{"gene":"POPDC2","run_date":"2026-04-28T19:45:44","timeline":{"discoveries":[{"year":2012,"finding":"Morpholino oligonucleotide-mediated knockdown of popdc2 in zebrafish causes aberrant skeletal muscle development, cardiac morphogenesis defects (pericardial edema, abnormal looping), and cardiac arrhythmia with 2:1 or 3:1 atrial/ventricular conduction block, identifying popdc2 as required for striated muscle differentiation and cardiac conduction system development.","method":"Morpholino knockdown in zebrafish; optical recording of cardiac contractility; calcium transient imaging via transgenic calcium indicator (Tg(cmlc2:gCaMP)) and SPIM microscopy","journal":"Developmental biology","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple orthogonal phenotypic readouts (morphology, electrophysiology, calcium imaging) in zebrafish ortholog","pmids":["22290329"],"is_preprint":false},{"year":2015,"finding":"MEIS1 and NKX2-5 bind overlapping sites on the Popdc2 gene enhancer in a mutually exclusive manner; as cardiac progenitors differentiate, sequential high MEIS1 then increasing NKX2-5 levels provide spatiotemporal transcriptional regulation of Popdc2 expression during cardiogenesis.","method":"ChIP, reporter assays, in vivo cardiac differentiation models; binding site competition assays","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — reciprocal binding and functional enhancer validation with multiple orthogonal methods","pmids":["26411676"],"is_preprint":false},{"year":2020,"finding":"A loss-of-function nonsense mutation in POPDC2 (W188*, deleting part of the cAMP-binding domain) impairs TREK-1 potassium channel modulation; co-expression of POPDC2W188* with TREK-1 in Xenopus oocytes fails to increase TREK-1 current density, and knock-in mice display stress-induced sinus bradycardia and pauses, establishing POPDC2 as a cAMP-dependent regulator of TREK-1 required for normal cardiac conduction.","method":"Xenopus oocyte electrophysiology (co-expression); knock-in mouse model; quantitative PCR for expression profiling","journal":"Journal of molecular and cellular cardiology","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro electrophysiology with disease variant, corroborated by knock-in mouse phenotype","pmids":["32535041"],"is_preprint":false},{"year":2025,"finding":"Bi-allelic loss-of-function variants in POPDC2 abolish TREK-1 current upregulation in vitro; patient muscle biopsies show reduced abundance of both POPDC1 and POPDC2, indicating that pathogenic variants in either protein impair stability and/or membrane trafficking of the POPDC1-POPDC2 complex; single-cell RNA-seq shows co-expression of POPDC1 and POPDC2 is highest in AV node, AV node pacemaker, and AV bundle cells.","method":"In vitro electrophysiology (Xenopus oocyte co-expression); protein quantification from patient muscle biopsy; homology modeling; single-cell RNA sequencing of human hearts","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (electrophysiology, proteomics, scRNA-seq) in single rigorous study","pmids":["40409267"],"is_preprint":false},{"year":2004,"finding":"Popdc2 is a transmembrane plasma membrane protein expressed predominantly in myocardium and skeletal muscle during chick embryogenesis, with distinct spatial patterns across cardiac chambers and muscle-forming fields, identifying it as a myocyte-specific differentiation marker.","method":"Northern blot; whole-mount in situ hybridization; cDNA cloning of alternatively spliced isoforms","journal":"Developmental dynamics","confidence":"Medium","confidence_rationale":"Tier 3 — direct localization by expression analysis; no functional manipulation","pmids":["14991725"],"is_preprint":false},{"year":2008,"finding":"LacZ reporter knock-in into the Popdc2 locus shows expression in cardiac and extraembryonic mesoderm from E7.5, then heart, somites, coelomic mesothelium, skeletal muscle, bladder, gut, and smooth muscle postnatally, establishing the in vivo expression pattern linked to the Popdc2 locus.","method":"LacZ reporter gene knock-in; embryonic and postnatal whole-mount and section staining","journal":"Developmental dynamics","confidence":"Medium","confidence_rationale":"Tier 2 — direct reporter-based localization from endogenous locus; no functional consequence tested","pmids":["18189275"],"is_preprint":false}],"current_model":"POPDC2 is a transmembrane, striated-muscle-enriched protein whose conserved Popeye (cAMP-binding) domain directly binds cAMP and, in a cAMP-dependent manner, upregulates TREK-1 two-pore domain potassium channel current density; loss of this regulation—through knockout, morpholino knockdown, or disease-causing variants—causes sinus node dysfunction, atrioventricular conduction block, and cardiac arrhythmia, while pathogenic variants in either POPDC2 or its partner POPDC1 also impair membrane trafficking of the POPDC1–POPDC2 complex."