A significant number of autosomal dominantly inherited Parkinson's disease (PD) cases are due to mutations in the leucine-rich repeat kinase 2 (LRRK2) gene. In cells, these pathogenic mutations have a number of differing effects on LRRK2 enzymatic activity and protein stability. In particular, five of the six described pathogenic LRRK2 mutations ablate the constitutive phosphorylation of LRRK2 on Ser910 and Ser935, two residues required for binding of LRRK2 to 14-3-3 proteins. This suggests a potential pathogenic role for these residues. However, LRRK2 kinase inhibitors, which have shown early promise as neuroprotective agents, also ablate the phosphorylation of Ser910 and Ser935. Additionally, LRRK2 is phosphorylated on Ser910 and Ser935 following activation of the inflammatory toll-like receptor pathway and inflammatory cytokines are often increased in PD patients. Whether LRRK2 protein or phosphorylation is altered in idiopathic PD is unknown. We therefore measured LRRK2 protein and its phosporylation in peripheral blood mononuclear cells (PBMCs) from 33 idiopathic Parkinson's disease patients and 27 age-matched controls. We found no significant difference in total LRRK2 protein levels in PBMCs from PD patients compared to controls. Furthermore, total LRRK2 protein expression was not effected by age, disease duration, disease severity or levodopa medication. The amount of phosphorylation on LRRK2 at both Ser910 and Ser935 correlated highly with total LRRK2 levels and was also unchanged in PD patients. Therefore, changes in LRRK2 Ser910/Ser935 phosphorylation in PBMCs are unlikely to contribute to idiopathic Parkinson's disease or be of utility as a disease biomarker. However, the invariance of Ser910 and Ser935 phosphorylation in PD PBMC's suggests that these residues could be used as pharmacodynamic biomarkers for the effectiveness of LRRK2 kinase inhibitors in patients.
