{"gene":"SPAST","run_date":"2026-04-28T20:42:08","timeline":{"discoveries":[{"year":2003,"finding":"Spastin (encoded by SPAST/SPG4) is localized in both the perinuclear cytoplasm and the nucleus of neurons; protein and transcript analyses of patients with nonsense or frameshift mutations show neither truncated protein nor mutated transcripts, indicating these mutations cause loss of spastin function (haploinsufficiency) rather than dominant-negative mechanisms.","method":"Polyclonal antibody immunolabeling, western blotting, RT-PCR analysis of patient tissues","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization experiment with functional consequence, single lab but multiple methods","pmids":["12490534"],"is_preprint":false},{"year":2004,"finding":"Spastin contains two functional nuclear localisation sequences (NLS), located in exons 1 and 6, each independently capable of mediating nuclear entry, as demonstrated using a novel Tetra-GFP reporter system.","method":"Tetra-GFP reporter nuclear localisation assay with NLS-containing spastin fragments","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization experiment with defined structural elements, single lab","pmids":["15147984"],"is_preprint":false},{"year":2008,"finding":"A cryptic promoter within exon 1 of the SPAST gene selectively drives expression of the shorter 60-kDa (M87) spastin isoform in a tissue-regulated manner; the two isoforms (68 kDa/M1 and 60 kDa/M87) arise via alternative transcriptional initiation as well as alternative translation initiation from two AUG codons, and have different subcellular localizations and interact with distinct molecules.","method":"Promoter-less reporter constructs, luciferase assays, RT-PCR, promoter mapping in multiple cell lines","journal":"BMC biology","confidence":"High","confidence_rationale":"Tier 1 — reconstitution and functional reporter assays with mutagenesis, multiple cell types","pmids":["18613979"],"is_preprint":false},{"year":2009,"finding":"A missense mutation R560Q in the ATPase domain of bovine SPAST severely impairs ATPase activity of recombinant Spastin in vitro, demonstrating the causal relationship between ATPase domain integrity and spastin function.","method":"In vitro ATPase activity assay of recombinant R560Q mutant Spastin protein","journal":"Neurogenetics","confidence":"High","confidence_rationale":"Tier 1 — direct in vitro enzymatic assay with disease-causing mutation","pmids":["19714378"],"is_preprint":false},{"year":2010,"finding":"Four HSP-associated SPAST mutations located outside the AAA ATPase region do not affect the microtubule-severing enzymatic activity or expression of M1 or M87 spastin isoforms; three of these mutations (L195V, 46Stop, S44L) confer dominant-negative activity specifically to the M1 isoform, indicating disease mechanisms beyond loss of microtubule-severing activity.","method":"In vitro microtubule-severing assay, expression level measurement, dominant-negative activity assay in cells","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic assay plus cellular dominant-negative assay with multiple mutations tested","pmids":["20430936"],"is_preprint":false},{"year":2012,"finding":"SPAST transcription is positively regulated by transcription factors NRF1 and SOX11, and the SPAST mRNA is negatively regulated post-transcriptionally by miR-96 and miR-182, which reduce mRNA stability and protein levels.","method":"Molecular phylogenetic conservation analysis, luciferase reporter assays, mRNA stability assays, western blotting","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — multiple molecular methods in single lab identifying transcriptional and miRNA regulatory mechanisms","pmids":["22574173"],"is_preprint":false},{"year":2013,"finding":"In hiPSC-derived neurons from SPG4 patients with a nonsense mutation, all spastin isoforms are reduced; patient neurons show decreased neurite complexity, imbalanced axonal transport with less retrograde movement, neurite swellings with disrupted microtubules. Overexpression of either M1 or M87 spastin isoform restores neurite length, branching, and reduces swellings, demonstrating gene dosage-dependent neurite maintenance.","method":"hiPSC differentiation into neurons, live-cell imaging of axonal transport, electron microscopy, spastin isoform overexpression rescue experiments","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — patient-derived neurons with multiple orthogonal methods and rescue experiments","pmids":["24381312"],"is_preprint":false},{"year":2016,"finding":"In olfactory stem cell lines from SPAST-mutant patients, peroxisome transport speed is reduced due to fewer stable microtubules (not impaired peroxisome-microtubule interactions); treatment with epothilone D, which increases stable microtubules, restores peroxisome speed and reduces oxidative stress sensitivity, linking spastin-dependent microtubule stability to organelle trafficking and oxidative stress resistance.","method":"Time-lapse imaging of peroxisome movement, automated image analysis, epothilone D pharmacological rescue, oxidative stress assays","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods with mechanistic pharmacological rescue, patient-derived cells","pmids":["27229699"],"is_preprint":false},{"year":2017,"finding":"Truncating mutations N184X and S245X in the SPAST M1 isoform produce truncated proteins that accumulate to higher levels than truncated M87 or wild-type counterparts; truncated M1 is more detrimental to neurite outgrowth than truncated M87. The N184X mutation also triggers translation reinitiation at a third start codon producing a novel M187 isoform capable of microtubule severing.","method":"Western blotting for protein accumulation, neurite outgrowth assay, microtubule-severing assay for M187 isoform","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro enzymatic assay for M187 plus cellular assays for protein accumulation and neurite toxicity","pmids":["28495799"],"is_preprint":false},{"year":2007,"finding":"N-terminal missense variants in SPAST (S44L, E43Q, P45Q) enhance the stability of the M87 spastin isoform selectively, rather than acting through haploinsufficiency or dominant-negative mechanisms; their phenotypic effects may be mediated by altering isoform competition for interacting proteins or substrates.","method":"Protein stability assays, expression level measurement of M1 and M87 isoforms, functional assessment of known disease mechanisms","journal":"European journal of neurology","confidence":"Medium","confidence_rationale":"Tier 2 — protein stability experiments with multiple variants, single lab","pmids":["17916079"],"is_preprint":false},{"year":2006,"finding":"A c.1216A>G substitution in the SPAST ATPase domain that appears as a missense mutation actually causes aberrant in-frame splicing and destabilization of the mutant transcript; the resulting mutant protein lacks microtubule-severing activity but shows normal subcellular localization, indicating haploinsufficiency as the disease mechanism for this mutation.","method":"Splicing analysis, microtubule-severing activity assay, subcellular localization immunofluorescence","journal":"Neurology","confidence":"High","confidence_rationale":"Tier 1 — in vitro microtubule-severing assay combined with splicing and localization analyses","pmids":["16476945"],"is_preprint":false},{"year":2018,"finding":"The I344K missense mutation in the SPAST ATPase domain abolishes ATPase and microtubule-severing activity in vitro, prolongs protein half-life by altering post-translational modifications for proteasomal degradation, causes microtubule