BACKGROUND: Flat epithelial atypia (FEA) of the breast is characterised by a few 
layers of mildly atypical luminal epithelial cells. Genetic changes found in 
ductal carcinoma in situ (DCIS) and invasive ductal breast cancer (IDC) are also 
found in FEA, albeit at a lower concentration. So far, miRNA expression changes 
associated with invasive breast cancer, like miR-21, have not been studied in 
FEA.
METHODS: We performed miRNA in-situ hybridization (ISH) on 15 cases with 
simultaneous presence of normal breast tissue, FEA and/or DCIS and 17 additional 
cases with IDC. Expression of the miR-21 targets PDCD4, TM1 and PTEN was 
investigated by immunohistochemistry.
RESULTS: Two out of fifteen cases showed positive staining for miR-21 in normal 
breast ductal epithelium, seven out of fifteen cases were positive in the FEA 
component and nine out of twelve cases were positive in the DCIS component. A 
positive staining of miR-21 was observed in 15 of 17 IDC cases. In 12 cases all 
three components were present in one tissue block and an increase of miR-21 from 
normal breast to FEA and to DCIS was observed in five cases. In three cases the 
FEA component was negative, whereas the DCIS component was positive for miR-21. 
In three other cases, normal, FEA and DCIS components were negative for miR-21 
and in the last case all three components were positive. Overall we observed a 
gradual increase in percentage of miR-21 positive cases from normal, to FEA, 
DCIS and IDC. Immunohistochemical staining for PTEN revealed no obvious changes 
in staining intensities in normal, FEA, DCIS and IDC. Cytoplasmic staining of 
PDCD4 increased from normal to IDC, whereas, the nuclear staining decreased. TM1 
staining decreased from positive in normal breast to negative in most DCIS and 
IDC cases. In FEA, the staining pattern for TM1 was similar to normal breast 
tissue.
CONCLUSION: Upregulation of miR-21 from normal ductal epithelial cells of the 
breast to FEA, DCIS and IDC parallels morphologically defined carcinogenesis. No 
clear relation was observed between the staining pattern of miR-21 and its 
previously reported target genes.