{"gene":"STAG1","run_date":"2026-04-28T20:42:08","timeline":{"discoveries":[{"year":1997,"finding":"SA-1 (STAG1) was identified as a novel nuclear protein of ~120 kDa expressed in hematopoietic organs, cloned from mouse stromal cell and human thymus cDNA libraries. A closely related paralog SA-2 was also cloned, sharing 71% identity. Subcellular fractionation demonstrated nuclear localization of SA-1 protein.","method":"Lambda gt11 cDNA library screening with monoclonal antibody B92, RACE-PCR, immunoprecipitation/Western blotting, subcellular fractionation","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 — original cloning with protein identification and nuclear localization by fractionation, single lab","pmids":["9305759"],"is_preprint":false},{"year":2009,"finding":"Cohesin-SA1 and cohesin-SA2 have distinct, non-redundant roles in sister chromatid cohesion: SA1 is specifically required for telomere cohesion after DNA replication in S phase, while SA2 is required for centromere cohesion. Depletion of SA1 but not SA2 caused loss of sister telomere cohesion, inability to repair chromatid breaks, and sister telomere loss. The telomere cohesion function of SA1 was shown to be linked to the telomeric protein TIN2.","method":"siRNA depletion of SA1 or SA2 in human cells, fluorescence in situ hybridization (FISH) for telomere/centromere cohesion, live-cell imaging, cell cycle analysis","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal depletion experiments with specific phenotypic readouts replicated across multiple conditions, independently confirmed by other labs","pmids":["19822671"],"is_preprint":false},{"year":2012,"finding":"SA1 is largely responsible for cohesin accumulation at gene promoters and CTCF-bound sites genome-wide. SA1 ablation alters transcription of genes related to Cornelia de Lange syndrome, and cohesin-SA1 positively regulates expression of myc and protocadherin genes at their promoters — a function that cannot be compensated by cohesin-SA2.","method":"ChIP-seq in wild-type and SA1-null mouse embryonic fibroblasts, RNA-seq, SA1-null mouse embryo generation","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — genome-wide ChIP-seq and transcriptomics in genetic knockout model, multiple orthogonal methods","pmids":["22415368"],"is_preprint":false},{"year":2012,"finding":"Complete SA1 ablation in mice causes embryonic lethality; heterozygous SA1-deficient animals show shorter lifespan and earlier tumor onset. SA1-null MEFs display increased aneuploidy and chromosome segregation defects arising specifically from defective telomere replication, not from impaired centromeric cohesion (which depends on cohesin-SA2). SA1 is thus specifically required for telomere replication-associated cohesion.","method":"SA1-null mouse model generation, MEF culture, cytogenetic analysis, BrdU incorporation, metaphase spreads, telomere FISH","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — in vivo mouse knockout with multiple orthogonal phenotypic assays, mechanistic distinction from SA2 established","pmids":["22415365"],"is_preprint":false},{"year":2013,"finding":"SA1 binds directly to telomeric DNA through a unique AT-hook motif in its N-terminal domain (absent in SA2). Overexpression of SA1 alone is sufficient to induce cohesion at telomeres independently of the cohesin ring, and this requires the AT-hook-containing N-terminal domain. SA1 is highly enriched at telomeres by ChIP, decreases at mitosis when cohesion is resolved, and increases when cohesion persists.","method":"ChIP in human cells, overexpression of SA1 truncation mutants, AT-hook mutation analysis, telomere cohesion assays by FISH","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — domain mapping with mutagenesis, ChIP enrichment, functional overexpression assays with specific phenotypic readouts","pmids":["23729739"],"is_preprint":false},{"year":2013,"finding":"SA1 and SA2 interact with RAD21 (Rad21/Scc1) through two distinct SA-binding motifs on RAD21: an N-terminal motif (aa 60-81) and a middle-part α-helical motif (aa 383-392). Mutation of three conserved residues (L385, F389, T390) in the α-helical motif significantly disrupts Rad21-SA1/2 interaction without affecting Smc1-Smc3-Rad21 interaction. Mutant Rad21 fails to rescue precocious chromosome separation caused by endogenous Rad21 depletion.","method":"In vitro binding assays, co-immunoprecipitation (ex vivo), deletion mapping, site-directed mutagenesis, chromosome segregation rescue assay","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro mapping combined with mutagenesis and functional rescue, multiple orthogonal binding assays","pmids":["23874961"],"is_preprint":false},{"year":2013,"finding":"Cohesin-SA2 (not SA1) is the main complex co-recruited with the cohesin-loading factor NIPBL to DNA damage sites in an S/G2-phase-specific manner. Replacing the diverged C-terminal region of SA1 with the corresponding region of SA2 confers DNA damage site accumulation activity on SA1. SA2 depletion (not SA1) decreased sister chromatid homologous recombination repair and affected DNA repair pathway choice. Both cohesin complexes function in the intra-S checkpoint.","method":"Laser microirradiation with live-cell imaging, siRNA depletion, domain-swap chimera construction, sister chromatid exchange assay, cell cycle checkpoint analysis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal depletion, chimeric protein functional analysis, multiple orthogonal assays distinguishing SA1 vs SA2 functions","pmids":["24324008"],"is_preprint":false},{"year":2016,"finding":"SA1 displays two-state DNA binding behavior: non-specific 1D free diffusion for DNA searching, and subdiffusive sliding at telomeric regions for recognition. The AT-hook motif in SA1 modulates both non-specific DNA binding and subdiffusive dynamics over telomeric sequences. TRF1 tethers SA1 within telomeric regions, and together SA1 and TRF1 form longer DNA-DNA pairing tracts than TRF1 alone.","method":"Single-molecule fluorescence imaging, 1D diffusion tracking on DNA tightropes, AT-hook mutant SA1, atomic force microscopy","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 — single-molecule reconstitution with mutagenesis and AFM structural validation","pmids":["27298259"],"is_preprint":false},{"year":2017,"finding":"STAG1 and STAG2 display a strong synthetic lethal interaction: STAG1 loss abrogates sister chromatid cohesion specifically in STAG2-mutated cells (not wild-type), leading to mitotic catastrophe, defective cell division, and apoptosis. Restoration of STAG2 expression alleviates dependence on STAG1. This synthetic lethality was demonstrated across bladder cancer and Ewing sarcoma cell lines.","method":"CRISPR-Cas9 knockout, siRNA depletion, metaphase spread analysis, cell viability/proliferation assays, STAG2 re-expression rescue","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis across multiple cell line models with rescue experiment, replicated by independent lab","pmids":["28691904"],"is_preprint":false},{"year":2018,"finding":"Cohesin-SA1 and cohesin-SA2 have distinct genomic distributions and non-redundant contributions to 3D genome organization: cohesin-SA1 preferentially stabilizes topologically associating domain (TAD) boundaries together with CTCF, while cohesin-SA2 promotes cell-type-specific enhancer-promoter contacts independently of CTCF. Loss of SA2 rewires local chromatin contacts in a way that cannot be compensated by SA1.","method":"ChIP-seq for SA1 and SA2, Hi-C, RNA-seq in SA1- or SA2-depleted human cells, insulation score analysis","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 2 — genome-wide ChIP-seq, Hi-C, and transcriptomics with isogenic depletions, multiple orthogonal methods","pmids":["29867216"],"is_preprint":false},{"year":2019,"finding":"Stag1 and Stag2 bind a shared set of genomic loci in hematopoietic stem and progenitor cells, but a component of Stag2 binding sites is unoccupied by Stag1. Stag2 loss decreased chromatin accessibility and transcription of lineage-specification genes (Ebf1, Pax5), increasing self-renewal and reducing B cell commitment. Concurrent loss of both Stag1 and Stag2 abrogated hematopoiesis entirely, demonstrating partial redundancy.","method":"Conditional Stag2 deletion in mouse HSPCs, ChIP-seq for Stag1 and Stag2, ATAC-seq, RNA-seq, flow cytometry, hematopoietic reconstitution assay","journal":"Cell stem cell","confidence":"High","confidence_rationale":"Tier 2 — in vivo genetic model with genome-wide chromatin and transcriptional profiling, epistasis between Stag1 and Stag2","pmids":["31495782"],"is_preprint":false},{"year":2019,"finding":"In mouse embryonic stem cells, cohesin-SA1 disrupts Polycomb domain interaction networks and preserves TAD borders, whereas cohesin-SA2 facilitates Polycomb domain compaction through PRC1 recruitment and promotes long-range interactions between Polycomb-bound promoters. These two modes of action reflect distinct roles: SA1 in loop extrusion with CTCF for TAD organization, SA2 in local transcriptional hub organization.","method":"ChIP-seq, Hi-C, ChIA-PET, siRNA depletion of SA1 or SA2 in mESCs, PRC1 co-immunoprecipitation","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — multiple genome-wide methods in genetic depletion model with protein interaction validation","pmids":["31216471"],"is_preprint":false},{"year":2020,"finding":"ESCO1 acetyltransferase and CTCF protect a subset of cohesin-STAG1 complexes from removal by WAPL, enabling formation of long chromatin loops. During G1 phase, acetylated cohesin-STAG1 binds chromatin for hours while cohesin-STAG2 binds for minutes. ESCO1 and CTCF together contribute to boundary formation in chromatin looping, consistent with a nested loop extrusion model where stable cohesin-STAG1 loops demarcate boundaries for more transient cohesin-STAG2 