{"gene":"CCM2","run_date":"2026-06-09T22:57:17","timeline":{"discoveries":[{"year":2005,"finding":"CCM2 (malcavernin/OSM) directly interacts with CCM1 (KRIT1) via the phosphotyrosine binding (PTB) domain of CCM2, and this interaction is dependent on the PTB domain. A familial CCM2 missense mutation abrogates the CCM1/CCM2 interaction. CCM2 also forms a ternary complex with CCM1 and MEKK3. CCM2 and ICAP1 bound to CCM1 via their respective PTB domains differentially influence the subcellular localization of CCM1.","method":"Co-immunoprecipitation, fluorescence resonance energy transfer (FRET), subcellular localization assays, missense mutant analysis","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, FRET, and localization assays in a single study; PTB-domain dependence confirmed by mutation; ternary complex demonstrated","pmids":["16037064"],"is_preprint":false},{"year":2007,"finding":"CCM3 (PDCD10) coprecipitates and colocalizes with CCM2, indicating a physical interaction. STK25 also forms a protein complex with CCM2, linking CCM2 to GCKIII kinase signaling.","method":"Co-immunoprecipitation, colocalization by immunofluorescence, Yeast two-hybrid","journal":"Neurogenetics","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP and colocalization shown; replicated in part by other studies showing CCM1/CCM2/CCM3 complex","pmids":["17657516"],"is_preprint":false},{"year":2008,"finding":"An in-frame deletion of CCM2 exon 2 (p.P11_K68del) results in a protein that can bind CCM3 but loses the ability to interact with CCM1 and to form a CCM1/CCM2/CCM3 ternary complex, demonstrating that the N-terminal region of CCM2 is required for CCM1 binding and that full-length CCM2 is the essential core of the CCM1/CCM2/CCM3 complex.","method":"Cell culture expression of deletion mutant, co-immunoprecipitation","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — specific domain deletion mutant with Co-IP readout; single lab but two binding partners tested","pmids":["18300272"],"is_preprint":false},{"year":2009,"finding":"CCM2 is required in endothelial cells for proper vascular development; endothelial-specific deletion of CCM2 severely impairs angiogenesis, causing morphogenic defects in major arterial/venous blood vessels and heart, leading to embryonic lethality at mid-gestation. Deletion from neuroglial precursor cells does not produce cerebrovascular defects.","method":"Tissue-specific conditional knockout mice (Cre-lox), embryonic phenotype analysis","journal":"Disease models & mechanisms","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean tissue-specific KO with defined vascular phenotype; cell-type specificity established by comparison of endothelial vs. neuroglial KO","pmids":["19259391"],"is_preprint":false},{"year":2009,"finding":"CCM2 interacts with the juxtamembrane region of TrkA receptor tyrosine kinase via its PTB domain and mediates TrkA-induced cell death. Both the PTB domain (for interaction specificity) and the Karet domain (for linking to death pathways) of CCM2 are required for TrkA-dependent cell death. Downregulation of CCM2 attenuates TrkA-dependent death in medulloblastoma and neuroblastoma cells.","method":"Co-immunoprecipitation, domain deletion/mutation analysis, siRNA knockdown, cell death assays","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, domain mapping with multiple mutants, functional KD rescue; multiple cell types tested","pmids":["19755102"],"is_preprint":false},{"year":2011,"finding":"Endothelial-specific Ccm2 deletion at postnatal day 1 in mice results in vascular lesions mimicking human CCM, restricted to the venous bed in cerebellum and retina. The consequences of Ccm2 loss depend on the developmental timing of ablation.","method":"Inducible endothelial-specific conditional knockout mice (postnatal Cre-lox), histology, MRI","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — inducible KO with temporally controlled deletion; defined cellular and anatomical phenotype; replicated across CCM1/2/3","pmids":["21859843"],"is_preprint":false},{"year":2011,"finding":"In zebrafish, ccm2 (valentine) mutants display heart and circulation defects distinct from ccm3 mutants. ccm2 defects cannot be rescued by overexpression of stk25b (unlike ccm3), and additional loss of ccm3 in ccm2 mutants produces synergistic cranial vessel dilation, supporting CCM2 and CCM3 acting via distinct pathways.","method":"Zebrafish genetic mutants, morpholino knockdown, epistasis/genetic interaction analysis, overexpression rescue","journal":"Developmental biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with multiple mutant combinations; rescue experiments; clear distinction from CCM3/STK25 pathway","pmids":["22182521"],"is_preprint":false},{"year":2012,"finding":"STK25 (GCKIII kinase) interacts with CCM2 and is identified as a novel CCM2 interactor by affinity proteomics. STK25, but not STK24, mediates TrkA/CCM2-dependent cell death in medulloblastoma cells; CCM2 can be phosphorylated by STK25, and STK25 kinase activity is required for death signaling downstream of TrkA/CCM2.","method":"Affinity proteomics (BioID/AP-MS), co-immunoprecipitation, siRNA knockdown, kinase assay, cell death assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — affinity proteomics discovery validated by Co-IP; kinase activity requirement shown by catalytic mutant; functional rescue","pmids":["22782892"],"is_preprint":false},{"year":2012,"finding":"The C-terminal domain of CCM2 adopts a folded helical structure homologous to the N-terminal domain of harmonin, named the CCM2 harmonin-homology domain (HHD). The crystal structure was determined at 1.9 Å resolution; analytical ultracentrifugation showed this domain is monomeric.","method":"X-ray crystallography (1.9 Å), analytical ultracentrifugation","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure at near-atomic resolution; biophysical validation of oligomerization state; single lab","pmids":["23266514"],"is_preprint":false},{"year":2013,"finding":"ccm2-like (ccm2l) binds CCM1 (Ccm1) and acts as a component of the Heg-CCM pathway in zebrafish cardiovascular development. ccm2 overexpression can partially rescue ccm2l morphant defects. Deletion and mutational analyses defined the regions of CCM1 that mediate binding to CCM2l and CCM2.","method":"Morpholino knockdown in zebrafish, mRNA overexpression rescue, co-immunoprecipitation, deletion/mutational analysis","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP with domain mapping plus zebrafish genetic rescue; single lab","pmids":["23328253"],"is_preprint":false},{"year":2014,"finding":"The CCM2 PTB domain displays preferential binding to the third NPX(Y/F) motif of KRIT1. The 2.75 Å co-crystal structure of the CCM2 PTB domain with a KRIT1 NPX(Y/F)3 peptide reveals a Dab-like PTB fold for CCM2. Disease-associated CCM2 missense mutations can destabilize the CCM2 PTB domain and disrupt the KRIT1-CCM2 interaction.","method":"X-ray crystallography (co-crystal at 2.75 Å), binding preference mapping, missense mutant functional analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — co-crystal structure with peptide ligand; domain preference mapping; disease mutant functional validation; multiple orthogonal approaches","pmids":["25525273"],"is_preprint":false},{"year":2015,"finding":"Both CCM2 and CCM2L bind MEKK3 in a complex with CCM1 