{"gene":"CBX4","run_date":"2026-04-28T17:28:52","timeline":{"discoveries":[{"year":2003,"finding":"CBX4 (Pc2) functions as a SUMO E3 ligase, dramatically enhancing sumoylation of the corepressor CtBP by recruiting both CtBP and the SUMO E2 enzyme Ubc9 to polycomb bodies, thereby bringing substrate and E2 into proximity.","method":"In vitro sumoylation assay with purified components; co-immunoprecipitation; live-cell imaging of polycomb bodies","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro sumoylation plus cellular localization studies, foundational paper with high citations","pmids":["12679040"],"is_preprint":false},{"year":2005,"finding":"CBX4 (Pc2) acts as a SUMO E3 ligase for Smad-interacting protein 1 (SIP1), sumoylating it at Lys391 and Lys866; sumoylation disrupts SIP1 recruitment of CtBP and attenuates E-cadherin transcriptional repression in a promoter-context-dependent manner.","method":"In vivo and in vitro sumoylation assays; co-immunoprecipitation; luciferase reporter assays; site-directed mutagenesis of sumoylation sites","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro sumoylation reconstitution plus mutagenesis and functional reporter assays","pmids":["16061479"],"is_preprint":false},{"year":2006,"finding":"CBX4 (Pc2) serves as a SUMO E3 ligase for HIPK2; DNA damage-induced phosphorylation of Pc2 at multiple sites (including Thr495) by HIPK2 is required for Pc2-mediated sumoylation of HIPK2, establishing an autoregulatory feedback loop that enhances HIPK2-dependent transcriptional repression.","method":"Co-immunoprecipitation; in vitro kinase and sumoylation assays; site-directed mutagenesis; immunofluorescence","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic assays plus mutagenesis with defined functional readout","pmids":["17018294"],"is_preprint":false},{"year":2007,"finding":"CBX4 (Cbx4) functions as a SUMO E3 ligase for the de novo DNA methyltransferase Dnmt3a, promoting its SUMO-1 modification at the N-terminal regulatory region including the PWWP domain; purified Cbx4 directly promotes Dnmt3a sumoylation in vitro.","method":"Co-immunoprecipitation; in vitro sumoylation assay with purified Cbx4; transfection co-expression","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — in vitro sumoylation reconstitution with purified protein, confirmed in cells","pmids":["17439403"],"is_preprint":false},{"year":2010,"finding":"CBX4 (Pc2) contains two functional SUMO interaction motifs (SIMs) that are required for full E3 ligase activity; the SIMs contribute to Pc2 self-sumoylation, CtBP sumoylation, and enrichment of SUMO at polycomb foci, and interact with the SIM-binding surface of both SUMO1 and SUMO2.","method":"Mutagenesis of SIMs; in vivo sumoylation assays; immunofluorescence; SUMO isoform mutant analysis","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis combined with multiple orthogonal assays; independently corroborated by Yang & Sharrocks (PMID 20176810)","pmids":["20098713","20176810"],"is_preprint":false},{"year":2010,"finding":"CBX4 (Pc2) SIM2 is crucial for its E3 ligase activity by enabling subnuclear partitioning of the active SUMO~Ubc9 thioester intermediate and controlling lineage-specific gene expression during embryonic stem cell differentiation.","method":"SIM mutagenesis; in vivo sumoylation assays; Ubc9 interaction studies; ES cell differentiation assays","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with mechanistic dissection of SUMO~Ubc9 recruitment","pmids":["20176810"],"is_preprint":false},{"year":2004,"finding":"CBX4 (Pc2) E3 ligase activity requires two distinct domains: a C-terminal adapter domain that recruits both Ubc9 and CtBP, and a second N-terminal domain with intrinsic E3 activity in vitro; adapter function is required to selectively co-recruit E2 and substrate in vivo.","method":"Domain deletion and mutagenesis; in vitro sumoylation assays with domain fragments; in vivo sumoylation assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution plus domain mapping with mutagenesis","pmids":["15592428"],"is_preprint":false},{"year":2012,"finding":"CBX4 is recruited to DNA damage sites in a PARP-dependent manner (independent of H2AX, RNF8, BMI1, and PI3-related kinases) and acts as a SUMO E3 ligase for BMI1, sumoylating it at lysine 88; this sumoylation is required for BMI1 accumulation at DNA lesions and for cellular resistance to ionizing radiation.","method":"Laser micro-irradiation with live-cell imaging; co-immunoprecipitation; in vitro sumoylation; siRNA knockdown with clonogenic survival assay; site-directed mutagenesis of BMI1-K88","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 — multiple orthogonal methods including in vitro sumoylation, mutagenesis, and functional radiation resistance assay","pmids":["22402492"],"is_preprint":false},{"year":2017,"finding":"CBX4 SUMO E3 ligase constitutively sumoylates CtIP at lysine 896, which is required for CtIP recruitment to DNA double-strand breaks and for DNA end resection; CBX4 depletion or non-sumoylatable CtIP-K896R blocks homologous recombination and increases genomic instability; artificial CtIP-SUMO fusion rescues these defects.","method":"siRNA depletion; site-directed mutagenesis (K896R); resection assays; HR reporter assay; CtIP-SUMO fusion rescue; laser micro-irradiation imaging","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal assays with genetic rescue demonstrating pathway placement","pmids":["28740167"],"is_preprint":false},{"year":2016,"finding":"CBX4 suppresses colorectal carcinoma metastasis by repressing Runx2 transcription through recruiting HDAC3 to the Runx2 promoter, maintaining deacetylated H3K27; this function requires the CBX4-HDAC3 interaction but is independent of its SUMO E3 ligase activity, chromodomain, or PRC1 complex.","method":"Co-immunoprecipitation; chromatin immunoprecipitation; domain mutagenesis; migration/invasion assays; in vivo metastasis model","journal":"Cancer research","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, ChIP, domain dissection, and in vivo functional validation","pmids":["27864346"],"is_preprint":false},{"year":2020,"finding":"CBX4 promotes osteosarcoma metastasis by transcriptionally upregulating Runx2 via recruitment of the acetyltransferase GCN5 to the Runx2 promoter; CK1α phosphorylates CBX4 at T437, facilitating its ubiquitination at K178 and K280 and subsequent CHIP-mediated degradation; TNFα reduces this phosphorylation to stabilize CBX4.","method":"Co-immunoprecipitation; ChIP; phosphorylation and ubiquitination assays; site-directed mutagenesis; kinase assays; in vivo metastasis model","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro kinase assay, mutagenesis of phospho/ubiquitin sites, ChIP, and in vivo validation","pmids":["32111827"],"is_preprint":false},{"year":2019,"finding":"CBX4 counteracts human mesenchymal stem cell senescence by recruiting nucleolar proteins fibrillarin (FBL) and KAP1 to rDNA loci, limiting excessive rRNA expression, ribosome biogenesis, and protein translation; CBX4 knockout destabilizes nucleolar heterochromatin and accelerates senescence.","method":"CBX4 knockout in hMSCs; RNA-seq; ChIP; co-immunoprecipitation; rRNA quantification; ribosome profiling; mouse osteoarthritis model with CBX4 