{"gene":"FSIP2","run_date":"2026-06-09T23:54:44","timeline":{"discoveries":[{"year":2018,"finding":"Loss-of-function mutations in FSIP2 cause complete disorganization of the fibrous sheath (FS) and axonemal defects, including abnormal central-pair microtubules and inner/outer dynein arms, establishing FSIP2 as necessary for FS assembly and overall flagellar biogenesis. Importantly, FSIP2 mutations uniquely result in the absence of AKAP4 (A-kinase anchoring protein 4), distinguishing FSIP2-related MMAF from DNAH1/CFAP43/CFAP44-related MMAF.","method":"Whole-exome sequencing, Sanger sequencing, immunofluorescence (IF), transmission electron microscopy (TEM)","journal":"Human reproduction (Oxford, England)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — IF and TEM in patient sperm with loss-of-function mutations, single lab, two orthogonal methods; no formal protein-absence confirmation due to lack of reliable antibody","pmids":["30137358"],"is_preprint":false},{"year":2021,"finding":"Fsip2 physically interacts with Acrv1 (an acrosomal marker protein), and its expression is required at the acrosome during spermiogenesis. Knock-in (KI) truncating mutation in mice caused MMAF phenotype, while overexpression (OE) produced sperm tails with increased length. Proteomic analysis revealed changes in proteins at the fibrous sheath, mitochondrial sheath, and acrosomal vesicle, indicating dosage-dependent roles of Fsip2 in both sperm tail and acrosome formation.","method":"Knock-in and overexpression mouse models, single-cell RNA sequencing, co-immunoprecipitation (physical interaction with Acrv1), proteomic analysis, immunofluorescence","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP for Acrv1 interaction, KI mouse model with defined MMAF phenotype, OE model, proteomics, and scRNA-seq, multiple orthogonal methods in one study","pmids":["34125190"],"is_preprint":false},{"year":2022,"finding":"FSIP2 co-localizes with peanut agglutinin (PNA) at the acrosome during spermatogenesis and interacts (by co-immunoprecipitation) with proteins involved in acrosome biogenesis: DPY19L2, SPACA1, HSP90B1, KIAA1210, HSPA2, and CLTC. Loss of FSIP2 function leads to downregulated expression of DPY19L2, ZPBP, SPACA1, CCDC62, CCIN, SPINK2, and CSNK2A2, and results in acrosomal hypoplasia in addition to flagellar defects.","method":"Whole-exome sequencing, western blot, immunofluorescence, co-immunoprecipitation (Co-IP), liquid chromatography-tandem mass spectrometry (LC-MS/MS) proteomics","journal":"Journal of medical genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP with multiple acrosome biogenesis proteins validated by proteomics and IF, single lab, two orthogonal methods","pmids":["35654582"],"is_preprint":false},{"year":2023,"finding":"FSIP2 loss-of-function variants cause disassembly of the fibrous sheath and axonemal defects, as well as pathological 'super-length' mitochondrial sheaths with increased TOMM20 levels and decreased mitochondrial ATP consumption. Dislocation/deletion of the annulus and reduction of annulus protein SEPT4 were also observed, implicating FSIP2 in the termination of mitochondrial sheath extension and intra-flagellar transport during spermatogenesis.","method":"Exome sequencing, immunofluorescence, transmission electron microscopy, mitochondrial ATP consumption assay, western blot for TOMM20 and SEPT4","journal":"International journal of biological sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (TEM, IF, functional ATP assay) in patient samples, single lab","pmids":["36632462"],"is_preprint":false},{"year":2025,"finding":"Loss-of-function FSIP2 variants result in complete absence of FSIP2 protein from spermatozoa and co-absence of AKAP4, SPAG6, IFT20, and ACTL7A, establishing that FSIP2 is required for proper localization/stability of intraflagellar transport (IFT20) and acrosomal (ACTL7A) proteins in addition to fibrous sheath components.","method":"Whole-exome sequencing, immunofluorescence staining, RT-PCR, transmission electron microscopy","journal":"Asian journal of andrology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — immunofluorescence in patient sperm only, single lab, single method per protein target, no functional rescue experiments","pmids":["40968718"],"is_preprint":false},{"year":2026,"finding":"FSIP2 biallelic variants disrupt FSIP2 protein localization in sperm and dysregulate expression of key axonemal assembly factors, with ultrastructural consequences including pathological mitochondrial sheath elongation in the midpiece and fibrous sheath dysplasia or loss in the principal piece.","method":"Whole-exome sequencing, Sanger sequencing, third-generation