{"gene":"SPAST","run_date":"2026-06-10T07:46:39","timeline":{"discoveries":[{"year":2003,"finding":"Spastin is expressed as two isoforms of 75 and 80 kDa in human and mouse tissues, with tissue-specific variability in isoform ratio. Immunolabeling showed spastin is expressed in neurons but not glial cells. Protein and transcript analyses of patients with nonsense or frameshift mutations revealed neither truncated protein nor mutated transcripts, indicating these mutations cause loss of spastin function (haploinsufficiency).","method":"Western blotting, immunolabeling, RT-PCR, protein analysis from patient tissues","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (western blot, immunolabeling, RT-PCR) in single lab establishing protein isoforms, cellular localization, and loss-of-function mechanism","pmids":["12490534"],"is_preprint":false},{"year":2004,"finding":"Spastin contains two functional nuclear localization sequences (NLSs) located in exons 1 and 6, both independently capable of mediating nuclear entry. The native protein localizes to both the perinuclear cytoplasm and the nucleus.","method":"Polyclonal antibody staining, novel Tetra-GFP reporter system for NLS functionality, subcellular localization experiments","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization experiments combined with novel reporter system testing NLS functionality; single lab, two orthogonal approaches","pmids":["15147984"],"is_preprint":false},{"year":2008,"finding":"The 60-kDa (M87) spastin isoform is produced by a cryptic promoter located within exon 1 of the SPG4 gene (overlapping the 5'-UTR and coding region), which generates a transcript with a shorter 5'-UTR that selectively translates the 60-kDa isoform. The S44L polymorphism, located near this cryptic transcription start site, decreases the activity of the cryptic promoter in luciferase assays.","method":"Promoter-less constructs in cell lines (HeLa, HEK293, NSC34, SH-SY5Y), luciferase reporter assays, ruling out cap-independent translation","journal":"BMC biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — promoter activity demonstrated in multiple cell lines with promoter-less constructs and luciferase assays; single lab, multiple orthogonal approaches","pmids":["18613979"],"is_preprint":false},{"year":2010,"finding":"Four HSP-associated SPAST mutations outside the AAA region (L195V, 46Stop, S44L, E112K) were studied. None affected enzymatic activity or expression levels of M1 or M87. Three mutations (L195V, 46Stop, and surprisingly S44L) conferred dominant-negative activity specifically to the M1 isoform. E112K (symptomatic heterozygously) did not show dominant-negative activity, demonstrating that some HSP-SPG4 cases cannot be explained by reduced microtubule-severing activity.","method":"In vitro microtubule-severing assays, expression level analysis, dominant-negative activity assays in cells","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — in vitro enzymatic assays plus cell-based dominant-negative testing; single lab with multiple mutations tested across two orthogonal methods","pmids":["20430936"],"is_preprint":false},{"year":2007,"finding":"N-terminal missense variants (S44L, E43Q, P45Q) of spastin do not affect haploinsufficiency-based disease mechanisms but instead enhance the stability of the shorter (M87/60 kDa) spastin isoform. Their phenotypic effects may be mediated by increasing isoform competition for interacting proteins, substrates, or oligomerization partners.","method":"Protein stability assays, isoform-specific expression analysis in cells","journal":"European journal of neurology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, protein stability measured but mechanism of isoform competition not directly demonstrated; limited methodological detail in abstract","pmids":["17916079"],"is_preprint":false},{"year":2009,"finding":"In bovine spinal dysmyelination (a recessive SPAST disease model), the R560Q substitution at an invariant position in the ATPase domain of spastin severely impaired ATPase activity of recombinant spastin in vitro, demonstrating a causal relationship between this SPAST mutation and loss of enzymatic function.","method":"Recombinant protein expression, in vitro ATPase activity assay","journal":"Neurogenetics","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct in vitro reconstitution and enzymatic assay establishing that ATPase domain mutation abolishes ATPase activity; single lab but rigorous biochemical approach","pmids":["19714378"],"is_preprint":false},{"year":2013,"finding":"In hiPSC-derived neurons from SPG4 patients with a c.1684C>T nonsense mutation, all spastin isoforms were reduced, neurite complexity was decreased, axonal transport was imbalanced (less retrograde movement), and neurite swellings with disrupted microtubules were present ultrastructurally. Overexpression of either M1 or M87 spastin isoform restored neurite length, branching, number of primary neurites, and reduced swellings, demonstrating gene dosage-dependent rescue.","method":"hiPSC differentiation, live imaging of axonal transport, electron microscopy, overexpression rescue experiments","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — patient-derived neurons with multiple orthogonal readouts (transport, ultrastructure, morphology) and isoform-specific rescue experiments; rigorous experimental design","pmids":["24381312"],"is_preprint":false},{"year":2016,"finding":"In olfactory stem cells from SPAST-mutation patients, average peroxisome movement speed was slower due to reduced numbers of stable microtubules (not impaired peroxisome-microtubule interaction per se). Patient cells showed increased oxidative stress. Epothilone D, which increases stable microtubules, restored peroxisome speed and ameliorated hydrogen peroxide sensitivity, linking reduced stable microtubules to impaired peroxisome transport and oxidative stress.","method":"Time-lapse imaging with automated image analysis, epothilone D pharmacological rescue, oxidative stress assays","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple patient-derived cell lines, quantitative live imaging, pharmacological rescue; single lab with two orthogonal approaches","pmids":["27229699"],"is_preprint":false},{"year":2017,"finding":"Truncating SPAST mutations (N184X, S245X) produce truncated M1 spastin proteins that accumulate to notably higher levels than truncated M87 or wild-type counterparts. Truncated M1 was more detrimental to neurite outgrowth than truncated M87. The N184X mutation also triggers reinitiation of translation at a third start codon, producing