extrusion.","method":"Auxin-inducible degron (AID) system for acute protein depletion, Hi-C, ChIP-seq, FRAP, co-immunoprecipitation","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — inducible degron system with Hi-C, FRAP quantifying residence time, and ChIP-seq; multiple orthogonal methods","pmids":["32065581"],"is_preprint":false},{"year":2020,"finding":"Cohesin SA1 and SA2 are direct RNA-binding proteins. Both bind ssRNA, dsRNA, RNA:DNA hybrids, and R-loops in vitro, and localize to RNA-containing regions on dsDNA in single-molecule experiments. SA1 and SA2 binding sites overlap significantly with R-loop (DRIP-seq) sites in cells, with most R-loop-localized SA1/SA2 also binding other cohesin subunits.","method":"Electrophoretic mobility shift assay (EMSA), atomic force microscopy (AFM), DNA tightrope single-molecule assay, ChIP-seq/DRIP-seq overlap analysis","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 — direct in vitro biochemical binding assays with multiple RNA substrates, single-molecule validation, and genomic overlap analysis","pmids":["32352519"],"is_preprint":false},{"year":2020,"finding":"STAG1 was identified as the most prominent and selective genetic dependency of STAG2-deficient cells in genome-wide CRISPR screens. X-ray crystallography identified the STAG1 regions that interact with RAD21. STAG1 mutations that abrogate interaction with RAD21 selectively compromise viability of STAG2-deficient cells. Chemical-genetic degradation of STAG1 using an inducible degron causes loss of sister chromatid cohesion and rapid cell death specifically in STAG2-deficient cells.","method":"Genome-wide CRISPR screen, X-ray crystallography of STAG1-RAD21 interface, auxin-inducible degron system, metaphase spread, cell viability assay","journal":"Life science alliance","confidence":"High","confidence_rationale":"Tier 1 — X-ray crystal structure combined with genome-wide screen, inducible degradation system, and mutagenesis","pmids":["32467316"],"is_preprint":false},{"year":2012,"finding":"Nuclear import of human SA1/STAG1 depends on a functional N-terminal NLS. In contrast to SA2 (which has multiple functional NLS and NES signals), SA1 has only one functional NLS, located at its N-terminus, with no functional nuclear export signals identified, suggesting SA1 is constitutively nuclear.","method":"In silico NLS/NES prediction, expression of fluorescently tagged SA1/SA2 in Saccharomyces cerevisiae, validation in HeLa cells by confocal microscopy","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 3 — localization by fluorescence in two cell systems, but functional consequence not directly tested","pmids":["22715410"],"is_preprint":false},{"year":2023,"finding":"TRF1, TIN2, and SA1 act synergistically to compact telomeric DNA. SA1 alone can compact telomeric DNA, and compaction by the full ternary TRF1-TIN2-SA1 system is additive (TRF1-TIN2 component plus SA1 component). Atomic force microscopy confirmed that all three proteins together drive strong intermolecular aggregation as required for chromosome cohesion.","method":"Nanochannel DNA stretching to quantify compaction, atomic force microscopy of intermolecular aggregation, systematic comparison of protein subsets","journal":"Biophysical journal","confidence":"Medium","confidence_rationale":"Tier 1-2 — reconstitution with single-molecule imaging and AFM, but single lab","pmids":["37081787"],"is_preprint":false},{"year":2023,"finding":"STAG1 is the dominant paralog in mouse embryonic stem cells and is required for pluripotency. The N-terminus of STAG1 specifically represses the two-cell (2C) embryonic state by maintaining rRNA expression and nucleolar integrity. Skewing the balance of naturally occurring STAG1 isoforms impacts cell identity. This role in nucleolar structure is distinct from STAG1's role in chromatin loop organization.","method":"mESC STAG1 depletion, isoform-specific overexpression, RNA-seq for 2C gene markers, rRNA expression analysis, nucleolar morphology by immunofluorescence","journal":"Stem cell reports","confidence":"Medium","confidence_rationale":"Tier 2 — loss-of-function with domain-specific rescue, specific phenotypic readouts, single lab","pmids":["37802073"],"is_preprint":false},{"year":2024,"finding":"KPT-6566 was identified as a dual inhibitor of STAG1 and STAG2 that disrupts their interactions with SCC1 and double-stranded DNA. KPT-6566 causes premature chromosome separation, chromosome damage, impaired DNA damage repair, accumulation of double-strand breaks, and cell apoptosis in HeLa cells. It also sensitizes cells to PARP inhibitor Olaparib and NHEJ inhibitor UMI-77.","method":"Fluorescence polarization high-throughput screening, biochemical/biophysical binding assays, metaphase chromosome spread assay, γH2AX immunofluorescence, cell viability assay","journal":"DNA repair","confidence":"Medium","confidence_rationale":"Tier 2 — biochemical target engagement plus cellular functional assays, single lab","pmids":["39541712"],"is_preprint":false},{"year":2025,"finding":"STAG1 binds tightly to the trimeric SMC1/SMC3/SCC1 complex; this tetramer then weakly but cooperatively binds DNA. NIPBL binds DNA tightly and acts as a DNA anchor during the mechanochemical loop extrusion cycle. An ATP-modulated DNA binding site created by the STAG1-SMC1/SMC3/SCC1 interaction is important for repeated DNA grabbing and release critical to loop extrusion.","method":"Mass photometry to quantify biomolecular interactions, systematic measurement of cohesin subunit assembly and DNA binding affinities, cohesin mutant analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — quantitative single-molecule mass photometry of all key interactions in reconstituted system, multiple mutants tested","pmids":["40763028"],"is_preprint":false},{"year":2020,"finding":"Cohesin-STAG1 specifically contributes to stabilization of TAD boundaries together with CTCF, while cohesin-STAG2 promotes cell-type-specific contacts between enhancers and promoters independently of CTCF. These findings establish distinct 3D genome organization roles for the two cohesin variants.","method":"ChIP-seq, Hi-C in multiple human cell types with SA1/SA2 depletion, insulation score analysis","journal":"Current opinion in genetics & development","confidence":"Medium","confidence_rationale":"Tier 3 — review synthesizing existing data, no new experimental findings","pmids":["32294612"],"is_preprint":false},{"year":2025,"finding":"Cohesin-STAG1 extrudes DNA loops more efficiently than cohesin-STAG2 in single-molecule assays, despite comparable ATPase activity and topological DNA entrapment. The AT-hook motif unique to the STAG1 N-terminus promotes loop extrusion without altering ATPase activity or DNA binding. In human somatic cells, the AT-hook is required for stable cohesin-chromatin association during G1 phase (but dispensable for sister chromatid cohesion), and its mutation markedly impairs TAD and chromatin loop formation.","method":"Single-molecule DNA loop extrusion assays, ATPase activity assay, Hi-C in cells expressing AT-hook mutant STAG1, ChIP-seq for cohesin residence","journal":"bioRxiv","confidence":"High","confidence_rationale":"Tier 1 — single-molecule reconstitution with domain mutagenesis validated by Hi-C and ChIP-seq in cells","pmids":["bio_10.1101_2025.11.17.688764"],"is_preprint":true},{"year":2025,"finding":"Stag1 depletion has negligible impact on post-mitotic genome restructuring or transcription reactivation during mitosis-to-G1 transition, whereas Stag2 orchestrates early-G1 chromatin remodeling. Simultaneous depletion of both Stag proteins results in synergistic loss of virtually all structural loops and more severe transcriptional disruption than individual deletions, demonstrating that Stag1 provides compensatory support during genome refolding.","method":"Auxin-inducible degron for acute Stag1/Stag2 depletion during mitosis-to-G1 transition, Hi-C time course, RNA-seq, ChIP-seq","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — inducible degron system with Hi-C time course, preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.10.24.684407"],"is_preprint":true},{"year":2025,"finding":"Molecular dynamics simulations predict that STAG1, together with SMC1, SMC3, and NIPBL, forms a DNA clamping patch group that facilitates DNA bending and capture within the cohesin ring, identifying specific high-affinity DNA binding patches on STAG1 implicated in loop extrusion.","method":"Molecular dynamics simulations at amino-acid residue resolution, dissociation rate constant quantification as proxy for DNA affinity","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 4 — computational prediction only, not experimentally validated","pmids":["bio_10.1101_2024.09.17.613402"],"is_preprint":true},{"year":2025,"finding":"The CTCF N-terminal region contains two motifs (YDF and KTYQR) that hinder cohesin-mediated loop extrusion. The KTYQR motif fully impedes loop extrusion activity, while YDF converts cohesin into a unidirectional extruder by strengthening the affinity of STAG1 to DNA, revealing that CTCF stalls loop extrusion partly through modulating STAG1's DNA affinity.","method":"Single-molecule DNA loop extrusion assay with CTCF N-terminal region fragments, force measurements, directionality analysis","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 — single-molecule reconstitution with domain peptides, preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.01.26.634934"],"is_preprint":true},{"year":2025,"finding":"Loss of SA1 (the Drosophila STAG1/2 homolog) during brain development leads to defects in neural stem cell differentiation and promotion of tumorigenesis. PARP inhibition combined with SA1/STAG2 depletion results in apoptosis in vitro and in vivo, and reduction of PARP activity ameliorates tumor-associated phenotypes of SA1-deficient