{"gene":"PIGH","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":1996,"finding":"PIG-A is an ER transmembrane protein with a large cytoplasmic domain homologous to a bacterial GlcNAc transferase and a small lumenal domain; PIG-H is a cytoplasmically oriented, ER-associated protein; both form a protein complex in the endoplasmic reticulum, indicating they are subunits of the GPI GlcNAc transferase that transfers GlcNAc to PI on the cytoplasmic side of the ER.","method":"Subcellular fractionation, immunofluorescence, topology studies, co-immunoprecipitation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, direct localization with functional consequence, replicated across multiple subsequent studies","pmids":["8900170"],"is_preprint":false},{"year":1998,"finding":"PIG-A, PIG-H, PIG-C, and human GPI1 form a four-protein complex in the ER membrane that has GPI-GlcNAc transferase (GPI-GnT) activity in vitro, catalyzing transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol; PIG-L (involved in the second GPI synthesis step) did not associate with this complex. The complex preferentially utilizes bovine PI (~100-fold) over soybean PI, suggesting recognition of specific fatty acyl chains.","method":"Co-immunoprecipitation, in vitro GPI-GnT activity assay with substrate specificity analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution of enzymatic activity combined with co-IP complex identification, multiple orthogonal methods","pmids":["9463366"],"is_preprint":false},{"year":1999,"finding":"GPI1 stabilizes the GPI-GnT enzyme complex by tying PIG-C to a core PIG-A/PIG-H complex; disruption of GPI1 in F9 cells caused nearly undetectable PIG-A/PIG-H/PIG-C trimeric complex while PIG-A/PIG-H dimeric complex remained detectable, and caused partial decreases in PIG-C and PIG-H protein levels.","method":"Gene disruption (KO) in F9 embryonal carcinoma cells, co-immunoprecipitation, Western blot","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean KO with defined molecular phenotype, epistatic placement of GPI1 relative to PIG-H within the complex, replicated by multiple subsequent studies","pmids":["10373468"],"is_preprint":false},{"year":2000,"finding":"GPI-GnT requires a fifth component, PIG-P, which associates with PIG-A and GPI1; a cell lacking PIG-P is GPI-anchor negative, establishing PIG-P as essential. DPM2 associates with GPI-GnT (through interactions with PIG-A, PIG-C, and GPI1) and enhances enzyme activity 3-fold, but is not essential for the reaction. PIG-H is thus a subunit of this larger, regulated complex.","method":"Co-immunoprecipitation, cell-based GPI-anchor expression assay (flow cytometry), in vitro GPI-GnT activity assay","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — in vitro activity assay plus Co-IP plus loss-of-function cell lines, multiple orthogonal methods identifying complex composition and regulation","pmids":["10944123"],"is_preprint":false},{"year":2000,"finding":"PIG-A (the proposed catalytic subunit of the GPI-GnT complex) localizes to both perinuclear and mitochondria-associated lamellae of the ER; computer-aided alignment identified highly conserved residues in the membrane-proximal cytoplasmic domain (residues 230–340) of PIG-A potentially involved in catalysis. A topological model of the four partners (PIG-A, PIG-H, PIG-C, GPI1) was proposed.","method":"Immunofluorescence, immunoelectron microscopy, affinity chromatography, computational sequence comparison","journal":"The international journal of biochemistry & cell biology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct localization experiment with functional context, computational active-site prediction without mutagenesis validation; single lab","pmids":["10716631"],"is_preprint":false},{"year":2018,"finding":"A homozygous c.1A>T transversion in PIGH (disrupting the start codon) results in utilization of an in-frame start site at codon 63, producing a truncated protein that cannot efficiently restore GPI-anchored protein expression in PIGH-deficient CHO cells, demonstrating that the N-terminal 62 residues of PIGH are essential for GPI-GnT function.","method":"FACS analysis of GPI-AP surface expression in PIGH-deficient CHO cells transfected with cDNA bearing c.1A>T, Sanger sequencing","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-based