Abstract
The methylation state of lysine 20 on histone H4 (H4K20) has been linked to chromatin compaction, transcription, DNA repair and DNA replication. Monomethylation of H4K20 (H4K20me1) is mediated by the cell cycle-regulated histone methyltransferase PR-Set7. PR-Set7 depletion in mammalian cells results in defective S phase progression and the accumulation of DNA damage, which has been partially attributed to defects in origin selection and activation. However, these studies were limited to only a handful of mammalian origins, and it remains unclear how PR-Set7 and H4K20 methylation impact the replication program on a genomic scale. We employed genetic, cytological, and genomic approaches to better understand the role of PR-Set7 and H4K20 methylation in regulating DNA replication and genome stability in Drosophila cells. We find that deregulation of H4K20 methylation had no impact on origin activation throughout the genome. Instead, depletion of PR-Set7 and loss of H4K20me1 results in the accumulation of DNA damage and an ATR-dependent cell cycle arrest. Coincident with the ATR-dependent cell cycle arrest, we find increased DNA damage that is specifically limited to late replicating regions of the Drosophila genome, suggesting that PR-Set7-mediated monomethylation of H4K20 is critical for maintaining the genomic integrity of late replicating domains.
Highlights
Histone post-translational modifications (PTMs) regulate almost all DNA-templated processes including DNA replication, transcription and DNA repair
Deregulation of H4K20 methylation had no impact on origin activation throughout the genome; instead, we found that the DNA damage marker, phosphorylated H2A.v (γ-H2A.v), accumulated in late replicating domains in the absence of PR-Set7
A number of histone PTMs, including methylation of lysine 20 on histone H4 (H4K20), have been linked to the regulation of the replication program; the majority of these studies have been limited in scope, surveying only a few select loci for origin activity
Summary
Histone post-translational modifications (PTMs) regulate almost all DNA-templated processes including DNA replication, transcription and DNA repair. Deregulation of these epigenetic histone modifications has the potential to lead to catastrophic consequences at both the cellular and organismal level. PR-Set harbors a conserved PIP-box motif and undergoes PCNA- and CRL4Cdt2-mediated degradationduring S phase [8,9,10,11,12] This interaction between the PR-Set PIP-box and PCNA is conserved in Drosophilacell lines, where the levels of PR-Set and H4K20me display a similar cell cycle oscillation pattern as seen in mammalian systems [13]
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