Drosophila tissue, suggesting synthetic lethality between cohesin and PARP activity.","method":"Drosophila RNAi knockdown of SA1 in brain tissue, human cell STAG2 depletion, PARP inhibitor treatment, tumor phenotype scoring, apoptosis assays","journal":"Disease models & mechanisms","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo Drosophila model (ortholog) with pharmacological intervention and in vitro validation, cross-species","pmids":["41502392"],"is_preprint":false},{"year":2025,"finding":"A frameshift STAG1 variant (c.500dup) causes dramatic reduction in STAG1 mRNA and protein through nonsense-mediated mRNA decay (NMD), establishing haploinsufficiency as the underlying pathogenic mechanism for STAG1-associated intellectual developmental disorder (MRD47).","method":"Trio whole-exome sequencing, mutant STAG1 expression vector transfection into HEK293T cells, qRT-PCR for mRNA levels, Western blot for protein levels, NMD pathway analysis","journal":"Frontiers in pediatrics","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro functional study with mRNA and protein quantification demonstrating NMD mechanism, single lab","pmids":["41210242"],"is_preprint":false}],"current_model":"STAG1 (SA1) is a core subunit of one of two vertebrate cohesin variants (cohesin-STAG1), functioning as a nuclear protein that (1) mediates sister chromatid cohesion specifically at telomeres via a unique N-terminal AT-hook motif that binds telomeric DNA and cooperates with TRF1/TIN2, (2) drives DNA loop extrusion more efficiently than STAG2-containing cohesin through its AT-hook, enabling stable long chromatin loops at TAD boundaries in conjunction with CTCF and the acetyltransferase ESCO1 which protects it from WAPL-mediated eviction, (3) binds directly to RNA and R-loops, (4) interacts with RAD21/SCC1 through two defined binding motifs, and (5) is partially redundant with STAG2 for sister chromatid cohesion and cell survival, forming a synthetic lethal pair exploitable in STAG2-mutant cancers."},"narrative":{"teleology":[],"mechanism_profile":{"molecular_activity":[],"localization":[],"pathway":[],"complexes":[],"partners":[],"other_free_text":[]},"mechanistic_narrative":"Insufficient on-target evidence to synthesize a narrative — discovery timeline does not match the canonical STAG1 protein."},"prefetch_data":{"uniprot":{"accession":"Q8WVM7","full_name":"Cohesin subunit SA-1","aliases":["SCC3 homolog 1","Stromal antigen 1"],"length_aa":1258,"mass_kda":144.4,"function":"Component of cohesin complex, a complex required for the cohesion of sister chromatids after DNA replication. The cohesin complex apparently forms a large proteinaceous ring within which sister chromatids can be trapped. At anaphase, the complex is cleaved and dissociates from chromatin, allowing sister chromatids to segregate. The cohesin complex may also play a role in spindle pole assembly during mitosis","subcellular_location":"Nucleus; Chromosome; Chromosome, centromere","url":"https://www.uniprot.org/uniprotkb/Q8WVM7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/STAG1","classification":"Not 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COHESIN RELEASE FACTOR; WAPL","url":"https://www.omim.org/entry/610754"},{"mim_id":"608667","title":"NIPPED-B-LIKE; NIPBL","url":"https://www.omim.org/entry/608667"},{"mim_id":"608489","title":"STROMAL ANTIGEN 3; STAG3","url":"https://www.omim.org/entry/608489"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Nuclear bodies","reliability":"Additional"},{"location":"Primary cilium","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/STAG1"},"hgnc":{"alias_symbol":["SA-1","SCC3A","SA1"],"prev_symbol":[]},"alphafold":{"accession":"Q969W9","domains":[{"cath_id":"1.20.5","chopping":"29-70","consensus_level":"medium","plddt":82.4126,"start":29,"end":70}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q969W9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q969W9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q969W9-F1-predicted_aligned_error_v6.png","plddt_mean":56.72},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=STAG1","jax_strain_url":"https://www.jax.org/strain/search?query=STAG1"},"sequence":{"accession":"Q969W9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q969W9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q969W9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q969W9"}},"corpus_meta":[{"pmid":"22415365","id":"PMC_22415365","title":"Cohesin-SA1 deficiency 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centromere cohesion. Depletion of SA1 prevents establishment/maintenance of telomere cohesion, and loss of telomere cohesion impairs chromatid break repair and causes sister telomere loss.\",\n      \"method\": \"siRNA depletion in human cells, fluorescence imaging of telomere/centromere cohesion, sister chromatid cohesion assays\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal depletion experiments with specific phenotypic readouts, replicated with TIN2 co-depletion\",\n      \"pmids\": [\"19822671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SA1-null mouse embryonic fibroblasts show defective telomere replication (not impaired centromeric cohesion) leading to chromosome segregation defects and aneuploidy, establishing that cohesin-SA1 is specifically required for telomere replication.\",\n      \"method\": \"SA1 knockout mouse model, MEF proliferation assays, chromosome segregation analysis, telomere FISH\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with specific mechanistic readout (telomere replication vs. centromeric cohesion) distinguishing SA1 from SA2\",\n      \"pmids\": [\"22415365\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SA1 is largely responsible for cohesin accumulation at gene promoters and CTCF-binding sites genome-wide; loss of SA1 alters transcription of genes related to Cornelia de Lange syndrome, and cohesin-SA1 positively regulates expression of MYC and protocadherin genes in a manner that cannot be compensated by cohesin-SA2.\",\n      \"method\": \"SA1 knockout mouse model, genome-wide ChIP-seq of cohesin in wild-type vs SA1-null cells, transcriptomic analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-wide ChIP-seq with KO cells and transcriptomics, multiple orthogonal methods\",\n      \"pmids\": [\"22415368\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SA1 directly binds telomeric DNA through a unique N-terminal AT-hook motif (not present in SA2); overexpression of SA1 alone is sufficient to induce telomere cohesion independently of the cohesin ring, dependent on this AT-hook domain.\",\n      \"method\": \"ChIP showing SA1 enrichment at telomeres, overexpression of SA1 deletion mutants, AT-hook mutagenesis, cohesion assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain mapping, mutagenesis, overexpression rescue, ChIP; multiple orthogonal methods in single study\",\n      \"pmids\": [\"23729739\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Cohesin-SA2 (not SA1) is the main complex recruited to DNA double-strand break sites in S/G2 phase together with NIPBL; SA2 but not SA1 depletion reduces sister chromatid homologous recombination repair and affects repair pathway choice. Both complexes function in the intra-S checkpoint.\",\n      \"method\": \"siRNA depletion, laser-induced DSB assays, sister chromatid HR repair assays, C-terminal domain swap between SA1 and SA2\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain-swap mutagenesis and multiple functional readouts distinguishing SA1 from SA2\",\n      \"pmids\": [\"24324008\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SA1/SA2 interact with RAD21 through two SA-binding motifs on RAD21: an N-terminal motif (residues 60–81) and a middle-part α-helical motif (residues 383–392). Mutation of L385, F389, T390 in the MP motif disrupts Rad21–SA1/SA2 interaction without affecting SMC1–SMC3–Rad21 interaction; mutant Rad21 fails to rescue precocious chromosome separation.\",\n      \"method\": \"In vitro binding assays, co-immunoprecipitation, mutagenesis, analytical ultracentrifugation, complementation in cells\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with mutagenesis and cellular validation\",\n      \"pmids\": [\"23874961\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SA1 uses two-state binding on DNA: one-dimensional free diffusion for searching and subdiffusive sliding at telomeric regions for recognition. The AT-hook motif modulates non-specific DNA binding and subdiffusive dynamics. TRF1 tethers SA1 at telomeric regions, and together SA1 and TRF1 form longer DNA-DNA pairing tracts than TRF1 alone, promoting telomere cohesion.\",\n      \"method\": \"Single-molecule fluorescence imaging, atomic force microscopy, AT-hook mutagenesis, DNA tightrope assay\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — single-molecule reconstitution with mutagenesis and AFM structural validation\",\n      \"pmids\": [\"27298259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"STAG1 and STAG2 are synthetic lethal: STAG1 loss abrogates sister chromatid cohesion specifically in STAG2-mutated cells, leading to mitotic catastrophe, defective cell division, and apoptosis, while sparing STAG2 wild-type cells.\",\n      \"method\": \"CRISPR/RNAi knockouts in isogenic cell lines, mitotic catastrophe assays, chromosome spread analysis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — isogenic genetic models with mechanistic readout (cohesion loss → mitotic catastrophe), replicated across cancer contexts\",\n      \"pmids\": [\"28691904\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Cohesin-SA1 preferentially localizes to CTCF sites genome-wide and stabilizes topologically associating domain (TAD) boundaries together with CTCF, whereas cohesin-SA2 localizes to enhancers lacking CTCF and promotes cell-type-specific enhancer–promoter contacts.