{"gene":"FAN1","run_date":"2026-04-28T17:46:03","timeline":{"discoveries":[{"year":2010,"finding":"FAN1 (KIAA1018) possesses intrinsic 5'-3' exonuclease activity and endonuclease activity that cleaves nicked and branched DNA structures, and is recruited to sites of interstrand crosslink (ICL) damage through its ubiquitin-binding zinc finger (UBZ) domain binding to monoubiquitinated FANCD2.","method":"shRNA screen, in vitro nuclease assay, co-immunoprecipitation, immunofluorescence colocalization at DNA damage foci, UBZ domain mutant analysis","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (biochemical activity, domain mutagenesis, cellular localization), independently replicated by four simultaneous papers","pmids":["20603073"],"is_preprint":false},{"year":2010,"finding":"FAN1 interacts with and is recruited to DNA damage sites by the monoubiquitinated form of FANCD2; FAN1 exhibits 5' flap endonuclease activity and 5' exonuclease activity mediated by its conserved VRR_nuc domain.","method":"Co-immunoprecipitation, in vitro nuclease assay, siRNA knockdown with ICL sensitivity and genome instability readouts","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1-2 — biochemical reconstitution plus domain identification plus cellular phenotype, confirmed by multiple independent groups simultaneously","pmids":["20603015"],"is_preprint":false},{"year":2010,"finding":"FAN1 (KIAA1018) is a 5'→3' exonuclease and structure-specific endonuclease preferentially incising 5' flaps; its UBZ domain interacts with monoubiquitylated FANCD2 for recruitment to ICL damage sites; depletion causes ICL hypersensitivity and chromosomal instability.","method":"In vitro nuclease assay, co-immunoprecipitation, siRNA knockdown, chromosomal instability assay","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1-2 — reconstituted enzymatic activity plus domain interaction plus cellular phenotype, independently replicated","pmids":["20603016"],"is_preprint":false},{"year":2010,"finding":"FAN1 promotes ICL repair in a manner strictly dependent on its ability to accumulate at sites of DNA damage, and this accumulation relies on monoubiquitylation of the FANCI-FANCD2 (ID) complex.","method":"siRNA knockdown, fluorescence microscopy, epistasis analysis with ID complex mutants","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis plus localization experiment, replicated by multiple independent groups","pmids":["20671156"],"is_preprint":false},{"year":2010,"finding":"FAN1 localizes to stalled replication forks (colocalizing with RPA) in a UBZ domain-dependent manner, and is a nuclear protein forming DNA-damage-induced foci.","method":"Immunofluorescence colocalization with RPA, UBZ domain mutant analysis, siRNA knockdown","journal":"Cell cycle","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization experiment with domain mutant, single lab","pmids":["20935496"],"is_preprint":false},{"year":2010,"finding":"FAN1 null DT40 cells are sensitive to cisplatin and MMC but not ionizing/UV radiation, MMS, or camptothecin; double knockouts of FAN1 with FANCC or FANCJ show increased cisplatin sensitivity, indicating FAN1 participates in ICL repair both within and independently of the classical FA pathway.","method":"Gene targeting in chicken DT40 cells, drug sensitivity assays, double-mutant epistasis analysis, sister chromatid exchange assay","journal":"PNAS","confidence":"High","confidence_rationale":"Tier 2 — clean genetic knockout plus epistasis analysis with multiple FA pathway components","pmids":["21115814"],"is_preprint":false},{"year":2014,"finding":"Crystal structures of human FAN1 reveal it cleaves DNA successively at every third nucleotide via exonuclease activity, requiring a 5'-terminal phosphate anchor at a nick or 1-2 nucleotide flap, augmented by a 3' flap; this mechanism allows FAN1 to excise an ICL from one strand through flanking incisions.","method":"X-ray crystallography (FAN1-DNA co-crystal structures), in vitro biochemical assay, active-site mutagenesis","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with in vitro biochemical validation and mutagenesis","pmids":["25430771"],"is_preprint":false},{"year":2014,"finding":"Human FAN1 forms a head-to-tail dimer in complex with 5' flap DNA; two FAN1 molecules cooperate to locate the lesion, orient the DNA, and unwind a 5' flap for subsequent incision; structure-informed mutations disrupting dimerization, substrate orientation, or flap unwinding impair nuclease activity.","method":"X-ray crystallography (three crystal structures), biochemical nuclease assay, structure-guided mutagenesis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — multiple crystal structures plus mutagenesis plus biochemical validation","pmids":["25500724"],"is_preprint":false},{"year":2014,"finding":"FAN1 VRR_nuc domain is monomeric (unlike bacterial/viral homologs which dimerize) due to an insertion that packs against the dimerization interface, and cleaves 5' flap structures but not Holliday junctions.","method":"X-ray crystallography of three VRR_nuc representatives, in vitro nuclease assay, structural modeling","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1 — crystal structures plus biochemical activity, structural basis for substrate specificity established","pmids":["24981866"],"is_preprint":false},{"year":2014,"finding":"FAN1 is recruited to aphidicolin-stalled replication forks via a FANCD2-dependent (but FA core complex- and UBZ domain-independent) mechanism; FAN1 joins the BLM-FANCD2 complex and uses its nuclease activity for fork restart; in the absence of FANCD2, MRE11-promoted FAN1 access leads to nucleolytic degradation of nascent DNA.","method":"Co-immunoprecipitation, chromatin fractionation, siRNA knockdown, DNA fiber analysis (replication fork assay), nuclease-defective mutant analysis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (Co-IP, chromatin fractionation, fiber assay, domain mutants) in a single study","pmids":["25135477"],"is_preprint":false},{"year":2014,"finding":"Crystal structure of Pseudomonas aeruginosa FAN1 in complex with 5' flap DNA shows all four domains participate in DNA recognition with each playing a specific role; a six-helix bundle connecting to the VRR_nuc domain enables incision several bases from the junction; a clamping motion facilitates nucleolytic cleavage.","method":"X-ray crystallography, biochemical nuclease assay","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with biochemical validation; ortholog structure consistent with human FAN1 mechanism","pmids":["25319828"],"is_preprint":false},{"year":2015,"finding":"FAN1 efficiently promotes endonucleolytic incision of 5' flap DNA complexed with RPA at the proper site, demonstrating that RPA-coated single-stranded DNA does not block FAN1 activity at stalled replication forks.","method":"In vitro nuclease assay with purified recombinant human FAN1 and RPA on 5'-flapped DNA substrates","journal":"Journal of biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 — reconstituted in vitro assay, single lab, single method","pmids":["25922199"],"is_preprint":false},{"year":2016,"finding":"Fan1 recruitment to ICLs by ubiquitinated Fancd2 is dispensable for ICL repair per se; instead, Fan1 recruitment and nuclease activity restrain replication fork progression and prevent chromosome abnormalities when forks stall; Fan1 nuclease-defective knockin mice are cancer-prone; a cancer-associated Fan1 variant abolishing Ub-Fancd2 recruitment causes genetic instability without affecting ICL repair.","method":"Fan1 knockin mouse (nuclease-defective), cancer incidence monitoring, DNA fiber analysis, epistasis with FANCD2 ubiquitination, chromosome aberration assay","journal":"Science","confidence":"High","confidence_rationale":"Tier 1-2 — knockin mouse model with nuclease-defective mutation, multiple readouts, separates ICL repair from fork-stalling functions","pmids":["26797144"],"is_preprint":false},{"year":2016,"finding":"In Fan1-deficient mice, the UBZ domain (required for FANCD2 interaction) is not required for initial rapid recruitment of FAN1 to ICLs or for ICL resistance; epistasis analyses show FAN1 has ICL repair activities independent of Fanconi anemia proteins, with activity redundant with the 5'-3' exonuclease SNM1A.","method":"Fan1-deficient mouse, UBZ domain mutant knockin, drug sensitivity assays, epistasis with FA pathway mutants and SNM1A","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 — clean mouse knockout plus knockin domain mutant plus epistasis analysis","pmids":["26980189"],"is_preprint":false},{"year":2016,"finding":"Fan1 nuclease-defective knockin (Fan1nd/nd) mice develop karyomegalic interstitial nephritis; karyomegalic nuclei in kidneys are