{"gene":"IHO1","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":2016,"finding":"IHO1 (CCDC36) was identified as a direct interactor of HORMAD1 and shown to be essential for meiotic DNA double-strand break (DSB) formation in mice. IHO1, together with SPO11-auxiliary proteins MEI4 and REC114, assembles chromatin-bound recombinosomes (pre-DSB complexes) on meiotic chromosome axes. HORMAD1 is required for robust recruitment of IHO1 to unsynapsed axes, and this HORMAD1-IHO1 interaction provides a mechanism for selective promotion of DSB formation along unsynapsed chromosome axes.","method":"Co-immunoprecipitation, yeast two-hybrid, cytological localization (immunofluorescence), mouse knockout","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, in vivo KO with defined phenotype, multiple orthogonal methods in a highly-cited foundational paper","pmids":["27723721"],"is_preprint":false},{"year":2017,"finding":"CXXC1, a member of the COMPASS complex and ortholog of yeast Spp1, interacts with IHO1. This interaction was identified by yeast two-hybrid and suggests that the molecular link between DSB sites (via PRDM9-CXXC1) and the DSB machinery on the chromosome axis (via IHO1) is conserved in mouse meiosis.","method":"Yeast two-hybrid assay","journal":"Chromosoma","confidence":"Medium","confidence_rationale":"Tier 3 — single yeast two-hybrid interaction, supported by evolutionary conservation argument","pmids":["28527011"],"is_preprint":false},{"year":2017,"finding":"Mer2 (the IHO1 ortholog in Sordaria/fungi) is evolutionarily conserved from fungi to plants (PRD3/PAIR1) and mammals (IHO1). Beyond its role in DSB formation, Mer2 functions in homolog spatial juxtaposition for pairing, transfer/maintenance of recombination complexes to the synaptonemal complex central region, and global chromosome compaction post-recombination. SUMOylation is implicated in the compaction role.","method":"Genetic analysis of 13 mer2 mutants, sequential localization of Mer2 to axis/SC/chromatin in Sordaria, cytological and functional assays","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 — extensive mutant analysis with orthogonal functional readouts in a conserved ortholog, replicated across multiple alleles","pmids":["29021238"],"is_preprint":false},{"year":2018,"finding":"Mouse REC114 is required for meiotic DSB formation and forms a stable complex with MEI4 and IHO1 in spermatocytes. In vitro, the REC114 C-terminal domain forms a stable complex with the MEI4 N-terminal domain. The REC114 N-terminal domain has structural similarity to Pleckstrin homology (PH) domains.","method":"Mouse knockout, co-immunoprecipitation, in vitro complex reconstitution, X-ray crystallography","journal":"Life science alliance","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution and crystal structure combined with in vivo Co-IP and KO phenotype","pmids":["30569039"],"is_preprint":false},{"year":2019,"finding":"ANKRD31 is a key component of DSB-promoting protein complexes on meiotic chromosome axes and interacts with the same complexes as IHO1. Loss of ANKRD31 causes genome-wide delay in assembling DSB-promoting proteins (including IHO1) on autosome axes and abolishes the specialized pseudoautosomal region (PAR)-axis domain enriched for DSB-promoting proteins, resulting in failure of X-Y crossover formation.","method":"Mouse knockout, immunofluorescence co-localization, chromosome spread analysis","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined molecular and cellular phenotype, IHO1 axis recruitment directly measured","pmids":["31000436"],"is_preprint":false},{"year":2019,"finding":"PRDM9 interacts with cohesin subunits STAG3 and REC8 in a cooperative relationship that promotes meiotic DSB formation. STAG3 and REC8 promote axis localization of HORMAD1, IHO1, and MEI4, demonstrating that meiotic cohesin complexes are required upstream of IHO1 axis recruitment.","method":"Co-immunoprecipitation, genetic epistasis (double mutants), immunofluorescence in spermatocytes","journal":"Current biology : CB","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP plus genetic epistasis with direct measurement of IHO1 localization","pmids":["30853435"],"is_preprint":false},{"year":2021,"finding":"IHO1 serves as the main anchor for pre-DSB recombinosomes (containing MEI4 and REC114) on chromosome axes. DSBs negatively feed back on the DSB machinery through four distinct pathways: (1) ATM activation restricts pre-DSB recombinosome numbers without affecting IHO1 levels; (2) ATR triggers IHO1 depletion locally near DSBs; (3) synapsis (enabled by DSBs) promotes depletion of IHO1 and pre-DSB recombinosomes from synapsed axes; (4) ATM, ATR, and PRKDC together enable stage-specific depletion of IHO1 from all axes.","method":"Mouse genetics (kinase inhibitors and mutants), immunofluorescence, chromosome spread analysis","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic interventions dissecting four distinct pathways with direct IHO1 localization readout","pmids":["33619545"],"is_preprint":false},{"year":2021,"finding":"In zebrafish, Iho1 dissociation from chromosome axes occurs in a DSB-dependent manner, as persistent Iho1 foci are observed in spo11 mutant spermatocytes. This demonstrates that SPO11-dependent DSB formation is required for IHO1 removal from the axis.","method":"Zebrafish mutant analysis (sycp1 and spo11 mutants), immunofluorescence","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis in zebrafish ortholog with direct localization readout, single lab","pmids":["33842489"],"is_preprint":false},{"year":2022,"finding":"REC114 directly interacts with TOPOVIBL (the SPO11 partner in the TOPOVIL catalytic complex), and point mutations disrupting this interaction strongly reduce DSB activity genome-wide in oocytes and in sub-telomeric regions in spermatocytes. Since REC114 is a direct partner of both IHO1 and TOPOVIBL, REC114 serves as a key bridge linking IHO1-containing pre-DSB recombinosomes to the catalytic TOPOVIL complex.","method":"Structural analysis (crystallography/AlphaFold), co-immunoprecipitation, mouse point-mutant knockins, DSB quantification by immunofluorescence and sequencing","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — structural identification of interaction domains combined with in vivo point-mutant knockin and quantitative DSB assays","pmids":["36396648"],"is_preprint":false},{"year":2022,"finding":"SKP1, a constitutive subunit of the SCF ubiquitin E3 ligase, restrains accumulation of the IHO1-REC114-MEI4 pre-DSB complex on the chromosome axis. Mechanistically, the meiosis-specific F-box protein FBXO47 interacts with SKP1 and HORMAD1, targeting HORMAD1 for polyubiquitination and proteasomal degradation, which in turn modulates the pre-DSB complex including IHO1.","method":"Mouse conditional knockout, immunofluorescence, co-immunoprecipitation, ubiquitination assay in HEK293T cells","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 — KO with defined molecular phenotype plus biochemical ubiquitination assay establishing mechanism","pmids":["35489071"],"is_preprint":false},{"year":2023,"finding":"IHO1 forms coiled-coil-based tetramers. IHO1 directly interacts with the PH domain of REC114 using the same surface as TOPOVIBL and ANKRD31, indicating mutually exclusive interactions. Combined with AlphaFold2 modeling and biochemical