Mammary epithelium‐specific conditional expression of human mutant Ki‐Ras in mice generates estrogen receptor alpha (ERα)‐positive mammary adenocarcinomas

Background Several genetically modified mouse models (GEMMs) have been generated for mechanistic and therapeutic studies of mammary tumors. Although a number of cancer GEMMs have been developed, an individual model may recapitulate only some aspects of this complex disease. Indeed, breast cancer features include at least five biologically and clinically distinct intrinsic subtypes, including luminal A, luminal B, human epidermal growth factor receptor 2 (HER2)‐enriched, basal‐like, and normal‐like, with luminal A and B subtypes representing ~ 70% of breast cancers, and thus being of major clinical interest. We have engineered a novel transgenic mouse line expressing human Ki‐Ras bearing an activating mutation (Ki‐Ras (G12V) ) selectively in the mammary epithelium and characterized them. Methods To allow conditional Ki‐Ras (G12V) expression in mammary epithelium, we generated C‐Ki‐Ras (G12V) transgenic mice that express, after Cre‐mediated DNA recombination during lactation, Ki‐Ras (G12V) under the control of the mammary tumor virus long terminal repeat (MMTV‐LTR) promoter. Tumor development was followed in 48 females after the start of lactation. Tumor characterization was performed by histopathological analysis, immunohistochemistry and real time PCR. Two cell lines were isolated from mouse mammary tumors and their phenotypes were evaluated by immunoblotting, ‘in vitro’ cell growth assays and ‘in vivo’ tumor xenograft studies. Results All mice developed homogenous invasive ductal adenocarcinomas with 100% incidence within 13–35 weeks after Ki‐Ras (G12V) expression. Immunohistochemical analysis revealed high levels of estrogen receptor (ER) α in more than 90% of epithelial cells and focal progesterone receptor (PR) staining in neoplastic epithelial cells, while no HER2 staining was observed. Mammary tumor cells exhibited less than 14% nuclei positive for the proliferation marker Ki‐67, a feature that can segregate the luminal A subtype. The cytokeratin profile, as well as quantification of transcript levels of specific markers in the tumor cells, also reflected a luminal cell phenotype. Cell lines isolated from such tumors have similar characteristics as the primary tumors, are estrogen‐responsive and transplantable. Importantly, palpable tumors were detected faster than those derived from the human luminal breast cancer MCF‐7 cells (one week versus one month). Conclusions MG‐Ki‐Ras (G12V) mice develop within a couple of months at full penetrance mammary tumors that recapitulate the most common human breast cancer subtype, classified as ductal ER‐positive invasive adenocarcinoma. Thus, this animal model together with the derived primary cancer cell line represent valuable tools with high potential of translational relevance to investigate the biology of estrogen‐dependent mammary tumors and to test various therapeutic agents. Support or Funding Information This work was supported by: PRIN2015 #2015B7M39T (to SA), Fondazione Italiana per la Ricerca sul Cancro (AIRC) grants: IG no. 11595 (to SA), MFAG no. 16899 (to IB), PROGRAMMA ‘FUTURO IN RICERCA’ Anno 2012 no.RBFR12FI27 (to IB), AIRC Fellowships for abroad no. 19552 (to DR), EMBO ASTF no. 18‐2010 (to RM), European Social Fund operational programme of the Calabria region (to SP), Faculté de Médecine, Université de Strasbourg (to CE) and by the Centre Nationalpour la Recherche Scientifique (CNRS), the Institut National de la Santé et de laRecherche Médicale (INSERM), the Université de Strasbourg and French state funds through the Agence Nationale de la Recherche ANR‐10‐LABX‐0030‐INRT under the frame programme Investissements d'Avenir labelled ANR‐10‐IDEX‐0002‐02. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .