{"gene":"LENG8","run_date":"2026-04-28T18:30:27","timeline":{"discoveries":[{"year":2025,"finding":"LENG8 forms a novel trimeric complex called TREX-2.1 (composed of LENG8, PCID2, and DSS1) that is localized in the nucleus and facilitates the release of DDX39B (UAP56) from mRNPs. Cryo-EM structures of TREX-2.1/DDX39B revealed a conserved trigger loop in LENG8 (analogous to the trigger loop in GANP of the canonical TREX-2 complex) that is critical for DDX39B regulation. LENG8 knockdown alters the nucleocytoplasmic ratio of a subset of mRNAs with high GC content.","method":"Cryo-EM structure determination, co-immunoprecipitation, RNA sequencing from knockdown cells","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure with functional mutagenesis validation combined with transcriptomic readout","pmids":["40595470"],"is_preprint":false},{"year":2026,"finding":"LENG8 is a conserved RNA nuclear retention and degradation factor. It is recruited to pre-mRNAs by splicing factors including the U1 snRNP. LENG8 associates with PCID2 and SEM1 to form the REX (Repressor of EXport) complex, conserved from yeast to humans. The REX complex causes RNA nuclear retention by acting as a dominant-negative factor for the essential mRNA export factor TREX-2. Loss of LENG8 results in cytoplasmic leakage of misprocessed mRNAs (including intronically polyadenylated and intron-retained mRNAs) and noncoding RNAs. LENG8 also promotes nuclear RNA degradation through direct interaction with the RNA exosome adaptor PAXT.","method":"Co-immunoprecipitation, loss-of-function (siRNA/KD) with RNA-seq readout, biochemical reconstitution of the REX complex, genetic epistasis with TREX-2 and exosome pathway","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (complex reconstitution, LOF with defined molecular phenotypes, pathway epistasis) in a single rigorous study","pmids":["41861815"],"is_preprint":false},{"year":2025,"finding":"LENG8 is recruited to pre-mRNAs by splicing factors including U1 snRNP, and LENG8 depletion leads to leakage of misprocessed mRNAs and noncoding RNAs into the cytoplasm; LENG8 promotes RNA degradation by recruiting PAXT and the RNA exosome (preprint version of the peer-reviewed Molecular Cell study).","method":"Co-immunoprecipitation, knockdown with RNA-seq, biochemical complex reconstitution","journal":"bioRxiv","confidence":"High","confidence_rationale":"Tier 2 — same study as peer-reviewed version; preprint with multiple orthogonal methods","pmids":["40832163"],"is_preprint":true},{"year":2025,"finding":"The LENG8-PCID2-SEM1 (LENG8-PS) trimer is structurally and functionally equivalent to the central GANP-PCID2-SEM1 (GANP-PS) trimer of TREX-2. The LENG8-PS module is embedded within the PAXT connection and releases polyadenylated RNAs from UAP56 for decay by the nuclear exosome, acting in competition with NPC-associated TREX-2 that directs RNAs toward export.","method":"Mutagenesis, transcriptomics, biochemical reconstitution, structural homology analysis","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — preprint with mutagenesis and transcriptomic data; not yet peer-reviewed","pmids":["bio_10.1101_2025.09.16.676470"],"is_preprint":true},{"year":2026,"finding":"PCID2 acts as a scaffold for mutually exclusive SAC3-based subcomplexes with GANP, LENG8, and SAC3D1. LENG8 localizes specifically to nuclear speckles (distinct from the nuclear envelope localization of GANP-containing TREX-2), and LENG8 depletion alters mRNA processing and polyadenylation site usage, placing LENG8 upstream of the canonical TREX-2 complex in mRNA processing.","method":"Immunoprecipitation/interactome analysis, subcellular localization (immunofluorescence), siRNA knockdown with RNA-seq for polyadenylation site analysis","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — preprint with direct localization experiments and functional KD readout; multiple orthogonal approaches","pmids":["42039562"],"is_preprint":true}],"current_model":"LENG8 is a conserved nuclear RNA quality-control factor that is recruited to pre-mRNAs via U1 snRNP, localizes to nuclear speckles, and forms the REX/TREX-2.1 complex with PCID2 and SEM1/DSS1; this complex uses a trigger loop in LENG8 to release DDX39B from mRNPs, acts as a dominant-negative inhibitor of the export-competent TREX-2 complex to enforce nuclear retention of misprocessed mRNAs and noncoding RNAs, and promotes their degradation by recruiting the PAXT–nuclear exosome pathway."