{"gene":"SHLD1","run_date":"2026-06-10T07:46:31","timeline":{"discoveries":[{"year":2018,"finding":"SHLD1 (C20orf196) is a core subunit of the shieldin complex (SHLD1-SHLD2-SHLD3-REV7), which localizes to DNA double-strand break sites in a 53BP1- and RIF1-dependent manner to protect DNA ends and promote non-homologous end joining (NHEJ). Loss of SHLD1 impairs NHEJ, leads to defective immunoglobulin class switching, causes hyper-resection, and restores homologous recombination in BRCA1-deficient cells, conferring PARP inhibitor resistance.","method":"CRISPR screens, Co-IP, epistasis analysis, cellular DNA repair assays, immunofluorescence localization","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — independently replicated across multiple labs (PMID:30022168, 30022158, 30022119, 30046110) using reciprocal co-IP, epistasis, loss-of-function phenotypes, and localization experiments","pmids":["30022168","30022158","30022119","30046110"],"is_preprint":false},{"year":2018,"finding":"SHLD1 (C20orf196) forms a complex with FAM35A (SHLD2) and REV7, and this complex is recruited to DSBs downstream of RIF1 in vivo. The complex prevents DNA end resection in BRCA1-mutant cells. FAM35A preferentially binds single-strand DNA (ssDNA) in vitro, consistent with a direct DNA-end protection mechanism.","method":"Mass spectrometry-based interactome, Co-IP, in vitro ssDNA binding assay, epistasis analysis, live-cell localization","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — replicated across multiple independent studies (PMID:30254264, 30154076, 30022168) with orthogonal methods including in vitro binding, Co-IP, and epistasis","pmids":["30254264","30154076"],"is_preprint":false},{"year":2018,"finding":"Shieldin (including SHLD1) acts downstream of 53BP1-RIF1 and recruits the CST complex, which in turn recruits Polymerase-α (Polα)-primase to DSB sites to perform fill-in synthesis, thereby limiting single-stranded DNA at DSBs and counteracting resection.","method":"Co-IP of CST with shieldin, immunofluorescence co-localization of Polα at DSBs, depletion epistasis, BrdU incorporation at DSBs","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — replicated in two independent studies (PMID:30022158, 35027730) using Co-IP, localization, epistasis, and functional rescue experiments","pmids":["30022158","35027730"],"is_preprint":false},{"year":2020,"finding":"Crystal structure of the SHLD3-REV7-SHLD2 ternary complex revealed that shieldin assembly requires an unexpected conformational dimer of REV7 in closed (C-REV7) and open (O-REV7) states mediated by SHLD3, with SHLD2 interacting with O-REV7. The conserved FXPWFP motif of SHLD3 binds to C-REV7 and blocks REV1 binding, thereby excluding shieldin from the REV1/Pol-ζ translesion synthesis complex. Disruption of the REV7 conformational dimer abolishes shieldin assembly and impairs NHEJ.","method":"Crystal structure determination, mutagenesis, NHEJ efficiency assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure with functional mutagenesis, single lab but multiple orthogonal methods (PMID:32332881), replicated structurally (PMID:33597306)","pmids":["32332881","33597306"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structures of the SHLD2-SHLD3-REV7-TRIP13 complex revealed that the N-terminus of REV7 inserts into the central channel of the TRIP13 hexamer, and ATP hydrolysis-triggered rotatory motions of TRIP13 pull the C-REV7 safety-belt segment through the channel, causing conformational disassembly of the shieldin complex. This TRIP13-mediated remodeling of shieldin promotes homology-directed repair at the expense of NHEJ.","method":"Cryo-EM structure, crystal structure of SHLD3-REV7 binary complex, in vitro disassembly assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM and crystal structures with functional validation in a single rigorous study (PMID:33597306), consistent with independent biochemical findings (PMID:31915374, 34521823)","pmids":["33597306","31915374"],"is_preprint":false},{"year":2020,"finding":"TRIP13 ATPase catalyzes the transition of REV7 from an active 'closed' conformation to an inactive 'open' conformation, thereby dissociating the REV7-shieldin (including SHLD1-3) complex to promote homologous recombination and confer PARP inhibitor resistance.","method":"Biochemical conformation assays, Co-IP, loss-of-function and overexpression experiments, DNA repair pathway choice assays","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods across independent studies (PMID:31915374, 34521823, 33597306)","pmids":["31915374","34521823"],"is_preprint":false},{"year":2021,"finding":"MAD2L2 (REV7) dimerization within shieldin is mediated by SHLD2 and is required for MAD2L2-SHLD3 interaction and proper shieldin complex assembly. Dimerization-defective