{"gene":"C2CD3","run_date":"2026-04-28T17:12:38","timeline":{"discoveries":[{"year":2014,"finding":"C2CD3 localizes to the distal end of centrioles and physically associates with OFD1; C2CD3 acts as a positive regulator of centriole elongation while OFD1 antagonizes C2CD3 activity as a negative regulator. Loss of C2CD3 results in short centrioles lacking subdistal and distal appendages, whereas C2CD3 overexpression triggers centriole hyperelongation.","method":"Co-immunoprecipitation, immunofluorescence colocalization, overexpression and loss-of-function in human cells, patient mutation analysis","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP and multiple cellular phenotype readouts, replicated in patient and model systems","pmids":["24997988"],"is_preprint":false},{"year":2008,"finding":"C2cd3 (mouse ortholog) is required for ciliogenesis and Hedgehog signal transduction; loss of C2cd3 disrupts cilia formation, prevents proteolytic processing of Gli3, and impairs intracellular transduction of the Hedgehog signal. C2cd3 protein localizes to the basal body, consistent with its role in ciliogenesis.","method":"Forward genetic screen, genetic mapping, second allele characterization, target gene expression analysis, double-mutant epistasis, immunofluorescence localization","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — epistasis with Hedgehog pathway, direct localization, and multiple mutant alleles","pmids":["19004860"],"is_preprint":false},{"year":2019,"finding":"CEP120 interacts with C2CD3 and is required for proper recruitment of C2CD3 to the distal ends of centrioles; a disease-associated CEP120 mutant (I975S) has reduced affinity for C2CD3 binding. Loss of CEP120 results in impaired C2CD3 recruitment, failed centriole appendage assembly, and defective ciliogenesis.","method":"Co-immunoprecipitation, CRISPR/Cas9 knockout in RPE1 cells, immunofluorescence, disease mutant binding assay","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP with disease mutant validation and clean KO phenotype","pmids":["30988386"],"is_preprint":false},{"year":2016,"finding":"Loss of C2CD3 function in human patient-derived fibroblasts reduces the ability to form cilia and impairs basal body maturation; however, distal appendage protein CEP164 is still recruited, indicating CEP164 recruitment is not sufficient for efficient basal body maturation in C2CD3-defective backgrounds.","method":"Patient fibroblast culture analysis, immunofluorescence for basal body and appendage markers, ciliation assay","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — direct patient-cell functional assay with specific marker readout, single lab","pmids":["27094867"],"is_preprint":false},{"year":2021,"finding":"Conditional deletion of the C-terminal PKC-C2 domains or the N-terminal C2CD3N-C2 domain of C2cd3 in mice reveals that the N-terminal C2CD3N-C2 domain is critical for early embryonic development and a craniofacial-specific role, indicating domain-specific functional requirements.","method":"Conditional allelic series in mouse, tissue-specific isoform analysis, phenotypic characterization","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 — domain deletion in vivo with defined phenotypic readout, single lab","pmids":["34211969"],"is_preprint":false},{"year":2025,"finding":"C2CD3 adopts a radially symmetric 9-fold organization within the centriole's distal lumen, forming a ~100 nm luminal ring structure (~27 nodes). The C-terminal region localizes near the luminal ring, while the N-terminal region localizes near a hook-like structure that attaches to the A-microtubule and is adjacent to the DISCO complex (MNR/CEP90/OFD1). C2CD3 depletion disrupts DISCO complex recruitment via direct interaction with MNR, destabilizes the luminal ring network (C2CD3/SFI1/centrin-2/CEP135/NA14), and impairs the distal microtubule tip protein CEP162, functioning as an architectural scaffold for distal centriole assembly and appendage formation.","method":"Ultrastructure Expansion Microscopy (U-ExM), iterative U-ExM, in situ cryo-electron tomography (cryo-ET), C2CD3 depletion with immunofluorescence for multiple markers, direct interaction assay with MNR","journal":"PLoS biology","confidence":"High","confidence_rationale":"Tier 1 — structural determination by cryo-ET and U-ExM combined with functional depletion and direct binding experiments","pmids":["41364719"],"is_preprint":false},{"year":2025,"finding":"C2CD3 acts upstream of SSNA1 and LRRCC1 in a hierarchical targeting network in the distal centriolar lumen; C2CD3 is required for SSNA1 localization to the distal lumen of centrioles.","method":"Knockout