P2-12-01: Immunohistochemical (IHC) BAG1 Expression Improves the Estimation of Residual Risk (RR) by IHC4 in Postmenopausal Patients Treated with Anastrozole or Tamoxifen: A TransATAC Study.
Authors: Dowsett M , Afentakis M , Pineda S , Salter J , Howell A , Buzdar A , Forbes JF , Cuzick J 
Published: June 2012

Aim: To determine whether the incorporation of BAG1 staining improves the estimate of RR after endocrine therapy in postmenopausal patients with ER+ve tumours treated with endocrine therapy. Background: BAG1 encodes a protein (BCL2-associated athanogene 1) that binds to BCL2 and enhances its anti-apoptotic effects. BAG1 is included as a separate subgroup in the 21-gene OncotypeDx Recurrence Score (RS) that is used to assess RR after endocrine therapy in primary ER+ breast cancer. IHC4 is a 4-panel set of IHC markers (ER, PgR, HER2, Ki67) that was shown to provide as much prognostic accuracy as RS in the translational arm of the ATAC trial (TransATAC) of anastrozole versus tamoxifen alone or combined and subsequently independently validated (Cuzick et al, JCO, 2011, in press). Addition of extra markers such as BAG1 to IHC4 may improve the accuracy of the IHC4 and provide extra discriminatory power for oncologists.

Methods: Samples in triplicate TMAs from the TransATAC cohort were stained for BAG1 using the Genetex 3.10G3E2 antibody after validation using siRNA knockdown. Staining was scored separately as nuclear or cytoplasmic and categorized by intensity as 0, 1, 2 or 3. BAG1 IHC values were assessed for their correlation with BAG1 mRNA levels. The statistical analysis plan was pre-specified and tested possible additional information from BAG1 expression to the IHC4 in patients not treated with chemotherapy by change in the likelihood ratio chi-square (ΔLR-X2). Results were included only if there was also complete data for ER, PgR, Ki67 and HER2. Primary analysis was on the HER2−ve node-negative (N-neg) population; secondary analysis was on all N-neg patients. Follow-up was to 10 years and the primary end-point was time to distant recurrence (TTDR).

Results: Data on both nuclear and cytoplasmic BAG1 as well as the other 4 IHC parameters was available on 961 cases of which 855 were HER2−ve. There was a significant correlation between cytoplasmic and nuclear BAG1 (p=0.23, p&lt;0.0001) but the nuclear staining correlated better with mRNA levels and was therefore considered further. Weak but significant correlations were also seen with ER, PgR and tumour grade. In the univariate analysis nuclear BAG1 was significantly associated with worse TTDR in HER2−ve and all N-neg cases (X2=7.91, p=0.005 and X2=10.63, p=0.001 respectively). Nuclear BAG1 also contributed significantly in multivariate analyses in the 2 populations firstly when added to the clinical model (X2=4.99, p=0.02 and X2=5.93, p=0.015 respectively) and secondly when subtracted from clinical plus the IHC4 parameters (X2=5.55, p=0.02 and X2=4.50, p=0.03 respectively).

Conclusions: Nuclear BAG1 expression has significant value for estimating RR that is independent of standard clinical and IHC parameters and it improves the prediction of TTDR in the TransATAC population beyond that with the validated IHC4 score. Unlike IHC4 markers, BAG1 is not commonly measured in pathology work-up of breast cancers. The clinical utility of its addition to IHC4 will be tested by measuring its discrimination of high and low risk patients in clinical practice.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-12-01.