Point mutations in IMPDH2 which cause early-onset neurodevelopmental disorders disrupt enzyme regulation and filament structure.

Inosine 5' monophosphate dehydrogenase (IMPDH) is a critical regulatory enzyme in purine nucleotide biosynthesis that is inhibited by the downstream product GTP. Multiple point mutations in the human isoform IMPDH2 have recently been associated with dystonia and other neurodevelopmental disorders, but the effect of the mutations on enzyme function has not been described. Here, we report identification of two additional affected individuals with missense variants in IMPDH2 and show that all of the disease-associated mutations disrupt GTP regulation. Cryo-EM structures of one IMPDH2 mutant suggest this regulatory defect arises from a shift in the conformational equilibrium toward a more active state. This structural and functional analysis provides insight into IMPDH2-associated disease mechanisms that point to potential therapeutic approaches and raises new questions about fundamental aspects of IMPDH regulation.

SPTSSA variants alter sphingolipid synthesis and cause a complex hereditary spastic paraplegia.

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.

A recessive S174X mutation in Optineurin causes amyotrophic lateral sclerosis through a loss of function via allele-specific nonsense-mediated decay.

Loss of function (LoF) mutations in Optineurin can cause recessive amyotrophic lateral sclerosis (ALS) with some heterozygous LoF mutations associated with dominant ALS. The molecular mechanisms underlying the variable inheritance pattern associated with OPTN mutations have remained elusive. We identified that affected members of a consanguineous Middle Eastern ALS kindred possessed a novel homozygous p.S174X OPTN mutation. Analysis of these primary fibroblast lines from family members identified that the p.S174X mutation reduces OPTN mRNA expression in an allele-dependent fashion by nonsense mediated decay. Western blotting correlated a reduced expression in heterozygote carriers but a complete absence of OPTN protein in the homozygous carrier. This data suggests that the p.S174X truncation mutation causes recessive ALS through LoF. However, functional analysis detected a significant increase in mitophagy markers TOM20 and COXIV, and higher rates of mitochondrial respiration and ATP levels in heterozygous carriers only. This suggests that heterozygous LoF OPTN mutations may not be causative in a Mendelian manner but may potentially behave as contributory ALS risk factors.