SPTLC2

SPTLC2 is the PLP-binding catalytic subunit of serine palmitoyltransferase (SPT), the heterodimeric enzyme that catalyzes the rate-limiting condensation of serine and palmitoyl-CoA to 3-ketosphinganine in de novo sphingolipid biosynthesis. SPTLC2 forms an obligate heterodimer with SPTLC1, with the active site residing at the subunit interface where a conserved lysine in SPTLC2 forms a Schiff base with the PLP cofactor (PMID:8058731, PMID:9837968, PMID:11781309). Missense mutations in the PLP-binding region of SPTLC2 cause hereditary sensory and autonomic neuropathy type I (HSAN-I) by shifting substrate specificity from serine toward alanine, resulting in accumulation of neurotoxic 1-deoxysphingolipids, whereas gain-of-function variants in the transmembrane domain near the ORMDL3-interaction region cause juvenile ALS through unrestrained sphingolipid overproduction due to loss of ORMDL3-mediated feedback inhibition (PMID:20920666, PMID:38041684, PMID:40849231). SPTLC2 transcription is induced by NF-κB during inflammation and by sXBP1 during ER stress, and SPTLC2-dependent sphingolipid synthesis is required for CD8+ T-cell metabolic fitness and pancreatic β-cell survival (PMID:21167294, PMID:35513574, PMID:30952607).

1. 1994 High

   Establishing that LCB2 encodes a required subunit of SPT resolved the gene identity behind the first committed step of sphingolipid biosynthesis, showing that both LCB1 and LCB2 are needed for activity.

   Evidence Yeast genetics with LCB1/LCB2 co-expression and enzymatic activity assays

   PMID:8058731

   Open questions at the time

   • Mammalian ortholog not yet characterized
   • Subunit stoichiometry unknown
   • Active-site architecture unresolved
2. 1998 High

   Demonstrating that SPTLC2 and SPTLC1 form a physical heterodimer essential for SPT activity in mammalian cells established the minimal enzyme composition and confirmed the yeast paradigm in mammals.

   Evidence Reciprocal co-immunoprecipitation and SPT activity assays in CHO cell mutants with cDNA rescue

   PMID:9837968

   Open questions at the time

   • No structural information on the heterodimer
   • Contribution of each subunit to catalysis not resolved
3. 2002 High

   Mapping the PLP cofactor Schiff-base linkage to a specific lysine in LCB2 and showing that HSAN1-like mutations cluster near this residue resolved how the active site spans the subunit interface and identified the catalytic residues.

   Evidence Site-directed mutagenesis of yeast LCB1/LCB2 with SPT activity assays and structural modeling

   PMID:11781309

   Open questions at the time

   • No high-resolution crystal structure of mammalian SPT
   • Substrate-binding determinants for serine vs. alternative amino acids unknown
4. 2010 High

   Identification of SPTLC2 missense mutations (V359M, G382V, I504F) as the cause of HSAN-I, with demonstration that these mutations shift substrate specificity toward alanine to produce neurotoxic 1-deoxysphingolipids, established a gain-of-aberrant-function pathomechanism distinct from simple loss of activity.

   Evidence Patient genetics combined with cell-free and cell-based SPT activity assays and LC-MS metabolite profiling

   PMID:20920666

   Open questions at the time

   • Structural basis of altered substrate selectivity not determined
   • Mechanism of 1-deoxysphingolipid neurotoxicity not established
5. 2010 Medium

   Showing that NF-κB directly activates the Sptlc2 promoter in response to LPS established a transcriptional mechanism linking innate immune activation to de novo sphingolipid biosynthesis.

   Evidence ChIP, promoter analysis, pharmacological NF-κB inhibition, and p65 overexpression in macrophages

   PMID:21167294

   Open questions at the time

   • Relevance to in vivo inflammatory sphingolipid accumulation not tested
   • Other transcription factors potentially involved not examined
6. 2013 Medium

   Structural mapping of HSAN-I mutations to the PLP-binding pocket using a bacterial SPT mimic, and identification of additional HSAN-I mutations (A182P, R183W) that recapitulate the substrate-shift mechanism, consolidated the shared pathomechanism across multiple SPTLC2 HSAN-I variants.