overexpression","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function with multiple orthogonal molecular readouts and in vivo rescue","pmids":["30917318"],"is_preprint":false},{"year":2018,"finding":"Cbx4 is a SUMO E3 ligase for Prdm16 in brown/beige adipocytes; Cbx4-mediated sumoylation of Prdm16 at K917 blocks its ubiquitination-mediated degradation, stabilizes Prdm16, augments thermogenic gene expression, and is required for white fat browning; this sumoylation also primes Prdm16 for further stabilization by methyltransferase Ehmt1.","method":"In vitro sumoylation assay; site-directed mutagenesis (K917R); ubiquitination assay; fat-specific Cbx4 knockout and overexpression; cold-exposure experiments; thermogenic gene expression analysis","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1 — in vitro sumoylation reconstitution, mutagenesis, and in vivo fat-specific genetic models","pmids":["29539416"],"is_preprint":false},{"year":2013,"finding":"Cbx4 is essential for thymic epithelial cell (TEC) proliferation and thymus organogenesis; cell-specific deletion shows the defect is intrinsic to TECs; Cbx4 physically interacts with p63 transcription factor, and the two functionally cooperate to maintain TEC stemness and proliferative capacity.","method":"Conditional Cbx4 knockout mice; BrdU proliferation assay; co-immunoprecipitation of Cbx4 with p63; histology and flow cytometry of thymic compartments","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 — conditional KO with defined cellular phenotype plus Co-IP of binding partner","pmids":["23362346"],"is_preprint":false},{"year":2015,"finding":"Cbx4 maintains epithelial lineage identity and keratinocyte proliferation in developing epidermis by repressing neuronal gene programs and cyclin-dependent kinase inhibitors (p16/p19, p57); its chromodomain and SUMO E3 ligase activities differentially regulate proliferation, differentiation, and nonepidermal gene repression; Cbx4 expression is directly regulated by p63.","method":"Cbx4 ablation in mice; domain-specific mutant rescue experiments; ChIP; RNA-seq; p63 binding to Cbx4 promoter","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — in vivo KO with domain dissection and upstream regulatory mapping","pmids":["26711500"],"is_preprint":false},{"year":2005,"finding":"CBX4 (HPC2) interacts with the LIM domain protein KyoT2 through its C-terminal fragment; this interaction enables CBX4 to inhibit RBP-J-mediated transcriptional activation by Notch intracellular domain (NIC) and by constitutively active RBP-J-VP16.","method":"Yeast two-hybrid; GST pulldown; co-immunoprecipitation; mammalian two-hybrid; luciferase reporter assay","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 — multiple binding assays plus functional reporter, single lab","pmids":["15710417"],"is_preprint":false},{"year":2017,"finding":"CBX4 acts as a ubiquitin E3 ligase for HDAC7, mediating its ubiquitin-dependent degradation in the dorsal hippocampus after contextual fear conditioning; this degradation derepresses Nur77 and promotes long-term memory formation.","method":"Co-immunoprecipitation; ubiquitination assay identifying CBX4 as E3 for HDAC7; in vivo hippocampal manipulation; contextual fear conditioning behavioral assay","journal":"The Journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP plus in vivo functional assay, single lab, moderate mechanistic depth","pmids":["28283560"],"is_preprint":false},{"year":2020,"finding":"CBX4 transcriptionally suppresses KLF6 in clear cell renal cell carcinoma by physically binding HDAC1 and maintaining HDAC1 localization on the KLF6 promoter; disruption of CBX4-HDAC1 interaction attenuates CBX4-mediated tumor growth and migration.","method":"Co-immunoprecipitation; ChIP; luciferase reporter assay; domain disruption experiments; in vivo xenograft model","journal":"EBioMedicine","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP, ChIP, and in vivo validation, single lab","pmids":["32113161"],"is_preprint":false},{"year":2024,"finding":"CBX4 sumoylates YAP1 at K97 and K280 (SUMO1 modification), which competitively inhibits YAP1-S127 phosphorylation, preserves YAP1 stability, and promotes its cytoplasm-to-nucleus transport, thereby inactivating the Hippo pathway and countering cellular senescence and chemoresistance in gastric cancer; CEBPB transcriptionally downregulates CBX4 in response to low-dose cisplatin.","method":"Co-immunoprecipitation; ChIP; sumoylation assays; high-throughput sequencing; site-directed mutagenesis of SUMO sites; in vitro and in vivo tumor models","journal":"Drug resistance updates","confidence":"Medium","confidence_rationale":"Tier 2 — multiple molecular assays with in vivo validation, single lab","pmids":["39154499"],"is_preprint":false}],"current_model":"CBX4 (also called Pc2/HPC2) is a polycomb group protein that functions primarily as a SUMO E3 ligase—using two functional SUMO interaction motifs (SIMs) and two distinct catalytic/adapter domains to sumoylate substrates including CtBP, SIP1, HIPK2, Dnmt3a, BMI1, CtIP, Prdm16, and YAP1—and also acts as a ubiquitin E3 ligase (e.g., for HDAC7), recruits HDAC3/HDAC1 to gene promoters to repress transcription independently of its SUMO ligase activity, localizes to polycomb bodies in a PARP-dependent manner at DNA damage sites to coordinate homologous recombination, maintains nucleolar heterochromatin homeostasis to restrain ribosome biogenesis and cellular senescence, and is itself regulated by CK1α-mediated phosphorylation that triggers its ubiquitin-dependent degradation."},"narrative":{"teleology":[{"year":2003,"claim":"Establishing that a Polycomb group protein could function as a SUMO E3 ligase resolved how sumoylation substrates are brought together with the E2 enzyme Ubc9 at discrete nuclear foci (Polycomb bodies).","evidence":"In vitro sumoylation reconstitution with purified CBX4/Ubc9/CtBP plus live-cell imaging of Polycomb bodies","pmids":["12679040"],"confidence":"High","gaps":["Structural basis of CBX4-Ubc9-substrate ternary complex unresolved","Whether SUMO E3 activity is required for canonical PRC1-mediated gene silencing unclear"]},{"year":2004,"claim":"Domain mapping revealed that CBX4 uses two mechanistically distinct regions—an N-terminal domain with intrinsic E3 activity and a C-terminal adapter that co-recruits E2 and substrate—explaining how a single protein achieves selective SUMO conjugation.","evidence":"Domain deletion/mutagenesis with in vitro and in vivo sumoylation assays","pmids":["15592428"],"confidence":"High","gaps":["Which domain contacts Ubc9 catalytic cysteine not mapped","No crystal structure of either E3-active domain"]},{"year":2005,"claim":"Expanding the substrate repertoire to SIP1 and demonstrating that sumoylation disrupts a corepressor interaction (SIP1-CtBP) showed CBX4-mediated SUMO conjugation as a transcriptional regulatory switch rather than a single-substrate phenomenon.","evidence":"In vitro/in vivo sumoylation with site-directed mutagenesis plus E-cadherin promoter reporter assays","pmids":["16061479"],"confidence":"High","gaps":["In vivo chromatin-level effects of SIP1 sumoylation not assessed","Interaction with KyoT2/Notch pathway studied only by single lab (PMID:15710417)"]},{"year":2006,"claim":"Identification of an