Abstract A004: Expansion of immature mammary epithelial populations promotes Wnt1-driven mammary tumorigenesis in an autophagy-deficient mouse model

Autophagy is a catabolic process, whereby intracellular organelles and proteins are degraded in the lysosomes. Basal autophagy protects cellular homeostasis, whereas stress-induced autophagy acts as a temporary cell survival mechanism by internally recycling energy and amino acids. The essential autophagy regulator Beclin1 (BECN1) has been implicated as a haploinsufficient tumor suppressor in breast cancer, as 50% of breast cancers exhibit allelic BECN1 deletions and Becn1+/- mice develop mammary hyperplasias, while retaining a wild type allele. Furthermore, Becn1+/- immortalized mouse mammary epithelial cells (iMMECs) are partially autophagy-defective in response to stress and are more tumorigenic orthotopically in nude mice, in association with higher levels of DNA damage and genomic instability. Low BECN1 mRNA levels are commonly observed in basal-like human breast cancers, which often result from transformation of immature stem or progenitor cells that are negative for estrogen and progesterone receptors. A unique aspect of the mammary gland (MG) is its cyclic nature, in that it repeatedly undergoes vast epithelial cell proliferation, differentiation, and cell death. This process makes the MG heavily reliant on immature mammary cells for continued tissue repopulation. Blocks in mammary differentiation have been implicated in expansion of progenitor and stem cell populations in vivo, which potentially provide prime targets for malignant transformation. In the MMTV-Wnt1 mouse model, studies have shown that resultant tumors have increased Keratin 6 (K6)-positive cell populations and these cells are considered the tumor-initiating cells (TICs), giving rise to spontaneous mammary tumors in these mice. Further studies have revealed that K6-positive cells are bipotent mammary progenitor cells and isolation of K6a-positive cell populations have been shown to have increased tumorigenic potential, generating mammary tumors with normal-like subtype characteristics. In order to understand whether the correlation between low BECN1 and the basal-like subtype is the result of the transformation of immature stem/progenitor cell populations, we sought to characterize the differentiation status of Becn1+/- vs. Becn1+/+ mouse MGs. Examination of MGs from Becn1+/- mice has revealed enrichment in K6-positive cells, which co-localize with markers used for mammary stem cell (MaSC) and mammary progenitor cell isolation. Increases in K6 mRNA and protein levels have also been observed in both apoptosis-competent and -defective iMMECs, as well as in the resultant tumors from the orthotopic implantation of the apoptosis-defective Becn1+/- iMMECs into the mammary fat pad. In vivo and in vitro assays have revealed increased MaSC and progenitor cell populations in the mammary glands of Becn1+/- mice, suggesting that Becn1 heterozygocity, and possibly defective autophagy in general, leads to mammary TIC expansion and higher susceptibility to malignant transformation. In support of this hypothesis, we found that MMTV-Wnt1;Becn1+/- mice developed mammary tumors faster than MMTV-Wnt1;Becn1+/+ mice. Additionally, analysis of human breast cancer data sets revealed that patients having both Wnt pathway activation and low BECN1 had a shorter disease-free survival, compared to patients having Wnt pathway activation and high BECN1. Together this data suggests that allelic loss of BECN1 promotes the expansion of immature stem/progenitor cell populations in the MG, providing increased targets for oncogenic transformation. Citation Format: Michelle Cicchini, Rumela Chakarbarti, Sameera Kongara, Ritu Nahar, Fred Lozy, Alexei Vasquez, Yibin Kang, Vassiliki Karantza. Expansion of immature mammary epithelial populations promotes Wnt1-driven mammary tumorigenesis in an autophagy-deficient mouse model. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A004.