complementation assay with defined molecular variant; single lab, single method","pmids":["29573052"],"is_preprint":false},{"year":2018,"finding":"Loss of GPI-anchor–negative phenotype in B-ALL cells results from epigenetic silencing of PIGH mRNA expression (rather than gene mutation or deletion), leading to defective first-step GPI biosynthesis and loss of GPI-anchored protein (including CD52) surface expression.","method":"RT-PCR/mRNA expression analysis, flow cytometry for GPI-AP surface expression, methylation/epigenetic analysis","journal":"American journal of hematology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct demonstration that PIGH mRNA loss (not mutation) causes GPI-AP loss; single lab, multiple methods","pmids":["30370942"],"is_preprint":false},{"year":2021,"finding":"Epigenetic silencing of PIGH in BLaER1 monocyte-model cells causes GPI-anchor deficiency, loss of CD14 surface expression, and diminished LPS/TLR4 signaling (but not TLR7/TLR8 signaling); overexpressing PIGH restored GPI-anchored protein (including CD14) expression and LPS responsiveness, placing PIGH upstream of CD14-dependent TLR4 activation.","method":"Flow cytometry for GPI-AP/CD14 surface expression, LPS stimulation assay, PIGH overexpression rescue experiment","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — rescue experiment with defined functional readout (TLR4 signaling), single lab, multiple orthogonal readouts","pmids":["34294787"],"is_preprint":false},{"year":2015,"finding":"A splice-site variant (c211-10C>G) in bovine PIGH causes skipping of exon 2, producing a non-functional PIGH protein lacking essential domains; this loss-of-function is associated with autosomal recessive arthrogryposis in Belgian Blue cattle, confirming PIGH is essential for normal development in mammals.","method":"Genome-wide association mapping, next-generation DNA sequencing, RNA-Seq, RT-PCR confirmation of exon skipping","journal":"BMC genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — RNA-Seq plus RT-PCR confirmation of splicing defect; functional consequence inferred from loss of essential domains; single study","pmids":["25895751"],"is_preprint":false}],"current_model":"PIGH (PIG-H) is an essential, cytoplasmically oriented ER-associated protein that forms a core subcomplex with PIG-A in the endoplasmic reticulum; together with PIG-C, GPI1, PIG-P, and the regulatory subunit DPM2, it constitutes the GPI-N-acetylglucosaminyltransferase (GPI-GnT) complex that catalyzes the first committed step of GPI-anchor biosynthesis—transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol on the cytoplasmic face of the ER—with GPI1 stabilizing the complex by bridging PIG-C to the PIG-A/PIG-H core, and loss of PIGH (by mutation, truncation, or epigenetic silencing) abolishing GPI-anchored protein surface expression and downstream GPI-dependent signaling such as CD14-mediated TLR4 activation."},"narrative":{"mechanistic_narrative":"PIGH is an essential, cytoplasmically oriented ER-associated subunit of the GPI-N-acetylglucosaminyltransferase (GPI-GnT) complex that catalyzes the first committed step of GPI-anchor biosynthesis—transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol on the cytoplasmic face of the ER [PMID:8900170, PMID:9463366]. PIG-H forms a core protein complex with the catalytic-class subunit PIG-A in the ER, and together with PIG-C and GPI1 constitutes a four-protein complex with reconstituted GPI-GnT activity in vitro that preferentially recognizes specific PI fatty-acyl species [PMID:8900170, PMID:9463366]. Within this assembly GPI1 stabilizes the enzyme by bridging PIG-C to the PIG-A/PIG-H core, while a stable PIG-A/PIG-H dimer persists in its absence, defining PIG-H as part of the irreducible catalytic scaffold [PMID:10373468]. The complex is further completed by the essential component PIG-P and regulated by DPM2, which enhances activity without being required for catalysis [PMID:10944123]. Loss of PIGH function—through start-codon disruption that removes the essential N-terminal 62 residues, epigenetic silencing of its mRNA, or splice-disrupting variants—abolishes surface expression of GPI-anchored proteins and the downstream signaling they support, including CD14-mediated LPS/TLR4 activation [PMID:29573052, PMID:30370942, PMID:34294787]. A loss-of-function splice variant