Sarah Lehman Abstracts

  Sarah Lehman
  Ph.D. Candidate
  Physiological Sciences GIDP

  Biophysical Society Annual Meeting
  Los Angeles, CA
  February 27 – March 2, 2016

"DIFFERENTIAL EFFECTS OF CAMKII ACTIVITY IN HCM-LINKED TNT MUTATIONS"

Sarah J. Lehman1, Lauren Tal-Grinspan2, Melissa Lynn1, Mark E. Anderson3, Jil C. Tardiff4. 1Physiology, University of Arizona, Tucson, AZ, USA, 2Albert Einstein College of Medicine, Bronx, NY, USA, 3University of Iowa, Iowa City, IA, USA, 4University of Arizona, Tucson, AZ, USA.

Abstract:

Mutations in human cardiac troponin T (hcTnT) account for approximately 5-10% of all Hypertrophic Cardiomyopathy (HCM). Residue 92 (Arg) in cTnT is a mutational hotspot. Despite differing by only a single amino acid, Arg92Leu (R92L) and Arg92Trp (R92W) are associated with varying clinical presentations. In the course of studying the differential effects of the R92 mutations on Ca2+ homeostasis, an increase in phospholamban (PLB) phosphorylation at Thr17 was observed only in R92W mice, suggesting a potential mutation-specific role for CaMKII activation in cTnT-linked HCM. To directly test this hypothesis we crossed CaMKII peptide inhibitor mice (AC3I) to both R92W and R92L transgenic mice to evaluate whether a partial inhibition of CaMKII activity could specifically alter the natural history of HCM via modulation of SERCA2 activity. In the AC3IxR92W mice, a compensatory increase in PKA- mediated PLB-Ser16 phosphorylation led to an increase SERCA2 activity coupled with a decrease in atrial mass, indicating improved cardiac function. AC3IxR92L mice did not show  any change in SERCA2 activity or morphology, suggesting minimal role of CaMKII in the progression of R92L-linked HCM. R92W has been shown to cause an increase in the Ca2+ dissociation rate from the thin filament, leading to an increase in resting Ca2+ concentration. This increase in Ca2+ initiates aberrant and likely pathophysiologic CaMKII activity, contributing to the progression of HCM in these patients. Thus, targeting the activity of CaMKII may be a potential drug therapy for patients expressing the R92W mutation. Ongoing work aims to elucidate the effects of conventional drug therapies on mice expressing these two mutations to differentiate treatment for patients.

Abstract for Lay Audience

Hypertrophic Cardiomyopathy (HCM) is a genetic form of heart failure that affects 1 in 500 Americans and is the leading cause of sudden cardiac death in young people and athletes. This disease commonly arises from mutations found in the contractile proteins of the heart. However, the progression of disease differs for each mutation, complicating clinical treatment of these patients. Therefore, we aim to determine the mechanisms by which cardiac protein mutations lead to HCM in an effort to develop differential therapeutics. We studied two mutations found within the cardiac protein troponin T at site arginine 92 (TnT R92L and TnT R92W) which are known to cause varying degrees of disease severity in the human population. Two mouse models (R92L and R92W) that have been previously characterized were utilized to study the calcium handling within the hearts as calcium plays a critical role in activation of contraction. Specifically, we monitored calcium removal from the cardiac myocyte via SERCA2a Ca2+-ATPase pump activity to better define the alterations in contraction and relaxation caused by the R92L or R92W mutations. Determining the pump activity will provide insight into the intracellular calcium storage of the cardiomyocyte, and thus a better understanding of how calcium is regulated on a beat-to-beat basis in these hearts.  Although they differ by only a single amino acid, we hypothesized that hearts from these animals will have differential effects on SERCA2a pump activity. This mutation-specific phenomenon is likely the result of differences in the regulation of the pump by secondary proteins that are down-stream of the TnT mutations. Thus, it is important to not only understand the primary disruptions caused within the mutated protein, but also examine the down- stream effects throughout the entire heart that also result from the mutation. Our ultimate goal is to define the primary insult to the cardiac proteins and track the progression of these mutations in order to developed therapeutic interventions. These therapies will be designed to delay or prevent disease progression and if possible, reverse the disease in patients, thus greatly reducing the risk of sudden cardiac death and improving the patient’s quality of life.