{"gene":"TTC8","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2004,"finding":"C. elegans BBS-8 protein localizes predominantly at the base of cilia and moves bidirectionally along the ciliary axoneme, similar to IFT proteins. BBS-8 is required for the normal localization and motility of IFT proteins OSM-5/Polaris and CHE-11 (and to a lesser extent CHE-2), demonstrating that BBS-8 plays a selective role in the assembly and/or function of IFT particle components.","method":"Loss-of-function mutations in C. elegans bbs-8; fluorescence microscopy of protein localization; functional IFT motility assays","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean loss-of-function genetic model with multiple orthogonal readouts (localization, motility, structural cilia defects), replicated across BBS-7 and BBS-8","pmids":["15231740"],"is_preprint":false},{"year":2010,"finding":"A retina-specific alternative exon of BBS8 (exon 2A) encodes 10 amino acids and represents the major BBS8 mRNA isoform in mammalian photoreceptors. A splice-site mutation eliminating this exon causes nonsyndromic retinitis pigmentosa restricted to the retina, establishing that this photoreceptor-specific sequence is pivotal for BBS8 function in the retina.","method":"Genome-wide linkage mapping; candidate gene sequencing; RT-PCR and expression analysis of retina-specific exon; segregation analysis","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — human genetic mapping combined with tissue-specific expression characterization, single lab","pmids":["20451172"],"is_preprint":false},{"year":2011,"finding":"BBS8 localizes in olfactory sensory neurons (OSNs) at the dendritic knob in a shell-like structure surrounding basal bodies. Loss of BBS8 causes near-complete loss of cilia from OSNs, mislocalization of ciliary-enriched proteins, and aberrant axon targeting of OSNs to the olfactory bulb, demonstrating BBS8 is required for ciliary protein localization and axon targeting in OSNs.","method":"Bbs8-null mouse with fluorescent reporter knock-in; immunohistochemistry; ex vivo live imaging of SLP3-eGFP knock-in; electroolfactogram","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout with multiple orthogonal methods (reporter localization, IHC, electrophysiology, live imaging), single lab but rigorous","pmids":["21646512"],"is_preprint":false},{"year":2015,"finding":"BBS8 directly interacts with the core planar cell polarity (PCP) molecule Vangl2, as demonstrated by co-immunoprecipitation. Loss of Bbs8 disrupts asymmetric accumulation of Vangl2 in cochlear cells and causes mis-oriented hair cell stereociliary bundles, placing BBS8 upstream of core PCP asymmetry. BBS8 also localizes to filamentous actin as well as microtubules.","method":"Co-immunoprecipitation (BBS8–Vangl2); Bbs8 conditional knockout mouse; immunofluorescence of cochlear PCP markers; subcellular fractionation/localization","journal":"Development (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP plus genetic loss-of-function with defined PCP phenotype, single lab","pmids":["25605782"],"is_preprint":false},{"year":2010,"finding":"Bbs8 and the core PCP protein Vangl2 interact and act synergistically in zebrafish to establish left-right asymmetry; loss of bbs8 disrupts cilia at Kupffer's vesicle, causes defective actin organization, and leads to nuclear mislocalisation implying defective centrosome/basal body migration and apical docking.","method":"Zebrafish morpholino knockdown of bbs8 and vangl2; genetic interaction/epistasis; cilia imaging; actin staining","journal":"Developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis in vivo with multiple cellular readouts, single lab","pmids":["20643117"],"is_preprint":false},{"year":2015,"finding":"Cell-type-specific alternative splicing of BBS8 exon 2A (the IVS1-2A>G mutation) causes photoreceptor-specific loss of BBS8 protein because: (1) in photoreceptors, exon 2A is included and the mutation causes a frameshift eliminating the protein; (2) in all other cell types, exon 2A is skipped, rendering those cells immune to the mutation. Splicing of exon 2A in photoreceptors is directed exclusively by redundant splicing enhancers in the flanking introns.","method":"RT-PCR in multiple cell types; minigene splicing reporters; analysis of intronic splicing enhancer sequences; patient and control cell RNA analysis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct mechanistic dissection of splicing with minigene reporters and mutagenesis of regulatory elements, multiple orthogonal methods in one study","pmids":["25776555"],"is_preprint":false},{"year":2018,"finding":"BBS8 is required for normal elaboration of photoreceptor outer segments. Loss of BBS8 leads to: concomitant decrease in BBSome subunits BBS2 and BBS5 (and increase in BBS1 and BBS4); disorganized and shortened outer segments; altered distribution of photoreceptor axonemal proteins; hyper-acetylation of ciliary microtubules; and mislocalization of syntaxin3 to the outer segment (a protein normally absent from OS).","method":"Bbs8 conditional knockout mouse (rod- and cone-specific); Western blot for BBSome subunit levels; immunofluorescence; ERG; histology","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic knockout with multiple orthogonal readouts including protein-level BBSome stability, localization studies, and functional ERG, single lab