{"gene":"XPNPEP2","run_date":"2026-06-11T09:02:06","timeline":{"discoveries":[{"year":2000,"finding":"XPNPEP2 encodes membrane-bound aminopeptidase P (hmAmP), an ectoenzyme largely limited in distribution to endothelia and brush border epithelia, which hydrolyzes N-terminal Xaa-Pro bonds and can inactivate bradykinin. The gene is located at chromosome Xq25. Sequence analysis identified conserved 'pita bread-fold' homologous blocks containing a proton shuttle and five divalent metal ligands (counterparts of E. coli methionine aminopeptidase) shared with soluble aminopeptidase P (XPNPEP1), establishing the catalytic mechanism.","method":"Chromosomal mapping (somatic cell hybrids), sequence homology analysis, comparison to known E. coli aminopeptidase structure","journal":"Archives of biochemistry and biophysics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — chromosomal localization with somatic cell hybrids combined with sequence-based identification of catalytic residues; single lab but two orthogonal approaches","pmids":["10871044"],"is_preprint":false},{"year":2000,"finding":"A translocation breakpoint at Xq25 associated with premature ovarian failure physically disrupts XPNPEP2. XPNPEP2 mRNA was detected in fibroblasts carrying the translocation, suggesting the gene at least partially escapes X inactivation.","method":"Breakpoint mapping using somatic cell hybrids and densely spaced markers; RT-PCR for XPNPEP2 mRNA in translocation-bearing fibroblasts","journal":"Cytogenetics and cell genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct molecular mapping of breakpoint to XPNPEP2 locus plus mRNA detection showing partial X-inactivation escape; single lab, two orthogonal methods","pmids":["10894934"],"is_preprint":false},{"year":2005,"finding":"A C-2399A SNP upstream of XPNPEP2 and a coding deletion in XPNPEP2 are associated with reduced plasma aminopeptidase P (APP) activity, and the C-2399A variant is also associated with ACE inhibitor-induced angioedema. Linkage analysis showed significant LOD score (3.75) to the XPNPEP2 locus for plasma APP activity as a quantitative trait.","method":"Variance-component QTL analysis with genome-wide microsatellite scan; measured genotype association analysis; case-control study","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — genome-wide linkage plus measured-genotype association in pedigrees plus independent case-control replication; multiple labs","pmids":["16175507"],"is_preprint":false},{"year":2006,"finding":"The XPNPEP2 C-2399A SNP is a significant predictor of plasma APP activity in hemodialysis patients, and recombinant APP is non-specifically inhibited by some ACE inhibitors in vitro, establishing a direct enzymatic interaction between ACEi and APP activity.","method":"Plasma APP activity assay; variance component genetic analysis; in vitro enzymatic inhibition assay with recombinant APP","journal":"Kidney international","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — in vitro enzymatic assay of recombinant APP inhibition by ACEi plus genetic association with APP activity; single lab but two orthogonal methods","pmids":["17003818"],"is_preprint":false},{"year":2007,"finding":"Activity studies and MALDI-TOF mass spectrometry in brain extracts and human plasma demonstrated that XPNPEP2 (membrane-bound APP) is among at least five peptidases potentially capable of cleaving NPY at N-terminal Xaa-Pro bonds, contributing to N-terminal truncation of neuropeptide Y.","method":"Selective inhibitor-based activity assays in brain extracts and plasma; MALDI-TOF mass spectrometry of cleavage products","journal":"Peptides","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two orthogonal methods (activity assay + MS) in biological samples; single lab","pmids":["17223229"],"is_preprint":false},{"year":2011,"finding":"Nested deletion analysis of the XPNPEP2 promoter identified a minimal promoter region (-338 bp to -147 bp) and an enhancer region (-2,502 bp to -2,238 bp). The C-2399A SNP lies within the enhancer region and was predicted to differentially bind hepatic nuclear factor 4 (HNF4). Overexpression of HNF4 increased activation of the ATG haplotype. A functional ATG haplotype (c.-2399C>A, c.-1612G>T, c.-393G>A) was associated with reduced reporter gene activity and reduced plasma APP activity.","method":"Nested deletion/reporter gene assays; electrophoretic mobility/binding prediction for HNF4; HNF4 overexpression reporter assay; plasma APP activity measurement; case-control study","journal":"Human mutation","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — promoter deletion mapping, reporter assay, transcription factor overexpression, and plasma activity phenotype; multiple orthogonal methods in single rigorous study","pmids":["21898657"],"is_preprint":false},{"year":2010,"finding":"The XPNPEP2 C-2399A genotype associates with serum APP activity in both men and women; however, serum APP activity is lower in men than in women independent of genotype, indicating sex-dependent regulation of APP activity. The A/ genotype associated with ACE inhibitor-associated angioedema specifically in men (particularly black men), but not women.","method":"Genotyping of XPNPEP2 C-2399A SNP; serum APP activity assay; case-control study with multivariate analysis","journal":"Pharmacogenetics and genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — APP activity measurement combined with genotyping in a case-control study; single lab, two orthogonal methods","pmids":["20625347"],"is_preprint":false},{"year":2015,"finding":"Gestational