{"gene":"ATP6V0A4","run_date":"2026-04-28T17:12:37","timeline":{"discoveries":[{"year":2002,"finding":"ATP6V0A4 (a4 subunit) mutations cause autosomal recessive distal renal tubular acidosis (rdRTA) with preserved hearing, and ATP6V0A4 is expressed within the human inner ear (cochlea), with some patients developing hearing loss in young adulthood.","method":"Linkage analysis, mutation screening, RT-PCR expression analysis of human inner ear tissue","journal":"Journal of medical genetics","confidence":"High","confidence_rationale":"Tier 2 — direct mutation identification in patients, expression confirmed by RT-PCR in cochlear tissue, replicated across multiple kindreds","pmids":["12414817"],"is_preprint":false},{"year":2003,"finding":"The ATP6V0A4 (a4) subunit localizes to the apical compartment of proximal tubule (S1/S2 segment), loop of Henle, intercalated cells of distal convoluted tubule, connecting segment, and all intercalated cells of the entire collecting duct in human and mouse kidney. Under acid-loading (NH4Cl) or K+ depletion, a4 undergoes subcellular redistribution to pronounced apical localization without changing total protein expression levels, suggesting regulation by trafficking rather than expression to adapt V-H+-ATPase activity.","method":"RT-PCR on dissected nephron segments, immunohistochemistry, Western blotting, acid-base/electrolyte loading experiments in mice","journal":"Journal of the American Society of Nephrology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (IHC, Western blot, RT-PCR, functional perturbation) in single rigorous study","pmids":["14638902"],"is_preprint":false},{"year":2018,"finding":"Disease-causing missense mutations in ATP6V0A4 (a4 subunit): a4R449H causes protein instability and is degraded predominantly via the proteasomal pathway, with retention in the endoplasmic reticulum, defective cell-surface expression, increased association with the V0 assembly factor VMA21, and reduced association with V1 sector subunit ATP6V1B1 (B1). a4G820R has relatively normal stability and trafficking but 3D molecular modeling suggests it blocks the proton translocation pathway.","method":"Transient expression of FLAG-tagged WT and mutant a4 in HEK293 cells, cycloheximide chase assays, endoglycosidase treatment, immunofluorescence, co-immunoprecipitation, 3D molecular modeling","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal biochemical methods (CHX chase, co-IP, IF, glycosylation assay, modeling) in single study with disease-relevant mutations","pmids":["29311258"],"is_preprint":false},{"year":1999,"finding":"Genetic linkage analysis mapped autosomal recessive distal renal tubular acidosis with normal hearing (rdRTA2) to chromosome 7q33-34, establishing that genes causing rdRTA with normal hearing versus rdRTA with sensorineural hearing loss (ATP6B1/ATP6V1B1) are distinct, and predicting the location of the ATP6V0A4 gene locus.","method":"Genome-wide linkage analysis with multipoint LOD score of 8.84 in 13 kindreds, molecular exclusion of ATP6B1","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis/linkage with strong LOD score in multiple independent kindreds","pmids":["10577919"],"is_preprint":false},{"year":2021,"finding":"Seven exonic variants in ATP6V0A4 (including c.322C>T, p.Gln108* and c.1572G>A, p.Pro524Pro) alter pre-mRNA splicing by disrupting exonic splicing enhancers (ESEs) or generating exonic splicing silencers, or interfering with classic splicing site recognition, resulting in complete or incomplete exon skipping as demonstrated by minigene assay.","method":"Minigene splicing assay, bioinformatics ESE/ESS prediction, direct sequencing","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 2 — functional minigene assay demonstrates mechanistic splicing consequence, single lab study","pmids":["34157794"],"is_preprint":false},{"year":2012,"finding":"ATP6V0A4 (a4 subunit) is expressed in specific subtypes of human gliomas including grade III oligodendrogliomas (~34%), pilocytic astrocytomas (~68%), and gangliogliomas (~37%), and is absent in epileptic brain tissue. In pilocytic astrocytomas, a4 expression is associated with a tandem duplication of the 7q34 chromosomal region, 0.5 Mb from the ATP6V0A4 gene locus.","method":"RT-PCR/qPCR of 205 glioma biopsies and 11 epileptic brain biopsies, chromosomal