accumulation and inhibits neurite growth in neurons; these effects are rescued by overexpression of wild-type M1 spastin, which physically interacts with the mutant protein.","method":"In vitro ATPase assay, microtubule-severing assay, protein half-life measurement, co-immunoprecipitation, neurite outgrowth assay, overexpression rescue in neuroblastoma and primary neurons","journal":"Biochimica et biophysica acta. Molecular basis of disease","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic assays with mutagenesis plus cellular and interaction data in single rigorous study","pmids":["30006150"],"is_preprint":false},{"year":2020,"finding":"iPSC-derived forebrain neurons from SPAST-mutant patients show reduced stabilized microtubules, reduced peroxisome transport speed, fewer peroxisomes, and more axon swellings; patient axons are more susceptible to oxidative stress-induced fragmentation than controls, and treatment with epothilone D or noscapine rescues peroxisome transport and protects against axon fragmentation.","method":"iPSC differentiation, live-cell axonal transport imaging, axon fragmentation assay, pharmacological rescue with tubulin-binding drugs","journal":"Frontiers in neuroscience","confidence":"High","confidence_rationale":"Tier 2 — patient-derived neurons with multiple orthogonal methods and pharmacological rescue","pmids":["32457567"],"is_preprint":false},{"year":2021,"finding":"A novel frameshift SPAST mutation (c.985dupA) produces truncated M1 and M87 isoforms that accumulate to higher levels than wild-type; truncated M1 decorates and stabilizes microtubules (rendering them resistant to depolymerization), while truncated M87 is diffusely localized and cannot decorate or sever microtubules, demonstrating isoform-specific toxic gain-of-function independent of haploinsufficiency.","method":"Western blotting, immunofluorescence microtubule decoration and depolymerization assays in transfected cells","journal":"Movement disorders : official journal of the Movement Disorder Society","confidence":"High","confidence_rationale":"Tier 2 — multiple cellular assays with mechanistic isoform-specific analysis, strong functional conclusions","pmids":["34927746"],"is_preprint":false},{"year":2022,"finding":"In transgenic mouse models, expression of human mutant spastin (C448Y) causes corticospinal dieback and gait deficiencies, while Spast knockout causes axonal swellings; crossbreeding the two lines produces both axonal swellings and earlier, more severe corticospinal dieback, indicating that reduced spastin function exacerbates toxic effects of mutant spastin, and that each component (haploinsufficiency and gain-of-function) contributes separately to disease pathology.","method":"Transgenic mouse generation, histological analysis, behavioral gait analysis, histone deacetylase 6 and tubulin modification analysis","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis via transgenic mouse crossbreeding with behavioral and histological readouts","pmids":["34935948"],"is_preprint":false},{"year":2019,"finding":"miR-33a directly targets the SPAST 3'-UTR (binding site conserved in humans but not mice) and negatively regulates spastin levels; inhibition of miR-33a using locked nucleic acid anti-miR ameliorated pathological phenotypes in SPG4 patient iPSC-derived cortical neurons.","method":"miRNA target site analysis, anti-miR LNA treatment, iPSC-derived cortical neuron phenotype rescue assay","journal":"Clinical science","confidence":"Medium","confidence_rationale":"Tier 2 — functional rescue in patient-derived neurons, single lab","pmids":["30777884"],"is_preprint":false},{"year":2015,"finding":"M1 spastin, due to its hydrophobic N-terminal domain not shared by M87, can insert into the endoplasmic reticulum membrane and interact with reticulons, atlastin, and REEP1 to play a role in ER morphogenesis; M87 is more abundant in neuronal and non-neuronal tissues, while M1 is predominantly detected in adult spinal cord.","method":"Isoform expression analysis by western blot across tissues, ER membrane association experiments (fractionation), protein interaction studies","journal":"Brain : a journal of neurology","confidence":"Medium","confidence_rationale":"Tier 2-3 — review synthesizing multiple experimental findings; primary data cited from multiple labs","pmids":["26094131"],"is_preprint":false}],"current_model":"Spastin (encoded by SPAST) is a microtubule-severing ATPase expressed as two major isoforms (M1 and M87) from alternative transcriptional and translational start sites; M87 is ubiquitous while M1 is enriched in adult spinal cord and can insert into the ER membrane via its unique N-terminal hydrophobic domain; pathogenic SPAST mutations act primarily through haploinsufficiency (loss of microtubule-severing activity) but also through isoform-specific toxic gain-of-function, particularly involving truncated or missense M1 proteins that accumulate, stabilize microtubules, impair axonal transport of organelles such as peroxisomes, increase oxidative stress vulnerability, and ultimately cause retrograde degeneration of corticospinal tract axons."},"narrative":{"teleology":[{"year":2003,"claim":"Establishing the primary disease mechanism: whether SPAST mutations in HSP patients produce toxic truncated protein or simply reduce functional spastin levels was unknown; analysis of patient tissues showed that nonsense/frameshift mutations cause transcript degradation and no truncated protein, establishing haploinsufficiency as the predominant disease mechanism and revealing dual nuclear–cytoplasmic localization of spastin.","evidence":"Polyclonal antibody immunolabeling, western blotting, and RT-PCR in patient tissues","pmids":["12490534"],"confidence":"Medium","gaps":["Single lab study; haploinsufficiency model does not account for all mutation types","Nuclear function of spastin not characterized"]},{"year":2004,"claim":"Identifying structural determinants of nuclear localization: spastin was shown to harbor two independent NLS motifs (in exons 1 and 6), each sufficient for nuclear import, defining the structural basis for the nuclear pool of spastin.","evidence":"Tetra-GFP reporter nuclear localization assay with NLS-containing spastin fragments","pmids":["15147984"],"confidence":"Medium","gaps":["Nuclear function of spastin remains unknown","Physiological role of dual NLS redundancy not tested"]},{"year":2006,"claim":"Demonstrating that apparent missense mutations can act through splicing defects: a c.1216A>G ATPase-domain substitution was shown to cause aberrant splicing and transcript destabilization, with the resulting protein lacking microtubule-severing activity, reinforcing haploinsufficiency as the mechanism for this mutation class.","evidence":"Splicing analysis, microtubule-severing activity assay, and immunofluorescence localization","pmids":["16476945"],"confidence":"High","gaps":["Only one mutation characterized at this level of splicing detail","Whether other apparent missense mutations act similarly is unknown"]},{"year":2007,"claim":"Revealing an alternative disease mechanism for N-terminal variants: S44L and related N-terminal missense mutations were found to selectively enhance M87 isoform stability rather than cause haploinsufficiency or dominant-negative effects, suggesting isoform-specific protein stabilization as a distinct pathogenic pathway.","evidence":"Protein stability assays and expression-level measurements of M1 and M87 isoforms","pmids":["17916079"],"confidence":"Medium","gaps":["Downstream consequences of increased M87 stability not defined","Single lab; no in vivo