PROJECT SUMMARY/ABSTRACT tRNA-derived RNA Fragments (tRFs), a newly discovered family of non-coding RNAs (ncRNAs), are emerging as essential disease biomarkers and regulators. Our recent publication demonstrated that tRFs are the most impacted small ncRNAs (sncRNAs) by Alzheimer's disease (AD) in the hippocampus. The changes are mainly from five tRFs, with one proven to correlate with the disease severity of AD experimentally. The correlation between an AD-impacted tRF and AD severity was also present in serum samples , supporting tRFs as promising AD indicators and potential prognostic/diagnostic biomarkers. Herein, we propose exploring a combination of an unbiased discovery method and a newly developed biomedical quantification approach to investigate whether the expression level of tRFs in the peripheral serum reflects the onset and progress of AD. During the discovery R61 phase, we will determine the clinical reliability of serum biomarkers to differentiate AD subjects from healthy individuals or individuals with non-AD dementia or non-dementia neurodegenerative diseases (Aim 1). We will then determine the lead tRF signatures or signature compositions and investigate the correlation between their changes with AD severity (Aim 2). The studies of the R61 phase will be mainly cross-sectional. During the R33 phase, we will investigate whether the AD biomarkers and their quantification assay can distinguish AD from its early mild cognitive impairment (MCI) stage. We will determine biomarker changes in disease progression in patients between baseline and subsequent follow-up patient visits. The patients who are healthy or have stable MCI over the years will be used as controls. The culmination of these aims will benefit both prognosis and diagnosis of AD. The feasibility of this approach is established by sample availability and our extensive research experience in tRFs and clinical service experience in AD. The overall goal of our research is to discover AD molecular biomarkers that could improve AD prevention and diagnosis and monitor the therapeutic efficacy in the future.