{"gene":"B3GLCT","run_date":"2026-04-28T17:12:37","timeline":{"discoveries":[{"year":2006,"finding":"B3GALTL (B3GLCT) encodes a putative β1,3-galactosyltransferase-like glycosyltransferase; loss-of-function mutations cause Peters Plus syndrome, placing B3GLCT on the list of congenital malformation syndromes caused by glycosylation defects.","method":"Array-based comparative genomic hybridization, mutation analysis in 20 patients with biallelic truncating mutations","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — original disease-gene discovery with multiple patients and orthogonal genetic methods, independently replicated across studies","pmids":["16909395"],"is_preprint":false},{"year":2017,"finding":"B3GLCT catalyzes the transfer of glucose via a β1-3 glycosidic linkage to O-linked fucose on thrombospondin type 1 repeats (TSRs); in vitro glucosylation assays using UDP-glucose and O-fucosylated TSR substrate confirmed this enzymatic activity is conserved in vertebrates.","method":"In vitro glucosylation assay with wildtype and b3glct knockout zebrafish embryo extracts; TALEN-mediated knockout demonstrating complete loss of activity","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic assay with substrate and knockout control demonstrating loss of activity","pmids":["28926587"],"is_preprint":false},{"year":2019,"finding":"B3GLCT works sequentially with POFUT2 to add an O-linked glucose-β1-3-fucose disaccharide to TSRs; B3GLCT inactivation differentially affects ADAMTS superfamily substrates, with ADAMTS20 being highly sensitive and ADAMTS9 partially sensitive, linking specific substrate loss to distinct developmental defects (hydrocephalus/white spotting from ADAMTS20 loss; eye abnormalities from partial ADAMTS9 reduction; cleft palate from combined loss).","method":"Mouse B3glct knockout models, genetic epistasis, biochemical secretion assays","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 1–2 — genetic epistasis combined with biochemical evidence across two knockout alleles in mouse model","pmids":["31600785"],"is_preprint":false},{"year":2021,"finding":"B3GLCT adds glucose to O-linked fucose on TSRs in the endoplasmic reticulum, stabilizing the TSR fold and promoting secretion; loss of B3GLCT reduces secretion of SCO-spondin (SSPO) in cultured cells, increases intracellular BiP levels indicating a folding defect, and results in secreted SSPO colocalizing with BiP, suggesting abnormal extracellular assembly.","method":"Cultured cell secretion assays with B3glct mutant mouse model, immunofluorescence, BiP co-localization, in situ hybridization","journal":"Glycobiology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (secretion assay, BiP colocalization, mRNA localization) in mouse knockout model","pmids":["33909046"],"is_preprint":false},{"year":2021,"finding":"Loss of B3GLCT in mouse B3glct mutants causes ependymal cell abnormalities including fewer cilia basal bodies and altered translational polarity, contributing to hydrocephalus; this implicates B3GLCT-mediated TSR glycosylation in ependymal cell ciliogenesis.","method":"Mouse B3glct knockout, cilia basal body quantification, immunofluorescence, mRNA localization","journal":"Glycobiology","confidence":"Medium","confidence_rationale":"Tier 2 — clean knockout with specific cellular phenotype, but mechanistic link to ciliogenesis inferred from substrate expression rather than direct reconstitution","pmids":["33909046"],"is_preprint":false},{"year":2021,"finding":"B3GLCT-mediated glucose-β1,3-fucose modification was confirmed on thrombospondin 1 (TSP1) in retinal pigment epithelial cells by glycopeptide analysis; loss of B3GLCT in RPE knockout cells abolished this modification but did not affect TSP1 secretion, and increased C-mannosylation on TSR domains 1 and 3.","method":"B3GLCT knockout RPE cells, glycopeptide mass spectrometry analysis, secretion assay, HEK293T overexpression","journal":"Experimental eye research","confidence":"High","confidence_rationale":"Tier 1 — glycopeptide MS confirmation of substrate modification, knockout with orthogonal secretion and glycosylation assays","pmids":["34695439"],"is_preprint":false},{"year":2013,"finding":"A c.597-2A>G splice site mutation in B3GALTL causes complete skipping of exon 8, altering