AIB1 (amplified in breast cancer 1) is an estrogen receptorα (ERα) co-activator, known to be amplified and overexpressed in a fraction of breast cancers. It has been linked to prognosis and tamoxifen resistance. However, results have been ambiguous. The different functions of AIB1 in ERα-positive and -negative disease are poorly understood. Therefore, we analyzed the clinical significance of AIB1 in breast cancer with respect to ERα-status and characterized the subgroups. 2,197 breast carcinomas sampled on a pre-existing tissue microarray (TMA) were analyzed for AIB1 expression and amplification by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH).
Results: AIB1 expression was detected in 60 % of the tumors. It was associated with tumor size (p = 0.003), high histological grade (p < 0.0001), poor disease-specific, and overall survival (p = 0.0018 and p = 0.003). There was a strong inverse relationship between AIB1 and ERα expression (p < 0.0001). AIB1 overexpression was associated with increased Ki67 labeling index (p < 0.0001), even if analyzed for different ER expression levels. AIB1 amplification was found in 11 % of the carcinomas. It was associated with high histological grade (p = 0.0012), lymph node involvement (p = 0.0163), and poor disease-specific survival (p = 0.0032) but not with overall survival (p = 0.1672) or ER status (p = 0.4456). If ER-positive tumors were stratified according to their AIB1 amplification status, there was a significant worse disease-specific survival in cases showing AIB1 amplification (p = 0.0017). AIB1 expression is associated with unfavorable prognosis and tumor phenotype. It seems to unfold its oncogenic potential at least in part independent from its role as an ERα co-activator. AIB1 has an impact on cell cycle regulation in ERα-positive as well as ERα-negative tumors. Furthermore, AIB1 amplification characterizes a subgroup of ERα-positive breast cancer with worse outcome. Therefore, AIB1 might be helpful to identify those ERα-positive breast cancers patients who are candidates for adjuvant chemotherapy.