in bovine PIGH causes autosomal recessive arthrogryposis, establishing PIGH as essential for normal mammalian development [PMID:25895751].","teleology":[{"year":1996,"claim":"Established that PIG-H is a cytoplasmically oriented ER-associated protein that physically partners with PIG-A, defining the topology and subunit pairing of the GPI GlcNAc transferase before its activity was reconstituted.","evidence":"Subcellular fractionation, topology studies, immunofluorescence, and co-immunoprecipitation in mammalian cells","pmids":["8900170"],"confidence":"High","gaps":["Did not reconstitute enzymatic activity","Catalytic versus structural role of PIG-H within the complex not resolved"]},{"year":1998,"claim":"Showed that PIG-A, PIG-H, PIG-C and GPI1 together form a four-protein complex carrying GPI-GnT activity, placing PIG-H in a defined enzyme rather than an inferred association.","evidence":"Co-immunoprecipitation plus in vitro GPI-GnT activity assay with PI substrate-specificity analysis","pmids":["9463366"],"confidence":"High","gaps":["Which subunit performs catalysis not assigned","Did not test whether additional components are required for full activity"]},{"year":1999,"claim":"Resolved the internal architecture of the complex by showing GPI1 bridges PIG-C to a stable PIG-A/PIG-H core, defining PIG-H as part of the irreducible catalytic dimer.","evidence":"GPI1 gene disruption in F9 cells with co-IP and Western blot analysis of remaining subassemblies","pmids":["10373468"],"confidence":"High","gaps":["Structural basis of the PIG-A/PIG-H interaction not determined","Did not establish whether PIG-A/PIG-H dimer alone has residual activity"]},{"year":2000,"claim":"Expanded the complex to five essential subunits plus a regulator, showing PIG-P is required and DPM2 enhances but is dispensable for activity, contextualizing PIG-H within a larger regulated machine.","evidence":"Co-IP, flow-cytometry GPI-anchor expression assays in loss-of-function cells, and in vitro GPI-GnT activity assays","pmids":["10944123"],"confidence":"High","gaps":["Specific contribution of PIG-H to substrate binding or catalysis still undefined","No structural model of the assembled complex"]},{"year":2000,"claim":"Localized the catalytic-class subunit to perinuclear and mitochondria-associated ER lamellae and proposed a topological model, refining where the PIG-H-containing complex operates.","evidence":"Immunofluorescence, immunoelectron microscopy, affinity chromatography, and computational sequence alignment","pmids":["10716631"],"confidence":"Medium","gaps":["Predicted active-site residues not validated by mutagenesis","Single-lab topological model"]},{"year":2015,"claim":"Demonstrated that loss-of-function PIGH is incompatible with normal development, linking a splice-disrupting variant to autosomal recessive arthrogryposis in cattle.","evidence":"GWAS mapping, NGS, RNA-Seq, and RT-PCR confirmation of exon-2 skipping in Belgian Blue cattle","pmids":["25895751"],"confidence":"Medium","gaps":["Functional consequence inferred from domain loss rather than direct enzyme assay","Single study; mechanism linking GPI deficiency to arthrogryposis not detailed"]},{"year":2018,"claim":"Mapped a functionally essential region of PIGH by showing that start-codon disruption removing the N-terminal 62 residues yields a truncated protein unable to restore GPI-anchor expression.","evidence":"FACS of GPI-AP surface expression in PIGH-deficient CHO cells complemented with c.1A>T cDNA, with Sanger sequencing","pmids":["29573052"],"confidence":"Medium","gaps":["Single-lab cell-based complementation only","Molecular reason the N-terminus is required not defined"]},{"year":2018,"claim":"Showed that PIGH loss can be epigenetic rather than genetic, with mRNA silencing causing defective first-step GPI biosynthesis and loss of GPI-anchored proteins such as CD52.","evidence":"RT-PCR/mRNA expression, methylation analysis, and flow cytometry for GPI-AP surface expression in B-ALL cells","pmids":["30370942"],"confidence":"Medium","gaps":["Mechanism of silencing not fully defined","Single lab"]},{"year":2021,"claim":"Connected PIGH-dependent GPI synthesis to a specific signaling output by showing silencing abolishes CD14 surface expression and LPS/TLR4 responses, rescued by PIGH overexpression.","evidence":"Flow cytometry for