Objectives: The Cancer Stem Cell theory suggests that there is a small subpopulation of cells called Cancer Stem Cells (CSCs), which are responsible for tumour recurrence. CSCs are inherently radioresistant thus evading current treatment modalities. The aims of this project were to study the role of high LET alpha particle and low LET x-ray radiation on CSCs including cell survival, DNA damage, invasion and expression of stem cell markers.
Methods: CSCs and differentiated cells were developed from G7 and E2 glioblastoma cell lines, derived from freshly resected patients’ tumours. Immunoflourescence staining for CSC markers CD133 and Nestin was used to confirm the presence of CSCs. The effects of low LET x-ray radiation, delivered using an X-RAD 225 radiation unit, and high LET alpha particle radiation from an americium-241 radioisotope source, on cell function was determined in terms of clonogenicity, cell invasion and DNA repair.
Results: Clonogenic assays indicated that CSCs were more radioresistant than differentiated cells, exhibiting a higher surviving fraction at all doses. Alpha particle radiation was more effective at reducing cell survival of CSCs compared to x-ray radiation (2Gy p<0.0004, 4Gy p< 0.0415). Using transwell invasion chambers, alpha particle radiation reduced CSC invasion at increasing doses while treatment with x-ray radiation resulted in increased cell invasion (G7 2Gy p<0.0019), E2 2Gy p<0.0062). DNA repair assays showed that CSCs display statistically significant ability to repair DNA double strand breaks (DSBs) over a 24-hour period compared to differentiated cells (24 hours p<0.0076). Compared to x-ray radiation, alpha radiation was able to inhibit the resolution of DSBs in CSCs (24 hours p<0.009).
Conclusions: Targeting of the CSC population in tumours is vital to reduce the risk of recurrence. High LET alpha particle radiation has the potential to overcome radioresistance and be used as an effective treatment strategy to target the CSC population and improve response.