Epigenetic Silencing of THY1 Tracks the Acquisition of the Notch1-EGFR Signaling in a Xenograft Model of CD44+/CD24low/CD90+ Myoepithelial Cells.

The surface glycoprotein THY is a marker of myoepithelial precursor cells, which are basal cells with epithelial-mesenchymal intermediate phenotype originating from the ectoderm. Myoepithelial precursor cells are lost during progression from in situ to invasive carcinoma. To define the functional role of Thy1-positive cells within the myoepithelial population, we tracked Thy1 expression in human breast cancer samples, isolated THY1-positive myoepithelial progenitor cells (CD44+/CD24low/CD90+), and established long-term cultures (parental cells). Parental cells were used to generate a xenograft model to examine Thy1 expression during tumor formation. Post-transplantation cell cultures lost THY1 expression through methylation at the THY1 locus and this is associated with an increase in EGFR and NOTCH1 transcript levels. Thy1-low cells are sensitive to the EGFR/HER2 dual inhibitor lapatinib. High THY1 expression is associated with poorer relapse-free survival in patients with breast cancer. THY1 methylation may track the shift of bipotent progenitors into differentiated cells. Thy1 is a good candidate biomarker in basal-like breast cancer. IMPLICATIONS: Our findings provide evidence that THY1 expression is lost in xenografts due to promoter methylation. Thy1-low cells with increased EGFR and Notch1 expression are responsive to target therapy. Because DNA methylation is often altered in early cancer development, candidate methylation markers may be exploited as biomarkers for basal-like breast cancer.