PRECLINICAL STUDY The microRNA maturation regulator Drosha is an independent predictor of outcome in breast cancer patients Sarkawt M. Khoshnaw • Emad A. Rakha • Tarek Abdel-Fatah • Christopher C. Nolan • Zsolt Hodi • R. Douglas Macmillan • Ian O. Ellis • Andrew R. Green Received: 21 November 2012 / Accepted: 22 November 2012 / Published online: 6 December 2012 Ó Springer Science+Business Media New York 2012 Abstract Drosha is a protein that plays a key role in the biogenesis of microRNAs which are well known to be deranged in human breast cancer (BC). The purpose of the current study was to assess the biological and prognostic value of Drosha protein expression in BC. Drosha protein expression was assessed immunohistochemically in two sets of BC: (1) full-face sections of selected BC series with distinct stages of tumour progression (Normal parenchymal cells, ductal carcinoma in situ (DCIS), primary invasive BC and nodal metastases) to evaluate its differential expression, (2) tissue microarray comprising a large and well-characterised series of unselected clinically annotated invasive BC to investigate its correlation with clinico- pathological features and patient outcome. A gradual loss of Drosha cytoplasmic expression was observed along tumour progression from DCIS, to invasive and to meta- static cancer cells. In invasive BC, loss of Drosha cytoplasmic expression was associated with BRCA1 and ER expression and with shorter BC specific survival (BCSS), disease free interval (DFI) and distant metastasis free interval (DMFI). This correlation was maintained in ER negative, HER2 negative, triple negative and LN negative cases. Moreover, loss of cytoplasmic Drosha was predictive of better response to chemotherapy and endocrine therapy. This study provides evidence that Drosha protein potentially plays an important role in BC progression and assessment of its expression provides an independent predictor of patient outcome. These observations provide further evidence that alterations in miRNA regulation influence tumour behaviour. Keywords Breast cancer  microRNA  Drosha  Formalin-fixed paraffin embedded  FFPE Introduction MicroRNAs (miRNAs) are a recently described class of non-protein coding RNA molecules (18–25 nucleotides in length) involved in the regulation of key cellular processes such as proliferation, differentiation and apoptosis [1–4]. Mature miRNAs are produced through a multistage process that begins in the nucleus where long primary miRNA transcripts (pri-miRNAs), several hundred to a thousand nucleotides (nt) in length, are transcribed by RNA poly- merase II [5]. Pri-miRNAs are processed to shorter (70–85 nt) stem loop precursors (pre-miRNAs) mediated by a microprocessor complex which consists of Drosha, an endonuclease RNase III enzyme, and its cofactor DGCR8, the product of a gene deleted in DiGeorge syndrome [6–8]. Pre-miRNAs are exported to the cytoplasm by exportin 5 [9] and then cleaved by Dicer, another RNase III enzyme, to produce *22 nt mature miRNAs [10, 11]. Processing of miRNAs is modulated by other molecules through their interaction with Drosha protein and Drosha microprocessor complex, e.g. Breast Cancer 1 (BRCA1), a tumour suppressor, promotes processing of pri-miRNAs through its direct association with Drosha and DEAD-box RNA helicase p68 (also known as DDX5) of the Drosha S. M. Khoshnaw (&)  E. A. Rakha  T. Abdel-Fatah  C. C. Nolan  Z. Hodi  R. D. Macmillan  I. O. Ellis  A. R. Green Department of Histopathology, School of Molecular Medical Sciences, University of Nottingham, Nottingham, UK e-mail: mrxsmok8@nottingham.ac.uk S. M. Khoshnaw  E. A. Rakha  T. Abdel-Fatah  C. C. Nolan  Z. Hodi  R. D. Macmillan  I. O. Ellis  A. R. Green Nottingham University Hospitals NHS Trust, Nottingham NG7 2UH, UK 123 Breast Cancer Res Treat (2013) 137:139–153 DOI 10.1007/s10549-012-2358-0