validation"]},{"year":2008,"claim":"Defining the dual-isoform architecture of SPAST: a cryptic promoter within exon 1 was shown to drive tissue-regulated expression of the shorter M87 isoform, establishing that M1 and M87 arise from both alternative transcription initiation and alternative translation, have distinct subcellular localizations, and interact with different protein partners.","evidence":"Promoter-less reporter constructs, luciferase assays, RT-PCR, and promoter mapping in multiple cell lines","pmids":["18613979"],"confidence":"High","gaps":["Tissue-specific transcription factors driving the cryptic promoter not fully identified","Functional consequences of isoform-specific interactions only partially characterized"]},{"year":2009,"claim":"Directly linking ATPase domain integrity to enzymatic function: the R560Q disease-causing mutation was shown to severely impair recombinant spastin ATPase activity in vitro, providing biochemical proof that AAA domain mutations cause loss of catalytic function.","evidence":"In vitro ATPase activity assay of recombinant R560Q mutant spastin","pmids":["19714378"],"confidence":"High","gaps":["Microtubule-severing activity not directly measured in this study","Structural basis of impaired catalysis not resolved"]},{"year":2010,"claim":"Expanding the disease mechanism beyond loss of severing: four HSP mutations outside the ATPase domain were shown to retain normal microtubule-severing activity, with three conferring M1-specific dominant-negative effects, proving that spastin pathogenesis extends beyond simple loss of catalytic function.","evidence":"In vitro microtubule-severing assay and cellular dominant-negative activity assay with multiple mutations","pmids":["20430936"],"confidence":"High","gaps":["Molecular targets of M1 dominant-negative effects not identified","Whether dominant-negative mechanism involves ER-membrane interactions or cytoskeletal functions not resolved"]},{"year":2013,"claim":"Validating the haploinsufficiency model in patient neurons and demonstrating rescue: iPSC-derived neurons from SPG4 patients showed reduced spastin, decreased neurite complexity, disrupted microtubules with swellings, and imbalanced axonal transport; overexpression of either M1 or M87 rescued these phenotypes, confirming gene-dosage dependence.","evidence":"hiPSC differentiation into neurons, live-cell transport imaging, electron microscopy, spastin isoform overexpression rescue","pmids":["24381312"],"confidence":"High","gaps":["Which cargo classes are most affected by transport imbalance not fully defined","Long-term axon degeneration not modeled in vitro"]},{"year":2015,"claim":"Establishing M1 spastin as an ER-shaping protein: the M1-specific N-terminal hydrophobic domain was shown to mediate ER membrane insertion and interaction with reticulons, atlastin, and REEP1, with M1 enriched in adult spinal cord, connecting spastin to the ER-morphogenesis pathway disrupted in multiple HSP subtypes.","evidence":"Western blot tissue expression profiling, ER membrane fractionation, protein interaction studies","pmids":["26094131"],"confidence":"Medium","gaps":["Evidence synthesized primarily from review; direct biochemical reconstitution of ER-shaping by M1 not demonstrated in a single study","Relative contributions of microtubule severing vs. ER shaping to disease not separated"]},{"year":2016,"claim":"Linking spastin to peroxisome trafficking and oxidative stress: in SPAST-mutant patient cells, peroxisome transport speed was reduced due to fewer stable microtubules; epothilone D treatment restored transport and reduced oxidative stress vulnerability, establishing a mechanistic chain from microtubule severing to organelle transport to cellular stress resistance.","evidence":"Time-lapse peroxisome tracking, automated image analysis, epothilone D pharmacological rescue, oxidative stress assays in patient olfactory stem cells","pmids":["27229699"],"confidence":"High","gaps":["Whether peroxisome deficiency is the primary driver of neurodegeneration vs. other cargo defects unknown","Mechanism by which reduced microtubule dynamics decreases peroxisome speed not fully elucidated"]},{"year":2017,"claim":"Demonstrating isoform-specific toxicity of truncated spastin and discovery of a third functional isoform: truncating mutations produced M1 truncated proteins that accumulated to higher levels and were more toxic to neurites than truncated M87; one mutation triggered translation reinitiation producing a novel M187 isoform with microtubule-severing capacity.","evidence":"Western blotting, neurite outgrowth assay, microtubule-severing assay for M187 isoform","pmids":["28495799"],"confidence":"High","gaps":["Physiological relevance of M187 isoform in vivo not tested","Mechanism of differential accumulation of truncated M1 vs. M87 not defined"]},{"year":2018,"claim":"Demonstrating that ATPase-dead spastin is toxic through protein stabilization: the I344K mutation abolished ATPase and severing activity, prolonged protein half-life by altering proteasomal targeting modifications, caused microtubule accumulation, and inhibited neurite growth; these effects were rescued by wild-type M1, which physically interacts with the mutant.","evidence":"In vitro ATPase and severing assays, protein half-life measurement, co-immunoprecipitation, neurite outgrowth rescue in neuroblastoma and primary neurons","pmids":["30006150"],"confidence":"High","gaps":["Identity of post-translational modifications governing spastin turnover not fully mapped","Whether mutant–wild-type hexamer mixing underlies rescue not determined structurally"]},{"year":2020,"claim":"Extending the peroxisome–oxidative stress axis to axon degeneration: iPSC-derived forebrain neurons from patients showed fewer peroxisomes, slower peroxisome transport, axon swellings, and heightened susceptibility to oxidative stress–induced axon fragmentation; microtubule-stabilizing drugs rescued transport and protected axons.","evidence":"iPSC-derived cortical neuron differentiation, live-cell axonal transport imaging, axon fragmentation assay, epothilone D and noscapine pharmacological rescue","pmids":["32457567"],"confidence":"High","gaps":["Optimal drug dosing window and long-term safety for translation not addressed","Contribution of other organelle transport deficits (e.g., mitochondria, lysosomes) not quantified"]},{"year":2021,"claim":"Providing direct evidence for isoform-specific toxic gain-of-function: a frameshift mutation produced truncated M1 that decorated and hyperstabilized microtubules (rendering them resistant to depolymerization), while truncated M87 was diffusely localized and could not sever or decorate microtubules—demonstrating that M1-specific toxic effects are independent of haploinsufficiency.","evidence":"Western blotting, immunofluorescence microtubule decoration and depolymerization assays in transfected cells","pmids":["34927746"],"confidence":"High","gaps":["Whether M1-mediated microtubule stabilization involves ER-anchored vs. cytosolic pools not resolved","In vivo demonstration of this mechanism in spinal cord neurons lacking"]},{"year":2022,"claim":"Establishing genetic epistasis between haploinsufficiency and gain-of-function in vivo: in transgenic mice, mutant spastin expression caused corticospinal dieback while Spast knockout caused axonal swellings; combining both produced earlier, more severe degeneration, proving that reduced spastin function synergizes with toxic mutant protein effects to drive disease.","evidence":"Transgenic mouse crossbreeding, histological analysis, behavioral gait analysis","pmids":["34935948"],"confidence":"High","gaps":["Whether pharmacological rescue (e.g., microtubule-stabilizing drugs) works in this compound model not tested","Cell-type specificity of the synergistic mechanism (upper motor neurons vs. other populations) not resolved"]},{"year":null,"claim":"Key unresolved questions include: the precise structural basis of spastin hexamer assembly and how mutant subunits poison the oligomer; the nuclear function of spastin; the relative contributions of microtubule severing versus ER morphogenesis to corticospinal axon maintenance; and whether tubulin-binding drug rescue translates to clinical benefit in SPG4 patients.