{"gene":"B3GLCT","run_date":"2026-06-09T22:02:44","timeline":{"discoveries":[{"year":2006,"finding":"B3GALTL (B3GLCT) encodes a beta-1,3-galactosyltransferase-like glycosyltransferase; biallelic truncating mutations cause Peters Plus syndrome, placing it on the list of congenital malformation syndromes caused by glycosylation defects.","method":"Array-based comparative genomic hybridization (aCGH) and mutation analysis identifying biallelic truncating mutations in 20/20 patients","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic identification with strong disease association across 20 patients; enzymatic function inferred from sequence homology, not direct biochemical assay in this paper","pmids":["16909395"],"is_preprint":false},{"year":2017,"finding":"B3GLCT (b3glct in zebrafish) catalyzes the transfer of glucose via a β1-3 glycosidic linkage to O-linked fucose on thrombospondin type 1 repeats (TSRs); this enzymatic activity is conserved between zebrafish and human B3GLCT.","method":"In vitro glucosylation assay using embryo extracts transferring glucose from UDP-glucose to an O-fucosylated TSR substrate; TALEN-generated knockouts showed complete loss of this activity in double homozygous mutants","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct in vitro enzymatic assay with substrate and knockout controls confirming loss of activity; functional conservation demonstrated","pmids":["28926587"],"is_preprint":false},{"year":2019,"finding":"B3GLCT works sequentially with Protein O-fucosyltransferase 2 (POFUT2) to add an O-linked glucose β1-3 fucose disaccharide to properly folded thrombospondin type 1 repeats (TSRs); B3GLCT-mediated extension to the disaccharide is essential for only a subset of POFUT2 targets, with ADAMTS20 being highly sensitive and ADAMTS9 partially sensitive to B3GLCT loss.","method":"Mouse B3glct knockout models (two alleles); genetic epistasis combined with biochemical analysis of ADAMTS9 and ADAMTS20 secretion and function; rescue/interaction experiments","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — two independent knockout alleles, genetic epistasis, and biochemical evidence across multiple substrates in a single rigorous study","pmids":["31600785"],"is_preprint":false},{"year":2021,"finding":"B3GLCT adds glucose to O-linked fucose on TSRs in the endoplasmic reticulum; loss of B3GLCT reduces secretion of SCO-spondin (SSPO) in cultured cells and causes intracellular accumulation with increased BiP levels, indicating a folding/quality control defect. TSRs of SSPO were confirmed to be modified with O-linked glucose-fucose by B3GLCT.","method":"Cell culture secretion assays with B3glct mutant cells; glycosylation analysis of SSPO TSRs; BiP co-localization; mRNA expression analysis of ADAMTS substrates in ependymal cells and subcommissural organ","journal":"Glycobiology","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — direct biochemical demonstration of B3GLCT modification of SSPO TSRs, secretion assays with KO cells, and BiP upregulation as folding quality control readout","pmids":["33909046"],"is_preprint":false},{"year":2021,"finding":"Loss of B3GLCT in RPE cells abolishes glucose-β1,3-fucose modification on thrombospondin 1 (TSP1) TSR domains (confirmed by glycopeptide analysis), and increases C-mannosylation on TSR domains 1 and 3, but does not impair TSP1 secretion.","method":"B3GLCT knockout RPE cells generated by gene editing; glycopeptide mass spectrometry analysis of TSP1 modifications; secretion assays in KO vs. wildtype cells including TNFα stimulation and HEK293T overexpression","journal":"Experimental eye research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct glycopeptide analysis confirms substrate modification, KO cells with secretion assay; negative result (no secretion impairment for TSP1) is experimentally rigorous","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, generating a premature termination codon in exon 9, and triggering nonsense-mediated mRNA decay (NMD).","method":"Ex vivo mRNA analysis of patient-derived cells; RT-PCR demonstrating exon skipping; bioinformatics prediction of NMD","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — ex vivo functional RNA analysis from single lab demonstrating splicing consequence; NMD inferred bioinformatically","pmids":["23954224"],"is_preprint":false},{"year":2021,"finding":"Loss of B3GLCT in mouse ependymal cells results in fewer cilia basal bodies and altered translational polarity, with reduced ADAMTS9 and ADAMTS20 mRNA localization contributing to these ependymal abnormalities; this implicates B3GLCT substrates in cilia organization relevant to CSF flow.","method":"Mouse