polyploid; fibroblasts from Fan1nd/nd mice become polyploid upon ICL induction, indicating FAN1 nuclease activity controls ploidy through ICL repair.","method":"Knockin mouse model (nuclease-defective point mutation), histology, flow cytometry (ploidy), drug treatment assays","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1-2 — nuclease-defective knockin mouse with mechanistic polyploidy readout","pmids":["26980188"],"is_preprint":false},{"year":2018,"finding":"Structural analysis reveals that despite lacking a basic pocket and dimerization features of human FAN1, Pseudomonas aeruginosa FAN1 cleaves substrates at ~3-nt intervals and resolves ICLs; a conserved Arg/Lys patch recognizes phosphates near the 5' terminus; in human FAN1 the Arg/Lys patch and basic pocket play complementary roles in ICL resolution.","method":"X-ray crystallography of PaFAN1-DNA complexes, site-directed mutagenesis, in vitro ICL resolution assay","journal":"Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structures plus mutagenesis plus biochemical ICL assay","pmids":["29514982"],"is_preprint":false},{"year":2019,"finding":"FAN1 binds to the expanded HTT CAG repeat DNA and suppresses CAG repeat expansion in a nuclease-independent manner; FAN1 overexpression reduces CAG repeat expansion whereas FAN1 knockdown increases it in patient-derived stem cells and neurons; stabilizing effect is FAN1 concentration- and CAG length-dependent.","method":"FAN1 overexpression and siRNA knockdown in human cells and patient-derived iPSCs/neurons, repeat-length PCR assay, DNA binding assay, nuclease-dead mutant analysis","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (overexpression, knockdown, nuclease-dead mutant, direct DNA binding), multiple cell models","pmids":["30358836"],"is_preprint":false},{"year":2020,"finding":"Fan1 knockout increases somatic CAG repeat expansion of Htt knock-in CAG repeats in mice; simultaneous knockout of Mlh1 blocks Fan1-KO-induced acceleration of somatic CAG expansion, establishing a genetic interaction where MLH1 is required for the CAG repeat-destabilizing effect of FAN1 loss.","method":"Fan1 knockout mice crossed with Htt CAG knock-in mice, Mlh1 knockout double mutant, somatic repeat instability quantification","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — clean mouse genetic epistasis with three-way knockout analysis","pmids":["32876667"],"is_preprint":false},{"year":2020,"finding":"FAN1 missense variants p.Arg507His and p.Arg377Trp (within or near the DNA-binding domain) reduce FAN1's DNA-binding activity and its capacity to rescue MMC-induced cytotoxicity; FAN1 knockout increases CAG repeat expansion in HD iPSCs.","method":"In vitro DNA-binding assay, MMC complementation assay, FAN1 knockout iPSC CAG repeat expansion assay","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — multiple biochemical and cellular assays, disease-relevant mutations functionally characterized","pmids":["32589923"],"is_preprint":false},{"year":2020,"finding":"FAN1 is loaded onto chromatin through interaction with MLH1 following O6-methylguanine damage; FAN1 produces single-stranded DNA by its exonuclease activity contributing to DNA damage response and apoptosis; FAN1 interacts with both MLH1 and MSH2 after alkylation damage; FAN1 focus formation requires MLH1 but not FANCD2.","method":"Co-immunoprecipitation, immunofluorescence (FAN1 foci with MLH1, ssDNA), siRNA knockdown, sub-G1 and caspase-9 apoptosis assays","journal":"Genes to cells","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP plus localization plus knockdown phenotype, single lab","pmids":["31955481"],"is_preprint":false},{"year":2021,"finding":"FAN1 contains a conserved SPYF motif at its N-terminus that binds MLH1; FAN1 restricts MLH1 recruitment by MSH3 to inhibit assembly of a functional MMR complex that would otherwise promote CAG repeat expansion; FAN1 also promotes accurate repair via its nuclease activity.","method":"Mutagenesis of SPYF motif, co-immunoprecipitation, CAG repeat expansion assays in patient-derived cells","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — motif mutagenesis plus Co-IP plus functional repeat expansion assay","pmids":["34469738"],"is_preprint":false},{"year":2021,"finding":"Specific amino acid residues in two adjacent FAN1 motifs are critical for MLH1 binding; disruption of the FAN1-MLH1 interaction causes ICL hypersensitivity and defective repair of CAG/CTG slip-outs; FAN1-S126 phosphorylation by cyclin-dependent kinase activity hinders FAN1-MLH1 association and is attenuated upon ICL induction.","method":"Site-directed mutagenesis, co-immunoprecipitation, ICL sensitivity assay, CAG/CTG slip-out repair assay, phosphorylation site mapping, CDK inhibitor treatment","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis plus biochemical interaction plus functional ICL and repeat repair assays plus phosphorylation mechanism","pmids":["34330701"],"is_preprint":false},{"year":2021,"finding":"FAN1 exonuclease (but not endonuclease) activity pauses at specific sites within CAG and CTG slip-out repeats (5'-C↓A↓GC↓A↓G-3' and 5'-C↓T↓G↓C↓T↓G-3'); FAN1 binds, dimerizes, and cleaves slipped DNAs through iterative cycles; nuclease-ligand naphthyridine-azaquinolone protecting slip-outs from FAN1 exo- but not endo-nucleolytic digestion requires FAN1 for in vivo repeat contraction.","method":"In vitro nuclease assay with purified FAN1 on slipped-DNA substrates, cleavage pattern mapping, dimerization analysis, ligand competition assay","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1 — detailed biochemical reconstitution with sequence-specific cleavage mapping and domain-specific activity separation","pmids":["34879276"],"is_preprint":false},{"year":2022,"finding":"Rare FAN1 coding variants in the DNA-binding and nuclease domains are associated with earlier HD onset; nuclease activities of purified variants in vitro correlate with residual age at motor onset; mutating endogenous FAN1 to nuclease-inactive form in iPSCs leads to CAG expansion rates similar to complete FAN1 knockout, establishing nuclease activity as the key protective function.","method":"Exome sequencing, in vitro nuclease assay with purified FAN1 variants, CRISPR knockin of nuclease-inactive FAN1 in iPSCs, CAG repeat expansion assay","journal":"Nature neuroscience","confidence":"High","confidence_rationale":"Tier 1-2 — purified protein biochemistry correlated with clinical phenotype plus CRISPR knockin functional validation","pmids":["35379994"],"is_preprint":false},{"year":2023,"finding":"FAN1 nuclease function on CAG triplet repeat extrusions is activated by RFC, PCNA, and ATP at physiological ionic strength; RFC-PCNA confer strand directionality to FAN1; PCNA-FAN1 physical interaction is required; FAN1-dependent CAG extrusion removal in cell extracts proceeds via a short patch excision-repair mechanism competing with MutSβ-dependent MMR.","method":"In vitro nuclease assay with purified FAN1, RFC, PCNA on extrahelical extrusion substrates, cell extract assay, PCNA-FAN1 interaction assay","journal":"PNAS","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro assay with multiple defined components plus cell extract validation","pmids":["37549289"],"is_preprint":false},{"year":2012,"finding":"FAN1 protein levels are regulated during cell cycle; FAN1 is degraded during mitotic exit as a substrate of APC/CCdh1 (not APC/CCdc20) through KEN box and D-box degrons; FAN1 levels affect progression to mitotic exit.","method":"Western blotting during cell cycle, Cdh1/Cdc20 overexpression, KEN box/D-box mutant analysis, flow cytometry","journal":"Chinese journal of cancer","confidence":"Medium","confidence_rationale":"Tier 2-3 — degron mutagenesis plus APC/C component specificity, single lab","pmids":["22854063"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM structures of PCNA-FAN1-DNA complex reveal that FAN1 R507 directly contacts PCNA D232; the R507H mutation attenuates FAN1-PCNA complex assembly on CAG extrahelical extrusions and abolishes PCNA-FAN1-dependent cleavage; PCNA modulates FAN1 activity upon ternary complex formation.","method":"Cryo-EM structure determination, biophysical interaction studies, in vitro nuclease assay with R507H mutant and PCNA","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure plus biophysical and biochemical validation, two independent groups","pmids":["40368897","40368883"],"is_preprint":false},{"year":2025,"finding":"FAN1 preferentially targets the looped strand of CAG extrahelical extrusions; RFC-PCNA stimulate and direct FAN1 nuclease to the 3' boundary of the loop while restricting exonuclease activity; no pre-existing nick is required; FAN1 action followed by Polδ causes repeat contraction; FAN1 also directly inhibits MutLγ, preventing its activation by MutSβ.","method":"In vitro reconstitution with purified human proteins (FAN1, RFC, PCNA, MutLγ, MutSβ, Polδ), nuclease assay, repeat expansion/contraction assay","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — fully reconstituted system with purified human proteins establishing complete mechanistic pathway","pmids":["41145416"],"is_preprint":false},{"year":2026,"finding":"USP7 deubiquitinase interacts with FAN1 and stabilizes FAN1 protein levels by preventing its proteasomal degradation through deubiquitination; USP7 depletion reduces FAN1 chromatin association, increases ICL sensitivity, and accelerates CAG repeat expansion.","method":"Co-immunoprecipitation (USP7-FAN1 interaction), western blot (protein stability), chromatin fractionation, ICL sensitivity assay, CAG repeat expansion assay in RPE-1 cells","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — Co-IP plus functional knockdown with multiple mechanistic readouts","pmids":["41786746"],"is_preprint":false},{"year":2024,"finding":"miR-124-3p selectively targets the reference allele at rs3512 in the FAN1 3'-UTR, reducing FAN1 mRNA stability and levels; the alternative allele at rs3512 renders FAN1 mRNA less susceptible to miR-124-3p-mediated degradation, resulting in increased FAN1 expression.","method":"Antagomir treatment, 3'-UTR reporter assays with swapped alleles, allelic imbalance analysis","journal":"PNAS","confidence":"High","confidence_rationale":"Tier 2 — reporter assay plus antagomir validation plus allelic imbalance, multiple orthogonal approaches","pmids":["38607933"],"is_preprint":false},{"year":2021,"finding":"In a Fragile X mouse model, Fan1 nuclease domain point mutation has the same effect on CGG repeat expansion as a null mutation; FAN1 and EXO1 have additive effects protecting against MSH3-dependent expansions; loss of FANCD2 has no effect on expansions, indicating FAN1 protects against repeat expansion independently of the canonical FA pathway and requires its nuclease domain.","method":"FAN1 nuclease-domain point mutant knockin mouse, Fan1/Exo1 double knockout, Fancd2 knockout, somatic repeat instability quantification","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 — clean mouse genetic models (knockin + double knockouts) with epistasis establishing nuclease requirement and FA-independence","pmids":["34718701"],"is_preprint":false}],"current_model":"FAN1 is a structure-specific nuclease with 5' flap endonuclease and 5'→3' exonuclease activities (mediated by its VRR_nuc domain) that is recruited to DNA interstrand crosslink (ICL) sites via its UBZ domain binding monoubiquitinated FANCD2, and to stalled replication forks via a FANCD2-dependent but UBZ-independent mechanism; at ICLs it excises lesions by cleaving every ~3 nucleotides; it is also activated by PCNA-RFC on extrahelical CAG/CTG repeat extrusions to promote their removal and prevent somatic repeat expansion (competing with pro-expansion MutSβ/MutLγ MMR), an activity requiring its physical interaction with PCNA (through R507) and its nuclease function; additionally, FAN1 interacts with MLH1 through a conserved SPYF motif to restrict MMR complex assembly, its protein levels are regulated by APC/CCdh1-mediated proteasomal degradation and by USP7 deubiquitination, and its S126 is phosphorylated by CDKs to modulate MLH1 binding in a cell cycle-regulated manner."},"narrative":{"teleology":[{"year":2010,"claim":"Four simultaneous studies established FAN1 as a novel structure-specific nuclease with 5' flap endonuclease and 5'→3' exonuclease activities recruited to ICL damage sites via its UBZ domain binding monoubiquitinated FANCD2, resolving how cells engage a dedicated nuclease for crosslink processing.","evidence":"Independent shRNA/siRNA screens, in vitro nuclease assays, Co-IP of UBZ–ubFANCD2 interaction, immunofluorescence foci, domain mutagenesis in human cells and DT40 knockouts","pmids":["20603073","20603015","20603016","20671156","21115814"],"confidence":"High","gaps":["Structural basis of how FAN1 engages ICL substrates was unknown","Whether FAN1 acted solely within or also outside the canonical FA pathway was unresolved","The mechanistic basis for 5' flap specificity was not explained"]},{"year":2014,"claim":"Crystal structures of human and Pseudomonas FAN1 revealed the molecular basis for its ~3-nt iterative cleavage mechanism, head-to-tail dimerization on flap DNA, and a 5'-phosphate anchor requirement, explaining how FAN1 excises ICL lesions through flanking incisions.","evidence":"X-ray crystallography of human FAN1–DNA and PaFAN1–DNA complexes, active-site mutagenesis, in vitro nuclease assays","pmids":["25430771","25500724","24981866","25319828"],"confidence":"High","gaps":["How FAN1 operates on chromatin substrates in vivo was not addressed","The role of FAN1 dimerization in cellular ICL repair was not tested","No structural data on FAN1 with an actual crosslinked substrate existed"]},{"year":2014,"claim":"FAN1 was shown to act at stalled replication forks through a FANCD2-dependent but UBZ-independent mechanism involving the BLM–FANCD2 complex, separating its fork-protective role from its ICL recruitment mechanism.","evidence":"Co-IP, chromatin fractionation, DNA fiber analysis, nuclease-dead mutant in human cells with siRNA knockdown of FANCD2, BLM, MRE11","pmids":["25135477"],"confidence":"High","gaps":["The molecular basis of UBZ-independent FANCD2 recruitment at forks was unknown","How MRE11 gates FAN1 access to nascent DNA in the absence of FANCD2 was not structurally resolved"]},{"year":2016,"claim":"Mouse knockin and knockout models demonstrated that FAN1 nuclease activity restrains replication fork progression and prevents cancer, and that its UBZ–ubFANCD2 interaction is dispensable for ICL repair per se but critical for fork stability, fundamentally reframing FAN1's primary physiological role.","evidence":"Fan1 nuclease-defective knockin mice, cancer incidence monitoring, UBZ mutant knockin, DNA fiber analysis, epistasis with FA genes and SNM1A","pmids":["26797144","26980189"],"confidence":"High","gaps":["The non-ICL substrates processed by FAN1 at stalled forks were not identified","Whether FAN1's fork-protective function extends to repeat sequences was unexplored"]},{"year":2016,"claim":"FAN1 nuclease-defective mice developed karyomegalic interstitial nephritis with polyploid renal nuclei, establishing the disease mechanism as a failure of ICL repair leading to endoreduplication.","evidence":"Fan1 nuclease-dead knockin mouse histology, flow cytometry for ploidy, ICL-induced polyploidy in fibroblasts","pmids":["26980188"],"confidence":"High","gaps":["The identity of the endogenous ICL-inducing agent in kidney was not established","Whether other tissues show subclinical polyploidy was not fully assessed"]},{"year":2019,"claim":"FAN1 was identified as a suppressor of somatic CAG repeat expansion in Huntington disease models, with overexpression reducing and knockdown increasing expansion, revealing an unexpected genome stability role at trinucleotide repeats.","evidence":"FAN1 overexpression/knockdown in HD patient-derived iPSCs and neurons, repeat-length PCR, nuclease-dead mutant analysis, direct DNA binding assay","pmids":["30358836"],"confidence":"High","gaps":["The mechanism by which FAN1 prevented expansion (binding vs. cleavage) was debated—nuclease-independent stabilization was reported","Whether FAN1 acted directly on repeat extrusions or indirectly through MMR was unknown"]},{"year":2020,"claim":"Mouse genetic epistasis showed that MLH1 is required for the repeat-destabilizing effect of FAN1 loss, and FAN1 was shown to interact with MLH1 and MSH2 on chromatin after alkylation damage, linking FAN1 to mismatch repair machinery.","evidence":"Fan1/Mlh1 double-knockout mice with Htt CAG knock-in, Co-IP of FAN1–MLH1/MSH2 after O6-meG damage, immunofluorescence","pmids":["32876667","31955481"],"confidence":"High","gaps":["The molecular interface between FAN1 and MLH1 was not mapped","Whether FAN1–MLH1 interaction directly inhibited MMR or redirected it was unclear"]},{"year":2021,"claim":"Mapping of the FAN1 SPYF motif for MLH1 binding, CDK-dependent S126 phosphorylation modulating this interaction, and detailed biochemistry of FAN1 cleavage patterns on slipped CAG/CTG DNA established dual mechanisms—sequestration of MLH1 from pro-expansion MMR plus direct nucleolytic removal of slip-outs.","evidence":"SPYF motif and S126 mutagenesis, Co-IP, ICL and CAG/CTG slip-out repair assays, CDK inhibitor treatment, in vitro cleavage pattern mapping on slipped DNA","pmids":["34469738","34330701","34879276","34718701"],"confidence":"High","gaps":["How S126 phosphorylation is coordinated with replication timing at repeat loci was not resolved","Whether FAN1 dimerization on slip-outs has the same architecture as on ICL substrates was unknown","In vivo contribution of SPYF-MLH1 versus nuclease activity to repeat protection was not quantitatively separated"]},{"year":2022,"claim":"Rare FAN1 coding variants in the nuclease and DNA-binding domains were shown to correlate with reduced nuclease activity in vitro and earlier Huntington disease onset, with CRISPR knockin of nuclease-dead FAN1 phenocopying complete FAN1 loss for CAG expansion, establishing nuclease activity as the critical protective function.","evidence":"Exome sequencing, purified FAN1 variant biochemistry, CRISPR knockin of nuclease-inactive FAN1 in iPSCs, CAG expansion quantification","pmids":["35379994"],"confidence":"High","gaps":["Whether partial nuclease activity from hypomorphic alleles yields proportional clinical benefit was not established in vivo","Structural basis for how specific variants impair nuclease function was not determined"]},{"year":2023,"claim":"Reconstitution showed RFC–PCNA activate FAN1 nuclease on extrahelical CAG extrusions at physiological ionic strength, conferring strand directionality and enabling a short-patch excision-repair mechanism that competes with MutSβ-dependent MMR, explaining how FAN1 and MMR compete for the same substrate.","evidence":"In vitro reconstitution with purified FAN1, RFC, PCNA on extrahelical extrusion substrates; cell extract assay","pmids":["37549289"],"confidence":"High","gaps":["Structural basis of PCNA–FAN1 interaction was not resolved","Whether RFC–PCNA similarly regulate FAN1 at ICL substrates was not tested"]},{"year":2025,"claim":"Cryo-EM structures of the PCNA–FAN1–DNA ternary complex revealed R507 as the direct PCNA contact; reconstitution with Polδ showed FAN1 incision followed by polymerase fill-in causes repeat contraction; FAN1 was additionally shown to directly inhibit MutLγ activation by MutSβ, establishing a complete pathway from lesion recognition to contraction.","evidence":"Cryo-EM structure determination, R507H mutant biochemistry, full reconstitution with FAN1/RFC/PCNA/MutSβ/MutLγ/Polδ","pmids":["40368897","40368883","41145416"],"confidence":"High","gaps":["In vivo validation of the R507-PCNA interface for repeat protection is lacking","Whether FAN1 inhibition of MutLγ occurs through direct protein–protein contact or substrate competition was not fully distinguished","No in vivo structure of FAN1 operating at an endogenous repeat locus exists"]},{"year":2026,"claim":"USP7 was identified as a deubiquitinase that stabilizes FAN1 protein levels by counteracting proteasomal degradation, with USP7 depletion reducing FAN1 chromatin association and accelerating both ICL sensitivity and CAG repeat expansion.","evidence":"Co-IP of USP7–FAN1, western blot protein stability, chromatin fractionation, ICL and CAG expansion assays in RPE-1 cells","pmids":["41786746"],"confidence":"High","gaps":["The specific ubiquitin sites on FAN1 targeted by USP7 were not mapped","How APC/C^Cdh1-mediated degradation and USP7 stabilization are coordinated across the cell cycle is unresolved"]},{"year":null,"claim":"Key unresolved questions include the identity of endogenous ICL-generating agents in kidney tissue, the in vivo structural dynamics of FAN1 at endogenous repeat loci, the quantitative partitioning of FAN1's protective effect between nuclease activity and MLH1 sequestration, and the therapeutic potential of modulating FAN1 protein levels or activity to delay trinucleotide repeat expansion diseases.","evidence":"","pmids":[],"confidence":"Low","gaps":["No endogenous crosslinking agent in kidney has been identified","Relative contribution of SPYF-MLH1 sequestration versus nuclease activity to repeat protection has not been quantitatively determined in vivo","No pharmacological activator of FAN1 has been reported"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140097","term_label":"catalytic activity, acting on DNA","supporting_discovery_ids":[0,1,2,6,7,8,22,23,24,27]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,1,2,6,22,24]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[16,18,22]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,4,19]},{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[9,19,28]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,1,2,3,5,6,12,13,14]},{"term_id":"R-HSA-69306","term_label":"DNA Replication","supporting_discovery_ids":[9,12]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[25]}],"complexes":["PCNA-RFC-FAN1","BLM-FANCD2-FAN1"],"partners":["FANCD2","MLH1","PCNA","BLM","USP7","MSH2","RFC"],"other_free_text":[]},"mechanistic_narrative":"FAN1 is a structure-specific nuclease that maintains genome integrity at interstrand crosslinks, stalled replication forks, and trinucleotide repeat sequences. Its VRR_nuc domain confers 5' flap endonuclease and 5'→3' exonuclease activities, cleaving DNA at ~3-nucleotide intervals; at ICL sites it is recruited via its UBZ domain binding monoubiquitinated FANCD2, while at stalled forks it acts through a FANCD2-dependent but UBZ-independent mechanism involving the BLM–FANCD2 complex [PMID:20603073, PMID:25135477, PMID:25430771]. FAN1 prevents somatic CAG/CTG trinucleotide repeat expansion by two converging mechanisms: its nuclease activity, activated by RFC–PCNA loading on extrahelical repeat extrusions, excises the looped strand to promote contraction by Polδ, and its N-terminal SPYF motif sequesters MLH1 to restrict pro-expansion MutSβ/MutLγ MMR complex assembly—activities whose loss accelerates Huntington disease and Fragile X repeat expansion in mouse and iPSC models [PMID:34469738, PMID:37549289, PMID:41145416, PMID:34718701]. FAN1 protein levels are cell-cycle regulated through APC/C^Cdh1-mediated proteasomal degradation and stabilized by USP7 deubiquitination, while CDK-dependent phosphorylation at S126 modulates its MLH1 interaction [PMID:22854063, PMID:34330701, PMID:41786746]. Loss-of-function mutations in FAN1 cause karyomegalic interstitial nephritis, linked to failed ICL repair and resultant polyploidy in renal tubular cells [PMID:26980188]."},"prefetch_data":{"uniprot":{"accession":"Q9Y2M0","full_name":"Fanconi-associated nuclease 1","aliases":["FANCD2/FANCI-associated nuclease 1","hFAN1","Myotubularin-related protein 15"],"length_aa":1017,"mass_kda":114.2,"function":"Nuclease required for the repair of DNA interstrand cross-links (ICL) recruited at sites of DNA damage by monoubiquitinated FANCD2. Specifically involved in repair of ICL-induced DNA breaks by being required for efficient homologous recombination, probably in the resolution of homologous recombination intermediates (PubMed:20603015, PubMed:20603016, PubMed:20603073, PubMed:20671156, PubMed:24981866, PubMed:25430771). Not involved in DNA double-strand breaks resection (PubMed:20603015, PubMed:20603016). Acts as a 5'-3' exonuclease that anchors at a cut end of DNA and cleaves DNA successively at every third nucleotide, allowing to excise an ICL from one strand through flanking incisions. Probably keeps excising with 3'-flap annealing until it reaches and unhooks the ICL (PubMed:25430771). Acts at sites that have a 5'-terminal phosphate anchor at a nick or a 1- or 2-nucleotide flap and is augmented by a 3' flap (PubMed:25430771). Also has endonuclease activity toward 5'-flaps (PubMed:20603015, PubMed:20603016, PubMed:24981866)","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9Y2M0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FAN1","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FAN1","total_profiled":1310},"omim":[{"mim_id":"614817","title":"INTERSTITIAL NEPHRITIS, KARYOMEGALIC; KMIN","url":"https://www.omim.org/entry/614817"},{"mim_id":"613984","title":"FANCD2 GENE; FANCD2","url":"https://www.omim.org/entry/613984"},{"mim_id":"613534","title":"FANCD2/FANCI-ASSOCIATED NUCLEASE 1; FAN1","url":"https://www.omim.org/entry/613534"},{"mim_id":"612024","title":"OTU DOMAIN-CONTAINING PROTEIN 7A; OTUD7A","url":"https://www.omim.org/entry/612024"},{"mim_id":"612001","title":"CHROMOSOME 15q13.3 DELETION SYNDROME","url":"https://www.omim.org/entry/612001"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Cytosol","reliability":"Approved"},{"location":"Cytokinetic bridge","reliability":"Additional"},{"location":"Primary cilium","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in many","driving_tissues":[],"url":"https://www.proteinatlas.org/search/FAN1"},"hgnc":{"alias_symbol":[],"prev_symbol":["KIAA1018","MTMR15"]},"alphafold":{"accession":"Q9Y2M0","domains":[{"cath_id":"-","chopping":"375-506_520-542","consensus_level":"medium","plddt":92.5458,"start":375,"end":542},{"cath_id":"1.25.40.10","chopping":"592-767","consensus_level":"medium","plddt":94.5783,"start":592,"end":767},{"cath_id":"-","chopping":"780-1010","consensus_level":"medium","plddt":91.8765,"start":780,"end":1010}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y2M0","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y2M0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y2M0-F1-predicted_aligned_error_v6.png","plddt_mean":69.