{"gene":"LIG1","run_date":"2026-06-10T02:59:49","timeline":{"discoveries":[{"year":2022,"finding":"Human LIG1 uses two PCNA-interacting motifs (PIPs) — one at its disordered N-terminus (PIPN-term) and one in its DNA binding domain (PIPDBD) — to recruit PCNA to nicked DNA. Cryo-EM structures showed that once LIG1 and PCNA assemble as two-stack rings encircling DNA, PIPN-term is released and only PIPDBD is required for ligation, facilitating substrate handoff from FEN1 via a toolbelt mechanism on an unoccupied PCNA monomer.","method":"Cryo-EM structures combined with functional ligation assays and PCNA-interacting motif mutagenesis","journal":"Nature Communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple cryo-EM structures plus functional assays and mutagenesis in one rigorous study","pmids":["36539424"],"is_preprint":false},{"year":2022,"finding":"X-ray structures of LIG1 bound to nick DNA containing G:T (wobble) and A:C mismatches revealed that LIG1 can accommodate a G:T mismatch and transfer AMP to the 5'-phosphate (DNA-AMP intermediate), while with an A:C mismatch the AMP remains on the LIG1-AMP intermediate. APE1 was shown to interact with LIG1 at the final BER steps and remove mismatched bases as a compensatory proofreading enzyme.","method":"X-ray crystallography of LIG1/nick-DNA complexes with mismatches; in vitro ligation and abortive ligation assays; Co-IP/interaction assay with APE1","journal":"Nature Communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures plus multiple biochemical assays in one study","pmids":["35790757"],"is_preprint":false},{"year":2021,"finding":"High-resolution X-ray structures and pre-steady-state kinetics of LIG1 disease-associated variants R771W and R641L (LIG1 syndrome) revealed a cooperative network of DNA-LIG1 interactions connecting DNA substrate engagement with productive Mg2+ cofactor binding. These mutations destabilize the network, reduce Mg2+ binding affinity, decrease ligation efficiency, and increase abortive ligation.","method":"X-ray crystallography, steady-state and pre-steady-state kinetics, systematic mutagenesis","journal":"Nucleic Acids Research","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal methods (structure + kinetics + mutagenesis) in one rigorous study","pmids":["33444456"],"is_preprint":false},{"year":2019,"finding":"Crystal structure of the UHRF1 tandem Tudor domain (TTD) bound to a LIG1 K126me3 peptide showed that methylated LIG1 K126 binds the TTD with nanomolar affinity (higher than histone H3K9me2/3). This interaction switches UHRF1 from a closed (auto-inhibited) to a flexible open conformation, relieving auto-inhibition and permitting UHRF1 recruitment to chromatin for DNA methylation maintenance.","method":"X-ray crystallography of UHRF1 TTD–LIG1 K126me3 peptide complex; binding affinity measurements; conformational analysis","journal":"Structure","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with functional conformational analysis and affinity measurements in one rigorous study","pmids":["30639225"],"is_preprint":false},{"year":2024,"finding":"X-ray structures of LIG1 bound to 3'-ribonucleotide-containing nick DNA (3'-rA:T and 3'-rG:C) at pre- and post-step-3 ligation stages revealed that Asp570 and Arg871 side chains interact with the 2'-OH of the ribose at the nick, but these interactions do not prevent ligation. LIG1 was shown to ligate nick substrates with pre-inserted 3'-ribonucleotides as efficiently as Watson-Crick base-paired DNA ends in vitro, demonstrating a lack of sugar discrimination at the 3'-end.","method":"X-ray crystallography; in vitro ligation assays","journal":"Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures plus in vitro biochemical assays in one study","pmids":["38522520"],"is_preprint":false},{"year":2024,"finding":"X-ray structures of LIG1 bound to a nick with a 5'-ribonucleotide (5'-rG:C) at the initial ligation step uncovered a large conformational change downstream of the nick with a shift in Arg871 in the adenylation domain. Functionally, ligation of nick DNA with a 5'-ribonucleotide was significantly diminished compared with efficient ligation of a nick with a 3'-ribonucleotide, demonstrating proficient sugar discrimination at the 5'-end during ribonucleotide excision repair.","method":"X-ray crystallography; in vitro ligation assays; active-site mutagenesis","journal":"Journal of Biological Chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures plus functional assays and mutagenesis in one study","pmids":["39159820"],"is_preprint":false},{"year":2024,"finding":"Single-molecule fluorescence (C-Trap and TIRF) measurements showed that full-length LIG1 exhibits 1D diffusion along DNA and becomes enriched at nick sites with longer binding lifetimes, whereas the LIG1 C-terminal mutant (catalytic core + DNA-binding domain) binds non-specifically and for shorter durations throughout DNA. This demonstrated that the N-terminal domain promotes 1D diffusion and efficient nick searching.","method":"Single-molecule fluorescence microscopy (C-Trap optical tweezers + TIRF); comparison of full-length vs. C-terminal LIG1 mutant","journal":"Nucleic Acids Research","confidence":"High","confidence_rationale":"Tier 2 / Strong — two orthogonal single-molecule methods with domain-deletion mutant controls in one study","pmids":["39404052"],"is_preprint":false},{"year":2024,"finding":"Biochemical analyses demonstrated that when polβ fails to fill a one-nucleotide gap repair intermediate, LIG1 ligates the resulting gap DNA, generating single-nucleotide deletion products (aberrant/mutagenic nick sealing). APE1 showed distinct substrate specificity for exonuclease removal of 3'-mismatched bases and ribonucleotides from nick repair intermediates, providing a proofreading role.","method":"In vitro ligation assays with polβ-defective substrates; biochemical substrate-specificity assays for APE1","journal":"Nucleic Acids Research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean in vitro reconstitution with defined substrates, single lab","pmids":["38366780"],"is_preprint":false},{"year":2024,"finding":"X-ray structures of LIG1/nick complexes containing 3'-8oxodG and 3'-8oxorG opposite templating C or A, captured at steps 2 and 3 of the ligation reaction, revealed structural adjustments (differences in distances at +1/+2 nucleotides and template base position shifts depending on 8-oxoG syn/anti conformation) that underlie mutagenic ligation or non-mutagenic end joining. LIG1 wild-type and disease-associated variants seal 3'-8oxodG:A and 3'-8oxorG:A nicks mutagenically in