THE USE OF ASYMMETRIC-FIELD INVERSION GEL-ELECTROPHORESIS TO PREDICT TUMOR-CELL RADIOSENSITIVITY

The success of a predictive assay for radiotherapy relies on the use of one or more tumor cell traits that equate with tumor radioresistance or radiosensitivity. These traits can be divided into intrinsic (genetic) and extrinsic (epi-genetic) factors. Most probably, a tumor's response to radiotherapy will be influenced by both of these sets of traits. Radiobiological analysis of cultured cells derived from explanted tumors of head and neck patients has shown that in vitro survival of tumor cells is not the only factor affecting tumor radiocurability. Two possible reasons are the high degree of selection involved in growing the cells in vitro and the inability to assess the contribution of the cell-cell contact effect with cultured cells. A possible means of overcoming both of these problems would be an assessment of the radiosensitivity of the cell population immediately after removal from the tumor. Since a good correlation exists between intrinsic cellular radioresistance and DNA double-strand break repair (DSBR) as assayed by the Neutral Elution technique [21], we have investigated the feasibility of using asymmetric field inversion gel electrophoresis (AFIGE) in identifying resistant tumor cells in vitro. AFIGE has several advantages over neutral elution in that it is faster (approximately 60-80 samples can be run on the same agarose gel) and, most importantly, one can visualize DNA damage and repair by staining the DNA with ethidium bromide. Analysis of five squamous cell carcinoma cell lines derived from head and neck cancers and one normal human fibroblast fine showed a significant correlation (r2 = 0.92, p less-than-or-equal-to 0.006) between their extent of DSBR after one hour of repair following 10 Gy of ionizing radiation and their radiosensitivity (D0, SF2 Gy). This correlation between DSBR kinetics and radiosensitivity was independent of dose, and there was no correlation between radiosensitivity and damage induction. Thus, AFIGE may overcome some of the problems of clonogenic assays in determining tumor radiosensitivity, particularly if combined with a technique to determine the oxygen profile of individual tumors.