{"gene":"CRB2","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":1997,"finding":"Fission yeast Crb2 (S. pombe ortholog) physically interacts with Cut5 (TopBP1 homolog) and Chk1 in two-hybrid assays; Crb2 is required for checkpoint arrests induced by irradiation and polymerase mutations; upon UV damage Crb2 is transiently phosphorylated upstream of Chk1; moderate overexpression of Chk1 suppresses crb2 mutant phenotypes, placing Crb2 upstream of Chk1 in a Cut5-Crb2-Chk1 checkpoint sensor-transmitter pathway.","method":"Two-hybrid interaction assay, genetic epistasis (suppressor analysis), phosphorylation analysis by gel mobility shift, checkpoint arrest assays","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal genetic and biochemical evidence across multiple methods, foundational study replicated by subsequent work","pmids":["9407031"],"is_preprint":false},{"year":1999,"finding":"Cdc2 kinase directly phosphorylates Crb2 at threonine-215 in vitro; this phosphorylation is required for cells to re-enter the cell cycle after DNA damage-induced checkpoint arrest; a non-phosphorylatable T215A mutant remains arrested even after DNA repair and maintains phosphorylated Chk1.","method":"In vitro kinase assay with purified Cdc2; site-directed mutagenesis (T215A); checkpoint re-entry assays","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro kinase assay combined with mutagenesis and cellular phenotypic analysis, replicated in subsequent studies","pmids":["10488332"],"is_preprint":false},{"year":2002,"finding":"In fission yeast, Cdc2-cyclin B kinase activity influences recombinational repair at two stages: early Rhp51 focus formation (redundant with Rad50) and late topoisomerase III (Top3) regulation; the effect on Top3 function is mediated through the BRCT-domain checkpoint protein Crb2, directly linking checkpoint proteins to recombinational repair in G2.","method":"Genetic epistasis, ionizing radiation survival assays, Rhp51 focus analysis by fluorescence microscopy","journal":"Genes & development","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with defined cellular phenotype, single lab, two orthogonal readouts","pmids":["12023299"],"is_preprint":false},{"year":2003,"finding":"Crb2 localizes to distinct nuclear foci at sites of DNA double-strand breaks (DSBs) in live fission yeast cells; Crb2 co-localizes with Rad22 at persistent foci; damage-induced Crb2 foci still form in cells lacking Rad1, Rad3, and Rad17, but do not persist, indicating that Rad1/Rad3 complexes are required for retention but not initial recruitment of Crb2 at DSBs.","method":"Live-cell fluorescence microscopy, co-localization analysis, genetic deletion analysis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct localization experiment with functional consequence, multiple genetic backgrounds tested, replicated conceptually by subsequent studies","pmids":["12917337"],"is_preprint":false},{"year":2004,"finding":"The SET domain protein Set9 is responsible for H4-K20 methylation in S. pombe; genetic experiments link Set9 to Crb2; Set9 is required for Crb2 localization to sites of DNA damage; loss of Set9 or mutation of H4-K20 impairs cell survival after genotoxic challenge and compromises checkpoint-mediated cell cycle arrest.","method":"Genetic epistasis, ChIP, fluorescence microscopy of Crb2 foci, checkpoint survival assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods, independently replicated by subsequent biochemical studies","pmids":["15550243"],"is_preprint":false},{"year":2004,"finding":"Crb2 regulates DNA damage checkpoint through temporal and dynamic interactions with Rad3, Chk1, and replication factor Cut5; active complex formation between Chk1 and Crb2 is regulated by Rad3 and is maximal during checkpoint arrest; Chk1 activation requires two steps involving loss of Rad3-Chk1 and Rad3-Crb2 interactions and subsequent association of hyperphosphorylated Chk1 with Crb2; an in vitro Chk1 assay demonstrates activation requires Crb2 BRCT domain; direct Rad3-Crb2 interaction is inhibitory to Rad3 activity.","method":"Co-immunoprecipitation, in vitro Chk1 kinase assay, two-hybrid interaction, genetic analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro kinase assay combined with co-IP and genetic analysis, multiple orthogonal methods","pmids":["14739927"],"is_preprint":false},{"year":2004,"finding":"Crb2 undergoes homo-oligomerization through its tandem BRCT domains; truncated Crb2 lacking BRCT domains is defective in DNA damage checkpoint signaling, but addition of heterologous dimerization motifs largely restores function; replacement of BRCT domains with a dimerization motif also rescues cells from the dominant-negative effect of overexpressed BRCT domains, demonstrating that the essential function of tandem BRCT domains is to oligomerize Crb2.","method":"Domain truncation and replacement, dimerization motif substitution, checkpoint signaling assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple domain-swap experiments with genetic rescue, clear mechanistic conclusion","pmids":["15229228"],"is_preprint":false},{"year":2004,"finding":"Gamma-H2A (phosphorylated histone H2A at the SQE motif by Rad3/Tel1 ATR/ATM-related kinases) is required for large-scale recruitment of Crb2 to damaged DNA and for proper checkpoint phosphorylation of Crb2 and Chk1; H2A-AQE mutations cause defective Crb2 phosphorylation and failure to recruit Crb2 to DSBs; H2A-AQE mutations partially suppress IR hypersensitivity of crb2Δ cells in a mechanism requiring Rqh1 helicase.","method":"Site-directed mutagenesis of histone H2A, fluorescence microscopy of Crb2 foci, checkpoint phosphorylation assays, genetic epistasis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal approaches (mutagenesis, microscopy, phosphorylation assays, epistasis), foundational study replicated by subsequent work","pmids":["15226425"],"is_preprint":false},{"year":2005,"finding":"Cooperative control of Crb2 by ATM-family kinases and Cdc2 is essential for the DNA damage checkpoint; crb2-T215A cells can initiate but not sustain a checkpoint response; gamma-H2A is essential for checkpoint function specifically in crb2-T215A cells; inactivation of Cdc2 in gamma-H2A-defective cells impairs Crb2-dependent Chk1 signaling, demonstrating that full Crb2 activity requires Cdc2-mediated phosphorylation of T215.","method":"Genetic epistasis, checkpoint kinase assays, site-directed mutagenesis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis combined with biochemical checkpoint assays, multiple mutant combinations tested","pmids":["16314498"],"is_preprint":false},{"year":2005,"finding":"Crb2 mediates Chk1 activation in fission yeast; Crb2 BRCT domain mutants that experience damaged replication forks activate a Chk1 pathway that induces sustained spindle checkpoint activation in a Mad2-dependent fashion, delaying metaphase-to-anaphase transition.","method":"BRCT domain mutant analysis, checkpoint assays, Mad2 genetic epistasis","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — single lab, genetic epistasis with defined molecular pathway, multiple mutant alleles","pmids":["16107732"],"is_preprint":false},{"year":2006,"finding":"Using X-ray crystallography and NMR, both 53BP1 and fission yeast Crb2 contain tandem tudor domains that directly bind histone H4 specifically dimethylated at Lys20 (H4-K20me2); a unique five-residue binding cage in 53BP1 is structurally conserved in Crb2 and accommodates dimethyllysine but excludes trimethyllysine, establishing the molecular basis of methylation state-specific recognition.","method":"X-ray crystallography, NMR spectroscopy, in vitro binding assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with NMR validation and direct binding demonstration, independently replicated","pmids":["17190600"],"is_preprint":false},{"year":2006,"finding":"Crb2 IRIF formation requires two histone modifications (H2A C-terminal phosphorylation and H4-K20 methylation) that cooperate in the same recruitment pathway and require Crb2's Tudor and BRCT motifs; an alternative, parallel recruitment pathway sufficient for checkpoint activation requires a CDK phosphorylation site (T215) in Crb2 that mediates association with Cut5 (TopBP1 homolog); Cut5 also accumulates at HO-induced DSBs.","method":"Fluorescence microscopy of Crb2 foci, genetic epistasis with histone modification mutants, HO endonuclease-induced DSB assay, domain mutation analysis","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal genetic and imaging approaches, clear epistasis between two independent pathways","pmids":["16778077"],"is_preprint":false},{"year":2008,"finding":"Crystal structure of Crb2-BRCT2 domain alone and in complex with a phosphorylated H2A.1 peptide reveals the structural basis for Crb2 dimerization and direct interaction with gamma-H2A.1 at IRIF; mutational analysis shows that dimerization mutants are genotoxin-sensitive and defective in checkpoint signaling and Chk1 phosphorylation, while phosphopeptide-binding mutants are only slightly IR-sensitive, have extended checkpoint delays, and form IRIF but show reduced Rpa1 and Rad22 foci indicating a DNA repair defect.","method":"X-ray crystallography, site-directed mutagenesis, in vitro binding, checkpoint phosphorylation assays, fluorescence microscopy of foci","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure combined with mutagenesis and multiple functional assays, single rigorous study with orthogonal methods","pmids":["18676809"],"is_preprint":false},{"year":2008,"finding":"The tandem tudor domains of Crb2 preferentially bind di-methylated H4K20 over other methylation states; disruption of the tudor-binding motif or loss of Set9/Kmt5 (the H4K20 methylating enzyme) ablates Crb2 localization to DSBs and impairs checkpoint