SPTLC1 Mutations Associated with Early Onset Amyotrophic Lateral Sclerosis Impair ORMDL Regulation

Abstract

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal neurodegenerative disorder characterized by selective degeneration of lower and upper motor neurons leading to progressive muscle weakness, swallowing difficulties, and respiratory insufficiency. The majority of ALS cases occur sporadically, but a significant number (>10%) are hereditary. Although a variety of cellular processes have been implicated in ALS, a specific monogenic metabolic disruption has not been reported. Thus, the recent discovery of four dominantly inherited SPTLC1 mutations that resulting in childhood-onset ALS in seven families is significant. SPTLC1 encodes a subunit of serine palmitoyltransferase (SPT), the committed and rate-limiting enzyme of sphingolipid synthesis. The ALS mutations flank the membrane spanning domain of SPTLC1, which is not required for ER targeting, association with the SPTLC2 and ssSPT subunits, or enzymatic activity. Rather, this domain is critical for binding of the ORMDL proteins that regulate SPT. This suggested that the SPTLC1 mutations likely result in unrestrained SPT activity. Our studies reveal that these highly penetrant SPTLC1 mutations do indeed abrogate negative regulation of SPT by the ORMDL proteins and result in elevated levels of sphingolipids. Using cultured cells, patient fibroblasts, iPSC-induced MNs, and mouse models, we are investigating the importance of ORMDL regulation in the maintenance of sphingolipid homeostasis and potential mechanisms of sphingolipid mediated motor neuron death. Significantly, the ALS SPTLC1 mutations confer disease pathology by a distinctly different mechanism than SPTLC1 and SPTLC2 mutations associated with hereditary sensory and autonomic neuropathy, type 1 (HSAN1). Whereas the ALS mutations result in elevated levels of canonical sphingolipids, the HSAN1 mutations, which compromise amino acid substrate selectivity of SPT, cause accumulation of atypical deoxy-sphingoid bases that are implicated in neuronal cell death.