   Evidence Bacterial SPT mimic mutagenesis with enzymatic assays; cell-based assays and LC-MS sphingolipid profiling for new variants

   PMID:23658386 PMID:24175284

   Open questions at the time

   • Role of small subunits ssSPTa/b in modulating mutant phenotype incompletely defined
   • No mammalian SPT crystal structure to validate modeling
7. 2019 High

   Conditional T-cell knockout of Sptlc2 revealed that de novo sphingolipid synthesis is required for CD8+ T-cell survival by restraining mTORC1 hyperactivation and ER stress, establishing a cell-autonomous metabolic requirement for SPTLC2 in adaptive immunity.

   Evidence T-cell-specific Sptlc2 conditional knockout mice, viral infection model, mTORC1/ER stress readouts, sphingolipid rescue experiments

   PMID:30952607

   Open questions at the time

   • Specific sphingolipid species responsible not identified
   • Role in other lymphocyte lineages not tested
8. 2022 Medium

   Demonstrating that sXBP1 transactivates the Sptlc2 promoter during ER stress, linking ceramide overproduction to hepatic insulin resistance in transgenic mice, established a second transcriptional induction axis and a metabolic disease consequence.

   Evidence Sptlc2 promoter analysis, liver-specific Sptlc2 transgenic mice, insulin signaling assays, ceramide measurements

   PMID:35513574

   Open questions at the time

   • Relative contribution of sXBP1 vs. NF-κB to Sptlc2 induction in different tissues unknown
   • Causal role of specific ceramide species in insulin resistance not delineated
9. 2024 Medium

   Discovery that the recurrent SPTLC2 p.Glu260Lys variant causes juvenile ALS through unrestrained SPT activity—mechanistically distinct from HSAN-I substrate shifts—by impairing ORMDL3-mediated feedback inhibition revealed a second, gain-of-function disease mechanism operating through the SPT regulatory interface.

   Evidence Whole-genome/exome sequencing, sphingolipidomics in patient plasma and fibroblasts, mutant HEK cell expression assays across multiple families

   PMID:38041679 PMID:38041684 PMID:38316966

   Open questions at the time

   • Direct biophysical evidence that ORMDL3 binding is disrupted by E260K not shown
   • Downstream pathway from excess sphingolipids to motor neuron death not elucidated
10. 2025 Medium

    Additional transmembrane-domain variants (T66R) confirmed the ORMDL3-derepression mechanism, while binding of SPTLC2 to EGFR and requirement of SPT catalytic activity for EGFR-FAK-HBEGF oncogenic signaling revealed an unexpected role in receptor tyrosine kinase signaling.

    Evidence Mutant cell-line sphingolipid profiling for T66R; co-immunoprecipitation and knockdown/overexpression with xenograft model for EGFR interaction

    PMID:39645550 PMID:40849231

    Open questions at the time

    • SPTLC2-EGFR interaction demonstrated by single Co-IP without reciprocal validation or domain mapping
    • Generality of EGFR interaction beyond ovarian cancer unknown
    • Mechanism by which sphingolipid products activate FAK-HBEGF axis not defined

• A high-resolution structure of the mammalian SPT heterodimer (with ORMDL3) explaining both HSAN-I substrate-shift mutations and ALS-associated derepression mutations, and the identity of specific sphingolipid species mediating T-cell fitness, β-cell survival, and motor neuron toxicity, remain to be determined.
• No published mammalian SPT-ORMDL3 co-structure defining the regulatory interface
• Specific sphingolipid species mediating downstream cellular phenotypes not identified
• Relative contribution of canonical vs. deoxy-sphingolipid toxicity to ALS pathogenesis unresolved