function; dimethylation but not trimethylation of H4K20 is specifically required for Crb2 localization, checkpoint function, and cell survival after DNA damage.","method":"In vitro tudor binding assays with differentially methylated peptides, fluorescence microscopy, checkpoint function assays, genetic knockout","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro binding combined with cellular localization and functional assays, methylation-state specificity rigorously demonstrated","pmids":["18826944"],"is_preprint":false},{"year":2010,"finding":"Crb2 BRCT2 domain residues serine-548 and lysine-619 make polar interactions with the phosphate group of phospho-H2A (gamma-H2A); mutations of these residues critically impair Crb2 IRIF formation and checkpoint function with additive effects when combined; combining either mutation with the T215A CDK phosphorylation site mutation completely abrogates Crb2 IRIF and function, demonstrating cooperative phosphate interactions between BRCT2 and gamma-H2A coupled with Tudor domain interactions with H4-K20me2.","method":"Site-directed mutagenesis, fluorescence microscopy, checkpoint kinase assays, double-mutant epistasis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — precise mutagenesis of binding residues with multiple orthogonal functional readouts, additive/epistatic mutations tested","pmids":["20679485"],"is_preprint":false},{"year":2010,"finding":"Loss of Crb2 BRCT domain phospho-H2AX binding activity severely impairs Crb2 accumulation at IR-induced DSBs, compromises checkpoint signaling, and disrupts checkpoint-mediated cell cycle arrest; combined ablation of both tudor (H4K20me2-binding) and BRCT (pH2AX-binding) modules yields a strikingly additive reduction in Crb2 activity.","method":"BRCT domain mutagenesis, fluorescence microscopy, checkpoint arrest assays, double-mutant analysis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — defined domain mutagenesis with multiple functional readouts, cooperative binding mechanism established","pmids":["20679488"],"is_preprint":false},{"year":2010,"finding":"Human CRB2 (the most abundant Crumbs ortholog in the brain) inhibits gamma-secretase cleavage of APP to reduce Abeta and AICD production; CRB2 knockdown increases gamma-secretase cleavage products; CRB2 interacts with the gamma-secretase complex but is not a competitive substrate; the transmembrane domain of CRB2 is indispensable for Abeta inhibition and mediates binding to the gamma-secretase complex; co-overexpression of presenilin-1 or APH-1 abrogates inhibition.","method":"Transfection/knockdown in HEK293 and SH-SY5Y cells, cell-free gamma-secretase assay, co-immunoprecipitation, domain deletion analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — cell-free in vitro assay combined with Co-IP, knockdown, overexpression, and domain deletion in two cell lines","pmids":["20299451"],"is_preprint":false},{"year":2011,"finding":"Germline knockout of mouse Crb2 causes embryonic lethality by E12.5; the primary defect is disturbed epiblast cell polarity at the primitive streak affecting epithelial-to-mesenchymal transition (EMT) during gastrulation, resulting in impaired mesoderm and endoderm formation, demonstrating an essential role for CRB2 in early embryonic epithelial polarity.","method":"Gene targeting/knockout, histological and developmental analysis of embryos","journal":"Developmental dynamics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KO with defined developmental phenotype, single lab, phenotype description without full molecular mechanism","pmids":["22072575"],"is_preprint":false},{"year":2012,"finding":"Phosphorylation-dependent interactions between Crb2 and Chk1 are essential for the DNA damage checkpoint; Crb2 recruits Chk1 to DSBs through direct physical interaction; a pair of conserved SQ/TQ motifs in Crb2 (Rad3 phosphorylation consensus) is required for Chk1 recruitment and activation; tethering Crb2 and Chk1 together rescues the SQ/TQ mutation; a 19-amino-acid phosphorylated peptide containing these motifs is sufficient for Chk1 binding in vitro; tethering this peptide to DSBs via Rad22/Rad52 or Rad4/Cut5 fusion can rescue crb2Δ checkpoint defect and even bypass the need for the 9-1-1 complex.","method":"In vitro binding with phosphopeptides, site-directed mutagenesis, fusion protein tethering, checkpoint assays, co-immunoprecipitation","journal":"PLoS genetics","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro binding, mutagenesis, and synthetic rescue experiments with multiple orthogonal approaches","pmids":["22792081"],"is_preprint":false},{"year":2012,"finding":"Conditional knockout of CRB2 in the developing mouse retina causes progressive disorganization, thinning of the photoreceptor layer, disruption of adherens junctions, increased numbers of late-born rod photoreceptors and Müller glia with concomitant programmed cell death, demonstrating an essential role for CRB2 in retinal lamination and suppression of late-born progenitor proliferation.","method":"Conditional gene knockout, confocal scanning laser ophthalmoscopy, OCT, electroretinography, histology","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean conditional KO with multiple functional readouts, single lab","pmids":["23001562"],"is_preprint":false},{"year":2013,"finding":"Combined ablation of Crb1 and Crb2 in retinal progenitor cells causes severe retinal dysfunction and dysregulation of Notch1 and YAP/Hippo signaling pathway target genes and increased P120-catenin levels; CRB1 and CRB2 restrain proliferation of retinal progenitor cells and control cell cycle distribution.","method":"Conditional double knockout, gene expression profiling, cell cycle analysis, electroretinography","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — double KO with pathway analysis, single lab, signaling pathway changes inferred from expression changes","pmids":["24339791"],"is_preprint":false},{"year":2014,"finding":"Recessive mutations in human CRB2 cause steroid-resistant nephrotic syndrome; zebrafish crb2b loss-of-function mutation shows that crb2b is required for podocyte foot process arborization, slit diaphragm formation, and proper nephrin trafficking; complementation experiments in zebrafish confirm that human CRB2 mutations result in loss of function.","method":"Homozygosity mapping, whole exome sequencing, zebrafish loss-of-function mutant analysis, complementation assay, nephrin trafficking analysis","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — complementation experiments in zebrafish combined with cellular phenotyping across multiple functional readouts","pmids":["25557779"],"is_preprint":false},{"year":2014,"finding":"Targeted ablation of CRB2 specifically in photoreceptors causes progressive photoreceptor degeneration (RP-like phenotype) with early abnormal lamination onset; ablation in Müller cells causes late-onset retinal disorganization; short-term loss of CRB2 in adult photoreceptors but not in Müller glial cells causes sporadic loss of adhesion between photoreceptors and Müller cells.","method":"Cell-type specific conditional deletion, AAV-Cre and shRNA-mediated ablation, OCT, electroretinography, histology","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-type specific KO with multiple in vivo functional readouts, single lab","pmids":["24493795"],"is_preprint":false},{"year":2014,"finding":"Hyperactive Cdc2 (cdc2.1w) mutation causes cells to bypass Chk1 and enter extended cell-cycle arrest dependent on Cds1 kinase in the presence of broken replication forks; this Chk1 bypass requires the mitotic Cdc2 phosphorylation site Crb2-T215, suggesting that presence of the T215 phosphorylation channels Rad3 activity towards Cds1 instead of Chk1; hyperactive Cdc2 locks cells in a G1-like repair mode favoring NHEJ over recombination.","method":"Genetic analysis with cdc2.1w mutant, checkpoint kinase assays, recombination assays","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — single lab, genetic analysis with CDK mutant, multiple pathway readouts","pmids":["24861625"],"is_preprint":false},{"year":2015,"finding":"CRB2 protein is expressed in mouse retinal pigment epithelium (RPE) and is restricted to the apicolateral membrane, specifically at tight junctions; CRB2 completes a fully expressed Crumbs complex in the RPE apico-lateral membrane together with other CRB complex components.","method":"Custom antibody generation, confocal immunofluorescence microscopy, subcellular fractionation","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct localization with custom antibody, multiple imaging methods, no functional consequence tested","pmids":["26404741"],"is_preprint":false},{"year":2018,"finding":"The 53BP1 ortholog Crb2 and Rev7 specifically repress long-range resection of DSBs through the RecQ helicase-dependent pathway, thereby preventing hyper-resection; this is established using a single-cell microscopy assay for resection phases in live S. pombe cells.","method":"Live-cell single-cell microscopy resection assay, genetic deletion, pathway dissection","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — novel assay with single lab, defined molecular pathway, genetic epistasis","pmids":["29697047"],"is_preprint":false},{"year":2021,"finding":"Knockdown of CRB2 in human RPE cells disrupts tight junction maintenance, affects cell cycle arrest, and in vivo knockdown of CRB2 in mouse RPE perturbs distribution of apical polarity proteins and results in invasion of activated microglial cells into the subretinal space, demonstrating CRB2 is required for development and maintenance of polarized RPE epithelium.","method":"siRNA knockdown, in vitro polarization assay, AAV-mediated in vivo knockdown, immunofluorescence","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KD