EMT associated transcription factors Slug and Snail demonstrate specific expression patterns in breast cancer subtypes, involving tumor or stroma components and correlating with proliferation status and complex clinical prognosis.

EMT associated transcription factors (EMTaTF) controls the epithelial-mesenchymal transition (EMT) process, but their detailed expression pattern in cancer is poorly documented at the cellular level. Here, we located two major EMTaTF: Snail and Slug by immunochemistry using validated antibodies in a cohort of 569 invasive breast carcinomas. We screened all tumor molecular types using TMA analysis in addition to full tumor sections, identifying cell types and structures involved in their expression, correlated to morphological, functional, and clinical characteristics. Briefly, Slug was expressed in Luminal A/B and HER2 breast cancer subtypes by almost all cells from the basal layer in normal-looking tubule structures and was very sporadically seen in transformed cells in the in situ or invasive components. In contrast, Slug was visibly expressed in 28% of our triple-negative (TN) tumor samples, significantly expressed by invasive tumor cells. Slug was also strongly expressed in a large subpopulation of stroma fibroblast-like cells in all breast carcinoma subtypes. Conversely, Snail was frequently expressed in transformed cells in all invasive breast carcinomas subtypes, up to 54 and 64% of tumor cells in TN and HER2 tumors respectively. Expression pattern was heterogeneous and included invasive areas and in situ component. Some stroma cells, particularly endothelial cells from the tumor microenvironment were also found to express Snail. Slug and Snail proteins were also detected in metastatic foci, sometimes with an increase in the expression level, particularly for Snail. Unexpectedly, we found a significant positive correlation between cell proliferation and Slug stroma cell expression in luminal A and B subtypes. This link bolstered the relative proximity we uncovered between CK- KI67- Slug+ stoma cells and CK8+ KI67+ Slug- tumor cells in LumB samples. However, survival long-term studies failed to demonstrate a link between slug tumor or stromal expression and long-term survival. On the other hand, Snail protein expression in tumor cells was surprisingly and significantly linked to a better survival fate overall. In contrast, Snail overexpression in stroma cells from primary tumors was significantly linked to a time-dependent tendency for relapses, metastasis and poorer survival, arising when post-surgery time lapse increased. In conclusion, these findings offer new and unsuspected understanding of the complex localization pattern and clinical involvement of Snail genes in breast cancer, beyond classic EMT pathways.