{"gene":"SPTLC1","run_date":"2026-04-28T20:42:08","timeline":{"discoveries":[{"year":1991,"finding":"LCB1 (yeast ortholog of SPTLC1) encodes serine palmitoyltransferase or a subunit thereof, as demonstrated by molecular cloning and complementation of lcb1-defective yeast strains that lack serine palmitoyltransferase activity. The protein has predicted transmembrane helices and homology to pyridoxal 5'-phosphate-dependent alpha-oxoamine synthases.","method":"Molecular cloning, yeast complementation assay, sequence analysis","journal":"Journal of bacteriology","confidence":"High","confidence_rationale":"Tier 2 — genetic complementation with functional enzyme activity restoration, foundational study replicated by subsequent work","pmids":["2066332"],"is_preprint":false},{"year":1994,"finding":"Yeast LCB1 and LCB2 (orthologs of SPTLC1 and SPTLC2) both encode subunits of serine palmitoyltransferase; overproduction of both together is required for increased SPT activity, establishing a heterodimeric complex.","method":"Yeast genetics, overexpression, enzyme activity assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — functional complementation with activity assay; replicated across multiple studies","pmids":["8058731"],"is_preprint":false},{"year":1997,"finding":"Mammalian LCB1 (SPTLC1) is a component of serine palmitoyltransferase; transfection of SPT-defective CHO cells (LY-B) with CHO LCB1 cDNA restored both SPT activity and de novo sphingolipid synthesis; SPT activity co-purified with His6-tagged LCB1 on Ni2+ resin.","method":"CHO cell complementation, affinity purification, enzyme activity assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — direct biochemical complementation of enzyme-deficient cells with activity assay and affinity purification","pmids":["9405408"],"is_preprint":false},{"year":1998,"finding":"SPTLC1 (LCB1) and LCB2 form a heterodimeric SPT complex in mammalian cells; anti-LCB2 antibody co-immunoprecipitated both SPT activity and LCB1; affinity-tagged LCB1 pulled down endogenous LCB2, demonstrating direct physical interaction.","method":"Co-immunoprecipitation, affinity pulldown, enzyme activity assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — reciprocal co-IP and affinity pulldown demonstrating complex formation, replicated independently","pmids":["9837968"],"is_preprint":false},{"year":2001,"finding":"Missense mutations in SPTLC1 (C133Y, C133W, V144D) cause hereditary sensory neuropathy type I (HSN1) and are associated with increased de novo glucosyl ceramide synthesis in lymphoblast cell lines, suggesting altered SPT enzyme activity.","method":"Mutation screening, lipid biosynthesis assay in patient lymphoblasts","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — functional biochemical readout in patient cells; replicated by second simultaneous independent study","pmids":["11242114","11242106"],"is_preprint":false},{"year":2002,"finding":"HSN1-corresponding mutations in yeast LCB1 (equivalent to SPTLC1) dominantly reduce SPT activity by ~50% when co-expressed with wild-type, establishing a dominant-negative mechanism. The mutant Lcb1p retains interaction with Lcb2p. Modeling indicates SPT has a single active site at the LCB1-LCB2 interface, and the mutations reside near the PLP-binding lysine in LCB2.","method":"Yeast genetics, enzyme activity assay, co-expression dominance test, structural modeling, co-immunoprecipitation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including enzyme assay, dominance test, and protein interaction analysis","pmids":["11781309"],"is_preprint":false},{"year":2002,"finding":"SPTLC1 is an integral ER membrane protein with a single transmembrane domain near the N-terminus, with its N-terminus oriented to the ER lumen and C-terminus to the cytosol. SPTLC1 is required for maintenance of LCB2 protein levels; LY-B cells lacking SPTLC1 have drastically reduced LCB2, which is restored by SPTLC1 transfection.","method":"Indirect immunofluorescence with N- and C-terminal epitope tags, subcellular fractionation, Western blotting, stable transfection of LY-B cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — direct localization experiment with functional consequence; multiple epitope-tag orientations tested","pmids":["12464627"],"is_preprint":false},{"year":2005,"finding":"Transgenic mice overexpressing mutant SPTLC1 (C133W) dominantly inhibit SPT activity in vivo and develop age-dependent peripheral neuropathy with loss of myelinated axons and myelin thinning, confirming the dominant inhibition mechanism in a mammalian model.","method":"Transgenic mouse model, SPT activity assay, neuropathological analysis, axon counting","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — in vivo genetic model with biochemical and pathological readouts; corroborated by cell culture and yeast studies","pmids":["16210380"],"is_preprint":false},{"year":2008,"finding":"SPTLC1 (but not SPTLC2) physically interacts with ABCA1 and negatively regulates ABCA1's cholesterol efflux activity by blocking ABCA1 exit from the ER. This interaction was confirmed by co-immunoprecipitation in THP-1 macrophages and mouse liver. siRNA knockdown of SPTLC1 and pharmacologic inhibition with myriocin both increased ABCA1 efflux activity ~60%.","method":"Affinity purification/mass spectrometry, co-immunoprecipitation, siRNA knockdown, dominant-negative SPTLC1 mutants, ABCA1 trafficking assay","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP in endogenous settings plus functional assays with multiple perturbations","pmids":["18484747"],"is_preprint":false},{"year":2009,"finding":"SPTLC1 HSN1 mutations (C133W) alter the amino acid substrate selectivity of SPT such that palmitate is condensed with alanine and glycine in addition to serine, generating neurotoxic deoxysphingoid bases (1-deoxy-sphingolipids). Overexpression of wild-type SPTLC1 rescues the neuropathy phenotype and reduces deoxysphingolipid levels in a mouse model.","method":"Transgenic mouse genetics (double transgenic rescue), lipid mass spectrometry, SPT activity assay","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 — double transgenic rescue experiment with biochemical validation of substrate shift mechanism","pmids":["19923297"],"is_preprint":false},{"year":2009,"finding":"Crystal structures and kinetic analysis of bacterial SPT (Sphingomonas paucimobilis) mimicking HSN1 mutations (N100Y, N100W equivalent to SPTLC1 C133Y/W) reveal structural changes around the active site that impair movement of catalytic Arg378, alter PLP chemistry, and destabilize a quinonoid intermediate, providing mechanistic insight into how disease mutations perturb enzyme activity.","method":"X-ray crystallography, kinetic assays, spectroscopic analysis, site-directed mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus kinetics plus mutagenesis in bacterial ortholog that directly models human disease mutations","pmids":["19376777"],"is_preprint":false},{"year":2009,"finding":"SPTLC1 interacts with the PDZ protein Par3 via a conserved C-terminal type II PDZ-binding motif. Par3 associates with the SPTLC1/2 holoenzyme, and Par3 knockdown by siRNA reduces