{"gene":"PKP1","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":2017,"finding":"RIPK4 (receptor-interacting serine-threonine kinase 4) directly phosphorylates the N-terminal domain of PKP1 during epidermal differentiation; loss of function of either Pkp1 or Ripk4 impairs skin differentiation and enhances epidermal carcinogenesis in vivo, and phosphorylation of PKP1's N-terminal domain by RIPK4 is essential for their role in epidermal differentiation.","method":"Quantitative phosphoproteomics, mammalian kinome cDNA library screen, genome-editing (loss-of-function), mouse genetics, in vivo carcinogenesis models","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (phosphoproteomics, kinome screen, genome editing, mouse genetics) in a single rigorous study establishing PKP1 as a RIPK4 substrate with in vivo functional validation","pmids":["28507225"],"is_preprint":false},{"year":2022,"finding":"PKP1 enhances MYC protein translation by binding to the 5'-UTR of MYC mRNA in conjunction with the translation initiation complex; conversely, MYC directly binds specific sequences within the PKP1 promoter to transcriptionally activate PKP1, forming a feedforward loop in squamous cell lung cancer.","method":"Chromatin immunoprecipitation (ChIP), promoter mutagenesis with luciferase assays, gain/loss-of-function models, mRNA/protein expression analysis","journal":"Cellular oncology (Dordrecht, Netherlands)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal functional relationship established by ChIP, promoter mutagenesis, and gain/loss-of-function in a single lab with multiple orthogonal methods","pmids":["35182388"],"is_preprint":false},{"year":2025,"finding":"PKP1 stabilizes platelet-type phosphofructokinase (PFKP), a key glycolytic enzyme, by binding to TRIM21 and preventing PFKP ubiquitination and proteasomal degradation; PKP1 depletion selectively reduces PFKP levels by enhancing its ubiquitination, and PFKP mediates the proliferative role of PKP1 in lung squamous cell carcinoma.","method":"Genome-wide CRISPR knockout screening, metabolic assays (OCR/ECAR), ubiquitination assays, functional rescue experiments, multiple LUSC cell lines","journal":"Biomarker research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR screen plus biochemical ubiquitination assays and functional rescue in multiple cell lines, single lab","pmids":["40890861"],"is_preprint":false},{"year":2011,"finding":"PKP1 knockdown in Barrett's esophagus cell lines (which normally express PKP1) resulted in increased cell motility, indicating that PKP1 loss promotes cell migration and may contribute to disease progression via decreased desmosome assembly.","method":"siRNA knockdown, cell motility assay in CP-A and CP-D cell lines","journal":"Genes, chromosomes & cancer","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single lab, loss-of-function with defined cellular phenotype (motility), two cell lines tested","pmids":["22170739"],"is_preprint":false},{"year":2021,"finding":"High expression of PKP1 (and DSC2) facilitates cancer cell cluster formation in circulation, activates PI3K/AKT/Bcl-2-mediated cell survival signaling, and maintains high vimentin expression to stimulate fibronectin/integrin β1/FAK/Src/MEK/ERK/ZEB1-mediated metastasis; PKP1 overexpression enhances resistance to fluid shear stress.","method":"Microfluidic circulatory system selection of shear-stress-resistant cells, protein knockdown/overexpression, pathway inhibitor studies, in vivo metastasis in mice","journal":"Science advances","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional experiments with defined pathway readouts and in vivo validation, single lab","pmids":["34586853"],"is_preprint":false},{"year":2004,"finding":"Homozygous splice site mutations in PKP1 result in complete loss of plakophilin-1 protein in the epidermis, leading to intraepidermal separation, widening of intercellular spaces, and abnormal desmosome ultrastructure, establishing PKP1 as a critical desmosomal plaque component required for normal epidermal cohesion.","method":"Molecular genetics (mutation analysis), immunohistochemistry, electron microscopy of patient skin biopsies","journal":"The Journal of investigative dermatology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — human loss-of-function genetics with direct protein and ultrastructural readout, two independent families","pmids":["15086548"],"is_preprint":false},{"year":2012,"finding":"A homozygous splice donor site mutation in intron 1 of canine PKP1 results in a premature stop codon and complete absence of plakophilin-1 protein, causing desmosomal abnormality (reduced number of partially formed desmosomes with detached keratin intermediate filaments) and redistribution of desmoplakin and keratins 10/14, phenocopying human ectodermal dysplasia-skin fragility syndrome.","method":"DNA sequencing, immunostaining, electron microscopy, histopathology in a natural animal model","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — natural loss-of-function model with protein-level and ultrastructural validation, independent of human studies","pmids":["22384142"],"is_preprint":false}],"current_model":"PKP1 (plakophilin-1) is a desmosomal plaque component whose N-terminal domain is phosphorylated by RIPK4 to drive epidermal differentiation; it stabilizes the glycolytic enzyme PFKP by binding TRIM21 to prevent ubiquitin-mediated degradation, enhancing metabolic activity in lung squamous cell carcinoma; it promotes MYC translation via 5'-UTR binding (while MYC reciprocally transcribes PKP1), and its loss—whether through mutation, splice-site defects, or knockdown—disrupts desmosome assembly, increases cell motility, and impairs epithelial cohesion."},"narrative":{"mechanistic_narrative":"PKP1 (plakophilin-1) is a desmosomal plaque component required for epithelial cohesion and a regulator of epidermal differentiation, with additional roles in squamous carcinoma growth and metastasis [PMID:28507225, PMID:15086548]. Complete loss of PKP1 in human and canine epidermis through splice-site mutations abolishes the protein, producing intraepidermal separation, widened intercellular spaces, and abnormal desmosome ultrastructure with detached keratin filaments and redistributed desmoplakin, establishing it as essential for desmosome assembly and epithelial cohesion [PMID:15086548, PMID:22384142]. During epidermal differentiation, the kinase RIPK4 directly phosphorylates the N-terminal domain of PKP1, and this phosphorylation is essential for their shared role in differentiation and suppression of epidermal carcinogenesis [PMID:28507225]. Beyond adhesion, PKP1 carries out cytoplasmic functions in cancer cells: it enhances MYC protein translation by binding the MYC 5'-UTR with the translation initiation complex while MYC reciprocally transactivates the PKP1 promoter, forming a feedforward loop in squamous lung cancer [PMID:35182388], and it stabilizes the glycolytic enzyme PFKP by binding TRIM21 to block PFKP ubiquitination and proteasomal degradation, with PFKP mediating PKP1-driven proliferation [PMID:40890861]. In epithelial cancers, loss of PKP1 increases cell motility [PMID:22170739], whereas high PKP1 expression supports circulating tumor-cell cluster formation, shear-stress resistance, and PI3K/AKT- and integrin/FAK/ERK/ZEB1-mediated metastasis [PMID:34586853].","teleology":[{"year":2004,"claim":"Establishing whether PKP1 is genuinely required for human epidermal integrity, this work showed that complete loss of the protein disrupts desmosome structure and epithelial cohesion.","evidence":"Mutation analysis, immunohistochemistry, and electron microscopy of patient skin biopsies carrying homozygous PKP1 splice-site mutations","pmids":["15086548"],"confidence":"Medium","gaps":["Does not define which desmosomal partners PKP1 directly binds","Mechanism linking plaque loss to intercellular separation not resolved at the molecular level"]},{"year":2011,"claim":"Addressing whether PKP1 loss has consequences beyond adhesion failure, knockdown showed it actively restrains cell migration in epithelial cells.","evidence":"siRNA knockdown and cell motility assays in Barrett's esophagus CP-A and CP-D cell lines","pmids":["22170739"],"confidence":"Medium","gaps":["Molecular pathway coupling PKP1 loss to increased motility not identified","Only two cell lines tested; no in vivo confirmation"]},{"year":2012,"claim":"Confirming the human phenotype was a direct consequence of PKP1 loss, a natural canine model recapitulated the desmosomal and keratin defects.","evidence":"DNA