cell lines, super-resolution imaging with expansion microscopy, epistasis analysis","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — KO epistasis with super-resolution imaging, preprint single lab","pmids":["bio_10.1101_2025.04.28.648957"],"is_preprint":true},{"year":2025,"finding":"Hypomorphic missense variants in C2CD3 reduce C2CD3 localization to the basal body, shorten cilia length, reduce ciliation frequency (especially in kidney epithelial cells), and dysregulate SHH signaling (reduced GLI3 activator and GLI1 mRNA); rescue experiments in C2CD3 KO RPE-1 cells confirm these variants have a reduced capacity to restore ciliogenesis.","method":"Patient-derived fibroblast and urinary renal epithelial cell assays, C2CD3 KO RPE-1 rescue experiments, immunofluorescence, qRT-PCR for GLI1 and GLI3","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 — KO rescue with functional readout in multiple cell types, single lab","pmids":["39690811"],"is_preprint":false},{"year":2025,"finding":"C. elegans SAS-1 (homolog of human C2CD3) is essential for centriole integrity during oogenesis, spermatogenesis, and early embryogenesis, and is required for SSNA-1 localization to centrioles; SAS-1 epistasis places it upstream of SSNA-1 in centriole organization.","method":"Null allele generation, U-Ex-STED microscopy, molecular epistasis with null alleles, heterologous human cell recruitment assay","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — null allele epistasis with super-resolution imaging in C. elegans ortholog, preprint","pmids":["bio_10.1101_2025.04.22.650004"],"is_preprint":true}],"current_model":"C2CD3 is an architectural scaffold protein that adopts a 9-fold radially symmetric organization in the distal centriolar lumen, where it forms and stabilizes a luminal ring network (with SFI1, centrin-2, CEP135, and NA14), extends radially via a hook-like N-terminal structure to attach to A-microtubules, recruits the DISCO complex (via direct interaction with MNR) to initiate distal appendage assembly, and promotes centriole elongation in opposition to OFD1; loss of C2CD3 results in short centrioles lacking appendages, failed ciliogenesis, and impaired Hedgehog signal transduction."},"narrative":{"teleology":[{"year":2008,"claim":"A forward genetic screen established that C2cd3 is required for ciliogenesis and acts as an intracellular component of Hedgehog signal transduction, resolving whether the gene functions in cilia formation versus signaling per se by showing loss of both cilia and Gli3 processing.","evidence":"Forward genetic screen with multiple mutant alleles in mouse, epistasis with Hedgehog pathway components","pmids":["19004860"],"confidence":"High","gaps":["Molecular mechanism by which C2cd3 supports ciliogenesis was unknown","Direct localization within the centriole substructure was not resolved","No interacting partners were identified"]},{"year":2014,"claim":"C2CD3 was placed at the distal end of centrioles and shown to physically associate with OFD1 in an antagonistic regulatory axis controlling centriole length, establishing that centriole elongation is actively balanced by opposing activities.","evidence":"Reciprocal co-immunoprecipitation, overexpression and loss-of-function in human cells, patient mutation analysis","pmids":["24997988"],"confidence":"High","gaps":["How C2CD3 mechanistically drives centriole elongation was not determined","Structural organization of C2CD3 at the distal centriole was unresolved","Relationship to appendage assembly beyond marker loss was unclear"]},{"year":2016,"claim":"Patient fibroblast analysis showed that C2CD3 loss impairs basal body maturation even when the distal appendage protein CEP164 is still recruited, separating C2CD3 function from simple appendage protein targeting.","evidence":"Patient-derived fibroblast immunofluorescence and ciliation assay","pmids":["27094867"],"confidence":"Medium","gaps":["Single-lab patient cell study; not replicated independently","Which step of basal body maturation requires C2CD3 was not defined","Whether CEP164 at appendages is fully functional in C2CD3-deficient cells was untested"]},{"year":2019,"claim":"CEP120 was identified as a physical partner required for C2CD3 recruitment to distal centrioles, and a disease-associated CEP120 mutation reduced C2CD3 binding, establishing a recruitment hierarchy upstream of C2CD3.","evidence":"Reciprocal co-IP, CRISPR KO in RPE1 cells, disease mutant binding assay","pmids":["30988386"],"confidence":"High","gaps":["Whether CEP120 directly deposits C2CD3 or acts indirectly was not resolved","Domains of C2CD3 mediating CEP120 interaction