the open reading frame and generating a premature termination codon in exon 9, which triggers nonsense-mediated mRNA decay (NMD).","method":"Ex vivo mRNA splicing analysis, cDNA sequencing, bioinformatics splice prediction","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 — functional ex vivo splicing assay confirming NMD mechanism, single lab","pmids":["23954224"],"is_preprint":false}],"current_model":"B3GLCT is an ER-resident β1,3-glucosyltransferase that, acting sequentially with POFUT2, adds a glucose-β1,3-fucose disaccharide to O-linked fucose on properly folded thrombospondin type 1 repeats (TSRs), thereby stabilizing TSR folding and promoting secretion of TSR-containing proteins (especially ADAMTS family members and SCO-spondin), with differential sensitivity among substrates accounting for the specific developmental defects seen in Peters Plus syndrome."},"narrative":{"teleology":[{"year":2006,"claim":"Identifying B3GLCT as the gene mutated in Peters Plus syndrome established that a glycosyltransferase-like enzyme is essential for normal craniofacial and ocular development, but its enzymatic activity and substrate were unknown.","evidence":"Array-based CGH and mutation analysis in 20 patients with biallelic truncating mutations","pmids":["16909395"],"confidence":"High","gaps":["Enzymatic activity not demonstrated — gene annotated only by homology","Substrate and acceptor sugar unknown","No animal model to dissect developmental phenotypes"]},{"year":2013,"claim":"Demonstrating that a common splice-site mutation triggers nonsense-mediated mRNA decay clarified the molecular mechanism of allele loss but did not address enzyme function.","evidence":"Ex vivo mRNA splicing analysis and cDNA sequencing of c.597-2A>G variant","pmids":["23954224"],"confidence":"Medium","gaps":["Single lab study; not independently confirmed in additional patient cohorts","Protein-level consequence not directly measured","Genotype–phenotype correlation across mutation spectrum not addressed"]},{"year":2017,"claim":"In vitro glucosylation assays resolved B3GLCT's catalytic activity as a β1,3-glucosyltransferase that transfers glucose to O-fucosylated TSRs, correcting the earlier annotation as a galactosyltransferase.","evidence":"In vitro glucosylation with UDP-glucose and O-fucosylated TSR substrate using wildtype and TALEN-generated b3glct knockout zebrafish extracts","pmids":["28926587"],"confidence":"High","gaps":["Structural basis for donor (UDP-glucose) and acceptor specificity unknown","Full repertoire of TSR-containing substrates not determined","Mammalian in vitro reconstitution not yet performed"]},{"year":2019,"claim":"Mouse knockouts revealed that B3GLCT acts sequentially with POFUT2 and that individual TSR-containing substrates differ in their dependence on glucosylation for secretion, linking specific substrate sensitivities (ADAMTS20, ADAMTS9) to discrete developmental defects.","evidence":"Mouse B3glct knockout models with genetic epistasis and biochemical secretion assays","pmids":["31600785"],"confidence":"High","gaps":["Molecular basis for differential substrate sensitivity not defined","Whether other ADAMTS family members are affected remains untested","Direct structural evidence for glucose stabilizing TSR fold not available"]},{"year":2021,"claim":"B3GLCT loss was shown to impair SCO-spondin secretion, elevate ER chaperone BiP, and reduce ependymal cilia basal bodies, linking the glycosylation defect to ER stress, abnormal protein quality control, and hydrocephalus.","evidence":"Mouse B3glct knockout cultured cell secretion assays, BiP colocalization, cilia basal body quantification","pmids":["33909046"],"confidence":"High","gaps":["Direct mechanism connecting TSR underglycosylation to cilia basal body loss not reconstituted","Contribution of ER stress versus substrate loss to developmental phenotypes not dissected","Whether BiP elevation reflects specific ERAD targeting of misfolded substrates is untested"]},{"year":2021,"claim":"Glycopeptide mass spectrometry confirmed B3GLCT-dependent glucose-β1,3-fucose on TSP1, but TSP1 secretion was unaffected, demonstrating that not all TSR-containing proteins require this modification for secretion and revealing compensatory increases in C-mannosylation.","evidence":"B3GLCT