sequencing, immunostaining, electron microscopy, and histopathology of a dog carrying a homozygous PKP1 splice donor mutation","pmids":["22384142"],"confidence":"Medium","gaps":["Does not address PKP1 regulatory or signaling functions","Mechanism of desmoplakin/keratin redistribution unresolved"]},{"year":2017,"claim":"Identifying an upstream regulator of PKP1, this study showed RIPK4 directly phosphorylates the PKP1 N-terminus and that this is essential for epidermal differentiation and tumor suppression.","evidence":"Quantitative phosphoproteomics, kinome cDNA library screen, genome editing, mouse genetics, and in vivo carcinogenesis models","pmids":["28507225"],"confidence":"High","gaps":["Specific phosphosites and how phosphorylation alters PKP1 function not fully mapped","Downstream effectors of phosphorylated PKP1 in differentiation not defined"]},{"year":2021,"claim":"Reframing PKP1 as a metastasis driver, high expression was shown to promote tumor-cell cluster formation, shear-stress resistance, and survival/metastatic signaling.","evidence":"Microfluidic selection of shear-resistant cells, knockdown/overexpression, pathway inhibitor studies, and in vivo metastasis in mice","pmids":["34586853"],"confidence":"Medium","gaps":["Direct molecular link between PKP1 and the PI3K/AKT and integrin/FAK/ERK pathways not biochemically defined","Apparent contrast with loss-of-function promoting motility not reconciled"]},{"year":2022,"claim":"Revealing a non-adhesive cytoplasmic function, PKP1 was shown to enhance MYC translation via 5'-UTR binding while MYC transactivates PKP1, forming a feedforward loop in squamous lung cancer.","evidence":"ChIP, promoter mutagenesis with luciferase assays, and gain/loss-of-function with mRNA/protein analysis","pmids":["35182388"],"confidence":"Medium","gaps":["Direct PKP1 binding to MYC 5'-UTR not demonstrated by structural or crosslinking methods","Components of the initiation complex engaged by PKP1 not identified"]},{"year":2025,"claim":"Extending PKP1's cytoplasmic role to metabolism, it was shown to stabilize the glycolytic enzyme PFKP by binding TRIM21 and blocking PFKP ubiquitination.","evidence":"Genome-wide CRISPR knockout screen, OCR/ECAR metabolic assays, ubiquitination assays, and functional rescue in multiple LUSC cell lines","pmids":["40890861"],"confidence":"Medium","gaps":["Whether PKP1 sequesters TRIM21 away from PFKP or otherwise inhibits the ligase not resolved","Direct binary interactions among PKP1, TRIM21, and PFKP not structurally defined"]},{"year":null,"claim":"How PKP1's desmosomal adhesion role mechanistically integrates with its cytoplasmic functions in translation and metabolism, and how RIPK4 phosphorylation toggles between these roles, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified model linking adhesion, MYC translation, and PFKP stabilization","Phosphorylation-dependent switch between adhesive and signaling pools undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[1]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[1]},{"term_id":"GO:0045182","term_label":"translation regulator activity","supporting_discovery_ids":[1]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[5,6]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[5,6]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1,2]}],"pathway":[{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[5,6]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[0]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[2,4]}],"complexes":["desmosome"],"partners":["RIPK4","TRIM21","PFKP","MYC"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q13835","full_name":"Plakophilin-1","aliases":["Band 6 protein","B6P"],"length_aa":747,"mass_kda":82.9,"function":"A component of desmosome cell-cell junctions which are required for positive regulation of cellular adhesion (PubMed:23444369). Plays a role in desmosome protein expression regulation and localization to the desmosomal plaque, thereby maintaining cell sheet integrity and anchorage of desmosomes to intermediate filaments (PubMed:10852826, PubMed:23444369). Required for localization of DSG3 and YAP1 to the cell membrane in keratinocytes in response to mechanical strain, via the formation of an interaction complex composed of DSG3, YAP1, PKP1 