},"narrative":{"teleology":[{"year":2004,"claim":"Before any functional data existed, expression profiling established that Popdc2 encodes a transmembrane protein with striking restriction to myocardium and skeletal muscle during embryogenesis, framing it as a candidate striated-muscle regulator.","evidence":"Northern blot and whole-mount in situ hybridization in chick embryos","pmids":["14991725"],"confidence":"Medium","gaps":["Expression data only; no functional manipulation or loss-of-function phenotype","Mammalian expression pattern not yet mapped from endogenous locus"]},{"year":2008,"claim":"A LacZ knock-in at the endogenous Popdc2 locus confirmed cardiac and somitic expression beginning at E7.5 and revealed postnatal expression in smooth muscle and visceral organs, extending the known expression domains to mammalian development.","evidence":"LacZ reporter knock-in mouse with whole-mount and section staining across embryonic and postnatal stages","pmids":["18189275"],"confidence":"Medium","gaps":["Reporter knock-in disrupts one allele but heterozygous phenotype was not assessed","No functional consequence of loss tested"]},{"year":2012,"claim":"The first loss-of-function study showed that popdc2 is required for cardiac conduction and striated muscle development: morpholino knockdown in zebrafish produced AV block, arrhythmia, and skeletal muscle disorganization, establishing a direct role in cardiac rhythm.","evidence":"Morpholino knockdown in zebrafish with optical contractility mapping and calcium transient imaging (Tg(cmlc2:gCaMP))","pmids":["22290329"],"confidence":"High","gaps":["Morpholino off-target effects not fully excluded; genetic mutant confirmation not yet available","Molecular mechanism (channel target, cAMP dependence) not identified"]},{"year":2015,"claim":"Transcriptional control of Popdc2 was resolved: MEIS1 and NKX2-5 bind its enhancer in a mutually exclusive manner, providing a competitive switch that times Popdc2 activation as cardiac progenitors differentiate.","evidence":"ChIP, enhancer reporter assays, and binding-site competition experiments in cardiac differentiation models","pmids":["26411676"],"confidence":"High","gaps":["Upstream signals controlling the MEIS1-to-NKX2-5 transition are not defined","Whether this enhancer logic applies to POPDC1 or POPDC3 is unknown"]},{"year":2020,"claim":"The molecular mechanism was identified: POPDC2 upregulates TREK-1 potassium channel current in a cAMP-dependent manner, and a human disease-causing nonsense variant (W188*) that truncates the cAMP-binding domain abolishes this modulation and causes stress-induced sinus bradycardia in knock-in mice.","evidence":"Xenopus oocyte two-electrode voltage clamp co-expressing POPDC2 and TREK-1; W188* knock-in mouse with ECG telemetry","pmids":["32535041"],"confidence":"High","gaps":["Direct cAMP-binding affinity of POPDC2 was not measured in this study","Whether POPDC2 interacts physically with TREK-1 or acts indirectly was not resolved","Contribution of other K2P channels not tested"]},{"year":2025,"claim":"Bi-allelic POPDC2 variants were confirmed as a cause of human cardiac conduction disease; pathogenic mutations abolish TREK-1 modulation and destabilize the POPDC1–POPDC2 complex, with both proteins reduced in patient muscle biopsies and co-expression enriched in AV node cells.","evidence":"Xenopus oocyte electrophysiology; protein quantification from patient muscle biopsy; homology modeling; single-cell RNA-seq of human hearts","pmids":["40409267"],"confidence":"High","gaps":["Stoichiometry and structural basis of the POPDC1–POPDC2 complex remain undetermined","Whether POPDC2 loss affects channels other than TREK-1 in human conduction tissue is unknown","No rescue or gene-therapy experiments have been reported"]},{"year":null,"claim":"Key unresolved questions include the structural basis of cAMP binding and TREK-1 modulation by POPDC2, whether POPDC2 directly contacts TREK-1, and whether additional effectors beyond TREK-1 contribute to conduction phenotypes.","evidence":"","pmids":[],"confidence":"High","gaps":["No crystal or cryo-EM structure of POPDC2 or the POPDC1–POPDC2 complex","Direct physical