SPT activity and de novo ceramide synthesis by ~40% in THP-1 monocytes, and also reduces MCP-1-induced chemotaxis in a manner dependent on SPT activity.","method":"PDZ domain protein array screen, co-immunoprecipitation, siRNA knockdown, enzyme activity assay, chemotaxis assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — PDZ array identification confirmed by co-IP in endogenous settings with functional enzyme and cell migration readouts","pmids":["19592499"],"is_preprint":false},{"year":2011,"finding":"SPTLC1 mutations p.S331F and p.A352V reduce SPT activity in vitro and increase deoxysphingoid base (1-deoxy-sphinganine, 1-deoxymethyl-sphinganine) levels in patients' plasma and in stably expressing HEK293T cells, confirming the substrate shift to L-alanine as a shared pathogenic mechanism across HSAN1 mutations.","method":"In vitro SPT activity assay, stable cell expression, plasma lipid mass spectrometry","journal":"Human mutation","confidence":"High","confidence_rationale":"Tier 2 — functional enzymatic assay combined with patient plasma validation and cell model","pmids":["21618344"],"is_preprint":false},{"year":2013,"finding":"SPTLC1 is phosphorylated at Tyr164 by the tyrosine kinase ABL, which inhibits SPT enzyme activity. Phosphoproteomic identification in ER microsomes, validated by in vitro kinase assay. Y164F mutation of SPTLC1 increased SPT activity, increased ceramide-driven apoptosis, and sensitized BCR-ABL-expressing cells to imatinib-induced cell death.","method":"Phosphoproteomic analysis, in vitro ABL kinase assay, site-directed mutagenesis (Y164F), SPT activity assay, apoptosis assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — phosphoproteomics plus in vitro kinase validation plus mutagenesis with functional enzyme and cell viability readouts","pmids":["23629659"],"is_preprint":false},{"year":2015,"finding":"Systematic enzymatic profiling of 11 SPTLC1 and 6 SPTLC2 HSAN1 mutants using isotope-labelling revealed that HSAN1 mutations cause increased 1-deoxysphingolipid synthesis without reducing canonical serine activity. A homology model based on prokaryotic SPT structure grouped mutations into structural clusters that predict clinical severity.","method":"Stable isotope labelling SPT activity assay, homology structural modeling, plasma sphingoid base profiling","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 1-2 — comprehensive enzymatic profiling with multiple mutants plus structural modeling, strong evidence for substrate-shift mechanism","pmids":["26681808"],"is_preprint":false},{"year":2015,"finding":"SPTLC2 Ser384 is a phosphorylation site (confirmed by isoelectric focusing); phosphomimetic S384D mutation increased 1-deoxysphingolipid formation, suggesting that phosphorylation at this position regulates SPT substrate specificity in wild-type enzyme.","method":"Isoelectric focusing, site-directed mutagenesis, SPT activity assay in HEK293 cells","journal":"Neuromolecular medicine","confidence":"Medium","confidence_rationale":"Tier 2 — phosphorylation confirmed biochemically, functional consequence assessed by mutagenesis in single lab","pmids":["25567748"],"is_preprint":false},{"year":2018,"finding":"The first transmembrane domain (TMD1) of Lcb1 (yeast SPTLC1 ortholog) is required for ORM protein binding to SPT. Loss of ORM binding abolishes ORM-dependent SPT oligomerization and partially alters SPT redistribution within the ER, establishing TMD1 as the ORM interaction interface that mediates homeostatic regulation.","method":"Co-immunoprecipitation, in vivo fluorescence imaging, yeast genetics, deletion/replacement mutagenesis of TMD1","journal":"Biochimica et biophysica acta. Molecular and cell biology of lipids","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP plus in vivo imaging plus genetic mutagenesis with multiple functional readouts","pmids":["30529276"],"is_preprint":false},{"year":2019,"finding":"Sptlc1 deletion in adult bone marrow cells causes defective myeloid differentiation, expansion of multipotent progenitors, and ER stress due to accumulation of fatty acid substrate. ER stress (also induced by thapsigargin or palmitate) independently compromises myeloid differentiation, establishing that sphingolipid biosynthesis via SPTLC1 is required for normal myeloid cell development.","method":"Conditional bone marrow knockout, chimeric mouse transplant assay, ER stress marker analysis, pharmacological ER stress induction","journal":"Blood advances","confidence":"High","confidence_rationale":"Tier 2 — clean conditional KO with defined cellular and molecular phenotypes including mechanistic ER stress link","pmids":["31751474"],"is_preprint":false},{"year":2021,"finding":"SPTLC1 ALS-linked variants (p.Ala20Ser, p.Ser331Tyr, p.Leu39del) are de novo mutations that disrupt the sphingolipid synthesis pathway, broadening the pathogenic mechanism of SPTLC1 mutations beyond sensory neuropathy to include motor neuron disease.","method":"Trio whole-exome sequencing, genetic analysis of de novo variants","journal":"JAMA neurology","confidence":"Medium","confidence_rationale":"Tier 3 — genetic identification only in this paper; mechanistic consequence inferred from known SPT biology","pmids":["34459874"],"is_preprint":false},{"year":2022,"finding":"Endothelial-specific knockout of SPTLC1 in mice reduces EC sphingolipid de novo synthesis, impairs lipid raft formation, decreases VEGF signaling efficiency, reduces EC proliferation and tip/stalk cell differentiation, causing delayed retinal vascular development and reduced neovascularization. Postnatal EC-specific deletion also rapidly reduces sphingolipid metabolites in plasma and peripheral organs but not CNS.","method":"Endothelial-specific conditional knockout (Cre-lox), retinal vascular imaging, VEGF signaling assay, lipid raft analysis, lipidomics","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — tissue-specific KO with multiple mechanistic readouts including lipid raft formation and growth factor signaling","pmids":["36197001"],"is_preprint":false},{"year":2023,"finding":"Novel SPTLC1 p.L38R mutation (in the first transmembrane domain/exon 2) impedes interaction with the regulatory ORMDL subunit, releasing homeostatic suppression of SPT activity. This results in globally increased sphingolipid levels and particularly elevated dihydrosphingolipids in patient plasma and p.L38R-expressing HEK293 cells.","method":"Plasma lipidomics, HEK293 cell expression, SPT activity assay, clinical/genetic characterization","journal":"Biochimica et biophysica acta. Molecular and cell biology of lipids","confidence":"Medium","confidence_rationale":"Tier 2-3 — functional assay in cells confirming ORMDL interaction loss as mechanism, single lab","pmids":["37348646"],"is_preprint":false},{"year":2025,"finding":"ATF4, induced by cocaine-triggered ER stress in D1-MSNs, directly targets the SPTLC1 promoter and upregulates SPTLC1 expression, remodeling sphingolipid metabolism. D1-MSN-specific knockdown of SPTLC1 markedly reduced cocaine-induced behavioral and neuroplastic changes, establishing an ATF4-SPTLC1 signaling axis in ER stress-driven neuroadaptation.","method":"Promoter