Lesson of the Month - Expression of small breast epithelial mucin (SBEM) protein in tissue microarrays (TMAs) of primary invasive breast cancers

Aims: Small breast epithelial mucin (SBEM) is a recently described gene product that shows promise as a new breast biomarker. The aim was to investigate for the first time SBEM protein expression in a large cohort (n = 300) of invasive breast cancers, its relationship to established clinical variables and its association with clinical outcome. Methods and results: Immunohistochemical analysis was performed on tissue microarrays consisting of 149 oestrogen receptor (ER) alpha- and 151 ER alpha+ breast cancers. Overall, 18% of tumours were SBEM+ (n = 53/300). However, SBEM protein was more frequently observed in ER- (22%) than in ER+ cancers (13%; P = 0.049). A significant association with psoriasin/S100A7 expression (P <= 0.0001) was observed in the entire cohort. SBEM was also positively associated with HER-2 (P = 0.046) in ER- cancers, and increased levels of SBEM were strongly associated with higher tumour grade (P = 0.0015). Furthermore, SBEM expression showed a trend towards an association with reduced overall survival and relapse-free survival in the ER+ cohort (P = 0.063 and P = 0.072, respectively). Conclusions: Our results suggest that SBEM may identify a unique subset of breast cancers with poor prognosis and may have future implications for therapeutic management of this disease.