AIMS: To determine, by in situ immunohistochemistry, whether ovarian carcinomas have increased expression of DNA topoisomerase I. METHODS: Paraffin wax blocks obtained from 15 samples of normal human tissues and from 14 cases of ovarian cancer were cut on to glass slides and immunohistochemically stained for topoisomerase I. The primary antibody was a mouse monoclonal that recognises topoisomerase I in western blots. Colour was detected using a peroxidase system with diaminobenzidine as the chromogen. The expression of topoisomerase I in the tissues and tumours was graded subjectively from 0 to 3+ based on the colour intensity of the immunostain. RESULTS: In normal tissues, topoisomerase I expression was strongest in the mucosal lymphocytes in the gastrointestinal tract and in the germinal centres of the tonsil. Weak topoisomerase I staining was found in the columnar epithelium of the gastrointestinal tract and in squamous mucosa. In the series of ovarian carcinomas, raised topoisomerase I was observed in 43% (6 of 14) of the tumours. Of the tumours with raised topoisomerase I, only three contained a population of rapidly cycling cells. Therefore, 21% of our series of ovarian carcinomas (3 of 14) had raised topoisomerase I expression and were proliferating rapidly. CONCLUSIONS: Topoisomerase I expression in formalin fixed, paraffin wax embedded human tissues can be evaluated by immunohistochemical staining. Increases in topoisomerase I occur in some cases of ovarian cancer.
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