Abstract P2-09-09: Loss of BTG2 Expression in Breast Cancer Predicts Tamoxifen Resistance

Abstract

Abstract Background: The B-cell translocation gene-2 (BTG2) belongs to a class of proteins known as the Tob and BTG anti-proliferative protein family. It was shown that estrogen and progesterone suppress BTG2 mRNA for the development of mammary gland. (Kawakubo et al., Oncogene 2004) Expression analysis of BTG2 protein in estrogen receptor (ER) positive human breast cancer was revealed the loss of nuclear expression in 46% of tumors and showed a significant inverse correlation with cyclinD1 expression. (Kawakubo, Hayashida et al., Cancer Res 2006) According to the Oncomine database online, loss of BTG2 expression significantly correlated with increased recurrence rate after tamoxifen monotherapy. Based on these results, we hypothesized that BTG2 expression might be a novel biomarker for predicting the clinical outcome in tamoxifen treatment and analyzed if BTG2 expression affects the sensitivity against tamoxifen both in vitro and in vivo. Method and Results: The cohort of 60 patients treated with adjuvant tamoxifen monotherapy was available online. (Ma et al., Cancer Cell 2004) Comparison of disease-free survival (DFS) of patients whose laser-captured tumors showed positive BTG2 expression versus patients with loss of BTG2 expression revealed an univariate association in the dataset. (log-rank probability, p=0.029) Multivariate analysis indicated that BTG2 resulted independent prognostic factor for DFS (HR, 0.69; 95% CI, 0.495 to 0.963; p=0.029). In vitro cellular proliferation assay to evaluate the cytotoxicity induced by tamoxifen against ER positive cell lines revealed that endogenous BTG2 positive cell line, MCF7 and T47D, showed drug sensitivity, whereas MDA-MB468 expressing loss of BTG2 showed tamoxifen resistance. (MCF7; IC50=7.35uM, T47D; =3.79uM, MDA-MB468; >25.0uM) Tetracycline-inducible BTG2 expression model in MCF7 was developed to assess the cytotoxicity by tamoxifen treatment under the condition of BTG2 overexpression in vitro and in vivo. The cellular proliferation was strongly inhibited by the concomitant administration of tetracyclin and tamoxifen in vitro. The cell line was injected into the mammary fad pad of immunodeficient mice and these were administered water including tetracyclin and/or tamoxifen pellet at the time when the xenografted tumor grew to be over 125mm3 for 3 weeks. The transgene expression was confirmed by both RT-PCR and immunohistochemistry. Tumor growth ratio was significantly suppressed in the concomitant administration of tetracyclin and tamoxifen in comparison with single treatment of tamoxifen or of tetracyclin. Consistently, tumor weight and Ki67 expression were also significantly suppressed in the mice administered both tamoxifen and tetracyclin. Conclusion: Despite further validation studies need to be conducted, BTG2 expression may be useful biomarker to identify patients appropriate for tamoxifen treatment. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P2-09-09.