STAT 5a expression in the breast is maintained in secretory carcinoma, in contrast to other histologic types.

The 7 signal transducer and activator of transcription (STAT) molecules are responsible for the transcription of a variety of regulatory and differentiation proteins. STAT 5a is activated through a variety of mechanisms; in the breast, this is predominantly through binding of prolactin to its receptor. Previously, we showed that STAT 5a expression is decreased in atypical and malignant breast ductal epithelial cells. Interestingly, STAT 5a overexpression was observed in cells undergoing secretory change. In this study, secretory carcinomas were examined by immunohistochemistry for the presence of STAT 5a. In contrast to usual in situ or invasive ductal carcinoma, which lacked STAT 5a expression, all secretory carcinomas (11 invasive and 7 in situ, including 4 cases with both) expressed STAT 5a. No expression was seen in apocrine metaplasia or in other specialized breast carcinomas, such as mucinous or clear cell carcinoma. This retention of signal in the secretory carcinomas may be explained by the higher STAT 5a concentration present in cells undergoing secretory changes in general. Alternatively, STAT 5a expression may be related to the t(12;15)(p13;q25) chromosomal translocation, associated with certain pediatric tumors and recently demonstrated in many secretory carcinomas of the breast, which results in the expression of a tyrosine kinase through ETV6 and NTRK3 fusion. ETV6 also has been associated with the STAT 5a signaling pathway in another gene translocation and may be altering STAT 5a expression in secretory carcinomas. Breast cancer causes significant morbidity and mortality, and, regardless of the mechanism for retention of STAT 5a expression in this uncommon variant, the examination of STAT 5a will aid our understanding of normal and abnormal breast tissues.