B3glct knockout; immunofluorescence and imaging of ependymal cilia basal bodies and translational polarity; in situ hybridization for substrate mRNAs","journal":"Glycobiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization and phenotypic analysis in KO mouse with cellular resolution; single lab study","pmids":["33909046"],"is_preprint":false}],"current_model":"B3GLCT is an endoplasmic reticulum-resident β1,3-glucosyltransferase that acts sequentially with POFUT2 to add glucose via a β1-3 linkage to O-linked fucose on properly folded thrombospondin type 1 repeats (TSRs), forming a glucose-fucose disaccharide that stabilizes the TSR fold and promotes secretion of a subset of TSR-containing proteins (notably ADAMTS20 and SSPO, and partially ADAMTS9), with differential sensitivity among targets; loss of B3GLCT causes the congenital glycosylation disorder Peters Plus syndrome through disruption of these substrate-specific quality control functions."},"narrative":{"mechanistic_narrative":"B3GLCT is an endoplasmic reticulum-resident β1,3-glucosyltransferase that operates within a substrate quality-control pathway for thrombospondin type 1 repeat (TSR)-containing proteins [PMID:28926587, PMID:31600785]. It catalyzes transfer of glucose via a β1-3 linkage onto O-linked fucose attached to TSRs, acting sequentially after POFUT2 to complete a glucose-β1,3-fucose disaccharide on properly folded TSRs [PMID:28926587, PMID:31600785]. This disaccharide stabilizes the TSR fold and promotes secretion of a specific subset of TSR-containing proteins: ADAMTS20 is highly dependent on B3GLCT, ADAMTS9 is partially dependent, and SCO-spondin (SSPO) requires the modification for efficient secretion, with loss causing intracellular accumulation and elevated BiP indicative of a folding/quality-control defect [PMID:31600785, PMID:33909046]. The dependence is substrate-specific — thrombospondin 1 (TSP1) loses its glucose-fucose modification upon B3GLCT loss yet is secreted normally, with a compensatory increase in TSR C-mannosylation [PMID:34695439]. Through this role, B3GLCT loss perturbs ADAMTS9/ADAMTS20-dependent ependymal cilia basal body organization and translational polarity [PMID:33909046]. Biallelic truncating mutations in B3GLCT cause Peters Plus syndrome, a congenital disorder of glycosylation, including via splice mutations that trigger nonsense-mediated decay [PMID:16909395, PMID:23954224].","teleology":[{"year":2006,"claim":"Established B3GLCT as a disease gene by linking biallelic truncating mutations to Peters Plus syndrome, framing it as a glycosylation-defect malformation syndrome before its catalytic activity was directly tested.","evidence":"aCGH and mutation analysis identifying biallelic truncating mutations in 20/20 patients","pmids":["16909395"],"confidence":"Medium","gaps":["Enzymatic activity inferred from homology, not measured","No substrate identified at this stage","Mechanism connecting gene loss to phenotype unknown"]},{"year":2013,"claim":"Clarified one molecular route to loss of function by showing a splice-site mutation causes exon skipping, frameshift, and predicted nonsense-mediated decay, defining how patient mutations abolish the gene product.","evidence":"Ex vivo RT-PCR of patient-derived cells with bioinformatic NMD prediction","pmids":["23954224"],"confidence":"Medium","gaps":["NMD inferred bioinformatically rather than measured directly","Single-lab, single-allele analysis"]},{"year":2017,"claim":"Provided the first direct biochemical proof that B3GLCT is a glucosyltransferase, resolving whether the homology-based annotation reflected real catalytic activity on TSRs.","evidence":"In vitro glucosylation assay transferring glucose from UDP-glucose to an O-fucosylated TSR substrate, with TALEN knockouts in zebrafish showing complete loss of activity","pmids":["28926587"],"confidence":"High","gaps":["Did not define which physiological proteins depend on the modification","Structural basis of TSR recognition not addressed"]},{"year":2019,"claim":"Placed B3GLCT in a sequential POFUT2-then-B3GLCT pathway and revealed substrate selectivity, showing that disaccharide completion is essential for only a subset of TSR proteins.","evidence":"Two independent mouse B3glct knockout alleles with genetic epistasis and biochemical analysis of ADAMTS9 and ADAMTS20 secretion","pmids":["31600785"],"confidence":"High","gaps":["Molecular basis for differential substrate sensitivity not defined","Range of full substrate repertoire incomplete"]},{"year":2021,"claim":"Defined the cellular consequence of B3GLCT loss as an ER folding/quality-control defect for SSPO and connected substrate misregulation to ependymal