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No high-resolution structure of full-length M1 spastin in ER membrane context","Nuclear substrates/functions unknown","No clinical trial data for microtubule-stabilizing drugs in SPG4"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[3,11]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[4,10,11]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[4,10,13]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,1]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[16]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[4,13]}],"pathway":[{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[16]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[6,12,14]}],"complexes":[],"partners":["RTN1","ATL1","REEP1"],"other_free_text":[]},"mechanistic_narrative":"Spastin is a microtubule-severing AAA ATPase whose loss-of-function mutations cause hereditary spastic paraplegia type 4 (SPG4), the most common form of autosomal dominant HSP. Two major isoforms (M1, 68 kDa; M87, 60 kDa) arise from alternative transcriptional and translational initiation: M87 is ubiquitously expressed, whereas M1 is enriched in adult spinal cord and inserts into the endoplasmic reticulum membrane via a unique N-terminal hydrophobic domain, where it interacts with reticulons, atlastin, and REEP1 to participate in ER morphogenesis [PMID:18613979, PMID:26094131]. Disease-causing mutations act through haploinsufficiency—ATPase-domain mutations abolish microtubule-severing activity and reduce spastin dosage [PMID:19714378, PMID:12490534]—and through isoform-specific toxic gain-of-function, whereby truncated or missense M1 proteins accumulate, decorate and hyperstabilize microtubules, and impair axonal organelle transport [PMID:34927746, PMID:28495799, PMID:34935948]. Reduced spastin function in patient-derived neurons leads to diminished stabilized microtubules, slowed peroxisome transport, axonal swellings, and increased vulnerability to oxidative stress, phenotypes that are rescued by microtubule-stabilizing drugs or restoration of spastin expression [PMID:24381312, PMID:32457567, PMID:27229699]."},"prefetch_data":{"uniprot":{"accession":"Q9UBP0","full_name":"Spastin","aliases":["Spastic paraplegia 4 protein"],"length_aa":616,"mass_kda":67.2,"function":"ATP-dependent microtubule severing protein that specifically recognizes and cuts microtubules that are polyglutamylated (PubMed:11809724, PubMed:15716377, PubMed:16219033, PubMed:17389232, PubMed:20530212, PubMed:22637577, PubMed:26875866). Preferentially recognizes and acts on microtubules decorated with short polyglutamate tails: severing activity increases as the number of glutamates per tubulin rises from one to eight, but decreases beyond this glutamylation threshold (PubMed:26875866). Severing activity is not dependent on tubulin acetylation or detyrosination (PubMed:26875866). Microtubule severing promotes reorganization of cellular microtubule arrays and the release of microtubules from the centrosome following nucleation. It is critical for the biogenesis and maintenance of complex microtubule arrays in axons, spindles and cilia. SPAST is involved in abscission step of cytokinesis and nuclear envelope reassembly during anaphase in cooperation with the ESCRT-III complex (PubMed:19000169, PubMed:21310966, PubMed:26040712). Recruited at the midbody, probably by IST1, and participates in membrane fission during abscission together with the ESCRT-III complex (PubMed:21310966). Recruited to the nuclear membrane by IST1 and mediates microtubule severing, promoting nuclear envelope sealing and mitotic spindle disassembly during late anaphase (PubMed:26040712). Required for membrane traffic from the endoplasmic reticulum (ER) to the Golgi and endosome recycling (PubMed:23897888). Recruited by IST1 to endosomes and regulates early endosomal tubulation and recycling by mediating microtubule severing (PubMed:23897888). Probably plays a role in axon growth and the formation of axonal branches (PubMed:15716377) Involved in lipid metabolism by regulating the size and distribution of lipid droplets","subcellular_location":"Cytoplasm; Endosome; Nucleus membrane; Cytoplasm, cytoskeleton, microtubule organizing center, centrosome","url":"https://www.uniprot.org/uniprotkb/Q9UBP0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SPAST","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000021574","cell_line_id":"CID000784","localizations":[{"compartment":"vesicles","grade":3},{"compartment":"cytoplasmic","grade":1},{"compartment":"nucleoplasm","grade":1}],"interactors":[{"gene":"CAPZB","stoichiometry":0.2},{"gene":"CLTA","stoichiometry":0.2},{"gene":"PSPC1","stoichiometry":0.2},{"gene":"RPS16","stoichiometry":0.2},{"gene":"SVIL","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID000784","total_profiled":1310},"omim":[{"mim_id":"612539","title":"SPASTIC PARAPLEGIA 42, AUTOSOMAL DOMINANT; SPG42","url":"https://www.omim.org/entry/612539"},{"mim_id":"610882","title":"SS NUCLEAR AUTOANTIGEN 1; SSNA1","url":"https://www.omim.org/entry/610882"},{"mim_id":"609347","title":"RECEPTOR EXPRESSION-ENHANCING PROTEIN 2; REEP2","url":"https://www.omim.org/entry/609347"},{"mim_id":"608145","title":"NIPA MAGNESIUM TRANSPORTER 1; NIPA1","url":"https://www.omim.org/entry/608145"},{"mim_id":"607259","title":"SPASTIC PARAPLEGIA 7, AUTOSOMAL RECESSIVE, WITH OR WITHOUT CEREBELLAR ATAXIA; SPG7","url":"https://www.omim.org/entry/607259"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SPAST"},"hgnc":{"alias_symbol":["FSP2","ADPSP","KIAA1083"],"prev_symbol":["SPG4"]},"alphafold":{"accession":"Q9UBP0","domains":[{"cath_id":"1.20.58.80","chopping":"110-197","consensus_level":"high","plddt":90.1181,"start":110,"end":197},{"cath_id":"3.40.50.300","chopping":"321-504","consensus_level":"high","plddt":90.104,"start":321,"end":504},{"cath_id":"1.10.8.60","chopping":"510-595","consensus_level":"high","plddt":94.9894,"start":510,"end":595}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UBP0","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UBP0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UBP0-F1-predicted_aligned_error_v6.png","plddt_mean":76.