analysis, functional validation, D1-MSN-specific viral knockdown, behavioral assays, sphingolipid synthesis assay","journal":"Frontiers in pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — promoter-binding analysis plus cell-type-specific KD with behavioral readout in single study","pmids":["41378204"],"is_preprint":false}],"current_model":"SPTLC1 is an ER-resident integral membrane protein that forms the catalytic heterodimeric core of serine palmitoyltransferase (SPT) together with SPTLC2/LCB2, where it contributes to a single active site at the subunit interface for PLP-dependent condensation of L-serine and palmitoyl-CoA to generate 3-ketosphinganine, the first committed step of sphingolipid biosynthesis; its N-terminal transmembrane domain mediates interaction with regulatory ORMDL proteins that control SPT oligomerization and activity, while its C-terminal PDZ motif binds Par3 to modulate activity and cell migration; SPTLC1 is phosphorylated at Tyr164 by ABL kinase to inhibit its activity; HSN1/HSAN1 disease mutations shift substrate specificity toward L-alanine to produce neurotoxic 1-deoxysphingolipids via a dominant gain-of-function mechanism, whereas ALS-linked mutations in the transmembrane domain disrupt ORMDL-mediated homeostatic regulation, causing unrestrained sphingolipid synthesis."},"narrative":{"teleology":[{"year":1991,"claim":"Identification of LCB1 as an SPT component resolved the genetic basis of a previously known enzyme-deficient yeast mutant and revealed that the sphingolipid biosynthetic enzyme belongs to the PLP-dependent alpha-oxoamine synthase family.","evidence":"Molecular cloning and complementation of lcb1-defective S. cerevisiae strains restoring SPT activity","pmids":["2066332"],"confidence":"High","gaps":["Whether LCB1 alone is sufficient for catalysis or requires additional subunits was unknown","Mammalian ortholog not yet cloned"]},{"year":1994,"claim":"Demonstrating that SPT requires co-overexpression of both LCB1 and LCB2 for elevated activity established the obligate heterodimeric architecture of the enzyme, resolving why single-subunit overexpression was insufficient.","evidence":"Yeast co-overexpression of LCB1 and LCB2 with enzyme activity measurements","pmids":["8058731"],"confidence":"High","gaps":["Stoichiometry and active-site architecture unresolved","Mammalian complex not yet characterized"]},{"year":1998,"claim":"Reciprocal co-immunoprecipitation of mammalian SPTLC1 and SPTLC2 with co-purifying SPT activity confirmed that the heterodimeric enzyme architecture is conserved from yeast to mammals, enabling biochemical studies in human cells.","evidence":"Co-IP and affinity pulldown of endogenous SPTLC1/2 from mammalian cells with activity assays","pmids":["9405408","9837968"],"confidence":"High","gaps":["Active-site location within the heterodimer not mapped","Membrane topology and ER retention mechanism unknown"]},{"year":2002,"claim":"Determining that SPTLC1 has a single N-terminal transmembrane domain with lumen-facing N-terminus and that it is required for SPTLC2 protein stability resolved both the membrane topology and the structural basis for obligate heterodimerization.","evidence":"Epitope-tag orientation analysis, subcellular fractionation, and SPTLC1-null CHO cell rescue","pmids":["12464627"],"confidence":"High","gaps":["ER retention signals not identified","Relationship of topology to catalytic mechanism unresolved"]},{"year":2002,"claim":"HSN1 mutations in SPTLC1 (C133Y/W, V144D) were shown to dominantly reduce SPT activity by ~50% and to cluster near the predicted PLP-binding site at the LCB1–LCB2 interface, establishing the first mechanistic link between SPTLC1 mutations, active-site disruption, and hereditary neuropathy.","evidence":"Yeast dominance tests, enzyme activity assays, structural modeling, co-IP confirming retained LCB2 interaction","pmids":["11242114","11242106","11781309"],"confidence":"High","gaps":["No high-resolution structure of the eukaryotic enzyme","Whether activity reduction alone explains neuronal toxicity was unclear"]},{"year":2009,"claim":"Discovery that HSN1 mutations shift SPT substrate specificity from serine to alanine/glycine, generating neurotoxic 1-deoxysphingolipids, fundamentally reframed the disease mechanism from simple loss-of-function to a toxic gain-of-function, and rescue by wild-type SPTLC1 overexpression validated the model in vivo.","evidence":"Transgenic mouse double-transgenic rescue, lipid mass spectrometry, bacterial SPT crystal structures of equivalent mutations","pmids":["19923297","19376777"],"confidence":"High","gaps":["No crystal structure of mammalian SPT to confirm active-site rearrangement","Downstream mechanisms of deoxysphingolipid neurotoxicity unknown"]},{"year":2009,"claim":"Identification of a C-terminal PDZ-binding motif in SPTLC1 that recruits Par3 to regulate SPT activity and MCP-1-driven chemotaxis revealed a previously unrecognized link between sphingolipid biosynthesis and cell polarity/migration signaling.","evidence":"PDZ domain array screen, co-IP of endogenous Par3–SPTLC1/2 complex, siRNA knockdown reducing SPT activity and chemotaxis in monocytes","pmids":["19592499"],"confidence":"High","gaps":["Whether Par3 modulates SPT at the active-site level or via localization is unknown","Relevance in non-immune cell types not tested"]},{"year":2013,"claim":"ABL kinase-dependent phosphorylation of SPTLC1 at Tyr164 was shown to inhibit SPT activity, connecting sphingolipid metabolism to oncogenic tyrosine kinase signaling and explaining a mechanism by which BCR-ABL protects cells from ceramide-driven apoptosis.","evidence":"ER microsome phosphoproteomics, in vitro ABL kinase assay, Y164F mutagenesis with SPT activity and apoptosis readouts","pmids":["23629659"],"confidence":"High","gaps":["Whether other kinases phosphorylate this site in vivo","Structural basis for how Y164 phosphorylation inhibits catalysis unknown"]},{"year":2015,"claim":"Systematic profiling of 11 HSAN1 SPTLC1 mutants confirmed that deoxysphingolipid overproduction occurs without proportional loss of canonical serine-based activity, establishing the substrate-shift as a universal feature of HSAN1 mutations and enabling structure–severity correlations.","evidence":"Stable isotope labelling SPT activity assay across mutant panel, homology modeling","pmids":["26681808"],"confidence":"High","gaps":["No mammalian SPT structure for definitive active-site mapping","Mutant-specific differences in substrate preference not fully explained structurally"]},{"year":2018,"claim":"Mapping the ORMDL interaction interface to the first transmembrane domain of Lcb1/SPTLC1 resolved how homeostatic sphingolipid sensing controls SPT: ORM proteins bind the TMD to promote SPT oligomerization and suppress activity, a mechanism later found disrupted in ALS-linked mutations.","evidence":"TMD1 deletion/replacement mutagenesis in yeast, co-IP for ORM binding, fluorescence imaging of SPT oligomers","pmids":["30529276"],"confidence":"High","gaps":["High-resolution structure of the SPTLC1–ORMDL complex not yet available","Whether ORMDL regulation involves allosteric