{"gene":"TOP6BL","run_date":"2026-04-28T21:42:59","timeline":{"discoveries":[{"year":2016,"finding":"TOPOVIBL (mouse) physically interacts with SPO11 and forms a complex required for meiotic DNA double-strand break (DSB) formation. TOPOVIBL shares strong structural similarity to the TopoVIB subunit of archaeal TopoVI topoisomerase, and its loss abolishes meiotic DSBs.","method":"Co-immunoprecipitation, structural similarity analysis, mouse knockout with meiotic DSB phenotyping","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — reciprocal complex identification with functional KO phenotype, replicated by multiple subsequent studies","pmids":["26917764"],"is_preprint":false},{"year":2022,"finding":"TOPOVIBL directly interacts with REC114 through conserved interacting domains; this interaction is required for the efficiency and timing of meiotic DSB formation genome-wide in oocytes and in sub-telomeric regions of spermatocytes.","method":"Structural analysis of interacting domains, Co-IP, point-mutation knock-in mice with genome-wide DSB monitoring","journal":"Nature Communications","confidence":"High","confidence_rationale":"Tier 1–2 — structural domain mapping combined with in vivo point-mutation mouse models and genome-wide DSB assays","pmids":["36396648"],"is_preprint":false},{"year":2020,"finding":"An autosomal recessive loss-of-function mutation in TOP6BL causes failure of programmed meiotic DSB formation and meiotic arrest prior to the pachytene stage in humans; mouse models with equivalent mutations recapitulate the spermatogenic arrest and absence of DSBs in oocytes.","method":"Whole-exome sequencing, Sanger sequencing, histopathology of patient testis biopsy, knock-in mouse models with DSB assays","journal":"Science Bulletin","confidence":"High","confidence_rationale":"Tier 2 — human genetics confirmed by mouse KI models with mechanistic DSB readout","pmids":["36732965"],"is_preprint":false},{"year":2024,"finding":"Purified TOPOVIBL (TOPOVIBLΔC25) is monomeric in solution, does not bind ATP (unlike its archaeal TopoVIB ancestor), interacts with DNA with a preference for specific geometries (e.g., supercoiled or branched substrates), and adopts a dynamic conformation, suggesting TOPOVIBL senses specific DNA architectures rather than hydrolyzing ATP.","method":"Protein purification, size-exclusion chromatography, ATPase assay, SAXS/structural analysis, DNA-binding assays with different DNA geometries","journal":"Nucleic Acids Research","confidence":"High","confidence_rationale":"Tier 1 — in vitro biochemical and structural characterization with multiple orthogonal methods","pmids":["38966985"],"is_preprint":false},{"year":2025,"finding":"Purified recombinant mouse SPO11-TOP6BL complex reconstitutes DNA cleavage in vitro, forming covalent 5' attachments to broken DNA ends. The complex is monomeric (1:1) in solution; cleavage requires SPO11 dimerization (2:2 assemblies), active-site residues, and divalent metal ions (Mg2+), but is ATP-independent. SPO11 can also reseal nicked DNA. AlphaFold 3 modeling suggests DNA bending prior to cleavage. In vitro cleavage shows a rotationally symmetric sequence bias that partially explains in vivo DSB site preferences.","method":"In vitro reconstitution with purified recombinant proteins, covalent complex assays, active-site mutagenesis, AlphaFold 3 structural modelling, deep sequencing of cleavage products, artificial dimerization module fusion","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — full in vitro reconstitution with mutagenesis and structural modelling, replicated independently by two other labs in the same journal issue","pmids":["39972129","39972130","39972125"],"is_preprint":false},{"year":2025,"finding":"Mouse SPO11-TOP6BL forms a 1:1 complex that catalyzes DNA cleavage in vitro with activity similar to SPO11 alone, but the complex binds DNA ends with higher affinity than SPO11 alone, suggesting TOP6BL's main role may be post-cleavage (e.g., stabilizing the DSB complex).","method":"In vitro reconstitution, DNA cleavage assays, DNA end-binding affinity measurements","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with quantitative affinity comparisons","pmids":["39972130"],"is_preprint":false},{"year":2025,"finding":"In vitro reconstitution with mouse SPO11-TOP6BL shows that Mg2+ is essential for DNA cleavage activity; a knock-in point mutation in SPO11 disrupting Mg2+ binding abolishes meiotic DSB formation in vivo. The SPO11 complex activity is ATP-independent and biochemically distinct from archaeal TopoVI.","method":"In vitro cleavage assays with metal ion chelation, knock-in mouse