but comprehensive","pmids":["29126234"],"is_preprint":false},{"year":2021,"finding":"Loss of Bbs8 in the retinal pigment epithelium (RPE) causes changes in cytoskeletal and cell adhesion molecules, defective cellular polarization and morphology, and an epithelial-to-mesenchymal transition (EMT)-like phenotype, demonstrating a role for BBS8 in RPE homeostasis beyond ciliary function.","method":"Bbs8-null mouse RPE; proteomics and transcriptomics (combinatorial omics); immunofluorescence for cytoskeletal markers; cell polarity assays","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — combined proteomic/transcriptomic and in vivo cellular phenotype analysis, single lab","pmids":["33681195"],"is_preprint":false},{"year":2022,"finding":"A missense mutation c.1347G>C at the last base of exon 13 of TTC8 disrupts the canonical donor splice site, activating a cryptic splice site 77 bases into intron 13, causing intron retention, a frameshift, and a premature termination codon in exon 14. The mutation also disrupts the binding site for SC35 splicing factor and causes duplication and fusion of exon 15.","method":"RT-PCR and Sanger sequencing of patient cDNA; bioinformatic splice site prediction (SpliceAid2); minigene/functional RNA analysis","journal":"Molecular genetics and genomics : MGG","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct RNA sequencing from patient material establishing aberrant splicing mechanism, single lab, single method","pmids":["35939099"],"is_preprint":false},{"year":2025,"finding":"Loss of BBS8 in adipocyte precursor cells (APCs) induces ectopic ciliary Hedgehog signaling, driving a phenotypic switch from the stem-cell-like P1 APC subpopulation to a fibrogenic progenitor state (characterized by ECM remodeling and upregulation of CD9), bypassing the committed P2 progenitor state and altering adipogenesis.","method":"Bbs8-/- mouse model; single-cell RNA sequencing; flow cytometry; Hedgehog pathway reporter assays; in vitro differentiation assays","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — scRNA-seq trajectory analysis plus functional Hedgehog pathway measurements in vivo and in vitro, single lab","pmids":["40836034"],"is_preprint":false},{"year":2025,"finding":"BBS8 physically interacts with HDAC2. Loss of BBS8 upregulates HDAC2 in kidneys, mouse embryonic fibroblasts, and patient-derived urine renal epithelial cells, leading to reduced K40 acetylation of α-tubulin within primary cilia and compromised ciliary stability. Pharmacological HDAC2 inhibition restores tubulin acetylation in BBS8-deficient cells.","method":"Bbs8-/- mouse kidney phenotyping; proteomics; co-immunoprecipitation (BBS8–HDAC2); immunofluorescence for acetylated tubulin; pharmacological rescue with HDAC inhibitor in patient-derived cells","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP interaction plus pharmacological rescue in patient-derived cells, preprint not yet peer-reviewed","pmids":["40667253"],"is_preprint":true},{"year":2025,"finding":"Musashi proteins (MSI1/MSI2) promote inclusion of the photoreceptor-specific alternative exon of Ttc8 by binding to proximal downstream intronic sequences. A single Musashi allele (from either Msi1 or Msi2) is sufficient to maintain high inclusion levels of the Ttc8 photoreceptor-specific exon and photoreceptor function.","method":"Combined Msi1/Msi2 knockout mice with graded allele reduction; RT-PCR splicing analysis of Ttc8 photoreceptor-specific exon; photoreceptor function assays (ERG)","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic allele-series with direct splicing readout for Ttc8 exon, preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.11.26.690869"],"is_preprint":true}],"current_model":"TTC8/BBS8 is a component of the BBSome octameric complex that localizes at the base of cilia and is required for intraflagellar transport (IFT) particle assembly and bidirectional ciliary movement; it directly interacts with the PCP protein Vangl2 to regulate planar cell polarity and left-right asymmetry, interacts with HDAC2 to control ciliary tubulin acetylation and stability, and in photoreceptors its inclusion of a tissue-specific alternative exon (directed by intronic Musashi-binding splicing enhancers) is essential for outer segment development and photoreceptor survival, while its loss also triggers ectopic Hedgehog signaling in adipocyte precursors that drives fibrogenesis over adipogenesis."},"narrative":{"mechanistic_narrative":"TTC8/BBS8 is a ciliary protein that localizes at the base of cilia and moves bidirectionally along the axoneme, where it acts selectively in the assembly and motility of intraflagellar transport (IFT) particles and is required for ciliary protein localization and cilium maintenance across diverse cell types including sensory neurons and photoreceptors [PMID:15231740, PMID:21646512, PMID:29126234]. It functions as a BBSome-associated component, and its loss destabilizes other BBSome subunits and disorganizes photoreceptor outer segments [PMID:29126234]. BBS8 directly interacts with the core planar cell polarity protein Vangl2 to control asymmetric Vangl2 accumulation, stereociliary bundle orientation, and left-right asymmetry, linking it to cytoskeletal organization at both actin and microtubule structures [PMID:25605782, PMID:20643117]. It also interacts with HDAC2, and BBS8 loss elevates HDAC2 to reduce