a novel M187 isoform capable of severing microtubules.","method":"Western blotting for protein accumulation, neurite outgrowth assays, microtubule-severing assays, translation initiation analysis","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple mutations studied, protein accumulation quantified, functional assays performed; single lab with multiple orthogonal methods","pmids":["28495799"],"is_preprint":false},{"year":2018,"finding":"A missense mutation I344K in the AAA domain of spastin (I344K-SPAST) abolished ATPase activity and microtubule-severing activity in vitro and in cells. The mutant protein showed prolonged half-life compared to wild-type SPAST due to altered post-translational modifications for proteasomal degradation. Mutant M1 isoform localized to microtubules but failed to sever them, causing microtubule accumulation and inhibited neurite outgrowth. Overexpression of wild-type M1 SPAST reduced pathogenic effects in a dose-dependent manner, with WT-SPAST shown to interact with I344K-SPAST.","method":"In vitro ATPase activity assay, microtubule-severing assay, co-immunoprecipitation, protein stability/half-life assay, neurite outgrowth in neuroblastoma/neural progenitor/primary cortical neuron cells","journal":"Biochimica et biophysica acta. Molecular basis of disease","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — in vitro enzymatic assays combined with cell-based functional assays, protein interaction, and post-translational modification analysis; multiple orthogonal methods in one study","pmids":["30006150"],"is_preprint":false},{"year":2012,"finding":"In patient-derived olfactory mucosa neural progenitor cells with SPAST mutations, acetylated α-tubulin was reduced by 50% and stathmin (a microtubule-destabilizing enzyme) was increased by 150%, with altered intracellular distribution and slower movement of peroxisomes and mitochondria. Sub-nanomolar concentrations of microtubule-binding drugs (paclitaxel, vinblastine) restored acetylated α-tubulin to control levels.","method":"Patient-derived neural progenitor cell culture, immunostaining, live imaging of organelle transport, pharmacological intervention","journal":"Disease models & mechanisms","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — patient-derived cells with multiple cellular readouts and pharmacological rescue; single lab, multiple orthogonal methods","pmids":["23264559"],"is_preprint":false},{"year":2020,"finding":"In iPS-derived forebrain neurons from SPAST-mutation patients, there were reduced levels of stable microtubules, reduced peroxisome transport speed, reduced peroxisome numbers, higher density of axon swellings, and increased fragmentation after hydrogen peroxide. Treatment with epothilone D and noscapine rescued peroxisome transport and protected against oxidative stress-induced axon fragmentation, establishing that SPAST patient axons are vulnerable to oxidative stress as a consequence of reduced axonal microtubule-dependent transport.","method":"hiPSC differentiation into neurons, live peroxisome tracking, microtubule stability assays, oxidative stress challenge, pharmacological rescue with epothilone D and noscapine","journal":"Frontiers in neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — three patient lines, multiple orthogonal functional readouts in patient neurons, pharmacological rescue establishing causal mechanism","pmids":["32457567"],"is_preprint":false},{"year":2021,"finding":"A novel c.985dupA (p.Met329Asnfs*3) SPAST mutation produced truncated M1 and M87 isoforms that accumulated to higher levels than wild-type. Truncated M1 localized on microtubules and rendered them resistant to depolymerization (dominant-negative effect on microtubule dynamics). Truncated M87 was diffusely distributed in nucleus and cytoplasm, could not decorate microtubules, and did not promote microtubule disassembly.","method":"Western blotting, immunofluorescence, microtubule depolymerization assay in cells","journal":"Movement disorders","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-based functional assays with both isoforms studied, protein accumulation and localization characterized; single lab, multiple orthogonal methods","pmids":["34927746"],"is_preprint":false},{"year":2012,"finding":"SPAST transcription is positively regulated by transcription factors NRF1 and SOX11. miR-96 and miR-182 negatively regulate SPAST by effects on mRNA stability and protein level, providing post-transcriptional regulation.","method":"Molecular phylogenetic conservation analysis, transcription factor binding assays, miRNA reporter/overexpression experiments, RT-PCR and western blotting for mRNA and protein levels","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple transcription factors and miRNAs tested with functional validation of both mRNA stability and protein level effects; single lab, multiple orthogonal methods","pmids":["22574173"],"is_preprint":false},{"year":2006,"finding":"A nucleotide substitution c.1216A>G in the ATPase domain of SPG4 (apparent missense) causes aberrant in-frame splicing and destabilization of the mutated transcript. The resulting mutant protein is deficient in microtubule-severing activity but shows regular subcellular localization, and the disease mechanism is haploinsufficiency rather than dominant-negative effect.","method":"Mutation analysis, RT-PCR for aberrant splicing, microtubule-severing activity assay, subcellular localization analysis","journal":"Neurology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — microtubule-severing activity assay combined with splicing analysis and localization; single lab, multiple methods","pmids":["16476945"],"is_preprint":false},{"year":2022,"finding":"hSPAST-C448Y transgenic mice (expressing human mutant spastin) display corticospinal dieback and gait deficiencies but not axonal swellings, while Spast-KO mice display axonal swellings but not dieback or gait deficiencies. Crossbreeding showed KO background worsened gait deficiencies and produced earlier onset plus axonal swellings in hSPAST-C448Y mice, with changes in HDAC6 and tubulin modifications in axons—indicating reduced spastin function exacerbates toxic gain-of-function properties of mutant spastin.","method":"Transgenic mouse models (hSPAST-C448Y, Spast-KO), crossbreeding, histological analysis, behavioral gait analysis, immunostaining for HDAC6 and tubulin modifications","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis using two independent mouse models with crossbreeding, behavioral and histological readouts, replicated across multiple methods in single rigorous study","pmids":["34935948"],"is_preprint":false}],"current_model":"SPAST encodes spastin, a microtubule-severing AAA-ATPase expressed as two major isoforms (M1/80 kDa and M87/60 kDa) generated from two translation start codons—with M87 driven additionally by a cryptic exon-1 promoter—that localizes to both nucleus and cytoplasm via two independent NLS sequences; most pathogenic mutations cause haploinsufficiency by abolishing ATPase/severing activity or destabilizing transcripts, but some truncating or missense mutations produce toxic M1 isoforms that accumulate, decorate microtubules, and impair their dynamics in a dominant-negative fashion, while reduced stable microtubule pools secondarily impair axonal transport of peroxisomes and mitochondria, increase oxidative stress vulnerability, and cause neurite swellings—with disease severity reflecting a combination of haploinsufficiency and isoform-specific gain-of-function toxicity."