{"gene":"TFR2","run_date":"2026-04-28T21:42:59","timeline":{"discoveries":[{"year":2000,"finding":"Homozygous nonsense mutation in TFR2 causes hereditary hemochromatosis (type 3), establishing TFR2 as essential for iron homeostasis in humans.","method":"Genetic mapping and mutation analysis in affected families","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — foundational genetic discovery, replicated across multiple populations","pmids":["10802645"],"is_preprint":false},{"year":2004,"finding":"TFR2 is upstream of hepcidin in the iron regulatory pathway; TfR2 mutant mice show reduced hepcidin mRNA expression and elevated duodenal DMT1, and inflammatory stimuli (IL-6, LPS) can induce hepcidin independently of TfR2.","method":"Northern blot analysis of hepcidin and DMT1 mRNA in TfR2(Y245X) mutant vs wild-type mice; isolated hepatocyte stimulation with IL-6 and LPS","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — genetic KO model with specific molecular readouts, replicated by independent groups","pmids":["15345587"],"is_preprint":false},{"year":2004,"finding":"TFR2 mutations in patients result in low or absent urinary hepcidin, indicating TFR2 is a modulator of hepcidin production in response to iron.","method":"Urinary hepcidin measurement in patients homozygous for TFR2 mutations","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — direct patient-based functional readout, consistent with mouse model data","pmids":["15486069"],"is_preprint":false},{"year":2006,"finding":"BMP2, BMP4, and BMP9 stimulate hepcidin expression independently of Tfr2 (and Hfe, IL-6), as shown by equivalent BMP responses in isolated hepatocytes from wild-type and Tfr2 mutant mice, placing Tfr2 outside the BMP-direct signaling axis.","method":"Primary hepatocyte stimulation assays with BMPs in wild-type vs Tfr2 mutant mice; comparison of hepcidin transcription responses","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — epistasis established by genetic model + in vitro stimulation assay","pmids":["16801541"],"is_preprint":false},{"year":2006,"finding":"TfR2 localizes in lipid raft (low-density Triton-insoluble) plasma membrane domains, co-immunoprecipitates with caveolin-1 and CD81, and activation by holotransferrin or anti-TfR2 antibody triggers ERK1/ERK2 and p38 MAPK signaling in a lipid raft-dependent manner. TfR2 is also exported via exosomes.","method":"Lipid raft biochemical fractionation, co-immunoprecipitation, subcellular fractionation, MAPK phosphorylation assays, raft disruption, exosome isolation from HepG2 and K562 cells","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (fractionation, Co-IP, signaling assay, raft disruption) in single study","pmids":["17046995"],"is_preprint":false},{"year":2009,"finding":"TfR2 contains a mitochondrial targeting sequence and localizes to mitochondria of substantia nigra dopamine neurons, where it participates in a transferrin/TfR2-mediated mitochondrial iron transport pathway that can deliver iron to respiratory complex I; this pathway is redox-sensitive and is disrupted in Parkinson's disease.","method":"Subcellular fractionation, mitochondrial targeting sequence identification, iron delivery assays to mitochondria and complex I, rotenone PD model, human SN immunolocalization","journal":"Neurobiology of disease","confidence":"Medium","confidence_rationale":"Tier 2 — multiple methods but single lab, novel and not independently replicated","pmids":["19250966"],"is_preprint":false},{"year":2010,"finding":"Hepatocyte-specific expression of Tfr2 in Tfr2-deficient mice increases hepcidin mRNA and lowers hepatic iron and transferrin saturation, confirming the liver as the site of TfR2 action on hepcidin. HFE and TFR2 have non-redundant roles: expressing HFE in Tfr2-deficient mice or Tfr2 in HFE-null mice has no effect, suggesting both are required together (likely as a complex).","method":"AAV2/8-mediated hepatocyte-specific gene delivery of Hfe or Tfr2 in knockout mice; measurement of hepcidin mRNA, liver iron, transferrin saturation","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — clean genetic rescue experiment with specific molecular and biochemical readouts","pmids":["20177050"],"is_preprint":false},{"year":2010,"finding":"The two TFR2 isoforms (alpha and beta) have distinct functions: hepatic alpha-Tfr2 senses diferric transferrin and activates hepcidin, while beta-Tfr2 (expressed in spleen) specifically controls splenic iron efflux by regulating ferroportin-1 expression.","method":"Comparison of Tfr2 knockout vs. beta-isoform knockin mice; measurement of iron parameters, hepcidin, BMP6, ferroportin-1 in liver and spleen; liver-specific Tfr2 conditional knockout in knockin mice","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple mouse models with tissue-specific conditional knockouts, multiple molecular readouts","pmids":["20179178"],"is_preprint":false},{"year":2010,"finding":"TFR2-hemochromatosis patients show absent hepcidin response to oral iron challenge, whereas HFE-hemochromatosis patients show a blunted but detectable response, establishing TFR2 as playing a prominent role (and HFE a contributory role) in acute hepcidin induction by iron.","method":"Oral iron challenge test with time-course measurement of serum iron, transferrin saturation, and serum hepcidin by ELISA and mass spectrometry","journal":"Haematologica","confidence":"High","confidence_rationale":"Tier 2 — controlled clinical challenge experiment with quantitative hormone measurement at multiple time points","pmids":["21173098"],"is_preprint":false},{"year":2012,"finding":"HFE, TfR2, and HJV form a multi-protein membrane complex on hepatocytes. TfR2 residues 120-139 of the extracellular domain are critical for binding both HFE and HJV. HJV competes with TfR1 for HFE binding (as does TfR2). RGMA (a CNS homologue of HJV) can substitute for HJV in the complex.","method":"Glycerol gradient sedimentation assays, co-immunoprecipitation in transfected HuH7 hepatoma cells, domain mapping with TfR2 mutant constructs","journal":"Journal of hepatology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP with domain mapping, multiple orthogonal methods","pmids":["22728873"],"is_preprint":false},{"year":2012,"finding":"Hfe and Tfr2 are not substrates for Tmprss6 (matriptase-2); double mutant mice lacking both Hfe or Tfr2 and Tmprss6 show the Tmprss6-null phenotype for hepcidin but with greater erythropoiesis, indicating Hfe and Tfr2 allow increased erythropoiesis independently of liver hepcidin regulation.","method":"Generation of