Abstract 5104: Prolactin-induced mammary tumors: A preclinical model of luminal B breast cancer which exhibits mutations in the RAS pathway

Metastatic therapy-resistant breast cancers expressing estrogen receptor alpha (ER+) account for the majority of breast cancer deaths. Understanding of the underlying biology of aggressive luminal B cancers would improve preventive/treatment strategies. However, this has been limited by the paucity of preclinical immunocompetent models. Strong epidemiologic data link prolactin (PRL) exposure to development of ER+ metastatic breast cancer. We have capitalized on this relationship to generate a murine model which overexpresses PRL in mammary epithelial cells (NRL-PRL). Transgenic mammary PRL expression leads to the development of spontaneous metastatic ER+ carcinomas with characteristics of luminal B human cancers. In order to understand the mechanisms that underlie the ability of PRL to drive the development of cancers in this model, we used functional genomic analyses to identify genomic alterations and patterns of gene expression with cancer progression. Following development of spontaneous tumors, ER+ tumors and matched adjacent mammary glands and tail samples (N=5) were collected and examined by whole genome sequencing and RNA-seq analysis. A set of mammary cell preparations from the caudal glands of 12-week-old NRL-PRL females (N=5) were also analyzed. Unexpectedly, we found that all of the tumors contained somatic alterations of Kras, including four with canonical hotspot mutations (G12 and Q61) and one with an amplification of the Kras locus. In contrast to the tumors, no Kras mutations were detected in the matched mammary glands and tails, or the cell preparations. Our findings were further validated by targeted sequencing of an extension set consisting of frozen archived spontaneous ER+ tumors from NRL-PRL females (N=15), additional samples from mice with matched tumor, adjacent mammary gland and tails (N=7) and ER+ cell lines generated from PRL-induced spontaneous tumors (N=4). The Kras hotspot mutations were detected in 14 of 22 of the extension tumors and all four cell lines. One additional tumor contained a hotspot activating mutation in Nras. Immunohistochemistry demonstrated that RAS downstream effectors, pAKT and pERK, were upregulated, whereas nuclear STAT5A was decreased, in the tumors compared to the adjacent normal gland. Moreover, RNA-Seq analyses showed that acquisition of RAS mutations by the tumors was associated with alterations in multiple pathways compared to non-tumor glands, including distinct immune cell subpopulations. Although few clinical breast cancers display RAS protein mutations, studies have shown that the RAS pathway is aberrantly activated in more than 50% of all breast lesions. This enhanced RAS activity confers many benefits to these tumors resulting in a poor prognosis. The NRL-PRL model provides a valuable tool to dissect mechanisms that drive metastatic luminal B breast cancer and identify therapeutic opportunities. Citation Format: Kathleen A. O9Leary, Katie M. Campbell, Debra E. Rugowski, Kilannin Krysiak, William A. Mulligan, Malachi Griffith, Obi L. Griffith, Linda A. Schuler. Prolactin-induced mammary tumors: A preclinical model of luminal B breast cancer which exhibits mutations in the RAS pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5104.