},"narrative":{"mechanistic_narrative":"SPAST encodes spastin, a microtubule-severing AAA-ATPase expressed in neurons whose enzymatic activity maintains the pool of stable, dynamically regulated axonal microtubules [PMID:12490534, PMID:19714378]. The gene produces two principal isoforms from distinct translation start sites—the longer M1 (80 kDa) and the shorter M87 (60 kDa), the latter driven additionally by a cryptic promoter within exon 1 that generates a short 5'-UTR transcript [PMID:18613979]—and the native protein partitions between perinuclear cytoplasm and the nucleus via two independent nuclear localization signals encoded in exons 1 and 6 [PMID:15147984]. Microtubule severing requires an intact AAA ATPase domain, and disease-associated substitutions in this domain (R560Q, I344K) abolish ATPase and severing activity [PMID:19714378, PMID:30006150]. Two genetically distinct disease mechanisms converge on the same axonal pathology: most loss-of-function mutations cause haploinsufficiency by eliminating protein, destabilizing transcripts, or inactivating severing without dominant-negative effect [PMID:12490534, PMID:16476945], whereas certain truncating and missense alleles produce stabilized M1 protein that decorates microtubules, renders them resistant to depolymerization, and impairs their dynamics in a dominant-negative manner [PMID:20430936, PMID:28495799, PMID:34927746]. The downstream consequence is a reduced pool of stable, acetylated microtubules that slows microtubule-dependent transport of peroxisomes and mitochondria, increases vulnerability to oxidative stress, and produces neurite swellings and reduced neurite complexity—deficits rescued by restoring spastin dosage or by microtubule-stabilizing drugs such as epothilone D [PMID:24381312, PMID:27229699, PMID:23264559, PMID:32457567]. Genetic epistasis in mouse models shows that reduced spastin function and gain-of-function toxicity act in combination to determine disease severity [PMID:34935948]. SPAST is transcriptionally activated by NRF1 and SOX11 and post-transcriptionally repressed by miR-96 and miR-182 [PMID:22574173].","teleology":[{"year":2003,"claim":"Established that SPAST nonsense and frameshift mutations produce neither truncated protein nor mutant transcript, defining haploinsufficiency as a primary disease mechanism and localizing spastin expression to neurons.","evidence":"Western blotting, immunolabeling, and RT-PCR on patient tissues, identifying two isoforms (75/80 kDa)","pmids":["12490534"],"confidence":"Medium","gaps":["Did not resolve the molecular origin of the two isoforms","Did not test whether any mutations act by gain-of-function"]},{"year":2004,"claim":"Answered where spastin acts by showing it carries two functional NLSs and distributes between nucleus and perinuclear cytoplasm, broadening the protein's potential roles beyond cytoplasmic microtubules.","evidence":"Polyclonal antibody staining plus a Tetra-GFP reporter assay testing NLS functionality","pmids":["15147984"],"confidence":"Medium","gaps":["Nuclear function of spastin not defined","Relative contribution of each NLS to physiological localization unknown"]},{"year":2008,"claim":"Explained the origin of the shorter isoform by identifying a cryptic exon-1 promoter that selectively drives M87 translation, providing a mechanistic basis for isoform-specific regulation.","evidence":"Promoter-less constructs and luciferase assays across HeLa, HEK293, NSC34, SH-SY5Y cells","pmids":["18613979"],"confidence":"Medium","gaps":["Physiological tissue contexts favoring cryptic-promoter activity not mapped","Functional difference between M1 and M87 not addressed here"]},{"year":2007,"claim":"Proposed that some N-terminal missense variants act not through haploinsufficiency but by stabilizing the M87 isoform, introducing isoform-competition as a candidate mechanism.","evidence":"Protein stability and isoform-specific expression assays in cells","pmids":["17916079"],"confidence":"Low","gaps":["Isoform-competition mechanism not directly demonstrated","Functional consequence of M87 stabilization untested"]},{"year":2009,"claim":"Demonstrated directly that an invariant ATPase-domain substitution (R560Q) abolishes spastin's enzymatic activity, causally linking the AAA domain to severing function and disease.","evidence":"Recombinant protein expression and in vitro ATPase activity assay in a bovine recessive model","pmids":["19714378"],"confidence":"High","gaps":["Did not assay microtubule severing directly","Cellular consequences of ATPase loss not examined"]},{"year":2010,"claim":"Showed that HSP mutations outside the AAA domain can act by conferring dominant-negative activity specifically to M1, establishing that not all disease cases derive from reduced severing activity.","evidence":"In vitro severing assays plus cell-based dominant-negative testing of four mutations","pmids":["20430936"],"confidence":"Medium","gaps":["Molecular basis of M1-specific dominant-negative effect unresolved","E112K pathogenic mechanism left unexplained"]},{"year":2012,"claim":"Connected reduced spastin to a measurable microtubule defect by showing patient progenitor cells lose acetylated tubulin and gain stathmin, slowing organelle transport, with rescue by sub-nanomolar microtubule-binding drugs.","evidence":"Patient-derived neural progenitor cells, immunostaining, organelle live imaging, pharmacological rescue","pmids":["23264559"],"confidence":"Medium","gaps":["Causal link between stathmin increase and transport defect not isolated","In vivo relevance not tested"]},{"year":2012,"claim":"Defined the regulatory inputs controlling SPAST levels by identifying NRF1/SOX11 as