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FAN1","jax_strain_url":"https://www.jax.org/strain/search?query=FAN1"},"sequence":{"accession":"Q9Y2M0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9Y2M0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9Y2M0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y2M0"}},"corpus_meta":[{"pmid":"20603073","id":"PMC_20603073","title":"A 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immunofluorescence colocalization at DNA damage foci, UBZ domain mutant analysis\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (biochemical activity, domain mutagenesis, cellular localization), independently replicated by four simultaneous papers\",\n      \"pmids\": [\"20603073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"FAN1 interacts with and is recruited to DNA damage sites by the monoubiquitinated form of FANCD2; FAN1 exhibits 5' flap endonuclease activity and 5' exonuclease activity mediated by its conserved VRR_nuc domain.\",\n      \"method\": \"Co-immunoprecipitation, in vitro nuclease assay, siRNA knockdown with ICL sensitivity and genome instability readouts\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — biochemical reconstitution plus domain identification plus cellular phenotype, confirmed by multiple independent groups simultaneously\",\n      \"pmids\": [\"20603015\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"FAN1 (KIAA1018) is a 5'→3' exonuclease and structure-specific endonuclease preferentially incising 5' flaps; its UBZ domain interacts with monoubiquitylated FANCD2 for recruitment to ICL damage sites; depletion causes ICL hypersensitivity and chromosomal instability.\",\n      \"method\": \"In vitro nuclease assay, co-immunoprecipitation, siRNA knockdown, chromosomal instability assay\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstituted enzymatic activity plus domain interaction plus cellular phenotype, independently replicated\",\n      \"pmids\": [\"20603016\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"FAN1 promotes ICL repair in a manner strictly dependent on its ability to accumulate at sites of DNA damage, and this accumulation relies on monoubiquitylation of the FANCI-FANCD2 (ID) complex.\",\n      \"method\": \"siRNA knockdown, fluorescence microscopy, epistasis analysis with ID complex mutants\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis plus localization experiment, replicated by multiple independent groups\",\n      \"pmids\": [\"20671156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"FAN1 localizes to stalled replication forks (colocalizing with RPA) in a UBZ domain-dependent manner, and is a nuclear protein forming DNA-damage-induced foci.\",\n      \"method\": \"Immunofluorescence colocalization with RPA, UBZ domain mutant analysis, siRNA knockdown\",\n      \"journal\": \"Cell cycle\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment with domain mutant, single lab\",\n      \"pmids\": [\"20935496\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"FAN1 null DT40 cells are sensitive to cisplatin and MMC but not ionizing/UV radiation, MMS, or camptothecin; double knockouts of FAN1 with FANCC or FANCJ show increased cisplatin sensitivity, indicating FAN1 participates in ICL repair both within and independently of the classical FA pathway.\",\n      \"method\": \"Gene targeting in chicken DT40 cells, drug sensitivity assays, double-mutant epistasis analysis, sister chromatid exchange assay\",\n      \"journal\": \"PNAS\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean genetic knockout plus epistasis analysis with multiple FA pathway components\",\n      \"pmids\": [\"21115814\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Crystal structures of human FAN1 reveal it cleaves DNA successively at every third nucleotide via exonuclease activity, requiring a 5'-terminal phosphate anchor at a nick or 1-2 nucleotide flap, augmented by a 3' flap; this mechanism allows FAN1 to excise an ICL from one strand through flanking incisions.\",\n      \"method\": \"X-ray crystallography (FAN1-DNA co-crystal structures), in vitro biochemical assay, active-site mutagenesis\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with in vitro biochemical validation and mutagenesis\",\n      \"pmids\": [\"25430771\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Human FAN1 forms a head-to-tail dimer in complex with 5' flap DNA; two FAN1 molecules cooperate to locate the lesion, orient the DNA, and unwind a 5' flap for subsequent incision; structure-informed mutations disrupting dimerization, substrate orientation, or flap unwinding impair nuclease activity.\",\n      \"method\": \"X-ray crystallography (three crystal structures), biochemical nuclease assay, structure-guided mutagenesis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple crystal structures plus mutagenesis plus biochemical validation\",\n      \"pmids\": [\"25500724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"FAN1 VRR_nuc domain is monomeric (unlike bacterial/viral homologs which dimerize) due to an insertion that packs against the dimerization interface, and cleaves 5' flap structures but not Holliday junctions.\",\n      \"method\": \"X-ray crystallography of three VRR_nuc representatives, in vitro nuclease assay, structural modeling\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures plus biochemical activity, structural basis for substrate specificity established\",\n      \"pmids\": [\"24981866\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"FAN1 is recruited to aphidicolin-stalled replication forks via a FANCD2-dependent (but FA core complex- and UBZ domain-independent) mechanism; FAN1 joins the BLM-FANCD2 complex and uses its nuclease activity for fork restart; in the absence of FANCD2, MRE11-promoted FAN1 access leads to nucleolytic degradation of nascent DNA.\",\n      \"method\": \"Co-immunoprecipitation, chromatin fractionation, siRNA knockdown, DNA fiber analysis (replication fork assay), nuclease-defective mutant analysis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (Co-IP, chromatin fractionation, fiber assay, domain mutants) in a single study\",\n      \"pmids\": [\"25135477\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Crystal structure of Pseudomonas aeruginosa FAN1 in complex with 5' flap DNA shows all four domains participate in DNA recognition with each playing a specific role; a six-helix bundle connecting to the VRR_nuc domain enables incision several bases from the junction; a clamping motion facilitates nucleolytic cleavage.\",\n      \"method\": \"X-ray crystallography, biochemical nuclease assay\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with biochemical validation; ortholog structure consistent with human FAN1 mechanism\",\n      \"pmids\": [\"25319828\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FAN1 efficiently promotes endonucleolytic incision of 5' flap DNA complexed with RPA at the proper site, demonstrating that RPA-coated single-stranded DNA does not block FAN1 activity at stalled replication forks.\",\n      \"method\": \"In vitro nuclease assay with purified recombinant human FAN1 and RPA on 5'-flapped DNA substrates\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro assay, single lab, single method\",\n      \"pmids\": [\"25922199\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Fan1 recruitment to ICLs by ubiquitinated Fancd2 is dispensable for ICL repair per se; instead, Fan1 recruitment and nuclease activity restrain replication fork progression and prevent chromosome abnormalities when forks stall; Fan1 nuclease-defective knockin mice are cancer-prone; a cancer-associated Fan1 variant abolishing Ub-Fancd2 recruitment causes genetic instability without affecting ICL repair.\",\n      \"method\": \"Fan1 knockin mouse (nuclease-defective), cancer incidence monitoring, DNA fiber analysis, epistasis with FANCD2 ubiquitination, chromosome aberration assay\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — knockin mouse model with nuclease-defective mutation, multiple readouts, separates ICL repair from fork-stalling functions\",\n      \"pmids\": [\"26797144\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In Fan1-deficient mice, the UBZ domain (required for FANCD2 interaction) is not required for initial rapid recruitment of FAN1 to ICLs or for ICL resistance; epistasis analyses show FAN1 has ICL repair activities independent of Fanconi anemia proteins, with activity redundant with the 5'-3' exonuclease SNM1A.