knockout RPE cells, glycopeptide MS, secretion assay","pmids":["34695439"],"confidence":"High","gaps":["Functional consequence of increased C-mannosylation on TSP1 domains unknown","Whether TSP1 function (e.g., TGF-β activation) is affected despite normal secretion not tested","Systematic mapping of all affected versus unaffected substrates in vivo not performed"]},{"year":null,"claim":"The structural determinants within TSRs that dictate dependence on B3GLCT-mediated glucosylation for folding and secretion remain undefined, as does the mechanism by which underglycosylated substrates are recognized and routed to ERAD versus secretion.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal structure of B3GLCT or B3GLCT–TSR complex available","ERAD pathway for misfolded TSR substrates not identified","Comprehensive substrate catalogue across tissues and developmental stages lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[1,2,5]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[3]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[1,2,3,5]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[2,3]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,2]}],"complexes":[],"partners":["POFUT2","ADAMTS20","ADAMTS9","SSPO","THBS1"],"other_free_text":[]},"mechanistic_narrative":"B3GLCT is an endoplasmic reticulum-resident β1,3-glucosyltransferase that transfers glucose to O-linked fucose on thrombospondin type 1 repeats (TSRs), functioning sequentially with POFUT2 to generate a glucose-β1,3-fucose disaccharide that stabilizes TSR folding and promotes secretion of TSR-containing proteins [PMID:28926587, PMID:31600785, PMID:33909046]. Loss of B3GLCT differentially impairs secretion of ADAMTS superfamily members and SCO-spondin, with ADAMTS20 being highly sensitive and ADAMTS9 partially sensitive, whereas thrombospondin 1 secretion is unaffected despite loss of the disaccharide modification [PMID:31600785, PMID:34695439]. B3GLCT inactivation triggers ER stress (elevated BiP), reduces ependymal cilia basal bodies, and causes hydrocephalus, eye abnormalities, and cleft palate in mouse models, recapitulating features of Peters Plus syndrome caused by biallelic loss-of-function mutations in humans [PMID:16909395, PMID:33909046]."},"prefetch_data":{"uniprot":{"accession":"Q6Y288","full_name":"Beta-1,3-glucosyltransferase","aliases":["Beta 3-glucosyltransferase","Beta-3-glycosyltransferase-like"],"length_aa":498,"mass_kda":56.6,"function":"Beta-1,3-glucosyltransferase involved in one of the two pathways responsible for protein O-linked fucosylation, a unique post-translational modification of cysteine-knotted proteins that regulates various biological processes. This pathway targets proteins with Thrombospondin type-1 (TSP1) repeats (TSR) in the endoplasmic reticulum. It starts with POFUT2, which attaches fucose via an O-glycosidic bond to a conserved serine or threonine residue. B3GLCT extends this modification by transferring a glucose molecule from UDP-glucose to the fucose","subcellular_location":"Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/Q6Y288/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/B3GLCT","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/B3GLCT","total_profiled":1310},"omim":[{"mim_id":"610308","title":"BETA-3-GLUCOSYLTRANSFERASE; B3GLCT","url":"https://www.omim.org/entry/610308"},{"mim_id":"610249","title":"PROTEIN O-FUCOSYLTRANSFERASE 2; POFUT2","url":"https://www.omim.org/entry/610249"},{"mim_id":"261540","title":"PETERS-PLUS SYNDROME; PTRPLS","url":"https://www.omim.org/entry/261540"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/B3GLCT"},"hgnc":{"alias_symbol":["B3GTL","B3Glc-T"],"prev_symbol":["B3GALTL"]},"alphafold":{"accession":"Q6Y288","domains":[{"cath_id":"3.90.550.50","chopping":"55-254","consensus_level":"high","plddt":93.9016,"start":55,"end":254},{"cath_id":"3.90.550.50","chopping":"267-498","consensus_level":"high","plddt":91.6998,"start":267,"end":498}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6Y288","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q6Y288-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q6Y288-F1-predicted_aligned_error_v6.png","plddt_mean":86.