Objective: In this study, we investigate how the PD risk factor GBA would affect the cellular phenotypes associated with homozygous mutations in Parkin, causing an early onset form of PD. We aim to understand the synergistic or reductive effects of the combination of these two PD-associated genes and their respective contributions to the phenotypic alterations.

Background: It is increasingly recognized that the classical one-gene one-trait model may not catch the full picture of genetic contribution to PD pathophysiology. Therefore, a polygenic model where multiple genes would influence the disease risk and the phenotypic traits in PD should be investigated. Mutations in GBA are the most common genetic risk factor for PD. Patients carrying GBA mutations present an earlier age of onset, a more rapid disease progression and are at greater risk to develop cognitive impairment. The mutated GBA enzyme, glucocerebrosidase (GCase), has been shown to be implicated in PD-linked cellular phenotypes as α-synuclein accumulation.

Method: Fibroblasts derived from two PD patients carrying either both a deletion in Parkin and a point mutation in GBA or only a deletion in Parkin were reprogrammed into induced pluripotent stem cells (iPSC). At iPSC stage, the cells were genetically edited by CRISPR Cas9, to correct or insert the GBA mutation, respectively. iPSCs were then differentiated to obtain midbrain dopaminergic neurons subjected to cellular phenotyping. To untangle the contribution of Parkin to cellular phenotypes, lentiviral-mediated overexpression of Parkin was performed.

Results: In contrast with the expected phenotype in GBA mutant lines, the iPSC-derived neurons harbouring both Parkin and GBA mutations present decreased levels of intracellular α-synuclein. Nevertheless, an increase of α-synuclein release was observed. Isogenic correction of the GBA mutation led to a reversal of this α-synuclein phenotype. Interestingly, normalizing Parkin status was also affecting α-synuclein levels.

Conclusion: We show here that the observed neuronal phenotypes in Parkin-GBA-double-mutant cells do not correspond to a simple additive effect of both mutations. The outcome of the rescue of both GCase and Parkin protein levels on the α-synuclein phenotype points to a more complex underlying mechanism. This study shows how precision medicine approaches for PD should be developed to take into account specificities in terms of cellular phenotypes of each patient.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/dissecting-the-genetic-complexity-of-parkinsons-disease-a-parkin-gba-study/