and interfere with MEKK3 activation and its ability to phosphorylate MEK5 (downstream target). In endothelial cells, CCM2 deletion leads to activation of ERK5 and a MEKK3-dependent transcriptional program. In zebrafish, silencing of mekk3 rescues the big heart and body axis phenotype caused by ccm2l/ccm2 knockdown.","method":"Co-immunoprecipitation (in vitro), MEKK3 kinase assay (MEK5 phosphorylation), siRNA knockdown in endothelial cells, zebrafish morpholino knockdown and genetic rescue","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vitro binding plus kinase assay plus in vivo genetic epistasis in zebrafish; multiple orthogonal methods","pmids":["26540726"],"is_preprint":false},{"year":2016,"finding":"The CCM1-CCM2 complex acts as a scaffold to promote ROCK2 interactions with VE-cadherin and to limit ROCK1 kinase activity. Loss of CCM2 leads to excessive ROCK1-dependent actin stress fibers and destabilized intercellular junctions. Silencing of ROCK1 (but not ROCK2) restores adhesive and mechanical homeostasis of CCM1/CCM2-depleted endothelial monolayers and rescues cardiovascular defects in ccm1 mutant zebrafish.","method":"siRNA knockdown, Co-IP, traction force microscopy, zebrafish genetic rescue, ROCK1/ROCK2 isoform-specific knockdown","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Strong — Co-IP for scaffold function; isoform-specific KD with mechanical readout; zebrafish in vivo rescue; multiple orthogonal methods","pmids":["30030370"],"is_preprint":false},{"year":2016,"finding":"Micro-CT imaging in mice showed that Mekk3 heterozygosity prevents CCM lesion formation in Ccm2-deficient neonatal endothelium, placing MEKK3 downstream of CCM2 in a genetic epistasis relationship in vivo.","method":"Micro-CT imaging of mouse brains, Ccm2 endothelial-specific deletion combined with Mekk3 heterozygous KO","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean genetic epistasis in vivo with quantitative imaging; single lab, single method for rescue","pmids":["27513872"],"is_preprint":false},{"year":2017,"finding":"CCM2 and PAK4 are required downstream of ANP/GC-A signaling for phosphorylation of myosin light chain (MLC) and promotion of endothelial cell spreading. siRNA knockdown of CCM2 abolishes ANP-induced MLC phosphorylation and cell spreading in GC-A-expressing mouse endothelial cells.","method":"siRNA knockdown, MLC phosphorylation assay (Western blot), cell spreading assay","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — siRNA KD with defined phosphorylation and morphological readout; CCM2 placed downstream of ANP/GC-A; single lab","pmids":["28432261"],"is_preprint":false},{"year":2019,"finding":"CCM2 has an atypical PTB (aPTB) domain identified from alternatively spliced isoforms. Both CCM1 and CCM3 can bind competitively to this aPTB domain. Some long CCM2 isoforms contain both a typical PTB and the aPTB domain, making CCM2 a dual PTB domain-containing protein. CCM2 isoforms show distinct subcellular compartmentalization and cell/tissue-specific expression patterns.","method":"Molecular cloning, subcellular localization assays, co-immunoprecipitation (competitive binding), splice variant characterization","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP for competitive binding; subcellular localization by imaging; novel domain identified; single lab","pmids":["31676827"],"is_preprint":false},{"year":2021,"finding":"CCM2-silenced endothelial cells undergo a ROCK-dependent reprogramming into senescence-associated secretory phenotype (SASP), driving extracellular matrix invasion and chemoattraction of surrounding wild-type endothelial and immune cells. This SASP is driven by cytoskeletal, molecular, and transcriptomic disorders provoked by ROCK dysfunction.","method":"siRNA knockdown of CCM2, senescence assays (SA-β-gal), traction force microscopy, transcriptomic analysis, ECM invasion assay","journal":"Angiogenesis","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KD with multiple orthogonal phenotypic readouts (senescence, ECM invasion, transcriptomics); mechanistic link to ROCK established; single lab","pmids":["34342749"],"is_preprint":false},{"year":2023,"finding":"CCM2 in epicardial cells is required for cardiac development and regeneration. Loss of epicardial Ccm2 delays cardiac function recovery and aggravates cardiac fibrosis after myocardial infarction. Mechanistically, CCM2 targets production of cytoskeletal and matrix proteins to maintain epicardial cell adhesion, polarity, spreading, and migration.","method":"Epicardial-specific Ccm2 conditional knockout mice, ex vivo epicardial cell assays (adhesion, polarity, spreading, migration), proteomics/Western blot for cytoskeletal proteins, myocardial infarction model","journal":"JACC. Basic to translational science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-type-specific KO with defined functional phenotype and molecular mechanism; multiple cellular readouts; single lab","pmids":["38510716"],"is_preprint":false},{"year":2020,"finding":"CRISPR/Cas9-mediated knockout of CCM1 or CCM2 (as well as CCM3) in human endothelial cells impairs fibronectin expression and reduces fibronectin fiber formation in the extracellular matrix, demonstrating that impaired production of a functional fibronectin matrix is a common feature of CCM1-, CCM2-, and CCM3-deficient endothelial cells. Fibronectin supplementation rescued aberrant spheroid formation and altered EC morphology, and suppressed actin stress fiber formation.","method":"CRISPR/Cas9 KO in human endothelial cells, fibronectin rescue experiments, spheroid assay, actin imaging","journal":"FASEB journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KO with defined molecular and cellular phenotype; rescue experiment; single lab","pmids":["32515053"],"is_preprint":false},{"year":2026,"finding":"To restrain KLF4 expression in endothelial cells, two CCM2 molecules must cluster on a single KRIT1, with the PTB domain of each CCM2 binding either the second or third NPxF motif within KRIT1. This dual PTB-domain recruitment to a single peptide scaffold is a previously unobserved mechanism. Loss of either KRIT1 or CCM2 causes over-expression of KLF4.","method":"Knockdown and reconstitution in endothelial cell lines, co-immunoprecipitation, biophysical analysis of purified proteins, co-crystallography","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — co-crystal structure combined with biophysical analysis and functional cellular reconstitution; multiple orthogonal methods in a single rigorous study","pmids":["41688454"],"is_preprint":false},{"year":2008,"finding":"Complete localized loss of CCM2 protein expression (but not CCM1 or CCM3) is observed specifically in cavernous endothelial cells of CCM2 germline mutation carriers, establishing endothelial cell-specific somatic second-hit loss of function and identifying the endothelial cell as the cell of disease origin.","method":"Immunohistochemistry of human CCM tissue from germline mutation carriers, protein expression analysis","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 3 / Strong — IHC-based protein loss in relevant tissue; replicated across CCM1/2/3; but IHC is a lower-tier method","pmids":["19088124"],"is_preprint":false}],"current_model":"CCM2 (malcavernin/OSM) is a scaffold/adaptor protein that forms a core ternary complex with CCM1 (KRIT1) and CCM3 (PDCD10): CCM2 binds KRIT1 via its PTB domain (with two CCM2 PTB domains clustering on one KRIT1 to suppress KLF4 expression), and its N-terminal region is required for CCM1 binding while its C-terminal harmonin-homology domain (HHD) provides structural integrity; within this complex, CCM2 restrains MEKK3 kinase activity (thereby suppressing MEK5/ERK5 and KLF4/KLF2 transcriptional programs), orchestrates ROCK1 versus ROCK2 activity to maintain endothelial cell-cell junctions and prevent actin stress fiber formation, and in endothelial cells is required for proper angiogenesis, fibronectin matrix production, and prevention of ROCK-dependent senescence-associated secretory phenotype; CCM2 also mediates TrkA receptor tyrosine kinase-induced apoptosis via its PTB (interaction specificity) and Karet (death pathway linkage) domains in partnership with STK25 kinase."