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=B3GLCT","jax_strain_url":"https://www.jax.org/strain/search?query=B3GLCT"},"sequence":{"accession":"Q6Y288","fasta_url":"https://rest.uniprot.org/uniprotkb/Q6Y288.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q6Y288/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6Y288"}},"corpus_meta":[{"pmid":"16909395","id":"PMC_16909395","title":"Peters Plus syndrome is caused by mutations in B3GALTL, a putative glycosyltransferase.","date":"2006","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/16909395","citation_count":142,"is_preprint":false},{"pmid":"18798333","id":"PMC_18798333","title":"Mutation analysis of B3GALTL in Peters Plus syndrome.","date":"2008","source":"American journal of medical genetics. Part A","url":"https://pubmed.ncbi.nlm.nih.gov/18798333","citation_count":50,"is_preprint":false},{"pmid":"23889335","id":"PMC_23889335","title":"Novel B3GALTL mutations in classic Peters plus syndrome and lack of mutations in a large cohort of patients with similar phenotypes.","date":"2013","source":"Clinical genetics","url":"https://pubmed.ncbi.nlm.nih.gov/23889335","citation_count":39,"is_preprint":false},{"pmid":"31600785","id":"PMC_31600785","title":"ADAMTS9 and ADAMTS20 are differentially affected by loss of B3GLCT in mouse model of Peters plus syndrome.","date":"2019","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/31600785","citation_count":25,"is_preprint":false},{"pmid":"23161355","id":"PMC_23161355","title":"Hydrocephalus, agenesis of the corpus callosum, and cleft lip/palate represent frequent associations in fetuses with Peters' plus syndrome and B3GALTL mutations. Fetal PPS phenotypes, expanded by Dandy Walker cyst and encephalocele.","date":"2012","source":"Prenatal diagnosis","url":"https://pubmed.ncbi.nlm.nih.gov/23161355","citation_count":21,"is_preprint":false},{"pmid":"21067481","id":"PMC_21067481","title":"A novel nonsense B3GALTL mutation confirms Peters plus syndrome in a patient with multiple malformations and Peters anomaly.","date":"2010","source":"Ophthalmic genetics","url":"https://pubmed.ncbi.nlm.nih.gov/21067481","citation_count":20,"is_preprint":false},{"pmid":"33909046","id":"PMC_33909046","title":"Hydrocephalus in mouse B3glct mutants is likely caused by defects in multiple B3GLCT substrates in ependymal cells and subcommissural organ.","date":"2021","source":"Glycobiology","url":"https://pubmed.ncbi.nlm.nih.gov/33909046","citation_count":13,"is_preprint":false},{"pmid":"28926587","id":"PMC_28926587","title":"Functional characterization of zebrafish orthologs of the human Beta 3-Glucosyltransferase B3GLCT gene mutated in Peters Plus Syndrome.","date":"2017","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/28926587","citation_count":11,"is_preprint":false},{"pmid":"34085516","id":"PMC_34085516","title":"High-Throughput miRFluR Platform Identifies miRNA Regulating B3GLCT That Predict Peters' Plus Syndrome Phenotype, Supporting the miRNA Proxy Hypothesis.","date":"2021","source":"ACS chemical biology","url":"https://pubmed.ncbi.nlm.nih.gov/34085516","citation_count":8,"is_preprint":false},{"pmid":"22759511","id":"PMC_22759511","title":"Two Tunisian patients with Peters plus syndrome harbouring a novel splice site mutation in the B3GALTL gene that modulates the mRNA secondary structure.","date":"2012","source":"Gene","url":"https://pubmed.ncbi.nlm.nih.gov/22759511","citation_count":7,"is_preprint":false},{"pmid":"16907644","id":"PMC_16907644","title":"Murine ortholog of the novel glycosyltransferase, B3GTL: primary structure, characterization of the gene and transcripts, and expression in tissues.","date":"2006","source":"DNA and cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/16907644","citation_count":7,"is_preprint":false},{"pmid":"23954224","id":"PMC_23954224","title":"First functional analysis of a novel splicing mutation in the B3GALTL gene by an ex vivo approach in Tunisian patients with typical Peters plus syndrome.","date":"2013","source":"Gene","url":"https://pubmed.ncbi.nlm.nih.gov/23954224","citation_count":6,"is_preprint":false},{"pmid":"34695439","id":"PMC_34695439","title":"Loss of the AMD-associated B3GLCT gene affects glycosylation of TSP1 without impairing secretion in retinal pigment epithelial cells.","date":"2021","source":"Experimental