K40 acetylation of ciliary α-tubulin and compromise ciliary stability, a defect reversible by HDAC inhibition [PMID:40667253]. A photoreceptor-specific alternative exon (exon 2A), whose inclusion is governed by redundant intronic splicing enhancers bound by Musashi proteins, is essential for BBS8 function in photoreceptors; mutations disrupting its splicing cause nonsyndromic retinitis pigmentosa confined to the retina [PMID:20451172, PMID:25776555, PMID:bio_10.1101_2025.11.26.690869]. Beyond ciliary roles, BBS8 loss drives an EMT-like phenotype in retinal pigment epithelium and induces ectopic ciliary Hedgehog signaling in adipocyte precursors that biases them toward a fibrogenic rather than adipogenic fate [PMID:33681195, PMID:40836034].","teleology":[{"year":2004,"claim":"Established that BBS8 is not merely ciliary but functionally embedded in the IFT machinery, defining its core molecular role.","evidence":"Loss-of-function mutations in C. elegans bbs-8 with fluorescence localization and IFT motility assays","pmids":["15231740"],"confidence":"High","gaps":["Does not define which IFT subcomplex BBS8 assembles","No biochemical reconstitution of BBS8 within a complex","Mechanism of bidirectional movement unresolved"]},{"year":2010,"claim":"Showed that a retina-specific BBS8 alternative exon is the dominant photoreceptor isoform and its loss causes tissue-restricted disease, revealing tissue-specific function encoded by splicing.","evidence":"Linkage mapping, candidate sequencing, and RT-PCR of exon 2A in human nonsyndromic retinitis pigmentosa","pmids":["20451172"],"confidence":"Medium","gaps":["What the 10 extra amino acids do biochemically is unknown","Splicing regulators not yet identified at this stage","Single family/lab"]},{"year":2010,"claim":"Connected BBS8 to planar cell polarity and left-right asymmetry, expanding its role from IFT to PCP and basal body positioning.","evidence":"Zebrafish morpholino knockdown and genetic interaction of bbs8 and vangl2 with cilia and actin imaging","pmids":["20643117"],"confidence":"Medium","gaps":["Morpholino-based, no genetic mutant","Direct physical interaction not shown here","Mechanism of centrosome migration defect unclear"]},{"year":2011,"claim":"Demonstrated BBS8 is required for ciliogenesis, ciliary protein trafficking, and neuronal axon targeting in olfactory neurons, showing broad sensory dependence on BBS8.","evidence":"Bbs8-null mouse with reporter knock-in, IHC, live imaging, and electroolfactogram","pmids":["21646512"],"confidence":"High","gaps":["Link between ciliary defect and axon mistargeting mechanistically unresolved","Trafficking substrates not enumerated"]},{"year":2015,"claim":"Resolved the molecular basis of retina-restricted BBS8 disease: photoreceptor-specific exon 2A inclusion makes only photoreceptors vulnerable to a splice mutation, governed by redundant intronic enhancers.","evidence":"RT-PCR across cell types, minigene splicing reporters, and intronic enhancer analysis","pmids":["25776555"],"confidence":"High","gaps":["Identity of trans-acting splicing factors not determined here","Functional consequence of the encoded peptide not defined"]},{"year":2015,"claim":"Provided direct biochemical evidence that BBS8 binds Vangl2 and positions BBS8 upstream of core PCP asymmetry, with localization to both actin and microtubules.","evidence":"Reciprocal Co-IP, Bbs8 conditional knockout mouse, and cochlear PCP marker immunofluorescence","pmids":["25605782"],"confidence":"Medium","gaps":["Whether interaction is direct or complex-mediated not fully resolved","Single lab","Mechanism linking BBS8 to Vangl2 asymmetry unclear"]},{"year":2018,"claim":"Showed BBS8 loss destabilizes the BBSome and disrupts photoreceptor outer segment architecture and ciliary microtubule acetylation, linking BBSome integrity to outer segment elaboration.","evidence":"Rod/cone-specific Bbs8 conditional knockout with Western blot of BBSome subunits, IF, ERG, and histology","pmids":["29126234"],"confidence":"High","gaps":["Mechanism of differential subunit stabilization unknown","Cause of syntaxin3 mislocalization unresolved"]},{"year":2021,"claim":"Revealed a non-ciliary BBS8 role in RPE homeostasis, where loss produces EMT-like changes in polarity and adhesion.","evidence":"Bbs8-null mouse RPE proteomics/transcriptomics with cytoskeletal IF and polarity assays","pmids":["33681195"],"confidence":"Medium","gaps":["Whether EMT phenotype is cilium-dependent unclear","No direct molecular effector identified","Single lab"]},{"year":2022,"claim":"Defined an additional disease mechanism whereby an exon 13 donor-site mutation causes cryptic splicing and premature termination, broadening the spectrum of TTC8 splicing pathology.","evidence":"Patient cDNA RT-PCR/Sanger sequencing and bioinformatic splice prediction","pmids":["35939099"],"confidence":"Medium","gaps":["Single method, single lab","Protein-level consequence not directly measured","SC35 disruption inferred bioinformatically"]},{"year":2025,"claim":"Linked BBS8 loss to ectopic ciliary Hedgehog signaling that redirects adipocyte precursors toward fibrogenesis, connecting ciliary dysfunction to a tissue fate switch.","evidence":"Bbs8-/- mouse scRNA-seq trajectory analysis, flow cytometry, Hedgehog reporters, and in vitro differentiation","pmids":["40836034"],"confidence":"Medium","gaps":["Direct molecular link