{"gene":"PKP1","run_date":"2026-04-28T19:45:44","timeline":{"discoveries":[{"year":2017,"finding":"RIPK4 (receptor-interacting serine-threonine kinase 4) directly phosphorylates PKP1's N-terminal domain during epidermal differentiation, and this phosphorylation is essential for PKP1's role in epidermal differentiation and suppression of epidermal carcinogenesis.","method":"Quantitative phosphoproteomics, mammalian kinome cDNA library screen, genome-editing (loss-of-function), mouse genetics, in vitro kinase assay","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including phosphoproteomics, kinase screen, genome-editing, and in vivo mouse genetics in a single study","pmids":["28507225"],"is_preprint":false},{"year":2004,"finding":"Loss-of-function mutations in PKP1 (splice site mutations) result in complete absence of plakophilin-1 protein in the epidermis, causing intraepidermal separation, widened intercellular spaces, and abnormal desmosome ultrastructure, demonstrating PKP1's essential role in desmosomal integrity and epidermal cohesion.","method":"Mutation analysis, immunostaining, electron microscopy, skin biopsy histopathology","journal":"The Journal of investigative dermatology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (genetics, protein absence, ultrastructure) replicated across multiple families and species","pmids":["15086548"],"is_preprint":false},{"year":2022,"finding":"PKP1 enhances MYC translation by binding to the 5'-UTR of MYC mRNA in conjunction with the translation initiation complex in squamous cell lung cancer, while MYC in turn acts as a direct transcription factor for PKP1 by binding to specific sequences in its promoter, forming a feedforward loop.","method":"ChIP, promoter mutagenesis with luciferase assay, gain/loss of function models, mRNA correlation analysis","journal":"Cellular oncology (Dordrecht, Netherlands)","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods (ChIP, luciferase, KD/OE) but single lab study","pmids":["35182388"],"is_preprint":false},{"year":2011,"finding":"Knockdown of PKP1 in Barrett's esophagus cell lines results in increased cell motility, indicating PKP1 suppresses migration and that loss of PKP1 (secondary to promoter methylation) may promote progression to esophageal adenocarcinoma.","method":"siRNA knockdown, cell motility assay, methylation analysis of primary tissue samples","journal":"Genes, chromosomes & cancer","confidence":"Medium","confidence_rationale":"Tier 2/3 — functional KD with defined phenotype (motility) plus epigenetic mechanism, single lab","pmids":["22170739"],"is_preprint":false},{"year":2021,"finding":"Elevated PKP1 and DSC2 expression in cancer cells facilitates cluster formation under fluid shear stress in circulation, activates PI3K/AKT/Bcl-2-mediated survival pathway, and maintains high vimentin expression to stimulate fibronectin/integrin β1/FAK/Src/MEK/ERK/ZEB1-mediated metastasis.","method":"Microfluidic circulatory system selection, siRNA knockdown, western blot, mouse metastasis models","journal":"Science advances","confidence":"Medium","confidence_rationale":"Tier 2 — functional KD with defined pathway readouts and in vivo validation, but PKP1 studied together with DSC2","pmids":["34586853"],"is_preprint":false},{"year":2025,"finding":"PKP1 stabilizes the glycolytic enzyme PFKP by binding to TRIM21 and preventing TRIM21-mediated ubiquitination and proteasomal degradation of PFKP, thereby promoting a hyperactive metabolic state (elevated OCR and ECAR) in lung squamous cell carcinoma cells.","method":"CRISPR knockout screen, metabolic assays (OCR/ECAR), ubiquitination assays, functional rescue experiments, co-immunoprecipitation","journal":"Biomarker research","confidence":"Medium","confidence_rationale":"Tier 2 — multiple orthogonal methods (CRISPR screen, metabolic assays, ubiquitination, rescue) in single lab study","pmids":["40890861"],"is_preprint":false},{"year":2012,"finding":"A homozygous splice donor site mutation within intron 1 of PKP1 in dogs causes a premature stop codon, resulting in complete absence of plakophilin-1 protein, reduced desmosome number, and detached keratin intermediate filaments, confirming the structural role of PKP1 in desmosome assembly.","method":"Sequencing, immunostaining, electron microscopy, histopathology","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — ortholog (canine) with loss-of-function genetic + protein + ultrastructural evidence","pmids":["22384142"],"is_preprint":false},{"year":2020,"finding":"LncRNA APPAT acts as a sponge for miR-328a, relieving miR-328a-mediated suppression of PKP1 protein expression; PKP1 functions downstream of this axis to regulate breast cancer cell proliferation, migration, and invasion.","method":"Luciferase reporter assay, western blot, siRNA knockdown, qPCR","journal":"European review for medical and pharmacological sciences","confidence":"Low","confidence_rationale":"Tier 3 — single lab, mechanistic follow-up is partial, PKP1 positioned as downstream target without direct functional domain interrogation","pmids":["32495884"],"is_preprint":false}],"current_model":"PKP1 (plakophilin-1) is a desmosomal plaque protein whose N-terminal domain is phosphorylated by RIPK4 to regulate epidermal differentiation; it also functions outside the desmosome by binding the 5'-UTR of MYC mRNA to enhance its translation, stabilizing PFKP from TRIM21-mediated ubiquitin-proteasomal degradation to support glycolytic metabolism, and loss of PKP1 disrupts desmosome assembly and increases cell motility, collectively establishing PKP1 as both a structural adhesion component and a multifunctional regulator of translation and metabolism."},"narrative":{"teleology":[{"year":2004,"claim":"Whether PKP1 is indispensable for desmosomal adhesion in vivo was resolved: human loss-of-function mutations showed complete absence of plakophilin-1 causes intraepidermal separation and aberrant desmosome ultrastructure, proving PKP1 is required for epidermal cohesion.","evidence":"Mutation analysis, immunostaining, and electron microscopy in skin biopsies from affected families","pmids":["15086548"],"confidence":"High","gaps":["Molecular mechanism by which PKP1 recruits desmosomal components or anchors intermediate filaments was not defined","Whether other plakophilins can partially compensate in specific epidermal layers remained unclear"]},{"year":2011,"claim":"PKP1's role beyond structural adhesion was expanded: knockdown in Barrett's esophagus cells increased motility, linking PKP1 loss to a migratory phenotype relevant to cancer progression.","evidence":"siRNA knockdown with cell motility assay and promoter methylation analysis in esophageal cell lines","pmids":["22170739"],"confidence":"Medium","gaps":["Downstream signaling pathways mediating motility suppression by PKP1 were not identified","In vivo validation of motility phenotype was lacking"]},{"year":2012,"claim":"Cross-species conservation of PKP1's desmosomal function was confirmed: a canine PKP1 splice-site mutation phenocopied the human disease with reduced desmosomes and detached keratin filaments.","evidence":"Sequencing, immunostaining, and electron microscopy in affected dog epidermis","pmids":["22384142"],"confidence":"Medium","gaps":["Single breed/pedigree study; broader allelic series not available","Mechanistic basis for filament detachment versus desmosome assembly failure not dissected"]},{"year":2017,"claim":"How PKP1 is activated during differentiation was answered: RIPK4 was identified as a direct kinase that phosphorylates the PKP1 N-terminal domain, and this phosphorylation is essential for epidermal differentiation and tumor suppression.","evidence":"Phosphoproteomics, kinome library screen, in vitro kinase assay, genome-editing, and mouse genetics","pmids":["28507225"],"confidence":"High","gaps":["Specific phosphoresidues and their individual contributions to differentiation versus adhesion not fully mapped","Whether RIPK4-PKP1 axis operates outside epidermis is unknown"]},{"year":2021,"claim":"PKP1's contribution to metastatic circulating tumor cell survival was defined: together with DSC2, elevated PKP1 supports cluster formation under shear stress and activates PI3K/AKT and MEK/ERK survival signaling.","evidence":"Microfluidic circulatory selection, siRNA knockdown, western blot, and mouse metastasis models","pmids":["34586853"],"confidence":"Medium","gaps":["PKP1's individual contribution versus DSC2 was not isolated","Whether PKP1's adhesion or signaling function drives the survival advantage is unresolved"]},{"year":2022,"claim":"A non-desmosomal RNA-regulatory function for PKP1 was established: PKP1 binds the 5′-UTR of MYC mRNA and enhances MYC translation, while