Hfe/Tmprss6 and Tfr2/Tmprss6 double knockout mice; measurement of hepcidin, iron parameters, and erythropoiesis","journal":"Blood cells, molecules & diseases","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with double knockout models and specific molecular readouts","pmids":["22244935"],"is_preprint":false},{"year":2013,"finding":"In stably co-expressing cells, HFE and TFR2 do not directly interact (no signal detected by proximity ligation assay), whereas HFE-TFR1 and TFR1-TFR2 heterodimer interactions are detected, suggesting HFE and TFR2 can regulate hepcidin independently.","method":"Proximity ligation assay in stably co-expressing cell lines; comparison with known HFE-TfR1 interaction as positive control","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — single lab with rigorous proximity ligation assay, contradicts some prior Co-IP data","pmids":["24155934"],"is_preprint":false},{"year":2013,"finding":"Tfr2 is required for upregulation of Bmp6 in response to hepatocyte iron (parenchymal iron), but not non-parenchymal iron; Hfe is not required for Bmp6 upregulation but is required for efficient downstream transmission of the regulatory signal.","method":"Dietary vs parenteral iron loading in wild-type, Hfe-/-, Tfr2-/-, and Hfe-/-/Tfr2-/- mice; measurement of Bmp6, hepcidin, and iron parameters","journal":"American journal of physiology. Gastrointestinal and liver physiology","confidence":"High","confidence_rationale":"Tier 2 — genetic double KO with differential iron-loading protocols and multiple molecular readouts","pmids":["24284962"],"is_preprint":false},{"year":2015,"finding":"CD81 interacts with TfR2 via both the cytoplasmic and ecto-transmembrane domains, promotes TfR2 degradation (increasing TfR2 half-life when knocked down), and the TfR2/CD81 complex is required for maintenance of hepcidin mRNA expression independently of BMP and ERK1/2 pathways. CD81 is itself targeted for degradation by the ubiquitin E3 ligase GRAIL.","method":"Yeast two-hybrid screen, co-precipitation with TfR2 domain constructs, siRNA knockdown of CD81 and GRAIL, hepcidin expression assay, BMP6 stimulation, ID1 and SMAD7 expression, ERK1/2 phosphorylation in Hep3B-TfR2 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — yeast two-hybrid plus multiple orthogonal biochemical assays and functional readouts in single study","pmids":["25635054"],"is_preprint":false},{"year":2016,"finding":"In hereditary tyrosinemia type 1 mice, transcription factor Sp1 regulates Tfr2 expression; downregulation of Sp1 reduces Tfr2, which decreases hepcidin, leading to iron overload. Forced expression of Tfr2 in the liver reduces iron accumulation.","method":"qRT-PCR, immunoblotting, adenovirus-mediated Tfr2 overexpression in Fah-/- mice; transferrin-sensitive hepcidin induction assay in hepatocytes","journal":"Journal of hepatology","confidence":"Medium","confidence_rationale":"Tier 2 — mechanistic pathway placement with rescue experiment, single lab","pmids":["27013087"],"is_preprint":false},{"year":2018,"finding":"TfR2 binds holotransferrin (holo-Tf) through a mechanism different from TfR1: the helical domain differences account for differences in Tf on-rate, and conserved apical arm junction residues are critical for TfR2-Tf (but not TfR1-Tf) interaction stabilization. Apo-Tf binds TfR2 only very weakly at serum pH.","method":"Binding studies with full-length receptors, TfR2-TfR1 chimeras containing the TfR1 helical domain, mutagenesis of apical arm junction residues; kinetic binding analysis","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro binding assay with chimera and mutagenesis, mechanistic domain mapping","pmids":["29388418"],"is_preprint":false},{"year":2003,"finding":"HFE and TfR2 co-localize in duodenal crypt cells (but not mature enterocytes) and in a CD63-negative early endosomal compartment in Caco-2 cells; HFE deficiency disrupts TfR2 cellular distribution; signal enhancement occurs on exposure to iron-saturated transferrin, indicating HFE preferentially interacts with TfR2 in a specialized early endosomal iron-transferrin transport pathway in intestinal crypts.","method":"Confocal microscopy with specific peptide antisera in human/mouse duodenum and Caco-2 cells; comparison with HFE-deficient tissue; iron-saturated transferrin stimulation","journal":"The journal of histochemistry and cytochemistry","confidence":"Medium","confidence_rationale":"Tier 3 — co-localization with functional context, single lab, no direct biochemical interaction assay","pmids":["12704209"],"is_preprint":false},{"year":2023,"finding":"DENND3 p.L708V activating variant causes TFR2 degradation in lysosomes via upregulation of RAB12 (a GTPase), leading to decreased TFR2 and reduced pSMAD1/5 signaling and hepcidin expression, causing hereditary hemochromatosis.","method":"Cell transfection with DENND3 p.L708V vector; measurement of RAB12 expression, TfR2 lysosomal degradation, pSMAD1/5, hepcidin; adeno-associated virus mouse model","journal":"Hepatology international","confidence":"Medium","confidence_rationale":"Tier 2 — mechanistic in vitro and in vivo data, single lab, pathway placement established","pmids":["36729283"],"is_preprint":false},{"year":2024,"finding":"EZH2 suppresses TFR2 expression via H3K27me3 epigenetic modification at the TFR2 promoter, reducing RNA polymerase II binding, which in turn suppresses ferroptosis in hepatocellular carcinoma.","method":"EZH2 overexpression/knockdown, H3K27me3 ChIP-like analysis, RNA polymerase II binding at TFR2 promoter, ferroptosis assays in HCC cells and sorafenib-resistant cells","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 2 — epigenetic mechanism with chromatin-level evidence and functional ferroptosis readout, single lab","pmids":["38623968"],"is_preprint":false},{"year":2023,"finding":"PFOS exposure causes ATP5B to interact with TFR2 and both proteins to translocate from the plasma membrane to mitochondria, leading to mitochondrial iron overload and hepatic insulin resistance; inhibiting TfR2 translocation or stabilizing ATP5B on the plasma membrane prevents this.","method":"Co-immunoprecipitation of ATP5B and TfR2, subcellular fractionation, TfR2 knockdown/overexpression, pharmacological inhibition of mitochondrial iron, PFOS-treated L-O2 cells and mouse liver","journal":"Ecotoxicology and environmental safety","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP with functional intervention, single lab","pmids":["36801541"],"is_preprint":false},{"year":2026,"finding":"Tfr2 is required for acute hepcidin induction in