cilia organization, linking the molecular activity to tissue phenotype.","evidence":"B3glct mutant cell secretion assays with SSPO glycosylation analysis and BiP co-localization; mouse KO ependymal imaging and substrate mRNA in situ hybridization","pmids":["33909046"],"confidence":"High","gaps":["BiP upregulation establishes folding stress but not the precise quality-control mechanism","Causal chain from substrate loss to cilia defects partly correlative"]},{"year":2021,"claim":"Demonstrated that B3GLCT modification is not universally required for secretion by showing TSP1 loses the glucose-fucose modification yet is secreted normally, sharpening the substrate-specific model.","evidence":"B3GLCT knockout RPE cells with glycopeptide mass spectrometry and secretion assays including TNFα stimulation and HEK293T overexpression","pmids":["34695439"],"confidence":"High","gaps":["Why TSP1 escapes secretion dependence is unexplained","Functional significance of compensatory C-mannosylation increase unknown"]},{"year":null,"claim":"It remains unresolved what structural or sequence features of individual TSRs determine whether a substrate requires B3GLCT for folding and secretion versus tolerating its loss.","evidence":"","pmids":[],"confidence":"High","gaps":["No structural model of TSR discrimination","Full substrate repertoire and ranking of sensitivities incomplete","Mechanistic link from substrate-specific failure to specific Peters Plus features not fully defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[1,2,3,4]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[3,4]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,3]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,5]}],"complexes":[],"partners":["POFUT2"],"other_free_text":[]}},"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":14,"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":"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":"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":"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":"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},{"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}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":9461,"output_tokens":1910,"usd":0.028517,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8816,"output_tokens":2361,"usd":0.051553,"stage2_stop_reason":"end_turn"},"total_usd":0.08007,"stage1_batch_id":"msgbatch_0179Z2pxRYBsXi2xADneafRw","stage2_batch_id":"msgbatch_01LaY9ZaVdgjWArcZo9eqQE5","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2006,\n      \"finding\": \"B3GALTL (B3GLCT) encodes a beta-1,3-galactosyltransferase-like glycosyltransferase; biallelic truncating mutations cause Peters Plus syndrome, placing it on the list of congenital malformation syndromes caused by glycosylation defects.\",\n      \"method\": \"Array-based comparative genomic hybridization (aCGH) and mutation analysis identifying biallelic truncating mutations in 20/20 patients\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic identification with strong disease association across 20 patients; enzymatic function inferred from sequence homology, not direct biochemical assay in this paper\",\n      \"pmids\": [\"16909395\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"B3GLCT (b3glct in zebrafish) catalyzes the transfer of glucose via a β1-3 glycosidic linkage to O-linked fucose on thrombospondin type 1 repeats (TSRs); this enzymatic activity is conserved between zebrafish and human B3GLCT.\",\n      \"method\": \"In vitro glucosylation assay using embryo extracts transferring glucose from UDP-glucose to an O-fucosylated TSR substrate; TALEN-generated knockouts showed complete loss of this activity in double homozygous mutants\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct in vitro enzymatic assay with substrate and knockout controls confirming loss of activity; functional conservation demonstrated\",\n      \"pmids\": [\"28926587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"B3GLCT works sequentially with Protein O-fucosyltransferase 2 (POFUT2) to add an O-linked glucose β1-3 fucose disaccharide to properly folded thrombospondin type 1 repeats (TSRs); B3GLCT-mediated extension to the disaccharide is essential for only a subset of POFUT2 targets, with ADAMTS20 being highly sensitive and ADAMTS9 partially sensitive to B3GLCT loss.