interaction between POPDC2 and TREK-1 not demonstrated","Role of POPDC2 in skeletal muscle disease beyond cardiac conduction not mechanistically defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[2,3]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4,5]}],"pathway":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4]}],"complexes":["POPDC1-POPDC2 complex"],"partners":["POPDC1","TREK-1"],"other_free_text":[]},"mechanistic_narrative":"POPDC2 is a transmembrane, striated-muscle-enriched cAMP-effector protein essential for cardiac conduction system function. It is expressed from early cardiogenesis under sequential regulation by MEIS1 and NKX2-5 [PMID:26411676], with highest abundance in the atrioventricular node and bundle [PMID:40409267]. POPDC2 upregulates TREK-1 two-pore domain potassium channel current density in a cAMP-dependent manner; loss-of-function variants abolish this modulation and cause sinus bradycardia, atrioventricular conduction block, and arrhythmia in zebrafish, mouse, and human [PMID:22290329, PMID:32535041, PMID:40409267]. POPDC2 forms a complex with POPDC1, and pathogenic variants in either gene reduce abundance and membrane trafficking of both proteins, establishing bi-allelic POPDC2 loss-of-function as a cause of hereditary cardiac conduction disease [PMID:40409267]."},"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":"12859897","id":"PMC_12859897","title":"Pollen 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\"Morpholino knockdown in zebrafish; optical recording of cardiac contractility; calcium transient imaging via transgenic calcium indicator (Tg(cmlc2:gCaMP)) and SPIM microscopy\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple orthogonal phenotypic readouts (morphology, electrophysiology, calcium imaging) in zebrafish ortholog\",\n      \"pmids\": [\"22290329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"MEIS1 and NKX2-5 bind overlapping sites on the Popdc2 gene enhancer in a mutually exclusive manner; as cardiac progenitors differentiate, sequential high MEIS1 then increasing NKX2-5 levels provide spatiotemporal transcriptional regulation of Popdc2 expression during cardiogenesis.\",\n      \"method\": \"ChIP, reporter assays, in vivo cardiac differentiation models; binding site competition assays\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal binding and functional enhancer validation with multiple orthogonal methods\",\n      \"pmids\": [\"26411676\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"A loss-of-function nonsense mutation in POPDC2 (W188*, deleting part of the cAMP-binding domain) impairs TREK-1 potassium channel modulation; co-expression of POPDC2W188* with TREK-1 in Xenopus oocytes fails to increase TREK-1 current density, and knock-in mice display stress-induced sinus bradycardia and pauses, establishing POPDC2 as a cAMP-dependent regulator of TREK-1 required for normal cardiac conduction.\",\n      \"method\": \"Xenopus oocyte electrophysiology (co-expression); knock-in mouse model; quantitative PCR for expression profiling\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro electrophysiology with disease variant, corroborated by knock-in mouse phenotype\",\n      \"pmids\": [\"32535041\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Bi-allelic loss-of-function variants in POPDC2 abolish TREK-1 current upregulation in vitro; patient muscle biopsies show reduced abundance of both POPDC1 and POPDC2, indicating that pathogenic variants in either protein impair stability and/or membrane trafficking of the POPDC1-POPDC2 complex; single-cell RNA-seq shows co-expression of POPDC1 and POPDC2 is highest in AV node, AV node pacemaker, and AV bundle cells.\",\n      \"method\": \"In vitro electrophysiology (Xenopus oocyte co-expression); protein quantification from patient muscle biopsy; homology modeling; single-cell RNA sequencing of human hearts\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (electrophysiology, proteomics, scRNA-seq) in single rigorous study\",\n      \"pmids\": [\"40409267\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Popdc2 is a transmembrane plasma membrane protein expressed predominantly in myocardium and skeletal muscle during chick embryogenesis, with distinct spatial patterns across cardiac chambers and muscle-forming fields, identifying it as a myocyte-specific differentiation marker.