bcl-2/bax ratio as a predictive marker for therapeutic response to radiotherapy in patients with rectal cancer

Combined radiation therapy and chemotherapy are adjuvant treatments given after surgery to patients with rectal carcinoma. Because apoptosis seems to play a role in tumor response to radiotherapy, the current study investigates whether there is a correlation between the ratio of bcl-2 oncoprotein and bax expression in rectal adenocarcinoma and the clinical response to radiotherapy. Elective colectomy for primary rectal adenocarcinoma followed by adjuvant radiotherapy and chemotherapy was performed on 35 patients. Tumors were staged as B2 (n = 30) and C (n = 5), and were classified as radiation resistant (n = 19, group A) and radiation nonresistant (n = 16, group B). Immunohistochemical study, using the streptavidin-biotin complex technique and monoclonal antibody to bcl-2 and polyclonal antibody to bax protein was used on paraffin sections. Cases were considered positive if at least 5% of tumor cells displayed cytoplasmic staining for bcl-2 or bax. In each tumor, the bcl-2/bax ratio was calculated dividing the percentage of bcl-2-positive cells by the percentage of bax-positive cells. For statistical analysis, the Mann-Whitney rank sum test and Kruskal-Wallis analysis of variance test were used. Rectal tumors of group A displayed significantly greater bcl-2 immunoreactivity (40.2 +/- 4.2) compared with group B (20.2 +/- 3.8). In contrast, expression of bax protein was less in group A (30.3 +/- 3.3) compared with group B (41.3 +/- 2.3). The bcl-2/bax ratio was greater in group A (1.3 +/- 0.1) compared with group B (0.49 +/- 0.1), and was correlated with poor responsiveness to radiotherapy. The current study indicates that in patients with rectal carcinoma an elevated bcl-2/bax ratio in tissue specimens suggests increased tumor resistance to adjuvant radiotherapy. Thus, in such patients, the bcl-2/bax ratio may serve as a potential molecular marker for prediction of tumor prognosis.