{"gene":"STAG1","run_date":"2026-06-10T07:46:42","timeline":{"discoveries":[{"year":2009,"finding":"Cohesin-SA1 is specifically required for telomere cohesion in human cells, whereas cohesin-SA2 is required for centromere cohesion. Cells depleted of SA1 fail to establish or maintain cohesion between sister telomeres after DNA replication in S phase, leading to inability to repair chromatid breaks and sister telomere loss.","method":"siRNA depletion of SA1 or SA2 in human cells with cytological analysis of telomere and centromere cohesion","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — replicated across multiple papers with clean KD and defined cellular phenotypes; foundational finding confirmed in subsequent studies","pmids":["19822671"],"is_preprint":false},{"year":2012,"finding":"SA1-null mouse embryonic fibroblasts show defective telomere replication (not centromeric cohesion) leading to chromosome segregation defects and aneuploidy. Complete SA1 ablation causes embryonic lethality; heterozygous mice show shorter lifespan and earlier tumourigenesis.","method":"SA1 knockout mouse model; analysis of MEFs by FISH, cytogenetics, and BrdU incorporation; telomere FISH","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo knockout model with multiple orthogonal phenotypic readouts, replicated finding of SA1-specific telomere role","pmids":["22415365"],"is_preprint":false},{"year":2012,"finding":"SA1 is largely responsible for cohesin accumulation at gene promoters and CTCF-binding sites in mouse cells. Loss of SA1 alters transcription of genes related to Cornelia de Lange syndrome, including myc and protocadherin genes, a function that cannot be assumed by cohesin-SA2.","method":"ChIP-seq of cohesin in wild-type vs SA1-null MEFs; RNA-seq; comparison of genome-wide cohesin distribution","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — genome-wide ChIP-seq with SA1-null cells, multiple orthogonal methods, single rigorous study","pmids":["22415368"],"is_preprint":false},{"year":1997,"finding":"SA1 (stromal antigen 1) is a nuclear protein of ~120 kDa encoded by a novel gene family. It is ubiquitously expressed at the mRNA level but the protein is predominantly detected in hematopoietic organs, suggesting post-transcriptional regulation.","method":"Molecular cloning from mouse/human cDNA libraries; subcellular fractionation; immunoprecipitation + Western blot with polyclonal antiserum","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — original cloning paper with subcellular fractionation showing nuclear localization; single lab, two methods","pmids":["9305759"],"is_preprint":false},{"year":2013,"finding":"SA1 binds directly to telomeric DNA through a unique AT-hook motif in its N-terminal domain (not present in SA2). Overexpression of SA1 alone is sufficient to induce telomere cohesion independent of the cohesin ring, dependent on this AT-hook. SA1 is highly enriched at telomeres by ChIP and enrichment decreases at mitosis when cohesion is resolved.","method":"ChIP of SA1 at telomeres; overexpression of SA1 truncation/domain mutants; FISH-based cohesion assay; identification of AT-hook by sequence analysis and mutagenesis","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — direct DNA binding via domain mutagenesis, ChIP, and functional cohesion assay; single lab with multiple orthogonal methods","pmids":["23729739"],"is_preprint":false},{"year":2013,"finding":"Cohesin-SA2, not cohesin-SA1, is the main complex recruited to DNA double-strand break sites in S/G2 phase, co-recruited with the cohesin-loading factor NIPBL. Replacing the diverged C-terminal region of SA1 with the corresponding SA2 region confers damage-site recruitment on SA1. SA2 but not SA1 depletion decreases sister chromatid homologous recombination repair and affects repair pathway choice. Both SA1 and SA2 function in the intra-S checkpoint.","method":"Laser micro-irradiation with live-cell imaging; siRNA depletion; SA1/SA2 chimeric protein constructs; HR repair assay","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — multiple orthogonal methods including chimeric constructs, live imaging, and functional repair assays in a single rigorous study","pmids":["24324008"],"is_preprint":false},{"year":2013,"finding":"SA1/SA2 interact with Rad21 through two SA-binding motifs on Rad21: an N-terminal motif (residues 60–81) and a middle-part 10-amino-acid α-helical motif (residues 383–392). Mutation of conserved residues L385, F389, T390 in the middle motif significantly hinders Rad21 from binding SA1/2. SA2 interacts with Rad21 through a broad region (301–750 aa). Mutant Rad21 fails to rescue precocious chromosome separation caused by Rad21 depletion.","method":"In vitro binding assays; co-immunoprecipitation; analytical ultracentrifugation; site-directed mutagenesis; chromosome segregation rescue assay","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with mutagenesis and functional rescue; single lab with multiple orthogonal methods","pmids":["23874961"],"is_preprint":false},{"year":2016,"finding":"SA1 uses two-state binding on DNA: 1D free diffusion for searching and subdiffusive sliding for recognition at telomeric regions. The AT-hook motif modulates both non-specific DNA binding and subdiffusive dynamics at telomeric sequences. TRF1 tethers SA1 within telomeric regions. SA1 and TRF1 together form longer DNA-DNA pairing tracts than TRF1 alone.","method":"Single-molecule fluorescence imaging; DNA tightrope assay; atomic force microscopy; SA1 AT-hook mutant analysis","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — single-molecule and AFM imaging with domain mutant; multiple orthogonal biophysical methods in one study","pmids":["27298259"],"is_preprint":false},{"year":2017,"finding":"STAG1 loss abrogates sister chromatid cohesion specifically in STAG2-mutated cells, leading to mitotic catastrophe, defective cell division, and apoptosis — demonstrating a synthetic lethal interaction between STAG1 and STAG2. Restoration of STAG2 expression in a STAG2-mutated cell line alleviates STAG1 dependency.","method":"RNAi/CRISPR-mediated STAG1 inactivation in isogenic STAG2-wildtype vs. mutant cell lines; metaphase spread analysis; cell viability assays; STAG2 re-expression rescue","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis via isogenic cell lines with multiple cancer contexts, rescue experiment, replicated by independent lab (PMID:28430577)","pmids":["28691904","28430577"],"is_preprint":false},{"year":2018,"finding":"Cohesin-SA1 preferentially stabilizes topologically associating domain (TAD) boundaries together with CTCF, while cohesin-SA2 promotes cell-type-specific contacts between enhancers and promoters independently of CTCF. Both variants are found at CTCF sites, but SA2-cohesin also localizes to enhancers lacking CTCF, which SA1-cohesin cannot replace when SA2 is absent.","method":"ChIP-seq and Hi-C in human cells with SA1 or SA2 downregulation; comparison of genomic distributions; RNA-seq","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genome-wide ChIP-seq and Hi-C with depletion of each variant; multiple orthogonal methods; confirmed by multiple independent studies","pmids":["29867216"],"is_preprint":false},{"year":2019,"finding":"Stag2 and Stag1 bind a shared set of genomic loci in hematopoietic stem and progenitor cells, but a component of Stag2 binding sites is unoccupied by Stag1 even in Stag2-deficient cells. Stag2 loss alone decreases chromatin accessibility and transcription of lineage-specification genes; concurrent loss of both Stag2 and Stag1 abrogates hematopoiesis. Both share a role in chromosomal segregation.","method":"ChIP-seq for Stag1 and Stag2 in HSPCs; ATAC-seq; conditional Stag2 knockout and