with defined cellular and in vivo phenotypes, single lab, multiple readouts","pmids":["34262913"],"is_preprint":false},{"year":2021,"finding":"Podocyte-specific Crb2 knockout mice develop massive albuminuria and focal segmental glomerulosclerosis; human podocytes lacking CRB2 show significantly decreased F-actin positive area and increased susceptibility to apoptosis, demonstrating that CRB2 is required for proper actin cytoskeleton organization and podocyte survival.","method":"Podocyte-specific conditional knockout, electron microscopy, siRNA knockdown in human podocytes, F-actin quantification, apoptosis assay","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — both in vivo conditional KO and in vitro human cell knockdown with multiple orthogonal readouts","pmids":["34654837"],"is_preprint":false},{"year":2022,"finding":"CRB2 protein transport from the ER to the plasma membrane requires proper formation of disulfide bridges; disease-associated CRB2 missense variants and variants with mutated putative disulfide bridge-forming cysteines accumulate predominantly at the ER rather than reaching the plasma membrane; CRB2 is retained at the ER in cells lacking protein disulfide isomerase A3 (PDIA3), identifying post-translational disulfide bridge formation as a crucial step for CRB2 surface transport.","method":"Live-cell imaging of CRB2-GFP fusion proteins, BFP-labeled plasma membrane reporter, PDIA3 knockout cells, in silico variant selection","journal":"Life science alliance","confidence":"High","confidence_rationale":"Tier 2 / Strong — live-cell imaging with multiple variant alleles, genetic KO of PDIA3, functional readout of surface localization","pmids":["36549870"],"is_preprint":false},{"year":2022,"finding":"Anti-Crb2 autoantibodies cause nephrotic syndrome in mice; immunization with the recombinant extracellular domain of Crb2 generates autoantibodies that colocalize with Crb2 at podocyte foot processes; anti-Crb2 antibody treatment of podocytes increases phosphorylation of ezrin (which connects Crb2 to the cytoskeleton), alters Crb2 localization, and disturbs actin distribution.","method":"Mouse immunization model, anti-Crb2 antibody incubation of podocyte cell line, phosphorylation assays, immunofluorescence colocalization","journal":"Journal of the American Society of Nephrology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo immunization model combined with cell-based mechanistic assays, single lab","pmids":["35985815"],"is_preprint":false},{"year":2024,"finding":"Human CRB1 and CRB2 co-localize in the human retina and iPSC-derived retinal organoids; retina-specific pull-downs show CRB1 is enriched in CRB2 samples; co-immunoprecipitation demonstrates human canonical CRB1 interacts with CRB1 and CRB2 but not with CRB3 (which lacks an extracellular domain), indicating the extracellular domain mediates CRB1-CRB2 homo- and heterodimerization.","method":"Co-immunoprecipitation, retina-specific pull-down, co-localization in human retina and retinal organoids","journal":"Life science alliance","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP with domain specificity (CRB3 negative control), single lab","pmids":["38570189"],"is_preprint":false},{"year":2025,"finding":"CRB2 knockdown in podocytes induces YAP transcriptional activity and target gene expression, upregulates YAP-mediated mechanosignaling, and increases density of focal adhesions and F-actin; using ERISM, CRB2 knockdown enhances podocyte contractility in a substrate stiffness-dependent manner with impaired mechanosensing at low substrate stiffness, demonstrating CRB2 regulates podocyte mechanotransduction via YAP signaling.","method":"siRNA knockdown, YAP reporter assays, elastic resonator interference stress microscopy (ERISM), F-actin quantification, YAP inhibitor treatment","journal":"American journal of physiology. Renal physiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — novel biophysical assay (ERISM) combined with YAP signaling analysis, single lab, multiple readouts","pmids":["40062402"],"is_preprint":false},{"year":2026,"finding":"CRB2 inhibits transcription of the ferroptosis suppressor SLC7A11 by increasing dimethylation of histone H4 lysine 20 (H4K20me2) at its promoter; mechanistically, CRB2 hinders the interaction between the LSD1(E8A) histone lysine demethylase isoform and the deubiquitinase USP7, facilitating LSD1(E8A) degradation and thereby increasing H4K20me2 levels; this epigenetic axis sensitizes HNSCC cells to ferroptosis.","method":"CRB2 overexpression/knockdown, ferroptosis assays (erastin/RSL3), ChIP for H4K20me2, co-immunoprecipitation of LSD1-USP7 interaction, protein stability assays","journal":"Cancer research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP combined with ChIP and ferroptosis functional assays, single lab, multiple orthogonal methods","pmids":["40991301"],"is_preprint":false}],"current_model":"CRB2 is a multifunctional protein operating in two major contexts: (1) as a fission yeast DNA damage checkpoint mediator (ortholog Crb2/Rhp9) that is recruited to DSBs through cooperative binding of its tandem Tudor domain to H4-K20me2 and its BRCT2 domain to gamma-H2A, undergoes phosphorylation by Rad3/ATR and Cdc2/CDK1 at distinct sites to control checkpoint initiation, maintenance, and termination, and recruits/activates Chk1 by direct phosphorylation-dependent interaction; and (2) as a mammalian Crumbs family transmembrane polarity protein that localizes to the subapical/apicolateral membrane of epithelial cells (retina, RPE, kidney podocytes), where it is essential for podocyte foot process formation and nephrin trafficking, retinal lamination and progenitor cell proliferation control, and forms homo- and heteromeric complexes with CRB1 via extracellular domains, with its surface transport dependent on disulfide bridge formation by PDIA3, and its loss in podocytes activating YAP mechanosignaling and disrupting actin cytoskeleton organization."},"narrative":{"mechanistic_narrative":"The CRB2 timeline describes two coherent but distinct protein contexts that share the gene symbol: the fission yeast DNA damage checkpoint mediator Crb2 (a 53BP1-related ortholog) and the mammalian Crumbs-family transmembrane polarity protein CRB2. In S. pombe, Crb2 is a checkpoint adaptor that operates in a Cut5(TopBP1)–Crb2–Chk1 sensor-transmitter pathway, acting upstream of Chk1 to enforce DNA damage-induced arrest [PMID:9407031]. Crb2 is recruited to double-strand breaks through cooperative reading of two histone marks: its tandem Tudor domain binds H4-K20me2 (deposited by Set9/Kmt5), specifically discriminating the dimethyl over trimethyl state, and its BRCT2 domain binds γ-H2A phosphorylated by Rad3/Tel1, with the two modules contributing additively to focus formation and checkpoint signaling [PMID:15550243, PMID:15226425, PMID:17190600, PMID:18826944, PMID:20679485, PMID:20679488]. A parallel CDK-dependent pathway, requiring Cdc2 phosphorylation of Crb2-T215 and association with Cut5, can recruit Crb2 to breaks independently of the histone-reading route [PMID:16778077]. Crb2 function further requires BRCT-mediated homo-oligomerization [PMID:15229228, PMID:18676809], and once positioned it recruits and activates Chk1 via Rad3-phosphorylated SQ/TQ motifs that directly bind Chk1 [PMID:14739927, PMID:22792081]. Cdc2 phosphorylation of T215 governs the temporal switch between checkpoint maintenance and recovery, controlling re-entry into the cycle after repair and channeling checkpoint output [PMID:10488332, PMID:16314498, PMID:24861625], and Crb2 also restrains long-range DSB resection to balance repair pathway choice [PMID:12023299, PMID:29697047]. In mammals, CRB2 is an apicolateral/subapical membrane polarity protein required for epiblast polarity during gastrulation [PMID:22072575], retinal lamination and suppression of late-born progenitor proliferation through regulation of Notch and YAP/Hippo targets [PMID:23001562, PMID:24339791], and for development and maintenance of polarized retinal pigment epithelium [PMID:26404741, PMID:34262913]. In kidney podocytes, recessive CRB2 mutations cause steroid-resistant nephrotic syndrome, and CRB2 is required for foot process arborization, slit diaphragm formation, nephrin trafficking, actin cytoskeleton organization, and podocyte survival, with its loss activating YAP mechanosignaling [PMID:25557779, PMID:34654837, PMID:40062402]. CRB2 surface delivery depends on PDIA3-mediated disulfide bridge formation, and disease variants are retained in the ER [PMID:36549870]; its extracellular domain mediates homo- and heterodimerization with CRB1 [PMID:38570189].","teleology":[{"year":1997,"claim":"Established Crb2 as a checkpoint mediator acting upstream of Chk1, answering how DNA damage signals are transmitted to cell cycle arrest in fission yeast.","evidence":"Two-hybrid interaction, genetic suppressor analysis, and phosphorylation/checkpoint arrest assays placing Crb2 in a Cut5-Crb2-Chk1 pathway","pmids":["9407031"],"confidence":"High","gaps":["Did not define the molecular basis of Crb2 recruitment to damage","Direct vs indirect Chk1 activation not resolved"]},{"year":1999,"claim":"Identified CDK phosphorylation of Crb2-T215 as the switch enabling checkpoint recovery, distinguishing checkpoint maintenance from termination.","evidence":"In vitro Cdc2 kinase assay, T215A mutagenesis, and checkpoint re-entry assays","pmids":["10488332"],"confidence":"High","gaps":["Mechanism by which T215 phosphorylation silences Chk1 not defined","Whether T215 also affects recruitment unaddressed at this stage"]},{"year":2002,"claim":"Linked Crb2 to recombinational repair downstream of Cdc2, showing the checkpoint protein also regulates the repair machinery (Top3) in G2.","evidence":"Genetic epistasis, IR survival, and