{"gene":"MSH6","run_date":"2026-06-10T02:59:51","timeline":{"discoveries":[{"year":1995,"finding":"MSH6 (GTBP) forms a heterodimeric mismatch-binding complex with hMSH2; both proteins are required for mismatch-specific DNA binding activity in human cells, as demonstrated by loss of binding activity in tumor cell lines lacking either protein.","method":"Biochemical purification, co-immunoprecipitation, gel-shift/mismatch-binding assays, tumor cell line analysis","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 / Strong — original biochemical reconstitution of the heterodimer; independently replicated across two concurrent papers (PMID 7604265, 7604266) using complementary methods","pmids":["7604265","7604266"],"is_preprint":false},{"year":1996,"finding":"In S. cerevisiae, MSH6 binds to DNA mismatches as a heterodimer with MSH2, mirroring the human GTBP-hMSH2 complex; yeast MSH2-MSH6 provides the primary pathway for repair of base-base and single-nucleotide insertion/deletion mismatches, while MSH2-MSH3 provides a partially redundant pathway for insertion/deletion repair.","method":"Yeast genetics (null mutations, mutation-rate assays, co-immunoprecipitation), biochemical mismatch-binding assays","journal":"Genes & development / Current biology / Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — replicated across three independent yeast studies using genetic and biochemical methods (PMID 8600025, 8723353, 8631743)","pmids":["8600025","8723353","8631743"],"is_preprint":false},{"year":1995,"finding":"Loss-of-function mutations in GTBP/MSH6 in human tumor cell lines produce a mutator phenotype characterized primarily by instability at mononucleotide (rather than dinucleotide) repeat tracts, distinguishing it from defects in other MMR genes.","method":"Microsatellite instability analysis, gene sequencing of hypermutable tumor cell lines","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct sequencing of inactivating mutations in multiple cell lines with MSI phenotyping; independently confirmed","pmids":["7604266"],"is_preprint":false},{"year":1997,"finding":"Msh6-null mouse cells are specifically defective in repair of single-nucleotide mismatches but retain repair of 1-, 2-, and 4-nucleotide insertion/deletion mismatches, establishing that MSH6 is required for single-base mismatch correction in vivo.","method":"Gene targeting (knockout mouse), cell-free mismatch repair assay on extracts","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted repair assay from knockout cells with specific substrate discrimination; clean genetic model","pmids":["9390556"],"is_preprint":false},{"year":2000,"finding":"MSH3 and MSH6 contain N-terminal PCNA-binding motifs (PIP boxes) that mediate direct interaction with PCNA; mutation of conserved residues in these motifs elevates mutation rates in yeast, and addition of MSH6 or MSH3 PCNA-binding peptides to human cell extracts inhibits mismatch repair prior to DNA resynthesis.","method":"Peptide-PCNA binding assays, alanine substitution mutagenesis, yeast mutation-rate assays, human cell extract MMR inhibition assay","journal":"Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods (peptide binding, mutagenesis, genetic, biochemical inhibition) in a single study","pmids":["11005803"],"is_preprint":false},{"year":2002,"finding":"PCNA and MSH2-MSH6 form a stable ternary complex on homoduplex DNA, but mismatch recognition by MSH2-MSH6 disrupts its binding to PCNA; ATP or ATPγS restores PCNA binding by releasing MSH2-MSH6 from the mismatch. This supports a model in which MSH2-MSH6 is transferred from PCNA to mispaired bases.","method":"In vitro protein-DNA binding assays (gel shift, filter binding), nucleotide addition experiments","journal":"Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — rigorous in vitro reconstitution with defined substrates and nucleotide conditions, single lab","pmids":["12435741"],"is_preprint":false},{"year":2005,"finding":"Analysis of the S. cerevisiae MSH2-MSH6 complex using a reversible DNA end-blocking system showed that ATP binding converts the mispair-bound MSH2-MSH6 into a rapidly sliding clamp that dissociates via DNA ends; MLH1-PMS1 forms mispair-dependent ternary complexes with MSH2-MSH6 in an ATP- and magnesium-dependent manner on end-blocked DNA.","method":"IAsys biosensor real-time binding assay with lac repressor-operator end-blocking system","journal":"Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — real-time quantitative binding with mechanistic controls (ATP, end-blocking), single lab with multiple substrate conditions","pmids":["15811858"],"is_preprint":false},{"year":2005,"finding":"Single-molecule DNA-unzipping force analysis of S. cerevisiae MSH2-MSH6 revealed a high-affinity mismatch-recognition state not detectable in bulk assays, and an ATP-dependent sliding clamp state on end-blocked DNA, providing direct physical evidence for the two-state model of MSH2-MSH6 action.","method":"Single-molecule DNA-unzipping force assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Moderate — novel single-molecule method providing direct mechanistic evidence; rigorous controls with end-blocking and ATP variants","pmids":["16337600"],"is_preprint":false},{"year":1999,"finding":"A phenylalanine-to-alanine substitution (msh6-F337A) in the mismatch recognition domain of yeast MSH6 disrupts mismatch DNA binding by the MSH2-MSH6 heterodimer without preventing MSH2 heterodimerization, identifying this residue as critical for mismatch recognition.","method":"Site-directed mutagenesis, UV cross-linking, filter binding, gel retardation, ATPase assays, yeast genetics","journal":"Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — multiple biochemical assays plus genetic validation in a single study","pmids":["10347163"],"is_preprint":false},{"year":2005,"finding":"ATPase activity of the S. cerevisiae Msh2-Msh6 heterodimer is asymmetric: Msh6 is the rapid ATPase subunit (analogous to MutS S1) and makes base-specific mismatch contact via a conserved phenylalanine; Msh2 is the slower ATPase and strongly influences Msh6 ATPase; mismatch binding by Msh6 suppresses its own ATP hydrolysis ('cis' coupling), placing both subunits in an ATP-bound state that alters mismatch interactions.","method":"ATPase site mutagenesis, ATPase kinetics, DNA binding assays","journal":"DNA repair","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic ATPase mutagenesis with kinetic analyses; mechanistically definitive for subunit asymmetry","pmids":["16214425"],"is_preprint":false},{"year":2002,"finding":"MSH2 and MSH6 proteins are phosphorylated in vitro by protein kinase C and casein kinase II; in vivo phosphorylation of MSH6 (more extensively than MSH2) is required for mismatch-binding activity of the MutSα complex and for methylation-induced nuclear translocation of the repair complex.","method":"In vitro kinase assays, phosphate depletion, kinase inhibitors, phosphatase treatment, mismatch-binding assay, nuclear translocation assay","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple methods (kinase assay, inhibitors, functional binding assay), single lab","pmids":["11972333"],"is_preprint":false},{"year":2008,"finding":"The N-terminal region of human MSH6 contains a PWWP domain that binds double-stranded DNA without mismatch preference; the disease-associated S144I mutation is located within the DNA-binding surface of this domain but does not abolish DNA binding in vitro.","method":"NMR structural characterization, DNA binding assays, mutagenesis","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — structural identification with functional validation, but S144I result is a negative/weak finding; single lab","pmids":["18484749"],"is_preprint":false},{"year":2010,"finding":"MSH6 physically interacts with Ku70 (identified by yeast two-hybrid and confirmed by co-immunoprecipitation); this interaction is enhanced by DNA double-strand break (DSB)-inducing agents; MSH6 forms nuclear foci colocalizing with γ-H2AX at DSB sites, MSH6 depletion increases persistent DSBs, and impairs non-homologous end-joining (NHEJ), which is rescued by MSH6 overexpression.","method":"Yeast