vitro.","method":"X-ray crystallography; in vitro ligation assays with oxidatively damaged substrates; comparison with LIG3α and disease variants","journal":"bioRxiv (preprint)","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — crystal structures plus biochemical assays but preprint, single lab","pmids":["38766188"],"is_preprint":true},{"year":2025,"finding":"X-ray structures of LIG1 with 3'-8oxodG and 3'-8oxorG at pre- and post-catalytic steps demonstrated that 8-oxoG accommodates Hoogsteen or Watson-Crick pairing in syn or anti conformation, leading to mutagenic ligation or non-mutagenic end joining respectively. These structural adjustments explain how LIG1 processes oxidatively damaged nick ends during DNA repair.","method":"X-ray crystallography; in vitro ligation assays","journal":"Nucleic Acids Research","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple crystal structures plus biochemical validation in peer-reviewed publication","pmids":["41370201"],"is_preprint":false},{"year":2025,"finding":"The Huntington's disease-associated LIG1 K845N variant (lysine to asparagine) was shown by in vitro ligase assays and enzyme kinetics to enhance mismatch substrate discrimination and increase ligation fidelity. The mouse orthologue (K843N) suppressed somatic CAG repeat expansion in HD knock-in mice. Cell-based assays showed K845N confers protection against oxidative stress.","method":"In vitro ligase assays; enzyme kinetics; HD knock-in mouse model; cell-based oxidative stress assays","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods (in vitro biochemistry, enzyme kinetics, in vivo mouse model, cell assays) in one study","pmids":["41770933"],"is_preprint":false},{"year":2025,"finding":"Biochemical, X-ray crystallographic, and TIRF single-molecule analyses of LIG1 K845N showed reduced ligation efficiency for nicks with mismatches, 8-oxoG, and 3'-ribonucleotides compared to wild-type, with K845N exhibiting a lack of discrimination against 3'-ribonucleotide-containing nicks. Structural comparison showed similar active-site conformation but altered distances between the K/N845 residue and DNA ends. Single-molecule data revealed less frequent and shorter-lived nick binding by K845N.","method":"X-ray crystallography; in vitro ligation assays; TIRF single-molecule microscopy","journal":"NAR Molecular Medicine","confidence":"High","confidence_rationale":"Tier 1 / Strong — three orthogonal methods (structure, biochemistry, single-molecule) in one peer-reviewed study","pmids":["41346861"],"is_preprint":false},{"year":2025,"finding":"Active site residues Phe635 and Phe872 of LIG1 are required for faithful ligation: F635A/L and F872A/L substitutions abolished ligation of all 12 non-canonical mismatches. Structures of LIG1 F635A and F872A mutants with mismatched and 8-oxoG-containing nick DNA demonstrated that these residues govern DNA end rigidity and alignment at the active site, and their mutation causes a shift in a flexible loop near the 5'-end of the nick that increases the barrier to adenylate transfer.","method":"X-ray crystallography; in vitro ligation assays; active-site mutagenesis; single-molecule TIRF","journal":"bioRxiv (preprint)","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures plus biochemical mutagenesis and single-molecule experiments in one study","pmids":["39574773"],"is_preprint":true},{"year":2024,"finding":"CRISPR/Cas9 screening in prostate cancer cell lines identified that LIG1 loss combined with PARP inhibition induces replication stress, DNA double-strand breaks, and apoptosis (synthetic lethality). This synthetic lethal interaction was confirmed in vivo in xenograft models and was shown to depend on LIG1's role in sealing ssDNA nicks, as catalytically dead LIG1 K568A failed to rescue viability in BRCA1 mutant cells.","method":"CRISPR/Cas9 genome-wide screen; CRISPRn/CRISPRi/RNAi/protein degradation; xenograft in vivo model; catalytic mutant rescue assay","journal":"Journal of Clinical Investigation / Molecular Cancer Therapeutics","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple loss-of-function approaches, in vivo validation, and catalytic-dead mutant rescue across two independent studies","pmids":["39718835","39868490"],"is_preprint":false},{"year":2025,"finding":"Single-molecule TIRF measurements comparing LIG1 and LIG3α showed that LIG3α binds less frequently but forms longer-lived complexes than LIG1 for canonical and mismatch-containing nicks. LIG1 can stably bind to and ligate gap DNA substrates (including one-nucleotide and larger gaps), with a higher percentage of LIG1 molecules forming stable long-lived complexes on gap DNA compared to LIG3α.","method":"TIRF single-molecule fluorescence microscopy; in vitro ligation assays","journal":"bioRxiv (preprint)","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — two orthogonal methods but preprint, single lab","pmids":["40666977"],"is_preprint":true},{"year":2025,"finding":"In the context of Okazaki fragment maturation, LIG1 is recruited to PCNA only after JMJD1B-mediated demethylation of FEN1 R192 causes FEN1 to dissociate from PCNA. This sequential (not simultaneous) binding of Polδ, FEN1, and LIG1 to PCNA was demonstrated in mammalian cells; disruption leads to unprocessed 5' flaps and induction of a PARP1-LIG3-dependent alternative mutagenic pathway.","method":"Cell-based Co-IP/chromatin fractionation; mutant cell lines (FEN1 R192Q, Jmjd1b-/-); replication intermediate analysis","journal":"bioRxiv (preprint)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple genetic models and biochemical approaches in preprint, multiple orthogonal methods","pmids":["bio_10.1101_2025.10.06.680735"],"is_preprint":true},{"year":2018,"finding":"The RNA-binding protein SRSF1 binds to LIG1 mRNA, increases its stability, and enhances its translation in an mTOR-dependent manner in non-small cell lung cancer cells. siRNA-mediated LIG1 knockdown reduced proliferation and increased apoptosis of NSCLC cells.","method":"RIP (RNA immunoprecipitation) for SRSF1-LIG1 mRNA binding; mRNA stability assays; siRNA knockdown with proliferation/apoptosis readout; mTOR inhibitor experiments","journal":"Laboratory Investigation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — RIP assay plus functional siRNA knockdown with multiple readouts, single lab","pmids":["30181552"],"is_preprint":false},{"year":2025,"finding":"LIG1 seals the nick repair product after polβ mutagenic dATP insertion opposite 8-oxoG, whereas LIG3α cannot ligate the dCTP:8-oxoG polβ insertion product. Ribonucleotide insertions by polβ during 8-oxoG bypass