were not mapped"]},{"year":2021,"claim":"Domain-specific conditional alleles in mouse revealed that the N-terminal C2 domain of C2cd3 is critical for early embryonic and craniofacial development, establishing that C2CD3 has separable domain-specific functions in vivo.","evidence":"Conditional allelic series with tissue-specific phenotyping in mouse","pmids":["34211969"],"confidence":"Medium","gaps":["Single-lab study; molecular partners of each domain were not identified","How N-terminal versus C-terminal domains relate to centriolar substructures was unknown"]},{"year":2025,"claim":"Cryo-ET and expansion microscopy resolved C2CD3's 9-fold radially symmetric architecture in the distal centriolar lumen, showed its N-terminus forms a hook attaching to A-microtubules while its C-terminus anchors a luminal ring with SFI1/centrin-2/CEP135/NA14, and demonstrated that C2CD3 directly binds MNR to recruit the DISCO complex for appendage initiation—unifying its roles in centriole structure, elongation, and appendage assembly.","evidence":"U-ExM, cryo-ET, C2CD3 depletion with multi-marker IF, direct MNR interaction assay","pmids":["41364719"],"confidence":"High","gaps":["Atomic-resolution structure of C2CD3 or its domain interfaces is not available","How the luminal ring contributes to centriole elongation mechanistically is unclear","Whether the 9-fold ring is dynamically remodeled during the cell cycle is unknown"]},{"year":2025,"claim":"Hypomorphic patient variants were shown to reduce C2CD3 basal body localization, shorten cilia, and attenuate SHH signaling output, directly linking variant severity to functional deficits via rescue in KO cells.","evidence":"Patient fibroblast and renal epithelial cell assays, C2CD3 KO RPE-1 rescue, qRT-PCR for GLI1/GLI3","pmids":["39690811"],"confidence":"Medium","gaps":["Single-lab study with limited variant panel","Whether reduced localization reflects folding defects or impaired CEP120 interaction is untested"]},{"year":2025,"claim":"Cross-species studies in C. elegans (SAS-1) and human cells placed C2CD3 upstream of SSNA1 and LRRCC1 in the distal luminal hierarchy, establishing a conserved epistatic relationship for centriole integrity.","evidence":"Null allele epistasis in C. elegans with U-Ex-STED, KO super-resolution imaging in human cells (preprints)","pmids":["bio_10.1101_2025.04.28.648957","bio_10.1101_2025.04.22.650004"],"confidence":"Medium","gaps":["Both studies are preprints awaiting peer review","Whether C2CD3–SSNA1 interaction is direct or mediated by the luminal ring is unresolved","Functional consequence of SSNA1 loss downstream of C2CD3 in ciliogenesis not fully characterized"]},{"year":null,"claim":"The atomic structure of C2CD3, the mechanism by which it promotes centriole elongation, and the dynamic regulation of the luminal ring during centriole maturation and the cell cycle remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of C2CD3 or its individual domains is available","How the C2CD3–OFD1 antagonism is biochemically regulated is unknown","Whether C2CD3 has enzymatic activity through its C2 domains or acts purely as a scaffold is untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,5]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[5]}],"localization":[{"term_id":"GO:0005815","term_label":"microtubule organizing center","supporting_discovery_ids":[0,1,2,5]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1,7]},{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[0,5]}],"complexes":["C2CD3/SFI1/centrin-2/CEP135/NA14 luminal ring"],"partners":["OFD1","CEP120","MNR","SFI1","CETN2","CEP135","SSNA1"],"other_free_text":[]},"mechanistic_narrative":"C2CD3 is a centriolar scaffold protein that organizes the distal lumen of centrioles and is essential for ciliogenesis, centriole appendage assembly, and Hedgehog signal transduction. It adopts a 9-fold radially symmetric arrangement forming a ~100 nm luminal ring composed of C2CD3, SFI1, centrin-2, CEP135, and NA14, with its N-terminal hook-like domain attaching to A-microtubules and its C-terminal region anchoring the luminal ring; depletion disrupts this network, abolishes DISCO complex (MNR/CEP90/OFD1) recruitment via direct interaction with MNR, and prevents distal appendage formation [PMID:41364719]. C2CD3 promotes centriole elongation in opposition to OFD1, and its loss yields short centrioles lacking subdistal and distal appendages, while its overexpression drives centriole hyperelongation; CEP120 is required for proper C2CD3 recruitment to the distal centriole [PMID:24997988, PMID:30988386]. Loss-of-function mutations in C2CD3 cause ciliopathy phenotypes including defective Gli3 processing and impaired Hedgehog signaling, and hypomorphic patient variants reduce ciliation frequency and SHH pathway output in rescue experiments [PMID:19004860, PMID:39690811]."