GPI-AP/CD14, LPS stimulation assays, and PIGH overexpression rescue in BLaER1 monocyte-model cells","pmids":["34294787"],"confidence":"Medium","gaps":["Specificity to TLR4 versus broader GPI-AP loss not fully dissected","Single-lab functional model"]},{"year":null,"claim":"The precise catalytic contribution of PIG-H within the GPI-GnT complex and a structural model of how it scaffolds PIG-A remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No atomic structure of the PIG-A/PIG-H core","Whether PIG-H contributes residues to catalysis or only stabilizes the complex is unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1,3]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[0,1,4]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[1,3]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[1,7]}],"complexes":["GPI-GlcNAc transferase (GPI-GnT) complex"],"partners":["PIGA","PIGC","GPI1","PIGP","DPM2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q14442","full_name":"Phosphatidylinositol N-acetylglucosaminyltransferase subunit H","aliases":["Phosphatidylinositol-glycan biosynthesis class H protein","PIG-H"],"length_aa":188,"mass_kda":21.1,"function":"Part of the glycosylphosphatidylinositol-N-acetylglucosaminyltransferase (GPI-GnT) complex that catalyzes the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to phosphatidylinositol and participates in the first step of GPI biosynthesis","subcellular_location":"Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/Q14442/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/PIGH","classification":"Common Essential","n_dependent_lines":414,"n_total_lines":1208,"dependency_fraction":0.34271523178807944},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/PIGH","total_profiled":1310},"omim":[{"mim_id":"618010","title":"GLYCOSYLPHOSPHATIDYLINOSITOL BIOSYNTHESIS DEFECT 17; GPIBD17","url":"https://www.omim.org/entry/618010"},{"mim_id":"610293","title":"GLYCOSYLPHOSPHATIDYLINOSITOL BIOSYNTHESIS DEFECT 1; GPIBD1","url":"https://www.omim.org/entry/610293"},{"mim_id":"605754","title":"PHOSPHATIDYLINOSITOL GLYCAN ANCHOR BIOSYNTHESIS CLASS Q PROTEIN; PIGQ","url":"https://www.omim.org/entry/605754"},{"mim_id":"601730","title":"PHOSPHATIDYLINOSITOL GLYCAN ANCHOR BIOSYNTHESIS CLASS C PROTEIN; PIGC","url":"https://www.omim.org/entry/601730"},{"mim_id":"600154","title":"PHOSPHATIDYLINOSITOL GLYCAN ANCHOR BIOSYNTHESIS CLASS H PROTEIN; PIGH","url":"https://www.omim.org/entry/600154"}],"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/PIGH"},"hgnc":{"alias_symbol":["GPI-H"],"prev_symbol":[]},"alphafold":{"accession":"Q14442","domains":[{"cath_id":"2.30.29","chopping":"1-35_85-185","consensus_level":"medium","plddt":90.2499,"start":1,"end":185},{"cath_id":"1.10.287","chopping":"37-82","consensus_level":"medium","plddt":81.49,"start":37,"end":82}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q14442","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q14442-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q14442-F1-predicted_aligned_error_v6.png","plddt_mean":86.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PIGH","jax_strain_url":"https://www.jax.org/strain/search?query=PIGH"},"sequence":{"accession":"Q14442","fasta_url":"https://rest.uniprot.org/uniprotkb/Q14442.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q14442/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q14442"}},"corpus_meta":[{"pmid":"15853884","id":"PMC_15853884","title":"Biosynthesis 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The complex preferentially utilizes bovine PI (~100-fold) over soybean PI, suggesting recognition of specific fatty acyl chains.\",\n      \"method\": \"Co-immunoprecipitation, in vitro GPI-GnT activity assay with substrate specificity analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution of enzymatic activity combined with co-IP complex identification, multiple orthogonal methods\",\n      \"pmids\": [\"9463366\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"GPI1 stabilizes the GPI-GnT enzyme complex by tying PIG-C to a core PIG-A/PIG-H complex; disruption of GPI1 in F9 cells caused nearly undetectable PIG-A/PIG-H/PIG-C trimeric complex while PIG-A/PIG-H dimeric complex remained detectable, and caused partial decreases in PIG-C and PIG-H protein levels.