Stag1/Stag2 double knockout mouse models","journal":"Cell stem cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — ChIP-seq, ATAC-seq, and in vivo knockout models with defined phenotypes; multiple orthogonal methods","pmids":["31495782"],"is_preprint":false},{"year":2019,"finding":"In mouse embryonic stem cells, cohesin-SA1 preserves TAD borders and disrupts Polycomb domain interaction networks, while cohesin-SA2 facilitates Polycomb domain compaction through PRC1 recruitment and promotes long-range interaction networks between Polycomb-bound promoters.","method":"Hi-C, ChIP-seq, and RNA-seq in mESCs with SA1 or SA2 depletion; comparison of 3D genome organization","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — Hi-C and ChIP-seq with individual depletions; multiple orthogonal methods; rigorous mechanistic dissection","pmids":["31216471"],"is_preprint":false},{"year":2020,"finding":"During G1 phase, acetylated cohesin-STAG1 binds chromatin for hours (long residence time), whereas cohesin-STAG2 binds for minutes. CTCF and the acetyltransferase ESCO1 protect a subset of cohesin-STAG1 from WAPL-mediated removal, enabling formation of long and long-lived chromatin loops. ESCO1 contributes to boundary formation in chromatin looping similarly to CTCF.","method":"FRAP, ChIP-seq, Hi-C, auxin-inducible degron system for ESCO1/WAPL; acetylation analysis; live-cell imaging","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — FRAP for chromatin residence time, degron system, ChIP-seq and Hi-C; multiple orthogonal methods in single rigorous study","pmids":["32065581"],"is_preprint":false},{"year":2020,"finding":"SA1 (STAG1) and SA2 (STAG2) directly bind RNA, including ssRNA, dsRNA, RNA:DNA hybrids, and R-loops, using bulk biochemical assays and single-molecule techniques. SA1 and SA2 localize to regions on dsDNA that contain RNA; SA1/SA2 binding sites significantly overlap with R-loop sites genome-wide by ChIP-seq/DRIP-seq comparison.","method":"Electrophoretic mobility shift assay (EMSA), atomic force microscopy, DNA tightrope assay with RNA substrates; ChIP-seq and DRIP-seq data comparison","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro biochemical binding assays with multiple RNA substrates plus single-molecule imaging; multiple orthogonal methods in one study","pmids":["32352519"],"is_preprint":false},{"year":2020,"finding":"STAG1 is required for sister chromatid cohesion specifically in STAG2-deficient cells. Chemical-genetic degradation of STAG1 protein (inducible degron) results in loss of sister chromatid cohesion and rapid cell death selectively in STAG2-deficient cells. X-ray crystallography identifies STAG1 regions that interact with the RAD21 subunit; STAG1 mutations abrogating this interaction selectively compromise viability of STAG2-deficient cells.","method":"Genome-wide CRISPR screens; auxin-inducible degron for STAG1 degradation; X-ray crystallography of STAG1–RAD21 interaction domain; metaphase spread assay; biochemical interaction assays","journal":"Life science alliance","confidence":"High","confidence_rationale":"Tier 1 / Strong — X-ray crystallography plus degron system plus CRISPR screen and functional rescue; multiple orthogonal methods","pmids":["32467316"],"is_preprint":false},{"year":2012,"finding":"SA1 and SA2 contain nuclear localization signals (NLS) and export signals (NES). Only the N-terminal NLS of SA1 is functional for nuclear import (validated in both yeast and HeLa cells), whereas SA2 has at least two functional NLS and two functional NES allowing nucleocytoplasmic shuttling. This represents a difference in nuclear transport organization between the two paralogs.","method":"Expression of fluorescently tagged SA1/SA2 in S. cerevisiae and HeLa cells; deletion/mutation analysis of putative NLS and NES sequences; fluorescence microscopy","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional NLS/NES mapping in two cell systems; single lab, two orthogonal systems","pmids":["22715410"],"is_preprint":false},{"year":2023,"finding":"TRF1, TIN2, and SA1 work synergistically to compact telomeric DNA. SA1 alone can drive DNA compaction. In the ternary TRF1-TIN2-SA1 system, compaction is an additive effect of TRF1-TIN2 and SA1-based compaction pathways. Compaction by TRF1 and TIN2 proceeds via TRF1 binding followed by TIN2 recognition of TRF1.","method":"Nanochannel DNA stretching to quantify compaction; atomic force microscopy of intermolecular aggregation; systematic comparison of protein combinations","journal":"Biophysical journal","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro single-molecule reconstitution with multiple protein combinations; 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 regulates repeat (rRNA) expression, nucleolar integrity, and repression of the two-cell (2C) state to maintain mESC identity. Skewing the balance of naturally occurring Stag1 isoforms impacts cell identity.","method":"STAG1 depletion in mESCs; rescue with N-terminal domain constructs; RNA-seq for repeat expression; immunofluorescence of nucleolar markers; STAG1 isoform characterization","journal":"Stem cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with domain rescue experiments and multiple readouts; single lab","pmids":["37802073"],"is_preprint":false},{"year":2025,"finding":"STAG1 binds tightly to the trimeric SMC1/SMC3/SCC1 complex; this tetramer then weakly but cooperatively binds DNA. Full-length NIPBL acts as a DNA anchor during loop extrusion. An ATP-modulated DNA binding site created by STAG1's interaction with SMC1/SMC3/SCC1 is important for repeated grabbing and release of DNA critical to extrusion. Cohesin mutants lacking DNA-binding domains in ATPase heads show negligible differences in overall DNA affinity.","method":"Mass photometry to quantify biomolecular interactions; systematic binding measurements of cohesin subcomplexes; ATPase-dead and head-engagement mutants","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mass photometry with comprehensive quantification of all key interactions, multiple cohesin mutants; single rigorous study","pmids":["40763028"],"is_preprint":false},{"year":2025,"finding":"Cohesin-STAG1 extrudes DNA loops more efficiently than cohesin-STAG2 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 is dispensable for sister chromatid cohesion. Mutation of the AT-hook markedly impairs TAD and chromatin loop formation.","method":"Single-molecule loop extrusion assay comparing cohesin-STAG1 vs cohesin-STAG2; AT-hook mutant constructs; ChIP-seq; Hi-C; ATPase assay; cohesion assay in human cells","journal":"bioRxiv","confidence":"High","confidence_rationale":"Tier 1 / Moderate — single-molecule reconstitution with domain mutants, supported by genome-wide assays; multiple orthogonal methods in one study","pmids":[],"is_preprint":true},{"year":2024,"finding":"KPT-6566 directly binds to both STAG1 and STAG2, disrupting their interactions with SCC1 and double-stranded DNA. KPT-6566 causes premature chromosome separation and chromosome damage in HeLa cells, impairs DNA damage repair, and accumulates DSBs. KPT-6566 sensitizes cells to PARP inhibitor Olaparib and NHEJ inhibitor UMI-77.","method":"Fluorescence polarization-based high-throughput screening; biochemical and biophysical binding assays; metaphase chromosome spread; γH2AX foci assay; cell viability synergy assay","journal":"DNA repair","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct biochemical binding confirmation plus functional cellular assays; single lab, multiple methods","pmids":["39541712"],"is_preprint":false},{"year":2025,"finding":"The CTCF N-terminal region (NTR) contains two motifs, YDF and KTYQR, that hinder cohesin-mediated loop extrusion. YDF hinders loop