transcriptional activators and miR-96/miR-182 as post-transcriptional repressors.","evidence":"Conservation analysis, transcription factor binding assays, miRNA reporter/overexpression, RT-PCR and western blot","pmids":["22574173"],"confidence":"Medium","gaps":["Physiological conditions engaging these regulators unknown","Whether dysregulation contributes to disease untested"]},{"year":2013,"claim":"Used patient iPSC neurons to show that reduced spastin lowers all isoforms, decreases neurite complexity, imbalances axonal transport, and produces microtubule-disrupted swellings, with isoform-specific overexpression rescuing the phenotype in a dosage-dependent manner.","evidence":"hiPSC-derived neurons, live axonal transport imaging, electron microscopy, overexpression rescue","pmids":["24381312"],"confidence":"High","gaps":["Did not separate haploinsufficiency from gain-of-function in this allele","Mechanism linking transport imbalance to swellings not fully resolved"]},{"year":2016,"claim":"Pinpointed reduced stable microtubule number (not impaired organelle-microtubule binding) as the cause of slowed peroxisome transport and linked this to elevated oxidative stress, rescued by epothilone D.","evidence":"Patient olfactory stem cells, automated time-lapse imaging, epothilone D rescue, oxidative stress assays","pmids":["27229699"],"confidence":"Medium","gaps":["Mechanism connecting transport deficit to oxidative vulnerability not detailed","Generalizability beyond peroxisomes unclear here"]},{"year":2017,"claim":"Showed that truncating mutations produce M1 truncation products that over-accumulate and are more detrimental to neurite outgrowth than truncated M87, and can trigger reinitiation to a severing-competent M187 isoform.","evidence":"Western blotting, neurite outgrowth assays, severing assays, translation initiation analysis","pmids":["28495799"],"confidence":"Medium","gaps":["Why truncated M1 accumulates preferentially not mechanistically resolved","In vivo significance of M187 reinitiation unknown"]},{"year":2018,"claim":"Provided a detailed gain-of-function mechanism: the AAA-domain mutant I344K loses ATPase/severing activity, gains a prolonged half-life via altered degradation, decorates but cannot sever microtubules, and physically interacts with wild-type spastin, with WT overexpression dose-dependently mitigating toxicity.","evidence":"In vitro ATPase and severing assays, co-IP, half-life assays, neurite outgrowth in multiple neuronal cell types","pmids":["30006150"],"confidence":"High","gaps":["Specific post-translational modifications altering degradation not identified","Stoichiometry of WT/mutant interaction not quantified"]},{"year":2020,"claim":"Confirmed in iPSC forebrain neurons that reduced stable microtubules lower peroxisome transport and number, increase axon swellings, and sensitize axons to oxidative-stress fragmentation, with rescue by epothilone D and noscapine.","evidence":"Three patient hiPSC lines, peroxisome tracking, microtubule stability assays, oxidative challenge, pharmacological rescue","pmids":["32457567"],"confidence":"High","gaps":["Did not establish whether oxidative damage is cause or consequence of axon degeneration","Long-term therapeutic durability untested"]},{"year":2021,"claim":"Distinguished the isoform-specific behavior of a frameshift allele by showing truncated M1 decorates microtubules and confers depolymerization resistance whereas truncated M87 is diffuse and inert, refining the dominant-negative model to M1.","evidence":"Western blotting, immunofluorescence, microtubule depolymerization assay in cells","pmids":["34927746"],"confidence":"Medium","gaps":["Endogenous expression level of the truncated isoforms in patient neurons not measured","Contribution to clinical phenotype not assessed"]},{"year":2022,"claim":"Demonstrated through genetic epistasis that haploinsufficiency and gain-of-function are distinct but interacting contributions, since a KO background worsened and accelerated mutant-spastin phenotypes alongside altered HDAC6 and tubulin modifications.","evidence":"hSPAST-C448Y transgenic and Spast-KO mouse models, crossbreeding, gait/histology, HDAC6 and tubulin immunostaining","pmids":["34935948"],"confidence":"High","gaps":["Molecular role of HDAC6 changes in pathology not mechanistically resolved","Whether human disease combines both mechanisms in the same patient not established"]},{"year":null,"claim":"The nuclear function of spastin and the precise molecular link between reduced stable-microtubule-dependent transport and oxidative-stress vulnerability remain undefined.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No defined nuclear substrate or role for spastin despite two functional NLSs","Mechanism converting transport deficits into oxidative damage not isolated","Direct interacting partners and oligomerization stoichiometry incompletely mapped"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[5,9]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[3,5,9,8,14]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[9,12]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[5,9]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[1]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1,12]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[9,12]}],"pathway":[],"complexes":[],"partners":["SPAST"],"other_free_text":[]}},"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":"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":"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":9,"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":"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":8,"is_preprint":false},{"pmid":"34753439","id":"PMC_34753439","title":"A p.Arg499His mutation in SPAST is associated with infantile-onset complicated spastic paraplegia: a case report and review of the literature.","date":"2021","source":"BMC neurology","url":"https://pubmed.ncbi.nlm.nih.gov/34753439","citation_count":7,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":45660,"output_tokens":4320,"usd":0.10089,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":12312,"output_tokens":4606,"usd":0.088355,"stage2_stop_reason":"end_turn"},"total_usd":0.189245,"stage1_batch_id":"msgbatch_013LWZZT6MWaqB9qyTSeQBmU","stage2_batch_id":"msgbatch_0194ZzQaTGNpruoNJBbEMbt3","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2003,\n      \"finding\": \"Spastin is expressed as two isoforms of 75 and 80 kDa in human and mouse tissues, with tissue-specific variability in isoform ratio. Immunolabeling showed spastin is expressed in neurons but not glial cells. Protein and transcript analyses of patients with nonsense or frameshift mutations revealed neither truncated protein nor mutated transcripts, indicating these mutations cause loss of spastin function (haploinsufficiency).