},"narrative":{"mechanistic_narrative":"CCM2 (malcavernin/OSM) is a scaffold/adaptor protein that organizes a core ternary complex with CCM1 (KRIT1) and CCM3 (PDCD10) to maintain endothelial vascular integrity [PMID:16037064, PMID:18300272]. CCM2 binds KRIT1 through its phosphotyrosine-binding (PTB) domain, which adopts a Dab-like fold and preferentially engages the third NPX(Y/F) motif of KRIT1; disease-associated CCM2 missense mutations destabilize this domain and disrupt the interaction [PMID:16037064, PMID:25525273]. Its N-terminal region is required for CCM1 binding and ternary-complex assembly, while its C-terminal harmonin-homology domain provides structural integrity [PMID:18300272, PMID:23266514]; long isoforms additionally carry an atypical PTB domain that CCM1 and CCM3 bind competitively [PMID:31676827]. CCM2 is required in endothelial cells for proper angiogenesis and vascular development, and endothelial loss produces CCM-like venous lesions, with the endothelial cell established as the cell of disease origin [PMID:19259391, PMID:21859843, PMID:19088124]. Mechanistically, the CCM1-CCM2 complex restrains MEKK3 kinase activity, thereby suppressing MEK5/ERK5 signaling and KLF4 transcription—the latter requiring two CCM2 PTB domains to cluster on a single KRIT1 scaffold [PMID:26540726, PMID:27513872, PMID:41688454]—and acts as a scaffold favoring ROCK2 association with VE-cadherin while limiting ROCK1 activity to preserve cell-cell junctions and prevent actin stress fibers, fibronectin matrix defects, and ROCK-dependent senescence-associated secretory phenotype [PMID:30030370, PMID:34342749, PMID:32515053]. Independent of its vascular role, CCM2 mediates TrkA receptor-induced apoptosis through its PTB and Karet domains in partnership with the GCKIII kinase STK25, which phosphorylates CCM2 to drive death signaling [PMID:19755102, PMID:22782892].","teleology":[{"year":2005,"claim":"Established the physical basis of the CCM complex by showing CCM2 binds CCM1 through its PTB domain and seeds a ternary complex, explaining how distinct CCM gene products converge on one pathway.","evidence":"Co-IP, FRET, and localization assays with a familial missense mutant in cells","pmids":["16037064"],"confidence":"High","gaps":["Did not define the KRIT1 motif bound","MEKK3 role within the complex not functionally tested"]},{"year":2007,"claim":"Extended the complex to include CCM3 and linked CCM2 to GCKIII signaling by identifying STK25 as a binding partner.","evidence":"Co-IP, immunofluorescence colocalization, and yeast two-hybrid","pmids":["17657516"],"confidence":"Medium","gaps":["No functional consequence of CCM2-STK25 binding tested","Stoichiometry of CCM2/CCM3 interaction undefined"]},{"year":2008,"claim":"Mapped domain requirements for complex assembly, showing the N-terminal region is needed for CCM1 binding while CCM3 binding is retained, and that full-length CCM2 is the essential core of the ternary complex.","evidence":"Exon-2 deletion mutant expression with Co-IP for two partners","pmids":["18300272"],"confidence":"Medium","gaps":["Single lab","Structural impact of deletion not resolved"]},{"year":2008,"claim":"Identified the endothelial cell as the cell of disease origin by demonstrating endothelial-specific second-hit loss of CCM2 protein in human cavernous lesions.","evidence":"IHC of CCM tissue from germline mutation carriers","pmids":["19088124"],"confidence":"Medium","gaps":["IHC is a lower-tier method","Does not establish downstream mechanism of lesion formation"]},{"year":2009,"claim":"Defined the in vivo requirement for CCM2 in endothelial vascular morphogenesis and showed cell-type specificity by contrasting endothelial versus neuroglial deletion.","evidence":"Tissue-specific conditional KO mice with embryonic phenotyping","pmids":["19259391"],"confidence":"High","gaps":["Molecular effector of the angiogenic defect not identified in this study","Embryonic lethality limits adult lesion analysis"]},{"year":2009,"claim":"Revealed a vascular-independent function: CCM2 transmits TrkA receptor-induced apoptosis via its PTB and Karet domains, expanding its role beyond endothelial scaffolding.","evidence":"Co-IP, domain mutation mapping, siRNA, and cell death assays in tumor cells","pmids":["19755102"],"confidence":"High","gaps":["Kinase coupling death signal not yet identified at this stage","Relevance to endothelial complex unclear"]},{"year":2011,"claim":"Showed that timing of CCM2 loss determines outcome, with postnatal endothelial deletion producing venous CCM-like lesions, modeling the human disease.","evidence":"Inducible postnatal endothelial-specific KO mice with histology and MRI","pmids":["21859843"],"confidence":"High","gaps":["Why lesions are restricted to the venous bed not explained","Downstream signaling not addressed here"]},{"year":2011,"claim":"Genetically distinguished CCM2 from the CCM3/STK25 pathway, showing ccm2 defects are not rescued by stk25b and synergize with ccm3 loss.","evidence":"Zebrafish mutants, morpholino knockdown, epistasis, and overexpression rescue","pmids":["22182521"],"confidence":"High","gaps":["Molecular basis of CCM2/CCM3 divergence undefined","Does not reconcile with shared ternary complex"]},{"year":2012,"claim":"Solidified the CCM2-STK25 axis in apoptosis by showing STK25 (not STK24) phosphorylates CCM2 and its kinase activity is required for TrkA/CCM2 death signaling.","evidence":"Affinity proteomics, Co-IP, kinase assay with catalytic mutant, siRNA, cell death assays","pmids":["22782892"],"confidence":"High","gaps":["CCM2 phosphosites not mapped","Connection to endothelial CCM function unresolved"]},{"year":2012,"claim":"Provided atomic-level architecture of the C-terminus, identifying a monomeric harmonin-homology domain that contributes structural integrity to CCM2.","evidence":"X-ray crystallography at 1.9 Å and analytical ultracentrifugation","pmids":["23266514"],"confidence":"High","gaps":["No binding partner assigned to the HHD","Functional role of the domain in vivo untested"]},{"year":2013,"claim":"Placed CCM2 within the Heg-CCM cardiovascular pathway and mapped the CCM1 regions binding CCM2 and the paralog CCM2L.","evidence":"Zebrafish morpholino, mRNA rescue, Co-IP, and deletion mapping","pmids":["23328253"],"confidence":"Medium","gaps":["Functional redundancy between CCM2 and CCM2L not fully resolved","Single lab"]},{"year":2014,"claim":"Defined the molecular recognition code, showing the Dab-like CCM2 PTB domain preferentially