Rhp51 focus analysis","pmids":["12023299"],"confidence":"Medium","gaps":["Direct biochemical link between Crb2 and Top3 not shown","Single-lab genetic inference"]},{"year":2003,"claim":"Demonstrated Crb2 forms DSB-localized nuclear foci and separated initial recruitment from Rad1/Rad3/Rad17-dependent retention.","evidence":"Live-cell fluorescence microscopy, co-localization with Rad22, deletion analysis","pmids":["12917337"],"confidence":"High","gaps":["Molecular determinants of initial recruitment not yet identified","Retention mechanism via 9-1-1 not mechanistically defined"]},{"year":2004,"claim":"Defined the histone-mark code for Crb2 recruitment by identifying Set9/H4-K20 methylation and γ-H2A as the marks read at breaks, and showed BRCT-mediated oligomerization is the essential BRCT function.","evidence":"Genetic epistasis, ChIP, histone mutagenesis, Crb2 focus imaging, domain-swap rescue, and co-IP/in vitro Chk1 assays across four studies","pmids":["15550243","15226425","15229228","14739927"],"confidence":"High","gaps":["Structural basis of mark recognition not yet solved","Methylation-state specificity not resolved"]},{"year":2006,"claim":"Solved the structural basis of dimethyl-state-specific H4-K20me2 reading by the tandem Tudor domain and showed histone-reading and CDK/Cut5 pathways are parallel recruitment routes.","evidence":"X-ray crystallography, NMR, in vitro binding, and HO-induced DSB genetic epistasis","pmids":["17190600","16778077"],"confidence":"High","gaps":["Relative contribution of each pathway to checkpoint output in vivo not quantified"]},{"year":2008,"claim":"Provided the structural and mutational basis for cooperative bivalent recruitment, showing BRCT2 dimerization and γ-H2A phosphopeptide binding have separable roles in checkpoint versus repair.","evidence":"Crystal structure of BRCT2-phospho-H2A complex, mutagenesis, binding and focus/checkpoint assays","pmids":["18676809","18826944"],"confidence":"High","gaps":["How dimerization couples to downstream Chk1 activation not fully resolved"]},{"year":2010,"claim":"Established that Tudor (H4K20me2) and BRCT (γ-H2A) modules contribute additively to recruitment, defining the bivalent-anchoring logic of Crb2.","evidence":"Precise mutagenesis of binding residues (S548, K619), double-mutant epistasis, focus imaging, and checkpoint kinase assays","pmids":["20679485","20679488"],"confidence":"High","gaps":["Stoichiometry of dual-mark engagement at a single break unknown"]},{"year":2012,"claim":"Resolved how Crb2 activates Chk1, showing Rad3-phosphorylated SQ/TQ motifs directly recruit Chk1 and can bypass upstream 9-1-1 when tethered.","evidence":"Phosphopeptide binding, mutagenesis, fusion-protein tethering rescue, and co-IP","pmids":["22792081"],"confidence":"High","gaps":["Conformational change in Chk1 upon binding not defined"]},{"year":2010,"claim":"Extended CRB2 biology to mammals by showing human CRB2 inhibits γ-secretase cleavage of APP via its transmembrane domain.","evidence":"Knockdown/overexpression in HEK293 and SH-SY5Y, cell-free γ-secretase assay, co-IP, domain deletion","pmids":["20299451"],"confidence":"High","gaps":["Physiological relevance in brain not established","Relationship to CRB2 polarity function unclear"]},{"year":2014,"claim":"Identified CRB2 as a developmental polarity protein essential in early embryo, retina, and kidney, with mutations causing human steroid-resistant nephrotic syndrome.","evidence":"Mouse germline and conditional knockouts, zebrafish loss-of-function and complementation, human exome sequencing, and nephrin trafficking analysis (spanning 2011–2014)","pmids":["22072575","23001562","24493795","25557779"],"confidence":"Medium","gaps":["Direct molecular link between CRB2 and nephrin trafficking not defined","How CRB2 controls progenitor proliferation mechanistically unresolved"]},{"year":2021,"claim":"Defined CRB2 cytoskeletal and survival functions in podocytes and RPE, linking its loss to actin disorganization, apoptosis, and epithelial polarity failure.","evidence":"Podocyte- and RPE-specific knockouts, human cell knockdown, F-actin quantification, apoptosis assays, and polarity protein imaging","pmids":["34654837","34262913","24339791"],"confidence":"Medium","gaps":["Direct effector linking CRB2 to actin not identified","Single-lab signaling inferences"]},{"year":2022,"claim":"Established post-translational control of CRB2 trafficking, showing PDIA3-mediated disulfide bridge formation is required for ER-to-surface transport and that disease variants are ER-retained, and that the extracellular domain mediates CRB1-CRB2 dimerization.","evidence":"Live-cell imaging of CRB2-GFP, PDIA3 knockout, disease-variant analysis, autoantibody immunization model, and reciprocal co-IP with CRB1/CRB3 (2022–2024)","pmids":["36549870","35985815","38570189"],"confidence":"Medium","gaps":["Functional consequence of CRB1-CRB2 heterodimers in vivo not defined","Autoantibody mechanism vs genetic loss not fully reconciled"]},{"year":2025,"claim":"Connected CRB2 loss to YAP mechanotransduction and, separately, identified a nuclear CRB2 role in H4K20me2-mediated transcriptional repression of SLC7A11 controlling ferroptosis.","evidence":"siRNA knockdown with YAP reporters and ERISM contractility measurement; CRB2 over/knockdown with ChIP for H4K20me2, LSD1-USP7 co-IP, and ferroptosis assays (2025–2026)","pmids":["40062402","40991301"],"confidence":"Medium","gaps":["How a membrane polarity protein influences nuclear H4K20me2 mechanistically unclear","Single-lab findings, awaiting independent confirmation"]},{"year":null,"claim":"It remains unresolved whether and how the chromatin-reading checkpoint functions of yeast Crb2 mechanistically relate to the membrane-polarity functions of mammalian CRB2, and how CRB2 transduces polarity loss into YAP, actin, and transcriptional outcomes.","evidence":"No discovery in the timeline bridges the checkpoint mediator and transmembrane polarity contexts","pmids":[],"confidence":"Low","gaps":["No unified molecular model across the two contexts","Effectors linking CRB2 surface signaling to nuclear/cytoskeletal outputs not defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0042393","term_label":"histone binding","supporting_discovery_ids":[10,13]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,5,18]},{"term_id":"GO:0098772","term_label":"molecular function regulator 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Inhibits gamma-secretase-dependent cleavage of APP and secretion of amyloid-beta peptide 40 and amyloid-beta peptide 42, and thereby inhibits gamma-secretase-dependent Notch transcription (PubMed:20299451)","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/Q5IJ48/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CRB2","classification":"Not Classified","n_dependent_lines":20,"n_total_lines":1208,"dependency_fraction":0.016556291390728478},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CRB2","total_profiled":1310},"omim":[{"mim_id":"616220","title":"FOCAL SEGMENTAL GLOMERULOSCLEROSIS 9; FSGS9","url":"https://www.omim.org/entry/616220"},{"mim_id":"611730","title":"ERYTHROCYTE MEMBRANE PROTEIN BAND 4.1-LIKE 5; EPB41L5","url":"https://www.omim.org/entry/611730"},{"mim_id":"609720","title":"CRUMBS CELL POLARITY COMPLEX COMPONENT 2; CRB2","url":"https://www.omim.org/entry/609720"},{"mim_id":"605230","title":"TUMOR PROTEIN p53-BINDING PROTEIN 1; TP53BP1","url":"https://www.omim.org/entry/605230"},{"mim_id":"603278","title":"FOCAL SEGMENTAL GLOMERULOSCLEROSIS 1; FSGS1","url":"https://www.omim.org/entry/603278"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"brain","ntpm":6.8},{"tissue":"choroid 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Section D, Biological crystallography","url":"https://pubmed.ncbi.nlm.nih.gov/12832769","citation_count":2,"is_preprint":false},{"pmid":"40991301","id":"PMC_40991301","title":"CRB2 Activates an Epigenetic Axis to Promote Ferroptosis in Head and Neck Squamous Cell Carcinoma.","date":"2026","source":"Cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/40991301","citation_count":0,"is_preprint":false},{"pmid":"39484460","id":"PMC_39484460","title":"CRB2 Depletion Induces YAP Signaling and Disrupts Mechanosensing in Podocytes.","date":"2024","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/39484460","citation_count":0,"is_preprint":false},{"pmid":"40456931","id":"PMC_40456931","title":"Expanded CRB2-related disease phenotype: multisystem involvement and post-transplant complications in monozygotic twins.","date":"2025","source":"Pediatric nephrology (Berlin, Germany)","url":"https://pubmed.ncbi.nlm.nih.gov/40456931","citation_count":0,"is_preprint":false},{"pmid":"40761226","id":"PMC_40761226","title":"Heterozygous variants of uncertain significance in NPHS1 and CRB2 in a newborn with congenital nephrotic syndrome of the Finnish type and multiple fetal anomalies: a case report.","date":"2025","source":"AME case reports","url":"https://pubmed.ncbi.nlm.nih.gov/40761226","citation_count":0,"is_preprint":false},{"pmid":"42169774","id":"PMC_42169774","title":"A Novel CRB2 Mutation Associated With FSGS and ESRD in an Adult Patient.","date":"2026","source":"Case reports in nephrology","url":"https://pubmed.ncbi.nlm.nih.gov/42169774","citation_count":0,"is_preprint":false},{"pmid":"39987590","id":"PMC_39987590","title":"CRISPR/Cas9-mediated generation of a homozygous CRB2 knockout H1 human embryonic stem cell line.","date":"2025","source":"Stem cell research","url":"https://pubmed.ncbi.nlm.nih.gov/39987590","citation_count":0,"is_preprint":false},{"pmid":"40734719","id":"PMC_40734719","title":"CRB2-Related Syndrome in 2 New Patients: Three Novel Variants.","date":"2025","source":"Molecular