sequencing, immunofluorescence, western blotting, scanning electron microscopy, transmission electron microscopy","journal":"Asian journal of andrology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — patient-based study with IF and WB, multiple morphological methods, single lab, no functional reconstitution or epistasis","pmids":["42169611"],"is_preprint":false}],"current_model":"FSIP2 is a fibrous sheath protein essential for sperm flagellar assembly and acrosome biogenesis: it physically interacts with acrosomal proteins (including Acrv1, DPY19L2, SPACA1, HSP90B1, HSPA2, CLTC) and its loss causes disorganization of the fibrous sheath, axonemal defects (including absent/abnormal dynein arms and central-pair microtubules), absence of AKAP4 and other flagellar/acrosomal proteins, pathological mitochondrial sheath elongation, and acrosomal hypoplasia, indicating dosage-dependent roles in both sperm tail and acrosome formation during spermiogenesis."},"narrative":{"mechanistic_narrative":"FSIP2 is a fibrous sheath protein required for sperm flagellar assembly and acrosome biogenesis during spermiogenesis, and its loss causes the multiple morphological abnormalities of the sperm flagella (MMAF) phenotype [PMID:30137358, PMID:34125190]. Loss-of-function variants produce complete disorganization of the fibrous sheath together with axonemal defects, including abnormal central-pair microtubules and inner/outer dynein arms, and uniquely abolish AKAP4 from the flagellum, distinguishing FSIP2-related MMAF from other MMAF genes [PMID:30137358]. Mouse knock-in and overexpression models establish a dosage-dependent role in both tail and acrosome formation, with overexpression elongating tails and truncation reproducing MMAF, while FSIP2 physically interacts with the acrosomal protein Acrv1 [PMID:34125190]. FSIP2 co-localizes at the acrosome and interacts with acrosome-biogenesis proteins including DPY19L2, SPACA1, HSP90B1, HSPA2, and CLTC; its loss downregulates DPY19L2, ZPBP, and SPACA1 and produces acrosomal hypoplasia alongside flagellar defects [PMID:35654582]. FSIP2 deficiency also drives pathological 'super-length' mitochondrial sheaths with elevated TOMM20 and reduced annulus protein SEPT4, implicating it in terminating mitochondrial sheath extension [PMID:36632462].","teleology":[{"year":2018,"claim":"Established FSIP2 as a causal MMAF gene by showing its loss disorganizes the fibrous sheath and disrupts axonemal architecture, defining its place in flagellar biogenesis.","evidence":"Whole-exome sequencing with IF and TEM in patient sperm carrying loss-of-function mutations","pmids":["30137358"],"confidence":"Medium","gaps":["No protein-absence confirmation due to lack of reliable antibody","Mechanism by which FSIP2 loss abolishes AKAP4 not defined","Patient-only observation without functional rescue"]},{"year":2021,"claim":"Demonstrated that FSIP2 acts in a dosage-dependent manner on both sperm tail and acrosome formation and physically engages an acrosomal protein, extending its role beyond the fibrous sheath.","evidence":"Knock-in and overexpression mouse models, scRNA-seq, reciprocal co-IP with Acrv1, and proteomics","pmids":["34125190"],"confidence":"High","gaps":["Direct binding interface with Acrv1 not mapped","How overexpression lengthens tails mechanistically unresolved","Causal hierarchy among proteomic changes not established"]},{"year":2022,"claim":"Connected FSIP2 directly to the acrosome-biogenesis machinery by identifying multiple interacting partners and linking its loss to acrosomal hypoplasia.","evidence":"Co-IP and LC-MS/MS proteomics with western blot and IF in patient samples","pmids":["35654582"],"confidence":"Medium","gaps":["Co-IP interactions not validated reciprocally for all partners","Whether interactions are direct or complex-mediated unknown","No in vivo rescue of acrosomal defects"]},{"year":2023,"claim":"Implicated FSIP2 in terminating mitochondrial sheath extension, revealing a structural role in midpiece organization beyond the fibrous sheath.","evidence":"Exome sequencing, TEM, IF, mitochondrial ATP consumption assay, and western blot for TOMM20 and SEPT4 in patient sperm","pmids":["36632462"],"confidence":"Medium","gaps":["Molecular link between FSIP2 and SEPT4/annulus not defined","Cause of decreased mitochondrial ATP consumption unclear","No functional reconstitution"]},{"year":2025,"claim":"Showed FSIP2 absence coincides with loss of intraflagellar transport (IFT20) and acrosomal (ACTL7A) proteins, broadening the set of proteins dependent on FSIP2 for localization or stability.","evidence":"IF, RT-PCR, and TEM in patient spermatozoa","pmids":["40968718"],"confidence":"Low","gaps":["Single