KDSR

KDSR (FVT-1) is an NADPH-dependent 3-ketodihydrosphingosine reductase that catalyzes the reduction of 3-ketodihydrosphingosine to dihydrosphingosine (sphinganine), a committed step in de novo ceramide biosynthesis (PMID:15328338). The enzyme resides in the endoplasmic reticulum membrane with its catalytic domain oriented toward the cytosol, and it serves as the principal 3-ketosphinganine reductase in mammalian cells, as demonstrated by siRNA-mediated depletion that directly correlates with loss of reductase activity (PMID:15328338, PMID:19141869). When KDSR is impaired by loss-of-function mutations, ceramide synthases utilize the accumulating 3-ketodihydrosphingosine substrate to generate aberrant keto-type ceramides, disrupting epidermal barrier formation and causing progressive symmetric erythrokeratoderma, and separately impairing proplatelet formation from megakaryocytes through defective sphingolipid-dependent cytoskeletal organization (PMID:28774589, PMID:28575652, PMID:30467204, PMID:34686882).

1. 2004 High

   The molecular function of KDSR was established: purified recombinant human FVT-1 catalyzes NADPH-dependent reduction of 3-ketodihydrosphingosine to dihydrosphingosine, and the protein localizes to the ER with its catalytic domain facing the cytosol, assigning both enzymatic identity and subcellular topology to a previously uncharacterized gene.

   Evidence In vitro enzyme assay with purified recombinant protein, yeast TSC10-null complementation, immunofluorescence and proteinase K topology assay

   PMID:15328338

   Open questions at the time

   • Whether FVT-1 is the sole or dominant KDS reductase in mammalian cells was not determined
   • No structural model of the active site was available
   • Physiological consequence of KDSR loss in mammals remained unknown
2. 2007 High

   A naturally occurring bovine missense mutation (Ala175Thr) was shown to abolish KDSR reductase activity in vitro, identifying a critical catalytic residue and providing the first genotype–activity relationship for the enzyme.

   Evidence In vitro enzyme assay comparing wild-type and A175T variant; yeast complementation

   PMID:17420465

   Open questions at the time

   • Whether this residue participates directly in catalysis or affects protein folding was not distinguished
   • No human disease phenotype had yet been linked to KDSR mutations
3. 2009 High

   KDSR was established as the principal 3-ketosphinganine reductase in mammalian cells, resolving whether redundant enzymes existed, and active-site mutagenesis revealed mechanistic differences from yeast Tsc10p despite functional conservation.

   Evidence siRNA knockdown with enzymatic activity correlation, GFP-fusion localization, active-site mutagenesis with activity measurement

   PMID:19141869

   Open questions at the time

   • The precise catalytic mechanism and structural basis for differences from Tsc10p remained unresolved
   • No loss-of-function animal or human model had been characterized
4. 2017 High

   Biallelic KDSR loss-of-function mutations were identified as the cause of progressive symmetric erythrokeratoderma in humans, linking KDSR enzymatic deficiency to impaired acylceramide synthesis and defective epidermal barrier formation.

   Evidence Whole-exome sequencing of patient families, ceramide metabolite measurement in patient skin, enzymatic activity assays in patient cells, yeast complementation, immunohistochemistry, and splicing assays across two independent cohorts

   PMID:28575652 PMID:28774589

   Open questions at the time

   • How specific ceramide species downstream of KDSR control keratinocyte differentiation was not defined
   • Whether partial loss of KDSR activity produces milder phenotypes was unknown
5. 2018 High

   KDSR was shown to be required for normal megakaryocyte proplatelet formation, extending its physiological role beyond epidermis to hematopoiesis and demonstrating that sphingolipid-dependent cytoskeletal organization underlies the platelet defect.

   Evidence iPSC-derived megakaryocyte differentiation from KDSR-deficient patient, proplatelet formation assay with genetic rescue, zebrafish kdsr depletion, sphingolipid metabolomics

   PMID:30467204

   Open questions at the time

   • Which specific sphingolipid species regulate cytoskeletal dynamics in megakaryocytes was not identified
   • Whether KDSR deficiency affects other hematopoietic lineages was not explored
6. 2022 Medium

   Lipidomic analysis of KDSR-deficient patient skin revealed that accumulating 3-ketodihydrosphingosine is shunted into novel keto-type ceramides by ceramide synthases, identifying a metabolic bypass pathway that contributes to pathology.

   Evidence Mass spectrometry-based lipidomics of patient stratum corneum

   PMID:34686882

   Open questions at the time

   • Whether keto-type ceramides are directly pathogenic or merely markers of substrate accumulation is unresolved
   • Not independently confirmed in a second cohort or model system
   • Whether therapeutic inhibition of ceramide synthase activity on keto-substrates could ameliorate disease was not tested

• No high-resolution structure of mammalian KDSR exists, the precise catalytic mechanism and cofactor-binding geometry remain unresolved, and the downstream sphingolipid species that mediate cytoskeletal and differentiation phenotypes in skin and megakaryocytes have not been identified.
• No crystal or cryo-EM structure of KDSR
• Specific sphingolipid effectors downstream of dihydrosphingosine in keratinocytes and megakaryocytes are unknown
• Whether KDSR has non-catalytic or regulatory functions has not been investigated