leading to increased cell surface expression and enhanced TREK-1 current density, both of which are negatively modulated by cAMP. Popdc2 null mice develop stress-induced sinus node dysfunction and age-dependent bradyarrhythmia.","method":"cAMP binding assays (Popeye domain), co-immunoprecipitation/interaction studies with TREK-1, Xenopus oocyte electrophysiology, ECG analysis of Popdc2 knockout mice","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — cAMP binding established biochemically, TREK-1 interaction confirmed by co-IP, functional electrophysiology in oocytes, replicated across Popdc1 and Popdc2 null models with multiple orthogonal methods","pmids":["22354168"],"is_preprint":false},{"year":2012,"finding":"Morpholino oligonucleotide-mediated knockdown of popdc2 in zebrafish causes aberrant skeletal muscle development (irregularly shaped muscle segments, reduced craniofacial muscles) and cardiac abnormalities including pericardial edema, elongated heart chambers, abnormal looping, and severe cardiac arrhythmia with 2:1 or 3:1 atrial/ventricular conduction ratios confirmed by calcium transient recordings.","method":"Morpholino knockdown in zebrafish, optical recordings of cardiac contractility, SPIM microscopy with transgenic calcium indicator line","journal":"Developmental biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — morpholino knockdown with multiple orthogonal functional readouts (contractility, calcium imaging, conduction ratio), independently consistent with mouse knockout phenotype","pmids":["22290329"],"is_preprint":false},{"year":2014,"finding":"POPDC2 interacts with all three Popdc family members, and in Xenopus oocytes co-expression of Popdc proteins with TREK-1 causes enhanced membrane transport leading to increased TREK-1 current, which is blocked when cAMP levels are increased. POPDC1 interaction with caveolin 3 was also established, and loss of Popdc1 affects caveolar size.","method":"Xenopus oocyte electrophysiology, interaction studies, caveolae analysis in Popdc1 null mice","journal":"Biochemical Society transactions","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional electrophysiology in oocytes and knockout phenotype, single lab review summarizing prior data, no new independent replication described","pmids":["24646234"],"is_preprint":false},{"year":2015,"finding":"The POPDC1(S201F) disease mutation reduces cAMP affinity by ~50%; in patient skeletal muscle, both POPDC1(S201F) and wild-type POPDC2 display impaired membrane trafficking, establishing that POPDC2 membrane trafficking is affected by pathogenic variants in the POPDC1-POPDC2 complex.","method":"Whole-exome sequencing (variant identification), cAMP affinity assays, immunostaining of patient muscle biopsies, zebrafish homologous mutation expression","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — direct biochemical cAMP affinity measurement, patient biopsy immunostaining, zebrafish in vivo model with homologous mutation, multiple orthogonal methods","pmids":["26642364"],"is_preprint":false},{"year":2015,"finding":"NKX2-5 and MEIS1 bind to an overlapping motif in the Popdc2 gene enhancer in a mutually exclusive manner, providing a regulatory mechanism for spatiotemporal control of Popdc2 expression during cardiogenesis as cardiac progenitors from the anterior heart field differentiate.","method":"Transcription factor binding assays, reporter assays, chromatin studies during cardiac differentiation","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding and reporter assays with two transcription factors, single lab, Popdc2 used as a model locus","pmids":["26411676"],"is_preprint":false},{"year":2020,"finding":"A heterozygous nonsense mutation in POPDC2 (W188*, deleting part of the cAMP-binding domain) causes loss-of-function with impaired TREK-1 modulation in Xenopus oocytes. POPDC2W188* knock-in mice display stress-induced sinus bradycardia and pauses. In the human heart, the POPDC2-modulated K2P channel TREK-1 is preferentially expressed in the atrioventricular node.","method":"Whole-exome sequencing, Xenopus oocyte co-expression electrophysiology, knock-in mouse ECG analysis, quantitative PCR","journal":"Journal of molecular and cellular cardiology","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro electrophysiology demonstrating loss-of-function, knock-in mouse model with ECG phenotype, consistent with prior TREK-1 modulation mechanism, multiple orthogonal approaches","pmids":["32535041"],"is_preprint":false},{"year":2020,"finding":"POPDC1 and POPDC2 interact with XIRP1 (Xin actin-binding repeat-containing protein 1) and actin; both XIRP1 and POPDC1/2 co-localize at intercalated discs and T-tubules in adult rat and human heart, and the POPDC1–XIRP1 interaction was confirmed in adult rat heart extracts.","method":"Pulldown from cultured human skeletal myotubes with proteomic analysis, co-immunoprecipitation from adult rat heart, immunofluorescence co-localization in human and rat heart sections and isolated cardiomyocytes","journal":"BMC molecular and cell biology","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — reciprocal pulldown and co-IP with co-localization, two tissue types, single