characterization, this demonstrates a ternary IHO1-REC114-MEI4 complex architecture where REC114 acts as a regulatory platform for mutually exclusive partner interactions.","method":"AlphaFold2 modeling, biochemical reconstitution, analytical ultracentrifugation, multi-angle light scattering, crosslinking mass spectrometry","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — structural modeling combined with multiple orthogonal biochemical methods defining complex architecture","pmids":["37431931"],"is_preprint":false},{"year":2024,"finding":"Efficient biogenesis of DSB-machinery clusters on chromosome axes requires seeding by axial IHO1 platforms. IHO1 phosphorylation and formation of axial IHO1 platforms are diminished by chemical inhibition of DBF4-dependent kinase (DDK), implicating DDK as a regulator of IHO1 axis assembly. IHO1-HORMAD1 interaction mediates the seeding of DSB-machinery on axes; without this interaction, residual DSBs depend on ANKRD31, which enhances both seeding and growth of DSB-machinery clusters.","method":"Mouse genetics (IHO1-HORMAD1 interaction mutants), DDK chemical inhibition, super-resolution microscopy, quantitative immunofluorescence","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — genetic interaction mutants with multiple orthogonal readouts including kinase inhibition and super-resolution imaging","pmids":["38580643"],"is_preprint":false},{"year":2024,"finding":"PRDM9 binding sites promote recruitment of MEI4 and IHO1 to chromatin. IHO1 in turn anchors DSB sites to the chromosome axis components HORMAD1 and SYCP3. Additionally, IHO1, HORMAD1, and SYCP3 remain associated at DSB ends during DSB repair, linking DSB site identity to axis structure throughout the recombination process.","method":"ChIP-seq, immunofluorescence, co-localization analysis in spermatocytes","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 — genome-wide ChIP-seq combined with cytological co-localization establishing hierarchical recruitment","pmids":["38657614"],"is_preprint":false},{"year":2026,"finding":"MEI1 variants that cause non-obstructive azoospermia disrupt MEI1 interactions with ANKRD31, IHO1, REC114, and MEI4, as demonstrated by co-immunoprecipitation, establishing that MEI1 is a physical interaction partner of IHO1 within the meiotic DSB machinery.","method":"Co-immunoprecipitation in HEK293T cells with patient-derived MEI1 mutants","journal":"Journal of assisted reproduction and genetics","confidence":"Medium","confidence_rationale":"Tier 3 — single Co-IP demonstrating interaction, in disease-context mutants","pmids":["41706353"],"is_preprint":false}],"current_model":"IHO1 (CCDC36) is the mammalian ortholog of yeast Mer2 and functions as the primary axis-anchoring platform for pre-DSB recombinosomes on meiotic chromosome cores: it directly interacts with HORMAD1 (recruited via DDK-modulated phosphorylation), forms coiled-coil-based tetramers, and binds REC114 (via REC114's PH domain) to nucleate heterooligomeric IHO1-REC114-MEI4 complexes that activate SPO11/TOPOVIBL-dependent DSB formation; DSBs then negatively feed back on IHO1 through at least four pathways involving ATM, ATR, and PRKDC kinases and synapsis-coupled axis remodeling, while upstream regulators including meiotic cohesins, PRDM9-CXXC1, ANKRD31, and SCF-FBXO47-mediated ubiquitination collectively tune the spatiotemporal distribution and abundance of axis-associated IHO1 to ensure appropriate DSB numbers and distribution for homologous chromosome pairing."},"narrative":{"teleology":[{"year":2016,"claim":"Identification of IHO1 as a HORMAD1 interactor essential for meiotic DSB formation established the first known molecular link between the chromosome axis protein HORMAD1 and the SPO11-accessory DSB machinery in mammals.","evidence":"Co-IP, yeast two-hybrid, immunofluorescence, and Iho1-knockout mouse showing complete loss of DSBs","pmids":["27723721"],"confidence":"High","gaps":["Structural basis of IHO1–HORMAD1 interaction unknown","Whether IHO1 has roles beyond DSB initiation not tested","Mechanism linking HORMAD1 to IHO1 axis loading unresolved"]},{"year":2017,"claim":"Discovery that CXXC1 interacts with IHO1, paralleling the yeast Spp1–Mer2 axis-tethering mechanism, suggested a conserved loop-axis tethering module connecting PRDM9-marked hotspots to the DSB machinery on axes.","evidence":"Yeast two-hybrid assay detecting CXXC1–IHO1 interaction","pmids":["28527011"],"confidence":"Medium","gaps":["Interaction not confirmed by reciprocal Co-IP or in vivo disruption","Functional consequence of disrupting the CXXC1–IHO1 link not tested in mice"]},{"year":2017,"claim":"Analysis of the fungal ortholog Mer2 revealed conserved roles extending beyond DSB formation to homolog juxtaposition, recombination complex transfer, and chromosome compaction, broadening the functional repertoire attributable to IHO1-family proteins.","evidence":"Thirteen Sordaria mer2 mutants with sequential cytological and functional assays","pmids":["29021238"],"confidence":"High","gaps":["Whether mammalian IHO1 retains the compaction and SC-transfer roles demonstrated for fungal Mer2 is untested","SUMOylation of mammalian IHO1 not examined"]},{"year":2018,"claim":"Reconstitution of a stable REC114–MEI4 sub-complex and demonstration that REC114, MEI4, and IHO1 co-immunoprecipitate in vivo defined the tripartite pre-DSB recombinosome and identified REC114's PH-domain fold as a critical interaction surface.","evidence":"Mouse Rec114 knockout, in vitro reconstitution, X-ray crystallography of REC114 C-terminal/MEI4 N-terminal complex","pmids":["30569039"],"confidence":"High","gaps":["Direct IHO1–REC114 binding interface not yet mapped","Stoichiometry of the ternary complex unknown"]},{"year":2019,"claim":"Placing meiotic cohesins (REC8–STAG3) and ANKRD31 upstream of IHO1 axis loading resolved the hierarchical dependency for pre-DSB complex assembly: cohesins recruit HORMAD1/IHO1, and ANKRD31 ensures timely accumulation especially at the PAR.","evidence":"Mouse knockouts of ANKRD31 and cohesin subunits with direct IHO1 localization readout by immunofluorescence","pmids":["31000436","30853435"],"confidence":"High","gaps":["Whether ANKRD31 acts directly on IHO1 or indirectly through other axis components not resolved","Biochemical mechanism by which cohesins promote IHO1 loading unknown"]},{"year":2021,"claim":"Dissection of four distinct negative-feedback pathways—ATM restricting recombinosome numbers, ATR locally depleting IHO1 near DSBs, synapsis removing IHO1 from synapsed axes, and combined ATM/ATR/PRKDC enabling global IHO1 depletion—established IHO1 as the central regulatory target ensuring DSB homeostasis.","evidence":"Mouse kinase mutants and inhibitors with quantitative immunofluorescence of IHO1 on chromosome spreads; zebrafish spo11 mutants showing DSB-dependent IHO1 removal","pmids":["33619545","33842489"],"confidence":"High","gaps":["Direct phosphorylation sites on IHO1 targeted by ATM/ATR/PRKDC not identified","Mechanistic basis of synapsis-coupled IHO1 stripping unresolved"]},{"year":2022,"claim":"Structural and genetic evidence that REC114 bridges IHO1-containing pre-DSB recombinosomes to the catalytic TOPOVIL complex via its PH domain, and that SCF-FBXO47 ubiquitinates HORMAD1 to restrain axis-associated IHO1, defined both the downstream catalytic connection and an upstream proteolytic