SIX1 regulates aberrant endometrial epithelial cell differentiation and cancer latency following developmental estrogenic chemical exposure.

Early-life exposure to estrogenic chemicals can increase cancer risk, likely by disrupting normal patterns of cellular differentiation. Female mice exposed neonatally to the synthetic estrogen diethylstilbestrol (DES) develop metaplastic and neoplastic uterine changes as adults. Abnormal endometrial glands express the oncofetal protein sine oculis homeobox 1 (SIX1) and contain cells with basal (cytokeratin [CK]14+/18-) and poorly differentiated features (CK14+/18+), strongly associating SIX1 with aberrant differentiation and cancer. Here we tested whether SIX1 expression is necessary for abnormal endometrial differentiation and DES-induced carcinogenesis by using Pgr-cre to generate conditional knockout mice lacking uterine Six1 (Six1d/d). Interestingly, corn oil (CO) vehicle treated Six1d/d mice develop focal endometrial glandular dysplasia and features of carcinoma in situ as compared with CO wildtype Six1 (Six1+/+) mice. Furthermore, Six1d/d mice neonatally exposed to DES had a 42% higher incidence of endometrial cancer relative to DES Six1+/+ mice. While DES Six1d/d mice had >10-fold fewer CK14+/18- basal cells within the uterine horns as compared with DES Six1+/+ mice, the appearance of CK14+/18+ cells remained a feature of neoplastic lesions. These findings suggest that SIX1 is required for normal endometrial epithelial differentiation, CK14+/18+ cells act as a cancer progenitor population, and SIX1 delays DES-induced endometrial carcinogenesis by promoting basal differentiation of CK14+/18+ cells. In human endometrial biopsies, 35% of malignancies showed CK14+/18+ expression, which positively correlated with tumor stage and grade and was not present in normal endometrium. Implications: Aberrant epithelial differentiation is a key feature in both the DES mouse model of endometrial cancer and human endometrial cancer. The association of CK14+/18+ cells with human endometrial cancer provides a novel cancer biomarker and could lead to new therapeutic strategies.