MYC transcriptionally activates PKP1, forming a feedforward loop.","evidence":"ChIP, promoter mutagenesis with luciferase, gain/loss-of-function in squamous cell lung cancer lines","pmids":["35182388"],"confidence":"Medium","gaps":["Direct RNA-binding domain or motif in PKP1 responsible for 5′-UTR interaction not mapped","Whether this translational role extends beyond MYC or beyond lung squamous carcinoma is unknown"]},{"year":2025,"claim":"A metabolic regulatory mechanism for PKP1 was uncovered: PKP1 binds TRIM21 and prevents TRIM21-mediated ubiquitination and proteasomal degradation of PFKP, promoting glycolysis and oxidative metabolism in lung squamous cell carcinoma.","evidence":"CRISPR knockout screen, OCR/ECAR metabolic assays, ubiquitination assays, co-immunoprecipitation, and rescue experiments","pmids":["40890861"],"confidence":"Medium","gaps":["Structural basis of PKP1–TRIM21 interaction not determined","Whether this metabolic role is cancer-specific or relevant in normal epithelial physiology is unknown","Independent replication in additional cancer types or labs is lacking"]},{"year":null,"claim":"How the desmosomal adhesion, translational regulation, and metabolic stabilization functions of PKP1 are coordinated—and whether they are regulated by distinct post-translational modifications or subcellular pools—remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structural model of PKP1 in complex with its diverse binding partners exists","Whether RIPK4 phosphorylation modulates PKP1's non-desmosomal functions is unknown","Comprehensive identification of PKP1 RNA targets beyond MYC has not been performed"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[2]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[1,6]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[5]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,6]}],"pathway":[{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[1,6]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[0]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[5]}],"complexes":["desmosome"],"partners":["RIPK4","TRIM21","PFKP","DSC2","MYC"],"other_free_text":[]},"mechanistic_narrative":"PKP1 (plakophilin-1) is a desmosomal plaque protein essential for epidermal cell–cell adhesion that also functions outside the desmosome as a regulator of translation and metabolism. Loss-of-function mutations in PKP1 abolish protein expression, reduce desmosome number, detach keratin intermediate filaments, and cause intraepidermal separation, establishing PKP1 as a structural cornerstone of desmosomal integrity [PMID:15086548, PMID:22384142]. RIPK4-mediated phosphorylation of the PKP1 N-terminal domain is required for its function during epidermal differentiation and suppression of epidermal carcinogenesis [PMID:28507225]. Beyond adhesion, PKP1 binds the 5′-UTR of MYC mRNA to enhance its translation in a feedforward loop with MYC-driven PKP1 transcription [PMID:35182388], and stabilizes the glycolytic enzyme PFKP by sequestering TRIM21 to prevent PFKP ubiquitination and proteasomal degradation, thereby promoting glycolytic and oxidative metabolism [PMID:40890861]."},"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 antibodies PG-B6a and PG-B6p recognize, respectively, a highly conserved and a formol-resistant epitope on the human BCL-6 protein amino-terminal region.","date":"1996","source":"The American journal of pathology","url":"https://pubmed.ncbi.nlm.nih.gov/8623923","citation_count":90,"is_preprint":false},{"pmid":"12827610","id":"PMC_12827610","title":"Immunohistochemical localization of plakophilins (PKP1, PKP2, PKP3, and p0071) in primary oropharyngeal tumors: correlation with clinical parameters.","date":"2003","source":"Human pathology","url":"https://pubmed.ncbi.nlm.nih.gov/12827610","citation_count":62,"is_preprint":false},{"pmid":"28507225","id":"PMC_28507225","title":"Phosphorylation of Pkp1 by RIPK4 regulates epidermal differentiation and skin tumorigenesis.","date":"2017","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/28507225","citation_count":50,"is_preprint":false},{"pmid":"34586853","id":"PMC_34586853","title":"Desmosomal proteins of DSC2 and PKP1 promote 