Syngeneic mammary models of intraductal carcinoma reveal progression to invasiveness in the absence of a non-obligatory in situ stage

Background: The incidence of ductal carcinoma in situ (DCIS), a pre-invasive malignancy of the breast, has increased sharply in the U.S. following routine mammographic screening. Since not all DCIS progress to invasive ductal cancer (IDC), identifying prognostic markers predictive of progression may reduce potential for overtreatment. Mammary myoepithelial cells serve a barrier function in the progression of DCIS to IDC, and associations between myoepithelial cell differentiation status, inflammation, and risk of DCIS progression are reported. However, immune-competent mouse models of DCIS are limited. To address this gap, we interrogated whether orthotopic transplant models using syngeneic cell lines in immune-competent mice recapitulate early breast cancer progression from in situ to invasive disease. Methods: : In immune-competent BALB/c and C57BL/6 mice, we used a non-surgical method to deliver six different mouse mammary tumor cell lines (D2.OR, D2A1, 4T1, EMT6, EO771, and Py230) directly into the main lactiferous duct. Mammary glands were dissected 11-35 days later and analyzed for lesion type (DCIS and IDC), myoepithelial cell differentiation state, and immune cell influx. Results: Intraductal injection of murine mammary cancer cell lines resulted in the formation of IDC at the earliest time-points in the absence of DCIS, which associated with immune cell infiltrate and loss of myoepithelial cell differentiation markers p63, SMA, and calponin. In contrast, intraductal delivery of human breast cancer cell lines (HCC70, MCF7) into immunocompromised mice readily formed DCIS-like lesions with intact myoepithelium. Meanwhile, murine mammary tumor cells intraductally injected into immunocompromised mice still failed to develop DCIS, and rapidly progressed to IDC. These data suggest that loss of the myoepithelial barrier function and progression to invasion is not exclusively dependent on an intact immune system. Conclusions: Six different mouse mammary tumor cells lines failed to form DCIS-like lesions after intraductal injection, and instead rapidly progressed to invasive disease, even in immunocompromised hosts. To interrogate the role of the immune system in DCIS progression, improved immune-competent murine models of DCIS are needed. Of potential significance, these isogenic intraductal models may be useful for the study of invasive breast cancers that occur in the absence of a non-obligatory DCIS stage.