control layer.","evidence":"Crystal structure/AlphaFold of REC114–TOPOVIBL interface with in vivo point-mutant knockins; SKP1/FBXO47 conditional KO with ubiquitination assay and IHO1 immunofluorescence","pmids":["36396648","35489071"],"confidence":"High","gaps":["Whether IHO1 itself is ubiquitinated or only indirectly regulated via HORMAD1 turnover not determined","In vivo stoichiometric competition between TOPOVIBL and ANKRD31 for REC114 PH domain not quantified"]},{"year":2023,"claim":"Determination that IHO1 forms coiled-coil tetramers and that its REC114-binding surface is shared with TOPOVIBL and ANKRD31 revealed the oligomeric architecture of the pre-DSB complex and a mutually exclusive interaction logic governing complex composition.","evidence":"AlphaFold2 modeling, analytical ultracentrifugation, multi-angle light scattering, crosslinking mass spectrometry","pmids":["37431931"],"confidence":"High","gaps":["No high-resolution experimental structure of full-length IHO1 tetramer","How mutually exclusive interactions are dynamically regulated in vivo is unclear"]},{"year":2024,"claim":"Super-resolution imaging and DDK inhibition demonstrated that DDK-dependent phosphorylation of IHO1 seeds axial platforms from which DSB-machinery clusters grow, with ANKRD31 providing a parallel seeding route when IHO1–HORMAD1 interaction is abolished, and PRDM9-bound sites promoting IHO1/MEI4 chromatin recruitment.","evidence":"IHO1–HORMAD1 interaction-disrupting mouse mutants, DDK chemical inhibition, super-resolution microscopy, ChIP-seq for IHO1 and MEI4","pmids":["38580643","38657614"],"confidence":"High","gaps":["Specific DDK phosphorylation sites on IHO1 not mapped","Whether IHO1 persists at DSB repair intermediates functionally or as a passive marker is unknown"]},{"year":null,"claim":"Key open questions include the identification of direct phosphorylation sites on IHO1 targeted by DDK, ATM, and ATR; whether IHO1 has post-DSB functional roles (as suggested for fungal Mer2) beyond serving as a platform for DSB initiation; and how the mutually exclusive REC114–partner interactions are dynamically coordinated in vivo.","evidence":"","pmids":[],"confidence":"High","gaps":["No phosphosite mapping on mammalian IHO1","Post-DSB roles of IHO1 in mammals untested","No high-resolution experimental structure of intact IHO1 tetramer"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,10,11]}],"localization":[{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[0,2,5,6,10,11,12]}],"pathway":[{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[0,3,6,8,10,11]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,6,8,12]}],"complexes":["IHO1–REC114–MEI4 pre-DSB recombinosome"],"partners":["HORMAD1","REC114","MEI4","ANKRD31","CXXC1","MEI1","TOPOVIBL"],"other_free_text":[]},"mechanistic_narrative":"IHO1 (CCDC36/Mer2 ortholog) is the principal axis-anchoring platform for pre-DSB recombinosomes in mammalian meiosis, nucleating heterooligomeric IHO1–REC114–MEI4 complexes on chromosome cores to activate SPO11/TOPOVIBL-dependent DNA double-strand break formation required for homologous chromosome pairing [PMID:27723721, PMID:30569039, PMID:37431931]. IHO1 forms coiled-coil-based tetramers and directly binds the PH domain of REC114, which in turn bridges to the catalytic TOPOVIL complex and to mutually exclusive partners ANKRD31 and TOPOVIBL [PMID:37431931, PMID:36396648]. Recruitment of IHO1 to unsynapsed axes depends on HORMAD1, meiotic cohesins (REC8–STAG3), and DDK-mediated phosphorylation, while PRDM9-bound sites promote IHO1 and MEI4 loading onto chromatin [PMID:27723721, PMID:30853435, PMID:38580643, PMID:38657614]. Once DSBs are formed, at least four feedback pathways—mediated by ATM, ATR, PRKDC, and synapsis-coupled axis remodeling—progressively deplete IHO1 from synapsed axes, and SCF-FBXO47-directed ubiquitination of HORMAD1 further restrains the pre-DSB complex, ensuring appropriate DSB number and distribution [PMID:33619545, PMID:35489071]."},"prefetch_data":{"uniprot":{"accession":"Q8IYA8","full_name":"Interactor of HORMAD1 protein 1","aliases":["Cancer/testis antigen 74","CT74","Coiled-coil domain-containing protein 36"],"length_aa":594,"mass_kda":66.3,"function":"Required for DNA double-strand breaks (DSBs) formation in unsynapsed regions during meiotic recombination. Probably acts by forming a complex with MEI4 and REC114, which activates DSBs formation in unsynapsed regions, an essential step to ensure completion of synapsis. Not required for HORMAD1 functions in pairing-independent synaptonemal complex formation, ATR recruitment to unsynapsed axes, meiotic silencing of unsynapsed chromatin (MSUC) or meiotic surveillance","subcellular_location":"Chromosome","url":"https://www.uniprot.org/uniprotkb/Q8IYA8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/IHO1","classification":"Not Classified","n_dependent_lines":28,"n_total_lines":1208,"dependency_fraction":0.023178807947019868},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/IHO1","total_profiled":1310},"omim":[{"mim_id":"619190","title":"INTERACTOR OF HORMAD1 1; IHO1","url":"https://www.omim.org/entry/619190"},{"mim_id":"618421","title":"REC114 MEIOTIC RECOMBINATION PROTEIN; REC114","url":"https://www.omim.org/entry/618421"},{"mim_id":"609824","title":"HORMA DOMAIN-CONTAINING PROTEIN 1; HORMAD1","url":"https://www.omim.org/entry/609824"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"}],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"testis","ntpm":24.1}],"url":"https://www.proteinatlas.org/search/IHO1"},"hgnc":{"alias_symbol":["FLJ25320","CT74","LELA1"],"prev_symbol":["CCDC36"]},"alphafold":{"accession":"Q8IYA8","domains":[{"cath_id":"1.20.5","chopping":"116-216","consensus_level":"medium","plddt":96.7772,"start":116,"end":216}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IYA8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IYA8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IYA8-F1-predicted_aligned_error_v6.png","plddt_mean":54.66},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=IHO1","jax_strain_url":"https://www.jax.org/strain/search?query=IHO1"},"sequence":{"accession":"Q8IYA8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8IYA8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8IYA8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IYA8"}},"corpus_meta":[{"pmid":"27723721","id":"PMC_27723721","title":"Meiotic 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IHO1, MEI4, and REC114 assemble chromatin-bound recombinosomes (pre-DSB complexes) that are predicted activators of SPO11-mediated DSB formation. HORMAD1 is required for robust recruitment of IHO1 to unsynapsed chromosome axes and efficient formation/stabilization of these recombinosomes.\",\n      \"method\": \"Co-immunoprecipitation, yeast two-hybrid, cytological co-localization, mouse knockout (IHO1-null spermatocytes lack DSBs), fractionation\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, KO with defined DSB phenotype, replicated in multiple subsequent studies\",\n      \"pmids\": [\"27723721\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"REC114, MEI4, and IHO1 form a ternary complex in mouse spermatocytes. The REC114 C-terminal domain forms a stable complex with the MEI4 N-terminal domain in vitro, and the REC114 N-terminal domain adopts a Pleckstrin homology (PH) domain fold.