\",\n      \"method\": \"ChIP-seq, Hi-C, siRNA depletion of SA1 vs SA2 in human cells\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-wide ChIP-seq and Hi-C with orthogonal depletions, multiple cell types\",\n      \"pmids\": [\"29867216\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"In embryonic stem cells, cohesin-SA1 preserves TAD boundaries and disrupts Polycomb domain long-range interaction networks, consistent with a loop extrusion mode of action together with CTCF. Cohesin-SA1 and cohesin-SA2 have opposing effects on Polycomb domain compaction.\",\n      \"method\": \"SA1/SA2 depletion in mESCs, Hi-C, ChIP-seq, RNA-seq\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-wide Hi-C and ChIP-seq with KO cells and multiple orthogonal methods\",\n      \"pmids\": [\"31216471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Stag1 and Stag2 bind a shared set of genomic loci in hematopoietic stem and progenitor cells, but a subset of Stag2 binding sites is not occupied by Stag1 even when Stag2 is absent; concurrent loss of both Stag1 and Stag2 abrogates hematopoiesis whereas Stag2 loss alone primarily affects chromatin accessibility and lineage-specification gene transcription.\",\n      \"method\": \"Conditional KO in HSPCs, ChIP-seq, ATAC-seq, differentiation assays\",\n      \"journal\": \"Cell stem cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with ChIP-seq and ATAC-seq, isogenic comparison\",\n      \"pmids\": [\"31495782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"During G1 phase, cohesin-STAG1 is acetylated by ESCO1 and protected from WAPL-dependent removal, resulting in hours-long chromatin residence time (vs. minutes for cohesin-STAG2). CTCF and ESCO1 together enable cohesin-STAG1 to form long, stable chromatin loops demarcating TAD boundaries.\",\n      \"method\": \"Auxin-inducible degron depletion, FRAP, Hi-C, ChIP-seq, cohesin acetylation assays\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (FRAP, Hi-C, biochemistry) with inducible depletion system\",\n      \"pmids\": [\"32065581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"SA1 and SA2 directly bind RNA-containing substrates (ssRNA, dsRNA, RNA:DNA hybrids, R-loops) and localize to R-loop regions on dsDNA, with SA1/SA2 ChIP-seq sites significantly overlapping DRIP-seq R-loop sites.\",\n      \"method\": \"Bulk biochemical RNA binding assays, AFM, single-molecule DNA tightrope assay, ChIP-seq/DRIP-seq overlap analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with single-molecule imaging plus genomic overlap analysis\",\n      \"pmids\": [\"32352519\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Chemical-genetic degradation of STAG1 protein via an inducible degron system causes loss of sister chromatid cohesion and rapid cell death selectively in STAG2-deficient cells. X-ray crystallography identified the STAG1 regions that interact with the RAD21 subunit of cohesin; mutations abrogating this interaction selectively compromise viability of STAG2-deficient cells.\",\n      \"method\": \"Inducible degron system, X-ray crystallography of STAG1–RAD21 interface, biochemical interaction assays, cell viability assays\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure combined with biochemical validation and cell-based functional assays\",\n      \"pmids\": [\"32467316\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The N-terminus of STAG1 is required for maintaining rRNA expression, nucleolar integrity, and repression of the two-cell (2C) state in mouse embryonic stem cells. Multiple naturally occurring Stag1 isoforms with different N-termini exist, and skewing their balance impacts cell identity.\",\n      \"method\": \"Stag1 isoform overexpression/depletion in mESCs, RNA-seq, immunofluorescence of nucleolar markers, 2C reporter assays\",\n      \"journal\": \"Stem cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KD/OE with specific phenotypic readouts but single study\",\n      \"pmids\": [\"37802073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"TRF1, TIN2, and SA1 work synergistically to compact telomeric DNA; SA1 alone can drive DNA compaction, and maximal compaction requires all three proteins together. Compaction by TRF1-TIN2 and SA1 is additive, and the ternary system drives strong intermolecular aggregation as required for chromosome cohesion.\",\n      \"method\": \"Nanochannel-stretched DNA compaction assay, atomic force microscopy of intermolecular aggregation\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with single-molecule imaging, single study\",\n      \"pmids\": [\"37081787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"SA1 (STAG1) is a nuclear protein of ~120 kDa encoded by a novel gene family; its mRNA is ubiquitously expressed but protein is predominantly detected in hematopoietic organs, suggesting post-transcriptional regulation. A closely related paralog SA2 shares 71% homology.\",\n      \"method\": \"cDNA cloning, Northern blot, immunoprecipitation/Western blot, subcellular fractionation\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — molecular cloning with subcellular fractionation identifying nuclear localization\",\n      \"pmids\": [\"9305759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SA1 contains functional N-terminal nuclear localization signal (NLS) that directs nuclear import; this NLS is functional in both yeast and HeLa cells, whereas SA2 has multiple NLS and NES signals enabling nucleocytoplasmic shuttling.\",\n      \"method\": \"Fluorescent fusion protein expression in S. cerevisiae and HeLa cells, NLS/NES mutagenesis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mutagenesis with localization readout validated across two cell systems\",\n      \"pmids\": [\"22715410\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"STAG1 binds tightly to the trimeric SMC1/SMC3/SCC1 complex and together they weakly but cooperatively bind DNA. NIPBL tightly binds DNA as an anchor during loop extrusion. STAG1 creates an ATP-modulated DNA binding site upon interaction with SMC1/SMC3/SCC1 that is important for repeated DNA grabbing and release during loop extrusion.\",\n      \"method\": \"Mass photometry, quantitative biomolecular interaction measurements, cohesin mutant analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — quantitative single-molecule mass photometry reconstitution with mutant analysis\",\n      \"pmids\": [\"40763028\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"KPT-6566 identified as a dual inhibitor of STAG1 and STAG2 that directly binds both proteins and disrupts their interactions with SCC1 and double-stranded DNA, causing premature chromosome separation and impaired DNA damage repair.\",\n      \"method\": \"Fluorescence polarization high-throughput screening, biochemical/biophysical binding assays, metaphase chromosome spread assay, DNA damage assays\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding demonstrated with FP assay and functional cellular validation, single study\",\n      \"pmids\": [\"39541712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The AT-hook motif unique to the STAG1 N-terminus promotes DNA loop extrusion efficiency without altering ATPase activity or DNA binding; in human somatic cells the AT-hook is required for stable cohesin-chromatin association during G1 phase and for TAD and chromatin loop formation, but is dispensable for sister chromatid cohesion.\",\n      \"method\": \"Single-molecule loop extrusion assays, AT-hook mutagenesis, Hi-C, ChIP-seq, cohesion assays in human cells\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — single-molecule reconstitution with mutagenesis, validated in cells with Hi-C and ChIP-seq\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CTCF N-terminal region contains two motifs (YDF and KTYQR) that impede cohesin-STAG1-mediated loop extrusion by different mechanisms: KTYQR fully impedes loop extrusion while YDF converts cohesin into a unidirectional extruder by strengthening STAG1 affinity to DNA.\",\n      \"method\": \"Single-molecule loop extrusion assays with CTCF NTR fragments, cohesin-STAG1 containing complexes\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — single-molecule reconstitution, preprint not yet peer-reviewed\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"STAG1 is a subunit of cohesin-STAG1 that confers unique properties to the complex: its N-terminal AT-hook motif directly binds telomeric DNA to mediate sister telomere cohesion and promotes more efficient DNA loop extrusion than cohesin-STAG2; ESCO1-mediated acetylation and CTCF together protect cohesin-STAG1 from WAPL removal, enabling hours-long chromatin residence and formation of stable, long-range TAD-boundary loops; it also binds RNA and R-loops, interacts with RAD21 through defined SA-binding motifs, and its N-terminus regulates nucleolar integrity and rRNA expression in stem cells.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll systematically classify each paper before extracting discoveries.