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":43,"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":"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":40,"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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toxicology","url":"https://pubmed.ncbi.nlm.nih.gov/39853870","citation_count":0,"is_preprint":false},{"pmid":"41514414","id":"PMC_41514414","title":"Multidimensional characterization of a novel porcine Klebsiella pneumoniae phage Pkp-1.","date":"2026","source":"BMC microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/41514414","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":12392,"output_tokens":1855,"usd":0.032501},"stage2":{"model":"claude-opus-4-6","input_tokens":5128,"output_tokens":2095,"usd":0.117023},"total_usd":0.149524,"stage1_batch_id":"msgbatch_01SUR8NDzVrNmyszWyJeCct7","stage2_batch_id":"msgbatch_01G7Yfutko3cDWzejrwH5AMx","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 PKP1's N-terminal domain during epidermal differentiation, and this phosphorylation is essential for PKP1's role in epidermal differentiation and suppression of epidermal carcinogenesis.\",\n      \"method\": \"Quantitative phosphoproteomics, mammalian kinome cDNA library screen, genome-editing (loss-of-function), mouse genetics, in vitro kinase assay\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including phosphoproteomics, kinase screen, genome-editing, and in vivo mouse genetics in a single study\",\n      \"pmids\": [\"28507225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Loss-of-function mutations in PKP1 (splice site mutations) result in complete absence of plakophilin-1 protein in the epidermis, causing intraepidermal separation, widened intercellular spaces, and abnormal desmosome ultrastructure, demonstrating PKP1's essential role in desmosomal integrity and epidermal cohesion.\",\n      \"method\": \"Mutation analysis, immunostaining, electron microscopy, skin biopsy histopathology\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (genetics, protein absence, ultrastructure) replicated across multiple families and species\",\n      \"pmids\": [\"15086548\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PKP1 enhances MYC translation by binding to the 5'-UTR of MYC mRNA in conjunction with the translation initiation complex in squamous cell lung cancer, while MYC in turn acts as a direct transcription factor for PKP1 by binding to specific sequences in its promoter, forming a feedforward loop.\",\n      \"method\": \"ChIP, promoter mutagenesis with luciferase assay, gain/loss of function models, mRNA correlation analysis\",\n      \"journal\": \"Cellular oncology (Dordrecht, Netherlands)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (ChIP, luciferase, KD/OE) but single lab study\",\n      \"pmids\": [\"35182388\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Knockdown of PKP1 in Barrett's esophagus cell lines results in increased cell motility, indicating PKP1 suppresses migration and that loss of PKP1 (secondary to promoter methylation) may promote progression to esophageal adenocarcinoma.\",\n      \"method\": \"siRNA knockdown, cell motility assay, methylation analysis of primary tissue samples\",\n      \"journal\": \"Genes, chromosomes & cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2/3 — functional KD with defined phenotype (motility) plus epigenetic mechanism, single lab\",\n      \"pmids\": [\"22170739\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Elevated PKP1 and DSC2 expression in cancer cells facilitates cluster formation under fluid shear stress in circulation, activates PI3K/AKT/Bcl-2-mediated survival pathway, and maintains high vimentin expression to stimulate fibronectin/integrin β1/FAK/Src/MEK/ERK/ZEB1-mediated metastasis.\",\n      \"method\": \"Microfluidic circulatory system selection, siRNA knockdown, western blot, mouse metastasis models\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional KD with defined pathway readouts and in vivo validation, but PKP1 studied together with DSC2\",\n      \"pmids\": [\"34586853\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"PKP1 stabilizes the glycolytic enzyme PFKP by binding to TRIM21 and preventing TRIM21-mediated ubiquitination and proteasomal degradation of PFKP, thereby promoting a hyperactive metabolic state (elevated OCR and ECAR) in lung squamous cell carcinoma cells.