Characterization of human tumour cell lines with different radiosensitivities

Failure of radiotherapy for treatment of neoplastic disease can be due to selection of radioresistant S-phase cells within tumours. However, S-phase cells can be targeted with a type-1 DNA topoisomerase inhibitor, camptothecin (CPT) or one of its analogs. The first objective of this study was to understand the mechanism of cellular death of human tumour and normal cells following treatment to CPT and single dose (nonfractionated) X-radiation. We found that necrotic cell death was more important than apoptotic cell death during concurrent CPT and radiation treatment in melanoma (SkMel-3) cells, but not in normal fibroblast (AG1522) cells.; Further experiments involving fractionated X-radiation pre-treatment gave rise to the presence of radioresistant cells in the human tumour colorectal cell line (HCT116). Therefore, the second objective of this study was to assess whether the radioresistant cells occurred by selection or adaptation to the X-radiation treatment. We discovered that the resistant subpopulation of cells surviving a previous fractionated irradiation exposure was likely due to the selection of a radioresistant subpopulation, whereas the radiationsensitive clone manifested a potential inducible radiation response. The third objective was to produce a model system for distinguishing the genetic factors that may be involved in the radiosensitive or radioresistant phenotype. To this end, a set of untreated genetically-related human cell clones of varying radioresponses, with non-overlapping drug sensitivities, were generated from the HCT116 cell line, suggesting the presence of clonal heterogeneity and providing an excellent model. Lastly, using the clones derived from objective 3, the fourth objective was to exploit this model system and determine the genetic basis of radiosensitivity. To do this, we used human cDNA microarrays containing 19,200 ESTs and verified these gene expression changes with Q-PCR. Of a number of genes identified by the array, only spermidine/spermine N1-acetyltransferase (SSAT), an enzyme that catabolizes radioprotective polyamines had a known potential link to radiosensitivity. Our findings establish the potential importance of intrinsic gene expression in radioresponsiveness, and the identification of SSAT as a possible modulator of radiosensitivity, providing a potential tool for understanding and predicting X-radiation response.