Analysis of cardiac cell turnover in humans by radiocarbon dating and mathematical modeling

Abstract: Cardiovascular disease is the largest cause of morbidity and mortality in the Western World. Disease progression often involves a loss of contracting cells, cardiomyocytes, which leads to cardiac failure and the need for heart transplantation with time. However the shortage of donor hearts is a large problem and a strong motivator for finding alternative solutions; this is the focus of regenerative heart medicine. For new treatment strategies to be effective we first need to better understand the potential and capacity of the heart and its cells. This thesis addresses two questions specifically: 1) Do cardiomyocytes renew in human hearts during healthy aging? 2) How does cardiac disease affect cardiomyocyte renewal? Studies in experimental animals and to a small extent in humans had previously not been able to resolve these questions, mainly because limitations in methods and ethical restrictions. We employed primarily two methodologies, 14C birth dating and mathematical modeling. 14C birth dating is a method developed within the Frisén group that exploits the changes in atmospheric 14C levels due to testing of nuclear weapons during the Cold War. The 14C concentration in the genomic DNA of a cell reflects when the cell was born, and hence the level of renewal. The core part of the mathematical model is a first order partial differential equation (PDE). It describes cells according to their age and how the distribution of ages changes as the individual grows older. We found that human cardiomyocytes in healthy hearts indeed renew throughout life, with a declining turnover not exceeding 1% per year in adult life, and that the cell number is established already at birth. Endothelial and mesenchymal cardiac cells are more dynamic, both in terms of changes in cell number and baseline turnover (Paper II and IV). Preliminary results indicate that ischemic heart disease and dilated cardiomyopathy can increase the renewal rate to 2.7% per year; however it is likely that individual turnover estimates differ from this, which may reflect the differences in disease etiology and patient specific manifestation (Paper I). In order to reach these conclusions we developed a method to isolate cardiomyocyte nuclei, based on the molecular markers, PCM-1, cTroponin T, and cTroponin I (Paper III). This work shows that adult cardiomyocytes in healthy and diseased hearts have a measurable regenerative capacity, suggesting that it can be exploited for developing new therapeutic strategies to treat heart disease.