Role and regulation of cyclin F in the cellular responses to ionizing radiation (IR).

Each cell in our body duplicates itself by copying its main information hub: the DNA. To ensure that the information contained in the DNA is not altered during the process of cell division, cells have evolved checkpoints that monitor the correct execution of DNA replication. Studies from the laboratory of Dr Vincenzo D’Angiolella have revealed how the balance of the building blocks composing the DNA molecule is maintained in the cell cycle and how this is coordinated with the execution of checkpoints. The laboratory discovered that the organized synthesis and removal of the enzymes responsible for the production of the building blocks is crucial to ensure the fidelity of DNA replication and cell survival.
A characteristic feature cancer cells is the loss of checkpoints. The lack of checkpoint control provides cancer cells with a proliferative advantage at the cost of increased susceptibility to agents damaging the DNA molecule. This is exploited in cancer therapy through the use of radiation therapy.
Future studies will focus on elucidating the function of genes that modulate the checkpoint responses of cancer cells to radiation therapy. These studies will reveal novel potential weakness to increase the efficacy of radiation therapy for cancer treatment.

A feature of cells exposed to IR is the activation of a G2/M checkpoint, which controls cell cycle progression, allows damage repair or drives cell death. From work in my laboratory, cyclin F has emerged as a crucial regulator of multiple aspects of the G2 phase (D’Angiolella et al., Trends in Cell Biol, 2013). I found that cyclin F act as an E3 ubiquitin ligase and induces the ubiquitin-mediated proteolysis of the Ribonucleotide Reductase subunit (RRM2) (D’Angiolella et al., Cell, 2012), which controls production of dNTPs, the building blocks of DNA. After the induction of the G2/M checkpoint by IR, cyclin F is proteolyzed to allow production of dNTPs for DNA repair and to modulate multiple aspects of the IR response. How cyclin F activity and levels are regulated by the checkpoint, however, is not understood. We have identified discrete phosphorylation events on cyclin F, which change dynamically upon checkpoint induction and we have shown that cyclin F is phosphorylated directly by cyclin-dependent kinases, Wee1 and Chk1. Furthermore, we have identified the E3 ubiquitin ligase ßtrcp as an interacting partner and regulator of cyclin F. We will establish how cyclin F activity and levels are controlled during the checkpoint induced by IR and the impact on the production of dNTPs (Aim1). These studies will reveal how DNA replication and checkpoint responses induced by IR in S and G2 phases of the cell cycle are coordinated with dNTP production. Since altered cyclin F levels have been described in hepatocellular carcinoma and ovarian cancer, where cyclin F levels correlate with survival outcome (Carlucci, D’Angiolella, Br J Cancer, 2015), our studies may also predict patient responses to IR and checkpoint inhibitors in the presence of altered cyclin F levels.
In addition to RRM2, preliminary studies from my laboratory have identified Scaper as an additional substrate of Cyclin F. Scaper is a poorly characterized regulator of cell cycle progression that regulates cyclin A localization (Tsang et al., J Cell Biol, 2007). We have observed that Scaper has a crucial role in regulating cell survival after IR. We propose to further investigate the role of Scaper and its regulation by ubiquitin mediated proteolysis mediated by cyclin F and APC/C (Aim2). Interestingly, deletion and mutations of Scaper are frequent in cancer cell lines and tumors. Our studies may predict patient responses to IR and checkpoint inhibitors in cancers bearing Scaper mutations.