exposure to hexavalent chromium (CrVI) in rats increased Xpnpep2 expression during germ cell nest breakdown, decreased it during postnatal follicle development, and increased colocalization of Xpnpep2 with Col3 and Col4. Xpnpep2 was found to inversely regulate expression of Col1, Col3, and Col4 across developmental stages, placing Xpnpep2 in a pathway controlling extracellular matrix composition and follicle development.","method":"In vivo gestational exposure model; immunofluorescence/colocalization analysis; protein/mRNA expression profiling across developmental stages; apoptosis assays","journal":"Biology of reproduction","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct in vivo exposure model with multiple orthogonal readouts (colocalization, expression profiling); single lab","pmids":["25568306"],"is_preprint":false},{"year":2017,"finding":"Overexpression of XPNPEP2 in SiHa and HeLa cervical cancer cells promoted cell invasion and migration without affecting proliferation or apoptosis, and facilitated epithelial-mesenchymal transition (EMT). XPNPEP2 also promoted tumor metastasis in a xenograft mouse model.","method":"Overexpression in cancer cell lines; invasion/migration assays; EMT marker analysis; xenograft mouse model","journal":"Tumour biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional cell-based assays plus in vivo xenograft model; single lab, multiple orthogonal methods","pmids":["28670957"],"is_preprint":false},{"year":2018,"finding":"Mutagenesis studies of bacterial small aminopeptidases-P illuminate the importance of the DXRY sequence motif for Xaa-Pro aminopeptidase activity, and structural/sequence analyses suggest common evolutionary origin of these bacterial enzymes with human XPNPEP1 and XPNPEP2, supporting shared catalytic mechanism and metal-binding architecture.","method":"Crystal structure determination; in vitro enzymatic activity assays; site-directed mutagenesis of DXRY motif","journal":"Proteins","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — crystal structure + mutagenesis in bacterial orthologs with extrapolation to human XPNPEP2; direct experiments on human XPNPEP2 not performed","pmids":["30536999"],"is_preprint":false},{"year":2023,"finding":"XPNPEP2 was identified as a receptor for the tumor-homing peptide TMTP1, which selectively targets highly metastatic tumor cells. XPNPEP2 has a secreted form detectable in serum (confirmed by Western blot), in addition to its membrane-bound form.","method":"Western blot for secreted XPNPEP2 in serum; ELISA quantification in patient serum; prior receptor identification by published work cited","journal":"Scientific reports","confidence":"Low","confidence_rationale":"Tier 3 / Weak — receptor identification cited from prior work, secreted form demonstrated by Western blot in a single study; no direct binding assay reported in this abstract","pmids":["31296901"],"is_preprint":false},{"year":2026,"finding":"XPNPEP2 is essential for endothelial cell (EC) function and angiogenesis via modulation of mitochondrial function. XPNPEP2 deletion in mice led to pathological changes in pulmonary artery wall and renal tissue, decreased venous blood vessel density in retinal vessels, and slowed wound healing and tumor growth. In vitro, XPNPEP2 deficiency impaired EC proliferation, migration, and tubulogenesis, accompanied by diminished mitochondria-associated membranes, insufficient ATP, excessive mitochondrial ROS, and disrupted respiratory chain function. XPNPEP2 was found to interact with SLC25A6 (a mitochondrial ADP/ATP translocator), and XPNPEP2 ablation downregulated SLC25A6 via SIAH1-mediated ubiquitin-proteasome degradation. Overexpression of XPNPEP2 restored impaired EC angiogenesis and SLC25A6 levels.","method":"In vivo XPNPEP2 knockout mouse model; in vitro EC proliferation/migration/tubulogenesis assays; Co-immunoprecipitation of XPNPEP2 and SLC25A6; mitochondrial functional assays (ATP, mROS, respiration); SIAH1-mediated degradation assay; SLC25A6 silencing rescue experiments; XPNPEP2 overexpression rescue experiments","journal":"Frontiers in cell and developmental biology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — in vivo KO model combined with in vitro mechanistic experiments, Co-IP of binding partner, ubiquitin degradation pathway identified, rescue experiments; multiple orthogonal methods in single rigorous study","pmids":["41573684"],"is_preprint":false}],"current_model":"XPNPEP2 encodes membrane-bound aminopeptidase P (APP), a GPI-anchored ectoenzyme expressed on vascular endothelia and brush border epithelia that cleaves N-terminal Xaa-Pro bonds to inactivate bradykinin and related vasoactive peptides; its promoter is regulated by HNF4 through an upstream enhancer, and a functional ATG haplotype reduces APP expression and plasma activity, predisposing to ACE inhibitor-induced angioedema; beyond peptide catabolism, XPNPEP2 regulates angiogenesis by interacting with the mitochondrial ADP/ATP translocator SLC25A6 and preventing its SIAH1-mediated degradation, thereby maintaining mitochondrial function in endothelial cells, and it also promotes epithelial-mesenchymal transition and cancer cell invasion."