\",\n      \"method\": \"Co-immunoprecipitation from spermatocytes, in vitro reconstitution of REC114-MEI4 complex, X-ray crystallography of REC114 N-terminal domain\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution plus crystal structure plus reciprocal Co-IP\",\n      \"pmids\": [\"30569039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"IHO1 interacts with CXXC1 (ortholog of yeast Spp1), a member of the COMPASS complex, linking meiotic DSB sites to the DSB machinery on the chromosome axis. This interaction was identified via yeast two-hybrid screening of PRDM9 KRAB domain interactors.\",\n      \"method\": \"Yeast two-hybrid assay\",\n      \"journal\": \"Chromosoma\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single yeast two-hybrid identification, no in vitro reconstitution or reciprocal Co-IP\",\n      \"pmids\": [\"28527011\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The Mer2/IHO1 ortholog in Sordaria (Asy2/Mer2) mediates assembly of recombination-initiation complexes and DSBs, and additionally functions in homolog pairing, transfer/maintenance of recombination complexes to/from the SC central region, and global chromosome compaction potentially dependent on SUMOylation. This establishes IHO1 as an evolutionarily conserved mediator from fungi to mammals.\",\n      \"method\": \"Genetic analysis of 13 mer2 mutants, cytological localization (successive localization to axis, SC, and chromatin), functional epistasis in Sordaria\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple mutant alleles with defined phenotypes, orthogonal localization and genetic epistasis in conserved ortholog\",\n      \"pmids\": [\"29021238\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"ANKRD31 forms complexes with IHO1 and other DSB-promoting proteins on meiotic chromosome axes. ANKRD31 deficiency delays recruitment of IHO1 and recombinosome components to autosome axes and abolishes the specialized PAR-axis domain enriched for DSB-promoting proteins, demonstrating ANKRD31 controls spatiotemporal patterning of IHO1-containing recombinosomes.\",\n      \"method\": \"Co-immunoprecipitation, mouse knockout, cytological quantification of IHO1 foci, immunofluorescence\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus KO with defined IHO1 localization phenotype\",\n      \"pmids\": [\"31000436\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"STAG3 (meiotic cohesin) promotes axis localization of IHO1, HORMAD1, and MEI4. Loss of STAG3 reduces axis-associated IHO1 levels and diminishes DSB numbers, placing cohesin complexes upstream of IHO1 axis recruitment.\",\n      \"method\": \"Immunofluorescence quantification of IHO1 foci in Stag3-null spermatocytes, genetic epistasis with Prdm9 mutants\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined IHO1 localization phenotype, single lab\",\n      \"pmids\": [\"30853435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"IHO1 serves as the main anchor for pre-DSB recombinosomes on chromosome axes. DSBs restrict the DSB machinery through four distinct negative feedback pathways: (1) ATM activation restricts recombinosome numbers without affecting IHO1 levels; (2) locally, DSBs trigger IHO1 depletion via ATR; (3) DSB-enabled synapsis promotes depletion of IHO1 and recombinosomes from synapsed axes; (4) DSBs and ATM/ATR/PRKDC enable stage-specific global depletion of IHO1.\",\n      \"method\": \"Mouse knockouts and chemical inhibitors (ATM, ATR, PRKDC inhibitors), immunofluorescence quantification of IHO1 foci, epistasis analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic/chemical perturbations with defined IHO1 phenotypes, four orthogonal pathways identified\",\n      \"pmids\": [\"33619545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"In zebrafish, Iho1 dissociation from chromosome axes occurs in a DSB-dependent (Spo11-dependent) manner, as persistent Iho1 foci are observed in spo11 mutant spermatocytes.\",\n      \"method\": \"Immunofluorescence in zebrafish spo11 and sycp1 mutants\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined IHO1 localization phenotype in zebrafish ortholog\",\n      \"pmids\": [\"33842489\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SKP1, a subunit of the SCF ubiquitin E3 ligase, restrains accumulation of IHO1 (as part of the IHO1-REC114-MEI4 pre-DSB complex) on the chromosome axis. The F-box protein FBXO47 interacts with SKP1 and HORMAD1, targeting HORMAD1 for polyubiquitination and degradation, thereby modulating the pre-DSB complex levels during meiosis.\",\n      \"method\": \"Conditional SKP1 knockout (immunofluorescence quantification of IHO1 foci), co-immunoprecipitation (FBXO47-SKP1-HORMAD1), ubiquitination assay in HEK293T cells\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined IHO1 phenotype plus Co-IP plus in vitro ubiquitination assay\",\n      \"pmids\": [\"35489071\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"REC114 directly interacts with TOPOVIBL (SPO11 catalytic complex partner) through conserved domains identified by structural analysis. This REC114-TOPOVIBL interaction is required for efficient DSB activity genome-wide and for DSB timing in spermatocytes, placing IHO1-REC114-MEI4 complex as a direct regulatory module of the TOPOVIL catalytic complex.\",\n      \"method\": \"Co-immunoprecipitation, X-ray crystallography/structural analysis of interacting domains, mouse point mutants of TOPOVIBL disrupting REC114 binding\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structural identification of interacting domains plus mouse point mutants with defined DSB phenotype\",\n      \"pmids\": [\"36396648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"IHO1 forms coiled-coil-based tetramers. IHO1 directly interacts with the PH domain of REC114, recognizing the same surface as TOPOVIBL and ANKRD31, suggesting REC114 acts as a regulatory platform mediating mutually exclusive interactions with IHO1, TOPOVIBL, and ANKRD31. The REC114-MEI4 complex is a 2:1 heterotrimer that further dimerizes.\",\n      \"method\": \"AlphaFold2 modeling, biochemical characterization (size-exclusion chromatography, analytical ultracentrifugation), in vitro binding assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structural modeling validated by multiple biochemical assays, mutually exclusive binding demonstrated\",\n      \"pmids\": [\"37431931\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"IHO1 forms axial platforms on chromosome axes through direct interaction with HORMAD1, and these axial IHO1 platforms seed the biogenesis of DSB-machinery clusters (recombinosomes). IHO1 phosphorylation and formation of axial IHO1 platforms are diminished by chemical inhibition of DBF4-dependent kinase (DDK), indicating DDK contributes to regulating axial IHO1 platform formation. Without IHO1-HORMAD1 interaction, residual DSBs depend on ANKRD31.\",\n      \"method\": \"Mouse genetics (IHO1 mutants disrupting HORMAD1 interaction), DDK chemical inhibitor, immunofluorescence quantification, epistasis with ANKRD31\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple mutants plus chemical inhibition plus genetic epistasis, replicated from preprint\",\n      \"pmids\": [\"38580643\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PRDM9 promotes recruitment of MEI4 and IHO1 to DSB sites. In turn, IHO1 anchors DSB sites to the axis components HORMAD1 and SYCP3. IHO1, HORMAD1, and SYCP3 are associated at DSB ends during DSB repair, revealing IHO1's role in tethering recombination sites to the chromosome axis throughout the DSB repair process.