Age-related loss of function in the human haematopoietic system is well documented, manifesting as reduced regenerative capacity, age-related cytopenias and immune dysfunction. However, the cellular and population level changes that underpin both this functional decline and the increased risk of clonal haematopoiesis and blood cancer in the elderly remain elusive. Here we performed whole genome sequencing on >3350 single haematopoietic stem cell / multipotent progenitors (HSC/MPP) derived colonies across 10 haematologically normal subjects aged 0 to 81. We found that HSC/MPPs accumulated 17 single nucleotide variants per year post birth and had a reduction in telomere length of 50bp per year throughout young adult life. We reconstructed phylogenies of the sampled HSC/MPPs to interrogate changes in clonal dynamics through life. Haematopoiesis in adults aged less than 65 was predominantly polyclonal, with few known driver mutations. In contrast, individuals aged over 75 displayed a profound change in clonal structure, with frequent clonal expansions, many unexplained by known driver mutations. The ratio of non-synonymous to synonymous mutations revealed widespread positive selection, estimating around 1000 driver mutations in the dataset (10-fold more than the number of known drivers). We identified novel genes ZNF318 and HIST2H3D as being under positive selection, despite not being enriched in myeloid malignancies. Our data show that HSC clonal dynamics is more complex than previously thought. One implication is that by old age, the majority of HSCs carry at least one of a number of largely undescribed driver mutations, which may underlie aspects of their functional decline.
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