models, cytological DSB assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — in vitro biochemistry validated by KI mouse models","pmids":["39972125"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structures of yeast Spo11 core complex (with Rec102/Rec104/Ski8, the yeast Top6BL-containing complex) bound to DNA at up to 3.3 Å resolution reveal that monomeric core complexes make extensive contacts with the DNA backbone, the recessed 3'-OH, and the first 5' overhanging nucleotide, defining molecular determinants of DNA end-binding and cleavage preference. Structures reveal unexpected structural variation in homologs of the Top6BL component.","method":"Cryo-electron microscopy, functional assays in yeast","journal":"Nature Structural & Molecular Biology","confidence":"High","confidence_rationale":"Tier 1 — high-resolution cryo-EM structure with functional validation in yeast","pmids":["39304764"],"is_preprint":false},{"year":2025,"finding":"TOP6BL variants (p.Arg515Ter and p.Pro356Arg) cause non-obstructive azoospermia by distinct mechanisms: the truncation variant (p.Arg515Ter) impairs binding to REC114 and disrupts interaction with SPO11, while the missense variant (p.Pro356Arg) impairs TOP6BL self-dimerization without affecting protein binding partners. Deletion of the TOP6BL central region in mice causes meiotic arrest at the zygotene stage.","method":"Whole-exome sequencing, Sanger sequencing, Co-IP/binding assays for REC114 and SPO11 interactions, self-dimerization assays, mouse central-region deletion model with meiotic staging","journal":"Reproduction","confidence":"High","confidence_rationale":"Tier 2 — multiple distinct variants with mechanistically distinct functional consequences tested by binding and mouse KO experiments","pmids":["41211863"],"is_preprint":false},{"year":2023,"finding":"IHO1 directly interacts with the PH domain of REC114 by recognizing the same surface as TOPOVIBL (and ANKRD31), indicating that REC114 acts as a regulatory platform mediating mutually exclusive interactions among IHO1, TOPOVIBL, and ANKRD31 to control DSB complex assembly.","method":"AlphaFold2 modeling, biochemical co-purification, pull-down assays, SEC-MALS for complex stoichiometry","journal":"The EMBO Journal","confidence":"High","confidence_rationale":"Tier 1–2 — structural modelling with biochemical validation of competitive binding surfaces","pmids":["37431931"],"is_preprint":false},{"year":2022,"finding":"Phylogenetic analysis demonstrates that TOPOVIBL has undergone extensive divergence compared to SPO11, with ATP hydrolysis mutations or truncations in multiple eukaryotic lineages, consistent with loss of ATPase function and evolution toward a meiotic DSB-formation role distinct from ancestral topoisomerase activity.","method":"Phylogenetic analysis, model structure comparisons","journal":"Molecular Biology and Evolution","confidence":"Medium","confidence_rationale":"Tier 4 — evolutionary/computational analysis supporting functional inference, consistent with biochemical data from other studies","pmids":["36256608"],"is_preprint":false}],"current_model":"TOP6BL (TOPOVIBL) is an evolutionarily derived, ATP-hydrolysis-deficient paralog of the archaeal TopoVIB subunit that forms a monomeric 1:1 complex with SPO11; together they constitute the catalytic TOPOVIL complex that introduces meiotic DNA double-strand breaks via a covalent 5'-phosphotyrosyl intermediate requiring Mg2+, SPO11 dimerization (2:2), and DNA bending, but not ATP hydrolysis, while TOP6BL additionally senses DNA geometry, binds DNA ends with high affinity post-cleavage, and serves as a regulatory scaffold that recruits accessory factors REC114 (through a conserved domain) and controls the efficiency, timing, and genomic distribution of DSBs essential for meiotic recombination and fertility."},"narrative":{"teleology":[{"year":2016,"claim":"Identification of TOPOVIBL as the long-sought partner of SPO11 established that meiotic DSB formation requires a TopoVI-like heteromeric complex, resolving how SPO11 is activated in vivo.","evidence":"Co-IP of TOPOVIBL–SPO11, structural homology to archaeal TopoVIB, and mouse knockout abolishing meiotic DSBs","pmids":["26917764"],"confidence":"High","gaps":["Whether TOPOVIBL retains enzymatic (ATPase) activity was unknown","How TOPOVIBL connects to upstream DSB-promoting factors was unresolved","Whether the complex is sufficient for DNA cleavage in vitro was untested"]},{"year":2022,"claim":"Demonstration that TOPOVIBL directly recruits REC114 through conserved domains revealed the molecular link between the catalytic core and the DSB-regulatory