\",\n      \"method\": \"Western blotting, immunolabeling, RT-PCR, protein analysis from patient tissues\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (western blot, immunolabeling, RT-PCR) in single lab establishing protein isoforms, cellular localization, and loss-of-function mechanism\",\n      \"pmids\": [\"12490534\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Spastin contains two functional nuclear localization sequences (NLSs) located in exons 1 and 6, both independently capable of mediating nuclear entry. The native protein localizes to both the perinuclear cytoplasm and the nucleus.\",\n      \"method\": \"Polyclonal antibody staining, novel Tetra-GFP reporter system for NLS functionality, subcellular localization experiments\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiments combined with novel reporter system testing NLS functionality; single lab, two orthogonal approaches\",\n      \"pmids\": [\"15147984\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"The 60-kDa (M87) spastin isoform is produced by a cryptic promoter located within exon 1 of the SPG4 gene (overlapping the 5'-UTR and coding region), which generates a transcript with a shorter 5'-UTR that selectively translates the 60-kDa isoform. The S44L polymorphism, located near this cryptic transcription start site, decreases the activity of the cryptic promoter in luciferase assays.\",\n      \"method\": \"Promoter-less constructs in cell lines (HeLa, HEK293, NSC34, SH-SY5Y), luciferase reporter assays, ruling out cap-independent translation\",\n      \"journal\": \"BMC biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — promoter activity demonstrated in multiple cell lines with promoter-less constructs and luciferase assays; single lab, multiple orthogonal approaches\",\n      \"pmids\": [\"18613979\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Four HSP-associated SPAST mutations outside the AAA region (L195V, 46Stop, S44L, E112K) were studied. None affected enzymatic activity or expression levels of M1 or M87. Three mutations (L195V, 46Stop, and surprisingly S44L) conferred dominant-negative activity specifically to the M1 isoform. E112K (symptomatic heterozygously) did not show dominant-negative activity, demonstrating that some HSP-SPG4 cases cannot be explained by reduced microtubule-severing activity.\",\n      \"method\": \"In vitro microtubule-severing assays, expression level analysis, dominant-negative activity assays in cells\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — in vitro enzymatic assays plus cell-based dominant-negative testing; single lab with multiple mutations tested across two orthogonal methods\",\n      \"pmids\": [\"20430936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"N-terminal missense variants (S44L, E43Q, P45Q) of spastin do not affect haploinsufficiency-based disease mechanisms but instead enhance the stability of the shorter (M87/60 kDa) spastin isoform. Their phenotypic effects may be mediated by increasing isoform competition for interacting proteins, substrates, or oligomerization partners.\",\n      \"method\": \"Protein stability assays, isoform-specific expression analysis in cells\",\n      \"journal\": \"European journal of neurology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, protein stability measured but mechanism of isoform competition not directly demonstrated; limited methodological detail in abstract\",\n      \"pmids\": [\"17916079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"In bovine spinal dysmyelination (a recessive SPAST disease model), the R560Q substitution at an invariant position in the ATPase domain of spastin severely impaired ATPase activity of recombinant spastin in vitro, demonstrating a causal relationship between this SPAST mutation and loss of enzymatic function.\",\n      \"method\": \"Recombinant protein expression, in vitro ATPase activity assay\",\n      \"journal\": \"Neurogenetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct in vitro reconstitution and enzymatic assay establishing that ATPase domain mutation abolishes ATPase activity; single lab but rigorous biochemical approach\",\n      \"pmids\": [\"19714378\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In hiPSC-derived neurons from SPG4 patients with a c.1684C>T nonsense mutation, all spastin isoforms were reduced, neurite complexity was decreased, axonal transport was imbalanced (less retrograde movement), and neurite swellings with disrupted microtubules were present ultrastructurally. Overexpression of either M1 or M87 spastin isoform restored neurite length, branching, number of primary neurites, and reduced swellings, demonstrating gene dosage-dependent rescue.\",\n      \"method\": \"hiPSC differentiation, live imaging of axonal transport, electron microscopy, overexpression rescue experiments\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — patient-derived neurons with multiple orthogonal readouts (transport, ultrastructure, morphology) and isoform-specific rescue experiments; rigorous experimental design\",\n      \"pmids\": [\"24381312\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In olfactory stem cells from SPAST-mutation patients, average peroxisome movement speed was slower due to reduced numbers of stable microtubules (not impaired peroxisome-microtubule interaction per se). Patient cells showed increased oxidative stress. Epothilone D, which increases stable microtubules, restored peroxisome speed and ameliorated hydrogen peroxide sensitivity, linking reduced stable microtubules to impaired peroxisome transport and oxidative stress.\",\n      \"method\": \"Time-lapse imaging with automated image analysis, epothilone D pharmacological rescue, oxidative stress assays\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple patient-derived cell lines, quantitative live imaging, pharmacological rescue; single lab with two orthogonal approaches\",\n      \"pmids\": [\"27229699\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Truncating SPAST mutations (N184X, S245X) produce truncated M1 spastin proteins that accumulate to notably higher levels than truncated M87 or wild-type counterparts. Truncated M1 was more detrimental to neurite outgrowth than truncated M87. The N184X mutation also triggers reinitiation of translation at a third start codon, producing a novel M187 isoform capable of severing microtubules.