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SPAST","jax_strain_url":"https://www.jax.org/strain/search?query=SPAST"},"sequence":{"accession":"Q9UBP0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9UBP0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9UBP0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UBP0"}},"corpus_meta":[{"pmid":"10699187","id":"PMC_10699187","title":"Spectrum 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Le journal canadien des sciences neurologiques","url":"https://pubmed.ncbi.nlm.nih.gov/17598599","citation_count":11,"is_preprint":false},{"pmid":"27334366","id":"PMC_27334366","title":"SPAST mutation spectrum and familial occurrence among Czech patients with pure hereditary spastic paraplegia.","date":"2016","source":"Journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/27334366","citation_count":10,"is_preprint":false},{"pmid":"25421405","id":"PMC_25421405","title":"High frequency of SPG4 in Taiwanese families with autosomal dominant hereditary spastic paraplegia.","date":"2014","source":"BMC neurology","url":"https://pubmed.ncbi.nlm.nih.gov/25421405","citation_count":10,"is_preprint":false},{"pmid":"22192498","id":"PMC_22192498","title":"Peripheral neuropathy in hereditary spastic paraplegia due to spastin (SPG4) mutation--a neurophysiological study using excitability techniques.","date":"2011","source":"Clinical neurophysiology : official journal of the International Federation of Clinical Neurophysiology","url":"https://pubmed.ncbi.nlm.nih.gov/22192498","citation_count":10,"is_preprint":false},{"pmid":"16476945","id":"PMC_16476945","title":"Unexpected pathogenic mechanism of a novel mutation in the coding sequence of SPG4 (spastin).","date":"2006","source":"Neurology","url":"https://pubmed.ncbi.nlm.nih.gov/16476945","citation_count":10,"is_preprint":false},{"pmid":"17122756","id":"PMC_17122756","title":"Identification of a novel mutation in the spastin gene (SPG4) in an Italian family with hereditary spastic paresis.","date":"2006","source":"Panminerva medica","url":"https://pubmed.ncbi.nlm.nih.gov/17122756","citation_count":9,"is_preprint":false},{"pmid":"34353391","id":"PMC_34353391","title":"Anticipation Can Be More Common in Hereditary Spastic Paraplegia with SPAST Mutations Than It Appears.","date":"2021","source":"The Canadian journal of neurological sciences. Le journal canadien des sciences neurologiques","url":"https://pubmed.ncbi.nlm.nih.gov/34353391","citation_count":8,"is_preprint":false},{"pmid":"15164410","id":"PMC_15164410","title":"Prenatal diagnosis of autosomal dominant hereditary spastic paraplegia (SPG4) using direct mutation detection.","date":"2004","source":"Prenatal diagnosis","url":"https://pubmed.ncbi.nlm.nih.gov/15164410","citation_count":8,"is_preprint":false},{"pmid":"12460147","id":"PMC_12460147","title":"A Japanese SPG4 family with a novel missense mutation of the SPG4 gene: intrafamilial variability in age at onset and clinical severity.","date":"2002","source":"Acta neurologica Scandinavica","url":"https://pubmed.ncbi.nlm.nih.gov/12460147","citation_count":8,"is_preprint":false},{"pmid":"30777884","id":"PMC_30777884","title":"MiR-33a is a therapeutic target in SPG4-related hereditary spastic paraplegia human neurons.","date":"2019","source":"Clinical science (London, England : 1979)","url":"https://pubmed.ncbi.nlm.nih.gov/30777884","citation_count":7,"is_preprint":false},{"pmid":"31486053","id":"PMC_31486053","title":"A p.Arg499His Mutation in SPAST Is Associated with Infantile Onset Ascending Spastic Paralysis Complicated with Dysarthria and Anarthria.","date":"2019","source":"Neuropediatrics","url":"https://pubmed.ncbi.nlm.nih.gov/31486053","citation_count":7,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":44795,"output_tokens":3928,"usd":0.096653},"stage2":{"model":"claude-opus-4-6","input_tokens":7360,"output_tokens":3832,"usd":0.1989},"total_usd":0.295553,"stage1_batch_id":"msgbatch_011rkcQdVV1RaFq9XhcG6df6","stage2_batch_id":"msgbatch_01RzgrJafdF4EWHhj5mNi6dh","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2003,\n      \"finding\": \"Spastin (encoded by SPAST/SPG4) is localized in both the perinuclear cytoplasm and the nucleus of neurons; protein and transcript analyses of patients with nonsense or frameshift mutations show neither truncated protein nor mutated transcripts, indicating these mutations cause loss of spastin function (haploinsufficiency) rather than dominant-negative mechanisms.\",\n      \"method\": \"Polyclonal antibody immunolabeling, western blotting, RT-PCR analysis of patient tissues\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment with functional consequence, single lab but multiple methods\",\n      \"pmids\": [\"12490534\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Spastin contains two functional nuclear localisation sequences (NLS), located in exons 1 and 6, each independently capable of mediating nuclear entry, as demonstrated using a novel Tetra-GFP reporter system.\",\n      \"method\": \"Tetra-GFP reporter nuclear localisation assay with NLS-containing spastin fragments\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment with defined structural elements, single lab\",\n      \"pmids\": [\"15147984\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"A cryptic promoter within exon 1 of the SPAST gene selectively drives expression of the shorter 60-kDa (M87) spastin isoform in a tissue-regulated manner; the two isoforms (68 kDa/M1 and 60 kDa/M87) arise via alternative transcriptional initiation as well as alternative translation initiation from two AUG codons, and have different subcellular localizations and interact with distinct molecules.\",\n      \"method\": \"Promoter-less reporter constructs, luciferase assays, RT-PCR, promoter mapping in multiple cell lines\",\n      \"journal\": \"BMC biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution and functional reporter assays with mutagenesis, multiple cell types\",\n      \"pmids\": [\"18613979\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"A missense mutation R560Q in the ATPase domain of bovine SPAST severely impairs ATPase activity of recombinant Spastin in vitro, demonstrating the causal relationship between ATPase domain integrity and spastin function.\",\n      \"method\": \"In vitro ATPase activity assay of recombinant R560Q mutant Spastin protein\",\n      \"journal\": \"Neurogenetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct in vitro enzymatic assay with disease-causing mutation\",\n      \"pmids\": [\"19714378\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Four HSP-associated SPAST mutations located outside the AAA ATPase region do not affect the microtubule-severing enzymatic activity or expression of M1 or M87 spastin isoforms; three of these mutations (L195V, 46Stop, S44L) confer dominant-negative activity specifically to the M1 isoform, indicating disease mechanisms beyond loss of microtubule-severing activity.\",\n      \"method\": \"In vitro microtubule-severing assay, expression level measurement, dominant-negative activity assay in cells\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic assay plus cellular dominant-negative assay with multiple mutations tested\",\n      \"pmids\": [\"20430936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SPAST transcription is positively regulated by transcription factors NRF1 and SOX11, and the SPAST mRNA is negatively regulated post-transcriptionally by miR-96 and miR-182, which reduce mRNA stability and protein levels.\",\n      \"method\": \"Molecular phylogenetic conservation analysis, luciferase reporter assays, mRNA stability assays, western blotting\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple molecular methods in single lab identifying transcriptional and miRNA regulatory mechanisms\",\n      \"pmids\": [\"22574173\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In hiPSC-derived neurons from SPG4 patients with a nonsense mutation, all spastin isoforms are reduced; patient neurons show decreased neurite complexity, imbalanced axonal transport with less retrograde movement, neurite swellings with disrupted microtubules. Overexpression of either M1 or M87 spastin isoform restores neurite length, branching, and reduces swellings, demonstrating gene dosage-dependent neurite maintenance.