Quantification of tau phosphorylated at threonine 217 using a novel ultrasensitive immunoassay distinguishes Alzheimer’s disease from healthy controls

Abstract Background One of the major neuropathological hallmarks of a brain affected by Alzheimer’s disease (AD) is neurofibrillary tangles (NFTs), which are composed of aggregated, and sometimes truncated, hyperphosphorylated tau protein. Clinical diagnosis of AD is often aided by the use of biomarkers. One of the three core cerebrospinal fluid (CSF) biomarkers for AD, tau phosphorylated at amino acid 181 (p‐tauT181), shows quite a high sensitivity and specificity for AD but there is also a relatively large overlap between AD non‐demented subjects. Recently, tau species phosphorylated at amino acid 217 (p‐tauT217) quantified by mass spectrometry was shown to correlate with amyloid and tau lesions in the brain and clinical disease progression, and thus seem to be a new potential AD biomarker. Here, we present a novel immunoassay used to quantify p‐tauT217 in CSF and show that it outperforms the classical AD biomarkers. Method CSF samples from AD, healthy controls (Co) (cohort1 [Lund University] and cohort2 [TRIAD cohort, McGill University]), frontotemporal dementia (FTD) and mild cognitive impairment (MCI) (cohort2) were analysed by a novel ultrasensitive immunoassay on the Single molecule array (Simoa) platform. The Simoa assay was based on two in‐house generated monoclonal antibodies. Result CSF levels of p‐tauT217 were significantly increased in AD compared with Co in both cohort1 (p<0.0001, 4.3‐fold increase) and cohort2 (p=0.0003, 3.5‐fold increase). In cohort2, p‐tauT217 was also increased in AD compared with FTD (p=0.0059) and amyloid PET‐negative MCI (p=0.0203, 4.8‐fold increase), and in amyloid PET‐positive MCI compared with Co (p<0.0001, 3.9‐fold increase), amyloid PET‐negative MCI (p=0.0035, 5.2‐fold increase) and FTD (p=0.0011). Conclusion We present performance data on a novel ultrasensitive immunoassay capable of quantifying p‐tauT217 in CSF. There was much less overlap between AD patients and healthy controls, as well as between amyloid PET‐positive and ‐negative MCI when using p‐tauT217 compared with p‐tauT181 in both cohorts, thus indicating that p‐tauT217 reflects the presence of AD pathology better. In conclusion, p‐tauT217 is a very promising new biomarker for AD, which potentially could be used to aid clinical diagnosis, even at pre‐dementia stages of the disease.