Heterogeneity of O(6)-alkylguanine DNA-alkyltransferase expression in human breast tumours

An important determinant of cellular resistance to chemotherapeutic O(6)-alkylating agents, which comprise methylating and chloroethylating agents, is the ability of cells to repair alkylation damage at the O(6)-position of guanine in DNA. This is achieved by a specific DNA repair enzyme O(6)-alkylguanine DNA-alkyltransferase. In this study O(6)-alkylguanine DNA-alkyltransferase expression was measured in human breast tumours using both biochemical and immunohistochemical techniques. O(6)-alkylguanine DNA-alkyltransferase activity was then compared with known clinical prognostic indices to assess the potential role of O(6)-alkylguanine DNA-alkyltransferase in predicting the behaviour of this common malignancy. The application of both biochemical and immunohistochemical techniques was feasible and practical. Most breast tumours expressed high levels of O(6)-alkylguanine DNA-alkyltransferase. Immunohistochemical analysis showed marked variation in expression not only between individuals but also within individual tumours, and in the same patient, between metastases and between primary tumour and metastatic site. O(6)-alkylguanine DNA-alkyltransferase activity in tissue extracts significantly correlated not only with immunohistochemical staining intensity determined by subjective quantitation, but also with measures of protein levels using a computerised image analysis system including mean grey (P<0.001), percentage of cells positive for O(6)-alkylguanine DNA-alkyltransferase (P<0.001), and integrated optical density (P<0.001). O(6)-alkylguanine DNA-alkyltransferase expression did not correlate with any of the established clinical prognostic indicators for current treatment regimens. However, immunohistochemical offers a rapid and convenient method for assessing potential utility of O(6)-alkylating agents or O(6)-alkylguanine DNA-alkyltransferase inactivating agents in future studies of breast cancer treatment. British Journal of Cancer (2002) 86, 1797–1802. doi:10.1038/sj.bjc.6600324 www.bjcancer.com © 2002 Cancer Research UK