Highly metastatic claudin-low mammary cancers can originate from luminal epithelial cells. Highly metastatic claudin-low mammary cancers can originate from luminal epithelial cells

Claudin-low breast cancer represents an aggressive molecular subtype that is comprised of mostly triple-negative mammary tumor cells that possess stem cell-like and mesenchymal features. Little is known about the cellular origin and oncogenic drivers that promote claudin-low breast cancer. In this study, we show that persistent oncogenic RAS signaling causes highly metastatic triple-negative mammary tumors in mice. More importantly, the activation of endogenous mutant KRAS and expression of exogenous KRAS specifically in luminal epithelial cells in a continuous and differentiation stage-independent manner induces preneoplastic lesions that evolve into basal-like and claudin-low mammary cancers. Further investigations demonstrate that the continuous signaling of oncogenic RAS as well as regulators of EMT play a crucial role in the cellular plasticity and maintenance of the mesenchymal and stem cell characteristics of claudin-low mammary cancer cells. Overall design: The RNA of 6 mammary tumor cell lines was sequenced. 3 off_dox cell lines that were left untreated and their 3 isogenic cell lines that were treated with Doxycycline. The 3 cell lines represent biological replicates and are derived from distinct mammary tumors. Additionally, the RNA of 10 primary mammary tumors [from the MMTV-Flp FSF-Kras(G12D) p53(R172H)] was sequenced. The tumors all originate from distinct animals and were not treated. The tumors all represent biological replicates.

低紧密连接蛋白乳腺癌（claudin-low breast cancer）属于一类侵袭性分子亚型，其肿瘤细胞绝大多数为三阴性乳腺肿瘤细胞，兼具干细胞样与间质表型。目前学界对该亚型乳腺癌的细胞起源及促癌驱动因子仍所知有限。

本研究证实，持续性致癌RAS信号通路可在小鼠体内诱导出高转移性三阴性乳腺肿瘤。尤为关键的是，以持续且不依赖分化阶段的方式，特异性在腔上皮细胞中激活内源性突变KRAS并表达外源性KRAS，可诱导癌前病变，最终进展为基底样型与低紧密连接蛋白型乳腺癌。

进一步研究表明，致癌RAS的持续信号通路及上皮间质转化（EMT, epithelial-mesenchymal transition）调控因子，在低紧密连接蛋白乳腺癌细胞的细胞可塑性维持以及其间质表型与干细胞特性的保持中发挥关键作用。

实验整体设计：本研究对6株乳腺肿瘤细胞系的RNA进行了测序：其中3株为未接受处理的off_dox细胞系，另外3株为与其同基因背景、经多西环素（Doxycycline）处理的细胞系。该3株细胞系均来自不同的原发性乳腺肿瘤，属于生物学重复样本。此外，本研究还对10株原发性乳腺肿瘤[源自MMTV-Flp FSF-Kras(G12D) p53(R172H)模型]的RNA进行了测序，这些肿瘤均来自不同个体且未接受任何处理，同样属于生物学重复样本。