autoregulatory feedback loop—HIPK2 phosphorylates CBX4 to activate its SUMO E3 ligase toward HIPK2 itself—demonstrated that DNA damage signaling dynamically tunes CBX4 activity.","evidence":"In vitro kinase/sumoylation assays with phosphosite mutagenesis","pmids":["17018294"],"confidence":"High","gaps":["Which phosphatase reverses HIPK2-mediated CBX4 phosphorylation unknown","Physiological conditions activating this loop beyond DNA damage not tested"]},{"year":2007,"claim":"Demonstration that CBX4 sumoylates the DNA methyltransferase Dnmt3a linked Polycomb SUMO ligase activity to epigenetic DNA methylation regulation.","evidence":"In vitro sumoylation with purified CBX4 and Dnmt3a plus cellular co-expression","pmids":["17439403"],"confidence":"High","gaps":["Functional consequence of Dnmt3a sumoylation on methyltransferase activity or genomic targeting not determined","No in vivo genetic validation"]},{"year":2010,"claim":"Discovery of two functional SUMO-interaction motifs (SIMs) in CBX4—especially SIM2—resolved how the enzyme partitions the active SUMO~Ubc9 thioester to Polycomb foci and controls lineage-specific gene expression during ES cell differentiation.","evidence":"SIM mutagenesis with in vivo sumoylation assays, Ubc9 interaction studies, and ES cell differentiation","pmids":["20098713","20176810"],"confidence":"High","gaps":["Genome-wide targets of SIM-dependent SUMO conjugation in differentiating ES cells not mapped","Whether SIM1 and SIM2 engage different SUMO paralogs in vivo remains unclear"]},{"year":2012,"claim":"Showing that CBX4 is recruited to DNA damage sites in a PARP-dependent manner and sumoylates BMI1 at K88 to enable its damage-site accumulation placed CBX4 upstream in the Polycomb-mediated DNA damage response.","evidence":"Laser micro-irradiation, in vitro sumoylation, BMI1-K88 mutagenesis, and clonogenic survival assays","pmids":["22402492"],"confidence":"High","gaps":["Which PAR-binding domain on CBX4 mediates PARP-dependent recruitment not identified","Relationship between BMI1 sumoylation and ubiquitin ligase activity of PRC1 at damage sites not clarified"]},{"year":2013,"claim":"Conditional knockout demonstrated that CBX4 is cell-intrinsically required for thymic epithelial cell proliferation and thymus organogenesis, acting through physical interaction with the transcription factor p63.","evidence":"TEC-specific Cbx4 conditional knockout mice; BrdU assays; Co-IP of Cbx4-p63","pmids":["23362346"],"confidence":"High","gaps":["Whether p63 is a SUMO substrate of CBX4 in TECs not tested","Transcriptional targets co-regulated by CBX4-p63 not identified genome-wide"]},{"year":2015,"claim":"Domain-specific rescue in Cbx4-null epidermis dissected the contributions of the chromodomain (repression of non-epidermal genes) versus SUMO E3 ligase domain (proliferation control via CDK inhibitor repression), and showed p63 directly regulates Cbx4 transcription, completing a reciprocal regulatory circuit.","evidence":"Cbx4 ablation in mouse skin with domain-mutant rescue; ChIP; RNA-seq; p63 binding at Cbx4 promoter","pmids":["26711500"],"confidence":"High","gaps":["SUMO substrates mediating CDK inhibitor repression in epidermis unidentified","Whether chromodomain-dependent gene repression involves canonical PRC1 H2AK119ub not resolved"]},{"year":2016,"claim":"Discovery that CBX4 represses Runx2 transcription by recruiting HDAC3 to its promoter—independently of SUMO ligase activity, chromodomain, or PRC1—revealed a non-canonical, HDAC-dependent transcriptional repression function.","evidence":"ChIP, reciprocal Co-IP, domain mutagenesis, and in vivo colorectal cancer metastasis model","pmids":["27864346"],"confidence":"High","gaps":["How CBX4 is targeted to the Runx2 promoter without chromodomain engagement unknown","Whether HDAC3 recruitment is a general mechanism or specific to Runx2"]},{"year":2017,"claim":"Two parallel advances linked CBX4 to homologous recombination (via CtIP sumoylation enabling DNA end resection) and to memory formation (via ubiquitin-dependent HDAC7 degradation), broadening CBX4's enzymatic repertoire to include ubiquitin E3 ligase activity.","evidence":"CtIP-K896R mutagenesis with HR reporter and resection assays (PMID:28740167); Co-IP and in vivo hippocampal manipulation for HDAC7 ubiquitination (PMID:28283560)","pmids":["28740167","28283560"],"confidence":"High","gaps":["Whether CBX4 ubiquitin ligase activity uses a RING-dependent mechanism or distinct catalytic site unknown","CtIP sumoylation by CBX4 not confirmed in non-cancer cell lines","HDAC7 ubiquitination finding from single lab"]},{"year":2018,"claim":"Identification of Prdm16 as a CBX4 SUMO substrate in adipocytes—where sumoylation at K917 blocks ubiquitin-mediated Prdm16 degradation—established CBX4 as a regulator of thermogenic fat programming and white fat browning.","evidence":"In vitro sumoylation, K917R mutagenesis, fat-specific Cbx4 knockout/overexpression, and cold-exposure experiments","pmids":["29539416"],"confidence":"High","gaps":["Which ubiquitin ligase targets Prdm16 in competition with SUMO not identified","Contribution of other CBX family members in adipocytes not assessed"]},{"year":2019,"claim":"Demonstrating that CBX4 maintains nucleolar heterochromatin by recruiting fibrillarin and KAP1 to rDNA loci, thereby restraining rRNA synthesis and ribosome biogenesis, revealed a mechanism linking Polycomb function to senescence control.","evidence":"CBX4 knockout in human mesenchymal stem cells; ChIP at rDNA; RNA-seq; ribosome profiling; mouse osteoarthritis model rescue","pmids":["30917318"],"confidence":"High","gaps":["Whether SUMO ligase activity is required for fibrillarin/KAP1 recruitment not tested","Mechanism of CBX4 targeting to rDNA repeats not elucidated"]},{"year":2020,"claim":"CK1α was identified as the kinase that phosphorylates CBX4 at T437, triggering CHIP-mediated ubiquitination and degradation, establishing the first defined post-translational pathway controlling CBX4 protein turnover; TNFα stabilizes CBX4 by suppressing this phosphorylation.","evidence":"In vitro kinase assays, T437 mutagenesis, ubiquitination assays identifying K178/K280, and in vivo osteosarcoma model","pmids":["32111827"],"confidence":"High","gaps":["Whether CK1α-mediated degradation operates in non-cancer cell types unknown","Context-dependent switch between CBX4-mediated Runx2 repression (colorectal) and activation (osteosarcoma) not mechanistically resolved"]},{"year":2024,"claim":"Sumoylation of YAP1 by CBX4 at K97/K280 was shown to competitively block S127 phosphorylation, stabilize YAP1, and promote its nuclear entry, linking CBX4's SUMO ligase activity to Hippo pathway inactivation.","evidence":"Sumoylation assays, SUMO-site mutagenesis, high-throughput sequencing, and gastric cancer tumor models","pmids":["39154499"],"confidence":"Medium","gaps":["Mechanism by which SUMO conjugation blocks LATS-mediated phosphorylation not structurally defined","Single lab; independent confirmation needed","Whether this applies outside gastric cancer contexts untested"]},{"year":null,"claim":"No structural model exists for CBX4 in complex with Ubc9 or any substrate, and the mechanistic basis for substrate selectivity among its growing list of SUMO targets remains undefined; additionally, how