Cell cycle re‐entry from quiescence promotes genome instability

Abstract

Animal cells transition between periods of active cell division cycles and periods of quiescence without cell division. Prior to cell division, DNA replication initiates at thousands of origins across the genome during S phase of every cell cycle. Each origin that initiates must have been licensed for replication in the preceding G1 phase by the regulated loading of Minichromosome maintenance (MCM) complexes onto DNA. Cells with insufficient MCM loading are underlicensed, risk incomplete replication, and are hypersensitive to replication stress and DNA damage. Licensing is tightly restricted to G1 phase, but the length of G1 varies. How mammalian cells ensure sufficient licensing in cells with either long or short G1 phases is still poorly understood. We recently reported that cycling cells with naturally short G1 phases license origins faster than cells with long G1s and that the fast loading is important for a short G1. The first G1 phase of cells re‐entering the cell cycle from quiescence is longer than G1 phase in actively cycling cells. We hypothesized that the differences in G1 length during cell cycle re‐entry and G1 length in dividing cells will result in differences in the amount or rate of origin licensing in the different G1 phases. We used single cell flow cytometry and live cell imaging to explore the relationships among cell cycle re‐entry, G1 length, origin licensing, and the activity of the family of cyclin‐dependent kinases that drive the G1‐to‐S phase transition in untransformed human cells. Despite the extra time in G1 phase, cells re‐entering the cell cycle are significantly underlicensed in the first cell cycle but not the second or subsequent cell cycles. Moreover, cells in the first S phase are hypersensitive to replication stress. Thus, G1 phase upon cell cycle re‐entry is characterized by slow (or delayed) origin licensing combined with S phase entry before all cells are fully licensed. Overproducing cyclin E or cyclin A can uncouple G1 length from origin licensing by driving premature S phase entry. We predict that the normal coupling of origin licensing and Cdk2 activation is weaker at the first G1/S transition. Cells re‐entering the cell cycle from quiescence have increased levels of Cdk inhibitor proteins which may contribute to the uncoupling of licensing timing or speed and S phase entry. The transition from quiescence to active division is a particularly sensitive time for DNA damage and the number of transitions into and out of quiescence may play a role in long term cell viability and genome stability.Support or Funding InformationThis work was supported by a fellowship from the NSF (DGE‐1144081) to J.P.M and by the NIH to J.G.C. (GM083024 and GM102413). Additional funding was provided by the W.M. Keck Foundation.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.