\",\n      \"method\": \"hiPSC differentiation into neurons, live-cell imaging of axonal transport, electron microscopy, spastin isoform overexpression rescue experiments\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — patient-derived neurons with multiple orthogonal methods and rescue experiments\",\n      \"pmids\": [\"24381312\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In olfactory stem cell lines from SPAST-mutant patients, peroxisome transport speed is reduced due to fewer stable microtubules (not impaired peroxisome-microtubule interactions); treatment with epothilone D, which increases stable microtubules, restores peroxisome speed and reduces oxidative stress sensitivity, linking spastin-dependent microtubule stability to organelle trafficking and oxidative stress resistance.\",\n      \"method\": \"Time-lapse imaging of peroxisome movement, automated image analysis, epothilone D pharmacological rescue, oxidative stress assays\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods with mechanistic pharmacological rescue, patient-derived cells\",\n      \"pmids\": [\"27229699\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Truncating mutations N184X and S245X in the SPAST M1 isoform produce truncated proteins that accumulate to higher levels than truncated M87 or wild-type counterparts; truncated M1 is more detrimental to neurite outgrowth than truncated M87. The N184X mutation also triggers translation reinitiation at a third start codon producing a novel M187 isoform capable of microtubule severing.\",\n      \"method\": \"Western blotting for protein accumulation, neurite outgrowth assay, microtubule-severing assay for M187 isoform\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro enzymatic assay for M187 plus cellular assays for protein accumulation and neurite toxicity\",\n      \"pmids\": [\"28495799\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"N-terminal missense variants in SPAST (S44L, E43Q, P45Q) enhance the stability of the M87 spastin isoform selectively, rather than acting through haploinsufficiency or dominant-negative mechanisms; their phenotypic effects may be mediated by altering isoform competition for interacting proteins or substrates.\",\n      \"method\": \"Protein stability assays, expression level measurement of M1 and M87 isoforms, functional assessment of known disease mechanisms\",\n      \"journal\": \"European journal of neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — protein stability experiments with multiple variants, single lab\",\n      \"pmids\": [\"17916079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A c.1216A>G substitution in the SPAST ATPase domain that appears as a missense mutation actually causes aberrant in-frame splicing and destabilization of the mutant transcript; the resulting mutant protein lacks microtubule-severing activity but shows normal subcellular localization, indicating haploinsufficiency as the disease mechanism for this mutation.\",\n      \"method\": \"Splicing analysis, microtubule-severing activity assay, subcellular localization immunofluorescence\",\n      \"journal\": \"Neurology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro microtubule-severing assay combined with splicing and localization analyses\",\n      \"pmids\": [\"16476945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The I344K missense mutation in the SPAST ATPase domain abolishes ATPase and microtubule-severing activity in vitro, prolongs protein half-life by altering post-translational modifications for proteasomal degradation, causes microtubule accumulation and inhibits neurite growth in neurons; these effects are rescued by overexpression of wild-type M1 spastin, which physically interacts with the mutant protein.\",\n      \"method\": \"In vitro ATPase assay, microtubule-severing assay, protein half-life measurement, co-immunoprecipitation, neurite outgrowth assay, overexpression rescue in neuroblastoma and primary neurons\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular basis of disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic assays with mutagenesis plus cellular and interaction data in single rigorous study\",\n      \"pmids\": [\"30006150\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"iPSC-derived forebrain neurons from SPAST-mutant patients show reduced stabilized microtubules, reduced peroxisome transport speed, fewer peroxisomes, and more axon swellings; patient axons are more susceptible to oxidative stress-induced fragmentation than controls, and treatment with epothilone D or noscapine rescues peroxisome transport and protects against axon fragmentation.\",\n      \"method\": \"iPSC differentiation, live-cell axonal transport imaging, axon fragmentation assay, pharmacological rescue with tubulin-binding drugs\",\n      \"journal\": \"Frontiers in neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — patient-derived neurons with multiple orthogonal methods and pharmacological rescue\",\n      \"pmids\": [\"32457567\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"A novel frameshift SPAST mutation (c.985dupA) produces truncated M1 and M87 isoforms that accumulate to higher levels than wild-type; truncated M1 decorates and stabilizes microtubules (rendering them resistant to depolymerization), while truncated M87 is diffusely localized and cannot decorate or sever microtubules, demonstrating isoform-specific toxic gain-of-function independent of haploinsufficiency.\",\n      \"method\": \"Western blotting, immunofluorescence microtubule decoration and depolymerization assays in transfected cells\",\n      \"journal\": \"Movement disorders : official journal of the Movement Disorder Society\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple cellular assays with mechanistic isoform-specific analysis, strong functional conclusions\",\n      \"pmids\": [\"34927746\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In transgenic mouse models, expression of human mutant spastin (C448Y) causes corticospinal dieback and gait deficiencies, while Spast knockout causes axonal swellings; crossbreeding the two lines produces both axonal swellings and earlier, more severe corticospinal dieback, indicating that reduced spastin function exacerbates toxic effects of mutant spastin, and that each component (haploinsufficiency and gain-of-function) contributes separately to disease pathology.\",\n      \"method\": \"Transgenic mouse generation, histological analysis, behavioral gait analysis, histone deacetylase 6 and tubulin modification analysis\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via transgenic mouse crossbreeding with behavioral and histological readouts\",\n      \"pmids\": [\"34935948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"miR-33a directly targets the SPAST 3'-UTR (binding site conserved in humans but not mice) and negatively regulates spastin levels; inhibition of miR-33a using locked nucleic acid anti-miR ameliorated pathological phenotypes in SPG4 patient iPSC-derived cortical neurons.\",\n      \"method\": \"miRNA target site analysis, anti-miR LNA treatment, iPSC-derived cortical neuron phenotype rescue assay\",\n      \"journal\": \"Clinical science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional rescue in patient-derived neurons, single lab\",\n      \"pmids\": [\"30777884\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"M1 spastin, due to its hydrophobic N-terminal domain not shared by M87, can insert into the endoplasmic reticulum membrane and interact with reticulons, atlastin, and REEP1 to play a role in ER morphogenesis; M87 is more abundant in neuronal and non-neuronal tissues, while M1 is predominantly detected in adult spinal cord.