binds the third KRIT1 NPX(Y/F) motif and that disease mutants destabilize this interface.","evidence":"Co-crystal structure at 2.75 Å, binding preference mapping, missense mutant analysis","pmids":["25525273"],"confidence":"High","gaps":["Stoichiometry of PTB engagement not resolved at this stage","Downstream effect of binding loss not measured here"]},{"year":2015,"claim":"Established the effector mechanism of the vascular complex: CCM2 (and CCM2L) bind MEKK3 to suppress its kinase activity and the ERK5 transcriptional program, with mekk3 silencing rescuing ccm2 phenotypes.","evidence":"In vitro Co-IP, MEKK3-MEK5 kinase assay, endothelial siRNA, and zebrafish epistasis","pmids":["26540726"],"confidence":"High","gaps":["Did not define the transcription factors downstream of ERK5","Mechanism of MEKK3 inhibition structurally undefined"]},{"year":2016,"claim":"Confirmed MEKK3 as the genetically epistatic effector in vivo, with Mekk3 heterozygosity preventing CCM lesions in Ccm2-deficient endothelium.","evidence":"Micro-CT of mouse brains combining endothelial Ccm2 deletion with Mekk3 heterozygosity","pmids":["27513872"],"confidence":"Medium","gaps":["Single rescue method","Does not address the cytoskeletal arm of CCM2 function"]},{"year":2016,"claim":"Defined a parallel cytoskeletal mechanism: the CCM1-CCM2 scaffold promotes ROCK2-VE-cadherin association and limits ROCK1, with ROCK1 silencing restoring junction stability and rescuing zebrafish defects.","evidence":"siRNA, Co-IP, traction force microscopy, isoform-specific knockdown, zebrafish rescue","pmids":["30030370"],"confidence":"High","gaps":["How CCM2 differentially partitions ROCK1 vs ROCK2 structurally unknown","Integration with the MEKK3 arm not resolved"]},{"year":2017,"claim":"Connected CCM2 to ANP/GC-A signaling, showing it is required downstream for PAK4-dependent MLC phosphorylation and endothelial cell spreading.","evidence":"siRNA, MLC phosphorylation Western blot, and cell spreading assay","pmids":["28432261"],"confidence":"Medium","gaps":["Direct CCM2-PAK4 interaction not established","Single lab"]},{"year":2019,"claim":"Revealed isoform diversity by identifying an atypical PTB domain in long CCM2 isoforms that binds CCM1 and CCM3 competitively, making CCM2 a dual-PTB protein with distinct subcellular distributions.","evidence":"Cloning, subcellular localization, and competitive-binding Co-IP of splice variants","pmids":["31676827"],"confidence":"Medium","gaps":["Functional consequence of competitive CCM1/CCM3 binding untested","Tissue specificity of isoform roles unknown"]},{"year":2020,"claim":"Identified a shared downstream output, showing CCM2 loss (like CCM1/CCM3) impairs fibronectin matrix production and that fibronectin supplementation rescues morphology and suppresses stress fibers.","evidence":"CRISPR/Cas9 KO in human endothelial cells, fibronectin rescue, spheroid and actin imaging","pmids":["32515053"],"confidence":"Medium","gaps":["Mechanism linking CCM2 loss to reduced fibronectin not defined","Single lab"]},{"year":2021,"claim":"Extended the cytoskeletal phenotype to cell-state reprogramming, showing CCM2 silencing drives a ROCK-dependent senescence-associated secretory phenotype that recruits neighboring cells.","evidence":"siRNA, SA-β-gal senescence assays, traction force microscopy, transcriptomics, ECM invasion","pmids":["34342749"],"confidence":"Medium","gaps":["Causal transcriptional drivers of SASP not pinpointed","Single lab"]},{"year":2023,"claim":"Broadened the developmental role beyond endothelium, showing epicardial CCM2 maintains cytoskeletal/matrix protein production needed for cardiac development and post-infarction regeneration.","evidence":"Epicardial-specific conditional KO mice, ex vivo cell assays, proteomics, MI model","pmids":["38510716"],"confidence":"Medium","gaps":["Molecular pathway in epicardium not linked to MEKK3 or ROCK arms","Single lab"]},{"year":2026,"claim":"Resolved the stoichiometry of transcriptional control, showing two CCM2 PTB domains must cluster on a single KRIT1 (binding the second and third NPxF motifs) to restrain KLF4 expression, a novel dual-PTB recruitment mechanism.","evidence":"Endothelial knockdown/reconstitution, Co-IP, biophysics of purified proteins, co-crystallography","pmids":["41688454"],"confidence":"High","gaps":["How KLF4 restraint integrates with MEKK3/ERK5 signaling not fully mapped","Relevance of dual-PTB clustering to ROCK arm untested"]},{"year":null,"claim":"How CCM2's distinct functional arms—MEKK3/KLF4 transcriptional restraint, ROCK1/ROCK2 cytoskeletal control, fibronectin matrix production, and TrkA/STK25 apoptosis—are coordinated within or partitioned across its domains and isoforms remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified structural model integrating MEKK3 and ROCK functions","Functional division of labor among CCM2 isoforms undefined","Mechanistic link between scaffold loss and reduced fibronectin not established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,2,11,12,19]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[11,12,19]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[11]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0,15]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[12,4]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[11,12,14]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[3,5,17]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[4,7]},{"term_id":"R-HSA-1474244","term_label":"Extracellular matrix organization","supporting_discovery_ids":[18,17]}],"complexes":["CCM complex (CCM1-CCM2-CCM3)"],"partners":["KRIT1","PDCD10","STK25","MAP3K3","ROCK1","ROCK2","NTRK1","KLF4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BSQ5","full_name":"Cerebral cavernous malformations 2 protein","aliases":["Malcavernin"],"length_aa":444,"mass_kda":48.8,"function":"Component of the CCM signaling pathway which is a crucial regulator of heart and vessel formation and integrity. May act through the stabilization of endothelial cell junctions (By similarity). May function as a scaffold protein for MAP2K3-MAP3K3 signaling. Seems to play a major role in the modulation of MAP3K3-dependent p38 activation induced by hyperosmotic shock (By similarity)","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q9BSQ5/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CCM2","classification":"Not Classified","n_dependent_lines":26,"n_total_lines":1208,"dependency_fraction":0.02152317880794702},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CCM2","total_profiled":1310},"omim":[{"mim_id":"621032","title":"CEREBRAL CAVERNOUS MALFORMATIONS 5; CCM5","url":"https://www.omim.org/entry/621032"},{"mim_id":"619538","title":"CEREBRAL CAVERNOUS MALFORMATIONS 4; CCM4","url":"https://www.omim.org/entry/619538"},{"mim_id":"619402","title":"CARDIOMYOPATHY, FAMILIAL HYPERTROPHIC, 28; CMH28","url":"https://www.omim.org/entry/619402"},{"mim_id":"615466","title":"TALIN ROD DOMAIN-CONTAINING PROTEIN 1; TLNRD1","url":"https://www.omim.org/entry/615466"},{"mim_id":"614182","title":"HEART DEVELOPMENT PROTEIN WITH EGF-LIKE DOMAINS 1; HEG1","url":"https://www.omim.org/entry/614182"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Mitochondria","reliability":"Enhanced"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in 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A familial CCM2 missense mutation abrogates the CCM1/CCM2 interaction. CCM2 also forms a ternary complex with CCM1 and MEKK3. CCM2 and ICAP1 bound to CCM1 via their respective PTB domains differentially influence the subcellular localization of CCM1.