\",\n      \"method\": \"Gene disruption (KO) in F9 embryonal carcinoma cells, co-immunoprecipitation, Western blot\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean KO with defined molecular phenotype, epistatic placement of GPI1 relative to PIG-H within the complex, replicated by multiple subsequent studies\",\n      \"pmids\": [\"10373468\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"GPI-GnT requires a fifth component, PIG-P, which associates with PIG-A and GPI1; a cell lacking PIG-P is GPI-anchor negative, establishing PIG-P as essential. DPM2 associates with GPI-GnT (through interactions with PIG-A, PIG-C, and GPI1) and enhances enzyme activity 3-fold, but is not essential for the reaction. PIG-H is thus a subunit of this larger, regulated complex.\",\n      \"method\": \"Co-immunoprecipitation, cell-based GPI-anchor expression assay (flow cytometry), in vitro GPI-GnT activity assay\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — in vitro activity assay plus Co-IP plus loss-of-function cell lines, multiple orthogonal methods identifying complex composition and regulation\",\n      \"pmids\": [\"10944123\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PIG-A (the proposed catalytic subunit of the GPI-GnT complex) localizes to both perinuclear and mitochondria-associated lamellae of the ER; computer-aided alignment identified highly conserved residues in the membrane-proximal cytoplasmic domain (residues 230–340) of PIG-A potentially involved in catalysis. A topological model of the four partners (PIG-A, PIG-H, PIG-C, GPI1) was proposed.\",\n      \"method\": \"Immunofluorescence, immunoelectron microscopy, affinity chromatography, computational sequence comparison\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct localization experiment with functional context, computational active-site prediction without mutagenesis validation; single lab\",\n      \"pmids\": [\"10716631\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"A homozygous c.1A>T transversion in PIGH (disrupting the start codon) results in utilization of an in-frame start site at codon 63, producing a truncated protein that cannot efficiently restore GPI-anchored protein expression in PIGH-deficient CHO cells, demonstrating that the N-terminal 62 residues of PIGH are essential for GPI-GnT function.\",\n      \"method\": \"FACS analysis of GPI-AP surface expression in PIGH-deficient CHO cells transfected with cDNA bearing c.1A>T, Sanger sequencing\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-based complementation assay with defined molecular variant; single lab, single method\",\n      \"pmids\": [\"29573052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Loss of GPI-anchor–negative phenotype in B-ALL cells results from epigenetic silencing of PIGH mRNA expression (rather than gene mutation or deletion), leading to defective first-step GPI biosynthesis and loss of GPI-anchored protein (including CD52) surface expression.\",\n      \"method\": \"RT-PCR/mRNA expression analysis, flow cytometry for GPI-AP surface expression, methylation/epigenetic analysis\",\n      \"journal\": \"American journal of hematology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct demonstration that PIGH mRNA loss (not mutation) causes GPI-AP loss; single lab, multiple methods\",\n      \"pmids\": [\"30370942\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Epigenetic silencing of PIGH in BLaER1 monocyte-model cells causes GPI-anchor deficiency, loss of CD14 surface expression, and diminished LPS/TLR4 signaling (but not TLR7/TLR8 signaling); overexpressing PIGH restored GPI-anchored protein (including CD14) expression and LPS responsiveness, placing PIGH upstream of CD14-dependent TLR4 activation.\",\n      \"method\": \"Flow cytometry for GPI-AP/CD14 surface expression, LPS stimulation assay, PIGH overexpression rescue experiment\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — rescue experiment with defined functional readout (TLR4 signaling), single lab, multiple orthogonal readouts\",\n      \"pmids\": [\"34294787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"A splice-site variant (c211-10C>G) in bovine PIGH causes skipping of exon 2, producing a non-functional PIGH protein lacking essential domains; this loss-of-function is associated with autosomal recessive arthrogryposis in Belgian Blue cattle, confirming PIGH is essential for normal development in mammals.