\",\n      \"method\": \"Fan1-deficient mouse, UBZ domain mutant knockin, drug sensitivity assays, epistasis with FA pathway mutants and SNM1A\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean mouse knockout plus knockin domain mutant plus epistasis analysis\",\n      \"pmids\": [\"26980189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Fan1 nuclease-defective knockin (Fan1nd/nd) mice develop karyomegalic interstitial nephritis; karyomegalic nuclei in kidneys are polyploid; fibroblasts from Fan1nd/nd mice become polyploid upon ICL induction, indicating FAN1 nuclease activity controls ploidy through ICL repair.\",\n      \"method\": \"Knockin mouse model (nuclease-defective point mutation), histology, flow cytometry (ploidy), drug treatment assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — nuclease-defective knockin mouse with mechanistic polyploidy readout\",\n      \"pmids\": [\"26980188\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Structural analysis reveals that despite lacking a basic pocket and dimerization features of human FAN1, Pseudomonas aeruginosa FAN1 cleaves substrates at ~3-nt intervals and resolves ICLs; a conserved Arg/Lys patch recognizes phosphates near the 5' terminus; in human FAN1 the Arg/Lys patch and basic pocket play complementary roles in ICL resolution.\",\n      \"method\": \"X-ray crystallography of PaFAN1-DNA complexes, site-directed mutagenesis, in vitro ICL resolution assay\",\n      \"journal\": \"Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures plus mutagenesis plus biochemical ICL assay\",\n      \"pmids\": [\"29514982\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FAN1 binds to the expanded HTT CAG repeat DNA and suppresses CAG repeat expansion in a nuclease-independent manner; FAN1 overexpression reduces CAG repeat expansion whereas FAN1 knockdown increases it in patient-derived stem cells and neurons; stabilizing effect is FAN1 concentration- and CAG length-dependent.\",\n      \"method\": \"FAN1 overexpression and siRNA knockdown in human cells and patient-derived iPSCs/neurons, repeat-length PCR assay, DNA binding assay, nuclease-dead mutant analysis\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (overexpression, knockdown, nuclease-dead mutant, direct DNA binding), multiple cell models\",\n      \"pmids\": [\"30358836\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Fan1 knockout increases somatic CAG repeat expansion of Htt knock-in CAG repeats in mice; simultaneous knockout of Mlh1 blocks Fan1-KO-induced acceleration of somatic CAG expansion, establishing a genetic interaction where MLH1 is required for the CAG repeat-destabilizing effect of FAN1 loss.\",\n      \"method\": \"Fan1 knockout mice crossed with Htt CAG knock-in mice, Mlh1 knockout double mutant, somatic repeat instability quantification\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean mouse genetic epistasis with three-way knockout analysis\",\n      \"pmids\": [\"32876667\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FAN1 missense variants p.Arg507His and p.Arg377Trp (within or near the DNA-binding domain) reduce FAN1's DNA-binding activity and its capacity to rescue MMC-induced cytotoxicity; FAN1 knockout increases CAG repeat expansion in HD iPSCs.\",\n      \"method\": \"In vitro DNA-binding assay, MMC complementation assay, FAN1 knockout iPSC CAG repeat expansion assay\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple biochemical and cellular assays, disease-relevant mutations functionally characterized\",\n      \"pmids\": [\"32589923\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FAN1 is loaded onto chromatin through interaction with MLH1 following O6-methylguanine damage; FAN1 produces single-stranded DNA by its exonuclease activity contributing to DNA damage response and apoptosis; FAN1 interacts with both MLH1 and MSH2 after alkylation damage; FAN1 focus formation requires MLH1 but not FANCD2.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence (FAN1 foci with MLH1, ssDNA), siRNA knockdown, sub-G1 and caspase-9 apoptosis assays\",\n      \"journal\": \"Genes to cells\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP plus localization plus knockdown phenotype, single lab\",\n      \"pmids\": [\"31955481\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FAN1 contains a conserved SPYF motif at its N-terminus that binds MLH1; FAN1 restricts MLH1 recruitment by MSH3 to inhibit assembly of a functional MMR complex that would otherwise promote CAG repeat expansion; FAN1 also promotes accurate repair via its nuclease activity.\",\n      \"method\": \"Mutagenesis of SPYF motif, co-immunoprecipitation, CAG repeat expansion assays in patient-derived cells\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — motif mutagenesis plus Co-IP plus functional repeat expansion assay\",\n      \"pmids\": [\"34469738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Specific amino acid residues in two adjacent FAN1 motifs are critical for MLH1 binding; disruption of the FAN1-MLH1 interaction causes ICL hypersensitivity and defective repair of CAG/CTG slip-outs; FAN1-S126 phosphorylation by cyclin-dependent kinase activity hinders FAN1-MLH1 association and is attenuated upon ICL induction.\",\n      \"method\": \"Site-directed mutagenesis, co-immunoprecipitation, ICL sensitivity assay, CAG/CTG slip-out repair assay, phosphorylation site mapping, CDK inhibitor treatment\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis plus biochemical interaction plus functional ICL and repeat repair assays plus phosphorylation mechanism\",\n      \"pmids\": [\"34330701\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FAN1 exonuclease (but not endonuclease) activity pauses at specific sites within CAG and CTG slip-out repeats (5'-C↓A↓GC↓A↓G-3' and 5'-C↓T↓G↓C↓T↓G-3'); FAN1 binds, dimerizes, and cleaves slipped DNAs through iterative cycles; nuclease-ligand naphthyridine-azaquinolone protecting slip-outs from FAN1 exo- but not endo-nucleolytic digestion requires FAN1 for in vivo repeat contraction.\",\n      \"method\": \"In vitro nuclease assay with purified FAN1 on slipped-DNA substrates, cleavage pattern mapping, dimerization analysis, ligand competition assay\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — detailed biochemical reconstitution with sequence-specific cleavage mapping and domain-specific activity separation\",\n      \"pmids\": [\"34879276\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Rare FAN1 coding variants in the DNA-binding and nuclease domains are associated with earlier HD onset; nuclease activities of purified variants in vitro correlate with residual age at motor onset; mutating endogenous FAN1 to nuclease-inactive form in iPSCs leads to CAG expansion rates similar to complete FAN1 knockout, establishing nuclease activity as the key protective function.\",\n      \"method\": \"Exome sequencing, in vitro nuclease assay with purified FAN1 variants, CRISPR knockin of nuclease-inactive FAN1 in iPSCs, CAG repeat expansion assay\",\n      \"journal\": \"Nature neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — purified protein biochemistry correlated with clinical phenotype plus CRISPR knockin functional validation\",\n      \"pmids\": [\"35379994\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FAN1 nuclease function on CAG triplet repeat extrusions is activated by RFC, PCNA, and ATP at physiological ionic strength; RFC-PCNA confer strand directionality to FAN1; PCNA-FAN1 physical interaction is required; FAN1-dependent CAG extrusion removal in cell extracts proceeds via a short patch excision-repair mechanism competing with MutSβ-dependent MMR.\",\n      \"method\": \"In vitro nuclease assay with purified FAN1, RFC, PCNA on extrahelical extrusion substrates, cell extract assay, PCNA-FAN1 interaction assay\",\n      \"journal\": \"PNAS\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro assay with multiple defined components plus cell extract validation\",\n      \"pmids\": [\"37549289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"FAN1 protein levels are regulated during cell cycle; FAN1 is degraded during mitotic exit as a substrate of APC/CCdh1 (not APC/CCdc20) through KEN box and D-box degrons; FAN1 levels affect progression to mitotic exit.