Development of a clinically relevant mouse model of ER+ breast cancer

这个被提议的R21项目的目的是开发和验证一种新的、临床相关的小鼠模型 用于雌激素受体阳性(ER)的乳腺癌。大多数人的乳腺癌是ER，可以用来治疗 内分泌治疗。目前治疗ER乳腺癌的临床挑战包括：1)一些患者不 对内分泌治疗有反应或产生抵抗；2)一些患者最终进展为急诊室 转移性乳腺癌。可以忠实地概括ER乳腺癌生物学的动物模型是 对于在临床前环境中应对这些挑战至关重要。然而，目前的活体模型是 不够充分。更真实地建立ER乳腺癌模型仍然是一个尚未得到满足的临床需求 生理环境。未能在动物(如小鼠)中准确地建立ER乳腺癌模型可能是由于： 1)致癌事件没有针对正确的细胞来源；2)特定的致癌突变 与人类ER相关的乳腺肿瘤很少在小鼠身上建模。考虑到这些因素，我们 最近开发了一种新的ER乳腺癌小鼠模型，该模型基于一种独特的重复基因突变 在人类ER乳腺肿瘤中发现，即雌激素/ER信号的功能丧失突变或缺失- 相关转录因子基因RUNX1。该模型是基于导管内注射表达Cre的 角蛋白8启动子控制下的重组腺病毒(Ad-Cre)对携带K8启动子的雌性小鼠的作用 RUNX1和P53的条件性基因敲除Ad-K8-Cre诱导K8细胞RUNX1和P53的缺失 乳腺上皮细胞(MECs)导致核糖体过度活跃的ER乳腺肿瘤 生物发生和免疫反应，这也在人类RUNX1缺陷的ER乳腺癌中观察到。 然而，由于Ad-K8-Cre同时针对ER和ER-腔内微血管内皮细胞，由此产生的肿瘤表现为 ER和Claudin-Low(即具有间叶样外观的肿瘤细胞)的特征，它是 不清楚混合肿瘤表型是否是由于它们不同的细胞来源(例如，ER与ER-Lumina MECs)和/或由于P53丢失(促进细胞命运可塑性)。我们假设为了使这一点 与人ER乳腺癌更具临床相关性的模型，可通过诱导RUNX1- 丢失及其协同致癌事件(即，不同于P53-丢失)，在内质网腔微血管内皮细胞(即， ER乳腺癌的潜在细胞来源)。为了解决这一问题，提出了两个具体目标。目标1 将专注于开发一种新的Ad-Cre工具，专门针对内质网腔亚系的Cre表达， 因此，乳腺肿瘤的发生只能从内质网腔细胞开始。在目标2中，我们将测试致癌因素 影响Rb-E2F途径的事件(因为过度活跃的核糖体生物发生可能导致核糖体应激， 可能通过此途径导致细胞周期停滞)会与RUNX1-Lost协同，导致内质网 来自腔内微血管内皮细胞的乳腺肿瘤。这种模式的成功开发将提供一种 为更好地了解ER乳腺癌的生物学和治疗提供了宝贵的工具。

The purpose of this proposed R21 project is to develop and validate a novel, clinically relevant mouse model for estrogen receptor-positive (ER+) breast cancer. Most human breast cancers are ER+ and are treated by endocrine therapy. Current clinical challenges for treating ER+ breast cancer include: 1) some patients do not respond to or develop resistance to endocrine therapy; 2) some patients eventually progress to ER+ metastatic breast cancer. Animal models that can faithfully recapitulate the biology of ER+ breast cancer are essential for addressing these challenges in a preclinical setting. However, current in vivo models are inadequate. It remains an unmet clinical need to more faithfully model ER+ breast cancer under the physiological setting. The failure to model ER+ breast cancer faithfully in animals (e.g., mice) may be due to: 1) oncogenic events are not targeted to the correct cellular origin; 2) oncogenic mutations specifically associated with human ER+ breast tumors are rarely modeled in mice. Taking these into consideration, we recently developed a new mouse model for ER+ breast cancer based on a recurrent genetic mutation uniquely found in human ER+ breast tumors, i.e., loss-of-function mutation or deletion of an estrogen/ER signaling- related transcription factor gene, RUNX1. The model is based on intraductal injection of a Cre-expressing adenovirus (Ad-Cre) under the control of the Keratin 8 promoter (Ad-K8-Cre) to female mice carrying conditional knockout alleles of Runx1 and p53. Ad-K8-Cre-induced loss of RUNX1 and p53 in K8+ luminal mammary epithelial cells (MECs) led to development of ER+ mammary tumors with hyperactive ribosome biogenesis and immune responses, which are also observed in human RUNX1-deficient ER+ breast cancers. However, since Ad-K8-Cre targets both ER+ and ER- luminal MECs and the resulting tumors exhibited features of both ER+ and Claudin-low (i.e., tumor cells with mesenchymal-like appearance) cancers, it is unclear whether the mixed tumor phenotype is due to their different cellular origins (e.g., ER+ vs. ER- luminal MECs) and/or due to p53-loss (which promotes cell fate plasticity). We hypothesize that in order to make this model more clinically relevant to human ER+ breast cancer, it can be further improved by inducing RUNX1- loss and its cooperating oncogenic events (i.e., other than p53-loss) specifically in ER+ luminal MECs (i.e., the potential cellular origin of ER+ breast cancer). To address this, two Specific Aims are proposed. Aim 1 will focus on developing a new Ad-Cre tool to specifically target Cre expression to the ER+ luminal sublineage, so that mammary tumorigenesis is initiated only from ER+ luminal cells. In Aim 2, we will test if oncogenic events affecting the RB-E2F pathway (as hyperactive ribosome biogenesis may lead to ribosomal stress, which may cause cell cycle arrest via this pathway) would cooperate with RUNX1-loss, leading to ER+ mammary tumors from luminal MECs. The successful development of such model would provide an invaluable tool for better understanding the biology and therapy of ER+ breast cancer.