or steric inhibition unresolved"]},{"year":2022,"claim":"Endothelial-specific SPTLC1 knockout demonstrated that de novo sphingolipid synthesis is essential for lipid raft integrity, VEGF receptor signaling, and developmental angiogenesis, extending SPTLC1 function beyond neurobiology to vascular biology.","evidence":"EC-specific Cre-lox knockout in mice, retinal vascular imaging, VEGF signaling assays, lipidomics","pmids":["36197001"],"confidence":"High","gaps":["Whether sphingolipid species other than ceramide mediate the vascular phenotype is unclear","Relevance to adult pathological angiogenesis not tested"]},{"year":2023,"claim":"An ALS-linked SPTLC1 p.L38R mutation in the first transmembrane domain was shown to disrupt ORMDL binding and release homeostatic suppression, causing globally elevated sphingolipid synthesis — establishing a gain-of-function mechanism mechanistically distinct from the HSAN1 substrate shift.","evidence":"Plasma lipidomics, HEK293 expression with SPT activity assay, genetic characterization","pmids":["37348646"],"confidence":"Medium","gaps":["ORMDL interaction loss inferred from activity data rather than direct binding assay","Whether all ALS-linked TMD mutations share this mechanism needs confirmation","No in vivo model for SPTLC1-ALS"]},{"year":null,"claim":"A high-resolution structure of the human SPTLC1–SPTLC2–ORMDL holoenzyme complex and the molecular basis for how TMD mutations versus active-site-proximal mutations produce mechanistically distinct disease outcomes remain to be determined.","evidence":"","pmids":[],"confidence":"Low","gaps":["No cryo-EM or crystal structure of mammalian SPT holoenzyme with ORMDL bound","Mechanism of 1-deoxysphingolipid neurotoxicity at the cellular level unresolved","Whether pharmacological correction of substrate specificity or ORMDL binding is therapeutically feasible is untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1,2,3,5,9,14]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[8,16,20]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[6,13]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,1,2,9,14,19]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[4,9,12,18,20]}],"complexes":["SPT (serine palmitoyltransferase heterodimer: SPTLC1-SPTLC2)","SPT-ORMDL regulatory complex"],"partners":["SPTLC2","ORMDL3","PARD3","ABCA1","ABL1"],"other_free_text":[]},"mechanistic_narrative":"SPTLC1 encodes the LCB1 subunit of serine palmitoyltransferase (SPT), an ER-resident heterodimeric enzyme that catalyzes the PLP-dependent condensation of L-serine with palmitoyl-CoA to form 3-ketosphinganine — the first and rate-limiting step of de novo sphingolipid biosynthesis [PMID:2066332, PMID:8058731, PMID:9405408]. SPTLC1 is an integral ER membrane protein with a single N-terminal transmembrane domain that mediates interaction with ORMDL regulatory proteins controlling SPT oligomerization and homeostatic activity, while its cytosolic C-terminal PDZ-binding motif recruits Par3 to modulate enzyme activity and cell migration [PMID:12464627, PMID:30529276, PMID:19592499]. SPTLC1 activity is negatively regulated by ABL kinase-mediated phosphorylation at Tyr164, and conditional deletion in specific tissues disrupts lipid raft formation, VEGF signaling in endothelial cells, and myeloid differentiation in bone marrow progenitors [PMID:23629659, PMID:36197001, PMID:31751474]. Missense mutations in SPTLC1 cause hereditary sensory and autonomic neuropathy type I (HSAN1) through a gain-of-function shift in substrate specificity toward L-alanine, generating neurotoxic 1-deoxysphingolipids, whereas transmembrane-domain mutations linked to ALS disrupt ORMDL-mediated suppression and cause unrestrained sphingolipid synthesis [PMID:19923297, PMID:26681808, PMID:37348646]."},"prefetch_data":{"uniprot":{"accession":"O15269","full_name":"Serine palmitoyltransferase 1","aliases":["Long chain base biosynthesis protein 1","LCB 1","Serine-palmitoyl-CoA transferase 1","SPT 1","SPT1"],"length_aa":473,"mass_kda":52.7,"function":"Component of the serine palmitoyltransferase multisubunit enzyme (SPT) that catalyzes the initial and rate-limiting step in sphingolipid biosynthesis by condensing L-serine and activated acyl-CoA (most commonly palmitoyl-CoA) to form long-chain bases. The SPT complex is also composed of SPTLC2 or SPTLC3 and SPTSSA or SPTSSB. Within this complex, the heterodimer with SPTLC2 or SPTLC3 forms the catalytic core (PubMed:19416851, PubMed:33558762, PubMed:36170811). The composition of the serine palmitoyltransferase (SPT) complex determines the substrate preference (PubMed:19416851, PubMed:33558762). The SPTLC1-SPTLC2-SPTSSA complex shows a strong preference for C16-CoA substrate, while the SPTLC1-SPTLC3-SPTSSA isozyme uses both C14-CoA and C16-CoA as substrates, with a slight preference for C14-CoA (PubMed:19416851, PubMed:19648650). The SPTLC1-SPTLC2-SPTSSB complex shows a strong preference for C18-CoA substrate, while the SPTLC1-SPTLC3-SPTSSB isozyme displays an ability to use a broader range of acyl-CoAs, without apparent preference (PubMed:19416851, PubMed:19648650, PubMed:33558761, PubMed:33558762). Required for adipocyte cell viability and metabolic homeostasis (By similarity)","subcellular_location":"Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/O15269/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/SPTLC1","classification":"Common Essential","n_dependent_lines":1022,"n_total_lines":1208,"dependency_fraction":0.8460264900662252},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000090054","cell_line_id":"CID000458","localizations":[{"compartment":"er","grade":3},{"compartment":"vesicles","grade":1}],"interactors":[{"gene":"SPTLC2","stoichiometry":10.0},{"gene":"COPG2","stoichiometry":10.0},{"gene":"COPB2","stoichiometry":4.0},{"gene":"ARCN1","stoichiometry":4.0},{"gene":"COPB1","stoichiometry":4.0},{"gene":"COPA","stoichiometry":4.0},{"gene":"CANX","stoichiometry":0.2},{"gene":"DDOST","stoichiometry":0.2},{"gene":"OST4","stoichiometry":0.2},{"gene":"PGRMC1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID000458","total_profiled":1310},"omim":[{"mim_id":"620285","title":"AMYOTROPHIC LATERAL SCLEROSIS 27, JUVENILE; ALS27","url":"https://www.omim.org/entry/620285"},{"mim_id":"613640","title":"NEUROPATHY, HEREDITARY SENSORY AND AUTONOMIC, TYPE IC; HSAN1C","url":"https://www.omim.org/entry/613640"},{"mim_id":"613540","title":"SERINE PALMITOYLTRANSFERASE, SMALL SUBUNIT, A; SPTSSA","url":"https://www.omim.org/entry/613540"},{"mim_id":"611120","title":"SERINE PALMITOYLTRANSFERASE, LONG-CHAIN BASE SUBUNIT 3; SPTLC3","url":"https://www.omim.org/entry/611120"},{"mim_id":"610412","title":"SERINE PALMITOYLTRANSFERASE, SMALL SUBUNIT, B; SPTSSB","url":"https://www.omim.org/entry/610412"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Endoplasmic reticulum","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SPTLC1"},"hgnc":{"alias_symbol":["LCB1","SPTI","HSAN1","hLCB1"],"prev_symbol":["HSN1"]},"alphafold":{"accession":"O15269","domains":[{"cath_id":"3.90.1150.10","chopping":"83-112_362-473","consensus_level":"high","plddt":96.7444,"start":83,"end":473},{"cath_id":"3.40.640.10","chopping":"116-359","consensus_level":"high","plddt":97.379,"start":116,"end":359}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O15269","model_url":"https://alphafold.ebi.ac.uk/files/AF-O15269-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O15269-F1-predicted_aligned_error_v6.png","plddt_mean":94.12},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SPTLC1","jax_strain_url":"https://www.jax.org/strain/search?query=SPTLC1"},"sequence":{"accession":"O15269","fasta_url":"https://rest.uniprot.org/uniprotkb/O15269.