\n\n**Classification decisions:**\n\n**KEEP (canonical STAG1/cohesin subunit):**\n- [1] PMID:22415365 - SA1 (STAG1) cohesin, mouse model, telomere replication\n- [2] PMID:19822671 - SA1/SA2 cohesins, telomere/centromere cohesion\n- [3] PMID:32387133 - EXCLUDE: circRNA STAG1 (alt-locus product, circular RNA)\n- [4] PMID:29867216 - STAG1/STAG2 cohesin, 3D genome\n- [5] PMID:32065581 - cohesinSTAG1, chromatin loops\n- [6] PMID:31495782 - Stag1/Stag2, HSC, cohesin\n- [7] PMID:22415368 - SA1 cohesin, gene regulation\n- [8] PMID:32571217 - EXCLUDE: Bacillus cereus SA1 (bacterium)\n- [9] PMID:28691904 - STAG1/STAG2 synthetic lethality\n- [10] PMID:31216471 - cohesin-SA1/SA2, TADs, ESCs\n- [11] PMID:24324008 - cohesin-SA1/SA2, DSB repair\n- [12] PMID:2837486 - EXCLUDE: cystatin SA-1 (alias collision)\n- [13] PMID:11568975 - EXCLUDE: STAG1/PMEPA1 alias collision (different gene - solid tumor associated gene 1 = PMEPA1)\n- [14] PMID:14639658 - EXCLUDE: STAG1/PMEPA1 alias collision\n- [15] PMID:10188880 - EXCLUDE: SA-1 tumor model (mouse sarcoma cell line)\n- [16] PMID:28430577 - STAG1/STAG2 synthetic lethality\n- [17] PMID:9305759 - SA-1 stromal antigen, KEEP (original cloning of STAG1)\n- [18] PMID:20971822 - EXCLUDE: GRAALL-SA1 clinical trial\n- [19] PMID:32294612 - STAG1/STAG2, 3D genome review\n- [20] PMID:32352519 - cohesin SA1/SA2 RNA binding\n- [21] PMID:27374909 - EXCLUDE: bacteriophage SA1\n- [22] PMID:28119487 - STAG1 mutations, cohesinopathy\n- [23] PMID:36825945 - EXCLUDE: ML-SA1 (TRPML1 agonist)\n- [24] PMID:15361841 - EXCLUDE: STAG1/PMEPA1 alias collision (this is PMEPA1)\n- [25] PMID:23874961 - Rad21-SA1/SA2 interaction, cohesin\n- [26] PMID:32353077 - EXCLUDE: Bacillus cereus SA1\n- [27] PMID:9305759 - KEEP (original SA-1/STAG1 cloning)\n- [28] PMID:23811524 - EXCLUDE: microalga SA1\n- [29] PMID:32858116 - EXCLUDE: ML-SA1 TRPML agonist\n- [30] PMID:23729739 - SA1 AT-hook, telomere cohesion\n- [31] PMID:1830833 - EXCLUDE: SA-1 anti-DNA antibody idiotype (SLE)\n- [32] PMID:25266962 - EXCLUDE: ML-SA1 TRPML agonist\n- [33] PMID:32467316 - STAG1/STAG2, synthetic lethality, RAD21 interaction, X-ray crystallography\n- [34] PMID:16636007 - EXCLUDE: Thermus thermophilus SA1\n- [35] PMID:24865325 - EXCLUDE: microalga SA1\n- [36] PMID:30455068 - EXCLUDE: Phytoplasma SA1\n- [37] PMID:14990328 - EXCLUDE: SA-1 tumor cell line electrotransfection\n- [38] PMID:14991566 - EXCLUDE: SA1 sensory neurons (different entity)\n- [39] PMID:27298259 - SA1 DNA binding, TRF1 interaction, telomere cohesion\n- [40] PMID:23122533 - EXCLUDE: cystatin SA1\n- [41] PMID:29481647 - SA1/SA2 cohesins, telomere homeostasis\n- [42] PMID:582093 - EXCLUDE: Vibrio SA1\n- [43] PMID:27851894 - EXCLUDE: Novosphingobium SA1\n- [44] PMID:39423944 - STAG1/TP53/P21 signaling - KEEP (involves STAG1 protein in chondrocytes)\n- [45] PMID:24068240 - EXCLUDE: Clostridium SA-1\n- [46] PMID:35572667 - EXCLUDE: bacteriophage SA1\n- [47] PMID:33793924 - EXCLUDE: SA 1-hydroxylase (plant enzyme)\n- [48] PMID:34687820 - EXCLUDE: ML-SA1 TRPML agonist\n- [49] PMID:34440290 - STAG1 frameshift, cohesinopathy - clinical, no mechanism\n- [50] PMID:33365313 - Stag1/Stag2 zebrafish, haematopoiesis - KEEP\n- [51] PMID:17318295 - EXCLUDE: STAG1/PMEPA1 alias collision\n- [52] PMID:29908000 - EXCLUDE: Serpin A1 peptide SA1-III\n- [53] PMID:39435653 - EXCLUDE: bacteriophage SA1\n- [54] PMID:27549371 - cohesin SA1, colorectal cancer, chromatin - KEEP\n- [55] PMID:29051988 - EXCLUDE: Novosphingobium SA1\n- [56] PMID:1765979 - EXCLUDE: SA1 anti-DNA idiotype\n- [57] PMID:27341316 - STAG1/STAG2 in OSCC - KEEP (LOH at STAG1 locus)\n- [58] PMID:9844765 - EXCLUDE: SA1 idiotype SLE\n- [59] PMID:22715410 - SA1/STAG1 nuclear import/export signals - KEEP\n- [60] PMID:39336775 - STAG1 variant, twins - clinical, minimal mechanism\n- [61] PMID:8099047 - EXCLUDE: SA-1 monoclonal antibody antigen\n- [62] PMID:37802073 - STAG1 N-terminus, nucleolar integrity, 2C state - KEEP\n- [63] PMID:16233594 - EXCLUDE: Acidovorax SA1 bacterium\n- [64] PMID:12732949 - EXCLUDE: Anabaena SA1 mutant\n- [65] PMID:18297793 - EXCLUDE: Sphingomonas SA1\n- [66] PMID:12070691 - EXCLUDE: Acidovorax SA1\n- [67] PMID:38923811 - EXCLUDE: ML-SA1 TRPML1 agonist\n- [68] PMID:36014008 - EXCLUDE: jumbo phage SA1\n- [69] PMID:39224759 - STAG1 variant - clinical, no mechanism\n- [70] PMID:36029 - EXCLUDE: Vibrio SA1\n- [71] PMID:17166409 - EXCLUDE: STAG1/PMEPA1 alias\n- [72] PMID:28334389 - EXCLUDE: aphid biotype SA1\n- [73] PMID:17672324 - EXCLUDE: bacterial strain SA1\n- [74] PMID:29900221 - EXCLUDE: fungal isolate Sa-1\n- [75] PMID:29748404 - EXCLUDE: archaeal division SA1\n- [76] PMID:8187053 - EXCLUDE: SA-1 tumor cell line\n- [77] PMID:8533299 - EXCLUDE: Cryptosporidium SA-1 antigen\n- [78] PMID:32622987 - EXCLUDE: bacterial TA system\n- [79] PMID:23405302 - EXCLUDE: Simiduia SA1 bacterium\n- [80] PMID:40763028 - STAG1/NIPBL/cohesin loop extrusion - KEEP\n- [81] PMID:37081787 - TRF1/TIN2/SA1 telomere compaction - KEEP\n- [82] PMID:39541712 - STAG1/2 inhibitor KPT-6566 - KEEP\n- [83] PMID:40625213 - STAG1 variant - clinical\n- [84] PMID:32446378 - EXCLUDE: YoeBSa1/YefMSa1 S. aureus toxin\n- [85] PMID:36694417 - STAG1::USP6 fusion - KEEP (novel fusion)\n- [86] PMID:40769226 - EXCLUDE: bacteriophage ECP-SA1\n- [87] PMID:4081640 - EXCLUDE: SA-1 monoclonal antibody\n- [88] PMID:40899458 - STAG1 splicing variant - KEEP (functional splicing study)\n- [89] PMID:40222411 - EXCLUDE: ML-SA1 TRPML1 agonist\n- [90] PMID:41502392 - STAG2/SA1 Drosophila, PARP - KEEP\n- [91] PMID:40361893 - STAG1 clinical case\n- [92] PMID:41669812 - STAG1 haploinsufficiency functional study - KEEP\n- [93] PMID:41210242 - STAG1 haploinsufficiency, NMD - KEEP\n- [94] PMID:2628751 - EXCLUDE: vaccinia virus DNA fragment\n- [95] bio_10.1101_2025.11.17.688764 - STAG1 AT-hook, loop extrusion - KEEP (preprint)\n- [96] bio_10.1101_2025.10.24.684407 - Stag1/Stag2 post-mitotic genome refolding - KEEP (preprint)\n- [97] bio_10.1101_2025.04.18.649500 - Drosophila SA1 (STAG1/2 homolog), PARP - KEEP (preprint, same findings as [90])\n- [98] bio_10.1101_2024.09.17.613402 - STAG1 DNA binding patches, loop extrusion - KEEP (preprint)\n- [99] bio_10.1101_2025.01.26.634934 - STAG1 DNA affinity, CTCF stalling - KEEP (preprint)\n- [100] bio_10.1101_2024.10.14.24314843 - STAG1 rare coding variants, schizophrenia - no mechanism\n\n**Gene2pubmed additional papers:**\n- Most are PMEPA1/TMEPAI papers (alias collision) or general proteomics - EXCLUDE most\n- PMID:9305759 already covered\n- PMID:32296183, 33961781 - general interactome, not mechanistic for STAG1 specifically\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1997,\n      \"finding\": \"SA-1 (STAG1) was identified as a novel nuclear protein of ~120 kDa expressed in hematopoietic organs, cloned from mouse stromal cell and human thymus cDNA libraries. A closely related paralog SA-2 was also cloned, sharing 71% identity. Subcellular fractionation demonstrated nuclear localization of SA-1 protein.\",\n      \"method\": \"Lambda gt11 cDNA library screening with monoclonal antibody B92, RACE-PCR, immunoprecipitation/Western blotting, subcellular fractionation\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — original cloning with protein identification and nuclear localization by fractionation, single lab\",\n      \"pmids\": [\"9305759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Cohesin-SA1 and cohesin-SA2 have distinct, non-redundant roles in sister chromatid cohesion: SA1 is specifically required for telomere cohesion after DNA replication in S phase, while SA2 is required for centromere cohesion. Depletion of SA1 but not SA2 caused loss of sister telomere cohesion, inability to repair chromatid breaks, and sister telomere loss. The telomere cohesion function of SA1 was shown to be linked to the telomeric protein TIN2.\",\n      \"method\": \"siRNA depletion of SA1 or SA2 in human cells, fluorescence in situ hybridization (FISH) for telomere/centromere cohesion, live-cell imaging, cell cycle analysis\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal depletion experiments with specific phenotypic readouts replicated across multiple conditions, independently confirmed by other labs\",\n      \"pmids\": [\"19822671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SA1 is largely responsible for cohesin accumulation at gene promoters and CTCF-bound sites genome-wide. SA1 ablation alters transcription of genes related to Cornelia de Lange syndrome, and cohesin-SA1 positively regulates expression of myc and protocadherin genes at their promoters — a function that cannot be compensated by cohesin-SA2.\",\n      \"method\": \"ChIP-seq in wild-type and SA1-null mouse embryonic fibroblasts, RNA-seq, SA1-null mouse embryo generation\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-wide ChIP-seq and transcriptomics in genetic knockout model, multiple orthogonal methods\",\n      \"pmids\": [\"22415368\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Complete SA1 ablation in mice causes embryonic lethality; heterozygous SA1-deficient animals show shorter lifespan and earlier tumor onset. SA1-null MEFs display increased aneuploidy and chromosome segregation defects arising specifically from defective telomere replication, not from impaired centromeric cohesion (which depends on cohesin-SA2). SA1 is thus specifically required for telomere replication-associated cohesion.\",\n      \"method\": \"SA1-null mouse model generation, MEF culture, cytogenetic analysis, BrdU incorporation, metaphase spreads, telomere FISH\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo mouse knockout with multiple orthogonal phenotypic assays, mechanistic distinction from SA2 established\",\n      \"pmids\": [\"22415365\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SA1 binds directly to telomeric DNA through a unique AT-hook motif in its N-terminal domain (absent in SA2). Overexpression of SA1 alone is sufficient to induce cohesion at telomeres independently of the cohesin ring, and this requires the AT-hook-containing N-terminal domain. SA1 is highly enriched at telomeres by ChIP, decreases at mitosis when cohesion is resolved, and increases when cohesion persists.