\",\n      \"method\": \"Co-immunoprecipitation, fluorescence resonance energy transfer (FRET), subcellular localization assays, missense mutant analysis\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, FRET, and localization assays in a single study; PTB-domain dependence confirmed by mutation; ternary complex demonstrated\",\n      \"pmids\": [\"16037064\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"CCM3 (PDCD10) coprecipitates and colocalizes with CCM2, indicating a physical interaction. STK25 also forms a protein complex with CCM2, linking CCM2 to GCKIII kinase signaling.\",\n      \"method\": \"Co-immunoprecipitation, colocalization by immunofluorescence, Yeast two-hybrid\",\n      \"journal\": \"Neurogenetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP and colocalization shown; replicated in part by other studies showing CCM1/CCM2/CCM3 complex\",\n      \"pmids\": [\"17657516\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"An in-frame deletion of CCM2 exon 2 (p.P11_K68del) results in a protein that can bind CCM3 but loses the ability to interact with CCM1 and to form a CCM1/CCM2/CCM3 ternary complex, demonstrating that the N-terminal region of CCM2 is required for CCM1 binding and that full-length CCM2 is the essential core of the CCM1/CCM2/CCM3 complex.\",\n      \"method\": \"Cell culture expression of deletion mutant, co-immunoprecipitation\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — specific domain deletion mutant with Co-IP readout; single lab but two binding partners tested\",\n      \"pmids\": [\"18300272\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"CCM2 is required in endothelial cells for proper vascular development; endothelial-specific deletion of CCM2 severely impairs angiogenesis, causing morphogenic defects in major arterial/venous blood vessels and heart, leading to embryonic lethality at mid-gestation. Deletion from neuroglial precursor cells does not produce cerebrovascular defects.\",\n      \"method\": \"Tissue-specific conditional knockout mice (Cre-lox), embryonic phenotype analysis\",\n      \"journal\": \"Disease models & mechanisms\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean tissue-specific KO with defined vascular phenotype; cell-type specificity established by comparison of endothelial vs. neuroglial KO\",\n      \"pmids\": [\"19259391\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"CCM2 interacts with the juxtamembrane region of TrkA receptor tyrosine kinase via its PTB domain and mediates TrkA-induced cell death. Both the PTB domain (for interaction specificity) and the Karet domain (for linking to death pathways) of CCM2 are required for TrkA-dependent cell death. Downregulation of CCM2 attenuates TrkA-dependent death in medulloblastoma and neuroblastoma cells.\",\n      \"method\": \"Co-immunoprecipitation, domain deletion/mutation analysis, siRNA knockdown, cell death assays\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, domain mapping with multiple mutants, functional KD rescue; multiple cell types tested\",\n      \"pmids\": [\"19755102\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Endothelial-specific Ccm2 deletion at postnatal day 1 in mice results in vascular lesions mimicking human CCM, restricted to the venous bed in cerebellum and retina. The consequences of Ccm2 loss depend on the developmental timing of ablation.\",\n      \"method\": \"Inducible endothelial-specific conditional knockout mice (postnatal Cre-lox), histology, MRI\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — inducible KO with temporally controlled deletion; defined cellular and anatomical phenotype; replicated across CCM1/2/3\",\n      \"pmids\": [\"21859843\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In zebrafish, ccm2 (valentine) mutants display heart and circulation defects distinct from ccm3 mutants. ccm2 defects cannot be rescued by overexpression of stk25b (unlike ccm3), and additional loss of ccm3 in ccm2 mutants produces synergistic cranial vessel dilation, supporting CCM2 and CCM3 acting via distinct pathways.\",\n      \"method\": \"Zebrafish genetic mutants, morpholino knockdown, epistasis/genetic interaction analysis, overexpression rescue\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with multiple mutant combinations; rescue experiments; clear distinction from CCM3/STK25 pathway\",\n      \"pmids\": [\"22182521\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"STK25 (GCKIII kinase) interacts with CCM2 and is identified as a novel CCM2 interactor by affinity proteomics. STK25, but not STK24, mediates TrkA/CCM2-dependent cell death in medulloblastoma cells; CCM2 can be phosphorylated by STK25, and STK25 kinase activity is required for death signaling downstream of TrkA/CCM2.\",\n      \"method\": \"Affinity proteomics (BioID/AP-MS), co-immunoprecipitation, siRNA knockdown, kinase assay, cell death assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — affinity proteomics discovery validated by Co-IP; kinase activity requirement shown by catalytic mutant; functional rescue\",\n      \"pmids\": [\"22782892\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The C-terminal domain of CCM2 adopts a folded helical structure homologous to the N-terminal domain of harmonin, named the CCM2 harmonin-homology domain (HHD). The crystal structure was determined at 1.9 Å resolution; analytical ultracentrifugation showed this domain is monomeric.\",\n      \"method\": \"X-ray crystallography (1.9 Å), analytical ultracentrifugation\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure at near-atomic resolution; biophysical validation of oligomerization state; single lab\",\n      \"pmids\": [\"23266514\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"ccm2-like (ccm2l) binds CCM1 (Ccm1) and acts as a component of the Heg-CCM pathway in zebrafish cardiovascular development. ccm2 overexpression can partially rescue ccm2l morphant defects. Deletion and mutational analyses defined the regions of CCM1 that mediate binding to CCM2l and CCM2.\",\n      \"method\": \"Morpholino knockdown in zebrafish, mRNA overexpression rescue, co-immunoprecipitation, deletion/mutational analysis\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP with domain mapping plus zebrafish genetic rescue; single lab\",\n      \"pmids\": [\"23328253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The CCM2 PTB domain displays preferential binding to the third NPX(Y/F) motif of KRIT1. The 2.75 Å co-crystal structure of the CCM2 PTB domain with a KRIT1 NPX(Y/F)3 peptide reveals a Dab-like PTB fold for CCM2. Disease-associated CCM2 missense mutations can destabilize the CCM2 PTB domain and disrupt the KRIT1-CCM2 interaction.\",\n      \"method\": \"X-ray crystallography (co-crystal at 2.75 Å), binding preference mapping, missense mutant functional analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — co-crystal structure with peptide ligand; domain preference mapping; disease mutant functional validation; multiple orthogonal approaches\",\n      \"pmids\": [\"25525273\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Both CCM2 and CCM2L bind MEKK3 in a complex with CCM1 and interfere with MEKK3 activation and its ability to phosphorylate MEK5 (downstream target). In endothelial cells, CCM2 deletion leads to activation of ERK5 and a MEKK3-dependent transcriptional program. In zebrafish, silencing of mekk3 rescues the big heart and body axis phenotype caused by ccm2l/ccm2 knockdown.