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O15269/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O15269"}},"corpus_meta":[{"pmid":"11242114","id":"PMC_11242114","title":"Mutations in SPTLC1, encoding serine palmitoyltransferase, long chain base subunit-1, cause hereditary sensory neuropathy type I.","date":"2001","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/11242114","citation_count":330,"is_preprint":false},{"pmid":"11242106","id":"PMC_11242106","title":"SPTLC1 is mutated in hereditary sensory neuropathy, type 1.","date":"2001","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/11242106","citation_count":233,"is_preprint":false},{"pmid":"8058731","id":"PMC_8058731","title":"The LCB2 gene of Saccharomyces and the related LCB1 gene encode subunits of serine palmitoyltransferase, the initial enzyme in sphingolipid synthesis.","date":"1994","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/8058731","citation_count":187,"is_preprint":false},{"pmid":"2066332","id":"PMC_2066332","title":"Cloning and characterization of LCB1, a Saccharomyces gene required for biosynthesis of the long-chain base component of sphingolipids.","date":"1991","source":"Journal of bacteriology","url":"https://pubmed.ncbi.nlm.nih.gov/2066332","citation_count":175,"is_preprint":false},{"pmid":"9837968","id":"PMC_9837968","title":"Mammalian cell mutants resistant to a sphingomyelin-directed cytolysin. 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The protein has predicted transmembrane helices and homology to pyridoxal 5'-phosphate-dependent alpha-oxoamine synthases.\",\n      \"method\": \"Molecular cloning, yeast complementation assay, sequence analysis\",\n      \"journal\": \"Journal of bacteriology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic complementation with functional enzyme activity restoration, foundational study replicated by subsequent work\",\n      \"pmids\": [\"2066332\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Yeast LCB1 and LCB2 (orthologs of SPTLC1 and SPTLC2) both encode subunits of serine palmitoyltransferase; overproduction of both together is required for increased SPT activity, establishing a heterodimeric complex.\",\n      \"method\": \"Yeast genetics, overexpression, enzyme activity assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional complementation with activity assay; replicated across multiple studies\",\n      \"pmids\": [\"8058731\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Mammalian LCB1 (SPTLC1) is a component of serine palmitoyltransferase; transfection of SPT-defective CHO cells (LY-B) with CHO LCB1 cDNA restored both SPT activity and de novo sphingolipid synthesis; SPT activity co-purified with His6-tagged LCB1 on Ni2+ resin.\",\n      \"method\": \"CHO cell complementation, affinity purification, enzyme activity assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct biochemical complementation of enzyme-deficient cells with activity assay and affinity purification\",\n      \"pmids\": [\"9405408\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"SPTLC1 (LCB1) and LCB2 form a heterodimeric SPT complex in mammalian cells; anti-LCB2 antibody co-immunoprecipitated both SPT activity and LCB1; affinity-tagged LCB1 pulled down endogenous LCB2, demonstrating direct physical interaction.\",\n      \"method\": \"Co-immunoprecipitation, affinity pulldown, enzyme activity assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reciprocal co-IP and affinity pulldown demonstrating complex formation, replicated independently\",\n      \"pmids\": [\"9837968\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Missense mutations in SPTLC1 (C133Y, C133W, V144D) cause hereditary sensory neuropathy type I (HSN1) and are associated with increased de novo glucosyl ceramide synthesis in lymphoblast cell lines, suggesting altered SPT enzyme activity.\",\n      \"method\": \"Mutation screening, lipid biosynthesis assay in patient lymphoblasts\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional biochemical readout in patient cells; replicated by second simultaneous independent study\",\n      \"pmids\": [\"11242114\", \"11242106\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"HSN1-corresponding mutations in yeast LCB1 (equivalent to SPTLC1) dominantly reduce SPT activity by ~50% when co-expressed with wild-type, establishing a dominant-negative mechanism. The mutant Lcb1p retains interaction with Lcb2p. Modeling indicates SPT has a single active site at the LCB1-LCB2 interface, and the mutations reside near the PLP-binding lysine in LCB2.\",\n      \"method\": \"Yeast genetics, enzyme activity assay, co-expression dominance test, structural modeling, co-immunoprecipitation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including enzyme assay, dominance test, and protein interaction analysis\",\n      \"pmids\": [\"11781309\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"SPTLC1 is an integral ER membrane protein with a single transmembrane domain near the N-terminus, with its N-terminus oriented to the ER lumen and C-terminus to the cytosol. SPTLC1 is required for maintenance of LCB2 protein levels; LY-B cells lacking SPTLC1 have drastically reduced LCB2, which is restored by SPTLC1 transfection.\",\n      \"method\": \"Indirect immunofluorescence with N- and C-terminal epitope tags, subcellular fractionation, Western blotting, stable transfection of LY-B cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment with functional consequence; multiple epitope-tag orientations tested\",\n      \"pmids\": [\"12464627\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Transgenic mice overexpressing mutant SPTLC1 (C133W) dominantly inhibit SPT activity in vivo and develop age-dependent peripheral neuropathy with loss of myelinated axons and myelin thinning, confirming the dominant inhibition mechanism in a mammalian model.\",\n      \"method\": \"Transgenic mouse model, SPT activity assay, neuropathological analysis, axon counting\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic model with biochemical and pathological readouts; corroborated by cell culture and yeast studies\",\n      \"pmids\": [\"16210380\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SPTLC1 (but not SPTLC2) physically interacts with ABCA1 and negatively regulates ABCA1's cholesterol efflux activity by blocking ABCA1 exit from the ER. This interaction was confirmed by co-immunoprecipitation in THP-1 macrophages and mouse liver. siRNA knockdown of SPTLC1 and pharmacologic inhibition with myriocin both increased ABCA1 efflux activity ~60%.