response to dietary iron challenge: in Tfr1-deficient hepatocytes, 'liberated' HFE requires Tfr2 to become functionally active for Smad1/5/9 phosphorylation and hepcidin induction. Under chronic iron loading, Tfr2 and Hfe have non-redundant functions; Tfr2 is dispensable for hepatocellular transferrin-bound iron uptake.","method":"Hepatocyte-specific double Tfr1/Tfr2 knockout mice; fluorescent holo-transferrin uptake assays in primary hepatocytes; dietary iron restriction/challenge experiments; Hamp mRNA and Smad1/5/9 phosphorylation measurement","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic models with epistasis analysis and orthogonal molecular readouts","pmids":["41662592"],"is_preprint":false},{"year":2014,"finding":"In erythroid cells, TFR2 is a binding partner of the erythropoietin receptor (EPOR) and stabilizes EPOR on the cell surface, modulating EPO signaling and red blood cell production in response to iron availability.","method":"Review/synthesis citing erythroid TFR2-EPOR interaction studies; Tfr2-Tmprss6 double knockout phenotype analysis","journal":"Frontiers in pharmacology","confidence":"Medium","confidence_rationale":"Tier 3 — cited interaction described in review context; underlying original data not in this abstract","pmids":["24847265"],"is_preprint":false}],"current_model":"TFR2 is a hepatic iron sensor that, as part of a membrane complex with HFE and HJV, detects circulating diferric transferrin levels and activates hepcidin transcription via the BMP-SMAD signaling pathway (requiring Tfr2-mediated Bmp6 upregulation and cooperative Smad1/5/9 phosphorylation); in erythroid cells TFR2 also partners with the erythropoietin receptor to modulate erythropoiesis in accordance with iron availability, while a beta isoform expressed in the spleen controls ferroportin-mediated iron egress."},"narrative":{"teleology":[{"year":2000,"claim":"Identifying TFR2 as a hemochromatosis gene established that it is essential for human iron homeostasis, opening the question of where it acts in the regulatory hierarchy.","evidence":"Homozygous nonsense mutation identified by genetic mapping in affected families","pmids":["10802645"],"confidence":"High","gaps":["Downstream mechanism by which TFR2 loss causes iron overload was unknown","Relationship to hepcidin not yet established"]},{"year":2004,"claim":"Positioning TFR2 upstream of hepcidin resolved its place in the iron regulatory hierarchy and showed that inflammatory hepcidin induction operates independently of TFR2.","evidence":"Hepcidin mRNA measurement in Tfr2 mutant vs wild-type mice and isolated hepatocytes stimulated with IL-6/LPS; urinary hepcidin in TFR2-mutant patients","pmids":["15345587","15486069"],"confidence":"High","gaps":["Molecular mechanism linking TFR2 to hepcidin transcription was unknown","Whether TFR2 acts via BMP-SMAD signaling was unresolved"]},{"year":2006,"claim":"Demonstrating that BMP-stimulated hepcidin induction occurs independently of TFR2 positioned TFR2 outside the direct BMP receptor axis, while discovery of TFR2 in lipid rafts with ERK/p38 MAPK signaling identified a candidate downstream pathway.","evidence":"BMP stimulation of wild-type vs Tfr2-mutant hepatocytes; lipid raft fractionation, co-IP with caveolin-1/CD81, MAPK phosphorylation assays in HepG2/K562 cells","pmids":["16801541","17046995"],"confidence":"High","gaps":["Whether MAPK signaling downstream of TFR2 is required for hepcidin induction in vivo was untested","Nature of TFR2's relationship with HFE at the molecular level remained unclear"]},{"year":2010,"claim":"Hepatocyte-specific rescue and clinical iron-challenge experiments established that TFR2 and HFE are both required in liver for hepcidin regulation but are non-redundant, and that distinct isoforms (alpha in liver, beta in spleen) serve tissue-specific iron-regulatory functions.","evidence":"AAV-mediated hepatocyte-specific gene delivery in KO mice; oral iron challenge with serum hepcidin measurement in patients; comparison of Tfr2 KO vs beta-knockin mice with conditional liver KO","pmids":["20177050","21173098","20179178"],"confidence":"High","gaps":["Whether HFE and TFR2 physically interact as a complex was unresolved","Mechanism by which beta-TFR2 controls splenic ferroportin was unclear"]},{"year":2012,"claim":"Identification of an HFE–TFR2–HJV ternary membrane complex and mapping of the critical TFR2 interaction domain (residues 120–139) provided a structural basis for the cooperative hepcidin regulation; epistasis with Tmprss6 revealed TFR2 additionally promotes erythropoiesis independently of hepcidin.","evidence":"Glycerol gradient sedimentation and reciprocal co-IP with TFR2 mutant constructs in hepatoma cells; Hfe/Tmprss6 and Tfr2/Tmprss6 double KO mice","pmids":["22728873","22244935"],"confidence":"High","gaps":["Whether HFE directly contacts TFR2 was challenged by subsequent PLA data","Structural resolution of the ternary complex was lacking"]},{"year":2013,"claim":"Demonstrating that TFR2 is required for parenchymal iron–induced Bmp6 upregulation placed TFR2 upstream of BMP6 ligand production, revising the initial view that TFR2 acts wholly outside the BMP pathway; meanwhile, proximity ligation data questioned direct HFE–TFR2 contact.","evidence":"Dietary vs parenteral iron loading in WT, Hfe−/−, Tfr2−/−, and double KO mice; proximity ligation assay in stably co-expressing cells","pmids":["24284962","24155934"],"confidence":"High","gaps":["Whether TFR2 and HFE physically interact or signal through parallel arms remains debated","Mechanism by which TFR2 senses parenchymal iron to induce Bmp6 is unknown"]},{"year":2015,"claim":"Identification of CD81 as a TFR2 partner that controls TFR2 turnover and sustains hepcidin expression independently of BMP and ERK pathways revealed an additional layer of TFR2-dependent hepcidin regulation.","evidence":"Yeast two-hybrid, co-precipitation domain mapping, CD81/GRAIL siRNA knockdown with hepcidin and signaling readouts in Hep3B-TfR2 cells","pmids":["25635054"],"confidence":"High","gaps":["In vivo relevance of CD81-dependent TFR2 regulation not demonstrated","Whether GRAIL-mediated CD81 degradation is iron-responsive is unknown"]},{"year":2018,"claim":"Structural dissection of holo-transferrin binding showed that TFR2 uses a mechanistically distinct recognition mode from TFR1, with critical apical arm junction residues, explaining TFR2's selective role as a transferrin saturation sensor rather than an iron uptake receptor.","evidence":"Binding kinetics with full-length receptors, TFR2–TFR1 