},"prefetch_data":{"uniprot":{"accession":"Q4AC94","full_name":"C2 domain-containing protein 3","aliases":[],"length_aa":2353,"mass_kda":260.4,"function":"Component of the centrioles that acts as a positive regulator of centriole elongation (PubMed:24997988). Promotes assembly of centriolar distal appendage, a structure at the distal end of the mother centriole that acts as an anchor of the cilium, and is required for recruitment of centriolar distal appendages proteins CEP83, SCLT1, CEP89, FBF1 and CEP164. Not required for centriolar satellite integrity or RAB8 activation. Required for primary cilium formation (PubMed:23769972). Required for sonic hedgehog/SHH signaling and for proteolytic processing of GLI3","subcellular_location":"Cytoplasm, cytoskeleton, cilium basal body; Cytoplasm, cytoskeleton, microtubule organizing center, centrosome, centriole","url":"https://www.uniprot.org/uniprotkb/Q4AC94/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/C2CD3","classification":"Not Classified","n_dependent_lines":3,"n_total_lines":1208,"dependency_fraction":0.0024834437086092716},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/C2CD3","total_profiled":1310},"omim":[{"mim_id":"615948","title":"OROFACIODIGITAL SYNDROME XIV; OFD14","url":"https://www.omim.org/entry/615948"},{"mim_id":"615944","title":"C2 CALCIUM-DEPENDENT DOMAIN-CONTAINING PROTEIN 3; C2CD3","url":"https://www.omim.org/entry/615944"},{"mim_id":"300170","title":"OFD1 CENTRIOLE AND CENTRIOLAR SATELLITE PROTEIN; OFD1","url":"https://www.omim.org/entry/300170"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Centrosome","reliability":"Supported"},{"location":"Basal body","reliability":"Supported"},{"location":"Acrosome","reliability":"Additional"},{"location":"Equatorial segment","reliability":"Additional"},{"location":"Principal piece","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/C2CD3"},"hgnc":{"alias_symbol":["DKFZP586P0123"],"prev_symbol":[]},"alphafold":{"accession":"Q4AC94","domains":[{"cath_id":"2.60.40.150","chopping":"29-169","consensus_level":"high","plddt":80.5744,"start":29,"end":169},{"cath_id":"2.60.40.150","chopping":"533-551_576-702","consensus_level":"high","plddt":80.2621,"start":533,"end":702},{"cath_id":"2.60.40.150","chopping":"804-969","consensus_level":"medium","plddt":80.0045,"start":804,"end":969},{"cath_id":"2.60.40","chopping":"1023-1041_1054-1162_1171-1187","consensus_level":"medium","plddt":76.7797,"start":1023,"end":1187},{"cath_id":"2.60.40.150","chopping":"1191-1303_1315-1347_1363-1389","consensus_level":"medium","plddt":82.2339,"start":1191,"end":1389},{"cath_id":"2.60.40.150","chopping":"1404-1562","consensus_level":"medium","plddt":79.6753,"start":1404,"end":1562},{"cath_id":"2.60.40.150","chopping":"1634-1652_1660-1777","consensus_level":"high","plddt":79.1327,"start":1634,"end":1777}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q4AC94","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q4AC94-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q4AC94-F1-predicted_aligned_error_v6.png","plddt_mean":55.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=C2CD3","jax_strain_url":"https://www.jax.org/strain/search?query=C2CD3"},"sequence":{"accession":"Q4AC94","fasta_url":"https://rest.uniprot.org/uniprotkb/Q4AC94.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q4AC94/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q4AC94"}},"corpus_meta":[{"pmid":"24997988","id":"PMC_24997988","title":"The oral-facial-digital syndrome gene C2CD3 encodes a positive regulator of centriole elongation.","date":"2014","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/24997988","citation_count":115,"is_preprint":false},{"pmid":"19004860","id":"PMC_19004860","title":"C2cd3 is required for cilia formation and Hedgehog signaling in mouse.","date":"2008","source":"Development (Cambridge, England)","url":"https://pubmed.ncbi.nlm.nih.gov/19004860","citation_count":85,"is_preprint":false},{"pmid":"30988386","id":"PMC_30988386","title":"CEP120 interacts with C2CD3 and Talpid3 and is required for centriole appendage assembly and ciliogenesis.","date":"2019","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/30988386","citation_count":34,"is_preprint":false},{"pmid":"27094867","id":"PMC_27094867","title":"Mutations in human C2CD3 cause skeletal dysplasia and provide new insights into phenotypic and cellular consequences of altered C2CD3 