and YWHAG (PubMed:31835537). Positively regulates differentiation of keratinocytes, potentially via promoting localization of DSG1 at desmosome cell junctions (By similarity). Required for calcium-independent development and maturation of desmosome plaques specifically at lateral cell-cell contacts in differentiating keratinocytes (By similarity). Plays a role in the maintenance of DSG3 protein abundance, DSG3 clustering and localization of these clusters to the cell membrane in keratinocytes (By similarity). May also promote keratinocyte proliferation and morphogenesis during postnatal development (PubMed:9326952). Required for tight junction inside-out transepidermal barrier function of the skin (By similarity). Promotes Wnt-mediated proliferation and differentiation of ameloblasts, via facilitating TJP1/ZO-1 localization to tight junctions (By similarity). Binds single-stranded DNA (ssDNA), and may thereby play a role in sensing DNA damage and promoting cell survival (PubMed:20613778). Positively regulates cap-dependent translation and as a result cell proliferation, via recruitment of EIF4A1 to the initiation complex and promotion of EIF4A1 ATPase activity (PubMed:20156963, PubMed:23444369). Regulates the mRNA stability and protein abundance of desmosome components PKP2, PKP3, DSC2 and DSP, potentially via its interaction with FXR1 (PubMed:25225333)","subcellular_location":"Nucleus; Cytoplasm, perinuclear region; Cytoplasm; Cell junction, desmosome; Cell membrane; Cytoplasm, Stress granule","url":"https://www.uniprot.org/uniprotkb/Q13835/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PKP1","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"PMVK","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/PKP1","total_profiled":1310},"omim":[{"mim_id":"605561","title":"PLAKOPHILIN 3; PKP3","url":"https://www.omim.org/entry/605561"},{"mim_id":"604536","title":"ECTODERMAL DYSPLASIA/SKIN FRAGILITY SYNDROME; EDSFS","url":"https://www.omim.org/entry/604536"},{"mim_id":"604276","title":"PLAKOPHILIN 4; PKP4","url":"https://www.omim.org/entry/604276"},{"mim_id":"604275","title":"CATENIN, DELTA-2; CTNND2","url":"https://www.omim.org/entry/604275"},{"mim_id":"602861","title":"PLAKOPHILIN 2; PKP2","url":"https://www.omim.org/entry/602861"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Plasma membrane","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"esophagus","ntpm":404.9},{"tissue":"skin 1","ntpm":665.0},{"tissue":"vagina","ntpm":192.1}],"url":"https://www.proteinatlas.org/search/PKP1"},"hgnc":{"alias_symbol":["B6P"],"prev_symbol":[]},"alphafold":{"accession":"Q13835","domains":[{"cath_id":"1.25.10.10","chopping":"242-361","consensus_level":"medium","plddt":94.3249,"start":242,"end":361}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13835","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q13835-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q13835-F1-predicted_aligned_error_v6.png","plddt_mean":69.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PKP1","jax_strain_url":"https://www.jax.org/strain/search?query=PKP1"},"sequence":{"accession":"Q13835","fasta_url":"https://rest.uniprot.org/uniprotkb/Q13835.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q13835/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13835"}},"corpus_meta":[{"pmid":"8623923","id":"PMC_8623923","title":"Monoclonal 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cancer cells survival and metastasis by increasing cluster formation in circulatory system.","date":"2021","source":"Science advances","url":"https://pubmed.ncbi.nlm.nih.gov/34586853","citation_count":47,"is_preprint":false},{"pmid":"21194493","id":"PMC_21194493","title":"Expression of plakophilins (PKP1, PKP2, and PKP3) in gastric cancers.","date":"2011","source":"Diagnostic pathology","url":"https://pubmed.ncbi.nlm.nih.gov/21194493","citation_count":41,"is_preprint":false},{"pmid":"15086548","id":"PMC_15086548","title":"Homozygous splice site mutations in PKP1 result in loss of epidermal plakophilin 1 expression and underlie ectodermal dysplasia/skin fragility syndrome in two consanguineous families.","date":"2004","source":"The Journal of investigative dermatology","url":"https://pubmed.ncbi.nlm.nih.gov/15086548","citation_count":41,"is_preprint":false},{"pmid":"19016709","id":"PMC_19016709","title":"Novel