lab","pmids":["33261556"],"is_preprint":false},{"year":2023,"finding":"POPDC proteins interact with each other through a helix-helix interface located at the C-terminus of the Popeye domain. Site-directed mutagenesis of ultra-conserved hydrophobic residues demonstrated that some are required for membrane trafficking of the POPDC1-POPDC2 complex. Pathogenic mutations that impair membrane localization also disrupt POPDC complex formation, while mutations that leave protein-protein interaction intact likely affect another essential function.","method":"Co-precipitation, proximity ligation assay, bioluminescence resonance energy transfer (BRET), bimolecular fluorescence complementation (BiFC), site-directed mutagenesis, patient muscle biopsy immunostaining","journal":"Acta neuropathologica communications","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — four orthogonal interaction assays (co-IP, PLA, BRET, BiFC) plus mutagenesis mapping interaction domain and membrane trafficking consequences, single lab but highly rigorous","pmids":["36624536"],"is_preprint":false},{"year":2024,"finding":"POPDC proteins downregulate cardiac Nav1.5 (SCN5A) currents in a cAMP-dependent manner by reducing the surface expression of the channel. POPDC2 co-immunoprecipitates with Nav1.5 from native cardiac tissue. Knockdown of popdc2 in embryonic zebrafish caused increased upstroke velocity and overshoot of cardiac action potentials, consistent with increased Nav1.5 surface expression.","method":"Functional co-expression screen in Xenopus oocytes, co-immunoprecipitation from native murine cardiac tissue, zebrafish popdc2 knockdown with action potential recordings","journal":"iScience","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — co-IP from native tissue, in vitro electrophysiology establishing cAMP-dependent current modulation, in vivo zebrafish knockdown with functional readout, multiple orthogonal methods in one study","pmids":["38689644"],"is_preprint":false},{"year":2017,"finding":"POPDC2 was identified as a novel binding partner of the cardiac sodium-calcium exchanger NCX1 by affinity purification-mass spectrometry from rat left ventricle lysates, with the interaction validated by co-precipitation.","method":"Affinity purification coupled with mass spectrometry, co-precipitation validation from rat left ventricle lysates","journal":"Proteomics","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — AP-MS discovery plus co-precipitation validation, single lab, no functional consequence tested for POPDC2-NCX1 interaction","pmids":["28755400"],"is_preprint":false},{"year":2016,"finding":"POPDC2 localizes to the cardiac intercalated disk, confirmed by co-localization with n-cadherin in human and rat heart cryo-sections and isolated dog cardiomyocytes.","method":"Quantitative proteomics of membrane-enriched fractions, immunofluorescence co-localization in human/rat cryo-sections and isolated cardiomyocytes","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — proteomics-based discovery confirmed by immunofluorescence in multiple species, single lab, subcellular localization without direct functional consequence tested","pmids":["27148881"],"is_preprint":false},{"year":2025,"finding":"Bi-allelic loss-of-function variants in POPDC2 cause an autosomal recessive cardiac syndrome. In vitro electrophysiological studies showed that wild-type POPDC2 co-expressed with TREK-1 increases TREK-1 current density, whereas pathogenic POPDC2 variants fail to do so. Patient muscle biopsies showed significantly reduced abundance of both POPDC1 and POPDC2, indicating impaired stability and/or membrane trafficking of the POPDC1-POPDC2 complex.","method":"In vitro electrophysiology (TREK-1 co-expression), patient muscle biopsy protein quantification, homology modeling, single-cell RNA sequencing from human hearts","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — direct in vitro electrophysiology demonstrating loss of TREK-1 modulation by patient variants, biopsy evidence of membrane trafficking impairment, consistent with prior mechanistic work, multiple orthogonal approaches","pmids":["40409267"],"is_preprint":false}],"current_model":"POPDC2 is a plasma membrane-localized, high-affinity cAMP-binding protein (via its conserved Popeye domain) that acts as a cAMP effector in cardiac and skeletal muscle by modulating the membrane trafficking and surface expression of interacting ion channel partners — principally the K2P potassium channel TREK-1 (increasing its surface expression and current density in a cAMP-suppressible manner) and the cardiac sodium channel Nav1.5 (reducing its surface expression in a cAMP-dependent manner); POPDC2 forms heteromeric complexes with POPDC1 through a C-terminal helix-helix interface whose integrity is required for correct membrane trafficking of both proteins, and also interacts with XIRP1 at intercalated discs/T-tubules and with NCX1; loss of POPDC2 function in mice and zebrafish causes stress-induced sinus bradycardia, atrioventricular block, and cardiac arrhythmia, phenotypes recapitulated by human bi-allelic loss-of-function mutations that abolish TREK-1 current augmentation."