Detection of mild cognitive impairment of Alzheimer’s disease and tauopathy differentiation by phospho‐tau396

Abstract Background In recent years, CSF and blood‐based biomarkers have been developed for premortem diagnosis of AD. However, no well‐established biomarkers are currently available for early detection of AD in patients with mild cognitive impairment (MCI), a transitional stage between normal cognition and dementia. As tau aggregates in AD brains are highly phosphorylated, we sought to develop novel biomarkers for early diagnosis of AD and differential diagnosis of tauopathies by detecting site‐specific phospho‐tau (p‐tau) epitopes. Method A panel of site‐specific p‐tau antibodies were screened for their ability to detect high molecular weight “smear‐like” tau aggregates from brain extracts of AD, PSP, CBD, and Pick’s disease patients by Western blotting. Selection of tau phosphorylation sites is based on the high frequencies of these sites reported in AD patients from recent high‐resolution quantitative post‐translational modification mapping. Selected candidates were further tested for their ability to differentiate subjects with AD vs. other tauopathies and MCI vs. cognitively normal controls by ELISA assays and immunohistochemical staining of postmortem brain tissue followed by machine learning‐based image analysis. Result Among the antibodies screened, p‐tau396 was able to differentiate between AD vs. controls (AUC: 0.983, P<0.0001), AD vs, CBD (AUC: 0.986, p = 0.0001), and AD vs. PSP (AUC: 0.967, p<0.0001). More importantly, ptau396 was able to detect subjects with MCI vs. cognitively normal controls (AUC: 0.835, p = 0.0032). Immunohistochemical staining of postmortem brain tissue with p‐tau396 antibody also showed increased neurofibrillary tangle pathology in the hippocampus and temporal cortex of subjects with MCI. Machine learning‐based image analysis reliably detected more severe tau burden in subjects with MCI vs. cognitively normal controls, even among subjects with the same Braak stages. Conclusion p‐tau396 is a promising novel biomarker for early detection of AD and differential diagnosis of AD from other tauopathies. Ongoing work is aimed at developing ultrasensitive assays, such as SIMOA, to detect p‐tau396 from patient blood and CSF.