between BBS8 and Hedgehog activation unresolved","Whether fibrogenesis is reversible untested","Single lab"]},{"year":2025,"claim":"Identified Musashi proteins as the trans-acting regulators that drive photoreceptor-specific Ttc8 exon inclusion, closing the loop on the splicing mechanism inferred earlier.","evidence":"Graded Msi1/Msi2 knockout allele series with RT-PCR splicing readout and ERG (preprint)","pmids":["bio_10.1101_2025.11.26.690869"],"confidence":"Medium","gaps":["Preprint, not peer-reviewed","Direct Musashi binding to Ttc8 introns shown by sequence proximity not in vivo footprinting","Other regulators not excluded"]},{"year":2025,"claim":"Identified HDAC2 as a BBS8 partner whose deregulation explains reduced ciliary tubulin acetylation, offering a pharmacologically reversible mechanism of ciliary destabilization.","evidence":"Bbs8-/- mouse kidney phenotyping, Co-IP, acetylated-tubulin IF, and HDAC-inhibitor rescue in patient cells (preprint)","pmids":["40667253"],"confidence":"Medium","gaps":["Preprint, not peer-reviewed","Whether interaction is direct or via BBSome unclear","Mechanism of HDAC2 upregulation upon BBS8 loss unknown"]},{"year":null,"claim":"How BBS8 mechanistically couples its BBSome/IFT role to its diverse downstream effects (Vangl2-PCP asymmetry, HDAC2-tubulin acetylation, Hedgehog activation) remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified structural model placing BBS8 within the BBSome","Causal hierarchy among ciliary, PCP, and signaling defects undefined","Function of the photoreceptor-specific peptide unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[3]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,3]}],"localization":[{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[0,2,6,10]},{"term_id":"GO:0005815","term_label":"microtubule organizing center","supporting_discovery_ids":[2,4]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[3]}],"pathway":[{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[0,2,6]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[3,4,9]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[3,4,9]}],"complexes":["BBSome"],"partners":["VANGL2","HDAC2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8TAM2","full_name":"Tetratricopeptide repeat protein 8","aliases":["Bardet-Biedl syndrome 8 protein"],"length_aa":541,"mass_kda":61.5,"function":"The BBSome complex is thought to function as a coat complex required for sorting of specific membrane proteins to the primary cilia. The BBSome complex is required for ciliogenesis but is dispensable for centriolar satellite function. This ciliogenic function is mediated in part by the Rab8 GDP/GTP exchange factor, which localizes to the basal body and contacts the BBSome. Rab8(GTP) enters the primary cilium and promotes extension of the ciliary membrane. Firstly the BBSome associates with the ciliary membrane and binds to RAB3IP/Rabin8, the guanosyl exchange factor (GEF) for Rab8 and then the Rab8-GTP localizes to the cilium and promotes docking and fusion of carrier vesicles to the base of the ciliary membrane. The BBSome complex, together with the LTZL1, controls SMO ciliary trafficking and contributes to the sonic hedgehog (SHH) pathway regulation. Required for proper BBSome complex assembly and its ciliary localization","subcellular_location":"Cytoplasm, cytoskeleton, microtubule organizing center, centrosome; Cell projection, cilium membrane; Cytoplasm; Cytoplasm, cytoskeleton, microtubule organizing center, centrosome, centriolar satellite; Cell projection, cilium","url":"https://www.uniprot.org/uniprotkb/Q8TAM2/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TTC8","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TTC8","total_profiled":1310},"omim":[{"mim_id":"616475","title":"CENTROSOMAL PROTEIN, 72-KD; CEP72","url":"https://www.omim.org/entry/616475"},{"mim_id":"615985","title":"BARDET-BIEDL SYNDROME 8; BBS8","url":"https://www.omim.org/entry/615985"},{"mim_id":"613605","title":"BBS PROTEIN COMPLEX-INTERACTING PROTEIN 1; BBIP1","url":"https://www.omim.org/entry/613605"},{"mim_id":"613464","title":"RETINITIS PIGMENTOSA 51; RP51","url":"https://www.omim.org/entry/613464"},{"mim_id":"610142","title":"CENTROSOMAL PROTEIN, 290-KD; CEP290","url":"https://www.omim.org/entry/610142"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoli fibrillar center","reliability":"Approved"},{"location":"Basal body","reliability":"Approved"},{"location":"Perinuclear theca","reliability":"Additional"},{"location":"Flagellar centriole","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/TTC8"},"hgnc":{"alias_symbol":["BBS8","RP51"],"prev_symbol":[]},"alphafold":{"accession":"Q8TAM2","domains":[{"cath_id":"-","chopping":"2-64","consensus_level":"medium","plddt":80.2341,"start":2,"end":64},{"cath_id":"1.25.40.10","chopping":"465-541","consensus_level":"medium","plddt":93.2564,"start":465,"end":541}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8TAM2","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8TAM2-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8TAM2-F1-predicted_aligned_error_v6.png","plddt_mean":84.25},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TTC8","jax_strain_url":"https://www.jax.org/strain/search?query=TTC8"},"sequence":{"accession":"Q8TAM2","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8TAM2.