truncating mutations in PKP1 and DSP cause similar skin phenotypes in two Brazilian families.","date":"2008","source":"The British journal of dermatology","url":"https://pubmed.ncbi.nlm.nih.gov/19016709","citation_count":37,"is_preprint":false},{"pmid":"21947748","id":"PMC_21947748","title":"Expression of Plakophilins (PKP1, PKP2, and PKP3) in breast cancers.","date":"2011","source":"Medical oncology (Northwood, London, England)","url":"https://pubmed.ncbi.nlm.nih.gov/21947748","citation_count":34,"is_preprint":false},{"pmid":"22170739","id":"PMC_22170739","title":"Aberrantly methylated PKP1 in the progression of Barrett's esophagus to esophageal adenocarcinoma.","date":"2011","source":"Genes, chromosomes & cancer","url":"https://pubmed.ncbi.nlm.nih.gov/22170739","citation_count":29,"is_preprint":false},{"pmid":"22309335","id":"PMC_22309335","title":"Ectodermal dysplasia-skin fragility syndrome due to a new homozygous internal deletion mutation in the PKP1 gene.","date":"2011","source":"The Australasian journal of dermatology","url":"https://pubmed.ncbi.nlm.nih.gov/22309335","citation_count":22,"is_preprint":false},{"pmid":"22384142","id":"PMC_22384142","title":"Deficient plakophilin-1 expression due to a mutation in PKP1 causes ectodermal dysplasia-skin fragility syndrome in Chesapeake Bay retriever dogs.","date":"2012","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/22384142","citation_count":21,"is_preprint":false},{"pmid":"11180229","id":"PMC_11180229","title":"Preimplantation genetic diagnosis of compound heterozygous mutations leading to ablation of plakophilin-1 (PKP1) and resulting in skin fragility ectodermal dysplasia syndrome: a case report.","date":"2000","source":"Prenatal diagnosis","url":"https://pubmed.ncbi.nlm.nih.gov/11180229","citation_count":20,"is_preprint":false},{"pmid":"16159729","id":"PMC_16159729","title":"Compound heterozygosity for new splice site mutations in the plakophilin 1 gene (PKP1) in a Chinese case of ectodermal dysplasia-skin fragility syndrome.","date":"2005","source":"Acta dermato-venereologica","url":"https://pubmed.ncbi.nlm.nih.gov/16159729","citation_count":18,"is_preprint":false},{"pmid":"35182388","id":"PMC_35182388","title":"PKP1 and MYC create a feedforward loop linking transcription and translation in squamous cell lung cancer.","date":"2022","source":"Cellular oncology (Dordrecht, Netherlands)","url":"https://pubmed.ncbi.nlm.nih.gov/35182388","citation_count":13,"is_preprint":false},{"pmid":"36186467","id":"PMC_36186467","title":"Integrated analysis of bulk and single-cell RNA sequencing reveals the interaction of PKP1 and tumor-infiltrating B cells and their therapeutic potential for nasopharyngeal carcinoma.","date":"2022","source":"Frontiers in genetics","url":"https://pubmed.ncbi.nlm.nih.gov/36186467","citation_count":12,"is_preprint":false},{"pmid":"24073657","id":"PMC_24073657","title":"Ectodermal dysplasia-skin fragility syndrome: a novel mutation in the PKP1 gene.","date":"2013","source":"Clinical and experimental dermatology","url":"https://pubmed.ncbi.nlm.nih.gov/24073657","citation_count":10,"is_preprint":false},{"pmid":"37114302","id":"PMC_37114302","title":"Elucidation of the inhibitory potential of flavonoids against PKP1 protein in non-small cell lung cancer.","date":"2022","source":"Cellular and molecular biology (Noisy-le-Grand, France)","url":"https://pubmed.ncbi.nlm.nih.gov/37114302","citation_count":8,"is_preprint":false},{"pmid":"32495884","id":"PMC_32495884","title":"LncRNA APPAT regulated miR-328a/Pkp1 signal pathway to participate in breast cancer.","date":"2020","source":"European review for medical and pharmacological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/32495884","citation_count":6,"is_preprint":false},{"pmid":"40612005","id":"PMC_40612005","title":"KRT6A, KRT6B, PKP1, and PKP3 as key hub genes in esophageal cancer: A combined bioinformatics and experimental study.","date":"2025","source":"Biochemistry and biophysics 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Tumor Progression in Melanoma: Analysis of a Cell Adhesion-Related Model.","date":"2025","source":"Environmental