Plasma extracellular vesicles and phosphorylated tau 181 as early biomarkers of cognitive impairment in Alzheimer’s dementia

BACKGROUND: Timely and accurate diagnosis of Alzheimer’s disease (AD) in clinical practice is a great challenge, especially during early disease stages with subtle or mild symptoms of cognitive decline. Moreover, robust and more accessible blood-based screening tests for early diagnosis are needed. In this study, we investigated the core AD blood biomarkers — amyloid beta 42 (Aβ42) and 40 (Aβ40) peptides, phosphorylated tau 181 (p-Tau181), neurofilament light chain (NfL), and total tau (t-Tau) — and extracellular vesicle (EVs) size and concentration in individuals characterized by different stages of cognitive decline to identify biochemical markers of dementia for early diagnosis. METHODS: A total of n = 800 human plasma samples were analyzed. Plasma levels of NfL, t-Tau, p-Tau181, Aβ42, Aβ40 and plasma EVs were evaluated in n = 217 elderly healthy subjects (CTRL), in individuals with subjective cognitive complaints (SCC, n = 48), pre-mild cognitive impairment (pre-MCI, n = 58) and mild cognitive impairment (MCI, n = 426), and in n = 51 probable AD dementia patients (AD-dem), using ultrasensitive Single Molecule Array technology (Simoa®) and nanoparticle tracking analysis (NTA). Logistic regression and Receiver Operating Characteristic (ROC) analyses were employed. RESULTS: Plasma NfL displayed increased levels in AD-dem and MCI patients, while p-Tau181, Aβ42/Aβ40 ratio, Aβ42/p-Tau181 ratio, and EVs plasma levels were altered since the early stages of the pathology: in particular, p-Tau181 levels increased as cognitive symptoms worsened, already in the SCC and pre-MCI groups compared to CTRL, while the ratio of EVs concentration and size (EVs ratio) was decreased in all groups compared to CTRL. Plasma p-Tau181 best classified AD-dem patients from CTRL with an area under the curve (AUC) equal to 0.87, while EVs ratio best differentiated SCC from CTRL (AUC = 0.78). Combining p-Tau181 and EVs ratio with Aβ42/Aβ40 ratio and NfL, respectively, significantly improved the classification of pre-MCI and MCI from CTRL (AUCcomb = 0.79 and AUCcomb = 0.85). Combining biomarkers did not improve accuracy in discriminating MCI from SCC, pre-MCI and AD-dem. CONCLUSIONS: p-Tau181 and EVs ratio are promising biomarkers for the identification of individuals at risk of degenerative dementia. Combining the core AD plasma biomarkers with EVs ratio can aid in diagnosing the early stages of AD dementia.