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Advances in understanding the molecular basis of breast cancer has necessitated a definition of improved indicators of prognosis that are central to the underlying cancer biology and that reflect the heterogeneous nature of the disease. This study investigates the pattern of expression of the steroid receptor co-regulators NCOA1/SRC1, NCOA3/RAC3, NCOR2/SMRT, and CBP/p300 in breast cancer. The aims were to identify whether their expression was related to patient outcome, their relationships to known prognostic factors and to provide a basis for further research into the mechanistic significance of such associations.The protein levels of steroid receptor co-regulators were assessed by immunohistochemistry in a large well-characterised series of breast carcinomas prepared as tissue microarrays. Relationships between these targets, other clinicopathological variables and patients' outcome were examined.NCOR2/SMRT was an independent prognostic indicator of overall patient survival (OS) and disease free interval (DFI) and was significantly correlated with distant metastases and local recurrence whereas tumours expressing NCOA1/SRC1 had a significantly longer OS and DFI. There were also significant correlations between co-regulator expression of NCOA1/SRC1, CBP/p300 and NCOA3/RAC3, which were associated with lower tumour grade. NCOA1/SRC1 was also correlated with smaller tumour size. Furthermore, the co-activators had a significant association with steroid receptors, particularly ERα.NCOR2/SMRT is associated with poor patient outcome, independent of other prognostic factors. In contrast, steroid receptor co-activator expression is generally associated with a good prognosis. Further investigations are needed to establish the mechanisms of these links between the steroid receptor co-regulator system and patient outcome.