method per protein target with no rescue","Cannot distinguish loss of stability from loss of localization","No direct interaction data for IFT20 or ACTL7A"]},{"year":2026,"claim":"Reinforced that FSIP2 variants dysregulate axonemal assembly factors and produce combined midpiece and principal-piece ultrastructural defects.","evidence":"Whole-exome and third-generation sequencing with IF, WB, SEM, and TEM in patient sperm","pmids":["42169611"],"confidence":"Low","gaps":["No functional reconstitution or epistasis","Specific dysregulated factors and their direct dependence on FSIP2 not resolved","Patient-based correlative evidence only"]},{"year":null,"claim":"The biochemical mechanism by which FSIP2 organizes the fibrous sheath and stabilizes its diverse partners remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structural model of FSIP2 or its complexes","Direct versus indirect basis of partner dependence not separated","No reconstitution of FSIP2-dependent assembly steps"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1,3]}],"localization":[{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[0,3]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[0,1,2]}],"complexes":["fibrous sheath"],"partners":["ACRV1","DPY19L2","SPACA1","HSP90B1","HSPA2","CLTC","AKAP4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q5CZC0","full_name":"Fibrous sheath-interacting protein 2","aliases":[],"length_aa":6907,"mass_kda":780.6,"function":"Plays a role in spermatogenesis","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/Q5CZC0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FSIP2","classification":"Not Classified","n_dependent_lines":3,"n_total_lines":1208,"dependency_fraction":0.0024834437086092716},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FSIP2","total_profiled":1310},"omim":[{"mim_id":"618153","title":"SPERMATOGENIC FAILURE 34; SPGF34","url":"https://www.omim.org/entry/618153"},{"mim_id":"615796","title":"FIBROUS SHEATH-INTERACTING PROTEIN 2; FSIP2","url":"https://www.omim.org/entry/615796"},{"mim_id":"615795","title":"FIBROUS SHEATH-INTERACTING PROTEIN 1; FSIP1","url":"https://www.omim.org/entry/615795"},{"mim_id":"258150","title":"SPERMATOGENIC FAILURE 1; SPGF1","url":"https://www.omim.org/entry/258150"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Microtubules","reliability":"Approved"},{"location":"Cytokinetic bridge","reliability":"Additional"},{"location":"Mitotic spindle","reliability":"Additional"},{"location":"Primary cilium","reliability":"Additional"},{"location":"Basal body","reliability":"Additional"},{"location":"Mid piece","reliability":"Additional"}],"tissue_specificity":"Group enriched","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"intestine","ntpm":1.3},{"tissue":"stomach 1","ntpm":2.4},{"tissue":"testis","ntpm":5.0}],"url":"https://www.proteinatlas.org/search/FSIP2"},"hgnc":{"alias_symbol":["FLJ34780"],"prev_symbol":[]},"alphafold":{"accession":"Q5CZC0","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5CZC0","model_url":"","pae_url":"","plddt_mean":null},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FSIP2","jax_strain_url":"https://www.jax.org/strain/search?query=FSIP2"},"sequence":{"accession":"Q5CZC0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q5CZC0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q5CZC0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5CZC0"}},"corpus_meta":[{"pmid":"30137358","id":"PMC_30137358","title":"Whole-exome sequencing identifies mutations in FSIP2 as a recurrent cause of multiple morphological abnormalities of the sperm flagella.","date":"2018","source":"Human reproduction (Oxford, England)","url":"https://pubmed.ncbi.nlm.nih.gov/30137358","citation_count":95,"is_preprint":false},{"pmid":"34125190","id":"PMC_34125190","title":"Hypomorphic and hypermorphic mouse models of Fsip2 indicate its dosage-dependent roles in sperm tail and acrosome formation.","date":"2021","source":"Development (Cambridge, England)","url":"https://pubmed.ncbi.nlm.nih.gov/34125190","citation_count":25,"is_preprint":false},{"pmid":"36632462","id":"PMC_36632462","title":"Novel FSIP2 Variants Induce Super-Length Mitochondrial Sheath and Asthenoteratozoospermia in Humans.","date":"2023","source":"International journal of biological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/36632462","citation_count":22,"is_preprint":false},{"pmid":"35654582","id":"PMC_35654582","title":"FSIP2 plays a role in the acrosome development during spermiogenesis.","date":"2022","source":"Journal of medical genetics","url":"https://pubmed.ncbi.nlm.nih.gov/35654582","citation_count":18,"is_preprint":false},{"pmid":"35672654","id":"PMC_35672654","title":"Novel