syndromology","url":"https://pubmed.ncbi.nlm.nih.gov/40734719","citation_count":0,"is_preprint":false},{"pmid":"41906249","id":"PMC_41906249","title":"CRB2 Facilitates Epithelial Ovarian Cancer Progression by Inducing Polarity Changes via Activation of the Wnt/PCP Signalling Pathway.","date":"2026","source":"Journal of cellular and molecular medicine","url":"https://pubmed.ncbi.nlm.nih.gov/41906249","citation_count":0,"is_preprint":false},{"pmid":"39527864","id":"PMC_39527864","title":"Biochemical characterization and structure prediction of the Cerrado soil CRB2(1) metagenomic dioxygenase.","date":"2024","source":"Enzyme and microbial technology","url":"https://pubmed.ncbi.nlm.nih.gov/39527864","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.09.06.674608","title":"Extrafollicular B cell responses are associated with autoreactivity against the slit diaphragm in idiopathic podocytopathies","date":"2025-09-10","source":"bioRxiv","url":"https://doi.org/10.1101/2025.09.06.674608","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":34394,"output_tokens":8515,"usd":0.115454,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":18386,"output_tokens":4889,"usd":0.107078,"stage2_stop_reason":"end_turn"},"total_usd":0.222532,"stage1_batch_id":"msgbatch_01ACAxJQabDsht39JXeWG22d","stage2_batch_id":"msgbatch_01QEU22A9U5sbPTgsMzTmvSf","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1997,\n      \"finding\": \"Fission yeast Crb2 (S. pombe ortholog) physically interacts with Cut5 (TopBP1 homolog) and Chk1 in two-hybrid assays; Crb2 is required for checkpoint arrests induced by irradiation and polymerase mutations; upon UV damage Crb2 is transiently phosphorylated upstream of Chk1; moderate overexpression of Chk1 suppresses crb2 mutant phenotypes, placing Crb2 upstream of Chk1 in a Cut5-Crb2-Chk1 checkpoint sensor-transmitter pathway.\",\n      \"method\": \"Two-hybrid interaction assay, genetic epistasis (suppressor analysis), phosphorylation analysis by gel mobility shift, checkpoint arrest assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal genetic and biochemical evidence across multiple methods, foundational study replicated by subsequent work\",\n      \"pmids\": [\"9407031\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Cdc2 kinase directly phosphorylates Crb2 at threonine-215 in vitro; this phosphorylation is required for cells to re-enter the cell cycle after DNA damage-induced checkpoint arrest; a non-phosphorylatable T215A mutant remains arrested even after DNA repair and maintains phosphorylated Chk1.\",\n      \"method\": \"In vitro kinase assay with purified Cdc2; site-directed mutagenesis (T215A); checkpoint re-entry assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro kinase assay combined with mutagenesis and cellular phenotypic analysis, replicated in subsequent studies\",\n      \"pmids\": [\"10488332\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"In fission yeast, Cdc2-cyclin B kinase activity influences recombinational repair at two stages: early Rhp51 focus formation (redundant with Rad50) and late topoisomerase III (Top3) regulation; the effect on Top3 function is mediated through the BRCT-domain checkpoint protein Crb2, directly linking checkpoint proteins to recombinational repair in G2.\",\n      \"method\": \"Genetic epistasis, ionizing radiation survival assays, Rhp51 focus analysis by fluorescence microscopy\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with defined cellular phenotype, single lab, two orthogonal readouts\",\n      \"pmids\": [\"12023299\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Crb2 localizes to distinct nuclear foci at sites of DNA double-strand breaks (DSBs) in live fission yeast cells; Crb2 co-localizes with Rad22 at persistent foci; damage-induced Crb2 foci still form in cells lacking Rad1, Rad3, and Rad17, but do not persist, indicating that Rad1/Rad3 complexes are required for retention but not initial recruitment of Crb2 at DSBs.\",\n      \"method\": \"Live-cell fluorescence microscopy, co-localization analysis, genetic deletion analysis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct localization experiment with functional consequence, multiple genetic backgrounds tested, replicated conceptually by subsequent studies\",\n      \"pmids\": [\"12917337\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The SET domain protein Set9 is responsible for H4-K20 methylation in S. pombe; genetic experiments link Set9 to Crb2; Set9 is required for Crb2 localization to sites of DNA damage; loss of Set9 or mutation of H4-K20 impairs cell survival after genotoxic challenge and compromises checkpoint-mediated cell cycle arrest.\",\n      \"method\": \"Genetic epistasis, ChIP, fluorescence microscopy of Crb2 foci, checkpoint survival assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods, independently replicated by subsequent biochemical studies\",\n      \"pmids\": [\"15550243\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Crb2 regulates DNA damage checkpoint through temporal and dynamic interactions with Rad3, Chk1, and replication factor Cut5; active complex formation between Chk1 and Crb2 is regulated by Rad3 and is maximal during checkpoint arrest; Chk1 activation requires two steps involving loss of Rad3-Chk1 and Rad3-Crb2 interactions and subsequent association of hyperphosphorylated Chk1 with Crb2; an in vitro Chk1 assay demonstrates activation requires Crb2 BRCT domain; direct Rad3-Crb2 interaction is inhibitory to Rad3 activity.\",\n      \"method\": \"Co-immunoprecipitation, in vitro Chk1 kinase assay, two-hybrid interaction, genetic analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro kinase assay combined with co-IP and genetic analysis, multiple orthogonal methods\",\n      \"pmids\": [\"14739927\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Crb2 undergoes homo-oligomerization through its tandem BRCT domains; truncated Crb2 lacking BRCT domains is defective in DNA damage checkpoint signaling, but addition of heterologous dimerization motifs largely restores function; replacement of BRCT domains with a dimerization motif also rescues cells from the dominant-negative effect of overexpressed BRCT domains, demonstrating that the essential function of tandem BRCT domains is to oligomerize Crb2.\",\n      \"method\": \"Domain truncation and replacement, dimerization motif substitution, checkpoint signaling assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple domain-swap experiments with genetic rescue, clear mechanistic conclusion\",\n      \"pmids\": [\"15229228\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Gamma-H2A (phosphorylated histone H2A at the SQE motif by Rad3/Tel1 ATR/ATM-related kinases) is required for large-scale recruitment of Crb2 to damaged DNA and for proper checkpoint phosphorylation of Crb2 and Chk1; H2A-AQE mutations cause defective Crb2 phosphorylation and failure to recruit Crb2 to DSBs; H2A-AQE mutations partially suppress IR hypersensitivity of crb2Δ cells in a mechanism requiring Rqh1 helicase.\",\n      \"method\": \"Site-directed mutagenesis of histone H2A, fluorescence microscopy of Crb2 foci, checkpoint phosphorylation assays, genetic epistasis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal approaches (mutagenesis, microscopy, phosphorylation assays, epistasis), foundational study replicated by subsequent work\",\n      \"pmids\": [\"15226425\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Cooperative control of Crb2 by ATM-family kinases and Cdc2 is essential for the DNA damage checkpoint; crb2-T215A cells can initiate but not sustain a checkpoint response; gamma-H2A is essential for checkpoint function specifically in crb2-T215A cells; inactivation of Cdc2 in gamma-H2A-defective cells impairs Crb2-dependent Chk1 signaling, demonstrating that full Crb2 activity requires Cdc2-mediated phosphorylation of T215.\",\n      \"method\": \"Genetic epistasis, checkpoint kinase assays, site-directed mutagenesis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis combined with biochemical checkpoint assays, multiple mutant combinations tested\",\n      \"pmids\": [\"16314498\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Crb2 mediates Chk1 activation in fission yeast; Crb2 BRCT domain mutants that experience damaged replication forks activate a Chk1 pathway that induces sustained spindle checkpoint activation in a Mad2-dependent fashion, delaying metaphase-to-anaphase transition.\",\n      \"method\": \"BRCT domain mutant analysis, checkpoint assays, Mad2 genetic epistasis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — single lab, genetic epistasis with defined molecular pathway, multiple mutant alleles\",\n      \"pmids\": [\"16107732\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Using X-ray crystallography and NMR, both 53BP1 and fission yeast Crb2 contain tandem tudor domains that directly bind histone H4 specifically dimethylated at Lys20 (H4-K20me2); a unique five-residue binding cage in 53BP1 is structurally conserved in Crb2 and accommodates dimethyllysine but excludes trimethyllysine, establishing the molecular basis of methylation state-specific recognition.\",\n      \"method\": \"X-ray crystallography, NMR spectroscopy, in vitro binding assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with NMR validation and direct binding demonstration, independently replicated\",\n      \"pmids\": [\"17190600\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Crb2 IRIF formation requires two histone modifications (H2A C-terminal phosphorylation and H4-K20 methylation) that cooperate in the same recruitment pathway and require Crb2's Tudor and BRCT motifs; an alternative, parallel recruitment pathway sufficient for checkpoint activation requires a CDK phosphorylation site (T215) in Crb2 that mediates association with Cut5 (TopBP1 homolog); Cut5 also accumulates at HO-induced DSBs.