Repair of DNA double-strand breaks and radiosensitivity to killing in an isogenic group of p53 mutant cell lines.

PURPOSE: Accumulation of the p53 protein can result in G1 arrest that may facilitate DNA repair, or alternatively, it may lead to apoptosis. Mutations that alter p53's ability to mediate these responses are expected to alter cell radiosensitivity to killing. However, the relationship between p53 status and cell radiosensitivity has proven to be complex. Several studies have suggested that p53 mutations are associated with increased radioresistance to killing, while others have shown no such correlation. These differences may be derived from the fact that different mutations of p53 exert different effects on cell radiosensitivity. METHODS AND MATERIALS: To address this question, we examined a group of isogenic cell lines that express different "hot spot" mutant forms of p53. These cells were generated from human osteosarcoma (SAOS) cells, a p53 null cell line, by transfection with vectors expressing different p53 mutants. Vectors with the following p53 mutations were utilized: 143Ala, 175His, 248Try, 273His, and 281Gly. As controls, we used the original SAOS cells and cells transfected with the vector alone. Results were compared to those obtained with a cell line expressing wild-type p53 (wt p53). Radiosensitivity to killing was determined in the exponential phase of growth by measuring loss of colony-forming ability. Induction and repair of DNA double-strand breaks (dsb) was measured in irradiated cells using pulsed-field gel electrophoresis. Apoptosis was assessed using morphologic evaluation of DAPI-stained cells after treatment either with radiation or paclitaxel.
RESULTS: Transfected SAOS-2 cell lines expressed a mutant form of p53 that could not be induced by radiation, and which was transcriptionally inactive. Among the 7 cell lines studied, we observed no difference in cellular radiosensitivity to killing (p = NS). When examining DNA repair, no difference in either the induction or repair of DNA dsb was noted in any of the cell lines studied (p = NS). Also, induction of apoptosis, either after exposure to radiation or paclitaxel, was low, and similar in all cell lines (p = NS). Non-isogenic cells expressing wt p53 were more radioresistant to killing by radiation, but showed similar kinetics of dsb rejoining.
CONCLUSION: The results suggest that expression of different p53 mutants does not alter the yields of radiation-induced dsb, or the ability of cells to repair this type of lesion. In addition, the same p53 mutants do not affect cellular radiosensitivity to killing, or the induction of apoptosis after exposure to radiation or paclitaxel.