toxicology","url":"https://pubmed.ncbi.nlm.nih.gov/39853870","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":13147,"output_tokens":1845,"usd":0.033558,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8746,"output_tokens":2895,"usd":0.058052,"stage2_stop_reason":"end_turn"},"total_usd":0.09161,"stage1_batch_id":"msgbatch_01CNoDbkChHEkgrJnL7BwtGJ","stage2_batch_id":"msgbatch_016Y4x9E9Wg5VQDANj9quSQD","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2017,\n      \"finding\": \"RIPK4 (receptor-interacting serine-threonine kinase 4) directly phosphorylates the N-terminal domain of PKP1 during epidermal differentiation; loss of function of either Pkp1 or Ripk4 impairs skin differentiation and enhances epidermal carcinogenesis in vivo, and phosphorylation of PKP1's N-terminal domain by RIPK4 is essential for their role in epidermal differentiation.\",\n      \"method\": \"Quantitative phosphoproteomics, mammalian kinome cDNA library screen, genome-editing (loss-of-function), mouse genetics, in vivo carcinogenesis models\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (phosphoproteomics, kinome screen, genome editing, mouse genetics) in a single rigorous study establishing PKP1 as a RIPK4 substrate with in vivo functional validation\",\n      \"pmids\": [\"28507225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PKP1 enhances MYC protein translation by binding to the 5'-UTR of MYC mRNA in conjunction with the translation initiation complex; conversely, MYC directly binds specific sequences within the PKP1 promoter to transcriptionally activate PKP1, forming a feedforward loop in squamous cell lung cancer.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), promoter mutagenesis with luciferase assays, gain/loss-of-function models, mRNA/protein expression analysis\",\n      \"journal\": \"Cellular oncology (Dordrecht, Netherlands)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal functional relationship established by ChIP, promoter mutagenesis, and gain/loss-of-function in a single lab with multiple orthogonal methods\",\n      \"pmids\": [\"35182388\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"PKP1 stabilizes platelet-type phosphofructokinase (PFKP), a key glycolytic enzyme, by binding to TRIM21 and preventing PFKP ubiquitination and proteasomal degradation; PKP1 depletion selectively reduces PFKP levels by enhancing its ubiquitination, and PFKP mediates the proliferative role of PKP1 in lung squamous cell carcinoma.\",\n      \"method\": \"Genome-wide CRISPR knockout screening, metabolic assays (OCR/ECAR), ubiquitination assays, functional rescue experiments, multiple LUSC cell lines\",\n      \"journal\": \"Biomarker research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR screen plus biochemical ubiquitination assays and functional rescue in multiple cell lines, single lab\",\n      \"pmids\": [\"40890861\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"PKP1 knockdown in Barrett's esophagus cell lines (which normally express PKP1) resulted in increased cell motility, indicating that PKP1 loss promotes cell migration and may contribute to disease progression via decreased desmosome assembly.\",\n      \"method\": \"siRNA knockdown, cell motility assay in CP-A and CP-D cell lines\",\n      \"journal\": \"Genes, chromosomes & cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single lab, loss-of-function with defined cellular phenotype (motility), two cell lines tested\",\n      \"pmids\": [\"22170739\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"High expression of PKP1 (and DSC2) facilitates cancer cell cluster formation in circulation, activates PI3K/AKT/Bcl-2-mediated cell survival signaling, and maintains high vimentin expression to stimulate fibronectin/integrin β1/FAK/Src/MEK/ERK/ZEB1-mediated metastasis; PKP1 overexpression enhances resistance to fluid shear stress.\",\n      \"method\": \"Microfluidic circulatory system selection of shear-stress-resistant cells, protein knockdown/overexpression, pathway inhibitor studies, in vivo metastasis in mice\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional experiments with defined pathway readouts and in vivo validation, single lab\",\n      \"pmids\": [\"34586853\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Homozygous splice site mutations in PKP1 result in complete loss of plakophilin-1 protein in the epidermis, leading to intraepidermal separation, widening of intercellular spaces, and abnormal desmosome ultrastructure, establishing PKP1 as a critical desmosomal plaque component required for normal epidermal cohesion.