},"narrative":{"teleology":[{"year":2025,"claim":"Structural determination of the LENG8–PCID2–DSS1 (TREX-2.1) complex revealed how LENG8 uses a trigger loop to release DDX39B from mRNPs, establishing LENG8 as a direct regulator of mRNP remodeling analogous to GANP in canonical TREX-2.","evidence":"Cryo-EM structure of TREX-2.1/DDX39B complex with trigger-loop mutagenesis and RNA-seq in knockdown cells","pmids":["40595470"],"confidence":"High","gaps":["Substrate specificity beyond GC-content enrichment is not defined","Whether trigger-loop activity is regulated by post-translational modifications is unknown","No in vivo reconstitution of the full retention-to-degradation pathway"]},{"year":2026,"claim":"Characterization of the REX complex demonstrated that LENG8 serves dual, coupled roles — nuclear retention via dominant-negative competition with TREX-2 and RNA degradation via direct recruitment of PAXT and the nuclear exosome — thereby establishing LENG8 as a central checkpoint factor for misprocessed and noncoding RNA disposal.","evidence":"Biochemical reconstitution of the REX complex, siRNA knockdown with RNA-seq, genetic epistasis with TREX-2 and exosome components (Molecular Cell)","pmids":["41861815"],"confidence":"High","gaps":["How selectivity for misprocessed versus correctly processed mRNAs is achieved at the molecular level is unresolved","Whether LENG8 functions redundantly with other nuclear retention factors is not tested","Direct structural basis for the LENG8–PAXT interaction has not been determined"]},{"year":2026,"claim":"Localization and interactome studies placed LENG8 at nuclear speckles — distinct from the nuclear-envelope residence of GANP-containing TREX-2 — and showed that LENG8 depletion alters polyadenylation site usage, positioning REX upstream of TREX-2 in the mRNA processing hierarchy.","evidence":"Immunofluorescence for subcellular localization, interactome analysis, siRNA knockdown with poly(A)-site usage profiling (preprint)","pmids":["42039562"],"confidence":"Medium","gaps":["Preprint; not yet peer-reviewed","Mechanism by which nuclear speckle localization is achieved is unknown","Whether polyadenylation site changes are a direct or indirect consequence of LENG8 depletion is unclear"]},{"year":null,"claim":"Key unresolved questions include the molecular basis for selective recognition of misprocessed transcripts, the structural interface between REX and PAXT, and whether LENG8 loss contributes to disease through aberrant cytoplasmic accumulation of noncoding or intron-containing RNAs.","evidence":"","pmids":[],"confidence":"Low","gaps":["No disease association has been established for LENG8","No structural model of the full LENG8–PAXT interface exists","In vivo physiological consequences of LENG8 loss in animal models have not been reported"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[1,2]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,1]},{"term_id":"GO:0005654","term_label":"nucleoplasm","supporting_discovery_ids":[0,4]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,1]}],"complexes":["REX (TREX-2.1; LENG8–PCID2–SEM1/DSS1)"],"partners":["PCID2","SEM1","DDX39B","SNRNP70"],"other_free_text":[]},"mechanistic_narrative":"LENG8 is a conserved nuclear RNA quality-control factor that enforces retention and degradation of misprocessed mRNAs and noncoding RNAs. It is recruited to pre-mRNAs by splicing factors including U1 snRNP and assembles with PCID2 and SEM1/DSS1 into the REX (TREX-2.1) complex, which is structurally analogous to the canonical TREX-2 export complex but acts as its dominant-negative antagonist: a conserved trigger loop in LENG8 releases DDX39B (UAP56) from mRNPs, diverting polyadenylated transcripts away from nuclear export [PMID:40595470, PMID:41861815]. LENG8 localizes to nuclear speckles and promotes degradation of retained RNAs by directly recruiting the PAXT–nuclear exosome pathway; loss of LENG8 results in cytoplasmic leakage of intron-retained, intronically polyadenylated, and noncoding RNAs [PMID:41861815, PMID:40595470]."