Characterization of Hematopoietic Stem Cell Function and Sensitivity in the Homologous Recombination Deficient Exonuclease1^mut Mouse Model

Abstract 1293

Hematopoietic stem cell (HSC) maintenance is essential for sustained longevity and tissue function. The HSC population has lifelong self-renewing capabilities and gives rise to subsets of multipotent progenitor cells, and in turn a progeny of terminally differentiated mature cells consisting of all subtypes of the myeloid and lymphoid lineages. Long term reconstituting HSCs are necessary to replace these differentiated cells after losses caused by normal degradation or damage accumulation, with failure to replenish these stores being linked to a variety of human pathogeneses as well as aging phenotypes.

HSC populations require functional DNA repair pathways in order to maintain their reconstitution capabilities but little is known about the pathways involved or the mechanism of regulation. While the majority of HSCs are quiescent at steady state, endogenous or exogenous stress can force these cells into proliferation, and previous evidence has suggested that the HSC reliance on DNA repair changes with this mobilization. Quiescent HSCs are believed to depend on non-homologous end joining (NHEJ) for repair but prior literature has shown that once forced into cycle, the DNA repair dependency shifts and is shared between homologous recombination (HR) and NHEJ.

We use Exo1 deficiency as a model for homologous recombination loss in mice and demonstrate in vivo that HR is dispensable in quiescent HSCs. This is in contrast to loss of the complementary double strand break repair pathway NHEJ which has been shown to result in severe defects in HSC function. However when we force mobilize HSCs into cycle in vivo using the anti metabolite 5-fluorouracil we are able to demonstrate that the HR defects become detrimental to the animal as shown by increased cellular IR sensitivity and subsequent animal death. Additionally we use competitive repopulation studies to show that indeed the Exo1^mut HSC population is more radiation sensitive after forced mobilization. This work begins to elucidate the consequences of the loss of homologous recombination in hematopoietic stem cells as well as the interplay between cell cycle status and DNA repair dependency.