analysis","journal":"Glia","confidence":"Medium","confidence_rationale":"Tier 3 — expression study with mechanistic chromosomal link (gene duplication), large sample set but no direct functional experiment","pmids":["22460948"],"is_preprint":false}],"current_model":"ATP6V0A4 encodes the a4 isoform of the vacuolar H+-ATPase (V-ATPase) V0 sector, which localizes apically to intercalated cells throughout the kidney collecting duct and proximal tubule where it drives proton secretion; loss-of-function mutations cause autosomal recessive distal renal tubular acidosis by destabilizing the protein (via proteasomal degradation and ER retention) or by blocking the proton translocation pathway, while acid-base challenges regulate a4 activity through subcellular trafficking rather than changes in total protein expression."},"narrative":{"teleology":[{"year":1999,"claim":"Establishing that rdRTA with normal hearing maps to a distinct locus on 7q33-34, separate from ATP6V1B1, predicted the existence of a second V-ATPase gene (later identified as ATP6V0A4) causing distal renal tubular acidosis.","evidence":"Genome-wide linkage analysis with LOD score 8.84 across 13 kindreds with molecular exclusion of ATP6B1","pmids":["10577919"],"confidence":"High","gaps":["The causative gene at the 7q33-34 locus was not yet identified","Whether the mapped gene encoded a V-ATPase subunit was unknown"]},{"year":2002,"claim":"Identification of ATP6V0A4 mutations as the cause of rdRTA with preserved hearing resolved the 7q34 locus and revealed unexpected expression in the human cochlea, explaining late-onset hearing loss in some patients.","evidence":"Mutation screening in rdRTA kindreds, RT-PCR of human inner ear tissue","pmids":["12414817"],"confidence":"High","gaps":["Precise subcellular localization of a4 along the nephron was unresolved","Mechanism by which mutations cause disease (protein stability vs. function) was unknown"]},{"year":2003,"claim":"Detailed nephron mapping showed a4 localizes apically across multiple nephron segments and demonstrated that acid-base regulation of V-ATPase occurs through a4 subcellular redistribution rather than changes in protein abundance, establishing a trafficking-based regulatory mechanism.","evidence":"Immunohistochemistry, Western blotting, and RT-PCR on dissected nephron segments; NH4Cl acid-loading and K⁺ depletion in mice","pmids":["14638902"],"confidence":"High","gaps":["The molecular signals and machinery directing a4 apical trafficking remain unidentified","Whether other V-ATPase subunits co-traffic with a4 was not tested"]},{"year":2012,"claim":"Detection of ectopic ATP6V0A4 expression in specific glioma subtypes, associated with a 7q34 tandem duplication, raised the possibility that a4 may contribute to tumor biology outside its normal renal and cochlear expression domains.","evidence":"RT-PCR/qPCR of 205 glioma biopsies with chromosomal analysis","pmids":["22460948"],"confidence":"Medium","gaps":["No functional experiments tested whether a4 expression contributes to glioma biology","Whether ectopic a4 assembles into functional V-ATPase complexes in glioma cells is unknown"]},{"year":2018,"claim":"Biochemical dissection of disease-causing missense mutations revealed two distinct pathomechanisms: R449H causes protein instability with proteasomal degradation and ER retention, while G820R preserves stability but is predicted to block the proton translocation pathway, providing the first mechanistic classification of ATP6V0A4 mutations.","evidence":"Cycloheximide chase, co-immunoprecipitation, immunofluorescence, endoglycosidase assays, and 3D molecular modeling of FLAG-tagged a4 variants in HEK293 cells","pmids":["29311258"],"confidence":"High","gaps":["Proton translocation blockade by G820R was inferred from modeling, not measured directly","Whether ER-retained R449H mutant can be pharmacologically rescued is untested","Role of VMA21 assembly factor in normal a4 biogenesis versus mutant retention is not fully defined"]},{"year":2021,"claim":"Demonstration that multiple exonic ATP6V0A4 variants disrupt pre-mRNA splicing through ESE/ESS modulation expanded the mutational spectrum and revealed that apparently synonymous or missense variants can be pathogenic through splicing rather than protein-level effects.","evidence":"Minigene splicing assays with bioinformatic