Aims Dicer and Drosha are components of the miRNA-producing machinery and their altered expression may play a role in cancer progression. The main purpose of this study was a detailed investigation of Dicer and Drosha expression and localisation in triple-negative breast cancers.

Methods Thirty-one triple-negative breast cancers and several breast cancer cell lines were investigated. Expression of Dicer and Drosha was evaluated at the mRNA level by quantitative reverse transcription PCR and at the protein level by immunohistochemistry or western blot.

Results Compared with normal breast tissues, a wide variation of Dicer and Drosha mRNA levels was detected in triple-negative breast cancers. As a group, Drosha mRNA levels in triple-negative breast cancers were significantly higher than those in normal breast tissues. Immunohistochemical data confirmed higher expression of Drosha protein in triple-negative breast cancers. In normal breast tissues Dicer was detectable predominantly in the cytoplasm of basal/myoepithelial cells only. In contrast, in the majority of triple-negative breast cancers, intense Dicer staining was detectable also in the nuclear compartment. Detection of Dicer and Drosha mRNA and protein levels in breast cancer cell lines confirmed the nuclear localisation of Dicer, suggesting, in addition, that the steady-state protein levels could be controlled by post-mRNA regulatory events.

Conclusions These findings indicate that Dicer and Drosha expression is deregulated in triple-negative breast cancers.