},"narrative":{"mechanistic_narrative":"POPDC2 is a plasma membrane-associated cAMP effector that controls cardiac and skeletal muscle excitability by regulating the surface expression of ion channel partners through its high-affinity cAMP-binding Popeye domain [PMID:22354168]. It interacts with the two-pore potassium channel TREK-1 to increase TREK-1 surface expression and current density, an effect reversed when cAMP is elevated [PMID:22354168, PMID:24646234], and conversely downregulates the cardiac sodium channel Nav1.5 (SCN5A) by reducing its surface expression in a cAMP-dependent manner [PMID:38689644]. POPDC2 functions within a heteromeric POPDC complex, assembling with the other Popeye-domain proteins through a helix-helix interface at the C-terminus of the Popeye domain whose integrity is required for correct membrane trafficking of the POPDC1-POPDC2 complex [PMID:36624536]. At the cardiomyocyte membrane, POPDC2 localizes to the intercalated disc and T-tubules, where it associates with the Xin actin-binding protein XIRP1 and actin [PMID:33261556, PMID:27148881] and with the sodium-calcium exchanger NCX1 [PMID:28755400]. Loss of POPDC2 function in mice and zebrafish produces stress-induced sinus node dysfunction, conduction block, and arrhythmia [PMID:22354168, PMID:22290329], and bi-allelic loss-of-function variants in POPDC2 that abolish TREK-1 current augmentation cause an autosomal recessive cardiac syndrome in humans [PMID:32535041, PMID:40409267]. Expression of Popdc2 during cardiogenesis is controlled by mutually exclusive binding of NKX2-5 and MEIS1 to an overlapping enhancer motif [PMID:26411676].","teleology":[{"year":2012,"claim":"Established the founding molecular function: that the POPDC2 Popeye domain is a cAMP sensor and that POPDC2 couples cAMP signaling to TREK-1 channel surface expression, defining it as a membrane cAMP effector in the cardiac conduction system.","evidence":"cAMP binding assays, co-IP with TREK-1, Xenopus oocyte electrophysiology, and ECG of Popdc2 null mice","pmids":["22354168"],"confidence":"High","gaps":["Did not resolve the structural basis of cAMP-regulated trafficking","Mechanism by which cAMP suppresses TREK-1 surface delivery not defined"]},{"year":2012,"claim":"Demonstrated in vivo that POPDC2 is required for both skeletal muscle morphogenesis and cardiac conduction, independently corroborating the conduction phenotype seen in mouse knockouts.","evidence":"Morpholino knockdown in zebrafish with optical contractility and calcium-transient imaging","pmids":["22290329"],"confidence":"High","gaps":["Morpholino specificity not orthogonally controlled by mutant","Molecular effector linking knockdown to conduction defect not identified in this study"]},{"year":2014,"claim":"Extended the channel-trafficking model by showing all three Popdc family members enhance TREK-1 membrane transport in a cAMP-blockable manner and linking the family to caveolar organization.","evidence":"Xenopus oocyte electrophysiology, interaction studies, caveolae analysis in Popdc1 null mice","pmids":["24646234"],"confidence":"Medium","gaps":["Review summarizing prior data without new independent replication","POPDC2-specific contribution to caveolar effects not separated from POPDC1"]},{"year":2015,"claim":"Connected POPDC dysfunction to human disease by showing a POPDC1 cAMP-affinity-reducing mutation impairs membrane trafficking of both POPDC1 and POPDC2, establishing complex interdependence.","evidence":"Exome sequencing, cAMP affinity assays, patient muscle biopsy immunostaining, zebrafish homologous mutation","pmids":["26642364"],"confidence":"High","gaps":["Did not test whether POPDC2 mutations reciprocally affect POPDC1 trafficking","Functional channel consequence in patient tissue not directly measured"]},{"year":2015,"claim":"Identified the transcriptional control of Popdc2 expression, showing NKX2-5 and MEIS1 compete at an overlapping enhancer motif to time expression during cardiac progenitor differentiation.","evidence":"Transcription factor binding assays, reporter assays, and chromatin studies during cardiac differentiation","pmids":["26411676"],"confidence":"Medium","gaps":["In vivo requirement of this enhancer for endogenous Popdc2 expression not established","Single lab, Popdc2 used as model locus"]},{"year":2016,"claim":"Pinpointed the subcellular site of POPDC2 action by localizing it to the cardiac intercalated disc.","evidence":"Quantitative membrane proteomics with immunofluorescence co-localization with n-cadherin across species","pmids":["27148881"],"confidence":"Medium","gaps":["No direct functional consequence of intercalated-disc localization tested","Localization-trafficking link not mechanistically resolved"]},{"year":2017,"claim":"Expanded the POPDC2 interactome with the sodium-calcium exchanger NCX1, hinting at broader regulation of cardiac ion handling beyond potassium and sodium channels.","evidence":"Affinity purification-mass spectrometry from rat left ventricle with co-precipitation validation","pmids":["28755400"],"confidence":"Medium","gaps":["Functional consequence of POPDC2-NCX1 interaction not tested","Single lab, no cAMP dependence examined"]},{"year":2020,"claim":"Provided the first direct human genetic evidence for POPDC2 as a disease gene, linking