\",\n      \"method\": \"Northern blot; whole-mount in situ hybridization; cDNA cloning of alternatively spliced isoforms\",\n      \"journal\": \"Developmental dynamics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — direct localization by expression analysis; no functional manipulation\",\n      \"pmids\": [\"14991725\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"LacZ reporter knock-in into the Popdc2 locus shows expression in cardiac and extraembryonic mesoderm from E7.5, then heart, somites, coelomic mesothelium, skeletal muscle, bladder, gut, and smooth muscle postnatally, establishing the in vivo expression pattern linked to the Popdc2 locus.\",\n      \"method\": \"LacZ reporter gene knock-in; embryonic and postnatal whole-mount and section staining\",\n      \"journal\": \"Developmental dynamics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct reporter-based localization from endogenous locus; no functional consequence tested\",\n      \"pmids\": [\"18189275\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"POPDC2 is a transmembrane, striated-muscle-enriched protein whose conserved Popeye (cAMP-binding) domain directly binds cAMP and, in a cAMP-dependent manner, upregulates TREK-1 two-pore domain potassium channel current density; loss of this regulation—through knockout, morpholino knockdown, or disease-causing variants—causes sinus node dysfunction, atrioventricular conduction block, and cardiac arrhythmia, while pathogenic variants in either POPDC2 or its partner POPDC1 also impair membrane trafficking of the POPDC1–POPDC2 complex.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"POPDC2 is a transmembrane, striated-muscle-enriched cAMP-effector protein essential for cardiac conduction system function. It is expressed from early cardiogenesis under sequential regulation by MEIS1 and NKX2-5 [PMID:26411676], with highest abundance in the atrioventricular node and bundle [PMID:40409267]. POPDC2 upregulates TREK-1 two-pore domain potassium channel current density in a cAMP-dependent manner; loss-of-function variants abolish this modulation and cause sinus bradycardia, atrioventricular conduction block, and arrhythmia in zebrafish, mouse, and human [PMID:22290329, PMID:32535041, PMID:40409267]. POPDC2 forms a complex with POPDC1, and pathogenic variants in either gene reduce abundance and membrane trafficking of both proteins, establishing bi-allelic POPDC2 loss-of-function as a cause of hereditary cardiac conduction disease [PMID:40409267].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Before any functional data existed, expression profiling established that Popdc2 encodes a transmembrane protein with striking restriction to myocardium and skeletal muscle during embryogenesis, framing it as a candidate striated-muscle regulator.\",\n      \"evidence\": \"Northern blot and whole-mount in situ hybridization in chick embryos\",\n      \"pmids\": [\"14991725\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Expression data only; no functional manipulation or loss-of-function phenotype\",\n        \"Mammalian expression pattern not yet mapped from endogenous locus\"\n      ]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"A LacZ knock-in at the endogenous Popdc2 locus confirmed cardiac and somitic expression beginning at E7.5 and revealed postnatal expression in smooth muscle and visceral organs, extending the known expression domains to mammalian development.\",\n      \"evidence\": \"LacZ reporter knock-in mouse with whole-mount and section staining across embryonic and postnatal stages\",\n      \"pmids\": [\"18189275\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Reporter knock-in disrupts one allele but heterozygous phenotype was not assessed\",\n        \"No functional consequence of loss tested\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"The first loss-of-function study showed that popdc2 is required for cardiac conduction and striated muscle development: morpholino knockdown in zebrafish produced AV block, arrhythmia, and skeletal muscle disorganization, establishing a direct role in cardiac rhythm.\",\n      \"evidence\": \"Morpholino knockdown in zebrafish with optical contractility mapping and calcium transient imaging (Tg(cmlc2:gCaMP))\",\n      \"pmids\": [\"22290329\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Morpholino off-target effects not fully excluded; genetic mutant confirmation not yet available\",\n        \"Molecular mechanism (channel target, cAMP dependence) not identified\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Transcriptional control of Popdc2 was resolved: MEIS1 and NKX2-5 bind its enhancer in a mutually exclusive manner, providing a competitive switch that times Popdc2 activation as cardiac progenitors differentiate.