An integrated top-down and bottom-up proteomic platform to reveal potential salivary biomarkers of the rare disorders SAPHO syndrome, Wilson’s disease and Hereditary angioedema

Wilson’s disease, SAPHO syndrome and Hereditary angioedema are three rare disorders characterized by a wide spectrum of different clinical manifestations, which involve several organs and apparatus, making the diagnosis extremely difficult. In this study, the salivary proteome and peptidome of subjects affected by these pathologies has been investigated using mass spectrometry, through an integrated top-down and bottom-up platform, and compared with groups of healthy controls, with the aim to assess whether qualitative and quantitative variations of salivary proteins and peptides could be associated to the immune derangement distinctive of each disease and in order to have suggestions on potential specific salivary biomarkers. The analysis of the salivary proteome from patients affected by Wilson’s disease allowed to characterize new oxidized proteoforms of S100A8 and S100A9 and two fragments of the polymeric immunoglobulin receptor named ASVD and AVAD. Higher levels of these proteins and peptides observed in the patients are most likely connected to the oxidative stress, the activation of the inflammatory processes, and the hepatic damage caused by the altered copper transport and its subsequent accumulation in the organism, which is at the origin of the pathology. The observed increase of the level of α-defensins 2 and 4 may give a contribution to the development of the disease by the improvement of the free copper. The proteome of patients affected by SAPHO syndrome revealed a significant decrease of cystatins, histatins, and aPRPs, which are involved in the protection against infections, suggesting a reduced ability of these subjects to contrast bacteria colonization, in particular P. acnes which is a possible trigger of this disease. In particular, the lower levels of histatins and the higher frequency of S100A12 observed in patients with respect to controls, may be connected with the dysregulation of the innate immunity and the neutrophil response typical of SAPHO syndrome. Cystatin SN abundance decrease correlated with the disease duration, suggesting its reduced production during the chronic phase of the ~ 5 ~ disease, while histatins showed positive correlation with serum levels of the C reactive protein. In saliva of Hereditary angioedema patients, the increased percentage of peptides generated by the proteolytic cleavage by metalloproteinases indicates the intense metalloproteinase activity possibly connected to the activation of inflammatory pathways. Interestingly, in consideration of the possible role of cystatin B in enhancing the production of nitric oxide, and the higher salivary levels measured in the patients, we suggest that cystatin B may give a contribution to the vasodilatation and the vasopermeability responsible for the oedema formation, which is the main feature of this pathology. In conclusion, the results obtained in these studies clearly highlighted that the salivary proteome showed some features specific of the three pathologies. Even though these results have been obtained in a small cohort of patients, due to the difficult recruitment of subjects affected by rare disorders, and need further validation by using orthogonal techniques, they strongly suggest that saliva, with easy and non-invasive collection characteristics, could be a biofluid suitable for diagnostic applications.