axis, explaining how DSB number and distribution are controlled genome-wide.","evidence":"Structural domain mapping, Co-IP, and point-mutation knock-in mice with genome-wide DSB profiling in oocytes and spermatocytes","pmids":["36396648"],"confidence":"High","gaps":["Whether REC114 binding is mutually exclusive with other regulatory factors was not yet known","The precise stoichiometry of the TOPOVIBL–REC114 interaction in vivo was not determined"]},{"year":2022,"claim":"Phylogenetic analysis revealed that TOPOVIBL has diverged far more than SPO11, with ATPase-inactivating mutations across eukaryotes, framing the functional shift from topoisomerase to meiotic DSB catalyst.","evidence":"Phylogenetic reconstruction and model structure comparisons across eukaryotic lineages","pmids":["36256608"],"confidence":"Medium","gaps":["Computational inference without direct biochemical confirmation of ATP independence at that time","Whether structural divergence correlates with gain of novel regulatory interactions was untested"]},{"year":2023,"claim":"Discovery that REC114's PH domain mediates mutually exclusive binding of TOPOVIBL, IHO1, and ANKRD31 established a competitive-interaction model for stepwise DSB complex assembly.","evidence":"AlphaFold2 modeling validated by co-purification, pull-downs, and SEC-MALS stoichiometry measurements","pmids":["37431931"],"confidence":"High","gaps":["In vivo dynamics of the competitive exchange between IHO1 and TOPOVIBL on REC114 were not captured","Whether additional post-translational modifications regulate the switch was not addressed"]},{"year":2024,"claim":"Biochemical characterization showed TOPOVIBL is monomeric, does not bind ATP, preferentially engages supercoiled/branched DNA, and adopts a dynamic conformation, establishing it as a DNA-geometry sensor rather than an ATPase.","evidence":"Protein purification, SEC, ATPase assays, SAXS, and DNA-binding assays with varied substrates","pmids":["38966985"],"confidence":"High","gaps":["How DNA-geometry sensing translates to site selection in vivo was not determined","Whether conformational dynamics are regulated by accessory factors was unknown"]},{"year":2024,"claim":"Cryo-EM structures of the yeast Spo11 core complex (containing Top6BL homologs Rec102/Rec104) bound to DNA defined the molecular contacts underlying DNA end recognition, providing the first high-resolution view of the meiotic DSB machinery on its substrate.","evidence":"Cryo-EM at up to 3.3 Å resolution with functional yeast assays","pmids":["39304764"],"confidence":"High","gaps":["Structures captured post-cleavage complexes; the pre-cleavage DNA-engaged state was not resolved","Whether mammalian TOPOVIBL makes equivalent DNA contacts was not established"]},{"year":2025,"claim":"In vitro reconstitution of the SPO11–TOP6BL complex demonstrated that the heterodimer is catalytically competent for DNA cleavage via covalent 5′-phosphotyrosyl intermediates, requires Mg²⁺ and SPO11 dimerization but not ATP, and that TOP6BL enhances DNA-end binding post-cleavage — three independent studies converged on this mechanism.","evidence":"Purified recombinant protein reconstitution, covalent complex assays, active-site mutagenesis, AlphaFold 3 modeling, deep sequencing of cleavage products, knock-in mouse validation","pmids":["39972129","39972130","39972125"],"confidence":"High","gaps":["How accessory factors (REC114, IHO1, ANKRD31) modulate catalytic activity in vitro was not tested","The structural basis for DNA bending prior to cleavage lacks experimental atomic-resolution data","Whether the rotationally symmetric sequence bias fully explains in vivo hotspot selection is unclear"]},{"year":2025,"claim":"Identification of distinct human TOP6BL pathogenic variants causing non-obstructive azoospermia through separable mechanisms — loss of REC114/SPO11 binding versus impaired self-dimerization — demonstrated that TOP6BL is essential for human fertility and has multiple functional surfaces.","evidence":"Whole-exome sequencing of infertile patients, Co-IP/binding assays for REC114 and SPO11, self-dimerization assays, mouse central-region deletion model","pmids":["36732965","41211863"],"confidence":"High","gaps":["The functional role of TOP6BL self-dimerization in the cleavage mechanism is not understood","Whether heterozygous carriers have subtle meiotic defects has not been assessed"]},{"year":null,"claim":"Key unresolved questions include the structural basis of the full mammalian SPO11–TOP6BL complex on DNA, how accessory factors modulate catalytic activity, and the mechanism by which TOP6BL self-dimerization contributes to DSB