two-hybrid, co-immunoprecipitation, immunofluorescence (foci formation, γ-H2AX colocalization), comet assay, NHEJ reporter assay, clonogenic survival","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal interactions confirmed plus functional NHEJ assay and rescue; single lab with multiple orthogonal methods","pmids":["21075794"],"is_preprint":false},{"year":2014,"finding":"PCNA activates the Mlh1-Pms1 endonuclease in an Exo1-independent MMR pathway; mutations disrupting the Msh2-Msh6 interaction with PCNA (via the MSH6 PIP box) are specifically defective in this pathway, demonstrating that MSH2-MSH6 localizes PCNA to repair sites to activate Mlh1-Pms1 endonuclease activity.","method":"Yeast genetics (PCNA mutant screen, epistasis), biochemical PCNA trimerization/Msh2-Msh6 binding assays, Mlh1-Pms1 endonuclease assay, Mlh1-Pms1 focus quantification","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — genetic screen plus multiple biochemical assays establishing pathway position; rigorous epistasis analysis","pmids":["24981171"],"is_preprint":false},{"year":2000,"finding":"The MSH2-MSH6 heterodimer (but not MSH2-MSH3) is required for the second phase of somatic hypermutation that generates A:T substitutions in immunoglobulin V genes; Msh6-deficient mice show reduced A:T mutations and reduced class-switch recombination, while Msh3-deficient mice are normal.","method":"Msh6-/- and Msh3-/- mouse models, immunoglobulin V-region and switch-region sequencing, antibody response assays","journal":"Journal of Experimental Medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — two independent studies (PMID 10662804, 15238605) using knockout mice with sequence-level readout","pmids":["10662804","15238605"],"is_preprint":false},{"year":2006,"finding":"Msh6 plays a scaffolding role in the first phase of somatic hypermutation (independent of its enzymatic MMR function) to direct AID targeting to WRC motifs in vivo; a Msh6 point-mutant knockin (Msh6TD/TD) that abolishes the second phase of SHM reveals that AID targeting to non-WRC motifs increases when Msh6 enzymatic activity is lost, and that Msh6 also mediates proper recombination site targeting in CSR.","method":"Msh6 point-mutant knockin mice (Msh6TD/TD), SHM sequencing, CSR analysis","journal":"Immunity","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — knockin mouse model distinguishing enzymatic from structural role, single lab","pmids":["16618598"],"is_preprint":false},{"year":2009,"finding":"MSH6 mutations arise in glioblastomas during temozolomide therapy (absent in matched pre-treatment tumors); in vitro temozolomide selection generates MSH6 mutations in wild-type lines; MSH6 knockdown increases temozolomide resistance and MSH6 reconstitution in MSH6-null cells restores cytotoxicity, establishing that MSH6 loss causally mediates temozolomide resistance.","method":"Matched pre/post-treatment tumor sequencing, in vitro temozolomide selection, lentiviral shRNA knockdown, MSH6 reconstitution, clonogenic survival","journal":"Clinical Cancer Research","confidence":"High","confidence_rationale":"Tier 2 / Strong — loss-of-function plus reconstitution rescue plus in vivo and in vitro selection; independently validated in multiple studies (PMID 19584161, 25078279)","pmids":["19584161","25078279"],"is_preprint":false},{"year":2018,"finding":"MSH6 haploinsufficiency causes thiopurine resistance by failing to recognize thioguanine-nucleotide mismatches and initiate apoptosis/cell-cycle arrest (CHK1 and γ-H2AX activation defect), rather than by generating secondary mutations; knockdown in MMR-proficient cell lines increases IC50 to 6-TG and 6-MP with increased intracellular TGN accumulation.","method":"MSH6 shRNA knockdown in cell lines, IC50 assays, CHK1/γ-H2AX Western blot, microsatellite instability analysis, in vivo treatment model","journal":"Haematologica","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — knockdown with mechanistic readout (checkpoint markers) and functional rescue controls, single lab","pmids":["29449434"],"is_preprint":false},{"year":2003,"finding":"Msh6-deficient primary mouse fibroblasts undergo significantly less apoptosis following UV-B irradiation compared to wild-type cells, and UV-B increases Msh6 protein levels in wild-type cells independently of p53, establishing a role for Msh6 in the UV-B-induced apoptotic response.","method":"Primary Msh6-/- and Msh6+/+ mouse embryonic fibroblasts, UV-B irradiation, apoptosis assays, Western blot for Msh6 protein","journal":"Journal of Investigative Dermatology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — knockout cells with specific phenotypic readout; single lab","pmids":["14632208"],"is_preprint":false},{"year":1999,"finding":"Simultaneous loss of Msh3 and Msh6 in mice produces a cancer predisposition and mismatch repair deficiency equivalent to Msh2 deficiency, demonstrating that MSH2-MSH6 and MSH2-MSH3 provide fully redundant coverage of the MSH2-dependent repair pathway; Msh6 alone is required for mismatch-directed anti-recombination and sensitivity to methylating agents.","method":"Msh3-/-, Msh6-/-, and Msh3-/-Msh6-/- double-knockout mouse models; tumor incidence, MMR assays, anti-recombination assays, methylating agent sensitivity","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — double-knockout epistasis with multiple functional readouts; independently replicated (PMID 10545954, 10706084)","pmids":["10545954","10706084"],"is_preprint":false},{"year":2022,"finding":"S6K1 directly phosphorylates MSH6 at serine 309, enhancing mismatch repair activity; this phosphorylation event is part of a coordinated DNA repair response in which S6K1 also phosphorylates Cdk1 at serine 39 to cause G2/M arrest enabling HR repair.","method":"In vitro kinase assay (S6K1 phosphorylating MSH6), phosphosite identification, MMR activity assays, cell-cycle analysis","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 1–2 / Weak — in vitro kinase assay with site identification, but functional validation of phospho-MSH6 in MMR is limited to single lab","pmids":["36189922"],"is_preprint":false},{"year":2023,"finding":"Lysine crotonylation of MSH6 at K544 (Kcr) regulates its association with Ku70; mutation of K544cr impairs MSH6-Ku70 association and promotes NHEJ over homologous recombination (HR) in fetal oocytes during meiosis; DBP exposure reduces MSH6 crotonylation and mimics Msh6 knockout meiotic defects.","method":"Targeted disruption of Msh6 in fetal ovaries, K544 crotonylation site mutation, co-immunoprecipitation (MSH6-Ku70), HR/NHEJ assays, meiotic progression analysis","journal":"Journal of Hazardous Materials","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic disruption plus crotonylation site mutation with Co-IP and functional HR/NHEJ readout; single lab","pmids":["37167869"],"is_preprint":false},{"year":2012,"finding":"Biochemical analysis of human MSH2(G674A)-MSH6 and MSH2-MSH6(T1219D) mutants showed that both retain mismatch recognition but fail MMR in vitro by different mechanisms: MSH6(T1219D) fails to couple nucleotide binding with mismatch recognition, while MSH2(G674A) has a partial nucleotide-binding defect; both remain mismatch-bound and inhibit excision in a dominant manner.","method":"In vitro MMR assay, DNA binding kinetics, ATPase assay, excision step assay with defined mutant proteins","journal":"Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted in vitro MMR with purified mutant proteins and multiple orthogonal biochemical assays; single lab","pmids":["22277660"],"is_preprint":false},{"year":2001,"finding":"MSH6 deficiency (or MLH1 deficiency) elevates the rate of CAD gene amplification 50–100-fold in human cells, with amplification events occurring via unequal sister chromatid exchange and translocations, implicating MutSα in suppression of gene amplification.","method":"PALA selection for CAD amplification, FISH analysis, mismatch repair-deficient human cell lines","journal":"PNAS","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional assay in MMR-deficient cells with cytogenetic confirmation; single lab","pmids":["11717437"],"is_preprint":false},{"year":2002,"finding":"The