\",\n      \"method\": \"ChIP-seq for axis proteins, immunofluorescence, Co-IP, analysis of axis-associated genomic sites in mouse spermatocytes\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP-seq plus Co-IP plus cytological localization with functional inference\",\n      \"pmids\": [\"38657614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"MEI1 interacts with IHO1 (as well as ANKRD31, REC114, and MEI4) as shown by co-immunoprecipitation. Biallelic MEI1 mutations in infertile men disrupt these interactions, implicating MEI1 as a component of the meiotic DSB machinery complex containing IHO1.\",\n      \"method\": \"Co-immunoprecipitation of MEI1 with IHO1 in patient mutation context\",\n      \"journal\": \"Journal of assisted reproduction and genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP experiment, no reciprocal pulldown or in vitro reconstitution\",\n      \"pmids\": [\"41706353\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"IHO1 (CCDC36/Mer2 ortholog) functions as the central axis-anchoring scaffold of the meiotic pre-DSB recombinosome: it directly binds HORMAD1 to form axial platforms that seed assembly of the IHO1-REC114-MEI4 hetero-oligomeric complex on chromosome axes, with REC114 acting as a regulatory hub mediating mutually exclusive interactions with IHO1, TOPOVIBL (SPO11 co-factor), and ANKRD31; DDK-mediated phosphorylation of IHO1 promotes its axial platform formation, while ATM, ATR, PRKDC, and SCF-FBXO47-mediated degradation of HORMAD1 provide multi-layered negative feedback to ensure DSBs are formed in the correct number, distribution, and timing.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2016,\n      \"finding\": \"IHO1 (CCDC36) was identified as a direct interactor of HORMAD1 and shown to be essential for meiotic DNA double-strand break (DSB) formation in mice. IHO1, together with SPO11-auxiliary proteins MEI4 and REC114, assembles chromatin-bound recombinosomes (pre-DSB complexes) on meiotic chromosome axes. HORMAD1 is required for robust recruitment of IHO1 to unsynapsed axes, and this HORMAD1-IHO1 interaction provides a mechanism for selective promotion of DSB formation along unsynapsed chromosome axes.\",\n      \"method\": \"Co-immunoprecipitation, yeast two-hybrid, cytological localization (immunofluorescence), mouse knockout\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, in vivo KO with defined phenotype, multiple orthogonal methods in a highly-cited foundational paper\",\n      \"pmids\": [\"27723721\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"CXXC1, a member of the COMPASS complex and ortholog of yeast Spp1, interacts with IHO1. This interaction was identified by yeast two-hybrid and suggests that the molecular link between DSB sites (via PRDM9-CXXC1) and the DSB machinery on the chromosome axis (via IHO1) is conserved in mouse meiosis.\",\n      \"method\": \"Yeast two-hybrid assay\",\n      \"journal\": \"Chromosoma\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single yeast two-hybrid interaction, supported by evolutionary conservation argument\",\n      \"pmids\": [\"28527011\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Mer2 (the IHO1 ortholog in Sordaria/fungi) is evolutionarily conserved from fungi to plants (PRD3/PAIR1) and mammals (IHO1). Beyond its role in DSB formation, Mer2 functions in homolog spatial juxtaposition for pairing, transfer/maintenance of recombination complexes to the synaptonemal complex central region, and global chromosome compaction post-recombination. SUMOylation is implicated in the compaction role.\",\n      \"method\": \"Genetic analysis of 13 mer2 mutants, sequential localization of Mer2 to axis/SC/chromatin in Sordaria, cytological and functional assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — extensive mutant analysis with orthogonal functional readouts in a conserved ortholog, replicated across multiple alleles\",\n      \"pmids\": [\"29021238\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Mouse REC114 is required for meiotic DSB formation and forms a stable complex with MEI4 and IHO1 in spermatocytes. In vitro, the REC114 C-terminal domain forms a stable complex with the MEI4 N-terminal domain. The REC114 N-terminal domain has structural similarity to Pleckstrin homology (PH) domains.\",\n      \"method\": \"Mouse knockout, co-immunoprecipitation, in vitro complex reconstitution, X-ray crystallography\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution and crystal structure combined with in vivo Co-IP and KO phenotype\",\n      \"pmids\": [\"30569039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"ANKRD31 is a key component of DSB-promoting protein complexes on meiotic chromosome axes and interacts with the same complexes as IHO1. Loss of ANKRD31 causes genome-wide delay in assembling DSB-promoting proteins (including IHO1) on autosome axes and abolishes the specialized pseudoautosomal region (PAR)-axis domain enriched for DSB-promoting proteins, resulting in failure of X-Y crossover formation.\",\n      \"method\": \"Mouse knockout, immunofluorescence co-localization, chromosome spread analysis\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined molecular and cellular phenotype, IHO1 axis recruitment directly measured\",\n      \"pmids\": [\"31000436\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"PRDM9 interacts with cohesin subunits STAG3 and REC8 in a cooperative relationship that promotes meiotic DSB formation. STAG3 and REC8 promote axis localization of HORMAD1, IHO1, and MEI4, demonstrating that meiotic cohesin complexes are required upstream of IHO1 axis recruitment.\",\n      \"method\": \"Co-immunoprecipitation, genetic epistasis (double mutants), immunofluorescence in spermatocytes\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus genetic epistasis with direct measurement of IHO1 localization\",\n      \"pmids\": [\"30853435\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"IHO1 serves as the main anchor for pre-DSB recombinosomes (containing MEI4 and REC114) on chromosome axes. DSBs negatively feed back on the DSB machinery through four distinct pathways: (1) ATM activation restricts pre-DSB recombinosome numbers without affecting IHO1 levels; (2) ATR triggers IHO1 depletion locally near DSBs; (3) synapsis (enabled by DSBs) promotes depletion of IHO1 and pre-DSB recombinosomes from synapsed axes; (4) ATM, ATR, and PRKDC together enable stage-specific depletion of IHO1 from all axes.\",\n      \"method\": \"Mouse genetics (kinase inhibitors and mutants), immunofluorescence, chromosome spread analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic interventions dissecting four distinct pathways with direct IHO1 localization readout\",\n      \"pmids\": [\"33619545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"In zebrafish, Iho1 dissociation from chromosome axes occurs in a DSB-dependent manner, as persistent Iho1 foci are observed in spo11 mutant spermatocytes. This demonstrates that SPO11-dependent DSB formation is required for IHO1 removal from the axis.