\",\n      \"method\": \"Molecular genetics (mutation analysis), immunohistochemistry, electron microscopy of patient skin biopsies\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — human loss-of-function genetics with direct protein and ultrastructural readout, two independent families\",\n      \"pmids\": [\"15086548\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A homozygous splice donor site mutation in intron 1 of canine PKP1 results in a premature stop codon and complete absence of plakophilin-1 protein, causing desmosomal abnormality (reduced number of partially formed desmosomes with detached keratin intermediate filaments) and redistribution of desmoplakin and keratins 10/14, phenocopying human ectodermal dysplasia-skin fragility syndrome.\",\n      \"method\": \"DNA sequencing, immunostaining, electron microscopy, histopathology in a natural animal model\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — natural loss-of-function model with protein-level and ultrastructural validation, independent of human studies\",\n      \"pmids\": [\"22384142\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PKP1 (plakophilin-1) is a desmosomal plaque component whose N-terminal domain is phosphorylated by RIPK4 to drive epidermal differentiation; it stabilizes the glycolytic enzyme PFKP by binding TRIM21 to prevent ubiquitin-mediated degradation, enhancing metabolic activity in lung squamous cell carcinoma; it promotes MYC translation via 5'-UTR binding (while MYC reciprocally transcribes PKP1), and its loss—whether through mutation, splice-site defects, or knockdown—disrupts desmosome assembly, increases cell motility, and impairs epithelial cohesion.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PKP1 (plakophilin-1) is a desmosomal plaque component required for epithelial cohesion and a regulator of epidermal differentiation, with additional roles in squamous carcinoma growth and metastasis [#0, #5]. Complete loss of PKP1 in human and canine epidermis through splice-site mutations abolishes the protein, producing intraepidermal separation, widened intercellular spaces, and abnormal desmosome ultrastructure with detached keratin filaments and redistributed desmoplakin, establishing it as essential for desmosome assembly and epithelial cohesion [#5, #6]. During epidermal differentiation, the kinase RIPK4 directly phosphorylates the N-terminal domain of PKP1, and this phosphorylation is essential for their shared role in differentiation and suppression of epidermal carcinogenesis [#0]. Beyond adhesion, PKP1 carries out cytoplasmic functions in cancer cells: it enhances MYC protein translation by binding the MYC 5'-UTR with the translation initiation complex while MYC reciprocally transactivates the PKP1 promoter, forming a feedforward loop in squamous lung cancer [#1], and it stabilizes the glycolytic enzyme PFKP by binding TRIM21 to block PFKP ubiquitination and proteasomal degradation, with PFKP mediating PKP1-driven proliferation [#2]. In epithelial cancers, loss of PKP1 increases cell motility [#3], whereas high PKP1 expression supports circulating tumor-cell cluster formation, shear-stress resistance, and PI3K/AKT- and integrin/FAK/ERK/ZEB1-mediated metastasis [#4].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Establishing whether PKP1 is genuinely required for human epidermal integrity, this work showed that complete loss of the protein disrupts desmosome structure and epithelial cohesion.\",\n      \"evidence\": \"Mutation analysis, immunohistochemistry, and electron microscopy of patient skin biopsies carrying homozygous PKP1 splice-site mutations\",\n      \"pmids\": [\"15086548\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not define which desmosomal partners PKP1 directly binds\", \"Mechanism linking plaque loss to intercellular separation not resolved at the molecular level\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Addressing whether PKP1 loss has consequences beyond adhesion failure, knockdown showed it actively restrains cell migration in epithelial cells.