\",\n      \"method\": \"Isoform expression analysis by western blot across tissues, ER membrane association experiments (fractionation), protein interaction studies\",\n      \"journal\": \"Brain : a journal of neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — review synthesizing multiple experimental findings; primary data cited from multiple labs\",\n      \"pmids\": [\"26094131\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Spastin (encoded by SPAST) is a microtubule-severing ATPase expressed as two major isoforms (M1 and M87) from alternative transcriptional and translational start sites; M87 is ubiquitous while M1 is enriched in adult spinal cord and can insert into the ER membrane via its unique N-terminal hydrophobic domain; pathogenic SPAST mutations act primarily through haploinsufficiency (loss of microtubule-severing activity) but also through isoform-specific toxic gain-of-function, particularly involving truncated or missense M1 proteins that accumulate, stabilize microtubules, impair axonal transport of organelles such as peroxisomes, increase oxidative stress vulnerability, and ultimately cause retrograde degeneration of corticospinal tract axons.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"Spastin is a microtubule-severing AAA ATPase whose loss-of-function mutations cause hereditary spastic paraplegia type 4 (SPG4), the most common form of autosomal dominant HSP. Two major isoforms (M1, 68 kDa; M87, 60 kDa) arise from alternative transcriptional and translational initiation: M87 is ubiquitously expressed, whereas M1 is enriched in adult spinal cord and inserts into the endoplasmic reticulum membrane via a unique N-terminal hydrophobic domain, where it interacts with reticulons, atlastin, and REEP1 to participate in ER morphogenesis [PMID:18613979, PMID:26094131]. Disease-causing mutations act through haploinsufficiency—ATPase-domain mutations abolish microtubule-severing activity and reduce spastin dosage [PMID:19714378, PMID:12490534]—and through isoform-specific toxic gain-of-function, whereby truncated or missense M1 proteins accumulate, decorate and hyperstabilize microtubules, and impair axonal organelle transport [PMID:34927746, PMID:28495799, PMID:34935948]. Reduced spastin function in patient-derived neurons leads to diminished stabilized microtubules, slowed peroxisome transport, axonal swellings, and increased vulnerability to oxidative stress, phenotypes that are rescued by microtubule-stabilizing drugs or restoration of spastin expression [PMID:24381312, PMID:32457567, PMID:27229699].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Establishing the primary disease mechanism: whether SPAST mutations in HSP patients produce toxic truncated protein or simply reduce functional spastin levels was unknown; analysis of patient tissues showed that nonsense/frameshift mutations cause transcript degradation and no truncated protein, establishing haploinsufficiency as the predominant disease mechanism and revealing dual nuclear–cytoplasmic localization of spastin.\",\n      \"evidence\": \"Polyclonal antibody immunolabeling, western blotting, and RT-PCR in patient tissues\",\n      \"pmids\": [\"12490534\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab study; haploinsufficiency model does not account for all mutation types\", \"Nuclear function of spastin not characterized\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identifying structural determinants of nuclear localization: spastin was shown to harbor two independent NLS motifs (in exons 1 and 6), each sufficient for nuclear import, defining the structural basis for the nuclear pool of spastin.\",\n      \"evidence\": \"Tetra-GFP reporter nuclear localization assay with NLS-containing spastin fragments\",\n      \"pmids\": [\"15147984\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Nuclear function of spastin remains unknown\", \"Physiological role of dual NLS redundancy not tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrating that apparent missense mutations can act through splicing defects: a c.1216A>G ATPase-domain substitution was shown to cause aberrant splicing and transcript destabilization, with the resulting protein lacking microtubule-severing activity, reinforcing haploinsufficiency as the mechanism for this mutation class.\",\n      \"evidence\": \"Splicing analysis, microtubule-severing activity assay, and immunofluorescence localization\",\n      \"pmids\": [\"16476945\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Only one mutation characterized at this level of splicing detail\", \"Whether other apparent missense mutations act similarly is unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Revealing an alternative disease mechanism for N-terminal variants: S44L and related N-terminal missense mutations were found to selectively enhance M87 isoform stability rather than cause haploinsufficiency or dominant-negative effects, suggesting isoform-specific protein stabilization as a distinct pathogenic pathway.\",\n      \"evidence\": \"Protein stability assays and expression-level measurements of M1 and M87 isoforms\",\n      \"pmids\": [\"17916079\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Downstream consequences of increased M87 stability not defined\", \"Single lab; no in vivo validation\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Defining the dual-isoform architecture of SPAST: a cryptic promoter within exon 1 was shown to drive tissue-regulated expression of the shorter M87 isoform, establishing that M1 and M87 arise from both alternative transcription initiation and alternative translation, have distinct subcellular localizations, and interact with different protein partners.\",\n      \"evidence\": \"Promoter-less reporter constructs, luciferase assays, RT-PCR, and promoter mapping in multiple cell lines\",\n      \"pmids\": [\"18613979\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific transcription factors driving the cryptic promoter not fully identified\", \"Functional consequences of isoform-specific interactions only partially characterized\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Directly linking ATPase domain integrity to enzymatic function: the R560Q disease-causing mutation was shown to severely impair recombinant spastin ATPase activity in vitro, providing biochemical proof that AAA domain mutations cause loss of catalytic function.\",\n      \"evidence\": \"In vitro ATPase activity assay of recombinant R560Q mutant spastin\",\n      \"pmids\": [\"19714378\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Microtubule-severing activity not directly measured in this study\", \"Structural basis of impaired catalysis not resolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Expanding the disease mechanism beyond loss of severing: four HSP mutations outside the ATPase domain were shown to retain normal microtubule-severing activity, with three conferring M1-specific dominant-negative effects, proving that spastin pathogenesis extends beyond simple loss of catalytic function.\",\n      \"evidence\": \"In vitro microtubule-severing assay and cellular dominant-negative activity assay with multiple mutations\",\n      \"pmids\": [\"20430936\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular targets of M1 dominant-negative effects not identified\", \"Whether dominant-negative mechanism involves ER-membrane interactions or cytoskeletal functions not resolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Validating the haploinsufficiency model in patient neurons and demonstrating rescue: iPSC-derived neurons from SPG4 patients showed reduced spastin, decreased neurite complexity, disrupted microtubules with swellings, and imbalanced axonal transport; overexpression of either M1 or M87 rescued these phenotypes, confirming gene-dosage dependence.