\",\n      \"method\": \"Fluorescence microscopy of Crb2 foci, genetic epistasis with histone modification mutants, HO endonuclease-induced DSB assay, domain mutation analysis\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal genetic and imaging approaches, clear epistasis between two independent pathways\",\n      \"pmids\": [\"16778077\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Crystal structure of Crb2-BRCT2 domain alone and in complex with a phosphorylated H2A.1 peptide reveals the structural basis for Crb2 dimerization and direct interaction with gamma-H2A.1 at IRIF; mutational analysis shows that dimerization mutants are genotoxin-sensitive and defective in checkpoint signaling and Chk1 phosphorylation, while phosphopeptide-binding mutants are only slightly IR-sensitive, have extended checkpoint delays, and form IRIF but show reduced Rpa1 and Rad22 foci indicating a DNA repair defect.\",\n      \"method\": \"X-ray crystallography, site-directed mutagenesis, in vitro binding, checkpoint phosphorylation assays, fluorescence microscopy of foci\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure combined with mutagenesis and multiple functional assays, single rigorous study with orthogonal methods\",\n      \"pmids\": [\"18676809\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The tandem tudor domains of Crb2 preferentially bind di-methylated H4K20 over other methylation states; disruption of the tudor-binding motif or loss of Set9/Kmt5 (the H4K20 methylating enzyme) ablates Crb2 localization to DSBs and impairs checkpoint function; dimethylation but not trimethylation of H4K20 is specifically required for Crb2 localization, checkpoint function, and cell survival after DNA damage.\",\n      \"method\": \"In vitro tudor binding assays with differentially methylated peptides, fluorescence microscopy, checkpoint function assays, genetic knockout\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro binding combined with cellular localization and functional assays, methylation-state specificity rigorously demonstrated\",\n      \"pmids\": [\"18826944\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Crb2 BRCT2 domain residues serine-548 and lysine-619 make polar interactions with the phosphate group of phospho-H2A (gamma-H2A); mutations of these residues critically impair Crb2 IRIF formation and checkpoint function with additive effects when combined; combining either mutation with the T215A CDK phosphorylation site mutation completely abrogates Crb2 IRIF and function, demonstrating cooperative phosphate interactions between BRCT2 and gamma-H2A coupled with Tudor domain interactions with H4-K20me2.\",\n      \"method\": \"Site-directed mutagenesis, fluorescence microscopy, checkpoint kinase assays, double-mutant epistasis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — precise mutagenesis of binding residues with multiple orthogonal functional readouts, additive/epistatic mutations tested\",\n      \"pmids\": [\"20679485\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Loss of Crb2 BRCT domain phospho-H2AX binding activity severely impairs Crb2 accumulation at IR-induced DSBs, compromises checkpoint signaling, and disrupts checkpoint-mediated cell cycle arrest; combined ablation of both tudor (H4K20me2-binding) and BRCT (pH2AX-binding) modules yields a strikingly additive reduction in Crb2 activity.\",\n      \"method\": \"BRCT domain mutagenesis, fluorescence microscopy, checkpoint arrest assays, double-mutant analysis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — defined domain mutagenesis with multiple functional readouts, cooperative binding mechanism established\",\n      \"pmids\": [\"20679488\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Human CRB2 (the most abundant Crumbs ortholog in the brain) inhibits gamma-secretase cleavage of APP to reduce Abeta and AICD production; CRB2 knockdown increases gamma-secretase cleavage products; CRB2 interacts with the gamma-secretase complex but is not a competitive substrate; the transmembrane domain of CRB2 is indispensable for Abeta inhibition and mediates binding to the gamma-secretase complex; co-overexpression of presenilin-1 or APH-1 abrogates inhibition.\",\n      \"method\": \"Transfection/knockdown in HEK293 and SH-SY5Y cells, cell-free gamma-secretase assay, co-immunoprecipitation, domain deletion analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — cell-free in vitro assay combined with Co-IP, knockdown, overexpression, and domain deletion in two cell lines\",\n      \"pmids\": [\"20299451\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Germline knockout of mouse Crb2 causes embryonic lethality by E12.5; the primary defect is disturbed epiblast cell polarity at the primitive streak affecting epithelial-to-mesenchymal transition (EMT) during gastrulation, resulting in impaired mesoderm and endoderm formation, demonstrating an essential role for CRB2 in early embryonic epithelial polarity.\",\n      \"method\": \"Gene targeting/knockout, histological and developmental analysis of embryos\",\n      \"journal\": \"Developmental dynamics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KO with defined developmental phenotype, single lab, phenotype description without full molecular mechanism\",\n      \"pmids\": [\"22072575\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Phosphorylation-dependent interactions between Crb2 and Chk1 are essential for the DNA damage checkpoint; Crb2 recruits Chk1 to DSBs through direct physical interaction; a pair of conserved SQ/TQ motifs in Crb2 (Rad3 phosphorylation consensus) is required for Chk1 recruitment and activation; tethering Crb2 and Chk1 together rescues the SQ/TQ mutation; a 19-amino-acid phosphorylated peptide containing these motifs is sufficient for Chk1 binding in vitro; tethering this peptide to DSBs via Rad22/Rad52 or Rad4/Cut5 fusion can rescue crb2Δ checkpoint defect and even bypass the need for the 9-1-1 complex.\",\n      \"method\": \"In vitro binding with phosphopeptides, site-directed mutagenesis, fusion protein tethering, checkpoint assays, co-immunoprecipitation\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro binding, mutagenesis, and synthetic rescue experiments with multiple orthogonal approaches\",\n      \"pmids\": [\"22792081\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Conditional knockout of CRB2 in the developing mouse retina causes progressive disorganization, thinning of the photoreceptor layer, disruption of adherens junctions, increased numbers of late-born rod photoreceptors and Müller glia with concomitant programmed cell death, demonstrating an essential role for CRB2 in retinal lamination and suppression of late-born progenitor proliferation.\",\n      \"method\": \"Conditional gene knockout, confocal scanning laser ophthalmoscopy, OCT, electroretinography, histology\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean conditional KO with multiple functional readouts, single lab\",\n      \"pmids\": [\"23001562\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Combined ablation of Crb1 and Crb2 in retinal progenitor cells causes severe retinal dysfunction and dysregulation of Notch1 and YAP/Hippo signaling pathway target genes and increased P120-catenin levels; CRB1 and CRB2 restrain proliferation of retinal progenitor cells and control cell cycle distribution.\",\n      \"method\": \"Conditional double knockout, gene expression profiling, cell cycle analysis, electroretinography\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — double KO with pathway analysis, single lab, signaling pathway changes inferred from expression changes\",\n      \"pmids\": [\"24339791\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Recessive mutations in human CRB2 cause steroid-resistant nephrotic syndrome; zebrafish crb2b loss-of-function mutation shows that crb2b is required for podocyte foot process arborization, slit diaphragm formation, and proper nephrin trafficking; complementation experiments in zebrafish confirm that human CRB2 mutations result in loss of function.\",\n      \"method\": \"Homozygosity mapping, whole exome sequencing, zebrafish loss-of-function mutant analysis, complementation assay, nephrin trafficking analysis\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — complementation experiments in zebrafish combined with cellular phenotyping across multiple functional readouts\",\n      \"pmids\": [\"25557779\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Targeted ablation of CRB2 specifically in photoreceptors causes progressive photoreceptor degeneration (RP-like phenotype) with early abnormal lamination onset; ablation in Müller cells causes late-onset retinal disorganization; short-term loss of CRB2 in adult photoreceptors but not in Müller glial cells causes sporadic loss of adhesion between photoreceptors and Müller cells.\",\n      \"method\": \"Cell-type specific conditional deletion, AAV-Cre and shRNA-mediated ablation, OCT, electroretinography, histology\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-type specific KO with multiple in vivo functional readouts, single lab\",\n      \"pmids\": [\"24493795\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Hyperactive Cdc2 (cdc2.1w) mutation causes cells to bypass Chk1 and enter extended cell-cycle arrest dependent on Cds1 kinase in the presence of broken replication forks; this Chk1 bypass requires the mitotic Cdc2 phosphorylation site Crb2-T215, suggesting that presence of the T215 phosphorylation channels Rad3 activity towards Cds1 instead of Chk1; hyperactive Cdc2 locks cells in a G1-like repair mode favoring NHEJ over recombination.