Salivary inflammatory biomarkers are predictive of mild cognitive impairment and Alzheimer's disease in a feasibility study

Abstract

Alzheimer's disease (AD) is an insidious disease. Its distinctive pathology forms over a considerable length of time without symptoms. There is a need to detect this disease, before even subtle changes occur in cognition. Hallmark AD biomarkers, tau and amyloid-β, have shown promising results in CSF and blood. However, detecting early changes in these biomarkers and others will involve screening a wide group of healthy, asymptomatic individuals. Saliva is a feasible alternative. Sample collection is economical, non-invasive and saliva is an abundant source of proteins including tau and amyloid-β. This work sought to extend an earlier promising untargeted mass spectrometry study in saliva from individuals with mild cognitive impairment (MCI) or AD with age- and gender-matched cognitively normal from the South Australian Neurodegenerative Disease cohort. Five proteins, with key roles in inflammation, were chosen from this study and measured by ELISA from individuals with AD (n = 16), MCI (n = 15) and cognitively normal (n = 29). The concentrations of Cystatin-C, Interleukin-1 receptor antagonist, Stratifin, Matrix metalloproteinase 9 and Haptoglobin proteins had altered abundance in saliva from AD and MCI, consistent with the earlier study. Receiver operating characteristic analysis showed that combinations of these proteins demonstrated excellent diagnostic accuracy for distinguishing both MCI (area under curve = 0.97) and AD (area under curve = 0.97) from cognitively normal. These results provide evidence for saliva being a valuable source of biomarkers for early detection of cognitive impairment in individuals on the AD continuum and potentially other neurodegenerative diseases.

Keywords: Alzheimer’s disease; biomarker; cognitive impairment; dementia; inflammation; saliva.

FIGURE 1

Levels of total protein and five biomarkers in the SANDs cohort showing reduced abundance of CST-C, IL-1RN, and SFN in MCI and AD and increased abundance of total protein, MMP-9 and HP in MCI and AD. Scatter dot plots show the mean, standard deviation and all points for each biomarker adjusted for total protein with AD in orange (n = 16), MCI in blue (n = 15) and CN in black (n = 29) for (A) CST-C (B) IL-1RN (C) SFN (D) MMP-9 (E) Hp (F) Total protein. *Unadjusted p-values shown from log transformed data for cohorts CN vs. AD with *P < 0.05 and ^**P < 0.01. AD, Alzheimer’s disease; CN, Cognitively Normal; CST-C, Cystatin-C; Hp, Haptoglobin; IL-1RN, Interleukin-1 receptor antagonist protein; Matrix metalloproteinase 9, MMP-9; MCI, Mild cognitive impairment; SFN, Stratifin.

FIGURE 2

Receiver operating characteristic curve analyses for the top three models. In the comparison CN vs. MCI (blue dashed line), AUC = 0.97 and top model is base model with SFN (μg/mL) and total protein (μg/mL), accuracy = 91%. For CN vs. AD (orange solid line), AUC = 0.97 and top model is base model with CST-C (total protein ratio) and IL-1RN (total protein ratio), accuracy = 96%. For CN vs. combined MCI/AD (pink dotted line), AUC = 0.97 and top model is base model with CST-C (total protein ratio), IL-1RN (total protein ratio), MMP-9 (total protein ratio) and total protein (μg/mL), accuracy = 92%. AD, Alzheimer’s disease; CN, Cognitively Normal; CST-C, Cystatin-C; IL-1RN, Interleukin-1 receptor antagonist protein; Matrix metalloproteinase 9, MMP-9; MCI, Mild cognitive impairment; SFN, Stratifin.