extrusion step cycles and converts cohesin into a unidirectional extruder by strengthening STAG1's affinity to DNA. KTYQR fully impedes loop extrusion activity.","method":"Single-molecule DNA loop extrusion assay with CTCF NTR fragments and motif mutants; measurement of STAG1-DNA affinity","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — single-molecule reconstitution with NTR peptide fragments; preprint, single lab","pmids":[],"is_preprint":true},{"year":2024,"finding":"Molecular dynamics simulations predict DNA binding patches on STAG1 that, together with patches on SMC1, SMC3, and NIPBL, form a DNA clamping patch group. This group is predicted to facilitate DNA bending and capture of DNA within the cohesin ring, critical steps in loop extrusion.","method":"Molecular dynamics simulations with amino-acid-residue resolution structural models of cohesin subunits including STAG1; quantification of DNA-binding affinities via dissociation rate constants","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 4 / Weak — computational prediction only, no experimental validation reported in this paper","pmids":[],"is_preprint":true},{"year":2020,"finding":"In zebrafish, stag1a (but not stag1b) contributes to primitive embryonic haematopoiesis. Loss of stag1a expands the lateral plate mesoderm zone of scl-positive haematopoietic/vascular progenitors, reduces kidney progenitors, and increases spi1-positive myeloid-biased cells, consistent with skewing toward primitive myelopoiesis.","method":"CRISPR-Cas9 zebrafish loss-of-function mutants; in situ hybridization for haematopoietic markers; phenotypic analysis at 24 hpf","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo CRISPR knockout with defined haematopoietic phenotype readouts; single lab","pmids":["33365313"],"is_preprint":false},{"year":2016,"finding":"SA1 modulates high-order chromatin structure in colorectal cancer cells. SA1 knockdown in HT29 cells increases chromatin accessibility (by MNase assay and nanocytology) and increases cell proliferation, supporting a tumor suppressor role through chromatin organization.","method":"siRNA knockdown of SA1 in HT29 cells; micrococcal nuclease (MNase) chromatin accessibility assay; nanocytology (biophotonic); WST-1 proliferation assay; colony formation assay","journal":"Cancer prevention research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with chromatin accessibility and proliferation readouts; single lab, multiple methods","pmids":["27549371"],"is_preprint":false},{"year":2025,"finding":"A frameshift STAG1 variant (c.500dup; p.Gly168TrpfsTer13) leads to dramatic reduction in mutant mRNA levels due to nonsense-mediated mRNA decay, resulting in impaired production of functional STAG1 protein. This provides functional evidence that STAG1 haploinsufficiency is the pathogenic mechanism underlying MRD47.","method":"Mutant expression vector transfection into HEK293T cells; RT-qPCR of mutant mRNA; Western blot of STAG1 protein; comparison with wild-type construct","journal":"Frontiers in pediatrics","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — in vitro functional expression study with mRNA and protein quantification; single lab, limited to one variant","pmids":["41210242"],"is_preprint":false}],"current_model":"STAG1 is a core subunit of cohesin-SA1, a variant of the cohesin complex (SMC1/SMC3/SCC1/STAG1), which is mechanistically distinct from cohesin-SA2: STAG1 confers preferential binding at CTCF sites and gene promoters to stabilize TAD boundaries and mediate long-lived chromatin loop extrusion via its unique AT-hook motif that directly binds telomeric/AT-rich DNA and strengthens cohesin-chromatin interaction; cohesin-SA1 is protected from WAPL-mediated eviction by acetylation (via ESCO1) and CTCF binding, enabling hours-long chromatin residence in G1; it specifically mediates sister telomere cohesion (not centromere cohesion) through its AT-hook and cooperative interaction with TRF1 and TIN2, and supports DNA-DNA pairing at telomeres; both SA1 and SA2 can bind RNA and R-loops, and SA1 interacts with the RAD21 subunit through defined binding motifs (characterised by X-ray crystallography); loss of SA1 in STAG2-mutant cells abrogates residual cohesion, establishing a synthetic lethal interaction exploitable in cancer."},"narrative":{"mechanistic_narrative":"STAG1 (SA1) is a core HEAT-repeat subunit of the cohesin complex that defines a functionally distinct variant, cohesin-SA1, mechanistically separable from the paralogous cohesin-SA2 [PMID:19822671, PMID:29867216]. STAG1 binds tightly to the SMC1/SMC3/SCC1(RAD21) trimer to form a tetramer that cooperatively engages DNA, and an ATP-modulated DNA-binding site created at this interface supports the repeated DNA grabbing-and-release cycles of loop extrusion [PMID:40763028]; the STAG1–RAD21 interface has been resolved crystallographically and depends on defined SA-binding motifs on RAD21 [PMID:23874961, PMID:32467316]. A unique N-terminal AT-hook motif absent from SA2 lets STAG1 bind AT-rich and telomeric DNA directly and drives efficient loop extrusion and stable G1 chromatin association without changing ATPase activity [PMID:23729739]. Through these properties cohesin-SA1 preferentially accumulates at CTCF sites and gene promoters to stabilize TAD boundaries and long-lived chromatin loops, a role SA2-cohesin cannot replace [PMID:22415368, PMID:29867216, PMID:32065581]; ESCO1-mediated acetylation and CTCF protect cohesin-SA1 from WAPL eviction, conferring hours-long chromatin residence in G1 [PMID:32065581]. Functionally, STAG1 is specifically required for sister telomere cohesion—not centromere cohesion—via its AT-hook and cooperation with the shelterin proteins TRF1 and TIN2 to pair and compact telomeric DNA [PMID:19822671, PMID:23729739, PMID:27298259, PMID:37081787], and SA1-null cells show telomere replication defects, aneuploidy, and altered transcription of Cornelia de Lange-associated genes [PMID:22415365, PMID:22415368]. Loss of STAG1 abrogates residual sister chromatid cohesion selectively in STAG2-mutant cells, establishing a synthetic-lethal vulnerability exploitable in cancer [PMID:28691904, PMID:28430577, PMID:32467316]. STAG1 haploinsufficiency, via nonsense-mediated decay of a frameshift allele, is a pathogenic mechanism underlying the intellectual disability disorder MRD47 [PMID:41210242].","teleology":[{"year":1997,"claim":"Established STAG1 as a discrete nuclear protein and gene, providing the molecular entity later placed within cohesin.","evidence":"cDNA cloning with subcellular fractionation and immunoprecipitation/Western blot","pmids":["9305759"],"confidence":"Medium","gaps":["No functional or complex assignment","Discrepancy between ubiquitous mRNA and hematopoietic-restricted protein left unexplained"]},{"year":2009,"claim":"Resolved whether the two cohesin SA paralogs are functionally redundant by showing SA1 governs telomere cohesion while SA2 governs centromere cohesion, defining variant-specific roles.","evidence":"siRNA depletion of SA1 vs SA2 in human cells with cytological cohesion analysis","pmids":["19822671"],"confidence":"High","gaps":["Molecular basis of telomere specificity not yet identified","No structural detail on SA1-chromatin engagement"]},{"year":2012,"claim":"Defined the in vivo consequences of SA1 loss and its genome-wide chromatin role, linking SA1 to telomere replication, CTCF/promoter occupancy, and Cornelia de Lange-associated transcription.","evidence":"SA1 knockout mouse, MEF cytogenetics/FISH, cohesin ChIP-seq and RNA-seq; separate NLS/NES mapping in yeast and HeLa","pmids":["22415365","22415368","22715410"],"confidence":"High","gaps":["Direct DNA-binding determinant not yet identified","Mechanism connecting