eye research","url":"https://pubmed.ncbi.nlm.nih.gov/34695439","citation_count":2,"is_preprint":false},{"pmid":"24427506","id":"PMC_24427506","title":"An Unusual Case of Peters Plus Syndrome with Sexual Ambiguity and Absence of Mutations in the B3GALTL Gene.","date":"2013","source":"Iranian journal of pediatrics","url":"https://pubmed.ncbi.nlm.nih.gov/24427506","citation_count":2,"is_preprint":false},{"pmid":"31795264","id":"PMC_31795264","title":"Contribution of a Novel B3GLCT Variant to Peters Plus Syndrome Discovered by a Combination of Next-Generation Sequencing and Automated Text Mining.","date":"2019","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/31795264","citation_count":2,"is_preprint":false},{"pmid":"32204707","id":"PMC_32204707","title":"Peters plus syndrome and Chorioretinal findings associated with B3GLCT gene mutation - a case report.","date":"2020","source":"BMC ophthalmology","url":"https://pubmed.ncbi.nlm.nih.gov/32204707","citation_count":2,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8686,"output_tokens":1705,"usd":0.025816},"stage2":{"model":"claude-opus-4-6","input_tokens":4979,"output_tokens":1919,"usd":0.109305},"total_usd":0.135121,"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\": 2006,\n      \"finding\": \"B3GALTL (B3GLCT) encodes a putative β1,3-galactosyltransferase-like glycosyltransferase; loss-of-function mutations cause Peters Plus syndrome, placing B3GLCT on the list of congenital malformation syndromes caused by glycosylation defects.\",\n      \"method\": \"Array-based comparative genomic hybridization, mutation analysis in 20 patients with biallelic truncating mutations\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — original disease-gene discovery with multiple patients and orthogonal genetic methods, independently replicated across studies\",\n      \"pmids\": [\"16909395\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"B3GLCT catalyzes the transfer of glucose via a β1-3 glycosidic linkage to O-linked fucose on thrombospondin type 1 repeats (TSRs); in vitro glucosylation assays using UDP-glucose and O-fucosylated TSR substrate confirmed this enzymatic activity is conserved in vertebrates.\",\n      \"method\": \"In vitro glucosylation assay with wildtype and b3glct knockout zebrafish embryo extracts; TALEN-mediated knockout demonstrating complete loss of activity\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic assay with substrate and knockout control demonstrating loss of activity\",\n      \"pmids\": [\"28926587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"B3GLCT works sequentially with POFUT2 to add an O-linked glucose-β1-3-fucose disaccharide to TSRs; B3GLCT inactivation differentially affects ADAMTS superfamily substrates, with ADAMTS20 being highly sensitive and ADAMTS9 partially sensitive, linking specific substrate loss to distinct developmental defects (hydrocephalus/white spotting from ADAMTS20 loss; eye abnormalities from partial ADAMTS9 reduction; cleft palate from combined loss).\",\n      \"method\": \"Mouse B3glct knockout models, genetic epistasis, biochemical secretion assays\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic epistasis combined with biochemical evidence across two knockout alleles in mouse model\",\n      \"pmids\": [\"31600785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"B3GLCT adds glucose to O-linked fucose on TSRs in the endoplasmic reticulum, stabilizing the TSR fold and promoting secretion; loss of B3GLCT reduces secretion of SCO-spondin (SSPO) in cultured cells, increases intracellular BiP levels indicating a folding defect, and results in secreted SSPO colocalizing with BiP, suggesting abnormal extracellular assembly.\",\n      \"method\": \"Cultured cell secretion assays with B3glct mutant mouse model, immunofluorescence, BiP co-localization, in situ hybridization\",\n      \"journal\": \"Glycobiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (secretion assay, BiP colocalization, mRNA localization) in mouse knockout model\",\n      \"pmids\": [\"33909046\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Loss of B3GLCT in mouse B3glct mutants causes ependymal cell abnormalities including fewer cilia basal bodies and altered translational polarity, contributing to hydrocephalus; this implicates B3GLCT-mediated TSR glycosylation in ependymal cell ciliogenesis.