MAD2L2 impairs shieldin assembly and fails to promote NHEJ. MAD2L2 dimerization together with SHLD3 enables shieldin to interact with the TRIP13 ATPase.","method":"Co-IP, mutagenesis of MAD2L2 dimerization surface, NHEJ functional assays, CSR assays","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP and mutagenesis with functional readout, single lab (PMID:34521823)","pmids":["34521823"],"is_preprint":false},{"year":2022,"finding":"In BRCA1-deficient cells, SHLD1 acts primarily by recruiting CST to DSBs, as CST tethered near DSBs bypassed the requirement for shieldin; a SHLD1 mutant defective in CST binding (SHLD1Δ) was non-functional in BRCA1-deficient cells. However, SHLD1Δ was fully functional at dysfunctional telomeres and during class switch recombination, where CST can be recruited independently of SHLD1.","method":"Mutagenesis of SHLD1 CST-binding domain, artificial tethering of CST to DSBs, functional rescue assays in BRCA1-deficient cells, CSR assays","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis plus functional rescue with multiple orthogonal experimental contexts in a single rigorous study (PMID:35027730)","pmids":["35027730"],"is_preprint":false},{"year":2022,"finding":"SHLD1 is dispensable for lymphocyte development and RAG-mediated V(D)J recombination (even in XLF-deficient cells), but is essential for restricting resection at AID-induced DSB ends during class switch recombination in both NHEJ-proficient and NHEJ-deficient B cells, and is required for orientation-specific joining of AID-initiated DSBs.","method":"SHLD1 knockout mice, B cell CSR assays, V(D)J recombination assays, resection quantification","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean knockout with defined cellular phenotypes and multiple functional assays, single lab (PMID:35764636)","pmids":["35764636"],"is_preprint":false},{"year":2021,"finding":"The transcription factors THAP1, YY1, and HCF1 bind directly to the SHLD1 promoter and cooperatively maintain the low basal expression of SHLD1, thereby controlling the balance between end protection and resection. Loss of THAP1-dependent SHLD1 expression confers PARP inhibitor and cisplatin cross-resistance in BRCA1-deficient cells. Ablation of SHLD1 rescues embryonic lethality and PARPi sensitivity of BRCA1-deficient mice.","method":"Chromatin immunoprecipitation, promoter reporter assays, knockout mouse models, PARP inhibitor sensitivity assays","journal":"Molecular cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP binding and functional KO rescue, single lab (PMID:33857404)","pmids":["33857404"],"is_preprint":false},{"year":2025,"finding":"CTC1 (of CST) and SHLD1 are epistatic in preventing exacerbated DNA end resection and genetic instability during class switch recombination. Notably, a SHLD1 mutant defective in CST binding via the LDLP motif (SHLD1ΔLDLP) is fully proficient for CSR, demonstrating that the direct SHLD1-CTC1 interaction through this specific motif is dispensable for CST and SHLD functions in promoting productive CSR.","method":"CRISPR knockout of CTC1 and SHLD1 in AID-inducible B cell lines, complementation with SHLD1ΔLDLP mutant, CSR assays, resection quantification","journal":"European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis complementation with functional CSR readout, single lab (PMID:40178294)","pmids":["40178294"],"is_preprint":false},{"year":2025,"finding":"In the absence of NHEJ (XRCC4 deficiency) or DSB end protection (SHLD1 deficiency) during class switch recombination, Polymerase theta (Pol θ) mediates an alternative end-joining pathway characterized by end resection, inversion, and microhomology usage. This Pol θ-mediated repair occurs at the G1-to-S phase transition and is independent of RHINO and PLK1.","method":"Knockout of XRCC4, SHLD1, and/or Pol θ in primary B cells, CSR assays, sequencing of switch junctions","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean triple knockout with defined pathway placement and functional assays, single lab (PMID:41298353)","pmids":["41298353"],"is_preprint":false}],"current_model":"SHLD1 (C20orf196) is a core subunit of the four-component shieldin complex (SHLD1-SHLD2-SHLD3-REV7), which is recruited to DNA double-strand breaks downstream of 53BP1-RIF1 to protect DNA ends from nucleolytic resection; shieldin promotes NHEJ and immunoglobulin class switch recombination by binding single-stranded DNA (via SHLD2's OB-fold domains), recruiting the CST complex which in turn engages Polα-primase for fill-in synthesis, with the complex assembled through a REV7 closed/open conformational dimer regulated by SHLD3 and actively disassembled by the TRIP13 ATPase to shift repair pathway choice toward homologous recombination."