Objective: To determine the interaction between glucocerebrosidase deficiency and acid sphingomyelinase deficiency in a tractable vertebrate model system of Parkinson’s disease.

Background: Heterozygous glucocerebrosidase (GBA) mutations are the strongest and most common risk factor for PD. An excessive burden of other lysosomal storage disorder gene variants, in particular for SMPD1 (encoding for acid sphingomyelinase) has also been reported [1]. However, it is unclear whether GBA and SMPD1 interact genetically. Zebrafish are an ideal vertebrate system to test for such interactions. We previously observed early microglial activation and alpha-synuclein independent neuronal cell death in gba1-/- zebrafish [2]. The aim of this study was to study the effect of gba_smpd1 gene-gene interaction in vivo.

Methods: An smpd1 mutant zebrafish line was generated using the Crispr/Cas9 technology. smpd-/- were crossed to our previously described gba-/- zebrafish line. A wide range of different parameters including survival, spontaneous motor behaviour, mass-spectrometry based glycolipid quantification and mitochondrial respiratory chain activity was assessed across four different genotypes (gba-/-, smpd-/-, gba-/-_smpd1-/-, wt).

Results: Sphingomyelinase activity was reduced by 93% in smpd-/- zebrafish which developed normally and did not have a shortened life span or altered spontaneous motor behaviour. A synergistic effect of combined glucocerebrosidase (GCase) and acid sphingomyelinase deficiency was observed for a range of different sphingolipid metabolites with highly significant increases (p < 0.0001) in gba-/-_smpd1-/- compared to gba-/- for sphingosine, sphinganine, ceramide, GM3 ganglioside and glucosylceramide. Unexpectedly, the characteristic "barrel-role" motor phenotype observed in adult gba-/- zebrafish was rescued in gba-/-_smpd1-/- double mutant zebrafish and survival of gba-/-_smpd1-/- double mutant zebrafish compared to gba-/- was significantly increased by ~ 25% from 102 days in gba-/- to 125 days in gba-/-_smpd1-/- (p = 0.0055). Mitochondrial respiratory chain complex IV activity was decreased in gba-/-, but normalized in gba-/-_smpd1 double mutant zebrafish.

Conclusions: Our work highlights the importance of a functional validation in tractable model systems for any gene-gene interaction rather than readily assuming an additive effect. Similar studies will be particularly important to study the biological interaction of PD risk genes identified in genome-wide association (GWA) studies.

References: 1. Robak LA, Jansen IE, van Rooji J, et al (2017): Excessive burden of lysosomal storage disorder gene variants in Parkinson’s disease. Brain 140:3191-3203. 2. Keatinge M, Bui H, Menke A, et al (2015): Glucocerebrosidase 1 deficient Danio rerio mirror key pathological aspects of human Gaucher disease and provide evidence of early microglial activation preceding alpha-synuclein-independent neuronal cell death. Hum Mol Genet 24:6640-6652.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/acid-sphingomyelinase-deficiency-rescues-mitochondrial-dysfunction-in-gba-zebrafish-danio-rerio/