\",\n      \"method\": \"Western blotting for protein accumulation, neurite outgrowth assays, microtubule-severing assays, translation initiation analysis\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple mutations studied, protein accumulation quantified, functional assays performed; single lab with multiple orthogonal methods\",\n      \"pmids\": [\"28495799\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"A missense mutation I344K in the AAA domain of spastin (I344K-SPAST) abolished ATPase activity and microtubule-severing activity in vitro and in cells. The mutant protein showed prolonged half-life compared to wild-type SPAST due to altered post-translational modifications for proteasomal degradation. Mutant M1 isoform localized to microtubules but failed to sever them, causing microtubule accumulation and inhibited neurite outgrowth. Overexpression of wild-type M1 SPAST reduced pathogenic effects in a dose-dependent manner, with WT-SPAST shown to interact with I344K-SPAST.\",\n      \"method\": \"In vitro ATPase activity assay, microtubule-severing assay, co-immunoprecipitation, protein stability/half-life assay, neurite outgrowth in neuroblastoma/neural progenitor/primary cortical neuron cells\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular basis of disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — in vitro enzymatic assays combined with cell-based functional assays, protein interaction, and post-translational modification analysis; multiple orthogonal methods in one study\",\n      \"pmids\": [\"30006150\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In patient-derived olfactory mucosa neural progenitor cells with SPAST mutations, acetylated α-tubulin was reduced by 50% and stathmin (a microtubule-destabilizing enzyme) was increased by 150%, with altered intracellular distribution and slower movement of peroxisomes and mitochondria. Sub-nanomolar concentrations of microtubule-binding drugs (paclitaxel, vinblastine) restored acetylated α-tubulin to control levels.\",\n      \"method\": \"Patient-derived neural progenitor cell culture, immunostaining, live imaging of organelle transport, pharmacological intervention\",\n      \"journal\": \"Disease models & mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — patient-derived cells with multiple cellular readouts and pharmacological rescue; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"23264559\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In iPS-derived forebrain neurons from SPAST-mutation patients, there were reduced levels of stable microtubules, reduced peroxisome transport speed, reduced peroxisome numbers, higher density of axon swellings, and increased fragmentation after hydrogen peroxide. Treatment with epothilone D and noscapine rescued peroxisome transport and protected against oxidative stress-induced axon fragmentation, establishing that SPAST patient axons are vulnerable to oxidative stress as a consequence of reduced axonal microtubule-dependent transport.\",\n      \"method\": \"hiPSC differentiation into neurons, live peroxisome tracking, microtubule stability assays, oxidative stress challenge, pharmacological rescue with epothilone D and noscapine\",\n      \"journal\": \"Frontiers in neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — three patient lines, multiple orthogonal functional readouts in patient neurons, pharmacological rescue establishing causal mechanism\",\n      \"pmids\": [\"32457567\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"A novel c.985dupA (p.Met329Asnfs*3) SPAST mutation produced truncated M1 and M87 isoforms that accumulated to higher levels than wild-type. Truncated M1 localized on microtubules and rendered them resistant to depolymerization (dominant-negative effect on microtubule dynamics). Truncated M87 was diffusely distributed in nucleus and cytoplasm, could not decorate microtubules, and did not promote microtubule disassembly.\",\n      \"method\": \"Western blotting, immunofluorescence, microtubule depolymerization assay in cells\",\n      \"journal\": \"Movement disorders\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-based functional assays with both isoforms studied, protein accumulation and localization characterized; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"34927746\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SPAST transcription is positively regulated by transcription factors NRF1 and SOX11. miR-96 and miR-182 negatively regulate SPAST by effects on mRNA stability and protein level, providing post-transcriptional regulation.\",\n      \"method\": \"Molecular phylogenetic conservation analysis, transcription factor binding assays, miRNA reporter/overexpression experiments, RT-PCR and western blotting for mRNA and protein levels\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple transcription factors and miRNAs tested with functional validation of both mRNA stability and protein level effects; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"22574173\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A nucleotide substitution c.1216A>G in the ATPase domain of SPG4 (apparent missense) causes aberrant in-frame splicing and destabilization of the mutated transcript. The resulting mutant protein is deficient in microtubule-severing activity but shows regular subcellular localization, and the disease mechanism is haploinsufficiency rather than dominant-negative effect.\",\n      \"method\": \"Mutation analysis, RT-PCR for aberrant splicing, microtubule-severing activity assay, subcellular localization analysis\",\n      \"journal\": \"Neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — microtubule-severing activity assay combined with splicing analysis and localization; single lab, multiple methods\",\n      \"pmids\": [\"16476945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"hSPAST-C448Y transgenic mice (expressing human mutant spastin) display corticospinal dieback and gait deficiencies but not axonal swellings, while Spast-KO mice display axonal swellings but not dieback or gait deficiencies. Crossbreeding showed KO background worsened gait deficiencies and produced earlier onset plus axonal swellings in hSPAST-C448Y mice, with changes in HDAC6 and tubulin modifications in axons—indicating reduced spastin function exacerbates toxic gain-of-function properties of mutant spastin.