Acid Sphingomyelinase Deficiency Normalizes Neuronal Function in GCase Deficiency - Unexpected Biological Rescue Effect of Combined Genetic Risk Factors for Parkinson’s Disease

Abstract BackgroundThe additive mechanistic effect of genetic risk variants for Parkinson’s disease (PD) is a plausible but largely unproven hypothesis. We investigated the mechanistic interaction between the two lysosomal PD risk genes glucocerebrosidase 1 (GBA1) and sphingomyelinase 1 (SMPD1) in complementing model systems.MethodsUsing CRISPR/Cas gene editing, smpd1 mutant (smpd1-/-) zebrafish were generated and crossed to our previously characterised gba1-/- zebrafish line, generating double knockouts (gba1-/-;smpd1-/-). Spontaneous motor behaviour and survival were assessed in WT, single mutants and double mutants. HPLC-based sphingolipid quantification was combined with RNAseq based pathway analysis, assessment of the mitochondrial respiratory chain and quantification of lipid membrane oxidation for in-depth assessment of cellular health across all four genotypes. We also determined the effect of combined glucocerebrosidase (GCase) and acid sphingomyelinase (ASM) inactivation on autophagy and alpha-synuclein homeostasis in the human neuronal cell line SH-SY5Y.ResultsUnexpectedly, ASM deficiency rescued the marked behavioural phenotype and prolonged survival in gba1-/-;smpd1-/- double-mutant zebrafish compared to gba1-/-. RNAseq-based pathway analysis confirmed a profound rescue of neuronal function and intracellular homeostasis. We identified complete reciprocal rescue of mitochondrial respiratory chain function and abolished lipid membrane oxidation in gba1-/-;smpd1-/- compared to gba1-/- or smpd1-/- as the underlying rescue mechanism. The complementing in vitro experiments demonstrated an unexpected reduction of α-synuclein levels in human cell lines with combined GCase and ASM deficiency.ConclusionsOur study highlights the importance of functional validation for any putative mechanistic interactions between genetic risk factors and their overall effect on disease-relevant mechanisms rather than readily assuming an additive effect.