It is widely recognized that the lack of early diagnostic protocols for Alzheimer’s disease means that the disorder is rarely diagnosed before the onset of significant clinical dementia and that, without early diagnosis, effective treatment of the disease is seriously hampered. For this reason, the search for Alzheimer’s disease biomarkers is a ripe area of proteomics study.
Researchers have previously identified potential biomarkers in cerebrospinal fluid (CSF). These biomarkers are total tau (t-tau), phosporylated tau (p-tau), and amyloid beta peptide 1-42 (AB42). The good news is that these biomarkers demonstrate high sensitivity and specificity when identifying AD patients from non-AD controls, as well as when predicting which patients with mild cognitive impairment (MCI) will progress to AD dementia. However, one of the serious drawbacks of this potential method of diagnosis using CSF biomarkers is that the protocol requires the patient to undergo lumbar puncture, an expensive procedure that results in discomfort and even side effects for the patient. Studies have been performed on blood plasma to determine if this human fluid may serve as a source for Alzheimer’s disease biomarkers. However, to date, the results of studies on AB42 levels in plasma are unclear, making this an ineffective biological fluid for the measure of both AB42 and tau, which is not detectable in plasma.
One exciting possibility is that these biomarkers may be found in human saliva, a bodily fluid that is easy to procure without excessive expense, discomfort, or side effects. Previous research indicates that tau mRNA is indeed expressed by the salivary glands and that AB precursor proteins are present in salivary epithelial cells. This makes sense since the submandibular gland is known to exhibit impairment in patients with AD. Additionally, recent positive findings of potential salivary biomarkers for Parkinson’s disease give researchers reason to investigate further.
Shi et al. used controls comprised of 21 AD patients and 38 cognitively healthy individuals to evaluate for the presence of t-tau, p-tau, and AB42 in human saliva. To do this, the researchers utilized immunoprecipitation to enrich tau and/or AB42 in saliva samples using mouse antibodies (Thermo Scientific). They then used an LTQ-Orbitrap (Thermo Scientific) to perform mass spectrometry and identify five unique tau proteins. Using a 3-plex assay, the researchers quantified the t-tau and p-tau levels. Generally, they found a significant increase in p-tau as well as an increase in the p-tau/t-tau ratio for patients with AD. Findings regarding t-tau were less clear, and the researchers were unable to identify AB42 in the saliva samples using this method.
It is known that the formation of characteristic tangles in neurofibers amongst AD patients requires the hyperphosphorylation of tau. The number and distribution of these neurofibrillary tangles are now used to determine the progressive stage of AD. The findings of Shi et al.– that the p-tau/t-tau ratio is significantly increased amongst AD patients versus cognitively healthy controls– indicates that as the disease progresses, the salivary tau levels show an increase in the phosphorylated form. The researchers report the possibility of using tau biomarkers in saliva for not only diagnostic purposes, but also in the monitoring of the progression of AD in general and in response to medication therapy.
The work highlights some issues in this area of study, particularly in the differences they noted in their study versus the findings of studies related to biomarkers in CSF. Specifically, the researchers found that while, t-tau and p-tau levels in CSF both increase for AD patients, salivary t-tau is not demonstrated to change or may even decrease. The researchers also noted higher p-tau levels versus t-tau levels in the same patient when they evaluated the saliva, which is not the same as CSF findings. It is possible that a subsequent study of Alzheimer’s disease biomarkers in saliva that utilizes an alternative method for saliva collection may prove useful. Shi et al. also call for further investigation into the origin of tau species in the saliva as well as why their study did not identify AB42 in the saliva while another study using a different assay kit did indicate AB42 presence in saliva.
Shi, M., et al. (2011) ‘Salivary tau species are potential biomarkers of Alzheimer’s disease‘, Journal of Alzheimer’s Disease, 27 (2), (pp 299-305)