},"narrative":{"mechanistic_narrative":"SHLD1 (C20orf196) is a core subunit of the four-component shieldin complex (SHLD1-SHLD2-SHLD3-REV7) that protects DNA double-strand break ends to dictate repair pathway choice [PMID:30022168, PMID:30022158, PMID:30022119, PMID:30046110, PMID:30254264, PMID:30154076]. Recruited to breaks downstream of 53BP1-RIF1, shieldin counteracts nucleolytic end resection and thereby promotes non-homologous end joining and immunoglobulin class switch recombination; its loss causes hyper-resection, defective class switching, and restoration of homologous recombination in BRCA1-deficient cells, conferring PARP inhibitor resistance [PMID:30022168, PMID:30022158, PMID:30022119, PMID:30046110, PMID:30254264, PMID:30154076]. SHLD1 executes end protection chiefly by recruiting the CST complex to breaks, which engages Polα-primase for fill-in synthesis that limits single-stranded DNA at break ends [PMID:30022158, PMID:35027730]. Complex assembly depends on a conformational dimer of REV7 in closed and open states organized by SHLD3 and SHLD2, an architecture that is actively dismantled by the TRIP13 ATPase to release shieldin and shift repair toward homologous recombination [PMID:32332881, PMID:33597306, PMID:31915374, PMID:34521823]. The contribution of the direct SHLD1-CST contact is context-dependent: it is essential in BRCA1-deficient cells but dispensable at dysfunctional telomeres and during class switch recombination, where CST is recruited independently [PMID:35027730, PMID:40178294]. SHLD1 expression is held at a low basal level by the THAP1-YY1-HCF1 transcription factor module, tuning the balance between end protection and resection [PMID:33857404].","teleology":[{"year":2018,"claim":"Established SHLD1 as a previously uncharacterized factor that acts downstream of 53BP1-RIF1 to protect DNA ends, defining its role in repair pathway choice and PARP inhibitor response.","evidence":"CRISPR screens, Co-IP, epistasis, and DNA repair/localization assays in human cells","pmids":["30022168","30022158","30022119","30046110"],"confidence":"High","gaps":["Molecular basis of end protection not resolved at this stage","How SHLD1 within the complex is recruited to break-localized 53BP1-RIF1 not defined"]},{"year":2018,"claim":"Defined SHLD1 as part of a SHLD2-REV7 complex with direct ssDNA-binding activity (via SHLD2), pointing to a physical end-protection mechanism.","evidence":"Mass spectrometry interactome, Co-IP, in vitro ssDNA binding, epistasis, and live-cell localization","pmids":["30254264","30154076"],"confidence":"High","gaps":["Did not establish SHLD1's own biochemical activity within the complex","Downstream effector of end protection not yet identified"]},{"year":2018,"claim":"Connected shieldin to a downstream effector pathway by showing it recruits CST and Polα-primase for fill-in synthesis that opposes resection.","evidence":"Co-IP of CST with shieldin, Polα localization at DSBs, depletion epistasis, BrdU incorporation","pmids":["30022158","35027730"],"confidence":"High","gaps":["Precise SHLD1 residues/motif contacting CST not yet mapped here","Stoichiometry of CST/Polα engagement at breaks unresolved"]},{"year":2020,"claim":"Resolved how shieldin assembles, showing a SHLD3-organized REV7 closed/open conformational dimer required for complex integrity and NHEJ, and explaining mutual exclusivity with REV1/Pol-ζ.","evidence":"Crystal structure of SHLD3-REV7-SHLD2 with mutagenesis and NHEJ assays","pmids":["32332881","33597306"],"confidence":"High","gaps":["SHLD1's position within the assembled complex not structurally resolved","Trigger that controls REV7 conformational state in vivo not defined"]},{"year":2020,"claim":"Identified TRIP13 as the ATPase that converts REV7 from closed to open, dissociating shieldin and shifting repair toward homologous recombination, linking the complex to PARPi resistance.","evidence":"Biochemical conformation assays, Co-IP, loss/gain-of-function, pathway choice assays; cryo-EM of SHLD2-SHLD3-REV7-TRIP13 (2021)","pmids":["31915374","34521823","33597306"],"confidence":"High","gaps":["Whether SHLD1 influences TRIP13-mediated disassembly not directly tested","Regulation of TRIP13 activity at break sites unknown"]},{"year":2021,"claim":"Showed REV7 dimerization mediated by SHLD2 is required for shieldin assembly, NHEJ, CSR, and TRIP13 engagement, refining the assembly mechanism.","evidence":"Co-IP, dimerization-surface mutagenesis, NHEJ and CSR functional assays","pmids":["34521823"],"confidence":"Medium","gaps":["Single lab","Direct contribution of SHLD1 to dimerization-dependent steps not isolated"]},{"year":2022,"claim":"Demonstrated SHLD1's principal function in BRCA1-deficient cells is to recruit CST, and that this requirement is context-specific (dispensable at telomeres and during CSR).","evidence":"SHLD1 