chimeras, and mutagenesis of apical arm junction residues","pmids":["29388418"],"confidence":"High","gaps":["Atomic-resolution structure of TFR2–holo-Tf complex not available","How conformational change upon Tf binding propagates intracellularly is unknown"]},{"year":2023,"claim":"Discovery that DENND3-activated RAB12 targets TFR2 for lysosomal degradation, reducing pSMAD1/5 and hepcidin, identified a new degradation pathway controlling TFR2 protein levels and causing hemochromatosis.","evidence":"DENND3 variant transfection with RAB12, TFR2 lysosomal degradation, pSMAD1/5, and hepcidin readouts; AAV mouse model","pmids":["36729283"],"confidence":"Medium","gaps":["Whether RAB12-mediated TFR2 degradation is a physiological iron-responsive mechanism is unknown","Single-lab finding, not independently replicated"]},{"year":2026,"claim":"Epistasis analysis in hepatocyte-specific Tfr1/Tfr2 double knockouts demonstrated that liberated HFE requires TFR2 to activate Smad1/5/9 phosphorylation and acute hepcidin induction, while TFR2 is dispensable for transferrin-bound iron uptake, cementing TFR2's role as a signaling sensor rather than an iron transporter.","evidence":"Hepatocyte-specific double Tfr1/Tfr2 KO mice; fluorescent holo-Tf uptake assays; dietary iron challenge with Hamp mRNA and pSmad1/5/9 measurement","pmids":["41662592"],"confidence":"High","gaps":["Molecular mechanism by which TFR2 activates Smad1/5/9 phosphorylation upon Tf binding remains unresolved","Whether TFR2 signaling requires kinase activity or acts as a scaffold is unknown"]},{"year":null,"claim":"The signal transduction mechanism linking holotransferrin-bound TFR2 to Smad1/5/9 phosphorylation and Bmp6 upregulation—including the identity of intermediary kinases or scaffolds, the atomic structure of the TFR2–HFE–HJV complex, and the mechanism of beta-TFR2 control of splenic ferroportin—remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structural model of the TFR2–HFE–HJV ternary complex exists","Signaling intermediates between TFR2 and Smad phosphorylation are unidentified","Mechanism of beta-TFR2 regulation of ferroportin in spleen is unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[1,2,8,15,20]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[7,12,13]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[9,21]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[4,9,15,16]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[16]},{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[5]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1,4,9,12,13,17,20]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[0,2,6,7,8]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,8,17]}],"complexes":["HFE-TFR2-HJV complex","TFR2-CD81 complex","TFR2-EPOR complex"],"partners":["HFE","HJV","CD81","EPOR","TFR1","ATP5B","RGMA"],"other_free_text":[]},"mechanistic_narrative":"TFR2 is a hepatocyte membrane iron sensor that transduces circulating diferric transferrin signals into transcriptional activation of hepcidin, the master regulator of systemic iron homeostasis. The alpha isoform forms a multi-protein complex with HFE and HJV on hepatocyte membranes—via extracellular residues 120–139—and is required for both Bmp6 upregulation in response to parenchymal iron and downstream Smad1/5/9 phosphorylation leading to hepcidin induction, while HFE and TFR2 exert non-redundant functions in this pathway [PMID:15345587, PMID:20177050, PMID:22728873, PMID:24284962, PMID:41662592]. A distinct beta isoform expressed in the spleen controls ferroportin-mediated iron egress, and in erythroid cells TFR2 partners with the erythropoietin receptor to couple iron availability with erythropoiesis [PMID:20179178, PMID:24847265]. Loss-of-function mutations in TFR2 cause type 3 hereditary hemochromatosis, characterized by absent hepcidin response to iron and progressive iron overload [PMID:10802645, PMID:21173098]."},"prefetch_data":{"uniprot":{"accession":"Q9UP52","full_name":"Transferrin receptor protein 2","aliases":[],"length_aa":801,"mass_kda":88.8,"function":"Mediates cellular uptake of transferrin-bound iron in a non-iron dependent manner. May be involved in iron metabolism, hepatocyte function and erythrocyte differentiation","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q9UP52/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TFR2","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TFR2","total_profiled":1310},"omim":[{"mim_id":"615517","title":"HEMOCHROMATOSIS, TYPE 5; HFE5","url":"https://www.omim.org/entry/615517"},{"mim_id":"613609","title":"HOMEOSTATIC IRON REGULATOR; HFE","url":"https://www.omim.org/entry/613609"},{"mim_id":"608374","title":"HEMOJUVELIN BMP CORECEPTOR; HJV","url":"https://www.omim.org/entry/608374"},{"mim_id":"607733","title":"SCRIBBLE PLANAR CELL POLARITY PROTEIN; SCRIB","url":"https://www.omim.org/entry/607733"},{"mim_id":"606464","title":"HEPCIDIN ANTIMICROBIAL PEPTIDE; HAMP","url":"https://www.omim.org/entry/606464"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Golgi apparatus","reliability":"Approved"}],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"liver","ntpm":313.5}],"url":"https://www.proteinatlas.org/search/TFR2"},"hgnc":{"alias_symbol":["HFE3","TFRC2"],"prev_symbol":[]},"alphafold":{"accession":"Q9UP52","domains":[{"cath_id":"3.40.630.10","chopping":"138-202_413-636","consensus_level":"high","plddt":95.2645,"start":138,"end":636},{"cath_id":"3.50.30.30","chopping":"209-402","consensus_level":"high","plddt":91.1391,"start":209,"end":402},{"cath_id":"1.20.930.40","chopping":"645-754_767-797","consensus_level":"high","plddt":90.8135,"start":645,"end":797}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UP52","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UP52-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UP52-F1-predicted_aligned_error_v6.png","plddt_mean":83.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TFR2","jax_strain_url":"https://www.jax.org/strain/search?query=TFR2"},"sequence":{"accession":"Q9UP52","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9UP52.