\",\n      \"method\": \"Western blotting during cell cycle, Cdh1/Cdc20 overexpression, KEN box/D-box mutant analysis, flow cytometry\",\n      \"journal\": \"Chinese journal of cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — degron mutagenesis plus APC/C component specificity, single lab\",\n      \"pmids\": [\"22854063\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM structures of PCNA-FAN1-DNA complex reveal that FAN1 R507 directly contacts PCNA D232; the R507H mutation attenuates FAN1-PCNA complex assembly on CAG extrahelical extrusions and abolishes PCNA-FAN1-dependent cleavage; PCNA modulates FAN1 activity upon ternary complex formation.\",\n      \"method\": \"Cryo-EM structure determination, biophysical interaction studies, in vitro nuclease assay with R507H mutant and PCNA\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure plus biophysical and biochemical validation, two independent groups\",\n      \"pmids\": [\"40368897\", \"40368883\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"FAN1 preferentially targets the looped strand of CAG extrahelical extrusions; RFC-PCNA stimulate and direct FAN1 nuclease to the 3' boundary of the loop while restricting exonuclease activity; no pre-existing nick is required; FAN1 action followed by Polδ causes repeat contraction; FAN1 also directly inhibits MutLγ, preventing its activation by MutSβ.\",\n      \"method\": \"In vitro reconstitution with purified human proteins (FAN1, RFC, PCNA, MutLγ, MutSβ, Polδ), nuclease assay, repeat expansion/contraction assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — fully reconstituted system with purified human proteins establishing complete mechanistic pathway\",\n      \"pmids\": [\"41145416\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"USP7 deubiquitinase interacts with FAN1 and stabilizes FAN1 protein levels by preventing its proteasomal degradation through deubiquitination; USP7 depletion reduces FAN1 chromatin association, increases ICL sensitivity, and accelerates CAG repeat expansion.\",\n      \"method\": \"Co-immunoprecipitation (USP7-FAN1 interaction), western blot (protein stability), chromatin fractionation, ICL sensitivity assay, CAG repeat expansion assay in RPE-1 cells\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP plus functional knockdown with multiple mechanistic readouts\",\n      \"pmids\": [\"41786746\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"miR-124-3p selectively targets the reference allele at rs3512 in the FAN1 3'-UTR, reducing FAN1 mRNA stability and levels; the alternative allele at rs3512 renders FAN1 mRNA less susceptible to miR-124-3p-mediated degradation, resulting in increased FAN1 expression.\",\n      \"method\": \"Antagomir treatment, 3'-UTR reporter assays with swapped alleles, allelic imbalance analysis\",\n      \"journal\": \"PNAS\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reporter assay plus antagomir validation plus allelic imbalance, multiple orthogonal approaches\",\n      \"pmids\": [\"38607933\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"In a Fragile X mouse model, Fan1 nuclease domain point mutation has the same effect on CGG repeat expansion as a null mutation; FAN1 and EXO1 have additive effects protecting against MSH3-dependent expansions; loss of FANCD2 has no effect on expansions, indicating FAN1 protects against repeat expansion independently of the canonical FA pathway and requires its nuclease domain.\",\n      \"method\": \"FAN1 nuclease-domain point mutant knockin mouse, Fan1/Exo1 double knockout, Fancd2 knockout, somatic repeat instability quantification\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean mouse genetic models (knockin + double knockouts) with epistasis establishing nuclease requirement and FA-independence\",\n      \"pmids\": [\"34718701\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FAN1 is a structure-specific nuclease with 5' flap endonuclease and 5'→3' exonuclease activities (mediated by its VRR_nuc domain) that is recruited to DNA interstrand crosslink (ICL) sites via its UBZ domain binding monoubiquitinated FANCD2, and to stalled replication forks via a FANCD2-dependent but UBZ-independent mechanism; at ICLs it excises lesions by cleaving every ~3 nucleotides; it is also activated by PCNA-RFC on extrahelical CAG/CTG repeat extrusions to promote their removal and prevent somatic repeat expansion (competing with pro-expansion MutSβ/MutLγ MMR), an activity requiring its physical interaction with PCNA (through R507) and its nuclease function; additionally, FAN1 interacts with MLH1 through a conserved SPYF motif to restrict MMR complex assembly, its protein levels are regulated by APC/CCdh1-mediated proteasomal degradation and by USP7 deubiquitination, and its S126 is phosphorylated by CDKs to modulate MLH1 binding in a cell cycle-regulated manner.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"FAN1 is a structure-specific nuclease that maintains genome integrity at interstrand crosslinks, stalled replication forks, and trinucleotide repeat sequences. Its VRR_nuc domain confers 5' flap endonuclease and 5'→3' exonuclease activities, cleaving DNA at ~3-nucleotide intervals; at ICL sites it is recruited via its UBZ domain binding monoubiquitinated FANCD2, while at stalled forks it acts through a FANCD2-dependent but UBZ-independent mechanism involving the BLM–FANCD2 complex [PMID:20603073, PMID:25135477, PMID:25430771]. FAN1 prevents somatic CAG/CTG trinucleotide repeat expansion by two converging mechanisms: its nuclease activity, activated by RFC–PCNA loading on extrahelical repeat extrusions, excises the looped strand to promote contraction by Polδ, and its N-terminal SPYF motif sequesters MLH1 to restrict pro-expansion MutSβ/MutLγ MMR complex assembly—activities whose loss accelerates Huntington disease and Fragile X repeat expansion in mouse and iPSC models [PMID:34469738, PMID:37549289, PMID:41145416, PMID:34718701]. FAN1 protein levels are cell-cycle regulated through APC/C^Cdh1-mediated proteasomal degradation and stabilized by USP7 deubiquitination, while CDK-dependent phosphorylation at S126 modulates its MLH1 interaction [PMID:22854063, PMID:34330701, PMID:41786746]. Loss-of-function mutations in FAN1 cause karyomegalic interstitial nephritis, linked to failed ICL repair and resultant polyploidy in renal tubular cells [PMID:26980188].\",\n  \"teleology\": [\n    {\n      \"year\": 2010,\n      \"claim\": \"Four simultaneous studies established FAN1 as a novel structure-specific nuclease with 5' flap endonuclease and 5'→3' exonuclease activities recruited to ICL damage sites via its UBZ domain binding monoubiquitinated FANCD2, resolving how cells engage a dedicated nuclease for crosslink processing.\",\n      \"evidence\": \"Independent shRNA/siRNA screens, in vitro nuclease assays, Co-IP of UBZ–ubFANCD2 interaction, immunofluorescence foci, domain mutagenesis in human cells and DT40 knockouts\",\n      \"pmids\": [\"20603073\", \"20603015\", \"20603016\", \"20671156\", \"21115814\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of how FAN1 engages ICL substrates was unknown\",\n        \"Whether FAN1 acted solely within or also outside the canonical FA pathway was unresolved\",\n        \"The mechanistic basis for 5' flap specificity was not explained\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Crystal structures of human and Pseudomonas FAN1 revealed the molecular basis for its ~3-nt iterative cleavage mechanism, head-to-tail dimerization on flap DNA, and a 5'-phosphate anchor requirement, explaining how FAN1 excises ICL lesions through flanking incisions.\",\n      \"evidence\": \"X-ray crystallography of human FAN1–DNA and PaFAN1–DNA complexes, active-site mutagenesis, in vitro nuclease assays\",\n      \"pmids\": [\"25430771\", \"25500724\", \"24981866\", \"25319828\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How FAN1 operates on chromatin substrates in vivo was not addressed\",\n        \"The role of FAN1 dimerization in cellular ICL repair was not tested\",\n        \"No structural data on FAN1 with an actual crosslinked substrate existed\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"FAN1 was shown to act at stalled replication forks through a FANCD2-dependent but UBZ-independent mechanism involving the BLM–FANCD2 complex, separating its fork-protective role from its ICL recruitment mechanism.\",\n      \"evidence\": \"Co-IP, chromatin fractionation, DNA fiber analysis, nuclease-dead mutant in human cells with siRNA knockdown of FANCD2, BLM, MRE11\",\n      \"pmids\": [\"25135477\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The molecular basis of UBZ-independent FANCD2 recruitment at forks was unknown\",\n        \"How MRE11 gates FAN1 access to nascent DNA in the absence of FANCD2 was not structurally resolved\"\n      ]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Mouse knockin and knockout models demonstrated that FAN1 nuclease activity restrains replication fork progression and prevents cancer, and that its UBZ–ubFANCD2 interaction is dispensable for ICL repair per se but critical for fork stability, fundamentally reframing FAN1's primary physiological role.