The intrinsic radiation sensitivity of normal and tumour tissue is a major determinant of the outcome of radiotherapy. There is currently no established test that can be used routinely to measure the radiosensitivity of the cells in an individual patient's cancer in a manner that can inform treatment planning. The purpose of this study was to evaluate, in four human colorectal adenocarcinoma cell lines, two possible end points as surrogate markers of radiation response -apoptosis and induction of DNA single-strand breaksand to compare the results with those of a conventional clonogenic assay. Cell lines (SW707 SW480, SW48 and HT29) known to differ in radiosensitivity were exposed to single doses of X-rays ranging from 0.5 to 5 Gy and cell survival was measured using the clonogenic assay. Apoptosis was determined on the basis of morphology under fluorescent microscopy and DNA damage/repair was measured, as tail moment, using an adaptation of the alkaline comet assay. The relationship between surviving fraction at 2 Gy (SF 2 ) and the percentage of apoptotic cells 24 h after the same dose was complex, but apoptosis accurately predicted the order of radiosensitivities as measured by SF 2 . Initial damage measured after 2 Gy using the alkaline comet assay gave a close correlation with SF 2 (r 2 ¼ 0.95), whereas there was no correlation between initial DNA damage repair rate and SF 2 .

We have isolated a novel gene, DIR1, from L132 cells that is transiently repressed after exposure to low radiation doses and has a potential role in induced radioresistance. Molecular and cellular characterization of this gene reveals that it is unique but has similarities to a family of heat-shock-related proteins known as immunophilins. These have been implicated in various cellular functions including general stress responses and control of the cell cycle. Antisense strategies have demonstrated that the DIR1 gene also appears to have some involvement in the control of the cell cycle. Furthermore, there appears be a potential role for this gene product in the phenomenon of induced radioresistance through a mechanism that increases the rate of DNA repair in cells exposed to X rays and subsequently increases the cells' resistance to radiation. This is the first description of an immunophilin-like gene that has a possible role in adaptive/inducible responses to X rays in mammalian cells.

Bladder tumours show a variable response to radiotherapy with only about 50% showing good local control; currently there is no test to predict outcome prior to treatment. We have used five bladder tumour cell lines (T24, UM-UC-3, TCC-SUP, RT112, HT1376) to investigate the potential of the alkaline comet assay (ACA) to predict radiosensitivity. Radiation-induced DNA damage and repair were compared to clonogenic survival. When the five cell lines were irradiated and initial DNA damage was plotted against cell survival, at all doses (0 -6 Gy), a significant correlation was found (r 2 ¼ 0.9514). Following 4 Gy X-irradiation, all cell lines, except T24, showed a correlation between SF2 vs half-time for repair and SF2 vs residual damage at 5, 10, 20 and 30 min. The T24 cell line showed radioresistance at low doses (0 -2 Gy) and radiosensitivity at higher doses (4 -6 Gy) using both cell survival and ACA end points, explaining the lack of correlation observed for this cell line. These data indicate that initial DNA damage and residual damage can be used to predict for radiosensitivity. Our data suggest that predictive tests of radiosensitivity, appropriate to the clinical situation, may require the use of test doses in the clinical range.

There is increasing concern about the adverse health effects associated with the use of sunbeds, particularly with respect to skin photocarcinogenesis. The induction of mutagenic DNA damage is a prerequisite for the development of skin tumours, and it is well established that direct types of damage such as cyclobutane pyrimidine dimers (CPDs) give rise to mutations in tumour suppressor genes and oncogenes. In addition, ultraviolet radiation may induce indirect types of DNA damage, including oxidative products, which are also potentially mutagenic. By using specific DNA repair enzymes (T4 endonuclease V and endonuclease III) and the comet assay we have been able to detect the induction of CPDs, oxidized or hydrated pyrimidine bases and single-strand breaks in cultured human fibroblasts (MRC-5) after exposure for between 15 s and 20 min on two different commercial sunbeds containing Philips 'Performance' 100W-R or Philips TL80W/10R lamps. The ratio of endonuclease III to T4 endonuclease V sensitive sites varied substantially between the two lamps and was 3.3% and 18%, respectively. The sunbed containing the 'Performance' 100W-R lamps was as potent at inducing CPDs as was natural sunlight in fine weather. These results establish that commercial tanning lamps produce the types of DNA damage associated with photocarcinogenesis in human cells, and complement epidemiological evidence indicating the potential risk of using sunbeds.

There is increasing concern about the adverse health effects associated with the use of sunbeds, particularly with respect to skin photocarcinogenesis. The induction of mutagenic DNA damage is a prerequisite for the development of skin tumours, and it is well established that direct types of damage such as cyclobutane pyrimidine dimers (CPDs) give rise to mutations in tumour suppressor genes and oncogenes. In addition, ultraviolet radiation may induce indirect types of DNA damage, including oxidative products, which are also potentially mutagenic. By using specific DNA repair enzymes (T4 endonuclease V and endonuclease III) and the comet assay we have been able to detect the induction of CPDs, oxidized or hydrated pyrimidine bases and single-strand breaks in cultured human fibroblasts (MRC-5) after exposure for between 15 s and 20 min on two different commercial sunbeds containing Philips 'Performance' 100W-R or Philips TL80W/10R lamps. The ratio of endonuclease III to T4 endonuclease V sensitive sites varied substantially between the two lamps and was 3.3% and 18%, respectively. The sunbed containing the 'Performance' 100W-R lamps was as potent at inducing CPDs as was natural sunlight in fine weather. These results establish that commercial tanning lamps produce the types of DNA damage associated with photocarcinogenesis in human cells, and complement epidemiological evidence indicating the potential risk of using sunbeds.