Serine palmitoyltransferase (SPT) inhibition in SPTSSA-related complex hereditary spastic paraplegia.

Defective feedback inhibition of serine palmitoyltransferase (SPT) caused by pathogenic SPTSSA variants underlies childhood-onset complex hereditary spastic paraplegia, yet the developmental timing and therapeutic reversibility of sphingolipid dysregulation remain unclear. We generated a knock-in mouse carrying the disease-associated SptssaT51I variant and show that heterozygous animals exhibit preserved intrinsic SPT activity but impaired ORMDL-mediated regulation, leading to sustained elevation of bioactive sphingolipid intermediates that peak during postnatal myelination. Although gross myelin formation was initially maintained, excess sphingolipid flux rendered oligodendrocytes and neurons selectively vulnerable. Dietary L-serine, which augments SPT substrate availability, amplified sphingolipid accumulation in mutant but not wild-type mice, unmasking progressive spasticity, axonal injury, myelin ultrastructural defects, and, when administered during early postnatal development, severe pulmonary pathology likely responsible for lethality. Pharmacologic SPT inhibition with myriocin normalized sphingolipid synthesis, prevented serine-induced lethality, and reversed neurological and metabolic abnormalities. Translating these findings, treatment of a child with SPTSSA-T51I-associated complex hereditary spastic paraplegia using the FDA approved SPT inhibitor D-Cycloserine resulted in sustained improvement in spasticity, reduced baclofen requirement, and decreased plasma levels of neurofilament light chain (NFL). These data define dysregulated sphingolipid biosynthesis as a developmentally and metabolically sensitive driver of neurodegeneration and suggest SPT inhibition as a mechanistically grounded therapeutic strategy.