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8TAM2/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8TAM2"}},"corpus_meta":[{"pmid":"15231740","id":"PMC_15231740","title":"Loss of C. elegans BBS-7 and BBS-8 protein function results in cilia defects and compromised intraflagellar transport.","date":"2004","source":"Genes & development","url":"https://pubmed.ncbi.nlm.nih.gov/15231740","citation_count":285,"is_preprint":false},{"pmid":"20451172","id":"PMC_20451172","title":"A splice-site mutation in a retina-specific exon of BBS8 causes nonsyndromic retinitis pigmentosa.","date":"2010","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/20451172","citation_count":99,"is_preprint":false},{"pmid":"3915776","id":"PMC_3915776","title":"Effect of RP51 gene dosage alterations on ribosome synthesis in Saccharomyces cerevisiae.","date":"1985","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/3915776","citation_count":92,"is_preprint":false},{"pmid":"21646512","id":"PMC_21646512","title":"Loss of Bardet-Biedl syndrome protein-8 (BBS8) perturbs olfactory function, protein localization, and axon 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reports","url":"https://pubmed.ncbi.nlm.nih.gov/31406157","citation_count":1,"is_preprint":false},{"pmid":"40667253","id":"PMC_40667253","title":"Loss of Bbs8 leads to cystic kidney disease in mice with reduced acetylation of ciliary alpha-tubulin through HDAC2.","date":"2025","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/40667253","citation_count":0,"is_preprint":false},{"pmid":"41686921","id":"PMC_41686921","title":"A novel protein truncating mutation of TTC8 causes Bardet-Biedl Syndrome (BBS) in a Pakistani family.","date":"2026","source":"Genetics and molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/41686921","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.11.26.690869","title":"A single  <i>Musashi</i>  gene allele is sufficient to maintain mouse photoreceptor cells","date":"2025-12-01","source":"bioRxiv","url":"https://doi.org/10.1101/2025.11.26.690869","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":13623,"output_tokens":3292,"usd":0.045124,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10821,"output_tokens":3911,"usd":0.07594,"stage2_stop_reason":"end_turn"},"total_usd":0.121064,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2004,\n      \"finding\": \"C. elegans BBS-8 protein localizes predominantly at the base of cilia and moves bidirectionally along the ciliary axoneme, similar to IFT proteins. BBS-8 is required for the normal localization and motility of IFT proteins OSM-5/Polaris and CHE-11 (and to a lesser extent CHE-2), demonstrating that BBS-8 plays a selective role in the assembly and/or function of IFT particle components.\",\n      \"method\": \"Loss-of-function mutations in C. elegans bbs-8; fluorescence microscopy of protein localization; functional IFT motility assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean loss-of-function genetic model with multiple orthogonal readouts (localization, motility, structural cilia defects), replicated across BBS-7 and BBS-8\",\n      \"pmids\": [\"15231740\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"A retina-specific alternative exon of BBS8 (exon 2A) encodes 10 amino acids and represents the major BBS8 mRNA isoform in mammalian photoreceptors. A splice-site mutation eliminating this exon causes nonsyndromic retinitis pigmentosa restricted to the retina, establishing that this photoreceptor-specific sequence is pivotal for BBS8 function in the retina.\",\n      \"method\": \"Genome-wide linkage mapping; candidate gene sequencing; RT-PCR and expression analysis of retina-specific exon; segregation analysis\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — human genetic mapping combined with tissue-specific expression characterization, single lab\",\n      \"pmids\": [\"20451172\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"BBS8 localizes in olfactory sensory neurons (OSNs) at the dendritic knob in a shell-like structure surrounding basal bodies. Loss of BBS8 causes near-complete loss of cilia from OSNs, mislocalization of ciliary-enriched proteins, and aberrant axon targeting of OSNs to the olfactory bulb, demonstrating BBS8 is required for ciliary protein localization and axon targeting in OSNs.\",\n      \"method\": \"Bbs8-null mouse with fluorescent reporter knock-in; immunohistochemistry; ex vivo live imaging of SLP3-eGFP knock-in; electroolfactogram\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout with multiple orthogonal methods (reporter localization, IHC, electrophysiology, live imaging), single lab but rigorous\",\n      \"pmids\": [\"21646512\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"BBS8 directly interacts with the core planar cell polarity (PCP) molecule Vangl2, as demonstrated by co-immunoprecipitation. Loss of Bbs8 disrupts asymmetric accumulation of Vangl2 in cochlear cells and causes mis-oriented hair cell stereociliary bundles, placing BBS8 upstream of core PCP asymmetry. BBS8 also localizes to filamentous actin as well as microtubules.