SUMO ligase-dependent and HDAC-dependent functions are coordinated at specific genomic loci is unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No crystal or cryo-EM structure of CBX4 E3 ligase domain or CBX4-Ubc9 complex","Genome-wide mapping of CBX4-SUMO versus CBX4-HDAC target gene sets not performed","Whether ubiquitin E3 ligase activity uses the same or distinct catalytic surface as SUMO E3 activity unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,1,2,3,4,5,6,7,8,12,18]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,2,3,7,8,12,16,18]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[9,14,17]},{"term_id":"GO:0042393","term_label":"histone binding","supporting_discovery_ids":[9,17]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,4,7,8,9,11,14,17]},{"term_id":"GO:0005730","term_label":"nucleolus","supporting_discovery_ids":[11]},{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[7,8,9,17]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[7,8]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[0,4,9,11,14]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,2,3,7,8,12,16,18]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[9,14,17]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[13,14]}],"complexes":["PRC1"],"partners":["UBC9","BMI1","CTBP1","HDAC3","HDAC1","TP63","HIPK2","CTIP"],"other_free_text":[]},"mechanistic_narrative":"CBX4 (Pc2/HPC2) is a Polycomb group protein that functions as a SUMO E3 ligase and transcriptional regulator with diverse roles in chromatin silencing, DNA damage repair, lineage specification, and senescence control. As a SUMO E3 ligase, CBX4 uses two SUMO-interaction motifs and dual catalytic/adapter domains to recruit Ubc9 and substrates—including CtBP, SIP1, HIPK2, Dnmt3a, BMI1, CtIP, Prdm16, and YAP1—to Polycomb bodies, thereby controlling processes ranging from homologous recombination (via CtIP sumoylation enabling DNA end resection) and ionizing-radiation resistance (via BMI1 sumoylation at damage sites) to thermogenic fat programming (via Prdm16 stabilization) [PMID:12679040, PMID:22402492, PMID:28740167, PMID:29539416, PMID:39154499]. Independently of its SUMO ligase activity, CBX4 represses transcription by recruiting HDAC3 or HDAC1 to target promoters, and it also possesses ubiquitin E3 ligase activity toward HDAC7 [PMID:27864346, PMID:32113161, PMID:28283560]. CBX4 maintains nucleolar heterochromatin to restrain ribosome biogenesis and counteract cellular senescence, is essential for epithelial lineage identity through cooperation with p63, and is itself regulated by CK1α-mediated phosphorylation that triggers its ubiquitin-dependent degradation [PMID:30917318, PMID:26711500, PMID:32111827]."},"prefetch_data":{"uniprot":{"accession":"O00257","full_name":"E3 SUMO-protein ligase CBX4","aliases":["Chromobox protein homolog 4","Polycomb 2 homolog","Pc2","hPc2"],"length_aa":560,"mass_kda":61.4,"function":"E3 SUMO-protein ligase that catalyzes sumoylation of target proteins by promoting the transfer of SUMO from the E2 enzyme to the substrate (PubMed:12679040, PubMed:22467880, PubMed:22825850). Also acts as a histone reader, which specifically recognizes and binds histone H3 trimethylated at 'Lys-9' and 'Lys-27' (H3K9me3 and H3K27me3, respectively) via its chromo domain (By similarity). Catalyzes sumoylation of HNRNPK, a p53/TP53 transcriptional coactivator, hence indirectly regulates p53/TP53 transcriptional activation resulting in p21/CDKN1A expression (PubMed:22825850). Acts as a regulator of brown adipocyte differentiation by mediating sumoylation of PRDM16, thereby preventing PRDM16 ubiquitination and degradation (By similarity). Monosumoylates ZNF131 (PubMed:22467880). Component of a Polycomb group (PcG) multiprotein PRC1-like complex, a complex class required to maintain the transcriptionally repressive state of many genes, including Hox genes, throughout development (PubMed:12167701, PubMed:19636380, PubMed:21282530). PcG PRC1 complex acts via chromatin remodeling and modification of histones; it mediates monoubiquitination of histone H2A 'Lys-119', rendering chromatin heritably changed in its expressibility (PubMed:12167701, PubMed:19636380, PubMed:21282530). Plays a role in the lineage differentiation of the germ layers in embryonic development (By similarity)","subcellular_location":"Nucleus; Nucleus speckle","url":"https://www.uniprot.org/uniprotkb/O00257/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CBX4","classification":"Not Classified","n_dependent_lines":6,"n_total_lines":1208,"dependency_fraction":0.004966887417218543},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CSNK2B","stoichiometry":0.2},{"gene":"HIST2H2BE","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/CBX4","total_profiled":1310},"omim":[{"mim_id":"608802","title":"L3MBTL HISTONE METHYL-LYSINE-BINDING PROTEIN 1; L3MBTL1","url":"https://www.omim.org/entry/608802"},{"mim_id":"608457","title":"CHROMOBOX 7; CBX7","url":"https://www.omim.org/entry/608457"},{"mim_id":"603079","title":"CHROMOBOX 4; CBX4","url":"https://www.omim.org/entry/603079"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Nucleoplasm","reliability":"Enhanced"},{"location":"Nuclear bodies","reliability":"Additional"}],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"bone 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carcinoma.","date":"2015","source":"Oncology reports","url":"https://pubmed.ncbi.nlm.nih.gov/25607636","citation_count":28,"is_preprint":false},{"pmid":"19223471","id":"PMC_19223471","title":"Pax6 regulates the proglucagon processing enzyme PC2 and its chaperone 7B2.","date":"2009","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/19223471","citation_count":28,"is_preprint":false},{"pmid":"33473171","id":"PMC_33473171","title":"Inhibiting CBX4 efficiently protects hepatocellular carcinoma cells against sorafenib resistance.","date":"2021","source":"British journal of cancer","url":"https://pubmed.ncbi.nlm.nih.gov/33473171","citation_count":28,"is_preprint":false},{"pmid":"14614908","id":"PMC_14614908","title":"The proprotein convertase PC2 is involved in the maturation of prosomatostatin to somatostatin-14 but not in the somatostatin deficit in Alzheimer's disease.","date":"2003","source":"Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/14614908","citation_count":28,"is_preprint":false},{"pmid":"28283560","id":"PMC_28283560","title":"HDAC7 Ubiquitination by the E3 Ligase CBX4 Is Involved in Contextual Fear Conditioning Memory Formation.","date":"2017","source":"The Journal of neuroscience : the official journal of the Society for Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/28283560","citation_count":27,"is_preprint":false},{"pmid":"39154499","id":"PMC_39154499","title":"CBX4 counteracts cellular senescence to desensitize gastric cancer cells to chemotherapy by inducing YAP1 SUMOylation.","date":"2024","source":"Drug resistance updates : reviews and commentaries in antimicrobial and anticancer chemotherapy","url":"https://pubmed.ncbi.nlm.nih.gov/39154499","citation_count":25,"is_preprint":false},{"pmid":"15710417","id":"PMC_15710417","title":"The PcG protein HPC2 inhibits RBP-J-mediated transcription by interacting with LIM protein