\",\n      \"method\": \"Affinity purification/mass spectrometry, co-immunoprecipitation, siRNA knockdown, dominant-negative SPTLC1 mutants, ABCA1 trafficking assay\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP in endogenous settings plus functional assays with multiple perturbations\",\n      \"pmids\": [\"18484747\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"SPTLC1 HSN1 mutations (C133W) alter the amino acid substrate selectivity of SPT such that palmitate is condensed with alanine and glycine in addition to serine, generating neurotoxic deoxysphingoid bases (1-deoxy-sphingolipids). Overexpression of wild-type SPTLC1 rescues the neuropathy phenotype and reduces deoxysphingolipid levels in a mouse model.\",\n      \"method\": \"Transgenic mouse genetics (double transgenic rescue), lipid mass spectrometry, SPT activity assay\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — double transgenic rescue experiment with biochemical validation of substrate shift mechanism\",\n      \"pmids\": [\"19923297\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Crystal structures and kinetic analysis of bacterial SPT (Sphingomonas paucimobilis) mimicking HSN1 mutations (N100Y, N100W equivalent to SPTLC1 C133Y/W) reveal structural changes around the active site that impair movement of catalytic Arg378, alter PLP chemistry, and destabilize a quinonoid intermediate, providing mechanistic insight into how disease mutations perturb enzyme activity.\",\n      \"method\": \"X-ray crystallography, kinetic assays, spectroscopic analysis, site-directed mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus kinetics plus mutagenesis in bacterial ortholog that directly models human disease mutations\",\n      \"pmids\": [\"19376777\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"SPTLC1 interacts with the PDZ protein Par3 via a conserved C-terminal type II PDZ-binding motif. Par3 associates with the SPTLC1/2 holoenzyme, and Par3 knockdown by siRNA reduces SPT activity and de novo ceramide synthesis by ~40% in THP-1 monocytes, and also reduces MCP-1-induced chemotaxis in a manner dependent on SPT activity.\",\n      \"method\": \"PDZ domain protein array screen, co-immunoprecipitation, siRNA knockdown, enzyme activity assay, chemotaxis assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — PDZ array identification confirmed by co-IP in endogenous settings with functional enzyme and cell migration readouts\",\n      \"pmids\": [\"19592499\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"SPTLC1 mutations p.S331F and p.A352V reduce SPT activity in vitro and increase deoxysphingoid base (1-deoxy-sphinganine, 1-deoxymethyl-sphinganine) levels in patients' plasma and in stably expressing HEK293T cells, confirming the substrate shift to L-alanine as a shared pathogenic mechanism across HSAN1 mutations.\",\n      \"method\": \"In vitro SPT activity assay, stable cell expression, plasma lipid mass spectrometry\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional enzymatic assay combined with patient plasma validation and cell model\",\n      \"pmids\": [\"21618344\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SPTLC1 is phosphorylated at Tyr164 by the tyrosine kinase ABL, which inhibits SPT enzyme activity. Phosphoproteomic identification in ER microsomes, validated by in vitro kinase assay. Y164F mutation of SPTLC1 increased SPT activity, increased ceramide-driven apoptosis, and sensitized BCR-ABL-expressing cells to imatinib-induced cell death.\",\n      \"method\": \"Phosphoproteomic analysis, in vitro ABL kinase assay, site-directed mutagenesis (Y164F), SPT activity assay, apoptosis assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — phosphoproteomics plus in vitro kinase validation plus mutagenesis with functional enzyme and cell viability readouts\",\n      \"pmids\": [\"23629659\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Systematic enzymatic profiling of 11 SPTLC1 and 6 SPTLC2 HSAN1 mutants using isotope-labelling revealed that HSAN1 mutations cause increased 1-deoxysphingolipid synthesis without reducing canonical serine activity. A homology model based on prokaryotic SPT structure grouped mutations into structural clusters that predict clinical severity.\",\n      \"method\": \"Stable isotope labelling SPT activity assay, homology structural modeling, plasma sphingoid base profiling\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — comprehensive enzymatic profiling with multiple mutants plus structural modeling, strong evidence for substrate-shift mechanism\",\n      \"pmids\": [\"26681808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"SPTLC2 Ser384 is a phosphorylation site (confirmed by isoelectric focusing); phosphomimetic S384D mutation increased 1-deoxysphingolipid formation, suggesting that phosphorylation at this position regulates SPT substrate specificity in wild-type enzyme.\",\n      \"method\": \"Isoelectric focusing, site-directed mutagenesis, SPT activity assay in HEK293 cells\",\n      \"journal\": \"Neuromolecular medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — phosphorylation confirmed biochemically, functional consequence assessed by mutagenesis in single lab\",\n      \"pmids\": [\"25567748\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The first transmembrane domain (TMD1) of Lcb1 (yeast SPTLC1 ortholog) is required for ORM protein binding to SPT. Loss of ORM binding abolishes ORM-dependent SPT oligomerization and partially alters SPT redistribution within the ER, establishing TMD1 as the ORM interaction interface that mediates homeostatic regulation.\",\n      \"method\": \"Co-immunoprecipitation, in vivo fluorescence imaging, yeast genetics, deletion/replacement mutagenesis of TMD1\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular and cell biology of lipids\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP plus in vivo imaging plus genetic mutagenesis with multiple functional readouts\",\n      \"pmids\": [\"30529276\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Sptlc1 deletion in adult bone marrow cells causes defective myeloid differentiation, expansion of multipotent progenitors, and ER stress due to accumulation of fatty acid substrate. ER stress (also induced by thapsigargin or palmitate) independently compromises myeloid differentiation, establishing that sphingolipid biosynthesis via SPTLC1 is required for normal myeloid cell development.\",\n      \"method\": \"Conditional bone marrow knockout, chimeric mouse transplant assay, ER stress marker analysis, pharmacological ER stress induction\",\n      \"journal\": \"Blood advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with defined cellular and molecular phenotypes including mechanistic ER stress link\",\n      \"pmids\": [\"31751474\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SPTLC1 ALS-linked variants (p.Ala20Ser, p.Ser331Tyr, p.Leu39del) are de novo mutations that disrupt the sphingolipid synthesis pathway, broadening the pathogenic mechanism of SPTLC1 mutations beyond sensory neuropathy to include motor neuron disease.