\",\n      \"evidence\": \"ChIP, enhancer reporter assays, and binding-site competition experiments in cardiac differentiation models\",\n      \"pmids\": [\"26411676\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Upstream signals controlling the MEIS1-to-NKX2-5 transition are not defined\",\n        \"Whether this enhancer logic applies to POPDC1 or POPDC3 is unknown\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"The molecular mechanism was identified: POPDC2 upregulates TREK-1 potassium channel current in a cAMP-dependent manner, and a human disease-causing nonsense variant (W188*) that truncates the cAMP-binding domain abolishes this modulation and causes stress-induced sinus bradycardia in knock-in mice.\",\n      \"evidence\": \"Xenopus oocyte two-electrode voltage clamp co-expressing POPDC2 and TREK-1; W188* knock-in mouse with ECG telemetry\",\n      \"pmids\": [\"32535041\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct cAMP-binding affinity of POPDC2 was not measured in this study\",\n        \"Whether POPDC2 interacts physically with TREK-1 or acts indirectly was not resolved\",\n        \"Contribution of other K2P channels not tested\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Bi-allelic POPDC2 variants were confirmed as a cause of human cardiac conduction disease; pathogenic mutations abolish TREK-1 modulation and destabilize the POPDC1–POPDC2 complex, with both proteins reduced in patient muscle biopsies and co-expression enriched in AV node cells.\",\n      \"evidence\": \"Xenopus oocyte electrophysiology; protein quantification from patient muscle biopsy; homology modeling; single-cell RNA-seq of human hearts\",\n      \"pmids\": [\"40409267\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Stoichiometry and structural basis of the POPDC1–POPDC2 complex remain undetermined\",\n        \"Whether POPDC2 loss affects channels other than TREK-1 in human conduction tissue is unknown\",\n        \"No rescue or gene-therapy experiments have been reported\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of cAMP binding and TREK-1 modulation by POPDC2, whether POPDC2 directly contacts TREK-1, and whether additional effectors beyond TREK-1 contribute to conduction phenotypes.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No crystal or cryo-EM structure of POPDC2 or the POPDC1–POPDC2 complex\",\n        \"Direct physical interaction between POPDC2 and TREK-1 not demonstrated\",\n        \"Role of POPDC2 in skeletal muscle disease beyond cardiac conduction not mechanistically defined\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [2, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"complexes\": [\"POPDC1-POPDC2 complex\"],\n    \"partners\": [\"POPDC1\", \"TREK-1\"],\n    \"other_free_text\": []\n  }\n}\n```\n\nWait, I need to fix the pathway section - I accidentally used a GO term instead of a Reactome pathway. Let me correct this.\n\n```json\n{\n  \"mechanistic_narrative\": \"POPDC2 is a transmembrane, striated-muscle-enriched cAMP-effector protein essential for cardiac conduction system function. It is expressed from early cardiogenesis under sequential regulation by MEIS1 and NKX2-5 [PMID:26411676], with highest abundance in the atrioventricular node and bundle [PMID:40409267]. POPDC2 upregulates TREK-1 two-pore domain potassium channel current density in a cAMP-dependent manner; loss-of-function variants abolish this modulation and cause sinus bradycardia, atrioventricular conduction block, and arrhythmia in zebrafish, mouse, and human [PMID:22290329, PMID:32535041, PMID:40409267]. POPDC2 forms a complex with POPDC1, and pathogenic variants in either gene reduce abundance and membrane trafficking of both proteins, establishing bi-allelic POPDC2 loss-of-function as a cause of hereditary cardiac conduction disease [PMID:40409267].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Before any functional data existed, expression profiling established that Popdc2 encodes a transmembrane protein with striking restriction to myocardium and skeletal muscle during embryogenesis, framing it as a candidate striated-muscle regulator.