\",\n      \"method\": \"Co-immunoprecipitation (BBS8–Vangl2); Bbs8 conditional knockout mouse; immunofluorescence of cochlear PCP markers; subcellular fractionation/localization\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP plus genetic loss-of-function with defined PCP phenotype, single lab\",\n      \"pmids\": [\"25605782\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Bbs8 and the core PCP protein Vangl2 interact and act synergistically in zebrafish to establish left-right asymmetry; loss of bbs8 disrupts cilia at Kupffer's vesicle, causes defective actin organization, and leads to nuclear mislocalisation implying defective centrosome/basal body migration and apical docking.\",\n      \"method\": \"Zebrafish morpholino knockdown of bbs8 and vangl2; genetic interaction/epistasis; cilia imaging; actin staining\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis in vivo with multiple cellular readouts, single lab\",\n      \"pmids\": [\"20643117\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Cell-type-specific alternative splicing of BBS8 exon 2A (the IVS1-2A>G mutation) causes photoreceptor-specific loss of BBS8 protein because: (1) in photoreceptors, exon 2A is included and the mutation causes a frameshift eliminating the protein; (2) in all other cell types, exon 2A is skipped, rendering those cells immune to the mutation. Splicing of exon 2A in photoreceptors is directed exclusively by redundant splicing enhancers in the flanking introns.\",\n      \"method\": \"RT-PCR in multiple cell types; minigene splicing reporters; analysis of intronic splicing enhancer sequences; patient and control cell RNA analysis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct mechanistic dissection of splicing with minigene reporters and mutagenesis of regulatory elements, multiple orthogonal methods in one study\",\n      \"pmids\": [\"25776555\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"BBS8 is required for normal elaboration of photoreceptor outer segments. Loss of BBS8 leads to: concomitant decrease in BBSome subunits BBS2 and BBS5 (and increase in BBS1 and BBS4); disorganized and shortened outer segments; altered distribution of photoreceptor axonemal proteins; hyper-acetylation of ciliary microtubules; and mislocalization of syntaxin3 to the outer segment (a protein normally absent from OS).\",\n      \"method\": \"Bbs8 conditional knockout mouse (rod- and cone-specific); Western blot for BBSome subunit levels; immunofluorescence; ERG; histology\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic knockout with multiple orthogonal readouts including protein-level BBSome stability, localization studies, and functional ERG, single lab but comprehensive\",\n      \"pmids\": [\"29126234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Loss of Bbs8 in the retinal pigment epithelium (RPE) causes changes in cytoskeletal and cell adhesion molecules, defective cellular polarization and morphology, and an epithelial-to-mesenchymal transition (EMT)-like phenotype, demonstrating a role for BBS8 in RPE homeostasis beyond ciliary function.\",\n      \"method\": \"Bbs8-null mouse RPE; proteomics and transcriptomics (combinatorial omics); immunofluorescence for cytoskeletal markers; cell polarity assays\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — combined proteomic/transcriptomic and in vivo cellular phenotype analysis, single lab\",\n      \"pmids\": [\"33681195\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"A missense mutation c.1347G>C at the last base of exon 13 of TTC8 disrupts the canonical donor splice site, activating a cryptic splice site 77 bases into intron 13, causing intron retention, a frameshift, and a premature termination codon in exon 14. The mutation also disrupts the binding site for SC35 splicing factor and causes duplication and fusion of exon 15.\",\n      \"method\": \"RT-PCR and Sanger sequencing of patient cDNA; bioinformatic splice site prediction (SpliceAid2); minigene/functional RNA analysis\",\n      \"journal\": \"Molecular genetics and genomics : MGG\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct RNA sequencing from patient material establishing aberrant splicing mechanism, single lab, single method\",\n      \"pmids\": [\"35939099\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Loss of BBS8 in adipocyte precursor cells (APCs) induces ectopic ciliary Hedgehog signaling, driving a phenotypic switch from the stem-cell-like P1 APC subpopulation to a fibrogenic progenitor state (characterized by ECM remodeling and upregulation of CD9), bypassing the committed P2 progenitor state and altering adipogenesis.\",\n      \"method\": \"Bbs8-/- mouse model; single-cell RNA sequencing; flow cytometry; Hedgehog pathway reporter assays; in vitro differentiation assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — scRNA-seq trajectory analysis plus functional Hedgehog pathway measurements in vivo and in vitro, single lab\",\n      \"pmids\": [\"40836034\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"BBS8 physically interacts with HDAC2. Loss of BBS8 upregulates HDAC2 in kidneys, mouse embryonic fibroblasts, and patient-derived urine renal epithelial cells, leading to reduced K40 acetylation of α-tubulin within primary cilia and compromised ciliary stability. Pharmacological HDAC2 inhibition restores tubulin acetylation in BBS8-deficient cells.