},"narrative":{"mechanistic_narrative":"XPNPEP2 encodes membrane-bound aminopeptidase P (APP), an X-linked (Xq25) ectoenzyme largely restricted to vascular endothelia and brush border epithelia that hydrolyzes N-terminal Xaa-Pro bonds through a metal-dependent 'pita-bread fold' catalytic architecture, enabling it to inactivate vasoactive peptides including bradykinin [PMID:10871044]. Beyond bradykinin, APP participates in the N-terminal truncation of neuropeptide Y as one of several Xaa-Pro peptidases [PMID:17223229]. Its expression is governed by an upstream enhancer (-2,502 to -2,238 bp) acting on a minimal promoter (-338 to -147 bp), and a functional ATG haplotype carrying the C-2399A variant — which falls within an HNF4-responsive enhancer element — lowers reporter activity and plasma APP activity [PMID:21898657]. Reduced APP activity from this genetic variation, together with non-specific inhibition of recombinant APP by ACE inhibitors, predisposes to ACE inhibitor-induced angioedema, an association linked genetically to the XPNPEP2 locus and showing sex-dependent penetrance [PMID:16175507, PMID:17003818, PMID:20625347]. Independent of its peptidase role, XPNPEP2 is required for endothelial function and angiogenesis: it interacts with the mitochondrial ADP/ATP translocator SLC25A6 and protects it from SIAH1-mediated ubiquitin-proteasome degradation, thereby sustaining mitochondria-associated membranes, ATP production, and respiratory chain integrity, and its loss in mice impairs vascular development, wound healing, and tumor growth [PMID:41573684]. XPNPEP2 also promotes epithelial-mesenchymal transition and cancer cell invasion and metastasis [PMID:28670957].","teleology":[{"year":2000,"claim":"Established the molecular identity, chromosomal location, and catalytic basis of XPNPEP2, defining it as a metal-dependent Xaa-Pro ectopeptidase capable of inactivating bradykinin.","evidence":"Chromosomal mapping by somatic cell hybrids and sequence-homology identification of catalytic residues against E. coli aminopeptidase","pmids":["10871044"],"confidence":"Medium","gaps":["No experimental crystal structure of human APP","Physiological substrate repertoire beyond bradykinin not defined","Tissue distribution inferred, not quantified"]},{"year":2000,"claim":"Showed that an Xq25 translocation physically disrupts XPNPEP2 and that the gene at least partially escapes X inactivation, addressing its genomic regulation.","evidence":"Breakpoint mapping and RT-PCR in translocation-bearing fibroblasts","pmids":["10894934"],"confidence":"Medium","gaps":["Causal link between XPNPEP2 disruption and the premature ovarian failure phenotype not established","Extent of X-inactivation escape not quantified"]},{"year":2005,"claim":"Linked XPNPEP2 genetic variation to a heritable plasma APP activity trait and to ACE inhibitor-induced angioedema, connecting enzyme dosage to a clinical adverse drug reaction.","evidence":"Genome-wide linkage (LOD 3.75), measured-genotype association in pedigrees, and case-control study","pmids":["16175507"],"confidence":"High","gaps":["Mechanism by which the SNP reduces activity not yet resolved at this stage","Causal variant versus tagged variant not distinguished"]},{"year":2006,"claim":"Demonstrated a direct enzymatic interaction by showing recombinant APP is inhibited by ACE inhibitors, providing a biochemical basis for the angioedema association.","evidence":"In vitro enzymatic inhibition assay with recombinant APP plus genetic association in hemodialysis patients","pmids":["17003818"],"confidence":"Medium","gaps":["Inhibition described as non-specific; physiological relevance of in vitro potency unclear","Single-lab data"]},{"year":2007,"claim":"Expanded the substrate scope of APP to include N-terminal truncation of neuropeptide Y, positioning it among several Xaa-Pro peptidases in neuropeptide metabolism.","evidence":"Selective inhibitor activity assays and MALDI-TOF MS of cleavage products in brain extracts and plasma","pmids":["17223229"],"confidence":"Medium","gaps":["Relative contribution of XPNPEP2 versus other peptidases not quantified","In vivo relevance of NPY truncation by APP not tested"]},{"year":2010,"claim":"Revealed sex-dependent regulation of APP activity and sex-specific penetrance of the angioedema-associated genotype, refining the genotype-phenotype relationship.","evidence":"Genotyping and serum APP activity assay in a case-control study with multivariate analysis","pmids":["20625347"],"confidence":"Medium","gaps":["Molecular basis of sex difference not identified","Population-specific effects (black men) not mechanistically explained"]},{"year":2011,"claim":"Defined the cis-regulatory architecture of XPNPEP2 and identified HNF4 as a trans-activator acting on the enhancer where the functional C-2399A variant resides, explaining how the ATG haplotype lowers expression and plasma activity.","evidence":"Promoter deletion/reporter assays, HNF4 binding prediction and overexpression reporter assay, and plasma APP activity in a case-control study","pmids":["21898657"],"confidence":"High","gaps":["Direct HNF4 occupancy at the endogenous enhancer not shown (prediction-based)","Contribution of the other haplotype variants not individually dissected"]},{"year":2015,"claim":"Placed Xpnpep2 in a pathway controlling extracellular matrix composition and ovarian follicle development, suggesting roles beyond circulating peptide catabolism.","evidence":"In vivo gestational CrVI exposure model with colocalization and expression profiling across developmental stages","pmids":["25568306"],"confidence":"Medium","gaps":["Mechanism linking APP to collagen regulation not defined","Causality (regulator vs correlate) not established"]},{"year":2017,"claim":"Demonstrated a pro-metastatic function for XPNPEP2, showing it drives invasion, migration, and EMT in cancer cells independent of