Mechanisms of massive genome reactivation during quiescence exit

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ABSTRACT Many cells in our body are alive, but reversibly dormant, in a conserved stage of the cell cycle called quiescence. Quiescent cells can inhibit DNA-templated processes via the establishment of a repressive chromatin environment, and it was recently discovered that widespread histone deacetylation is required for quiescence. How cells overcome this repressive chromatin state for quiescence exit is unknown. The goal of this proposal is to elucidate fundamental, conserved mechanisms of chromatin regulation of transcription activation and DNA replication in quiescence exit, an understudied area with broad applications to human health. Intriguingly, I have uncovered persisting histone acetylation at inactive genes in quiescent cells that correspond to genes induced immediately during quiescence exit. However, the mechanism underlying this unexpected phenomenon is undefined. This proposal will shed light on the roles of histone acetylation in poising genes for rapid activation and will determine the mechanism for how poised promoter architecture is established in quiescent cells. Downstream of genome reprogramming events during early exit, cells globally reposition nucleosomes. In cycling cells, it has been shown that nucleosome positioning around DNA replication origins (origin) can strongly influence origin activity. However, a model system to investigate mechanisms and functions of nucleosome repositioning around origins in a physiologically relevant context has not been available. Preliminary data from the lab indicate that nucleosomes reposition substantially around origins during quiescence exit compared to cycling cells. Specifically, dramatic changes in the width of nucleosome-depleted regions (NDRs) around origins occur by the first G1 after quiescence exit. Preliminary data suggest that the essential, SWI/SNF family chromatin remodeling enzyme RSC is responsible for this repositioning and that RSC plays a role in regulating S phase. Based on these results, I will elucidate the role of RSC and nucleosome positioning in DNA replication as cells exit quiescence. To determine the conservation of these mechanisms, these models will be tested in human cells. Cumulatively, this proposal will address outstanding questions on how the genome is reprogrammed for DNA-templated processes during quiescence exit.