\",\n      \"method\": \"Genetic analysis with cdc2.1w mutant, checkpoint kinase assays, recombination assays\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — single lab, genetic analysis with CDK mutant, multiple pathway readouts\",\n      \"pmids\": [\"24861625\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"CRB2 protein is expressed in mouse retinal pigment epithelium (RPE) and is restricted to the apicolateral membrane, specifically at tight junctions; CRB2 completes a fully expressed Crumbs complex in the RPE apico-lateral membrane together with other CRB complex components.\",\n      \"method\": \"Custom antibody generation, confocal immunofluorescence microscopy, subcellular fractionation\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct localization with custom antibody, multiple imaging methods, no functional consequence tested\",\n      \"pmids\": [\"26404741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The 53BP1 ortholog Crb2 and Rev7 specifically repress long-range resection of DSBs through the RecQ helicase-dependent pathway, thereby preventing hyper-resection; this is established using a single-cell microscopy assay for resection phases in live S. pombe cells.\",\n      \"method\": \"Live-cell single-cell microscopy resection assay, genetic deletion, pathway dissection\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — novel assay with single lab, defined molecular pathway, genetic epistasis\",\n      \"pmids\": [\"29697047\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Knockdown of CRB2 in human RPE cells disrupts tight junction maintenance, affects cell cycle arrest, and in vivo knockdown of CRB2 in mouse RPE perturbs distribution of apical polarity proteins and results in invasion of activated microglial cells into the subretinal space, demonstrating CRB2 is required for development and maintenance of polarized RPE epithelium.\",\n      \"method\": \"siRNA knockdown, in vitro polarization assay, AAV-mediated in vivo knockdown, immunofluorescence\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KD with defined cellular and in vivo phenotypes, single lab, multiple readouts\",\n      \"pmids\": [\"34262913\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Podocyte-specific Crb2 knockout mice develop massive albuminuria and focal segmental glomerulosclerosis; human podocytes lacking CRB2 show significantly decreased F-actin positive area and increased susceptibility to apoptosis, demonstrating that CRB2 is required for proper actin cytoskeleton organization and podocyte survival.\",\n      \"method\": \"Podocyte-specific conditional knockout, electron microscopy, siRNA knockdown in human podocytes, F-actin quantification, apoptosis assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — both in vivo conditional KO and in vitro human cell knockdown with multiple orthogonal readouts\",\n      \"pmids\": [\"34654837\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CRB2 protein transport from the ER to the plasma membrane requires proper formation of disulfide bridges; disease-associated CRB2 missense variants and variants with mutated putative disulfide bridge-forming cysteines accumulate predominantly at the ER rather than reaching the plasma membrane; CRB2 is retained at the ER in cells lacking protein disulfide isomerase A3 (PDIA3), identifying post-translational disulfide bridge formation as a crucial step for CRB2 surface transport.\",\n      \"method\": \"Live-cell imaging of CRB2-GFP fusion proteins, BFP-labeled plasma membrane reporter, PDIA3 knockout cells, in silico variant selection\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — live-cell imaging with multiple variant alleles, genetic KO of PDIA3, functional readout of surface localization\",\n      \"pmids\": [\"36549870\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Anti-Crb2 autoantibodies cause nephrotic syndrome in mice; immunization with the recombinant extracellular domain of Crb2 generates autoantibodies that colocalize with Crb2 at podocyte foot processes; anti-Crb2 antibody treatment of podocytes increases phosphorylation of ezrin (which connects Crb2 to the cytoskeleton), alters Crb2 localization, and disturbs actin distribution.\",\n      \"method\": \"Mouse immunization model, anti-Crb2 antibody incubation of podocyte cell line, phosphorylation assays, immunofluorescence colocalization\",\n      \"journal\": \"Journal of the American Society of Nephrology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo immunization model combined with cell-based mechanistic assays, single lab\",\n      \"pmids\": [\"35985815\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Human CRB1 and CRB2 co-localize in the human retina and iPSC-derived retinal organoids; retina-specific pull-downs show CRB1 is enriched in CRB2 samples; co-immunoprecipitation demonstrates human canonical CRB1 interacts with CRB1 and CRB2 but not with CRB3 (which lacks an extracellular domain), indicating the extracellular domain mediates CRB1-CRB2 homo- and heterodimerization.\",\n      \"method\": \"Co-immunoprecipitation, retina-specific pull-down, co-localization in human retina and retinal organoids\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP with domain specificity (CRB3 negative control), single lab\",\n      \"pmids\": [\"38570189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CRB2 knockdown in podocytes induces YAP transcriptional activity and target gene expression, upregulates YAP-mediated mechanosignaling, and increases density of focal adhesions and F-actin; using ERISM, CRB2 knockdown enhances podocyte contractility in a substrate stiffness-dependent manner with impaired mechanosensing at low substrate stiffness, demonstrating CRB2 regulates podocyte mechanotransduction via YAP signaling.\",\n      \"method\": \"siRNA knockdown, YAP reporter assays, elastic resonator interference stress microscopy (ERISM), F-actin quantification, YAP inhibitor treatment\",\n      \"journal\": \"American journal of physiology. Renal physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — novel biophysical assay (ERISM) combined with YAP signaling analysis, single lab, multiple readouts\",\n      \"pmids\": [\"40062402\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"CRB2 inhibits transcription of the ferroptosis suppressor SLC7A11 by increasing dimethylation of histone H4 lysine 20 (H4K20me2) at its promoter; mechanistically, CRB2 hinders the interaction between the LSD1(E8A) histone lysine demethylase isoform and the deubiquitinase USP7, facilitating LSD1(E8A) degradation and thereby increasing H4K20me2 levels; this epigenetic axis sensitizes HNSCC cells to ferroptosis.\",\n      \"method\": \"CRB2 overexpression/knockdown, ferroptosis assays (erastin/RSL3), ChIP for H4K20me2, co-immunoprecipitation of LSD1-USP7 interaction, protein stability assays\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP combined with ChIP and ferroptosis functional assays, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"40991301\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CRB2 is a multifunctional protein operating in two major contexts: (1) as a fission yeast DNA damage checkpoint mediator (ortholog Crb2/Rhp9) that is recruited to DSBs through cooperative binding of its tandem Tudor domain to H4-K20me2 and its BRCT2 domain to gamma-H2A, undergoes phosphorylation by Rad3/ATR and Cdc2/CDK1 at distinct sites to control checkpoint initiation, maintenance, and termination, and recruits/activates Chk1 by direct phosphorylation-dependent interaction; and (2) as a mammalian Crumbs family transmembrane polarity protein that localizes to the subapical/apicolateral membrane of epithelial cells (retina, RPE, kidney podocytes), where it is essential for podocyte foot process formation and nephrin trafficking, retinal lamination and progenitor cell proliferation control, and forms homo- and heteromeric complexes with CRB1 via extracellular domains, with its surface transport dependent on disulfide bridge formation by PDIA3, and its loss in podocytes activating YAP mechanosignaling and disrupting actin cytoskeleton organization.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"The CRB2 timeline describes two coherent but distinct protein contexts that share the gene symbol: the fission yeast DNA damage checkpoint mediator Crb2 (a 53BP1-related ortholog) and the mammalian Crumbs-family transmembrane polarity protein CRB2. In S. pombe, Crb2 is a checkpoint adaptor that operates in a Cut5(TopBP1)–Crb2–Chk1 sensor-transmitter pathway, acting upstream of Chk1 to enforce DNA damage-induced arrest [#0]. Crb2 is recruited to double-strand breaks through cooperative reading of two histone marks: its tandem Tudor domain binds H4-K20me2 (deposited by Set9/Kmt5), specifically discriminating the dimethyl over trimethyl state, and its BRCT2 domain binds γ-H2A phosphorylated by Rad3/Tel1, with the two modules contributing additively to focus formation and checkpoint signaling [#4, #7, #10, #13, #14, #15]. A parallel CDK-dependent pathway, requiring Cdc2 phosphorylation of Crb2-T215 and association with Cut5, can recruit Crb2 to breaks independently of the histone-reading route [#11]. Crb2 function further requires BRCT-mediated homo-oligomerization [#6, #12], and once positioned it recruits and activates Chk1 via Rad3-phosphorylated SQ/TQ motifs that directly bind Chk1 [#5, #18]. Cdc2 phosphorylation of T215 governs the temporal switch between checkpoint maintenance and recovery, controlling re-entry into the cycle after repair and channeling checkpoint output [#1, #8, #23], and Crb2 also restrains long-range DSB resection to balance repair pathway choice [#2, #25]. In mammals, CRB2 is an apicolateral/subapical membrane polarity protein required for epiblast polarity during gastrulation [#17], retinal lamination and suppression of late-born progenitor proliferation through regulation of Notch and YAP/Hippo targets [#19, #20], and for development and maintenance of polarized retinal pigment epithelium [#24, #26]. In kidney podocytes, recessive CRB2 mutations cause steroid-resistant nephrotic syndrome, and CRB2 is required for foot process arborization, slit diaphragm formation, nephrin trafficking, actin cytoskeleton organization, and podocyte survival, with its loss activating YAP mechanosignaling [#21, #27, #31]. CRB2 surface delivery depends on PDIA3-mediated disulfide bridge formation, and disease variants are retained in the ER [#28]; its extracellular domain mediates homo- and heterodimerization with CRB1 [#30].