proliferation.","evidence":"Overexpression in cervical cancer cell lines with invasion/migration and EMT assays plus xenograft metastasis model","pmids":["28670957"],"confidence":"Medium","gaps":["Molecular effectors of EMT downstream of XPNPEP2 not identified","Whether catalytic activity is required not tested"]},{"year":2018,"claim":"Reinforced the conserved Xaa-Pro catalytic mechanism by mutagenesis of the DXRY motif in bacterial orthologs, extrapolated to human XPNPEP2.","evidence":"Crystal structure, enzymatic assays, and DXRY motif mutagenesis in bacterial small aminopeptidases-P","pmids":["30536999"],"confidence":"Medium","gaps":["Experiments performed on bacterial enzymes, not human XPNPEP2","Functional importance of the motif in the human enzyme not directly verified"]},{"year":2023,"claim":"Identified XPNPEP2 as the receptor for the tumor-homing peptide TMTP1 and documented a secreted serum form, linking the protein to selective targeting of metastatic cells.","evidence":"Western blot for secreted XPNPEP2 and ELISA in patient serum; receptor identity cited from prior work","pmids":["31296901"],"confidence":"Low","gaps":["No direct TMTP1 binding assay reported in this study","Origin and processing of the secreted form not characterized"]},{"year":2026,"claim":"Established a peptidase-independent role for XPNPEP2 in endothelial mitochondrial homeostasis and angiogenesis through stabilization of SLC25A6, defining a new mechanistic axis.","evidence":"Knockout mouse model, endothelial functional assays, XPNPEP2-SLC25A6 Co-IP, SIAH1 degradation assay, and rescue experiments","pmids":["41573684"],"confidence":"High","gaps":["Reciprocal validation and direct/indirect nature of the XPNPEP2-SLC25A6 interaction not fully resolved","Whether aminopeptidase catalytic activity is needed for SLC25A6 protection not determined","Mechanism by which XPNPEP2 blocks SIAH1-mediated ubiquitination unknown"]},{"year":null,"claim":"It remains unresolved whether the catalytic aminopeptidase activity of XPNPEP2 is required for its non-enzymatic roles in mitochondrial stabilization, EMT, and angiogenesis, and how its membrane-bound, secreted, and receptor functions are mechanistically integrated.","evidence":"","pmids":[],"confidence":"Low","gaps":["No study dissociates catalytic from scaffolding functions","Relationship between secreted and membrane forms uncharacterized","Structural basis of SLC25A6 binding unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,4,9]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,4]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,11]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[10]}],"pathway":[{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[2,8]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[11]}],"complexes":[],"partners":["SLC25A6","SIAH1","HNF4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O43895","full_name":"Xaa-Pro aminopeptidase 2","aliases":["Aminoacylproline aminopeptidase","Membrane-bound aminopeptidase P","Membrane-bound APP","Membrane-bound AmP","mAmP","X-Pro aminopeptidase 2"],"length_aa":674,"mass_kda":75.6,"function":"Membrane-bound metalloprotease which catalyzes the removal of a penultimate prolyl residue from the N-termini of peptides, such as Arg-Pro-Pro. May play a role in the metabolism of the vasodilator bradykinin","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/O43895/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/XPNPEP2","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/XPNPEP2","total_profiled":1310},"omim":[{"mim_id":"613553","title":"X-PROLYL AMINOPEPTIDASE 3; XPNPEP3","url":"https://www.omim.org/entry/613553"},{"mim_id":"610618","title":"ANGIOEDEMA, HEREDITARY, 3; HAE3","url":"https://www.omim.org/entry/610618"},{"mim_id":"602443","title":"X-PROLYL AMINOPEPTIDASE 1; XPNPEP1","url":"https://www.omim.org/entry/602443"},{"mim_id":"300909","title":"ANGIOEDEMA INDUCED BY ACE INHIBITORS, SUSCEPTIBILITY TO; AEACEI","url":"https://www.omim.org/entry/300909"},{"mim_id":"300145","title":"X-PROLYL AMINOPEPTIDASE 2; XPNPEP2","url":"https://www.omim.org/entry/300145"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"},{"location":"Cytosol","reliability":"Approved"}],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"intestine","ntpm":156.9},{"tissue":"kidney","ntpm":145.6}],"url":"https://www.proteinatlas.org/search/XPNPEP2"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"O43895","domains":[{"cath_id":"3.40.350.10","chopping":"51-196","consensus_level":"high","plddt":96.3274,"start":51,"end":196},{"cath_id":"3.40.350.10","chopping":"206-356","consensus_level":"medium","plddt":97.0611,"start":206,"end":356},{"cath_id":"3.90.230.10","chopping":"360-645","consensus_level":"medium","plddt":96.9926,"start":360,"end":645}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O43895","model_url":"https://alphafold.ebi.ac.uk/files/AF-O43895-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O43895-F1-predicted_aligned_error_v6.png","plddt_mean":91.56},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=XPNPEP2","jax_strain_url":"https://www.jax.org/strain/search?query=XPNPEP2"},"sequence":{"accession":"O43895","fasta_url":"https://rest.uniprot.org/uniprotkb/O43895.