\",\n      \"method\": \"ChIP in human cells, overexpression of SA1 truncation mutants, AT-hook mutation analysis, telomere cohesion assays by FISH\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain mapping with mutagenesis, ChIP enrichment, functional overexpression assays with specific phenotypic readouts\",\n      \"pmids\": [\"23729739\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SA1 and SA2 interact with RAD21 (Rad21/Scc1) through two distinct SA-binding motifs on RAD21: an N-terminal motif (aa 60-81) and a middle-part α-helical motif (aa 383-392). Mutation of three conserved residues (L385, F389, T390) in the α-helical motif significantly disrupts Rad21-SA1/2 interaction without affecting Smc1-Smc3-Rad21 interaction. Mutant Rad21 fails to rescue precocious chromosome separation caused by endogenous Rad21 depletion.\",\n      \"method\": \"In vitro binding assays, co-immunoprecipitation (ex vivo), deletion mapping, site-directed mutagenesis, chromosome segregation rescue assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro mapping combined with mutagenesis and functional rescue, multiple orthogonal binding assays\",\n      \"pmids\": [\"23874961\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Cohesin-SA2 (not SA1) is the main complex co-recruited with the cohesin-loading factor NIPBL to DNA damage sites in an S/G2-phase-specific manner. Replacing the diverged C-terminal region of SA1 with the corresponding region of SA2 confers DNA damage site accumulation activity on SA1. SA2 depletion (not SA1) decreased sister chromatid homologous recombination repair and affected DNA repair pathway choice. Both cohesin complexes function in the intra-S checkpoint.\",\n      \"method\": \"Laser microirradiation with live-cell imaging, siRNA depletion, domain-swap chimera construction, sister chromatid exchange assay, cell cycle checkpoint analysis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal depletion, chimeric protein functional analysis, multiple orthogonal assays distinguishing SA1 vs SA2 functions\",\n      \"pmids\": [\"24324008\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SA1 displays two-state DNA binding behavior: non-specific 1D free diffusion for DNA searching, and subdiffusive sliding at telomeric regions for recognition. The AT-hook motif in SA1 modulates both non-specific DNA binding and subdiffusive dynamics over telomeric sequences. TRF1 tethers SA1 within telomeric regions, and together SA1 and TRF1 form longer DNA-DNA pairing tracts than TRF1 alone.\",\n      \"method\": \"Single-molecule fluorescence imaging, 1D diffusion tracking on DNA tightropes, AT-hook mutant SA1, atomic force microscopy\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — single-molecule reconstitution with mutagenesis and AFM structural validation\",\n      \"pmids\": [\"27298259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"STAG1 and STAG2 display a strong synthetic lethal interaction: STAG1 loss abrogates sister chromatid cohesion specifically in STAG2-mutated cells (not wild-type), leading to mitotic catastrophe, defective cell division, and apoptosis. Restoration of STAG2 expression alleviates dependence on STAG1. This synthetic lethality was demonstrated across bladder cancer and Ewing sarcoma cell lines.\",\n      \"method\": \"CRISPR-Cas9 knockout, siRNA depletion, metaphase spread analysis, cell viability/proliferation assays, STAG2 re-expression rescue\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis across multiple cell line models with rescue experiment, replicated by independent lab\",\n      \"pmids\": [\"28691904\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Cohesin-SA1 and cohesin-SA2 have distinct genomic distributions and non-redundant contributions to 3D genome organization: cohesin-SA1 preferentially stabilizes topologically associating domain (TAD) boundaries together with CTCF, while cohesin-SA2 promotes cell-type-specific enhancer-promoter contacts independently of CTCF. Loss of SA2 rewires local chromatin contacts in a way that cannot be compensated by SA1.\",\n      \"method\": \"ChIP-seq for SA1 and SA2, Hi-C, RNA-seq in SA1- or SA2-depleted human cells, insulation score analysis\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-wide ChIP-seq, Hi-C, and transcriptomics with isogenic depletions, multiple orthogonal methods\",\n      \"pmids\": [\"29867216\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Stag1 and Stag2 bind a shared set of genomic loci in hematopoietic stem and progenitor cells, but a component of Stag2 binding sites is unoccupied by Stag1. Stag2 loss decreased chromatin accessibility and transcription of lineage-specification genes (Ebf1, Pax5), increasing self-renewal and reducing B cell commitment. Concurrent loss of both Stag1 and Stag2 abrogated hematopoiesis entirely, demonstrating partial redundancy.\",\n      \"method\": \"Conditional Stag2 deletion in mouse HSPCs, ChIP-seq for Stag1 and Stag2, ATAC-seq, RNA-seq, flow cytometry, hematopoietic reconstitution assay\",\n      \"journal\": \"Cell stem cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic model with genome-wide chromatin and transcriptional profiling, epistasis between Stag1 and Stag2\",\n      \"pmids\": [\"31495782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"In mouse embryonic stem cells, cohesin-SA1 disrupts Polycomb domain interaction networks and preserves TAD borders, whereas cohesin-SA2 facilitates Polycomb domain compaction through PRC1 recruitment and promotes long-range interactions between Polycomb-bound promoters. These two modes of action reflect distinct roles: SA1 in loop extrusion with CTCF for TAD organization, SA2 in local transcriptional hub organization.\",\n      \"method\": \"ChIP-seq, Hi-C, ChIA-PET, siRNA depletion of SA1 or SA2 in mESCs, PRC1 co-immunoprecipitation\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genome-wide methods in genetic depletion model with protein interaction validation\",\n      \"pmids\": [\"31216471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ESCO1 acetyltransferase and CTCF protect a subset of cohesin-STAG1 complexes from removal by WAPL, enabling formation of long chromatin loops. During G1 phase, acetylated cohesin-STAG1 binds chromatin for hours while cohesin-STAG2 binds for minutes. ESCO1 and CTCF together contribute to boundary formation in chromatin looping, consistent with a nested loop extrusion model where stable cohesin-STAG1 loops demarcate boundaries for more transient cohesin-STAG2 extrusion.\",\n      \"method\": \"Auxin-inducible degron (AID) system for acute protein depletion, Hi-C, ChIP-seq, FRAP, co-immunoprecipitation\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — inducible degron system with Hi-C, FRAP quantifying residence time, and ChIP-seq; multiple orthogonal methods\",\n      \"pmids\": [\"32065581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cohesin SA1 and SA2 are direct RNA-binding proteins. Both bind ssRNA, dsRNA, RNA:DNA hybrids, and R-loops in vitro, and localize to RNA-containing regions on dsDNA in single-molecule experiments. SA1 and SA2 binding sites overlap significantly with R-loop (DRIP-seq) sites in cells, with most R-loop-localized SA1/SA2 also binding other cohesin subunits.\",\n      \"method\": \"Electrophoretic mobility shift assay (EMSA), atomic force microscopy (AFM), DNA tightrope single-molecule assay, ChIP-seq/DRIP-seq overlap analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro biochemical binding assays with multiple RNA substrates, single-molecule validation, and genomic overlap analysis\",\n      \"pmids\": [\"32352519\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"STAG1 was identified as the most prominent and selective genetic dependency of STAG2-deficient cells in genome-wide CRISPR screens. X-ray crystallography identified the STAG1 regions that interact with RAD21. STAG1 mutations that abrogate interaction with RAD21 selectively compromise viability of STAG2-deficient cells. Chemical-genetic degradation of STAG1 using an inducible degron causes loss of sister chromatid cohesion and rapid cell death specifically in STAG2-deficient cells.\",\n      \"method\": \"Genome-wide CRISPR screen, X-ray crystallography of STAG1-RAD21 interface, auxin-inducible degron system, metaphase spread, cell viability assay\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — X-ray crystal structure combined with genome-wide screen, inducible degradation system, and mutagenesis\",\n      \"pmids\": [\"32467316\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Nuclear import of human SA1/STAG1 depends on a functional N-terminal NLS. In contrast to SA2 (which has multiple functional NLS and NES signals), SA1 has only one functional NLS, located at its N-terminus, with no functional nuclear export signals identified, suggesting SA1 is constitutively nuclear.\",\n      \"method\": \"In silico NLS/NES prediction, expression of fluorescently tagged SA1/SA2 in Saccharomyces cerevisiae, validation in HeLa cells by confocal microscopy\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — localization by fluorescence in two cell systems, but functional consequence not directly tested\",\n      \"pmids\": [\"22715410\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"TRF1, TIN2, and SA1 act synergistically to compact telomeric DNA. SA1 alone can compact telomeric DNA, and compaction by the full ternary TRF1-TIN2-SA1 system is additive (TRF1-TIN2 component plus SA1 component). Atomic force microscopy confirmed that all three proteins together drive strong intermolecular aggregation as required for chromosome cohesion.\",\n      \"method\": \"Nanochannel DNA stretching to quantify compaction, atomic force microscopy of intermolecular aggregation, systematic comparison of protein subsets\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstitution with single-molecule imaging and AFM, but single lab\",\n      \"pmids\": [\"37081787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"STAG1 is the dominant paralog in mouse embryonic stem cells and is required for pluripotency. The N-terminus of STAG1 specifically represses the two-cell (2C) embryonic state by maintaining rRNA expression and nucleolar integrity. Skewing the balance of naturally occurring STAG1 isoforms impacts cell identity. This role in nucleolar structure is distinct from STAG1's role in chromatin loop organization.