completely diminish coordination with both LIG1 and LIG3α. APE1 proofreads 3'-mismatches and ribonucleotides templating 8-oxoG.","method":"In vitro ligation assays with defined polβ insertion products; substrate-specificity analysis comparing LIG1 vs. LIG3α; APE1 exonuclease assays","journal":"Journal of Biological Chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro reconstitution with defined substrates comparing two ligases, single lab","pmids":["40286853"],"is_preprint":false}],"current_model":"Human DNA Ligase I (LIG1) is an ATP-dependent nick-sealing enzyme that finalizes DNA replication (Okazaki fragment joining) and most DNA repair pathways by catalyzing phosphodiester bond formation in three steps; it is recruited to PCNA-bound nicked DNA via two PIP motifs (PIPN-term and PIPDBD), forms a two-ring clamp with PCNA encircling DNA, searches for nicks via 1D diffusion promoted by its N-terminal domain, discriminates against mismatches and 5'-ribonucleotides (but not 3'-ribonucleotides) through a cooperative network of active-site residues including F635, F872, Asp570, and Arg871, is regulated post-translationally by K126 methylation (read by the UHRF1 tandem Tudor domain to recruit UHRF1 for epigenetic maintenance), and is recruited sequentially after FEN1 in Okazaki fragment maturation through JMJD1B-mediated FEN1 demethylation, with disease-associated variants (R641L, R771W, K845N) and synthetic-lethal interactions with PARP inhibitors defining its roles in immunodeficiency, Huntington's disease modification, and cancer therapy."},"narrative":{"mechanistic_narrative":"LIG1 is the principal ATP-dependent DNA ligase that seals nicks to complete DNA replication and base excision repair, catalyzing phosphodiester bond formation through a multi-step adenylation reaction at single-strand breaks [PMID:35790757, PMID:38522520]. It is recruited to nicked DNA via two PCNA-interacting motifs—one in its disordered N-terminus and one in its DNA-binding domain—forming a two-stack ring with PCNA that encircles DNA; once assembled, the N-terminal PIP is released and only the DBD PIP is required for ligation, enabling a toolbelt handoff of substrate from FEN1 on an unoccupied PCNA monomer [PMID:36539424]. Its N-terminal domain promotes one-dimensional diffusion along DNA so it becomes enriched at nicks with long binding lifetimes, supporting efficient nick searching [PMID:39404052]. Fidelity is governed by an active-site network: residues Phe635 and Phe872 enforce DNA-end rigidity and alignment, and their loss abolishes faithful ligation of all mismatched ends [PMID:39574773], while Asp570 and Arg871 sense the 2'-OH of ribose at the nick, conferring proficient sugar discrimination against 5'-ribonucleotides but not 3'-ribonucleotides [PMID:38522520, PMID:39159820]. LIG1 accommodates mismatched and oxidatively damaged termini such as G:T and 8-oxoG, sealing them mutagenically or non-mutagenically depending on base-pairing geometry, with APE1 acting as a compensatory 3'-proofreading exonuclease at the final repair steps [PMID:35790757, PMID:41370201, PMID:40286853]. Beyond catalysis, methylated LIG1 K126 is read by the UHRF1 tandem Tudor domain with nanomolar affinity, relieving UHRF1 auto-inhibition and recruiting it to chromatin for DNA methylation maintenance, linking the ligase to epigenetic inheritance [PMID:30639225]. Disease-associated variants define its physiological roles: R771W and R641L destabilize a DNA–LIG1 cooperative network that couples substrate engagement to Mg2+ binding, reducing ligation efficiency (LIG1 syndrome) [PMID:33444456], and the K845N variant enhances mismatch discrimination, increases fidelity, and suppresses somatic CAG-repeat expansion in a Huntington's disease mouse model [PMID:41770933, PMID:41346861]. LIG1 loss is synthetically lethal with PARP inhibition through accumulation of unsealed nicks, replication stress, and double-strand breaks [PMID:39718835, PMID:39868490].","teleology":[{"year":2018,"claim":"Before this work it was unclear how LIG1 expression is controlled post-transcriptionally; the finding established a translational regulatory input linking LIG1 abundance to proliferation in cancer cells.","evidence":"RIP, mRNA stability assays, and siRNA knockdown with proliferation/apoptosis readouts in NSCLC cells","pmids":["30181552"],"confidence":"Medium","gaps":["Single-lab study confined to NSCLC","Does not address whether SRSF1 regulation operates in normal tissues or other cancers"]},{"year":2019,"claim":"It was unknown how LIG1 could connect replication/repair to epigenetic maintenance; structural work showed methylated LIG1 K126 is a high-affinity ligand for the UHRF1 tandem Tudor domain that switches UHRF1 to an open, active conformation.","evidence":"X-ray crystallography of the UHRF1 TTD–LIG1 K126me3 peptide complex with affinity and conformational analysis","pmids":["30639225"],"confidence":"High","gaps":["The writer methyltransferase for LIG1 K126 is not identified here","Cellular consequences for global methylation maintenance not directly quantified"]},{"year":2021,"claim":"The molecular basis of LIG1 syndrome was unresolved; structures and kinetics of R771W and R641L revealed a cooperative DNA–LIG1 interaction network coupling substrate engagement to productive Mg2+ binding that these mutations disrupt.","evidence":"X-ray crystallography plus steady-state and pre-steady-state kinetics with systematic mutagenesis","pmids":["33444456"],"confidence":"High","gaps":["Does not establish the in vivo immunodeficiency mechanism at the cellular level","Patient-derived cell phenotypes not assayed"]},{"year":2022,"claim":"How LIG1 engages PCNA and receives substrate from upstream factors was undefined; cryo-EM showed a two-PIP, two-ring toolbelt mechanism enabling FEN1-to-LIG1 handoff on PCNA.","evidence":"Cryo-EM structures with functional ligation assays and PIP-motif mutagenesis","pmids":["36539424"],"confidence":"High","gaps":["Dynamics of the handoff in cells not directly visualized","Stoichiometry on stalled or damaged forks not addressed"]},{"year":2022,"claim":"Whether LIG1 discriminates mismatched ends and how errors are corrected was unclear; structures of mismatch-containing nicks plus APE1 interaction data showed LIG1 can mis-seal certain mismatches while APE1 proofreads them.","evidence":"X-ray crystallography of LIG1/mismatch-nick complexes, ligation/abortive-ligation assays, and Co-IP with APE1","pmids":["35790757"],"confidence":"High","gaps":["APE1 recruitment mechanism at the final BER step not defined","In vivo contribution of APE1 proofreading not