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O43895/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O43895"}},"corpus_meta":[{"pmid":"16175507","id":"PMC_16175507","title":"A variant in XPNPEP2 is associated with angioedema induced by angiotensin I-converting enzyme inhibitors.","date":"2005","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/16175507","citation_count":101,"is_preprint":false},{"pmid":"25401373","id":"PMC_25401373","title":"Diagnosis and treatment of bradykinin-mediated angioedema: outcomes from an angioedema expert consensus meeting.","date":"2014","source":"International archives of allergy and immunology","url":"https://pubmed.ncbi.nlm.nih.gov/25401373","citation_count":81,"is_preprint":false},{"pmid":"17223229","id":"PMC_17223229","title":"Neuropeptide Y (NPY) cleaving enzymes: structural and functional homologues of dipeptidyl peptidase 4.","date":"2007","source":"Peptides","url":"https://pubmed.ncbi.nlm.nih.gov/17223229","citation_count":75,"is_preprint":false},{"pmid":"20625347","id":"PMC_20625347","title":"Sex-dependent and race-dependent association of XPNPEP2 C-2399A polymorphism with angiotensin-converting enzyme inhibitor-associated angioedema.","date":"2010","source":"Pharmacogenetics and genomics","url":"https://pubmed.ncbi.nlm.nih.gov/20625347","citation_count":68,"is_preprint":false},{"pmid":"10894934","id":"PMC_10894934","title":"Physical mapping of nine Xq translocation breakpoints and identification of XPNPEP2 as a premature ovarian failure candidate gene.","date":"2000","source":"Cytogenetics and cell genetics","url":"https://pubmed.ncbi.nlm.nih.gov/10894934","citation_count":62,"is_preprint":false},{"pmid":"31040866","id":"PMC_31040866","title":"Identification of DNA-Methylated CpG Islands Associated With Gene Silencing in the Adult Body Tissues of the Ogye Chicken Using RNA-Seq and Reduced Representation Bisulfite Sequencing.","date":"2019","source":"Frontiers in genetics","url":"https://pubmed.ncbi.nlm.nih.gov/31040866","citation_count":53,"is_preprint":false},{"pmid":"19178938","id":"PMC_19178938","title":"Genetic analysis of Factor XII and bradykinin 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distribution to endothelia and brush border epithelia, which hydrolyzes N-terminal Xaa-Pro bonds and can inactivate bradykinin. The gene is located at chromosome Xq25. Sequence analysis identified conserved 'pita bread-fold' homologous blocks containing a proton shuttle and five divalent metal ligands (counterparts of E. coli methionine aminopeptidase) shared with soluble aminopeptidase P (XPNPEP1), establishing the catalytic mechanism.\",\n      \"method\": \"Chromosomal mapping (somatic cell hybrids), sequence homology analysis, comparison to known E. coli aminopeptidase structure\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — chromosomal localization with somatic cell hybrids combined with sequence-based identification of catalytic residues; single lab but two orthogonal approaches\",\n      \"pmids\": [\"10871044\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"A translocation breakpoint at Xq25 associated with premature ovarian failure physically disrupts XPNPEP2. XPNPEP2 mRNA was detected in fibroblasts carrying the translocation, suggesting the gene at least partially escapes X inactivation.\",\n      \"method\": \"Breakpoint mapping using somatic cell hybrids and densely spaced markers; RT-PCR for XPNPEP2 mRNA in translocation-bearing fibroblasts\",\n      \"journal\": \"Cytogenetics and cell genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct molecular mapping of breakpoint to XPNPEP2 locus plus mRNA detection showing partial X-inactivation escape; single lab, two orthogonal methods\",\n      \"pmids\": [\"10894934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"A C-2399A SNP upstream of XPNPEP2 and a coding deletion in XPNPEP2 are associated with reduced plasma aminopeptidase P (APP) activity, and the C-2399A variant is also associated with ACE inhibitor-induced angioedema. Linkage analysis showed significant LOD score (3.75) to the XPNPEP2 locus for plasma APP activity as a quantitative trait.\",\n      \"method\": \"Variance-component QTL analysis with genome-wide microsatellite scan; measured genotype association analysis; case-control study\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genome-wide linkage plus measured-genotype association in pedigrees plus independent case-control replication; multiple labs\",\n      \"pmids\": [\"16175507\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The XPNPEP2 C-2399A SNP is a significant predictor of plasma APP activity in hemodialysis patients, and recombinant APP is non-specifically inhibited by some ACE inhibitors in vitro, establishing a direct enzymatic interaction between ACEi and APP activity.\",\n      \"method\": \"Plasma APP activity assay; variance component genetic analysis; in vitro enzymatic inhibition assay with recombinant APP\",\n      \"journal\": \"Kidney international\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — in vitro enzymatic assay of recombinant APP inhibition by ACEi plus genetic association with APP activity; single lab but two orthogonal methods\",\n      \"pmids\": [\"17003818\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Activity studies and MALDI-TOF mass spectrometry in brain extracts and human plasma demonstrated that XPNPEP2 (membrane-bound APP) is among at least five peptidases potentially capable of cleaving NPY at N-terminal Xaa-Pro bonds, contributing to N-terminal truncation of neuropeptide Y.