Mutations in the Parkinson’s disease (PD) gene Leucine-rich repeat kinase 2 (LRRK2) are dominantly inherited with reduced penetrance. To date, little is known about the molecular mechanisms that trigger the onset of LRRK2-PD. To allow for better monitoring of progression and, ultimately, for the identification of penetrance-associated cellular pathways, in ProtectMove I, we investigated markers of LRRK2-PD manifestation. In fibroblasts from manifesting (LRRK2+/PD+; n=10) and non-manifesting carriers (LRRK2+/PD-; n=21) of the G2019S mutation, first, we confirmed a link between PD onset and LRRK2 phosphorylation. Second, inspired by a report connecting LRRK2 kinase activity with mitochondrial DNA (mtDNA) lesions, we assessed mtDNA integrity and mitochondrial function in LRRK2+/PD- and LRRK2+/PD+ fibroblasts and observed an accumulation of mtDNA deletions and decreased complex I activity in the latter. In light of a study showing reduced urate in LRRK2+/PD+ individuals, we explored urate-induced Nrf2-ARE antioxidant signaling. Interestingly, the Nrf2-ARE target TFAM, which acts as mtDNA transcription and packaging factor was reduced in LRRK2+/PD+ neurons, increasing mtDNA exposure to reactive oxygen species. We hypothesize that a vicious cycle between environment, mitochondrial function and LRRK2 kinase activity, where the mtDNA acts as central interface, defines LRRK2-PD penetrance. To assess environmental factors contributing to penetrance and to identify relevant toxins (Objective 1), we will collect environmental exposure, diet and medication information from the LIPAD cohort (LRRK2+/PD+, n=1,500; LRRK2+/PD-, n=500; controls, n=500). Moreover, in selected persons, we will perform untargeted toxicology. To investigate penetrance-associated (epi-)genetic mtDNA alterations (Objective 2), we will perform sequencing and quantitative mtDNA analyses in blood samples from LIPAD. To determine the contribution of rare variants in nuclear genes involved in antioxidant signaling and mitochondrial function to penetrance (Objective 3), whole genome sequencing and polygenic risk score analyses will be applied to individuals from LIPAD. Finally, to test the causal link between toxin exposure, mtDNA disintegration, mitochondrial dysfunction, LRRK2 kinase activity, and antioxidant signaling (Objective 4), we will conduct RNA- Seq and functional studies in LRRK2+/PD+ and LRRK2+/PD- neurons before and after exposure to oxidative stressors, kinase inhibitors and antioxidants. The results from our study will help to guide personalized medicine approaches, which aim at delaying the onset of LRRK2-PD. Project P1 is led by Principal Investigators well-versed in epidemiological, clinical, genetic and cellular studies. Furthermore, they benefit from (inter-)national collaborations allowing to expand P1 to epigenetic and toxicology analyses. P1 is embedded in a close network of interactions within ProtectMove II. This includes projects P2-P4, P8-P10, INF, Z2 and all Cores.