formation.","evidence":"","pmids":[],"confidence":"High","gaps":["No high-resolution structure of the mammalian SPO11–TOP6BL complex exists","In vitro reconstitution with the full accessory factor complement has not been achieved","The functional significance of TOP6BL self-dimerization versus the obligate 2:2 SPO11 dimer is unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[3,5,7]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[4,5,6]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,3,9]}],"localization":[{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[0,1,2]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,4,6]},{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[0,1,2,8]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[0,2]}],"complexes":["SPO11-TOPOVIBL (TOPOVIL)"],"partners":["SPO11","REC114","IHO1","ANKRD31"],"other_free_text":[]},"mechanistic_narrative":"TOP6BL (TOPOVIBL) is the regulatory subunit of the meiotic DNA double-strand break (DSB) machinery, partnering with SPO11 to form the TOPOVIL complex that initiates meiotic recombination. TOP6BL derives from the archaeal TopoVIB topoisomerase subunit but has lost ATP-binding and hydrolysis capacity; instead it exists as a monomer that senses DNA geometry, preferentially engaging supercoiled or branched substrates, and stabilizes DSB ends with high affinity after SPO11-mediated cleavage, which requires Mg²⁺-dependent SPO11 dimerization (2:2) but is ATP-independent [PMID:39972129, PMID:39972130, PMID:38966985]. TOP6BL additionally serves as a regulatory scaffold by recruiting REC114 through conserved interacting domains—an interaction that is mutually exclusive with IHO1 and ANKRD31 binding to REC114—thereby controlling the efficiency, timing, and genomic distribution of meiotic DSBs [PMID:36396648, PMID:37431931]. Autosomal recessive loss-of-function mutations in TOP6BL cause meiotic arrest and non-obstructive azoospermia in humans [PMID:36732965, PMID:41211863]."},"prefetch_data":{"uniprot":{"accession":"Q8N6T0","full_name":"Type 2 DNA topoisomerase 6 subunit B-like","aliases":["TOP6B like initiator of meiotic double strand breaks","Type 2 DNA topoisomerase VI subunit B-like","TOPOVIBL"],"length_aa":511,"mass_kda":57.0,"function":"Component of a topoisomerase 6 complex specifically required for meiotic recombination. Together with SPO11, mediates DNA cleavage that forms the double-strand breaks (DSB) that initiate meiotic recombination. The complex promotes relaxation of negative and positive supercoiled DNA and DNA decatenation through cleavage and ligation cycles","subcellular_location":"Chromosome","url":"https://www.uniprot.org/uniprotkb/Q8N6T0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TOP6BL","classification":"Not Classified","n_dependent_lines":10,"n_total_lines":1208,"dependency_fraction":0.008278145695364239},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TOP6BL","total_profiled":1310},"omim":[{"mim_id":"618432","title":"HYDATIDIFORM MOLE, RECURRENT, 4; HYDM4","url":"https://www.omim.org/entry/618432"},{"mim_id":"616109","title":"TOP6B-LIKE INITIATOR OF MEIOTIC DOUBLE STRAND BREAKS; TOP6BL","url":"https://www.omim.org/entry/616109"},{"mim_id":"605114","title":"SPO11 INITIATOR OF MEIOTIC DOUBLE-STRANDED BREAKS; SPO11","url":"https://www.omim.org/entry/605114"},{"mim_id":"231090","title":"HYDATIDIFORM MOLE, RECURRENT, 1; HYDM1","url":"https://www.omim.org/entry/231090"}],"hpa":{"profiled":true,"resolved_as":"C11ORF80","reliability":"Approved","locations":[{"location":"Centrosome","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/C11ORF80"},"hgnc":{"alias_symbol":["FLJ22531","TOPOVIBL"],"prev_symbol":["C11orf80"]},"alphafold":{"accession":"Q8N6T0","domains":[{"cath_id":"3.30.565","chopping":"2-95_103-161","consensus_level":"medium","plddt":78.3662,"start":2,"end":161},{"cath_id":"-","chopping":"163-312_321-382","consensus_level":"medium","plddt":81.5739,"start":163,"end":382},{"cath_id":"-","chopping":"386-449","consensus_level":"medium","plddt":85.2025,"start":386,"end":449}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N6T0","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N6T0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8N6T0-F1-predicted_aligned_error_v6.png","plddt_mean":67.