Pregnancy and breast cancer: the other side of the coin

Molecular definition of breast tumor heterogeneity

Cells with distinct phenotypes including stem-cell-like properties have been proposed to exist in normal human mammary epithelium and breast carcinomas, but their detailed molecular characteristics and clinical significance are unclear. We determined gene expression and genetic profiles of cells purified from cancerous and normal breast tissue using markers previously associated with stem-cell-like properties. CD24+ and CD44+ cells from individual tumors were clonally related but not always identical. CD44+ cell-specific genes included many known stem-cell markers and correlated with decreased patient survival. The TGF-beta pathway was specifically active in CD44+ cancer cells, where its inhibition induced a more epithelial phenotype. Our data suggest prognostic relevance of CD44+ cells and therapeutic targeting of distinct tumor cell populations.

Fog2 excision in mice leads to premature mammary gland involution and reduced Esr1 gene expression

The critical role for GATA family proteins in maintaining the normal (non-transformed) cell state is corroborated by the recent findings of mutations or methylation in GATA genes both in primary cancers and tumor lines including breast. Previously, microarray profiling studies determined that the highest expression of both GATA3 and ESR1 (estrogen receptor alpha) is seen in tumors associated with the most favorable survival outcomes, whereas the lowest expression of GATA3 is detected in tumor subtypes showing the worst outcomes. At this time, genes and pathways that are regulated by GATA3 in the mammary gland are not well defined. We have previously established a requirement for FOG (Friend Of GATA) cofactors during mouse development. Here we report that in the murine mammary gland Fog2 gene expression is upregulated upon pregnancy and lactation with prominent expression in the epithelial cells of the gland during post-lactational regression. Mammary-specific deletion of Fog2 identified a role for this gene during gland involution; excision of the Fog2 gene leads to the accelerated involution of the gland despite diminished levels of the remodeling enzymes. Importantly, the levels of several genes linked to the control of cancerous transformation in the breast (Esr1, Prg and Foxa1) are significantly reduced upon Fog2 excision. This implicates FOG2 in the maintenance of epithelial cell differentiation in the mammary gland and in performing a protective role in breast cancer.