The global clonal complexity of the murine blood system declines throughout life and after serial transplantation.

Although many recent studies describe the emergence and prevalence of 'clonal-hematopoiesis of indeterminate-potential' (CHIP) in aged human populations, a systematic analysis of the numbers of clones supporting steady-state hematopoiesis throughout mammalian life is lacking. Previous efforts relied on transplantation of 'barcoded' hematopoietic stem cells (HSC) to track the contribution of HSC clones to reconstituted blood. However, ex vivo manipulation and transplantation alter HSC function and thus may not reflect the biology of steady-state hematopoiesis. Using a non-invasive in vivo color-labeling system, we report the first comprehensive analysis of the changing global clonal complexity of steady-state hematopoiesis during the natural murine lifespan. We observed that the number of clones (i.e. clonal complexity) supporting the major blood and bone marrow hematopoietic compartments decline with age by about 30% and 60%, respectively. Aging dramatically reduced HSC in vivo repopulating activity and lymphoid potential while increasing functional heterogeneity. Continuous challenge of the hematopoietic system by serial transplantation provoked the clonal collapse of both young and aged hematopoietic systems. Whole exome sequencing of serially transplanted aged and young hematopoietic clones confirmed oligoclonal hematopoiesis and revealed mutations in at least 27 genes, including nonsense, missense and deletion mutations in Bcl11b , Hist1h2ac , Npy2r , Notch3 , Ptprr and Top2b .