CST-binding mutant, artificial CST tethering, functional rescue in BRCA1-deficient cells, CSR assays","pmids":["35027730"],"confidence":"High","gaps":["Alternative CST-recruitment routes during CSR/at telomeres not mechanistically defined","Single lab"]},{"year":2022,"claim":"Separated SHLD1's roles in adaptive immunity, showing it is dispensable for V(D)J recombination but essential for restricting resection and enforcing orientation-specific joining at AID-induced breaks during CSR.","evidence":"SHLD1 knockout mice, B cell CSR and V(D)J assays, resection quantification","pmids":["35764636"],"confidence":"Medium","gaps":["Mechanism of orientation-specific joining control not resolved","Single lab"]},{"year":2021,"claim":"Identified transcriptional control of SHLD1 by THAP1-YY1-HCF1 as a tuning point for end protection versus resection and a determinant of PARPi/cisplatin resistance.","evidence":"ChIP, promoter reporter assays, knockout mouse rescue of BRCA1-deficient phenotypes, drug sensitivity assays","pmids":["33857404"],"confidence":"Medium","gaps":["Upstream signals regulating THAP1/YY1/HCF1 occupancy unknown","Single lab"]},{"year":2025,"claim":"Clarified that the direct SHLD1-CTC1 (LDLP-motif) interaction is dispensable for productive CSR despite SHLD1 and CTC1 being epistatic in restraining resection.","evidence":"CRISPR knockout of CTC1/SHLD1 in AID-inducible B cells, SHLD1ΔLDLP complementation, CSR and resection assays","pmids":["40178294"],"confidence":"Medium","gaps":["How CST is engaged independently of the LDLP contact during CSR unknown","Single lab"]},{"year":2025,"claim":"Placed SHLD1 within pathway-choice logic by showing that loss of SHLD1-mediated end protection unleashes Polθ-dependent alternative end joining during CSR.","evidence":"XRCC4/SHLD1/Polθ knockouts in primary B cells, CSR assays, switch-junction sequencing","pmids":["41298353"],"confidence":"Medium","gaps":["Determinants steering breaks to Polθ versus NHEJ when SHLD1 is lost not fully defined","Single lab"]},{"year":null,"claim":"How SHLD1 itself is structurally positioned within assembled shieldin and what upstream signals govern its recruitment and conformational dynamics at breaks remain open.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No high-resolution structure placing SHLD1 in the full shieldin assembly","Post-translational regulation of SHLD1 at break sites uncharacterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[2,7]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,5]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,1]},{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,1,2]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[8,10,11]}],"complexes":["shieldin (SHLD1-SHLD2-SHLD3-REV7)"],"partners":["SHLD2","SHLD3","REV7","CTC1","TRIP13"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8IYI0","full_name":"Shieldin complex subunit 1","aliases":["RINN1-REV7-interacting novel NHEJ regulator 3","Shield complex subunit 1"],"length_aa":205,"mass_kda":22.9,"function":"Component of the shieldin complex, which plays an important role in repair of DNA double-stranded breaks (DSBs). During G1 and S phase of the cell cycle, the complex functions downstream of TP53BP1 to promote non-homologous end joining (NHEJ) and suppress DNA end resection. Mediates various NHEJ-dependent processes including immunoglobulin class-switch recombination, and fusion of unprotected telomeres","subcellular_location":"Chromosome","url":"https://www.uniprot.org/uniprotkb/Q8IYI0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SHLD1","classification":"Not Classified","n_dependent_lines":162,"n_total_lines":1208,"dependency_fraction":0.13410596026490065},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SHLD1","total_profiled":1310},"omim":[{"mim_id":"618030","title":"SHIELD COMPLEX, SUBUNIT 3; SHLD3","url":"https://www.omim.org/entry/618030"},{"mim_id":"618029","title":"SHIELD COMPLEX, SUBUNIT 2; SHLD2","url":"https://www.omim.org/entry/618029"},{"mim_id":"618028","title":"SHIELD COMPLEX, SUBUNIT 1; SHLD1","url":"https://www.omim.org/entry/618028"},{"mim_id":"605230","title":"TUMOR PROTEIN p53-BINDING PROTEIN 1; TP53BP1","url":"https://www.omim.org/entry/605230"},{"mim_id":"604094","title":"MITOTIC ARREST-DEFICIENT 2 LIKE 2; MAD2L2","url":"https://www.omim.org/entry/604094"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Endoplasmic reticulum","reliability":"Approved"},{"location":"Golgi apparatus","reliability":"Approved"},{"location":"Vesicles","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SHLD1"},"hgnc":{"alias_symbol":["FLJ25067","RINN3"],"prev_symbol":["C20orf196"]},"alphafold":{"accession":"Q8IYI0","domains":[{"cath_id":"-","chopping":"118-165_184-205","consensus_level":"medium","plddt":82.2679,"start":118,"end":205}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IYI0","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IYI0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IYI0-F1-predicted_aligned_error_v6.png","plddt_mean":66.