a loss-of-function nonsense variant to impaired TREK-1 modulation and a sinus node phenotype reproduced in knock-in mice.","evidence":"Exome sequencing, Xenopus oocyte co-expression electrophysiology, knock-in mouse ECG, qPCR of TREK-1 in AV node","pmids":["32535041"],"confidence":"High","gaps":["Mechanism of stress-dependence of the bradycardia not resolved","Whether heterozygous loss is sufficient in humans not fully established"]},{"year":2023,"claim":"Defined the structural basis of POPDC complex assembly, mapping a C-terminal helix-helix interface required for membrane trafficking and separating trafficking-disrupting from interaction-preserving pathogenic mutations.","evidence":"Co-IP, PLA, BRET, BiFC, site-directed mutagenesis, and patient muscle biopsy immunostaining","pmids":["36624536"],"confidence":"High","gaps":["The 'other essential function' of interaction-preserving mutants not identified","No high-resolution structure of the interface"]},{"year":2024,"claim":"Broadened the effector model by establishing that POPDC proteins also suppress Nav1.5 surface expression in a cAMP-dependent manner, showing POPDC2 reciprocally tunes both repolarizing and depolarizing currents.","evidence":"Functional co-expression screen in oocytes, co-IP from native cardiac tissue, zebrafish knockdown with action potential recordings","pmids":["38689644"],"confidence":"High","gaps":["Trafficking machinery mediating Nav1.5 internalization not identified","Relative in vivo contribution of Nav1.5 vs TREK-1 modulation to arrhythmia unresolved"]},{"year":2025,"claim":"Confirmed POPDC2 as a recessive disease gene, showing bi-allelic loss-of-function variants fail to augment TREK-1 current and reduce abundance of the POPDC1-POPDC2 complex in patient muscle.","evidence":"In vitro TREK-1 co-expression electrophysiology, patient muscle protein quantification, homology modeling, single-cell RNA-seq of human heart","pmids":["40409267"],"confidence":"High","gaps":["Whether reduced complex abundance reflects degradation or trafficking failure not distinguished","Skeletal muscle phenotype mechanism in patients not fully defined"]},{"year":null,"claim":"How cAMP binding to the Popeye domain is mechanistically transduced into altered channel trafficking, and what the trafficking machinery and 'other essential function' of interaction-competent POPDC2 mutants are, remain unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No structure of the cAMP-bound Popeye domain or POPDC complex","Identity of the membrane trafficking adaptors engaged by POPDC2 unknown","Functional consequence of NCX1 and XIRP1 interactions for channel regulation untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,8]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,10]}],"pathway":[{"term_id":"R-HSA-397014","term_label":"Muscle contraction","supporting_discovery_ids":[0,8]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[7,8]}],"complexes":["POPDC1-POPDC2 complex"],"partners":["POPDC1","TREK-1","SCN5A","XIRP1","NCX1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9HBU9","full_name":"Popeye domain-containing protein 2","aliases":[],"length_aa":364,"mass_kda":40.4,"function":"Important for the maintenance of cardiac function. Plays a regulatory function in heart rate dynamics mediated, at least in part, through cAMP-binding and, probably, by increasing cell surface expression of the potassium channel KCNK2 and enhancing current density","subcellular_location":"Membrane; Cell membrane, sarcolemma","url":"https://www.uniprot.org/uniprotkb/Q9HBU9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/POPDC2","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/POPDC2","total_profiled":1310},"omim":[{"mim_id":"621367","title":"CARDIAC CONDUCTION DISEASE WITH OR WITHOUT CARDIOMYOPATHY 2; CCCM2","url":"https://www.omim.org/entry/621367"},{"mim_id":"616812","title":"MUSCULAR DYSTROPHY, LIMB-GIRDLE, AUTOSOMAL RECESSIVE 25; LGMDR25","url":"https://www.omim.org/entry/616812"},{"mim_id":"605824","title":"POPEYE DOMAIN-CONTAINING PROTEIN 3; POPDC3","url":"https://www.omim.org/entry/605824"},{"mim_id":"605823","title":"POPEYE DOMAIN-CONTAINING PROTEIN 2; POPDC2","url":"https://www.omim.org/entry/605823"},{"mim_id":"605739","title":"KYPHOSCOLIOSIS PEPTIDASE; KY","url":"https://www.omim.org/entry/605739"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"heart muscle","ntpm":378.0}],"url":"https://www.proteinatlas.org/search/POPDC2"},"hgnc":{"alias_symbol":["POP2"],"prev_symbol":[]},"alphafold":{"accession":"Q9HBU9","domains":[{"cath_id":"2.60.120.10","chopping":"107-257","consensus_level":"high","plddt":92.6866,"start":107,"end":257},{"cath_id":"1.20.58","chopping":"26-103","consensus_level":"high","plddt":91.1101,"start":26,"end":103}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9HBU9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9HBU9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9HBU9-F1-predicted_aligned_error_v6.png","plddt_mean":75.