\",\n      \"method\": \"Zebrafish mutant analysis (sycp1 and spo11 mutants), immunofluorescence\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in zebrafish ortholog with direct localization readout, single lab\",\n      \"pmids\": [\"33842489\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"REC114 directly interacts with TOPOVIBL (the SPO11 partner in the TOPOVIL catalytic complex), and point mutations disrupting this interaction strongly reduce DSB activity genome-wide in oocytes and in sub-telomeric regions in spermatocytes. Since REC114 is a direct partner of both IHO1 and TOPOVIBL, REC114 serves as a key bridge linking IHO1-containing pre-DSB recombinosomes to the catalytic TOPOVIL complex.\",\n      \"method\": \"Structural analysis (crystallography/AlphaFold), co-immunoprecipitation, mouse point-mutant knockins, DSB quantification by immunofluorescence and sequencing\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structural identification of interaction domains combined with in vivo point-mutant knockin and quantitative DSB assays\",\n      \"pmids\": [\"36396648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SKP1, a constitutive subunit of the SCF ubiquitin E3 ligase, restrains accumulation of the IHO1-REC114-MEI4 pre-DSB complex on the chromosome axis. Mechanistically, the meiosis-specific F-box protein FBXO47 interacts with SKP1 and HORMAD1, targeting HORMAD1 for polyubiquitination and proteasomal degradation, which in turn modulates the pre-DSB complex including IHO1.\",\n      \"method\": \"Mouse conditional knockout, immunofluorescence, co-immunoprecipitation, ubiquitination assay in HEK293T cells\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined molecular phenotype plus biochemical ubiquitination assay establishing mechanism\",\n      \"pmids\": [\"35489071\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"IHO1 forms coiled-coil-based tetramers. IHO1 directly interacts with the PH domain of REC114 using the same surface as TOPOVIBL and ANKRD31, indicating mutually exclusive interactions. Combined with AlphaFold2 modeling and biochemical characterization, this demonstrates a ternary IHO1-REC114-MEI4 complex architecture where REC114 acts as a regulatory platform for mutually exclusive partner interactions.\",\n      \"method\": \"AlphaFold2 modeling, biochemical reconstitution, analytical ultracentrifugation, multi-angle light scattering, crosslinking mass spectrometry\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — structural modeling combined with multiple orthogonal biochemical methods defining complex architecture\",\n      \"pmids\": [\"37431931\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Efficient biogenesis of DSB-machinery clusters on chromosome axes requires seeding by axial IHO1 platforms. IHO1 phosphorylation and formation of axial IHO1 platforms are diminished by chemical inhibition of DBF4-dependent kinase (DDK), implicating DDK as a regulator of IHO1 axis assembly. IHO1-HORMAD1 interaction mediates the seeding of DSB-machinery on axes; without this interaction, residual DSBs depend on ANKRD31, which enhances both seeding and growth of DSB-machinery clusters.\",\n      \"method\": \"Mouse genetics (IHO1-HORMAD1 interaction mutants), DDK chemical inhibition, super-resolution microscopy, quantitative immunofluorescence\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic interaction mutants with multiple orthogonal readouts including kinase inhibition and super-resolution imaging\",\n      \"pmids\": [\"38580643\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PRDM9 binding sites promote recruitment of MEI4 and IHO1 to chromatin. IHO1 in turn anchors DSB sites to the chromosome axis components HORMAD1 and SYCP3. Additionally, IHO1, HORMAD1, and SYCP3 remain associated at DSB ends during DSB repair, linking DSB site identity to axis structure throughout the recombination process.\",\n      \"method\": \"ChIP-seq, immunofluorescence, co-localization analysis in spermatocytes\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-wide ChIP-seq combined with cytological co-localization establishing hierarchical recruitment\",\n      \"pmids\": [\"38657614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"MEI1 variants that cause non-obstructive azoospermia disrupt MEI1 interactions with ANKRD31, IHO1, REC114, and MEI4, as demonstrated by co-immunoprecipitation, establishing that MEI1 is a physical interaction partner of IHO1 within the meiotic DSB machinery.\",\n      \"method\": \"Co-immunoprecipitation in HEK293T cells with patient-derived MEI1 mutants\",\n      \"journal\": \"Journal of assisted reproduction and genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP demonstrating interaction, in disease-context mutants\",\n      \"pmids\": [\"41706353\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"IHO1 (CCDC36) is the mammalian ortholog of yeast Mer2 and functions as the primary axis-anchoring platform for pre-DSB recombinosomes on meiotic chromosome cores: it directly interacts with HORMAD1 (recruited via DDK-modulated phosphorylation), forms coiled-coil-based tetramers, and binds REC114 (via REC114's PH domain) to nucleate heterooligomeric IHO1-REC114-MEI4 complexes that activate SPO11/TOPOVIBL-dependent DSB formation; DSBs then negatively feed back on IHO1 through at least four pathways involving ATM, ATR, and PRKDC kinases and synapsis-coupled axis remodeling, while upstream regulators including meiotic cohesins, PRDM9-CXXC1, ANKRD31, and SCF-FBXO47-mediated ubiquitination collectively tune the spatiotemporal distribution and abundance of axis-associated IHO1 to ensure appropriate DSB numbers and distribution for homologous chromosome pairing.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"IHO1 (CCDC36) is the central axis-anchoring scaffold of the meiotic pre-DSB recombinosome, essential for SPO11-mediated double-strand break formation during meiosis. IHO1 forms coiled-coil-based tetramers that directly bind HORMAD1 on chromosome axes to create axial platforms, which seed assembly of the IHO1–REC114–MEI4 hetero-oligomeric complex; DDK-mediated phosphorylation of IHO1 promotes this platform formation, while REC114 serves as a regulatory hub mediating mutually exclusive interactions with IHO1, TOPOVIBL, and ANKRD31 to control DSB catalysis [PMID:27723721, PMID:37431931, PMID:38580643, PMID:36396648]. ANKRD31 controls the spatiotemporal patterning of IHO1-containing recombinosomes, particularly at the pseudoautosomal region, and PRDM9 promotes IHO1 recruitment to DSB hotspots where IHO1 tethers recombination sites to axis components throughout DSB repair [PMID:31000436, PMID:38657614]. Multiple negative feedback pathways restrict IHO1 levels: ATR triggers local IHO1 depletion, ATM/ATR/PRKDC enable stage-specific global depletion, synapsis removes IHO1 from synapsed axes, and the SCF-FBXO47 ubiquitin ligase degrades HORMAD1 to limit pre-DSB complex accumulation [PMID:33619545, PMID:35489071].\",\n  \"teleology\": [\n    {\n      \"year\": 2016,\n      \"claim\": \"Establishing that IHO1 is essential for meiotic DSB formation resolved how axis-associated pre-DSB complexes are assembled: IHO1 directly binds HORMAD1 and is required for recruitment of MEI4 and REC114 into chromatin-bound recombinosomes.