function.","date":"2016","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/27094867","citation_count":30,"is_preprint":false},{"pmid":"30097616","id":"PMC_30097616","title":"Characterization of three ciliopathy pedigrees expands the phenotype associated with biallelic C2CD3 variants.","date":"2018","source":"European journal of human genetics : EJHG","url":"https://pubmed.ncbi.nlm.nih.gov/30097616","citation_count":22,"is_preprint":false},{"pmid":"34211969","id":"PMC_34211969","title":"Centriolar Protein C2cd3 Is Required for Craniofacial Development.","date":"2021","source":"Frontiers in cell and developmental biology","url":"https://pubmed.ncbi.nlm.nih.gov/34211969","citation_count":6,"is_preprint":false},{"pmid":"33805906","id":"PMC_33805906","title":"Mutation in the Ciliary Protein C2CD3 Reveals Organ-Specific Mechanisms of Hedgehog Signal Transduction in Avian Embryos.","date":"2021","source":"Journal of developmental biology","url":"https://pubmed.ncbi.nlm.nih.gov/33805906","citation_count":5,"is_preprint":false},{"pmid":"40667239","id":"PMC_40667239","title":"The Luminal Ring Protein C2CD3 Acts as a Radial In-to-Out Organizer of the Distal Centriole and Appendages.","date":"2025","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/40667239","citation_count":4,"is_preprint":false},{"pmid":"38810379","id":"PMC_38810379","title":"The ciliary protein C2cd3 is required for mandibular musculoskeletal tissue patterning.","date":"2024","source":"Differentiation; research in biological diversity","url":"https://pubmed.ncbi.nlm.nih.gov/38810379","citation_count":1,"is_preprint":false},{"pmid":"41364719","id":"PMC_41364719","title":"The luminal ring protein C2CD3 acts as a radial in-to-out organizer of the distal centriole and appendages.","date":"2025","source":"PLoS biology","url":"https://pubmed.ncbi.nlm.nih.gov/41364719","citation_count":1,"is_preprint":false},{"pmid":"39734585","id":"PMC_39734585","title":"Multiple Fibrolipomas of the Tongue: A Rare Case Report of a Pediatric Patient With Whole Exome Sequencing of the C2CD3 Gene.","date":"2024","source":"Case reports in dentistry","url":"https://pubmed.ncbi.nlm.nih.gov/39734585","citation_count":1,"is_preprint":false},{"pmid":"39690811","id":"PMC_39690811","title":"Use of patient-derived cell models for characterization of compound heterozygous hypomorphic C2CD3 variants in a patient with isolated nephronophthisis.","date":"2025","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/39690811","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.04.28.648957","title":"SSNA1 organizes the distal luminal centriolar network and promotes ciliogenesis without microtubule association","date":"2025-04-28","source":"bioRxiv","url":"https://doi.org/10.1101/2025.04.28.648957","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2025.04.22.650004","title":"<i>C. elegans</i> SAS-1 ensures centriole integrity and ciliary function, and operates with SSNA-1","date":"2025-04-24","source":"bioRxiv","url":"https://doi.org/10.1101/2025.04.22.650004","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8495,"output_tokens":2302,"usd":0.030007},"stage2":{"model":"claude-opus-4-6","input_tokens":5591,"output_tokens":2472,"usd":0.134632},"total_usd":0.164639,"stage1_batch_id":"msgbatch_01TqNM6p6FsoMBT2jzQTwKBr","stage2_batch_id":"msgbatch_01Ya473eco8sTHbvRymQaU8o","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2014,\n      \"finding\": \"C2CD3 localizes to the distal end of centrioles and physically associates with OFD1; C2CD3 acts as a positive regulator of centriole elongation while OFD1 antagonizes C2CD3 activity as a negative regulator. Loss of C2CD3 results in short centrioles lacking subdistal and distal appendages, whereas C2CD3 overexpression triggers centriole hyperelongation.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence colocalization, overexpression and loss-of-function in human cells, patient mutation analysis\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP and multiple cellular phenotype readouts, replicated in patient and model systems\",\n      \"pmids\": [\"24997988\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"C2cd3 (mouse ortholog) is required for ciliogenesis and Hedgehog signal transduction; loss of C2cd3 disrupts cilia formation, prevents proteolytic processing of Gli3, and impairs intracellular transduction of the Hedgehog signal. C2cd3 protein localizes to the basal body, consistent with its role in ciliogenesis.