\",\n      \"method\": \"Bbs8-/- mouse kidney phenotyping; proteomics; co-immunoprecipitation (BBS8–HDAC2); immunofluorescence for acetylated tubulin; pharmacological rescue with HDAC inhibitor in patient-derived cells\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP interaction plus pharmacological rescue in patient-derived cells, preprint not yet peer-reviewed\",\n      \"pmids\": [\"40667253\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Musashi proteins (MSI1/MSI2) promote inclusion of the photoreceptor-specific alternative exon of Ttc8 by binding to proximal downstream intronic sequences. A single Musashi allele (from either Msi1 or Msi2) is sufficient to maintain high inclusion levels of the Ttc8 photoreceptor-specific exon and photoreceptor function.\",\n      \"method\": \"Combined Msi1/Msi2 knockout mice with graded allele reduction; RT-PCR splicing analysis of Ttc8 photoreceptor-specific exon; photoreceptor function assays (ERG)\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic allele-series with direct splicing readout for Ttc8 exon, preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.11.26.690869\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"TTC8/BBS8 is a component of the BBSome octameric complex that localizes at the base of cilia and is required for intraflagellar transport (IFT) particle assembly and bidirectional ciliary movement; it directly interacts with the PCP protein Vangl2 to regulate planar cell polarity and left-right asymmetry, interacts with HDAC2 to control ciliary tubulin acetylation and stability, and in photoreceptors its inclusion of a tissue-specific alternative exon (directed by intronic Musashi-binding splicing enhancers) is essential for outer segment development and photoreceptor survival, while its loss also triggers ectopic Hedgehog signaling in adipocyte precursors that drives fibrogenesis over adipogenesis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"TTC8/BBS8 is a ciliary protein that localizes at the base of cilia and moves bidirectionally along the axoneme, where it acts selectively in the assembly and motility of intraflagellar transport (IFT) particles and is required for ciliary protein localization and cilium maintenance across diverse cell types including sensory neurons and photoreceptors [#0, #2, #6]. It functions as a BBSome-associated component, and its loss destabilizes other BBSome subunits and disorganizes photoreceptor outer segments [#6]. BBS8 directly interacts with the core planar cell polarity protein Vangl2 to control asymmetric Vangl2 accumulation, stereociliary bundle orientation, and left-right asymmetry, linking it to cytoskeletal organization at both actin and microtubule structures [#3, #4]. It also interacts with HDAC2, and BBS8 loss elevates HDAC2 to reduce K40 acetylation of ciliary α-tubulin and compromise ciliary stability, a defect reversible by HDAC inhibition [#10]. A photoreceptor-specific alternative exon (exon 2A), whose inclusion is governed by redundant intronic splicing enhancers bound by Musashi proteins, is essential for BBS8 function in photoreceptors; mutations disrupting its splicing cause nonsyndromic retinitis pigmentosa confined to the retina [#1, #5, #11]. Beyond ciliary roles, BBS8 loss drives an EMT-like phenotype in retinal pigment epithelium and induces ectopic ciliary Hedgehog signaling in adipocyte precursors that biases them toward a fibrogenic rather than adipogenic fate [#7, #9].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Established that BBS8 is not merely ciliary but functionally embedded in the IFT machinery, defining its core molecular role.\",\n      \"evidence\": \"Loss-of-function mutations in C. elegans bbs-8 with fluorescence localization and IFT motility assays\",\n      \"pmids\": [\"15231740\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not define which IFT subcomplex BBS8 assembles\", \"No biochemical reconstitution of BBS8 within a complex\", \"Mechanism of bidirectional movement unresolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Showed that a retina-specific BBS8 alternative exon is the dominant photoreceptor isoform and its loss causes tissue-restricted disease, revealing tissue-specific function encoded by splicing.\",\n      \"evidence\": \"Linkage mapping, candidate sequencing, and RT-PCR of exon 2A in human nonsyndromic retinitis pigmentosa\",\n      \"pmids\": [\"20451172\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"What the 10 extra amino acids do biochemically is unknown\", \"Splicing regulators not yet identified at this stage\", \"Single family/lab\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Connected BBS8 to planar cell polarity and left-right asymmetry, expanding its role from IFT to PCP and basal body positioning.\",\n      \"evidence\": \"Zebrafish morpholino knockdown and genetic interaction of bbs8 and vangl2 with cilia and actin imaging\",\n      \"pmids\": [\"20643117\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Morpholino-based, no genetic mutant\", \"Direct physical interaction not shown here\", \"Mechanism of centrosome migration defect unclear\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Demonstrated BBS8 is required for ciliogenesis, ciliary protein trafficking, and neuronal axon targeting in olfactory neurons, showing broad sensory dependence on BBS8.