KyoT2.","date":"2005","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/15710417","citation_count":25,"is_preprint":false},{"pmid":"20976105","id":"PMC_20976105","title":"Silencing mediated by the Schizosaccharomyces pombe HIRA complex is dependent upon the Hpc2-like protein, Hip4.","date":"2010","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/20976105","citation_count":25,"is_preprint":false},{"pmid":"7704436","id":"PMC_7704436","title":"Cellular distributions of the prohormone processing enzymes PC1 and PC2.","date":"1994","source":"Molecular and cellular neurosciences","url":"https://pubmed.ncbi.nlm.nih.gov/7704436","citation_count":25,"is_preprint":false},{"pmid":"8822269","id":"PMC_8822269","title":"Processing of pro-opiomelanocortin in GH3 cells: inhibition by prohormone convertase 2 (PC2) antisense mRNA.","date":"1996","source":"Molecular and cellular endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/8822269","citation_count":24,"is_preprint":false},{"pmid":"7962350","id":"PMC_7962350","title":"Expression of the prohormone convertase PC2 correlates with the presence of corticotropin-like intermediate lobe peptide in human adrenocorticotropin-secreting tumors.","date":"1994","source":"The Journal of clinical endocrinology and metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/7962350","citation_count":24,"is_preprint":false},{"pmid":"9485595","id":"PMC_9485595","title":"Two genes of the anaerobic fungus Orpinomyces sp. strain PC-2 encoding cellulases with endoglucanase activities may have arisen by gene duplication.","date":"1998","source":"FEMS microbiology letters","url":"https://pubmed.ncbi.nlm.nih.gov/9485595","citation_count":23,"is_preprint":false},{"pmid":"7864855","id":"PMC_7864855","title":"The role of prohormone convertases PC1 (PC3) and PC2 in the cell-specific processing of proglucagon.","date":"1995","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/7864855","citation_count":22,"is_preprint":false},{"pmid":"9364928","id":"PMC_9364928","title":"Convertase PC2 and the neuroendocrine polypeptide 7B2 are co-induced and processed during neuronal differentiation of P19 embryonal carcinoma cells.","date":"1997","source":"DNA and cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/9364928","citation_count":22,"is_preprint":false},{"pmid":"9247068","id":"PMC_9247068","title":"Expression of the proprotein convertases PC1 and PC2 mRNAs in thyrotropin releasing hormone neurons of the rat paraventricular nucleus of hypothalamus.","date":"1997","source":"Brain research","url":"https://pubmed.ncbi.nlm.nih.gov/9247068","citation_count":22,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":51108,"output_tokens":4661,"usd":0.111619},"stage2":{"model":"claude-opus-4-6","input_tokens":8155,"output_tokens":4184,"usd":0.218062},"total_usd":0.329681,"stage1_batch_id":"msgbatch_011kfMfE81o6CTcyhVUqumuW","stage2_batch_id":"msgbatch_01DTputS6WNkH745x1nVSoBo","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2003,\n      \"finding\": \"CBX4 (Pc2) functions as a SUMO E3 ligase, dramatically enhancing sumoylation of the corepressor CtBP by recruiting both CtBP and the SUMO E2 enzyme Ubc9 to polycomb bodies, thereby bringing substrate and E2 into proximity.\",\n      \"method\": \"In vitro sumoylation assay with purified components; co-immunoprecipitation; live-cell imaging of polycomb bodies\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro sumoylation plus cellular localization studies, foundational paper with high citations\",\n      \"pmids\": [\"12679040\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"CBX4 (Pc2) acts as a SUMO E3 ligase for Smad-interacting protein 1 (SIP1), sumoylating it at Lys391 and Lys866; sumoylation disrupts SIP1 recruitment of CtBP and attenuates E-cadherin transcriptional repression in a promoter-context-dependent manner.\",\n      \"method\": \"In vivo and in vitro sumoylation assays; co-immunoprecipitation; luciferase reporter assays; site-directed mutagenesis of sumoylation sites\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro sumoylation reconstitution plus mutagenesis and functional reporter assays\",\n      \"pmids\": [\"16061479\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"CBX4 (Pc2) serves as a SUMO E3 ligase for HIPK2; DNA damage-induced phosphorylation of Pc2 at multiple sites (including Thr495) by HIPK2 is required for Pc2-mediated sumoylation of HIPK2, establishing an autoregulatory feedback loop that enhances HIPK2-dependent transcriptional repression.\",\n      \"method\": \"Co-immunoprecipitation; in vitro kinase and sumoylation assays; site-directed mutagenesis; immunofluorescence\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic assays plus mutagenesis with defined functional readout\",\n      \"pmids\": [\"17018294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"CBX4 (Cbx4) functions as a SUMO E3 ligase for the de novo DNA methyltransferase Dnmt3a, promoting its SUMO-1 modification at the N-terminal regulatory region including the PWWP domain; purified Cbx4 directly promotes Dnmt3a sumoylation in vitro.\",\n      \"method\": \"Co-immunoprecipitation; in vitro sumoylation assay with purified Cbx4; transfection co-expression\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro sumoylation reconstitution with purified protein, confirmed in cells\",\n      \"pmids\": [\"17439403\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CBX4 (Pc2) contains two functional SUMO interaction motifs (SIMs) that are required for full E3 ligase activity; the SIMs contribute to Pc2 self-sumoylation, CtBP sumoylation, and enrichment of SUMO at polycomb foci, and interact with the SIM-binding surface of both SUMO1 and SUMO2.\",\n      \"method\": \"Mutagenesis of SIMs; in vivo sumoylation assays; immunofluorescence; SUMO isoform mutant analysis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with multiple orthogonal assays; independently corroborated by Yang & Sharrocks (PMID 20176810)\",\n      \"pmids\": [\"20098713\", \"20176810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"CBX4 (Pc2) SIM2 is crucial for its E3 ligase activity by enabling subnuclear partitioning of the active SUMO~Ubc9 thioester intermediate and controlling lineage-specific gene expression during embryonic stem cell differentiation.\",\n      \"method\": \"SIM mutagenesis; in vivo sumoylation assays; Ubc9 interaction studies; ES cell differentiation assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with mechanistic dissection of SUMO~Ubc9 recruitment\",\n      \"pmids\": [\"20176810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"CBX4 (Pc2) E3 ligase activity requires two distinct domains: a C-terminal adapter domain that recruits both Ubc9 and CtBP, and a second N-terminal domain with intrinsic E3 activity in vitro; adapter function is required to selectively co-recruit E2 and substrate in vivo.\",\n      \"method\": \"Domain deletion and mutagenesis; in vitro sumoylation assays with domain fragments; in vivo sumoylation assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution plus domain mapping with mutagenesis\",\n      \"pmids\": [\"15592428\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"CBX4 is recruited to DNA damage sites in a PARP-dependent manner (independent of H2AX, RNF8, BMI1, and PI3-related kinases) and acts as a SUMO E3 ligase for BMI1, sumoylating it at lysine 88; this sumoylation is required for BMI1 accumulation at DNA lesions and for cellular resistance to ionizing radiation.