Natural and induced epithelial-mesenchymal transition results in epigenetic silencing of HER2 overexpression

Epithelial-mesenchymal transition (EMT) is a well-known phenomenon that has been implicated in diverse biological processes ranging from embryonal development to cancer invasion and metastasis. In epithelial-derived cancers which both invade and metastasize as epithelial clumps or clusters, EMT would have to be followed by MET (mesenchymal-epithelial transition) since both the initial cancer and the metastasis appear epithelial in nature. There is a rare subset of breast carcinomas, however, that exhibit biphasic epithelial and mesenchymal differentiation, so-called metaplastic carcinomas. Our initial studies were designed to examine whether EMT was indeed occurring in this unique subset of metaplastic breast carcinomas. Based on both RT-PCR and immunocytochemical studies, EMT was naturally occurring. Once this was confirmed, we wanted to investigate the effects of EMT beyond the immediate gene expression pattern that traditionally defined it. Although approximately 90% of metaplastic breast carcinomas are triple negative, 5-10% amplify and overexpress HER2. We then conducted both observational studies in these biphasic HER2 overexpressing metaplastic breast carcinomas and experimental studies with a HER2 overexpressing cell line, the HTB20, where TGFβ1 induced EMT. In the observational studies, HER2 gene amplification was equally present in both the epithelial and mesenchymal phases but both HER2 mRNA and protein levels were essentially silenced in the areas having undergone EMT. Similarly in the experimental studies where TGFβ1 induced EMT, HER2 gene amplification persisted but HER2 mRNA and protein levels were similarly silenced. These studies provide direct evidence that both naturally occurring and induced EMT results in epigenetically silencing of HER2 overexpression.