\",\n      \"evidence\": \"IHO1-null mouse spermatocytes lack DSBs; reciprocal Co-IP and yeast two-hybrid confirm direct HORMAD1 interaction\",\n      \"pmids\": [\"27723721\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of IHO1–HORMAD1 interaction not defined\", \"How IHO1 activates SPO11 catalytic activity unclear\", \"Stoichiometry of the recombinosome unknown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Functional analysis of the Sordaria ortholog (Asy2/Mer2) demonstrated that IHO1's role in DSB initiation is evolutionarily conserved from fungi to mammals and revealed additional functions in homolog pairing and chromosome compaction.\",\n      \"evidence\": \"Genetic analysis of 13 mer2 mutant alleles with cytological localization and epistasis in Sordaria macrospora\",\n      \"pmids\": [\"29021238\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether pairing and compaction functions are conserved in mammals not tested\", \"SUMOylation-dependent mechanism not confirmed in vivo\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Biochemical reconstitution of the REC114–MEI4 binary complex and determination of the REC114 PH domain structure established that IHO1 operates within a defined ternary complex with known domain architecture.\",\n      \"evidence\": \"In vitro reconstitution of REC114–MEI4 complex, X-ray crystallography of REC114 N-terminal PH domain\",\n      \"pmids\": [\"30053726\", \"30569039\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct IHO1–REC114 binding interface not structurally resolved at this stage\", \"Ternary complex not reconstituted in vitro\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Identification of ANKRD31 as a spatiotemporal regulator of IHO1-containing recombinosomes, and STAG3 (meiotic cohesin) as an upstream factor for IHO1 axis recruitment, defined the hierarchy of axis assembly upstream of DSB formation.\",\n      \"evidence\": \"ANKRD31-KO and STAG3-KO mice with quantification of IHO1 foci; reciprocal Co-IP of ANKRD31 with IHO1\",\n      \"pmids\": [\"31000436\", \"30853435\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ANKRD31 directly contacts IHO1 or acts through REC114 not resolved\", \"Cohesin-dependent mechanism of IHO1 axis loading unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Dissection of four distinct DSB-dependent negative feedback pathways controlling IHO1 levels (ATM restricts recombinosomes; ATR depletes IHO1 locally; synapsis removes IHO1; ATM/ATR/PRKDC enable global depletion) revealed how meiotic cells ensure correct DSB number and timing.\",\n      \"evidence\": \"Mouse knockouts and chemical inhibitors of ATM, ATR, PRKDC with immunofluorescence quantification of IHO1 foci; zebrafish spo11 mutants showing persistent Iho1\",\n      \"pmids\": [\"33619545\", \"33842489\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct phosphorylation targets on IHO1 by ATR/ATM not identified\", \"Mechanism of synapsis-triggered IHO1 removal unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Discovery that REC114 directly contacts TOPOVIBL (SPO11 cofactor) through the same PH domain surface, and that SCF-FBXO47 degrades HORMAD1 to limit IHO1 complex accumulation, defined two additional regulatory layers connecting the pre-DSB recombinosome to catalysis and turnover.\",\n      \"evidence\": \"Crystal structure of REC114–TOPOVIBL interface; TOPOVIBL point mutants in mice with DSB phenotype; conditional SKP1-KO showing IHO1 accumulation; FBXO47–HORMAD1 ubiquitination assay\",\n      \"pmids\": [\"36396648\", \"35489071\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether IHO1 contacts the SPO11–TOPOVIBL catalytic complex directly not resolved\", \"In vivo ubiquitination of HORMAD1 by SCF-FBXO47 not demonstrated in spermatocytes\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Structural and biochemical characterization revealed IHO1 forms coiled-coil tetramers and that REC114 mediates mutually exclusive binding with IHO1, TOPOVIBL, and ANKRD31 on the same PH domain surface, establishing a competitive switch model for DSB regulation.\",\n      \"evidence\": \"AlphaFold2 modeling validated by SEC, analytical ultracentrifugation, and in vitro competition binding assays\",\n      \"pmids\": [\"37431931\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution experimental structure of full-length IHO1\", \"Dynamics of the mutually exclusive switch in vivo not measured\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Genetic dissection demonstrated that IHO1–HORMAD1 direct interaction is required for axial platform formation and recombinosome seeding, that DDK phosphorylation promotes this platform assembly, and that IHO1 tethers DSB sites to axis components throughout the repair process.\",\n      \"evidence\": \"Mouse IHO1 mutants disrupting HORMAD1 binding; DDK chemical inhibition; ChIP-seq for axis proteins at DSB sites\",\n      \"pmids\": [\"38580643\", \"38657614\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific DDK phosphorylation sites on IHO1 not mapped\", \"How IHO1 persists at DSB ends during repair mechanistically unclear\", \"Role of IHO1 in crossover/non-crossover resolution not addressed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the high-resolution experimental structure of full-length IHO1, the identity of DDK and ATR phosphorylation sites on IHO1, the mechanism by which synapsis triggers IHO1 removal, and whether IHO1 directly contacts the SPO11–TOPOVIBL catalytic complex.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No experimental atomic structure of IHO1\", \"DDK/ATR phosphosites on IHO1 unmapped\", \"Mechanism of synapsis-triggered IHO1 removal unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 10, 11]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 10, 11, 12]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [0, 3, 4, 5, 6, 7, 11, 12]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [0, 6, 9, 11]}\n    ],\n    \"complexes\": [\n      \"IHO1–REC114–MEI4 pre-DSB recombinosome\",\n      \"IHO1–HORMAD1 axial platform\"\n    ],\n    \"partners\": [\n      \"HORMAD1\",\n      \"REC114\",\n      \"MEI4\",\n      \"ANKRD31\",\n      \"CXXC1\",\n      \"MEI1\",\n      \"STAG3\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"IHO1 (CCDC36/Mer2 ortholog) is the principal axis-anchoring platform for pre-DSB recombinosomes in mammalian meiosis, nucleating heterooligomeric IHO1–REC114–MEI4 complexes on chromosome cores to activate SPO11/TOPOVIBL-dependent DNA double-strand break formation required for homologous chromosome pairing [PMID:27723721, PMID:30569039, PMID:37431931]. IHO1 forms coiled-coil-based tetramers and directly binds the PH domain of REC114, which in turn bridges to the catalytic TOPOVIL complex and to mutually exclusive partners ANKRD31 and TOPOVIBL [PMID:37431931, PMID:36396648]. Recruitment of IHO1 to unsynapsed axes depends on HORMAD1, meiotic cohesins (REC8–STAG3), and DDK-mediated phosphorylation, while PRDM9-bound sites promote IHO1 and MEI4 loading onto chromatin [PMID:27723721, PMID:30853435, PMID:38580643, PMID:38657614]. Once DSBs are formed, at least four feedback pathways—mediated by ATM, ATR, PRKDC, and synapsis-coupled axis remodeling—progressively deplete IHO1 from synapsed axes, and SCF-FBXO47-directed ubiquitination of HORMAD1 further restrains the pre-DSB complex, ensuring appropriate DSB number and distribution [PMID:33619545, PMID:35489071].