In search of Parkinson's disease biomarkers - is the answer in our mouths? A systematic review of the literature on salivary biomarkers of Parkinson's disease.

The identification of reliable biomarkers of Parkinson's disease (PD) is a pivotal step in the introduction of causal therapies. Saliva is a biofluid which may be involved in synuclein pathology in PD. We have reviewed current studies on salivary proteins and compounds in PD patients and healthy controls, and their potential application as biomarkers. A systematic literature search of the Pubmed and Scopus databases was performed. A total of 198 studies were screened, of which 20 were included in our qualitative analysis. We conclude that the oligomeric form of salivary alpha synuclein is higher in PD patients, and that this may serve as a potential biomarker of PD. Salivary DJ-1 concentrations fail to differentiate PD patients from controls. Other enzymes and substances (heme oxygenase-1, nitric oxide, acetylcholinesterase) have been assessed in single studies. Salivary cortisol levels are higher in PD than in healthy subjects. Further validation of these findings is needed. Saliva may be a promising source of biomarkers in PD.