Intrinsic checkpoint deficiency during cell cycle re-entry from quiescence

Jacob Peter Matson; Amy M. House; Gavin D. Grant; Huaitong Wu; Joanna Perez; Jeanette Gowen Cook

Quiescent hematopoietic stem cells re-enter the cell cycle in response to stresses such as viral infection and then return to quiescence (Cheung and Rando, 2013) . Interestingly, hematopoietic stem cells acquire DNA damage in the first cycle after G0, accumulate DNA damage over time in older stem cells, and become depleted as they age from repeated cell cycle re-entry (Beerman et al., Walter et al., 2015) . Additionally, quiescent human T cells re-enter the first S phase even when treated with siRNA to substantially reduce the amount of loaded MCM to ~5-10% of their normal loaded amount (Orr et al., 2010) . Taken together, these data suggest that at least some quiescent cells in vivo lack a licensing checkpoint in the first cycle, although it is unknown if the first cell cycle is naturally underlicensed compared to the second cycle in vivo. Old hematopoietic stem cells experience more replication stress in the first S phase after G0 compared to young stem cells, possibly because they express less MCM protein. In that regard, a defective licensing checkpoint would be particularly toxic . CC-BY-NC-ND 4.0 International license is made available under a The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/558783 doi: bioRxiv preprint to old quiescent cells re-entering G1 because they may enter S phase with even less loaded MCM than young cells (Flach et al., 2014) . These in vivo studies are consistent with our observation that artificially inducing repeated rounds of quiescence and cell cycle re-entry enhanced replication stress sensitivity (Fig. S2 ). We suggest that over many rounds of quiescence and re-entry, incremental DNA damage accrues from unresolved replication stress or incomplete replication that may pass through mitosis into the next cell generation. The ability of unresolved replication stress to carry forward into subsequent cell cycles was recently established both by artificially inducing underlicensing and by monitoring spontaneous replication stress passing from mother to daughter cells (Moreno et al., 2016; Yang et al., 2017; Arora et al., 2017; Ahuja et al., 2016) . Repeated rounds of underlicensed cell cycle re-entry could contribute to the genome damage that drives both aging and oncogenesis. Such accumulating damage could be analogous to the enhanced wear on an automobile that is routinely driven under stop-and-start conditions vs one driven at steady speeds. By that analogy, the most important consequences of entering a high-risk underlicensed cell cycle may appear not in a single cycle, but rather over an organism's lifetime.