Syeda and colleagues used an elegant combination of genetics, meticulous clinical evaluation and cell biology to demonstrate that disrupting serine palmitoyl transferase (SPT) can cause amyotrophic lateral sclerosis (ALS) in very young children.1 SPT is the initial rate-limiting step in sphingolipid biosynthesis, joining serine and palmitoyl CoA in a decarboxylating condensation reaction that ultimately creates ceramide, sphingosine, glycosphingolipids and gangliosides.2 SPT resides in the endoplasmic reticulum membrane as a dimer of two subunits, SPTLC1 and SPTLC2, which are in turn attached to two small subunits ssSPTa and ssSPTb that activate enzyme activity and orosomucoid-like proteins (ORMDLs) that repress SPT activity to ensure sphingolipid homeostasis. Resultant sphingolipids are subsequently processed through the Golgi apparatus and inserted into plasma membranes where they are involved in multiple signalling pathways including those that sometimes cause opposing effects. For example, ceramide frequently induces apoptosis in cells, whereas sphingosine1-phosphate has anti-apoptotic effects. Relative levels of sphingolipids are tightly regulated, and disturbances in these levels have been associated with multiple pathological conditions including cancers, heart disease and type 2 diabetes.2

SPT variants in SPTLC1 and SPTLC2 that disrupt the enzyme activity of SPT are known to cause hereditary sensory and autonomic neuropathies type 1 and 2 (HSAN1 and HSAN2), most likely by allowing L-alanine or glycine to replace L-serine in the SPT reaction.3 4 This creates deoxysphingolipids that cannot be further metabolised and are hypothesised to be toxic to sensory neurons.3 Dietary supplementation with L-serine is currently being investigated as a therapeutic approach for such patients, by increasing serine utilisation in the SPT reaction.5 However, in the current manuscript, Syeda and colleagues identified six separate families with a heterozygous p.Glu260Lys variant in SPTLC2 that causes an aggressive early child onset of ALS rather than HSAN.1 Symptoms in all six children presented prior to the age of 4 years. They lost the ability to walk by 10 years of age, developed upper and lower motor neuron signs, with bulbar involvement, tongue fasciculations, respiratory compromise and no sensory involvement; they met Escorial criteria for ALS. Interestingly, early developmental motor milestones were normal up past a year of age for most of these children. Toxic deoxysphingolipids were not generated. Rather, plasma levels of sphingosine, ceramides and other sphingolipids were elevated compared with normal values, suggesting overactive SPT activity.1

The authors then used in silico mapping of SPTLC2 E260 to show that the glycine at residue 260 is not on the enzyme active site of SPTLC2 where many HSAN1 SPTLC1 and SPTLC2 cluster. Rather, it resides in the transmembrane domain where it makes direct contact with the histidine at codon 7 of ORMDL3. Studies with patient fibroblasts demonstrated that mutant SPTLC2 no longer interacted with ORDL3 and could no longer reduce sphingolipid biosynthesis. This resulted in increased sphingolipid levels, including ceramide and a dysregulation of sphingolipid biosynthesis. This same group recently published similar findings in another group of children with pathogenic variants in SPTLC1 who also developed juvenile ALS, though not as severely or as early on as with the patients in the current study.6 ALS-causing variants in SPTLC1 also disrupted interactions with ORDL, increased SPT activity and increased sphingolipid levels. These SPTLC1 variants also did not produce toxic deoxysphingolipids. Taken together, the authors suggested in both studies that increased sphingolipid synthesis caused the ALS, and providing L-serine to patients with ALS-causing variants in SPTLC1 and SPTLC2 would likely exacerbate their disease.

In summary, the current manuscript confirms and extends our knowledge that disruption of the homeostasis of SPT activity can cause ALS in children by increased sphingolipid synthesis. How increased sphingolipid synthesis causes ALS and where in the sphingolipid pathway pathogenic mechanisms occur remain unknown. Ceramide is known to promote apoptosis2 and perhaps increased ceramide may play a role in motor neuron death. SPTLC2 and SPTLC1 now become members of the group of genes known to cause hereditary ALS. They also provide a window into a metabolic pathway that can cause ALS. Whether disruptions in sphingolipid synthesis have implications for other forms of ALS will be exciting to discover. Why the Glu260Lys variant in SPTLC2 causes a more aggressive form of ALS than variants in SPTLC1 and why there is disease specificity for motor neurons remain a mystery. These SPTLC2 cases provide the most recent example of the power of genetics, careful clinical investigation and cell biology to unravel complex neurodegenerative diseases in neurology.