\",\n      \"method\": \"Co-immunoprecipitation (in vitro), MEKK3 kinase assay (MEK5 phosphorylation), siRNA knockdown in endothelial cells, zebrafish morpholino knockdown and genetic rescue\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vitro binding plus kinase assay plus in vivo genetic epistasis in zebrafish; multiple orthogonal methods\",\n      \"pmids\": [\"26540726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The CCM1-CCM2 complex acts as a scaffold to promote ROCK2 interactions with VE-cadherin and to limit ROCK1 kinase activity. Loss of CCM2 leads to excessive ROCK1-dependent actin stress fibers and destabilized intercellular junctions. Silencing of ROCK1 (but not ROCK2) restores adhesive and mechanical homeostasis of CCM1/CCM2-depleted endothelial monolayers and rescues cardiovascular defects in ccm1 mutant zebrafish.\",\n      \"method\": \"siRNA knockdown, Co-IP, traction force microscopy, zebrafish genetic rescue, ROCK1/ROCK2 isoform-specific knockdown\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — Co-IP for scaffold function; isoform-specific KD with mechanical readout; zebrafish in vivo rescue; multiple orthogonal methods\",\n      \"pmids\": [\"30030370\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Micro-CT imaging in mice showed that Mekk3 heterozygosity prevents CCM lesion formation in Ccm2-deficient neonatal endothelium, placing MEKK3 downstream of CCM2 in a genetic epistasis relationship in vivo.\",\n      \"method\": \"Micro-CT imaging of mouse brains, Ccm2 endothelial-specific deletion combined with Mekk3 heterozygous KO\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean genetic epistasis in vivo with quantitative imaging; single lab, single method for rescue\",\n      \"pmids\": [\"27513872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"CCM2 and PAK4 are required downstream of ANP/GC-A signaling for phosphorylation of myosin light chain (MLC) and promotion of endothelial cell spreading. siRNA knockdown of CCM2 abolishes ANP-induced MLC phosphorylation and cell spreading in GC-A-expressing mouse endothelial cells.\",\n      \"method\": \"siRNA knockdown, MLC phosphorylation assay (Western blot), cell spreading assay\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — siRNA KD with defined phosphorylation and morphological readout; CCM2 placed downstream of ANP/GC-A; single lab\",\n      \"pmids\": [\"28432261\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CCM2 has an atypical PTB (aPTB) domain identified from alternatively spliced isoforms. Both CCM1 and CCM3 can bind competitively to this aPTB domain. Some long CCM2 isoforms contain both a typical PTB and the aPTB domain, making CCM2 a dual PTB domain-containing protein. CCM2 isoforms show distinct subcellular compartmentalization and cell/tissue-specific expression patterns.\",\n      \"method\": \"Molecular cloning, subcellular localization assays, co-immunoprecipitation (competitive binding), splice variant characterization\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP for competitive binding; subcellular localization by imaging; novel domain identified; single lab\",\n      \"pmids\": [\"31676827\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CCM2-silenced endothelial cells undergo a ROCK-dependent reprogramming into senescence-associated secretory phenotype (SASP), driving extracellular matrix invasion and chemoattraction of surrounding wild-type endothelial and immune cells. This SASP is driven by cytoskeletal, molecular, and transcriptomic disorders provoked by ROCK dysfunction.\",\n      \"method\": \"siRNA knockdown of CCM2, senescence assays (SA-β-gal), traction force microscopy, transcriptomic analysis, ECM invasion assay\",\n      \"journal\": \"Angiogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KD with multiple orthogonal phenotypic readouts (senescence, ECM invasion, transcriptomics); mechanistic link to ROCK established; single lab\",\n      \"pmids\": [\"34342749\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CCM2 in epicardial cells is required for cardiac development and regeneration. Loss of epicardial Ccm2 delays cardiac function recovery and aggravates cardiac fibrosis after myocardial infarction. Mechanistically, CCM2 targets production of cytoskeletal and matrix proteins to maintain epicardial cell adhesion, polarity, spreading, and migration.\",\n      \"method\": \"Epicardial-specific Ccm2 conditional knockout mice, ex vivo epicardial cell assays (adhesion, polarity, spreading, migration), proteomics/Western blot for cytoskeletal proteins, myocardial infarction model\",\n      \"journal\": \"JACC. Basic to translational science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-type-specific KO with defined functional phenotype and molecular mechanism; multiple cellular readouts; single lab\",\n      \"pmids\": [\"38510716\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CRISPR/Cas9-mediated knockout of CCM1 or CCM2 (as well as CCM3) in human endothelial cells impairs fibronectin expression and reduces fibronectin fiber formation in the extracellular matrix, demonstrating that impaired production of a functional fibronectin matrix is a common feature of CCM1-, CCM2-, and CCM3-deficient endothelial cells. Fibronectin supplementation rescued aberrant spheroid formation and altered EC morphology, and suppressed actin stress fiber formation.\",\n      \"method\": \"CRISPR/Cas9 KO in human endothelial cells, fibronectin rescue experiments, spheroid assay, actin imaging\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KO with defined molecular and cellular phenotype; rescue experiment; single lab\",\n      \"pmids\": [\"32515053\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"To restrain KLF4 expression in endothelial cells, two CCM2 molecules must cluster on a single KRIT1, with the PTB domain of each CCM2 binding either the second or third NPxF motif within KRIT1. This dual PTB-domain recruitment to a single peptide scaffold is a previously unobserved mechanism. Loss of either KRIT1 or CCM2 causes over-expression of KLF4.\",\n      \"method\": \"Knockdown and reconstitution in endothelial cell lines, co-immunoprecipitation, biophysical analysis of purified proteins, co-crystallography\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — co-crystal structure combined with biophysical analysis and functional cellular reconstitution; multiple orthogonal methods in a single rigorous study\",\n      \"pmids\": [\"41688454\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Complete localized loss of CCM2 protein expression (but not CCM1 or CCM3) is observed specifically in cavernous endothelial cells of CCM2 germline mutation carriers, establishing endothelial cell-specific somatic second-hit loss of function and identifying the endothelial cell as the cell of disease origin.