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9UP52/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UP52"}},"corpus_meta":[{"pmid":"10802645","id":"PMC_10802645","title":"The gene TFR2 is mutated in a new type of haemochromatosis mapping to 7q22.","date":"2000","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/10802645","citation_count":626,"is_preprint":false},{"pmid":"15486069","id":"PMC_15486069","title":"Hepcidin is decreased in TFR2 hemochromatosis.","date":"2004","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/15486069","citation_count":305,"is_preprint":false},{"pmid":"16801541","id":"PMC_16801541","title":"Bone morphogenetic proteins 2, 4, and 9 stimulate murine hepcidin 1 expression independently of Hfe, transferrin receptor 2 (Tfr2), and IL-6.","date":"2006","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/16801541","citation_count":259,"is_preprint":false},{"pmid":"15345587","id":"PMC_15345587","title":"Expression of hepcidin is down-regulated in TfR2 mutant mice manifesting a phenotype of hereditary hemochromatosis.","date":"2004","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/15345587","citation_count":175,"is_preprint":false},{"pmid":"19250966","id":"PMC_19250966","title":"A novel transferrin/TfR2-mediated mitochondrial iron transport system is disrupted in Parkinson's disease.","date":"2009","source":"Neurobiology of disease","url":"https://pubmed.ncbi.nlm.nih.gov/19250966","citation_count":154,"is_preprint":false},{"pmid":"17046995","id":"PMC_17046995","title":"TfR2 localizes in lipid raft domains and is released in exosomes to activate signal transduction along the MAPK pathway.","date":"2006","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/17046995","citation_count":146,"is_preprint":false},{"pmid":"22728873","id":"PMC_22728873","title":"The hemochromatosis proteins HFE, TfR2, and HJV form a membrane-associated protein complex for hepcidin regulation.","date":"2012","source":"Journal of 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multiple populations\",\n      \"pmids\": [\"10802645\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"TFR2 is upstream of hepcidin in the iron regulatory pathway; TfR2 mutant mice show reduced hepcidin mRNA expression and elevated duodenal DMT1, and inflammatory stimuli (IL-6, LPS) can induce hepcidin independently of TfR2.\",\n      \"method\": \"Northern blot analysis of hepcidin and DMT1 mRNA in TfR2(Y245X) mutant vs wild-type mice; isolated hepatocyte stimulation with IL-6 and LPS\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO model with specific molecular readouts, replicated by independent groups\",\n      \"pmids\": [\"15345587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"TFR2 mutations in patients result in low or absent urinary hepcidin, indicating TFR2 is a modulator of hepcidin production in response to iron.\",\n      \"method\": \"Urinary hepcidin measurement in patients homozygous for TFR2 mutations\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct patient-based functional readout, consistent with mouse model data\",\n      \"pmids\": [\"15486069\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"BMP2, BMP4, and BMP9 stimulate hepcidin expression independently of Tfr2 (and Hfe, IL-6), as shown by equivalent BMP responses in isolated hepatocytes from wild-type and Tfr2 mutant mice, placing Tfr2 outside the BMP-direct signaling axis.\",\n      \"method\": \"Primary hepatocyte stimulation assays with BMPs in wild-type vs Tfr2 mutant mice; comparison of hepcidin transcription responses\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis established by genetic model + in vitro stimulation assay\",\n      \"pmids\": [\"16801541\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"TfR2 localizes in lipid raft (low-density Triton-insoluble) plasma membrane domains, co-immunoprecipitates with caveolin-1 and CD81, and activation by holotransferrin or anti-TfR2 antibody triggers ERK1/ERK2 and p38 MAPK signaling in a lipid raft-dependent manner. TfR2 is also exported via exosomes.\",\n      \"method\": \"Lipid raft biochemical fractionation, co-immunoprecipitation, subcellular fractionation, MAPK phosphorylation assays, raft disruption, exosome isolation from HepG2 and K562 cells\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (fractionation, Co-IP, signaling assay, raft disruption) in single study\",\n      \"pmids\": [\"17046995\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"TfR2 contains a mitochondrial targeting sequence and localizes to mitochondria of substantia nigra dopamine neurons, where it participates in a transferrin/TfR2-mediated mitochondrial iron transport pathway that can deliver iron to respiratory complex I; this pathway is redox-sensitive and is disrupted in Parkinson's disease.\",\n      \"method\": \"Subcellular fractionation, mitochondrial targeting sequence identification, iron delivery assays to mitochondria and complex I, rotenone PD model, human SN immunolocalization\",\n      \"journal\": \"Neurobiology of disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple methods but single lab, novel and not independently replicated\",\n      \"pmids\": [\"19250966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Hepatocyte-specific expression of Tfr2 in Tfr2-deficient mice increases hepcidin mRNA and lowers hepatic iron and transferrin saturation, confirming the liver as the site of TfR2 action on hepcidin. HFE and TFR2 have non-redundant roles: expressing HFE in Tfr2-deficient mice or Tfr2 in HFE-null mice has no effect, suggesting both are required together (likely as a complex).\",\n      \"method\": \"AAV2/8-mediated hepatocyte-specific gene delivery of Hfe or Tfr2 in knockout mice; measurement of hepcidin mRNA, liver iron, transferrin saturation\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic rescue experiment with specific molecular and biochemical readouts\",\n      \"pmids\": [\"20177050\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The two TFR2 isoforms (alpha and beta) have distinct functions: hepatic alpha-Tfr2 senses diferric transferrin and activates hepcidin, while beta-Tfr2 (expressed in spleen) specifically controls splenic iron efflux by regulating ferroportin-1 expression.\",\n      \"method\": \"Comparison of Tfr2 knockout vs. beta-isoform knockin mice; measurement of iron parameters, hepcidin, BMP6, ferroportin-1 in liver and spleen; liver-specific Tfr2 conditional knockout in knockin mice\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple mouse models with tissue-specific conditional knockouts, multiple molecular readouts\",\n      \"pmids\": [\"20179178\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"TFR2-hemochromatosis patients show absent hepcidin response to oral iron challenge, whereas HFE-hemochromatosis patients show a blunted but detectable response, establishing TFR2 as playing a prominent role (and HFE a contributory role) in acute hepcidin induction by iron.