\",\n  \"teleology\": [\n    {\n      \"year\": 2016,\n      \"claim\": \"Identification of IHO1 as a HORMAD1 interactor essential for meiotic DSB formation established the first known molecular link between the chromosome axis protein HORMAD1 and the SPO11-accessory DSB machinery in mammals.\",\n      \"evidence\": \"Co-IP, yeast two-hybrid, immunofluorescence, and Iho1-knockout mouse showing complete loss of DSBs\",\n      \"pmids\": [\"27723721\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of IHO1–HORMAD1 interaction unknown\", \"Whether IHO1 has roles beyond DSB initiation not tested\", \"Mechanism linking HORMAD1 to IHO1 axis loading unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Discovery that CXXC1 interacts with IHO1, paralleling the yeast Spp1–Mer2 axis-tethering mechanism, suggested a conserved loop-axis tethering module connecting PRDM9-marked hotspots to the DSB machinery on axes.\",\n      \"evidence\": \"Yeast two-hybrid assay detecting CXXC1–IHO1 interaction\",\n      \"pmids\": [\"28527011\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Interaction not confirmed by reciprocal Co-IP or in vivo disruption\", \"Functional consequence of disrupting the CXXC1–IHO1 link not tested in mice\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Analysis of the fungal ortholog Mer2 revealed conserved roles extending beyond DSB formation to homolog juxtaposition, recombination complex transfer, and chromosome compaction, broadening the functional repertoire attributable to IHO1-family proteins.\",\n      \"evidence\": \"Thirteen Sordaria mer2 mutants with sequential cytological and functional assays\",\n      \"pmids\": [\"29021238\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether mammalian IHO1 retains the compaction and SC-transfer roles demonstrated for fungal Mer2 is untested\", \"SUMOylation of mammalian IHO1 not examined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Reconstitution of a stable REC114–MEI4 sub-complex and demonstration that REC114, MEI4, and IHO1 co-immunoprecipitate in vivo defined the tripartite pre-DSB recombinosome and identified REC114's PH-domain fold as a critical interaction surface.\",\n      \"evidence\": \"Mouse Rec114 knockout, in vitro reconstitution, X-ray crystallography of REC114 C-terminal/MEI4 N-terminal complex\",\n      \"pmids\": [\"30569039\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct IHO1–REC114 binding interface not yet mapped\", \"Stoichiometry of the ternary complex unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Placing meiotic cohesins (REC8–STAG3) and ANKRD31 upstream of IHO1 axis loading resolved the hierarchical dependency for pre-DSB complex assembly: cohesins recruit HORMAD1/IHO1, and ANKRD31 ensures timely accumulation especially at the PAR.\",\n      \"evidence\": \"Mouse knockouts of ANKRD31 and cohesin subunits with direct IHO1 localization readout by immunofluorescence\",\n      \"pmids\": [\"31000436\", \"30853435\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ANKRD31 acts directly on IHO1 or indirectly through other axis components not resolved\", \"Biochemical mechanism by which cohesins promote IHO1 loading unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Dissection of four distinct negative-feedback pathways—ATM restricting recombinosome numbers, ATR locally depleting IHO1 near DSBs, synapsis removing IHO1 from synapsed axes, and combined ATM/ATR/PRKDC enabling global IHO1 depletion—established IHO1 as the central regulatory target ensuring DSB homeostasis.\",\n      \"evidence\": \"Mouse kinase mutants and inhibitors with quantitative immunofluorescence of IHO1 on chromosome spreads; zebrafish spo11 mutants showing DSB-dependent IHO1 removal\",\n      \"pmids\": [\"33619545\", \"33842489\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct phosphorylation sites on IHO1 targeted by ATM/ATR/PRKDC not identified\", \"Mechanistic basis of synapsis-coupled IHO1 stripping unresolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Structural and genetic evidence that REC114 bridges IHO1-containing pre-DSB recombinosomes to the catalytic TOPOVIL complex via its PH domain, and that SCF-FBXO47 ubiquitinates HORMAD1 to restrain axis-associated IHO1, defined both the downstream catalytic connection and an upstream proteolytic control layer.\",\n      \"evidence\": \"Crystal structure/AlphaFold of REC114–TOPOVIBL interface with in vivo point-mutant knockins; SKP1/FBXO47 conditional KO with ubiquitination assay and IHO1 immunofluorescence\",\n      \"pmids\": [\"36396648\", \"35489071\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether IHO1 itself is ubiquitinated or only indirectly regulated via HORMAD1 turnover not determined\", \"In vivo stoichiometric competition between TOPOVIBL and ANKRD31 for REC114 PH domain not quantified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Determination that IHO1 forms coiled-coil tetramers and that its REC114-binding surface is shared with TOPOVIBL and ANKRD31 revealed the oligomeric architecture of the pre-DSB complex and a mutually exclusive interaction logic governing complex composition.\",\n      \"evidence\": \"AlphaFold2 modeling, analytical ultracentrifugation, multi-angle light scattering, crosslinking mass spectrometry\",\n      \"pmids\": [\"37431931\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution experimental structure of full-length IHO1 tetramer\", \"How mutually exclusive interactions are dynamically regulated in vivo is unclear\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Super-resolution imaging and DDK inhibition demonstrated that DDK-dependent phosphorylation of IHO1 seeds axial platforms from which DSB-machinery clusters grow, with ANKRD31 providing a parallel seeding route when IHO1–HORMAD1 interaction is abolished, and PRDM9-bound sites promoting IHO1/MEI4 chromatin recruitment.\",\n      \"evidence\": \"IHO1–HORMAD1 interaction-disrupting mouse mutants, DDK chemical inhibition, super-resolution microscopy, ChIP-seq for IHO1 and MEI4\",\n      \"pmids\": [\"38580643\", \"38657614\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific DDK phosphorylation sites on IHO1 not mapped\", \"Whether IHO1 persists at DSB repair intermediates functionally or as a passive marker is unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include the identification of direct phosphorylation sites on IHO1 targeted by DDK, ATM, and ATR; whether IHO1 has post-DSB functional roles (as suggested for fungal Mer2) beyond serving as a platform for DSB initiation; and how the mutually exclusive REC114–partner interactions are dynamically coordinated in vivo.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No phosphosite mapping on mammalian IHO1\", \"Post-DSB roles of IHO1 in mammals untested\", \"No high-resolution experimental structure of intact IHO1 tetramer\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 10, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [0, 2, 5, 6, 10, 11, 12]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [0, 3, 6, 8, 10, 11]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 6, 8, 12]}\n    ],\n    \"complexes\": [\n      \"IHO1–REC114–MEI4 pre-DSB recombinosome\"\n    ],\n    \"partners\": [\n      \"HORMAD1\",\n      \"REC114\",\n      \"MEI4\",\n      \"ANKRD31\",\n      \"CXXC1\",\n      \"MEI1\",\n      \"TOPOVIBL\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}