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TOP6BL","jax_strain_url":"https://www.jax.org/strain/search?query=TOP6BL"},"sequence":{"accession":"Q8N6T0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8N6T0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8N6T0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8N6T0"}},"corpus_meta":[{"pmid":"26917764","id":"PMC_26917764","title":"The TopoVIB-Like protein family is required for meiotic DNA double-strand break formation.","date":"2016","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/26917764","citation_count":236,"is_preprint":false},{"pmid":"36396648","id":"PMC_36396648","title":"TOPOVIBL-REC114 interaction regulates meiotic DNA double-strand breaks.","date":"2022","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/36396648","citation_count":34,"is_preprint":false},{"pmid":"35413094","id":"PMC_35413094","title":"Whole-exome sequencing in patients with maturation arrest: a potential additional diagnostic tool for prevention of recurrent negative testicular sperm extraction outcomes.","date":"2022","source":"Human reproduction (Oxford, England)","url":"https://pubmed.ncbi.nlm.nih.gov/35413094","citation_count":31,"is_preprint":false},{"pmid":"36732965","id":"PMC_36732965","title":"A TOP6BL mutation abolishes meiotic DNA double-strand break formation and causes human 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controls meiotic DNA double-strand break formation","date":"2024-11-20","source":"bioRxiv","url":"https://doi.org/10.1101/2024.11.20.624454","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":12636,"output_tokens":2672,"usd":0.038994},"stage2":{"model":"claude-opus-4-6","input_tokens":6012,"output_tokens":2504,"usd":0.13899},"total_usd":0.177984,"stage1_batch_id":"msgbatch_01RPbVc9PDH9zQmYK5Hs6srm","stage2_batch_id":"msgbatch_01JVbVLzeKN2yVrsGp3zC8aM","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2016,\n      \"finding\": \"TOPOVIBL (mouse) physically interacts with SPO11 and forms a complex required for meiotic DNA double-strand break (DSB) formation. TOPOVIBL shares strong structural similarity to the TopoVIB subunit of archaeal TopoVI topoisomerase, and its loss abolishes meiotic DSBs.\",\n      \"method\": \"Co-immunoprecipitation, structural similarity analysis, mouse knockout with meiotic DSB phenotyping\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal complex identification with functional KO phenotype, replicated by multiple subsequent studies\",\n      \"pmids\": [\"26917764\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"TOPOVIBL directly interacts with REC114 through conserved interacting domains; this interaction is required for the efficiency and timing of meiotic DSB formation genome-wide in oocytes and in sub-telomeric regions of spermatocytes.\",\n      \"method\": \"Structural analysis of interacting domains, Co-IP, point-mutation knock-in mice with genome-wide DSB monitoring\",\n      \"journal\": \"Nature Communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — structural domain mapping combined with in vivo point-mutation mouse models and genome-wide DSB assays\",\n      \"pmids\": [\"36396648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"An autosomal recessive loss-of-function mutation in TOP6BL causes failure of programmed meiotic DSB formation and meiotic arrest prior to the pachytene stage in humans; mouse models with equivalent mutations recapitulate the spermatogenic arrest and absence of DSBs in oocytes.\",\n      \"method\": \"Whole-exome sequencing, Sanger sequencing, histopathology of patient testis biopsy, knock-in mouse models with DSB assays\",\n      \"journal\": \"Science Bulletin\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — human genetics confirmed by mouse KI models with mechanistic DSB readout\",\n      \"pmids\": [\"36732965\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Purified TOPOVIBL (TOPOVIBLΔC25) is monomeric in solution, does not bind ATP (unlike its archaeal TopoVIB ancestor), interacts with DNA with a preference for specific geometries (e.g., supercoiled or branched substrates), and adopts a dynamic conformation, suggesting TOPOVIBL senses specific DNA architectures rather than hydrolyzing ATP.\",\n      \"method\": \"Protein purification, size-exclusion chromatography, ATPase assay, SAXS/structural analysis, DNA-binding assays with different DNA geometries\",\n      \"journal\": \"Nucleic Acids Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro biochemical and structural characterization with multiple orthogonal methods\",\n      \"pmids\": [\"38966985\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Purified recombinant mouse SPO11-TOP6BL complex reconstitutes DNA cleavage in vitro, forming covalent 5' attachments to broken DNA ends. The complex is monomeric (1:1) in solution; cleavage requires SPO11 dimerization (2:2 assemblies), active-site residues, and divalent metal ions (Mg2+), but is ATP-independent. SPO11 can also reseal nicked DNA. AlphaFold 3 modeling suggests DNA bending prior to cleavage. In vitro cleavage shows a rotationally symmetric sequence bias that partially explains in vivo DSB site preferences.