Remarkably, the mouse CH driver mutations confer about the same fitness advantages as in humans, suggesting conserved evolutionary pressures. Nevertheless, major clonal expansions remain rare in mice – the large stem cell pool and rapid turnover set a high drift threshold. 13/n

Lab mice live in a highly protected environment, unlike humans. We found a variety of exposures stimulated clonal expansions. For instance, chemo-treated mice developed larger Trp53-mutant clones, paralleling what we see in human CH. 12/n

While rare, these clones exhibited clear signs of positive selection; the proportion of nonsynonymous to synonymous mutations was elevated – but each clone’s growth is constrained. 11/n

To probe this further, we ultra-deep sequenced 24 key blood cell genes in young and old mice . This revealed clonal expansions driven by classic CH drivers like Dnmt3a, Tet2, and Bcor. The clones were very small (~0.02% of blood cells). 10/n

Unlike older humans where massive clones can overtake blood production, mice maintained a diverse stem cell pool, with thousands of clones actively contributing to blood production and minimal loss of stem cell diversity. 8/n

The pattern of branchpoints (i.e. cell divisions) indicates that the murine HSC/MPP pool continuously grows throughout aging, expanding to ~70,000 cells with age. Each HSC divides every ~6 weeks, with one cell leaving the pool every ~18 weeks. 7/n

We constructed phylogenetic “family trees” of the single cells based on their relatedness determined by shared mutations. The striking “banding” pattern of blue and red indicates HSCs and MPPs are largely separate pools that work in parallel to regenerate blood. 6/n

First big result: mutation rates. Mouse HSCs accumulate ~45 mutations per year – by end of life an average HSC harbors ~160 mutations. This rate is only ~3× higher than in humans, despite their ~30x difference in lifespan! 5/n

We isolated hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs) from young and old mice and performed WGS on over 1800 colonies grown from single cells, mapping around 222,000 total mutations. 4/n

How conserved are these aging processes across species, especially in short-lived mammals like mice? We studied blood stem cell evolution in mice across their lifespan, coupling colony whole genome sequencing (WGS) and targeted duplex-corrected sequencing. 3/n

As we age, our stem cells accumulate DNA mutations. Some mutations give growth advantage leading to greater cellular expansion – a process called clonal hematopoiesis (CH) in the blood. Such somatic mosaicism happens in all organs – we become mosaic patchworks with age! 2/n