human MSH6 promoter is a classical housekeeping gene promoter; MSH6 transcription is upregulated during late G1 phase, but protein levels remain essentially constant during the cell cycle, suggesting post-transcriptional regulation.","method":"Promoter cloning and functional luciferase reporter assays, cell-cycle synchronization with Northern/Western blot","journal":"Genes, Chromosomes & Cancer","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct promoter functional analysis with cell-cycle readout; single lab","pmids":["11746986"],"is_preprint":false},{"year":2018,"finding":"Reduced MSH6 (and MSH2) expression directly promotes pituitary tumor cell proliferation by decreasing ATR expression in the ATR-Chk1 pathway; siRNA-mediated knockdown of MSH6 and/or MSH2 in AtT-20ins cells and primary human pituitary adenoma cultures significantly increased cell proliferation and decreased ATR expression.","method":"siRNA knockdown, real-time qPCR, cell proliferation assays in cell lines and primary cultures, correlation analysis in human tumor specimens","journal":"Journal of Clinical Endocrinology and Metabolism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA in two cell systems with mechanistic readout (ATR expression/proliferation); single lab","pmids":["29342268"],"is_preprint":false}],"current_model":"MSH6 forms the obligate heterodimeric MutSα complex with MSH2; within this complex, MSH6 is the subunit that makes base-specific contact with mismatched DNA via a conserved phenylalanine residue, hydrolyzes ATP rapidly ('cis' coupling) to transition into an ATP-bound sliding clamp state, recruits PCNA via an N-terminal PIP-box to localize the complex to replication forks and to activate Mlh1-Pms1 endonuclease for downstream excision, and also carries a PWWP domain that binds double-stranded DNA non-specifically; MSH6 loss specifically abolishes single-base mismatch repair while leaving insertion/deletion repair partially intact (covered by MSH2-MSH3), participates in immunoglobulin somatic hypermutation and class-switch recombination through the MSH2-MSH6 heterodimer, contributes to UV-B-induced apoptosis, associates with Ku70 to support non-homologous end-joining, and is subject to regulatory phosphorylation (by PKC, CK2, and S6K1) and crotonylation (K544) that modulate its activity and protein interactions."},"narrative":{"mechanistic_narrative":"MSH6 is the mismatch-recognition subunit of the eukaryotic DNA mismatch repair (MMR) machinery, functioning as an obligate heterodimer with MSH2 (the MutSα complex) that initiates correction of replication errors [PMID:7604265, PMID:7604266, PMID:8600025, PMID:8723353, PMID:8631743]. Within this heterodimer MSH6 makes the base-specific contact with mispaired DNA through a conserved phenylalanine, whose substitution abolishes mismatch binding without disrupting heterodimerization [PMID:10347163]; the two subunits are functionally asymmetric, with MSH6 acting as the rapid ATPase whose mismatch binding suppresses its own ATP hydrolysis ('cis' coupling) to drive the complex into an ATP-bound state [PMID:16214425]. ATP binding converts the mispair-bound complex into a rapidly diffusing sliding clamp, a two-state mechanism resolved by both real-time and single-molecule analyses [PMID:15811858, PMID:16337600]. MSH6 carries an N-terminal PIP box that recruits PCNA, localizing the complex to replication sites and activating the Mlh1-Pms1 endonuclease for downstream excision; mismatch recognition transiently releases PCNA, supporting an ordered handoff to the mispair [PMID:11005803, PMID:12435741, PMID:24981171]. Genetically, MSH6 loss specifically abolishes single-base mismatch repair while insertion/deletion repair persists through the redundant MSH2-MSH3 pathway, producing mononucleotide-repeat instability, and combined loss of MSH3 and MSH6 phenocopies MSH2 deficiency [PMID:7604266, PMID:9390556, PMID:10545954, PMID:10706084]. Beyond replication-error correction, the MSH2-MSH6 heterodimer drives the A:T-mutation phase of immunoglobulin somatic hypermutation and class-switch recombination [PMID:10662804, PMID:15238605, PMID:16618598], contributes to UV-B-induced apoptosis [PMID:14632208], and associates with Ku70 to support non-homologous end-joining at double-strand breaks [PMID:21075794]. MSH6 activity is further tuned by regulatory phosphorylation (PKC, CK2, and S6K1 at Ser309) and by lysine crotonylation at K544 that governs the MSH6-Ku70 interaction [PMID:11972333, PMID:36189922, PMID:37167869]. Loss of MSH6-dependent damage sensing causally mediates resistance to temozolomide and thiopurines by failing to engage apoptotic and checkpoint responses [PMID:19584161, PMID:25078279, PMID:29449434].","teleology":[{"year":1995,"claim":"Established that mismatch recognition in human cells requires a defined heterodimer rather than a single protein, defining MSH6's obligate partnership with MSH2.","evidence":"Biochemical purification, co-IP and gel-shift mismatch-binding assays in tumor cell lines lacking either subunit","pmids":["7604265","7604266"],"confidence":"High","gaps":["Did not resolve which subunit contacts the mismatch","No structural basis for heterodimerization"]},{"year":1995,"claim":"Connected MSH6 loss to a specific genome-instability signature, showing it is not functionally interchangeable with other MMR genes.","evidence":"Microsatellite instability analysis and sequencing of inactivating mutations in hypermutable tumor lines","pmids":["7604266"],"confidence":"High","gaps":["Mechanistic basis for mononucleotide-specific instability not yet defined"]},{"year":1996,"claim":"Showed the heterodimer mechanism is conserved and defined the division of labor between MutSα and MutSβ pathways.","evidence":"Yeast genetics (null mutants, mutation-rate assays), co-IP and biochemical mismatch-binding assays","pmids":["8600025","8723353","8631743"],"confidence":"High","gaps":["Extent of MSH3/MSH6 redundancy in vivo not quantified at this stage"]},{"year":1997,"claim":"Demonstrated in a clean genetic system that MSH6 is specifically required for single-base mismatch correction, separating its role from insertion/deletion repair.","evidence":"Msh6-null mouse cells with cell-free mismatch repair assay on defined substrates","pmids":["9390556"],"confidence":"High","gaps":["Did not address non-MMR functions"]},{"year":1999,"claim":"Pinpointed the residue responsible for mismatch recognition, separating DNA-contact function from heterodimer assembly.","evidence":"msh6-F337A site-directed mutagenesis with UV cross-linking, binding, ATPase assays and yeast genetics","pmids":["10347163"],"confidence":"High","gaps":["Structural geometry of the phenylalanine-mismatch contact not directly visualized"]},{"year":1999,"claim":"Established quantitatively that MSH2-MSH6 and MSH2-MSH3 jointly cover the MSH2 pathway, while MSH6 alone governs anti-recombination and methylator sensitivity.","evidence":"Msh3/Msh6 single and double knockout mice with tumor, MMR, anti-recombination and drug-sensitivity readouts","pmids":["10545954","10706084"],"confidence":"High","gaps":["Molecular basis of MSH6-specific anti-recombination not defined"]},{"year":2000,"claim":"Identified the PCNA-recruitment motif in MSH6 and placed it functionally upstream of DNA resynthesis.","evidence":"PIP-box peptide-PCNA binding, alanine mutagenesis, yeast mutation-rate assays and human extract MMR inhibition","pmids":["11005803"],"confidence":"High","gaps":["Did not establish how PCNA binding couples to downstream endonuclease activation"]},{"year":2000,"claim":"Defined a non-canonical immune role: MSH2-MSH6 specifically drives the A:T-mutation phase of somatic hypermutation and class-switch recombination.","evidence":"Msh6-/- and Msh3-/- mice with Ig V-region/switch-region sequencing and antibody assays","pmids":["10662804","15238605"],"confidence":"High","gaps":["Mechanism linking mismatch recognition to mutagenic processing not resolved"]},{"year":2002,"claim":"Revealed an ordered PCNA-to-mismatch handoff, mechanistically connecting replication-fork loading to mismatch recognition.","evidence":"In vitro gel-shift/filter-binding ternary complex