The p97/VCP system in Ionising Radiation Response

Maintaining the integrity of our cells' DNA and proteins is essential to remain healthy. However, cellular metabolic processes, external factors like UV or medical treatments like ionizing radiation constantly damage our DNA and proteins. If not repaired, such DNA damage can lead to the development of a range of diseases including accelerated ageing, neurodegeneration and cancer or even cell and organismal death. Cells have evolved specialised DNA repair and protein quality control mechanisms to detect and repair DNA and protein damage and thus protect us from the associated diseases. Cancer cells suffer from severe DNA and protein damage but upregulate various cellular mechanisms to allow their survival. One of these mechanisms is upregulation of p97 ATPase, an enzyme that is involved in both DNA repair and removal of damaged proteins. Indeed, it has been shown that altering the functions of p97 enhances selective lethality of various cancer cells, and that the use of p97 inhibitors combined with ionising radiation renders some tumours more radiosensitive. Although this is currently a promising clinical research avenue, the molecular mechanisms underlying these p97 increased dependencies in cancer cells remain poorly understood. Before translating this concept into the clinic, we have to understand the molecular details of why p97 inactivation causes radiosensitivity and cancer cell death. Therefore, we aim to further study p97 ATPase, the essential enzyme in DNA repair and removal of damaged proteins, with the special focus on its role in cellular response to ionizing radiation. This is a basic science research programme that has a strong potential to improve the outcome of ionizing radiation therapy.

The highly conserved AAA+ ATPase p97 converts its ATPase-driven chemical energy into mechanical force to segregate proteins labelled with ubiquitin (Ub) and/or SUMO from various macromolecules or cellular locations, including chromatin. During this process, p97 turns highly folded substrates into linearised polypeptides that are either presented for proteasome-dependent degradation or recycled. Thus, p97 controls two cellular processes, namely protein homeostasis and DNA repair/genome stability, that are ultimately essential for cellular survival. There are several thousands of p97 substrates and the specificity of p97 towards its substrates is tightly regulated through about 40 known p97 cofactors (cofactors). By forming unknown number of p97 complexes and sub-complexes (p97 with a combination of different cofactors), the cofactors specifically bridge p97 with designated and mostly ubiquitinated and/or SUMOylated substrates. Besides its well-known "unfoldase/segregase" function, p97 can also serve as a platform for substrate processivity, as it co-assembles several E3-Ub ligases, deubiquitinating enzymes or the SPRTN protease that additionally regulate the fate of various substrates. The p97 system is highly relevant for cancer cells as these are constantly on the verge of suffering from lethal effect of proteotoxic stress and an unstable genome. Indeed, numerous reports have shown that p97 overexpression correlates with poor prognosis of various cancer types. The overarching goal of my work is to address the p97 system and its role in proteostasis and genome stability in response to IR. This question is important if we want to understand and explore how the inactivation of the p97 system and consequently induction of both proteotoxic stress and inactivation of DNA repair could potentially cure cancer.