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=POPDC2","jax_strain_url":"https://www.jax.org/strain/search?query=POPDC2"},"sequence":{"accession":"Q9HBU9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9HBU9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9HBU9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9HBU9"}},"corpus_meta":[{"pmid":"22354168","id":"PMC_22354168","title":"Popeye 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Heart and circulatory physiology","url":"https://pubmed.ncbi.nlm.nih.gov/41632968","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2024.07.04.24309755","title":"Biallelic variants in<i>POPDC2</i>cause a novel autosomal recessive syndrome presenting with cardiac conduction defects and variable hypertrophic cardiomyopathy","date":"2024-07-05","source":"bioRxiv","url":"https://doi.org/10.1101/2024.07.04.24309755","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":21259,"output_tokens":3317,"usd":0.056766,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10928,"output_tokens":3714,"usd":0.073745,"stage2_stop_reason":"end_turn"},"total_usd":0.130511,"stage1_batch_id":"msgbatch_01CNoDbkChHEkgrJnL7BwtGJ","stage2_batch_id":"msgbatch_016Y4x9E9Wg5VQDANj9quSQD","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2012,\n      \"finding\": \"The conserved Popeye domain of POPDC2 functions as a high-affinity cAMP-binding site; POPDC2 interacts with the potassium channel TREK-1, leading to increased cell surface expression and enhanced TREK-1 current density, both of which are negatively modulated by cAMP. Popdc2 null mice develop stress-induced sinus node dysfunction and age-dependent bradyarrhythmia.\",\n      \"method\": \"cAMP binding assays (Popeye domain), co-immunoprecipitation/interaction studies with TREK-1, Xenopus oocyte electrophysiology, ECG analysis of Popdc2 knockout mice\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — cAMP binding established biochemically, TREK-1 interaction confirmed by co-IP, functional electrophysiology in oocytes, replicated across Popdc1 and Popdc2 null models with multiple orthogonal methods\",\n      \"pmids\": [\"22354168\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Morpholino oligonucleotide-mediated knockdown of popdc2 in zebrafish causes aberrant skeletal muscle development (irregularly shaped muscle segments, reduced craniofacial muscles) and cardiac abnormalities including pericardial edema, elongated heart chambers, abnormal looping, and severe cardiac arrhythmia with 2:1 or 3:1 atrial/ventricular conduction ratios confirmed by calcium transient recordings.\",\n      \"method\": \"Morpholino knockdown in zebrafish, optical recordings of cardiac contractility, SPIM microscopy with transgenic calcium indicator line\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — morpholino knockdown with multiple orthogonal functional readouts (contractility, calcium imaging, conduction ratio), independently consistent with mouse knockout phenotype\",\n      \"pmids\": [\"22290329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"POPDC2 interacts with all three Popdc family members, and in Xenopus oocytes co-expression of Popdc proteins with TREK-1 causes enhanced membrane transport leading to increased TREK-1 current, which is blocked when cAMP levels are increased. POPDC1 interaction with caveolin 3 was also established, and loss of Popdc1 affects caveolar size.\",\n      \"method\": \"Xenopus oocyte electrophysiology, interaction studies, caveolae analysis in Popdc1 null mice\",\n      \"journal\": \"Biochemical Society transactions\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional electrophysiology in oocytes and knockout phenotype, single lab review summarizing prior data, no new independent replication described\",\n      \"pmids\": [\"24646234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The POPDC1(S201F) disease mutation reduces cAMP affinity by ~50%; in patient skeletal muscle, both POPDC1(S201F) and wild-type POPDC2 display impaired membrane trafficking, establishing that POPDC2 membrane trafficking is affected by pathogenic variants in the POPDC1-POPDC2 complex.\",\n      \"method\": \"Whole-exome sequencing (variant identification), cAMP affinity assays, immunostaining of patient muscle biopsies, zebrafish homologous mutation expression\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — direct biochemical cAMP affinity measurement, patient biopsy immunostaining, zebrafish in vivo model with homologous mutation, multiple orthogonal methods\",\n      \"pmids\": [\"26642364\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"NKX2-5 and MEIS1 bind to an overlapping motif in the Popdc2 gene enhancer in a mutually exclusive manner, providing a regulatory mechanism for spatiotemporal control of Popdc2 expression during cardiogenesis as cardiac progenitors from the anterior heart field differentiate.\",\n      \"method\": \"Transcription factor binding assays, reporter assays, chromatin studies during cardiac differentiation\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding and reporter assays with two transcription factors, single lab, Popdc2 used as a model locus\",\n      \"pmids\": [\"26411676\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"A heterozygous nonsense mutation in POPDC2 (W188*, deleting part of the cAMP-binding domain) causes loss-of-function with impaired TREK-1 modulation in Xenopus oocytes. POPDC2W188* knock-in mice display stress-induced sinus bradycardia and pauses. In the human heart, the POPDC2-modulated K2P channel TREK-1 is preferentially expressed in the atrioventricular node.