Compound Heterozygous Mutation in FSIP2 Causes Multiple Morphological Abnormalities of the Sperm Flagella (MMAF) and Male Infertility.","date":"2022","source":"Reproductive sciences (Thousand Oaks, Calif.)","url":"https://pubmed.ncbi.nlm.nih.gov/35672654","citation_count":13,"is_preprint":false},{"pmid":"34935173","id":"PMC_34935173","title":"Successful outcomes of intracytoplasmic sperm injection-embryo transfer using ejaculated spermatozoa from two Chinese asthenoteratozoospermic brothers with a compound heterozygous FSIP2 mutation.","date":"2021","source":"Andrologia","url":"https://pubmed.ncbi.nlm.nih.gov/34935173","citation_count":9,"is_preprint":false},{"pmid":"34113648","id":"PMC_34113648","title":"Association Between FSIP2 Mutation and an Improved Efficacy of Immune Checkpoint Inhibitors in Patients With Skin Cutaneous Melanoma.","date":"2021","source":"Frontiers in molecular biosciences","url":"https://pubmed.ncbi.nlm.nih.gov/34113648","citation_count":4,"is_preprint":false},{"pmid":"39753944","id":"PMC_39753944","title":"Novel variants of FSIP2 and SPEF2 cause varying degrees of spermatozoa damage in MMAF patients and favorable ART outcomes.","date":"2025","source":"Journal of assisted reproduction and genetics","url":"https://pubmed.ncbi.nlm.nih.gov/39753944","citation_count":4,"is_preprint":false},{"pmid":"40968718","id":"PMC_40968718","title":"Novel mutations in FSIP2 cause male infertility through multiple morphological abnormalities of the sperm flagella.","date":"2025","source":"Asian journal of andrology","url":"https://pubmed.ncbi.nlm.nih.gov/40968718","citation_count":1,"is_preprint":false},{"pmid":"42169611","id":"PMC_42169611","title":"Novel biallelic FSIP2 variants cause male infertility with multiple morphological abnormalities of sperm flagella in humans.","date":"2026","source":"Asian journal of andrology","url":"https://pubmed.ncbi.nlm.nih.gov/42169611","citation_count":0,"is_preprint":false},{"pmid":"40788209","id":"PMC_40788209","title":"[Clinical and genetic analysis of a patient with FSIP2 compound heterozygous variants causing multiple morphological abnormalities of sperm flagella].","date":"2025","source":"Zhonghua nan ke xue = National journal of andrology","url":"https://pubmed.ncbi.nlm.nih.gov/40788209","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8703,"output_tokens":1863,"usd":0.027027,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8706,"output_tokens":2341,"usd":0.051028,"stage2_stop_reason":"end_turn"},"total_usd":0.078055,"stage1_batch_id":"msgbatch_01AsR7g8dGTEiEw9U83bJeWn","stage2_batch_id":"msgbatch_01YV5ZyxtFNUZEjxRcpd9jzb","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2018,\n      \"finding\": \"Loss-of-function mutations in FSIP2 cause complete disorganization of the fibrous sheath (FS) and axonemal defects, including abnormal central-pair microtubules and inner/outer dynein arms, establishing FSIP2 as necessary for FS assembly and overall flagellar biogenesis. Importantly, FSIP2 mutations uniquely result in the absence of AKAP4 (A-kinase anchoring protein 4), distinguishing FSIP2-related MMAF from DNAH1/CFAP43/CFAP44-related MMAF.\",\n      \"method\": \"Whole-exome sequencing, Sanger sequencing, immunofluorescence (IF), transmission electron microscopy (TEM)\",\n      \"journal\": \"Human reproduction (Oxford, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — IF and TEM in patient sperm with loss-of-function mutations, single lab, two orthogonal methods; no formal protein-absence confirmation due to lack of reliable antibody\",\n      \"pmids\": [\"30137358\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Fsip2 physically interacts with Acrv1 (an acrosomal marker protein), and its expression is required at the acrosome during spermiogenesis. Knock-in (KI) truncating mutation in mice caused MMAF phenotype, while overexpression (OE) produced sperm tails with increased length. Proteomic analysis revealed changes in proteins at the fibrous sheath, mitochondrial sheath, and acrosomal vesicle, indicating dosage-dependent roles of Fsip2 in both sperm tail and acrosome formation.\",\n      \"method\": \"Knock-in and overexpression mouse models, single-cell RNA sequencing, co-immunoprecipitation (physical interaction with Acrv1), proteomic analysis, immunofluorescence\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP for Acrv1 interaction, KI mouse model with defined MMAF phenotype, OE model, proteomics, and scRNA-seq, multiple orthogonal methods in one study\",\n      \"pmids\": [\"34125190\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"FSIP2 co-localizes with peanut agglutinin (PNA) at the acrosome during spermatogenesis and interacts (by co-immunoprecipitation) with proteins involved in acrosome biogenesis: DPY19L2, SPACA1, HSP90B1, KIAA1210, HSPA2, and CLTC. Loss of FSIP2 function leads to downregulated expression of DPY19L2, ZPBP, SPACA1, CCDC62, CCIN, SPINK2, and CSNK2A2, and results in acrosomal hypoplasia in addition to flagellar defects.