ESE/ESS prediction","pmids":["34157794"],"confidence":"Medium","gaps":["Splicing defects were shown in minigene context, not confirmed in patient-derived cells","Whether partial exon skipping produces any residual functional protein is unknown"]},{"year":null,"claim":"The molecular signals that trigger a4 apical trafficking in response to acid-base perturbation, the structural basis of proton translocation through a4, and whether pharmacological chaperones can rescue ER-retained mutants remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of a4 is available","Trafficking adaptors or kinases regulating a4 redistribution have not been identified","Functional role of ectopic a4 in glioma has not been tested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[1,2]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,2]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[2]}],"pathway":[{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[1,2]}],"complexes":["V-ATPase (V0 sector)"],"partners":["ATP6V1B1","VMA21"],"other_free_text":[]},"mechanistic_narrative":"ATP6V0A4 encodes the a4 isoform of the V0 sector of the vacuolar H⁺-ATPase (V-ATPase), which localizes to the apical compartment of intercalated cells throughout the kidney collecting duct, connecting segment, distal convoluted tubule, loop of Henle, and proximal tubule (S1/S2), where it mediates electrogenic proton secretion required for urinary acidification [PMID:14638902]. Under acid-loading or potassium depletion, a4 undergoes subcellular redistribution to the apical membrane without changes in total protein expression, indicating that V-ATPase activity is regulated by trafficking rather than transcriptional control [PMID:14638902]. Loss-of-function mutations in ATP6V0A4 cause autosomal recessive distal renal tubular acidosis (rdRTA), with some patients developing late-onset sensorineural hearing loss; disease-causing missense mutations destabilize the protein via proteasomal degradation and ER retention (e.g., R449H) or block the proton translocation pathway while preserving protein stability and trafficking (e.g., G820R) [PMID:12414817, PMID:29311258]."},"prefetch_data":{"uniprot":{"accession":"Q9HBG4","full_name":"V-type proton ATPase 116 kDa subunit a 4","aliases":["Vacuolar proton translocating ATPase 116 kDa subunit a isoform 4","Vacuolar proton translocating ATPase 116 kDa subunit a kidney isoform"],"length_aa":840,"mass_kda":96.4,"function":"Subunit of the V0 complex of vacuolar(H+)-ATPase (V-ATPase), a multisubunit enzyme composed of a peripheral complex (V1) that hydrolyzes ATP and a membrane integral complex (V0) that translocates protons (By similarity). V-ATPase is responsible for acidifying and maintaining the pH of intracellular compartments and in some cell types, is targeted to the plasma membrane, where it is responsible for acidifying the extracellular environment (By similarity). Involved in normal vectorial acid transport into the urine by the kidney (PubMed:10973252, PubMed:12414817)","subcellular_location":"Apical cell membrane; Basolateral cell membrane","url":"https://www.uniprot.org/uniprotkb/Q9HBG4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ATP6V0A4","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ATP6V0A4","total_profiled":1310},"omim":[{"mim_id":"605239","title":"ATPase, H+ TRANSPORTING, LYSOSOMAL, V0 SUBUNIT A, ISOFORM 4; ATP6V0A4","url":"https://www.omim.org/entry/605239"},{"mim_id":"602722","title":"RENAL TUBULAR ACIDOSIS, DISTAL, 3, WITH OR WITHOUT SENSORINEURAL HEARING LOSS; DRTA3","url":"https://www.omim.org/entry/602722"},{"mim_id":"267300","title":"RENAL TUBULAR ACIDOSIS, DISTAL, 2, WITH PROGRESSIVE SENSORINEURAL HEARING LOSS; DRTA2","url":"https://www.omim.org/entry/267300"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Uncertain","locations":[{"location":"Cytosol","reliability":"Uncertain"}],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"kidney","ntpm":59.4},{"tissue":"salivary gland","ntpm":73.9}],"url":"https://www.proteinatlas.org/search/ATP6V0A4"},"hgnc":{"alias_symbol":["RDRTA2","VPP2","RTADR","a4","Vph1","Stv1"],"prev_symbol":["ATP6N1B","ATP6N2","RTA1C"]},"alphafold":{"accession":"Q9HBG4","domains":[{"cath_id":"3.30.70.260","chopping":"1-38_312-366","consensus_level":"medium","plddt":86.7075,"start":1,"end":366},{"cath_id":"-","chopping":"371-480_516-663_716-815","consensus_level":"medium","plddt":88.7334,"start":371,"end":815}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9HBG4","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9HBG4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9HBG4-F1-predicted_aligned_error_v6.png","plddt_mean":82.