\",\n      \"method\": \"Forward genetic screen, genetic mapping, second allele characterization, target gene expression analysis, double-mutant epistasis, immunofluorescence localization\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis with Hedgehog pathway, direct localization, and multiple mutant alleles\",\n      \"pmids\": [\"19004860\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CEP120 interacts with C2CD3 and is required for proper recruitment of C2CD3 to the distal ends of centrioles; a disease-associated CEP120 mutant (I975S) has reduced affinity for C2CD3 binding. Loss of CEP120 results in impaired C2CD3 recruitment, failed centriole appendage assembly, and defective ciliogenesis.\",\n      \"method\": \"Co-immunoprecipitation, CRISPR/Cas9 knockout in RPE1 cells, immunofluorescence, disease mutant binding assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP with disease mutant validation and clean KO phenotype\",\n      \"pmids\": [\"30988386\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Loss of C2CD3 function in human patient-derived fibroblasts reduces the ability to form cilia and impairs basal body maturation; however, distal appendage protein CEP164 is still recruited, indicating CEP164 recruitment is not sufficient for efficient basal body maturation in C2CD3-defective backgrounds.\",\n      \"method\": \"Patient fibroblast culture analysis, immunofluorescence for basal body and appendage markers, ciliation assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct patient-cell functional assay with specific marker readout, single lab\",\n      \"pmids\": [\"27094867\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Conditional deletion of the C-terminal PKC-C2 domains or the N-terminal C2CD3N-C2 domain of C2cd3 in mice reveals that the N-terminal C2CD3N-C2 domain is critical for early embryonic development and a craniofacial-specific role, indicating domain-specific functional requirements.\",\n      \"method\": \"Conditional allelic series in mouse, tissue-specific isoform analysis, phenotypic characterization\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — domain deletion in vivo with defined phenotypic readout, single lab\",\n      \"pmids\": [\"34211969\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"C2CD3 adopts a radially symmetric 9-fold organization within the centriole's distal lumen, forming a ~100 nm luminal ring structure (~27 nodes). The C-terminal region localizes near the luminal ring, while the N-terminal region localizes near a hook-like structure that attaches to the A-microtubule and is adjacent to the DISCO complex (MNR/CEP90/OFD1). C2CD3 depletion disrupts DISCO complex recruitment via direct interaction with MNR, destabilizes the luminal ring network (C2CD3/SFI1/centrin-2/CEP135/NA14), and impairs the distal microtubule tip protein CEP162, functioning as an architectural scaffold for distal centriole assembly and appendage formation.\",\n      \"method\": \"Ultrastructure Expansion Microscopy (U-ExM), iterative U-ExM, in situ cryo-electron tomography (cryo-ET), C2CD3 depletion with immunofluorescence for multiple markers, direct interaction assay with MNR\",\n      \"journal\": \"PLoS biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structural determination by cryo-ET and U-ExM combined with functional depletion and direct binding experiments\",\n      \"pmids\": [\"41364719\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"C2CD3 acts upstream of SSNA1 and LRRCC1 in a hierarchical targeting network in the distal centriolar lumen; C2CD3 is required for SSNA1 localization to the distal lumen of centrioles.\",\n      \"method\": \"Knockout cell lines, super-resolution imaging with expansion microscopy, epistasis analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO epistasis with super-resolution imaging, preprint single lab\",\n      \"pmids\": [\"bio_10.1101_2025.04.28.648957\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Hypomorphic missense variants in C2CD3 reduce C2CD3 localization to the basal body, shorten cilia length, reduce ciliation frequency (especially in kidney epithelial cells), and dysregulate SHH signaling (reduced GLI3 activator and GLI1 mRNA); rescue experiments in C2CD3 KO RPE-1 cells confirm these variants have a reduced capacity to restore ciliogenesis.\",\n      \"method\": \"Patient-derived fibroblast and urinary renal epithelial cell assays, C2CD3 KO RPE-1 rescue experiments, immunofluorescence, qRT-PCR for GLI1 and GLI3\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO rescue with functional readout in multiple cell types, single lab\",\n      \"pmids\": [\"39690811\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"C. elegans SAS-1 (homolog of human C2CD3) is essential for centriole integrity during oogenesis, spermatogenesis, and early embryogenesis, and is required for SSNA-1 localization to centrioles; SAS-1 epistasis places it upstream of SSNA-1 in centriole organization.