quantified"]},{"year":2024,"claim":"It was unknown whether LIG1 discriminates ribonucleotides at nick ends; paired structural studies showed Asp570/Arg871 sense the 2'-OH but tolerate 3'-ribonucleotides while strongly rejecting 5'-ribonucleotides.","evidence":"X-ray crystallography of LIG1 with 3'- and 5'-ribonucleotide nicks plus in vitro ligation assays and active-site mutagenesis","pmids":["38522520","39159820"],"confidence":"High","gaps":["Physiological consequences of 3'-ribonucleotide tolerance during RER not measured in cells","Coupling to upstream ribonucleotide excision factors not addressed"]},{"year":2024,"claim":"How LIG1 locates nicks among bulk DNA was unresolved; single-molecule imaging showed the N-terminal domain enables 1D diffusion and nick enrichment with long dwell times.","evidence":"C-Trap optical tweezers and TIRF single-molecule microscopy comparing full-length vs C-terminal LIG1","pmids":["39404052"],"confidence":"High","gaps":["Behavior in the context of PCNA/chromatin not measured","Quantitative search kinetics in cells unknown"]},{"year":2024,"claim":"Whether LIG1 can mis-seal incomplete repair intermediates was unclear; biochemistry showed LIG1 ligates pol-beta-defective gap intermediates to generate single-nucleotide deletions, with APE1 proofreading 3'-errors.","evidence":"In vitro ligation assays with pol-beta-defective substrates and APE1 substrate-specificity assays","pmids":["38366780"],"confidence":"Medium","gaps":["Single-lab in vitro reconstitution","In vivo frequency of such mutagenic events not established"]},{"year":2024,"claim":"Targeting LIG1 therapeutically required a synthetic-lethal rationale; CRISPR screens and xenograft validation showed LIG1 loss with PARP inhibition causes replication stress and apoptosis dependent on its catalytic nick-sealing activity.","evidence":"Genome-wide CRISPR screen, multiple loss-of-function approaches, xenograft models, and catalytic-dead K568A rescue","pmids":["39718835","39868490"],"confidence":"High","gaps":["Tumor-genotype determinants of sensitivity beyond BRCA1 not fully mapped","Resistance mechanisms not characterized"]},{"year":2025,"claim":"How LIG1 processes oxidatively damaged 8-oxoG ends was unresolved; pre- and post-catalytic structures explained mutagenic vs non-mutagenic sealing via syn/anti Hoogsteen or Watson-Crick pairing.","evidence":"X-ray crystallography of LIG1 with 3'-8oxodG/8oxorG nicks plus in vitro ligation assays","pmids":["41370201"],"confidence":"High","gaps":["Cellular mutagenic burden from this ligation not quantified","Interplay with downstream mismatch repair not addressed"]},{"year":2025,"claim":"The mechanistic consequences of the HD-modifier K845N variant were unknown; combined biochemistry, structure, kinetics and a mouse model showed it raises fidelity, reduces ligation of damaged/mismatched/ribonucleotide nicks, and suppresses somatic CAG expansion.","evidence":"In vitro ligase assays, enzyme kinetics, X-ray crystallography, TIRF single-molecule, HD knock-in mouse, and cell-based oxidative stress assays","pmids":["41770933","41346861"],"confidence":"High","gaps":["Mechanistic link between ligation fidelity and repeat-instability suppression not fully resolved","Human relevance of the mouse phenotype not directly tested"]},{"year":2025,"claim":"Active-site determinants of fidelity were undefined; structural and biochemical analysis identified Phe635 and Phe872 as residues enforcing end rigidity required for faithful ligation of mismatched ends.","evidence":"X-ray crystallography of F635A/F872A mutants, in vitro ligation assays, mutagenesis, and single-molecule TIRF (preprint)","pmids":["39574773"],"confidence":"High","gaps":["Preprint, single lab","In vivo fidelity contribution of these residues not tested"]},{"year":2025,"claim":"The temporal order of Okazaki fragment maturation factors on PCNA was unclear; cell-based work showed JMJD1B demethylates FEN1 R192 to release FEN1, allowing sequential LIG1 recruitment, with disruption activating a mutagenic PARP1-LIG3 pathway.","evidence":"Cell-based Co-IP/chromatin fractionation with FEN1 R192Q and Jmjd1b-/- mutants and replication intermediate analysis (preprint)","pmids":["bio_10.1101_2025.10.06.680735"],"confidence":"Medium","gaps":["Preprint, single lab","Direct demonstration of LIG1 timing independent of FEN1 release not isolated"]},{"year":null,"claim":"How LIG1 nick-sealing fidelity, PCNA toolbelt dynamics, and K126 methylation are coordinately regulated in living cells across replication and the multiple repair pathways remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No in vivo measurement integrating search, handoff and catalysis","Methyltransferase writing LIG1 K126 not identified in the corpus","Pathway-specific regulation of fidelity in cells not established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[0,1,4,5,9]},{"term_id":"GO:0140097","term_label":"catalytic activity, acting on DNA","supporting_discovery_ids":[1,4,12]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[6,12]},{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[3,15]},{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[3]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[1,4,9,17]},{"term_id":"R-HSA-69306","term_label":"DNA Replication","supporting_discovery_ids":[0,15]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[3]}],"complexes":["LIG1-PCNA two-ring clamp"],"partners":["PCNA","FEN1","APE1","UHRF1","SRSF1","POLB"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q96JA1","full_name":"Leucine-rich repeats and immunoglobulin-like domains protein 1","aliases":[],"length_aa":1093,"mass_kda":119.1,"function":"Acts as a feedback negative regulator of signaling by receptor tyrosine kinases, through a mechanism that involves enhancement of receptor ubiquitination and accelerated intracellular degradation","subcellular_location":"Cell membrane","url":"https://www.uniprot.org/uniprotkb/Q96JA1/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/LIG1","classification":"Not Classified","n_dependent_lines":269,"n_total_lines":1208,"dependency_fraction":0.222682119205298},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000105486","cell_line_id":"CID000810","localizations":[{"compartment":"nuclear_punctae","grade":3},{"compartment":"nucleoplasm","grade":3}],"interactors":[{"gene":"UBE3B","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID000810","total_profiled":1310},"omim":[{"mim_id":"619774","title":"IMMUNODEFICIENCY 96; 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Cryo-EM structures showed that once LIG1 and PCNA assemble as two-stack rings encircling DNA, PIPN-term is released and only PIPDBD is required for ligation, facilitating substrate handoff from FEN1 via a toolbelt mechanism on an unoccupied PCNA monomer.