A Novel Epigenetic Function of WEE1

We discovered that WEE1 kinase phosphorylates histone H2B at tyrosine37 in a short window of 30-40 minutes in late S phase. Strikingly, this phosphorylation occurred upstream of histone cluster 1, Hist1 where majority of histone genes are located.

All eukaryotic cells precisely regulates histone levels by shutting off histone transcription at the end of DNA synthesis to avoid burdening histone turnover machinery to downregulate histones. However, the precise mechanism of this process is not fully understood. We discovered that WEE1 kinase deposit pY37-H2B epigenetic marks upstream of Hist1 cluster, suppressing global histone transcription in both yeast and mammalian cells.

Our data provides crucial evidence that in addition to mRNA turnover, histone transcription shut off would efficiently lower histone transcript levels once DNA synthesis is complete, thus eliminating overproduction of core histones.

Thus, we uncover previously unknown function of WEE1, a cell cycle regulator that has a dual role as an epigenetic modifier that maintains histone transcript levels

  • Reference:
  • Mahajan K, Fang B, Koomen JM and Mahajan NP*. H2B Tyr37 phosphorylation suppresses expression of replication-dependent core histone genes. Nature Structural & Molecular biology 2012. Pubmed ID: 22885324

 

WEE1 epigenetically regulates 5hmC levels

WEE1 kinase has been reported to be aberrantly expressed in melanomas, glioblastoma multiforme (GBMs), triple negative breast cancers (TNBCs) and some prostate cnacers. Dependence of various cancer cells on WEE1 signaling suggests that targeting epigenentic activity of WEE1 is a viable strategy for overcoming tumor proliferation.

 

Clinical significance of WEE1-H2B epigenetic signaling in Melanoma, GBM and prostate cancer

WEE1-H2B epigenetic signaling plays a crucial role in various malignancies such as melanoma, GBM and prostate cancer

H2B Tyr37 phosphorylation upstream of Hist1 suppresses histone mRNA transcription: a model. WEE1 phosphorylates H2B at Tyr37 upstream of histone cluster 1, Hist1. NPAT is excluded from binding to the Hist1 cluster because of its inability to recognize Tyr37-phosphorylated H2B, which in turn inhibits RNA polymerase II (Pol II) recruitment. Subsequently, HIRA is recruited to Tyr37-phosphorylated H2B, preventing NPAT rebinding and effectively resulting in the suppression of histone mRNA synthesis.

A model for WEE1 in chromatin integrity during replication stress Replication protein A (RPA) (small magenta circles) coats ssDNA formed during DNA replication and DNA repair which facilitates the localization of Ataxia-telangiectasia mutated and RAD3-related (ATR) kinase to sites of DNA damage in both human and yeast systems. Activated ATR phosphorylates the downstream checkpoint kinase-1 (CHK1) which in turn phosphorylates and activates WEE1. In addition to WEE1 activation, CHK1 also phosphorylates and inactivates CDC25, a phosphatase that dephosphorylates CDK1. WEE1 plays a crucial role in maintaining chromatin integrity during DNA damage or replication stress by performing two distinct phosphorylations; first, it phosphorylates CDK1 at Y15 preventing entry into mitosis so as to allow repair of damaged DNA. Second, it actively downregulates histone gene transcription by marking H2B at Tyr37 in nucleosomes upstream of histone cluster 1 (Hist1). H2B Tyr37 phosphorylation prevents binding of the transcriptional coactivator NPAT and RNA polymerase II and recruits the histone chaperone HIRA upstream of the Hist1 cluster thereby avoiding overproduction of histones. The collective outcome of these events is initiation of intra-S phase and G2 checkpoints in response to DNA damage or replication stress and maintenance of chromatin integrity during repair of damaged DNA, prior to entry into mitosis.