Six1 expands the mouse mammary epithelial stem/progenitor cell pool and induces mammary tumors that undergo epithelial-mesenchymal transition
Erica L. McCoy, … , Alana L. Welm, Heide L. Ford
Erica L. McCoy, … , Alana L. Welm, Heide L. Ford
Published August 24, 2009
Citation Information: J Clin Invest. 2009;119(9):2663-2677. https://doi.org/10.1172/JCI37691.
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Research Article

Six1 expands the mouse mammary epithelial stem/progenitor cell pool and induces mammary tumors that undergo epithelial-mesenchymal transition

Abstract

Six1 is a developmentally regulated homeoprotein with limited expression in most normal adult tissues and frequent misexpression in a variety of malignancies. Here we demonstrate, using a bitransgenic mouse model, that misexpression of human Six1 in adult mouse mammary gland epithelium induces tumors of multiple histological subtypes in a dose-dependent manner. The neoplastic lesions induced by Six1 had an in situ origin, showed diverse differentiation, and exhibited progression to aggressive malignant neoplasms, as is often observed in human carcinoma of the breast. Strikingly, the vast majority of Six1-induced tumors underwent an epithelial-mesenchymal transition (EMT) and expressed multiple targets of activated Wnt signaling, including cyclin D1. Interestingly, Six1 and cyclin D1 coexpression was found to frequently occur in human breast cancers and was strongly predictive of poor prognosis. We further show that Six1 promoted a stem/progenitor cell phenotype in the mouse mammary gland and in Six1-driven mammary tumors. Our data thus provide genetic evidence for a potent oncogenic role for Six1 in mammary epithelial neoplasia, including promotion of EMT and stem cell–like features.

Authors

Erica L. McCoy, Ritsuko Iwanaga, Paul Jedlicka, Nee-Shamo Abbey, Lewis A. Chodosh, Karen A. Heichman, Alana L. Welm, Heide L. Ford

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Figure 1

Characterization of the inducible, mammary-specific Six1 transgenic mouse model.

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Characterization of the inducible, mammary-specific Six1 transgenic mous...
(A) Schematic representation of the inducible, bitransgenic mouse model system. Tet-O, tet operator. (B) Southern blot analysis shows varying copy numbers in TetSix animal lines (4910, 4922, 6239, and 6245) as compared with spiked-plasmid control. (C) qPCR, using transgene-specific primers and probe, reveals that HASix1 is not expressed in the MTB +dox control animals but is expressed at low levels in the uninduced TOSix mammary glands and at high levels in the induced TOSix mammary glands. Differences in expression between –dox and +dox mammary glands are much greater than differences between transgenic lines (4922 and 6239). Values were transformed using log10(value+1) equation and plotted using a linear axis. Each point represents the value for 1 mammary gland. The middle horizontal lines represent the mean, and error bars represent mean ± SEM. Analysis was performed on multiparous animals. (D) Immunohistochemistry using Six1 antibody reveals no Six1 protein in the control MTB +dox mammary glands (No Six1), low levels of protein in the TOSix –dox mammary glands (Low Six1), and higher levels of Six1 protein in the TOSix +dox mammary glands (High Six1) (scale bar: 100 μm; original magnification, ×40). Clear nuclear staining is shown in insets at higher magnification (arrows) (original magnification, ×100).