\",\n      \"method\": \"Cryo-EM structures combined with functional ligation assays and PCNA-interacting motif mutagenesis\",\n      \"journal\": \"Nature Communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple cryo-EM structures plus functional assays and mutagenesis in one rigorous study\",\n      \"pmids\": [\"36539424\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"X-ray structures of LIG1 bound to nick DNA containing G:T (wobble) and A:C mismatches revealed that LIG1 can accommodate a G:T mismatch and transfer AMP to the 5'-phosphate (DNA-AMP intermediate), while with an A:C mismatch the AMP remains on the LIG1-AMP intermediate. APE1 was shown to interact with LIG1 at the final BER steps and remove mismatched bases as a compensatory proofreading enzyme.\",\n      \"method\": \"X-ray crystallography of LIG1/nick-DNA complexes with mismatches; in vitro ligation and abortive ligation assays; Co-IP/interaction assay with APE1\",\n      \"journal\": \"Nature Communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures plus multiple biochemical assays in one study\",\n      \"pmids\": [\"35790757\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"High-resolution X-ray structures and pre-steady-state kinetics of LIG1 disease-associated variants R771W and R641L (LIG1 syndrome) revealed a cooperative network of DNA-LIG1 interactions connecting DNA substrate engagement with productive Mg2+ cofactor binding. These mutations destabilize the network, reduce Mg2+ binding affinity, decrease ligation efficiency, and increase abortive ligation.\",\n      \"method\": \"X-ray crystallography, steady-state and pre-steady-state kinetics, systematic mutagenesis\",\n      \"journal\": \"Nucleic Acids Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal methods (structure + kinetics + mutagenesis) in one rigorous study\",\n      \"pmids\": [\"33444456\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Crystal structure of the UHRF1 tandem Tudor domain (TTD) bound to a LIG1 K126me3 peptide showed that methylated LIG1 K126 binds the TTD with nanomolar affinity (higher than histone H3K9me2/3). This interaction switches UHRF1 from a closed (auto-inhibited) to a flexible open conformation, relieving auto-inhibition and permitting UHRF1 recruitment to chromatin for DNA methylation maintenance.\",\n      \"method\": \"X-ray crystallography of UHRF1 TTD–LIG1 K126me3 peptide complex; binding affinity measurements; conformational analysis\",\n      \"journal\": \"Structure\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with functional conformational analysis and affinity measurements in one rigorous study\",\n      \"pmids\": [\"30639225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"X-ray structures of LIG1 bound to 3'-ribonucleotide-containing nick DNA (3'-rA:T and 3'-rG:C) at pre- and post-step-3 ligation stages revealed that Asp570 and Arg871 side chains interact with the 2'-OH of the ribose at the nick, but these interactions do not prevent ligation. LIG1 was shown to ligate nick substrates with pre-inserted 3'-ribonucleotides as efficiently as Watson-Crick base-paired DNA ends in vitro, demonstrating a lack of sugar discrimination at the 3'-end.\",\n      \"method\": \"X-ray crystallography; in vitro ligation assays\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures plus in vitro biochemical assays in one study\",\n      \"pmids\": [\"38522520\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"X-ray structures of LIG1 bound to a nick with a 5'-ribonucleotide (5'-rG:C) at the initial ligation step uncovered a large conformational change downstream of the nick with a shift in Arg871 in the adenylation domain. Functionally, ligation of nick DNA with a 5'-ribonucleotide was significantly diminished compared with efficient ligation of a nick with a 3'-ribonucleotide, demonstrating proficient sugar discrimination at the 5'-end during ribonucleotide excision repair.\",\n      \"method\": \"X-ray crystallography; in vitro ligation assays; active-site mutagenesis\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures plus functional assays and mutagenesis in one study\",\n      \"pmids\": [\"39159820\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Single-molecule fluorescence (C-Trap and TIRF) measurements showed that full-length LIG1 exhibits 1D diffusion along DNA and becomes enriched at nick sites with longer binding lifetimes, whereas the LIG1 C-terminal mutant (catalytic core + DNA-binding domain) binds non-specifically and for shorter durations throughout DNA. This demonstrated that the N-terminal domain promotes 1D diffusion and efficient nick searching.\",\n      \"method\": \"Single-molecule fluorescence microscopy (C-Trap optical tweezers + TIRF); comparison of full-length vs. C-terminal LIG1 mutant\",\n      \"journal\": \"Nucleic Acids Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two orthogonal single-molecule methods with domain-deletion mutant controls in one study\",\n      \"pmids\": [\"39404052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Biochemical analyses demonstrated that when polβ fails to fill a one-nucleotide gap repair intermediate, LIG1 ligates the resulting gap DNA, generating single-nucleotide deletion products (aberrant/mutagenic nick sealing). APE1 showed distinct substrate specificity for exonuclease removal of 3'-mismatched bases and ribonucleotides from nick repair intermediates, providing a proofreading role.\",\n      \"method\": \"In vitro ligation assays with polβ-defective substrates; biochemical substrate-specificity assays for APE1\",\n      \"journal\": \"Nucleic Acids Research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean in vitro reconstitution with defined substrates, single lab\",\n      \"pmids\": [\"38366780\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"X-ray structures of LIG1/nick complexes containing 3'-8oxodG and 3'-8oxorG opposite templating C or A, captured at steps 2 and 3 of the ligation reaction, revealed structural adjustments (differences in distances at +1/+2 nucleotides and template base position shifts depending on 8-oxoG syn/anti conformation) that underlie mutagenic ligation or non-mutagenic end joining. LIG1 wild-type and disease-associated variants seal 3'-8oxodG:A and 3'-8oxorG:A nicks mutagenically in vitro.