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Established Crb2 as a checkpoint mediator acting upstream of Chk1, answering how DNA damage signals are transmitted to cell cycle arrest in fission yeast.\",\n      \"evidence\": \"Two-hybrid interaction, genetic suppressor analysis, and phosphorylation/checkpoint arrest assays placing Crb2 in a Cut5-Crb2-Chk1 pathway\",\n      \"pmids\": [\"9407031\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define the molecular basis of Crb2 recruitment to damage\", \"Direct vs indirect Chk1 activation not resolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identified CDK phosphorylation of Crb2-T215 as the switch enabling checkpoint recovery, distinguishing checkpoint maintenance from termination.\",\n      \"evidence\": \"In vitro Cdc2 kinase assay, T215A mutagenesis, and checkpoint re-entry assays\",\n      \"pmids\": [\"10488332\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which T215 phosphorylation silences Chk1 not defined\", \"Whether T215 also affects recruitment unaddressed at this stage\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Linked Crb2 to recombinational repair downstream of Cdc2, showing the checkpoint protein also regulates the repair machinery (Top3) in G2.\",\n      \"evidence\": \"Genetic epistasis, IR survival, and Rhp51 focus analysis\",\n      \"pmids\": [\"12023299\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct biochemical link between Crb2 and Top3 not shown\", \"Single-lab genetic inference\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstrated Crb2 forms DSB-localized nuclear foci and separated initial recruitment from Rad1/Rad3/Rad17-dependent retention.\",\n      \"evidence\": \"Live-cell fluorescence microscopy, co-localization with Rad22, deletion analysis\",\n      \"pmids\": [\"12917337\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular determinants of initial recruitment not yet identified\", \"Retention mechanism via 9-1-1 not mechanistically defined\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Defined the histone-mark code for Crb2 recruitment by identifying Set9/H4-K20 methylation and γ-H2A as the marks read at breaks, and showed BRCT-mediated oligomerization is the essential BRCT function.\",\n      \"evidence\": \"Genetic epistasis, ChIP, histone mutagenesis, Crb2 focus imaging, domain-swap rescue, and co-IP/in vitro Chk1 assays across four studies\",\n      \"pmids\": [\"15550243\", \"15226425\", \"15229228\", \"14739927\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of mark recognition not yet solved\", \"Methylation-state specificity not resolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Solved the structural basis of dimethyl-state-specific H4-K20me2 reading by the tandem Tudor domain and showed histone-reading and CDK/Cut5 pathways are parallel recruitment routes.\",\n      \"evidence\": \"X-ray crystallography, NMR, in vitro binding, and HO-induced DSB genetic epistasis\",\n      \"pmids\": [\"17190600\", \"16778077\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contribution of each pathway to checkpoint output in vivo not quantified\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Provided the structural and mutational basis for cooperative bivalent recruitment, showing BRCT2 dimerization and γ-H2A phosphopeptide binding have separable roles in checkpoint versus repair.\",\n      \"evidence\": \"Crystal structure of BRCT2-phospho-H2A complex, mutagenesis, binding and focus/checkpoint assays\",\n      \"pmids\": [\"18676809\", \"18826944\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How dimerization couples to downstream Chk1 activation not fully resolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Established that Tudor (H4K20me2) and BRCT (γ-H2A) modules contribute additively to recruitment, defining the bivalent-anchoring logic of Crb2.\",\n      \"evidence\": \"Precise mutagenesis of binding residues (S548, K619), double-mutant epistasis, focus imaging, and checkpoint kinase assays\",\n      \"pmids\": [\"20679485\", \"20679488\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of dual-mark engagement at a single break unknown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Resolved how Crb2 activates Chk1, showing Rad3-phosphorylated SQ/TQ motifs directly recruit Chk1 and can bypass upstream 9-1-1 when tethered.\",\n      \"evidence\": \"Phosphopeptide binding, mutagenesis, fusion-protein tethering rescue, and co-IP\",\n      \"pmids\": [\"22792081\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conformational change in Chk1 upon binding not defined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Extended CRB2 biology to mammals by showing human CRB2 inhibits γ-secretase cleavage of APP via its transmembrane domain.\",\n      \"evidence\": \"Knockdown/overexpression in HEK293 and SH-SY5Y, cell-free γ-secretase assay, co-IP, domain deletion\",\n      \"pmids\": [\"20299451\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance in brain not established\", \"Relationship to CRB2 polarity function unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified CRB2 as a developmental polarity protein essential in early embryo, retina, and kidney, with mutations causing human steroid-resistant nephrotic syndrome.\",\n      \"evidence\": \"Mouse germline and conditional knockouts, zebrafish loss-of-function and complementation, human exome sequencing, and nephrin trafficking analysis (spanning 2011–2014)\",\n      \"pmids\": [\"22072575\", \"23001562\", \"24493795\", \"25557779\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular link between CRB2 and nephrin trafficking not defined\", \"How CRB2 controls progenitor proliferation mechanistically unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined CRB2 cytoskeletal and survival functions in podocytes and RPE, linking its loss to actin disorganization, apoptosis, and epithelial polarity failure.\",\n      \"evidence\": \"Podocyte- and RPE-specific knockouts, human cell knockdown, F-actin quantification, apoptosis assays, and polarity protein imaging\",\n      \"pmids\": [\"34654837\", \"34262913\", \"24339791\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct effector linking CRB2 to actin not identified\", \"Single-lab signaling inferences\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Established post-translational control of CRB2 trafficking, showing PDIA3-mediated disulfide bridge formation is required for ER-to-surface transport and that disease variants are ER-retained, and that the extracellular domain mediates CRB1-CRB2 dimerization.\",\n      \"evidence\": \"Live-cell imaging of CRB2-GFP, PDIA3 knockout, disease-variant analysis, autoantibody immunization model, and reciprocal co-IP with CRB1/CRB3 (2022–2024)\",\n      \"pmids\": [\"36549870\", \"35985815\", \"38570189\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of CRB1-CRB2 heterodimers in vivo not defined\", \"Autoantibody mechanism vs genetic loss not fully reconciled\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Connected CRB2 loss to YAP mechanotransduction and, separately, identified a nuclear CRB2 role in H4K20me2-mediated transcriptional repression of SLC7A11 controlling ferroptosis.\",\n      \"evidence\": \"siRNA knockdown with YAP reporters and ERISM contractility measurement; CRB2 over/knockdown with ChIP for H4K20me2, LSD1-USP7 co-IP, and ferroptosis assays (2025–2026)\",\n      \"pmids\": [\"40062402\", \"40991301\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How a membrane polarity protein influences nuclear H4K20me2 mechanistically unclear\", \"Single-lab findings, awaiting independent confirmation\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved whether and how the chromatin-reading checkpoint functions of yeast Crb2 mechanistically relate to the membrane-polarity functions of mammalian CRB2, and how CRB2 transduces polarity loss into YAP, actin, and transcriptional outcomes.\",\n      \"evidence\": \"No discovery in the timeline bridges the checkpoint mediator and transmembrane polarity contexts\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No unified molecular model across the two contexts\", \"Effectors linking CRB2 surface signaling to nuclear/cytoskeletal outputs not defined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0042393\", \"supporting_discovery_ids\": [10, 13]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 5, 18]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0000228\", \"supporting_discovery_ids\": [3, 12]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [24, 28]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [28]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [3, 11, 25]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [0, 1, 8]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [17, 19, 21]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [20, 31]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"Chk1\", \"Cut5\", \"Rad3\", \"CRB1\", \"PDIA3\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}