\",\n      \"method\": \"Whole-exome sequencing, Xenopus oocyte co-expression electrophysiology, knock-in mouse ECG analysis, quantitative PCR\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro electrophysiology demonstrating loss-of-function, knock-in mouse model with ECG phenotype, consistent with prior TREK-1 modulation mechanism, multiple orthogonal approaches\",\n      \"pmids\": [\"32535041\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"POPDC1 and POPDC2 interact with XIRP1 (Xin actin-binding repeat-containing protein 1) and actin; both XIRP1 and POPDC1/2 co-localize at intercalated discs and T-tubules in adult rat and human heart, and the POPDC1–XIRP1 interaction was confirmed in adult rat heart extracts.\",\n      \"method\": \"Pulldown from cultured human skeletal myotubes with proteomic analysis, co-immunoprecipitation from adult rat heart, immunofluorescence co-localization in human and rat heart sections and isolated cardiomyocytes\",\n      \"journal\": \"BMC molecular and cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — reciprocal pulldown and co-IP with co-localization, two tissue types, single lab\",\n      \"pmids\": [\"33261556\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"POPDC proteins interact with each other through a helix-helix interface located at the C-terminus of the Popeye domain. Site-directed mutagenesis of ultra-conserved hydrophobic residues demonstrated that some are required for membrane trafficking of the POPDC1-POPDC2 complex. Pathogenic mutations that impair membrane localization also disrupt POPDC complex formation, while mutations that leave protein-protein interaction intact likely affect another essential function.\",\n      \"method\": \"Co-precipitation, proximity ligation assay, bioluminescence resonance energy transfer (BRET), bimolecular fluorescence complementation (BiFC), site-directed mutagenesis, patient muscle biopsy immunostaining\",\n      \"journal\": \"Acta neuropathologica communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — four orthogonal interaction assays (co-IP, PLA, BRET, BiFC) plus mutagenesis mapping interaction domain and membrane trafficking consequences, single lab but highly rigorous\",\n      \"pmids\": [\"36624536\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"POPDC proteins downregulate cardiac Nav1.5 (SCN5A) currents in a cAMP-dependent manner by reducing the surface expression of the channel. POPDC2 co-immunoprecipitates with Nav1.5 from native cardiac tissue. Knockdown of popdc2 in embryonic zebrafish caused increased upstroke velocity and overshoot of cardiac action potentials, consistent with increased Nav1.5 surface expression.\",\n      \"method\": \"Functional co-expression screen in Xenopus oocytes, co-immunoprecipitation from native murine cardiac tissue, zebrafish popdc2 knockdown with action potential recordings\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — co-IP from native tissue, in vitro electrophysiology establishing cAMP-dependent current modulation, in vivo zebrafish knockdown with functional readout, multiple orthogonal methods in one study\",\n      \"pmids\": [\"38689644\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"POPDC2 was identified as a novel binding partner of the cardiac sodium-calcium exchanger NCX1 by affinity purification-mass spectrometry from rat left ventricle lysates, with the interaction validated by co-precipitation.\",\n      \"method\": \"Affinity purification coupled with mass spectrometry, co-precipitation validation from rat left ventricle lysates\",\n      \"journal\": \"Proteomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — AP-MS discovery plus co-precipitation validation, single lab, no functional consequence tested for POPDC2-NCX1 interaction\",\n      \"pmids\": [\"28755400\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"POPDC2 localizes to the cardiac intercalated disk, confirmed by co-localization with n-cadherin in human and rat heart cryo-sections and isolated dog cardiomyocytes.\",\n      \"method\": \"Quantitative proteomics of membrane-enriched fractions, immunofluorescence co-localization in human/rat cryo-sections and isolated cardiomyocytes\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — proteomics-based discovery confirmed by immunofluorescence in multiple species, single lab, subcellular localization without direct functional consequence tested\",\n      \"pmids\": [\"27148881\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Bi-allelic loss-of-function variants in POPDC2 cause an autosomal recessive cardiac syndrome. In vitro electrophysiological studies showed that wild-type POPDC2 co-expressed with TREK-1 increases TREK-1 current density, whereas pathogenic POPDC2 variants fail to do so. Patient muscle biopsies showed significantly reduced abundance of both POPDC1 and POPDC2, indicating impaired stability and/or membrane trafficking of the POPDC1-POPDC2 complex.