\",\n      \"method\": \"Immunohistochemistry of human CCM tissue from germline mutation carriers, protein expression analysis\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Strong — IHC-based protein loss in relevant tissue; replicated across CCM1/2/3; but IHC is a lower-tier method\",\n      \"pmids\": [\"19088124\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CCM2 (malcavernin/OSM) is a scaffold/adaptor protein that forms a core ternary complex with CCM1 (KRIT1) and CCM3 (PDCD10): CCM2 binds KRIT1 via its PTB domain (with two CCM2 PTB domains clustering on one KRIT1 to suppress KLF4 expression), and its N-terminal region is required for CCM1 binding while its C-terminal harmonin-homology domain (HHD) provides structural integrity; within this complex, CCM2 restrains MEKK3 kinase activity (thereby suppressing MEK5/ERK5 and KLF4/KLF2 transcriptional programs), orchestrates ROCK1 versus ROCK2 activity to maintain endothelial cell-cell junctions and prevent actin stress fiber formation, and in endothelial cells is required for proper angiogenesis, fibronectin matrix production, and prevention of ROCK-dependent senescence-associated secretory phenotype; CCM2 also mediates TrkA receptor tyrosine kinase-induced apoptosis via its PTB (interaction specificity) and Karet (death pathway linkage) domains in partnership with STK25 kinase.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CCM2 (malcavernin/OSM) is a scaffold/adaptor protein that organizes a core ternary complex with CCM1 (KRIT1) and CCM3 (PDCD10) to maintain endothelial vascular integrity [#0, #2]. CCM2 binds KRIT1 through its phosphotyrosine-binding (PTB) domain, which adopts a Dab-like fold and preferentially engages the third NPX(Y/F) motif of KRIT1; disease-associated CCM2 missense mutations destabilize this domain and disrupt the interaction [#0, #10]. Its N-terminal region is required for CCM1 binding and ternary-complex assembly, while its C-terminal harmonin-homology domain provides structural integrity [#2, #8]; long isoforms additionally carry an atypical PTB domain that CCM1 and CCM3 bind competitively [#15]. CCM2 is required in endothelial cells for proper angiogenesis and vascular development, and endothelial loss produces CCM-like venous lesions, with the endothelial cell established as the cell of disease origin [#3, #5, #20]. Mechanistically, the CCM1-CCM2 complex restrains MEKK3 kinase activity, thereby suppressing MEK5/ERK5 signaling and KLF4 transcription—the latter requiring two CCM2 PTB domains to cluster on a single KRIT1 scaffold [#11, #13, #19]—and acts as a scaffold favoring ROCK2 association with VE-cadherin while limiting ROCK1 activity to preserve cell-cell junctions and prevent actin stress fibers, fibronectin matrix defects, and ROCK-dependent senescence-associated secretory phenotype [#12, #16, #18]. Independent of its vascular role, CCM2 mediates TrkA receptor-induced apoptosis through its PTB and Karet domains in partnership with the GCKIII kinase STK25, which phosphorylates CCM2 to drive death signaling [#4, #7].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Established the physical basis of the CCM complex by showing CCM2 binds CCM1 through its PTB domain and seeds a ternary complex, explaining how distinct CCM gene products converge on one pathway.\",\n      \"evidence\": \"Co-IP, FRET, and localization assays with a familial missense mutant in cells\",\n      \"pmids\": [\"16037064\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define the KRIT1 motif bound\", \"MEKK3 role within the complex not functionally tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Extended the complex to include CCM3 and linked CCM2 to GCKIII signaling by identifying STK25 as a binding partner.\",\n      \"evidence\": \"Co-IP, immunofluorescence colocalization, and yeast two-hybrid\",\n      \"pmids\": [\"17657516\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No functional consequence of CCM2-STK25 binding tested\", \"Stoichiometry of CCM2/CCM3 interaction undefined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Mapped domain requirements for complex assembly, showing the N-terminal region is needed for CCM1 binding while CCM3 binding is retained, and that full-length CCM2 is the essential core of the ternary complex.\",\n      \"evidence\": \"Exon-2 deletion mutant expression with Co-IP for two partners\",\n      \"pmids\": [\"18300272\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Structural impact of deletion not resolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identified the endothelial cell as the cell of disease origin by demonstrating endothelial-specific second-hit loss of CCM2 protein in human cavernous lesions.\",\n      \"evidence\": \"IHC of CCM tissue from germline mutation carriers\",\n      \"pmids\": [\"19088124\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"IHC is a lower-tier method\", \"Does not establish downstream mechanism of lesion formation\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Defined the in vivo requirement for CCM2 in endothelial vascular morphogenesis and showed cell-type specificity by contrasting endothelial versus neuroglial deletion.\",\n      \"evidence\": \"Tissue-specific conditional KO mice with embryonic phenotyping\",\n      \"pmids\": [\"19259391\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular effector of the angiogenic defect not identified in this study\", \"Embryonic lethality limits adult lesion analysis\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Revealed a vascular-independent function: CCM2 transmits TrkA receptor-induced apoptosis via its PTB and Karet domains, expanding its role beyond endothelial scaffolding.\",\n      \"evidence\": \"Co-IP, domain mutation mapping, siRNA, and cell death assays in tumor cells\",\n      \"pmids\": [\"19755102\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinase coupling death signal not yet identified at this stage\", \"Relevance to endothelial complex unclear\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Showed that timing of CCM2 loss determines outcome, with postnatal endothelial deletion producing venous CCM-like lesions, modeling the human disease.\",\n      \"evidence\": \"Inducible postnatal endothelial-specific KO mice with histology and MRI\",\n      \"pmids\": [\"21859843\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why lesions are restricted to the venous bed not explained\", \"Downstream signaling not addressed here\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Genetically distinguished CCM2 from the CCM3/STK25 pathway, showing ccm2 defects are not rescued by stk25b and synergize with ccm3 loss.\",\n      \"evidence\": \"Zebrafish mutants, morpholino knockdown, epistasis, and overexpression rescue\",\n      \"pmids\": [\"22182521\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of CCM2/CCM3 divergence undefined\", \"Does not reconcile with shared ternary complex\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Solidified the CCM2-STK25 axis in apoptosis by showing STK25 (not STK24) phosphorylates CCM2 and its kinase activity is required for TrkA/CCM2 death signaling.\",\n      \"evidence\": \"Affinity proteomics, Co-IP, kinase assay with catalytic mutant, siRNA, cell death assays\",\n      \"pmids\": [\"22782892\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"CCM2 phosphosites not mapped\", \"Connection to endothelial CCM function unresolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Provided atomic-level architecture of the C-terminus, identifying a monomeric harmonin-homology domain that contributes structural integrity to CCM2.