assays with ATP/ATPγS","pmids":["12435741"],"confidence":"High","gaps":["Single lab in vitro; kinetics of the handoff in vivo unknown"]},{"year":2002,"claim":"Introduced post-translational control of MMR, showing MSH6 phosphorylation is required for mismatch binding and damage-induced nuclear translocation.","evidence":"In vitro PKC/CK2 kinase assays, inhibitors, phosphatase treatment, binding and translocation assays","pmids":["11972333"],"confidence":"Medium","gaps":["Phosphosites not mapped","Single lab; in vivo relevance partial"]},{"year":2002,"claim":"Showed MSH6 is constitutively expressed with post-transcriptional regulation, framing how MMR capacity is maintained across the cell cycle.","evidence":"Promoter luciferase reporter assays plus cell-cycle synchronization with Northern/Western blot","pmids":["11746986"],"confidence":"Medium","gaps":["Post-transcriptional regulatory mechanism not identified"]},{"year":2001,"claim":"Linked MutSα to genome-stability functions beyond point-mutation correction by showing it suppresses gene amplification.","evidence":"PALA selection for CAD amplification with FISH in MMR-deficient human cells","pmids":["11717437"],"confidence":"Medium","gaps":["Direct molecular role of MSH6 in suppressing unequal exchange not defined","Single lab"]},{"year":2003,"claim":"Established MSH6 as an effector of UV-B-induced apoptosis independent of p53-mediated induction.","evidence":"Msh6-/- vs wild-type primary MEFs with UV-B, apoptosis assays and Msh6 Western blot","pmids":["14632208"],"confidence":"Medium","gaps":["Apoptotic signaling pathway downstream of MSH6 not mapped","Single lab"]},{"year":2005,"claim":"Provided direct mechanistic evidence for the two-state model: ATP converts the mispair-bound complex into a sliding clamp, with mispair-dependent MLH1-PMS1 recruitment.","evidence":"IAsys biosensor and single-molecule DNA-unzipping assays with end-blocking and ATP variants","pmids":["15811858","16337600"],"confidence":"High","gaps":["In vivo lifetime of the sliding clamp state unknown"]},{"year":2005,"claim":"Defined subunit ATPase asymmetry and the 'cis' coupling rule whereby mismatch binding by MSH6 suppresses its own hydrolysis.","evidence":"ATPase-site mutagenesis with kinetic and DNA-binding analyses in yeast Msh2-Msh6","pmids":["16214425"],"confidence":"High","gaps":["How asymmetry feeds into MLH1 activation not fully resolved"]},{"year":2006,"claim":"Separated MSH6's enzymatic and structural roles in immunity, identifying a scaffolding function directing AID targeting.","evidence":"Msh6TD/TD point-mutant knockin mice with SHM and CSR sequencing","pmids":["16618598"],"confidence":"Medium","gaps":["Molecular basis of AID-targeting scaffolding not defined","Single lab"]},{"year":2009,"claim":"Established MSH6 loss as a causal driver of chemoresistance, demonstrating its role in damage-induced cytotoxic signaling.","evidence":"Matched pre/post-treatment tumor sequencing, in vitro selection, shRNA knockdown and MSH6 reconstitution","pmids":["19584161","25078279"],"confidence":"High","gaps":["Signaling steps from MSH6 to cell death incompletely defined"]},{"year":2010,"claim":"Extended MSH6 function into double-strand break repair through a physical Ku70 interaction supporting NHEJ.","evidence":"Yeast two-hybrid, reciprocal co-IP, γ-H2AX foci colocalization, comet and NHEJ reporter assays with rescue","pmids":["21075794"],"confidence":"Medium","gaps":["Whether this requires MSH2 heterodimer not established","Single lab"]},{"year":2012,"claim":"Dissected how disease-associated mutations uncouple nucleotide binding from mismatch recognition, explaining dominant repair failure.","evidence":"Reconstituted in vitro MMR, DNA-binding kinetics, ATPase and excision assays with purified mutant proteins","pmids":["22277660"],"confidence":"High","gaps":["In vivo dominance of these mutants not tested","Single lab"]},{"year":2014,"claim":"Resolved how MSH6-bound PCNA activates downstream excision, placing MSH2-MSH6 as the localizer of the Mlh1-Pms1 endonuclease.","evidence":"Yeast PCNA-mutant genetic screen with epistasis plus biochemical PCNA-binding and endonuclease assays","pmids":["24981171"],"confidence":"High","gaps":["Structural arrangement of the PCNA-MutSα-MutLα assembly not resolved"]},{"year":2018,"claim":"Showed MSH6 mediates thiopurine cytotoxicity through damage sensing and checkpoint/apoptosis activation rather than secondary mutagenesis.","evidence":"shRNA knockdown, IC50 assays, CHK1/γ-H2AX Western blot, MSI analysis and in vivo model","pmids":["29449434"],"confidence":"Medium","gaps":["Mechanistic link from mismatch recognition to checkpoint activation not detailed","Single lab"]},{"year":2018,"claim":"Connected reduced MSH6 to a proliferative phenotype via downregulation of the ATR-Chk1 axis in pituitary tumor cells.","evidence":"siRNA knockdown with qPCR, proliferation assays in cell lines and primary cultures, tumor correlation","pmids":["29342268"],"confidence":"Medium","gaps":["Direct molecular link between MSH6 and ATR transcription unknown","Single lab"]},{"year":2022,"claim":"Added a kinase-driven activation arm, showing S6K1 phosphorylates MSH6 at Ser309 to enhance MMR within a coordinated DNA-repair response.","evidence":"In vitro S6K1 kinase assay with phosphosite identification, MMR activity and cell-cycle assays","pmids":["36189922"],"confidence":"Medium","gaps":["Functional validation of phospho-Ser309 in MMR limited","Single lab"]},{"year":2023,"claim":"Identified a crotonylation switch (K544) controlling the MSH6-Ku70 interaction and the choice between NHEJ and HR in meiosis.","evidence":"Fetal-ovary Msh6 disruption, K544 crotonylation-site mutation, MSH6-Ku70 co-IP and HR/NHEJ assays","pmids":["37167869"],"confidence":"Medium","gaps":["Generality beyond fetal oocytes unknown","Single lab"]},{"year":null,"claim":"How MSH6's diverse post-translational modifications and DSB-repair/immune scaffolding functions are mechanistically integrated with its core MutSα mismatch-repair activity remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified structural model linking modification states to activity","Crosstalk between MMR, NHEJ and SHM roles undefined","In vivo significance of individual modification sites not established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[0,8,11]},{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[9,6,7]},{"term_id":"GO:0140097","term_label":"catalytic activity, acting on DNA","supporting_discovery_ids":[3,13]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[13,4]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[10,12]},{"term_id":"GO:0000228","term_label":"nuclear chromosome","supporting_discovery_ids":[12]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,3,13]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[14,15]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[16,17,25]}],"complexes":["MutSα (MSH2-MSH6)"],"partners":["MSH2","PCNA","MLH1","PMS1","KU70"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P52701","full_name":"DNA mismatch repair protein Msh6","aliases":["G/T mismatch-binding protein","GTBP","GTMBP","MutS protein homolog 6","MutS-alpha 160 kDa subunit","p160"],"length_aa":1360,"mass_kda":152.8,"function":"Component of the post-replicative DNA mismatch repair system (MMR). 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both proteins are required for mismatch-specific DNA binding activity in human cells, as demonstrated by loss of binding activity in tumor cell lines lacking either protein.\",\n      \"method\": \"Biochemical purification, co-immunoprecipitation, gel-shift/mismatch-binding assays, tumor cell line analysis\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — original biochemical reconstitution of the heterodimer; independently replicated across two concurrent papers (PMID 7604265, 7604266) using complementary methods\",\n      \"pmids\": [\"7604265\", \"7604266\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"In S. cerevisiae, MSH6 binds to DNA mismatches as a heterodimer with MSH2, mirroring the human GTBP-hMSH2 complex; yeast MSH2-MSH6 provides the primary pathway for repair of base-base and single-nucleotide insertion/deletion mismatches, while MSH2-MSH3 provides a partially redundant pathway for insertion/deletion repair.