P53 has been reported to be one of tumor suppressor genes that play a major role in signal transduction following many kinds of stresses, including ionizing radiation. Changes in p53 expression during radiation therapy in tumor tissues have not yet been reported. We determined whether radiotherapy changes p53 expression in human squamous cell carcinomas of the head and neck, and established the possible correlations between p53 expression and the therapeutic effects of radiation therapy. 30 patients with tumors of the oral cavity, oropharynx, and maxillary sinus were examined, and all the tumors were confirmed as squamous cell carcinomas. Biopsies were performed on the cancer tissues before treatment and at doses of 4, 10, and 20 Gy of radiotherapy, and the specimens were preserved in liquid nitrogen for further examination. Samples were immunohistochemically stained using streptoavidin-biotin peroxidase method and a monoclonal antibody against p53 (Ab-3, mutant type). For all the samples p53 PCR-SSCP (polymerase chain reaction-single strand conformation polymorphism) assays were performed. 14 of the 30 patients with squamous cell carcinomas showed expression of p53 in their tumor cells before and/or at 4 Gy or 10 Gy of radiotherapy. Eleven of the 14 tumors showed high radiosensitivity. Results of the p53 PCR-SSCP assays revealed mutations of p53 in 13 of 30 patients examined, and percentages of mutated p53 DNA varied at radiation doses of 4 Gy and 10 Gy. Ten of 12 patients with mutated p53 in their tumors showed decreased percentages of mutated p53 DNA during radiotherapy. The relationship between the immunohistochemical findings and the antitumor effect of a radiation dose of 20 Gy was examined on the correspondent hematoxylin-eosin sections. In patients whose p53 expressions in tumor cells were grades + or ++ or before radiotherapy and/or at 4 Gy of radiotherapy, the tumors responded significantly well to radiation therapy but the patients responded with significantly unfavorable clinical courses. The high radiosensitivity of squamous cell carcinomas in our samples could be explained by an overexpression of mutant type p53 in the tumor cells, and these mutant type p53-positive tumor cells possibly showed radioresponsiveness.

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