Notch1-Cyclin E-p27kip1 and RAD17 form a network of proteins which control cellular proliferation and DNA damage response in cholangiocarcinoma

Cholangiocellular cancers are characterized by a high degree of genetic instability, which is believed to contribute to the heterogeneous malignant phenotype and the resistance towards therapeutic interventions. Previously we had shown that expression of the intracellular domain of Notch1 (NICD) protein leads to the formation of progenitor derived CCC in mice. NICD overexpressing CCC are characterized by high expression of Cyclin E and greatly increased levels of DNA damage. Cyclin E exerts its oncogenic function through the acceleration of the cell cycle (together with cdk2) and through its ability to induce replicative stress and DNA damage. Cyclin E/cdk2 complexes are in turn inhibited by the cyclin kinase inhibitor (cki) p27kip1. p27 is an established prognostic marker in CCC and many tumors show reduced expression of this protein which correlates with poor prognosis. We recently identified a new function of p27 in CCC cells in directly regulating the response to DNA damage. p27 binds RAD17 an essential protein which loads DNA damage response complexes (9 – 1-1 and MRN) onto DNA. Upon DNA damage the p27-RAD17 complex dissociates after phosphorylation of rad17 by the ATM&ATR kinase. Dissociation from p27 is an essential step in the DNA damage response pathway. We therefore hypothesize that Notch1-Cyclin E-p27 and RAD17 together form a network of proteins which control cellular proliferation and DNA damage response in cholangiocarcinoma.

Together our study allow us to reveal the mechanism which control cell proliferation and DNA damage responses in Notch1 driven CCC and identify potential targets for therapeutic interventions.