\",\n      \"method\": \"Genome-wide association mapping, next-generation DNA sequencing, RNA-Seq, RT-PCR confirmation of exon skipping\",\n      \"journal\": \"BMC genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — RNA-Seq plus RT-PCR confirmation of splicing defect; functional consequence inferred from loss of essential domains; single study\",\n      \"pmids\": [\"25895751\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PIGH (PIG-H) is an essential, cytoplasmically oriented ER-associated protein that forms a core subcomplex with PIG-A in the endoplasmic reticulum; together with PIG-C, GPI1, PIG-P, and the regulatory subunit DPM2, it constitutes the GPI-N-acetylglucosaminyltransferase (GPI-GnT) complex that catalyzes the first committed step of GPI-anchor biosynthesis—transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol on the cytoplasmic face of the ER—with GPI1 stabilizing the complex by bridging PIG-C to the PIG-A/PIG-H core, and loss of PIGH (by mutation, truncation, or epigenetic silencing) abolishing GPI-anchored protein surface expression and downstream GPI-dependent signaling such as CD14-mediated TLR4 activation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PIGH is an essential, cytoplasmically oriented ER-associated subunit of the GPI-N-acetylglucosaminyltransferase (GPI-GnT) complex that catalyzes the first committed step of GPI-anchor biosynthesis—transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol on the cytoplasmic face of the ER [#0, #1]. PIG-H forms a core protein complex with the catalytic-class subunit PIG-A in the ER, and together with PIG-C and GPI1 constitutes a four-protein complex with reconstituted GPI-GnT activity in vitro that preferentially recognizes specific PI fatty-acyl species [#0, #1]. Within this assembly GPI1 stabilizes the enzyme by bridging PIG-C to the PIG-A/PIG-H core, while a stable PIG-A/PIG-H dimer persists in its absence, defining PIG-H as part of the irreducible catalytic scaffold [#2]. The complex is further completed by the essential component PIG-P and regulated by DPM2, which enhances activity without being required for catalysis [#3]. Loss of PIGH function—through start-codon disruption that removes the essential N-terminal 62 residues, epigenetic silencing of its mRNA, or splice-disrupting variants—abolishes surface expression of GPI-anchored proteins and the downstream signaling they support, including CD14-mediated LPS/TLR4 activation [#5, #6, #7]. A loss-of-function splice variant in bovine PIGH causes autosomal recessive arthrogryposis, establishing PIGH as essential for normal mammalian development [#8].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Established that PIG-H is a cytoplasmically oriented ER-associated protein that physically partners with PIG-A, defining the topology and subunit pairing of the GPI GlcNAc transferase before its activity was reconstituted.\",\n      \"evidence\": \"Subcellular fractionation, topology studies, immunofluorescence, and co-immunoprecipitation in mammalian cells\",\n      \"pmids\": [\"8900170\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Did not reconstitute enzymatic activity\",\n        \"Catalytic versus structural role of PIG-H within the complex not resolved\"\n      ]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Showed that PIG-A, PIG-H, PIG-C and GPI1 together form a four-protein complex carrying GPI-GnT activity, placing PIG-H in a defined enzyme rather than an inferred association.\",\n      \"evidence\": \"Co-immunoprecipitation plus in vitro GPI-GnT activity assay with PI substrate-specificity analysis\",\n      \"pmids\": [\"9463366\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Which subunit performs catalysis not assigned\",\n        \"Did not test whether additional components are required for full activity\"\n      ]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Resolved the internal architecture of the complex by showing GPI1 bridges PIG-C to a stable PIG-A/PIG-H core, defining PIG-H as part of the irreducible catalytic dimer.