\",\n      \"method\": \"Yeast genetics (null mutations, mutation-rate assays, co-immunoprecipitation), biochemical mismatch-binding assays\",\n      \"journal\": \"Genes & development / Current biology / Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — replicated across three independent yeast studies using genetic and biochemical methods (PMID 8600025, 8723353, 8631743)\",\n      \"pmids\": [\"8600025\", \"8723353\", \"8631743\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Loss-of-function mutations in GTBP/MSH6 in human tumor cell lines produce a mutator phenotype characterized primarily by instability at mononucleotide (rather than dinucleotide) repeat tracts, distinguishing it from defects in other MMR genes.\",\n      \"method\": \"Microsatellite instability analysis, gene sequencing of hypermutable tumor cell lines\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct sequencing of inactivating mutations in multiple cell lines with MSI phenotyping; independently confirmed\",\n      \"pmids\": [\"7604266\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Msh6-null mouse cells are specifically defective in repair of single-nucleotide mismatches but retain repair of 1-, 2-, and 4-nucleotide insertion/deletion mismatches, establishing that MSH6 is required for single-base mismatch correction in vivo.\",\n      \"method\": \"Gene targeting (knockout mouse), cell-free mismatch repair assay on extracts\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted repair assay from knockout cells with specific substrate discrimination; clean genetic model\",\n      \"pmids\": [\"9390556\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"MSH3 and MSH6 contain N-terminal PCNA-binding motifs (PIP boxes) that mediate direct interaction with PCNA; mutation of conserved residues in these motifs elevates mutation rates in yeast, and addition of MSH6 or MSH3 PCNA-binding peptides to human cell extracts inhibits mismatch repair prior to DNA resynthesis.\",\n      \"method\": \"Peptide-PCNA binding assays, alanine substitution mutagenesis, yeast mutation-rate assays, human cell extract MMR inhibition assay\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods (peptide binding, mutagenesis, genetic, biochemical inhibition) in a single study\",\n      \"pmids\": [\"11005803\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PCNA and MSH2-MSH6 form a stable ternary complex on homoduplex DNA, but mismatch recognition by MSH2-MSH6 disrupts its binding to PCNA; ATP or ATPγS restores PCNA binding by releasing MSH2-MSH6 from the mismatch. This supports a model in which MSH2-MSH6 is transferred from PCNA to mispaired bases.\",\n      \"method\": \"In vitro protein-DNA binding assays (gel shift, filter binding), nucleotide addition experiments\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — rigorous in vitro reconstitution with defined substrates and nucleotide conditions, single lab\",\n      \"pmids\": [\"12435741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Analysis of the S. cerevisiae MSH2-MSH6 complex using a reversible DNA end-blocking system showed that ATP binding converts the mispair-bound MSH2-MSH6 into a rapidly sliding clamp that dissociates via DNA ends; MLH1-PMS1 forms mispair-dependent ternary complexes with MSH2-MSH6 in an ATP- and magnesium-dependent manner on end-blocked DNA.\",\n      \"method\": \"IAsys biosensor real-time binding assay with lac repressor-operator end-blocking system\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — real-time quantitative binding with mechanistic controls (ATP, end-blocking), single lab with multiple substrate conditions\",\n      \"pmids\": [\"15811858\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Single-molecule DNA-unzipping force analysis of S. cerevisiae MSH2-MSH6 revealed a high-affinity mismatch-recognition state not detectable in bulk assays, and an ATP-dependent sliding clamp state on end-blocked DNA, providing direct physical evidence for the two-state model of MSH2-MSH6 action.\",\n      \"method\": \"Single-molecule DNA-unzipping force assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — novel single-molecule method providing direct mechanistic evidence; rigorous controls with end-blocking and ATP variants\",\n      \"pmids\": [\"16337600\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"A phenylalanine-to-alanine substitution (msh6-F337A) in the mismatch recognition domain of yeast MSH6 disrupts mismatch DNA binding by the MSH2-MSH6 heterodimer without preventing MSH2 heterodimerization, identifying this residue as critical for mismatch recognition.\",\n      \"method\": \"Site-directed mutagenesis, UV cross-linking, filter binding, gel retardation, ATPase assays, yeast genetics\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — multiple biochemical assays plus genetic validation in a single study\",\n      \"pmids\": [\"10347163\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"ATPase activity of the S. cerevisiae Msh2-Msh6 heterodimer is asymmetric: Msh6 is the rapid ATPase subunit (analogous to MutS S1) and makes base-specific mismatch contact via a conserved phenylalanine; Msh2 is the slower ATPase and strongly influences Msh6 ATPase; mismatch binding by Msh6 suppresses its own ATP hydrolysis ('cis' coupling), placing both subunits in an ATP-bound state that alters mismatch interactions.\",\n      \"method\": \"ATPase site mutagenesis, ATPase kinetics, DNA binding assays\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic ATPase mutagenesis with kinetic analyses; mechanistically definitive for subunit asymmetry\",\n      \"pmids\": [\"16214425\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"MSH2 and MSH6 proteins are phosphorylated in vitro by protein kinase C and casein kinase II; in vivo phosphorylation of MSH6 (more extensively than MSH2) is required for mismatch-binding activity of the MutSα complex and for methylation-induced nuclear translocation of the repair complex.\",\n      \"method\": \"In vitro kinase assays, phosphate depletion, kinase inhibitors, phosphatase treatment, mismatch-binding assay, nuclear translocation assay\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple methods (kinase assay, inhibitors, functional binding assay), single lab\",\n      \"pmids\": [\"11972333\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The N-terminal region of human MSH6 contains a PWWP domain that binds double-stranded DNA without mismatch preference; the disease-associated S144I mutation is located within the DNA-binding surface of this domain but does not abolish DNA binding in vitro.\",\n      \"method\": \"NMR structural characterization, DNA binding assays, mutagenesis\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — structural identification with functional validation, but S144I result is a negative/weak finding; single lab\",\n      \"pmids\": [\"18484749\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"MSH6 physically interacts with Ku70 (identified by yeast two-hybrid and confirmed by co-immunoprecipitation); this interaction is enhanced by DNA double-strand break (DSB)-inducing agents; MSH6 forms nuclear foci colocalizing with γ-H2AX at DSB sites, MSH6 depletion increases persistent DSBs, and impairs non-homologous end-joining (NHEJ), which is rescued by MSH6 overexpression.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, immunofluorescence (foci formation, γ-H2AX colocalization), comet assay, NHEJ reporter assay, clonogenic survival\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal interactions confirmed plus functional NHEJ assay and rescue; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"21075794\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"PCNA activates the Mlh1-Pms1 endonuclease in an Exo1-independent MMR pathway; mutations disrupting the Msh2-Msh6 interaction with PCNA (via the MSH6 PIP box) are specifically defective in this pathway, demonstrating that MSH2-MSH6 localizes PCNA to repair sites to activate Mlh1-Pms1 endonuclease activity.