New insights into the cellular response to complex DNA damage induced by proton beam therapy

Radiotherapy is still the most effective treatment for a number of human cancers, and the emergence of proton beam therapy (PBT) facilities in the UK, particularly in Manchester and London (operational from late 2018 and 2021, respectively), is likely to signal a new era for cancer treatment. This is due to the fact that PBT is a precision technique able to deliver the majority of the radiation dose directly to the cancer, thus sparing the surrounding normal tissues of any unwanted dose of radiation and reducing the adverse side-effects that are commonly observed with conventional radiotherapy. However whilst radiotherapy, including PBT, is known to act by causing damage to the DNA of cancer cells causing them to die, there is still not a complete understanding of the mechanisms which the cells use to repair certain types of DNA damage and that can cause significant resistance to treatment.

This proposal therefore aims to reveal new knowledge into the basic biological mechanisms that cancer cells use to repair DNA damage caused by PBT, with the future goal of providing leads to the development of new strategies in combination with PBT for effective cancer treatment. Indeed, this study will largely employ cells derived from patients with head and neck cancer, which is a priority tumour research area within the University of Liverpool due to the high local incidence, but also a tumour which is increasingly being treated with PBT worldwide. Therefore, this proposal has the significant potential to devise future optimal treatments using PBT for head and neck cancer patients, leading to reductions in adverse side-effects and improvement in quality of life, but more importantly to increases in overall survival rates.

Proton beam therapy (PBT) is a cutting-edge precision technique for cancer treatment that specifically targets the tumour and spares the surrounding normal tissues. The therapeutic effect of PBT is largely attributable to DNA damage, particularly complex DNA damage (CDD) where several DNA lesions are generated in close proximity by a single radiation track. Since PBT is not monoenergetic and energy is deposited via the Bragg peak, the amount and complexity of CDD can vary which can have a significant impact on the biological response to PBT. However little is known of the molecular and cellular mechanisms that recognise and process CDD in cells.

Recent evidence published by the Principal Investigator has demonstrated that a specific cellular DNA damage response is triggered in response to CDD induced by PBT, and that specific enzymes (e.g. USP6 and PARP-1) control cell survival under these conditions. Utilising PBT at different energies (and thus linear energy transfer; LET) which creates CDD in different proportions, in combination with various biochemical, molecular and cellular biology techniques, this proposal aims to expand on this knowledge by focusing on the specific DNA repair pathways and enzymes, particularly of the base excision/single strand break repair pathways, that are critical for resolving of PBT-induced CDD sites. Furthermore, data provided from recent comprehensive siRNA screening has identified proteins in the cellular DNA damage response (e.g. OGG1 and PARG), and ultimately utilised novel inhibitors against these enzymes (TH5487 and PDD00017273) that can enhance cellular radiosensitivity in response to CDD-induced by PBT, which require further detailed investigation. The overall goal is to improve our understanding of the radiobiology of PBT linked to CDD and repair, and ultimately in the identification of novel strategies using targeted drugs/small molecule inhibitors that may enhance the efficacy of PBT in cancer treatment.