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ATP6V0A4","jax_strain_url":"https://www.jax.org/strain/search?query=ATP6V0A4"},"sequence":{"accession":"Q9HBG4","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9HBG4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9HBG4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9HBG4"}},"corpus_meta":[{"pmid":"12172542","id":"PMC_12172542","title":"Multi-pronged inhibition of airway hyper-responsiveness and inflammation by lipoxin A(4).","date":"2002","source":"Nature medicine","url":"https://pubmed.ncbi.nlm.nih.gov/12172542","citation_count":286,"is_preprint":false},{"pmid":"8415676","id":"PMC_8415676","title":"Protein phosphorylation inhibits production of Alzheimer amyloid beta/A4 peptide.","date":"1993","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/8415676","citation_count":257,"is_preprint":false},{"pmid":"12414817","id":"PMC_12414817","title":"Novel ATP6V1B1 and ATP6V0A4 mutations in autosomal recessive distal renal tubular acidosis with new evidence for hearing loss.","date":"2002","source":"Journal of medical genetics","url":"https://pubmed.ncbi.nlm.nih.gov/12414817","citation_count":223,"is_preprint":false},{"pmid":"1385813","id":"PMC_1385813","title":"The VPH1 gene encodes a 95-kDa integral membrane polypeptide required for in vivo assembly and activity of the yeast vacuolar H(+)-ATPase.","date":"1992","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/1385813","citation_count":220,"is_preprint":false},{"pmid":"10360957","id":"PMC_10360957","title":"Lipoxin A4: a new class of ligand for the Ah receptor.","date":"1999","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10360957","citation_count":207,"is_preprint":false},{"pmid":"15330748","id":"PMC_15330748","title":"Combretastatin A4 phosphate: background and current clinical status.","date":"2004","source":"Expert opinion on investigational drugs","url":"https://pubmed.ncbi.nlm.nih.gov/15330748","citation_count":149,"is_preprint":false},{"pmid":"11905799","id":"PMC_11905799","title":"Targeting tumour vasculature: the development of combretastatin A4.","date":"2001","source":"The Lancet. 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Under acid-loading (NH4Cl) or K+ depletion, a4 undergoes subcellular redistribution to pronounced apical localization without changing total protein expression levels, suggesting regulation by trafficking rather than expression to adapt V-H+-ATPase activity.\",\n      \"method\": \"RT-PCR on dissected nephron segments, immunohistochemistry, Western blotting, acid-base/electrolyte loading experiments in mice\",\n      \"journal\": \"Journal of the American Society of Nephrology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (IHC, Western blot, RT-PCR, functional perturbation) in single rigorous study\",\n      \"pmids\": [\"14638902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Disease-causing missense mutations in ATP6V0A4 (a4 subunit): a4R449H causes protein instability and is degraded predominantly via the proteasomal pathway, with retention in the endoplasmic reticulum, defective cell-surface expression, increased association with the V0 assembly factor VMA21, and reduced association with V1 sector subunit ATP6V1B1 (B1). a4G820R has relatively normal stability and trafficking but 3D molecular modeling suggests it blocks the proton translocation pathway.\",\n      \"method\": \"Transient expression of FLAG-tagged WT and mutant a4 in HEK293 cells, cycloheximide chase assays, endoglycosidase treatment, immunofluorescence, co-immunoprecipitation, 3D molecular modeling\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal biochemical methods (CHX chase, co-IP, IF, glycosylation assay, modeling) in single study with disease-relevant mutations\",\n      \"pmids\": [\"29311258\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Genetic linkage analysis mapped autosomal recessive distal renal tubular acidosis with normal hearing (rdRTA2) to chromosome 7q33-34, establishing that genes causing rdRTA with normal hearing versus rdRTA with sensorineural hearing loss (ATP6B1/ATP6V1B1) are distinct, and predicting the location of the ATP6V0A4 gene locus.