\",\n      \"method\": \"In vitro reconstitution with purified recombinant proteins, covalent complex assays, active-site mutagenesis, AlphaFold 3 structural modelling, deep sequencing of cleavage products, artificial dimerization module fusion\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — full in vitro reconstitution with mutagenesis and structural modelling, replicated independently by two other labs in the same journal issue\",\n      \"pmids\": [\"39972129\", \"39972130\", \"39972125\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Mouse SPO11-TOP6BL forms a 1:1 complex that catalyzes DNA cleavage in vitro with activity similar to SPO11 alone, but the complex binds DNA ends with higher affinity than SPO11 alone, suggesting TOP6BL's main role may be post-cleavage (e.g., stabilizing the DSB complex).\",\n      \"method\": \"In vitro reconstitution, DNA cleavage assays, DNA end-binding affinity measurements\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with quantitative affinity comparisons\",\n      \"pmids\": [\"39972130\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In vitro reconstitution with mouse SPO11-TOP6BL shows that Mg2+ is essential for DNA cleavage activity; a knock-in point mutation in SPO11 disrupting Mg2+ binding abolishes meiotic DSB formation in vivo. The SPO11 complex activity is ATP-independent and biochemically distinct from archaeal TopoVI.\",\n      \"method\": \"In vitro cleavage assays with metal ion chelation, knock-in mouse models, cytological DSB assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro biochemistry validated by KI mouse models\",\n      \"pmids\": [\"39972125\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structures of yeast Spo11 core complex (with Rec102/Rec104/Ski8, the yeast Top6BL-containing complex) bound to DNA at up to 3.3 Å resolution reveal that monomeric core complexes make extensive contacts with the DNA backbone, the recessed 3'-OH, and the first 5' overhanging nucleotide, defining molecular determinants of DNA end-binding and cleavage preference. Structures reveal unexpected structural variation in homologs of the Top6BL component.\",\n      \"method\": \"Cryo-electron microscopy, functional assays in yeast\",\n      \"journal\": \"Nature Structural & Molecular Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution cryo-EM structure with functional validation in yeast\",\n      \"pmids\": [\"39304764\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"TOP6BL variants (p.Arg515Ter and p.Pro356Arg) cause non-obstructive azoospermia by distinct mechanisms: the truncation variant (p.Arg515Ter) impairs binding to REC114 and disrupts interaction with SPO11, while the missense variant (p.Pro356Arg) impairs TOP6BL self-dimerization without affecting protein binding partners. Deletion of the TOP6BL central region in mice causes meiotic arrest at the zygotene stage.\",\n      \"method\": \"Whole-exome sequencing, Sanger sequencing, Co-IP/binding assays for REC114 and SPO11 interactions, self-dimerization assays, mouse central-region deletion model with meiotic staging\",\n      \"journal\": \"Reproduction\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple distinct variants with mechanistically distinct functional consequences tested by binding and mouse KO experiments\",\n      \"pmids\": [\"41211863\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"IHO1 directly interacts with the PH domain of REC114 by recognizing the same surface as TOPOVIBL (and ANKRD31), indicating that REC114 acts as a regulatory platform mediating mutually exclusive interactions among IHO1, TOPOVIBL, and ANKRD31 to control DSB complex assembly.\",\n      \"method\": \"AlphaFold2 modeling, biochemical co-purification, pull-down assays, SEC-MALS for complex stoichiometry\",\n      \"journal\": \"The EMBO Journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — structural modelling with biochemical validation of competitive binding surfaces\",\n      \"pmids\": [\"37431931\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Phylogenetic analysis demonstrates that TOPOVIBL has undergone extensive divergence compared to SPO11, with ATP hydrolysis mutations or truncations in multiple eukaryotic lineages, consistent with loss of ATPase function and evolution toward a meiotic DSB-formation role distinct from ancestral topoisomerase activity.