\",\n      \"method\": \"Transgenic mouse models (hSPAST-C448Y, Spast-KO), crossbreeding, histological analysis, behavioral gait analysis, immunostaining for HDAC6 and tubulin modifications\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis using two independent mouse models with crossbreeding, behavioral and histological readouts, replicated across multiple methods in single rigorous study\",\n      \"pmids\": [\"34935948\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SPAST encodes spastin, a microtubule-severing AAA-ATPase expressed as two major isoforms (M1/80 kDa and M87/60 kDa) generated from two translation start codons—with M87 driven additionally by a cryptic exon-1 promoter—that localizes to both nucleus and cytoplasm via two independent NLS sequences; most pathogenic mutations cause haploinsufficiency by abolishing ATPase/severing activity or destabilizing transcripts, but some truncating or missense mutations produce toxic M1 isoforms that accumulate, decorate microtubules, and impair their dynamics in a dominant-negative fashion, while reduced stable microtubule pools secondarily impair axonal transport of peroxisomes and mitochondria, increase oxidative stress vulnerability, and cause neurite swellings—with disease severity reflecting a combination of haploinsufficiency and isoform-specific gain-of-function toxicity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SPAST encodes spastin, a microtubule-severing AAA-ATPase expressed in neurons whose enzymatic activity maintains the pool of stable, dynamically regulated axonal microtubules [#0, #5]. The gene produces two principal isoforms from distinct translation start sites\\u2014the longer M1 (80 kDa) and the shorter M87 (60 kDa), the latter driven additionally by a cryptic promoter within exon 1 that generates a short 5'-UTR transcript [#2]\\u2014and the native protein partitions between perinuclear cytoplasm and the nucleus via two independent nuclear localization signals encoded in exons 1 and 6 [#1]. Microtubule severing requires an intact AAA ATPase domain, and disease-associated substitutions in this domain (R560Q, I344K) abolish ATPase and severing activity [#5, #9]. Two genetically distinct disease mechanisms converge on the same axonal pathology: most loss-of-function mutations cause haploinsufficiency by eliminating protein, destabilizing transcripts, or inactivating severing without dominant-negative effect [#0, #14], whereas certain truncating and missense alleles produce stabilized M1 protein that decorates microtubules, renders them resistant to depolymerization, and impairs their dynamics in a dominant-negative manner [#3, #8, #12]. The downstream consequence is a reduced pool of stable, acetylated microtubules that slows microtubule-dependent transport of peroxisomes and mitochondria, increases vulnerability to oxidative stress, and produces neurite swellings and reduced neurite complexity\\u2014deficits rescued by restoring spastin dosage or by microtubule-stabilizing drugs such as epothilone D [#6, #7, #10, #11]. Genetic epistasis in mouse models shows that reduced spastin function and gain-of-function toxicity act in combination to determine disease severity [#15]. SPAST is transcriptionally activated by NRF1 and SOX11 and post-transcriptionally repressed by miR-96 and miR-182 [#13].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Established that SPAST nonsense and frameshift mutations produce neither truncated protein nor mutant transcript, defining haploinsufficiency as a primary disease mechanism and localizing spastin expression to neurons.\",\n      \"evidence\": \"Western blotting, immunolabeling, and RT-PCR on patient tissues, identifying two isoforms (75/80 kDa)\",\n      \"pmids\": [\"12490534\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not resolve the molecular origin of the two isoforms\", \"Did not test whether any mutations act by gain-of-function\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Answered where spastin acts by showing it carries two functional NLSs and distributes between nucleus and perinuclear cytoplasm, broadening the protein's potential roles beyond cytoplasmic microtubules.\",\n      \"evidence\": \"Polyclonal antibody staining plus a Tetra-GFP reporter assay testing NLS functionality\",\n      \"pmids\": [\"15147984\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Nuclear function of spastin not defined\", \"Relative contribution of each NLS to physiological localization unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Explained the origin of the shorter isoform by identifying a cryptic exon-1 promoter that selectively drives M87 translation, providing a mechanistic basis for isoform-specific regulation.\",\n      \"evidence\": \"Promoter-less constructs and luciferase assays across HeLa, HEK293, NSC34, SH-SY5Y cells\",\n      \"pmids\": [\"18613979\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological tissue contexts favoring cryptic-promoter activity not mapped\", \"Functional difference between M1 and M87 not addressed here\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Proposed that some N-terminal missense variants act not through haploinsufficiency but by stabilizing the M87 isoform, introducing isoform-competition as a candidate mechanism.\",\n      \"evidence\": \"Protein stability and isoform-specific expression assays in cells\",\n      \"pmids\": [\"17916079\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Isoform-competition mechanism not directly demonstrated\", \"Functional consequence of M87 stabilization untested\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Demonstrated directly that an invariant ATPase-domain substitution (R560Q) abolishes spastin's enzymatic activity, causally linking the AAA domain to severing function and disease.\",\n      \"evidence\": \"Recombinant protein expression and in vitro ATPase activity assay in a bovine recessive model\",\n      \"pmids\": [\"19714378\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not assay microtubule severing directly\", \"Cellular consequences of ATPase loss not examined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Showed that HSP mutations outside the AAA domain can act by conferring dominant-negative activity specifically to M1, establishing that not all disease cases derive from reduced severing activity.\",\n      \"evidence\": \"In vitro severing assays plus cell-based dominant-negative testing of four mutations\",\n      \"pmids\": [\"20430936\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of M1-specific dominant-negative effect unresolved\", \"E112K pathogenic mechanism left unexplained\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Connected reduced spastin to a measurable microtubule defect by showing patient progenitor cells lose acetylated tubulin and gain stathmin, slowing organelle transport, with rescue by sub-nanomolar microtubule-binding drugs.