\",\n      \"method\": \"Oral iron challenge test with time-course measurement of serum iron, transferrin saturation, and serum hepcidin by ELISA and mass spectrometry\",\n      \"journal\": \"Haematologica\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — controlled clinical challenge experiment with quantitative hormone measurement at multiple time points\",\n      \"pmids\": [\"21173098\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"HFE, TfR2, and HJV form a multi-protein membrane complex on hepatocytes. TfR2 residues 120-139 of the extracellular domain are critical for binding both HFE and HJV. HJV competes with TfR1 for HFE binding (as does TfR2). RGMA (a CNS homologue of HJV) can substitute for HJV in the complex.\",\n      \"method\": \"Glycerol gradient sedimentation assays, co-immunoprecipitation in transfected HuH7 hepatoma cells, domain mapping with TfR2 mutant constructs\",\n      \"journal\": \"Journal of hepatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP with domain mapping, multiple orthogonal methods\",\n      \"pmids\": [\"22728873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Hfe and Tfr2 are not substrates for Tmprss6 (matriptase-2); double mutant mice lacking both Hfe or Tfr2 and Tmprss6 show the Tmprss6-null phenotype for hepcidin but with greater erythropoiesis, indicating Hfe and Tfr2 allow increased erythropoiesis independently of liver hepcidin regulation.\",\n      \"method\": \"Generation of Hfe/Tmprss6 and Tfr2/Tmprss6 double knockout mice; measurement of hepcidin, iron parameters, and erythropoiesis\",\n      \"journal\": \"Blood cells, molecules & diseases\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with double knockout models and specific molecular readouts\",\n      \"pmids\": [\"22244935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In stably co-expressing cells, HFE and TFR2 do not directly interact (no signal detected by proximity ligation assay), whereas HFE-TFR1 and TFR1-TFR2 heterodimer interactions are detected, suggesting HFE and TFR2 can regulate hepcidin independently.\",\n      \"method\": \"Proximity ligation assay in stably co-expressing cell lines; comparison with known HFE-TfR1 interaction as positive control\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — single lab with rigorous proximity ligation assay, contradicts some prior Co-IP data\",\n      \"pmids\": [\"24155934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Tfr2 is required for upregulation of Bmp6 in response to hepatocyte iron (parenchymal iron), but not non-parenchymal iron; Hfe is not required for Bmp6 upregulation but is required for efficient downstream transmission of the regulatory signal.\",\n      \"method\": \"Dietary vs parenteral iron loading in wild-type, Hfe-/-, Tfr2-/-, and Hfe-/-/Tfr2-/- mice; measurement of Bmp6, hepcidin, and iron parameters\",\n      \"journal\": \"American journal of physiology. Gastrointestinal and liver physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic double KO with differential iron-loading protocols and multiple molecular readouts\",\n      \"pmids\": [\"24284962\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"CD81 interacts with TfR2 via both the cytoplasmic and ecto-transmembrane domains, promotes TfR2 degradation (increasing TfR2 half-life when knocked down), and the TfR2/CD81 complex is required for maintenance of hepcidin mRNA expression independently of BMP and ERK1/2 pathways. CD81 is itself targeted for degradation by the ubiquitin E3 ligase GRAIL.\",\n      \"method\": \"Yeast two-hybrid screen, co-precipitation with TfR2 domain constructs, siRNA knockdown of CD81 and GRAIL, hepcidin expression assay, BMP6 stimulation, ID1 and SMAD7 expression, ERK1/2 phosphorylation in Hep3B-TfR2 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — yeast two-hybrid plus multiple orthogonal biochemical assays and functional readouts in single study\",\n      \"pmids\": [\"25635054\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In hereditary tyrosinemia type 1 mice, transcription factor Sp1 regulates Tfr2 expression; downregulation of Sp1 reduces Tfr2, which decreases hepcidin, leading to iron overload. Forced expression of Tfr2 in the liver reduces iron accumulation.\",\n      \"method\": \"qRT-PCR, immunoblotting, adenovirus-mediated Tfr2 overexpression in Fah-/- mice; transferrin-sensitive hepcidin induction assay in hepatocytes\",\n      \"journal\": \"Journal of hepatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic pathway placement with rescue experiment, single lab\",\n      \"pmids\": [\"27013087\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"TfR2 binds holotransferrin (holo-Tf) through a mechanism different from TfR1: the helical domain differences account for differences in Tf on-rate, and conserved apical arm junction residues are critical for TfR2-Tf (but not TfR1-Tf) interaction stabilization. Apo-Tf binds TfR2 only very weakly at serum pH.\",\n      \"method\": \"Binding studies with full-length receptors, TfR2-TfR1 chimeras containing the TfR1 helical domain, mutagenesis of apical arm junction residues; kinetic binding analysis\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro binding assay with chimera and mutagenesis, mechanistic domain mapping\",\n      \"pmids\": [\"29388418\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"HFE and TfR2 co-localize in duodenal crypt cells (but not mature enterocytes) and in a CD63-negative early endosomal compartment in Caco-2 cells; HFE deficiency disrupts TfR2 cellular distribution; signal enhancement occurs on exposure to iron-saturated transferrin, indicating HFE preferentially interacts with TfR2 in a specialized early endosomal iron-transferrin transport pathway in intestinal crypts.\",\n      \"method\": \"Confocal microscopy with specific peptide antisera in human/mouse duodenum and Caco-2 cells; comparison with HFE-deficient tissue; iron-saturated transferrin stimulation\",\n      \"journal\": \"The journal of histochemistry and cytochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — co-localization with functional context, single lab, no direct biochemical interaction assay\",\n      \"pmids\": [\"12704209\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"DENND3 p.L708V activating variant causes TFR2 degradation in lysosomes via upregulation of RAB12 (a GTPase), leading to decreased TFR2 and reduced pSMAD1/5 signaling and hepcidin expression, causing hereditary hemochromatosis.