\",\n      \"method\": \"Trio whole-exome sequencing, genetic analysis of de novo variants\",\n      \"journal\": \"JAMA neurology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — genetic identification only in this paper; mechanistic consequence inferred from known SPT biology\",\n      \"pmids\": [\"34459874\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Endothelial-specific knockout of SPTLC1 in mice reduces EC sphingolipid de novo synthesis, impairs lipid raft formation, decreases VEGF signaling efficiency, reduces EC proliferation and tip/stalk cell differentiation, causing delayed retinal vascular development and reduced neovascularization. Postnatal EC-specific deletion also rapidly reduces sphingolipid metabolites in plasma and peripheral organs but not CNS.\",\n      \"method\": \"Endothelial-specific conditional knockout (Cre-lox), retinal vascular imaging, VEGF signaling assay, lipid raft analysis, lipidomics\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — tissue-specific KO with multiple mechanistic readouts including lipid raft formation and growth factor signaling\",\n      \"pmids\": [\"36197001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Novel SPTLC1 p.L38R mutation (in the first transmembrane domain/exon 2) impedes interaction with the regulatory ORMDL subunit, releasing homeostatic suppression of SPT activity. This results in globally increased sphingolipid levels and particularly elevated dihydrosphingolipids in patient plasma and p.L38R-expressing HEK293 cells.\",\n      \"method\": \"Plasma lipidomics, HEK293 cell expression, SPT activity assay, clinical/genetic characterization\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular and cell biology of lipids\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — functional assay in cells confirming ORMDL interaction loss as mechanism, single lab\",\n      \"pmids\": [\"37348646\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ATF4, induced by cocaine-triggered ER stress in D1-MSNs, directly targets the SPTLC1 promoter and upregulates SPTLC1 expression, remodeling sphingolipid metabolism. D1-MSN-specific knockdown of SPTLC1 markedly reduced cocaine-induced behavioral and neuroplastic changes, establishing an ATF4-SPTLC1 signaling axis in ER stress-driven neuroadaptation.\",\n      \"method\": \"Promoter analysis, functional validation, D1-MSN-specific viral knockdown, behavioral assays, sphingolipid synthesis assay\",\n      \"journal\": \"Frontiers in pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — promoter-binding analysis plus cell-type-specific KD with behavioral readout in single study\",\n      \"pmids\": [\"41378204\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SPTLC1 is an ER-resident integral membrane protein that forms the catalytic heterodimeric core of serine palmitoyltransferase (SPT) together with SPTLC2/LCB2, where it contributes to a single active site at the subunit interface for PLP-dependent condensation of L-serine and palmitoyl-CoA to generate 3-ketosphinganine, the first committed step of sphingolipid biosynthesis; its N-terminal transmembrane domain mediates interaction with regulatory ORMDL proteins that control SPT oligomerization and activity, while its C-terminal PDZ motif binds Par3 to modulate activity and cell migration; SPTLC1 is phosphorylated at Tyr164 by ABL kinase to inhibit its activity; HSN1/HSAN1 disease mutations shift substrate specificity toward L-alanine to produce neurotoxic 1-deoxysphingolipids via a dominant gain-of-function mechanism, whereas ALS-linked mutations in the transmembrane domain disrupt ORMDL-mediated homeostatic regulation, causing unrestrained sphingolipid synthesis.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SPTLC1 encodes the LCB1 subunit of serine palmitoyltransferase (SPT), an ER-resident heterodimeric enzyme that catalyzes the PLP-dependent condensation of L-serine with palmitoyl-CoA to form 3-ketosphinganine — the first and rate-limiting step of de novo sphingolipid biosynthesis [PMID:2066332, PMID:8058731, PMID:9405408]. SPTLC1 is an integral ER membrane protein with a single N-terminal transmembrane domain that mediates interaction with ORMDL regulatory proteins controlling SPT oligomerization and homeostatic activity, while its cytosolic C-terminal PDZ-binding motif recruits Par3 to modulate enzyme activity and cell migration [PMID:12464627, PMID:30529276, PMID:19592499]. SPTLC1 activity is negatively regulated by ABL kinase-mediated phosphorylation at Tyr164, and conditional deletion in specific tissues disrupts lipid raft formation, VEGF signaling in endothelial cells, and myeloid differentiation in bone marrow progenitors [PMID:23629659, PMID:36197001, PMID:31751474]. Missense mutations in SPTLC1 cause hereditary sensory and autonomic neuropathy type I (HSAN1) through a gain-of-function shift in substrate specificity toward L-alanine, generating neurotoxic 1-deoxysphingolipids, whereas transmembrane-domain mutations linked to ALS disrupt ORMDL-mediated suppression and cause unrestrained sphingolipid synthesis [PMID:19923297, PMID:26681808, PMID:37348646].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Identification of LCB1 as an SPT component resolved the genetic basis of a previously known enzyme-deficient yeast mutant and revealed that the sphingolipid biosynthetic enzyme belongs to the PLP-dependent alpha-oxoamine synthase family.\",\n      \"evidence\": \"Molecular cloning and complementation of lcb1-defective S. cerevisiae strains restoring SPT activity\",\n      \"pmids\": [\"2066332\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether LCB1 alone is sufficient for catalysis or requires additional subunits was unknown\", \"Mammalian ortholog not yet cloned\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Demonstrating that SPT requires co-overexpression of both LCB1 and LCB2 for elevated activity established the obligate heterodimeric architecture of the enzyme, resolving why single-subunit overexpression was insufficient.\",\n      \"evidence\": \"Yeast co-overexpression of LCB1 and LCB2 with enzyme activity measurements\",\n      \"pmids\": [\"8058731\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and active-site architecture unresolved\", \"Mammalian complex not yet characterized\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Reciprocal co-immunoprecipitation of mammalian SPTLC1 and SPTLC2 with co-purifying SPT activity confirmed that the heterodimeric enzyme architecture is conserved from yeast to mammals, enabling biochemical studies in human cells.\",\n      \"evidence\": \"Co-IP and affinity pulldown of endogenous SPTLC1/2 from mammalian cells with activity assays\",\n      \"pmids\": [\"9405408\", \"9837968\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Active-site location within the heterodimer not mapped\", \"Membrane topology and ER retention mechanism unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Determining that SPTLC1 has a single N-terminal transmembrane domain with lumen-facing N-terminus and that it is required for SPTLC2 protein stability resolved both the membrane topology and the structural basis for obligate heterodimerization.