Piwil2 is expressed in various stages of breast cancers and has the potential to be used as a novel biomarker.

Piwil2, a member of AGO/PIWI family of proteins, has been reported to be expressed in precancerous stem cells (pCSCs), tumor cell lines and various types of human cancers. However, the significance of piwil2 expression in breast cancer has not been investigated. In this study, archival formalin-fixed, paraffin-embedded breast cancer specimens at various developmental stages were prepared as tissue microarrays (TMAs) and examined for the expressions of piwil2, estrogen receptor (ER), progesterone receptor (PR) and a cell proliferation marker Ki67 by immunohistochemical (IHC) staining and human epidermal growth factor receptor 2 (HER2) by fluorescence in situ hybridization (FISH). The correlation of piwil2 expression with ER, PR and Ki67 were analyzed statistically. The piwil2 was detected in all of breast cancer TMA cores. In contrast, ER, PR, HER2, and Ki67 were detected only in 66.1%, 54.5%, 36.0%, and 47% of the TMA cores, respectively. Piwil2 was expressed in cytoplasm (Cyt), nucleus (N) or both cytoplasm and nucleus (C-N). The N pattern was less observed in breast precancers, whereas all three patterns were observed in invasive and metastatic cancers. While the Cyt pattern was significantly correlated with ER expression (p = 0.002); N pattern was significantly correlated with Ki67 expression (p =0.001). ER and Ki67 expressions were reduced and increased, respectively, with the expression patterns being shifted from Cyt --> C-N --> N. In conclusion, piwil2 is expressed in various stages of breast cancers and has the potential to be used a novel biomarker.