\",\n      \"method\": \"Genome-wide linkage analysis with multipoint LOD score of 8.84 in 13 kindreds, molecular exclusion of ATP6B1\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis/linkage with strong LOD score in multiple independent kindreds\",\n      \"pmids\": [\"10577919\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Seven exonic variants in ATP6V0A4 (including c.322C>T, p.Gln108* and c.1572G>A, p.Pro524Pro) alter pre-mRNA splicing by disrupting exonic splicing enhancers (ESEs) or generating exonic splicing silencers, or interfering with classic splicing site recognition, resulting in complete or incomplete exon skipping as demonstrated by minigene assay.\",\n      \"method\": \"Minigene splicing assay, bioinformatics ESE/ESS prediction, direct sequencing\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional minigene assay demonstrates mechanistic splicing consequence, single lab study\",\n      \"pmids\": [\"34157794\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"ATP6V0A4 (a4 subunit) is expressed in specific subtypes of human gliomas including grade III oligodendrogliomas (~34%), pilocytic astrocytomas (~68%), and gangliogliomas (~37%), and is absent in epileptic brain tissue. In pilocytic astrocytomas, a4 expression is associated with a tandem duplication of the 7q34 chromosomal region, 0.5 Mb from the ATP6V0A4 gene locus.\",\n      \"method\": \"RT-PCR/qPCR of 205 glioma biopsies and 11 epileptic brain biopsies, chromosomal analysis\",\n      \"journal\": \"Glia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — expression study with mechanistic chromosomal link (gene duplication), large sample set but no direct functional experiment\",\n      \"pmids\": [\"22460948\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ATP6V0A4 encodes the a4 isoform of the vacuolar H+-ATPase (V-ATPase) V0 sector, which localizes apically to intercalated cells throughout the kidney collecting duct and proximal tubule where it drives proton secretion; loss-of-function mutations cause autosomal recessive distal renal tubular acidosis by destabilizing the protein (via proteasomal degradation and ER retention) or by blocking the proton translocation pathway, while acid-base challenges regulate a4 activity through subcellular trafficking rather than changes in total protein expression.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"ATP6V0A4 encodes the a4 isoform of the V0 sector of the vacuolar H⁺-ATPase (V-ATPase), which localizes to the apical compartment of intercalated cells throughout the kidney collecting duct, connecting segment, distal convoluted tubule, loop of Henle, and proximal tubule (S1/S2), where it mediates electrogenic proton secretion required for urinary acidification [PMID:14638902]. Under acid-loading or potassium depletion, a4 undergoes subcellular redistribution to the apical membrane without changes in total protein expression, indicating that V-ATPase activity is regulated by trafficking rather than transcriptional control [PMID:14638902]. Loss-of-function mutations in ATP6V0A4 cause autosomal recessive distal renal tubular acidosis (rdRTA), with some patients developing late-onset sensorineural hearing loss; disease-causing missense mutations destabilize the protein via proteasomal degradation and ER retention (e.g., R449H) or block the proton translocation pathway while preserving protein stability and trafficking (e.g., G820R) [PMID:12414817, PMID:29311258].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Establishing that rdRTA with normal hearing maps to a distinct locus on 7q33-34, separate from ATP6V1B1, predicted the existence of a second V-ATPase gene (later identified as ATP6V0A4) causing distal renal tubular acidosis.\",\n      \"evidence\": \"Genome-wide linkage analysis with LOD score 8.84 across 13 kindreds with molecular exclusion of ATP6B1\",\n      \"pmids\": [\"10577919\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The causative gene at the 7q33-34 locus was not yet identified\",\n        \"Whether the mapped gene encoded a V-ATPase subunit was unknown\"\n      ]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identification of ATP6V0A4 mutations as the cause of rdRTA with preserved hearing resolved the 7q34 locus and revealed unexpected expression in the human cochlea, explaining late-onset hearing loss in some patients.