Clonal fitness decline in somatic differentiation hierarchies

The concept of clonal fitness is fundamental to describe the evolutionary dynamics in somatic tissues. It is now well established that otherwise healthy somatic tissues become increasingly populated by expanding clones with age. However, the dynamic properties and respective fitnesses of these clones are less well understood. Here we show, that in somatic tissues organised as a differentiation hierarchy, theory predicts a natural decline of effective clonal fitness over time in the absence of additional driver events. This decline is intrinsic to the tissue organisation and can be captured quantitatively by a simple heuristic equation that is proportional to 1/time. We also show that the expected fitness decline is directly observable in human haematopoiesis. The predicted short and long term dynamics agree with in vivo observations using data of Neutrophil recovery after bone marrow transplants and naturally progressing Chronic Lymphocyte Leukemia (CLL). We further show that theory predicts the existence of a long term equilibrium fitness. All CLL patients transition into a stable equilibrium fitness eventually. We find significant inter-patient variation of long term fitness and a strong correlation with disease aggressiveness. Interestingly, CLL long term fitness can be forecast based on the early stages of disease progression, suggesting a Big Bang like model for CLL evolution.