\",\n      \"method\": \"Mouse B3glct knockout models (two alleles); genetic epistasis combined with biochemical analysis of ADAMTS9 and ADAMTS20 secretion and function; rescue/interaction experiments\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two independent knockout alleles, genetic epistasis, and biochemical evidence across multiple substrates in a single rigorous study\",\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; loss of B3GLCT reduces secretion of SCO-spondin (SSPO) in cultured cells and causes intracellular accumulation with increased BiP levels, indicating a folding/quality control defect. TSRs of SSPO were confirmed to be modified with O-linked glucose-fucose by B3GLCT.\",\n      \"method\": \"Cell culture secretion assays with B3glct mutant cells; glycosylation analysis of SSPO TSRs; BiP co-localization; mRNA expression analysis of ADAMTS substrates in ependymal cells and subcommissural organ\",\n      \"journal\": \"Glycobiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — direct biochemical demonstration of B3GLCT modification of SSPO TSRs, secretion assays with KO cells, and BiP upregulation as folding quality control readout\",\n      \"pmids\": [\"33909046\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Loss of B3GLCT in RPE cells abolishes glucose-β1,3-fucose modification on thrombospondin 1 (TSP1) TSR domains (confirmed by glycopeptide analysis), and increases C-mannosylation on TSR domains 1 and 3, but does not impair TSP1 secretion.\",\n      \"method\": \"B3GLCT knockout RPE cells generated by gene editing; glycopeptide mass spectrometry analysis of TSP1 modifications; secretion assays in KO vs. wildtype cells including TNFα stimulation and HEK293T overexpression\",\n      \"journal\": \"Experimental eye research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct glycopeptide analysis confirms substrate modification, KO cells with secretion assay; negative result (no secretion impairment for TSP1) is experimentally rigorous\",\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, generating a premature termination codon in exon 9, and triggering nonsense-mediated mRNA decay (NMD).\",\n      \"method\": \"Ex vivo mRNA analysis of patient-derived cells; RT-PCR demonstrating exon skipping; bioinformatics prediction of NMD\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — ex vivo functional RNA analysis from single lab demonstrating splicing consequence; NMD inferred bioinformatically\",\n      \"pmids\": [\"23954224\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Loss of B3GLCT in mouse ependymal cells results in fewer cilia basal bodies and altered translational polarity, with reduced ADAMTS9 and ADAMTS20 mRNA localization contributing to these ependymal abnormalities; this implicates B3GLCT substrates in cilia organization relevant to CSF flow.\",\n      \"method\": \"Mouse B3glct knockout; immunofluorescence and imaging of ependymal cilia basal bodies and translational polarity; in situ hybridization for substrate mRNAs\",\n      \"journal\": \"Glycobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization and phenotypic analysis in KO mouse with cellular resolution; single lab study\",\n      \"pmids\": [\"33909046\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"B3GLCT is an endoplasmic reticulum-resident β1,3-glucosyltransferase that acts sequentially with POFUT2 to add glucose via a β1-3 linkage to O-linked fucose on properly folded thrombospondin type 1 repeats (TSRs), forming a glucose-fucose disaccharide that stabilizes the TSR fold and promotes secretion of a subset of TSR-containing proteins (notably ADAMTS20 and SSPO, and partially ADAMTS9), with differential sensitivity among targets; loss of B3GLCT causes the congenital glycosylation disorder Peters Plus syndrome through disruption of these substrate-specific quality control functions.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"B3GLCT is an endoplasmic reticulum-resident β1,3-glucosyltransferase that operates within a substrate quality-control pathway for thrombospondin type 1 repeat (TSR)-containing proteins [#1, #2]. It catalyzes transfer of glucose via a β1-3 linkage onto O-linked fucose attached to TSRs, acting sequentially after POFUT2 to complete a glucose-β1,3-fucose disaccharide on properly folded TSRs [#1, #2]. This disaccharide stabilizes the TSR fold and promotes secretion of a specific subset of TSR-containing proteins: ADAMTS20 is highly dependent on B3GLCT, ADAMTS9 is partially dependent, and SCO-spondin (SSPO) requires the modification for efficient secretion, with loss causing intracellular accumulation and elevated BiP indicative of a folding/quality-control defect [#2, #3]. The dependence is substrate-specific — thrombospondin 1 (TSP1) loses its glucose-fucose modification upon B3GLCT loss yet is secreted normally, with a compensatory increase in TSR C-mannosylation [#4]. Through this role, B3GLCT loss perturbs ADAMTS9/ADAMTS20-dependent ependymal cilia basal body organization and translational polarity [#6]. Biallelic truncating mutations in B3GLCT cause Peters Plus syndrome, a congenital disorder of glycosylation, including via splice mutations that trigger nonsense-mediated decay [#0, #5].