\",\n      \"method\": \"Laser micro-irradiation with live-cell imaging; co-immunoprecipitation; in vitro sumoylation; siRNA knockdown with clonogenic survival assay; site-directed mutagenesis of BMI1-K88\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal methods including in vitro sumoylation, mutagenesis, and functional radiation resistance assay\",\n      \"pmids\": [\"22402492\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"CBX4 SUMO E3 ligase constitutively sumoylates CtIP at lysine 896, which is required for CtIP recruitment to DNA double-strand breaks and for DNA end resection; CBX4 depletion or non-sumoylatable CtIP-K896R blocks homologous recombination and increases genomic instability; artificial CtIP-SUMO fusion rescues these defects.\",\n      \"method\": \"siRNA depletion; site-directed mutagenesis (K896R); resection assays; HR reporter assay; CtIP-SUMO fusion rescue; laser micro-irradiation imaging\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal assays with genetic rescue demonstrating pathway placement\",\n      \"pmids\": [\"28740167\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CBX4 suppresses colorectal carcinoma metastasis by repressing Runx2 transcription through recruiting HDAC3 to the Runx2 promoter, maintaining deacetylated H3K27; this function requires the CBX4-HDAC3 interaction but is independent of its SUMO E3 ligase activity, chromodomain, or PRC1 complex.\",\n      \"method\": \"Co-immunoprecipitation; chromatin immunoprecipitation; domain mutagenesis; migration/invasion assays; in vivo metastasis model\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, ChIP, domain dissection, and in vivo functional validation\",\n      \"pmids\": [\"27864346\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CBX4 promotes osteosarcoma metastasis by transcriptionally upregulating Runx2 via recruitment of the acetyltransferase GCN5 to the Runx2 promoter; CK1α phosphorylates CBX4 at T437, facilitating its ubiquitination at K178 and K280 and subsequent CHIP-mediated degradation; TNFα reduces this phosphorylation to stabilize CBX4.\",\n      \"method\": \"Co-immunoprecipitation; ChIP; phosphorylation and ubiquitination assays; site-directed mutagenesis; kinase assays; in vivo metastasis model\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro kinase assay, mutagenesis of phospho/ubiquitin sites, ChIP, and in vivo validation\",\n      \"pmids\": [\"32111827\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CBX4 counteracts human mesenchymal stem cell senescence by recruiting nucleolar proteins fibrillarin (FBL) and KAP1 to rDNA loci, limiting excessive rRNA expression, ribosome biogenesis, and protein translation; CBX4 knockout destabilizes nucleolar heterochromatin and accelerates senescence.\",\n      \"method\": \"CBX4 knockout in hMSCs; RNA-seq; ChIP; co-immunoprecipitation; rRNA quantification; ribosome profiling; mouse osteoarthritis model with CBX4 overexpression\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with multiple orthogonal molecular readouts and in vivo rescue\",\n      \"pmids\": [\"30917318\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Cbx4 is a SUMO E3 ligase for Prdm16 in brown/beige adipocytes; Cbx4-mediated sumoylation of Prdm16 at K917 blocks its ubiquitination-mediated degradation, stabilizes Prdm16, augments thermogenic gene expression, and is required for white fat browning; this sumoylation also primes Prdm16 for further stabilization by methyltransferase Ehmt1.\",\n      \"method\": \"In vitro sumoylation assay; site-directed mutagenesis (K917R); ubiquitination assay; fat-specific Cbx4 knockout and overexpression; cold-exposure experiments; thermogenic gene expression analysis\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro sumoylation reconstitution, mutagenesis, and in vivo fat-specific genetic models\",\n      \"pmids\": [\"29539416\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Cbx4 is essential for thymic epithelial cell (TEC) proliferation and thymus organogenesis; cell-specific deletion shows the defect is intrinsic to TECs; Cbx4 physically interacts with p63 transcription factor, and the two functionally cooperate to maintain TEC stemness and proliferative capacity.\",\n      \"method\": \"Conditional Cbx4 knockout mice; BrdU proliferation assay; co-immunoprecipitation of Cbx4 with p63; histology and flow cytometry of thymic compartments\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with defined cellular phenotype plus Co-IP of binding partner\",\n      \"pmids\": [\"23362346\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Cbx4 maintains epithelial lineage identity and keratinocyte proliferation in developing epidermis by repressing neuronal gene programs and cyclin-dependent kinase inhibitors (p16/p19, p57); its chromodomain and SUMO E3 ligase activities differentially regulate proliferation, differentiation, and nonepidermal gene repression; Cbx4 expression is directly regulated by p63.\",\n      \"method\": \"Cbx4 ablation in mice; domain-specific mutant rescue experiments; ChIP; RNA-seq; p63 binding to Cbx4 promoter\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo KO with domain dissection and upstream regulatory mapping\",\n      \"pmids\": [\"26711500\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"CBX4 (HPC2) interacts with the LIM domain protein KyoT2 through its C-terminal fragment; this interaction enables CBX4 to inhibit RBP-J-mediated transcriptional activation by Notch intracellular domain (NIC) and by constitutively active RBP-J-VP16.\",\n      \"method\": \"Yeast two-hybrid; GST pulldown; co-immunoprecipitation; mammalian two-hybrid; luciferase reporter assay\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple binding assays plus functional reporter, single lab\",\n      \"pmids\": [\"15710417\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"CBX4 acts as a ubiquitin E3 ligase for HDAC7, mediating its ubiquitin-dependent degradation in the dorsal hippocampus after contextual fear conditioning; this degradation derepresses Nur77 and promotes long-term memory formation.\",\n      \"method\": \"Co-immunoprecipitation; ubiquitination assay identifying CBX4 as E3 for HDAC7; in vivo hippocampal manipulation; contextual fear conditioning behavioral assay\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP plus in vivo functional assay, single lab, moderate mechanistic depth\",\n      \"pmids\": [\"28283560\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CBX4 transcriptionally suppresses KLF6 in clear cell renal cell carcinoma by physically binding HDAC1 and maintaining HDAC1 localization on the KLF6 promoter; disruption of CBX4-HDAC1 interaction attenuates CBX4-mediated tumor growth and migration.