Molecular Determinants of Radiosensitivity

Radiotherapy uses radiation to kill tumour cells and is given to about 40% of cancer patients as part of their treatment. There is a maximum radiation dose that can be given to a patient because radiation also damages the healthy tissues that surround the tumour. This radiation dose may thus not be sufficient to kill the tumour. The aim of our research is to improve radiotherapy by making tumour cells more sensitive to radiation. We have identified particular proteins that are important for a tumour cell to survive radiation. We showed that when these proteins are inhibited, the radiation kills tumour cells more effectively without affecting healthy cells.
The outcome of radiotherapy is also influenced by the tumour environment. Tumours often have core areas with low oxygen levels. As the absence of oxygen greatly decreases the efficacy of radiation, these regions are much more resistant to radiotherapy. One way to improve the availability of oxygen in these areas is to reduce the amount of oxygen that is consumed by cancer cells throughout the tumour. We have identified an anti-malarial drug that can do exactly that and showed that it improves radiotherapy in mice. We are now conducting a clinical trial to test whether this drug improves oxygen levels in patients’ tumours.

The maximum radiation dose that can safely be delivered to patients is limited by the side effects caused by damage to the healthy tissues that surround the tumour. The aim of our research is to improve radiotherapy treatment by making the tumour cells more sensitive to ionising radiation without altering the sensitivity of normal tissues. Tumour radiosensitivity is determined by intrinsic sensitivity, the inherent sensitivity of the tumour cells, and extrinsic sensitivity, resistance to killing that is imparted through the tumour environment. Our work has identified several targets that allow manipulation of both of these parameters in order to improve the radiation response. To identify novel intrinsic radiosensitisation targets, we developed a high throughput colony formation assay that closely reflected the clinical use of radiation. Using this assay to screen large siRNA libraries, we identified several genes not previously identified with radiation sensitivity. These included SRP72 (signal recognition particle 72), which is part of the signal recognition particle complex responsible for transporting secretory proteins, and thiamine pyrophosphokinase-1 (TPK1). The discovery that thiamine metabolism is associated with tumour radiosensitivity may represent a new clinical strategy for augmenting radiotherapy.
So far the most promising target we identified in our high throughput screens is POLQ, a DNA polymerase involved in microhomology-mediated end-joining repair of double-strand DNA breaks. We demonstrated that POLQ mediated tumour radioresistance, and that its overexpression is a highly significant prognostic factor in breast cancer. The subsequent demonstration that it is synthetically lethal in cells lacking functional BRCA1 has greatly increased the interest of these studies.
One key factor determining extrinsic radiosensitivity is tumour hypoxia. Solid tumour often have large hypoxic regions, which can be up to three times more radioresistant. A possible strategy to reduce tumour hypoxia is to decrease the oxygen consumption rate in the normoxic tumour cells thereby increasing the availability of oxygen to the hypoxic areas. We developed a high throughput screen to measure oxygen consumption in cancer cells and screened library of 1,697 FDA-approved compounds. This screen identified the anti-malarial drug atovaquone as a potent reducer of oxygen consumption. Atovaquone was subsequently demonstrated to reduce hypoxia in spheroids and xenograft tumours. Most importantly, atovaquone was shown to cause significant tumour growth delay in combination with radiation in these models. We are now conducting clinical trials to assess whether atovaquone reduces tumour hypoxia in patients.