\",\n      \"method\": \"Phylogenetic analysis, model structure comparisons\",\n      \"journal\": \"Molecular Biology and Evolution\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 4 — evolutionary/computational analysis supporting functional inference, consistent with biochemical data from other studies\",\n      \"pmids\": [\"36256608\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"TOP6BL (TOPOVIBL) is an evolutionarily derived, ATP-hydrolysis-deficient paralog of the archaeal TopoVIB subunit that forms a monomeric 1:1 complex with SPO11; together they constitute the catalytic TOPOVIL complex that introduces meiotic DNA double-strand breaks via a covalent 5'-phosphotyrosyl intermediate requiring Mg2+, SPO11 dimerization (2:2), and DNA bending, but not ATP hydrolysis, while TOP6BL additionally senses DNA geometry, binds DNA ends with high affinity post-cleavage, and serves as a regulatory scaffold that recruits accessory factors REC114 (through a conserved domain) and controls the efficiency, timing, and genomic distribution of DSBs essential for meiotic recombination and fertility.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"TOP6BL (TOPOVIBL) is the regulatory subunit of the meiotic DNA double-strand break (DSB) machinery, partnering with SPO11 to form the TOPOVIL complex that initiates meiotic recombination. TOP6BL derives from the archaeal TopoVIB topoisomerase subunit but has lost ATP-binding and hydrolysis capacity; instead it exists as a monomer that senses DNA geometry, preferentially engaging supercoiled or branched substrates, and stabilizes DSB ends with high affinity after SPO11-mediated cleavage, which requires Mg²⁺-dependent SPO11 dimerization (2:2) but is ATP-independent [PMID:39972129, PMID:39972130, PMID:38966985]. TOP6BL additionally serves as a regulatory scaffold by recruiting REC114 through conserved interacting domains—an interaction that is mutually exclusive with IHO1 and ANKRD31 binding to REC114—thereby controlling the efficiency, timing, and genomic distribution of meiotic DSBs [PMID:36396648, PMID:37431931]. Autosomal recessive loss-of-function mutations in TOP6BL cause meiotic arrest and non-obstructive azoospermia in humans [PMID:36732965, PMID:41211863].\",\n  \"teleology\": [\n    {\n      \"year\": 2016,\n      \"claim\": \"Identification of TOPOVIBL as the long-sought partner of SPO11 established that meiotic DSB formation requires a TopoVI-like heteromeric complex, resolving how SPO11 is activated in vivo.\",\n      \"evidence\": \"Co-IP of TOPOVIBL–SPO11, structural homology to archaeal TopoVIB, and mouse knockout abolishing meiotic DSBs\",\n      \"pmids\": [\"26917764\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether TOPOVIBL retains enzymatic (ATPase) activity was unknown\",\n        \"How TOPOVIBL connects to upstream DSB-promoting factors was unresolved\",\n        \"Whether the complex is sufficient for DNA cleavage in vitro was untested\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstration that TOPOVIBL directly recruits REC114 through conserved domains revealed the molecular link between the catalytic core and the DSB-regulatory axis, explaining how DSB number and distribution are controlled genome-wide.\",\n      \"evidence\": \"Structural domain mapping, Co-IP, and point-mutation knock-in mice with genome-wide DSB profiling in oocytes and spermatocytes\",\n      \"pmids\": [\"36396648\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether REC114 binding is mutually exclusive with other regulatory factors was not yet known\",\n        \"The precise stoichiometry of the TOPOVIBL–REC114 interaction in vivo was not determined\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Phylogenetic analysis revealed that TOPOVIBL has diverged far more than SPO11, with ATPase-inactivating mutations across eukaryotes, framing the functional shift from topoisomerase to meiotic DSB catalyst.\",\n      \"evidence\": \"Phylogenetic reconstruction and model structure comparisons across eukaryotic lineages\",\n      \"pmids\": [\"36256608\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Computational inference without direct biochemical confirmation of ATP independence at that time\",\n        \"Whether structural divergence correlates with gain of novel regulatory interactions was untested\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Discovery that REC114's PH domain mediates mutually exclusive binding of TOPOVIBL, IHO1, and ANKRD31 established a competitive-interaction model for stepwise DSB complex assembly.