\",\n      \"method\": \"Whole-exome sequencing, western blot, immunofluorescence, co-immunoprecipitation (Co-IP), liquid chromatography-tandem mass spectrometry (LC-MS/MS) proteomics\",\n      \"journal\": \"Journal of medical genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP with multiple acrosome biogenesis proteins validated by proteomics and IF, single lab, two orthogonal methods\",\n      \"pmids\": [\"35654582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FSIP2 loss-of-function variants cause disassembly of the fibrous sheath and axonemal defects, as well as pathological 'super-length' mitochondrial sheaths with increased TOMM20 levels and decreased mitochondrial ATP consumption. Dislocation/deletion of the annulus and reduction of annulus protein SEPT4 were also observed, implicating FSIP2 in the termination of mitochondrial sheath extension and intra-flagellar transport during spermatogenesis.\",\n      \"method\": \"Exome sequencing, immunofluorescence, transmission electron microscopy, mitochondrial ATP consumption assay, western blot for TOMM20 and SEPT4\",\n      \"journal\": \"International journal of biological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (TEM, IF, functional ATP assay) in patient samples, single lab\",\n      \"pmids\": [\"36632462\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Loss-of-function FSIP2 variants result in complete absence of FSIP2 protein from spermatozoa and co-absence of AKAP4, SPAG6, IFT20, and ACTL7A, establishing that FSIP2 is required for proper localization/stability of intraflagellar transport (IFT20) and acrosomal (ACTL7A) proteins in addition to fibrous sheath components.\",\n      \"method\": \"Whole-exome sequencing, immunofluorescence staining, RT-PCR, transmission electron microscopy\",\n      \"journal\": \"Asian journal of andrology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — immunofluorescence in patient sperm only, single lab, single method per protein target, no functional rescue experiments\",\n      \"pmids\": [\"40968718\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"FSIP2 biallelic variants disrupt FSIP2 protein localization in sperm and dysregulate expression of key axonemal assembly factors, with ultrastructural consequences including pathological mitochondrial sheath elongation in the midpiece and fibrous sheath dysplasia or loss in the principal piece.\",\n      \"method\": \"Whole-exome sequencing, Sanger sequencing, third-generation sequencing, immunofluorescence, western blotting, scanning electron microscopy, transmission electron microscopy\",\n      \"journal\": \"Asian journal of andrology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — patient-based study with IF and WB, multiple morphological methods, single lab, no functional reconstitution or epistasis\",\n      \"pmids\": [\"42169611\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FSIP2 is a fibrous sheath protein essential for sperm flagellar assembly and acrosome biogenesis: it physically interacts with acrosomal proteins (including Acrv1, DPY19L2, SPACA1, HSP90B1, HSPA2, CLTC) and its loss causes disorganization of the fibrous sheath, axonemal defects (including absent/abnormal dynein arms and central-pair microtubules), absence of AKAP4 and other flagellar/acrosomal proteins, pathological mitochondrial sheath elongation, and acrosomal hypoplasia, indicating dosage-dependent roles in both sperm tail and acrosome formation during spermiogenesis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"FSIP2 is a fibrous sheath protein required for sperm flagellar assembly and acrosome biogenesis during spermiogenesis, and its loss causes the multiple morphological abnormalities of the sperm flagella (MMAF) phenotype [#0, #1]. Loss-of-function variants produce complete disorganization of the fibrous sheath together with axonemal defects, including abnormal central-pair microtubules and inner/outer dynein arms, and uniquely abolish AKAP4 from the flagellum, distinguishing FSIP2-related MMAF from other MMAF genes [#0]. Mouse knock-in and overexpression models establish a dosage-dependent role in both tail and acrosome formation, with overexpression elongating tails and truncation reproducing MMAF, while FSIP2 physically interacts with the acrosomal protein Acrv1 [#1]. FSIP2 co-localizes at the acrosome and interacts with acrosome-biogenesis proteins including DPY19L2, SPACA1, HSP90B1, HSPA2, and CLTC; its loss downregulates DPY19L2, ZPBP, and SPACA1 and produces acrosomal hypoplasia alongside flagellar defects [#2]. FSIP2 deficiency also drives pathological 'super-length' mitochondrial sheaths with elevated TOMM20 and reduced annulus protein SEPT4, implicating it in terminating mitochondrial sheath extension [#3].