\",\n      \"evidence\": \"Northern blot and whole-mount in situ hybridization in chick embryos\",\n      \"pmids\": [\"14991725\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Expression data only; no functional manipulation or loss-of-function phenotype\",\n        \"Mammalian expression pattern not yet mapped from endogenous locus\"\n      ]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"A LacZ knock-in at the endogenous Popdc2 locus confirmed cardiac and somitic expression beginning at E7.5 and revealed postnatal expression in smooth muscle and visceral organs, extending the known expression domains to mammalian development.\",\n      \"evidence\": \"LacZ reporter knock-in mouse with whole-mount and section staining across embryonic and postnatal stages\",\n      \"pmids\": [\"18189275\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Reporter knock-in disrupts one allele but heterozygous phenotype was not assessed\",\n        \"No functional consequence of loss tested\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"The first loss-of-function study showed that popdc2 is required for cardiac conduction and striated muscle development: morpholino knockdown in zebrafish produced AV block, arrhythmia, and skeletal muscle disorganization, establishing a direct role in cardiac rhythm.\",\n      \"evidence\": \"Morpholino knockdown in zebrafish with optical contractility mapping and calcium transient imaging (Tg(cmlc2:gCaMP))\",\n      \"pmids\": [\"22290329\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Morpholino off-target effects not fully excluded; genetic mutant confirmation not yet available\",\n        \"Molecular mechanism (channel target, cAMP dependence) not identified\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Transcriptional control of Popdc2 was resolved: MEIS1 and NKX2-5 bind its enhancer in a mutually exclusive manner, providing a competitive switch that times Popdc2 activation as cardiac progenitors differentiate.\",\n      \"evidence\": \"ChIP, enhancer reporter assays, and binding-site competition experiments in cardiac differentiation models\",\n      \"pmids\": [\"26411676\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Upstream signals controlling the MEIS1-to-NKX2-5 transition are not defined\",\n        \"Whether this enhancer logic applies to POPDC1 or POPDC3 is unknown\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"The molecular mechanism was identified: POPDC2 upregulates TREK-1 potassium channel current in a cAMP-dependent manner, and a human disease-causing nonsense variant (W188*) that truncates the cAMP-binding domain abolishes this modulation and causes stress-induced sinus bradycardia in knock-in mice.\",\n      \"evidence\": \"Xenopus oocyte two-electrode voltage clamp co-expressing POPDC2 and TREK-1; W188* knock-in mouse with ECG telemetry\",\n      \"pmids\": [\"32535041\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct cAMP-binding affinity of POPDC2 was not measured in this study\",\n        \"Whether POPDC2 interacts physically with TREK-1 or acts indirectly was not resolved\",\n        \"Contribution of other K2P channels not tested\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Bi-allelic POPDC2 variants were confirmed as a cause of human cardiac conduction disease; pathogenic mutations abolish TREK-1 modulation and destabilize the POPDC1–POPDC2 complex, with both proteins reduced in patient muscle biopsies and co-expression enriched in AV node cells.\",\n      \"evidence\": \"Xenopus oocyte electrophysiology; protein quantification from patient muscle biopsy; homology modeling; single-cell RNA-seq of human hearts\",\n      \"pmids\": [\"40409267\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Stoichiometry and structural basis of the POPDC1–POPDC2 complex remain undetermined\",\n        \"Whether POPDC2 loss affects channels other than TREK-1 in human conduction tissue is unknown\",\n        \"No rescue or gene-therapy experiments have been reported\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of cAMP binding and TREK-1 modulation by POPDC2, whether POPDC2 directly contacts TREK-1, and whether additional effectors beyond TREK-1 contribute to conduction phenotypes.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No crystal or cryo-EM structure of POPDC2 or the POPDC1–POPDC2 complex\",\n        \"Direct physical interaction between POPDC2 and TREK-1 not demonstrated\",\n        \"Role of POPDC2 in skeletal muscle disease beyond cardiac conduction not mechanistically defined\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [2, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"complexes\": [\"POPDC1-POPDC2 complex\"],\n    \"partners\": [\"POPDC1\", \"KCNK2\"],\n    \"other_free_text\": []\n  }\n}\n```"}