On Wednesday, April 23, 2025 Gregory Milburn successfully defended his dissertation and earned his doctoral degree in physiology. Congratulations, Dr. Milburn!
Biochemical and biophysical properties of failing myocardium in rare patient populations
Heart failure is one of the leading causes of morbidity and mortality in the United States, yet the presence of diverse underlying etiologies complicates clinical and translational research on heart failure. While some forms of heart failure, such as ischemic disease or inherited mutations, have been extensively studied, research on rarer forms of heart failure and the patients they impact has lagged behind. Improving our understanding of the molecular drivers of disease in these rarer patient groups may help to improve diagnostic techniques and therapeutic strategies. This work examines two rare patient populations: patients with ATTR cardiac amyloidosis and patients receiving mechanical assist device support. The studies presented here use a mix of biochemical and biophysical techniques to better understand molecular changes in the myocardium of these patients.
In the first study, we show that myocardium from patients with cardiac amyloidosis displays dramatically increased fibrosis and reduced production without significant changes in cross-bridge kinetics. This would suggest the decrease in force generation at the tissue level is driven by decreased contractile tissue per unit area due to the deposition of fibrosis and amyloid. Additionally, we show that the passive stiffness of amyloidosis myocardium is not significantly different from non-failing myocardium, suggesting increased ventricular wall thickness rather than stiffer myocardium may be responsible for the restrictive filling seen in amyloidosis. Lastly, we show decreased phosphorylation of sarcomeric regulatory proteins such as troponin I (TnI) and myosin binding protein-C (MyBPC). These changes are similar to those observed in ischemic and non-ischemic heart failure, suggesting hypophosphorylation of sarcomeric proteins may be present in heart failure independent of underlying etiology.
Our second study used paired samples from patients before and after the implantation of mechanical assist devices to examine the impact of unloading on the myocardium. We show increased phosphorylation of multiple regulatory proteins, including TnI and MyBPC, suggesting unloading by these devices promotes some degree of sarcomere-level reverse remodeling. Additional analysis shows that patients with elevated BMI and HbA1C do not exhibit increased phosphorylation of these proteins suggesting that metabolic factors may modulate the effect unloading has on sarcomere phosphorylation. We also report changes in gene expression in sarcomere and adrenergic signaling protein suggesting altered transcriptional regulation and activity in unloaded myocardium.
The results in this dissertation expand our understanding of cardiac amyloidosis disease pathogenesis at the tissue and sarcomere level and highlight the impact of mechanical unloading on sarcomere regulation through increased phosphorylation and altered transcription.