Assessing the Biomarker Potential of LRRK2, GCase and Cysteine Cathepsins in Parkinson's Disease Monocytes

Abstract

Better Parkinson’s disease (PD) biomarkers are needed to enable early detection, confirm diagnosis, monitor prognosis, or inform on stratification of PD patients into clinical trials. Leucine-rich repeat kinase 2 (LRRK2) and glucocerebrosidase (GCase) are two enzymes highly expressed in peripheral blood monocytes, and evidence implicates the involvement of both enzymes in the regulation of lysosome function and inflammation. Mutations in LRRK2 are the most common cause of familial PD, and heterozygous mutations in GBA1, the gene encoding GCase, are associated with increased risk of developing PD. Antigen presentation and lysosomal cathepsin activity are also of interest due to their potential role in PD pathogenesis. A peripheral blood mononuclear cell (PBMC) processing protocol with downstream parallel flow cytometry assays was developed to standardise the measurement of these PD-relevant proteins. The optimised workflow was then applied to PBMCs from PD patients with and without a LRRK2 or GBA1 mutation and matched controls. This revealed decreased GCase activity in the GBA-PD group, elevated cathepsin activity in the GBA-PD and LRRK2-PD groups, and decreased antigen presentation capacity in the LRRK2-PD group. Higher LRRK2 kinase activity was detected in iPD patient monocytes compared to other groups, suggesting a central role for increased LRRK2 kinase activity in PD pathogenesis even in the absence of a pathogenic mutation. The altered activity levels of LRRK2, GCase and cathepsins in different patient groups revealed in this thesis may help inform clinical trials targeting these enzymes and help monitor responses to therapeutic intervention. Further research is needed to determine how these readouts may relate to disease progression or severity, and to further elucidate the mechanisms underlying the altered expression and activities of these PD-associated proteins.