},"prefetch_data":{"uniprot":{"accession":"Q96PV6","full_name":"Leukocyte receptor cluster member 8","aliases":[],"length_aa":800,"mass_kda":88.2,"function":"","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/Q96PV6/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/LENG8","classification":"Common 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all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/LENG8"},"hgnc":{"alias_symbol":["KIAA1932","MGC40108","pp13842"],"prev_symbol":[]},"alphafold":{"accession":"Q96PV6","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96PV6","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96PV6-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96PV6-F1-predicted_aligned_error_v6.png","plddt_mean":55.56},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=LENG8","jax_strain_url":"https://www.jax.org/strain/search?query=LENG8"},"sequence":{"accession":"Q96PV6","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96PV6.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96PV6/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96PV6"}},"corpus_meta":[{"pmid":"26910819","id":"PMC_26910819","title":"Early Loss of Blood-Brain Barrier Integrity Precedes NOX2 Elevation in the Prefrontal Cortex of an Animal Model of 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the release of DDX39B (UAP56) from mRNPs; cryo-EM structures of TREX-2.1/DDX39B identified a conserved 'trigger loop' in LENG8 that is critical for DDX39B regulation.\",\n      \"method\": \"Cryo-EM structure determination, complex identification by co-immunoprecipitation/mass spectrometry, LENG8 knockdown + RNA sequencing\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with mutagenesis of trigger loop, supported by functional knockdown RNA-seq data\",\n      \"pmids\": [\"40595470\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"LENG8 forms the REX (Repressor of EXport) complex with PCID2 and SEM1, conserved from yeast to humans; LENG8 is recruited to pre-mRNAs by splicing factors including U1 snRNP, acts as a dominant-negative factor for the TREX-2 mRNA export complex to cause nuclear RNA retention, promotes degradation of retained RNAs by recruiting the PAXT exosome adaptor, and its loss leads to cytoplasmic leakage of misprocessed mRNAs (intronically polyadenylated, intron-retained) and noncoding RNAs.\",\n      \"method\": \"Co-immunoprecipitation, LENG8 depletion (loss-of-function) with RNA-seq, dominant-negative assay, interaction mapping with PAXT and exosome\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (Co-IP, LOF with defined phenotype, pathway epistasis) in peer-reviewed publication; corroborated by preprint from same group\",\n      \"pmids\": [\"41861815\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LENG8 localizes to nuclear speckles, and LENG8 depletion alters mRNA processing and polyadenylation site usage; PCID2 forms mutually exclusive subcomplexes with GANP, LENG8, and SAC3D1, with LENG8-containing subcomplex having distinct localization from the canonical TREX-2 complex at the nuclear envelope.\",\n      \"method\": \"Interactome characterization (proteomics/Co-IP), subcellular localization imaging, LENG8 depletion with RNA processing analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — localization and interactome data with functional consequence from depletion; preprint, single lab\",\n      \"pmids\": [\"42039562\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The LENG8-PCID2-SEM1 (LENG8-PS) trimer is structurally and functionally equivalent to the central GANP-PCID2-SEM1 trimer of TREX-2, and acts as a TREX-2-like module within the PAXT complex to release polyadenylated RNAs from UAP56/DDX39B for nuclear exosome-mediated decay; mutagenesis and transcriptomic data show that PAXT and TREX-2 compete to determine the nuclear fate (decay vs. export) of polyadenylated RNPs.\",\n      \"method\": \"Structural/mutagenesis analysis, transcriptomics, biochemical reconstitution of LENG8-PS trimer\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis and transcriptomics supporting structural equivalence; preprint, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.09.16.676470\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"LENG8 is the defining subunit of the nuclear REX/TREX-2.1 complex (with PCID2 and SEM1) that localizes to nuclear speckles, is recruited to pre-mRNAs via splicing factors (including U1 snRNP), acts as a dominant-negative inhibitor of the TREX-2 mRNA export complex through a conserved trigger loop that regulates DDX39B/UAP56, thereby retaining incompletely or aberrantly processed mRNAs and noncoding RNAs in the nucleus, and promotes their degradation by recruiting the PAXT–nuclear exosome machinery.