\",\n      \"method\": \"Yeast genetics (PCNA mutant screen, epistasis), biochemical PCNA trimerization/Msh2-Msh6 binding assays, Mlh1-Pms1 endonuclease assay, Mlh1-Pms1 focus quantification\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — genetic screen plus multiple biochemical assays establishing pathway position; rigorous epistasis analysis\",\n      \"pmids\": [\"24981171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The MSH2-MSH6 heterodimer (but not MSH2-MSH3) is required for the second phase of somatic hypermutation that generates A:T substitutions in immunoglobulin V genes; Msh6-deficient mice show reduced A:T mutations and reduced class-switch recombination, while Msh3-deficient mice are normal.\",\n      \"method\": \"Msh6-/- and Msh3-/- mouse models, immunoglobulin V-region and switch-region sequencing, antibody response assays\",\n      \"journal\": \"Journal of Experimental Medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two independent studies (PMID 10662804, 15238605) using knockout mice with sequence-level readout\",\n      \"pmids\": [\"10662804\", \"15238605\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Msh6 plays a scaffolding role in the first phase of somatic hypermutation (independent of its enzymatic MMR function) to direct AID targeting to WRC motifs in vivo; a Msh6 point-mutant knockin (Msh6TD/TD) that abolishes the second phase of SHM reveals that AID targeting to non-WRC motifs increases when Msh6 enzymatic activity is lost, and that Msh6 also mediates proper recombination site targeting in CSR.\",\n      \"method\": \"Msh6 point-mutant knockin mice (Msh6TD/TD), SHM sequencing, CSR analysis\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knockin mouse model distinguishing enzymatic from structural role, single lab\",\n      \"pmids\": [\"16618598\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"MSH6 mutations arise in glioblastomas during temozolomide therapy (absent in matched pre-treatment tumors); in vitro temozolomide selection generates MSH6 mutations in wild-type lines; MSH6 knockdown increases temozolomide resistance and MSH6 reconstitution in MSH6-null cells restores cytotoxicity, establishing that MSH6 loss causally mediates temozolomide resistance.\",\n      \"method\": \"Matched pre/post-treatment tumor sequencing, in vitro temozolomide selection, lentiviral shRNA knockdown, MSH6 reconstitution, clonogenic survival\",\n      \"journal\": \"Clinical Cancer Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — loss-of-function plus reconstitution rescue plus in vivo and in vitro selection; independently validated in multiple studies (PMID 19584161, 25078279)\",\n      \"pmids\": [\"19584161\", \"25078279\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"MSH6 haploinsufficiency causes thiopurine resistance by failing to recognize thioguanine-nucleotide mismatches and initiate apoptosis/cell-cycle arrest (CHK1 and γ-H2AX activation defect), rather than by generating secondary mutations; knockdown in MMR-proficient cell lines increases IC50 to 6-TG and 6-MP with increased intracellular TGN accumulation.\",\n      \"method\": \"MSH6 shRNA knockdown in cell lines, IC50 assays, CHK1/γ-H2AX Western blot, microsatellite instability analysis, in vivo treatment model\",\n      \"journal\": \"Haematologica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knockdown with mechanistic readout (checkpoint markers) and functional rescue controls, single lab\",\n      \"pmids\": [\"29449434\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Msh6-deficient primary mouse fibroblasts undergo significantly less apoptosis following UV-B irradiation compared to wild-type cells, and UV-B increases Msh6 protein levels in wild-type cells independently of p53, establishing a role for Msh6 in the UV-B-induced apoptotic response.\",\n      \"method\": \"Primary Msh6-/- and Msh6+/+ mouse embryonic fibroblasts, UV-B irradiation, apoptosis assays, Western blot for Msh6 protein\",\n      \"journal\": \"Journal of Investigative Dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — knockout cells with specific phenotypic readout; single lab\",\n      \"pmids\": [\"14632208\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Simultaneous loss of Msh3 and Msh6 in mice produces a cancer predisposition and mismatch repair deficiency equivalent to Msh2 deficiency, demonstrating that MSH2-MSH6 and MSH2-MSH3 provide fully redundant coverage of the MSH2-dependent repair pathway; Msh6 alone is required for mismatch-directed anti-recombination and sensitivity to methylating agents.\",\n      \"method\": \"Msh3-/-, Msh6-/-, and Msh3-/-Msh6-/- double-knockout mouse models; tumor incidence, MMR assays, anti-recombination assays, methylating agent sensitivity\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — double-knockout epistasis with multiple functional readouts; independently replicated (PMID 10545954, 10706084)\",\n      \"pmids\": [\"10545954\", \"10706084\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"S6K1 directly phosphorylates MSH6 at serine 309, enhancing mismatch repair activity; this phosphorylation event is part of a coordinated DNA repair response in which S6K1 also phosphorylates Cdk1 at serine 39 to cause G2/M arrest enabling HR repair.\",\n      \"method\": \"In vitro kinase assay (S6K1 phosphorylating MSH6), phosphosite identification, MMR activity assays, cell-cycle analysis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Weak — in vitro kinase assay with site identification, but functional validation of phospho-MSH6 in MMR is limited to single lab\",\n      \"pmids\": [\"36189922\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Lysine crotonylation of MSH6 at K544 (Kcr) regulates its association with Ku70; mutation of K544cr impairs MSH6-Ku70 association and promotes NHEJ over homologous recombination (HR) in fetal oocytes during meiosis; DBP exposure reduces MSH6 crotonylation and mimics Msh6 knockout meiotic defects.\",\n      \"method\": \"Targeted disruption of Msh6 in fetal ovaries, K544 crotonylation site mutation, co-immunoprecipitation (MSH6-Ku70), HR/NHEJ assays, meiotic progression analysis\",\n      \"journal\": \"Journal of Hazardous Materials\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic disruption plus crotonylation site mutation with Co-IP and functional HR/NHEJ readout; single lab\",\n      \"pmids\": [\"37167869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Biochemical analysis of human MSH2(G674A)-MSH6 and MSH2-MSH6(T1219D) mutants showed that both retain mismatch recognition but fail MMR in vitro by different mechanisms: MSH6(T1219D) fails to couple nucleotide binding with mismatch recognition, while MSH2(G674A) has a partial nucleotide-binding defect; both remain mismatch-bound and inhibit excision in a dominant manner.\",\n      \"method\": \"In vitro MMR assay, DNA binding kinetics, ATPase assay, excision step assay with defined mutant proteins\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted in vitro MMR with purified mutant proteins and multiple orthogonal biochemical assays; single lab\",\n      \"pmids\": [\"22277660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"MSH6 deficiency (or MLH1 deficiency) elevates the rate of CAD gene amplification 50–100-fold in human cells, with amplification events occurring via unequal sister chromatid exchange and translocations, implicating MutSα in suppression of gene amplification.\",\n      \"method\": \"PALA selection for CAD amplification, FISH analysis, mismatch repair-deficient human cell lines\",\n      \"journal\": \"PNAS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional assay in MMR-deficient cells with cytogenetic confirmation; single lab\",\n      \"pmids\": [\"11717437\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The human MSH6 promoter is a classical housekeeping gene promoter; MSH6 transcription is upregulated during late G1 phase, but protein levels remain essentially constant during the cell cycle, suggesting post-transcriptional regulation.