Abstract Body: In the heart, Ca/Calmodulin Kinase IIδ (CaMKIIδ) functions to maintain electromechanical and Ca²⁺ homeostasis. Post-translational modifications (PTMs) of CaMKIIδ render its activity autonomous of Ca/CaM. In hypertrophic cardiomyopathy (HCM), increased levels of autonomous CaMKIIδ are linked to disease progression. We have shown that the pathogenic role of CaMKIIδ in HCM is mutation-specific: autonomous CaMKIIδ is elevated in cardiac troponin T (cTnT)-R92W mice, but not in cTnT-R92L mice. These results pose a clinically relevant and critical question: what is the trigger for CaMKIIδ dysregulation in HCM, such that it is only found with certain mutations? We hypothesize that distinct modifications in thin filament (TF) biophysical properties altered by point mutations result in early Ca²⁺ dysregulation and thus CaMKIIδ overactivity. Specifically, cTnT-R92W TFs display faster Ca²⁺ dissociation kinetics compared to wild type TFs, whereas cTnT-R92L TFs have slower dissociation kinetics, thus we propose accelerated Ca²⁺ dissociation kinetics as a mutation-specific trigger of CaMKIIδ dysregulation. To test this, we are using targeted mass spectrometry following CaMKIIδ immunoprecipitation to assess Thr-287 phosphorylation and Met-281/282 oxidation levels in mice expressing the R94H, I79N, or R92W mutations in cTnT, which exhibit differing Ca²⁺ dissociation kinetics. Preliminary data shows that both PTMs can be readily detected in our samples. As CaMKIIδ dysregulation plays a pathogenic role in HCM, we also aim to test the efficacy of small molecule-based CaMKIIδ inhibition in cTnT-R92W mice using two candidate small molecules, hesperadin and ruxolitinib. Both molecules acutely inhibit CaMKIIδ activity in vivo upon isoproterenol stimulation, and two-week treatment of cTnT-R92W mice with ruxolitinib improves systolic and diastolic function with no effect in control mice, implicating small molecule inhibition of CaMKIIδ as a potential therapeutic option in HCM. To continue these studies, we will explore Ca²⁺ handling in the three HCM mouse models to further establish potential mechanistic pathways for CaMKIIδ dysregulation and will extend small molecule treatment to longer timepoints to further evaluate its efficacy as an HCM therapeutic.