\",\n      \"method\": \"In vitro electrophysiology (TREK-1 co-expression), patient muscle biopsy protein quantification, homology modeling, single-cell RNA sequencing from human hearts\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — direct in vitro electrophysiology demonstrating loss of TREK-1 modulation by patient variants, biopsy evidence of membrane trafficking impairment, consistent with prior mechanistic work, multiple orthogonal approaches\",\n      \"pmids\": [\"40409267\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"POPDC2 is a plasma membrane-localized, high-affinity cAMP-binding protein (via its conserved Popeye domain) that acts as a cAMP effector in cardiac and skeletal muscle by modulating the membrane trafficking and surface expression of interacting ion channel partners — principally the K2P potassium channel TREK-1 (increasing its surface expression and current density in a cAMP-suppressible manner) and the cardiac sodium channel Nav1.5 (reducing its surface expression in a cAMP-dependent manner); POPDC2 forms heteromeric complexes with POPDC1 through a C-terminal helix-helix interface whose integrity is required for correct membrane trafficking of both proteins, and also interacts with XIRP1 at intercalated discs/T-tubules and with NCX1; loss of POPDC2 function in mice and zebrafish causes stress-induced sinus bradycardia, atrioventricular block, and cardiac arrhythmia, phenotypes recapitulated by human bi-allelic loss-of-function mutations that abolish TREK-1 current augmentation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"POPDC2 is a plasma membrane-associated cAMP effector that controls cardiac and skeletal muscle excitability by regulating the surface expression of ion channel partners through its high-affinity cAMP-binding Popeye domain [#0]. It interacts with the two-pore potassium channel TREK-1 to increase TREK-1 surface expression and current density, an effect reversed when cAMP is elevated [#0, #2], and conversely downregulates the cardiac sodium channel Nav1.5 (SCN5A) by reducing its surface expression in a cAMP-dependent manner [#8]. POPDC2 functions within a heteromeric POPDC complex, assembling with the other Popeye-domain proteins through a helix-helix interface at the C-terminus of the Popeye domain whose integrity is required for correct membrane trafficking of the POPDC1-POPDC2 complex [#7]. At the cardiomyocyte membrane, POPDC2 localizes to the intercalated disc and T-tubules, where it associates with the Xin actin-binding protein XIRP1 and actin [#6, #10] and with the sodium-calcium exchanger NCX1 [#9]. Loss of POPDC2 function in mice and zebrafish produces stress-induced sinus node dysfunction, conduction block, and arrhythmia [#0, #1], and bi-allelic loss-of-function variants in POPDC2 that abolish TREK-1 current augmentation cause an autosomal recessive cardiac syndrome in humans [#5, #11]. Expression of Popdc2 during cardiogenesis is controlled by mutually exclusive binding of NKX2-5 and MEIS1 to an overlapping enhancer motif [#4].\",\n  \"teleology\": [\n    {\n      \"year\": 2012,\n      \"claim\": \"Established the founding molecular function: that the POPDC2 Popeye domain is a cAMP sensor and that POPDC2 couples cAMP signaling to TREK-1 channel surface expression, defining it as a membrane cAMP effector in the cardiac conduction system.\",\n      \"evidence\": \"cAMP binding assays, co-IP with TREK-1, Xenopus oocyte electrophysiology, and ECG of Popdc2 null mice\",\n      \"pmids\": [\"22354168\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the structural basis of cAMP-regulated trafficking\", \"Mechanism by which cAMP suppresses TREK-1 surface delivery not defined\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Demonstrated in vivo that POPDC2 is required for both skeletal muscle morphogenesis and cardiac conduction, independently corroborating the conduction phenotype seen in mouse knockouts.\",\n      \"evidence\": \"Morpholino knockdown in zebrafish with optical contractility and calcium-transient imaging\",\n      \"pmids\": [\"22290329\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Morpholino specificity not orthogonally controlled by mutant\", \"Molecular effector linking knockdown to conduction defect not identified in this study\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Extended the channel-trafficking model by showing all three Popdc family members enhance TREK-1 membrane transport in a cAMP-blockable manner and linking the family to caveolar organization.\",\n      \"evidence\": \"Xenopus oocyte electrophysiology, interaction studies, caveolae analysis in Popdc1 null mice\",\n      \"pmids\": [\"24646234\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Review summarizing prior data without new independent replication\", \"POPDC2-specific contribution to caveolar effects not separated from POPDC1\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Connected POPDC dysfunction to human disease by showing a POPDC1 cAMP-affinity-reducing mutation impairs membrane trafficking of both POPDC1 and POPDC2, establishing complex interdependence.