\",\n      \"method\": \"Mouse B3glct knockout, cilia basal body quantification, immunofluorescence, mRNA localization\",\n      \"journal\": \"Glycobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean knockout with specific cellular phenotype, but mechanistic link to ciliogenesis inferred from substrate expression rather than direct reconstitution\",\n      \"pmids\": [\"33909046\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"B3GLCT-mediated glucose-β1,3-fucose modification was confirmed on thrombospondin 1 (TSP1) in retinal pigment epithelial cells by glycopeptide analysis; loss of B3GLCT in RPE knockout cells abolished this modification but did not affect TSP1 secretion, and increased C-mannosylation on TSR domains 1 and 3.\",\n      \"method\": \"B3GLCT knockout RPE cells, glycopeptide mass spectrometry analysis, secretion assay, HEK293T overexpression\",\n      \"journal\": \"Experimental eye research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — glycopeptide MS confirmation of substrate modification, knockout with orthogonal secretion and glycosylation assays\",\n      \"pmids\": [\"34695439\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"A c.597-2A>G splice site mutation in B3GALTL causes complete skipping of exon 8, altering the open reading frame and generating a premature termination codon in exon 9, which triggers nonsense-mediated mRNA decay (NMD).\",\n      \"method\": \"Ex vivo mRNA splicing analysis, cDNA sequencing, bioinformatics splice prediction\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional ex vivo splicing assay confirming NMD mechanism, single lab\",\n      \"pmids\": [\"23954224\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"B3GLCT is an ER-resident β1,3-glucosyltransferase that, acting sequentially with POFUT2, adds a glucose-β1,3-fucose disaccharide to O-linked fucose on properly folded thrombospondin type 1 repeats (TSRs), thereby stabilizing TSR folding and promoting secretion of TSR-containing proteins (especially ADAMTS family members and SCO-spondin), with differential sensitivity among substrates accounting for the specific developmental defects seen in Peters Plus syndrome.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"B3GLCT is an endoplasmic reticulum-resident β1,3-glucosyltransferase that transfers glucose to O-linked fucose on thrombospondin type 1 repeats (TSRs), functioning sequentially with POFUT2 to generate a glucose-β1,3-fucose disaccharide that stabilizes TSR folding and promotes secretion of TSR-containing proteins [PMID:28926587, PMID:31600785, PMID:33909046]. Loss of B3GLCT differentially impairs secretion of ADAMTS superfamily members and SCO-spondin, with ADAMTS20 being highly sensitive and ADAMTS9 partially sensitive, whereas thrombospondin 1 secretion is unaffected despite loss of the disaccharide modification [PMID:31600785, PMID:34695439]. B3GLCT inactivation triggers ER stress (elevated BiP), reduces ependymal cilia basal bodies, and causes hydrocephalus, eye abnormalities, and cleft palate in mouse models, recapitulating features of Peters Plus syndrome caused by biallelic loss-of-function mutations in humans [PMID:16909395, PMID:33909046].\",\n  \"teleology\": [\n    {\n      \"year\": 2006,\n      \"claim\": \"Identifying B3GLCT as the gene mutated in Peters Plus syndrome established that a glycosyltransferase-like enzyme is essential for normal craniofacial and ocular development, but its enzymatic activity and substrate were unknown.\",\n      \"evidence\": \"Array-based CGH and mutation analysis in 20 patients with biallelic truncating mutations\",\n      \"pmids\": [\"16909395\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Enzymatic activity not demonstrated — gene annotated only by homology\",\n        \"Substrate and acceptor sugar unknown\",\n        \"No animal model to dissect developmental phenotypes\"\n      ]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Demonstrating that a common splice-site mutation triggers nonsense-mediated mRNA decay clarified the molecular mechanism of allele loss but did not address enzyme function.