\",\n      \"method\": \"Promoter cloning and functional luciferase reporter assays, cell-cycle synchronization with Northern/Western blot\",\n      \"journal\": \"Genes, Chromosomes & Cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct promoter functional analysis with cell-cycle readout; single lab\",\n      \"pmids\": [\"11746986\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Reduced MSH6 (and MSH2) expression directly promotes pituitary tumor cell proliferation by decreasing ATR expression in the ATR-Chk1 pathway; siRNA-mediated knockdown of MSH6 and/or MSH2 in AtT-20ins cells and primary human pituitary adenoma cultures significantly increased cell proliferation and decreased ATR expression.\",\n      \"method\": \"siRNA knockdown, real-time qPCR, cell proliferation assays in cell lines and primary cultures, correlation analysis in human tumor specimens\",\n      \"journal\": \"Journal of Clinical Endocrinology and Metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA in two cell systems with mechanistic readout (ATR expression/proliferation); single lab\",\n      \"pmids\": [\"29342268\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MSH6 forms the obligate heterodimeric MutSα complex with MSH2; within this complex, MSH6 is the subunit that makes base-specific contact with mismatched DNA via a conserved phenylalanine residue, hydrolyzes ATP rapidly ('cis' coupling) to transition into an ATP-bound sliding clamp state, recruits PCNA via an N-terminal PIP-box to localize the complex to replication forks and to activate Mlh1-Pms1 endonuclease for downstream excision, and also carries a PWWP domain that binds double-stranded DNA non-specifically; MSH6 loss specifically abolishes single-base mismatch repair while leaving insertion/deletion repair partially intact (covered by MSH2-MSH3), participates in immunoglobulin somatic hypermutation and class-switch recombination through the MSH2-MSH6 heterodimer, contributes to UV-B-induced apoptosis, associates with Ku70 to support non-homologous end-joining, and is subject to regulatory phosphorylation (by PKC, CK2, and S6K1) and crotonylation (K544) that modulate its activity and protein interactions.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"MSH6 is the mismatch-recognition subunit of the eukaryotic DNA mismatch repair (MMR) machinery, functioning as an obligate heterodimer with MSH2 (the MutSα complex) that initiates correction of replication errors [#0, #1]. Within this heterodimer MSH6 makes the base-specific contact with mispaired DNA through a conserved phenylalanine, whose substitution abolishes mismatch binding without disrupting heterodimerization [#8]; the two subunits are functionally asymmetric, with MSH6 acting as the rapid ATPase whose mismatch binding suppresses its own ATP hydrolysis ('cis' coupling) to drive the complex into an ATP-bound state [#9]. ATP binding converts the mispair-bound complex into a rapidly diffusing sliding clamp, a two-state mechanism resolved by both real-time and single-molecule analyses [#6, #7]. MSH6 carries an N-terminal PIP box that recruits PCNA, localizing the complex to replication sites and activating the Mlh1-Pms1 endonuclease for downstream excision; mismatch recognition transiently releases PCNA, supporting an ordered handoff to the mispair [#4, #5, #13]. Genetically, MSH6 loss specifically abolishes single-base mismatch repair while insertion/deletion repair persists through the redundant MSH2-MSH3 pathway, producing mononucleotide-repeat instability, and combined loss of MSH3 and MSH6 phenocopies MSH2 deficiency [#2, #3, #19]. Beyond replication-error correction, the MSH2-MSH6 heterodimer drives the A:T-mutation phase of immunoglobulin somatic hypermutation and class-switch recombination [#14, #15], contributes to UV-B-induced apoptosis [#18], and associates with Ku70 to support non-homologous end-joining at double-strand breaks [#12]. MSH6 activity is further tuned by regulatory phosphorylation (PKC, CK2, and S6K1 at Ser309) and by lysine crotonylation at K544 that governs the MSH6-Ku70 interaction [#10, #20, #21]. Loss of MSH6-dependent damage sensing causally mediates resistance to temozolomide and thiopurines by failing to engage apoptotic and checkpoint responses [#16, #17].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Established that mismatch recognition in human cells requires a defined heterodimer rather than a single protein, defining MSH6's obligate partnership with MSH2.\",\n      \"evidence\": \"Biochemical purification, co-IP and gel-shift mismatch-binding assays in tumor cell lines lacking either subunit\",\n      \"pmids\": [\"7604265\", \"7604266\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve which subunit contacts the mismatch\", \"No structural basis for heterodimerization\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Connected MSH6 loss to a specific genome-instability signature, showing it is not functionally interchangeable with other MMR genes.\",\n      \"evidence\": \"Microsatellite instability analysis and sequencing of inactivating mutations in hypermutable tumor lines\",\n      \"pmids\": [\"7604266\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanistic basis for mononucleotide-specific instability not yet defined\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Showed the heterodimer mechanism is conserved and defined the division of labor between MutSα and MutSβ pathways.\",\n      \"evidence\": \"Yeast genetics (null mutants, mutation-rate assays), co-IP and biochemical mismatch-binding assays\",\n      \"pmids\": [\"8600025\", \"8723353\", \"8631743\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Extent of MSH3/MSH6 redundancy in vivo not quantified at this stage\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Demonstrated in a clean genetic system that MSH6 is specifically required for single-base mismatch correction, separating its role from insertion/deletion repair.\",\n      \"evidence\": \"Msh6-null mouse cells with cell-free mismatch repair assay on defined substrates\",\n      \"pmids\": [\"9390556\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address non-MMR functions\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Pinpointed the residue responsible for mismatch recognition, separating DNA-contact function from heterodimer assembly.\",\n      \"evidence\": \"msh6-F337A site-directed mutagenesis with UV cross-linking, binding, ATPase assays and yeast genetics\",\n      \"pmids\": [\"10347163\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural geometry of the phenylalanine-mismatch contact not directly visualized\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Established quantitatively that MSH2-MSH6 and MSH2-MSH3 jointly cover the MSH2 pathway, while MSH6 alone governs anti-recombination and methylator sensitivity.\",\n      \"evidence\": \"Msh3/Msh6 single and double knockout mice with tumor, MMR, anti-recombination and drug-sensitivity readouts\",\n      \"pmids\": [\"10545954\", \"10706084\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of MSH6-specific anti-recombination not defined\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Identified the PCNA-recruitment motif in MSH6 and placed it functionally upstream of DNA resynthesis.\",\n      \"evidence\": \"PIP-box peptide-PCNA binding, alanine mutagenesis, yeast mutation-rate assays and human extract MMR inhibition\",\n      \"pmids\": [\"11005803\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish how PCNA binding couples to downstream endonuclease activation\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Defined a non-canonical immune role: MSH2-MSH6 specifically drives the A:T-mutation phase of somatic hypermutation and class-switch recombination.\",\n      \"evidence\": \"Msh6-/- and Msh3-/- mice with Ig V-region/switch-region sequencing and antibody assays\",\n      \"pmids\": [\"10662804\", \"15238605\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism linking mismatch recognition to mutagenic processing not resolved\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Revealed an ordered PCNA-to-mismatch handoff, mechanistically connecting replication-fork loading to mismatch recognition.