cohesin distribution to specific gene regulation incomplete"]},{"year":2013,"claim":"Identified the molecular determinants of SA1 function: a unique AT-hook for direct telomeric DNA binding, defined RAD21-binding motifs, and a C-terminal region dictating damage-site recruitment versus SA2.","evidence":"Domain mutagenesis and ChIP for the AT-hook; in vitro binding, AUC and rescue for RAD21 motifs; chimeric SA1/SA2 constructs and laser micro-irradiation for DSB recruitment","pmids":["23729739","23874961","24324008"],"confidence":"High","gaps":["AT-hook DNA-binding dynamics not yet resolved at single-molecule level","How AT-hook contribution differs from cohesin ring entrapment unclear"]},{"year":2016,"claim":"Provided biophysical and chromatin-level mechanism: single-molecule imaging showed SA1 searches DNA by 1D diffusion and is tethered at telomeres by TRF1, and SA1 modulates higher-order chromatin compaction relevant to tumor suppression.","evidence":"DNA tightrope/single-molecule imaging and AFM with AT-hook mutants; MNase accessibility, nanocytology and proliferation assays in HT29 cells","pmids":["27298259","27549371"],"confidence":"High","gaps":["In vitro telomere pairing not yet linked to cellular cohesion quantitatively","Tumor suppressor mechanism in colorectal cells limited to single line"]},{"year":2017,"claim":"Demonstrated a therapeutically relevant synthetic-lethal dependency: STAG1 loss selectively kills STAG2-mutant cells by abrogating residual cohesion, reversible by STAG2 restoration.","evidence":"RNAi/CRISPR STAG1 inactivation in isogenic STAG2-WT vs mutant lines, metaphase spreads, viability and rescue assays","pmids":["28691904","28430577"],"confidence":"High","gaps":["Druggable interface not yet defined","Breadth across cancer genotypes incompletely mapped"]},{"year":2018,"claim":"Separated the genome-organization functions of the two variants, assigning cohesin-SA1 to CTCF-anchored TAD boundaries and cohesin-SA2 to CTCF-independent enhancer-promoter contacts.","evidence":"ChIP-seq and Hi-C with individual SA1 or SA2 depletion in human cells","pmids":["29867216"],"confidence":"High","gaps":["Mechanism of SA1 preference for CTCF sites not yet molecular","Polycomb-related contributions addressed only later"]},{"year":2019,"claim":"Extended variant-specific roles to development: SA1/SA2 cooperate in HSPC genome occupancy and segregation, while SA1 specifically preserves TAD borders and limits Polycomb domain interactions in stem cells.","evidence":"ChIP-seq/ATAC-seq and conditional single/double knockout mouse models in HSPCs; Hi-C/ChIP-seq in mESCs","pmids":["31495782","31216471"],"confidence":"High","gaps":["Direct biochemical basis of SA1 versus SA2 boundary preference still unresolved","Polycomb antagonism mechanism not fully defined"]},{"year":2020,"claim":"Mechanistically explained SA1's preferential boundary role through chromatin kinetics (hours-long acetylation/CTCF/ESCO1-protected residence), defined RNA/R-loop binding by both paralogs, and crystallized the STAG1–RAD21 interface underpinning the synthetic-lethal dependency.","evidence":"FRAP, degron, ChIP-seq, Hi-C for residence/acetylation; EMSA/AFM/tightrope and DRIP-seq for RNA binding; X-ray crystallography plus degron/CRISPR for STAG1–RAD21","pmids":["32065581","32352519","32467316"],"confidence":"High","gaps":["Functional consequence of SA1 RNA/R-loop binding in vivo unclear","How acetylation status is set specifically on SA1-cohesin not resolved"]},{"year":2023,"claim":"Defined SA1's telomere compaction biochemistry with TRF1/TIN2 and its developmental role at the nucleolus, showing the N-terminus regulates rRNA/repeat expression and pluripotency in mESCs.","evidence":"Nanochannel DNA stretching and AFM for TRF1-TIN2-SA1 compaction; STAG1 depletion with N-terminal rescue, RNA-seq and nucleolar imaging in mESCs","pmids":["37081787","37802073"],"confidence":"Medium","gaps":["Telomere compaction reconstituted in vitro, not in cells","Link between nucleolar/repeat regulation and cohesin activity unestablished"]},{"year":2024,"claim":"Advanced toward pharmacology and structural mechanism: a small molecule (KPT-6566) directly disrupts STAG1/STAG2 binding to SCC1 and DNA, and simulations predict STAG1 DNA-clamping patches contributing to loop extrusion.","evidence":"FP-based screening and biophysical binding plus chromosome/γH2AX and synergy assays for KPT-6566; molecular dynamics simulations for DNA-binding patches","pmids":["39541712"],"confidence":"Medium","gaps":["KPT-6566 selectivity between paralogs not established","MD-predicted clamping patches lack experimental validation"]},{"year":2025,"claim":"Provided quantitative biochemical and physiological mechanism: STAG1 builds an ATP-modulated DNA-binding site within the SMC1/SMC3/SCC1 tetramer for extrusion, the AT-hook enhances extrusion efficiency and G1 chromatin retention, and STAG1 haploinsufficiency via NMD causes MRD47.","evidence":"Mass photometry of cohesin subcomplexes with mutants; single-molecule loop extrusion comparing SA1 vs SA2 with AT-hook mutants (preprint); mutant expression with RT-qPCR/Western for the MRD47 frameshift variant","pmids":["40763028","41210242"],"confidence":"High","gaps":["AT-hook extrusion data from preprint single-molecule assay","How CTCF asymmetrically halts SA1 extrusion still being mapped (preprint)"]},{"year":null,"claim":"How cohesin-SA1's biochemical features (AT-hook DNA binding, ATP-modulated DNA site, RNA/R-loop binding, acetylation-dependent residence) are integrated and selectively regulated to specify telomere cohesion, TAD boundary maintenance, and developmental gene control remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No unified structure of chromatin-engaged cohesin-SA1 during extrusion","Functional role of SA1 RNA/R-loop binding in vivo undefined","Mechanism linking nucleolar/repeat regulation to cohesin activity unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[4,7,18,19]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[13]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[6,18]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,4]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[3,15]},{"term_id":"GO:0000228","term_label":"nuclear chromosome","supporting_discovery_ids":[0,4,9]},{"term_id":"GO:0005730","term_label":"nucleolus","supporting_discovery_ids":[17]}],"pathway":[{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[9,11,12]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[0,1,8]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[5]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[2,9]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[10,17,23]}],"complexes":["cohesin complex (cohesin-SA1)"],"partners":["RAD21","SMC1A","SMC3","CTCF","ESCO1","TRF1","TIN2","NIPBL"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q969W9","full_name":"Protein TMEPAI","aliases":["Prostate transmembrane protein androgen induced 1","Solid tumor-associated 1 protein","Transmembrane prostate androgen-induced protein"],"length_aa":287,"mass_kda":31.6,"function":"Functions as a negative regulator of TGF-beta signaling and thereby probably plays a role in cell proliferation, differentiation, apoptosis, motility, extracellular matrix production and immunosuppression. In the canonical TGF-beta pathway, ZFYVE9/SARA recruits the intracellular signal transducer and transcriptional modulators SMAD2 and SMAD3 to the TGF-beta receptor. Phosphorylated by the receptor, SMAD2 and SMAD3 then form a heteromeric complex with SMAD4 that translocates to the nucleus to regulate transcription. Through interaction with SMAD2 and SMAD3, LDLRAD4 may compete with ZFYVE9 and SMAD4 and prevent propagation of the intracellular signal (PubMed:20129061, PubMed:24627487). Also involved in down-regulation of the androgen receptor (AR), enhancing ubiquitination and proteasome-mediated degradation of AR, probably by recruiting NEDD4 (PubMed:18703514)","subcellular_location":"Early endosome membrane; Golgi apparatus membrane","url":"https://www.uniprot.org/uniprotkb/Q969W9/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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Cells depleted of SA1 fail to establish or maintain cohesion between sister telomeres after DNA replication in S phase, leading to inability to repair chromatid breaks and sister telomere loss.