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Loss of Ezh2 function reshapes DNA replication initiation landscape

ABSTRACT: DNA replication initiation is made possible by the assembly of pre- replication complexes (Pre-RCs) at genomic locations called DNA replication initiation sites (origins) during G1-phase. Although there are no conserved characteristics of metazoan origins on which Pre-RCs form, previous findings suggest that origins are strongly associated with open, active chromatin regions of the genome such as promoters, enhancers, and active histone marks. However, over a third of transcriptionally silent genes associate with DNA replication initiation activity in mESC cells, and most of these genes are bound and repressed by the Polycomb repressive complex 2 (PRC2) through the repressive H3K27me3 mark. This is the strongest observed association of an epigenetic regulator element with replication origin activity. Thus, I wished to ask whether Polycomb-mediated gene repression plays a direct role in the recruitment of DNA replication origins activity to silent genes. I comprehensively characterized the consequences of PRC2 catalytic activity (EZH2 subunit) depletion on the activity of DNA replication origins in mESC through a genome-wide evaluation. My findings suggest that absence of EZH2 results in increased DNA replication initiation activity at EZH2-bound sites. Interestingly, the increase in DNA replication initiation activity does not correlate with transcriptional de-repression or acquisition of the activating marks H3K4me3 and H3K27ac but loss of the repressive H3K27me3 mark leading to increased accessibility of chromatin.

ORGANISM(S): Mus musculus

PROVIDER: GSE205756 | GEO | 2023/04/19

REPOSITORIES: GEO

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Project description:Background: Eukaryotic cells must inhibit re-initiation of DNA replication at each of the thousands of origins in their genome because re-initiation events can generate genomic alterations with extraordinary frequency. To minimize the probability of re-initiation from so many origins, cells use a battery of regulatory mechanisms that reduce the activity of replication initiation proteins. Given the global nature of these mechanisms, it has been presumed that all origins are inhibited identically. However, transiently disabling these mechanisms causes replication origins to re-initiate with diverse efficiencies, which do not correlate with known differences in the efficiency or timing of origin initiation during normal DNA replication. These observations suggest an additional local layer of replication control that can differentially influence how re-initiation is regulated at distinct origins. Principal Findings: We have identified novel genetic elements that are necessary for preferential re-initiation of two origins and sufficient to confer preferential re-initiation on heterologous origins when the control of re-initiation is partially deregulated. The elements do not enhance the S phase timing or efficiency of adjacent origins and thus are specifically acting as re-initiation promoters (RIPs). We have mapped the two RIPs to ~60bp AT rich sequences that act in a distance- and sequence-dependent manner. During the induction of re-replication, Mcm2-7 re-associates both with origins that preferentially re-initiate and origins that do not, suggesting that the RIP elements can overcome a block to re-initiation imposed after Mcm2-7 associates with origins. Conclusions/Significance: We have uncovered a local level of control in the block to re-initiation. This local control creates a complex genomic landscape of re-replication potential that is revealed when global mechanisms preventing re-replication are peeled away. Hence, if re-replication does contribute to genomic alterations, as has been speculated for cancer cells, some regions of the genome may be more susceptible to these alterations than others. 142 array CGH experiments are presented. Each experiment is done in experimental replicate. Cells grown as described in Richardson CD and Li JJ PLoS Genetics 2014. Ch1 samples taken from replicating (S phase) or re-replicating (M phase + 3/6 hour induced). Ch2 samples taken from M phase arrested DNA.

Project description:Eukaryotic cells must inhibit re-initiation of DNA replication at each of the thousands of origins in their genome because re-initiation can generate genomic alterations with extraordinary frequency. To minimize the probability of re-initiation from so many origins, cells use a battery of regulatory mechanisms that reduce the activity of replication initiation proteins. Given the global nature of these mechanisms, it has been presumed that all origins are inhibited identically. However, origins re-initiate with diverse efficiencies when these mechanisms are disabled, and this diversity cannot be explained by differences in the efficiency or timing of origin initiation during normal S phase replication. This observation raises the possibility of an additional layer of replication control that can differentially regulate re-initiation at distinct origins. We have identified novel genetic elements that are necessary for preferential re-initiation of two origins and sufficient to confer preferential re-initiation on heterologous origins when the control of re-initiation is partially deregulated. The elements do not enhance the S phase timing or efficiency of adjacent origins and thus are specifically acting as re-initiation promoters (RIPs). We have mapped the two RIPs to ~60 bp AT rich sequences that act in a distance- and sequence-dependent manner. During the induction of re-replication, Mcm2-7 reassociates both with origins that preferentially re-initiate and origins that do not, suggesting that the RIP elements can overcome a block to re-initiation imposed after Mcm2-7 associates with origins. Our findings identify a local level of control in the block to re-initiation. This local control creates a complex genomic landscape of re-replication potential that is revealed when global mechanisms preventing re-replication are compromised. Hence, if re-replication does contribute to genomic alterations, as has been speculated for cancer cells, some regions of the genome may be more susceptible to these alterations than others.

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Loss of Ezh2 function reshapes DNA replication initiation landscape

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