\",\n      \"method\": \"Null allele generation, U-Ex-STED microscopy, molecular epistasis with null alleles, heterologous human cell recruitment assay\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — null allele epistasis with super-resolution imaging in C. elegans ortholog, preprint\",\n      \"pmids\": [\"bio_10.1101_2025.04.22.650004\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"C2CD3 is an architectural scaffold protein that adopts a 9-fold radially symmetric organization in the distal centriolar lumen, where it forms and stabilizes a luminal ring network (with SFI1, centrin-2, CEP135, and NA14), extends radially via a hook-like N-terminal structure to attach to A-microtubules, recruits the DISCO complex (via direct interaction with MNR) to initiate distal appendage assembly, and promotes centriole elongation in opposition to OFD1; loss of C2CD3 results in short centrioles lacking appendages, failed ciliogenesis, and impaired Hedgehog signal transduction.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"C2CD3 is a centriolar scaffold protein that organizes the distal lumen of centrioles and is essential for ciliogenesis, centriole appendage assembly, and Hedgehog signal transduction. It adopts a 9-fold radially symmetric arrangement forming a ~100 nm luminal ring composed of C2CD3, SFI1, centrin-2, CEP135, and NA14, with its N-terminal hook-like domain attaching to A-microtubules and its C-terminal region anchoring the luminal ring; depletion disrupts this network, abolishes DISCO complex (MNR/CEP90/OFD1) recruitment via direct interaction with MNR, and prevents distal appendage formation [PMID:41364719]. C2CD3 promotes centriole elongation in opposition to OFD1, and its loss yields short centrioles lacking subdistal and distal appendages, while its overexpression drives centriole hyperelongation; CEP120 is required for proper C2CD3 recruitment to the distal centriole [PMID:24997988, PMID:30988386]. Loss-of-function mutations in C2CD3 cause ciliopathy phenotypes including defective Gli3 processing and impaired Hedgehog signaling, and hypomorphic patient variants reduce ciliation frequency and SHH pathway output in rescue experiments [PMID:19004860, PMID:39690811].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"A forward genetic screen established that C2cd3 is required for ciliogenesis and acts as an intracellular component of Hedgehog signal transduction, resolving whether the gene functions in cilia formation versus signaling per se by showing loss of both cilia and Gli3 processing.\",\n      \"evidence\": \"Forward genetic screen with multiple mutant alleles in mouse, epistasis with Hedgehog pathway components\",\n      \"pmids\": [\"19004860\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular mechanism by which C2cd3 supports ciliogenesis was unknown\",\n        \"Direct localization within the centriole substructure was not resolved\",\n        \"No interacting partners were identified\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"C2CD3 was placed at the distal end of centrioles and shown to physically associate with OFD1 in an antagonistic regulatory axis controlling centriole length, establishing that centriole elongation is actively balanced by opposing activities.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation, overexpression and loss-of-function in human cells, patient mutation analysis\",\n      \"pmids\": [\"24997988\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How C2CD3 mechanistically drives centriole elongation was not determined\",\n        \"Structural organization of C2CD3 at the distal centriole was unresolved\",\n        \"Relationship to appendage assembly beyond marker loss was unclear\"\n      ]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Patient fibroblast analysis showed that C2CD3 loss impairs basal body maturation even when the distal appendage protein CEP164 is still recruited, separating C2CD3 function from simple appendage protein targeting.\",\n      \"evidence\": \"Patient-derived fibroblast immunofluorescence and ciliation assay\",\n      \"pmids\": [\"27094867\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single-lab patient cell study; not replicated independently\",\n        \"Which step of basal body maturation requires C2CD3 was not defined\",\n        \"Whether CEP164 at appendages is fully functional in C2CD3-deficient cells was untested\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"CEP120 was identified as a physical partner required for C2CD3 recruitment to distal centrioles, and a disease-associated CEP120 mutation reduced C2CD3 binding, establishing a recruitment hierarchy upstream of C2CD3.