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2025,\n      \"finding\": \"LENG8 forms a novel trimeric complex called TREX-2.1 (composed of LENG8, PCID2, and DSS1) that is localized in the nucleus and facilitates the release of DDX39B (UAP56) from mRNPs. Cryo-EM structures of TREX-2.1/DDX39B revealed a conserved trigger loop in LENG8 (analogous to the trigger loop in GANP of the canonical TREX-2 complex) that is critical for DDX39B regulation. LENG8 knockdown alters the nucleocytoplasmic ratio of a subset of mRNAs with high GC content.\",\n      \"method\": \"Cryo-EM structure determination, co-immunoprecipitation, RNA sequencing from knockdown cells\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with functional mutagenesis validation combined with transcriptomic readout\",\n      \"pmids\": [\"40595470\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"LENG8 is a conserved RNA nuclear retention and degradation factor. It is recruited to pre-mRNAs by splicing factors including the U1 snRNP. LENG8 associates with PCID2 and SEM1 to form the REX (Repressor of EXport) complex, conserved from yeast to humans. The REX complex causes RNA nuclear retention by acting as a dominant-negative factor for the essential mRNA export factor TREX-2. Loss of LENG8 results in cytoplasmic leakage of misprocessed mRNAs (including intronically polyadenylated and intron-retained mRNAs) and noncoding RNAs. LENG8 also promotes nuclear RNA degradation through direct interaction with the RNA exosome adaptor PAXT.\",\n      \"method\": \"Co-immunoprecipitation, loss-of-function (siRNA/KD) with RNA-seq readout, biochemical reconstitution of the REX complex, genetic epistasis with TREX-2 and exosome pathway\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (complex reconstitution, LOF with defined molecular phenotypes, pathway epistasis) in a single rigorous study\",\n      \"pmids\": [\"41861815\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"LENG8 is recruited to pre-mRNAs by splicing factors including U1 snRNP, and LENG8 depletion leads to leakage of misprocessed mRNAs and noncoding RNAs into the cytoplasm; LENG8 promotes RNA degradation by recruiting PAXT and the RNA exosome (preprint version of the peer-reviewed Molecular Cell study).\",\n      \"method\": \"Co-immunoprecipitation, knockdown with RNA-seq, biochemical complex reconstitution\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — same study as peer-reviewed version; preprint with multiple orthogonal methods\",\n      \"pmids\": [\"40832163\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The LENG8-PCID2-SEM1 (LENG8-PS) trimer is structurally and functionally equivalent to the central GANP-PCID2-SEM1 (GANP-PS) trimer of TREX-2. The LENG8-PS module is embedded within the PAXT connection and releases polyadenylated RNAs from UAP56 for decay by the nuclear exosome, acting in competition with NPC-associated TREX-2 that directs RNAs toward export.\",\n      \"method\": \"Mutagenesis, transcriptomics, biochemical reconstitution, structural homology analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — preprint with mutagenesis and transcriptomic data; not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.09.16.676470\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"PCID2 acts as a scaffold for mutually exclusive SAC3-based subcomplexes with GANP, LENG8, and SAC3D1. LENG8 localizes specifically to nuclear speckles (distinct from the nuclear envelope localization of GANP-containing TREX-2), and LENG8 depletion alters mRNA processing and polyadenylation site usage, placing LENG8 upstream of the canonical TREX-2 complex in mRNA processing.\",\n      \"method\": \"Immunoprecipitation/interactome analysis, subcellular localization (immunofluorescence), siRNA knockdown with RNA-seq for polyadenylation site analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — preprint with direct localization experiments and functional KD readout; multiple orthogonal approaches\",\n      \"pmids\": [\"42039562\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"LENG8 is a conserved nuclear RNA quality-control factor that is recruited to pre-mRNAs via U1 snRNP, localizes to nuclear speckles, and forms the REX/TREX-2.1 complex with PCID2 and SEM1/DSS1; this complex uses a trigger loop in LENG8 to release DDX39B from mRNPs, acts as a dominant-negative inhibitor of the export-competent TREX-2 complex to enforce nuclear retention of misprocessed mRNAs and noncoding RNAs, and promotes their degradation by recruiting the PAXT–nuclear exosome pathway.