\",\n      \"method\": \"Cell transfection with DENND3 p.L708V vector; measurement of RAB12 expression, TfR2 lysosomal degradation, pSMAD1/5, hepcidin; adeno-associated virus mouse model\",\n      \"journal\": \"Hepatology international\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic in vitro and in vivo data, single lab, pathway placement established\",\n      \"pmids\": [\"36729283\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"EZH2 suppresses TFR2 expression via H3K27me3 epigenetic modification at the TFR2 promoter, reducing RNA polymerase II binding, which in turn suppresses ferroptosis in hepatocellular carcinoma.\",\n      \"method\": \"EZH2 overexpression/knockdown, H3K27me3 ChIP-like analysis, RNA polymerase II binding at TFR2 promoter, ferroptosis assays in HCC cells and sorafenib-resistant cells\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epigenetic mechanism with chromatin-level evidence and functional ferroptosis readout, single lab\",\n      \"pmids\": [\"38623968\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"PFOS exposure causes ATP5B to interact with TFR2 and both proteins to translocate from the plasma membrane to mitochondria, leading to mitochondrial iron overload and hepatic insulin resistance; inhibiting TfR2 translocation or stabilizing ATP5B on the plasma membrane prevents this.\",\n      \"method\": \"Co-immunoprecipitation of ATP5B and TfR2, subcellular fractionation, TfR2 knockdown/overexpression, pharmacological inhibition of mitochondrial iron, PFOS-treated L-O2 cells and mouse liver\",\n      \"journal\": \"Ecotoxicology and environmental safety\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP with functional intervention, single lab\",\n      \"pmids\": [\"36801541\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Tfr2 is required for acute hepcidin induction in response to dietary iron challenge: in Tfr1-deficient hepatocytes, 'liberated' HFE requires Tfr2 to become functionally active for Smad1/5/9 phosphorylation and hepcidin induction. Under chronic iron loading, Tfr2 and Hfe have non-redundant functions; Tfr2 is dispensable for hepatocellular transferrin-bound iron uptake.\",\n      \"method\": \"Hepatocyte-specific double Tfr1/Tfr2 knockout mice; fluorescent holo-transferrin uptake assays in primary hepatocytes; dietary iron restriction/challenge experiments; Hamp mRNA and Smad1/5/9 phosphorylation measurement\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic models with epistasis analysis and orthogonal molecular readouts\",\n      \"pmids\": [\"41662592\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In erythroid cells, TFR2 is a binding partner of the erythropoietin receptor (EPOR) and stabilizes EPOR on the cell surface, modulating EPO signaling and red blood cell production in response to iron availability.\",\n      \"method\": \"Review/synthesis citing erythroid TFR2-EPOR interaction studies; Tfr2-Tmprss6 double knockout phenotype analysis\",\n      \"journal\": \"Frontiers in pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — cited interaction described in review context; underlying original data not in this abstract\",\n      \"pmids\": [\"24847265\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"TFR2 is a hepatic iron sensor that, as part of a membrane complex with HFE and HJV, detects circulating diferric transferrin levels and activates hepcidin transcription via the BMP-SMAD signaling pathway (requiring Tfr2-mediated Bmp6 upregulation and cooperative Smad1/5/9 phosphorylation); in erythroid cells TFR2 also partners with the erythropoietin receptor to modulate erythropoiesis in accordance with iron availability, while a beta isoform expressed in the spleen controls ferroportin-mediated iron egress.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"TFR2 is a hepatocyte membrane iron sensor that transduces circulating diferric transferrin signals into transcriptional activation of hepcidin, the master regulator of systemic iron homeostasis. The alpha isoform forms a multi-protein complex with HFE and HJV on hepatocyte membranes—via extracellular residues 120–139—and is required for both Bmp6 upregulation in response to parenchymal iron and downstream Smad1/5/9 phosphorylation leading to hepcidin induction, while HFE and TFR2 exert non-redundant functions in this pathway [PMID:15345587, PMID:20177050, PMID:22728873, PMID:24284962, PMID:41662592]. A distinct beta isoform expressed in the spleen controls ferroportin-mediated iron egress, and in erythroid cells TFR2 partners with the erythropoietin receptor to couple iron availability with erythropoiesis [PMID:20179178, PMID:24847265]. Loss-of-function mutations in TFR2 cause type 3 hereditary hemochromatosis, characterized by absent hepcidin response to iron and progressive iron overload [PMID:10802645, PMID:21173098].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Identifying TFR2 as a hemochromatosis gene established that it is essential for human iron homeostasis, opening the question of where it acts in the regulatory hierarchy.\",\n      \"evidence\": \"Homozygous nonsense mutation identified by genetic mapping in affected families\",\n      \"pmids\": [\"10802645\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream mechanism by which TFR2 loss causes iron overload was unknown\", \"Relationship to hepcidin not yet established\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Positioning TFR2 upstream of hepcidin resolved its place in the iron regulatory hierarchy and showed that inflammatory hepcidin induction operates independently of TFR2.\",\n      \"evidence\": \"Hepcidin mRNA measurement in Tfr2 mutant vs wild-type mice and isolated hepatocytes stimulated with IL-6/LPS; urinary hepcidin in TFR2-mutant patients\",\n      \"pmids\": [\"15345587\", \"15486069\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism linking TFR2 to hepcidin transcription was unknown\", \"Whether TFR2 acts via BMP-SMAD signaling was unresolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrating that BMP-stimulated hepcidin induction occurs independently of TFR2 positioned TFR2 outside the direct BMP receptor axis, while discovery of TFR2 in lipid rafts with ERK/p38 MAPK signaling identified a candidate downstream pathway.