Abstract P2071: Mutation-Specific Calmodulin-Kinase II Activation In Hypertrophic Cardiomyopathy

Abstract

Autophosphorylation of Calmodulin-Kinase IIδ (CaMKIIδ) at Thr-287 leads to autoactivation such that kinase activity is autonomous of Ca 2+ /CaM. In the heart, CaMKIIδ functions to regulate Ca 2+ homeostasis by targeting Ca 2+ handling proteins, including phospholamban (PLB). In sarcomeric hypertrophic cardiomyopathy (HCM), disruption of Ca 2+ homeostasis and aberrant autoactivation of CaMKIIδ has been linked to disease progression. We have previously shown that the pathogenic role of autonomous CaMKIIδ in HCM is mutation-specific. Transgenic mice expressing the HCM mutation R92W in cardiac troponin T (cTnT) exhibited an increase in autonomous CaMKIIδ while cTnT-R92L mice did not. The potentially clinically relevant mechanism(s) underlying this mutation specificity are unresolved. We hypothesize that a potential trigger of autonomous CaMKIIδ is accelerated myofilament Ca 2+ dissociation kinetics. We have previously shown that cTnT-R92W accelerates Ca 2+ dissociation, while cTnT-R92L decelerates Ca 2+ dissociation. Here, we extended our study to include cTnT-I79N and cTnT-R94H to determine if Ca 2+ dissociation rates can be linked to autonomous CaMKIIδ activity and thus provide a potential novel target in HCM. Our data show that cTnT-I79N accelerates Ca 2+ dissociation, and cTnT-R94H decelerates Ca 2+ dissociation, similar to our earlier findings. To determine whether the mutation-specific changes in Ca 2+ dissociation alter CaMKIIδ activity, transgenic mice expressing cTnT-R94H and cTnT-I79N were probed for autophosphorylated CaMKIIδ and Thr-17 phosphorylated PLB. At 2 months, PLB phosphorylation is unchanged in cTnT-R94H and cTnT-I79N mice. At 4 months, cTnT-I79N mice exhibit a trending increase in autonomous CaMKIIδ and cTnT-R94H mice exhibit a modest increase, similar to cTnT-R92W positive controls. These trends are matched by PLB phosphorylation levels. At 6 months, cTnT-R94H mice exhibit normal autonomous CaMKIIδ levels. While future and ongoing work will explore changes in myocellular Ca 2+ homeostasis in more depth, these preliminary data suggest that, unlike cTnT-R94H mice, cTnT-I79N mice exhibit elevated autonomous CaMKIIδ and these mutation-specific differences may underlie observed phenotypic variation in patients.