\",\n      \"evidence\": \"Fan1 nuclease-defective knockin mice, cancer incidence monitoring, UBZ mutant knockin, DNA fiber analysis, epistasis with FA genes and SNM1A\",\n      \"pmids\": [\"26797144\", \"26980189\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The non-ICL substrates processed by FAN1 at stalled forks were not identified\",\n        \"Whether FAN1's fork-protective function extends to repeat sequences was unexplored\"\n      ]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"FAN1 nuclease-defective mice developed karyomegalic interstitial nephritis with polyploid renal nuclei, establishing the disease mechanism as a failure of ICL repair leading to endoreduplication.\",\n      \"evidence\": \"Fan1 nuclease-dead knockin mouse histology, flow cytometry for ploidy, ICL-induced polyploidy in fibroblasts\",\n      \"pmids\": [\"26980188\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The identity of the endogenous ICL-inducing agent in kidney was not established\",\n        \"Whether other tissues show subclinical polyploidy was not fully assessed\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"FAN1 was identified as a suppressor of somatic CAG repeat expansion in Huntington disease models, with overexpression reducing and knockdown increasing expansion, revealing an unexpected genome stability role at trinucleotide repeats.\",\n      \"evidence\": \"FAN1 overexpression/knockdown in HD patient-derived iPSCs and neurons, repeat-length PCR, nuclease-dead mutant analysis, direct DNA binding assay\",\n      \"pmids\": [\"30358836\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The mechanism by which FAN1 prevented expansion (binding vs. cleavage) was debated—nuclease-independent stabilization was reported\",\n        \"Whether FAN1 acted directly on repeat extrusions or indirectly through MMR was unknown\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Mouse genetic epistasis showed that MLH1 is required for the repeat-destabilizing effect of FAN1 loss, and FAN1 was shown to interact with MLH1 and MSH2 on chromatin after alkylation damage, linking FAN1 to mismatch repair machinery.\",\n      \"evidence\": \"Fan1/Mlh1 double-knockout mice with Htt CAG knock-in, Co-IP of FAN1–MLH1/MSH2 after O6-meG damage, immunofluorescence\",\n      \"pmids\": [\"32876667\", \"31955481\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The molecular interface between FAN1 and MLH1 was not mapped\",\n        \"Whether FAN1–MLH1 interaction directly inhibited MMR or redirected it was unclear\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Mapping of the FAN1 SPYF motif for MLH1 binding, CDK-dependent S126 phosphorylation modulating this interaction, and detailed biochemistry of FAN1 cleavage patterns on slipped CAG/CTG DNA established dual mechanisms—sequestration of MLH1 from pro-expansion MMR plus direct nucleolytic removal of slip-outs.\",\n      \"evidence\": \"SPYF motif and S126 mutagenesis, Co-IP, ICL and CAG/CTG slip-out repair assays, CDK inhibitor treatment, in vitro cleavage pattern mapping on slipped DNA\",\n      \"pmids\": [\"34469738\", \"34330701\", \"34879276\", \"34718701\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How S126 phosphorylation is coordinated with replication timing at repeat loci was not resolved\",\n        \"Whether FAN1 dimerization on slip-outs has the same architecture as on ICL substrates was unknown\",\n        \"In vivo contribution of SPYF-MLH1 versus nuclease activity to repeat protection was not quantitatively separated\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Rare FAN1 coding variants in the nuclease and DNA-binding domains were shown to correlate with reduced nuclease activity in vitro and earlier Huntington disease onset, with CRISPR knockin of nuclease-dead FAN1 phenocopying complete FAN1 loss for CAG expansion, establishing nuclease activity as the critical protective function.\",\n      \"evidence\": \"Exome sequencing, purified FAN1 variant biochemistry, CRISPR knockin of nuclease-inactive FAN1 in iPSCs, CAG expansion quantification\",\n      \"pmids\": [\"35379994\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether partial nuclease activity from hypomorphic alleles yields proportional clinical benefit was not established in vivo\",\n        \"Structural basis for how specific variants impair nuclease function was not determined\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Reconstitution showed RFC–PCNA activate FAN1 nuclease on extrahelical CAG extrusions at physiological ionic strength, conferring strand directionality and enabling a short-patch excision-repair mechanism that competes with MutSβ-dependent MMR, explaining how FAN1 and MMR compete for the same substrate.\",\n      \"evidence\": \"In vitro reconstitution with purified FAN1, RFC, PCNA on extrahelical extrusion substrates; cell extract assay\",\n      \"pmids\": [\"37549289\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of PCNA–FAN1 interaction was not resolved\",\n        \"Whether RFC–PCNA similarly regulate FAN1 at ICL substrates was not tested\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Cryo-EM structures of the PCNA–FAN1–DNA ternary complex revealed R507 as the direct PCNA contact; reconstitution with Polδ showed FAN1 incision followed by polymerase fill-in causes repeat contraction; FAN1 was additionally shown to directly inhibit MutLγ activation by MutSβ, establishing a complete pathway from lesion recognition to contraction.\",\n      \"evidence\": \"Cryo-EM structure determination, R507H mutant biochemistry, full reconstitution with FAN1/RFC/PCNA/MutSβ/MutLγ/Polδ\",\n      \"pmids\": [\"40368897\", \"40368883\", \"41145416\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"In vivo validation of the R507-PCNA interface for repeat protection is lacking\",\n        \"Whether FAN1 inhibition of MutLγ occurs through direct protein–protein contact or substrate competition was not fully distinguished\",\n        \"No in vivo structure of FAN1 operating at an endogenous repeat locus exists\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"USP7 was identified as a deubiquitinase that stabilizes FAN1 protein levels by counteracting proteasomal degradation, with USP7 depletion reducing FAN1 chromatin association and accelerating both ICL sensitivity and CAG repeat expansion.\",\n      \"evidence\": \"Co-IP of USP7–FAN1, western blot protein stability, chromatin fractionation, ICL and CAG expansion assays in RPE-1 cells\",\n      \"pmids\": [\"41786746\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The specific ubiquitin sites on FAN1 targeted by USP7 were not mapped\",\n        \"How APC/C^Cdh1-mediated degradation and USP7 stabilization are coordinated across the cell cycle is unresolved\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the identity of endogenous ICL-generating agents in kidney tissue, the in vivo structural dynamics of FAN1 at endogenous repeat loci, the quantitative partitioning of FAN1's protective effect between nuclease activity and MLH1 sequestration, and the therapeutic potential of modulating FAN1 protein levels or activity to delay trinucleotide repeat expansion diseases.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No endogenous crosslinking agent in kidney has been identified\",\n        \"Relative contribution of SPYF-MLH1 sequestration versus nuclease activity to repeat protection has not been quantitatively determined in vivo\",\n        \"No pharmacological activator of FAN1 has been reported\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140097\", \"supporting_discovery_ids\": [0, 1, 2, 6, 7, 8, 22, 23, 24, 27]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 1, 2, 6, 22, 24]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [16, 18, 22]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 4, 19]},\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [9, 19, 28]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 1, 2, 3, 5, 6, 12, 13, 14]},\n      {\"term_id\": \"R-HSA-69306\", \"supporting_discovery_ids\": [9, 12]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [25]}\n    ],\n    \"complexes\": [\n      \"PCNA-RFC-FAN1\",\n      \"BLM-FANCD2-FAN1\"\n    ],\n    \"partners\": [\n      \"FANCD2\",\n      \"MLH1\",\n      \"PCNA\",\n      \"BLM\",\n      \"USP7\",\n      \"MSH2\",\n      \"RFC\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}