\",\n      \"method\": \"Selective inhibitor-based activity assays in brain extracts and plasma; MALDI-TOF mass spectrometry of cleavage products\",\n      \"journal\": \"Peptides\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two orthogonal methods (activity assay + MS) in biological samples; single lab\",\n      \"pmids\": [\"17223229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Nested deletion analysis of the XPNPEP2 promoter identified a minimal promoter region (-338 bp to -147 bp) and an enhancer region (-2,502 bp to -2,238 bp). The C-2399A SNP lies within the enhancer region and was predicted to differentially bind hepatic nuclear factor 4 (HNF4). Overexpression of HNF4 increased activation of the ATG haplotype. A functional ATG haplotype (c.-2399C>A, c.-1612G>T, c.-393G>A) was associated with reduced reporter gene activity and reduced plasma APP activity.\",\n      \"method\": \"Nested deletion/reporter gene assays; electrophoretic mobility/binding prediction for HNF4; HNF4 overexpression reporter assay; plasma APP activity measurement; case-control study\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — promoter deletion mapping, reporter assay, transcription factor overexpression, and plasma activity phenotype; multiple orthogonal methods in single rigorous study\",\n      \"pmids\": [\"21898657\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The XPNPEP2 C-2399A genotype associates with serum APP activity in both men and women; however, serum APP activity is lower in men than in women independent of genotype, indicating sex-dependent regulation of APP activity. The A/ genotype associated with ACE inhibitor-associated angioedema specifically in men (particularly black men), but not women.\",\n      \"method\": \"Genotyping of XPNPEP2 C-2399A SNP; serum APP activity assay; case-control study with multivariate analysis\",\n      \"journal\": \"Pharmacogenetics and genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — APP activity measurement combined with genotyping in a case-control study; single lab, two orthogonal methods\",\n      \"pmids\": [\"20625347\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Gestational exposure to hexavalent chromium (CrVI) in rats increased Xpnpep2 expression during germ cell nest breakdown, decreased it during postnatal follicle development, and increased colocalization of Xpnpep2 with Col3 and Col4. Xpnpep2 was found to inversely regulate expression of Col1, Col3, and Col4 across developmental stages, placing Xpnpep2 in a pathway controlling extracellular matrix composition and follicle development.\",\n      \"method\": \"In vivo gestational exposure model; immunofluorescence/colocalization analysis; protein/mRNA expression profiling across developmental stages; apoptosis assays\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct in vivo exposure model with multiple orthogonal readouts (colocalization, expression profiling); single lab\",\n      \"pmids\": [\"25568306\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Overexpression of XPNPEP2 in SiHa and HeLa cervical cancer cells promoted cell invasion and migration without affecting proliferation or apoptosis, and facilitated epithelial-mesenchymal transition (EMT). XPNPEP2 also promoted tumor metastasis in a xenograft mouse model.\",\n      \"method\": \"Overexpression in cancer cell lines; invasion/migration assays; EMT marker analysis; xenograft mouse model\",\n      \"journal\": \"Tumour biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional cell-based assays plus in vivo xenograft model; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"28670957\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Mutagenesis studies of bacterial small aminopeptidases-P illuminate the importance of the DXRY sequence motif for Xaa-Pro aminopeptidase activity, and structural/sequence analyses suggest common evolutionary origin of these bacterial enzymes with human XPNPEP1 and XPNPEP2, supporting shared catalytic mechanism and metal-binding architecture.\",\n      \"method\": \"Crystal structure determination; in vitro enzymatic activity assays; site-directed mutagenesis of DXRY motif\",\n      \"journal\": \"Proteins\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — crystal structure + mutagenesis in bacterial orthologs with extrapolation to human XPNPEP2; direct experiments on human XPNPEP2 not performed\",\n      \"pmids\": [\"30536999\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"XPNPEP2 was identified as a receptor for the tumor-homing peptide TMTP1, which selectively targets highly metastatic tumor cells. XPNPEP2 has a secreted form detectable in serum (confirmed by Western blot), in addition to its membrane-bound form.\",\n      \"method\": \"Western blot for secreted XPNPEP2 in serum; ELISA quantification in patient serum; prior receptor identification by published work cited\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — receptor identification cited from prior work, secreted form demonstrated by Western blot in a single study; no direct binding assay reported in this abstract\",\n      \"pmids\": [\"31296901\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"XPNPEP2 is essential for endothelial cell (EC) function and angiogenesis via modulation of mitochondrial function. XPNPEP2 deletion in mice led to pathological changes in pulmonary artery wall and renal tissue, decreased venous blood vessel density in retinal vessels, and slowed wound healing and tumor growth. In vitro, XPNPEP2 deficiency impaired EC proliferation, migration, and tubulogenesis, accompanied by diminished mitochondria-associated membranes, insufficient ATP, excessive mitochondrial ROS, and disrupted respiratory chain function. XPNPEP2 was found to interact with SLC25A6 (a mitochondrial ADP/ATP translocator), and XPNPEP2 ablation downregulated SLC25A6 via SIAH1-mediated ubiquitin-proteasome degradation. Overexpression of XPNPEP2 restored impaired EC angiogenesis and SLC25A6 levels.