SPTLC1 variants associated with childhood onset amyotrophic lateral sclerosis produce distinct sphingolipid signatures through impaired interaction with ORMDL proteins
Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease affecting motor neurons. Mutations in the SPTLC1 subunit of serine-palmitoyltransferase (SPT), which catalyzes the first step in the de novo synthesis of sphingolipids cause childhood-onset ALS. SPTLC1-ALS variants map to a transmembrane domain that interacts with ORMDL proteins, negative regulators of SPT activity. We show that ORMDL binding to the holoenzyme complex is impaired in cells expressing pathogenic SPTLC1-ALS alleles, resulting in increased sphingolipid synthesis and a distinct lipid signature. C-terminal SPTLC1 variants cause the peripheral sensory neuropathy HSAN1 due to the synthesis of 1-deoxysphingolipids (1-deoxySLs) that form when SPT metabolizes L-alanine instead of L-serine. Limiting L-serine availability in SPTLC1-ALS expressing cells increased 1-deoxySL and shifted the SL profile from an ALS to an HSAN1-like signature. This effect was corroborated in an SPTLC1-ALS pedigree in which the index patient uniquely presented with an HSAN1 phenotype, increased 1-deoxySL levels, and an L-serine deficiency. These data demonstrate how pathogenic variants in different domains of SPTLC1 give rise to distinct clinical presentations that are nonetheless modifiable by substrate availability.