Alzheimer’s disease (AD) is a prevalent age-related neurodegenerative disorder affecting millions of people worldwide. Clearly AD represents a critical public health issue for societies worldwide. Pathologically, the brains of individuals with AD are characterised by the accumulation of extracellular plaques composed of amyloid-b-peptide (Ab) and intracellular neurofibrillary tangles composed of the microtubule associated phosphoprotein tau. Though plaques and tangles within the AD brain are considered hallmarks of the disease, little is known about the early molecular changes that initiate plaque and tangle formation. Importantly, while most AD diagnosis are made following the onset of clinical symptoms, there appears to be a long asymptomatic phase of disease progression, where memory and cognition are not affected. Current diagnostic practices that focus on imaging techniques and cerebral spinal fluid (CSF) analysis identify the advanced disease state, thus there is a pressing need for diagnostic measures able to identify individuals at risk of developing AD as current drug treatments are not effective. In this study, the researching team applied a proteomics discovery approach to identify early molecular biomarkers of AD in saliva. Saliva as a screening tool has many advantages, in that it is non-invasive, self sampleable, and has shown to reflect the physiological function of the body. Shot-gun-filter aided sample preparation (FASP) proteomics and liquid chromatography mass spectrometry (LC-MS/MS) was used to identify neural derived proteins in saliva and CSF, from cognitively normal, mild cognitive impairment (MCI) and AD individuals. The researchers identified a novel salivary biomarker, transthyretin (TTR) that was shown to be reduced in the saliva of both MCI and AD individuals. The reduction in TTR in the saliva of MCI participants suggests this reduction occurs with the onset of cognitive symptoms and may represent a unique salivary biomarker for early detection of AD.
How was PEAKS used?
The raw MS data were filtered at the peptide and protein level based on there identification score, sequence length, replication rate and false discover rate using PEAKS Studio Xpro. PEAKS DB was used for protein identification in both saliva and CSF. The data was searched using the UniProt/Swiss-Pro human database. Label free quantification was used to determine the amount of each protein per sample according to their relative intestines. Protein intensities were normalised and compared between biofluids and participant condition. Post translational modifications between the proteins observed in both saliva and CSF were also examined in PEAKS.
Eldem E, Barve A, Sallin O, Foucras S, Annoni JM, Schmid AW, and Auber A. (2022) Salivary proteomics identifies transthyretin as a biomarker of early dementia conversion. J. Alzheimer’s. Dis. 6, 31-41. doi:10.3233/ADR-210056.
Abstract
Background:
Alzheimer’s disease (AD) remains to date an incurable disease with a long asymptomatic phase. Early diagnosis in peripheral biofluids has emerged as key for identifying subjects at risk and developing therapeutics and preventative approaches.
Objective:
We apply proteomics discovery to identify salivary diagnostic biomarkers for AD, which are suitable for self-sampling and longitudinal biomonitoring during aging.
Methods:
57 participants were recruited for the study and were categorized into Cognitively normal (CNh) (n = 19), mild cognitive impaired (MCI) (n = 21), and Alzheimer’s disease (AD) (n = 17). On a subset of subjects, 3 CNh and 3 mild AD, shot-gun filter aided sample preparation (FASP) proteomics and liquid chromatography mass spectroscopy (LC-MS/MS) was employed in saliva and cerebrospinal fluid (CSF) to identify neural-derived proteins. The protein level of salivary Transthyretin (TTR) was validated using western blot analysis across groups.
Results:
We found that 19.8% of the proteins in saliva are shared with CSF. When we compared the saliva and CSF proteome, 24 hits were decreased with only one protein expressed more. Among the differentially expressed proteins, TTR with reported function in amyloid misfolding, shows a significant drop in AD samples, confirmed by western blot showing a 0.5-fold reduction in MCI and AD compared to CNh.
Conclusion:
A reduction in salivary TTR appears with the onset of cognitive symptoms. More in general, the proteomic profiling of saliva shows a plethora of biomarkers worth pursuing as non-invasive hallmarks of dementia in the preclinical stage.