Abstract Alzheimer's disease (AD), the most common form of dementia in older adults, is a neurodegenerative disorder characterized by progressive decline of memory and cognition. AD is multifaceted and heterogeneous, which prevents clear mechanistic understanding of AD pathogenesis and therefore hinders development of effective therapeutics. Meanwhile, post-mortem epidemiology indicated that AD onset was associated with elevated activity of the histone methyltransferase G9a (EHMT2) in diseased brain, implicating G9a activity- associated pathways in AD pathogenesis. Using our c hromatin-activity-based chemoproteomic (ChaC) approach that enables dissection of AD heterogeneity , we (the Chen lab) have discovered a noncanonical, translation regulatory function of G9a in AD pathogenesis. Further, we deduced the mechanism of action of a brain-penetrant G9a-targetd drug, MS1262, w here G9a inhibition by MS1262 reversed AD patient proteomes, particularly the AD-disturbed expression or phosphorylation of proteins related to cognition and learning, synaptic transmission, synaptogenesis, and hyperactive behavior. Correspondingly, intermittent MS1262 treatment restored cognitive and affective functions in mid/late-stage 5xFAD mice to the healthy level (p <0.0027). Thus, we will develop a new mechanism-based AD therapeutics. TransChromix, LLC, a startup company created by the NC Kick-Start program, and Professor Xian Chen at the UNC School of Medicine, will conduct this project. Our preliminary results show that MS1262 therapy of AD is clinically practical: Comparison of proteomic analyses of MS1262-treated AD mice with human data from large cohorts of AD patients revealed that MS1262 reversed the patient proteomic landscapes that were highly correlated with AD pathology and cognitive decline. This mouse-to-human conservation of G9a-translated AD proteomes suggests that the therapeutic effects of MS1262 in mice could extend to AD patients. Thus, we will test the hypothesis that targeting G9a-mediated translational mechanisms using brain-penetrant drugs is a specific and effective strategy to prevent and/or reverse AD progression. In Phase I, we will test the hypothesis that targeting G9a-mediated translational mechanisms using MS1262 is a specific and effective strategy to prevent and/or reverse AD progression. Specifically, in the MS1262 -treated AD mouse models including 5xFAD mice and other established AD mouse models at different stages of AD progression we will (1) comprehensively determine the long-term efficacy and the specificity of MS1262 on synaptic and cognitive function, (2) measure inhibitor toxicity and brain specificity by dose range finding and pharmacokinetic studies, and (3) validate the clinical accuracy of G9a inhibitory mechanism. In Phase II we will use the Phase I-optimized doses with low toxicity to test the treatment efficacy for AD patients. Meanwhile, we will develop companion diagnostic assays to stratify patients for enhanced th...