\",\n      \"evidence\": \"AlphaFold2 modeling validated by co-purification, pull-downs, and SEC-MALS stoichiometry measurements\",\n      \"pmids\": [\"37431931\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"In vivo dynamics of the competitive exchange between IHO1 and TOPOVIBL on REC114 were not captured\",\n        \"Whether additional post-translational modifications regulate the switch was not addressed\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Biochemical characterization showed TOPOVIBL is monomeric, does not bind ATP, preferentially engages supercoiled/branched DNA, and adopts a dynamic conformation, establishing it as a DNA-geometry sensor rather than an ATPase.\",\n      \"evidence\": \"Protein purification, SEC, ATPase assays, SAXS, and DNA-binding assays with varied substrates\",\n      \"pmids\": [\"38966985\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How DNA-geometry sensing translates to site selection in vivo was not determined\",\n        \"Whether conformational dynamics are regulated by accessory factors was unknown\"\n      ]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Cryo-EM structures of the yeast Spo11 core complex (containing Top6BL homologs Rec102/Rec104) bound to DNA defined the molecular contacts underlying DNA end recognition, providing the first high-resolution view of the meiotic DSB machinery on its substrate.\",\n      \"evidence\": \"Cryo-EM at up to 3.3 Å resolution with functional yeast assays\",\n      \"pmids\": [\"39304764\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structures captured post-cleavage complexes; the pre-cleavage DNA-engaged state was not resolved\",\n        \"Whether mammalian TOPOVIBL makes equivalent DNA contacts was not established\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"In vitro reconstitution of the SPO11–TOP6BL complex demonstrated that the heterodimer is catalytically competent for DNA cleavage via covalent 5′-phosphotyrosyl intermediates, requires Mg²⁺ and SPO11 dimerization but not ATP, and that TOP6BL enhances DNA-end binding post-cleavage — three independent studies converged on this mechanism.\",\n      \"evidence\": \"Purified recombinant protein reconstitution, covalent complex assays, active-site mutagenesis, AlphaFold 3 modeling, deep sequencing of cleavage products, knock-in mouse validation\",\n      \"pmids\": [\"39972129\", \"39972130\", \"39972125\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How accessory factors (REC114, IHO1, ANKRD31) modulate catalytic activity in vitro was not tested\",\n        \"The structural basis for DNA bending prior to cleavage lacks experimental atomic-resolution data\",\n        \"Whether the rotationally symmetric sequence bias fully explains in vivo hotspot selection is unclear\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identification of distinct human TOP6BL pathogenic variants causing non-obstructive azoospermia through separable mechanisms — loss of REC114/SPO11 binding versus impaired self-dimerization — demonstrated that TOP6BL is essential for human fertility and has multiple functional surfaces.\",\n      \"evidence\": \"Whole-exome sequencing of infertile patients, Co-IP/binding assays for REC114 and SPO11, self-dimerization assays, mouse central-region deletion model\",\n      \"pmids\": [\"36732965\", \"41211863\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The functional role of TOP6BL self-dimerization in the cleavage mechanism is not understood\",\n        \"Whether heterozygous carriers have subtle meiotic defects has not been assessed\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of the full mammalian SPO11–TOP6BL complex on DNA, how accessory factors modulate catalytic activity, and the mechanism by which TOP6BL self-dimerization contributes to DSB formation.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No high-resolution structure of the mammalian SPO11–TOP6BL complex exists\",\n        \"In vitro reconstitution with the full accessory factor complement has not been achieved\",\n        \"The functional significance of TOP6BL self-dimerization versus the obligate 2:2 SPO11 dimer is unresolved\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [3, 5, 7]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [4, 5, 6]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 3, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 4, 6]},\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [0, 1, 2, 8]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"complexes\": [\n      \"SPO11-TOPOVIBL (TOPOVIL)\"\n    ],\n    \"partners\": [\n      \"SPO11\",\n      \"REC114\",\n      \"IHO1\",\n      \"ANKRD31\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}