\",\n      \"method\": \"siRNA depletion of SA1 or SA2 in human cells with cytological analysis of telomere and centromere cohesion\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — replicated across multiple papers with clean KD and defined cellular phenotypes; foundational finding confirmed in subsequent studies\",\n      \"pmids\": [\"19822671\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SA1-null mouse embryonic fibroblasts show defective telomere replication (not centromeric cohesion) leading to chromosome segregation defects and aneuploidy. Complete SA1 ablation causes embryonic lethality; heterozygous mice show shorter lifespan and earlier tumourigenesis.\",\n      \"method\": \"SA1 knockout mouse model; analysis of MEFs by FISH, cytogenetics, and BrdU incorporation; telomere FISH\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo knockout model with multiple orthogonal phenotypic readouts, replicated finding of SA1-specific telomere role\",\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 in mouse cells. Loss of SA1 alters transcription of genes related to Cornelia de Lange syndrome, including myc and protocadherin genes, a function that cannot be assumed by cohesin-SA2.\",\n      \"method\": \"ChIP-seq of cohesin in wild-type vs SA1-null MEFs; RNA-seq; comparison of genome-wide cohesin distribution\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genome-wide ChIP-seq with SA1-null cells, multiple orthogonal methods, single rigorous study\",\n      \"pmids\": [\"22415368\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"SA1 (stromal antigen 1) is a nuclear protein of ~120 kDa encoded by a novel gene family. It is ubiquitously expressed at the mRNA level but the protein is predominantly detected in hematopoietic organs, suggesting post-transcriptional regulation.\",\n      \"method\": \"Molecular cloning from mouse/human cDNA libraries; subcellular fractionation; immunoprecipitation + Western blot with polyclonal antiserum\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — original cloning paper with subcellular fractionation showing nuclear localization; single lab, two methods\",\n      \"pmids\": [\"9305759\"],\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 (not present in SA2). Overexpression of SA1 alone is sufficient to induce telomere cohesion independent of the cohesin ring, dependent on this AT-hook. SA1 is highly enriched at telomeres by ChIP and enrichment decreases at mitosis when cohesion is resolved.\",\n      \"method\": \"ChIP of SA1 at telomeres; overexpression of SA1 truncation/domain mutants; FISH-based cohesion assay; identification of AT-hook by sequence analysis and mutagenesis\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — direct DNA binding via domain mutagenesis, ChIP, and functional cohesion assay; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"23729739\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Cohesin-SA2, not cohesin-SA1, is the main complex recruited to DNA double-strand break sites in S/G2 phase, co-recruited with the cohesin-loading factor NIPBL. Replacing the diverged C-terminal region of SA1 with the corresponding SA2 region confers damage-site recruitment on SA1. SA2 but not SA1 depletion decreases sister chromatid homologous recombination repair and affects repair pathway choice. Both SA1 and SA2 function in the intra-S checkpoint.\",\n      \"method\": \"Laser micro-irradiation with live-cell imaging; siRNA depletion; SA1/SA2 chimeric protein constructs; HR repair assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — multiple orthogonal methods including chimeric constructs, live imaging, and functional repair assays in a single rigorous study\",\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 10-amino-acid α-helical motif (residues 383–392). Mutation of conserved residues L385, F389, T390 in the middle motif significantly hinders Rad21 from binding SA1/2. SA2 interacts with Rad21 through a broad region (301–750 aa). Mutant Rad21 fails to rescue precocious chromosome separation caused by Rad21 depletion.\",\n      \"method\": \"In vitro binding assays; co-immunoprecipitation; analytical ultracentrifugation; site-directed mutagenesis; chromosome segregation rescue assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with mutagenesis and functional rescue; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"23874961\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SA1 uses two-state binding on DNA: 1D free diffusion for searching and subdiffusive sliding for recognition at telomeric regions. The AT-hook motif modulates both non-specific DNA binding and subdiffusive dynamics at telomeric sequences. TRF1 tethers SA1 within telomeric regions. SA1 and TRF1 together form longer DNA-DNA pairing tracts than TRF1 alone.\",\n      \"method\": \"Single-molecule fluorescence imaging; DNA tightrope assay; atomic force microscopy; SA1 AT-hook mutant analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — single-molecule and AFM imaging with domain mutant; multiple orthogonal biophysical methods in one study\",\n      \"pmids\": [\"27298259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"STAG1 loss abrogates sister chromatid cohesion specifically in STAG2-mutated cells, leading to mitotic catastrophe, defective cell division, and apoptosis — demonstrating a synthetic lethal interaction between STAG1 and STAG2. Restoration of STAG2 expression in a STAG2-mutated cell line alleviates STAG1 dependency.\",\n      \"method\": \"RNAi/CRISPR-mediated STAG1 inactivation in isogenic STAG2-wildtype vs. mutant cell lines; metaphase spread analysis; cell viability assays; STAG2 re-expression rescue\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis via isogenic cell lines with multiple cancer contexts, rescue experiment, replicated by independent lab (PMID:28430577)\",\n      \"pmids\": [\"28691904\", \"28430577\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Cohesin-SA1 preferentially stabilizes topologically associating domain (TAD) boundaries together with CTCF, while cohesin-SA2 promotes cell-type-specific contacts between enhancers and promoters independently of CTCF. Both variants are found at CTCF sites, but SA2-cohesin also localizes to enhancers lacking CTCF, which SA1-cohesin cannot replace when SA2 is absent.\",\n      \"method\": \"ChIP-seq and Hi-C in human cells with SA1 or SA2 downregulation; comparison of genomic distributions; RNA-seq\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genome-wide ChIP-seq and Hi-C with depletion of each variant; multiple orthogonal methods; confirmed by multiple independent studies\",\n      \"pmids\": [\"29867216\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Stag2 and Stag1 bind a shared set of genomic loci in hematopoietic stem and progenitor cells, but a component of Stag2 binding sites is unoccupied by Stag1 even in Stag2-deficient cells. Stag2 loss alone decreases chromatin accessibility and transcription of lineage-specification genes; concurrent loss of both Stag2 and Stag1 abrogates hematopoiesis. Both share a role in chromosomal segregation.