\",\n      \"evidence\": \"In vitro gel-shift/filter-binding ternary complex assays with ATP/ATPγS\",\n      \"pmids\": [\"12435741\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Single lab in vitro; kinetics of the handoff in vivo unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Introduced post-translational control of MMR, showing MSH6 phosphorylation is required for mismatch binding and damage-induced nuclear translocation.\",\n      \"evidence\": \"In vitro PKC/CK2 kinase assays, inhibitors, phosphatase treatment, binding and translocation assays\",\n      \"pmids\": [\"11972333\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Phosphosites not mapped\", \"Single lab; in vivo relevance partial\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Showed MSH6 is constitutively expressed with post-transcriptional regulation, framing how MMR capacity is maintained across the cell cycle.\",\n      \"evidence\": \"Promoter luciferase reporter assays plus cell-cycle synchronization with Northern/Western blot\",\n      \"pmids\": [\"11746986\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Post-transcriptional regulatory mechanism not identified\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Linked MutSα to genome-stability functions beyond point-mutation correction by showing it suppresses gene amplification.\",\n      \"evidence\": \"PALA selection for CAD amplification with FISH in MMR-deficient human cells\",\n      \"pmids\": [\"11717437\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular role of MSH6 in suppressing unequal exchange not defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Established MSH6 as an effector of UV-B-induced apoptosis independent of p53-mediated induction.\",\n      \"evidence\": \"Msh6-/- vs wild-type primary MEFs with UV-B, apoptosis assays and Msh6 Western blot\",\n      \"pmids\": [\"14632208\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Apoptotic signaling pathway downstream of MSH6 not mapped\", \"Single lab\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Provided direct mechanistic evidence for the two-state model: ATP converts the mispair-bound complex into a sliding clamp, with mispair-dependent MLH1-PMS1 recruitment.\",\n      \"evidence\": \"IAsys biosensor and single-molecule DNA-unzipping assays with end-blocking and ATP variants\",\n      \"pmids\": [\"15811858\", \"16337600\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo lifetime of the sliding clamp state unknown\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Defined subunit ATPase asymmetry and the 'cis' coupling rule whereby mismatch binding by MSH6 suppresses its own hydrolysis.\",\n      \"evidence\": \"ATPase-site mutagenesis with kinetic and DNA-binding analyses in yeast Msh2-Msh6\",\n      \"pmids\": [\"16214425\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How asymmetry feeds into MLH1 activation not fully resolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Separated MSH6's enzymatic and structural roles in immunity, identifying a scaffolding function directing AID targeting.\",\n      \"evidence\": \"Msh6TD/TD point-mutant knockin mice with SHM and CSR sequencing\",\n      \"pmids\": [\"16618598\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of AID-targeting scaffolding not defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Established MSH6 loss as a causal driver of chemoresistance, demonstrating its role in damage-induced cytotoxic signaling.\",\n      \"evidence\": \"Matched pre/post-treatment tumor sequencing, in vitro selection, shRNA knockdown and MSH6 reconstitution\",\n      \"pmids\": [\"19584161\", \"25078279\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling steps from MSH6 to cell death incompletely defined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Extended MSH6 function into double-strand break repair through a physical Ku70 interaction supporting NHEJ.\",\n      \"evidence\": \"Yeast two-hybrid, reciprocal co-IP, γ-H2AX foci colocalization, comet and NHEJ reporter assays with rescue\",\n      \"pmids\": [\"21075794\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether this requires MSH2 heterodimer not established\", \"Single lab\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Dissected how disease-associated mutations uncouple nucleotide binding from mismatch recognition, explaining dominant repair failure.\",\n      \"evidence\": \"Reconstituted in vitro MMR, DNA-binding kinetics, ATPase and excision assays with purified mutant proteins\",\n      \"pmids\": [\"22277660\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo dominance of these mutants not tested\", \"Single lab\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Resolved how MSH6-bound PCNA activates downstream excision, placing MSH2-MSH6 as the localizer of the Mlh1-Pms1 endonuclease.\",\n      \"evidence\": \"Yeast PCNA-mutant genetic screen with epistasis plus biochemical PCNA-binding and endonuclease assays\",\n      \"pmids\": [\"24981171\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural arrangement of the PCNA-MutSα-MutLα assembly not resolved\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showed MSH6 mediates thiopurine cytotoxicity through damage sensing and checkpoint/apoptosis activation rather than secondary mutagenesis.\",\n      \"evidence\": \"shRNA knockdown, IC50 assays, CHK1/γ-H2AX Western blot, MSI analysis and in vivo model\",\n      \"pmids\": [\"29449434\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic link from mismatch recognition to checkpoint activation not detailed\", \"Single lab\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Connected reduced MSH6 to a proliferative phenotype via downregulation of the ATR-Chk1 axis in pituitary tumor cells.\",\n      \"evidence\": \"siRNA knockdown with qPCR, proliferation assays in cell lines and primary cultures, tumor correlation\",\n      \"pmids\": [\"29342268\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular link between MSH6 and ATR transcription unknown\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Added a kinase-driven activation arm, showing S6K1 phosphorylates MSH6 at Ser309 to enhance MMR within a coordinated DNA-repair response.\",\n      \"evidence\": \"In vitro S6K1 kinase assay with phosphosite identification, MMR activity and cell-cycle assays\",\n      \"pmids\": [\"36189922\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional validation of phospho-Ser309 in MMR limited\", \"Single lab\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified a crotonylation switch (K544) controlling the MSH6-Ku70 interaction and the choice between NHEJ and HR in meiosis.\",\n      \"evidence\": \"Fetal-ovary Msh6 disruption, K544 crotonylation-site mutation, MSH6-Ku70 co-IP and HR/NHEJ assays\",\n      \"pmids\": [\"37167869\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Generality beyond fetal oocytes unknown\", \"Single lab\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How MSH6's diverse post-translational modifications and DSB-repair/immune scaffolding functions are mechanistically integrated with its core MutSα mismatch-repair activity remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified structural model linking modification states to activity\", \"Crosstalk between MMR, NHEJ and SHM roles undefined\", \"In vivo significance of individual modification sites not established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [0, 8, 11]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [9, 6, 7]},\n      {\"term_id\": \"GO:0140097\", \"supporting_discovery_ids\": [3, 13]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [13, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [10, 12]},\n      {\"term_id\": \"GO:0000228\", \"supporting_discovery_ids\": [12]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 3, 13]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [14, 15]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [16, 17, 25]}\n    ],\n    \"complexes\": [\"MutSα (MSH2-MSH6)\"],\n    \"partners\": [\"MSH2\", \"PCNA\", \"MLH1\", \"PMS1\", \"KU70\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}