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SHLD1","jax_strain_url":"https://www.jax.org/strain/search?query=SHLD1"},"sequence":{"accession":"Q8IYI0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8IYI0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8IYI0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IYI0"}},"corpus_meta":[{"pmid":"30022168","id":"PMC_30022168","title":"The shieldin 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biologiia","url":"https://pubmed.ncbi.nlm.nih.gov/41090333","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":16195,"output_tokens":3430,"usd":0.050017,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10847,"output_tokens":3553,"usd":0.07153,"stage2_stop_reason":"end_turn"},"total_usd":0.121547,"stage1_batch_id":"msgbatch_013LWZZT6MWaqB9qyTSeQBmU","stage2_batch_id":"msgbatch_0194ZzQaTGNpruoNJBbEMbt3","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2018,\n      \"finding\": \"SHLD1 (C20orf196) is a core subunit of the shieldin complex (SHLD1-SHLD2-SHLD3-REV7), which localizes to DNA double-strand break sites in a 53BP1- and RIF1-dependent manner to protect DNA ends and promote non-homologous end joining (NHEJ). Loss of SHLD1 impairs NHEJ, leads to defective immunoglobulin class switching, causes hyper-resection, and restores homologous recombination in BRCA1-deficient cells, conferring PARP inhibitor resistance.\",\n      \"method\": \"CRISPR screens, Co-IP, epistasis analysis, cellular DNA repair assays, immunofluorescence localization\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — independently replicated across multiple labs (PMID:30022168, 30022158, 30022119, 30046110) using reciprocal co-IP, epistasis, loss-of-function phenotypes, and localization experiments\",\n      \"pmids\": [\"30022168\", \"30022158\", \"30022119\", \"30046110\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"SHLD1 (C20orf196) forms a complex with FAM35A (SHLD2) and REV7, and this complex is recruited to DSBs downstream of RIF1 in vivo. The complex prevents DNA end resection in BRCA1-mutant cells. FAM35A preferentially binds single-strand DNA (ssDNA) in vitro, consistent with a direct DNA-end protection mechanism.\",\n      \"method\": \"Mass spectrometry-based interactome, Co-IP, in vitro ssDNA binding assay, epistasis analysis, live-cell localization\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — replicated across multiple independent studies (PMID:30254264, 30154076, 30022168) with orthogonal methods including in vitro binding, Co-IP, and epistasis\",\n      \"pmids\": [\"30254264\", \"30154076\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Shieldin (including SHLD1) acts downstream of 53BP1-RIF1 and recruits the CST complex, which in turn recruits Polymerase-α (Polα)-primase to DSB sites to perform fill-in synthesis, thereby limiting single-stranded DNA at DSBs and counteracting resection.\",\n      \"method\": \"Co-IP of CST with shieldin, immunofluorescence co-localization of Polα at DSBs, depletion epistasis, BrdU incorporation at DSBs\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — replicated in two independent studies (PMID:30022158, 35027730) using Co-IP, localization, epistasis, and functional rescue experiments\",\n      \"pmids\": [\"30022158\", \"35027730\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Crystal structure of the SHLD3-REV7-SHLD2 ternary complex revealed that shieldin assembly requires an unexpected conformational dimer of REV7 in closed (C-REV7) and open (O-REV7) states mediated by SHLD3, with SHLD2 interacting with O-REV7. The conserved FXPWFP motif of SHLD3 binds to C-REV7 and blocks REV1 binding, thereby excluding shieldin from the REV1/Pol-ζ translesion synthesis complex. Disruption of the REV7 conformational dimer abolishes shieldin assembly and impairs NHEJ.\",\n      \"method\": \"Crystal structure determination, mutagenesis, NHEJ efficiency assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure with functional mutagenesis, single lab but multiple orthogonal methods (PMID:32332881), replicated structurally (PMID:33597306)\",\n      \"pmids\": [\"32332881\", \"33597306\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures of the SHLD2-SHLD3-REV7-TRIP13 complex revealed that the N-terminus of REV7 inserts into the central channel of the TRIP13 hexamer, and ATP hydrolysis-triggered rotatory motions of TRIP13 pull the C-REV7 safety-belt segment through the channel, causing conformational disassembly of the shieldin complex. This TRIP13-mediated remodeling of shieldin promotes homology-directed repair at the expense of NHEJ.