\",\n      \"evidence\": \"Epitope-tag orientation analysis, subcellular fractionation, and SPTLC1-null CHO cell rescue\",\n      \"pmids\": [\"12464627\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"ER retention signals not identified\", \"Relationship of topology to catalytic mechanism unresolved\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"HSN1 mutations in SPTLC1 (C133Y/W, V144D) were shown to dominantly reduce SPT activity by ~50% and to cluster near the predicted PLP-binding site at the LCB1–LCB2 interface, establishing the first mechanistic link between SPTLC1 mutations, active-site disruption, and hereditary neuropathy.\",\n      \"evidence\": \"Yeast dominance tests, enzyme activity assays, structural modeling, co-IP confirming retained LCB2 interaction\",\n      \"pmids\": [\"11242114\", \"11242106\", \"11781309\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure of the eukaryotic enzyme\", \"Whether activity reduction alone explains neuronal toxicity was unclear\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Discovery that HSN1 mutations shift SPT substrate specificity from serine to alanine/glycine, generating neurotoxic 1-deoxysphingolipids, fundamentally reframed the disease mechanism from simple loss-of-function to a toxic gain-of-function, and rescue by wild-type SPTLC1 overexpression validated the model in vivo.\",\n      \"evidence\": \"Transgenic mouse double-transgenic rescue, lipid mass spectrometry, bacterial SPT crystal structures of equivalent mutations\",\n      \"pmids\": [\"19923297\", \"19376777\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No crystal structure of mammalian SPT to confirm active-site rearrangement\", \"Downstream mechanisms of deoxysphingolipid neurotoxicity unknown\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Identification of a C-terminal PDZ-binding motif in SPTLC1 that recruits Par3 to regulate SPT activity and MCP-1-driven chemotaxis revealed a previously unrecognized link between sphingolipid biosynthesis and cell polarity/migration signaling.\",\n      \"evidence\": \"PDZ domain array screen, co-IP of endogenous Par3–SPTLC1/2 complex, siRNA knockdown reducing SPT activity and chemotaxis in monocytes\",\n      \"pmids\": [\"19592499\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Par3 modulates SPT at the active-site level or via localization is unknown\", \"Relevance in non-immune cell types not tested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"ABL kinase-dependent phosphorylation of SPTLC1 at Tyr164 was shown to inhibit SPT activity, connecting sphingolipid metabolism to oncogenic tyrosine kinase signaling and explaining a mechanism by which BCR-ABL protects cells from ceramide-driven apoptosis.\",\n      \"evidence\": \"ER microsome phosphoproteomics, in vitro ABL kinase assay, Y164F mutagenesis with SPT activity and apoptosis readouts\",\n      \"pmids\": [\"23629659\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other kinases phosphorylate this site in vivo\", \"Structural basis for how Y164 phosphorylation inhibits catalysis unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Systematic profiling of 11 HSAN1 SPTLC1 mutants confirmed that deoxysphingolipid overproduction occurs without proportional loss of canonical serine-based activity, establishing the substrate-shift as a universal feature of HSAN1 mutations and enabling structure–severity correlations.\",\n      \"evidence\": \"Stable isotope labelling SPT activity assay across mutant panel, homology modeling\",\n      \"pmids\": [\"26681808\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No mammalian SPT structure for definitive active-site mapping\", \"Mutant-specific differences in substrate preference not fully explained structurally\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Mapping the ORMDL interaction interface to the first transmembrane domain of Lcb1/SPTLC1 resolved how homeostatic sphingolipid sensing controls SPT: ORM proteins bind the TMD to promote SPT oligomerization and suppress activity, a mechanism later found disrupted in ALS-linked mutations.\",\n      \"evidence\": \"TMD1 deletion/replacement mutagenesis in yeast, co-IP for ORM binding, fluorescence imaging of SPT oligomers\",\n      \"pmids\": [\"30529276\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"High-resolution structure of the SPTLC1–ORMDL complex not yet available\", \"Whether ORMDL regulation involves allosteric or steric inhibition unresolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Endothelial-specific SPTLC1 knockout demonstrated that de novo sphingolipid synthesis is essential for lipid raft integrity, VEGF receptor signaling, and developmental angiogenesis, extending SPTLC1 function beyond neurobiology to vascular biology.\",\n      \"evidence\": \"EC-specific Cre-lox knockout in mice, retinal vascular imaging, VEGF signaling assays, lipidomics\",\n      \"pmids\": [\"36197001\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether sphingolipid species other than ceramide mediate the vascular phenotype is unclear\", \"Relevance to adult pathological angiogenesis not tested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"An ALS-linked SPTLC1 p.L38R mutation in the first transmembrane domain was shown to disrupt ORMDL binding and release homeostatic suppression, causing globally elevated sphingolipid synthesis — establishing a gain-of-function mechanism mechanistically distinct from the HSAN1 substrate shift.\",\n      \"evidence\": \"Plasma lipidomics, HEK293 expression with SPT activity assay, genetic characterization\",\n      \"pmids\": [\"37348646\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"ORMDL interaction loss inferred from activity data rather than direct binding assay\", \"Whether all ALS-linked TMD mutations share this mechanism needs confirmation\", \"No in vivo model for SPTLC1-ALS\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A high-resolution structure of the human SPTLC1–SPTLC2–ORMDL holoenzyme complex and the molecular basis for how TMD mutations versus active-site-proximal mutations produce mechanistically distinct disease outcomes remain to be determined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No cryo-EM or crystal structure of mammalian SPT holoenzyme with ORMDL bound\", \"Mechanism of 1-deoxysphingolipid neurotoxicity at the cellular level unresolved\", \"Whether pharmacological correction of substrate specificity or ORMDL binding is therapeutically feasible is untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1, 2, 3, 5, 9, 14]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [8, 16, 20]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [6, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 1, 2, 9, 14, 19]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [4, 9, 12, 18, 20]}\n    ],\n    \"complexes\": [\n      \"SPT (serine palmitoyltransferase heterodimer: SPTLC1-SPTLC2)\",\n      \"SPT-ORMDL regulatory complex\"\n    ],\n    \"partners\": [\n      \"SPTLC2\",\n      \"ORMDL3\",\n      \"PARD3\",\n      \"ABCA1\",\n      \"ABL1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}