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"LENG8 is the defining subunit of a nuclear RNA quality-control complex (TREX-2.1/REX, composed of LENG8, PCID2, and SEM1) that retains and degrades aberrantly processed mRNAs and noncoding RNAs in the nucleus. LENG8 localizes to nuclear speckles, is recruited to pre-mRNAs by splicing factors including U1 snRNP, and acts as a dominant-negative counterpart to the TREX-2 mRNA export complex by engaging the RNA helicase DDX39B/UAP56 through a conserved trigger loop, thereby releasing polyadenylated RNAs from export-competent mRNPs [PMID:40595470, PMID:41861815]. Retained transcripts—including intronically polyadenylated, intron-retained, and noncoding RNAs—are then channeled for degradation through LENG8-mediated recruitment of the PAXT–nuclear exosome machinery, and loss of LENG8 leads to cytoplasmic leakage of these misprocessed species [PMID:41861815].\",\n  \"teleology\": [\n    {\n      \"year\": 2025,\n      \"claim\": \"Identification of LENG8 as a subunit of a novel TREX-2-related complex (TREX-2.1) resolved how DDX39B/UAP56 is released from mRNPs independently of the canonical NPC-associated TREX-2, revealing a conserved trigger loop in LENG8 as the structural basis for helicase regulation.\",\n      \"evidence\": \"Cryo-EM structure of TREX-2.1/DDX39B complex, co-IP/mass spectrometry for complex identification, LENG8 knockdown with RNA-seq in human cells\",\n      \"pmids\": [\"40595470\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The precise catalytic step by which the trigger loop stimulates DDX39B ATPase activity or RNA release is not fully resolved\",\n        \"Whether TREX-2.1 acts on all mRNPs or only specific transcript classes remains unclear\",\n        \"In vivo dynamics of TREX-2.1 assembly and disassembly at nuclear speckles are not established\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Localization of LENG8 to nuclear speckles and demonstration that PCID2 partitions into mutually exclusive subcomplexes (with GANP, LENG8, or SAC3D1) established that LENG8-containing complexes are spatially and functionally distinct from the canonical TREX-2 export machinery at the nuclear envelope.\",\n      \"evidence\": \"Proteomics/co-IP interactome mapping with subcellular imaging and LENG8 depletion RNA processing analysis (preprint)\",\n      \"pmids\": [\"42039562\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Preprint not yet peer-reviewed; independent replication of mutual exclusivity of PCID2 subcomplexes needed\",\n        \"How PCID2 allocation among GANP, LENG8, and SAC3D1 complexes is regulated is unknown\",\n        \"Whether speckle localization is required for LENG8 function has not been tested\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Defining LENG8 as a dominant-negative repressor of mRNA export that couples nuclear RNA retention to PAXT–exosome degradation resolved the mechanism by which aberrantly processed transcripts are triaged: LENG8 is recruited co-transcriptionally via splicing factors, blocks TREX-2-mediated export, and hands retained RNAs to the nuclear exosome.\",\n      \"evidence\": \"Co-IP interaction mapping with PAXT/exosome components, LENG8 depletion with RNA-seq showing cytoplasmic leakage of misprocessed mRNAs and ncRNAs, dominant-negative assays in human cells\",\n      \"pmids\": [\"41861815\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The structural interface between LENG8 and the PAXT complex has not been resolved at atomic resolution\",\n        \"Whether LENG8-mediated retention is reversible under specific conditions is unknown\",\n        \"The relative contribution of LENG8-mediated decay versus other nuclear RNA surveillance pathways is not quantified\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unknown how cells regulate the balance between LENG8/REX-mediated retention/decay and TREX-2-mediated export, whether signaling pathways modulate LENG8 activity, and whether LENG8 dysfunction contributes to human disease.