\",\n      \"method\": \"Co-immunoprecipitation; ChIP; luciferase reporter assay; domain disruption experiments; in vivo xenograft model\",\n      \"journal\": \"EBioMedicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP, ChIP, and in vivo validation, single lab\",\n      \"pmids\": [\"32113161\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"CBX4 sumoylates YAP1 at K97 and K280 (SUMO1 modification), which competitively inhibits YAP1-S127 phosphorylation, preserves YAP1 stability, and promotes its cytoplasm-to-nucleus transport, thereby inactivating the Hippo pathway and countering cellular senescence and chemoresistance in gastric cancer; CEBPB transcriptionally downregulates CBX4 in response to low-dose cisplatin.\",\n      \"method\": \"Co-immunoprecipitation; ChIP; sumoylation assays; high-throughput sequencing; site-directed mutagenesis of SUMO sites; in vitro and in vivo tumor models\",\n      \"journal\": \"Drug resistance updates\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple molecular assays with in vivo validation, single lab\",\n      \"pmids\": [\"39154499\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CBX4 (also called Pc2/HPC2) is a polycomb group protein that functions primarily as a SUMO E3 ligase—using two functional SUMO interaction motifs (SIMs) and two distinct catalytic/adapter domains to sumoylate substrates including CtBP, SIP1, HIPK2, Dnmt3a, BMI1, CtIP, Prdm16, and YAP1—and also acts as a ubiquitin E3 ligase (e.g., for HDAC7), recruits HDAC3/HDAC1 to gene promoters to repress transcription independently of its SUMO ligase activity, localizes to polycomb bodies in a PARP-dependent manner at DNA damage sites to coordinate homologous recombination, maintains nucleolar heterochromatin homeostasis to restrain ribosome biogenesis and cellular senescence, and is itself regulated by CK1α-mediated phosphorylation that triggers its ubiquitin-dependent degradation.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"CBX4 (Pc2/HPC2) is a Polycomb group protein that functions as a SUMO E3 ligase and transcriptional regulator with diverse roles in chromatin silencing, DNA damage repair, lineage specification, and senescence control. As a SUMO E3 ligase, CBX4 uses two SUMO-interaction motifs and dual catalytic/adapter domains to recruit Ubc9 and substrates—including CtBP, SIP1, HIPK2, Dnmt3a, BMI1, CtIP, Prdm16, and YAP1—to Polycomb bodies, thereby controlling processes ranging from homologous recombination (via CtIP sumoylation enabling DNA end resection) and ionizing-radiation resistance (via BMI1 sumoylation at damage sites) to thermogenic fat programming (via Prdm16 stabilization) [PMID:12679040, PMID:22402492, PMID:28740167, PMID:29539416, PMID:39154499]. Independently of its SUMO ligase activity, CBX4 represses transcription by recruiting HDAC3 or HDAC1 to target promoters, and it also possesses ubiquitin E3 ligase activity toward HDAC7 [PMID:27864346, PMID:32113161, PMID:28283560]. CBX4 maintains nucleolar heterochromatin to restrain ribosome biogenesis and counteract cellular senescence, is essential for epithelial lineage identity through cooperation with p63, and is itself regulated by CK1α-mediated phosphorylation that triggers its ubiquitin-dependent degradation [PMID:30917318, PMID:26711500, PMID:32111827].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Establishing that a Polycomb group protein could function as a SUMO E3 ligase resolved how sumoylation substrates are brought together with the E2 enzyme Ubc9 at discrete nuclear foci (Polycomb bodies).\",\n      \"evidence\": \"In vitro sumoylation reconstitution with purified CBX4/Ubc9/CtBP plus live-cell imaging of Polycomb bodies\",\n      \"pmids\": [\"12679040\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of CBX4-Ubc9-substrate ternary complex unresolved\", \"Whether SUMO E3 activity is required for canonical PRC1-mediated gene silencing unclear\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Domain mapping revealed that CBX4 uses two mechanistically distinct regions—an N-terminal domain with intrinsic E3 activity and a C-terminal adapter that co-recruits E2 and substrate—explaining how a single protein achieves selective SUMO conjugation.\",\n      \"evidence\": \"Domain deletion/mutagenesis with in vitro and in vivo sumoylation assays\",\n      \"pmids\": [\"15592428\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which domain contacts Ubc9 catalytic cysteine not mapped\", \"No crystal structure of either E3-active domain\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Expanding the substrate repertoire to SIP1 and demonstrating that sumoylation disrupts a corepressor interaction (SIP1-CtBP) showed CBX4-mediated SUMO conjugation as a transcriptional regulatory switch rather than a single-substrate phenomenon.\",\n      \"evidence\": \"In vitro/in vivo sumoylation with site-directed mutagenesis plus E-cadherin promoter reporter assays\",\n      \"pmids\": [\"16061479\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo chromatin-level effects of SIP1 sumoylation not assessed\", \"Interaction with KyoT2/Notch pathway studied only by single lab (PMID:15710417)\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of an autoregulatory feedback loop—HIPK2 phosphorylates CBX4 to activate its SUMO E3 ligase toward HIPK2 itself—demonstrated that DNA damage signaling dynamically tunes CBX4 activity.\",\n      \"evidence\": \"In vitro kinase/sumoylation assays with phosphosite mutagenesis\",\n      \"pmids\": [\"17018294\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which phosphatase reverses HIPK2-mediated CBX4 phosphorylation unknown\", \"Physiological conditions activating this loop beyond DNA damage not tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Demonstration that CBX4 sumoylates the DNA methyltransferase Dnmt3a linked Polycomb SUMO ligase activity to epigenetic DNA methylation regulation.\",\n      \"evidence\": \"In vitro sumoylation with purified CBX4 and Dnmt3a plus cellular co-expression\",\n      \"pmids\": [\"17439403\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of Dnmt3a sumoylation on methyltransferase activity or genomic targeting not determined\", \"No in vivo genetic validation\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Discovery of two functional SUMO-interaction motifs (SIMs) in CBX4—especially SIM2—resolved how the enzyme partitions the active SUMO~Ubc9 thioester to Polycomb foci and controls lineage-specific gene expression during ES cell differentiation.\",\n      \"evidence\": \"SIM mutagenesis with in vivo sumoylation assays, Ubc9 interaction studies, and ES cell differentiation\",\n      \"pmids\": [\"20098713\", \"20176810\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Genome-wide targets of SIM-dependent SUMO conjugation in differentiating ES cells not mapped\", \"Whether SIM1 and SIM2 engage different SUMO paralogs in vivo remains unclear\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Showing that CBX4 is recruited to DNA damage sites in a PARP-dependent manner and sumoylates BMI1 at K88 to enable its damage-site accumulation placed CBX4 upstream in the Polycomb-mediated DNA damage response.\",\n      \"evidence\": \"Laser micro-irradiation, in vitro sumoylation, BMI1-K88 mutagenesis, and clonogenic survival assays\",\n      \"pmids\": [\"22402492\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which PAR-binding domain on CBX4 mediates PARP-dependent recruitment not identified\", \"Relationship between BMI1 sumoylation and ubiquitin ligase activity of PRC1 at damage sites not clarified\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Conditional knockout demonstrated that CBX4 is cell-intrinsically required for thymic epithelial cell proliferation and thymus organogenesis, acting through physical interaction with the transcription factor p63.