PD10-06: Loss of the Retinoblastoma Tumor Suppressor (RB) in Triple Negative Breast Cancer Is Associated with a Favorable Prognosis.

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Abstract Introduction: The retinoblastoma tumor suppressor gene (RB), initially identified in the pediatric tumor retinoblastoma, has been shown to be functionally inactivated in a variety of other tumor types. RB functions as a key regulator of cell cycle progression and modulates the response to a variety of physiological and clinically relevant stresses. In breast cancer, loss of RB has been shown to have prognostic significance and influence therapeutic response; however, a comprehensive analyses of RB in triple negative breast cancer (TNBC) has not been performed. Although TNBC represents only 15–20% of breast cancers it accounts for approximately half of breast cancer deaths. A subset of patients with TNBC will have a dramatic and durable response to standard chemotherapy. However, currently there are no markers that identify this patient subset. Since RB loss impinges on the response to chemotherapy, the goal of this study was to investigate the impact of RB-status on clinical management of TNBC. Material and Methods: A cohort of 220 patients diagnosed and treated at Thomas Jefferson University Hospital was included in the study. RB-status was evaluated by immunohistochmistry (IHC) using 3 markers: p16ink4a (MTM Laboratories, Cat #: 9518,1:50 dilution), Ki67 (AbCam, Cat #: ab16667, 1:600 dilution) and RB (Thermoscientific, Cat #: MS-107-B, 1:50 dilution) through automated image analysis (Aperio). Cases were considered negative for RB when no neoplastic cell nuclei demonstrated labeling in sections in which stromal cells and endothelial cells stained; Ki67 and p16ink4 were scored using established published criteria. Gene expression profiling was performed on 12 cases (6 RB positive and 6 RB negative by IHC) and RB pathway status in these samples was evaluated using an RB-loss signature developed by our group. Parallel analyses of TNBC expression data sets were also employed to define the relationship of the RB pathway with clinical outcome. Overall survival was analyzed using LogRank test and Cox proportional hazard model. Data were analyzed in SAS 9.1 (SAS Institute Inc., Cary, NC, USA). Results: 176 patients had samples that could be scored for RB, p16ink4 and Ki67 expression. 46% (71/176) showed RB loss. RB loss was associated with high p16ink4a expression and high Ki67 proliferation index in 93% (66/71) of samples and correlated with RB loss signature in all but one sample. RB loss and high p16ink4 expression was associated with a longer overall survival in this cohort (p=0.003). In contrast, patients with TNBC showing RB positivity and high p16ink4 had the worse survival (median survival 4 months, 10 year survival 23%). These findings were consistent with gene expression profiling data that indicate specific loss of RB function in TNBC is associated with improved response to chemotherapy and improved survival. Conclusion: RB loss appears to be a predictor of a favorable clinical outcome in TNBC treated with conventional adjuvant chemotherapy. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD10-06.