\",\n      \"method\": \"X-ray crystallography; in vitro ligation assays with oxidatively damaged substrates; comparison with LIG3α and disease variants\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — crystal structures plus biochemical assays but preprint, single lab\",\n      \"pmids\": [\"38766188\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"X-ray structures of LIG1 with 3'-8oxodG and 3'-8oxorG at pre- and post-catalytic steps demonstrated that 8-oxoG accommodates Hoogsteen or Watson-Crick pairing in syn or anti conformation, leading to mutagenic ligation or non-mutagenic end joining respectively. These structural adjustments explain how LIG1 processes oxidatively damaged nick ends during DNA repair.\",\n      \"method\": \"X-ray crystallography; in vitro ligation assays\",\n      \"journal\": \"Nucleic Acids Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple crystal structures plus biochemical validation in peer-reviewed publication\",\n      \"pmids\": [\"41370201\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The Huntington's disease-associated LIG1 K845N variant (lysine to asparagine) was shown by in vitro ligase assays and enzyme kinetics to enhance mismatch substrate discrimination and increase ligation fidelity. The mouse orthologue (K843N) suppressed somatic CAG repeat expansion in HD knock-in mice. Cell-based assays showed K845N confers protection against oxidative stress.\",\n      \"method\": \"In vitro ligase assays; enzyme kinetics; HD knock-in mouse model; cell-based oxidative stress assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods (in vitro biochemistry, enzyme kinetics, in vivo mouse model, cell assays) in one study\",\n      \"pmids\": [\"41770933\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Biochemical, X-ray crystallographic, and TIRF single-molecule analyses of LIG1 K845N showed reduced ligation efficiency for nicks with mismatches, 8-oxoG, and 3'-ribonucleotides compared to wild-type, with K845N exhibiting a lack of discrimination against 3'-ribonucleotide-containing nicks. Structural comparison showed similar active-site conformation but altered distances between the K/N845 residue and DNA ends. Single-molecule data revealed less frequent and shorter-lived nick binding by K845N.\",\n      \"method\": \"X-ray crystallography; in vitro ligation assays; TIRF single-molecule microscopy\",\n      \"journal\": \"NAR Molecular Medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — three orthogonal methods (structure, biochemistry, single-molecule) in one peer-reviewed study\",\n      \"pmids\": [\"41346861\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Active site residues Phe635 and Phe872 of LIG1 are required for faithful ligation: F635A/L and F872A/L substitutions abolished ligation of all 12 non-canonical mismatches. Structures of LIG1 F635A and F872A mutants with mismatched and 8-oxoG-containing nick DNA demonstrated that these residues govern DNA end rigidity and alignment at the active site, and their mutation causes a shift in a flexible loop near the 5'-end of the nick that increases the barrier to adenylate transfer.\",\n      \"method\": \"X-ray crystallography; in vitro ligation assays; active-site mutagenesis; single-molecule TIRF\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures plus biochemical mutagenesis and single-molecule experiments in one study\",\n      \"pmids\": [\"39574773\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"CRISPR/Cas9 screening in prostate cancer cell lines identified that LIG1 loss combined with PARP inhibition induces replication stress, DNA double-strand breaks, and apoptosis (synthetic lethality). This synthetic lethal interaction was confirmed in vivo in xenograft models and was shown to depend on LIG1's role in sealing ssDNA nicks, as catalytically dead LIG1 K568A failed to rescue viability in BRCA1 mutant cells.\",\n      \"method\": \"CRISPR/Cas9 genome-wide screen; CRISPRn/CRISPRi/RNAi/protein degradation; xenograft in vivo model; catalytic mutant rescue assay\",\n      \"journal\": \"Journal of Clinical Investigation / Molecular Cancer Therapeutics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple loss-of-function approaches, in vivo validation, and catalytic-dead mutant rescue across two independent studies\",\n      \"pmids\": [\"39718835\", \"39868490\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Single-molecule TIRF measurements comparing LIG1 and LIG3α showed that LIG3α binds less frequently but forms longer-lived complexes than LIG1 for canonical and mismatch-containing nicks. LIG1 can stably bind to and ligate gap DNA substrates (including one-nucleotide and larger gaps), with a higher percentage of LIG1 molecules forming stable long-lived complexes on gap DNA compared to LIG3α.\",\n      \"method\": \"TIRF single-molecule fluorescence microscopy; in vitro ligation assays\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — two orthogonal methods but preprint, single lab\",\n      \"pmids\": [\"40666977\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In the context of Okazaki fragment maturation, LIG1 is recruited to PCNA only after JMJD1B-mediated demethylation of FEN1 R192 causes FEN1 to dissociate from PCNA. This sequential (not simultaneous) binding of Polδ, FEN1, and LIG1 to PCNA was demonstrated in mammalian cells; disruption leads to unprocessed 5' flaps and induction of a PARP1-LIG3-dependent alternative mutagenic pathway.\",\n      \"method\": \"Cell-based Co-IP/chromatin fractionation; mutant cell lines (FEN1 R192Q, Jmjd1b-/-); replication intermediate analysis\",\n      \"journal\": \"bioRxiv (preprint)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple genetic models and biochemical approaches in preprint, multiple orthogonal methods\",\n      \"pmids\": [\"bio_10.1101_2025.10.06.680735\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The RNA-binding protein SRSF1 binds to LIG1 mRNA, increases its stability, and enhances its translation in an mTOR-dependent manner in non-small cell lung cancer cells. siRNA-mediated LIG1 knockdown reduced proliferation and increased apoptosis of NSCLC cells.\",\n      \"method\": \"RIP (RNA immunoprecipitation) for SRSF1-LIG1 mRNA binding; mRNA stability assays; siRNA knockdown with proliferation/apoptosis readout; mTOR inhibitor experiments\",\n      \"journal\": \"Laboratory Investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — RIP assay plus functional siRNA knockdown with multiple readouts, single lab\",\n      \"pmids\": [\"30181552\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LIG1 seals the nick repair product after polβ mutagenic dATP insertion opposite 8-oxoG, whereas LIG3α cannot ligate the dCTP:8-oxoG polβ insertion product. Ribonucleotide insertions by polβ during 8-oxoG bypass completely diminish coordination with both LIG1 and LIG3α. APE1 proofreads 3'-mismatches and ribonucleotides templating 8-oxoG.