\",\n      \"evidence\": \"Mutation screening in rdRTA kindreds, RT-PCR of human inner ear tissue\",\n      \"pmids\": [\"12414817\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Precise subcellular localization of a4 along the nephron was unresolved\",\n        \"Mechanism by which mutations cause disease (protein stability vs. function) was unknown\"\n      ]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Detailed nephron mapping showed a4 localizes apically across multiple nephron segments and demonstrated that acid-base regulation of V-ATPase occurs through a4 subcellular redistribution rather than changes in protein abundance, establishing a trafficking-based regulatory mechanism.\",\n      \"evidence\": \"Immunohistochemistry, Western blotting, and RT-PCR on dissected nephron segments; NH4Cl acid-loading and K⁺ depletion in mice\",\n      \"pmids\": [\"14638902\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The molecular signals and machinery directing a4 apical trafficking remain unidentified\",\n        \"Whether other V-ATPase subunits co-traffic with a4 was not tested\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Detection of ectopic ATP6V0A4 expression in specific glioma subtypes, associated with a 7q34 tandem duplication, raised the possibility that a4 may contribute to tumor biology outside its normal renal and cochlear expression domains.\",\n      \"evidence\": \"RT-PCR/qPCR of 205 glioma biopsies with chromosomal analysis\",\n      \"pmids\": [\"22460948\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No functional experiments tested whether a4 expression contributes to glioma biology\",\n        \"Whether ectopic a4 assembles into functional V-ATPase complexes in glioma cells is unknown\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Biochemical dissection of disease-causing missense mutations revealed two distinct pathomechanisms: R449H causes protein instability with proteasomal degradation and ER retention, while G820R preserves stability but is predicted to block the proton translocation pathway, providing the first mechanistic classification of ATP6V0A4 mutations.\",\n      \"evidence\": \"Cycloheximide chase, co-immunoprecipitation, immunofluorescence, endoglycosidase assays, and 3D molecular modeling of FLAG-tagged a4 variants in HEK293 cells\",\n      \"pmids\": [\"29311258\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Proton translocation blockade by G820R was inferred from modeling, not measured directly\",\n        \"Whether ER-retained R449H mutant can be pharmacologically rescued is untested\",\n        \"Role of VMA21 assembly factor in normal a4 biogenesis versus mutant retention is not fully defined\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstration that multiple exonic ATP6V0A4 variants disrupt pre-mRNA splicing through ESE/ESS modulation expanded the mutational spectrum and revealed that apparently synonymous or missense variants can be pathogenic through splicing rather than protein-level effects.\",\n      \"evidence\": \"Minigene splicing assays with bioinformatic ESE/ESS prediction\",\n      \"pmids\": [\"34157794\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Splicing defects were shown in minigene context, not confirmed in patient-derived cells\",\n        \"Whether partial exon skipping produces any residual functional protein is unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The molecular signals that trigger a4 apical trafficking in response to acid-base perturbation, the structural basis of proton translocation through a4, and whether pharmacological chaperones can rescue ER-retained mutants remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No high-resolution structure of a4 is available\",\n        \"Trafficking adaptors or kinases regulating a4 redistribution have not been identified\",\n        \"Functional role of ectopic a4 in glioma has not been tested\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"complexes\": [\n      \"V-ATPase (V0 sector)\"\n    ],\n    \"partners\": [\n      \"ATP6V1B1\",\n      \"VMA21\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}