\",\n  \"teleology\": [\n    {\n      \"year\": 2006,\n      \"claim\": \"Established B3GLCT as a disease gene by linking biallelic truncating mutations to Peters Plus syndrome, framing it as a glycosylation-defect malformation syndrome before its catalytic activity was directly tested.\",\n      \"evidence\": \"aCGH and mutation analysis identifying biallelic truncating mutations in 20/20 patients\",\n      \"pmids\": [\"16909395\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Enzymatic activity inferred from homology, not measured\", \"No substrate identified at this stage\", \"Mechanism connecting gene loss to phenotype unknown\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Clarified one molecular route to loss of function by showing a splice-site mutation causes exon skipping, frameshift, and predicted nonsense-mediated decay, defining how patient mutations abolish the gene product.\",\n      \"evidence\": \"Ex vivo RT-PCR of patient-derived cells with bioinformatic NMD prediction\",\n      \"pmids\": [\"23954224\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"NMD inferred bioinformatically rather than measured directly\", \"Single-lab, single-allele analysis\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Provided the first direct biochemical proof that B3GLCT is a glucosyltransferase, resolving whether the homology-based annotation reflected real catalytic activity on TSRs.\",\n      \"evidence\": \"In vitro glucosylation assay transferring glucose from UDP-glucose to an O-fucosylated TSR substrate, with TALEN knockouts in zebrafish showing complete loss of activity\",\n      \"pmids\": [\"28926587\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define which physiological proteins depend on the modification\", \"Structural basis of TSR recognition not addressed\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Placed B3GLCT in a sequential POFUT2-then-B3GLCT pathway and revealed substrate selectivity, showing that disaccharide completion is essential for only a subset of TSR proteins.\",\n      \"evidence\": \"Two independent mouse B3glct knockout alleles with genetic epistasis and biochemical analysis of ADAMTS9 and ADAMTS20 secretion\",\n      \"pmids\": [\"31600785\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis for differential substrate sensitivity not defined\", \"Range of full substrate repertoire incomplete\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined the cellular consequence of B3GLCT loss as an ER folding/quality-control defect for SSPO and connected substrate misregulation to ependymal cilia organization, linking the molecular activity to tissue phenotype.\",\n      \"evidence\": \"B3glct mutant cell secretion assays with SSPO glycosylation analysis and BiP co-localization; mouse KO ependymal imaging and substrate mRNA in situ hybridization\",\n      \"pmids\": [\"33909046\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"BiP upregulation establishes folding stress but not the precise quality-control mechanism\", \"Causal chain from substrate loss to cilia defects partly correlative\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstrated that B3GLCT modification is not universally required for secretion by showing TSP1 loses the glucose-fucose modification yet is secreted normally, sharpening the substrate-specific model.\",\n      \"evidence\": \"B3GLCT knockout RPE cells with glycopeptide mass spectrometry and secretion assays including TNFα stimulation and HEK293T overexpression\",\n      \"pmids\": [\"34695439\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why TSP1 escapes secretion dependence is unexplained\", \"Functional significance of compensatory C-mannosylation increase unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved what structural or sequence features of individual TSRs determine whether a substrate requires B3GLCT for folding and secretion versus tolerating its loss.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural model of TSR discrimination\", \"Full substrate repertoire and ranking of sensitivities incomplete\", \"Mechanistic link from substrate-specific failure to specific Peters Plus features not fully defined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [1, 2, 3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [3, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"POFUT2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}