\",\n      \"method\": \"In vitro ligation assays with defined polβ insertion products; substrate-specificity analysis comparing LIG1 vs. LIG3α; APE1 exonuclease assays\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro reconstitution with defined substrates comparing two ligases, single lab\",\n      \"pmids\": [\"40286853\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Human DNA Ligase I (LIG1) is an ATP-dependent nick-sealing enzyme that finalizes DNA replication (Okazaki fragment joining) and most DNA repair pathways by catalyzing phosphodiester bond formation in three steps; it is recruited to PCNA-bound nicked DNA via two PIP motifs (PIPN-term and PIPDBD), forms a two-ring clamp with PCNA encircling DNA, searches for nicks via 1D diffusion promoted by its N-terminal domain, discriminates against mismatches and 5'-ribonucleotides (but not 3'-ribonucleotides) through a cooperative network of active-site residues including F635, F872, Asp570, and Arg871, is regulated post-translationally by K126 methylation (read by the UHRF1 tandem Tudor domain to recruit UHRF1 for epigenetic maintenance), and is recruited sequentially after FEN1 in Okazaki fragment maturation through JMJD1B-mediated FEN1 demethylation, with disease-associated variants (R641L, R771W, K845N) and synthetic-lethal interactions with PARP inhibitors defining its roles in immunodeficiency, Huntington's disease modification, and cancer therapy.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"LIG1 is the principal ATP-dependent DNA ligase that seals nicks to complete DNA replication and base excision repair, catalyzing phosphodiester bond formation through a multi-step adenylation reaction at single-strand breaks [#1, #4]. It is recruited to nicked DNA via two PCNA-interacting motifs—one in its disordered N-terminus and one in its DNA-binding domain—forming a two-stack ring with PCNA that encircles DNA; once assembled, the N-terminal PIP is released and only the DBD PIP is required for ligation, enabling a toolbelt handoff of substrate from FEN1 on an unoccupied PCNA monomer [#0]. Its N-terminal domain promotes one-dimensional diffusion along DNA so it becomes enriched at nicks with long binding lifetimes, supporting efficient nick searching [#6]. Fidelity is governed by an active-site network: residues Phe635 and Phe872 enforce DNA-end rigidity and alignment, and their loss abolishes faithful ligation of all mismatched ends [#12], while Asp570 and Arg871 sense the 2'-OH of ribose at the nick, conferring proficient sugar discrimination against 5'-ribonucleotides but not 3'-ribonucleotides [#4, #5]. LIG1 accommodates mismatched and oxidatively damaged termini such as G:T and 8-oxoG, sealing them mutagenically or non-mutagenically depending on base-pairing geometry, with APE1 acting as a compensatory 3'-proofreading exonuclease at the final repair steps [#1, #9, #17]. Beyond catalysis, methylated LIG1 K126 is read by the UHRF1 tandem Tudor domain with nanomolar affinity, relieving UHRF1 auto-inhibition and recruiting it to chromatin for DNA methylation maintenance, linking the ligase to epigenetic inheritance [#3]. Disease-associated variants define its physiological roles: R771W and R641L destabilize a DNA–LIG1 cooperative network that couples substrate engagement to Mg2+ binding, reducing ligation efficiency (LIG1 syndrome) [#2], and the K845N variant enhances mismatch discrimination, increases fidelity, and suppresses somatic CAG-repeat expansion in a Huntington's disease mouse model [#10, #11]. LIG1 loss is synthetically lethal with PARP inhibition through accumulation of unsealed nicks, replication stress, and double-strand breaks [#13].\",\n  \"teleology\": [\n    {\n      \"year\": 2018,\n      \"claim\": \"Before this work it was unclear how LIG1 expression is controlled post-transcriptionally; the finding established a translational regulatory input linking LIG1 abundance to proliferation in cancer cells.\",\n      \"evidence\": \"RIP, mRNA stability assays, and siRNA knockdown with proliferation/apoptosis readouts in NSCLC cells\",\n      \"pmids\": [\"30181552\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Single-lab study confined to NSCLC\", \"Does not address whether SRSF1 regulation operates in normal tissues or other cancers\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"It was unknown how LIG1 could connect replication/repair to epigenetic maintenance; structural work showed methylated LIG1 K126 is a high-affinity ligand for the UHRF1 tandem Tudor domain that switches UHRF1 to an open, active conformation.\",\n      \"evidence\": \"X-ray crystallography of the UHRF1 TTD–LIG1 K126me3 peptide complex with affinity and conformational analysis\",\n      \"pmids\": [\"30639225\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"The writer methyltransferase for LIG1 K126 is not identified here\", \"Cellular consequences for global methylation maintenance not directly quantified\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"The molecular basis of LIG1 syndrome was unresolved; structures and kinetics of R771W and R641L revealed a cooperative DNA–LIG1 interaction network coupling substrate engagement to productive Mg2+ binding that these mutations disrupt.\",\n      \"evidence\": \"X-ray crystallography plus steady-state and pre-steady-state kinetics with systematic mutagenesis\",\n      \"pmids\": [\"33444456\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Does not establish the in vivo immunodeficiency mechanism at the cellular level\", \"Patient-derived cell phenotypes not assayed\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"How LIG1 engages PCNA and receives substrate from upstream factors was undefined; cryo-EM showed a two-PIP, two-ring toolbelt mechanism enabling FEN1-to-LIG1 handoff on PCNA.\",\n      \"evidence\": \"Cryo-EM structures with functional ligation assays and PIP-motif mutagenesis\",\n      \"pmids\": [\"36539424\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Dynamics of the handoff in cells not directly visualized\", \"Stoichiometry on stalled or damaged forks not addressed\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Whether LIG1 discriminates mismatched ends and how errors are corrected was unclear; structures of mismatch-containing nicks plus APE1 interaction data showed LIG1 can mis-seal certain mismatches while APE1 proofreads them.