\",\n      \"method\": \"CRISPR knockout screen, metabolic assays (OCR/ECAR), ubiquitination assays, functional rescue experiments, co-immunoprecipitation\",\n      \"journal\": \"Biomarker research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (CRISPR screen, metabolic assays, ubiquitination, rescue) in single lab study\",\n      \"pmids\": [\"40890861\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A homozygous splice donor site mutation within intron 1 of PKP1 in dogs causes a premature stop codon, resulting in complete absence of plakophilin-1 protein, reduced desmosome number, and detached keratin intermediate filaments, confirming the structural role of PKP1 in desmosome assembly.\",\n      \"method\": \"Sequencing, immunostaining, electron microscopy, histopathology\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ortholog (canine) with loss-of-function genetic + protein + ultrastructural evidence\",\n      \"pmids\": [\"22384142\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"LncRNA APPAT acts as a sponge for miR-328a, relieving miR-328a-mediated suppression of PKP1 protein expression; PKP1 functions downstream of this axis to regulate breast cancer cell proliferation, migration, and invasion.\",\n      \"method\": \"Luciferase reporter assay, western blot, siRNA knockdown, qPCR\",\n      \"journal\": \"European review for medical and pharmacological sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, mechanistic follow-up is partial, PKP1 positioned as downstream target without direct functional domain interrogation\",\n      \"pmids\": [\"32495884\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PKP1 (plakophilin-1) is a desmosomal plaque protein whose N-terminal domain is phosphorylated by RIPK4 to regulate epidermal differentiation; it also functions outside the desmosome by binding the 5'-UTR of MYC mRNA to enhance its translation, stabilizing PFKP from TRIM21-mediated ubiquitin-proteasomal degradation to support glycolytic metabolism, and loss of PKP1 disrupts desmosome assembly and increases cell motility, collectively establishing PKP1 as both a structural adhesion component and a multifunctional regulator of translation and metabolism.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PKP1 (plakophilin-1) is a desmosomal plaque protein essential for epidermal cell–cell adhesion that also functions outside the desmosome as a regulator of translation and metabolism. Loss-of-function mutations in PKP1 abolish protein expression, reduce desmosome number, detach keratin intermediate filaments, and cause intraepidermal separation, establishing PKP1 as a structural cornerstone of desmosomal integrity [PMID:15086548, PMID:22384142]. RIPK4-mediated phosphorylation of the PKP1 N-terminal domain is required for its function during epidermal differentiation and suppression of epidermal carcinogenesis [PMID:28507225]. Beyond adhesion, PKP1 binds the 5′-UTR of MYC mRNA to enhance its translation in a feedforward loop with MYC-driven PKP1 transcription [PMID:35182388], and stabilizes the glycolytic enzyme PFKP by sequestering TRIM21 to prevent PFKP ubiquitination and proteasomal degradation, thereby promoting glycolytic and oxidative metabolism [PMID:40890861].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Whether PKP1 is indispensable for desmosomal adhesion in vivo was resolved: human loss-of-function mutations showed complete absence of plakophilin-1 causes intraepidermal separation and aberrant desmosome ultrastructure, proving PKP1 is required for epidermal cohesion.\",\n      \"evidence\": \"Mutation analysis, immunostaining, and electron microscopy in skin biopsies from affected families\",\n      \"pmids\": [\"15086548\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular mechanism by which PKP1 recruits desmosomal components or anchors intermediate filaments was not defined\",\n        \"Whether other plakophilins can partially compensate in specific epidermal layers remained unclear\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"PKP1's role beyond structural adhesion was expanded: knockdown in Barrett's esophagus cells increased motility, linking PKP1 loss to a migratory phenotype relevant to cancer progression.