\",\n      \"method\": \"mESC STAG1 depletion, isoform-specific overexpression, RNA-seq for 2C gene markers, rRNA expression analysis, nucleolar morphology by immunofluorescence\",\n      \"journal\": \"Stem cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with domain-specific rescue, specific phenotypic readouts, single lab\",\n      \"pmids\": [\"37802073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"KPT-6566 was identified as a dual inhibitor of STAG1 and STAG2 that disrupts their interactions with SCC1 and double-stranded DNA. KPT-6566 causes premature chromosome separation, chromosome damage, impaired DNA damage repair, accumulation of double-strand breaks, and cell apoptosis in HeLa cells. It also sensitizes cells to PARP inhibitor Olaparib and NHEJ inhibitor UMI-77.\",\n      \"method\": \"Fluorescence polarization high-throughput screening, biochemical/biophysical binding assays, metaphase chromosome spread assay, γH2AX immunofluorescence, cell viability assay\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — biochemical target engagement plus cellular functional assays, single lab\",\n      \"pmids\": [\"39541712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"STAG1 binds tightly to the trimeric SMC1/SMC3/SCC1 complex; this tetramer then weakly but cooperatively binds DNA. NIPBL binds DNA tightly and acts as a DNA anchor during the mechanochemical loop extrusion cycle. An ATP-modulated DNA binding site created by the STAG1-SMC1/SMC3/SCC1 interaction is important for repeated DNA grabbing and release critical to loop extrusion.\",\n      \"method\": \"Mass photometry to quantify biomolecular interactions, systematic measurement of cohesin subunit assembly and DNA binding affinities, cohesin mutant analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — quantitative single-molecule mass photometry of all key interactions in reconstituted system, multiple mutants tested\",\n      \"pmids\": [\"40763028\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cohesin-STAG1 specifically contributes to stabilization of TAD boundaries together with CTCF, while cohesin-STAG2 promotes cell-type-specific contacts between enhancers and promoters independently of CTCF. These findings establish distinct 3D genome organization roles for the two cohesin variants.\",\n      \"method\": \"ChIP-seq, Hi-C in multiple human cell types with SA1/SA2 depletion, insulation score analysis\",\n      \"journal\": \"Current opinion in genetics & development\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — review synthesizing existing data, no new experimental findings\",\n      \"pmids\": [\"32294612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cohesin-STAG1 extrudes DNA loops more efficiently than cohesin-STAG2 in single-molecule assays, despite comparable ATPase activity and topological DNA entrapment. The AT-hook motif unique to the STAG1 N-terminus promotes loop extrusion without altering ATPase activity or DNA binding. In human somatic cells, the AT-hook is required for stable cohesin-chromatin association during G1 phase (but dispensable for sister chromatid cohesion), and its mutation markedly impairs TAD and chromatin loop formation.\",\n      \"method\": \"Single-molecule DNA loop extrusion assays, ATPase activity assay, Hi-C in cells expressing AT-hook mutant STAG1, ChIP-seq for cohesin residence\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — single-molecule reconstitution with domain mutagenesis validated by Hi-C and ChIP-seq in cells\",\n      \"pmids\": [\"bio_10.1101_2025.11.17.688764\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Stag1 depletion has negligible impact on post-mitotic genome restructuring or transcription reactivation during mitosis-to-G1 transition, whereas Stag2 orchestrates early-G1 chromatin remodeling. Simultaneous depletion of both Stag proteins results in synergistic loss of virtually all structural loops and more severe transcriptional disruption than individual deletions, demonstrating that Stag1 provides compensatory support during genome refolding.\",\n      \"method\": \"Auxin-inducible degron for acute Stag1/Stag2 depletion during mitosis-to-G1 transition, Hi-C time course, RNA-seq, ChIP-seq\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — inducible degron system with Hi-C time course, preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.10.24.684407\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Molecular dynamics simulations predict that STAG1, together with SMC1, SMC3, and NIPBL, forms a DNA clamping patch group that facilitates DNA bending and capture within the cohesin ring, identifying specific high-affinity DNA binding patches on STAG1 implicated in loop extrusion.\",\n      \"method\": \"Molecular dynamics simulations at amino-acid residue resolution, dissociation rate constant quantification as proxy for DNA affinity\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational prediction only, not experimentally validated\",\n      \"pmids\": [\"bio_10.1101_2024.09.17.613402\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The CTCF N-terminal region contains two motifs (YDF and KTYQR) that hinder cohesin-mediated loop extrusion. The KTYQR motif fully impedes loop extrusion activity, while YDF converts cohesin into a unidirectional extruder by strengthening the affinity of STAG1 to DNA, revealing that CTCF stalls loop extrusion partly through modulating STAG1's DNA affinity.\",\n      \"method\": \"Single-molecule DNA loop extrusion assay with CTCF N-terminal region fragments, force measurements, directionality analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — single-molecule reconstitution with domain peptides, preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.01.26.634934\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Loss of SA1 (the Drosophila STAG1/2 homolog) during brain development leads to defects in neural stem cell differentiation and promotion of tumorigenesis. PARP inhibition combined with SA1/STAG2 depletion results in apoptosis in vitro and in vivo, and reduction of PARP activity ameliorates tumor-associated phenotypes of SA1-deficient Drosophila tissue, suggesting synthetic lethality between cohesin and PARP activity.\",\n      \"method\": \"Drosophila RNAi knockdown of SA1 in brain tissue, human cell STAG2 depletion, PARP inhibitor treatment, tumor phenotype scoring, apoptosis assays\",\n      \"journal\": \"Disease models & mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo Drosophila model (ortholog) with pharmacological intervention and in vitro validation, cross-species\",\n      \"pmids\": [\"41502392\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"A frameshift STAG1 variant (c.500dup) causes dramatic reduction in STAG1 mRNA and protein through nonsense-mediated mRNA decay (NMD), establishing haploinsufficiency as the underlying pathogenic mechanism for STAG1-associated intellectual developmental disorder (MRD47).\",\n      \"method\": \"Trio whole-exome sequencing, mutant STAG1 expression vector transfection into HEK293T cells, qRT-PCR for mRNA levels, Western blot for protein levels, NMD pathway analysis\",\n      \"journal\": \"Frontiers in pediatrics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro functional study with mRNA and protein quantification demonstrating NMD mechanism, single lab\",\n      \"pmids\": [\"41210242\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"STAG1 (SA1) is a core subunit of one of two vertebrate cohesin variants (cohesin-STAG1), functioning as a nuclear protein that (1) mediates sister chromatid cohesion specifically at telomeres via a unique N-terminal AT-hook motif that binds telomeric DNA and cooperates with TRF1/TIN2, (2) drives DNA loop extrusion more efficiently than STAG2-containing cohesin through its AT-hook, enabling stable long chromatin loops at TAD boundaries in conjunction with CTCF and the acetyltransferase ESCO1 which protects it from WAPL-mediated eviction, (3) binds directly to RNA and R-loops, (4) interacts with RAD21/SCC1 through two defined binding motifs, and (5) is partially redundant with STAG2 for sister chromatid cohesion and cell survival, forming a synthetic lethal pair exploitable in STAG2-mutant cancers.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"STAG1 is a cohesin subunit that confers distinct genome-organizing and telomere-protective functions to the cohesin-STAG1 complex, acting through its unique N-terminal AT-hook motif to bind telomeric DNA, promote efficient DNA loop extrusion, and stabilize TAD boundaries in cooperation with CTCF. Cohesin-STAG1 is specifically required for sister telomere cohesion after DNA replication, where SA1 directly binds telomeric repeats via its AT-hook and cooperates with TRF1 to compact and pair telomeric DNA [PMID:19822671, PMID:23729739, PMID:27298259, PMID:37081787]. ESCO1-mediated acetylation protects cohesin-STAG1 from WAPL-dependent removal, conferring hours-long chromatin residence that enables formation of stable, long-range CTCF-anchored chromatin loops demarcating TAD boundaries; STAG1 also creates an ATP-modulated DNA-binding site on the SMC1/SMC3/SCC1 trimer critical for repeated DNA grabbing during loop extrusion [PMID:32065581, PMID:29867216, PMID:40763028]. STAG1 additionally binds RNA and R-loops, regulates Polycomb domain organization and gene expression at promoters and CTCF sites, maintains nucleolar integrity and rRNA expression in stem cells, and is synthetic lethal with STAG2 because simultaneous loss abrogates sister chromatid cohesion [PMID:32352519, PMID:22415368, PMID:31216471, PMID:37802073, PMID:28691904].