\",\n      \"evidence\": \"Reciprocal co-IP, CRISPR KO in RPE1 cells, disease mutant binding assay\",\n      \"pmids\": [\"30988386\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether CEP120 directly deposits C2CD3 or acts indirectly was not resolved\",\n        \"Domains of C2CD3 mediating CEP120 interaction were not mapped\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Domain-specific conditional alleles in mouse revealed that the N-terminal C2 domain of C2cd3 is critical for early embryonic and craniofacial development, establishing that C2CD3 has separable domain-specific functions in vivo.\",\n      \"evidence\": \"Conditional allelic series with tissue-specific phenotyping in mouse\",\n      \"pmids\": [\"34211969\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single-lab study; molecular partners of each domain were not identified\",\n        \"How N-terminal versus C-terminal domains relate to centriolar substructures was unknown\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Cryo-ET and expansion microscopy resolved C2CD3's 9-fold radially symmetric architecture in the distal centriolar lumen, showed its N-terminus forms a hook attaching to A-microtubules while its C-terminus anchors a luminal ring with SFI1/centrin-2/CEP135/NA14, and demonstrated that C2CD3 directly binds MNR to recruit the DISCO complex for appendage initiation—unifying its roles in centriole structure, elongation, and appendage assembly.\",\n      \"evidence\": \"U-ExM, cryo-ET, C2CD3 depletion with multi-marker IF, direct MNR interaction assay\",\n      \"pmids\": [\"41364719\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Atomic-resolution structure of C2CD3 or its domain interfaces is not available\",\n        \"How the luminal ring contributes to centriole elongation mechanistically is unclear\",\n        \"Whether the 9-fold ring is dynamically remodeled during the cell cycle is unknown\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Hypomorphic patient variants were shown to reduce C2CD3 basal body localization, shorten cilia, and attenuate SHH signaling output, directly linking variant severity to functional deficits via rescue in KO cells.\",\n      \"evidence\": \"Patient fibroblast and renal epithelial cell assays, C2CD3 KO RPE-1 rescue, qRT-PCR for GLI1/GLI3\",\n      \"pmids\": [\"39690811\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single-lab study with limited variant panel\",\n        \"Whether reduced localization reflects folding defects or impaired CEP120 interaction is untested\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Cross-species studies in C. elegans (SAS-1) and human cells placed C2CD3 upstream of SSNA1 and LRRCC1 in the distal luminal hierarchy, establishing a conserved epistatic relationship for centriole integrity.\",\n      \"evidence\": \"Null allele epistasis in C. elegans with U-Ex-STED, KO super-resolution imaging in human cells (preprints)\",\n      \"pmids\": [\"bio_10.1101_2025.04.28.648957\", \"bio_10.1101_2025.04.22.650004\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Both studies are preprints awaiting peer review\",\n        \"Whether C2CD3–SSNA1 interaction is direct or mediated by the luminal ring is unresolved\",\n        \"Functional consequence of SSNA1 loss downstream of C2CD3 in ciliogenesis not fully characterized\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The atomic structure of C2CD3, the mechanism by which it promotes centriole elongation, and the dynamic regulation of the luminal ring during centriole maturation and the cell cycle remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No high-resolution structure of C2CD3 or its individual domains is available\",\n        \"How the C2CD3–OFD1 antagonism is biochemically regulated is unknown\",\n        \"Whether C2CD3 has enzymatic activity through its C2 domains or acts purely as a scaffold is untested\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 5]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005815\", \"supporting_discovery_ids\": [0, 1, 2, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 7]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"complexes\": [\n      \"C2CD3/SFI1/centrin-2/CEP135/NA14 luminal ring\"\n    ],\n    \"partners\": [\n      \"OFD1\",\n      \"CEP120\",\n      \"MNR\",\n      \"SFI1\",\n      \"CETN2\",\n      \"CEP135\",\n      \"SSNA1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}