\",\n      \"evidence\": \"siRNA knockdown with cell motility assay and promoter methylation analysis in esophageal cell lines\",\n      \"pmids\": [\"22170739\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Downstream signaling pathways mediating motility suppression by PKP1 were not identified\",\n        \"In vivo validation of motility phenotype was lacking\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Cross-species conservation of PKP1's desmosomal function was confirmed: a canine PKP1 splice-site mutation phenocopied the human disease with reduced desmosomes and detached keratin filaments.\",\n      \"evidence\": \"Sequencing, immunostaining, and electron microscopy in affected dog epidermis\",\n      \"pmids\": [\"22384142\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single breed/pedigree study; broader allelic series not available\",\n        \"Mechanistic basis for filament detachment versus desmosome assembly failure not dissected\"\n      ]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"How PKP1 is activated during differentiation was answered: RIPK4 was identified as a direct kinase that phosphorylates the PKP1 N-terminal domain, and this phosphorylation is essential for epidermal differentiation and tumor suppression.\",\n      \"evidence\": \"Phosphoproteomics, kinome library screen, in vitro kinase assay, genome-editing, and mouse genetics\",\n      \"pmids\": [\"28507225\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Specific phosphoresidues and their individual contributions to differentiation versus adhesion not fully mapped\",\n        \"Whether RIPK4-PKP1 axis operates outside epidermis is unknown\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"PKP1's contribution to metastatic circulating tumor cell survival was defined: together with DSC2, elevated PKP1 supports cluster formation under shear stress and activates PI3K/AKT and MEK/ERK survival signaling.\",\n      \"evidence\": \"Microfluidic circulatory selection, siRNA knockdown, western blot, and mouse metastasis models\",\n      \"pmids\": [\"34586853\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"PKP1's individual contribution versus DSC2 was not isolated\",\n        \"Whether PKP1's adhesion or signaling function drives the survival advantage is unresolved\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"A non-desmosomal RNA-regulatory function for PKP1 was established: PKP1 binds the 5′-UTR of MYC mRNA and enhances MYC translation, while MYC transcriptionally activates PKP1, forming a feedforward loop.\",\n      \"evidence\": \"ChIP, promoter mutagenesis with luciferase, gain/loss-of-function in squamous cell lung cancer lines\",\n      \"pmids\": [\"35182388\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Direct RNA-binding domain or motif in PKP1 responsible for 5′-UTR interaction not mapped\",\n        \"Whether this translational role extends beyond MYC or beyond lung squamous carcinoma is unknown\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"A metabolic regulatory mechanism for PKP1 was uncovered: PKP1 binds TRIM21 and prevents TRIM21-mediated ubiquitination and proteasomal degradation of PFKP, promoting glycolysis and oxidative metabolism in lung squamous cell carcinoma.\",\n      \"evidence\": \"CRISPR knockout screen, OCR/ECAR metabolic assays, ubiquitination assays, co-immunoprecipitation, and rescue experiments\",\n      \"pmids\": [\"40890861\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Structural basis of PKP1–TRIM21 interaction not determined\",\n        \"Whether this metabolic role is cancer-specific or relevant in normal epithelial physiology is unknown\",\n        \"Independent replication in additional cancer types or labs is lacking\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the desmosomal adhesion, translational regulation, and metabolic stabilization functions of PKP1 are coordinated—and whether they are regulated by distinct post-translational modifications or subcellular pools—remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No structural model of PKP1 in complex with its diverse binding partners exists\",\n        \"Whether RIPK4 phosphorylation modulates PKP1's non-desmosomal functions is unknown\",\n        \"Comprehensive identification of PKP1 RNA targets beyond MYC has not been performed\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"complexes\": [\n      \"desmosome\"\n    ],\n    \"partners\": [\n      \"RIPK4\",\n      \"TRIM21\",\n      \"PFKP\",\n      \"DSC2\",\n      \"MYC\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}