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Identification of SA1 as a nuclear protein and paralog of SA2 established the existence of two distinct STAG subunits in mammalian cohesin, raising the question of whether they have non-redundant functions.\",\n      \"evidence\": \"cDNA cloning, Northern blot, immunoprecipitation, and subcellular fractionation in human tissues\",\n      \"pmids\": [\"9305759\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No functional role assigned; only expression and localization determined\", \"Post-transcriptional regulation mechanism not identified\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Reciprocal depletion experiments revealed that cohesin-SA1 is specifically required for sister telomere cohesion while cohesin-SA2 mediates centromere cohesion, establishing the first non-redundant function for STAG1.\",\n      \"evidence\": \"siRNA depletion in human cells with fluorescence imaging of telomere/centromere cohesion\",\n      \"pmids\": [\"19822671\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of telomere specificity unknown\", \"Whether SA1 contacts telomeric DNA directly was untested\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Genetic knockout confirmed SA1 is required for telomere replication and showed that SA1 is largely responsible for cohesin accumulation at CTCF sites and gene promoters, linking it to transcriptional regulation of Cornelia de Lange syndrome-related genes and protocadherins.\",\n      \"evidence\": \"SA1 knockout mouse model with MEF analysis, genome-wide ChIP-seq, transcriptomics\",\n      \"pmids\": [\"22415365\", \"22415368\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which SA1 preferentially localizes to CTCF sites versus enhancers unknown\", \"Whether transcriptional effects are direct or secondary to chromatin architecture changes unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Domain mapping revealed that SA1's unique N-terminal AT-hook motif directly binds telomeric DNA and is sufficient to induce telomere cohesion independently of the cohesin ring, and structural analysis defined the SA1–RAD21 binding interface through two discrete SA-binding motifs.\",\n      \"evidence\": \"AT-hook mutagenesis and overexpression cohesion assays; in vitro binding, co-IP, mutagenesis and analytical ultracentrifugation of RAD21–SA1 interaction; domain swap between SA1/SA2\",\n      \"pmids\": [\"23729739\", \"23874961\", \"24324008\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No atomic-resolution structure of SA1–DNA complex\", \"How AT-hook cooperates with full cohesin ring during loop extrusion untested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Single-molecule imaging revealed SA1 uses two-state DNA binding — free diffusion for searching and subdiffusive sliding at telomeres — with TRF1 tethering SA1 to form extended DNA-DNA pairing tracts, providing a biophysical mechanism for telomere cohesion.\",\n      \"evidence\": \"Single-molecule fluorescence DNA tightrope assay, AFM, AT-hook mutagenesis\",\n      \"pmids\": [\"27298259\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Contribution of TIN2 and full shelterin complex not addressed\", \"In vivo single-molecule dynamics uncharacterized\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"The discovery of STAG1–STAG2 synthetic lethality established that simultaneous loss abrogates cohesion and causes mitotic catastrophe, defining STAG1 as a therapeutic vulnerability in STAG2-mutant cancers.\",\n      \"evidence\": \"CRISPR/RNAi in isogenic STAG2-mutant vs wild-type cell lines with cohesion and viability assays\",\n      \"pmids\": [\"28691904\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Therapeutic window in vivo not determined\", \"Contribution of non-cohesion functions (loop extrusion, transcription) to lethality unclear\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Genome-wide profiling demonstrated that cohesin-STAG1 preferentially occupies CTCF-bound sites and stabilizes TAD boundaries, while cohesin-STAG2 occupies enhancers, explaining the non-redundant chromatin-organizing roles of the two complexes.\",\n      \"evidence\": \"ChIP-seq and Hi-C after siRNA depletion of SA1 vs SA2 in human cells\",\n      \"pmids\": [\"29867216\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular determinants directing STAG1 to CTCF vs STAG2 to enhancers not identified\", \"Role of STAG1 acetylation in this partitioning not tested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Studies in embryonic stem cells and hematopoietic progenitors showed cohesin-STAG1 disrupts Polycomb domain long-range interactions and that STAG1 and STAG2 share many binding sites but have unique targets essential for hematopoiesis, revealing context-dependent functional specialization.\",\n      \"evidence\": \"SA1/SA2 depletion/conditional KO in mESCs and HSPCs with Hi-C, ChIP-seq, ATAC-seq, differentiation assays\",\n      \"pmids\": [\"31216471\", \"31495782\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Polycomb domain effects are direct or secondary to TAD boundary changes unknown\", \"Lineage-specific STAG1 contributions beyond hematopoiesis unexplored\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"ESCO1-mediated acetylation and CTCF together protect cohesin-STAG1 from WAPL removal, conferring hours-long chromatin residence and enabling stable long-range loop formation; SA1/SA2 also directly bind RNA and R-loops, adding a nucleic acid recognition dimension beyond DNA.\",\n      \"evidence\": \"Auxin-degron depletion with FRAP, Hi-C, ChIP-seq, cohesin acetylation assays; in vitro RNA binding, AFM, DRIP-seq overlap\",\n      \"pmids\": [\"32065581\", \"32352519\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of R-loop binding by STAG1 in vivo unknown\", \"Whether RNA binding is independent of or cooperative with DNA binding unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"X-ray crystallography of the STAG1–RAD21 interface and chemical-genetic degron experiments validated that disrupting this interface selectively kills STAG2-deficient cells, providing structural basis for therapeutic targeting.\",\n      \"evidence\": \"X-ray crystallography, inducible degron, biochemical interaction assays, cell viability assays\",\n      \"pmids\": [\"32467316\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No small-molecule inhibitor with clinical-grade selectivity demonstrated\", \"Whether allosteric sites on STAG1 are druggable untested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"The STAG1 N-terminus was shown to maintain nucleolar integrity and rRNA expression in stem cells, with naturally occurring Stag1 isoforms differing in their N-termini impacting cell identity and 2C-state repression, revealing a non-canonical role beyond chromosome cohesion.\",\n      \"evidence\": \"Stag1 isoform overexpression/depletion in mESCs, RNA-seq, nucleolar immunofluorescence, 2C reporter assays\",\n      \"pmids\": [\"37802073\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single study; independent replication needed\", \"Mechanism linking STAG1 N-terminus to nucleolar function not defined\", \"Whether this function operates through cohesin ring or independently unclear\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Quantitative reconstitution showed STAG1 creates an ATP-modulated DNA-binding site on the SMC1/SMC3/SCC1 trimer essential for repeated DNA grabbing during loop extrusion, providing a biochemical mechanism for STAG1's role in chromatin loop formation.\",\n      \"evidence\": \"Mass photometry, quantitative biomolecular interaction measurements, cohesin mutant analysis\",\n      \"pmids\": [\"40763028\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How STAG1 vs STAG2 quantitatively differ in this DNA-grabbing mechanism not directly compared\", \"Structural basis of ATP modulation at the STAG1 site unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the structural basis of STAG1 AT-hook interaction with telomeric DNA at atomic resolution, the functional significance of STAG1 RNA/R-loop binding in vivo, how STAG1 N-terminal isoforms regulate nucleolar function mechanistically, and whether STAG1-selective therapeutic targeting can achieve clinical efficacy in STAG2-mutant cancers.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No atomic structure of STAG1 AT-hook bound to telomeric DNA\", \"In vivo function of STAG1 RNA binding uncharacterized\", \"STAG1-selective inhibitor with therapeutic index not demonstrated\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [3, 6, 12, 18, 20]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [12]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [5, 13]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [5, 13, 18]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [16, 17]},\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [0, 1, 3, 7, 8]},\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [14]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [0, 1, 7]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [8, 9, 11, 18, 20]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [2, 14]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [4, 19]}\n    ],\n    \"complexes\": [\n      \"cohesin-STAG1 (SMC1/SMC3/RAD21/STAG1)\"\n    ],\n    \"partners\": [\n      \"RAD21\",\n      \"SMC1A\",\n      \"SMC3\",\n      \"CTCF\",\n      \"TRF1\",\n      \"TIN2\",\n      \"ESCO1\",\n      \"WAPL\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"Insufficient on-target evidence to synthesize a narrative — discovery timeline does not match the canonical STAG1 protein.\",\n  \"teleology\": [],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [],\n    \"localization\": [],\n    \"pathway\": [],\n    \"complexes\": [],\n    \"partners\": [],\n    \"other_free_text\": []\n  }\n}\n```"}