\",\n      \"evidence\": \"siRNA knockdown and cell motility assays in Barrett's esophagus CP-A and CP-D cell lines\",\n      \"pmids\": [\"22170739\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular pathway coupling PKP1 loss to increased motility not identified\", \"Only two cell lines tested; no in vivo confirmation\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Confirming the human phenotype was a direct consequence of PKP1 loss, a natural canine model recapitulated the desmosomal and keratin defects.\",\n      \"evidence\": \"DNA sequencing, immunostaining, electron microscopy, and histopathology of a dog carrying a homozygous PKP1 splice donor mutation\",\n      \"pmids\": [\"22384142\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not address PKP1 regulatory or signaling functions\", \"Mechanism of desmoplakin/keratin redistribution unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identifying an upstream regulator of PKP1, this study showed RIPK4 directly phosphorylates the PKP1 N-terminus and that this is essential for epidermal differentiation and tumor suppression.\",\n      \"evidence\": \"Quantitative phosphoproteomics, kinome cDNA library screen, genome editing, mouse genetics, and in vivo carcinogenesis models\",\n      \"pmids\": [\"28507225\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific phosphosites and how phosphorylation alters PKP1 function not fully mapped\", \"Downstream effectors of phosphorylated PKP1 in differentiation not defined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Reframing PKP1 as a metastasis driver, high expression was shown to promote tumor-cell cluster formation, shear-stress resistance, and survival/metastatic signaling.\",\n      \"evidence\": \"Microfluidic selection of shear-resistant cells, knockdown/overexpression, pathway inhibitor studies, and in vivo metastasis in mice\",\n      \"pmids\": [\"34586853\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular link between PKP1 and the PI3K/AKT and integrin/FAK/ERK pathways not biochemically defined\", \"Apparent contrast with loss-of-function promoting motility not reconciled\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Revealing a non-adhesive cytoplasmic function, PKP1 was shown to enhance MYC translation via 5'-UTR binding while MYC transactivates PKP1, forming a feedforward loop in squamous lung cancer.\",\n      \"evidence\": \"ChIP, promoter mutagenesis with luciferase assays, and gain/loss-of-function with mRNA/protein analysis\",\n      \"pmids\": [\"35182388\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct PKP1 binding to MYC 5'-UTR not demonstrated by structural or crosslinking methods\", \"Components of the initiation complex engaged by PKP1 not identified\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Extending PKP1's cytoplasmic role to metabolism, it was shown to stabilize the glycolytic enzyme PFKP by binding TRIM21 and blocking PFKP ubiquitination.\",\n      \"evidence\": \"Genome-wide CRISPR knockout screen, OCR/ECAR metabolic assays, ubiquitination assays, and functional rescue in multiple LUSC cell lines\",\n      \"pmids\": [\"40890861\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether PKP1 sequesters TRIM21 away from PFKP or otherwise inhibits the ligase not resolved\", \"Direct binary interactions among PKP1, TRIM21, and PFKP not structurally defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How PKP1's desmosomal adhesion role mechanistically integrates with its cytoplasmic functions in translation and metabolism, and how RIPK4 phosphorylation toggles between these roles, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model linking adhesion, MYC translation, and PFKP stabilization\", \"Phosphorylation-dependent switch between adhesive and signaling pools undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0045182\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [5, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [5, 6]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [5, 6]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [2, 4]}\n    ],\n    \"complexes\": [\"desmosome\"],\n    \"partners\": [\"RIPK4\", \"TRIM21\", \"PFKP\", \"MYC\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}