\",\n      \"evidence\": \"X-ray crystallography at 1.9 Å and analytical ultracentrifugation\",\n      \"pmids\": [\"23266514\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No binding partner assigned to the HHD\", \"Functional role of the domain in vivo untested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Placed CCM2 within the Heg-CCM cardiovascular pathway and mapped the CCM1 regions binding CCM2 and the paralog CCM2L.\",\n      \"evidence\": \"Zebrafish morpholino, mRNA rescue, Co-IP, and deletion mapping\",\n      \"pmids\": [\"23328253\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional redundancy between CCM2 and CCM2L not fully resolved\", \"Single lab\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defined the molecular recognition code, showing the Dab-like CCM2 PTB domain preferentially binds the third KRIT1 NPX(Y/F) motif and that disease mutants destabilize this interface.\",\n      \"evidence\": \"Co-crystal structure at 2.75 Å, binding preference mapping, missense mutant analysis\",\n      \"pmids\": [\"25525273\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry of PTB engagement not resolved at this stage\", \"Downstream effect of binding loss not measured here\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Established the effector mechanism of the vascular complex: CCM2 (and CCM2L) bind MEKK3 to suppress its kinase activity and the ERK5 transcriptional program, with mekk3 silencing rescuing ccm2 phenotypes.\",\n      \"evidence\": \"In vitro Co-IP, MEKK3-MEK5 kinase assay, endothelial siRNA, and zebrafish epistasis\",\n      \"pmids\": [\"26540726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define the transcription factors downstream of ERK5\", \"Mechanism of MEKK3 inhibition structurally undefined\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Confirmed MEKK3 as the genetically epistatic effector in vivo, with Mekk3 heterozygosity preventing CCM lesions in Ccm2-deficient endothelium.\",\n      \"evidence\": \"Micro-CT of mouse brains combining endothelial Ccm2 deletion with Mekk3 heterozygosity\",\n      \"pmids\": [\"27513872\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single rescue method\", \"Does not address the cytoskeletal arm of CCM2 function\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Defined a parallel cytoskeletal mechanism: the CCM1-CCM2 scaffold promotes ROCK2-VE-cadherin association and limits ROCK1, with ROCK1 silencing restoring junction stability and rescuing zebrafish defects.\",\n      \"evidence\": \"siRNA, Co-IP, traction force microscopy, isoform-specific knockdown, zebrafish rescue\",\n      \"pmids\": [\"30030370\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How CCM2 differentially partitions ROCK1 vs ROCK2 structurally unknown\", \"Integration with the MEKK3 arm not resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Connected CCM2 to ANP/GC-A signaling, showing it is required downstream for PAK4-dependent MLC phosphorylation and endothelial cell spreading.\",\n      \"evidence\": \"siRNA, MLC phosphorylation Western blot, and cell spreading assay\",\n      \"pmids\": [\"28432261\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct CCM2-PAK4 interaction not established\", \"Single lab\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Revealed isoform diversity by identifying an atypical PTB domain in long CCM2 isoforms that binds CCM1 and CCM3 competitively, making CCM2 a dual-PTB protein with distinct subcellular distributions.\",\n      \"evidence\": \"Cloning, subcellular localization, and competitive-binding Co-IP of splice variants\",\n      \"pmids\": [\"31676827\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of competitive CCM1/CCM3 binding untested\", \"Tissue specificity of isoform roles unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identified a shared downstream output, showing CCM2 loss (like CCM1/CCM3) impairs fibronectin matrix production and that fibronectin supplementation rescues morphology and suppresses stress fibers.\",\n      \"evidence\": \"CRISPR/Cas9 KO in human endothelial cells, fibronectin rescue, spheroid and actin imaging\",\n      \"pmids\": [\"32515053\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking CCM2 loss to reduced fibronectin not defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Extended the cytoskeletal phenotype to cell-state reprogramming, showing CCM2 silencing drives a ROCK-dependent senescence-associated secretory phenotype that recruits neighboring cells.\",\n      \"evidence\": \"siRNA, SA-β-gal senescence assays, traction force microscopy, transcriptomics, ECM invasion\",\n      \"pmids\": [\"34342749\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal transcriptional drivers of SASP not pinpointed\", \"Single lab\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Broadened the developmental role beyond endothelium, showing epicardial CCM2 maintains cytoskeletal/matrix protein production needed for cardiac development and post-infarction regeneration.\",\n      \"evidence\": \"Epicardial-specific conditional KO mice, ex vivo cell assays, proteomics, MI model\",\n      \"pmids\": [\"38510716\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular pathway in epicardium not linked to MEKK3 or ROCK arms\", \"Single lab\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Resolved the stoichiometry of transcriptional control, showing two CCM2 PTB domains must cluster on a single KRIT1 (binding the second and third NPxF motifs) to restrain KLF4 expression, a novel dual-PTB recruitment mechanism.\",\n      \"evidence\": \"Endothelial knockdown/reconstitution, Co-IP, biophysics of purified proteins, co-crystallography\",\n      \"pmids\": [\"41688454\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How KLF4 restraint integrates with MEKK3/ERK5 signaling not fully mapped\", \"Relevance of dual-PTB clustering to ROCK arm untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CCM2's distinct functional arms—MEKK3/KLF4 transcriptional restraint, ROCK1/ROCK2 cytoskeletal control, fibronectin matrix production, and TrkA/STK25 apoptosis—are coordinated within or partitioned across its domains and isoforms remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified structural model integrating MEKK3 and ROCK functions\", \"Functional division of labor among CCM2 isoforms undefined\", \"Mechanistic link between scaffold loss and reduced fibronectin not established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 2, 11, 12, 19]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [11, 12, 19]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0, 15]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [12, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [11, 12, 14]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [3, 5, 17]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [4, 7]},\n      {\"term_id\": \"R-HSA-1474244\", \"supporting_discovery_ids\": [18, 17]}\n    ],\n    \"complexes\": [\"CCM complex (CCM1-CCM2-CCM3)\"],\n    \"partners\": [\"KRIT1\", \"PDCD10\", \"STK25\", \"MAP3K3\", \"ROCK1\", \"ROCK2\", \"NTRK1\", \"KLF4\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}