\",\n      \"evidence\": \"GPI1 gene disruption in F9 cells with co-IP and Western blot analysis of remaining subassemblies\",\n      \"pmids\": [\"10373468\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of the PIG-A/PIG-H interaction not determined\",\n        \"Did not establish whether PIG-A/PIG-H dimer alone has residual activity\"\n      ]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Expanded the complex to five essential subunits plus a regulator, showing PIG-P is required and DPM2 enhances but is dispensable for activity, contextualizing PIG-H within a larger regulated machine.\",\n      \"evidence\": \"Co-IP, flow-cytometry GPI-anchor expression assays in loss-of-function cells, and in vitro GPI-GnT activity assays\",\n      \"pmids\": [\"10944123\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Specific contribution of PIG-H to substrate binding or catalysis still undefined\",\n        \"No structural model of the assembled complex\"\n      ]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Localized the catalytic-class subunit to perinuclear and mitochondria-associated ER lamellae and proposed a topological model, refining where the PIG-H-containing complex operates.\",\n      \"evidence\": \"Immunofluorescence, immunoelectron microscopy, affinity chromatography, and computational sequence alignment\",\n      \"pmids\": [\"10716631\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Predicted active-site residues not validated by mutagenesis\",\n        \"Single-lab topological model\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrated that loss-of-function PIGH is incompatible with normal development, linking a splice-disrupting variant to autosomal recessive arthrogryposis in cattle.\",\n      \"evidence\": \"GWAS mapping, NGS, RNA-Seq, and RT-PCR confirmation of exon-2 skipping in Belgian Blue cattle\",\n      \"pmids\": [\"25895751\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Functional consequence inferred from domain loss rather than direct enzyme assay\",\n        \"Single study; mechanism linking GPI deficiency to arthrogryposis not detailed\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Mapped a functionally essential region of PIGH by showing that start-codon disruption removing the N-terminal 62 residues yields a truncated protein unable to restore GPI-anchor expression.\",\n      \"evidence\": \"FACS of GPI-AP surface expression in PIGH-deficient CHO cells complemented with c.1A>T cDNA, with Sanger sequencing\",\n      \"pmids\": [\"29573052\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single-lab cell-based complementation only\",\n        \"Molecular reason the N-terminus is required not defined\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showed that PIGH loss can be epigenetic rather than genetic, with mRNA silencing causing defective first-step GPI biosynthesis and loss of GPI-anchored proteins such as CD52.\",\n      \"evidence\": \"RT-PCR/mRNA expression, methylation analysis, and flow cytometry for GPI-AP surface expression in B-ALL cells\",\n      \"pmids\": [\"30370942\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism of silencing not fully defined\",\n        \"Single lab\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Connected PIGH-dependent GPI synthesis to a specific signaling output by showing silencing abolishes CD14 surface expression and LPS/TLR4 responses, rescued by PIGH overexpression.\",\n      \"evidence\": \"Flow cytometry for GPI-AP/CD14, LPS stimulation assays, and PIGH overexpression rescue in BLaER1 monocyte-model cells\",\n      \"pmids\": [\"34294787\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Specificity to TLR4 versus broader GPI-AP loss not fully dissected\",\n        \"Single-lab functional model\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The precise catalytic contribution of PIG-H within the GPI-GnT complex and a structural model of how it scaffolds PIG-A remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No atomic structure of the PIG-A/PIG-H core\",\n        \"Whether PIG-H contributes residues to catalysis or only stabilizes the complex is unknown\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [0, 1, 4]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [1, 3]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [1, 7]}\n    ],\n    \"complexes\": [\"GPI-GlcNAc transferase (GPI-GnT) complex\"],\n    \"partners\": [\"PIGA\", \"PIGC\", \"GPI1\", \"PIGP\", \"DPM2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}