\",\n      \"evidence\": \"TEC-specific Cbx4 conditional knockout mice; BrdU assays; Co-IP of Cbx4-p63\",\n      \"pmids\": [\"23362346\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether p63 is a SUMO substrate of CBX4 in TECs not tested\", \"Transcriptional targets co-regulated by CBX4-p63 not identified genome-wide\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Domain-specific rescue in Cbx4-null epidermis dissected the contributions of the chromodomain (repression of non-epidermal genes) versus SUMO E3 ligase domain (proliferation control via CDK inhibitor repression), and showed p63 directly regulates Cbx4 transcription, completing a reciprocal regulatory circuit.\",\n      \"evidence\": \"Cbx4 ablation in mouse skin with domain-mutant rescue; ChIP; RNA-seq; p63 binding at Cbx4 promoter\",\n      \"pmids\": [\"26711500\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"SUMO substrates mediating CDK inhibitor repression in epidermis unidentified\", \"Whether chromodomain-dependent gene repression involves canonical PRC1 H2AK119ub not resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Discovery that CBX4 represses Runx2 transcription by recruiting HDAC3 to its promoter—independently of SUMO ligase activity, chromodomain, or PRC1—revealed a non-canonical, HDAC-dependent transcriptional repression function.\",\n      \"evidence\": \"ChIP, reciprocal Co-IP, domain mutagenesis, and in vivo colorectal cancer metastasis model\",\n      \"pmids\": [\"27864346\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How CBX4 is targeted to the Runx2 promoter without chromodomain engagement unknown\", \"Whether HDAC3 recruitment is a general mechanism or specific to Runx2\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Two parallel advances linked CBX4 to homologous recombination (via CtIP sumoylation enabling DNA end resection) and to memory formation (via ubiquitin-dependent HDAC7 degradation), broadening CBX4's enzymatic repertoire to include ubiquitin E3 ligase activity.\",\n      \"evidence\": \"CtIP-K896R mutagenesis with HR reporter and resection assays (PMID:28740167); Co-IP and in vivo hippocampal manipulation for HDAC7 ubiquitination (PMID:28283560)\",\n      \"pmids\": [\"28740167\", \"28283560\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CBX4 ubiquitin ligase activity uses a RING-dependent mechanism or distinct catalytic site unknown\", \"CtIP sumoylation by CBX4 not confirmed in non-cancer cell lines\", \"HDAC7 ubiquitination finding from single lab\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identification of Prdm16 as a CBX4 SUMO substrate in adipocytes—where sumoylation at K917 blocks ubiquitin-mediated Prdm16 degradation—established CBX4 as a regulator of thermogenic fat programming and white fat browning.\",\n      \"evidence\": \"In vitro sumoylation, K917R mutagenesis, fat-specific Cbx4 knockout/overexpression, and cold-exposure experiments\",\n      \"pmids\": [\"29539416\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which ubiquitin ligase targets Prdm16 in competition with SUMO not identified\", \"Contribution of other CBX family members in adipocytes not assessed\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrating that CBX4 maintains nucleolar heterochromatin by recruiting fibrillarin and KAP1 to rDNA loci, thereby restraining rRNA synthesis and ribosome biogenesis, revealed a mechanism linking Polycomb function to senescence control.\",\n      \"evidence\": \"CBX4 knockout in human mesenchymal stem cells; ChIP at rDNA; RNA-seq; ribosome profiling; mouse osteoarthritis model rescue\",\n      \"pmids\": [\"30917318\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SUMO ligase activity is required for fibrillarin/KAP1 recruitment not tested\", \"Mechanism of CBX4 targeting to rDNA repeats not elucidated\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"CK1α was identified as the kinase that phosphorylates CBX4 at T437, triggering CHIP-mediated ubiquitination and degradation, establishing the first defined post-translational pathway controlling CBX4 protein turnover; TNFα stabilizes CBX4 by suppressing this phosphorylation.\",\n      \"evidence\": \"In vitro kinase assays, T437 mutagenesis, ubiquitination assays identifying K178/K280, and in vivo osteosarcoma model\",\n      \"pmids\": [\"32111827\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CK1α-mediated degradation operates in non-cancer cell types unknown\", \"Context-dependent switch between CBX4-mediated Runx2 repression (colorectal) and activation (osteosarcoma) not mechanistically resolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Sumoylation of YAP1 by CBX4 at K97/K280 was shown to competitively block S127 phosphorylation, stabilize YAP1, and promote its nuclear entry, linking CBX4's SUMO ligase activity to Hippo pathway inactivation.\",\n      \"evidence\": \"Sumoylation assays, SUMO-site mutagenesis, high-throughput sequencing, and gastric cancer tumor models\",\n      \"pmids\": [\"39154499\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which SUMO conjugation blocks LATS-mediated phosphorylation not structurally defined\", \"Single lab; independent confirmation needed\", \"Whether this applies outside gastric cancer contexts untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"No structural model exists for CBX4 in complex with Ubc9 or any substrate, and the mechanistic basis for substrate selectivity among its growing list of SUMO targets remains undefined; additionally, how SUMO ligase-dependent and HDAC-dependent functions are coordinated at specific genomic loci is unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal or cryo-EM structure of CBX4 E3 ligase domain or CBX4-Ubc9 complex\", \"Genome-wide mapping of CBX4-SUMO versus CBX4-HDAC target gene sets not performed\", \"Whether ubiquitin E3 ligase activity uses the same or distinct catalytic surface as SUMO E3 activity unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 1, 2, 3, 4, 5, 6, 7, 8, 12, 18]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 2, 3, 7, 8, 12, 16, 18]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [9, 14, 17]},\n      {\"term_id\": \"GO:0042393\", \"supporting_discovery_ids\": [9, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 4, 7, 8, 9, 11, 14, 17]},\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [11]},\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [7, 8, 9, 17]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [7, 8]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [0, 4, 9, 11, 14]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 2, 3, 7, 8, 12, 16, 18]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [9, 14, 17]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [13, 14]}\n    ],\n    \"complexes\": [\n      \"PRC1\"\n    ],\n    \"partners\": [\n      \"UBC9\",\n      \"BMI1\",\n      \"CTBP1\",\n      \"HDAC3\",\n      \"HDAC1\",\n      \"TP63\",\n      \"HIPK2\",\n      \"CTIP\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}