\",\n      \"evidence\": \"BMP stimulation of wild-type vs Tfr2-mutant hepatocytes; lipid raft fractionation, co-IP with caveolin-1/CD81, MAPK phosphorylation assays in HepG2/K562 cells\",\n      \"pmids\": [\"16801541\", \"17046995\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether MAPK signaling downstream of TFR2 is required for hepcidin induction in vivo was untested\", \"Nature of TFR2's relationship with HFE at the molecular level remained unclear\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Hepatocyte-specific rescue and clinical iron-challenge experiments established that TFR2 and HFE are both required in liver for hepcidin regulation but are non-redundant, and that distinct isoforms (alpha in liver, beta in spleen) serve tissue-specific iron-regulatory functions.\",\n      \"evidence\": \"AAV-mediated hepatocyte-specific gene delivery in KO mice; oral iron challenge with serum hepcidin measurement in patients; comparison of Tfr2 KO vs beta-knockin mice with conditional liver KO\",\n      \"pmids\": [\"20177050\", \"21173098\", \"20179178\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether HFE and TFR2 physically interact as a complex was unresolved\", \"Mechanism by which beta-TFR2 controls splenic ferroportin was unclear\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Identification of an HFE–TFR2–HJV ternary membrane complex and mapping of the critical TFR2 interaction domain (residues 120–139) provided a structural basis for the cooperative hepcidin regulation; epistasis with Tmprss6 revealed TFR2 additionally promotes erythropoiesis independently of hepcidin.\",\n      \"evidence\": \"Glycerol gradient sedimentation and reciprocal co-IP with TFR2 mutant constructs in hepatoma cells; Hfe/Tmprss6 and Tfr2/Tmprss6 double KO mice\",\n      \"pmids\": [\"22728873\", \"22244935\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether HFE directly contacts TFR2 was challenged by subsequent PLA data\", \"Structural resolution of the ternary complex was lacking\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Demonstrating that TFR2 is required for parenchymal iron–induced Bmp6 upregulation placed TFR2 upstream of BMP6 ligand production, revising the initial view that TFR2 acts wholly outside the BMP pathway; meanwhile, proximity ligation data questioned direct HFE–TFR2 contact.\",\n      \"evidence\": \"Dietary vs parenteral iron loading in WT, Hfe−/−, Tfr2−/−, and double KO mice; proximity ligation assay in stably co-expressing cells\",\n      \"pmids\": [\"24284962\", \"24155934\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether TFR2 and HFE physically interact or signal through parallel arms remains debated\", \"Mechanism by which TFR2 senses parenchymal iron to induce Bmp6 is unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Identification of CD81 as a TFR2 partner that controls TFR2 turnover and sustains hepcidin expression independently of BMP and ERK pathways revealed an additional layer of TFR2-dependent hepcidin regulation.\",\n      \"evidence\": \"Yeast two-hybrid, co-precipitation domain mapping, CD81/GRAIL siRNA knockdown with hepcidin and signaling readouts in Hep3B-TfR2 cells\",\n      \"pmids\": [\"25635054\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of CD81-dependent TFR2 regulation not demonstrated\", \"Whether GRAIL-mediated CD81 degradation is iron-responsive is unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Structural dissection of holo-transferrin binding showed that TFR2 uses a mechanistically distinct recognition mode from TFR1, with critical apical arm junction residues, explaining TFR2's selective role as a transferrin saturation sensor rather than an iron uptake receptor.\",\n      \"evidence\": \"Binding kinetics with full-length receptors, TFR2–TFR1 chimeras, and mutagenesis of apical arm junction residues\",\n      \"pmids\": [\"29388418\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of TFR2–holo-Tf complex not available\", \"How conformational change upon Tf binding propagates intracellularly is unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Discovery that DENND3-activated RAB12 targets TFR2 for lysosomal degradation, reducing pSMAD1/5 and hepcidin, identified a new degradation pathway controlling TFR2 protein levels and causing hemochromatosis.\",\n      \"evidence\": \"DENND3 variant transfection with RAB12, TFR2 lysosomal degradation, pSMAD1/5, and hepcidin readouts; AAV mouse model\",\n      \"pmids\": [\"36729283\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether RAB12-mediated TFR2 degradation is a physiological iron-responsive mechanism is unknown\", \"Single-lab finding, not independently replicated\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Epistasis analysis in hepatocyte-specific Tfr1/Tfr2 double knockouts demonstrated that liberated HFE requires TFR2 to activate Smad1/5/9 phosphorylation and acute hepcidin induction, while TFR2 is dispensable for transferrin-bound iron uptake, cementing TFR2's role as a signaling sensor rather than an iron transporter.\",\n      \"evidence\": \"Hepatocyte-specific double Tfr1/Tfr2 KO mice; fluorescent holo-Tf uptake assays; dietary iron challenge with Hamp mRNA and pSmad1/5/9 measurement\",\n      \"pmids\": [\"41662592\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which TFR2 activates Smad1/5/9 phosphorylation upon Tf binding remains unresolved\", \"Whether TFR2 signaling requires kinase activity or acts as a scaffold is unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The signal transduction mechanism linking holotransferrin-bound TFR2 to Smad1/5/9 phosphorylation and Bmp6 upregulation—including the identity of intermediary kinases or scaffolds, the atomic structure of the TFR2–HFE–HJV complex, and the mechanism of beta-TFR2 control of splenic ferroportin—remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structural model of the TFR2–HFE–HJV ternary complex exists\", \"Signaling intermediates between TFR2 and Smad phosphorylation are unidentified\", \"Mechanism of beta-TFR2 regulation of ferroportin in spleen is unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [1, 2, 8, 15, 20]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [7, 12, 13]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [9, 21]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [4, 9, 15, 16]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [16]},\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 4, 9, 12, 13, 17, 20]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [0, 2, 6, 7, 8]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 8, 17]}\n    ],\n    \"complexes\": [\n      \"HFE-TFR2-HJV complex\",\n      \"TFR2-CD81 complex\",\n      \"TFR2-EPOR complex\"\n    ],\n    \"partners\": [\n      \"HFE\",\n      \"HJV\",\n      \"CD81\",\n      \"EPOR\",\n      \"TFR1\",\n      \"ATP5B\",\n      \"RGMA\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}