\",\n      \"evidence\": \"Ex vivo mRNA splicing analysis and cDNA sequencing of c.597-2A>G variant\",\n      \"pmids\": [\"23954224\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single lab study; not independently confirmed in additional patient cohorts\",\n        \"Protein-level consequence not directly measured\",\n        \"Genotype–phenotype correlation across mutation spectrum not addressed\"\n      ]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"In vitro glucosylation assays resolved B3GLCT's catalytic activity as a β1,3-glucosyltransferase that transfers glucose to O-fucosylated TSRs, correcting the earlier annotation as a galactosyltransferase.\",\n      \"evidence\": \"In vitro glucosylation with UDP-glucose and O-fucosylated TSR substrate using wildtype and TALEN-generated b3glct knockout zebrafish extracts\",\n      \"pmids\": [\"28926587\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis for donor (UDP-glucose) and acceptor specificity unknown\",\n        \"Full repertoire of TSR-containing substrates not determined\",\n        \"Mammalian in vitro reconstitution not yet performed\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Mouse knockouts revealed that B3GLCT acts sequentially with POFUT2 and that individual TSR-containing substrates differ in their dependence on glucosylation for secretion, linking specific substrate sensitivities (ADAMTS20, ADAMTS9) to discrete developmental defects.\",\n      \"evidence\": \"Mouse B3glct knockout models with genetic epistasis and biochemical secretion assays\",\n      \"pmids\": [\"31600785\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular basis for differential substrate sensitivity not defined\",\n        \"Whether other ADAMTS family members are affected remains untested\",\n        \"Direct structural evidence for glucose stabilizing TSR fold not available\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"B3GLCT loss was shown to impair SCO-spondin secretion, elevate ER chaperone BiP, and reduce ependymal cilia basal bodies, linking the glycosylation defect to ER stress, abnormal protein quality control, and hydrocephalus.\",\n      \"evidence\": \"Mouse B3glct knockout cultured cell secretion assays, BiP colocalization, cilia basal body quantification\",\n      \"pmids\": [\"33909046\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Direct mechanism connecting TSR underglycosylation to cilia basal body loss not reconstituted\",\n        \"Contribution of ER stress versus substrate loss to developmental phenotypes not dissected\",\n        \"Whether BiP elevation reflects specific ERAD targeting of misfolded substrates is untested\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Glycopeptide mass spectrometry confirmed B3GLCT-dependent glucose-β1,3-fucose on TSP1, but TSP1 secretion was unaffected, demonstrating that not all TSR-containing proteins require this modification for secretion and revealing compensatory increases in C-mannosylation.\",\n      \"evidence\": \"B3GLCT knockout RPE cells, glycopeptide MS, secretion assay\",\n      \"pmids\": [\"34695439\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Functional consequence of increased C-mannosylation on TSP1 domains unknown\",\n        \"Whether TSP1 function (e.g., TGF-β activation) is affected despite normal secretion not tested\",\n        \"Systematic mapping of all affected versus unaffected substrates in vivo not performed\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural determinants within TSRs that dictate dependence on B3GLCT-mediated glucosylation for folding and secretion remain undefined, as does the mechanism by which underglycosylated substrates are recognized and routed to ERAD versus secretion.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No crystal structure of B3GLCT or B3GLCT–TSR complex available\",\n        \"ERAD pathway for misfolded TSR substrates not identified\",\n        \"Comprehensive substrate catalogue across tissues and developmental stages lacking\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [1, 2, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [1, 2, 3, 5]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"POFUT2\",\n      \"ADAMTS20\",\n      \"ADAMTS9\",\n      \"SSPO\",\n      \"THBS1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}