\",\n      \"evidence\": \"X-ray crystallography of LIG1/mismatch-nick complexes, ligation/abortive-ligation assays, and Co-IP with APE1\",\n      \"pmids\": [\"35790757\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"APE1 recruitment mechanism at the final BER step not defined\", \"In vivo contribution of APE1 proofreading not quantified\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"It was unknown whether LIG1 discriminates ribonucleotides at nick ends; paired structural studies showed Asp570/Arg871 sense the 2'-OH but tolerate 3'-ribonucleotides while strongly rejecting 5'-ribonucleotides.\",\n      \"evidence\": \"X-ray crystallography of LIG1 with 3'- and 5'-ribonucleotide nicks plus in vitro ligation assays and active-site mutagenesis\",\n      \"pmids\": [\"38522520\", \"39159820\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Physiological consequences of 3'-ribonucleotide tolerance during RER not measured in cells\", \"Coupling to upstream ribonucleotide excision factors not addressed\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"How LIG1 locates nicks among bulk DNA was unresolved; single-molecule imaging showed the N-terminal domain enables 1D diffusion and nick enrichment with long dwell times.\",\n      \"evidence\": \"C-Trap optical tweezers and TIRF single-molecule microscopy comparing full-length vs C-terminal LIG1\",\n      \"pmids\": [\"39404052\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Behavior in the context of PCNA/chromatin not measured\", \"Quantitative search kinetics in cells unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Whether LIG1 can mis-seal incomplete repair intermediates was unclear; biochemistry showed LIG1 ligates pol-beta-defective gap intermediates to generate single-nucleotide deletions, with APE1 proofreading 3'-errors.\",\n      \"evidence\": \"In vitro ligation assays with pol-beta-defective substrates and APE1 substrate-specificity assays\",\n      \"pmids\": [\"38366780\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Single-lab in vitro reconstitution\", \"In vivo frequency of such mutagenic events not established\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Targeting LIG1 therapeutically required a synthetic-lethal rationale; CRISPR screens and xenograft validation showed LIG1 loss with PARP inhibition causes replication stress and apoptosis dependent on its catalytic nick-sealing activity.\",\n      \"evidence\": \"Genome-wide CRISPR screen, multiple loss-of-function approaches, xenograft models, and catalytic-dead K568A rescue\",\n      \"pmids\": [\"39718835\", \"39868490\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Tumor-genotype determinants of sensitivity beyond BRCA1 not fully mapped\", \"Resistance mechanisms not characterized\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"How LIG1 processes oxidatively damaged 8-oxoG ends was unresolved; pre- and post-catalytic structures explained mutagenic vs non-mutagenic sealing via syn/anti Hoogsteen or Watson-Crick pairing.\",\n      \"evidence\": \"X-ray crystallography of LIG1 with 3'-8oxodG/8oxorG nicks plus in vitro ligation assays\",\n      \"pmids\": [\"41370201\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Cellular mutagenic burden from this ligation not quantified\", \"Interplay with downstream mismatch repair not addressed\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"The mechanistic consequences of the HD-modifier K845N variant were unknown; combined biochemistry, structure, kinetics and a mouse model showed it raises fidelity, reduces ligation of damaged/mismatched/ribonucleotide nicks, and suppresses somatic CAG expansion.\",\n      \"evidence\": \"In vitro ligase assays, enzyme kinetics, X-ray crystallography, TIRF single-molecule, HD knock-in mouse, and cell-based oxidative stress assays\",\n      \"pmids\": [\"41770933\", \"41346861\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Mechanistic link between ligation fidelity and repeat-instability suppression not fully resolved\", \"Human relevance of the mouse phenotype not directly tested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Active-site determinants of fidelity were undefined; structural and biochemical analysis identified Phe635 and Phe872 as residues enforcing end rigidity required for faithful ligation of mismatched ends.\",\n      \"evidence\": \"X-ray crystallography of F635A/F872A mutants, in vitro ligation assays, mutagenesis, and single-molecule TIRF (preprint)\",\n      \"pmids\": [\"39574773\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Preprint, single lab\", \"In vivo fidelity contribution of these residues not tested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"The temporal order of Okazaki fragment maturation factors on PCNA was unclear; cell-based work showed JMJD1B demethylates FEN1 R192 to release FEN1, allowing sequential LIG1 recruitment, with disruption activating a mutagenic PARP1-LIG3 pathway.\",\n      \"evidence\": \"Cell-based Co-IP/chromatin fractionation with FEN1 R192Q and Jmjd1b-/- mutants and replication intermediate analysis (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.10.06.680735\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Preprint, single lab\", \"Direct demonstration of LIG1 timing independent of FEN1 release not isolated\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How LIG1 nick-sealing fidelity, PCNA toolbelt dynamics, and K126 methylation are coordinately regulated in living cells across replication and the multiple repair pathways remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No in vivo measurement integrating search, handoff and catalysis\", \"Methyltransferase writing LIG1 K126 not identified in the corpus\", \"Pathway-specific regulation of fidelity in cells not established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [0, 1, 4, 5, 9]},\n      {\"term_id\": \"GO:0140097\", \"supporting_discovery_ids\": [1, 4, 12]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [6, 12]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [3, 15]},\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [1, 4, 9, 17]},\n      {\"term_id\": \"R-HSA-69306\", \"supporting_discovery_ids\": [0, 15]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [3]}\n    ],\n    \"complexes\": [\n      \"LIG1-PCNA two-ring clamp\"\n    ],\n    \"partners\": [\n      \"PCNA\",\n      \"FEN1\",\n      \"APE1\",\n      \"UHRF1\",\n      \"SRSF1\",\n      \"POLB\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}