\",\n      \"method\": \"ChIP-seq for Stag1 and Stag2 in HSPCs; ATAC-seq; conditional Stag2 knockout and Stag1/Stag2 double knockout mouse models\",\n      \"journal\": \"Cell stem cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — ChIP-seq, ATAC-seq, and in vivo knockout models with defined phenotypes; multiple orthogonal methods\",\n      \"pmids\": [\"31495782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"In mouse embryonic stem cells, cohesin-SA1 preserves TAD borders and disrupts Polycomb domain interaction networks, while cohesin-SA2 facilitates Polycomb domain compaction through PRC1 recruitment and promotes long-range interaction networks between Polycomb-bound promoters.\",\n      \"method\": \"Hi-C, ChIP-seq, and RNA-seq in mESCs with SA1 or SA2 depletion; comparison of 3D genome organization\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — Hi-C and ChIP-seq with individual depletions; multiple orthogonal methods; rigorous mechanistic dissection\",\n      \"pmids\": [\"31216471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"During G1 phase, acetylated cohesin-STAG1 binds chromatin for hours (long residence time), whereas cohesin-STAG2 binds for minutes. CTCF and the acetyltransferase ESCO1 protect a subset of cohesin-STAG1 from WAPL-mediated removal, enabling formation of long and long-lived chromatin loops. ESCO1 contributes to boundary formation in chromatin looping similarly to CTCF.\",\n      \"method\": \"FRAP, ChIP-seq, Hi-C, auxin-inducible degron system for ESCO1/WAPL; acetylation analysis; live-cell imaging\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — FRAP for chromatin residence time, degron system, ChIP-seq and Hi-C; multiple orthogonal methods in single rigorous study\",\n      \"pmids\": [\"32065581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"SA1 (STAG1) and SA2 (STAG2) directly bind RNA, including ssRNA, dsRNA, RNA:DNA hybrids, and R-loops, using bulk biochemical assays and single-molecule techniques. SA1 and SA2 localize to regions on dsDNA that contain RNA; SA1/SA2 binding sites significantly overlap with R-loop sites genome-wide by ChIP-seq/DRIP-seq comparison.\",\n      \"method\": \"Electrophoretic mobility shift assay (EMSA), atomic force microscopy, DNA tightrope assay with RNA substrates; ChIP-seq and DRIP-seq data comparison\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro biochemical binding assays with multiple RNA substrates plus single-molecule imaging; multiple orthogonal methods in one study\",\n      \"pmids\": [\"32352519\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"STAG1 is required for sister chromatid cohesion specifically in STAG2-deficient cells. Chemical-genetic degradation of STAG1 protein (inducible degron) results in loss of sister chromatid cohesion and rapid cell death selectively in STAG2-deficient cells. X-ray crystallography identifies STAG1 regions that interact with the RAD21 subunit; STAG1 mutations abrogating this interaction selectively compromise viability of STAG2-deficient cells.\",\n      \"method\": \"Genome-wide CRISPR screens; auxin-inducible degron for STAG1 degradation; X-ray crystallography of STAG1–RAD21 interaction domain; metaphase spread assay; biochemical interaction assays\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — X-ray crystallography plus degron system plus CRISPR screen and functional rescue; multiple orthogonal methods\",\n      \"pmids\": [\"32467316\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SA1 and SA2 contain nuclear localization signals (NLS) and export signals (NES). Only the N-terminal NLS of SA1 is functional for nuclear import (validated in both yeast and HeLa cells), whereas SA2 has at least two functional NLS and two functional NES allowing nucleocytoplasmic shuttling. This represents a difference in nuclear transport organization between the two paralogs.\",\n      \"method\": \"Expression of fluorescently tagged SA1/SA2 in S. cerevisiae and HeLa cells; deletion/mutation analysis of putative NLS and NES sequences; fluorescence microscopy\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional NLS/NES mapping in two cell systems; single lab, two orthogonal systems\",\n      \"pmids\": [\"22715410\"],\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. In the ternary TRF1-TIN2-SA1 system, compaction is an additive effect of TRF1-TIN2 and SA1-based compaction pathways. Compaction by TRF1 and TIN2 proceeds via TRF1 binding followed by TIN2 recognition of TRF1.\",\n      \"method\": \"Nanochannel DNA stretching to quantify compaction; atomic force microscopy of intermolecular aggregation; systematic comparison of protein combinations\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro single-molecule reconstitution with multiple protein combinations; 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 regulates repeat (rRNA) expression, nucleolar integrity, and repression of the two-cell (2C) state to maintain mESC identity. Skewing the balance of naturally occurring Stag1 isoforms impacts cell identity.\",\n      \"method\": \"STAG1 depletion in mESCs; rescue with N-terminal domain constructs; RNA-seq for repeat expression; immunofluorescence of nucleolar markers; STAG1 isoform characterization\",\n      \"journal\": \"Stem cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with domain rescue experiments and multiple readouts; single lab\",\n      \"pmids\": [\"37802073\"],\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. Full-length NIPBL acts as a DNA anchor during loop extrusion. An ATP-modulated DNA binding site created by STAG1's interaction with SMC1/SMC3/SCC1 is important for repeated grabbing and release of DNA critical to extrusion. Cohesin mutants lacking DNA-binding domains in ATPase heads show negligible differences in overall DNA affinity.\",\n      \"method\": \"Mass photometry to quantify biomolecular interactions; systematic binding measurements of cohesin subcomplexes; ATPase-dead and head-engagement mutants\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mass photometry with comprehensive quantification of all key interactions, multiple cohesin mutants; single rigorous study\",\n      \"pmids\": [\"40763028\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cohesin-STAG1 extrudes DNA loops more efficiently than cohesin-STAG2 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 is dispensable for sister chromatid cohesion. Mutation of the AT-hook markedly impairs TAD and chromatin loop formation.\",\n      \"method\": \"Single-molecule loop extrusion assay comparing cohesin-STAG1 vs cohesin-STAG2; AT-hook mutant constructs; ChIP-seq; Hi-C; ATPase assay; cohesion assay in human cells\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — single-molecule reconstitution with domain mutants, supported by genome-wide assays; multiple orthogonal methods in one study\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"KPT-6566 directly binds to both STAG1 and STAG2, disrupting their interactions with SCC1 and double-stranded DNA. KPT-6566 causes premature chromosome separation and chromosome damage in HeLa cells, impairs DNA damage repair, and accumulates DSBs. KPT-6566 sensitizes cells to PARP inhibitor Olaparib and NHEJ inhibitor UMI-77.\",\n      \"method\": \"Fluorescence polarization-based high-throughput screening; biochemical and biophysical binding assays; metaphase chromosome spread; γH2AX foci assay; cell viability synergy assay\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct biochemical binding confirmation plus functional cellular assays; single lab, multiple methods\",\n      \"pmids\": [\"39541712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The CTCF N-terminal region (NTR) contains two motifs, YDF and KTYQR, that hinder cohesin-mediated loop extrusion. YDF hinders loop extrusion step cycles and converts cohesin into a unidirectional extruder by strengthening STAG1's affinity to DNA. KTYQR fully impedes loop extrusion activity.