Amplified in breast cancer 1 expression in breast cancer

Aims: The amplified in breast cancer 1 (AIB1), steroid receptor co‐activator family member, acts as an oestrogen receptor (ER) co‐activator. Acting with HER‐2, it is thought to play a role in endocrine resistance by facilitating ER–growth factor crosstalk. The aim was to analyse AIB1 expression by immunohistochemistry and study its correlations with other prognostic variables in breast cancer and its effect on survival. Methods: A tissue microarray comprising tumours from 438 patients with 15.4 years’ median follow‐up was used. Interpretable AIB1 expression obtained in 395 patients was analysed along with other prognostic factors in breast cancer. Results: AIB1 expression scores ranged from 0 to 30; positive AIB1 expression (score > 14) was seen in 146/395 breast cancers; it correlated negatively with ER (P = 0.003) and progesterone receptor (PR) (P = 0.007), and positively with HER‐2 (P = 0.005) and tumour grade (P = 0.014). It did not correlate with nodal status (P = 0.437). Among ER+ patients, AIB1 expression showed a trend towards loss of PR expression (29% versus 20%; P = 0.14). AIB1 did not predict survival on univariate or multivariate analysis. Conclusions: AIB1 expression correlates with HER‐2 expression in breast cancer and shows a trend of association with loss of PR expression in ER+ tumours. Our study supports the postulated role of AIB1 in ER–growth factor interactions.