\",\n      \"method\": \"In vivo XPNPEP2 knockout mouse model; in vitro EC proliferation/migration/tubulogenesis assays; Co-immunoprecipitation of XPNPEP2 and SLC25A6; mitochondrial functional assays (ATP, mROS, respiration); SIAH1-mediated degradation assay; SLC25A6 silencing rescue experiments; XPNPEP2 overexpression rescue experiments\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — in vivo KO model combined with in vitro mechanistic experiments, Co-IP of binding partner, ubiquitin degradation pathway identified, rescue experiments; multiple orthogonal methods in single rigorous study\",\n      \"pmids\": [\"41573684\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"XPNPEP2 encodes membrane-bound aminopeptidase P (APP), a GPI-anchored ectoenzyme expressed on vascular endothelia and brush border epithelia that cleaves N-terminal Xaa-Pro bonds to inactivate bradykinin and related vasoactive peptides; its promoter is regulated by HNF4 through an upstream enhancer, and a functional ATG haplotype reduces APP expression and plasma activity, predisposing to ACE inhibitor-induced angioedema; beyond peptide catabolism, XPNPEP2 regulates angiogenesis by interacting with the mitochondrial ADP/ATP translocator SLC25A6 and preventing its SIAH1-mediated degradation, thereby maintaining mitochondrial function in endothelial cells, and it also promotes epithelial-mesenchymal transition and cancer cell invasion.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"XPNPEP2 encodes membrane-bound aminopeptidase P (APP), an X-linked (Xq25) ectoenzyme largely restricted to vascular endothelia and brush border epithelia that hydrolyzes N-terminal Xaa-Pro bonds through a metal-dependent 'pita-bread fold' catalytic architecture, enabling it to inactivate vasoactive peptides including bradykinin [#0]. Beyond bradykinin, APP participates in the N-terminal truncation of neuropeptide Y as one of several Xaa-Pro peptidases [#4]. Its expression is governed by an upstream enhancer (-2,502 to -2,238 bp) acting on a minimal promoter (-338 to -147 bp), and a functional ATG haplotype carrying the C-2399A variant — which falls within an HNF4-responsive enhancer element — lowers reporter activity and plasma APP activity [#5]. Reduced APP activity from this genetic variation, together with non-specific inhibition of recombinant APP by ACE inhibitors, predisposes to ACE inhibitor-induced angioedema, an association linked genetically to the XPNPEP2 locus and showing sex-dependent penetrance [#2, #3, #6]. Independent of its peptidase role, XPNPEP2 is required for endothelial function and angiogenesis: it interacts with the mitochondrial ADP/ATP translocator SLC25A6 and protects it from SIAH1-mediated ubiquitin-proteasome degradation, thereby sustaining mitochondria-associated membranes, ATP production, and respiratory chain integrity, and its loss in mice impairs vascular development, wound healing, and tumor growth [#11]. XPNPEP2 also promotes epithelial-mesenchymal transition and cancer cell invasion and metastasis [#8].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Established the molecular identity, chromosomal location, and catalytic basis of XPNPEP2, defining it as a metal-dependent Xaa-Pro ectopeptidase capable of inactivating bradykinin.\",\n      \"evidence\": \"Chromosomal mapping by somatic cell hybrids and sequence-homology identification of catalytic residues against E. coli aminopeptidase\",\n      \"pmids\": [\"10871044\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No experimental crystal structure of human APP\", \"Physiological substrate repertoire beyond bradykinin not defined\", \"Tissue distribution inferred, not quantified\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Showed that an Xq25 translocation physically disrupts XPNPEP2 and that the gene at least partially escapes X inactivation, addressing its genomic regulation.\",\n      \"evidence\": \"Breakpoint mapping and RT-PCR in translocation-bearing fibroblasts\",\n      \"pmids\": [\"10894934\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal link between XPNPEP2 disruption and the premature ovarian failure phenotype not established\", \"Extent of X-inactivation escape not quantified\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Linked XPNPEP2 genetic variation to a heritable plasma APP activity trait and to ACE inhibitor-induced angioedema, connecting enzyme dosage to a clinical adverse drug reaction.\",\n      \"evidence\": \"Genome-wide linkage (LOD 3.75), measured-genotype association in pedigrees, and case-control study\",\n      \"pmids\": [\"16175507\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which the SNP reduces activity not yet resolved at this stage\", \"Causal variant versus tagged variant not distinguished\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrated a direct enzymatic interaction by showing recombinant APP is inhibited by ACE inhibitors, providing a biochemical basis for the angioedema association.\",\n      \"evidence\": \"In vitro enzymatic inhibition assay with recombinant APP plus genetic association in hemodialysis patients\",\n      \"pmids\": [\"17003818\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Inhibition described as non-specific; physiological relevance of in vitro potency unclear\", \"Single-lab data\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Expanded the substrate scope of APP to include N-terminal truncation of neuropeptide Y, positioning it among several Xaa-Pro peptidases in neuropeptide metabolism.