\",\n      \"evidence\": \"Exome sequencing, cAMP affinity assays, patient muscle biopsy immunostaining, zebrafish homologous mutation\",\n      \"pmids\": [\"26642364\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not test whether POPDC2 mutations reciprocally affect POPDC1 trafficking\", \"Functional channel consequence in patient tissue not directly measured\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identified the transcriptional control of Popdc2 expression, showing NKX2-5 and MEIS1 compete at an overlapping enhancer motif to time expression during cardiac progenitor differentiation.\",\n      \"evidence\": \"Transcription factor binding assays, reporter assays, and chromatin studies during cardiac differentiation\",\n      \"pmids\": [\"26411676\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo requirement of this enhancer for endogenous Popdc2 expression not established\", \"Single lab, Popdc2 used as model locus\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Pinpointed the subcellular site of POPDC2 action by localizing it to the cardiac intercalated disc.\",\n      \"evidence\": \"Quantitative membrane proteomics with immunofluorescence co-localization with n-cadherin across species\",\n      \"pmids\": [\"27148881\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct functional consequence of intercalated-disc localization tested\", \"Localization-trafficking link not mechanistically resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Expanded the POPDC2 interactome with the sodium-calcium exchanger NCX1, hinting at broader regulation of cardiac ion handling beyond potassium and sodium channels.\",\n      \"evidence\": \"Affinity purification-mass spectrometry from rat left ventricle with co-precipitation validation\",\n      \"pmids\": [\"28755400\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of POPDC2-NCX1 interaction not tested\", \"Single lab, no cAMP dependence examined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Provided the first direct human genetic evidence for POPDC2 as a disease gene, linking a loss-of-function nonsense variant to impaired TREK-1 modulation and a sinus node phenotype reproduced in knock-in mice.\",\n      \"evidence\": \"Exome sequencing, Xenopus oocyte co-expression electrophysiology, knock-in mouse ECG, qPCR of TREK-1 in AV node\",\n      \"pmids\": [\"32535041\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of stress-dependence of the bradycardia not resolved\", \"Whether heterozygous loss is sufficient in humans not fully established\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Defined the structural basis of POPDC complex assembly, mapping a C-terminal helix-helix interface required for membrane trafficking and separating trafficking-disrupting from interaction-preserving pathogenic mutations.\",\n      \"evidence\": \"Co-IP, PLA, BRET, BiFC, site-directed mutagenesis, and patient muscle biopsy immunostaining\",\n      \"pmids\": [\"36624536\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The 'other essential function' of interaction-preserving mutants not identified\", \"No high-resolution structure of the interface\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Broadened the effector model by establishing that POPDC proteins also suppress Nav1.5 surface expression in a cAMP-dependent manner, showing POPDC2 reciprocally tunes both repolarizing and depolarizing currents.\",\n      \"evidence\": \"Functional co-expression screen in oocytes, co-IP from native cardiac tissue, zebrafish knockdown with action potential recordings\",\n      \"pmids\": [\"38689644\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trafficking machinery mediating Nav1.5 internalization not identified\", \"Relative in vivo contribution of Nav1.5 vs TREK-1 modulation to arrhythmia unresolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Confirmed POPDC2 as a recessive disease gene, showing bi-allelic loss-of-function variants fail to augment TREK-1 current and reduce abundance of the POPDC1-POPDC2 complex in patient muscle.\",\n      \"evidence\": \"In vitro TREK-1 co-expression electrophysiology, patient muscle protein quantification, homology modeling, single-cell RNA-seq of human heart\",\n      \"pmids\": [\"40409267\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether reduced complex abundance reflects degradation or trafficking failure not distinguished\", \"Skeletal muscle phenotype mechanism in patients not fully defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How cAMP binding to the Popeye domain is mechanistically transduced into altered channel trafficking, and what the trafficking machinery and 'other essential function' of interaction-competent POPDC2 mutants are, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of the cAMP-bound Popeye domain or POPDC complex\", \"Identity of the membrane trafficking adaptors engaged by POPDC2 unknown\", \"Functional consequence of NCX1 and XIRP1 interactions for channel regulation untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 8]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-397014\", \"supporting_discovery_ids\": [0, 8]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [7, 8]}\n    ],\n    \"complexes\": [\"POPDC1-POPDC2 complex\"],\n    \"partners\": [\"POPDC1\", \"TREK-1\", \"SCN5A\", \"XIRP1\", \"NCX1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}