\",\n  \"teleology\": [\n    {\n      \"year\": 2018,\n      \"claim\": \"Established FSIP2 as a causal MMAF gene by showing its loss disorganizes the fibrous sheath and disrupts axonemal architecture, defining its place in flagellar biogenesis.\",\n      \"evidence\": \"Whole-exome sequencing with IF and TEM in patient sperm carrying loss-of-function mutations\",\n      \"pmids\": [\"30137358\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No protein-absence confirmation due to lack of reliable antibody\", \"Mechanism by which FSIP2 loss abolishes AKAP4 not defined\", \"Patient-only observation without functional rescue\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstrated that FSIP2 acts in a dosage-dependent manner on both sperm tail and acrosome formation and physically engages an acrosomal protein, extending its role beyond the fibrous sheath.\",\n      \"evidence\": \"Knock-in and overexpression mouse models, scRNA-seq, reciprocal co-IP with Acrv1, and proteomics\",\n      \"pmids\": [\"34125190\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct binding interface with Acrv1 not mapped\", \"How overexpression lengthens tails mechanistically unresolved\", \"Causal hierarchy among proteomic changes not established\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Connected FSIP2 directly to the acrosome-biogenesis machinery by identifying multiple interacting partners and linking its loss to acrosomal hypoplasia.\",\n      \"evidence\": \"Co-IP and LC-MS/MS proteomics with western blot and IF in patient samples\",\n      \"pmids\": [\"35654582\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Co-IP interactions not validated reciprocally for all partners\", \"Whether interactions are direct or complex-mediated unknown\", \"No in vivo rescue of acrosomal defects\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Implicated FSIP2 in terminating mitochondrial sheath extension, revealing a structural role in midpiece organization beyond the fibrous sheath.\",\n      \"evidence\": \"Exome sequencing, TEM, IF, mitochondrial ATP consumption assay, and western blot for TOMM20 and SEPT4 in patient sperm\",\n      \"pmids\": [\"36632462\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular link between FSIP2 and SEPT4/annulus not defined\", \"Cause of decreased mitochondrial ATP consumption unclear\", \"No functional reconstitution\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Showed FSIP2 absence coincides with loss of intraflagellar transport (IFT20) and acrosomal (ACTL7A) proteins, broadening the set of proteins dependent on FSIP2 for localization or stability.\",\n      \"evidence\": \"IF, RT-PCR, and TEM in patient spermatozoa\",\n      \"pmids\": [\"40968718\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single method per protein target with no rescue\", \"Cannot distinguish loss of stability from loss of localization\", \"No direct interaction data for IFT20 or ACTL7A\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Reinforced that FSIP2 variants dysregulate axonemal assembly factors and produce combined midpiece and principal-piece ultrastructural defects.\",\n      \"evidence\": \"Whole-exome and third-generation sequencing with IF, WB, SEM, and TEM in patient sperm\",\n      \"pmids\": [\"42169611\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No functional reconstitution or epistasis\", \"Specific dysregulated factors and their direct dependence on FSIP2 not resolved\", \"Patient-based correlative evidence only\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The biochemical mechanism by which FSIP2 organizes the fibrous sheath and stabilizes its diverse partners remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structural model of FSIP2 or its complexes\", \"Direct versus indirect basis of partner dependence not separated\", \"No reconstitution of FSIP2-dependent assembly steps\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [0, 3]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"complexes\": [\"fibrous sheath\"],\n    \"partners\": [\"ACRV1\", \"DPY19L2\", \"SPACA1\", \"HSP90B1\", \"HSPA2\", \"CLTC\", \"AKAP4\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"faith_supported":5,"faith_total":5,"faith_pct":100.0}}