SPTSSA Variants Alter Sphingolipid Synthesis and Cause a Complex Hereditary Spastic Paraplegia

Authors

Siddharth Srivastava
Hagar Mor Shaked
Kenneth Gable
Sita D Gupta
Xueyang Pan
Niranjanakumari Somashekarappa
Gongshe Han
Payam Mohassel
Marc Gotkine
Elizabeth Doney
Paula Goldenberg
Queenie K G Tan
Yi Gong
Benjamin Kleinstiver
Brian Wishart
Heidi Cope
Claudia Brito Pires
Hannah Stutzman
Rebecca C Spillmann
Undiagnosed Disease Network
Reza Sadjadi
Orly Elpeleg
Chia-Hsueh Lee
Hugo J Bellen
Simon Edvardson
Florian Eichler
Teresa M Dunn

Publication Date

4-19-2023

Journal

Brain

DOI

10.1093/brain/awac460

PMID

36718090

PMCID

PMC10319774

PubMedCentral® Posted Date

1-30-2023

PubMedCentral® Full Text Version

Post-print

Published Open-Access

yes

Keywords

Animals, Child, Humans, Spastic Paraplegia, Hereditary, Serine C-Palmitoyltransferase, Sphingolipids, Cell Membrane, Mammals, serine palmitoyltransferase, SPTSSA, sphingolipids, ORMDLs, hereditary spastic paraplegia

Abstract

Sphingolipids are a diverse family of lipids with critical structural and signalling functions in the mammalian nervous system, where they are abundant in myelin membranes. Serine palmitoyltransferase, the enzyme that catalyses the rate-limiting reaction of sphingolipid synthesis, is composed of multiple subunits including an activating subunit, SPTSSA. Sphingolipids are both essential and cytotoxic and their synthesis must therefore be tightly regulated. Key to the homeostatic regulation are the ORMDL proteins that are bound to serine palmitoyltransferase and mediate feedback inhibition of enzymatic activity when sphingolipid levels become excessive. Exome sequencing identified potential disease-causing variants in SPTSSA in three children presenting with a complex form of hereditary spastic paraplegia. The effect of these variants on the catalytic activity and homeostatic regulation of serine palmitoyltransferase was investigated in human embryonic kidney cells, patient fibroblasts and Drosophila. Our results showed that two different pathogenic variants in SPTSSA caused a hereditary spastic paraplegia resulting in progressive motor disturbance with variable sensorineural hearing loss and language/cognitive dysfunction in three individuals. The variants in SPTSSA impaired the negative regulation of serine palmitoyltransferase by ORMDLs leading to excessive sphingolipid synthesis based on biochemical studies and in vivo studies in Drosophila. These findings support the pathogenicity of the SPTSSA variants and point to excessive sphingolipid synthesis due to impaired homeostatic regulation of serine palmitoyltransferase as responsible for defects in early brain development and function.