\",\n      \"evidence\": \"hiPSC differentiation into neurons, live-cell transport imaging, electron microscopy, spastin isoform overexpression rescue\",\n      \"pmids\": [\"24381312\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which cargo classes are most affected by transport imbalance not fully defined\", \"Long-term axon degeneration not modeled in vitro\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Establishing M1 spastin as an ER-shaping protein: the M1-specific N-terminal hydrophobic domain was shown to mediate ER membrane insertion and interaction with reticulons, atlastin, and REEP1, with M1 enriched in adult spinal cord, connecting spastin to the ER-morphogenesis pathway disrupted in multiple HSP subtypes.\",\n      \"evidence\": \"Western blot tissue expression profiling, ER membrane fractionation, protein interaction studies\",\n      \"pmids\": [\"26094131\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Evidence synthesized primarily from review; direct biochemical reconstitution of ER-shaping by M1 not demonstrated in a single study\", \"Relative contributions of microtubule severing vs. ER shaping to disease not separated\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Linking spastin to peroxisome trafficking and oxidative stress: in SPAST-mutant patient cells, peroxisome transport speed was reduced due to fewer stable microtubules; epothilone D treatment restored transport and reduced oxidative stress vulnerability, establishing a mechanistic chain from microtubule severing to organelle transport to cellular stress resistance.\",\n      \"evidence\": \"Time-lapse peroxisome tracking, automated image analysis, epothilone D pharmacological rescue, oxidative stress assays in patient olfactory stem cells\",\n      \"pmids\": [\"27229699\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether peroxisome deficiency is the primary driver of neurodegeneration vs. other cargo defects unknown\", \"Mechanism by which reduced microtubule dynamics decreases peroxisome speed not fully elucidated\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrating isoform-specific toxicity of truncated spastin and discovery of a third functional isoform: truncating mutations produced M1 truncated proteins that accumulated to higher levels and were more toxic to neurites than truncated M87; one mutation triggered translation reinitiation producing a novel M187 isoform with microtubule-severing capacity.\",\n      \"evidence\": \"Western blotting, neurite outgrowth assay, microtubule-severing assay for M187 isoform\",\n      \"pmids\": [\"28495799\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance of M187 isoform in vivo not tested\", \"Mechanism of differential accumulation of truncated M1 vs. M87 not defined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrating that ATPase-dead spastin is toxic through protein stabilization: the I344K mutation abolished ATPase and severing activity, prolonged protein half-life by altering proteasomal targeting modifications, caused microtubule accumulation, and inhibited neurite growth; these effects were rescued by wild-type M1, which physically interacts with the mutant.\",\n      \"evidence\": \"In vitro ATPase and severing assays, protein half-life measurement, co-immunoprecipitation, neurite outgrowth rescue in neuroblastoma and primary neurons\",\n      \"pmids\": [\"30006150\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of post-translational modifications governing spastin turnover not fully mapped\", \"Whether mutant–wild-type hexamer mixing underlies rescue not determined structurally\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Extending the peroxisome–oxidative stress axis to axon degeneration: iPSC-derived forebrain neurons from patients showed fewer peroxisomes, slower peroxisome transport, axon swellings, and heightened susceptibility to oxidative stress–induced axon fragmentation; microtubule-stabilizing drugs rescued transport and protected axons.\",\n      \"evidence\": \"iPSC-derived cortical neuron differentiation, live-cell axonal transport imaging, axon fragmentation assay, epothilone D and noscapine pharmacological rescue\",\n      \"pmids\": [\"32457567\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Optimal drug dosing window and long-term safety for translation not addressed\", \"Contribution of other organelle transport deficits (e.g., mitochondria, lysosomes) not quantified\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Providing direct evidence for isoform-specific toxic gain-of-function: a frameshift mutation produced truncated M1 that decorated and hyperstabilized microtubules (rendering them resistant to depolymerization), while truncated M87 was diffusely localized and could not sever or decorate microtubules—demonstrating that M1-specific toxic effects are independent of haploinsufficiency.\",\n      \"evidence\": \"Western blotting, immunofluorescence microtubule decoration and depolymerization assays in transfected cells\",\n      \"pmids\": [\"34927746\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether M1-mediated microtubule stabilization involves ER-anchored vs. cytosolic pools not resolved\", \"In vivo demonstration of this mechanism in spinal cord neurons lacking\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Establishing genetic epistasis between haploinsufficiency and gain-of-function in vivo: in transgenic mice, mutant spastin expression caused corticospinal dieback while Spast knockout caused axonal swellings; combining both produced earlier, more severe degeneration, proving that reduced spastin function synergizes with toxic mutant protein effects to drive disease.\",\n      \"evidence\": \"Transgenic mouse crossbreeding, histological analysis, behavioral gait analysis\",\n      \"pmids\": [\"34935948\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether pharmacological rescue (e.g., microtubule-stabilizing drugs) works in this compound model not tested\", \"Cell-type specificity of the synergistic mechanism (upper motor neurons vs. other populations) not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the precise structural basis of spastin hexamer assembly and how mutant subunits poison the oligomer; the nuclear function of spastin; the relative contributions of microtubule severing versus ER morphogenesis to corticospinal axon maintenance; and whether tubulin-binding drug rescue translates to clinical benefit in SPG4 patients.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No high-resolution structure of full-length M1 spastin in ER membrane context\", \"Nuclear substrates/functions unknown\", \"No clinical trial data for microtubule-stabilizing drugs in SPG4\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [3, 11]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [4, 10, 11]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [4, 10, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [16]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [4, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [16]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [6, 12, 14]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"RTN1\",\n      \"ATL1\",\n      \"REEP1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}