\",\n      \"method\": \"Single-molecule DNA loop extrusion assay with CTCF NTR fragments and motif mutants; measurement of STAG1-DNA affinity\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — single-molecule reconstitution with NTR peptide fragments; preprint, single lab\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Molecular dynamics simulations predict DNA binding patches on STAG1 that, together with patches on SMC1, SMC3, and NIPBL, form a DNA clamping patch group. This group is predicted to facilitate DNA bending and capture of DNA within the cohesin ring, critical steps in loop extrusion.\",\n      \"method\": \"Molecular dynamics simulations with amino-acid-residue resolution structural models of cohesin subunits including STAG1; quantification of DNA-binding affinities via dissociation rate constants\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 / Weak — computational prediction only, no experimental validation reported in this paper\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In zebrafish, stag1a (but not stag1b) contributes to primitive embryonic haematopoiesis. Loss of stag1a expands the lateral plate mesoderm zone of scl-positive haematopoietic/vascular progenitors, reduces kidney progenitors, and increases spi1-positive myeloid-biased cells, consistent with skewing toward primitive myelopoiesis.\",\n      \"method\": \"CRISPR-Cas9 zebrafish loss-of-function mutants; in situ hybridization for haematopoietic markers; phenotypic analysis at 24 hpf\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo CRISPR knockout with defined haematopoietic phenotype readouts; single lab\",\n      \"pmids\": [\"33365313\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"SA1 modulates high-order chromatin structure in colorectal cancer cells. SA1 knockdown in HT29 cells increases chromatin accessibility (by MNase assay and nanocytology) and increases cell proliferation, supporting a tumor suppressor role through chromatin organization.\",\n      \"method\": \"siRNA knockdown of SA1 in HT29 cells; micrococcal nuclease (MNase) chromatin accessibility assay; nanocytology (biophotonic); WST-1 proliferation assay; colony formation assay\",\n      \"journal\": \"Cancer prevention research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with chromatin accessibility and proliferation readouts; single lab, multiple methods\",\n      \"pmids\": [\"27549371\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"A frameshift STAG1 variant (c.500dup; p.Gly168TrpfsTer13) leads to dramatic reduction in mutant mRNA levels due to nonsense-mediated mRNA decay, resulting in impaired production of functional STAG1 protein. This provides functional evidence that STAG1 haploinsufficiency is the pathogenic mechanism underlying MRD47.\",\n      \"method\": \"Mutant expression vector transfection into HEK293T cells; RT-qPCR of mutant mRNA; Western blot of STAG1 protein; comparison with wild-type construct\",\n      \"journal\": \"Frontiers in pediatrics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — in vitro functional expression study with mRNA and protein quantification; single lab, limited to one variant\",\n      \"pmids\": [\"41210242\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"STAG1 is a core subunit of cohesin-SA1, a variant of the cohesin complex (SMC1/SMC3/SCC1/STAG1), which is mechanistically distinct from cohesin-SA2: STAG1 confers preferential binding at CTCF sites and gene promoters to stabilize TAD boundaries and mediate long-lived chromatin loop extrusion via its unique AT-hook motif that directly binds telomeric/AT-rich DNA and strengthens cohesin-chromatin interaction; cohesin-SA1 is protected from WAPL-mediated eviction by acetylation (via ESCO1) and CTCF binding, enabling hours-long chromatin residence in G1; it specifically mediates sister telomere cohesion (not centromere cohesion) through its AT-hook and cooperative interaction with TRF1 and TIN2, and supports DNA-DNA pairing at telomeres; both SA1 and SA2 can bind RNA and R-loops, and SA1 interacts with the RAD21 subunit through defined binding motifs (characterised by X-ray crystallography); loss of SA1 in STAG2-mutant cells abrogates residual cohesion, establishing a synthetic lethal interaction exploitable in cancer.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"STAG1 (SA1) is a core HEAT-repeat subunit of the cohesin complex that defines a functionally distinct variant, cohesin-SA1, mechanistically separable from the paralogous cohesin-SA2 [#0, #9]. STAG1 binds tightly to the SMC1/SMC3/SCC1(RAD21) trimer to form a tetramer that cooperatively engages DNA, and an ATP-modulated DNA-binding site created at this interface supports the repeated DNA grabbing-and-release cycles of loop extrusion [#18]; the STAG1\\u2013RAD21 interface has been resolved crystallographically and depends on defined SA-binding motifs on RAD21 [#6, #14]. A unique N-terminal AT-hook motif absent from SA2 lets STAG1 bind AT-rich and telomeric DNA directly and drives efficient loop extrusion and stable G1 chromatin association without changing ATPase activity [#4, #19]. Through these properties cohesin-SA1 preferentially accumulates at CTCF sites and gene promoters to stabilize TAD boundaries and long-lived chromatin loops, a role SA2-cohesin cannot replace [#2, #9, #12]; ESCO1-mediated acetylation and CTCF protect cohesin-SA1 from WAPL eviction, conferring hours-long chromatin residence in G1 [#12]. Functionally, STAG1 is specifically required for sister telomere cohesion\\u2014not centromere cohesion\\u2014via its AT-hook and cooperation with the shelterin proteins TRF1 and TIN2 to pair and compact telomeric DNA [#0, #4, #7, #16], and SA1-null cells show telomere replication defects, aneuploidy, and altered transcription of Cornelia de Lange-associated genes [#1, #2]. Loss of STAG1 abrogates residual sister chromatid cohesion selectively in STAG2-mutant cells, establishing a synthetic-lethal vulnerability exploitable in cancer [#8, #14]. STAG1 haploinsufficiency, via nonsense-mediated decay of a frameshift allele, is a pathogenic mechanism underlying the intellectual disability disorder MRD47 [#25].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Established STAG1 as a discrete nuclear protein and gene, providing the molecular entity later placed within cohesin.\",\n      \"evidence\": \"cDNA cloning with subcellular fractionation and immunoprecipitation/Western blot\",\n      \"pmids\": [\"9305759\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No functional or complex assignment\", \"Discrepancy between ubiquitous mRNA and hematopoietic-restricted protein left unexplained\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Resolved whether the two cohesin SA paralogs are functionally redundant by showing SA1 governs telomere cohesion while SA2 governs centromere cohesion, defining variant-specific roles.\",\n      \"evidence\": \"siRNA depletion of SA1 vs SA2 in human cells with cytological cohesion analysis\",\n      \"pmids\": [\"19822671\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of telomere specificity not yet identified\", \"No structural detail on SA1-chromatin engagement\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defined the in vivo consequences of SA1 loss and its genome-wide chromatin role, linking SA1 to telomere replication, CTCF/promoter occupancy, and Cornelia de Lange-associated transcription.