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

这个拟议的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 雌激素受体阳性（ER+）乳腺癌。大多数人乳腺癌是ER+， 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耐药性; 2）一些患者最终进展为ER + 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，目前的体内模型 不足在乳腺癌模型下更忠实地建模ER+乳腺癌仍然是未满足的临床需求。 生理环境未能在动物中忠实地建模ER+乳腺癌（例如，小鼠）可能是由于： 1)致癌事件不是针对正确的细胞起源; 2）致癌突变特异性 与人ER+乳腺肿瘤相关的肿瘤很少在小鼠中建模。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复发性基因突变， 在人ER+乳腺肿瘤中发现，即，雌激素/ER信号传导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Cr 在角蛋白8启动子（Ad-K8-Cre）的控制下，将腺病毒（Ad-Cre）转染至携带 Runx1和p53的条件敲除等位基因。Ad-K8-Cre诱导的K8+细胞中RUNX1和p53的丢失 乳腺上皮细胞（MEC）导致ER+乳腺肿瘤的发展，具有高度活跃的核糖体 生物发生和免疫应答，这也在人RUNX1缺陷型ER+乳腺癌中观察到。 然而，由于Ad-K8-Cre靶向ER+和ER-管腔MEC两者，并且所产生的肿瘤表现出与ER+和ER-管腔MEC的相似性。 ER+和紧密连接蛋白-低的特征（即，具有间充质样外观的肿瘤细胞）癌症， 不清楚混合肿瘤表型是否是由于它们的不同细胞起源（例如，ER + vs. ER-管腔 MEC）和/或由于p53缺失（其促进细胞命运可塑性）。我们假设为了让这个 在临床上与人ER+乳腺癌更相关的模型中，它可以通过诱导RUNX1- 损失及其协同致癌事件（即，而不是p53-缺失）在ER+管腔MEC中特异性地表达（即， ER+乳腺癌的潜在细胞起源）。为此，提出了两个具体目标。要求1 将专注于开发一种新的Ad-Cre工具，以特异性地将Cre表达靶向ER+管腔亚系， 因此乳腺肿瘤发生仅从ER+管腔细胞开始。在目标2中，我们将测试是否致癌 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， 这可能会导致细胞周期停滞通过这一途径）将与RUNX1的损失，导致ER + 来自管腔MEC的乳腺肿瘤。这种模式的成功发展将提供一个 更好地了解ER+乳腺癌的生物学和治疗的宝贵工具。

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.