\",\n      \"evidence\": \"Selective inhibitor activity assays and MALDI-TOF MS of cleavage products in brain extracts and plasma\",\n      \"pmids\": [\"17223229\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Relative contribution of XPNPEP2 versus other peptidases not quantified\", \"In vivo relevance of NPY truncation by APP not tested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Revealed sex-dependent regulation of APP activity and sex-specific penetrance of the angioedema-associated genotype, refining the genotype-phenotype relationship.\",\n      \"evidence\": \"Genotyping and serum APP activity assay in a case-control study with multivariate analysis\",\n      \"pmids\": [\"20625347\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of sex difference not identified\", \"Population-specific effects (black men) not mechanistically explained\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Defined the cis-regulatory architecture of XPNPEP2 and identified HNF4 as a trans-activator acting on the enhancer where the functional C-2399A variant resides, explaining how the ATG haplotype lowers expression and plasma activity.\",\n      \"evidence\": \"Promoter deletion/reporter assays, HNF4 binding prediction and overexpression reporter assay, and plasma APP activity in a case-control study\",\n      \"pmids\": [\"21898657\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct HNF4 occupancy at the endogenous enhancer not shown (prediction-based)\", \"Contribution of the other haplotype variants not individually dissected\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Placed Xpnpep2 in a pathway controlling extracellular matrix composition and ovarian follicle development, suggesting roles beyond circulating peptide catabolism.\",\n      \"evidence\": \"In vivo gestational CrVI exposure model with colocalization and expression profiling across developmental stages\",\n      \"pmids\": [\"25568306\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking APP to collagen regulation not defined\", \"Causality (regulator vs correlate) not established\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrated a pro-metastatic function for XPNPEP2, showing it drives invasion, migration, and EMT in cancer cells independent of proliferation.\",\n      \"evidence\": \"Overexpression in cervical cancer cell lines with invasion/migration and EMT assays plus xenograft metastasis model\",\n      \"pmids\": [\"28670957\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular effectors of EMT downstream of XPNPEP2 not identified\", \"Whether catalytic activity is required not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Reinforced the conserved Xaa-Pro catalytic mechanism by mutagenesis of the DXRY motif in bacterial orthologs, extrapolated to human XPNPEP2.\",\n      \"evidence\": \"Crystal structure, enzymatic assays, and DXRY motif mutagenesis in bacterial small aminopeptidases-P\",\n      \"pmids\": [\"30536999\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Experiments performed on bacterial enzymes, not human XPNPEP2\", \"Functional importance of the motif in the human enzyme not directly verified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified XPNPEP2 as the receptor for the tumor-homing peptide TMTP1 and documented a secreted serum form, linking the protein to selective targeting of metastatic cells.\",\n      \"evidence\": \"Western blot for secreted XPNPEP2 and ELISA in patient serum; receptor identity cited from prior work\",\n      \"pmids\": [\"31296901\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No direct TMTP1 binding assay reported in this study\", \"Origin and processing of the secreted form not characterized\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Established a peptidase-independent role for XPNPEP2 in endothelial mitochondrial homeostasis and angiogenesis through stabilization of SLC25A6, defining a new mechanistic axis.\",\n      \"evidence\": \"Knockout mouse model, endothelial functional assays, XPNPEP2-SLC25A6 Co-IP, SIAH1 degradation assay, and rescue experiments\",\n      \"pmids\": [\"41573684\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Reciprocal validation and direct/indirect nature of the XPNPEP2-SLC25A6 interaction not fully resolved\", \"Whether aminopeptidase catalytic activity is needed for SLC25A6 protection not determined\", \"Mechanism by which XPNPEP2 blocks SIAH1-mediated ubiquitination unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved whether the catalytic aminopeptidase activity of XPNPEP2 is required for its non-enzymatic roles in mitochondrial stabilization, EMT, and angiogenesis, and how its membrane-bound, secreted, and receptor functions are mechanistically integrated.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No study dissociates catalytic from scaffolding functions\", \"Relationship between secreted and membrane forms uncharacterized\", \"Structural basis of SLC25A6 binding unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 4, 9]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 11]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [2, 8]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [11]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"SLC25A6\",\n      \"SIAH1\",\n      \"HNF4\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":6,"faith_total":6,"faith_pct":100.0}}