\",\n      \"evidence\": \"SA1 knockout mouse, MEF cytogenetics/FISH, cohesin ChIP-seq and RNA-seq; separate NLS/NES mapping in yeast and HeLa\",\n      \"pmids\": [\"22415365\", \"22415368\", \"22715410\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct DNA-binding determinant not yet identified\", \"Mechanism connecting cohesin distribution to specific gene regulation incomplete\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Identified the molecular determinants of SA1 function: a unique AT-hook for direct telomeric DNA binding, defined RAD21-binding motifs, and a C-terminal region dictating damage-site recruitment versus SA2.\",\n      \"evidence\": \"Domain mutagenesis and ChIP for the AT-hook; in vitro binding, AUC and rescue for RAD21 motifs; chimeric SA1/SA2 constructs and laser micro-irradiation for DSB recruitment\",\n      \"pmids\": [\"23729739\", \"23874961\", \"24324008\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"AT-hook DNA-binding dynamics not yet resolved at single-molecule level\", \"How AT-hook contribution differs from cohesin ring entrapment unclear\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Provided biophysical and chromatin-level mechanism: single-molecule imaging showed SA1 searches DNA by 1D diffusion and is tethered at telomeres by TRF1, and SA1 modulates higher-order chromatin compaction relevant to tumor suppression.\",\n      \"evidence\": \"DNA tightrope/single-molecule imaging and AFM with AT-hook mutants; MNase accessibility, nanocytology and proliferation assays in HT29 cells\",\n      \"pmids\": [\"27298259\", \"27549371\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vitro telomere pairing not yet linked to cellular cohesion quantitatively\", \"Tumor suppressor mechanism in colorectal cells limited to single line\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrated a therapeutically relevant synthetic-lethal dependency: STAG1 loss selectively kills STAG2-mutant cells by abrogating residual cohesion, reversible by STAG2 restoration.\",\n      \"evidence\": \"RNAi/CRISPR STAG1 inactivation in isogenic STAG2-WT vs mutant lines, metaphase spreads, viability and rescue assays\",\n      \"pmids\": [\"28691904\", \"28430577\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Druggable interface not yet defined\", \"Breadth across cancer genotypes incompletely mapped\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Separated the genome-organization functions of the two variants, assigning cohesin-SA1 to CTCF-anchored TAD boundaries and cohesin-SA2 to CTCF-independent enhancer-promoter contacts.\",\n      \"evidence\": \"ChIP-seq and Hi-C with individual SA1 or SA2 depletion in human cells\",\n      \"pmids\": [\"29867216\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of SA1 preference for CTCF sites not yet molecular\", \"Polycomb-related contributions addressed only later\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Extended variant-specific roles to development: SA1/SA2 cooperate in HSPC genome occupancy and segregation, while SA1 specifically preserves TAD borders and limits Polycomb domain interactions in stem cells.\",\n      \"evidence\": \"ChIP-seq/ATAC-seq and conditional single/double knockout mouse models in HSPCs; Hi-C/ChIP-seq in mESCs\",\n      \"pmids\": [\"31495782\", \"31216471\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct biochemical basis of SA1 versus SA2 boundary preference still unresolved\", \"Polycomb antagonism mechanism not fully defined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Mechanistically explained SA1's preferential boundary role through chromatin kinetics (hours-long acetylation/CTCF/ESCO1-protected residence), defined RNA/R-loop binding by both paralogs, and crystallized the STAG1\\u2013RAD21 interface underpinning the synthetic-lethal dependency.\",\n      \"evidence\": \"FRAP, degron, ChIP-seq, Hi-C for residence/acetylation; EMSA/AFM/tightrope and DRIP-seq for RNA binding; X-ray crystallography plus degron/CRISPR for STAG1\\u2013RAD21\",\n      \"pmids\": [\"32065581\", \"32352519\", \"32467316\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of SA1 RNA/R-loop binding in vivo unclear\", \"How acetylation status is set specifically on SA1-cohesin not resolved\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Defined SA1's telomere compaction biochemistry with TRF1/TIN2 and its developmental role at the nucleolus, showing the N-terminus regulates rRNA/repeat expression and pluripotency in mESCs.\",\n      \"evidence\": \"Nanochannel DNA stretching and AFM for TRF1-TIN2-SA1 compaction; STAG1 depletion with N-terminal rescue, RNA-seq and nucleolar imaging in mESCs\",\n      \"pmids\": [\"37081787\", \"37802073\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Telomere compaction reconstituted in vitro, not in cells\", \"Link between nucleolar/repeat regulation and cohesin activity unestablished\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Advanced toward pharmacology and structural mechanism: a small molecule (KPT-6566) directly disrupts STAG1/STAG2 binding to SCC1 and DNA, and simulations predict STAG1 DNA-clamping patches contributing to loop extrusion.\",\n      \"evidence\": \"FP-based screening and biophysical binding plus chromosome/\\u03b3H2AX and synergy assays for KPT-6566; molecular dynamics simulations for DNA-binding patches\",\n      \"pmids\": [\"39541712\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"KPT-6566 selectivity between paralogs not established\", \"MD-predicted clamping patches lack experimental validation\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Provided quantitative biochemical and physiological mechanism: STAG1 builds an ATP-modulated DNA-binding site within the SMC1/SMC3/SCC1 tetramer for extrusion, the AT-hook enhances extrusion efficiency and G1 chromatin retention, and STAG1 haploinsufficiency via NMD causes MRD47.\",\n      \"evidence\": \"Mass photometry of cohesin subcomplexes with mutants; single-molecule loop extrusion comparing SA1 vs SA2 with AT-hook mutants (preprint); mutant expression with RT-qPCR/Western for the MRD47 frameshift variant\",\n      \"pmids\": [\"40763028\", \"41210242\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"AT-hook extrusion data from preprint single-molecule assay\", \"How CTCF asymmetrically halts SA1 extrusion still being mapped (preprint)\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How cohesin-SA1's biochemical features (AT-hook DNA binding, ATP-modulated DNA site, RNA/R-loop binding, acetylation-dependent residence) are integrated and selectively regulated to specify telomere cohesion, TAD boundary maintenance, and developmental gene control remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No unified structure of chromatin-engaged cohesin-SA1 during extrusion\", \"Functional role of SA1 RNA/R-loop binding in vivo undefined\", \"Mechanism linking nucleolar/repeat regulation to cohesin activity unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [4, 7, 18, 19]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [13]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [6, 18]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [3, 15]},\n      {\"term_id\": \"GO:0000228\", \"supporting_discovery_ids\": [0, 4, 9]},\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [17]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [9, 11, 12]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [0, 1, 8]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [2, 9]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [10, 17, 23]}\n    ],\n    \"complexes\": [\"cohesin complex (cohesin-SA1)\"],\n    \"partners\": [\"RAD21\", \"SMC1A\", \"SMC3\", \"CTCF\", \"ESCO1\", \"TRF1\", \"TIN2\", \"NIPBL\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}