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No post-translational regulation of LENG8 has been identified\",\n        \"No disease-associated mutations in LENG8 have been reported\",\n        \"The full repertoire of LENG8 RNA substrates across cell types is uncharacterized\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005654\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1, 3]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"complexes\": [\n      \"TREX-2.1 (REX)\",\n      \"PAXT complex (recruited)\"\n    ],\n    \"partners\": [\n      \"PCID2\",\n      \"SEM1\",\n      \"DDX39B\",\n      \"GANP\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"LENG8 is a conserved nuclear RNA quality-control factor that enforces retention and degradation of misprocessed mRNAs and noncoding RNAs. It is recruited to pre-mRNAs by splicing factors including U1 snRNP and assembles with PCID2 and SEM1/DSS1 into the REX (TREX-2.1) complex, which is structurally analogous to the canonical TREX-2 export complex but acts as its dominant-negative antagonist: a conserved trigger loop in LENG8 releases DDX39B (UAP56) from mRNPs, diverting polyadenylated transcripts away from nuclear export [PMID:40595470, PMID:41861815]. LENG8 localizes to nuclear speckles and promotes degradation of retained RNAs by directly recruiting the PAXT–nuclear exosome pathway; loss of LENG8 results in cytoplasmic leakage of intron-retained, intronically polyadenylated, and noncoding RNAs [PMID:41861815, PMID:40595470].\",\n  \"teleology\": [\n    {\n      \"year\": 2025,\n      \"claim\": \"Structural determination of the LENG8–PCID2–DSS1 (TREX-2.1) complex revealed how LENG8 uses a trigger loop to release DDX39B from mRNPs, establishing LENG8 as a direct regulator of mRNP remodeling analogous to GANP in canonical TREX-2.\",\n      \"evidence\": \"Cryo-EM structure of TREX-2.1/DDX39B complex with trigger-loop mutagenesis and RNA-seq in knockdown cells\",\n      \"pmids\": [\"40595470\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Substrate specificity beyond GC-content enrichment is not defined\",\n        \"Whether trigger-loop activity is regulated by post-translational modifications is unknown\",\n        \"No in vivo reconstitution of the full retention-to-degradation pathway\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Characterization of the REX complex demonstrated that LENG8 serves dual, coupled roles — nuclear retention via dominant-negative competition with TREX-2 and RNA degradation via direct recruitment of PAXT and the nuclear exosome — thereby establishing LENG8 as a central checkpoint factor for misprocessed and noncoding RNA disposal.\",\n      \"evidence\": \"Biochemical reconstitution of the REX complex, siRNA knockdown with RNA-seq, genetic epistasis with TREX-2 and exosome components (Molecular Cell)\",\n      \"pmids\": [\"41861815\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How selectivity for misprocessed versus correctly processed mRNAs is achieved at the molecular level is unresolved\",\n        \"Whether LENG8 functions redundantly with other nuclear retention factors is not tested\",\n        \"Direct structural basis for the LENG8–PAXT interaction has not been determined\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Localization and interactome studies placed LENG8 at nuclear speckles — distinct from the nuclear-envelope residence of GANP-containing TREX-2 — and showed that LENG8 depletion alters polyadenylation site usage, positioning REX upstream of TREX-2 in the mRNA processing hierarchy.\",\n      \"evidence\": \"Immunofluorescence for subcellular localization, interactome analysis, siRNA knockdown with poly(A)-site usage profiling (preprint)\",\n      \"pmids\": [\"42039562\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Preprint; not yet peer-reviewed\",\n        \"Mechanism by which nuclear speckle localization is achieved is unknown\",\n        \"Whether polyadenylation site changes are a direct or indirect consequence of LENG8 depletion is unclear\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the molecular basis for selective recognition of misprocessed transcripts, the structural interface between REX and PAXT, and whether LENG8 loss contributes to disease through aberrant cytoplasmic accumulation of noncoding or intron-containing RNAs.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No disease association has been established for LENG8\",\n        \"No structural model of the full LENG8–PAXT interface exists\",\n        \"In vivo physiological consequences of LENG8 loss in animal models have not been reported\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0005654\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"complexes\": [\n      \"REX (TREX-2.1; LENG8–PCID2–SEM1/DSS1)\"\n    ],\n    \"partners\": [\n      \"PCID2\",\n      \"SEM1\",\n      \"DDX39B\",\n      \"SNRNP70\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}