\",\n      \"method\": \"Cryo-EM structure, crystal structure of SHLD3-REV7 binary complex, in vitro disassembly assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM and crystal structures with functional validation in a single rigorous study (PMID:33597306), consistent with independent biochemical findings (PMID:31915374, 34521823)\",\n      \"pmids\": [\"33597306\", \"31915374\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"TRIP13 ATPase catalyzes the transition of REV7 from an active 'closed' conformation to an inactive 'open' conformation, thereby dissociating the REV7-shieldin (including SHLD1-3) complex to promote homologous recombination and confer PARP inhibitor resistance.\",\n      \"method\": \"Biochemical conformation assays, Co-IP, loss-of-function and overexpression experiments, DNA repair pathway choice assays\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods across independent studies (PMID:31915374, 34521823, 33597306)\",\n      \"pmids\": [\"31915374\", \"34521823\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"MAD2L2 (REV7) dimerization within shieldin is mediated by SHLD2 and is required for MAD2L2-SHLD3 interaction and proper shieldin complex assembly. Dimerization-defective MAD2L2 impairs shieldin assembly and fails to promote NHEJ. MAD2L2 dimerization together with SHLD3 enables shieldin to interact with the TRIP13 ATPase.\",\n      \"method\": \"Co-IP, mutagenesis of MAD2L2 dimerization surface, NHEJ functional assays, CSR assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP and mutagenesis with functional readout, single lab (PMID:34521823)\",\n      \"pmids\": [\"34521823\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In BRCA1-deficient cells, SHLD1 acts primarily by recruiting CST to DSBs, as CST tethered near DSBs bypassed the requirement for shieldin; a SHLD1 mutant defective in CST binding (SHLD1Δ) was non-functional in BRCA1-deficient cells. However, SHLD1Δ was fully functional at dysfunctional telomeres and during class switch recombination, where CST can be recruited independently of SHLD1.\",\n      \"method\": \"Mutagenesis of SHLD1 CST-binding domain, artificial tethering of CST to DSBs, functional rescue assays in BRCA1-deficient cells, CSR assays\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis plus functional rescue with multiple orthogonal experimental contexts in a single rigorous study (PMID:35027730)\",\n      \"pmids\": [\"35027730\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SHLD1 is dispensable for lymphocyte development and RAG-mediated V(D)J recombination (even in XLF-deficient cells), but is essential for restricting resection at AID-induced DSB ends during class switch recombination in both NHEJ-proficient and NHEJ-deficient B cells, and is required for orientation-specific joining of AID-initiated DSBs.\",\n      \"method\": \"SHLD1 knockout mice, B cell CSR assays, V(D)J recombination assays, resection quantification\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean knockout with defined cellular phenotypes and multiple functional assays, single lab (PMID:35764636)\",\n      \"pmids\": [\"35764636\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The transcription factors THAP1, YY1, and HCF1 bind directly to the SHLD1 promoter and cooperatively maintain the low basal expression of SHLD1, thereby controlling the balance between end protection and resection. Loss of THAP1-dependent SHLD1 expression confers PARP inhibitor and cisplatin cross-resistance in BRCA1-deficient cells. Ablation of SHLD1 rescues embryonic lethality and PARPi sensitivity of BRCA1-deficient mice.\",\n      \"method\": \"Chromatin immunoprecipitation, promoter reporter assays, knockout mouse models, PARP inhibitor sensitivity assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP binding and functional KO rescue, single lab (PMID:33857404)\",\n      \"pmids\": [\"33857404\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CTC1 (of CST) and SHLD1 are epistatic in preventing exacerbated DNA end resection and genetic instability during class switch recombination. Notably, a SHLD1 mutant defective in CST binding via the LDLP motif (SHLD1ΔLDLP) is fully proficient for CSR, demonstrating that the direct SHLD1-CTC1 interaction through this specific motif is dispensable for CST and SHLD functions in promoting productive CSR.