Investigating a Wnt-telomere feedback loop in the colorectal adenoma-carcinoma sequence

摘要： 结直肠腺癌（CRC）是全球癌症死亡的主要原因。CRC肿瘤发生 通过结肠上皮从良性腺瘤逐步转化为侵袭性腺瘤的模型进行， carcinoma.这种遗传过程通常始于APC的功能丧失突变， Wnt信号通路，并需要p53的最终突变，肿瘤的进展和侵袭。多数 的CRC病例的特征还在于染色体不稳定性（CIN），其在早期出现并通过 肿瘤进展，并使突变驱动致癌作用。尽管实现了转型 由于CIN，CIN发生的机制尚未完全了解。强有力的候选人贡献者， 早期CRC中的CIN是端粒功能障碍，当染色体末端变短时发生， 脱帽并激活导致染色体末端融合的DNA损伤反应（DDR）。端粒 在腺瘤和癌活检中观察到侵蚀和融合， 这与端粒可能作为致癌作用的驱动因素的可能性相一致。鉴于这些 观察，更深入地了解调节端粒帽的机制及其后果， 在CRC进展过程中端粒脱帽是必要的。我们的实验室以前发现了一种新的反馈 端粒加帽和Wnt信号之间的环有助于小鼠肠道的稳态维持 还有人类通过这个环，端粒帽的丢失导致Wnt途径活性的广泛抑制 在肠腺上皮和下面的基质中。我们确定，在小鼠中，这种调节涉及p53- 介导表达miR-34 a，一种靶向Wnt通路组分的p53激活的microRNA。此外，委员会认为， Wnt途径的激活通过上调shelterin蛋白TRF 2来加强端粒加帽。 因此，肠道中的端粒加帽和Wnt信号是相互支持的，并且这种反馈的失调 CRC中的环可能允许具有功能失调的端粒的细胞。本文提出的研究旨在 了解可能损害Wnt-端粒反馈的致癌CRC突变如何影响端粒 CRC进展期间的状态，以及端粒加帽干扰促进肿瘤进展的程度 增长我的中心假设是，Wnt信号和端粒加帽之间的调节回路是 在CRC中被破坏，使得功能失调的端粒增强而不是抑制肿瘤发生。在目标1中，我将 使用人类结肠类器官模型来确定APC和p53突变如何破坏Wnt-端粒反馈。 在目标2中，我将描述这些突变与Wnt信号传导/TRF 2表达之间的关系， 人CRC样本，并确定是否在APC和p53丢失的情况下诱导端粒功能障碍 增强小鼠异种移植物中的类器官生长。综合起来，这些研究将提供前所未有的 深入了解端粒功能障碍作为早期CRC肿瘤发生的驱动因素，并揭示了关于端粒功能障碍的新信息。 结肠肿瘤发展的突变景观。

ABSTRACT: Colorectal adenocarcinoma (CRC) is a leading cause of cancer deaths globally. CRC tumorigenesis proceeds through a model of stepwise transformation of the colonic epithelium from benign adenoma to invasive carcinoma. This genetic process generally initiates from loss-of-function mutations in APC, which hyperactivate the Wnt signaling pathway, and requires eventual mutations in p53 for tumor progression and invasion. A majority of CRC cases is also characterized by chromosomal instability (CIN), which arises early and increases through tumor progression and which enables mutations that drive carcinogenesis. Despite enablement of transformation by CIN, the mechanisms by which CIN arises are not completely understood. A strong candidate contributor to CIN in early CRC is telomere dysfunction, which occurs when chromosome ends become shortened and uncapped and activate a DNA damage response (DDR) that leads to chromosome end-fusions. Telomere erosion and fusions have been observed in adenoma and carcinoma biopsies and have been shown to correlate with CIN, consistent with the possibility that telomeres may function as a driver of carcinogenesis. Given these observations, a deeper understanding of the mechanisms that regulate telomere capping and the consequences of telomere uncapping during CRC progression is needed. Our laboratory previously uncovered a novel feedback loop between telomere capping and Wnt signaling that contributes to homeostatic intestinal maintenance in mice and humans. Through this loop, loss of telomere capping leads to a broad suppression of Wnt pathway activity in intestinal crypt epithelia and underlying stroma. We determined that in mice this regulation involves p53- mediated expression miR-34a, a p53-activated microRNA which targets Wnt pathway components. Moreover, activation of the Wnt pathway reinforces telomere capping through upregulation of the shelterin protein TRF2. Thus, telomere capping and Wnt signaling in the gut are mutually supportive, and dysregulation of this feedback loop in CRC may be permissive for cells harboring dysfunctional telomeres. The study proposed herein aims to understand how oncogenic CRC mutations that might compromise Wnt-telomere feedback impact telomere status during CRC progression, as well as the extent to which perturbed telomere capping promotes tumor growth. My central hypothesis is that the regulatory loop between Wnt signaling and telomere capping is disrupted in CRC such that dysfunctional telomeres enhance, rather than inhibit, tumorigenesis. In Aim 1, I will use a human colonic organoid model to determine how APC and p53 mutations disrupt Wnt-telomere feedback. In Aim 2, I will characterize the relationship between these mutations and Wnt signaling/TRF2 expression in human CRC samples and determine whether induced telomere dysfunction in the setting of APC and p53 loss enhances organoid growth in murine xenografts. Taken together, these studies will provide unprecedented insight into telomere dysfunction as a driver of early CRC tumorigenesis and reveal new information about the mutational landscape of developing colon tumors.