Defining the role of Notch signalling in the formation of cholangiocarcinoma
Cholangiocarcinoma (CC) is characterized by a high degree of genetic instability, which is believed to contribute to its resistance towards therapeutic interventions. We have recently shown that expression of the intracellular domain of the Notch1 receptor (NICD) in mice leads to the formation of intrahepatic cholangiocarcinoma (ICC) which originates from hepatic progenitor cells. These ICC are characterized by their high expression of Cyclin E and a dramatically increased level of DNA damage. We showed that Notch directly transactivates the Cyclin E promoter and that Cyclin E is the critical downstream target of Notch induced transformation in ICC. Cyclin E exerts its oncogenic function through an acceleration of the cell cycle (in complex 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. While the ability of p27 to act as a cki is well established, we recently identified in cholangiocarcinoma cells an additional new function of this protein which we believe is of central importance for its activity as a tumor suppressor protein. We found that p27 is directly regulating the response to DNA damage through its ability to bind and regulate the activity of the rad17 protein. Rad17 is an essential protein which loads DNA damage response complexes (9-1-1 and MRN) onto DNA. Loss of this activity results in unresolved DNA damage, genetic instability and tumor formation. We therefore hypothesize that Notch, Cyclin E, p27 and rad17 form a network of proteins which controls cellular proliferation and DNA damage in ICC. Importantly, the degradation of the Notch and Cyclin Eproteins is under the control of the Fbox protein FBXW7 which is mutated or downregulated in a significant fraction of human ICC. In this project we will model the genetic changes observed in human ICC, in order to characterize the cellular and molecular changes that drive cholangiocarcinogenesis under conditions of increased replicative stress in vivo. We will extend these studies by analyzing the DNA damage response mechanisms which are controlled by the interaction of p27 and rad17 in human CC cell lines in vitro and by conducting a comprehensive in vivo screen for collaborating genetic changes. Together our study will reveal the mechanisms which control cell proliferation and DNA damage responses in Notch driven ICC and will help to identify potential targets for therapeutic interventions.