\",\n      \"evidence\": \"Bbs8-null mouse with reporter knock-in, IHC, live imaging, and electroolfactogram\",\n      \"pmids\": [\"21646512\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Link between ciliary defect and axon mistargeting mechanistically unresolved\", \"Trafficking substrates not enumerated\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Resolved the molecular basis of retina-restricted BBS8 disease: photoreceptor-specific exon 2A inclusion makes only photoreceptors vulnerable to a splice mutation, governed by redundant intronic enhancers.\",\n      \"evidence\": \"RT-PCR across cell types, minigene splicing reporters, and intronic enhancer analysis\",\n      \"pmids\": [\"25776555\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of trans-acting splicing factors not determined here\", \"Functional consequence of the encoded peptide not defined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Provided direct biochemical evidence that BBS8 binds Vangl2 and positions BBS8 upstream of core PCP asymmetry, with localization to both actin and microtubules.\",\n      \"evidence\": \"Reciprocal Co-IP, Bbs8 conditional knockout mouse, and cochlear PCP marker immunofluorescence\",\n      \"pmids\": [\"25605782\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether interaction is direct or complex-mediated not fully resolved\", \"Single lab\", \"Mechanism linking BBS8 to Vangl2 asymmetry unclear\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showed BBS8 loss destabilizes the BBSome and disrupts photoreceptor outer segment architecture and ciliary microtubule acetylation, linking BBSome integrity to outer segment elaboration.\",\n      \"evidence\": \"Rod/cone-specific Bbs8 conditional knockout with Western blot of BBSome subunits, IF, ERG, and histology\",\n      \"pmids\": [\"29126234\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of differential subunit stabilization unknown\", \"Cause of syntaxin3 mislocalization unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Revealed a non-ciliary BBS8 role in RPE homeostasis, where loss produces EMT-like changes in polarity and adhesion.\",\n      \"evidence\": \"Bbs8-null mouse RPE proteomics/transcriptomics with cytoskeletal IF and polarity assays\",\n      \"pmids\": [\"33681195\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether EMT phenotype is cilium-dependent unclear\", \"No direct molecular effector identified\", \"Single lab\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Defined an additional disease mechanism whereby an exon 13 donor-site mutation causes cryptic splicing and premature termination, broadening the spectrum of TTC8 splicing pathology.\",\n      \"evidence\": \"Patient cDNA RT-PCR/Sanger sequencing and bioinformatic splice prediction\",\n      \"pmids\": [\"35939099\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single method, single lab\", \"Protein-level consequence not directly measured\", \"SC35 disruption inferred bioinformatically\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Linked BBS8 loss to ectopic ciliary Hedgehog signaling that redirects adipocyte precursors toward fibrogenesis, connecting ciliary dysfunction to a tissue fate switch.\",\n      \"evidence\": \"Bbs8-/- mouse scRNA-seq trajectory analysis, flow cytometry, Hedgehog reporters, and in vitro differentiation\",\n      \"pmids\": [\"40836034\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular link between BBS8 and Hedgehog activation unresolved\", \"Whether fibrogenesis is reversible untested\", \"Single lab\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified Musashi proteins as the trans-acting regulators that drive photoreceptor-specific Ttc8 exon inclusion, closing the loop on the splicing mechanism inferred earlier.\",\n      \"evidence\": \"Graded Msi1/Msi2 knockout allele series with RT-PCR splicing readout and ERG (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.11.26.690869\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, not peer-reviewed\", \"Direct Musashi binding to Ttc8 introns shown by sequence proximity not in vivo footprinting\", \"Other regulators not excluded\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified HDAC2 as a BBS8 partner whose deregulation explains reduced ciliary tubulin acetylation, offering a pharmacologically reversible mechanism of ciliary destabilization.\",\n      \"evidence\": \"Bbs8-/- mouse kidney phenotyping, Co-IP, acetylated-tubulin IF, and HDAC-inhibitor rescue in patient cells (preprint)\",\n      \"pmids\": [\"40667253\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, not peer-reviewed\", \"Whether interaction is direct or via BBSome unclear\", \"Mechanism of HDAC2 upregulation upon BBS8 loss unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How BBS8 mechanistically couples its BBSome/IFT role to its diverse downstream effects (Vangl2-PCP asymmetry, HDAC2-tubulin acetylation, Hedgehog activation) remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified structural model placing BBS8 within the BBSome\", \"Causal hierarchy among ciliary, PCP, and signaling defects undefined\", \"Function of the photoreceptor-specific peptide unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [0, 2, 6, 10]},\n      {\"term_id\": \"GO:0005815\", \"supporting_discovery_ids\": [2, 4]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [0, 2, 6]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 4, 9]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [3, 4, 9]}\n    ],\n    \"complexes\": [\"BBSome\"],\n    \"partners\": [\"VANGL2\", \"HDAC2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}