\",\n      \"method\": \"CRISPR knockout of CTC1 and SHLD1 in AID-inducible B cell lines, complementation with SHLD1ΔLDLP mutant, CSR assays, resection quantification\",\n      \"journal\": \"European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis complementation with functional CSR readout, single lab (PMID:40178294)\",\n      \"pmids\": [\"40178294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In the absence of NHEJ (XRCC4 deficiency) or DSB end protection (SHLD1 deficiency) during class switch recombination, Polymerase theta (Pol θ) mediates an alternative end-joining pathway characterized by end resection, inversion, and microhomology usage. This Pol θ-mediated repair occurs at the G1-to-S phase transition and is independent of RHINO and PLK1.\",\n      \"method\": \"Knockout of XRCC4, SHLD1, and/or Pol θ in primary B cells, CSR assays, sequencing of switch junctions\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean triple knockout with defined pathway placement and functional assays, single lab (PMID:41298353)\",\n      \"pmids\": [\"41298353\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SHLD1 (C20orf196) is a core subunit of the four-component shieldin complex (SHLD1-SHLD2-SHLD3-REV7), which is recruited to DNA double-strand breaks downstream of 53BP1-RIF1 to protect DNA ends from nucleolytic resection; shieldin promotes NHEJ and immunoglobulin class switch recombination by binding single-stranded DNA (via SHLD2's OB-fold domains), recruiting the CST complex which in turn engages Polα-primase for fill-in synthesis, with the complex assembled through a REV7 closed/open conformational dimer regulated by SHLD3 and actively disassembled by the TRIP13 ATPase to shift repair pathway choice toward homologous recombination.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SHLD1 (C20orf196) is a core subunit of the four-component shieldin complex (SHLD1-SHLD2-SHLD3-REV7) that protects DNA double-strand break ends to dictate repair pathway choice [#0, #1]. Recruited to breaks downstream of 53BP1-RIF1, shieldin counteracts nucleolytic end resection and thereby promotes non-homologous end joining and immunoglobulin class switch recombination; its loss causes hyper-resection, defective class switching, and restoration of homologous recombination in BRCA1-deficient cells, conferring PARP inhibitor resistance [#0, #1]. SHLD1 executes end protection chiefly by recruiting the CST complex to breaks, which engages Pol\\u03b1-primase for fill-in synthesis that limits single-stranded DNA at break ends [#2, #7]. Complex assembly depends on a conformational dimer of REV7 in closed and open states organized by SHLD3 and SHLD2, an architecture that is actively dismantled by the TRIP13 ATPase to release shieldin and shift repair toward homologous recombination [#3, #5]. The contribution of the direct SHLD1-CST contact is context-dependent: it is essential in BRCA1-deficient cells but dispensable at dysfunctional telomeres and during class switch recombination, where CST is recruited independently [#7, #10]. SHLD1 expression is held at a low basal level by the THAP1-YY1-HCF1 transcription factor module, tuning the balance between end protection and resection [#9].\"\n  ,\n  \"teleology\": [\n    {\n      \"year\": 2018,\n      \"claim\": \"Established SHLD1 as a previously uncharacterized factor that acts downstream of 53BP1-RIF1 to protect DNA ends, defining its role in repair pathway choice and PARP inhibitor response.\",\n      \"evidence\": \"CRISPR screens, Co-IP, epistasis, and DNA repair/localization assays in human cells\",\n      \"pmids\": [\"30022168\", \"30022158\", \"30022119\", \"30046110\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of end protection not resolved at this stage\", \"How SHLD1 within the complex is recruited to break-localized 53BP1-RIF1 not defined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Defined SHLD1 as part of a SHLD2-REV7 complex with direct ssDNA-binding activity (via SHLD2), pointing to a physical end-protection mechanism.\",\n      \"evidence\": \"Mass spectrometry interactome, Co-IP, in vitro ssDNA binding, epistasis, and live-cell localization\",\n      \"pmids\": [\"30254264\", \"30154076\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish SHLD1's own biochemical activity within the complex\", \"Downstream effector of end protection not yet identified\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Connected shieldin to a downstream effector pathway by showing it recruits CST and Pol\\u03b1-primase for fill-in synthesis that opposes resection.\",\n      \"evidence\": \"Co-IP of CST with shieldin, Pol\\u03b1 localization at DSBs, depletion epistasis, BrdU incorporation\",\n      \"pmids\": [\"30022158\", \"35027730\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise SHLD1 residues/motif contacting CST not yet mapped here\", \"Stoichiometry of CST/Pol\\u03b1 engagement at breaks unresolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Resolved how shieldin assembles, showing a SHLD3-organized REV7 closed/open conformational dimer required for complex integrity and NHEJ, and explaining mutual exclusivity with REV1/Pol-\\u03b6.\",\n      \"evidence\": \"Crystal structure of SHLD3-REV7-SHLD2 with mutagenesis and NHEJ assays\",\n      \"pmids\": [\"32332881\", \"33597306\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"SHLD1's position within the assembled complex not structurally resolved\", \"Trigger that controls REV7 conformational state in vivo not defined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identified TRIP13 as the ATPase that converts REV7 from closed to open, dissociating shieldin and shifting repair toward homologous recombination, linking the complex to PARPi resistance.\",\n      \"evidence\": \"Biochemical conformation assays, Co-IP, loss/gain-of-function, pathway choice assays; cryo-EM of SHLD2-SHLD3-REV7-TRIP13 (2021)\",\n      \"pmids\": [\"31915374\", \"34521823\", \"33597306\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SHLD1 influences TRIP13-mediated disassembly not directly tested\", \"Regulation of TRIP13 activity at break sites unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Showed REV7 dimerization mediated by SHLD2 is required for shieldin assembly, NHEJ, CSR, and TRIP13 engagement, refining the assembly mechanism.\",\n      \"evidence\": \"Co-IP, dimerization-surface mutagenesis, NHEJ and CSR functional assays\",\n      \"pmids\": [\"34521823\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Direct contribution of SHLD1 to dimerization-dependent steps not isolated\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrated SHLD1's principal function in BRCA1-deficient cells is to recruit CST, and that this requirement is context-specific (dispensable at telomeres and during CSR).\",\n      \"evidence\": \"SHLD1 CST-binding mutant, artificial CST tethering, functional rescue in BRCA1-deficient cells, CSR assays\",\n      \"pmids\": [\"35027730\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Alternative CST-recruitment routes during CSR/at telomeres not mechanistically defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Separated SHLD1's roles in adaptive immunity, showing it is dispensable for V(D)J recombination but essential for restricting resection and enforcing orientation-specific joining at AID-induced breaks during CSR.\",\n      \"evidence\": \"SHLD1 knockout mice, B cell CSR and V(D)J assays, resection quantification\",\n      \"pmids\": [\"35764636\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of orientation-specific joining control not resolved\", \"Single lab\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified transcriptional control of SHLD1 by THAP1-YY1-HCF1 as a tuning point for end protection versus resection and a determinant of PARPi/cisplatin resistance.\",\n      \"evidence\": \"ChIP, promoter reporter assays, knockout mouse rescue of BRCA1-deficient phenotypes, drug sensitivity assays\",\n      \"pmids\": [\"33857404\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Upstream signals regulating THAP1/YY1/HCF1 occupancy unknown\", \"Single lab\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Clarified that the direct SHLD1-CTC1 (LDLP-motif) interaction is dispensable for productive CSR despite SHLD1 and CTC1 being epistatic in restraining resection.\",\n      \"evidence\": \"CRISPR knockout of CTC1/SHLD1 in AID-inducible B cells, SHLD1\\u0394LDLP complementation, CSR and resection assays\",\n      \"pmids\": [\"40178294\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How CST is engaged independently of the LDLP contact during CSR unknown\", \"Single lab\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Placed SHLD1 within pathway-choice logic by showing that loss of SHLD1-mediated end protection unleashes Pol\\u03b8-dependent alternative end joining during CSR.\",\n      \"evidence\": \"XRCC4/SHLD1/Pol\\u03b8 knockouts in primary B cells, CSR assays, switch-junction sequencing\",\n      \"pmids\": [\"41298353\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Determinants steering breaks to Pol\\u03b8 versus NHEJ when SHLD1 is lost not fully defined\", \"Single lab\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How SHLD1 itself is structurally positioned within assembled shieldin and what upstream signals govern its recruitment and conformational dynamics at breaks remain open.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No high-resolution structure placing SHLD1 in the full shieldin assembly\", \"Post-translational regulation of SHLD1 at break sites uncharacterized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [2, 7]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [8, 10, 11]}\n    ],\n    \"complexes\": [\"shieldin (SHLD1-SHLD2-SHLD3-REV7)\"],\n    \"partners\": [\"SHLD2\", \"SHLD3\", \"REV7\", \"CTC1\", \"TRIP13\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}