\",\n      \"evidence\": \"Patient-derived neural progenitor cells, immunostaining, organelle live imaging, pharmacological rescue\",\n      \"pmids\": [\"23264559\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal link between stathmin increase and transport defect not isolated\", \"In vivo relevance not tested\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defined the regulatory inputs controlling SPAST levels by identifying NRF1/SOX11 as transcriptional activators and miR-96/miR-182 as post-transcriptional repressors.\",\n      \"evidence\": \"Conservation analysis, transcription factor binding assays, miRNA reporter/overexpression, RT-PCR and western blot\",\n      \"pmids\": [\"22574173\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological conditions engaging these regulators unknown\", \"Whether dysregulation contributes to disease untested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Used patient iPSC neurons to show that reduced spastin lowers all isoforms, decreases neurite complexity, imbalances axonal transport, and produces microtubule-disrupted swellings, with isoform-specific overexpression rescuing the phenotype in a dosage-dependent manner.\",\n      \"evidence\": \"hiPSC-derived neurons, live axonal transport imaging, electron microscopy, overexpression rescue\",\n      \"pmids\": [\"24381312\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not separate haploinsufficiency from gain-of-function in this allele\", \"Mechanism linking transport imbalance to swellings not fully resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Pinpointed reduced stable microtubule number (not impaired organelle-microtubule binding) as the cause of slowed peroxisome transport and linked this to elevated oxidative stress, rescued by epothilone D.\",\n      \"evidence\": \"Patient olfactory stem cells, automated time-lapse imaging, epothilone D rescue, oxidative stress assays\",\n      \"pmids\": [\"27229699\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism connecting transport deficit to oxidative vulnerability not detailed\", \"Generalizability beyond peroxisomes unclear here\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Showed that truncating mutations produce M1 truncation products that over-accumulate and are more detrimental to neurite outgrowth than truncated M87, and can trigger reinitiation to a severing-competent M187 isoform.\",\n      \"evidence\": \"Western blotting, neurite outgrowth assays, severing assays, translation initiation analysis\",\n      \"pmids\": [\"28495799\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Why truncated M1 accumulates preferentially not mechanistically resolved\", \"In vivo significance of M187 reinitiation unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Provided a detailed gain-of-function mechanism: the AAA-domain mutant I344K loses ATPase/severing activity, gains a prolonged half-life via altered degradation, decorates but cannot sever microtubules, and physically interacts with wild-type spastin, with WT overexpression dose-dependently mitigating toxicity.\",\n      \"evidence\": \"In vitro ATPase and severing assays, co-IP, half-life assays, neurite outgrowth in multiple neuronal cell types\",\n      \"pmids\": [\"30006150\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific post-translational modifications altering degradation not identified\", \"Stoichiometry of WT/mutant interaction not quantified\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Confirmed in iPSC forebrain neurons that reduced stable microtubules lower peroxisome transport and number, increase axon swellings, and sensitize axons to oxidative-stress fragmentation, with rescue by epothilone D and noscapine.\",\n      \"evidence\": \"Three patient hiPSC lines, peroxisome tracking, microtubule stability assays, oxidative challenge, pharmacological rescue\",\n      \"pmids\": [\"32457567\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish whether oxidative damage is cause or consequence of axon degeneration\", \"Long-term therapeutic durability untested\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Distinguished the isoform-specific behavior of a frameshift allele by showing truncated M1 decorates microtubules and confers depolymerization resistance whereas truncated M87 is diffuse and inert, refining the dominant-negative model to M1.\",\n      \"evidence\": \"Western blotting, immunofluorescence, microtubule depolymerization assay in cells\",\n      \"pmids\": [\"34927746\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endogenous expression level of the truncated isoforms in patient neurons not measured\", \"Contribution to clinical phenotype not assessed\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrated through genetic epistasis that haploinsufficiency and gain-of-function are distinct but interacting contributions, since a KO background worsened and accelerated mutant-spastin phenotypes alongside altered HDAC6 and tubulin modifications.\",\n      \"evidence\": \"hSPAST-C448Y transgenic and Spast-KO mouse models, crossbreeding, gait/histology, HDAC6 and tubulin immunostaining\",\n      \"pmids\": [\"34935948\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular role of HDAC6 changes in pathology not mechanistically resolved\", \"Whether human disease combines both mechanisms in the same patient not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The nuclear function of spastin and the precise molecular link between reduced stable-microtubule-dependent transport and oxidative-stress vulnerability remain undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No defined nuclear substrate or role for spastin despite two functional NLSs\", \"Mechanism converting transport deficits into oxidative damage not isolated\", \"Direct interacting partners and oligomerization stoichiometry incompletely mapped\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [5, 9]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [3, 5, 9, 8, 14]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [9, 12]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [5, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [1]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1, 12]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [9, 12]}\n    ],\n    \"pathway\": [],\n    \"complexes\": [],\n    \"partners\": [\"SPAST\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":7,"faith_total":7,"faith_pct":100.0}}